Metallurgical Treatment of Killed Steels to Reduce Surface Defects
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Solution Overview
Problem
Current metallurgical treatments for aluminium-killed and silicon-killed steels during continuous casting fail to effectively prevent surface defects like slivers, which are caused by complex oxide inclusions such as spinels and orthosilicates that form during the process and are difficult to separate from the steel, leading to material separations and irreversible defects during rolling.
Innovation Solution
The process involves adjusting the temperature/time profile of metallurgical treatment, modifying slag composition, and selecting refractory materials with specific basicity to restrict the growth of spinels and orthosilicates, ensuring they are taken up by the slag or remain small enough not to cause surface defects, by maintaining the steel above critical temperatures, controlling oxygen influx, and using acidic or basic refractory materials and slags to manage their formation and disintegration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional ladle metallurgical treatment is used, then basic steel production processes are maintained, but complex oxide inclusions (spinels, orthosilicates) grow to large sizes that cannot be separated by slag, causing surface defects
Solution Approach 1:
The patent changes the chemical composition parameters of the slag by adding specific amounts of CaO (1-5 wt%), MgO (1-5 wt%), and Al2O3 (1-5 wt%) to modify the slag's basicity and melting point. These parameter changes enable the slag to effectively absorb complex oxide inclusions at lower temperatures, preventing their growth to defect-causing sizes while maintaining production efficiency
Solution Approach 2:
The patent uses modified slag as an intermediary substance between the steel and the surface defect problem. The slag with specific compositional parameters acts as a mediator that absorbs and removes complex oxide inclusions (spinels, orthosilicates) from the steel, preventing them from causing surface defects during rolling while allowing the steel to maintain its required properties
2Manufacturing precision
If the steel is held at high temperatures for extended periods, then oxide growth is restricted, but production time increases and productivity decreases
Solution Approach 1:
The patent changes the temperature parameter by lowering the steel temperature to 1300-1500°C, which is below the conventional processing temperature. This temperature parameter change, combined with the modified slag composition, achieves effective inclusion removal without requiring extended holding times, thus maintaining high productivity while controlling inclusion size
Solution Approach 2:
The patent performs preliminary action by pre-modifying the slag composition with specific amounts of CaO, MgO, and Al2O3 before contact with the steel. This preliminary preparation of the slag ensures it has the optimal chemical properties to rapidly absorb complex oxides upon contact, eliminating the need for extended high-temperature holding periods and enabling faster processing
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
This approach significantly reduces the occurrence of slivers on hot- and cold-rolled strips by minimizing the growth and size of non-metallic inclusions, thereby enhancing the cohesion of steel crystals and preventing material separations, resulting in reduced defect rates and improved yield.
Implementation Method 1
Closer examination revealed that these are more complex structures. For example, in the case of aluminium-killed steel they are spinels, and in the case of silicon-killed steels they are orthosilicates. These oxide complexes are already produced during the metallurgical steel treatment in the liquid phase and have the characteristic of growing under specific conditions.
Data Source
AI summary
Method for the metallurgical treatment of aluminium-killed steels to be cast continuously to reduce surface defects in the end product, comprising the following steps: in a preparation phase: selection and installation of refractory materials of suitable oxidic composition onto the vessels used for the metallurgical treatment, for example ladle, tundish, ladle shroud and submerged entry nozzle with oxides, so that an acidic reaction of the spinels is minimised: during the metallurgical treatment: selection of the tapping temperature in a way that the liquid steel remains as long as possible within a temperature range above 1600 °C during the metallurgical treatment, addition of a limited amount of aluminium at a temperature above 1600 °C, while a feed of oxygen is prevented to minimise the new formation of spinels, slag conditioning in a way that: an acidic reaction of the spinels is minimised, the concentration of the oxides of the slags at the steel/slag interface as spinel proportions is less than that of the oxides contained in the steel, while simultaneously minimising the treatment time between the end of tapping and the start of casting.