Coated Steel Sheet Microstructure for Strength and Bendability
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Solution Overview
Problem
Existing high strength press-hardened steel parts face challenges in achieving a combination of high mechanical strength, impact resistance, and good bendability, particularly due to limitations in microstructural composition and manufacturing processes that affect weldability, coatability, and bendability.
Innovation Solution
A coated steel sheet with a specific composition and microstructure, including a decarburized layer with a ferrite layer and a bulk microstructure of ferrite and martensite-austenite islands, combined with an aluminum coating, is produced through controlled annealing and hot forming to achieve tensile strength above 1350 MPa and a bending angle greater than 70°.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thin ferrite layer is formed in the substrate to inhibit crack propagation, then corrosion resistance is improved, but bendability deteriorates with bending angle lower than 70°
Solution Approach 1:
The patent applies local quality by creating distinct microstructural zones with different properties: a bulk region with 60-90% ferrite for overall strength and corrosion resistance, and a surface decarburized layer (1-100 μm) with refined ferrite structure for improved bendability. This spatial differentiation of material properties allows simultaneous achievement of high corrosion resistance and bendability above 70°.
2Strength
If the steel sheet is annealed in an atmosphere with dew point temperature from 50°C to 90°C to achieve high strength and bendability, then mechanical properties are improved, but the aluminum alloy coating is damaged
Solution Approach 1:
The patent resolves the contradiction by changing the annealing atmosphere parameter from high dew point (50-90°C) to low dew point (-10 to -50°C). This parameter change prevents aluminum alloy coating oxidation while still achieving the desired microstructure (60-90% ferrite in bulk with decarburized surface layer) through controlled carbon diffusion at the surface.
3Strength
If high carbon content steel is used to achieve tensile strength above 1350 MPa, then strength is improved, but weldability and coatability deteriorate
Solution Approach 1:
The patent applies local quality by creating a carbon gradient through the decarburized surface layer (1-100 μm thickness). The bulk material maintains high carbon content (0.20-0.50% C) for tensile strength above 1350 MPa, while the surface layer has reduced carbon content for improved weldability and coatability. This spatial variation in carbon concentration allows simultaneous achievement of high strength and manufacturability.
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 steel parts with enhanced mechanical properties, including a tensile strength of 1350 MPa and a bending angle of over 70°, while maintaining good weldability and coatability, addressing the limitations of previous technologies.
Implementation Method 1
the cold rolled steel sheet is annealed in an atmosphere with a dew point temperature comprises from 50° C. to 90° C.
Implementation Method 2
a decarburized layer comprising in upper part a ferrite layer having a thickness from 1 μm to 100 μm
Implementation Method 3
heating a steel material to a temperature at which an austenite single phase may be formed
Implementation Method 4
quenching and hot forming thereof using a mold
Data Source
AI summary
A coated steel sheet and press hardened steel part having a composition including, by weight percent: C 0.15-0.25%, Mn 0.5-1.8%, Si 0.1-1.25%, Al 0.01-0.1%, Cr 0.1-1.0%, Ti 0.01-0.1%, B 0.001-0.004%, P≤0.020%, S≤0.010%, N≤0.010% the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The press hardened steel part includes a bulk having a microstructure including, in surface fraction, more than 95% of martensite and less than 5% of bainite, a coating layer at the surface of the steel part, a ferritic interdiffusion layer between the coating layer and the bulk, and a ratio between the ferritic grain width in the interdiffusion layer GWint over prior austenite grain size in the bulk PAGSbulk, satisfying following equation (GWint/PAGSbulk)−1≥30%.