The distribution of hardness through the whole section of hot-rolled large-sized steel plate is of vital importance for plastic mould, sice it affects the machinability, the polishability and the service life of the mould and as a result, the quality of the manufactured plastic products, among other things. The currently-applied steels for plastic mould are the quenched and tempered steels, such as P20 and P20 +Ni. The higher alloy contents, such as 1.8wt percent chromium, 1wt percent nickel and 0.25wt percent molybdemun are incorporated in such steels with carbon content around 0.40wt percent to ensure the full martensite transformation during oil quenching through the whole section of large-sized steel plate. After oil quenching, such steels must be tempered at temperature of around 650 deg C. The production of such steels is energy-wasting and needs too long production cycle. More importantly, the distribution of hardness through the whole section of large-sized steel plate is actually far from uniform. For example, the difference of hardness between the surface and the centre of the plate with a thickness of 400 mm is more than 30 VHN of P20 + Ni and 60 VHN for P20. In order to improve the hardness distribution of the large-sized steel plate for plastic mold, the bainitic steel B30 was developed in Shanghai Baosteel. The continuous cooling transformation behaviour in the laboratory scale was studied, based on which, the industrial trial test of B30 steel plate was made with the emphasis on the distribution of both hardness and microstructure through the whole section.
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