KR101182052B1 - Controlling method for surface defection of stainless steel and stainless steel manufactured using the same - Google Patents

Abstract

The present invention relates to a method for controlling surface defects of stainless steel that can improve surface defects, and to stainless steels produced using the same.
According to the present invention, during the continuous casting process of stainless steel for producing cast steel from molten steel using a machine, the immersion nozzle in the machine so that the temperature deviation of the center portion and the outer portion in the width direction of the cast steel ranges from 0 ℃ to 5 ℃ Provided are a method for controlling surface defects of stainless steel in which an ejection angle is controlled, and a stainless steel manufactured using the same.

Description

Controlling method for surface defection of stainless steel and stainless steel manufactured using the same}

The present invention relates to a method for controlling surface defects of stainless steel and a stainless steel manufactured using the same, and more particularly, to a method for controlling surface defects of stainless steel that can improve surface defects and a stainless steel manufactured using the same.

In general, stainless steel should have a beautiful surface and excellent surface compared to general carbon steel, and therefore, surface defects should be reduced, particularly during continuous casting of stainless steel.

In the molten steel containing a significant proportion of sulfur (S), the surface tension is lowered by the sulfur, and thus tensile stress acts in the width direction of the slab in the mold, thereby causing cracks vertically on the upper surface of the slab.

If the cracks are intensified due to such duty free, it is impossible to make stainless steel products and the entire scrap process is required, so proper continuous casting technology is required.

In particular, the high sulfur-containing martensitic stainless steel is used as a mold for manufacturing high hardness and high strength materials, and thus the demand for using high sulfur-containing martensitic stainless steel having excellent corrosion resistance is increasing. Steel continuous casting technology is required.

It is an object of the present invention to provide a method capable of controlling surface defects of stainless steel.

Another object of the present invention is to provide a stainless steel with improved quality by reducing surface defects.

According to an aspect of the present invention, in the continuous casting process of the stainless steel to manufacture the cast from molten steel using a machine, the temperature deviation of the center and the outer portion in the width direction of the cast in the range of 0 ℃ to 5 ℃ A surface defect control method of stainless steel in which the discharge angle of an immersion nozzle is controlled is provided.

Here, the discharge angle may be controlled in the range of 0 ° to 5 °.

In addition, the casting temperature of the cast can be controlled in the range of 15 ℃ to 25 ℃.

In addition, the molten steel may be manufactured into the cast by being cooled by the primary cooling by the mold in the machine and the secondary cooling by the cooling water supplied from the cooling water supply while descending to the vertical curved shape.

On the other hand, the specific amount of the cooling water supplied in the secondary cooling may be controlled in the range of 0.40l / kg to 0.45l / kg.

Here, the stainless steel is C: 0wt% to 0.07wt%, Cr: 12wt% to 15wt%, Ni: 0wt% to 0.8wt%, Mn: 0wt% to 1.5wt%, P: 0wt% to 0.05wt%, S: 0.1wt% to 0.2wt%, Si: 0wt% to 0.7wt%, N: 0wt% to 0.06wt%, may be martensitic stainless steel comprising the remaining Fe and other unavoidable impurities.

According to another aspect of the invention, C: 0wt% to 0.07wt%, Cr: 12wt% to 15wt%, Ni: 0wt% to 0.8wt%, Mn: 0wt% to 1.5wt%, P: 0wt% to 0.05wt %, S: 0.1wt% to 0.2wt%, Si: 0wt% to 0.7wt%, N: 0wt% to 0.06wt%, and the remaining Fe and other unavoidable impurities. The stainless steel produced by controlling the discharge angle of the immersion nozzle in the machine is controlled so that the temperature deviation of the center portion and the outer portion in the width direction of the cast steel in the continuous casting process of the stainless steel to be produced is in the range of 0 ℃ to 5 ℃.

Here, the discharge angle may be controlled in the range of 0 ° to 5 °.

According to the present invention, it is possible to provide a method for controlling surface defects of stainless steel that can improve surface defects such as cracks in duty free, and stainless steel manufactured using the same.

In addition, by appropriately controlling the secondary cooling during continuous casting to minimize the air gap between the mold and the cast, and to minimize the air gap that is not solved by minimizing the air gap casting temperature 15 ℃ to 25 ℃ compared to the intrinsic melting point of the metal It is possible to provide the effect of reducing the cracks in the duty-free of stainless steel by controlling the widthwise temperature deviation of the cast steel to be in the range of 0 ℃ to 5 ℃.

