Selective Oxidation of a C-2Mn-1.3Si (Wt Pct) Advanced High-Strength Steel During Continuous Galvanizing Heat Treatments
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INTRODUCTION
ADVANCED high-strength steels (AHSS) are of great interest to the automotive industry due to legislative requirements for improving fuel efficiency, where reducing vehicle weight is a partial solution to achieving this objective.[1] Dual-phase (DP) and Transformation-Induced Plasticity (TRIP) steels are among the so-called first generation (1G) of AHSSs and possess a unique combination of high strength, good formability, and high-energy absorption capacity, making it possible to reduce the thickness (and, therefore, the mass) of a part without compromising performance. The promising properties of DP and TRIP steels are brought about by carefully designed heat treatments and through the proper addition of alloying elements such as carbon, manganese, and silicon.[2] Carbon is added to stabilize austenite and is a potent solid-solution strengthener. Manganese is an austenite stabilizer and a moderate solid-solution strengthener. Silicon acts as a solid-solution strengthener and also delays carbide formation during isothermal bainitic transformation annealing,[3–5]
G. SEYED MOUSAVI and J.R. MCDERMID are with the Steel Research Centre, McMaster University, Hamilton, ON, L8S 4L8 Canada. Contact e-mail: [email protected] Manuscript submitted December 15, 2017. Article published online August 3, 2018 5546—VOLUME 49A, NOVEMBER 2018
a role critical in the microstructural development of TRIP alloys. In order to be used in commonly encountered road environments, it is essential to protect the steel from corrosion to ensure its long-term stability, to achieve which the continuous hot-dip galvanizing process is one of the most cost-effective techniques.[6,7] However, galvanizing DP and TRIP steels can be challenging. This difficulty arises because, although the commonly-used industrial N2-(5 to 20 vol pct) H2-xH2O annealing atmosphere is reducing for Fe oxides, the oxygen potential is sufficient to selectively oxidize alloying elements such as Si and Mn. Selective oxidation can occur externally, on the surface, or internally, below the surface.[8–10] It has been well established that external oxides can deteriorate the reactive wetting of the steel strip by the molten Zn-Al-Fe bath during the continuous hot-dip galvanizing process.[8,11] Furthermore, it has been advocated by several authors that the shape, thickness, spatial distribution, and chemistry of the external oxides also affect the reactive wetting of the steel strip by the molten zinc bath.[6,10,12–16] For example, the study of Bellhouse and McDermid[17] on the effect of process atmosphere oxygen partial pressure on the selective oxidation and reactive wetting of a C-1.5Mn-1.5Al (wt pct) TRIP-assisted steels showed that the widely spaced external oxide nodules with relatively thin internodular MnO films resulted in a fully developed and homogeneous g-Fe2Al5ZnX interfacial METALLURGICAL AND MATERIALS TRANSACTIONS A
layer being formed, indicative of reactive wetting, upon dipping in a 733 K (460 C) Zn-0.2Al-0.01Fe (wt pct) continuous galvanizing bath. This res
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