State of the Art in Evaluation and Control of Steel Cleanliness

The demand for cleaner steels increases every year. In addition to lowering non-metallic oxide inclusions and controlling their size distribution, morphology, and composition, clean steel requires control of sulfur, phosphorus, hydrogen, nitrogen and even carbon, and minimizing metallic impurity elements such as As, Sn, Sb, Se, Cu, Zn, Pb, Cd, Te and Bi. These requirements vary with steel grade and its end use, as shown in Table 1. Thus, clean steel for one application is often unacceptable for a different application. Metallic impurity elements, which are traditionally found only in trace amounts, are becoming an increasing problem due to their accumulation in the scrap supply. These elements cause intergranular segregation leading to cracks, detrimental precipitates and other problems, which are often manifested as slivers in the final product. These elements can be difficult to remove in steelmaking and refining, but can be lowered by carefully controlling the scrap charge, or by charging blast furnace iron, direct-reduced iron, or other relatively pure iron source. These trace element aspects of cleanliness are reviewed elsewhere, so the remainder of this paper reviews oxide inclusion cleanliness, focusing on Low Carbon Al-Killed steel (LCAK steel). Inclusions can generate many defects in the steel product. For example, LCAK steel cans suffer from cracked flanges due to lack of formability, while axels and bearings suffer fatigue life problems. Both formability and fatigue life are highly affected by sulfide and oxide inclusions in the steel. Sliver defects occur as lines along the steel strip surface parallel to the rolling direction. Slivers plague LCAK steel sheet for automotive applications, causing both cosmetic surface imperfections and formability problems. They usually consist of aluminates originating from deoxidation or from complex non-metallic inclusions from entrained mold slag, as documented in many studies, such as at Inland Steel, National Steel, and Kawasaki Steel. In addition to the amount of inclusions, steel cleanliness depends greatly on the size distribution, morphology and composition of non-metallic inclusions in the steel. The inclusion size distribution is particularly important, because large macroinclusions are the most harmful to mechanical properties. One kg of typical LCAK steel contains 10–10 inclusions, including only 400 80–130 mm inclusions, ten 130–200 mm inclusions and less than one 200–270 mm sized inclusion. Obviously, detecting the rare large inclusions is very difficult. Though the large inclusions are far outnumbered by the small ones, their total volume fraction ISIJ International, Vol. 43 (2003), No. 3, pp. 271–291