High-Strength Steel Plate for Structural Pipes
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Solution Overview
Problem
Existing high-strength steel plates for structural pipes or tubes, such as those of API X100 grade, face challenges in maintaining strength and material homogeneity after post-weld heat treatment (PWHT), often requiring large amounts of alloying elements that increase costs and compromise weldability and toughness.
Innovation Solution
A steel plate with a specific chemical composition and microstructure, primarily composed of bainite with a low martensite austenite constituent, is produced through hot rolling and accelerated cooling, followed by rapid reheating, to achieve high strength and homogeneity without excessive alloying elements, ensuring stability during PWHT.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If large amounts of Cr are added to compensate for strength decrease during PWHT, then strength after PWHT is improved, but material cost increases and weldability and toughness deteriorate
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting Cr content to 0.05-1.00% (preferably 0.10-0.50%) and optimizing combinations with Mo, Ti, and Nb. This parameter optimization resolves the contradiction by achieving sufficient strength after PWHT through controlled alloying rather than excessive Cr addition, thereby maintaining weldability and toughness.
Solution Approach 2:
The invention creates a composite alloying system combining Cr (0.05-1.00%), Mo (0.10-0.50%), Ti (0.005-0.025%), and Nb (0.005-0.080%) in specific proportions. This composite approach allows the synergistic effect of multiple elements to provide the necessary strength after PWHT without relying on excessive Cr, thus avoiding weldability and toughness deterioration.
2Strength
If high-strength steel plate is used to maintain base metal strength after PWHT, then strength is improved, but the difference between strength in rolling direction and perpendicular direction increases
Solution Approach 1:
The invention optimizes chemical composition parameters (C: 0.060-0.100%, Si: 0.01-0.50%, Mn: 1.50-2.50%, Mo: 0.10-0.50%, Ti: 0.005-0.025%, Nb: 0.005-0.080%) and microstructural parameters (bainite proportion, martensite austenite constituent area fraction <3.0%) to achieve high strength with minimal anisotropy. This resolves the contradiction by creating a composition that produces isotropic microstructure.
Solution Approach 2:
The invention achieves material homogeneity by controlling the microstructure to be mainly bainite with minimal martensite austenite constituent (<3.0% area fraction). This homogeneous microstructure ensures that strength properties are consistent in both rolling and perpendicular directions, resolving the contradiction between high strength and material homogeneity.
3Strength
If Cr content is increased to maintain strength during PWHT, then strength retention is improved, but manufacturing cost increases
Solution Approach 1:
The invention optimizes the Cr content parameter to a moderate range (0.05-1.00%, preferably 0.10-0.50%) rather than using high Cr content. This parameter optimization, combined with synergistic alloying elements, achieves sufficient strength retention during PWHT at lower material cost, resolving the contradiction between strength retention and manufacturing cost.
Solution Approach 2:
The invention employs a composite alloying strategy combining Cr (0.05-1.00%), Mo (0.10-0.50%), Ti (0.005-0.025%), and Nb (0.005-0.080%) in optimized proportions. This composite material approach achieves strength retention during PWHT through synergistic effects, reducing reliance on expensive high Cr content and thereby lowering manufacturing cost.
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 steel plate with high strength in the rolling direction and minimal strength difference in directions perpendicular to it, maintaining excellent mechanical properties before and after PWHT, thus enhancing safety and reducing material costs.
Implementation Method 1
hot-rolling a steel to which 0.30 % to 1.00 % of Cr, 0.005 % to 0.0030 % of Ti, and 0.060 % or less of Nb are added, and then subjecting it to accelerated cooling
Implementation Method 2
a microstructure that is mainly composed of bainite and that contains martensite austenite constituent in an area fraction of less than 3.0 %
Implementation Method 3
For a forged product subjected to welding, post weld heat treatment (PWHT) is performed to remove the residual stress caused by the welding from the forged product
Implementation Method 4
a steel plate for structural pipes or tubes that can exhibit excellent strength even after subjection to stress relief (SR) annealing, which is one type of PWHT
Data Source
AI summary
Disclosed is, as a high-strength steel plate of API X100 grade or higher, a steel plate for structural pipes or tubes that exhibits high strength in a rolling direction and that has only a small difference between strength in a rolling direction and strength in a direction perpendicular to the rolling direction (exhibiting high material homogeneity) without addition of large amounts of alloying elements. The steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure mainly composed of bainite and containing martensite austenite constituent in an area fraction of less than 3.0 %; a tensile strength in the rolling direction of 760 MPa or more; and TSC - TSL being 30 MPa or less in terms of absolute value, where TSC denotes a tensile strength in a direction perpendicular to the rolling direction.