大锻件热加工过程混晶形成机理及控制研究

The mixed crystals, as the main microstructure defects of heavy forgings, degrade the forgings properties significantly, if severely, which cause waste product. The project takes 30Cr2Ni4MoV steel used for rotor as the research project. The refining mechanism and thermodynamics conditions of coarse grains and second phases during as-cast microstructure transforming into forging microstructure are studied in this project to determine the technology conditions of as-cast microstructure transforming into forging microstructure completely by integrating physical simulation and numerical simulation and technology tests. Meanwhile, the quantitative relationships between grain nonuniform degree in the inner of forgings and deformation temperature and loading way and forging ratio and nonuniform deformation degree and so on in forging process are studied to ascertain formation mechanism of mixed crystals. The technology tolerance condition is obtained under certain condition of mixed crystals. The phase transformation law and grain genetic mechanism of forging microstructure under the different heat treatment technology are studied to determine the acceptable tolerance of heat treatment after forging on original forging microstructure and high efficient heat treatment method after forging of cutting off structure heredity. The breakthrough in the key scientific problems of formation mechanism of mixed crystals and technology tolerance condition and so on in the primary progress of hot processing of heavy forgings is achieved, which provides scientific basis for adjustment and control of mixed crystals in the process of hot processing of heavy forgings. There is important scientific significance and application value to improve the manufacturing quality and level of heavy forgings.

混晶作为大锻件主要组织缺陷，会使锻件性能显著降低、严重时造成废品。本项目以转子钢30Cr2Ni4MoV为研究对象，采用物理模拟、数值模拟及工艺试验集成方法，研究钢锭铸态组织向锻态组织转变过程中粗大晶粒和第二相的碎化机制和热力学条件，确定其完全转变为锻态组织的工艺条件。研究成形锻造过程中锻件内部晶粒不均匀程度与变形温度、加载方式、锻比及不均匀变形程度等参数之间的定量关系，探明混晶形成机理，获得一定混晶度条件下工艺容限条件。研究锻态组织在不同热处理工艺条件下的相变规律及晶粒遗传机理，确定锻后热处理对原始锻造组织的接受容限及切断组织遗传的高效锻后热处理方法。在大锻件热加工主要环节中的混晶形成机理和工艺容限条件等关键科学问题上取得突破，为大锻件热加工过程的混晶调控提供科学依据，对于提高大锻件的制造质量与水平具有重要的科学意义和应用价值。

混晶作为大锻件主要组织缺陷，会使锻件性能显著降低、严重时造成废品。本项目以转子钢30Cr2Ni4MoV作为研究对象，采用物理模拟、数值模拟及工艺实验相结合的方法，建立了30Cr2Ni4MoV钢铸态材料再加热过程和热变形过程晶粒演变模型、热锻过程塑性本构关系模型、动态再结晶模型、静态再结晶模型及晶粒长大模型，深入研究钢锭铸态组织压实锻造、多火次成形锻造及热处理过程等热加工全流程中主要组织演变机制，获得了研究结果如下：（1）借助于本项目所提出的基于多样本检测的混晶评价方法，获得了30Cr2Ni4MoV钢铸态组织均匀化的加热工艺条件：加热温度1250℃、保温时间大于20h；热变形过程中粗大铸态晶粒转变为锻态细匀晶粒是以动态再结晶演变机理为主，并且与变形温度、应变率及塑性应变等紧密相关，当铸态材料加热1250℃保温20h之后，进行两镦两拔的制坯工艺可获得较细匀化的锻态组织。（2）对于上平下V型砧拔长工艺，以锻件内部晶粒非均匀程度及混晶为控制目标，变形温度、拔长锻比及砧型结构是影响锻件组织非均匀变化的主要因素，其工艺容限条件为：V型砧角度为120°、砧宽比为0.8，锻造温度为1250℃，最小锻比为1.2～1.3。（3）锻件心部粗大组织热变形过程演变机理主要为动态再结晶，动态再结晶体积百分数大小是表征其能否锻透的主要指标，随着变形温度升高其锻透的临界塑性应变降低，当变形温度为1000℃、1100℃和1200℃时，临界应变分别为0.52，0.41和0.35，并据此分析、获得适于实验材料的临界锻透模型。（4）30Cr2Ni4MoV钢组织遗传机理为：奥氏体晶核在长大过程中，针状奥氏体具有遗传效应，当α相未发生充分再结晶，球状奥氏体晶核在一定程度也会产生遗传效应；只有当α相充分再结晶，打乱原始α相的有序空间取向，才能有效切断粗大组织遗传。采用多次高温回火及多次降梯高温加热正火，以及高温回火与稍低于Ac3温度的正火联合使用，能够有效切断粗大组织遗传和细化锻件内部晶粒。