Converter blowing method and converter equipment

Abstract

To provide a converter blowing method capable of suppressing nitrogen absorption and phosphorus condensation of hot metal or molten steel.SOLUTION: A converter blowing method satisfies the following requirements 1-1 and 1-2. Requirement 1-1: A top-blown gas with a nitrogen concentration of 0.3 mol% or less is blown from a top-blown lance to hot metal or molten steel from the start point of blowing TS to TA1. Here, the total amount of oxygen sprayed from the top blowing lance from the start point of blowing TS to TA1 is more than 60% of the total amount of oxygen sprayed from the top blowing lance from the start point of blowing TS to the end point of blowing TE. Requirement 1-2: A dephosphorizing agent comprising Ca is sprayed from the top blowing lance to hot metal or molten steel from at least time point TA1 to the end of blowing, with low nitrogen concentration gas as the top blowing gas. The low nitrogen concentration gas contains oxygen and carrier gas and has a nitrogen concentration of 0.3 mol% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application discloses a converter blowing method and converter equipment.

Background Art

[0002] As a method for smelting low-phosphorus steel from blast furnace hot metal, as disclosed in Patent Document 1, a method is known in which the hot metal is dephosphorized in a low-temperature and high-carbon concentration range that is thermodynamically advantageous for dephosphorization, and then the hot metal is decarburized and blown. Since the hot metal subjected to the dephosphorization treatment has a lower phosphorus concentration than the untreated hot metal, the dephosphorization load in the subsequent decarburization blowing process is reduced. On the other hand, the hot metal subjected to the dephosphorization treatment inevitably contains dephosphorization slag generated by the dephosphorization treatment. That is, a part of the dephosphorization slag generated by the dephosphorization treatment is brought into the decarburization blowing in the next step. Here, in decarburization blowing, since it becomes a high-temperature and low-carbon concentration range that is thermodynamically disadvantageous for dephosphorization, rephosphorization from the dephosphorization slag to the hot metal or molten steel is likely to occur. In this regard, in order to suppress rephosphorization from the dephosphorization slag to the molten steel during decarburization blowing, it is necessary to add a dephosphorizing agent during decarburization blowing.

[0003] Examples of the dephosphorizing agent added during decarburization blowing include dephosphorizing agents containing Ca. For example, the dephosphorizing agent may contain CaO or CaCO3. These dephosphorizing agents are slagged at a high-temperature ignition point and efficiently cause a dephosphorization reaction by a transient reaction. As a method for adding a dephosphorizing agent during decarburization blowing, a method of placing a lump-shaped dephosphorizing agent on the hot metal or molten steel is common, but there is also a method of spraying the dephosphorizing agent together with an upward blowing gas from an upward blowing lance into the hot metal or molten steel. In this case, the dephosphorizing agent carried by the carrier gas and the oxygen gas are merged and then upward blowing is performed. In other words, the upward blowing gas contains oxygen gas and carrier gas.

[0004] As the carrier gas for the dephosphorizing agent, nitrogen gas is usually used from the viewpoints of cost and safety. However, when nitrogen gas is used as the carrier gas, in decarburizing blowing, the upper blowing gas with a high nitrogen concentration is blown from the upper blowing lance onto the molten steel, increasing the nitrogen concentration in the molten steel. Since nitrogen in steel reduces the toughness of the steel, it is desirable to keep the nitrogen concentration in the finally produced molten steel as low as possible.

[0005] In response to the above problem, Patent Document 2 discloses a method of spraying a dephosphorizing agent together with the upper blowing gas when the amount of sprayed oxygen is 10% or more and 50% or less of the total oxygen amount in decarburizing blowing. That is, by spraying the dephosphorizing agent in the first half of the blowing, the nitrogen absorption into the hot metal or molten steel in the second half of the blowing can be suppressed, and the nitrogen concentration in the finally produced molten steel can be kept low.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] According to the findings of the present inventor, as disclosed in Patent Document 2, by spraying the dephosphorizing agent in the first half of the blowing, it is possible to suppress the nitrogen absorption into the hot metal or molten steel and promote the dephosphorization of the hot metal or molten steel. However, rephosphorization occurs in the second half of the blowing when the temperature is higher, and the dephosphorization promoting effect in the first half of the blowing is reduced. In this regard, in converter blowing, a new technology capable of suppressing the nitrogen absorption and rephosphorization of the hot metal or molten steel is required.

Means for Solving the Problems

[0008] This application provides, as one of the means for solving the above problems, A converter blowing method, comprising blowing an oxygen-containing upward blowing gas from an upward blowing lance onto hot metal or molten steel in a converter, satisfying the following requirements 1-1 and 1-2, Converter blowing method is disclosed.

[0009] Requirement 1-1: At the start time point T S to time point T A1 an upward blowing gas having a nitrogen concentration of 0.3 mol% or less is blown from the upward blowing lance onto the hot metal or the molten steel. Here, the total amount of oxygen blown from the upward blowing lance from the start time point T S to the time point T A1 is 60% or more of the total amount of oxygen blown from the upward blowing lance from the start time point T S to the end time point T E of the total amount of oxygen blown from the upward blowing lance.

[0010] Requirement 1-2: At least from the time point T A1 to the end of the blowing period, a dephosphorizing agent containing Ca is blown from the upward blowing lance onto the hot metal or the molten steel together with a low-nitrogen concentration gas as the upward blowing gas. Here, the low-nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

[0011] The converter blowing method of the present disclosure may satisfy the following requirements 1-1a and 1-1b.

[0012] Requirement 1-1a: From the start time point T S to any time point T B1 after the start of blowing, an upward blowing gas having a nitrogen concentration of 0.3 mol% or less is blown from the upward blowing lance onto the hot metal or the molten steel, while the dephosphorizing agent is not blown.

[0013] Requirement 1-1b: From the time point T B1 to the time point T A1Up to that point, the dephosphorizing agent is sprayed from the top-blowing lance onto the hot metal or the molten steel, together with the low-nitrogen concentration gas as the top-blowing gas.

[0014] As one of the means for solving the above problems, the present application is a converter blowing method, including spraying an oxygen-containing top-blowing gas from a top-blowing lance onto the hot metal or molten steel in a converter, satisfying the following requirement 2-1: Converter blowing method is disclosed.

[0015] Requirement 2-1: From the time point T after the start of blowing A2 to the end of blowing, a dephosphorizing agent containing Ca is sprayed from the top-blowing lance onto the hot metal or the molten steel, together with the low-nitrogen concentration gas as the top-blowing gas. Here, from the start time point T of blowing S to the time point T A2 the total amount of oxygen sprayed from the top-blowing lance is less than 60% of the total amount of oxygen sprayed from the top-blowing lance from the start time point T of blowing S to the end time point T of blowing E and the low-nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

[0016] The converter blowing method of the present disclosure may satisfy the following requirement 2-2.

[0017] Requirement 2-2: From the start time point T of blowing S to the time point T A2 or at any time point T after the start of blowing B2 to the time point T A2 a top-blowing gas having a nitrogen concentration of more than 0.3 mol% is sprayed from the top-blowing lance onto the hot metal or the molten steel, and at the time point T A2 the top-blowing gas is switched to the low-nitrogen concentration gas.

[0018] The converter blowing method of the present disclosure may satisfy the following requirements 2-2a and 2-2b.

