Gradient Outer Coating for Delayed-Fracture-Resistant Press-Formed Steel
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Solution Overview
Problem
High strength steel sheets with a tensile strength of 1180 MPa or greater are prone to delayed fracture, particularly at the sheared end face, due to residual tensile stress and hydrogen brittleness, which existing technologies struggle to effectively inhibit.
Innovation Solution
Applying an outer coating with an organic resin and an inorganic material to the steel sheet or coated steel sheet, where the concentration of inorganic material in a specific region of the coating is higher than in another region, effectively reduces strains on the surface of the sheared end portion, thereby inhibiting cracking and delayed fracture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased to 1180 MPa or greater, then vehicle safety and CO2 emission reduction are improved, but delayed fracture resistance deteriorates due to residual tensile stress and hydrogen brittleness
Solution Approach 1:
The invention applies a gradient coating structure where the inorganic material concentration varies through the coating thickness. The first region (near steel sheet) has higher inorganic material concentration to provide strong adhesion and stress resistance at the critical interface, while the second region (outer surface) has lower inorganic material concentration to maintain flexibility and reduce hydrogen embrittlement susceptibility. This local quality variation resolves the contradiction between strength and delayed fracture resistance.
Solution Approach 2:
The invention uses a composite coating system combining organic resin and inorganic materials in a gradient structure. This composite approach allows the coating to simultaneously provide adhesion promotion, stress distribution, and hydrogen barrier properties, thereby maintaining high steel sheet strength while improving delayed fracture resistance through the synergistic effects of the composite material system.
2Object-affected harmful factors
If conventional coatings are applied to high strength steel sheets, then corrosion protection is provided, but delayed fracture resistance remains insufficient due to inadequate strain reduction on the sheared end face
Solution Approach 1:
The gradient coating structure concentrates inorganic material in the first region near the steel sheet surface, creating a localized zone of enhanced properties that effectively reduces strain on the sheared end face. This local quality enhancement at the critical steel-coating interface provides both corrosion protection and improved delayed fracture resistance, overcoming the limitations of conventional uniform coatings.
Solution Approach 2:
The invention changes the physical parameter of inorganic material concentration through the coating thickness, creating a gradient from high concentration at the steel interface to low concentration at the outer surface. This parameter change optimizes the coating's mechanical properties, enabling effective strain reduction on the sheared end face while maintaining corrosion protection.
3Reliability
If shot peening is applied to the sheared end face to improve delayed fracture resistance, then cracking is reduced, but the manufacturing process becomes more complex and costly
Solution Approach 1:
The gradient coating is applied to the steel sheet before shearing and forming operations. This preliminary action pre-reduces the strain on the sheared end face, preventing crack initiation before the component undergoes subsequent processing. By performing the protective action in advance, the invention eliminates the need for additional post-processing steps like shot peening, thereby reducing manufacturing complexity while maintaining delayed fracture resistance.
Solution Approach 2:
The gradient coating acts as an intermediary layer between the steel sheet and the external environment, mediating the stress and strain distribution at the sheared end face. The inorganic material-rich first region serves as a buffer zone that reduces strain concentration, effectively replacing the need for mechanical surface treatment processes like shot peening and simplifying the manufacturing process.
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 proposed solution significantly reduces the occurrence of cracking in the sheared end face of bent portions, effectively inhibiting delayed fractures in high strength steel sheets and coated steel sheets, even in the presence of rust-preventive oil or press oil.
Implementation Method 1
the delayed fracture is caused by a residual tensile stress and hydrogen brittleness of steel at a stress concentration area
Implementation Method 2
a concentration of the inorganic material in a region of the outer coating, which is a region extending to 20% of a thickness of the outer coating from a steel-sheet-side, is higher than a concentration of the inorganic material in another region of the outer coating
Data Source
AI summary
A member that is a press-formed product formed from a high strength steel sheet, and a coated steel sheet including a high strength steel sheet as a substrate, are disclosed.The steel sheet includes an outer coating disposed on a surface, and the outer coating includes an organic resin and an inorganic material. The steel sheet has a tensile strength of 1180 MPa or greater. In the outer coating, a concentration of the inorganic material in a region of the outer coating, which is a region extending to 20% of a thickness of the outer coating from a steel-sheet-side, is higher than a concentration of the inorganic material in another region of the outer coating, which is a region extending to 20% of the thickness from an outer-surface-side.