Hot Formed Steel Coating for Spot Weldability
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Solution Overview
Problem
Existing hot formed steel products face challenges with spot weldability and corrosion protection, particularly during high-temperature forming processes, where oxidation and zinc loss lead to increased contact resistance, making them unweldable and compromising corrosion protection.
Innovation Solution
A metal-phosphate layer, predominantly zinc-phosphate with additional elements like manganese and calcium, is applied to the hot formable steel substrate, reducing oxidation and zinc evaporation, thereby enhancing spot weldability and corrosion protection by maintaining a low contact resistance and effective coating performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a zinc coating is applied to hot formable steel for corrosion protection, then corrosion protection is improved, but oxidation and zinc loss occur during hot forming, leading to increased contact resistance and poor spot weldability
Solution Approach 1:
A metal-phosphate layer (intermediary substance) is applied between the zinc coating and the hot forming environment. This phosphate layer acts as a protective barrier that prevents direct oxidation of the zinc coating and reduces zinc evaporation during hot forming, thereby maintaining low contact resistance and good spot weldability while preserving the corrosion protection function
Solution Approach 2:
The coating system is transformed from a single-layer zinc coating to a composite structure consisting of a zinc layer combined with a metal-phosphate layer. This composite coating provides both corrosion protection (from the zinc layer) and oxidation resistance during hot forming (from the phosphate layer), resolving the contradiction between maintaining corrosion protection and preventing oxidation/zinc loss
2Reliability
If the zinc coating is thick enough to provide good corrosion protection, then corrosion protection is improved, but zinc evaporation during hot forming increases, leading to poor spot weldability
Solution Approach 1:
The metal-phosphate layer serves as an intermediary barrier that reduces zinc evaporation during hot forming. This allows the use of adequate zinc coating thickness for corrosion protection without suffering from excessive zinc loss during the hot forming process, thereby maintaining both corrosion protection and spot weldability
3Ease of manufacture
If no phosphate layer is applied, then the production process is simpler and more economical, but spot weldability after hot forming is poor due to high contact resistance
Solution Approach 1:
A thin metal-phosphate layer is applied that serves its purpose during hot forming (maintaining low contact resistance for good spot weldability) and can be applied using cost-effective processes. The phosphate layer is a relatively simple, inexpensive coating that provides the necessary functionality without requiring complex production equipment or materials
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 zinc-phosphate layer significantly reduces oxidation and zinc loss, ensuring good spot weldability and improved corrosion protection, even at high temperatures, with contact resistance below 1 mOhm, suitable for automotive applications.
Implementation Method 1
the metal-phosphate layer is very suitable to greatly reduce the extent of oxidation during the hot forming of a steel strip
Implementation Method 2
The metal-phosphate layer also reduces zinc losses due to evaporation
Implementation Method 3
When hot formed, the phosphated material has a low contact resistance and consequently good weldability
Data Source
Figure 1A~3
Figure 4A~4C
Figure 4D~4E
AI summary
The invention relates to a hot formable strip, sheet or blank which is formable at a temperature of 600°C or above, comprising a substrate of hot formable steel coated with a corrosion protective coating. According to the invention, the coated steel substrate is covered by a metal-phosphate solution containing zinc and optionally at least one element of manganese, aluminium, nickel, magnesium, titanium, copper and calcium. The invention also relates to a process to produce such strip, sheet or blank, a method for hot forming a product using such a strip, sheet or blank and a hot formed product using such a strip, sheet or blank.