JP3241910B2 - Manufacturing method of extremely low sulfur steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing extremely low sulfur steel having excellent workability.

[0002]

2. Description of the Related Art In order to improve the workability of a metal, it is important to increase the manganese content (hereinafter referred to as [Mn]) in the metal and to reduce the sulfur content (hereinafter referred to as [S]). is there. The general method of producing extremely low sulfur steel is to perform desulfurization and dephosphorization of the hot metal in the hot metal pretreatment step, then decarburize and dephosphorize by converter blowing, and at the end of blowing. (Hereinafter referred to as blow stop), the carbon content is set to 0.1.
After reducing the temperature to about 05%, the steel is poured into a ladle, then a large amount of FeMn is added, the slag is removed, the steel is deoxidized by secondary refining, molten steel desulfurization is performed, desulfurization is performed, and vacuum degassing is performed. Continuous casting is performed after degassing and component adjustment such as acceleration of inclusion floating by a gas treatment device.

[0003] The removal of slag by this method is performed even if slag generated in a converter at the time of tapping is cut by slag, slag which partially flows into a ladle or FeMn during or after tapping.
If the ratio of FeO and MnO in the slag generated by the addition is large and the oxygen potential in the slag during the desulfurization of molten steel is high, the progress of the desulfurization reaction is poor, and a simple and safe slag dragger is generally used for deslagging. Used.

As a method for desulfurizing molten steel, a method of inserting a lance into molten steel in a ladle and injecting flux for desulfurization, stirring the molten steel with an inert gas blown through a porous plug installed at the bottom of the ladle. At the same time, LF or the like is used which heats the desulfurization flux added to the upper part of the molten steel with an electrode to accelerate the desulfurization reaction.

[0005]

In the conventional molten steel desulfurization process of removing slag after tapping from a converter, equipment and personnel for slag removal are required, and heat loss due to the slag removal is extremely large. There is a disadvantage that a large amount of a slag-making agent for coating the molten steel is required after removing the slag. If the desulfurization treatment of molten steel is performed without removing slag, the desulfurization efficiency is low, and in order to achieve ultra-low sulfurization, a large amount of flux is required, the flux cost increases, heat loss increases, and refractories are increased. There is a problem that hinders the operation such as erosion of slag, and even if a large amount of slag deoxidizer is added to the ladle slag to reduce the oxygen potential of the slag, most of it remains unreacted,
There was a problem that sufficient slag reforming was not performed.

A large amount of FeMn added during or after tapping from a converter partially reacts with oxygen or air in the ladle slag to form MnO, and all of the MnO is added to the slag. It is absorbed and causes a decrease in the desulfurization efficiency of molten steel. An object of the present invention is to eliminate deslagging of slag in a ladle in the conventional method, reduce the addition of manganese added after tapping, lower the oxygen potential in the ladle slag, and improve desulfurization efficiency. It is an object of the present invention to provide a method for producing an extremely low sulfur steel.

[0007]

The method for producing ultra-low sulfur steel according to the present invention is characterized in that in a hot metal pretreatment step, a converter is blown using hot metal that has been desulfurized and dephosphorized, and the molten steel at the time of blow-off is blown. carbon content of After steel out by (hereinafter [C] hereinafter) was suppressed to 0.1 5%, the slag deoxidant is added to the ladle slag, then decarburization without performing Jokasu Then, after adjusting to a predetermined carbon content, a deoxidizing treatment is performed, and then a molten steel desulfurizing treatment is performed.

When the converter is blown, the concentration of [C] decreases at first, but when the concentration of [C] decreases to some extent, [Mn]
Becomes oxidized. FIG. 1 shows the [C] concentration at the time of blow-off and the concentrations of FeO and MnO in slag (hereinafter referred to as (FeO),
(Referred to as (MnO)). The higher the [C] concentration, the lower the (FeO) and (MnO), and the lower the [C] <0.
At 1%, (FeO) and (MnO) rapidly increase. Since the desulfurization reaction proceeds as the (FeO) and (MnO) become lower, it is desirable that the [C] at the time of the blow-off is 0.1% or more, and the [C] at the time of the blow-off is increased in order to increase the desulfurization efficiency.
Considering variation in control accuracy, [C] ≧ 0.15%
Is more desirable.

