Galvanized Steel Sheet Carbide Control for Oxide Prevention
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Solution Overview
Problem
High strength galvanized steel sheets face issues with coating adhesiveness, corrosion resistance, and surface appearance, particularly during bending and stretch flange forming, due to the formation of internal oxides and insufficient carbide precipitation, which affects their formability and weldability.
Innovation Solution
Control the concentration ratio of C to Ti in the surface portion of the steel sheet, manage the coiling temperature, and regulate water vapor and hydrogen partial pressures during annealing to precipitate Ti carbides, ensuring a balanced chemical composition and microstructure that prevents oxide formation and enhances carbide dispersion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If coiling is performed at a high coiling temperature (550°C or higher) to precipitate fine carbides and achieve high strength, then tensile strength is improved, but internal oxides are formed in the surface portion, causing decreased coating adhesiveness and surface appearance
Solution Approach 1:
The patent applies inert atmosphere protection during the coiling process by controlling the atmosphere to prevent oxidation. Specifically, the coiling is performed in an atmosphere with controlled oxygen potential, using methods such as nitrogen atmosphere coiling or vacuum coiling, which prevents the formation of internal oxides in the surface portion while still allowing carbide precipitation at high coiling temperatures (550°C or higher). This resolves the contradiction by creating an oxidization-resistant environment that enables both high strength and clean surface.
Solution Approach 2:
The patent changes the atmospheric parameters during coiling by controlling oxygen partial pressure, carbon potential, and temperature profile. By adjusting these parameters - specifically maintaining high carbon potential and low oxygen potential in the coiling atmosphere - the patent enables carbide precipitation while suppressing oxide formation. This parameter control allows achieving tensile strength of 590 MPa or higher without the harmful effect of internal oxides that would compromise coating adhesiveness.
2Ease of manufacture
If the concentration ratio of C to Ti in the surface portion is not controlled, then manufacturing is simpler, but coating adhesiveness and corrosion resistance deteriorate due to excessive oxide formation or insufficient carbide precipitation
Solution Approach 1:
The patent establishes specific parameter ranges for the concentration ratio of C to Ti in the surface portion (within 10 μm from the surface) to be 0.8 or more and 1.5 or less in atomic ratio. This parameter control ensures optimal carbide precipitation while preventing excessive oxide formation. The patent also controls the total oxide amount to 0.05 g/m² or less. These precise parameter specifications transform a complex manufacturing process into a controllable one with clear criteria, improving both coating adhesiveness and corrosion resistance while maintaining manufacturing feasibility through defined composition ranges.
3Strength
If high strength is achieved through conventional methods, then tensile strength increases, but formability and weldability deteriorate due to microstructure and oxide issues
Solution Approach 1:
The patent uses inert atmosphere coiling to prevent internal oxide formation while achieving high strength through controlled carbide precipitation. The clean microstructure obtained by preventing oxide formation during coiling in a nitrogen or vacuum atmosphere maintains both high strength (590 MPa or higher) and excellent formability. The absence of internal oxides eliminates stress concentration points that would otherwise compromise formability during bending and stretch flange forming operations.
Solution Approach 2:
The patent optimizes multiple parameters simultaneously: coiling temperature (550°C or higher), concentration ratio of C to Ti (0.8-1.5 atomic ratio), and total oxide amount (0.05 g/m² or less). This coordinated parameter control achieves a microstructure with dispersed fine carbides in a clean ferrite matrix, providing both high strength through carbide reinforcement and excellent formability through the absence of oxide-related defects and uniform microstructure.
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 solution results in a high strength galvanized steel sheet with improved coating adhesiveness, corrosion resistance, and surface appearance, maintaining good formability while achieving tensile strengths of 590 MPa or higher.
Implementation Method 1
precipitates containing one or more of Ti, Mo and W and having a size of less than 10 nm are dispersed
Implementation Method 2
internal oxides containing easily oxidized chemical elements are formed in the surface portion of the hot-rolled steel sheet
Data Source
AI summary
A high strength galvanized steel sheet has a composition including, C: 0.02% or more and 0.30% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.003% or more and 0.08% or less, S: 0.01% or less, Al: 0.001% or more and 0.20% or less, Ti: 0.03% or more and 0.40% or less and the balance being Fe and inevitable impurities, and a zinc-coated layer having a coating weight per surface of 20 g/m2 or more and 120 g/m2 or less. The concentration ratio of C to Ti (C/Ti) in a portion within 10 μm from the surface of the base steel sheet is 0.8 or more and 1.5 or less, and the total amount of oxides of one or more selected from Fe, Si, Mn, P, Al and Ti formed in a surface portion within 100 μm from the surface of the base steel sheet is 0.05 g/m2 or less.