KR20140084721A - Twin Roll Strip Caster - Google Patents

Abstract

The present invention relates to a twin roll strip type strip casting machine having a gas discharging unit formed on a casting roll to prevent inert gas discharged from a molten steel pool from remaining between the casting roll and a solidified shell when molten steel is coagulated and pressed, wherein the twin roll type strip casting machine comprises: a pair of casting rolls rotating in opposite directions; an edge dam installed on both surfaces of the casting rolls to form a molten steel pool; a meniscus shield provided to prevent the molten steel pool from becoming in contact with air by covering the upper part of the molten steel pool; and a gas supplying hole which is arranged on the meniscus shield and provided to supply the inert gas to the surface of the molten steel pool. According to the present invention, when a steel having the high content of nitrogen such as super austenite stainless steel (S32050) and the likes is manufactured, the twin roll type strip casting machine can have an effect of producing an excellent cast-piece by efficiently removing inert gas remaining between the casting roll and the solidified shell such as nitrogen gas and the likes.

Description

Twin Roll Strip Caster < RTI ID = 0.0 >

The present invention relates to a twin roll type thin plate casting machine and more particularly to an apparatus and a method for effectively removing inert gas such as nitrogen gas remaining between a casting roll and a solidification shell during production of a steel sheet having a high nitrogen content such as super austenitic stainless steel (S32050) Rolled sheet metal casting machine.

Conventionally, in order to manufacture super-austenitic stainless steel, as shown in FIG. 1A, a slab S is manufactured through continuous casting and hot rolled to produce a thin plate having a thickness of 2 to 6 mm.

In this conventional continuous casting and hot rolling process, molten steel having undergone the steelmaking process and refined is transported in the ladle 110, and then flows into the tundish 120 from the ladle 110. At this time, when the tundish 120 is positioned between the ladle 110 and the mold 130, the tundish 120 serves as a buffer for flowing molten steel.

 The molten steel introduced into the tundish 120 is supplied to the mold 130 through the immersion nozzle disposed at the lower end of the tundish 120 and the molten steel supplied to the mold 130 is cross- And passes through a plurality of segments 140 disposed under the mold 130 in a rectangular shape to be cooled and turned into a slab S.

Thereafter, the slab S is reheated in the heating furnace 150 and subjected to rough rolling (rough rolling) 160 and finishing rolling (finish rolling) 170 to be changed into thin plates of a desired size by the customer, And finally wound in the form of a coil at the winder 190.

However, the production of super-austenitic stainless steels using the continuous casting and hot rolling processes does not have a serious problem until the step of producing the slab S through the continuous casting process, but a serious problem occurs in the hot rolling process.

That is, a crack is generated at an edge portion of the thin plate during hot rolling. If a crack is severely generated at the edge portion, a thin plate may not be used even if the portion is cut, and the productivity of super austenitic stainless steel To a very low level.

In order to solve such a problem, the operation conditions of the manufacture of the thin plate of the super-austenitic stainless steel are such that super-austenitic stainless steels generally have a high content of alloying elements and are poor in hot workability. Therefore, The phosphorus content is minimized to 0.0003% or less, and the reduction rate of the thin plate in the rough rolling is set to 20% or less.

However, such operating conditions are not easy to implement, and refining time is long, which causes manufacturing cost and production delay of the product.

Accordingly, recently, as shown in FIG. 1B, a method of manufacturing super-austenitic stainless steel through a twin roll thin plate casting process has been studied. Such a twin roll thin sheet casting process is a new process which is produced by casting a cast steel (S) having a thickness of 3 to 6 mm directly from the melt, and can omit the hot rolling process in the slab continuous casting process.

That is, molten steel is supplied from the ladle 210 to the tundish 220 in the same manner as the slab continuous casting method, but molten steel is directly introduced into the twin roll type thin plate casting roll 230 from the immersion nozzle of the tundish 220, The casting rolls 230 are cooled down and directly discharged to a lower portion of the pair of casting rolls 230 and finally cooled in the cooling equipment 240 and then wound in a winder 250 to be produced as a coiled product.

