JPS61279614A - Production of extra-low sulfur alloy steel - Google Patents

Abstract

PURPOSE:To decrease efficiently the sulfur component of a high chronium steel by forming slag having specific basicity on the surface of a molten steel contg. a specific ratio of Cr and blowing an additive for desulfurization refining to the surface of the molten steel under specific conditions while subjecting the molten steel to bottom stirring. CONSTITUTION:The slag having >=1 basicity =(CaO+MgO)/(SiO2+Al2O3) is preliminarily formed on the surface of the molten steel contg. 10-35% Cr in the stage of producing the extra-low sulfur alloy steel from the molten steel. The powder of the additive for desulfurization refining is then blown to the surface of the molten steel under the reduced pressure at the rate at which the powder is substantially intruded into the molten steel while the molten steel is stirred by the inert gas blown from the bottom. The slag is formed on the surface of the molten steel at about 1-100kg/molten steel ton and the bottom blowing inert gas is blown at about 0.5-10Nl/min molten steel ton. The powder of the additive for desulfurization refining consists essentially of CaO and the inside of the refining vessel is reduced to about 200-0.3Torr pressure.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing ultra-low sulfur alloy steel from molten steel containing 10 to 35% chromium, such as stainless steel. The present invention relates to a method for improving desulfurization efficiency in the reductive refining stage below.

! Hiki 1ura, :.

Reduces molten steel that has been refined in advance using equipment such as converters and electric furnaces.
It is widely practiced to produce molten steel with a desired chemical composition by further refining under one pressure. Typical examples are the ■○D process in stainless steel manufacturing, the VAD process used in the manufacturing of some high-grade steels and alloy steels, and the DH method and RH method, which are mainly used for degassing molten steel. It is also a type of steel refining under reduced pressure.

FIG. 2 is a flowchart of the VOD process in stainless steel manufacturing. As shown in the figure, in this method, scrap and ferroalloy are first melted and reduced in an electric furnace to produce chromium-containing crude molten steel, which is tapped and slag removed, then charged into a VOD furnace and refined under reduced pressure. In refining in this VOD furnace, first, oxygen is top-blown under vacuum to perform decarburization and reduce the amount of C to a desired value. During the reduction (degassing) stage, the odor is oxidized during the previous vacuum decarburization, becomes chromium oxide, and is dissolved.
(In order to reduce the chromium oxide generated as slag on the steel surface, deoxidizers such as Si and AI and CaO,
A slag-forming agent such as CaF2 is added, and agitation is performed by means of bottom blowing, and desulfurization is also performed. Next, the molten steel is deoxidized, and then tapped and cast.

In the conventional VOD process, when the S content in the steel is particularly low, the basicity of the flux used in the reduction stage is increased, and in order to promote the formation of flux and the contact reaction with molten steel, ^r gas is The molten steel was strongly stirred by bottom blowing.

However, when high basicity flux is applied to the surface of molten steel, it tends to fuse together and form lumps, making it difficult to form into slag. On the other hand, vigorously stirring the molten steel to promote slag formation causes damage to the refractories in the container and a drop in the temperature of the molten steel, making the production of ultra-low sulfur chromium alloy steel complicated and expensive.

On the other hand, it is generally known that if S, which is one of the impurities in ferritic or austenitic stainless steel, is reduced to the order of several ppm, the corrosion resistance will be dramatically improved. is shown in Figure 3.

FIG. 3 is a graph showing the results of investigating the effect of the amount of S in stainless steel on the corrosion rate using a boiling hydrochloric acid test. The test used a stainless steel specimen of 2t XIOW The temperature was measured and the results are shown in Figure 3.

As understood from Figure 3, the amount of S in the steel is 20 p, pm.
If it is below, the corrosion rate will drop sharply, and stainless steel with extremely excellent corrosion resistance will be obtained.

In this way, it has long been desired to efficiently reduce the 8% of high chromium steel such as stainless steel.
1 was there.

