US-12624413-B2 - Steel sheet, member, and method for producing steel sheet, and method for producing member

US12624413B2US 12624413 B2US12624413 B2US 12624413B2US-12624413-B2

Abstract

A steel sheet including: a chemical composition containing, by mass %, C: 0.12-0.40%, Si: 1.5% or less, Mn: more than 1.7% and 3.5% or less, P: 0.05% or less, S: 0.010% or less, sol. Al: 1.00% or less, N: 0.010% or less, Ti: 0.002-0.080%, and B: 0.0002-0.0050%, with the balance being Fe and inevitable impurities; a metallic structure in which an area ratio of martensite to an entire microstructure is 85% or more, and a ratio L S /L B satisfies a predetermined formula (1), where L S denotes a length of a sub-block boundary and L B denotes a length of a block boundary; and a tensile strength of 1310 MPa or more.

Inventors

Shimpei Yoshioka
Shinjiro Kaneko
Yuma HONDA

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260512
Application Date: 20220228
Priority Date: 20210331

Claims (6)

1 . A steel sheet comprising: a chemical composition containing, by mass %, C: 0.12% or more and 0.40% or less, Si: 1.5% or less, Mn: more than 1.7% and 3.2% or less, P: 0.05% or less, S: 0.010% or less, sol. Al: 1.00% or less, N: 0.010% or less, Ti: 0.002% or more and 0.080% or less, and B: 0.0002% or more and 0.0050% or less, and optionally further containing, by mass %, at least one selected from the group consisting of Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo: 0.005% or more and 0.350% or less, Cr: 0.005% or more and 0.350% or less, Zr: 0.005% or more and 0.350% or less, Ca: 0.0002% or more and 0.0050% or less, Nb: 0.002% or more and 0.060% or less, V: 0.005% or more and 0.500% or less, W: 0.005% or more and 0.200% or less, Sb: 0.001% or more and 0.100% or less, Sn: 0.001% or more and 0.100% or less, Mg: 0.0002% or more and 0.0100% or less, and REM: 0.0002% or more and 0.0100% or less with the balance being Fe and inevitable impurities; a metallic structure in which an area ratio of martensite to an entire microstructure is 85% or more, and a ratio L S /L B satisfies formula (1), where L S denotes a length of a sub-block boundary and L B denotes a length of a block boundary: 0.06/[C %] 0.8 ≤L S /L B ≤0.13/[C %] 0.8 (1), where [C %] represents a C content in mass %; a tensile strength of 1683 MPa or more, and the Mn has a standard deviation of concentration of 0.24% or less.
2 . The steel sheet according to claim 1 , comprising a galvanized layer on a surface thereof.
3 . A member obtainable by subjecting the steel sheet as recited in claim 1 to at least one of forming or welding.
4 . A method for producing the steel sheet according to claim 1 , comprising: subjecting a steel material having the chemical composition as recited in claim 1 to hot rolling to obtain a hot-rolled steel sheet, and then subjecting the hot-rolled steel sheet to cold rolling to obtain a cold-rolled steel sheet; and subjecting the cold-rolled steel sheet to soaking treatment at or above Ac 3 point for 240 seconds or more, followed by primary cooling in which the cold-rolled steel sheet is cooled at an average cooling rate of 10° C./s or higher and 1500° C./s or lower in a temperature range from a cooling start temperature of 680° C. or higher to Ms point, followed by secondary cooling in which the cold-rolled steel sheet is cooled at an average cooling rate of 240° C./s or higher and 330° C./s or lower in a temperature range from the Ms point to a temperature of (the Ms point—50° C.), followed by tertiary cooling in which the cold-rolled steel sheet is cooled to a temperature of 50° C. or lower at an average cooling rage of 70° C./s or higher and 230° C./s or lower, wherein optionally, after the tertiary cooling, reheating is performed in which the cold-rolled steel sheet is held in a temperature range from 150° C. to 300° C. for 20 seconds to 1500 seconds, when the reheating is performed, optionally, coating or plating treatment is performed after the reheating, and optionally, the secondary cooling uses water as a refrigerant and has a water flux density of 0.5 m 3 /m 2 /min or more and 10.0 m 3 /m 2 /min or less, wherein the hot rolling includes rolling the steel material at a rolling finish temperature of 840° C. or higher, then cooling the steel material to a temperature of 640° C. or lower within 3 seconds, then holding the steel material in a temperature range from 600° C. to 500° C. for 5 seconds or more, and then coiling the steel material at a temperature of 550° C. or lower.
5 . A method for producing a member, comprising subjecting the steel sheet produced by the method as recited in claim 4 to at least one of forming or welding.
6 . A member obtainable by subjecting the steel sheet as recited in claim 2 to at least one of forming or welding.

