DZ55冷拔钢管断裂原因分析与控制措施

Analysis of Causes for Crack of DZ55 Cold-drawn Steel Tube and Related Preventive Measures

采用金相检验、硬度检测、扫描电镜及能谱分析、热处理试验等手段,分析了DZ55冷拔钢管在拔制过程中产生横向裂口的原因,并提出控制措施。分析认为:由于元素偏析,导致塑性低于基体的马氏体更容易在靠近内表面的Mn元素聚集区域产生,从而引起材料塑性降低,导致DZ55冷拔钢管在冷加工中出现横向裂口。采取低温浇铸,设法扩大铸坯中心等轴晶区,以减轻或消除坯料偏析,从而减少内裂口的出现;采用高温回火+缓冷的热处理工艺,可以降低DZ55钢的强度,消除其组织中的马氏体带,从而提高了材料塑性,有利于后续的冷加工。

Causes for the transverse cracks of the DZ55 cold-drawn pipe as occurrred during the drawing process are analyzed by means of metallographic examination, hardness testing, scanning electron microscopy, and energy spectrum analysis as well as heat treatment tests. Accordingly preventive measures are proposed. The analysis result identifies that owing to element segregation, it becomes easier for the martensite matrix with plasticity lower than that of the base metal to produce in the Mn-aggregation area that close to the inner surface, which leads to reduction of the plasticity of the material, and thus causes the development of the said crack during the cold drawing process. Therefore taking low temperature casting of the billet to expand the central equiaxed zone of the billet can reduce or even eliminate the segregation so as to reduce development of the inner crack. Moreover adopting the heat treatment process of hi-temperature tempering ＋slow-cooling can decrease strength of the DZ55 steel, and eliminate the martensite band so as to enhance plasticity of the material, which is helpful for the cold-work process as follows.