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CN-117062934-B - Steel sheet, component, and method for producing same

CN117062934BCN 117062934 BCN117062934 BCN 117062934BCN-117062934-B

Abstract

The present invention provides a steel sheet having a tensile strength of 1310MPa or more, which is excellent in press formability, in a steel mainly composed of a martensitic structure having excellent delayed fracture resistance. The steel sheet is characterized by comprising, in mass%, 0.12-0.40% of C, 1.5% or less of Si, more than 1.7% and 3.5% or less of Mn, 0.05% or less of P, 0.010% or less of S, 1.00% or less of sol.Al, 0.010% or less of N, 0.002-0.080% of Ti, 0.0002-0.0050% of B, and the balance Fe and unavoidable impurities, wherein the area ratio of martensite in the metal structure to the whole structure is 85% or more, the ratio L S /L B of the length L S of the sub-lath block boundary to the length L B of the lath block boundary in the martensite satisfies the prescribed formula (1), and the tensile strength is 1310MPa or more.

Inventors

  • Yoshioka Mahira
  • KANEKO SHINJIRO
  • HONDA YUMA

Assignees

  • 杰富意钢铁株式会社

Dates

Publication Date
20260512
Application Date
20220228
Priority Date
20210331

Claims (11)

  1. 1. A steel sheet having the following composition and metallic structure, The composition comprises, by mass%, 0.12% -0.40% of C, 1.5% or less of Si, more than 1.7% and 3.5% or less of Mn, 0.05% or less of P, 0.010% or less of S, 1.00% or less of sol.Al, 0.010% or less of N, 0.002% -0.080% of Ti, 0.0002% -0.0050% of B, and the balance of Fe and unavoidable impurities, In the metal structure, the area ratio of martensite relative to the whole structure is more than 85 percent, the ratio L S /L B of the length L S of the sub-lath block boundary to the length L B of the lath block boundary satisfies the following formula (1), And a tensile strength of 1310MPa or more, 0.06/[C%] 0.8 ≦L S /L B ≦0.13/[C％] 0.8 ···(1) Wherein, [ C% ]: the content of C in mass%, The steel sheet is obtained by hot-rolling a steel slab having the above-mentioned composition to obtain a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet, soaking the cold-rolled steel sheet at a temperature range of at least 680 ℃ and at least 240 seconds to cool the steel sheet at an average cooling rate of at least 10 ℃ per second in a temperature range of at least 680 ℃ to at least Ms, then cooling the steel sheet at an average cooling rate of at least 100 ℃ and at most 380 ℃ per second in a temperature range of at least Ms to (Ms-50 ℃), and then cooling the steel sheet at an average cooling rate of at least 70 ℃ and at most 290 ℃ per second to at most 3 times.
  2. 2. The steel sheet according to claim 1, wherein the composition further comprises one or more selected from Cu：0.01％～1.00％、Ni：0.01％～1.00％、Mo：0.005％～0.350％、Cr：0.005％～0.350％、Zr：0.005％～0.350％、Ca：0.0002％～0.0050％、Nb：0.002％～0.060％、V：0.005％～0.500％、W：0.005％～0.200％、Sb：0.001％～0.100％、Sn：0.001％～0.100％、Mg：0.0002％～0.0100％ and REM 0.0002% -0.0100% by mass.
  3. 3. The steel sheet according to claim 1 or 2, wherein the standard deviation of the concentration of Mn is 0.35% or less.
  4. 4. A steel sheet according to any one of claims 1 to 3, wherein a zinc plating layer is provided on the surface.
  5. 5. A member obtained by at least one of forming and welding the steel sheet according to any one of claims 1 to 4.
  6. 6. A method for producing a steel sheet, comprising hot-rolling a steel slab having the composition of claim 1 or 2 to produce a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet, soaking the cold-rolled steel sheet at a temperature range of at least the Ac 3 point and at least 240 seconds to cool at least 1 cooling time at an average cooling rate of at least 10 ℃ per second from a cooling start temperature of at least 680 ℃ to a temperature range of at least the Ms point, subsequently cooling at least 2 cooling times at an average cooling rate of at least 100 ℃ per second and at most 380 ℃ per second from the Ms point to (Ms point-50 ℃) and subsequently cooling at least 3 cooling times at an average cooling rate of at least 70 ℃ per second and at most 290 ℃ per second to at most 50 ℃.
  7. 7. The method for producing a steel sheet according to claim 6, wherein reheating is performed for 20 to 1500 seconds in a temperature range of 150 to 300 ℃ after the 3 times of cooling.
  8. 8. The method for manufacturing a steel sheet according to claim 6 or 7, wherein the refrigerant used in the 2-pass cooling is water, and the water flow density in the 2-pass cooling is 0.5m 3 /m 2 /min～10.0m 3 /m 2 /min.
  9. 9. The method for producing a steel sheet according to any one of claims 6 to 8, wherein in the hot rolling, after rolling is performed at a finish rolling temperature of 840 ℃ or higher, cooling is performed to 640 ℃ or lower within 3 seconds, the steel sheet is maintained at a temperature range of 600 ℃ to 500 ℃ for 5 seconds or longer, and then a coiling treatment is performed at a temperature of 550 ℃ or lower.
  10. 10. The method for manufacturing a steel sheet according to claim 7, wherein the reheating is followed by a plating treatment.
  11. 11. A method for manufacturing a component, comprising at least one of forming and welding a steel sheet manufactured by the method for manufacturing a steel sheet according to any one of claims 6 to 10.

