WO2016047023A1 - 電気抵抗溶接鋼管用鋼帯および電気抵抗溶接鋼管ならびに電気抵抗溶接鋼管用鋼帯の製造方法 - Google Patents
電気抵抗溶接鋼管用鋼帯および電気抵抗溶接鋼管ならびに電気抵抗溶接鋼管用鋼帯の製造方法 Download PDFInfo
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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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
- F16L9/00—Rigid pipes
- F16L9/16—Rigid pipes wound from sheets or strips, with or without reinforcement
- F16L9/165—Rigid pipes wound from sheets or strips, with or without reinforcement of metal
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- 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
- B21B1/24—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 in a continuous or semi-continuous process
- B21B1/26—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 in a continuous or semi-continuous process by hot-rolling, e.g. Steckel hot mill
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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- 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
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- 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
- B21C37/083—Supply, or operations combined with supply, of strip material
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/08—Seam welding not restricted to one of the preceding subgroups
- B23K11/087—Seam welding not restricted to one of the preceding subgroups for rectilinear seams
- B23K11/0873—Seam welding not restricted to one of the preceding subgroups for rectilinear seams of the longitudinal seam of tubes
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/001—Heat treatment of ferrous alloys containing Ni
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- C21D6/00—Heat treatment of ferrous alloys
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- 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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- 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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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- 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
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16L—PIPES; JOINTS OR FITTINGS FOR PIPES; SUPPORTS FOR PIPES, CABLES OR PROTECTIVE TUBING; MEANS FOR THERMAL INSULATION IN GENERAL
- F16L9/00—Rigid pipes
- F16L9/17—Rigid pipes obtained by bending a sheet longitudinally and connecting the edges
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/04—Tubular or hollow articles
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- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/04—Tubular or hollow articles
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- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2103/00—Materials to be soldered, welded or cut
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- 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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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
- C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
Definitions
- the present invention relates to a steel strip for an electric resistance welded steel pipe, and more particularly to a steel strip for an electric resistance welded steel pipe excellent in sour resistance, which is suitable as a material for a line pipe for transporting oil, natural gas, or the like. Moreover, this invention relates to the manufacturing method of the steel pipe manufactured using the said steel strip for electrical resistance welding steel pipes, and the said steel strip for electrical resistance welding steel pipes.
- a high-strength electric resistance welded steel pipe is a pipe obtained by electric resistance welding of a hot-rolled steel strip, and high-strength steel is used as a material to increase the strength.
- HIC Hydrogen Induced Cracking
- SSC Sulfide Stress Cracking
- Patent Document 1 in a high-strength hot-rolled steel sheet for line pipes having a yield strength of 450 MPa or more, sour resistance is improved by setting the area ratio of the bainite phase or bainitic ferrite phase to 95% or more. Is described.
- Patent Document 2 describes that in a hot-rolled steel sheet for high-strength electric resistance welded steel pipe, sour resistance is improved by controlling the steel structure.
- the steel structure disclosed in Patent Literature 2 is mainly bainitic ferrite or a mixed structure of bainitic ferrite and polygonal ferrite, and has a pearlite occupancy of 2% by volume or less.
- an object of the present invention is to provide a steel strip for an electric resistance welded steel pipe having strength of X70 grade or higher and having extremely excellent HIC resistance and SSC resistance.
- Another object of the present invention is to provide an electric resistance welded steel pipe formed using the steel strip as a material, and a method for manufacturing the steel strip.
- X70 grade is a grade of line pipe material defined in API (American Petroleum Institute) standard, and means that the yield strength (YS) is 485 MPa or more.
- the bainite structure has superior sour resistance compared to the pearlite structure, but it is the same as the pearlite structure in that it consists of a ferrite phase and a cementite phase, so the sour resistance is sufficient. There is no such thing.
- the steel structure is mainly composed of bainite or bainitic ferrite, the structure tends to vary depending on the temperature conditions during production, and it is difficult to obtain high HIC resistance over the entire steel strip.
- the gist configuration of the present invention is as follows. (1) In mass%, C: 0.02 to 0.06%, Si: 0.1 to 0.3%, Mn: 0.8 to 1.3%, P: 0.01% or less, S: 0.001% or less, V: 0.04 to 0.07%, Nb: 0.04 to 0.07%, Ti: 0.01 to 0.04%, Cu: 0.1 to 0.3%, Ni: 0.1 to 0.3%, Ca: 0.001 to 0.005%, Al: 0.01 to 0.07%, N: 0.007% or less, The balance Fe and inevitable impurities, And the content of C, Nb, V, and Ti has a component composition that satisfies the condition of the following formula (1), Steel strip for electric resistance welded steel pipe with a ferrite area ratio of 90% or more. [C] -12 ([Nb] /92.9+ [V] /50.9+ [Ti] /47.9) ⁇ 0.03% (1) However, [M] is the content of element M expressed in mass% units.
- the steel material is mass%, C: 0.02 to 0.06%, Si: 0.1 to 0.3%, Mn: 0.8 to 1.3%, P: 0.01% or less, S: 0.001% or less, V: 0.04 to 0.07%, Nb: 0.04 to 0.07%, Ti: 0.01 to 0.04%, Cu: 0.1 to 0.3%, Ni: 0.1 to 0.3%, Ca: 0.001 to 0.005%, Al: 0.01 to 0.07%, N: 0.007% or less,
- the content of C, Nb, V, and Ti has a component composition that satisfies the condition of the following formula (1), and in the hot rolling, rough rolling and finish rolling are performed,
- the finish rolling start temperature is 950 ° C.
- the finish temperature of the finish rolling is 780 to 850 ° C.
- the cooling rate in the cooling is 20 to 100 ° C./s
- [C] -12 [Nb] /92.9+ [V] /50.9+ [Ti] /47.9) ⁇ 0.03% (1)
- [M] is the content of element M expressed in mass% units.
- a steel strip for an electric resistance welded steel pipe having a yield strength (YS) of 485 MPa or more and having excellent HIC resistance can be obtained. Since the steel strip of the present invention has a structure consisting essentially of a ferrite single phase, there is less variation in HIC resistance than a steel strip mainly composed of a bainite phase or a bainitic ferrite phase. Further, by using a high concentration precipitation strengthening element, it is possible to stably improve the strength by forming carbide and improve the sour resistance by precipitation of ferrite without reducing the coiling temperature so much.
