WO2024100939A1 - Hot-rolled steel sheet, electric resistance welded steel pipe, rectangular steel pipe, line pipe, and building structure - Google Patents
Hot-rolled steel sheet, electric resistance welded steel pipe, rectangular steel pipe, line pipe, and building structure Download PDFInfo
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- WO2024100939A1 WO2024100939A1 PCT/JP2023/027298 JP2023027298W WO2024100939A1 WO 2024100939 A1 WO2024100939 A1 WO 2024100939A1 JP 2023027298 W JP2023027298 W JP 2023027298W WO 2024100939 A1 WO2024100939 A1 WO 2024100939A1
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Images
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
- B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
- B21C37/08—Making tubes with welded or soldered seams
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
- B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
- B21C37/15—Making tubes of special shape; Making tube fittings
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
Definitions
- the present invention relates to electric resistance welded steel pipes and square steel pipes, as well as hot-rolled steel sheets used as the raw materials for these pipes, and line pipes and building structures that use these pipes.
- Electric resistance welded steel pipes and roll-formed square steel pipes used in line pipes and building structures are required to have high strength to withstand the internal pressure of the fluid flowing inside and external loads. At the same time, they are also required to have high buckling resistance from the perspective of earthquake resistance.
- Electric welded steel pipes and roll-formed rectangular steel pipes are made from hot-rolled steel plate (hot-rolled steel strip). This is cold rolled to form a cylindrical open pipe, and the butt joint is electric resistance welded (sometimes referred to as electric resistance welding) to form a round steel pipe.
- Electric welded steel pipes are manufactured by adjusting the outer diameter and roundness using a forming roll placed on the outside of this round steel pipe.
- Square steel pipes are manufactured by further roll-forming this round steel pipe into a square shape using a roll with a desired polygonal hole shape. This method of manufacturing rectangular steel pipes by roll forming has the advantage of being more productive than the method of manufacturing steel pipes by press bending.
- Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent uniform elongation after cold working, which contains, by weight, 0.04-0.25% C, 0.0050-0.0150% N, and 0.003-0.050% Ti, has a carbon equivalent (Ceq.) of 0.10-0.45% calculated by a specific formula, has a pearlite phase with an area fraction of 5-20%, and further has TiN with an average particle size of 1-30 ⁇ m dispersed in the steel at a ratio of 0.0008-0.015% by weight.
- C carbon equivalent
- Patent Document 2 discloses a thick, hot-rolled steel sheet for rectangular steel pipes for architectural structural members with a low yield ratio, which has a composition, by mass%, of C: 0.07-0.18%, Mn: 0.3-1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.06%, N: 0.006% or less, with the balance being Fe and unavoidable impurities, and has a structure in which ferrite is the main phase and pearlite or pearlite and bainite are present as the second phase, the second phase frequency defined by a specific formula is 0.20-0.42, and the average crystal grain size including the main phase and the second phase is 7-15 ⁇ m.
- Patent Document 3 discloses an electric resistance welded steel pipe for line pipe with a low yield ratio, characterized in that dislocations introduced during the forming process are pinned by carbon atom clusters, fine carbides, and Nb carbides due to tempering after pipe making.
- Patent Document 4 discloses a square steel pipe with a low yield ratio, made from a hot-rolled steel sheet characterized by having ferrite as the main phase, a second phase frequency of 0.05 to 0.15, a second phase area ratio of 3 to 15%, and an average crystal grain size of the main phase and the second phase at 1/4 the thickness of the steel sheet of 10 to 25 ⁇ m.
- Patent Document 5 discloses a square steel pipe manufactured by hot forming, characterized by high deformability and toughness.
- the present invention has been made in consideration of the above circumstances, and has an object to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes. Another object of the present invention is to provide a line pipe and a building structure using the above-mentioned electric resistance welded steel pipe and square steel pipe.
- t represents the wall thickness (mm) of the electric resistance welded steel pipe or square steel pipe
- D represents the outer diameter (mm) of the electric resistance welded steel pipe
- B represents the side length (mm) of the square steel pipe
- ⁇ max represents the maximum stress intensity (N/mm 2 ) in the axial compression test.
- the hot-rolled steel plate of the above-mentioned material includes a hot-rolled steel strip.
- the buckling resistance of cold-formed electric resistance welded steel pipes and cold-formed rectangular steel pipes can be improved by giving them a low yield ratio and reducing the logarithmic standard deviation of the equivalent plastic strain distribution during deformation.
- the smaller the logarithmic standard deviation the smaller the variation in plastic strain during deformation, the more uniform the distribution of plastic strain, and the less likely strain will concentrate in a specific part, making it less likely that local buckling will occur.
- the electric resistance welded steel pipes and rectangular steel pipes can be obtained by using hot-rolled steel sheets with a small logarithmic standard deviation of the equivalent plastic strain distribution during deformation as their raw material.
- the present invention has been completed based on these findings and comprises the following: [1]
- the component composition is, in mass%, C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
- the steel structure at the center of the plate thickness is The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
- the balance is one or more selected from pearlite, martensite, and austenite,
- the average crystal grain size is 15.0 ⁇ m or less,
- the CP value calculated by the following formula (1) is 0.090 or less,
- the tensile strength is 400 MPa or more, A hot-rolled steel sheet having a yield ratio of 90% or less.
- CP total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 ⁇ m) / (total length of high-angle grain boundaries) (1)
- Nb 0.100% or less
- V 0.100% or less
- Ti 0.150% or less
- Cr 0.50% or less
- Mo 0.0% or less
- Cu 0.50% or less
- Ni 0.50% or less
- Ca 0.0050% or less
- B 0.0050% or less
- Mg 0.020% or less
- Zr 0.020% or less
- REM 0.020% or less
- the hot-rolled steel sheet according to [1] comprising one or more selected from the following: [3]
- the hot-rolled steel sheet according to [1] or [2], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is 0.70 or less.
- An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion The composition is in mass percent: C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, the balance being Fe and unavoidable impurities;
- the steel structure in the center of the wall thickness is The total volume fraction of ferrite and bainite is 70% or more and 98% or less, The balance is one or more selected from pearlite, martensite, and austenite, The average crystal grain size is 15.0 ⁇ m or less,
- the CP value calculated by the following formula (1) is 0.090 or less, The tensile strength of the base material is 400 MPa or more, An electric welded steel pipe having a base metal portion with a yield ratio of 97% or less.
- a line pipe in which the electric resistance welded steel pipe according to [4] or [5] is used.
- a line pipe in which the electric resistance welded steel pipe according to [6] is used.
- a square steel pipe having a flat portion and a corner portion, The composition is in mass percent: C: 0.030% or more and 0.300% or less, Si: 0.010% or more and 0.500% or less, Mn: 0.30% or more and 2.50% or less, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
- the steel structure in the center of the wall thickness is The total volume fraction of ferrite and bainite is 70% or more and 98% or less, The balance is one or more selected from pearlite, martensite, and austenite, The average crystal grain size is 15.0 ⁇ m or less,
- the CP value calculated by the following formula (1) is 0.090
- CP total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 ⁇ m) / (total length of high-angle grain boundaries) (1)
- Nb 0.100% or less
- V 0.100% or less
- Ti 0.150% or less
- Cr 0.50% or less
- Mo 0.0% or less
- Cu 0.50% or less
- Ni 0.50% or less
- Ca 0.0050% or less
- B 0.0050% or less
- Mg 0.020% or less
- Zr 0.020% or less
- REM 0.020% or less
- the square steel pipe according to [9] comprising one or more selected from the following: [11] A square steel pipe according to [9] or [10], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the flat portion is 0.60 or less.
- the present invention it is possible to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes. Furthermore, according to the present invention, it is possible to provide a line pipe and a building structure using the electric resistance welded steel pipe and the square steel pipe.
- FIG. 1 is a schematic diagram of a tensile test piece used for measuring the distribution of equivalent plastic strain.
- the hot-rolled steel sheet of the present invention has a composition, in mass%, of C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% to 0.100% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities.
- the steel structure at the center of the sheet thickness has a total of 70% to 98% by volume of ferrite and bainite, the balance being one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 ⁇ m or less, and the CP value calculated by the following formula (1) is 0.090 or less.
- the tensile strength is 400 MPa or more, and the yield ratio is 90% or less.
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is preferably 0.70 or less.
- the electric resistance welded steel pipe of the present invention has a base material portion and an electric resistance welded portion, and contains, by mass%, C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% to 0.0200%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, with the balance being Fe and unavoidable impurities.
- the steel structure at the center of the wall thickness has a total of 70% to 98% by volume of ferrite and bainite, with the balance being one or more types selected from pearlite, martensite, and austenite, has an average crystal grain size of 15.0 ⁇ m or less, and has a CP value calculated by the above formula (1) of 0.090 or less.
- the tensile strength of the base material is 400 MPa or more, and the yield ratio of the base material is 97% or less.
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the base material is preferably 0.60 or less.
- the square steel pipe of the present invention has a flat portion and a corner portion, and contains, by mass%, C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% to 0.0200%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, with the balance being Fe and unavoidable impurities.
- the steel structure at the center of the wall thickness is, by volume, 70% to 98% in total of ferrite and bainite, with the balance being one or more types selected from pearlite, martensite, and austenite, with an average crystal grain size of 15.0 ⁇ m or less, and a CP value calculated by the above formula (1) of 0.090 or less.
- the tensile strength of the flat plate portion is 400 MPa or more, and the yield ratio of the flat plate portion is 97% or less.
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the flat plate portion is preferably 0.60 or less.
- C 0.030% or more and 0.300% or less C is an element that increases the strength of steel by solid solution strengthening.
- C promotes the formation of pearlite, improves hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and therefore also contributes to the formation of a hard phase.
- C needs to be contained at 0.030% or more.
- the ratio of the hard phase increases and the yield ratio targeted in the present invention cannot be obtained.
- the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained.
- the C content is set to 0.030% or more and 0.300% or less.
- the C content is preferably 0.035% or more, more preferably 0.040% or more.
- the C content is preferably 0.250% or less, more preferably 0.200% or less.
- Si 0.010% or more and 0.500% or less Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, it is desirable to contain Si at 0.010% or more. However, if the Si content exceeds 0.500%, the ratio of hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Si content is set to 0.500% or less. The Si content is preferably 0.020% or more, more preferably 0.030% or more. The Si content is preferably 0.400% or less, more preferably 0.300% or less.
- Mn 0.30% or more and 2.50% or less
- Mn is an element that increases the strength of steel by solid solution strengthening.
- Mn is an element that contributes to fine structure by lowering the transformation start temperature.
- Mn needs to be contained at 0.30% or more.
- the Mn content is set to 0.30% or more and 2.50% or less.
- the Mn content is preferably 0.40% or more, more preferably 0.50% or more.
- the Mn content is preferably 2.30% or less, more preferably 2.10% or less.
- P 0.050% or less Since P segregates at grain boundaries and causes inhomogeneity in the material, it is preferable to reduce it as much as possible as an inevitable impurity, but a content of 0.050% or less is acceptable. Therefore, the P content is 0.050% or less.
- the P content is preferably 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit for the P content, but since excessive reduction leads to high smelting costs, the P content is preferably 0.002% or more.
- S 0.0200% or less S is usually present in steel as MnS, but MnS is thinly drawn in the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.0200% or less is acceptable. For this reason, the S content is 0.0200% or less.
- the S content is preferably 0.0150% or less, and more preferably 0.0100% or less. Although there is no particular lower limit for the S content, since excessive reduction leads to an increase in smelting costs, the S content is preferably 0.0002% or more.
- Al 0.005% or more and 0.100% or less
- Al is an element that acts as a strong deoxidizer when added to molten steel. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, and the amount of alumina-based inclusions increases, resulting in poor surface properties. For this reason, the Al content is set to 0.005% or more and 0.100% or less.
- the Al content is preferably 0.010% or more, and more preferably 0.020% or more. Moreover, the Al content is preferably 0.080% or less, and more preferably 0.060% or less.
- N 0.0100% or less
- N is an inevitable impurity and an element that has the effect of increasing the yield ratio by firmly fixing the movement of dislocations.
- the N content is set to 0.0100% or less.
- the N content is preferably 0.0090% or less, and more preferably 0.0080% or less. Note that excessive reduction leads to an increase in smelting costs, so the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.
- the balance can be Fe and unavoidable impurities.
- unavoidable impurities in the balance include Sn, As, Sb, Bi, Co, Pb, Zn, and O.
- this does not exclude the inclusion of 0.1% or less Sn, 0.05% or less As, Sb, and Co, and 0.005% or less Bi, Pb, Zn, and O.
- the above components are the basic composition of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in this invention.
- the properties desired in this invention can be obtained with the essential elements listed above, but the following elements can be included as necessary within the content ranges listed below.
- Nb 0.100% or less
- V 0.100% or less
- Ti 0.150% or less
- Cr 0.50% or less
- Mo 0.50% or less
- Cu 0.50% or less
- Ni 0.50% or less
- Ca 0.0050% or less
- B 0.0050% or less
- Mg 0.020% or less
- Zr 0.020% or less
- REM 0.020% or less
- Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through strengthening by precipitation, and can be contained as necessary.
- the contents of Nb, Ti, and V may be 0%, but when Nb, Ti, and V are contained, the preferred contents are Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more, respectively. More preferred contents are Nb: 0.008% or more, V: 0.008% or more, and Ti: 0.008% or more, respectively.
- excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution.
- Nb, Ti, and V when Nb, Ti, and V are contained, it is preferable to set the Nb content to 0.100% or less, the V content to 0.100% or less, and the Ti content to 0.150% or less, respectively. More preferable contents are Nb: 0.070% or less, V: 0.070% or less, and Ti: 0.110% or less, respectively. Note that when two or more selected from Nb, Ti, and V are contained, there is a risk of increasing the yield ratio and increasing the logarithmic standard deviation of the equivalent plastic strain distribution, so it is preferable to set the total content (the total content of Nb + Ti + V) to 0.150% or less.
- Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are elements that increase the hardenability of steel and increase the strength of steel, and can be contained as necessary.
- the contents of Cr and Mo may be 0%, but when Cr and Mo are contained, the preferred contents are Cr: 0.01% or more and Mo: 0.01% or more, respectively. More preferred contents are Cr: 0.10% or more and Mo: 0.10% or more, respectively.
- excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Cr and Mo are contained, it is preferable to set Cr: 0.50% or less and Mo: 0.50% or less, respectively. More preferred contents are Cr: 0.30% or less and Mo: 0.30% or less, respectively.
- Cu: 0.50% or less, Ni: 0.50% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained as necessary.
- the contents of Cu and Ni may be 0%, but when Cu and Ni are contained, the preferred contents are Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferred contents are Cu: 0.10% or more and Ni: 0.10% or more, respectively.
- excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Cu and Ni are contained, it is preferable to set Cu: 0.50% or less and Ni: 0.50% or less, respectively. More preferred contents are Cu: 0.35% or less and Ni: 0.35% or less, respectively.
- Ca 0.0050% or less
- Ca is an element that contributes to improving the ductility and toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as necessary.
- the Ca content may be 0%, but when Ca is contained, the preferred content is 0.0002% or more.
- a more preferred content is Ca: 0.0010% or more.
- the Ca content is preferably 0.0050% or less.
- a more preferred content is Ca: 0.0040% or less.
- B 0.0050% or less
- B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature.
- the content of B may be 0%, but when B is contained, the preferred content is 0.0001% or more. A more preferred content is B: 0.0005% or more. However, if the B content exceeds 0.0050%, there is a risk of an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when B is contained, the B content is preferably 0.0050% or less. A more preferred content is B: 0.0040% or less.
- Mg, Zr, and REM are elements that increase the strength of steel through grain refinement, and can be contained as necessary.
- the contents of Mg, Zr, and REM may each be 0%, but when Mg, Zr, and REM are contained, the preferred contents are Mg: 0.0005% or more, Zr: 0.0005% or more, and REM: 0.0005% or more, respectively.
- excessive content may cause an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution.
- Mg, Zr, and REM when Mg, Zr, and REM are contained, it is preferable to set the contents to Mg: 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less, respectively. More preferable contents are Mg: 0.010% or less, Zr: 0.010% or less, and REM: 0.010% or less.
- REM is a general term for 17 elements in total, including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained in the steel, and the REM content means the total content of these elements.
- the limited steel structure described below refers to the steel structure at the center of the plate thickness or the center of the wall thickness, and exists at the 1/2t position of the plate thickness.
- the 1/2t position of the plate thickness means the position 1/2 (middle) of the plate thickness t in the plate thickness direction.
- Total volume fraction of ferrite and bainite 70% or more and 98% or less
- Ferrite and bainite are soft structures, and by mixing them with other hard structures, the yield ratio can be reduced.
- the total volume fraction of ferrite and bainite needs to be 70% or more.
- the total volume fraction of ferrite and bainite is preferably 75% or more, more preferably 80% or more.
- the total volume fraction of ferrite and bainite exceeds 98%, the tensile strength targeted in the present invention cannot be obtained, so the total volume fraction of ferrite and bainite needs to be 98% or less.
- the total volume fraction of ferrite and bainite is preferably 97% or less, more preferably 95% or less.
- Remainder one or more selected from pearlite, martensite, and austenite
- Pearlite, martensite, and austenite are hard structures, and in particular they increase the strength of steel, and when mixed with soft ferrite, they can achieve a low yield ratio.
- the remainder other than ferrite and bainite is one or more selected from pearlite, martensite, and austenite.
- the total volume fraction of pearlite, martensite, and austenite is 2% or more and 30% or less.
- the total of the volume fractions is preferably 3% or more, and more preferably 5% or more.
- the total of the volume fractions is preferably 25% or less, and more preferably 20% or less.
- volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.
- Average grain size 15.0 ⁇ m or less If the average grain size of the grains exceeds 15.0 ⁇ m, the tensile strength targeted in the present invention cannot be obtained. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. This is because, when the average grain size is large, the degree of connectivity between the coarse grains increases, so that the strains generated in the coarse grains during deformation are connected to each other, and the distribution of the strain becomes more non-uniform as the deformation progresses. For this reason, the average grain size of the grains is set to 15.0 ⁇ m or less. The average grain size of the grains is preferably set to 13.0 ⁇ m or less, more preferably set to 10.0 ⁇ m or less. Since the yield ratio increases when the average grain size is small, the average grain size is preferably 2.0 ⁇ m or more. The average grain size is more preferably 3.0 ⁇ m or more.
- the CP value is a value representing the degree of connectivity between coarse grains having a grain size of 20 ⁇ m or more, and is calculated by the following formula (1).
- the larger the CP value the higher the proportion of grain boundaries between coarse crystal grains, so that the coarse grains are more connected to each other. If the CP value exceeds 0.090, the strain generated in the coarse grains during deformation will be connected to each other, and the distribution of strain will become more non-uniform as the deformation progresses, so that the logarithmic standard deviation of the equivalent plastic strain distribution, which is the object of the present invention, cannot be obtained. For this reason, the CP value is set to 0.090 or less.
- the CP value is preferably 0.080 or less, and more preferably 0.070 or less.
- the total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 ⁇ m refers to the total length of high-angle grain boundaries in the portion where crystal grains with a grain size of 20 ⁇ m or more are adjacent to each other.
- the average crystal grain size and CP value can be measured by the SEM/EBSD method, and can be measured by the method described in the examples below.
- Tensile strength of hot-rolled steel sheet 400 MPa or more If the tensile strength of the hot-rolled steel sheet is less than 400 MPa, the tensile strength of the electric resistance welded steel pipe and the tensile strength of the square steel pipe targeted in the present invention cannot be obtained. Therefore, the tensile strength of the hot-rolled steel sheet is set to 400 MPa or more.
- the tensile strength of the hot-rolled steel sheet is preferably 420 MPa or more, more preferably 450 MPa or more.
- the upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited, but as an example, the tensile strength of the hot-rolled steel sheet is 700 MPa or less.
- the yield ratio of the hot-rolled steel sheet is set to 90% or less.
- the yield ratio of the hot-rolled steel sheet is preferably 88% or less, more preferably 85% or less.
- the lower limit of the yield ratio of the hot-rolled steel sheet is not particularly limited, but as an example, the yield ratio of the hot-rolled steel sheet is 60% or more.
- the equivalent plastic strain distribution can be approximated by a logarithmic normal distribution with the horizontal axis representing equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %).
- the logarithm of the variable (horizontal axis) follows a normal distribution. Therefore, it can be approximated by a normal distribution with the horizontal axis representing the natural logarithm of equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %).
- the standard deviation at this time is defined as the "logarithmic standard deviation". The smaller the logarithmic standard deviation, the smaller the spread of the peak of the equivalent plastic strain distribution, and the more uniform the distribution of plastic strain becomes.
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain to the hot-rolled steel sheet is 0.70 or less, it becomes easier to obtain the suitable logarithmic standard deviation of the equivalent plastic strain distribution of the electric resistance welded steel pipe and the logarithmic standard deviation of the equivalent plastic strain distribution of the square steel pipe that are the objective of the present invention. Therefore, it is preferable that the logarithmic standard deviation after applying 8.0% tensile strain to the hot-rolled steel sheet is 0.70 or less.
- the logarithmic standard deviation is more preferably 0.68 or less, and even more preferably 0.65 or less. The smaller the logarithmic standard deviation, the better, and no lower limit is specified, but since excessive reduction leads to increased manufacturing costs and manufacturing load, it is preferable that the logarithmic standard deviation is 0.050 or more.
- Tensile strength of the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe 400 MPa or more If the tensile strength of the base material of the electric resistance welded steel pipe and the tensile strength of the flat plate of the square steel pipe are less than 400 MPa, the buckling resistance performance decreases. Therefore, the tensile strength is set to 400 MPa or more.
- the tensile strength is preferably 420 MPa or more, and more preferably 450 MPa or more.
- the upper limit of the tensile strength is not particularly limited, but as an example, the tensile strength is 700 MPa or less.
- the yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe 97% or less If the yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe exceed 97%, the buckling resistance performance decreases. Therefore, the yield ratio is set to 97% or less.
- the yield ratio is preferably 96% or less, and more preferably 95% or less.
- the lower limit of the yield ratio is not particularly limited, but as an example, the yield ratio is 75% or more.
