Abstract
High-strength steel is widely used in hot forging products for application to the oil and gas industry because it has good mechanical properties under severe environment. In order to apply to the extreme environment industry requiring high temperature and high pressure, heat treatments such as austenitizing, quenching and tempering are required. The microstructure of high-strength steel after heat treatment has various microstructures such as Granular Bainite (GB), Acicular Ferrite (AF), Bainitic Ferrite (BF), and Martensite (M) depending on the heat treatment conditions and cooling rate. Especially in large forged products, the difference in microstructure occurs due to the difference in the forging ratio depending on the location and the temperature gradient according to the thickness during post-heat treatment. Therefore, this study attempted to quantitatively analyze various phases of F70 high-strength steel according to the austenitizing temperature and hot forging ratio using the existing EBSD analysis method. In addition, the correlation between microstructure and mechanical properties was investigated through various phase analysis and fracture behavior of high-strength steel. We found that various microstructures of strength steel depend on the austenitizing temperature and hot forging ratio, and influence the mechanical properties and fracture behavior.
Highlights
F70 High-strength steel is widely used in hot forging products for extreme environment applications such as oil pipeline systems, offshore plants, and gas industries owing to its good formability, economical, and high strength even in high-pressure environments achieved by post-heat treatments
In Granular Bainite (GB) and Bainitic Ferrite (BF), grains with a coarse high angle grain boundaries (HAGBs) and Low angle grain boundaries (LAGBs) were observed in the form of island and lath structure, respectively
In the M, both grain boundaries and packet had HAGBs, and a LAGBs were inside the packet
Summary
F70 High-strength steel is widely used in hot forging products for extreme environment applications such as oil pipeline systems, offshore plants, and gas industries owing to its good formability, economical, and high strength even in high-pressure environments achieved by post-heat treatments. Polygonal ferrite (PF) formed during cooling in the austenite-ferrite twophase region has low strength but excellent toughness, while the structure formed at lowtemperature transformation by rapid cooling depends on the heat treatment temperature and cooling rate such as Granular Bainite (GB), Acicular Ferrite (AF), Bainitic Ferrite (BF), and Martensite (M) as the faster cooling rate, the higher strength of the structure formed [6,7,8]. Studies on quantitative analysis of microstructures according to various hot forging ratios and post-heat treatment of high-strength steel are still limited. This study attempted to quantitatively analyze various phases of high-tensile steel according to the austenitizing temperature and hot forging ratio using the existing EBSD analysis method. The correlation between microstructure and mechanical properties was investigated through various phase analysis and fracture behavior of high-strength steel
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