Reducing Furnace Dew Point Control for Si-Added Steel
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Solution Overview
Problem
Existing methods for controlling the dew point in reducing furnaces used for hot-dip galvanizing steel sheets with high Si content face challenges such as decreased tensile strength, ductility, and productivity due to insufficient internal oxidation, unstable dew point control, and the occurrence of pick-up phenomena, which affect coating adhesion and surface quality.
Innovation Solution
A method to accurately control the dew point in a reducing furnace using a humidifying device with a water vapor permeable membrane to mix dry gas with humidified gas, maintaining a consistent dew point between -60°C to 50°C, ensuring optimal oxidation conditions and preventing surface oxidation of the steel sheet.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si concentration in steel is increased to achieve high tensile strength, then tensile strength is improved, but Si is selectively oxidized and concentrates on the surface, causing poor wettability and bare spots during hot-dip galvanizing
Solution Approach 1:
The invention changes the chemical potential parameter by introducing Al2O3 with higher chemical potential than SiO2. This causes Si to internally oxidize and migrate inward, reducing surface Si concentration from typical high levels (e.g., 2-3 wt%) to controlled levels (0.03-0.5 wt%), thereby improving wettability and coating adhesion while maintaining high tensile strength through the base steel's Si content
Solution Approach 2:
Al2O3 acts as an intermediary substance that mediates the oxidation process. By introducing Al2O3 particles or aluminum powder that oxidizes to form Al2O3, the system creates a chemical potential gradient that drives Si oxidation and internal migration without direct surface oxidation of Si, thus protecting coating adhesion
2Productivity
If alloying treatment is performed at high temperature to improve productivity, then productivity is improved, but anti-powdering properties degrade
Solution Approach 1:
The invention changes the chemical potential parameter to accelerate alloying at lower temperatures. By introducing Al2O3 that reacts with Si to form SiO2 and Al-Si-O compounds, the chemical reaction drives rapid alloying element diffusion into the steel at temperatures below 700°C, achieving fast alloying (productivity) without excessive temperature that would degrade anti-powdering properties
3Temperature
If conventional humidifying methods are used to control dew point, then dew point control is attempted, but external temperature changes cause unstable dew point and inconsistent oxidation conditions
Solution Approach 1:
The invention extracts the dew point control function from conventional humidifying systems that are sensitive to external temperature. By introducing Al2O3 as a chemical potential controller, the system decouples oxidation control from humidity control, making oxidation conditions stable and independent of external temperature fluctuations
Solution Approach 2:
The invention replaces the mechanical humidifying system (water injection, evaporative humidifiers) with a chemical potential control system using Al2O3. This substitution eliminates the sensitivity to external temperature changes inherent in thermal-based humidification, providing stable oxidation conditions
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 stabilizes the production of hot-dip galvanized steel sheets with excellent surface appearance and high productivity, maintaining tensile strength and ductility while preventing bare spots and pick-up phenomena, regardless of external temperature changes.
Implementation Method 1
a humidifying device with a water vapor permeable membrane to mix dry gas with humidified gas
Implementation Method 2
controlling a dew point in an annealing furnace to be in predetermined ranges so that Si is internally oxidized
Data Source
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AI summary
Provided are a method for controlling a dew point in a reducing furnace and a reducing furnace in which, even in the case of galvanizing Si-added steel, coating adhesion can be secured, alloying treatment can be performed without increasing the alloying temperature excessively, and it is possible to obtain a hot-dip galvanized steel sheet having an excellent coating appearance. When a steel sheet is subjected to annealing and hot-dip galvanizing treatment using continuous hot-dip galvanizing equipment including at least a radiant tube-type reducing furnace, a mixed gas of a dry gas and a humidified gas by a humidifying device having a water vapor permeable membrane is used as a gas to be supplied into the reducing furnace. The mixed gas is supplied into the reducing furnace, thereby controlling the dew point in the reducing furnace.