Molten Steel Denitrification With Controlled Stirring
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Solution Overview
Problem
Conventional methods for removing nitrogen from molten steel are costly, inefficient, or impractical, especially when producing low-nitrogen steel with low carbon concentration, and often result in AlN remaining in the steel, leading to cracking during casting or requiring complex control conditions.
Innovation Solution
A denitrification method using CaO-and-Al2O3-containing slag, with controlled stirring and atmospheric conditions, including a stirring power density of 60 W/t or higher, an atmosphere of 1.0 × 10^5 Pa or lower, and limited MgO concentration in the slag, to stabilize nitrogen removal to extremely low concentrations without top-blown gas.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If Al concentration in molten steel is increased to form AlN for denitrification, then nitrogen removal is improved, but AlN remains in molten steel and causes cracking during casting
Solution Approach 1:
The patent introduces CaO-containing slag as an intermediary substance to transfer nitrogen from molten steel to slag phase. The slag acts as a mediator that absorbs nitrogen through chemical reactions (forming compounds like Ca3N2) while preventing AlN inclusion formation in the steel, thus resolving the contradiction between nitrogen removal and cracking prevention
Solution Approach 2:
The patent changes the chemical environment parameters by controlling CaO concentration in slag (5-20 mass%), Al concentration in steel (0.03-0.15 mass%), and temperature (1600-1700°C) to optimize the nitrogen transfer reaction from steel to slag while preventing AlN formation, thereby achieving both low nitrogen content and high casting reliability
2Quantity of substance
If conventional denitrification methods using AlN formation are used, then nitrogen removal is achieved, but the process becomes extremely costly and is applicable only to steel with large amounts of dissolved nitrogen
Solution Approach 1:
The patent optimizes key parameters including reducing Al concentration to 0.03-0.15 mass% (much lower than conventional methods), controlling CaO concentration in slag to 5-20 mass%, and maintaining temperature at 1600-1700°C. These parameter changes enable cost-effective nitrogen removal by minimizing expensive alloy additions while achieving nitrogen concentrations below 10 ppm
Solution Approach 2:
The patent adopts and adapts the slag-based nitrogen transfer concept from existing research (Patent Literatures 3, 4, 5) but optimizes the specific composition parameters (CaO/Al2O3 ratio, CaO concentration) and process conditions (stirring power density, temperature) to create a more cost-effective and universally applicable method for various steel types
3Productivity
If stirring power density is increased to enhance nitrogen removal speed, then denitrification efficiency is improved, but energy consumption increases
Solution Approach 1:
The patent identifies and optimizes the stirring power density parameter to a specific range (10-100 W/t) that achieves maximum nitrogen transfer efficiency from steel to slag. This optimized parameter range provides sufficient mixing to maintain contact between steel and slag phases while avoiding excessive energy consumption, resolving the contradiction between denitrification speed and energy use
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 extremely low concentrations in a short time, avoiding AlN formation and cracking, while being cost-effective and applicable to various steel types.
Implementation Method 1
adding a metal-Al-containing substance to molten steel to deoxidize and turn the molten steel into Al-containing molten steel
Implementation Method 2
form aluminum nitride (hereinafter 'AlN') and thereby remove nitrogen
Implementation Method 3
nitrogen in molten steel charged in a reaction vessel, such as a ladle, by bringing the molten steel into contact with slag that is added and formed on top of the molten steel
Implementation Method 4
CaO-and-Al2O3-containing slag formed by a combination of an Al addition step... and a CaO addition step
Implementation Method 5
the molten steel is stirred at a stirring power density ε of 60 W/t or higher
Implementation Method 6
the molten steel is stirred at a stirring power density ε of 60 W/t or higher
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
A method by which an extremely low nitrogen concentration range can be stably reached in a short time without use of a top-blown gas is proposed. A molten steel denitrification method is a denitrification process in which CaO- and-Al2O3-containing slag 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 is brought into contact with the Al-containing molten steel to remove nitrogen in the molten steel, in which the molten steel is stirred at a stirring power density ε of 60 W/t or higher. It is preferable that, in the denitrification process, a surface of the molten steel or the slag is subjected to an atmosphere of 1.0 × 105 Pa or lower. In a steel production method, the obtained molten steel is cast after the components are adjusted.