【摘要】：以微合金化鋼S355及新鋼特厚板坯連鑄機為研究對象,首先通過熱膨脹試驗研究了冷卻速度對組織轉(zhuǎn)變規(guī)律的影響、通過高溫拉伸試驗研究了溫度履歷對鋼種熱塑性的影響,分析得到了 S355鋼第三脆性區(qū)的高溫脆性機理、明確了冷卻速率及冷卻溫度對高溫組織及塑性的影響規(guī)律;其次通過冷卻后返溫再降溫試樣的高溫拉伸試驗研究了返溫制度下溫度履歷對鋼種熱塑性的改善效果,提出了特厚板坯連鑄強冷最佳冷卻速率及冷卻溫度;最后在鋼種連續(xù)冷卻過程中的相變規(guī)律及高溫塑性特征的研究基礎(chǔ)上,以鑄坯凝固傳熱計算分析為基礎(chǔ),提出了特厚板坯連鑄機垂直段與彎曲段強冷的新工藝,并以鑄機二冷噴淋系統(tǒng)的結(jié)構(gòu)分析為基礎(chǔ)提出了強冷區(qū)域的噴嘴選型優(yōu)化,制定了特厚板坯連鑄二冷強冷工藝試驗方案并組織了生產(chǎn)驗證試驗,獲得了細化鑄坯表層組織、降低表面裂紋率的效果。以微合金鋼S355為研究對象,利用熱膨脹分析儀、金相顯微鏡、掃描電鏡、透射電鏡及高溫激光共聚焦顯微鏡等設(shè)備研究了微合金鋼S355在連續(xù)冷卻過程中微觀組織的轉(zhuǎn)變規(guī)律,研究表明,微合金鋼S355發(fā)生貝氏體轉(zhuǎn)變的臨界冷卻速率為2℃/s、發(fā)生馬氏體相變的臨界冷卻速率為15℃/s,而當冷卻速率達到7℃/s以上時不發(fā)生珠光體相變;原始奧氏體晶粒尺寸不均勻很容易造成混晶組織的出現(xiàn)。利用Gleeble熱模擬設(shè)備系統(tǒng)研究了不同溫度履歷條件下微合金鋼S355的高溫塑性行為、利用掃描電鏡和透射電鏡觀察了斷口組織與析出物分布,研究表明:原始奧氏體晶界處形成的薄膜狀鐵素體是導致S355鋼第三脆性區(qū)塑性低谷的首要因素;經(jīng)強冷-返溫-再冷卻過程后,材料的熱塑性明顯提高,且斷口附近網(wǎng)狀薄膜鐵素體明顯減少、奧氏體晶粒內(nèi)部形成了鐵素體;大多數(shù)析出物在晶內(nèi)均勻分布,位錯交叉位置析出物尺寸較大。應用數(shù)學模型模擬分析了強冷工藝對連鑄坯表面溫度的影響,校核了現(xiàn)有的扇形段噴淋系統(tǒng)、優(yōu)化了強冷區(qū)的噴嘴選型,確定了連鑄機內(nèi)強冷的最佳位置及冷卻速度等強冷參數(shù),提出了二冷強冷工藝方案并組織了生產(chǎn)試驗。試驗結(jié)果證明,鑄坯表面溫度在強冷后達到了 720℃以下,鑄坯的表層組織得到了細化、晶粒度均勻性得到提高,基本消除了粗大的枝晶結(jié)構(gòu),強冷鑄坯的表層晶粒度達到了 11級、表層細晶區(qū)深度達到了 5mm,同時鑄坯的表面裂紋率由原來的7.53%降低到了 3.29%,實現(xiàn)了鑄坯表層組織和表面裂紋控制的目的。強冷鑄坯的對比、熱送軋制試驗結(jié)果說明,特厚板坯及鋼板表層晶粒度得以細化、特厚鋼板的心部質(zhì)量得到了顯著改善,說明強冷工藝的開發(fā)及強冷區(qū)的噴淋系統(tǒng)改造取得了成功。形成的創(chuàng)新點如下:1)提出并驗證了利用二冷局部強冷工藝,細化鑄坯表面組織、降低裂紋發(fā)生率的技術(shù)思想;2)揭示了 S355鋼先共析鐵素體膜及其厚度是影響鋼種第三脆性區(qū)塑性的關(guān)鍵因素,通過鑄坯熱履歷的控制,實現(xiàn)了鑄坯表面鐵素體膜及析出物的有效控制;3)以S355鋼特厚板坯連鑄為對象,開展了工業(yè)規(guī)模的二冷局部強冷試驗,證明了特厚板表面鐵素體化控制工藝的可行性。
[Abstract]:The effect of cooling rate on the law of microstructure transformation was studied by thermal expansion test. The effect of temperature record on the thermal plasticity of steel was studied by thermal expansion test. The mechanism of high temperature brittleness in third brittle zone of S355 steel was obtained, and the cooling rate was clarified. The effect of rate and cooling temperature on the microstructure and plasticity of high temperature was studied. Secondly, the improvement effect of temperature on the thermal plasticity of steel was studied by the high temperature tensile test of the back temperature and temperature drop after cooling, and the best cooling rate and cooling temperature of the heavy plate slab continuous casting were put forward. On the basis of the study of the phase transition and high temperature plastic characteristics, based on the calculation and analysis of the solidification heat transfer of the cast billet, a new technology for the strong cold of the vertical section and the bending section of the thick slab caster is put forward. Based on the structure analysis of the two cold spray system of the casting machine, the selection and optimization of the nozzle of the strong cold zone are put forward, and the two cold strength of the thick slab continuous casting is formulated. The cold process test scheme and the production verification test were organized. The effect of refining the surface structure and reducing the surface crack rate was obtained. Microalloy steel S355 was used as the research object. The microalloy steel S355 was studied by the thermal expansion analyzer, metallographic microscope, scanning electron microscope, transmission electron microscope and high temperature laser confocal microscope. However, the transformation law of microstructures in the process shows that the critical cooling rate of bainite transformation in microalloy steel S355 is 2 /s, and