Mechanical Properties and Anisotropies of 0.2% Carbon Steel with Bimodal Microstructure Subjected to Heavy‐Reduction Controlled‐Rolling Process

Abstract

The mechanical properties, anisotropies, and fracture behavior of 0.2% carbon steel sheets produced by a heavy‐reduction single‐pass controlled‐rolling process with approximately 75% thickness reduction are investigated via a tensile test, field‐emission scanning electron microscopy, and electron backscattering diffraction. The 900‐ and 1000 °C‐heated specimens with less well‐developed textures show relatively homogeneous anisotropies, whereas the 700‐ and 800 °C‐heated specimens show strong V‐shaped anisotropies, where the relative strength decreases and the relative elongation increases at 45° from the rolling direction in comparison with other directions. The uniform and total elongations of the 900‐ and 1000 °C‐heated specimens in their rolling direction of roughly 12–14% and 23–28% are higher than those of the 700‐ and 800 °C‐heated specimens, respectively, while maintaining high strength (610–670 MPa), as clarified from the nominal stress–strain curves. Additionally, the bimodal structure on the fracture surface consisting of submicron‐size (<1 μm) and fine (2–5 μm) dimples lead to higher ductility while retaining outstanding strength. This is attributed to the submicron‐size grains having excessive strength and the micron‐size grains having high ductility.

Relationships between Microstructural and Mechanical Performance on Example of an Air‐Hardening Steel

“…2) Hot deformation slightly above the A C3 ‐temperature, allowing for a partial strain‐induced γ→α transformation, and thus, the formation of a heterogeneous distribution of ferrite grain sizes after a subsequent controlled cooling . 3) Hot deformation between the A C1 ‐ and A C3 ‐temperatures, recrystallizing fine ferrite grains due to strain accumulation, and transforming austenite into coarse ferrite grains during a subsequent controlled cooling . 4) Hot deformation at 1050 °C, followed by a warm deformation at 550 °C or by a cold deformation with a subsequent rapid annealing between the A C1 ‐ and A C3 ‐temperatures.…”

“…In the last decade, the topic of bimodal ferritic microstructures in bulk low‐carbon steels has been researched extensively. The following processing techniques lead to a bimodal grain size in ferrite at room temperature: 1) Hot deformation at a temperature of 1000 °C, resulting in a partial recrystallization of austenite grains, and thus, in a heterogeneous grain growth of the ferrite during the subsequent controlled cooling . 2) Hot deformation slightly above the A C3 ‐temperature, allowing for a partial strain‐induced γ→α transformation, and thus, the formation of a heterogeneous distribution of ferrite grain sizes after a subsequent controlled cooling .…”

“…The following processing techniques lead to a bimodal grain size in ferrite at room temperature: 1) Hot deformation at a temperature of 1000 °C, resulting in a partial recrystallization of austenite grains, and thus, in a heterogeneous grain growth of the ferrite during the subsequent controlled cooling . 2) Hot deformation slightly above the A C3 ‐temperature, allowing for a partial strain‐induced γ→α transformation, and thus, the formation of a heterogeneous distribution of ferrite grain sizes after a subsequent controlled cooling . 3) Hot deformation between the A C1 ‐ and A C3 ‐temperatures, recrystallizing fine ferrite grains due to strain accumulation, and transforming austenite into coarse ferrite grains during a subsequent controlled cooling .…”