Therefore, there is an advantage that the manufacturing cost of the steel sheet is reduced as compared with the slab continuous casting method, and the internal quality of the cast steel is advantageous due to finer grain size and inclusion fineness even in the unbalanced state due to rapid solidification.

However, in the conventional twin roll type thin plate casting process, a dimple A, which is a concavo-convex shape by shot blasting or the like, is formed on the surface of the casting roll 230 as shown in FIG. 2A in order to produce a good cast product free from cracks, dents, And an inert gas such as nitrogen, argon or the like is supplied to the surface of the molten metal so as to prevent the molten steel from coming into contact with the outside air.

However, when the nitrogen [N ₂ ] content in the molten steel is 2000 ppm or more, as in super-austenitic stainless steel (S32050), nitrogen gas (G) supersaturated in the spool M for casting is discharged And is present between the dimple A of the casting roll 230 and the solidification shell B to increase the nitrogen gas pressure in accordance with the increase in temperature to generate the solidification shell lifting phenomenon C, Cracks (Y), dent (X) and the like on the cast steel as shown in Fig.

Therefore, in order to produce super-austenitic stainless steels having excellent quality using the twin roll thin plate casting process, nitrogen gas discharged from the molten steel during casting is effectively removed from the casting rolls and the solidified shells It is necessary.

The present invention has been proposed in order to solve the above-mentioned conventional problems, and it is an object of the present invention to effectively remove an inert gas such as nitrogen gas remaining between a casting roll and a solidification shell in the production of a high nitrogen content steel such as super- And to provide a device for producing a cast steel of excellent quality.

In order to achieve the above object, an embodiment of the present invention provides a twin roll laminate casting machine as described below.

In an embodiment of the present invention, there is provided a casting machine comprising: a pair of casting rolls rotating in opposite directions to each other; an edge dam installed on both sides of the pair of casting rolls to form a molten steel pool; And a gas supply port provided in the meniscus shield for supplying an inert gas to the bath surface of the molten metal pool. The molten metal is discharged from the molten steel pool when the molten steel is solidified or compressed. So that the inert gas is prevented from remaining between the casting roll and the solidification shell.

In one embodiment, the gas discharging portion may be formed of a band-shaped valley formed on the surface of the casting roll.

In one embodiment, the valleys may be configured to be formed around the rotational direction of the casting roll.

In one embodiment, the surface of the casting roll is formed with a concavo-convex portion by a shot blasting process, and the concavo-convex portion is arranged on the convexo-concave portion.

In one embodiment, the depth of the concave-convex portion may be in the range of 15 to 25 um.

In one embodiment, the depth of the bone may be in the range of 10-200 um.

In one embodiment, the width of the valleys may range from 50 to 200 um.

In one embodiment, the plurality of troughs are disposed along the periphery of the casting roll, and the intervals between the troughs may be in the range of 100 to 500 um.

In one embodiment of the roll-type thin plate casting machine of the present invention, the inert gas such as nitrogen gas discharged from the molten steel is effectively removed between the casting roll and the solidification shell of the molten steel through the belt-shaped valley formed along the rotating direction of the casting roll , And super-austenitic stainless steels, it is possible to prevent defects on the surface of the cast steel such as dents and cracks and to produce cast steel of excellent quality.

1A is a schematic view of a conventional slab-casting process.
1B is a schematic view of a conventional twin roll type thin plate manufacturing process.
FIG. 2A is a view showing the surface roughness of the casting roll shown in FIG. 1B. FIG.
FIG. 2B is a view showing a state where an inert gas remains between the casting roll shown in FIG. 1B and the solidification shell of molten steel.
FIGS. 3A and 3B are diagrams showing surface defects such as dents and cracks of super-austenitic stainless steel produced by the process shown in FIG. 1B.
Fig. 4 is a schematic view showing an embodiment of the twin roll type sheet metal casting machine of the present invention.
FIG. 5A is a view showing the surface roughness and the score of the casting roll in the invention shown in FIG. 4. FIG.
FIG. 5B is a view showing a state in which inert gas between the casting roll and the solidification shell of the molten steel is discharged through the valley in the invention shown in FIG.
6A and 6B are diagrams showing a state in which the surface quality of the austenitic stainless steel produced by the process shown in FIG. 4 is improved.

