Abstract
It is well-known that grain refinement is one of the most effective ways to improve strength of metals without addition of alloying elements. In order to obtain bulky metals having ultrafine grained (UFG) microstructures with average grain sizes smaller than 1 μm, severe plastic deformation (SPD) processes have made a great success. However, there are still big barriers to realize UFG metallic materials, especially UFG steels, in large scale industries, since severe plastic deformation processes usually need special techniques and equipment, and large deformation forces are required for heavy plastic deformations. Cyclic heat treatments to repeat martensitic transformation and austenitization have been known as a simple way to fabricate fine-grained austenitic structures in steels. In the present study, we tried to make final ferrite microstructures ultrafine in a low-C steel by means of the cyclic heat treatment. Evolution of microstructures during the cyclic heat treatment was systematically investigated, putting stress on the change of grain sizes of austenite and ferrite. The austenite grain size decreased with increasing the number of heat treatment cycles, and the minimum average austenite grain size obtained was 11 μm. By having furnace-cooling from austenite states with various grain sizes, ferrite microstructures with different mean grain sizes were fabricated. We could successfully obtain a fine-grained ferrite structure with a mean grain size of 4.5 μm and nearly a random texture through the heat treatment without deformation. Microstructural features and mechanical properties of the obtained fine-grained ferritic structures were investigated by scanning electron microscope/electron back-scattering diffraction measurements and a tensile test at room temperature. The specimens with ferrite + pearlite microstructure with the smallest average ferrite grain size of 4.5 μm managed both high strength (yield strength of 375 MPa and tensile strength of 500 MPa) and large tensile ductility (uniform elongation of 20% and total elongation of 39%) in the simple 2Mn-0.1C steel.
Highlights
Almost all the metallic materials used for practical applications are polycrystalline materials composed of a number of grains
We investigate the possibility of grain refinement of austenite and ferrite microstructures in a lowcarbon steel by a simple cyclic heat treatment without plastic deformation
It was confirmed that all the specimens had fully martensitic microstructures of typical lath martensite consisting of lathes, blocks, packets, all formed within each prior-austenite grain
Summary
Almost all the metallic materials used for practical applications are polycrystalline materials composed of a number of grains. Shibata et al (2013) used the cyclic heat treatment for refining austenite grain sizes in medium carbon steels and could obtain the austenite grain size of 4.5 and 2.4 μm in a 0.44C-0.86Mn (mass %) steel and a 0.46C-0.84Mn-0.3V steel, respectively. Furuhara et al (2008) used a cyclic heat treatment followed by air-cooling and reported that fine-grained ferrite having a grain size of 2.2 μm was successfully obtained in a medium-carbon steel with a chemical composition of Fe-0.35C-1.05Cr-0.17Mo (in mass%). Fine grained microstructures have been successfully obtained in medium carbon steels by the cyclic heat treatments, there are few studies on microstructure refinement in low-carbon steels with ∼0.1 mass% C contents by such cyclic processes, probably due to their higher Ae3 temperatures. Microstructure evolution during the cyclic heat treatment is systematically observed, and mechanical properties of the obtained low-C steel with fine-grained microstructures are compared with those of coarse-grained counterparts
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