- Logarithmic standard deviation of equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe 0.60 or less
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe is 0.60 or less
- the logarithmic standard deviation is more preferably 0.58 or less, and even more preferably 0.55 or less.
- the tensile strength and yield ratio can be measured by the tensile test described in the Examples below.
- the logarithmic standard deviation of the equivalent plastic strain distribution can be measured by combining the tensile test described in the Examples below and the SEM-DIC method. More specifically, the logarithmic standard deviation of the equivalent plastic strain distribution can be determined by the method described in the Examples below.
- the hot-rolled steel sheet of the present invention can be obtained, for example, by subjecting a steel material having the above-mentioned composition to a heating process in which the material is heated to a temperature of 1100°C or higher and 1300°C or lower, followed by a hot rolling process in which the material is rolled to a finish rolling end temperature of 750°C or higher and 850°C or lower, and at an average cooling rate of 1.0°C/s or higher in the temperature range of 900°C or higher at the center of the plate thickness, to obtain a hot-rolled sheet, and after the hot rolling process, a cooling process is performed in which the material is cooled at an average cooling rate of 5°C/s or higher and 50°C/s or lower from the start of cooling to the end of cooling at the center of the plate thickness, and at a cooling end temperature of 400°C or higher and 650°C or lower, and after the cooling process, a winding process is performed in which the hot-rolled sheet is wound into a coil.
- the electric resistance welded steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, butting both circumferential ends of the cylindrical shape together and electric resistance welding, and then adjusting the outer diameter and roundness by cold forming using a roll with a circular hole shape.
- the square steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, butting both circumferential ends of the cylinder and electric resistance welding them, and then forming the flat plate portion and the corner portion by cold forming using a roll having a hole shape of the desired polygonal shape.
- the square steel pipe of the present invention includes regular polygons (equilateral triangle, square, regular pentagon, etc.), equilateral polygons with different combinations of interior angles (rhombus, star, etc.), and polygons with different combinations of side lengths (isosceles triangle, rectangle, parallelogram, trapezoid, etc.).
- beams are usually joined on all four sides at 90 degree intervals, so it is preferable that the cross section is square or rectangular.
- the cylindrical shape mentioned above refers to a circumferential cross section of the pipe that is "C” shaped.
- the temperature indicated in “°C” refers to the surface temperature of the steel material or steel plate (hot-rolled plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be found by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis, and correcting the result by the surface temperature of the steel plate.
- the method of melting the steel material is not particularly limited, and any of the known melting methods such as converter, electric furnace, and vacuum melting furnace are suitable.
- the casting method is also not particularly limited, and the desired dimensions are produced by known casting methods such as continuous casting. Note that there is no problem in applying the ingot casting-blooming rolling method instead of the continuous casting method.
- the molten steel may further be subjected to secondary refining such as ladle refining.
- the obtained steel material (steel slab) is heated to a heating temperature of 1100°C or higher and 1300°C or lower, and then subjected to a hot rolling process in which the finish rolling end temperature is 750°C or higher and 850°C or lower, and the average cooling rate in the temperature range of 900°C or higher at the center of the plate thickness is 1.0°C/s or higher, to produce a hot-rolled plate.
- Heating temperature 1100°C or more and 1300°C or less
- the heating temperature is less than 1100°C
- the deformation resistance of the rolled material (steel slab) increases, making rolling difficult.
- the heating temperature exceeds 1300°C
- the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), making it difficult to ensure the average grain size aimed at in the present invention.
- the heating temperature in the heating furnace before hot rolling is 1100°C or more and 1300°C or less.
- the heating temperature is more preferably 1120°C or more.
- the heating temperature is more preferably 1280°C or less.
- the present invention can also be used to easily apply energy-saving direct rolling processes in which the hot slab is loaded into the heating furnace without being cooled to room temperature, or is rolled immediately after a short period of heat retention.
- Finish rolling end temperature 750°C or more and 850°C or less
- the finish rolling end temperature is set to 750°C or more and 850°C or less.
- the finish rolling end temperature is more preferably 760°C or more.
- the finish rolling end temperature is more preferably 840°C or less.
- Average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature 1.0°C/s or more
- the average cooling rate in hot rolling by increasing the average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature (hereinafter, sometimes referred to as the average cooling rate in hot rolling), it is possible to suppress the coarsening of austenite in the austenite recrystallization temperature range and obtain the average grain size and CP value targeted in the present invention.
- the rolled material may be cooled using water cooling equipment during rolling. If the average cooling rate is less than 1.0°C/s, austenite will coarsen in the austenite recrystallization temperature range, making it difficult to ensure the average grain size targeted in the present invention.
- the average cooling rate is preferably 1.2°C/s or more, more preferably 1.5°C/s or more. If the average cooling rate exceeds 5.0°C/s, the equipment load increases, so the average cooling rate is preferably 5.0°C/s or less.
- the average cooling rate in the temperature range where the temperature at the center of the plate thickness is 900°C or higher is calculated as the average cooling rate at the center of the plate thickness from when the steel material (steel slab) is extracted from the heating furnace until the temperature at the center of the plate thickness reaches 900°C.
- the average cooling rate is calculated as [(Temperature at the center of the plate thickness when the steel material is extracted from the heating furnace (°C) - 900 (°C)) / Time (s) from when the steel material is extracted from the heating furnace until the temperature at the center of the plate thickness of the steel material reaches 900°C].
- the upper limit of the finished plate thickness is not specified, but from the viewpoint of ensuring the necessary cooling rate and controlling the steel plate temperature, it is preferable that the thickness is 32 mm or less.
- the lower limit of the finished plate thickness is not particularly limited, but as an example, the plate thickness is 5 mm or more.
- the hot-rolled sheet is subjected to a cooling process.
- the average cooling rate from the start of cooling to the end of cooling is 5°C/s or more and 50°C/s or less, and the cooling end temperature is 400°C or more and 650°C or less.
- Average cooling rate from start of cooling to end of cooling (end of cooling) 5°C/s or more and 50°C/s or less
- the average cooling rate in the temperature range from start of cooling to end of cooling (hereinafter, sometimes referred to as the average cooling rate in the cooling process) at the center temperature of the thickness of the hot-rolled sheet is less than 5°C/s
- the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention.
- the average cooling rate is preferably 7°C/s or more, more preferably 10°C/s or more.
- the average cooling rate is preferably 45°C/s or less, more preferably 40°C/s or less.
- the start of intentional cooling such as water cooling is regarded as the start of cooling, and air cooling before that is not included in the cooling.
- Cooling stop temperature 400°C or more and 650°C or less At the thickness center temperature of the hot-rolled sheet, if the cooling stop temperature is less than 400°C, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite targeted in the present invention cannot be obtained. On the other hand, if the cooling stop temperature exceeds 650°C, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention.
- the cooling stop temperature is preferably 420°C or more, more preferably 450°C or more. In addition, the cooling stop temperature is preferably 620°C or less, more preferably 600°C or less.
- the average cooling rate in the cooling process is a value calculated by ((temperature at the center of thickness of the hot-rolled sheet before cooling - temperature at the center of thickness of the hot-rolled sheet after cooling) / cooling time).
- cooling methods include water cooling, such as spraying water from a nozzle, and cooling by spraying cooling gas.
- the hot-rolled sheet After the cooling process, the hot-rolled sheet is coiled and then cooled in the coiling process.
- the hot-rolled steel sheet of the present invention is manufactured in this manner.
- the hot-rolled steel sheet of the present invention has the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 90% or less. Furthermore, it can have the characteristics of a logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% of 0.70 or less.
- electric resistance welded steel pipes and square steel pipes manufactured using the above-mentioned hot-rolled steel plate as a material have the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 97% or less. Furthermore, they can have the characteristic of a logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain of 0.60 or less.
- the electric resistance welded steel pipes and square steel pipes of the present invention have excellent buckling resistance.
- line pipes and architectural structures using the electric resistance welded steel pipes and square steel pipes can have high buckling resistance.
- the architectural structures have high buckling resistance and can withstand external loads, making them suitable for use as pillar materials for architectural structures.
- Steel material steel slab with the composition shown in Table 1 was melted and subjected to the heating process, hot rolling process, and cooling process under the conditions shown in Table 2 to produce hot-rolled steel sheet with the finished thickness (mm) shown in Table 2.
- the hot-rolled steel sheet thus obtained was formed into a cylindrical open pipe (round steel pipe) by cold rolling, and the butt joints of the open pipe were electric resistance welded to produce steel pipe material.
- the steel pipe material was then formed using rolls arranged above, below, left and right, to obtain electric resistance welded steel pipes with the outer diameter D (mm) and wall thickness t (mm) shown in Table 3, or square steel pipes with the side length B (mm) and wall thickness t (mm) shown in Table 3.
- the cross-sectional shape of the square steel pipes is square.
- Test pieces were taken from the obtained hot-rolled steel sheets, electric resistance welded steel pipes, and square steel pipes, and the following structural observations, tensile tests, and equivalent plastic strain distribution measurements were carried out.
- the test pieces for microstructure observation were prepared by taking the specimens so that the observation surface was a cross section in the rolling direction during hot rolling and at the 1/2t position of the plate thickness, polishing them, and then etching them with nital.
- an optical microscope magnification: 1000 times
- a scanning electron microscope SEM, magnification: 1000 times
- the area ratios of ferrite, bainite, and the remaining structure pearlite, martensite, austenite
- the area ratios of each structure were calculated as the average value of the values obtained in five visual fields by observing the structure.
- the area ratios obtained by the microstructure observation were taken as the volume ratios of each structure.
- ferrite is a product of diffusion transformation, and has a low dislocation density and a nearly restored structure.
- Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density.
- Pearlite is a structure in which cementite and ferrite are arranged in layers. Austenite does not contain cement, unlike bainite. Martensite and austenite were distinguished from each other by the brighter contrast of the SEM image compared to bainite.
- the austenite volume fraction was measured by X-ray diffraction. Test pieces for microstructural observation were prepared by grinding so that the diffraction surface was at 1/2t of the plate thickness, then chemically polishing to remove the surface treatment layer. Mo K ⁇ radiation was used for the measurement, and the austenite volume fraction was calculated from the integrated intensity of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.
- the average grain size and CP value were measured using the SEM/EBSD method.
- the measurement area was 500 ⁇ m x 500 ⁇ m, and the measurement step size was 0.5 ⁇ m.
- the crystal orientation analysis software OIM Analysis (trademark) was used to obtain the grain boundary distribution, with boundaries with an orientation difference of 15° or more considered as grain boundaries (high-angle grain boundaries).
- the average grain size was calculated as the arithmetic mean of the circle equivalent diameter (grain size) of each grain.
- the CP value was calculated by calculating the total length of the high-angle grain boundaries in the area excluding grains with a grain size of less than 20 ⁇ m, and the total length of the high-angle grain boundaries, and then calculating the ratio of these.
- the total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 ⁇ m is the sum of the lengths of high-angle grain boundaries measured in the region excluding crystal grains with a grain size of less than 20 ⁇ m from the distribution of the grain boundaries in the measurement region
- the total length of high-angle grain boundaries is the sum of the lengths of high-angle grain boundaries measured from the distribution of the grain boundaries in the measurement region. Note that, in calculating the average grain size and CP value, grains with a grain size of 2.0 ⁇ m or less were excluded as measurement noise.
- the tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011), and the yield stress ⁇ y and tensile strength were measured, and the yield ratio defined as (yield stress ⁇ y)/(tensile strength) was calculated.
- the DIC method is a method of measuring the displacement and strain at various points on the observation surface by comparing the random patterns on the surface of an object before and after deformation. Specifically, a square area called a subset is defined in the image before deformation, and the subset is tracked before and after deformation based on the random pattern inside the subset, and the displacement of the center point of the subset is calculated. This operation is performed comprehensively throughout the entire image to obtain the displacement distribution and strain distribution.
- the nital corrosion marks of the metal structure are used as a random pattern
- the subset size is 80 pixels x 80 pixels (3.6 ⁇ m x 3.6 ⁇ m) and the measurement interval is 10 pixels (0.45 ⁇ m) for an image of 1910 pixels x 2560 pixels.
- the horizontal axis represents the natural logarithm of the obtained equivalent plastic strain (unit: none), and the vertical axis represents the ratio (area ratio) (unit: %).
- the standard deviation at this time was approximated by a normal distribution, and the logarithmic standard deviation (logarithmic standard deviation of the equivalent plastic strain distribution) was used. Specifically, the logarithmic standard deviation was calculated by the following method. First, the ratio (area ratio) (unit: %) of each class was calculated with a class width of 0.02 within the range of equivalent plastic strain from 0 to 0.20.
- the class with equivalent plastic strain of 0 to less than 0.02 was set as the first class
- the class with equivalent plastic strain of 0 to less than 0.02 was set as the second class
- the class with equivalent plastic strain of 0 to less than 0.20 was set as the tenth class.
- the logarithmic standard deviation was calculated by the following formulas (4) and (5), with x i as the natural logarithm of the class value of the i-th class and x 0 as the average value of the natural logarithm of the equivalent plastic strain.
- Nos. 1 to 6 are examples of the present invention, and Nos. 7 to 12 are comparative examples.
- the hot-rolled steel plate of the present invention had a steel structure at the center of the plate thickness with a total volume fraction of ferrite and bainite of 70% to 98%, with the remainder consisting of one or more types selected from pearlite, martensite and austenite, an average crystal grain size of 15.0 ⁇ m or less, and a CP value calculated by the specified formula (1) of 0.090 or less.
- the tensile strength was 400 MPa or more, and the yield ratio was 90% or less.
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% was 0.70 or less.
- the electric resistance welded steel pipe and square steel pipe of the present invention were manufactured from the hot rolled steel plate of the present invention, and the steel structure at the center of the wall thickness had a total volume fraction of ferrite and bainite of 70% to 98%, with the remainder consisting of one or more types selected from pearlite, martensite and austenite, an average crystal grain size of 15.0 ⁇ m or less, and a CP value calculated by the specified formula (1) of 0.090 or less. Furthermore, the tensile strength of the base material or flat plate was 400 MPa or more, and the yield ratio of the base material or flat plate was 97% or less.
- the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the base material or flat plate was 0.60 or less.
- Table 5 the values obtained by substituting t and D for electric resistance welded steel pipes and t and B for square steel pipes into the right-hand sides of the above equations (2) and (3) are listed as the required lower limit value of ⁇ .
- Comparative example No. 8 had a C content exceeding the range of the present invention, so the total volume ratio of ferrite and bainite was below the range of the present invention. As a result, the yield ratio was outside the range of the present invention, and the logarithmic standard deviation was outside the preferred range, so the yield strength increase rate did not reach the desired value.
- Comparative example No. 9 had a Si and Mn content below the range of the present invention, so the total volume fraction of ferrite and bainite exceeded the range of the present invention, and the average crystal grain size exceeded the range of the present invention. As a result, the tensile strength was outside the range of the present invention.
- Comparative example No. 10 had a Si and Mn content exceeding the range of the present invention, so the total volume fraction of ferrite and bainite was below the range of the present invention. As a result, the yield ratio was outside the range of the present invention, and the logarithmic standard deviation was outside the preferred range, so the yield strength increase rate did not reach the desired value.
- Comparative example No. 12 had a cooling stop temperature in the hot rolling process that exceeded the range of the preferred manufacturing method, so the CP value exceeded the range of the present invention. As a result, the logarithmic standard deviation exceeded the preferred range of the present invention, and the yield strength increase rate did not reach the desired value.
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Abstract
The present invention provides an electric resistance welded steel pipe and a rectangular steel pipe which have excellent anti-buckling performance, a hot-rolled steel sheet which is used as a material for the same, and a line pipe and a building structure which use the same. Provided is a hot-rolled steel sheet having a specific component composition, wherein: in the steel structure in the plate thickness center, the total of ferrite and bainite by volume ratio is 70-98%, with the remainder being one or more structures selected from pearlite, martensite, and austenite; the average crystal grain size is not more than 15.0 μm; the value of CP as determined by a specific formula is not more than 0.090; tensile strength is not less than 400 MPa; and yield ratio is not more than 90%. Also provided are an electric resistance welded steel pipe and a rectangular steel pipe which use the hot-rolled steel sheet, and a line pipe and a building structure which use these.
Description
本発明は、電縫鋼管および角形鋼管、ならびにそれらの素材として用いられる熱延鋼板、ならびにそれらを用いたラインパイプおよび建築構造物に関する。
The present invention relates to electric resistance welded steel pipes and square steel pipes, as well as hot-rolled steel sheets used as the raw materials for these pipes, and line pipes and building structures that use these pipes.
ラインパイプや建築構造物に用いられる電縫鋼管およびロール成形角形鋼管には、内部を流れる流体の内圧や、外部からの荷重に耐えるために、高い強度を備えることが求められる。同時に、耐震性の観点から高い耐座屈性能を備えることも求められる。
Electric resistance welded steel pipes and roll-formed square steel pipes used in line pipes and building structures are required to have high strength to withstand the internal pressure of the fluid flowing inside and external loads. At the same time, they are also required to have high buckling resistance from the perspective of earthquake resistance.
電縫鋼管、およびロール成形角形鋼管(以下、「角形鋼管」と称する場合もある)は、熱延鋼板(熱延鋼帯)を素材とする。これを冷間でロール成形することによって円筒状のオープン管とし、突合せ部を電縫溶接(電気抵抗溶接と称する場合もある。)することで丸型の鋼管とする。電縫鋼管は、この丸型の鋼管の外側に配置された成形ロールによって外径および真円度が調整され、製造される。角形鋼管は、この丸型の鋼管を目的の多角形形状の孔形を有するロールによってさらに角形にロール成形することにより製造される。このロール成形による角形鋼管の製造方法は、プレス曲げ成形による鋼管の製造方法と比較して生産性が高いという利点がある。しかし、ロール成形の際には管軸方向に大きな引張ひずみが付与されるため、電縫鋼管およびロール成形角形鋼管は管軸方向の延性が低く、耐座屈性能が低いという問題がある。また、ロール成形を施す素材には、ロール成形による延性の低下を考慮して適切な熱延鋼板(熱延鋼帯)を選択することが要求される。
Electrical welded steel pipes and roll-formed rectangular steel pipes (hereinafter sometimes referred to as "rectangular steel pipes") are made from hot-rolled steel plate (hot-rolled steel strip). This is cold rolled to form a cylindrical open pipe, and the butt joint is electric resistance welded (sometimes referred to as electric resistance welding) to form a round steel pipe. Electric welded steel pipes are manufactured by adjusting the outer diameter and roundness using a forming roll placed on the outside of this round steel pipe. Square steel pipes are manufactured by further roll-forming this round steel pipe into a square shape using a roll with a desired polygonal hole shape. This method of manufacturing rectangular steel pipes by roll forming has the advantage of being more productive than the method of manufacturing steel pipes by press bending. However, since a large tensile strain is applied in the axial direction of the pipe during roll forming, electric welded steel pipes and roll-formed rectangular steel pipes have a problem of low ductility in the axial direction and low buckling resistance. In addition, it is required to select an appropriate hot-rolled steel plate (hot-rolled steel strip) for the material to be rolled, taking into account the decrease in ductility caused by roll forming.
さらに、電縫鋼管およびロール成形角形鋼管は肉厚が大きいほどロール成形時の加工ひずみが大きくなるため、延性はより低下し、耐座屈性能はより低下する。
Furthermore, the thicker the wall of electric resistance welded steel pipes and roll-formed square steel pipes, the greater the processing strain during roll forming, resulting in a further decrease in ductility and a further decrease in buckling resistance.
このような要求に対し、例えば、特許文献1には、重量でC:0.04~0.25%、N:0.0050~0.0150%およびTi:0.003~0.050%を含有し、かつ所定の式で求められる炭素当量(Ceq.)が0.10~0.45%の鋼であって、かつパーライト相が面積分率で5~20%の範囲にあり、さらに鋼中に粒径の平均が1~30μmのTiNが重量で0.0008~0.015%の割合で分散させた、冷間加工後の一様伸びに優れる高強度熱延鋼板が開示されている。
In response to such demands, for example, Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent uniform elongation after cold working, which contains, by weight, 0.04-0.25% C, 0.0050-0.0150% N, and 0.003-0.050% Ti, has a carbon equivalent (Ceq.) of 0.10-0.45% calculated by a specific formula, has a pearlite phase with an area fraction of 5-20%, and further has TiN with an average particle size of 1-30 μm dispersed in the steel at a ratio of 0.0008-0.015% by weight.
特許文献2には、質量%で、C:0.07~0.18%、Mn:0.3~1.5%、P:0.03%以下、S:0.015%以下、Al:0.01~0.06%、N:0.006%以下を含み、残部Feおよび不可避的不純物からなる組成と、フェライトを主相とし、第二相として、パーライト、または、パーライトおよびベイナイトを有し、所定の式で定義される第二相頻度が0.20~0.42であり、主相と第二相とを含む平均結晶粒径が7~15μmである組織を有する、低降伏比の建築構造部材向け角形鋼管用厚肉熱延鋼板が開示されている。
Patent Document 2 discloses a thick, hot-rolled steel sheet for rectangular steel pipes for architectural structural members with a low yield ratio, which has a composition, by mass%, of C: 0.07-0.18%, Mn: 0.3-1.5%, P: 0.03% or less, S: 0.015% or less, Al: 0.01-0.06%, N: 0.006% or less, with the balance being Fe and unavoidable impurities, and has a structure in which ferrite is the main phase and pearlite or pearlite and bainite are present as the second phase, the second phase frequency defined by a specific formula is 0.20-0.42, and the average crystal grain size including the main phase and the second phase is 7-15 μm.
特許文献3には、造管後の焼戻しにより、成形過程で導入された転位が炭素原子クラスター、微細炭化物、及びNb炭化物によりピンニングされていることを特徴とする、低降伏比のラインパイプ用電縫鋼管が開示されている。
Patent Document 3 discloses an electric resistance welded steel pipe for line pipe with a low yield ratio, characterized in that dislocations introduced during the forming process are pinned by carbon atom clusters, fine carbides, and Nb carbides due to tempering after pipe making.
