Abstract
The continuous cooling transformation diagrams in case of maximum heating temperature 1350°C were constructed fo three kinds of rimmed-, semi-killed-and kliled-mild steel of which ultimate tensile strengths are 35, 42 and 49 kg/mm2. As the critical cooling times of those steels are generally remarkably smaller, it is difficult to construct those continuous cooling transformation diagrams. However, when the new apparatus for thermal analysis manufactured by the authors is used, the transformation tempe-ratures and the cooling curves can be determined accurately even in the case of cooling time from 800°C to 500°C being shotter than 1 sec. The following results were obtained:
1. For the rimmed steel having ultimate tensile strength of about 35 kg/mm2, of which A3-transfo-rmation-point is 871°C and A1-point 729°C, critical cooling times from A3-point to 500°C are determined as follows; Cz'=0.17 sec, Cf' =0.27 sec, C p'=2.0 sec, Ce'=23 sec. The cooling time corresponding to 50% martensite is 0.47 sec and Ms-temperature 435°C.
2. For the semi-killed steel having ultimate tensile strength of about 42 kg/mm2, of which A3-point is 842°C and Al-point 719°C, critical cooling times are determined as follows; Cz'=0.26 sec, Cf=0.40 sec, Cp'=2.0 sec, Ce'=55 sec. The cooling time corresponding to 50% martensite is 0.88 sec and Ms-tem-perature 420°C.
3. For the killed steel having ultimate tensile strength of about 49 kg/mm2, of which A3 point is 832°C and A1-point 718°C, critical cooling times are determined as follows; Cz'=0.68 sec, Cf' =1.04 sec, Cp' =10.0 sec, Ce'=140 sec. The cooling time corresponding to 50% martensite is 1.5 sec and Ms-tempera-ture 415°C.
4. Each critical cooling times and the cooling time corresponding to 50% martensite tend to become large generally with increase of carbon, manganese and silicon contents of a steel.
