Hot-Pressed Steel Coating Composition for LME and Microcrack Control
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Solution Overview
Problem
Hot press forming of galvanized steel sheets often results in cracking due to liquid metal embrittlement (LME) at bent portions, and the corrosion resistance of existing hot pressed members is insufficient when zirconium chemical conversion treatment is applied.
Innovation Solution
A hot pressed member is developed with a steel sheet coated or plated with a layer containing FeAl, Fe2Al5, and Zn, where the solute Zn content in FeAl is less than 10 mass %, and a Zn-containing oxide layer is distributed over the coated or plated layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Zn—Ni alloy coated or plated layer is used to suppress LME cracking, then macrocrack resistance is improved, but microcracks occur due to zinc vapor penetration into the base steel sheet
Solution Approach 1:
The invention changes the compositional parameters of the coated layer by strictly controlling solute Zn content in FeAl to less than 10 mass%, and specifies the presence of Fe2Al5 phase with controlled Fe content (5-20 mass%). This parameter optimization reduces zinc vapor generation during hot press forming while maintaining LME crack suppression, thereby eliminating microcracks without sacrificing macrocrack resistance.
Solution Approach 2:
The invention creates a composite coated layer structure containing multiple phases (FeAl with controlled solute Zn, Fe2Al5 with specific Fe content, and Zn) that work synergistically. The FeAl phase suppresses LME cracking, the Fe2Al5 phase controls zinc vapor generation, and the Zn phase provides sacrificial protection, achieving both macrocrack and microcrack resistance simultaneously.
2Strength
If conventional Zn—Ni alloy coating is used for hot press forming, then LME cracking is suppressed, but corrosion resistance after coating deteriorates
Solution Approach 1:
The invention optimizes the compositional parameters of the coated layer by controlling solute Zn content in FeAl to less than 10 mass% and specifying Fe2Al5 phase with 5-20 mass% Fe content. This parameter control creates a coated layer that provides both LME crack resistance and excellent corrosion resistance after zirconium chemical conversion treatment and coating, resolving the contradiction between crack resistance and corrosion resistance.
3Object-affected harmful factors
If rapid intermediate cooling is introduced to eliminate microcracks, then microcrack formation is suppressed, but production cost increases due to additional equipment
Solution Approach 1:
The invention extracts the microcrack formation mechanism (zinc vapor penetration) by controlling the coated layer composition, specifically limiting solute Zn content in FeAl to less than 10 mass% and incorporating Fe2Al5 phase. This compositional control eliminates the root cause of microcracks without requiring additional cooling equipment, thereby reducing production costs while maintaining microcrack suppression.
Solution Approach 2:
The invention enables the coated layer to self-regulate zinc vapor generation through its controlled composition (low solute Zn in FeAl, presence of Fe2Al5 phase), preventing microcrack formation during hot press forming without requiring external intervention or additional processing equipment, thus eliminating the need for rapid intermediate cooling systems.
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 proposed solution achieves excellent corrosion resistance after coating, particularly when zirconium chemical conversion treatment is applied, while reducing microcracks, and does so without requiring additional equipment or processes that increase production costs.
Implementation Method 1
hot press forming of galvanized steel sheets has the problem of cracking caused by liquid metal embrittlement (LME) at bent portions
Implementation Method 2
Microcracks are thought to be caused by the penetration of zinc vapor generated during hot press forming into the base steel sheet
Implementation Method 3
the corrosion resistance evaluated in PTL 1 is corrosion resistance without coating, not corrosion resistance under actual use conditions of the components
Data Source
AI summary
A hot pressed member that has excellent corrosion resistance after coating, in particular when a zirconium chemical conversion treatment is applied, and with reduced microcracks. The hot pressed member includes a steel sheet, a coated or plated layer containing FeAl, Fe2Al5, and Zn distributed over at least one side of the steel sheet, and a Zn-containing oxide layer distributed over the coated or plated layer. The solute Zn content in the FeAl is less than 10 mass %.