| With the development of high-speed and heavy-load railways,high strength is the most important requirement for the new generation of weathering steels.Strength and toughness can be impoved by vanadium nitrogen microalloying with a certain amount of nitrogen and vanadium,however,excessive nitrogen may lead to the formation of subsurface pinholes during solidification of slab.Corrosion resistance can be improved by adding chromium,but excessive chromium may lead to cracking during rolling.Therefore,it is significant for both engineering application and foundmental theory to develope the new weathering steel through the materials compositon design,the process optimization,the defect prevetion and microstructure as well as property control.First,the composition of the new weathering steel was designed based on the steel of YQ450NQR1,by the combined screening method with different isothermal phase diagrams calculation,and the addition amount of C and N was determined.The equilibrium phase diagram of the new weathering steel with different N/C ratios during the solidification and cooling process was calculated,and the relationship between the precipitation of VN/V?C,N?and the N/C ratio was obtained.The continuous cooling transformation?CCT?curves of the new weathering steel were calculated.The micorsture with bainite and ferrite was obtained in the room temperature after cooling with the cooling rate of 1 oC/s.At the same time,the thermodynamic model of the nitrogen solubility in the liquid and solid phase of Fe-Cr-Mn-V-N alloys was established based on Hillert's model.The nitrogen contents in phases as a function of the temperature were calculated and validated,while the effect of the alloy elements of Cr,V and Mn as well as the nitrogen partial pressure on the solubility of nitrogen was studied.According to the solubility and segregation of nitrogen during the solidification process,the release pressure of nitrogen bubbles was obtained and the preventive measures for the subsurface pinholes were proposed.Then,the strain-induced precipitation kinetics model for V?C,N?precipitates in V-N microalloyed weathering steels was established based on the Johnson-Mehl-Avrami equation.The precipitation-temperature-time?PTT?curves of V?C,N?precipitates with the influence of nitrogen content were studied using this validated model.With the increase of nitrogen content,the tip temperature of PTT curves increased significantly,and the curves shifted to the left.With the decrease of the nitrogen content,the PTT curves presented the characteristics of“S”shape,because of the nonlinear free energy of phase transformation.Finally,the processing maps of the new weathering steel in the strain range of0.10.9 were constructed based on the dynamic material model and the high temperature compression test with the temperatures of 8501050 oC and strain rates between 0.01 s-1 and 5 s-1.The microscopic deformation mechanism of the stable zone and the unstable zone in the maps was analyzed.The hot working characteristics of the weathering steel were studied combining with the microstructure observation of EBSD,and the optimum working window of hot rolling was established.At the same time,the microstructure and mechanical properties of the new high strength weathering steel were experimentally studied.The formation of bainite was in situ observed by the high-temperature confocal laser scanning microscopy?HT-CLSM?,and the formation mechanism of bainite on the second phase as well as the pathway of co-precipitation of nanoscale precipitates were obtained.Results showed that with the increase of N/C ratio,the yield strength and the tensile strength of the new high strength weathering steel increase.With N of 0.036%and C of 0.036%,the yield strength and the tensile strength reach 585MPa and 830MPa,respectively.It is resonable to develop the new high strength weathering steel by increasing the nitrogen content and promoting the formation of the bainite matrix as well as the nanoscale precipitates. |
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