WO2021039776A1 - 鋼板、部材及びそれらの製造方法 - Google Patents
鋼板、部材及びそれらの製造方法 Download PDFInfo
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
- C21D9/54—Furnaces for treating strips or wire
- C21D9/56—Continuous furnaces for strip or wire
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Definitions
- the present invention relates to steel plates and members used for automobile parts and the like, and methods for manufacturing them. More specifically, the present invention relates to steel sheets, members, and methods for producing them, which have high strength and are excellent in shape uniformity and delayed fracture resistance.
- shape uniformity and delayed fracture resistance are improved by controlling the area ratio of ferrite and martensite. Specifically, by forming a composite structure steel in which the metal structure contains a tempered martensite phase having a volume fraction of 50 to 80% and a ferrite phase having a volume fraction of 20 to 50%, hydrogen intrusion is suppressed and shape uniformity is suppressed. Also, an ultra-high strength steel plate having good delayed fracture resistance is provided.
- Patent Document 2 provides a technique for suppressing deterioration of the shape of a steel sheet due to martensitic transformation that occurs during water quenching by restraining the steel sheet with a roll in water.
- the steel plate used for the automobile body is pressed and used, good shape uniformity is a necessary characteristic. Furthermore, the amount of high-strength steel sheets used for materials for automobile parts is increasing recently, and it is necessary to have good delayed fracture resistance, which is a concern with the increase in strength. Since automobile parts are collected at the total width of the steel sheet (hereinafter, also referred to as the total width of the steel sheet), the delayed fracture resistance must be excellent without variation even at the total width of the steel sheet. Therefore, it is necessary to have high strength, excellent shape uniformity, and excellent delayed fracture resistance in the entire width of the steel sheet.
- Patent Document 1 provides a technique having excellent shape uniformity and delayed fracture resistance by microstructure control, the shape is deteriorated due to transformation expansion occurring during martensitic transformation, and therefore the shape is uniform from the present invention. The effect of improving sexuality seems to be inferior.
- Patent Document 2 provides a technique for improving shape uniformity, it is not a technique having excellent delayed fracture resistance.
- An object of the present invention is to provide a steel sheet, a member, and a method for manufacturing the same, which have high strength and excellent shape uniformity and delayed fracture resistance.
- the present inventors have conducted extensive research on the requirements for a steel sheet having a tensile strength of 750 MPa or more and excellent shape uniformity and delayed fracture resistance. As a result, it was found that it is necessary to set the residual stress at the center of the plate width to 800 MPa or less in order to obtain excellent delayed fracture resistance. In addition, the present inventors have found that the strength is increased by increasing the martensite fraction to 20% or more in terms of area ratio by rapid cooling. On the other hand, since the martensitic transformation during water cooling occurs rapidly and non-uniformly, the transformation strain deteriorates the shape uniformity of the steel sheet.
- the present inventors have obtained a steel sheet having excellent delayed fracture resistance by reducing the residual stress at the center of the plate width.
- a steel sheet having excellent shape uniformity and delayed fracture resistance over the entire width of the steel sheet can be obtained, and the present invention has been completed.
- the gist of the present invention is as follows. [1] It has a steel structure having martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the balance: 5% or less in terms of area ratio.
- the residual stress at the center of the plate width when V-shaped bending is 800 MPa or less.
- the residual stress at the edge of the plate width is 90% or more and 110% or less of the residual stress at the center of the plate width.
- the component composition is further increased by mass%.
- the component composition is further increased by mass%.
- the component composition is further increased by mass%.
- the component composition is further increased by mass%.
- the hot-rolled steel sheet obtained in the hot rolling step is held at an annealing temperature of 1 AC or higher for 30 seconds or longer, then water quenching is started at Ms or higher, water-cooled to 100 ° C or lower, and then 100 ° C or higher. It has an annealing step of heating again at 300 ° C. or lower.
- the steel sheet is restrained from the front surface and the back surface of the steel sheet by two rolls installed sandwiching the steel sheet.
- a method for manufacturing a steel sheet in which the restraint pressure ratio at the center of the plate width to the end of the plate width at the restraint positions of the two rolls is 1.05 or more and 2.0 or less.
- the cold-rolled steel sheet obtained in the cold rolling step is held at an annealing temperature of 1 AC or higher for 30 seconds or longer, then water quenching is started at Ms or higher, water-cooled to 100 ° C or lower, and then 100 ° C or higher. It has an annealing step of heating again at 300 ° C. or lower.
- the steel sheet is restrained from the front surface and the back surface of the steel sheet by two rolls installed sandwiching the steel sheet.
- a method for manufacturing a steel sheet in which the restraint pressure ratio at the center of the plate width to the end of the plate width at the restraint positions of the two rolls is 1.05 or more and 2.0 or less.
- the present invention it is possible to provide steel sheets, members, and methods for manufacturing them, which have high strength and excellent shape uniformity and delayed fracture resistance.
- the steel sheet of the present invention By applying the steel sheet of the present invention to a structural member for automobiles, it is possible to achieve both high strength of the steel sheet for automobiles and improvement of delayed fracture resistance. That is, according to the present invention, the performance of the automobile body is improved.
- the steel sheet of the present invention has a steel structure having martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the balance: 5% or less in terms of area ratio, and when V-shaped bending is performed.
- the residual stress at the center of the plate width (hereinafter, also simply referred to as "residual stress at the center of the plate width") is 800 MPa or less, and when V-shaped bending is performed, the residual stress at the end of the plate width becomes the residual stress at the center of the plate width.
- it is 90% or more and 110% or less, and the maximum warp amount of the steel sheet when sheared with a length of 1 m in the longitudinal direction of the steel sheet is 15 mm or less.
- the effect of the present invention can be obtained, so that the composition of the steel sheet is not particularly limited.
- the thickness of the steel plate is preferably 0.2 mm or more and 3.2 mm or less.
- the steel structure of the steel sheet of the present invention has martensite: 20% or more and 100% or less, ferrite: 0% or more and 80% or less, and the balance: 5% or less in terms of area ratio.
- Area ratio of martensite 20% or more and 100% or less In order to obtain high strength of TS ⁇ 750 MPa, the area ratio of martensite is 20% or more. If the area ratio of martensite is less than 20%, either ferrite, retained austenite, pearlite, or bainite increases, and the strength decreases. The total area ratio of martensite may be 100%. From the viewpoint of improving the strength, the area ratio of martensite is preferably 30% or more. Martensite is the sum of as-quenched fresh martensite and tempered tempered martensite.
- martensite refers to a hard structure generated from austenite below the martensitic transformation point (also simply referred to as Ms point), and tempered martensite refers to a structure that is tempered when martensite is reheated. ..
- Area ratio of ferrite 0% or more and 80% or less From the viewpoint of ensuring the strength of the steel sheet, the area ratio of ferrite is 80% or less. The area ratio may be 0%.
- ferrite is a structure formed by transformation from austenite at a relatively high temperature and composed of BCC lattice crystal grains.
- the steel structure of the steel sheet of the present invention may contain a metallic phase inevitably contained as a remaining portion other than martensite and ferrite.
- the area ratio of the remaining portion is acceptable if it is 5% or less.
- Phases contained in the balance include, for example, retained austenite, pearlite, and bainite.
- retained austenite refers to austenite that remains at room temperature without being transformed into martensite.
- the pearlite referred to in the present invention is a structure composed of ferrite and needle-like cementite.
- the bainite referred to in the present invention refers to a hard structure formed from austenite at a relatively low temperature (above the martensitic transformation point) and in which fine carbides are dispersed in needle-shaped or plate-shaped ferrite.
- Residual stress at the center of the plate width during V-shaped bending is 800 MPa or less
- the V-shaped bending in the present invention is bending at a bending angle of 90 ° so that the bending ridge direction is parallel to the width direction of the steel sheet. Means to do.
- the steel sheet of the present invention has excellent delayed fracture resistance.
- the critical load stress obtained when the delayed fracture test described in the examples is performed is equal to or higher than the yield strength (hereinafter, also simply referred to as YS).
- the critical load stress is preferably (YS + 100) MPa or more, more preferably (YS + 200) MPa or more.
- the residual stress is preferably 780 MPa or less, more preferably 700 MPa or less, and further preferably 600 MPa or less.
- the residual stress at the edge of the plate width is 90% or more and 110% or less of the residual stress at the center of the plate width.
