JPH02197513A - Production of steel - Google Patents

Abstract

PURPOSE:To decrease the output of slag, etc., and to reduce the refining cost of low-P and high-Mn steel by using two converters consisting of a dephosphorization furnace and a decarburization furnace, adding the converter slag generated in the decarburization furnace, a refining agent and a carbonaceous material to molten iron and up-blowing gaseous oxygen. CONSTITUTION:Molten iron is refined by the two converters consisting of the dephosphorization furnace 1 and the decarburization furnace 2. The converter slag 4 generated in the decarburization furnace 2, a refining agent consisting essentially of Mn ore, and a carbonaceous material are added to the molten iron 3 injected into the dephosphorization furnace 1. Gaseous oxygen is then up-blown from a nozzle 5, and the molten iron is agitated to control the temp. of the molten iron after dephosphorization to 1200-1400 deg.C. Consequently, the molten iron is dephosphorized, and the Mn concn. in the molten iron is increased. A slag making agent and Mn ore are charged into the obtained dephosphorized molten iron, which is refined in the decarburization furnace 2. The molten iron is thus decarburized, and the Mn concn. in the refined molten steel is increased.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention achieves highly efficient dephosphorization (hereinafter referred to as "deP") while minimizing the amount of slag-forming agents (quicklime, etc.) used throughout the entire steelmaking process. At the same time, manganese ore (hereinafter also referred to as "iron-manganese ore") is added, and this is melted and reduced to the maximum extent to increase the end point [Mnl t7/s degrees] in the converter, thereby improving the quality. This invention relates to a method for producing high-quality steel at low cost.

<Prior art and its issues> In response to the increasing demand for quality stabilization and cost reduction of thick steel plates in recent years, various measures have been studied and implemented to produce high Mnfi at the lowest possible price. One of these efforts is to reduce the amount of slag-forming agents (quicklime, dolomite, etc.) used in the steelmaking process, and to increase the Mnl concentration at the end point of the converter (manganese alloy (ferromanganese).

In order to reduce the amount of silicomanganese, etc. used, first, a pre-film of hot metal is introduced by injecting a quicklime-based dephosphorizing agent (mainly "quicklime-iron oxide-fluorite base") in a torpedo or hot metal transfer pot. A method of performing P treatment, then pouring the iron into a converter, adding a small amount of ordinary slag-forming agent (quicklime, etc.) and manganese ore, and performing blowing has been tried, and good results have been obtained.

However, on the other hand, the following problems were also pointed out in the above conventional method. That is, a) Although the amount of dephosphorization agent used during converter blowing can be reduced, quicklime-based flux is still used both during hot metal dephosphorization treatment (preliminary dephosphorization treatment) and converter blowing.
In the end, the amount of slag forming agent used throughout the entire steelmaking process does not decrease much.

b) The amount of manganese ore that can be smelted and reduced during converter blowing for dephosphorized pig iron varies depending on the target converter end temperature.
The thermal limit is approximately 15 to 20 kg/l, and therefore the end point [Mn] concentration that can be reached is at most about 0.5 to 0.9%. However, in this case, it is possible to increase the temperature of the hot metal by charging carbonaceous material such as coke as a heat source and increase the amount of manganese ore to be smelted and reduced, but with this method, the pickup of [S] from the coke is a problem. Therefore, there was still a limit to the increase in the converter end point [Mn]fI by 1 degree.

Therefore, with the aim of solving this problem, the inventors of the present invention first developed a system in which one of the two converter-type furnaces with both upper and lower blowing functions was used as a dephosphorizing furnace and the other as a decarburizing furnace (hereinafter referred to as First, a refining agent mainly composed of converter slag and manganese ore generated in the carbon removal furnace is added to the hot metal poured into the carbon removal furnace. Refining is carried out to increase the hot metal mp and the hot metal [Mn] concentration while keeping the hot metal temperature below 1400°C by blowing oxygen gas upward while stirring the blowing gas.Then, the resulting dephosphorized hot metal is usually slag-formed. He proposed a steelmaking method in which the hot metal is decarbonized and the end point [Mn] concentration of the hot metal is increased by smelting it in a decarbonizing furnace by adding manganese ore to the hot metal (Japanese Patent Application No. 62-301262).

