JP3505142B2 - Casting method of high clean steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting operation technique for producing a slab for producing a steel material having excellent cleanliness.

[0002]

2. Description of the Related Art In recent years, ultra-low carbon steel having excellent workability and containing C of 0.04% or less and containing Ti has been widely used for automobile outer plates, surface-treated steel plates and the like. .
The properties required for these ultra deep drawn materials are not only excellent in mechanical property values such as r value and elongation, but also in addition to cleanliness, especially the cleanliness of the surface layer that has been conventionally required,
It is required that the internal slab cleanliness, which is the starting point of cracks during processing, be excellent.

As a conventional means for improving the surface cleanliness of a cast slab,
For example, a technique for improving the surface cleanliness of a slab using an electromagnetic force has been reported, as disclosed in Japanese Patent Laid-Open No. 6-000606. Surface cleanliness is improved by imparting a uniform molten steel flow to the interface between the molten steel in the mold and the solidified shell independently of the discharge flow from the immersion nozzle. However, in the case of a thin plate material with severe workability, it not only improves the cleanability of the surface layer and prevents the occurrence of flaws, but also improves the cleanability of the inside of the slab,
It is important to improve the cleanliness of the inside of the slab, which is the starting point of cracks during processing, especially the upper surface integrated part of the bending die and vertical bending die continuous casting equipment.

[0004]

In order to improve the cleanliness of the inside of the slab, electromagnetic brake equipment is provided to prevent inclusions from penetrating into the inside of the slab, or the upper limit of the casting speed is set to levitate. Countermeasures such as lengthening the time are effective, but new equipment investment is required for molten steel flow control by the electromagnetic brake, and if the casting speed is limited, productivity will naturally deteriorate and internal cleanliness will improve. However, there is a problem that the cleanliness of the surface layer may not be improved. The present invention is to solve the above-mentioned problems, excellent slab surface layer cleanliness, and to provide a continuous casting operation technology for producing a slab excellent in slab internal cleanability. Has an aim.

[0005]

[Means for Solving the Problems] (1) When casting molten steel using a continuous casting facility equipped with an electromagnetic stirring device in a mold, the stirring condition of electromagnetic force to the molten steel in the mold is expressed by the following formula. When the molten steel injection amount index (Qc) into the mold from the molten steel distribution container per unit time and unit thrust obtained is smaller than 0.45, the electromagnetic force application time (T1) and the application stop time (T
2. A continuous casting method for molten steel, characterized in that intermittent stirring with a relationship of 2) with T1 / (T1 + T2) ≧ 0.7, and alternating stirring with 0.45 or more. Here, the specific gravity of molten steel is 7.0
[T / m ³ ] and the stirring thrust is the stirring thrust at a position 15 mm inside from the copper plate surface. Qc = [amount of molten steel injected per unit time from molten steel distribution container (t / min)] / [stirring thrust of electromagnetic stirrer (kN / m ³ )] (2) In-mold electromagnetic stirrer is installed When casting molten steel using the continuous casting equipment described above, the amount of molten steel injected into the mold from the molten steel distribution container per unit time and unit thrust is calculated by Index (Q
If c) is less than 0.45, intermittent stirring is performed. If c) is 0.45 or more, the alternating time interval (T3) of electromagnetic force is 3 seconds or more 1
A continuous casting method for molten steel, characterized in that alternating stirring is performed for 5 seconds or less. Here, the molten steel specific gravity is 7.0 [t / m ³ ] and the stirring thrust is the stirring thrust at a position 15 mm inside from the copper plate surface. Qc = [amount of molten steel injected from the molten steel distribution container into the mold per unit time (t / min)] / [stirring thrust of the electromagnetic stirrer (kN / m ³ )]

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.

In the casting method of the present invention, when casting a molten steel melted to a predetermined composition by a continuous casting facility, a refractory material is passed from a molten steel container through a molten steel distribution container such as a tundish through a refractory injection pipe. The molten steel is supplied into the water-cooled copper mold by the immersion nozzle of.

