Abstract: Microstructure and properties of 120 mm thick F460 steel quenched and tempered plate with the same quenching and tempering temperature and different quenching cooling rates were compared to find the best heat treatment process of the tested steel. The steel plate quenched with the 2 ℃/s cooling rate has the highest strength after tempering, but the impact property is poor. After rationally reducing the quenching cooling rate, the strength of the steel plate after tempering decreases to a certain extent, but the impact toughness is significantly improved. Further reducing the quenching cooling rate further reduces the strength of the steel plate after tempering, but the improvement of impact property is limited. The microstructure analysis shows that when the steel plate is quenched at 2 ℃/s cooling rate, the microstructure after tempering is ferrite and bainite, and bainite is mainly in the microstructure. When the cooling rate is reduced, the structure of the steel plate after tempering is ferrite and degenerate pearlite. The increase of ferrite content is conducive to the improvement of toughness of the steel plate. The pearlite structure formed after tempering from retained austenite is relatively small, which can effectively ensure the strength of the steel plate. By analyzing the continuous cooling transformation curve of the steel plate, the steel plate begins to undergo ferrite transformation firstly and solute elements migrate to austenite during cooling. When the cooling rate of steel plate is fast, the carbides in ferrite migrate less, and austenite transforms into martensite or bainite at lower temperature. When the cooling rate of steel plate is slow, carbides migrate into austenite, which improves the stability of austenite and retains it to room temperature to form retained austenite. Retained austenite is transformed into pearlite in the subsequent high-temperature tempering process. The ferrite structure formed by massive transformation and the fine pearlite formed during tempering are conducive to the strength toughness matching of the steel plate.

Key words: F460 steel thick plate, low temperature toughness, quenching, degenerate perlite