Stability Of Retained Austenite In High Strength Offshore Steel And Its Effect On Mechanical Properties

At present,China Offshore Steel has met most of the domestic market demand,but some high-strength,resistance to lamellar tearing,high heat input welding,ultra-low temperature toughness,high crack arresting and other performance steel plates still rely on imports.After heat treatment process 1: 830 ° C quenching + 640tempering(QT process),heat treatment process 2: 830 quenching + 720 two phase zone annealing(QC process)and heat treatment process 3: 830 quenching + 720 two phase zone annealing + 580/600/620/640 tempering(QCT process),the microstructure of the tested steel was observed by means of optical microscope,scanning electron microscope and transmission electron microscope.Determination and calculation of retained austenite content by X-ray diffractometer,and the carbon content of retained austenite was measured by energy dispersive electron microscope.The results show that after three heat treatment processes,the matrix structure of the test steel is ferrite,martensite,and retained austenite.Residual austenite is thin-film / lamellar and lumpy.Among them,thin-film / lamellar retained austenite has higher stability.The stability of retained austenite distributed at the boundary of the structure is higher than that of the retained austenite inside the structure.The smaller the grain size,the higher the stability of retained austenite.The increased C content in retained austenite increases its stability.During the tensile process,the test steel produces a TRIP effect,and the retained austenite transforms to martensite,absorbing energy and dislocations,and improving plasticity.The content of stable retained austenite increases,the content of martensite and bainite decreases,and its tensile strength decreases.In the low-temperature impact,the increase of the residual austenite content increases the low-temperature impact work.When the residual austenite content is close,the better the stability of the residual austenite,the higher the low-temperature impact work.After different heat treatment processes,the obtained residual austenite content and stability are not the same.Therefore,in the actual production process of steel,changing the heat treatment process according to different needs can better control the stability and content of residual austenite.