JPH11267810A - Steel continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a slab cast piece of excellent interior quality in steel continuous casting, while preventing the central segregation. SOLUTION: In this continuos casting method of steel, in order to stably produce a slab of the central segregation, molten steel superheat degree in a tundish in excellent casting should be at 20 deg.C or less to generate a minute equiaxed grain. In addition, the crater end angle obtained from the following equation should be over 1 deg. or more. Equation: θ=2.arctan[k.k/(Vc.t)+Δt/2] (In the equation, Q; crater end angle (deg.), K; the solidification constant (mm/√ min), Vc; the casting speed (mm/min), t; the casting slab thickness (mm), Δt; roll squeezing gradient in a solidification advanced stage division (mm)). In addition, it is the more desirable condition that a vertical continuous casting machine is used and the casting slab thickness is 300 mm or more.

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 slab cast piece having a broken internal quality while preventing center segregation in continuous casting of steel.

[0002]

2. Description of the Related Art Generally, in continuous casting of steel, when molten steel is cooled by a water-cooled mold, a solidified shell is formed around the slab, and the slab that has passed through the mold retains unsolidified molten steel therein. As it is pulled out by a group of rolls. In the process of drawing the slab, spray water is sprayed on the slab to promote solidification inside the slab. After the slab is completely solidified in this way, the slab is cut into a predetermined length by a cutter and transported to the next step.

[0003] However, the completely solidified portion of the slab (hereinafter, referred to as the slab)
In the vicinity of the crater end), component elements such as C, Mn, and P contained in the molten steel are concentrated in the unsolidified molten steel and directly solidify. This phenomenon is observed as an abnormal solidification structure (center segregation) in the structure observation of the slab cross section.
Significantly impairs the quality characteristics of the product. Further, when the solidification shell swells due to the molten steel static pressure (hereinafter referred to as bulging), the concentrated molten steel concentrates near the crater end, which promotes center segregation. Furthermore, when solidification proceeds, the flow of concentrated molten steel due to solidification shrinkage,
Center segregation is further promoted.

As a method of improving the center segregation, there is a technique in which a slab is drawn down by a specific amount by a light reduction device consisting of a group of rolls having a specified roll pitch in the vicinity of a crater end and reduced in pressure. No. 6,086,045. Also, the degree of superheat of molten steel during casting, ΔT should be 50-100
Japanese Patent Application Laid-Open No. 1-178356 discloses a method in which low-temperature casting is performed at a high temperature limited to ° C., the casting speed is limited, and light reduction is performed.

[0005]

However, these methods do not always completely resolve center segregation. Specifically, it is difficult to always maintain appropriate conditions with the light reduction technology that specifies the narrowing amount, considering the fluctuations in casting speed during operation and the accuracy of setting the roll profile during casting. Sex was inadequate.

In the method in which the degree of superheat of molten steel in a tundish during casting is set at ΔT of 50 to 100 ° C. and light reduction is carried out, the solidified structure in the slab becomes columnar, and the solidified structure is coarse and segregated. Since the spots and marks are coarse, it is difficult to stably obtain a good segregation score.

[0007] As a method of alleviating the fluctuation of the center segregation due to the fluctuation of the operation, a method of stirring the unsolidified portion by an electromagnetic stirrer in the secondary cooling zone so that the solidified structure becomes an equiaxed crystal is common. However, since the equiaxed crystal is coarse, this central equiaxed crystal causes a bridging phenomenon between the solidified shells and may cause abnormal center segregation. Absent.

[0008]

In order to solve the above-mentioned problems, the present inventors have observed the microstructure of the cross section of a slab cast under various casting conditions using various continuous slab casters. In order to stably produce slabs with good quality, the degree of superheat of molten steel during casting should be kept low to generate fine equiaxed crystals, and the opening angle at the solidification completed part (crater end) should be large. Invented that is important. Furthermore, it has been found that it is important to promote the deposition of the fine equiaxed crystals at the center of the slab thickness when the fine equiaxed crystals settle in the mold by using a vertical continuous caster.

