Microstructure Design And Property Evaluation Of Three Low Carbon Transformation Induced Plasticity Steels

Transformation induced plasticity steel is one of the advanced high strength auto steels. As the retained austenite in microstructure can induce martensitic phase transformation during plastic deformation, the balance of strength and elongation is pretty well, and it can meet the requirement of auto lighting wight and security. Thus, the steel is widely used in auto manufacturing. While, different microstructure design will cause different properties of transformation induced plasticity steel, so it’s necessary to have a comprehensive evaluation of mechanical property. On the premise of alloy composition is constant, the study obtained different microstrtures by process control, which include ferrite+bainite+retained austenite (TRIP steel), bainitic bainite+retained austenite (TRIP-aided BF steel), Martensite+retained austenite (Q&P steel). Based on thermal simulation and hot-rolled online heat treatment, the paper studied the effect of process parameters on microstructure evolution and evaluated the properity of tested steels. The main results are as follows:(1) Continuous cooling transformation rule was researched. Static CCT curve and dynamic CCT curve were obtained. Ferrite, bainite and martensite phase transformation temperature range were cleared out.(2) By thermal simulation, the microstructure evolution of TRIP steel was studied, and the optimum parameters wrer determined. Based on thermal simulation, the hot-rolled online treatment of TRIP steel was conducted. The results are as follows:tensile strength is above700MPa, total elongation is above25%, limited hole-expanding ratio is about80%. Thus, the balance of strength and elongation of TRIP steel is pretty well.(3) By thermal simulation, the microstructure evolution of BF steel was studied, and the optimum parameters were determined. The best process is at425℃holding30s, when the volume fraction of retained austenite and the carbon in austenite both reach the highest. Based on thermal simulation, the hot-rolled online treatment of TRIP steel was conducted. The results are as follows:when austempered at425℃and holding for5min, total elongation and the product of strength and elongation reach the highest, which are16.6%and18924MPa%. The limited hole-expanding ratio of BF steel is about110%. Thus the stretch-formability of BF steel is excellent.(4) By thermal simulation, the microstructure evolution of Q&P steel was studied, and the optimum parameters were determined. The best process is quenched to250℃and at400℃partitioning30s, when the volume fraction of retained austenite and the carbon in austenite both reach the highest. Based on thermal simulation, the hot-rolled online treatment of TRIP steel was conducted. When quenching to250℃and at400℃partitioning15min, the total elongation and the product of strength and elongation reach the highest, which are17.2%and20884MPa%. What’s more, when quenched and coiled at360℃, although the tensile strength decrease, the total elongation is as high as23.9%, and the product of strength and elongation is20217MPa%.(5) The mechanical properties have their advantage of three kinds of transformation plasticity steels. TRIP steel has excellent extensibility and deep drawability. BF steel has high strength and excellent stretch-formability. Q&P steel has highest strength among the there studied steels, and it’s extensibility is relatively good.