JPH0533036A - Continuously casting slab and method for reforming its surface - Google Patents

Abstract

PURPOSE:To facilitate grinding work and plastic workability executed later by refoming surface defect generated on the surface layer part of Fe based, Ni based or Co based alloy continuously casting slab and restraining lowering of the yield in a minimum limit. CONSTITUTION:In the continuously casting slab having recrystal structure, diffusion structure or melting and resolidified structure at least on the surface layer part in structure non-continuous corresponding part in the cast slab, what is called, witness mark or oscillation mark, as the concrete surface refoming method for the casting slab, it can be achieved that heating treatment exceeding the ordinary hot working temp. is executed or only the surface layer part is melted or semi-melted.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to Fe group, Ni group or C group.
The present invention relates to a continuously cast slab in which a discontinuous structure called a witness mark or an oscillation mark formed on a surface layer part of a continuously cast slab of a base alloy is modified, and a surface modification method thereof.

[0002]

2. Description of the Related Art Surface defects, called witness marks or oscillation marks, are generated in the surface layer portion of a slab produced by a continuous casting method, which is accompanied by discontinuous solidification.
This occurs at the joint between the old solidified shell by intermittent drawing and the new solidified shell newly generated in the drawing process during continuous casting, and the surface of the slab is almost perpendicular to the longitudinal direction of the slab. It is observed as a linear discontinuous mark, and depending on the casting conditions, this discontinuous mark portion may open as a V-groove as an unpressed portion. Further, a discontinuous surface of the cast structure exists immediately below the discontinuous mark portion, and particularly in the case of alloy steel having a high carbon content, significant segregation of carbides occurs near this discontinuous surface. Above all, in the case of horizontal continuous casting method where lubricant such as powder cannot be used, the depth of discontinuous solidification part reaches several mm, and there are cracks due to unwelded discontinuous surface called cold shut crack. The tendency becomes remarkable.

FIG. 1 is a schematic diagram of solidification growth in the horizontal continuous casting method. A black line which is seen vertically in the center of the surface is a discontinuous structure surface called a witness mark. It explains that there is a region where solidification grows in the opposite direction across the surface. Figure 2 is SUS304, Figure 3 is SKD61,
Figure 4 shows the actual microstructure of the typical witness mark part of SKH51. It can be clearly seen that the solidification grows in the opposite directions at the discontinuous surface. In addition, high carbon content
In SKH51, it can be seen that carbide is segregated on this discontinuous surface.

[0004]

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, if the discontinuously solidified portion is open without welding, it not only causes stress concentration during plastic working, but even if the discontinuously solidified portion is not open, the discontinuous portion of the cast structure causes ductility. This is to significantly reduce it.
Further, in the case of a high carbon alloy, since the carbide segregates in the discontinuous portion, the ductility of this region becomes extremely poor, and the cracking susceptibility during plastic working tends to become remarkable.
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 become larger or a new opening may be created due to the processing heat.
In the case of cutting, the discontinuously solidified portion is not opened, but there is a problem that the bending time and deformation of the slab significantly increase the processing time and reduce the yield.

Further, there are methods such as hot scarfing and cold scarfing as a method for removing surface flaws. In this method, about 5 mm of the surface is melted and blown away, and a slab with a small cross-sectional shape is obtained. Is significantly reduced in yield. For example, the yield when the slab with a cross section of 100 × 100 mm is 5 mm scarfed, the yield is 81%, further grinding to remove surface irregularities is required, and the yield until rolling is 80 There is a problem that it becomes less than%. The object of the present invention is to improve surface defects such as discontinuity of structure, segregation of carbides, etc. which occur in the surface layer portion of a continuously cast slab of Fe-based, Ni-based or Co-based alloy, and further, the yield reduction is minimized. And to provide a continuously cast slab that can be easily ground and plastically worked thereafter, and a method for surface modification thereof.

[0006]

Means for Solving the Problems The present inventor
In the process of studying continuous casting such as base or Co base, plastic working by reforming without substantially removing surface casting defects called witness mark or oscillation mark which significantly inhibits plastic workability The inventors have found a slab that is capable of improving the yield and is effective in improving the yield, and a method for surface modification thereof. As a method of modifying the surface, it is effective to change the witness mark or the oscillation mark in some way as a result of various examinations, and to make the difference with the sound part as small as possible. It was found to be effective to partially heat the slab to change the structure. The object of the present invention is achieved when at least a part of the structure discontinuous portion of the cast slab has a recrystallized structure, a diffusion structure or a melt resolidified structure depending on the degree of heating.