- the steel plate of the present invention has excellent delayed fracture resistance even at the entire width of the steel plate. Specifically, when the steel plate is V-bent and the critical load stress is obtained at the center of the plate width and the end of the plate width, the critical load stress at the end of the plate width is 90% of the critical load stress at the center of the plate width. It is 110% or more, preferably 92% or more and 108% or less, and more preferably 95% or more and 105% or less.
- the residual stress at the edge of the plate width In order for the critical load stress at the edge of the plate width to be 90% or more and 110% or less of the critical load stress at the center of the plate width, the residual stress at the edge of the plate width remains at the center of the plate width during V-shaped bending. It needs to be 90% or more and 110% or less with respect to the stress. From the viewpoint of improving the excellent delayed fracture resistance, the residual stress at the edge of the plate width is preferably 92% or more and 108% or less with respect to the residual stress at the center of the plate width during V-shaped bending, more preferably. It is 95% or more and 105% or less.
- the maximum amount of warpage of a steel sheet when sheared at a length of 1 m in the longitudinal direction of the steel sheet is 15 mm or less.
- the steel sheet of the present invention has good shape uniformity. Specifically, the maximum amount of warpage of a steel sheet when sheared with a length of 1 m in the longitudinal direction (rolling direction) of the steel sheet is 15 mm or less.
- the maximum amount of warpage is preferably 13 mm or less, more preferably 12 mm or less, still more preferably 10 mm or less.
- the lower limit of the maximum warp amount is not limited, and 0 mm is most preferable.
- the "maximum amount of warpage of a steel sheet when sheared at a length of 1 m in the longitudinal direction of the steel sheet" in the present invention means that after the steel sheet is sheared at the original width of the steel sheet at the original width of the steel sheet in the longitudinal direction of the steel sheet (rolling direction) at a length of 1 m.
- the distance here is a distance in a direction (vertical direction) perpendicular to the horizontal plane of the horizontal table.
- the amount of warpage is measured with the other side of the steel sheet on the upper side, and the maximum value of the measured amount of warpage is defined as the maximum amount of warpage.
- the sheared steel plate is placed on the horizontal table so that the corners of the steel sheet and the horizontal table have more contact points (two or more points). The amount of warpage is determined by lowering the horizontal plate from a position above the steel plate until it contacts the steel plate, and at the position where it contacts the steel plate, the thickness of the steel plate is determined from the distance between the horizontal table and the horizontal plate. Pull and ask.
- the steel sheet of the present invention has high strength.
- the high strength referred to in the present invention means that the tensile strength measured by the method described in Examples is 750 MPa or more.
- the tensile strength of the steel sheet is preferably 950 MPa or more, more preferably 1150 MPa or more, still more preferably 1300 MPa or more.
- the upper limit of the tensile strength of the steel sheet is not particularly limited, but is preferably 2500 MPa or less from the viewpoint of easy balancing with other characteristics.
- % which is a unit of the content of the component, means “mass%”.
- C 0.05% or more and 0.60% or less
- C is an element that improves hardenability and is necessary for securing a predetermined area ratio of martensite. Further, C is necessary from the viewpoint of increasing the strength of martensite and ensuring the strength.
- the amount of C is preferably 0.05% or more from the viewpoint of maintaining excellent delayed fracture resistance and obtaining a predetermined strength. From the viewpoint of obtaining TS ⁇ 950 MPa, the lower limit of the C content is more preferably 0.11% or more. Further, from the viewpoint of further increasing the tensile strength, the lower limit of the C content is more preferably 0.125% or more.
- the C content is preferably 0.60% or less.
- the C content is more preferably 0.50% or less, still more preferably 0.40% or less.
- Si 0.01% or more and 2.0% or less Si is a strengthening element by solid solution strengthening.
- the Si content is preferably 0.01% or more.
- the Si content is more preferably 0.02% or more, still more preferably 0.03% or more.
- the Si content is preferably 2.0% or less.
- the Si content is more preferably 1.7% or less, still more preferably 1.5% or less.
- Mn 0.1% or more and 3.2% or less Mn is contained in order to improve the hardenability of steel and secure the area ratio of a predetermined martensite. If the Mn content is less than 0.1%, the strength tends to decrease due to the formation of ferrite on the surface layer of the steel sheet. Therefore, the Mn content is preferably 0.1% or more. The Mn content is more preferably 0.2% or more, still more preferably 0.3% or more. On the other hand, Mn is an element that particularly promotes the formation and coarsening of MnS, and when the Mn content exceeds 3.2%, the residual stress at the center of the plate width increases due to the increase in coarse inclusions. Delayed fracture resistance tends to deteriorate. Therefore, the Mn content is preferably 3.2% or less. The Mn content is more preferably 3.0% or less, still more preferably 2.8% or less.
- P 0.050% or less
- P is an element that reinforces steel, but if its content is high, it segregates at the grain boundaries and the residual stress at the center of the plate width increases, which tends to deteriorate the delayed fracture resistance. There is. Therefore, the P content is preferably 0.050% or less.
- the P content is more preferably 0.030% or less, still more preferably 0.010% or less.
- the lower limit of the P content is not particularly limited, but at present, the lower limit that can be industrially implemented is about 0.003%.
- S 0.0050% or less
- inclusions such as MnS, TiS, Ti (C, S) are excessively formed, and the delayed fracture resistance tends to deteriorate.
- the S content is preferably 0.0050% or less from the viewpoint that the delayed fracture resistance suppresses deterioration.
- the S content is more preferably 0.0020% or less, still more preferably 0.0010% or less, and particularly preferably 0.0005% or less.
- the lower limit of the S content is not particularly limited, but at present, the lower limit that can be industrially implemented is about 0.0002%.
- Al 0.005% or more and 0.10% or less Al is added to sufficiently deoxidize and reduce coarse inclusions in steel.
- the Al content is preferably 0.005% or more.
- the Al content is more preferably 0.010% or more.
- the amount of Al is preferably 0.10% or less.
- the Al content is more preferably 0.08% or less, still more preferably 0.06% or less.
- N 0.010% or less
- N is an element that forms a nitride such as TiN, (Nb, Ti) (C, N), AlN, and a carbonitride-based coarse inclusion in steel. If N is excessively contained, the delayed fracture resistance deteriorates due to the formation of coarse inclusions.
- the N content is preferably 0.010% or less in order to prevent deterioration of the delayed fracture resistance.
- the N content is preferably 0.007% or less, more preferably 0.005% or less.
- the lower limit of the N content is not particularly limited, but at present, the lower limit that can be industrially implemented is about 0.0006%.
- the steel sheet of the present invention contains the above-mentioned components as a basic component, and the balance is composed of Fe (iron) and unavoidable impurities.
- the steel sheet of the present invention may contain the following components as optional components as long as the actions of the present invention are not impaired. If the following optional components are contained below the lower limit, the components shall be included in the unavoidable impurities.
- Cr At least one selected from 0.01% or more and 0.50% or less, Mo: 0.01% or more and less than 0.15%, V: 0.001% or more and 0.05% or less Cr, Mo , V can be contained for the purpose of obtaining the effect of improving the hardenability of steel.
- the Cr content and the Mo content are both 0.01% or more.
- the Cr content and the Mo content are more preferably 0.02% or more, still more preferably 0.03% or more, respectively.
- V content it is preferably 0.001% or more, more preferably 0.002% or more, still more preferably 0.003% or more.
- the Cr content is preferably 0.50% or less, and more preferably 0.1% or less.
- the Mo content is preferably less than 0.15%, more preferably 0.10% or less.
- the V content is preferably 0.05% or less, more preferably 0.04% or less, still more preferably 0.03% or less.
- Nb At least one selected from 0.001% or more and 0.020% or less and Ti: 0.001% or more and 0.020% or less Nb and Ti have increased strength through miniaturization of old ⁇ grains. Contribute to. In order to obtain such an effect, it is preferable to contain Nb and Ti in an amount of 0.001% or more, respectively.
- the contents of Nb and Ti are more preferably 0.002% or more, still more preferably 0.003% or more, respectively.
- Nb-based materials such as NbN, Nb (C, N), (Nb, Ti) (C, N) that remain undissolved during slab heating in the hot rolling process Coarse precipitates, TiN, Ti (C, N), Ti (C, S), TiS and other Ti-based coarse precipitates increase, and the residual stress at the center of the plate width increases, resulting in delayed fracture resistance. Deteriorates. Therefore, the Nb and Ti contents are preferably 0.020% or less, respectively. The Nb and Ti contents are more preferably 0.015% or less, still more preferably 0.010% or less, respectively.
