결정립계 정합성 조절을 통한 Fe-Mn-C 트윕강의 수소 취성 저항성 향상

Abstract

Twinning-induced plasticity (TWIP) steels containing more than 15 wt. % Mn haven been considered as the promising materials in construction and automobile industries due to the excellent combination of tensile strength and elongation. Dynamic grain refinement by deformation-induced twinning and dynamic strain aging (DSA) by interaction between dislocations and Mn-C complexes suppressed the dislocation mobility, leading to the additional strengthening. However, during the development of high strength steels, the catastrophic failure frequently observed in the production process or the service environment due to internal hydrogen elements. Hydrogen-induced degradation, which is generally called as hydrogen embrittlement (HE), comprehensively refers to the deterioration of mechanical properties such as strength, elongation, and time to failure by internal or environmental hydrogen. Therefore, the high strength steels including TWIP steels must guarantee the safety against HE. HE in TWIP steels occurs in the form of intergranular fractures at the area highly concentrated by hydrogen. The prevention of those intergranular fractures is the major objective in this research. At first, the sensitivity to intergranular fracture depending on grain boundary characters (GBCs) was investigated in Fe−17Mn−0.8 (wt. %). The grain boundaries can be categorized depending on the coincidence site lattice (CSL). The CSL denotes the overlapped super-lattice created by the differently oriented neighboring grains. As Σ value representing the volume ratio of CSL and unit lattice is decreased, the boundary become more ordered structure. The boundaries of Σ ≤ 29 classified as high-ordered special boundaries, and the others were classified as random boundaries. Comparing the resistance to hydrogen-induced intergranular fracture of special and random boundaries, the intergranular cracking was rarely observed in special boundaries, especially Σ3 boundaries. The Σ3 coherent twin boundaries presented significantly lower energy states compared to other random boundaries due to the high fraction of stable bonding at the boundary interface. For those boundaries, the hydrogen segregation was less preferred and also the interfacial bonding strength was stronger Therefore, the interface was highly resistant to hydrogen-induced boundary failure. Based on the above results, we aimed to modify the GBCs to improve HE resistance. The grain boundary engineering consisting of straining of 15% and post annealing at 1073 K for 15 min was applied in Base (Fe−17Mn−0.8C). After the process, the fraction of special boundaries was increased from 48% to 59%. The results were ascribed to the evolution of random boundaries to Σ3-realted special boundaries (i.e., Σ3, Σ9, and Σ27 boundaries). The resistance to HE of Base subjected grain boundaries engineering (GBE) was compared with Base and Al-added TWIP steel (Fe−17Mn−0.8C−1Al and Fe−17Mn−0.8C−2Al). As a result, GBE exhibited the HE resistance comparable to Al-added samples. The replacement of random boundaries with special boundaries can reduce the probable sites of hydrogen-induced cracking and also disturb the crack propagation along the random boundaries. In TWIP steel, the sensitivity of HE is affected not only by deformation mode governed by the chemical composition and initial microstructures but also by deformation condition such as strain mode and strain rate. In Base, with increasing the strain rate, the strain hardening rate was gradually decreased, revealing the negative strain rate sensitivity. The results was attributed to the suppression of deformation-induced twinning and DSA at a higher strain rate. Meanwhile, the degradation of mechanical properties by HE was more severe with decreasing the strain rate, shown as larger loss of fracture strength and reduction of area. As the strain rate was decreased from 10-3 to 10-5 s-1, the crack propagation became promoted because the more hydrogen atoms can be diffused into crack tips by the lattice diffusion and the transportation by mobile dislocations. To conserve the fracture strength and reduction of area, we tried to retard the crack propagation by modification of GBCs. The application of grain boundary engineering to Base can replace the pre-existing random boundaries with special boundaries. As shown from the above research, the special boundaries were highly resistant to boundary embrittlement. When the hydrogen-induced cracking propagated along the random boundaries met junction only consisting of special boundaries, the further propagation was retarded due to absence of additional crack path. Since the effect of crack propagation was alleviated, the tensile properties at a low strain rate of 10-5 s-1 were conserved comparable to those at a high strain rate of 10-3 s-1.