JP2013241654A - Method for producing medium and high carbon steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable medium and high carbon steel to be surely produced even when fluorite is not used.SOLUTION: A basicity of slag upon starting of blowing is set to be 0.8-1.2 and an oxygen feed speed is set to be ≤1.8 Nm/min/t, the oxygen feed speed is changed to 2.2 Nm/min/t-3.2 Nm/min/t while an oxygen quantity during the blowing is 3-10% of the total oxygen quantity of the total blowing after starting the blowing, Cao source is charged twice or more while the oxygen quantity during the blowing is 40-70% of the total oxygen quantity, a charging quantity of the Cao source per one time is set to be ≤16 kg/t, the oxygen feed speed during charging period of the Cao source is set to be ≤2 Nm/min/t, an interval of charging the Cao source is set to be ≥1 min, and the basicity of the slag when the blowing is finished is set to be 2.5-4.

Description

  The present invention relates to a method for producing medium-high carbon steel that produces medium-high carbon steel using a converter, for example.

Conventionally, as a technique for refining and manufacturing medium carbon steel, high carbon steel, and the like in a converter, there are, for example, those of Patent Documents 1 to 3.
In Patent Document 1, in decarburization refining by a refining apparatus having an upper bottom blowing function using hot metal in which the phosphorus concentration [P] is reduced to 0.050% or less by dephosphorization treatment, lime powder together with top blowing oxygen gas When the blowing time is t0 (minutes) and the blowing end time is t (minutes), t / t0 is set to 0.7 or more, and the fluorine concentration in the blowing slag is set to 0.5% or less. .

In Patent Document 2, a low-melting slag is formed by adding a CaO and Si source at a ratio of basicity of 1.0 to 2.0 at the initial stage of blowing, and then the basicity of the generated slag is set to 3.0 to 4 A sufficient amount of CaO is added.
In Patent Document 3, using a top blow converter or a top blow converter, after adding dephosphorization flux mainly composed of quick lime, hatching accelerator and iron oxide, medium high carbon steel is obtained by performing oxygen blowing. In the production method, the same or different hatching accelerator and / or iron oxide as those described above are additionally added during blowing, and the dephosphorization reaction is advanced by increasing the flow rate of the top blowing acid.

  In addition, there are methods shown in Patent Documents 4 to 7 as a method of refining in a converter or the like.

JP 2002-275520 A Japanese Patent Laid-Open No. 58-016014 JP 2000-008113 A Japanese Patent Laid-Open No. 03-153811 JP 2002-047508 A JP-A-11-006006 Japanese Patent Laid-Open No. 11-062212

  Although Patent Document 1 discloses a method for producing high carbon steel without using fluorite, it is necessary to blow lime powder instead of using fluorite, and a large-scale facility for blowing lime powder. (Lump lime grinding equipment, powder storage tank, high-pressure gas equipment, etc.) are required. In addition, in the facility for blowing lime powder, since the lime powder is pumped with high-pressure gas, the wear of the piping is large, so frequent maintenance work is necessary, and maintenance work is difficult. That is, in the technique of Patent Document 1, in order to manufacture high carbon steel without using fluorite, a large-scale apparatus must be used, and it is difficult to manufacture high carbon steel by this method. It was. Further, although Patent Documents 2 to 7 disclose methods for producing medium-high carbon steel and the like, it is difficult to say that the technique can reliably produce medium-high carbon steel without using fluorite.

  Then, in view of the said problem, this invention aims at providing the manufacturing method of medium-high carbon steel which can manufacture medium-high carbon steel simply and reliably, without using a fluorite.

In order to achieve the above object, the present invention has taken the following measures.
That is, the technical means for solving the problems in the present invention is that, when producing a medium-high carbon steel using a converter, the basicity of slag at the start of blowing is 0.8 to 1.2 and the acid feed While the speed is 1.8 Nm ³ / min / t or less and the oxygen amount during blowing is 3% to 10% with respect to the total oxygen amount of the entire blowing, the change below 2.2Nm ³ / min / t or more 3.2Nm ³ / min / t, while the amount of oxygen in the blowing is a 40% to 70% relative to the total oxygen amount, twice The CaO source is charged as described above, the charging amount of the CaO source per one time is 16 kg / t or less, the acid feed rate during the charging period of the CaO source is 2 Nm ³ / min / t or less, The CaO source charging interval is 1 minute or more, and the basicity of the slag at the end of blowing is 2.5 to 4 Characterized by way becomes.

