Galvanized Steel Sheet Dual-Layer Microstructure for Bendability
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Solution Overview
Problem
High strength galvanized steel sheets with tensile strength of 980 MPa or larger face challenges in achieving excellent bendability and weldability, particularly in applications requiring severe bending, as existing technologies fail to maintain fatigue strength and plane-bending fatigue properties while ensuring sufficient strength and workability.
Innovation Solution
A high strength galvanized steel sheet composition with a surface layer containing more than 70% ferrite phase up to 10 μm depth and an inner layer with 20-70% ferrite phase, along with specific elements like C, P, S, Si, Mn, Al, Cr, Mo, Ti, and Nb, optimized through a two-stage annealing process and hot rolling, to achieve improved bendability and weldability without compromising tensile strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of the steel sheet is increased to 980 MPa or larger, then the collision safety and structural strength are improved, but the workability and bendability deteriorate, causing fracturing during press forming
Solution Approach 1:
The steel sheet employs a dual-layer microstructure: a surface layer (0-10 μm depth) with >70% ferrite phase for ductility and bendability, and an inner layer with 20-70% ferrite phase for strength. This local differentiation allows the surface to deform plastically during bending while the core maintains high tensile strength, resolving the contradiction between workability and strength.
Solution Approach 2:
The invention creates a composite microstructure within the steel sheet by combining two distinct phase compositions: a soft ferrite-rich surface layer and a stronger mixed-phase inner layer. This internal composite structure enables the material to exhibit both high strength and good formability, as the ferrite phase provides ductility while the combined structure maintains overall strength at 980 MPa or higher.
2Ease of operation
If the steel sheet is softened in the surface layer to improve bendability, then the workability is improved, but the fatigue strength deteriorates
Solution Approach 1:
The surface layer is selectively softened with >70% ferrite phase to enable excellent bendability and a critical bend radius of 0.3t or less, while the inner layer maintains 20-70% ferrite phase to preserve fatigue strength and overall structural integrity. This localized property differentiation resolves the contradiction between bendability and fatigue strength.
3Shape
If the steel sheet is subjected to severe bending process, then the complex shape formation is achieved, but the fracturing occurs due to poor workability
Solution Approach 1:
The surface layer with >70% ferrite phase provides the necessary ductility and plastic deformability to undergo severe bending and form complex shapes without fracturing, while the inner layer maintains strength. This microstructure enables the steel sheet to achieve complex geometries with a critical bend radius of 0.3t or less.
4Strength
If the steel sheet requires resistance spot welding in the assembly process, then the structural integrity is maintained, but the weldability becomes challenging due to high strength
Solution Approach 1:
The surface layer with >70% ferrite phase and controlled composition (C: 0.05-0.12%, Si: 0.01-1.6%, Mn: 2.0-3.5%) provides good weldability by reducing hardening tendency and improving ductility during welding, while the overall high strength of 980 MPa or more is maintained through the combined dual-layer structure and alloying elements.
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 steel sheet exhibits excellent bendability with a critical bend radius not exceeding 0.3t and good weldability, as evidenced by base material fracture at a nugget diameter of 4t1/2 mm or larger, while maintaining a durability ratio of 0.35 or higher, thus suitable for severe bending applications.
Implementation Method 1
the steel sheet has been subjected to alloy annealing treatment subsequent to hot-dip galvanization
Implementation Method 2
a steel sheet surface layer, constituting a portion of the steel sheet up to a depth of 10 μm measured from each surface of the steel sheet, has a structure containing more than 70% of ferrite phase by a volume fraction
Data Source
AI summary
A method of manufacturing a galvanized steel sheet includes a two-stage temperature raising process which includes: primary heating the sheet from 200° C. to an intermediate temperature of 500 to 800° C. at a primary average heating rate of 5 to 50° C./second at an excess air ratio of 1.10 to 1.20 maintained up to the intermediate temperature; secondary heating the sheet from the intermediate temperature to an annealing temperature of 730 to 900° C. at a secondary average heating rate of 0.1 to 10° C./second at an excess air ratio of less than 1.10 maintained up to the annealing temperature; holding the sheet to the annealing temperature for 10 to 500 seconds; cooling the sheet to 450 to 550° C. at an average cooling rate of 1 to 30° C./second; and subjecting the sheet to a galvanizing process and, optionally, an alloying process.