Effect of Strain Rate on Hot Ductility Behavior of a High Nitrogen Cr-Mn Austenitic Steel

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NITROGEN is an effective solid-solution strengthener, a pitting resistance enhancer, and also a strong austenite stabilizer.[1] Owing to these beneficial effects of nitrogen, high nitrogen Cr-Mn austenitic steels possess an interesting combination of mechanical, chemical, and physical properties: high strength and toughness, good corrosion resistance, and low magnetic susceptibility.[1,2] These steels have been widely used in engineering[3] with a typical example being heavy retaining rings.[4] However, high nitrogen Cr-Mn austenitic steels are sensitive to hot cracking during the hot working process, leading to high production costs and low efficiency.[4–6] Several process variables must be considered to avoid hot cracking in steel, such as the preheating temperature, deformation temperature, and strain rate. An excessively high preheating temperature results in a large initial grain size, which reduces the hot ductility of the steel.[5,7] A higher preheating temperature also leads to the formation of delta ferrite and promotes the nucleation, propagation, and linkage of cracks.[6] A high ZHENHUA WANG, Associate Professor, is with the School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, P.R. China, and also with the Hebei Iron & Steel Technology Research Institute, Shijiazhuang 050023, P.R. China, and State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, P.R. China. Contact e-mail: wangzhenhua@ ysu.edu.cn QING MENG, Student, is with the School of Mechanical Engineering, Yanshan University. MINGGUI QU, Research Assistant, ZEAN ZHOU and BO WANG, Students, and WANTANG FU, Professor, are with the State Key Laboratory of Metastable Materials Science and Technology, Yanshan University. Manuscript submitted June 26, 2015. Article published online January 4, 2016 1268—VOLUME 47A, MARCH 2016

deformation temperature is beneficial for dynamic recrystallization (DRX),[8] which allows high hot ductility to be achieved.[9] In contrast, at lower deformation temperatures, Cr2N may precipitate and have a negative effect on the hot ductility.[9,10] Although process parameters such as the preheating and deformation temperatures have been well studied, few studies have focused on the effect of strain rate on the hot ductility behavior of high nitrogen Cr-Mn austenitic steels. The strain rate is known to have a pronounced effect on the hot ductility of metallic materials. In AZ31 alloys, the elongation at fracture increases with decreasing strain rate.[11,12] Similar trends have been observed in Csf/AZ91D[13] and AZ81.[14] The effect of strain rate on hot ductility of steels is more complex. No obvious strain rate effect is observed in super 304H austenitic steel.[15] However, the equivalent fracture strain increases with decreasing strain rate for 30Cr2Ni4MoV steel[16] and 2205 steel.[17] In contrast, a low strain rate decreases the hot ductility of C-Mn-Nb-Al steel[18] and Cr17Mn6Ni4Cu2N steel.[19] In other austenitic steels, a detrimental effect of low strain rate