JP2006283154A - Method for heating molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the service life of a refractory in a vacuum vessel without reducing the heating efficiency, when molten steel is heated by blowing oxygen gas in an RH-vacuum degassing apparatus provided with two sets of immersion tubes and the vacuum vessel. <P>SOLUTION: In the method for heating the molten steel by blowing the oxygen gas to the surface of the molten steel having 0.01-0.1% Al concentration and reacting Al in the molten steel and the blown oxygen gas with the RH-vacuum degassing apparatus provided with two sets of the immersion tubes and the vacuum vessel; in the case of being 0.2-0.5(V/Q)(Nm<SP>3</SP>/ton) to the ratio of V(Nm<SP>3</SP>/min): the flowing rate of the blown oxygen gas and Q(ton/min): the reflux volume of the molten metal, the pressure in the inner part of the vacuum vessel is controlled according with the following (1)-(3) formulas, A1≤t/T≤A2: 5.3kPa≤P≤13kPa...(1), A3≤t/T≤A4: 2.7kPa≤P≤4.7kPa...(2) and A5≤t/T≤A6: P≤1.3kPa...(3). Wherein, A1=0.2, A2=0.55, A3=0.6, A4=0.75, A5=0.8, A6=1.0 and (t): time from the starting of heating (oxygen blowing time) and T: the total time for heat treating time (total time for oxygen top-blown) and P: the pressure in the vacuum vessel. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for heating molten steel. Specifically, when heating molten steel by spraying oxygen gas in an RH vacuum degassing apparatus including two dip tubes and a vacuum tank, heating efficiency is reduced. The present invention relates to a method for improving the life of a refractory in a vacuum chamber.

  One of the main purposes of secondary refining is the adjustment of molten steel temperature. The molten steel delivered from the converter, etc. to the ladle is subjected to secondary refining for component adjustment and degassing, and at the same time, the temperature is adjusted to a suitable temperature for casting, which is the next process of secondary refining. Is done.

  When the temperature of the molten steel is higher than the temperature appropriate for casting, it is easy to decrease the temperature to an appropriate temperature by extending the time or introducing a cold iron material. On the other hand, there are heating methods such as electrode heating and induction heating to raise the molten steel to an appropriate temperature when the temperature of the molten steel is lower than the appropriate temperature, but for economic reasons, oxygen gas is used in the RH vacuum degassing apparatus. A method of spraying molten steel is generally used.

  That is, a metal such as Al or Si is added to the molten steel, and oxygen gas is sprayed onto the molten steel in the vacuum chamber of the RH vacuum degassing apparatus. The blown oxygen gas reacts with Al in the molten steel to generate heat of oxidation. This oxidation heat is used to raise the molten steel temperature. This treatment is often used because of its easy operation and high temperature adjustment accuracy.

  Further, this process has been improved in cleanliness and efficiency by various improvements so far. For example, in Patent Document 1, by controlling the amount of Al flowing into the vacuum chamber calculated from the product of the Al concentration in the molten steel and the ring flow rate, and the amount of acid sent from the nozzle as an index, An invention is disclosed in which the generation of MnO and FeO is suppressed and the molten steel is heated with an RH vacuum degassing apparatus without deteriorating the cleanliness of the molten steel.

However, this process of raising the temperature of the molten steel by blowing oxygen gas in the RH vacuum degassing apparatus has a problem that the refractory in the vacuum tank tends to wear out. In order to solve this problem, Patent Document 2 describes the ratio of the lining structure of the vacuum chamber of the RH vacuum degassing apparatus, the side walls of which are magnesia-carbonaceous unfired bricks, and the floor and the reflux pipe in the refractory aggregate. The invention is a castable refractory having a content of less than 1% (“%” means “mass%” unless otherwise specified) in this specification. Further, the invention is further disclosed in which the lining structure of the lower tank is made of magnesia carbon brick having at least a molten steel contact portion with a C content of less than 5% and a magnesia carbon brick with a C content of 5 to 9%. In Document 4, the central axis of a vertical lance that is disposed in a vacuum chamber of an RH degassing device so as to be able to move up and down and blows the oxygen-containing gas upward is located within the region of the inner diameter of the reflux pipe, and oxygen-containing gas is injected. By , The invention prevents wear of the refractory of the vacuum tank is disclosed, respectively.
JP-A-9-249910 JP 2001-89808 A JP 2002-285228 A JP-A-9-143546

  The inventions disclosed in Patent Documents 2 to 4 certainly reduce the wear of the refractory in the vacuum chamber. However, the wear of the refractory has not been eliminated, and further improvement in the life of the refractory is desired.

