SUPPRESSION OF AXIAL NONUNIFORMITY IN CONTINUOUS-CAST STEEL BY INOCULATING IT WITH A CLAD POWDER INOCULANT

Abstract

Improving the quality of rolled plates is one of the main goals of the metallurgical industry. This goal can be achieved through treatment of the steel outside the furnace ‐ especially in the pouring ladle or tundish ‐ with the use of chemically active elements that will refine, inoculate, and micro-alloy the melt. Such a practice is most efficiently carried out by introducing master alloys in the form of clad powder inoculants (cored wires). These materials consist of an infinite thin-walled metal tube packed with a pressed powder of suitable chemical and granulometric composition. The use of cored wires with chemically active elements allows measured, controlled introduction of refining and alloying components with a high degree of assimilation and without adverse environmental consequences [1]. Plates of low-alloy stainless steels are characterized by anomalously low ductility in the Z-direction (through their thickness). The nonuniform cooling of the semifinished product and an increase in the concentration of certain elements (carbon, the alloying elements manganese, silicon, etc.) and impurities (sulfur, phosphorus) in the liquid phase ahead of the crystallization front lead to the formation of an axial segregation zone. The formation of this zone makes the structure of the st eel more nonuniform through the thickness of the plate and affects the morphology of the nonmetallic inclusions in the axial zone. The structural features of the resulting slab are reflected in the structure of the finished rolled product, which is cha racterized by structural and chemical axial nonuniformity. As a result, the properties of the axial region differ significantly from the properties of the matrix of the metal. Despite the thinness of the segregation zone (10‐250 μm), it has a decisive effect on the mechanical characteristics of the product in the Z-direction in connection with the formation of microcracks in that zone. The presence of the microcracks can lead to low-energy fracture. One method of alleviating axial nonuniformity is microalloying the melt with master alloys containing rare-earth metals (REMs) and calcium [2]. This article examines the effect of calcium and cerium introduced into steel through cored wire on the structure of the steel in the axial segregation zone and the mechanical characteristics of the rolled product in the Z-direction. We studied series of trial heats of two steels: steel 09G2S (0.09% C; 1.48% Mn; 0.56% Si; 0.008% S; 0.017% P; 0.03% Ti; 0.043% Al; 0.01% Nb; 0.009% N; 0.02% Cu); steel St 52.3 (0.20% C; 1.38% Mn; 0.36% Si; 0.006% S; 0.013% P; 0.04% Ti; 0.035% Al; 0.03% Nb; 0.011% N; 0.03% Cu). Each steel was treated with 10-mm-diam. cored wire during casting on a continuous caster. The wire contained REMs and calcium-silicon in a 1:1 ratio. The coefficient characterizing the fullness of the tube had a value of 42‐46%. Pinch rollers were used to introduce the wire into a tundish near the stopper at a rate of 0.6‐0.7 m/sec, which was sufficient to produce finished steel with 0.003‐0.004% Ca and 0.008‐0.010% Ce. To evaluate the effect of the treatment of steel with cerium and calcium, we determined the rates of crystallization of experimental and comparison (non-inoculated) continuous-cast metal from the change in the density of the dendritic structure. The results show that microalloying had almost no effect on crystallization rate in the axial zone of the