KR101372741B1 - Method for manufacturing a duplex stainless thin steel sheet containing high nitrogen by using strip caster with twin roll - Google Patents

Abstract

The present invention relates to a method of manufacturing a high nitrogen duplex stainless steel sheet using a twin roll thin plate casting machine. The present invention in one embodiment, a pair of thin plate casting roll to rotate; A pair of edge dams forming a sump which is a space in which molten steel is accommodated together with the thin sheet casting roll; And casting the thin cast pieces using a sheet casting machine including a side molten steel discharge port for discharging molten steel toward the sheet casting roll and an immersion nozzle for supplying molten steel through the front and rear molten steel discharge ports for discharging the molten steel toward the edge dam. The method of manufacturing a high nitrogen duplex stainless steel sheet by in-line hot-rolling the thin slab, the supply of molten steel through the immersion nozzle is discharged through the discharge amount of the molten steel discharged through the side molten steel discharge port and the front, rear molten steel discharge port Provided is a method for producing a high nitrogen duplex stainless steel sheet, wherein the ratio of the discharge amount of molten steel is 4 to 9.
According to the present invention, it is possible to provide a method for producing a high nitrogen duplex stainless steel sheet using a twin roll sheet caster that can significantly suppress the occurrence of cracks or surface defects by achieving uniform solidification of molten steel.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a high nitrogen duplex stainless steel thin plate using a twin roll thin plate casting machine,

The present invention relates to a method of manufacturing a high nitrogen duplex stainless steel sheet using a twin roll thin plate casting machine.

In order to further increase the corrosion resistance of stainless steels, many stainless steels containing a large amount of nitrogen at 1500 ppm or more (including up to 3000 ppm in consideration of nitrogen solubility) have been developed, and a representative example thereof is a duplex stainless steel. .

Duplex stainless steel means a stainless steel having a structure in which austenite and a ferrite phase are mixed. These duplex stainless steels not only have higher strength and corrosion resistance than ordinary stainless steels, but also have strong properties against chlorine ions and seawater. Therefore, duplex stainless steel is widely used in fields requiring high strength and high corrosion resistance, and is mainly used for thermal power plants, flue gas desulfurizer (FGD) ducts, seawater cooling pipes for nuclear power plants, chemical tanks for shipbuilding, and seawater / chemical facilities. Its use is gradually increasing.

In the duplex stainless steel, a slab is manufactured by a continuous casting method, and the obtained slab is hot rolled to produce a desired thickness. However, when the slab is manufactured and rolled by the conventional method, there is a problem that cracks are severely generated at the edge and the surface of the steel sheet during rolling as shown in FIG. 1. In some cases, it is necessary to provide a method for producing a duplex stainless steel in which the occurrence of cracks is severe enough to prevent the occurrence of cracks, so that cracks are reduced.

Cracks are generated in the duplex stainless steel because austenite phase and ferrite phase coexist in the steel. The austenite phase has a higher hardness at high temperature than the ferrite phase, and therefore, when stress acts, the amount of deformation of the two phases becomes different. In this case, the stress is concentrated at the interface between the austenite phase and the ferrite phase, and cracks are generated.

For this purpose, techniques have been proposed, such as limiting the reduction rate per pass at high temperature and strictly restricting the content of impurity such as S, which affects cracks. However, limiting the reduction rate per pass at high temperature may not only cause a problem in productivity, but also cause difficulty in securing a standard, and in the case of limiting the content of impurity components, there may be a problem of increasing operation burden.

The sheet casting method shown in Fig. 2 is an effective method for solving this problem. That is, in the thin plate casting method, a method of casting a thin plate by supplying molten steel to a space between revolutions while rotating one or a pair of rolls, in which a thin plate having a thickness close to the final thickness is manufactured by a casting method, There is no. However, even with this method, there is a problem that cracks generated in the steel sheet can not be completely prevented.

The present invention is to provide a method for producing a high-nitrogen duplex stainless steel sheet using a twin roll sheet caster that can reduce the occurrence of surface defects such as edge cracks, dents, depressions when manufacturing the duplex stainless steel by the sheet casting method .

