| The low alloy high strength wear-resistant steel NM500is used under the service environment of low or medium impact load condition, where high hardness and strength are required for NM500, as well as excellent impact toughness. Nowadays, NM500has poor impact toughness which is produced in domestic companies, which leads to the bad wear-resistant property. Therefore, the focus of the development for domestic NM500is to improve its impact toughness and service life without sacrificing its hardness and strength. In this work, based on the chemical composition of domestic NM500, the effect of microalloying element Nb on the thermodynamics of precipitation and phase transformation, heat treatment technology, the mechanism of strengthening arid toughening, and the impact wear resistance is systematically investigated. Simultaneously, the optimal chemical composition and the corresponding heat treatment process of the experimental steel are obtained, which achieves a good combination of hardness, strength, toughness and impact wear resistance.The type, temperature, and quantity of precipitation in the Nb containing NM500were calculated by the Thermo-calc software. The result shows that the precipitation temperature of Nb-rich MC carbides obviously increases from1150℃to1300℃, as well as the precipitation quantity, when the content of Nb increases from0.018%to0.078%, which is benefit to the impact toughness of the experimental steel through grain refinement. In the process of low temperature tempering, the phases of MC, M7C3, MC_ETA and MC_SHP precipitating dispersively on the matrix of quenching martensite, lead to secondary hardening, which can compensate for the hardness decrease due to tempering to some degree.The austenite grain growing rule of the experimental steel shows that with the increase of Nb content, the grain refinement is relatively obvious at the austenitizing temperature range of850～950℃. As the austenitizing temperature is further increased from1000to1200℃, the grain refinement of Nb is weakened. The austenite coarsening temperature of the experimental steel is950℃. Moreover, the hardness of quenching microstructure first rises (10～20min)and then drops(excess of20min), with the increase of holding time. In order to satisfy the requirement of NM500, the holding time of austenization is limited to40min. The result from the dynamic continuous cooling transformation (CCT) curves, shows that the main phases are proeutectoid ferrite, pearlite, bainite, and martensite. To obtain the appropriate volume fraction of bainite and martensite, the cooling rate should be controlled in the range of5～8℃/s.After being quenched and low temperature tempering, the microstructure of the experimental steel is tempered lath martensite and a little retained austenite. An excellent combination of hardness, strength, and toughness can be achieved when the experimental steel is soaked at850～890℃for20-40min, then water-quenched and tempered at200～250℃for30-60min. Under this condition of heat treatment, all the experimental steels can reach the mechanical properties of the abroad NM500.When it is heating at850℃for30min, then water-quenching and tempering at250℃for60min, followed by air-cooling to room temperature, the microstructure of steel2#containing0.043%Nb is tempering lath martensite and some of retained austenite.TheBrinell hardness of the microstructure is484. The tensile strength, yield strength and elongation after fracture are1652MPa,1412MPa and10.8%, respectively. The impact energy values at room temperature and-40℃are53.3J and51.3J.All the above results reach the requirement of NM500. And its impact toughness exceeds that of the gradeforeign production.After being isothermally quenched and low temperature tempered, the microstructure of the experimental steel is tempering martensite and needle-like lower bainite. When the experimental steel is heating at850～930℃for20-40min, then it was quenched at330～370℃(in50%NaNO2+50%KNOamolten salt) for5-20s, finally water-quenching and tempering at200℃for30min, followed by air-cooling to the room temperature, a better combination of hardness, strength, and toughness is achieved, compared to that obtained from quenching and low temperature tempering. Under this condition of heat treatment, the experimental steel containing0.043%Nb, has good mechanical properties, stability and the heat-treating process is easily controlled. Therefore, the optimal addition of Nb is0.043%.The result obtained from the impact abrasive wear experiment, shows that under the condition of medium impact load with abrasive wear, the plastic deformation area accounted for90%wear surface. The fatigue and spalling caused by plastic deformation resulted in the agravity, so the steel with a good impact toughness as well as high hardness and strength, exhibits a better impact abrasion resistance. From the wear result of different microstructures, it is evident that the microstructure composed of tempered lath martensite and needle-like lower bainite exhibited a better impact abrasion resistance than that of tempering lath martensite and some of retained austenite. Based on the chemical composition of this work, steel2#containing0.043%exhibits optimal combination of hardness, strength, toughness and stability under the process of quenching and low temperature tempering or isothermal quenching and low temperature tempering. Therefore, the Nb content of0.043%is the best choice. |
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