- C 0.02 to 0.06%
- C is an element having an effect of increasing the strength of steel by forming precipitates with elements such as Nb, V, and Ti.
- steel needs to contain 0.02% or more of C.
- C content of steel needs to be 0.06% or less.
- the C content is preferably 0.03 to 0.05%. The C content must be adjusted in accordance with the amounts of Nb, V, and Ti as will be described later.
- Si 0.1 to 0.3%
- Si is a ferrite-forming element, but in order to precipitate fine precipitates from ferrite, it is necessary to add an amount corresponding to other additive elements.
- the Si content is 0.1 to 0.3%.
- the Si content is preferably 0.15 to 0.25%.
- Mn 0.8 to 1.3%
- Mn is an element that has the effect of delaying the ferrite transformation and precipitating fine precipitates during rapid cooling after finish rolling.
- steel needs to contain 0.8% or more of Mn.
- Mn content is 1.3% or less.
- the Mn content is preferably 0.9 to 1.1%.
- P 0.01% or less
- P is an element that easily segregates in steel, and the segregation deteriorates sour resistance. Therefore, in the present invention, it is important that the P content is 0.01% or less.
- the P content is preferably 0.006% or less.
- the lower limit of the P content is not limited and may be 0%, but industrially it exceeds 0%. Moreover, excessively low P leads to an increase in refining time and cost, so the P content is preferably 0.001% or more.
- S 0.001% or less S forms sulfides in the steel and lowers sour resistance. In order to prevent this, it is important in the present invention that the S content is 0.001% or less.
- the S content is preferably 0.0006% or less. Note that the lower limit of the S content is not limited and may be 0%, but industrially it is over 0%. Moreover, since excessively low S causes an increase in refining time and cost, the S content is preferably 0.0003% or more.
- V 0.04 to 0.07%
- V is an element having a property of forming a carbide with C in the steel and precipitating. This carbide precipitation can increase the strength of the steel (precipitation strengthening). Moreover, the effective C concentration in steel falls by precipitation of the said carbide
- Nb 0.04 to 0.07% Nb, like V, is an element that contributes to increasing the strength of steel by precipitation strengthening. Moreover, Nb also has the effect
- Ti 0.01 to 0.04% Ti is also a carbide forming element, but has a property of forming nitride by reacting with N in steel preferentially over V and Nb. Therefore, by adding an appropriate amount of Ti to the steel, it is possible to prevent Nb and V from reacting with N and to reliably form Nb and V carbides. In order to obtain such an effect, it is important in the present invention that the Ti content is 0.01% or more. When the Ti content is less than 0.01%, coarse precipitates such as Nb (CN) and V (CN) are formed, and the sour resistance of the steel is lowered.
- Ti content shall be 0.04% or less.
- the Ti content is preferably 0.02 to 0.03%.
- Cu 0.1 to 0.3%
- Cu is an element having an action of delaying the ferrite transformation and causing fine precipitation of carbides such as Nb, Ti, and V.
- Cu is also an element that suppresses corrosion in a corrosive environment and improves sour resistance by reducing the amount of hydrogen penetration.
- Cu content shall be 0.1% or more in this invention.
- the effect is saturated even if Cu is added excessively.
- excess Cu increases the roughness of the surface of the steel strip, and as a result, the amount of hydrogen intrusion in the corrosive environment increases and the sour resistance of the steel decreases. Therefore, in this invention, Cu content shall be 0.3% or less.
- the Cu content is preferably 0.2 to 0.3%.
- Ni 0.1-0.3%
- Ni is an element having an action of delaying the ferrite transformation together with Cu and finely depositing carbides such as Nb, Ti, and V.
- Ni like Cu, has the action of suppressing sour resistance in a corrosive environment and improving the sour resistance by reducing the amount of hydrogen penetration. This effect is further enhanced when the steel contains both Cu and Ni.
- Ni content shall be 0.1% or more in this invention.
- the Ni content is 0.3% or less.
- the Ni content is preferably 0.1 to 0.2%.
- Ca 0.001 to 0.005%
- Ca is an element having a function of making sulfide contained in steel spherical and improving sour resistance. In order to obtain such an effect, it is necessary to determine the Ca content according to the S content.
- the steel needs to contain 0.001% or more of Ca.
- the Ca content is less than 0.001%, S spheroidization is not sufficient.
- the Ca content is 0.005% or less.
- the Ca content is preferably 0.002 to 0.003%.
- Al 0.01 to 0.07%
- Al is an element added as a deoxidizer. If the Al content is less than 0.01%, Ca forms an oxide, so that the sulfide spheroidizing effect of Ca cannot be sufficiently obtained. On the other hand, if the Al content is more than 0.07%, coarse alumina is produced and sour resistance is lowered. Therefore, in the present invention, the Al content is set to 0.01 to 0.07%.
- the Al content is preferably 0.02 to 0.04%.
- N 0.007% or less
- N is an element that forms a nitride with Ti or the like.
- it is necessary to form fine carbides in order to obtain a high strength equivalent to X70 grade.
- N content shall be 0.007% or less.
- the N content is preferably 0.005% or less.
- the lower limit is not limited and may be 0%, but industrially it is more than 0%.
- N content in steel with low C content like this invention, in order to suppress the grain growth of a weld part and to ensure the intensity
- the steel strip for electric resistance welded steel pipe of the present invention is composed of inevitable impurities and the remaining Fe in addition to the above components.
- it is important that the steel has the above component composition in order to achieve both the conflicting properties of strength and sour resistance of the steel.
- a steel strip does not contain the following elements.
- their concentration is preferably in the following range: Cr: 0.05% or less, Mo: 0.03% or less, B: 0.0005% or less. More preferably, Cr: 0.02% or less, Mo: 0.01% or less, B: 0.0002% or less.
- steel containing 0.05% or less of Sn can also be used.
- the Sn content is more preferably 0.02% or less.
- W has an action of excessively delaying the ferrite transformation.
- W is an element having an effect of improving the hardenability, so that when it is added in a large amount, the strength locally increases depending on the cooling conditions and the like. In that case, it is necessary to remove the portion, and the yield decreases. Accordingly, in the present invention, the smaller the amount of W contained in the steel, the better.
- the W content is preferably 0.03% or less, and more preferably 0.01% or less.
