High-Si Hot-Forming Steel Sheet for Scale Adhesion Control
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Solution Overview
Problem
Existing methods for hot press forming of steel sheets face challenges in reliably suppressing scale shedding/peeling, especially under fluctuating heating conditions, leading to surface scratches and die contamination, and result in variability in hardness and appearance of the formed steel members.
Innovation Solution
A steel sheet composition with a high Si content and controlled surface-layer oxygen concentration, combined with optimized pickling and heating conditions, to enhance internal oxide formation and adhesion, ensuring reliable scale adhesion during hot press forming without restricting forming conditions to a narrow range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a steel sheet with enhanced strength is subjected to cold working, then high strength is achieved, but press forming force increases and dimensional accuracy degrades
Solution Approach 1:
The patent applies phase transition by heating the steel sheet to austenite phase before forming, then rapidly cooling it during die pressing to transform to martensite phase. This phase transition enables the steel sheet to be formed at high temperature with low force, and then hardened to high strength through quenching, resolving the contradiction between strength and formability.
Solution Approach 2:
The patent changes the temperature parameter from room temperature (cold working) to high temperature (hot press forming). By performing forming at elevated temperature where the steel sheet has superior ductility and formability, then cooling to achieve high strength, the contradiction between strength enhancement and manufacturing precision is resolved.
2Strength
If hot press forming is performed to achieve high strength and good dimensional accuracy, then forming force is reduced, but scale forms on the steel sheet surface and adheres to dies
Solution Approach 1:
The patent applies local quality by creating a surface layer with different composition (higher Si and Al content) compared to the base material. This surface layer specifically suppresses scale formation during heating, while the base material maintains the required mechanical properties. The local modification at the surface resolves the scale adhesion problem without compromising overall strength.
Solution Approach 2:
The patent applies preliminary action by pre-forming a protective surface layer with high Si and Al content before the hot press forming process. This surface layer is created during steel sheet production through controlled rolling and cooling, providing scale resistance before the steel sheet undergoes hot forming, thus preventing die contamination from the outset.
3Strength
If Si content is increased to enhance strength and suppress self-tempering, then hardness variability is reduced, but scale generation increases during heating
Solution Approach 1:
The patent resolves this contradiction by creating a local quality difference: the surface layer has high Si and Al content to suppress scale formation, while the base material has the required Si content for strength and hardness stability. This spatial differentiation allows both functions to coexist without conflict.
Solution Approach 2:
The patent effectively creates a composite structure with a surface layer and base material having different compositions. The surface layer (high Si-Al) provides oxidation resistance, while the base material provides mechanical strength. This composite approach allows the steel sheet to simultaneously achieve scale resistance and hardness stability.
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 effectively suppresses scale shedding/peeling, achieving high-strength steel members with excellent appearance and reduced die contamination, while maintaining flexibility in hot press forming conditions, and allows for subsequent welding and coating without issues.
Implementation Method 1
forming a steel sheet in a high-temperature state, using a punch and/or a die, and maintaining cooling at a forming bottom dead point, to elicit quenching through removal of heat from the steel sheet to the die, and harden thus the material
Implementation Method 2
eliciting quenching through removal of heat from the steel sheet to the die
Data Source
AI summary
Provided is a method for manufacturing a steel member through hot press forming using a steel sheet that contains a comparatively large amount of Si. The steel sheet includes, in mass ratio, C: 0.15% to 0.35%, Si: 1.0% to 3.0%, Mn: 1.0% to 3.0%, Al: more than 0% up to 0.10%, Ti: ([N]×48/14)% to 0.10% (where [N] denotes the amount of N in the steel sheet), B: 5 ppm to 50 ppm, P: more than 0% to less than 0.015%, S: more than 0% up to 0.010%, and N: more than 0% up to 0.010%, the balance being iron and unavoidable impurities. The average oxygen concentration from an outermost surface of the steel sheet down to a depth of 10 μm in a sheet thickness direction is 0.70 mass % or higher.