Effective Grain Size and Charpy Impact Properties of High-Toughness X70 Pipeline Steels

Author: Hwang Byoungchul   Kim Yang Gon   Lee Sunghak   Kim Young Min   Kim Nack J.   Yoo Jang Yong  

Publisher: Springer Publishing Company

ISSN: 1543-1940

Source: Metallurgical and Materials Transactions A, Vol.36, Iss.8, 2005-08, pp. : 2107-2114

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Abstract

The correlation of microstructure and Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel was investigated in this study. Six kinds of steel were fabricated by varying the hot-rolling conditions, and their microstructures, effective grain sizes, and CVN impact properties were analyzed. The CVN impact test results indicated that the steels rolled in the single-phase region had higher upper-shelf energies (USEs) and lower energy-transition temperatures (ETTs) than the steels rolled in the two-phase region because their microstructures were composed of acicular ferrite (AF) and fine polygonal ferrite (PF). The decreased ETT in the steels rolled in the single-phase region could be explained by the decrease in the overall effective grain size due to the presence of AF having a smaller effective grain size. On the other hand, the absorbed energy of the steels rolled in the two-phase region was considerably lower because a large amount of dislocations were generated inside PFs during rolling. It was further decreased when coarse martensite or cementite was formed during the cooling process.

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