Over the past few decades, high-nitrogen austenitic steels have steadily received greater attention since they provide a unique combination of high strength and ductility, good corrosion resistance, and non-magnetic properties. Recently, highnitrogen 18Mn-18Cr austenitic steels with enhanced strength have been developed and widely used for generator retaining rings in order to prevent the copper wiring from being displaced by the centrifugal forces occurring during high-speed rotation. The high-nitrogen austenitic steels for generator retaining ring should be expanded at room temperature and then stress relief annealed at around 400˚C to achieve the required mechanical properties. In this study, four kinds of high-nitrogen 18Mn-18Cr austenitic steels with different nitrogen content were fabricated by using a pressurized vacuum induction melting furnace, and then the effects of nitrogen content, cold working, and stress relieving on tensile properties were investigated. The yield and tensile strengths increased proportionally with increasing nitrogen content and cold working, and they further increased after stress relieving treatment. Based on these results, a semi-empirical equation was proposed to predict the tensile strength of highnitrogen 18Mn-18Cr austenitic steels for generator retaining rings. It will be a useful for the effective fabrication of high-nitrogen 18Mn-18Cr austenitic steels for generator retaining rings with the required tensile properties.

Ultra high performance concrete which has recently been studied was developed to complement the brittle behaviour and dynamic uppermost limit of high strength concrete. Fiber reinforced concrete which mixed steel fiber is receiving attention as an alternative about this and is being developed to complement the disadvantages of high strength concrete including lower toughness coefficients and crack resistance and spalling in fires. Review about fiber reinforced ultra high strength concrete that this study tries to treat includes reduction of self shrinkage generated by high cement content per unit volume of concrete, evaluation of compression and tension strength to lower internal and external spalling resistance and fragility factors of member of framework, and flow characteristics of concrete which doesn't harden according to steel fiber amounts and used materials. As the result, the more fiber reinforcement increases, the more compression and tensile strength increase and deformation control of cement matrix and improvement of energy absorption ability showed the great effect in shrinkage reduction.

In this study, Rebar of SD500, SD600 is applied to the designed or constructed in domestic apartment. Analyzed the variation of the rebar according to the strength of the rebar and the construction cost of the change. Reinforced volume reduction of the apartment building is less than the other. Because, A low wall of rebar reduction is more than 50% of the total. The reason for the previous, Reduction of the quantity of horizontal reinforcement does not appear and Vertical rebar reinforcement than the stress is determined by the placement of rebar in which the upper floor.

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.

First, a purified sludge was calcined at various temperatures viz. 800, 900, 1000, 1100 and 1200˚C per hour. Subsequently 100 wt% of ware from Geryong mountain was mixed with 5~25 wt% of a purified sludge. Then the ware was treated at 1250˚C in an electric kiln to test a bending strength. The physicochemical property of the prepared ware was characterized by XRD, Raman and SEM analysis. Among the different percentage, 25 wt% of a ware in a purified sludge calcined at 1000˚C showed 689 kg/cm2 strength. Also the purified sludge calcined at 1000˚C was adequate physical properties than the other composites. Further to enhance the physical strength, 3 wt% TiO2 (a mineralizer) was added into the ware and the strength was increased up to 731 kg/cm2. The composites exhibit absorption and porosity rates of 0.17% and 0.39% respectively.

현재 국내에서 생산되는 구조용 강재의 강도는 크게 5가지로 구분할 수 있다. 경제적인 구조설계를 하기 위하여 적절한 강도의 선정이 우선적으로 요구되는데 현재는 이와 관련된 기존 자료가 충분치 않은 상태이다. 최근, 국내에서 항복강도가 650MPa인 고강도 강재가 개발되어 구조용 강재의 강도 범위가 더 커졌기 때문에 부재 종류별 강재의 선택에 따른 경제성의 차이도 더 커졌을 것으로 예상된다. 본 논문에서는 경제적인 구조설계에 도움이 되도록, 항복강도 235MPa, 325MPa 및 650MPa 강재를 다양한 구조부재에 적용함으로서, 고강도 강재 적용에 따른 부재 종류별 경제성을 분석하였다.

This paper intends to review possible application in the high strength area through compressive strength estimation of the simulated high strength concrete member using Rock Test Hammer and suggest it as a reference data for the strength estimation technique of the ultra high strength concrete in the future. From the results of test, in the low strength area less than 15MPa and normal strength area in 15∼60MPa, as shown on the existing studies, it is indicated that P Type Schmidt Hammer in the low strength area and N Type Schmidt Hammer in the normal strength area have high correlation of rebound-compressive strength. As the Rock Test Hammer indicated more or less reduced accuracy in the low strength area and the normal strength area but high correlation on the high strength area (50∼100MPa) defined on this test, it is determined that it would be possible to make the fastest and simplest compressive strength estimation on the site where the high strength concrete is applied. As was resulted above the experiments, on the basis of the result of Rebound Hardness Method and Compressive Strength on the high strength area which is over 50MPa, the conclusion is drawn which is Fc=4.409e0.059R through regression analizing the relations of Compressive Strength which is based on Rebound Hardness Method.