[0019] Requirement 2-2a: the starting point T of the blowing S From the above time point T B2 Until the above time point T, while spraying the top blowing gas having a nitrogen concentration of more than 0.3 mol% or the top blowing gas having a nitrogen concentration of 0.3 mol% or less from the top blowing lance onto the hot metal or the molten steel, the dephosphorizing agent is not sprayed.

[0020] Requirement 2-2b: the time point T B2 From the above time point T A2 Until the above time point T, while spraying the dephosphorizing agent together with the high nitrogen concentration gas as the top blowing gas from the top blowing lance onto the hot metal or the molten steel, the type of the top blowing gas is switched at the time point T. Here, the high nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of more than 0.3 mol%. A2

[0021] As one of the means for solving the above problems, the present application A converter facility, comprising a converter, a top blowing lance, a first supply line, a second supply line, a first supply source, a second supply source, a third supply source, and a fourth supply source. The first supply line is connected to the top blowing lance or the second supply line. The second supply line is connected to the top blowing lance or the first supply line. The first supply source is connected to the first supply line and is configured to be able to supply oxygen gas to the first supply line. The second supply source is connected to the second supply line and is configured to be able to supply a first carrier gas to the second supply line. The third supply source is connected to the second supply line and is configured to be able to supply a second carrier gas to the second supply line. The fourth supply source is connected to the second supply line and is configured to be able to supply a dephosphorizing agent containing Ca to the second supply line. The dephosphorizing agent supplied to the second supply line is configured to be supplied to the top blowing lance or the first supply line together with the first carrier gas or the second carrier gas via the second supply line. The first carrier gas or the second carrier gas and the dephosphorizing agent supplied to the top blowing lance or the first supply line are configured to be blown upward from the top blowing lance into the converter together with the oxygen. The carrier gas supplied to the second supply line is configured to be switchable between the first carrier gas and the second carrier gas. By switching the carrier gas, the top blowing gas blown from the top blowing lance into the converter is configured to be switchable between a low nitrogen concentration gas having a nitrogen concentration of 0.3 mol% or less and a high nitrogen concentration gas having a nitrogen concentration exceeding 0.3 mol%. Converter equipment is disclosed.

Advantages of the Invention

[0022] According to the technology of the present disclosure, in converter blowing, it is possible to suppress nitrogen absorption and phosphorus reversion in hot metal or molten steel.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0024] 1. Converter blowing method In the converter blowing method of the present disclosure, when blowing an oxygen-containing upper blowing gas from an upper blowing lance onto hot metal or molten steel in a converter, a carrier gas that does not contain nitrogen or has a low nitrogen gas concentration is used to perform upper blowing of a dephosphorizing agent together with the above oxygen, thereby suppressing the absorption of phosphorus and the rephosphorization into hot metal or molten steel and producing low-phosphorus steel. Hereinafter, two forms of the converter blowing method of the present disclosure will be exemplified.

[0025] 1.1 First form The converter blowing method according to the first form includes blowing an oxygen-containing upper blowing gas from an upper blowing lance onto hot metal or molten steel in a converter, and as shown in FIG. 1, satisfies the following requirements 1-1 and 1-2.

[0026] Requirement 1-1: Blowing start time point T S From time point T A1 To time point T S To time point T A1 To time point T S To blowing end time point T E To blowing end time point T

[0027] Requirement 1-2: At least from the time point T A1 To the end of blowing, a dephosphorizing agent containing Ca is blown from the upper blowing lance onto the hot metal or the molten steel together with a low-nitrogen concentration gas as the upper blowing gas. Here, the low-nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

[0028] 1.1.1 Requirement 1-1 As shown in FIG. 1, in the converter blowing method according to the first form, from the blowing start time point T S To time point T A1Up to the time point, an upward blowing gas having a nitrogen concentration of 0.3 mol% or less is blown from the upward blowing lance onto the hot metal or molten steel. Here, at the start time point T of the blowing operation S to the time point T A1 the total amount of oxygen blown from the upward blowing lance is 60% or more of the total amount of oxygen blown from the upward blowing lance from the start time point T of the blowing operation S to the end time point T of the blowing operation E The "start time point T of the blowing operation" refers to the time point when the upward blowing of oxygen from the upward blowing lance is started in the converter blowing operation (the time point when the upward blowing oxygen amount is 0%). For example, it may be the time point when the upward blowing of oxygen in the decarburization blowing operation is started.

[0029] The "end time point T of the blowing operation" refers to the time point when the upward blowing of oxygen from the upward blowing lance is completed in the converter blowing operation (the time point when the upward blowing oxygen amount is 100%). For example, in the decarburization blowing operation, it may be the time point when the upward blowing of oxygen is completed and the molten steel is produced. S

[0030] The "time point T" refers to the time point when 60% or more of the total amount of oxygen blown from the upward blowing lance from the start time point T of the blowing operation to the end time point T of the blowing operation is blown in. The time point T E

[0031] The "time point T" may be the time point when 65% or more, 70% or more, 75% or more, or 80% or more of the total amount of oxygen blown from the upward blowing lance from the start time point T of the blowing operation to the end time point T of the blowing operation is blown in. A1 is from the start time point T of the blowing operation S to the end time point T of the blowing operation E A1 is from the start time point T of the blowing operation S to the end time point T of the blowing operation E

[0032] ​​​​"The top-blown gas having a nitrogen concentration of 0.3 mol% or less" may be only oxygen or a mixed gas of oxygen and a gas other than oxygen, as shown in Fig. 1. Examples of the gas other than oxygen in this case include argon, carbon dioxide, and air. In particular, when argon or carbon dioxide is used, it is easier to further reduce the nitrogen concentration of the top-blown gas. The mixing ratio of oxygen and the gas other than oxygen in the mixed gas is not particularly limited and may be appropriately adjusted according to the intended blowing conditions. The gas other than oxygen may also be a carrier gas for carrying the dephosphorizing agent described later. That is, as shown in Fig. 1, in the converter blowing method according to the first embodiment, between the start time point T S and the time point T A1 , the top blowing of the dephosphorizing agent may or may not be performed together with the top-blown gas.

[0033] As shown in Fig. 1, in the converter blowing method according to the first embodiment, from the start time point T S to the time point T A1 , the top-blown gas having a nitrogen concentration exceeding 0.3 mol% is not blown from the top-blown lance to the hot metal or molten steel, and the top-blown gas having a nitrogen concentration of 0.3 mol% or less is blown. According to the findings of the present inventor, when the top-blown gas having a nitrogen concentration exceeding 0.3 mol% is blown between the start time point T S and the time point T A1 , nitrogen absorption into the hot metal or molten steel occurs, and the nitrogen taken into the hot metal or molten steel is not sufficiently removed by the end of the blowing, resulting in an increase in the nitrogen concentration of the finally produced steel. On the other hand, like the converter blowing method according to the first embodiment, by blowing the top-blown gas having a nitrogen concentration of 0.3 mol% or less from the top-blown lance to the hot metal or molten steel from the start time point T S to the time point T A1 , nitrogen absorption into the hot metal or molten steel in the first half of the blowing can be suppressed, and the nitrogen concentration of the finally produced steel is likely to be reduced.

