KR101510506B1 - Twin roll strip casting method for reducing scum formation - Google Patents

Abstract

The present invention relates to a method of manufacturing a thin plate using a twin roll type thin plate casting machine, and more particularly, to a twin roll thin plate casting method capable of reducing the occurrence of scum on the surface of a molten metal.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a twin roll type casting method,

The present invention relates to a method of manufacturing a thin plate using a twin roll type thin plate casting machine, and more particularly, to a twin roll thin plate casting method capable of reducing the occurrence of scum on the surface of a molten metal.

In general, a strip casting apparatus refers to a facility for continuously supplying molten steel between a pair of rotating casting rolls and continuously producing a thin plate of several mm thickness from the molten steel.

A method of manufacturing a thin plate by using the strip casting apparatus is characterized in that a molten steel supplied through a pair of casting rolls rotated in opposite directions while being cooled by cooling water is supplied to a molten pool surrounded by an edge dam The molten paste is solidified by contact with the surface of the casting roll to form a thin solidification cell. These solidification cells are integrated at a recent contact point and are continuously cast to a constant thickness to produce a thin plate.

On the other hand, it is very important that the austenitic stainless steel produced by such a twin roll thin sheet casting process is manufactured to have excellent surface quality.

However, oxidizing inclusions such as Al ₂ O ₃ may be included in the molten steel, and such molten steel may solidify during the twin roll type thin plate casting process, and oxidized inclusions may be generated as the molten steel surface of the molten steel comes into contact with the atmosphere. Since such an oxidative inclusion is smaller in specific gravity than molten steel, it floats on the surface of the molten pool to form a kind of oxide film. As a result, as the molten steel flow in the molten metal surface becomes less smooth as the casting process progresses, scum).

As a result, such scum may adhere to the casting roll at the time of casting and may enter the solidifying cell, thereby causing surface defects of the cast thin plate or causing cracks to be generated.

Since scum is inevitably generated through processes such as deoxidation of molten steel, temperature control, and injection of ferroalloy, scum generation itself can not be prevented. Therefore, it is necessary to minimize the generation of oxidative inclusions that may be contained in molten steel.

Conventionally, as a method for minimizing the occurrence of scum on the surface of the steel bath and preventing the inclusion of the steel into the cast steel, the molten steel pool is firstly oxidized by the oxygen contained in the gas contacting the surface of the molten metal pool. A method of providing a non-oxidizing gas to the space between the lower surface and the bath surface is provided. In this case, since the molten steel pool of the bath surface comes into contact with the non-oxidizing gas, the oxidation of the molten metal can be suppressed as much as possible. However, since the sump portion of the casting roll is opened for preheating the edge dam and the molten steel nozzle before casting, Scum is generated due to the oxidizing atmosphere. In order to prevent such scum from being mixed, a pair of scum weirs were installed in the width direction at a slight distance from both rolls on the surface of the molten bath to prevent the scum on the molten surface from being introduced into the meniscus. At this time, a pair of scum weirs float at a constant interval from both edge dams, and this interval is called a side scum outlet. By forming the scum outlet, the angle and the flow rate of the molten steel supply nozzle are set such that the molten steel flow between the roll and the scum weir flows to the both edge dam, so that the scum between the roll and the scum weir is discharged through the scum outlet. However, since there is a limit to making a strong flow by only the supply nozzle of molten steel, when the amount of scum is large, there is a problem that the scum is mixed due to the back flow. Thus, a gas nozzle was disposed so as to assist the discharge flow of the scum by blowing an inert gas onto the bath surface. However, when an inert gas is blown into the bath surface to assist the discharge flow of the scum, the flow can be stagnated because the flow of the inert gas in the center portion has to be divided in the right and left edge directions. In this case, .

As a technique for preventing scum entanglement, in the case of Patent Document 1, in order to prevent cracking of a cast steel by scum present on a hot-dip pan surface of a double-roll type thin plate casting machine, scum is artificially mixed However, if the scum is continuously generated, it is possible to produce a large number of coils that can not be produced in the initial stage of production.

In Patent Document 2, when casting per one coils of the casting member is finished, gas is blown from the gas nozzle to blow scum floating in the molten metal to one side of the meniscus, and the scum that is blown is blown onto the surface of the casting member, However, since the thin plate produced during the discharge of the scum in the middle of the coil and the coil can not be commercialized, there is a problem in that the rate of water drop is low.

Korean Patent No. 10-0605683 Korean Patent No. 10-0618002

An aspect of the present invention is to provide a twin roll type thin sheet casting method capable of suppressing the generation of scum on the surface of a molten steel during a twin roll type thin plate casting process to produce a high quality cast product.

