• Corrosion Science and Technology
• /
• 제7권5호
• /
• pp.288-295
• /
• 2008

The increase use of natural gas as an energy source has been continuous demand for ever-increasing strength in gas transmission pipeline materials in order to achieve safe and economic transportation of natural gas. In particular, linepipe material for sour gas service primarily needs to have crack resistant property. However, applications of sour linepipes are expanding toward deep water or cold region, which require higher toughness and/or heavier wall thickness as well as higher strength. To improve the crack resistance of linepipe steel in sour environment, low alloy steel are produced by controlled rolling subsequently followed by the accelerated cooling process. This paper summarizes the design concepts for controlling crack resistant property low alloy linepipe steels for sour gas service.

• Journal of Welding and Joining
• /
• 제8권4호
• /
• pp.46-57
• /
• 1990

TMCP steel manufactured by controlled rolling followed by accelerated cooling process is known to have extra-ordinary mechanical properties such as tensile strength and toughness. However, there is much uncertainty about the fatigue fracture characteristics, especially, in the welded state of this steel. In case of this steel, the softening zone by welding is generated in heat affected zone in contrast with the case of conventional normalized high strength steel. This softening zone is considered to play significant roles in low cycle fatigue fracture of the welded part of this steel. In this paper, the low cycle fatigue behaviors of TMCP steel were inspected in as-received and welded state using the smooth specimen. The fatigue life-time was seperately investigated on the basis of failure of the specimen and crack initiation which is detected by differential strain method. Moreover, the low cycle fatigue characteristics of TMCP steel were quantitatively compared with those of the conventional normalized steel of same strength level.

• 한국재료학회지
• /
• 제23권9호
• /
• pp.525-530
• /
• 2013

This study investigated the continuous cooling transformation, microstructure, and mechanical properties of highstrength low-alloy steels containing B and Cu. Continuous cooling transformation diagrams under non-deformed and deformed conditions were constructed by means of dilatometry, metallographic methods, and hardness data. Based on the continuous cooling transformation behaviors, six kinds of steel specimens with different B and Cu contents were fabricated by a thermomechanical control process comprising controlled rolling and accelerated cooling. Then, tensile and Charpy impact tests were conducted to examine the correlation of the microstructure with mechanical properties. Deformation in the austenite region promoted the formation of quasi-polygonal ferrite and granular bainite with a significant increase in transformation start temperatures. The mechanical test results indicate that the B-added steel specimens had higher strength and lower upper-shelf energy than the B-free steel specimens without deterioration in low-temperature toughness because their microstructures were mostly composed of lower bainite and lath martensite with a small amount of degenerate upper bainite. On the other hand, the increase of Cu content from 0.5 wt.% to 1.5 wt.% noticeably increased yield and tensile strengths by 100 MPa without loss of ductility, which may be attributed to the enhanced solid solution hardening and precipitation hardening resulting from veryfine Cu precipitates formed during accelerated cooling.

• 한국재료학회지
• /
• 제22권1호
• /
• pp.16-23
• /
• 2012

Research into the development of high strength (1 GPa) and superior formability, such as total elongation (10%), and stretch-flangeability (50%) in hot-rolled steel was conducted with a thermomechanically controlled hot-rolling process. To improve the overall mechanical properties simultaneously, low-carbon steel using precipitation hardening of Ti-Nb-V multimicroalloying elements was employed. And, ideal microstructural characteristics for the realization of balanced mechanical properties were determined using SEM, EBSD, and TEM analyses. The developed steel, 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V), consisted of ferrite as the matrix phase and second phase of granular bainite with fine carbides (20-50 nm) in both phases. The significant factor of the microstructural characteristics that affect stretch-flangeability was found to be the microstructural homogeneity. The microstructural homogeneity, manifest in such characteristics as low localization of plastic strain and internally stored energy, was identified by grain average misorientation method, analyzed by electron backscattered diffraction (EBSD) and hardness deviation between the phases. In summar, a hot-rolled steel having a composition 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V) demonstrated a tensile strength of 998 MPa, a total elongation of 19%, and a hole expansion ratio of 65%. The most important factors to satisfy the mechanical property were the presence of fine carbides and the microstructural homogeneity, which provided low hardness deviation between the phases.

• 제어로봇시스템학회:학술대회논문집
• /
• 제어로봇시스템학회 2000년도 제15차 학술회의논문집
• /
• pp.180-180
• /
• 2000

The shape of cold strip for the stainless process has been become issue in quality recently, and hence POSCO (Pohang Iron & Steel Co., Ltd) developed an automatic control system for strip shape in the sendzimir mill. The strip shape is measured by an outward measuring roll and is controlled by As_U roll and first intermediate roll. As_U roll consists of 8 saddles, which are controlled vertically. The fist intermediate rolls, which are controlled horizontally, consist of two pairs of rolls up and down. A developed shape control system is applied to real plant by using fuzzy logic and neural network method to control actuators; As_U roll and first intermediate roll. This system composes mainly of three parts as a real-time system, input to output conditioner board, and man-machine interface. The actual shape is recognized by neural network and converted into symmetric shape. The fuzzy controller, based on the shape from neural network and sensor, controls positions of the As_U roll and first intermediate roll. This paper verifies the shape controller performance. The experiments are made on line for the sendzimir mill. The shape control performance shows very efficient for the target tracking, shape symmetry, and fluctuation of shape.

