Method for producing a high strength steel sheet having improved strength and formability and obtained sheet

Patent metadata

Field	Value
Publication number	US-2017137907-A1
Application number	US-201515322712-A
Country	US
Kind code	A1
Filing date	Jul 3, 2015
Priority date	Jul 3, 2014
Publication date	May 18, 2017
Grant date	—

Abstract

A method for producing a high strength steel sheet having a yield strength YS>850 MPa, a tensile strength TS>1180 MPa, a total elongation >13% and a hole expansion ratio HER>30%, by heat treating a steel sheet wherein the chemical composition of the steel contains: 0.13%≦C≦0.22%, 1.2%≦Si≦1.8%, 1.8%≦Mn≦2.2%, 0.10%≦Mo≦0.20%, Nb≦0.05%, Ti<0.05%, Al≦0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at an annealing temperature TA>865° C. and <1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 275° C. and 375° C., at a cooling speed >30° C./s in order to have, just after quenching, a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 370° C. and 470° C. and maintained at this temperature for a time Pt between 50 s and 150 s, then cooled to the room temperature

First claim

1 - 12 . (canceled)

13 . A method for producing a high strength steel sheet having an improved strength and an improved formability, the steel sheet having a yield strength YS of at least 850 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 13% and a hole expansion ratio HER of at least 30%, comprising the steps of:
providing a steel sheet having a chemical composition containing in weight %:
0.13%≦C≦0.22%;
1.2%≦Si≦1.8%;
1.8%≦Mn≦2.2%;
0.10%≦Mo≦0.20%;
Nb≦0.05%;
Ti≦0.05%; and
Al≦0.5%;
a remainder being Fe and unavoidable impurities;
annealing the sheet at an annealing temperature TA higher than 865° C. but less than 1000° C. for a time of more than 30 s; quenching the sheet by cooling the sheet down to a quenching temperature QT between 310° C. and 375° C., at a cooling speed of at least 30° C./s in order to have, just after quenching, a structure consisting of austenite and at least 50% of martensite, with an austenite content such that the steel sheet has a final structure after heat treatment and cooling to room temperature, comprising between 3% and 15% of residual austenite and between 85% and 97% of a sum of martensite and bainite, the structure not including ferrite; heating the sheet up to a partitioning temperature PT between 370° C. and 470° C. and maintaining the sheet at the partitioning temperature for a partitioning time Pt between 50 s and 150 s; and cooling the sheet down to the room temperature.

14 . The method according to claim 13 , wherein the chemical composition of the steel includes Al≦0.05%.

15 . The method according to claim 13 , wherein the quenching temperature QT is between 310° C. and 340° C.

16 . The method according to claim 13 , further comprising, after the sheet is quenched to the quenching temperature QT and before heating the sheet up to the partitioning temperature PT, a step of holding the sheet at the quenching temperature QT for a holding time between 2 s and 8 s.

17 . The method according to claim 16 , wherein the holding time is between 3 s and 7 s.

18 . A steel sheet comprising:
a chemical composition of the steel including in weight %:
0.13%≦C≦0.22%;
1.2%≦Si≦1.8%:
1.8%≦Mn≦2.2%;
0.10%≦Mo≦0.20%;
Nb≦0.05%;
Ti<0.05%; and
Al≦0.5%;
a remainder being Fe and unavoidable impurities,
a yield strength of at least 850 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 13% and a hole expansion ratio HER of at least 30%; a structure of the steel sheet comprising between 3% and 15% of residual austenite and between 85% and 97% of a sum of martensite and bainite, the structure not including ferrite, and an average austenitic grain size of the residual austenite being 5 μm or less.

19 . The steel sheet according to claim 18 , wherein the chemical composition of the steel includes Al≦0.05%.

20 . The steel sheet according to claim 18 , wherein the total elongation is at least 14%.

21 . The steel sheet according to claim 18 , wherein the hole expansion ratio is at least 50%.

22 . The steel sheet according to claim 18 , wherein an average size of grains or blocks of martensite and bainite is 10 μm or less.

Assignees

• Arcelormittal

Inventors

• Mohanty Rashmi Ranjan
• Jun Hyun Jo
• Fan Dongwei
• Venkatasurya Pavan K C

Classifications

• C21D8/0242 Primary

  Flattening; Dressing; Flexing · CPC title

• C21D2211/008

  Martensite · CPC title

• C21D1/20

  Isothermal quenching, e.g. bainitic hardening · CPC title

• C21D1/18

  Hardening (C21D1/02 takes precedence); Quenching with or without subsequent tempering (quenching devices C21D1/62) · CPC title

• C21D9/46 Primary

  for sheet metals · CPC title