In this study, effects of carbon and nickel on microstructure and low temperature Charpy impact properties of HSLA (high strength low alloy) steels are investigated. To understand the complex phase transformation behavior of HSLA steels with high strength and toughness before and after welding processes, three kinds of HSLA steels are fabricated by varying the carbon and nickel content. Microstructure analysis, low temperature Charpy impact test, and Vickers hardness test are performed for the base metals and CGHAZ (coarse-grain heat affected zone) specimens. The specimens with the lowest carbon and nickel content have the highest volume fraction of AF, the lowest volume fraction of GB, and the smallest GB packet size. So, the low temperature Charpy absorbed energy of the CGHAZ specimen is the highest. The specimens with increased carbon and nickel content have the lowest volume fraction of AF, the highest volume fraction of GB, and the largest GB packet size. So, the low temperature Charpy absorbed energy of the CGHAZ specimen is the lowest.

In the present study the microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes was characterized and identified based on various microstructure analysis methods including optical and scanning electron microscopy, and electron backscatter diffraction(EBSD). Although low-carbon steels are usually composed of α-ferrite and cementite(Fe3C) phases, they can have complex microstructures consisting of ferrites with different size, morphology, and dislocation density, and secondary phases dependent on rolling and accelerated cooling conditions. The microstructure of lowcarbon steels investigated in this study was basically classified into polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite based on the inverse pole figure, image quality, grain boundary, kernel average misorientation(KAM), and grain orientation spread(GOS) maps, obtained from EBSD analysis. From these results, it can be said that the EBSD analysis provides a valuable tool to identify and quantify the complex microstructure of low-carbon steels fabricated by controlled rolling and accelerated cooling processes.

This present study deals with the effect of micro-alloying elements and transformation temperature on the correlation of microstructure and tensile properties of low-carbon steels with ferrite-pearlite microstructure. Six kinds of lowcarbon steel specimens were fabricated by adding micro-alloying elements of Nb, Ti and V, and by varying isothermal transformation temperature. Ferrite grain size of the specimens containing mirco-alloying elements was smaller than that of the Base specimens because of pinning effect by the precipitates of carbonitrides at austenite grain boundaries. The pearlite interlamellar spacing and cementite thickness decreased with decreasing transformation temperature, while the pearlite volume fraction was hardly affected by micro-alloying elements and transformation temperature. The room-temperature tensile test results showed that the yield strength increased mostly with decreasing ferrite grain size and elongation was slightly improved as the ferrite grain size and pearlite interlamellar spacing decreased. All the specimens exhibited a discontinuous yielding behavior and the yield point elongation of the Nb4 and TiNbV specimens containing micro-alloying elements was larger than that of the Base specimens, presumably due to repetitive pinning and release of dislocation by the fine precipitates of carbonitrides.

In this study, six kinds of low-carbon steel specimens with different ferrite-pearlite microstructures were fabricated by varying the Nb content and the transformation temperature. The microstructural factors of ferrite grain size, pearlite fraction, interlamellar spacing, and cementite thickness were quantitatively measured based on optical and scanning electron micrographs; then, Charpy impact tests were conducted in order to investigate the correlation of the microstructural factors with the impact toughness and the ductile-brittle transition temperature (DBTT). The microstructural analysis results showed that the Nb4 specimens had ferrite grain size smaller than that of the Nb0 specimens due to the pinning effect resulting from the formation of carbonitrides. The pearlite interlamellar spacing and the cementite thickness also decreased as the transformation temperature decreased. The Charpy impact test results indicated that the impact-absorbed energy increased and the ductile-brittle transition temperature decreased with addition of Nb content and decreasing transformation temperature, although all specimens showed ductile-brittle transition behaviour.