[0009] With吹止when [C] ≧ 0.1 5%, (FeO) , also be (MnO) is lowered, when spraying is [Mn] concentration is kept high, manganese step added It means that the pool gets higher. When the yield of manganese to be added is improved, it is required for products after tapping [M
n] The amount of manganese added when manganese is added to near the level can be reduced, thereby reducing the MnO produced thereby, and the addition of slag deoxidizing agent reduces (MnO) to the extent required for molten steel desulfurization It becomes easy to do.

In order to reach the [Mn] level required for a product, FeMn is produced during tapping as in the conventional method.
Alternatively, a large amount may be added after tapping. However, when it is necessary to raise the temperature of the converter molten steel, FeMn during or after tapping is required.
It is desirable to add SiMn to near the [Mn] level required for the product during the converter blowing instead of adding.

[0011] In order to compensate for heat loss due to the flux added in the subsequent molten steel desulfurization step, the temperature of molten steel at the time of blow-off of ultra-low sulfurized steel is higher than that of general steel types. Carbon-based heating agents such as graphite generally contain sulfur as an impurity, so that sulfur is picked up during blowing and the [S] in molten steel increases, whereas SiMn generates heat by oxidation of Si. Since the reaction occurs, the use of the carbon-based heating agent can be eliminated or the amount thereof can be reduced, so that the sulfur in the molten steel does not increase or the increase of the sulfur can be controlled.

(MnO) is increased by adding a large amount of SiMn at the time of blowing, but by using a slag cutting device at the time of tapping, it is necessary to prevent a large amount of slag in the converter from flowing into the ladle. Can be. In order to add a deoxidizer to the ladle slag after tapping, the slag flowing into the ladle contains 5 to 15% of FeO and 3 to 10% of MnO. This is to increase the desulfurization efficiency. The slag deoxidizer in this case is usually Al-Ca
CO ₃ , Al—Al ₂ O ₃ or the like is used.

FIG. 2 shows the relationship between (FeO) + (MnO) and the desulfurization rate (%). As can be seen from FIG.
(FeO) + (Mn) after deoxidation by adding a slag deoxidizer
O) By setting the concentration to 5% or less, the desulfurization rate can be improved. The decarburizing treatment after the addition of the slag deoxidizing agent is usually performed by using an RH vacuum degassing apparatus, and decarburizing to a predetermined [C] level. At this time, if the oxygen necessary for decarburization is not dissolved in the molten steel, oxygen spraying is performed, but if oxygen blowing is performed excessively, not only oxidation of carbon but also oxidation of Mn is performed. And MnO is increased. As shown in FIG. 3, the oxidation of Mn can be controlled by controlling the acid supply rate so that the dissolved oxygen in the molten steel at the end of decarburization is 100 ppm or less.

As the desulfurization method of molten steel, a method of adjusting components in a vacuum degassing step and performing deoxidation of molten steel, followed by desulfurization by injection or LF, and deoxidation of molten steel by a vacuum degassing apparatus, A method of adding a flux, a method of blowing a flux using an inert gas as a carrier gas from a lower portion of a lower tank of the RH vacuum degassing apparatus, and the like can be used. It is preferable to perform the process without complicating the process.

[0015] When the process up to desulfurization is performed only with the RH vacuum degassing device, the flux addition method uses a normal alloy addition device, so no equipment remodeling is necessary. Because of the drop, when selecting the particle size, a massive desulfurizing agent must be used so as not to be sucked by the vacuum exhaust device. For this reason, the area of the desulfurization reaction interface becomes small, and improvement of the desulfurization reaction can be expected only by a method of increasing the desulfurizing agent basic unit.

In order to improve the desulfurization effect, it is desirable to use a powdery desulfurizing agent. As a method for this, there is a method of blowing an inert gas as a carrier gas from the lower part of the RH degassing apparatus. There are drawbacks such as abrasion and the necessity of constant gas purging. In order to solve the above-mentioned problem of the molten steel desulfurization method, it is desirable to spray a powdery desulfurizing agent into the circulating molten steel from a lance inserted into the RH vacuum degassing tank from above. According to this method, there is no increase in the proportional cost excluding the equipment cost and the desulfurizing agent cost.

The thermal compensation for the temperature drop due to the addition of flux during desulfurization is usually performed by oxygen spraying after Al addition. In this case, Al (hereinafter referred to as [S
Ol.Al]) is 0.05% or more.
It was found that [Mn] oxidation during the heating could be prevented. Therefore, the addition of Al before the temperature rise is based on [SOl
[Al] is desirably increased to 0.05% or more. FIG. 4 shows the relationship between the [SOl.Al] after the temperature rise and the [Mn] oxidation rate during the temperature rise.

