# US 7,608,156 B2 · High strength cold rolled steel sheet and method for manufacturing the same

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- Number: US 7,608,156 B2
- Document ID: US7608156B2
- Status: Granted
- Kind: B2
- Application number: 10/549,164
- Filed: 2004-11-26
- Published: 2006-08-03
- Granted: 2009-10-27
- Earliest claimed: 2003-12-05
- Assignees: Jfe Steel Corporation
- Inventors: Yoshihiko Ono; Yasunobu Nagataki; Yasushi Tanaka; Kozo Harada; Hisanori Ando
- Web page: https://patentgrep.com/patent/US7608156B2

## CPC

- C22C 38/02: containing silicon
- C21D 8/0273: {Final recrystallisation annealing}
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C21D 8/0236: {Cold rolling}
- C21D 2211/005: Ferrite
- C22C: ALLOYS (treatment of alloys C21D, C22F)
- 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)

## Abstract

This disclosure relates to a high strength cold rolled steel sheet composed of ferrite grains having an average grain diameter of 10 μm or less, in which the average number per unit area of Nb(C, N) precipitates having a diameter of 50 nm or more is 7.0×10−2/μm2 or less, and a zone having a width of 0.2 to 2.4 μm and an average area density of NbC precipitates of 60% or less of that of the central portion of the ferrite grains is formed along grain boundaries of the ferrite grains, for example, the steel sheet consisting of 0.004 to 0.02% of C, 1.5% or less of Si, 3% or less of Mn, 0.15% or less of P, 0.02% or less of S, 0.1 to 1.5% of sol.Al, 0.001 to 0.007% of N, 0.03 to 0.2% of Nb, by mass, and the balance of Fe and inevitable impurities. The steel sheet is most preferably used for automobile panel parts since it has the TS of 340 MPa or more and the superior surface strain resistance and press formability.

## Claims

### Claim 1

A high strength cold rolled steel sheet consisting of 0.004 to 0.02% of C, 1.5% or less of Si, 3% or less of Mn, 0.15% or less of P, 0.02% or less of S, 0.001 to 0.007% of N, 0.03 to 0.2% of Nb, 0.2 to 0.6% of sol.Al by mass, and the balance of Fe and inevitable impurities and composed of ferrite grains having an average grain diameter of 10 μm or less, in which the average number per unit area (hereinafter referred to as “average area density”) of Nb(C, N) precipitates having a diameter of 50 nm or more in the ferrite grains is 7.0×10−2/μm2 or less, and a zone having a width of 0.2 to 2.4 μm and an average area density of NbC precipitates of 60% or less of that of the central portion of the ferrite grains is formed along grain boundaries of the ferrite grains.

### Claim 2 (depends on 1)

The high strength cold roiled steel sheet according to claim 1, wherein the following equation (1) is satisfied;
([Nb]/[C])×(12/93)≧1 (1),
where [Nb] and [C] represent the amounts of Nb and C (mass %), respectively.

### Claim 3 (depends on 1)

The high strength cold roiled steel sheet according to claim 1, further containing 0.0001 to 0.003% of B.

### Claim 4 (depends on 2)

The high strength cold rolled steel sheet according to claim 2 further containing 0.0001 to 0.003% of B.

### Claim 5 (depends on 1)

The high strength cold rolled steel sheet according to claim 1 further containing at least one element selected from the group consisting of 0.5% or less of Cu, 0.5% or less of Ni, 0.3% or less of Mo, 0.5% or less of Cr, and 0.04% or less of Ti.

### Claim 6 (depends on 1)

The high strength cold roiled steel sheet according to claim 1 further containing at least one element selected from the group consisting of 0.5% or less of Cu, 0.5% or less of Ni, 0.3% or less of Mo, 0.5% or less of Cr, and 0.04% or less of Ti.

### Claim 7 (depends on 1)

The high strength cold rolled steel sheet according to claim 1 further containing at least one element selected front the group consisting of 0.2% or less of Sb and 0.2% or less of Sn, wherein the following equation (2) is satisfied;
0.002≦[Sb]+½×[Sn]≦0.2 (2),
where [Sb] and [Sn] represent the amounts of Sb and Sn (mass %), respectively.

### Claim 8 (depends on 6)

The high strength cold rolled steel sheet according to claim 6 further containing at least one element selected from the group consisting of 0.2% or less of Sb and 0.2% or less of Sn, wherein the following equation (2) is satisfied;
0.002≦[Sb]+½×[Sn]≦0.2 (2),
where [Sb] and [Sn] represent the amounts of Sb and Sn (mass %), respectively.

### Claim 9 (depends on 1)

A method for manufacturing a high strength cold rolled steel sheet comprising the steps of:
hot rolling a steel slab having the chemical composition according to claim 1 into a hot rolled steel sheet after heating the steel slab at a heating temperature SRT which satisfies the following equations (3) and (4); and
pickling and cold rolling the hot rolled steel sheet, followed, by annealing within a temperature range of a ferrite phase above the recrystallization temperature,
SRT≦1350° C. (3)
1050° C.≦SRT≦{770+([sol.Al]−0.085)0.24×820}° C. (4),
where [sol.Al] represents the amount of sol.Al (mass %).

### Claim 10 (depends on 1)

The high strength cold rolled steel sheet according to claim 1 having an n1-10 of 0.20 or more.

### Claim 11 (depends on 1)

The high strength cold rolled steel sheet according to claim 1 having a YS of 270 MPa or less.

### Claim 12 (depends on 1)

The high strength cold rolled steel sheet of claim 1 having a TS of 340 MPa or more.