| How to further compromise the intrinsic contradiction between strength and plasticity in metal structure materials to ensure that they have high strength and obtain excellent plasticity to meet the needs of safe service of components has always been a hot topic and frontier in the field of materials science.Therefore,it is of great theoretical and practical significance to develop a suitable steel grade and design a matching microstructure regulation process for promoting the green development of high-strength steel.The material used in this dissertation is a novel medium carbon low alloy 55Si2 Mn Mo V steel and its performance optimization has been beneficially explored.The main research contents and conclusions are as follows:(1)A series of austempering heat treatments are carried out on the designed steel to study the influence of various parameters on its microstructure and mechanical properties.Excellent comprehensive performance can be obtained at 275 °C for 30 min,with ultimate tensile strength(UTS)and product of strength and elongation(PSE)being 2144 MPa and 25.26 GPa%,respectively,which is mainly determined by the reasonable distribution of fine microstructure at low temperature.Transmission electron microscope reveals that the multiphase microstructure is composed of lath bainite ferrite,martensite,and retained austenite(RA),and the filmy RA is distributed between bainite laths,while the fine carbide is embedded in the matrix.As the austempering temperature increases,the grain size increases gradually,while the strength decreases slowly.Austempering at 325 °C will produce a long-term transformation induced plasticity(TRIP)effect during tensile strain,making total elongation reach the maximum.(2)The effects of isothermal treatment below Ms temperature on the microstructure and mechanical properties of tested steel are explored by improving low temperature quenching parameters.After isothermal decomposition at 150 ℃,180 ℃,and 210 ℃,their microstructures consist of this part of the martensite formed first during the isothermal holding that undergoes a tempering stage to form TM but its microstructure may be confused with BF and can be identified as TM/BF,and the subsequent formation of fresh martensite and RA.The quantitative results indicate that the RA of the three investigated steel increased with the increase of isothermal temperature,which are 23.76%,26.19%,and27.59%,respectively.Among the three samples,S-150 has the highest UTS and the PSE reaches a peak of 29.33 GPa% at the same time.Although the YS is slightly lower than S-210,the UTS is 9.5% higher,and its strength reaches 2.7 GPa,which is difficult to reach for the same type of steel.The yield ratios of the three samples are all below 0.72,which is ideal for good fatigue resistance and the fracture failure modes are all ductile fracture.(3)To ensure the strength of the tested steel while greatly improving the plasticity,three samples with different initial microstructures(annealing,quenching,and austempering)are subjected to intercritical annealing(820 °C)and austempering(325 °C)for 25 min,respectively.The effects of different original microstructures on the final microstructure and mechanical properties were systematically studied.The volume of RA in the quenched sample is reduced compared with the annealed sample and significantly decreased near the tensile fracture surface.The conversion proportion and carbon content of RA in the quenched sample are 16.5% and 1.906%,respectively,which are the highest values among the three investigated samples,indicating that RA continuously and stably transforms into martensite during tensile,thus delaying the occurrence of necking.The ferrite grains of the pretreated samples are significantly refined,which are uniformly distributed in the quenched sample,while the spherical martensite is caused by the decrease of the net surface energy.In addition,the grains of the austempered sample are mostly lath because of the inheritance of microstructure.Compared with the annealed sample,the UTS and PSE of the quenched sample reached 1093.61 MPa and a peak value of 67.05 GPa%,respectively,with a slight decrease in strength but a significant increase in plasticity by 26.22%.The strength ratio of the three samples fluctuates between 0.67-0.71,and their failure modes are all ductile fracture. |
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