Coated High-Strength Steel Sheet Microstructure for Bendability
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Solution Overview
Problem
High-strength steel sheets with a tensile strength of 1180 MPa or higher face issues with delayed fracture resistance under atmospheric corrosion conditions in a coated state, particularly at bent portions and sheared end surfaces, which compromises their application in automotive parts.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure, including controlled carbon concentration, martensite content, and plastic deformation initiation stress, combined with a production process involving hot rolling, pickling, cold rolling, and controlled annealing, followed by coating or plating, to enhance strain dispersibility, ductility, and bendability while improving delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets with tensile strength of 1180 MPa or higher are used to reduce vehicle weight and improve crashworthiness, then strength and weight reduction are achieved, but delayed fracture resistance under atmospheric corrosion conditions deteriorates
Solution Approach 1:
The patent applies local quality by creating distinct microstructure regions: a first region near the surface with lower martensite content (5-30%) and higher ferrite/bainite content (70-95%), and a second region in the interior with higher martensite content (85-95%). This gradient structure provides different properties at different locations - the surface region resists hydrogen penetration and corrosion initiation, while the interior region provides high strength, thereby resolving the contradiction between strength and delayed fracture resistance.
Solution Approach 2:
The patent creates a composite microstructure within the steel sheet by combining multiple phases (martensite, ferrite, bainite) in specific proportions and distributions. The dual-region microstructure acts as an internal composite system where each region contributes different functional properties, enabling the material to simultaneously achieve high tensile strength and improved delayed fracture resistance under atmospheric corrosion conditions.
2Strength
If high-strength steel sheets are used for automotive structural parts, then vehicle weight reduction and crashworthiness improvement are achieved, but strain dispersibility and ductility are compromised
Solution Approach 1:
The patent uses local quality by creating a surface region with lower martensite content (5-30%) and higher ferrite/bainite content (70-95%). Ferrite and bainite phases provide better ductility and strain dispersibility compared to martensite. This surface region with improved ductility allows the steel sheet to maintain good strain dispersibility during forming operations while the interior region provides the required high strength.
3Strength
If high-strength steel sheets are used for automotive parts, then strength and weight reduction are achieved, but bendability at bent portions deteriorates
Solution Approach 1:
The patent applies local quality by creating a surface region with lower martensite content (5-30%) and higher ferrite/bainite content (70-95%). The ferrite and bainite phases in the surface region provide better ductility and bendability, allowing the steel sheet to be bent without excessive cracking or deformation. The interior region maintains high martensite content for strength, thus resolving the contradiction between strength and bendability.
4Object-affected harmful factors
If high-strength steel sheets are used in coated state for atmospheric corrosion environments, then protection against corrosion is achieved, but hydrogen penetration and delayed fracture risk increase at sheared end surfaces
Solution Approach 1:
The patent applies local quality by creating a surface region with lower martensite content (5-30%) and higher ferrite/bainite content (70-95%). This surface region acts as a protective layer that reduces hydrogen penetration from the coating defects and corrosive environment. The softer ferrite and bainite phases are less susceptible to hydrogen embrittlement, thereby protecting the sheared end surfaces and improving delayed fracture resistance while maintaining corrosion protection.
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 steel sheet achieves excellent strain dispersibility, ductility, and bendability, with enhanced resistance to delayed fracture under atmospheric corrosion conditions, making it suitable for automotive parts.
Implementation Method 1
at a 1/4 thickness position of the steel sheet... an area ratio of martensite is 70% or more
Implementation Method 2
a holding time in a temperature region T3 of 400°C or higher and lower than 500°C is 10 s or more and 150 s or less
Data Source
AI summary
Provided is a high-strength steel sheet that is excellent in all of strain dispersibility, ductility, bendability, and delayed fracture resistance under atmospheric corrosion conditions in the coated state. The steel sheet has a predetermined chemical composition and microstructure, where an average value [τq] of plastic deformation initiation stress τq measured by a nanoindentation method at the 1/4 thickness position of the steel sheet is 1.70 GPa or more and 3.30 GPa or less, standard deviation σq of the τq is 0.42 GPa or less, and when an average value of plastic deformation initiation stress τs measured by a nanoindentation method at the 10 µm position from the steel sheet surface is defined as [τs], the proportion of measurement points having values less than 0.85 × [τs] is 30.0 % or less.