KR101449178B1 - Strip casting roll for manufacturing high nitrogen stainless steel having excellent edge part quality and manufacturing method for high nitrogen stainless steel using the same - Google Patents

Abstract

The present invention relates to a thin sheet casting roll for producing a high nitrogen stainless steel sheet having excellent edge quality and a method for producing a high nitrogen stainless steel sheet using the same.
According to an embodiment of the present invention, there is provided a thin sheet casting roll for casting a high nitrogen stainless steel sheet, comprising: concave and convex portions and fine grooves on a surface, the fine grooves having an occupied area ratio of an area from an edge to 100 mm Is less than 80% (excluding 0), and the occupied area ratio is linearly increased from the edge, and a method of manufacturing a high nitrogen stainless steel sheet using the same.
According to the present invention, by improving the surface shape of the thin plate casting roll, more uniform solidification can be achieved, and the problem of cracking of the product can be greatly reduced, and in particular, edge breakage and plate breakage can be fundamentally solved have.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-nitrogen stainless steel sheet for producing a high-nitrogen stainless steel sheet having excellent edge quality, and a method for manufacturing a high-nitrogen stainless steel sheet using the same. TECHNICAL FIELD [0001] THE SAME}

The present invention relates to a thin sheet casting roll for producing a high nitrogen stainless steel sheet having excellent edge quality and a method for producing a high nitrogen stainless steel sheet using the same.

In order to increase the corrosion resistance of the stainless steel material, for example, a large number of stainless steel plates containing a large amount of nitrogen at 1000 ppm or more (including up to 3000 ppm when considering the solubility of nitrogen) have been developed. As a representative example thereof, duplex stainless steel plates .

Duplex stainless steel means a stainless steel having a structure in which austenite and a ferrite phase are mixed. These duplex stainless steels have higher strength and corrosion resistance than ordinary stainless steels, and have strong resistance to 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, FGD (Flue Gas Desulfurizer) ducts, nuclear power plant seawater cooling pipes, chemical tank for shipbuilding, And 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 such a conventional method, cracks are severely generated at the edge and surface of the thin plate during rolling. 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 a high temperature may cause problems in productivity as well as difficulty in securing specifications, and if the content of impurity components is limited, there is a problem that the burden of operation increases.

The thin plate casting method shown in Fig. 1 is an effective method for solving this problem. The thin plate casting method is a method of casting a thin plate between gaps rotated while rotating one or a pair of rolls. Since a thin plate having a thickness close to the final thickness (2 to 6 mm) is manufactured by a casting method, none. In addition, since the hot rolling process in the slab continuous casting process can be omitted, the thin plate casting process is advantageous in that the manufacturing cost is reduced as compared with the slab continuous casting process, and furthermore, The internal quality of cast steel is also excellent.

However, even with this method, there is a problem that cracks generated in the steel sheet can not be completely prevented, and there is a problem that it is difficult to obtain a good quality at the edge portion.

The present invention is to provide a thin sheet casting roll capable of reducing occurrence of cracks when a high nitrogen stainless steel sheet is manufactured by a thin plate casting method, and in particular, obtaining a good material at an edge portion, and a method of casting a thin plate using the same.

According to one embodiment of the present invention, there is provided a thin sheet casting roll for casting a high nitrogen stainless steel sheet, comprising a surface unevenness and fine grooves, the fine grooves having an area The present invention provides a thin sheet casting roll for casting a high nitrogen stainless steel sheet having an occupied area ratio of 80% or less (excluding 0) and an occupied area ratio linearly increasing from the edge.

According to still another aspect of the present invention, there is provided a method of manufacturing high nitrogen stainless steel comprising the steps of: supplying molten steel having a composition of duplex stainless steel between a pair of rotating thin sheet casting rolls; And casting the thin plate by discharging it through a casting roll, characterized in that a thin plate casting roll having the above-mentioned characteristics is used in the production of the steel plate.

According to the present invention, by improving the surface shape of the thin plate casting roll, more uniform solidification can be achieved, and the problem of cracking of the product can be greatly reduced, and in particular, edge breakage and plate breakage can be fundamentally solved have.

