Research On Microstructure Evolution Of Cr-Mn-Si Low Alloy Steel During Hot Processing

The Cr-Mn-Si low alloy steel produced based on TMCP multiphase microstructure control process has obtained"multiphase,sub-stable,multi-scale"M3tissue,which has obtained good plasticity while improving strength,effectively solving the problem of strength-plasticity mutual exclusion of low alloy steel,showing good application prospects in aerospace,defense,naval,automotive and other fields.It has shown good application prospects in the fields of aerospace,defense,naval and automotive.Therefore,it is very meaningful to study the hot forming behavior under the premise of understanding its microstructure properties.The constitutive equation and processing map are the main means to study the hot forming behavior,which is also the most basic process to guide the actual production.In this paper,the Cr-Mn-Si low alloy steel is used as the research object,the strain hardening and fracture mechanisms during the DEFORMation process are systematically investigated by using the comprehensive research method of theoretical calculation,experimental characterization and simulation;the influence of thermal DEFORMation parameters on its microstructure and mechanical properties was explored;the influence of heat treatment on its toughening was initially explored;several representative constitutive models were constructed and their predicted performance was evaluated by correlation coefficients;the thermal processing map was constructed to guide the hot forming process,the microstructure of the safe and unstable zones during the thermal processing was investigated,reasonable processing parameters were determined,and the forming simulation was carried out by using DEFORM software.The main results include the following:（1）The Cr-Mn-Si high-strength steel has good mechanical properties,under room temperature DEFORMation conditions,its microstructure appears to gradually refine with the increase of strain,which is caused by the rearrangement and migration of dislocations during the DEFORMation process,resulting in the formation of small-angle grain boundaries inside the alloy steel grains and gradually transformed into large-angle grain boundaries.The overall strain hardening pattern was expressed in four stages,and the strain hardening rate and the real stress in each stage were influenced by the microstructure evolution behavior.（2）Under the heat DEFORMation condition,the microstructure of Cr-Mn-Si high-strength steel consists of martensite and residual austenite,and the higher the DEFORMation temperature,the larger the size of martensite and the greater the number of slats of martensite;the size of martensite microstructure increases with the increase of strain rate.The dynamic recrystallization of Cr-Mn-Si naval steel can be improved by increasing the DEFORMation temperature and decreasing the strain rate.The thermoplastic DEFORMation at higher strain rates results in a microstructure with more fine recrystallized grains.（3）Cr-Mn-Si high strength steel was quenched at 900℃and tempered at 150℃-650℃,the tensile strength gradually decreased with the increase of tempering temperature;yield strength and microhardness were slowly increased with the increase of tempering temperature and then a rapid decline;when the tempering temperature of250℃can be obtained the best comprehensive performance,at this time the microstructure was tempered martensite and residual austenite,residual austenite is distributed on the original austenite grain boundaries as thin film,which helps to improve the toughness of the material.;when the tempering temperature of 350-550℃,the impact absorbed energy decreased significantly,the first type of tempering brittleness was appeared,this was due to the dissolution of martensite and residual austenite,and the gradual replacement of residual austenite by cementite on the original austenite grain boundaries,resulting in a significant decrease of toughness;dislocation density was reduced with the increase of tempering temperature,but in the tempering brittleness temperature zone there was an abnormal large increase.（4）A series of rheological stress-strain data within certain DEFORMation parameters were obtained by thermal DEFORMation experiments.At the same strain rate,the higher the heating temperature,the smaller the value of peak stress;at the same DEFORMation temperature,the higher the strain rate,the higher the peak stress.On this basis,four constitutive models of Cr-Mn-Si low-alloy steel were established,including the strain-compensated Arrhenius model,the modified Johnson-Cook model,the dislocation density-based constitutive model and the back-propagation artificial neural network-based constitutive model.The prediction accuracy was also verified,and it was found that the artificial neural network-based constitutive model had the highest prediction accuracy for the rheological stress of Cr-Mn-Si low-alloy steel.（5）The processing map of Cr-Mn-Si low-alloy steel was established,and the results showed that the power dissipation coefficient was lower in the low-temperature high-strain region and the high-temperature high-strain region,was higher in the low-strain region,and the maximum value occurred around 1050°C and 0.1s^-1.The rheological instability map was different from the power dissipation map,which is more influenced by the strain variables,and the main distribution areas were the same as the power dissipation coefficient distribution.（6）Based on the above established constitutive equations and the processing map,the hot forging parameters were determined,finite element simulations were carried out using DEFORM software.The simulation results showed that a fully filled forging could be obtained on this basis,the stress,strain and temperature fields were analyzed,the distribution in the forging was reasonable,proving that the above analysis was a good guideline for the hot forming of Cr-Mn-Si low alloy steel.The above research can provide a theoretical basis for the toughening of the new low-alloy steel produced by TMCP process and its thermoforming process,and can provide a technical reference for realizing the industrial application of this type of high-performance material.