# US 11,078,552 B2 · High-strength steel sheet and method for manufacturing the same

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- Number: US 11,078,552 B2
- Document ID: US11078552B2
- Status: Granted
- Kind: B2
- Application number: 16/082,158
- Filed: 2017-01-30
- Published: 2019-03-28
- Granted: 2021-08-03
- Earliest claimed: 2016-03-07
- Assignees: Jfe Steel Corporation
- Applicants: Jfe Steel Corporation
- Inventors: Fusae Shiimori; Shinjiro Kaneko; Yasunobu Nagataki
- Web page: https://patentgrep.com/patent/US11078552B2

## CPC

- C21D 8/0247: characterised by the heat treatment
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 9/46: for sheet metals
- C22C 38/00: Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00)
- C22C 38/001: {containing N}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/38: with more than 1.5% by weight of manganese
- C23C 2/02: Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence)
- C23C 2/0224: {Two or more thermal pretreatments}
- C23C 2/024: {by cleaning or etching}
- C23C 2/06: Zinc or cadmium or alloys based thereon
- C23C 2/12: Aluminium or alloys based thereon
- C23C 2/28: Thermal after-treatment, e.g. treatment in oil bath
- C21D 2211/001: Austenite
- C21D 2211/002: Bainite
- C21D 2211/005: Ferrite
- C21D 2211/008: Martensite
- 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 38/08: 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 38/22: with molybdenum or tungsten
- 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)
- 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 exhibiting excellent ductility and stretch-flangeability, and a method for manufacturing such a high-strength steel sheet. The high-strength steel sheet has a chemical composition including specific proportions of components in which C/Mn is 0.08 to 0.20, the balance being iron and inevitable impurities, and includes microstructures including, in terms of area fraction relative to all the microstructures, 40% to 70% total of ferrite and bainitic ferrite, 5% to 35% martensite and 5% to 30% retained austenite. The proportion of martensite (including retained austenite) adjacent to bainitic ferrite is not less than 60% of all martensite (including retained austenite). The proportion of 4.0 GPa and smaller differences in microhardness measured at 0.5 μm intervals is not less than 70%. The proportion of microstructures with 8.0 GPa or smaller microhardness is not less than 85% of all the microstructures.

## Claims

### Claim 1

A high-strength steel sheet having a chemical composition comprising:
C: 0.10% to 0.35%, by mass %,
Si: 0.5% to 2.0%, by mass %,
Mn: 1.5% to 3.0%, by mass %,
P: not more than 0.050%, by mass %,
S: not more than 0.0100%, by mass %,
Al: 0.001% to 1.00%, by mass %,
N: 0.0005% to 0.0200%, by mass %, and
iron and inevitable impurities,
wherein:
a ratio of C/Mn is from 0.08 to 0.20,
the steel sheet has microstructures including, in terms of area fraction relative to all the microstructures:
less than 10% ferrite,
40% to 70% total of ferrite and bainitic ferrite,
5% to 35% martensite, and
5% to 30% retained austenite,
a proportion of martensite including retained austenite adjacent to bainitic ferrite being not less than 60% of all martensite including retained austenite,
a proportion of 4.0 GPa and smaller differences in microhardness measured at 0.5 μm intervals being not less than 70% of all indentations, and
a proportion of microstructures with 8.0 GPa or smaller microhardness being not less than 85% of all the microstructures.

### Claim 2 (depends on 1)

The high-strength steel sheet according to claim 1, wherein the chemical composition further comprises at least one element selected from at least one of groups A to C:
Group A:
one or more selected from:
Ti: 0.005% to 0.100%, by mass %,
Nb: 0.005% to 0.100%, by mass %, and
V: 0.005% to 0.100%, by mass %,
Group B:
one or more selected from:
Cr: 0.05% to 1.0%, by mass %,
Ni: 0.05% to 0.50%, by mass %,
Mo: 0.05% to 1.0%, by mass %,
Cu: 0.005% to 0.500%, by mass %, and
B: 0.0001% to 0.0100%, by mass %, and
Group C:
either one or both of:
Ca: 0.0001% to 0.0050%, by mass %, and
REM: 0.0005% to 0.0050%, by mass %.