特許文献4には、フェライトを主相とし、第二相頻度が0.05~0.15であり、かつ、第二相面積率が3~15%であり、鋼板の1/4厚における主相と第二相の平均結晶粒径が10~25μmであることを特徴とする熱延鋼板を素材とした、低降伏比の角形鋼管が開示されている。
Patent Document 4 discloses a square steel pipe with a low yield ratio, made from a hot-rolled steel sheet characterized by having ferrite as the main phase, a second phase frequency of 0.05 to 0.15, a second phase area ratio of 3 to 15%, and an average crystal grain size of the main phase and the second phase at 1/4 the thickness of the steel sheet of 10 to 25 μm.
特許文献5には、熱間成形により製造され、高い変形性能と靭性を有することを特徴とした角形鋼管が開示されている。
Patent Document 5 discloses a square steel pipe manufactured by hot forming, characterized by high deformability and toughness.
しかしながら、これらの技術は引張変形時の特性、すなわち引張部のくびれや破断の抑制に関して検討されたものであり、電縫鋼管および角形鋼管の曲げ変形や圧縮変形における局部座屈に関する検討は十分にはなされていなかった。
However, these technologies were developed with a view to preventing necking and breakage in tensile sections, and no sufficient research was done into local buckling during bending or compressive deformation of electric resistance welded steel pipes and square steel pipes.
また、特許文献3および5のように、造管後に熱処理を施した鋼管や、熱間成形により製造した鋼管は、降伏伸びが大きいため、不均一変形が生じやすく、耐座屈性能を十分に発揮できなかった。
Furthermore, as in Patent Documents 3 and 5, steel pipes that have been heat-treated after pipe-making or steel pipes manufactured by hot forming have a large yield elongation, so they are prone to non-uniform deformation and are unable to fully demonstrate their buckling resistance.
本発明は、上記の事情を鑑みてなされたものであり、耐座屈性能に優れた電縫鋼管および角形鋼管、ならびにそれらの素材として用いられる熱延鋼板を提供することを目的とする。
また、本発明は、上記電縫鋼管および角形鋼管を用いたラインパイプおよび建築構造物を提供することを目的とする。 The present invention has been made in consideration of the above circumstances, and has an object to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes.
Another object of the present invention is to provide a line pipe and a building structure using the above-mentioned electric resistance welded steel pipe and square steel pipe.
また、本発明は、上記電縫鋼管および角形鋼管を用いたラインパイプおよび建築構造物を提供することを目的とする。 The present invention has been made in consideration of the above circumstances, and has an object to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes.
Another object of the present invention is to provide a line pipe and a building structure using the above-mentioned electric resistance welded steel pipe and square steel pipe.
ここで、本発明でいう「耐座屈性能に優れる」とは、軸圧縮試験における耐力上昇率τ(=σmax/σy)が、電縫鋼管においてはτ≧4.0×(t/D)+0.85を、角形鋼管においてはτ≧3.0×(t/B)+0.85をそれぞれ満足することを指す。ただし、tは電縫鋼管または角形鋼管の肉厚(mm)、Dは電縫鋼管の外径(mm)、Bは角形鋼管の辺長(mm)、σyは電縫鋼管の母材部または角形鋼管の平板部の降伏応力(N/mm2(=MPa))、σmaxは軸圧縮試験における最大応力度(N/mm2)をそれぞれ表す。ただし、角形鋼管の断面形状が、異なる辺長の多角形である場合は、各辺長の平均値を、角形鋼管の辺長Bとする。なお、本発明では、上記素材の熱延鋼板には熱延鋼帯を含むものとする。
Here, "excellent buckling resistance" in the present invention means that the yield strength increase rate τ (=σmax/σy) in the axial compression test satisfies τ≧4.0×(t/D)+0.85 for electric resistance welded steel pipes and τ≧3.0×(t/B)+0.85 for square steel pipes. Here, t represents the wall thickness (mm) of the electric resistance welded steel pipe or square steel pipe, D represents the outer diameter (mm) of the electric resistance welded steel pipe, B represents the side length (mm) of the square steel pipe, σy represents the yield stress (N/mm 2 (=MPa)) of the base material part of the electric resistance welded steel pipe or the flat part of the square steel pipe, and σmax represents the maximum stress intensity (N/mm 2 ) in the axial compression test. However, when the cross-sectional shape of the square steel pipe is a polygon with different side lengths, the average value of each side length is taken as the side length B of the square steel pipe. In the present invention, the hot-rolled steel plate of the above-mentioned material includes a hot-rolled steel strip.
上記課題を解決すべく鋭意検討を行った結果、冷間成形電縫鋼管および冷間成形角形鋼管を低降伏比とし、かつ、変形時の相当塑性ひずみ分布の対数標準偏差を小さくすることで、それらの耐座屈性能を向上させることができることを見出した。すなわち、前記対数標準偏差が小さいほど、変形時の塑性ひずみのばらつきが小さく、塑性ひずみが均一に分布し、特定の部分にひずみが集中し難いため、局部座屈が生じにくいことを知見した。また、前記電縫鋼管および角形鋼管は、変形時の相当塑性ひずみ分布の対数標準偏差が小さい熱延鋼板を素材とすることで得られることも知見した。
本発明は、これらの知見に基づいて完成されたものであり、下記の要旨からなる。
[1] 成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
板厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
引張強度が400MPa以上であり、
降伏比が90%以下である、熱延鋼板。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
[2] 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、[1]に記載の熱延鋼板。
[3] 8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.70以下である、[1]または[2]に記載の熱延鋼板。
[4] 母材部と電縫溶接部を有する電縫鋼管であって、
成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
肉厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
母材部の引張強度が400MPa以上であり、
母材部の降伏比が97%以下である、電縫鋼管。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
[5] 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、[4]に記載の電縫鋼管。
[6] 母材部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である、[4]または[5]に記載の電縫鋼管。
[7] [4]または[5]に記載の電縫鋼管が使用されている、ラインパイプ。
[8] [6]に記載の電縫鋼管が使用されている、ラインパイプ。
[9] 平板部と角部を有する角形鋼管であって、
成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
肉厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
平板部の引張強度が400MPa以上であり、
平板部の降伏比が97%以下である、角形鋼管。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
[10] 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、[9]に記載の角形鋼管。
[11] 平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である、[9]または[10]に記載の角形鋼管。
[12] [9]または[10]に記載の角形鋼管が、柱材として使用されている、建築構造物。
[13] [11]に記載の角形鋼管が、柱材として使用されている、建築構造物。 As a result of intensive research to solve the above problems, it was found that the buckling resistance of cold-formed electric resistance welded steel pipes and cold-formed rectangular steel pipes can be improved by giving them a low yield ratio and reducing the logarithmic standard deviation of the equivalent plastic strain distribution during deformation. In other words, it was found that the smaller the logarithmic standard deviation, the smaller the variation in plastic strain during deformation, the more uniform the distribution of plastic strain, and the less likely strain will concentrate in a specific part, making it less likely that local buckling will occur. It was also found that the electric resistance welded steel pipes and rectangular steel pipes can be obtained by using hot-rolled steel sheets with a small logarithmic standard deviation of the equivalent plastic strain distribution during deformation as their raw material.
The present invention has been completed based on these findings and comprises the following:
[1] The component composition is, in mass%,
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
The steel structure at the center of the plate thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength is 400 MPa or more,
A hot-rolled steel sheet having a yield ratio of 90% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[2] In addition to the above-mentioned component composition, further, in mass%,
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The hot-rolled steel sheet according to [1], comprising one or more selected from the following:
[3] The hot-rolled steel sheet according to [1] or [2], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is 0.70 or less.
[4] An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the base material is 400 MPa or more,
An electric welded steel pipe having a base metal portion with a yield ratio of 97% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[5] In addition to the above-mentioned component composition, further, in mass%,
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The electric welded steel pipe according to [4], comprising one or more selected from the following:
[6] An electric resistance welded steel pipe according to [4] or [5], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the base metal portion is 0.60 or less.
[7] A line pipe, in which the electric resistance welded steel pipe according to [4] or [5] is used.
[8] A line pipe, in which the electric resistance welded steel pipe according to [6] is used.
[9] A square steel pipe having a flat portion and a corner portion,
The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the flat portion is 400 MPa or more,
A square steel pipe having a yield ratio of 97% or less in the flat portion.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[10] In addition to the above-mentioned component composition, further, in mass%,
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The square steel pipe according to [9], comprising one or more selected from the following:
[11] A square steel pipe according to [9] or [10], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the flat portion is 0.60 or less.
[12] An architectural structure in which the square steel pipe according to [9] or [10] is used as a pillar material.
[13] An architectural structure in which the square steel pipe according to [11] is used as a pillar material.
本発明は、これらの知見に基づいて完成されたものであり、下記の要旨からなる。
[1] 成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
板厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
引張強度が400MPa以上であり、
降伏比が90%以下である、熱延鋼板。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
[2] 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、[1]に記載の熱延鋼板。
[3] 8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.70以下である、[1]または[2]に記載の熱延鋼板。
[4] 母材部と電縫溶接部を有する電縫鋼管であって、
成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
肉厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
母材部の引張強度が400MPa以上であり、
母材部の降伏比が97%以下である、電縫鋼管。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
[5] 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、[4]に記載の電縫鋼管。
[6] 母材部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である、[4]または[5]に記載の電縫鋼管。
[7] [4]または[5]に記載の電縫鋼管が使用されている、ラインパイプ。
[8] [6]に記載の電縫鋼管が使用されている、ラインパイプ。
[9] 平板部と角部を有する角形鋼管であって、
成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
肉厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
平板部の引張強度が400MPa以上であり、
平板部の降伏比が97%以下である、角形鋼管。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
[10] 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、[9]に記載の角形鋼管。
[11] 平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である、[9]または[10]に記載の角形鋼管。
[12] [9]または[10]に記載の角形鋼管が、柱材として使用されている、建築構造物。
[13] [11]に記載の角形鋼管が、柱材として使用されている、建築構造物。 As a result of intensive research to solve the above problems, it was found that the buckling resistance of cold-formed electric resistance welded steel pipes and cold-formed rectangular steel pipes can be improved by giving them a low yield ratio and reducing the logarithmic standard deviation of the equivalent plastic strain distribution during deformation. In other words, it was found that the smaller the logarithmic standard deviation, the smaller the variation in plastic strain during deformation, the more uniform the distribution of plastic strain, and the less likely strain will concentrate in a specific part, making it less likely that local buckling will occur. It was also found that the electric resistance welded steel pipes and rectangular steel pipes can be obtained by using hot-rolled steel sheets with a small logarithmic standard deviation of the equivalent plastic strain distribution during deformation as their raw material.
The present invention has been completed based on these findings and comprises the following:
[1] The component composition is, in mass%,
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
The steel structure at the center of the plate thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength is 400 MPa or more,
A hot-rolled steel sheet having a yield ratio of 90% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[2] In addition to the above-mentioned component composition, further, in mass%,
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The hot-rolled steel sheet according to [1], comprising one or more selected from the following:
[3] The hot-rolled steel sheet according to [1] or [2], wherein the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is 0.70 or less.
[4] An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the base material is 400 MPa or more,
An electric welded steel pipe having a base metal portion with a yield ratio of 97% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[5] In addition to the above-mentioned component composition, further, in mass%,
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The electric welded steel pipe according to [4], comprising one or more selected from the following:
[6] An electric resistance welded steel pipe according to [4] or [5], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the base metal portion is 0.60 or less.
[7] A line pipe, in which the electric resistance welded steel pipe according to [4] or [5] is used.
[8] A line pipe, in which the electric resistance welded steel pipe according to [6] is used.
[9] A square steel pipe having a flat portion and a corner portion,
The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the flat portion is 400 MPa or more,
A square steel pipe having a yield ratio of 97% or less in the flat portion.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
[10] In addition to the above-mentioned component composition, further, in mass%,
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The square steel pipe according to [9], comprising one or more selected from the following:
[11] A square steel pipe according to [9] or [10], in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the flat portion is 0.60 or less.
[12] An architectural structure in which the square steel pipe according to [9] or [10] is used as a pillar material.
[13] An architectural structure in which the square steel pipe according to [11] is used as a pillar material.
本発明によれば、耐座屈性能に優れた電縫鋼管および角形鋼管、ならびにそれらの素材として用いられる熱延鋼板を提供することができる。
また、本発明によれば、前記電縫鋼管および角形鋼管を用いたラインパイプおよび建築構造物を提供することができる。 According to the present invention, it is possible to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes.
Furthermore, according to the present invention, it is possible to provide a line pipe and a building structure using the electric resistance welded steel pipe and the square steel pipe.
また、本発明によれば、前記電縫鋼管および角形鋼管を用いたラインパイプおよび建築構造物を提供することができる。 According to the present invention, it is possible to provide electric resistance welded steel pipes and square steel pipes having excellent buckling resistance, as well as hot-rolled steel sheets used as raw materials for these pipes.
Furthermore, according to the present invention, it is possible to provide a line pipe and a building structure using the electric resistance welded steel pipe and the square steel pipe.
以下に、本発明を詳細に説明する。
The present invention is described in detail below.
本発明の熱延鋼板は、質量%で、C:0.030%以上0.300%以下、Si:0.010%以上0.500%以下、Mn:0.30%以上2.50%以下、P:0.050%以下、S:0.0200%以下、Al:0.005%以上0.100%以下、N:0.0100%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成を有する。板厚中央の鋼組織は、体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、平均結晶粒径が15.0μm以下であり、下記(1)式で求められるCPの値が0.090以下である。また、引張強度が400MPa以上であり、降伏比が90%以下である。また、8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が好ましくは0.70以下である。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1) The hot-rolled steel sheet of the present invention has a composition, in mass%, of C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% to 0.100% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities. The steel structure at the center of the sheet thickness has a total of 70% to 98% by volume of ferrite and bainite, the balance being one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value calculated by the following formula (1) is 0.090 or less. In addition, the tensile strength is 400 MPa or more, and the yield ratio is 90% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is preferably 0.70 or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1) The hot-rolled steel sheet of the present invention has a composition, in mass%, of C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% or less, S: 0.0200% or less, Al: 0.005% to 0.100% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities. The steel structure at the center of the sheet thickness has a total of 70% to 98% by volume of ferrite and bainite, the balance being one or more selected from pearlite, martensite, and austenite, the average grain size is 15.0 μm or less, and the CP value calculated by the following formula (1) is 0.090 or less. In addition, the tensile strength is 400 MPa or more, and the yield ratio is 90% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is preferably 0.70 or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
本発明の電縫鋼管は、母材部と電縫溶接部を有し、質量%で、C:0.030%以上0.300%以下、Si:0.010%以上0.500%以下、Mn:0.30%以上2.50%以下、P:0.050%以下、S:0.0200%以下、Al:0.005%以上0.100%以下、N:0.0100%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成を有する。肉厚中央の鋼組織は、体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、平均結晶粒径が15.0μm以下であり、上述の(1)式で求められるCPの値が0.090以下である。また、母材部の引張強度が400MPa以上であり、母材部の降伏比が97%以下である。また、母材部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が好ましくは0.60以下である。
The electric resistance welded steel pipe of the present invention has a base material portion and an electric resistance welded portion, and contains, by mass%, C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% to 0.0200%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, with the balance being Fe and unavoidable impurities. The steel structure at the center of the wall thickness has a total of 70% to 98% by volume of ferrite and bainite, with the balance being one or more types selected from pearlite, martensite, and austenite, has an average crystal grain size of 15.0 μm or less, and has a CP value calculated by the above formula (1) of 0.090 or less. In addition, the tensile strength of the base material is 400 MPa or more, and the yield ratio of the base material is 97% or less. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the base material is preferably 0.60 or less.
本発明の角形鋼管は、平板部と角部を有し、質量%で、C:0.030%以上0.300%以下、Si:0.010%以上0.500%以下、Mn:0.30%以上2.50%以下、P:0.050%以下、S:0.0200%以下、Al:0.005%以上0.100%以下、N:0.0100%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成を有する。肉厚中央の鋼組織は、体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、平均結晶粒径が15.0μm以下であり、上述の(1)式で求められるCPの値が0.090以下である。また、平板部の引張強度が400MPa以上であり、平板部の降伏比が97%以下である。また、平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が好ましくは0.60以下である。
The square steel pipe of the present invention has a flat portion and a corner portion, and contains, by mass%, C: 0.030% to 0.300%, Si: 0.010% to 0.500%, Mn: 0.30% to 2.50%, P: 0.050% to 0.0200%, Al: 0.005% to 0.100%, N: 0.0100% to 0.0100%, with the balance being Fe and unavoidable impurities. The steel structure at the center of the wall thickness is, by volume, 70% to 98% in total of ferrite and bainite, with the balance being one or more types selected from pearlite, martensite, and austenite, with an average crystal grain size of 15.0 μm or less, and a CP value calculated by the above formula (1) of 0.090 or less. In addition, the tensile strength of the flat plate portion is 400 MPa or more, and the yield ratio of the flat plate portion is 97% or less. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the flat plate portion is preferably 0.60 or less.
まず、本発明において、熱延鋼板、電縫鋼管および角形鋼管の成分組成を限定した理由を以下に説明する。本明細書において、特に断りがない限り、各成分の含有量を示す「%」は、「質量%」を意味する。
First, the reasons for limiting the chemical composition of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in the present invention will be explained below. In this specification, unless otherwise specified, "%" indicating the content of each component means "mass %."
C:0.030%以上0.300%以下
Cは、固溶強化により鋼の強度を上昇させる元素である。また、Cは、パーライトの生成を促進し、かつ焼入れ性を高めてマルテンサイトの生成に寄与し、かつオーステナイトの安定化に寄与することから、硬質相の形成にも寄与する元素である。本発明で目的とする強度を確保するため、Cは0.030%以上含有することを必要とする。しかし、C含有量が0.300%を超えると、硬質相の割合が高くなり本発明で目的とする降伏比が得られない。さらに、引張ひずみ付与時の変形中のひずみ分布が不均一となり、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、C含有量は0.030%以上0.300%以下とする。C含有量は、好ましくは0.035%以上であり、より好ましくは0.040%以上である。また、C含有量は、好ましくは0.250%以下であり、より好ましくは0.200%以下である。 C: 0.030% or more and 0.300% or less C is an element that increases the strength of steel by solid solution strengthening. In addition, C promotes the formation of pearlite, improves hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and therefore also contributes to the formation of a hard phase. In order to ensure the strength targeted in the present invention, C needs to be contained at 0.030% or more. However, if the C content exceeds 0.300%, the ratio of the hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the C content is set to 0.030% or more and 0.300% or less. The C content is preferably 0.035% or more, more preferably 0.040% or more. The C content is preferably 0.250% or less, more preferably 0.200% or less.
Cは、固溶強化により鋼の強度を上昇させる元素である。また、Cは、パーライトの生成を促進し、かつ焼入れ性を高めてマルテンサイトの生成に寄与し、かつオーステナイトの安定化に寄与することから、硬質相の形成にも寄与する元素である。本発明で目的とする強度を確保するため、Cは0.030%以上含有することを必要とする。しかし、C含有量が0.300%を超えると、硬質相の割合が高くなり本発明で目的とする降伏比が得られない。さらに、引張ひずみ付与時の変形中のひずみ分布が不均一となり、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、C含有量は0.030%以上0.300%以下とする。C含有量は、好ましくは0.035%以上であり、より好ましくは0.040%以上である。また、C含有量は、好ましくは0.250%以下であり、より好ましくは0.200%以下である。 C: 0.030% or more and 0.300% or less C is an element that increases the strength of steel by solid solution strengthening. In addition, C promotes the formation of pearlite, improves hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and therefore also contributes to the formation of a hard phase. In order to ensure the strength targeted in the present invention, C needs to be contained at 0.030% or more. However, if the C content exceeds 0.300%, the ratio of the hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the C content is set to 0.030% or more and 0.300% or less. The C content is preferably 0.035% or more, more preferably 0.040% or more. The C content is preferably 0.250% or less, more preferably 0.200% or less.
Si:0.010%以上0.500%以下
Siは、固溶強化により鋼の強度を上昇させる元素である。このような効果を得るためには、Siは0.010%以上含有することが望ましい。しかし、Si含有量が0.500%を超えると硬質相の割合が高くなり本発明で目的とする降伏比が得られない。さらに、引張ひずみ付与時の変形中のひずみ分布が不均一となり、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、Si含有量は0.500%以下とする。Si含有量は、好ましくは0.020%以上であり、より好ましくは0.030%以上である。また、Si含有量は、好ましくは0.400%以下であり、より好ましくは0.300%以下である。 Si: 0.010% or more and 0.500% or less Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, it is desirable to contain Si at 0.010% or more. However, if the Si content exceeds 0.500%, the ratio of hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Si content is set to 0.500% or less. The Si content is preferably 0.020% or more, more preferably 0.030% or more. The Si content is preferably 0.400% or less, more preferably 0.300% or less.
Siは、固溶強化により鋼の強度を上昇させる元素である。このような効果を得るためには、Siは0.010%以上含有することが望ましい。しかし、Si含有量が0.500%を超えると硬質相の割合が高くなり本発明で目的とする降伏比が得られない。さらに、引張ひずみ付与時の変形中のひずみ分布が不均一となり、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、Si含有量は0.500%以下とする。Si含有量は、好ましくは0.020%以上であり、より好ましくは0.030%以上である。また、Si含有量は、好ましくは0.400%以下であり、より好ましくは0.300%以下である。 Si: 0.010% or more and 0.500% or less Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, it is desirable to contain Si at 0.010% or more. However, if the Si content exceeds 0.500%, the ratio of hard phase increases and the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Si content is set to 0.500% or less. The Si content is preferably 0.020% or more, more preferably 0.030% or more. The Si content is preferably 0.400% or less, more preferably 0.300% or less.
Mn:0.30%以上2.50%以下
Mnは、固溶強化により鋼の強度を上昇させる元素である。また、Mnは、変態開始温度を低下させることで組織の微細化に寄与する元素である。本発明で目的とする強度および組織を確保するためには、Mnは0.30%以上含有することを必要とする。しかし、Mn含有量が2.50%を超えると本発明で目的とする降伏比が得られない。さらに、引張ひずみ付与時の変形中のひずみ分布が不均一となり、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、Mn含有量は0.30%以上2.50%以下とする。Mn含有量は、好ましくは0.40%以上であり、より好ましくは0.50%以上である。また、Mn含有量は、好ましくは2.30%以下であり、より好ましくは2.10%以下である。 Mn: 0.30% or more and 2.50% or less Mn is an element that increases the strength of steel by solid solution strengthening. In addition, Mn is an element that contributes to fine structure by lowering the transformation start temperature. In order to ensure the strength and structure targeted in the present invention, Mn needs to be contained at 0.30% or more. However, if the Mn content exceeds 2.50%, the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Mn content is set to 0.30% or more and 2.50% or less. The Mn content is preferably 0.40% or more, more preferably 0.50% or more. The Mn content is preferably 2.30% or less, more preferably 2.10% or less.
