A New Cooling Strategy in Curved Continuous Casting Process of Vanadium Micro-alloyed YQ450NQR1 Steel Bloom Combining Experimental and Modeling Approach

Kun Dou,Qing Liu

doi:10.1007/s11661-020-05819-9

Abstract

In the continuous casting process of steel, the bloom surfaces would experience intensive cooling from the water-cooled copper mold to secondary cooling water spray. If the cooling process is not controlled properly, hot ductility of the bloom surface microstructures would deteriorate, and bloom surface cracks would form easily under straightening deformation in a curved caster. Considering the above facts, the cooling scheme for the continuous casting of YQ450NQR1 steel bloom, a kind of vanadium-containing micro-alloyed steel, is studied with both experimental investigation and mathematical modeling in this work. The authors first investigate the hot ductility of bloom surface microstructures at various cooling rates using a Gleeble thermal simulator. Then, the precipitation of V(C, N) particles and its influence on ferrite formation during continuous cooling are studied and characterized using High-Temperature Laser Scanning Confocal Microscopy. Based on these, the preferred cooling rate for surface microstructures at the straightening position in the caster is obtained. To further reduce the solute macro-segregation through enlargement of the equiaxed crystal zone, a cellular automaton-finite element model is used to calculate heat transfer and solidification structure evolution during the continuous casting process. After calibration with industrial trials, the model is utilized to determine the critical position for columnar to equiaxed transition and to adjust the pouring temperature for the melt. Combining the above research, a new cooling strategy for YQ450NQR1 steel bloom is obtained, which can improve crack resistance of bloom surface microstructures and reduce solute macro-segregation at the same time.

Highlights

IN the steel continuous casting process, the cooling condition in the mold region and the secondary cooling zone have predominant influence on the solidification structure and surface quality in the casting bloom.[1,2] This phenomenon has been extensively researched in such works by Liu[3] and Thomas.[4]
In a curved continuous caster, due to large cross section dimension and high thermal gradient from the bloom surface to center, the solidification structure tends to vary from the surface to center,[5,6] with a thin layer of fine equiaxed grains initiated in the outer region[7] and a columnar grain region stretching towards the bloom center.[8,9]
With the cooling rates of 0.5, 1, and 3 °C/s, respectively. It can be seen from the fracture morphologies at 1050 °C, the sample fracture is trans-granular type, while at 800 °C where the a-ferrite has formed, the sample fracture is intergranular type

Summary

Introduction

IN the steel continuous casting process, the cooling condition in the mold region and the secondary cooling zone have predominant influence on the solidification structure and surface quality in the casting bloom.[1,2] This phenomenon has been extensively researched in such works by Liu[3] and Thomas.[4]. With the decrease of the superheat and the forced convection in the molten steel, coarse equiaxed grains form eventually in the central area.[10,11,12] According to the research from Dong,[13,14] the columnar to equiaxed grain transition (CET) usually occurs in the secondary cooling zone during steel continuous casting, and it affects the solute macro-segregation behavior. Proper enlargement and refinement of the central equiaxed grain zone would reduce macro-segregation of the solute

Methods

Results

Conclusion