With the rapid development of China's plastics industry, the market demand for large plastic mold steel, especially P20 steel, is increasing. The metallurgical production process of P20 steel for plastic mould is simple, high yield, has good processing performance within the pre hardened hardness range, good hardenability, can make large cross-section steel obtain uniform hardness, especially the mirror polishing performance is obviously better than the ordinary carbon steel and low-alloy steel, can be widely used in the manufacture of large plastic mould high precision and complicated cavity plastic mold. The traditional heat treatment process is quenching / normalizing + tempering, which not only has many procedures, but also brings energy loss, labor intensity and other issues. Therefore, in the production of die steel, the process of controlled rolling, controlled cooling and low temperature tempering is a good way to save energy. On the basis of small sample experiment, the feasibility of direct tempering after controlled rolling and controlled cooling of P20 steel was analyzed and studied.
The chemical composition (mass fraction,%): C0.35, Si0.38, Mn1.12, Cr1.70, Mo0.40, B and Ti of P20 steel produced by medium plate plant of a steel Co., Ltd. was measured. The phase change point was determined by dilatometer, and the phase transformation point of steel was determined by GB5056-1985. The heating rate was 200 /h, and the Ac1 and Ac3 were 740 and 800, respectively.
The hot rolled samples were quenched at different temperatures. The quenching temperature was 820, 860, 900, 940, and the holding time was 40min. After quenching, the samples were tempered at different temperatures, and the tempering temperature was 400, 500, 550 and 600. In order to simulate the heat treatment process and the properties of steel plate in large scale production, hot rolling direct tempering test was also done. Metallographic specimens were prepared according to the above process. The specimens were observed by optical microscope after etching with nitric acid alcohol, and the Rockwell hardness of each sample was measured by Rockwell hardness tester.
The results show that:
(1) the addition of Trace Boron in P20 prolongs the incubation period of ferrite transformation, inhibits ferrite formation, improves hardenability, and makes the section hardness of the plate more uniform, which has a good application value.
(2) after tempering at 900 C for P20, the size of martensite lath increases gradually with the increase of tempering temperature at different temperatures, and some dispersed carbide particles are dispersed. At 550 C, some carbide particles begin to grow. With the increase of temperature, tempering hardness decreases, and decreases faster at 550.
(3) the controlled rolling, controlled cooling and low temperature tempering process is adopted in the field trial, and the structure is tempered bainite, which has good strength, toughness and enough hardness, and the Rockwell hardness is between 32~36. By normalizing rolling instead of off-line normalizing, great economic and social benefits can be brought to the enterprise.