KR100544430B1 - A method for manufacturing continuous casting slab of high nickel containing alloy - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a continuous casting slab of high-Ni alloys used in bimetals, liquid nitrogen storage tanks, shadow masks of color TVs, computer monitors, etc., by appropriately controlling the casting temperature in tundish and the amount of cooling water in the mold. An object of the present invention is to provide a method of manufacturing a continuous cast slab that can prevent the surface of the slab and internal cracks generated during continuous casting of high-Ni alloy steel.

The present invention for achieving the above object,

A method for producing a continuous casting slab of a high Ni alloy in which the Ni content is 30 to 45% by weight and starts to solidify into the primary austenite phase during continuous casting and completes solidification without phase transformation during the solidification process. The difference between the molten steel temperature and the theoretical solidification temperature (ΔT) is 10 to 40 ° C, and the amount of cooling water in the mold is 5,500 to 7,000 l / min for the long side of the mold and 700 to 900 l / min for the short side of the mold. The manufacturing method of the continuous casting slab of high Ni alloy characterized by setting and operating is a technical subject matter.

High Ni Alloy, Continuous Casting, Slabs, Cracks, Line Defects

Description

Manufacturing method of continuous casting slab of high Ni alloy {A METHOD FOR MANUFACTURING CONTINUOUS CASTING SLAB OF HIGH NICKEL CONTAINING ALLOY}

1 is a schematic view showing a continuous casting apparatus

* Explanation of symbols for main parts of drawing *

One… Molten steel 2. Ladle 3... Shrouding nozzle 4... Tundish

5... Tundish dam 6... Tundish stopper 7... Immersion nozzle

8… Mold 9... Secondary cooling stand

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing continuous cast slabs of high Ni alloys used in bi-metal, liquid nitrogen (LNG) storage tanks, shadow masks of color TVs, and computer monitors. The present invention relates to a method for manufacturing a high-Ni alloy continuous casting slab free of cracks on the surface and inside during continuous casting.

The high Ni alloy generally refers to an alloy containing 30% or more of Ni, and has a very low thermal expansion characteristic of 1.2 × 10 ⁻⁶ ° C or less at a temperature range of −96 to 180 ° C., and shows excellent toughness at extremely low temperatures. Due to these characteristics, they are used for bi-metal, liquid nitrogen (LNG) storage tanks and shadow masks of color TVs and computer monitors, and the demand is increasing as the industry is advanced and advanced. .

Such high Ni alloys are preferably manufactured by inexpensive continuous casting. However, since continuous solidification starts to solidify onto primary austenite phase and completes solidification without phase transformation, cracks are generated on the slab surface and inside during continuous casting. I have a problem. Accordingly, research on continuous casting technology has been conducted for bimetals and LNG tanks, but production by ingot casting has become common. In addition, since the high Ni alloy has a high internal oxidation in the heating furnace, hot workability is degraded during hot rolling and surface defects are likely to occur. In order to solve this problem, an oxidation-free furnace was used, or in Japanese Patent Laid-Open No. 55-94791, a technique of applying an antioxidant to a slab surface and covering carbon steel or stainless steel sheet was applied.

On the other hand, surface defects such as surface and internal cracks also occur in continuous casting slabs of ordinary carbon steel, but such defects are prevented by improvement of mold powder, improvement of mold cooling, improvement of secondary cooling pattern, and the like. However, the defect of the slab generated in the high Ni alloy steel containing 30% or more of Ni is different from that of the conventional continuous casting slab, and the type and characteristics of such a defect will be described below.

1) Surface transverse crack: This defect occurs at the low density of spray nozzles in the secondary cooling zone. It is remarkably observed in the ungrinded slab and periodic in the slab width direction.

2) Subsurface crack: It occurs at the position of 3 ~ 30mm from the surface of the slab, so the grinding tends to increase with grinding.

3) Corner Crack: It occurs at about 50mm from the edge and is a kind of duty free crack.

