Process for producing hot-rolled steel strip

Patent metadata

Field	Value
Publication number	US-10053757-B2
Application number	US-201314415448-A
Country	US
Kind code	B2
Filing date	Jul 15, 2013
Priority date	Aug 3, 2012
Publication date	Aug 21, 2018
Grant date	Aug 21, 2018

Abstract

A process for producing hot-rolled steel strip with a tensile strength of between 760 and 940 MPa and a steel produced therewith, suitable for producing parts by working such as press forming, bending or stretch flanging.

First claim

The invention claimed is:

1. A process for producing a hot-rolled steel strip with a tensile strength of between 760 and 940 MPa and a hole expansion ratio of at least 50%, the steel having a final microstructure comprising ferrite, bainite and at least 3% of martensite, wherein the total volume of a ferrite phase and a bainite phase is not smaller than 80%, optionally also tempered martensite, retained austenite and/or fine carbides having an average diameter of not larger than 30 nm, and wherein the final microstructure does not contain pearlite and/or coarse Fe 3 C and has a steel composition comprising, in weight %:
0.07 to 0.13% C,
0.75 to 1.30% Mn,
0.6 to 1.4% Cr,
0.1 to 0.49% Si,
0.03% P or less,
up to 0.05% S,
up to 0.001% B,
0.07 to 0.15% Ti,
0.003 to 0.6% Al,
up to 0.01% N,
optionally calcium in an amount consistent with a calcium treatment for MnS inclusion control or REM in an amount consistent with a treatment for MnS inclusion control, and
remainder iron and inevitable impurities associated with the steelmaking process;
the process comprising:
providing a steel slab or thick strip of the steel composition, optionally calcium treated, by:
continuous casting,
thin slab casting,
belt casting, or
strip casting;
optionally followed by reheating the steel slab or thick strip at a reheating temperature of at most 1300° C.;
hot-rolling the steel slab or thick strip at a hot-rolling finishing temperature above Ar 3 so the steel composition is still austenitic during a last hot-rolling pass;
cooling the hot-rolled steel strip by means of continuous cooling or interrupted cooling at a cooling rate of at least 20° C./s to a coiling temperature between Ms and Bs which is between 440° C. and 600° C.; and
coiling the hot-rolled steel strip at said coiling temperature.

2. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises at least 0.03% aluminum.

3. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises at most 1.0% manganese.

4. The process for producing a hot-rolled steel strip according to claim 1 , wherein cooling the hot-rolled steel strip by means of continuous cooling or interrupted cooling is at a cooling rate of at least 30° C./s.

5. The process for producing a hot-rolled steel strip according to claim 1 , wherein cooling the hot-rolled steel strip by means of continuous cooling or interrupted cooling is at a cooling rate of at most 150° C./s.

6. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises 0.005 to 0.35% Si by weight.

7. The process for producing a hot-rolled steel strip according to claim 1 , wherein cooling the hot-rolled steel strip takes place by means of interrupted cooling.

8. The process for producing a hot-rolled steel strip according to claim 1 , wherein the martensite is embedded within the bainite phase.

9. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises at most 0.002% P by weight.

10. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises at most 1.0% Cr by weight.

11. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises:
0.07 to 0.104% C,
0.75 to 1% Mn,
0.79 to 1.01% Cr,
0.1 to 0.49% Si,
0.03% P or less,
up to 0.05% S,
up to 0.001% B,
0.08 to 0.15% Ti,
0.003 to 0.6% Al,
up to 0.01% N,
optionally calcium in an amount consistent with a calcium treatment for MnS inclusion control or REM in an amount consistent with a treatment for MnS inclusion control, and
remainder iron and inevitable impurities associated with the steelmaking process;
wherein said cooling the hot-rolled steel strip is by means of continuous cooling at the cooling rate of at least 20° C./s to the coiling temperature between Ms and Bs which is between 440° C. and 600° C.

12. The process for producing a hot-rolled steel strip according to claim 1 , wherein said cooling the hot-rolled steel strip is by means of continuous cooling at the cooling rate of at least 20° C./s to the coiling temperature between Ms and Bs which is between 440° C. and 600° C.

13. The process for producing a hot-rolled steel strip according to claim 1 , wherein the steel composition comprises 0.003 to 0.03% Al by weight.

14. The process for producing a hot-rolled steel strip according to claim 11 , wherein the steel composition comprises 0.003 to 0.03% Al by weight.

Assignees

• Tata Steel Ijmuiden Bv

Inventors

• Hanlon David Neal
• Kop Theo Arnold
• Van Bohemen Stefanus Matheus Cornelis

Classifications

• C22C38/38 Primary

  with more than 1.5% by weight of manganese · CPC title

• C22C38/32

  with boron · CPC title

• C22C38/28

  with titanium or zirconium · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C22C38/06

  containing aluminium · CPC title