JPH04266470A - Method for continuous casting extremely low carbon steel - Google Patents

Abstract

PURPOSE:To stably obtain a cast slab for good quality product by changing operational condition in the case the prescribed correlation between an index showing producing quantity of non-metallic inclusion and an index showing leaned flowing degree is not satisfied. CONSTITUTION:Molten steel 4 executing reducing treatment for non-metallic inclusion is poured into a mold 1 through a tundish 2 and continuously drawn to obtain the cast slab having high cleanness. Then, the index showing the producing quantity of non-metallic inclusion in the molten steel 4 is obtd. as oxidizing degree in slag covering the molten steel 4 by using an inclusion measuring instrument 9. On the other hand, flowing condition of the molten steel in the mold 1 is investigated with thermometers 12-15, temp. difference calculating parts 16, 17, decision part 20. Then, the index showing the leaned flowing degree is obtd. and in the case the prescribed correlation between both indexes is not satisfied, the operational condition related to pouring of the molten steel 4 into the mold 1 is changed. By this method, lowering of the productivity due to the change of useless operational condition is cancelled.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is carried out in order to obtain high-quality product slabs with excellent cleanliness by reducing as much as possible the introduction of non-metallic inclusions contained in molten steel into slabs. Concerning a continuous casting method for ultra-low carbon steel.

0002

[Prior art] Carbon content is extremely low (0.00
3% or less) Ultra-low carbon steel usually contains 20 to 30% of unstable metal oxides in the slag that covers its surface during the tapping stage from a smelting furnace such as a converter or electric furnace. (FeO
, MnO), and these oxidize Al in the molten steel, forming non-metallic inclusions Al2O3 (alumina).
Therefore, if continuous casting is carried out using this directly, a product slab containing many non-metallic inclusions and low cleanliness will be obtained, making it difficult to ensure the specified quality. arise.

Therefore, in the continuous casting of ultra-low carbon steel, for example, as disclosed in Japanese Patent Application Laid-Open No. 1-301814, when the tapped steel from the smelting furnace is vacuum refined, it is necessary to An oxidation degree inhibitor such as MgO or CaO is added to the slag to suppress the oxidation reaction that occurs at the interface between the slag and molten steel, and a reduction treatment is performed to reduce the amount of nonmetallic inclusions produced.
By supplying the molten steel after this reduction treatment to continuous casting equipment, the amount of non-metallic inclusions mixed into the slab is reduced and the cleanliness is improved.

On the other hand, in continuous casting equipment that uses such ultra-low carbon steel, nonmetallic inclusions that enter the mold as molten steel is injected are carried into the slab that is pulled out from the mold. In order to prevent this and improve the quality of the product slab, we examine the flow condition inside the mold, and if the degree of flow deviation is large, it is determined that the amount of molten metal taken out is large, and the pouring nozzle is replaced or relocated. Changes are being made to the operating conditions related to molten steel injection, such as reducing the amount of molten steel injection, and reducing the amount of molten steel injection.

[0005] At this time, it is necessary to evaluate the flow state in order to find out the degree of unbalanced flow within the mold, but this evaluation method uses a large number of thermocouples embedded in both short sides of the mold. A method of measuring the temperature distribution in the depth direction at the mold and making an evaluation based on this result, and a method of measuring the melt level fluctuation that occurs inside the mold near both short sides of the mold, and based on a comparison of these. A method of evaluation has been widely adopted, and Japanese Patent Application Laid-Open No. 150762/1986 discloses that a thermocouple housed in a refractory case is inserted into the mold and faced to the pouring spout. The temperature of the molten steel flowing out from the spout is measured with a thermocouple, and the dynamic pressure of the molten steel flowing out from the spout is measured with a strain gauge attached to the supporting part of the case, and the flow is determined based on these measurement results. A method for evaluating the condition is disclosed, and furthermore, Japanese Patent Application Laid-Open No. 2-207955 detects the temperature difference between the inlet temperature and the outlet temperature of cooling water supplied to both short sides of the mold, A method for determining the degree of biased flow by comparing these is disclosed.

