A new mechanism of hook formation during continuous casting of ultra-low-carbon steel slabs

Abstract

The initial stages of solidification near the meniscus during continuous casting of steel slabs involve many complex inter-related transient phenomena, which cause periodic oscillation marks (OMs), subsurface hooks, and related surface defects. This article presents a detailed mechanism for the formation of curved hooks and their associated OMs, based on a careful analysis of numerous specially etched samples from ultra-low-carbon steel slabs combined with previous measurements, observations, and theoretical modeling results. It is demonstrated that hooks form by solidification and dendritic growth at the liquid meniscus during the negative strip time. Oscillation marks form when molten steel overflows over the curved hook and solidifies by nucleation of undercooled liquid. The mechanism has been justified by its explanation of several plant observations, including the variability of hook and OM characteristics under different casting conditions, and the relationships with mold powder consumption and negative/positive strip times.

Transient Fluid Flow during Steady Continuous Casting of Steel Slabs: Part I. Measurements and Modeling of Two-phase Flow

“…2,3) Abnormal high surface velocity and velocity variations, leading to asymmetric surface flow, vortex formation, 4,5) and instability at the interface between the molten steel and slag, [6][7][8] could entrain slag into the molten steel, causing both surface and internal defects in the steel product. On the other hand, abnormal slow surface flow could result in low and non-uniform surface temperature, inducing insufficient slag melting and infiltration, meniscus freezing, hook formation, 9,10) and surface defects related to initial solidification problems. Argon gas is injected to prevent nozzle clogging in continuous steel casting, but may cause complexity and instability of transient flow pattern.…”

Optimization of Chemical Composition of the Mould Powder for Casting Ø 170 mm Billets from C45 Steel

“…The following equations were used to calculate the physical parameters of the powder 1) The amount of crystalline phase of the slag (NBO/T) [7]: (3) where: X -the mole fraction of slag components…”

“…3) The equivalent thermal conductivity λsys at temperature 1200 °C for mould powder, [W/mK] [9]: (5) where: %"CaO" = %CaO+%MgO+%MnO%+%K 2 O+%Na 2 O+%liO 2 %"SiO 2 " = %SiO 2 +%B 2 O 3 In order to determine the current lubrication conditions of Ø 170 mm billets cast under mould powder Scorialit SPH-C 189/e1, the following measurements were conducted:…”

“…1. General figure of the mould flux in continuous casting [2,3] Careful selection and monitoring of these processes determine the possibility of obtaining defects-free billets.…”

Simulation of fluid flow and solididification in the funnel type crystalizer of thin slab continuous cast

“…On the other hand, increasing the submergence depth and contact of the fluid from lower nozzle port to the lower part of the solidified shell can cause the breakout. In contrary, at the top surface the flow field is weaker and meniscus is more likely to solidify, giving a chance for the formation of defects like blisters and oscillation mark on the surface of the slabs and rolling products [21][22][23][24]. The steel jets traverse the liquid pool to impinge against the narrow faces, splitting into two flows along the narrow face.…”