Effect of carbon content on formation of bimodal microstructure and mechanical properties of low-carbon steels subjected to heavy-reduction single-pass hot/warm deformation

Abstract

A compression test simulating heavy-reduction single-pass rolling was conducted to investigate the microstructural evolution based on the formation of a bimodal structure and the mechanical properties of 0.01% and 0.1% carbon steels and niobium steel. When thermomechanical processing was conducted near and above the critical transformation temperature (A c3 ), microstructures of all steels were significantly refined and consisted of equiaxed grains without elongated grains. Nevertheless, these microstructures showed weak or no formation of the bimodal structure or coarse grains with decreasing carbon content, while they showed bimodal structure formation when 0.2% carbon steel was used in our previous © 2014. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ research. The average grain size of Nb steel was about 2 m and its microstructure was uniformly refined. These may be attributed to a decrease in the number of nucleation sites with decreasing carbon content in low-carbon steels and the occurrence of nucleation at grain boundaries as well as in grain interiors in Nb steel during processing. Mechanical properties of all steels deformed above the critical transformation temperature exhibited high performance characteristics with superior strength and marked elongation. Their fractographs indicated ductile fracture, which was revealed by SEM observation after a tensile test.

Microstructural evolution and formation mechanism of bimodal structure of 0.2% carbon steel subjected to the heavy-reduction controlled rolling process

“…Elongated structures were mostly observed and the fraction of elongated ferrites decreased with increasing heating temperature from 700 to 800 The initial coarse structure with an average ferrite grain size of about 41 µm consisting of coarse ferrite and pearlite structures was transformed into fine ferrite and finely dispersed pearlite or cementite structures, and the ferrite grains were considerably refined after the single-pass heavy-reduction controlled rolling process near or above the critical transformation temperature (A c3 ). This result shows good agreement with our previous studies using 0.01, 0.1, and 0.2% carbon steels as well as a niobium steel subjected to the PSC test near or above A c3 , simulating heavy-reduction single-pass controlled rolling [42,43]. Furthermore, it has been well known that grain size was significantly reduced by the strain-induced transformation (γ→α) in various steels during the thermomechanical treatment near A c3 or A r3 [46][47][48][49].…”

“…There were many dislocations, which were introduced inside the microstructure through the controlled rolling process. This process formed fine ferrites, and fine pearlite or cementite particles from the coarse pearlite lamellae as well as substructures such as subgrains in the ferrites instead of generating new grains with HAGBs because of the high stacking fault energy of ferrite [42,43,57]. This led to not only an increase in the fraction of Fig.…”

“…We have attempted to resolve these issues through a heavy-reduction single-pass controlled rolling process, which can be used to mass-produce steel strip sheets. As the first step, a heavy-reduction single-pass plane-strain compression (PSC) test, which simulates the hot-rolling process, was performed to investigate the formation mechanism of the bimodal structure and its mechanical properties using a variety of steels such as 0.01, 0.1, and 0.2% carbon steel as well as niobium steel (0.16%C-1.41%Mn-0.03%Nb) with different deformation temperatures [42,43]. It was found that high-performance steels composed of equiaxed fine or ultrafine grains with uniformly dispersed cementite or fine pearlite grains were produced in all the steels.…”

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

“…A ferrite having a high stacking fault energy (SFE) caused an increase in the fraction of LAGBs during warm forming around the transformation temperature ( A c1 ), leading to a decrease in ductility despite an increase in strength . Saray et al reported that the heavily elongated grain morphology in IF steel resulted in low formability owing to the increased tendency for cracking to occur through elongated grain boundaries.…”

“…We have proposed a new concept for manufacturing bimodal steel sheets having micron‐size grains (1–5 μm) distributed in a submicron‐size matrix (<1 μm) in a low‐carbon steel with an outstanding balance between strength and elongation by inducing a phase transformation (γ→α) after SPD slightly above the critical transformation temperature ( A c3 ) (Figure ). In our previous investigations, the formation of a bimodal structure with coexisting submicron‐size (<1 μm) and micron‐size grains (1–4 μm) in 0.2% carbon steel and its excellent mechanical properties were revealed by conducting plane‐strain compression (PSC) tests and single‐pass heavy‐reduction rolling. In our hot rolling test with one‐pass reduction of 75%, using 0.2% carbon steel with different heating temperatures, it was demonstrated that the proposed ‐rolling process is capable of producing a bimodal steel strip with an excellent balance between cold formability and strength.…”