The present invention in one embodiment, a pair of thin plate casting roll to rotate; A pair of edge dams forming a sump which is a space in which molten steel is accommodated together with the thin sheet casting roll; And casting the thin cast pieces using a sheet casting machine including a side molten steel discharge port for discharging molten steel toward the sheet casting roll and an immersion nozzle for supplying molten steel through the front and rear molten steel discharge ports for discharging the molten steel toward the edge dam. The method of manufacturing a high nitrogen duplex stainless steel sheet by in-line hot-rolling the thin slab, the supply of molten steel through the immersion nozzle is discharged through the discharge amount of the molten steel discharged through the side molten steel discharge port and the front, rear molten steel discharge port Provided is a method for producing a high nitrogen duplex stainless steel sheet, wherein the ratio of the discharge amount of molten steel is 4 to 9.

According to the present invention, it is possible to provide a method for producing a high nitrogen duplex stainless steel sheet using a twin roll sheet caster that can significantly suppress the occurrence of cracks or surface defects by achieving uniform solidification of molten steel.

1 is a photograph showing an example of a thin steel sheet in which an edge crack has occurred.
2 is a schematic diagram schematically showing a steel sheet casting method.
Figure 3 is a schematic diagram for explaining the phenomenon that the gas layer is formed between the molten steel and the sheet casting roll during the sheet steel casting of high nitrogen steel.
4 is an enlarged perspective view of the immersion nozzle.
5 is a photograph showing an example of a thin steel sheet in which dents and depressions occur.
Figure 6 is a result of observing the shape of the irregularities and fine grooves formed on the surface of the thin plate casting roll according to an embodiment of the present invention.
7 is a photograph of the surface of the thin steel sheet of Inventive Example 2 according to an embodiment of the present invention, (a) shows a state where no edge crack occurs, and (b) shows a state where a depression defect does not occur.

Hereinafter, the present invention will be described in detail. In the twin roll thin sheet casting method, two rolls are used, and molten steel is supplied to the space between the rolls and cast by rotation of the two rolls. The cast strip is manufactured through the casting, and the thin strip is hot-rolled to produce a thin steel sheet. In the present invention, duplex stainless steel containing high nitrogen is used as a main target steel. The duplex stainless steel is a well-known steel material in the technical field of the present invention, and the distinction between the duplex stainless steel and the non-duplex stainless steel can be clearly made. However, one example of the duplex stainless steel to be used in the present invention may include the composition shown in Table 1 below.

Alloy component C Si Mn P S Cr Ni Mo N Content (wt%) 0.02 to 0.1 0.5 to 2.0 1-5 0.03 or less 0.02 or less 19-23 0.5 to 6.5 0 to 3.5 0.1 to 0.3

The inventors of the present invention have studied to solve the problem that defects such as dent and depression occur on the surface of steel sheet when manufacturing duplex stainless steel containing high nitrogen by sheet casting. One of the major causes of surface defects at the time was found to be that the solidification shell is lifted on the roll surface or uneven solidification occurs, leading to the present invention.

Duplex stainless steels play an equal role in the content of Cr (qq), elemental elements compared to conventional 304 stainless steels (including about 18 wt% Cr and 8 wt% Ni and classified as austenitic stainless steel) (In terms of Cr content) is high and has a composition of Nieq (Ni equivalent) reduced. Due to these compositional characteristics, the coagulation and cooling of the duplex stainless steel undergoes a phase transformation process of L (liquid phase) → L + delta (delta ferrite) → delta → delta + gamma. In addition, duplex stainless steels manage the variable PREN (= Cr + 3.3Mo + 16N) as high as possible to improve corrosion resistance. As a result, the nitrogen content in steels may become very high as compared with a normal austenitic stainless steel, and in some cases, it may exceed 1500 ppm. However, when duplex stainless steel containing such a large amount of nitrogen is produced, when the molten steel is solidified, the solidified shell does not closely adhere to the surface of the roll and appears to float or unevenly solidify.