This method is a steelmaking method in which hot metal is refined by using one of two converter-type furnaces with both upper and lower blowing functions as a de-P furnace and the other as a de-C furnace. It is possible to stably produce low-P steel with low-cost operation that minimizes the amount of additives, and is extremely advantageous in that it can stably provide high-quality steel.''[S]
It is extremely effective as a high-Mn steel melting method, as it is possible to effectively increase the end point [Mn] concentration in the decarbonizing furnace without requiring a heat source such as "coke addition" that involves pick-up of carbon. It was an advantageous means.

However, as a result of further study by the present inventors through actual operation, the present inventors were able to apply for patent application No. 62-301.
It has been strongly recognized that the method proposed as No. 262 has room for improvement as described below.

That is, even when the method previously proposed by the present inventors is applied, the increase in [MnltM degree in the deP furnace is usually 0.5 to
It is about 0.7%, and the end point [M
n] The concentration is about 1.0 to 1.1%.

However, the [Mn] concentration required for products is still high, at 1.5 to 1.6% depending on the steel type, and therefore, with the above method, "operation without adding any manganese alloy" or "operation close to this" cannot be performed. , it is necessary to further add manganese alloy iron in subsequent steps (during converter tapping or RH treatment, etc.), which inevitably increases production costs.

Therefore, the purpose of the present invention is to produce steel with a product [Mn] concentration of 1.5% or more without using any manganese alloy without using a large amount of slag forming agent or causing [,S] pick-up. The aim was to establish a means for melting in an operation without additives or in a similar manner.

In order to achieve the above object, the present inventors conducted new studies from various viewpoints, and firstly,
The reason why the steelmaking method proposed as No. 262 requires the addition of manganese alloy iron in subsequent steps (during converter tapping, RH, etc.) is that the main components of the steelmaking method are converter slag and manganese ore. At the end of de-P furnace refining, which is performed by adding a refining agent, hot metal [C
] It is necessary to secure a concentration of about 4.0%, which inevitably puts a limit on the amount of manganese ore that can be added, and also requires no heat source such as carbon material to be input in the subsequent decarbonizing furnace blowing. After confirming that "it is difficult to secure enough heat to sufficiently melt and reduce manganese ore," we continued further research with the goal of solving this problem, and as a result, we obtained the following knowledge. It's arrived.

In other words, in order to sufficiently raise the decarbonization furnace end point [Mn]t1 degree in the previously proposed "method of mesh making using two converter-type furnaces," it is necessary to use carbonaceous materials during blowing. It is necessary to introduce a heat source such as However, since coke, which is commonly used as a carbonaceous material, contains about 0.6% S, an attempt was made to increase the amount of smelting and reduction of manganese ore by introducing it into the decarbonization furnace. This causes the problem that S is picked up from the carbonaceous material and the [S] in the molten iron increases.

However, by using an upper and lower blowing converter, sufficient dephosphorization agent (
In the case of the above-mentioned steelmaking method, in which hot metal is blown and dephosphorized in advance (refining agent mainly composed of converter slag and manganese ore), carbonaceous materials such as coke are introduced as a heat source in the dephosphorization furnace. Then, as long as the basicity (CaO / SiO As a result, it becomes possible to sufficiently improve the C removal furnace end point [Mn] ta degree.

The present invention has been made based on the above knowledge, and is based on the following: ``As shown in FIG. In a steelmaking method in which hot metal is refined using decarburization furnace 2, hot metal 3 is injected into the dephosphorization furnace 1 together with converter slag 4 generated in the decarburization furnace 2 and a refining agent mainly composed of manganese ore. While adding carbonaceous material and stirring the bottom blowing gas using the stirring gas blowing nozzle 5, the temperature of the hot metal after dephosphorization is 1200 to 1400°C.
Oxygen gas is blown upward from lance 6 so that the hot metal is dephosphorized.
and a step of increasing the hot metal [Mn] concentration, and adding a normal slag forming agent and manganese ore to the obtained dephosphorized hot metal to decarburize the decarburizer 2.
By decarburizing the hot metal and increasing the refining end point [Mn] concentration of the molten steel, it is possible to produce low-P, high-Mn-containing steel at a low cost with less slag-forming agent usage. ”.

In other words, the present invention provides a method for manufacturing steel using two converter-type furnaces having both upper and lower blowing functions, one of which is used as a dephosphorization furnace and the other as a carbon removal furnace. By adding it to efficiently melt and reduce manganese ore and significantly increasing the [Mn] concentration in the dephosphorized pig iron, the final point [Mn] in the subsequent decarbonizing furnace is
] The main point is to improve the concentration, but the reason why there has been no concept of using a heat source such as coke during hot metal dephosphorization is as follows.