In addition, these cracks are described metallurgically as follows.

1) Since high Ni alloy steel (36% Ni steel) has aortic phase in austenitic phase and completes solidification in austenite phase without phase transformation during solidification process, segregation of impurities such as P and S occurs severely in the austenite grain boundary. In addition, coagulated tissue is formed coarse and has a weak characteristic against thermal stress.

2) Slab transverse cracks are cracked due to thermal stress caused by uneven cooling during initial solidification and propagate along the austenite grain boundary.

3) Subsurface cracks and corner cracks are caused by thermal stress by cooling-recuperation in molds and secondary cooling zones. All of these cracks progress at the austenite boundary, which is a weak part.

These cracks that determine whether they can be manufactured are linear defects that are detected after pickling in the hot rolling process, which is a fatal defect in continuous casting slabs and stainless steel where surface quality is important. They must be removed through the correction process. However, since the addition of such a process is a factor of the price increase, various studies have been made from the continuous casting process to the hot rolling and annealing process in order to prevent ship defects.

Meanwhile, as shown in FIG. 1, the continuous casting process serves as a buffer between the ladle 2 and the ladle 2 and the mold 8 containing the molten steel 1 completed in the steelmaking process. It consists of a tundish 4, a mold 8 for producing a slab and a secondary cooling table 9. There is a shrouding nozzle 3 between the ladle 2 and the tundish 4, and a tundish dam for controlling the flow of the molten steel 1 inside the tundish 4. (5), there is an immersion nozzle (7) between the tundish (4) and the mold (8), a tundish stopper (6) for adjusting the flow rate of the molten steel is installed above the immersion nozzle (7) It is. In the continuous casting process as described above, the linear defect is caused when the solidified structure is formed unevenly, so that the mold 8 and the secondary cooling zone 9 in which the slab is formed become the occurrence location of the linear defect. Therefore, Japanese Patent Laid-Open No. 57-26141 discloses a method of adding Ca and Ti from the viewpoint of increasing the toughness of the material in order to prevent the occurrence of linear defects. However, the removal of impurities and the addition of trace elements that increase toughness for such materials require additional steps in the refining process, which is a preliminary casting process, and thus are a factor of cost increase such as temperature compensation, refining material and alloy. In addition, in 鐵 and 鋼 67-1980 and S738, water density is uniformly adjusted in the cooling zone to suppress surface cracking without adding special elements, and for this purpose, an oval type spray nozzle is used. have. In addition, a method of uniformizing the slab temperature by using an air / water nozzle is also disclosed. In Japanese Patent Laid-Open No. 57-32862, the slab surface temperature is not affected by a weak toughness in a curved machine. In order to avoid the high temperature side, the method of applying the slow cooling pattern which cools the area | region below 950 degreeC in the cooling rate of 20 degree-C / min or less in a secondary cooling zone is proposed. However, since the above method requires additional processing and increases the process load, it is difficult to completely remove the surface and internal cracks.

Accordingly, the inventors of the present invention have repeatedly conducted research and experiments to solve the above problems, and propose the present invention based on the results, and the present invention appropriately controls the casting temperature in the tundish and the amount of cooling water in the mold. An object of the present invention is to provide a method for manufacturing a continuous cast slab that can prevent the surface of a slab and internal cracks generated during continuous casting of high-Ni alloy steel by controlling.

The present invention for achieving the above object,

In the method of manufacturing a continuous cast slab of high Ni alloy in which the Ni content is 30 to 45% by weight and starts to solidify in the primary austenite phase during continuous casting and completes the solidification without phase transformation during the solidification process.