In this study, expedite curing period at curing temperature being 40, 60, 80℃ similar to strength of standard 28days curing temperature to confirm methods and applicability of early estimation of strength by warm water curing. Also, checked the effect related to different conditions such as using various kinds of material, differentiating quantity of material, with or without fiber mixing at same rate of water-bonding material. Existing method proposed by KS and JIS to estimate strength of 28 days standard curing temperature curing for 7days at 40℃ is not relevant because it takes so long to estimate strength. Already known method of estimating strength of 28days standard curing temperature curing for 3days in 60℃ warm water, too, is not relevant to apply 3day cycle of super high-rise. It also had the problem which didn’t consider fiber mixed concrete. According to the result of experiment, traits and rate of strength revelation were different relating to the kinds and quantity of bonding material. Strength value of mix without fiber was higher than that of mix with fiber, but without any relation to that, over 96% of similar confidence level with strength of 28 days standard curing temperature was shown in 7 days at 40℃, 3 days at 60℃, 2days at 80℃. Confidence level of estimation method of 2 days at 80℃ was similar to those of estimation methods of 4 days at 40℃, 3 days at 60℃, so it(2 days at 80℃) is regarded as the most relevant method because it’s possible to estimate strength fastly. As a result it can be said that early quality control of recently increasing super high-rise may be possible through estimation method of 2 days at 80℃.

In this study, Rebar of SD500, SD600 is applied to the designed or constructed in domestic underground parking. Analyzed the variation of the rebar according to the strength of the rebar and the construction cost of the change. Most of underground parking appear reduction of Rebar ratio. In this case, the load acting on the underground parking garage is large. So, Rebar placing is dominated by element's stress. Therefore, Large amounts material of slabs, beams and foundations are reduced. In case of columns and wall, Ratio of Material reduction is less than former. Because Splice and Anchorage of rebar amounts are increased and Hoop and horizontal rebar amounts are fixed. Percentage of rebar of the column and wall are 15%. So, Showed a less impact on the reduction of total material.

In this study, prediction of later-age compressive strength of ultra-high strength concrete, based on the accelerated strength of concrete cured in hot water was investigated. Comparing other acceleration method, hot water curing method is relatively easy and intuitive to use in the real construction site. The amount of time for evaluation of the concrete strength using the hot water curing method in KS and JIS is too long to predict the strength of the ultra-high strength concrete that are used in the tall building structure. For that reason, curing temperature of 40, 50, 60℃ 3 levels were examined to shorten the amount of time for the evaluation of the strength. When curing in warm water, different strength characteristics are verified from the experiment. In case of F3 substituting 30% fly ash in combination, because of the curing temperature sensitivity of fly ash, differences of strength expression velocity was verified according to the curing temperature at the same age. In case of B4 substituting 40% ground granulated blast furnace slag, there were no big strength expression velocity differences of the specimen cured in 3 different level of curing temperature(40, 50, 60℃). The results show reliable accuracy by regression relation between 28day strength cured by standard curing method and accelerated strength of concrete cured in warm water.(y=1x-0.0002 R2=0.9866) As a result, the feasibility of 3day-prediction was confirmed using warm water curing method with accelerated strength of concrete cured for three days in warm water.

Strength bar has merits in workability improvement, construction time shortening and connection details to be easy. But, lap splice length tends to be longer. In this study, high strength threaded bar will be reviewed whether it is appropriate in domestic standards by KCI concrete structural criteria and propose the mechanical splice.

이 연구에서는 프리스트레싱용 고강도 강연선의 정착장치 중 강연선을 직접 정착하는 앵커헤드(anchor head)에 대해 거 동특성을 분석하고, 앵커헤드의 제원을 결정하는 단계에 있어서 해석적 검토에 요구되는 프로세스에 대해 정립하였다. 앵 커헤드는 쐐기와의 접촉(contact)을 통해 강연선으로부터의 힘이 전달되고 거동변화에 따라 접촉상태 또한 변하게 된다. 이 를 고려한 상세 거동분석을 위해 쐐기와 헤드 사이의 접촉(contact)조건을 설정하였으며, 앵커헤드의 비선형 재료모델을 적 용하여 기하 및 재료 비선형성을 고려한 구조해석을 수행하였다. 해석결과로부터 다음의 결과를 얻을 수 있었다. 앵커헤드 의 거동은 앵커헤드와 쐐기 간의 상호거동에 크게 영향을 받기 때문에 초기 설계단계부터 상대 영향을 고려해야 한다. 쐐 기홀(wedge hole)의 배치는 층배열(layered) 보다는 원형배열(circular)이 보다 응력분배에 효과적이고, 쐐기홀의 간격을 증 가시키고 헤드 하면 구멍의 크기를 줄여 구멍사이 강재의 두께를 다소 늘이는 것이 구조거동에 효과적이다.

