Thick Steel Plate Microstructure for High Strength and Toughness
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Solution Overview
Problem
Existing high-strength steel plates for structural pipes or tubes, such as those of API X80 grade, face challenges in maintaining strength and Charpy properties after post-weld heat treatment (PWHT), often requiring large amounts of alloying elements that increase material costs and compromise weldability and toughness.
Innovation Solution
A thick steel plate with a specific chemical composition and microstructure, featuring a dual-phase microstructure of ferrite and bainite at the mid-thickness part, achieved through controlled hot rolling and accelerated cooling, which maintains high strength and excellent Charpy properties without excessive alloying elements, ensuring strength and toughness even after PWHT.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If large amounts of Cr are added to compensate for strength decrease after PWHT, then strength is maintained, but material cost increases and weldability and toughness deteriorate
Solution Approach 1:
The invention changes the chemical composition parameters by limiting Cr to 0.05-1.00% (avoiding excessive Cr addition) while optimizing other alloying elements (Ti: 0.005-0.030%, Nb: 0.005-0.080%, V: 0.005-0.100%, Mo: 0.05-0.50%) to achieve the desired strength-toughness balance without compromising weldability
Solution Approach 2:
The invention creates a composite microstructure consisting of ferrite and bainite phases through controlled hot rolling and accelerated cooling, where the dual-phase microstructure provides both strength and toughness without requiring large amounts of Cr
2Length of stationary object
If the steel plate has a thickness of 38 mm or more, then it meets structural pipe requirements, but Charpy properties at the mid-thickness part deteriorate
Solution Approach 1:
The invention achieves local quality optimization by creating a dual-phase microstructure (ferrite and bainite) specifically at the mid-thickness region through controlled hot rolling and accelerated cooling, ensuring excellent Charpy properties at the critical mid-thickness location while maintaining the required plate thickness of 38 mm or more
Solution Approach 2:
The invention changes the microstructural parameters by controlling the phase composition (ferrite + bainite dual-phase structure) and grain size distribution through hot rolling and accelerated cooling processes, thereby improving Charpy properties at the mid-thickness part of thick plates
3Strength
If Cr carbide precipitation is induced during PWHT to compensate for strength decrease, then strength is recovered, but alloying element addition increases and material cost rises
Solution Approach 1:
The invention optimizes the chemical composition parameters by limiting Cr to 0.05-1.00% and carefully controlling Ti, Nb, V, and Mo contents, achieving strength recovery after PWHT through controlled precipitation of carbides and nitrides without requiring large amounts of alloying elements
Solution Approach 2:
The invention achieves strength recovery by creating a dual-phase microstructure (ferrite and bainite) that can be replicated through controlled hot rolling and accelerated cooling processes, eliminating the need for excessive Cr carbide precipitation
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 high-strength steel plate with API X80 grade or higher that retains excellent mechanical properties, including tensile strength and Charpy absorption energy, at the mid-thickness part, effectively addressing the limitations of existing technologies by avoiding the need for large alloy additions and maintaining performance post-PWHT.
Implementation Method 1
a microstructure at a mid-thickness part of the thick steel plate that is mainly a dual-phase microstructure of ferrite and bainite
Implementation Method 2
by 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 3
subjecting it to accelerated cooling
Data Source
AI summary
Disclosed is, as a high-strength steel plate of API X80 grade or higher with a thickness of 38 mm or more, a thick steel plate for structural pipes or tubes that exhibits high strength in the rolling direction and excellent Charpy properties at its mid-thickness part without addition of large amounts of alloying elements. The thick steel plate for structural pipes or tubes disclosed herein has: a specific chemical composition; a microstructure at its mid-thickness part that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50%, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80% or more with respect to the whole area of the ferrite; a tensile strength of 620 MPa or more; and a Charpy absorption energy vE−20+ C. at −20° C. at the mid-thickness part of 100 J or more.