JPH057989A - Continuously cast billet and its surface reforming method and continuous casting device - Google Patents

Abstract

PURPOSE:To provide a continuously cast billet excellent in plastic workability such as grinding work, hot working, successively casting in a steel having no crystallization of eutectic carbide. CONSTITUTION:This cast billet has resolidified layer in a discontinuous solidified corresponding part on a surface layer and as the concrete surface reforming method for cast billet, this is obtd. by melting or half-melting the discontinuous solidified part. Further, in a continuous casting device by using the continuous casting device providing a heating means between a cooling zone and a cutting device, the discontinuous solidified corresponding part in the surface layer part on the cast billet is made to the resolidified layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention refers to a relatively low C steel such as carbon tool steel and alloy tool steel in addition to stainless steel and heat resistant steel, and is a continuous steel that does not crystallize so-called eutectic carbide. The present invention relates to a cast slab, a surface modification method thereof, and a surface casting apparatus.

[0002]

2. Description of the Related Art The steel targeted by the present invention refers to steel that does not crystallize eutectic carbides, such as ferritic stainless steel, martensitic stainless steel, austenitic stainless steel and heat-resistant steel for internal combustion engine members. In addition, JIS
Among steels defined as carbon tool steels and alloy tool steels in foreign standards such as and ASTM, tool steels having a relatively low C and steels generally used industrially as their improved steels are meant. Among the continuous casting, in horizontal continuous casting (hereinafter referred to as horizontal continuous casting), the slab is wit at the joint between the old solidified shell that was present at the start of drawing by intermittent drawing and the new solidified shell that was newly created during the drawing process. Surface defects with discontinuous solidification called Nesmark or cold shut occur.

Also, in vertical continuous casting, new and old solidified shells are generated as the mold is vibrated, and the horizontal continuous casting referred to as an oscillation mark is formed in the joint portion of the surface layer of the slab. Similar surface defects are generated (hereinafter, typically referred to as witness mark). These surfaces are
It is observed as a linear discontinuous mark in a direction perpendicular to the longitudinal direction of the slab, and depending on the casting conditions, this discontinuous mark portion may open as a V-groove as an unbonded portion. Further, immediately below the discontinuous mark portion, there is a discontinuous surface of the cast structure.

In particular, in the case of a horizontal continuous casting method in which a lubricant such as powder cannot be used, the discontinuous solidification portion tends to open in a V-groove shape due to no pressure bonding. As an example, a schematic view of the microstructure of the discontinuous solidification portion generated in the surface layer of the slab when horizontal continuous casting is performed on SKD61, which is a typical alloy tool steel, is shown. In FIG. 1, it can be seen that the discontinuous solidification portion opens in a V-groove shape, and the cast structure is discontinuous immediately below it. Further, in FIG. 2, the discontinuous solidification portion is not opened, but the cast structure is discontinuous as in FIG. 1. As shown in FIGS. 1 and 2, there is a discontinuous surface of the cast structure immediately below the discontinuous mark generated on the surface of the slab, and the dendritic structure is growing in the opposite direction to this surface. Recognize.

[0005]

DISCLOSURE OF THE INVENTION Generally, the discontinuously solidified portion of a continuously cast slab is particularly fragile and significantly hinders subsequent plastic working. That is, when the discontinuous solidification portion is open, not only causes stress concentration during plastic working, but even when the discontinuous solidification portion is not open, the discontinuous portion of the cast structure significantly reduces ductility. This is because. Therefore, conventionally, the discontinuous solidified portion of the cast piece after continuous casting has been completely removed by grinding or cutting.
However, in the case of grinding, the opening may be expanded or newly opened due to the processing heat. In the case of cutting, the discontinuous solidification part is not opened, but bending or deformation of the slab, etc. Therefore, there is a problem that the processing time becomes remarkably long and the yield decreases.

Further, if it is attempted to intensively remove only the discontinuously solidified portion of the slab, the surface of the slab becomes uneven, which hinders the subsequent plastic working. There is a problem in terms of material yield as well as increase. An object of the present invention is to provide a continuous casting slab excellent in plastic workability such as hot working performed following casting particularly for steel that does not crystallize eutectic carbide, a surface reforming method, and a continuous casting apparatus. Is.

[0007]

In the process of studying continuous casting of steel, the inventor of the present invention has a discontinuous surface of the cast structure immediately below the witness mark portion and has a V groove as an unbonded portion. It was found that there was a high frequency of opening in a shape. And, the depth of the structure discontinuity surface of the witness mark part, in addition to the composition to be cast, the melt temperature, the slab size, it depends on the casting conditions such as the drawing cycle and the frequency of the mold, but it is usually 1 to 3 mm. Depth. Therefore, if a part or all of this depth is melted or semi-melted, the discontinuous solidification equivalent part (witness mark part) is eliminated as a resolidification layer, and the opening part is closed at the same time, so in the next step. It turned out that some grinding and hot working can be done without any problems.

