CASTING ROLL FOR TWIN ROLL STRIP CASTER

Abstract

Disclosed is a casting roll for a twin roll strip caster, and particularly, a casting roll for a twin roll strip caster, in which a continuous gas channel, for preventing the generation of dents in the surface of a strip when casting TWIP steel in order to improve the mechanical properties of the strip, including increasing the tensile strength and elongation, is formed in the surface of the casting roll, thus discharging gas mixed when the strip is cast, thereby preventing the generation of dents in the surface of the strip when casting the TWIP steel, in particular, a continuous gas channel is formed, thereby decreasing a conventional dent index and discouraging the generation of dents.

Description

TECHNICAL FIELD

The present invention relates to a casting roll for a twin roll strip caster, and more particularly, to a casting roll for a twin roll strip caster, in which a continuous gas channel, for preventing the generation of dents in the surface of a strip when casting TWIP steel in order to improve the mechanical properties of the strip, including increasing the tensile strength and elongation, is formed on the surface of the casting roll, thus discharging gas mixed when the strip is cast, thereby preventing the generation of dents in the surface of the strip when casting the TWIP steel, in particular, a continuous gas channel is formed, thereby decreasing a conventional dent index and discouraging the generation of dents.

BACKGROUND ART

In a general twin roll strip casting process, as illustrated in FIG. 1, molten steel 140 in a tundish 130 is supplied between two water-cooling type casting rolls 110, which are rotated in opposite directions, and an edge dam 150 attached to opposite surfaces thereof, through an immersion nozzle 120, and the casting rolls 110 are rotated such that the molten steel 140 is brought into contact with the casting rolls to transfer heat into the casting rolls. Thereby, the molten steel is rapidly solidified to thus form a solidification shell, which is then subjected to rolling reduction using a roll nip, consequently producing a strip 160.

The rolls are provided on both sides thereof with the edge dam 150, which is formed of ceramic, to prevent the leakage of the molten steel from the sides thereof. Further, a closed meniscus shield 180 is provided above the rolls so that no oxides or the like are produced on the surface of the melt, and is filled with inert gas.

Further, in the casting roll 110, there is a cooling water hole for supplying cooling water to cool the casting roll. The surface of the Cu roll, formed of copper or copper alloy, may be plated with a wear resistant material such as Ni or Ni+NiW, or may be subjected to deposit welding, in which the outer surface of the roll is thickly coated with a material (weldclad) containing Cr for wear resistance as a welding layer. The surface of the roll is subjected to surface treatment such as shot blasting.

The strip, which is cast while passing through the casting rolls 110, is subjected to in-line hot rolling of rolling reduction 10˜50%, water cooling, and then winding at a predetermined winding temperature.

TWIP (Twin Induced Plasticity) steel is steel which exhibits twin transformation behavior in the course of molding to drastically increase the elongation thereof, thus assuring high strength and high plasticity, such that it may be applied to automobile steel plates. Generally, TWIP steel is high Mn steel, composed of 1.5% C or less to increase the strength thereof and maintain a solid-liquid zone, 15.0˜35.0% Mn to stabilize austenite in a microstructure, and 0.1˜6.0% Al to stabilize austenite during the processing and induce twin transformation. In the case where such TWIP steel is manufactured through a conventional continuous casting process, many cracks are created in the casting process, making it difficult to realize continuous casting. In particular, upon in-line hot rolling, the production of oxidation scale is non-uniform due to the Mn and Al components contained in greater amounts in the steel, and selective oxidation takes place. Furthermore, an oxide layer is not completely removed, but adheres on the surface of the roll, undesirably generating many dents. Therefore, it is very favorable to conduct twin strip casting without the need for the hot rolling process.

As material having high strength and high plasticity, TWIP steel should satisfy strength of 80 kgf/mm² or more and an elongation of 40% or more. In order to satisfy such high strength and high plasticity, as disclosed in U.S. Pat. No. 5,431,753, the steel should be composed of 1.5% or less C, 15.0˜35.0% Mn, and 0.1˜6.0% Al, and very few defects should be present in the strip.

