Development Of The Third Generation Advanced High Strength SRQP980 Steel For Automobile

In order to realize the lightweight of cold-rolled automobile steel and achieve the purpose of energy saving and environmental protection,the development and promotion of advanced high strength and high plasticity steel are being accelerated.In this context,considering the welding,forming and other processing requirements,according to the new quenching and partioning(Q&P)process proposed recently,a C-Si-Mn SRQP980 steel with low carbon and less alloy was designed in this study.The effect of intercritical anneaing temperature,soaking time,quenching temperature and partioning time on microstructural evolution and mechanical properties of SRQP980 tested steel under the one-step Q&P process was investigated by means of thermal dilatometer,metallurgical microscope,electron probe(EMPA),scanning electron microscopy(SEM),tensile tests and continuous annealing simulation.The major conclusions are as follows:(1)Based on the static CCT curve and TTT curve,the phase transformation type and temperature of tested steel was obtained,and the annealing treatment in dual-phase region was determined in continuous annealing process simulation.The content of retained austenite after partitioning could be effectively controlled by adjusting the ratio of ferrite to initial austenite in the critical zone through the changes of the soaking temperature and time.(2)The comprehensive mechanical properties of tested steel were closely related to the soaking temperature and soaking time of annealing in dual-phase region.The tensile strength and elongation of tested steel first synchronously increased with the increase of annealing temperature,both of which reached the highest value at 8300C.As the annealing temperature continuing rising,the tensile strength and elongation began to decrease.During the whole experiment,the yield strength of tested steel ranged from 530 to 753MPa.Although the tensile strength increased gradually with the increase of soaking time,it was controlled steadily at 1067?1136 MPa during the whole test process.With the increase of annealing time,the elongation increased sharply first and then decreased slightly,while the yield strength decreased sharply first and then increased slightly.(3)The comprehensive mechanical properties of tested steel were closely related to the partitioning temperature and partitioning time.The tensile strength of tested steel decreased first with the increase of partitioning temperature.And the elongation increased first with the increase of the partitioning temperature,which reached the maximum value of 26.5%at 380?.As the partitioning temperature continuing to rise to 400?,the product of strength and elongation slumped due to the formation of bainite.The tensile strength of tested steel decreased with the increase of partitioning time,and the elongation increased first with the increase of partition temperature,which reached the maximum value of 33.5%at 500s.At the same time,the product of strength and elongation reached the maximum value of 30.25 GPa%.As the partitioning time continuing to rise to 1000s,the product of strength and elongation showed a smaller decline due to the precipitation of granular carbides.(4)SRQP980 tested steel showed the optimum comprehensive mechanical properties(the yield strength,tensile strength,elongation and product of strength and elongation was in a range of 600?677MPa,981?1116MPa,22.0?30.0%and 22.1?29.5 GPa%respectively)when soaking time was 60s at 830? and partitioning time was 100?200s at 380?400?,which meant SRQP980 steel really belonged to the typical third generation advanced cold-rolled high strength steel,and this also meant SRQP980 steel was sufficient for the design requiremens of this subject.(5)The strength indexes of hot-rolled and cold-rolled plates processed by the experimental rolling mill were within the design capacity of the production line of Rizhao Company,Shandong Steel.The optimum strip velocity obtained by continuous annealing process simulation was 220.5m/min?245m/min.In a word,SRQP980 tested steel had the basic conditions for industrial trial production in each process,such as HSM,PLTCM and CAL.