In order to facilitate understanding of the features of the present invention as described above, the twin roll type thin plate casting machine related to the embodiment of the present invention will be described in more detail.

In order to facilitate understanding of the embodiments described below, in the reference numerals shown in the accompanying drawings, the related components among the components that perform the same function in the respective embodiments are denoted by the same or an extension line number.

The embodiments related to the present invention basically remove inert gas such as nitrogen gas remaining between the casting roll and the solidification shell effectively in producing a steel having a high nitrogen content such as super-austenitic stainless steel, It is based on production.

Hereinafter, a specific embodiment of the present invention will be described with reference to the accompanying drawings.

Fig. 4 is a schematic view showing an embodiment of the twin roll type sheet metal casting machine of the present invention. FIG. 5A is a view showing the surface roughness and the valleys of the casting roll in the invention shown in FIG. 4, FIG. 5B is a graph showing the surface roughness and the valleys of the casting roll in the invention shown in FIG. Fig.

4 to 5B, an embodiment of the twin roll type thin sheet casting machine of the present invention includes a pair of casting rolls 30 rotated in opposite directions and a pair of rollers 30 on both sides of the pair of casting rolls 30, M and a meniscus shield 50 provided to cover the upper portion of the spool M to block the contact between the outer shell and the spool M, And a gas supply port (60) which is disposed in the discharge port (50) and is provided to supply an inert gas (G) to the bath surface of the welding spool (M) The gas discharge portion H may be formed in the casting roll 30 so that the inert gas G is prevented from remaining between the casting roll 30 and the solidification shell B.

The present invention can be applied to the production of a steel having a high nitrogen content using a twin roll thin plate casting machine, but is not limited thereto. However, for the sake of clarity, the present invention will be described with reference to super-austenitic stainless steel (S32050), which is one of the steel materials having a high nitrogen content.

The molten steel having been subjected to the steelmaking process is carried in the ladle 21 to be transported to the upper portion of the tundish 22 and introduced into the tundish 22 through the molten steel discharge port disposed at the lower end of the ladle 21. [ The molten steel introduced into the tundish 22 is then discharged through the immersion nozzle 23 disposed at the lower end of the tundish 22 by adjusting the discharge amount of the molten steel by the stopper 22a, And is supplied to a forming sump space.

Wherein the casting rolls 30 are arranged in pairs under the tundish 22 and configured to rotate in opposite directions. The edge dams 40 are mounted on both sides of the casting roll 30 to form the sump space.

The molten steel pool M discharged from the tundish 22 is formed in the sump space formed by the casting roll 30 and the edge dam 40 in a predetermined space. Thereafter, the molten steel is cooled down by the cooler (not shown) mounted on the casting roll 30 while being pressed down between the pair of casting rolls 30 within about 0.2 second, and solidified and cast into the casting.

Thereafter, the slab is guided by the pinch roll 25, and finally wound in the form of a coil in the take-up machine 27 via the cooling facility 24.

At least one of the pair of casting rolls 30 may be connected to a driving unit 80, which may be implemented as a hydraulic cylinder or the like, so that the distance between the pair of casting rolls 30 may be adjusted. It is possible to adjust the thickness of the cast steel to be cast.

The pair of casting rolls 30 are attached to the periphery of the casting roll 30 to remove impurities such as an oxide scale adhered to the surface of the casting roll 30, (70) can be further disposed.

In order to prevent the occurrence of defects such as cracks and dents in the edge portion of the thin plate, it is necessary to uniformly coagulate the entire length of the cast steel during the production of the cast steel in order to manufacture super-austenitic stainless steel using the twin roll type thin plate casting machine.

This can be achieved by solidifying the welding spool M in a state in which the welding spool M is shielded from the outside. To this end, the meniscus shield 50 is connected to the pair of casting rolls 30 and the edge dam 40 and blocks the surface of the welding spool M from the outside.