[0018]

【Example】

Example 1 In a hot metal pretreatment step, hot metal that had been desulfurized and dephosphorized was put into a converter, and as shown in Table 1, per 1 ton of hot metal, SiM
35 kg of n and 9 kg of Mn ore were added and blowing was performed. Blowing is performed until [C] at the time of the blow-off becomes 0.20%, [Mn] at the time of the blow-off is 1.80%, and (FeO) is 8.
2% and (MnO) were 6.0%. Thereafter, the molten steel was tapped into a ladle while slag cutting with a slag cutting device, and a slag deoxidizer Al-Al ₂ O ₃ was added to the ladle slag at a rate of 2 kg per ton of molten steel to perform deoxidation.
H was transferred to a H vacuum degassing tank and sprayed with oxygen from above at an acid feed rate of 2.3 Nm ³ / t to remove [C] from 0.08
%. Then, 2.5 kg of Al per ton of molten steel
The mixture was added and heated by blowing oxygen from above. After the molten steel Atsushi Nobori, after component adjustment and FeMn per molten steel 1t was added 3 kg, the desulfurization flux 70CaO-30CaF ₂ was added 10 kg / t, 0.2 kg of Al / t performing desulfurization with a desulfurization flux bath additives Was.

Example 2 In the same manner as in Example 1, the hot metal subjected to desulfurization and dephosphorization in the hot metal pretreatment step was placed in a converter, and as shown in Table 1, 1 ton of hot metal was obtained.
Blowing was performed by adding 35 kg of SiMn and 9 kg of Mn ore per on. [C] at the time of blowing is 0.20%
Until [Mn] at the time of blow-off is 1.80%, (F
eO) was 7.4%, and (MnO) was 6.2%. After that, the molten steel is poured onto the ladle while slag cutting with a slag cutting device, and the slag deoxidizer Al-A is placed on the slag ladle.
Deoxidation was performed by adding 2 kg of 1 ₂ O ₃ per 1 ton of molten steel, and then transferred to an RH vacuum degassing tank and 2.3 Nm ₃
Oxygen was sprayed from above with an acid feed rate of / t to remove [C] to 0.08% by decarburization. After that, 1 ton of molten steel
2.5 kg per unit, and heated by spraying oxygen from above. After the temperature of the molten steel was raised, 3 kg of FeMn was added per 1 ton of the molten steel to adjust the composition, and then the desulfurization flux 70
5 kg / t of CaO-30CaF ₂ and 0.2 kg of Al
/ T was added to perform desulfurization.

[0020]

[Table 1] Comparative Example 1 Similarly, in the hot metal pretreatment step, hot metal subjected to desulfurization and dephosphorization was put into a converter, and as shown in Table 1, 25 kg of a carbon-based heating agent (graphite) and 18 kg of manganese ore were added per 1 ton of hot metal. And blew. [C] at the time of blowing is 0.
05%, [Mn] is 0.40%, and (FeO) is 21.0%.
% And (MnO) were 7.5%. Thereafter, the molten steel was poured into a ladle while slag cutting with a slag cutting device, and 22 kg of FeMn was added per 1 ton of molten steel during tapping. And after tapping, slag deoxidizer A is put on ladle slag
The deoxidation was performed by the addition of l-Al ₂ O _3. Then this is R
After transferring to a H vacuum degassing tank, 1.8 kg of Al was added per 1 ton of molten steel, and heated by blowing oxygen from above. After the temperature of the molten steel was raised, 3 FeMn / ton of molten steel was added.
After components adjusted kg added, the desulfurization flux 70CaO-30CaF ₂ in the desulfurization flux bath additives 10
kg and 1.4 kg of Al were added for desulfurization.

Comparative Example 2 Similarly, in the hot metal pretreatment step, hot metal that had been desulfurized and dephosphorized was put into a converter, and as shown in Table 1, 25 kg of a carbon-based heating agent (graphite) per ton of hot metal, manganese ore Was added and the mixture was blown. [C] at the time of blowing is 0.
05%, [Mn] 0.40%, (FeO) 19.8
% And (MnO) were 7.6%. Thereafter, the molten steel was tapped into a ladle while slag cutting with a slag cutting device, and 22 kg of FeMn was added per ton of molten steel during tapping. And after tapping, slag deoxidizer A is put on ladle slag
The deoxidation was performed by the addition of l-Al ₂ O _3. Then this is R
After transferring to a H vacuum degassing tank, 1.8 kg of Al was added per 1 ton of molten steel, and heated by blowing oxygen from above. After the temperature of the molten steel was raised, 3 FeMn per ton of molten steel
After adding kg and adjusting the components, 5 kg of desulfurization flux 70CaO-30CaF ₂ was added by flux spraying, and A
l was added to carry out desulfurization.

