Nb Physical Metallurgy Principles, High-carbon Steel

High carbon steel is widely used in manufacturing. A large number of gears, springs, bearings, shafts, tools etc.are produced mainly using the high-carbon steel in machinery, automobile, railways, shipping, aerospace, chemical, electronics etc. With the national economic development, there is a change from infrastructure to equipment manufacturing, especially the change in the high-end manufacturing industry.The demand for high carbon steel and quality performance of high carbon steel will significantly increase and the focus of iron and steel industry will be gradually transferred from the structural engineering steel to machinery manufacturing steel. The mechanism of Niobium in high-carbon steel as well as the applied technology of niobium in high-carbon steel need in-depth study urgently.As a result of the solute elements of system increases the interaction when the carbon content increases, the activity of the solute elements have a change.We study by the thermodynamic calculation of the solubility product formula of NbC in austenite of high carbon steel when considering the interaction of Wagner between the solute elements and gives the solubility product formula which relative to C content .The study found that the solubility product formula of NbC in austenite increase with the C content rising. Also the physical -chemical phase analysis gives the solubility product formula of NbC in austenite of 0.75% C steel contained Nb. By comparing the calculated values and experimental values are in good agreement in the condition of same C content.The results show that the amount of pro-eutectoid ferrite in the steels with micro-addition Nb (0.04%Nb,0.064%Nb) increased compared with steel without Nb addition. That is probably because that the carbon content at eutectoid point increased with addition of Nb.Meanwhile, the orientation difference of ferrite which is greater than 15 degree increase in 0.04%Nb steel, however, the orientation difference is mainly in the range of 5-15 degree when there is no Nb steel.what's more,solute Nb also made the morphology of transformed pearlite changed obviously, namely the aspect ratio of cementite reduced markedly. The analysis of transformation kinetics shows that 0.04%Nb could raise the nose temperature of TTT curve by about 50℃ and retard the eutectoid transformation start time for about one order of magnitude, which imply that addition of Nb retard the nose temperature and the pearlite transformation at lower temperature, then the hardenability is improved.To obtain the less aspect ratio of cementite the process of contunous spheroidization was study, by confirming the best deformation temperature via the deformation narby the A1 temperature, This for quenched/tempered steel, that obtaining the tempered sorbite, equivalent to direct realize the real meaning non-quenched/tempered.According to consider C content in the impact of solubility product formula of NbC in austenite,the precipitation kinetics of NbC in austenite with deferent C content was calculated at the three deferent mechanisms(homogeneous nucleation,grain boundary nucleation, dislocation nucleation).And the effects of C contents on critical nucleation diameter,critical nucleation energy,nucleate ratio,nose temperature of PTT cruves of NbC was calculated and analysed.Additional,the effect of stored deformation energy on the PTT also was considered.