High-Strength Galvanized Steel Sheet Bending Workability
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Solution Overview
Problem
High-strength galvanized steel sheets with excellent bending workability face issues with appearance quality due to streaky undulation and decreased coating capability, particularly when alloy chemical elements segregate, leading to problems in forming processes.
Innovation Solution
A high-strength galvanized steel sheet with a chemical composition of C: 0.07% to 0.25%, Si: 0.01% to 3.00%, Mn: 1.5% to 4.0%, and controlled microstructure, including a martensite phase area ratio of 25% or more, and a manufacturing process involving hot rolling, cold rolling, annealing, galvanizing, and alloying treatments to achieve optimal strength and bending workability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If high-strength galvanized steel sheet is formed by bending, then bending workability is improved, but streaky undulation appears on bending ridge line due to alloy chemical element segregation, decreasing appearance quality and coating capability
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting alloy element contents (Si: 0.01-3.00%, Mn: 1.5-4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01-1.50%) and controlling microstructural parameters (martensite phase area ratio: 25% or more, average crystal grain diameter: 20 μm or less, Vickers hardness variation: 20 or less). These parameter changes prevent alloy segregation during bending while maintaining high strength and excellent bending workability with improved appearance quality.
2Weight of moving object
If the thickness of steel sheet is reduced to reduce automobile body weight, then weight is reduced, but strength must be maintained, requiring high-strength steel sheet
Solution Approach 1:
The invention creates a composite microstructure consisting of martensite phase (25% or more area ratio) as the primary strengthening phase, combined with controlled amounts of ferrite and bainite phases. This composite microstructure, along with the specific chemical composition range, achieves high strength (tensile strength 980 MPa or more) that enables thinning of steel sheets while maintaining required strength levels for automobile body weight reduction.
3Strength
If alloy chemical elements are added in large amounts to increase strength, then strength is improved, but streaky undulation appears due to segregation, decreasing coating capability and appearance quality
Solution Approach 1:
The invention optimizes the chemical composition parameters by setting specific ranges for alloy elements (Si: 0.01-3.00%, Mn: 1.5-4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01-1.50%) that provide sufficient strength through solid solution strengthening and microstructural control without excessive amounts that would cause segregation. This parameter optimization maintains coating capability and appearance quality while achieving the required strength level.
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 bending workability and appearance quality, suitable for use in automobile parts, achieving tensile strengths of 980 MPa or more while maintaining excellent formability and reducing weight in automotive applications.
Implementation Method 1
a microstructure including, in terms of area ratio in a cross section located at 1/4 of the thickness from the surface of a base steel sheet, a ferrite phase in an amount of 70% or less (including 0%), a bainite phase in an amount of 20% or less (including 0%), a martensite phase in an amount of 25% or more
Implementation Method 2
the relationship Ti>4N is satisfied, and a microstructure including, in terms of area ratio in a cross section located at 1/4 of the thickness from the surface of a base steel sheet
Implementation Method 3
High-strength galvanized steel sheet and method for manufacturing the same
Data Source
AI summary
A high-strength galvanized steel sheet having a chemical composition containing, by mass %, C: 0.07% to 0.25%, Si: 0.01% to 3.00%, Mn: 1.5% to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 1.50%, N: 0.001% to 0.008%, Ti: 0.003% to 0.200%, B: 0.0003% to 0.0050%, and the balance being Fe and inevitable impurities, in which the relationship Ti>4N is satisfied, and a microstructure including, in terms of area ratio in a cross section located at ¼ of the thickness from the surface of a base steel sheet, a ferrite phase in an amount of 70% or less (including 0%), a bainite phase in an amount of 20% or less (including 0%), a martensite phase in an amount of 25% or more, and a retained austenite phase in an amount of less than 3% (including 0%), in which the average crystal grain diameter of the martensite phase is 20 μm or less, and in which a variation in the Vickers hardness of the martensite phase is 20 or less in terms of standard deviation, as well as a method for manufacturing the steel sheet, is disclosed.