Mnは、固溶強化により鋼の強度を上昇させる元素である。また、Mnは、変態開始温度を低下させることで組織の微細化に寄与する元素である。本発明で目的とする強度および組織を確保するためには、Mnは0.30%以上含有することを必要とする。しかし、Mn含有量が2.50%を超えると本発明で目的とする降伏比が得られない。さらに、引張ひずみ付与時の変形中のひずみ分布が不均一となり、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、Mn含有量は0.30%以上2.50%以下とする。Mn含有量は、好ましくは0.40%以上であり、より好ましくは0.50%以上である。また、Mn含有量は、好ましくは2.30%以下であり、より好ましくは2.10%以下である。 Mn: 0.30% or more and 2.50% or less Mn is an element that increases the strength of steel by solid solution strengthening. In addition, Mn is an element that contributes to fine structure by lowering the transformation start temperature. In order to ensure the strength and structure targeted in the present invention, Mn needs to be contained at 0.30% or more. However, if the Mn content exceeds 2.50%, the yield ratio targeted in the present invention cannot be obtained. Furthermore, the strain distribution during deformation when tensile strain is applied becomes non-uniform, and the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. For this reason, the Mn content is set to 0.30% or more and 2.50% or less. The Mn content is preferably 0.40% or more, more preferably 0.50% or more. The Mn content is preferably 2.30% or less, more preferably 2.10% or less.
P:0.050%以下
Pは、粒界に偏析し材料の不均質を招くため、不可避的不純物としてできるだけ低減することが好ましいが、0.050%以下の含有量までは許容できる。このため、P含有量は0.050%以下とする。P含有量は、好ましくは0.040%以下であり、より好ましくは0.030%以下である。なお、特にP含有量の下限は規定しないが、過度の低減は製錬コストの高騰を招くため、P含有量は0.002%以上とすることが好ましい。 P: 0.050% or less Since P segregates at grain boundaries and causes inhomogeneity in the material, it is preferable to reduce it as much as possible as an inevitable impurity, but a content of 0.050% or less is acceptable. Therefore, the P content is 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit for the P content, but since excessive reduction leads to high smelting costs, the P content is preferably 0.002% or more.
Pは、粒界に偏析し材料の不均質を招くため、不可避的不純物としてできるだけ低減することが好ましいが、0.050%以下の含有量までは許容できる。このため、P含有量は0.050%以下とする。P含有量は、好ましくは0.040%以下であり、より好ましくは0.030%以下である。なお、特にP含有量の下限は規定しないが、過度の低減は製錬コストの高騰を招くため、P含有量は0.002%以上とすることが好ましい。 P: 0.050% or less Since P segregates at grain boundaries and causes inhomogeneity in the material, it is preferable to reduce it as much as possible as an inevitable impurity, but a content of 0.050% or less is acceptable. Therefore, the P content is 0.050% or less. The P content is preferably 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit for the P content, but since excessive reduction leads to high smelting costs, the P content is preferably 0.002% or more.
S:0.0200%以下
Sは、鋼中では通常、MnSとして存在するが、MnSは、熱間圧延工程で薄く延伸され、延性および靭性に悪影響を及ぼす。このため、本発明ではSをできるだけ低減することが好ましいが、0.0200%以下の含有量までは許容できる。このため、S含有量は0.0200%以下とする。S含有量は、好ましくは0.0150%以下であり、より好ましくは0.0100%以下である。なお、特にS含有量の下限は規定しないが、過度の低減は製錬コストの高騰を招くため、S含有量は0.0002%以上とすることが好ましい。 S: 0.0200% or less S is usually present in steel as MnS, but MnS is thinly drawn in the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.0200% or less is acceptable. For this reason, the S content is 0.0200% or less. The S content is preferably 0.0150% or less, and more preferably 0.0100% or less. Although there is no particular lower limit for the S content, since excessive reduction leads to an increase in smelting costs, the S content is preferably 0.0002% or more.
Sは、鋼中では通常、MnSとして存在するが、MnSは、熱間圧延工程で薄く延伸され、延性および靭性に悪影響を及ぼす。このため、本発明ではSをできるだけ低減することが好ましいが、0.0200%以下の含有量までは許容できる。このため、S含有量は0.0200%以下とする。S含有量は、好ましくは0.0150%以下であり、より好ましくは0.0100%以下である。なお、特にS含有量の下限は規定しないが、過度の低減は製錬コストの高騰を招くため、S含有量は0.0002%以上とすることが好ましい。 S: 0.0200% or less S is usually present in steel as MnS, but MnS is thinly drawn in the hot rolling process and has a negative effect on ductility and toughness. For this reason, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.0200% or less is acceptable. For this reason, the S content is 0.0200% or less. The S content is preferably 0.0150% or less, and more preferably 0.0100% or less. Although there is no particular lower limit for the S content, since excessive reduction leads to an increase in smelting costs, the S content is preferably 0.0002% or more.
Al:0.005%以上0.100%以下
Alは、溶鋼に対して添加すると強力な脱酸剤として作用する元素である。このような効果を得るためには、Alは0.005%以上含有することを必要とする。しかし、Al含有量が0.100%を超えると溶接性が悪化するとともに、アルミナ系介在物が多くなり、表面性状が悪化する。このため、Al含有量は0.005%以上0.100%以下とする。Al含有量は、好ましくは0.010%以上であり、より好ましくは0.020%以上である。また、Al含有量は、好ましくは0.080%以下であり、より好ましくは0.060%以下である。 Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizer when added to molten steel. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, and the amount of alumina-based inclusions increases, resulting in poor surface properties. For this reason, the Al content is set to 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, and more preferably 0.020% or more. Moreover, the Al content is preferably 0.080% or less, and more preferably 0.060% or less.
Alは、溶鋼に対して添加すると強力な脱酸剤として作用する元素である。このような効果を得るためには、Alは0.005%以上含有することを必要とする。しかし、Al含有量が0.100%を超えると溶接性が悪化するとともに、アルミナ系介在物が多くなり、表面性状が悪化する。このため、Al含有量は0.005%以上0.100%以下とする。Al含有量は、好ましくは0.010%以上であり、より好ましくは0.020%以上である。また、Al含有量は、好ましくは0.080%以下であり、より好ましくは0.060%以下である。 Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizer when added to molten steel. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, and the amount of alumina-based inclusions increases, resulting in poor surface properties. For this reason, the Al content is set to 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, and more preferably 0.020% or more. Moreover, the Al content is preferably 0.080% or less, and more preferably 0.060% or less.
N:0.0100%以下
Nは、不可避的不純物であり、転位の運動を強固に固着することで降伏比を上昇させる作用を有する元素である。本発明では、Nは不純物としてできるだけ低減することが望ましいが、Nの含有量は0.0100%までは許容できる。このため、N含有量は0.0100%以下とする。N含有量は、好ましくは0.0090%以下であり、より好ましくは0.0080%以下である。なお、過度の低減は製錬コストの高騰を招くため、N含有量は0.0010%以上とすることが好ましく、0.0015%以上とすることがより好ましい。 N: 0.0100% or less N is an inevitable impurity and an element that has the effect of increasing the yield ratio by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the N content can be tolerated up to 0.0100%. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0090% or less, and more preferably 0.0080% or less. Note that excessive reduction leads to an increase in smelting costs, so the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.
Nは、不可避的不純物であり、転位の運動を強固に固着することで降伏比を上昇させる作用を有する元素である。本発明では、Nは不純物としてできるだけ低減することが望ましいが、Nの含有量は0.0100%までは許容できる。このため、N含有量は0.0100%以下とする。N含有量は、好ましくは0.0090%以下であり、より好ましくは0.0080%以下である。なお、過度の低減は製錬コストの高騰を招くため、N含有量は0.0010%以上とすることが好ましく、0.0015%以上とすることがより好ましい。 N: 0.0100% or less N is an inevitable impurity and an element that has the effect of increasing the yield ratio by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the N content can be tolerated up to 0.0100%. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0090% or less, and more preferably 0.0080% or less. Note that excessive reduction leads to an increase in smelting costs, so the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.
残部はFeおよび不可避的不純物とすることができる。残部における不可避的不純物としては、例えば、Sn、As、Sb、Bi、Co、Pb、Zn、Oが挙げられる。ただし、本発明の効果を損なわない範囲においては、Snを0.1%以下、As、SbおよびCoをそれぞれ0.05%以下、Bi、Pb、ZnおよびOをそれぞれ0.005%以下含有することを拒むものではない。
The balance can be Fe and unavoidable impurities. Examples of unavoidable impurities in the balance include Sn, As, Sb, Bi, Co, Pb, Zn, and O. However, within the range that does not impair the effects of the present invention, this does not exclude the inclusion of 0.1% or less Sn, 0.05% or less As, Sb, and Co, and 0.005% or less Bi, Pb, Zn, and O.
上記の成分が本発明における熱延鋼板、電縫鋼管および角形鋼管の基本の成分組成である。上記した必須元素で本発明で目的とする特性は得られるが、必要に応じて下記の元素を下記含有量の範囲で含有することができる。
The above components are the basic composition of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in this invention. The properties desired in this invention can be obtained with the essential elements listed above, but the following elements can be included as necessary within the content ranges listed below.
Nb:0.100%以下、V:0.100%以下、Ti:0.150%以下、Cr:0.50%以下、Mo:0.50%以下、Cu:0.50%以下、Ni:0.50%以下、Ca:0.0050%以下、B:0.0050%以下、Mg:0.020%以下、Zr:0.020%以下、REM:0.020%以下のうちから選ばれた1種または2種以上
One or more selected from the following: Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.0050% or less, B: 0.0050% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less
Nb:0.100%以下、V:0.100%以下、Ti:0.150%以下
Nb、Ti、Vは、いずれも鋼中で微細な炭化物、窒化物を形成し、析出物による強化を通じて鋼の強度向上に寄与する元素であり、必要に応じて含有することができる。Nb、Ti、Vの含有量はそれぞれ0%でもよいが、Nb、Ti、Vを含有する場合には、好ましい含有量はそれぞれNb:0.001%以上、Ti:0.001%以上、V:0.001%以上である。より好ましい含有量はそれぞれ、Nb:0.008%以上、V:0.008%以上、Ti:0.008%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Nb、Ti、Vを含有する場合には、それぞれNb:0.100%以下、V:0.100%以下、Ti:0.150%以下とすることが好ましい。より好ましい含有量はそれぞれ、Nb:0.070%以下、V:0.070%以下、Ti:0.110%以下である。なお、Nb、Ti、Vのうちから選ばれた2種以上を含有する場合、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがあるため、合計量(Nb+Ti+Vの合計含有量)を0.150%以下とすることが好ましい。 Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through strengthening by precipitation, and can be contained as necessary. The contents of Nb, Ti, and V may be 0%, but when Nb, Ti, and V are contained, the preferred contents are Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more, respectively. More preferred contents are Nb: 0.008% or more, V: 0.008% or more, and Ti: 0.008% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Nb, Ti, and V are contained, it is preferable to set the Nb content to 0.100% or less, the V content to 0.100% or less, and the Ti content to 0.150% or less, respectively. More preferable contents are Nb: 0.070% or less, V: 0.070% or less, and Ti: 0.110% or less, respectively. Note that when two or more selected from Nb, Ti, and V are contained, there is a risk of increasing the yield ratio and increasing the logarithmic standard deviation of the equivalent plastic strain distribution, so it is preferable to set the total content (the total content of Nb + Ti + V) to 0.150% or less.
Nb、Ti、Vは、いずれも鋼中で微細な炭化物、窒化物を形成し、析出物による強化を通じて鋼の強度向上に寄与する元素であり、必要に応じて含有することができる。Nb、Ti、Vの含有量はそれぞれ0%でもよいが、Nb、Ti、Vを含有する場合には、好ましい含有量はそれぞれNb:0.001%以上、Ti:0.001%以上、V:0.001%以上である。より好ましい含有量はそれぞれ、Nb:0.008%以上、V:0.008%以上、Ti:0.008%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Nb、Ti、Vを含有する場合には、それぞれNb:0.100%以下、V:0.100%以下、Ti:0.150%以下とすることが好ましい。より好ましい含有量はそれぞれ、Nb:0.070%以下、V:0.070%以下、Ti:0.110%以下である。なお、Nb、Ti、Vのうちから選ばれた2種以上を含有する場合、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがあるため、合計量(Nb+Ti+Vの合計含有量)を0.150%以下とすることが好ましい。 Nb: 0.100% or less, V: 0.100% or less, Ti: 0.150% or less Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through strengthening by precipitation, and can be contained as necessary. The contents of Nb, Ti, and V may be 0%, but when Nb, Ti, and V are contained, the preferred contents are Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more, respectively. More preferred contents are Nb: 0.008% or more, V: 0.008% or more, and Ti: 0.008% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Nb, Ti, and V are contained, it is preferable to set the Nb content to 0.100% or less, the V content to 0.100% or less, and the Ti content to 0.150% or less, respectively. More preferable contents are Nb: 0.070% or less, V: 0.070% or less, and Ti: 0.110% or less, respectively. Note that when two or more selected from Nb, Ti, and V are contained, there is a risk of increasing the yield ratio and increasing the logarithmic standard deviation of the equivalent plastic strain distribution, so it is preferable to set the total content (the total content of Nb + Ti + V) to 0.150% or less.
Cr:0.50%以下、Mo:0.50%以下
Cr、Moはそれぞれ、鋼の焼入れ性を高め、鋼の強度を上昇させる元素であり、必要に応じて含有することができる。Cr、Moの含有量はそれぞれ0%でもよいが、Cr、Moを含有する場合には、好ましい含有量はそれぞれCr:0.01%以上、Mo:0.01%以上である。より好ましい含有量はそれぞれ、Cr:0.10%以上、Mo:0.10%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Cr、Moを含有する場合には、それぞれCr:0.50%以下、Mo:0.50%以下とすることが好ましい。より好ましい含有量はそれぞれ、Cr:0.30%以下、Mo:0.30%以下である。 Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are elements that increase the hardenability of steel and increase the strength of steel, and can be contained as necessary. The contents of Cr and Mo may be 0%, but when Cr and Mo are contained, the preferred contents are Cr: 0.01% or more and Mo: 0.01% or more, respectively. More preferred contents are Cr: 0.10% or more and Mo: 0.10% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Cr and Mo are contained, it is preferable to set Cr: 0.50% or less and Mo: 0.50% or less, respectively. More preferred contents are Cr: 0.30% or less and Mo: 0.30% or less, respectively.
Cr、Moはそれぞれ、鋼の焼入れ性を高め、鋼の強度を上昇させる元素であり、必要に応じて含有することができる。Cr、Moの含有量はそれぞれ0%でもよいが、Cr、Moを含有する場合には、好ましい含有量はそれぞれCr:0.01%以上、Mo:0.01%以上である。より好ましい含有量はそれぞれ、Cr:0.10%以上、Mo:0.10%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Cr、Moを含有する場合には、それぞれCr:0.50%以下、Mo:0.50%以下とすることが好ましい。より好ましい含有量はそれぞれ、Cr:0.30%以下、Mo:0.30%以下である。 Cr: 0.50% or less, Mo: 0.50% or less Cr and Mo are elements that increase the hardenability of steel and increase the strength of steel, and can be contained as necessary. The contents of Cr and Mo may be 0%, but when Cr and Mo are contained, the preferred contents are Cr: 0.01% or more and Mo: 0.01% or more, respectively. More preferred contents are Cr: 0.10% or more and Mo: 0.10% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Cr and Mo are contained, it is preferable to set Cr: 0.50% or less and Mo: 0.50% or less, respectively. More preferred contents are Cr: 0.30% or less and Mo: 0.30% or less, respectively.
Cu:0.50%以下、Ni:0.50%以下
Cu、Niはそれぞれ、固溶強化により鋼の強度を上昇させる元素であり、必要に応じて含有することができる。Cu、Niの含有量はそれぞれ0%でもよいが、Cu、Niを含有する場合には、好ましい含有量はそれぞれCu:0.01%以上、Ni:0.01%以上である。より好ましい含有量はそれぞれ、Cu:0.10%以上、Ni:0.10%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Cu、Niを含有する場合には、それぞれCu:0.50%以下、Ni:0.50%以下とすることが好ましい。より好ましい含有量はそれぞれ、Cu:0.35%以下、Ni:0.35%以下である。 Cu: 0.50% or less, Ni: 0.50% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained as necessary. The contents of Cu and Ni may be 0%, but when Cu and Ni are contained, the preferred contents are Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferred contents are Cu: 0.10% or more and Ni: 0.10% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Cu and Ni are contained, it is preferable to set Cu: 0.50% or less and Ni: 0.50% or less, respectively. More preferred contents are Cu: 0.35% or less and Ni: 0.35% or less, respectively.
Cu、Niはそれぞれ、固溶強化により鋼の強度を上昇させる元素であり、必要に応じて含有することができる。Cu、Niの含有量はそれぞれ0%でもよいが、Cu、Niを含有する場合には、好ましい含有量はそれぞれCu:0.01%以上、Ni:0.01%以上である。より好ましい含有量はそれぞれ、Cu:0.10%以上、Ni:0.10%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Cu、Niを含有する場合には、それぞれCu:0.50%以下、Ni:0.50%以下とすることが好ましい。より好ましい含有量はそれぞれ、Cu:0.35%以下、Ni:0.35%以下である。 Cu: 0.50% or less, Ni: 0.50% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained as necessary. The contents of Cu and Ni may be 0%, but when Cu and Ni are contained, the preferred contents are Cu: 0.01% or more and Ni: 0.01% or more, respectively. More preferred contents are Cu: 0.10% or more and Ni: 0.10% or more, respectively. On the other hand, excessive content may lead to an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Cu and Ni are contained, it is preferable to set Cu: 0.50% or less and Ni: 0.50% or less, respectively. More preferred contents are Cu: 0.35% or less and Ni: 0.35% or less, respectively.
Ca:0.0050%以下
Caは、熱間圧延工程で薄く延伸されるMnS等の硫化物を、球状化することで鋼の延性および靭性の向上に寄与する元素であり、必要に応じて含有することができる。Caの含有量は0%でもよいが、Caを含有する場合には、好ましい含有量は0.0002%以上である。より好ましい含有量は、Ca:0.0010%以上である。しかし、Ca含有量が0.0050%を超えると、鋼中にCa酸化物クラスターが形成され、延性および靭性が悪化する場合がある。このため、Caを含有する場合は、Ca含有量は0.0050%以下とすることが好ましい。より好ましい含有量は、Ca:0.0040%以下である。 Ca: 0.0050% or less Ca is an element that contributes to improving the ductility and toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as necessary. The Ca content may be 0%, but when Ca is contained, the preferred content is 0.0002% or more. A more preferred content is Ca: 0.0010% or more. However, when the Ca content exceeds 0.0050%, Ca oxide clusters are formed in the steel, and ductility and toughness may deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.0050% or less. A more preferred content is Ca: 0.0040% or less.
Caは、熱間圧延工程で薄く延伸されるMnS等の硫化物を、球状化することで鋼の延性および靭性の向上に寄与する元素であり、必要に応じて含有することができる。Caの含有量は0%でもよいが、Caを含有する場合には、好ましい含有量は0.0002%以上である。より好ましい含有量は、Ca:0.0010%以上である。しかし、Ca含有量が0.0050%を超えると、鋼中にCa酸化物クラスターが形成され、延性および靭性が悪化する場合がある。このため、Caを含有する場合は、Ca含有量は0.0050%以下とすることが好ましい。より好ましい含有量は、Ca:0.0040%以下である。 Ca: 0.0050% or less Ca is an element that contributes to improving the ductility and toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as necessary. The Ca content may be 0%, but when Ca is contained, the preferred content is 0.0002% or more. A more preferred content is Ca: 0.0010% or more. However, when the Ca content exceeds 0.0050%, Ca oxide clusters are formed in the steel, and ductility and toughness may deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.0050% or less. A more preferred content is Ca: 0.0040% or less.
B:0.0050%以下
Bは、フェライト変態開始温度を低下させることで組織の微細化に寄与する元素である。Bの含有量は0%でもよいが、Bを含有する場合には、好ましい含有量は0.0001%以上である。より好ましい含有量は、B:0.0005%以上である。しかし、B含有量が0.0050%を超えると、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。このため、Bを含有する場合は、B含有量は0.0050%以下とすることが好ましい。より好ましい含有量は、B:0.0040%以下である。 B: 0.0050% or less B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. The content of B may be 0%, but when B is contained, the preferred content is 0.0001% or more. A more preferred content is B: 0.0005% or more. However, if the B content exceeds 0.0050%, there is a risk of an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when B is contained, the B content is preferably 0.0050% or less. A more preferred content is B: 0.0040% or less.
Bは、フェライト変態開始温度を低下させることで組織の微細化に寄与する元素である。Bの含有量は0%でもよいが、Bを含有する場合には、好ましい含有量は0.0001%以上である。より好ましい含有量は、B:0.0005%以上である。しかし、B含有量が0.0050%を超えると、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。このため、Bを含有する場合は、B含有量は0.0050%以下とすることが好ましい。より好ましい含有量は、B:0.0040%以下である。 B: 0.0050% or less B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. The content of B may be 0%, but when B is contained, the preferred content is 0.0001% or more. A more preferred content is B: 0.0005% or more. However, if the B content exceeds 0.0050%, there is a risk of an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when B is contained, the B content is preferably 0.0050% or less. A more preferred content is B: 0.0040% or less.