Generally, when no electromagnetic force is applied to the molten steel in the mold, the molten steel in the mold has a flow as shown in FIG.
By applying an electromagnetic force to this state, an external force is applied to the mold independently of the molten steel discharge flow 5 supplied from the immersion nozzle 1, a molten steel flow is generated, and continuous rotation is possible,
The molten steel in the mold 3 has a uniform flow as shown in FIG. 2, and the surface layer cleanliness is improved by the inclusion removing effect by the molten steel flow. However, a uniform swirl flow collides with the short piece,
It was found that the steady downward flow 7 was obtained, the reaching depth 11 of the downward flow was increased, and the floating of inclusions was hindered.

Then, the inventors of the present invention conducted research, and as a fundamental solution thereof, as shown in FIG. 3, discharge of molten steel supplied from the immersion nozzle 1 into the mold 3 by the electromagnetic stirring device 10 was performed. Flow of molten steel 9
Flow rate at the solidification interface necessary for improving surface cleanliness by applying electromagnetic force intermittently as shown in FIG. It was found that it is possible to reduce the reaching depth 11 of the downward flow and improve the cleanliness of the inside of the slab while ensuring the above.

However, when the amount of molten steel injected into the mold from the molten steel distribution container per unit time and unit thrust becomes large, the reaching depth of the downward flow during continuous stirring becomes the same as that during intermittent stirring, as shown in FIG. Therefore, the effect of improving the internal cleanliness of the cast slab due to the intermittent stirring is not observed. Here, the slab internal cleanliness index is a value obtained by dividing the number of slab internal inclusions by the number of slab internal inclusions during continuous stirring. This is because the electromagnetic force applied to molten steel per unit time decreases as the amount of molten steel injected from the molten steel distribution container into the mold per unit time and thrust increases, and changes in electromagnetic force affect molten steel flow. This is because the effect is small. Therefore, when the present researchers proceeded with further research, when an electromagnetic force was applied as shown in Fig. 5, when the molten steel injection amount per unit time and per unit thrust from the molten steel distribution container into the mold was large. However, it was found that the reaching depth of the downward flow was small, and as shown in FIG. 6, the internal cleanability of the slab was improved as compared with continuous stirring and intermittent stirring. This is because both continuous stirring and intermittent stirring have only one stirring direction, whereas alternating stirring reverses the stirring direction, and therefore the flow in the mold is different. This is because the short-side collision flow velocity is significantly smaller than that during continuous application or intermittent application, and thus the downward flow generated therefrom is small.

On the other hand, the relationship between the surface cleanliness of the slab and the average and time-averaged meniscus flow velocity in the width direction of the mold is 10 cm / s or more, and the surface cleanliness of the slab is secured. Here, the linear flaw occurrence rate is the linear flaw occurrence index (the linear flaw occurrence index is a value obtained by dividing one linear flaw by 1 m and dividing by the total coil length), and the mold width direction average and time average meniscus flow velocity are 0.
It is a value divided by the linear flaw generation index at m / min. As shown in FIG. 8, when alternating injection stirring is performed when the molten steel injection amount per unit time and per unit thrust from the molten steel distribution container is small, the mold width direction average and time average meniscus flow velocity become 10 cm / s or less. However, it becomes difficult to secure the cleanliness of the surface layer of the cast slab to a level at which there is no problem in terms of quality. Here, the time average during continuous stirring is an arbitrary time average, the time average during intermittent stirring is the time average of (T1 + T2), and the time average during alternating stirring is the time average of T3.

Therefore, in order to always produce a slab having excellent surface cleanliness and internal cleanliness regardless of the molten steel injection amount per unit time and thrust into the mold from the molten steel distribution container, In addition, when the molten steel injection amount index Qc per unit stirring thrust is 0.45 or less, intermittent stirring is performed, and when it is 0.45 or more, alternating stirring is performed. It was found that this makes it possible to achieve both high cleanliness of the surface layer of the slab and the inside.

