Low phosphorus steel smelting method

Abstract

To provide a smelting method of low phosphorus steel with more improved dephosphorization efficiency during decarburization blowing.SOLUTION: In a smelting method of low phosphorus steel, in a converter type smelter equipped with a top blowing lance, dephosphorization blowing is carried out to separate the dephosphorization slag produced by dephosphorization blowing, and then decarburization blowing is carried out by feeding a lime source containing CaO or CaCO3 in the range of 30-70 kg / t-steel in total CaO equivalent mass, and the C concentration in the molten iron is reduced to less than 0.4 mass%. In the decarburization blowing, at the time of starting the blowing, 60-80% of the total CaO equivalent mass is put into the lime source, oxygen is sprayed from the top blowing lance at an acid feeding rate in the range of 150-250 Nm3 / (h t-steel) to start the blowing, and at the time when 80-90% oxygen is supplied at a ratio of flow rate in the total oxygen amount from the time of starting the blowing, the rest of the lime source is put in and the acid feeding rate is reduced to 40-85% before the change.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing low phosphorus steel with improved dephosphorization efficiency during decarburization blowing. [Background technology]

[0002] Conventionally, dephosphorization blowing is performed to remove P from molten pig iron, but when producing low-phosphorus steel, it is important to efficiently remove P in the subsequent decarburization blowing as well. In addition, to prevent rephosphorization from the dephosphorization slag, it is necessary to remove as much dephosphorization slag as possible before the decarburization blowing. However, since some dephosphorization slag remains as carryover slag, if new lime source is added to adjust the slag basicity before the decarburization blowing to prevent rephosphorization, the amount of lime becomes excessive and the liquid phase ratio of the slag decreases. As a result, the dephosphorization rate becomes excessively slow, and the dephosphorization efficiency during the decarburization blowing is low, and dephosphorization cannot be performed sufficiently.

[0003] Therefore, various proposals have been made to perform dephosphorization efficiently. Patent Document 1 discloses a technique of changing the oxygen supply rate or charging a CaO source depending on the blowing time. Patent Document 2 discloses a method of charging a first slag agent before and after the start of oxygen blowing, and injecting a second slag agent using a carrier gas during decarburization blowing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6053570 [Patent Document 2] JP 2020-105562 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the technology described in the above Patent Document 1, the slag basicity at the start of blowing is excessively lowered compared to the slag basicity in the decarburization blowing of a normal converter process for low phosphorus, prioritizing the dissolution of the slag, and the dephosphorization ability of the slag at the beginning of blowing is significantly reduced. This results in an excessive amount of CaO source added in the middle of blowing, which is not fully dissolved during blowing, and a stable phosphorus reduction effect cannot be obtained. The technology described in Patent Document 2 requires equipment modification to perform top blowing of the second slag agent in the decarburization blowing. In addition, the nitrogen gas that serves as a carrier during top blowing of the second slag agent increases the N concentration in the molten iron, so there are restrictions on the injection timing when applying this technology to low-nitrogen steel.

[0006] In view of the above problems, an object of the present invention is to provide a method for producing low phosphorus steel which can improve the efficiency of dephosphorization during decarburization blowing by a simple operation. [Means for solving the problem]

[0007] If dephosphorization slag is separated and new lime source is added before decarburization blowing, the amount of lime becomes excessive and the slag liquid fraction decreases. As a result, the dephosphorization rate becomes excessively slow until the middle of decarburization blowing, and rephosphorization from the carried-over slag occurs. On the other hand, in the final stage of decarburization blowing, the C concentration in the molten iron is difficult to decrease, and a large amount of FeO is generated. When a large amount of FeO is generated, the liquid fraction of the slag increases and the dephosphorization reaction becomes easier to occur, but from the viewpoint of iron yield, the time in the final stage of decarburization blowing cannot be extended excessively.

[0008] The inventors therefore considered that the dephosphorization efficiency would increase if the liquid phase rate of the slag could be secured up to the end of the decarburization blowing, and a longer time could be secured at the end of the blowing while suppressing a decrease in yield. They discovered that the amount of lime source charged at the start of blowing could be suppressed, and the remaining lime source could be charged at the end of the decarburization blowing while reducing the oxygen supply rate, thereby arriving at the present invention.

