Hot-dip Galvanized Steel Sheet Surface Quality and Low Temperature Fracture Resistance
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Solution Overview
Problem
Hot-dip galvanized steel sheets often suffer from poor surface qualities due to the spangle pattern caused by zinc solidification and are prone to low temperature brittle fracturing, especially when the {0001} plane of the zinc crystal structure has a strong preferred orientation.
Innovation Solution
A hot-dip galvanized steel sheet with a zinc-based plating layer having an average equivalent circular diameter of 20 μm to 100 μm and a {0001} plane orientation of 70% or less, optimized by adjusting the composition and manufacturing conditions to include Al, Mg, Ca, Mn, and Be within specific ranges, and using a phosphate solution to control solidification nuclei and cooling rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If zinc solidifies with dendrite growth forming spangle pattern, then solidification process is natural and simple, but surface quality deteriorates due to convex and concave portions
Solution Approach 1:
The patent applies preliminary action by introducing solidification nuclei (phosphate solution droplets) before the zinc solidification process begins. These pre-introduced nuclei serve as predetermined sites for crystal growth, controlling the solidification process to produce fine-grained structure rather than allowing natural dendrite formation. This preliminary intervention prevents the formation of large spangle patterns while maintaining manufacturing simplicity.
Solution Approach 2:
The patent changes the parameters of the solidification process by controlling the average grain size of zinc grains to be 0.1 mm or less through the introduction of solidification nuclei. This parameter change transforms the natural coarse dendritic structure into a fine-grained structure, improving surface quality without complicating the manufacturing process.
2Manufacturing precision
If phosphate solution droplets are sprayed to provide solidification nuclei and reduce grain size to 0.1 mm or less, then surface quality improves, but low temperature brittle fracturing occurs due to excessive {0001} plane orientation
Solution Approach 1:
The patent adjusts the parameters of the phosphate solution treatment, specifically controlling the concentration (0.01-10 wt%), temperature (20-100°C), and the amount of droplets sprayed. By optimizing these parameters, the patent achieves a balance where the zinc grain size is reduced to improve surface quality while the {0001} plane orientation is controlled to remain at 70% or less, preventing low temperature brittle fracturing.
Solution Approach 2:
The patent applies partial action by using a controlled amount of phosphate solution droplets rather than excessive amounts. The droplets are sprayed at specific intervals and in controlled quantities, providing just enough solidification nuclei to achieve the desired grain size and orientation balance without over-treating the surface, which would cause excessive {0001} plane orientation and brittle fracturing.
3Duration of action of moving object
If dendrites grow consuming surrounding molten zinc, then solidification progresses naturally, but plating layer becomes uneven with poor surface quality
Solution Approach 1:
The patent introduces solidification nuclei (phosphate solution droplets) before solidification begins, creating predetermined sites for crystal growth. This preliminary action distributes nucleation sites uniformly across the molten zinc surface, preventing the natural tendency of dendrites to grow unevenly and consume molten zinc irregularly, thereby producing a more uniform plating layer.
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 solution results in improved surface qualities, enhanced galling resistance, and resistance to low temperature brittle fracturing, ensuring a ductile fracture mode even at -40°C, as demonstrated by the T-peel test and lab shear test comparisons.
Implementation Method 1
when the zinc solidifies, dendrites shaped like a tree branch grow from solidification nuclei as a framework of a plating structure
Implementation Method 2
spraying droplets of a phosphate solution onto a surface of a molten zinc plating layer to provide the droplets as solidification nuclei when the molten zinc plating layer solidifies
Data Source
AI summary
Provided is a hot-dip galvanized steel sheet including a base steel sheet and a hot-dip zinc-based plating layer formed on the base steel sheet. The hot-dip zinc-based plating layer includes a Zn single phase having an average equivalent circular diameter of 120 μm or less as a microstructure. In the Zn single phase, a Zn single phase having a crystal structure of which a {0001} plane is parallel to a surface of the steel sheet, is provided in an area fraction of 70% or less.