JP7196598B2 - Hot metal decarburization method - Google Patents

Description

The present invention relates to a method for decarburizing hot metal.

In the molten iron refining process, P is first removed from the molten iron by dephosphorization refining. P is incorporated into the slag as P ₂ O ₅ . After the slag containing P ₂ O ₅ is removed from the hot metal, excess C is then removed from the hot metal by decarburization refining. There are various means of removing slag from hot metal. For example, when it is necessary to reduce the phosphorus content in the final product, molten iron and slag are separated by tapping the molten iron from the converter after dephosphorization and refining (LD-ORP method). The molten pig iron discharged from the converter can be moved to another furnace for decarburization refining (dedicated furnace type LD-ORP), or returned to the dephosphorization refining converter from which the slag has been removed for decarburization refining. (same furnace type LD-ORP).

However, it has been difficult to completely remove the slag generated during dephosphorization and refining. For example, in a dedicated furnace type LD-ORP, when molten iron is transferred from the converter, a small amount of slag is transferred together. In the same furnace type LD-ORP, a small amount of slag remaining in the dephosphorization refining converter is returned to the hot metal.
P contained in the slag present in the furnace together with the molten iron after dephosphorization refining returns to the molten steel during decarburization refining. This phenomenon is called re-phosphorization or re-P. In some cases, the phosphorus content of molten steel after decarburization and refining could not be sufficiently reduced due to rephosphorization. In refining methods other than the LD-ORP method, the problem of rephosphorization due to residual slag may also occur.

As a means for solving this problem, for example, in Patent Document 1, while enjoying the merits of performing dephosphorization refining and decarburization refining in the same converter, ultra-low phosphorus steel with particularly strict P standards As a converter refining method that can stably melt, when refining steel using a top and bottom blown converter, molten iron is charged into the converter in the first step and flux is used in the second step. Molten iron is dephosphorized by top-bottom blowing refining in a converter, the converter is tilted in the third step to discharge part or all of the slag generated in the second step, and flux is added in the fourth step to add flux to the converter. Hot metal is dephosphorized by top and bottom blow refining, in the fifth step the converter is tilted to discharge part or all of the slag generated in the fourth step, and in the sixth step decarburization is performed by top and bottom blow refining of the converter. A converter refining method characterized by performing

According to this technique, it is said that the P concentration in the molten steel after decarburization refining can be sufficiently reduced to the level of ultra-low P steel. However, in this technique, it is essential to perform hot metal dephosphorization twice. Therefore, according to the technique of Patent Literature 1, it is inevitable that the operating time is prolonged and the manufacturing cost is increased.

Due to the above circumstances, it has been necessary to sufficiently remove the dephosphorization slag by performing hot metal dephosphorization twice as in Patent Document 1 for stable production of ultra-low phosphorus steel. However, in this case, the refining time and amount of heat required for refining are large, and there is a problem regarding operational efficiency. Therefore, there is a demand for a refining method capable of sufficiently reducing the P content of molten steel after decarburization refining in a short time and with a small amount of heat.

JP 2011-144415 A

The present invention sufficiently reduces the P content of molten steel after dephosphorization refining in a short time and with a small amount of heat by efficiently suppressing rephosphorization from slag present in the furnace together with molten iron after dephosphorization refining. An object of the present invention is to provide a refining method capable of