[0034] 1.1.2 Requirement 1-2 As shown in Fig. 1, in the converter blowing method according to the first embodiment, at least at the time point T A1From the end stage of blowing to the end of blowing, a dephosphorizing agent containing Ca is sprayed from the top blowing lance onto the hot metal or molten steel together with a low nitrogen concentration gas as the top blowing gas. Here, the low nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

[0035] The "end stage of blowing" refers to the period from immediately before the end of blowing to the end of blowing. Specifically, it is the period when more than 95% of the total amount of oxygen sprayed from the top blowing lance from the start time point T S to the end time point T E of blowing has been blown in, and can be regarded as the end stage of blowing.

[0036] The "low nitrogen concentration gas" is sprayed as the top blowing gas from the top blowing lance onto the hot metal or molten steel. The low nitrogen concentration gas contains oxygen and a carrier gas. The nitrogen concentration in the low nitrogen concentration gas is 0.3 mol% or less, and may be 0.2 mol% or less. The lower limit of the nitrogen concentration in the low nitrogen concentration gas is not particularly limited and may be substantially 0. The mixing ratio of oxygen and the carrier gas in the low nitrogen concentration gas is not particularly limited and may be appropriately adjusted according to the intended blowing conditions so as to satisfy the above nitrogen concentration.

[0037] The "carrier gas" is used to carry the dephosphorizing agent and can form part of the top blowing gas. In order to form a low concentration nitrogen gas with a nitrogen concentration of 0.3 mol% or less, a gas that does not contain nitrogen or has a low nitrogen concentration is used as the carrier gas. Examples of such a carrier gas include argon, carbon dioxide, and air. In particular, when argon or carbon dioxide is used, it is easier to further reduce the nitrogen concentration. Also, when air is used as the carrier gas, it is better to combine it with argon or carbon dioxide in order to dilute the nitrogen in the air. Note that it is better to avoid using oxygen as the carrier gas from the viewpoint of safety. In this regard, the carrier gas may have an oxygen concentration of 50 mol% or less, 40 mol% or less, 30 mol% or less, 20 mol% or less, 10 mol% or less, 5 mol% or less, 1 mol% or less, or 0.1 mol% or less.

[0038] Any "phosphorus-removing agent" containing Ca and capable of exhibiting a phosphorus-removing function can be adopted. For example, a phosphorus-removing agent containing CaO, a phosphorus-removing agent containing CaCO3, etc. can be adopted. The phosphorus-removing agent may contain a SiO2 source. Examples of the SiO2 source include silica stone, kanran stone, etc. As shown in FIG. 1, the phosphorus-removing agent is sprayed onto hot metal or molten steel together with the low-nitrogen-concentration gas as the above-mentioned top-blown gas. That is, in the converter blowing method of the present disclosure, instead of adding the lump-shaped phosphorus-removing agent on top of the hot metal or molten steel, the powdered phosphorus-removing agent is sprayed onto the hot metal or molten steel together with the top-blown gas. As long as spraying from the top-blown lance is possible, the shape and size (particle diameter) of the phosphorus-removing agent are not particularly limited.

[0039] In the converter blowing method according to the first embodiment, for example, when the phosphorus-removing agent carried by the above-mentioned carrier gas and the top-blown oxygen merge, the phosphorus-removing agent is sprayed from the top-blown lance onto the hot metal or molten steel together with the low-nitrogen-concentration gas (mixed gas of oxygen and carrier gas) as the top-blown gas. The nitrogen concentration in the low-nitrogen-concentration gas is 0.3 mol% or less, whereby nitrogen absorption into the hot metal or molten steel is suppressed.

[0040] As shown in FIG. 1, in the converter blowing method according to the first embodiment, at least from time point T A1 to the end of the blowing period, the phosphorus-removing agent may be top-blown together with the low-nitrogen-concentration gas. That is, the starting point of the top-blowing of the phosphorus-removing agent is not limited to time point T A1 , and as shown in FIG. 1, the spraying of the phosphorus-removing agent may be carried out before time point T A1 . Also, the ending point of the spraying of the phosphorus-removing agent is not limited to the end point of the blowing period, and as shown in FIG. 1, the spraying of the phosphorus-removing agent may be ended before the end of the blowing period. From the viewpoint of further enhancing the phosphorus-removing effect, the spraying of the phosphorus-removing agent may be carried out before time point T A1 , and from the viewpoint of cost reduction, the spraying of the phosphorus-removing agent may not be carried out before time point T A1 .

[0041] 1.1.3 Requirement 1-1a and 1-1b As shown in Fig. 2, the converter blowing method according to the first embodiment may satisfy the following requirements 1-1a and 1-1b.

[0042] Requirement 1-1a: At the start point in time T S to any point in time T B1 after the start of blowing, while blowing an upper blowing gas having a nitrogen concentration of 0.3 mol% or less from the upper blowing lance onto the hot metal or the molten steel, the dephosphorizing agent is not blown.

[0043] Requirement 1-1b: From the point in time T B1 to the point in time T A1 while blowing the low-nitrogen concentration gas as the upper blowing gas from the upper blowing lance onto the hot metal or the molten steel, the dephosphorizing agent is blown.

[0044] As shown in Figs. 1 and 2, Requirements 1-1a and 1-1b correspond to an example of the above Requirement 1-1. That is, in the converter blowing method according to the first embodiment, only the blowing of the upper blowing gas with a low nitrogen concentration is performed from the start of blowing to a certain point in time T B1 and the upper blowing of the dephosphorizing agent is not performed. From a certain point in time T B1 the upper blowing of the dephosphorizing agent may be started together with the low-nitrogen concentration gas. In this way, by not performing the upper blowing of the dephosphorizing agent from the start point in time T S to the point in time T B1 and starting the upper blowing of the dephosphorizing agent from the point in time T B1 the dephosphorizing agent can be stably blown.

[0045] As shown in Fig. 2, the point in time T B1 is the point in time when less than 60% of the total amount of oxygen blown from the upper blowing lance from the start point in time T S to the end point in time T E of blowing has been blown in. For example, the point in time T B1 is from the start point in time T S to the end point in time T EIt may be the time when 1% or more, 5% or more, or 10% or more, and 50% or less, 40% or less, 30% or less, or 20% or less of the total amount of oxygen blown from the top blowing lance has been blown in.

[0046] As described above, in the converter blowing method according to the first embodiment, by not blowing the top blowing gas with a high nitrogen concentration from the first half to the middle stage of blowing, nitrogen absorption in the hot metal or molten steel can be suppressed. Further, in the converter blowing method according to the first embodiment, from the latter half to the final stage of blowing when the hot metal or molten steel becomes high temperature and low carbon concentration, by blowing the dephosphorizing agent together with the top blowing gas with a low nitrogen concentration, while suppressing nitrogen absorption into the hot metal or molten steel, rephosphorization into the hot metal or molten steel can be suppressed (dephosphorization can be promoted). As a result, the nitrogen concentration and phosphorus concentration of the finally produced steel are likely to be reduced.

[0047] 1.2 Second Embodiment The converter blowing method according to the second embodiment includes blowing a top blowing gas containing oxygen from the top blowing lance into the hot metal or molten steel in the converter, and as shown in FIG. 3, satisfies the following requirement 2-1.