- Cu At least one selected from 0.001% or more and 0.20% or less and Ni: 0.001% or more and 0.10% or less
- Cu and Ni improve the corrosion resistance in the usage environment of automobiles.
- the corrosion product coats the surface of the steel sheet and has the effect of suppressing hydrogen intrusion into the steel sheet.
- the Cu content and the Ni content are each more preferably 0.002% or more.
- the Cu content is preferably 0.20% or less.
- the Cu content is more preferably 0.15% or less, still more preferably 0.10% or less.
- the Ni content is preferably 0.10% or less.
- the Ni content is more preferably 0.08% or less, still more preferably 0.06% or less.
- B 0.0001% or more and less than 0.0020%
- B is an element that improves the hardenability of steel, and by containing B, martensite having a predetermined area ratio is generated even when the Mn content is small. The effect of making it is obtained.
- the B content is preferably 0.0001% or more.
- the B content is preferably 0.0002% or more, and more preferably 0.0003% or more.
- the B content is 0.0020% or more, the solid solution rate of cementite at the time of annealing is delayed, and carbides containing Fe as a main component such as unsolidified cementite remain. As a result, the residual stress at the center of the plate width increases, and the delayed fracture resistance deteriorates. Therefore, the B content is preferably less than 0.0020%.
- the B content is more preferably 0.0015% or less, still more preferably 0.0010% or less.
- Sb At least one selected from 0.002% or more and 0.1% or less and Sn: 0.002% or more and 0.1% or less Sb and Sn suppress oxidation and nitriding of the surface layer of the steel sheet. It suppresses the reduction of C and B due to oxidation and nitriding of the surface layer of the steel sheet. Further, by suppressing the reduction of C and B, the formation of ferrite on the surface layer of the steel sheet is suppressed, which contributes to high strength.
- the Sb content and the Sn content are preferably 0.002% or more in either case.
- the Sb content and Sn content are more preferably 0.003% or more, still more preferably 0.004% or more, respectively.
- the content exceeds 0.1% in both the Sb content and the Sn content, Sb and Sn segregate at the old ⁇ grain boundaries and the residual stress at the center of the plate width increases, resulting in delayed fracture resistance.
- the characteristics deteriorate. Therefore, it is preferably 0.1% or less in both the Sb content and the Sn content.
- the Sb content and Sn content are more preferably 0.08% or less, still more preferably 0.06% or less, respectively.
- the steel sheet of the present invention may contain Ta, W, Ca, Mg, Zr, and REM as other elements as long as the effects of the present invention are not impaired, and the contents of these elements are different. If it is 0.1% or less, it is acceptable.
- the method for producing a steel sheet of the present invention includes a hot rolling step, a cold rolling step performed as needed, and an annealing step.
- the method for producing a steel sheet of the present invention includes, for example, a hot rolling step of heating a steel slab having the above-mentioned preferable composition and then hot rolling, a cold rolling step performed as needed, and hot rolling.
- the hot-rolled steel sheet obtained in the process or the cold-rolled steel sheet obtained in the cold rolling process is held at a quenching temperature of AC 1 point or higher for 30 seconds or longer, and then water-hardening is started at Ms point or higher to 100 ° C. It has a annealing step of water-cooling to the following and then heating again at 100 ° C.
- the steel plate is restrained from the front and back surfaces of the steel plate by two rolls installed sandwiching the steel plate, and the restraint pressure ratio at the center of the plate width to the plate width end at the restraint position of the two rolls is 1.05 or more and 2.0 or less. ..
- the temperature at which the steel slab, steel plate, etc. shown below is heated or cooled means the surface temperature of the steel slab, steel plate, etc.
- Hot rolling step is a step of heating a steel slab having the above-mentioned composition and then hot rolling.
- the steel slab having the above-mentioned composition is subjected to hot rolling.
- the slab heating temperature is not particularly limited, but by setting it to 1200 ° C. or higher, it is possible to promote the solid solution of sulfide and reduce Mn segregation, reduce the amount of coarse inclusions and carbides described above, and tolerate delayed fracture. The characteristics are improved. Therefore, the slab heating temperature is preferably 1200 ° C. or higher. It is more preferably 1230 ° C. or higher, and even more preferably 1250 ° C. or higher.
- the upper limit of the slab heating temperature is not particularly limited, but is preferably 1400 ° C. or lower.
- the heating rate during slab heating is not particularly limited, but is preferably 5 to 15 ° C./min.
- the slab heating time during slab heating is not particularly limited, but is preferably 30 to 100 minutes.
- the finish rolling temperature is not particularly limited, but is preferably 840 ° C. or higher. If the finish rolling temperature is less than 840 ° C, it takes time for the temperature to drop, and inclusions and coarse carbides may be generated, which not only deteriorates the delayed fracture resistance but also deteriorates the internal quality of the steel sheet. is there. Therefore, the finish rolling temperature is preferably 840 ° C. or higher. The finish rolling temperature is more preferably 860 ° C. or higher. On the other hand, the upper limit of the finish rolling temperature is not particularly limited, but the finish rolling temperature is preferably 950 ° C. or lower because it becomes difficult to cool down to the subsequent winding temperature. The finish rolling temperature is more preferably 920 ° C. or lower.
- the winding temperature is preferably 630 ° C. or lower. More preferably, it is 600 ° C. or lower.
- the lower limit is not particularly limited, but is preferably 500 ° C. or higher in order to prevent deterioration of cold rollability.
- the hot-rolled steel sheet after winding may be pickled.
- the pickling conditions are not particularly limited.
- the cold rolling process is a process of cold rolling a hot-rolled steel sheet obtained in the hot rolling process.
- the reduction rate of cold rolling is not particularly limited, but if the reduction rate is less than 20%, the flatness of the surface may be poor and the structure may become uneven, so the reduction rate should be 20% or more. preferable.
- the cold rolling step is not an essential step, and the cold rolling step may be omitted as long as the steel structure and mechanical properties satisfy the scope of the present invention.
- the annealing step is to hold a cold-rolled steel sheet or a hot-rolled steel sheet at a quenching temperature of 1 AC or higher for 30 seconds or longer, then start water quenching at Ms or higher, water-cool to 100 ° C or lower, and then 100. This is a step of heating again at ° C. or higher and 300 ° C. or lower.
- the steel sheet is restrained from the front surface and the back surface by two rolls installed sandwiching the steel sheet, and the two rolls.
- the restraint pressure ratio at the center of the plate width to the end of the plate width at the restraint position of is 1.05 or more and 2.0 or less.
- the annealing temperature needs to be AC 1 point or more. It is preferably (AC 1 point + 10 ° C.) or higher.
- the upper limit of the annealing temperature is not particularly limited, but the annealing temperature is preferably 900 ° C. or lower from the viewpoint of optimizing the temperature at the time of water quenching and preventing deterioration of shape uniformity.
- the AC1 point referred to here is calculated by the following formula. Further, in the following formula, (% element symbol) means the content (mass%) of each element.
- AC 1 point (° C) 723 + 22 (% Si) -18 (% Mn) + 17 (% Cr) + 4.5 (% Mo) + 16 (% V)
- Retention time at annealing temperature is 30 seconds or more If the annealing retention time is less than 30 seconds, the dissolution of carbides and austenite transformation do not proceed sufficiently, so the carbides remaining during the subsequent heat treatment become coarse and the plate Delayed fracture resistance deteriorates due to the increase in residual stress at the center of the width. Therefore, the annealing holding time is 30 seconds or more, preferably 35 seconds or more.
- the upper limit of the annealing holding time is not particularly limited, but the annealing holding time is preferably 900 seconds or less from the viewpoint of suppressing coarsening of the austenite particle size and preventing an increase in the amount of invading hydrogen.
- Water quenching start temperature is above Ms point Quenching start temperature is an important factor for determining the martensite fraction, which is the controlling factor of strength.
- the quenching start temperature is less than the Ms point, martensite transformation occurs before quenching, so that self-tempering of martensite occurs and the shape uniformity deteriorates. Therefore, the water quenching start temperature is above the Ms point. It is preferably (Ms point + 50 ° C.) or higher.
- the upper limit of the water quenching start temperature is not particularly limited, and the annealing temperature may be used.
- the Ms point referred to here is calculated by the following formula.