The phenomenon found by the present inventors will be described with reference to FIGS. 2 and 3. As shown in the drawing, the molten steel which has been subjected to the refining process taken in the ladle 1 is supplied to the sump 4 through the turn-dish 2 and the submerged entry nozzle (SEN) 3. The molten steel supplied to the sump 4 is cast into a thin steel plate passing between the thin sheet casting rolls.

The sump 4 is a space enclosed by two casting rolls 5 and an edge dam 6, and means a space where molten steel is received. However, when the molten steel in the sump 4 is exposed to the atmosphere, the molten steel is oxidized, and thus the produced oxide significantly affects the quality of the product. Therefore, A meniscus shield 7 is disposed on the upper portion of the housing. Therefore, the upper portion of the sump is provided with a free space formed by the molten steel bath surface, the casting roll 5, and the edge dam 6, and the atmosphere gas is supplied to the free space to prevent oxidation of molten steel. If the pressure of the atmospheric gas is insufficient, air may be introduced into the free space from the outside, so that the pressure of the atmospheric gas needs to be maintained at 150-250 mmH ₂ O higher than atmospheric pressure. For this reason, the atmospheric gas having a high pressure is not only located at the upper surface of the molten steel, but also penetrates between the casting roll and the solidification shell due to the pressure (see 11 in FIG. 3).

As such, when a layer of atmospheric gas is formed between the solidification shell and the casting roll, a phenomenon in which the solidification shell is separated from the casting roll occurs, and in this case, the dents may be formed on the surface of the thin plate due to the thermal and physical effects of the molten steel supplied from the immersion nozzle. Defects such as dents, depressions, and cracks occur. This defect will be described in more detail with reference to FIG. 4. 4 is an enlarged perspective view of the immersion nozzle. As shown in FIG. 4, in the twin roll type sheet casting, the immersion nozzle 3 includes a plurality of molten steel discharge ports, and the molten steel discharge ports face side molten steel discharge ports 32 and edge dams for discharging molten steel toward the thin plate casting rolls. It can be divided into the front and rear molten steel discharge port 34 for discharging molten steel. In this case, conventionally, molten steel injected through the side molten steel discharge port 32 is discharged at a large amount and at a high pressure to give a strong thermal and physical impact to the solidified shell, and thus the thin plate manufactured is dent, as shown in FIG. 5. It will have defects such as depression or cracks.

Accordingly, in the present invention, in order to solve the problem caused by the remaining of the gas layer, molten steel discharged through the immersion nozzle is first discharged through the side molten steel discharge port (hereinafter referred to as 'side discharge amount') and before, The molten steel discharged through the rear molten steel discharge port (hereinafter referred to as 'front, rear discharge amount') is characterized in that the supply so that the ratio of 4 to 9. While the ratio of the existing side discharge amount / front and rear discharge amount is about 9.1 to 19, the total amount of molten steel supplied to the sump is about the same level as before, but the discharge amount of the front and rear sides of the molten steel is as described above. By increasing and decreasing the side discharge amount, it is possible to reduce the thermal physical influence that the solidified shell receives from the molten steel to suppress surface defects such as depressions and streaks. If the ratio of side discharge amount / front and rear discharge amount is less than 4, the flow of molten steel in the sump is uneven due to the imbalance between the front and rear discharge amount and the side discharge amount, and when it exceeds 9, the molten steel discharge amount toward the edge dam direction is exceeded. This becomes so small that the temperature at the edges decreases, resulting in a piece of skull that is partially overcoagulated. In addition, since the molten steel discharged in the casting roll direction increases, thermal and physical effects increase in the solidification shell, and defects such as depression may occur.

On the other hand, when controlling the ratio of the side discharge amount / front, rear discharge amount as described above can express the desired effect to obtain the present invention, for more preferable control, the side discharge amount of molten steel is 114,075 ~ 163,215㎥ / min, The front and rear discharge amount is preferably 10,238 ~ 38,610 ㎥ / min.

