METHOD FOR THE MANUFACTURE OF A HARDENED PART WHICH DOES NOT HAVE LME ISSUES

摘要

A method for the manufacture of a hardened part comprising the following steps:A) the provision of a steel sheet pre-coated with a metallic coating comprising from 2.0 to 24.0% by weight of zinc, from 1.1 to 7.0% by weight of silicon, optionally from 0.5 to 3.0% by weight of magnesium when the amount of silicon is between 1.1 and 4.0%, and optionally additional elements chosen from Pb, Ni, Zr, or Hf, the content by weight of each additional element being less than 0.3% by weight, the balance being aluminum, unavoidable impurities and Fe in a content of up to 3.0% by weight as a residual element, wherein the ratio Al/Zn is above 2.9,B) the cutting of the coated steel sheet to obtain a blank,C) the thermal treatment of the blank at a temperature between 840 and 950°C to obtain a fully austenitic microstructure in the steel,D) the transfer of the blank into a press tool,E) the hot-forming of the blank to obtain a part,F) the cooling of the part obtained at step E) in order to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.