JP3082834B2 - Continuous casting method for round slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method having a circular cross section, such as a material for a seamless steel pipe or a material for forging, in a continuous casting machine having an arc-shaped mold.

[0002]

2. Description of the Related Art In a continuous casting method, as shown in FIG.
3 contacts the mold 91 and first forms a thin solidified shell 94, which is initially in contact with the mold 91, but eventually the solidified shell 94 shrinks with the temperature drop due to cooling and the mold 91
You can get away from it. However, since the amount of shrinkage of the solidified shell 94 differs depending on the location, there is a difference in the gap between the outer surface of the solidified shell 94 and the inner surface of the mold 91, resulting in an excess or deficiency in the supply amount of the powder 95. For this reason, an air gap is formed in the gap between the outer surface of the solidified shell 94 and the inner surface of the mold 91, and a difference in the powder thickness is generated, so that a solidified shell having an uneven thickness is formed.

[0003] When the thickness of the solidified shell becomes uneven, thermal stress concentrates on a thin portion, causing a breakout and making it impossible to cast. To prevent this breakout, the casting speed must be reduced, which reduces productivity. Further, unevenness of thickness of solidified shell, evil surface texture of the slab
Will be converted. Such a problem occurs remarkably especially in a round section slab.

[0004] As a method of solving the above-mentioned problem occurring at the time of casting the slab of round cross section, a tundish and a mold are connected by a nozzle, and the mold is swung in a direction intersecting with the direction of drawing the slab at an early stage of solidification. A method in which a solidified shell is brought into close contact with the inner wall of a mold by static pressure of molten steel to uniformly solidify (JP-A-53-137825), and the inner surface of the mold is moved in two steps in the casting direction so that the inner surface of the mold is as close as possible to the shrinking solidified shell. Squeezed mold (Japanese Utility Model Application Laid-open No. 59-165)
748), in a continuous casting method in which steel is cast into a mold having at least two tapered steps that are continuous in the casting direction, in order to adapt the shrinkage behavior of the formed product to instantaneous casting parameters, Method of changing molten steel level within a range of a plurality of taper steps (Japanese Patent Publication No. 59-392)
No. 20) has been proposed.

[0005]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Application Laid-Open No. 53-13 / 1979
In the method disclosed in Japanese Patent No. 7825, an apparatus for injecting powder into molten steel is required because the tundish and the mold are connected. Further, in order to swing the mold in a direction intersecting with the direction of drawing the slab, a significant design change must be made. Further, the mold disclosed in Japanese Utility Model Laid-Open Publication No. 59-165748, the amount of shrinkage of the solidified shell greatly varies depending on conditions such as the steel composition and the casting speed, and therefore, when the taper is determined based on a specific steel composition and casting speed. Insufficient or excessive taper occurs for other casting conditions. further,
Even the method disclosed in Japanese Patent Publication No. 59-39220 cannot cope with steels of various compositions unless the taper set value is appropriate.

In general, the subperitectic steel having a C concentration of about 0.08 to 0.15% in which the shrinkage behavior of the solidified shell greatly affects the quality of the slab and the occurrence of casting troubles. In the hypoperitectic steel, the shell immediately after complete solidification largely shrinks due to the δ → γ transformation, so that the solidified shell separates from the mold during casting and is likely to undergo uneven solidification. In order to solve such a problem peculiar to the hypoperitectic steel, it is required to set the taper based on the hypoperitectic steel having the largest solidification shrinkage.

On the other hand, in the casting of steel having a small solidification shrinkage, when a mold having a large taper based on hypoperitectic steel is used, a compressive stress acts from the mold to the solidified shell, and depending on the steel composition, the solidified shell may be formed. Buckling sometimes occurred.
No prior art focuses on buckling deformation of a solidified shell generated in steel having a specific composition, and does not limit a mold taper for the purpose of preventing buckling deformation.

SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method of a round section slab which can solve the above-mentioned drawbacks of the prior art and can prevent buckling deformation of a solidified shell which occurs when continuously casting a round section slab of a specific component. To provide.