The gas supply port 60 is disposed in the meniscus shield 50 and supplies an inert gas G such as nitrogen gas or argon gas to the upper surface of the welding spool M to prevent contact with the outside air do.

The inert gas G prevents the oxidation of the molten steel and changes the interface energy of the molten steel's meniscus at the point where the molten steel comes into contact with the casting roll 30 and the edge dam 40 upon solidification of the molten steel. Thereby performing a function as an atmospheric gas for uniformizing solidification.

At this time, the atmospheric gas is sufficiently supplied to prevent the oxidation of the molten steel, so that the oxygen concentration on the upper surface of the welding spool M should be kept at 0.1% or less of the total outside air.

The atmosphere gas is to be sufficiently transferred to the casting roll 30 by using a soluble gas which is well dissolved in the molten steel. In order for the solidifying cell to be sufficiently transferred to the surface of the casting roll 30, atmospheric gas contained in the molten steel must be smoothly discharged to the outside without remaining between the solidifying cell and the casting roll 30.

In particular, super-austenitic stainless steels have a nitrogen content of at least 2000 ppm, which is higher than that of ordinary steels. When nitrogen gas is used as the atmospheric gas, the proportion of nitrogen remaining in the molten steel may be high. ), The rate at which the solidification cell is transferred to the casting roll 30 during solidification is reduced.

This may lead to nonuniform solidification of the entire molten steel, which ultimately leads to surface defects of the cast steel in the region where the solidification unevenness occurs.

Therefore, in an embodiment of the present invention, when the molten steel is solidified or compressed by forming the gas discharge portion H on the surface of the casting roll 30, the inert gas G discharged from the spool M Is prevented from remaining between the casting roll (30) and the solidification shell (B), thereby improving the rate at which the solidification shell (B) is transferred to the surface of the casting roll (30).

The gas discharge unit H may be formed of a band-shaped valley H on the surface of the casting roll 30. At this time, the belt-like valleys H may be formed in a circumferential direction along the circumference of the casting roll 30, and may be arranged in a plurality of rows along the width direction of the casting roll 30. The band-like valley (H) can be processed by using a laser.

As shown in FIG. 5A, the belt-like valleys H are formed on the surface of the casting roll 30 on a dimple-shaped concavo-convex portion A formed by a shoot blasting process .

That is, in the conventional casting rolls in which the band-like valleys H are not formed, the inert gas G generated in the molten steel is not smoothly discharged between the conventional casting roll and the solidifying cell due to dimple-shaped surface roughness So that the solidification shell (B) of molten steel interfered with the transfer to the conventional casting roll surface.

However, in the casting roll 30 of the present invention, since the band-like valley H is formed on the dimple-shaped protrusive portion A along the circumferential direction of rotation, the inert gas G is formed on the band- It can move along the valley (H) of the shape and be discharged to the outside air.

5B, the belt-like corrugation H is formed on the solidification shell B region of the molten steel and the concavo-convex portion A of the casting roll 30 along the circumferential direction of rotation of the casting roll 30 So that the inert gas G discharged to the molten steel does not remain between the solidification shell B and the casting roll 30 and is prevented from rotating by the rotation of the casting roll 30, It can be confirmed that it is discharged to the outside by taking the shape of the valley (H).

Since the present invention is particularly applied to the production of a steel having a high nitrogen content, when the supplied inert gas G is nitrogen gas, the score of the belt formation will be more significant. However, since the inert gas (G) is prevented from remaining in the space between the casting roll and the solidifying shell, even if the inert gas (G) is another stabilized gas such as argon gas,

As a result, the surface transfer rate of the casting roll 30 of the solidification shell (B) is improved to prevent lifting of the solidification shell (B), thereby improving the surface quality of the cast steel.

The effective depth (S), width (W), and spacing distance (L) of the strip-shaped valleys (H) will be examined through experimental data obtained through actual experiments.

The composition table of the super-austenitic stainless steels used in this experiment is shown in Table 1 below. Cr, Ni, Mo and N are contained in the molten steel under the maximum ratio (%) shown in Table 1 below, and the contents of Cr, Ni, Mo, . Unless otherwise indicated for each alloy element, the ratio is pure Fe.

