Sang Yong Shin

Effects of Aluminum Addition on Tensile and Cup Forming Properties of Three Twinning Induced Plasticity Steels

In the present study, a high Mn twinning induced plasticity (TWIP) steel and two Al-added TWIP steels were fabricated, and their microstructures, tensile properties, and cup formability were analyzed to investigate the effects of Al addition on deformation mechanisms in tensile and cup forming tests. In the high Mn steel, the twin formation was activated to increase the strain hardening rate and ultimate tensile strength, which needed the high punch load during the cup forming test. In the Al-added TWIP steels, the twin formation was reduced, while the slip activation increased, thereby leading to the decrease in strain hardening rate and ultimate tensile strength. 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.

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.

Serration Phenomena Occurring During Tensile Tests of Three High-Manganese Twinning-Induced Plasticity (TWIP) Steels

In this study, 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 localized Portevin–Le Chatelier 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 gage 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. This serration behavior was generally explained by dynamic strain aging, which was closely related with the formation of deformation bands.

Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Strength Bainitic Steels

In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fast-cooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensite-austenite.

Effects of Inclusions on Delayed Fracture Properties of Three TWinning Induced Plasticity (TWIP) Steels

In the present study, delayed fracture properties of a high-Mn TWinning Induced Plasticity (TWIP) steel and two Al-added TWIP steels were examined by dipping tests of cup specimens in the boiled water, after which the microcrack formation behavior was analyzed. The TWIP steels contained a small amount of elongated MnS inclusions, spherical-shaped AlN particles, and submicron-sized (Fe,Mn)3C carbides. 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 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.

Effects of microstructure and pre-strain on Bauschinger effect in API X70 and X80 linepipe steels

In this study, effects of microstructure and pre-strain on the Bauschinger effect were investigated in two API X70 and two API X80 linepipe steel sheets fabricated by controlling the cooling condition, and their yield strength and Bauschinger parameters were measured by the tension-compression test with varying tension pre-strain. The fast-cooled steels had the higher fraction of acicular ferrite, granular bainite and martensite-austenite (MA) constituents and smaller grain sizes. The reduction in yield stress (ΔYS) of the steels having a higher fraction of MA and smaller grain sizes was higher than that of the steels having a lower fraction of MA and larger grain sizes. The ΔYS was smallest at the pre-strain of 1%, reached the maximum at the pre-strain of 2%, and then decreased with increasing pre-strain. This result could be explained by the amounts of mobile dislocations and back stress, which affected the Bauschinger effect and strain hardening effect simultaneously. Since these two effects affected the yield strength on a competing basis, the Bauschinger stress and hardening parameter were used to separately analyze these effects. It could be confirmed that the Bauschinger effect and strain hardening effect were activated at pre-strains of 1–2% and 3–4%, respectively.

Charpy Impact Properties of Heat Affected Zones of API X80 Linepipe Steels Containing Complex Oxides

This study assessed the Charpy impact properties of the heat-affected zones (HAZs) of API X80 linepipe steels containing complex oxides. Three types of steel were fabricated by adding Mg and O2 to form complex oxides and their microstructures and Charpy impact properties were investigated. The number of complex oxides increased with the amount of excess Mg and O2 that was included in the steels. Simulated HAZs containing a number of oxides showed a high volume fraction of acicular ferrite (AF) because the oxides acted as nucleation sites for AF, thereby leading to an improvement in the Charpy impact properties. According to a correlation study between the heat input, the volume fraction of the AF, and the Charpy impact properties, ductile fractures occurred predominantly when the fraction of the AF was 20% or higher; moreover, the Charpy absorbed energy was excellent at more than 100 J. These findings suggest that the improvement of the Charpy impact properties of the HAZs was associated with the active nucleation of AF in the oxidecontaining steel HAZs. (Received March 17, 2010)

Dynamic Deformation Behavior of Zr-Based Amorphous Alloy Matrix Composites Reinforced with STS304 or Tantalum Fibers

The Zr-based amorphous alloy matrix composites reinforced with stainless steel (STS) or tantalum (Ta) continuous fibers were fabricated without pores or defects by liquid pressing process, and their dynamic deformation behaviors were investigated. Dynamic compressive tests were conducted by a split Hopkinson pressure bar, and then the test data were analyzed in relation to the microstructures and the deformation modes. In the STS-fiber–reinforced composite, the STS fibers could interrupt the propagation of cracks initiated in the matrix and promoted the continuous deformation without fracture according to the strain-hardening effect of the fibers themselves. The Ta-fiber–reinforced composite showed the higher yield strength than the STS-fiber–reinforced composite, but the cracks were not interrupted properly by the Ta fibers according to the lower ductility and strain hardening of the Ta fibers. Both the Ta and STS fibers favorably affected the strength and ductility of the composites by interrupting the propagation of cracks formed in the amorphous matrix, by dispersing the stress applied to the matrix, and by promoting deformation mechanisms such as fiber buckling. The STS-fiber–reinforced composite showed the higher compressive strength and ductility than the Ta-fiber–reinforced composite because the STS fibers were higher in the resistance to deformation and fracture than the tantalum fibers.

Correlation of Microstructure with Tensile and Crack Tip Opening Displacement Properties at Low Temperatures in API Linepipe Steels

The correlations of the microstructural factors with the tensile and crack tip opening displacement (CTOD) properties at −20 and −60 °C for three kinds of API linepipe steels were investigated. The C steel composed mainly of small-sized acicular ferrite exhibited excellent tensile and CTOD properties. On the other hand, the A and B steels with large-sized polygonal ferrite or granular bainite exhibited low CTOD properties at −60 °C. The effective grain size was inversely proportional to the CTOD value at low temperatures. In the A and B steels, the values of the plastic deformation area and the CTOD were low because the crack tips of the steels opened under a low maximum force due to the fracture mode of the unstable brittle crack extension behavior. In the C steel, however, the values of the plastic deformation area and the CTOD were high because the crack tip of the steel opened under a high maximum force due to the fracture mode of almost fully plastic behavior. The C steel showed the widest stretch zone and the highest CTOD value. The CTOD values and the portions of post elongation in the A and B steels decreased with decreasing test temperature.

Effects of C and Si on strain aging of strain-based API X60 pipeline steels

Four types of strain-based API X60 pipeline steels were fabricated by varying the C and Si contents, and the effects of C and Si on strain aging were investigated. The 0.05 wt% C steels consisted mainly of polygonal ferrite (PF), whereas the 0.08 wt% C steels consisted of acicular ferrite (AF). The volume fraction of AF increased with increasing C content because C is an austenite stabilizer element. The volume fractions of bainitic ferrite (BF) of the 0.15 wt% Si steels were higher than those of the 0.25 wt% Si steels, whereas the volume fractions of the secondary phases were lower. From the tensile properties before and after the aging process of the strainbased API X60 pipeline steels, the yield strength increased and the uniform and total elongation decreased, which is the strain aging effect. The strain aging effect in the strain-based API X60 pipeline steels was minimized when the volume fraction of AF was increased and secondary phases were distributed uniformly. On the other hand, an excessively high C content formed fine precipitates, and the strain aging effect occurred because of the interactions among dislocations and fine precipitates.