铜合金压铸/挤压用奥氏体热作模具钢强韧化机制及工艺调控

The hot work die steel used for copper die-casting and extrusion dies is always martensitic steel, such as H13 and 3Cr2W8V. However, the service temperature of dies during die-casting and extruding is higher than 700 ℃. During this die-casting and extruding process, martensitic matrix of the steel decomposes and carbides coarsen, resulting in a great decrease in strength and toughen of die steel, as well as premature failure and cracking of dies. Therefore, it is urgent to develop die steel for copper die-casting and extrusion, and master production technology of the steel. The developed Cr-Ni-Mo austenitic hot-work die steel will be chosen as research object in this work. Based on a systematic study on the effect of process parameters of electroslag remelting, high temperature homogenization treatment, forging, annealing, solid solution and aging on precipitates, microstructure, strengthen and toughness, the strengthening-toughening mechanism and processing of an austenitic hot-work die steel used for copper die-casting and extrusion dies will be established. The research content includes: (1) control of the precipitates and dendritic structure of austenitic hot-work die steel during electroslag remelting, (2) the effect of B on the microstructure of austenitic hot-work die steel and its mechanism, (3) the precipitates and its effect on the microstructure of austenitic hot-work die steel, and (4) the evolution and processing of the precipitates in austenitic hot-work die steel. The study in this project will provide related theoretical basis for the strengthening-toughening, processing and industrial manufacture of austenitic hot-work die steel used for copper die-casting and extrusion dies.

铜合金压铸/挤压用模具一直以来使用的是马氏体热作模具钢，如H13、3Cr2W8V等。但铜合金压铸/挤压时模具服役温度超过700℃，在服役过程中马氏体基体分解、碳化物聚集长大，造成模具钢强韧性急剧降低，模具早期开裂失效。因此，亟待开发铜合金压铸/挤压用模具钢，并掌握其生产工艺要点。本项目以自主开发的Cr-Ni-Mo系奥氏体热作模具钢为研究对象，通过系统研究电渣重熔、高温均匀化处理、锻造、退火、固溶和时效工艺参数对钢中析出相、组织和强韧性的影响，最终明晰该模具钢强韧化机制及工艺调控原理。主要研究内容包括：（1）奥氏体模具钢电渣重熔过程析出相和枝晶组织的控制；（2）B对奥氏体模具钢组织的影响及其作用机制；（3）钢中的析出相及其对奥氏体组织的影响；（4）奥氏体模具钢中析出相的演变规律及调控工艺。本项目开展的研究将为铜合金压铸/挤压用奥氏体热作模具钢的强韧化、工艺调控和工业生产提供相关的理论依据。

本项目以自主设计的Cr-Ni-Mo系奥氏体热作模具钢为研究对象，通过系统研究电渣重熔、锻造、固溶和时效工艺参数对钢组织、析出相和强韧性的影响，明确了电渣重熔对奥氏体模具钢凝固组织和析出相控制的影响，B和稀土Ce对奥氏体模具钢组织的影响及其作用机制，Nb/Ti比对钢组织和析出相影响，奥氏体模具钢中析出相在热处理过程中演变规律，强韧化机制及调控工艺。主要研究成果包括：（1）提高冷速可以细化15Cr-22Ni-1Nb奥氏体钢的凝固组织和钢中析出相的尺寸。定向凝固电渣重熔可使整个电渣铸锭获得完全平行生长的柱状晶，细化枝晶组织，减轻电渣铸锭中合金元素的偏析程度；（2）15Cr-22Ni-1Nb钢中添加微量的硼元素不能细化钢的凝固组织。含铈15Cr-22Ni-1Nb奥氏体钢中稀土夹杂物的类型为Ce2O3，Ce2O2S和CeAlO3。15Cr-22Ni-1Nb钢中添加稀土铈可以细化枝晶组织，减轻了合金元素的偏析程度，减少了钢中大尺寸的NbC和Laves相；（3）Nb含量增加显著促进了15Cr-22Ni奥氏体钢中Laves相析出，且共晶NbC析出温度降低。15Cr-22Ni奥氏体钢最佳热加工窗口为1050-1150 oC/ 0.01-0.1 s-1。钢中固溶Nb和含Nb析出相含量增加后引起动态再结晶延迟，钢的变形抗力增加；（4）不同Nb含量奥氏体钢的奥氏体晶粒长大激活能（210.477~268.796 kJ/mol）与Nb在奥氏体晶界的扩散激活能（225kJ/mol）非常接近。奥氏体晶粒随着保温时间和固溶温度的提高明显长大。钢中Nb含量分别为0.64wt%和1.00wt%时，Laves相在1170 ℃~1230 ℃的固溶温度和0.5 h~4 h的保温时间下均完全溶解，共晶NbC的溶解度随固溶温度的增加而增大。钢中Nb含量为1.40wt%时，Laves相和共晶NbC的溶解存在竞争关系。钢中Nb含量增加后可以抑制固溶过程中奥氏体晶粒的长大；（5）15Cr-22Ni奥氏体钢在820 ℃时效处理4 h后具有最高的室温抗拉强度574 MPa。亚微米级的Laves相对屈服强度的贡献度小于纳米NbC。随着时效温度的提高和保温时间的延长，Laves体积分数的增加导致奥氏体钢塑性降低。奥氏体钢在780 ℃和820 ℃时效处理后，具有稳定的高温强度（346 MPa~391 MPa）。