[0009] The present invention is as follows. (1) A method for producing low phosphorus steel, comprising the steps of: performing dephosphorization blowing in a converter-type refining apparatus equipped with a top-blowing lance; separating the dephosphorization slag produced by the dephosphorization blowing; feeding a lime source containing CaO or CaCO3 in a range of 30 to 70 kg / t-steel in terms of total CaO; and performing decarburization blowing to reduce the C concentration in molten iron to less than 0.4% by mass, In the decarburization blowing, the lime source is charged at 60 to 80% of the total CaO converted mass at the time of starting blowing, and the oxygen flow rate from the top blowing lance is 150 to 250 Nm 3 / (h t-steel) and starting blowing, and when 80 to 90% of the total amount of oxygen has been supplied in terms of a flow rate from the start of blowing, the remainder of the lime source is charged and the oxygen supply rate is changed to the range of formula (1). 0.40≦F O2_after / F O2_before ≦0.85 (1) Here, F O2_before : The oxygen supply rate (Nm m) before the remaining lime source is added and the oxygen supply rate is changed 3 / (h·t-steel)), F O2_after : The oxygen supply rate (Nm) after the remaining lime source is added and the oxygen supply rate is changed 3 / (h·t-steel)). Effect of the Invention

[0010] According to the present invention, it is possible to provide a method for producing low phosphorus steel with improved dephosphorization efficiency during decarburization blowing. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram for explaining the change in P concentration in molten iron during decarburization blowing. [Diagram 2] FIG. 2 is a diagram for explaining the change in the liquid phase ratio of decarburization slag during decarburization blowing. [Diagram 3] FIG. 13 is a diagram for explaining the extension of the time at the end of decarburization blowing by changing the oxygen supply rate. [Figure 4]FIG. 2 is a diagram for explaining the amount of FeO produced at the end of decarburization blowing. [Diagram 5] FIG. 13 is a graph showing the relationship between the dephosphorization rate before and after decarburization blowing and the ratio FO2_after / FO2_before. [Figure 6] FIG. 11 is a diagram showing the relationship between the oxygen supply rate and the oxygen supply amount per unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Here, the low phosphorus steel produced in the present invention is a steel having a P concentration of 0.01 mass % or less.

[0013] First, before the decarburization blowing, the hot metal is subjected to dephosphorization blowing to reduce the P concentration in the hot metal in advance. In the dephosphorization blowing before the decarburization blowing, for example, it is preferable to reduce the P concentration in the hot metal to 0.04 mass% or less. After the dephosphorization blowing, the dephosphorization slag generated by the dephosphorization blowing is separated from the hot metal. There is no particular limitation on the method for separating the hot metal and the dephosphorization slag after the dephosphorization blowing, but in the converter blowing process for low phosphorus steel, it is necessary to separate the dephosphorization slag and the hot metal as much as possible. Therefore, as a method for minimizing the amount of slag carried over, for example, a method for draining the dephosphorization slag after the hot metal is tapped and charging the tapped hot metal back into the converter can be mentioned. It is preferable to separate 90% or more of the dephosphorization slag by these methods.

[0014] After the dephosphorization slag is separated, a lime source containing CaO or CaCO3 is charged into the converter to start the decarburization blowing. In this embodiment, the liquid phase rate of the decarburization slag is ensured to promote the dephosphorization reaction until the middle stage of the decarburization blowing, as will be described later in detail, and in the final stage of the decarburization blowing when a large amount of FeO is produced, the time for the dephosphorization reaction is ensured as much as possible while suppressing a decrease in the iron yield.

[0015] Figure 1 is a diagram for explaining the transition of the P concentration in molten iron during decarburization blowing. A part of the dephosphorization slag generated by the dephosphorization blowing inevitably remains as carryover slag. As described above, in order to separate the dephosphorization slag from the molten pig iron as much as possible, the decarburization slag contains carryover slag but has a low SiO2 content, and the liquid phase ratio of the decarburization slag decreases and the dephosphorization rate becomes excessively slow until the middle stage of decarburization blowing, when decarburization progresses due to excessive input of lime source. Therefore, in the conventional example shown by the dotted line in Figure 1, rephosphorization from the decarburization slag containing carryover slag is greater than the dephosphorization reaction until the middle stage of decarburization blowing, and the P concentration in molten iron tends to increase slightly.

