JPH0431016B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) A process for manufacturing chromium-containing stainless steel high carbon molten metal (referred to as "stainless steel mother molten metal" in this specification) for manufacturing stainless steel to efficiently and inexpensively melt stainless steel. Regarding improvement. (Conventional technology) Scrap and other methods of melting stainless steel are used.
The most common process is the electric furnace-AOD (VOD) process, in which ferroalloys such as Fe, Cr, and FeNi are used as main raw materials and are melted in an electric furnace, followed by decarburization and reduction refining in AOD or VOD to produce stainless steel. I will do it. However, this process using an electric furnace has problems as an inexpensive method for producing stainless steel because it requires the use of expensive electrical energy and there are restrictions on the selection of raw materials. On the other hand, hot metal is charged into a top-bottom blowing converter without using an electric furnace, and scrap and ferroalloys (FeCr and FeNi) are added during or before decarburization blowing to become the components of stainless steel. For example, in the literature (Tetsu to Hagane (1985), Vol. 71 ，
S180). However, even with this method, the amount of scrap and ferroalloy that can be added to the hot metal is limited due to heat balance. In addition, in the method of adding carbonaceous material as a heat source to remove this restriction, the blowing time is extended and the amount of dust generated increases, causing problems in terms of yield and in terms of matching with continuous casting. Furthermore, it cannot be said that efficient stainless steel melting using inexpensive materials is sufficient. In recent years, chromium ore or partially reduced chromium ore pellets (hereinafter referred to as "pre-reduced pellets") are melted and reduced to produce stainless steel instead of ferrochrome as part of the stainless steel melting process. Techniques have been developed; see, for example, the literature Tetsu to Hagane (1987). In Vol.73 and S875, dephosphorized hot metal is charged in a top-bottom blowing converter, carbonaceous material is added and the temperature is raised, then chromium ore or pre-reduced pellets and carbonaceous material are charged and melted and reduced. A technique has been disclosed in which a chromium-containing molten metal is produced by continuously supplying materials, and after removing slag and slag, decarburization is subsequently performed. On the other hand, Japanese Patent Publication No. 62-50545 also discloses an improvement in a technique in which ore is smelted and reduced in the same furnace, chromium-containing molten metal is melted, and only the slag is removed, followed by decarburization. In these methods, smelting reduction and oxidation refining are performed in the same furnace, so even if slag is removed during the process, it is not always sufficient, and a large amount of sulfur impurity remains even after decarburization refining. Another drawback was that the refractories inside the furnace were subject to significant melting. Note that Japanese Patent Publication No. 62-50545 cites the technology of using scrap in the decarburization of chromium-containing molten metal, so when decarburizing molten stainless steel, stainless steel scrap is appropriately added and the scrap is used as a raw material. It is also a conventionally known technique that can be used. However, even if stainless steel scrap is used as a cooling material during decarburization of molten stainless steel, the amount used must be limited, and from this point of view, it is still difficult to use as a process that can use inexpensive raw materials. It leaves a lot to be desired. Particularly in the production of stainless steel, a process that can use large quantities of inexpensive chromium-containing and nickel-containing scrap is required, and in this respect, with conventional technology, an electric furnace that can use large quantities of scrap was absolutely necessary. (Problem to be solved by the invention) It is an object to provide a fundamental solution to the conventional technology as described above, and to provide an efficient process for stainless steel melting that guarantees high quality. The process uses hot metal, Cr ore, large amounts of scrap and ferroalloy, and has a wide range of choices in terms of the use of inexpensive raw materials. Moreover, impurity components such as phosphorus and sulfur can be sufficiently controlled, and The first object of the present invention is to provide a method for melting a stainless steel mother metal, which causes less erosion of refractories in a furnace, has excellent cleaning properties, and has an extremely low concentration of gas components as impurities. In particular, instead of melting scrap in an electric furnace, which requires expensive electrical energy, it is possible to use a large amount of scrap using only a top-bottom blowing converter using the energy of cheap carbonaceous materials. The present invention provides a process in which semi-reduced pellets can be used. It is a further object to provide a reaction vessel bottom tuyere arrangement and bottom tuyeres that can be advantageously utilized in carrying out this process. (Means for solving the problem) The above purpose is achieved by the following configuration. To produce stainless steel mother metal from hot metal together with chromium-containing substances and carbon-containing substances, scrap is charged together with hot metal into a reaction vessel containing slag, and the scrap is charged while supplying carbon-containing substances and oxygen-containing gas. Following the melting process to melt the scrap, the temperature is raised, and then chromium-containing substances and carbon-containing substances are added to the hot metal while supplying oxygen-containing gas, and a smelting reduction process is carried out until the chromium content reaches the target value. A method for producing molten stainless steel mother metal, which is characterized by performing a finishing reduction process by blowing oxygen-containing gas, and tapping the molten metal with an appropriate amount of slag left behind. (First invention). In this method, the amount of slag present in the reaction vessel during the scrap melting process is at least 50 kg/t.
