JPH02190420A - Melting method for ultra-low carbon steel - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention removes carbon [C] contained in molten steel to an extremely small amount, for example, 0.001 wt% or less, thereby making ultra-low carbon steel molten. The present invention relates to an efficient, simple and inexpensive decarburization method for manufacturing.

(Prior art) In the case of steel sheets used as thin steel sheets for automobiles, thin steel sheets for beverage cans, etc., the carbon concentration in steel must be extremely small to improve workability and prevent aging. It is.

Generally, in the steel industry, decarburization of molten steel is carried out using various vacuum decarburization equipment as shown in, for example, 3rd Edition Iron and Steel Handbook ■ Pig Making/Steel Making, pages 671-685.

In this case, oxygen [01] contained in the molten steel, or iron ore Fe, 0. 1G of carbon contained in molten steel by the following reaction using an oxidation source such as , oxygen gas 0□]
is being removed.

[CI + [0] = Co,,. .

y [CI + FexOy = V CO (*
*sl + xFe ”' (1) [CI +
1/201 = CO (was > (Problem to be solved by the invention)) However, when the carbon concentration of molten steel becomes 0.015 wt% or less, the decarburization rate begins to decrease, and in addition, the [C] concentration becomes 0.00 wt% or less.
The decarburization rate further decreases from about 5 wt%, and in order to continue decarburization, the decarburization treatment time must be extended.

Moreover, in such a case, the temperature of the molten steel decreases.

For this purpose, in the next step, the molten steel is reheated, or the temperature of the molten steel to be decarburized when tapped from a converter or electric furnace is raised to a high temperature to compensate for the temperature drop corresponding to the decarburization time. I am dealing with this by doing this. This goes against energy conservation and is not efficient.

Moreover, when the tapping temperature becomes high, the refractories in the converter or electric furnace are eroded, the unit consumption of refractories increases, the cost for decarburization increases, and the cost of the reaction vessel used for decarburization increases. The unit consumption of refractories also increases, which is both inefficient and uneconomical, and it is extremely difficult to stably produce ultra-low carbon steel.

An object of the present invention is to provide an efficient and simple method for producing ultra-low carbon molten steel.

(Means for Solving the Problems) In order to achieve this object, the present invention has the following objectives: In the region where the carbon concentration [%C1] of molten steel to be decarburized under reduced pressure is Q, 015wt% or less, the grain size is 0.1% or less. The method is characterized in that the molten steel is decarburized while supplying limestone or dolomite in the range of 20 to 20 g, either singly or as a mixture, to the molten steel. moreover[
This is a method for producing ultra-low carbon steel characterized by maintaining %C1 in a range of 0.005 tvt% or less.

((1+72 ・-1% [S])/1801 (1-
0,25)≦wt% [01≦((1+72 ・-t%
(S])/180) (1+0.25)・・・・・・(
2) The origin of the technical idea of the present invention is that when molten steel is decarburized according to the above formula (1), (1) the bubble generation nuclei necessary for the generation of CO gas bubbles from inside the molten steel are provided, and (2)
The added substance itself contacts the molten steel and is heated, producing CO□
Generates gas and reacts with EC in the reaction shown in equation (3) below to promote decarburization, and furthermore, CO or C
The purpose of this method is to increase the contact interface area (gas-liquid calculated area) between O□ gas and molten steel and promote the decarburization reaction at the gas-liquid interface expressed by the following equation (4).

COz + [CI = 2CO(9□, ・・
・・・・・・・・・・・・・・・(3) [ols,,fac
e: Resistance of molten iron surface in contact with gas 01 The inventors have
In the region of low carbon concentration, the decarburization reaction due to the generation of CO gas bubbles from inside the molten steel is extremely inhibited, and the decarburization reaction mainly occurs at the surface where the molten steel and gas come into contact, as shown by equation (4). I figured out that it was becoming dominant. Furthermore, a nucleus for producing CO gas is required to promote the decarburization reaction inside the molten steel expressed by equation (1).

