The Thermodynamic Of Carbon Partitioning And Its Application In Q&P Steel

Q&P steel is a kind of new high-strength steel with high elongation. Its excellent performance is mainly from a mixed phase structure of martensite and austenite in steels. Martensite and solid solution strengthening alloying elements contribute to the high strength, while the high plasticity depends on the quantity and distribution of the residual austenite. Thus Q&P steel character to high yield strength, high tensile strength, high strain hardening rate and relatively high elongation. During the Q&P process, carbon partitioning is the most crucial process. Carbon diffusing from the supersaturated martensite into residual austenite results in carbon-rich austenite which can greatly improves the stability of the austenite so that austenite can exist stably at room temperature. Carbon partitioning between martensite and retained austenite is a spontaneous process during which the free energy of the whole system reduces. How much carbon partitions into the austenite directly affects the final quantity and stability of austinite, thus ultimately affect the mechanical performance of steel. Therefore, the study of carbon partitioning process is necessary.The constrained paraquilibrium model established by Speer in Fe-C binary system describes the endpoint of carbon partitioning between quenched martensite and retained austenite. The model assumes a stationary a/y interface, and requires a uniformed chemical potential for carbon, but not iron. The precipitation of carbide is completely inhibited during the carbon partitioning process. With the experimental conditions are limited that the concentration of the carbon can only measured by atom probe tomography (ATP), the carbon concentration of martensite and residual austenite can not be directly measured, compared to the calculated results of the CPE model.A new model called CPEMA (constrained paraequilibrium between martensite and austenite) is founded on the basis of CPE model established by Speer, which take the effects of alloying elements and the dislocation of carbon on partitioning process into account in this paper. The model can predict the carbon content of quenched martensite and residual austenite in the steels cantaining multiple alloy elements with huge quantities of dislocations under the condition of constrained paraquilibrium. Combined the new model with computer which has huge capacity and fast calculation speed, we can numerically simulated the model using visual Visual Basic programming language, and make a foundation for developing thermodynamic software in Q&P process guiding for the actual production process.We designed a kind of steel treated by Q&P process in this study. The tensile strength, elongation, and other mechanical properties are measured using the universal electronic testing machine. After carbon partitioning, the strength of the first martensite reduces because it is carbon-depleted, while the strength of the second martensite increases because of carbon-richness. Moreover, with the extension of the time spended on partitioning, the strength of the first martensite declines since the number of dislocations reduce. By measuring the intensity of quenched martensite and the contribution of carbon reditrution and dislocation density to intensity of martensite, we can calculate the weighted tensile strength of martensite. The results show that the caculated values of tensile strength corresbonding to the actually measuread valus. All of these prove the correctness of the CPEMA model. When the alloying elements do not exist and there is no dislocation, the new model is the famous Speer CPE model. Speer model is a special case of the CPEMA model and the new CPE model can be considered as a general model about thermodynamics of Q&P process, without limited to Fe-C binary system.