[0016] Therefore, in this embodiment, the total amount of lime source charged during decarburization blowing is set to a range of 30 to 70 kg / t-steel in terms of total CaO equivalent mass, and in order to ensure the liquid phase rate of the decarburization slag, 60 to 80% of the total CaO equivalent mass of the lime source is charged at the start of decarburization blowing.

[0017] Here, the total amount of the lime source mentioned above is the amount of lime source generally charged in decarburization blowing for low phosphorus steel. If the total amount of the lime source charged is less than 30 kg / t-steel in total CaO equivalent mass, the desired low phosphorus steel cannot be produced. If the total amount of the lime source charged is more than 70 kg / t-steel in total CaO equivalent mass, the dephosphorization effect is saturated, the amount of slag increases, and the cost of the lime source becomes high. If the amount of the lime source charged at the start of decarburization blowing is less than 60% of the total CaO equivalent mass, the CaO content of the decarburization slag decreases, and the lime source added later cannot be dissolved in time, resulting in a small final amount of dephosphorization. If the amount of the lime source charged at the start of decarburization blowing is more than 80% of the total CaO equivalent mass, the liquid phase ratio of the decarburization slag decreases in the first half of the decarburization blowing, and the amount of dephosphorization decreases, especially in the middle of the decarburization blowing.

[0018] Fig. 2 is a diagram for explaining the transition of the liquid phase ratio of the decarburization slag during decarburization blowing. In this embodiment, the liquid phase ratio of the decarburization slag is ensured by adding 60 to 80% of the total CaO converted mass of lime source at the start of decarburization blowing, and the dephosphorization reaction efficiency is increased, particularly up to the middle of decarburization blowing. As a result, as shown in Fig. 1, the P concentration in molten iron, particularly in the middle of decarburization blowing, can be kept low compared to the conventional method.

[0019] As described above, at the start of decarburization blowing, a part of the lime source is left and the oxygen supply rate is set to 150-250 Nm 3 When the flow rate of the total oxygen supply reaches 80-90% from the start of blowing, the remaining lime source is added and the oxygen supply rate is changed to the range of the following formula (1). 0.40≦F O2_after / F O2_before ≦0.85 (1)

[0020] Here, F O2_before is the oxygen flow rate before changing the oxygen flow rate (Nm 3 / (h t-steel)), and F O2_after is the oxygen supply rate (Nm 3 / (h·t-steel)).

[0021] Here, the above-mentioned 150 to 250 Nm 3 The oxygen flow rate range of / (h·t-steel) is the oxygen flow rate generally used in decarburization blowing. 3 If the oxygen supply rate is less than 250Nm / (h·t-steel), decarburization is not sufficient and a lot of time is wasted. 3 Increasing the steelmaking efficiency beyond 100% may be difficult due to equipment specifications, or may cause operational problems such as molten iron splashing and refractory wear.

[0022] In addition, when decarburization blowing is performed under the above conditions, if the flow rate of the total oxygen is 80-90%, most of the C in the molten iron is removed, and FeO is easily formed. When FeO is formed, the liquid phase rate of the decarburization slag increases as shown in Figure 2, and the dephosphorization reaction is more likely to occur. If the remaining lime source is added at a stage where the flow rate of the total oxygen is less than 80% from the start of blowing and the oxygen supply rate is changed to the range of formula (1), the liquid phase rate of the decarburization slag will excessively decrease due to the additionally added lime source, and the dephosphorization efficiency will decrease. Furthermore, since the oxygen supply rate is suppressed at the stage where the decarburization reaction is still occurring significantly (the stage where FeO is not yet generated much), excessive oxidation of Fe occurs and the iron yield decreases. In addition, if the remaining lime source is added when the flow rate of the total oxygen exceeds 90% from the start of blowing, and the oxygen supply rate is changed to the range of equation (1), it is not possible to secure sufficient time for the final stage of decarburization blowing, when the proportion of FeO in the decarburization slag is high, and the additionally added lime source is not sufficiently slag-formed, resulting in insufficient dephosphorization effect.

[0023] Fig. 3 is a diagram for explaining the extension of the time in the final stage of decarburization blowing by changing the oxygen flow rate. In Fig. 3, period II indicates the middle stage of decarburization blowing where the decarburization reaction is proceeding remarkably, and period III indicates the final stage of decarburization blowing where the decarburization reaction is slowing down and the generation of FeO is increasing. For example, if the oxygen flow rate is increased to 60,000 Nm 3 / h, and by lowering the oxygen supply rate when additional lime source is added, the time for the final stage of decarburization blowing can be extended if the total amount of oxygen is the same.