or more, and the thickness on the hot metal surface is 100mm or more,
In the scrap dissolution process, from the bottom of the reaction vessel.
It is more desirable to suck in stirring gas of 1.0Nm ³ /min/t or more. Next, there is a bottom tuyere arrangement of a reaction vessel exclusively used in the method of the first invention, in which bottom blowing tuyeres are arranged in a row at intervals from each other at the bottom of the furnace, and the number of the arrangement is four or more, and A straight line in which the ratio d/D of the distance d between the two tuyeres closest to the center and the diameter D of the bottom of the reactor vessel is 0.1 to 0.5, and the sum of the distances to the tuyeres is the minimum. A tuyere arrangement at the bottom of a reaction vessel for melting stainless steel mother metal, in which the ratio l/D of the distance l to the diameter parallel to this straight line and the diameter D of the furnace bottom portion is 0.1 or less. (Second invention). The bottom tuyere of the reaction vessel used exclusively in the method of the first invention is a concentric double tube consisting of an outer tube and an inner tube inserted into the outer tube, and at least at the tip of the inner tube a plurality of oxygen gas jets are provided. A bottom blowing tuyere of a reaction vessel for melting a stainless steel mother melt, which has a structure in which molten metal is spouted in a spiral shape. (Third invention). In the method of the first invention, first, the hot metal is deSi and P removed in a torpedo car, and the Si and P after the treatment are
The concentrations are set to 0.02% by weight or less and 0.015% by weight or less, respectively, and the materials are charged into the first top-bottom blowing converter at a temperature of 1180 to 1270°C. At that time, before or after that, the scrap is charged into the reaction vessel and oxygen is supplied from the top blowing lance and bottom blowing tuyere, and the scrap is melted by adding carbonaceous material such as coke. After establishing a period (period), the hot metal is heated at 1550℃ or higher and 1620℃ or lower while continuously maintaining the carbon concentration in the hot metal in the reaction vessel at least 4% by supplying carbonaceous materials and oxygen. A period (period) is provided in which the temperature is raised to the state of . Subsequently, chromium ore or prereduced pellets, carbonaceous material, and oxygen are supplied to melt and reduce the chromium ore. In the melt reduction, (%CaO)/(%SiO ₂ ) in the slag is kept at 1.5 or more and 3.5 or less, and {(MgO) + (Al ₂ O ₃ )}/{(CaO) +
(SiO ₂ ) + (MgO) + (Al ₂ O ₃ )} is maintained at 45% or more and 55% or less (this period is called the period). Once the chromium ore has been added to achieve a predetermined chromium concentration, a period is set in which oxygen gas is supplied from the top blowing lance and the bottom blowing tuyere (this period is called the first period). At the end of the first period, the supply of oxygen from the top blowing lance and the bottom blowing tuyere is stopped, and first, the slag and chromium-containing molten iron from the first top and bottom blowing converter are discharged. At this time, when the first top-bottom blowing converter is repeatedly operated, at least 10% or more of the slag from the previous charge remains. Furthermore, the bottom blowing gas flow rate range Q in the third stage operation is set to 1.0 (Nm ³ /min/t) or more. However, Q = 2・Q _p2 +Q _I +4Q _Pr / (Charging hot metal weight + scrap charging amount) Q _p2 : Bottom blowing oxygen flow rate (Nm ³ /min) Q _I : Bottom blowing inert gas flow rate (Nm ³ /min) min) Q _Pr : bottom-blown propane flow rate (Nm ³ /min) The process procedure of the first invention described above is as shown in FIG. (Function) In order to efficiently produce stainless steel mother molten metal, according to the first invention, first, a pig iron mixing car (torpedo car) is used.
