JP2000197953A - Manufacture of continuously cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the surface crack on corner parts by executing rolling reduction to a protruded part of oscillation mark developed on the corner parts in a cast slab at the upper position of a bending part and flattening. SOLUTION: The position for executing the rolling reduction to the oscillation mark in a continuous casting line, is at the upper part of the bending part in the continuous caster. This reason is because it is effective to beforehand eliminate or reduce a notch causing the concentration of stress, in the early stage of developing, propagation and growth of the crack due to the bending. When the rolling reduction is executed to the oscillation mark, the surface temp. of the cast slab is desirable to be >=500 deg.C. The flattening is desirable to reduce the depth of the oscillation mark to <=0.2 mm. In this way, the notching action caused by the oscillation mark is reduced and the surface crack can be prevented. Thereafter, the cast slab can straightly be conveyed and a straight-conveyed rolling and a hot-charge rolling can ideally be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuous cast slab having a small number of surface cracks. The present invention relates to a method for manufacturing a slab.

[0002]

2. Description of the Related Art A general continuous casting method for steel will be described with reference to the schematic diagram shown in FIG. Molten steel 2 is injected from ladle 1 into tundish 4 via long nozzle 3. The injected molten steel 2 passes through a submerged nozzle 5 to a mold 6.
Injected continuously into. The molten steel 2 in the mold is rapidly cooled to form a solidified shell 7 on the inner surface of the mold 6. The solidified shell 7 is further cooled by the secondary cooling water while being supported by the guide roll 8. Then, the slab 9 that has been completely solidified is continuously pulled out by the pinch roll 10. At this time, in order to reduce the friction between the mold and the slab formed by the solidified shell, prevent seizure on the mold and perform stable casting, mold powder 11 is added into the mold, and the mold is added. A vertical movement called an oscillation is operated at 6. The regions a and b in FIG. 1 are a bent portion and a straightened portion, respectively. In this region, the slab undergoes deformation at a strain rate of 0.0001 to 0.001 sec ^-1 .

[0003] By the way, continuous cast slabs have conventionally been
It is known that various surface cracks occur. This surface cracking, especially when performing direct-feed rolling or hot-charge rolling using continuous cast slabs, requires the detection or care of defects during hot working. The above obstacles were caused, and the productivity was significantly impaired. Such surface cracks, which cause harmful effects, are likely to occur during deformation at a low strain rate.
Very easily occurs in the embrittlement temperature range of 0 ° C.

[0004] The embrittlement of the slab during low strain rate deformation is mainly caused by precipitation of carbonitrides such as AlN, Nb (C, N), V (C, N) and (Fe, Mn) S. This is due to sulfide precipitation. Therefore, as a countermeasure for suppressing the surface cracking conventionally taken, when continuously casting a low-alloy steel containing these alloying elements at 1% or less, in order to reduce the above-mentioned harmful precipitates, , Nb, V, B, and N, or adding Ti to preferentially precipitate more harmless TiN to improve the high-temperature ductility.

[0005]

However, due to the requirements of the material properties of steel, the addition of elements such as NB and V is indispensable, and depending on the type of steel, the low-temperature toughness may decrease due to the addition of Ti. Yes, the above measures were not drastic. In addition, as another measure to suppress surface cracking, in order to avoid the brittle temperature range in the bent and straightened parts due to secondary cooling during continuous casting, casting of high-temperature slabs and temperature reduction by reducing the specific water content Attempts have also been made to cut the cooling water at the corners where the width is large, such as by cutting the width. Although the occurrence of surface cracks has been significantly reduced by these measures, the notch (notch) effect of the oscillation mark is particularly large at the slab corners, and it has not yet been possible to completely prevent surface cracks. is there.

It is considered that such a surface crack attributable to the oscillation mark at the slab corner is generated by a mechanism as shown in FIG. That is, when the oscillation mark is formed, as shown in FIG. 2, in a series of continuous casting steps, the frictional force between the solidified shell and the mold, the thermal stress due to uneven cooling, the deformation stress in the bent portion and the straightening portion, etc. Works, the concave portion of the oscillation mark acts as a notch for stress concentration, and a crack is generated from the bottom of the concave portion, which gradually propagates and grows into a surface crack. Surface cracks due to the notch effect as described above are particularly likely to occur at the slab corners because the difference in mold powder penetration at the corners and the effect of shrinkage during cooling are large. This is because the ration mark tends to be deep.

[0007] Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to completely prevent surface cracks at the slab corners. And the present invention can withstand direct rolling or hot charge rolling,
The purpose is to obtain a sound slab without surface cracks.

[0008]

SUMMARY OF THE INVENTION The inventors of the present invention squeezed and flattened an oscillation mark, particularly a projection of an oscillation mark at a corner of a slab, at the earliest possible stage before crack growth. The present inventors have found that relaxing the notch action of the concave portion is extremely effective in preventing surface cracking, and completed the present invention. The summary configuration is as follows. (1) From the molten steel injected into the mold, through the bending section and straightening section of the continuous casting machine, in the method of manufacturing a continuous cast slab to obtain a slab, the convex portion of the oscillation mark generated in the corner of the slab A method of producing a continuous cast slab by flattening by rolling down at a position above a bent portion.

