Steel strip having high strength and high formability, the steel strip having a hot dip zinc based coating

Patent metadata

Field	Value
Publication number	US-2017191150-A1
Application number	US-201515312929-A
Country	US
Kind code	A1
Filing date	Jul 6, 2015
Priority date	Jul 7, 2014
Publication date	Jul 6, 2017
Grant date	—

Abstract

A steel strip having a hot dip zinc based coating, the steel strip having the following composition, in weight %:
C: 0.17-0.24 Mn: 1.8-2.5 Si: 0.65-1.25 Al: ≦0.3 optionally: Nb: ≦0.1 and/or V: ≦0.3 and/or Ti: ≦0.15 and/or Cr: ≦0.5 and/or Mo: ≦0.3, the remainder being iron and unavoidable impurities;

with a Si/Mn ratio ≦0.5 and a Si/C ratio ≧3.0, with an Mn equivalent ME of at most 3.5, wherein ME=Mn+Cr+2 Mo (in wt. %); having a microstructure with (in vol. %): ferrite: 0-40, bainite: 20-70, martensite: 7-30, retained austenite: 5-20, pearlite: ≦2, cementite: ≦1; having a tensile strength in the range of 960-1100 MPa, a yield strength of at least 500 MPa, and a uniform elongation of at least 12%.

First claim

1 . A steel strip having a hot dip zinc based coating, the steel strip having the following composition, in weight %:
C: 0.17-0.24 Mn: 1.8-2.5 Si: 0.65-1.25 Al: ≦0.3 optionally at least one member of the group consisting of Nb: ≦0.1, V: ≦0.3, Ti: ≦0.15, Cr: ≦0.5, and Mo: ≦0.3, the remainder being iron and unavoidable impurities, with a Si/Mn ratio ≦0.5 and a Si/C ratio ≧3.0, with an Mn equivalent ME of at most 3.5, wherein ME=Mn+Cr+2 Mo (in wt. %) having a microstructure with (in vol. %): ferrite: 0-40 bainite: 20-70 martensite: 7-30 retained austenite: 5-20 pearlite: ≦2 cementite: ≦1 having a tensile strength in the range of 960-1100 MPa, a yield strength of at least 500 MPa, and a uniform elongation of at least 12%.

2 . The steel strip according to claim 1 , wherein C: 0.18-0.22 wt. %.

3 . The steel strip according to claim 1 , wherein Si: 0.8-1.2 wt %.

4 . The steel strip according to claim 1 , wherein Si/C ratio ≧4.0.

5 . The steel strip according to claim 1 , wherein the zinc based coating is a galvanised or galvannealed coating.

6 . The steel strip according to claim 1 , wherein the zinc based coating is a coating containing 0.5-3.8 wt. % Al, 0.5-3.0 wt % Mg, optionally at most 0.2% of one or more additional elements selected from the group of Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi the balance being zinc and unavoidable impurities.

7 . The steel strip according to claim 1 , wherein element Nb is present in an amount of 0.01-0.04 wt. %.

8 . A method for producing a high strength hot dipped zinc coated steel strip in a continuous way, comprising the following steps:
1) providing a steel strip having the following composition in wt. %:
C: 0.17-0.24
Mn: 1.8-2.5
Si: 0.65-1.25
Al: ≦0.3
optionally at least one member of the group consisting of Nb: ≦0.1, V: ≦0.3, Ti: ≦0.15, Cr: ≦0.5, and Mo: ≦0.3
the remainder being iron and unavoidable impurities,
with a Si/Mn ratio ≦0.5 and a Si/C ratio ≧3.0,
with an Mn equivalent ME of at most 3.5, wherein ME=Mn+Cr+2 Mo (in wt. %);
2) heating the strip to a temperature T1 (in ° C.) in the range of (Ac3+20)-(Ac3−30) to form a fully or partially austenitic microstructure; 3) slow cooling of the strip with a cooling rate in the range of 2-4° C./s to a temperature T2 in the range of 620-680° C.; 4) rapid cooling of the strip with a cooling rate in the range of 25-50° C./s to a temperature T3 (in ° C.) in the range of (Ms−20)-(Ms+100); 5) keeping the strip at a hold or slow cool temperature T4 in the range of 420-550° C. for a time period of 30-220 seconds; 6) hot dip coating the steel strip in a zinc bath to provide the strip with a zinc based coating; 7) cooling the coated steel strip at a cooling rate of at least 5° C./s to a temperature below 300° C.

9 . The method according to claim 8 , wherein the hold or slow cool temperature T4 is in the range of 440-480° C.

10 . The method according to claim 8 , wherein in step 5) the temperature variation is ±20° C.

11 . The method according to claim 8 , wherein in step 5) the time period t is in the range of 30-80 seconds.

12 . The method according to claim 8 , wherein in step 6) the steel strip temperature upon entry into the zinc bath is at most 30° C. above the bath temperature.

13 . The method according to claim 8 , wherein the zinc bath contains 0.10-0.35 wt. % Al, the balance being zinc and inevitable impurities.

14 . The method according to claim 8 , wherein the zinc bath contains, in weight %, 0.5-3.8 Al, 0.5-3.0 Mg, unavoidable impurities, the balance being zinc.

15 . The steel strip according to claim 1 , wherein the steel strip comprises at least one member of the group consisting of Nb: ≦0.1, V: ≦0.3, Ti: ≦0.15, Cr: ≦0.5, and Mo: ≦0.3.

16 . The steel strip according to claim 1 , wherein the level of C is 0.20-0.22 wt. %.

Assignees

• Tata Steel Ijmuiden Bv

Inventors

• Hanlon David Neal
• Zuijderwijk Marga Josina
• Van Bohemen Stefanus Matheus Cornelis

Classifications

• C22C38/02

  containing silicon · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C18/00

  Alloys based on zinc · CPC title

• C22C38/04 Primary

  containing manganese · CPC title

• C23C2/40

  Plates; Strips · CPC title