The Evolution Of Properties And Microstructure Of Carbon Steels Subjected To Intercritically Caliber-rolling

In this paper a new process,multi-pass caliber-rolling in two-phase region carbon steel,was researched systematically.The influence of different rolling temperatures and different carbon content on the microstructure and properties of the rolled bars was the main aspect of this study.Round bars of 45 steel with a diameter of 30 mm were heated at 720~820 ? for 40 min,and then subsequently subjected to the multi-pass caliber-rolling(square-diamond-square)with an accumulative reduction in area of approximately 70% and followed by air cooled.It should be noted that the rolled bars were held in the furnace at prior temperature for 5min after every three pass rolling.While,20,35 and T10 steels were subjected to the multi-pass caliber-rolling at 760 ? to investigate the influence of carbon content.The charpy impact tests,hardness tests and tensile tests were carried out according to national standards.The microstructure of caliber rolled bars was investigated by optical microscopy(OM)and scanning electron microscopy(SEM).The means of electron backscattered diffraction(EBSD)is utilized to analyze the substructure and the texture.Fractographs of V-notch Charpy impact test specimens were investigated by SEM to analysis the change of impact toughness.It could be concluded that with the decrease of rolling temperature,the grains were turned to fibrous and the lamellar cementite were spheroidized gradually.What's more,the intensity of <101> fiber texture which parallel to rolling direction was dramatically increased.The properties of rolled bars could comparable to the process of quenching and tempering according to GB/T 699-1999.With the decrease of rolling temperature,the yield stress of the rolled bars decreased to 541 MPa and then rised to 587 MPa.All bars exhibit excellent plasticity.The most prominent feature in performance is that the impact absorb energy showed a sharp increase to almost 100 J from 50 J when the rolling tamperature below 760 ?.After multi-pass caliber-rolling at 720~760 ?,cracks appeared on the fractographs of Charpy impact specimens,and secondary cracks were mainly extended along interconnected large angle grain boundary and the extended region is mainly concentrated in the aggregation region of the spherical cementite.With the increase of carbon content from 0.2% to 1.0%,the value of yield stress increased to the maximum 553 MPa from 447 MPa.Furthermore,the yield strength ratio of the steel was gradually reduced and linear with the carbon content.The elongation of hypoeutectoid steel can reach more than 25% and the impact absorbed energy could reach up to 100 J,while,the elongation of T10 steel dropped to 19% and impact absorbed energy decrease to 20 J.A lot of cracks appeared on the fractographs of sample 35 steel and 45 steel,while there was little on the fractographs of sample 20 steel and T10 steel.