압력용기용 Ni-Mo-Cr계 고강도 저합금강의 합금원소 함량 변화에 따른 미세조직학적 특성변화의 열역학 계산 및 평가 (Thermodynamic Calculation and Observation of Microstructural Change in Ni-Mo-Cr High Strength Low Alloy RPV Steels with Alloying Elements)

서지정보

· 발행기관 : 대한금속·재료학회
· 수록지 정보 : 대한금속·재료학회지 / 46권 / 12호 / 771 ~ 779페이지
· 저자명 : 박상규, 김민철, 이봉상, 위당문

초록

An effective way of increasing the strength and fracture toughness of reactor pressure vessel steels
is to change the material specification from that of Mn-Mo-Ni low alloy steel(SA508 Gr.3) to Ni-Mo-Cr low
alloy steel(SA508 Gr.4N). In this study, we evaluate the effects of alloying elements on the microstructural
characteristics of Ni-Mo-Cr low alloy steel. The changes in the stable phase of the SA508 Gr.4N low alloy steel
with alloying elements were evaluated by means of a thermodynamic calculation conducted with the software
ThermoCalc. The changes were then compared with the observed microstructural results. The calculation of
Ni-Mo-Cr low alloy steels confirms that the ferrite formation temperature decreases as the Ni content
increases because of the austenite stabilization effect. Consequently, in the microscopic observation, the lath
martensitic structure becomes finer as the Ni content increases. However, Ni does not affect the carbide
phases such as M23C6 and M7C3. When the Cr content decreases, the carbide phases become unstable and
carbide coarsening can be observed. With an increase in the Mo content, the M2C phase becomes stable instead
of the M7C3 phase. This behavior is also observed in TEM. From the calculation results and the observation
results of the microstructure, the thermodynamic calculation can be used to predict the precipitation behavior.

영어초록

An effective way of increasing the strength and fracture toughness of reactor pressure vessel steels
is to change the material specification from that of Mn-Mo-Ni low alloy steel(SA508 Gr.3) to Ni-Mo-Cr low
alloy steel(SA508 Gr.4N). In this study, we evaluate the effects of alloying elements on the microstructural
characteristics of Ni-Mo-Cr low alloy steel. The changes in the stable phase of the SA508 Gr.4N low alloy steel
with alloying elements were evaluated by means of a thermodynamic calculation conducted with the software
ThermoCalc. The changes were then compared with the observed microstructural results. The calculation of
Ni-Mo-Cr low alloy steels confirms that the ferrite formation temperature decreases as the Ni content
increases because of the austenite stabilization effect. Consequently, in the microscopic observation, the lath
martensitic structure becomes finer as the Ni content increases. However, Ni does not affect the carbide
phases such as M23C6 and M7C3. When the Cr content decreases, the carbide phases become unstable and
carbide coarsening can be observed. With an increase in the Mo content, the M2C phase becomes stable instead
of the M7C3 phase. This behavior is also observed in TEM. From the calculation results and the observation
results of the microstructure, the thermodynamic calculation can be used to predict the precipitation behavior.

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