Hot-Rolled Steel Sheet Microstructure for Stable ERW Pipe Expansion
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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, which affects the pipe's expandability and weld integrity.
Innovation Solution
A hot-rolled steel sheet with a specific alloy composition (0.15-0.22% C, 0.1-1.0% Si, 0.8-1.8% Mn, 0.001-0.03% P, 0.001-0.01% S, 0.001-0.19% Sol.Al, 0.3-1.0% Cr, 0.01-0.05% Ti, 0.025% or less Nb, 0.035% or less V, and 0.001-0.01% N) and a microstructure comprising a ferrite phase with a mixed martensite and bainite phase fraction of 50% or more, optimized through a reheating, hot rolling, and controlled cooling process to achieve a low yield ratio and enhanced expandability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large amount of Si is added to stabilize ferrite phase during cooling, then ferrite fraction is secured, but Si oxides are generated in welding zone causing penetrator defects
Solution Approach 1:
The patent optimizes the Si content parameter to a specific range (0.1-1.0 wt%) and establishes a quantitative relationship with Mn content through Expression 1 (4[Mn]-12[Si]=2 to 10). This parameter optimization resolves the contradiction by providing sufficient Si for ferrite stabilization while limiting Si oxide generation in the welding zone.
2Strength
If the material is rapidly cooled below Ms to obtain martensite, then strength is increased, but hardness drop in welding heat-affected zone exceeds 30
Solution Approach 1:
The patent creates a composite microstructure where different phases are distributed throughout the material: ferrite provides ductility and reduces hardness drop, while martensite provides strength. This local quality distribution resolves the contradiction between achieving high strength and maintaining hardness stability in the HAZ.
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and martensite phases. The ferrite phase acts as a buffer that reduces the hardness drop in the HAZ while the martensite phase provides the necessary strength, thus resolving the contradiction between strength and hardness stability.
3Stability of the object's composition
If pure bainite phase is obtained by rapid cooling below Bs, then hardness drop is reduced, but yield strength increases and elongation decreases
Solution Approach 1:
Instead of creating a uniform bainite structure, the patent creates a composite structure with ferrite and martensite phases distributed throughout. The ferrite phase provides ductility and hardness stability, while martensite provides strength, resolving the contradiction between hardness stability and yield 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
The solution provides a hot-rolled steel sheet with a tensile strength of 980 MPa or more, reduced yield ratio, and minimized defects in the welding zone, ensuring excellent cold formability and expandability of the steel pipes without cracking.
Implementation Method 1
a microstructure includes a ferrite phase as a matrix structure and a hard phase composed of martensite and bainite phases
Implementation Method 2
reheating a steel slab satisfying the alloy composition described above and the Relational Expression 1 in a temperature range of 1180 to 1300° C.
Implementation Method 3
finish hot rolling the reheated steel slab at a temperature of Ar3 or higher to prepare a hot-rolled steel sheet
Data Source
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.