On 0.15C-0.25Si-1.10Mn steel containing 0.15wt.% carbon, during deformation, austenite-to-ferrite transformation behavior have been studied by high temperature compression test using Gleeble 1500 tester. The deformation conditions were 0 to 0.8 in strain and 0.001/s to 1/s in strain rate. After deformation, specimen was immediately quenched in ice brine water and the variation of microstructures was observed. From this, it was tried to find out transformation mechanism during deformation. $Ae_3$ temperature and $T_0$ temperature were calculated by Thermo-calc program and $Ar_3$ temperature was measured by dilatation test. These temperatures became the criteria to set an experiment condition. Deformation temperatures were from 700℃ to 875℃, strain rates was from 0.001/s to 1.0/s and strains were from 0.1 to 0.8. the specimens for test were machined to cylindrical sample by φ10 x 15mmL. On the specimen deformed, microstructure was observed by optical microscope , SEM and TEM. Also ferrite grain size and transformed fraction were analyzed quantitatively by 'INSPECTOR' program. In low carbon steel, dynamic transformation by deformation was occurred when under-cooled austenite was deformed. That is, at the temperature region which is not transformed without deformation, over $Ar_3$ temperature, under-cooled austenite was transformed to ferrite by deformation. However, at over $Ae_3$ temperature, transformation by deformation never occurred. The transformation mechanism by deformation was changed at $T_0$ temperature. At the temperature higher than $T_0$ temperature, transformation was by strain induced transformation(SIT), which occurred right after deformation. On the other hand, at the temperature lower than T0 temperature, transformation was by massive transformation(MT), which occurred during deformation. On the transformation induced by strain(SIT), two points were considered, the one was that it is transformed right after deformation at the temperature lower than $A_d$ temperature, the other was that it needs time for transformation at the temperature higher than $A_d$ temperature. $A_d$ temperature means strain induced transformation starting temperature. The shape of the interface of ferrite structure formed by MT was serrated, and that formed by SIT was the type of widmanstaten structure. The higher deformation temperature and the faster strain rate, the critical strain for MT increased more. At very fast strain rate, in spite of low temperature, the MT may not occur. In case of occurrence of MT, as strain increases, ferrite grain comes to be refined by continuous dynamic recrystallization of ferrite. As strain rate is faster deformation temperature is higher, critical strain for SIT increases, As strain rate is faster, train induced transformation starting temperature, $A_d$, is decreases, and as strain is larger, $A_d$, is increases. The most important point is that the mechanism of transformation during deformation is MT. The grain formed by MT is refined by the continuous dynamic recrystallization of ferrite.
본 연구는 .15C-1.1Mn-0.25Si 순저탄소강을 오스테나이트-페라이트 2상 영역에서 고온 가공할 때 발생하는 오스테나이트->페라이트 동적 상변태 거동을 Gleeble 1500을 이용하여 변형률 0-0.8, 변형률속도 0.001/s~1/s 변형 조건에서 연구하였다. 본 연구에서 특별히 제작한 Gleeble 부속 급냉장치(냉각속도:400℃/s이상)와 자체 개발한 전해연마법을 이용하여 변형 중에 발생하는 페라이트 상변태 거동을 성공적으로 관찰할 수 있음을 보여주었다. 이로부터 2상 영역 내에서 변형온도에 따른 동적상변태 거동을 분석할 수 있었다. 이 결과 본 연구는 $T_0$ 온도를 기준으로 하여 그 이상과 그 이하의 온도에서 서로 다른 페라이트 형성기구가 작동한다는 사실을 처음 밝힐 수 있었다. 즉 $T_0$ 온도 이상에서는, 종래에 알려져 오던 변태기구, 즉 변형 유기변태에 의해 페라이트가 형성되나 $T_o$ 온도 이하에서는 새로운 변태기구 즉 매씨브 변태에 의해 페라이트가 형성된다는 사실을 규명할 수 있었다. 이러한 매씨브 변태를 위해서는 임계 변형률이 필요한데 이 임계 변형률은 변형률 속도가 증가할수록 증가하는 경향을 보였다, 더욱이 이 경우 페라이트 결정립 미세화는 매씨브 페라이트의 연속 동적재결정에 의해 일어난다는 사실을 아울러 새로이 규명하였다.
