A COLD-ROLLED STEEL SHEET HAVING A TENSILE STRENGTH OF 780 MPA OR MORE, AN EXCELLENT LOCAL FORMABILITY AND A SUPPRESSED INCREASE IN WELD HARDNESS

• US 20070071997A1
• Filed: 01/09/2004
• Published: 03/29/2007
• Est. Priority Date: 05/21/2003
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation
• Inventors: Taniguchi, Hirokazu, Goto, Koichi, Okamoto, Riki
• Granted Patent: US 7,780,799 B2
• Time in Patent Office: Days
• US Class Current: 428/659
• CPC Class Codes: C21D 2211/001 Austenite
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 8/02 during manufacturing of plates or strips C21D8/12 takes precedence
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/14 containing titanium or zirconium
• CPC Class Codes: Y10T 428/12799 Next to Fe-base component [e.g., galvanized]
• CPC Class Codes: Y10T 428/12965 Both containing 0.01-1.7% carbon [i.e., steel]

Abstract

The present invention provides a high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase and being characterized by: said steel sheets containing, in weight, C: 0.05 to 0.09%, Si: 0.4 to 1.3%, Mn: 2.5 to 3.2%, P: 0.001 to 0.05%, N: 0.0005 to 0.006%, Al: 0.005 to 0.1%, Ti: 0.001 to 0.045%, and S in the range stipulated by the following expression (A), with the balance consisting of Fe and unavoidable impurities; the microstructures of said steel sheets being composed of bainite of 7% or more in terms of area percentage and the balance consisting of one or more of ferrite, martensite, tempered martensite and retained austenite; and said components in said steel sheets satisfying the following expressions (C) and (D) when Mneq. is defined by the following expression (B); S≦0.08×(Ti(%)−3.43×N(%)+0.004 . . . (A), where, when a value of the member Ti(%)−3.43×N(%) of said expression (A) is negative, the value is regarded as zero. MIneq.=Mn(%)−0.29 ×Si(%)+6.24×C(%) . . . (B), 950≦(Mneq./(C(%)−(Si(%)/75)))×bainite area percentage (%) . . . (C), C(%)+(Si(%)/20)+(Mn(%)/18)⁵0.30 . . . (D).

Claims

1. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase, characterized by:

said steel sheets containing, in weight,

C;

0.05 to 0.09%,

Si;

0.4 to 1.3%,

Mn;

2.5 to 3.2%,

P;

0.001 to 0.05%,

N;

0.0005 to 0.006%,

Al;

0.005 to 0.1%,

Ti;

0.001 to 0.045%, and

S in the range stipulated by the following expression (A), with the balance consisting of Fe and unavoidable impurities;

the microstructures of said steel sheets being composed of bainite of 7% or more in terms of area percentage and the balance consisting of one or more of ferrite, martensite, tempered martensite and retained austenite; and

said components in said steel sheets satisfying the following expressions (C) and (D) when Mneq. is defined by the following expression (B);

S≦

0.08×

(Ti(%)−

3.43×

N(%))+0.004  

 

(A),

where, when a value of the member Ti(%)−

3.43×

N(%) of said expression (A) is negative, the value is regarded as zero,

Mneq.=Mn(%)−

0.29×

Si(%)+6.24×

C(%)  

 

(B),
950≦

(Mneq./(C(%)−

(Si(%)/75)))×

bainite area percentage (%)  

 

(C),
C(%)+(Si(%)/20)+(Mn(%)/18)≦

0.30  

 

(D).

2. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase according to claim 1, characterized by said steel sheets containing, as additional chemical components, one or more of

Nb:

0.001 to 0.04%,

B;

0.0002 to 0.0015%, and

Mo;

0.05 to 0.50%.

3. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase according to claim 1, characterized by said steel sheets containing 0.0003 to 0.01% Ca as a further additional chemical component.

4. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase according to claim 1, characterized by said steel sheets containing 0.0002 to 0.01% Mg as a further additional chemical component.

5. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase according to claim 1, characterized by said steel sheets containing 0.0002 to 0.01% REM as further additional chemical components.

6. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase according to claim 1, characterized by said steel sheets containing 0.2 to 2.0% Cu and 0.05 to 2.0% Ni as further additional chemical components.

7. A high-strength cold-rolled steel sheet and a high-strength surface treated steel sheet 780 MPa or more in tensile strength, said steel sheets having excellent local formability and suppressed weld hardness increase according to claim 1, characterized by said surface treated steel sheet being coated with zinc, or an alloy thereof, as the surface treatment.