EP 1595622 discloses a surface treatment method, in which the surface of a casting roll is subjected to laser dimple processing to decrease the surface defects of a steel plate in the case where steel is manufactured through a strip casting process. Such surface treatment may be conducted through laser dimple processing, or dimple processing may be performed through shot blasting. When such surface treatment is conducted on the surface of the roll and STS 304 steel is cast, cracks are prevented from occurring in the surface of the strip, thus enabling the manufacture of a strip having no defects.

However, in the case where such roll surface treatment is conducted and TWIP steel is subjected to strip casting, dents 161 are generated in the surface of the strip, as shown in FIG. 2. FIG. 2(a) is a scanning electron micrograph (SEM) illustrating the surface of the dent 161 in the surface of the strip, and FIG. 2(b) is an optical micrograph illustrating the section of the dent 161 in the surface of the strip. The dent 161, which is a hemispherical pit having a diameter of 0.5˜3 mm and a depth of 0.1˜1 mm generated in the surface of the strip, partially remains in the surface of the strip even after the rolling process, and is thus a surface defect, undesirably resulting in considerably decreased tensile strength and elongation.

DISCLOSURE OF THE INVENTION

Technical Tasks to be Solved by the Invention

Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a casting roll for a twin roll strip caster, which prevents the generation of dents in the surface of a strip when casting TWIP steel, thus improving the mechanical properties of the strip, including increasing the tensile strength and elongation.

Technical Solution

In order to accomplish the above object, the present invention is characterized in that a continuous gas channel 113 is formed in the surface of a casting roll 110.

The gas channel 113 may have a depth ranging from 0.03 mm to 0.3 mm, a width ranging from 0.1 mm to 2 mm, and a channel interval ranging from 0.1 mm to 0.6 mm. Further, the gas channel 113 may be formed in a grid type, a grid type continuous gas channel, formed in the dimple formed through shot blasting, a continuous gas channel, which is longitudinally formed, or a continuous gas channel which is longitudinally finely formed using a laser in the dimple formed through shot blasting.

In the present invention, when a strip of TWIP steel having a thickness of 1˜10 mm is manufactured using a strip casting process, in the casting of TWIP steel composed of 1.5% or less C, 15.0˜35.0% Mn, and 0.1˜6.0% Al, with the balance of Fe and inevitable impurities in a melting material, a gas channel is formed in the surface of the roll to decrease the generation of dents on the surface of the strip.

Advantageous Effects

According to the present invention, the casting roll for a twin roll strip caster has a continuous gas channel formed in the surface thereof to discharge gas mixed when casting the strip, thereby preventing the dents from being generated on the surface of the strip when casting the TWIP steel. In particular, because the continuous gas channel is formed, a dent index may be decreased from 3.36%, which is a conventional value, to 0.04% or less, and the sigma level of the generation of dents may be increased from 0.7 to 4.0.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating a general twin roll strip caster;

FIG. 2(a) is a scanning electron micrograph (SEM) illustrating the surface of the dent generated in the surface of a conventional strip;

FIG. 2(b) is an optical micrograph illustrating the section of the dent generated in the surface of a conventional strip;

FIG. 3(a) is a schematic view illustrating the shape of a casting roll, which is surface treated through shot blasting;

FIG. 3(b) is a photograph illustrating the results of experiment on dents in the surface of a strip manufactured using the casting roll which is surface treated through shot blasting;

FIG. 4(a) is a photograph illustrating the shape of a casting roll surface treated using LBT;

FIG. 4(b) is a photograph illustrating the result of experiment on dents in the surface of the strip, manufactured using the casting roll surface treated through LBT;

FIG. 5(a) is a photograph illustrating the dimples formed in the surface of the casting roll through shot blasting;

FIG. 5(b) is a photograph illustrating the dents in the surface of the strip, cast using the casting roll having the dimples formed through shot blasting;