### Claim 3

A method for manufacturing a high-strength steel sheet, the method comprising:
providing a steel sheet having a chemical composition comprising:
C: 0.10% to 0.35%, by mass %,
Si: 0.5% to 2.0%, by mass %,
Mn: 1.5% to 3.0%, by mass %,
P: not more than 0.050%, by mass %,
S: not more than 0.0100%, by mass %,
Al: 0.001% to 1.00%, by mass %,
N: 0.0005% to 0.0200%, by mass %, and
iron and inevitable impurities,
wherein:
a ratio of C/Mn is from 0.08 to 0.20, and
the steel sheet includes microstructures in which a total of bainite and martensite, both having a grain size of 1 μm to 25 μm and a block interval of not more than 3 μm, represents not less than 80% of all the microstructures,
heating the steel sheet to 700° C. at an average heating rate of not less than 15° C./sec,
holding the steel sheet at a temperature in the range of 740° C. to 860° C. for 60 seconds to 600 seconds,
cooling the steel sheet to a temperature in the range of 350° C. to 550° C. at an average cooling rate of not more than 50° C./sec, and
after cooling the steel sheet, subsequently holding the steel sheet at a temperature in the range of 350° C. to 550° C. for 30 seconds to 1200 seconds,
where the high-strength steel sheet has microstructures including less than 10% ferrite in terms of area fraction relative to all the microstructures.

### Claim 4 (depends on 3)

The method for manufacturing a high-strength steel sheet according to claim 3, wherein the chemical composition further comprises at least one element selected from at least one of groups A to C:
Group A:
one or more selected from:
Ti: 0.005% to 0.100%, by mass %,
Nb: 0.005% to 0.100%, by mass %, and
V: 0.005% to 0.100%, by mass %,
Group B:
one or more selected from:
Cr: 0.05% to 1.0%, by mass %,
Ni: 0.05% to 0.50%, by mass %,
Mo: 0.05% to 1.0%, by mass %,
Cu: 0.005% to 0.500%, by mass %, and
B: 0.0001% to 0.0100%, by mass %, and
Group C:
either one or both of:
Ca: 0.0001% to 0.0050%, by mass %, and
REM: 0.0005% to 0.0050%, by mass %.

### Claim 5 (depends on 3)

The method for manufacturing a high-strength steel sheet according to claim 3, further comprising performing a coating treatment.

### Claim 6 (depends on 4)

The method for manufacturing a high-strength steel sheet according to claim 4, further comprising performing a coating treatment.

### Claim 7 (depends on 5)

The method for manufacturing a high-strength steel sheet according to claim 5, wherein the coating treatment is a hot dip coating or an electrocoating.

### Claim 8 (depends on 6)

The method for manufacturing a high-strength steel sheet according to claim 6, wherein the coating treatment is a hot dip coating or an electrocoating.

### Claim 9 (depends on 5)

The method for manufacturing a high-strength steel sheet according to claim 5, further comprising performing an alloying treatment at an alloying temperature of 450 to 600° C. after the coating treatment.

### Claim 10 (depends on 6)

The method for manufacturing a high-strength steel sheet according to claim 6, further comprising performing an alloying treatment at an alloying temperature of 450 to 600° C. after the coating treatment.

### Claim 11 (depends on 7)

The method for manufacturing a high-strength steel sheet according to claim 7, further comprising performing an alloying treatment at an alloying temperature of 450 to 600° C. after the coating treatment.

### Claim 12 (depends on 8)

The method for manufacturing a high-strength steel sheet according to claim 8, further comprising performing an alloying treatment at an alloying temperature of 450 to 600° C. after the coating treatment.

### Claim 13 (depends on 1)

The high-strength steel sheet according to claim 1, wherein the steel sheet has 20% to 35% martensite.

### Claim 14 (depends on 1)

The high-strength steel sheet according to claim 1, wherein the steel sheet has not more than 8% ferrite.

### Claim 15 (depends on 1)

The high-strength steel sheet according to claim 1, wherein the steel sheet has 2% to 8% ferrite.