Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Formability and Obtained Sheet

Patent metadata

Field	Value
Publication number	US-2017137908-A1
Application number	US-201515322789-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 coated steel sheet having an improved ductility and an improved formability, the chemical composition of the steel containing: 0.13%≦C≦0.22%, 1.9%≦Si≦2.3%, 2.4%≦Mn≦3%, Al≦0.5%, Ti<0.05%, Nb<0.05%, the remainder being Fe and unavoidable impurities. The sheet is annealed at temperature TA higher than Ac3 but less than 1000° C. for a time of more than 30 s, quenched by cooling to a quenching temperature QT between 200° C. and 280° C. in order to obtain 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, heated up to a partitioning temperature PT between 430° C. and 490° C. and maintained at this temperature for a time Pt between 10 s and 100 s, hot dip coated and cooled to the room temperature.

First claim

1 - 18 . (canceled)

19 . A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the coated steel sheet having a yield strength YS of at least 800 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%, comprising the steps of:
providing a steel sheet having a chemical composition including:
0.13%≦C≦0.22%;

1.9%≦Si≦2.3%;

2.4%≦Mn≦3%;

Al≦0.5%;

Ti≦0.05%; and

Nb≦0.05%;

a remainder being Fe and unavoidable impurities;
annealing the sheet at an annealing temperature TA higher than Ac3 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 200° C. and 280° C., at a cooling speed sufficient to obtain a structure consisting of austenite and at least 50% of martensite, with an austenite content such that the coated steel sheet has a final structure after treatment, coating and cooling to room temperature, comprising between 3% and 15% of retained 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 430° C. and 490° C. and maintaining the sheet at this temperature for a partitioning time Pt between 10 s and 100 s, this step being a partitioning step, hot dip coating the sheet; and cooling the sheet down to room temperature.

20 . The method according to claim 19 , wherein at least one of the following conditions is satisfied:
PT≧455° C.; and
PT≦485° C.

21 . The method according to claim 19 , wherein during the partitioning step, the temperature of the sheet remains between PT−20° C. and PT+20° C.

22 . The method according to claim 19 , wherein during the partitioning step, the temperature of the sheet linearly decreases from the temperature of reheating to a temperature between 455° C. and 465° C.

23 . The method according to claim 19 , wherein the chemical composition of the steel satisfies at least one of the following conditions:
C≧0.16%;
C≦0.20%;
Si≧2.0%;
Si≦2.2%;
Mn≧2.6%; and
Mn≦2.8%.

24 . The method according to claim 19 , wherein, after the sheet is quenched to the quenching temperature QT and before the sheet is heated to the partitioning temperature PT, the sheet is held at the quenching temperature QT for a holding time between 2 s and 8 s.

25 . The method according to claim 24 , wherein the holding time is between 3 s and 7 s.

26 . The method according to claim 19 , wherein the annealing temperature is higher than 875° C.

27 . The method according to claim 19 , wherein the partitioning time Pt is between 10 and 90 s.

28 . The method according to claim 19 , wherein the hot dip coating step is a galvanizing step.

29 . The method according to claim 19 , wherein the hot dip coating step is a galvannealing step with an alloying temperature TGA between 490° C. and 530° C.

30 . The method according to claim 29 , wherein the alloying temperature satisfies at least one of the following conditions:
TGA>515° C.; and
TGA<525° C.

31 . A coated steel sheet comprising:
a chemical composition of the steel including in weight %:
0.13%≦C≦0.22%;

2.0%≦Si≦2.2%;

2.4%≦Mn≦3%;

Al≦0.5%;

Ti≦0.05%; and

Nb≦0.05%;

a remainder being Fe and unavoidable impurities;
a structure comprising 3% to 15% of retained austenite and between 85% and 97% of a sum of martensite and bainite, the structure not including ferrite; at least one face of the coated steel sheet including a metallic coating; a yield strength of at least 800 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%, and an average size of grains or blocks of bainite and martensite being 10 μm or less.

32 . The coated steel sheet according to claim 31 , wherein the structure consists of 3% to 15% of retained austenite and between 85% and 97% of the sum of martensite and bainite.

33 . The coated steel sheet according to claim 31 , wherein the hole expansion ratio HER is greater than 40%.

34 . The coated steel sheet according to claim 31 , wherein the chemical composition of the steel satisfies at least one of the following conditions:
C≧0.16%;
C≦0.20%;
Si≧2.0%;
Si≦2.2%;
Mn≧2.6%; and
Mn≦2.8%.

35 . The coated steel sheet according to claim 31 , wherein the at least one face of the coated steel sheet is galvanized.

36 . The coated steel sheet according to claim 31 , wherein the at least one face of the coated steel sheet is galvannealed.

37 . The coated steel sheet according to claim 31 , wherein the retained austenite has a C content of at least 0.9%.

38 . The coated steel sheet according to claim 37 , wherein the C content is at least 1.0%.

39 . The coated steel sheet according to claim 31 , wherein the retained austenite has an average grain size of 5 μm or less.

Assignees

• Arcelormittal

Inventors

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

Classifications

• C21D8/02

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

• C21D1/18 Primary

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

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C23C2/40

  Plates; Strips · CPC title