| The service life of engineering machinery and equipment can be improved by using higher strength wear resistance plate. It’s also meaningful to reduce the equipment weight and save materials and energy. Compared with world famous wear resistance steel plates, domestic products are not strong enough and less of toughness in Chinese market. Therefore, Chinese Ministry of Science and Technology supported a high technology research and development program to realized technical breakthrough in the manufacture of wear resistance steel. This thesis focused on the laboratory R&D of NM600 grade wear resistance steel plate. Based on this, microstructure control technology was explored through composition adjustment and process parameters control. As a result, steel plate with ultrahigh strength, good toughness and better wear resistant ability was obtained, and the mechanism of this microstructure control technology was discussed as well. The details are as follows:The medium carbon (-0.4%C) steels with 0.030%,0.062% (weight percentage) Nb and without Nb were studied by Thermo-Calc software, thermal dilation method, thermal-mechanical simulation and laboratory trail-produce. Nb addition was proved to impede the grain growth of prior austenite and restrain the recrystallization of deformed austenite grains, which are both significant to refine the final microstructure. Experimental steel plates, prepared by two-stage hot rolling and water quenching after 900℃ austenization, were tempered at 200～400℃. Mechanical properties test showed that 200℃ tempered 0.062%-Nb steel reached the requirements for NM600 grade ultrahigh strength wear resistance steel plate.Steel with 0.062% Nb was processed by two-stage control rolling followed by water cooling, low temperature relaxation of super cooled austenite, direct partitioning (TMCP-DP, Thermo-mechanical rolling process-Direct partitioning). Steel plate with up to 2200 MPa strength,620 HBW hardness and 12% elongation was obtained. On the basis of mechanical properties test, the microstructures of the tested steel at different states were characterized by means of OM, SEM, TEM and XRD. During the direct partitioning after low temperature relaxation, the slow transformation of the super cooled austenite phase to martensite phase allows the partitioning of carbon from a to y phase, and that will stabilize the retained austenite (RA) to room temperature. In addition, the observed microstructures of all the experimental samples in this work are consist of lath martensite and slight twinning structure.Nanostructured carbide precipitates with size range of 20-50 nm, which contain Nb, Ti and Mo were observed in the investigated ultrahigh strength martensitic steel obtained by TMCP-DP, and confirmed to be MC-type carbides with B1 structure. The compositional characteristics revealed by Energy dispersive X-ray (EDX) spectrometer mapping and three-dimensional atom probe tomography (APT) reconstruction indicated that Mo evenly incorporated into the precipitates without enrichment in the carbide-matrix interface and probably substituted for Nb and Ti to form the (Nb,Ti,Mo)C carbides.Gleeble-3500 was utilized to conduct the TMCP-DP and one-step Q&P (Quenching & Partitioning) process. The results show that with the decrease of cooling rate in DP, the volume fraction of obtained RA increased to higher value at first, and then decreased, and the change of which with the increase of partitioning time is similar in one-step Q&P. Higher carbon content of austenite was detected at higher RA volume fraction after both processes, which proved the occurrence of carbon partitioning. Steel samples with different RA content were studied by impact wear tester. The results showed that the weight loss after wear increased slightly with the RA content increasing. That’s an evidence that RA phase was harmful to the wear resistant ability. The wear mechanism of the studied samples supposed to be brittle spalling and fatigue, which caused by the continuously impact of the abrasive particles to the steel surface. Considered the benefits of RA to toughness, it’s suggested to keep RA phase in the final microstructure. |
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