In addition, in order to improve the defect suppression effect more, it is preferable to control so that the thickness of the cast thin piece may be 3-6 mm. As such, when the thickness of the thin cast steel is ensured to be 3 mm or more, the thermal and physical effects of the solidified shell are reduced even though the amount of molten steel discharged to the solidified shell side is the same. Through this, it is possible to produce a thin steel sheet having excellent surface quality from which edge cracks or surface defects are removed. On the other hand, when the thickness of the thin slab exceeds 6mm, the slab formation becomes difficult, the productivity may be sharply lowered. On the other hand, the thin steel sheet is finally produced by in-line hot rolling the thin slab may have a thickness of 1.5 ~ 3mm.

As described above, in order to manufacture a thin cast steel having a thicker thickness as compared with the usual thin cast steel, it is preferable to control the casting speed. The control of the casting speed is to control the contact time between the solidification shell and the casting roll, and when the distance between the casting roll and the immersion nozzle is the same, the thickness of the solidification shell becomes thicker as the contact time of the solidification shell and the casting roll becomes longer. . In the present invention, the casting speed is preferably controlled to have a range of 25 ~ 50m / min. If the casting speed is less than 25m / min, the thickness may be too thick to form the cast steel, if it exceeds 50m / min, the thickness of the thin cast steel can be thinned due to the excessive casting speed, and eventually Since the influence may not be reduced, the inhibitory effect of the surface defect may be lowered.

In addition to the above conditions, in order to solve the problem of the surface defects of the thin plate caused by the gas layer, in the present invention, the transfer of the molten steel to the casting roll surface is facilitated by performing shot blasting on the surface of the sheet casting roll to form as many irregularities as possible. It is desirable to induce uniform coagulation.

Shot blasting is for imparting fine concavity and convexity to the surface. When the molten steel and the casting roll come into contact with each other by the irregularities generated by the shot blasting, the convex portions of the irregularities serve as seeds of solidification, In addition, it is possible to suppress the occurrence of surface cracks due to solidification shrinkage due to the effect of reducing the surface tensile force generated at the time of solidification shrinkage. For this purpose, it is preferable that the irregularities have an average roughness Ra of 15 mu m or more. However, when the depth of the irregularities is too deep, the recesses of the irregularities of the casting roll may not contact the molten steel, and therefore, it is preferable that the irregularities have an average roughness Ra of 25 占 퐉 or less.

The irregularities have an effect of suppressing the occurrence of surface cracks occurring during the solidification shrinkage. However, in the case of a high nitrogen duplex steel type in which nitrogen is supersaturated and difficulty in adsorption, a gas layer is formed between the solidification shell of the molten steel and the casting roll, Transfer of the shell and the casting roll is disturbed, and the gas layer expands, causing intaglio type dent defects in the solidified shell and defective defects due to lifting. Therefore, in order to produce high-nitrogen-content duplex steel using strip casting, a method of discharging the trapped gas in the gas layer is required in order to facilitate the transfer of the solidification shell and the casting roll while suppressing the occurrence of surface cracking during coagulation shrinkage.

In order to discharge the gas in the gas layer, fine grooves are additionally formed in the casting direction on the surface of the thin plate casting roll on which the unevenness is formed. The fine grooves serve as discharge passages for allowing the formed gas layer to be easily discharged out of the contact interface between the roll and the molten steel by the positive pressure of the molten steel. It is preferable that the fine grooves have an average width of 50 to 400 mu m and an average depth of 50 to 300 mu m in order to obtain a sufficient gas discharging effect and an average interval between the fine grooves is in the range of 100 to 700 mu m. Such fine grooves can be manufactured by various known methods and are not particularly limited in the present invention. However, one example is a laser or an etching process in which a laser having a high energy is irradiated onto a surface and a groove is formed by chemical etching. The photograph which observed an example of the surface of the thin cast steel roll processed as mentioned above was shown in FIG. Meanwhile, the interval between the adjacent fine grooves may be variously defined, but in the present invention, the distance between the centers of adjacent grooves and the center. If the interval between the fine grooves is too wide, the effect of discharging the gas may not be sufficient. On the contrary, when the distance between the fine grooves is too narrow, the contact area required for static pressure and heat transfer of molten steel becomes too small, which may cause scratches on the surface of the product.