  The wear mechanism of the refractory during the heating of the molten steel using the RH degassing apparatus is considered as follows. The oxygen gas sprayed on the surface of the molten steel is absorbed by the molten steel and mainly reacts with Al in the molten steel. If all the oxygen sprayed reacts with Al in the molten steel, only alumina will be produced. However, oxygen that could not react with Al reacts with the molten steel to form FeOx. When this FeOx comes into contact with the refractory, it reacts with MgO, which is the main component of the refractory, and lowers its melting point to 1600 ° C. or lower. In general, since the temperature of the molten steel inside the vacuum chamber during heating of the molten steel is 1600 ° C. or higher, the refractory is easily melted.

Absorption of oxygen gas into molten steel: (1/2) O ₂ (g) → [O] (6)
Reaction of absorbed oxygen: 3 [O] +2 [Al] → Al ₂ O ₃ (7)
Fe + x [O] → FeO _x (8)
For this reason, in order to reduce the wear of the refractory during heating of the molten steel, the reaction shown in the equation (8) should be suppressed as much as possible and the reaction shown in the equation (7) should be promoted.

  However, until now, heating the molten steel based on such a concept has not been studied. Therefore, the present inventors completed the present invention as a result of intensive studies on the premise of such a concept.

In the present invention, an RH vacuum degassing apparatus including two dip tubes and a vacuum tank is used to blow oxygen gas onto the surface of molten steel having an Al concentration of 0.01% or more and 0.1% or less. A method of heating molten steel by reacting Al with a sprayed oxygen gas, the ratio of the flow rate V (Nm ³ / min) of the sprayed oxygen gas to the ring flow rate Q (ton / min) of the molten steel (V / Q) When the (Nm ³ / ton) is 0.2 or more and 0.5 or less, the heating of the molten steel is characterized in that the pressure inside the vacuum chamber is controlled according to the following formulas (1) to (3): Is the method.

A1 ≦ t / T ≦ A2: 5.3 kPa ≦ P ≦ 13 kPa (1)
A3 ≦ t / T ≦ A4: 2.7 kPa ≦ P ≦ 4.7 kPa (2)
A5 ≦ t / T ≦ A6: P ≦ 1.3 kPa (3)
However, A1 = 0.2, A2 = 0.55, A3 = 0.6, A4 = 0.75, A5 = 0.8, A6 = 1.0, and t is heating start (oxygen top blowing start) , T is the total heat treatment time (total oxygen blowing time, time from the start of oxygen blowing to the end of oxygen blowing) (min), P is the pressure in the vacuum chamber ( kPa).

  By heating the molten steel by spraying oxygen gas in an RH vacuum degassing apparatus comprising two dip tubes and a vacuum chamber, the heating method of the molten steel according to the present invention reduces the heating efficiency without reducing the heating efficiency. The life of the refractory can be improved.

Hereinafter, the best mode for carrying out the method for heating molten steel according to the present invention will be described in detail with reference to the accompanying drawings.
The present invention will be described by taking as an example the case of manufacturing steel using a converter, an RH vacuum degassing apparatus, and a continuous casting machine.

  After performing decarburization processing in the converter, the molten steel is taken out into a ladle and the ladle is moved to an RH vacuum degasser. Although the molten steel is heat-treated in the RH vacuum degassing apparatus, this heat treatment may be performed at any time of the initial stage, the middle stage, or the end stage of the RH vacuum degassing process.

  Usually, immediately after the start of the RH vacuum degassing treatment, oxygen is blown over the surface of the molten steel containing a predetermined amount of Al to heat the molten steel. It is desirable that Al in the molten steel is added in advance before oxygen top blowing. Although it is possible to add Al appropriately while blowing oxygen, it is not preferable because the speed balance will be lost if the optimum time is incorrect.

