Hot-dip Galvanized Steel Sheets with Internal Oxide Layer
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Solution Overview
Problem
High-Si-content steel sheets face challenges in producing hot-dip galvanized steel sheets and galvannealed steel sheets with good mechanical characteristics and coating quality due to surface defects like uncoated areas and indentation flaws, which are difficult to consistently produce using existing oxidation-reduction techniques.
Innovation Solution
A method involving heating steel sheets in a direct heating furnace with specific gas ratios and temperatures to form Si oxides containing Fe and/or Mn, which improve wettability and suppress uncoated areas by reducing oxygen potential and promoting internal oxidation, resulting in a consistent production of defect-free hot-dip galvanized steel sheets and galvannealed steel sheets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si is added to steel sheets to increase strength, then mechanical characteristics are improved, but surface oxides form during annealing that decrease coating wettability and adhesion
Solution Approach 1:
The invention changes the chemical composition parameters by adding specific elements (Al: 0.03-3 mass%, Ti: 0.003-0.1 mass%, Nb: 0.003-0.1 mass%, V: 0.003-0.1 mass%) to the steel sheet. These compositional modifications suppress oxide formation on the steel sheet surface during annealing, thereby maintaining coating wettability and adhesion while preserving the strength-enhancing effect of Si addition.
2Manufacturing precision
If oxidation-reduction techniques are used to improve wettability, then coating adhesion is enhanced, but indentation flaws and uncoated areas occur
Solution Approach 1:
The invention performs preliminary action by suppressing oxide formation on the steel sheet surface before the hot-dip galvanizing process through controlled addition of Al, Ti, Nb, and V elements. By preventing oxide formation in advance rather than attempting to remove or reduce oxides afterward, the method eliminates the root cause of wettability problems and avoids the induction of surface defects like indentation flaws and uncoated areas.
3Productivity
If alloying treatment is performed at high temperatures to increase productivity, then production speed is improved, but powdering resistance decreases
Solution Approach 1:
The invention modifies the chemical composition parameters by adding specific elements (Ti, Nb, V) that suppress oxide formation. This compositional change allows the alloying process to be conducted at lower temperatures while maintaining productivity, thereby preserving powdering resistance. The elements Ti, Nb, and V form stable compounds that prevent surface oxidation, enabling controlled alloying at reduced temperatures.
4Productivity
If high temperatures are used for alloying treatment, then production efficiency is improved, but retained austenite phase stability is reduced
Solution Approach 1:
The invention changes the temperature parameter of the alloying treatment by conducting it at lower temperatures made possible through the addition of Ti, Nb, and V elements. These elements suppress surface oxidation, allowing efficient alloying at reduced temperatures that preserve the stability of the retained austenite phase while maintaining production efficiency.
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 method consistently produces hot-dip galvanized steel sheets and galvannealed steel sheets with no surface defects, good adhesion, and improved coating quality, enhancing yield and mechanical characteristics, particularly for high-Si-content steel sheets.
Implementation Method 1
because of its high affinity for oxygen, Si in steel is selectively oxidized even in a reducing atmosphere and forms oxides on the surface of steel sheets
Implementation Method 2
The steel sheet is then heated in a nonoxidizing or reducing atmosphere for recrystallization annealing
Implementation Method 3
The steel sheet is then cooled in a nonoxidizing or reducing atmosphere to a temperature suitable for coating and is immersed in a hot-dip galvanizing bath without exposed to the air
Implementation Method 4
Alloying treatment at high temperatures for the purpose of high productivity may lower powdering resistance
Data Source
Figure 1
Figure 2(a)~2(b)
AI summary
There are provided hot-dip galvanized steel sheets and galvannealed steel sheets that have no surface defects and have a good appearance and good adhesion to the coating and methods for producing the hot-dip galvanized steel sheets and galvannealed steel sheets. The hot-dip galvanized steel sheets and galvannealed steel sheets are produced from high-Si-content steel sheets. A hot-dip galvanized steel sheet having a good appearance and good adhesion to a coating, the hot-dip galvanized steel sheet having a composition comprising, on a mass basis: C: 0.20% to 0.50%, Si: 0.1% to 3.0%, Mn: 0.5% to 3.0%, P: 0.001% to 0.10%, Al: 0.01% to 3. 00%, and S: 0.200% or less, a remainder being Fe and incidental impurities, wherein the hot-dip galvanized steel sheet includes an internal oxidation layer and a decarburized layer, the internal oxidation layer having a thickness of 4 µm or less on a ferrite side from an interface between ferrite and a galvanized layer, the decarburized layer having a thickness of 16 µm or less on the ferrite side from the interface between the ferrite and the galvanized layer, and 50% or more by area of the internal oxidation layer is composed of a Si oxide containing Fe and/or Mn represented by Fe2xMn2-2xSiOy, wherein X ranges from 0 to 1, and Y is 3 or 4.