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Busan, South Korea

Eom J.G.,MFRC | Son Y.H.,Youngsin Metal Industrial Co. | Jeong S.W.,Gyeongsang National University | Ahn S.T.,Samhwa Steel | And 3 more authors.
Materials and Design | Year: 2014

A comparative study of a pre-heat-treated steel PHTS and the SCM435 steel is made by simulations and experiments. PHTS is characterized by high initial yield strength and negligible strain-hardening capability while SCM435 is a typical strain hardening material. Material identification is first conducted. To characterize the plastic deformation behaviors of the two materials, predictions of cone indentation are compared. Predictions and experiments of axial compression of cylindrical specimens are also compared. In addition, a forward extrusion process, a backward extrusion process, a heading process, a forward and backward extrusion process and a four-stage forging process are simulated to examine the difference between the two steels. The findings has shown significant differences in metal flow between PHTS and SCM435, which is due to the difference in strain-hardening capabilities, indicating that experience-oriented design rules for common commercial materials may lead to failures in process design if the new material PHTS is used without considering its plastic deformation behavior. © 2013 Elsevier Ltd.

Lee J.B.,Pusan National University | Kang N.,Pusan National University | Park J.T.,Samhwa Steel | Ahn S.-T.,Samhwa Steel | And 4 more authors.
Journal of Korean Institute of Metals and Materials | Year: 2010

Various types of steel, namely, 0.35C, 0.2C-Cr, and 0.2C-Cr-Mo steels, were quenched and tempered by high-frequency induction heat treatment. The type, size, and spheroidization of the carbides varied depending on the tempering temperatures (450-720°C). During the tempering process, the carbide was precipitated in the martensite matrix. The 0.35C, 0.2C-Cr, and 0.2C-Cr-Mo steels contained carbides that were smaller than 120 nm. The carbide was spheroidized as the tempering temperature increased. Owing to the fine microstructure and spheroidization of the carbides, all three steels had a high tensile strength as well as yield ratio and reduction of area. In the case of the 0.2C-Cr steel, the use of Cr as an alloying element facilitated the precipitation of alloyed carbides with an extremely small particle and resulted in an increase in the spheroidization rate of the carbides. As a result, a large reduction of area was achieved (>70%). The 0.2C-Cr-Mo steel had the highest tensile strength because of the high hardenability that can be attributed to the presence of alloying elements (Cr and Mo). Quenching and tempering steels by induction heat treatment resulted in a high strength of over 1 GPa and a large reduction of area (>70%) because of the rapid heating and cooling rates.

Kang H.J.,Pusan National University | Yoo J.S.,Pusan National University | Park J.T.,Samhwa Steel | Ahn S.T.,Samhwa Steel | And 2 more authors.
Materials Science and Engineering A | Year: 2012

The amount, size, and spheroidization of nano-carbides were evaluated to determine their effect on the hydrogen-delayed fracture (HDF) properties of high-strength steel wires produced by quenching and tempering (QT) during high-frequency induction heat treatment. The steel wires had a microstructure of tempered martensite showing a tensile strength of 1.2. GPa. The size of the carbide decreased and the spheroidization increased with the tempering temperature. The 0.3C-Si steels that have needle-like carbides were more susceptible to HDF than the other steels (0.2C-Cr and 0.2C-Cr-Mo) that have carbides of high spheroidization rate. Calculation of the activation energy using thermal desorption spectrometry (TDS) defined the hydrogen trapping site. The trapping sites of diffusible hydrogen were the grain boundary and lath boundary for all three wires. Carbide/matrix interfaces trapped nondiffusible hydrogen for all specimens. The 0.2C-Cr steel exhibited the largest resistance to HDF because of the nano-sized and spheroidized carbide. © 2012 Elsevier B.V..

Lee J.B.,Pusan National University | Kang N.,Pusan National University | Park J.T.,Samhwa Steel | Ahn S.-T.,Samhwa Steel | And 4 more authors.
Materials Chemistry and Physics | Year: 2011

The amount, size, and spheroidization of carbides precipitated during quenching and tempering of steels were analyzed as a function of the tempering temperatures and alloying elements during high-frequency induction heat treatment. The size of the carbides decreased and their spheroidization increased as the tempering temperature increased. Strong carbide formers such as Cr and Mo produced nano sized alloyed carbides (60-150 nm) that were significantly spheroidized. The amount of carbides produced by Cr and Mo increased as the temperature increased. However, 0.45C steel with no carbide-forming elements maintained its carbide amount to 700 °C; this was followed by a significant decrease in the carbide amount at 720 °C. Induction heating reduced the tempering time for carbide spheroidization, and alloying elements such as Cr and Mo played an important role in reducing the spheroidization time. © 2011 Elsevier B.V.

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