The present invention has been made based on this finding. Specifically, in continuous casting of a slab, it is necessary to cast the superheat degree of molten steel in a tundish at the time of casting, ΔT, at 20 ° C. or less. A continuous casting method characterized in that casting is performed under casting conditions such that the crater end angle determined by the following equation is 1 ° or more. Further, in order to more effectively exert the effects of the present invention, it is more preferable that a vertical continuous casting machine is used and that the thickness of the casting slab is 300 mm or more.

Θ = 2 · arctan [k · k / (Vc · t) + △ t / 2] where θ: crater end angle (deg) k: solidification constant (mm / √min) Vc: casting Speed (mm / min) t: Cast slab thickness (mm) Δt: Roll drawing gradient at the end of solidification (mm / mm) Strictly speaking, this solidification constant is determined by steel grade components and secondary cooling conditions. Although it varies in the range of 23 to 30, there is no particular problem even if it is assumed that k 27.5 mm / √min under normal cooling conditions.

[0010]

FIG. 1 is an explanatory view showing an embodiment of the present invention. Generally, in order to cast steel with a vertical continuous casting machine, as shown in FIG. 1, molten steel produced in a converter or a secondary refining process is transferred to a continuous casting machine in a molten steel pot 1. A continuous casting mold 5 from a molten steel pot via a refractory long nozzle 2 using a tundish 3 and a submerged nozzle 4
Injected into The poured molten steel is cooled by the mold 5 and the subsequent secondary cooling nozzle to form a solidified shell 6 from the surface. The solidified shell is pulled out while being held by the roll 7, and the unsolidified molten steel 8 is completely solidified in the continuous casting machine. In this way, the completely solidified steel is cut into a predetermined length by a cutting machine, and is sent as a slab to a lower rolling process.

At this time, in the vicinity of the completely solidified portion, the slab may be slightly reduced (with a reduction gradient, Δt = tan α) using the roll as a measure against segregation. As a result, in the final solidification part, the crater end angle = θ
(Degree) to form a final solidified shape. The opening angle θ of the crater end formed at this time is geometrically obtained by the following equation.

Θ = 2 · arctan [k · k / (Vc · t) + △ t / 2] where θ: crater end angle (deg) k: solidification constant (mm / √min) Vc: casting Speed (mm / min) t: Cast slab thickness (mm) △ t: Roll drawing gradient at the end of solidification (mm / mm)

In the continuous casting process of steel, the degree of superheat of molten steel in a tundish at the time of casting is an important operation management index for stabilizing operation and ensuring quality. Normal,
There is a close relationship between the degree of superheat of the molten steel in the tundish during casting, ΔT, and the solidification structure in the slab, and as shown in FIG. It is well known that axial crystals are likely to occur. However, the present inventor has found that, at the time of low-temperature casting, not only a large equiaxed crystal ratio can be ensured, but also the size of the equiaxed crystal can be made extremely small as shown in FIG. In other words, by casting at the superheat degree of molten steel in the tundish at the time of casting, ΔT ≦ 20 ° C, fine equiaxed crystals can be obtained, and the size of segregation spots and zags can be suppressed to a very small value. To maintain a good level.

On the other hand, the equiaxed crystals settle from the mold to the final solidified portion in the unsolidified molten steel 8 in the continuous casting machine. In a curved continuous casting machine or a vertical bending continuous casting machine, since the final solidification position is a horizontal portion, the settled equiaxed crystals are deposited on the solidified shell on the lower surface of the slab, and are hardly present in the final solidified portion.

On the other hand, in the vertical continuous casting machine, almost all of the settled equiaxed crystals are deposited on the final solidified portion, so that the final solidified portion tends to have an equiaxed crystal structure. In other words, in order to form fine equiaxed crystals in the final solidified part,
It is desirable to perform low-temperature casting using a vertical continuous casting machine.