Fig. 1 is a schematic view schematically showing a thin casting method.
2 is a schematic view for explaining a phenomenon that a gas layer is formed between a molten steel and a thin sheet casting roll during thin plate casting of a high nitrogen steel.
3 is a photograph showing an example of a thin steel plate in which dent and depression are generated.
FIG. 4 is a result of observing a shape in which concaves and convexes and fine grooves are formed on the surface of a thin-film casting roll according to an embodiment of the present invention.
5 is a photograph showing an example of a thin steel plate in which an edge portion breakage occurred.
FIG. 6 is a schematic view showing a surface machining mode according to a position of a casting roll according to a conventional method of treating a thin plate casting roll.
FIG. 7 is a photograph of microstructure of a thin plate produced according to the conventional method.
8 is a schematic view showing a surface machining mode according to a position of a casting roll according to an embodiment of the present invention.
9 is a photograph showing a microstructure of a thin plate produced according to an embodiment of the present invention

Hereinafter, the present invention will be described in detail. Usually, the thin plate casting method is divided into a single roll method and a twin roll method, but the present invention does not particularly discriminate the kind thereof. However, two rolls are used, and molten steel is supplied to the space between the rolls and is used more effectively in the twin-roll method in which casting is performed by rotation of two rolls. In the present invention, high nitrogen stainless steel is used as a main target steel, and one example thereof is described by taking a duplex stainless 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 1.5 1-5 0.03 or less 0.02 or less 19-23 1 to 6.5 0.5 to 3.5 0.1 to 0.3

DISCLOSURE OF THE INVENTION The inventors of the present invention have conducted research to solve the problem of dents, depressions, and cracks on the surface of a steel sheet when producing duplex stainless steel containing high nitrogen by thin sheet casting As a result, it was found that one of the major causes of surface defects during thin sheet casting is that the solidification shell is lifted from the roll surface or non-uniform solidification occurs. Especially, It has been found that breakage of plate edge, breakage of plate, and the like may occur when incorporated, 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 above phenomenon discovered by the present inventors will be described with reference to Figs. 1 and 2. Fig. 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 part of the molten steel but also penetrates into between the casting roll and the solidifying shell due to the pressure (refer to 11 in FIG. 2).

When the atmosphere gas layer is formed between the solidification shell and the casting roll, the solidification shell is separated from the casting roll. In this case, due to the thermal physical effects of the molten steel supplied from the immersion nozzle, Defects such as dent, depression, and crack are generated on the surface of the thin plate.

Therefore, in order to solve the remaining problem of the gas layer in the present invention, the surface of the casting roll is shot-blasted to form as many irregularities as possible, thereby facilitating the transfer from the casting roll surface of the molten steel, Thereby forming a fine groove in the casting direction on the surface of the roll.

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 400 mu m in order to obtain a sufficient gas discharging effect and an average interval between fine grooves is in a range of 100 to 700 mu m. When the average width or depth of the fine grooves is less than 50 탆, the gas discharging effect is lowered. Therefore, it is necessary to have a range of 50 탆 or more. However, considering the workability of the grooves, the width or depth does not exceed 400 탆 . On the other hand, if the distance between the fine grooves is too narrow, the contact area required for static pressure and heat transfer of the molten steel becomes too small, and the surface of the product It is preferable that the average interval between the fine grooves has a range of 100 to 700 mu m. The fine grooves of this type can be manufactured by various known methods, and the method of forming the fine grooves in the present invention is not particularly limited. However, for example, a laser or an etching process may be used in which a laser having a high energy is irradiated onto a surface thereof and a groove is formed by chemical etching. A photograph of the surface of the thin plate casting roll thus processed is 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.

On the other hand, when concave and convex portions and fine grooves are formed on the entire surface of the thin plate casting roll as described above, sticking to the edge of the casting roll, sticking occurs. As a result, as shown in Fig. 5, the edge portion is broken. In addition, when the strip pieces adhering extremely to the surface of the casting roll enter the sump accommodating the molten steel, the thickness of the solidified strip becomes thick. At this time, in order to protect the casting roll, when the casting roll is spread in the opposite direction A hot band is generated and the plate is broken. As a result, there arises a problem that the edge portion of the thin plate must be cut and used, and as a result, the productivity is lowered due to the reduction in the yield. Accordingly, in order to prevent edge breakage and sheet breakage which may occur due to sticking due to excessive coagulation scarring, it is possible to prevent the edge breakage and plate breakage from occurring at the edges of the casting rolls 50 to 100 mm A method of not forming stripe-shaped fine grooves is proposed.