At the time of intermittent stirring, there was a concern that the surface cleanliness would deteriorate due to the absence of electromagnetic force.
According to the study by the present inventors, under the same casting conditions, if the ratio of the application time; T1 and the stop time; T2 is controlled within the range shown in the following formula (2), the average in the mold width direction and Although the time-averaged meniscus flow velocity decreases to some extent,
As shown in 0, it was found that the cleanliness of the inside of the slab can be ensured while ensuring the cleanliness of the surface of the slab. Here, the slab surface layer cleanliness index is a value obtained by dividing the number of slab surface layer inclusions by the number of slab surface layer inclusions during continuous stirring. T1 / (T1 + T2) ≧ 0.7 (2) In this case, if T1 is too long, it will be in the same state as when continuously stirring and stirring, and the downward reaching depth of the molten steel becomes deep, so 10 seconds The following is desirable. If T2 is too short, the effect of stopping the electromagnetic stirring to prevent the formation of a steady flow is lost, and the descending flow velocity reducing effect is lost.

During alternating application, since there is an inertial flow immediately after reversing the stirring direction, as shown in FIG. 11, when the alternating stirring cycle T3 is less than 3 seconds, the cycle is too short and the average in the mold width direction and the time average are obtained. A meniscus flow rate of 10 cm / s or more cannot be obtained. On the other hand, if it exceeds 15 seconds, the surface flow velocity will be the same as when continuously stirring, and the reaching depth will be deeper, and as shown in FIG. 12, the internal cleanability of the slab will deteriorate. Therefore, it has been found that by setting the alternating stirring cycle T3 to 3 seconds or more and 15 seconds or less, the cleanliness of the inside of the slab can be ensured while ensuring the cleanliness of the surface of the slab.

When changing the application conditions in an actual machine,
The following delay times such as rise time and fall time of electromagnetic force can be considered, but as long as these times occur steadily,
In the present invention, there is no problem even if these times are included in T1 and T3.

[0016]

The present invention will be described in more detail based on the following examples. In order to produce Ti-added ultra-low carbon steel for automobiles, 300 tons of molten steel was melted in a converter and RH was used.
The composition was adjusted to the predetermined composition shown in (1) and cast in a vertical bending continuous casting facility. The results are shown in Table 2.

As is clear from Table 2, in the case of the cast product produced under the conditions of the present invention, the amount of inclusions in the surface layer is equivalent to that of continuous stirring while maintaining the cleanliness of the surface layer, while maintaining the cleanliness inside the cast product. It was possible to obtain an excellent slab. As shown in Table 2, in the cold-rolled sheet after hot-rolling or cold-rolling these slabs by a conventional method, continuous stirring was performed even if evaluated by linear flaws caused by inclusions visually determined. A surface quality similar to that of the case was obtained. It was also confirmed that the rate of cracking caused by inclusions was significantly lower than that of the continuously stirred cold-rolled sheet.

Further, in the present embodiment, an example of producing a slab for a thin steel sheet for an automobile has been described, but the essence of the present technique is to optimize the electromagnetic stirring application method in the mold. , Is to optimize the flow of molten steel in the mold, and is also effective when manufacturing slabs of other steel types such as thick plates and steel pipes.
There is no restriction on the type of applied steel.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

As described above, according to the present invention,
It is possible to provide a steel material having good formability and capable of producing a good workable thin steel plate in which linear defects due to inclusions in the steel material are less likely to occur. Not only can the cold rolled steel sheet be manufactured using the present steel material, but it is also possible to produce an electrogalvanized or galvannealed steel sheet after annealing, and further to manufacture a base plate of an organic coated copper steel sheet. Further, as long as the continuous annealing conditions are satisfied, it can be used as a steel material for continuous annealing hot-dip galvanizing alloy hot-dip galvanizing steel plate. Therefore, since it can be applied to a wide range of household appliances and automobiles, the effect on the industry is extremely large.

[Brief description of drawings]

FIG. 1 is a front view (a) and a plan view (b) showing a flow of molten steel without electromagnetic stirring in a mold.

FIG. 2 is a front view (a) and a plan view (b) showing a flow of molten steel when electromagnetic stirring is performed in a mold.

FIG. 3 is a front view showing the flow of molten steel during intermittent stirring (a).
And a plan view (b).

FIG. 4 is a diagram showing an application pattern of intermittent stirring.

FIG. 5 is a diagram showing an application pattern of alternating stirring.

