钒微合金化钢连铸方坯凝固特性与组织性能研究

发布时间：2018-06-06 21:03

本文选题：连铸 + 微合金钢　；参考：《北京科技大学》2017年博士论文

【摘要】：自从钢的微合金化技术问世以来,微合金化钢在交通、建筑及机械制造等各个领域获得了广泛的应用。鉴于钢种成分的特殊性,钢中第二相粒子的析出是微合金钢质量的主要制约因素。国内外众多研究者对微合金化钢加热及轧制过程进行了深入研究分析并取得了较好的应用效果。然而,微合金化钢轧制成品的质量在很大程度上取决于其对应铸坯的凝固质量。若连铸过程冷却工艺控制不当,极易造成连铸坯产生裂纹、偏析等凝固缺陷并最终遗留至终轧成品造成轧材边裂、带状组织等缺陷。因此,有必要针对微合金化钢连铸坯凝固冷却过程中的凝固特性与组织性能进行研究,深入分析连铸过程中的铸坯组织演变规律与第二相粒子析出的相关作用关系,阐明连铸坯凝固缺陷的形成机理,最终为微合金化钢连铸过程冷却工艺优化奠定基础。为此,本文以钒微合金化钢YQ450NQR1连铸方坯为对象,针对其连铸过程凝固特性与组织性能进行了系统研究与深入分析。针对YQ450NQR1钢连铸方坯的高温性能,通过差示扫描量热法结合计算材料学相关方法确定了YQ450NQR1钢的液相线温度为1511℃,固相线温度为1466℃；在考虑微合金元素含量变化的条件下,运用热模拟方法对不同温度下YQ450NQR1钢连铸方坯的断面收缩率及应力-应变曲线进行测定,研究发现：当YQ450NQR1钢中的[%V][%N]从1.5×10-3增加到2.02x 10-3时,奥氏体再结晶温度从950℃提升至1000℃,钢的塑性槽由700℃～917℃扩展至700℃-970℃。对于YQ450NQR1钢,在热制度不变的条件下,塑性槽的高温端温度受第二相粒子的开始析出温度影响,塑性槽的深度受第二相粒子的体积分数的影响,而塑性槽的宽度取决于大量晶内铁素体的形成温度。针对YQ450NQR1钢方坯在结晶器附近区域初始凝固过程中冷却速率变化幅度大、内裂纹敏感性高这一特点,结合实际钢种成分和工艺条件建立了溶质微观偏析模型,并结合相关文献及元素偏析度原位分析对该模型进行了验证,分析了冷却速率对方坯凝固过程溶质元素C, Si, Mn, P, S的微观偏析行为及凝固过程零强度温度ZST,零塑性温度ZDT和黏滞性温度LIT的影响机制。在此基础上,针对冷却速率(CR)与零塑性温度(ZDT),零强度温度(ZST)以及黏滞性温度(LIT)分别进行非线性拟合,建立了描述冷却速率与各凝固特征温度的定量模型并根据ZDT, ZST及LIT的相互关系提出了内裂纹敏感性指数IICS,最终构建了YQ450NQR1钢方坯内裂纹敏感性模型。针对YQ450NQR1钢连铸方坯矫直过程中表面横裂纹发生率较高与非平衡固态相变的紧密联系,通过Gleeble热模拟实验机、热膨胀仪、高温共聚焦显微镜、扫描电镜结合能谱仪研究了YQ450NQR1钢连铸方坯冷却过程中的γ→α相变和第二相粒子析出规律及其对高温热塑性的影响机制,阐述了冷却速率变化对YQ450NQR1钢固态相变的影响,运用多元非线性回归方法得到了α析出量与冷却速率、α析出所用时间及a相开始析出温度之间的定量关系。通过研究可知,连铸过程中微合金元素V对促进铁素体析出的作用主要体现在两个方面：(1)V(C,N)充当孕育粒子促进了晶界铁素体的形核。(2)V(C,N)析出过程中奥氏体晶界附近形成溶质贫化区,其存在导致奥氏体晶内局部C元素浓度降低,促进了丫→α相转变。为了研究YQ450NQR1钢连铸冷却工艺参数对铸坯凝固组织及溶质元素偏析的影响作用,运用元胞自动机-有限元法建立了YQ450NQR1钢方坯宏观传热-凝固组织生长耦合模型,研究了不同钢液过热度、二次冷却强度条件下方坯中心等轴晶率和平均晶粒尺寸的变化规律。在深入剖析YQ450NQR1钢连铸过程凝固特征及组织演变规律的基础上,以优化铸坯凝固组织结构与提高铸坯表层微观组织强度为目的,提出了YQ450NQR1钢连铸冷却精益控制策略“低过热度浇注+二冷三区弱冷十二冷四区强冷”,并进一步制定了相应的控制方案,即“过热度23℃(原过热度37℃),二冷三区水量2.6L/min(原二冷三区水量51.5L/min),二冷四区水量165.6 L/min(原二冷四区水量18.4 L/min) ",构建了基于YQ450NQR1钒微合金化钢特性的连铸二次冷却调控新机制。综合考虑到YQ450NQR1钢在后续轧制过程中变形复杂,流变应力变化剧烈的特点,通过热模拟实验获得了不同的压下量(30%、60%)、应变速率(1/s、10/s、30/s)和变形温度条件下的YQ450NQR1钢流变应力曲线。在此基础上,建立了YQ450NQR1钢流变应力预测模型。通过与实验数据进行对比分析可知,预测流变应力值与相同条件下的实测值之间相关性系数为0.998,平均相对误差为8.40%。模型的预测精度较高,可为YQ450NQR1钢热轧过程轧制力的确定及热轧工艺参数的制定提供参考。
[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.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TF777

【相似文献】