Furthermore, the decarburization reaction in the region of low carbon concentration is mainly a reaction at the gas-liquid interface, and the decarburization reaction is caused by the surface-active element [S
] and [011 degrees are promoted when the relationship of equation (2) is satisfied, and this has already been disclosed in Japanese Patent Application Laid-Open No. 82239/1983 as a method for producing ultra-low carbon steel. According to the present invention, the decarburization reaction site is enlarged by the generation of air bubbles, and at the same time, the [0] concentration is adjusted according to the IS] concentration by adding a deoxidizing agent to reduce the inhibitory factor of the decarburization reaction. Decarburization speed can be increased.

In the present invention, the substance that is blown or sprayed into the molten steel is of great importance. In other words, when it comes into contact with molten steel, ■
It is necessary to generate gas; (1) it does not melt easily and can become a nucleus for generating CO gas; and (2) it must not melt into molten steel as carbon. Limestone or dolomite is most suitable as a material having the above three properties.

Hereinafter, limestone or dolomite and a mixture thereof will be referred to as a carbonate compound. The above carbonate compound is (5) at the molten steel processing temperature.
It decomposes into solid oxide and CO2 gas by the reaction shown in equation (6).

CaC0= = CaO+ Cot ・Old...old” (5) CaCO+ ・MgC01= Ca
O+ MgO+ 2COz −= ”・= (6) That is, CaO and MgO are oxides with high melting points and do not melt at the processing temperature of molten steel, and are effective because they produce CO gas bubbles. This increases the gas-liquid interface and does not supply carbon to the molten steel.Of course, chemically synthesized CaCO and CaCO3/MgCO3 have the same effect.

The particle size of the carbonate compound used is an extremely important point in order to disperse the generation nuclei of COO gas or CO2 gas generated in molten steel and to increase the effect of increasing the gas-liquid interface area.

In other words, if the particle size of the carbonate compound is too large, the size of the generated CO□ gas will increase, and the effect of increasing the gas-liquid interface area per unit weight of the substance used will be small, which will also reduce the effect of increasing the decarburization rate. It is small (and uneconomical). Therefore, the upper limit of the particle size of the carbonate compound used is 20 m11.

On the other hand, if the particle size of the sulfuric acid compound is too small, the efficiency with which it blows through or scatters and reacts with molten steel decreases, and the effect of increasing the decarburization rate decreases. Therefore, the lower limit of the particle size of the carbonate compound is set to 0.1 tm.

The method of supplying the carbonate compound to the molten steel in the present invention includes a method of spraying the carbonate compound onto the molten steel from above the surface of the molten steel together with a carrier gas (inert gas such as Ar) using a top blowing lance;
Alternatively, the carbonate compound can be injected into the molten steel together with a carrier gas (inert gas such as Ar) using an immersed lance, or the carbonate compound can be added by dropping it onto the molten steel using an electromagnetic feeder. No matter which method is used, there is no difference in the decarburization rate of molten steel, which is good.

In the present invention, when decarburizing molten steel using a carbonate compound, the carbon weight concentration [%C1] of the molten steel to be decarburized is 0.
In the range of 005% or less, the decarburization rate is improved by maintaining [011 degrees] within the range shown by the above formula (2) depending on the [S] concentration contained in the molten steel.

When the oxygen concentration of the molten steel to be decarburized is too low, it is effective to add, spray, or inject iron oxide powder, iron ore powder, etc. to the molten steel. In this case, it may be mixed with a carbonate compound and added to the molten steel, or it may be sprayed or injected into the molten steel. Furthermore, oxygen gas can be used to increase the oxygen concentration in molten steel. On the other hand, if the oxygen concentration is too high, the deoxidizing agent 1, for example, aluminum or the like and a carbonate compound may be mixed together or separately added to the molten steel.

Furthermore, when an inert gas is sprayed onto the surface of the molten steel in carrying out the present invention, the COO gas movement speed near the interface between the molten steel and the gas phase can be increased, and the decarburization speed can be improved.

The method of the present invention can be applied to current vacuum refining equipment such as DH,R
It can also be applied when decarburizing molten steel using H, VOD, VAD, etc. equipment.

(Examples) The present invention will be described below with reference to drawings based on examples.

Example】.

