JPH04220140A - Method and mold for continuously casting round billet or beam blank - Google Patents

Abstract

PURPOSE:To directly cast the round billet or beam blank with the continuous casting without generating surface defect. CONSTITUTION:The inner surface temperature of the mold is kept within the temperature over than the solidifying temperature of the mold lubricant and less than solidification point of the casting metal. For this purpose, the cooling water passage whose to the inner surface of the mold is 2-15mm is provided at the back surface by using the Ni-Cr-Fe alloy. For example, the cooling water passage 10 is formed between the back surface of the casting mold material 1 and the back frame 9. This cooling water passage 10 is supplied the cooling water from the cooling water supplying path and it is discharged from the cooling water discharging passage 13. The cooling water passage is sealed with the O-ring 11. The 'Inconel(R)' 718 is used as the mold material 1.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a cast slab (round billet or beam blank) with a circular or beam blank cross-sectional area.
The present invention relates to continuous casting methods and continuous casting molds for obtaining the same, and to technology for continuously casting high quality slabs with few surface cracks.

0002

[Prior Art] In recent years, from the viewpoint of energy saving and process saving, conventionally round billets or H-beams have been manufactured from bloom continuous cast slabs through a rolling process, but instead of using slabs that more closely resemble the final product. For the purpose of omitting processing steps, attempts are being made to obtain round billets or beam blanks directly by continuous casting. In this case, in order to obtain productivity equivalent to that of the conventional method, the casting speed of the round billet is required to be several times the casting speed of the bloom. However, in steels that are prone to surface defects, such as peritectic composition steels, this increase in casting speed promotes surface crack defects, resulting in breakouts caused by cracks and roundness problems due to uneven solidification. There were problems such as defects.

Since beam blank slabs have a complicated shape, they have the disadvantage that surface defects such as vertical cracks are more likely to occur than slabs with simple cast cross sections such as slabs and billet blooms. It has been reported that the cause of this type of vertical cracking is due to uneven cooling within the mold (Kawasaki Steel Technical Report Vol.
l9, No. 3, p11). In general, as a means to prevent nonuniformity of the solidified shell, from the viewpoint of relaxing the cooling of the slab, (a) means to process grooves etc. on the inner surface of the water-cooled mold (Japanese Patent Laid-Open No. 51-50819) (b) Means for reducing heat transfer by thermally spraying ceramics on the inner surface of a water-cooled copper mold (Japanese Patent Laid-Open No. 153550/1983)
Other proposals have been made. However, in (a) above, the slow cooling effect is insufficient and the surface defects cannot be improved. Also(
In b), due to the difference in thermal expansion between the copper plate and the ceramic phase, peeling easily occurs at the joint between the copper plate and the ceramic phase, making it unsuitable for actual use.

The present inventor has studied in detail the cooling process of continuous casting using a mold lubricant. As a result, it was found that the main causes of uneven solidification were as follows. The contents will be explained with reference to FIG. FIG. 2 shows a sectional view of continuous casting from the immersion nozzle 7 into the mold 2. The solidified shell 4 is guided down by support rolls 8. The mold lubricant 5 on the meniscus flows between the mold material 1 and the solidified shell 4 of the mold 2, forming a mold lubricant 3 between the mold material 1 and the solidified shell 4. The solidification temperature of the mold lubricant 3 is generally 1000 to 1
On the other hand, since the conventional mold material 1 uses a copper alloy with high thermal conductivity, the surface temperature is designed to be 400° C. or less in consideration of its high temperature yield strength. Therefore, the mold lubricant 3 solidifies on the mold material side, and at this time, an air gap layer 6 is formed between the mold lubricant 3 and the mold 1. When this air gap layer 6 is formed, the amount of heat removed from the mold is greatly reduced. This resulted in non-uniform coagulation.