• 한국강구조학회 논문집
• /
• 제15권3호
• /
• pp.331-339
• /
• 2003

Thermo Mechanical Control Process(TMCP) 강재는 열간압연시에 압연 온도를 제어하면서 경우에 따라 압연직후 냉각, 열처리하여 안정된 조직으로 압연, 제조된 강재이다. 본 연구에서는 극후판 TMCP강재의 소재특성과 건축구조용 강재로서의 적합성과 특성을 밝히기 위하여 화학성분 및 조직특성, 내력 및 기계적 특성, 사용성 및 인성, 등으로 분류하여 소재특성과 용접특성을 분석하였다. 실험결과, 대상강재는 극후판에서도 설계기준강도를 만족하고 낮은 탄소당량 ($C_{eq}$) 및 용접갈라짐 감수성조성($P_cm$)과 저항복비 등이 확보되었다. 또한, 기준온도(${\pm}0^{\circ}C$)는 물론 $-60^{\circ}C$의 극저온에서도 충분한 충격흡수에너지값으로 양호한 인성의 소재특성을 나타냈고, 용접부에서도 경화현상이 저감되고, 용접부의 인성 및 내력이 충분한 것으로 나타났다.

• 대한금속재료학회지
• /
• 제48권9호
• /
• pp.798-806
• /
• 2010

The effects of alloying elements and the cooling condition on the microstructure, tensile properties, and Charpy impact properties of high-strength bainitic steel plates fabricated by a controlled rolling process were investigated in the present study. Eight kinds of steel plates were fabricated by varying C, Cr, and Nb additions under two different cooling rates, and their microstructures and tensile and Charpy impact properties were evaluated. The microstructures present in the steels increased in the order of granular bainite, acicular ferrite, bainitic ferrite, and martensite as the carbon equivalent or cooling rate increased, which resulted in a decrease in the ductility and Charpy absorbed energy. The steels containing a considerable amount of bainitic ferrite or martensite showed very high strengths, together with good ductility and Charpy absorbed energy. In order to achieve the best combination of strength, ductility, and Charpy absorbed energy, granular bainite and acicular ferrite were properly included in the high-strength bainitic steels by controlling the carbon equivalent and cooling rate, while about 50 vol.% of bainitic ferrite or martensite was maintained to maintain the high strength.

• 한국해양공학회지
• /
• 제11권3호
• /
• pp.39-47
• /
• 1997

Recently developed TMCP steels, which were manufactured by controlled rolling followed by accelerated cooling process, were examined to study their characteristics and weldability. Accelerated cooling type TMCP steel's hardness test result exhibited high value on weld zone. On the contrary, base metal and HAZ exhibited comparatively the similar value. On this experiment result Softening of HAZ is not occurred. in the-heat affected zone, grain size repression be caused by chemical composition properties which a small quantity Al-Ti-B-N. Changing stress ratio near-threshold fatigue crack propagation experiments were carried out. According to this result, crack propagation velocity of the HAZ exhibited slower than the base metal and near-threshold value had increased at the HAZ. Finally accelerated cooling type TMCP steels were exhibited excellent mechanical properties in both strength and toughness.

• 한국산학기술학회논문지
• /
• 제19권5호
• /
• pp.184-189
• /
• 2018

최근에 안전기준과 환경규제 강화의 증대로 인해, 차체의 경량화에 대한 연구가 지속적으로 증가하고 있다. 고강도강의 응용은 가장 보편화 된 차체 경량화의 효과적인 방법 중에 하나이다. TWB(Tailor Welded Blank)는 다양한 두께와 용접을 사용하여 부분적인 강도와 경량화를 만족시키는 주요한 공법으로 활용되고 있다. 그러나 추가적인 용접공정으로 인해 판재간의 용접성이 중요하고 용접부에 대한 품질관리가 중요하다. 이점을 개선하기 위해, TRB(Tailor Rollded Blank)공법이 제안되었다. TRB는 롤러 압력을 조절하여 판재의 두께를 다르게 하고, 용접을 사용하는 TWB 공법보다 추가적인 용접공정이 없기 때문에 생산 비용적인 측면에서 훨씬 더 효율적이다. 본 연구에서는 Blank의 압연 영향을 조사하기 위해 스탬핑용 고강도강 TRB 판재의 성형성을 분석하였다. 성형성을 분석하기 위해, TRB 판재에서 0.8 mm, 1 mm의 두께를 갖는 부분을 채취하여 시험하였다. 시편에 새겨진 그리드마킹의 변형도를 분석하여 변형률을 조사하였고, 0.8 mm와 1.0 mm의 두께를 갖는 TRB 시편으로 Erichsen 시험을 한 후에 FE분석을 활용하여 성형성과 고유진동수를 비교하였다.

• 한국재료학회지
• /
• 제27권10호
• /
• pp.524-529
• /
• 2017

The effect of strain aging on the tensile properties of API X70 linepipe steel was investigated in this study. The API X70 linepipe steel was fabricated by controlled rolling and accelerated cooling processes, and the microstructure was analyzed using optical and scanning electron microscopes and electron backscatter diffraction. Strain aging tests consisting of 1 % pre-strain and thermal aging at $200^{\circ}C$ and $250^{\circ}C$ were conducted to simulate U-forming, O-forming, Expansion(UOE) pipe forming and anti-corrosion coating processes. The API X70 linepipe steel was composed of polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite whose volume fraction was dependent on the chemical composition and process conditions. As the thermal aging temperature increased, the steel specimens showed more clearly discontinuous type yielding behavior in the tensile stress-strain curve due to the formation of a Cottrell atmosphere. After pre-strain and thermal aging, the yield and tensile strengths increased and the yield-to-tensile strength ratio decreased because yielding and aging behaviors significantly affected work hardening. On the other hand, uniform and total elongations decreased after pre-strain and thermal aging since dislocation gliding was restricted by increased dislocation density after a 1 % pre-strain.