Microstructure、Mechanical Properties And Production Technology Of High-strength BHT Steel

Recently, bake-hardening steel has excellent formability before forming and in the later paint treatment is strengthened so that it is widely used in automobile industry. On the one hand, new high strength BHT steel can meet the thinning plate thickness and reduce the weight of vehicle, on the other hand, it can be applied to the automotive body structure parts. The microstructure, and mechanical properties of new high strength BHT steel were investigated in this work, especially the tensile strength, yield strength, BH value and BHT value under the different cold rolled continuous annealing conditions and prestrain. Experimental works were aimed to obtain the best possible strengthening parameters as well as provided the theories and practice references for the application of the new high strength BHT steel. The main works involved as follows:(1) The curves of continuous cooling transformation of BHT steel were gained by thermal simulation experiment of CCT and with the cooling rate increasing, the microstructure grain was refined under set processes. The recrystallization behaviors were investigated by single and double pass thermal simulation experiment with deformation resistance model and the softening rate curve of BHT steel.(2) A number of micrographs, SEM and TEM results about the BHT steel showed that the microstructure of the BHT steel was ferrite and pearlite with more ferrite. TEM analysis of hot-rolled BHT steel ferrite dislocation density, state under different conditions proved yield strength and tensile strength rising mechanism of the BHT steel and verified that the high hot finishing rolling process temperature, rapid cooling after rolling and the low temperature in take-up inhibited the precipitation of A1N in steel. At the same time, they were beneficial for improving the bake hardening performance of BHT steel. The maximum BH value and BHT values of hot rolled BHT steel reached60MPa and127MPa.(3) By designing new high strength cold-rolled BHT steel annealing process, the influence of heating rate, annealing temperature, cooling rate and aging temperature on BHT steel was studyed to get a reasonable test parameters for guiding the production. The optimized process parameters for the continuous annealing were as fllows:①heating rate at50℃/s;②annealing temperature at840℃;cooling rate at50℃/s;④aging temperature at450℃. The BH value and BHT values of cold rolled BHT steel reached190MPa and96MPa under the optimized process parameters.(4) The trend of BH and BHT value of cold rolled BHT steel was basically the same as the trend of the yield strength and tensile strength under different prestrain.With the prestrain increases, BH value and BHT value rised.