Hot-dip Galvanizing Apparatus Soaking Zone Humidified Gas Control
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Solution Overview
Problem
Existing methods for producing hot-dip galvanized steel sheets with high Si content face challenges in achieving good coating adhesion properties and preventing pick-up defects, as Si oxidation leads to insufficient internal oxidation and increased alloying temperatures, causing decreased tensile strength and productivity.
Innovation Solution
A continuous hot-dip galvanizing apparatus with strategically positioned humidified gas supply ports in the soaking zone ensures uniform dew point distribution, promoting Si internal oxidation and preventing pick-up defects by dispersing humidified gas along the steel sheet's accompanying gas flow, thereby improving coating adhesion properties.
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 coating adhesion deteriorates due to Si oxidation and concentration at the surface
Solution Approach 1:
The patent applies preliminary oxidation action by introducing humidified gas into the soaking zone before hot-dip galvanizing. This pre-oxidizes the Si in the steel substrate, converting it to SiO2 that remains distributed within the steel rather than concentrating at the surface. By performing this oxidation treatment in advance during the soaking zone processing, the steel surface maintains good wettability with molten zinc while the Si is already transformed, thus preventing adhesion deterioration.
Solution Approach 2:
The patent changes the chemical state of Si by controlling the oxidation environment in the soaking zone. By adjusting the dew point of the atmosphere through humidified gas introduction, Si is oxidized to SiO2 which has different properties than metallic Si. This parameter change in Si's chemical state prevents its harmful concentration at the surface while maintaining the strength benefits of high Si content steel.
2Reliability
If Si is selectively oxidized to improve coating wettability, then coating adhesion is improved, but alloying process is delayed leading to lower productivity
Solution Approach 1:
The patent performs preliminary oxidation of Si in the soaking zone before the hot-dip galvanizing and alloying processes. By pre-converting Si to SiO2 and distributing it uniformly within the steel matrix, the subsequent alloying process does not need to wait for slow oxidation reactions. This preliminary action eliminates the alloying delay that would otherwise occur, thereby maintaining high productivity while ensuring good coating adhesion.
Solution Approach 2:
The patent skips the slow oxidation step during alloying by performing it in advance in the soaking zone. The oxidation process is rushed through during the soaking zone residence time with controlled humidified gas atmosphere, rather than waiting for it to occur slowly during the alloying process. This skipping of the oxidation step from the alloying sequence maintains production speed.
3Reliability
If reducing atmosphere annealing is used to prevent Si surface concentration, then coating adhesion is improved, but Si internal oxidation is insufficient requiring higher alloying temperatures
Solution Approach 1:
The patent changes the atmosphere parameter in the soaking zone from a simple reducing atmosphere to a controlled humidified atmosphere with specific dew point. This parameter change enables Si internal oxidation to occur effectively during soaking, transforming Si to SiO2 that distributes uniformly in the steel. As a result, subsequent alloying can proceed at lower temperatures because the Si is already in its oxidized state and properly distributed, eliminating the need for high alloying temperatures.
4Reliability
If humidified gas is supplied to increase Si internal oxidation, then coating adhesion is improved, but pick-up defects occur on the steel sheet surface
Solution Approach 1:
The patent applies local quality control by strategically positioning the humidified gas supply ports and dry gas supply ports at different locations. Humidified gas is supplied in the soaking zone where oxidation is desired, while dry gas is supplied near the steel sheet surface in other zones to prevent excessive surface oxidation. This spatial differentiation of gas properties ensures Si internal oxidation occurs without creating harmful surface oxides that cause pick-up defects.
Solution Approach 2:
The patent uses dry gas as an intermediary substance to protect the steel sheet surface from excessive oxidation. By introducing dry gas in strategic locations, it acts as a protective medium that prevents humidified gas from causing harmful surface oxidation. This intermediary dry gas layer allows the beneficial Si internal oxidation to occur while blocking the harmful effect of surface oxide formation that leads to pick-up defects.
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 achieves good coating appearance and high coating adhesion properties for hot-dip galvanized steel sheets with Si content above 0.2 mass %, while preventing pick-up defects and maintaining tensile strength.
Implementation Method 1
dispersing humidified gas along the steel sheet's accompanying gas flow
Implementation Method 2
the steel sheet is subjected to annealing by conveying the steel sheet through a heating zone, a soaking zone, and a cooling zone in this order
Implementation Method 3
heating zone
Implementation Method 4
soaking the steel sheet at a steel sheet temperature in the range of at least 300° C.
Data Source
AI summary
Provided is a continuous hot-dip galvanizing apparatus comprising: a vertical annealing furnace having heating, soaking zone, and cooling zones therein; and a hot-dip galvanizing line downstream of the cooling zone. The heating, soaking, and cooling zones each have, in its upper portion, at least one upper hearth roll and, in its lower portion, at least one lower hearth roll. The soaking zone has a first and second humidified gas supply ports to supply a humidified gas having a dew point of 10° C. to 30° C. to the soaking zone. The first and second humidified gas supply ports are 1.0 m to 5.0 m lower than the center of the lower and upper hearth rolls, respectively, and overlap the steel sheet. The first humidified gas supply port is provided only for an ascending pass and the second humidified gas supply port is provided only for a descending pass.