Formation Of Ultrafine Ferrite Research Articles

Formation of ultra-fine ferrite in hot rolled strip: potential mechanisms for grain refinement

A novel single-pass hot strip rolling process has been developed in which ultra-fine (<2 μm) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 μm. Two experiments were performed on strip that had been re-heated to 1250°C for 300 s and air-cooled to the rolling temperatures. The first involved hot rolling a sample of 0.09 wt.%C–1.68Mn–0.22Si–0.27Mo steel (steel A) at 800°C, which was just above the Ar 3 of this sample, while the second involved hot rolling a sample of 0.11C–1.68Mn–0.22Si steel (steel B) at 675°C, which is just below the Ar 3 temperature of the sample. After air cooling, the surface regions of strip of both steel A and B consisted of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure. In the case of steel B, a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section. Based on results from optical microscopy and scanning/transmission electron microscopy, as well as bulk X-ray texture analysis and microtextural analysis using Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra-fine ferrite most likely forms by a process of rapid intragranular nucleation during, or immediately after, deformation. This process of inducing intragranular nucleation of ferrite by deformation is referred to as strain-induced transformation.

Ultrafine ferrite formation during hot strip rolling

A rolling process has been developed to produce ultrafine ferrite in the surface layers of steel strip. Microscopic examination of the rolled steel revealed that the ultrafine ferrite formed by means of a strain induced transformation. Comparison with a model Ni–30 wt-%Fe alloy subjected to the same rolling schedule suggested that the ultrafine ferrite formed on the substructure in the deformed material. This rolling process is termed strain induced transformation rolling.

The Effect of Chemistry on the Formation of Ultrafine Ferrite in Steel.

A thermomechanical process has been developed to produce ultrafine (1-2 μm) ferrite grains in hot rolled steel strip. This process involves rolling the steel at or just above the austenite to ferrite transformation temperature, and was applied to a variety of steel chemistries, including plain low and high carbon grades and steels containing microalloying elements such as Nb, Ti, B and Mo. Significant volume fractions of ultrafine grains were observed in all samples. However, it was found that the chemical composition of the steel slightly influenced the morphology and volume fraction of ultrafine grains formed in the surface layers of the strip, and significantly altered the microstructure formed in the core of the strip. In plain carbon grades, the level of ferrite refinement increased slightly as carbon content increased. In the case of the microalloyed steels, those containing Nb and Ti additions produced the greatest level of refinement. The volume fraction and level of refinement of the ultrafine ferrite grains also influenced the strengths of the steels.

In situ observation of γ → α transformation during hot deformation in an Fe-Ni alloy by an X-ray diffraction method

The formation of ultra-fine ferrite of 1 to 4 {micro}m in diameter was reported in various low carbon steels and in Fe-Ni-C alloys when heavy hot deformation of more than 40 pct is given at the temperature range around A{sub e3}, followed by rapid quenching. It is known that in various alloys including pure iron and iron alloys, there exists a massive transformation with which product phases grow at a relatively rapid velocity and involve a composition-invariance. It is widely accepted that this transformation is interface-controlled. High transition velocity and no susceptible partition of C between {alpha} and prior {gamma} in the above mentioned transformation during hot deformation coincides with the feature of massive transformation. It was inferred, therefore, that the interface-controlled transformation was enhanced by plastic deformation in the above experiments. Although the above assumption agreed with many experimental results, it remains ambiguous as all the ferrite structures were observed after cooling. Considering this fact, therefore, it is imperative to provide direct or on-line evidences for the formation of ferrite in high temperature during hot deformation process.