고망간 쌍정유기소성(TWIP)강의 지연파괴 현상에 미치는 합금원소 Mn, Al 첨가의 영향

Abstract

In the present study, Effects of Alloying Elements Mn and Al addition on delayed fracture phenomena in Fe-Mn-C TWIP steels were investigated. TWIP steels with different Mn and Al contents were fabricated, and delayed fracture properties of TWIP steels were examined by exposed to air or dipping tests of cup specimens in the boiling water. The addition of Mn increases the delayed fracture phenomenon, but the addition of Al considerably suppresses the delayed fracture phenomena. However, exact mechanisms behind the delayed fracture are not sufficiently understood. This is because detailed analyses on amounts of twinning and slip, precipitation or segregation on grain boundaries, distribution of residual stresses varied with deformed locations, and hydrogen embrittlement are essentially needed. Microstructure, fracture surface analysis, tensile properties, distribution of residual stresses, and cup formability of high Mn TWIP steels were investigated. The mechanical properties of TWIP steels related with twin formation were investigated. In the high Mn steel, the twin formation was activated to increase the strain hardening rate and ultimate tensile strength. The SFE increases with increasing Mn and Al contents. Especially, the SFE is dramatically increased by Al addition. The yield and ultimate tensile strengths decreased with increasing Mn content because the addition of Mn slightly raised the stacking fault energy and decreased the twin formation. In the Al-added steel, the huge decrease in tensile strength was explained by the increase in stacking fault energy due to the small addition of Al, as the increased stacking fault energy considerably restrained the twin formation.TWIP steels show a peculiar characteristic of serration phenomenon occurring during tensile deformation. The serration phenomena of two high-Mn TWIP steels and an Al-added TWIP steel were examined by tensile tests, and were explained by the microstructural evolution including formation of deformation bands and twins. In stress-strain curves of the high-Mn steels, serrations started in a fine and short shape, and their height and periodic interval increased with increasing strain, whereas the Al-added steel did not show any serrations. According to digital images of strain rate and strain obtained from a vision strain gauge system, deformation bands were initially formed at the upper region of the gage section, and moved downward along the tensile loading direction. The time when the band formation started was matched with the time when one serration occurred in the stress-time curve. In the Al added TWIP steels, deformation bands were hardly formed as the deformation occurred homogeneously. This was because the twin formation was restrained, and because the movement and reorientation of C atoms were reduced by the addition of Al. The formation of deformation bands reduces the formability. The cup properties of TWIP steels related with twin formation, cup formability and distribution of residual stress were investigated. Cup forming tests were conducted, and tensile properties of the specimen located at the distance of 4.25 mm, 10.25 mm, and 16.25 mm from the cup edge were evaluated to investigate the susceptibility against the cracking and to compare mechanical properties which are varied with the steels and distance from the cup edge. The distribution of residual stresses of the cup specimens was measured by Finite Element Method (FEM). In the Al-added TWIP steels, as twins and slips were homogeneously formed during the tensile or cup forming test, the punch load required for the cup forming and residual stresses were relatively low, and the tensile ductility was sufficiently high even after the cup forming test. This indicated that making use of twins and slips simultaneously in TWIP steels by the Al addition was an effective way to improve overall properties including cup formability.The delayed fracture behavior related with secondary crack formation was investigated. The TWIP steels contained a small amount of elongated MnS inclusions. Since MnS inclusions worked as crack initiation sites, longitudinal cracks were formed along the cup forming direction mostly by MnS inclusions. These cracks were readily grown when high tensile residual stresses affected to the cracking or hydrogen atoms were gathered inside cracks, which resulted in the delayed fracture. In the Al-added steels, MnS inclusions acted as crack initiation and propagation sites during cup forming or boiled-water dipping test, but residual stresses applied to MnS might be low for the crack initiation and growth. Thus, longitudinal cracks formed by MnS inclusions did not work much for delayed fracture. AlN particles present in the Al-added steels hardly acted as crack initiation or growth sites for the delayed fracture because of their spherical shape.The delayed fracture behavior related with intergranular carbide precipitation of three TWinning Induced Plasticity (TWIP) steels was investigated. According to the microstructural analysis, nano-sized (Fe,Mn)3C cementites were precipitated along grain boundaries in the 0.6C-22Mn and 0.6C-18Mn steels, whereas their precipitation was hardly observed in the 0.6C-18Mn-2Al steel, which was confirmed by equilibrium phase diagrams calculated from a ThermoCalc program. When cup specimens were dipped in the boiled water, the 0.6C-22Mn, 0.6C-18Mn, and 0.6C-18Mn-2Al steel cups were cracked after 5.5, 15, and 169 hours, respectively. The delayed fracture regions consisted of intergranular facets, and the tendency of intergranular facture decreased in the order of 0.6C-22Mn, 0.6C-18Mn, and 0.6C-18Mn-2Al steels. Thus, the delayed fracture behavior was closely related with the intergranular fracture mode caused by grain boundary cementites. The addition of Al remarkably increased the resistance to delayed fracture because it suppressed the formation of grain boundary cementites and reduced the residual stresses in the cup specimen. On the contrary, the addition of Mn decreased the resistance to delayed fracture because it raised the precipitation of grain boundary cementites.