That is, hot metal dephosphorization has conventionally been carried out in a torpedo or a transfer pot, and therefore it has not been possible to use a large amount of oxygen due to the equipment. Therefore, even if coke is added, it cannot be burned in a short period of time, and there is a problem in that only a long processing time is required.In addition, the heat removed during processing is large, making it difficult to provide substantial heat. In addition, if coke is used in such equipment where only a small amount of 0□ can be used even if 02 injection is performed, depending on the processing method, the dephosphorized slag may be reduced too much by the coke before the coke is combusted by oxygen. Therefore, there was a problem that the P removal rate itself deteriorated.

On the other hand, when an upper and lower blowing converter is used as the deP refining vessel as in the present invention, for example, 2 to 4 N
rrr/I! Since a large amount of Ot (l1n-t) can be used, it is possible to burn and heat carbonaceous materials in a short time.
When manganese ore is added, efficient melt reduction is performed.

By the way, in the hot metal deP treatment in the present invention, a refining agent (mainly composed of converter slag generated in a carbon removal furnace and manganese ore) is used.
Usually, fluorite is added as a solvent, and a carbon material such as coke is added to the mixture for blowing. The amount of manganese ore used as a dephosphorizing agent as an oxidizing agent originally required for dephosphorization is 10 to 15 liters per ton of hot metal.
kg/l, and therefore manganese ore exceeding this amount is theoretically added only to increase the [Mn] concentration, but here we will include it as a “refining agent (de-P7flll)”.
)”.

The hot metal film P treatment process in the present invention basically involves removing P.
The above dephosphorization agent and coke are added to the hot metal stored in the furnace, and the amount of 02 required for combustion of coke and the amount of Ot required for dephosphorization (5
~10N rrr/win-t) is blown upward, and the temperature of the hot metal after deP treatment is 1.
It is sufficient to adjust the temperature to 200 to 1400°C, and specifically, for example, one of the following methods may be used.

i) Converter slag and fluorspar are added to the hot metal, and coke is also added to produce 2 to 4 N, which is the normal converter blowing level.
rrr/n+1n-t of oxygen was blown upward to raise the temperature by burning coke, and then manganese ore was added (then the amount of top-blown oxygen was 0.5 to lNn?/m1n-
The aim is to reduce P to a certain level and remove P along with the slag that has already formed during the temperature rising period and further increase [Mnl].

ii) Add converter slag, fluorite, and manganese ore used only for increasing Mnl to hot metal, and add 2 to 4 N r
After heating by burning coke and melting and reducing manganese ore by top-blowing oxygen at rr/5 in-t, the amount of top-blowing oxygen is reduced to 0.5 to I N rrr/m1n-, The remaining manganese ore (10 to 15 kg/l) is added to remove P and further increase Mnl.

The converter slag used here is generated in the decarbonization furnace, and the amount added is the amount necessary to make the slag basicity after treatment 2.5 or more, preferably 3 or more. It's basic. Of course, as a CaO source, not only the converter slag but also quicklime may be added. The specific amount of these can be determined by considering the [Si] concentration of the hot metal used and the SiO□ source such as manganese ore or coke gangue, but usually the converter slag is 25 to 35 kg/ degree is necessary.

The reason why it is better to set the basicity of the slag to 2.5 or higher, preferably 3 or higher is because Mn, P, and S
As is clear from Figure 2, which shows the relationship between the distribution ratio of and the basicity of slag, in order to effectively melt and reduce manganese ore, it is necessary to reduce the (MnO)/[Mnl ratio and (
P)/[P] ratio and (S)/[S] ratio are increased to remove P.
Improves the ability to remove S from slag and removes [S] from coke, etc.
] The purpose is to prevent pickup. For example, CaO/
When 5iOt=3, the reduction yield of manganese ore is 60
%, and the S removal rate is about 70%. Therefore, although it varies depending on the hot metal used, the [S] concentration before treatment is 0.01
%, there is no [S] pickup from coke at all, and rather [3] after treatment is 0.006 to 0.
008%, to the extent that the film S has progressed slightly.

In this way, it is useful to increase the basicity of the dephosphorized slag to 2.5% or more, but in that sense, the basicity of the hot metal [s+
]? The lower the Jf1 degree, the better, preferably 0.30%.
It is desirable to use hot metal that has been preliminarily desiliconized to the following level.