FIG. 5(c) is a schematic view illustrating the state in which molten steel is brought into contact with the casting roll having the formed dimples;

FIG. 5(d) is a schematic view illustrating the state in which the dent is generated due to the thermal expansion of the gas in the dimple;

FIG. 6 is a photograph illustrating the state in which the portions around the dimples are closed;

FIG. 7 is a schematic view illustrating the rapid solidification test apparatus;

FIG. 8(a) is a photograph illustrating the surface of the casting roll in which a grid type continuous gas channel is formed, according to the present invention;

FIG. 8(b) is a photograph illustrating the surface of the strip, cast using the casting roll in which the grid type continuous gas channel is formed, according to the present invention;

FIG. 9(a) is a photograph illustrating the surface of the casting roll to which roughness is imparted through shot blasting and then a grid type continuous gas channel is finely formed, according to the present invention;

FIG. 9(b) is a photograph illustrating the surface of the strip, cast using the casting roll to which roughness is imparted through shot blasting and then the grid type continuous gas channel is finely formed, according to the present invention;

FIG. 10(a) is a photograph illustrating the surface of the casting roll in which a continuous gas channel is longitudinally formed, according to the present invention;

FIG. 10(b) is a photograph illustrating the surface of the strip, cast using the casting roll in which the continuous gas channel is longitudinally formed, according to the present invention;

FIG. 11(a) is a photograph illustrating the surface of the casting roll in which shot blasting is conducted and then a continuous gas channel is longitudinally formed;

FIG. 11(b) is a photograph illustrating the surface of the strip, cast using the casting roll in which shot blasting is conducted and then the continuous gas channel is longitudinally formed;

FIG. 12(a) is a photograph illustrating the surface of the casting roll in which shot blasting is conducted and then a continuous gas channel is finely formed using a laser;

FIG. 12(b) is a photograph illustrating the surface of the strip, cast using the casting roll in which shot blasting is conducted and then the continuous gas channel is finely formed using the laser;

FIG. 13(a) is a photograph illustrating the surface of the casting roll in which a continuous gas channel is longitudinally formed through mechanical processing and then shot blasting is conducted;

FIG. 13(b) is a photograph illustrating the surface of the strip, cast using the casting roll in which the continuous gas channel is longitudinally formed through mechanical processing and then shot blasting is conducted;

FIG. 14 is a graph illustrating the decrease in the dent index of the surface of the strip and the improvement of dispersion when the continuous gas channel is formed, according to the present invention; and

FIG. 15 is a graph illustrating the dimension of the gas channel, according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a detailed description will be given of the present invention, with reference to the appended drawings.

When casting TWIP steel to a strip, both the case of using a casting roll which is surface treated through shot blasting, as illustrated in FIG. 3, and the case of using a casting roll which is surface treated through LBT, as illustrated in FIG. 4, generate dents in the surface of the strip. Such dents are formed in the shape of a hemispherical pit, having a diameter of 0.5˜3 mm and a depth of 0.1˜1 mm, in the surface of the strip, and are not completely removed even by in-line hot rolling of rolling reduction 10˜50%.

From FIG. 5, it is apparent that the dent is generated by the dimple in the surface of the casting roll.

FIG. 5(a) illustrates the dimples formed in the surface of the casting roll through shot blasting, and FIG. 5(b) illustrates the dents in the surface of the strip, which is cast using the casting roll having the dimples formed through shot blasting. The dents in the surface of the strip correspond to the dimples, which are formed through shot blasting, in the surface of the casting roll. FIG. 5(c) illustrates the state in which the molten steel 160 is brought into contact with the casting roll 110 having the formed dimples 111. The dimple 111 is not completely filled with the molten steel, rather, some gas 162 remains therein. FIG. 5(d) illustrates the state in which the gas 162 remaining in the dimple is brought into contact with hot molten steel to thus thermally expands, undesirably generating the dent. Generally, in the case of stainless steel, having high nitrogen solubility, nitrogen gas is dissolved in the molten steel and the thermal expansion thereof does not occur, thereby preventing the generation of dents. However, in the case of TWIP steel, having low nitrogen solubility, the dents 161 are generated. Further, as for the TWIP steel, Mn is contained in a large amount in the molten steel and is greatly volatilized. The Mn thus volatilized is attached to the surface of the roll, and thereby, the close contact between the molten steel and the roll is obtained, so that the protrusions between the dimples are completely closed to thus prohibit the residual gas from escaping, enhancing the thermal expansion effect.

