EP-4741517-A1 - METHOD FOR DEPHOSPHORIZING MOLTEN IRON

EP4741517A1EP 4741517 A1EP4741517 A1EP 4741517A1EP-4741517-A1

Abstract

To provide a method for dephosphorizing molten iron that can effectively improve the phosphorus distribution and thus promote a dephosphorization reaction of molten iron by providing electric energy using an electrochemical method. Specifically, provided is a method for dephosphorizing molten iron, applying a current between molten slag and molten iron via two electrodes, with one in contact with the molten iron serving as an anode and the other in contact with only the molten slag serving as a cathode, characterized in that Relational Expression (1) below is satisfied among the density I of the applied current, the molten steel temperature T of the molten iron, the phosphorus distribution Lp in the molten slag, and the required phosphorus distribution Lp' for the molten slag: I ≥ α × ln Lp ′ / Lp × T where in Relational Expression (1), I represents the density (A/m 2 ) of the applied current, α represents the constant, Lp represents the phosphorus distribution (-) in the molten slag, Lp' represents the required phosphorus distribution (-) for the molten slag, and T represents the molten steel temperature (K) of the molten iron.

Inventors

KIYOIZUMI, KOTA
NEGISHI, Hidemitsu
MURAI, TAKESHI
MIZOBATA, Keisuke

Assignees

JFE Steel Corporation

Dates

Publication Date: 20260513
Application Date: 20240328

Claims (5)

A method for dephosphorizing molten iron, comprising: applying a current between molten slag and molten iron via two electrodes, with one electrode in contact with the molten iron serving as an anode and the other in contact with only the molten slag serving as a cathode, characterized in that a density I of the applied current satisfies Relational Expression (1) below, in relation to a molten steel temperature T of the molten iron, a phosphorus distribution LP in the molten slag, and a required phosphorus distribution LP' in the molten slag: [Math. 1] I ≥ α × ln Lp ′ / Lp × T where, in Relational Expression (1), I represents the density (A/m 2 ) of the applied current, α represents a constant, LP represents the phosphorus distribution (-) in the molten slag, LP' represents the required phosphorus distribution (-) in the molten slag, and T represents the molten steel temperature T (K) of the molten iron.
The method for dephosphorizing molten iron according to claim 1, wherein a concentration [C] of carbon contained in the molten iron is 4.0 mass% or less, and the density I (A/m 2 ) of the applied current satisfies Relational Expression (2) below: [Math. 2] I ≥ 5.264 × 10 − 2 ln Lp ′ / Lp × T where in Relational Expression (2), I represents the density (A/m 2 ) of the applied current, Lp represents the phosphorus distribution (-) in the molten slag, Lp' represents the required phosphorus distribution (-) in the molten slag, and T represents the molten steel temperature (K) of the molten iron.
The method for dephosphorizing molten iron according to claim 1 or 2, wherein arc discharge is not generated by applying a current between the molten slag and the molten iron.
The method for dephosphorizing molten iron according to claim 3, wherein a value of the current is 5000 (A) or less.
The method for dephosphorizing molten iron according to claim 1 or 2, wherein a liquid phase ratio of the molten slag is 60 vol.% or greater.

Description

Technical Field The present invention relates to a method for dephosphorizing molten iron. In particular, the present invention relates to a method for dephosphorizing molten iron that can promote dephosphorization reactions. Background Art Phosphorus contained in molten steel tends to be segregated at grain boundaries in a material formed from the molten steel, resulting in reduced strength and toughness of steel formed from the molten steel. Accordingly, when producing steel from molten steel, the phosphorus content in the molten steel is typically controlled to remain low. In recent years, there has been growing demand for the production of high-grade steel, and further reduction of the phosphorus content in molten steel has been desired. However, the burden on dephosphorizing molten iron has become increasingly severe due to factors such as degraded quality of iron ore, which serves as a raw material for molten steel. From such a technical perspective, it is essential to develop a technology for reducing the phosphorus concentration in molten steel after the molten steel has been processed. In a conventional technology for dephosphorizing molten steel, since phosphorus oxide contained in the molten steel is acidic, lime-based flux such as CaO is added to the molten steel to increase the basicity of molten slag formed during the production of the molten steel. Adding the lime-based flux, such as CaO, into the molten steel enhances the dephosphorization capability of molten slag, thereby reducing the equilibrium phosphorus concentration in the molten steel. However, molten slag with higher basicity has a higher melting temperature. Therefore, if the molten steel temperature drops during the preliminary treatment of molten pig iron, the viscosity of the molten slag increases. This may result in insufficient slag formation from CaO, which is contained in the lime-based flux introduced into the molten steel, leading to a reduced dephosphorization efficiency. To increase such low dephosphorization efficiency in order to compensate for the efficiency of dephosphorization reactions, it is necessary to use an excess refining material for the molten steel. However, using an excess refining material for the molten steel increases cost for refining the molten steel. Further, if dephosphorization is performed to reduce the concentration of phosphorus in the molten steel to an extremely low level, the amount of the molten slag becomes excessive. This may lead to problems such as insufficient capacity at a molten steel refining facility or a slag removal facility. To address this challenge, in dephosphorization of molten steel aiming to achieve an extremely low phosphorus concentration, a method has been adopted in which a halide, such as fluorite (CaF2), is added to promote slag formation from CaO contained in a lime-based flux and thereby reduce the amount of flux to be charged. This method enables both high dephosphorization capability and a reduced amount of molten slag, thereby maintaining a high dephosphorization rate even in a region of extremely low phosphorus concentration. However, adding halide, such as fluorite (CaF2), into the molten steel results in an increased content of fluorine (F) in the resulting molten slag. In recent years, there have been growing social concerns about environmental issues, and the use that evokes concerns about the elution of fluorine (F) has been regulated. From this perspective, the use of halide such as fluorite (CaF2) for molten steel has become difficult. Therefore, there is a need for a technology that can efficiently dephosphorize molten steel without increasing the basicity of slag. In view of the foregoing circumstances, research has been conducted on a technology for efficiently dephosphorizing molten steel without increasing the basicity of molten slag, with a focus on electric energy. For example, Non Patent Literature 1 discloses a method for promoting dephosphorization reactions where a reaction that occurs at the interface between slag and molten iron (hereinafter referred to as a "slag-metal reaction") is arranged based on the concept of electrochemistry. Specifically, Non Patent Literature 1 presents the concept of a reaction promoted by electric energy, citing an example where polarizing the potential of molten iron toward the noble side causes iron to be removed through oxidation into the slag. Conversely, polarizing it toward the less noble side reduces iron ions in the slag, returning them to the molten iron. Various research related to slag-metal reactions has been reported based on Non Patent Literature 1. Patent Literature 1 discloses a method of applying a current, using one electrode in contact with molten slag and another in contact with an iron bath, thereby reducing the content of metallic iron droplets in molten slag and variation in the metallic iron content in the slag per charge. Patent Literature 2 discloses a method in which impurities such as