1 is a cross-sectional view schematically showing a continuous casting process according to an embodiment of the present invention.
FIG. 2 is a perspective view schematically showing an enlarged immersion nozzle of FIG. 1. FIG.
3 is a cross-sectional view of the immersion nozzle according to an embodiment of the present invention, which is taken along AA of FIG. 2.
4 is a cross-sectional view schematically illustrating the immersion nozzle according to another embodiment of the present invention, which is taken along AA of FIG. 2.
5 is a graph showing the relationship between the discharge angle of the immersion nozzle and the temperature difference in the width direction of the cast steel according to the embodiment of the present invention.
6A is an enlarged photograph of a surface of stainless steel according to a comparative example of the present invention.
Figure 6b is an enlarged photograph of the surface of the stainless steel according to an embodiment of the present invention.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention and other matters required by those skilled in the art will be described in detail with reference to the accompanying drawings. However, the present invention may be embodied in various different forms within the scope of the claims, and thus the embodiments described below are merely exemplary, regardless of expression.

In describing the present embodiment, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. It should be noted that the same elements in the drawings are represented by the same reference numerals and symbols as much as possible even though they are shown in different drawings. In addition, the thickness or size of each layer in the drawings may be exaggerated for convenience and clarity of description and may be different from the actual layer thickness or size.

In an embodiment of the present invention, the stainless steel may be martensitic stainless steel, the martensitic stainless steel is C: 0wt% to 0.07wt%, Cr: 12wt% to 15wt%, Ni: 0wt% to 0.8wt% , Mn: 0wt% to 1.5wt%, P: 0wt% to 0.05wt%, S: 0.1wt% to 0.2wt%, Si: 0wt% to 0.7wt%, N: 0wt% to 0.06wt%, remaining Fe and It may contain other unavoidable impurities.

Martensitic stainless steel as an example in the embodiment of the present invention is a stainless steel containing sulfur at a high concentration, the embodiment of the present invention is not limited thereto. Recently, the use of high sulfur-containing martensitic stainless steel with excellent corrosion resistance, which is used as a mold for manufacturing high hardness and high strength materials, has been increasing. Therefore, there is a demand for continuous casting process of high sulfur-containing martensitic stainless steel without cracks.

Duty-free cracking occurs for the first time in the mold, so it is necessary to control the casting temperature. In addition, the cooling characteristics of the secondary cooling during continuous casting is an important factor affecting the surface and internal quality of the cast steel, because the secondary cooling determines the solidification thickness, the temperature gradient in the solidification layer and the strength of the solidification layer. Because. The duty free cracks generated in the cast steel can be controlled according to the conditions of the secondary cooling, and in particular, the small duty free cracks generated in the mold may be deteriorated or reduced according to the conditions of the secondary cooling.

In general, the main cause of the duty-free crack generation is due to the air gap phenomenon between the solidification layer and the mold due to the δ → γ transformation and the hot water level hunting. Thus, conventionally, the use of mold powder and grooved mold with low heat transfer reduced duty free cracking. However, caution is required when casting high sulfur-containing martensitic stainless steel in this way, if the thermal conditions are not optimized, cracks may occur in the duty free and severely lead to accidents during performance such as breakout. In addition, since high sulfur-containing martensitic stainless steel contains a large amount of sulfur, the strength and ductility of the solidified layer tends to be easily cracked in the solidified layer, and the surface tension of molten steel containing a large amount of sulfur is very low. Since the meniscus shape is very inadequate and the mold powder inflow state is bad, it is difficult to solve the crack in duty free, so it is not established as a systematic technique.

In order to solve the above problems, in the embodiment of the present invention, to implement a surface defect control process of stainless steel that can improve the cracks duty-free and proposes a method for providing a stainless steel manufactured using the same as follows.

Hereinafter, a process for controlling surface defects of stainless steel according to an embodiment of the present invention will be described with reference to FIGS. 1 to 4.