  According to the present invention, medium and high carbon steel can be produced easily and reliably without using fluorite.

It is a general view of the converter which performs refining. It is the figure which showed the condition of the refining in a converter.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a converter that performs primary refining when manufacturing medium-high carbon steel. For convenience of explanation, hot metal and molten steel are referred to as molten metal.
When producing medium and high carbon steel, first, the molten metal 1 for medium carbon steel is charged into the converter 2, and after refining such as decarburization in the converter 2, the secondary refining process is a lower process. To remove inclusions and adjust ingredients.

  In primary refining in the converter, a charging device 4 such as a hopper provided on the upper side of the converter 2 is used to charge a CaO source (slagging material) such as burned lime from the furnace port 3 and into the molten metal 1. On the other hand, oxygen blowing is performed using the top blowing lance 5 to remove [C] and [P] in the hot metal. Although FIG. 1 shows a top blow converter, an upper bottom blow converter may be used when producing medium-high carbon steel.

  In producing medium-high carbon steel, it is preferable to set [C] at the time of blowing stop to 0.5 mass% or more and [P] to 0.015 mass% or less. That is, after the converter is blown (after primary refining), it is desired to increase [C] and decrease [P]. However, it is well known that in blowing in a converter, both decarburization and dephosphorization proceed, and it is difficult to increase [C] and decrease [P]. For this reason, in the conventional technology, the dephosphorization reaction is once promoted in the blowing of the converter (primary refining) to reduce the [C] to less than 0.5% by mass, and the molten metal is then carburized after the steel is discharged. By doing this, medium and high carbon steel is manufactured.

In order to increase the dephosphorization ability in the converter (to promote the dephosphorization reaction), it is considered that fluorite (CaF ₂ ) is introduced into the converter to promote the hatching of slag. When fluorite is used for promotion, a large amount of fluorine or the like is contained in the slag after refining. Since slag containing a large amount of fluorine or the like is difficult to reuse and has a large environmental impact and load, there is a demand to promote dephosphorization without using fluorite in a converter.

  Moreover, when carburizing after making [C] less than 0.5 mass% in a converter, it becomes difficult to adjust the composition of the molten metal due to S or N contained unavoidably in the carburized material. There is a concern that the quality may be deteriorated, and there is a concern that productivity is lowered due to an increase in cost or an extension of processing time. For this reason, it is desired to produce medium and high carbon steel without using fluorite in the blowing of the converter or performing a carburizing treatment after the blowing of the converter.

  Here, Japanese Patent No. 4701727 discloses a method in which fluorite is not used in the converter by promoting dissolution of CaO by increasing FeO in the slag. Therefore, it is difficult to increase FeO in the slag while [C] in the molten metal is high in, for example, blowing with a small amount of the iron oxide source charged. Even if FeO in the slag can be increased, an undesirable phenomenon may occur in which the CO gas generated by the reaction between FeO and C suddenly escapes from the slag.

That is, since it is difficult to stably produce medium carbon steel without using fluorite only by using the conventional technique such as Japanese Patent No. 4701727, in the present invention, the base at the start of blowing The medium-high carbon steel is easily manufactured by specifying the degree, the acid feeding speed, the acid feeding speed during blowing, the number of times of charging the CaO source (number of times of charging), and the like. In particular, according to the present invention, medium-high carbon steel is easily and reliably produced using a conventional charging device 4 such as a hopper without using a large-scale facility.

Hereinafter, the manufacturing method of the medium-high carbon steel of the present invention will be described in detail.
As described above, when the CaO source is supplied, if there is a large amount of FeO in the slag, hatching of the slag proceeds. In the present invention, first, the basicity (CaO / SiO ₂ ) of the slag at the initial stage of blowing is reduced to suppress the activity of FeO in the slag so that FeO in the slag is not reduced as much as possible. Promoting hatching. Specifically, the basicity of the slag at the start of blowing is 0.8 or more and 1.2 or less.

  When the basicity of the slag at the start of blowing is less than 0.8, the amount of CaO source (lime) additionally charged in the middle of blowing is increased as compared with the molten slag, resulting in insufficient dissolution of lime. As a result, [P] of the molten metal cannot be made 0.015% by mass or less. On the other hand, if the basicity of the slag exceeds 1.2, the activity of FeO in the slag increases, the reaction between FeO in the slag and [C] in the molten metal is promoted, and the FeO in the slag The amount is reduced. As a result, the melting of lime does not proceed and the [P] of the molten metal cannot be made 0.015% by mass or less.