Table 2 shows [S], [Mn], (FeO) at the time of the blow-off and RH degassing treatments of Example 1 and Comparative Example 1.
And (MnO). Table 3 shows [S] at the time of the blow stop and the RH degassing treatment of Example 2 and Comparative Example 2.
[Mn], (FeO) and (MnO) are shown. The former Table 2 is based on the addition in the RH desulfurization flux tank, and the latter Table 3 is based on the RH desulfurization flux spraying.

FIG. 5 also shows [Mn] and [Sol. Al] and (MnO). As can be seen from FIG.
At the time of blowing, the [Mn] level has reached the level required for the product and (MnO) is slightly higher than that in the comparative example. (MnO) before the RH treatment and the degassing treatment is reduced by half because there is no addition. Also, almost no increase in (MnO) after heating was observed.

Further, as shown in Tables 2 and 3, [S] after the hot metal pretreatment was not provided in Examples 1 and 2, whereas in Comparative Examples 1 and 2, [S] was 10 pp.
m was also found to have increased. Example 1,
Compared to the addition of the RH desulfurization flux of Comparative Example 1, the spraying of the RH desulfurization flux of Example 2 and Comparative Example 2 provided about half the desulfurization flux basic unit, and a low [S] equivalent or equal to or more than that was obtained.

[0025]

[Table 2]

[0026]

[Table 3] FIG. 6 shows the desulfurization behavior after the desulfurization flux was added. In Example 1, the desulfurization efficiency was increased as compared with Comparative Example 1 by reducing (MnO), and [S] was 20 to 30 pp.
[S] was able to be stabilized to 10 ppm or less in 10 minutes by desulfurization.

Further, in Examples and Comparative Example 2, the desulfurization efficiency was increased by spraying the fine powder of the desulfurization flux. In Example 2, [S] was reduced to 10 ppm or less in about half the time. In Comparative Example 2, [S] 15 pp
m or less could be obtained. Example 3 As shown in Table 1, the addition amount of the carbon-based heating agent was 27 k
g, and blowing was performed under the same conditions as in the comparative example, except that [C] at the time of blowing was set to 0.20%. [Mn] after blowing is 0.1.
55%. Next, as in Comparative Example 1, FeM
After adding 20 kg of n, the slag deoxidizer Al-
Deoxidation was performed by adding 2 kg of Al ₂ O ₃ , transferred to an RH vacuum degassing tank, and sprayed with oxygen from above at an acid feed rate of 2.3 Nm ³ / t as in Example 1. Carbonized, and [C] was set to 0.08%. Then, after adding 2.5 kg of Al and spraying oxygen, adding 3 kg of FeMn after the temperature rise and adjusting the components, the desulfurization flux 70caO-30C
Desulfurization was performed by adding 10 kg of aF ₂ and 0.2 kg of Al.

Table 3 shows [S], [Mn], (FeO) and (MnO) at the time of blow-off and at the time of RH treatment. [S] after desulfurization was 15 ppm.

[0029]

The present invention is configured as described above and has the following effects. According to claim 1 process according, by the [C] ≧ 0.1 5% during吹止, during吹止
[C] Even if the control accuracy varies, (Fe
O) and (MnO) can be lowered, and the Mn yield during converter blowing can be improved to reduce the amount of Mn added for [Mn] adjustment during tapping.

Thus, the resulting (Mn
O) can also be reduced, so that the addition of the slag deoxidizer (FeO)
(MnO) can be reduced to a level required for molten steel desulfurization, and desulfurization can be performed with an increased desulfurization efficiency.

According to the second aspect of the present invention, if SiMn is added during converter blowing, the use of a carbon-based heating agent for raising the temperature due to the exothermic reaction of oxidation of Si can be eliminated.
Alternatively, the number of sulfur pickups can be reduced or reduced, so that [S] does not increase or the increase of [S] can be reduced by increasing the temperature because sulfur is reduced. Claim
3 (FeO) + (MnO) ≦ 5
%, The desulfurization rate can be improved.

