KR100770341B1 - Casting roll for the twin roll strip casting apparatus and fabrication method of the same - Google Patents

Abstract

A casting roll for a twin roll strip casting apparatus and a fabrication method thereof are provided to enhance an amount reduction and widthwise distribution of a delta ferrite of an edge of casting by preventing build-up of the delta ferrite left on both ends of the casting. A casting roll for a twin roll strip casting apparatus includes a main dimple and a sub dimple. The main dimple is big and deep. The sub dimple is small and shallow. Density of the sub dimple is getting reduced toward an edge part over a central part. The casting roll for a twin roll strip casting apparatus improves shaping quality of casting and actual yield of the casting by preventing hot band or bulging in a casting edge part and by enhancing an amount reduction and widthwise distribution of a delta ferrite of the casting edge part.

Description

Casting roll for the twin roll strip casting machine and manufacturing method therefor {Casting roll for the twin roll strip casting apparatus and fabrication method of the same}

1 is a schematic diagram of a conventional general double roll type sheet casting apparatus,

Figure 2 is a photograph of observation of the hot band (hot band) of the end of the conventional cast,

3 is a delta ferrite distribution chart of a conventional slab width direction,

4 is a width direction thickness profile of a cast steel in which a conventional hot band is generated;

5 is a surface shape and a profile of the main dimple of the stochastic structure formed by the shot blasting method according to the present invention;

Figure 6 (a) is a photograph of the surface shape of the main dimple and the laser beam processing auxiliary dimples imprinted on the main dimples according to the present invention,

6 (b) is a cross-sectional view showing the cross-sectional shape of the main dimple and the laser beam processing auxiliary dimples stamped in the secondary dimples according to the present invention;

Figure 7 (a) is a schematic diagram of a laser beam processing apparatus used to give a secondary dimple density differential according to the present invention,

7 (b) is a schematic diagram of P1, P2 and control variables for determining the interval between the dimples of the laser beam processing apparatus used for providing the secondary dimple density differential according to the present invention;

8 is a control variable and algorithm used to reduce the secondary dimple density from the roll barrel to the edge in accordance with the present invention;

9 is a laser beam auxiliary dimple density change table according to the present invention;

10 is a hot strip photograph when casting with a laser beam auxiliary dimple width direction density difference provision roll according to the present invention,

11 is a roll width direction delta ferrite distribution and thickness profile of the cast of FIG. 10 according to the present invention;

<Description of Symbols for Main Parts of Drawings>

601: main dimple 602: auxiliary dimple

700: laser beam auxiliary dimple processing apparatus 801, 802, 803: auxiliary dimple processing process

The present invention relates to a cast roll for a twin roll sheet caster and a method of manufacturing the same. More specifically, a large and deep main dimple and a small and shallow minor dimple are formed on a surface thereof. It improves the shape quality and error rate of casts by preventing hot bands or bulgings, which are internal quality such as reducing the amount of delta ferrite and improving the distribution in the width direction, etc. The present invention relates to a casting roll and a casting roll manufacturing method for a twin roll sheet casting machine.

As shown in FIG. 1, the twin roll-type sheet casting machine 100 used for general strip casting has two casting rolls 101 and an edge dam 102 attached to the side from the tundish 103 through the nozzle 104. After the molten steel is supplied to the space formed by the molten steel pool 105 to be formed, the casting roll 101 is rotated through the contact between the casting roll 101 and the molten steel while rotating the casting roll 101 in opposite directions. It is a device for rapidly casting the molten steel by the heat leakage of the furnace to produce the cast steel (106).

The thickness direction cross-sectional shape of the slab 106 manufactured by the twin roll-shaped sheet casting machine 100 is closely related to the shape of the roll outer surface in the casting space. In order for the cast to be rolled in the cold rolling step, which is a post-treatment process, to improve the flatness of the final product, the cross-sectional shape is ideally rectangular or slightly convex in the center. In order to have such a cross-sectional shape of the cast steel, it is preferable that the bus bar of each casting roll is straight or slightly concave in a roll nip where two rolls are closest in the casting space.

