Abstract：In order to investigate the continuous cooling transformation law of the novel 18CrNiMo medium plate steel and its optimal heat treatment processes,the continuous cooling transformation curve (CCT curve) of the experimental steel is drawn. According to it,the heat treatment process of the steel plate is industrially tested and the strength and toughness of the trial steel are analyzed. The CCT curves of the experimental steel are plotted using the thermal expansion transformation instrument,combined with Optical Microscope(OM),Scanning Electron Microscope(SEM),Electron Backscatter Diffraction(EBSD) for microstructure observation,and Vickers Hardness (HV) test. The results show that when the cooling rate of the experimental steel becomes ≤2 ℃/s,the room temperature microstructure is mainly composed of ferrite and granular bainite. The cooling rate increases to 2-30 ℃/s,at which time the room temperature microstructure from the main bainite microstructure,gradually transformed into the martensite. With the increase in cooling rate,the subcooling degree increases,and the microstructure of martensite further refinement. When the cooling rate increases upon 30 ℃/s,the microstructure is mainly martensite. The hardness of the experimental steel shows two stages. Microstructure gradually transforms from polygonal ferrite to bainite/martensite,where the hardness increases rapidly from 121HV to 356HV. Then,the cooling rate continues to increase and the microstructure refines,rising smoothly from 366HV to 407HV. The heat treatment process of 18CrNiMo steel is developed according to the CCT curve,i.e. quenching at 900 ℃ (cooling rate 10-30 ℃/s) and tempering at 650 ℃. The heat-treated steel is composed of martensitic/bainitic dual-phase with yield strength Rp0.2≥900 MPa and low-temperature impact KV2≥120 J at -40 ℃. The proportion of martensite and bainite in heat-treated steel plates is about 61% and 39%,respectively. The pre-formed bainite refines the size of the martensite,effectively hindering crack extension and improving the toughness of the steel.
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2023, Vol. 58 
