Cold rolled and heat-treated steel sheet and method of manufacturing the same

Patent metadata

Field	Value
Publication number	US-2022017985-A1
Application number	US-201917311808-A
Country	US
Kind code	A1
Filing date	Dec 17, 2019
Priority date	Dec 18, 2018
Publication date	Jan 20, 2022
Grant date	—

Abstract

A cold rolled and heat-treated steel sheet having a composition including, by weight percent: C 0.3-0.4%, Mn 2.0-2.6%, Si: 0.8-1.6%, Al 0.01-0.6%, Mo 0.15-0.5%, Cr 0.3-1.0%, Nb≤0.06%, Ti≤0.06%, Ni≤0.8%, S≤0.010%, P≤0.020% and N≤0.008%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction: between 15% and 30% of retained austenite, said retained austenite having a carbon content of at least 0.7%, between 70% and 85% of tempered martensite, at most 5% of fresh martensite and at most 5% of bainite. It also deals with a manufacturing method thereof.

First claim

1 - 17 . (canceled)

18 . A cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:
C: 0.3-0.4% Mn: 2.0-2.6% Si: 0.8-1.6% Al: 0.01-0.6% Mo: 0.15-0.5% Cr: 0.3-1.0% Nb≤0.06% Ti≤0.06% Ni≤0.8% S≤0.010% P≤0.020% N≤0.008% Cu≤0.03% and optionally one or more of the following elements, in weight percentage: B: 0.0003-0.005% V≤0.2% a remainder of the composition being iron and unavoidable impurities resulting from processing, the steel sheet having a microstructure consisting of, in surface fraction: between 15% and 30% of retained austenite, said retained austenite having a carbon content of at least 0.7%; between 70% and 85% of tempered martensite; at most 5% of fresh martensite; and at most 5% of bainite.

19 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the chromium content is between 0.6% and 0.8%.

20 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the silicon content is below 1.5%.

21 . The cold-rolled and heat-treated steel sheet as recited in claim 20 wherein the silicon content is below 1.4%.

22 . The cold-rolled and heat-treated steel sheet as recited in claim 21 wherein the silicon content is below 1.3%.

23 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the cumulated amount of silicon and aluminum is equal to or above 1.6%.

24 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the aluminum content is between 0.2% and 0.5%.

25 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the molybdenum content is between 0.20% and 0.40%.

26 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the microstructure includes at most 2% of fresh martensite.

27 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the microstructure includes at most 2% of bainite.

28 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the microstructure includes no bainite and no fresh martensite.

29 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the cold-rolled and heat-treated steel sheet is coated with Zn or a Zn alloy or with Al or an Al alloy.

30 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the cold-rolled and heat-treated steel sheet has a yield strength YS of at least 1100 MPa, a tensile strength TS of at least 1470 MPa, a total elongation TE of at least 13%, a hole expansion ratio HER of at least 15% and an LME index of less than 0.70.

31 . A method for manufacturing a cold-rolled and heat-treated steel sheet, comprising the following successive steps:
casting a steel to obtain a slab, said steel having a composition comprising, by weight percent:
C: 0.3-0.4%
Mn: 2.0-2.6%
Si: 0.8-1.6%
Al : 0.01-0.6%
Mo: 0.15-0.5%
Cr: 0.3-1.0%
Nb≤0.06%
Ti≤0.06%
Ni≤0.8%
S≤0.010%
P≤0.020%
N≤0.008%
Cu≤0.03%
and optionally one or more of the following elements, in weight percentage: B: 0.0003-0.005% V≤0.2% a remainder of the composition being iron and unavoidable impurities resulting from processing; reheating the slab at a temperature T reheat comprised between 1150° C. and 1300° C., hot rolling the reheated slab at a temperature higher than Ar3 to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a coiling temperature T coil between 200° C. and 700° C.; optionally pickling said hot rolled steel sheet; optionally annealing the hot rolled steel sheet, to obtain a hot-rolled and annealed steel sheet; optionally pickling said hot-rolled and annealed steel sheet; cold rolling the hot-rolled and annealed steel sheet to obtain a cold rolled steel sheet; reheating the cold-rolled steel sheet to an annealing temperature between Ac3 and Ac3+100° C. and maintaining the cold-rolled steel sheet at the annealing temperature for a holding time between 30 s and 600 s, to obtain, upon annealing, a fully austenitic structure; quenching the cold-rolled steel sheet at a cooling rate comprised between 0.1° C./s and 200° C./s, to a quenching temperature Tq between (Ms-140° C.) and (Ms-75° C.) and optionally maintaining it at Tq for a holding time between 1 and 200 s; reheating the cold-rolled steel sheet to a partitioning temperature between 350° C. and 500° C., and maintaining the cold-rolled steel sheet at the partitioning temperature for a partitioning time between 30 s and 2000 s; and cooling the cold-rolled steel and heat-treated sheet to room temperature.

32 . The method as recited in claim 31 wherein the coiling temperature T coil is between 450° C. and 650° C.

33 . The method as recited in claim 31 wherein the hot rolled steel sheet after coiling comprises a grain boundary oxidation layer having a maximum thickness of 5 μm.

34 . The method as recited in claim 31 wherein the hot rolled steel sheet is annealed at a temperature between 500 and 800° C., during 1000 s to 108000 s.

Assignees

• Arcelormittal

Inventors

• Alexandre Patrice
• Bouzat Magali
• Chakraborty Anirban
• Ghassemi-Armaki Hassan
• Girina Olga
• Jacolot Ronan
• Koltsov Alexey
• Nadler Aude
• Panahi Damon
• Soler Michel

Classifications

• C21D8/02

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

• C22C38/38

  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

• C22C38/26

  with niobium or tantalum · CPC title