FIG. 6 illustrates the state in which the portions around the dimples are closed.

As shown in this drawing, the protrusions between the dimples in the surface of the casting roll are brought into complete contact with the molten steel to thus close the contact portions, so that the gas in the dimple does not escape but thermally expands therein, undesirably resulting in dents.

So, in order to prevent the generation of the dents in the TWIP steel, a gas channel for discharging the residual gas in the dimple 111 is formed, and the gas 162 is discharged through the gas channel, thereby inhibiting thermal expansion.

With reference to FIG. 7, the rapid solidification test apparatus is schematically shown.

As shown in this drawing, in the rapid solidification test apparatus, a small amount of molten steel 140 is loaded, and an atmospheric gas 185 is further loaded thereon, after which a substrate 112 formed with the material for a casting roll is immersed in the molten steel for a short period of time, thus enabling observation of the surface of the solidification shell formed on the surface of the substrate. This apparatus enables the simulation of the strip casting process. In the present invention, in order to establish the proccessability of the gas channel and set appropriate dimensions thereof, experiments were conducted using the substrate 112 in which the gas channel was formed through various methods, with the use of the above rapid solidification apparatus.

The method of forming the gas channel includes etching, laser, and mechanical methods.

The etching method is conducted by applying a photoresist on the surface of the roll, radiating light to photosensitize a gas channel, and spraying an etchant to etch the portion of the roll corresponding to the gas channel.

The laser method is conducted by radiating laser light onto the surface of the roll to thus melt and vaporize the portion of the roll corresponding to the gas channel, thus processing the gas channel.

The mechanical method is conducted by processing the gas channel using a fine bite or cutter.

FIG. 8(a) shows the photograph of the surface of the casting roll in which a grid type continuous gas channel is formed, according to the present invention, and FIG. 8(b) shows the photograph of the surface of the strip which is cast using the casting roll having the grid type continuous gas channel formed therein, according to the present invention.

As shown in these drawings, as the results of processing of the grid type gas channel 113, no dents were generated in the surface of the strip 160, and the strip had a surface shape complementary to the surface shape of the casting roll. This is considered to be because the residual gas 162 is discharged through the formed grid type gas channel 113, thereby eliminating the thermal expansion of the gas, consequently preventing the generation of dents 161.

FIG. 9(a) shows the photograph of the surface of the casting roll to which roughness is imparted through shot blasting and then a grid type continuous gas channel is finely formed, according to the present invention, and FIG. 9(b) shows the photograph of the surface of the strip which is cast using the casting roll to which roughness is imparted through shot blasting and then the grid type continuous gas channel is finely formed, according to the present invention.

As shown in these drawings, the residual gas in the dimples of the casting roll, formed through shot blasting, was discharged through the gas channel, and thereby no dents were generated in the surface of the strip.

FIG. 10(a) shows the photograph of the surface of the casting roll in which a continuous gas channel is longitudinally formed, according to the present invention, and FIG. 10(b) shows the photograph of the surface of the strip which is cast using the casting roll having the continuous gas channel formed longitudinally therein, according to the present invention.

As shown in these drawings, as the results of the longitudinal formation of the continuous gas channel in the surface of the casting roll, no dents were generated in the surface of the strip.

