Hot-Rolled Steel Sheet Composition for Stable ERW Pipe Expandability
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Solution Overview
Problem
Conventional hot-rolled steel sheets for electric resistance welded steel pipes face challenges in maintaining strength and elongation while minimizing the drop in hardness at the welding heat-affected zone, leading to defects and reduced formability during pipemaking.
Innovation Solution
A hot-rolled steel sheet with a specific alloy composition and optimized manufacturing conditions, including carbon, silicon, manganese, phosphorus, sulfur, aluminum, chromium, titanium, niobium, vanadium, and nitrogen, is developed to achieve a high tensile strength of 980 MPa or more with a low yield ratio, ensuring excellent electrical resistance weldability and cold formability without cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large amount of Si is added to stabilize ferrite fraction during cooling, then ferrite transformation is improved and elongation is enhanced, but Si oxides are generated in the welding zone causing penetrator defects
Solution Approach 1:
The patent optimizes the Si content parameter within a specific range (0.01-2.00 wt%) and combines it with controlled Mn content (0.50-2.50 wt%) to achieve stable ferrite transformation while minimizing oxide formation. This parameter optimization resolves the contradiction by finding the optimal balance point where ferrite stability is maintained without excessive oxide generation.
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and martensite phases with specific proportions (ferrite: 5-50%, martensite: 50-95%). This composite structure achieves both the ferrite stability needed for elongation and the strength required to minimize welding defects, resolving the contradiction through phase composition control.
2Strength
If the material is high-strength steel with high yield strength, then strength is improved, but the yield ratio increases causing severe spring back during roll forming and difficulty in securing out-of-roundness
Solution Approach 1:
The patent controls the yield ratio parameter within a specific range (0.70-0.85) by adjusting the microstructure composition (ferrite-martensite dual phase) and alloying elements. This parameter control allows the material to maintain high strength while reducing spring back during roll forming, thereby improving out-of-roundness.
3Strength
If pure martensite is obtained by rapid cooling below Ms temperature, then strength is maximized, but a drop in hardness of the welding heat-affected zone occurs in excess of 30
Solution Approach 1:
The patent creates a non-uniform microstructure where ferrite (5-50%) is distributed throughout the martensite matrix. The ferrite phase acts as a buffer in the welding heat-affected zone, preventing excessive hardness drop during welding while maintaining high overall strength. This local quality distribution resolves the contradiction between strength and hardness stability.
Solution Approach 2:
The ferrite-martensite composite microstructure combines the high strength of martensite with the stability and ductility of ferrite. During welding, the ferrite phase prevents excessive hardness drop in the HAZ while the martensite provides the required tensile strength, resolving the contradiction through phase combination.
4Stability of the object's composition
If pure bainite phase is obtained by rapid cooling below Bs temperature, then hardness drop during welding is reduced, but yield strength increases and elongation is lowered
Solution Approach 1:
The patent controls the microstructure to contain primarily ferrite and martensite phases rather than bainite, by adjusting cooling rates and tempering parameters. This parameter selection achieves hardness stability during welding while maintaining the required yield strength and elongation, avoiding the trade-off associated with pure bainite structures.
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 effectively suppresses defects in the welding zone and minimizes the hardness drop in the welding heat-affected zone, ensuring high strength, excellent expandability, and maintaining elongation, thereby enhancing the overall performance of the steel pipes.
Implementation Method 1
electric resistance welding
Implementation Method 2
roll forming properties
Implementation Method 3
cold forming
Data Source
Figure 1~1(b)
Figure 2~2(b)
AI summary
The present invention relates to steel used for a sash component and the like of a vehicle and, more specifically, to a hot-rolled steel sheet for a high-strength electric resistance welded steel pipe having excellent expandability and a method for manufacturing same, the hot-rolled steel sheet having a smaller decrease in the strength of a welding heat-affected zone (HAZ) formed during electric resistance welding, in comparison with a base material.