Microstructure Evolution during Recrystallization in Dual-Phase Steels

Abstract

The microstructure and texture of rolled and annealed dual-phase steels with 0.147 wt. % C, 1.868 wt. % Mn, and 0.403 wt. % Si were analyzed using SEM, EDX, and EBSD. Hot rolled sheets showed a ferritic-pearlitic microstructure with a pearlite volume fraction of about 40 % and ferrite grain size of about 6 µm. Ferrite and pearlite were heterogeneously distributed at the surface and distributed in bands at the center of the sheets. The hot rolled sheets revealed a through-thickness texture inhomogeneity with a plane-strain texture with strong α-fiber and γ-fiber at the center and a shear texture at the surface. After cold rolling, the ferrite grains showed elongated morphology and larger orientation gradients, the period of the ferrite-pearlite band structure at the center of the sheets was decreased, and the plane-strain texture components were strengthened in the entire sheet. Recrystallization, phase transformation, and the competition of both processes were of particular interest with respect to the annealing experiments. For this purpose, various annealing techniques were applied, i.e., annealing in salt bath, conductive annealing, and industrial hot-dip coating. The sheets were annealed in the ferritic, intercritical, and austenitic temperature regimes in a wide annealing time range including variation of heating and cooling rates.

A Model and Experimental Validation to Predict Heating Rates for Overlap Between Ferrite Recrystallization and Austenite Transformation in Dual Phase Steel Manufacture

“…11 The amount of overlap between these thermally activated processes strongly depend on the initial microstructures, stored deformation energy, type and alloy contents and the employed heating rates during the continuous annealing process. 12 In the past decade, researchers have employed high heating rates 13 to increase the percentage of this overlap and therefore increase the complex interplay between these mechanisms, and consequently achieved interesting results such as ferrite 14 and prior austenite 15 grain refinement, 16 increase in martensite volume fraction, 17 change in ferrite/martensite morphology, variation in texture evolution, 18,19 texture memory effect, 20 increase in ultimate tensile strength and enhanced % elongation, 21 etc. However, the chosen heating rates were mostly arbitrary and inconsistent, as the sole aim of the most researchers was to choose as high heating rate as possible to achieve maximum overlap.…”

Interaction Between Ferrite Recrystallization and Austenite Formation in Dual-Phase Steel Manufacture

“…This can be explained by the slow austenite formation kinetics and/or requirement of high incubation time at lower IA temperatures. [17] Figure 2(c) shows the dependency of ferrite grain aspect ratio on increasing heating rates and holding times. Aspect ratio (AR) is related to the recrystallization process, where a higher aspect ratio represents less progress in recrystallization process.…”

“…[16] As a result, additional high temperature mechanisms such as annihilation of defects through ferrite recovery, ferrite recrystallization and grain growth, play a role during the heating step of CR material. [17,18] Generally these processes occur below the start temperature of austenite formation (A c1 ). [18,19] However, to fulfill the ever rising demand for higher strength materials (while maintaining formability), addition of precipitate forming elements like vanadium, titanium, molybdenum and niobium and/or increase in the amount of substitutional alloying elements like manganese, chromium, silicon is becoming a common practice in steel manufacture.…”

Effect of Silicon, Manganese and Heating Rate on the Ferrite Recrystallization Kinetics

“…This size range is approximately equivalent to the pearlite band spacing of the alloys, which is measured and published in Ref. 18,44) Because the ferrite growth rates in RD and TD directions are comparable and much higher than the growth rate observed for ND direction, only the former was considered in the subsequent analysis of the recrystallization process. It was then compared with the growth rates obtained from the fitting of non-isothermal JMAK model to the volume fraction recrystallized measured by XRD (method described in Section 3.2.).…”