Multi-phase steel, cold-rolled flat product produced from such a multi-phase steel and method for producing it

Patent metadata

Field	Value
Publication number	US-9970088-B2
Application number	US-201113877782-A
Country	US
Kind code	B2
Filing date	Sep 22, 2011
Priority date	Oct 5, 2010
Publication date	May 15, 2018
Grant date	May 15, 2018

Abstract

A multi-phase steel including in % wt. C: 0.14-0.25%, Mn: 1.7-2.5%, Si: 0.2-0.7%, Al: 0.5-1.5%, Cr: <0.1%, Mo: <0.05%, Nb: 0.02-0.06%, S: up to 0.01%, P: up to 0.02%, N: up to 0.01% and optionally at least one of Ti, B, and V according to the following stipulation: Ti: up to 0.1%, B: up to 0.002%, V: up to 0.15%, with the remainder iron and unavoidable impurities, wherein the microstructure has at least 10% vol. ferrite and at least 6% vol. residual austenite and the steel has a tensile strength R m of at least 950 MPa, a yield point R eL of at least 500 MPa and an elongation at break A 80 measured in the transverse direction of at least 15%. A method of producing the multi-phase steel.

First claim

The invention claimed is:

1. A multi-phase steel consisting of, in % wt.,
C: 0.14-0.25%
Mn: 1.7-2.5%
Si: 0.2-0.7%
Al: 0.5-1.5%
Cr: <0.1%
Mo: <0.05%
Nb: 0.02-0.06%
S: up to 0.01%
P: up to 0.02%
N: up to 0.01%
and optionally at least one element from the group Ti, B, and V according to the following stipulation:
Ti: up to 0.1%
B: up to 0.002%
V: up to 0.15%
with the remainder iron and unavoidable impurities, wherein, in the microstructure of the steel, at least 10% vol. ferrite, 6 to 20% vol. residual austenite, 5 to 40% vol. martensite, and, optionally, 5 to 40% vol. bainite are present and the steel has a tensile strength, R m , of at least 950 MPa, a yield point, R eL , of at least 500 MPa and an elongation at break, A 80 , measured in the transverse direction of at least 15%.

2. The multi-phase steel according to claim 1 , wherein the carbon content of the residual austenite, C inRA , calculated according to formula [1] is more than 0.6% wt.:
C inRA =( a RA −a γ )/0.0044  [1]

with a γ : 0.3578 nm, a lattice constant of the austenite;
a RA : a lattice parameter of the residual austenite in the finished multi-phase steel after final cooling in nm.

3. The multi-phase steel according to claim 1 , wherein the sum of the Al and Si contents is 1.2-2.0% wt.

4. The multi-phase steel according to claim 1 , wherein the Si content is less than 0.6% wt.

5. The multi-phase steel according to claim 1 , wherein the Al content is 0.7-1.4% wt.

6. The multi-phase steel according to claim 1 , wherein the Ti content is up to 0.02% wt.

7. The multi-phase steel according to claim 1 , wherein the Ti content % Ti fulfils the condition [3]:
% Ti≥3.4×% N  [3]

with % N: the N content of the multi-phase steel.

8. The multi-phase steel according to claim 1 , wherein the B content is at least 0.0005% wt.

9. The multi-phase steel according to claim 1 , wherein the V content is at least 0.06% wt.

10. A cold flat product produced from the multi-phase steel of claim 1 .

11. A multi-phase steel according to claim 2 , wherein the multi-phase steel has a grade, G RA , of the residual austenite calculated according to formula [2], for which G RA >6 applies:
G RA =% RA×C inRA   [2]

with % RA: the residual austenite content of the multi-phase steel in % vol.;
C inRA : the carbon content of the residual austenite calculated according to formula [1].

12. A method for producing a cold flat product, comprising:
melting and casting the multi-phase steel of claim 1 into a semi-finished product;
hot rolling the semi-finished product into a hot strip starting from an initial temperature of 1100-1300° C. and ending at a final temperature of 820-950° C.;
coiling the hot strip at a coiling temperature of 400-750° C.;
optionally annealing the hot strip to improve its ability to be cold rolled;
after coiling, cold rolling the hot strip into the cold flat product at cold rolling degrees of 30-80%;
continuously annealing the cold flat product at an annealing temperature of 750-900° C.;
optionally accelerated cooling at a cooling rate of at least 5° C./s of the continuously annealed cold flat product; and
overageing the cold flat product at an overageing temperature of 350-500° C.

13. The method according to claim 12 , characterised in that the coiling temperature is 530-600° C., the cold-rolling degree is 50-70%, the annealing temperature is 800-830° C. or the overageing temperature is 370-460° C.

14. The method according to claim 12 , wherein the annealing optionally performed after the coiling and before the cold rolling is carried out as batch annealing or as continuous annealing at an annealing temperature of 400-700° C.

Assignees

• Bocharova Ekaterina
• Mattissen Dorothea
• Sebald Roland
• Krizan Daniel
• Pichler Andreas
• Thyssenkrupp Steel Europe Ag

Inventors

• Bocharova Ekaterina
• Mattissen Dorothea
• Sebald Roland
• Krizan Daniel
• Pichler Andreas

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/04 Primary

  containing manganese · CPC title

• C21D2211/005

  Ferrite · CPC title

• C22C38/02

  containing silicon · CPC title

• C21D8/0263

  following hot rolling · CPC title