Analysis of Microstructure Effects on Edge Crack of Thin Strip During Cold Rolling
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ETALS tend to develop edge cracks during hot and cold rolling occasionally. Cracks in metal products decrease the strength, rigidity, toughness, plasticity, and residual life, which affect the thin strip quality and productivity significantly. The phenomenon of edge cracking in cold rolling of steel is complicated, depending on rolling conditions, as well as the microscopic factors such as the microstructure, inclusions, and pores. From the viewpoint of an engineer who is applying metal forming processes to the production line, it is important to define the fracture event because the ultimate stage of the rolled strip is preceded by or corresponds to crack initiation and propagation. Ductile fracture has been the subject of considerable interest by fracture mechanics researchers because of its practical engineering importance. Such interest has prompted the development of several ductile fracture criteria including notably crack opening displaces (CODs), and J-integral and strain-energy density.[1,2] Before the cracks grow to a critical length in successive rolling passes, edge cracks need to be removed by a trimming operation to prevent the possible rupture of the sheet in the rolling mill. The HAIBO XIE, Associate Research Fellow, and ZHENGYI JIANG, Professor, are with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia. Contact e-mail: [email protected] W.Y. DANIEL YUEN, Manager Research, is with the BlueScope Steel Research, BlueScope Steel Limited, Port Kembla, NSW 2505, Australia. Manuscript submitted December 14, 2010. Article published online June 28, 2011. 1244—VOLUME 42B, DECEMBER 2011
situation becomes critical when the remaining width of the strip does not meet the requirement of the next stage of the process. Therefore, the microstructure factors that may influence the strip cracking must be investigated simultaneously with the evolution mechanism of edge cracks. There are numerous publications about the research on cracks during metal forming. Physical defects are important factors that affect the material fracture strength.[3] Damage mechanics and fracture models of ductile materials have been introduced into crack evolution analysis.[1,4,5] Stored energies were applied to validate the damage mechanism,[6] and void growth study was conducted for plasticity and ductile fracture damage.[7] The research on crack initiation was carried out in steel,[8–10] and phase transformation contributed to toughening around the crack tip.[11] Previous studies showed that the fracture behavior of materials was influenced greatly by their microstructures.[12] The surface and longitudinal cracks have been reported in continuous casting.[13,14] Edge crack evolution during rolling was analyzed,[15–18] and microstructure influences on crack formation were investigated also.[19,20] Additionally, the influences of the grain size on crack initiation and crack growth were investigated.[21–23] The fracture toughness of low-carbon steel is governed largely by the
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