Effect of boron on the hot ductility of Nb-containing steel
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M, Graduate Student, and NACK J. KIM, Professor, are with the Department of Materials Science and Engineering and Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] JUNE SUNG KIM, Manager, is with the Quality Technology Department, POSCO, Pohang 790-360, Korea. Manuscript submitted July 10, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Gleeble tests were carried out in order to simulate the thermal-strain history of the continuous casting process. The specimens were heated to 1350 ⬚C and cooled at 1 ⬚C/s to the test temperature and subjected to tensile testing at a strain rate of 5 ⫻ 10⫺3/s. The test temperatures were from 700 ⬚C to 900 ⬚C with 50 ⬚C intervals. After tensile testing, specimens were quenched by spraying with compressed air to preserve the proeutectoid ferrite formed at the test temperature. The reduction of area (RA) was measured as an evaluation of hot ductility. The cooling rate of 1 ⬚C/s used in the Gleeble tests is the same as that used in the continuous caster at the Pohang Iron and Steel Co. Transmission electron microscopy (TEM) was employed to investigate the details of microstructural features. Specimens for TEM were made by electropolishing in a solution of 8 pct perchloric acid and 92 pct acetic acid. Figure 1 shows the microstructures of the steels quenched after fracture at 750 ⬚C. It shows that both steels have similar prior austenite grain sizes of 1 mm. Proeutectoid ferrite is present along the prior austenite grain boundaries in all steels. However, there are differences in the microstructures between the base steel (A steel) and the boron-containing steel (B steel). The B steel has a smaller amount of ferrite films along the prior austenite grain boundaries than the A steel (12.9 vol pct in the B steel vs 23.3 vol pct in the A steel). Also, the B steel contains proeutectoid ferrite within the prior austenite grains (10.9 vol pct). In addition to such differences in the morphology of ferrite between the A and B steels, the boron-containing B steel has 5-m-sized coarse precipitates along the prior austenite grain boundaries as well as within the prior austenite grains (arrows in Figure 1). The TEM analysis was conducted to identify the nature of these precipitates, as shown in Figure 2. Selected area diffraction patterns (SADPs) of the precipitates show that they are Fe23(B,C)6. Scanning Auger microscopy (SAM) analysis was also conducted for the detailed identification of the composition of precipitates and it shows that they contain N besides Fe, C, and B (Figure 2(d)). The presence of intragranular ferrite in the B steel is worth noting. It has been previously shown that Fe23(B,C)6 particles have varying effects on the nucleation of proeutectoid ferrite, depending on their sizes;[10] fine particles retard the nucleation of ferrite and coarse ones promote the nucleation of ferrite, although the critical size is not known. As shown in Figure 1(b), there are many coarse Fe23(B,C)6 particle
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