FIG. 11(a) shows the photograph of the surface of the casting roll in which shot blasting is conducted and then a continuous gas channel is longitudinally formed, and FIG. 11(b) shows the photograph of the surface of the strip which is cast using the casting roll in which shot blasting is conducted and then the continuous gas channel is longitudinally formed.

As shown in these drawings, as the results in which shot blasting was conducted on the surface of the casting roll and then the continuous gas channel was longitudinally formed, no dents were generated on the surface of the strip.

FIG. 12(a) shows the photograph of the surface of the casting roll in which shot blasting is conducted and then a continuous gas channel is longitudinally finely formed using a laser, and FIG. 12(b) shows the photograph of the surface of the strip which is cast using the casting roll in which shot blasting is conducted, and then the continuous gas channel is finely formed using the laser.

As shown in these drawings, as the results in which shot blasting was conducted on the surface of the casting roll and then the continuous gas channel was longitudinally finely formed using the laser, no dents were generated in the surface of the strip.

FIG. 13(a) shows the photograph of the surface of the casting roll in which a continuous gas channel is longitudinally formed through mechanical processing and then shot blasting is conducted, and FIG. 13(b) shows the photograph of the surface of the strip which is cast using the casting roll in which the continuous gas channel is longitudinally formed through mechanical processing, and then shot blasting is conducted.

As shown in these drawings, after the continuous gas channel was longitudinally formed in the surface of the casting roll using mechanical processing and then shot blasting was conducted, no dents were generated in the surface of the strip.

From these results, it can be confirmed that, when the continuous gas channel is formed in the surface of the casting roll, the generation of the dents can be prevented.

TABLE 1
	Dimension of Gas	Presence of	Generation of
Surface Treatment of	Channel	Continuous	Dents in Surface	Inventive
Casting Roll	(pitch/depth/width)	Gas Channel	of Strip	Range
Shot Blasting Dimple	—	X	Yes	X
Laser Dimple	—	X	Yes	X
Grid Processing	0.5 mm/0.1 mm/0.4 mm	◯	No	◯
Shot Blasting + Grid	0.5 mm/0.1 mm/0.4 mm	◯	No	◯
Processing
Longitudinal Line	0.5 mm/0.15 mm/0.5 mm	◯	No	◯
Processing
Shot Blasting +	0.4 mm/0.1 mm/0.6 mm	◯	No	◯
Longitudinal Line
Processing
Shot Blasting + Laser	0.33 mm/0.07 mm/0.17 mm	◯	No	◯
Longitudinal Fine
Line Processing
Longitudinal Line	0.5 mm/0.15 mm/0.5 mm	◯	No	◯
Processing + Shot
Blasting

As summarized in Table 1, in the present invention, the continuous gas channel is formed in the surface of the casting roll, thus discharging the gas mixed when casting the strip. Thereby, when TWIP steel is cast, dents may be prevented from being generated in the surface of the strip.

Referring to FIG. 14, there is shown a graph illustrating the decrease in the dent index of the surface of the strip and the improvement of dispersion, when the continuous gas channel is imparted, according to the present invention.

When the continuous gas channel is formed in the surface of the casting roll, the dent index can be decreased from 3.36%, which is a conventional value, to 0.04% or less, and thus it can be decreased by 99%. The sigma level of generation of the dents may be increased from 0.7 to 4.0. The dent index is calculated using the following equation. The sigma level is a value obtained by calculating how many more times a distance between a specification mean and a specification limit is than a standard deviation, based on short term process capability, and is a common index indicating the capability of the process. The calculation of the sigma level is based on the number of defects per million opportunities (DPMO). As the results of application of the present invention, the standard deviation of the dent index was decreased from 3.68% to 0.06%, and the sigma level was increased from 0.7 to 4.0.