- the lower limit of the W content is not particularly limited and may be 0%, but industrially it may be more than 0%.
- the carbide forming elements Nb, V, and Ti form carbides with C in the steel and precipitate. Therefore, in addition to the effect
- the effective C concentration means the C content in steel excluding the amount formed by precipitation by forming alloy elements and carbides.
- Nb, V, and Ti mainly form MC type carbide having an atomic ratio of 1: 1 with C. Therefore, assuming that all Nb, V, and Ti contained in the steel form carbides, the effective C concentration can be calculated using the atomic weight of each element, [C] -12 ([Nb] /92.9+ [ V] /50.9+ [Ti] /47.9). In the present invention, as described later, since the steel structure needs to be a ferrite single phase, the effective C concentration needs to be 0.03% or less. The value of 0.03% corresponds to the amount of C that can be dissolved in the ferrite during ferrite precipitation.
- the value on the left side of the formula (1) is preferably 0.02% or less from the viewpoint of reducing the carbides precipitated in the ferrite during cooling from ferrite precipitation to room temperature.
- the value on the left side of the above equation (1) is preferably more than 0% in order to suppress the coarsening of the crystal grains in the weld.
- carbide precipitation does not occur only by the presence of elements such as Nb, V, and Ti.
- Proper carbide precipitation and formation of a ferrite structure can be achieved for the first time by producing a steel strip under an appropriate temperature condition at the same time that the steel component composition satisfies the above conditions. The temperature conditions during production will be described in detail later.
- Ti and N Furthermore, in this invention, it is preferable that Ti content and N content of steel satisfy the conditions of following (2) Formula. [Ti] /47.9 ⁇ [N] / 14 (2) However, [M] is the content of element M expressed in units of mass%.
- Ti has a property of easily forming a nitride as compared with V and Nb. Therefore, by adding a sufficient amount of Ti to the steel, it is possible to prevent Nb and V from reacting with N and forming coarse precipitates. In order to obtain such an effect, it is preferable that Ti contained in the steel is larger in atomic equivalent ratio than N.
- the above expression (2) expresses this using the atomic weights of both elements.
- the steel strip was manufactured according to the procedure described below. First, molten steel having a predetermined composition was melted in a converter and formed into a slab having a thickness of 250 mm by a continuous casting method. Next, the obtained slab was heated to 1220 to 1240 ° C. and hot rolled. In the hot rolling, first, the slab was roughly rolled into a 45 mm thick sheet bar, and then finish rolling was performed to obtain a steel strip having a thickness of 11.3 mm and a width of 1080 mm.
- the obtained steel strip was water-cooled on a run-out table (ROT) and then wound into a coil.
- the manufacturing conditions are as follows. Finish rolling start temperature: 890 to 910 ° C., finish rolling end temperature: 785 to 805 ° C., finish rolling time (time from start to finish of finish rolling): 4 seconds, cooling (water cooling) rate at ROT: 24 to 37 ° C. / S, coiling temperature: 585-615 ° C.
- the V content was varied from 0.002% (no addition) to 0.081%.
- the content of elements other than V is as follows. C: 0.033 to 0.045%, Si: 0.16 to 0.23%, Mn: 0.92 to 1.07%, P: 0.003 to 0.005%, S: 0.0003 to 0.0007%, Nb: 0.050 to 0.058%, Ti: 0.021 to 0.029%, Cu: 0.22 to 0.28%, Ni: 0.12 to 0.18%, Ca: 0.0022 to 0.0029%, Al: 0.023 to 0.038%, N: 0.0036 to 0.0045%, Cr: 0.02%, Mo: 0.01%, B: Less than 0.0001%, and the balance Fe and inevitable impurities.
- FIG. 1 is a plot of CLR values measured for each steel strip against V content.
- the V content is within the range defined in the present invention.
- the deviation of the V content is larger.
- the degree of HIC was large.
- the reason for limitation of the steel structure in this invention is demonstrated.
- the ferrite area ratio in the steel structure is 90% or more.
- the pearlite structure is inferior in sour resistance, it is preferably not contained in the steel.
- the bainite structure has superior sour resistance compared to the pearlite structure, but it is the same as the pearlite structure in that it consists of a ferrite phase and a cementite phase, so the sour resistance is sufficient. That's not true. Therefore, in the present invention, the steel structure is substantially a ferrite single phase.
- the ferrite single phase substantially means that the ferrite area ratio is 90% or more.
- the ferrite area ratio is preferably 95% or more. Such a high ferrite area ratio can be achieved by controlling the composition of steel as described above and manufacturing a steel strip under specific temperature conditions. On the other hand, the upper limit of the ferrite area ratio is not particularly limited, and may be 100%.
- the “ferrite” in the present invention does not include “bainitic ferrite” generated at a low temperature of about 500 ° C. close to the martensitic transformation point. This is because the bainitic ferrite generated at such a low temperature has a small amount of C that can be dissolved, and C that cannot be dissolved forms cementite (Fe 3 C) and degrades sour resistance.
- the lower limit of the area ratio of the structure other than the ferrite phase is not particularly limited, and may be 0%.
- a steel material having the above component composition is melted in accordance with a conventional method.
- the steel material is preferably manufactured by a continuous casting method in order to prevent the formation of a pearlite structure due to segregation, particularly center segregation.
- the thickness of the slab obtained by the continuous casting method is preferably 200 mm or more. Thereby, many recrystallization can be caused at the time of rough rolling in a hot rolling process, and formation of a pearlite structure by segregation can be suppressed.
- the slab thickness is preferably 300 mm or less. A more preferable range of the slab thickness is 240 to 260 mm.
- the slab is heated to a predetermined heating temperature, and hot rolling consisting of rough rolling and finish rolling is performed.
- the heating temperature is preferably 1200 ° C. or higher in order to dissolve precipitates in the steel.
- the heating temperature is preferably 1250 ° C. or lower.
- the steel material is roughly rolled into a sheet bar.
- the thickness of the sheet bar is preferably 40 mm or more in order to increase the reduction ratio in the subsequent finish rolling.
- the sheet bar thickness is preferably 60 mm or less in order to secure a certain rolling reduction and suppress segregation in rough rolling.
- the obtained sheet bar is further finish-rolled to form a steel strip.
