• 비파괴검사학회지
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• 제16권1호
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• pp.10-18
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• 1996

본 연구에서는 SS41 불림재와 M.E.F. 복합 조직강에 대해서 파괴 역학 파라미터와 X-선 파라미터 사이의 관계를 규명하였다. 피로 균열 진전 실험이 수행되었고, X-선 회절 방법이 피로 파단면에 적용되었다. 피로 파단면을 순차적으로 전해 연마하면서 깊이에 따른 X-선 파라미터(잔류응력, 반가폭)의 변화를 관찰하였다. 최대 소성역 깊이 $w_y$는 SS41 불림재에서는 반가폭의 분포로, M.E.F. 복합 조직강에 대해서는 잔류 응력의 분포로 결정할 수 있었다. 피로 파단면의 깊이 방향에 따른 X-선 파라미터의 분포로부터 $K_{max}$과 $w_y$의 관계식이 실험 데이타로부터 얻어지고, 이 식들으로부터 $K_{max}$은 $w_y$의 측정에 의하여 추정되어질 수 있다.

• 소성∙가공
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• 제29권1호
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• pp.11-19
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• 2020

For microstructural analysis of a friction stir welded (FSWed) joint of advanced high-strength steels, dual phase (DP) and complex phase (CP) steels, are studied. FSWed joints are successfully fabricated in the following four cases: (i) DP/DP; (ii) CP/CP; (iii) DP/CP, where the advancing side is DP and the retreating side is CP; (iv) CP/DP, where the advancing side is CP and the retreating side is DP. The stir zone (SZ) of (i) the DP/DP joint mainly consists of lath martensite, while the stir zone of (ii) the CP/CP joint consists not only of lath martensite but also of bainite. In the case of (iii) DP/CP and (iv) CP/DP, they exhibit a similar microstructure including acicular-shaped phases in the joints; however, cross-sections of the joints show differences in material mixing in each case. In (iv) the CP/DP joint, temperature towards the CP steel is sufficient to cause softening, thus leading to better mixing than that in (iii) DP/CP. The phases of the SZ in each of the four cases are formed by phase transformation during the FSWed process; however, the transformed phase volume fraction of CP steel is lower than that of DP steel, indicating that dynamic recrystallization occurs mainly in CP steel. The hardness values of the SZ are significantly higher than those of the base materials, especially, the SZ of (iii) the DP/CP joint has the highest value due to highest fraction of lath martensite.

• 한국해양공학회지
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• 제5권2호
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• pp.198-198
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• 1991

Not only difference of fatigue crack growth and propagation behavior resulted from the grain size, the hardness ratio and volume fraction in M.E.F. dual phase steel composed of martensite in hard phase and ferrite in soft phase, but also the effects of the plastic constraint were investigated by fracture mechanics and microstructural method. The main results obtained are as follows: 1) The fatigue endurance of M.E.F. steel increases with decreasing the grain size, increasing the ratio of hardness and volume fraction. 2) The initiation of slip and crack occures faster as the stress level goes higher. These phenomena result from the plastic constraint effect of the second phase. 3) The crack propagation rate in the constant stress level is faster as the grain size gets larger, the ratio of hardness lower and volume fraction smaller.

• Corrosion Science and Technology
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• 제5권1호
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• pp.23-26
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• 2006

Recent development of ultrafine grained (UFG) low carbon steels by using equal channel angular pressing (ECAP) and their room temperature tensile properties are reviewed, focusing on the strategies overcoming their inherent mechanical drawbacks. In addition to ferrite grain refinement, when proper post heat treatments are imposed, carbon atom dissolution from pearlitic cementite during ECAP can be utilized for microstructural modification such as uniform distribution of nano-sized cementite particles or microalloying element carbides inside UFG ferrite grains and fabrication of UFG ferrite/martensite dual phase steel. The utilization of nano-sized particles is effective on improving thermal stability of UFG low carbon ferrite/pearlite steel but less effective on improving its tensile properties. By contrast, UFG ferrite/martensite dual phase steel exhibits an excellent combination of ultrahigh strength, large uniform elongation and extensive strain hardenability.

• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part 1
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• pp.169-170
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• 2006

Advanced melting technology is now being employed in the manufacture of stainless steel powders. The new process currently includes electric arc furnace (EAF) technology in concert with Argon Oxygen Decarburization (AOD), High Performance Atomizing (HPA) and hydrogen annealing. The new high performance-processing route has allowed the more consistent production of existing products, and has allowed enhanced properties, such as improved green strength and green density. This paper will review these processing changes along with the potential new products that are being developed utilizing this technology. These include high strength stainless steels such as duplex and dual phase as well as stainless steel powders used in high temperature applications such as diesel filters and fuel cells.