The present invention solves the technical limitations of steel refining under reduced pressure.
) industry, especially in terms of efficiently supplying and reacting additives for desulfurization and refining, which go beyond the limits of conventional technology.
The purpose is to propose new refining technology.

More specifically, the object of the present invention is to contain chromium from 10 to
- In a method for desulfurizing molten steel containing 35%,
- To provide ultra-low sulfur chromium alloy steel by promoting the reaction 1 between desulfurization refining additives and molten steel without vigorously stirring the molten steel.

Means for solving the problem As mentioned above, in order to particularly reduce the S content in steel containing chromium, the basicity of the flux used in the reduction (degassing) stage is increased and the flux is placed on the surface of the molten steel. In the conventional method of adding molten steel, desulfurization does not proceed sufficiently if the stirring force of the molten steel is weak.
In addition, strong stirring causes damage to refractories and a drop in molten steel temperature, increasing the manufacturing cost of ultra-low sulfur steel. In order to solve these problems, the inventors of the present invention have discovered the following after various experiments and studies.

That is, as a means to promote slag formation of flux and contact reaction with molten steel, during the reduction period, when powder of a desulfurization refining additive is blown into molten steel using refining gas and/or other carrier gas, If the contact area with molten steel is large and desulfurization proceeds efficiently, and if the basicity of the slag is increased, it is possible to reduce the S content in the steel to the order of several ppm.

The present invention was completed based on the above findings, and includes a method for producing an ultra-low sulfur alloy network from molten steel containing 10 to 35% chromium.
) / (SiO2+A1ao3) dissolves 1 or more Noslag:.

Generates on the steel surface and blows inert gas from the bottom
i! While stirring the molten steel, add powder of additive for desulfurization refining.
The method is characterized in that the powder is blown onto the surface of the molten steel under reduced pressure using a refining gas and/or other carrier gas at a speed that allows the powder to sufficiently penetrate into the molten steel.

The amount of slag generated on the surface of molten steel is 1 to +00 kg.
/ ton of molten steel, the amount of inert gas blown from the bottom is 0.5
~10 N 1 /min molten steel tons is preferred.

The powder of the desulfurization refining additive used contains CaO, and the degree of vacuum in the refining vessel is S U 5.
304 refining ■ oD process, 2oo ~ 0, 3T
It is in the range of orr.

Further, the carrier gas into which the powder of the desulfurization refining additive is blown may be a refining gas, that is, O2 gas, an inert gas such as Ar, or a mixture thereof.

The reasons for limiting each condition of the method of the present invention will be explained below.

The reason for limiting the range of chromium content in molten steel that is subject to the method of the present invention is that steel types that are subject to ultra-low sulfur in the VOD process contain 10% or more of chromium, whereas currently chromium contains 35% or more. Steel types containing chromium are practically impractical.

In addition, slag with a basicity of 1 or more is generated on the surface of the molten steel in advance (Cr = C) generated in the VOD vacuum decarburization process.
In order to turn slag into slag in the reduction process of reducing oxides such as This is because the effect of applying it cannot be obtained.

Hereinafter, the present invention will be explained in detail with reference to Examples.

It should be noted that these Examples are merely illustrative of the present invention and do not limit the technical scope of the present invention in any way.

EXAMPLE Crude molten steel having the chemical composition shown in Table 1 was subjected to a decarburization process using oxygen top blowing. At this time, in the low carbon region, some Cr was oxidized and turned into chromium oxides, which were generated as slag on the surface of the molten steel. Next, for the purpose of deoxidizing the molten steel and reducing chromium, deoxidizers such as Sl and A1 and slag forming agents such as CaO and CaF2 are added, stirred by bottom blowing etc., and the chemical composition shown in Table 1 below is added. The composition is as follows.

Next, according to the present invention, desulfurization was performed by top blowing powder while blowing ^r gas from the bottom of the furnace.

