Process for manufacturing a medium-carbon steel plate with improved formability and weldability

• US 5,454,887 A
• Filed: 07/23/1993
• Issued: 10/03/1995
• Est. Priority Date: 09/29/1992
• Status: Expired due to Fees

• Current Assignee: Sumitomo Metal Industries Limited (Nippon Steel Corporation)
• Original Assignee: Sumitomo Metal Industries Limited (Nippon Steel Corporation)
• Inventors: Fukui, Kiyoshi
• Primary Examiner(s): Silverman, Stanley S.
• Assistant Examiner(s): Ip, Sikyin
• Application Number:
  US08/095,340
• Time in Patent Office: 802 Days
• Field of Search: 148/601, 148/603
• US Class Current: 148/603
• CPC Class Codes: C22C 38/001 containing N

Abstract

A process for manufacturing a medium-carbon steel plate having a graphitization of 50% or more with improved formability and weldability is disclosed, the process comprises the steps of:

hot rolling a steel with a finishing temperature of 700°-900° C., the alloy composition of the steel consisting essentially of, by weight %:

C: 0.20-0.70%,

Si: more than 0.20 but not more than 2.00%,

Mn: 0.05-0.50%, P: not more than 0.020%,

S: not more than 0.010%, sol. Al: 0.01-1.00%,

B: 0.0003-0.0050%, N: 0.002-0.010%,

B/N: 0.2-0.8,

Cu: 0-1.00%, Ni: 0-2.00%, Ca: 0-0.010%, and

Fe and incidental impurities: balance,

cooling the resulting hot-rolled steel plate at a cooling rate of 5°-50° C./s,

coiling the steel plate at a temperature of 400°-650° C., and

optionally, cold rolling the hot-rolled steel plate with a reduction in thickness of 20-85%, and

annealing the cold-rolled steel plate at a temperature of 600°- Ac₁ for 1 hour or longer.

Claims

1. A process for manufacturing a medium-carbon steel plate having a graphitization of 50% or more with improved formability and weldability, which comprises the steps of:

hot rolling a steel with a finishing temperature of 700°

-900°

C., the steel consisting essentially of, by weight %;

C;

0.20-0.70%,

Si;

more than 0.20 but not more than 2.00%,

Mn;

0.05-0.50%, P;

not more than 0.020%,

S;

not more than 0.010%, sol. Al;

0.01-1.00%,

B;

0.0003-0.0050%, N;

0.002-0.010%,

B/N;

0.2-0.8,

Cu;

0-1.00%, Ni;

0-2.00%, Ca;

0-0.010%, and

Fe and incidental impurities;

balance, cooling the resulting hot-rolled steel plate at a cooling rate of 5°

-50°

C./s,

coiling the steel plate at a temperature of 400°

-650°

C.,

cold rolling the hot-rolled steel plate with a reduction in thickness of 20-85%, and

annealing the cold-rolled steel plate at a temperature of 600°

- Ac₁ for 1 hour or longer.

2. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 wherein before carrying out the cold rolling a preliminary annealing is applied to the hot-rolled steel plate at a temperature of 600°

- Ac₁ for 6 hours or longer.

3. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 wherein the annealing temperature is 670°

-740°

C.

4. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 wherein at least the annealing is carried out in an atmosphere containing 80% or more of hydrogen.

5. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 in which

C:

0.20-0.40%.

6. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 in which

Si:

more than 0.20% but not more than 1.00%.

7. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 in which

sol. Al:

0.05-1.00%.

8. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 in which

Cu:

0.05-1.00%.

9. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 in which

Ni:

0.05-2.00%.

10. A process for manufacturing a medium-carbon steel plate as set forth in claim 1 in which

Ca:

0.001-0.010%.

11. A process for manufacturing a medium-carbon steel plate as set forth in claim 1, the steel including at least 0.26% Si and the annealing step effecting graphitization of 50% or more of cementite in the steel plate.

12. A process for manufacturing a medium-carbon steel plate as set forth in claim 1, the steel having B/N:

0.34 - 0.8 and the annealing step effecting graphitization of 50% to 90% of cementite in the steel plate.