Cubic boron nitride (c-BN) is a promising material for use in many potential applications because of its outstanding physical properties such as high thermal stability, high abrasive wear resistance, and super hardness. Even though 316L austenitic stainless steel (STS) has poor wear resistance causing it to be toxic in the body due to wear and material chips, 316L STS has been used for implant biomaterials in orthopedics due to its good corrosion resistance and mechanical properties. Therefore, in the present study, c-BN films with a B4C layer were applied to a 316L STS specimen in order to improve its wear resistance. The deposition of the c-BN films was performed using an r.f. (13.56 MHz) magnetron sputtering system with a B4C target. The coating layers were characterized using XPS and SEM, and the mechanical properties were investigated using a nanoindenter. The friction coefficient of the c-BN coated 316L STS steel was obtained using a pin-on-disk according to the ASTM G163-99. The thickness of the obtained c-BN and B4C were about 220 nm and 630 nm, respectively. The high resolution XPS spectra analysis of B1s and N1s revealed that the c-BN film was mainly composed of sp3 BN bonds. The hardness and elastic modulus of the c-BN measured by the nanoindenter were 46.8 GPa and 345.7 GPa, respectively. The friction coefficient of the c-BN coated 316L STS was decreased from 3.5 to 1.6. The wear property of the c-BN coated 316L STS was enhanced by a factor of two.

The effects of Quenching and Partitioning (Q&P) and Annealed Martensite (AM) heat treatment on the microstructure and tensile properties were investigated for 0.24C-0.5Si-1.5Mn-1Al steels. The Q&P steels were annealed at a single phase (γ) or a dual phase (γ+α), followed by quenching to a temperature between Ms and Mf. Then, enriching carbon was conducted to stabilize the austenite through the partitioning, followed by water quenching. The AM steels were intercritically annealed at a dual phase (γ+α) temperature and austempered at Ms and Ms±50˚C, followed by cooling in oil quenching. The dual phase Q&P steels showed lower tensile strength and yieldyield strength than those of the single phase Q&P steels, and tThe elongation for the dual phase Q&P steel was partitioning 100s higher than that of that for the single phase Q&P steels as the partitioning time was less than 100s up to partitioning 100s. For AM steels, the tensile/yield strength decreased and the total elongation increased as the austempering temperature increased. The stability of the retained austenite controlled the elongation for Q&P steels and the volume fraction of the retained austenite controlled the elongation for AM steels.

Austenite precipitation behavior was studied with solidification rates and alloying contents, N and Cr, in duplex stainless steels by directional solidification. Directional solidification experiments were carried out with solidification rates, 1~100mm/s, and N and Cr contents, 0~0.27wt.%, 25~28wt.% respectively, in a duplex stainless steel, CD4MCU. As the solidification rate increases, the dendrite spacing reduced and the austenite phase in the ferrite matrix became finer. The volume fraction of austenite phase increased and its shape went to be round with increasing nitrogen contents in duplex stainless alloys. The Cr alloying element, even though it is a ferrite former, showed to enhance the nitrogen solubility in the alloy and caused the austenite round and finer. Also, Cr was supposed to decrease the austenite volume fraction, but it increased the austenite slightly due to increasing nitrogen solubility during solidification.

Densificationbehavior of conventional austenitic stainless steel powder compacts was studied by comparing the relative density of sintered compact(Ds)with that of green compacts(Dg)prepared with various catbon contents and P/M process. Dg of 304and 316 powders by warm compaction under pressure of 686 MPa at heating temperature of powder(553K) and dies (573K) were 80% and 81%, repectively, whichwere 2 and 3% higher than those of conventional green compacts at the same pressure. Ds of 304 compacts sintered at 1373K in H2 gas has the same value of 84% max. regardless of compacting temperature, and Ds of 316 compacts at the same sintering conditions were 80% by conventional compaction and 83% by warm compaction. Oxygen contents of 304 and 316 sintered compacts were increased 1.43∼2.94% and 0.010∼0.921% higher than those of raw powders and warm green compacts, respectively. In other case, Ds of 316 compacts sintered at 1573K in vacuum had the same value of 86%max. And Ds of 316 compacts at the same sintering conditions were 83% and 86% by conventional and warm compaction, respectively. Oxygen contents of 304 sintered compacts were 0.321% and 0.360%, and in case of 316, they were 0.419% and 0.182% by the respective compating condition. With carbon additions in the range 0.1∼0.6% Ds increased to the extent of 86∼89% in 304 sintered compacts, and to 82∼84% and 85∼87% in 316 according to different two compacting peocesses compared to those of sintered compacts without carbon addition.