That is, the first invention of the present invention is Fe
At least the surface layer portion of the structure corresponding to the discontinuous structure of the slab called the witness mark or oscillation mark of the continuously cast slab of the base, Ni-based or Co-based alloy,
A continuously cast slab having a diffusion structure or a melt resolidification structure. The second aspect of the present invention heats the surface of a continuously cast slab of Fe-based, Ni-based or Co-based alloy to re-create at least the surface layer portion of the discontinuous structure of the slab called a witness mark or oscillation mark. Crystallization,
Or a surface modification method for a continuously cast slab, which comprises diffusing a phase precipitated in the discontinuous structure portion,
The present invention heats the surface of a continuously cast slab of Fe-based, Ni-based or Co-based alloy to melt or semi-melt at least the surface layer portion of the discontinuous structure of the slab called witness mark or oscillation mark. It is a surface modification method of a continuously cast slab, which is characterized by re-solidifying.

The term "recrystallized structure" as used in the present invention means that by heating the surface of the cast slab, new crystal grains are generated on the discontinuous surface of the old structure to form a fragile discontinuous surface partially or entirely. It refers to the newly formed crystal structure that has been resolved. Further, the above-mentioned heating is not intended to remove high-temperature materials such as hot scarfing which is a conventional technique, but is intended to modify the surface of the slab and is intended to be hot-worked thereafter. Heating is heating performed for another purpose. Therefore,
The heating of the surface of the present invention is usually added as a separate step prior to hot working, but it is of course possible to utilize the retained heat at the time of heating for the purpose of the present invention and utilize it as the heat of the next step. I do not care.

[0009]

As described above, since the discontinuous structure surface of the cast slab surface layer portion generated by continuous casting is a flat joint surface between the old solidified shell and the new solidified shell, the subsequent grinding and plastic working It easily becomes a propagation path for openings or cracks against such thermal stress and deformation stress. The present invention eliminates the discontinuity structure and enhances ductility by newly forming crystal grains on the above-mentioned planar joint surface, that is, by forming a recrystallized structure. Furthermore, when the material to be continuously cast contains a relatively large amount of carbon, segregation of carbides becomes remarkable in the vicinity of this discontinuous surface, and the as-cast slab synergistically becomes brittle. The ductility is improved mainly by solidifying the segregation of carbide in the surface layer of the cast slab into the matrix to form a diffusion structure. In order to form the above-mentioned recrystallized structure or a diffusion structure in which a carbide is solid-solved, generally only the step of heating for plastic working is not sufficient, as a separate step the normal hot working temperature or higher, and the solidification end temperature. It can be achieved by heating to the following temperature.

On the other hand, when the Fe-based, Ni-based or Co-based alloy continuous casting conditions are unsuitable, the witness mark portion and the oscillation mark portion on the surface of the slab become uncompressed and the V groove is formed. Open in a circle. Further, in the case of a high carbon-based high alloy that crystallizes eutectic carbide, segregation of eutectic carbide is remarkable in the discontinuous structure. The above-mentioned V-groove-shaped slab or a slab in which eutectic carbide is segregated in the discontinuous structure even when the slab is not opened is not sufficient for heating below the above-mentioned solidification end temperature, and thus the surface layer of the slab By making the part have a molten or semi-molten solidification structure, the segregation of the opening or the eutectic carbide is eliminated, and the grinding process and the plastic working performed thereafter become good. The heating for forming the resolidified structure is more advantageous in terms of energy when concentrated on only the discontinuous portion, but it is industrially preferable to perform it on the entire surface of the slab for controlling the heating means. It will be easier. Further, the heating for forming the recrystallized structure can be carried out in a normal burner heating furnace or an electric furnace.