[0009]

Means for Solving the Problems The present inventors have found that in continuous casting of a slab of round cross section, solidification shell buckling deformation occurs in steel of a specific composition, and as a result of investigating the occurrence tendency and cause thereof. Reached the following conclusions.

The buckling deformation of the solidified shell is C: 0.04 to
0.60, Mn: In continuous casting of molten steel of 0.6 to 1.7%, M in steel of round cast slab having a diameter of 225 mm and 292 mm
As shown in FIG. 4 showing the relationship between the n concentration and the buckling deformation occurrence rate of the solidified shell, the Mn concentration in steel: [Mn] ≧ 1.0% and a high M
n steel shows a high incidence rate.

The cause of the high buckling deformation occurrence rate of the solidified shell in the high Mn steel is as follows: [C]: 0.08 to 0.40%, [Mn]: 0.6 to 1.6% As shown in FIG. 5 showing the relationship, when the Mn concentration in the steel is high, the increase in the deformation stress (strength) due to the temperature drop is steep in a region of 1200 ° C. or less. Therefore, a diagram schematically showing the occurrence of buckling deformation
6 (a) to cage shown strike, and accidentally resulting solidified delayed site 1 (high temperature region), the relative solidified shell strength difference between healthy coagulation site 2 (low temperature portion) increases. At this time, the solidified shell is as shown in FIG.
To indicate cage strike, subjected to pressure (compressive stress) 3 from the mold, or solidified shell itself shrinks 5 Then, strain coagulation delay unit 1 is buckling 4 is generated intensively.

The buckling deformation position of the solidified shell was calculated by calculating the compressive stress acting on the solidified shell in the mold in continuous casting of a high carbon / high Mn steel and a 0.2% carbon steel having the chemical compositions shown in Table 1. As a result, as shown in FIG. 7, it was found that the compressive stress increased in the lower part of the mold 400 mm below the meniscus. This calculation confirmed that the buckling deformation of the solidified shell occurred in the lower part of the mold 400 mm below the meniscus.

[0013]

[Table 1]

In order to prevent the buckling deformation of the solidified shell generated in the high Mn steel, it is effective to limit the upper limit of the taper of the lower part of the mold at 400 mm or less below the meniscus. In view of maintaining close contact and achieving uniform solidification, it is clear that there is an appropriate range for the mold taper at the bottom of the mold and at the meniscus.

That is, the present invention provides a method for producing C: 0.04 to 0.60%, M
n: In a method of continuously casting a round section slab from molten steel containing 1.0 to 1.7%, the taper at the meniscus portion is 4.0
In ~19.0% / m, and the average value of the taper of the mold bottom from subsequent meniscus under 400mm is 0.4 to 0.7% / m Ihensa
Continuous casting is performed using one type of mold per size . Thus, the molten steel containing C: 0.04 to 0.60%, Mn: 1.0 to 1.7%, the taper in the meniscus is 4.0 to 19.0%
In / m, and one from the meniscus under 400mm mean value of the taper of the mold lower end for each slab size is 0.4 to 0.7% / m
Continuous casting using molds of the same type can suppress the occurrence of buckling deformation of the solidified shell, can suppress the occurrence of vertical cracks due to uneven solidification, and provide excellent surface quality with few defects A round section slab can be obtained.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the Mn concentration in molten steel is set to 1.0 to 1.7% in the present invention is that when it is less than 1.0%, buckling deformation of the solidified shell is low regardless of the mold taper. Also, if it exceeds 1.7%, buckling deformation of the solidified shell occurs even if the mold taper is defined.

In the present invention, the taper K (% /
m) is represented by the following equation. K (% / m) = (ΔB / Bu) · (100 / L) where ΔB: difference between upper and lower diameters at the tapered portion (mm), B
u: lower width (mm) of the hollow in the tapered portion, L: length (m) of the same tapered portion.

In the present invention, the taper of the meniscus portion is set to 4 to 19% / m, as shown in FIG.
If it is less than m, the mold taper becomes insufficient with respect to the shrinkage amount of the solidified shell, so that a gap is formed between the mold and the solidified shell, and heat removal from the mold is greatly reduced in the gap. Solidification of the solidified shell at that portion causes a defect such as a crack due to a delay in solidification. On the other hand, if it exceeds 19% / m, on the contrary, the mold taper becomes excessive, so that the occurrence rate of the phenomenon (restriction) of seizure of the solidified shell on the mold increases, and the operation becomes difficult.

