WO2018110152A1 - 低降伏比角形鋼管用熱延鋼板およびその製造方法並びに低降伏比角形鋼管およびその製造方法 - Google Patents
低降伏比角形鋼管用熱延鋼板およびその製造方法並びに低降伏比角形鋼管およびその製造方法 Download PDFInfo
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- WO2018110152A1 WO2018110152A1 PCT/JP2017/040169 JP2017040169W WO2018110152A1 WO 2018110152 A1 WO2018110152 A1 WO 2018110152A1 JP 2017040169 W JP2017040169 W JP 2017040169W WO 2018110152 A1 WO2018110152 A1 WO 2018110152A1
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- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21D5/00—Bending sheet metal along straight lines, e.g. to form simple curves
- B21D5/06—Bending sheet metal along straight lines, e.g. to form simple curves by drawing procedure making use of dies or forming-rollers, e.g. making profiles
- B21D5/10—Bending sheet metal along straight lines, e.g. to form simple curves by drawing procedure making use of dies or forming-rollers, e.g. making profiles for making tubes
- B21D5/12—Bending sheet metal along straight lines, e.g. to form simple curves by drawing procedure making use of dies or forming-rollers, e.g. making profiles for making tubes making use of forming-rollers
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- 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
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- 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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- 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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- 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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
Definitions
- the present invention relates to a hot rolled steel sheet for a low yield ratio square steel pipe, and a square steel pipe (square column) manufactured by cold forming from the hot rolled steel sheet and having a low yield ratio and low temperature toughness.
- the present invention relates to a rectangular steel pipe that can be applied as a building member for a medium-rise building having a height of more than 20 m.
- a square steel pipe is usually manufactured by cold forming using a hot-rolled steel sheet (hot-rolled steel strip) or a thick plate as a raw material.
- a cold forming method used for manufacturing a square steel pipe there are press forming and roll forming.
- When manufacturing a square steel pipe using hot-rolled steel sheet as a raw material using roll forming first form the hot-rolled steel sheet into a round steel pipe, and then cold-form the round steel pipe to obtain a square steel pipe. It is common.
- This square steel pipe manufacturing method using roll forming has the advantage of higher productivity than the square steel pipe manufacturing method using press forming.
- Patent Document 1 contains 0.20% or less of C by weight%, Mn: 0.40 to 0.90%, Nb: 0.005 to 0.040%, and Ti.
- a steel material containing one or two of 0.005 to 0.050% has a rolling reduction in the non-recrystallization temperature range of 55% or more, a rolling end temperature of 730 to 830 ° C, and a coiling temperature of 550 ° C or less.
- the yield ratio is 90% or less and the Charpy absorbed energy at a test temperature of 0 ° C. is 27 J or more.
- a square steel pipe is obtained.
- the steel containing C: 0.07 to 0.18% and Mn: 0.3 to 1.5% by mass% is heated to a heating temperature of 1100 to 1300 ° C., and then the rough rolling is completed.
- the hot-rolled steel sheet as a raw material needs to have a steel structure that suppresses an increase in the yield ratio during forming and excellent low-temperature toughness that can withstand the deterioration of toughness due to large working strain.
- the square steel pipes manufactured by the methods disclosed in Patent Documents 1 to 3 have a problem that the yield ratio becomes high and the yield ratio of 90% or less cannot be satisfied particularly when the plate thickness exceeds 25 mm. is there. That is, in the prior art, a rectangular steel pipe manufactured by cold roll forming cannot be applied as a building member for a middle-rise building having a height of more than 20 m.
- An object of the present invention is to provide a hot-rolled steel sheet for low yield ratio square steel pipe that can have low temperature toughness with absorbed energy of Charpy impact test at ⁇ 20 ° C. of 27 J or more, and a method for producing the same.
- the present invention is a low yield ratio square steel pipe made of a hot-rolled steel sheet having the above-mentioned characteristics, and in the pipe axis direction, the yield strength is 295 MPa or more, the tensile strength is 400 MPa or more, and 90% or less.
- An object of the present invention is to provide a low yield ratio square steel pipe that can exhibit low-temperature toughness that exhibits a low yield ratio and has an absorption energy of 27 J or more in a Charpy impact test at a test temperature of 0 ° C. and a method for producing the same. .
- the inventors investigated a steel structure suitable for suppressing the increase in yield ratio. Specifically, the easiness of work hardening of a ferrite single phase structure (including a bainitic ferrite single phase structure), a structure composed of ferrite and pearlite, a martensite structure, and an upper bainite structure was examined. Note that the higher the work hardening is, the higher the yield ratio is due to the work strain introduced during cold forming. As a result, the ferrite single-phase structure (including bainitic ferrite single-phase structure) is the hardest to work harden, followed by the ferrite and pearlite structures are hard to harden, and the martensite structure and upper bainite structure are the most work hardened. I found it easy to do.
- the inventors have conducted further detailed studies and have come up with an invention.
- the gist of the present invention is as follows.
- the steel structure at the center of the plate thickness is composed of a main phase composed of ferrite and a second phase composed of one or more selected from pearlite, pseudo pearlite and upper bainite and having an area fraction of 8 to 20%.
- a hot-rolled steel sheet for a low yield ratio square steel pipe characterized in that the steel structure on the front and back surfaces of the plate is a single phase of ferrite or a single phase of bainitic ferrite and has an average crystal grain size of 2 to 20 ⁇ m.
- the composition further contains one or more selected from mass%, Nb: 0.04% or less, Ti: 0.02% or less, and V: 0.10% or less.
