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Analysis and Estimation of Yield Strength of API X80 Linepipe Steel Pipe by Low-Cycle Fatigue Tests
손석수,Seung Youb Han,Sang Yong Shin,Jin-ho Bae,Kisoo Kim,김낙준,김형섭,이성학 대한금속·재료학회 2012 METALS AND MATERIALS International Vol.18 No.4
In the present study, spiral piping was conducted on API X80 linepipe steel, and the outer and inner wall pipe yield strengths were measured. A low-cycle fatigue test was conducted on a leveled X80 steel sheet to simulate piping and flattening processes, and the strain hardening and Bauschinger effects, induced from different strain histories, were evaluated and combined using Swift’s equation and the Bauschinger stress parameter, respectively. By analyzing the stress-strain curves obtained from the low-cycle fatigue test, the yield strengths of the outer and inner walls were estimated to be 592 MPa and 492 MPa, respectively,which are lower by 20-80 MPa than that of the actual pipe used. Possible reasons for measured and estimated yield strength differences could be the simulation determining procedure of the pre-strain and Bauschinger stress parameters, preposition of same strain hardening behavior depending on strain history,and pre-strain differences depending on thickness location in the steel sheet during piping.
손석수,이병주,이성학,곽재현 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.1
An investigation was conducted into the cracking phenomenon occurring during the cold rolling of lightweightsteel plates. Four steels of varying C contents were fabricated and steel plates containing C contents of 0.5wt% orhigher were cracked during the initial stage of the cold rolling. The steels were basically composed of ferritegrains and -carbides in a band shape, but the volume fraction and thickness of κ-carbide band increased asthe C content increased. Microstructural observation of the deformed region of fractured tensile specimens revealedthat deformation bands were homogeneously formed in wide areas of ferrite matrix in the steels containingC contents of 0.3 wt% or lower, while κ-carbide bands were hardly deformed or cracked. In the steels containinghigh C contents of 0.5 wt% or higher, on the other hand, microcracks were initiated mostly at fine proeutectoidferrite located within κ-carbide bands, and were grown further to coalesce with other microcracksto form long cracks. To prevent the cracking, thus, the proeutectoid ferrite should be minimized by the hotrolling in the (α+γ) two phase region. As practical methods, the content of C below 0.5% or Al above 5%was suggested to expand the (α+γ) phase region.
정승문,손석수,최원미,이병주,오용준,장성식,이성학 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.1
High-temperature tensile properties of three heat-resistant austenitic cast steels fabricated by varying W, Mo, and Al contents were interpreted by thermodynamically calculated equilibrium phase diagrams of austenite, ferrite, and carbides as well as microstructural analyses. A two-step calculation method was adopted to cast steel microstructures below the liquid dissolution temperature because the casting route was not an equilibrium state. Thermodynamically calculated fractions of equilibrium phases were well matched with experimentally measured fractions. Ferrites existed at room and high temperatures in both equilibrium phase diagrams and actual microstructures, which has not been reported in previous researches on austenitic cast steels. In the W2Mo1Al1 steel, 38% and 12% of ferrite existed in the equilibrium phase diagram and actual microstructure, respectively, and led to the void initiation and coalescence at ferrites and consequently to the serious deterioration of high-temperature strengths. The present equilibrium phase diagrams, besides detailed microstructural analyses, effectively evaluated the high-temperature performance by estimating high-temperature equilibrium phases, and provided an important idea on whether ferrite were formed or not in the heat-resistant austenitic cast steels.
강민주,박재영,손석수,김현민,김관호,이성학 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.6
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.