| High-strength steel is an important basic material for economic development.Due to its excellent strength,toughness and cold bending performance,it is widely used in the manufacture of automobile frame longitudinals,beams and other fasteners.In the design of high-strength steel,in order to reduce the alloy content,controlled rolling and rapid cooling technology are widely used.The rapid cooling process introduces huge residual stress in high-strength steel,which restricts the expansion of high-strength steel applications.Tempering is an effective heat treatment method to reduce residual stress and improve plate shape in the actual production process.In the previous study on the evolution of residual stress during 700 L tempering,the research group proposed that under the action of initial residual stress,the precipitation plasticity caused by carbide precipitation is the main reason for the reduction of tempering residual stress.At present,there are few studies on tempering precipitation plasticity,and the formation mechanism of precipitation plasticity is still unclear.This paper takes 700 L low-carbon microalloyed high-strength steel as the research object,focusing on the micro-influence mechanism of precipitation plasticity,using gleeble thermal simulation experiment,non-isothermal double tempering experiment,SEM,TEM,XRD structure analysis and other experimental methods,from different carbides The preparation of the structure and the influence of carbide morphology on the evolution of tempering precipitation plastic behavior are systematically studied.The main conclusions are as follows:（1）The size of the precipitated carbides during the tempering process has a significant influence on the precipitation plasticity behavior.In the case of the same carbide volume percentage,the smaller the size of the carbide,the more diffuse the distribution,and the more significant the precipitation plastic strain.Pre-deformation significantly increases the precipitation plastic strain by refining the size of precipitated carbides.Under the action of a heating rate of0.5℃/s and an external load of 100 MPa,when the size of the alloy carbides in the structure are11.6nm,10.8nm and 10.1nm,the precipitation plastic strains are:-5.86×10-3,-6.12×10-3,and-6.94×10-3 respectively.（2）For ultra-low carbon steel 700 L,pre-deformation increases the density of dislocations,sub-grain boundaries,vacancies and other defects in the structure to increase the nucleation rate of alloy carbides and reduce the average diameter of precipitates.At 0MPa,the carbide size of the undeformed and tempered sample is 13.9nm,while the carbide sizes of the 5% and 10%pre-deformed and tempered samples are 12.9nm and 11.5nm,which is 7.2% and 17.2% less than that of the undeformed sample.（3）For 700 L,during the precipitation stage of alloy carbide,the precipitation plastic strain is positively correlated with the applied load.For the 0% pre-deformed,10℃/s heating rate tempered sample,when the applied load increases from 50 MPa to 100 MPa,the precipitation plastic strain also increases from-8.28×10-3 to-9.23×10-3.（4）The reason that the precipitation plasticity increases with the increase of the applied stress is: during the tempering process,the applied stress can promote the nucleation and precipitation of carbides and reduce the size of the carbides.For the sample with 0% pre-deformation and tempering at 10℃/s heating rate,when the applied load is zero,the carbide size is 13.9nm too large.After 50 MPa and 100 MPa stress tempering,the number of carbides in the structure increased significantly,and the size of the precipitated carbides was 11.2nm and 10.9nm,and the precipitation position of the carbides also changed from being dominated by grain boundaries to intragranular dispersion.（5）Increasing the heating rate can reduce the precipitation size of alloy carbides,but at the same time increase the precipitation and growth temperature of alloy carbides.The result of the competition of the two factors determines the precipitation plastic behavior of the tempering process.（6）In this paper,the Greenwood-Johnson model and the power-law creep model are used to fit the precipitation plasticity equation,and the normalized carbide size is taken into account in the precipitation plasticity equation to describe the effect of pre-deformation on the precipitation plastic strain.The results show that for 700 L,during the precipitation stage of alloy carbides,the power law model can more accurately describe the change trend of precipitation plastic strain with applied load and pre-deformation.This paper enriches the theoretical basis of precipitation plasticity by studying the influence of different carbide distributions on the precipitation plasticity behavior of 700 L.A theoretical model of precipitation plasticity based on the structural characteristics of carbides is established,which provides a theoretical basis for the reasonable formulation of processing techniques to reduce the residual stress of high-strength steel in the subsequent heat treatment process. |
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