Study On The Effect Of Heat Treatment On Microstructure And Properties Of Q&P Steel

The research of the third generation of advanced high strength steel is based on the first generation of high-strength steel under the condition that does not significantly increase the alloy content.While the strength and plasticity reach a higher level,so as to strengthen the formability and impact performance of the steel.Otherwise by reducing the alloy content of the second generation of advanced high-strength steel,the production cost is reduced,and its process performance and performance are improved.The third generation advanced high strength steel with excellent comprehensive performance has become a hot spot of scientific research institutions and enterprises.According to the quenching and partitioning(Q&P)process proposed by Professor Speer,the complete austenitizing Q&P process and intercritical annealing quenching and partitioning process(IQ&P)process for the experimental steel is carried out.The effects of quenching temperature,partitioning time and intercritical annealing time on the microstructure and mechanical properties of Q&P steel were studied.Quite a few experimental tests are used to study,which including optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),X ray diffractometer(XRD),and room temperature tensile test.The mechanism of high strength and high plasticity of Q&P steel was analyzed,especially attach importance to the influence of retained austenite volume fraction on the properties of Q&P steel.This paper draws the following conclusions:(1)The experimental steel is treated by Q&P process with different quenching temperature which microstructure is composed of lath martensite and retained austenite.With the increasing of quenching temperature,the martensite gradually decreases at the first quenching,while the retained austenite tends to increase first and subsequent decrease.When the quenching temperature is high,blocky retained austenite and twin martensite appear,with the increase of quenching temperature,the tensile strength of Q&P steel decreased gradually,while the elongation increases first and afterward decreases.The quenching temperature is 280?,the content of retained austenite reaches the maximum of 6.4%,tensile strength 1269 MPa,elongation 16.7% and strength elongation product reached 21.2GPa·%.From the perspective of comprehensive mechanical performance,280? is the best quenching temperature for the Q&P steel.(2)The experimental steel is treated by Q&P process with different partitioning time,which microstructure is composed of lath martensite,tempered troostite and retained austenite.The distribution of carbon to the retained austenite is not sufficient when the time of partitioning is short,which microstructure is lath martensite and retained austenite.With the prolongation of the partitioning time,the partially quenched martensite is decomposed into ferrite and carbide(tempered troostite)during the middle temperature tempering process,the retained austenite increased first and after that decreased.The tensile strength of the experimental steel is mainly determined by the volume fraction of martensite and the carbon content in martensite.When the best quenching temperature of 280?and the best partition time 60 s,retained austenite content reached the maximum 6.4%,tensile strength 1269 MPa,elongation 16.7% and strong elongation product reached the maximum 21.2 GPa·%.(3)The experimental steel is treated by IQ&P process,which microstructure is mainly composed of lath Martensitic,tempered troostite,massive ferrite and retained austenite.With the prolongation of intercritical annealing time,the grain of martensite and ferrite grow up,and the tempered troostite gradually increases.The retained austenite content tends to increase first and then decrease with the changing trend of intercritical annealing time,but the change of tensile strength is little.When the intercritical annealing time 15 min,retained austenite content reached the maximum 7.8%,tensile strength 1074 MPa,elongation 22.9%,strength elongation product reached the maximum 24.6GPa·%.(4)The mechanical properties of Q&P steel can be effectively improved by the treatment of IQ&P process.Although the tensile strength decreased by about 200 MPa compared with the complete austenitizing Q&P steel,the elongation increased by 37%,and the strength elongation product increased by 16%,achieving the maximum 24.6GPa%.It has better performance than complete austenitic Q&P steel.The maximum strength elongation product is always obtained when the volume fraction of retained austenite is maximum when only one process parameter is changed.