After desilicification and dephosphorization treatment, the hot metal is charged into a smelting reduction furnace. At this time, the amount of hot metal charged should be 65% or less of the amount of steel tapped in the RH degasser so that the specified stainless steel mother metal can be obtained by using Cr ore, alloy, and scrap in the next process. Here, the amount of inexpensive scrap used can be significantly increased compared to the conventional method, and the amount of scrap that meets the needs is used. Therefore, the weight of hot metal used in the smelting reduction furnace is
The weight can be calculated by subtracting the amount of scrap used in the smelting reduction furnace from 65% of the amount of steel tapped in the RH degasser.
In addition, the P concentration in the dephosphorization treatment is set to 0.015% by weight or less in order to cope with the increase in phosphorus from input raw materials in the next process. In addition, the temperature of the hot metal after treatment is controlled at 1180-1270°C to maintain high dephosphorization efficiency. Hot metal is charged into the smelting reduction furnace with the above considerations in mind, but before that, scrap can be charged in advance or after the hot metal is charged, considering the economical cost. It is efficient to use scraps. However, it is of course also applicable to cases where a larger amount of scrap is used. After charging the above-mentioned scrap, a second stage is required in which only carbonaceous material and oxygen gas are supplied. This is because if chromium oxide is added immediately, the dissolution of the scrap will be delayed, and the rise in steel bath temperature will also be delayed, which will worsen the reduction of the chromium oxide and prolong the blowing time due to undissolved scrap. . Furthermore, at that time, it is also necessary to always maintain the carbon concentration in the steel bath at 4% or higher. This is because scrap can be melted quickly by not only simply supplying heat but also by carburizing it with carbon in a steel bath. This is also because when melting scrap containing chromium, it is necessary to increase the carbon concentration during the scrap melting stage, to suppress chromium oxidation during the scrap melting stage, and to make the next smelting reduction stage advantageous. Furthermore, the inventors have tried various improvements when dissolving scrap and have found that there are suitable conditions for the bottom blowing tuyere flow rate in order to quickly dissolve the scrap. In other words, Fig. 2 shows the relationship between the amount of oxygen supplied and the scrap dissolution rate in the stage of dissolving the scrap.Furthermore, for example, if sampling is performed after the amount of oxygen supplied is 100Nm ³ /min, the bottom blowing tuyere flow rate Q will be
It was found that when the dissolution rate was 1.0 Nm ³ /min/t or more, the scrap dissolution rate (the proportion of scrap input that had been completely dissolved) was almost 100%. This is because during scrap melting, it is important to increase the bottom blowing stirring force to increase the chances of contact between the scrap surface and the molten steel surface. Next, as shown in Table 1, as a result of experiments with various tuyere arrangements, it was found that the number of tuyeres is 4 or more, preferably 6 to 8, as shown in Figure 3, in order to dissolve scrap quickly. If the ratio d/D of the distance d between the innermost tuyeres and the hearth bottom diameter D is set to 0.1 to 0.5, more preferably 0.2 to 0.4, the sum of the distances to the tuyere group The ratio l/D between the distance l and the diameter parallel to this line L and the diameter parallel to this line L is 0.1.
Hereinafter, it was found that it is necessary to set it preferably to 0.006 or less.