During continuous casting, the difference between the molten steel temperature and the theoretical solidification temperature (ΔT) in the tundish is 10 to 40 ° C, and the amount of cooling water in the mold is 5,500 to 7,000 l / min with respect to the long side of the mold and the short side of the mold. The method relates to a method for producing a continuous cast slab of high Ni alloy, characterized by operating at 700 to 900 l / min.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The inventors of the present invention, when manufacturing high Ni alloy steel containing 30 to 45% by weight of Ni in continuous casting, studies and examines in order to prevent the surface and internal cracks of the slab, the coil line defect during hot rolling of the continuous casting slab The present invention has been accomplished by investigating the correlation between the occurrence position of the and slab cracks and setting the appropriate operating conditions for continuous casting.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

Cracks in slab during continuous casting are different depending on the amount of alloying elements added. Low alloy steels containing some special elements such as Nb and W have a solidification form that solidifies into a ferrite phase and transforms into an austenite phase after completion of solidification into a ferrite phase, and thus the austenite grain boundary and the final solidification position are different. Impurities such as P and S hardly segregate at the grain boundaries, and the coagulation structure becomes fine. For this reason, low alloy steels do not have high slab surface and internal crack susceptibility during continuous casting.

However, since high Ni alloy steel solidifying into austenite phase coincides with the austenite grain boundary and the final solidification position, impurities such as P and S are severely generated at the austenite grain boundary, and coagulation structure is easily formed coarsely. The system is significantly brittle and has very high crack susceptibility to thermal stress. Therefore, in order to prevent the surface of the slab and internal cracks in such a high Ni alloy steel, the segregation amount of the solute element should be reduced and the solidified structure should be finely formed. The method is as follows.

The temperature range from the liquidus temperature to the solidification temperature, the solidification temperature, is a section in which solutes are concentrated. If the cooling rate is slowed in this section, the content of solutes concentrated in the liquid phase decreases because the solutes diffuse. do. Therefore, slowing down the cooling rate can suppress the occurrence of coagulation cracks. On the other hand, if the cooling rate is increased, the size of the dendrite becomes fine, and thus, the effect of suppressing the coagulation crack can be suppressed, but the diffusion of the solute element is also delayed. Based on this fact, the inventors of the present invention determined that the solidification crack of the slab can be controlled during continuous casting by appropriately adjusting the cooling conditions during the solidification process, and at the casting temperature and mold cooling set to the molten steel superheat in the tundish. It is presented how to control the amount of cooling water. However, in the process of adjusting the casting temperature and the amount of cooling water, the quality and operation problems other than the surface of the slab and the internal crack may be caused, so that the limits and ranges of the casting temperature and the amount of cooling water are set.

Accordingly, in the present invention, the difference between the molten steel temperature and the theoretical solidification temperature (ΔT) in the tundish is set in the range of 10 to 40 ° C, and the amount of cooling water in the mold is 5,500 to 7,000 (l / min) with respect to the mold long side. It is preferable to set and operate to 700-900 L / min with respect to a short side. Here, the size of the mold is about 1200 ~ 1300mm in the long side, about 200mm in the short side, it is preferable to manufacture the slab mainly in such a mold.

When the difference between the molten steel temperature in the tundish and the theoretical solidification difference ΔT (° C.) exceeds the upper limit of 40 ° C., the solidification rate is slowed during the solidification and the coagulation structure becomes coarse. Defects are likely to occur. On the other hand, when ΔT is lower than the lower limit of 10 ° C., the lubricating ability of the mold powder is degraded during the playing process, thereby causing inclusion defects in the playing slab.

In addition, if the amount of cooling water in the mold is 7,000 L / min or more for the mold long side and 900 L / min or more for the mold short side, coagulation structure can be finely formed, but impurities segregated between the coagulation textures during continuous casting Since the time for the diffusion is reduced, cracks are generated vertically on the surface of the slab, and the slab is re-arranged so that not only cracks due to thermal stress, but also residual stress is excessively generated on the surface. On the other hand, when the amount of cooling water in the mold is less than 5,500 l / min for the mold long side and less than 700 l / min for the mold short side, the solidification structure becomes coarse to generate the slab surface and internal cracks and the slab solidification shell during casting. The strength of the shell is reduced, causing cracks due to slab bulging.