That is, the first aspect of the present invention is a continuous cast slab characterized in that a portion corresponding to discontinuous solidification of a continuous cast slab of steel that does not crystallize eutectic carbide has a resolidification layer. According to a second aspect of the present invention, in the method for continuously modifying a continuously cast slab of steel in which eutectic carbide is not crystallized, a part or all of the surface layer of the continuously cast slab is melted or semi-molten. The method for surface modification of continuously cast slabs is
The invention is between a cooling zone and a cutting device of a continuous casting device for steel that does not crystallize eutectic carbides, and melts or semi-melts part or all of the witness mark part or the oscillation mark part of the slab. It is a continuous casting apparatus characterized by comprising heating means.

[0009]

In the continuously cast slab of the present invention in which the portion corresponding to discontinuous solidification has the resolidified layer, the discontinuous solidified portion is melted or semi-melted by the heating means to form the resolidified layer. As a result, not only the portion opened to the discontinuous solidification portion is closed, but also the discontinuous portion of the cast structure disappears, and the plastic workability such as hot working thereafter becomes good. In the present invention, the target steel is limited to the steel which does not crystallize eutectic carbides, because when the steel crystallizing eutectic carbides is reheated to a temperature higher than the eutectic temperature, the carbides in the molten or semi-molten layer are regenerated. By crystallization, the uniformity of carbide grain size and distribution may be impaired, but in steels that do not have eutectic carbides, such concerns are reduced,
This is because the range of heatable temperature is widened, which is advantageous in terms of process. Further, the heating for forming the re-solidified layer is preferably concentrated only in the discontinuous solidified portion, and is preferably melted or semi-molten, but it is industrially preferable to carry out the entire circumference of the slab surface layer portion. Therefore, it becomes easy to control the heating means.

In order to melt or semi-melt the surface layer of the cast slab, local heating that can raise the temperature in a short time and lower the temperature in a short time is effective. As a heat source, high frequency, plasma arc or laser beam is used. Should be adopted. Furthermore, the time for melting or semi-melting the surface layer is
Although it is possible to reheat the slab at room temperature after the continuous casting is completed, it is not possible due to the transformation stress in the case of a material that undergoes transformation on the low temperature side such as pearlite transformation or martensite transformation during cooling after continuous casting. The frequency at which the continuous solidification part opens is significantly increased.

Therefore, it is preferable to carry out immediately after the end of continuous casting on the high temperature side where the transformation of the slab does not begin, which is advantageous in preventing the loss of thermal energy by utilizing the heat during casting. As a concrete device, it is effective to melt or semi-melt the surface layer part of the slab in a continuous line using a continuous casting device provided with heating means between the cooling zone of the continuous casting device and the cutting device. . The heating means of the surface modification method and apparatus of the present invention can be applied to both a vertical continuous casting apparatus and a horizontal continuous casting apparatus. FIG. 3 shows an example in which the heating means is applied to a horizontal continuous casting apparatus as an example. Further, the depth to be melted or semi-melted is the depth of the discontinuous solidification layer, that is, it is desirable to make the entire witness mark portion into a resolidification layer, but in the case where there is an opening in the slab surface layer portion, The effect is recognized because the opening is closed only by making a part of the witness mark part a re-solidified layer.

[0012]

【Example】

(Example 1) An alloy tool steel corresponding to JIS standard SKD61 was melted, and a slab having a cross section of 120 mm square was manufactured using a horizontal continuous casting machine. This slab was annealed at 830 ° C for 3 hours. After preheating this slab to 1200 ° C, a high-frequency power source of 150 KW and frequency of 400 KHW was used.
Using a coil whose inside is water-cooled with a 3 mm copper square pipe,
A high-frequency power source was applied to the discontinuous solidification part of the slab with a gap of 5 mm, power of 35 KW, current of 1500 A, and heating time of 0.5 sec. The moving distance of the slab was adjusted to the pitch interval of the discontinuous solidification portion, and local heating was carried out on the discontinuous solidification portion of the billet length of 1500 mml. As a result of visual observation during the surface modification, it was confirmed that the discontinuous solidified portion had a liquid phase slightly blurring and was in a semi-molten state.