[0005]

[Problems to be Solved by the Invention] Peritectal composition steel has a large amount of shrinkage during solidification, tends to generate an air gap layer 6, and is unevenly formed, resulting in significant non-uniform solidification. As a result, thermal stress was added, causing problems such as surface cracking and breakout of the slab, and poor roundness of the round billet slab. Since the shape of the beam blank is complex, non-uniformity tends to occur and vertical cracking defects are likely to occur.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a method for continuous casting of round billets or beam blanks, in which a uniform solidified shell is formed, there are no surface defects, and high-speed casting is possible, and a mold thereof. That is.

[0007]

[Means for Solving the Problems] The method of the present invention, when continuously casting round billets or beam blanks, keeps the inner surface temperature of the mold above the solidification temperature of the mold lubricant used and below the freezing point of the cast metal. Characterized by maintaining and casting. The apparatus of the present invention which can suitably carry out the method of the present invention described above uses Ni-Cr, which has excellent high-temperature yield strength, as a mold material.
-Fe-based alloy is used, and a cooling water passage is provided on the back side of the mold. In this case, the thickness of the mold material is 2
It is suitable to set it to mm - 15 mm.

[0008]

[Operation] The present invention has been made for the purpose of reducing the air gap layer between the mold and the mold lubricant. The feature of the present invention is that the material of the mold material is not a copper alloy but a Ni-Cr-Fe alloy with low thermal conductivity (approximately 1/20) and high high temperature strength, and the surface temperature of the mold material is raised to 700℃ or higher. The objective is to use a mold lubricant that has a freezing point below the surface temperature of the mold material. Therefore, the mold 1 and the solidified shell 4
Since the mold lubricant 3 in between disappears from a solid state and is kept in a liquid state, even if the slab shrinks due to solidification, the air gap phase 6 is significantly reduced compared to the conventional method. This has made it possible to solve various problems caused by uneven solidification.

Next, the thickness of the mold of the present invention will be explained. Mold material surface temperature: Ts (°C) Mold material thickness: t (m) Mold material thermal conductivity: λ (kcal/mhr°C)
Heat transfer coefficient between cooling water and mold material: hw (kcal/
mhr℃) Cooling water temperature: Tw (℃) Heat flux: q (kcal/m2 hr) Ts=(t/λ+1/hw)q+Tw where, Tw=30℃ hw=20,000kcal/m2 hr℃
FIG. 3 shows the relationship between the heat flux q depending on the thickness t of the mold material and the surface temperature Ts of the mold material, assuming λ=16 kcal/mhr°C.

[0010] At a casting speed of 1.8 m/min, the heat flow rate q in the meniscus part is approximately 240 x 104 kcal/m2.
hr, and in high-speed casting at a casting speed of about 4 m/min, the heat flux q is predicted to be 400 x 104 kcal/m2 hr. In addition, the heat flux q at the mold outlet is 100 x 104 kcal/m at a casting speed of 1.0 m/min.
It takes about 2 hours. Therefore, the distance from the cooling water passage to the surface of the mold material, that is, the thickness t of the mold material is 2 mm to 15 mm.
About mm is required. Generally, the heat flux is large near the meniscus and small at the bottom of the mold, so in order to obtain a uniform surface temperature of the mold material in the casting direction, the thickness of the mold material t
must be thinner at the meniscus and thicker at the bottom of the mold.

[0011]

[Example 1] Fig. 1 shows a longitudinal sectional view of a mold of the present invention. A cooling water passage 10 is formed between the back surface of the mold material 1 and the back claim 9. This cooling water passage 1
0 supplies cooling water from a cooling water supply path 12 and drains it from a cooling water drainage path 13. The cooling water passage is sealed with an O-ring 11. Inconel 718 was used as the mold material 1. As a result of numerical heat transfer analysis of the temperature profile of the mold when this mold material is used, the mold surface temperature reaches a maximum of 825.
It was ℃. Under these casting conditions, the amount of heat removed from the mold was increased by 30% compared to the conventional copper alloy mold, and the aforementioned reduction in the air gap layer was confirmed. Furthermore, in the FeS addition test during casting, it was observed that the non-uniformity of the solidified shell thickness in the circumferential direction was improved and the roundness of the round billet material was improved.

