Effect of high-temperature reduction pretreatment on internal quality of 42CrMo casting billet
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ORIGINAL PAPER
Effect of high-temperature reduction pretreatment on internal quality of 42CrMo casting billet Hong-qiang Liu1 • Zhi-cheng Cheng1
•
Wei Yu2 • Zhao-yuan Zhou1 • Lei Cheng2 • Qing-wu Cai1
Received: 19 March 2020 / Revised: 17 June 2020 / Accepted: 17 June 2020 Ó China Iron and Steel Research Institute Group 2020
Abstract Comparative analysis was conducted to illustrate the effect of high-temperature reduction pretreatment on the internal quality of 42CrMo casting billet. Analysis of macrostructure clarifies that reduction pretreatment could reduce the size of central cavities and equiaxed crystals and improve the segregation degree and porosity on statistics. Lath bainite matrix and granular structure on segregation bands mainly dominate the microstructure of original billet, while pro-eutectoid ferrite and its accompanying troostite are added due to the application of reduction pretreatment. Additionally, C, Mn, and Mo are suggested to be enriched in segregation bands where pores and inclusions tend to be generated. By clarifying the microstructure evolution during cooling process, physical compression and multiplication of prior austenite grain boundaries caused by recrystallization are regarded as the main reasons in improving the internal quality of 42CrMo billet. Keywords 42CrMo billet Reduction pretreatment Continuous casting Recrystallization
1 Introduction Increased production tempo makes the quality control of billets to be a new challenge, especially for the improvement in internal defects, such as central crack and segregation. Different strategies have been proposed to improve the quality of continuous casting billets, such as electromagnetic stirring (EMS), low superheat pouring, and soft reduction (SR) process. Although these processes have improved the quality of continuous casting billets, some problems remain. For example, ‘‘white band’’ defect was difficult to be eliminated during EMS process even when the stirrer location was adjusted [1]. On the other hand, temperature range available for the soft reduction process is very narrow and excess reduction can easily cause inversed V-type segregation, negative segregation (white band), and internal cracks [2]. Formation of internal cracks & Zhi-cheng Cheng [email protected] 1
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
2
Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China
during soft reduction is induced by the applied stress exceeding the critical fracture stress during solidification, which generally initiates in the mushy zone [3]. Additionally, the porosity will reappear due to volume shrinkage while continuing to solidify because EMS and SR are generally applied at the end of solidification of a slab. Both extrinsic (reduction ratio, healing temperature, and holding time) and intrinsic (chemical composition and crack shape) parameters influence th
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