- the steel material is subjected to a hot rolling process, a cooling process and a winding process in this order to form a hot rolled steel sheet.
- the steel material is a steel material having the composition according to any one of [1] to [4],
- the hot rolling step after the steel material is heated to a heating temperature of 1100 to 1300 ° C., the heated steel material is subjected to rough rolling to a rough rolling end temperature of 1150 to 950 ° C., and finish rolling start temperature: 1100 to 850 ° C., finish rolling finish temperature: 900 to 750 ° C.
- the cooling step is a step of performing hot rolling to obtain a hot rolled sheet
- the cooling step is a step of cooling the hot-rolled sheet to a cooling stop temperature: 580 ° C. or less at a cooling rate at which the average cooling rate from the start of cooling to the stop of cooling is 4 to 25 ° C./s at the plate thickness center temperature.
- the initial cooling step which is between 10s from the start of cooling, has at least one cooling step of 0.2s or more and less than 3.0s
- a low yield ratio square steel pipe characterized by using the hot rolled steel sheet for low yield ratio square steel pipe as described in any one of [1] to [5] above.
- a hot-rolled steel sheet obtained by the method for producing a hot-rolled steel sheet for a low yield ratio rectangular steel pipe according to [6] or [7] is obtained by cold-rolling to obtain a rectangular steel pipe.
- the steel structure at the center of the plate thickness is composed of a main phase composed of ferrite and a second phase composed of one or more selected from pearlite, pseudo pearlite and upper bainite and having an area fraction of 8 to 20%.
- a low-yield-ratio rectangular steel pipe characterized in that the steel structure on the front and back sides of the plate thickness is a single phase of ferrite or a single phase of bainitic ferrite and has an average crystal grain size of 2 to 20 ⁇ m.
- the yield strength 200 MPa or more
- the tensile strength: 400 MPa or more a low yield ratio of 75% or less
- a low temperature at which the absorbed energy of the Charpy impact test at a test temperature of ⁇ 20 ° C. is 27 J or more.
- a hot-rolled steel sheet for low yield ratio square steel pipe having toughness can be provided. And even if this hot-rolled steel sheet has a thickness of more than 25 mm, in a rectangular steel pipe manufactured by cold roll forming using this as a raw material, the yield strength: 295 MPa or more in the tube axis direction
- the hot-rolled steel sheet for low yield ratio square steel pipe of the present invention is in mass%, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, having a component composition consisting of the balance Fe and inevitable impurities, And a main phase composed of ferrite and a second phase composed of one or more selected from pearlite, pseudo-pearlite and upper bainite and having an area fraction of 8 to 20%.
- the steel structure including the phase has an average crystal grain size of 7 to 20 ⁇ m
- the steel structure on the front and back sides of the plate thickness is a ferrite single phase or a bainitic ferrite single phase
- the average crystal grain size is 2 to 20 ⁇ m. It is characterized by this.
- the “hot rolled steel sheet” includes a hot rolled steel sheet and a hot rolled steel strip.
- C 0.07 to 0.20%
- C is an element that contributes to the formation of pearlite, which is one of the second phases, while increasing the strength of the steel sheet by solid solution strengthening.
- a content of 0.07% or more is required.
- the content exceeds 0.20%, there is a concern that a martensite structure is generated during on-site welding of square steel pipes (for example, when welding square steel pipes), which causes weld cracking. Therefore, C is limited to the range of 0.07 to 0.20%.
- the lower limit of C is preferably 0.09%, and the upper limit is preferably 0.18%.
- Mn 0.3 to 2.0%
- Mn is an element that increases the strength of the steel sheet through solid solution strengthening, and needs to be contained in an amount of 0.3% or more in order to ensure a desired steel sheet strength. If the content is less than 0.3%, the ferrite transformation start temperature rises and the structure tends to become excessively coarse. On the other hand, if the content exceeds 2.0%, the hardness of the central segregation part increases, which may cause cracks during field welding of the square steel pipe. Therefore, Mn is limited to the range of 0.3 to 2.0%.
- the upper limit of Mn is preferably 1.6%. More preferably, the upper limit is 1.4%.
- P 0.03% or less
- P is an element that has the effect of segregating at the ferrite grain boundaries and lowering toughness.
- P is preferably reduced as much as possible.
- excessive reduction leads to an increase in refining costs, so 0.002% or more is preferable.
- 0.03% is acceptable. For this reason, P was limited to 0.03% or less.
- P is preferably 0.025% or less.
- S 0.015% or less S is present as sulfide in steel, and is mainly present as MnS within the composition range of the present invention. Since MnS is thinly stretched in the hot rolling step and adversely affects ductility and toughness, it is desirable to reduce MnS as much as possible in the present invention. However, excessive reduction leads to an increase in refining costs, so S is preferably 0.0002% or more. In addition, up to 0.015% is acceptable. For this reason, S was limited to 0.015% or less. S is preferably 0.010% or less.
- Al 0.01 to 0.06%
- Al is an element that acts as a deoxidizer and has the effect of fixing N as AlN. In order to acquire such an effect, 0.01% or more of content is required. If it is less than 0.01%, the deoxidizing power is insufficient when Si is not added, the oxide inclusions increase, and the cleanliness of the steel sheet decreases. On the other hand, when the content exceeds 0.06%, the amount of solute Al increases, and the welded portion is increased during the longitudinal welding of the square steel pipe (when welding the square steel pipe), particularly in the atmosphere. The risk of forming an oxide increases, and the toughness of the welded portion of the square steel pipe decreases. For this reason, Al was limited to 0.01 to 0.06%. Al preferably has a lower limit of 0.02% and an upper limit of 0.05%.
