Thick-Walled Stainless Seamless Pipe With Refined Ferrite Grains
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Solution Overview
Problem
Heavy-walled stainless steel seamless tubes or pipes face challenges in achieving high yield strength and low-temperature toughness, particularly in the central portion of the wall thickness, due to coarsened microstructures and reduced strain accumulation during hot working, leading to degraded toughness and strength.
Innovation Solution
The method involves heating the steel to 1,100°C to 1,300°C, performing elongating rolling with a hot working temperature of 700°C to 1,200°C to achieve a 35% or more austenitic phase fraction, followed by quenching or quenching and tempering in the austenite-ferrite dual-phase region, to refine ferrite grains and optimize phase transformation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional hot working is used for heavy-walled steel, then production efficiency is maintained, but ferrite grains coarsen in the wall thickness center leading to degraded toughness and strength
Solution Approach 1:
The patent applies parameter changes by precisely controlling hot working temperature (700-1200°C) and phase fraction (35% or more austenite) to transform the microstructure evolution behavior. This temperature and phase fraction control prevents ferrite grain coarsening in the wall thickness center while maintaining production efficiency through optimized hot working parameters.
Solution Approach 2:
The patent utilizes phase transitions by maintaining 35% or more austenite phase during hot working at 700-1200°C. This phase transition control ensures that the steel microstructure transforms appropriately during processing, preventing coarse ferrite grain formation and improving both strength and toughness in heavy-walled steel.
2Strength
If alloy elements are increased to improve strength and corrosion resistance, then material performance is enhanced, but manufacturing cost increases
Solution Approach 1:
The patent optimizes alloy element parameters within specific ranges (Cr: 15.5-18.0%, Ni: 1.5-5.0%, Mo: 2.0-3.5%) rather than using excessive amounts. This parameter optimization achieves the required strength and corrosion resistance while controlling manufacturing costs through efficient alloy design.
Solution Approach 2:
The patent creates a composite microstructure consisting of martensitic phase (50% or more) and ferritic phase (20-50%), with controlled carbide precipitates. This composite material structure achieves high strength and corrosion resistance through synergistic phase combinations and precipitation hardening, reducing dependence on excessive alloying.
3Strength
If wall thickness is increased for heavy-walled applications, then structural strength is improved, but strain accumulation during hot working decreases leading to coarsened microstructure
Solution Approach 1:
The patent utilizes phase transitions by maintaining 35% or more austenite phase during hot working of heavy-walled steel. This phase transition behavior ensures uniform strain distribution and prevents coarse ferrite grain formation even in thick sections, achieving fine microstructure throughout the wall thickness.
Solution Approach 2:
The patent applies parameter changes by controlling hot working temperature (700-1200°C) and accumulated reduction ratio (10% or more) to ensure uniform microstructure in heavy-walled steel. These parameter optimizations compensate for reduced strain accumulation in thick sections, maintaining microstructure uniformity.
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 results in high-strength heavy-walled stainless steel seamless tubes or pipes with excellent low-temperature toughness and corrosion resistance, ensuring the ferrite grains are fine even at the wall thickness center, enhancing both yield strength and absorbed energy at -10°C.
Implementation Method 1
the steel microstructure of the hollow base steel at the above-described hot working temperature contains 35% or more of austenite on an area fraction basis, and after hot working performing quenching, or quenching and tempering, or a solution heat treatment as a heat treatment in the dual-phase region of austenite and ferrite
Implementation Method 2
heating a steel to a temperature of 1,100°C to 1,300°C
Implementation Method 3
performing cooling after the pipe-making to room temperature at a cooling rate larger than or equal to that of air cooling
Data Source
AI summary
A high-strength heavy-walled stainless steel seamless tube or pipe with a wall thickness central portion having excellent yield strength and low-temperature toughness and a method for manufacturing the same are provided. The high-strength heavy-walled stainless steel seamless tube or pipe exhibiting excellent low-temperature toughness is characterized by having a chemical composition containing Cr: 15.5% to 18.0% and a steel microstructure containing a ferritic phase and a martensitic phase, wherein the maximum value of the areas of the ferrite grains in the steel microstructures in a circumferential direction cross-section and an L direction (rolling direction) cross-section of the steel tube or pipe is 3,000 µm2 or less and the content of ferrite grains having areas of 800 µm2 or less is 50% or more on an area fraction basis, where when adjacent ferrite grains are present in the steel microstructure and the crystal misorientation between one ferrite grain and the other ferrite grain is 15° or more, the adjacent grains are assumed to be grains different from each other.