Microstructures and Age Hardening Characteristics of Direct Quenched Cu Bearing HSLA Steel.

Abstract

Microstructures and age hardening characteristics of 0.04C-3.5Ni-0.6Cr-1.8Cu-0.4Mo-0.03Nb Cu bearing HSLA steel manufactured by reheat quenching (RQ) and direct quenching (DQ) processes, were investigated. Especially, the effect of DQ treatment on quenched microstructures and niobium carbide and e-Cu precipitation were examined. A finer quenched microstructure was obtained in DQ processed steel compared with RQ processed steel, due to the heavily deformed austenite introduced by DQ. In DQ processed steel, lath and block sizes of martensite decreased and internal dislocation density increased with the increase of austenite deformation in the non-recystallization region. Hence, the strength of quenched DQ steel increased with the amount of deformation in non-recrystallization region. e-Cu precipitation characteristics in the aging stage changed very sensitively to the quenched microstructures. The size and growth rate of e-Cu precipitate decreased with the amount of deformation in the non-recrystallization region, because heavily dislocated structure supplied more preferred nucleation sites of e-Cu precipitates and retarded the precipitate growth. Also, DQ process affected the niobium carbide precipitation in this steel. In RQ steel, most of niobium precipitated as a coarse carbide in the reheating stage for austenitization. Whereas, in DQ steel, most of niobium precipitated very finely in the aging stage and thus highly contributed to the increase of the strength and tempering resistance. Therefore, it was councluded that DQ processed steel had higher strength and tempering resistance than RQ processed steel due to the finer e-Cu and niobium carbide precipitates.