- the finish rolling start temperature (finishing temperature of finish rolling) is lowered to 950 ° C. or lower, and finish rolling is performed in the austenite non-recrystallization temperature range.
- the finish rolling start temperature exceeds 950 ° C., the formation of precipitates such as Nb, V and Ti is not sufficient, the strength is lowered, the ferrite phase ratio is lowered, and the sour resistance is deteriorated.
- the finish rolling start temperature is more preferably 910 ° C. or lower.
- the lower limit of the finish rolling start temperature is not particularly limited, but is preferably 850 ° C. or higher.
- the finish rolling is preferably performed for 3 to 15 seconds using a tandem rolling mill.
- the finish rolling finishing temperature (finishing exit temperature) is 780 to 850 ° C. If the finish rolling finish temperature is too low, ferrite precipitates on the steel strip surface layer during finish rolling, and sour resistance deteriorates. Further, when the finish rolling finish temperature exceeds 850 ° C., precipitates such as Nb, V, and Ti are not sufficiently formed. As a result, the strength of the steel strip is lowered and the ferrite phase ratio is lowered due to a decrease in ferrite precipitation nuclei, so Deteriorates.
- the finish rolling finish temperature is preferably 780 to 830 ° C. More preferably, it is 780 to 810 ° C.
- finish rolling in order to ensure the finish temperature, the entire sheet bar or only the edge of the sheet bar is heated, or the sheet bar is once wound into a coil before finish rolling, and then finish rolling is performed. It is forgiven.
- the steel strip obtained by the finish rolling is cooled in order to precipitate fine carbides and improve the strength.
- the said cooling can be performed by water-cooling a steel strip on ROT, for example.
- the cooling rate in the cooling is set to 20 ° C./s or more.
- the cooling rate is set to 100 ° C./s or less.
- the cooling rate is preferably 20 to 50 ° C./s.
- the cooling is performed immediately after finishing rolling is finished until the temperature of the steel strip reaches a predetermined coiling temperature.
- the cooling is performed by water cooling, it is possible to perform air cooling or water cooling again as necessary after completion of the water cooling in consideration of recuperation and the like. Not included in the cooling rate.
- the surface temperature of the steel strip does not become 500 ° C. or lower, in other words, the surface temperature of the steel strip is maintained above 500 ° C. It is preferable to do. This is because the closer the steel temperature is to the A 1 transformation point, the greater the amount of C that can be dissolved in the ferrite. By causing the ferrite to precipitate at a relatively high temperature, it is possible to suppress the precipitation of cementite (pearlite or bainite).
- the cooled steel strip is wound into a coil shape.
- the coiling temperature it is important to set the coiling temperature at 550 to 700 ° C. in order to improve the strength of the steel strip by precipitating fine carbides and to form a structure mainly composed of ferrite.
- the coiling temperature is less than 550 ° C., sufficient precipitation strengthening does not occur, and it is difficult to obtain a structure mainly composed of ferrite.
- the coiling temperature is higher than 700 ° C., coarse precipitates are formed, so that the strength of the steel strip is lowered.
- the winding temperature is preferably 580 to 620 ° C.
- the coiling temperature in this invention is the surface temperature of the steel strip just before a coiling start.
- the steel strip for electric resistance welded steel pipe of the present invention can be manufactured. Furthermore, an electrical resistance welded steel pipe can be manufactured by molding the obtained steel strip and then performing electrical resistance welding.
- the conditions for processing and welding to the electric resistance welded steel pipe are not particularly limited, and for example, conditions known in this technical field can be adopted.
- the electric resistance welding method since the steel strip edge portion is joined together without using a weld metal, the influence of processing appears strongly in the welded portion (steel strip edge portion). Therefore, it is important that steel strips for electric resistance welded steel pipes have excellent properties such as HIC resistance up to the edge.
- Molten steel having the composition shown in Table 1 was melted in a converter and formed into a slab having a thickness of 250 mm by a continuous casting method. Next, the obtained slab was heated to 1230 ° C. and hot rolled. In the hot rolling, first, the slab was roughly rolled into a sheet bar having a thickness of 50 mm, and then finish rolling was performed to obtain a steel strip having a thickness of 12.5 mm and a width of 1260 mm. The obtained steel strip was intermittently water-cooled on the ROT until reaching a predetermined winding temperature, and then wound in a coil shape. The conditions for finish rolling, water cooling, and winding are as shown in Table 2. The finish rolling time (the time from the start to the end of finish rolling) was 5 seconds. In the above manufacturing process, the surface temperature of the steel strip was kept above 500 ° C. during the period from finish rolling to winding.
- the ferrite area ratio, yield strength, tensile strength, HIC resistance, and SSC resistance were measured for each of the obtained steel strips.
- a processing strain was applied to the test piece in advance.
- the processing strain was imparted by a method simulating the introduction of pipe-forming strain in an electric resistance welded steel pipe, in which bending and unbending were performed with an R150 press.
- HIC and SSC are liable to occur by applying processing strain. Therefore, it can be said that the test conditions of the HIC resistance and the SSC resistance performed here are stricter than the test performed in a state where no processing strain is applied.
- the processing strain the elongation at the yield point disappears, and the yield strength equivalent to that after pipe forming can be obtained.
- the measurement method and measurement conditions were as follows.
- the rectangular tensile test piece based on ASTM A370 standard was extract
- a tensile test was performed using the test piece, and the yield strength (YS) and the tensile strength (TS) were measured.
- the distance between test marks (GL) of the test piece was 50 mm.
- test HIC test HIC test specimens from the three positions of the steel strip in the width direction center, 1/4 width, and edge so that the length direction of the specimen is the rolling direction of the steel strip, respectively. Were collected. The dimensions of the test piece were 20 mm wide ⁇ 100 mm long, and the thickness was the same as that of the steel strip (polishing only). The obtained test piece was imparted with processing strain by the method described above, and then an HIC test was performed according to NACE-TM0284. In the test, the test piece was immersed in solution A (a solution in which an aqueous solution of 5.0% NaCl + 0.50% CH 3 COOH was saturated with H 2 S) for 96 hours.
- solution A a solution in which an aqueous solution of 5.0% NaCl + 0.50% CH 3 COOH was saturated with H 2 S
- the cracks generated in the test piece were measured by an ultrasonic flaw detection method, and the crack length defined as “total length of the measured cracks / test piece length ⁇ 100%” for the three cross sections with the largest cracks. Ratio (CLR) was calculated.
