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1. (WO2018203111) METHOD FOR PRODUCING A HIGH STRENGTH STEEL SHEET HAVING HIGH DUCTILITY, FORMABILITY AND WELDABILITY, AND OBTAINED STEEL SHEET

Pub. No.:    WO/2018/203111    International Application No.:    PCT/IB2017/052631
Publication Date: Fri Nov 09 00:59:59 CET 2018 International Filing Date: Sat May 06 01:59:59 CEST 2017
IPC: C21D 6/00
C23C 2/02
C21D 9/46
C22C 38/02
Applicants: ARCELORMITTAL
Inventors: VENKATASURYA, Pavan C
CHAKRABORTY, Anirban
GHASSEMI-ARMAKI, Hassan
Title: METHOD FOR PRODUCING A HIGH STRENGTH STEEL SHEET HAVING HIGH DUCTILITY, FORMABILITY AND WELDABILITY, AND OBTAINED STEEL SHEET
Abstract:
The method comprises providing a cold-rolled sheet, with a composition containing: 0.15% ≤ C ≤ 0.23%, 1.4 % ≤ Mn ≤ 2.6%, 0.6% ≤ Si ≤ 1.3%, with C+Si/10 ≤ 0.30%, 0.4% ≤ Al ≤ 1.0%, with Al ≥ 6(C+Mn/10) – 2.5%, 0.010% ≤ Nb ≤ 0.035%, 0.1% ≤ Mo ≤ 0.5%, annealing the sheet at 860 °C-900 °C to obtain a structure consisting of at least 90% austenite and at least 2 % intercritical ferrite, quenching to a temperature between Ms- 0 °C and Ms-60 °C at a rate Vc higher than 30° C/s, heating to a temperature PT between 410 °C and 470 °C for 60s to 130s, hot-dip coating the sheet, and cooling to room temperature. The microstructure comprises 45 % to 68 % of martensite, consisting of 85 % to 95 % partitioned martensite having a C content of at most 0.45 %, and fresh martensite; 0 % to 15 % retained austenite; 2 % to 10 % intercritical ferrite; 20 % to 30% lower bainite.