Effect of cooling procedure on microstructures and mechanical properties of hot rolled Nb–Ti bainitic high strength steel

For the purpose of obtaining the appropriate cooling procedure to produce the commercial low cost hot-rolled Nb–Ti bainitic high strength automotive wheel steel, the microstructures and mechanical properties of the experimental steels at three different cooling procedures were investigated. The results indicate that the yield strength and tensile strength increase with the cooling rate increasing and coiling temperature decreasing, while the elongation is slightly reduced. The hole-expansion ratio first increases and then decreases. The optimum microstructure is granular bainite and a small amount of polygonal ferrite with fine M/A constituents and dispersive nano-scaled (Nb,Ti)C precipitates. The best cooling procedure is at moderate cooling rate and coiling temperature. The strengthening mechanisms of the high yield strength bainitic steel at different cooling procedures are discussed. The bainitic hardening is an important way to reduce the production cost.