# US 2021/0340641 A1 · High-strength steel sheet and method for producing same

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- Number: US 2021/0340641 A1
- Document ID: US20210340641A1
- Status: Published application
- Kind: A1
- Application number: 17/271,663
- Filed: 2019-08-22
- Published: 2021-11-04
- Earliest claimed: 2018-08-31
- Assignees: Jfe Steel Corporation
- Applicants: JFE Steel Corporation
- Inventors: Yoshihiko Ono; Yuma Honda; Shimpei Yoshioka; Koichi Taniguchi; Nobuyuki Nakamura; Takeshi Murai
- Web page: https://patentgrep.com/patent/US20210340641A1

## CPC

- C21D 9/46: for sheet metals
- B22D 11/16: Controlling or regulating processes or operations
- B22D 11/182: {by measuring temperature}
- C21D 6/005: {containing Mn}
- C21D 6/008: {containing Si}
- C21D 6/02: Hardening by precipitation
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/021: involving particular fabrication steps or treatments of ingots or slabs
- C21D 8/0226: {Hot rolling}
- C21D 8/0236: {Cold rolling}
- C21D 8/0247: characterised by the heat treatment
- C21D 8/0263: {following hot rolling}
- C21D 8/0273: {Final recrystallisation annealing}
- 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 38/008: {containing tin}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/08: containing nickel {(C22C38/105 takes precedence)}
- C22C 38/12: containing tungsten, tantalum, molybdenum, vanadium, or niobium
- C22C 38/14: containing titanium or zirconium
- C22C 38/16: containing copper
- C22C 38/42: with copper
- 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
- C22C 38/60: containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
- C23C 2/06: Zinc or cadmium or alloys based thereon
- C23C 2/40: Plates; Strips
- C21D 2211/002: Bainite
- C21D 2211/004: Dispersions; Precipitations
- 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)
- B22D: CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES (shaping of plastics or substances in a plastic state B29C; metallurgical processing, selection of substances to be added to metal C21, C22)
- C22C: ALLOYS (treatment of alloys C21D, C22F)
- C23C: COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL (making metal-coated products by extrusion B21C23/22; covering with metal by connecting pre-existing layers to articles, see the relevant places, e.g. B21D39/00, B23K; metallising of glass C03C; metallising mortars, concrete, artificial stone, ceramics or natural stone C04B41/00; enamelling of, or applying a vitreous layer to, metals C23D; treating metal surfaces or coating of metals by electrolysis or electrophoresis C25D; single-crystal film growth C30B; by metallising textiles D06M11/83; decorating textiles by locally metallising D06Q1/04)

## Abstract

A high-strength steel sheet of the present invention has a specific chemical composition. Furthermore, in the steel sheet, a degree of Mn segregation in a specific region is 1.5 or less; a maximum P concentration in a specific region is 0.08 mass % or less; in a specific region, the number of specific MnS particle groups is 2.0 or fewer per 1 mm2, and the number of specific oxide-based inclusions is 8 or fewer per 1 mm2; of all of the oxide-based inclusions, oxide-based inclusions having a specified composition are present in a number ratio of 80% or greater; the microstructure includes, in terms of a volume fraction, 30 to 95% martensite and bainite in total, 5 to 70% ferrite phase, and less than 3% (and 0% or greater) austenite phase; and a tensile strength is 980 MPa or greater.

## Claims

### Claim at position 1

1-10. (canceled)

### Claim 11

A high-strength steel sheet, the high-strength steel sheet having a chemical composition containing, in mass %,
C: 0.10 to 0.35%,
Si: 0.01 to 2.0%,
Mn: 2.2 to 3.5%,
P: 0.015% or less (and greater than 0%),
S: 0.0015% or less (and greater than 0%),
Sol. Al: 0.01 to 1.0%,
N: 0.0055% or less (and greater than 0%),
O: 0.0025% or less (and greater than 0%), and
Ca: 0.0005% or less (and 0% or greater), with a balance of Fe and incidental impurities, wherein
in a region within 100 μm of a surface of the high-strength steel sheet in a sheet thickness direction, a degree of Mn segregation is 1.5 or less,
in a region within 100 μm of the surface in the sheet thickness direction, a maximum P concentration is 0.08 mass % or less,
in a region within 100 μm of the surface in the sheet thickness direction, at least one MnS particle group formed of one or more MnS particles having a major axis of 0.3 μm or greater is present, the one or more MnS particles being elongated and/or distributed in a form of a sequence of dots in a rolling direction of the steel sheet, a distance between adjacent MnS particles being 40 μm or less in a case where the at least one MnS particle group is formed of two or more MnS particles, and a number of MnS particle groups having a longitudinal dimension of 150 μm or greater is 2.0 or fewer per 1 mm2, as viewed in a cross section in a sheet thickness direction and parallel to the rolling direction,
in a region within 100 μm of the surface in the sheet thickness direction, a number of oxide-based inclusions having a particle diameter of 5 μm or greater is 8 or fewer per 1 mm2 as viewed in a plane parallel to a sheet surface,
of all the number of the oxide-based inclusions having a particle diameter of 5 μm or greater, oxide-based inclusions having a composition in which an alumina content is 50 mass % or greater, a silica content is 20 mass % or less, and a calcia content is 40 mass % or less are present in the number ratio of 80% or greater,
the high-strength steel sheet has a microstructure that includes, in terms of a volume fraction, 30 to 95% martensite and bainite in total, 5 to 70% ferrite phase, and less than 3% (and 0% or greater) austenite phase, and
the high-strength steel sheet has a tensile strength of 980 MPa or greater.

