本文選題：連鑄 + 微合金鋼��； 參考：《北京科技大學(xué)》2017年博士論文

【摘要】：自從鋼的微合金化技術(shù)問世以來,微合金化鋼在交通、建筑及機械制造等各個領(lǐng)域獲得了廣泛的應(yīng)用。鑒于鋼種成分的特殊性,鋼中第二相粒子的析出是微合金鋼質(zhì)量的主要制約因素。國內(nèi)外眾多研究者對微合金化鋼加熱及軋制過程進行了深入研究分析并取得了較好的應(yīng)用效果。然而,微合金化鋼軋制成品的質(zhì)量在很大程度上取決于其對應(yīng)鑄坯的凝固質(zhì)量。若連鑄過程冷卻工藝控制不當(dāng),極易造成連鑄坯產(chǎn)生裂紋、偏析等凝固缺陷并最終遺留至終軋成品造成軋材邊裂、帶狀組織等缺陷。因此,有必要針對微合金化鋼連鑄坯凝固冷卻過程中的凝固特性與組織性能進行研究,深入分析連鑄過程中的鑄坯組織演變規(guī)律與第二相粒子析出的相關(guān)作用關(guān)系,闡明連鑄坯凝固缺陷的形成機理,最終為微合金化鋼連鑄過程冷卻工藝優(yōu)化奠定基礎(chǔ)。為此,本文以釩微合金化鋼YQ450NQR1連鑄方坯為對象,針對其連鑄過程凝固特性與組織性能進行了系統(tǒng)研究與深入分析。針對YQ450NQR1鋼連鑄方坯的高溫性能,通過差示掃描量熱法結(jié)合計算材料學(xué)相關(guān)方法確定了YQ450NQR1鋼的液相線溫度為1511℃,固相線溫度為1466℃；在考慮微合金元素含量變化的條件下,運用熱模擬方法對不同溫度下YQ450NQR1鋼連鑄方坯的斷面收縮率及應(yīng)力-應(yīng)變曲線進行測定,研究發(fā)現(xiàn)：當(dāng)YQ450NQR1鋼中的[%V][%N]從1.5×10-3增加到2.02x 10-3時,奧氏體再結(jié)晶溫度從950℃提升至1000℃,鋼的塑性槽由700℃～917℃擴展至700℃-970℃。對于YQ450NQR1鋼,在熱制度不變的條件下,塑性槽的高溫端溫度受第二相粒子的開始析出溫度影響,塑性槽的深度受第二相粒子的體積分?jǐn)?shù)的影響,而塑性槽的寬度取決于大量晶內(nèi)鐵素體的形成溫度。針對YQ450NQR1鋼方坯在結(jié)晶器附近區(qū)域初始凝固過程中冷卻速率變化幅度大、內(nèi)裂紋敏感性高這一特點,結(jié)合實際鋼種成分和工藝條件建立了溶質(zhì)微觀偏析模型,并結(jié)合相關(guān)文獻及元素偏析度原位分析對該模型進行了驗證,分析了冷卻速率對方坯凝固過程溶質(zhì)元素C, Si, Mn, P, S的微觀偏析行為及凝固過程零強度溫度ZST,零塑性溫度ZDT和黏滯性溫度LIT的影響機制。在此基礎(chǔ)上,針對冷卻速率(CR)與零塑性溫度(ZDT),零強度溫度(ZST)以及黏滯性溫度(LIT)分別進行非線性擬合,建立了描述冷卻速率與各凝固特征溫度的定量模型并根據(jù)ZDT, ZST及LIT的相互關(guān)系提出了內(nèi)裂紋敏感性指數(shù)IICS,最終構(gòu)建了YQ450NQR1鋼方坯內(nèi)裂紋敏感性模型。針對YQ450NQR1鋼連鑄方坯矯直過程中表面橫裂紋發(fā)生率較高與非平衡固態(tài)相變的緊密聯(lián)系,通過Gleeble熱模擬實驗機、熱膨脹儀、高溫共聚焦顯微鏡、掃描電鏡結(jié)合能譜儀研究了YQ450NQR1鋼連鑄方坯冷卻過程中的γ→α相變和第二相粒子析出規(guī)律及其對高溫?zé)崴苄缘挠绊憴C制,闡述了冷卻速率變化對YQ450NQR1鋼固態(tài)相變的影響,運用多元非線性回歸方法得到了α析出量與冷卻速率、α析出所用時間及a相開始析出溫度之間的定量關(guān)系。通過研究可知,連鑄過程中微合金元素V對促進鐵素體析出的作用主要體現(xiàn)在兩個方面：(1)V(C,N)充當(dāng)孕育粒子促進了晶界鐵素體的形核。(2)V(C,N)析出過程中奧氏體晶界附近形成溶質(zhì)貧化區(qū),其存在導(dǎo)致奧氏體晶內(nèi)局部C元素濃度降低,促進了丫→α相轉(zhuǎn)變。為了研究YQ450NQR1鋼連鑄冷卻工藝參數(shù)對鑄坯凝固組織及溶質(zhì)元素偏析的影響作用,運用元胞自動機-有限元法建立了YQ450NQR1鋼方坯宏觀傳熱-凝固組織生長耦合模型,研究了不同鋼液過熱度、二次冷卻強度條件下方坯中心等軸晶率和平均晶粒尺寸的變化規(guī)律。在深入剖析YQ450NQR1鋼連鑄過程凝固特征及組織演變規(guī)律的基礎(chǔ)上,以優(yōu)化鑄坯凝固組織結(jié)構(gòu)與提高鑄坯表層微觀組織強度為目的,提出了YQ450NQR1鋼連鑄冷卻精益控制策略“低過熱度澆注+二冷三區(qū)弱冷十二冷四區(qū)強冷”,并進一步制定了相應(yīng)的控制方案,即“過熱度23℃(原過熱度37℃),二冷三區(qū)水量2.6L/min(原二冷三區(qū)水量51.5L/min),二冷四區(qū)水量165.6 L/min(原二冷四區(qū)水量18.4 L/min) ",構(gòu)建了基于YQ450NQR1釩微合金化鋼特性的連鑄二次冷卻調(diào)控新機制。綜合考慮到Y(jié)Q450NQR1鋼在后續(xù)軋制過程中變形復(fù)雜,流變應(yīng)力變化劇烈的特點,通過熱模擬實驗獲得了不同的壓下量(30%、60%)、應(yīng)變速率(1/s、10/s、30/s)和變形溫度條件下的YQ450NQR1鋼流變應(yīng)力曲線。在此基礎(chǔ)上,建立了YQ450NQR1鋼流變應(yīng)力預(yù)測模型。通過與實驗數(shù)據(jù)進行對比分析可知,預(yù)測流變應(yīng)力值與相同條件下的實測值之間相關(guān)性系數(shù)為0.998,平均相對誤差為8.40%。模型的預(yù)測精度較高,可為YQ450NQR1鋼熱軋過程軋制力的確定及熱軋工藝參數(shù)的制定提供參考。
[Abstract]:Since the advent of the microalloying technology of steel, microalloyed steel has been widely used in various fields such as transportation, construction and mechanical manufacturing. In view of the particularity of the steel composition, the precipitation of the second phase particles in the steel is the main restriction factor for the quality of the microalloyed steel. However, the quality of the finished products of microalloyed steel depends largely on the solidification quality of the slab. If the cooling process is not properly controlled in the continuous casting process, it is very easy to cause cracks in continuous casting billets, segregation and other solidification defects. Therefore, it is necessary to study the solidification and microstructure of the continuous casting billet during the solidification and