Mg:0.020%以下、Zr:0.020%以下、REM:0.020%以下
Mg、Zr、およびREMはそれぞれ、結晶粒微細化を通じて鋼の強度を上昇させる元素であり、必要に応じて含有することができる。Mg、Zr、およびREMの含有量はそれぞれ0%でもよいが、Mg、Zr、およびREMを含有する場合には、好ましい含有量はそれぞれMg:0.0005%以上、Zr:0.0005%以上、REM:0.0005%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Mg、Zr、およびREMを含有する場合には、それぞれMg:0.020%以下、Zr:0.020%以下、REM:0.020%以下とすることが好ましい。より好ましい含有量はそれぞれ、Mg:0.010%以下、Zr:0.010%以下、REM:0.010%以下である。なお、ここで、REMは、Sc、Y、およびランタノイド元素の合計17元素の総称である。これらの17元素のうちの1種以上を鋼に含有させることができ、REM含有量は、これらの元素の合計含有量を意味する。 Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less Mg, Zr, and REM are elements that increase the strength of steel through grain refinement, and can be contained as necessary. The contents of Mg, Zr, and REM may each be 0%, but when Mg, Zr, and REM are contained, the preferred contents are Mg: 0.0005% or more, Zr: 0.0005% or more, and REM: 0.0005% or more, respectively. On the other hand, excessive content may cause an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Mg, Zr, and REM are contained, it is preferable to set the contents to Mg: 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less, respectively. More preferable contents are Mg: 0.010% or less, Zr: 0.010% or less, and REM: 0.010% or less. Here, REM is a general term for 17 elements in total, including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained in the steel, and the REM content means the total content of these elements.
Mg、Zr、およびREMはそれぞれ、結晶粒微細化を通じて鋼の強度を上昇させる元素であり、必要に応じて含有することができる。Mg、Zr、およびREMの含有量はそれぞれ0%でもよいが、Mg、Zr、およびREMを含有する場合には、好ましい含有量はそれぞれMg:0.0005%以上、Zr:0.0005%以上、REM:0.0005%以上である。一方、過度の含有は、降伏比の上昇、および相当塑性ひずみ分布の対数標準偏差の増加を招く恐れがある。よって、Mg、Zr、およびREMを含有する場合には、それぞれMg:0.020%以下、Zr:0.020%以下、REM:0.020%以下とすることが好ましい。より好ましい含有量はそれぞれ、Mg:0.010%以下、Zr:0.010%以下、REM:0.010%以下である。なお、ここで、REMは、Sc、Y、およびランタノイド元素の合計17元素の総称である。これらの17元素のうちの1種以上を鋼に含有させることができ、REM含有量は、これらの元素の合計含有量を意味する。 Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less Mg, Zr, and REM are elements that increase the strength of steel through grain refinement, and can be contained as necessary. The contents of Mg, Zr, and REM may each be 0%, but when Mg, Zr, and REM are contained, the preferred contents are Mg: 0.0005% or more, Zr: 0.0005% or more, and REM: 0.0005% or more, respectively. On the other hand, excessive content may cause an increase in the yield ratio and an increase in the logarithmic standard deviation of the equivalent plastic strain distribution. Therefore, when Mg, Zr, and REM are contained, it is preferable to set the contents to Mg: 0.020% or less, Zr: 0.020% or less, and REM: 0.020% or less, respectively. More preferable contents are Mg: 0.010% or less, Zr: 0.010% or less, and REM: 0.010% or less. Here, REM is a general term for 17 elements in total, including Sc, Y, and lanthanoid elements. One or more of these 17 elements can be contained in the steel, and the REM content means the total content of these elements.
次に、本発明における熱延鋼板、電縫鋼管および角形鋼管の鋼組織を限定した理由を説明する。また、後述の限定した鋼組織は、板厚中央または肉厚中央の鋼組織であり、板厚1/2t位置に存在していることを指す。なお、本発明において板厚1/2t位置とは、板厚方向における板厚tの1/2(中間)の位置を意味する。
Next, we will explain the reasons for limiting the steel structure of the hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in this invention. The limited steel structure described below refers to the steel structure at the center of the plate thickness or the center of the wall thickness, and exists at the 1/2t position of the plate thickness. In this invention, the 1/2t position of the plate thickness means the position 1/2 (middle) of the plate thickness t in the plate thickness direction.
フェライトとベイナイトの合計の体積率:70%以上98%以下
フェライトおよびベイナイトは軟質な組織であり、他の硬質な組織と混合させることで、降伏比を低くすることができる。このような効果により本発明で目的とする低降伏比を得るためには、フェライトとベイナイトの合計の体積率は70%以上とする必要がある。フェライトとベイナイトの合計の体積率は、好ましくは75%以上であり、より好ましくは80%以上である。しかし、フェライトとベイナイトの合計の体積率が98%を超えると、本発明で目的とする引張強度が得られないため、フェライトとベイナイトの合計の体積率は98%以下とする必要がある。フェライトとベイナイトの合計の体積率は、好ましくは97%以下であり、より好ましくは95%以下である。 Total volume fraction of ferrite and bainite: 70% or more and 98% or less Ferrite and bainite are soft structures, and by mixing them with other hard structures, the yield ratio can be reduced. In order to obtain the low yield ratio targeted in the present invention by such an effect, the total volume fraction of ferrite and bainite needs to be 70% or more. The total volume fraction of ferrite and bainite is preferably 75% or more, more preferably 80% or more. However, if the total volume fraction of ferrite and bainite exceeds 98%, the tensile strength targeted in the present invention cannot be obtained, so the total volume fraction of ferrite and bainite needs to be 98% or less. The total volume fraction of ferrite and bainite is preferably 97% or less, more preferably 95% or less.
フェライトおよびベイナイトは軟質な組織であり、他の硬質な組織と混合させることで、降伏比を低くすることができる。このような効果により本発明で目的とする低降伏比を得るためには、フェライトとベイナイトの合計の体積率は70%以上とする必要がある。フェライトとベイナイトの合計の体積率は、好ましくは75%以上であり、より好ましくは80%以上である。しかし、フェライトとベイナイトの合計の体積率が98%を超えると、本発明で目的とする引張強度が得られないため、フェライトとベイナイトの合計の体積率は98%以下とする必要がある。フェライトとベイナイトの合計の体積率は、好ましくは97%以下であり、より好ましくは95%以下である。 Total volume fraction of ferrite and bainite: 70% or more and 98% or less Ferrite and bainite are soft structures, and by mixing them with other hard structures, the yield ratio can be reduced. In order to obtain the low yield ratio targeted in the present invention by such an effect, the total volume fraction of ferrite and bainite needs to be 70% or more. The total volume fraction of ferrite and bainite is preferably 75% or more, more preferably 80% or more. However, if the total volume fraction of ferrite and bainite exceeds 98%, the tensile strength targeted in the present invention cannot be obtained, so the total volume fraction of ferrite and bainite needs to be 98% or less. The total volume fraction of ferrite and bainite is preferably 97% or less, more preferably 95% or less.
残部:パーライト、マルテンサイト、オーステナイトから選択される1種または2種以上
パーライト、マルテンサイト、およびオーステナイトは硬質な組織であり、特に鋼の強度を上昇させるとともに、軟質なフェライトと混合させることで低降伏比を実現できる。このような効果を得るためには、フェライトおよびベイナイト以外の残部を、パーライト、マルテンサイト、オーステナイトから選択される1種または2種以上とする。パーライト、マルテンサイト、およびオーステナイトは、各体積率の合計で2%以上30%以下である。前記体積率の合計は、好ましくは3%以上であり、より好ましくは5%以上である。また、前記体積率の合計は、好ましくは25%以下であり、より好ましくは20%以下である。 Remainder: one or more selected from pearlite, martensite, and austenite Pearlite, martensite, and austenite are hard structures, and in particular they increase the strength of steel, and when mixed with soft ferrite, they can achieve a low yield ratio. In order to obtain such an effect, the remainder other than ferrite and bainite is one or more selected from pearlite, martensite, and austenite. The total volume fraction of pearlite, martensite, and austenite is 2% or more and 30% or less. The total of the volume fractions is preferably 3% or more, and more preferably 5% or more. The total of the volume fractions is preferably 25% or less, and more preferably 20% or less.
パーライト、マルテンサイト、およびオーステナイトは硬質な組織であり、特に鋼の強度を上昇させるとともに、軟質なフェライトと混合させることで低降伏比を実現できる。このような効果を得るためには、フェライトおよびベイナイト以外の残部を、パーライト、マルテンサイト、オーステナイトから選択される1種または2種以上とする。パーライト、マルテンサイト、およびオーステナイトは、各体積率の合計で2%以上30%以下である。前記体積率の合計は、好ましくは3%以上であり、より好ましくは5%以上である。また、前記体積率の合計は、好ましくは25%以下であり、より好ましくは20%以下である。 Remainder: one or more selected from pearlite, martensite, and austenite Pearlite, martensite, and austenite are hard structures, and in particular they increase the strength of steel, and when mixed with soft ferrite, they can achieve a low yield ratio. In order to obtain such an effect, the remainder other than ferrite and bainite is one or more selected from pearlite, martensite, and austenite. The total volume fraction of pearlite, martensite, and austenite is 2% or more and 30% or less. The total of the volume fractions is preferably 3% or more, and more preferably 5% or more. The total of the volume fractions is preferably 25% or less, and more preferably 20% or less.
なお、フェライト、ベイナイト、パーライト、マルテンサイト、およびオーステナイトの体積率は、後述する実施例に記載の方法で測定することができる。
The volume fractions of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in the examples below.
平均結晶粒径:15.0μm以下
結晶粒の平均結晶粒径が15.0μm超の場合、本発明で目的とする引張強度が得られない。また、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。これは、平均結晶粒径が大きいと粗大粒同士の連結度が高くなることから、変形時に粗大粒に発生したひずみが互いに連結し、変形の進行とともにひずみの分布がより不均一になるためである。このため、結晶粒の平均結晶粒径は15.0μm以下とする。結晶粒の平均結晶粒径は、好ましくは13.0μm以下とし、より好ましくは10.0μm以下とする。なお平均結晶粒径が小さいと降伏比が高くなるため、平均結晶粒径は2.0μm以上が好ましい。平均結晶粒径は、より好ましくは3.0μm以上である。 Average grain size: 15.0 μm or less If the average grain size of the grains exceeds 15.0 μm, the tensile strength targeted in the present invention cannot be obtained. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. This is because, when the average grain size is large, the degree of connectivity between the coarse grains increases, so that the strains generated in the coarse grains during deformation are connected to each other, and the distribution of the strain becomes more non-uniform as the deformation progresses. For this reason, the average grain size of the grains is set to 15.0 μm or less. The average grain size of the grains is preferably set to 13.0 μm or less, more preferably set to 10.0 μm or less. Since the yield ratio increases when the average grain size is small, the average grain size is preferably 2.0 μm or more. The average grain size is more preferably 3.0 μm or more.
結晶粒の平均結晶粒径が15.0μm超の場合、本発明で目的とする引張強度が得られない。また、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。これは、平均結晶粒径が大きいと粗大粒同士の連結度が高くなることから、変形時に粗大粒に発生したひずみが互いに連結し、変形の進行とともにひずみの分布がより不均一になるためである。このため、結晶粒の平均結晶粒径は15.0μm以下とする。結晶粒の平均結晶粒径は、好ましくは13.0μm以下とし、より好ましくは10.0μm以下とする。なお平均結晶粒径が小さいと降伏比が高くなるため、平均結晶粒径は2.0μm以上が好ましい。平均結晶粒径は、より好ましくは3.0μm以上である。 Average grain size: 15.0 μm or less If the average grain size of the grains exceeds 15.0 μm, the tensile strength targeted in the present invention cannot be obtained. In addition, the logarithmic standard deviation of the equivalent plastic strain distribution targeted in the present invention cannot be obtained. This is because, when the average grain size is large, the degree of connectivity between the coarse grains increases, so that the strains generated in the coarse grains during deformation are connected to each other, and the distribution of the strain becomes more non-uniform as the deformation progresses. For this reason, the average grain size of the grains is set to 15.0 μm or less. The average grain size of the grains is preferably set to 13.0 μm or less, more preferably set to 10.0 μm or less. Since the yield ratio increases when the average grain size is small, the average grain size is preferably 2.0 μm or more. The average grain size is more preferably 3.0 μm or more.
CP値:0.090以下
CP値は、粒径20μm以上の粗大粒同士の連結度を表す数値であり、下記(1)式によって求められる。CP値が大きいほど、粗大結晶粒間の粒界の割合が高くなるため、粗大粒同士がより連結した状態となる。CP値が0.090を超えると、変形時に粗大粒に発生したひずみが互いに連結し、変形の進行とともにひずみの分布がより不均一になるため、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、CP値は0.090以下とする。CP値は、好ましくは0.080以下であり、より好ましくは0.070以下である。なお、CP値は小さいほど好ましく、特に下限は規定しないが、過度の低減は製造コストや製造負荷の増大を招くため、CP値は0.001以上とすることが好ましい。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
なお、(1)式における「粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ」とは、粒径20μm以上の結晶粒同士が隣接する部分の大角粒界の総長さとする。 CP value: 0.090 or less The CP value is a value representing the degree of connectivity between coarse grains having a grain size of 20 μm or more, and is calculated by the following formula (1). The larger the CP value, the higher the proportion of grain boundaries between coarse crystal grains, so that the coarse grains are more connected to each other. If the CP value exceeds 0.090, the strain generated in the coarse grains during deformation will be connected to each other, and the distribution of strain will become more non-uniform as the deformation progresses, so that the logarithmic standard deviation of the equivalent plastic strain distribution, which is the object of the present invention, cannot be obtained. For this reason, the CP value is set to 0.090 or less. The CP value is preferably 0.080 or less, and more preferably 0.070 or less. The smaller the CP value, the more preferable it is, and there is no particular lower limit specified, but since excessive reduction leads to an increase in manufacturing costs and manufacturing load, the CP value is preferably set to 0.001 or more.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
In addition, in formula (1), "the total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm" refers to the total length of high-angle grain boundaries in the portion where crystal grains with a grain size of 20 μm or more are adjacent to each other.
CP値は、粒径20μm以上の粗大粒同士の連結度を表す数値であり、下記(1)式によって求められる。CP値が大きいほど、粗大結晶粒間の粒界の割合が高くなるため、粗大粒同士がより連結した状態となる。CP値が0.090を超えると、変形時に粗大粒に発生したひずみが互いに連結し、変形の進行とともにひずみの分布がより不均一になるため、本発明で目的とする、好適な相当塑性ひずみ分布の対数標準偏差が得られない。このため、CP値は0.090以下とする。CP値は、好ましくは0.080以下であり、より好ましくは0.070以下である。なお、CP値は小さいほど好ましく、特に下限は規定しないが、過度の低減は製造コストや製造負荷の増大を招くため、CP値は0.001以上とすることが好ましい。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1)
なお、(1)式における「粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ」とは、粒径20μm以上の結晶粒同士が隣接する部分の大角粒界の総長さとする。 CP value: 0.090 or less The CP value is a value representing the degree of connectivity between coarse grains having a grain size of 20 μm or more, and is calculated by the following formula (1). The larger the CP value, the higher the proportion of grain boundaries between coarse crystal grains, so that the coarse grains are more connected to each other. If the CP value exceeds 0.090, the strain generated in the coarse grains during deformation will be connected to each other, and the distribution of strain will become more non-uniform as the deformation progresses, so that the logarithmic standard deviation of the equivalent plastic strain distribution, which is the object of the present invention, cannot be obtained. For this reason, the CP value is set to 0.090 or less. The CP value is preferably 0.080 or less, and more preferably 0.070 or less. The smaller the CP value, the more preferable it is, and there is no particular lower limit specified, but since excessive reduction leads to an increase in manufacturing costs and manufacturing load, the CP value is preferably set to 0.001 or more.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1)
In addition, in formula (1), "the total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm" refers to the total length of high-angle grain boundaries in the portion where crystal grains with a grain size of 20 μm or more are adjacent to each other.
なお、平均結晶粒径およびCP値は、SEM/EBSD法によって測定することが可能であり、ここでは後述する実施例に記載の方法で測定することができる。
The average crystal grain size and CP value can be measured by the SEM/EBSD method, and can be measured by the method described in the examples below.
次に、本発明における熱延鋼板、電縫鋼管および角形鋼管の引張試験における特性を限定した理由を説明する。
Next, we will explain the reasons for limiting the properties in the tensile tests of the hot-rolled steel plate, electric resistance welded steel pipe, and square steel pipe in this invention.
熱延鋼板の引張強度:400MPa以上
熱延鋼板の引張強度が400MPa未満であると、本発明で目的とする電縫鋼管の引張強度および角形鋼管の引張強度が得られない。そのため、熱延鋼板の引張強度は400MPa以上とする。熱延鋼板の引張強度は、好ましくは420MPa以上であり、より好ましくは450MPa以上である。熱延鋼板の引張強度の上限は、特に限定されないが、一例としては、熱延鋼板の引張強度は、700MPa以下である。 Tensile strength of hot-rolled steel sheet: 400 MPa or more If the tensile strength of the hot-rolled steel sheet is less than 400 MPa, the tensile strength of the electric resistance welded steel pipe and the tensile strength of the square steel pipe targeted in the present invention cannot be obtained. Therefore, the tensile strength of the hot-rolled steel sheet is set to 400 MPa or more. The tensile strength of the hot-rolled steel sheet is preferably 420 MPa or more, more preferably 450 MPa or more. The upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited, but as an example, the tensile strength of the hot-rolled steel sheet is 700 MPa or less.
熱延鋼板の引張強度が400MPa未満であると、本発明で目的とする電縫鋼管の引張強度および角形鋼管の引張強度が得られない。そのため、熱延鋼板の引張強度は400MPa以上とする。熱延鋼板の引張強度は、好ましくは420MPa以上であり、より好ましくは450MPa以上である。熱延鋼板の引張強度の上限は、特に限定されないが、一例としては、熱延鋼板の引張強度は、700MPa以下である。 Tensile strength of hot-rolled steel sheet: 400 MPa or more If the tensile strength of the hot-rolled steel sheet is less than 400 MPa, the tensile strength of the electric resistance welded steel pipe and the tensile strength of the square steel pipe targeted in the present invention cannot be obtained. Therefore, the tensile strength of the hot-rolled steel sheet is set to 400 MPa or more. The tensile strength of the hot-rolled steel sheet is preferably 420 MPa or more, more preferably 450 MPa or more. The upper limit of the tensile strength of the hot-rolled steel sheet is not particularly limited, but as an example, the tensile strength of the hot-rolled steel sheet is 700 MPa or less.
熱延鋼板の降伏比:90%以下
熱延鋼板の降伏比が90%超であると、本発明で目的とする電縫鋼管の降伏比および角形鋼管の降伏比が得られない。そのため、熱延鋼板の降伏比は90%以下とする。熱延鋼板の降伏比は、好ましくは88%以下であり、より好ましくは85%以下である。熱延鋼板の降伏比の下限は、特に限定されないが、一例としては、熱延鋼板の降伏比は、60%以上である。 Yield ratio of hot-rolled steel sheet: 90% or less If the yield ratio of the hot-rolled steel sheet exceeds 90%, the yield ratio of the electric resistance welded steel pipe and the yield ratio of the square steel pipe targeted in the present invention cannot be obtained. Therefore, the yield ratio of the hot-rolled steel sheet is set to 90% or less. The yield ratio of the hot-rolled steel sheet is preferably 88% or less, more preferably 85% or less. The lower limit of the yield ratio of the hot-rolled steel sheet is not particularly limited, but as an example, the yield ratio of the hot-rolled steel sheet is 60% or more.
熱延鋼板の降伏比が90%超であると、本発明で目的とする電縫鋼管の降伏比および角形鋼管の降伏比が得られない。そのため、熱延鋼板の降伏比は90%以下とする。熱延鋼板の降伏比は、好ましくは88%以下であり、より好ましくは85%以下である。熱延鋼板の降伏比の下限は、特に限定されないが、一例としては、熱延鋼板の降伏比は、60%以上である。 Yield ratio of hot-rolled steel sheet: 90% or less If the yield ratio of the hot-rolled steel sheet exceeds 90%, the yield ratio of the electric resistance welded steel pipe and the yield ratio of the square steel pipe targeted in the present invention cannot be obtained. Therefore, the yield ratio of the hot-rolled steel sheet is set to 90% or less. The yield ratio of the hot-rolled steel sheet is preferably 88% or less, more preferably 85% or less. The lower limit of the yield ratio of the hot-rolled steel sheet is not particularly limited, but as an example, the yield ratio of the hot-rolled steel sheet is 60% or more.
熱延鋼板に8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差:0.70以下
相当塑性ひずみ分布は、横軸を相当塑性ひずみ(単位:無し)、縦軸を割合(面積率)(単位:%)として、対数正規分布で近似することができる。対数正規分布は、変数(横軸)の対数が正規分布に従う。よって、横軸を相当塑性ひずみ(単位:無し)の自然対数、縦軸を割合(面積率)(単位:%)とすると正規分布で近似することができる。本発明では、このときの標準偏差を「対数標準偏差」と定義する。対数標準偏差が小さいほど、相当塑性ひずみ分布のピークの広がりが小さくなり、塑性ひずみの分布がより均一になる。 Logarithmic standard deviation of equivalent plastic strain distribution after applying 8.0% tensile strain to hot-rolled steel sheet: 0.70 or less. The equivalent plastic strain distribution can be approximated by a logarithmic normal distribution with the horizontal axis representing equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %). In the logarithmic normal distribution, the logarithm of the variable (horizontal axis) follows a normal distribution. Therefore, it can be approximated by a normal distribution with the horizontal axis representing the natural logarithm of equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %). In the present invention, the standard deviation at this time is defined as the "logarithmic standard deviation". The smaller the logarithmic standard deviation, the smaller the spread of the peak of the equivalent plastic strain distribution, and the more uniform the distribution of plastic strain becomes.
相当塑性ひずみ分布は、横軸を相当塑性ひずみ(単位:無し)、縦軸を割合(面積率)(単位:%)として、対数正規分布で近似することができる。対数正規分布は、変数(横軸)の対数が正規分布に従う。よって、横軸を相当塑性ひずみ(単位:無し)の自然対数、縦軸を割合(面積率)(単位:%)とすると正規分布で近似することができる。本発明では、このときの標準偏差を「対数標準偏差」と定義する。対数標準偏差が小さいほど、相当塑性ひずみ分布のピークの広がりが小さくなり、塑性ひずみの分布がより均一になる。 Logarithmic standard deviation of equivalent plastic strain distribution after applying 8.0% tensile strain to hot-rolled steel sheet: 0.70 or less. The equivalent plastic strain distribution can be approximated by a logarithmic normal distribution with the horizontal axis representing equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %). In the logarithmic normal distribution, the logarithm of the variable (horizontal axis) follows a normal distribution. Therefore, it can be approximated by a normal distribution with the horizontal axis representing the natural logarithm of equivalent plastic strain (unit: none) and the vertical axis representing the ratio (area ratio) (unit: %). In the present invention, the standard deviation at this time is defined as the "logarithmic standard deviation". The smaller the logarithmic standard deviation, the smaller the spread of the peak of the equivalent plastic strain distribution, and the more uniform the distribution of plastic strain becomes.