The desulfurization refining additive powder to be sprayed onto the surface of the molten steel is a mixed powder with a particle size of 50 mesh or less, which has a composition of 6% Cab Ni, 17% CaFz, and 5iOz Ni. The molten steel was sprayed at high speed with Ar as a carrier gas. This top-blowing lance was a straight nozzle, and the powder was blown out with a Matsuha 1 using Ar as a carrier gas.

In addition, the Ar gas pressure is 5Kg/ca! , gas flow rate is 2N
The powder supply rate was 2 km/min/ton, the supply amount was 12 kg/min, and
The distance between the top blowing lance and the molten steel surface was maintained at 500n+m. At the same time, Ar was blown through a porous plug for bottom-blowing stirring at a rate of 1N1/min/ton. During this time, the inside of the furnace was maintained at a reduced pressure of 2 Torr.

Table 1 shows the chemical composition of the chromium-containing steel after powder top-blowing desulfurization according to the present invention.

Table 1 (% by weight) FIG. 1 is a graph showing the sulfur distribution ratio in the desulfurization treatment according to the conventional technique and the method of the present invention. This sulfur distribution ratio (S)/l:S) is the ratio of the amount of S in the slag to the amount of S in the steel, and is an index of desulfurization. Naturally, the higher the value, the better the desulfurization progresses. .

In FIG. 1, the marks indicate the results obtained by the conventional over-blowing method, the Δ marks indicate the results obtained by vigorous stirring in the conventional method, and the marks indicate the results obtained by top-blowing powder according to the present invention.

As shown in Table 1 and Figure 1, in the case of the conventional overlay method, when the slag basicity reaches 2.0 or more, the sulfur distribution ratio becomes constant and the desulfurization efficiency does not improve any further. In the powder top blowing method according to the invention, even if the slag basicity becomes 2.0 or more, the sulfur distribution ratio does not remain constant and becomes even higher.
A higher sulfur distribution ratio than the conventional method using stronger stirring power was obtained, and it was confirmed that the present invention has a much superior desulfurization effect compared to the conventional method.

Effects of the Invention As explained in detail above, the present invention involves forming a slag with a basicity of 1 or more on the surface of molten steel, and stirring the molten steel by blowing inert gas from the bottom under reduced pressure. The present invention is characterized in that the powder of the desulfurization refining additive is blown onto the surface of the molten steel using a refining gas and/or another carrier gas at a rate that allows the powder to sufficiently penetrate into the molten steel.

By the method of the present invention, the amount of S in steel can be reduced to several ppm, and stainless steel with excellent corrosion resistance can be provided. In addition, in the method of the present invention, it is not necessary to vigorously stir the molten steel with inert gas from the bottom of the furnace. Since there is no waste, the unit consumption of refractories in the furnace is improved, and desulfurization can be carried out with a better heat balance.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the sulfur distribution ratio in desulfurization treatment according to the prior art and the method of the present invention, Fig. 2 is a flowchart of the VOD process used in stainless steel production, and Fig. 3 is a graph showing the sulfur distribution ratio in desulfurization treatment by the conventional technology and the method of the present invention. 2 is a graph showing the results of a boiling hydrochloric acid test to examine the effect of amount on corrosion rate. Patent applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Masahiko Arai Figure 1

Claims (1)

[Claims] In a method for producing ultra-low sulfur alloy steel from molten steel containing 10 to 35% chromium, slag having a basicity of 1 or more according to the following formula is formed on the surface of the molten steel, and an inert gas is blown from the bottom. While stirring the molten steel, the powder of the desulfurization refining additive is applied to the surface of the molten steel under reduced pressure using a refining gas and/or other carrier gas at a speed that allows the powder to sufficiently penetrate into the molten steel. A method for producing ultra-low sulfur alloy steel, characterized by blowing. Basicity = (CaO + MgO) / (SiO_2 + Al_2
O_3)