Minju Kang

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.

Tensile property improvement of TWIP-cored three-layer steel sheets fabricated by hot-roll-bonding with low-carbon steel or interstitial-free steel

TWIP-cored three-layer steel sheets were newly fabricated by hot rolling of TWIP steel sheet surrounded by low-carbon (LC) or interstitial-free (IF) steel sheets. TWIP/LC or TWIP/IF interfaces were well bonded without pores or voids, while a few pearlites were thinly formed along the interfaces. The strengths and elongation of the TWIP-cored sheets increased as the volume fraction of TWIP-cored region increased, and were also well matched with the ones calculated by a rule of mixtures based on volume fraction or force fraction. According to digital image correlation and electron back-scatter diffraction analyses, very high strain hardening effect in the initial deformation stage and active twin formation in the interfacial region beneficially affected the overall homogeneous deformation in the TWIP-cored sheets without any yield point phenomenon occurring in the LC sheet and serrations occurring in the TWIP sheet, respectively. These TWIP-cored sheets can cover a wide range of yield strength, tensile strength, and ductility levels, e.g., 320~498 MPa, 545~878 MPa, and 48~54%, respectively, by controlling the volume fraction of TWIP-cored region, and thus present new applications to multi-functional automotive steel sheets requiring excellent properties.

Adiabatic shear banding and cracking phenomena occurring during cold-forging simulation tests of plain carbon steel wire rods by using a split Hopkinson’s pressure bar

Adiabatic shear banding and cracking phenomena occurring during cold forging of plain carbon steel wire rods, whose carbon content was varied from 0.2 to 0.8 wt%, were analyzed by forging simulation test using a split Hopkinson’s pressure bar. The test results indicated that the 0.2C and 0.3C steels were dynamically compressed without surface defects after the fifth hit, whereas a deep crack was formed along the 45° direction in the 0.8C steel. In all the steels, adiabatic shear bands were formed diagonally inside forging-simulated specimens, and grains were extremely elongated within shear bands. The higher the volume fraction of pearlite was, the easier was the adiabatic shear banding. Particularly in the 0.8C steel, the shear band was white-colored and narrow, along which a long crack was formed. After the spheroidization treatment of the 0.8C steel, adiabatic shear bands or cracks were not found during the forging simulation test as the steel was relatively homogeneously deformed, which indicated that the spheroidization effectively prevented the adiabatic shear banding or cracking. The present forging simulation test plausibly evaluated the cold-forging performance by controlling the number and amount of hit, and provided an important idea on whether the spheroidization was needed or not.

The Mechanism of Hot Ductility Loss and Recovery in Nb-Bearing Low Alloy Steels

In low alloy steels containing Nb, the poor hot ductility is basically due to the austenite grain boundary segregation of sulfur and the additional matrix strengthening of Nb(C,N) precipitates, both of which decrease the equicohesive temperature. The recovery of hot ductility is therefore attributed to not only the clean grain boundaries that the segregated sulfur is scavenged through the MnS reaction but also the matrix softening by the coarse Nb(C,N) precipitates.

Effects of Dynamic Strain Hardening Exponent on Abnormal Cleavage Fracture Occurring During Drop Weight Tear Test of API X70 and X80 Linepipe Steels

In this study, drop weight tear tests (DWTT) were conducted on API X70 and X80 linepipe steels fabricated with various compositions and rolling and cooling conditions in order to correlate the strain hardening with the abnormal cleavage fracture occurring in the hammer-impacted area. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, Charpy impact energy, and strain hardening. All the steels consisted of fine acicular ferrite, together with some bainitic ferrite, granular bainite, and martensite-austenite constituent. As the volume fraction of acicular ferrite increased, the area fraction of DWTT abnormal cleavage fracture decreased because the toughness of acicular ferrite was higher than other microstructures. The area fraction of abnormal cleavage fracture was weakly related with strain hardening exponents obtained from the quasi-static tensile and compressive tests, but showed better correlation with those obtained from the dynamic compressive test. This tendency could be more clearly observed when steels having similar Charpy impact energy levels were grouped. Since the DWTT was performed under a dynamic loading condition, thus, the abnormal cleavage fracture behavior should be related with the strain hardening analyzed under a dynamic loading condition.

Dynamic compressive properties obtained from a split Hopkinson pressure bar test of Boryeong shale

Dynamic compressive properties of a Boryeong shale were evaluated by using a split Hopkinson pressure bar, and were compared with those of a Hwangdeung granite which is a typical hard rock. The results indicated that the dynamic compressive loading reduced the resistance to fracture. The dynamic compressive strength was lower in the shale than in the granite, and was raised with increasing strain rate by microcracking effect as well as strain rate strengthening effect. Since the number of microcracked fragments increased with increasing strain rate in the shale having laminated weakness planes, the shale showed the better fragmentation performance than the granite at high strain rates. The effect of transversely isotropic plane on compressive strength decreased with increasing strain rate, which was desirable for increasing the fragmentation performance. Thus, the shale can be more reliably applied to industrial areas requiring good fragmentation performance as the striking speed of drilling or hydraulic fracturing machines increased. The present dynamic compressive test effectively evaluated the fragmentation performance as well as compressive strength and strain energy density by controlling the air pressure, and provided an important idea on which rock was more readily fragmented under dynamically processing conditions such as high-speed drilling and blasting.

Correlation of Microstructure, Chip-Forming Properties, and Dynamic Torsional Properties in Free-Machining Steels

Four free-machining steels were fabricated by varying volume fractions of MnS and soft metal additives of Pb and Bi, and their microstructures, tensile properties, chip-forming properties, and dynamic torsional properties were analyzed. Machining and dynamic torsional tests were conducted on the four steels to investigate chip-forming and dynamic torsional properties, respectively. In the Pb-S- and Bi-S-based steels, the chip thickness and ridge area of the 1st chip obtained from the machining test were smaller than in the S-based steels and were not changed much after repeated machining processes. These chip-forming properties were closely related with dynamic torsional properties. Dynamic maximum shear strains of the Pb-S- and Bi-S-based steels were higher than those of the S-based steels, while dynamic maximum shear stresses were lower, thereby leading to the relatively homogeneous dynamic shear deformation and to the better chip-forming properties and machinability.