Abstract
The microstructure and properties of 600MPa grade steels produced by isothermal treatment has been studied. The influence of isothermal treatment temperature on the microstructure of steels has been investigated by the aid of Optical microscopy and TEM. The results show that after various isothermal treatments the microstructure of steels varies with the isothermal temperature. When the temperature is lower than 530°C the microstructure is mainly composed of granular bainite and lath bainite. Whereas when the temperature is higher than 580°C the polygonal ferrite and quasi polygonal ferrite appears and the microstructure consists of ferrite and fine lath bainite. Since the microstructures are complex and the yield ratio of steels under these conditions are all lower than 0.8. The strength of sample for 630°C isothermal treatment is the highest. The yield strength is over than 800MPa and the tensile strength is over than 1000MPa due to the fine precipitates containing copper. These copper containing precipitates formed during the isothermal transformation by the way of inter-phase precipitation.
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