the critical cooling rate of martensitic transformation is 15 C /s, while the phase transformation of pearlite does not occur when the cooling rate is above 7 C /s, and the unhomogeneous grain size of the original austenite is easy to cause the mixed crystal group. The Gleeble thermal simulation equipment system was used to study the high temperature plastic behavior of microalloy steel S355 under different temperature records. The distribution of fracture tissue and precipitates was observed by scanning electron microscope and transmission electron microscope. The study showed that the thin film ferrite formed at the grain boundary of the original austenite resulted in the low plasticity of the third brittle zone in the S355 steel. The main factor of the valley is that the thermal plasticity of the material is obviously improved after the strong cold return temperature recooling process, and the ferrite in the net film near the fracture surface is obviously reduced. The ferrite is formed inside the austenite grain; most of the precipitates are evenly distributed in the crystal and the dislocations are larger in the intersection position. The strong cold work is simulated and analyzed by the mathematical model. The effect of art on the surface temperature of continuous casting billet is checked. The existing fan section spray system is checked, the nozzle selection of the strong cold zone is optimized, the best position and cooling speed of the strong cooling in the continuous casting machine are determined. The two cold strong cooling process scheme is put forward and the production test is organized. The test results show that the surface temperature of the slab is reached after the strong cold. Under 720 centigrade, the surface structure of the slab is refined, the grain degree uniformity is improved, the coarse dendrite structure is basically eliminated, the surface grain size of the strong cold cast billet reaches 11, the depth of the surface fine crystal area reaches 5mm, and the surface crack rate of the slab is reduced from 7.53% to 3.29%, and the surface structure of the slab is realized and the surface structure of the cast billet is realized. The purpose of control of surface crack. Contrast of strong cold cast billet, the result of hot rolling test shows that the grain size of the thick slab and steel plate can be refined, and the quality of the heart of the special thick steel plate has been greatly improved. It shows that the development of the strong cold process and the renovation of the spray system in the strong cold zone have been successful. The innovation points are as follows: 1) put forward and verified. Two cold local strong cooling technology is used to refine the surface structure of the slab and reduce the rate of crack occurrence. 2) it is revealed that the preeutectoid ferrite film and its thickness of S355 steel are the key factors affecting the ductility of the steel third brittle zone, and the effective control of the ferrite film and precipitates on the surface of the billet is realized through the control of the heat record of the billet. 3) 55 steel thick slab continuous casting is taken as an example. An industrial scale two cold local cold test is carried out, which proves the feasibility of the ferrite control technology on the surface of the heavy plate.
【學位授予單位】：鋼鐵研究總院
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TF777.1

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