High-Strength Steel Sheet with Edge Cracking Resistance and Bendability
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Solution Overview
Problem
Existing high strength steel sheets with a tensile strength of 1310 MPa or higher face challenges in maintaining a stable sheared edge due to edge cracking during shearing and bending processes, which affects tool life and subsequent press forming, and current technologies are insufficient in addressing this issue.
Innovation Solution
A steel sheet with a specified chemical composition and microstructure, including controlled inclusion and carbide densities, is produced through precise slab heating and rolling processes to achieve excellent edge cracking resistance and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheet with tensile strength of 1310 MPa or higher is used, then strength is improved, but edge cracking resistance deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters by strictly controlling C content to 0.12-0.40%, Si to 0.01-1.5%, Mn to 0.2-1.7%, and adding specific amounts of Nb (0.010-0.050%) and Ti (0.010-0.050%). These parameter changes create a microstructure with martensite area ratio of 90-100% that achieves both high strength (1310 MPa or higher) and excellent edge cracking resistance by optimizing the balance between strength and ductility at the microstructural level
Solution Approach 2:
The invention creates a composite microstructure consisting primarily of martensite (90-100% area ratio) with controlled inclusions of Nb and Ti carbides/nitrides. This composite microstructure combines the high strength of martensite with the crack-arresting capability of finely dispersed precipitates, resolving the contradiction between achieving high tensile strength and maintaining edge cracking resistance
2Strength
If high strength steel sheet with tensile strength of 1310 MPa or higher is used, then strength is improved, but bendability deteriorates
Solution Approach 1:
The invention optimizes chemical composition parameters including C (0.12-0.40%), Si (0.01-1.5%), Mn (0.2-1.7%), and adds Nb (0.010-0.050%) and Ti (0.010-0.050%) to create a martensitic microstructure with 90-100% area ratio. This parameter optimization achieves tensile strength of 1310 MPa or higher while maintaining bendability through controlled microstructure that reduces internal stress concentration during bending
Solution Approach 2:
The invention applies local quality by creating a uniform martensitic microstructure throughout the steel sheet with consistent distribution of Nb and Ti precipitates. This uniform local microstructure ensures that the material properties are homogeneous, allowing the steel to achieve both high strength and good bendability without localized weak points that would cause cracking during forming operations
3Productivity
If conventional steel sheet production method is used, then productivity is maintained, but manufacturing precision deteriorates due to edge cracking
Solution Approach 1:
The invention applies preliminary action by pre-optimizing the chemical composition and microstructure of the steel sheet before shearing operations. By controlling C content to 0.12-0.40%, adding Nb (0.010-0.050%) and Ti (0.010-0.050%), and ensuring martensite area ratio of 90-100%, the steel sheet is prepared in advance to resist edge cracking during shearing, thereby maintaining both productivity and manufacturing precision without requiring post-processing or adjusted shearing parameters
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 a tensile strength of 1310 MPa with reduced edge cracking and improved bendability, suitable for cold press forming, enhancing part strength and weight reduction in automotive applications.
Implementation Method 1
a steel microstructure containing martensite at an area ratio of 70% or more, bainite at an area ratio of 30% or less, and ferrite and retained austenite at a total area ratio of 5% or less
Implementation Method 2
a slab heating step that involves heating a slab having the chemical composition under such conditions that an average heating rate in a slab surface temperature range from 300°C to 1220°C is 0.10°C/s or more and that an average temperature ratio (Tc/Ts) of a slab center temperature Tc to a slab surface temperature Ts in this temperature range is 0.85 or less
Implementation Method 3
at a 1/4 thickness position of the steel sheet, a number density of carbides having long axes of 0.5 μm or more is 60000 carbides/mm2 or less
Data Source
AI summary
A steel sheet has a tensile strength of 1310 MPa or higher, a specified chemical composition, and a steel microstructure containing martensite at an area ratio of 70% or more, bainite at an area ratio of 30% or less, and ferrite and retained austenite at a total area ratio of 5% or less, in which, at a ¼ thickness position of the steel sheet, a number density of carbides having long axes of 0.5 μm or more is 60000 carbides/mm2 or less, in a ¼-to-¾ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 10 grains/mm2 or more and 30 grains/mm2 or less, and, in a surface-to-¼ thickness region of the steel sheet, a number density of inclusion grains having equivalent circle diameters of 4.0 μm or more is 27 grains/mm2 or less.