Correlation between Microstructure and Charpy Impact Properties of High-Strength Bainitic Steels

Abstract

This study is concerned with effect of chemical elements and rolling and cooling conditions on microstructure and Charpy impact properties of high-strength bainitic Steels. The effects of carbon equivalent and cooling rate on tensile and Charpy impact properties of high-strength bainitic steels were investigated. Eight steel plates were fabricated with varying C, Cr, and Nb additions under two different cooling rates, and their microstructures, tensile, and Charpy impact properties were evaluated. Volume fractions of microstructural components present in the steels increased in the order of granular bainite, acicular ferrite, bainitic ferrite, and martensite as the carbon equivalent or cooling rate increased which resulted in decreased ductility and upper shelf energy and increased energy transition temperature in spite of increased strength. In the steels containing about 50 vol pct of bainitic ferrite and martensite, the tensile strength was about 900 MPa, while the elongation and upper shelf energy were about 20 pct and 200 J, respectively. In order to achieve the best combination of tensile strength, ductility, and upper shelf energy, e.g., 860~900 MPa, 20 pct, and 200 J, respectively, granular bainite, and acicular ferrite were produced by controlling the carbon equivalent and cooling rate, while about 50 vol pct of bainitic ferrite and martensite were maintained to keep the high strength.Effects of B and Cu addition and cooling rate on microstructure and mechanical properties of low-carbon high-strength bainitic steels were investigated in this study. The steel specimens were composed mostly of bainitic ferrite, together with small amounts of acicular ferrite, granular bainite, and martensite. The yield and tensile strengths of all the specimens were higher than 1000 MPa and 1150 MPa, respectively, while the upper shelf energy was higher than 160 J and energy transition temperature was lower than 208 K (-65 °C) in most of the specimens. The slow-cooled specimens tended to have the lower strengths, higher elongation, and lower energy transition temperature than the fast-cooled specimens. The Charpy notch toughness was improved with increasing volume fraction of acicular ferrite because acicular ferrites favorably worked for Charpy notch toughness even when other low-toughness microstructures such as bainitic ferrite and martensite were mixed together. In order to develop high-strength bainitic steels having excellent combination of strength and toughness, the formation of bainitic microstructures mixed with acicular ferrite was needed, while the formation of granular bainite was prevented.Most of the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituent. The specimens fabricated with the higher finish cooling temperature had the lower volume fraction of martensite-austenite constituent than the specimens fabricated with the lower finish cooling temperature. The fast-cooled specimens had twice higher volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed lowest energy transition temperature among the four specimens because of the least content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, even though there existed some hard microstructures such as bainitic ferrite and martensite-austenite constituent.The microstructural evolution was more critically affected by start cooling temperature and cooling rate than by finish rolling temperature. Bainitic microstructures such as granular bainites and bainitic ferrites were well developed as the start cooling temperature decreased or the cooling rate increased. When the steels cooled from 973 K (700 °C) or 873 K (600 °C) were compared under the same cooling rate of 10 K/s (10 °C/s), the steels cooled from 973 K (700 °C) consisted mainly of coarse granular bainites, while the steels cooled from 873 K (600 °C) contained a considerable amount of bainitic ferrites having high strength, thereby resulting in the higher strength but the lower ductility and upper shelf energy. When the steels cooled at from 673 K (400 °C) at cooling rate of 10 K/s (10 °C/s) or 0.1 K/s (0.1 °C/s) were compared under the same start cooling temperature of 873 K (600 °C), the fast cooled specimens were composed mainly of coarse granular bainites or bainitic ferrites, while the slowly cooled specimens were composed mainly of acicular ferrites. Since acicular ferrites had small effective grain size and contained secondary phases finely distributed at grain boundaries, the slowly cooled specimens had the good combination of strength, ductility, and upper shelf energy, together with very low energy transition temperature.