Hot-dip Coating Cooling Rate Control for Steel Sheet Discoloration
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Solution Overview
Problem
The challenge in producing hot-dip aluminized steel sheets is the difficulty in achieving stable formation of fine spangles due to substances with higher specific gravity precipitating at the bottom of the Al-based hot-dip coating bath, leading to uneven distribution and potential discoloration of the surface, which affects the product's value.
Innovation Solution
A cooling rate determining device is employed to calculate and control the average cooling rate of the molten coating material-coated steel sheet using coefficients derived from the concentration of specific elements in the coating bath, ensuring the yellowness index remains below a predetermined value to prevent discoloration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If substances with higher specific gravity (Ti, Nb, TiC, TiB2, TiAl3) are added to the Al-based hot-dip coating bath to increase spangle nuclei and reduce spangle size, then the spangle density increases and fine spangles are formed, but these substances precipitate at the bottom of the coating bath due to specific gravity difference, causing uneven distribution and making stable formation of fine spangles difficult
Solution Approach 1:
The patent introduces boron (B) as an intermediary substance that acts as a carrier for the heavy spangle-nucleating substances. Boron forms compounds (such as TiB2) that have intermediate density between aluminum and the heavy substances, enabling them to remain suspended in the coating bath rather than precipitating to the bottom. This intermediary approach allows the heavy substances to be uniformly distributed while still providing their spangle-nucleating function.
Solution Approach 2:
The patent changes the density parameter of the spangle-nucleating substance by combining heavy substances (Ti, Nb) with lighter elements (B, Al) to form compounds with intermediate density. This parameter change transforms the physical state from precipitated particles to suspended compounds, enabling uniform distribution in the coating bath while maintaining the spangle-nucleating capability.
2Productivity
If the cooling rate is increased to solidify the coating material quickly, then productivity improves and production time is reduced, but discoloration occurs on the surface of the hot-dip coated steel sheet
Solution Approach 1:
The patent establishes a feedback relationship between cooling rate and yellowness index (a measure of discoloration). By determining the appropriate cooling rate based on the yellowness index criteria, the system can adjust cooling parameters to prevent discoloration while maintaining efficient production. The cooling rate is determined as a function of bath temperature and composition to achieve the desired surface appearance.
Solution Approach 2:
The patent optimizes the cooling rate parameter within a specific range (4°C/s to 20°C/s) that balances productivity and surface quality. By precisely controlling this parameter and relating it to bath temperature and composition, the system achieves both high production efficiency and acceptable surface appearance without excessive discoloration.
3Quantity of substance
If substances with higher specific gravity are added to the coating bath, then more spangle nuclei are obtained and fine spangles are formed, but the substances easily precipitate at the bottom of the Al-based hot-dip coating bath due to specific gravity difference, causing difficulty in stable formation of fine spangles
Solution Approach 1:
The patent creates composite spangle-nucleating substances by combining heavy elements (Ti, Nb) with lighter elements (B, Al) to form compounds such as TiB2, NbB2, and AlB2. These composite materials have intermediate density that prevents precipitation while maintaining the spangle-nucleating function. The composite structure allows the substance to remain suspended in the coating bath stably.
Solution Approach 2:
Boron acts as an intermediary that bridges the density gap between aluminum coating material and heavy spangle-nucleating substances. By forming borides and other compounds with intermediate density, boron enables the heavy substances to remain uniformly distributed in the coating bath, ensuring stable and reliable formation of fine spangles throughout production.
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 device effectively prevents surface discoloration of hot-dip coated steel sheets by optimizing the cooling rate, maintaining the desired appearance and quality of the hot-dip aluminized steel sheets.
Implementation Method 1
a cooling rate determining device for determining a cooling rate of a molten coating material-coated steel sheet
Implementation Method 2
an average cooling rate in a time period from immediately after taking the molten coating material-coated steel sheet out of the hot-dip coating bath until completion of solidification of the molten coating material
Data Source
AI summary
A hot-dip coating equipment includes a cooling rate determining device that includes: a coefficient obtaining section configured to obtain coefficients α, β, γ, and δ associated with the hot-dip coating bath, in the following equation that expresses a degree of discoloration YI of the surface of a hot-dip coated steel sheet; and a rate determining section configured to determine an average cooling rate A so as to allow the YI of the following Expression (1) to be not more than a predetermined reference value, YI =(α×[SE]+β)×{(T−ST)/A}1/2+γ×[SE]+δ . . . (1), where: [SE] is a concentration (mass %) of a specific element contained in the hot-dip coating bath; T is a temperature (° C.) of the hot-dip coating bath; and ST is a solidification temperature (° C.) at which the hot-dip coating bath is solidified.