In the modified surface layer portion of the cast slab obtained by the method of the present invention, as compared with the non-modified material of FIGS. 1 and 2, the unbonded portion of the outermost surface layer portion is closed and the discontinuous structure of the cast structure is It was confirmed that it had disappeared. Next, both the modified material according to the present invention and the non-modified material that had not been modified were reannealed at 830 ° C. First, as the first step, a step of removing the surface oxide film of the slab of the modified material and the non-modified material with a cold grinder was carried out. Since the modified material also had irregularities on the surface of the cast slab, removal of about 1 mm of the flesh gave a metallic surface, and no crack-like defects could be found over the entire length by color checking.

On the other hand, in the non-modified material, many defects were detected by the same removal of 2 mm. A color check was performed every 1 mm polishing, but the surface defects could not be completely removed, and it was necessary to finally grind to a depth of about 3 mm on each side. The polishing yield of the modified material was 95%, while that of the non-modified material was 89%.
The yield improvement was 6%.

(Example 2) A slab under the same conditions as in Example 1 was used.
After rolling to a size of 90 mm square, the state of defects on the surface layer was compared between the modified material and the non-modified material. The surface of the slab annealed at 830 ° C after rolling was removed with a 0.5 to 1.0 mm grinder, and the surface flaw was examined by color check. It was confirmed that the unmodified material was opened at 8 crack discontinuous solidification portions in the entire circumference of 1500 mml and the entire length. In contrast, no defects could be detected in the modifier. In the non-modified material, defects could be removed with a grinder with a thickness of 1.5 mm. When combined with the grinding amount of Example 1, the non-modified material had a total yield of 83%. On the other hand, the yield of the modified material according to the present invention was 90%, which was a significant improvement.

Example 3 A billet under the same conditions as in Example 1 was rolled into a 7 mmφ coil. After coil annealing at 830 ° C,
After pickling, scratches on the coil surface were compared and investigated between the modified material and the non-modified material. It was found that the unmodified material had fog defects with a depth of 0.2 to 0.5 mm on 1/12 of the total length of the coil. On the other hand, no surface flaw was detected in the modified material. As described above, by adding the surface modification of the present invention, crack opening of the discontinuous solidification portion at the time of grinder, hot rolling, and crack susceptibility at the time of small diameter material finish rolling can be significantly improved. confirmed.

(Example 4) The discontinuous solidified portion of the cast piece annealed after the continuous casting of Example 1 was melted by TIG arc heat. With the infrared thermometer attached, the temperature of the heating part is red-heated so that it falls within the range of 1490 to 1540 ℃, and the cast length is 1500.
The gradual modification treatment was performed on mml. At a current of 60 A, the depth of the molten layer was about 3.0 mm from the surface. The microstructure of the portion corresponding to the discontinuous solidification after the modification was the resolidified layer structure after remelting, and the discontinuous solidification portion was completely disappeared by this treatment.
The grinding yield of this material after the glider was measured as in Example 1. The modified material eliminated crack-like defects by grinding 1 mm on one side. The grinding yield was 97%, which was about 10% higher than that of the unmodified material.

(Embodiment 5) Between the cooling zone 6 and the cutting device 9 in the horizontal continuous casting facility line shown in FIG. 3, an output of 150 KW, a frequency of 400 KHZ, a coil cross section of 3 mm × 10 mm, and a gap between the slab 7 and the coil 2 A 7 mm high frequency power source 1 was attached. After horizontal continuous casting, when the slab temperature is 1100 ° C, output 33KW, power 1500A
High frequency. The moving speed of the slab is 1.4 m / min,
A method of heating the entire circumference with high frequency was used. The surface temperature of the slab at this time was 1380 ° C. The material of the slab was equivalent to JIS SKS3, and its cross-sectional dimension was 120 mm square.

After the surface-modified slab was annealed at 780 ° C., the surface black skin was removed with a grinder. The effect of surface modification was compared by the amount of grinder removed until the surface defects disappeared. Since the surface modified material was 96% and the non-modified material was 87%, the yield improvement of 9% could be achieved by the continuous casting of SKS3 by the practice of the present invention as compared with the conventional method. As described above, by melting or semi-melting the discontinuously solidified portion of the continuously cast slab, the grindability of the slab with respect to the grinder and the hot working performance were improved.

Example 6 Between the cooling zone 6 and the cutting device 9 in the horizontal continuous casting equipment line shown in FIG. 3, an output of 150 KW, a frequency of 400 KHZ, a coil cross section of 3 mm × 10 mm, a slab 7 and a coil 2 were used.
A high frequency power source 1 having a distance of 7 mm was attached. After horizontal continuous casting, when the material temperature of the slab was 1150 ° C, a high frequency power of 33KW output and 1500A power was applied. The moving speed of the slab was 1.6 m / min, and the entire circumference was subjected to high frequency heating. The surface temperature of the slab at this time was 1430 ° C. The material of the slab is JIS S
It is equivalent to US304, and its cross-sectional size was 120 mm square.