Steel grade

ingredient
C
Si
Mn
P
S
Cr
Ni
Mo
Cu
N
Remarks
S32050
Spec
0.03
max
1.0
max
1.5
max
0.035
max
0.02
max 22.0
~
24.0 20.0
~
23.0 6.0
~
6.8
0.4
max 0.21
~
0.32
ASTM

The quality characteristics of the thin plate produced between the conventional casting roll 30 and the casting roll 30 of the present invention are shown in Table 2 through an experiment using the super-austenitic stainless steel shown in Table 1 above.

The conditions of the test casting rolls corresponding to Experimental Examples 1 to 5 in Table 2 are as follows.

- Overall width: 1300mm

- Edge edge machining: Shot blasting with pressure of 7 bar with 0 ~ 30mm and 1270 ~ 1300mm part and 1.2mm (diameter) ball to form dimples (concave and convex parts) of Ra 17um (diameter).

- Central processing: Shot blasting at a pressure of 7 bar with 30 to 1270 mm part and 2.5 mm (diameter) ball to form dimples (concave and convex parts) of Ra 20 um (diameter).

division

Casting conditions

Quality Characteristics of Sheet
Casting roll surface treatment
(Presence or absence of band-shaped bone formation)


Coiling temperature (캜)

Crack and dent defect presence
Degree of precipitation of sigma phase
Experimental Example 1
Untreated
740
existence
Bad
(Excessive precipitation)
Experimental Example 2
Untreated
650
existence
usually
(Reduction in precipitation amount)
Experimental Example 3 Band-shaped bone formation by laser
(Depth: 10 to 200 um, width: 50 to 200 mm, gap: 100 to 500 mm)
650
Not existing
usually
(Reduction in precipitation amount)
Experimental Example 4 Band-shaped bone formation by laser
(Depth: 10 to 200 um, width: 50 to 200 mm, gap: 100 to 500 mm)
650
Not existing
usually
(Reduction in precipitation amount)
Experimental Example 5 Band-shaped bone formation by laser
(Depth: 10 to 200 um, width: 50 to 200 mm, gap: 100 to 500 mm)
580
Not existing
Good
(Under precipitation)

As can be seen from the above Table 2, in the case of Experimental Examples 3, 4 and 5 in which the belt-shaped corrugations H were formed on the surface of the casting roll 30, the casting rolls 30 and the solidification shells B The discharged inert gas G is not inflated but escapes to the outside air along the belt-like valley H formed in the casting direction as shown in FIG. 5B to thereby prevent lifting of the solidifying shell B, As a result, defects such as cracks and dents on the surface of the thin plate are not present.

However, in the case of Experiments 1 and 2 having the casting rolls in which the belt-like valleys H were not formed, the inert gas remained between the casting rolls and the solidifying shell and expanded, and the solidifying shells were transferred to the surface of the casting roll It is ultimately found that the solidification shell lifting phenomenon occurs and the surface quality of the thin plate is deteriorated.

It is preferable that the depth S of the concavo-convex portion A of the casting roll 30 is in the range of 15 to 25 um and the depth S of the band-like corrugation H is in the range of 10 to 200 um. When the depth S of the irregular portion A and the valley H formed by the shot blasting process and the laser processing is less than 10 um, a considerable amount of the inert gas G discharged from the molten steel does not flow into the valley H If the depth S of the irregular portion A and the valley H is more than 200 μm, the surface of the produced cast steel may be roughened.