- Ms point (°C) 550-350 ⁇ (% C) / (% V M) ⁇ 100 ⁇ -40 (% Mn) -17 (% Ni) -17 (% Cr) -21 (% Mo)
- Restraining the steel sheet from the front and back surfaces of the steel sheet with two rolls during water cooling in water quenching is an important factor for obtaining the shape correction effect, and the residual stress in the center of the sheet width and the residual stress fluctuation in the entire width of the steel sheet are affected. Control of constraint conditions is an important factor for reduction.
- the present invention is characterized in that the deformation strain during water cooling is corrected by restraint to improve the shape uniformity of the steel sheet, and the leveler correction and the correction by skin pass rolling, which increase the residual stress and deteriorate the delayed fracture resistance, are not required. There is. Residual stress can be reduced because leveler processing and skin pass rolling, which are performed when correcting the deterioration of shape uniformity, are not required. Further, since the residual stress fluctuation in the entire width of the steel sheet is reduced under the restraint condition, the delayed fracture resistance is improved in the entire width of the steel sheet.
- the restraint temperature When the surface temperature of the steel sheet (hereinafter referred to as the restraint temperature) when restraining the steel sheet from the front and back surfaces of the steel sheet with two rolls is (Ms point + 150 ° C) or less, the restraint temperature exceeds (Ms point + 150 ° C). Since the martensitic transformation occurs later, the deterioration of the shape uniformity due to the transformation expansion of the martensitic transformation cannot be suppressed, and the shape uniformity deteriorates. Therefore, the restraint temperature is (Ms point + 150 ° C.) or less, preferably (Ms point + 100 ° C.) or less, and more preferably (Ms point + 50 ° C.) or less.
- the lower limit of the restraint temperature is not particularly limited, and it is preferably 0 ° C. or higher at which water does not freeze.
- the restraint pressure ratio of the center of the plate width to the restraint position of the two rolls is 1.05 or more and 2.0 or less.
- the restraint pressure ratio of the center of the plate width to the restraint position of the two rolls is 1.
- the restraint pressure ratio is 1.05 or more.
- the restraint pressure ratio is preferably 1.10 or more.
- the restraint pressure ratio exceeds 2.0, the residual stress value at the center of the plate width becomes high, so that the delayed fracture resistance deteriorates. Therefore, the restraint pressure ratio is 2.0 or less.
- the confining pressure ratio is preferably 1.7 or less, more preferably 1.5 or less.
- the restraint pressure is not particularly limited, but from the viewpoint of more effectively obtaining the effect of the present invention, the restraint pressure (load load) per 1 mm 2 of the steel sheet is 50 to It is preferably 300 N / mm 2.
- the restraining pressure (load) referred to here is the total pressure applied to the steel sheet from the front surface and the back surface of the steel sheet. Further, it is preferable that the restraining pressure by the two rolls is uniformly applied from the front surface and the back surface of the steel sheet.
- the means for restraining the steel plate at the center of the plate width with a restraining pressure larger than that at the end of the plate width is not particularly limited.
- the roll diameter of the portion in contact with the center of the plate width is made larger than the roll diameter of the portion in contact with the end of the plate width.
- the roll diameter of the portion in contact with the center of the plate width can be increased due to the difference in the coefficient of thermal expansion.
- a restraining pressure can be stably applied to the steel sheet.
- a roll is used in which the temperature at the center of the plate width and the end of the plate width are constant and the roll diameter of the portion in contact with the center of the plate width is larger than the roll diameter of the portion in contact with the end of the plate width. You may decide.
- the guide roll for pressing the roll that comes into direct contact with the steel plate is divided into the center portion of the plate width of the steel plate and the plate width end portion of the steel plate, and the force of pressing from the guide roll is applied to the plate. It is possible to control the size at the center of the plate width rather than at the width end.
- the temperature of the steel sheet after being discharged from the water tank needs to be 100 ° C. or lower.
- the temperature after water cooling is preferably 80 ° C. or lower.
- the reheating temperature is 100 ° C. or higher.
- the reheating temperature is preferably 130 ° C. or higher.
- the shape uniformity deteriorates due to transformation shrinkage due to tempering. From the above, the reheating temperature is 300 ° C. or lower.
- the reheating temperature is preferably 260 ° C. or lower.
- the hot-rolled steel sheet after the hot-rolling process may be heat-treated to soften the structure. Further, after the annealing step, temper rolling for shape adjustment may be performed. Further, the surface of the steel sheet may be plated with Zn, Al or the like.
- the center of the plate width in the present invention is set to ⁇ 200 mm with respect to the position in the width direction of 1/2 of the width of the manufactured steel plate, assuming that the width end portion of the steel plate is removed by trimming during manufacturing. Good.
- the member of the present invention is formed by subjecting the steel sheet of the present invention to at least one of molding and welding.
- the method for manufacturing a member of the present invention includes a step of performing at least one of molding and welding on the steel sheet manufactured by the method for manufacturing a steel sheet of the present invention.
- the steel sheet of the present invention has high strength and is excellent in shape uniformity and delayed fracture resistance.
- the member obtained by using the steel sheet of the present invention has high strength, good dimensional accuracy, and excellent delayed fracture resistance, and therefore can be suitably used for, for example, a structural member for automobiles.
- general processing methods such as press processing can be used without limitation.
- welding general welding such as spot welding and arc welding can be used without limitation.
- the front surface and the back surface of the steel sheet are provided with two rolls installed sandwiching the steel sheet so that the restraining pressure (load load) per 1 m 2 of the roll steel sheet is 50 to 300 N / mm 2.
- a restraining pressure (load) was applied to the steel sheet at a uniform pressure.
- the area ratio was the average value of the three area ratios obtained from separate SEM images with a magnification of 1500 times.
- the measurement location was 1/4 of the plate thickness.
- Martensite has a white structure, and tempered martensite has fine carbides precipitated inside.
- Ferrite has a black structure. Further, depending on the surface orientation of the block grains and the degree of etching, it may be difficult for carbides to appear inside. In that case, it is necessary to sufficiently perform etching to confirm. Further, the area ratio of the remaining portion other than ferrite and martensite was calculated by subtracting the total area ratio of ferrite and martensite from 100%.
- Residual stress was measured by X-ray diffraction. Specifically, a sample of 100 mm in the rolling direction and 30 mm in the width direction is taken from the center of the plate width and the end of the plate width of each steel plate, and the steel plate sample is placed on a die having an angle of 90 °. By pressing the steel sheet with a punch having an angle of °, V-shaped bending was performed so that the bending ridge direction was parallel to the width direction of the steel sheet. The V-shaped bending process was performed under the conditions of punch moving speed: 30 mm / min, load: 15 ton, and holding time (pushing time) at maximum load: 5 seconds. Next, using bolts, nuts and taper washers, the bent steel plate (member) was bolted from both sides of the plate surface. The tightening amount was 30 mm.
- the measurement point of the residual stress in the steel sheet after the bending process was the center of the thickness of the end face of the bending ridge, and the X-ray irradiation diameter was 150 ⁇ m.
- the measurement direction was perpendicular to the plate thickness direction and perpendicular to the bending ridge line direction.
- the sample at the plate width end was taken at a position of 0 to 30 mm in the width direction from the plate width end of the steel plate.
- the steel sheet is manufactured by applying a load so as to be symmetrical with respect to the center of the plate width in the plate width direction, the residual stress is measured only at one end and the residual stress is measured at the other end. Residual stress was considered to be the same value.
- the critical load stress was measured by a delayed fracture test. Specifically, a sample of 100 mm in the rolling direction and 30 mm in the width direction was taken from the center of the plate width of each steel plate, and the sample was V-shaped bent by the same method as the above-mentioned residual stress measuring method. Next, using bolts, nuts and taper washers, the bent steel plate (member) was bolted from both sides of the plate surface. By CAE analysis using the YU model, the relationship between the load stress and the tightening amount was calculated from the stress-strain curve obtained from the tensile test, and molded members having various load stresses were produced.
- the molded member was immersed in hydrochloric acid having a pH of 1 (25 ° C.), and the maximum load stress that did not cause delayed fracture was evaluated as the critical load stress.
- the judgment of delayed fracture was performed visually and with an image magnified to a magnification of 20 with a stereomicroscope, and the case where the image was immersed for 96 hours and no cracks occurred was regarded as no destruction.
- the term “crack” as used herein refers to the case where a crack having a crack length of 200 ⁇ m or more occurs.
- the steel sheet obtained in each example was sheared in the longitudinal direction of the steel sheet (rolling direction) at the original width of the steel sheet to a length of 1 m, and the sheared steel sheet was placed on a horizontal table.
- the sheared steel sheet was placed on a horizontal table so that the corners of the steel sheet and the horizontal table had more contact points (two or more points).