FIG. 7 is a photograph of microstructure of a thin plate produced according to the conventional method. According to the above-described conventional methods, sticking is prevented, but it is impossible to overcome the difference in microstructure caused by a rapid difference in surface treatment as shown in FIG. 7, So that the breakdown easily occurs at the boundary of the region. In addition, surface defects such as dent and depression are caused by not machining the fine grooves.

8 is a schematic view showing a surface machining mode according to a position of a casting roll according to an embodiment of the present invention. In order to solve the above-mentioned problem, in the present invention, the area occupied by a region (hereinafter also referred to as an edge portion) from the edge to the edge of the fine groove in the width direction is 80% or less ), And at the same time, the occupied area ratio is linearly increased from the edge portion. The edge can be determined depending on whether the skull is mixed or not. FIG. 9 is a photograph of a microstructure of a thin plate manufactured according to an embodiment of the present invention. As shown in FIG. 9, by using the thin plate casting roll of the present invention provided above, And continuity of the formed region can be imparted, thereby preventing edge breakage and plate breakage when the skull is mixed. Also, it is possible to prevent surface defects that may occur due to grooving through grooving, thereby greatly improving the product realization rate, simplifying the manufacturing process, and reducing manufacturing costs. On the other hand, the area ratio of the edge portion is more preferably in the range of 30 to 80% in order to prevent surface defects, defects such as edge deterioration and plate breakage. The occupied area ratio means the area ratio of the fine grooves to the entire roll surface area as viewed from the top of the roll surface. On the other hand, the occupied area ratio of the central region of the thin plate casting roll except for the edge portion preferably has an occupied area ratio of about 80%, more preferably in the range of 50 to 80% .

8 shows that the roughness of the irregularities is also linear in the edge portion. This is because the edge portion of the casting roll usually has a shortage of the flow of cooling water, that is, the amount of cooling water, By making the roughness of the unevenness to have linearity, it is possible to ensure a uniform cooling ability in the manufacture of thin plates.

The method for producing a duplex stainless steel sheet containing a high nitrogen according to the present invention uses the above-described sheet metal casting roll and follows the following procedure.

That is, the method for producing a duplex stainless steel sheet of the present invention includes the steps of supplying molten steel having a composition of duplex stainless steel between a pair of rotating casting rolls (thin sheet casting rolls), and rotating the pair of rotating molten steel And casting the thin plate by discharging it through the casting rolls.

At this time, in order to prevent cracking of the duplex stainless steel produced as described above, it is preferable that the thin plate casting roll has a surface with concavities and convexities and fine grooves. In addition, it is preferable that the occupied area ratio of the fine grooves is 80% or less (excluding 0) from the edge to 50-100 mm with respect to the width direction, and the occupied area ratio linearly increases from the edge. The irregularities are formed by shot blasting as described above, and the details thereof are in accordance with the above-described method. Further, the fine grooves are formed by various methods such as laser machining as described above, and the detailed shapes and conditions can be determined in accordance with the above-described contents.

1: ladle
2: Turn Dish
3: immersion nozzle
4: Sump
5: casting roll
6: Edge dam
7: Meniscus shield
8: Inline hot rolling
9: Cooling unit
10: Coiling
11: Gas layer

Claims (5)

A thin sheet casting roll for casting a high nitrogen stainless steel sheet,
The surface includes concavities and convexities and fine grooves,
The roughness of the concavities and convexities linearly increases from the edge to 100 mm with respect to the width direction,
The fine grooves have a occupied area ratio of 80% or less (excluding 0) in the region from 50 to 100 mm from the edge with respect to the width direction,
Wherein the occupied area ratio is linearly increased from an edge of the thin sheet casting roll for casting a high nitrogen stainless steel sheet. The method according to claim 1,
Wherein the unevenness has an average depth of 15 to 25 占 퐉. The method according to claim 1,
Wherein the fine grooves have an average width of 50 to 400 mu m and an average depth of 50 to 400 mu m,
Wherein the average interval between the fine grooves is 100 to 700 占 퐉. delete Comprising the steps of: supplying molten steel having a composition of duplex stainless steel between a pair of rotating thin sheet casting rolls; and casting the thin plate by discharging the molten steel into a rotating pair of thin sheet casting rolls while solidifying the molten steel A method for producing a stainless steel sheet,
A method for producing a high-nitrogen stainless steel sheet, characterized by using the thin-film casting roll according to any one of claims 1 to 3.

Images