[Table] ×: Scrap undissolved ○: Scrap dissolution completed This is because if there are fewer than 4 tuyeres, there will be localized areas in the molten metal where stirring is insufficient, and the amount of oxygen supplied will be 100Nm ³ / When the inside of the furnace was observed after min/ton (hot metal + scrap) had elapsed, unmelted scrap was observed. In addition, unmelted scrap often exists mainly in the center, so even if the number of tuyeres is four or more,
When the distance d between the tuyeres in the center exceeded 0.5 relative to D, unmelted scrap was observed between the tuyeres. If d/D is less than 0.1, there is no unmelted scrap, but the tuyere melting loss is the fourth.
It rose as shown in the figure. This is thought to be due to the fact that as the distance between the tuyeres becomes smaller, the interference between the tuyeres increases, causing greater erosion. Also, the tuyere is eccentric,
When l/D is larger than 0.1, scrap remains undissolved, and a large l/D is also not suitable for rapid dissolution of scrap. Regarding the arrangement of the tuyeres, we took into consideration the fact that the slag volume is large, which is a characteristic of smelting reduction, and furthermore, in order to minimize damage to the refractories of the furnace wall caused by vibration of the furnace body, we arranged the tuyeres so that they are arranged in a straight line or in a neutral position. It is desirable to have a linear arrangement, and also an arrangement that is slightly offset from the center so that the tuyeres are not covered with slag when the furnace body is tilted. In addition, when we examined the yield during the scrap melting stage, we found that it was necessary to suppress the generation of dust (particularly fume caused by the top-blown lance). In other words, in the scrap dissolution stage (stage)
Dust of 1.5 to 2.5 (Kg/min/ton hot metal + scrap) is generated, which has a large negative impact on yield. Therefore, the inventors made various improvements, and as a result, the thickness of the slag was always maintained at 100 mm or more during the scrap melting stage, and the generation of fume from the hot spot formed by the top blowing lance was suppressed by covering the slag. We found that this is effective. Furthermore, in the smelting and reduction of chromium ore in the subsequent process, strengthening the stirring power using the bottom blowing tuyeres can improve the reduction rate of chromium ore, so securing a bottom blowing gas flow rate is essential. In this case, fine metal particles directly reduced from chromium ore by the carbonaceous material in the slag are suspended in the slag.
If the stirring is strengthened using a bottom-blown stirring gas, the number of metal particles will increase at the end of melting and reduction. As a result, if the entire amount of slag after melting and reduction is directly removed, metal particles will be discharged and the yield will decrease.
Therefore, instead of discharging the entire amount of slag, an attempt was made to discharge only a portion of the slag. Table 2 shows the results.
Shown below.

[Table] By keeping the slag thickness at 100 mm or more and the amount of slag remaining at 50 kg/t or more, the amount of dust generated during scrap melting can be suppressed, and the metal particles in the slag during slag discharge can be reduced to slag. It can be seen that chromium loss can be reduced. Furthermore, if the amount of bottom-blown gas is reduced, the loss of chromium to the slag due to metal particles in the slag can be reduced, but for scrap melting itself, as described above, it is more advantageous to increase the amount of bottom-blown gas and perform strong agitation. The most effective method is to leave the slag from the previous charge at 50 kg/ton or more without discharging the entire amount of slag, and to maintain the slag thickness at 100 mm or more during the next scrap melting. After the scrap melting is completed according to FIG. 1 in the process procedure of the first invention, there is a third stage in which the temperature of the molten steel is raised to 1500° C. or higher until the start of melt reduction. When this temperature rise is completed, chromium ore and carbonaceous materials are supplied together with oxygen gas, and the process moves to the stage of melting and reducing the chromium ore. At this time, it is important to add lime and lightly calcined dolomite to suppress MgO elution, which causes corrosion of refractories, and to form an appropriate slag composition during the smelting and reduction stage. Usually (CaO)/(SiO) is 1.5 or more and 3.5 or less, and {(MgO)+(Al ₂ O ₃ )}/{(CaO)+(SiO ₂ )+
(MgO) + (Al ₂ O ₃ )} is effective to maintain the slag composition between 0.45 and 0.55. because,
{(MgO)+(Al ₂ O ₃ )}/{(CaO)+(SiO ₂ )+
(MgO) + (Al ₂ O ₃ )}, the melting point of the slag decreases, but refractory erosion increases. On the other hand, {(MgO) + (Al ₂ O ₃ )}/{(CaO) +
(SiO ₂ ) + (MgO) + (Al ₂ O ₃ )} increases, the melting point of the slag increases, which is extremely disadvantageous for melting and reducing chromium ore. As shown in Conventional Tetsu to Hagane 69 (1984), S117,
(MgO) + (Al ₂ O ₃ ) was known to be disadvantageous for smelting reduction of chromium ore when it exceeds 45%, but (CaO) / (SiO ₂ ) increases the basicity further.