The temperature was 600°C, the weight was 100 kg, and the decarburization treatment of molten steel with a CI concentration of approximately 0.015 wt% was carried out under a low frequency vacuum at a vacuum degree of 2 to 10 ma Hg. Using an induction melting furnace, limestone and dolomite alone and a mixture of limestone and dolomite (mixed weight ratio: 1
) is carried out using a lance and spraying a quiet gas as a carrier gas, and the change in [C] concentration over time is shown in Fig. 1.

The particle size of the carbonic acid compound is 0.5-3I! The spraying amount is in the range of 20 to 65 (g/ml). In the comparative example, iron ore (particle size: 0.5 to 3) was
(g/win) These are the results when sprayed. Comparative example is 4
Even with 0mlnO decarburization treatment, [C] concentration is 0.0015wt
%, whereas according to the method of the present invention, the [C] concentration is 0.001 in approximately 20 ml decarburization treatment.
It can be decarburized to less than wt%, and ultra-low carbon molten steel can be produced.

Example 2 Molten steel was decarburized at a temperature of 1600°C, a weight of 100 kg, and a [C] concentration of approximately 0.015 wt% before decarburization under a vacuum degree of 0.5 to 2 mn+Hg. Using a low-frequency vacuum induction melting furnace, limestone and dolomite alone and a mixture of limestone and dolomite (mixed weight ratio:
1) was carried out by adding it to molten steel using a feeder.

20 The [C] concentrations before and after the decarburization treatment are shown in Table 1. The particle size of the carbonic acid compound is in the range of 2 to 5 flIIl, and the amount added is in the range of 20 to 65 (g/win). In the comparative example, iron ore (particle size: 2 to 5 ws) was
This is the result when spraying at a speed of (g/win). The comparative example is a 20 m in O decarburization treatment [C1 concentration is 0.00
4 wt%, whereas according to the method of the present invention, the [C] concentration was reduced to 0.001 in 20ml decarburization treatment.
It can decarburize to below ivt% and produce ultra-low carbon molten steel.

Table 1 Example 3 The temperature was 1600°C, the weight was 100 kg, and the [C] concentration before decarburization was approximately 0.015 wt%. Using a low-frequency vacuum induction melting furnace, limestone and dolomite alone and a mixture of limestone and dolomite (mixing weight ratio: 1) were injected into molten steel using a lance. The [C] concentrations before and after this decarburization treatment are shown in Table 2.

The particle size of the carbonic acid compound is in the range of 0.3 to 1.5 mm, and the amount of blowing is in the range of 32 to 60 mm (g/rain).

In the comparative example, iron ore (particle size: 0.3 to 1.5 on) was
This is the result when spraying (60 axes/win). The comparative example is a 20mlnO decarburization treatment [CI concentration is 0.0030i1
In contrast, according to the method of the present invention,
[C] concentration is 0.001 wt% with 20mlp decarburization treatment
It is possible to decarburize to the following levels and produce ultra-low carbon molten steel.

Table 2 Example 4 Temperature was 1600°C 1 weight was 100 kg, EC before decarburization] Concentration was approximately 0.015 wt%, [S]
Molten steel with a concentration of 0.015 wt% was decarburized using a low-frequency vacuum induction melting furnace with a degree of vacuum of 0.5 to 2 mmHg, and limestone and dolomite alone and a mixture of limestone and dolomite ( The mixture weight ratio: 1) was added to molten steel using a feeder, the oxygen concentration of the molten steel was measured with an oxygen sensor using a solid electrolyte, and the oxygen concentration was controlled at 0.013 wt% while adding Al. Table 3 shows the [C] concentrations before and after this decarburization treatment. The particle size of the above carbonate compound is 0
．． The value ranges from 3 to 1.5, and the amount added is 32 to 60 (g
/m1n). The comparative example is iron ore (particle size: 0
．． These are the results when 33 to 60 (g/mln) of 3 to 1.5 mal) were added. In the comparative example, the [C] concentration was approximately 0.0030 wt% with 20 ml nO decarburization treatment, whereas according to the method of the present invention, the [C] concentration was up to 0.001 wt% with 20 ml nO decarburization treatment. It can be decarburized and extremely low carbon molten steel can be produced.