cooling process of the microalloyed steel, and analyze the relationship between the microstructure evolution and the second phase particle precipitation in the continuous casting process. The formation mechanism of the solidification defects of Chan Minglian casting billet is the final microalloy. In this paper, the solidification and microstructure of the continuous casting process of the vanadium microalloyed steel YQ450NQR1 is studied and analyzed in this paper. In this paper, the high temperature properties of the continuous casting of YQ450NQR1 steel are analyzed. The correlation of the material science and the differential scanning calorimetry (differential scanning calorimetry) is used to calculate the material science. It is determined that the liquidus temperature of YQ450NQR1 steel is 1511, and the solid phase temperature is 1466. Under the condition of the change of the element content of the Microalloy, the thermal simulation method is used to determine the section shrinkage and stress strain curve of the continuous casting billet of YQ450NQR1 steel at different temperatures. It is found that when the [%V][%N] of YQ450NQR1 steel is from 1.5 The recrystallization temperature of the austenite is increased from 950 to 1000, and the plastic groove of the steel is expanded from 700 to 917 C to 700 -970 C at 2.02x 10-3. For YQ450NQR1 steel, the temperature of the high temperature end of the plastic groove is affected by the starting precipitation temperature of the second phase particles under the condition of the heat system unchanged. The depth of the plastic groove is affected by the second phase particles. With the effect of volume fraction, the width of the plastic groove depends on the formation temperature of a large number of intragranular ferrite. In the light of the large variation of the cooling rate and the high sensitivity of the internal crack in the initial solidification process of the YQ450NQR1 steel billet near the crystallizer, the micro segregation model of the solute is established by combining the actual steel composition and the technological conditions. The model was verified by the related literature and in situ analysis of element segregation degree. The microsegregation behavior of the solute elements C, Si, Mn, P, S in the cooling rate of the blank and the mechanism of the zero strength temperature ZST, the zero plastic temperature ZDT and the viscosity temperature LIT were analyzed. On this basis, the cooling rate (CR) and the zero plasticity were applied to the solidification process. The temperature (ZDT), zero strength temperature (ZST) and viscosity temperature (LIT) are nonlinear fitting, and a quantitative model describing the cooling rate and the characteristic temperature of each solidification is established. The internal crack sensitivity index IICS is proposed according to the relationship between ZDT, ZST and LIT. At the end, the sensitivity model of the internal crack in the square billet of YQ450NQR1 steel is constructed. In the process of straightening 0NQR1 steel, the incidence of surface transverse cracks is closely related to the non-equilibrium solid phase transformation. Through Gleeble thermal simulation test machine, thermal dilatometer, high temperature confocal microscope, scanning electron microscope combined energy spectrum analyzer, the law of gamma, alpha