However, in practice, since the roll is heated to a high temperature during casting, even when the roll has a completely linear busbar during cooling, the outer surface of the roll becomes convex due to thermal expansion, so that the plate thickness profile of the solidified cast slab is the gap between these rolls. Since the cross-sectional shape of the negative casting space is accurately reproduced, a slab with a cross-sectional shape is produced, which becomes slightly thicker from the center to the edge. The poor shape cast steel having such a plate thickness profile causes poor rolling during cold rolling, which greatly reduces the quality and the real rate of the final material.

In order to compensate for the thermal expansion phenomenon of the casting roll as described above, it is common to provide a roll crown in a shape in which the width center part is concave and both ends of the roll are convex in the roll processing step. Despite these measures, however, due to the so-called bulging phenomenon of thickening as the molten steel solidifies in the center of the plate thickness at both ends of the slab and the so-called hot band phenomenon in which the temperature at both ends of the slab is high. The center of the piece is flat, but both ends of the cast piece have a rather thick shape.

2 shows a picture of observing the hot band of the slab end 202, which is a picture taken using a hot strip observation camera at the bottom of the roll nip, the center 201 is red, and the slab end ( 202 is bright yellow, and yellow means a high temperature part.

As such, once bulging or hot band occurs at both ends of the cast, it causes a serious problem that results in deterioration of the quality of the sheet and a decrease in error rate.

In particular, in the case of the austenitic stainless steel STS304 steel, which is the main target steel of the twin roll type sheet casting process, the amount or distribution of residual delta ferrite varies greatly depending on the cooling rate during solidification. As little as possible from the stage, it should be kept as small as possible and have a uniform distribution throughout the roll width. The delta ferrite is preferentially produced during solidification, slowly decomposes through transformation as it cools down, and thus the final delta ferrite content is present. In the twin roll type sheet casting process, about 1 to 6% is maintained.

However, if a solidified phenomenon such as bulging or hot band occurs at the end of the slab, the amount of delta ferrite increases toward both ends as shown in FIG. 3 as compared to the center of the as-cast strip. Show an increasing distribution profile.

In Figure 3, the axis of abscissas is the distance from one end of the slab, and the total width is 1,300 mm, and the axis of ordinates represents the measurement of delta ferrite.

In the case of a large amount of delta ferrite remaining in the cast steel, especially at both ends of the cast steel, the internal quality of the final material is greatly degraded, so the study of countermeasures is of great concern.

In addition, as shown in the width direction thickness profile of the slab in which the hot band occurs in Figure 4, it can be seen that the slab has a thicker thickness at both ends of the slab than the average thickness or the center thickness, which means that the hot band It means that the severely generated, having such a thickness profile can not be rolled in the post-process, causing a serious problem even when winding the casting material, and the post-treatment process is also impossible. That is, when the initial solidification shell solidified on the roll surface of the non-uniform thickness causes a variety of defects, such as surface cracks and overlapping defects on the surface of the produced cast steel and affect the final product.

Therefore, not only in terms of stable operation but also in terms of improving the quality and error rate of the cast, preventing the non-solidification phenomenon of bulging or hot band, which is essential for the commercialization of the strip casting process.

In order to prevent the above-mentioned non-solidification phenomenon of both ends of the cast steel, many inventors have been studied in various angles.In the early stage of strip casting process development, the bulging or hot band phenomenon of both ends of the cast steel is basically roll on both ends of the roll itself. As a countermeasure, it is due to the relative decrease in the coagulation capacity of the rollers. As a countermeasure, it was attempted to solve the problem by adjusting the initial roll amount of the rolls and providing cooling differential in the width direction of the rolls.

In Japanese Patent Laid-Open Nos. 6-297108 and 6-328205, a method of adjusting the cooling capacity by dividing a cooling channel into several widthwise directions has been proposed. In the roll nip, a method of imparting a roll crown amount so as to be equal to or more than a predetermined solid phase ratio at the center of thickness of both ends of the slab is disclosed. As another method, as described in Japanese Patent Application Laid-open No. Hei 9-327753, There was also a method to adjust the cooling capacity in the roll width direction through the differential construction of the primary dimple.