Also, at the start of blowing, increasing the FeO in the slag by supplying oxygen to the slag by lowering the oxygen feed rate at which oxygen is blown from the top blowing lance 5 (soft blow) and suppressing stirring of the molten metal It is said. Specifically, the acid sending speed at the start of blowing is set to 1.8 Nm ³ / min / t or less. In addition, since the blowing time will increase and productivity may fall when an acid delivery rate is too small, it is preferable that an acid delivery rate shall be 0.5 or more and Nm < ³ > / min / t.

If the acid feed rate exceeds 1.8 Nm ³ / min / t, the blown oxygen reaches the molten metal and stirring of the molten metal becomes strong, so that FeO in the slag cannot be increased. As a result, the melting of lime does not proceed and the [P] of the molten metal cannot be made 0.015% by mass or less.
Therefore, at the start of blowing in the converter, 0.8 ≦ C / S (basicity) ≦ 1.2 and the acid feed rate is set to 1.8 Nm ³ / min / t or less.

After the start of blowing, the oxygen feed rate is set to 1.8 Nm ³ / min / t or less and oxygen is continuously blown. And while the amount of oxygen in the blown blowing is 3% to 10% with respect to the total amount of oxygen in the entire blowing (referred to as the total amount of blowing oxygen), the acid feed rate is set to 2.2 Nm ³ / Min / t to 3.2 Nm ³ / min / t or less. That is, in the present invention, when FeO of the slag increases, blowing is performed from soft blow (acid feed rate is 1.8 Nm ³ / min / t or less) to hard blow (acid feed rate is 2.2 Nm ³ / min). / T or more and 3.2 Nm ³ / min / t or less).

At an early stage when the oxygen amount during blowing is less than 3% of the total amount of blown oxygen, if the acid feed rate is increased to shift from soft blow to hard blow, the soft blow period is short, so oxygen Little increase in FeO in slag due to blowing.
On the other hand, when the oxygen feed rate is increased at a late stage in which the amount of oxygen during blowing exceeds 10% of the total amount of blown oxygen, FeO in the slag has already increased, but the FeO in the slag has increased. Too much. Therefore, immediately after increasing the acid feed rate, the generation rate of CO gas at the slag / metal interface is increased, the bubble density of the CO gas in the slag is increased, and the bubbles are coalesced and floated. The phenomenon of suddenly falling out of the slag occurs. Thus, if the phenomenon that CO gas suddenly escapes from the slag occurs, the slag becomes calm and the slag forming height cannot be obtained (the slag becomes difficult to form), and the formed slag is sufficient. Since it cannot heat up, decarburization progresses too much and there exists a possibility that [C] of molten metal cannot be made 0.5 mass% or more.

For example, as disclosed in Japanese Patent No. 4487812, when oxygen is blown in the blowing in the converter, secondary combustion occurs between oxygen blown by blowing and CO gas generated during blowing. CO ₂ gas is generated. If the forming slag is positioned in the secondary combustion zone where secondary combustion occurs, the heat generated by the secondary combustion will reach the forming slag, ensuring the slag temperature and transferring heat to the molten metal via the slag. It is possible to prevent excessive decarburization by blowing. In other words, in the present invention, it is necessary to form slag to some extent during blowing and heat from secondary combustion to reach the slag. The speed is specified.

When switching the blowing from soft blow to hard blow, if the acid feed rate is less than 2.2 Nm ³ / min / t, it is almost the same as the soft blow state, and FeO in the slag increases too much. There is a possibility that a phenomenon in which CO gas suddenly escapes from the slag may occur during smelting. In addition, when the acid feed rate is larger than 3.2 Nm ³ / min / t, it becomes almost impossible to supply oxygen to the slag and the stirring of the molten metal becomes strong, so that FeO in the slag is lowered too much and the slag is hatched. It becomes insufficient, and [P] of the molten metal cannot be made 0.015% by mass or less.

Therefore, after blowing the start, within the period the amount of oxygen in the blowing is 3-10% of the total blowing oxygen, 2.2 Nm the oxygen-flow-rate ^{3 /min/t~3.2Nm 3} / min / It is supposed to be changed to t. The total amount of blown oxygen is gaseous oxygen (the total amount of oxygen blown from the top blowing lance 5) and does not include solid oxygen. In medium-high carbon steel, the proportion of gaseous oxygen is larger than that of solid oxygen, and the influence of solid oxygen is negligible.