- N 0.006% or less
- N is an element having an action of lowering toughness by firmly fixing dislocation movement.
- N is preferably 0.005% or less.
- Si Less than 0.4% Si is an element that contributes to the increase in strength of the steel sheet by solid solution strengthening, and can be contained as necessary in order to ensure a desired steel sheet strength. In order to acquire such an effect, it is desirable to contain exceeding 0.01%. However, when the content is 0.4% or more, a firelight called red scale is easily formed on the surface of the steel sheet, and the appearance quality of the surface often decreases. For this reason, when it contains, it is preferable to set it as less than 0.4%. In particular, when Si is not added, Si is an inevitable impurity, and its level is 0.01% or less.
- Nb 0.04% or less
- Ti 0.02% or less
- V 0.10% or less
- Nb, Ti, and V are all fine carbides and nitriding in steel It is an element that forms an object and contributes to improving the strength of steel through precipitation strengthening. If contained, the yield ratio after steel pipe forming tends to be high. For this reason, it is desirable not to contain in this invention. However, as long as the yield ratio of the square steel pipe is 90% or less, it may be contained for the purpose of adjusting the strength.
- the ranges are Nb: 0.04% or less, Ti: 0.02% or less, and V: 0.10% or less, respectively. When Nb, Ti, or V is contained, it is preferable that Nb is 0.001% or more, Ti is 0.001% or more, and V is 0.001% or more.
- B 0.008% or less
- B is an element that delays the ferrite transformation in the cooling process, promotes the formation of low-temperature transformed ferrite, that is, the ash-like ferrite phase, and increases the steel sheet strength. Increases the yield ratio of the steel sheet and hence the square steel pipe. For this reason, in this invention, if it is a range which the yield ratio of a square steel pipe will be 90% or less, it can contain as needed in order to adjust an intensity
- B preferably has a lower limit of 0.0001% and an upper limit of 0.0015%. More preferably, the lower limit is 0.0003% and the upper limit is 0.0008%.
- the balance other than the above components is Fe and inevitable impurities.
- As an inevitable impurity for example, O: 0.005% or less is acceptable.
- the steel structure of the hot rolled steel sheet for low yield ratio square steel pipe of the present invention will be described.
- the steel structure at the center of the plate thickness is composed of a main phase and a second phase.
- the main phase is made of ferrite, and the area fraction of the main phase is 80 to 92%.
- the second phase is composed of one or more selected from pearlite, pseudo pearlite and upper bainite, and the area fraction of the second phase is 8 to 20%. If the area fraction of the second phase is less than 8%, the desired tensile strength cannot be satisfied. If the area fraction of the second phase exceeds 20%, the desired low temperature toughness cannot be ensured.
- the average crystal grain size of the steel structure including the main phase and the second phase which is the steel structure at the center of the plate thickness, is 7 to 20 ⁇ m.
- the “average grain size of the steel structure including the main phase and the second phase” as used herein refers to the ferrite phase constituting the main phase, the pearlite phase, the pseudo pearlite phase, and the upper bainite phase constituting the second phase. It means the average crystal grain size measured for all crystal grains. If the average crystal grain size is less than 7 ⁇ m, it is too fine to secure a yield ratio of 90% or less for the square steel pipe.
- the average crystal grain size exceeds 20 ⁇ m and becomes coarse, the toughness of the square steel pipe is lowered and the desired toughness cannot be secured.
- the average crystal grain size is preferably 15 ⁇ m or less.
- the steel structure in the central part of the plate thickness is observed by the following method to determine the type of main phase and second phase, the area fraction, and the average crystal grain size of the steel structure including the main phase and the second phase.
- the structure observation specimen taken from the hot-rolled steel sheet is polished so that the cross section in the rolling direction (L section) becomes the observation surface, subjected to nital corrosion, and from the surface of the structure observation specimen (hot-rolled steel sheet surface).
- the steel structure is observed and imaged using an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification: 500 times) with the plate thickness 1 / 2t position as the observation center.
- t is the thickness (plate thickness) of a steel plate.
- the steel structure of the thickness front and back surfaces of the hot rolled steel sheet (both surfaces of the hot rolled steel sheet) is a ferrite single phase or a bainitic ferrite single phase, and an average crystal
- the particle size is 2 to 20 ⁇ m.
- the single phase here refers to a case where the area fraction is 95% or more.
- board thickness front-and-back surface of a hot-rolled steel plate specifically means the area
- the average crystal grain size is less than 2 ⁇ m, the yield strength of the front and back surfaces of the plate will be excessively increased, the load during roll forming will increase, and it will be difficult to form round and square steel tubes. Moreover, when it coarsens exceeding 20 micrometers, the toughness of a square steel pipe will fall and it will become impossible to ensure desired toughness. Therefore, the average crystal grain size is limited to 2 to 20 ⁇ m. The upper limit of the average crystal grain size is preferably 15 ⁇ m.
- the steel structure on the front and back surfaces of the plate thickness is within the range of 1 mm from the surface of the hot-rolled steel sheet, instead of centering the position of the sheet thickness 1 / 2t from the surface of the specimen for structural observation (hot-rolled steel sheet surface). Except as described above, the type of steel structure and the average crystal grain size are determined in the same manner as in the method for observing and measuring the steel structure at the center of the plate thickness.
- all of the component composition, the type of steel structure at the center of the plate thickness, the area fraction and the average crystal grain size, and the type of steel structure on the front and back sides of the plate thickness and the average crystal grain size are as specified above. Yield strength: 200 MPa or more, tensile strength: 400 MPa or more, low yield ratio of 75% or less, and low temperature toughness at which the absorbed energy of Charpy impact test at a test temperature of ⁇ 20 ° C. is 27 J or more.