In addition, the inventors of the patent 1998-0057611, the method of controlling the cooling ability of the roll edge portion by supplying nitrogen gas, the patent 1999-0042986, the online control method of the thickness and composition of the gas film on the roll surface, and the patent 2000-0079600 casting Preventing the inflow of edge dam wear to the side of the roll prevents hot band phenomenon caused by the solidification delay occurring in the edge of the cast steel, Patent 2001-0083717, Patent 2002-0082840 control the thickness of the gas layer on the surface of the casting roll In order to control the shape of the slab end by means of a device, in particular, 2002-0082902 is concerned with hot band or delta ferrite build-up at the slab end by adjusting the gas layer thickness between the solidification shell and the roll in the roll width direction. A preventive measure has been disclosed.

However, the above-described inventions, namely, the method of adjusting the roll crown and the method of providing cooling differential in the roll width direction, can suppress the non-coagulation phenomenon such as bulging and hot bands at both ends of the slab, but it is not a complete solution. , The gas layer thickness adjusting device on the surface of the casting roll invented by the present inventors has a large effect of artificially giving coagulation ability in the width direction, and thus has a great effect in preventing bulging or hot bands at both ends of the cast steel. Difficulty in controlling the gas layer, which is difficult to observe, has made it difficult to secure reproducibility. In addition, it is possible to impart cooling differentials by adjusting the density of coarse primary dimples formed by photo etching or shot blasting during the surface treatment of rolls. Since it is directly related to the surface quality such as glossiness, clarity, etc., there is a limitation in adjusting the roll cooling ability.

Japanese Unexamined Patent Publication No. 60-184449 discloses processing numerous dimples on the surface of a cooling roll. The dimple forms a gas gap, which is an insulating layer, between the cooling roll and the solidification shell, thereby panning the cooling roll. By reducing the heat extraction to induce a gentle cooling of the solidification shell, while forming the embossed by the dimple on the surface of the slab and the solidification starts from the peripheral edge (dimple edge) of the relief It is to make it uniform in the plate width direction. In addition, in order to effectively prevent the surface cracks of cast steel, the technique that defines the formation of dimples on the surface of the cooling roll, the size of the dimples and the distribution of the dimples is disclosed in Japanese Patent Application Laid-Open Nos. 64-83340 and 64; -83342 and Japanese Patent Laid-Open No. 3-110044.

In the double roll type sheet casting device, the surface condition of the rolls in direct contact with the molten steel is a factor that greatly affects the quality of the cast steel. Therefore, in order to manufacture defect-free casts, the surface treatment of the rolls to effectively disperse the thermal stress of the cast steel is essential. .

The present invention is to overcome the above-mentioned conventional problems, an object of the present invention is a bulging, hot band and edge which is a non-solidified phenomenon at both ends when the austenitic stainless steel STS304 steel sheet is manufactured by a twin roll type strip casting process. In order to prevent secondary delta ferrite buildup, the hybrid roll surface treatment method combined with shot blasting method and laser beam processing method gives surface shape that can effectively disperse heat stress of cast steel and at the same time, unsolidification phenomenon of edge part As a means to overcome this problem, the roll surface treatment method was devised to reduce the density of laser beam processing fine dimples toward the edge portion as compared to the center portion of the roll, thereby contributing to differential cooling of the roll.

The present invention relates to a cast roll for a twin roll sheet caster and a method of manufacturing the same. More specifically, a large and deep main dimple 601 and a small and shallow auxiliary dimple 602 are formed on a surface thereof. And a pair that improves the shape quality and error rate of the slab by preventing hot bands and bulging, which are internal quality, such as reducing the amount of delta ferrite of the slab edge portion 1001 and improving the width direction distribution, and solidification delay phenomenon of the slab edge portion 1001. The present invention relates to a casting roll and a casting roll manufacturing method for a roll type thin casting machine.

Hereinafter, the configuration of the present invention will be described with reference to the accompanying drawings.

5 is a surface shape and a profile of the main dimple of the stochastic structure formed by the shot blasting method according to the present invention. As can be seen from the laser-type three-dimensional shape image 500, in general, when illuminance is given by the shot blasting method, an irregular and random illuminance is formed, which is called a stochastic structure. ), Which is in contrast to the deterministic structure, which is a deterministic structure of regular form.