  Next, after switching the acid feed rate, the CaO source is charged twice or more while the oxygen amount during blowing is 40 to 70% of the total amount of oxygen blown. And the basicity of the final slag (basicity of the slag after blowing) is set to 2.5 to 4.0. When the basicity of the slag is less than 2.5, the dephosphorization ability of the slag is insufficient, so that [P] of the molten metal cannot be made 0.015% by mass or less. On the other hand, when the basicity of the slag is greater than 4, slag failure due to uncalcification occurs, and [P] of the molten metal cannot be made 0.015 mass% or less.

Here, when a large amount of CaO source is supplied at once in order to make the basicity of the slag in the range of 2.5 to 4.0, the basicity suddenly increases, the viscosity of the slag decreases, and the slag calms down. May occur. In order to prevent the slag from calming down, the CaO source is charged twice or more.
In the state of continuing blowing, the basicity of slag will gradually increase, but the CaO source is charged at an early stage when the amount of oxygen during blowing becomes less than 40% of the total amount of blowing oxygen. If it does, the period when the basicity of slag will be around 2 becomes long. When the basicity of the slag is around 2, it is a time when the activity of FeO in the slag is high, and the reaction between FeO in the slag and [C] in the molten metal is promoted, and the FeO in the slag decreases, so the slag hatching However, there is a possibility that [P] of the molten metal cannot be made 0.015% by mass or less. On the other hand, at a late stage when the oxygen amount during blowing is greater than 70% of the total amount of blown oxygen, supplying a CaO source can shorten the period during which the basicity of the slag is around 2, There is a possibility that the time until hatching cannot be secured and the [P] of the molten metal cannot be made 0.015 mass% or less.

Therefore, the CaO source is charged twice or more within a period in which the amount of oxygen during blowing is 40 to 70% of the total amount of blowing oxygen.
Here, when the CaO source is charged twice or more, the charging amount of the CaO source per time is set to 16 kg / t or less. When the charged amount of the CaO source is larger than 16 kg / t, the charged material serving as the CaO source suppresses the slag in the forming state (physically destroys the slag bubbles). The slag calms down.

In addition, the acid feed rate during the charging period of the CaO source is 2 Nm ³ / min / t or less. When the CaO source is charged, the slag is temporarily cooled to increase the viscosity of the slag, and the pressure of the CO gas increases inside the slag. When the temperature of the slag rises and returns to its original state after the CaO source is charged, the viscosity of the slag becomes low, and high-pressure CO gas escapes from the slag. When the CaO source is charged, if the acid feed rate is higher than 2 Nm ³ / min / t, the CO gas generation rate is increased, and the CO gas pressure inside the slag becomes too high. As a result, when CO gas having a high pressure escapes from the slag, there is a possibility that the sudden elimination of the CO gas occurs and the slag is calmed down. For this reason, it is necessary to reduce the acid feed rate when supplying the CaO source as 2 Nm ³ / min / t or less.

The CaO source charging interval (interval from the end of the CaO source charging until the start of the CaO source charging) is set to 1 minute or longer. If the CaO source interval is less than 1 minute and is short, the basicity suddenly increases, the slag viscosity decreases, and slag calming may occur.
Therefore, the charging amount of the CaO source per one time is set to 16 kg / t or less, the acid feed rate during the charging period of the CaO source is set to 2 Nm ³ / min / t or less, and the charging interval of the CaO source is 1 minute. That's it.

FIG. 2 summarizes the state of secondary refining in a converter when producing medium-high carbon steel.
As shown in FIG. 2, in the first period in which the amount of oxygen during blowing after starting blowing is 3 to 10% of the total amount of blowing oxygen, the acid feeding rate is 1.8 Nm ³ / min / t. It is as follows. In the second period after the first period and when the acid delivery rate is switched, the acid delivery rate is set to 2.2 to 3.2 Nm ³ / min / t. In the second period, in the section until the amount of oxygen during blowing reaches 40 to 70% of the total amount of blowing oxygen, the CaO source is charged twice or more, and the charging period in which the CaO source is charged Then, the acid feed rate is 2.0 Nm ³ / min / t or less. Moreover, the interval which inserts a CaO source in the 2nd period is made into 1 minute or more.

  Tables 1 and 2 summarize an example in which the method for producing medium-high carbon steel of the present invention was performed and a comparative example in which a method for producing medium-high carbon steel different from the present invention was performed.

