The Effect of Heating Rate on Subsurface AlN Formation in Fe-Al Alloys in N 2 -O 2 Atmospheres

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INTRODUCTION

TRANSFORMATION induced plasticity (TRIP) steels are important to the automotive industry as a light-weight, high-strength, and high-ductility structural material. In TRIP steels, aluminum functions to stabilize a-ferrite and to prevent the formation of cementite, which leads to carbon enrichment in the retained c-austenite phase. This retained austenite produces the TRIP effect by transforming into martensite under strain.[1–3] However, aluminum is highly reactive in both liquid and solid steels and can form internal aluminum nitride and oxide particles which can affect the surface quality of the steel. Iron-aluminum alloys containing up to 8 wt pct aluminum were examined in previous work.[4,5] Heating conditions in the first study were set to replicate slab JUNE BOTT, Post-Doctoral Researcher, is with the Department of Materials Science & Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] HONGBIN YIN, Lead Engineer, is with the Casting, Process Research, East Chicago Center, ArcelorMittal Global R&D, East Chicago, IN 46321. SEETHARAMAN SRIDHAR, TATA Steel/RAEng Chair, is with the Low Carbon Materials Manufacturing, Warwick Manufacturing Group, International Digital Laboratory, University of Warwick, Coventry CV4 7AL, U.K. Manuscript submitted July 10, 2013. Article published online July 11, 2014. 2222—VOLUME 45B, DECEMBER 2014

reheating during the steelmaking process. A schematic of the steelmaking process is shown in Figure 1. After the molten steel is cast, it undergoes secondary cooling, where the slab is sprayed with water to reduce the temperature. Nitrogen and oxygen are both available from the air, and water vapor also contributes to oxidation. The slab is then cooled to room temperature in air in a slab yard. Before hot rolling processes, the slab must be reheated. This occurs in reheat furnaces in an atmosphere produced by the combustion of natural gas and contains carbon dioxide, water vapor, oxygen, and nitrogen. The primary focus of this and the previous study is on the formation of aluminum nitrides during reheating, but as seen in Figure 1, there are various other opportunities for nitrogen to react with the steel, e.g., during cooling during in the caster and hot-rolling. If there are sufficient levels of aluminum and nitrogen in solution in the system, they will combine to form aluminum nitride. However, oxide films are known to severely limit the penetration of adsorbed nitrogen into the bulk of the material, since the solubility of nitrogen is low in the oxide.[6] Oxides develop on and below the surface of the material. In particular, a-alumina is well known to form adherent and protective scales on iron-aluminum alloys.[7–12] Development of protective a-alumina scales is well known to be highly dependent on temperature METALLURGICAL AND MATERIALS TRANSACTIONS B

Fig. 1—Temperatures and atmospheres involved in the steelmaking process.

and aluminum content in the alloy.[8,11] For increasing aluminum content, oxidation of Fe-Al alloys will result i

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