Further, it is better to use fewer carbonaceous materials such as coke and gangue of manganese ore. This is because when the (Stoz) in slag increases, not only does the amount of converter slag (and quicklime) used for basicity adjustment increase, but also the amount of slag increases and the reduction yield of manganese ore decreases. It is.

The amount of manganese ore to be added is determined by considering (MnO) in the converter slag returned from the decarbonization furnace. For example, when the amount of converter slag used is 25 to 30 kg/l, , After P removal treatment, 15 to 25 kg/l of manganese ore is required to increase Mnl to about 0.8 to 0.9%. In addition, assuming that the end point of the decarbonization furnace [Mnl #1.5%, the converter slag contains a large amount of (MnO) of 30 to 50%;
Since O) is as low as 10 to 15%, the method of the present invention is considered to be an inherently advantageous method for incorporating the valuable metal Mn.

The amount of fluorite to be added is determined depending on the slag state of the slag, and is usually 2 to 15 kg, but most commonly it is about 5 to 8 kg.

Among these refining agents (dephosphorization agents), converter slag is extremely easy to turn into slag because it has been turned into slag once, and therefore the particle size may be around 1OO■lφ or even larger (
Of course, it may be molten.)

In addition, the particle size of manganese ore and fluorite that is normally used is sufficient.

Further, as the refining agent (dephosphorization agent), converter slag is used.

In addition to manganese ore, fluorspar, and quicklime, limestone, soda ash, and iron oxide may be added. Among these, it is best to inject 3 to 10 kg/l of soda ash into the deP pig iron together with the bottom stirring gas after the deP treatment, as this will further promote deS and produce ultra-low sulfur steel. It is an effective means of

Regarding the temperature of the P removal treatment, there is no problem even if the heating temperature rises to 1400°C or higher, but the temperature after the subsequent P removal period should be 1400°C or lower. This is because the temperature of hot metal at this time is 1
This is because when the temperature exceeds 400°C, decarbonization becomes active and the slag is reduced too much, resulting in a decrease in oxidizing power and deterioration of dephosphorization.

However, in order to reduce the loss of granular iron in the slag and increase the amount of manganese ore that can be smelted and reduced in the carbon removal furnace following the P removal treatment, the above temperature is preferably 1200°C or higher; It is preferable to adjust the temperature to 1320 to 1400°C if possible. In order to proceed with effective dephosphorization even at such a relatively high dephosphorization treatment temperature, it is preferable to adjust the basicity of the dephosphorization slag to 2.5 or more (see FIG. 2 above).

The amount of carbon materials used, such as coke, depends on the temperature of the hot metal used.

Amount of manganese ore to be melted and reduced, hot metal temperature after dephosphorization.

It also depends on the temperature of the deP furnace, but usually 1
Approximately 0 to 15 kg/. In addition, carbonaceous material (coke)
The particle size may be within the range normally used, but it is preferably as small as possible without scattering.

The top-blown oxygen is preferably soft-blown. In particular, when heating and raising the temperature, it is usually 2 to 4 N t+? /win-t, but this top-blown oxygen burns coke without reducing the molten metal [C], and furthermore, the generated CO gas is
In order to efficiently raise the temperature by secondary combustion to OZ, the ratio of the depth Lo of the molten iron to the depth L of the depression caused by the 02 jet (L/L
Soft blowing with o) of 0.1 or less is preferred.

Although the times of the heating temperature rising period and the P dephosphorization period vary depending on the conditions, the former is usually about 5 minutes, and the latter is about 8 to 15 minutes.

Note that the time for this deP period includes the rinsing time during which only the bottom gas is stirred after the top blowing oxygen is stopped.

By the way, in the steel making method according to the present invention, in addition to dephosphorization of hot metal and smelting reduction of manganese ore, scrap melting can also be carried out simultaneously. In this case, the scrap is mainly melted during the heating and temperature rising period, and a tentative guideline is that the amount of coke required for this purpose should be increased by 1 to 3 kg per 1% of the scrap ratio.

The stirring gas injected from the bottom of the deP furnace includes Ar, C,
It may be O+ COz, Nz+ 021 air, etc. The degree of bottom gas agitation in the deP furnace is the same as that in normal upper and lower double blowing combined blowing (0.03 to 0.0
．． 3 N n? /m1n-t), but it goes without saying that it may be greater than this in order to improve the P removal rate.

