Microstructure and mechanical properties of economical dual phase wheel steels based on CSP line with later ultra fast cooling

Abstract

To attain the economical wheel steels with high strength, the microstructure and properties of steel with microstructure consisting of ferrite and martensite were studied. The transformation behaviour of the economical wheel steels under continuous cooling condition and stepped cooling condition were researched and the 11·0-mm thickness wheel steel strips with simple chemical composition were developed on compact strip production (CSP) line with later ultra fast cooling (UFC) system. The results indicate that the dynamic continuous cooling transformation (CCT) curves of low carbon Mn–Cr steel is characterised by a large ferrite region, and the stepped cooling pattern is more beneficial to achieve ferrite–martensite microstructure than continuous cooling pattern. In order to obtain adequate polygonal ferrite matrix during stepped cooling process, the early cooling temperature should be controlled in the range of 700–660°C and the isothermal holding time should be ensured. The 11·0-mm thickness dual phase (DP) wheel steel strips are achieved on CSP line via control cooling technology based on later UFC system. Tensile strength increment of C–Mn–Cr steels can reach 130–170 MPa when the F-P microstucture is replaced by F-M microstucture. The wheel steel strips with tensile strength above 590 MPa, yield ratio under 0·62 and elongation above 28% have been used for wheel discs, whose weight achieve reduction of 16–22%.