- the hot-rolled steel sheet can be provided, and the hot-rolled steel sheet is very suitable as a material for a square steel pipe.
- the thickness of the hot rolled steel sheet for low yield ratio square steel pipe of the present invention is not particularly limited, and is, for example, 15 mm or more, preferably more than 25 mm, and more preferably 28 mm or more. If a hot rolled steel sheet for a low yield ratio square steel pipe having a thickness of more than 25 mm is formed into a square steel pipe by cold roll forming, the techniques of Patent Documents 1 to 3 have a problem that the yield ratio is high and insufficient. However, the hot-rolled steel sheet for a low yield ratio square steel pipe of the present invention can provide a square steel pipe with a yield ratio of 90% or less because the increase in the yield ratio is suppressed even if it is an extremely thick wall exceeding 25 mm.
- the method for producing a hot-rolled steel sheet for a low yield ratio square steel pipe according to the present invention is a hot-rolled steel sheet obtained by subjecting a steel material having the above-described component composition to a specific hot-rolling process, cooling process and winding process in this order.
- the method for producing a hot-rolled steel sheet for a low yield ratio square steel pipe according to the present invention includes subjecting a steel material to a hot-rolled steel sheet by subjecting a steel material to a hot-rolling process, a cooling process, and a winding process in this order.
- the material is a steel material having the above-described component composition, and after the hot rolling process, the steel material is heated to a heating temperature of 1100 to 1300 ° C., and then the rough rolling finish temperature is set to 1150 to 950 ° C. Rough rolling is performed, and the finish rolling start temperature is 1100 to 850 ° C. and the finish rolling finish temperature is 900 to 750 ° C. to form hot rolled sheets, and the cooling process is the thickness of the hot rolled sheets This is a process of cooling to a cooling stop temperature: 580 ° C.
- the temperature is the surface temperature of a steel material, a sheet bar, a hot-rolled plate, a steel plate, etc. unless otherwise specified.
- the surface temperature can be measured with a radiation thermometer or the like.
- the average cooling rate is ((temperature before cooling ⁇ temperature after cooling) / cooling time) unless otherwise specified.
- the manufacturing method of the steel material having the above-described component composition is not particularly limited, and is manufactured by a generally known casting method such as a converter, an electric furnace, a vacuum melting furnace, etc., and by a generally known casting method such as a continuous casting method. , Manufactured to the desired dimensions.
- the molten steel may be further subjected to secondary refining such as ladle refining. Further, there is no problem if an ingot-bundling method is applied instead of the continuous casting method.
- the steel material having the above-described composition is heated to a heating temperature of 1100 to 1300 ° C., and then the rough rolling finish temperature is set to 1150 to 950 ° C. Rough rolling is performed, and finish rolling start temperature (finish rolling entry temperature): 1100 to 850 ° C., finish rolling finish temperature (finish rolling exit temperature): 900 to 750 ° C. is performed to obtain a hot rolled sheet.
- Heating temperature 1100-1300 ° C
- the heating temperature of the steel material is 1100 to 1300 ° C., and preferably the upper limit is 1280 ° C.
- a heating temperature in the range of 1100 ° C. or lower and the Ar 3 transformation point or higher may be selected.
- the thickness of the steel material may be about 200 to 350 mm that is usually used, and is not particularly limited.
- the heated steel material is then roughly rolled into a sheet bar or the like.
- Rough rolling finish temperature 950-1150 ° C
- the heated steel material is refined by processing and recrystallizing austenite grains by rough rolling. If the rough rolling end temperature is less than 950 ° C., the load resistance and rolling torque of the rough rolling mill are likely to be insufficient. On the other hand, when the temperature exceeds 1150 ° C., the austenite grains become coarse, and it is difficult to secure a desired average crystal grain size of 20 ⁇ m or less even if finish rolling is performed thereafter. For this reason, the rough rolling end temperature is limited to a range of 950 to 1150 ° C. This rough rolling end temperature range can be achieved by adjusting the heating temperature of the steel material, the residence between passes of rough rolling, the thickness of the steel material, and the like.
- the lower limit of the rough rolling end temperature may be Ar 3 transformation point + 100 ° C. or higher.
- the thickness (thickness of the sheet bar, etc.) at the stage when the rough rolling is finished is not particularly limited as long as it can be a product plate (hot rolled steel plate) having a desired product thickness by finish rolling. About 32 to 60 mm is suitable.
- finish rolling is then performed by a tandem rolling mill to obtain a hot rolled steel sheet.
- Finish rolling start temperature finish rolling entrance temperature: 1100-850 ° C
- finish rolling entry temperature finish rolling entry temperature
- the processing strain introduced by the rolling process tends to remain, and the ⁇ grains can be easily refined.
- finishing rolling start temperature finishing rolling entry temperature
- the temperature in the vicinity of the steel sheet surface in the finishing mill becomes lower than the Ar 3 transformation point, increasing the risk of ferrite formation.
- the produced ferrite becomes ferrite grains elongated in the rolling direction by the subsequent finish rolling process, which causes a decrease in workability.
- finish rolling start temperature (finish rolling entry side temperature) exceeds 1100 ° C. and becomes high, the effect of refinement of ⁇ grains by the finish rolling described above is reduced, and the desired heat of average grain size: 20 ⁇ m or less. It becomes difficult to ensure the average crystal grain size of the rolled steel sheet. For this reason, the finish rolling start temperature is limited to a range of 1100 to 850 ° C.