The diameter of the main dimple 601 formed by the shot blasting method is 0.4 to 1.2 mm, and the depth is processed in the range of 0.05 to 0.15 mm.

501, 502, and 503 indicate the measurement portions of line profiles 501 ′, 502 ′, and 503 ′ representing a two-dimensional cross-sectional shape in the laser type three-dimensional shape image 500.

In the stochastic structure, the roughness profile varies according to the measurement site. However, the diameter and depth are within the above ranges, as can be seen from the line profiles 501 ', 502', and 503 '.

Figure 6 (a) is a photograph of the surface shape of the laser beam processing auxiliary dimples stamped secondary to the main dimple and the main dimple according to the present invention, 6 (b) is a cross-sectional view showing the cross-sectional shape of the main dimple and the auxiliary dimple. .

After the main dimple is formed by the shot blasting method, it is formed into a deterministic structure with a diameter ranging from 0.10 to 0.25 mm and a depth ranging from 0.03 to 0.10 mm using a laser beam of strong energy density.

Fig. 7 (a) is a schematic diagram of a laser beam processing apparatus used for providing sub dimple density differential according to the present invention.

As shown, the laser beam processing apparatus 700 includes a laser generator 701 for generating a laser of a strong energy density, a beam delivery system 702 for transmitting the generated laser to the laser head through an optical system, a transmission tube, and a laser. A driving system including a head 711, an eddy current sensor 712, an XZ stage 710 for controlling displacement of a processing part, a measuring part, and the like, and a target casting roll 721 and a rotating motor 722. consist of.

Fig. 7 (b) is a schematic diagram of P1, P2 and control variables for determining the interval between the dimples of the laser beam processing apparatus used for providing the auxiliary dimple density differential according to the present invention.

As shown, the density differential distribution of the laser processing auxiliary dimple in the casting roll width direction can be achieved by controlling the P1 value determined by the roll rotation speed and the P2 value determined by the feed speed of the X-Z stage.

8 is a control parameter and algorithm used for reducing the secondary dimple density from the roll barrel portion to the edge portion according to the present invention, and FIG. 9 is a secondary dimple density variation diagram.

The surface shape of the laser-assisted dimples constructed on the actual casting rolls by the control method shown in FIG. 8 is shown in FIG. 9, wherein the roll edge portion has a large distance between the auxiliary dimples (P1 and P2 values) for the purpose of increasing the cooling ability, The width center portion was made small so as to reduce the cooling ability according to the increase of the auxiliary dimple density. In addition, to prevent surface degradation such as glossiness and sensibility caused by the difference in roughness transfer rate caused by the rapid change of the auxiliary dimple in width direction, the density change was divided into narrow intervals.

FIG. 10 is a hot strip photograph when cast with a laser beam auxiliary dimple width direction density grant roll according to the present invention, and FIG. 11 is a roll width direction delta ferrite distribution and thickness profile of the cast slab of FIG. 10 according to the present invention.

As shown, it can be seen that a very good surface quality cast can be produced without any edge hot bands or bulging.

By the above configuration, the cast roll for a twin roll sheet caster according to the present invention and a method of manufacturing the same function as follows.

In the twin roll type sheet casting device, the surface state of the rolls in direct contact with the molten steel is a factor that greatly affects the quality of the cast steel. Therefore, in order to manufacture a defect-free cast, the surface treatment of the roll that can effectively disperse the thermal stress of the cast steel is essential. .

As shown in FIG. 5, the shot blasting method of forming dimples by projecting steel balls onto the roll surface with a high-pressure gas is advantageous in having a relatively large and deep dimple having a stochastic structure, while Laser beam processing is a method suitable for surface treatment of minute dimples in a deterministic structure by irradiating a laser beam with a strong energy density onto a roll surface to instantly heat, melt and evaporate an object surface, as shown in FIG. 6. As described above, a major dimple (601) having a diameter in the range of 0.4 to 1.2 mm and a depth in the range of 0.05 to 0.15 mm is first constructed by a stochastic structure, and then 0.10 to 0.25 mm by the laser beam processing method. Minor dimples (602) having a diameter in the range and a depth in the range of 0.03 to 0.10 mm will be constructed in a critical structure.