熱延鋼板に8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.70以下であると、本発明で目的とする、好適な電縫鋼管の相当塑性ひずみ分布の対数標準偏差および角形鋼管の相当塑性ひずみ分布の対数標準偏差が得られやすくなる。そのため、熱延鋼板に8.0%引張ひずみを付与した後の対数標準偏差は、0.70以下とすることが好ましい。前記対数標準偏差は、より好ましくは0.68以下であり、さらにより好ましくは0.65以下である。なお、前記対数標準偏差は小さいほど好ましく、特に下限は規定しないが、過度の低減は製造コストや製造負荷の増大を招くため、前記対数標準偏差は0.050以上とすることが好ましい。
If the logarithmic standard deviation of the equivalent plastic strain distribution after applying 8.0% tensile strain to the hot-rolled steel sheet is 0.70 or less, it becomes easier to obtain the suitable logarithmic standard deviation of the equivalent plastic strain distribution of the electric resistance welded steel pipe and the logarithmic standard deviation of the equivalent plastic strain distribution of the square steel pipe that are the objective of the present invention. Therefore, it is preferable that the logarithmic standard deviation after applying 8.0% tensile strain to the hot-rolled steel sheet is 0.70 or less. The logarithmic standard deviation is more preferably 0.68 or less, and even more preferably 0.65 or less. The smaller the logarithmic standard deviation, the better, and no lower limit is specified, but since excessive reduction leads to increased manufacturing costs and manufacturing load, it is preferable that the logarithmic standard deviation is 0.050 or more.
電縫鋼管の母材部の引張強度および角形鋼管の平板部の引張強度:400MPa以上
電縫鋼管の母材部の引張強度および角形鋼管の平板部の引張強度が400MPa未満であると、耐座屈性能が低下する。そのため、前記引張強度は、400MPa以上とする。前記引張強度は、好ましくは420MPa以上であり、より好ましくは450MPa以上である。前記引張強度の上限は、特に限定されないが、一例としては、前記引張強度は、700MPa以下である。 Tensile strength of the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe: 400 MPa or more If the tensile strength of the base material of the electric resistance welded steel pipe and the tensile strength of the flat plate of the square steel pipe are less than 400 MPa, the buckling resistance performance decreases. Therefore, the tensile strength is set to 400 MPa or more. The tensile strength is preferably 420 MPa or more, and more preferably 450 MPa or more. The upper limit of the tensile strength is not particularly limited, but as an example, the tensile strength is 700 MPa or less.
電縫鋼管の母材部の引張強度および角形鋼管の平板部の引張強度が400MPa未満であると、耐座屈性能が低下する。そのため、前記引張強度は、400MPa以上とする。前記引張強度は、好ましくは420MPa以上であり、より好ましくは450MPa以上である。前記引張強度の上限は、特に限定されないが、一例としては、前記引張強度は、700MPa以下である。 Tensile strength of the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe: 400 MPa or more If the tensile strength of the base material of the electric resistance welded steel pipe and the tensile strength of the flat plate of the square steel pipe are less than 400 MPa, the buckling resistance performance decreases. Therefore, the tensile strength is set to 400 MPa or more. The tensile strength is preferably 420 MPa or more, and more preferably 450 MPa or more. The upper limit of the tensile strength is not particularly limited, but as an example, the tensile strength is 700 MPa or less.
電縫鋼管の母材部の降伏比および角形鋼管の平板部の降伏比:97%以下
電縫鋼管の母材部の降伏比および角形鋼管の平板部の降伏比が97%超であると、耐座屈性能が低下する。そのため、前記降伏比は97%以下とする。前記降伏比は、好ましくは96%以下であり、より好ましくは95%以下である。前記降伏比の下限は、特に限定されないが、一例としては、前記降伏比は、75%以上である。 Yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe: 97% or less If the yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe exceed 97%, the buckling resistance performance decreases. Therefore, the yield ratio is set to 97% or less. The yield ratio is preferably 96% or less, and more preferably 95% or less. The lower limit of the yield ratio is not particularly limited, but as an example, the yield ratio is 75% or more.
電縫鋼管の母材部の降伏比および角形鋼管の平板部の降伏比が97%超であると、耐座屈性能が低下する。そのため、前記降伏比は97%以下とする。前記降伏比は、好ましくは96%以下であり、より好ましくは95%以下である。前記降伏比の下限は、特に限定されないが、一例としては、前記降伏比は、75%以上である。 Yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe: 97% or less If the yield ratio of the base material of the electric resistance welded steel pipe and the yield ratio of the flat plate of the square steel pipe exceed 97%, the buckling resistance performance decreases. Therefore, the yield ratio is set to 97% or less. The yield ratio is preferably 96% or less, and more preferably 95% or less. The lower limit of the yield ratio is not particularly limited, but as an example, the yield ratio is 75% or more.
電縫鋼管の母材部および角形鋼管の平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差:0.60以下
電縫鋼管の母材部および角形鋼管の平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下であると、耐座屈性能をより向上しやすくなる。そのため、前記対数標準偏差は0.60以下とすることが好ましい。前記対数標準偏差は、より好ましくは0.58以下であり、さらにより好ましくは0.55以下である。なお、前記対数標準偏差は小さいほど好ましく、特に下限は規定しないが、過度の低減は製造コストや製造負荷の増大を招くため、前記対数標準偏差は0.050以上とすることが好ましい。 Logarithmic standard deviation of equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe: 0.60 or less When the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe is 0.60 or less, the buckling resistance performance is more easily improved. Therefore, it is preferable that the logarithmic standard deviation is 0.60 or less. The logarithmic standard deviation is more preferably 0.58 or less, and even more preferably 0.55 or less. The smaller the logarithmic standard deviation, the more preferable, and there is no particular lower limit, but since excessive reduction leads to increased manufacturing costs and manufacturing loads, it is preferable that the logarithmic standard deviation is 0.050 or more.
電縫鋼管の母材部および角形鋼管の平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下であると、耐座屈性能をより向上しやすくなる。そのため、前記対数標準偏差は0.60以下とすることが好ましい。前記対数標準偏差は、より好ましくは0.58以下であり、さらにより好ましくは0.55以下である。なお、前記対数標準偏差は小さいほど好ましく、特に下限は規定しないが、過度の低減は製造コストや製造負荷の増大を招くため、前記対数標準偏差は0.050以上とすることが好ましい。 Logarithmic standard deviation of equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe: 0.60 or less When the logarithmic standard deviation of the equivalent plastic strain distribution after applying 4.0% tensile strain to the base material of the electric resistance welded steel pipe and the flat plate of the square steel pipe is 0.60 or less, the buckling resistance performance is more easily improved. Therefore, it is preferable that the logarithmic standard deviation is 0.60 or less. The logarithmic standard deviation is more preferably 0.58 or less, and even more preferably 0.55 or less. The smaller the logarithmic standard deviation, the more preferable, and there is no particular lower limit, but since excessive reduction leads to increased manufacturing costs and manufacturing loads, it is preferable that the logarithmic standard deviation is 0.050 or more.
なお、引張強度および降伏比は、後述する実施例に記載の引張試験によって測定することが可能である。また、相当塑性ひずみ分布の対数標準偏差は、後述する実施例に記載の引張試験およびSEM-DIC法を組み合わせることによって測定することが可能である。相当塑性ひずみ分布の対数標準偏差は、より具体的には、後述する実施例に記載の方法で求めることができる。
The tensile strength and yield ratio can be measured by the tensile test described in the Examples below. The logarithmic standard deviation of the equivalent plastic strain distribution can be measured by combining the tensile test described in the Examples below and the SEM-DIC method. More specifically, the logarithmic standard deviation of the equivalent plastic strain distribution can be determined by the method described in the Examples below.
次に、本発明の一実施形態における熱延鋼板、電縫鋼管および角形鋼管の製造方法を説明する。
Next, we will explain the manufacturing method of hot-rolled steel sheet, electric resistance welded steel pipe, and square steel pipe in one embodiment of the present invention.
本発明の熱延鋼板は、例えば、上記した成分組成を有する鋼素材を、加熱温度:1100℃以上1300℃以下に加熱する加熱工程を施した後、仕上圧延終了温度:750℃以上850℃以下、かつ、板厚中心温度で900℃以上の温度域の平均冷却速度:1.0℃/s以上で圧延する熱間圧延工程を施して熱延板とし、熱間圧延工程後に、板厚中心温度で冷却開始から冷却停止までの平均冷却速度:5℃/s以上50℃/s以下、冷却停止温度:400℃以上650℃以下で冷却する冷却工程を施し、冷却工程後に、熱延板をコイル状に巻き取る巻取工程を施すことで得られる。
The hot-rolled steel sheet of the present invention can be obtained, for example, by subjecting a steel material having the above-mentioned composition to a heating process in which the material is heated to a temperature of 1100°C or higher and 1300°C or lower, followed by a hot rolling process in which the material is rolled to a finish rolling end temperature of 750°C or higher and 850°C or lower, and at an average cooling rate of 1.0°C/s or higher in the temperature range of 900°C or higher at the center of the plate thickness, to obtain a hot-rolled sheet, and after the hot rolling process, a cooling process is performed in which the material is cooled at an average cooling rate of 5°C/s or higher and 50°C/s or lower from the start of cooling to the end of cooling at the center of the plate thickness, and at a cooling end temperature of 400°C or higher and 650°C or lower, and after the cooling process, a winding process is performed in which the hot-rolled sheet is wound into a coil.
さらに、本発明の電縫鋼管は、前記熱延鋼板を冷間ロール成形により円筒状に成形し、該円筒状の周方向両端部を突合せて電縫溶接し、次いで、真円形状の孔形を有するロールを用いた冷間成形により外径および真円度を調整することによって製造される。
Furthermore, the electric resistance welded steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, butting both circumferential ends of the cylindrical shape together and electric resistance welding, and then adjusting the outer diameter and roundness by cold forming using a roll with a circular hole shape.
また、本発明の角形鋼管は、前記熱延鋼板を冷間ロール成形により円筒状に成形し、該円筒状の周方向両端部を突合せて電縫溶接し、次いで、目的の多角形形状の孔形を有するロールを用いた冷間成形により平板部と角部を成形することによって製造される。なお、本発明の角形鋼管には、正多角形(正三角形、正方形、正五角形等)、異なる内角の組合せを有する等辺多角形(菱形、星形等)、および異なる辺長の組合せを有する多角形(二等辺三角形、長方形、平行四辺形、台形等)が含まれる。本発明の角形鋼管は、建築物の柱材として使用する際に、通常は90度間隔で四方に梁が接合されることから、断面が正方形または長方形であることが好ましい。
The square steel pipe of the present invention is manufactured by forming the hot-rolled steel sheet into a cylindrical shape by cold rolling, butting both circumferential ends of the cylinder and electric resistance welding them, and then forming the flat plate portion and the corner portion by cold forming using a roll having a hole shape of the desired polygonal shape. The square steel pipe of the present invention includes regular polygons (equilateral triangle, square, regular pentagon, etc.), equilateral polygons with different combinations of interior angles (rhombus, star, etc.), and polygons with different combinations of side lengths (isosceles triangle, rectangle, parallelogram, trapezoid, etc.). When the square steel pipe of the present invention is used as a pillar material for a building, beams are usually joined on all four sides at 90 degree intervals, so it is preferable that the cross section is square or rectangular.
なお、前記円筒状とは、管周断面が「C」形状であることを指す。また、以下の製造方法の説明において、温度に関する「℃」表示は、特に断らない限り、鋼素材や鋼板(熱延板)の表面温度とする。これらの表面温度は、放射温度計等で測定することができる。また、鋼板板厚中心の温度は、鋼板断面内の温度分布を伝熱解析により計算し、その結果を鋼板の表面温度によって補正することで求めることができる。
The cylindrical shape mentioned above refers to a circumferential cross section of the pipe that is "C" shaped. In addition, in the following explanation of the manufacturing method, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate (hot-rolled plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be found by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis, and correcting the result by the surface temperature of the steel plate.
本発明において、鋼素材(鋼スラブ)の溶製方法は特に限定されず、転炉、電気炉、真空溶解炉等の公知の溶製方法のいずれもが適合する。鋳造方法も特に限定されないが、連続鋳造法等の公知の鋳造方法により、所望寸法に製造される。なお、連続鋳造法に代えて、造塊-分塊圧延法を適用しても何ら問題はない。溶鋼にはさらに、取鍋精錬等の二次精錬を施してもよい。
In the present invention, the method of melting the steel material (steel slab) is not particularly limited, and any of the known melting methods such as converter, electric furnace, and vacuum melting furnace are suitable. The casting method is also not particularly limited, and the desired dimensions are produced by known casting methods such as continuous casting. Note that there is no problem in applying the ingot casting-blooming rolling method instead of the continuous casting method. The molten steel may further be subjected to secondary refining such as ladle refining.
次いで、得られた鋼素材(鋼スラブ)を、加熱温度:1100℃以上1300℃以下に加熱した後、仕上圧延終了温度:750℃以上850℃以下、かつ、板厚中心温度で900℃以上の温度域の平均冷却速度:1.0℃/s以上である熱間圧延工程を施して熱延板とする。
Then, the obtained steel material (steel slab) is heated to a heating temperature of 1100°C or higher and 1300°C or lower, and then subjected to a hot rolling process in which the finish rolling end temperature is 750°C or higher and 850°C or lower, and the average cooling rate in the temperature range of 900°C or higher at the center of the plate thickness is 1.0°C/s or higher, to produce a hot-rolled plate.
加熱温度:1100℃以上1300℃以下
加熱温度が1100℃未満である場合、被圧延材(鋼スラブ)の変形抵抗が大きくなり圧延が困難となる。一方、加熱温度が1300℃を超えると、オーステナイト粒が粗大化し、後の圧延(粗圧延、仕上圧延)において微細なオーステナイト粒が得られず、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが難しい。このため、熱間圧延前の加熱炉による加熱温度は、1100℃以上1300℃以下とする。前記加熱温度は、より好ましくは1120℃以上である。また、前記加熱温度は、より好ましくは1280℃以下である。 Heating temperature: 1100°C or more and 1300°C or less When the heating temperature is less than 1100°C, the deformation resistance of the rolled material (steel slab) increases, making rolling difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), making it difficult to ensure the average grain size aimed at in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, making it difficult to control the CP value to the range aimed at in the present invention. For this reason, the heating temperature in the heating furnace before hot rolling is 1100°C or more and 1300°C or less. The heating temperature is more preferably 1120°C or more. In addition, the heating temperature is more preferably 1280°C or less.
加熱温度が1100℃未満である場合、被圧延材(鋼スラブ)の変形抵抗が大きくなり圧延が困難となる。一方、加熱温度が1300℃を超えると、オーステナイト粒が粗大化し、後の圧延(粗圧延、仕上圧延)において微細なオーステナイト粒が得られず、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが難しい。このため、熱間圧延前の加熱炉による加熱温度は、1100℃以上1300℃以下とする。前記加熱温度は、より好ましくは1120℃以上である。また、前記加熱温度は、より好ましくは1280℃以下である。 Heating temperature: 1100°C or more and 1300°C or less When the heating temperature is less than 1100°C, the deformation resistance of the rolled material (steel slab) increases, making rolling difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), making it difficult to ensure the average grain size aimed at in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, making it difficult to control the CP value to the range aimed at in the present invention. For this reason, the heating temperature in the heating furnace before hot rolling is 1100°C or more and 1300°C or less. The heating temperature is more preferably 1120°C or more. In addition, the heating temperature is more preferably 1280°C or less.
なお、本発明では、鋼スラブを製造した後、一旦室温まで冷却し、その後再度加熱する従来法に加え、室温まで冷却しないで温片のままで加熱炉に装入する、あるいは、わずかの保熱を行った後に直ちに圧延する、これらの直送圧延の省エネルギープロセスも問題なく適用できる。
In addition to the conventional method of producing a steel slab, first cooling it to room temperature and then reheating it, the present invention can also be used to easily apply energy-saving direct rolling processes in which the hot slab is loaded into the heating furnace without being cooled to room temperature, or is rolled immediately after a short period of heat retention.
仕上圧延終了温度:750℃以上850℃以下
仕上圧延終了温度が750℃未満である場合、仕上圧延中に鋼板表面温度がフェライト変態開始温度以下になり、フェライトが生成し、その後の圧延により圧延方向に伸長した加工フェライト粒となり、降伏比上昇の原因となる。一方、仕上圧延終了温度が850℃を超えると、オーステナイト未再結晶温度域での圧下量が不足し、微細なオーステナイト粒が得られず、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが困難となる。このため、仕上圧延終了温度は、750℃以上850℃以下とする。仕上圧延終了温度は、より好ましくは760℃以上である。また、仕上圧延終了温度は、より好ましくは840℃以下である。 Finish rolling end temperature: 750°C or more and 850°C or less When the finish rolling end temperature is less than 750°C, the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during finish rolling, ferrite is generated, and the subsequent rolling results in processed ferrite grains elongated in the rolling direction, which causes an increase in the yield ratio. On the other hand, when the finish rolling end temperature exceeds 850°C, the reduction amount in the austenite non-recrystallization temperature range is insufficient, fine austenite grains cannot be obtained, and it becomes difficult to ensure the average grain size aimed at in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it becomes difficult to control the CP value to the range aimed at in the present invention. For this reason, the finish rolling end temperature is set to 750°C or more and 850°C or less. The finish rolling end temperature is more preferably 760°C or more. In addition, the finish rolling end temperature is more preferably 840°C or less.
仕上圧延終了温度が750℃未満である場合、仕上圧延中に鋼板表面温度がフェライト変態開始温度以下になり、フェライトが生成し、その後の圧延により圧延方向に伸長した加工フェライト粒となり、降伏比上昇の原因となる。一方、仕上圧延終了温度が850℃を超えると、オーステナイト未再結晶温度域での圧下量が不足し、微細なオーステナイト粒が得られず、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが困難となる。このため、仕上圧延終了温度は、750℃以上850℃以下とする。仕上圧延終了温度は、より好ましくは760℃以上である。また、仕上圧延終了温度は、より好ましくは840℃以下である。 Finish rolling end temperature: 750°C or more and 850°C or less When the finish rolling end temperature is less than 750°C, the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during finish rolling, ferrite is generated, and the subsequent rolling results in processed ferrite grains elongated in the rolling direction, which causes an increase in the yield ratio. On the other hand, when the finish rolling end temperature exceeds 850°C, the reduction amount in the austenite non-recrystallization temperature range is insufficient, fine austenite grains cannot be obtained, and it becomes difficult to ensure the average grain size aimed at in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it becomes difficult to control the CP value to the range aimed at in the present invention. For this reason, the finish rolling end temperature is set to 750°C or more and 850°C or less. The finish rolling end temperature is more preferably 760°C or more. In addition, the finish rolling end temperature is more preferably 840°C or less.
板厚中心温度で900℃以上の温度域の平均冷却速度:1.0℃/s以上
本発明では、板厚中心温度で900℃以上の温度域の平均冷却速度(以下、熱間圧延での平均冷却速度と称する場合もある。)を高くすることで、オーステナイト再結晶温度域でのオーステナイトの粗大化を抑制し、本発明で目的とする平均結晶粒径およびCP値を得ることができる。前記平均冷却速度を達成するためには、例えば、圧延中に水冷設備を用いて被圧延材を冷却すればよい。前記平均冷却速度が1.0℃/s未満である場合、オーステナイト再結晶温度域でオーステナイトが粗大化してしまい、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが困難となる。前記平均冷却速度は、好ましくは1.2℃/s以上であり、より好ましくは1.5℃/s以上である。前記平均冷却速度が5.0℃/sを超えると設備負荷が増大するため、前記平均冷却速度は5.0℃/s以下が好ましい。 Average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature: 1.0°C/s or more In the present invention, by increasing the average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature (hereinafter, sometimes referred to as the average cooling rate in hot rolling), it is possible to suppress the coarsening of austenite in the austenite recrystallization temperature range and obtain the average grain size and CP value targeted in the present invention. In order to achieve the average cooling rate, for example, the rolled material may be cooled using water cooling equipment during rolling. If the average cooling rate is less than 1.0°C/s, austenite will coarsen in the austenite recrystallization temperature range, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, making it difficult to control the CP value to the range targeted in the present invention. The average cooling rate is preferably 1.2°C/s or more, more preferably 1.5°C/s or more. If the average cooling rate exceeds 5.0°C/s, the equipment load increases, so the average cooling rate is preferably 5.0°C/s or less.
本発明では、板厚中心温度で900℃以上の温度域の平均冷却速度(以下、熱間圧延での平均冷却速度と称する場合もある。)を高くすることで、オーステナイト再結晶温度域でのオーステナイトの粗大化を抑制し、本発明で目的とする平均結晶粒径およびCP値を得ることができる。前記平均冷却速度を達成するためには、例えば、圧延中に水冷設備を用いて被圧延材を冷却すればよい。前記平均冷却速度が1.0℃/s未満である場合、オーステナイト再結晶温度域でオーステナイトが粗大化してしまい、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが困難となる。前記平均冷却速度は、好ましくは1.2℃/s以上であり、より好ましくは1.5℃/s以上である。前記平均冷却速度が5.0℃/sを超えると設備負荷が増大するため、前記平均冷却速度は5.0℃/s以下が好ましい。 Average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature: 1.0°C/s or more In the present invention, by increasing the average cooling rate in the temperature range of 900°C or more at the plate thickness center temperature (hereinafter, sometimes referred to as the average cooling rate in hot rolling), it is possible to suppress the coarsening of austenite in the austenite recrystallization temperature range and obtain the average grain size and CP value targeted in the present invention. In order to achieve the average cooling rate, for example, the rolled material may be cooled using water cooling equipment during rolling. If the average cooling rate is less than 1.0°C/s, austenite will coarsen in the austenite recrystallization temperature range, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, making it difficult to control the CP value to the range targeted in the present invention. The average cooling rate is preferably 1.2°C/s or more, more preferably 1.5°C/s or more. If the average cooling rate exceeds 5.0°C/s, the equipment load increases, so the average cooling rate is preferably 5.0°C/s or less.
