# US 12,110,568 B2 · Steel sheet and member, and methods for manufacturing same

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- Number: US 12,110,568 B2
- Document ID: US12110568B2
- Status: Granted
- Kind: B2
- Application number: 17/434,255
- Filed: 2020-02-28
- Published: 2022-05-19
- Granted: 2024-10-08
- Earliest claimed: 2019-02-28
- Assignees: Jfe Steel Corporation
- Applicants: Jfe Steel Corporation
- Inventors: Yoichiro Matsui; Yuka Miyamoto; Takeshi Yokota
- Web page: https://patentgrep.com/patent/US12110568B2

## CPC

- C21D 9/46: for sheet metals
- C21D 1/32: Soft annealing, e.g. spheroidising
- C21D 6/004: {containing Cr and Ni}
- C21D 6/005: {containing Mn}
- C21D 6/008: {containing Si}
- C21D 8/02: during manufacturing of plates or strips (C21D8/12 takes precedence)
- C21D 8/0226: {Hot rolling}
- C21D 8/0263: {following hot rolling}
- 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 38/008: {containing tin}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/18: containing chromium
- C22C 38/22: with molybdenum or tungsten
- C22C 38/24: with vanadium
- C22C 38/26: with niobium or tantalum
- C22C 38/28: with titanium or zirconium
- C22C 38/32: with boron
- C22C 38/40: with nickel
- C22C 38/54: with boron
- C22C 38/60: containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
- C21D 2211/004: Dispersions; Precipitations
- C21D 2211/005: Ferrite
- 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 steel sheet and a member excellent in cold workability, hardenability, and post-quenching surface layer hardness, and methods for manufacturing the steel sheet and the member. The steel sheet has a specified chemical composition and a microstructure containing ferrite and carbides, a ratio of the volume of ferrite and carbides to the volume of the entire microstructure is 90% or more, a ratio of the volume of proeutectoid ferrite to the volume of the entire microstructure is 20% or more and 80% or less, a Mn concentration in the carbides is 0.10 mass % or more and 0.50 mass % or less, and a ratio of the number of carbides with particle diameters of 1 μm or more to the total number of carbides is 30% or more and 60% or less.

## Claims

### Claim 1

A steel sheet having a chemical composition comprising, by mass %:
C: 0.10% or more and 0.33% or less;
Si: 0.01% or more and 0.50% or less;
Mn: 0.40% or more and 1.25% or less;
P: 0.03% or less;
S: 0.01% or less;
sol. Al: 0.10% or less;
N: 0.01% or less;
Cr: 0.50% or more and 1.50% or less; and
a balance being Fe and incidental impurities,
wherein the steel sheet has a microstructure containing ferrite and carbides, a ratio of a volume of the ferrite and the carbides to a volume of an entire microstructure is 90% or more, and a ratio of a volume of proeutectoid ferrite to the volume of the entire microstructure is in a range of 20% or more and 80% or less,
a Mn concentration in the carbides is in a range of 0.10 mass % or more and 0.50 mass % or less, and
a ratio of a number of carbides with particle diameters of 1 μm or more to a total number of carbides is in a range of 30% or more and 60% or less.

### Claim 2 (depends on 1)

The steel sheet according to claim 1, wherein the chemical composition further comprises, by mass %, at least one Group selected from the groups consisting of:
Group A: B: 0% or more and 0.01% or less,
Group B: at least one selected from the group consisting of Sb, Sn, Bi, Ge, Te, and Se: 0.002% or more and 0.03% or less in total,
Group C: at least one of Ni and Mo: 0.01% or more and 0.5% or less in total, and
Group D: at least one selected from the group consisting of Nb, Ti, and V: 0.001% or more and 0.05% or less in total.