- test piece for SSC test was extract
- the dimensions of the test piece were 15 mm wide ⁇ 120 mm long ⁇ 5 mm thick, and three test pieces having the same shape and size were taken from one steel strip.
- the specimen was collected by grinding one side of the steel strip (the unground surface remains the steel strip). After imparting a working strain to the obtained test piece by the method described above, a four-point bending test was performed under the conditions in accordance with NACE TM0177 so that the non-ground surface became the outside.
- test piece was immersed in the same solution A as used in the HIC test, and held for 720 hours in a state where a stress of 437 MPa was applied.
- the value of the stress corresponds to 90% of 485 MPa which is the standard minimum yield stress (SMYS) in API standard X70 class.
- STYS standard minimum yield stress
- the measurement results are as shown in Table 2.
- the steel strips Nos. 1 to 9 whose steel composition and ferrite area ratio satisfy the conditions of the present invention all showed YS and TS values of X70 grade or higher, and no SSC occurred.
- the CLR value was 0% in any of the test pieces collected from three locations of the steel strip. This result shows that the steel strip of the present invention has excellent sour resistance without variation throughout.
- the ferrite area ratio was less than 90%, and HIC and SSC were generated.
- No. 16 in which the S content is lower than the range defined in the present invention No. 18 in which the V content is greater than the range defined in the present invention, and No. 20 in which the Nb content is greater than the range defined in the present invention
- the ferrite area ratio was 90% or more, but sulfide (No. 16) and coarse precipitates (18, 20, 22). HIC and SSC occurred.
- the values of YS and TS were also low.
- the steel strip of No. 14 has a large C content, although the content of V, Nb, and Ti satisfies the conditions stipulated in the present invention, and as a result, satisfies the condition of the formula (1). Absent. Therefore, even though the manufacturing conditions satisfy the conditions specified in the present invention, sufficient sour resistance cannot be obtained.
- the No. 27 steel strip to which Mo was added by 0.12% had a 1/4 width and a slightly higher CLR value at the edge portion, and was inferior to the HIC resistance as compared with the steel strip satisfying the conditions of the present invention.
- the ferrite area ratio is low even though the steel composition satisfies the conditions defined in the present invention. And SSC occurred.
- the No. 26 steel strip having a coiling temperature of less than 550 ° C. had an extremely low ferrite area ratio of 27%.
- the content of individual elements in the steel satisfies the conditions of the present invention, and the ferrite area ratio is 90% by producing the steel under an appropriate temperature condition. It can be seen that a steel strip for an electric resistance welded steel pipe having little variation and extremely excellent HIC resistance and SSC resistance can be obtained.
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Abstract
Description
また、本発明は、上記電気抵抗溶接鋼管用鋼帯を用いて製造される鋼管と、上記電気抵抗溶接鋼管用鋼帯の製造方法に関するものである。
(1)ベイナイト組織は、パーライト組織に比べれば優れた耐サワー性を有しているが、フェライト相とセメンタイト相からなるという点においてパーライト組織と同一のものであるため、耐サワー性が十分であるとはいえない。