### Claim 12 (depends on 11)

The high-strength steel sheet according to claim 11, wherein the chemical composition further contains, in mass %, one or two or more selected from the following groups A to E:
Group A:
one or two or more selected from:
Ti: 0.003 to 0.05%,
Nb: 0.003 to 0.05%,
V: 0.001 to 0.1%, and
Zr: 0.001 to 0.1%.
Group B:
one or two or more selected from:
Cr: 0.01 to 1.0%,
Mo: 0.01 to 0.20%, and
B: 0.0001 to 0.0030%.
Group C:
one or two or more selected from:
Cu: 0.01 to 0.5%,
Ni: 0.01 to 0.5%, and
Sn: 0.001 to 0.1%.
Group D:
Sb: 0.001 to 0.1%.
Group E:
REMs and Mg in a total amount of 0.0002% or greater and 0.01% or less.

### Claim 13 (depends on 11)

The high-strength steel sheet according to claim 11, further comprising a galvanized layer on the surface.

### Claim 14 (depends on 12)

The high-strength steel sheet according to claim 12, further comprising a galvanized layer on the surface.

### Claim 15 (depends on 11)

A method for producing a high-strength steel sheet, the high-strength steel sheet being the high-strength steel sheet according to claim 11, the method comprising:
a casting step in which, after completion of refining, which is carried out in an RH vacuum degasser with a circulation time of 500 seconds or more, continuous casting is performed in a manner such that a difference between a casting temperature and a solidification temperature is 10° C. or greater and 35° C. or less, a flow rate of molten steel at a solidification interface near a mold meniscus is 0.5 to 1.5 m/min, and the steel is passed through a bending section and a straightening section at a temperature of 550° C. or higher and 1050° C. or lower;
a hot rolling step in which a steel starting material obtained in the casting step is heated directly after the casting step or after cooling, to a temperature of 1220° C. or higher and 1300° C. or lower and held for 80 minutes or more; and an amount of reduction for a first pass of rough rolling is 10% or greater, and an amount of reduction for a first pass of finish rolling is 20% or greater;
a cold rolling step in which, after a hot-rolled steel sheet obtained in the hot rolling step is pickled, the hot-rolled steel sheet is subjected to cold rolling; and
an annealing step in which a cold-rolled steel sheet obtained in the cold rolling step is annealed.

### Claim 16 (depends on 12)

A method for producing a high-strength steel sheet, the high-strength steel sheet being the high-strength steel sheet according to claim 12, the method comprising:
a casting step in which, after completion of refining, which is carried out in an RH vacuum degasser with a circulation time of 500 seconds or more, continuous casting is performed in a manner such that a difference between a casting temperature and a solidification temperature is 10° C. or greater and 35° C. or less, a flow rate of molten steel at a solidification interface near a mold meniscus is 0.5 to 1.5 m/min, and the steel is passed through a bending section and a straightening section at a temperature of 550° C. or higher and 1050° C. or lower;
a hot rolling step in which a steel starting material obtained in the casting step is heated directly after the casting step or after cooling, to a temperature of 1220° C. or higher and 1300° C. or lower and held for 80 minutes or more; and an amount of reduction for a first pass of rough rolling is 10% or greater, and an amount of reduction for a first pass of finish rolling is 20% or greater;
a cold rolling step in which, after a hot-rolled steel sheet obtained in the hot rolling step is pickled, the hot-rolled steel sheet is subjected to cold rolling; and
an annealing step in which a cold-rolled steel sheet obtained in the cold rolling step is annealed.

### Claim 17 (depends on 15)

The method for producing a high-strength steel sheet according to claim 15, wherein the annealing step is a step performed in a manner such that the cold-rolled steel sheet obtained in the cold rolling step is heated to a temperature range of 780 to 900° C.; thereafter, the steel sheet is soaked in the temperature range for 20 seconds or more; then, primary cooling, which is associated with a range from the soaking temperature to 350° C., is performed to cool the steel sheet to 350° C. or lower at an average rate of 3° C./sec or greater and less than 100° C./sec; then, the steel sheet is held under the conditions including a retention time for a temperature range of 450 to 130° C. of 10 to 1000 seconds; and further, secondary cooling is performed to cool the steel sheet over a temperature range of 130 to 50° C. at an average rate of 10° C./sec or greater.

### Claim 18 (depends on 16)

The method for producing a high-strength steel sheet according to claim 16, wherein the annealing step is a step performed in a manner such that the cold-rolled steel sheet obtained in the cold rolling step is heated to a temperature range of 780 to 900° C.; thereafter, the steel sheet is soaked in the temperature range for 20 seconds or more; then, primary cooling, which is associated with a range from the soaking temperature to 350° C., is performed to cool the steel sheet to 350° C. or lower at an average rate of 3° C./sec or greater and less than 100° C./sec; then, the steel sheet is held under the conditions including a retention time for a temperature range of 450 to 130° C. of 10 to 1000 seconds; and further, secondary cooling is performed to cool the steel sheet over a temperature range of 130 to 50° C. at an average rate of 10° C./sec or greater.

### Claim 19 (depends on 15)

The method for producing a high-strength steel sheet according to claim 15, further comprising a galvanizing step in which galvanizing is performed on the steel sheet resulting from the annealing step.

### Claim 20 (depends on 16)

The method for producing a high-strength steel sheet according to claim 16, further comprising a galvanizing step in which galvanizing is performed on the steel sheet resulting from the annealing step.

### Claim 21 (depends on 17)

The method for producing a high-strength steel sheet according to claim 17, further comprising a galvanizing step in which galvanizing is performed on the steel sheet resulting from the annealing step.

### Claim 22 (depends on 18)

The method for producing a high-strength steel sheet according to claim 18, further comprising a galvanizing step in which galvanizing is performed on the steel sheet resulting from the annealing step.