• 한국주조공학회지
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• 제12권5호
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• pp.403-411
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• 1992

The growth characteristics of bainite at early stage in the fast quenched spheroidal graphite cast irons containing 0.06%Mn and 0.45%Mn during austempering process, was investigated with optical and scanning electron microscope. The following results regarding the effects of Mn and isothermal heat treatment on the morphological variation of bainitic ferrite were obtained. The morphology of bainite varies from acicular below 350$^{\circ}C$ to feather shape above 350$^{\circ}C$. The period of isothermal treatment also affects the shape of bainite at the fixed temperature. At 350$^{\circ}C$, bainite is bamboo leaf-like up to 200 secs of isothermal holding time and with further increasing time up to 300 secs, changes to a mixed structure consisting of both feather and bamboo leaf and, finally becomes all feather shape at 900 secs. The morphology of bainitic ferrite formed at early stage of 300$^{\circ}C$ isothermal treatment is similar to that of bainitic ferrite formed at 250$^{\circ}C$ or 350$^{\circ}C$ with unbranched, linear ferrite. However, bainitic ferrite divides into branches with increasing isothermal treatment, which occurs more fast at 400$^{\circ}C$ than at 350$^{\circ}C$. The difference in adding amount of Mn influences the morphology of bainitic ferrite in upper bainite. The bainitic ferrite with 0.45%Mn is observed to be more stable than that with 0.06%Mn, remaining unbranched for a longer period at the same temperature.

• Corrosion Science and Technology
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• 제23권1호
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• pp.11-19
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• 2024

A Fe-Ni Bainitic steel as a weathering steel application was developed by combining its excellent mechanical properties and corrosion resistance in maritime environments. Nickel concentration (0.4-3 wt%) and inverted austempering multi-step (IAM) process were primary determinants of the microstructure of the Fe-Ni Bainitic steel. The initial austempering steel was performed at 300 ℃ for 600 seconds to obtain a partly bainitic transformation. The steel was heated again for 1800 s at 450 ℃. The microstructure was comprised of ferrite, a blocky martensite/austenite island, and a homogeneous lath-shape bainite structure with widths ranging from 4.67 to 6.89 ㎛. The maximum strength, 1480 MPa, was obtained with 3 wt% nickel. In this study, corrosion behavior was investigated utilizing potentiodynamic and electrochemical impedance spectroscopy (EIS) tests. A higher nickel content in Fe-Ni Bainitic steel refined the grain size, improved the bainite fraction, lowered the corrosion rate to 0.0257 mmpy, and increased the charge transfer of film resistance to 1369 Ω.

• 한국주조공학회지
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• 제22권5호
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• pp.231-237
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• 2002

The structure and properties of HSLA steel obtained by Osprey forming process were investigated. The porosities were generated at the bottom of specimen due to the quenching effect of cold substrate during forming. The steel had a bainitic structure. The grain size were different among positions due to the cooling rates. The elements such as Mn, S, Si, Cr, Mo and etc. were contained in inclusions. They were formed at grain boundries. The size of them was $1{\sim}2{\mu}m$ and has nothing to do with the chemical composition. MnS and NbC were precipitated during rolling and aging.

• 한국재료학회지
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• 제23권8호
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• pp.423-429
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• 2013

Recently, steel structures have increasingly been required to have sufficient deformability because they are subjected to progressive or abrupt displacement arising from structure loading itself, earthquake, and ground movement in their service environment. In this study, high-strength low-carbon bainitic steel specimens with enhanced deformability were fabricated by varying thermo-mechanical control process conditions consisting of controlled rolling and accelerated cooling, and then tensile and Charpy V-notch impact tests were conducted to investigate the correlation between microstructure and mechanical properties such as strength, deformability, and low-temperature toughness. Low-temperature transformation phases, i.e. granular bainite (GB), degenerate upper bainite(DUB), lower bainite(LB) and lath martensite(LM), together with fine polygonal ferrite(PF) were well developed, and the microstructural evolution was more critically affected by start and finish cooling temperatures than by finish rolling temperature. The steel specimens start-cooled at higher temperature had the best combination of strength and deformability because of the appropriate mixture of fine PF and low-temperature transformation phases such as GB, DUB, and LB/LM. On the other hand, the steel specimens start-cooled at lower temperature and finish-cooled at higher temperature exhibited a good low-temperature toughness because the interphase boundaries between the low-temperature transformation phases and/or PF act as beneficial barriers to cleavage crack propagation.

• 한국재료학회지
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• 제29권8호
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• pp.511-518
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• 2019

In this study, the effect of carbon equivalent and cooling rate on microstructure and hardness of A516 steels for pressure vessel is investigated. Six kinds of specimens are fabricated by varying carbon equivalent and cooling rate, and their microstructures and hardness levels are analyzed. Specimens with low carbon equivalent consist of ferrite and pearlite. As the cooling rate increases, the size of pearlite decreases slightly. The specimens with high carbon equivalent and rapid cooling rates of 10 and $20^{\circ}C/s$ consist of not only ferrite and pearlite but also bainite structure, such as granular bainite, acicular ferrite, and bainite ferrite. As the cooling rate increases, the volume fractions of bainite structure increase and the effective grain size decreases. The effective grain sizes of granular bainite, acicular ferrite, and bainitic ferrite are ~20, ~5, and ${\sim}10{{\mu}m$, respectively. In the specimens with bainite structure, the volume fractions of acicular ferrite and bainitic ferrite, with small effective grains, increase as cooling rate increases, and so the hardness increases significantly.