According to an aspect of the present invention, there is provided a method of manufacturing a steel plate, comprising: preparing molten steel subjected to a refining process; The molten steel is injected into a molten pool formed by a pair of casting rolls rotating in mutually opposite directions and an edge dam sealing the side of the casting rolls and then the molten steel is solidified and discharged between the pair of casting rolls, ; And rolling the thin plate with a rolling roll to produce a hot rolled sheet,

And a Mn and Si component ratio (Mn / Si) of the molten steel subjected to the refining process is 7 or more. The present invention also provides a method for producing an austenitic stainless steel.

In the present invention, the generation of scum formed on the molten steel bath surface can be reduced by controlling the Mn and Si component ratios (Mn / Si) in the molten steel during the refining process, and austenitic stainless steel having excellent surface quality can be obtained It can be obtained through thin sheet casting process.

1 is a schematic view of a twin roll thin plate casting process.
Fig. 2 shows the result of the scum of the molten steel bath surface being mixed with the cast material and causing defects.
FIG. 3 shows the equilibrium temperature and the equilibrium phase of each composition when three kinds of oxide phases of Al ₂ O ₃ , MnO, and SiO ₂ are combined to form an equilibrium state.
4 shows a process for producing molten steel of austenitic stainless steel.
FIG. 5 shows the results of measuring the time taken for the grade of the bath surface scum to become 1 or 2 according to the composition ratio of Mn and Si in molten steel.
Fig. 6 shows a comparison of occurrence of scum on the molten steel bath surfaces of the examples and comparative examples.

Hereinafter, a process for producing an austenitic stainless steel using a twin roll thin plate casting method will be described with reference to Fig.

As shown in Fig. 1, the twin roll type thin plate casting process is a process in which the molten steel fed into a pair of casting rolls 1 rotating in mutually opposite directions through the injection nozzle 4 attached to the tundish 3, (8) is formed by hot rolling with the surface of the casting roll (1) while forming a molten pool surrounded by the casting roll (1) and the edge dam (7) Rolled, and then wound by a winding coil 11.

In order to prevent the molten steel from being oxidized by contact with the atmosphere, an upper surface of the molten pool is provided with a maniscus shield 5 for blocking external air. The maniscus shield 5 is provided with an inert gas And a meniscus weir which is immersed in molten steel to control the flow of molten steel.

In carrying out the twin roll thin plate casting process, although the inside of the molten pool is maintained as an inert gas by blocking air from the meniscus shield 5 and supplying an inert gas through the gas injection nozzle, practically 100% air It is possible to form an oxide inclusion in part or to include an oxidizing inclusion such as Al ₂ O ₃ -MnO-SiO ₂ already in the molten steel which is charged with the molten steel. Since these oxidative inclusions have a smaller specific gravity than molten steel, they float on the surface of the molten pool. When the molten steel flows smoothly as the casting process proceeds, the bath surface is covered with scum, which is an oxidative inclusion.

If such scum is adhered to the casting roll 1 and mixed into the solidifying cell, it may cause defects in the casting plate material, and this phenomenon can be confirmed in FIG.

In the case of the austenitic stainless steel produced by the twin roll thin plate casting process, excellent surface quality characteristics are required. However, as described above, the inclusion of scum formed on the molten steel bath surface during the casting process causes surface defects or cracks There is a problem that arises.

Accordingly, the present inventors have found that a method of manufacturing an austenitic stainless steel by a twin roll thin plate casting process, a method capable of suppressing the generation of scum, which is an oxidative inclusion, on the surface of the molten steel supplied to a casting facility through a refining process , And that scum generation on the molten steel bath surface can be reduced by controlling the composition ratio (Mn / Si) of Mn and Si in the molten steel in the refining process, leading to the present invention.

Hereinafter, the twin roll type thin plate casting method of the present invention will be described in detail.

The present invention relates to a method of manufacturing a steel pipe, comprising: preparing molten steel subjected to a refining process; The molten steel is injected into a molten pool formed by a pair of casting rolls rotating in mutually opposite directions and an edge dam sealing the side of the casting rolls and then the molten steel is solidified and discharged between the pair of casting rolls, ; And rolling the thin plate with a rolling roll to produce a hot rolled sheet,

The Mn and Si component ratios (Mn / Si) of the molten steel subjected to the refining process are preferably 7 or more.

The present invention preferably controls the components of Mn and Si, which affect the formation of oxides among the molten steel components, in order to reduce scum on the molten steel bath surface during the casting process.

For this purpose, the time taken for the degree of the scum of the tepe according to the composition ratios of Mn and Si in the molten steel components to be 1 or 2 grades was measured and the correlation was observed (see FIG. 5).

