Thick Steel Plate Yield Strength via Center Temperature Control
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Solution Overview
Problem
Current methods fail to produce thick high-toughness high-strength steel plates with a yield strength of 620 MPa or above, as they either require excessive capital investment, uncertain strength, or compromise toughness due to center porosities and segregation issues, especially in thick steel plates with large amounts of alloying elements.
Innovation Solution
A method involving a continuously cast slab with specific chemical composition and microstructure, heated to 1200°C to 1350°C, hot worked with a strain rate of not more than 3/s, and subjected to multiple hot rolling passes with at least 4% reduction per pass, followed by quenching and tempering, to achieve a microstructure with an average prior austenite grain size of 50 µm or less and a martensite/bainite phase fraction of 80% or more throughout the plate thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If alloying elements are added in large amounts to ensure desired characteristics in thick steel plates, then strength is improved, but center porosities occur due to center segregation
Solution Approach 1:
The invention changes the temperature parameter during hot rolling, specifically maintaining the center temperature at 1000°C or higher during the rolling process. This temperature control prevents center segregation and porosity formation while allowing the use of alloying elements to achieve the desired yield strength of 620 MPa or higher without compromising internal quality.
2Productivity
If continuously cast slabs are used instead of ingot slabs, then production efficiency is improved, but the thickness of the slab is smaller causing small rolling reduction
Solution Approach 1:
The invention changes the temperature parameter during hot rolling to maintain the center temperature at 1000°C or higher, which enables effective hot rolling of continuously cast slabs with smaller initial thickness. This allows achieving the required rolling reduction and final product quality while maintaining the productivity benefits of continuous casting.
3Strength
If thick steel plates with 620 MPa yield strength are produced, then strength is improved, but toughness deteriorates due to center porosities
Solution Approach 1:
The invention changes the temperature parameter during hot rolling to maintain the center temperature at 1000°C or higher, which prevents center segregation and porosity formation. This enables the production of thick steel plates with 620 MPa yield strength while maintaining excellent toughness and uniform microstructure throughout the plate thickness.
4Ease of manufacture
If conventional hot rolling methods are used on continuously cast slabs, then manufacturing simplicity is maintained, but center segregation and porosity occur
Solution Approach 1:
The invention introduces a specific temperature control parameter during hot rolling, maintaining the center temperature at 1000°C or higher. This relatively simple temperature control measure effectively prevents center segregation and porosity in continuously cast slabs while maintaining manufacturing simplicity and avoiding complex process changes.
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 produces steel plates with a yield strength of 620 MPa or above and excellent toughness, enhancing the internal quality of thick steel plates, reducing production costs, and improving the safety and efficiency of steel structures.
Implementation Method 1
a continuously cast slab is heated to 1200°C to 1350°C
Implementation Method 2
hot worked at not less than 1000°C with a strain rate of not more than 3/s and a cumulative working reduction of not less than 15%
Implementation Method 3
air cooled and heated again to Ac3 point to 1200°C
Implementation Method 4
quenched to 350°C or below and tempered at 450°C to 700°C
Implementation Method 5
tempered at 450°C to 700°C
Data Source
AI summary
The invention provides thick high-toughness high-strength steel plates having excellent strength and toughness in the central area through the plate thickness, and methods for manufacturing such steel plates. The thick steel plate has a specific chemical composition and includes a microstructure having, throughout an entire region in the plate thickness direction, an average prior austenite grain size of not more than 50 µm and a martensite and/or bainite phase area fraction of not less than 80%. A continuously cast slab having the specific chemical composition is heated to 1200°C to 1350°C, hot worked with a strain rate of not more than 3/s and a cumulative working reduction of not less than 15%, and thereafter hot rolled and heat treated.