• 열처리공학회지
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• 제10권1호
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• pp.10-19
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• 1997

This study has been conducted to investigate the effects of initial structure on the microstructure and tensile properties of high strength trip steel sheet. The initial structure before austempering remarkably influenced the second phase. The specimen with normalized initial structure showed mainly bainitic ferrite and retained austenite, while the as rolled specimen and spherodized specimen showed martensite plus retained austenite and martensite plus bainitic ferrite with small retained austenite, respectively. Two type of retained austenite, film type and granual type were observed in all specimens. The as rolled specimen appeared the highest contents of retained austenite owing to the compressive stress by cold rolling. The contents of retained austenite increased with increasing intercritical annealing temperature and austempering time. Tensile strength showed the highest in the as rolled specimen, while the highest elongation were obtained in the normalized specimen. The maximum T.S.${\times}$El. Value showed in normalized initial structure and increased with increasing intercritical annealing and austempering time. The highest Value of T.S.${\times}$El. obtained at austempering temperature of $400^{\circ}C$ and retained austenite of 12%.

• 한국주조공학회지
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• 제16권2호
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• pp.109-115
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• 1996

Effect of various austempered structures on fracture characteristics of C/V graphite cast iron has studied. The tensile strength and hardness reached the maximum value of 971.4MPa and HB302 at the austempering temperature of $250^{\circ}C$, respectively. As the austempering temperature increased, the amount of retained austenite increased from 18% to 22, 29%, while $K_{IC}$ values ranged from the value of $65MPa{\cdot}m^{12} to 70MPa{\cdot}m^{1/2}, 66MPa{\cdot}m^{1/2}. This fact that $K_{IC}$ value was not sensitive to the increase of the amount of the retained austenite was that $K_{IC}$ was dependent on the matrix structure in lower bainitic matrix, while dependent on the notch effect from C/V graphite shape in upper bainitic matrix. Fractured surfaces showed a ductile fracture pattern at $300^{\circ}C$. Very large coalescence by C/V graphite and relatively small voids by spheroidal graphite were observed.

• 열처리공학회지
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• 제6권2호
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• pp.49-58
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• 1993

Isothermal transformation behavior during patenting and variations of microstructure and tensile strength of patented wires were investigated in Si-added high carbon steel. The TTT curves of the steels were made for two different austenitizing temperature. As the salt bath temperature was increased, the observed microstructures were bainite at $450^{\circ}C$, the mixture of bainite and pearlite at $500^{\circ}C$, and to pearlite at $600^{\circ}C$, The tensile strength of patented wire exhibited the highest value when the structure was pearlite. while the bainitic structure showed the lowest.

• 한국가스학회지
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• 제24권5호
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• pp.39-46
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• 2020

본 논문에서는 파이프라인에서 사용되는 API 5L 철강의 중심선 편석에 대해 다루고자 한다. Mn/S와 Nb 그리고 C는 파이프 두께 방향으로의 중심선에서 편석되는 요소로 알려져 있다. Mn은 일반적으로 긴 점성상을 이루는 S를 동반한다. 중심선의 미세조직은 MnS와 산소를 포함한 Nb/Ti로 구성되어 있는데 이 중심선 지역의 편석효과를 OM, SEM/EDS 및 마이크로비커스 경도기로 분석하였다. Mn, Nb 및 C는 오스테나이트가 페라이트 또는 마르텐사이트로 변태하는 것을 억제시키는 원소이다. 이러한 원소들은 마르텐사이트의 한 종류인 베이나이트 미세조직을 만들 수 있는데 이는 용융 및 열처리에서 얻어지는 중심선과 매트릭스 간의 원소 편석과는 다르다.

• 한국주조공학회지
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• 제11권4호
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• pp.323-330
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• 1991

The relationships between the microstructure changes, retained austenite volume and elevated temperature tensile properties of Mo-Ni-Cu ADI corresponding to various austempering temperatures and time were investigated, After the $250^{\circ}C$ tensile test for the test piece austempered at $270^{\circ}C$ the accicular bainite structure was observed blunted under room temperature microscope. In the case of $370^{\circ}C$ austempering, the feathery bainite lath spacing was observed broadened. But after the $450^{\circ}C$ tensile test, bainitic features could not be observed. As the testing temperature increased, retained austenite volume tested at room temperature decreased. Especially, after the $450^{\circ}C$ tensile test retained austenite volume approached nearly to zero. A little higher tensile properties appeared at $250^{\circ}C$ testing than those at room temperature.