The purpose of the present study is to investigate the effect of precipitation hardening on the properties of HSLA (high strength low alloy) controlled rolled steels containing vanadium and/or niobium.
Laboratory tests were carried out under various conditions of controlled rolling and heat treatment. Also, thermomechanical treatments such as direct quenching and tempering, rolling and tempering in conjunction with rolling conditions were studied.
The results revealed that there were two types of precipitation hardening. That is, the one which is called "secondary hardening" is accompanied by a large loss in toughness and the other called "dispersion hardening" is accompanied by little loss in toughness. The secondary hardening is attributable to fine coherent precipitates formed in the lower temperature range of ferrite region. On the other hand, the dispersion hardening is attributable to fine incoherent precipitates induced by strain in the lower temperature range of austenite region and/or in the high temperature range of ferrite region. The controlled rolling is supposed to accelerate the latter, while suppressing the former.