耦合糊状区变形的模铸钢锭宏观偏析数值模拟与实验研究

As a typical casting defect of steel ingots and castings, macrosegregation can not be eliminated via the downstream processes, and hence will cause a low qualification rate, low material utilization and poor quality stability. In terms of the advantages of controlling the common macrosegregation, shrinkage cavity and coarse grains in special steels by a novel processing of forging the solidifying ingot before its complete solidification, this project aims to provide a detailed study on the macrosegregation behavior coupling of mush deformation based on both numerical simulation and experimental characterization. First, a series of semisolid compression experiments of H13 low-alloy steel with a variable temperature, strain, and strain rate by Gleeble machine will be carried out, and hence the constitutive model of the mush can be built via the regression method. Then, based on our previous macrosegregation model and code, the mush and solid deformations induced by the external force, the solid movement of globular grains and the thermo-solutal natural convection will be coupled and solved in a new two-phase and multicomponent macrosegregation model. In addition, the dissection and characterization experiments of macrosegregation in the industrial steel ingots with different external loadings during solidification will be performed, and the distribution features and details of various macrosegregation types such as positive segregation, negative segregation, A-segregate and even V-segregate caused by the mush deformation can be analyzed. Finally, a full comparison between the simulation and experimental results of the dissected steel ingots will be carried out, by which the macrosegregation induced by mush deformation can reappear, and its evolution and formation mechanism can be also revealed. The current research is not only beneficial to the process optimization of the novel semisolid forming technology of special steel ingot, but also to extending the current macrosegregation theory and control methods.

宏观偏析是钢锭的常见缺陷之一，严重影响材料组织与性能的均匀性，甚至直接决定产品的最终质量。鉴于铸锻一体化软芯锻造技术在控制特殊钢偏析、疏松和粗晶等方面的潜在优势，本项目拟对未完全凝固的特殊钢模铸锭施加外力载荷，并基于数值模拟和实验表征，系统研究耦合糊状区变形的钢锭内部“流动-凝固-传热-传质”行为。设计一系列不同温度、应变量、应变速率下的H13低合金钢的半固态压缩实验，建立钢中糊状区超高温流变本构方程；基于经典宏观偏析模型，开发耦合糊状区变形、晶粒移动和自然对流等协同作用下的多组元偏析新模型。解剖并表征自然冷却与半凝固态变形下的工业级钢锭，掌握外力作用下的宏观偏析演化行为和特征。运用建立的宏观偏析新模型对实验钢锭进行数值模拟，再现不同加载方式下的钢中宏观偏析分布规律，揭示其形成原因、过程和机理，阐释特殊钢钢锭软芯锻造过程中偏析控制的原理和途径，推动宏观偏析理论的进一步发展。

宏观偏析是钢的常见缺陷，严重影响组织与性能的均匀性，甚至直接决定产品的最终质量。本项目基于数值仿真和实验表征，对固液两相区变形下的宏观偏析演化规律和行为进行了系统研究。通过不同加载方式、不同固相分数下的H13钢热模拟机两相区变形实验，构建了耦合液相分数的半固态本构方程。随着施加变形时固相分数的增加，两相区变形机制由滑移变形转变为塑性变形，而钢锭中心枝晶搭接以及变形作用下的凝固加速效应抑制了液体外排，使得元素分布更均匀。同时，百公斤级铁皮包套缩比件实验表明，相比于高液相分数，低液相分数变形促进了宏/微观组织和溶质均匀化，且有效抑制了一次碳化物的析出。上述结论在12Cr1MoV钢和M50高温轴承钢中同样适用。数值计算方面，开发了耦合变形的两相流多晶相场模型，实现了介观尺度下凝固过程中枝晶生长/变形/运动/断裂和两相区流动的模拟仿真。发现枝晶断裂经历了晶粒弯曲和晶界润湿两个阶段，而外加载荷加剧了局部液体从枝晶间挤出，导致严重的溶质富集和贫乏，半固态实验中观察到的排液现象证实了这一结论。此外，开发了耦合糊状区变形的宏观偏析计算模型，实现了温度场、成分场、应力应变场多物理场耦合计算，明确了开始施加变形时两相区液相分数的关键作用。高液相分数凝固变形时，钢液往端部和侧壁流动，导致中心区域溶质贫乏，H13钢中心区域成分最大降至0.23wt%。最后，基于钢锭液芯率计算、超高温热送及液芯锻造仿真，明确了初始锻造下的最佳液相分数约为10%，制备的19t液芯锻造钢锭的成分、组织和力学性能均匀性均达到使用要求。这一研究系统揭示了糊状区变形对钢锭偏析的影响规律和机制，对短流程铸锻一体化高品质钢制备提供了重要支撑，拓展了半固态理论内涵和应用领域。