- the amount of warpage is determined by lowering the horizontal plate from a position above the steel plate until it contacts the steel plate, and at the position where it contacts the steel plate, the thickness of the steel plate is determined from the distance between the horizontal table and the horizontal plate. I asked for it.
- the amount of warpage was measured with the other side of the steel sheet on the upper side, and the maximum value of the measured amount of warpage was taken as the maximum amount of warpage.
- the clearance of the blade of the shearing machine when cutting the steel sheet in the longitudinal direction is 4% (the upper limit of the control range is 10%).
- the critical load stress is YS or more
- the critical load stress ratio to the plate width end at the center of the plate width is 90% or more and 110% or less
- the length is 1 m in the longitudinal direction of the steel sheet.
- a steel sheet having a maximum warp of 15 mm or less was accepted, and Table 2 shows an example of the invention. Steel sheets that do not satisfy at least one of these were rejected and are shown as comparative examples in Table 2.
- the hot-rolled steel sheet and the cold-rolled steel sheet obtained as described above were annealed under the conditions shown in Table 4 or 5 to produce a steel sheet.
- the blanks in Table 3 indicate that they were not added intentionally, and include not only the case where they are not contained (0% by mass) but also the cases where they are unavoidably contained.
- the critical load stress is YS or more
- the critical load stress ratio to the plate width end at the center of the plate width is 90% or more and 110% or less
- the length is 1 m in the longitudinal direction of the steel sheet.
- Steel sheets having a maximum warp amount of 15 mm or less were accepted, and are shown as examples of inventions in Tables 6 and 7. Steel sheets that do not satisfy at least one of these were rejected and are shown as comparative examples in Tables 6 and 7.
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Abstract
Description
[1]面積率で、マルテンサイト:20%以上100%以下、フェライト:0%以上80%以下、及び残部:5%以下を有する鋼組織を有し、
V字曲げ加工した際の板幅中央の残留応力が800MPa以下であり、
V字曲げ加工した際に、板幅端の残留応力が板幅中央の残留応力に対して90%以上110%以下であり、
鋼板長手方向に長さ1mでせん断した際の鋼板の最大反り量が15mm以下である鋼板。
[2]質量%で、
C:0.05%以上0.60%以下、
Si:0.01%以上2.0%以下、
Mn:0.1%以上3.2%以下、
P:0.050%以下、
S:0.0050%以下、
Al:0.005%以上0.10%以下、及び
N:0.010%以下を含有し、残部はFe及び不可避的不純物からなる成分組成を有する[1]に記載の鋼板。
[3]前記成分組成は、さらに、質量%で、
Cr:0.01%以上0.50%以下、
Mo:0.01%以上0.15%未満、及び
V:0.001%以上0.05%以下のうちから選ばれた少なくとも1種を含有する[2]に記載の鋼板。
[4]前記成分組成は、さらに、質量%で、
Nb:0.001%以上0.020%以下及び
Ti:0.001%以上0.020%以下のうちから選ばれた少なくとも1種を含有する[2]又は[3]に記載の鋼板。
[5]前記成分組成は、さらに、質量%で、
Cu:0.001%以上0.20%以下及び
Ni:0.001%以上0.10%以下のうちから選ばれた少なくとも1種を含有する[2]~[4]のいずれか一つに記載の鋼板。
[6]前記成分組成は、さらに、質量%で、
B:0.0001%以上0.0020%未満を含有する[2]~[5]のいずれか一つに記載の鋼板。
[7]前記成分組成は、さらに、質量%で、
Sb:0.002%以上0.1%以下及び
Sn:0.002%以上0.1%以下のうちから選ばれた少なくとも1種を含有する[2]~[6]のいずれか一つに記載の鋼板。
[8][2]~[7]のいずれか一つに記載の成分組成を有する鋼スラブを加熱した後、熱間圧延する、熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板を、AC1点以上の焼鈍温度で30秒以上保持し、その後、Ms点以上で水焼入れを開始し、100℃以下まで水冷後、100℃以上300℃以下で再度加熱する焼鈍工程と、を有し、
前記焼鈍工程における前記水焼入れの水冷中、鋼板の表面温度が(Ms点+150℃)以下の領域において、鋼板を挟んで設置された2つのロールで鋼板の表面及び裏面から鋼板を拘束し、前記2つのロールの拘束位置での板幅端に対する板幅中央の拘束圧力比が1.05以上2.0以下である鋼板の製造方法。
[9][2]~[7]のいずれか一つに記載の成分組成を有する鋼スラブを加熱した後、熱間圧延する、熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板を冷間圧延する、冷間圧延工程と、
前記冷間圧延工程で得られた冷延鋼板を、AC1点以上の焼鈍温度で30秒以上保持し、その後、Ms点以上で水焼入れを開始し、100℃以下まで水冷後、100℃以上300℃以下で再度加熱する焼鈍工程と、を有し、
前記焼鈍工程における前記水焼入れの水冷中、鋼板の表面温度が(Ms点+150℃)以下の領域において、鋼板を挟んで設置された2つのロールで鋼板の表面及び裏面から鋼板を拘束し、前記2つのロールの拘束位置での板幅端に対する板幅中央の拘束圧力比が1.05以上2.0以下である鋼板の製造方法。
[10][1]~[7]のいずれか一つに記載の鋼板に対して、成形加工及び溶接の少なくとも一方を施してなる部材。
[11][8]又は[9]に記載の鋼板の製造方法によって製造された鋼板に対して、成形加工及び溶接の少なくとも一方を施す工程を有する部材の製造方法。
TS≧750MPaの高強度を得るため、マルテンサイトの面積率は20%以上である。マルテンサイトの面積率が20%未満であると、フェライト、残留オーステナイト、パーライト、ベイナイトのいずれかが多くなり、強度が低下する。なお、マルテンサイトの面積率は合計で100%であってもよい。強度向上の観点からは、マルテンサイトの面積率は30%以上であることが好ましい。マルテンサイトは焼入れままのフレッシュマルテンサイトと焼戻した焼戻しマルテンサイトの合計である。本発明において、マルテンサイトとは、マルテンサイト変態点(単にMs点ともいう。)以下でオーステナイトから生成した硬質な組織を指し、焼戻しマルテンサイトはマルテンサイトを再加熱した時に焼戻される組織を指す。