고력볼트의 초기 체결력은 미끄럼표면조건에 따라, 일정시간이 경과될 때까지, 축력저하가 발생한다. 이 연구는 미 끄럼접합부 표면에 도장이 되어있는 경우, 도장의 크리프현상에 따른 축력저하에 관한 예측 모델을 찾는 것이다. 이 실험연구 범위는 무기질 아연 프라이머로 도포된 볼트접합부의 장기축력저하에 한정한다. 실험에 적용된 볼트종 류는 다크로 도포된 토크쉬어 볼트이다. 대상 표면의 도막 두께는 각각 96, 168, 226㎛ 이었다. 도막두께가 증가될 수록, 초기 체결이후 축력이완율은 도막의 크리프 때문에 10%에서 18%로 증가되었다. 장기축력예측을 위한 정량적 인 모델은 도막두께에 따른 크리프 스트레인과 경과된 시간사이에 회귀분석 결과로 얻어진다. 이 실험연구를 통해 미끄럼표면 도막의 크리프 거동특성을 알 수 있다면, 일정시간 경과후 고력볼트 체결력은 초기 체결력으로부터 구 할 수 있다. 본 실험결과를 근거로 각 도막두께에 대한 장기축력이완이후의 고력볼트 체결력에 관한 정량적인 수식 을 제안하였다.

The hydrogen embrittlement of high strength steel for automobiles was evaluated by small punch (SP) test. The test specimens were fabricated to be 5 series, having various chemical compositions according to the processes of heat treatment and working. Hydrogen charging was electrochemically conducted for each specimen with varying of current density and charging time. It was shown that the SP energy and the maximum load decreased with increasing hydrogen charging time in every specimen. SEM investigation results for the hydrogen containing samples showed that the fracture behavior was a mixed fracture mode having 50% dimples and 50% cleavages. However, the fracture mode of specimens with charging hydrogen changed gradually to the brittle fracture mode, compared to the mode of other materials. All sizes and numbers of dimples decreased with increasing hydrogen charging time. These results indicate that hydrogen embrittlement is the major cause of fracture for high strength steels for automobiles; also, it is shown that the small punch test is a valuable test method for hydrogen embrittlement of high strength sheet steels for automobiles.

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.

주각부는 상부의 강구조와 철근콘크리트조 기초의 접합부로써 매우 중요한 부분이다. 현재 국내외 강구조 골조 공사에서 사용되는 주각부의 형식은 크게 구분하여 노출형 주각부 근권형 주각부, 매입형 주각부라고 할 수 있다. 이 중에서 부재의 재사용 및 재활용을 최우선으로 고려한다면 노출형 주각부가 가장 유리하다. 본 연구에서는 이러한 노출형 주각부의 시공 성능 및 역학적 성능을 획기적으로 개선할 수 있는 형태의 강구조 선형상 노출형 주각부를 개발하고, 구조 성능 평가를 위한 탄소성 재하실험을 수행하였다. 그 결과 개발된 신형상 노출형 주각부는 수평 방향과 수직 방향의 시공 오차를 현장에서 간단히 흡수하여 기존 노출형 주각부의 시공 불량을 사전에 방지함으로써 구조 성능을 발휘한다. 또한 에너지 흡수형 고력볼트를 사용함으로써 변형 성능이 증가함을 알 수 있었다.

The flat type automotive cross members with high strength steel have advantages in light weight and fewer parts compared to the hump type cross members. But the complex part shape of the flat type cross member and the poor formability of high strength steel make it difficult to form the parts without forming defects, such as splits and wrinkles. The purpose of this study is to develop the flat type automotive cross member with high strength steel. For that purpose, drawing processes are evaluated using PAM-STAMPTM and proper draw die and blank designs are proposed. Using the proposed die and blank design, the flat type upper and lower cross member could be formed successfully without forming defects.

The initial clamping forces of the high strength bolts depending on the different faying surface conditions drop within 1,000 hours regardless of loading any other external force or loosening of the nut. This study focused on the mathematical model for relaxation confined to creep on coated faying surface after initial clamping. The quantitative equation for estimating long term relaxation was derived from nonlinear regression analysis for relation between the creep strain of coated surface and the elapsed time.

In case of general concrete, autogenous shrinkage is about 10% of the drying shrinkage. Therefore it was not considered significant to be a subject matter about managing the crack control and design. It was reported that cracks can be generated from the autogenous shrinkage. Because of the low W/B rate and the high unit binder of the high strength concrete. In this study, autogenous shrinkage and drying shrinkage are examined which is the main reason of the cracks of the high strength concrete based on the previous studies. Comparing the data from this study and previous studies, we developed the shrinkage reduced concrete using shrinkage reducing agent. The purpose of this study is to provide the data for reducing and managing the column shortening of the high strength concrete structures.