[0048] Requirement 2-1: At time T after the start of blowing A2 From the start of blowing to the final stage of blowing, a dephosphorizing agent containing Ca is blown from the top blowing lance into the hot metal or the molten steel together with a low nitrogen concentration gas as the top blowing gas. Here, from the start time T of blowing S to the time T A2 the total amount of oxygen blown from the top blowing lance is less than 60% of the total amount of oxygen blown from the top blowing lance from the start time T of blowing S to the end time T of blowing E and the low nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

[0049] 1.2.1 Requirement 2-1 As shown in FIG. 3, in the converter blowing method according to the second embodiment, from the start time T of blowing S to time T A2The total amount of oxygen blown from the top lance by the time is less than 60% of the total amount of oxygen blown from the top lance from the start time point T S to the end time point T E of blowing. In other words, "time point T A2 " is the time point when less than 60% of the total amount of oxygen blown from the top lance from the start time point T S to the end time point T E of blowing is blown in. Time point T A2 may be the time point when 1% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, and 59% or less, 58% or less, 57% or less, 55% or less, 50% or less, 40% or less, 30% or less, or 20% or less of the total amount of oxygen blown from the top lance from the start time point T S to the end time point T E of blowing is blown in. Alternatively, as described later, if the high-nitrogen-concentration gas is blown in until time point T A2 and the top gas is switched from the high-nitrogen-concentration gas to the low-nitrogen concentration at time point T A2 , time point T A2 may be the time point when 40% or more, 45% or more, or 50% or more, and 59% or less, 58% or less, or 57% or less of the total amount of oxygen blown from the top lance from the start time point T S to the end time point T E of blowing is blown in. By setting time point T A2 as late as possible, the amount of use of the low-nitrogen-concentration carrier gas (argon, etc.), which is generally costly, can be reduced.

[0050] According to the findings of the present inventor, when a phosphorus-removing agent containing Ca is blown from the top lance into the hot metal or molten steel together with a low-nitrogen-concentration gas as the top gas from time point T A2 to the end stage of blowing, even if nitrogen absorption into the hot metal or molten steel occurred before time point T A2 , nitrogen in the hot metal or molten steel is removed from time point T A2 to the end of blowing, and the nitrogen concentration of the finally produced steel is likely to be reduced. Also, time point T A2By spraying the dephosphorizing agent until the end of the blowing process, it is possible to suppress the rephosphorization (promote dephosphorization) of the hot metal or molten steel from the latter half to the end of the blowing process when the hot metal or molten steel becomes at a high temperature and a low carbon concentration. As a result, the nitrogen concentration and phosphorus concentration of the finally produced steel are likely to be reduced.

[0051] 1.2.2 Requirement 2-2 As described above, in the converter blowing method according to the second embodiment, nitrogen absorption of the hot metal or molten steel may occur from the start time point T S to the time point T A2 and the nitrogen concentration in the hot metal or molten steel may be high. That is, as shown in FIG. 3, from the start time point T S to the time point T A2 during this period, only oxygen, a low nitrogen concentration gas, or a high nitrogen concentration gas may be used as the top blowing gas. In particular, before the time point T A2 a high nitrogen concentration gas is sprayed as the top blowing gas from the top blowing lance onto the hot metal or molten steel, and by switching the top blowing gas to a low nitrogen concentration gas at the time point T A2 it is possible to reduce the usage amount of a low nitrogen concentration carrier gas which is generally costly. That is, as shown in FIG. 4, the converter blowing method according to the second embodiment may satisfy the following requirement 2-2.

[0052] Requirement 2-2: From the start time point T S to the time point T A2 or, from any time point T B2 after the start of blowing to the time point T A2 a top blowing gas having a nitrogen concentration exceeding 0.3 mol% is sprayed from the top blowing lance onto the hot metal or the molten steel, and the top blowing gas is switched to the low nitrogen concentration gas at the time point T A2 .

[0053] The "top blowing gas having a nitrogen concentration exceeding 0.3 mol%" may be, for example, a mixed gas of oxygen and a gas with a high nitrogen concentration, or may be the "high nitrogen concentration gas" described later.

[0054] The method of switching the top-blowing gas from a gas with a high nitrogen concentration to a gas with a low nitrogen concentration is not particularly limited. For example, as will be described later, in a converter facility having an oxygen supply line (first supply line) and a carrier gas supply line (second supply line), by switching the type of carrier gas supplied to the carrier gas supply line, as a result, the top-blowing gas can be switched from a gas with a high nitrogen concentration to a gas with a low nitrogen concentration. Incidentally, when the carrier gas is switched from a gas with a high nitrogen concentration to a gas with a low nitrogen concentration during blowing, the carrier gas with a high nitrogen concentration remaining in the supply line and the fourth supply source (powder supply device) described later will be mixed with the gas with a low nitrogen concentration, and it is not necessarily the case that the gas with a low nitrogen concentration can be top-blown immediately after the switching. Taking this into account, it is preferable to manage the timing of switching the carrier gas and the nitrogen concentration in the top-blowing gas.

[0055] 1.2.3 Requirement 2-2a and 2-2b As shown in FIG. 3, in the converter blowing method according to the second embodiment, from the blowing start time point T S top-blowing of the dephosphorizing agent may be performed, or top-blowing of the dephosphorizing agent may be performed at any time point T B2 after the start of blowing. In particular, by not performing top-blowing of the dephosphorizing agent from the blowing start time point T S to the time point T B2 and starting top-blowing of the dephosphorizing agent from the time point T B2 the dephosphorizing agent can be stably top-blown. That is, the converter blowing method according to the second embodiment may satisfy the following requirements 2-2a and 2-2b, as shown in FIG. 4.

[0056] Requirement 2-2a: From the blowing start time point T S to the time point T B2 while blowing a top-blowing gas having a nitrogen concentration of more than 0.3 mol% or a top-blowing gas having a nitrogen concentration of 0.3 mol% or less from the top-blowing lance to the hot metal or the molten steel, the dephosphorizing agent is not blown.

[0057] Requirement 2-2b: The time point TB2 From the time point T A2 Until, into the hot metal or the molten steel from the top blowing lance, together with the high nitrogen concentration gas as the top blowing gas, the dephosphorizing agent is sprayed, at the time point T A2 Switch the type of the top blowing gas. Here, the high nitrogen concentration gas contains oxygen and a carrier gas, and has a nitrogen concentration exceeding 0.3 mol%.

[0058] "Time point T B2 " may be between the starting time point T of the blowing refining S and the above time point T A2 For example, the time point T B2 may be a time point when more than 0%, 1% or more, 3% or more, or 5% or more, less than 40%, 30% or less, 20% or less, or 15% or less of the total amount of oxygen sprayed from the top blowing lance from the starting time point T of the blowing refining S to the ending time point T of the blowing refining E is blown in.

[0059] The "high nitrogen concentration gas" contains oxygen and a carrier gas, and has a nitrogen concentration exceeding 0.3 mol%. That is, the carrier gas used for the high nitrogen concentration gas has a high nitrogen concentration. Examples of such a carrier gas include nitrogen and air. The nitrogen concentration in the high nitrogen concentration gas exceeds 0.3 mol%, and may be 0.5 mol% or more, 1.0 mol% or more, 1.5 mol% or more, or 2.0 mol% or more.

