The Solid Transformation Of High Strengthen Carbon Steels And Their Properties

At present the more and more industry parts demand both high strength and toughness. Although martensite structure of the steel can reached strength above 1000MPa, the steel usually showed a poor toughness. Currently high strength steel was mainly obtained by alloying, which increased the production cost. It was found in the carbon steel that the special structure have this properties, being martensite or the bainite microstructure with a few retained austenite. Therefore, three ordinary carbon steels were regarded as experimental materials in this paper, which were hot rolled and cold rolled to a thickness of 0.9mm. Effect of heat treatment process on microstructure of the steels was investigated by means of the optical microscope, TEM and SEM; Their mechanical properties were measured by tensile test, bending test. The transformation, quenched microstructure and the property were discussed in detail.The experimental results show: hot and cold rolling increased the nucleation rate of new phase during austenizing of the steels, which caused very fine cooled microstructures. After austenizing at temperature above 820℃, bainite transformation can detected at anstempering at temperature of 350℃. But at 400℃and 500℃, pearlite structure can be founded in the steels. Among the three kinds of steels by the heat treatment, the S steel had highest strength, while the Q steel showed the best ductility. It was obtained the structure, such as: lath martensite, bainite and a few retained austenite multiphase microstructure for the S steel by a treatment of 860℃austenizing and 350℃austempering, which had the steel to display high strength and high plastic characteristic.