  The oxygen supply method at the time of heat processing uses the method of spraying oxygen gas on the molten steel surface. It is exemplified that the spraying is performed by using an upper blowing lance disposed directly above the molten steel in the vacuum chamber, an oblique insertion lance disposed on the side wall of the vacuum chamber, or the like. The shape of the lance nozzle to be used is not particularly limited, but a single hole nozzle is desirable. This is because the hot spot area may vary greatly depending on the atmospheric pressure in the case of a multihole nozzle.

  Thus, the present embodiment is directed to a method in which oxygen gas is sprayed onto the surface of molten steel inside the vacuum chamber in an RH vacuum degassing apparatus including two dip tubes and a vacuum chamber. This is because the refractory wear mechanism in this method is as described above with reference to equations (6) to (8).

  In order to advance the reaction of formula (7) preferentially, it is better that the molten steel supply rate into the reaction part with oxygen, that is, the vacuum chamber, is high. This is to suppress the reaction of the formula (8) while giving priority to the reaction of the formula (7) by suppressing the Al deficiency in the reaction part.

In addition, the Al supply rate needs to take into account the balance between the oxygen supply rate, that is, the top blown oxygen flow rate. Since the supply rate of Al can be indicated by the ring flow rate Q (ton / min), and the oxygen supply amount is indicated by the top blowing oxygen flow rate V (Nm ³ / min), the ring flow rate Q (ton / min) And the balance of the top blowing oxygen flow rate V becomes important. Here, the top blowing oxygen flow rate V (Nm ³ / min) sprayed on the molten steel is obtained as V = total oxygen gas supply amount in the standard state (Nm ³ ) / top blowing time (min), and the ring flow rate Q (ton / min) ) Is Q = 11.4 × G ^1/3 × D ^4/3 × {ln (P1 / P0)} ^1/3 , where G: gas flow rate (Nl / min), D: dip tube inner diameter (m ), P1, P0: the blowing position and the pressure (Pa) in the vacuum chamber.

In this example, the ratio (V / Q) between the ring flow rate Q (ton / min) and the top blowing oxygen flow rate V (Nm ³ / min) is 0.2 (Nm ³ / ton) or more and 0.5 (Nm ³ / min). ton) The range which is below was examined. Moreover, since reaction of (6)-(8) type | formula may also receive the influence of Al concentration in molten steel, in this example, conditions with Al concentration in molten steel being 0.01% or more and 0.1% or less I examined it.

Although the above is the result of examination from the viewpoint of mass balance, the balance between the reaction of the equation (7) and the reaction of the equation (8) is influenced by more complicated factors.
The first factor is the oxygen absorption rate. As described above, so-called surplus oxygen that could not react with Al forms FeO _x . Although surplus oxygen depends on the ratio (V / Q) described above, it occurs when the reaction rate of the equation (7) exceeds the reaction rate of the equation (6). That is, the balance between the reaction of formula (7) and the reaction of formula (6) is also important. Here, the reaction rate of the equation (6) is influenced by the atmospheric pressure in addition to the oxygen flow rate. The reaction rate of the formula (6) is shown by the formula (9).

d [O] / dt = k ([O] e- [O]) (9)
In equation (9), k is a rate constant, and [O] e is an equilibrium oxygen concentration. The equilibrium oxygen concentration [O] e is expressed by the equation (10), and the equilibrium oxygen concentration [O] e becomes higher as the oxygen partial pressure _PO2 is higher. That is, the higher the atmospheric pressure, the greater the oxygen absorption rate.

K = [O] / P _O2 ^1/2 (10)
From the above, the reaction rate of the equation (6) can be controlled by controlling the pressure (atmospheric pressure) inside the vacuum chamber, and the reaction of the equation (8) can be suppressed by suppressing the generation of excess oxygen. However, excessive control also decreases the rate of the reaction of equation (7).

The second factor is the effect of reduction of the generated FeO _x by Al. FeO _x produced by the reaction of the formula (8) is reduced to Al according to the reaction of the formula (11).
FeOx + (2/3) xAl → (x / 3) Al ₂ O ₃ + Fe (11)
The reaction of formula (11) proceeds quickly when the Al concentration is high and when the stirring is strong. When the Al concentration in the molten steel is lowered, the reaction of the formula (11) is further reduced in speed, so that the stirring needs to be made stronger. However, unlike an ordinary ladle refining device, stirring cannot be freely controlled in the RH vacuum degassing device. In order to increase the stirring in the vacuum chamber, a method of increasing the stirring by the reflux gas with a higher vacuum is the simplest.

