High-Strength Hot-Dip Galvanized Steel Sheet for Spot Weldability
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Solution Overview
Problem
High-strength galvanized steel sheets face issues with spot weldability due to microcracks caused by liquid metal embrittlement during welding, which is exacerbated by the high tensile strength and yield strength of the steel, leading to zinc penetration and material destruction.
Innovation Solution
Control the difference in Mn/Si values between surface and internal oxides on the steel sheet by adjusting the reheating, rolling, annealing, and plating processes to inhibit microcrack formation, ensuring a Mn/Si difference of 0.5 or greater, typically between 0.8 to 1.5, to enhance weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel is used to reduce weight, then weight reduction is achieved, but spot weldability deteriorates due to microcrack formation and liquid metal embrittlement
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by strictly controlling the content ranges of alloying elements (C: 0.15-0.35%, Si: 1.00-2.50%, Mn: 1.50-4.00%, etc.) to achieve high strength while preventing microcrack formation during welding, thus resolving the contradiction between weight reduction and weldability
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (martensite, bainite, and retained austenite) with specific volume ratios (martensite: 30-80%, bainite: 10-50%, retained austenite: 3-20%) to simultaneously achieve high strength and improved spot weldability by reducing liquid metal embrittlement susceptibility
2Strength
If high-strength steel with high tensile strength is used, then strength is improved, but microcrack generation increases during welding due to inability to relieve tensile stress
Solution Approach 1:
The invention employs a composite microstructure with multiple phases (martensite for strength, bainite for toughness, and retained austenite for plasticity) that enables the steel to achieve high tensile strength (1320 MPa or more) while the retained austenite provides plastic deformation capability to relieve tensile stress during welding, preventing microcrack formation
Solution Approach 2:
The invention changes the microstructural parameters by controlling the volume ratios of different phases and the carbon equivalent content (0.35-0.55%) to achieve a balance between strength and crack resistance, allowing the steel to maintain high strength while reducing microcrack generation during welding
3Reliability
If zinc plating is applied to high-strength steel, then corrosion resistance is improved, but liquid metal embrittlement occurs during welding causing material destruction
Solution Approach 1:
The invention uses a composite microstructure with retained austenite (3-20% volume ratio) that reduces the susceptibility to liquid metal embrittlement, allowing zinc plating to be applied while maintaining corrosion resistance and preventing zinc penetration-induced embrittlement during spot welding of high-strength steel sheets
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 controlled Mn/Si oxide difference prevents microcrack generation, significantly improving spot weldability and reducing liquid metal embrittlement, while maintaining material integrity and corrosion resistance.
Implementation Method 1
reheating the slab to a temperature of 950 to 1300° C.; hot rolling the reheated slab to a finish rolling start temperature of 900 to 1,150° C. and a finish rolling end temperature of 850 to 1,050° C.
Implementation Method 2
cooling the cold-rolled steel sheet at a cooling rate of 5 to 30° C./s in a quenching zone
Implementation Method 3
recrystallization annealing the cold-rolled steel sheet by a process of heating the cold-rolled steel sheet under conditions of moist nitrogen containing 5 to 10 vol % of H2 as an atmospheric gas and a temperature and a dew point temperature at soaking zone being 650 to 900° C.
Implementation Method 4
a difference between an average of Mn/Si values of surface oxides present on a surface portion and an average of Mn/Si values of internal oxides
Data Source
AI summary
A hot-dip galvanized steel sheet according to one aspect of the present invention comprises a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the difference between the average of the Mn/Si values of surface oxides present on a surface portion, which is the region from the interface between the hot-dip galvanized layer and the base steel sheet to a depth of 15 nm, and the average of the Mn/Si values of internal oxides, which are present in the region from the interface to a depth of 50-100 nm, can be 0.5 or more. Mn and Si of each oxide mean the amounts (wt %) of Mn and Si components in the oxide, which are measured by EDS, and the average of Mn/Si values means the averaged value of the Mn/Si values measured for each oxide.