【摘要】：Ti微合金鋼的微觀組織和力學性能與鋼中各類含Ti第二相顆粒析出行為密切相關。熱軋后連續(xù)冷卻過程中相變鐵素體中形成的相間析出碳化物在強化鐵素體基體方面起關鍵作用�；鼗疬^程中納米碳化物的析出行為及碳化物的變化特征,顯著影響實驗鋼的力學性能。淬火、回火處理以及兩相區(qū)變形在改變實驗鋼微觀組織形態(tài)的同時顯著影響了碳化物的析出特征,從而影響實驗鋼的力學性能。鐵素體中均勻分布的納米碳化物析出可以顯著提高實驗鋼強度,為此研究了Ti微合金鋼軋后連續(xù)冷卻過程中含Ti析出相的析出行為及析出特征。結果表明,連續(xù)冷卻過程中形成的大量含Ti碳化物主要在鐵素體中以相間沉淀形式析出,析出相的形成與冷卻速率和Ti含量密切相關。而添加Mo元素推遲并促進了鐵素體中納米碳化物析出,使沉淀強化作用更顯著。為了探究回火工藝對相間析出與過飽和析出的影響,研究了不同溫度、不同時間條件下Ti微合金鋼中納米碳化物的析出行為和析出特征。結果表明,回火后屈服強度變化與回火過程中納米碳化物的析出、長大和粗化行為有關,在高溫長時間等溫過程中這種含Mo的納米碳化物具有較好的熱穩(wěn)定性,且相間析出顆粒較過飽和析出顆粒具有更優(yōu)越的抗粗化能力,析出強化作用更明顯。淬火、回火處理通過改變微觀組織形態(tài)顯著改善了鋼的低溫韌性,但也同時對熱軋鋼板中形成的納米碳化物顆粒產生影響。韌性改善主要得益于板條貝氏體組織的形成；而淬火、回火過程雖然弱化了熱軋態(tài)形成納米碳化物的強化作用但卻使這些碳化物充分保留下來并促進了過飽和碳化物析出。為了充分發(fā)揮細晶強化與沉淀強化的綜合作用,研究了終軋溫度對微觀組織演變及碳化物析出行為的影響。第二階段變形延續(xù)至兩相區(qū)時,不僅能夠細化晶粒,而且能生成由等軸晶粒及亞晶粒組成的混合組織。隨變形溫度降低,組織細化明顯,因此提高了鋼的低溫韌性。降低變形溫度抑制了相間析出形成,但應變累積促進了低溫鐵素體中過飽和碳化物析出,在一定程度上發(fā)揮了析出強化效應。
[Abstract]:The microstructure and mechanical properties of Ti microalloyed steel are closely related to the precipitation behavior of the second phase particles containing Ti. The precipitation of carbides from phase change ferrite during continuous cooling after hot rolling plays a key role in strengthening ferrite matrix. The precipitation behavior of nano-carbides and the changing characteristics of carbides during tempering have a significant effect on the mechanical properties of experimental steels. Quenching, tempering and deformation in the two-phase region have a significant effect on the precipitation of carbides while changing the microstructure of the experimental steels, thus affecting the mechanical properties of the experimental steels. The precipitation of nano-carbide in ferrite can improve the strength of experimental steel significantly. The precipitation behavior and characteristics of precipitated phase containing Ti in Ti microalloyed steel during continuous cooling after rolling are studied. The results show that a large number of Ti carbides formed during continuous cooling are precipitated in ferrite in the form of interphase precipitation. The formation of precipitates is closely related to the cooling rate and the content of Ti. The addition of Mo element delayed and promoted the precipitation of nano-carbides in ferrite, which made the precipitation strengthening more obvious. In order to investigate the effect of tempering process on the precipitation between phases and supersaturation, the precipitation behavior and precipitation characteristics of nano-carbides in Ti microalloyed steel at different temperatures and different time were studied. The results show that the change of yield strength after tempering is related to the precipitation, growth and coarsening behavior of nano-carbides during tempering, and the nano-carbides containing Mo have better thermal stability during high temperature and long time isothermal process. The interphase precipitated particles have better anti-coarsening ability than supersaturated precipitated particles, and the precipitation strengthening effect is more obvious. Quenching and tempering significantly improved the toughness of steel at low temperature by changing the microstructure, but also affected the formation of nano-carbides in hot rolled steel. The toughness improvement is mainly due to the formation of the lath bainite structure, while the hardening and tempering process weakens the strengthening effect of the hot rolled nano-carbides, but it makes these carbides fully retain and promote the precipitation of supersaturated carbides. In order to give full play to the comprehensive effect of fine grain strengthening and precipitation strengthening, the effect of finishing rolling temperature on microstructure evolution and carbide precipitation behavior was studied. When the second stage deformation continues to the two-phase region, it can not only refine the grain, but also produce the mixed microstructure composed of equiaxed grain and sub-grain. With the decrease of deformation temperature, the microstructure is refined obviously, so the low temperature toughness of the steel is improved. Lowering the deformation temperature inhibits the formation of interphase precipitation, but the accumulation of strain promotes the precipitation of supersaturated carbides in ferrite at low temperature, thus exerting the precipitation strengthening effect to some extent.
【學位授予單位】：北京科技大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG142.1

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