Coated Press-Hardened Steel With Ferrite Layer for 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, while also ensuring corrosion resistance, which are crucial for automotive applications, particularly in reducing vehicle weight for improved fuel efficiency and safety.
Innovation Solution
A steel sheet composition with specific carbon, manganese, silicon, aluminum, chromium, titanium, boron, and optional elements, combined with a controlled annealing process to form a decarburized layer and interdiffusion layer, enabling a microstructure that includes a ferrite layer and martensite-austenite islands, followed by hot forming and die-quenching to achieve a 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 at the surface 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 layers with different properties: a thin ferrite layer (5-50 μm) at the surface for corrosion resistance, and a bulk microstructure containing martensite-austenite islands for high strength. The ferrite layer thickness is precisely controlled to be sufficient for crack inhibition but thin enough to maintain bendability above 70°.
Solution Approach 2:
The patent uses composite materials by combining multiple microstructural phases (ferrite, martensite, austenite) within the steel sheet. The base steel contains martensite-austenite islands in a ferritic matrix, and the surface develops a ferrite-rich layer during annealing, creating a composite microstructure that simultaneously achieves high strength (1300-1500 MPa) and good bendability.
2Strength
If high carbon content is used to achieve high tensile strength above 1350 MPa, then mechanical strength is improved, but weldability and bendability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the carbon content within a narrow range (0.18-0.23%) rather than using high carbon content. This moderate carbon level, combined with specific amounts of alloying elements (Mn: 1.00-1.80%, Si: 0.10-1.25%, Cr: 0.10-1.00%), achieves the target tensile strength of 1350-1500 MPa while maintaining acceptable weldability and bendability.
Solution Approach 2:
The patent uses composite materials by creating a multi-phase microstructure with martensite-austenite islands dispersed in a ferritic matrix. This composite microstructure, along with the surface ferrite layer, enables high strength through martensite while the ferrite phases maintain ductility and weldability, avoiding the brittleness associated with high carbon steels.
3Reliability
If aluminium alloy coating is applied to ensure corrosion resistance, then corrosion resistance is improved, but the annealing atmosphere control becomes more critical to avoid coating damage
Solution Approach 1:
The patent applies parameter changes by precisely defining the annealing atmosphere dew point temperature range (-10°C to +20°C) and annealing temperature range (700-850°C). These controlled parameters enable the formation of the protective ferrite layer and the desired microstructure while preventing aluminium alloy coating degradation, oxidation, or excessive intermetallic compound formation.
Solution Approach 2:
The patent uses an intermediary approach by employing a controlled annealing atmosphere that acts as a mediator between the steel sheet and the aluminium alloy coating. The atmosphere composition (dew point control) prevents direct harmful interaction between the coating and the steel surface during annealing, allowing the ferrite layer to form protectively without coating damage.
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 yield strength of at least 1000 MPa and a bending angle exceeding 70°, while maintaining corrosion resistance, thus supporting weight reduction in vehicles without compromising safety.
Implementation Method 1
a decarburized layer comprising in upper part a ferrite layer having a thickness from 1μm to 100 μm
Implementation Method 2
The steel sheet is annealed in an atmosphere with a dew point temperature strictly higher than -10°C and below or equal to +20°C
Implementation Method 3
heating a steel material to a temperature at which an austenite single phase may be formed, and quenching and hot forming thereof
Implementation Method 4
a base steel sheet having a microstructure changing in the thickness direction, with a soft layer made of at least 90% of ferrite, a transition layer made of ferrite and martensite and a hard layer mainly martensitic
Data Source
Figure 1a~1b
Figure 2a~2b
Figure 3a~3b
AI summary
The invention deals with a coated steel sheet and press hardened steel part having a composition comprising, 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 comprises a bulk having a microstructure comprising, 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%.