[0060] 1.3 Supplementary In the converter blowing method according to the first and second embodiments, following requirements 1-2 and 2-1, even from the late stage of blowing to the end of blowing, it is preferable to blow an upper blowing gas having a nitrogen concentration of 0.3 mol% or less from the upper blowing lance onto the hot metal or molten steel. In other words, as shown in FIGS. 2 and 4, in the late stage of blowing, it is better not to blow an upper blowing gas having a nitrogen concentration exceeding 0.3 mol% from the upper blowing lance onto the hot metal or molten steel. By blowing an upper blowing gas having a nitrogen concentration of 0.3 mol% or less from the upper blowing lance onto the hot metal or molten steel in the late stage of blowing, nitrogen absorption into the hot metal or molten steel in the late stage of blowing can be suppressed, and it becomes easier to further reduce the nitrogen concentration of the finally melted steel. As described above, in the late stage of blowing, the upper blowing of the dephosphorizing agent may be performed together with the low-nitrogen concentration gas until the end of blowing, or the upper blowing of the dephosphorizing agent may be stopped immediately before the end of blowing, and only the upper blowing with the upper blowing gas having a nitrogen concentration of 0.3 mol% or less may be performed.

[0061] In the converter blowing method according to the first and second embodiments, the above requirements may be satisfied, and for other blowing conditions (such as the flow rate and flow velocity of the upper blowing gas from the upper blowing lance, the composition and temperature of the hot metal and molten steel, etc.), they may be the same as those in the prior art.

[0062] 1.4 Effects As described above, according to the converter blowing method of the present disclosure, by blowing the dephosphorizing agent onto the hot metal or molten steel in the latter half of blowing, a dephosphorization rate higher than the rephosphorization rate can be obtained at the flame point, so it is possible to eliminate the rephosphorization of the hot metal or molten steel that is a concern from the latter half to the end of the high-temperature blowing. Also, by using a carrier gas with a low nitrogen concentration, nitrogen absorption into the hot metal or molten steel can be suppressed. As a result, it becomes easier to melt steel with a low nitrogen concentration and a low phosphorus concentration.

[0063] 2. Converter Equipment The converter blowing method of the present disclosure can be implemented, for example, in a converter equipment capable of switching the type of the upper blowing gas. The technology of the present disclosure also has an aspect as converter equipment.

[0064] As shown in FIG. 5, a converter facility 100 according to an embodiment includes a converter 10, an upper blowing lance 20, a first supply line 31, a second supply line 32, a first supply source 41, a second supply source 42, a third supply source 43, and a fourth supply source 44. The first supply line 31 is connected to the upper blowing lance 20 or the second supply line 32. The second supply line 32 is connected to the upper blowing lance 20 or the first supply line 31. The first supply source 41 is connected to the first supply line 31 and is configured to supply oxygen gas to the first supply line 31. The second supply source 42 is connected to the second supply line 32 and is configured to supply a first carrier gas to the second supply line 32. The third supply source 43 is connected to the second supply line 32 and is configured to supply a second carrier gas to the second supply line 32. The fourth supply source 44 is connected to the second supply line 32 and is configured to supply a dephosphorizing agent containing Ca to the second supply line 32. As shown in FIG. 5, in the converter facility 100, the dephosphorizing agent supplied to the second supply line 32 can be supplied to the upper blowing lance 20 or the first supply line 31 together with the first carrier gas or the second carrier gas via the second supply line 32. The first carrier gas or the second carrier gas and the dephosphorizing agent supplied to the upper blowing lance 20 or the first supply line 31 can be blown upward from the upper blowing lance 20 into the converter 10 together with the oxygen. The carrier gas supplied to the second supply line 32 is configured to be switchable between the first carrier gas and the second carrier gas. By switching the carrier gas, the upper blowing gas blown from the upper blowing lance 20 into the converter 10 can be switched between a low nitrogen concentration gas having a nitrogen concentration of 0.3 mol% or less and a high nitrogen concentration gas having a nitrogen concentration exceeding 0.3 mol%.

[0065] As the converter 10, the same one as the conventional one may be used. In the converter facility of the present disclosure, the converter 10 may be either a top-blown converter or a top-bottom blown converter. The top-bottom blown converter may be provided with a plurality of flow paths for supplying bottom-blown gas into the furnace at its bottom. Further, the converter may be provided with a tapping hole or the like for tapping the molten steel after melting on its side portion.

[0066] As the hot metal 1 charged into the converter 10, for example, any general blast furnace hot metal can be adopted. The hot metal 1 contains P and C as impurities and may also contain Si. When the hot metal 1 contains Si, the desiliconization reaction of Si in the hot metal proceeds by oxidative refining with oxygen gas, and then the dephosphorization reaction proceeds. After desiliconization, the desiliconization slag in the converter 10 may be removed, or dephosphorization may be performed by the method of the present disclosure while leaving the desiliconization slag in the converter 10. The hot metal 1 may be a mixture of molten iron (for example, blast furnace hot metal) and an additive material such as scrap. The method of charging the hot metal 1 into the converter 10 is not particularly limited, and for example, a method of pouring it into the converter 10 using a known container such as a hot metal ladle can be mentioned. As described above, the converter blowing method of the present disclosure may be applied during decarburization blowing. In this case, the hot metal 1 in the converter 10 may be dephosphorized hot metal after dephosphorization treatment.

[0067] Regarding the top-blown lance 20, the same lance as the conventional one can be adopted. The top-blown lance 20 may be directly or indirectly connected to the first supply line 31 or the second supply line 32 at its upper end side (upstream side), for example. Thereby, during converter refining, oxygen, or oxygen and carrier gas, or oxygen, carrier gas and dephosphorizing agent are supplied from the first supply line 31 or the second supply line 32 to the top-blown lance 20, and the oxygen, carrier gas and dephosphorizing agent supplied to the top-blown lance 20 are blown out from the blowout port at the lower end side of the top-blown lance 20 into the hot metal 1 or the molten steel. By blowing the top-blown gas containing oxygen from the top-blown lance 20 into the hot metal 1, oxidative refining can proceed while stirring the hot metal 1. In addition, when the converter 10 is a top-bottom blown converter, the stirring of the hot metal 1 during refining can also be enhanced by continuously or intermittently blowing the bottom-blown gas from the bottom of the converter 10.

[0068] The first supply line 31 and the second supply line 32 can be constituted by, for example, known piping. In particular, by constituting at least a part of the first supply line 31 and the second supply line 32 with flexible pipes, it becomes easier for the first supply line 31 and the second supply line 32 to follow the elevation and descent of the up-blow lance 20. The first supply line 31 is connected to the up-blow lance 20 or the second supply line 32, and the second supply line 32 is connected to the up-blow lance 20 or the first supply line 31. That is, the first supply line 31 and the second supply line 32 may merge with each other upstream of the up-blow lance 20, or the first supply line 31 and the second supply line 32 may be separately connected to the up-blow lance 20 and merge within the lance.

[0069] The first supply source 41 is a supply source of oxygen gas, the second supply source 42 is a supply source of the first carrier gas, the third supply source 43 is a supply source of the second carrier gas, and the fourth supply source 44 is a supply source of a dephosphorizing agent. The first supply source 41 is connected to the first supply line 31 via piping or the like and is configured to be able to supply oxygen gas to the first supply line 31. The second supply source 42 is connected to the second supply line 32 via piping or the like and is configured to be able to supply the first carrier gas to the second supply line 32. The third supply source 43 is connected to the second supply line 32 via piping or the like and is configured to be able to supply the second carrier gas to the second supply line 32. The fourth supply source 44 is connected to the second supply line 32 via piping or the like and is configured to be able to supply a dephosphorizing agent containing Ca to the second supply line 32.

