Transformation of Inclusions in Linepipe Steels During Heat Treatment
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INTRODUCTION
LINEPIPE steels are usually used for pipeline transportation such as oil and natural gas with poor working environments and requires not only high strength, but also good toughness, corrosion resistance, and welding performance.[1–6] Non-metallic inclusions are undesired second-phase particles in linepipe steels that have a strong influence on mechanical properties and the surface quality of the steel.[7–11] Inclusions may also cause processing difficulties such as nozzle clogging during continuous casting.[12–15] Therefore, the control of inclusions is of great importance to achieve good properties of the steel. Extensive studies have been performed on the formation, removal, and modification of inclusions in molten steel;[14,16–27] different inclusions such as liquid calcium aluminate, CaO-CaS type, and Al2O3-CaS type also have been controlled and the main measures to control inclusions in linepipe steels include calcium treatment,[7,27–31] slag refining,[32–37] and argon blowing[38] to reduce their detrimental effects. Little attention has been paid to study the variation of inclusions in solid steels since little variation would be expected.
YANPING CHU, WEIFU LI, YING REN, and LIFENG ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing, 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 25, 2018. Article published online April 30, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
The concept of inclusions in solid steels was initiated by Japanese scholars as early as 1960s; Takahashi et al. reported that MnO-SiO2 oxide inclusions varied to MnO-Cr2O3 in Type 304 stainless steels during heat treatment, which was influenced by the content of Mn, Si, and Cr in the steel,[39,40] and after that, rare attention was paid to this issue until the year about 2011. Shibata[41] provided a supplemental issue on this topic. The same finding in Type 304 stainless steels was also confirmed by the current authors.[42] Li et al.[43] reported that Al-Ti oxide inclusions were divided into Al-rich part and Ti-rich part during heat treatment in solid Al-Ti-deoxidized steels. Wang et al.[44] reported the transformation of inclusions in Al-Ca-killed SUS400B stainless steels during heat treatment, which was mainly caused by the crystallization of inclusions themselves. Studies were also performed on the reaction between solid alloys and oxide particles under varied temperature.[45–47] Moreover, it has been reported that the equilibrium between steel and inclusions moves when temperature decreases since the equilibrium constant is closely related to the temperature, resulting in a change in the inclusion compositions between the tundish and the slab.[10] Thus, it is necessary to understand the characteristics of transformation of inclusions in solid steels for better inclusion controlling during the actual rolling process. However, there are few reports on the transformation of non-metallic inclusions in solid linep
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