Cold-Rolled Steel Sheet Oxidation Control for Convertibility
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing methods for producing high-strength cold-rolled steel sheets with high chemical convertibility face challenges such as variable oxide film thickness, insufficient oxidation, and operational controllability issues, leading to degradation of surface properties and chemical convertibility.
Innovation Solution
Control the oxygen concentration in the atmosphere during oxidation treatment to manage the oxidation amounts and coverage of reduced iron on the steel sheet surface, forming (Fe,Mn)2SiO4 oxides, which improves chemical convertibility without controlling the dew point or steam-hydrogen partial pressure ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si is added to increase the strength of cold-rolled steel sheets, then the tensile strength increases, but Si oxide forms on the surface during continuous annealing which degrades chemical convertibility
Solution Approach 1:
The patent applies preliminary action by forming an oxide film on the steel sheet surface before the continuous annealing process. Specifically, the steel sheet is heated to 500-700°C in an oxidizing atmosphere to create a controlled oxide layer that prevents Si oxidation during subsequent reducing atmosphere annealing, thereby maintaining chemical convertibility while preserving strength enhancement from Si addition
Solution Approach 2:
The patent utilizes parameter changes by controlling the oxidation-reduction potential and temperature parameters during different stages of the annealing process. By adjusting the atmosphere composition (oxidizing vs reducing) and temperature sequence, the patent prevents unwanted Si oxide formation while maintaining the strengthening effect of Si in the steel matrix
2Reliability
If the steel sheet temperature is controlled to form an oxide film in an oxidizing atmosphere to improve chemical convertibility, then chemical convertibility improves, but the oxide film thickness becomes variable and may remain or separate during subsequent annealing
Solution Approach 1:
The patent applies parameter changes by precisely controlling the temperature range (500-700°C) and atmosphere composition during the oxidation stage. This controlled parameter regime ensures formation of a uniform, thin oxide film that does not exceed desired thickness and remains stable during subsequent annealing processes, avoiding film separation while maintaining chemical convertibility
3Productivity
If a direct firing burner is used to heat the steel sheet in an oxidizing atmosphere, then oxidation treatment is effective, but furnace walls and in-furnace rolls deteriorate and scale defects occur on the steel sheet surface
Solution Approach 1:
The patent applies local quality by creating a localized oxidizing atmosphere only at the steel sheet surface through the oxidizing atmosphere heating method, while the bulk furnace environment remains less aggressive. This localized approach achieves effective oxidation treatment at the surface level without subjecting the entire furnace structure to harsh oxidizing conditions, thereby reducing furnace wall and roll deterioration and minimizing scale defects
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
Achieves high-strength cold-rolled steel sheets with tensile strength of 590 MPa or more and improved chemical convertibility, along with enhanced workability and reduced risk of furnace deterioration and scale defects.
Implementation Method 1
oxidation treatment to the cold-rolled steel sheet
Implementation Method 2
heating the steel sheet to a recrystallization temperature in a reducing atmosphere
Data Source
AI summary
A high-strength cold-rolled steel sheet having high chemical convertibility and a tensile strength of 590 MPa or more and a method for producing such a steel sheet are provided. The steel sheet contains, in terms of percent by mass, C: 0.05 to 0.3%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1%, N: 0.01% or less, and the balance being Fe and unavoidable impurities. The coverage ratio of reduced iron on a steel sheet surface is 40% or more. In order to produce such a steel sheet, an oxidation treatment is performed after cold rolling. In the oxidation treatment, first heating is conducted on a steel sheet in an atmosphere with an oxygen concentration of 1000 ppm or more until the steel sheet temperature reaches 630°C or higher and then second heating is conducted on the steel sheet in an atmosphere with an oxygen concentration of less than 1000 ppm until the steel sheet temperature reaches 700°C or higher. Subsequently, annealing is conducted in a furnace in a 1 to 10 vol% H2 + balance N2 gas atmosphere with a dew point of -25°C or less.