Research On Thermal Processing Map And Relaxation Characteristics Of F40MnVS Non-Quenched-Tempered Steel Bars Based On Cross-sectional Size Effect

Non-quenched and tempered steel eliminates quenching and tempering treatment(quenching and high temperature tempering),achieves the control of internal structure through controlled rolling and cooling technology,so that its performance reaches the level of quenched and tempered steel.For large-size bars,due to the large cross-sectional size,the temperature,strain and strain rate of different parts of the cross-section during hot rolling may be quite different.which puts higher requirements on the fine selection of rough rolling process to avoid rolling instability phenomena such as cracks in certain sections.Therefore,it is particularly important to construct heat processing drawings suitable for different parts of large-sized bar sections.In addition,in order to improve the uniformity of the performance of different parts of the large-sized finished rolled bar section,it is an effective way to reduce the temperature difference between the different parts of the large-sized bar section at the entrance of the finish rolling by using the holding temperature(relaxation)after rough rolling.The study of the evolution characteristics of the microstructure during the relaxation process is the key to predicting whether the finish rolling performance of the bar can meet the requirements and to formulate the holding temperature process.In this paper,F40MnVS non-quenched and tempered steel is taken as the research object,Using a combination of finite element simulation and Gleeble thermal simulation compression,the single-pass thermal simulation compression deformation behavior of the steel at 850?1050?and 0.01?10s^-1was studied.A hot working drawing suitable for the typical cross section of a large-size bar with a diameter of156 mm was constructed to provide a basis for the fine selection of rough rolling technology for large-size bars.Based on the above research work,combined with Gleeble thermal simulation compression test?electron backscatter diffraction analysis(EBSD)and transmission electron microscope(TEM)microscopic analysis,the relaxation characteristics of precipitated phase and static recrystallization of the steel after different rough rolling deformations and different time of holding temperatures were studied.Which provide a basis for whether the performance of finishing rolling can meet the requirements and the formulating the holding temperature process.The main research contents and conclusions of the paper are as follows:(1)The characteristics of F40MnVS non-quenched and tempered steel stress-strain curve and hot deformation behavior were systematically studied,and the constitutive equation and flow stress model of F40MnVS non-quenched and temperedsteel were established.The research results show that:under the conditions of higher deformation temperature and lower strain rate,the material is prone to dynamic recrystallization.The theoretical calculation results also show that F40MnVS steel is prone to dynamic recrystallization when Z?7.27038×10¹⁴.(2)Using MSC.Marc finite element simulation software to simulate the rough rolling process of?156 mm F40MnVS steel.The results show that:the equivalent strain and strain rate distribution gradually decrease from the surface to the core;the temperature distribution decreases from the core to the surface,and the temperature difference between the core and the surface is nearly 150?.(3)The thermal processing map of F40MnVS steel was established to obtain the best processing parameters:the deformation temperature is 1030?1050?,and the strain rate is 0.3?2.4 s^-1.Combined with the finite element simulation data,a thermal processing map suitable for?156 mm large-format F40MnVS non-quenched and tempered steel is constructed,and the safe processing area of F40MnVS steel with a size of?156 mm is obtained:the deformation temperature is 900?950?,and the strain rate is 0.03?0.1 s^-1.(4)The second phase precipitation behavior and static recrystallization behavior of the relaxation process after single-pass thermal compression were analyzed.The results show that:with the increase of relaxation time,the grain size of VC gradually increases,and its content is the highest at relaxation time of 7min.Moreover,during the relaxation process,the precipitation of VC can promote the occurrence of static recrystallization,but as the relaxation time increases,the storage energy is consumed,and the second phase begins to inhibit recrystallization,resulting in insufficient static recrystallization.