High-Strength Steel Sheet With Soft Surface Layer for Bendability
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Solution Overview
Problem
Existing high strength steel sheets face issues with bendability due to variations in hardness and hardness gradients, which affect their suitability for automotive applications, particularly in forming complex shapes and avoiding notching effects.
Innovation Solution
The steel sheets are produced with a soft surface layer and a hard inner layer, where the soft layer has controlled average hardness, minimal hardness variation, and a gradual hardness transition zone, achieved through specific welding, hot rolling, and annealing processes to suppress micro-hardness variations and gradients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a soft layer is provided at the surface layer to improve bendability, then the tensile stress and compressive stress at the surface are eased and bendability is improved, but the hardness variation within the soft layer causes insufficient bendability in some cases
Solution Approach 1:
The patent applies local quality by creating a surface layer with specifically controlled hardness properties. The soft layer at the surface has lower hardness (HV 150-350) compared to the inner layer, and this hardness gradient is precisely controlled to ensure uniformity within the soft layer itself (hardness variation ≤ 50 HV). This local differentiation resolves the contradiction by providing the necessary softness for bendability while maintaining internal uniformity to prevent stress concentration.
Solution Approach 2:
The patent utilizes parameter changes by controlling the hardness values and their distribution across the layer thickness. The soft layer hardness is maintained at HV 150-350 with minimal variation (≤ 50 HV), while the inner layer has higher hardness (HV 300-500). This precise parameter control ensures that the soft layer provides adequate stress relief during bending without developing internal stress concentrations that would compromise bendability.
2Ease of operation
If the average hardness of the soft layer is lowered and thickness is increased to improve bending, then bendability is improved, but the transition zone between soft layer and hard layer creates stress concentration if hardness gradient is too steep
Solution Approach 1:
The patent resolves this contradiction through precise parameter control of the hardness gradient. The soft layer thickness is controlled at 10-50 μm with hardness HV 150-350, while the inner layer has hardness HV 300-500. The hardness gradient is carefully managed so that the transition is gradual enough to avoid stress concentration but steep enough to maintain the soft layer's bending benefits. The hardness variation within the soft layer is limited to ≤ 50 HV, ensuring a controlled transition zone.
Solution Approach 2:
The patent applies preliminary action by pre-establishing the hardness gradient and transition zone characteristics before the bending operation. The surface layer is processed to achieve the specific hardness distribution (HV 150-350 in the soft layer, HV 300-500 in the inner layer) and thickness (10-50 μm) in advance, so that when bending occurs, the stress distribution is already optimized to prevent concentration at the transition zone.
3Strength
If ultra high strength cold rolled steel sheet is used to achieve high tensile strength, then tensile strength is improved, but bendability deteriorates due to large tensile stress at the outer circumference during bending
Solution Approach 1:
The patent applies local quality by creating a dual-layer structure where the surface layer (10-50 μm thick) has lower hardness (HV 150-350) to accommodate bending stresses, while the inner layer maintains higher hardness (HV 300-500) for overall strength. This local differentiation allows the steel sheet to achieve both high tensile strength and improved bendability, as the soft surface layer absorbs the large tensile stresses during bending while the hard inner layer provides structural integrity.
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
This approach enhances bendability and ductility, allowing for the production of steel sheets with tensile strengths of 800 MPa or more, while minimizing notching effects and improving formability for automotive parts.
Implementation Method 1
the steel sheet has a soft surface layer and a middle part in sheet thickness, and, between the soft surface layer and the middle part in sheet thickness, has a hardness transition zone with an average hardness change in the sheet thickness direction of 5000 (ΔHv/mm) or less
Implementation Method 2
achieved through specific welding, hot rolling, and annealing processes to suppress micro-hardness variations and gradients
Data Source
Figure 1
Figure 2~3
AI summary
High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 µm and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of 0.60 time or less the average Vickers hardness of the sheet thickness 1/2 position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.