[0024] Furthermore, as shown in Figure 4, when the oxygen flow rate is reduced, the FeO concentration in the decarburization slag is lower than when the oxygen flow rate is not reduced, but the dephosphorization reaction at the end of the decarburization blow is not rate-limited by the oxygen supply, but by the mass transfer of P between the molten iron and the decarburization slag. Therefore, since the amount of dephosphorization per unit time remains almost the same even if the oxygen flow rate is reduced, the P concentration in the molten iron decreases as the oxygen flow rate is reduced and the time is extended if the total amount of oxygen is the same. Therefore, the dephosphorization efficiency can be improved by avoiding excessive oxidation of Fe (reduced iron yield) and significant time extension due to an increase in the total amount of oxygen.

[0025] Figure 5 shows the dephosphorization rate and the ratio F before and after decarburization blowing. O2_after / F O2_before As shown in FIG. 5, by adjusting the oxygen supply rate so as to satisfy formula (1), the dephosphorization rate can be increased to 75% or more. O2_after / F O2_before If the ratio F is adjusted to more than 0.85, the time required for the final stage of decarburization blowing, when the dephosphorization reaction is likely to occur, is insufficient, and the dephosphorization effect cannot be obtained. O2_after / F O2_before If is adjusted to less than 0.40, the difference between the actual value and the equilibrium value of the P concentration, which is the driving force for the dephosphorization reaction, becomes small, and the dephosphorization effect of extending the time at the end of the decarburization blowing becomes saturated. In addition, the top-blown oxygen jet becomes an excessively soft blow, and the FeO reduction reaction rate in the decarburization slag caused by the bottom-blown stirring becomes high relative to the FeO generation rate, and the increase in the FeO concentration in the decarburization slag slows down, so that the dephosphorization efficiency rather decreases.

[0026] The oxygen flow rate may be changed as appropriate before or after the additional lime source is charged. Fig. 6 is a diagram showing the relationship between the oxygen flow rate and the oxygen flow rate consumption unit. When the oxygen flow rate is changed before the oxygen flow rate is changed at the stage of additionally charging the lime source, the average oxygen flow rate obtained by dividing the amount of oxygen supplied during that period by the blowing time is expressed as F. O2_before In the case where the oxygen supply rate is changed after the additional lime source is added, the average oxygen supply rate is also set to F O2_after Let us assume that.

[0027] In the decarburization blowing of this embodiment, the C concentration in the molten iron is reduced to less than 0.4 mass%. When producing high carbon steel with a C concentration of 0.4 mass% or more, it is necessary to stop the oxygen blowing at a stage where FeO is difficult to generate. Furthermore, the stirring conditions are not particularly limited, but the blowing is performed under general decarburization blowing conditions according to the type of bottom blown gas and the shape of the tuyere.

[0028] As described above, by adding 60 to 80% of the total CaO equivalent mass of lime source at the start of decarburization blowing, and adding the remaining lime source when 80 to 90% of the total oxygen amount has been supplied from the start of blowing in terms of flow rate, and changing the oxygen supply rate to the range of equation (1), it is possible to promote the dephosphorization reaction even up to the middle of the decarburization blowing, extend the final stage of the decarburization blowing when the dephosphorization reaction is likely to occur, and thereby improve the dephosphorization efficiency without reducing the iron yield. EXAMPLES

[0029] Next, an embodiment of the present invention will be described, but the conditions are merely examples of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of the embodiment. The present invention can be implemented in various ways to achieve the object of the present invention without departing from the gist of the present invention.

[0030] The hot metal after tapping from the blast furnace was charged into a converter, where dephosphorization blowing was performed to remove P until the P concentration in the hot metal was 0.040 mass% or less. The hot metal was then tapped and the dephosphorization slag was removed to separate the hot metal from the dephosphorization slag, yielding 320 tons of pre-treated hot metal. The treated hot metal was then returned to the converter, and decarburization blowing was performed by adding carbon, FeSi, and lime source (CaO) as a heating agent, to reduce the C concentration to 0.03 mass% or more and less than 0.4 mass%. The P concentration in the hot metal before the decarburization blowing was in the range of 0.02 to 0.04 mass%, the total amount of lime source newly added in the decarburization blowing was 50 to 60 kg per ton of hot metal in terms of total CaO mass, and the average oxygen flow rate before the change in the oxygen flow rate was F O2_before 191~213Nm 3 / (h·t-steel).