### Claim 3 (depends on 1)

A method for manufacturing a steel sheet according to claim 1, the method comprising:
performing rough hot rolling on a steel raw material having the chemical composition, subsequently performing finish rolling at a finishing temperature of 920° C. or less, and performing cooling such that an average cooling rate from the finishing temperature to 700° C. is 50° C./s or less,
subsequently performing coiling at a coiling temperature in a range of 550° C. or more and 700° C. or less and causing a ratio of a volume of proeutectoid ferrite grains with grain diameters of 3 μm or more to a volume of an entire microstructure to be in a range of 20% or more and 80% or less, and
subsequently performing annealing at an annealing temperature in a range of 700° C. or more and less than an Ac1 transformation temperature.

### Claim 4 (depends on 1)

A method for manufacturing a steel sheet according to claim 1, the method comprising:
performing rough hot rolling on a steel raw material having the chemical composition, subsequently performing finish rolling at a finishing temperature of 920° C. or less, and performing cooling such that an average cooling rate from the finishing temperature to 700° C. is 50° C./s or less,
subsequently performing coiling at a coiling temperature in a range of 550° C. or more and 700° C. or less and causing a ratio of a volume of proeutectoid ferrite grains with grain diameters of 3 μm or more to a volume of an entire microstructure to be in a range of 20% or more and 80% or less, and
subsequently performing annealing by performing heating to a temperature in a range of an Ac1 transformation temperature or more and 800° C. or less, performing holding for 0.5 hours or more, subsequently performing cooling to less than an Ar1 transformation temperature, and performing holding at a temperature in a range of 700° C. or more and less than the Ar1 transformation temperature for 20 hours or more.

### Claim 5 (depends on 1)

A member obtained by performing at least one of forming and heat treatment on the steel sheet according to claim 1.

### Claim 6 (depends on 3)

A method for manufacturing a member, the method comprising a step of performing at least one of forming and heat treatment on a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 3.

### Claim 7 (depends on 2)

A method for manufacturing a steel sheet according to claim 2, the method comprising:
performing rough hot rolling on a steel raw material having the chemical composition, subsequently performing finish rolling at a finishing temperature of 920° C. or less, and performing cooling such that an average cooling rate from the finishing temperature to 700° C. is 50° C./s or less,
subsequently performing coiling at a coiling temperature in a range of 550° C. or more and 700° C. or less and causing a ratio of a volume of proeutectoid ferrite grains with grain diameters of 3 μm or more to a volume of an entire microstructure to be in a range of 20% or more and 80% or less, and
subsequently performing annealing at an annealing temperature in a range of 700° C. or more and less than an Ac1 transformation temperature.

### Claim 8 (depends on 2)

A method for manufacturing a steel sheet according to claim 2, the method comprising:
performing rough hot rolling on a steel raw material having the chemical composition, subsequently performing finish rolling at a finishing temperature of 920° C. or less, and performing cooling such that an average cooling rate from the finishing temperature to 700° C. is 50° C./s or less,
subsequently performing coiling at a coiling temperature in a range of 550° C. or more and 700° C. or less and causing a ratio of a volume of proeutectoid ferrite grains with grain diameters of 3 μm or more to a volume of an entire microstructure to be in a range of 20% or more and 80% or less, and
subsequently performing annealing by performing heating to a temperature in a range of an Ac1 transformation temperature or more and 800° C. or less, performing holding for 0.5 hours or more, subsequently performing cooling to less than an Ar1 transformation temperature, and performing holding at a temperature in a range of 700° C. or more and less than the Ar1 transformation temperature for 20 hours or more.

### Claim 9 (depends on 2)

A member obtained by performing at least one of forming and heat treatment on the steel sheet according to claim 2.

### Claim 10 (depends on 4)

A method for manufacturing a member, the method comprising a step of performing at least one of forming and heat treatment on a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 4.

### Claim 11 (depends on 7)

A method for manufacturing a member, the method comprising a step of performing at least one of forming and heat treatment on a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 7.

### Claim 12 (depends on 8)

A method for manufacturing a member, the method comprising a step of performing at least one of forming and heat treatment on a steel sheet manufactured by the method for manufacturing a steel sheet according to claim 8.