Control of Stringer Shaped Non‐Metallic Inclusions of CaO–Al2O3 System in API X80 Linepipe Steel Plates

Abstract

The stringer shaped B type non‐metallic inclusions in linepipe steel plates deteriorates the properties of low temperature toughness, hydrogen induced crack, etc. At present the formation mechanism and controlling method of B type inclusions in mass production are rarely reported. In this paper, the characteristics, origin, and controlling method of stringer shaped B type non‐metallic inclusions were studied for linepipe steel. The stringer shaped B type non‐metallic inclusions in API X80 linepipe steel plates, produced through “BOF‐LF‐RH‐Ca treatment” steelmaking route, were mainly of CaO–Al2O3 system with lower melting temperatures, which formed because the presence of: (i) small sized liquid inclusions of CaO–Al2O3 system in liquid steel after the secondary refining and Ca treatment. These small inclusions could aggregate to larger ones of 10–20 μm in continuous casting and be deformed into stringer shaped inclusions in steel plates during rolling. (ii) The large sized low melting temperature CaO–Al2O3 inclusions, in Ca treatment, their surface layers could be modified into high melting temperature CaO, CaS, or CaO–CaS system, but the centers were still of CaO–Al2O3 system. During rolling, these inclusions could also be elongated to stringer typed ones because of their soft CaO–Al2O3 centers. A new strategy for control of B type inclusions in linepipe steel plates was adopted. The key of the control was shifted from removing low melting temperature inclusions of CaO–Al2O3 system after Ca treatment to removing as much as possible inclusions especially large sized inclusions before the Ca treatment. With the new strategy, amount of inclusions after RH refining was remarkably decreased and the efficiency of Ca treatment significantly improved. The inclusions found in steel plates were all of high melting temperature CaO–CaS system and the severities of B‐type inclusions have been lowered from “≤2.0” to “0”(ASTM E45 standard).