Thick Steel Plate Mid-Thickness Toughness Refinement
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Solution Overview
Problem
Producing steel plates with a thickness of 100 mm or greater that maintain excellent strength and toughness in the mid-thickness part is challenging due to the formation of coarse microstructures like ferrite, which is exacerbated by the addition of large amounts of alloying elements such as Mn, Ni, and Cr, leading to inadequate refinement of prior γ grain size during heat treatment.
Innovation Solution
A steel plate with a specific chemical composition and a production method involving multiple passes of hot rolling, reheating, rapid cooling, and tempering, which refines the prior γ grain size and achieves a microstructure with a high total area ratio of martensite and bainite, ensuring a yield strength of 620 MPa or greater and improved toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If large amounts of alloying elements (Mn, Ni, Cr, Mo) are added to inhibit ferrite formation and improve microstructure, then strength and toughness of mid-thickness part are improved, but prior γ grain size refinement is suppressed and coarse microstructure forms
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting alloying element contents (Mn: 0.5-2.0%, Ni: 0.5-3.0%, Cr: 0.1-2.0%, Mo: 0.05-1.0%) and controlling equivalent carbon content CeqIIW to 0.65% or greater through specific composition ratios, thereby resolving the contradiction between achieving adequate strength/toughness and enabling proper grain refinement during heat treatment
Solution Approach 2:
The invention focuses on achieving uniform microstructure quality throughout the plate thickness, particularly in the mid-thickness part, by controlling composition and heat treatment parameters to ensure prior γ grain size is 150 μm or less and martensite+bainite area ratio is 80% or greater, rather than optimizing only surface or average properties
2Length of stationary object
If plate thickness is increased to 100 mm or greater for large-scale structures, then structural scale and load-bearing capacity are improved, but cooling rate decreases and ferrite microstructure forms in mid-thickness part
Solution Approach 1:
The invention changes the thermal processing parameters by implementing controlled reheating to Ac3 transformation point or higher followed by cooling to obtain a specific microstructure (prior γ grain size ≤150 μm, martensite+bainite area ratio ≥80%), thereby achieving adequate strength and toughness in thick plates (100 mm or greater) without requiring excessive alloying
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 method results in steel plates with enhanced strength and toughness in the mid-thickness part, achieving a yield strength of 620 MPa or greater and Charpy impact toughness of 170 J or greater at -40°C, effectively addressing the limitations of existing techniques.
Implementation Method 1
nucleation and growth of γ grains normally occur from prior γ grain boundaries during heating of a steel material... prior γ grain size in a mid-thickness part of the steel plate has a maximum value, expressed as an equivalent circle diameter, of 150 μm or less... a total area ratio of martensite and bainite in the mid-thickness part is 80% or greater
Implementation Method 2
a shear-type reverse transformation may occur in which the prior γ grains themselves undergo a sudden reverse transformation to austenite
Implementation Method 3
it is necessary to form a bainite microstructure or a mixed microstructure of bainite and martensite in the mid-thickness part during quenching
Data Source
AI summary
A steel plate has excellent strength and toughness in a mid-thickness part thereof, despite having a plate thickness of 100 mm or greater. The steel plate has a chemical composition containing specific amounts of C, Si, Mn, P, S, Cr, Ni, Al, N, B, and O, with the balance being Fe and incidental impurities, and having an equivalent carbon content CeqIIW of 0.65 or greater. The steel plate has a yield strength of 620 MPa or greater, a plate thickness of 100 mm or greater, and has a microstructure in which prior γ grain size in a mid-thickness part of the steel plate has a maximum value, expressed as an equivalent circle diameter, of 150 μm or less, and a total area ratio of martensite and bainite in the mid-thickness part is 80% or greater.