(2)鋼の組織をベイナイトやベイニティックフェライト主体とする場合、製造時の温度条件によって組織のばらつきが発生しやすく、鋼帯の全体にわたって高い耐HIC性を得ることが難しい。
(3)鋼の組織をフェライト単相とすることにより、鋼帯の全体にわたって、ばらつきのない、優れた耐HIC性を実現できる。
(4)析出強化元素であるNb、V、およびTiを高い濃度で含有する鋼素材を使用し、製造時の温度条件を制御することによって、微細析出物を析出させて鋼帯の強度を上昇させるとともに、パーライト組織やベイナイト組織の生成を防止して、フェライト主体の組織とすることができる。
(1)質量%で、
C :0.02~0.06%、
Si:0.1~0.3%、
Mn:0.8~1.3%、
P :0.01%以下、
S :0.001%以下、
V :0.04~0.07%、
Nb:0.04~0.07%、
Ti:0.01~0.04%、
Cu:0.1~0.3%、
Ni:0.1~0.3%、
Ca:0.001~0.005%、
Al:0.01~0.07%、
N :0.007%以下、
残部Feおよび不可避的不純物からなり、
かつ、C、Nb、V、およびTiの含有量が下記(1)式の条件を満足する成分組成を有し、
フェライト面積率が90%以上である電気抵抗溶接鋼管用鋼帯。
記
[C]-12([Nb]/92.9+[V]/50.9+[Ti]/47.9)≦0.03% ……(1)
ただし、[M]は質量%単位で表した元素Mの含有量
記
[Ti]/47.9≧[N]/14 ……(2)
ただし、[M]は質量%単位で表した元素Mの含有量
Cr:0.05%以下、
Mo:0.03%以下、および
B :0.0005%以下
である前記(1)または(2)に記載の電気抵抗溶接鋼管用鋼帯。
前記鋼帯を冷却し、
冷却された前記鋼帯を巻き取ってコイル状とする、電気抵抗溶接鋼管用鋼帯の製造方法であって、
前記鋼素材が、質量%で、
C :0.02~0.06%、
Si:0.1~0.3%、
Mn:0.8~1.3%、
P :0.01%以下、
S :0.001%以下、
V :0.04~0.07%、
Nb:0.04~0.07%、
Ti:0.01~0.04%、
Cu:0.1~0.3%、
Ni:0.1~0.3%、
Ca:0.001~0.005%、
Al:0.01~0.07%、
N :0.007%以下、
残部Feおよび不可避的不純物からなり、
かつ、C、Nb、V、およびTiの含有量が下記(1)式の条件を満足する成分組成を有し、 前記熱間圧延において、粗圧延と仕上圧延とが行われ、
前記仕上圧延の開始温度が950℃以下であり、
前記仕上圧延の終了温度が780~850℃であり、
前記冷却における冷却速度が20~100℃/sであり、
前記巻取りが550~700℃の巻取り温度で行われる、電気抵抗溶接鋼管用鋼帯の製造方法。
記
[C]-12([Nb]/92.9+[V]/50.9+[Ti]/47.9)≦0.03% ……(1)
ただし、[M]は質量%単位で表した元素Mの含有量
本発明の電気抵抗溶接鋼管用鋼帯においては、鋼が所定の成分組成と組織を有することが重要である。そこで、まず、本発明において鋼の成分組成を上記のように限定する理由を説明する。なお、成分に関する「%」表示は、特に断らない限り「質量%」を意味するものとする。
Cは、Nb、V、Tiなどの元素と析出物を形成することによって、鋼の強度を高める作用を有する元素である。前記効果を得るためには、鋼が0.02%以上のCを含有する必要がある。一方、Cが多すぎると、析出物を形成せずに残存するCの量が増えるため、パーライト組織やベイナイト組織が生成し、耐サワー性が低下する。このため、鋼のC含有量は0.06%以下である必要がある。また、C含有量は0.03~0.05%とすることが好ましい。なお、C含有量は、後述するようにNb、V、およびTiの量に合わせて調整しなければならない。
Siはフェライト形成元素であるが、フェライトから微細析出物を析出させるため、他の添加元素に見合った量を添加する必要がある。本発明においては、Si含有量を0.1~0.3%とする。なお、Si含有量は0.15~0.25%とすることが好ましい。
Mnは、フェライト変態を遅滞させ、仕上圧延後の急冷時に微細析出物を析出させる効果を有する元素である。前記効果を得るためには、鋼が0.8%以上のMnを含有する必要がある。一方、Mnが多すぎると、パーライトが析出しやすくなる。この傾向は、偏析によりMnが濃化しやすい板厚中心部において顕著である。そのため、本発明においては、Mnの含有量を1.3%以下とすることが重要である。なお、Mnの含有量は0.9~1.1%とすることが好ましい。
Pは鋼中に偏析しやすい元素であり、偏析することにより耐サワー性を劣化させる。そのため、本発明においてはP含有量を0.01%以下とすることが重要である。また、P含有量は0.006%以下であることが好ましい。なお、P含有量の下限については限定されず、0%であってよいが、工業的には0%超である。また、過度の低P化は精錬時間の増加やコストの上昇を招くため、P含有量は0.001%以上とすることが好ましい
Sは鋼中で硫化物を形成し、耐サワー性を低下させる。これを防止するため、本発明ではS含有量を0.001%以下とすることが重要である。S含有量は、0.0006%以下であることが好ましい。なお、S含有量の下限については限定されず、0%であってよいが、工業的には0%超である。また、過度の低S化は精錬時間の増加やコストの上昇を招くため、S含有量は0.0003%以上とすることが好ましい。
Vは、鋼中のCと炭化物を形成して析出する性質を有する元素である。この炭化物の析出により、鋼の強度を高めることができる(析出強化)。また、前記炭化物の析出によって、鋼中の実効的なC濃度が低下し、パーライト組織やベイナイト組織の形成が抑制される。このような効果を得るために、本発明ではV含有量を0.04%以上とすることが重要である。一方、過剰のVは、他の析出物と粗大な複合析出物を形成し、かえって耐サワー性を劣化させる。このため、本発明ではV含有量を0.07%以下とする。なお、V含有量は、0.05~0.06%とすることが好ましい。
Nbは、Vと同様に、析出強化によって鋼の高強度化に寄与する元素である。また、Nbは、鋼中の実効的なC濃度を低下させ、パーライト組織やベイナイト組織の形成を抑制する作用も有している。このような効果を得るために、本発明ではNb含有量を0.04%以上とすることが重要である。一方、Nb含有量が多すぎても、析出強化の効果が飽和し、含有量に見合う強度上昇が得られない。また、過剰のNbは他の析出物と粗大な複合析出物を形成し、かえって耐サワー性を劣化させる。そのため、本発明ではNb含有量を0.07%以下とする。なお、Nb含有量は、0.05~0.06%とすることが好ましい。
Tiも炭化物形成元素であるが、VやNbよりも優先的に鋼中のNと反応して窒化物を形成する性質を有している。そのため、適切な量のTiを鋼に添加することにより、NbとVがNと反応することを防止して、確実にNbやVの炭化物を形成させることができる。このような効果を得るために、本発明ではTi含有量を0.01%以上とすることが重要である。Ti含有量が0.01%未満であると、Nb(CN)やV(CN)などの粗大析出物が形成され、鋼の耐サワー性が低下する。一方、Ti含有量が過剰であると、TiCの生成量が増加し、NbやVの析出物と共に粗大複合析出物を形成する結果、鋼の耐サワー性が低下する。したがって、本発明ではTi含有量を0.04%以下とする。なお、Ti含有量は0.02~0.03%とすることが好ましい。