鋼板の強度を確保する観点から、フェライトの面積率は80%以下である。当該面積率は、0%であってもよい。本発明において、フェライトとは比較的高温でのオーステナイトからの変態により生成し、BCC格子の結晶粒からなる組織である。
本発明の鋼板の鋼組織は、マルテンサイト及びフェライト以外の残部として、不可避的に含む金属相を含んでいてもよい。残部の面積率は、5%以下であれば許容される。残部に含まれる相としては、例えば、残留オーステナイト、パーライト、及びベイナイトが挙げられる。本発明でいう残留オーステナイトとは、マルテンサイト変態せずに室温まで残ったオーステナイトを指す。本発明でいうパーライトとは、フェライトと針状セメンタイトからなる組織である。本発明でいうベイナイトとは、比較的低温(マルテンサイト変態点以上)でオーステナイトから生成し、針状又は板状のフェライト中に微細な炭化物が分散した硬質な組織を指す。
本発明でいうV字曲げ加工とは、曲げ稜線方向が鋼板の幅方向と平行になるように、曲げ角度が90°で曲げ加工を行うことを意味する。本発明の鋼板は耐遅れ破壊特性に優れる。具体的には、実施例に記載の遅れ破壊試験を行ったときに求めた臨界負荷応力が降伏強度(以下、単にYSともいう。)以上である。臨界負荷応力は、好ましくは(YS+100)MPa以上、より好ましくは(YS+200)MPa以上である。臨界負荷応力をYS以上とするためには、鋼板をV字曲げ加工した際の板幅中央の残留応力を800MPa以下とする必要がある。優れた耐遅れ破壊特性を得る観点から、当該残留応力は、好ましくは780MPa以下、より好ましくは700MPa以下、さらに好ましくは600MPa以下である。
本発明の鋼板は鋼板全幅でも耐遅れ破壊特性に優れる。具体的には、鋼板をV字曲げ加工し、板幅中央と板幅端で臨界負荷応力を求めたときに、板幅端の臨界負荷応力が板幅中央の臨界負荷応力に対して90%以上110%以下であり、好ましくは92%以上108%以下であり、より好ましくは95%以上105%以下である。板幅端の臨界負荷応力が板幅中央の臨界負荷応力に対して90%以上110%以下とするためには、V字曲げ加工した際に、板幅端の残留応力を板幅中央の残留応力に対して90%以上110%以下とする必要がある。優れた耐遅れ破壊特性向上の観点からは、V字曲げ加工した際に、板幅端の残留応力が板幅中央の残留応力に対して好ましくは92%以上108%以下であり、より好ましくは95%以上105%以下である。
本発明の鋼板は形状均一性が良好である。具体的には、鋼板長手方向(圧延方向)に長さ1mでせん断した際の鋼板の最大反り量が15mm以下である。最大反り量は、好ましくは13mm以下、より好ましくは12mm以下、さらに好ましくは10mm以下である。最大反り量の下限は限定せず、0mmが最も好ましい。
Cは、焼入れ性を向上させる元素であり、所定のマルテンサイトの面積率を確保するために必要である。また、Cは、マルテンサイトの強度を上昇させ、強度を確保する観点から必要である。優れた耐遅れ破壊特性を維持して所定の強度を得る観点から、C量は0.05%以上であることが好ましい。なお、TS≧950MPaを得る観点からは、C含有量の下限は0.11%以上であることがより好ましい。また、引張強度をさらに高める観点からは、C含有量の下限は0.125%以上であることがさらに好ましい。一方、C含有量が0.60%を超えると、強度が過剰になりやすく、マルテンサイト変態による変態膨張を抑制しにくくなる傾向がある。そのため、形状均一性が劣化する傾向がある。したがって、C含有量は0.60%以下であることが好ましい。C含有量は、より好ましくは0.50%以下、さらに好ましくは0.40%以下である。
Siは固溶強化による強化元素である。このような効果を十分に得るには、Si含有量を0.01%以上であることが好ましい。Si含有量は、より好ましくは0.02%以上、さらに好ましくは0.03%以上である。一方、Si含有量が多くなりすぎると、板厚方向に粗大なMnSが生成しやすくなり、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する傾向がある。したがって、Si含有量は2.0%以下であることが好ましい。Si含有量は、より好ましくは1.7%以下、さらに好ましくは1.5%以下である。
Mnは、鋼の焼入れ性を向上させ、所定のマルテンサイトの面積率を確保するために含有させる。Mn含有量が0.1%未満では、鋼板表層部にフェライトが生成することで強度が低下する傾向がある。したがって、Mn含有量は0.1%以上であることが好ましい。Mn含有量は、より好ましくは0.2%以上、さらに好ましくは0.3%以上である。一方、Mnは、MnSの生成・粗大化を特に助長する元素であり、Mn含有量が3.2%を超えると、粗大な介在物の増加により、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する傾向がある。したがって、Mn含有量は3.2%以下であることが好ましい。Mn含有量は、より好ましくは3.0%以下、さらに好ましくは2.8%以下である。
Pは、鋼を強化する元素であるが、その含有量が多いと粒界に偏析し、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する傾向がある。したがって、P含有量は0.050%以下であることが好ましい。P含有量は、より好ましくは0.030%以下、さらに好ましくは0.010%以下である。なお、P含有量の下限は特に限定されるものではないが、現在、工業的に実施可能な下限は0.003%程度である。
Sが過剰に含有されていると、MnS、TiS、Ti(C、S)等の介在物が過剰に形成され、耐遅れ破壊特性が劣化する傾向がある。耐遅れ破壊特性が劣化を抑制する観点から、S含有量は0.0050%以下であることが好ましい。S含有量は、より好ましくは0.0020%以下、さらに好ましくは0.0010%以下、特に好ましくは0.0005%以下である。なお、S含有量の下限は特に限定されるものではないが、現在、工業的に実施可能な下限は0.0002%程度である。
Alは十分な脱酸を行い、鋼中の粗大介在物を低減するために添加される。その効果を十分に得る観点から、Al含有量0.005%以上であることが好ましい。Al含有量は、より好ましくは0.010%以上である。一方Al含有量が0.10%超となると、熱間圧延後の巻取り時に生成したセメンタイトなどのFeを主成分とする炭化物が焼鈍工程で固溶しにくくなり、粗大な介在物や炭化物が生成する傾向がある。そのため、鋼板の強度を低下させるのみならず、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する傾向がある。したがって、Al量は0.10%以下であることが好ましい。Al含有量は、より好ましくは0.08%以下、さらに好ましくは0.06%以下である。
Nは、鋼中でTiN、(Nb、Ti)(C、N)、AlN等の窒化物、炭窒化物系の粗大介在物を形成する元素である。Nが過剰に含有されていると粗大介在物の生成により耐遅れ破壊特性が劣化する。耐遅れ破壊特性の劣化を防止するため、N含有量は0.010%以下であることが好ましい。N含有量は、好ましくは0.007%以下、さらに好ましくは0.005%以下である。なお、N含有量の下限は特に限定されるものではないが、現在、工業的に実施可能な下限は0.0006%程度である。
Cr、Mo、Vは、鋼の焼入れ性の向上効果を得る目的で、含有させることができる。このような効果を得るには、Cr含有量及びMo含有量のいずれの場合でも0.01%以上にすることが好ましい。Cr含有量及びMo含有量は、それぞれ、より好ましくは0.02%以上、さらに好ましくは0.03%以上である。V含有量の場合は好ましくは0.001%以上、より好ましくは0.002%以上、さらに好ましくは0.003%以上である。しかしながら、いずれの元素も多くなりすぎると炭化物の粗大化により、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する。そのためCr含有量は、好ましくは0.50%以下であり、より好ましくは0.1%以下である。Mo含有量は好ましくは0.15%未満であり、より好ましくは0.10%以下である。V含有量は好ましくは0.05%以下であり、より好ましくは0.04%以下、さらに好ましくは0.03%以下である。
NbやTiは、旧γ粒の微細化を通じて、高強度化に寄与する。このような効果を得るためには、Nb及びTiを、それぞれ、0.001%以上で含有させることが好ましい。Nb及びTiの含有量は、それぞれ、より好ましくは0.002%以上、さらに好ましくは0.003%以上である。一方、NbやTiを多量に含有させると、熱間圧延工程のスラブ加熱時に未固溶で残存するNbN、Nb(C、N)、(Nb、Ti)(C、N)等のNb系の粗大な析出物、TiN、Ti(C、N)、Ti(C、S)、TiS等のTi系の粗大な析出物が増加し、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する。このため、Nb及びTi含有量は、それぞれ、0.020%以下であることが好ましい。Nb及びTi含有量は、それぞれ、より好ましくは0.015%以下、さらに好ましくは0.010%以下である。