Generally, in order to control the quality of a steel sheet to be manufactured, the grade of the pan surface scum is managed. First, the scum floats floating on the bath surface after the initial casting scum is produced is discharged with molten steel 2 to 3 times to partially remove the scum, and then a normal coil is formed. Thereafter, a normal coil is formed, Perform the discharge operation one more time. Through these operations, the time from when the amount of scum present on the surface of the tumbler reaches the grade 1 or 2, which is the level of the hot water surface, is measured.

As shown in Fig. 5, it can be seen that the longer the Mn / Si ratio is, the longer the time taken until the bath surface scum grade standard becomes 1 to 2 grade decreases. That is, when the Mn / Si ratio is controlled to 7 or more, the generation of scum on the molten steel bath surface can be effectively reduced, and the time required for securing the grade of the molten steel bath scum to one or two grades can be shortened to within a few minutes .

As described above, it can be seen that when the Mn / Si ratio is 7 or more, the time required for the scum grade to reach the grade 1 or 2 is very short. Thus, the time for the scum grade The decrease means that the amount of scum in the pan surface is reduced as a whole.

For example, in the case of an austenitic stainless steel 304 steel produced by a twin roll thin sheet casting process, the steel usually contains an average of 0.27 wt% Si and an average of 1.3 wt% Mn, so controlling the Mn / The Mn / Si component ratio can be increased to 7 or more by controlling the Mn content to 1.4 wt% or more, preferably 1.4-2.0 wt%, and reducing the Si content to 0.2 wt% or less.

As described above, the principle that scum generation is reduced by controlling the Mn / Si component ratio can be explained as follows. The scum, which has been continuously mentioned above, collectively refers to foreign substances such as oxides / nitrides floating on the bath surface during casting by the twin roll type thin plate casting process.

The origin of the scum generated on the bath surface can be seen as the floating of the inclusions in the molten steel and the reoxidation of the bath surface, which can be explained as the difference in composition between the inclusions and the scum. More specifically, the composition of the scum formed by the floating of the inclusion in the molten steel is mainly composed of Al ₂ O ₃ -MnO-SiO ₂ formed by the reaction of the oxidizing element with oxygen, and the composition of the scum formed by the reoxidation is Al ₂ O ₃ -MnO-SiO ₂ contains some Cr ₂ O ₃ -FeO oxides, often referred to as lower oxides. Generally, the composition of the bath surface scum is 20 to 30% Al ₂ O ₃ - 10 to 20% MnO - 40 to 50% SiO ₂ - 0 to 5% (Cr ₂ O _{3 -} FeO) , And these oxides of three kinds of Al ₂ O ₃ , MnO, and SiO ₂ have a ternary phase diagram as shown in FIG.

As shown in FIG. 3, Al ₂ O ₃ -MnO-SiO ₂ is found to be located at a position where a liquid phase and a solid phase are bounded. That is, the scum exists in a liquid phase depending on the composition of the scum, and exists in the middle boundary where it may coexist in the form of liquid + solid phase. Therefore, in the present invention, it is intended to utilize the principle that the composition of the bath surface scum oxide is reformed into a low melting point oxide to be liquefied, and such liquid materials are easily fused and the fused oxides are lifted up to the upper part by the difference in specific gravity with molten steel .

As described above, the bath surface scum in the scum 3 composition in order to modify a low-melting liquid phase oxide Al ₂ O ₃ -MnO-SiO ₂ as a ternary phase diagram is the direction to the left of oxide MnO and increase in the speed triangle or Al ₂ O ₃ , the liquefaction effect can be obtained by adjusting the composition in the direction in which the SiO ₂ oxide decreases. However, in order to control Al ₂ O ₃ , the Al component is not a component added separately in the production of molten steel but an impurity component mixed with other alloyed iron. Since it is a strong oxidizing element, an element forming an oxide So it is very difficult to control. Thus, in the present invention, the ratio of Mn / Si through control of the composition of Mn and Si is controlled to reduce the bath surface scum.

According to one embodiment of the present invention, the applicable stainless steel includes 0.06 to 0.075% of C, 0.035% or less of P, 0.01% or less of S, 8.0 to 18.4% or less of Cr, 8.0 to 8.3% %, Mo: not more than 0.5%, Cu: not more than 0.5%, N: 0.035 to 0.060%, the balance Fe and other unavoidable impurities.

In order to control the ratio of Mn / Si as described above, the Si content in the molten steel can be controlled, which can be performed in the refining process.

Typically, the process of making the austenitic stainless steel is through an electric furnace (EAF) - refining furnace (AOD) - component composition (LT) step followed by a tundish - strip casting , The Si content of the molten steel can be controlled in the step of performing the component composition (LT) in the process.

