Experimental Investigation On I&Q&P Process Of Cold Rolled DP780with A Large Product Of Strength And Elongation

In recent decades,with the increasingly serious environmental and energy problems,various countries have put forward higher requirements for automobile production in terms of weight reduction and safety all over the world.Dual phase steel is the most widely used steel material in automobile manufacturing industry,which makes the research of dual phase steel has strong practical value.In this paper,the technical concept of quenching and partitioning(Q&P)process is integrated into the preparation of traditional cold-rolled dual phase steel.And the paper focuses on analyzing the relationship between the content of Si,Nb,the process temperature of Q&P and the microstructure and properties of the test steel.By constructing a new constitutive model,the stress changes of the test steel after Q&P treatment are discussed.The specific researches are as follows:(1)It can be known from the study of 1# and 2# test steels under different quenching temperatures,the increase in Si content improved the tensile strength and yield strength of the test steel,while the elongation decreased significantly.The reason is that,after the Si content increased,the ?/?two-phase area moved up,the retained austenite content decreased,and the ?-Fe lattice distortion energy improved,so the strength increased and the plasticity decreased.When the quenching temperature increased from 240 ? to 280 ?,the elongation of 1# test steel with low Sicontent decreased firstly and then increased with the improving of quenching temperature,while the elongation of 2# test steel with high Si content increased firstly and then decreased,and the fluctuation of yield strength and tensile strength was not obvious.In contrast,adding 1.45% Si to the steel was more conducive to improving the economic benefit and optimizing the surface quality of the steel plate.(2)It can be known from the study of 1#,3#,4# test steels at different partitioning temperatures,the addition of Nb elements can refine the grain,promote the enrichment of carbon into austenite,and improve the strength and plasticity of the test steelsynchronously.The comprehensive mechanical properties of the tested steel were the best when the Nb content was 0.026%.However,excessive Nb content lead to the decrease of retained austenite and carbon content,which further worsened the properties of the material.When the partition temperature increased from 360 ? to400 ?,the yield strength of 1# test steel without Nb decreased,the tensile strength and elongation increased.However,due to the influence of Nb C precipitation and coarsening,the tensile strength,elongation and comprehensive mechanical properties of 3# and 4# test steels decreased.(3)It can be known from the study of 3# test steels at different annealing temperatures,the promoting of annealing temperature lead to the diminishing of ferrite content,the increase of martensite content and the dwindling of grain size;due to the multi-stage distribution of carbon,the volume fraction of retained austenite in the test steel was greatly increased and the carbon content of retained austenite was very high.In terms of mechanical properties,the average yield strength and elongation after fracturedincreased,while the tensile strength decreased slightly,but the change range was slightly.In contrast,the optimum comprehensive mechanical properties can be obtained when the steel is annealed at 860 ?.(4)By constructing a new constitutive model considering TRIP effect,the mechanical tensile behavior of 3# test steel was analyzed.The results show that,the strength of test steel in low strain stage mainly came from original martensite and ferrite,while the strength in high strain stage was mainly provided by deformed martensite.When annealed at 860 ? and quenched at 260 ?,the transformation of deformed martensite is the highest,and the corresponding true strain peak is also the largest.The reason lay in the work hardening of deformed martensite in the tensile process,which proved the introduction of retained austenite had a favorable contribution to the comprehensive properties of the 3# test steel.