In this way, when dephosphorized pig iron with increased [Mn] in a dephosphorizing furnace is blown in a decarbonizing furnace, the amount of manganese ore that can be added is:
Although it varies depending on the deP pig iron temperature, the end point temperature of the deC furnace, and the degree of warmth of the furnace, it is usually around 15 to 30 kg/hour. It goes without saying that in order to increase the amount of manganese ore added, a carbonaceous material such as coke may be added as a heat source to the extent that pick-up of [S] does not pose a problem. and,
In carbon-free furnace blowing, slag-forming agents, mainly quicklime and dolomite, are used together with manganese ore.

Next, the present invention will be explained in more detail using Examples and in comparison with Comparative Examples.

<Example> Comparative Example 1 250 tons of hot metal having the composition as shown in the upper row of Table 1, which had been subjected to de-S and de-t treatment in a torpedo, was poured into an upper and lower double blowing combined blowing furnace used as a de-P furnace, Similar to this, the de-C
25 kg/l of converter slag generated in the furnace is cooled and solidified and crushed to a particle size of 30 dragons or less, and 12 kg/l of manganese ore with a similar particle size.

8 kg of fluorspar and 4 kg/l of quicklime were added in a mixed state and dephosphorized for 12 minutes. Table 2 shows the composition of the converter slag used in this case and the basicity (Cab/5te2) of the dephosphorized slag containing this as the main component.

The de-P furnace and de-C furnace used were both 250 top and bottom blowing combined blowing converters capable of injecting and stirring gas from the bottom of the furnace, and the operating conditions shown in Table 3 were adopted. It was done.

The dephosphorized pig iron thus obtained (the composition is shown in the middle row of Table 1) was tapped into a hotpot and then poured into a decarbonizing furnace, and further 8 kg/l of quicklime and 5 kg of dolomite were poured into the decarbonizing furnace. /l,
Main blowing was carried out after adding 1 kg/l of fluorspar and 20 kg/l of manganese ore.

The converter slag generated at this time weighed 25 kg, and a series of operations were repeated in which it was added to the dephosphorization furnace as a raw material for the dephosphorization agent for the next charge along with manganese ore and fluorite to perform dephosphorization.

As a result, the total amount of quicklime and dolomite used in the entire steelmaking process was as low as 17 kg/L, and [P] in the steel was 0.012% as shown in the lower row of Table 1.

Molten steel with a [Mn] content of 1.04% was obtained.

Example 1 250 tons of de-S and de-Si hot metal having the components shown in the upper row of Table 4 were poured into a de-P furnace (composite converter with upper and lower blowing).
35 kg/l of converter slag and 8 kg of fluorite generated in the decarbonization furnace
/l, coke with particle size 3 (In or less) 10kg/l
was added and heated for 4 minutes with top-blown oxygen of 3 N rd7 min-t, then 20 kg/l of manganese ore was added and heated with top-blown oxygen of 0.7 N rrr/llll1 n-t for 12 minutes.
A minute-long deP treatment was performed. The composition of the converter slag used at this time and the basicity (C
a O/St Oz) is shown in Table 5.

Next, since the temperature of the dephosphorized pig iron obtained in this way (the composition is shown in the middle row of Table 4) could be raised slightly, it was poured into a decarbonized iron furnace, and 13 kg of quicklime/quicklime was added. l
, 5 kg/l of dolomite, 3 kg/l of fluorite and 30 kg of manganese ore were added before main blowing.

The converter slag generated at this time was 35 kg/l, and a series of operations were repeated in which it was added to the dephosphorization furnace as a raw material for the dephosphorization agent for the next charge together with fluorite and manganese ore to perform dephosphorization.

As a result, it was revealed that molten steel as shown in the lower row of Table 4 was obtained, and the end point [Mn] in the carbon removal furnace increased to 1.51%. Of course, as in the case of the comparative example, less quicklime and dolomite (fJ) were used in the entire steelmaking process.

As a result of heating using coke in a deP furnace,
The amount of manganese ore smelted and reduced was increased, and the [Mn] concentration could be increased by 0.47% compared to the comparative example. Note that no increase in [S] was observed despite the use of coke.