[0070] The first supply source 41, the second supply source 42, and the third supply source 43, which are gas supply sources, may be, for example, containers containing high-pressure gas. Alternatively, when air is used as the carrier gas, the supply source 42 or 43 of the air may be the atmosphere, that is, air can be supplied from the atmosphere via a pump or the like. Examples of the first carrier gas include gases with a high nitrogen concentration, specifically nitrogen and air. Examples of the second carrier gas include gases with a low nitrogen concentration, specifically argon and carbon dioxide. The supply source 44 of the phosphorus remover may be any device capable of supplying the powdered phosphorus remover to the second supply line 32, and a general powder supply device can be adopted.

[0071] The converter equipment 100 is configured such that the dephosphorizing agent supplied to the second supply line 32 can be supplied to the top blowing lance 20 or the first supply line 31 together with the carrier gas via the second supply line 32, and the carrier gas and the dephosphorizing agent supplied to the top blowing lance 20 or the first supply line 31 can be blown upward from the top blowing lance 20 into the converter 10 together with the oxygen from the first supply line. Further, by configuring the carrier gas supplied to the second supply line 32 to be switchable between a first carrier gas and a second carrier gas, the top blowing gas blown from the top blowing lance 20 into the converter 10 can be switched between a low nitrogen concentration gas having a nitrogen concentration of 0.3 mol% or less and a high nitrogen concentration gas having a nitrogen concentration exceeding 0.3 mol%. For example, by configuring a part of the supply source and the supply line to be openable and closable by a valve or the like, the supply of the carrier gas and the dephosphorizing agent from the second supply line 32 to the lance and the stop of the supply can be switched, and the carrier gas supplied to the second supply line 32 can be switched between the first carrier gas and the second carrier gas. Thus, according to the converter equipment 100, the first supply line 31, which is an oxygen supply line, and the second supply line 32, which is a supply line for the carrier gas and the dephosphorizing agent, are each connected to the top blowing lance 20, and the presence or absence of the supply of the carrier gas and the dephosphorizing agent from the second supply line 32 to the top blowing lance 20 and the type of the carrier gas supplied to the top blowing lance 20 can be switched. By being configured in this way, either of the converter blowing methods according to the first and second embodiments can be implemented.

Example

[0072] Hereinafter, the present invention will be further described while showing examples, but the present invention is not limited to the following examples. The present invention can adopt various conditions as long as it does not deviate from the gist and can achieve its purpose.

[0073] 1. Experiment 1 The upper limit of the nitrogen concentration in the top-blown gas when blowing CaO powder onto the molten iron to obtain molten steel with a sufficiently low nitrogen concentration was investigated using a test converter. Specifically, in the decarburization blowing experiment of molten iron using the test converter shown in Fig. 5, CaO-containing powder as a dephosphorizing agent was top-blown together with an oxygen-containing gas, and the phosphorus concentration and nitrogen concentration in the molten steel obtained after blowing were measured. The experimental conditions are as follows.

[0074] First, 2.0 t of molten iron with a temperature of 1300 °C to 1400 °C was charged into the upper-bottom blown test converter. The molten iron used had a carbon concentration equivalent to desiliconized and dephosphorized molten iron of 3.2 to 3.5 mass%, a silicon concentration of 0.05 mass% or less, a manganese concentration of 0.1 mass% or less, a phosphorus concentration of 0.03 to 0.05 mass%, and a sulfur concentration of 0.005 to 0.01 mass%. A 5-hole lance was used as the top-blown lance. Quicklime powder with a CaO pure content of 97% was used as the CaO-containing powder. The quicklime powder was transported from the powder supply device to the upper end of the lance by a carrier gas through a pipe and a flexible hose, connected to the oxygen supply pipe at the upper end of the lance, and merged with the top-blown oxygen gas. Then, in a state where the oxygen gas, carrier gas, and CaO-containing powder were mixed, they passed through the lance and were sprayed onto the molten iron from five nozzles provided at the tip of the lance. A total of 20 kg of quicklime powder was sprayed at a constant supply rate from the initial stage to the final stage of blowing by the above method. The top-blown oxygen gas was 6.5 Nm 3 / min, the carrier gas flow rate was 0.1 to 0.3 Nm 3 / min, the total gas flow rate was 6.6 to 6.8 Nm 3 / min, and the lance height was 450 mm for blowing. Various gases as shown in Table 1 below were used as the carrier gas, and the experiment was carried out in a state where the nitrogen concentration was 0 to 4.4 mol% when mixed with the oxygen gas. In addition to the above dephosphorizing agent, 5.0 kg of silica and 3.0 kg of MgO pellets were placed on top of the molten iron before blowing as a flux. Also, argon gas was flowed from each of the four bottom tuyeres at a flow rate of 0.1 Nm 3 / min.

[0075] After the blowing, the molten steel in the furnace was sampled, and the phosphorus concentration and nitrogen concentration in the molten steel were analyzed. As a result, the phosphorus concentration in the molten steel was 0.005% or less under any conditions. On the other hand, the nitrogen concentration varied depending on the conditions. Table 1 below shows the experimental conditions and experimental results.

[0076]

Table 1

[0077] As is clear from the results shown in Table 1, the higher the nitrogen concentration in the top-blown gas, the higher the nitrogen concentration in the molten steel. As shown in Table 1, in order to make the nitrogen concentration in the molten steel 0.0015 mass% or less, which is equivalent to the case of only top-blowing oxygen gas, it is necessary to make the nitrogen concentration in the top-blown gas 0.3 mol% or less.

[0078] 2. Experiment 2 In the above study, since quicklime powder was continuously top-blown during the blowing, it was assumed that the carrier gas was also continuously flowing. On the other hand, in order to keep the nitrogen concentration in the top-blown gas at 0.3 mol% or less, a large amount of expensive carrier gas with a low nitrogen concentration would be consumed, which might lead to a significant cost increase. Therefore, from the perspective of reducing the consumption of expensive carrier gas, conditions were studied where quicklime powder was not continuously top-blown from the beginning to the end of the blowing, but was top-blown after the latter half of the blowing when rephosphorization became significant, and conditions where an inexpensive carrier gas with a high nitrogen concentration was used in the first half of the blowing and an expensive carrier gas with a low nitrogen concentration was used in the latter half of the blowing. The experimental conditions are as follows.

[0079] In a converter facility as shown in Fig. 5, 290 to 310 t of hot metal was charged into an upper-bottom blown converter. After adding auxiliary raw materials, oxygen gas was blown onto the hot metal from an upper blowing lance to perform decarburization blowing. A 5-hole lance was used for the upper blowing lance. An oxygen gas supply pipe (first supply line) was connected to the upper end of the lance, and a powder supply pipe (second supply line) was connected to the oxygen gas supply pipe near the upper end of the lance. A valve was provided in the powder supply pipe near the connection part between the oxygen gas supply pipe and the powder gas supply pipe, and it was possible to open the valve when supplying powder and close it when not supplying powder, so as to suppress the waste of carrier gas when powder supply was not required. Also, an oxygen supply source (first supply source) was connected to the oxygen gas supply pipe to enable the flow of oxygen, while a powder supply device as the first carrier gas supply source (second supply source), the second carrier gas supply source (third supply source), and the powder supply source (fourth supply source) was connected to the powder supply pipe. Moreover, valves were provided near the first carrier gas supply source and near the second carrier gas supply source respectively, so that the carrier gas supplied to the powder supply pipe could be switched between the first carrier gas and the second carrier gas. The hot metal had been previously subjected to desiliconization and dephosphorization treatment in a dephosphorization converter and was the hot metal that had once flowed out from the dephosphorization converter into a hot metal ladle. The composition used had a carbon concentration of 3.2 to 3.5 mass%, a silicon concentration of 0.05 mass% or less, a manganese concentration of 0.1 mass% or less, a phosphorus concentration of 0.02 to 0.04 mass%, and a sulfur concentration of 0.005 to 0.01 mass%. As auxiliary raw materials, 2.4 to 3.4 t of lump quicklime, 0.5 to 0.8 t of silica stone, 0.6 to 1.0 t of peridotite, and up to 3.0 t of iron ore were added.

