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Control of Stringer Shaped Non‐ M etallic Inclusions of Ca O – A l 2 O 3 System in API X80 Linepipe Steel Plates
Author(s) -
Wang Xinhua,
Li Xiugang,
Li Qiang,
Huang Fuxiang,
Li Haibo,
Yang Jian
Publication year - 2014
Publication title -
steel research international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.603
H-Index - 49
eISSN - 1869-344X
pISSN - 1611-3683
DOI - 10.1002/srin.201300044
Subject(s) - stringer , non metallic inclusions , materials science , metallurgy , steelmaking , continuous casting , metal , liquid steel , composite material
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–Al 2 O 3 system with lower melting temperatures, which formed because the presence of: (i) small sized liquid inclusions of CaO–Al 2 O 3 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–Al 2 O 3 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–Al 2 O 3 system. During rolling, these inclusions could also be elongated to stringer typed ones because of their soft CaO–Al 2 O 3 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–Al 2 O 3 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).