Effect of boron on the microstructure and tensile properties of dual-phase steel

Abstract

The effect of boron on the hardenability of austenite during the production of dual-phase steel and on tensile properties has been studied. Increasing boron content from 0. 0005 to 0. 0029 wt pct decreased the volume fraction of austenite present at constant intercritical annealing temperature but increased the fraction of the austenite which transformed to martensite on cooling. Increasing boron also decreased the sensitivity of martensite content to cooling rate and, therefore, of tensile properties to cooling rate. The lower volume fraction of austenite present at constant intercritical annealing temperature, and the consequentially higher carbon content of the austenite, was in agreement with the known effect of boron on the phase diagram. The previously determined effect of carbon concentration in austenite on the austenite’s martensitic hardenability was sufficient to explain the observed effects of increasing boron content. The absence of a direct effect of boron was investigated using particle-tracking autoradiography (PTA) to follow changes in boron segregation. The pattern of segregation to prior austenite grain boundaries in the initial, normalized condition dispersed slightly on holding in the (α + γ ) phase field, but no evidence was found of resegregation to α/γ interfaces.