Continuous Casting Steel Slabs Using Combined AC DC Magnetic Fields
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Solution Overview
Problem
Current continuous casting methods for steel fail to adequately address the quality requirements for skin plates in automobiles, particularly in hot-dip galvannealed steel sheets, due to defects caused by minute bubbles, non-metallic inclusions, and mold flux, which result in irregularities in the plated layer and stripe-like surface defects.
Innovation Solution
A continuous casting method using a DC magnetic field to control the flow of extremely low carbon steel, optimizing the chemical composition and magnetic field intensities to prevent the entrainment of non-metallic inclusions and bubbles, and adjusting nozzle parameters to minimize defects, followed by pickling and cold rolling to produce high-quality steel sheets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a DC magnetic field is applied to brake the molten steel flow in the mold, then the entrainment of mold flux and non-metallic inclusions is reduced, but minute bubbles and fine inclusions are still caught in the solidified shell
Solution Approach 1:
The patent applies an AC magnetic field in addition to the DC magnetic field, changing the magnetic field parameters from static to dynamic. The AC magnetic field generates Lorentz forces that stir the molten steel, creating upward and downward flows that prevent minute bubbles and fine inclusions from being caught in the solidified shell, while the DC component continues to brake the overall flow to reduce mold flux entrainment.
Solution Approach 2:
The combined AC and DC magnetic field system performs multiple functions simultaneously: the DC magnetic field brakes the molten steel flow to reduce mold flux entrainment, while the AC magnetic field stirs the molten steel to prevent minute bubble and fine inclusion defects. This multi-functional approach resolves the contradiction between reducing coarse defects and preventing fine defects.
2Object-affected harmful factors
If the molten steel flow velocity is increased to prevent bubble entrapment, then bubble-caused defects are reduced, but mold flux entrainment increases
Solution Approach 1:
The AC magnetic field applies periodic electromagnetic forces to the molten steel, creating oscillating flow patterns. This periodic action generates localized upward and downward currents that actively transport bubbles away from the solidified shell interface without requiring an overall increase in flow velocity, thereby preventing bubble defects while avoiding increased mold flux entrainment.
Solution Approach 2:
The AC magnetic field induces vibrational motion in the molten steel through time-varying Lorentz forces. This vibration creates turbulent mixing and enhances bubble removal through the solidified shell, achieving bubble defect reduction without increasing the mean flow velocity that would cause mold flux entrainment.
3Object-affected harmful factors
If the DC magnetic field intensity is increased to reduce mold flux entrainment, then mold-flux-caused defects are reduced, but the cleaning effect on fine inclusions is insufficient
Solution Approach 1:
The DC magnetic field continuously brakes the molten steel flow to prevent mold flux entrainment, while the AC magnetic field continuously stirs the molten steel to maintain cleaning action on fine inclusions. Both fields operate simultaneously and continuously throughout the casting process, ensuring both types of defects are addressed without interruption.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method effectively reduces the number of defects in the steel sheets, ensuring high-quality slabs with minimal surface defects and blisters, suitable for stringent automotive applications.
Implementation Method 1
a magnetic field is applied to a molten steel flow in the mold and the flow of molten steel is controlled by making use of an electromagnetic force generated by a magnetic field
Implementation Method 2
an AC magnetic field is applied to the upper magnetic poles or the lower magnetic poles in a superimposed manner. This method provides the braking of the molten steel flow by the DC magnetic field in the same manner as Patent Document 1, and also aims at the acquisition of a cleaning effect of non-metallic inclusions or the like on an interface of a solidified shell due to stirring of molten steel by an AC magnetic field
Data Source
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AI summary
In a method for continuously casting an extremely low carbon steel using a continuous casting machine which includes a pair of upper magnetic poles and a pair of lower magnetic poles and an immersion nozzle having a molten steel discharge angle of 10° or more and 30° or less while braking a molten steel flow by DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively, by adjusting the chemical components of extremely low carbon steel within a specified range by taking into account an interface tension gradient in a concentration boundary layer on a front surface of a solidified shell, and also by optimizing intensities of the DC magnetic fields applied to the upper magnetic poles and the lower magnetic poles respectively corresponding to a slab width of a slab to be casted and a casting speed, it is possible to acquire the slab having high quality not only with the small number of defects caused by the entrainment of bubbles, non-metallic inclusion and a mold flux into the molten steel.