Galvanized Steel Sheet Internal Oxide Control for Plateability
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Solution Overview
Problem
High strength steel sheets used in automotive applications face issues with insufficient plateability and appearance due to the formation of external oxides and decarburization, which impede the interdiffusion of steel and plating constituents, leading to non-plating parts and uneven alloying, thereby affecting corrosion resistance and strength.
Innovation Solution
Control the thickness ratios of internal oxide and decarburized layers in a galvanized steel sheet with a thickness of 0.10 to 0.95 mm by adjusting the chemical composition and annealing conditions to promote internal oxide formation and minimize decarburization, ensuring adequate strength and plateability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external oxide layer is formed on the steel sheet surface during annealing, then the steel sheet gains corrosion resistance, but the plateability deteriorates due to impeded interdiffusion of steel and plating constituents
Solution Approach 1:
The patent converts the harmful effect of oxide formation by promoting internal oxidation instead of external oxidation. By controlling annealing conditions to form oxides within the steel sheet rather than on the surface, the harmful oxide layer is transformed into a beneficial internal structure that does not interfere with plating while still providing corrosion resistance.
Solution Approach 2:
Instead of allowing oxides to form on the external surface (conventional approach), the patent inverts the oxidation process to occur internally within the steel sheet. This inversion of the oxidation location eliminates the barrier to plating while maintaining the protective oxide structure within the material.
2Weight of moving object
If the steel sheet thickness is reduced to lighten weight, then fuel efficiency improves, but the strength near the surface deteriorates due to decarburization
Solution Approach 1:
The patent converts the harmful decarburization effect by controlling the annealing atmosphere to promote internal oxidation instead. This transforms the harmful loss of carbon at the surface into a beneficial internal oxide structure, allowing thin sheets to maintain surface strength while achieving weight reduction.
3Strength
If Si and Mn content is increased to improve strength, then the tensile strength increases, but the formation of external oxide layer is promoted which deteriorates plateability
Solution Approach 1:
The patent inverts the oxidation behavior by controlling annealing conditions to promote internal oxidation rather than external oxidation. This allows Si and Mn to form oxides within the steel sheet structure rather than on the surface, maintaining both high strength and good plateability.
Solution Approach 2:
The patent changes the parameters of the annealing process (temperature, atmosphere composition, heating rate) to control the oxidation behavior. By adjusting these parameters, the oxidation is directed internally rather than externally, allowing simultaneous achievement of high strength through Si-Mn addition and good plateability.
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 solution achieves a galvanized steel sheet with both high strength and excellent appearance properties by optimizing the thickness ratios of internal oxides and decarburized layers, enhancing plateability and corrosion resistance.
Implementation Method 1
bond with the oxygen in the atmosphere in the annealing step and sometimes form a layer including oxides near the surface of the steel sheet
Implementation Method 2
the carbon near the steel sheet surface bonds with the oxygen in the annealing atmosphere to become CO2 and is discharged to the outside of the system
Implementation Method 3
interdiffusion of the steel constituents (for example, Fe) and plating constituents (for example, Zn) is impeded
Data Source
Figure 1~2
AI summary
The present invention relates to a galvanized steel sheet comprising steel sheet containing C: 0.05 to 0.30%, Si: 0.01 to 3.00%, Mn: 0.80 to 3.00%, and Al: 0.010 to 2.000% and having a thickness of 0.10 to 0.95 mm, and a galvanized layer on the steel sheet, wherein a tensile strength is 550 to 1500 MPa, and, when defining a thickness of an internal oxide layer per surface of the steel sheet as "A" (µm), a thickness of a decarburized layer per surface of the steel sheet as "B" (µm), and a thickness of the steel sheet as "t" (µm), A/B: 0.01 to 0.50 and B/t: 0.001 to 0.200.