The present study is concerned with the influence of niobium(Nb) addition and austenitizing temperature on the hardenability of low-carbon boron steels. The steel specimens were austenitized at different temperatures and cooled with different cooling rates using dilatometry; their microstructures and hardness were analyzed to estimate the hardenability. The addition of Nb hardly affected the transformation start and finish temperatures at lower austenitizing temperatures, whereas it significantly decreased the transformation finish temperature at higher austenitizing temperatures. This could be explained by the non-equilibrium segregation mechanism of boron atoms. When the Nb-added boron steel specimens were austenitized at higher temperatures, it is possible that Nb and carbon atoms present in the austenite phase retarded the diffusion of carbon towards the austenite grain boundaries during cooling due to the formation of NbC precipitate and Nb-C clusters, thus preventing the precipitation of M23(C,B)6 along the austenite grain boundaries and thereby improving the hardenability of the boron steels. As a result, because it considerably decreases the transformation finish temperature and prohibits the nucleation of proeutectoid ferrite even at the slow cooling rate of 3 oC/s, irrespective of the austenitizing temperature, the addition of 0.05 wt.% Nb had nearly the same hardenability-enhancing effect as did the addition of 0.2 wt.% Mo.

The effect of tungsten (W) addition on the hardenability of low-carbon boron steels was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy. The hardenability was discussed with respect to transformation behaviour aspects depending on the segregation and precipitation of boron at austenite grain boundaries. A critical cooling rate producing a hardness corresponding to 90 % martensite structure was measured from a hardness distribution plot, and was used as a criterion to estimate hardenability at faster cooling rates. In the low-carbon boron steel, the addition of 0.50 wt.% W was comparable to that of 0.20 wt.% molybdenum in terms of critical cooling rate, indicating hardenability at faster cooling rates. However, the addition of 0.50 wt.% W was not more effective than the addition of .0.20 wt.% molybdenum at slower cooling rates. The addition of 0.20 wt.% molybdenum completely suppressed the formation of eutectoid ferrite even at the slow cooling rate of 0.2˚C/s, while the addition of 0.50 wt.% W did not, even at the cooling rate of 1.0˚C/s. Therefore, it was found that the effect of alloying elements on the hardenability of low-carbon boron steels can be differently evaluated according to cooling rate.

The hardenability of low-carbon boron steels with different molybdenum and chromium contents was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy (SIMS), and then discussed in terms of the segregation and precipitation behaviors of boron. The hardenability was quantitatively evaluated by a critical cooling rate obtained from the hardness distribution plotted as a function of cooling rate. It was found that the molybdenum addition was more effective than the chromium addition to increase the hardenability of boron steels, in contrast to boron-free steels. The addition of 0.2 wt.% molybdenum completely suppressed the formation of eutectoid ferrite, even at the slow cooling rate of 0.2˚C/s, while the addition of 0.5 wt.% chromium did this at cooling rates above 3˚C/s. The SIMS analysis results to observe the boron distribution at the austenite grain boundaries confirmed that the addition of 0.2 wt.% molybdenum effectively increased the hardenability of boron steels, as the boron atoms were significantly segregated to the austenite grain boundaries without the precipitation of borocarbide, thus retarding the austenite-to-ferrite transformation compared to the addition of 0.5 wt.% chromium. On the other hand, the synergistic effect of molybdenum and boron on the hardenability of boron steels could be explained from thermodynamic and kinetic perspectives.

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.

The present work was performed to investigate the effect of coiling temperature on the annealed texture in Cu/Nb-added ultra-low-carbon steels. The ultra-low-carbon steels were coiled at 650 and 720˚C, respectively. The result showed that the Cu-added ultra-low-carbon steel at a low coiling temperature produced a desirable annealed texture related to good formability. On the other hand, Nb-added ultra-low-carbon steel at a high coiling temperature also produced a desirable texture. This is attributed to the effect of Nb, which retards recrystallization during the coiling process.

The texture and microstructure in Cu/Nb added ultra low carbon steels through the different thickness layer were studied after hot rolling. It was found that the two ultra low carbon steels all show the inhomogeneity of hot rolling texture and the Cu-added ultra low carbon steel was far more inhomogeneous than Nb-added one. In the center layer, the strong α fibre, γ fibre textures and the shear textures including 001<110>, 111<112> were founded. Near the surface, the α fibre texture and the orientation texture caused by a typical plane-strain deformation condition of bcc metals were observed.

