Seong-Ju Kim

Influence of Ti addition on the hydrogen induced cracking of API 5L X70 hot-rolled pipeline steel in acid sour media

In this study, Hydrogen Induced Cracking (HIC) testing of high strength API 5L grade X70 linepipe hot rolled steel containing Ti was performed to investigate the effects of (Nb, Ti, V)(C, N) particles on HIC susceptibility. By controlling chemical composition and hot rolling parameters, experimental steel with Bainitic ferrite and Bainite microstructures was fabricated. HIC testing was carried out within an acidic condition (pH=2.7±0.1) according to NACE standards with test results showing cracking propagated along coarse (Nb, Ti, V)(C, N) particles at mid-thickness. This is mainly due to centerline segregation and hydrogen blistering between matrix and coarse (Nb, Ti, V)(C, N) particles without external stress.

Role of Ca treatment in hydrogen induced cracking of hot rolled API pipeline steel in acid sour media

The effect of Ca treatment on hydrogen-induced cracking (HIC) resistance of hot rolled pipeline steel was evaluated. HIC testing was carried out in acidic condition according to NACE standard; results clearly prove that HIC resistance is very sensitive to Ca/S ratio. When Ca/S ratio is below the stoichiometric ratio, HIC occurred at mid-thickness of the steel regardless of the S content. This is closely related to the formation of spherical CaS inclusion with Ca treatment instead of MnS inclusion, which acts on crack initiation sites.

Effects of Coiling Temperature and Carbides Behavior on Stretch-flangeability for 980 MPa Hot-rolled Steels

To analyze the factors on stretch-flangeability for 980 MPa-grade hot-rolled steels, two types of steels (Fe-Cr and Fe-Mo) were manufactured by hot-rolling. Manufactured steels at the low coiling temperature, such as 400 and 500°C, had poor stretch-flangeability due to un-uniformly distributed carbides and a large deviation of interphase hardness. However, when the coiling temperature was set at 650°C with Fe-Cr steel, 998 MPa of ultimate tensile strength, 19% of total elongation and 65% of the hole expanding ratio were achieved by microstructural constituents of polygonal ferrite (PF) and granular ferrite (GF) dispersed with fine carbides (<50 nm). Therefore, the material to attain 980 MPa with superior formability was the FeCr steel that was precipitation-hardened in polygonal ferrite and granular ferrite at the coiling temperature 650°C. (Received March 9, 2012)

Effect of the Tempered Martensite Matrix and Granular Bainite on Stretch-Flangeability for 980 MPa Hot-Rolled Steel

The main goal of the study is to obtain high strength and formability for automotive sheet steels such as wheel and chassis. Ferrite-based steels developed previously exhibited superior stretch-flangeability to 780 MPa and even to 980 MPa grade steels. However, it was difficult to achieve 980 MPa tensile strength sufficiently. To achieve a superior stretch-flangeability and to acquire a sufficiently high strength over 980 MPa, a composition of Fe-Cr-Ti-B was developed for hot rolling. The manufactured steel was coiled for 1 hour at 430, 450, and 470 ℃. Specifically, 998 MPa ultimate tensile strength, 11% total elongation, and a 59% excellent hole expansion ratio were obtained by microstructural constituents of a tempered martensite matrix and granular bainite when the coiling was conducted at 470 ℃. As the coiling temperature varied from 470 to 430 ℃, the fraction of granular bainite decreased and that of tempered martensite increased. Therefore, the deviation of phase fractions between two phases decreased at 470 ℃ coiling. Furthermore, as the hole expansion ratio (HER) increased, the hardness deviation of the two phases decreased. As the HER value increased, the crack propagation path dispersed on the second fracture surface and fracture was significantly delayed. Therefore, hot rolled steels using a tempered martensite matrix and granular bainite had the same behavior of stretch-flangeability with respect to the hardness deviation and crack propagation path as compared with the steels using a ferrite matrix. For high strength steels composed of the tempered martensite matrix and granular bainite, however, the minimal deviation between the two constituent phases was acknowledged as an important factor to increase stretch-flangeability. †(Received May 8, 2013)

Effect of Retained and Reversed Austenite on the Damping Capacity in High Manganese Stainless Steel

The effect of retained and reversed austenite on the damping capacity in high manganese stainless steel with two phases of martensite and austenite was studied. The two phase structure of martensite and retained austenite was obtained by deformation for various degrees of deformation, and a two phase structure of martensite and reverse austenite was obtained by reverse annealing treatment for various temperatures after 70 % cold rolling. With the increase in the degree of deformation, the retained austenite and damping capacity rapidly decreased, with an increase in the reverse annealing temperature, the reversed austenite and damping capacity rapidly increased. With the volume fraction of the retained and reverse austenite, the damping capacity increased rapidly. At same volume of retained and reversed austenite, the damping capacity of the reversed austenite was higher than the retained austenite. Thus, the damping capacity was affected greatly by the reversed austenite.

Microstructure and Mechanical Properties of High Strength and Stretch-Flangeability Hot-Rolled Steels

Research into the development of high strength (1 GPa) and superior formability, such as total elongation (10%), and stretch-flangeability (50%) in hot-rolled steel was conducted with a thermomechanically controlled hot-rolling process. To improve the overall mechanical properties simultaneously, low-carbon steel using precipitation hardening of Ti-Nb-V multimicroalloying elements was employed. And, ideal microstructural characteristics for the realization of balanced mechanical properties were determined using SEM, EBSD, and TEM analyses. The developed steel, 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V), consisted of ferrite as the matrix phase and second phase of granular bainite with fine carbides (20-50 nm) in both phases. The significant factor of the microstructural characteristics that affect stretch-flangeability was found to be the microstructural homogeneity. The microstructural homogeneity, manifest in such characteristics as low localization of plastic strain and internally stored energy, was identified by grain average misorientation method, analyzed by electron backscattered diffraction (EBSD) and hardness deviation between the phases. In summar, a hot-rolled steel having a composition 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V) demonstrated a tensile strength of 998 MPa, a total elongation of 19%, and a hole expansion ratio of 65%. The most important factors to satisfy the mechanical property were the presence of fine carbides and the microstructural homogeneity, which provided low hardness deviation between the phases.