Cuは、フェライト変態を遅滞させて、Nb、Ti、Vなどの炭化物を微細に析出させる作用を有する元素である。また、Cuは、腐食環境における腐食を抑制し、水素侵入量を減らすことにより耐サワー性を向上させる元素でもある。前記効果を得るために、本発明ではCu含有量を0.1%以上とする。一方、Cuを過剰に添加しても、その効果は飽和する。また、過剰のCuは、鋼帯表面の粗さを大きくし、その結果、腐食環境における水素の侵入量が増え、鋼の耐サワー性が低下する。そのため、本発明では、Cu含有量を0.3%以下とする。なお、Cu含有量は0.2~0.3%とすることが好ましい。
Niは、Cuと共にフェライト変態を遅滞させて、Nb、Ti、Vなどの炭化物を微細に析出させる作用を有する元素である。また、Niは、Cuと同様に、腐食環境における腐食を抑制し、水素侵入量を減らすことにより耐サワー性を向上させる作用を有している。この効果は、鋼がCuとNiをともに含有することにより、さらに高まる。前記効果を得るために、本発明ではNi含有量を0.1%以上とする。一方、Niを過剰に添加しても、その効果は飽和する。また、過剰のNiは、鋼帯表面の粗さを大きくし、その結果、腐食環境における水素の侵入量が増え、鋼の耐サワー性が低下する。そのため、本発明では、Ni含有量を0.3%以下とする。Ni含有量は0.1~0.2%とすることが好ましい。
Caは、鋼に含まれる硫化物を球状とし、耐サワー性を向上させ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、S含有量に応じてCa含有量を決定する必要がある。本発明においては、鋼が、0.001%以上のCaを含有する必要がある。Ca含有量が0.001%未満では、Sの球状化が十分ではない。一方、Ca含有量が多すぎると粗大な硫化物が形成され、かえって耐サワー性が低下する。そのため、Ca含有量は0.005%以下とする。なお、Ca含有量は0.002~0.003%とすることが好ましい。
Alは、脱酸剤として添加される元素である。Alの含有量が0.01%未満であると、Caが酸化物を形成してしまうため、Caの有する硫化物球状化効果を十分に得ることができない。一方、Al含有量が0.07%より多いと、粗大なアルミナが生成し、耐サワー性が低下する。したがって、本発明ではAl含有量を0.01~0.07%とする。なお、Al含有量は0.02~0.04%とすることが好ましい。
Nは、Tiなどと窒化物を形成する元素である。本発明では、X70級相当の高強度を得るために微細炭化物を形成する必要があるが、N含有量が多いと、Nb、Vなどの析出強化元素が炭化物では無く窒化物を形成してしまうため、十分な強度が得られない。よって、本発明ではN含有量を0.007%以下とする。なお、N含有量は0.005%以下とすることが好ましい。一方、下限については限定されず、0%であってよいが、工業的には0%超である。また、本発明のようにC含有量の少ない鋼においては、溶接部の粒成長を抑制し、溶接部の強度や、じん性を確保するために、N含有量を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含有量は、0.0035~0.0045%とすることがさらに好ましい。
Cr:0.05%以下、
Mo:0.03%以下、
B :0.0005%以下。
より好ましくは、
Cr:0.02%以下、
Mo:0.01%以下、
B :0.0002%以下である。
特に、Mo含有量が多いと、製造条件によっては、MoがTi、Nb、V等と粗大な複合析出物を形成し、耐HIC性が劣化することがある。そのため、より安定的に耐HIC性を確保するには、Cr、Mo、およびBの含有量を上記の範囲に抑えることが肝要である。なお、Cr、Mo、およびBの含有量は低いほどよく、それぞれ0%であってよいが、工業的にはそれぞれ0%超であってよい。
本発明においては、鋼に含まれる各元素の含有量がそれぞれ上記条件を満たすことに加えて、V、Nb、およびTiの3元素の含有量とC含有量とが、下記(1)式の条件を満足することが重要である。
記
[C]-12([Nb]/92.9+[V]/50.9+[Ti]/47.9)≦0.03% ……(1)
ただし、[M]は質量%単位で表した元素Mの含有量である。
さらに、本発明においては、鋼のTi含有量とN含有量とが、下記(2)式の条件を満足することが好ましい。
記
[Ti]/47.9≧[N]/14 ……(2)
ただし、[M]は質量%単位で表した元素Mの含有量である。
上述したように、本発明においては、鋼中の析出強化元素の含有量を制御することが重要である。その一例として、V含有量が鋼帯の耐HIC性に及ぼす影響を、以下に示す方法により、実験的に確認した。
C :0.033~0.045%、
Si:0.16~0.23%、
Mn:0.92~1.07%、
P :0.003~0.005%、
S :0.0003~0.0007%、
Nb:0.050~0.058%、
Ti:0.021~0.029%、
Cu:0.22~0.28%、
Ni:0.12~0.18%、
Ca:0.0022~0.0029%、
Al:0.023~0.038%、
N :0.0036~0.0045%、
Cr:0.02%、
Mo:0.01%、
B :0.0001%未満、および
残部Feおよび不可避的不純物。
次に、本発明における鋼の組織の限定理由について説明する。
本発明においては、鋼の組織に占めるフェライト面積率を90%以上とすることが重要である。パーライト組織は耐サワー性に劣るため、鋼中に含まれないことが好ましい。また、ベイナイト組織は、パーライト組織に比べれば優れた耐サワー性を有しているが、フェライト相とセメンタイト相からなるという点においてパーライト組織と同一のものであるため、耐サワー性が十分であるとはいえない。そこで、本発明においては鋼の組織を実質的にフェライト単相とする。ここで、実質的にフェライト単相とは、フェライト面積率が90%以上であることを意味する。また、フェライト面積率は95%以上であることが好ましい。このように高いフェライト面積率は、先に述べたように鋼の成分組成を制御するとともに、特定の温度条件で鋼帯を製造することによって達成することができる。一方、フェライト面積率の上限は特に限定されず、100%であってよい。なお、本発明における「フェライト」には、マルテンサイト変態点に近い500℃程度の低温で生成する「ベイニティックフェライト」は包含されない。このような低温で生成するベイニティックフェライトには、固溶できるC量が少なく、固溶できなかったCはセメンタイト(Fe3C)を形成し、耐サワー性を劣化させるためである。
次に、本発明の鋼帯の製造方法について説明する。
まず、上記の成分組成を有する鋼素材を、常法にしたがって溶製する。前記鋼素材は、偏析、特に中心偏析によってパーライト組織が形成されることを防ぐため、連続鋳造法によって製造することが好ましい。その際、連続鋳造法によって得られるスラブの厚さは200mm以上とすることが好ましい。それにより、熱間圧延工程における粗圧延時に再結晶を多く起こさせ、偏析によるパーライト組織の形成を抑制することができる。一方、スラブが厚すぎると、加熱時にスラブ全体の温度が上昇せず、析出物を十分に固溶させることが困難となる。そのため、スラブ厚は300mm以下とすることが好ましい。スラブ厚の、さらに好ましい範囲は、240~260mmである。
得られた鋼帯(加工ひずみなし)より、組織観察用試験片を作製し、ミクロ組織の観察を行った。観察は、試験片の圧延方向断面を研磨および腐食した面について実施した。観察は、光学顕微鏡(×400)、および走査型電子顕微鏡(×1000)を用いて行い、得られた画像を解析して、組織全体に占めるフェライトの比率を算出した。
前記鋼帯より、引張方向が圧延方向と直角となるように、ASTM A370規格に準拠した矩形引張試験片を採取した。前記試験片を用いて引張試験を実施し、降伏強度(YS)と引張強さ(TS)を測定した。試験片の標点間距離(GL)は、50mmとした。