When the auxiliary dimple 602 is processed using the laser beam processing apparatus 700 illustrated in FIG. 7, as can be seen in FIG. 8, in the first step process 801, the auxiliary dimple 602 is formed step by step. However, processing is performed while narrowing the pitch toward the roll center portion rather than the first edge portion. In the two-stage process 802, the pitch is processed to be constant at the center portion of the roll, and in the following three-stage process, the pitch is gradually processed toward the edge of the roll as opposed to the one-stage process.

A laser beam auxiliary dimple density change table according to the present invention is shown in FIG. 9 and details the one-step process 801 of FIG. 8. As shown, the processing of the auxiliary dimple 602 proceeds through about 20 steps, and the interval 902 of the auxiliary dimple 602 becomes 500 μm as the number of steps 904 is repeated to get closer to the roll center from the edge portion. It narrows to 250 mu m and the area ratio of the auxiliary dimple (903) increases from 9% to 37%.

The reason for reducing the density of the auxiliary dimples 602 toward the edge portion relative to the center portion of the roll is to differentially distribute the cooling ability of the roll toward the edge portion relative to the center portion of the roll to effectively disperse the thermal stress of the cast and to overcome the unsolidification phenomenon of the edge portion. For that.

FIG. 10 shows a hot strip photograph in the case of casting with a laser beam auxiliary dimple width direction density difference imparting roll according to the present invention. Compared with the photograph of the conventional hot band end of the slab end 202 of FIG. 2, it can be seen that the present invention has no hot band of the slab end 1001.

Also, as can be seen in the diagram of FIG. 11 (a), the delta ferrite distribution in the slab width direction was significantly reduced at the edge portion as compared with the conventional slab delta ferrite distribution in FIG. 3, and the diagram of (b) in FIG. As can be seen from the present invention, it can be seen that the thickness of the edge portion is thinner as compared with the conventional slab width direction thickness of FIG.

<Example>

In the present invention, in the case of the SBT process was constructed with a diameter of 0.6 ~ 1.0mm, depth 0.08 ~ 0.12mm, which is the most suitable roughness shape conditions for STS304 steel, the diameter of the steel ball (steel ball or bearing ball containing Cr element) used at this time is It was 2.0 ~ 2.5mm and the projection pressure was 7.0 ~ 8.0bar and it was constructed with stochastic structure.

In the LBT process, laser beams with strong energy density were used in the form of crystal structures with diameters ranging from 0.10 to 0.25 mm and depths ranging from 0.03 to 0.10 mm.

As a result of manufacturing the austenitic stainless steel STS304 steel sheet in the above process, it was possible to prevent the unsolidified phenomenon of bulging, hot band and edge delta ferrite buildup at both ends of the cast steel.

As described above, the casting roll for the twin roll sheet caster according to the present invention and the manufacturing method thereof prevents build-up of residual delta ferrite at both ends of the slab, thereby reducing the amount of delta ferrite at the edge of the slab and improving the distribution in the width direction. By preventing the hot band or bulging, which is the coagulation delay phenomenon of the slab edge, there was an effect of increasing the shape quality and the error rate of the slab.

Claims (5)

A pair of large and deep main dimples 601 and small and shallow sub dimples 602 are formed on the surface, and the auxiliary dimples 602 are characterized in that the density of the dimples decreases toward the edge of the roll center. Casting roll for roll type sheet casting machine. The method of claim 1, The main dimple (601) is formed by a shot blasting method, the auxiliary dimple (602) is a casting roll for a twin roll sheet caster, characterized in that formed by a laser beam processing method. delete The method of claim 1, The density of the auxiliary dimples (602) is a casting roll for a twin-roll sheet caster, characterized in that achieved by adjusting the distance between the dimples. In the method for forming a dimple on the surface of the casting roll for a twin roll sheet caster, After forming a relatively large and deep main dimple 601 into a stochastic structure on the surface of the casting roll, a small and shallow sub dimple 602 is formed on the main dimple in a deterministic structure by a laser beam processing method. The casting roll surface composite dimple forming method of the twin roll type sheet casting machine, characterized in that (602) is formed so as to decrease the density of the dimple toward the edge portion relative to the center portion of the roll.

Images