[0032] As according to claim 4 and 5, wherein the preparation, when the dissolved oxygen 100ppm or less, it is possible to suppress the oxidation of Mn. If the steps from decarburization to desulfurization are performed in the RH degassing tank as in the production method of the sixth and seventh aspects, the process is simplified. As of claim 8, wherein the preparation, when [Sol, Al] of temperature was raised to a 0.0 by 5% or more, it is possible to prevent oxidation of [Mn] of NoboriAtsushichu.

According to the ninth aspect of the present invention, in the RH vacuum degassing apparatus, the desulfurization reaction interface area can be increased by spraying a desulfurization flux to increase the desulfurization efficiency.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between [C] at the time of blowing, and (FeO) and (MnO) at the time of blowing.

FIG. 2 is a graph showing a relationship between (FeO) + (MnO) and a desulfurization rate.

FIG. 3 is a graph showing a relationship between dissolved oxygen in molten steel after decarburization and [Mn] decrease.

FIG. 4 [Sol. 5] A graph showing the relationship between [Al] and the [Mn] reduction rate due to oxidation during temperature rise.

FIG. 5 shows [M] before and after RH treatment in Example 1 and Comparative Example.
n] [Sol. 2] Graph showing [Al] and (MnO) concentrations.

FIG. 6 is a graph showing desulfurization behavior during desulfurization.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C21C 7/064 C21C 7/06 C21C 7/068

Claims (9)

(57) [Claims] In 1. A molten iron pretreatment step, performed converter吹練with hot metal subjected to desulfurization and dephosphorization, suppressing the carbon content in the molten steel at吹練ended 0.1 5% After tapping,
Add slag deoxidizer to ladle slag, then decarburize in vacuum degassing equipment without adjusting slag, adjust to predetermined carbon content, deoxidize, and then desulfurize molten steel A method for producing an ultra-low sulfur steel, comprising performing a treatment. 2. Desulfurization and dephosphorization are performed in a hot metal pretreatment step.
Blown the converter using hot metal that has been
After tapping the steel with the carbon content suppressed to 0.1% or more,
Add the slag deoxidizer to the pot slag, then carry out the slag
Decarburization with vacuum degassing equipment
After adjusting the content to the deoxidation treatment,
The method for producing ultra-low sulfur steel is characterized by performing sulfuration treatment.
There are, manufacturing method of extremely low 硫鋼, wherein the Mn level in the steel is added SiMn before blow to near the level required for the product. 3. A process according to claim 1 or 2 FeO + MnO concentration in the slag after deoxidation by slag deoxidant added 5% or less
The method for producing an extremely low sulfur steel according to claim 1. 4. Desulfurization and dephosphorization are performed in a hot metal pretreatment step.
Blown the converter using hot metal that has been
After tapping the steel with the carbon content suppressed to 0.1% or more,
Add the slag deoxidizer to the pot slag, then carry out the slag
Decarburization with vacuum degassing equipment
After adjusting the content to the deoxidation treatment,
The method for producing ultra-low sulfur steel is characterized by performing sulfuration treatment.
And the dissolved oxygen content in the molten steel at the end of decarburization is 100 ppm
A method for producing an ultra-low sulfur steel, comprising: 5. The amount of dissolved oxygen in molten steel at the end of decarburization is 100
The method for producing an ultra-low sulfur steel according to claim 1 or 2 , wherein the content is not more than ppm. 6. A desulfurization and dephosphorization step in a hot metal pretreatment step.
Blown the converter using hot metal that has been
After tapping the steel with the carbon content suppressed to 0.1% or more,
Add the slag deoxidizer to the pot slag, then carry out the slag
Decarburization with vacuum degassing equipment
After adjusting the content to the deoxidation treatment,
The method for producing ultra-low sulfur steel is characterized by performing sulfuration treatment.
There, in R H vacuum degassing apparatus, decarburization, deoxidation, Atsushi Nobori,
Manufacture of ultra-low sulfur steel characterized by component adjustment and desulfurization
Construction method. 7. An RH vacuum degassing apparatus for decarburizing, deoxidizing,
The method for producing an extremely low sulfur steel according to claim 1 or 2 , wherein the temperature is raised, the components are adjusted, and desulfurization is performed. 8. For thermal compensation by adding desulfurization flux,
After adding Al and raising the temperature, the Al concentration in the molten steel is set to 0.0
The method for producing an ultra-low sulfur steel according to claim 1 or 2 , wherein the content is 5% or more. 9. As a method for desulfurizing molten steel, a non-immersion lance installed above the RH vacuum degassing apparatus is inserted into the tank.
The method according to claim 6, wherein the desulfurizing agent is sprayed through an inert gas.