The gist of the present invention is as follows.
(1) A hot metal decarburization method according to an aspect of the present invention includes a step of blowing oxygen into the hot metal to decarburize the hot metal, and Before and after the start, a first slag agent containing CaO or CaCO ₃ is charged into the hot metal, and in the decarburization blowing, a powdery second slag agent containing CaO or CaCO ₃ is added to the above using a carrier gas. The amount of the oxygen that is blown into the hot metal between the start time of the oxygen blowing and the start time of the second slag agent blowing is defined as the total oxygen blowing in the decarburization blowing. 10% or more and 50% or less of the loading amount .
(2) In the hot metal decarburization method described in (1) above, the amount of oxygen to be blown between the start time of the oxygen blowing and the end time of the blowing of the second slag agent is , It may be 20% or more and 60% or less of the total amount of oxygen blown in the decarburization blowing.
(3) In the hot metal decarburization method according to any one of (1) or (2) above, the second slag agent may be injected at a CaO equivalent of 3 to 20 kg/t.
(4) In the hot metal decarburization method according to any one of (1) to (3) above, a third slag agent containing Si is added to the hot metal before the decarburization blowing, and the The input amount of the first slag agent and the input amount of the third slag agent are adjusted so that the charging basicity of the slag generated after the input of the first slag agent and before the blowing of the second slag agent is 3.5. It may be controlled to be ~4.5.
(5) In the method for decarburizing hot metal according to any one of (1) to (4) above, the first slag agent may be a lumpy slag agent.
(6) In the hot metal decarburization method according to any one of (1) to (5) above, one or both of the first slag agent and the second slag agent are made of quicklime, limestone, calcium ferrite, It may be one or more selected from dolomite-based lime, and converter slag or secondary refining slag containing CaO.
(7) In the hot metal decarburization method according to any one of (1) to (6) above, the carrier gas is one selected from the group consisting of Ar, N ₂ , CO ₂ and O ₂ . or more.

The present invention sufficiently reduces the P content of molten steel after decarburization refining in a short time and with a small amount of heat by efficiently suppressing rephosphorization from slag existing in a decarburization refining furnace. can provide a refining method capable of

FIG. 4 is a schematic diagram illustrating preferred start and end times of blowing the second slag agent. It is a figure which shows an example of the injection|throwing-in location of a 2nd slag agent. It is a graph which shows the relationship between the presence or absence of the injection|throwing-in of a 2nd slag agent, and blowing end Lp. It is a graph which shows the relationship between the timing of injection|throwing-in of a 2nd slag agent, and a slag-forming rate. It is a graph which shows the relationship between the timing of injection|throwing-in of a 2nd slag agent, and the amount of N picked up.

Dephosphorization slag is decarburized as a means for efficiently suppressing rephosphorization from slag containing P ₂ O ₅ (dephosphorization slag) produced by dephosphorization refining and carried over to the hot metal in the dephosphorization refining stage. It is conceivable to remove it from the furnace prior to smelting. However, in order to do this perfectly, it is inevitable that the operation time will be long and the manufacturing cost will be high. Therefore, the present inventors are not concerned with improving the process before the start of decarburization blowing to reduce the amount of slag, but rather with improving decarburization blowing to minimize the effect of slag. I made a thorough study. As a result, the following findings were obtained.

Usually, before the start of decarburization refining, slag is formed to cover the surface of molten iron for the purpose of suppressing spitting (a phenomenon in which molten iron blows out from the furnace port due to bumping of molten iron). This cover slag is formed by adding a slag-forming agent containing CaO (or CaCO ₂ ) as a main component into hot metal.

This cover slag also has dephosphorization ability. P produced by rephosphorization is removed from the hot metal by the following chemical reactions occurring between the cover slag and the hot metal.
2 [P] + 5 (FeO) → (P ₂ O ₅ ) + 5 [Fe]: Formula A
(P ₂ O ₅ )+3(CaO)→(3CaO·P ₂ O ₅ ): Formula B
Formulas enclosed in square brackets, as set forth in Formulas A and B, are the chemical formulas of the components in the hot metal, and the chemical formulas enclosed in parentheses are the chemical formulas of the components melted in the slag. First, as shown in Equation A, [P], ie P in hot metal, is oxidized to P ₂ O ₅ by (FeO), ie FeO in slag. This P ₂ O ₅ is then fixed to (CaO), the molten CaO in the slag, to produce the stabilized compound (3CaO.P ₂ O ₅ ), as shown in Equation B. .

