Numerical simulation of the deformation risk in thin slab continuous casting process with liquid core reduction

Abstract

The application of liquid core reduction (LCR) technology in thin slab continuous casting can refine the internal microstructures of slabs and improve their production efficiency. To avoid crack risks caused by large deformation during the LCR process and to minimize the thickness of the slab in bending segments, the maximum theoretical reduction amount and the corresponding reduction scheme for the LCR process must be determined. With SPA-H weathering steel as a specific research steel grade, the distributions of temperature and deformation fields of a slab with the LCR process were analyzed using a three-dimensional thermal–mechanical finite element model. High-temperature tensile tests were designed to determine the critical strain of corner crack propagation and intermediate crack initiation with various strain rates and temperatures, and a prediction model of the critical strain for two typical cracks, combining the effects of strain rate and temperature, was proposed by incorporating the Zener–Hollomon parameter. The crack risks with different LCR schemes were calculated using the crack risk prediction model, and the maximum theoretical reduction amount for the SPA-H slab with a transverse section of 145 mm × 1600 mm was 41.8 mm, with corresponding reduction amounts for Segment 0 to Segment 4 of 15.8, 7.3, 6.5, 6.4, and 5.8 mm, respectively.

1. Introduction

The liquid core reduction (LCR) technology of thin slab casting is typically applied to solving problems on thickness mismatching between thin slab continuous casting and rolling processes [1]. LCR technology applies continuous reduction in segments where the center of the slab contains a significant liquid core [2]. The advantages of LCR include enlarging mold cavities to improve inclusion removal, improving central segregation, refining slab microstructures, and enhancing productivity [3–4]. Therefore, in actual production processes, for minimizing to a desirable thickness of the slab in reduction segments, it is of great guiding significance to determine the maximum theoretical reduction amount. However, during the LCR process, corner crack propagation and intermediate crack initiation caused by large deformation significantly limit the effectiveness of this technology [5] and present a great challenge to the rationality of the total reduction amount and the corresponding reduction scheme.

The cracks of the continuous casting slab are generally believed to be caused mainly by two factors, as shown in Fig. 1(a). One is the poor hot ductility of the steel, which is caused by high-crack-sensitivity microstructures, such as coarse austenite [6], ferrite film on the grain boundary [7], and second-phase TiN precipitated [8] between dendrites. The other one is the local strain concentration at grain boundaries and dendrites during deformation, which is caused by mechanical stresses [9–10], such as reduction, bending, and thermal stress. In this respect, cracks inevitably form when the local strain exceeds the critical strain of the steel.

The formation mechanisms of corner crack propagation and intermediate crack initiation during the LCR process are shown in Fig. 1(b). Corner microcracks are mainly formed in the mold because of the hydrostatic pressure of molten steel and improper oscillation marks [11–13]. During the LCR process, tensile strain along the casting direction can lead to the propagation of microcracks along the grain boundary, resulting in transverse corner crack defects. Meanwhile, intermediate cracks are mostly formed at the solidification front. Large deformation during the LCR process can cause tensile strain (εzz) parallel to the width direction (i.e., z direction), resulting in the “tearing” of columnar dendrites at the solid–liquid interface, and intermediate cracks form without liquid filling or incomplete penetration by molten steel [14–16].