Effect of Two-Phase Region Quenching Temperature on Microstructure and Yield Ratio of HSLA100 Steel

Effects of secondary quenching temperature on microstructure and yield ratio of HSLA (high strength low alloy) 100 steel in the two-phase region were investigated. The results show that the mixed microstructure of ferrite and the M-A(martensite-austenite)islands were obtained in the two-phase region after secondary quenching. A small amount of island structure distributing along the lath ferrite was observed when quenching temperature was 700 ℃. With the increase of quenching temperature the island structure increased in the number and coarsened in the shape, and the ferrite morphology gradually changed from a single lath to the multilateral shape with the decrease of the dislocation density. When quenching temperature was 820 ℃, the microstructure reverted to lath bainite. The yield ratio of the test steel continually declined from 0.94 to 0.67 in the range of quenching temperature from 700 ℃ to 740 ℃, but kept a very small fluctuation in the range of 740-780 ℃ (about 0.67), and finally raised sharply at quenching temperatures above 780 ℃.