As shown in Equation A, FeO contained in the slag is required for removal of P in the hot metal. On the other hand, as the decarburization refining progresses, FeO in the slag is consumed by the following reaction.
(FeO) + [C] → [Fe] + CO ↑: Formula C
"CO↑" described in Formula C is CO gaseous and released from hot metal and slag.

The present inventors suspected that the chemical reaction represented by formula C would deplete the FeO in the slag and inhibit the dephosphorization reaction of formula A above. Accordingly, the present inventors have investigated means for suppressing the reduction of FeO during decarburization refining. The present inventors have found that the P content of molten steel after decarburization refining can be reduced by injecting powdered CaO into molten iron not only before the start of decarburization refining but also during decarburization refining. discovered. It is believed that this is because when powdered CaO is blown into hot metal during decarburization blowing, a reaction occurs to form a complex of CaO and FeO (CaO—FeO), which is represented by Formula D below. be done.
(CaO) + [Fe] + 1/2O ₂ → (CaO-FeO): Formula D
It is believed that this CaO—FeO functions as (FeO) in formula A and promotes the reaction of formula A. That is, CaO added to the hot metal during decarburization blowing maintains the dephosphorization ability of the cover slag and contributes to dephosphorization by replenishing FeO in the slag consumed during decarburization blowing. It is thought that

CaO is used as a dephosphorization agent in dephosphorization refining. However, according to the prior art, it was considered undesirable to inject CaO into hot metal during decarburization refining. Since CaO does not have decarburization ability, it does not contribute to the efficiency of decarburization refining. _In addition, the carrier gas when blowing _CaO is usually _N2 gas in consideration of cost and safety. is taken into molten iron and remains as undesirable impurities in molten steel after decarburization and refining. This phenomenon is called N-pickup. During dephosphorization refining, blowing powdered CaO with _N2 gas is not a problem because C in hot metal prevents N pick-up. However, as the decarburization of the hot metal progresses through decarburization refining, the hot metal tends to absorb nitrogen. In other words, in decarburization refining, the influence of N pick-up on the quality of molten steel cannot be ignored. Blowing with gases other than _N2 is problematic with respect to safety or operating costs. For the above reasons, CaO blowing in decarburization blowing has been avoided. According to the prior art, it was thought that rephosphorization should be prevented by improving the efficiency of removal of dephosphorization slag, as shown in Patent Document 1, for example.

However, as a result of further studies, the inventors also found that the influence of N pick-up can be suppressed by limiting the timing of blowing CaO.

The hot metal decarburization method according to the present embodiment based on the above-described technical concept includes a step of decarburizing the hot metal by blowing oxygen into the hot metal. Then, the first slag agent containing CaO or CaCO ₃ is charged into the hot metal, and in the decarburization blowing, immediately after the start of oxygen blowing, the first slag agent containing CaO or CaCO ₃ is charged into the hot metal, and further In decarburization blowing, a powdery secondary slag agent containing CaO or _CaCO3 is blown into the hot metal using a carrier gas. The hot metal decarburization method according to the present embodiment will be described in detail below.

In the hot metal decarburization method according to the present embodiment, the hot metal is decarburized and blown by blowing oxygen into the hot metal. During this decarburization blowing, the first slag agent and the second slag agent are put into the hot metal.

The first slag agent is a slag agent containing CaO or CaCO ₃ and is charged into the hot metal before and after the start of oxygen blowing and before the second slag agent, which will be described later. A first slag agent is added to form a cover slag on the surface of the hot metal to prevent spitting. The cover slag also has the function of removing P contained in the hot metal. The timing of charging the first slag agent is not particularly limited, and may be, for example, before or after the start of oxygen blowing in decarburization blowing. Note that CaCO ₃ is decomposed into CaO in a short period of time by the heat of hot metal, so it can be used as a supply source of cover slag like CaO.

The second slag agent is a powdery slag agent containing CaO or _CaCO3 , and is blown into the hot metal after the first slag agent is charged. The secondary slag agent forms FeO when blown into the hot metal during decarburization blowing. The FeO in the cover slag consumed by the progress of the decarburization reaction is replenished by blowing in the second slag agent, thereby recovering the dephosphorization ability of the cover slag.