Likelihood of failure for the external corrosion of carbon and low alloy steels, which affect to a risk of facilities, was analyzed quantitatively through the risk based inspection using API-581 BRD. We found that the technical module subfactor (TMSF) decreased as the inspection number increased and it increased as the Inspection effectiveness and the used year increased. In this condition, the TMSF showed high value for the case of the marine/cooling tower drift area as a corrosion driver, poor quality of coating, no insulation, and low insulation condition.

최근의 연구에 의하면 직접 소입강에서 미량의 베이나이트상의 생성이 확인되었다. 마르텐사이트 변태전에 생성된 베이나이트상은 마르텐사이트의 패킷을 미세화시키고 입도 미세화로 기계적 성질을 향상시킨다고 한다. 본 연구에서는 미량 합금 원소로 Mo, B 등을 첨가한 강을 1200˚C에서 단조하여 물에 직접 소입한 후, 베이나이트상의 분율을 조절하기 위해 베이나이트 변태 온도에서 일정시간 유지하는 열처리 과정을 거쳐 생성된 베이나이트상의 분율과 기계적 성질간의 관계를 고찰하였다. 이로써 마르텐사이트와 베이나이트 혼합 조직의 강도와 인성을 증가시키는 효과적인 베이나이트 분율을 조사한 후에, 직접 소입시에 이와 같은 분율의 베이나이트 함량으로 조절할 수 있는 방법을 제시하여 고강도 고인성형 직접 소입강의 개발에 활용하고자 한다.

공냉에 의한 중탄소 비조질강은 페라이트와 펄라이트의 미세구조를 갖는다. 기존의 조질강과 비교하여 이재료는 V, Nb, Ti을 첨가하여 강도는 거의 동등한 반면 인성에 있어서 조질강에 비해 취약하다. 본 연구에서는 직접 소입 저탄소 비조질강의 기계적 성질에 미치는 몰리브덴의 영향을 조사하였다. 몰리브덴 첨가 직접 소입 비조질강은 고강도와 고인성을 나타내었다. 몰리브덴은 변태 온도를 저하시켜 미세한 구조와 균일한 석출 구조를 효과적으로 생성시킨ㄷ. 본 실험의 결과로 0.15C+0.11V+0.035Nb+1.81Mo의 재료를 유냉시켜 121MPo의 강도와 127.5J의 인성을 얻었다.

공냉에 의한 중탄소 비조질강과 직접 소입 방법에 의한 저탄소 비조질강의 강도와 인성에 대하여 미량 첨가 원소와 온도 및 냉각 속도의 영향을 조사하였다. 공냉에 의한 중탄소 비조질강은 페라이트-펄라이트 조직으로 V+Nb 복합첨가로 강도와 인성의 조합을 이룰 수 있었으며 연구 결과 최적 조합은 0.40C+0.12V+0.07Nb에 의한 인장 강도 831MPa, 충격 52.1J이었다. 직접 소입 방법에 의한 저탄소 비조질강은 마르텐사이트 조직으로 Mo에 의한 강도와 인성의 조합을 이를 수 있었으며 가장 양호한 조합은 0.12C+0.10V+0.03Nb+1.13Mo에 의한 인장강도 855MPa, 충격인성 108J로써 중탄소 비조질강에 비해 충격 인성이 2배 정도 향상되었다. 가열온도 1100˚C가 1200˚C에 비해 더 양호한 강도 및 충격 인성을 나타내었고, 냉각속도는 1.2˚C/s가 가장 양호한 결과를 나타내었다. 또한 합금원소와 온도 및 냉각속도가 강도, 충격인성, 오스테나이트 결정립 크기 그리고 펄라이트 층간거리에 미치는 영향을 중회귀분석하여 계량화하였다.