Effects Of Alloy Elements Nb、Al And Heat Treatment Process On Microstructure And Mechanical Properties Of Q&P Steels

The concept of environmental protection is becoming more and more popular and the international development trend of carbon peaking and carbon neutralization is accelerated,the mainstream direction of vehicle development is to reduce harmful gas emissions and improve vehicle safety performance now.Quenching and partitioning(Q&P)steel can meet the requirements of light weight while taking into account safety.It has great development potential,but it also faces the challenge of performance upgrading,the reliable solutions are element addition and process improvement.In this work,the effects of Nb,Si and Al on the microstructure and mechanical properties of Q&P steel were systematically studied from the aspects of composition ratio and microstructure control.The phase transformation,phase composition,distribution and dislocation density of Q&P steel are regulated and designed by adjusting the quenching composition(Q&P)process parameters,so as to improve its mechanical properties.Further,through the analysis of mechanical equations,the physical relationships between microstructure,structural evolution and mechanical properties are established,and the influence mechanism of microstructure on macro properties is explained.Based on the above studies,the main conclusions of this paper are as follows:(1)The effects of Nb and annealing process were studied: With the increase of annealing temperature(840-920 °C),the morphology of lath martensite in the microstructure of 1#0.19C-1.45Si-1.91 Mn and 2#0.20C-1.48Si-1.92Mn-0.05 Nb test steels becomes more and more obvious.The strength and elongation of 1# and 2# test steels increased and decreased respectively.Moreever,with the quenching temperature increased from 240 °C to 280 °C,the content of the retained austenite in the test steels increased.The tensile strength and yield strength of 2# test steel under the same process are 30～150 MPa and 50～160 MPa higher than that of 1# Nb-free steel respectively,which is due to the precipitation strengthening and fine grain strengthening of Nb.Among them,The strong-plastic product(tensile strength and elongation product)of 1# and 2# test steels reached the maximμm at 840 °C annealing-240 °C quenching and 840 °C annealing-280 °C quenching,respectively,which were 12.2 GPa·% and 16.2 GPa·%.(2)The effects of Si,Al and partitioning parameters were studied: The increase of the partitioning temperature and the prolongation of partitioning time lead to the decrease of the strength of 2# and 3# test steels,and the plasticity of the steels first increased and then decreased.Compared with one-step partitioning process,the comprehensive properties of2#,3# steel treated by two-step partitioning process are better.Compared with 3#1.36 Al test steel,2#1.48 Si test steel contains more retained austenite.Under the same partitioning process,the tensile strength of 2# test steel is 50～130 MPa higher than that of 3# test steel,and the elongation is similar.The strength-plastic products of 2# and 3# test steels reached the maximμm at 450 °C in two-step partitioning process,which were 20.48 GPa·% and19.51 GPa·% respectively.(3)The strain hardening behavior of 2#,3# test steel after different partitioning processes is analyzed by incremental strain hardening index and modified crussard jaoul equation: The generation,annihilation and movement of dislocations affect the strain hardening ability of the test steel.The tensile process of the test steel is divided into three stages according to the different deformation mechanism: In the first stage,ferrite plastic deformation and martensite deformation work.In the second stage,the retained austenite underwent TRIP transformation to form martensite.And the work hardening mechanism generated in this process interferes with the deformation of ferrite and bainite.In the third stage,dynamic recovery occurs in the tissue,but the dislocation structure in the tissue is difficult to resist the increase in deformation.The Transformation-Induced Plasticity(TRIP)and Block Crack Propagation(BCP)effects of retained austenite and the pinning effect of second phase precipitates can effectively regulate the work hardening ability of each stage.