BAKE HARDENING STEEL WITH EXCELLENT SURFACE PROPERTIES AND RESISTANCE TO SECONDARY WORK EMBRITTLEMENT, AND PREPARATION METHOD THEREOF

摘要

Provided are a bake hardening steel having a crystalline grain size of ASTM No. 9 or more and a method for preparing the bake hardening steel by controlling the winding, rolling and cooling conditions. The bake hardening steel comprises, C:0.0016~0.0025%, Si:0.02% or less, P:0.01~0.05%, S:0.01% or less, sol.Al:0.08~0.12%, N:0.0025% or less, Ti:0.003% or less, Nb:0.003~0.011%, Mo:0.01~0.1%, B:0.0005~0.0015% or less, balance Fe and other inevitable impurities, wherein % is weight%, and Mn and P satisfy the relation of -30(?) = 803P - 24.4Mn - 58. Since the bake hardening steel has excellent surface properties, resistance to secondary work embrittlement, and resistance to natural aging as well as excellent baking hardenability and tensile strength, it can be used for various parts of vehicles.