| With the development of the methods of Niobium microalloying, the application of niobium in spring steels as a microalloying element has grown dramatically. Niobium microalloying has a significantly impact on the properties and the microstructures of spring steels, and plays a irreplaceable role in the development of spring steels towards high strength and lightweight. The purpose of this dissertation is to study the effects of niobium on the size of austenite, hardenability, decarburization sensitivity, mechanical properties and microstructures of60Si2Mn steel. Besides, the strengthening mechanism of niobium on60Si2Mn steel was discussed.The influence of niobium content on the growth of prior austenite with different temperatures and coarsening temperature of60Si2Mn steel were investigated systematically. The results show that niobium can inhibit the grain growth of60Si2Mn, which are able to refine austenitic grains. However, when the content of niobium increases from0.02%to0.04%, the inhibitation of niobium is not effective. Compare with60Si2Mn steel, niobium can increase the coarsening temperature of austenitic grains by150℃. Observation of the distribution of austenitic grains shows that niobium can improve the homogeneity of austenitic grains of60Si2Mn steel.The impact of niobium on the haedenability of60Si2Mn steel was investigated by jominy end quench test, and the influence of solid solution niobium content on hardenability was also investigated with different austenitic temperatures. The results show that when autenitic temperature is at860℃, trace niobium can decrease the hardenability. Besides, with the increase of austenitic temperature, the hardenability of low niobium steel can be improved.The decarburization layer of spring steels with different Nb contents were measured by metallographic microscopic method, the effect of niobium and its content on the surface decarburization sensitivity of60Si2Mn spring steel has been systematically investigated. The results show that niobium can effectively reduce the decarburization sensitivity of60Si2Mn spring steel. However, the depth of the decarburization does not decrease with the niobium content further increases. The diffusion coefficients of carbon in austenite in different temperatures were calculated by means of Thermo-Calc DICTRA software in which the effects of niobium and other elements on diffusion of carbon in austenite were considered. And the variation of decarburizing depths of the experimental steels was calculated by the Fick’s second law. It agrees with the measured datas and can be used to investigate the effects of other alloying elements on the decarburization more effectively.The mechanical properties of three as-rolled test steels were tested and the microstructures of test steels were observed. The results show that the mechanical properties of low niobium steel are better than that of comparative steel, and the properties of high niobium steel are also improved. As a consequence, niobium can improve the mechanical properties of as-rolled60Si2Mn steel which will not increase with the increase of niobium content. Niobium can increase the volume fraction of proeutectoid ferrite drastically, produce divorced pearlite, decrease the aspect ratio of cementites. Besides, through comparing the hardness of test steels, trace niobium can improve the homogeneity of60Si2Mn.The influence of niobium was investigated by comparing the mechanical properties and microstructures of tempered test steel. The result shows that the strength and hardness of steel decrease and the ductility of test steel rises with the increase of tempering temperature. In addition,0.02%niobium content can improve the mechanical properties of steel, but when the content increases to0.04%, the mechanical properties fluctuate sharply. With the increase of tempering temperature, the martensitic lath degrade gradually, the morphology of carbide will change. Niobium can refine martensitic lathes. The analysis on strengthening mechanism shows that fine-grain strengthening and precipitation strengthening are mainly strengthening methods of niobium, which can increase the yield strength by20MPa. |
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