Ultra-High-Strength Galvanized Steel With Resistivity Gradient for LME Control
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Solution Overview
Problem
Ultra-high-strength galvanized steel plates experience liquid metal embrittlement (LME) cracks during resistance spot welding, particularly Type B, Type C, and Type D cracks, which hinder their application in automobile body manufacturing due to increased welding failure risks.
Innovation Solution
A 100 kg grade ultra-high-strength galvanized steel plate with controlled chemical compositions and resistivity gradients, including C: 0.17-0.25%, Mn: 1.7-2.7%, Si: 0.35-1.5%, Al: 0.01-1.0%, and resistivity ranges R1: 0<R1≤50μΩ·cm, R2: 0<R2≤15μΩ·cm, R3: 0<R3≤30μΩ·cm, satisfying (R2/2+R3/3)≤(3.1R11/2−1.5), along with a manufacturing process involving smelting, continuous casting, hot rolling, pickling, cold rolling, and multi-stage annealing followed by galvanizing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If ultra-high-strength steel with plating layer is used to reduce body material thickness and improve corrosion resistance, then strength and corrosion resistance are improved, but liquid metal embrittlement (LME) cracks occur during resistance spot welding, increasing welding failure risk
Solution Approach 1:
The patent applies local quality by creating a specific microstructure distribution within the steel plate thickness. The microstructure is designed to have different characteristics at different depths: the surface region (0-0.035t) contains martensite and residual austenite for strength, while the intermediate region (0.035t-0.065t) contains bainite and martensite to control resistivity and prevent LME cracks. This localized microstructure design allows the material to simultaneously achieve high strength and resistance to liquid metal embrittlement during welding.
Solution Approach 2:
The patent utilizes parameter changes by precisely controlling the resistivity distribution through multi-stage heat treatment processes. The resistivity parameters are adjusted at different depth zones: the surface region has resistivity R1≤50μΩ·cm, the intermediate region has R2≤15μΩ·cm, and the deeper region has R3≤30μΩ·cm. These resistivity parameters are optimized to control heat generation during resistance spot welding, preventing excessive thermal input that would cause LME cracks while maintaining the required tensile strength of ≥980 MPa.
2Productivity
If welding current is increased to improve welding speed and productivity, then productivity is improved, but welding splashing increases and LME problems become more likely
Solution Approach 1:
The patent changes the electrical resistance parameter distribution within the steel plate to optimize welding performance. By controlling the resistivity gradient (R1≤50μΩ·cm at surface, R2≤15μΩ·cm in intermediate layer, R3≤30μΩ·cm in deeper region), the patent ensures uniform heat generation during welding, allowing higher welding currents to be applied without causing excessive splashing or LME. This resistivity parameter optimization enables improved welding speed while maintaining weld quality.
3Strength
If alloying elements are added to improve strength through phase change strengthening, then strength is improved, but the complexity of material composition increases
Solution Approach 1:
The patent employs composite material principles by creating a multi-phase microstructure within the steel plate. The material contains martensite, residual austenite, and bainite phases distributed in specific proportions and locations. This composite microstructure, achieved through controlled alloying (C: 0.17-0.25%, Si: 0.35-1.5%, Mn: 1.70-2.70%, Al: 0.01-1.00%) and multi-stage heat treatment, provides the required ultra-high strength (≥980 MPa) while managing the complexity through systematic phase distribution rather than excessive alloying.
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 inhibits LME cracks, ensuring no Type B or Type C cracks when welding current is below Isplash, and limits Type A and Type D cracks to less than 10% of the base metal thickness, achieving a tensile strength of ≥980 MPa, elongation at break of ≥20%, and hole expansion ratio of ≥20%, with excellent resistance spot welding performance.
Implementation Method 1
the resistivity R1 of the steel plate is 01≤50μΩ·cm; in the direction from the interface between the plating layer and the steel plate matrix to the steel plate matrix, the resistivity R2 of the steel plate in the range of 0.010 t to 0.035 t is 02≤15μΩ·cm, and the resistivity R3 of the steel plate in the range of greater than 0.035 t to less than or equal to 0.065 t is 03≤30μΩ·cm
Implementation Method 2
Ultra-high strength steels generally use the phase change strengthening mechanism to improve the strength of the material
Implementation Method 3
multi-stage annealing followed by galvanizing
Data Source
AI summary
The present invention provides a 100 kg-grade ultra-high-strength galvanized steel sheet having excellent resistance spot welding properties and excellent resistance to liquid metal embrittlement (LME) cracking. The steel sheet contains the following components in percentage by weight: C: 0.17-0.25%, Mn: 1.7-2.7%, Si: 0.35-1.5%, Al: 0.01-1.0%, 0.7%≤Si+Al≤1.7%, and at least one of Nb, Ti and B, with the balance being Fe and unavoidable impurities. When the thickness of the steel sheet is t, the resistivity thereof is 0<R1≤50 μΩ·cm; and, in the direction from the coating layer/steel sheet substrate interface to the steel sheet substrate, when the thickness of the steel sheet is in the range of greater than or equal to 0.010t to less than or equal to 0.035t the resistivity thereof is 0<R2≤15 μΩ·cm, and when the thickness of the steel sheet is in the range of greater than 0.035t to less than or equal to 0.065t the resistivity thereof is 0<R3≤30 μΩ·cm, wherein (R2/2+R3/3)≤(3.1R11/2−1.5) is satisfied.