ArcelorMittal S.A.
METHOD FOR PRODUCING HIGH-STRENGTH STEEL PARTS WITH IMPROVED DUCTILITY, AND PARTS OBTAINED BY SAID METHOD

Abstract:

A rolled steel sheet, for press hardening is provided, having a chemical composition where Ti/N>3.42, and the carbon, manganese, chromium and silicon contents satisfy: 2.6 C + Mn 5.3 + Cr 13 + Si 15 .gtoreq. 1.1 % . ##EQU00001## The sheet has a nickel content Ni.sub.surf at any point of the steel in the vicinity of the surface over a depth .DELTA., such that: Ni.sub.surf >Ni.sub.nom, Ni.sub.nom denoting the nominal nickel content of the steel, and such that, Ni.sub.max denoting the maximum nickel content within .DELTA.: ( Ni max + Ni nom ) 2 .times. ( .DELTA. ) .gtoreq. 0.6 , ##EQU00002## and such that: ( Ni max - Ni nom ) .DELTA. .gtoreq. 0.01 ##EQU00003## and the surface density of all of the particles D.sub.i and the surface density of the particles D.sub.(>2 .mu.m) larger than 2 micrometers satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said sheet: D.sub.i+6.75 D.sub.(>2 .mu.m) <270 D.sub.i and D.sub.(>2 .mu.m) being expressed as number of particles per square millimeter, and said particles denoting all the oxides, sulfides, and nitrides, either pure or combined such as oxysulfides and carbonitrides, present in the steel matrix.