In-situ observation of nonmetallic inclusions in steel using confocal scanning laser microscopy: A review

Abstract

The characteristics of nonmetallic inclusions formed during steel production have a significant influence on steel performance. In this paper, studies on inclusions using confocal scanning laser microscopy (CSLM) are reviewed and summarized, particularly the collision of various inclusions, dissolution of inclusions in liquid slag, and reactions between inclusions and steel. Solid inclusions exhibited a high collision tendency, whereas pure liquid inclusions exhibited minimal collisions because of the small attraction force induced by their <90° contact angle with molten steel. The collision of complex inclusions in molten steel was not included in the scope of this study and should be evaluated in future studies. Higher CaO/Al2O3 and CaO/SiO2 ratios in liquid slag promoted the dissolution of Al2O3-based inclusions. The formation of solid phases in the slag should be prevented to improve dissolution of inclusions. To accurately simulate the dissolution of inclusions in liquid slag, in-situ observation of the dissolution of inclusions at the steel–slag interface is necessary. Using a combination of CSLM and scanning electron microscopy–energy dispersive spectroscopy, the composition and morphological evolution of the inclusions during their modification by the dissolved elements in steel were observed and analyzed. Although the in-situ observation of MnS and TiN precipitations has been widely studied, the in-situ observation of the evolution of oxide inclusions in steel during solidification and heating processes has rarely been reported. The effects of temperature, heating and cooling rates, and inclusion characteristics on the formation of acicular ferrites (AFs) have been widely studied. At a cooling rate of 3–5 K/s, the order of AF growth rate induced by different inclusions, as reported in literature, is Ti–O < Ti–Ca–Zr–Al–O < Mg–O < Ti–Zr–Al–O < Mn–Ti–Al–O < Ti–Al–O < Zr–Ti–Al–O. Further comprehensive experiments are required to investigate the quantitative relationship between the formation of AFs and inclusions.

1. Introduction

The formation of nonmetallic inclusions during production of high-quality steels is a significant issue. Steel performance is related to quantity, size, composition, and distribution of such inclusions. The characteristics of these inclusions evolve during steel processing; for example, nucleation and collision occur after deoxidation during converter tapping [1–3]. During the refining process, inclusions float to the surface of the molten steel [4–6] and are dissolved in the refining slag [7–9]. Additionally, alloy treatment [10–13] and slag modification [14–16] can modify inclusion composition. Moreover, reoxidation and slag entrainment can be prevented to improve steel cleanliness [17–19]. The inclusion behavior during steelmaking, refining, and continuous casting processes, which cannot be directly observed because of the high temperature of molten steel, must be investigated.

Confocal scanning laser microscopy (CSLM) combines advanced technologies such as infrared heating, high-temperature stretching and compression, and confocal laser scanning. A schematic of CSLM equipment is shown in Fig. 1 [20]. The surface of samples can be observed in-situ under different conditions of temperature and pressure. CSLM is widely used in industries such as welding, materials, and metallurgy for purposes such as in-situ observation of the formation of acicular ferrites (AFs) induced by oxide metallurgy [21], phase transformation during cooling and heating of materials [22–23], and metal solidification and inclusion evolution on the surface of liquid metals [24–25].

In the 1950s, Minsky proposed the basic concept of confocal microscopy and applied it to technical patents. In the 1990s, the rapid development of optical, electronic, and computer technologies promoted the development and application of CSLM, which is commonly used to observe micro-morphology [26]. Chikama et al. [27] used a laser beam and infrared furnace to observe the dynamics of crystal growth in iron–carbon melts; CSLM has since been widely used in the study of steel. Particularly, CSLM has been widely applied for the in-situ observation of inclusion behaviors on molten steel surfaces. In 1997, Yin’s group [28–29] used CSLM to observe the collision of different inclusions on the surface of molten steel. In 1998, Shibata et al. [30] observed the engulfment and pushing of inclusions in molten steel at the melt–solid interface using CSLM. In 1999, Hanamura et al. [31] used CSLM