2022 年 108 巻 6 号 p. 370-382
Two kinds of ferrite were produced due to slow cooling after intercritical annealing, one being intercritically-annealed ferrite (αa phase), and the other transformed ferrite (αc phase). The effects of distributions of the αa and αc phases on the mechanical properties of a Fe–0.17C–1.5Si–1.7Mn dual-phase steel were examined experimentally at room temperature. Two types of intercritical annealing (IA) were conducted to control the distribution. The IA temperature and time are Ta = 800°C (1073 K) and ta = 0.5 h (1.8 ks), respectively, for the first type, and Ta = 740°C (1013 K) and ta = 4 h (14.4 ks), respectively, for the second one. For both types of IA, the steel was slowly cooled to 400°C (673 K) at a cooling rate of 10°C/s, followed by water quenching. While the total volume fraction fα of the αa and αc phases is close to 0.68–0.69 for both Ta = 800 and 740°C (1073 and 1013 K), the combination of the volume fraction of each α phase is different for Ta = 800 and 740°C (1073 and 1013 K). The volume fraction fαa of the αa phase and fαc of the αc phase is 0.33 and 0.36 for Ta = 800°C (1073 K), respectively, and 0.68 and 0 for Ta = 740°C (1013 K), respectively. The ultimate tensile strength su is about 970 MPa for fα = 0.68–0.69 independent of combination of fαa and fαc values. Thus, the effect of the α phase on su is close to each other between the αa and αc phases. In contrast, both the uniform strain eu and the local strain el increase with increasing volume fraction fαc of the αc phase. Such increase in eu is attributed to larger values of the strain hardening rate dσ/dε in the large strain region. Misfit strain stored at the boundary between the αa and αc phases causes to the larger values of dσ/dε. On the other hand, suppression of void formation deduces the increase in el. Acceleration of dynamic recovery of the α phase adjacent to martensite (M phase) and decrease in the hardness of the M phase adjacent to the α phase due to formation of the αc phase can suppress void formation. Consequently, formation of the αc phase contributes to improvement of the mechanical properties of the dual-phase steel.