1 is a cross-sectional view schematically showing a continuous casting process according to an embodiment of the present invention, Figure 2 is a perspective view showing an enlarged immersion nozzle of Figure 1, Figure 3 is a view along the AA of FIG. 4 is a cross-sectional view schematically showing an immersion nozzle according to an embodiment of the present invention, and FIG. 4 is a sectional view schematically showing an immersion nozzle according to another embodiment of the present invention. .

Referring to FIG. 1, the player 100 used to manufacture the cast steel 60 may include a ladle 10, a tundish 20, a mold 30, a transfer roll 50, and a coolant supply device 40. Include. The method of manufacturing the cast steel 60 using the player 100 having the above-described structure is as follows.

First, molten steel 5 which has been refined to the ladle 10 is provided, and the molten steel 5 filled in the ladle 10 is provided by the hydraulic or electric sliding gate nozzle G1 of the shrouding nozzle 15. The flow rate is adjusted and provided to the tundish 20 side. The tundish 20 serves as a buffer for temporarily storing the molten steel 5 between the ladle 10 and the mold 30, and the molten steel 5 provided to the tundish 20 is hydraulic or electric. The flow rate is controlled by the sliding gate nozzle G2 and is provided to the mold 30 through the discharge port 27 of the immersion nozzle 25.

The molten steel 5 provided to the mold 30 side is shaped into the slab 60 by the mold 30 and at the same time, primary cooling is performed on the slab 60.

The slab 60 shaped by the mold 30 and manufactured by primary cooling is conveyed by a feed roll 50. More specifically, the slab 60 withdrawn from the mold 30 is lowered to the ground surface 1 side by the feed roll 50, in the cross section, the slab 60 is parallel to the first direction (D1). Descending while drawing a vertical curved shape.

On the other hand, while the cast steel 60 is transported by the feed roll 50, the cast steel 60 is secondarily cooled by the cooling water provided from the cooling water supply device (40).

According to the embodiment of the present invention, during the continuous casting process of the stainless steel to manufacture the cast steel 60 from the molten steel 5 using the player 100, the temperature deviation of the center and the outer portion in the width direction of the cast steel 60 By controlling the discharge angle θ of the immersion nozzle 25 in the player 100 so that the DT ₁ is in the range of 0 ° C. to 5 ° C., it is possible to implement a process in which crack generation is controlled in a duty free manner of stainless steel.

Duty-free cracking occurs in the mold 30, and mainly occurs as the width of the cast slab 60 is increased. Focusing on this point, the temperature of the center portion and the outer portion of the width direction of the slab 60 was measured, and as the temperature deviation became larger, the depth of cracks also increased. When immersion cracks were observed by adjusting the discharge angle θ of the immersion nozzle 25 so that the temperature deviation of the center portion and the outer portion of the width direction of the cast slab was in the range of 0 ° C to 5 ° C. When the discharge angle θ of the nozzle 25 was in the range of 0 ° to 5 °, cracks did not occur vertically.

Here, the temperature deviation DT _{1 in} the width direction of the cast steel 60 in the mold 30 was defined as a temperature measured at a position of 10 mm from the surface of the molten steel 5 using a thermocouple of the molten steel. Can be calculated by

[Equation 1]

Width direction temperature deviation of cast steel (ΔT ₁ ) = intrinsic melting point of cast metal

1 and 2, the immersion nozzle 25 according to the exemplary embodiment of the present invention includes a hydraulic or electric sliding gate nozzle G2 at an upper portion thereof, and a discharge hole 27 having a slope formed at a lower portion thereof. The molten steel 5 flows into the mold 30 through the discharge port 27. At this time, according to the embodiment of the present invention, discharging by forming incline the discharge port 27 so that the widthwise temperature difference (ΔT ₁₎ is 0 ℃ to 5 ℃ range center and the outer portion in the width direction of the cast slab 60, each (θ) can be controlled. As described above, the duty free crack generation of the stainless steel may be controlled by controlling the discharge angle θ of the molten steel 5.

Referring to Table 2 and Figure 3, as the inclined surface (d1) is formed in the discharge port (27a) of the immersion nozzle (25a) according to an embodiment of the present invention with respect to the parallel plane (w) with the ground as shown in Table 2 Upward (+) discharge angle θ1 is formed.

Referring to Table 2 and Figure 4, as the inclined surface (d2) is formed in the discharge port (27b) of the immersion nozzle (25b) according to another embodiment of the present invention with respect to the parallel surface (w) of the ground as shown in Table 2 A downward (-) discharge angle θ2 is formed.