The width (W) of the valleys (H) may be in the range of 50 to 200 um. If the width W of the valley H is less than 50 탆, it may be difficult for a sufficient amount of the inert gas G to flow into the valley H. Here, a sufficient amount means that only the amount that does not prevent the solidifying shell (B) from being transferred to the surface of the casting roll (30) remains. That is, if the sufficient amount is not discharged, the transfer of the solidification shell B to the surface of the casting roll 30 becomes difficult. On the contrary, when the thickness is more than 200 μm, the width W of the valley H becomes too wide, and flow of the inert gas G to the outside of the valley H may occur during the movement of the inert gas G on the valley H . Which may again flow between the casting roll 30 and the solidification shell B. [

When the plurality of ribs H are arranged along the periphery of the casting roll 30, the distance L between the ribs H may preferably be in the range of 100 to 500 um. If the distance L between the valleys H is less than 100 탆, a considerable number of the band-shaped valleys H are formed on the concavo-convex portion A, and the function of the concavo- . That is, surface defects of the cast steel can be caused. On the contrary, if the distance L between the valleys H is more than 500 μm, the gap L between the valleys H becomes too wide, and the gap G between the casting roll 30 and the solidification shell B The emission effect may be minimal.

The thin plate S produced under the condition of forming the valleys H as described above has excellent surface quality without cracks or dents as shown in Figs. 6A and 6B.

When the cast steel is cast using the new casting roll 30 of the present invention and then the cast steel in the discharge process is cooled and wound by a water cooling device, And control it.

Generally, the sigma phase, which is most importantly controlled in super-austenitic stainless steel, contains Cr or Mo in an amount of more than 40%, and the crystal structure is orthorhombic. However, presence of a sigma phase in the thin sheet increases the brittleness of the material, so that the maximum precipitation must be suppressed.

Therefore, in order to suppress the precipitation of such sigma phase, the temperature of the winding coil should be strictly limited by the water-cooling facility in the discharge process. As shown in Table 2, when the coiling temperature was 580 ° C in Experimental Example 5, it was confirmed that the amount of precipitation of the sigma phase was the smallest. However, in other Experimental Examples 2, 3 and 4, as the coiling temperature exceeds 600 ° C, it can be seen that the deposition amount of the sigma phase is not significantly different from that of the conventional casting roll 30. In the case of Experimental Example 1 in which the coiling temperature exceeds 700 ° C, excessive precipitation occurs and the quality characteristics of the thin plate are inferior.

Therefore, in order to suppress the precipitation of the sigma phase and obtain sufficient toughness, it is preferable to wind at a temperature of 600 ^° C or lower.

The present invention has the above-described structure and effectively removes the inert gas remaining between the casting roll and the solidification shell in the production of super-austenitic stainless steel, thereby producing a cast steel of excellent quality.

The above is merely a specific example of a twin roll type thin plate casting machine for producing a high nitrogen content steel such as super austenitic stainless steel.

Therefore, it should be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. do.

21 ... Ladle 22 ... Tundish
23 ... immersion nozzle 24 ... cooling equipment
27 ... winder 30 ... casting roll
40 ... Edge Dam 50 ... Maniscus shield (meniscus shield)
60 ... gas supply port 70 ... brush roll
80 ... Drive

Claims (8)

A pair of casting rolls rotating in mutually opposite directions;
An edge dam installed on both sides of the pair of casting rolls to form a molten steel pool;
A meniscus shield covering an upper portion of the spool for blowing out the contact between the outside air and the spool; And
A gas supply port which is disposed in the meniscus shield and is provided to supply an inert gas to the bath surface of the heating coil; , ≪ / RTI &
Wherein a gas discharge portion is formed in the casting roll so that an inert gas discharged from the spool for molten steel when solidified or pressed down is prevented from remaining between the casting roll and the solidified shell.
The method according to claim 1,
Wherein the gas discharging portion is a strip-shaped groove formed on the surface of the casting roll.
3. The method of claim 2,
Wherein the ribs are formed along the rotation direction of the casting roll.
The method of claim 3,
Wherein a concavo-convex portion is formed on the surface of the casting roll by a shot blasting process, and the concavo-convex portion is disposed on the convexo-concave portion.
5. The method of claim 4,
Wherein the depth of the concave-convex portion is in the range of 15 to 25 um.
6. The method of claim 5,
Wherein the depth of the valleys is in the range of 10 to 200 um.
The method according to claim 6,
And the width of the valley is in the range of 50 to 200 um.
8. The method of claim 7,
Wherein the plurality of troughs are arranged along the periphery of the casting roll, and the interval between the troughs is in the range of 100 to 500 um.

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