[0031] The effect of the present invention was evaluated by the dephosphorization rate of the molten iron and the FeO concentration in the slag. The dephosphorization rate was calculated using the following formula (2) from the P concentration obtained by chemical analysis of metal samples taken before and after decarburization blowing, and the FeO concentration was analyzed from the chemical analysis of the slag after decarburization blowing. In this example, it was determined that the effect of the present invention was obtained when the dephosphorization rate was 75% or more and the FeO concentration was less than 30 mass%. Dephosphorization rate=100*([P]脱C前 -[P] 脱C後 ) / [P] 脱C前 (2) In addition, [P] 脱C前 represents the P concentration in the molten iron before decarburization, and [P] 脱C後 represents the P concentration in molten steel after decarburization blowing.

[0032] [Table 1]

[0033] As shown in Table 1, in all of Examples No. 1 to No. 4, the dephosphorization rate was 75% or more, and the FeO concentration in the decarburization slag was less than 30 mass%.

[0034] On the other hand, in Comparative Example No. 5, the lime source was not charged in portions and the oxygen supply rate was not changed, so the liquid phase rate of the decarburization slag was low until the middle of the decarburization blowing, and the final stage of the decarburization blowing, where the dephosphorization reaction is likely to proceed, could not be extended. As a result, the dephosphorization rate was low. In Comparative Example No. 6, the amount of lime source charged at the start of the decarburization blowing was small, so the amount of lime source charged at the end of the decarburization blowing was large, and as a result, the additionally charged lime source did not dissolve completely, and the dephosphorization rate was low. In Comparative Example No. 7, the amount of lime source charged at the start of the decarburization blowing was too large, so the liquid phase rate of the decarburization slag was low until the middle of the decarburization blowing. As a result, the dephosphorization rate was low.

[0035] In Comparative Example No. 8, the introduction of the remaining lime source and the change in the oxygen supply rate were too early, so the liquid phase rate of the decarburization slag in the middle of the decarburization blowing was low, and the dephosphorization rate was low. Furthermore, the FeO concentration in the decarburization slag was high due to excessive oxidation of Fe. In Comparative Example No. 9, the introduction of the remaining lime source and the change in the oxygen supply rate were too late, so there was not enough time at the end of the decarburization blowing, and the additionally introduced lime source was not sufficiently slag-formed, resulting in a low dephosphorization rate.

[0036] In Comparative Example No. 10, the oxygen flow rate was reduced too much when the remaining lime source was added, so the increase in the FeO concentration in the decarburization slag slowed down, and the increase in the liquid phase rate of the decarburization slag became too gradual, resulting in a low dephosphorization rate. In Comparative Example No. 11, the reduction in the oxygen flow rate when the remaining lime source was added was too small, so the effect of extending the time at the end of the decarburization blowing was not very great, resulting in a low dephosphorization rate.

Claims

[Claim 1] In a converter-type refining apparatus equipped with a top blowing lance, dephosphorization blowing is performed to separate the dephosphorization slag produced, and then CaO or CaCO 3 A method for producing low phosphorus steel, comprising the steps of: feeding a lime source containing the above-mentioned compound in an amount of 30 to 70 kg / t-steel in terms of total CaO; carrying out decarburization blowing; and reducing the C concentration in molten iron to less than 0.4 mass%, In the decarburization blowing, the lime source is charged at 60 to 80% of the total CaO converted mass at the time of starting blowing, and the oxygen supply rate from the top blowing lance is 150 to 250 Nm 3 / (h·t-steel) range to start blowing, and when 80 to 90% of the total amount of oxygen has been supplied in terms of a flow rate from the start of blowing, the remainder of the lime source is charged and the oxygen supply rate is changed to the range of formula (1). 0.40≦F O2_after / F O2_before ≦0.85 ... (1) Here, F O2_before : The oxygen supply rate (Nm m) before the remaining lime source is added and the oxygen supply rate is changed 3 / (h・t-steel)), F O2_after : The oxygen supply rate (Nm) after the remaining lime source is added and the oxygen supply rate is changed 3 / (h·t-steel).

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