[0012] The thickness of the mold material is selected in accordance with the heat flux during casting, and it is important to design the mold material so that the mold surface temperature is equal to or higher than the solidification temperature of the mold lubricant used. The heat flux value of the mold changes depending on the casting speed and mold lubricant, but
In the case of Ni-Cr-Fe alloy, 2 mm to 15 mm is required. When Inconel 718 is used as the mold material, the high temperature strength (0.2% proof stress) at 700℃ is also 95.
kg/mm2, which is sufficiently large, so it could be used for a long time without thermal deformation before or after casting.

[Example 2] Mold materials M1 and M2 shown in Table 1
(the above examples) and M3 (comparative example) were used to prepare medium carbon steel (
C/0.14%, Si/0.24%, Mn/1.36%
, P/0.018%, S/0.002%, Al/0.0
A round billet of 37%) was cast at a casting speed of 3.0 m/min. The round billet size is 210 mm in diameter. Continuous casting was performed using mold lubricants P1, P2, P3, and P4 shown in Table 2 at an oscillation cycle of 200 cpm and an oscillation stroke of 6 mm. The results are shown in Table 3. In Table 3, the surface crack index is the total length of vertical cracks (cm) per meter of slab length, and the roundness index is (minimum diameter)/(maximum diameter) of the billet cross section. No. which meets the requirements of the present invention. 1.No. 2.No. No. 5 shows good results in terms of surface cracking and roundness.

[Example 3] Using the mold materials shown in Table 1,
Beam blanks were cast using the mold lubricants shown in Table 2. Casting is medium carbon steel (C/0.14%, Si/0.15
%, Mn/1.40%, P/0.010%, S/0.0
10%, Al/0.035%) at a casting speed of 1.2 m/m
It was cast in. Mold size is flange height 400mm
, a beam blank cross-sectional shape with a width of 460 mm and a web thickness of 120 mm. Casting was performed under oscillation conditions of 150 cycles/min and a stroke of 6 mm. Table 4 shows the results of the casting experiments. In Table 4, longitudinal web cracking indicates the longitudinal crack length mm per 1 m of slab length. Experiment No. 10, No. 11
, No. 14 is a good result.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

[0019]

[Effects of the Invention] According to the present invention, uniform shell solidification is achieved in continuous casting of round billets, and high-quality round billets with few surface cracks and high roundness are stably cast without breakout. It became possible to do so.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a mold of the present invention.

FIG. 2 is a longitudinal cross-sectional view of a conventional mold.

FIG. 3 is a graph showing the relationship between plate thickness and heat flux.

[Explanation of symbols]

1 Mold material 2 Mold 3 Mold lubricant 4 Solidified shell 5 Mold lubricant 6 Air gap layer 7 Immersion nozzle 8 Support role 9 Back frame 10 Cooling water passage 11 O-ring 12 Cooling water supply path 13 Cooling water drain

Claims (3)

[Claims] Claim 1: When continuously casting slabs with a circular or beam blank cross-sectional area, the temperature of the inner surface of the mold is set to a temperature higher than the solidification temperature of the mold lubricant covering the surface of the molten steel and lower than the solidification point of the cast metal. A method for continuous casting of round billets or beam blanks, which is characterized by maintaining and casting the blanks. [Claim 2] Ni-Cr- as the mold material for the inner surface of the mold.
A round billet or beam blank continuous casting mold made of an Fe-based alloy and characterized in that a cooling water passage is provided on the back side of the mold material. 3. The continuous casting mold for a round billet or beam blank according to claim 2, wherein the distance from the cooling water passage to the inner surface of the mold is 2 mm to 15 mm.