| The high strength low alloy wear-resistant steel has many advantages, such as simple production process, relatively low of the production cost, and widely used in many impact load conditions, etc. This kind of steel has gradually replaced the traditional wear-resistant steels like high manganese steels and modified medium manganese steels, and it becomes the main steel that used in the engineering mechanical industry. However, the domestic production technology level of high strength low alloy wear-resistant steel is relatively lower than abroad at present, and the domestic research of the high level of wear-resistant steel is especially less than abroad. Therefore, it is very necessary that the domestic researchers should do more research of the high strength wear-resistant steel. In this paper, the compositions of the high strength steel in grade NM400is designed as one of the Cr-B series according to the design ideas of low cost and low alloy. The plates of NM400steel with excellent performances are successfully developed through the process of TMCP, off-line quenching and tempering in low temperature. The optimal parameters of rolling and heat treatment process are obtained by combining with the observation of microstructure and the results of mechanical properties’testes. The influence of the added micro-V on the wear-resistant steel’s microstructure and mechanical properties is analyzed. The main conclusions are as follows:(1) Micro-V which can form strong carbides is added in the test steel that based on Q345steel for micro-alloying. A few of Cr and B elements are added in order to improve quench-hardening ability, and there are no precious metal elements (such as Ni and Mo, etc) added for the sake of reducing material cost.(2) The test results of continuous cooling transforming (CCT) show that the steel has good hardenability and hardening capacity. When the cooling speed is5℃/s, the phase transformation temperature began to decline significantly; When the cooling speed is more than10℃/s, the phase transformation temperature began to close to martensite transformation temperature, the point of Ms. When the cooling speed reaches15℃/s or even more, the hardness of the test steel reach more than450HV. Along with the increase of the cooling speed, the hardness of the test steel is obviously improved.(3) Rolling process is in two-stage controlled by TMCP. The reasonable heat treatment process of the test steel is:Quenching at900～930℃, holding about10～20minutes; Low temperature of tempering at200～300℃, heat preservation for90minutes. The test results show that the tensile strength is1390～1460MPa, the elongation ratio is10～16%, the hardness value is420～490HV, and the impact energy at-20℃is40to70J. After tempered near250℃,the test steel can get the best comprehensive mechanics performance.(4) The microstructure of the tempered steel is mainly of lath martensite. The lath martensite is in width of0.1-0.2μm and the fine carbides (mainly V(C,N) which is in size of10～20nm) distribute uniformly. Micro-V make the organization refine. Its main function is solid solution strengthening, precipitation hardening and fine-grain strengthening. The precipitations which are in size of about10-20nm stop the moving of dislocations through the pinning effect, and improve the strength, hardness and toughness of the steel. |
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