The second slag agent is pulverized in its form and blown into the hot metal using a carrier gas to facilitate melting of the second slag agent. When the second slag agent is put into hot metal as a lump, the melting of the second slag agent does not proceed efficiently. An increase in environmental load occurs.

The point of time when the second slag agent starts blowing is not particularly limited in order to exhibit the dephosphorization effect. On the other hand, the present inventors have found that it is preferable to start blowing the second slag agent after the decarburization reaction by decarburization blowing has progressed to some extent. Specifically, the amount of oxygen blown between the start time of blowing oxygen and the start time of blowing the second slag agent is 10% of the total amount of oxygen blown in decarburization blowing. It is preferable to make it 50% or less.
A preferred time to start blowing the second slag agent is shown in FIG. FIG. 1 is a schematic graph showing the history of the oxygen blowing amount, with the horizontal axis representing the elapsed time from the start of oxygen blowing and the vertical axis representing the oxygen blowing amount (unit: mass % relative to the total blowing amount). be. In FIG. 1, for the sake of simplification, it is assumed that oxygen is blown at a constant speed, but the speed of blowing oxygen may of course be changed over time. In this case, the graph showing the relationship between the elapsed time and the oxygen blowing amount becomes a line or a curve. The left end of the double-headed arrow shown in FIG. This is the time when 50% of the injected amount has been injected.
According to the well-known technique, the higher the temperature of the cover slag, the more likely rephosphorization occurs. Therefore, it is not efficient to blow CaO after the decarburization blowing progresses and the temperature of the cover slag rises. It has been assumed not. However, the present inventors have found that the dephosphorization ability of the cover slag is further improved by delaying the blowing of the second slag agent. Although the cause of this is not clear, it is thought that the slag formation rate of the slag agent may be reduced when the second slag agent is blown too early. As a result of the experiment, in order to sufficiently secure the dephosphorization ability of the cover slag, the blowing of the second slag agent should be started at an amount corresponding to 10% of the total amount of oxygen blown for decarburization blowing. It was judged that it is more preferable to carry out after the blowing of oxygen. When the timing of the second slag agent is set within this range, the phosphorus content in the molten steel can be further reduced without increasing the amount of slag by increasing the distribution of phosphorus to the slag. The amount of oxygen blown between the start time of blowing oxygen and the start time of blowing the second slag agent is 15% or more, 20% or more of the total amount of oxygen blown in decarburization blowing , or 30% or more.

In addition, the amount of oxygen blown between the start time of blowing oxygen and the start time of blowing the second slag agent is set to 50% or less of the total amount of oxygen blown in decarburization blowing. is preferred. That is, it is preferable to start blowing the second slag agent before blowing oxygen in an amount corresponding to 50% of the total amount of oxygen blown for decarburization blowing. By setting the start of blowing of the second slag agent within the above range, the recovery of the dephosphorization ability of the cover slag due to the replenishment of FeO described above is caused early, and as a result, the P of the hot metal is more reliably removed. can do. In addition, since N ₂ is difficult to dissolve in hot metal before decarburization proceeds, the so-called N pick-up problem can be avoided when N ₂ is used as the carrier gas for injection. The amount of oxygen blown between the start time of blowing oxygen and the start time of blowing the second slag agent is 45% or less of the total amount of oxygen blown in decarburization blowing, 40% or less , or 35% or less. When O ₂ is used as the carrier gas, the problem of N pick-up does not occur, but the operational safety may be compromised. Using other inert gases as carrier gases increases operating costs.

