Development Of NM400Grade High Strength Low Alloy Wear-Resistant Steel And Study On Its Microstructure And Property

High strength low alloy wear-resistant steels are widely applied in construction machines and mining machines in wear working conditions to elevate resistance to wear and to extend the life of machines or equipments, because of high hardness, good toughness and weldability. In recent years, consumption of wear-resistant steels in China has increased dramatically, and the production has caused much attention of many steel companies. However, there are some technical problems for the domestic enterprises, such as high added quantity of alloy elements, unstable mechanical properties, low impact toughness value at low temperature, and so on.In view of the problems above, according to NM400grade high strength wear-resistant steel with greater demand, the evolution of microstructure and variation of mechanical properties during rolling and heat treatment process were systematically studied in this paper. Adding alloying elements such as Ti, Cr and B based on C-Mn steel, low cost NM400grade wear-resistant steel with high strength, high hardness, and great low-temperature impact toughness was successfully developed by means of the microstructure and property control. The major work is as follows:(1) The continuous cooling transformation (CCT) behavior of steel was investigated, and the feasibility analysis for producing NM400steel by conventional reheat quenching (RQ) or ultra fast cooling (UFC) process was conducted.The increasing cooling rate makes the microstructure with granular structure gradually transform into with finer lath structure. The additions of Mo/Ni lower the ferrite formation temperature and correspondingly shifte the CCT curve to the right. The hardness of steels is over400HV at the cooling rates above10℃/s, and it is feasible to produce NM400steel by DQ or UFC process.(2) The dynamic recrystallization behavior of hot deformation austenite, the effects of rolling and cooling parameters on rolling microstructure and subsequently RQ microstructure, as well as the effects of rolling and cooling processes on UFC microstructure transformation were analyzed and discussed.The activation energy of dynamic recrystallization is450.78kJ/mol, and the dynamic recrystallization constitutive equation is obtained through regression. At low cooling rate, granular bainite is obtained for low deformation temperature, whereas broad lath bainite is obtained for higher deformation temperature. At high cooling rate, the laths become finer. With the increasing strain, the interfaces of austenite grain increase, and the lath growth is suppressed. For RQ process, the prior austenite grain size after quenched decreases with the increasing phase interfaces, the thinning lath, the dispersing carbides and the decreasing austenite grain size of the microstructure before reheated. For UFC process, lath martensite is obtained by direct quenching (DQ). Bainite/martensite is obtained by cooling to the temperature below Ms at the cooling rate of10℃/s, and lath bainite is obtained when cooling to the temperature above Ms.(3) Effects of quenching temperature and holding time on microstructure, mechanical properties and hardenability were investigated according to RQ process. The morphology and substructure of martensite for different austenitizing conditions were analyzed.The hardenability of steel increases with the increase of the austenitizing temperature from830to910℃, but it decreases after austenitizing above910℃. When austenitizing below850℃, the quenching steel has high hardness and good toughness at830℃, whereas the optimum temperature is910℃when austenitizing above850℃. The strength and toughness of steel decrease by austenitizing above930℃or holding a longer time. The fraction of high angle boundaries which include prior austenite grain boundaries, martensite packet boundaries and block boundaries decreases with the increasing of prior austenite grain size, but the lath width is not evidently dependent on the prior austenite size.(4) The heat treatment process of intercritical quenching (IQ) was put forward, and the high hardness and better toughness were obtained by means of martensite/ferrite control through the method of IQ or RQ+IQ.The ferrite on the matrix of martensite is strengthened by its high density dislocation and precipitates. The toughness is improved by the fine and dispersed ferrite, but reduced by the massive ferrite on the matrix. The ferrite gradually becomes finer and more dispersed with the increase of the holding time at the quenching temperature, resulting in the increase of strength and toughness. As compared to IQ, finer ferrite and higher toughness are obtained after RQ+IQ.(5) According to UFC process, martensite strengthen by multiply strengthening mechanism was obtained by DQ, resulting in the increase of strength and hardness. It shows an advantage of saving resource and energy because of simplifying process.The prior austenite grain and substructure is influenced by the rolling process of DQ. The multi-pass deformation at the austenite temperature region decreases the packet size, increases the dislocation density in martensite, brokes and refines some martensite laths, improving the effects of dislocation strengthening and fine grain strengthening.(6) The variation of strength and impact toughness, dissolution of carbon, precipitation and of carbides, transformation of carbide type, resolution of retained austenite, and the evolution of matrix state during tempering were analyzed and discussed.Tempering at200℃and250℃resultes in a good combination of hardness and impact toughness due to the martensite laths with high dislocation density, interstitial solid solution of carbon atoms, tiny ε-carbide in the matrix, and the retained austenite films inter laths. The strength, hardness and toughness decrease at tempering temperature above300℃because of the cementite replacing ε-carbide and the carbide transformation from retained austenite, and the cementite is coarsened when tempering above350℃. Tempering at500～600℃, the cementite grows preferably and spheroidizates along the lath boundary and prior austenite boundary and the carbon content in matrix and the dislocation density decreases dramatically, leading to the obviously decrease of hardness and increase of toughness. Additions of Mo and Ni improve the resistance to temper softening. The embrittlement temperature region of300～400℃should be avoided when tempering.According to the experimental results, relying on the domestic roller quenching machine production line, NM400grade high strength low alloy wear-resistant steel with6-50mm thickness had been developed and produced in one of the iron and steel companies in China by recommended composition and production process. The product qualification is above99%. The industrial products with uniform and stable mechanical properties, excellent flatness and weldability can meet the application requirements.Moreover, impact abrasive wear resistance was tested for specimen of some typical process, NM400product and other steel products. The results show that the wear resistance could be improved by increase of hardness and toughness of steel. NM400steel has an excellent wear resistance that is6.36times as Q235steel,4.31times as Q345steel which has the similar composition, and1.37times as the same class imported wear-resistant steel.