Steel Sheet Surface Conversion for Yellowing Resistance and Phosphatability
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Solution Overview
Problem
Existing cold-rolled steel sheets face issues with non-uniform phosphate film formation due to oxide films, leading to poor coating quality, reduced corrosion resistance, and yellowing, particularly in high-strength steels with high silicon or manganese content, which existing techniques fail to adequately address.
Innovation Solution
A chemical conversion treatment is applied in the water-cooling or water-rinsing sections to enhance phosphatability and yellowing resistance by controlling the surface composition of the steel sheet with specific amounts of calcium, magnesium, phosphorus, carbon, and oxygen, along with optional additional elements, followed by treatments like flash plating, phosphating, and coating to improve surface quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the steel sheet is water-cooled or water-rinsed after annealing, then cooling efficiency is improved, but oxide film formation increases causing yellowing and poor phosphatability
Solution Approach 1:
A chemical conversion treatment is introduced as an intermediary substance between the water-cooling process and the steel sheet surface. This treatment layer prevents direct oxidation during water cooling while allowing efficient heat dissipation, thus mediating between cooling efficiency and oxide prevention
Solution Approach 2:
The chemical conversion treatment is applied preliminarily during the water-cooling or water-rinsing section before the steel sheet enters the phosphating process. This preliminary action prepares the surface by controlling oxide formation in advance, ensuring good phosphatability without requiring subsequent surface preparation
2Object-affected harmful factors
If the oxide film thickness is increased, then yellowing resistance is improved, but phosphate film coverage decreases
Solution Approach 1:
The invention changes the chemical composition parameters of the surface layer by controlling the content of Ca (40-100 ppm), Mg (40-100 ppm), P (100-500 ppm), C (10-50 ppm), and O (50-200 ppm). This parameter optimization allows the surface to resist yellowing while maintaining adequate phosphatability
3Strength
If high-strength steel with high Si or Mn content is used, then mechanical strength is improved, but oxidation resistance decreases causing yellowing
Solution Approach 1:
The invention converts the harmful oxidation tendency of high-Si and high-Mn steel into a beneficial surface composition. By controlling the oxide film formation and subsequently treating it with chemical conversion, the naturally formed oxides are transformed into a surface layer with good phosphatability and yellowing resistance
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 treatment effectively suppresses oxide film formation, promotes phosphate nucleation, and enhances phosphatability and yellowing resistance, resulting in improved surface quality and appearance of the steel sheet.
Implementation Method 1
promotes phosphate nucleation
Implementation Method 2
suppresses formation of an oxide film
Data Source
AI summary
The present invention relates to a steel sheet with improved yellowing resistance and phosphatability, wherein the steel sheet contains 0.5% by weight or more of Mn, and contains 0.01 to 10 mg/m2 of Ca+Mg, 0.01 to 10 mg/m2 of P, 0.01 to 20 mg/m2 of C, and 0.05 to 30 mg/m2 of O as components excluding a steel component on the surface of the steel sheet after pickling, water rinsing, and drying. According to the present invention, in a manufacturing process of the steel sheet, the surface of the steel sheet is subjected to a chemical conversion treatment for improving phosphatability and yellowing resistance in a water-cooling section or a water-washing section, thereby having an effect of improving the surface quality of products using same and various subsequently treated products.