Internal Crack Propagation in a Continuously Cast Austenitic Stainless Steel Analyzed by Actual Residual Stress Tensor D
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NTRODUCTION
GENERATION of stress and strain is inevitable in cast metals owing to the thermal gradient that is present during the casting process, which includes solidification of molten steel followed by cooling. Internal cracks are one of the most significant problems associated with the continuous casting process. It is widely recognized that cracks initiate and propagate when applied stress values are larger than tensile strength during casting. A number of studies have attempted to clarify the mechanisms that led to formation of internal cracks. An early study by Lankford[1] suggested that internal cracks propagated because of the bulging that repeatedly took place between rolls. This study recognized the three major factors that affect crack formation: residual liquid at the slab center, composition of the liquid phase, and stress loaded to the solidified shell. Brimacombe and Sorimachi[2] were the first to use finite element methods to model the effect of the stress on cracking. They proposed that the cooling condition at the corners of slabs should be properly controlled to prevent cracking. Phillion[3] developed a computational model and
YOUICHI SAITO, Graduate Student, is with the Graduate School of Engineering, Tohoku University, 6-6-02 Aza-Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Contact e-mail: youichi.saitou@ nyk.jp SHUN-ICHIRO TANAKA, Professor, is with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Manuscript submitted February 20, 2015. Article published online December 21, 2015. 882—VOLUME 47B, APRIL 2016
concluded that stress accumulated in the space between the porosities formed at the center of a slab, which resulted in the formation of cracks. These studies have demonstrated that stress is an essential factor for internal crack formation. However, the stress applied to the region where cracks tend to form has not been quantitatively measured. There are many difficulties associated with measuring the stress applied to the center portions of a slab, including the very high temperature of the slab material. A novel technique based on X-ray diffraction (XRD) has been developed to measure residual stress in a cast material containing coarse grains with strongly preferential orientations. In fact, the structure of steel slabs usually consists of columnar and equiaxed grains when applying electromagnetic stirrer (EMS). Some studies[4–7] have shown that the solidification structure begins with columnar grains grown from a mold followed by the transition to equiaxed grains after inducing electromagnetic power. The principle behind the new technique has been routinely used for stress measurement as it is accurate and non-destructive. One conventional approach, known as the sin2w method, is limited to applications where materials are relatively homogenized to measure a localized area, for example. Tanaka and coworkers successfully evaluated residual stress distributions of Si3N4 in an Si3N4/Cu/steel joint over a localiz
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