Effect Of Rolling Process On Structure Heredity And Hardness Of 40CrNiMo Steel

40CrNiMo steel is a typical low-alloy ultra-high-strength steel with high strength,high toughness,good hardenability and thermal stability.It is widely used in aviation.In the manufacturing process of 40CrNiMo steel,the coarse-grain genetic problem often occurs,and the coarse-grained structure is difficult to be refined through standardized heat treatment,showing strong structure heritability.On the other hand,due to that stable austenite of the alloying elements,the room temperature structure of 40CrNiMo steel is usually bainite and martensite,and the hardness is difficult to meet the supply standard.This paper is aimed at the production status and above problems of alloy steel bars in a large steel enterprise of China,the research on eliminating the structure heredity phenomenon of 40CrNiMo steel and improving the hardness after rolling was carried out.The effects of different rolling processes on austenite recrystallization,structure transformation and structure heritability were analyzed.On the basic of above all,a method for deformation-induced ferrite precipitation and incomplete annealing to improve the hardness after rolling was proposed.The main works involved as follows:1.Roughing the experimental steel to obtain the original coarse tissue.The austenite grain size of 40CrNiMo steel increases in an approximate exponential law with the increase of heating temperature.The carbides that affect the austenite grain growth tend to be evenly distributed with the increase of holding time,and the austenite growth trend becomes more and more stable.By collecting experimental data,the Sellars model of austenite grain growth of 40CrNiMo steel was established.Finally,the austenite roughening temperature of the experimental steel was determined to be 1100? and lh of heat preservation,then oil quenching.2.Experimental steel phase transition point obtained by thermo-mechanical simulator and full-automatic transformation test instrument.Because of the addition of alloying elements,the phase transition point of 40CrNiMo steel is relatively low,and Ac₁=710?,Ac₃=793?.During the dynamic deformation process,the pearlite transformation region is widened due to severe plastic deformation,which slows the pearlite transformation.High-density dislocation promotes formation of martensite and bainite.Actually,bainite and martensite structures can be formed even at lower cooling rates.3.Study on eliminating the mixed grains of experimental steel by rolling process using thermal simulation test machine.When the deformation temperature is controlled in the range of 950? to 1000?,the deformation is more than 30%,and the deformation rate is greater than is^-1,the austenite recrystallization is more sufficient,the grains are uniformly equiaxed,and the grains are remarkably refined.The level of granularity is above 8th.The experimental steel was subjected to quench experiments on this basis,and it was found that the mixed grains were basically eliminated.Increasing the deformation temperature is beneficial to the bainite transformation,which in turn increases the content of granular bainite,and the granular bainite can induce the formation of mixed grains.The deformation rate is inversely proportional to the bainite transformation.When deformation rate is increased from 0.1s^-1 to 10s^-1,the bainite transformation is reduced from 32%to 20%.4.Based on the study of improving experimental steel mixed crystals,the method of deformation-induced ferrite precipitation is used to improve the hardness.The results show that as the finishing temperature decreases,the amount of ferrite transformation increases and the hardness decreases.When the finish rolling temperature is 800?,the hardness of 40CrNiMo steel can be reduced to 397 HV.In the study of incomplete annealing to decrease the hardness,it is found that the second pass deformation temperature of the deformationinduced ferrite precipitation process is controlled at 800-825 ? with the annealing process of 760? × 0.5 h.The uniform balance structure of pearlite+ferrite+cementite can be obtained,and the hardness is as low as 285 HV or less,which can effectively eliminate the mixed grains while ensuring that the hardness of the product conforms to the national standard.