Effect of Cooling Rate on the Precipitation Behavior of Carbonitride in Microalloyed Steel Slab
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INTRODUCTION
THE surface transversal cracking in the process of continuous casting of microalloyed steel has always been a severe defect that the metallurgy scholars have obsessed over for decades. Many experts have proposed that transverse cracking could be prevented by keeping the slab surface temperature out of the brittle trough temperature range during bending or straightening operations. However, the effect of applying this method is not practical. Recently, Kato et al.[1,2] has managed to prevent the surface transversal cracking of continuous casting microalloyed steel through the control of the slab surface microstructure cooling. In this method, slab surface microstructure correlates with the precipitation behavior of carbonitrides in the microalloyed slab, and the precipitation behavior is controlled by the cooling rate. However, no research was conducted to investigate the effect of cooling rate on the carbonitrides precipitation. In the traditional continuous casting process, as the residence time of slab in the high-temperature zone is much longer, the carbonitrides in the microalloyed steel have enough time to precipitate and grow to form a FANJUN MA, PhD, formerly with School of Materials Science and Engineering, Chongqing University, Changsha 410083, P.R. China, is now with the School of Metallurgical Science and Engineering, Central South University, Changsha 410083, P.R. China. GUANGHUA WEN and PING TANG, Professors, are with the School of Materials and Science Engineering, Chongqing University. GUODONG XU, Principal Engineer, and FENG MEI, Engineer, are with the Baoshan Iron and Steel Co., Ltd., Shanghai 201900, P.R. China. WANLIN WANG, Shenghua Professor, is with the School of Metallurgical Science and Engineering, Central South University. Contact e-mail: wanlin.wang@ gmail.com Manuscript submitted August 23, 2010. Article published online November 24, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
chain-like distribution along the grain boundary. The precipitation of carbonitrides accelerates the nucleation of proeutectoid ferrite and forms a wide, net-like film of proeutectoid ferrite along the grain boundary.[3,4] The slab hot ductility would be significantly reduced by the formation of this kind of microstructure, and hence, the crack would be more likely to be introduced.[5,6] It also was studied that the hot ductility of microalloyed steel was influenced by the precipitation of carbonitride’s volume fraction, size, shape, and distribution, and the larger volume fraction, bigger size as well as the netor chain-like shape with nonuniform distribution are more harmful to the hot ductility.[7,8] The existence of carbonitride in steel is mainly related to its solid solubility in matrix and depends on the thermodynamic conditions when it precipitates. The distribution and size of precipitates in microalloyed steel is closely related to the cooling conditions.[9–12] Therefore, the study of the effect of cooling rates on the precipitation behavior of carbonitride in microalloyed steel in continuous c
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