CuやNiは、自動車の使用環境での耐食性を向上させ、かつ腐食生成物が鋼板表面を被覆して鋼板への水素侵入を抑制する効果がある。この効果を得るためには、Cu及びNiは、それぞれ、0.001%以上含有することが好ましい。Cu含有量及びNi含有量は、それぞれ、より好ましくは0.002%以上である。しかしながら、Cu含有量やNi含有量が多くなりすぎると表面欠陥の発生を招来し、めっき性や化成処理性を劣化させるので、Cu含有量は0.20%以下であることが好ましい。Cu含有量は、より好ましくは0.15%以下、さらに好ましくは0.10%以下である。Ni含有量は0.10%以下であることが好ましい。Ni含有量は、より好ましくは0.08%以下、さらに好ましくは0.06%以下である。
Bは、鋼の焼入れ性を向上させる元素であり、B含有により、Mn含有量が少ない場合であっても、所定の面積率のマルテンサイトを生成させる効果が得られる。このようなBの効果を得るには、B含有量を0.0001%以上にすることが好ましい。B含有量は、好ましくは0.0002%以上であり、さらに好ましくは0.0003%以上である。一方、B含有量が0.0020%以上になると、焼鈍時のセメンタイトの固溶速度を遅延させ、未固溶のセメンタイトなどのFeを主成分とする炭化物が残存することとなる。これにより、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する。したがって、B含有量は0.0020%未満であることが好ましい。B含有量は、より好ましくは0.0015%以下、さらに好ましくは0.0010%以下である。
SbやSnは、鋼板表層部の酸化や窒化を抑制し、鋼板表層部の酸化や窒化によるCやBの低減を抑制する。また、CやBの低減が抑制されることで、鋼板表層部のフェライト生成を抑制し、高強度化に寄与する。このような効果を得るためには、Sb含有量及びSn含有量はいずれの場合でも0.002%以上であることが好ましい。Sb含有量及びSn含有量は、それぞれ、より好ましくは0.003%以上、さらに好ましくは0.004%以上である。一方、Sb含有量及びSn含有量いずれの場合でも0.1%を超えて含有すると、旧γ粒界にSbやSnが偏析して、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する。このため、Sb含有量及びSn含有量いずれの場合でも0.1%以下であることが好ましい。Sb含有量及びSn含有量は、それぞれ、より好ましくは0.08%以下、さらに好ましくは0.06%以下である。
熱間圧延工程とは、上記成分組成を有する鋼スラブを加熱した後、熱間圧延する工程である。
冷間圧延工程とは、熱間圧延工程で得られた熱延鋼板を冷間圧延する工程である。冷間圧延の圧下率は特に限定されないが、圧下率が20%未満の場合、表面の平坦度が悪く、組織が不均一となる可能性があるため、圧下率は20%以上とするのが好ましい。なお、冷間圧延工程は必須の工程ではなく、鋼組織や機械的特性が本発明の範囲を満たせば、冷間圧延工程は省略しても構わない。
焼鈍工程とは、冷延鋼板又は熱延鋼板を、AC1点以上の焼鈍温度で30秒以上保持し、その後、Ms点以上で水焼入れを開始し、100℃以下まで水冷後、100℃以上300℃以下で再度加熱する工程である。焼鈍工程における水焼入れの水冷中、鋼板の表面温度が(Ms点+150℃)以下の領域において、鋼板を挟んで設置された2つのロールで鋼板の表面及び裏面から鋼板を拘束し、2つのロールの拘束位置での板幅端に対する板幅中央の拘束圧力比が1.05以上2.0以下である。
焼鈍温度がAC1点未満では、オーステナイトが生成しないため、20%以上のマルテンサイトを有する鋼板を得ることが難しくなり、所望の強度が得られなくなる。したがって、焼鈍温度はAC1点以上必要である。好ましくは(AC1点+10℃)以上である。焼鈍温度の上限は特に限定されないが、水焼入れ時の温度を適正化し、形状均一性の劣化を防ぐ観点から、焼鈍温度は900℃以下が好ましい。
AC1点(℃)=723+22(%Si)-18(%Mn)+17(%Cr)+4.5(%Mo)+16(%V)
焼鈍温度での保持時間(焼鈍保持時間)は30秒以上
焼鈍保持時間が30秒未満となると、炭化物の溶解とオーステナイト変態が十分に進行しないため、以降の熱処理時に残った炭化物が粗大化し、板幅中央の残留応力が増加することで耐遅れ破壊特性が劣化する。したがって、焼鈍保持時間は30秒以上、好ましくは35秒以上である。焼鈍保持時間の上限は特に限定されないが、オーステナイト粒径の粗大化を抑制し、侵入水素量の増加を防ぐ観点から、焼鈍保持時間は900秒以下とするのが好ましい。
焼入れ開始温度は強度の支配因子であるマルテンサイト分率を決めるために重要な因子である。焼入れ開始温度がMs点未満となると、焼入れ前にマルテンサイト変態するため、マルテンサイトの自己焼戻しが生じ、形状均一性が悪くなる。したがって、水焼入れ開始温度はMs点以上である。好ましくは(Ms点+50℃)以上である。水焼入れ開始温度の上限は特に限定せず、焼鈍温度でも構わない。
Ms点(℃)=550-350{(%C)/(%VM)×100}-40(%Mn)-17(%Ni)-17(%Cr)-21(%Mo)
水焼入れの水冷中、2つのロールで鋼板の表面及び裏面から鋼板を拘束することは形状矯正効果を得るために重要な因子であり、板幅中央の残留応力及び鋼板全幅での残留応力変動を低減するためには、拘束条件の制御が重要な因子となる。水冷中の変態ひずみを拘束により矯正することで鋼板の形状均一性を向上させ、残留応力を増加させ耐遅れ破壊特性を劣化させるレベラー矯正やスキンパス圧延による矯正を不要としたことに本発明は特徴がある。形状均一性の悪化を矯正する際に施されるレベラー加工やスキンパス圧延が不要となるため、残留応力の低減が可能となる。また、拘束条件で鋼板全幅での残留応力変動も低減するため、鋼板全幅で耐遅れ破壊特性が良好となる。
拘束温度が(Ms点+150℃)超となると、拘束後にマルテンサイト変態するため、マルテンサイト変態の変態膨張による形状均一性の劣化を抑制することができず、形状均一性が悪くなる。したがって、拘束温度は(Ms点+150℃)以下であり、好ましくは(Ms点+100℃)以下、より好ましくは(Ms点+50℃)以下である。拘束温度の下限は特に限定されず、水が凍らない0℃以上であることが好ましい。
2つのロールの拘束位置での板幅端に対する板幅中央の拘束圧力比を1.05以上とすることで、板幅中央に残存する残留応力を鋼板幅方向に分散することができ、板幅中央の残留応力及び鋼板全幅での残留応力変動を低減し、鋼板全幅で耐遅れ破壊特性に優れることが可能となる。したがって、拘束圧力比は1.05以上である。拘束圧力比は、好ましくは1.10以上である。一方、拘束圧力比が2.0超となると、板幅中央での残留応力値が高くなるため、耐遅れ破壊特性が劣化する。したがって、拘束圧力比は2.0以下である。拘束圧力比は、好ましくは1.7以下、より好ましくは1.5以下である。
水冷後の温度が100℃を超えると、形状均一性に悪影響をもたらすほどマルテンサイト変態が水冷後に進行する。そのため、水槽から出た後の鋼板温度は100℃以下である必要がある。水冷後の温度は、好ましくは80℃以下である。
水冷後は再加熱し、水冷時に生成したマルテンサイトを焼き戻すことで残留応力を低減することが可能となる。そうすることで耐遅れ破壊特性を良好にすることができる。再加熱温度が100℃未満では上記の効果が得られない。そこで、再加熱温度は100℃以上である。再加熱温度は、好ましくは130℃以上である。一方、300℃超で焼き戻すと焼戻しによる変態収縮により形状均一性を劣化させる。以上から、再加熱温度は300℃以下である。再加熱温度は、好ましくは260℃以下である。
本発明を、実施例を参照しながら具体的に説明する。
板厚1.4mmの冷延鋼板に、表1に示す条件で焼鈍を行い、表2に記載の特性を有する鋼板を製造した。なお、拘束ロール通過時の温度はロールに付随した接触式の温度計を用いて測定した。拘束圧力は、YUモデルを用いたCAE(Computer Aided Engineering)解析により、ロールの剛性、形状、押し込み量、鋼板の板厚、変態時のオーステナイト分率、応力-ひずみ曲線を用いて求めた。
各種製造条件で得られた鋼板に対して、鋼組織を解析することで組織分率を調査し、引張試験を実施することで引張強度等の引張特性を評価し、遅れ破壊試験によって耐遅れ破壊特性を評価し、鋼板形状の測定によって形状均一性を評価した。各評価の方法は次のとおりである。結果は表2に示す。
各鋼板の圧延方向及び圧延方向に対して垂直方向から試験片を採取し、圧延方向に平行な板厚L断面を鏡面研磨し、ナイタール液で組織現出した。組織現出したサンプルを、走査電子顕微鏡を用いて観察し、倍率1500倍のSEM像上の、実長さ82μm×57μmの領域上に4.8μm間隔の16×15の格子をおき、各相上にある点数を数えるポイントカウンティング法により、マルテンサイト及びフェライトの面積率を調査した。面積率は、倍率1500倍の別々のSEM像から求めた3つの面積率の平均値とした。測定場所は板厚1/4とした。マルテンサイトは白色の組織を呈しており、焼戻しマルテンサイトは内部に微細な炭化物が析出している。フェライトは黒色の組織を呈している。また、ブロック粒の面方位とエッチングの程度によっては、内部の炭化物が現出しにくい場合もあるので、その場合はエッチングを十分に行い確認する必要がある。また、フェライト及びマルテンサイト以外の残部の面積率を、100%からフェライト及びマルテンサイトの合計面積率を引いて算出した。