前記鋼帯の、幅方向中心部、1/4幅部、およびエッジ部の3つの位置から、それぞれ試験片の長さ方向が鋼帯の圧延方向となるようにHIC試験用試験片を採取した。前記試験片の寸法は、幅20mm×長さ100mm、厚さは鋼帯の厚さのまま(研磨のみ)とした。得られた試験片に、先に述べた方法で加工歪の付与を行った後、NACE-TM0284に準拠してHIC試験を実施した。試験においては、試験片をA液(5.0%NaCl+0.50%CH3COOHの水溶液を、H2Sで飽和させた溶液)に96時間浸漬した。試験終了後、試験片に生じた割れを超音波探傷法により測定し、割れの最も大きい3断面につき、「測定された割れの合計長さ/試験片長さ×100%」として定義されるCrack Length Ratio(CLR)を算出した。
鋼帯の幅方向と厚さ方向における中心部から、試験片の長さ方向が鋼帯の圧延方向となるようにSSC試験用試験片を採取した。前記試験片の寸法は、幅15mm×長さ120mm×厚さ5mmとし、1つの鋼帯から、同じ形状、寸法の試験片を3つ採取した。試験片の採取は、鋼帯の片面を研削して行った(非研削面は鋼帯まま)。得られた試験片に、先に述べた方法で加工歪の付与を行った後、NACE TM0177に準拠した条件で、非研削面が外側になるように4点曲げ試験を実施した。試験においては、HIC試験で用いたものと同様のA液に試験片を浸漬し、437MPaの応力を負荷した状態で、720時間保持した。前記応力の値は、API規格のX70級における規格最小降伏応力(SMYS)である485MPaの90%に相当する。試験終了後、光学顕微鏡を用いて倍率10倍で試験片の表面を観察し、割れがないものを○、1つでも割れを認めたものを×とした。
Claims (5)
- 質量%で、
C :0.02~0.06%、
Si:0.1~0.3%、
Mn:0.8~1.3%、
P :0.01%以下、
S :0.001%以下、
V :0.04~0.07%、
Nb:0.04~0.07%、
Ti:0.01~0.04%、
Cu:0.1~0.3%、
Ni:0.1~0.3%、
Ca:0.001~0.005%、
Al:0.01~0.07%、
N :0.007%以下、
残部Feおよび不可避的不純物からなり、
かつ、C、Nb、V、およびTiの含有量が下記(1)式の条件を満足する成分組成を有し、
フェライト面積率が90%以上である電気抵抗溶接鋼管用鋼帯。
記
[C]-12([Nb]/92.9+[V]/50.9+[Ti]/47.9)≦0.03% ……(1)
ただし、[M]は質量%単位で表した元素Mの含有量 - 前記成分組成が、下記(2)式の条件をさらに満足する、請求項1に記載の電気抵抗溶接鋼管用鋼帯。
記
[Ti]/47.9≧[N]/14 ……(2)
ただし、[M]は質量%単位で表した元素Mの含有量 - 前記不可避不純物としてのCr、Mo、およびBの含有量が、それぞれ質量%で、
Cr:0.05%以下、
Mo:0.03%以下、および
B :0.0005%以下
である請求項1または2に記載の電気抵抗溶接鋼管用鋼帯。 - 請求項1~3のいずれか一項に記載の電気抵抗溶接鋼管用鋼帯を材料として用いて形成された電気抵抗溶接鋼管。
- 鋼素材を熱間圧延して鋼帯とし、
前記鋼帯を冷却し、
冷却された前記鋼帯を巻き取ってコイル状とする、電気抵抗溶接鋼管用鋼帯の製造方法であって、
前記鋼素材が、質量%で、
C :0.02~0.06%、
Si:0.1~0.3%、
Mn:0.8~1.3%、
P :0.01%以下、
S :0.001%以下、
V :0.04~0.07%、
Nb:0.04~0.07%、
Ti:0.01~0.04%、
Cu:0.1~0.3%、
Ni:0.1~0.3%、
Ca:0.001~0.005%、
Al:0.01~0.07%、
N :0.007%以下、
残部Feおよび不可避的不純物からなり、
かつ、C、Nb、V、およびTiの含有量が下記(1)式の条件を満足する成分組成を有し、
前記熱間圧延において、粗圧延と仕上圧延とが行われ、
前記仕上圧延の開始温度が950℃以下であり、
前記仕上圧延の終了温度が780~850℃であり、
前記冷却における冷却速度が20~100℃/sであり、
前記巻取りが550~700℃の巻取り温度で行われる、電気抵抗溶接鋼管用鋼帯の製造方法。
記
[C]-12([Nb]/92.9+[V]/50.9+[Ti]/47.9)≦0.03% ……(1)
ただし、[M]は質量%単位で表した元素Mの含有量
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- 2015-08-06 US US15/509,266 patent/US20170307111A1/en not_active Abandoned
- 2015-08-06 KR KR1020177008568A patent/KR20170043662A/ko not_active Application Discontinuation
- 2015-08-06 CN CN201580051415.4A patent/CN106715744A/zh active Pending
- 2015-08-06 JP JP2015558291A patent/JP6179604B2/ja active Active
- 2015-08-06 CA CA2962370A patent/CA2962370C/en active Active
- 2015-08-06 WO PCT/JP2015/003972 patent/WO2016047023A1/ja active Application Filing
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WO2019070075A1 (ja) * | 2017-10-05 | 2019-04-11 | 新日鐵住金株式会社 | スポット溶接継手の製造方法、並びにスポット溶接用鋼板、及びスポット溶接用鋼板部材 |
JPWO2019070075A1 (ja) * | 2017-10-05 | 2020-10-22 | 日本製鉄株式会社 | スポット溶接継手の製造方法、並びにスポット溶接用鋼板、及びスポット溶接用鋼板部材 |
US11628511B2 (en) | 2017-10-05 | 2023-04-18 | Nippon Steel Corporation | Method of manufacture of spot welded joint, steel sheet for spot welding use, and steel sheet member for spot welding use |
Also Published As
Publication number | Publication date |
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CA2962370A1 (en) | 2016-03-31 |
EP3199657B1 (en) | 2019-02-13 |
KR20170043662A (ko) | 2017-04-21 |
EP3199657A4 (en) | 2017-08-23 |
EP3199657A1 (en) | 2017-08-02 |
JPWO2016047023A1 (ja) | 2017-04-27 |
CN106715744A (zh) | 2017-05-24 |
JP6179604B2 (ja) | 2017-08-16 |
US20170307111A1 (en) | 2017-10-26 |
WO2016047023A8 (ja) | 2017-03-30 |
CA2962370C (en) | 2019-07-02 |
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