As described above, after the step of manufacturing the thin steel sheet may further comprise the step of in-line hot rolling (8). The inline hot rolling is to remove pores interposed in the cast thin cast steel. In addition, when hot rolling is not performed, the cast structure of the thin cast steel remains as it is, and it is difficult to obtain the characteristics of the duplex steel type. For this effect, it is preferable that the reduction ratio during the inline hot rolling be 30% or more. However, the reduction ratio is difficult to exceed 50% due to the process limitations in consideration of the thickness of the thin cast steel cast.

Thereafter, the step of quenching the thin plate (the cooling device 9 of FIG. 2) may be further included. The step of winding 10 may be further included. That is, the duplex stainless steel is subjected to the phase transformation process of L (liquid phase) → L + δ (delta ferrite) → δ → δ + γ (austenite) as described above. That is, the austenite phase is formed in the delta ferrite. The ratio of austenite and ferrite phase is formed in a ratio of about 50:50, and since the steel contains high nitrogen, a sigma phase having more than 40% of nitrogen and (Ca, Mo) in the steel is generated when the cooling rate is slow. The sigma phase increases the brittleness of the steel sheet when present in a large amount in the steel sheet as a crystal of the tetragonal system.

Therefore, the present invention may further comprise the step of quenching after the production of a thin plate to suppress the generation of the sigma phase. As confirmed by the inventors, the cooling rate of the steel sheet for effectively preventing cracks by suppressing the generation of sigma phase is 2 ° C / sec or more. However, when the cooling rate is excessively increased, a difference in winding temperature may be generated for each part, and a problem occurs that a stain occurs because the strip cooling water does not evaporate on the surface, and thus the cooling rate has a range of 30 ° C / sec or less. It is preferable.

In addition, the winding operation is also preferably carried out at a temperature of 700 ° C or less, which is a temperature range in which no further sigma phase precipitation occurs, thereby giving sufficient toughness to the thin plate.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are only illustrative of the present invention in more detail and do not limit the scope of the present invention.

(Example)

Duplex stainless molten steel of the composition shown in Table 2 was prepared and used for sheet casting. In order to control the atmosphere during casting, the molten steel bath surface was sealed by using a meniscus shield having a shape shown in FIG. 1, and a nitrogen gas was supplied as an atmosphere gas to the pressure of about 200 mmAq. On the other hand, the detailed conditions for the production of the steel sheet is shown in Table 3 below, after casting the thin cast steel, in-line hot rolling at a reduction rate of 36% to produce a thin steel sheet, and the prepared steel sheet 20 ℃ / sec cooling rate After quenching to 700 ° C., it was wound up to obtain a wound steel sheet. At this time, the sheet casting roll used for sheet casting was formed by using a shot blasting fine irregularities having an average roughness of 20㎛, and then through the laser, etching process on the site where the fine irregularities are formed again in the form shown in FIG. Grooves were formed. At this time, the width of the fine grooves was 350 μm, the depth of the fine grooves was set to 150 μm, and the space between the grooves was set to 650 μm. After the observation with a naked eye and a stereo microscope whether the surface defects occurred with respect to the thin steel sheet thus prepared, the results are shown in Table 3 below.

Alloy component C Si Mn P S Cr Ni Mo N Content (wt%) 0.03 0.8 2.0 0.03 0.003 22 3 0.7 0.2

division Lateral discharge amount /
Front and rear discharge ratio Lateral discharge amount
(㎥ / min) Front and rear discharge
(㎥ / min) Casting speed
(m / min) Cast thickness
(mm) Surface defect
The presence or absence Comparative Example 1 3.55 136,890 38,610 45 3.0 U Comparative Example 2 3.55 114,075 32,175 25 4.5 U Comparative Example 3 5.67 152,490 26,910 60 2.3 U Comparative Example 4 5.67 149,175 26,325 50 2.7 U Inventory 1 5.67 149,175 26,325 45 3.0 radish Inventive Example 2 5.67 143,650 26,350 40 3.3 radish Inventory 3 5.67 139,230 24,570 35 3.6 radish Honorable 4 5.67 139,230 24,570 30 4.2 radish Inventory 5 5.67 124,313 21,938 25 4.5 radish Comparative Example 5 13.29 163,215 12,385 45 3.0 U Comparative Example 6 13.29 136,013 10,238 25 4.5 U

