In this report, the continuous cooling transformation diagram, in case of rapid heating maximum temperature 1350°C, was constructed with the new apparatus for "Wel-ten 55" containing 0.18% C, 0.47% Si, 1.40% Mn. Next, effects of chemical composition on the continuous cooling transformation diagram were investigated in regarding to three kinds of "Wel-ten 55" treated in the Reports 1-6.
Critical cooling time C
f', showing the limit over which primary ferrite appears, is changed remarkably by carbon content. When the carbon content decreases from 0.18% to 0.13% (amount of reduction : 0.05%), C
f' decreases from 8.5 to 3.8 sec. (amount of reduction : 4.7 sec.). The effects of other elements are very small in comparison with that of carbon.
The value of critical cooling time C
f' is very important in case of welding the steel showing it, since the structure and properties of the heat-affected zone, in the neighbourhood of fusion line, by welding show a remarkable increase of brittleness with decrease of cooling time when the steel is cooled with various cooling times in the neighbourhood of C
f'. The steel maker should make efforts to product steels whose C
f' are smaller as much as possible and which show the required strength, It is considered to be free from danger that the welding of the steel is carried out with a certain condition by which the cooling of heat-affected zone near the fusion line occurs with a larger cooling time than C
f'.
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