Continuous cooling transformation of deformed austenite in highly hole-expandable steels

The effects of Si and Mn contents on <b>transformation</b> temperature _r3, transformed microstructure and mechanical properties of three kinds of low-carbon <b>steels</b> during <b>continuous</b> <b>cooling</b> were investigated. A _r3 rises by 15-25°C when increasing Si content from 0.50% to 1.35%, and it drops by 30-50°C when increasing Mn content from 0.97% to 1.43%. The effect of Mn on A _r3 is more significant than Si. Si stimulates the precipitation of the high-temperature equiaxed ferrite to suppress the bainite <b>transformation</b>, but Mn not only provides the grain refining of transformed microstructure but also stimulates the forming of bainite. The homogeneous and grain refining diphase ferrite/bainite <b>steel</b> (w(Si)=0.56, w(Mn)=1.43) can be obtained after <b>deformed</b> at 850°C and cooled at the rate 30°C/s, of which the tensile strength is up to 654 MPa.