High strength steel and production method

US10697052B2 · kind B2 · utility

Assignee

• ArcelorMittal · LI

Inventors

• Josee Drillet · Rozérieulles, FR
• Véronique HEBERT · Metz, FR

Key dates

• Filing date: Jun 9, 2016
• Grant date: Jun 30, 2020
• Expiry date: Aug 28, 2036

Classification

• Technology area (CPC C): Chemistry; Metallurgy
• CPC primary: C21D2211/008
• WIPO field: Materials, metallurgy
• WIPO sector: Chemistry

Abstract

The present invention provides a cold-rolled and annealed steel sheet of thickness between 0.7 mm and 2 mm, mechanical strength ranging from 1180 MPa to 1320 MPa, wherein the hole expansion ratio Ac % is greater than 20% and the bending angle is greater than or equal to 40°. The chemical composition includes, the contents being expressed as weight percent: 0.09%≤C≤0.11%, 2.6%≤Mn≤2.8%, 0.20%≤Si≤0.55%, 0.25%≤Cr<0.5%, 0.025%≤Ti≤0.040%, 0.0015%≤B≤0.0025%, 0.005%≤Al≤0.18%, 0.08%≤Mo≤0.15%, 0.020%≤Nb≤0.040%, 0.002%≤N≤0.007%, 0.0005%≤S≤0.005%, 0.001%≤P≤0.020%, Ca≤0.003%, the remainder being iron and inevitable impurities resulting from processing. The sheet has a microstructure including martensite or lower bainite. A sum of the surface fractions of martensite and lower is between 40% and 70%, from 15% to 45% of the surface fraction is low-carbide bainite, from 5% to less than 20% of the surface fraction is ferrite, the non-recrystallized ferrite fraction relative to the total ferrite fraction is less than 15%, and less than 5% of the surface fraction of residual austenite is in the form of islands. The fraction of former austenitic grains in which the size is less than at least one microm…