본 연구에서는 오스테나이트계 스테인리스 강재(STS 304)에 대해 NaCl 환경 중에서 틈부식 특성을 연구하기 위해, 정전압 분극장치에 의해 분극특성시험을 실시하여 NaCl 용액의 농도에 따른 STS 304 강재의 틈부식에 의한 분극 거동에 대해 연구한 결과는 다음과 같은 결론을 얻었다. 1) 틈부위는 심하게 부식되고 틈의 인접한 외부 표면은 부동태화된다. 2) 오스테나이트계 스테인리스강재인 STS 304 강재는 분극거동에 있어서 부식 전위는 3.5% NaCl까지 농도가 증가할수록 귀전위화되다가 농도가 3.5%이상으로 증가할수록 오히려 비전위화된다. 3) 부식 전위하에 전류밀도는 NaCl 농도가 3.5%까지 증가할수록 더 많이 배류되다가 3.5% 이상으로 증가할수록 오히려 더 적게 배류된다.(이 논문의 결론(요약) 부분임)

LiCl 및 LiCl/Li2O 용융염분위기에서 오스테나이트 스테인레스강, SUS 316L과 SUS 304L의 부식특성을 650­~850˚C 온도범위에서 조사하였다. SUS 316L과 304L의 부식층은 외부 Li(CrFe)O2와 내부 Cr2O3의 2층 구조를 형성하였다. LiCl 용융염중에서는 균일한 부식충이 형성되지만, LiCl/Li2O 혼합용융염중에서는 균일한 부식충 형성외에 업계부식이 발생되는 것을 알 수 있었다. 750˚C까지 온도 증가에 따른 부식속도의 증가속도는 느리고, 750˚C 이상에서는 부식속도가 급격히 증가하였다. 용융염분위기에서 SUS 316L은 SUS 304L에 비하여 부식속도가 느려셔 보다 좋은 내식성을 나타내였다.

고크롬주철에 있어서 기지 및 탄화물의 조직제어는 내마모성의 향상을 위해 필수적이다. 특히 기지조직내의 잔류오스테나이트함량은 최적 내마모성을 얻기위해 반드시 조절되어야 한다. 본 연구에서는 3% C-18%(or 25%)Cr-Mo-Ni-Mn 고크롬주철을 800˚C, 900˚C, 1000˚C 및 1100˚C의 질소분위기 하에서 각각 20분 불안정화열처리를 행한 후 공냉시켰다. 잔류오스테나이트의 함량은 Xtjs 회절시험을 통해 측정하였으며 회절 peak는 α200, α220, γ220그리고 γ311을 이용하였다. 화학조성, 불안정화열처리의 온도 및 시간, 그리고 잔류오스테니이트함량강의 관계식을 도출하였다.

준안정 Fe-Cr-Ni 3원 오스테나이트 STS강을 90% 냉간압연한 후 여러온도에서 어니링하여 얻은 가공유기 마르텐사이트(α')와 역변태 오스테나이트(γ)와의2상 혼합조직가에 있어서 기게적 성질과 γ안정도와의 관계를 조사하였다. 모든 강들은 90% 냉간압연에 의하여 거의 α'단상조직이 되었고, 변태된 α'는 773~873K의 온도범위에서 급격하게 γ로 역변태하였다. Cr과 Ni함량이 많을수록, 그리고 결정립 미세화에 의해서γ는 더욱 안정화하여 Ms 점의 저하를 초래하였다. Cr량을 적게하여γ를 불안정하게 함으로써 많은 α'를 품는 고강도, 고연성의 초미세립강이 얻어졌다.

Concept of primary solidification mode control was adopted to obtain optimal solidification crack resistance, hot ductility, corrosion resistance and toughness for austenitic stainless steel. By controlling primary solidification phase as primary δ and containing no ferrite at room temperature, optimal solidification crack resistance, hot ductility, corrosion resistance and cryogenic toughness could be obtained. The optimum chemical composition of austenitic stainless steel ranges 1.46~1.55(Creq/Nieq ratio) calculated by Schaeffler's equation.