The amount of the second slag agent injected is not particularly limited. It is considered that a sufficient dephosphorization effect can be obtained by blowing the second slag agent into hot metal in an amount of 3 kg or more per ton of hot metal (i.e., 3 kg/t) in terms of CaO equivalent. It may be the lower limit of the blowing amount. In addition, if the second slag agent is injected at a CaO equivalent of 20 kg/t or more, the effect of the injection is saturated, but the amount of slag is considered to increase. may be The amount of CaO equivalent of the second slag agent injected may be 3.1 kg/t or more, 3.2 kg/t or more, or 4.0 kg/t or more. The amount of CaO equivalent of the second slag agent injected may be 10 kg/t or less, 5 kg/t or less, or 3.2 kg/t or less. The CaO equivalent of the slag agent is the CaO content of the slag agent, assuming that all the Ca in the slag agent forms CaO.
The point of time when the second slag agent blowing is finished is not particularly limited. On the other hand, from the viewpoint of avoiding the problem of N pick-up described above, the blowing of the second slag agent may be terminated before the decarburization blow progresses by about 60%. That is, the amount of oxygen blown between the start time of oxygen blowing and the end time of blowing the second slag agent is 60% or less of the total amount of oxygen blown in decarburization blowing. You may do so. The amount of oxygen blown between the start time of blowing oxygen and the end time of blowing the second slag agent is 55% or less of the total amount of oxygen blown in decarburization blowing, 50% or less , or 45% or less. Considering the above-mentioned preferable starting point of injection of the second slag agent and the ability of the decarburization refining unit to inject the second slag agent, the end point of the injection of the second slag agent is It may be after the time when the smelting progresses by about 20%. That is, the amount of oxygen blown between the start time of blowing oxygen and the end time of blowing the second slag agent is 20% or more of the total amount of oxygen blown in decarburization blowing. You may do so. The amount of oxygen blown between the start time of blowing oxygen and the end time of blowing the second slag agent is 25% or more, 30% or more of the total amount of oxygen blown in decarburization blowing , or 35% or more.
FIG. 1 shows the preferred end point of blowing of the second slag agent. The left end of the double arrow shown in FIG. 1, which indicates the preferred end time of blowing of the second slag agent, is the time point when 20% of the total amount of oxygen is blown, and the right end is the time point when oxygen is completely blown. This is the time when 60% of the injected amount has been injected.

A third slag agent containing Si may be added to the hot metal before decarburization blowing. Si contained in this third slag agent works as a heat source for heating molten iron in decarburization blowing. Moreover, the basicity of the cover slag can be controlled by the Si of the third slag agent. The slag basicity is the ratio between the amount of molten CaO and the amount of molten _SiO2 in the slag, and is a value calculated by the following formula E.
Basicity of slag = amount of molten CaO in slag / amount of molten _SiO2 in slag: formula E

The input amounts of the first slag agent and the third slag agent are controlled so that the charging basicity of the slag generated after the first slag agent is added and before the second slag agent is blown is 3.5 to 4.5. preferably. This is because if the charge basicity is too low, the slag becomes excessively viscous, and foaming (swelling caused by the gas generated by decarburization blowing lifting the highly viscous slag) is likely to occur. In this case, operational stability is impaired. On the other hand, if the charging basicity is too high, the viscosity of the slag becomes insufficient, and spitting may not be prevented. It should be noted that the charge basicity means that when the slag rate is assumed to be 100% (that is, all the Si of additives such as hot metal and slag agents becomes molten _SiO2 , and CaO of additives (or _CaCO3 etc.) were all turned into molten CaO). The input amounts of the first slag agent and the third slag agent are adjusted so that the charging basicity of the slag generated after the first slag agent is added and before the second slag agent is blown is 3.6 or more, 3.7 or more, or 3. .8 or more may be controlled. The input amounts of the first slag agent and the third slag agent are adjusted so that the charging basicity of the slag generated after the first slag agent is added and before the second slag agent is blown is 4.3 or less, 4.2 or less, or 4. .0 or less.

As described above, the form of the second slag agent is powdery, but the form of the first slag agent is not particularly limited. From the viewpoint of preventing input loss, the form of the first slag agent is preferably a lump.