By increasing the value to 1.5 or more, melting reduction can be carried out without problems even at (MgO) + (Al ₂ O ₃ ) concentrations, which were conventionally considered disadvantageous to melting reduction, and it is also advantageous in terms of protecting refractories. Furthermore, increasing the basicity is advantageous in terms of desulfurization. Furthermore, the inventors attempted to improve the tuyere in order to improve the melting and reduction of the chromium concentration and increase the yield.As shown in Figure 5, the inventors tried to improve the tuyere, which spews out multiple oxygen gas jets in a spiral shape so that they interfere with each other. It has been found that the reduction rate of chromium ore can be improved and the chromium yield can be increased by using chromium ore. This is because smelting reduction of chromium composition is thought to proceed through a slag-metal reaction in which the input chromium ore is once melted into slag, and the slag reacts with molten steel or coke, or through a reaction between coke and slag. For acceleration, using a tuyere in which the oxygen gas jets are ejected spirally and interfere with each other than a conventional tuyere, it is possible to form a large number of fine bubbles and promote the entrainment of slag into molten steel. It is believed that the slag-metal reaction rate is increased resulting in improved smelting reduction of the chromium ore. As such a tuyere, a tuyere formed by twisting a plurality of double-pipe tuyeres together (Fig. 5a) or a tuyere capable of spouting a plurality of spiral jets from an inner pipe (Fig. 5b) are suitable. After the chromium ore is melted and reduced until a predetermined chromium concentration is reached, the supply of carbonaceous material and chromium ore is stopped in the third stage, and oxygen is supplied. At this time, it is better to supply carbonaceous material in excess of the amount consumed by oxygen by the third stage, and not to add carbonaceous material at this stage. This is because, in the third stage, a large amount of metal grains produced by linear reduction of chromium ore are present in the slag. Therefore, if carbonaceous material is added at this time, metal particles will adhere to the surface of the added carbonaceous material,
This is because, as a result, precipitation of metal particles into the molten metal is inhibited, leading to a deterioration of the chromium yield. Furthermore, since the carbonaceous material remaining in the slag is discharged during slag removal, it is disadvantageous in terms of energy, so it is most desirable to consume the carbonaceous material input in the first stage in the second stage. In the third stage, oxygen is supplied to reduce chromium oxide, and the process continues until the temperature of the molten metal begins to rise.
After the molten metal temperature is in the range of 1,570 to 1,620°C, the molten metal is tapped and the slag is drained. At that time, the slag should be removed at a rate of 50 (Kg/t) or more, and the thickness of the slag should be adjusted during the next scrap melting.
Leave an appropriate amount of slag to keep it at least 100mm. After that, the stainless steel mother metal obtained as described above is charged into a decarburization furnace, and the amount of tapped steel is 30 to 40%, which is optimal for balancing the amount of heat supply and promoting the reduction rate of chromium oxide. A corresponding amount is added to the furnace in the form of ferroalloy or scrap. In the decarburization furnace, a mixed gas of O ₂ and inert gas is blown from the top blowing lance and bottom blowing double tuyere, and at the same time, hydrocarbon gas is blown from the bottom blowing tuyere at a ratio of 3 to 7% to bottom blowing oxygen. At the same time as flowing, the slag is decarburized to a predetermined carbon concentration, and in some cases, a ferroalloy containing Si is introduced into the furnace to reduce the chromium oxide in the slag and desulfurize at the same time.