오스테나이트계 스테인레스강 SUS316, SUS321 보다 용접성, 내식성 및 극저온인성이 양호하며 기존의 재료보다 고가 첨가 원소인 크롬과 니켈의 함유량을 절감한 새로운 강종의 개발을 목표로 연구를 수행한 결과, 다음과 같은 결론을 얻었다. 1) Trans-Varestraint 시험에 의하여 용해 제조강과 수입개의 고온 균열 감수성을 비교 평가한 결과, SUS321 수입재와 SuS321에 대응하여 개발한 강종 사이의 응고 균열 감수성이 거의 동등 내지는 용해 제조강의 고온 균열 저항력이 우수하였다. 따라서 Creq/Nieq의 값 1.43~1.48의 범위에서 합금 설계한 강종 M-1~M-9의 고온 균열 저항력은 기존의 문헌에 의한 테이타와 비교한 결과, 우수함을 알 수 있었다. 2) 샤르피(Charpy V notch) 표준 시험편에 의한 극저온 및 상온에서의 인성을 조사한 결과, 용해 제조강 M-1~M-9의 인성은 양호하였으며 특히 SUS321 수입재와 SUS321 대체재로서 개발한 M-7~M-9의 인성은 수입재의 그것보다 훨씬 높은 값을 나타내었다. 3) JIS G 0574에 의한 입계 부식 감수성을 조사한 결과, 입계 부식 감수성을 낮추기 위해서는 티타늄의 첨가가 매우 유효함을 알 수 있었다. 특히 SUS321 수입재와 SUS321 대응 강종(M-7~M-9)의 내식성은 수입재보다 개발 강종의 내식성이 훨씬 우수하였다. 4) 이상의 고온 균열 감수성과 극저온 및 상온에서의 인성, 내식성을 종합적으로 검토한 결과, 스테인리스강의 고가 첨가 원소인 크롬과 니켈의 첨가량을 절감할 수 있었다. 상기와 같은 관점에서 본 연구 수행에 의하여 새로운 합금 설계 개념에 의거, 새로운 강종의 개발과 생산 원가의 절감 그리고 제품의 품질 향상에 기여할 수 있는 기준을 확립할 수 있었다

The effect of primary solidification phase on the solidification cracking sensitivity, corrosion resistance and toughness at cryogenic temperature was investigated for the austenitic stainless steel welds. The conclusions were summarized as follows; 1. Soldification crack sensitivity of austenitic stainless steel welds depends on the primary solidification mode. 2. Austenitic stainless steels were very susceptible to solidification cracking in case of solidification as primary γ and immune when solidified as primary δ. 3. When the ratio of Creq/Nieq is in the range of 1.46 to 1.55, the most resistance against solidification cracking was obtained. These results agreed well with the relationship between primary solidification mode, corrosion resistance and toughness at cryogenic temperature. 4. Optimum toughness, corrosion and solidification cracking resistance can be obtained when alloys having chemical compositions described above and solidifies as primary δ containing no ferrite at room temperature.

스테인리스강은 최근에 다양한 구조 부재와 산업분야에서 스테인리스강의 상용이 증가하고 있다. 그러나 스테인리스강은 건축구조기준에 구조재료로 포함되어 있지 않고 해당 설계기준은 규정되어 있지 않다. 본 논문에서는 오스테나이트계 스테인리스강 STS304로 제작된 이면전단 볼트접합부에 대한 실험적을 수행되었다. 주요 변수는 볼트배열과 하중방향 연단거리로 설정하였다. 실험 결과, 순단면파단과 블록전단파단이 발생하였고, 블록전단파단 접합부의 경우연단거리가 증가함에 따라 내력이 증가하였다. 또한, 현행 기준식에 의한 예측내력과 실험 내력을 비교하였다.

Recently, since stainless steels have important properties such as superior durability, fire resistance and corrosion resistance, they are widely used as non-structural element as well as structural element in building construction. In this study, experimental studies have been performed to investigate the structural behaviors such as ultimate strength and fracture mode of double shear two-bolted connection and four-bolted connection with austenitic stainless steel(STS304) and varied end distance. In case of two-bolted connection, all of specimens showed block shear fracture. And four-bolted connection failed by block shear fracture or net section fracture at test end.