$\begin{array}{cc}\mathrm{Dent}\mathrm{Index}\left(\%\right)=\frac{\sum _{i=0}^mn_i\left(\frac{\pi D_i^2}{4}\right)}{50\times 50}\times 100& \mathrm{Equation}1\end{array}$

wherein n_i: the number of dents in the 50 mm×50 mm sized strip depending on the diameter thereof, and

D_i: the diameter of the dent in the 50 mm×50 mm sized strip

In FIG. 15, the dimension of the gas channel, according to the present invention, is graphed. The depth of the gas channel is set to at least 0.03 mm, and preferably at least 0.05 mm. There is no upper limit to the depth of the gas channel, in the side contour of the dent. However, as the depth of the gas channel is increased, solidification performance and surface quality are deteriorated, and thus the depth thereof should not be greater than 0.3 mm. Generally, in the case of TWIP steel, the molten steel infiltrates 50% of the gas channel. Hence, when processing the gas channel having a depth of 0.3 mm, a curvature 0.15 mm high is formed on the surface of the strip, but such curvature does not negatively affect surface quality after in-line hot rolling. On the other hand, when the depth of the gas channel is too low, to an extent below 0.03 mm, the surface of the casting roll is worn in the course of continuous strip casting, making it difficult to exhibit the function of the gas channel.

The width of the gas channel is at least 0.1 mm, and preferably 0.15 mm or more. When the width of the gas channel is less than 0.1 mm, impurities (molten steel vapor, molten steel slag, oxidation scale, etc.), which adhere on the surface of the casting roll in the course of strip casting, enter the gas channel, and thereby the gas channel is clogged and thus does not exhibit the gas channel effect. Further, although such impurities may be washed out using a brush roll, wires (diameter 0.1 mm) constituting the brush roll cannot enter a gas channel having a width less then 0.1 mm, and thus it is impossible to conduct the washing process, drastically decreasing the gas channel effect. The surface of the roll undergoes rolling pressure during the strip casting. In such a course, impurities may be compressed thereon, or part of the surface of the roll may be compressed, resulting in no gas channel effect. Further, the width of the gas channel is set to 2 mm or less, and preferably 0.7 mm or less. In the case where the width of the gas channel exceeds 2 mm, the molten steel completely infiltrates the gas channel, and thus the function of the gas channel is lost. That is, it is required that the width of the gas channel be 2 mm or less in order for the molten steel not to completely infiltrate the gas channel.

The gas channel should have a channel interval of 0.1 mm or more between neighboring gas channels. When rolling pressure is applied to the surface of the roll, the interval between the gas channels is a portion to which high force is applied. Thus, in the case where the rolling pressure is intensively applied to such a portion, if the channel interval is narrower than 0.1 mm, the durability of the gas channel is decreased. When the channel interval is narrow, many gas channels may be formed. In this case, considerable processing time and cost are required. As seen in FIG. 5, there are needed one or more gas channels per dimple formed in the surface of the roll through shot blasting. Thus, the channel interval should be set to be equal to or smaller than the minimum size of the dimple formed through shot blasting. When the diameter of the dimple formed in the surface of the roll through shot blasting is 0.6˜1 mm, the channel interval should be set to be equal to or smaller than 0.6 mm at a maximum. However, the size of the ball used for shot blasting may be various, and thus the channel interval should preferably be set to 0.5 mm or less.

INDUSTRIAL APPLICABILITY

As described above, the casting roll of the present invention may be applied to twin roll strip casting in order to prevent the generation of dents in the surface of a strip when casting TWIP steel.

Claims

1. A casting roll for a twin roll strip caster, comprising a dimple, formed through shot blasting, and a continuous gas channel (113), in a surface of the casting roll (110).

2. The casting roll according to claim 1, wherein the gas channel (113) is a grid type continuous gas channel.

3. The casting roll according to claim 1, wherein the gas channel (113) is a continuous gas channel which is longitudinally formed.

4. The casting roll according to any one of claims 1 to 3, wherein the gas channel (113) has a depth ranging from 0.03 mm to 0.3 mm, a width ranging from 0.1 mm to 2 mm, and a channel interval ranging from 0.1 mm to 0.6 mm.

5. The casting roll according to claim 4, wherein the gas channel (113) is finely formed using a laser.