The tapped molten metal is immediately transferred to the RH degassing equipment,
Process for 20 to 30 minutes at a vacuum level of 10 torr or less. The reason why RH treatment is performed under these conditions is that since multiple processes have been carried out before the previous process, the nitrogen concentration in the steel increases due to nitrogen absorption during tapping, and the hydrogen in the steel is reduced.
This is from the perspective of improving the cleanliness of steel by reducing oxygen in the steel. The RH degassed molten stainless steel is then cast using a regular continuous casting machine. (Example) Hot metal desiliconization and dephosphorization process A desiliconizing agent and a dephosphorizing agent are injected into hot metal using a powder suction lance 2 into a torpedo car 1 to treat the hot metal. Hot metal processing amount: 200t Hot metal component before treatment C: 4.5% by weight Si: 0.12% by weight Mn: 0.14% by weight P: 0.14% by weight S: 0.025% by weight Temperature before treatment: 1370℃ Desiliconizing agent: Dust generated from sintering furnace Raw material Unit: 25Kg/t Dephosphorizing agent: Sintering furnace dust (75% by weight) CaO (22% by weight) CaF ₂ (3% by weight) Basic unit: 60Kg/t Powder injection speed: 500Kg/min Post-treatment component C : 4.2 wt% Si: 0.01 wt% Mn: 0.10 wt% P: 0.015 wt% S: 0.024 wt% Temperature after treatment: 1240℃ 23.0 ton of stainless steel scrap was transferred to an 85 ton top-bottom blowing converter using a scrap chute. I loaded it.
After that, 42.2 tons of the above-mentioned dephosphorized pig iron was charged, and the furnace was turned into a furnace and blowing was performed. At that time, 3 tons of slag from the previous charge was left, and the slag thickness was 210 mm in the scrap melting and blowing (stage). Scrap melting was carried out while supplying carbonaceous agent and oxygen, and then heating blowing was carried out. The average dust generation rate during the scrap dissolution period was 22 kg/min. After the start of blowing, when the amount of oxygen supplied was 6000Nm ³ , temperature measurement and sampling were performed, and the temperature was 1560°C, indicating that melting and reduction was performed. Moreover, it was confirmed from the analytical values of the sampling at this time that the scraps were completely dissolved at this point. Moreover, [%C] was 4.9%. The top blowing acid rate was 220Nm ³ /min, the bottom blowing acid rate was 60Nm ³ /min, and the propane flow rate was 5.0Nm ³ /min. In addition, six tuyeres capable of spouting oxygen in a spiral shape as shown in Figure 5a were used as tuyeres, and in this experiment, operations were carried out at d/D = 0.25 and l/D = 0.06. As a result, the amount Q of bottom-blown gas generated during the scrap melting period was 60×2+5×4/(23+42.2)=2.15Nm ³ /min/t. Subsequently, chromium ore was melted and reduced using an oxygen supply amount of ^6200Nm3 . In addition, carbonaceous agent was supplied at a ratio of 1.8 Kg/Nm ³ O ₂ during the scrap melting and heating periods. Now, during the Cr oxide reduction period, half-reduced Cr pellets and carbonaceous agent were added to keep the iron bath temperature constant for thermal balance purposes. The components of the semi-reduced Cr pellets are shown in Table 3.

[Table] Total O ₂ after adding semi-reduced pellets
At 18300 Nm ³ it was transferred to the finishing reducing agent (stirring). In other words, the top blowing oxygen rate was reduced, top blowing _O2 : 60Nm ³ /min, bottom blowing O2: 60Nm ³ /min for 10 minutes.
I went there and took a bath. The amount of hot water tapped was 71.3 tons, and the refining time (from the start of blowing to the end of blowing) was 71 minutes. When the sublance was introduced before entering the final reduction period, the bath temperature was 1570°C, and it was confirmed that the iron bath temperature was kept almost constant during the Cr oxide reduction period. The bath temperature and components at the time of tapping were as shown in Table 4 below.

[Table] The slag composition at the time of tapping was as shown in Table 5 below.

