Microstructure Analysis of ZG310-570 Steel after Complete Annealing
ZG310-570 steel is a low carbon martensite-bainite steel that has been typically utilized for deep draw products. It has superior deep drawing ability and good anticorrosion performance. The mechanical properties of ZG310-570 could be improved by complete annealing process. In this research, ZG310-570 steel was heat treated at 780℃ and cooled to room temperature, then complete annealing was carried out. Microstructure observation and microhardness test were employed to study the microstructural evolution and the effect of complete annealing on the mechanical performance of the steel.
In the microstructure of the complete annealing ZG310-570 steel, ferrite, bainite and small amount of retained austenite were observed. The ferrite phase was machined smoother than the bainite phase, which was less well-defined and had a more granular structure imbricate with ferrite phase. The amount of retained austenite was very low, and its grains were small, compared with bainite phase. The average grains sizes of ferrite and bainite were 18.13μm and 23.54μm, respectively, which were larger than the acicular ferrite grains before annealing. This indicates that complete annealing was performanced fully and efficiently. In addition, a great amount of carbides were distributed in ferrite and bainite phase, which could enhance the strength and tensile properties of the steel.
Hardness tests were carried out on this steel samples. The results showed that the hardness of the complete annealing ZG310-570 steel increased from 248 HV20 to 290 HV20, and the increase is 42HV20. This demonstrated that the annealing process resulted in drastic improvements in mechanical performance.
In short, the microstructure of complete annealing ZG310-570 steel was composed of ferrite, bainite and some retained austenite, together with a great amount of carbides. The average grain sizes of ferrite and bainite increased, demonstrating the steel was completely annealed. After annealing, the hardness of the samples increased significantly from 248HV20 to 290 HV20, demonstrating that the complete annealing process had good results in improving the mechanical properties of the ZG310-570 steel.
