High-Strength Steel Sheet Surface Ferrite Layer for Bendability
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Solution Overview
Problem
Current high-strength steel sheets with tensile strengths above 1320 MPa face challenges in achieving both excellent bendability and fatigue properties, as existing methods either fail to sufficiently suppress micro cracks or significantly reduce fatigue strength by softening the surface layer.
Innovation Solution
A high-strength steel sheet with a chemical composition containing specific elements and a steel structure optimized to have a high area fraction of martensite and bainite, along with controlled ferrite distribution and hardness, is developed, allowing for improved bendability and fatigue properties through continuous annealing and overaging treatment processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel structure is set to contain martensite or bainite as main constituents to achieve high tensile strength, then tensile strength increases to 1320 MPa or more, but bendability deteriorates due to lower degree of elongation
Solution Approach 1:
The invention applies local quality by creating a dual-layer structure: a surface layer region (extending 5 μm or less from the surface) with a steel structure containing 5-30% ferrite for improved bendability, and a base material region with a steel structure containing 95-100% martensite and/or bainite for high strength. This allows different regions to have optimized properties for their specific functions.
Solution Approach 2:
The invention segments the steel sheet into two distinct regions based on depth from the surface: the surface layer region (0-5 μm) and the base material region (deeper than 5 μm). Each region has a controlled steel structure composition achieved through specific chemical composition ranges and heat treatment processes, allowing independent optimization of bendability and strength.
2Difficulty of detecting and measuring
If visual inspection is used to check cracks in bend tests, then detection is simple, but micro cracks of 1 mm or less are not detected and fatigue properties are degraded
Solution Approach 1:
The invention applies preliminary action by controlling the chemical composition and steel structure in advance to prevent micro crack formation during bending. By optimizing the surface layer region to contain 5-30% ferrite and controlling inclusion distribution, the steel sheet inherently resists micro crack initiation, eliminating the need for post-bending inspection while ensuring fatigue properties.
3Ease of operation
If the hardness of the surface layer region is reduced to 80% or less of the base material to improve bendability, then bendability improves, but fatigue strength is significantly reduced
Solution Approach 1:
The invention applies parameter changes by precisely controlling the chemical composition parameters (C: 0.10-0.40%, Si: 0.01-1.0%, Mn: 0.50-2.50%, etc.) and heat treatment parameters (cooling rate, tempering temperature) to achieve the desired steel structure distribution. This allows the surface layer to have reduced hardness for bendability while the base material maintains high hardness for fatigue strength.
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 achieves a high-strength steel sheet with both excellent bendability and fatigue properties, effectively suppressing micro cracks and maintaining fatigue strength, as demonstrated by the achieved tensile strength of 1320 MPa or more and favorable endurance ratios.
Implementation Method 1
continuous annealing and overaging treatment processes
Implementation Method 2
continuous annealing and overaging treatment processes
Data Source
AI summary
A high-strength steel sheet according to the present invention includes a specific chemical composition, and a steel structure in which a total area fraction of martensite and bainite in a position of ¼ of a sheet thickness is 95% or more and 100% or less, the balance in a case where the total area fraction is not 100% contains retained austenite, and an area fraction of ferrite in a region extending up to 10 μm in a sheet thickness direction from a surface is 10% or more and 40% or less, in which a tensile strength is 1320 MPa or more, and a Vickers hardness in a position of 15 μm in the sheet thickness direction from the surface satisfies a specified formula.