Further, when the thickness of the cast slab is large,
The time required for complete solidification is very long, which is advantageous for the deposition of equiaxed crystals. FIG. 4 shows the result of the present inventors examining the relationship between the thickness of the cast slab and the equiaxed crystal ratio. As can be seen from this figure, the greater the thickness of the cast slab, the higher the equiaxed crystal ratio in the vicinity of the final solidified part. In particular, a sufficient amount of equiaxed crystals can be obtained in extremely thick slabs of 300 mm or more. It is advantageous as

In general, in the case of equiaxed solidification by electromagnetic stirring, since the equiaxed crystal is coarse, a discontinuous abnormal structure due to bridging is exhibited, and the center segregation may be extremely deteriorated. However, in the case of the present invention, since the equiaxed crystal is made fine by low-temperature casting and the crater end angle is specified, no bridging phenomenon occurs.

Further, the above-mentioned extra-thick casting is advantageous from the viewpoint of preventing the occurrence of bridging since the crater end angle becomes large. FIG. 5 shows the relationship between the crater end angle and the segregation score, distinguishing between the case where the superheat degree of molten steel in the tundish during casting, ΔT is high (high-temperature casting) and the case where ΔT is low (low-temperature casting). Regardless of casting temperature, the larger the crater end angle, the smaller the segregation rating index,
Although the segregation is improved, the degree largely depends on the casting temperature. In other words, at the time of high-temperature casting, the segregation score is improved even under the condition that the crater end angle is secured large, but does not fall below the target level. On the other hand, when the crater end angle is small during low-temperature casting, bridging phenomena are likely to occur, and the segregation score deteriorates significantly.However, by ensuring a sufficient crater end angle, the segregation score is greatly improved. , The target level can be secured. From the above results, it is preferable to set the superheat degree of molten steel in a tundish during casting, ΔT to 20 ° C. or less, and to set the crater end angle to 1 ° or more, to produce a good cast piece with a stable segregation score. It is an important point to understand.

[0018]

Next, the present invention will be described by way of examples and comparative examples. Using molten steel having the composition shown in Table 1 as a representative chemical component, using a completely vertical continuous caster and a curved continuous caster having a curved radius of 10.5 m, various slab thicknesses and casting speeds, molten steel superheat during casting, Table 2 shows the results of casting under light pressure conditions.

[0019]

[Table 1]

[0020]

[Table 2]

A cut sample was taken from the cast slab, cut in the longitudinal direction at the center of the width, the cut surface was polished and corroded, and the result of structure observation was arranged as a center segregation index. As shown in Table 2, good segregation scores were obtained by setting the superheat of the molten steel in the tundish at the time of casting to 20 ° C. or less and ensuring the crater end angle of 1 ° or more. Adoption of a vertical continuous caster and an increase in the thickness of the casting slab promote this effect, and the center segregation index is further improved.

[0022]

As described above, by applying the present invention, a slab having good center segregation can be manufactured stably, and therefore, a material abnormality of a product obtained by rolling the slab can be prevented. At the same time, the casting yield and the product inspection yield can be significantly improved. Furthermore, soaking diffusion treatment of the cast slab as a countermeasure for center segregation mitigation is not required, so that the manufacturing cost is significantly reduced and the manufacturing period is shortened.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between a casting temperature and an equiaxed crystal ratio.

FIG. 3 is an explanatory diagram showing a relationship between a casting temperature and a size of an equiaxed crystal.

FIG. 4 is an explanatory diagram showing a relationship between a thickness of a cast slab and an equiaxed crystal ratio.

FIG. 5 is an explanatory diagram showing a relationship between a crater end angle and a center segregation index.

Claims (3)

[Claims] In a continuous casting of a slab, a superheat degree of molten steel in a tundish during casting, ΔT is set to 20 ° C. or less, and a crater end angle obtained by the following equation is set to 1
° Continuous casting method of steel, characterized by casting under casting conditions of not less than °. Equation 1 θ = 2 · arctan [k · k / (Vc · t) + Δt / 2] where θ: crater end angle (deg) k: solidification constant (mm / √min) Vc: casting Speed (mm / min) t: Cast slab thickness (mm) △ t: Roll drawing gradient at the end of solidification (mm / mm) 2. The continuous casting method for steel according to claim 1, wherein the casting is performed using a vertical continuous casting machine. 3. The continuous casting method for steel according to claim 1, wherein the thickness of the cast slab is 300 mm or more.