Cold-Rolled Steel Sheet Mn Concentration Profile for Galvanizing
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Solution Overview
Problem
The challenge is to produce a high-strength galvanized steel sheet with improved surface appearance and coating quality, as easily oxidizable elements like Si and Mn can form oxides that deteriorate wettability with molten zinc, leading to coating defects during the galvanizing process.
Innovation Solution
A cold-rolled steel sheet with a specific chemical composition and controlled Mn concentration profile, including 1.8% to 3.2% Mn, is annealed under controlled conditions to prevent oxide formation on the surface, ensuring good coatability and surface appearance, and subsequently coated to produce a high-strength galvanized steel sheet.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If solid solution strengthening chemical elements such as Si and Mn are added to increase the strength of a steel sheet, then the strength of the steel sheet is improved, but the wettability between the surface of the steel sheet and molten zinc deteriorates, resulting in coating defects
Solution Approach 1:
The patent applies local quality by creating a non-uniform Mn concentration distribution within the steel sheet thickness direction. Specifically, the Mn concentration is controlled to be higher in the surface layer (0.01-3.0 wt%) and lower in the inner layer (0.003-1.5 wt%), with the ratio of surface Mn to inner Mn being 1.05-5.00. This localized concentration gradient ensures that the surface has sufficient Mn to prevent oxidation and improve wettability with molten zinc, while the inner layer maintains the required strength through solid solution strengthening.
Solution Approach 2:
The patent applies parameter changes by precisely controlling the Mn concentration as a chemical parameter to resolve the contradiction. By adjusting the Mn concentration profile (surface Mn content, inner layer Mn content, and their ratio) and combining it with specific annealing conditions (temperature range of 500-750°C, time of 5-120 minutes), the patent transforms the uniform high-Mn composition into a differentiated concentration distribution that simultaneously achieves both high strength and good coating quality.
2Manufacturing precision
If heating is performed in an oxidizing atmosphere to form Fe oxide film on the surface, then the wettability between the surface of the steel sheet and molten zinc is improved, but iron oxides adhere to rolls in a furnace, resulting in pressing flaws on the surface of the steel sheet
Solution Approach 1:
The patent applies preliminary action by pre-forming a controlled oxide layer on the steel sheet surface through annealing in an oxidizing atmosphere at 500-750°C for 5-120 minutes before the galvanizing process. This preliminary oxidation creates a thin, uniform Fe oxide film that improves wettability with molten zinc. The controlled thickness and composition of this pre-formed oxide layer prevent excessive oxide formation during subsequent processing, thereby avoiding roll adhesion and pressing flaws while maintaining good coating quality.
3Strength
If Mn content is increased to improve strength, then the strength of the steel sheet is improved, but Mn oxides are formed on the surface during reduction annealing, which deteriorate the wettability
Solution Approach 1:
The patent applies local quality by creating a non-uniform Mn concentration distribution within the steel sheet thickness direction. Specifically, the Mn concentration is controlled to be higher in the surface layer (0.01-3.0 wt%) and lower in the inner layer (0.003-1.5 wt%), with the ratio of surface Mn to inner Mn being 1.05-5.00. This localized concentration gradient ensures that the surface has sufficient Mn to prevent oxidation and improve wettability with molten zinc, while the inner layer maintains the required strength through solid solution strengthening.
Solution Approach 2:
The patent applies parameter changes by precisely controlling the Mn concentration as a chemical parameter to resolve the contradiction. By adjusting the Mn concentration profile (surface Mn content, inner layer Mn content, and their ratio) and combining it with specific annealing conditions (temperature range of 500-750°C, time of 5-120 minutes), the patent transforms the uniform high-Mn composition into a differentiated concentration distribution that simultaneously achieves both high strength and good coating quality.
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 approach results in a coated steel sheet with excellent surface appearance and improved collision safety, contributing to enhanced fuel efficiency through weight reduction of automobile parts while minimizing coating defects.
Implementation Method 1
a cold-rolled steel sheet... in which the Mn concentration in a surface layer of the steel sheet satisfies relational expression (1) and relational expression (2) below... is annealed under controlled conditions
Implementation Method 2
Easily oxidizable chemical elements in steel are selectively oxidized even in a non-oxidizing atmosphere or a reducing atmosphere which is generally used, are concentrated on the surface of a steel sheet, and form oxides on the surface of the steel sheet
Data Source
AI summary
Provided are a cold-rolled steel sheet which can preferably be used for manufacturing a high-strength galvanized steel sheet and methods for manufacturing the steel sheets. The cold-rolled steel sheet has a specified chemical composition, in which the Mn concentration in a surface layer of the steel sheet satisfies relational expression (1) and relational expression (2) below. 8≤(Cp/Cc)×Mn . . . (1) (Cmin/Cc)×Mn≤2.5 . . . (2) where Cp: maximum Mn concentration in a region within 0.5 μm of the surface of a steel sheet in the thickness direction; Cc: average Mn concentration in a region from a position located 5 μm from a surface of a steel sheet in the thickness direction to a position located 5 μm from an opposite surface in the thickness direction; Cmin: minimum Mn concentration in a region from 0.5 μm to 5 μm from the surface of a steel sheet in the thickness direction; and Mn: Mn content (mass %).