The surface-modified black slab was removed from the surface-modified slab with a grinder. The effect of surface modification was compared by the amount of grinder removed until the surface defects disappeared. Since the surface modified material was 96% and the non-modified material was 92%, the continuous casting of SUS304 by the practice of the present invention was more than the conventional method.
A yield improvement of 4% was achieved. As described above, by melting or semi-melting the discontinuously solidified portion of the continuously cast slab, the grindability of the slab with respect to the grinder and the hot working performance were improved.

(Embodiment 7) Between the cooling zone 6 and the cutting device 9 in the horizontal continuous casting facility line shown in FIG. 3, an output of 150 KW, a frequency of 400 KHZ, a coil cross section of 3 mm × 10 mm, a slab 7 and a coil 2 were used.
A high frequency power source 1 having a distance of 7 mm was attached. After the horizontal continuous casting, when the material temperature of the slab was 1050 ° C, a high frequency power with an output of 33 KW and an electric power of 1500 A was applied. The moving speed of the slab was 1.0 m / min, and the entire circumference was subjected to high frequency heating. The surface temperature of the slab at this time was 1340 ° C. The material of the slab is JIS S
Equivalent to UH36, its cross-sectional size was 120 mm square.

The surface black skin of the slab subjected to this surface modification was removed by a grinder. The effect of surface modification was compared by the amount of grinder removed until the surface defects disappeared. Since the surface modified material was 91% and the non-modified material was 78%, the continuous casting of SUH36 by the practice of the present invention made it 13% less than the conventional method.
A yield improvement of% has been achieved. As described above, by melting or semi-melting the discontinuously solidified portion of the continuously cast slab, the grindability of the slab with respect to the grinder and the hot working performance were improved.

(Example 8) A JIS SUS430 equivalent material was melted using an experimental machine for vertical continuous casting to obtain a slab of 85 mm square. Two slabs each having a length of 1200 mm were prepared and annealed. The first slab has a temperature of 14 degrees due to TIG arc heat only on the oscillation that looks like a line.
A resolidified layer was formed while heating at 80 to 1500 ° C. A color check was performed on the first modified slab and the second non-modified slab, and grinding was continued until surface defects were completely eliminated. Subsequently, the slab after grinding was heated to carry out hot rolling with a rolling ratio of 1.5, and then grinding was carried out while confirming the presence or absence of a flaw. The slab yield and the grinding yield after hot rolling were evaluated by indexing the yield of the modified material with the yield of the non-modified material being 100.
Shown in.

[0025]

[Table 1]

[0026]

Industrial Applicability As described above, a cast product obtained by continuously casting a steel in which eutectic carbide is not crystallized has a resolidified layer in the surface layer corresponding to discontinuous solidification, and a specific surface modification. By melting or semi-melting part or all of the depth of the discontinuous solidification part as a method, the subsequent plastic working is significantly improved, and stable hot working in the next step can be performed, and material yield. From the above, it was confirmed to be extremely useful.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a casting structure showing a V-groove-shaped open discontinuous solidification portion generated in a surface layer portion of a slab in horizontal continuous casting of JIS SKD61 steel.

FIG. 2 is a schematic diagram of a casting structure showing a non-open discontinuous solidification portion generated in a surface layer of a slab in horizontal continuous casting of JIS SKD61 steel.

FIG. 3 is a conceptual diagram showing a horizontal continuous casting device as an example of the device of the present invention.

[Explanation of symbols]

1 high frequency power supply 2 coils 3 tundish 4 Molten steel 5 mold 6 spray 61 cooling zone 7 slab 8 pinch rolls 9 cutting device

Continued front page    (72) Inventor Hideki Nakamura             2 Hitachi, Kin, 2107 Yasugi-cho, Yasugi-shi, Shimane Prefecture             Yasugi factory

Claims (3)

[Claims] 1. A continuously cast slab, wherein a portion corresponding to discontinuous solidification of a continuously cast slab of steel that does not crystallize eutectic carbide has a resolidified layer. 2. A surface reforming method for a continuously cast slab of steel in which eutectic carbide is not crystallized, characterized in that part or all of the surface layer of the continuously cast slab is melted or semi-molten. Surface modification method for continuously cast slab. 3. A part or all of the depth of the witness mark part or oscillation mark part of the slab is melted or located between the cooling zone and the cutting device of the continuous casting device for steel that does not crystallize eutectic carbides. A continuous casting apparatus provided with heating means for semi-melting.