Cold Rolled Steel Surface Morphology for Phosphate Crystallization
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Solution Overview
Problem
High-strength cold rolled steel sheets with added Mo face deterioration in chemical conversion treatment properties, and existing methods to improve these properties are either ineffective or costly, such as controlling surface roughness or forming an iron coat through electric plating.
Innovation Solution
A high-strength cold rolled steel sheet with specific surface properties, including a maximum unevenness depth of 10 μm or more, average spacing of 30 μm or less, load length ratio of 20% or less, and a difference in load length ratios of 60% or more, along with cracks of 3 μm or less in width and 5 μm or more in depth, which enhances the formation and growth of zinc phosphate crystals, thereby improving chemical conversion treatment properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Mo is added to steel to increase strength, then tensile strength is improved, but chemical conversion treatment property deteriorates
Solution Approach 1:
The invention changes the surface morphology parameters of the steel sheet by controlling the maximum depth (Ry) and average spacing (Sm) of surface unevenness, as well as the load length ratios (tp40, tp60). These parameter changes create a surface structure that promotes uniform phosphate crystal deposition, compensating for the negative effect of Mo addition on chemical conversion treatment property while maintaining high tensile strength
2Reliability
If surface roughness is controlled to improve chemical conversion treatment property, then treatment property is improved, but the method is ineffective for Mo-added steel
Solution Approach 1:
The invention applies local quality control by specifying particular surface morphology characteristics (Ry ≥ 10μm, Sm ≤ 30μm, tp40 ≤ 20%, tp60 - tp40 ≥ 60%) that create favorable local conditions for phosphate crystal deposition. This localized surface structure optimization makes the chemical conversion treatment effective specifically for Mo-added steel, addressing the limitation of conventional roughness control methods
3Reliability
If an iron coat is formed on the surface to prevent deterioration of chemical conversion treatment property, then treatment property is maintained, but productivity and cost are reduced
Solution Approach 1:
The invention extracts the need for additional iron coating processes by directly controlling the surface morphology of the base steel sheet itself. By creating the appropriate surface unevenness parameters during steel sheet production, the invention eliminates the requirement for separate electric plating processes, thereby maintaining chemical conversion treatment property without sacrificing productivity or increasing cost
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 specified surface properties ensure excellent chemical conversion treatment properties in both Mo-added and non-Mo-added high-strength cold rolled steel sheets, providing a balance of strength and treatment quality at a lower cost, suitable for automotive structural parts.
Implementation Method 1
the present inventors developed a technology to effectively utilize the oxide generated on the steel sheet surface as a generating site of nuclei of phosphate crystal by controlling the shape of the oxide and to improve chemical conversion treatment property
Data Source
AI summary
The invention provides a high strength cold rolled steel sheet having excellent chemical conversion treatment property stably even Mo is added aiming high strengthening. The surface property of the cold rolled steel sheet satisfies that the characteristic of 10 μm or more of the maximum depth (Ry) of the unevenness and 30 μm or less of the average spacing (Sm) of the unevenness, and that either one or more preferably both of, the characteristic of the load length ratio (tp40) of the unevenness of the surface is 20% or less, and the characteristic of the difference of the load length ratios (tp60) and (tp40) is 60% or more, is satisfied, and the crack of 3 μm or less width and 5 μm or more depth does not exist on the surface.