A Microstructure Evolution Model for Hot Rolling of a Mo-TRIP Steel
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ADVANCED high-strength steels (AHSS) including dual-phase (DP) and transformation-induced-plasticity (TRIP) steels exhibit superior mechanical properties such as increased tensile strength with improved formability as compared to conventional high-strength lowalloy (HSLA) steels.[1] This makes AHSS an attractive choice for automotive manufacturers; the increase of strength enables weight reduction of the vehicles leading to better fuel efficiency, whereas the improvement of formability allows for more complex components to be produced. In particular, TRIP steels offer an excellent property combination due to the unique TRIP effect that is associated with the strain-induced transformation of retained austenite to martensite during deformation. However, the microstructure of TRIP steels is very complex, consisting of ferrite, bainite, martensite, and retained austenite. In addition to the volume fraction of these four components, other factors such as their size, distribution, and chemistry play an important role to attain a desired property combination. To consistently produce these steels on an industrial scale is still a challenge, and as a result, TRIP steels are presently a DONGSHENG LIU, Research Engineer, F. FAZELI, Postdoctoral Fellow, and M. MILITZER and W.J. POOLE, Professors, are with the The Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: [email protected] Manuscript submitted September 1, 2006. Article published online May 1, 2007. 894—VOLUME 38A, APRIL 2007
niche product; e.g., it has been projected that just 4 pct of an ultralight steel autobody would consist of TRIP steel as compared to 74 pct for DP steels.[2] There are two principal ways for producing TRIP steels: (1) hot strip rolling and (2) continuous annealing of cold-rolled material. The investigations on microstructure evolution during various thermomechanical processing conditions as well as the mechanical properties of these TRIP steels have been extensively conducted over the past decade.[3–17] A particular emphasis of these research efforts has been on two aspects: (1) retained austenite and its stability during deformation[11,12,13] and (2) the effects of steel chemistry on the TRIP effect and properties.[14–17] The latter aspect has primarily been driven to reduce the Si content of the classical TRIP steel, 0.2 wt pct C-1.5 wt pct Mn-1.5 wt pct Si, to mitigate adverse effects of Si in terms of steel making, surface quality, and coatability. As a result, partial or total replacement of Si by Al alloying has been proposed for commercial TRIP steels. For hot-rolled TRIP steels, controlled rolling in a hot strip mill and complex cooling on the run-out table are the key processing steps to develop the desired multiphase microstructure. The determination and optimization of these thermomechanical processes require the knowledge of the underlying metallurgical phenomena, i.e., hot deformation behavior, recrystallization, and austenite decomposition. Starting wit
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