[0011]

【Example】

(Example 1) JIS standards SUS304, SKD61, S
KH51, SKD11, SK3, SUH35 and Stellite No. 6 (Stellite is a trademark) were melted and a slab having a cross section of 120 mm was produced using a horizontal continuous casting device. FIG. 5 shows a horizontal continuous casting apparatus provided with a slab surface heating means showing an example of surface modification of a slab characterized by the present invention. More specifically, the horizontal continuous casting device has a high-frequency heating device for performing heating after completion of solidification to obtain a molten resolidified structure of the surface layer portion. The coil used in the experiment was 2 turns, and the gap between the coil and the slab was about 10
mm. Use frequency is 10KHz, output is 300 ~ 350K
It was W. The output was adjusted by the drawing speed of the slab and the melting point so that the melting depth was 2 mm. In this experiment, in the casting of each sample, after the slab was pulled out to a predetermined length without loading the high-frequency heating device to obtain the slab by the conventional method, the high-frequency heating power source was turned on to cast the sample according to the present invention. A piece was produced. The drawing speed of each sample is
SUS304 is 1.8m / min, SKD61 is 1.6m / min, S
1.2m / min for KH51 and SKD11, 1.6m / m for SK3
in, SUH35 is 1.6m / min, Stellite No.6 is 1.2m / mi
It was n. After casting, when observing each slab,
V-groove-like openings were partially observed in the surface layer of the cast pieces of SKD51, SKD11 and Stellite No. 6 which were not subjected to high-frequency heating, and eutectic carbide was observed in the discontinuous structure. However, no opening was observed in the surface layer of the slab where the surface layer was melted and resolidified by heating.

Next, each slab was solution treated at 1100 ° C. for SUS304, annealed at 750 ° C. × 3 hr for SK3, SKD61,
SKD11 and SKH51 are annealed at 830 ℃ × 3hr, and SU
H35 was subjected to solution treatment at 1200 ° C and Stellite No. 6 was subjected to solution treatment at 1230 ° C. Samples were indexed from the surface layers of the slabs subjected to the surface modification of the present invention and the slabs of the conventional method, and tensile test pieces and Charpy impact test pieces were prepared to determine the tensile strength and impact value. As a comparison method, a test piece indexed from the surface layer of a cast alloy having a re-solidified surface was set to 1 as the value of each alloy cast by the conventional method, that is, the cast without heating the surface of the cast by high frequency heating. Table 1 summarizes each value of.

[0013]

[Table 1]

It can be seen from Table 1 that both materials are significantly improved in strength and toughness by the surface modification of the present invention. On the other hand, each slab was ground to a thickness of about 0.5 mm, a penetrant flaw detection test was conducted, grinding was carried out until there were no flaws, and the penetrant flaw test was repeated, and the yields at the time when the flaws were completely eliminated were compared. It was As a comparison method, the slab yield obtained by the conventional method that did not perform high frequency heating was 1,
The yield of the cast slab which is the method of the present invention and which is obtained by heating the surface layer of the cast slab to form a resolidified structure is arranged for each sample, and the results are also shown in Table 1. From Table 1, SKH5 with high cracking sensitivity
It has been found that the cold grinding of 1, SKD11 and Stellite No. 6 can prevent the occurrence of cracks and obtain a high yield by the method of the present invention. SUS304 is an alloy having low cracking susceptibility, but in the conventional material, brittleness of the discontinuous portion is unavoidable, and the yield of grinding is lower than that of the material of the present invention due to cracking due to insufficient welding of the discontinuous portion. As described above, it is understood that the method of the present invention significantly improves the mechanical properties of the surface layer of the cast slab and improves the grinding yield. FIG. 6 shows a microstructure photograph of the old witness mark portion after SKH51 was subjected to the surface modification treatment of the present invention by heating the surface layer portion of the cast slab to form a melt resolidified structure. From FIG. 6, it can be seen that the tissue discontinuity immediately below the witness mark has completely disappeared.

(Embodiment 2) SU304, SUH35, S
US410L, NCF800, NCF750 is dissolved,
A slab having a cross section of 120 mm square was produced using a horizontal continuous casting device without a usual heating means. All withdrawal speeds are 1.6
m / min. After casting, a surface modification treatment of 1315 ° C. × 15 minutes each as a surface modification method for recrystallizing the structure discontinuous part of the surface part of the slab of the present invention leaving a part of the slab and a diffusion structure After being carried out, along with the slab that was not subjected to the above surface modification treatment, SUS410L was annealed at 800 ° C for 3 hours, and SKH35 was 12
Solution treatment at 00 ° C, solution treatment at 1100 ° C for SUS304 and NCF800, and solution treatment at 1150 ° C for NCF750. The solution treatment time was all set to 2 hours.