- the finish rolling start temperature is preferably 1050 to 850 ° C.
- Finish rolling finish temperature (finish rolling exit temperature): 900-750 ° C
- finish rolling finish temperature (finish rolling exit temperature) exceeds 900 ° C. and becomes a high temperature
- the processing strain applied at the time of finish rolling is insufficient, and the refinement of ⁇ grains is not achieved. Therefore, the average crystal grain size: It becomes difficult to ensure the average crystal grain size of a desired hot-rolled steel sheet of 20 ⁇ m or less.
- finish rolling finish temperature (finish rolling exit temperature) is less than 750 ° C., the temperature in the vicinity of the steel sheet surface is below the Ar 3 transformation point in the finish mill, and ferrite grains elongated in the rolling direction are formed. There is an increased risk of becoming mixed and reducing workability. For this reason, the finish rolling finish temperature (finish rolling exit temperature) is limited to the range of 900 to 750 ° C.
- the finish rolling finish temperature is preferably 850 ° C. at the upper limit.
- the hot-rolled sheet obtained by finish rolling is cooled at a cooling rate at which the average cooling rate from the start of cooling to the cooling stop (cooling end) is 4 to 25 ° C./s at the plate thickness center temperature: 580 Cool to below °C.
- the cooling performed in the cooling process is performed by water cooling (water cooling) such as water column cooling, spray cooling, mist cooling, or the like, or gas jet cooling for injecting a cooling gas.
- water cooling water cooling
- the average cooling rate at the steel sheet thickness center is less than 4 ° C./s, the frequency of ferrite grain formation decreases, the ferrite crystal grains become coarse, and the desired average crystal grain size: 20 ⁇ m or less at the center of the sheet thickness. The particle size cannot be secured.
- the average cooling rate at the center of the plate thickness is 4 to 25 ° C./s, more preferably the lower limit is 5 ° C./s and the upper limit is 15 ° C./s.
- the average cooling rate at the center of the plate thickness is obtained by ((temperature at the center of plate thickness at the start of cooling ⁇ temperature at the center of plate thickness at the time of cooling stop) / cooling time).
- the temperature at the thickness center of the steel sheet is obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis, and correcting the result by the temperatures of the actual outer surface and inner surface. If the cooling stop temperature exceeds 580 ° C., the desired average crystal grain size of 7 to 20 ⁇ m at the center of the plate thickness cannot be satisfied.
- the average cooling rate in the temperature range of 750 ° C. to 650 ° C. at the steel sheet surface temperature is preferably 20 ° C./s or more. Moreover, it is preferable to start the cooling process immediately (within 5 seconds) after finishing rolling.
- the initial cooling step is 10 s from the start of cooling, that is, the cooling step of 0.2 s or more and less than 3.0 s is performed for 10 seconds (between 10 s) after starting the cooling of the hot rolled sheet. Cool once at least once. This is performed to suppress the formation of a martensite structure or an upper bainite structure on the front and back surfaces of the plate.
- the steel structure on the front and back surfaces of the plate thickness becomes a martensite structure, a bainite structure, or an upper bainite structure. A tick ferrite single phase structure cannot be obtained.
- the time of the cooling process performed during the initial cooling process that is 10 seconds from the start of cooling is limited to 0.2 s or more and less than 3.0 s.
- the time for the cooling step is preferably 0.4 to 2.0 s.
- the number of cooling steps performed during the initial cooling step may be appropriately determined depending on the cooling equipment arrangement, the cooling stop temperature, and the like, and the upper limit is not particularly limited.
- winding is performed at a winding temperature of 580 ° C. or lower, and then allowed to cool. If the winding temperature exceeds 580 ° C., ferrite transformation and pearlite transformation proceed after winding, so that the desired average crystal grain size of 7 to 20 ⁇ m at the center of the plate thickness cannot be satisfied. Even if the coiling temperature is lowered, there will be no problem with the material. There may not be. For this reason, it is preferable that winding temperature shall be 400 degreeC or more.
- the hot rolled steel sheet for low yield ratio square steel pipe of the present invention is obtained by allowing to cool.
- the low yield ratio square steel pipe of the present invention is made from the above hot rolled steel sheet for low yield ratio square steel pipe of the present invention.
- the low yield ratio square steel pipe of the present invention exhibits a low yield ratio of 90% or less at a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, and a test temperature of 0 ° C. in the pipe axis direction.
- the low yield ratio square steel pipe of the present invention can be manufactured by cold forming the hot rolled steel sheet for the low yield ratio square steel pipe of the present invention.
- Cold roll forming means forming with a roll at room temperature without using a heating device or the like.
- the round steel pipe is A square steel pipe is manufactured by forming into a square by the roll forming method used.
- roll forming into a round steel pipe is carried out cold, a large working strain is introduced in the pipe axis direction, so that there is a problem that the yield ratio in the pipe axis direction tends to increase and the toughness tends to decrease.
- the hot rolled steel sheet for the low yield ratio square steel pipe of the present invention is used as a raw material, the above problem, that is, an increase in the yield ratio or the like is suppressed. Even a thick-walled one can have a low yield ratio and low temperature toughness.
