Effect Of Silicon On The Microstructure And Mechanical Properties Of Cold Rolled Q&P Steel

The development of AHSS has been highly valued as a promising method to realize the light-weight car body, into which a significant effort has been put. Quenching and partitioning steel has drew great attention once proposed in 2003, because of its excellent mechanical properties. So far, a lot of fundamental researches about Q&P steel have been carried out. However, there are still problems to be solved before the application of Q&P steel in industry, such as the optimization of processing parameters, chemical compositions and the stability of retained austenite.The influence of silicon on mechanical properties and microstructure of Q&P steel was studied. 3 kind of Q&P steels with different content of silicon were designed and hot-treated with a series of processing parameter, then cut into tensile specimens to evaluate the mechanical properties. OM, SEM, XRD and EBSD were used in the characterization of the microstructures and retained austenite fraction. The following conclusions come out:(1)Thermal simulation shows that the increase of Si content increases the Ac1 and Ac3 temperature. The Ac3 temperature rises to above 930℃ when the content of Si increased to 2.5%.( 2) The mechanical properties of experimental steels show different changing tendency with the processing parameter when the content of Si increase to 2.5%, which is that the strength increase and the elongation decrease with the rise of annealing temperature. This is caused by the acceleration effect of Si on bainitic transformation.(3)By increasing the content of Si, mechanical properties of Q&P steel are improved. Q&P steel with 1.9%Si has the strength of 1000 MPa and the elongation of 26.4%, and Q&P steel with 2.5% Si has the strength of 1200 MPa and the elongation of 17.9%.(4)The fraction of retained austenite is jointly affected by annealing temperature, quenching temperature and partitioning temperature. The enrichment of C in austenite at annealing stage must be taken into consideration when calculating the optimal quenching temperature.