Numerical Simulation Research On The Large Sized Rod Of Directly Cutting Non-quenched And Tempered Steel During TMCP

Special steel is high value-added and high-tech steel.It is an important mark for a country whether or nor it has developed iron and steel industry.As typical special steel,non-quenched and tempered steel for direct cutting has many advantages,for example,energy conservation,environment protection,high production efficiency,high cost performance.It has a promising furture in the market.However,the research on this steel is insufficient in our country.The evolution law of microstructure during production process is still unclear.The production efficiency of large sized steel rod is relatively low.The cooling process and the quality of large sized steel rod need to be further improved.Therefore,large sized rod of directly cutting non-quenched and tempered steel is focused in this thesis.A controlled cooling process which applicalbel to large sized steel rod is designed.The whole process of controlled rolling and controlled cooling process(TMCP)was researched by numerical simulation and experimental verification.Theoretical and application basis for actual production were provided to improve the production efficiency and product quality.The main contents and results are as follows:1.In order to investigate the microstructure evolution law of the tested steel during hot deformation process,physical simulation tests were performed by using Gleeble-1500 thermal simulation test machine.The behaviors of grain growth,dynamic recrystallization(DRX),metadynamic dynamic recrystallization(MDRX)and static recrystallization(SRX)were researched.And the machmatic models which can describe the grain growth and recrystallization behaviors were developed.The isothermal transformation diagram of the tested steel was measured by the Formastor-FII phase transformation tester.The phase transition law of the steel under different isothermal conditions was obtained and the microstructure after phase transformation was observed.The test results can provide data basis for the numerical simulation of phase transformation during cooling process.2.In order to research the mechanism of microstructure evolution of large steel rod during controlled rolling process,a thermo-mechanical-microstructure coupled finite element model of the tested steel,which combined with the mathematical model of austenite grain evolution,was developed by the finite element software MSC.Marc.The distribution and evolution of macroscopic physical field of different sized large rods during controlled rolling process were obtained.The distribution and evolution of the microstructure including the volume fraction of different types of recrystallization and the average grain size were also studied.The evolution law of austenite grains of large steel rod during controlled rolling was researched,and the simulation results were verified by experimental results.3.A controlled cooling process was designed for large steel rod of SG4201.And the process was verified by actual manufacturing.In order to further investigate the transformation law and evolution of temperature of large steel rod during controlled cooling process,a thermal-microstructure coupled finite element model combined with the austenite isothermal transformation diagram was established using MSC.Marc and its secondary development function.The evolutions of temperature and microstructure of different sized large rods during controlled cooling process were obtained.The phase transformation law of large sized steel rod SG4201 was researched.The simulation results were verified by experiments.4.The investigation results showed the main reason for low production efficiency of large sized steel rod is too long time for holding during rolling process and the large hardness of the surface of product is caused by martensite which is transformed during cooling process.According to the analysis,several optimized processes were proposed.And the optimized processes were compared by using finite element method.Finally,the discontinuous water cooling process was verified as the best solution for controlled rolling process.And the optimal process for controlled cooling is to increase the number of water tanks and reduce the amount of water in each tank.Using optimized process,the production efficiency of large sized steel rod of SG4201 can be increased by 16-23%,and almost all hardness layer on the surface of product can be eliminated,which can greatly improve the cutting performance of the product.