Effects of microalloying on stretch-flangeability of trip-aided martensitic sheet steel

Abstract

Synopsis : The effects of Cr, Mo and Ni additions on the microstructure and stretch-flangeability of 0.2%C-1.5%Si-1.5%Mn-0.05%Nb, in mass%, TRIPaided martensitic sheet steel produced by isothermal transformation process at a temperature below martensite-finish temperatures were investigated to develop the 3rd-generation sheet steel for automobile requiring high hardenability. When 0.5% or 1.0% Cr was added to the base steel, the combination of tensile strength of 1.5 GPa and hole-expanding ratio of 40% was attained. On the other hand, the Cr-Mo or Cr-Mo- Ni addition hardly influenced the stretch- flangeability and stretch-formability, although it increased the yield and tensile strengths, comparing to the base steel. The good balance of Cr-bearing steel was mainly caused by suitable combination of (1) volume fraction and (2) interparticle path of finely dispersed martensite-austenite complex phase (M-A phase), which suppressed a void initiation at the matrix/M-A phase interface on hole-punching and void coalescence or crack extension on hole-expanding.