Delamination Effect on Impact Properties of Ultrafine-Grained Low-Carbon Steel Processed by Warm Caliber Rolling
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STRONG and tough steels are always in demand to reduce weight and improve safety in transportation and improve performance in heavy machinery.[1] Recently, Kimura et al. developed stronger, tougher steel by combining grain refinement and delamination/splitting of the crack-arrester type shown in Figure 1(a) resulting from elongated microstructures.[2] It was produced by a caliber-rolling process of the square/square type[3,4] through the deformation of tempered martensite in a low-alloy steel containing 0.4 pct C, 2 pct Si, 1 pct Cr, and 1 pct Mo. The enhanced impact toughness was attributed to delamination, which was a result of crack branching on the aligned {100} cleavage planes in ultrafine elongated grains 260 nm in thickness strengthened by nanometer-sized carbides. The crack branching seen in Figure 1(a) is normally referred to as a lamella fracture in a research field of composite materials,[5–7] and it has been reported in the literature[8–13] as a method for enhancing toughness. However, a question arises as to whether this may be the case if the same process is applied to plain carbon steels in which there are numerous reports on the TADANOBU INOUE, FUXING YIN, and YUUJI KIMURA, Senior Researchers, and KANEAKI TSUZAKI, Managing Director, are with the National Institute for Materials Science, Tsukuba 3050047, Japan. Contact e-mail: [email protected] SHOJIRO OCHIAI, Professor, is with the Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. Manuscript submitted April 21, 2009. Article published online October 31, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
refinement from heavily deformed ferrite at warm working temperatures.[3,4,14–20] The delamination/splitting of the crack-divider type shown in Figure 1(b) has often been observed on fracture surfaces of impact specimens made from rolled steel pipe and plate products. Bourell[21] showed that the delamination of this type remarkably appeared with increasing rolling strain and decreasing rolling temperature, due to the development of f100gh110i texture;[22] furthermore, the upper-shelf energy decreased and the lower-shelf energy increased due to this cleavage delamination. Maehara et al.[23] reported that for low-carbon steel bars processed by caliber rolling of the oval/round type, the energy transition temperature became lower, and large longitudinal cracks, i.e., delamination of the type shown in Figure 1(a), were observed when the finishing temperature in the rolling was low (644 C). Several studies[4,17–19] have demonstrated the Charpy impact properties of ultrafine-grained (UFG) low-carbon steels produced by a caliber-rolling process of the square/ square type; however, no fractures that delaminate perpendicularly to the propagating crack tip (i.e., no lamella fracture) have been reported. The objective of this study was to confirm the appearance of a lamella fracture in a Charpy impact test on a UFG low-carbon steel fabricated by multipass caliber rolling at 500 C and to discuss the effect of its fract
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