[0080] For Comparative Examples 1 to 12 and Examples 1 to 6, decarburization blowing was carried out under the experimental conditions shown in Table 2 below. After the blowing was completed, the phosphorus concentration and nitrogen concentration in the molten steel were analyzed. When the phosphorus concentration in the molten steel was 0.005% by mass or less, it was evaluated as qualified (◎, ○), and when it exceeded 0.005% by mass, it was evaluated as unqualified (×). In particular, when the phosphorus concentration in the molten steel was 0.003% by mass or less, it was evaluated as good (◎). Also, when the nitrogen concentration in the molten steel was 0.0015% by mass or less, which was the nitrogen concentration in the molten steel obtained by decarburization blowing without top blowing of powder, it was evaluated as qualified (○), and when it exceeded 0.0015% by mass, it was evaluated as unqualified (×). The results are shown in Table 2 below.

[0081]

Table 2

[0082] From the results shown in Table 2 below, the following can be understood.

[0083] Comparative Example 1 is an example in which CaO powder was top-blown with oxygen at 0.8 kg / min / t from 10% (top-blown oxygen amount ratio) to the end of blowing of 100% of the blowing. N2 gas was used as the carrier gas, and the carrier gas flow rate was 2000 Nm 3 / min to supply CaO powder to the oxygen line. Also, the oxygen gas flow rate was 48000 - 65000 Nm 3 / h, the total flow rate of the top-blown gas was 50000 - 67000 Nm 3 / h, and the N2 concentration of the top-blown gas was 3.0 - 4.0 mol%. After the blowing was completed, when the phosphorus concentration and nitrogen concentration in the molten steel were analyzed, the phosphorus concentration in the molten steel was 0.003% by mass, and sufficient dephosphorization had been achieved. It is considered that rephosphorization was suppressed by the top blowing of powder performed until the end of blowing. On the other hand, the nitrogen concentration in the molten steel was 0.0042% by mass, which was significantly higher than 0.0015% by mass, the nitrogen concentration in the molten steel obtained by decarburization blowing without top blowing of powder. Since the nitrogen concentration in the top-blown gas during top blowing of powder was too high, nitrogen absorption by the molten steel occurred during blowing, and this is considered to have remained even after the blowing was completed.

[0084] Comparative Examples 2 to 5 are examples in which the carrier gas was air and decarburization blowing was performed with the timing of the end of powder top blowing being 70% to 100% of the blowing. After the blowing was completed, the phosphorus concentration and nitrogen concentration in the molten steel were analyzed. Similar to Comparative Example 1, the phosphorus concentration in the molten steel was 0.005% or less, and there was no problem with dephosphorization. However, the nitrogen concentration in the molten steel was significantly higher than 0.0015 mass% of the nitrogen concentration in the molten steel obtained by decarburization blowing without powder top blowing. Similar to Comparative Example 1, since the nitrogen concentration in the top blowing gas during powder top blowing was too high, nitrogen absorption by the molten steel occurred during blowing, and this was considered to remain even after the blowing was completed.

[0085] Example 1 is an example in which CaO powder was top blown together with oxygen until the end of blowing at 10% (ratio of top blown oxygen amount) to 100% of the blowing. However, argon not containing nitrogen was used as the carrier gas for the CaO powder. After the blowing was completed, the phosphorus concentration and nitrogen concentration in the molten steel were analyzed. The phosphorus concentration in the molten steel was 0.002 mass%, which was good. Also, the nitrogen concentration in the molten steel could be made as low as 0.0012 mass%, which was equivalent to the case without powder top blowing.

[0086] However, in Example 1, since powder top blowing was performed using argon as the carrier gas for a period of 60% or more of the blowing, there is a concern about an increase in manufacturing cost compared to the case where nitrogen or air is used as the carrier gas. To solve this, the following comparative examples and examples were examined.

[0087] Comparative Examples 6 to 8 are examples in which air is used as the carrier gas and CaO powder is blown from above only at the end of the blowing process. Specifically, the top blowing of CaO powder was started at 80% to 95% of the blowing time, and CaO powder was top blown together with oxygen at 0.8 to 0.9 kg / min / t until the end of the blowing. After the blowing was completed, the phosphorus concentration and nitrogen concentration in the molten steel were analyzed. As a result, although the phosphorus concentration in the molten steel was as low as 0.004 to 0.005 mass%, the nitrogen concentration in the molten steel was significantly increased compared to 0.0015 mass% of the nitrogen concentration in the molten steel obtained by decarburization blowing without top blowing of powder. Similar to Comparative Examples 1 to 5, since the nitrogen concentration in the top blowing gas during powder top blowing was too high, nitrogen absorption of the molten steel occurred during blowing, and it is considered that this remained even after the blowing was completed.

[0088] Examples 2 to 4 are examples in which a carrier gas containing no nitrogen is used and CaO powder is blown from above only at the end of the blowing process. Specifically, the top blowing of CaO powder was started at 80% to 95% of the blowing time, and CaO powder was top blown together with oxygen at 0.8 to 1.2 kg / min / t until the end of the blowing. After the blowing was completed, the phosphorus concentration and nitrogen concentration in the molten steel were analyzed. As a result, a very low result of 0.005 mass% was obtained for the phosphorus concentration in the molten steel, and the nitrogen concentration could also be made as low as 0.0010 to 0.0013 mass%, which is equivalent to the case where no top blowing of powder is performed. Thus, by performing top blowing with a top blowing gas having a low nitrogen concentration from the start to the end of the blowing and top blowing CaO powder at the end of the blowing, not only the powder cost but also the carrier gas cost could be suppressed compared to Example 1.

[0089] As described above, good dephosphorization results were also obtained in Examples 2 to 4 in which CaO powder was blown from above only at the end of the blowing, but the phosphorus concentration in the molten steel was slightly higher compared to Example 1 in which CaO powder was blown from above in the first half of the blowing. In this regard, from the viewpoint of dephosphorization, it can be said that it is preferable to blow CaO powder from the first half to the end of the blowing. In this regard, although CaO powder was blown from the first half of the blowing, switching of the carrier gas type during the blowing was examined.

