High-Si Cold Rolled Steel Sheet Oxidation Control
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Solution Overview
Problem
Existing methods for manufacturing high-Si cold rolled steel sheets face challenges in achieving consistent chemical conversion properties due to variable oxide film thickness, excessive oxidation, and operational controllability issues, particularly with high Si content and dew point control, leading to non-uniform surface quality and degradation of furnace components.
Innovation Solution
A method involving direct flame burners with specific air ratios and subsequent soak-annealing in a reducing atmospheric gas composition without dew point control, ensuring Si oxidation inside the steel sheet while forming a sufficient iron oxide layer for improved chemical conversion, maintaining tensile strength above 590 MPa and workability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si content is increased to enhance strength, then tensile strength is improved, but Si oxide formation on the surface increases which deteriorates chemical conversion properties
Solution Approach 1:
The patent applies local quality by creating a non-uniform oxide distribution where the surface layer contains controlled Fe oxide for good chemical conversion properties, while the interior contains Si oxide for strength enhancement. This spatial differentiation of oxide composition resolves the contradiction between strength and chemical conversion properties.
Solution Approach 2:
The patent changes the oxidation parameters by controlling the oxidation atmosphere and temperature to selectively form Fe oxide on the surface while limiting Si oxide formation. By adjusting the oxidation potential and temperature parameters, the patent achieves differential oxidation behavior that simultaneously satisfies strength and chemical conversion requirements.
2Reliability
If oxidation treatment is applied to improve chemical conversion properties, then surface oxide film is formed, but oxide film thickness becomes variable leading to non-uniform surface quality
Solution Approach 1:
The patent controls the oxidation process by adjusting temperature, atmosphere composition, and exposure time parameters to achieve a consistent, thin Fe oxide layer. This parameter optimization ensures uniform oxide formation across the surface while maintaining sufficient thickness for chemical conversion.
Solution Approach 2:
The patent applies preliminary oxidation treatment before final steel sheet processing to pre-form a uniform Fe oxide layer. This preliminary action ensures that the surface is properly prepared for subsequent chemical conversion treatment, achieving consistent results across all sheets.
3Reliability
If dew point control is implemented to prevent Si oxidation, then chemical conversion properties are maintained, but operational complexity and furnace degradation increase
Solution Approach 1:
The patent extracts the dew point control requirement from the oxidation process by using a different approach - controlling atmosphere composition and temperature directly. This eliminates the need for complex dew point monitoring and control systems while achieving the same protective effect against excessive Si oxidation.
Solution Approach 2:
The patent uses a controlled inert or reducing atmosphere during oxidation treatment to prevent excessive Si oxidation. By maintaining an atmosphere with controlled oxygen potential through composition control rather than dew point control, the patent simplifies the process while protecting the steel sheet surface.
4Strength
If high Si content is used to achieve tensile strength of 590 MPa or more, then workability is maintained, but oxide film formation inhibits chemical conversion treatment
Solution Approach 1:
The patent applies local quality by creating a non-uniform oxide distribution where the surface layer contains controlled Fe oxide for good chemical conversion properties, while the interior contains Si oxide for strength enhancement. This spatial differentiation of oxide composition resolves the contradiction between strength and chemical conversion properties.
Solution Approach 2:
The patent applies preliminary oxidation treatment before final steel sheet processing to pre-form a uniform Fe oxide layer. This preliminary action ensures that the surface is properly prepared for subsequent chemical conversion treatment, achieving consistent results across all sheets.
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 achieves stable and improved chemical conversion properties without dew point control, enhancing tensile strength and workability while preventing furnace degradation and scale defects, ensuring consistent high-Si cold rolled steel sheet quality.
Implementation Method 1
heating a cold rolled steel sheet with use of a direct flame burner (A) having an air ratio of not more than 0.89 when a temperature of the cold rolled steel sheet that is being increased is in a temperature range of not less than 300°C and less than Ta°C
Implementation Method 2
soak-annealing the cold rolled steel sheet in a furnace having an atmospheric gas composition which has a dew point of not more than -25°C and contains 1 to 10 volume% of H2 and the balance of N2
Implementation Method 3
silicon is oxidized even under an atmospheric gas composition including reducing N2 and H2 which does not induce the oxidation of Fe (which reduces Fe oxides). As a result, a thin film of Si oxide (SiO2) is formed on the outermost surface of a steel sheet
Data Source
AI summary
The invention provides methods for manufacturing high-Si cold rolled steel sheets which achieve good chemical conversion properties and exhibit a tensile strength of not less than 590 MPa and excellent workability with TS x EL being not less than 18000 MPa·%. A method includes a step of heating a cold rolled steel sheet that has a chemical composition containing C at 0.05 to 0.3 mass%, Si at 0.6 to 3.0 mass%, Mn at 1.0 to 3.0 mass%, P at not more than 0.1 mass%, S at not more than 0.05 mass%, Al at 0.01 to 1 mass%, and N at not more than 0.01 mass%, with the balance being represented by Fe and inevitable impurities, with a direct flame burner (A) having an air ratio of not more than 0.89 when the temperature of the cold rolled steel sheet that is being increased is in the temperature range of not less than 300°C and less than Ta°C, a step of subsequently heating the cold rolled steel sheet with a direct flame burner (B) having an air ratio of not less than 0.95 when the temperature of the cold rolled steel sheet is in the temperature range of not less than Ta°C and less than Tb°C, and subsequently soak-annealing the cold rolled steel sheet in a furnace having an atmospheric gas composition which has a dew point of not more than -25°C and contains 1 to 10 volume% of H2 and the balance of N2. 450°C ≤ Ta°C ≤ 550°C and 650°C ≤ Tb°C ≤ 800°C.