Hot-Pressed Steel Sheet Coating for Weldable Cut-Edge Corrosion Resistance
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Solution Overview
Problem
Conventional hot pressed members with Al—Zn coatings lack sufficient spot weldability and corrosion resistance at cut portions, due to issues such as chemical convertibility problems with Al coatings and liquid metal embrittlement cracking with Zn coatings.
Innovation Solution
Incorporating specific amounts of Si and Sr into the Al—Zn—Mg coated or plated layer suppresses alloying and oxide layer formation, enhancing spot weldability and corrosion resistance at cut portions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an Al coated steel sheet is used for hot press forming, then corrosion resistance is improved, but chemical conversion layer formation is prevented and corrosion resistance at cut portion is insufficient
Solution Approach 1:
The patent changes the chemical composition parameters of the coating layer by adding specific amounts of Si (1-10 mass%) and Sr (0.1-5 mass%) to the Al-Fe alloy layer. This parameter modification enables the coating to react with chemical conversion treatment liquids while maintaining corrosion resistance, resolving the contradiction between corrosion protection and chemical convertibility.
2Reliability
If a Zn coated or plated steel sheet is used for hot press forming, then painting layer adhesion and corrosion resistance are secured, but spot weldability deteriorates due to thick oxide layer formation
Solution Approach 1:
The patent modifies the coating composition by incorporating Si and Sr elements, which alter the oxidation behavior during hot press forming. This results in a thin oxide layer that does not interfere with spot welding, while the underlying Al-Zn-Si-Sr coating structure maintains excellent corrosion resistance and painting adhesion.
3Reliability
If a Zn coated or plated steel sheet is used for hot press forming, then painting layer adhesion is improved, but liquid metal embrittlement cracking occurs and fatigue resistance is insufficient
Solution Approach 1:
The patent creates a composite coating structure containing Al, Zn, Si, and Sr elements. The Al-Fe alloy layer provides a stable base that prevents LME cracking, while the Zn component ensures painting adhesion. The Si and Sr additions further enhance both weldability and resistance to embrittlement, achieving a balanced composite material system.
4Reliability
If an Al—Zn coated or plated steel sheet is used instead of Al or Zn coated sheets, then chemical convertibility and LME cracking issues are addressed, but spot weldability and corrosion resistance at cut portion remain insufficient
Solution Approach 1:
The patent optimizes the compositional parameters of the Al-Zn coating by adding Si (1-10 mass%) and Sr (0.1-5 mass%). These parameter changes suppress excessive alloying and oxide formation during hot press forming, enabling spot weldability while maintaining chemical convertibility and corrosion resistance at cut portions.
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 hot pressed member achieves excellent spot weldability and corrosion resistance at cut portions, combining high strength and improved post-painting corrosion resistance.
Implementation Method 1
Fe diffuses from the base steel sheet to a surface layer of the Al coated layer, forming an FeAl alloy layer
Implementation Method 2
a thick oxide layer is formed on a surface of the coated or plated layer
Data Source
AI summary
Provided is a hot pressed member having excellent spot weldability and corrosion resistance at cut portion. The hot pressed member includes a steel sheet, a coating layer on at least one side of the steel sheet, and an oxide layer having a thickness of 1.0 μm or less on the coating layer. The coating layer has a chemical composition containing, in mass %, Zn: 5.0% to 55.0%, Si: 1.1% to 8.0%, Mg: 2.0% to 6.0%, Sr: 0.01% to 2.0%, and Fe: 55.0% or less, with the balance being Al and inevitable impurity.