High-Strength Steel Sheet Gradient for LME Crack Suppression
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Solution Overview
Problem
Existing steel sheets face issues with liquid metal embrittlement (LME) cracks during spot welding due to molten zinc penetration, particularly when high strain is applied, and existing technologies do not effectively address this problem in high-strength dual phase steels.
Innovation Solution
A three-layer steel sheet structure is developed with a strong surface layer, a soft intermediate layer, and a harder inner layer, achieved through precise control of chemical composition and manufacturing processes like hot rolling and annealing, to manage strain and prevent LME cracks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel sheet is made high strength to improve structural performance, then the strength increases, but the weldability deteriorates due to LME cracks during spot welding
Solution Approach 1:
The invention applies local quality by creating a three-layer microstructure with different properties at different depths: a hard surface layer (1st depth region) for strength, a soft intermediate layer (2nd depth region) for strain absorption, and a hard core layer (3rd depth region) for overall strength. This localized differentiation allows the surface to resist LME cracks while the intermediate layer manages deformation, resolving the contradiction between high strength and weldability
Solution Approach 2:
The invention employs composite material principles by integrating three distinct microstructural regions within a single steel sheet. The composite structure combines hard martensitic regions with softer intermediate zones, creating a multi-phase material system that simultaneously achieves high tensile strength (>1320 MPa) and improved weldability by preventing LME crack propagation
2Strength
If the surface layer is made harder to increase strength, then the strength increases, but the strain concentration increases leading to more LME cracks
Solution Approach 1:
The invention implements beforehand cushioning by placing a soft intermediate layer (2nd depth region with Vickers hardness 150-350) between the hard surface layer and the core. This intermediate soft layer acts as a cushion that absorbs and distributes strain before it reaches the hard surface layer, preventing strain concentration and LME cracks while maintaining high overall strength
3Ease of manufacture
If a uniform microstructure is used to simplify production, then the manufacturing process is simpler, but the ability to suppress LME cracks is reduced
Solution Approach 1:
The invention applies parameter changes by controlling microstructural parameters (hardness, phase composition, grain size) at different depth regions through precise heat treatment parameters. By varying temperature, time, and atmosphere during annealing, the invention creates the desired three-layer structure with specific hardness gradients, achieving LME crack suppression through parameter optimization rather than complex mechanical processing
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 structured steel sheet exhibits high strength and improved weldability by minimizing strain at the surface, effectively reducing the occurrence of LME cracks during spot welding.
Implementation Method 1
The inventors discovered the method of imparting a difference in strength in a thickness direction so as to prevent an increase in strain at the surface layer of steel sheet. Specifically, they strongly controlled the surface-most layer (first depth region) by precipitation strengthening
Implementation Method 2
a step of cold rolling the hot rolled steel sheet, then annealing it
Data Source
AI summary
Provided is a steel sheet having a predetermined chemical composition, wherein precipitates having a diameter of less than 0.1 μm are present in a number density of 10 to 200/μm2 in a depth region of 1 to 10 μm from a surface, an amount of dissolved C in a depth region of 10 to 60 μm from the surface is less than 0.20 mass %, and a tensile strength is 1200 MPa or more. Further, provided is a method for producing a steel sheet comprising a step of hot rolling a steel slab having a predetermined chemical composition, then coiling it at 580° C. or less, a step of pickling the hot rolled steel sheet to remove oxide scale and remove the surface layer of the hot rolled steel sheet down to at least 5 μm, and a step of cold rolling the hot rolled steel sheet, then holding it in an atmosphere of a dew point of −20 to 20° C. at a temperature region of 200 to 400° C. for 20 to 180 seconds and holding it at a temperature region of 740 to 900° C. for 40 to 300 seconds.