  From the above, when the ratio (Q / V) is a certain value, the reaction of the formula (8) is suppressed and the reaction of the formula (7) is advanced preferentially and efficiently according to the Al concentration in the molten steel. It is important to (i) control the oxygen absorption rate and (ii) increase the agitation in the vacuum chamber.

  Several methods are conceivable to realize this, but the simplest method is to continuously change the atmospheric pressure during the heat treatment of the molten steel. In general, oxygen is blown at a constant atmospheric pressure, but it is effective to appropriately change the atmospheric pressure.

  However, in these methods, it is important to optimize the balance of each speed process, and it is difficult to estimate the conditions. Therefore, the degree of vacuum during the heat treatment of the molten steel was controlled, and the reaction efficiency and the refractory wear rate were investigated.

In the investigation, an RH vacuum degassing apparatus for treating 300 tons of molten steel was used, and oxygen was blown from the top blowing lance onto the surface of the molten steel. At this time, the ratio (V / Q) of the ring flow rate Q (ton / min) and the top blowing oxygen flow rate V (Nm ³ / min) is 0.35 to 0.42 (Nm ³ / ton). Adjusted. The Al concentration in the molten steel was set to 0.013% or more and 0.095% or less after the molten steel heat treatment.

  Furthermore, the degree of vacuum of the vacuum chamber during oxygen blowing was changed in several patterns, and the reaction efficiency and the refractory life were measured. In addition, a condition in which the atmospheric pressure during the heat treatment, which is a general treatment condition, was constant at 8 kPa was set as a reference condition (a). In addition, since the following arrangement was performed for the measured values, both the reaction efficiency index and the refractory life index under the reference condition A are 1.

Reaction efficiency index = {(amount of added oxygen gas reacted with Al) / (total amount of added oxygen gas)}
/ (Reaction efficiency of standard condition i)
Refractory life index = (Abrasion rate under standard conditions a) / (Abrasion rate under each condition)
FIG. 1 is a graph showing the atmospheric pressure control patterns a to f in the vacuum tank during the molten steel heat treatment time together with the reference condition A, and FIG. 2 is a graph showing the atmospheric pressure control patterns g to f in the vacuum tank during the molten steel heat treatment time. It is a graph which shows j. In addition, since it is not appropriate to use the length of the treatment time because the molten steel heat treatment time varies depending on the treatment, the molten steel heat treatment time is determined as follows: dimensionless time = actual time {start of heating (start of oxygen top blowing) Elapsed time} / {total molten steel heat treatment time (pure oxygen top blowing time)}. In addition, the molten steel heat processing time in this investigation is 5 to 12 minutes.

  FIG. 3 is a graph showing the reaction efficiency index and the refractory life index obtained by the atmospheric pressure control patterns a to j shown in FIGS. The reaction efficiency index and the refractory life index are data when 50 times of treatment are performed.

  As shown in the graph of FIG. 3, the atmospheric pressure control patterns a and h with the atmospheric pressure kept constant were close to the reference condition A. On the other hand, the atmospheric pressure control patterns e and f in which the atmospheric pressure was kept low and constant had a reaction efficiency lower than that of the reference condition A, but the wear of the refractory was suppressed. This is because the reaction of the formula (6) is suppressed because the atmospheric pressure is low, and as a result, the reaction efficiency decreases due to the decrease of the reaction of the formula (7), but the suppression of the reaction of the formula (6) (8 It is considered that the wear of the refractory was suppressed by suppressing the reaction of the formula. However, the reaction efficiency is low and not necessarily suitable for actual operation.

  On the other hand, the atmospheric pressure control patterns b, c, d in which the atmospheric pressure is changed in stages have high reaction efficiency and a low refractory wear rate. On the other hand, regarding the atmospheric pressure control patterns g, i, and j in which the atmospheric pressure is similarly fluctuated stepwise, the atmospheric pressure control pattern g has little effect, and the atmospheric pressure control patterns i and j have a slightly improved life index. The reaction efficiency is low. This indicates that although the stepwise change of the atmospheric pressure control pattern is effective, there are optimum conditions for the atmospheric pressure control pattern.