[0006]

However, in the continuous casting method for ultra-low carbon steel carried out as described above, the amount of non-metallic inclusions contained in the molten steel injected into the mold is reduced from the melting furnace. Despite changes in the processing conditions during the aforementioned reduction treatment performed on the tapped steel and the effects of various disturbances during this treatment, the operation of continuous casting equipment using this Since the procedure is to change the operating conditions based on the evaluation results of the flow state inside the mold, if there are many non-metallic inclusions in the molten steel, this effectively prevents them from being carried out into the slab. On the other hand, if there are few nonmetallic inclusions, unnecessary changes to operating conditions may be made based on the flow condition evaluation results, resulting in a decrease in the amount of molten steel injected. There was a problem in that productivity decreased due to careless replacement of the pouring nozzle.

The present invention has been made in view of the above circumstances, and it is possible to effectively prevent non-metallic inclusions from being taken out into slabs during continuous casting of ultra-low carbon steel, and to produce product castings with high cleanliness. It is an object of the present invention to provide a continuous casting method for ultra-low carbon steel, which makes it possible to stably obtain pieces, eliminates the risk of unnecessarily changing the operating conditions, and eliminates a decrease in productivity. purpose.

[0008]

[Means for Solving the Problems] The continuous casting method of ultra-low carbon steel according to the present invention provides ultra-low carbon steel that is produced by performing a treatment to reduce non-metallic inclusions during vacuum refining of tapped steel from a smelting furnace. In a continuous casting method for ultra-low carbon steel, in which molten steel is injected into a mold through a tundish and continuously pulled out from the mold to obtain a slab with high cleanliness, the molten steel is While extracting an index indicating the amount of nonmetallic inclusions produced in the molten steel as the degree of oxidation of the slag covering the surface of the molten steel,
The flow state of molten steel inside the mold is examined to obtain an index indicating the degree of unbalanced flow, and if a predetermined correlation is not satisfied between these two indexes, the operating conditions related to the injection of molten steel into the mold are determined. It is characterized by changing.

[0009]

[Operation] In the present invention, an indicator indicating the amount of nonmetallic inclusions formed inside the molten steel that has been subjected to nonmetallic inclusion reduction treatment after being tapped from the smelting furnace is coated on the surface of the molten steel. The degree of oxidation of the slag, that is, the amount of unstable metal oxides in the slag, is obtained, and the flow condition inside the mold is evaluated during the operation of continuous casting equipment using this molten steel. The operating conditions are changed only when a predetermined correlation determined based on operational results is not satisfied between these two indicators.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to drawings showing embodiments thereof. FIG. 1 is a schematic diagram showing the implementation state of the continuous casting method for ultra-low carbon steel according to the present invention (hereinafter referred to as the method of the present invention).

Reference numeral 1 in the figure is a mold for a slab, which has a cylindrical shape with a rectangular cross section and is installed with openings at both ends facing upward and downward.
A cross section along the long side of the rectangle is shown. Molten steel 4 is injected into the mold 1 from a tundish 2 placed above the mold 1 through an immersion nozzle 3. The immersion nozzle 3 has a pair of outlet ports 3a and 3b that open toward both short sides of the mold 1, respectively, in order to uniformly pour molten metal into the mold 1 having a rectangular cross section.
It is a two-hole type nozzle with a
The molten steel 4 flowing out from these outlets 3a, 3b toward both short sides of the mold 1 is cooled by contact with the water-cooled inner wall of the mold 1, and solidifies from the outside as it goes downward. The resulting slab 5 is covered with a solidified shell, and is continuously drawn downward from the lower opening of the mold 1.

Molten steel 4 in the tundish 2 is supplied from a ladle 6 located above the tundish 2. This ladle 6 stores therein molten steel 4 of ultra-low carbon steel produced in the converter 7, and is transported to the upper part of the tundish 2 by a transport means such as a crane (not shown).

A suction type vacuum refining device 8 is installed in the middle of the transport path of the ladle 6 from the converter 7 to the tundish 2, and the molten steel 4 stored in the ladle 6 and transported is is sucked up into the vacuum tank 80 in this vacuum refining device 8 through a dipping tube inserted from the top, and
During circulation, components are degassed, decarburized and stirred,
After being subjected to temperature equalization treatment, it is refluxed into the ladle 6.