• 열처리공학회지
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• 제10권2호
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• pp.128-137
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• 1997

The formation processes of the retained austenite(${\gamma}_R$) in SHCP100 steel sheets were investigated in order to improve the transformation induced plasticity(TRIP) effect of ${\gamma}_R$. An excellent combination of elongation about 23% and high strength over 830 MPa was achieved by processing of intercritical annealing and isothermal holding. The mechanical properties of TRIP-aided dual phase steel was found to depend on the volume ratio of each phase and the volume fraction of ${\gamma}_R$. It was also noted that the proper mechanical stability of ${\gamma}_R$ improved the mechanical properties. In this work, the best balance of strength-ductility was obtained by holding the steel at $420^{\circ}C$ for 500sec. after annealing at $730^{\circ}C$ for 300 sec.

• Journal of Welding and Joining
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• 제29권5호
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• pp.99-105
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• 2011

In this study, effect of Si content on nugget diameter in electric resistance spot welded dual-phase(DP) steel was investigated. The cold rolled DP steels with different Si content (0.5, 1.0, 1.5, 2.0 wt.%) were used and thickness of those sheet was 1.2mm. With increasing Si content, nugget diameter was increased at the same welding current. This is attributed to increase of heat input result from high resistivity. Also, nugget diameter was increased with an increase in Si content for the same heat input. For this reason, the melting point of DP steel is lowered with an increase in the Si content. And solid DP steel can easily be transformed to a liquid phase because the low melting point. Finally, a prediction formula for the nugget diameter(N.D.) could be obtained in terms of heat input(Q) and melting point(M.P) as follows: N.D.(mm) = 0.11Q(J) - 0.0031 M.P.($^{\circ}C$) + 0.32.

• 열처리공학회지
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• 제30권3호
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• pp.106-112
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• 2017

In this study dual-phase steels with different ferrite grain size and martensite fraction were fabricated by varying micro-alloying elements and intercritical anneling temperatures, and then the tensile properties were investigated in terms of yield and tensile strengths, elongation, and yield ratio. The addition of micro-alloying elements reduced ferrite grain size, and the increased intercritial transformation tempeature increased the martensite fracton. The tensile test results showed that yield and tensile strengths of all the steel specimens increased with increasing the martensite fraction. However, the elongation and yield ratio were differently changed according to variations in the morphology and carbon content of martensite, ferrite grain size, and precipitates resulting from the addition of micro-alloying elements and intercritical annealing.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2003년도 추계학술대회논문집
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• pp.269-273
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• 2003

There is a growing interest to replace the commercial steels with non-heat treated steels, which does not involve the spheroidization and quenching-tempering treatment. However, Non-heat treated steels should satisfy high strength and good formability without performing heat treatment. Therefore, it is important to investigate optimum materials showing a good combination of strength and formability after the drawing process. In this study, Dual-Phase Steels were studied as candidate materials for non-heat treated steels, which have different martensite morphologies and volume fractions obtained through heat-treatment of intercritical quenching (IcQ), intermediate quenching (ImQ) and step quenching (SQ). The mechanical properties of DP steels were measured by tension and compression tests. Also, the cold formability of three DP steels which have similar tensile strength value was investigated by estimating the deformation resistance and the forming limit. The deformation resistance which is important factor in determining die life was estimated by calculating the deformation energy. And the forming limit was estimated by measuring the critical strain revealing crack initiation at the notch tip of the specimens.

• 소성∙가공
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• 제13권1호
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• pp.84-89
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• 2004

There is a growing interest to replace the commercial steels with non-heat treated steels, which does not involve the spheroidization and quenching-tempering treatment. However, Non-heat treated steels should satisfy high strength and good formability without performing heat treatment. Therefore, it is important to investigate optimum materials showing a good combination of strength and formability after the drawing process. In this study, Dual-Phase Steels were studied as candidate materials for non-heat treated steels, which have different martensite morphologies and volume fractions obtained through heat-treatment of intercritical quenching (IcQ), intermediate quenching (ImQ) and step quenching (SQ). The mechanical properties of DP steels were measured by tension and compression tests. Also, the cold formability of three DP steels which have similar tensile strength value was investigated by estimating the deformation resistance and the forming limit. The deformation resistance which is important factor in determining die life was estimated by calculating the deformation energy. And the forming limit was estimated by measuring the critical strain revealing crack initiation at the notch tip of the specimens.