Fracture Properties of High-Toughness API X70 Pipeline Steels I. Correlation of Rolling Conditions, Microstructure, and Low-Temperature Toughness

摘要

This study is concerned with the effects of microstructure on fracture properties of a high-toughness API X70 pipeline steel. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot-rolling, and their microstructures were varied by the rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below -100 deg C when the finish cooling temperature range was 350-500 deg C, implying excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 percent, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture area occurred at some finish cooling temperatures.