The width of the sheet is 500~1500 mm; The width of the thickness is 600~3000 mm. Thin plates are classified by steel types, including ordinary steel, high quality steel, alloy steel, spring steel, stainless steel, tool steel, heat-resistant steel, bearing steel, silicon steel and industrial pure iron sheet, etc. According to the professional use, there are oil drum with board, enamel with board, bulletproof with board, etc. According to the surface coating points, there are galvanized sheet, tin plate, lead plate, plastic composite steel plate. High temperature, high pressure, low temperature, corrosion resistance. Thick steel plates are of roughly the same grade as thin ones. In all aspects, in addition to the bridge plate, boiler plate, automotive steel, pressure vessel steel plate and multilayer high pressure vessel steel and other varieties is thick plate, some varieties of steel such as automobile beam steel plate (2.5 ~ 10 mm thick), decorative pattern plate (2.5 ~ 8 mm thick), stainless steel, heat-resistant steel and other varieties with thin cross. In addition, steel plate and material said, not all steel plates are the same, the material is not the same, the place where the steel plate is used, is not the same.According to the content of alloying elements, it can be divided into: Low alloy steel (total alloying element less than 5%), Medium alloy steel (total alloying element 5%-10%), High alloy steel (total alloying element > 10%). According to the main alloying elements, they are divided into: Chromium steel (CR-Fe-C) Chrome-Nickel steel (CR-Ni-Fe-C), Manganese (Mn - Fe - C), Silicon Manganese steel (Si-Mn-Fe-C). According to the normalizing or as-cast structure of small samples, it can be divided into:  Pearlitic steel, Martensitic steel, Ferritic steel, Austenitic steel ,Ledeburite steel.(1) Low hardenability. In general, the maximum quenching diameter of carbon steel water quenching is only 10mm-20mm. (2) Relatively low strength and buckling. For example, the σ S of ordinary carbon steel Q235 steel is 235MPa, while the σ S of low alloy structural steel 16Mn is more than 360MPa. The σ S /σ B of 40 steel is only 0.43, which is much lower than that of alloy steel. (3) Poor tempering stability. Because of the poor tempering stability, carbon steel in the quenching and tempering treatment, in order to ensure higher strength to use a lower tempering temperature, so that the toughness of steel is low; In order to ensure better toughness, the strength of high tempering temperature is low, so the comprehensive mechanical properties of carbon steel is not high. (4) Can not meet the requirements of special performance. Carbon steel in oxidation resistance, corrosion resistance, heat resistance, low temperature resistance, wear resistance and special electromagnetic and other aspects are often poor, can not meet the needs of special performance.(1) Low hardenability. In general, the maximum quenching diameter of carbon steel water quenching is only 10mm-20mm. (2) Relatively low strength and buckling. For example, the σ S of ordinary carbon steel Q235 steel is 235MPa, while the σ S of low alloy structural steel 16Mn is more than 360MPa. The σ S /σ B of 40 steel is only 0.43, which is much lower than that of alloy steel. (3) Poor tempering stability. Because of the poor tempering stability, carbon steel in the quenching and tempering treatment, in order to ensure higher strength to use a lower tempering temperature, so that the toughness of steel is low; In order to ensure better toughness, the strength of high tempering temperature is low, so the comprehensive mechanical properties of carbon steel is not high. (4) Can not meet the requirements of special performance. Carbon steel in oxidation resistance, corrosion resistance, heat resistance, low temperature resistance, wear resistance and special electromagnetic and other aspects are often poor, can not meet the needs of special performance.