High-Si Cold-Rolled Steel Sheet Phosphatability via Oxidation-Reduction
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Solution Overview
Problem
High-Si cold rolled steel sheets face phosphatability issues due to silicon oxide formation during annealing, which inhibits chemical conversion and paint adhesion, and existing methods either fail to sufficiently oxidize the surface or result in insufficient reduced iron layers, leading to poor phosphatability.
Innovation Solution
Oxidizing the surface of high-Si cold rolled steel sheets at elevated temperatures to form a Fe oxide layer, followed by reduction in a controlled atmosphere to confine Si oxide inside, thereby improving phosphatability without requiring precise control of the annealing atmosphere.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-Si cold rolled steel sheets are annealed in a N2 + H2 gas atmosphere to prevent iron oxidation, then iron oxidation is prevented, but silicon oxide forms on the outermost surface deteriorating phosphatability
Solution Approach 1:
The invention applies preliminary oxidation of the steel sheet surface before the main annealing process. By oxidizing the surface in advance in a controlled manner, the silicon that would otherwise form harmful SiO2 during annealing is pre-treated, allowing it to form a beneficial oxide layer that improves phosphatability rather than deteriorating it
Solution Approach 2:
The invention changes the oxidation state parameters of the steel sheet surface by introducing a controlled oxidation step. The steel sheet is heated to 400-550°C in an oxidizing atmosphere to form Fe oxide on the surface, then subjected to reduction in a N2 + H2 atmosphere. This parameter change transforms the surface chemistry to prevent harmful SiO2 formation while maintaining phosphatability
2Reliability
If the steel sheet surface is oxidized at low temperatures (400-550°C) to form Fe oxide layer, then iron oxidation is promoted, but silicon suppresses Fe oxidation due to high Si content making the reduced Fe layer insufficient
Solution Approach 1:
The invention optimizes the temperature parameter within the 400-550°C range and controls the oxidation atmosphere composition and duration to achieve sufficient Fe oxide formation despite high Si content. The subsequent reduction step parameters are also optimized to ensure adequate reduced Fe layer formation that prevents SiO2 exposure
Solution Approach 2:
The invention applies a continuous two-step process where oxidation is immediately followed by reduction without exposing the steel sheet to conditions that would allow SiO2 formation. This continuous action ensures that the Fe oxide layer is formed and then reduced in sequence, maintaining surface integrity and phosphatability throughout the process
3Reliability
If conventional annealing is performed in N2 + H2 atmosphere, then iron oxidation is prevented, but silicon oxide remains on the surface after reduction deteriorating phosphatability
Solution Approach 1:
The invention performs preliminary oxidation of the steel sheet surface before the main annealing-reduction process. This pre-oxidation creates a controlled Fe oxide layer that serves as a template for subsequent reduction, ensuring that silicon remains in a state that forms beneficial oxides rather than harmful SiO2 during the annealing process
Solution Approach 2:
The invention converts the potentially harmful effect of silicon oxidation into a beneficial outcome. By controlling the oxidation-reduction sequence, the silicon that would normally form harmful SiO2 is instead guided to form a surface oxide composition that enhances phosphatability. The harmful SiO2 formation is transformed into a beneficial surface chemistry through controlled oxidation followed by reduction
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 high tensile strength of at least 590 MPa and excellent processability with TS x El exceeding 18000 MPa·%, while enhancing phosphatability and preventing furnace defects, such as scale formation and pickup issues.
Implementation Method 1
Fe on the surface of a high strength cold rolled steel sheet containing Si at 0.6% or more is oxidized
Implementation Method 2
whereafter the oxide layer on the surface of the steel sheet is reduced in an iron reducing atmosphere
Data Source
AI summary
A method for the manufacturing of high strength cold rolled steel sheets includes continuously annealing a cold rolled steel sheet that has a composition containing C: 0.05 to 0.3% by mass, Si: 0.6 to 3.0% by mass, Mn: 1.0 to 3.0% by mass, P: not more than 0.1% by mass, S: not more than 0.02% by mass, Al: 0.01 to 1% by mass, N: not more than 0.01% by mass, and Fe and inevitable impurities: balance, in a manner such that the cold rolled steel sheet is heated in a furnace using an oxidizing burner to a steel sheet temperature of not less than 700°C, thereafter the steel sheet is soak-annealed in a reducing atmosphere furnace at 750 to 900°C, and the steel sheet is cooled in a manner such that the average cooling rate between 500°C and 100°C is not less than 50°C/s. According to the method, high-Si cold rolled steel sheets that have high strength and good phosphatability while containing Si at 0.6% or more can be obtained without controlling conditions so as to increase the dew point in the reducing atmosphere in the soaking furnace or to increase the vapor hydrogen partial pressure ratio.