Effect of copper on microstructure, recrystallization and precipitation kinetics in strip cast low carbon steel

Abstract

Strip cast samples with different copper content have been developed by simulated direct strip casting. The as-cast microstructures were characterized by optical microscopy and electron microscopy. The microstructures mainly consisted of polygonal ferrite and Widmanstatten ferrite in as cast condition. X-ray diffraction showed that the copper was in solid solution in the as-cast condition. It was found that the equivalent tensile yield and maximum strengths, both increased with an increase in copper content. The strip cast samples were cold rolled to 70% reduction and then annealed isothermally at 600, 650 and 700 °C for various times to study the recrystallization and precipitation kinetics. After annealing, the hardness and strength were found to be decreased due to recovery and recrystallization. It was found that an increase in copper content retarded the recrystallization kinetics. During annealing, precipitation of copper was observed. The copper precipitates were bimodal with respect to particle size and distribution. The size and spacing of the particles were measured and the Zener pinning effect of the particles was calculated. High pinning forces of more than 2 MPa were predicted, and this correlated well with the observed decrease in recrystallization kinetics.

Effect of Copper on the Grain Size and Tensile Strength of Ultra-Low Carbon Steel

“…6, where the grain size has changed from 25 to 60 µm for the steel M-1 and 30 to 65 µm for the steel M-2 when the annealing temperature has increased from 700 to 900 °C, respectively. This finding is in agreement that recrystallization not only releases much larger amounts of stored energy but new, larger grains are formed by the combination of the nucleation of stressed grains and the joining of several grains to form larger ones [4,9]. Despite that, there are several factors that affect the rate of recrystallization for a given temperature, and the annealing steel must be decided and carefully controlled in practical production.…”

“…The study showed that by annealing the steel at 700 °C, a balance of strength and ductility in the Cu-containing steel can be attained. During the annealing process, two phenomena occurred: removal of residual stress caused by deformation and recrystallization [4,5]. Thus, both recrystallization and removal of the residual stress have been expected to occur when the steels were annealed in this study, resulting in the strength has been decreased for the steel annealed at a higher temperature.…”

“…Recrystallization may occur in a discontinuous way where distinct new grains form and grow, or in a continuous way where the microstructure gradually evolves into a recrystallized microstructure. During the annealing time, various thermally activated processes may occur, resulting in more and larger equiaxed grains forming but some small grains still stayed beside large ones when the steel was kept at a high temperature for a longer time [4]. Therefore, recrystallization is usually accompanied by a reduction in the strength and hardness of steel and a simultaneous increase in ductility.…”

“…By adding 0.20 to 0.35 wt.% Cu, corrosion resistant property of the structural steel has been improved [3]. The addition of Cu from 0.25 to 0.5 wt.% into carbon steel and low alloyed steel improves significantly corrosion resistance in the atmosphere; these steels are known as weather-resistant steel [4]. A lot of studies in Cu-bearing steel have been carried out to clarify the precipitation behavior and strengthening mechanism, and moreover, Cu-bearing steel strengthened by Cu-precipitation has been developed.…”

“…Although a small ratio of the scrap has been used to produce ULC steel in the BOF steelmaking route, the Cu content of the product may reach the level of 0.3 wt.% if the scrap has not been sorted carefully. It can be acknowledged that a few research works have been done to investigate the mechanical properties of Cu-containing ULC steel [4,10]. Therefore, this paper focuses on the effect of a small Cu content on the microstructure and tensile strength of the annealed ULC steel.…”