なお、板厚中心温度で900℃以上の温度域の平均冷却速度は、鋼素材(鋼スラブ)が加熱炉外に抽出されてから、板厚中心温度が900℃に達するまでの板厚中心の平均冷却速度として求める。すなわち、前記平均冷却速度は、[(鋼素材が加熱炉外に抽出されたときの板厚中心温度(℃)-900(℃))/鋼素材が加熱炉外に抽出されてから、鋼素材の板厚中心温度が900℃に達するまでの時間(s)]で求められる。
The average cooling rate in the temperature range where the temperature at the center of the plate thickness is 900°C or higher is calculated as the average cooling rate at the center of the plate thickness from when the steel material (steel slab) is extracted from the heating furnace until the temperature at the center of the plate thickness reaches 900°C. In other words, the average cooling rate is calculated as [(Temperature at the center of the plate thickness when the steel material is extracted from the heating furnace (°C) - 900 (°C)) / Time (s) from when the steel material is extracted from the heating furnace until the temperature at the center of the plate thickness of the steel material reaches 900°C].
本発明では、仕上板厚の上限は特に規定しないが、必要冷却速度の確保や鋼板温度管理の観点より、32mm以下が好ましい。また、仕上板厚の下限も特に限定されないが、一例としては、前記板厚は、5mm以上である。
In the present invention, the upper limit of the finished plate thickness is not specified, but from the viewpoint of ensuring the necessary cooling rate and controlling the steel plate temperature, it is preferable that the thickness is 32 mm or less. In addition, the lower limit of the finished plate thickness is not particularly limited, but as an example, the plate thickness is 5 mm or more.
熱間圧延工程後、熱延板に冷却工程を施す。冷却工程では、冷却開始から冷却停止までの平均冷却速度:5℃/s以上50℃/s以下、冷却停止温度:400℃以上650℃以下で冷却する。
After the hot rolling process, the hot-rolled sheet is subjected to a cooling process. In the cooling process, the average cooling rate from the start of cooling to the end of cooling is 5°C/s or more and 50°C/s or less, and the cooling end temperature is 400°C or more and 650°C or less.
冷却開始から冷却停止(冷却終了)までの平均冷却速度:5℃/s以上50℃/s以下
熱延板の板厚中心温度で、冷却開始から後述する冷却停止までの温度域における平均冷却速度(以下、冷却工程での平均冷却速度と称する場合もある。)が、5℃/s未満では、フェライトの核生成頻度が減少し、フェライト粒が粗大化するため、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが難しい。一方で、前記平均冷却速度が50℃/sを超えると、多量のマルテンサイトが生成し、本発明で目的とするフェライトとベイナイトの合計体積率が得られない。前記平均冷却速度は、好ましくは7℃/s以上であり、より好ましくは10℃/s以上である。また、前記平均冷却速度は、好ましくは45℃/s以下であり、より好ましくは40℃/s以下である。なお、冷却工程では、水冷等の意図的な冷却開始時点を冷却開始とし、それ以前の空冷は冷却には含めない。 Average cooling rate from start of cooling to end of cooling (end of cooling): 5°C/s or more and 50°C/s or less When the average cooling rate in the temperature range from start of cooling to end of cooling (hereinafter, sometimes referred to as the average cooling rate in the cooling process) at the center temperature of the thickness of the hot-rolled sheet is less than 5°C/s, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. On the other hand, when the average cooling rate exceeds 50°C/s, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite targeted in the present invention cannot be obtained. The average cooling rate is preferably 7°C/s or more, more preferably 10°C/s or more. In addition, the average cooling rate is preferably 45°C/s or less, more preferably 40°C/s or less. In the cooling process, the start of intentional cooling such as water cooling is regarded as the start of cooling, and air cooling before that is not included in the cooling.
熱延板の板厚中心温度で、冷却開始から後述する冷却停止までの温度域における平均冷却速度(以下、冷却工程での平均冷却速度と称する場合もある。)が、5℃/s未満では、フェライトの核生成頻度が減少し、フェライト粒が粗大化するため、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが難しい。一方で、前記平均冷却速度が50℃/sを超えると、多量のマルテンサイトが生成し、本発明で目的とするフェライトとベイナイトの合計体積率が得られない。前記平均冷却速度は、好ましくは7℃/s以上であり、より好ましくは10℃/s以上である。また、前記平均冷却速度は、好ましくは45℃/s以下であり、より好ましくは40℃/s以下である。なお、冷却工程では、水冷等の意図的な冷却開始時点を冷却開始とし、それ以前の空冷は冷却には含めない。 Average cooling rate from start of cooling to end of cooling (end of cooling): 5°C/s or more and 50°C/s or less When the average cooling rate in the temperature range from start of cooling to end of cooling (hereinafter, sometimes referred to as the average cooling rate in the cooling process) at the center temperature of the thickness of the hot-rolled sheet is less than 5°C/s, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. On the other hand, when the average cooling rate exceeds 50°C/s, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite targeted in the present invention cannot be obtained. The average cooling rate is preferably 7°C/s or more, more preferably 10°C/s or more. In addition, the average cooling rate is preferably 45°C/s or less, more preferably 40°C/s or less. In the cooling process, the start of intentional cooling such as water cooling is regarded as the start of cooling, and air cooling before that is not included in the cooling.
なお、本発明では、冷却前の鋼板表面におけるフェライト生成抑制の観点より、仕上圧延終了後直ちに冷却を開始することが好ましい。
In the present invention, it is preferable to start cooling immediately after the end of finish rolling in order to suppress the formation of ferrite on the steel sheet surface before cooling.
冷却停止温度:400℃以上650℃以下
熱延板の板厚中心温度で、冷却停止温度が400℃未満では、多量のマルテンサイトが生成し、本発明で目的とするフェライトとベイナイトの合計体積率が得られない。一方で、冷却停止温度が650℃を超えると、フェライトの核生成頻度が減少し、フェライト粒が粗大化するため、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが難しい。冷却停止温度は、好ましくは420℃以上であり、より好ましくは450℃以上である。また、冷却停止温度は、好ましくは620℃以下であり、より好ましくは600℃以下である。 Cooling stop temperature: 400°C or more and 650°C or less At the thickness center temperature of the hot-rolled sheet, if the cooling stop temperature is less than 400°C, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite targeted in the present invention cannot be obtained. On the other hand, if the cooling stop temperature exceeds 650°C, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. The cooling stop temperature is preferably 420°C or more, more preferably 450°C or more. In addition, the cooling stop temperature is preferably 620°C or less, more preferably 600°C or less.
熱延板の板厚中心温度で、冷却停止温度が400℃未満では、多量のマルテンサイトが生成し、本発明で目的とするフェライトとベイナイトの合計体積率が得られない。一方で、冷却停止温度が650℃を超えると、フェライトの核生成頻度が減少し、フェライト粒が粗大化するため、本発明で目的とする平均結晶粒径を確保することが困難となる。また、粗大粒の生成を抑制することが困難となり、CP値を本発明で目的とする範囲に制御することが難しい。冷却停止温度は、好ましくは420℃以上であり、より好ましくは450℃以上である。また、冷却停止温度は、好ましくは620℃以下であり、より好ましくは600℃以下である。 Cooling stop temperature: 400°C or more and 650°C or less At the thickness center temperature of the hot-rolled sheet, if the cooling stop temperature is less than 400°C, a large amount of martensite is generated, and the total volume ratio of ferrite and bainite targeted in the present invention cannot be obtained. On the other hand, if the cooling stop temperature exceeds 650°C, the frequency of ferrite nucleation decreases and the ferrite grains become coarse, making it difficult to ensure the average grain size targeted in the present invention. In addition, it becomes difficult to suppress the generation of coarse grains, and it is difficult to control the CP value within the range targeted in the present invention. The cooling stop temperature is preferably 420°C or more, more preferably 450°C or more. In addition, the cooling stop temperature is preferably 620°C or less, more preferably 600°C or less.
なお、本発明において、冷却工程での平均冷却速度は、特に断らない限り、((冷却前の熱延板の板厚中心温度-冷却後の熱延板の板厚中心温度)/冷却時間)で求められる値とする。冷却方法は、ノズルからの水の噴射等の水冷や、冷却ガスの噴射による冷却等が挙げられる。本発明では、熱延板の両面が同条件で冷却されるように、熱延板両面に冷却操作(処理)を施すことが好ましい。
In the present invention, unless otherwise specified, the average cooling rate in the cooling process is a value calculated by ((temperature at the center of thickness of the hot-rolled sheet before cooling - temperature at the center of thickness of the hot-rolled sheet after cooling) / cooling time). Examples of cooling methods include water cooling, such as spraying water from a nozzle, and cooling by spraying cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot-rolled sheet so that both sides are cooled under the same conditions.
冷却工程後に、熱延板を巻取り、その後放冷する巻取工程を施す。
After the cooling process, the hot-rolled sheet is coiled and then cooled in the coiling process.
以上により、本発明の熱延鋼板が製造される。本発明の熱延鋼板は、引張強度が400MPa以上であり、降伏比が90%以下である特性を備える。さらに、8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.70以下である特性を備えることができる。
The hot-rolled steel sheet of the present invention is manufactured in this manner. The hot-rolled steel sheet of the present invention has the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 90% or less. Furthermore, it can have the characteristics of a logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% of 0.70 or less.
また、前記熱延鋼板を素材として製造した電縫鋼管および角形鋼管は、引張強度が400MPa以上であり、降伏比が97%以下である特性を備える。さらに、4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である特性を備えることができる。本発明の電縫鋼管および角形鋼管は、優れた耐座屈性能を具備する。
In addition, electric resistance welded steel pipes and square steel pipes manufactured using the above-mentioned hot-rolled steel plate as a material have the characteristics of a tensile strength of 400 MPa or more and a yield ratio of 97% or less. Furthermore, they can have the characteristic of a logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain of 0.60 or less. The electric resistance welded steel pipes and square steel pipes of the present invention have excellent buckling resistance.
また、前記電縫鋼管および前記角形鋼管を使用したラインパイプおよび建築構造物は、高い耐座屈性能を具備することができる。これにより、前記建築構造物は、高い耐座屈性能を具備し、外部からの荷重に耐えることができるために、建築構造物の柱材として使用することに適している。
Furthermore, line pipes and architectural structures using the electric resistance welded steel pipes and square steel pipes can have high buckling resistance. As a result, the architectural structures have high buckling resistance and can withstand external loads, making them suitable for use as pillar materials for architectural structures.
以下、実施例に基づいてさらに本発明を詳細に説明する。なお、本発明は以下の実施例に限定されない。
The present invention will be described in more detail below based on examples. Note that the present invention is not limited to the following examples.
表1に示す成分組成を有する鋼素材(鋼スラブ)を溶製し、表2に示す条件の加熱工程、熱間圧延工程、冷却工程を施して、表2に示す仕上板厚(mm)の熱延鋼板とした。
Steel material (steel slab) with the composition shown in Table 1 was melted and subjected to the heating process, hot rolling process, and cooling process under the conditions shown in Table 2 to produce hot-rolled steel sheet with the finished thickness (mm) shown in Table 2.
かくして得られた熱延鋼板を、冷間ロール成形により円筒状のオープン管(丸型鋼管)に成形し、オープン管の突合せ部分を電縫溶接して鋼管素材とした。その後、該鋼管素材をその上下左右に配置したロールにより成形し、表3に示した外径D(mm)および肉厚t(mm)の電縫鋼管、または辺長B(mm)および肉厚t(mm)の角形鋼管を得た。なお、前記角形鋼管の断面形状は正方形である。
The hot-rolled steel sheet thus obtained was formed into a cylindrical open pipe (round steel pipe) by cold rolling, and the butt joints of the open pipe were electric resistance welded to produce steel pipe material. The steel pipe material was then formed using rolls arranged above, below, left and right, to obtain electric resistance welded steel pipes with the outer diameter D (mm) and wall thickness t (mm) shown in Table 3, or square steel pipes with the side length B (mm) and wall thickness t (mm) shown in Table 3. The cross-sectional shape of the square steel pipes is square.
得られた熱延鋼板、電縫鋼管および角形鋼管から試験片を採取して、以下に示す組織観察、引張試験、相当塑性ひずみ分布の測定を実施した。
Test pieces were taken from the obtained hot-rolled steel sheets, electric resistance welded steel pipes, and square steel pipes, and the following structural observations, tensile tests, and equivalent plastic strain distribution measurements were carried out.
〔組織観察〕
組織観察用の試験片は、観察面が熱間圧延時の圧延方向断面かつ板厚1/2t位置となるように採取し、研磨した後、ナイタール腐食して作製した。組織観察は、光学顕微鏡(倍率:1000倍)または走査型電子顕微鏡(SEM、倍率:1000倍)を用いて、鋼板の板厚1/2t位置における組織を観察し、撮像した。得られた光学顕微鏡像およびSEM像から、フェライト、ベイナイトおよび残部組織(パーライト、マルテンサイト、オーステナイト)の面積率を求めた。各組織の面積率は、5視野で観察を行い、各視野で得られた値の平均値として算出した。ここでは、組織観察により得られた面積率を、各組織の体積率とした。 [Structural Observation]
The test pieces for microstructure observation were prepared by taking the specimens so that the observation surface was a cross section in the rolling direction during hot rolling and at the 1/2t position of the plate thickness, polishing them, and then etching them with nital. For the microstructure observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) was used to observe and image the structure at the 1/2t position of the plate thickness of the steel plate. From the obtained optical microscope images and SEM images, the area ratios of ferrite, bainite, and the remaining structure (pearlite, martensite, austenite) were obtained. The area ratios of each structure were calculated as the average value of the values obtained in five visual fields by observing the structure. Here, the area ratios obtained by the microstructure observation were taken as the volume ratios of each structure.
組織観察用の試験片は、観察面が熱間圧延時の圧延方向断面かつ板厚1/2t位置となるように採取し、研磨した後、ナイタール腐食して作製した。組織観察は、光学顕微鏡(倍率:1000倍)または走査型電子顕微鏡(SEM、倍率:1000倍)を用いて、鋼板の板厚1/2t位置における組織を観察し、撮像した。得られた光学顕微鏡像およびSEM像から、フェライト、ベイナイトおよび残部組織(パーライト、マルテンサイト、オーステナイト)の面積率を求めた。各組織の面積率は、5視野で観察を行い、各視野で得られた値の平均値として算出した。ここでは、組織観察により得られた面積率を、各組織の体積率とした。 [Structural Observation]
The test pieces for microstructure observation were prepared by taking the specimens so that the observation surface was a cross section in the rolling direction during hot rolling and at the 1/2t position of the plate thickness, polishing them, and then etching them with nital. For the microstructure observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) was used to observe and image the structure at the 1/2t position of the plate thickness of the steel plate. From the obtained optical microscope images and SEM images, the area ratios of ferrite, bainite, and the remaining structure (pearlite, martensite, austenite) were obtained. The area ratios of each structure were calculated as the average value of the values obtained in five visual fields by observing the structure. Here, the area ratios obtained by the microstructure observation were taken as the volume ratios of each structure.
ここで、フェライトは、拡散変態による生成物のことであり、転位密度が低くほぼ回復した組織を呈する。ポリゴナルフェライトおよび擬ポリゴナルフェライトがこれに含まれる。また、ベイナイトは、転位密度が高いラス状のフェライトとセメンタイトの複相組織である。また、パーライトは、セメンタイトとフェライトが層状に並んだ組織である。また、オーステナイトは、ベイナイトと比較して、セメントを有さない。また、マルテンサイトおよびオーステナイトは、ベイナイトと比較して、SEM像のコントラストが明るいことから判別した。
Here, ferrite is a product of diffusion transformation, and has a low dislocation density and a nearly restored structure. This includes polygonal ferrite and pseudo-polygonal ferrite. Bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density. Pearlite is a structure in which cementite and ferrite are arranged in layers. Austenite does not contain cement, unlike bainite. Martensite and austenite were distinguished from each other by the brighter contrast of the SEM image compared to bainite.
なお、光学顕微鏡像およびSEM像ではマルテンサイトとオーステナイトの識別が難しいため、得られたSEM像からマルテンサイトあるいはオーステナイトとして観察された組織の面積率を測定し、それから後述する方法で測定したオーステナイトの体積率を差し引いた値をマルテンサイトの体積率とした。
Because it is difficult to distinguish between martensite and austenite in optical microscope images and SEM images, the area fraction of the structures observed as martensite or austenite in the obtained SEM images was measured, and the volume fraction of austenite, measured using the method described below, was subtracted from this to determine the volume fraction of martensite.
オーステナイトの体積率の測定は、X線回折により行った。組織観察用の試験片は、回折面が鋼板の板厚1/2t位置となるように研削した後、化学研磨をして表面加工層を除去して作製した。測定にはMoのKα線を使用し、fcc鉄の(200)、(220)、(311)面とbcc鉄の(200)、(211)面の積分強度からオーステナイトの体積率を求めた。
The austenite volume fraction was measured by X-ray diffraction. Test pieces for microstructural observation were prepared by grinding so that the diffraction surface was at 1/2t of the plate thickness, then chemically polishing to remove the surface treatment layer. Mo Kα radiation was used for the measurement, and the austenite volume fraction was calculated from the integrated intensity of the (200), (220), and (311) planes of fcc iron and the (200) and (211) planes of bcc iron.
平均結晶粒径およびCP値は、SEM/EBSD法を用いて測定した。測定領域は500μm×500μm、測定ステップサイズは0.5μmとした。得られたEBSDデータをもとに、結晶方位解析ソフトOIM Analysis(商標)を用いて、方位差が15°以上の境界を結晶粒界(大角粒界)として、粒界の分布を得た。平均結晶粒径は、各結晶粒の円相当径(粒径)の算術平均として求めた。また、CP値は、粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ、および大角粒界の総長さをそれぞれ算出し、これらの比として求めた。ここで、粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さは、測定領域における前記粒界の分布から粒径20μm未満の結晶粒を除いた領域で測定した大角粒界の長さの総和であり、大角粒界の総長さは、測定領域における前記粒界の分布から測定した大角粒界の長さの総和である。なお、平均結晶粒径およびCP値の算出においては、粒径が2.0μm以下の結晶粒は測定ノイズとして除外した。
The average grain size and CP value were measured using the SEM/EBSD method. The measurement area was 500 μm x 500 μm, and the measurement step size was 0.5 μm. Based on the obtained EBSD data, the crystal orientation analysis software OIM Analysis (trademark) was used to obtain the grain boundary distribution, with boundaries with an orientation difference of 15° or more considered as grain boundaries (high-angle grain boundaries). The average grain size was calculated as the arithmetic mean of the circle equivalent diameter (grain size) of each grain. The CP value was calculated by calculating the total length of the high-angle grain boundaries in the area excluding grains with a grain size of less than 20 μm, and the total length of the high-angle grain boundaries, and then calculating the ratio of these. Here, the total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm is the sum of the lengths of high-angle grain boundaries measured in the region excluding crystal grains with a grain size of less than 20 μm from the distribution of the grain boundaries in the measurement region, and the total length of high-angle grain boundaries is the sum of the lengths of high-angle grain boundaries measured from the distribution of the grain boundaries in the measurement region. Note that, in calculating the average grain size and CP value, grains with a grain size of 2.0 μm or less were excluded as measurement noise.
〔引張試験〕
引張方向が圧延方向と平行になるように、JIS5号の引張試験片を採取した。引張試験片は、熱延鋼板については幅方向端部から幅方向に1/4W(W:板幅)の位置から、電縫鋼管については電縫溶接部から周方向に90°離れた位置から、角形鋼管については電縫溶接部を含む平板部に隣接した平板部からそれぞれ採取した。引張試験は、JIS Z 2241(2011年)の規定に準拠して実施し、降伏応力σy、引張強度をそれぞれ測定して、(降伏応力σy)/(引張強度)で定義される降伏比を算出した。 [Tensile test]
Tensile test pieces according to JIS No. 5 were taken so that the tensile direction was parallel to the rolling direction. The tensile test pieces were taken from a position 1/4W (W: plate width) in the width direction from the end in the width direction for the hot-rolled steel plate, from aposition 90° away in the circumferential direction from the electric resistance welded part for the electric resistance welded steel pipe, and from a flat part adjacent to the flat part including the electric resistance welded part for the square steel pipe. The tensile test was carried out in accordance with the provisions of JIS Z 2241 (2011), and the yield stress σy and tensile strength were measured, and the yield ratio defined as (yield stress σy)/(tensile strength) was calculated.
