Continuously Cast Slab Microstructure for Thermal Crack Prevention
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Solution Overview
Problem
Conventional methods fail to adequately suppress slab thermal cracking and hole defects in high-strength steel slabs during cooling and rolling due to insufficient control over microstructure and grain size, particularly in slabs with high carbon, silicon, and manganese content, leading to surface defects and reduced yield.
Innovation Solution
A continuously cast slab with controlled microstructure and grain size, comprising specific chemical compositions and cooling processes to manage austenite grain size ratio and transformation phases, ensuring a balanced microstructure of ferrite and pearlite with controlled cooling rates and heat flux to prevent thermal cracking and defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel is highly alloyed to increase strength, then tensile strength is improved, but slab toughness deteriorates significantly
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.45%, Si: 0.50-2.00%, Mn: 1.20-4.50%) and cooling rate parameters to achieve the desired balance between strength and toughness. By adjusting these parameters, the invention resolves the contradiction by preventing slab thermal cracking while maintaining high tensile strength through controlled alloying and cooling processes.
2Reliability
If slow cooling is performed to suppress thermal cracking, then slab cracking is reduced, but low-temperature transformation phases (bainite, martensite) precipitate due to high quenching property
Solution Approach 1:
The patent applies parameter changes by optimizing the cooling rate parameters across different temperature ranges. The cooling rate is controlled to be 20°C/hr or less at 700-850°C and 10°C/hr or less at 500-700°C, which prevents thermal cracking while managing transformation phases. This parameter optimization resolves the contradiction by balancing cracking prevention with microstructure control.
Solution Approach 2:
The patent applies local quality by differentiating cooling rates at different locations and temperature ranges. Different cooling rates are applied at different temperature zones (700-850°C vs. 500-700°C) to address local microstructure requirements, preventing thermal cracking in critical zones while controlling transformation phases in other zones.
3Strength
If high carbon content is used to achieve high-strength steel, then tensile strength is improved, but slab thermal cracking occurs frequently due to deteriorated toughness
Solution Approach 1:
The patent applies parameter changes by precisely controlling the carbon content parameter within a specific range (0.15-0.45%) rather than using high carbon content indiscriminately. This parameter optimization, combined with controlled Si and Mn content and specific cooling rates, resolves the contradiction by achieving high tensile strength while preventing slab thermal cracking through balanced composition and 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
The solution effectively prevents slab thermal cracking and hole defects during cooling and rolling, maintaining high yield and quality of high-strength steel slabs by managing grain boundary embrittlement and transformation stress.
Implementation Method 1
a microstructure of the continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase
Implementation Method 2
there will occur a stress attributed to differences in heat contraction and transformation expansion between the surface of the slab and the inner region thereof
Implementation Method 3
there will occur a stress attributed to differences in heat contraction and transformation expansion between the surface of the slab and the inner region thereof
Data Source
Figure 1~2
Figure 3A
Figure 3B
AI summary
Provided are a continuously cast slab that exhibits no slab thermal cracking while being cooled even when the continuously cast slab has a low toughness; and a manufacturing method thereof. The continuously cast slab is a continuously cast slab for high-strength steel that has a given chemical composition, in which an average prior austenite grain size ratio (d20/d10) is 1.0 to 4.0, provided that d10 is an average prior austenite grain size at a position 10 mm below a surface layer of the continuously cast slab and d20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab, and in which a microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of ferrite and an area ratio of pearlite is 80% or more, and a microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of ferrite and an area ratio of pearlite is 60% or more, provided that a total of the area ratio of ferrite, the area ratio of pearlite, and an area ratio of a low-temperature transformation phase is 100%.