Effects of continuous cooling and isothermal cooling parameters on microstructure and mechanical properties of EH420 marine steel

This study aims to investigate the effects of continuous cooling after deformation at 900 °C and isothermal cooling after deformation at 830 °C on the microstructure and mechanical properties of EH420 steel, emphasizing ferrite structure evolution. As the continuous cooling rate increases from 0.3 °C/s to 50 °C/s at 900 °C, the fraction of ferrite and pearlite decreases, while the percentage of bainite and martensite increases. Correspondingly, the hardness increases from 180 HV5 to 323 HV5. Meanwhile, the ferrite undergoes a transformation from polygonal to an elongated morphology, and the average grain size decreases from 13.6 ± 6.3 μm to 3.7 ± 1.5 μm due to fast cooling, which limits grain growth. During the isothermal cooling process, the microstructure mainly consists of ferrite, pearlite, and a mixed structure of bainite and martensite. When the cooling rate increases from 0.3 °C/s to 3 °C/s at 830 °C, the average grain size of polygonal ferrite decreases from 8.7 ± 3.2 μm to 5.4 ± 3.5 μm, while the hardness increases from 199 HV5 to 233 HV5. The isothermal cooling process can result in equiaxed ferrite grains with a smaller size and promote the formation of pearlite and bainite with high dislocation density.