Research On Isothermal Transformation Kinetics And Microstructure And Properties Of High Strength Steel

This paper mainly studies the influence of the microstructure of Si-Mo-Cr-Mn steel on the macro mechanical properties.The isothermal transformation kinetic curve?TTT curve?is obtained from the thermal expansion curve and the volume fraction of martensite transformed at different temperatures is calculated,The martensite content obtained by kinetic calculation and the residual austenite content obtained by EBSD are calculated,Bainite ferrite content,After the TTT curve formulates the heat treatment process for the test,the best process is obtained through the analysis of organization and mechanical properties.In this experiment,pure Fe and high-purity metals were melted in a 20kg vacuum intermediate frequency induction furnace for ingot casting and forging.The obtained steel plate is processed into tensile specimens and strip specimens by reducing the hardness through annealing treatment and then used for the mechanical property testing of tissue observation.The TTT curve and critical point were determined by the combination of expansion method and metallography,Ac3=918.9?,Ac1=715.5?,Ms=318.4?,According to the TTT curve of the experimental steel,two heat treatment processes were designed:the austenitized experimental steel at 950?was air-cooled to 500?and placed into the same volume of KNO₃ and NaNO₃ salts?1?for 220,240,260,280,300?salt bath for 6h isothermal test,heat treatment,tensile,hardness,impact and other tests;?2?260?salt bath isothermal at different times,isothermal time is 0,0.6,5,30,60,90,120,240,360min.Through the test of the macroscopic mechanical properties of the sample and the observation of the microstructure,it can be obtained that the mechanical properties of the sample are the best when the quenching temperature is 260?,the maximum tensile strength is 1730MPa,and the average yield strength is 1049MPa,hardness is 49.2HRC and strong plastic product is 51GPa%,When the isothermal temperature approaches the Ms point,the bainite lath becomes larger,so the overall trend of impact toughness tends to increase first and then decrease.It reached 59.5J at 260?.The brittle transition temperature is 300?.Under the process of isothermal quenching temperature of 260?and different isothermal time,the hardness of the experimental steel showed an overall upward trend after decreasing.The experimental steel reached the maximum hardness value of 53.6 HRC at 5 minutes of isothermal,and the hardness remained basically unchanged after 120 minutes.As the isothermal quenching temperature approaches the Ms point,the elongation becomes smaller and the transformation rate becomes slower.At the same time,the rotation variable of bainite decreases accordingly.By isothermal transformation kinetics to obtain the content of martensite in each structure at different temperatures,and then by EBSD to obtain the residual austenite content at different isothermal temperatures,you can get the content of bainite in the structure,bainite content with isothermal The temperature increases and increases.At 260?,the bainite content is higher,which proves that bainite can provide excellent performance to the material.