- Molten steel was melted in a converter and slabs (steel material: wall thickness 250 mm) having the composition shown in Table 1 were obtained by a continuous casting method. After these slabs (steel materials) were heated to the heating temperature shown in Table 2, they were subjected to a hot rolling process, a cooling process, and a winding process under the conditions shown in Table 2, and then allowed to cool to obtain a sheet thickness: 19 A hot rolled steel sheet having a thickness of 32 mm was used. In addition, the cooling process was started immediately (within 5 seconds) after finishing rolling. Cooling was performed by water cooling. The cooling process was performed by providing a cooling section in which water cooling is not performed during the initial cooling process that is between 10s from the start of cooling. Moreover, the product plate
- the obtained hot-rolled steel sheet was used as a raw material to form a round steel pipe by cold roll forming, and then into a square steel pipe (400 to 550 mm square) by cold roll forming.
- Test pieces were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and impact test. The results are shown in Table 3.
- the structure observation was performed by the above-described method. About the central portion of the plate thickness, the type of the main phase and the second phase, the area fraction, the average grain size of the steel structure including the main phase and the second phase (" In the “steel structure at the center of the plate thickness” column, it is simply described as “average crystal grain size”), and the type of steel structure and the average crystal grain size were determined for the front and back surfaces of the plate thickness.
- test piece was extract
- the results are shown in Table 3.
- the test method was as follows. (3) Square steel pipe tensile test JIS No. 5 tensile test specimen was sampled from the flat part of the obtained square steel pipe so that the tensile direction would be the longitudinal direction of the pipe, and the tensile test was conducted in accordance with the provisions of JIS Z 2241. The yield strength YS and the tensile strength TS were measured, and the yield ratio YR (%) defined by (yield strength) / (tensile strength) ⁇ 100 (%) was calculated.
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Abstract
Description
板厚中心部の鋼組織が、フェライトからなる主相と、パーライト、擬似パーライトおよび上部ベイナイトから選択される1種または2種以上からなり面積分率が8~20%である第二相とを有し、主相と第二相とを含む鋼組織の平均結晶粒径が7~20μmであり、
板厚表裏面の鋼組織が、フェライト単相またはベイニティックフェライト単相であり、平均結晶粒径が2~20μmであることを特徴とする低降伏比角形鋼管用熱延鋼板。
前記鋼素材を、前記[1]~[4]のいずれかに記載の成分組成を有する鋼素材とし、
前記熱延工程が、前記鋼素材を加熱温度:1100~1300℃に加熱した後、該加熱された鋼素材に粗圧延終了温度:1150~950℃とする粗圧延を施し、仕上圧延開始温度:1100~850℃、仕上圧延終了温度:900~750℃とする仕上圧延を施し熱延板とする工程であり、
前記冷却工程が、前記熱延板を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が4~25℃/sとなる冷却速度で冷却停止温度:580℃以下まで冷却を施す工程であって、冷却開始から10s間である初期冷却工程において0.2s以上3.0s未満の放冷工程を1回以上有し、
前記巻取工程が、前記冷却工程後の熱延板を巻取温度:580℃以下で巻取り、その後放冷する工程であることを特徴とする低降伏比角形鋼管用熱延鋼板の製造方法。