  Furthermore, the atmospheric pressure control patterns b, c, and d obtained the highest effect under the present experimental conditions from the graph shown in FIG. Therefore, it can be considered that the optimum condition exists between the atmospheric pressure control pattern b and the atmospheric pressure control pattern d, and the three regions A and B set by the atmospheric pressure control pattern b and the atmospheric pressure control pattern d are set. , C at the same time is considered effective. The atmospheric pressure control patterns b, c, and d have different transition patterns to the regions A, B, and C, but a high effect can be obtained by passing the pressure regions in a certain time region through A, B, and C. From this, it can be seen that passing through all three regions A, B, and C is the most important condition, and the influence of the transition pattern between the regions A to C is small. Furthermore, in the atmospheric pressure control pattern f, h, i, j, since the effect cannot be obtained in one of the three regions A, B, C, or two passes, the three regions A, B, It can be seen that it is effective to satisfy all of C.

  From the above results, (a) it is necessary to control the atmospheric pressure as the process proceeds rather than controlling the atmospheric pressure to be constant, and (b) optimal for changing the atmospheric pressure control pattern as the process proceeds. It can be seen that the conditions exist, and (c) the optimum conditions are the atmospheric pressure control patterns b, c, d.

  And the conditions under which the atmospheric pressure control patterns b, c, d, that is, the balance of the formulas (6) to (8) and (11) can be optimized and high reaction efficiency and wear suppression can be simultaneously achieved are shown in FIG. The pressure in the atmosphere of the vacuum chamber during the heat treatment is controlled so that all of the regions A, B and C in the graphs 1 and 2 pass, and this is expressed by the equations (1) to (3). . However, the ratio (V / Q) is 0.2 or more and 0.5 or less, and the Al concentration in the molten steel is 0.01% or more and 0.1% or less.

A1 ≦ t / T ≦ A2: 5.3 kPa ≦ P ≦ 13 kPa (1)
A3 ≦ t / T ≦ A4: 2.7 kPa ≦ P ≦ 4.7 kPa (2)
A5 ≦ t / T ≦ A6: P ≦ 1.3 kPa (3)
However, A1 = 0.2, A2 = 0.55, A3 = 0.6, A4 = 0.75, A5 = 0.8, A6 = 1.0, and t is heating start (oxygen top blowing start) , T is the total heat treatment time (total oxygen top blowing time) (min), and P is the pressure in the vacuum chamber (kPa).

  Furthermore, as a result of arranging the conditions for obtaining the maximum values in the variations of the atmospheric pressure control patterns b, c, d in the graph of FIG. 3, the graph of FIG. 4 was obtained. If the expressions (1) to (3) are satisfied, the effect shown in the graph of FIG. 3 can be obtained. To further enhance the effect, the expressions (4) and (5) are satisfied from the graph shown in FIG. Is desirable.

A3 = 0.74 (V / Q) ^-0.12 (4)
A5 = 0.96 (V / Q) ^-0.95 (5)
As described above, since the ratio (V / Q) also affects the generation of surplus oxygen, by strictly controlling the pressure change in the atmosphere of the vacuum chamber according to the ratio (V / Q), It shows that the effect is further increased.

  In the present embodiment, the atmospheric pressure other than the heat treatment is not particularly limited, but it is essential that the atmospheric pressure during the heat treatment satisfies the expressions (1) to (3). It is desirable to satisfy both 4) and 5). Moreover, when the atmospheric pressure before heat processing is less than 5.3 kPa, it is desirable to raise to the pressure of 5.3 kPa or more before heat processing.

Control of the atmospheric pressure inside the vacuum chamber may be performed by a method of controlling the operation of the exhaust device of the vacuum chamber, a method of adjusting the amount of inert gas introduced into the vacuum chamber or the exhaust system, and the like.
The change to each atmospheric pressure is exemplified by the following procedure. The required temperature rise is obtained from the target temperature of the molten steel and the current temperature measurement value. The current temperature can be measured using, for example, a consumable thermocouple. Once the required temperature rise is determined, the required total oxygen amount and Al amount are determined accordingly. The relationship between the oxygen amount and the temperature increase amount may be obtained based on actual results. Since the equipment top blowing oxygen flow rate is known, the total oxygen top blowing time is calculated. This is T. Next, the atmospheric pressure is controlled so as to satisfy the expressions (1) to (3) using the ratio (t / T) of the actual processing time t and T.