Description

TECHNICAL FIELD This disclosure relates to high-strength steel sheets for cold press forming that are used in automobiles, home appliances, and other products through cold press forming, to members using the steel sheets, and to methods for producing them. BACKGROUND In recent years, the application of high-strength steel sheets with tensile strength (TS) of 1310 MPa or more to automotive body parts has been increasing due to the growing need for weight reduction of automotive body. In addition, from the viewpoint of further weight reduction, consideration is beginning to be given to increasing the strength to 1.8 GPa grade or higher. In the past, increased strength by hot pressing is being vigorously investigated. Recently, however, the application of cold pressing to high tensile strength steel is being reexamined from the viewpoint of cost and productivity. Since martensitic microstructure tends to provide higher strength than relatively soft microstructures such as ferrite and bainite, it is effective to use mainly martensitic microstructure in the microstructural design of high-strength steel sheets. However, martensite-dominant steels have lower ductility than multi-phase steels, which contain relatively soft microstructures such as ferrite and bainite. For this reason, martensite-dominant steels have been applied only to those parts with relatively simple shapes, such as door beams and bumpers, which are formed generally by bending. On the other hand, multi-phase steels have inferior delayed fracture resistance compared to martensite-dominant steels. In other words, to achieve the same strength in multi-phase steels as in martensite-dominant steels, it is necessary for multi-phase steels to contain a harder phase with a harder microstructure, which, however, acts as a starting point for delayed fracture due to the high stress concentration. Therefore, it has been difficult to simultaneously achieve high delayed fracture resistance and high formability in high-strength steel sheets. If the ductility of martensitic microstructure itself, which has high delayed fracture resistance, can be improved, it may be possible to achieve both high delayed fracture resistance and high formability without having a multi-phase structure. One method to improve the ductility of martensitic microstructure is to increase the tempering temperature. However, this method is less effective in improving ductility and significantly degrades bending properties due to the formation of coarse carbides. JP 6017341 B (PTL 1) describes a technology for a high-strength cold-rolled steel sheet having good bendability with a yield stress of 1180 MPa or more and a tensile strength of 1470 MPa or more, in which martensite is contained in an area ratio of 95% or more, while retained austenite and ferrite are less than 5% (inclusive of 0%) in total area ratio, and furthermore, the average size of carbide is 60 nm or less in equivalent diameter and the number density of carbide with an equivalent diameter of 25 nm or more is 0 per 1 mm2. JP 2019-2078 A (PTL 2) describes a technology for an ultra-high-strength steel sheet having a high yield ratio and high formability, the steel sheet having a microstructure containing 90% or more of martensite and 0.5% or more of retained austenite, in which regions where a local Mn concentration is 1.2 times or more than a Mn content of the entire steel sheet exist in an area ratio of 1% or more, the steel sheet having a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more. CITATION LIST Patent Literature PTL 1: JP 6017341 BPTL 2: JP 2019-2078 A SUMMARY Technical Problem In recent years, it has become possible to process even steel sheets with poor ductility into complex part shapes by utilizing press working technology. One of these methods is preforming technology, which suppresses the occurrence of cracking in a steel sheet by distributing strain over the entire steel sheet by preforming a portion of the steel sheet before forming it into the final shape, rather than forming it into the final shape in a single press working. In such a process, the introduction of strain is complicated. For example, deformation may occur in such a way that after uniaxial tension, strain may be applied in the biaxial direction in the next step, or in other words, the direction of strain applied in the first and second steps may be orthogonal. Press formability in such a process does not necessarily correlate with the property values evaluated in a uniaxial tensile test, which is a common formability evaluation test. The technology described in PTL 1 may be sufficient in terms of ductility against bending deformation, which is frequently used in the forming of parts, since it provides high bendability. However, this technology is considered to be insufficient for martensite-dominant steels in terms of ductility when machined into parts with more complex shapes. In the