引張方向が圧延方向と平行になるように、JIS5号の引張試験片を採取した。引張試験片は、熱延鋼板については幅方向端部から幅方向に1/4W(W:板幅)の位置から、電縫鋼管については電縫溶接部から周方向に90°離れた位置から、角形鋼管については電縫溶接部を含む平板部に隣接した平板部からそれぞれ採取した。引張試験は、JIS Z 2241(2011年)の規定に準拠して実施し、降伏応力σy、引張強度をそれぞれ測定して、(降伏応力σy)/(引張強度)で定義される降伏比を算出した。 [Tensile test]
Tensile test pieces according to JIS No. 5 were taken so that the tensile direction was parallel to the rolling direction. The tensile test pieces were taken from a position 1/4W (W: plate width) in the width direction from the end in the width direction for the hot-rolled steel plate, from a
〔相当塑性ひずみ分布〕
相当塑性ひずみ分布は、SEM-DIC法により測定した。引張方向が圧延方向と平行になるように、熱延鋼板の板厚中央、電縫鋼管の肉厚中央、および角形鋼管の肉厚中央から、図1に示す引張試験片を採取した。熱延鋼板の板幅方向については、幅方向端部から幅方向に1/4W(W:板幅)、電縫鋼管の周方向については電縫溶接部から周方向に90°離れた位置、角形鋼管については電縫溶接部を含む平板部に隣接した平板部で採取を行った。得られた引張試験片の一方の面を研磨し、ナイタール腐食して、SEM(倍率:1000倍)を用いて平行部(引張変形部)を5視野撮像した。その後、引張速度5mm/minで、熱延鋼板から採取した試験片には8.0%の引張ひずみ、電縫鋼管および角形鋼管から採取した試験片には4.0%の引張ひずみをそれぞれ付与し、除荷した。その後、SEM(倍率:1000倍)を用いて引張前(引張ひずみ付与前)と同一の視野を撮像した。得られた引張前後のSEM像をもとに、画像解析ソフトGOM Correlate(GOM社)を用いて、DIC法により撮像面の相当塑性ひずみ分布を算出した。DIC法は、物体表面のランダムパターンを変形前後で比較することで、観察面の各所における変位やひずみを測定する手法である。具体的には、変形前画像においてサブセットと呼ばれる正方形領域を定義し、サブセット内部のランダムパターンをもとにサブセットを変形前後で追跡し、サブセット中央点の変位を算出する。この操作を画像全体で網羅的に行い変位分布およびひずみ分布を得る。本発明では、金属組織のナイタール腐食痕をランダムパターンとして利用し、1910ピクセル×2560ピクセルの画像に対し、サブセットサイズを80ピクセル×80ピクセル(3.6μm×3.6μm)、測定間隔を10ピクセル(0.45μm)とした。横軸を得られた相当塑性ひずみ(単位:無し)の自然対数、縦軸を割合(面積率)(単位:%)としたものを正規分布で近似し、このときの標準偏差を対数標準偏差(相当塑性ひずみ分布の対数標準偏差)とした。具体的には以下の方法により、対数標準偏差を求めた。まず、相当塑性ひずみが0~0.20の範囲内において、階級幅を0.02として各階級の割合(面積率)(単位:%)を求めた。このとき、相当塑性ひずみが0以上0.02未満の階級を1番目の階級、0.02以上0.04未満の階級を2番目の階級、・・・、0.18以上0.20未満の階級を10番目の階級とした。xiをi番目の階級の階級値の自然対数、x0を相当塑性ひずみの自然対数の平均値として、下記(4)式と(5)式により対数標準偏差を求めた。 [Equivalent plastic strain distribution]
The equivalent plastic strain distribution was measured by the SEM-DIC method. Tensile test pieces shown in FIG. 1 were taken from the center of the plate thickness of the hot-rolled steel plate, the center of the plate thickness of the electric resistance welded steel pipe, and the center of the plate thickness of the square steel pipe so that the tensile direction was parallel to the rolling direction. For the plate width direction of the hot-rolled steel plate, 1/4W (W: plate width) in the width direction from the end in the width direction, for the circumferential direction of the electric resistance welded steel pipe, the sample was taken at aposition 90° away from the electric resistance weld in the circumferential direction, and for the square steel pipe, the sample was taken at a flat plate part adjacent to the flat plate part including the electric resistance weld. One side of the obtained tensile test piece was polished and etched with nital, and the parallel part (tensile deformation part) was imaged in five fields of view using an SEM (magnification: 1000 times). Then, at a tensile speed of 5 mm/min, a tensile strain of 8.0% was applied to the test pieces taken from the hot-rolled steel plate, and a tensile strain of 4.0% was applied to the test pieces taken from the electric resistance welded steel pipe and the square steel pipe, respectively, and the load was removed. Then, the same field of view as before tension (before tensile strain was applied) was imaged using an SEM (magnification: 1000 times). Based on the obtained SEM images before and after tension, the equivalent plastic strain distribution of the imaged surface was calculated by the DIC method using image analysis software GOM Correlate (GOM Co., Ltd.). The DIC method is a method of measuring the displacement and strain at various points on the observation surface by comparing the random patterns on the surface of an object before and after deformation. Specifically, a square area called a subset is defined in the image before deformation, and the subset is tracked before and after deformation based on the random pattern inside the subset, and the displacement of the center point of the subset is calculated. This operation is performed comprehensively throughout the entire image to obtain the displacement distribution and strain distribution. In the present invention, the nital corrosion marks of the metal structure are used as a random pattern, and the subset size is 80 pixels x 80 pixels (3.6 μm x 3.6 μm) and the measurement interval is 10 pixels (0.45 μm) for an image of 1910 pixels x 2560 pixels. The horizontal axis represents the natural logarithm of the obtained equivalent plastic strain (unit: none), and the vertical axis represents the ratio (area ratio) (unit: %). The standard deviation at this time was approximated by a normal distribution, and the logarithmic standard deviation (logarithmic standard deviation of the equivalent plastic strain distribution) was used. Specifically, the logarithmic standard deviation was calculated by the following method. First, the ratio (area ratio) (unit: %) of each class was calculated with a class width of 0.02 within the range of equivalent plastic strain from 0 to 0.20. At this time, the class with equivalent plastic strain of 0 to less than 0.02 was set as the first class, the class with equivalent plastic strain of 0 to less than 0.02 was set as the second class, ..., the class with equivalent plastic strain of 0 to less than 0.20 was set as the tenth class. The logarithmic standard deviation was calculated by the following formulas (4) and (5), with x i as the natural logarithm of the class value of the i-th class and x 0 as the average value of the natural logarithm of the equivalent plastic strain.
相当塑性ひずみ分布は、SEM-DIC法により測定した。引張方向が圧延方向と平行になるように、熱延鋼板の板厚中央、電縫鋼管の肉厚中央、および角形鋼管の肉厚中央から、図1に示す引張試験片を採取した。熱延鋼板の板幅方向については、幅方向端部から幅方向に1/4W(W:板幅)、電縫鋼管の周方向については電縫溶接部から周方向に90°離れた位置、角形鋼管については電縫溶接部を含む平板部に隣接した平板部で採取を行った。得られた引張試験片の一方の面を研磨し、ナイタール腐食して、SEM(倍率:1000倍)を用いて平行部(引張変形部)を5視野撮像した。その後、引張速度5mm/minで、熱延鋼板から採取した試験片には8.0%の引張ひずみ、電縫鋼管および角形鋼管から採取した試験片には4.0%の引張ひずみをそれぞれ付与し、除荷した。その後、SEM(倍率:1000倍)を用いて引張前(引張ひずみ付与前)と同一の視野を撮像した。得られた引張前後のSEM像をもとに、画像解析ソフトGOM Correlate(GOM社)を用いて、DIC法により撮像面の相当塑性ひずみ分布を算出した。DIC法は、物体表面のランダムパターンを変形前後で比較することで、観察面の各所における変位やひずみを測定する手法である。具体的には、変形前画像においてサブセットと呼ばれる正方形領域を定義し、サブセット内部のランダムパターンをもとにサブセットを変形前後で追跡し、サブセット中央点の変位を算出する。この操作を画像全体で網羅的に行い変位分布およびひずみ分布を得る。本発明では、金属組織のナイタール腐食痕をランダムパターンとして利用し、1910ピクセル×2560ピクセルの画像に対し、サブセットサイズを80ピクセル×80ピクセル(3.6μm×3.6μm)、測定間隔を10ピクセル(0.45μm)とした。横軸を得られた相当塑性ひずみ(単位:無し)の自然対数、縦軸を割合(面積率)(単位:%)としたものを正規分布で近似し、このときの標準偏差を対数標準偏差(相当塑性ひずみ分布の対数標準偏差)とした。具体的には以下の方法により、対数標準偏差を求めた。まず、相当塑性ひずみが0~0.20の範囲内において、階級幅を0.02として各階級の割合(面積率)(単位:%)を求めた。このとき、相当塑性ひずみが0以上0.02未満の階級を1番目の階級、0.02以上0.04未満の階級を2番目の階級、・・・、0.18以上0.20未満の階級を10番目の階級とした。xiをi番目の階級の階級値の自然対数、x0を相当塑性ひずみの自然対数の平均値として、下記(4)式と(5)式により対数標準偏差を求めた。 [Equivalent plastic strain distribution]
The equivalent plastic strain distribution was measured by the SEM-DIC method. Tensile test pieces shown in FIG. 1 were taken from the center of the plate thickness of the hot-rolled steel plate, the center of the plate thickness of the electric resistance welded steel pipe, and the center of the plate thickness of the square steel pipe so that the tensile direction was parallel to the rolling direction. For the plate width direction of the hot-rolled steel plate, 1/4W (W: plate width) in the width direction from the end in the width direction, for the circumferential direction of the electric resistance welded steel pipe, the sample was taken at a
〔軸圧縮試験〕
電縫鋼管および角形鋼管の両端に耐圧板を取り付け、大型圧縮試験装置により軸圧縮試験を実施した。圧縮荷重が最大になったときの応力を、最大応力度σmax(N/mm2)とした。また、前記引張試験により求めた降伏応力σyを用いて、耐力上昇率τ(=σmax/σy)を算出した。 [Axial compression test]
Pressure plates were attached to both ends of the electric resistance welded steel pipe and the square steel pipe, and an axial compression test was carried out using a large compression test device. The stress at which the compressive load was maximum was taken as the maximum stress intensity σmax (N/mm 2 ). The yield stress σy obtained from the tensile test was used to calculate the yield strength increase rate τ (=σmax/σy).
電縫鋼管および角形鋼管の両端に耐圧板を取り付け、大型圧縮試験装置により軸圧縮試験を実施した。圧縮荷重が最大になったときの応力を、最大応力度σmax(N/mm2)とした。また、前記引張試験により求めた降伏応力σyを用いて、耐力上昇率τ(=σmax/σy)を算出した。 [Axial compression test]
Pressure plates were attached to both ends of the electric resistance welded steel pipe and the square steel pipe, and an axial compression test was carried out using a large compression test device. The stress at which the compressive load was maximum was taken as the maximum stress intensity σmax (N/mm 2 ). The yield stress σy obtained from the tensile test was used to calculate the yield strength increase rate τ (=σmax/σy).
熱延鋼板について得られた結果を表4に示す。
The results obtained for hot-rolled steel sheets are shown in Table 4.
電縫鋼管および角形鋼管について得られた結果を表5に示す。
The results obtained for electric resistance welded steel pipes and square steel pipes are shown in Table 5.
表4および表5中、No.1~6は本発明例であり、No.7~12は比較例である。
In Tables 4 and 5, Nos. 1 to 6 are examples of the present invention, and Nos. 7 to 12 are comparative examples.
本発明例の熱延鋼板は、板厚中央の鋼組織は、体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、平均結晶粒径が15.0μm以下であり、所定の(1)式で求められるCPの値が0.090以下であった。また、引張強度が400MPa以上であり、降伏比が90%以下であった。さらに、8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.70以下であった。
The hot-rolled steel plate of the present invention had a steel structure at the center of the plate thickness with a total volume fraction of ferrite and bainite of 70% to 98%, with the remainder consisting of one or more types selected from pearlite, martensite and austenite, an average crystal grain size of 15.0 μm or less, and a CP value calculated by the specified formula (1) of 0.090 or less. In addition, the tensile strength was 400 MPa or more, and the yield ratio was 90% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% was 0.70 or less.
また、本発明例の電縫鋼管および角形鋼管は、前記本発明例の熱延鋼板から製造されたものであり、肉厚中央の鋼組織は、体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、平均結晶粒径が15.0μm以下であり、所定の(1)式で求められるCPの値が0.090以下であった。また、母材部または平板部の引張強度が400MPa以上であり、母材部または平板部の降伏比が97%以下であった。さらに、母材部または平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下であった。また、軸圧縮試験における耐力上昇率τ(=σmax/σy)が、電縫鋼管においてはτ≧4.0×(t/D)+0.85・・・(2)を、角形鋼管においてはτ≧3.0×(t/B)+0.85・・・(3)をそれぞれ満足していた。なお、表5においては、上記(2)式と(3)式の右辺にそれぞれ電縫鋼管のtとDおよび角形鋼管のtとBを代入した値を、τの必要下限値として記載している。
Furthermore, the electric resistance welded steel pipe and square steel pipe of the present invention were manufactured from the hot rolled steel plate of the present invention, and the steel structure at the center of the wall thickness had a total volume fraction of ferrite and bainite of 70% to 98%, with the remainder consisting of one or more types selected from pearlite, martensite and austenite, an average crystal grain size of 15.0 μm or less, and a CP value calculated by the specified formula (1) of 0.090 or less. Furthermore, the tensile strength of the base material or flat plate was 400 MPa or more, and the yield ratio of the base material or flat plate was 97% or less. Furthermore, the logarithmic standard deviation of the equivalent plastic strain distribution after applying a 4.0% tensile strain to the base material or flat plate was 0.60 or less. In addition, the yield strength increase rate τ (=σmax/σy) in the axial compression test satisfied τ≧4.0×(t/D)+0.85...(2) for electric resistance welded steel pipes, and τ≧3.0×(t/B)+0.85...(3) for square steel pipes. In Table 5, the values obtained by substituting t and D for electric resistance welded steel pipes and t and B for square steel pipes into the right-hand sides of the above equations (2) and (3) are listed as the required lower limit value of τ.
一方、比較例のNo.7は、Cの含有量が本発明の範囲を下回っていたため、引張強度が本発明の範囲外となった。
On the other hand, in the comparative example, No. 7, the C content was below the range of the present invention, so the tensile strength was outside the range of the present invention.
比較例のNo.8は、Cの含有量が本発明の範囲を上回っていたため、フェライトとベイナイトの合計の体積率が本発明の範囲を下回った。その結果、降伏比が本発明の範囲外となり、対数標準偏差が好ましい範囲の範囲外となったため、耐力上昇率が所望の値に達しなかった。
Comparative example No. 8 had a C content exceeding the range of the present invention, so the total volume ratio of ferrite and bainite was below the range of the present invention. As a result, the yield ratio was outside the range of the present invention, and the logarithmic standard deviation was outside the preferred range, so the yield strength increase rate did not reach the desired value.
比較例のNo.9は、SiおよびMnの含有量が本発明の範囲を下回っていたため、フェライトとベイナイトの合計の体積率が本発明の範囲を上回り、平均結晶粒径が本発明の範囲を上回った。その結果、引張強度が本発明の範囲外となった。
Comparative example No. 9 had a Si and Mn content below the range of the present invention, so the total volume fraction of ferrite and bainite exceeded the range of the present invention, and the average crystal grain size exceeded the range of the present invention. As a result, the tensile strength was outside the range of the present invention.
比較例のNo.10は、SiおよびMnの含有量が本発明の範囲を上回っていたため、フェライトとベイナイトの合計の体積率が本発明の範囲を下回った。その結果、降伏比が本発明の範囲外となり、対数標準偏差が好ましい範囲の範囲外となったため、耐力上昇率が所望の値に達しなかった。
Comparative example No. 10 had a Si and Mn content exceeding the range of the present invention, so the total volume fraction of ferrite and bainite was below the range of the present invention. As a result, the yield ratio was outside the range of the present invention, and the logarithmic standard deviation was outside the preferred range, so the yield strength increase rate did not reach the desired value.
比較例のNo.11は、熱間圧延工程における900℃以上の温度域の平均冷却速度が好適な製造方法の範囲を下回っていたため、平均結晶粒径が本発明の範囲を上回り、CP値が本発明の範囲を上回った。その結果、対数標準偏差が本発明の好ましい範囲を上回り、耐力上昇率が所望の値に達しなかった。また、引張強度が本発明の範囲を下回った。
In comparative example No. 11, the average cooling rate in the temperature range of 900°C or higher in the hot rolling process was below the range of the preferred manufacturing method, so the average crystal grain size exceeded the range of the present invention, and the CP value exceeded the range of the present invention. As a result, the logarithmic standard deviation exceeded the preferred range of the present invention, and the yield strength increase rate did not reach the desired value. In addition, the tensile strength was below the range of the present invention.
比較例のNo.12は、熱間圧延工程における冷却停止温度が好適な製造方法の範囲を上回っていたため、CP値が本発明の範囲を上回った。その結果、対数標準偏差が本発明の好ましい範囲を上回り、耐力上昇率が所望の値に達しなかった。
Comparative example No. 12 had a cooling stop temperature in the hot rolling process that exceeded the range of the preferred manufacturing method, so the CP value exceeded the range of the present invention. As a result, the logarithmic standard deviation exceeded the preferred range of the present invention, and the yield strength increase rate did not reach the desired value.
以上から、鋼組成、組織を本発明の範囲内とすることで、耐座屈性能に優れた電縫鋼管および角形鋼管、ならびにそれらの素材として用いられる熱延鋼板を提供することができる。また、前記電縫鋼管および角形鋼管を用いた高い耐座屈性能を有するラインパイプおよび建築構造物を提供することができる。
From the above, by setting the steel composition and structure within the range of the present invention, it is possible to provide electric resistance welded steel pipes and square steel pipes with excellent buckling resistance, as well as hot rolled steel sheets used as materials for these pipes. It is also possible to provide line pipes and building structures with high buckling resistance using the electric resistance welded steel pipes and square steel pipes.
From the above, by setting the steel composition and structure within the range of the present invention, it is possible to provide electric resistance welded steel pipes and square steel pipes with excellent buckling resistance, as well as hot rolled steel sheets used as materials for these pipes. It is also possible to provide line pipes and building structures with high buckling resistance using the electric resistance welded steel pipes and square steel pipes.
Claims (13)
- 成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
板厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
引張強度が400MPa以上であり、
降伏比が90%以下である、熱延鋼板。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1) The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, with the balance being Fe and unavoidable impurities;
The steel structure at the center of the plate thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength is 400 MPa or more,
A hot-rolled steel sheet having a yield ratio of 90% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1) - 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、請求項1に記載の熱延鋼板。 In addition to the above-mentioned component composition, the following is further added in mass%:
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The hot-rolled steel sheet according to claim 1 , comprising one or more selected from the following: - 8.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.70以下である、請求項1または2に記載の熱延鋼板。 The hot-rolled steel sheet according to claim 1 or 2, in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 8.0% is 0.70 or less.
- 母材部と電縫溶接部を有する電縫鋼管であって、
成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
肉厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
母材部の引張強度が400MPa以上であり、
母材部の降伏比が97%以下である、電縫鋼管。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1) An electric resistance welded steel pipe having a base metal portion and an electric resistance welded portion,
The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the base material is 400 MPa or more,
An electric welded steel pipe having a base metal portion with a yield ratio of 97% or less.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1) - 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、請求項4に記載の電縫鋼管。 In addition to the above-mentioned component composition, the following is further added in mass%:
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The electric resistance welded steel pipe according to claim 4, comprising one or more selected from the following: - 母材部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である、請求項4または5に記載の電縫鋼管。 An electric resistance welded steel pipe as described in claim 4 or 5, in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the base material is 0.60 or less.
- 請求項4または5に記載の電縫鋼管が使用されている、ラインパイプ。 A line pipe in which the electric resistance welded steel pipe according to claim 4 or 5 is used.
- 請求項6に記載の電縫鋼管が使用されている、ラインパイプ。 A line pipe in which the electric resistance welded steel pipe according to claim 6 is used.
- 平板部と角部を有する角形鋼管であって、
成分組成は、質量%で、
C :0.030%以上0.300%以下、
Si:0.010%以上0.500%以下、
Mn:0.30%以上2.50%以下、
P :0.050%以下、
S :0.0200%以下、
Al:0.005%以上0.100%以下、
N :0.0100%以下
を含有し、残部がFeおよび不可避的不純物からなり、
肉厚中央の鋼組織は、
体積率で、フェライトとベイナイトの合計が70%以上98%以下であり、
残部がパーライト、マルテンサイト、オーステナイトから選択される1種または2種以上からなり、
平均結晶粒径が15.0μm以下であり、
下記(1)式で求められるCPの値が0.090以下であり、
平板部の引張強度が400MPa以上であり、
平板部の降伏比が97%以下である、角形鋼管。
CP=(粒径20μm未満の結晶粒を除いた領域における大角粒界の総長さ)/(大角粒界の総長さ) ・・・(1) A square steel pipe having a flat portion and a corner portion,
The composition is in mass percent:
C: 0.030% or more and 0.300% or less,
Si: 0.010% or more and 0.500% or less,
Mn: 0.30% or more and 2.50% or less,
P: 0.050% or less,
S: 0.0200% or less,
Al: 0.005% or more and 0.100% or less,
N: 0.0100% or less, the balance being Fe and unavoidable impurities;
The steel structure in the center of the wall thickness is
The total volume fraction of ferrite and bainite is 70% or more and 98% or less,
The balance is one or more selected from pearlite, martensite, and austenite,
The average crystal grain size is 15.0 μm or less,
The CP value calculated by the following formula (1) is 0.090 or less,
The tensile strength of the flat portion is 400 MPa or more,
A square steel pipe having a yield ratio of 97% or less in the flat portion.
CP = (total length of high-angle grain boundaries in the region excluding crystal grains with a grain size of less than 20 μm) / (total length of high-angle grain boundaries) (1) - 前記成分組成に加えてさらに、質量%で、
Nb:0.100%以下、
V :0.100%以下、
Ti:0.150%以下、
Cr:0.50%以下、
Mo:0.50%以下、
Cu:0.50%以下、
Ni:0.50%以下、
Ca:0.0050%以下、
B :0.0050%以下、
Mg:0.020%以下、
Zr:0.020%以下、
REM:0.020%以下、
のうちから選ばれた1種または2種以上を含む、請求項9に記載の角形鋼管。 In addition to the above-mentioned component composition, the following is further added in mass%:
Nb: 0.100% or less,
V: 0.100% or less,
Ti: 0.150% or less,
Cr: 0.50% or less,
Mo: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Ca: 0.0050% or less,
B: 0.0050% or less,
Mg: 0.020% or less,
Zr: 0.020% or less,
REM: 0.020% or less,
The square steel pipe according to claim 9, comprising one or more selected from the following: - 平板部において4.0%引張ひずみを付与した後の相当塑性ひずみ分布の対数標準偏差が0.60以下である、請求項9または10に記載の角形鋼管。 The square steel pipe according to claim 9 or 10, in which the logarithmic standard deviation of the equivalent plastic strain distribution after applying a tensile strain of 4.0% to the flat portion is 0.60 or less.
- 請求項9または10に記載の角形鋼管が、柱材として使用されている、建築構造物。 An architectural structure in which the square steel pipe according to claim 9 or 10 is used as a pillar material.
- 請求項11に記載の角形鋼管が、柱材として使用されている、建築構造物。
An architectural structure, in which the square steel pipe according to claim 11 is used as a pillar material.
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