A predetermined amount of CaO is added to the vacuum chamber 80 of the vacuum refining apparatus 8 in order to suppress the formation of non-metallic inclusions in the molten steel 4 circulating therein. This Ca
After O flows back into the ladle 6 together with the molten steel 4, it floats to the surface of the molten steel 4, forms a solidified slag with low fluidity between it and the slag covering the surface, and is contained in the slag. FeO
, MnO, SiO2, and other unstable metal oxides and Al contained in the molten steel 4, and non-metallic inclusions (Al2O3) generated by this reaction.
acts to suppress the production of Note that the addition of CaO is
The process is carried out directly in the ladle 6 during and after tapping the steel from the converter 7, and the CaO added at the time of tapping and the CaO added in the vacuum chamber 80 are Due to the synergistic effect, the molten steel 4 conveyed onto the tundish 2 becomes high-quality molten steel with a low content of nonmetallic inclusions.

The amount of nonmetallic inclusions contained in the molten steel 4 is measured by an inclusion amount measuring device 9. This inclusion amount measuring device 9
The slag covering the surface of the molten steel 4 is transferred to the tundish 2.
A sample is collected before being supplied to the slag, and the collected slag is analyzed to determine the weight percent of FeO and MnO contained in the slag to the entire slag, in other words, the degree of oxidation of the slag, which indicates the amount of nonmetallic inclusions. This is determined as an inclusion index, and the result is given to the determination unit 20 that determines the suitability of the operating conditions in the above-mentioned continuous casting equipment.

FIG. 2 is a graph showing the relationship between the amount of CaO added into the vacuum chamber 80 of the vacuum refining apparatus 8 and the change in the inclusion index in the molten steel 4 obtained thereby. As shown in this figure, the inclusion index decreases as the amount of CaO added in the vacuum chamber 80 increases, indicating that the addition of CaO is effective in suppressing the formation of nonmetallic inclusions in the molten steel 4. I understand.

FIG. 3 also shows the number of inclusions contained in the product slab obtained by pouring molten steel 4 having various inclusion indicators into the mold 1 and performing continuous casting under the same conditions. This is a graph showing the results of the investigation. In the figure, ● indicates the case where inclusion suppression treatment was performed by adding CaO into the vacuum chamber 80 of the vacuum refining device 8, and ○ indicates the case where inclusion suppression treatment was performed. Each shows what was not done. It is clear from this figure that the number of inclusions in the product slab can be significantly reduced by using the suppressed molten steel 4. Note that the addition of CaO has the effect of spheroidizing nonmetallic inclusions and promoting the floating of the molten steel 4 to the surface, and also suppresses the reaction between the molten steel 4 and the slag covering the surface of the molten steel 4, and further reduces the slag. There is no diluting effect, and the synergistic effect of these suppresses the formation of inclusions.

The mold 1 is provided with a cooling water passage for water cooling the inner wall around its entire circumference, and is located on the downstream side in the flow direction of the molten steel from the outlet ports 3a and 3b of the immersion nozzle 3. Separate water passages 10 and 11 are provided on both short sides of 1 to allow cooling water to flow from the bottom to the top.
is formed. These water passages 10 and 11 are equipped with inlet thermometers 12 and 13 that detect the inlet temperature of the cooling water supplied to each channel, and outlet thermometers 14 and 15 that also detect the outlet temperature. It has been done. The temperature measurement results of the inlet thermometer 12 and the outlet thermometer 14 of one of the water passages 10 are given to the first temperature difference calculation unit 16, and The temperature measurement results of the exit side thermometer 15 are respectively given to the second temperature difference calculation unit 17, and as a result of cooling the molten steel 4 on both short sides of the mold 1, there is a difference between the inflow and outflow sides of each cooling water. The resulting temperature difference is calculated by these temperature difference calculating sections 16 and 17.

The calculation results of the first and second temperature difference calculation units 16 and 17 are sent to the determination unit 2 at predetermined sampling intervals.
0, and the determination unit 20 includes both calculation units 16
, 17 are compared with each other, and the flow state of the molten steel 4 in the mold 1 is evaluated based on the comparison result, and an index indicating the degree of bias of the flow toward either of the short sides. (one-sided flow index) is determined as described below.