The components of the first slag agent and the second slag agent are not particularly limited as long as they contain CaO (or CaCO ₃ ). Preferred examples of one or both of the first slag agent and the second slag agent are selected from quicklime, limestone, calcium ferrite, dolomite lime, and converter slag or secondary smelting slag containing CaO. The first slag agent and the second slag agent each contain 30 to 100% by mass of CaO, CaCO ₃ , and CaF ₂ in terms of CaO equivalents. . Here, the CaO equivalent refers to the CaO content when it is assumed that all Ca in the slag agent forms CaO. When converter slag or secondary refining slag is used as the first slag agent and/or the second slag agent, it is possible to reduce the amount of slag generated in the entire refining process, thereby reducing the environmental burden.

CaCO ₃ and CaF ₂ are decomposed into CaO in a short period of time by the heat of hot metal. Therefore, the components and input amounts of the first slag agent and the second slag agent are controlled by the CaO equivalent.

The type of carrier gas used for blowing the second slag agent is not particularly limited. Examples of carrier gases include one or more gases selected from the group consisting of Ar, _N2 , _CO2 , and _O2 . Among these, _N2 , which is an inert gas, is most preferable from the viewpoints of facility safety, operating costs, and the like. However, when N ₂ gas is used as the carrier gas, as described above, suppression of N pick-up of hot metal must be considered.

An apparatus for performing decarburization refining according to this embodiment is not particularly limited. For example, a top and bottom blowing converter 1 having a lance for blowing a powdery second slag agent 5 using a carrier gas, as illustrated in FIG. preferable. When decarburization refining of hot metal is performed using the top-bottom blown converter 1, the second slag agent 5 is preferably injected directly below and in the vicinity of the lance 4 for blowing top-blown oxygen 6 into the hot metal. Directly below the lance 4 and its vicinity is a region where the temperature is extremely high due to the heat of oxidation of Si, C, etc. in the hot metal, that is, a hot spot 7 . By injecting the second slag agent 5 into this region, the second slag agent 5 can be melted more efficiently. FIG. 2 shows an embodiment in which the second slag agent 5 is blown using the lance 4 for blowing the top-blown oxygen 6, but another lance for blowing the second slag agent 5 is used for top-bottom blowing. It may be provided in the converter 1 . In addition, by using the top and bottom blown converter 1 and performing converter refining in which dephosphorization refining, slag removal, and decarburization refining are performed in the same converter, the overall refining time is shortened, and heat loss in refining is reduced. can be reduced.

(1) Confirmation of the effect of blowing the second slag agent on dephosphorization In order to confirm the effect of blowing the second slag agent on dephosphorization, tests were conducted under the following two conditions.
Level A: A total of 1 ton of second slag agent was blown. The blowing of the second slag agent was started approximately 60 seconds after the start of decarburization blowing. _N2 was used as the carrier gas when blowing the second slag agent.
Level B: Blowing of the second slag agent was not carried out.
In the case of level A (60 seconds), the amount of oxygen blown between the start timing of blowing oxygen and the start timing of blowing the second slag agent is It was 10% of the total blowing amount. In the case of level A, the amount of oxygen blown between the start of blowing of oxygen and the end of blowing of the second slag agent was 20%. Under any conditions, the lumpy first slag agent was put into the hot metal immediately after the start of decarburization blowing. Light burnt dolomite (CaO: 65%, MgO: 32%) and quicklime (CaO: 98%) were used together as the first slag agent. The charge basicity after charging the first slag agent was within the range of 3.3 to 4.7. Also, quicklime (CaO: 98%) was used as the second slag agent. The amount of hot metal was 320±20 tons. Therefore, when the amount of the second slag agent injected (1 t in total) is converted into CaO equivalent per 1 ton of hot metal, it is 3.06 kg/t (=1000 kg×98%/320 t).