板厚中心部の鋼組織が、フェライトからなる主相と、パーライト、擬似パーライトおよび上部ベイナイトから選択される1種または2種以上からなり面積分率が8~20%である第二相とを有し、主相と第二相とを含む鋼組織の平均結晶粒径が7~20μmであり、
板厚表裏面の鋼組織が、フェライト単相またはベイニティックフェライト単相であり、平均結晶粒径が2~20μmであることを特徴とする低降伏比角形鋼管。
Cは、固溶強化により鋼板の強度を増加させるとともに、第二相の一つであるパーライトの形成に寄与する元素である。所望の引張特性、靭性、さらに所望の鋼板組織を確保するためには、0.07%以上の含有を必要とする。一方、0.20%を超える含有は、角形鋼管の現場溶接時(例えば、角形鋼管同士の溶接時)にマルテンサイト組織が生成し溶接割れの原因となる懸念がある。このため、Cは0.07~0.20%の範囲に限定した。Cは、好ましくは下限が0.09%であり、上限が好ましくは0.18%である。
Mnは、固溶強化を介して鋼板の強度を増加させる元素であり、所望の鋼板強度を確保するために、0.3%以上の含有を必要とする。0.3%未満の含有では、フェライト変態開始温度の上昇を招き、組織が過度に粗大化しやすい。一方、2.0%を超えて含有すると、中心偏析部の硬度が上昇し、角形鋼管の現場溶接時の割れの原因となる懸念がある。このため、Mnは0.3~2.0%の範囲に限定した。Mnは、好ましくは上限が1.6%である。より好ましくは、上限が1.4%である。
Pは、フェライト粒界に偏析して、靭性を低下させる作用を有する元素であり、本発明では、不純物としてできるだけ低減することが望ましい。しかし、過度の低減は、精錬コストの高騰を招くため、0.002%以上とすることが好ましい。なお、0.03%までは許容できる。このため、Pは0.03%以下に限定した。Pは、好ましくは0.025%以下である。
Sは、鋼中では硫化物として存在し、本発明の組成範囲であれば、主としてMnSとして存在する。MnSは、熱延工程で薄く延伸され、延性、靭性に悪影響を及ぼすため、本発明ではできるだけMnSは低減することが望ましい。しかし、過度の低減は、精錬コストの高騰を招くため、Sは0.0002%以上とすることが好ましい。なお、0.015%までは許容できる。このため、Sは0.015%以下に限定した。Sは、好ましくは0.010%以下である。
Alは、脱酸剤として作用するとともに、AlNとしてNを固定する作用を有する元素である。このような効果を得るためには、0.01%以上の含有を必要とする。0.01%未満では、Si無添加の場合に脱酸力が不足し、酸化物系介在物が増加し、鋼板の清浄度が低下する。一方、0.06%を超える含有は、固溶Al量が増加し、角形鋼管の長手溶接時(角形鋼管の製造時の溶接時)に、特に大気中での溶接の場合に、溶接部に酸化物を形成させる危険性が高くなり、角形鋼管溶接部の靭性が低下する。このため、Alは0.01~0.06%に限定した。Alは、好ましくは、下限が0.02%であり上限が0.05%である。
Nは、転位の運動を強固に固着することで靭性を低下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く、0.006%までは許容できる。このため、Nは0.006%以下に限定した。Nは、好ましくは0.005%以下である。
Siは、固溶強化で鋼板の強度増加に寄与する元素であり、所望の鋼板強度を確保するために、必要に応じて含有できる。このような効果を得るためには、0.01%を超えて含有することが望ましい。しかし、0.4%以上の含有は、鋼板表面に赤スケールと称するファイアライトが形成しやすくなり、表面の外観性状が低下する場合が多くなる。このため、含有する場合には、0.4%未満とすることが好ましい。なお、特にSiを添加しない場合は、Siは不可避的不純物として、そのレベルは0.01%以下である。
Nb、Ti、Vはいずれも、鋼中で微細な炭化物、窒化物を形成し、析出強化を通じて鋼の強度向上に寄与する元素である。含有すれば鋼管成形後の降伏比が高くなる傾向となる。このため、本発明では、含有しないことが望ましい。しかし、角形鋼管の降伏比が90%以下となるような範囲であれば、強度を調整する目的で含有してもよい。範囲はそれぞれ、Nb:0.04%以下、Ti:0.02%以下、V:0.10%以下である。なお、Nb、Ti、Vのいずれかを含有する場合、Nb:0.001%以上、Ti:0.001%以上、V:0.001%以上であることが好ましい。
Bは、冷却過程のフェライト変態を遅延させ、低温変態フェライト、すなわち、アシュキュラーフェライト相の形成を促進し、鋼板強度を増加させる作用を有する元素であり、Bの含有は、鋼板の降伏比、したがって角形鋼管の降伏比を増加させる。このため、本発明では、角形鋼管の降伏比が90%以下となるような範囲であれば、強度を調整する目的で必要に応じて含有できる。このような範囲はB:0.008%以下である。Bは、好ましくは、下限が0.0001%であり上限が0.0015%である。さらに好ましくは、下限が0.0003%であり上限が0.0008%である。
鋼素材の加熱温度が1100℃未満では、被圧延材の変形抵抗が大きくなりすぎて、粗圧延機、仕上圧延機の耐荷重、圧延トルクの不足が生じ、圧延が困難となる。一方、1300℃を超えると、オーステナイト結晶粒が粗大化し、粗圧延、仕上圧延でオーステナイト粒の加工・再結晶を繰返しても、細粒化することが困難となり、所望の熱延鋼板の平均結晶粒径を確保することが困難となる。このため、鋼素材の加熱温度は1100~1300℃であり、好ましくは、上限が1280℃である。また、圧延機の耐荷重、圧延トルクに余裕がある場合には、1100℃以下Ar3変態点以上の範囲の加熱温度を選択してもよい。鋼素材の厚さは、通常用いられる200~350mm程度でよく、特に限定されない。
加熱された鋼素材は、粗圧延により、オーステナイト粒が加工、再結晶されて微細化する。粗圧延終了温度が950℃未満では、粗圧延機の耐荷重、圧延トルクの不足が生じやすくなる。一方、1150℃を超えて高温となると、オーステナイト粒が粗大化し、その後に仕上圧延を施しても、平均結晶粒径:20μm以下という所望の平均結晶粒径を確保することが困難となる。このため、粗圧延終了温度は950~1150℃の範囲に限定する。この粗圧延終了温度範囲は、鋼素材の加熱温度、粗圧延のパス間での滞留、鋼素材厚さ等を調整することにより達成できる。なお、圧延機の耐荷重、圧延トルクに余裕がある場合には、粗圧延終了温度の下限を、Ar3変態点+100℃以上としてもよい。粗圧延が終了した段階での厚さ(シートバー等の厚さ)は、仕上圧延で、所望の製品厚さの製品板(熱延鋼板)とすることができればよく、特に限定する必要はなく、32~60mm程度が適当である。