[Table] Table 6 shows the Cr pellets and coke used.
Shown below.

[Table] In the above experiment, Cr yield: 98.7%,
The steel production yield was 95.7% and the Ni yield was 100%, which were extremely good. (Comparative Example) Exactly the same operation was carried out using a top-bottom blowing converter equipped with six ordinary double-tube tuyeres under conditions that no slag was left behind. The amount of scrap was 22.5 tons and the amount of hot metal charged was 41.5 tons. The other operating conditions were almost the same. After the completion of the final return (period), an investigation revealed that Cr
Yield was 83%, and steel production yield was 86.2%.
This means that the dust generation rate during the scrap dissolution period is 130%.
It is thought that the yield was reduced by the iron particles in the dust and slag because the discharged slag contained a large amount of granular iron with a high chromium concentration. (Effects of the Invention) The first invention relating to the melting of chromium-containing iron as a stainless steel mother melt allows the use of cheaper scrap raw materials in larger quantities than in the past, suppresses dust generation during scrap melting, and improves yield. Moreover, the tuyere arrangement of the second invention and the tuyere structure of the third invention were useful in carrying out the first invention.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing the process flow of the first invention, Figure 2 is a graph of the relationship between the bottom blowing gas flow rate during the scrap melting period and the scrap melting rate after the passage of 100Nm ³ /t of oxygen supply, and Figure 3 is a graph of the relationship between the scrap melting rate after the passage of 100 Nm 3 /t of oxygen FIG. 4 is an arrangement diagram showing the positional relationship of the bottom tuyeres, FIG. 4 is a graph showing the relationship between d/D and tuyere erosion, and FIG. 5 is an explanatory diagram of the tuyeres. d... Tuyere spacing, D... Hearth bottom diameter, l... Deviation from hearth bottom diameter, 1... Outer cylinder, 2... Inner cylinder, 3... Narrow gap, 4... Spiral hole, 7... ...outer pipe, 8...inner pipe.

Claims (1)

[Scope of Claims] 1. In melting a stainless steel mother metal from hot metal together with a chromium-containing substance and a carbon-containing substance, scrap is charged together with the hot metal into a reaction vessel containing molten slag, and the carbon-containing substance and oxygen Following the melting process in which the scrap is melted while supplying gas containing gas, the temperature is raised, and further, while supplying gas containing oxygen, chromium-containing substances and carbon-containing substances are added to the hot metal and melted until the chromium content reaches the target value. A method for producing molten stainless steel mother metal, which is characterized by performing a reduction process, then performing a final reduction process by blowing with oxygen-containing gas, and tapping the molten metal leaving an appropriate amount of slag. 2. The method according to claim 1, wherein the amount of slag present in the reaction vessel in the scrap melting process is at least 50 kg/t or more and has a thickness of 10 mm or more on the surface of the hot metal bath. 3. The method according to claim 1 or 2, wherein in the scrap melting step, a stirring gas of 1.0 Nm ³ /min/t or more is blown from the bottom of the reaction vessel. 4. A bottom tuyere arrangement of a reaction vessel exclusively used in the method according to claim 1, in which the bottom blowing tuyeres are arranged in rows at intervals from each other at the bottom of the furnace, and the number of the arrangement is 4 or more, and A straight line in which the ratio d/D of the distance d between the two tuyeres closest to the center and the diameter D of the bottom of the reactor vessel is 0.1 to 0.5, and the sum of the distances to the tuyeres is the minimum. A tuyere arrangement at the bottom of a reaction vessel for melting stainless steel mother metal, in which the ratio l/D of the distance l to the diameter parallel to this straight line and the diameter D of the furnace bottom portion is 0.1 or less. 5 The bottom tuyere of the reaction vessel used exclusively for the method according to claim 1 is a concentric double tube consisting of an outer tube and an inner tube inserted into the outer tube, with a plurality of concentric tubes at least at the tip of the inner tube. A bottom blowing tuyere of a reaction vessel for melting stainless steel mother metal, which has a structure in which an oxygen gas jet is spouted in a spiral manner.