Research On Hardness Prediction Model Of 20MnMoB In Cold Forging After Quenching And Tempering

20Mn Mo B forgings obtained by cold forging after quenching and tempering can not only meet the requirements of dimensional accuracy and mechanical properties,but also shorter the process chain,reduce cost,and they have a broad application prospect in the field of automotive parts manufacturing.In the actual engineering production,hardness is the key requirement in drawing design.To reduce the experiments and hardness testing,it is important to establish a reasonable hardness model to accurately predict the hardness of forgings.Currently,there are two kinds of hardness prediction models,one is based on macroscopic material parameters such as yield strength and shear modulus,and the other is based on microstructural characteristic parameters such as grain size and lattice constant.In fact,the hardness of forgings is affected by both macroscopic deformation and microscopic characteristic changes.This paper aims to study the variation law between hardness and equivalent strain,grain size and dislocation density of cold forgings,then establish a integrated hardness prediction model with these three factors.On this basis,the simulation software of ABAQUS was redeveloped to realize the numerical simulation of hardness prediction model.The model and algorithm were put into engineering application to predict the hardness of automobile locking set and cup forgings.The specific results of this paper are given as follows:(1)The relationship between the hardness and the equivalent strain of the material was studied by finite element simulation.It is found that for the materials under the same heat treatment condition,the hardness increases rapidly with the increase of equivalent strain at the initial deformation stage,and then tends to flatten out.For materials with different heat treatment conditions,the absolute value of hardness change is basically unchanged when the equivalent strain increases by a certain amount.(2)EBSD was used to analysis the microstructure changes at different tempering temperatures.With the increase of tempering temperature,ferrite grains recrystallized,grain size increased gradually,and the number of grain boundaries decreased,which was conducive to the continuation of slip between grains.Therefore,the initial hardness value also decreased gradually.(3)The hardness and dislocation density under different loading rates were studied by means of ring compression experiment and XRD.Hardness and dislocation density are not affected by loading rate but mainly determined by deformation.When dislocation density increases,irregular dislocation network distribution is formed,and the hardness gradually increases.(4)On the basis of the law of hardness change of forgings,the hardness prediction model combining equivalent strain,grain size and dislocation density was studied and established.After the numerical simulation of the model,the hardness of forgings of locking set and cup was predicted respectively.It can be well match with experimental data,and further shows that the hardness prediction model and algorithm are reliable,accurate and efficient,providing technical support for engineering application.