仕上圧延では、圧延加工-再結晶が繰り返され、オーステナイト(γ)粒の微細化が進行する。仕上圧延開始温度(仕上圧延入側温度)が低くなると、圧延加工により導入される加工歪が残存しやすくなり、γ粒の微細化を達成しやすい。仕上圧延開始温度(仕上圧延入側温度)が、850℃未満では、仕上圧延機内で鋼板表面近傍の温度がAr3変態点以下となりフェライトが生成する危険性が増大する。生成したフェライトは、その後の仕上圧延加工により圧延方向に伸長したフェライト粒となり、加工性低下の原因となる。一方、仕上圧延開始温度(仕上圧延入側温度)が、1100℃を超えて高温となると、上記した仕上圧延によるγ粒の微細化効果が低減し、平均結晶粒径:20μm以下の所望の熱延鋼板の平均結晶粒径を確保することが困難となる。このため、仕上圧延開始温度は1100~850℃の範囲に限定する。仕上圧延開始温度は、好ましくは1050~850℃である。
仕上圧延終了温度(仕上圧延出側温度)が900℃を超えて高温となると、仕上圧延時に付加される加工歪が不足し、γ粒の微細化が達成されず、したがって、平均結晶粒径:20μm以下の所望の熱延鋼板の平均結晶粒径を確保することが困難となる。一方、仕上圧延終了温度(仕上圧延出側温度)が750℃未満では、仕上圧延機内で鋼板表面近傍の温度がAr3変態点以下となり、圧延方向に伸長したフェライト粒が形成され、フェライト粒が混粒となり、加工性が低下する危険性が増大する。このため、仕上圧延終了温度(仕上圧延出側温度)は900~750℃の範囲に限定する。仕上圧延終了温度は、好ましくは、上限が850℃である。
(1)引張試験
得られた熱延鋼板から、引張方向が圧延方向となるように、JIS5号引張試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、降伏強さYS、引張強さTSを測定し、(降伏強さ)/(引張強さ)×100(%)で定義される降伏比YR(%)を算出した。
(2)シャルピー衝撃試験
得られた熱延鋼板の板厚1/2t位置から、試験片長手方向が圧延方向と直交する方向となるように、Vノッチ試験片を採取し、JIS Z 2242の規定に準拠して、試験温度:-20℃で、シャルピー衝撃試験を実施し、吸収エネルギー(J)を求めた。なお、試験片本数は各3本とし、その平均値を算出した。
(3)角形鋼管引張試験
得られた角形鋼管平坦部から、引張方向が管長手方向となるように、JIS5号引張試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、降伏強さYS、引張強さTSを測定し、(降伏強さ)/(引張強さ)×100(%)で定義される降伏比YR(%)を算出した。
(4)角形鋼管衝撃試験
得られた角形鋼管平坦部の板厚1/4t位置から、試験片長手方向が管周方向となるように、Vノッチ試験片を採取し、JIS Z 2242の規定に準拠して、試験温度:0℃で、シャルピー衝撃試験を実施し、吸収エネルギー(J)を求めた。なお、試験片本数は各3本の平均値とした。
Claims (10)
- 質量%で、C:0.07~0.20%、
Mn:0.3~2.0%、
P:0.03%以下、
S:0.015%以下、
Al:0.01~0.06%、
N:0.006%以下を含有し、残部Feおよび不可避的不純物からなる成分組成を有し、
板厚中心部の鋼組織が、フェライトからなる主相と、パーライト、擬似パーライトおよび上部ベイナイトから選択される1種または2種以上からなり面積分率が8~20%である第二相とを有し、主相と第二相とを含む鋼組織の平均結晶粒径が7~20μmであり、
板厚表裏面の鋼組織が、フェライト単相またはベイニティックフェライト単相であり、平均結晶粒径が2~20μmであることを特徴とする低降伏比角形鋼管用熱延鋼板。 - 前記成分組成に加えてさらに、質量%で、Si:0.4%未満を含有することを特徴とする請求項1に記載の低降伏比角形鋼管用熱延鋼板。
- 前記成分組成に加えてさらに、質量%で、Nb:0.04%以下、
Ti:0.02%以下
およびV:0.10%以下から選択される1種または2種以上を含有することを特徴とする請求項1または2に記載の低降伏比角形鋼管用熱延鋼板。 - 前記成分組成に加えてさらに、質量%で、B:0.008%以下を含有することを特徴とする請求項1~3のいずれか一項に記載の低降伏比角形鋼管用熱延鋼板。
- 板厚が25mm超であることを特徴とする請求項1~4のいずれか一項に記載の低降伏比角形鋼管用熱延鋼板。
- 鋼素材に、熱延工程、冷却工程および巻取工程をこの順に施して、熱延鋼板とするにあたり、
前記鋼素材を、請求項1~4のいずれか一項に記載の成分組成を有する鋼素材とし、
前記熱延工程が、前記鋼素材を加熱温度:1100~1300℃に加熱した後、該加熱された鋼素材に粗圧延終了温度:1150~950℃とする粗圧延を施し、仕上圧延開始温度:1100~850℃、仕上圧延終了温度:900~750℃とする仕上圧延を施し熱延板とする工程であり、
前記冷却工程が、前記熱延板を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が4~25℃/sとなる冷却速度で冷却停止温度:580℃以下まで冷却を施す工程であって、冷却開始から10s間である初期冷却工程において0.2s以上3.0s未満の放冷工程を1回以上有し、
前記巻取工程が、前記冷却工程後の熱延板を巻取温度:580℃以下で巻取り、その後放冷する工程であることを特徴とする低降伏比角形鋼管用熱延鋼板の製造方法。 - 前記熱延鋼板の板厚が25mm超であることを特徴とする請求項6に記載の低降伏比角形鋼管用熱延鋼板の製造方法。
- 請求項1~5のいずれか一項に記載の低降伏比角形鋼管用熱延鋼板を素材とすることを特徴とする低降伏比角形鋼管。
- 請求項6または7に記載の低降伏比角形鋼管用熱延鋼板の製造方法で得られた熱延鋼板を冷間でロール成形することにより角形鋼管を得ることを特徴とする低降伏比角形鋼管の製造方法。
- 質量%で、C:0.07~0.20%、
Mn:0.3~2.0%、
P:0.03%以下、
S:0.015%以下、
Al:0.01~0.06%、
N:0.006%以下を含有し、残部Feおよび不可避的不純物からなる成分組成を有し、
板厚中心部の鋼組織が、フェライトからなる主相と、パーライト、擬似パーライトおよび上部ベイナイトから選択される1種または2種以上からなり面積分率が8~20%である第二相とを有し、主相と第二相とを含む鋼組織の平均結晶粒径が7~20μmであり、
板厚表裏面の鋼組織が、フェライト単相またはベイニティックフェライト単相であり、平均結晶粒径が2~20μmであることを特徴とする低降伏比角形鋼管。
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