Effect Of Vanadium On The High Strength Steels Used For Railway Freight Cars

Weathering steel is one type of HSLA steels. There is a dense oxide-protective film formed on the surface after added some alloying elements such as P, Cu, Cr, and Ni into steel, thereby reducing corrosion of steel in depth. Nowadays weathering steel is widely applied in vehicle, bridge, building structure, and so on. The 450MPa grade high strength weathering steel is mostly applycation for railway freight cars in China. The common alloy system of 450MPa grade high strength weathering steel is Cu-Cr-Ni-Nb, and the researchs on the high strength weathering steel containing V and N were less.This research took full advantage of the V & Ti resources in Panzhihua, and the new kind of Cu-Cr-Ni-V-Ti-N alloy system was designed for high strength weathering steel contained 0.01-0.02%N. The effects of the complex addition of V and N on the V precipitation behavior, microstructure, and performance were studied. On the base of which establishing a small size specimen nitrogen quantitative analysis method used for high nitrogen steel, this study used thermal simulation test, precipitate phase quantitative analysis and microstructure analysis to investigate the V precipitation behavior of 450MPa grade high strength weathering steel with variable V, N contents. Meanwhile, this study investigated the effect of deformation processing on austenite-ferrite isothermal transformation and the effect of experiment parameters on the structure, performance and V precipitation behavior of steel. Furthermore, the regularity and mechanism of V precipitate in austenite and ferrite were analyzed by thermodynamics calculated.With investigations, a nitrogen quantitative analysis method of small size specimen called "measurement of fixed nitrogen in low alloy steel--electroextraction, distillation separation--indophenol blue spectrophotometry" was established in high-nitrogen steel laboratory to overcome the defect that N analytical precision of previous method was low. Through the comparison of precision of variable analysis method, this new method is able to do accurate quantitative analysis on nitrogen content in nitrogen containing high strength weathering steel.The research indicated that V(C,N) particles could precipitate in arstenite after the adding amount of N more than 0.01% in steels. The PTT curve of V(C,N) precipitation showed a typical C curve in tested steel. The nose-point temperature was the shortest time needed for V precipitation, corresponding temperature for high strength weathering steel was 870℃. Adding N into steel could accelerate V(C,N) precipitation. Deformation could accelerate the V(C,N) precipitation in austenite and impel the PTT curve moving to left. Compared with the effect of nitrogen content change on PTT curve, the effect of pre-strain on PTT curve is less.The result of thermodynamics calculation revealed that the temperature corresponding to maximum homogeneous nucleation ratio for V(C,N) precipitates in austenite was 820℃, while the nose-point temperature of PPT curve of V(C,N) homogeneous nucleation and nucleation at dislocation line were about 800℃. The PTT test results showed maximum homogeneous nucleation ratio temperature and nose-point temperature would be increased obviously since compound adding Nb and V into steels.The research indicated that the strength of steels would be raised obviously after V(C,-N) particles precipitated in ferrite. The PPT curve of V(C,N) precipitates in ferrite was a monotonous curve without a nose-point temperature. After pearlite transformation, the nucleation ratio of homogeneous nucleation or nucleation at dislocation line of V(C,N) in ferrite increased around 2-3 orders of magnitude compared to the corresponding nucleation of V(C,N) in austenite. Precipitation strengthening of V (C, N) in ferrite played the most important role in high strength weathering steels.The study indicated that V could play the role of promoting ferritic phase transformation in the alloy system of Cu-Cr-Ni-V-Ti-N weathering steel. Added the content of V, increased the percent by volume of tested steels. Alternatively, the microstructure would be refinement in size after adding N content into steel, but less effect on the pearlite phase transformation.The rust-layer of high strength wearthering steels contained V and N was mostly composed of α-FeOOH and γ-FeOOH. The protectived effect of rust-layer was enhanced, while the Ecorr values after accelerated corrosion test were raised by adding V and Nb into weathering steels. The corrosion would begin near the inclusions in the high strength weathering steels. V and Nb elements presented nearly equal distribution in the region of severe corrosion area. There showed a positive effect on weathering performance caused by adding V and Nb into steels.The critical technological parameters of 450MPa grade high strength weathering steel was confirmed, and the optimal coiling temperature of Cu-Cr-Ni-Nb-V-Ti-N steel was 650 ℃, which corresponds to the well effect of grain refinement and precipitation strengthening. The optimal finishing temperature of Cu-Cr-Ni-V-Ti-N steel was 870-890℉, which used for air-colling process. Considering the effect of impact value and aging index on performance, the ratio of V/N in V & N microalloying high strength weathering steel should be controlled to be a little higher than the ideal V/N chemical coordination ratio value(3.64) to achieve the best comprehensive property.This study designed low-cost high strength weathering steel with the new alloy system of Cu-Cr-N i-V-Nb-Ti-N, with the advantage of V and Ti resources. The new kind of weathering steel had a lower-cost of production, compared to the common alloy system of Cu-Cr-Ni-Nb steels.Based on experimental research, industrial application showed that the microstructure of experimental steels was fine polygonal ferrite and little bits of of pear lite, and the ferrite grain size was 11.5 grades (6.7μm). The average values of yield strength, tensile strength and elongation of tested steel with thickness less than 10mm were 508.3MPa,602.1 M Pa and 28.8% respectively, while those of tested steel with thickness more than 10mm reached 480.1MPa, 602.9MPa and 26.8% respectively. The weldability and corrosion resistance of experimental steels were excellent, and the average impact energy values of -40℃ and -70℃ low temperature impact test were 128.3J and 75.6J, respectively. These performance parameters justly satisfied the application requirement of steel for railway freight cars. The application of Q450NQR1 weathering steels was more than 110,000 ton. The new alloy system weathering steels can achieve better effect of popularization and application.This study provides a necessary technical support of developing high strength weathering steel contained vanadium, which develops at Pangang Steel latish.