| With the development of economy and society in our country, steel industry has been rapidly developed, people put forward higher requirements for the variety and quality of iron steel materials. Higher strength, higher toughness, lower cost and quantitative reduction are the future development direction of new generation steel. With the attention of environmental pollution and mineral resources depletion, reaearch and development of micro-alloyed high strength steel have been extensively studied. Fine grain strengthening and precipitation strengthening, which are the main ways to improve the mechanical properties, have been used in the development and production of micro-alloyed high strength steel. So, how to fully use the fole of micro-alloying elements in grain refinement and precipitation is the key problem of TMCP development. With the support of the fundamental research funds for the central universities of china (N110607006), this paper took the micro-alloyed steels, which are added Nb, Ti and Mo elements, as research object, and studied the high temperature deformation behavior of austenite and continuous cooling transformation behavior, ferrite transformation behavior and precipitation behavior of micro-alloy carbonitride by using of thermal simulation experiment, enriched and developed the basic theories of research on micro-alloyed steel, discussed the effect of precipitation on micro mechanical properties of ferrite, and also studied the influence of TMCP parameters on mechanical properties and precipitation behavior of micro-alloyed steel, provided theoretical basis for industrial production. Major researches and innovative achievements in this paper are as follows:(1) The studies of austenite high temperature deformation behavior of experimental steels were conducted by single pass compression experiments, and the effects of micro-alloying elements and deformation parameters on austenite dynamic recrystallization were analyzed, and the deformation resistance models of tested steels were established. The results showed that flow stress of austenite and the activation energy for dynamic recrystallization were enhanced by adding Nb, Ti and Mo micro-alloy elements. The activation energy for dynamic recrystallization of C-Mn steel, Ti micro-alloyed steel, Ti-Nb micro-alloyed steel and Ti-Nb-Mo micro-alloyed steel were351kJ/mol,458kJ/mol,483kJ/mol and497kJ/mol, respectively.(2) The austenite continuous cooling transformation behavior were conducted by continuous cooling phase transformation experiments. The continuous cooling transformation curves of experimental steels were established, and the effects of micro-alloying elements, cooling rate and deformation were studied. The results showed that Nb, Ti and Mo could improve the stability of austenite in continuous cooling process, inhibited ferrite and pearlite transformation, promoted the bainite transformation. Deformation decreased the stability of austenite and promoted ferrite transformation, made the C curve of ferrite transformation shift to left.(3) The effects of micro-alloying elements, holding time, cooling rate before coiling, coiling temperature, cooling rate after coiling and deformation on ferrite transformation and nano-precipitation behavior were investigated by using the thermal simulation experiment technology. The results indicated that at the same process conditions, Nb, Ti and Mo could refine the ferrite grain and formed a large number of nano-precipitate particles, which significantly increased the hardness of ferrite. When the isothermal temperature is640℃, the sheet spacing of interphase precipitation, size and volume fraction of ferrite increased with the increase of holding time. With the increasing of cooling rate before coiling, the size of ferrite and precipitate particles decreased and the hardness of ferrite increased. With the decreasing of coiling temperature, the size of ferrite grain decreased, the Vickers hardness of matrix first increased then decreased. When coiling temperature was640℃, it was highest. Interphase precipitation could be observed in ferrite when the coiling temperature were640℃and700℃. The sheet spacing of interphase precipitation increased with coiling temperature. With the increasing of cooling rate after coiling rate, microstructure of tested steel changed from ferrite and pearlite to bainite, the Vickers hardness of matrix decreased. Fine ferrite grain and a large number of nano-precipitates coule obtained in samller cooling rate after coiling. With the increasing of deformation, hardness of ferrite increased first then decreased, when deformation was0.22, the hardness of ferrite was highest. The sheet spacing of interphase precipitation increased with deformation.(4) Microscopic mechanical properties of ferrite with different precipitate particles were studied by nanoindentation experiments. The results showed that at same conditions, the nanohardness of ferrite for C-Mn steel and Ti-Nb steel were2.64GPa and4.19GPa. Nano-hardness of ferrite increased1.55GPa by nano-precipitate particles. When the coiling temperatures were600℃,640℃and700℃, the nano-hardness of ferrite were3.90GPa,4.19GPa and3.60GPa, respectively. Plateaus were formed in the early stages of load-depth curves for specimens with interphase precipitates. The variations of nanohardness in ferrite were different at different coiling temperatures. Nanohardness was highest near grain boundary and varied little in ferrite, when the coiling temperature was600℃. The nanohardness in ferrite almost was invariant when coiling temperature was640℃. The nanohardness decreased with distance from grain boundary, when coiling temperature was700℃.(5) Results of TMCP research indicated that tensile strength and yield strength first increased then desreased with the increasing of finishing rolling temperature. The strength was.highest when the coiling temperature was840℃. When the coiling temperature increased from460℃to675℃, tensile strength and yield strength first decreased then increased. When coiling temperature was675℃, because of high precipitation strengthening, the tested steel still had a good mechanical properties. Yield strength increased25-35MPa by adding Mo because of the finer grain size and precipitate particles. Yield strength and tensile strength were94MPa and54MPa haigher for insulation+furnace cooling than in asbestos. Different thickness of600MPa and700MPa micro-alloyed high strength steels have been successfully producted in one steel fractory. The steel products had an excellent mechanical properties, and can be strengthed more than300MPa by the nano-precipitate particles. |
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