When the structures of these cast pieces were observed, a discontinuous structure surface was clearly observed in the surface layer portion immediately below the witness mark which was not subjected to the surface modification treatment. In particular, the discontinuity structure of SUH35 was observed. Segregation of carbide was observed near the part. On the other hand, in each of the structures subjected to the surface modification treatment, a part or most of the discontinuous structure disappeared and a recrystallized structure was confirmed. Also, SUH3
It was a diffusion structure in which the carbide segregation of the portion corresponding to the structure discontinuity of No. 5 was almost eliminated. Samples were indexed from the surface layer portions of the slabs subjected to the surface modification treatment of the present invention and the slabs of the conventional method, tensile test pieces were prepared, and tensile strengths were compared. The test results are summarized in Table 2 with the strength of the conventional slab as 1.

[0017]

[Table 2]

It can be seen that the strength of any material is improved by performing the surface modification treatment of the present invention.
The strength improvement ratio does not reach that of the material of the present invention having the melt resolidified structure of Example 1. It is considered that this is because it is difficult to form a recrystallized structure in which the discontinuous structure can be completely disappeared, and it is difficult to completely eliminate the segregation of carbides. Then, the same grinding test as in Example 1 was performed, and the results are also shown in Table 2.

Regarding the grinding yield, it can be seen that the grinding yield is improved by modifying the surface layer of the cast slab of the present invention. Figure 7
Is a microstructure of a former witness mark part showing a recrystallized structure obtained by subjecting a horizontally continuous cast slab of SUS304 to a surface modification treatment (1315 ° C. × 15 minutes) which is the method of the present invention. Figure 7
From this, it is observed that the discontinuity of the structure is partially eliminated, and recrystallized grains having new witness marks are formed.

[0020]

As described above, in the continuous casting of Fe-based, Ni-based or Co-based alloys, the discontinuous structure called witness mark or oscillation mark generated in the surface layer of the slab is recrystallized. By using a structure, a diffusion structure, or a melt resolidified structure, the surface grinding yield in the next step was improved, and it became possible to significantly enhance the subsequent plastic workability.

[Brief description of drawings]

FIG. 1 is a view showing generation of a discontinuous structure surface by a horizontal continuous casting method,
It is a schematic diagram which shows the growth of a solidification structure.

FIG. 2 is a metallographic photograph showing a discontinuous structure of SUS304 produced by a continuous casting method.

FIG. 3 is a metallographic photograph showing a discontinuous structure of SKD61 by the continuous casting method.

FIG. 4 is a metallographic photograph showing a discontinuous structure portion of SKH51 by a continuous casting method.

FIG. 5 is a conceptual diagram showing an example of a horizontal continuous casting apparatus of the present invention.

FIG. 6 is a metallographic photograph showing a molten resolidified structure of a portion corresponding to a discontinuous structure of SKH51 according to the method of the present invention.

FIG. 7 is a metallographic photograph showing a recrystallized structure of a portion corresponding to a discontinuous structure of SUS304 according to the method 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 recrystallized structure or a diffusion structure at least in a surface layer portion of a cast piece called a witness mark or an oscillation mark of a continuously cast cast piece of an Fe-based, Ni-based or Co-based alloy. Alternatively, a continuously cast slab having a molten resolidified structure. 2. A surface of a continuously cast slab of Fe-based, Ni-based or Co-based alloy is heated to recrystallize at least the surface layer portion of the discontinuous portion of the slab called witness mark or oscillation mark. Or a surface modification method of a continuously cast slab, which comprises diffusing a phase precipitated in the discontinuous structure portion. 3. A surface of a continuously cast slab of Fe-based, Ni-based or Co-based alloy is heated to melt at least a surface layer portion of a discontinuous structure portion of the slab called a witness mark or oscillation mark or A method for modifying the surface of a continuously cast slab, which comprises semi-melting and re-solidifying.