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복합조직강의 연속어닐링과정에서 미세조직과 집합조직의 변화
정우창 ( Woo Chang Jeong ) 한국열처리공학회 2015 熱處理工學會誌 Vol.28 No.4
The variation in microstructure and texture during continuous annealing was examined in a series of 1.6% Mn-0.1% Cr-0.3% Mo-0.005% B steels with carbon contents in the range of 0.010 to 0.030%. It was found that microstructure of hot band consisted of ferrite and pearlite as a consequence of high coiling temperature, and eutectoid carbon content was between 0.011% and 0.016%. Martensite ranged in volume fraction from 1.5% to 4.0% when annealed at 820oC according to the typical continuous annealing cycle. The critical martensite content for the continuous yielding was about 4% from stress-strain curves. The continuous yielding was obtained in the 0.030% carbon steel and 0.010% to 0.020% carbon steels revealed some yield point elongation ranging from 0.8% to 2.2% in as-annealed conditions. Higher tensile strength in the higher carbon steel is due to both increase in the martensite volume fraction and ferrite grain refinement. Decreasing the carbon content to 0.01% strengthened the intensities of γ-fiber textures, resulting in the increase in the rm value, which was caused by the lower volume fraction of martensite. The higher carbon steels showed the lower rm value of about 1.0.
정우창 ( Woo Chang Jeong ) 한국열처리공학회 2014 熱處理工學會誌 Vol.27 No.2
The effect of carbon on the microstructure and texture of low carbon steels was investigated in a series of 1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon ranging from 0.021 to 0.048%. Intensity of {111} orientation increased with decreasing the carbon content, resulting in the increase in rm value. The highest rm value of1.30 was obtained in 0.021%C steel annealed at 820~850oC according to the typical galvannealing heat cycle. Marten site volume fraction was not substantially affected by the annealing temperature. It was found that the fine and uniformly distributed marten site particles which were present in amounts of about 5% volume fraction were desirable for the highest rm value. The other factor affecting the high rm value was the preferred epitaxial growth of retained ferrite with {111} orientation into austenite during cooling.
고장력 냉연강판에서 미세조직에 대한 연속어닐링 조건의 영향
정우창 ( Woo Chang Jeong ) 한국열처리공학회 2004 熱處理工學會誌 Vol.17 No.5
N/A The effect of the annealing parameters on microstructures were examined in a cold-rolled high strength steel containing 0.1% C, 0.5% Si, 1.5% Mn, and 0.04% Nb. It was impossible to avoid martensite in the microstructure even though the continuous annealing parameters were controlled. This indicates that the alloying elements such as silicon and manganese contributing to manganese equivalent (Mn_(eq)) should be reduced to pro-duce the ferrite-pearlite microstructure for the solid solution and precipitation hardened steel. It was found that a decrease in the rapid cooling temperature to 520℃ was effective to change the microstructure from ferrite-martensite to ferrite-pearlite-martensite. Typical dual-phase properties exhibiting a low yield ratio and a continuous yielding behavior were obtained when the rapid cooling temperature was in the range of 680℃ to 600℃. The critical volume fraction of martensite for the typical properties of dual-phase steel was about 11 percent.
인장강도 1200 MPa 급 자동차 서브 프레임의 합금성분 최적화 및 열변형 거동 연구
정우창 ( Woo Chang Jeong ) 한국열처리공학회 2020 熱處理工學會誌 Vol.33 No.3
Four air hardening steels with carbon, silicon, manganese, chromium, and molybdenum variations have been used in this study to find out the optimal chemical compositions of steels with over 1200 MPa tensile strength for automotive subframe. The dimensional changes after heat treatment were determined for two automotive parts with open and closed cross sections using 3D scanner. When four steels were austenitized at 900℃ for 30 seconds, cooled at 3℃/s, reheated to 450℃ for 10 seconds followed by air cooling to simulate hot-dip galvanizing treatment showed ultra high tensile strength over 1200 MPa. Rear floor cross member with open cross section revealed much bigger dimensional changes than subframe with closed cross section after heat treatment at 900℃ for 20 minutes followed by air cooling. (Received April 16, 2020; Revised April 27, 2020; Accepted April 28, 2020)
0.27% C-1.5% Mn-1.0% Cr 강의 미세조직과 기계적성질에 미치는 Si의 영향
정우창 ( Woo Chang Jeong ) 한국열처리공학회 2017 熱處理工學會誌 Vol.30 No.3
The variation in microstructure and mechanical properties during heat treatment was examined in a series of 0.27% C-1.5% Mn-1.0% Cr steels with silicon contents in the range of 0 to 1.0 wt%. It was found that addition of 0.5%~1.0% silicon increased both tensile strength and impact toughness through solid solution strengthening and microstructural refinement. 0.27% C-1.0% Si-1.5% Mn-1.0% Cr steel showed tensile strength of 1,700 MPa in the as-quenched condition and the steel revealed a full martensitic structure even after air cool-ing from 900℃ to room temperature, showing air hardening characteristics. Tempering at 150℃ which corre-sponds to the typical paint-baking temperature after painting of body in white, slightly decreased the tensile strength and increased elongation, but substantially increased the impact toughness compared to the as-quenched steel. (Received April 10, 2017; Revised April 21, 2017; Accepted April 27, 2017)
Al-Si 도금강의 통전 가열에 따른 미세조직과 도금층 변화
정우창 ( Woo Chang Jeong ) 한국열처리공학회 2021 熱處理工學會誌 Vol.34 No.3
Al-Si coated boron steel has been widely used as commercial hot stamping steel. When the steel is heated at 900~930℃ for 5 min in an electric furnace, thickness of the coating layer increases as a consequence of formation of intermetallic compounds and diffusion layer. The diffusion layer plays an important roll in blunting the propagation of crack from coating layer to base steel. Change in microstructure and coating layer of Al-Si coated boron steel after conductive heating with higher heating rate than electric furnace has been investigated in this study. Conductive-heated steel showed the martensitic structure with vickers hardness of 505~567. Both intermetallic compounds in coating layer and diffusion layer were not observed in conductive-heated steel due to rapid heating. It has been found that the conductive-heating consisting of rapid heating to 550℃ which is lower than melting point of Al-Si coating layer, slower heating to 900℃, and then 1 min holding at 900℃ is effective in forming intermetallic compound in coating layer and diffusion layer. (Received April 16, 2021; Revised April 23, 2021; Accepted April 30, 2021)