Galvanized Steel Sheet Surface Profile for Formability and Paint Clarity
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Solution Overview
Problem
Hot-dip galvanized steel sheets face challenges in press formability and image clarity after painting due to galling and surface unevenness, which are exacerbated by the omission of the primer painting process in 2-coat systems, leading to increased costs and reduced surface quality.
Innovation Solution
A method involving skin-pass-rolling with a roll having specific roughness skewness (Rsk) of −0.2 or lower, average roughness (Ra) of 2 μm or less, and a high number of peaks (RPc) in each unit length, combined with a lubricant-filled profile valley area (V2) of 350 nm or greater, to reduce friction and improve surface finish.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a hot-dip galvanized steel sheet with a smooth pure zinc coating is used, then the coating layer is soft and cohesive, but galling occurs easily during press-forming due to cohesion between the coating layer and press die
Solution Approach 1:
The patent applies local quality by creating specific surface roughness characteristics (Ra ≤ 2 μm, Rsk ≤ -0.2, RPc ≥ 140 count/cm) in the coating layer. This localized surface modification provides lubricant retention valleys while maintaining overall coating cohesion, preventing galling during press-forming without sacrificing the inherent strength and cohesiveness of the pure zinc coating.
Solution Approach 2:
The patent changes the surface parameter characteristics of the coating layer by controlling roughness skewness (Rsk ≤ -0.2), average roughness (Ra ≤ 2 μm), and peak density (RPc ≥ 140 count/cm). These parameter changes create a surface morphology that retains lubricants effectively, reducing friction and preventing galling while preserving the coating's cohesive properties.
2Ease of manufacture
If the primer painting process is omitted in a 2-coat painting system, then painting costs are reduced, but surface unevenness such as orange peel defects and dents remain exposed after painting, degrading image clarity
Solution Approach 1:
The patent applies preliminary action by pre-modifying the coating layer surface through controlled roughness characteristics (Ra ≤ 2 μm, Rsk ≤ -0.2, RPc ≥ 140 count/cm) before the painting process. This preliminary surface preparation creates lubricant-retaining valleys that prevent orange peel defects and surface unevenness during painting, eliminating the need for a primer coat while maintaining excellent image clarity after painting.
Solution Approach 2:
The patent changes the surface parameters of the coating layer to achieve Ra ≤ 2 μm, Rsk ≤ -0.2, and RPc ≥ 140 count/cm. These parameter changes create an optimal surface morphology that prevents painting defects directly, allowing the steel sheet to achieve excellent image clarity in a 2-coat painting system without requiring the intermediate primer process.
3Ease of operation
If skin-pass-rolling is performed to improve press formability, then surface ruggedness increases, but this may create orange peel defects that degrade image clarity after painting
Solution Approach 1:
The patent applies parameter changes by precisely controlling the skin-pass-rolling process to achieve specific surface characteristics: Ra ≤ 2 μm, Rsk ≤ -0.2, and RPc ≥ 140 count/cm. These controlled parameter changes create sufficient surface modification for improved press formability and lubricant retention while avoiding excessive ruggedness that would cause orange peel defects, thereby maintaining excellent image clarity after painting.
Solution Approach 2:
The patent creates a composite surface structure by combining controlled roughness features with a smooth overall appearance. The surface morphology comprises numerous small peaks (RPc ≥ 140 count/cm) with valleys that retain lubricants, creating a composite structure that provides both press formability and painting quality without the trade-off between the two properties.
Data Source
AI summary
Provided is a hot-dip galvanized steel sheet used for an automobile outer panel and so forth and, more specifically, to a method for producing a hot-dip galvanized steel sheet having excellent press formability and clarity after painting, and to a hot-dip galvanized steel sheet produced by the method.