Thin Steel Sheet Surface Chemistry for Plating and Strength
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Solution Overview
Problem
High strength steel sheets with thin thickness face issues of insufficient plateability and chemical convertibility due to the formation of oxide layers and decarburization, which affect the mutual diffusion of steel and plating constituents, leading to poor appearance and strength degradation.
Innovation Solution
Control the thickness ratio of the decarburized layer and internal oxidation layer in steel sheets with specific chemical compositions, ensuring a C concentration at the ½A position is Cb/2 or more, and apply a zinc-based plating layer with controlled thickness and composition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If annealing is performed to remove strain and improve workability, then plateability and chemical convertibility are improved, but oxide layers form on the surface impeding mutual diffusion between steel and plating constituents
Solution Approach 1:
The patent applies parameter changes by precisely controlling annealing temperature (700-950°C), holding time (1-10 minutes), and atmosphere composition (H2/CO/H2O partial pressures) to achieve internal oxidation layer formation while suppressing external oxidation. This transforms the annealing process parameters to simultaneously achieve strain relief and controlled oxide formation that promotes plating adhesion.
Solution Approach 2:
The patent converts the harmful effect of oxide layer formation into a beneficial phenomenon by controlling the oxidation process to create an internal oxidation layer that actually improves plating adhesion and chemical convertibility. The previously harmful external oxidation is transformed into a controlled internal oxidation that facilitates mutual diffusion between steel and plating constituents.
2Ease of manufacture
If internal oxidation layer is formed during annealing, then plateability is improved, but decarburization occurs near the surface reducing carbon content and strength
Solution Approach 1:
The patent controls the annealing parameters (temperature, time, atmosphere composition) to precisely regulate the formation of internal oxidation layer while suppressing excessive decarburization. By optimizing H2/CO partial pressures and temperature ranges, the process achieves controlled carbon diffusion that forms the desired oxidation layer without excessive carbon loss from the surface.
Solution Approach 2:
The patent applies local quality by creating a controlled gradient in carbon concentration and oxide distribution within the steel sheet. The internal oxidation layer forms with specific compositional characteristics that differ from the bulk steel, creating a localized region with enhanced plating adhesion while maintaining sufficient carbon content for strength.
3Weight of moving object
If steel sheet thickness is reduced to achieve lightweighting, then fuel economy is improved, but plateability and strength become insufficient
Solution Approach 1:
The patent transforms the surface chemistry and microstructure of thin steel sheets by controlling annealing parameters to form an internal oxidation layer. This parameter-controlled modification enhances plating adhesion and chemical convertibility specifically at the surface level, allowing thin sheets to achieve adequate plateability without increasing overall thickness.
Solution Approach 2:
The patent applies local quality by concentrating the oxidation layer formation at the surface region of thin steel sheets, creating a localized enhanced interface for plating adhesion. This allows the bulk material to remain thin for lightweighting while the surface layer provides the necessary properties for excellent plateability and strength.
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
Achieves both high strength and excellent plateability and chemical convertibility in steel sheets with a thickness of 0.4 to 2.0 mm, maintaining appearance properties and strength.
Implementation Method 1
the carbon near the surface of the steel sheet bonding with the oxygen in the annealing atmosphere to become CO2 which is then discharged outside the system
Implementation Method 2
Si and Mn, which are elements with strong affinity with oxygen among the elements typically included in high strength steel sheet, sometimes bond with the oxygen in the atmosphere in the annealing step and form a layer including oxides near the surface of the steel sheet
Implementation Method 3
before forming such a plating layer, the steel sheet (typically cold rolled steel sheet) is often annealed at a certain temperature or more so as to remove the strain from steel sheet and/or improve the workability
Data Source
AI summary
Steel sheet and plated steel sheet comprised of steel sheet with a thin thickness in which sufficient strength and plateability or chemical convertibility are both achieved, i.e., steel sheet having a chemical composition containing, by mass %,C: 0.05 to 0.30%,Si: 0.01 to 2.50%,Mn: 0.80 to 3.00%,Al: 0.010 to 2.000%, etc. and having a balance comprising Fe and impurities andhaving a thickness of 0.4 to 2.0 mm,in which steel sheet,a tensile strength is 550 to 1500 MPa,an internal oxidation layer and decarburized layer are contained,when a thickness of the decarburized layer per side of the steel sheet: A (μm), a bulk C concentration of the steel sheet: Cb (%), and a thickness of the steel sheet: t (mm),0.01≤A/t≤0.15, anda C concentration at the ½A position is Cb/2 or more andplated steel sheet using that steel sheet.