The dephosphorization effect was evaluated by blowing end Lp. The blow end Lp is the actual result Lp at the end of decarburization blowing. The actual Lp is the ratio of the measured value of phosphorus concentration (%P) in slag per unit mass% to the measured value of phosphorus concentration [%P] in hot metal per unit mass%, that is, the following formula F is the value represented. When the actual Lp is large, the amount of phosphorus distributed to the slag is large, so dephosphorization is achieved with high efficiency.
Actual Lp = (%P)/[%P]: Formula F

The results of the experiment are shown in FIG. FIG. 3 is a scatter diagram of the experimental results, with the horizontal axis representing the blow end performance (molten steel temperature at the end of decarburization blowing) and the vertical axis representing the blow end Lp. As is clear from FIG. 3, Level A, in which the second slag agent was blown, exhibited a higher phosphorus partition ratio than Level B. Furthermore, Table 1 shows the average value of blow end Lp under each condition. Table 1 also shows that level A had a higher phosphorus distribution ratio.

(2) Confirmation of Effect of Injection Timing of Second Slag Agent on Dephosphorization In order to confirm the effect of injection timing of second slag agent on dephosphorization, tests were conducted under the following three conditions. In all tests, the conditions other than those shown in the table below were the same as in the above experiment (1). That is, the charged amount of hot metal was in the range of 320±20 tons, and the compositions of the first and second slag agents were as described above.

Experimental results for these levels 1 to 3 are shown in FIG. Figure 4 shows the charge basicity at the end of the experiment (value calculated from the amount of material put into the furnace by the end of the experiment) and the actual basicity (the basicity of the cover slag at the end of the experiment). It is a graph showing the relationship between the obtained value). If the material charged into the furnace is all slag, charged basicity and actual basicity will be equal. Also, the larger the actual basicity, the higher the slag rate of the charged substance. As can be seen from FIG. 4, Level 2, in which the introduction of the second slag agent was delayed to some extent, showed a higher slag formation rate.

(3) Confirmation of effect of second slag injection timing on N pick-up In each of the above levels 1 to 3, [N] on the ladle (N concentration in molten steel) at the end of decarburization blowing was measured. The results are shown in FIG. As shown in FIG. 5, in level 1 and level 2, in which blowing was started early, [N] on the pan was low. This indicates that N pick-up was suppressed.

1 Converter 2 Hot metal 3 Slag 4 Lance 5 Second slag agent 6 Top-blown oxygen 7 Fire point

Claims (7)

A method for decarburizing hot metal,
A step of decarburizing and blowing the molten iron by blowing oxygen into the molten iron;
Before and after the start of the oxygen blowing in the decarburization blowing, a first slag agent containing CaO or CaCO ₃ is put into the hot metal,
Further, in the decarburization blowing, a powdery second slag agent containing CaO or CaCO ₃ is blown into the hot metal using a carrier gas ,
10% of the total amount of oxygen blown in the decarburization blowing A method for decarburizing hot metal, characterized in that the decarburization is 50% or less . 20% of the total amount of oxygen blown in the decarburization blowing 2. The method for decarburizing hot metal according to claim 1 , wherein the decarburization ratio is 60% or more. 3. The hot metal decarburization method according to claim 1, wherein the second slag agent is injected in an amount of 3 to 20 kg/t in terms of CaO equivalent. Before the decarburization blowing, a third slag agent containing Si is put into the hot metal,
The input amount of the first slag agent and the input amount of the third slag agent are adjusted so that the charging basicity of the slag generated after the input of the first slag agent and before the blowing of the second slag agent is 3. The method for decarburizing hot metal according to any one of claims 1 to 3 , characterized in that the ratio is controlled to be 5 to 4.5. The hot metal decarburization method according to any one of claims 1 to 4 , characterized in that the first slag agent is a massive slag agent. One or both of the first slag agent and the second slag agent are selected from quicklime, limestone, calcium ferrite, dolomite lime, and converter slag or secondary smelting slag containing CaO. The hot metal decarburization method according to any one of claims 1 to 5 , characterized by the above. The hot metal decarburization method according to any one of claims 1 to 6 , wherein the carrier gas is one or more selected from the group consisting of Ar, N ₂ , CO ₂ and O ₂ . .

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