| With the increasing demand for lightweight and energy-saving emission reduction,the development of advanced high-strength automotive steel with economical and practical performance has become an important research direction of steel materials under the premise of ensuring the safety performance of automobiles.As one of the most representative representatives of the third generation of advanced high-strength automotive steel,Q&P(Quenching and Partitioning)steel has attracted the widespread attention,because it had good comprehensive mechanical properties and low manufacturing cost.Premature austenite transformation during deformation will affect the strength and plasticity of Q&P steel.Therefore,finding a method that can suppress the premature stress concentration of retained austenite has become a research focus in recent years.For highstrength steels with a TRIP effect in ferritic matrix,without the addition of alloying elements,the ultra-fine grains are a direct and effective method to suppress the premature stress concentration of retained austenite.At the same time that the refined retained austenite has higher stability,the ultrafine crystal ferrite grains can also disperse the stress concentration of the retained austenite,so that the TRIP effect proceeds in a progressive manner,and the ultrafine crystal ferrite,martensite and retained austenite can also deform in coordination with each other,ensuring the simultaneous improvement of strength,plasticity and toughness.Under the support of the National Natural Science Foundation of China(51874088),four low-carbon and low-alloy steels with different alloy compositions are designed in this paper.The combination of grain refinement technology and Q&P heat treatment process is used to prepare high strength Q&P steel with ultrafine-grained ferrite matrix.The effects of alloy elements,rolling process and heat treatment process on the microstructure and properties of Q&P steel were systematically studied.The main research contents and results of the paper are as follows:(1)Based on the composition of traditional Q&P steels,the effects of different components on the microstructure and mechanical properties of experimental steels were studied by differentiating the content of Al and Si elements.The results show that the plasticity of the experimental steel increases significantly,and the strength gradually decreases with the increase of Al content(Al+Si=1.5%).Among them,0.5Si-1.0Al low carbon and low alloy Q&P steel has the best comprehensive mechanical properties,with an elongation of 27%and a strong plastic product of 19.1GPa·%.(2)The effect of asynchronous cold rolling process on the microstructure and mechanical properties of experimental steel was studied.The results show that the tensile strength and elongation of the experimental steel both increase and then decrease with the asynchronous ratio increases from 1.00 to 1.19.When the asynchronous ratio is 1.13,the experimental steel has good comprehensive mechanical properties,that is,the tensile strength is 750MPa,the elongation exceeds 28%,and the strong plastic product reaches 20.8GPa·%.(3)The effect of annealing temperature on the microstructure and mechanical properties of experimental steel was studied.The results show that pearlite,ferrite and carbide structures are obtained after quenching when annealing at 720℃,and the mechanical properties of the experimental steel are poor;when annealing at 750℃,the dissolution of carbides is reduced and the austenite content is lower,which is massive.The ferrite grain size is coarse;when annealing at 960℃,the martensite and a little retained austenite structure is obtained by complete austenitizing and quenching,and the plasticity of the experimental steel is poor.The initial hot-rolled matrix structure of ferrite and martensite is obtained through the control of the hot-rolling process.In the annealing at 840℃,the ultrafine-grain steel with an average ferrite grain size of about 1μm is prepared in combination with the Q&P heat treatment process.The tensile strength of the experimental steel is as high as 1000MPa.(4)The effect of partition heat treatment process on the microstructure and mechanical properties of experimental steel was studied.The results show that more stable retained austenite can be obtained at room temperature with the increase of the partition temperature from 280℃ to 440℃.When the partition temperature is 440℃,the retained austenite content is 14.7%;as the partition temperature increases,the tensile strength of the experimental steel decreases and the elongation increases significantly.(5)For the experimental steel with a composition of 0.18C-1.5Mn-0.5Si-1.0Al0.03Nb,the best process is as follows:After the hot-rolled two-phase zone is quenched,the experimental steel is annealed at 840℃ for 1min and quenched at 280℃ for 10s,then partitioned at 410℃ for 2min to obtain the Q&P steel structure of the ultra-fine grained ferrite matrix.At this time,the experimental steel has good comprehensive mechanical properties,the elongation exceeds 28%,and the strong plastic product reaches 21.2GPa·%. |
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