FORGED STEEL PART AND ITS MANUFACTURING METHOD

Abstract

A steel part for the transmission system of an automobile comprising a core made of the following elements0.15% ≦ C ≦ 0.25%;1.0% ≦ Mn ≦1.6%; 0.4% ≦ Si ≦ 0.9%; 0.001% ≦ Al ≦ 0.1 %; 0.10% ≦ Mo ≦ 0.5%; 1% ≦ Cr ≦ 1.5 %; 0 ≦ P ≦ 0.09%; 0 ≦ S ≦ 0.09%; 0% ≦ N ≦ 0.09%; 0 ≦ Nb ≦ 0.06%; 0% ≦ Ni ≦ 1%; 0% ≦ V≦ 0.2%; 0% ≦ Ti ≦ 0.1%; 0% ≦ Cu≦ 1%; 0% ≦ B ≦ 0.008%; 0% ≦ Sn≦ 0.1%; 0% ≦ Ce ≦ 0.1%; 0% ≦ Mg ≦ 0.10%; 0% ≦ Zr ≦ 0.10%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the core of said steel part comprising, by area percentage, at least 90% of Martensite, with a cumulative optional presence of any one or more from Residual Austenite, Pearlite, ferrite and Bainite from 1% to 10% and precipitates of Aluminum and / or Niobium in form of AlN and / or Nb (C,N), wherein the percentage of grains with prior austenite grains size from 3 to 5 is limited to a maximum of 10% of the total grains, such steel part comprising a martensite­deficient layer till the depth of 1mm or less on all its surfaces, such martensite­ deficient layer comprising from 55% to 72% of martensite, 10% to 40% of bainite and the remainder being any one or more from residual austenite, ferrite or cementite.