Application of Martensitic Transformation Fundamentals to Select Appropriate Alloys for Grain Refining Through Martensit
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INTRODUCTION
THE increasing stringency of the standards of automotive industry to enhance passenger safety in vehicle crashes and to reduce the exhaust emissions to conform to environmental regulations leads automakers toward the substitution of the traditional materials. In this context, new steel grades have been developed possessing the attractive combination of an increased formability with a high strength level. Application of these steels in vehicles makes them safer and lighter. Nano/ultrafine-grained austenitic steels feature promising results in this field, while their extraordinary properties can be controlled by alloying and processing. It is currently well known that in metastable austenitic steels, the reversion of deformation-induced martensite ( a) to austenite (c) enables a marked grain refinement.[1–4] This process, called martensite thermomechanical treatment, includes cold rolling to form a from c. The a-martensite volume fraction increases with deformation until complete transformation or saturates to a certain level. In the latter instance, the strain required for saturation is called saturation strain (es), as done in References 5, 14, and 15 (in this work the target fraction of martensite at es is 90 pct at minimum). The martensite crushes during deformation, increasing lattice defects inside a, and it provides suitable sites for c nucleation. Finally, the a is reverted to c during subsequent annealing at relatively low temperatures and short times, leading to noticeable grain refinement down to nano (d < 100 nm) and\or ultrafine (d < 500 nm) range.
In this process, the volume fraction of a-martensite and es play an important role in achieving a nano/ultrafinegrained structure.[5,6] In other words, the formation of a higher fraction of a-martensite at a lower strain is a favorite. It has been shown that in 204Cu and 201 austenitic stainless steels, the formation of ~30 to 40 pct a-martensite is effective for grain size refinement of these alloys down to 1 to 2 lm[7]; however, almost complete martensitic transformation is desirable when grain refinement to nanoscale is targeting. The volume fraction of a-martensite and es strongly depends on the chemical composition of c and deformation conditions (strain, strain rate, strain path, and rolling temperature).[8,9] The martensite reversion process has been applied to refine the microstructure of different commercial austenitic stainless steels such as 301,[1,3,5] 301LN,[10] 304,[11] 304L,[12] 316L,[13] 201,[14] 201L,[15,16] and 204Cu.[7] An appropriate chemical composition is required to have success in grain refining of the steel through application of the martensite reversion technique. Despite its importance, very few studies have been performed to develop a predictive model capable of assessing appropriation of a chemical composition for being used in the martensite treatment. Angel[17] proposed an empirical formula to describe the tendency of an austenitic alloy to deformation-induced martensite through its chemical composition. Later, Nohara
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