  Thereby, when heating molten steel using oxygen gas in an RH-type vacuum degassing apparatus having two dip tubes and a vacuum tank, the life of the refractory in the vacuum tank is improved without reducing the heating efficiency. It becomes possible to plan.

  In addition, although the heating method of the molten steel which concerns on this invention mentioned above is a technique which heats molten steel using Al and oxygen gas, molten steel contains other components, for example, C, Si, Mn, Ti, etc. Can be applied, and does not depend on its concentration. However, if the Cr content exceeds 10% and the Ni content exceeds 15%, the preferential oxidation of Cr, Ni, etc. also proceeds, so the Cr content of the molten steel is 10% or less and the Ni content is 15% or less. It is desirable that Moreover, it is applicable also to the molten steel which performed the ultra-low carbonization process by vacuum decarburization before the heat processing of molten steel.

  Move the molten steel after the converter steel having the molten steel amount and composition shown in Table 1 to an RH vacuum degassing apparatus that processes 300 tons of molten steel, or an RH vacuum degassing apparatus that processes 90 tons of molten steel, First, molten steel heat treatment was performed by an RH vacuum degassing apparatus, and then vacuum degassing treatment and component adjustment were performed, and samples 1 to 25 were obtained.

  The molten steel components and the heat treatment time shown in Table 1 showed average values under each treatment condition. In the table, A represents A1 in the formula (1) of the present invention, B represents A3 in the formula (2) of the present invention, and C represents A5 in the formula (3) of the present invention. The reaction efficiency index is an average value, and the refractory wear index is a value after about 50 treatments.

Note that samples Nos. 2, 5, 14, and 17 in Table 1 were obtained by performing vacuum decarburization with an RH vacuum degasser and then performing molten steel heat treatment after adding Al to the molten steel.
The reaction efficiency index and the refractory wear index were determined for the samples Nos. 1 to 25. The results are also shown in Table 1.

  As shown in Table 1, according to the present invention, it can be seen that the reaction efficiency is high and refractory wear can be suppressed at the same time. On the other hand, in the comparative example, it can be seen that the refractory wear suppression effect is hardly recognized and the reaction efficiency is not improved.

It is a graph which shows the atmospheric pressure control pattern in molten steel heat processing time. It is a graph which shows the atmospheric pressure control pattern in molten steel heat processing time. It is a graph which shows the reaction efficiency index | exponent and refractory life index | exponent which were obtained by the atmospheric pressure control pattern. It is a graph which shows the relationship between (V / Q) and the dimensionless processing time of atmospheric pressure switching.

Claims (1)

In an RH vacuum degassing apparatus including two dip tubes and a vacuum tank, oxygen gas is sprayed onto the surface of the molten steel having an Al concentration of 0.01% by mass or more and 0.1% by mass or less. A method of heating molten steel by reacting with blown oxygen gas, the ratio of the flow rate V (Nm ³ / min) of the blown oxygen gas to the ring flow rate Q (ton / min) of the molten steel (V / Q) When the (Nm ³ / ton) is 0.2 or more and 0.5 or less, the heating of the molten steel is characterized in that the pressure inside the vacuum chamber is controlled according to the equations (1) to (3). Method.
A1 ≦ t / T ≦ A2: 5.3 kPa ≦ P ≦ 13 kPa (1)
A3 ≦ t / T ≦ A4: 2.7 kPa ≦ P ≦ 4.7 kPa (2)
A5 ≦ t / T ≦ A6: P ≦ 1.3 kPa (3)
However, A1 = 0.2, A2 = 0.55, A3 = 0.6, A4 = 0.75, A5 = 0.8, A6 = 1.0,
t: Elapsed time from start of heating (start of oxygen top blowing) (min)
T: Total heat treatment time (total oxygen blowing time) (min)
P: Pressure in the vacuum chamber (kPa)
It is.

Images