# US 11,111,553 B2 · High-strength steel sheet and method for producing the same

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- Number: US 11,111,553 B2
- Document ID: US11111553B2
- Status: Granted
- Kind: B2
- Application number: 16/076,381
- Filed: 2017-02-07
- Published: 2019-02-07
- Granted: 2021-09-07
- Earliest claimed: 2016-02-10
- Assignees: Jfe Steel Corporation
- Applicants: Jfe Steel Corporation
- Inventors: Yusuke Kimata; Yoshihiko Ono; Kenji Kawamura
- Web page: https://patentgrep.com/patent/US11111553B2

## CPC

- C21D 8/0247: characterised by the heat treatment
- C21D 1/19: by interrupted quenching
- C21D 1/25: Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
- C21D 6/002: {containing Cr (C21D6/004 takes precedence)}
- C21D 6/005: {containing Mn}
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/0226: {Hot rolling}
- C21D 8/0236: {Cold rolling}
- C21D 9/46: for sheet metals
- C22C 38/001: {containing N}
- C22C 38/002: {containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60}
- C22C 38/005: {containing rare earths, i.e. Sc, Y, Lanthanides}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/42: with copper
- C22C 38/44: with molybdenum or tungsten
- C22C 38/46: with vanadium
- C22C 38/48: with niobium or tantalum
- C22C 38/50: with titanium or zirconium
- C22C 38/54: with boron
- C22C 38/58: with more than 1.5% by weight of manganese
- C21D 2211/001: Austenite
- C21D 2211/002: Bainite
- C21D 2211/005: Ferrite
- C21D 2211/008: Martensite
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)
- C22C: ALLOYS (treatment of alloys C21D, C22F)

## Abstract

A high-strength steel sheet having a tensile strength (TS) of 1,320 MPa or more and good workability. The high-strength steel sheet has a specific component composition and a steel microstructure containing, on an area-percentage basis with respect to the entire steel microstructure, 40% or more and less than 85% of a lower bainite, 5% or more and less than 40% martensite including tempered martensite, 10% or more and 30% or less retained austenite, and 10% or less (including 0%) polygonal ferrite, the retained austenite having an average C content of 0.60% by mass or more. Additionally, a Mn segregation value at a surface of the steel sheet is 0.8% or less, the ratio R/t of a limit bending radius (R) to a thickness (t) of the steel sheet is 2.0 or less, and tensile strength×total elongation of the steel sheet is 15,000 MPa % or more.

## Claims

### Claim 1

A high-strength steel sheet comprising:
a component composition including:
C: 0.15% to 0.40%, by mass %,
Si: 0.5% to 2.5%, by mass %,
Mn: 0.5% to 2.4%, by mass %,
P: 0.1% or lower, by mass %,
S: 0.01% or lower, by mass %,
Al: 0.01% to 0.5%, by mass %,
N: 0.010% or lower, by mass %, and
Fe and incidental impurities; and
a steel microstructure containing, on an area-percentage basis with respect to the entire steel microstructure, 40% or more and less than 85% of a lower bainite, 5% or more and less than 40% martensite including tempered martensite, 10% or more and 30% or less retained austenite, and 10% or less (including 0%) polygonal ferrite, the retained austenite having an average C content of 0.60% by mass or more,
wherein:
a Mn segregation value at a surface of the steel sheet is 0.8% or less, the Mn segregation value being defined as a difference between maximum and minimum values of a Mn concentration at the surface as measured by line analysis in a 1-mm-long portion of the surface perpendicular to a rolling direction using electron probe microanalysis (EPMA),
a tensile strength of the steel sheet is 1,320 MPa or more,
a ratio R/t of a limit bending radius (R) to a thickness (t) of the steel sheet is 2.0 or less,
tensile strength×total elongation of the steel sheet is 15,000 MPa·% or more, and
tensile strength×hole expansion ratio of the steel sheet is 50,000 MPa·% or more.

### Claim 2 (depends on 1)

The high-strength steel sheet according to claim 1, wherein the component composition further comprises one or more selected from the following groups A to D:
Group A:
one or more selected from:
Cr: 0.005% to 1.0%, by mass %,
V: 0.005% to 1.0%, by mass %,
Ni: 0.005% to 1.0%, by mass %,
Mo: 0.005% to 1.0%, by mass %, and
Cu: 0.01% to 2.0%, by mass %,
Group B:
one or more selected from:
Ti: 0.005% to 0.1%, by mass %, and
Nb: 0.005% to 0.1%, by mass %,
Group C:
B: 0.0003% to 0.0050%, by mass %, and
Group D:
one or more selected from:
Ca: 0.001% to 0.005%, by mass %, and
REM: 0.001% to 0.005%, by mass %.

### Claim 3 (depends on 1)

A method for producing the high-strength steel sheet according to claim 1, the method comprising:
subjecting a steel slab to hot rolling at a reduction ratio of a first pass in rough rolling of 10% or more and then cold rolling to form a cold-rolled steel sheet,
annealing the cold-rolled steel sheet in a single-phase austenite region for 200 seconds or more and 1,000 seconds or less,
cooling the steel sheet from an annealing temperature to Ac3—100° C. at an average cooling rate of 5° C./s or more, and cooling the steel sheet from Ac3—100° C. to a first temperature range of a martensitic transformation start temperature (Ms)—100° C. or higher and lower than Ms at an average cooling rate of 20° C./s or more,
after the cooling, increasing the temperature of the steel sheet to a second temperature range of 300° C. or higher, a bainitic transformation start temperature (Bs)—150° C. or lower, and 450° C. or lower, and
after the temperature increase, retaining the steel sheet in the second temperature range for 15 seconds or more and 1,000 seconds or less.

### Claim 4 (depends on 2)

A method for producing the high-strength steel sheet according to claim 2, the method comprising:
subjecting a steel slab to hot rolling at a reduction ratio of a first pass in rough rolling of 10% or more and then cold rolling to form a cold-rolled steel sheet,
annealing the cold-rolled steel sheet in a single-phase austenite region for 200 seconds or more and 1,000 seconds or less,
cooling the steel sheet from an annealing temperature to Ac3—100° C. at an average cooling rate of 5° C./s or more, and cooling the steel sheet from Ac3—100° C. to a first temperature range of a martensitic transformation start temperature (Ms)—100° C. or higher and lower than Ms at an average cooling rate of 20° C./s or more,
after the cooling, increasing the temperature of the steel sheet to a second temperature range of 300° C. or higher, a bainitic transformation start temperature (Bs)—150° C. or lower, and 450° C. or lower, and
after the temperature increase, retaining the steel sheet in the second temperature range for 15 seconds or more and 1,000 seconds or less.

### Claim 5 (depends on 1)

The high-strength steel sheet according to claim 1, wherein the Mn segregation value is in the range of from 0.34% to 0.8%.

### Claim 6 (depends on 2)

The high-strength steel sheet according to claim 2, wherein the Mn segregation value is in the range of from 0.34% to 0.8%.

### Claim 7 (depends on 3)

The method according to claim 3, wherein the Mn segregation value is in the range of from 0.34% to 0.8%.

### Claim 8 (depends on 4)

The method according to claim 4, wherein the Mn segregation value is in the range of from 0.34% to 0.8%.