Correlation Between MnS Precipitation, Sulfur Segregation Kinetics, and Hot Ductility in C-Mn Steel
- PDF / 1,230,262 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 52 Downloads / 237 Views
is known that Nb is the most detrimental to the hot ductility among the alloying elements Ti, Nb, Al, V, and B.[1–7] The mechanism of hot ductility loss is explained by the microvoid coalescence mechanism[2–7] and the grain boundary sliding mechanism.[2] Meanwhile, it has been reported that sulfur segregated to austenite grain boundaries accelerates the high-temperature embrittlement.[8,9] However, the mechanism is not still clear, regardless of much research on C-Mn-Nb low-alloy steels. In this study, the mechanism of hot ductility loss and recovery is investigated in the light of MnS precipitation and its effect on sulfur segregation kinetics, using a simple C-Mn steel without Nb.
M.H. KANG, Ph.D. Candidate, J.S. LEE, Y.M. KOO, S.-J. KIM, and N.H. HEO, Professors, are with the Graduate Institute of Ferrous Technology (GIFT), Pohang Institute of Science and Technology (POSTECH), Pohang 790-784, Korea. Contact e-mail: nhheo@postech. ac.kr Manuscript submitted April 3, 2014. Article published online August 29, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
The chemical composition of the prepared steel is listed in Table I. The ingot of 30 kg was hot rolled to plates of 12 mm thickness after soaking at 1423 K (1150 C) for 2 hours. Cylindrical tensile specimens with a dimension of 8 mm gage length and 7 mm gage diameter were machined from the hot-rolled plates in the direction normal to the hot-rolling direction. Specimens were induction heated to 1673 K (1400 C), using a thermomechanical simulator. The uniform temperature zone was 30 mm. Based on the result of ThermoCalc (TCFE7) shown in Figure 1, this temperature corresponds to the complete dissolution temperature of MnS particles formed during soaking at 1423 K (1150 C) into the austenite matrix. After solution treatment at 1673 K (1400 C) for 5 minutes, the specimens were cooled at a rate of 1 K/s or 20 K/s to 1273 K (1000 C) before tensile testing at a strain rate of 1/s. Changes in hot ductility with holding time at the test temperatures were also investigated. During the experiments, an argon atmosphere was employed to minimize the high-temperature oxidation. A schematic diagram that explains the experimental procedure is shown in Figure 2. After the tensile tests and the subsequent water quenching, the specimens showed a martensitic structure. The segregation behavior of sulfur to the prior austenite grain boundary (AGB) segregation was investigated, using Auger electron spectroscopy (AES). Notched AES specimens with dimensions of 3 mm diameter and 16 mm length, which were machined from the tensile-tested specimens, were chilled with liquid nitrogen and in situ fractured in a vacuum of about 7 9 10 8 Pa or better. Typical parameters used were the primary electron beam energy of 3 keV and an electron beam size of about 200 nm. Peak-to-peak height ratios, IS/IFe, were obtained from each differential AES spectra and then averaged. The AES peaks used were S150 and Fe703. Representative Auger spectra and changes in reduction of area (RA) and grain boundary se
Data Loading...