In Examples and Comparative Examples, primary refining was performed using a 250-ton class converter. The hot metal (molten metal) charged into the converter was preliminarily dephosphorized until [P] was 0.010 mass% to 0.035 mass%. As the CaO source, calcined lime (CaO: 95 mass%) and light calcined dolomite (CaO: 67 mass%) were used. As the CaO source to be charged, either calcined lime or light calcined dolomite may be used, or both calcined lime and light calcined dolomite may be used. Lightly burned dolomite was prepared by firing raw dolomite.

  Moreover, when producing medium and high carbon steel, the [C] of the molten metal at the time of stopping the blowing (at the time of blowing) needs to be 0.5% by mass or more. It was evaluated whether it was 5% by mass or more. P is an element that lowers toughness, and is preferably lower. For example, as described in JP-A-11-335773, by setting [P] to 0.015 mass% or less, toughness is ensured, so that [P] at the time of blowing is 0. It was evaluated whether the content was 015% by mass or less.

First, examples will be described.
In the examples, the basicity of the slag at the start of blowing is 0.8 or more as shown in the “basicity at the start of blowing” and the “acid feed rate at the start of blowing” column. 2 or less, and the acid feed rate is 1.8 Nm ³ / min / t or less. As shown in the column “Oxygen flow rate change” and “Oxygen feed rate after change”, the amount of oxygen during blowing is 3% to 10% of the total amount of oxygen in the entire blowing. during and has, and the oxygen-flow-rate to ^{2.2Nm 3 /min/t~3.2Nm 3 / min / t} .

Moreover, as shown in the column of “intermediate CaO source charging timing” and “intermediate CaO source charging frequency”, the oxygen amount during blowing is 40 to 70% of the total oxygen amount. During the time, the CaO source is charged twice or more. As shown in the columns of “1st half charge”, “2nd half charge” and “3rd half charge”, the charge amount of the CaO source per charge is set to 16 kg / t or less, The acid feed rate during the charging period is set to 2 Nm ³ / min / t or less, and the charging interval of the CaO source is set to 1 minute or more. As shown in the “basicity after blowing” column, the basicity of slag is 2.5-4.

Thus, in an example that satisfies all of the conditions defined in the present invention, [P] at the time of blowing is 0.015 mass% or less, and [C] at the time of blowing is 0.5 mass% or more. did it. Moreover, by using a charging device such as a hopper, medium-high carbon steel could be reliably produced simply by supplying a CaO source without using fluorite.
On the other hand, as shown in Comparative Examples 23, 24, and 31, [P] cannot be made 0.015% by mass or less when the basicity and acid feeding rate at the start of blowing do not satisfy the above-described conditions. It was.

As shown in Comparative Examples 33 to 34, [P] can be made 0.015% by mass or less when the timing for changing the acid feed rate after the start of blowing and the acid feed rate after the change do not satisfy the above-mentioned conditions. Or [C] could not be increased to 0.5% by mass or more.
As shown in Comparative Examples 27 and 28, when the CaO source charging timing and the CaO source charging frequency do not satisfy the above-described conditions, as shown in Comparative Examples 25, 26, 29, 30, and 37-40. Any one of the charging amount of the CaO source per one time, the acid feed rate during the charging period of the CaO source, the interval of charging the CaO source, and the basicity after blowing can satisfy the above-mentioned conditions. Even when it was not possible, [P] could not be made 0.015 mass% or less, or [C] could not be made 0.5 mass% or more.

  In the embodiment disclosed herein, matters not explicitly disclosed, for example, operating conditions, various parameters, dimensions, weights, volumes, etc. of the components do not deviate from the range normally practiced by those skilled in the art. However, matters that can be easily assumed by those skilled in the art are employed.

1 Molten Metal 2 Converter 3 Furnace Port 4 Charger

Claims (1)

When manufacturing medium and high carbon steel using a converter,
The basicity of the slag at the start of blowing is 0.8 to 1.2 and the acid feed rate is 1.8 Nm ³ / min / t or less,
After the start of blowing, while the oxygen amount during blowing is 3% to 10% with respect to the total oxygen amount of the entire blowing, the acid feed rate is 2.2 Nm ³ / min / t or more and 3.2 Nm Change to ³ / min / t or less,
While the amount of oxygen during blowing is 40 to 70% of the total amount of oxygen, the CaO source is charged twice or more, and the amount of the CaO source charged per time is 16 kg. / T or less, the acid feed rate during the charging period of the CaO source is 2 Nm ³ / min / t or less, the charging interval of the CaO source is 1 minute or more, and the basicity of slag at the end of blowing Is made to be 2.5 or more and 4 or less, the manufacturing method of the medium-high carbon steel characterized by the above-mentioned.

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