Example 2 250 tons of de-S and de-Si hot metal having the components shown in the upper row of Table 6 were poured into a de-P furnace (a combined upper and lower blowing converter).
@Converter slag generated in C furnace 25kg/l, fluorite 8kg
In addition to 10 kg/l of manganese ore, 15 kg/l of coke with a particle size of 30 Tsuru or less was added, and 3 liters of top-blown oxygen was added.
After reducing the manganese ore by heating at N rrf/n+1n-t for 5 minutes, 10 kg/l of manganese ore was added.

9 kg/l of quicklime and 1 kg/l of fluorspar were added, and dephosphorization treatment was carried out for 14 minutes with top-blown oxygen of 0.7 N rrr/n+1n-t. The composition of the converter slag used at this time and the basicity (CaO/
SI Oz> is shown in Table 7.

Next, the dephosphorized pig iron obtained at this time (middle row of Table 6) was decarbonized.
Pour into the furnace, add quicklime 8kg/l, and dolomite 4
kg/l, fluorite 3 kg/l and manganese ore 30 kg/l
kg/l was added and main blowing was carried out.

The converter slag generated at this time was 25 kg/l, and this was added to the deP furnace as a raw material for the deP agent for the next charge along with fluorite, manganese ore, and quicklime, and a series of operations were repeated to perform deP. Ta.

As a result, molten steel as shown in the lower row of Table 6 was obtained, and it was revealed that the end point [Mn] in the carbon removal furnace rose to 1.58% without picking up [S] from coke.

This is 0.54% higher than that of the comparative example [M
n] means that the increase has been secured.

Example 3 After 17 tons of scrap was charged into a deP furnace (upper and lower double blowing combined converter), 250 tons of de-S and de-St hot metal having the composition shown in the upper row of Table 8 was poured, and this was de-carbonized. 25 kg/l of converter slag generated in the furnace, 8 kg/l of fluorite, and 10 kg of manganese ore, as well as 26 kg of coke with a particle size of 30 tm or less
kg/l and top-blown oxygen 3Nn? DeP refining and main blowing were carried out in the same manner as in Example 2, except that the scrap was melted and the manganese ore reduced by heating at 7 m1 nt for 9 minutes. The composition of the converter slag used at this time and the basicity (CaO
/StOz) are shown in Table 9.

As a result, 6.8% of the scrap was melted and
Molten steel as shown in the lower row of Table 8 is obtained, and the decarbonization furnace end point [Mn] is 1 without [S] pink-up from coke.
．． It was confirmed that the rate had increased to 53%.

Comparative Example 2 Example 1 except that no coke was added in the deP furnace
Under the same conditions as above, de-P refining and main blowing of de-S and de-Si hot metal having the component compositions shown in the upper row of Table 10 were carried out.

Table 11 shows the composition of the converter slag used in this case and the basicity (Ca O/St Oz) of the dephosphorized slag containing this as the main component.

As a result, as shown in the middle row of Table 10, the [C] in the hot metal after the deP treatment decreased, and less than 10 kg/l of manganese ore was introduced in the next blowing in the deP furnace.

Summary of Effects> As explained above, according to the present invention, the amount of slag forming agent required throughout the entire steelmaking process is reduced, and furthermore, [
S] It brings about extremely useful effects industrially, such as being able to melt low P and high Mnfi at low cost without worrying about pickup.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of the process of the present invention. FIG. 2 is a graph showing the relationship between the distribution ratio of Mn, P, and S and the slag basicity in the deP furnace. In the drawings, 1... Dephosphorization furnace, 2... Decarburization furnace. 3... Hot metal, 4... Converter slag. 4'...Dephosphorization slag whose main component is converter slag. 5... Stirring gas blowing nozzle. 6... Lance.

Claims (1)

[Claims] In a steelmaking method in which hot metal is refined by using one of two converter type furnaces having upper and lower blowing functions as a dephosphorization furnace and the other as a decarburization furnace, the hot metal injected into the dephosphorization furnace is Carbon material is added together with the converter slag generated in the decarburization furnace and a refining agent mainly composed of manganese ore, and oxygen gas is added to the hot metal so that the temperature of the hot metal after dephosphorization is 1200 to 1400℃ while stirring the bottom blowing gas. A process of top blowing, dephosphorizing the hot metal and increasing the concentration of hot metal [Mn], and adding a normal slag forming agent and manganese ore to the resulting dephosphorized hot metal, refining it in a decarburizing furnace, and decarburizing the hot metal. and a step of increasing the refining end point [Mn] concentration of molten steel.