Effect of Zr-deoxidation on microstructure and mechanical behavior of microalloyed heavy plates with low impurity content

Abstract

The significance of different deoxidation practises on the ductility and impact toughness of next generation of microalloyed heavy plates was elucidated to explore the best deoxidation practice in obtaining required mechanical properties, which was judged by the combined effects of composition, size and number density of inclusions on the ductility of the experimental high-strength low-alloy steel. The impurity contents, i.e., total O + N + S contents, of 82 × 10⁻⁶ (Al-killed) and 118 × 10⁻⁶ (Zr-killed) have been induced to characterize both the steels. Ductility was characterized using tensile and Charpy V-notch testing. The number, size and composition of the inclusions were characterized using a field emission scanning electron microscope with an energy dispersive spectrometer. In the Al-killed steel, the inclusion structure consisted of titanium nitrides, stringer calcium aluminates and elongated manganese sulfides, whereas in the Zr-killed steel, the inclusion structure consisted of mainly fine spherical oxide inclusions with sulphide shells. The impurity content did not have a significant effect on the number density of inclusions, as with higher and lower impurity content, the number of inclusions was 83.7 and 78.8 mm⁻², respectively. However, the size distribution of the inclusions, especially the coarse inclusions with their longest length greater than 8 µm, differs much from each other. The number density of coarse inclusions differs from 0.8 to 1.1 mm⁻² with processing, and in Al-killed steel, 55.5% of the coarse inclusions were titanium nitrides or manganese sulfides, whereas in Zr-killed steel, only 22.5% of the coarse inclusions were titanium nitrides and manganese sulfides. Coarse titanium nitrides were especially detrimental to the impact toughness. The number density of them should be below 0.33 mm⁻² in order to guarantee the best possible toughness in the steel in question. The average crystallographic grain size detected by electron backscattered diffraction of Zr-killed steel (4.28 ± 2.70 μm) was smaller than that of Al-killed steel (6.00 ± 4.80 μm). As a result from the grain refinement and sulphide shape control, Zr-killed steel exhibited superior impact toughness (223 ± 70 J) at − 80 °C as compared with Al-killed steel (153 ± 68 J). Thus, Zr-killed steel was observed to provide good performance in terms of mechanical properties as compared with Al-killed steel.