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고망간 TWIP 강 기반 클래드 판재의 성형성

고망간 TWIP 강 기반 클래드 판재의 성형성
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최근 이종의 소재를 조합하여 기존 단일소재가 가질 수 없는 특성에 도달하는 것을 목표로 한 아키텍쳐드 소재에 대한 아이디어가 발표된 이후, 다양한 소재의 조합에 의한 아키텍쳐드 소재의 특성 평가가 이루어져 왔다. 특히 소성변형 시 계면상에서 발생하는 이종의 소재간 변형률 불합치성은 주위에 추가적인 전위의 형성을 유발하게 되어 추가적인 강화 현상을 유발하였다. 상기 계면에 의한 추가 강화현상에 의해 아키텍쳐드 소재의 기계적 특성은 종래 혼합비 법칙에 의해 예측되는 수치보다 더 우수한 결과를 보인다. 따라서, 상기 효과를 응용하여 기계적 특성 향상을 극대화하기 위한 2/3 차원의 아키텍쳐드 소재에 대한 연구는 많이 진행되어 왔으나, 실제 소재의 제품화에 가장 중요한 요소 중 하나인 성형성에 대한 연구는 많이 진행되지 못한 실정이다. 본 연구에서는 3 층 구조를 가지는 고 망간강 기반 층상소재에 대한 성형성 평가를 실시한 후, 모재의 혼합비에 따른 예측치 및 단일소재에서 적용되는 성형성과 인장/파괴특성 사이의 관계를 직접 적용해 비교하였다. 그 결과, 계면이 건전한 상태에서 층상소재는 단일소재와 유사하게 인장/파괴특성과 선형적 관계를 보이는 것을 확인할 수 있었으며 이는 층상소재의 인장/파괴시험 수치 만으로도 대략적인 성형성을 예측할 수 있다는 데에 의의를 둘 수 있다. 특히, 계면상에서의 추가적인 전위의 형성은 기계적 특성의 향상뿐 만 아니라 철강소재 내부의 전위-용질원자 상관관계에 의해 형성되는 항복점 연신 현상 및 세레이션을 완화시켜 기존의 단일소재보다도 우수한 신장 성형성을 보이는 것을 확인하였다. 정리하자면, 계면이 건전한 층상소재는 성형 시 단일 소재와 유사한 거동을 보임과 동시에 계면에서 추가적으로 형성되는 전위에 의한 기계적 특성 향상, 항복점 연신 현상 및 세레이션의 억제에 의해 더 우수한 성형성을 보이게 된다. 본 결과는 종래 아키텍쳐드 소재와 관련된 연구에서 부족한 소재의 특성과 성형성 간의 관계를 성공적으로 보여주었을 뿐 만 아니라 건전한 계면이 형성된 아키텍쳐드 소재가 종래의 단일소재보다도 우수한 성형성을 보일 수 있음을 보여준다.
The regulations on the safety and environment for automobile have tightened up recently. The essential points of the revised regulations are the reduction of green-house gases and the passenger survivability enhancement at the car accident. To achieve these requirements, advanced automobile steel sheets which have high strength, ductility and low density should be developed. The advanced high strength steel (AHSS) sheets have a potential to achieve ultra-high strength (more than 1 GPa) with a low production cost. Dual phase (DP), transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels are one of the representative AHSS, with an application to the automobile parts for crash energy absorption. Because of the specific chemical contents, TWIP steel has a lower stacking fault energy (SFE) and deformation twinning occurs during plastic deformation. These deformation twins act as an additional grain boundaries and the strength of TWIP steel drastically increase based on the dynamic Hall-Petch theorem. Although the outstanding mechanical properties of the TWIP steel, its formability is not enough to manufacture the automobile parts. Thus, TWIP steel-based architectured materials can be one of solution to overcome the drawback of the AHSS sheets. Recently, the architectured materials have a potential to expand the material’s properties charts which cannot be achieved by the present monolithic materials. The various properties of the architectured materials are originated from the selection of parent materials, the volume fraction of parent materials, and the configuration of architectured materials. Based on the strong relation between these factors and properties of architectured materials, the properties of architectured materials can be estimated in theoretically. The rule-of-mixtures, which considers both the properties of parent materials and the volume fraction of parent materials, successfully estimates the mechanical properties of architectured materials in the uniaxial tension. However, the formability of architectured materials, which has complex deformation modes compare with the uniaxial tension, no one gives clear explanation yet. Generally, the formability of monolithic materials is linked to the tensile parameters, i.e., strain hardening coefficient (n), strain rate sensitivity (m), mean R-value and elongation. To find the general relation between tensile parameters and formability of materials, several researchers reported some results from the monolithic materials. Based on these results, it can be assume that the formability of the architectured materials should be related to the tensile parameters while the tensile parameters follow the rule-of-mixtures and the interface is conserved during plastic forming. Therefore, in this thesis, the formability of the TWIP steel-based architectured material is investigated as follows: In the first category, the tensile parameters of architectured materials are evaluated from the tensile tests. From this investigation, the force-based ROM reproduces the experimental values. n, m and R-values are closed to the high strength TWIP steel due to the strength-dependent numerator in force-based ROM. Therefore, the tensile and anisotropic properties of layer architectured sheets are governed by not only volume fractions but also the strength of parent materials. In the second category, the deformation instability which gives negative effects on the formability of materials is investigated. This results shows that the deformation instability of the architectured materials can be suppressed by controlling its volume fractions, and the geometrically necessary dislocations (GNDs) accumulation occurs at the interface to cover the strain gradient at the interface. Thus, the properties of architectured materials not only controlled by the rule-of-mixtures, but also its constructed structure enhances the mechanical properties or suppresses the deformation instabilities of monolithic materials. In the third category, stretchability and drawability of the architectured materials are quantitatively investigated by comparing with the tensile parameters. The LDR of the TWCLS sheet is proportional to the mean R-value and m while the LDH of the TWCLS sheet, as a function of the TWIP steel volume fraction, was above the line connecting the LDHs of the LC and TWIP monolithic steel sheets. This LDH enhancement is originated from the deformation instability suppression as mentioned in the second category. This results shows that the layered sheets can improve the formability of TWIP steel sheet. In the last category, the stretch-flanageability of TWCLS sheet is evaluated by HET. The HER of TWCLS sheet is lower than the force-based ROM and linearly correlated to the fracture initiation energy as similar with the monolithic materials. Although the TWIP-IF interface is conserved after punching process, severely damaged fracture zone is induced at the TWIP steel-core. The IF steel-sheath cannot prevent the crack propagation during HET and SENT test, and results into the poor HER and fracture initiation energy.
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