# US 8,465,600 B2 · High-strength steel sheet having excellent workability

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- Number: US 8,465,600 B2
- Document ID: US8465600B2
- Status: Granted
- Kind: B2
- Application number: 12/278,204
- Filed: 2007-03-16
- Published: 2009-03-05
- Granted: 2013-06-18
- Earliest claimed: 2006-03-28
- Assignees: Kobe Steel, Ltd.
- Applicants: Seiko Watanabe; Masaaki Miura
- Inventors: Seiko Watanabe; Masaaki Miura
- Web page: https://patentgrep.com/patent/US8465600B2

## CPC

- C21D 9/46: for sheet metals
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- 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

According to the present invention, there is provided a high strength steel sheet, which has, for example, a tensile strength of 590 to 980 MPa or more, which has favorable workability, and which is useful for an automobile, etc. The high strength steel sheet of the present invention comprises 0.03 to 0.20% C (% by mass in chemical compositions; hereafter, the same holds true), 0.50 to 2.5% Si, 0.50 to 2.5% Mn, and further, preferably 0.02 to 0.2% Mo. Moreover, its metal structure includes ferrite and low temperature transformation phase. The mean grain size of the low temperature transformation phase is 3.0 μm or less. Further, grains whose size is 3.0 μm or less occupy 50% or more by area ratio of the low temperature transformation phase, and an average aspect ratio of the low temperature transformation phase is 0.35 or more.

## Claims

### Claim 1

A high strength steel sheet comprising
C: 0.03 to 0.20% (% by mass in chemical compositions; hereafter, the same holds true),
Si: 0.50 to 2.5%, and
Mn: 0.50 to 2.5%,
wherein a metal structure includes ferrite and low temperature transformation phase, a mean grain size of the low temperature transformation phase is 3.0 μm or less, grains having a grain size of 3.0 μm or less occupy 50% or more by area ratio of the low temperature transformation phase, and an average aspect ratio of the low temperature transformation phase is 0.35 or more,
wherein the steel sheet has a tensile strength of 780 MPa or more,
wherein the steel sheet is a cold rolled steel sheet, and
wherein the low temperature transformation phase contains martensite, bainite and pseudo pearlite.

### Claim 2 (depends on 1)

A high strength steel sheet according to claim 1, further containing 0.02 to 0.2% Mo.

### Claim 3 (depends on 1)

A high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of
Ti: 0.01 to 0.15%,
Nb: 0.01 to 0.15%,
Cr: 0.01 to 0.5%, and
V: 0.001 to 0.15%.

### Claim 4 (depends on 1)

A high strength steel sheet according to claim 1, which contains 0.05% to 0.17% of C.

### Claim 5 (depends on 1)

A high strength steel sheet according to claim 1, which contains 0.7 to 1.8% of Si.

### Claim 6 (depends on 1)

A high strength steel sheet according to claim 1, which contains 0.50% to 2.3% of Mn.

### Claim 7 (depends on 1)

A high strength steel sheet according to claim 1, wherein the low temperature transformation phase contains martensite occupying 10% to 80% by area ratio.

### Claim 8 (depends on 1)

A high strength steel sheet according to claim 1, wherein the low temperature transformation phase contains martensite occupying 20% to 70% by area ratio.

### Claim 9 (depends on 1)

A high strength steel sheet according to claim 1, wherein the low temperature transition phase contains martensite in 90% or more by area ratio.

### Claim 10 (depends on 1)

A high strength steel sheet according to claim 1, wherein the mean grain size of the low temperature transformation phase is 2.5 μm or less.

### Claim 11 (depends on 1)

A high strength steel sheet according to claim 1, wherein grains having a grain size of 3.0 μm or less occupy 65% or more by area ratio of the low temperature transformation phase

### Claim 12 (depends on 1)

A high strength steel sheet according to claim 1, wherein the average aspect ratio of the low temperature transformation phase is 0.45 or more.

### Claim 13 (depends on 1)

A high strength steel sheet according to claim 1, wherein the average aspect ratio of the low temperature transformation phase is 0.55 or more.

### Claim 14 (depends on 1)

A high strength steel sheet according to claim 1, which has a tensile strength of 1180 MPa or more.

### Claim 15 (depends on 1)

A high strength steel sheet according to claim 1, which has a tensile strength of 980 MPa or more.