(2) The method for producing a continuous cast slab according to the above (1), wherein the depth of the oscillation mark at the slab corner is flattened to be 0.2 mm or less.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. In the present invention, it is important that the convex portion of the oscillation mark at the corner of the slab is flattened above the bent portion of the continuous casting machine. First, the flattening of the oscillation mark sets the depth of the oscillation mark to 0.
If it can be reduced to 2 mm or less, a sufficient effect for preventing surface cracks can be obtained. Normally, the depth of the oscillation mark (the distance from the top of the projection to the bottom of the recess) is at most 2 m.
m, it is desirable to set it to 2 mm or more. However, even under a pressure of about 1 mm, if the depth of the oscillation mark can be reduced to 0.2 mm or less, a sufficient surface crack preventing effect can be obtained.

The position where the oscillation mark is reduced in the continuous casting line is, as exemplified by the dotted line in FIG.
It is preferably performed above the bending part of the continuous casting machine. This is because it is effective to eliminate or reduce notches, which cause stress concentration, in advance as early as possible to generate, propagate, and grow cracks due to bending. Further, the temperature at the time of rolling down the oscillation mark, from the rolling efficiency, it is preferable that the surface temperature of the slab is 500 ℃ or more,
The temperature is preferably set to 700 ° C. or higher in consideration of preventing precipitation of harmful precipitates on austenite grain boundaries (γ grain boundaries).

[0012] At the time of applying the reduction in slab, the strain rate can be expected to improve the hot ductility precipitate size is coarsened, if 0.01 sec ^-1 or higher, the surface-break preventing effect It is desirable because it increases. Although there is no particular limitation on the method of rolling, as shown in a plan view, for example, as shown in FIG. 3 (a), a corner portion 2 is formed by a spinning wheel-shaped roll having a tapered portion having an inner surface extending outward on both sides. As shown in FIG. 3 (b), there are a method of simultaneously rolling down portions, a method of rolling down each corner portion with an individual roll, and a method of continuous pressing (forging pressure).

[0013] In this manner, the corner portion of the continuous cast slab is pressed down above the bent portion to flatten the unevenness of the oscillation mark formed at the corner portion, so that the notch effect caused by the oscillation mark is obtained. And the occurrence of surface cracks can be prevented. As a result, it is possible to directly feed the slab without the need for hot defect detection or maintenance due to the occurrence of surface cracks, and it is possible to ideally carry out direct rolling or hot charge rolling. The present invention is applicable to all types of continuous casting of steel in which surface cracking is a problem. In particular, C content 0.02
When applied to low-medium carbon steel containing up to 0.22 wt% of Si and / or Mn, and low alloy steel containing alloying elements such as Al, Nb, V, B, Ti in a range of 1 wt% or less It is valid. In recent years, it has been confirmed that the present invention has an extremely large effect when applied to continuous casting of thin slabs, in which the development of casting technology is active.

[0014]

The present invention will be described more specifically with reference to the following examples. Using a vertical bending type continuous caster having a curvature radius of 10 m, a slab having a composition of medium carbon steel having a composition shown in Table 1 and a cross section of 220 mm x 1500 mm was cast at a casting speed of 1.2 to 1.6 m / min. At this time, at a position 3 m below the meniscus (0.2 m above the bent portion), a spinning wheel-shaped reduction roll as shown in FIG. 3A was installed on the short side of the slab, and the corner portion of the slab was reduced. Here, the slab pressure reduction device was composed of a pressure reduction roll and a hydraulic cylinder, and these were capable of controlling the reduction amount via a hydraulic unit. The amount of reduction in the slab corner was 5 mm, the average strain in the slab corner surface layer 5 mm was 20%, and the strain rate was 0.
It was 25 sec ^-1 . Note that this value is V
_R = 27 mm / sec, r = 45 mm, a = 5 mm, b = 21 mm, d ₀ =
Since was 25 mm, obtained by the strain amount _{ε = (a / d 0)} × 100 = 20, strain rate epsilon (dot) = (ε / 100) / (b / V R) = 0.25. Table 2 shows the casting conditions and the results. As shown in Table 2, when normal casting was performed without rolling down the corner portion above the bent portion during continuous casting, many surface cracks occurred at the slab corner portion. In the case of No., there was no surface crack at the corner of the slab.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

As described above, according to the present invention,
Since surface cracks at the corners of the continuous cast slab can be avoided, it is possible to go directly from continuous casting to the rolling process without the need for auxiliary steps such as detection and maintenance of surface cracks. Therefore, the present invention greatly contributes to improvement of productivity, energy saving, and cost reduction.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a continuous casting process.

FIG. 2 is a schematic view showing a mechanism of occurrence of a surface crack.

FIG. 3 is a plan view showing a method for rolling down a slab corner.

FIG. 4 is a diagram for explaining a distortion amount and a distortion speed.

[Description of Signs] 1 Ladle 2 Molten steel 3 Long nozzle 4 Tundish 5 Dipping nozzle 6 Mold 7 Solidification shell 8 Guide roll 9 Cast piece 10 Pinch roll 11 Mold powder 12 Roll-down roll a Bending part b Straightening part

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Koichi Tozawa 1-chome, Kawasaki-dori, Mizushima, Kurashiki-shi, Okayama Pref. Kawasaki Steel Corporation Mizushima Works

Claims (2)

[Claims] 1. A method of manufacturing a continuous cast slab which obtains a cast slab from a molten steel poured into a mold through a bending section and a straightening section of a continuous caster, wherein a projection of an oscillation mark generated at a corner of the cast slab. A method for producing a continuous cast slab, wherein a portion is flattened by rolling down a portion above a bent portion. 2. The method for producing a continuous cast slab according to claim 1, wherein the depth of the oscillation mark at the slab corner is flattened to 0.2 mm or less.