Description

Steel sheet, component, and method for producing same Technical Field The present invention relates to a high-strength steel sheet for cold press forming, which is used for automobiles, home appliances, and the like and requires a cold press forming step, a member using the steel sheet, and a method for producing the same. Background In recent years, as the demand for weight reduction of automobile bodies has further increased, the use of high-strength steel sheets having a Tensile Strength (TS) of 1310MPa or more in body frame members has been advancing. Further, from the viewpoint of further weight reduction, studies for increasing the strength of 1.8GPa or more are being started. Conventionally, high strength of hot pressing by pressing in hot sections has been actively studied, but recently, application of cold pressing to high strength steel has been newly studied from the viewpoint of cost and productivity. However, since the martensite structure is easier to obtain high strength than a relatively soft structure such as ferrite or bainite, it is effective to mainly use the martensite structure in the structure design of the high-strength steel sheet. However, the martensitic steel has a low ductility compared to the composite structure steel having a relatively soft structure such as ferrite and bainite. Therefore, the martensitic steel is only suitable for relatively simple shaped members such as door beams and bumpers which are mainly formed by bending. On the other hand, the delayed fracture resistance of the composite structure steel is inferior to that of the martensitic steel. That is, in order to achieve the same strength as that of the martensitic steel in the composite structure steel, a phase containing a hard structure having a higher hardness is required, but such a hard structure becomes a starting point of delayed fracture due to high stress concentration. Therefore, in a high-strength steel sheet, it is difficult to achieve both excellent delayed fracture resistance and formability. Here, if the ductility of the martensitic structure itself, which is excellent in delayed fracture resistance, can be improved, it is possible to achieve both excellent delayed fracture resistance and formability without performing the composite structure. One of the methods for improving the ductility of the martensitic structure is to raise the tempering temperature, but this method has a small effect of improving the ductility and significantly deteriorates the bendability due to the formation of coarse carbides. Patent document 1 discloses a technique for a high-strength cold-rolled steel sheet having excellent bendability, which has a yield strength of 1180MPa or more and a tensile strength of 1470MPa or more, and is characterized by containing 95% or more of martensite in terms of area ratio, a total of retained austenite and ferrite area ratio of less than 5% (including 0%), and further by having an average size of carbide of 60nm or less in terms of equivalent circle diameter and a number density of carbide of 25nm or more of 1/mm 2. Patent document 2 discloses a technique for an ultra-high strength steel sheet excellent in yield ratio and workability, which is characterized by having a structure composed of 90% or more of martensite and 0.5% or more of retained austenite, and by having a local Mn concentration of 1% or more in terms of area ratio in a region where the Mn content of the whole steel sheet is 1.2 times or more, and by having a tensile strength of 1470MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more. Prior art literature Patent literature Patent document 1 Japanese patent No. 6017341 Patent document 2 Japanese patent application laid-open No. 2019-2078 Disclosure of Invention In recent years, even a steel sheet lacking ductility can be processed into a complicated part shape by using a press working technique. As one of such manufacturing methods, there is a preforming technique of forming a part of a steel sheet before forming the steel sheet into a final shape, rather than forming the steel sheet into the final shape by press working 1 time, and of dispersing strain in the whole steel sheet, thereby controlling cracking of the steel sheet. In such a manufacturing method, for example, a strain may be applied in a biaxial direction in a subsequent step after uniaxial stretching, in other words, such a deformation that the strain application directions are orthogonal in the first step and the second step may be performed. The press workability in this manufacturing method is not necessarily related to the characteristic value evaluated in the uniaxial tension test, which is a general formability evaluation test. In the technique described in patent document 1, since excellent bendability can be obtained, ductility is sufficient for bending deformation which is commonly used in forming a member, but ductility is insufficient for a steel having a martensiti