Therefore, the amount of cooling water in the mold is preferably controlled at 5,500 to 7,000 (l / min) for the mold long side and 700 to 900 (l / min) for the mold short side.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

Continuous casting slab under the manufacturing conditions of Table 1 for the alloy steel, which is composed of weight%, C: 0.018%, Ni: 36.07%, Cr: 0.09%, Mn: 0.26%, P: 0.003%, S: 0.0007%. After the preparation, the surface and the internal cracks of the continuous casting slab were examined, and the results are shown in Table 1 below. At this time, the crack generation degree of the slab was expressed as ○: none, △: fine, X: severe.

division Superheat degree (℃) Mold cooling water amount (ℓ / min) Cracking degree of cast steel Long side Short side Inventive Example 1 10 6000 700 O Inventive Example 2 37 7000 880 O Comparative Example 1 20 5300 720 △ Comparative Example 2 30 7300 850 △ Comparative Example 3 25 6200 650 △ Comparative Example 4 35 7700 930 △ Comparative Example 5 7 6800 800 X Comparative Example 6 45 6500 750 X Comparative Example 7 50 7500 920 X

As shown in Table 1, in the case of the inventive examples (1) and (2), slab bulging and cracking did not occur during continuous casting, and line defects did not occur on the surface of the hot rolled coil. A good product could be obtained.

However, in the case of Comparative Example (1), the amount of mold long side of the mold cooling water was low, and slab bulging occurred during the play, and fine cracks were generated on the surface of the slab in a duty-free manner. Excessive formation caused linear defects on the hot rolled coil.

In the case of Comparative Example (2), the amount of cooling water for the mold long side of the mold cooling water was large, and there was no bulging during continuous casting. However, thermal stress acted on the surface of the slab during casting, and cracks occurred on the surface of the slab.

In addition, in the case of Comparative Example (3), the molten steel superheat degree in the tundish satisfies the condition of the present invention. Delayed, resulting in slab bulging of the short side portion, and microcracks occurred in the slab surface layer portion.

In the case of Comparative Example (4), the molten steel superheat degree in the tundish satisfies the conditions of the present invention, but the solidification process of the short side portion proceeds faster than the long side portion due to the large amount of cooling water in the mold short side portion. An air gap occurred at the corners. For this reason, heat transfer fell and microcracks generate | occur | produced in the slab corner part.

On the other hand, in Comparative Example 5, the molten steel superheat in the tundish was lower than the conditions of the present invention, resulting in poor melting of the mold powder, and the lubrication between the mold and the solidification layer was insufficient. As a result, cracks occur in the solidification layer, and large surface defects such as scabs occur on the surface of the slab at the end of the casting.

In Comparative Example (6), the molten steel superheat in the tundish was formed so that coarse solidification structure was formed on the surface of the slab, the surface of the slab and internal cracks were generated at the grain boundary, and slab bulging occurred during continuous casting. Operational instability such as hunting has occurred.

In addition, in Comparative Example (7), the casting temperature and the specific quantity were out of the standard values of the present invention, and cracks occurred in the continuous casting slab and the hot rolled coil.

According to the present invention as described above, since it is possible to prevent the occurrence of surface and internal cracks in the continuous casting slab to prevent the occurrence of linear defects during hot rolling, there is an effect that can produce high Ni alloy steel of high quality It is.

Claims (1)

In the method of manufacturing a continuous cast slab of high Ni alloy in which the Ni content is 30 to 45% by weight and starts to solidify in the primary austenite phase during continuous casting and completes the solidification without phase transformation during the solidification process. During continuous casting, the difference between the molten steel temperature and the theoretical solidification temperature (ΔT) in the tundish is 10 to 40 ° C, and the amount of cooling water in the mold is 5,500 to 7,000 l / min with respect to the long side of the mold and the short side of the mold. Method for producing a continuous cast slab of high Ni alloy, characterized in that the operation is set to 700 ~ 900l / min

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