Abstract: 05Cr17Ni4Cu4Nb steel is a martensitic precipitation-hardened stainless steel that frequently exhibits surface cracking during secondary forging， significantly compromising forging efficiency and product yield.In this study， the original austenite microstructure， martensite microstructure， phase composition and hardness of the secondary forging bar surface defect samples were tested. The results demonstrate that the observed surface cracking of secondary forging bar propagates along intergranular paths， with Cu embrittlement and mixed grain structure identified as primary contributing factors. Under high-temperature forging conditions， mixed grain structure and copper segregation exhibit a synergistic interaction. Abnormal growth of some prior austenitic grains within the mixed structure reduces the total grain boundary area， leading to increased Cu atom concentration per unit boundary area. Furthermore， inhomogeneous local strain rates during forging in mixed grain structures promote stress concentration at large/small grain ， accelerating Cu segregation to grain boundaries. At forging temperatures， locally enriched Cu atoms melt at grain boundaries， forming nanoscale liquid films. This liquid acts as a "lubricant，" significantly reducing grain boundary migration resistance and promoting rapid boundary movement， thereby facilitating abnormal grain growth and mixed structure formation. Besides Cu segregation， excessively high initial forging temperature also promotes mixed grains，excessively high initial forging temperature induces an oscillating "plunge-recovery" cycle in the billet surface temperature， causing dynamic thermal inhomogeneity. Significantly enhanced grain boundary mobility in localized high-temperature micro-regions allows preferential rapid growth of certain grains， forming mixed structures. This study mitigated copper segregation by reducing the electroslag remelting rate from 7.0-6.5 kg/min to 6.0-5.5 kg/min and suppressed mixed grain tendency by lowering the initial forging temperature from 1 180 ℃ to 1 160 ℃. The optimized forging process successfully eliminated surface cracking during secondary forging.

Key words: Martensitic Precipitation-Hardened Stainless steel, Secondary Forging, Surface Cracking, Cu Embrittlement, Mixed Grain

摘要： 05Cr17Ni4Cu4Nb钢是一种马氏体沉淀硬化不锈钢，在改锻过程中常发生表面开裂，导致锻件成形效率和合格率降低。本研究对改锻锻棒表面缺陷试样的原始奥氏体微观组织、马氏体微观组织、相组成和硬度进行检测。结果表明，改锻锻棒表面开裂形式为沿晶开裂，主要原因为锻棒外圈存在混晶和铜脆。混晶组织和铜偏析在高温锻造条件下存在相互促进作用。混晶中部分原始奥氏体晶粒异常长大，晶界总面积减少导致单位面积晶界对Cu原子的吸附量增加。并且混晶组织在锻造过程中存在局部应变速率差异，大晶粒与小晶粒交界处易形成应力集中区，加速Cu原子在晶界处的偏聚。在锻造的高温条件下，晶界处Cu原子局部熔化形成纳米级液相薄膜。液相可充当“润滑剂”，显著降低晶界迁移阻力，促进晶界快速移动，导致晶粒异常长大，促进混晶产生。除了Cu原子向原始奥氏体晶界偏聚会促进混晶产生以外，开锻温度过高也会促进混晶产生。开锻温度过高会使锻棒表面温度在锻造过程中出现“骤降-回升”振荡循环，导致表面温度发生动态不均匀，高温微区晶界迁移能力显著增强，局部晶粒优先快速长大，从而形成混晶。本研究将电渣重熔熔速由7.0~6.5 kg/min降低至6.0~5.5 kg/min，改善铜偏析；将开锻温度由1 180 ℃降低至1 160 ℃，降低混晶倾向。优化后的改锻工艺成功解决了锻棒改锻表面开裂问题。

关键词: 马氏体沉淀硬化不锈钢, 锻造, 表面开裂, 铜脆, 混晶