[0020] The evaluation of the flow state in the determination unit 20
The first and second temperature difference calculation units 16
, 17, the temperature difference T1 between the inlet and outlet sides of the cooling water
, T2 using the evaluation index H obtained by applying the following equation. H=k・(T1 −T2)×100(%)…(
1) Note that k in this formula is the reciprocal of the average value of both temperature differences T1 and T2, that is, k=2/(T1 +T2)
...(2), but may be an appropriate constant.

For example, one outlet 3a of the immersion nozzle 3
When a blockage occurs in the outlet 3a, the amount of molten steel 4 flowing out from the outlet 3a decreases, and the amount flowing out from the other outlet 3b relatively increases. Due to the imbalance in the amount of molten steel 4 flowing out from the mold 1, a biased flow toward the outlet 3b occurs, and the molten steel 4 flowing out from the outlet 3b flows into the mold 1.
This facilitates the introduction of non-metallic inclusions into the slab 5.

The temperature differences T1 and T2 occur as a result of heat exchange between the cooling water supplied to both short sides of the mold 1 and the molten steel 4 inside the mold 1, and the above-mentioned unbalanced flow occurs. If this occurs, the temperature difference T1 obtained on the side facing the outlet 3a will be smaller than the temperature difference T2 obtained on the side opposite the outlet 3b, and the evaluation index obtained by the above formula (1) H is a sign indicating the direction of biased flow;
It has an absolute value corresponding to the degree of occurrence of biased flow, and the degree of non-metallic inclusions brought into the slab 5 can be determined from this evaluation index H.

However, the evaluation index H obtained in this way is
Since the value is influenced by the relative positional relationship between the immersion nozzle 3 and the mold 1, in the evaluation of the flow state in the determination section 20, the evaluation index H generated within a predetermined time is used instead of the evaluation index H itself. variation, that is, standard deviation σH
Alternatively, the maximum fluctuation width Hmax is calculated and used as an index (unilateral flow index) indicating the degree of uneven flow. Also,
When large non-metallic inclusions contained in the molten steel 4 in the tundish 2 try to flow into the mold 1, they will be forced out after once blocking the outlet 3a or 3b of the submerged nozzle 3. At this time, the evaluation index H obtained by equation (1) varies greatly. This also means that flow evaluation, which is performed for the purpose of preventing inclusions from being brought into the slab 5, should be performed not by the evaluation index H itself, but by the standard deviation σH or maximum fluctuation width Hmax that indicates the fluctuation of this index. It is shown that.

[0024] Figure 4 shows the one-sided flow index σH calculated at each operation and the amount contained in the product slab obtained at each operation by performing multiple operations using molten steel 4 containing a certain amount of nonmetallic inclusions. It is a graph showing the results of examining the correlation with the number of inclusions. As is clear from this figure, the number of inclusions in the product slab increases with the increase in the one-sided flow index σH obtained as a result of flow evaluation in mold 1, and this one-sided flow index σH is maintained at an appropriate level. This shows that it is possible to modify the product slab.

However, the appropriate value of this one-sided flow index σH varies depending on the amount of nonmetallic inclusions contained in the molten steel 4 injected from the tundish 2 into the mold 1. 4, the amount of inclusions carried into the slab 5 can be kept small even in flow conditions where a relatively large one-sided flow index σH is obtained, and a product slab of good quality can be obtained. On the other hand, when molten steel 4 containing many non-metallic inclusions is used, it is necessary to keep the one-sided flow index σH small and to suppress the occurrence of unbalanced flow within the mold 1 as much as possible.

The determination unit 20 recognizes the amount of inclusions in the molten steel 4 supplied to the mold 1 based on the inclusion index given from the inclusion amount measuring device 9, and also recognizes the amount of inclusions in the molten steel 4 supplied to the mold 1. The degree of unbalanced flow in the mold 1 is recognized by the unbalanced flow index σH, and if a predetermined correlation is not satisfied between these two indices, the operating conditions are inappropriate, and the predetermined quality of the product slab is not satisfied. It is determined that it is difficult to secure.

