Hot-dip Galvanized Steel Sheet Edge Cracking Resistance
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Solution Overview
Problem
High-strength steel sheets for automotive components face challenges in delayed fracture resistance, particularly at sheared edge surfaces, due to hydrogen penetration and corrosive environments, which existing technologies have not adequately addressed.
Innovation Solution
A high-strength hot-dip galvanized steel sheet with a specific composition (C: 0.12% to 0.35%, Si: 0.01% to 3.0%, Mn: 2.0% to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 1.50%) and microstructure (90% to 100% martensite and carbide-containing bainite, prior austenite grains with an aspect ratio of 2.0 or less) is developed, along with a production method involving hot rolling, pickling, heat treatment, cold rolling, annealing, and hot-dip galvanizing, to enhance edge cracking resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased to improve crashworthiness and fuel economy, then the strength parameter is improved, but delayed fracture resistance deteriorates due to hydrogen penetration from corrosive environments
Solution Approach 1:
The patent applies local quality by creating a non-uniform carbon distribution within the steel sheet thickness. Specifically, the carbon concentration at the surface layer (position 5 μm from surface) is controlled to be 0.20 to 0.80 times the carbon concentration at the center layer (position 70 μm from surface). This gradient structure provides higher carbon content at the surface to prevent hydrogen penetration and delayed fracture, while maintaining appropriate strength in the interior region.
Solution Approach 2:
The patent employs parameter changes by precisely controlling multiple compositional and microstructural parameters simultaneously. The carbon content is maintained at 0.12% to 0.35%, silicon at 0.01% to 3.0%, and manganese at 2.0% to 4.0%. Additionally, the microstructure is controlled to contain 90% to 100% martensite and carbide-containing bainite with prior austenite grains having an aspect ratio of 2.0 or less. These parameter adjustments collectively achieve both high strength and improved delayed fracture resistance.
2Strength
If the strength of steel sheets is increased, then the strength parameter is improved, but edge cracking resistance at sheared edge surfaces deteriorates
Solution Approach 1:
The patent applies local quality by creating a non-uniform carbon distribution within the steel sheet thickness. Specifically, the carbon concentration at the surface layer (position 5 μm from surface) is controlled to be 0.20 to 0.80 times the carbon concentration at the center layer (position 70 μm from surface). This gradient structure provides higher carbon content at the surface to prevent hydrogen penetration and delayed fracture, while maintaining appropriate strength in the interior region.
Solution Approach 2:
The patent employs parameter changes by precisely controlling multiple compositional and microstructural parameters simultaneously. The carbon content is maintained at 0.12% to 0.35%, silicon at 0.01% to 3.0%, and manganese at 2.0% to 4.0%. Additionally, the microstructure is controlled to contain 90% to 100% martensite and carbide-containing bainite with prior austenite grains having an aspect ratio of 2.0 or less. These parameter adjustments collectively achieve both high strength and improved delayed fracture resistance.
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 edge cracking resistance, with no cracks larger than 1 mm forming after immersion in hydrochloric acid and tensile stress, demonstrating improved strength and resistance to delayed fracture from sheared edge surfaces.
Implementation Method 1
a high-strength hot-dip galvanized steel sheet
Implementation Method 2
a steel microstructure containing, on the basis of an area percentage, 90% to 100% of martensite and carbide-containing bainite
Implementation Method 3
the ratio of the average amount of C at a position 5 μm from a surface layer of the steel sheet in the thickness direction to the average amount of C at a position 70 μm from the surface layer of the steel sheet in the thickness direction is 0.20 to 0.80
Data Source
AI summary
A high-strength hot-dip galvanized steel sheet includes a hot-dip galvanized layer on a surface of the steel sheet and has a specific component composition and a steel microstructure containing, on an area percentage basis, 90% to 100% of martensite and carbide-containing bainite in total and 0% to 10% of retained austenite, and containing prior austenite grains having an aspect ratio of 2.0 or less, in a region extending from 300 μm to 400 μm from the surface layer, in which the ratio of the average amount of C at 5 μm from the surface layer to the average amount of C at 70 μm from the surface layer is 0.2 to 0.8, and the ratio of the standard deviation of the amount of C to the average amount of C in a region extending from 300 μm to 400 μm from the surface layer is 0.40 or less.