Hot-Stamping Steel Sheet Coating for Faster Heating and Spot Welding
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Solution Overview
Problem
The hot stamping method for steel sheets requires improvement in productivity, as existing techniques involve lengthy heating times and the elimination of organic materials at high temperatures, affecting the efficiency and spot weldability of the process.
Innovation Solution
A steel sheet with a surface-treated film containing carbon black and specific metal oxides, such as Zr, Zn, and Ti, is developed, which increases the emissivity and heating rate, allowing for quicker heating and improved spot weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a steel sheet is heated to Ac3 point or higher for hot stamping, then the material can be transformed into austenite and softened for easy molding, but the heating time becomes lengthy which reduces productivity
Solution Approach 1:
The patent applies parameter changes by modifying the surface properties of the steel sheet through coating with carbon black and metal oxides. This changes the emissivity parameter of the steel sheet surface, enabling more efficient heat absorption and reducing the heating time required to reach Ac3 transformation temperature, thus resolving the contradiction between ease of manufacture and productivity
Solution Approach 2:
The patent replaces conventional heating methods with radiative heating by optimizing the surface coating properties. The carbon black and metal oxide coating creates a surface that efficiently absorbs radiant heat energy, substituting traditional conductive or convective heating mechanisms and significantly reducing heating time while maintaining the austenite transformation capability
2Ease of manufacture
If organic materials such as carbon pigments are eliminated at high temperature of 750°C or more, then the heating process can be completed, but the productivity is reduced due to the extended heating time required
Solution Approach 1:
The patent changes the chemical composition parameters of the surface coating by using carbon black combined with specific metal oxides (Zr, Zn, Ti). This composition modification allows the coating to remain stable at high temperatures while maintaining high emissivity, enabling faster heating completion without the need to eliminate carbon-based materials, thus improving productivity
3Weight of moving object
If the sheet thickness of the steel sheet is reduced to decrease weight, then the weight of vehicle body can be reduced, but the strength of the structure is lowered
Solution Approach 1:
The patent utilizes phase transitions by heating the steel sheet to Ac3 transformation temperature to convert the microstructure into austenite, then rapidly cooling it during hot stamping to form martensite. This phase transition process enables thin steel sheets to achieve high strength properties, resolving the contradiction between weight reduction and strength maintenance
Solution Approach 2:
The patent applies composite materials by combining carbon black with specific metal oxides (ZrO2, ZnO, TiO2) in the surface coating. This composite coating provides both high emissivity for efficient heating and stability at high temperatures, enabling the thin steel sheet to be properly heat-treated to achieve desired strength levels
4Productivity
If a surface-treated film with high emissivity is applied to increase heating rate, then productivity can be improved, but the appearance and spot weldability may be affected
Solution Approach 1:
The patent optimizes the compositional parameters of the surface coating by selecting specific metal oxides (Zr, Zn, Ti) in controlled amounts. This parameter optimization ensures the coating provides high emissivity for improved heating rate while maintaining sufficient metal content to conduct heat to the base steel during spot welding, thus preserving weldability
Solution Approach 2:
The patent uses a thin surface coating layer that serves its primary function of heat absorption during the heating phase. The coating is designed to be thin enough to allow heat penetration to the base material for welding, effectively serving as a temporary functional layer that fulfills its purpose without compromising the underlying steel's weldability
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 enhances productivity, appearance, and spot weldability by increasing the heating rate and maintaining performance even at high temperatures, reducing the heating time and improving the mechanical strength of the steel sheet.
Implementation Method 1
a surface-treated film whose emissivity at a wavelength of 8.0 μm at 25° C. is 60% or more
Implementation Method 2
the surface-treated film contains carbon black, and one or more of oxides selected from a group consisting of a Zr oxide, a Zn oxide, and a Ti oxide
Data Source
AI summary
To provide a steel sheet for hot stamping that further improves productivity of a hot-stamped member, has excellent appearance, and has excellent spot weldability, and a hot-stamped member. A steel sheet for hot stamping according to the present invention includes, on the whole of at least one surface of the steel sheet, a surface-treated film whose emissivity at a wavelength of 8.0 μm at 25° C. is 60% or more, in which the surface-treated film contains carbon black, and one or more of oxides selected from a group consisting of a Zr oxide, a Zn oxide, and a Ti oxide, in which the carbon black and the oxides exist while being dispersed over the entire surface-treated film, the surface-treated film has a silica content of 0 to 0.3 g/m2, and when an adhesion amount of the carbon black and an adhesion amount of the oxides are set to XCB (g/m2) and XOxide (g/m2), respectively, an equation (1) is satisfied.