The controlling factors on metallurgical properties of advanced high strength steel sheets

摘要

Recent progress in production process for advanced high strength steel sheets(AHSS) is described focussing on Bainitic ferrite steel,transformation induced plasticity(TRIP) steel and precipitation hardened steel by NANO sized precipitates are reviewed. In higher tensile strength(TS) grade,elongation(El),hole expansion ratio(λ) and their balance are eagerly requested to improve by customers.Bainitic ferrite steel which has shear deformed lath with various distribution of iron carbides and retained austenite should be distinguished to produce different balance of El/λ. Chemical composition and heat cycle are reviewed to produce various kinds of metallurgical phase and mechanical property.5%Mn -2%Si TRIP steel is introduced in which a plenty of retained austenite at about 30%and larger absorbed energy(AE) more than 3 000 kgf/mm^2 TS·%El(gauge length:50 mm) can be obtained. That steel also showed excellent upper shelf energy in Charpy test of 140 J/cm^2,as well as excellent property of 20%El and 1 400 MPa TS after forging. A new type of precipitation hardened steel having NANO sized(Ti,Mo) C in ferrite matrix is introduced.It satisfies the both of excellent El andλat the same time. Bending property and holes expansion property obey the same kind of deforming criteria which depends on the forming capability of work hardened parts.Homogeneity of structure is their controlling factor and a new parameterαwhich is obtained by gauge length dependency of total elongation is proposed.