In the present invention, 400 mm below the meniscus
0.4 to 0.7% average taper from the end to the bottom of the mold
The reason for this is that if it is less than 0.4% / m, the rate of occurrence of vertical cracks will be high, and if it exceeds 0.7% / m, the taper at the bottom of the mold will be excessive, so that the buckling deformation of the solidified shell will occur. This is because the rate of occurrence of is increased.

[0020]

EXAMPLE A molten steel having a composition of A to Q shown in Table 2 was prepared by tapering the meniscus portion shown in Table 2 and 400 mm below the meniscus.
Thereafter, using a mold having an average taper up to the lower end, a diameter of 187 to
A 335 mm slab is cast at a steady part casting speed of 1.1 to 3.0 m /
min, and the buckling deformation rate of the solidified shell and the longitudinal cracking rate due to uneven solidification were investigated. The results are shown in Table 3 and FIGS. The “lower part of the mold” in the mold taper column in Table 2 is an average taper from 400 mm below the meniscus to the lower end.

[0021]

[Table 2]

[0022]

[Table 3]

As shown in Table 2 and FIGS.
In Example 1, the buckling occurrence rate of the solidified shell and the vertical cracking occurrence rate due to the uneven solidification were as small as 0.2% or less. On the other hand, in K, M, O, and Q of the comparative examples, since the mold taper in the meniscus portion is small, uneven solidification occurs, and the vertical crack generation rate becomes a high rate of 8.7 to 12.5%. I have. Further, in L, N, P, and Q of the comparative example, the buckling deformation occurrence rate of the solidified shell is as high as 0.7 to 1.5% because the taper in the lower part of the mold is excessive.

[0024]

According to the continuous casting method of the present invention, Mn
The buckling deformation rate of the solidified shell of high Mn steel of ≧ 1.0% can be suppressed to a low level, the rate of vertical cracking caused by non-uniform solidification is low, and the round surface has excellent surface quality with few defects. A cross section slab can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a mold taper in a meniscus portion and a vertical crack occurrence rate in an example.

FIG. 2 is a graph showing the relationship between the average taper at the bottom of the mold and the rate of occurrence of vertical cracks in Examples.

FIG. 3 is a graph showing a relationship between an average taper at a lower part of a mold and a buckling deformation occurrence rate of a solidified shell in an example.

FIG. 4: C: 0.30 to 0.60%, 0.18 to 0.2
9 is a graph showing the relationship between the Mn concentration in steel of 9% and 0.04 to 0.17% and the buckling deformation occurrence rate of the solidified shell.

FIG. 5 is a graph showing the relationship between temperature, deformation stress, and C concentration and Mn concentration in steel.

6A and 6B show the principle of buckling deformation of a solidified shell. FIG. 6A is a schematic diagram of generation of a starting point, and FIG. 6B is a schematic diagram of buckling deformation.

FIG. 7 is a graph showing a relationship between a compressive stress in a solidified shell of a high carbon / high manganese steel and a 0.2% carbon steel and a distance from a meniscus.

FIG. 8 is a graph showing a relationship between a mold taper and a constraint occurrence rate in a meniscus.

FIG. 9 is a schematic longitudinal sectional view showing the situation inside the mold.

[Explanation of symbols]

 Reference Signs List 1 solidification delay part 2 sound solidification part 3 pressure 4 buckling deformation 5 shrinkage 91 mold 92 immersion nozzle 93 molten steel 94 solidification shell 95 powder

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-8-132184 (JP, A) JP-A-59-165749 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/04 B22D 11/00

Claims (1)

(57) [Claims] 1. A method for continuously casting a round-section slab from molten steel containing C: 0.04 to 0.60% and Mn: 1.0 to 1.7%, wherein the meniscus has a taper of 4.0 to 19.0% / m and a meniscus. A continuous casting method for a round-section slab, wherein one slab is used for each slab size having an average taper of 0.4 to 0.7% / m at the lower end of the mold from the lower 400 mm or more.