[0090] Examples 5 and 6, and Comparative Examples 9 and 10 are examples in which CaO powder is blown from the upper part in the first half of the blowing process, and the carrier gas type is switched from a high-nitrogen-concentration gas to a low-nitrogen-concentration gas during the blowing process. As is clear from the results of Examples 5 and 6, by using air as the carrier gas in the first half of the blowing process and switching the carrier gas to Ar gas before reaching 60% of the blowing period, the phosphorus concentration in the molten steel can be reduced to 0.003% by mass or less, and the nitrogen concentration in the molten steel can also be suppressed to a low level. However, as is clear from the results of Comparative Examples 9 and 10, if the timing of switching the carrier gas is too late, the nitrogen concentration in the finally obtained molten steel will increase. It is considered that nitrogen absorption of the hot metal or molten steel occurs in the first half of the blowing process and remains after the blowing is completed. From the results of Examples 5 and 6, and Comparative Examples 9 and 10, it can be seen that by switching the carrier gas before 60% of the blowing process, the nitrogen concentration in the finally obtained molten steel can be reduced.

[0091] Comparative Examples 11 and 12 correspond to the prior art. That is, CaO powder is blown from the upper part in the first half of the blowing process, and in the second half of the blowing process, CaO powder is not blown from the upper part, and only the upper blowing of the upper blowing gas (oxygen) is performed. In this case, although the nitrogen concentration of the finally obtained molten steel can be reduced, the phosphorus concentration will increase significantly. It is considered that rephosphorization occurs in the second half of the blowing process where the temperature is higher, and the dephosphorization promoting effect in the first half of the blowing process is reduced.

[0092] 3. Summary From the above experiments, in a converter blowing method in which an upper blowing gas containing oxygen is blown from an upper blowing lance to hot metal or molten steel in a converter, when the following requirements 1-1 and 1-2 are satisfied, or when the following requirement 2-1 is satisfied, it can be said that nitrogen absorption and rephosphorization of hot metal or molten steel can be suppressed, and the phosphorus concentration and nitrogen concentration of the finally obtained molten steel can be sufficiently reduced.

[0093] Requirement 1-1: From the start time point T S to the time point T A1 until, an upper blowing gas having a nitrogen concentration of 0.3 mol% or less is blown from the upper blowing lance to hot metal or molten steel. Here, the start time point T of the blowingS From time point T A1 to the total amount of oxygen blown from the top lance is 60% or more of the total amount of oxygen blown from the top lance from the start time point T of the blowing process S to the end time point T of the blowing process E up to.

[0094] Requirement 1-2: At least from time point T A1 to the end stage of the blowing process, a dephosphorizing agent containing Ca is blown from the top lance into the hot metal or molten steel together with a low-nitrogen concentration gas as the top blowing gas. Here, the low-nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

[0095] Requirement 2-1: From the time point T after the start of the blowing process A2 to the end stage of the blowing process, a dephosphorizing agent containing Ca is blown from the top lance into the hot metal or molten steel together with a low-nitrogen concentration gas as the top blowing gas. Here, from the start time point T of the blowing process S to time point T A2 the total amount of oxygen blown from the top lance is less than 60% of the total amount of oxygen blown from the top lance from the start time point T of the blowing process S to the end time point T of the blowing process E up to, and the low-nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less.

Explanation of symbols

[0096] 100 Converter equipment 1 Hot metal (or molten steel) 10 Converter 20 Top lance 31 First supply line 32 Second supply line 41 First supply source 42 Second supply source 43 Third supply source 44 Fourth supply source

Claims

**Claim 1** A converter blowing method, comprising: blowing an oxygen-containing top blowing gas from a top blowing lance onto hot metal or molten steel in a converter; meeting the following requirements 1-1 and 1-2; making the phosphorus concentration in the molten steel after the blowing end 0.005 mass% or less and the nitrogen concentration 0.0015 mass% or less; A converter blowing method. Requirement 1-1: Blowing start time point T Ｓ to time point T Ａ１ until, from the top blowing lance to the hot metal or the molten steel, a top blowing gas having a nitrogen concentration of 0.3 mol% or less is blown. Here, the blowing start time point T Ｓ to the time point T Ａ１ until the total amount of oxygen blown from the top blowing lance is 60% or more of the total amount of oxygen blown from the top blowing lance from the blowing start time point T Ｓ to the blowing end time point T Ｅ until. Requirement 1-2: At least at the time point T Ａ１ From this point until the end of the blowing process, a phosphorus-removing agent containing Ca is sprayed from the top-blowing lance onto the hot metal or the molten steel, together with a low-nitrogen-concentration gas as the top-blowing gas. Here, the low-nitrogen-concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less. **Claim 2** meeting the following requirements 1-1a and 1-1b; The converter blowing method according to Claim 1. Requirement 1-1a: At the starting point in time T of the blowing Ｓ until any point in time T after the start of the blowing Ｂ１ while spraying the top blowing lance with a top blowing gas having a nitrogen concentration of 0.3 mol% or less onto the hot metal or the molten steel, the dephosphorizing agent is not sprayed. Here, the total amount of oxygen sprayed from the top blowing lance from the starting point in time T S to the point in time T B1 is less than 60% of the total amount of oxygen sprayed from the top blowing lance from the starting point in time T S to the ending point in time T E of the blowing. Requirement 1-1b: At the time point T Ｂ１ to the time point T Ａ１ until, the dephosphorizing agent is sprayed from the top blowing lance into the hot metal or the molten steel together with the low nitrogen concentration gas as the top blowing gas. **Claim 3** A converter blowing method, comprising: blowing an oxygen-containing top blowing gas from a top blowing lance onto hot metal or molten steel in a converter; meeting the following requirement 2-1; making the phosphorus concentration in the molten steel after the blowing end 0.005 mass% or less and the nitrogen concentration 0.0015 mass% or less; A converter blowing method. Requirement 2-1: Time point T after the start of blowing Ａ２ From the time point T after the start of blowing until the end of blowing, a dephosphorizing agent containing Ca is sprayed from the top blowing lance onto the hot metal or the molten steel together with a low nitrogen concentration gas as the top blowing gas. Here, at the start time point T of blowing Ｓ until the time point T Ａ２ the total amount of oxygen sprayed from the top blowing lance is less than 60% of the total amount of oxygen sprayed from the top blowing lance from the start time point T of blowing Ｓ until the end time point T of blowing Ｅ and the low nitrogen concentration gas contains oxygen and a carrier gas and has a nitrogen concentration of 0.3 mol% or less. **Claim 4** meeting the following requirement 2-2; The converter blowing method according to Claim 3. Requirement 2-2: The blowing start time point T Ｓ From the time point T Ａ２ Until, or at any time point T after the start of blowing Ｂ２ From the time point T Ａ２ Until, a top blowing gas having a nitrogen concentration of more than 0.3 mol% is blown from the top blowing lance onto the hot metal or the molten steel, and at the time point T Ａ２ The top blowing gas is switched to the low nitrogen concentration gas. **Claim 5** meeting the following requirements 2-2a and 2-2b; The converter blowing method according to Claim 4. Requirement 2-2a: At the start time point T of blowing Ｓ to the time point T Ｂ２ while blowing an upper blowing gas having a nitrogen concentration of more than 0.3 mol% or an upper blowing gas having a nitrogen concentration of 0.3 mol% or less from the upper blowing lance onto the hot metal or the molten steel, the dephosphorizing agent is not blown. Requirement 2-2b: the time point T Ｂ２ From the time point T Ａ２ Until the time point T Ａ２ Spray the dephosphorizing agent together with the high-nitrogen-concentration gas as the top blowing gas from the top blowing lance onto the hot metal or the molten steel, and switch the type of the top blowing gas at the time point T. Here, the high-nitrogen-concentration gas contains oxygen and a carrier gas and has a nitrogen concentration exceeding 0.3 mol%.

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