Clad Steel Plate Microstructure Control for Strength and Toughness
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Solution Overview
Problem
Clad steel plates with 1/2Mo steel base metal, normalized for coke drums, suffer from dual-phase microstructure with coarse ferrite and hard bainite, leading to significantly deteriorated toughness, making it difficult to achieve both high strength and toughness.
Innovation Solution
The clad steel plate is manufactured with a base steel composition and microstructure optimized to ensure an average ferrite grain size of 15 µm or less and an area fraction of bainite of 20% or more, along with appropriate chemical composition and normalizing heat treatment processes to enhance both strength and toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If normalizing heat treatment is applied to 1/2Mo steel base metal, then base steel strength is improved, but base steel toughness deteriorates due to coarse ferrite and hard bainite formation
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.25%, Si: 0.10-0.50%, Mn: 0.50-1.00%, Mo: 0.40-0.70%, Nb: 0.010-0.030%, Al: 0.005-0.050%) and heat treatment parameters (normalizing temperature, cooling rate) to transform the microstructure from coarse ferrite-hard bainite to fine ferrite with controlled bainite distribution, achieving both high strength and toughness
Solution Approach 2:
The patent creates a composite microstructure consisting of fine ferrite matrix with dispersed bainite phases, where each phase contributes different properties: fine ferrite provides toughness and ductility, while bainite provides strength. This composite microstructure resolves the contradiction between strength and toughness
2Reliability
If ferrite grain size is reduced to improve toughness, then base steel toughness is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent applies preliminary action by adding alloying elements (Nb, Al) before heat treatment that will precipitate during normalizing to pin austenite grain boundaries, preventing grain growth and ensuring fine ferrite grain size (15 μm or less) after transformation. This preliminary preparation simplifies the subsequent heat treatment process
Solution Approach 2:
The patent employs feedback control by establishing specific composition ranges and heat treatment parameters that create self-regulating conditions: the alloying elements control austenite grain size which directly determines ferrite grain size after transformation, providing automatic feedback control of microstructure without requiring complex process control
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 ensures high base steel toughness and strength, meeting the requirements for clad steel plates used in pressure vessels like coke drums by refining ferrite grains and controlling bainite area fraction, thereby improving resistance to brittle crack propagation and maintaining strength.
Implementation Method 1
A clad steel plate used for, for example, pressure vessels, a clad steel plate using a base metal, such as 1Cr-1/2Mo steel, having higher high-temperature strength than carbon steel
Implementation Method 2
a base metal composed of 1/2Mo steel in a clad steel plate manufactured by normalizing heat treatment, as described above, has a dual-phase microstructure containing coarse ferrite and hard bainite
Data Source
AI summary
[Object] [Solution] A clad steel plate including a cladding metal bonded to a base steel plate. The base steel plate has a chemical composition containing, by mass%, C: 0.120% to 0.180%, Si: 0.15% to 0.40%, Mn: 0.50% to 0.90%, P: 0.020% or less, S: 0.010% or less, Cu + Ni + Cr: 0.800% to 1.100% (where symbols of elements indicate amounts of the respective elements contained), Mo: 0.45% to 0.60%, Nb: 0.005% to 0.025%, Al: 0.005% to 0.050%, and N: 0.0010% to 0.0070%, the balance being Fe and unavoidable impurities. Bainite has an area fraction of 20% or more at the 1/2 thickness position of the base steel plate in the thickness direction. Ferrite has an average grain size of 15 µm or less at the 1/2 thickness position of the base steel plate in the thickness direction.