| With the deterioration of environment and energy crisis in the world, the main trend of modern car industry is to reduce weight, conserve energy, protect environment and improve safety. The automakers propose higher demands on the performance, costs and lightweight of the automotive steels. For this reason, Professor Speer, from Colorado School of Mines, put forward a novel process for producing the automotive steels with excellent performance, named quenching and partitioning (Q&P). Through the heat treatment design, this process was first carried out on the low alloying steels to obtain the steels with lath martensite and carbon-enriched retained austenite. In view of the transformation induced plasticity effect of retained austenite, the simultaneous enhancement of strength and ductility can be realized in Q&P steels.In this regard, four kinds of low carbon Si-Mn steels are designed in this study. In order to increase the strength of the experimental steels by microstructural refinement and carbide precipitation, the single or combined addition of Nb, Ni and Cr have been adopted. The Q&P processes with different heat treatment parameters are performed to obtain the steels with different microstructures and different volume fraction of retained austenite. By the combined analysis of the microstructures and the mechanical properties of the experimental steels, the influences of the compositions and heat treatments on the microstructural evolution of the Q&P steels are revealed. Also, the differences on the deformation mechanism of the experimental steels with different microstructures are investigated. The main achievements are expressed below.(1) Four kinds of low carbon Si-Mn based Q&P steels with different contents of Nb, Ni and Cr were designed in the present study. In order to design the reasonable heat treatment parameters, the volume fraction of retained austenite in the experimental steels was predicted by the carbon constrained equilibrium (CCE) model. The experimental results indicated that the low carbon Si-Mn based Q&P steels exhibited a good combination of high strength and ductility. The ultimate tensile strength of the experimental steels was more than 1000MPa and the optimal elongation and product of strength and elongation were 15% and 16GPa%, respectively. However, the mechanical properties of the steels with Nb were significantly improved compared with the steel without Nb. The ultimate tensile strength exceeded 1100MPa (1220MPa for the highest strength) and the optimal elongation and product of strength and elongation were 18% and 20GPa%, respectively. However, the Nb-content in low carbon Q&P steels was not the more the better. Due to the carbon consumption of precipitation of NbC, the excessive Nb-content in steels could result in low carbon enrichment in austenite, which decreased the stability of austenite and, therefore, leading to the decrease in volume fraction of retained austenite. Finally, the total elongation and the product of strength and elongation of the experimental steels decreased.(2) The experimental steel with Cr and Ni exhibited significant improvement in the ultimate tensile strength (400-500MPa) of the experimental steels with a few decrease in elongation (2-5%). The addition of Cr and Ni could refine the original austenite grain size of the heat treated steel and resulted in the decrease in the size of martensite packets. The precipitation of Cr and the twinned martensite were observed in the experimental steel by TEM analysis. Due to the co-work of soluted Cr and Ni increasing austenite stability and precipitated CrC decreasing carbon content in austenite, there was no obvious change in the volume fraction of retained austenite in these two steels.(3) The effects of cooling styles after partitioning on the microstructures and mechanical properties of the experimental steels were investigated. The results showed that air-cooling after partitioning was beneficial to obtain carbide free bainite, which increased the carbon enrichment of austenite and increased the volume fraction of retained austenite in steels at room temperature. The microstructures of the experimental steels quenched after partitioning consisted of lath martensite and retained austenite. In contrast, except for the two above phases, the carbide free bainite was observed in the experimental steels air-cooled after partitioning. Compared with the quenched steels, the elongation of the air-cooled steels was remarkably improved with a certain decrease in the ultimate tensile strength (200MPa drop) and the optimal product of strength and elongation reached 24.33GPa%. The steels after these two cooling styles exhibited a two-stage work hardening behavior. Due to the high volume fraction of retained austenite, the air-cooled steels exhibited more excellent work hardening behavior than the quenched ones.(4) Annealing with different temperatures were carried out on the experimental steels to investigate the effect of annealing temperature on the microstructure and mechanical properties by SEM, EMPA and tensile tests. Also, the work hardening behavior was analyzed by Hollomon equation. The results showed that the microstructure of the intercritical annealed steels consisted of ferrite, martensite and retained austenite. During intercritical annealing process, austenite stabilizing elements, such as C and Mn, enriched in austenite, which played an important role in stabilizing austenite and, finally, increased the volume fraction of retained austenite in steels at room temperature. Compared with the complete austenitizing samples, the ultimate tensile strength of the intercritical annealed samples decreased due to the exsitence of ferrite, but the total elongation had been significantly improved and the optimal total elongation reached 24%. The product of strength and elongation for the intercritical annealed samples was much higher than that for the complete austenitizing samples.(5) In the consideration of the theory of cyclic phase transformation in the maraging martensitic steels, a novel design, the pre-quenching from complete austenitization before Q&P treatment, was performed on the low carbon Si-Mn based steels to produce the Q&Q-P steels with high strength and high ductility. The microstructures of the Q&Q-P and Q&P steels both consisted of ferrite, lath martensite and retained austenite. The differences were the the size of martensite packets in the Q&Q-P samples were reduced compared with the intercritically annealed Q&P steels and ferrite in the Q&Q-P samples exhibited lath shape instead of blocky shape in the intercritical annealed Q&P samples. XRD results showed that the volume fraction of retained austenite in the Q&Q-P samples was higher that that in the intercritical annealed Q&P samples. Also, the strength of the Q&Q-P samples had been improved and the optimal ultimate tensile strength reached 1150MPa. The total elongation of all the Q&Q-P samples excessed 20% and the optimal product of strength and elongation reached 30GPa%, which exhibited excellent performance amount the low carbon Si-Mn based Q&P steels.(6) Combined with the changes in the volume fraction of retained austenite and the instantaneous work hardening exponents during strain, the TRIP effect of retained austenite in the experimental steels was explicitly proved. The volume fraction of retained austenite and the TRIP effect of retained austenite during deformation in the steels by different Q&P processes were investigated. The results showed that the retained austenite fraction in the experimental steels depended on its compositions, size and shapes. However, not only was the TRIP effect of retained austenite depended on the above factors, but also on the matrix of the steels. For the steels with martensitic matrix, a large amount of retained austenite transformed to martensite at a small strain stage, which led to the unsatisfactory elongation of the experimental steels. In contrast, for the steels with complex matrix with martensite and ferrite (or carbide free bainite), the transformation of retained austenite was postponed due to the delayed stress concentration caused by the compatible deformation capability of ferrite. At the large strain stage, there was still some retained austenite which contributed to the TRIP effect and, finally, led to the enhanced elongation. |
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