Table 3 Example 5 The temperature was 1600, the C1 weight was 100 kg, the [C] concentration before decarburization was approximately 0.015 i1t%, and the [S]
Molten steel with a concentration of 0.015 wt% was decarburized using a low-frequency vacuum induction melting furnace under a degree of vacuum of 0.5 to 2 mm/g. Weight ratio: 1) was added to molten steel using a feeder, the oxygen concentration of the molten steel was measured with an oxygen sensor using a solid electrolyte, and the oxygen concentration was reduced to 0.013 nt% while adding Ti or a mixture of Al and Ti. was controlled. Table 4 shows the [C] concentrations before and after decarburization. The particle size of the carbonate compound is in the range of 0.3 to 1.5 yn,
The amount added is in the range of 32 to 60 (glolin). In the comparative example, iron ore (particle size: 0.3 to 1.5 mm+) was
These are the results when 60 (g/mln) was added. The comparative example is a 20mlnO decarburization treatment and the [C] concentration is 0.0030w
In contrast, according to the method of the present invention,
With decarburization treatment of 20 ml, [CI concentration was 0.001 wt]
% or less, making it possible to produce ultra-low carbon molten steel.

Table 4 Example 6゜Atmospheric pressure 1 stroke Hg, temperature 1600'C1 weight 10
Decarburization treatment of molten steel was carried out in a low frequency vacuum induction furnace, and the capacity coefficient kc (min-') of the decarburization reaction was measured.

The size of the carbonate compound added from the top of the molten steel using a feeder is limestone, dolomite, and a mixture of limestone and dolomite (mixed weight ratio of limestone/dolomite of 20.5 to 1.5) in the range of 0.01 to 50 mm. It is.

One degree of [C] before decarburization is approximately 0.010 wt%. The capacity coefficient kc of the decarburization reaction obtained when the added carbonate compound has a particle size of 0.01 to 0.05 mm! t1fi・(
Figure 2 shows the relationship between kc/kc''ts++・ and the particle size of the carbonate compound, based on lIin-'). However, the addition rate of the carbonate compound is in the range of 40 to 65 (g/win). The value of kc/kc*ta- becomes large when the carbonate compound has a particle size in the range of 0.1 to 20 mm.This tendency depends on the type of carbonate compound and its mixing weight ratio. No differences observed.

(Effect of the invention) By the method of the invention, the [C] concentration of molten steel is 0.001w.
It has become possible to easily decarburize to an ultra-low carbon concentration of t% or less, making it easier to manufacture ultra-low carbon steel.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the change over time of [C] 1 degree, and FIG. 2 is a diagram showing the relationship between kc/kc""· and the particle size of the carbonate compound. Agent: Patent Attorney Masamitsu Aki Sawa and 1 other Figure 1 Processing B Benma (mirl) Voluntary procedure amendment document February 1, 1989 Daysai Figure 2! . Display of the case Special application No. 1 87 Address 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name Nippon Steel Corporation 4゜ Agent address Taiyo Building, 12-1 Nihonbashi Kabuto-cho, Chuo-ku, Tokyo 103 Phone number 00/ 002 005θ1 Diameter of parent carbonate compound (mm) 6゜Specification to be amended (detailed description of the invention) 7゜Contents of amendment Page 9, lines 7-8 of [mixed or J] delete.

Claims (2)

[Claims] (1) When decarburizing molten steel under reduced pressure, limestone with a particle size in the range of 0.1 to 20 mm is used in the region where the carbon concentration [%C] of the molten steel to be decarburized is 0.015 wt% or less. Alternatively, a method for producing ultra-low carbon steel, which comprises decarburizing the molten steel while supplying dolomite alone or in combination to the molten steel. (2) When decarburizing molten steel under reduced pressure, limestone with a particle size in the range of 0.1 to 20 mm is used in the region where the carbon concentration [%C] of the molten steel to be decarburized is 0.015 wt% or less. Alternatively, while decarburizing the molten steel while supplying dolomite alone or in a mixture, the oxygen concentration [%O] of the molten steel is reduced in a region where the carbon concentration [%C] of the molten steel is 0.005 wt% or less. A method for producing ultra-low carbon steel, characterized by maintaining the sulfur concentration wt% [S] contained in molten steel within a range limited by the following relational expression. {(1+72[%S])/180}(1-0.25)≦
[%O]≦{(1+72[%S])/180}(1+0
．． 25)