More specifically, referring to FIG. 4, molten metal produced by melting in an electric arc furnace (EAF), that is, molten metal in an electric furnace, is introduced into the loading ladle, the loading ladle is tilted, A part of the slag is removed and an electric furnace having the remaining slag removed from the discharge site is charged into the refining furnace. The molten steel is oxidized by decarburization in a refining furnace (AOD: Argon Oxygen Decarburization) to cause loss of molten steel components such as chromium and iron due to oxidation, and oxygen is blown into molten steel to remove carbon, (FeSi) as a deoxidizer is added together with a basic flux consisting mainly of calcium oxide (CaO) in order to reduce it again.

As described above, it is possible to control the Si content in the molten steel by adding alloyed iron such as iron (FeSi) added in the step of performing the component composition LT of the molten steel component.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

In the manufacturing process of the austenitic stainless steels through the EAF-AOD-component composition LT tundish-strip casting, Mn and Si (Mn / Si) were adjusted as shown in Table 1 below. At this time, it contains 0.065% of C, 0.028% of P, 0.0025% of S, 18.2% of Cr, 8.1% of Ni, 0.15% of Mo, 0.25% of Cu, 0.05% of N and the balance Fe and other unavoidable impurities And the molten steel was used.

Then, when molten steel adjusted to each component was supplied to the casting equipment and cast, the time for reaching grade 1 or 2 of the scum grade of the tumbling was measured and compared and analyzed.

division Mn (% by weight) Si (% by weight) Mn / Si ratio Tang mug scum 1 ~ 2 rating
Time to reach (minutes) Comparative Example 1 1.46 0.33 4.4 77 Comparative Example 2 1.29 0.29 4.5 82 Comparative Example 3 1.38 0.29 4.8 50 Comparative Example 4 1.37 0.30 4.6 48 Comparative Example 5 1.38 0.27 5.1 45 Comparative Example 6 1.30 0.26 5.0 40 Comparative Example 7 1.41 0.26 5.4 39 Comparative Example 8 1.41 0.24 5.8 25 Comparative Example 9 1.37 0.22 6.2 22 Inventory 1 1.40 0.20 7.0 11 Inventory 2 1.41 0.20 7.1 13 Inventory 3 1.40 0.20 7.0 11 Honorable 4 1.42 0.19 7.5 9 Inventory 5 1.43 0.19 7.5 10

As shown in Table 1, in the case of Comparative Examples 1 to 9 in which the composition ratio (Mn / Si) of Mn and Si in the molten steel was less than 7, the time required for the tumbling scum grade to reach the first and second grades was 20 to 85 Min. However, in the case of Examples 1 to 5, in which the Mn / Si ratio was 7 or more, it was confirmed to reach the desired grade 1 or 2 of scum grade within 13 minutes.

Fig. 6 shows the results of observing the bath surfaces of Comparative Example 5 and Inventive Example 2. Fig. 6 is a view showing the state of the tumbled scum of Comparative Example 5 at the same time point (13) after the start of casting (13 minutes) when the tumbling grade of Inventive Example 2 becomes 1 to 2 grade In the honor 2, 13 minutes after the start of the casting, the degree of the scum of the scum of the scum was 1 or 2, and the scum of the scum was reduced. In contrast, the scum of the scum of the scum At the branch, the grade of tangue scum was observed as 3 ~ 4 grade.

As a result, the Mn content in the molten steel is increased, the Si content is reduced, and the Mn / Si ratio is controlled to 7 or more, so that scum generation on the bath surface during the casting process can be reduced, An austenitic stainless steel excellent in surface quality can be produced.

1: casting roll
2: Ladle
3: Tundish
4: immersion nozzle
5: Meniscus Shield
6: Brush roll
7: Edge dam
8: Casting
9: Pinch roll
10: Water cooling device
11: Coil winding
12: Hot rolling mill
13: Load cell

Claims (4)

(Excluding 0%), S: not more than 0.01% (excluding 0%), Cr: 8.0 to 18.4%, Ni: 8.0 to 8.3%, Mo: 0.5% (Excluding 0%), Cu: not more than 0.5% (excluding 0%), N: 0.035 to 0.060%, the balance Fe and other unavoidable impurities;
Comprising the steps of: constituting the molten steel in such a manner that Mn and Si component ratios (Mn / Si) are 7 or more, Mn: 1.4 to 2.0% and Si: 0.2% or less (excluding 0%);
The molten steel having the constituent components is injected into a molten pool formed by a pair of casting rolls rotating in mutually opposite directions and an edge dam sealing the side of the casting rolls and then discharged while being solidified between the pair of casting rolls to form a thin plate step; And
And rolling the thin plate with a rolling roll to produce a hot rolled plate.
delete delete The method according to claim 1,
Wherein when the Mn / Si ratio of the molten steel is 7 or more, the time required for securing the scum grade of the molten steel bath surface to 1 to 2 grades is shortened to 15 minutes or less after the start of casting.

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