Effects of Microstructure on CVN Impact Toughness in Thermomechanically Processed High Strength Microalloyed Steel
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INTRODUCTION
TOUGHNESS, generally interpreted as the resistance to fracture, is considered as one of the most important mechanical properties for steel plate. It can be evaluated by drop weight tear test (DWTT), crack tip opening displacement (CTOD) test, or the more ordinary Charpy V-notch (CVN) impact test. Depending on the application scenario, the requirement for toughness varies. Taking the linepipe steel for example, besides the DWTT property, the requirement for the CVN impact absorbed energy at 273 K (0 C) which normally approaches the upper shelf energy (USE) increases with the increasing strength level. And the demand for low-temperature toughness is substantially increased when the oil exploration goes to the deep sea or abyssal region. Therefore, understanding the correlation between microstructures and toughness would be crucial for the microstructure design of high strength microalloyed steel plate. A large number of researches have been devoted to this field. Among the microstructural control measures, refining the grain size is of primary importance, since it enhances the yield strength and lowers the TAO JIA and ZHAODONG WANG are with the State Key Lab of Rolling and Automation, Northeastern University, 3-11 Wenhua Rd., Shenyang 110819, P.R. China. Conatct e-mail: [email protected] YANLEI ZHOU is with the Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Rd., Shenyang 110016, P.R. China. XIAOXIAO JIA is with the Advanced Engineering Materials, the University of Manchester, Oxford Rd., Manchester M139PL, U.K. Manuscript submitted July 29, 2016. Article published online December 20, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
ductile-to-brittle transition temperatures (DBTT) at the same time. Song et al.[1] investigated the CVN impact behavior of an ultrafine grained C-Mn steel with ‘‘ferrite + dispersed cementite’’ microstructure. Compared with the initial ‘‘ferrite + pearlite’’ microstructure, the lower-shelf energy (LSE) was significantly raised, and the DBTT was lowered by the joint contribution of the decrease in ferrite grain size from 6.8 to 1.3 lm and the occurrence of delamination crack. Calcagnotto et al.[2] has produced ‘‘ferrite + martensite’’ dual-phase steels with different grain size by large strain warm rolling and intercritical annealing. The grain refinement was found to promote the ductile fracture mechanism, lower the DBTT, and increase the USE and LSE. Martensite-austenite (M-A) constituent, which is formed during austenite to bainite transformation, affects the impact toughness mainly by its size. As done by Lan et al.,[3] the fracture stress of the M-A constituent can be calculated using the classical Griffith theory. The increase of M-A constituent size would decrease its fracture stress, and therefore increase the probability of cleavage fracture nucleated inside the M-A constituent or at the interface with the ductile matrix. The critical size that is able to suppress the cleavage fracture for different microstructures at room temperature was proposed,
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