On the other hand, the primary cooling in the mold 30 is more dependent on the temperature of the molten steel 5 in the mold 30 than the cooling by the mold cooling water (not shown), the casting temperature ΔT ₂ is as follows. Can evaluate

&Quot; (2) "

Casting temperature (ΔT ₂ ) = molten steel temperature-metal intrinsic melting point

Meanwhile, the molten steel 5 is first cooled by the mold 30 while descending to a vertical curved shape, and then secondly cooled by the cooling water supplied from the cooling water supply device 40 to produce the cast steel 60. .

Here, the degree of secondary cooling can be represented as a specific quantity (l / kg) that can be calculated by Equation 3 below, and the specific quantity is very useful not only for comparing the degree of cooling by steel type but also for designing a cooling table. .

&Quot; (3) "

S = Q / (V × W × D × G)

Where: S: specific quantity (l / kg), Q: total quantity of secondary cooling stand (l / min), W: slab width (m), D: slab thickness (m), G: density (kg / cu.m ), V: casting speed (m / min).

Comparative example  And Example

After preparing the high sulfur-containing martensitic stainless steel (5) containing the main elements in the range as shown in Table 1 below, the casting speed was 1.0mm / min through the above-described continuous casting process, the thickness of 200mm, width 1500mm or more To prepare a cast steel (60).

Hereinafter, with reference to Table 2 and 5 will be described examples and comparative examples of the high sulfur-containing martensitic stainless steel having a composition of Table 1.

Table 2 below, by appropriately adjusting the widthwise temperature deviation (ΔT ₁ ) and casting temperature (ΔT ₂ ) of the cast steel of high sulfur-containing martensitic stainless steel, by appropriately controlling the specific water amount (l / kg) during secondary cooling Test results of the manufactured examples and comparative examples are shown, and FIG. 5 is a graph showing the relationship between the discharge angle θ of the immersion nozzle 25 and the width direction temperature deviation ΔT ₁ of the slab 60.

As in Examples 1 to 4, the widthwise temperature deviation ΔT ₁ of the central portion and the outer portion of the slab 60 in the width direction is adjusted to 5 ° C or less, and the specific amount (l / kg) is 0.45 l / kg. It was adjusted to the following, and the crack did not generate | occur | produce duty-free when discharge angle (theta) is 5 degrees of downward (-) or 0 degrees of upward (+). On the other hand, as in Comparative Examples 1 to 5, the width direction temperature deviation ΔT ₁ of the center portion and the outer portion in the width direction of the slab 60 is greater than 5 ° C or the specific amount (l / kg) is 0.45 l / kg. If the discharge angle (θ) is greater than 5 degrees downward (-) or upward (+) 5 ° or more cracks occurred duty-free. Referring to FIG. 5, when the width direction temperature deviation ΔT ₁ of the center portion and the outer portion in the width direction of the slab 60 is in a range of 0 ° C. to 5 ° C., the discharge angle θ of the immersion nozzle 25 is downward. It can be seen that it is negative 5 ° or upward 0 °.

Here, in Comparative Example 1 was treated with the quantity ratio (l / ㎏) is carried casting temperature being the same conditions as in Example (ΔT ₂₎ If the high to 50 ℃ there by a crack defect caused by duty-free total amount of scrap. As described above, even when the stainless steel was cast by appropriate secondary cooling, crack defects occurred in a duty-free manner when the primary cooling conditions, that is, the casting temperature ΔT ₂ was high. The casting temperature (ΔT ₂ ) is generally low, but it is difficult to control the temperature below 15 °. Therefore, when the casting temperature is controlled to 25 °, there is an effect of reducing the crack vertically.

When SEM and EPMA were observed on high sulfur-containing martensitic specimens to investigate the components around the cracks, the powder component of the mold 30 was detected, which caused cracks in the mold 30. And it was found. If the mold 30 and the secondary cooling were not optimized as in the above embodiment, cracks were generated in the slab 60 in a duty-free manner, and powder components of the mold 30 were detected around the cracks in the duty-free crack, and the internal coagulation structure was fine. Positive MnS precipitated out.