As shown in Table 3, in the case of Inventive Examples 1 to 5 satisfying the casting speed and the thickness range of the cast steel, in addition to the manufacturing conditions proposed by the present invention, that is, the ratio of side discharge amount / front and rear discharge amount It can be seen that surface defects do not occur. 7 is a photograph observing the surface of the thin steel sheet of Inventive Example 2, (a) shows a state where no edge crack has occurred, and (b) shows a state where no depression defects have occurred. As shown in FIG. 7, when the conditions of the present invention are satisfied, a thin steel sheet having a very good surface state can be manufactured. On the other hand, as can be seen through Examples 1 to 5 of the present invention, it can be seen that the thickness of the cast slab increases as the casting speed decreases.

On the other hand, in Comparative Examples 1 and 2, which is less than the ratio of the amount of side discharge / front and rear discharge proposed by the present invention, it can be seen that the surface defect occurred due to the problem of uneven flow of molten steel in the sump. In the case of Comparative Examples 5 and 6 exceeding the proposed ratio of side discharge amount / front and rear discharge amount, it can be seen that the surface defect occurred due to the excessive amount of molten steel discharged in the casting roll direction.

In the case of Comparative Examples 3 and 4 as an example that does not satisfy the range of the thickness of the cast steel proposed by the present invention, it can be seen that a defect occurred on the surface of the thin steel sheet due to the thermal physical effect of the molten steel discharged because the thickness of the thin cast steel is too thin. .

1: ladle
2: Turn Dish
3: immersion nozzle
4: Sump
5: casting roll
6: Edge dam
7: Meniscus shield
8: Inline hot rolling
9: lamination cooling
10: Coiling
11: Gas layer
32: side molten steel outlet
34: Front and rear molten steel discharge

Claims (8)

A rotating pair of sheet metal casting rolls; A pair of edge dams forming a sump which is a space in which molten steel is accommodated together with the thin sheet casting roll; And casting the thin cast pieces using a sheet casting machine including a side molten steel discharge port for discharging molten steel toward the sheet casting roll and an immersion nozzle for supplying molten steel through the front and rear molten steel discharge ports for discharging the molten steel toward the edge dam. As a method for producing a high nitrogen duplex stainless steel sheet by in-line hot rolling the thin cast steel,
The supply of molten steel through the immersion nozzle is such that the ratio of the discharge amount of the molten steel discharged through the side molten steel discharge port and the discharge amount of the molten steel discharged through the front and rear molten steel discharge ports is 4 to 9, and the molten steel discharged through the side molten steel discharge port. Discharge amount of 114,075 ~ 163,215㎥ / min, the discharge amount of the molten steel discharged through the front and rear molten steel discharge port is 10,238 ~ 38,610㎥ / min Method for producing high nitrogen duplex stainless steel sheet.
delete The method according to claim 1,
The thickness of the thin cast steel is a method of manufacturing a high nitrogen duplex stainless steel sheet of 3 ~ 6mm.
The method of claim 3,
In the casting, the casting speed is 25 ~ 50m / min high nitrogen duplex stainless steel sheet production method.
The method according to claim 1,
The thin sheet casting roll includes irregularities on the surface, and the irregularities have a high nitrogen duplex stainless steel sheet having an average roughness (Ra) of 15 ~ 25㎛.
The method according to claim 1,
Wherein the thin sheet casting roll includes fine grooves on its surface,
Wherein the fine grooves have an average width of 50 to 400 mu m and an average depth of 50 to 300 mu m,
Wherein the average interval between the fine grooves is 100 to 700 mu m.
The method according to claim 1,
The inline hot rolling is a method for producing a high nitrogen duplex stainless steel sheet is carried out at a reduction rate of 30% or more.
The method according to claim 1,
After the production of the steel sheet, the method of producing a high nitrogen duplex stainless steel sheet comprising the step of cooling the thin steel sheet at a rate of 2 ° C / sec or more and winding the cooled steel sheet at a temperature of 700 ° C or less.

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