Effects Of Vanadium And Vanadium - Nitrogen Microalloying On The Microstructure And Properties Of Low Alloy Wear - Resistant Steel NM400

In this paper, four experimental steels were designed based on plain Cr-Mo-B low-alloy wear-resistant steel NM400:NM400steel (without V), V steel (V-res. N:0.082%V-0.0046%N), V-LN steel (V-low N steel:0.089%V-0.013%N), V-HN steel (V-high N steel:0.088%V-0.029%N), to investigate the effect of V and V-N microalloying on microstructure, mechanical properties and wear-resistance property of NM400steel. The CCT curves of V steel and V-HN steel which are with different N content were constructed using Formastor-FII automatic transformation measuring apparatus The effect of deformation temperature and holding time after deformation on refining martensite microstructure was studied through thermal simulation experiments on a Gleeble-1500D simulator. Then, the experimental steels were hot rolled into plates and directly quenched in water pool, followed by tempering at different temperatures. The microstructure, mechanical properties and wear resistance of the tempered plates of four kinds of steels were compared. The main results of this study are as follows:Increasing N content in V steel raises the starting temperature of γ→α phase transformation and shifts the critical cooling rate that the ferrite forms towards high cooling rate, accordingly, the critical cooling rate getting fully martensitic microstructure is increased significantly. The V-HN steel gets only about40%of martensite at the cooling rate of40℃/s, while V steel can obtain fully martensite at the cooling rates greater than10℃/s. It is deduced that the precipitation of BN in V-HN steel with enhanced N content weakens the beneficial effect of element B on hardenability of the steel, and the formation of V carbonitrides also reduces the content of elements V, C and/or N in solid solution in austenite, thus also makes the steel hardenability reduced.The results of the thermal simulation experiments show that holding a certain time after hot deformation is very important for V and V-HN steels to get refined martensite microstructure. For V steel, microstructure refinement can be achieved after deformation at850℃then relaxing for100s and at900℃for50s. For V-HN steel, it can be obtained after deformation at850℃and relaxing for100s.The plates of hot-rolled and directly quenched of four experimental steels achieve their best match between strength and toughness after tempering at200℃. The plates of NM400steel, V steel and V-HN steel show little difference on tensile strength and hardness, the plate of V-LN steel show slightly lower values. The-20℃impact energy value of three V-bearing plates is higher than that of NM400steel without V, and V-HN plate has the highest value. The V-HN plate achieves the best match of strength and toughness compared with other three plates. Microstructure analysis show that the four kinds of plates after hot-rolled and directly quenched are all with martensite microstructure. The V-bearing plates show refined microstructure compared with NM400plate without V. Among three V-bearing plates, the mastensite microstructure is refined as N content increases in the steel. It is thought that the V(C,N) particles which are formed during the relaxation after finish rolling at860℃promotes the formation of acicular ferrite inner austenite grains, thus the austenite grains are split and the transformed martenisite microstructure can be refined. The microstructure refinement is the main reason improving the toughness of the steel. The wear-resistance property of the plates of four experimental steels was evaluated by using pin-disc friction wear experiment and SRV-4friction wear experiment at ambient temperature. At lower contacting stress (15-20N) condition, the wear resistance depends on the hardness of the plate, therefore, there is no significant difference on wear resistance among the plates of NM400steel, V steel and V-HN steel because of their similar hardness, V-LN plate shows slightly lower wear-resistance. The micro-cutting wear is the main wear mechanism under this condition. Increasing the contacting load to50N, the wear-resistance is controlled by both hardness and toughness of the plates. The V-HN plate with good balance of strength and toughness shows the best wear-resistance, followed by V-LN plate and V plate, the NM400plate is the worst. The micro-cutting wear and fatigue wear are the main wear mechanisms under this condition.