FIG. 5 is a graph showing the relationship between the one-sided flow index σH obtained based on many operational results and the inclusion index, and ○ in this figure indicates the unit of the obtained product slab. The number of inclusions per weight is less than a predetermined amount, for example, 0.09 pieces/ton, and the desired quality standards are met, and ● indicates those that fail to meet the quality standards. ing. The determination unit 20 stores this correlation, and determines the current state based on the inclusion index given from the inclusion amount measuring device 9 and the one-sided flow index σH obtained as described above from the detection results of the temperature difference calculation units 16 and 17. The operating conditions of the equipment are determined on this graph, and the suitability of the current operating conditions is determined based on whether or not the operating conditions are within the appropriate range indicated by hatching in the diagram. Note that the criteria used in the determining section 20 differ depending on the quality standards required for the product slab.

The determination result by the determination unit 20 is outputted to the display unit 21, for example, and a predetermined display is made on the display unit 21, so that the operator can change the position of the immersion nozzle 3, change the position of the immersion nozzle 3, etc. It is used to prompt changes in operating conditions related to molten steel injection, such as replacement or reduction of the amount of molten metal poured from the immersion nozzle 3.

In this example, the flow inside the mold 1 is evaluated using the temperature difference between the inlet and outlet sides of the cooling water on both short sides of the mold 1, but the flow evaluation method is as follows. The present invention is not limited to this, and other evaluation methods such as fluctuations in the molten metal level on both short sides of the mold 1 may be adopted. Although the formation of is suppressed, other oxidation degree inhibitors such as MgO may also be used.

Furthermore, in this embodiment, an example of application to continuous slab casting equipment using a mold 1 with a rectangular cross section has been described, but it goes without saying that the method of the present invention can also be applied to other continuous casting equipment. .

[0031]

Effects of the Invention As detailed above, in the method of the present invention, an inclusion index indicating the amount of nonmetallic inclusions is determined for molten steel that has been treated to reduce nonmetallic inclusions before being poured into a mold, and
In addition, a one-sided flow index indicating the degree of unbalanced flow of this molten steel in the mold is obtained, and the suitability of the operating conditions is determined based on whether a predetermined correlation is satisfied between the two indicators, and changes in the operating conditions are made according to this. Because of this, proper operating conditions can always be obtained regardless of the amount of non-metallic inclusions in the molten steel, and the introduction of non-metallic inclusions into the slab can be effectively prevented, resulting in a high-quality product with excellent cleanliness. The present invention has excellent effects, such as being able to stably obtain slabs and eliminating the drop in productivity caused by unnecessary changes in operating conditions.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the implementation state of the method of the present invention.

FIG. 2 is a graph showing the effect of reducing inclusions by adding CaO.

FIG. 3 is a graph showing the effect of inclusion suppression treatment on the quality of product slabs.

FIG. 4 is a graph showing the relationship between the degree of unbalanced flow within the mold and the quality of the product slab obtained thereby.

FIG. 5 is a diagram showing an example of criteria for determining suitability of casting conditions in the method of the present invention.

[Explanation of symbols]

1 Mold 2 Tundish 3 Immersion nozzle 4 Molten steel 5 Slab 6 Ladle 7 Converter 8 Vacuum refining equipment 9 Inclusion amount measuring device 10,11 Waterway 12, 13 Inlet side thermometer 14, 15 Outlet side thermometer 20 Judgment section 80 Vacuum chamber

Claims (1)

[Claims] [Claim 1] Using molten steel of ultra-low carbon steel that has been subjected to a treatment to reduce nonmetallic inclusions during vacuum refining of tapped steel from a smelting furnace, this is injected into a mold through a tundish,
In a continuous casting method for ultra-low carbon steel, which is performed to obtain a highly clean slab by continuously drawing it from the mold, an index indicating the amount of non-metallic inclusions produced in the molten steel is determined based on the surface of the molten steel. While extracting the oxidation degree of the slag covering the mold, the flow state of the molten steel inside the mold is examined to obtain an index indicating the degree of unbalanced flow, and if a predetermined correlation between these two indexes is not satisfied, the mold 1. A continuous casting method for ultra-low carbon steel, characterized in that the operating conditions associated with the injection of molten steel into the steel are changed.