The slab 60 widthwise temperature deviation ΔT ₁ in the mold 30 is redissolved by the collision of the metal stream. At this time, when the slab 60 in the width direction temperature deviation ΔT ₁ in the mold 30 is large, the tensile stress in the width direction is increased at the center of the slab 60 to cause cracks. Due to the strong cooling of the upper end of the secondary cooling stand, the gap between the mold 30 and the solidification layer increases, so that the cracks do not disappear due to the duty-free duty generated in the mold 30 and the heat extraction of the thin slab is not smooth. The same duty-free duty as 2 shows cracks. In contrast, in the case of the first embodiment of FIG. 6B, no crack is generated in the duty-free manner.

The cause of the duty-free cracking is that the surface tension of the high sulfur-containing martensitic steel molten steel 5 is low, so that the powder inflow of the mold 30 is nonuniform, resulting in uneven solidification. As a result of this, an air gap is formed in the mold 30, and a fine amount of MnS is precipitated in the place where solidification is delayed due to the formation of the air gap. Due to the phenomenon in which the strength and ductility of the solidification layer are deteriorated, it is estimated that cracks are generated in the mold 30 due to the duty-free cracking of MnS in the internal solidification structure.

According to the present invention, it is possible to provide a method for controlling surface defects of stainless steel that can improve surface defects such as cracks in duty free, and stainless steel manufactured using the same.

In addition, by properly controlling the secondary cooling during continuous casting to minimize the air gap between the mold and the cast, and also to minimize the air gap is not solved by minimizing the air gap casting temperature is 15 ℃ to the intrinsic melting point of the metal By maintaining the temperature in the range of 25 ° C and controlling the widthwise temperature deviation of the cast steel to be in the range of 0 ° C to 5 ° C, it is possible to provide the effect of reducing cracks in the duty free of stainless steel.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention.

5: molten steel 10: ladle
15: shrouding nozzle 20: tundish
25: immersion nozzle 27: discharge port
30: mold 40: cooling water supply device
50: feed roll 100: player

Claims (8)

During the continuous casting process of stainless steel for producing cast steel from molten steel using a machine, the discharge angle of the immersion nozzle in the machine is controlled so that the temperature deviation of the center and the outer part in the width direction of the cast steel is in the range of 0 ° C to 5 ° C. Surface defect control method of stainless steel. The method of claim 1,
The discharge angle is a surface defect control method of stainless steel, characterized in that controlled in the range of 0 ° to 5 °. The method of claim 1,
Casting temperature of the cast steel surface defect control method of the stainless steel, characterized in that controlled in the range of 15 ℃ to 25 ℃. The method of claim 1,
The molten steel is lowered into a vertical curved shape and cooled by primary cooling by a mold in the machine and secondary cooling by cooling water supplied from a cooling water supply device, thereby manufacturing the surface defects of stainless steel. . The method of claim 4, wherein
The specific amount of the cooling water supplied from the secondary cooling is controlled in the range of 0.40l / kg to 0.45l / kg surface defect control method of stainless steel. The method of claim 1,
The stainless steel is C: 0wt% to 0.07wt%, Cr: 12wt% to 15wt%, Ni: 0wt% to 0.8wt%, Mn: 0wt% to 1.5wt%, P: 0wt% to 0.05wt%, S: Surface defect control of stainless steel, characterized in that martensitic stainless steel comprising 0.1wt% to 0.2wt%, Si: 0wt% to 0.7wt%, N: 0wt% to 0.06wt%, remaining Fe and other unavoidable impurities Way. C: 0wt% to 0.07wt%, Cr: 12wt% to 15wt%, Ni: 0wt% to 0.8wt%, Mn: 0wt% to 1.5wt%, P: 0wt% to 0.05wt%, S: 0.1wt% to 0.2 wt%, Si: 0wt% to 0.7wt%, N: 0wt% to 0.06wt%, remaining Fe and other unavoidable impurities, and in the continuous casting process of stainless steel to manufacture the cast steel from molten steel using a playing machine Stainless steel produced by controlling the discharge angle of the immersion nozzle in the machine so that the temperature deviation of the center portion and the outer portion in the width direction of the cast piece is in the range of 0 ℃ to 5 ℃. The method of claim 7, wherein
The discharge angle is stainless steel, characterized in that controlled in the range of 0 ° to 5 °.

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