Texture and microstructure development during intercritical rolling of low-carbon steels
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I. INTRODUCTION
THE properties of a material depend heavily on the combination of grain size, morphology, and crystallographic texture. The texture is particularly important to the formability of the material, more specifically, to deep-drawing properties.[1] There is a clear relationship between the intensities of certain crystallographic orientations and, for instance, the deep-drawing ratio or the r-value.[1] The texture also has consequences for other mechanical properties. In particular, in high-strength low-alloy steels, a retained deformation texture may result in an increased strength of the steels by promoting a smaller ferrite grain size.[2] Furthermore, in the production of high-strength plate in a plate mill, the rolling of thinner gages often results in intercritical deformation. In addition to the texture that results from the deformed and subsequently partially transformed austenite, intercritical deformation increases the amount of texture formation upon further deformation of the two-phase structure.[3,4] In most cases, the formability in terms of r-value is irrelevant for these kinds of steels, but the difference in mechanical properties in the longitudinal and transverse directions is highly relevant.[4] The changes in crystallographic orientation during deformation of grains are not random. They are a consequence of deformation occurring on the most favourably oriented slip or twinning systems. As a result, the deformed material acquires a preferred orientation or texture. If the metal subsequently recrystallizes, nucleation may occur preferentially in regions of a particular orientation.[5] The ability of the A. BODIN, Researcher, is with Corus Research, Development & Technology, IJmuiden Technology Centre, Steel Metallurgy, 1970 CA IJmuiden, The Netherlands. Contact e-mail: [email protected] J. SIETSMA, Associate Professor, and S. VAN DER ZWAAG, Professor, are with the Laboratory of Materials Science, Delft University of Technology, 2628 AL Delft, The Netherlands. Manuscript submitted August 14, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleus to grow is determined primarily by the orientations of adjacent regions in the microstructure. Together, these features (nucleation and growth) ensure that a texture also develops in the recrystallized material. Such a texture is called a recrystallization texture to distinguish it from the deformation texture from which it develops. In the case of deformation of austenite, the transformation of the deformed or recrystallized austenite into ferrite results in a transformation texture.[3] The relation between the orientation of the crystallites in the austenite and the resulting orientation in the ferrite can be described by an orientation relationship. In the case of the austenite-to-ferrite transformation, the Kurdjumov–Sachs orientation relationship is widely used and commonly accepted.[6] Transformation after rolling below the austenite recrystallization temperature (Tnr) results in a sharper austenite deformation texture than after
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