and second phase particles precipitation in the cooling process of YQ450NQR1 steel continuous casting billets is studied. The effect of the change of cooling rate on the solid phase transformation of YQ450NQR1 steel is expounded. The quantitative relationship between the amount of precipitation and the cooling rate, the time used for the precipitation and the starting temperature of the a phase are obtained by the multiple nonlinear regression method. Through the study, it is known that the microalloy element V is used to promote the iron in the process of continuous casting. The role of the precipitation of the vegetarian body is mainly embodied in two aspects: (1) V (C, N) acts as an inoculation particle to promote the nucleation of the grain boundary ferrite. (2) V (C, N) precipitates the solute dilution zone near the austenite grain boundary during the precipitation process, which leads to the decrease of the local C element concentration in the austenite grain and the transition of the bifurcation to the alpha phase. In order to study the continuous casting and cooling of YQ450NQR1 steel The effect of process parameters on the solidification structure and segregation of solute elements of the cast billet was affected. By using cellular automata finite element method, the coupling model of macro heat transfer and solidification structure of YQ450NQR1 steel billet was established, and the change regularity of the central equiaxial grain rate and average grain size under the two cooling strength conditions was studied. On the basis of in-depth analysis of solidification characteristics and microstructure evolution of YQ450NQR1 steel continuous casting process, in order to optimize the solidification structure of billet and improve the microstructure strength of the surface layer, the lean control strategy of continuous casting and cooling of YQ450NQR1 steel "low overheat pouring + two cold three zone and twelve cold four zone cold" is put forward, and further formulation is made. The corresponding control scheme, namely, "over heat 23 (37 degrees C), two cold and three region water 2.6L/min (original two cold three area water 51.5L/min), two cold four region water volume 165.6 L/min (original two cold four zone water 18.4)", constructed a new mechanism of two cooling regulation of continuous casting based on the characteristics of YQ450NQR1 vanadium microalloyed steel. Comprehensive consideration of YQ450NQR1 In the following rolling process, the deformation of the steel is complicated and the change of the rheological stress is intense. The rheological stress curves of the YQ450NQR1 steel under the conditions of 1/s, 10/s, 30/s and the deformation temperature are obtained by the thermal simulation experiment. On this basis, the prediction model of the rheological stress of YQ450NQR1 steel is established. According to the comparative analysis, the correlation coefficient between the predicted rheological stress values and the measured values under the same conditions is 0.998, and the average relative error is higher in the 8.40%. model, which can provide reference for the determination of rolling force in the hot rolling process of YQ450NQR1 steel and the formulation of hot rolling process parameters.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TF777

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