各鋼板の板幅中央部の圧延方向から、標点間距離50mm、標点間幅25mmのJIS5号試験片を採取し、JIS Z 2241(2011)の規定に準拠して、引張速度が10mm/分で引張試験を行い、引張強度(TS)及び降伏強度(YS)を測定した。
X線回折により残留応力を測定した。具体的には、各鋼板の板幅中央及び板幅端から、圧延方向に100mm、幅方向に30mmのサンプルを採取し、90°の角度を有するダイスの上に鋼板のサンプルを載せて、90°の角度を有するポンチによって鋼板をプレスすることで、曲げ稜線方向が鋼板の幅方向と平行になるように、V字曲げ加工を行った。V字曲げ加工は、ポンチ移動速度:30mm/min、荷重:15ton、最大荷重での保持時間(押し込み時間):5秒の条件で行った。次いで、ボルト、ナット及びテーパーワッシャーを用いて、曲げ加工後の鋼板(部材)を、板面の両側からボルトで締め込んだ。締め込み量は30mmとした。
遅れ破壊試験によって臨界負荷応力を測定した。具体的には、各鋼板の板幅中央から、圧延方向に100mm、幅方向に30mmのサンプルを採取し、上記残留応力測定方法と同じ方法で、サンプルをV字曲げ加工した。次いで、ボルト、ナット及びテーパーワッシャーを用いて、曲げ加工後の鋼板(部材)を、板面の両側からボルトで締め込んだ。YUモデルを用いたCAE解析によって、引張試験から求めた応力-ひずみ曲線から負荷応力と締込量の関係を算出し、様々な負荷応力を有する成形部材を作製した。その成形部材をpH=1(25℃)の塩酸中に浸漬し、遅れ破壊しない最大負荷応力を臨界負荷応力として評価した。遅れ破壊の判定は目視及び実体顕微鏡で倍率×20まで拡大した画像にて行い、96時間浸漬し割れが発生しなかった場合を破壊なしとした。ここでいう割れとは、亀裂長さが200μm以上の亀裂が発生した場合を指す。
各実施例で得た鋼板を鋼板長手方向(圧延方向)に鋼板の元幅にて長さ1mでせん断し、せん断後の鋼板を水平な台に置いた。なお、せん断後の鋼板は、鋼板の角部と水平な台がより多くの接触点(2点以上)が存在するように水平な台の上に置いた。反り量は、鋼板よりも上の位置から水平な板を鋼板に接触するまで降ろしていき、鋼板に接触した位置において、水平な台と水平な板との間の距離から、鋼板の板厚を引いて求めた。また、鋼板の一方の面を上側にして反り量を測定した後、鋼板の他方の面を上側にして反り量を測定し、測定した反り量のうち最大である値を最大反り量とした。なお、鋼板の長手方向の切断をする際のせん断機の刃のクリアランスは4%(管理範囲の上限は10%)で行っている。
上記評価結果を表2に示す。
本発明を、実施例を参照しながら具体的に説明する。
表3に示す成分組成を有し、残部がFe及び不可避的不純物よりなる鋼を真空溶解炉にて溶製後、分塊圧延し27mm厚の分塊圧延材を得た。得られた分塊圧延材を熱間圧延した。次いで、冷間圧延するサンプルは、熱延鋼板を研削加工した後、表4又は5に示す圧下率で冷間圧延して、表4又は5に記載の板厚となるように冷間圧延し、冷延鋼板を製造した。なお、一部のサンプルは、熱延鋼板を研削加工した後、冷間圧延しなかった。表中で圧下率「-」と記載したサンプルは、冷間圧延していないことを意味する。次いで、上記により得られた熱延鋼板及び冷延鋼板に、表4又は5に示す条件で焼鈍を行い、鋼板を製造した。なお、表3の空欄は、意図的に添加していないことを表しており、含有しない(0質量%)場合だけでなく、不可避的に含有する場合も含む。
各種製造条件で得られた鋼板に対して、鋼組織を解析することで組織分率を調査し、引張試験を実施することで引張強度等の引張特性を評価し、遅れ破壊試験によって耐遅れ破壊特性を評価し、鋼板形状の測定によって形状均一性を評価した。各評価の方法は実施例1に記載の方法と同じである。
上記評価結果を表6及び7に示す。
Claims (11)
- 面積率で、マルテンサイト:20%以上100%以下、フェライト:0%以上80%以下、及び残部:5%以下を有する鋼組織を有し、
V字曲げ加工した際の板幅中央の残留応力が800MPa以下であり、
V字曲げ加工した際に、板幅端の残留応力が板幅中央の残留応力に対して90%以上110%以下であり、
鋼板長手方向に長さ1mでせん断した際の鋼板の最大反り量が15mm以下である鋼板。 - 質量%で、
C:0.05%以上0.60%以下、
Si:0.01%以上2.0%以下、
Mn:0.1%以上3.2%以下、
P:0.050%以下、
S:0.0050%以下、
Al:0.005%以上0.10%以下、及び
N:0.010%以下を含有し、残部はFe及び不可避的不純物からなる成分組成を有する請求項1に記載の鋼板。 - 前記成分組成は、さらに、質量%で、
Cr:0.01%以上0.50%以下、
Mo:0.01%以上0.15%未満、及び
V:0.001%以上0.05%以下のうちから選ばれた少なくとも1種を含有する請求項2に記載の鋼板。 - 前記成分組成は、さらに、質量%で、
Nb:0.001%以上0.020%以下及び
Ti:0.001%以上0.020%以下のうちから選ばれた少なくとも1種を含有する請求項2又は3に記載の鋼板。 - 前記成分組成は、さらに、質量%で、
Cu:0.001%以上0.20%以下及び
Ni:0.001%以上0.10%以下のうちから選ばれた少なくとも1種を含有する請求項2~4のいずれか一項に記載の鋼板。 - 前記成分組成は、さらに、質量%で、
B:0.0001%以上0.0020%未満を含有する請求項2~5のいずれか一項に記載の鋼板。 - 前記成分組成は、さらに、質量%で、
Sb:0.002%以上0.1%以下及び
Sn:0.002%以上0.1%以下のうちから選ばれた少なくとも1種を含有する請求項2~6のいずれか一項に記載の鋼板。 - 請求項2~7のいずれか一項に記載の成分組成を有する鋼スラブを加熱した後、熱間圧延する、熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板を、AC1点以上の焼鈍温度で30秒以上保持し、その後、Ms点以上で水焼入れを開始し、100℃以下まで水冷後、100℃以上300℃以下で再度加熱する焼鈍工程と、を有し、
前記焼鈍工程における前記水焼入れの水冷中、鋼板の表面温度が(Ms点+150℃)以下の領域において、鋼板を挟んで設置された2つのロールで鋼板の表面及び裏面から鋼板を拘束し、前記2つのロールの拘束位置での板幅端に対する板幅中央の拘束圧力比が1.05以上2.0以下である鋼板の製造方法。 - 請求項2~7のいずれか一項に記載の成分組成を有する鋼スラブを加熱した後、熱間圧延する、熱間圧延工程と、
前記熱間圧延工程で得られた熱延鋼板を冷間圧延する、冷間圧延工程と、
前記冷間圧延工程で得られた冷延鋼板を、AC1点以上の焼鈍温度で30秒以上保持し、その後、Ms点以上で水焼入れを開始し、100℃以下まで水冷後、100℃以上300℃以下で再度加熱する焼鈍工程と、を有し、
前記焼鈍工程における前記水焼入れの水冷中、鋼板の表面温度が(Ms点+150℃)以下の領域において、鋼板を挟んで設置された2つのロールで鋼板の表面及び裏面から鋼板を拘束し、前記2つのロールの拘束位置での板幅端に対する板幅中央の拘束圧力比が1.05以上2.0以下である鋼板の製造方法。 - 請求項1~7のいずれか一項に記載の鋼板に対して、成形加工及び溶接の少なくとも一方を施してなる部材。
- 請求項8又は9に記載の鋼板の製造方法によって製造された鋼板に対して、成形加工及び溶接の少なくとも一方を施す工程を有する部材の製造方法。
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- 2020-08-25 CN CN202080059851.7A patent/CN114302978B/zh active Active
- 2020-08-25 WO PCT/JP2020/031994 patent/WO2021039776A1/ja unknown
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MX2022002441A (es) | 2022-06-02 |
EP4001447A1 (en) | 2022-05-25 |
JP2021181624A (ja) | 2021-11-25 |
CN114302978B (zh) | 2023-07-04 |
CN114302978A (zh) | 2022-04-08 |
EP4001447A4 (en) | 2022-06-15 |
JP6958752B2 (ja) | 2021-11-02 |
US20220298594A1 (en) | 2022-09-22 |
JPWO2021039776A1 (ja) | 2021-03-04 |
EP4001447B1 (en) | 2024-05-01 |
KR20220036975A (ko) | 2022-03-23 |
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