Tailored Hot-Pressed Steel Blank Welding Without Al-Induced Strength Loss
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Solution Overview
Problem
Existing methods for hot-press forming of tailored blank materials face challenges such as scale formation, reduced strength in weld metals due to Al mixing, and increased costs from filler wire additions, which compromise the strength and corrosion resistance of hot-pressed members.
Innovation Solution
Using Zn or Zn alloy coated steel sheets with a Zn or Zn—Ni alloy coated layer, where the coated layer thickness is optimized to prevent Al mixing and enhance quench hardenability, allowing for laser or plasma welding without additional layer removal steps, and ensuring sufficient strength and corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Al or Al alloy coated steel sheets are used for hot press forming, then scale formation is suppressed and corrosion resistance is improved, but Al mixes into the weld metal during laser welding, reducing the strength of the weld metal
Solution Approach 1:
The patent removes the Al or Al alloy coated layer from the steel sheets before laser welding to prevent Al from mixing into the weld metal. This extraction of the harmful coated layer resolves the contradiction by eliminating the source of Al contamination while preserving the base steel's corrosion resistance through subsequent Zn coating application.
Solution Approach 2:
The patent applies Zn coating to the steel sheets before laser welding as a preliminary action. This pre-coating prevents Al mixing during welding and ensures both weld strength and corrosion resistance, as the Zn layer acts as a barrier and the Zn-Al intermetallic compounds formed during welding do not adversely affect weld metal strength.
2Strength
If the coated layer is removed before welding, then Al mixing is prevented and weld metal strength is maintained, but additional processing steps are required, increasing manufacturing complexity
Solution Approach 1:
The patent applies Zn coating to the steel sheets before laser welding as a preliminary action. This pre-coating prevents Al mixing during welding and ensures both weld strength and corrosion resistance, as the Zn layer acts as a barrier and the Zn-Al intermetallic compounds formed during welding do not adversely affect weld metal strength.
3Strength
If filler wire is added during welding to compensate for strength loss, then weld metal strength is restored, but manufacturing cost and process complexity increase
Solution Approach 1:
The patent removes the Al or Al alloy coated layer from the steel sheets before laser welding to prevent Al from mixing into the weld metal. This extraction of the harmful coated layer resolves the contradiction by eliminating the source of Al contamination while preserving the base steel's corrosion resistance through subsequent Zn coating application.
4Strength
If steel sheets are strengthened to improve crashworthiness, then vehicle safety is improved, but press formability decreases, making it difficult to form the desired member shape
Solution Approach 1:
The patent utilizes hot press forming by heating the steel sheets to austenite transformation temperature (Ac3 point or higher) before forming. This temperature parameter change transforms the steel into a soft austenite phase that is easy to form, and subsequent rapid cooling produces martensite structure with high strength, thereby achieving both good formability and high strength.
Solution Approach 2:
The patent exploits the phase transition of steel from ferrite/pearlite to austenite during heating, and then from austenite to martensite during rapid cooling. This phase transition mechanism enables the steel to be soft and formable during the forming process, then hard and strong after forming, resolving the contradiction between formability and strength.
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
This approach stabilizes the strength of weld metals and prevents fractures, while maintaining corrosion resistance without the need for additional treatments like shot blasting or filler wire additions, improving the overall efficiency and cost-effectiveness of the hot-press forming process.
Implementation Method 1
a coated layer in a surface of each of the coated steel sheets is a Zn or Zn alloy coated layer
Implementation Method 2
heating the obtained tailored blank material to a temperature range of an Ac3 transformation point to 1000° C., and then cooling the tailored blank material and hot-pressing the cooled tailored blank material at a temperature of 600° C. or more
Implementation Method 3
butt joining respective ends of two or more coated steel sheets without removing their coated layers beforehand
Implementation Method 4
joining the ends by laser welding or plasma welding
Implementation Method 5
the thickness of the coated layer is optimized to prevent Al mixing and enhance quench hardenability
Data Source
AI summary
A hot-pressed member is formed using a tailored blank material obtained by butt joining respective ends of two or more coated steel sheets. The hot-pressed member has two or more sites formed by the respective coated steel sheets and at least one joining portion between the sites. Depending on a type of a coated layer of each of the coated steel sheets, tw/t0 is appropriately controlled where tw is a thickness of a thinnest portion in the joining portion and t0 is a thickness of a thinnest site of the sites. A tensile strength of each of the sites is 1180 MPa or more.