FIG. 6 is a view showing a relationship between an electromagnetic stirring application pattern and a cast slab internal cleanliness index. The slab internal cleanliness index is a value obtained by dividing the number of slab internal inclusions by the number of slab internal inclusions during continuous stirring.

FIG. 7 is a graph showing the relationship between the average widthwise and time-averaged meniscus flow velocity in the mold and the linear flaw occurrence rate. The linear flaw occurrence rate is the linear flaw occurrence index (the linear flaw occurrence index is a value obtained by dividing one linear flaw by 1 m and dividing by the total coil length) when the mold width direction average and time average meniscus flow velocity are 0 m / min. It is the value divided by the linear flaw generation index of.

FIG. 8 is a diagram showing a relationship between a molten steel injection amount index per unit time and a unit thrust, and a mold width direction average and a time average meniscus flow velocity. The time average during continuous stirring is an arbitrary time average, the time average during intermittent stirring is the time average of (T1 + T2), and the time average during alternating stirring is the time average of T3.

FIG. 9 is a diagram showing a relationship between an applied pattern at the time of intermittent stirring and a slab internal cleanliness index.

FIG. 10 is a view showing a relationship between an applied pattern at the time of intermittent stirring and a slab surface layer cleanliness index. The slab surface layer cleanliness index is a value obtained by dividing the number of slab surface layer inclusions by the number of slab surface layer inclusions during continuous stirring.

FIG. 11 is a diagram showing the relationship between the alternating stirring cycle and the mold width direction average and time average meniscus flow velocity.

FIG. 12 is a view showing a relationship between an alternating stirring cycle and a cast slab internal cleanliness index.

[Explanation of symbols]

1 immersion nozzle 2 Mold powder 3 molds 4 Meniscus reversal flow 5 Short side upflow 6 Molten steel 7 Downflow 8 updraft 9 Molten steel flow 10 Electromagnetic stirrer 11 Depth of reaching downward flow T1 Electromagnetic force application time for intermittent stirring T2 Application stop time for intermittent stirring T3 Alternating time interval of electromagnetic force during alternating stirring

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-10-249496 (JP, A) JP-A-10-85914 (JP, A) JP-A-8-71716 (JP, A) JP-A-7- 9099 (JP, A) International publication 99/029452 (WO, A1) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/00 B22D 11/04 311 B22D 11/115

Claims (2)

(57) [Claims] 1. When casting molten steel using a continuous casting facility equipped with an in-mold electromagnetic stirrer, the stirring conditions of electromagnetic force to the molten steel in the mold are calculated by the following equations per unit time and unit thrust. When the molten steel injection amount index (Qc) from the molten steel distribution container per mold to the mold is less than 0.45, the relationship between the electromagnetic force application time (T1) and the application stop time (T2) is T1 /
A continuous casting method for molten steel, characterized in that intermittent stirring with (T1 + T2) ≧ 0.7 and alternating stirring with 0.45 or more are carried out. Here, the molten steel specific gravity is 7.0 [t / m ³ ] and the stirring thrust is the stirring thrust at a position 15 mm inside from the copper plate surface. Qc = [amount of molten steel injected from the molten steel distribution container into the mold per unit time (t / min)] / [stirring thrust of the electromagnetic stirrer (kN / m ³ )] 2. When casting molten steel using a continuous casting facility equipped with an electromagnetic stirring device in a mold, the stirring conditions of the electromagnetic force to the molten steel in the mold are calculated by the following formulas per unit time and unit thrust. If the molten steel injection quantity index (Qc) from the molten steel distribution container to the mold is less than 0.45, intermittent stirring,
In the case of 0.45 or more, a continuous casting method for molten steel, characterized in that the alternating time interval (T3) of electromagnetic force is 3 seconds or more and 15 seconds or less to perform alternating stirring. Here, the specific gravity of molten steel is 7.0 [t / m
³ ], the stirring thrust is the stirring thrust at a position 15 mm inside from the copper plate surface. Qc = [amount of molten steel injected from the molten steel distribution container into the mold per unit time (t / min)] / [stirring thrust of the electromagnetic stirrer (kN / m ³ )]