Generation Mechanism of Unevenness of Ultra Low Carbon Steel at Initial Stage of Solidification

Abstract

Cooling curve of sample surface of ultra low carbon steel at initial stage of solidification was measured using a new temperature measurement system that consisted of a two-dimensional optical pyrometer and a chill plate which was made of transparent sapphire glass. The presence of recalescence phenomena was observed on the measured cooling curve of ultra low carbon steel sample and the recalescence temperature existed in the range of peritectic transformation temperature as well as low carbon and middle carbon steel samples. The difference of temperature at each measured point of a sample surface was very small until the temperature reached the recalescence temperature, but the difference of temperature became larger after recalescence because the thermal deformation of solidifying shell seemed to be generated. From experimental results for tensile strength and density during solidification, these values changed depending on the phase and the change of values between δ phase and γ phase was large. As the unevenness of solidified shell was generated by deformation accompanying peritectic transformation and the degree of unevenness could be arranged by both temperature ranges, there was a difference of tensile strength and difference of density during peritectic transformation.