Oxygen-Blown Slag Denitrification for Stable Low-Nitrogen Steel
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Solution Overview
Problem
Conventional methods for removing nitrogen from molten steel are costly, inefficient, and impractical for producing low-nitrogen steel due to high AlN formation, excessive slag requirements, and unclear control conditions, leading to slow denitrification speeds and potential cracking during casting.
Innovation Solution
A molten steel denitrification method using CaO-and-Al2O3-containing slag, controlled oxygen-containing gas supply, and specific conditions such as T.Fe concentration, Ls/Ls0 ratio, Al concentration, and atmospheric pressure to achieve rapid and stable nitrogen removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If Al concentration in molten steel is increased to form AlN for denitrification, then nitrogen removal efficiency is improved, but AlN remains in molten steel and causes cracking during casting
Solution Approach 1:
The patent introduces slag as an intermediary substance between molten steel and oxygen-containing gas. The slag layer absorbs nitrogen from molten steel through chemical reactions, preventing direct contact between oxygen-containing gas and molten steel. This mediator approach removes nitrogen efficiently while avoiding the formation of harmful AlN inclusions that cause cracking.
Solution Approach 2:
The patent changes the chemical composition parameters of slag (specifically controlling T.Fe concentration at 3.0 mass% or lower, and controlling the ratio of CaO to Al2O3) to optimize nitrogen removal efficiency. By adjusting these compositional parameters, the slag becomes more effective at absorbing nitrogen while preventing AlN formation in the steel.
2Productivity
If conventional AlN formation method is used for denitrification, then nitrogen removal is achieved, but the process becomes extremely costly and applicable only to steel with large amounts of dissolved nitrogen
Solution Approach 1:
The patent uses readily available and inexpensive materials for slag composition (CaO, Al2O3, and other common oxides) instead of expensive aluminum additions required by conventional methods. The slag serves as a disposable medium that can be easily adjusted and removed, making the process economically viable for various types of steel production.
3Productivity
If slag amount is increased to shield molten steel from oxidizing gas, then nitrogen removal is improved, but the amount of slag increases with vessel size making the process impractical
Solution Approach 1:
The patent optimizes the compositional parameters of slag (specifically controlling T.Fe concentration at 3.0 mass% or lower and the CaO/Al2O3 ratio) to enhance nitrogen removal efficiency per unit mass of slag. This allows effective denitrification with reduced slag quantities, making the process scalable regardless of vessel size.
4Productivity
If oxygen-containing gas is blown onto slag for denitrification, then nitrogen removal speed is improved, but control conditions become unclear and operation becomes difficult
Solution Approach 1:
The patent establishes specific parameter ranges for effective denitrification: T.Fe concentration in slag at 3.0 mass% or lower, controlled CaO/Al2O3 ratio, and regulated oxygen-containing gas flow rate. These quantified parameters provide clear operational guidelines, making the process easy to control and reproduce regardless of vessel size or production scale.
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
Stable removal of nitrogen to an extremely low concentration range in a short time, enabling efficient production of low-nitrogen steel with reduced costs and preventing cracking during casting.
Implementation Method 1
an oxygen-containing gas is blown onto the slag to perform a denitrification treatment
Implementation Method 2
CaO-and-Al2O3-containing slag is formed by a combination of an Al addition step of adding a metal-Al-containing substance to molten steel to deoxidize and turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel
Data Source
Figure 1
Figure 2
Figure 3(a)~3(b)
AI summary
A method by which an extremely low nitrogen concentration range can be stably reached in a short time is proposed. In this molten steel denitrification method, CaO-and-Al2O3-containing slag is formed by a combination of an Al addition step of adding a metal-Al-containing substance to molten steel to deoxidize and turn the molten steel into Al-containing molten steel and a CaO addition step of adding a CaO-containing substance to the molten steel, and then an oxygen-containing gas is blown from above the slag to perform a denitrification treatment. T.Fe in the slag after the denitrification treatment is set to 3.0 mass% or lower. It is preferable that, in the denitrification treatment, the oxygen-containing gas be supplied such that a ratio Ls/Ls0 between a thickness Ls0 of the slag and a depth Ls of a depression in the slag resulting from blowing of the oxygen-containing gas becomes 0.9 or lower. In this steel production method, the obtained molten steel is cast after its components are adjusted.