Flat Steel Ductile Surface Layer for Hot Forming
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Solution Overview
Problem
The production of high-strength or ultra-high-strength flat steel products for automotive components faces challenges in hot forming due to the risk of liquid metal embrittlement and limited ductility, which is exacerbated by the presence of metallic anti-corrosion coatings, and existing decarburization methods do not effectively address these issues for steels with C content above 0.1% by weight.
Innovation Solution
A method involving an annealing treatment in a continuous furnace under an atmosphere of 0.1-25 vol.% H2 and N2 with a controlled H2O/H2 ratio, achieving surface layer decarburization to create a ductile layer with enhanced ductility, which minimizes the risk of crack formation during forming and allows for subsequent corrosion protection without compromising mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel is heated above the melting temperature of the protective coating for hot forming, then the steel can be formed into high-strength components, but liquid metal embrittlement occurs when molten coating penetrates surface notches and reduces tensile and compressive strength
Solution Approach 1:
The patent applies preliminary decarburization action before hot forming by conducting a controlled annealing treatment in a continuous furnace with specific atmospheric composition (0.1-25 vol.% H2, H2O, and N2) and controlled H2O/H2 ratio. This creates a ductile surface layer with reduced carbon content (10-200 µm thick) that prevents liquid metal embrittlement during subsequent hot forming operations, allowing the steel to achieve high tensile and compressive strength without the harmful effects of molten coating penetration.
Solution Approach 2:
The patent creates a localized ductile surface layer with different properties from the core material. The surface layer has reduced carbon content and increased ductility compared to the underlying steel, forming a gradient structure where the surface layer prevents embrittlement while the core maintains high strength. This local quality differentiation allows the steel to withstand hot forming temperatures above the coating melting point without liquid metal embrittlement.
2Ease of operation
If conventional decarburization treatment is applied to reduce surface carbon content, then bending properties improve, but the method is not effective for steels with C content above 0.1% by weight and does not address hot forming applications with metallic coatings
Solution Approach 1:
The patent changes the parameters of the decarburization process by using a continuous furnace with a specifically controlled atmospheric composition (0.1-25 vol.% H2, H2O, and N2) and maintaining a controlled H2O/H2 ratio. The annealing is performed at temperatures of 600-1100°C for holding times of 10-360 seconds. These parameter changes enable effective decarburization of steels with C content above 0.1% by weight, creating a ductile surface layer that improves both bending properties and hot formability for applications with metallic coatings.
Solution Approach 2:
The patent employs a continuous annealing process in a continuous furnace, allowing the decarburization treatment to be applied continuously to steel strips or sheets as they pass through the furnace. This continuous action enables consistent creation of the ductile surface layer across the entire material, making the process suitable for high-volume production of hot-formable steel components with improved bending properties and adaptability to various carbon contents.
3Strength
If the steel has high carbon content (0.1-0.4% by weight) to achieve high strength, then the steel has limited ductility and tends to form cracks during forming, but reducing carbon content improves ductility and formability
Solution Approach 1:
The patent creates a localized ductile surface layer with reduced carbon content (10-200 µm thick) through controlled decarburization, while maintaining the high carbon content (0.1-0.4% by weight) in the core material. This spatial differentiation allows the surface layer to provide improved ductility and formability, preventing crack formation during forming operations, while the core retains high strength properties. The gradient structure resolves the contradiction between overall strength and local formability.
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 method produces a flat steel product with a 10-200 µm thick ductile surface layer, improving formability and strength, reducing the risk of embrittlement and crack formation, while enabling efficient integration into continuous manufacturing processes like hot-dip coating systems.
Implementation Method 1
a method in which a flat steel product with a C content of 0.1 - 0.4% by weight is subjected to an annealing treatment in a continuous furnace, thereby removing carbon from a surface layer of the flat steel product
Implementation Method 2
Due to its great mobility, the carbon dissolved in the lattice tends to effusion during heat treatment
Implementation Method 3
the flat steel product is subjected to an annealing treatment in a continuous furnace
Implementation Method 4
the respective steel strip is annealed at an annealing temperature of less than 780 °C
Data Source
Figure 1~3
Figure 4
AI summary
The invention relates to a method by means of which an easily deformable flat steel product having a carbon content of 0.1-0.4 wt % can be produced in a cost-effective manner. To this end, according to the invention, an annealing treatment is carried out under an annealing atmosphere comprising 0.1-25 vol % of H2, H2O, with the remainder being N2 and technically unavoidable impurities, and having a dew point of between -20°C and +60°C, wherein the ratio of H2O/H2 of the annealing atmosphere is no greater than 0.957. The flat steel product is thereby heated to a holding temperature of 600-1100°C in the course of the annealing treatment, at which it is held for a holding time of 10-360 s, so that the flat steel product obtained after the annealing treatment comprises a 10-200 µm thick ductile edge layer (R) adjacent to the free surface thereof, having a ductility that is greater than the interior core layer (K) of the flat steel product covered by the edge layer. The invention further relates to a correspondingly produced flat steel product particularly suitable for hot or cold forming, and to a method for producing components made of such a flat steel product.