Ferritic Stainless Steel Surface Quality and Coloration Control
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Solution Overview
Problem
Ferritic stainless steel sheets used for trim and car molding materials face challenges in achieving good surface quality due to surface defects from polishing marks and yellow coloration during high-frequency heating, which existing methods struggle to fully address.
Innovation Solution
A method involving a ferritic stainless steel sheet with a controlled area ratio of minute defects and a passivation film with specific thickness and composition, including Si, Al, Cr, and Fe, to prevent surface defects and coloration, achieved through a production process involving mechanical polishing, annealing, and controlled bright annealing conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If mechanical polishing is carried out to remove surface scratches, then surface quality is improved, but a hard layer is generated in polishing marks which causes minute surface cracking and scratch-shaped minute defects during subsequent rolling
Solution Approach 1:
The patent applies preliminary annealing treatment before mechanical polishing to soften the steel sheet surface. This preliminary action prevents the formation of hard layers during subsequent polishing and rolling operations, thereby eliminating the source of surface cracking and defects while maintaining smooth surface quality.
Solution Approach 2:
The patent changes the temperature parameter by conducting annealing treatment at specific temperature ranges (e.g., 800-950°C) to alter the material's physical properties. This parameter change softens the steel sheet, making it more ductile and resistant to forming hard layers during mechanical processing, thus preventing surface defects.
2Object-generated harmful factors
If annealing is carried out after mechanical polishing to soften the hard layer, then surface defects from polishing marks are reduced, but it is difficult to reduce other surface defects such as oil pits
Solution Approach 1:
The patent performs preliminary annealing before mechanical polishing rather than after. This preliminary action prepares the material in advance by softening it, which prevents the formation of hard layers and other surface defects during the polishing and subsequent rolling operations, achieving comprehensive surface quality improvement.
Solution Approach 2:
The patent applies beforehand cushioning by implementing annealing treatment prior to mechanical processing. This prior cushioning softens the material in advance, creating a more ductile state that prevents the formation of various surface defects including both polishing mark-related defects and other processing-induced defects like oil pits.
3Ease of manufacture
If high-frequency heating is carried out at low temperature to bond metal part and non-metal part, then bonding is achieved, but yellow coloration is produced on the surface of the metal part
Solution Approach 1:
The patent applies preliminary bright annealing treatment to form a protective passivation film on the steel sheet surface before bonding operations. This preliminary action creates a protective layer that prevents yellow coloration during subsequent high-frequency heating and bonding processes, allowing bonding to be performed without surface quality degradation.
Solution Approach 2:
The patent creates an inert protective environment by forming a passivation film through bright annealing in controlled atmospheres (e.g., hydrogen or nitrogen atmospheres). This inert environment prevents unwanted surface reactions during bonding operations, thereby preventing yellow coloration while allowing bonding to proceed effectively.
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 method effectively reduces surface defects and prevents coloration, ensuring a good surface quality and appearance suitable for trim and car molding materials by maintaining an optimal area ratio of minute defects and passivation film characteristics.
Implementation Method 1
a passivation film with a thickness of 1 nm or more and 10 nm or less is formed on the steel sheet surface
Implementation Method 2
a method for softening the hard layer in the polishing marks by annealing after mechanical polishing
Data Source
Figure 1(a)~3(b)
Figure 4~6
AI summary
Provided is a ferritic stainless steel sheet which has good surface quality and can prevent coloration due to heating, and a method for producing the same. The ferritic stainless steel sheet is a ferritic stainless steel sheet which is temper rolled after bright annealing and has a passivation film. The area ratio of minute defects having an area of 10 µm2 or more on the surface of the steel sheet is 0.2% or more and 1.5% or less. In addition, the thickness of the passivation film is 1 nm or more and 10 nm or less, the average concentration of Si in the passivation film is 10 at% or more and 20 at% or less, and the average concentration ratio of Si, Al, Cr and Fe in the passivation film component is (Si + Al + Cr) /Fe > 1.0. When producing this ferritic stainless steel sheet, a hot rolled pickled material is used as a starting material and the surface of the steel sheet is mechanical polished at least once or more in a step preceding finishing cold rolling, followed by annealing. After this annealing, finishing cold rolling, bright annealing and temper rolling are carried out in said order.