JP4222148B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a continuous casting method for maintaining a slab cross-sectional shape in a perfect circle in continuous casting of steel round slabs and reducing slab surface cracks.

  Even in the past, continuous casting of round slabs has already been performed, and if the cross-sectional shape (roundness) of continuous cast slabs deteriorates at that time, operational troubles such as equipment breakage occur in the next rolling process. To trigger. Moreover, when the surface crack has generate | occur | produced in the round slab, it becomes a surface defect in the rolling process of a lower process, and causes the deterioration of a yield and process inhibition.

  For example, Patent Document 1 and Patent Document 2 disclose a technique for maintaining the roundness by pressing a slab with a forming roll. However, in addition to a pinch roll, a reduction facility is required, which requires considerable equipment costs. .

  Further, Patent Document 3 discloses a technique for maintaining the roundness by making the cross-sectional shape of the upper end of the mold into a polygon and making the lower end of the mold circular, but if it is not corrected to a circle using a molding roll, The roundness may be out of specification.

In Table 3 of FIG. 2 and FIG. 5, the internal quality of the cast slab and the roundness of the slab are disclosed, which is related to the unsolidified reduction technology, Disclosure of the relationship between roundness and surface quality of round cast slabs in the as-cast state when the round cast slabs are flattened and then reduced in diameter by a rolling roll after solidification. Not what you want.
JP-A-9-201602 Japanese Patent Laid-Open No. 11-309552 JP-A-6-269903

  Accordingly, an object of the present invention is to provide a continuous casting method of steel that can prevent roundness deterioration of a round slab and prevent slab surface cracking in an as-cast state with a simple means that suppresses cost deterioration as much as possible. Is to provide.

  As a result of various studies to solve the above-mentioned problems, the present inventors have paid attention to the relationship between the roundness and the amount of secondary cooling water, and ensure the roundness and prevent slab surface cracks associated therewith. The technology has been established and the present invention has been achieved.

  In this specification, “roundness” refers to that in an as-cast state, and is distinguished from roundness of a round cast piece after processing such as reduction after solidification.

  Here, the present invention is as follows.

(1) When manufacturing round slabs of carbon steel and low alloy steel with a curved type or vertical curved type continuous casting machine, [Al%] and [N%] of the molten steel components injected into the continuous casting mold are set to [Al %] × [N%] ≦ 2 × 10 ⁻⁴ , casting was performed at a casting speed of 1.4 m / min or less, and the round slab drawn from the mold was subjected to a secondary cooling specific water amount of 0.4 L / A continuous casting method, wherein secondary cooling is performed at a rate of not less than kg and less than 1.0 L / kg .

  (2) The continuous casting method according to the above (1), wherein a slab diameter of the round slab is 300 mm or more.

  According to the present invention, when round cast pieces are continuously cast, roundness in an as-cast state and prevention of cracks on the cast piece surface can be achieved at the same time. Since the roundness can be secured and no surface cracks are observed, the practical significance of the present invention is great.

  Next, the reason why the continuous casting conditions are defined as described above in order to ensure the roundness in the present invention will be described with reference to examples.

  FIG. 1 shows the definition and specifications of roundness. In the figure, the slab cross-section of the round slab is shown on the top side and the ground side on the top and bottom. The vertical cross-sectional length in the drawing is called the top-and-bottom diameter a, and the cross-sectional length is called the oval diameter b. The roundness is defined by (ba) / b, and is a roundness specification that requires (ba) /b≦0.03. Therefore, also in this specification, it is said that the roundness is good when the above range is satisfied.

  Here, the deterioration of roundness is a phenomenon in which the top diameter is shortened and the oval diameter is lengthened in order to reduce the slab when the slab is pulled out with a pinch roll.

  When the roundness deteriorates from 0.03, there is a high possibility that an operation trouble such as a biting failure occurs at the time of rolling in the next process, so that it is necessary to downgrade the slab and the yield deteriorates. For the deterioration of the roundness, attempts have been made to improve the slab pressing pressure of the pinch rolls, but slab slipping has occurred, which is not a practical measure.

    FIG. 2 is a graph showing the relationship between secondary cooling and roundness. As can be seen from FIG. 2, the strength of the slab is a major factor in the roundness, and the roundness is improved by setting the secondary cooling specific water amount to 0.4 L / kg or more.

  However, when the specific water amount is increased in order to ensure the roundness, there are cases where minute surface cracks occur on the surface of the slab.

  FIG. 3 is a schematic explanatory view of the form of surface cracking at this time.

  Such surface cracks are presumed to be so-called cracks. When continuously casting steel with a curved or vertical bending type continuous casting machine, surface cracks occur on the top side of the slab when the slab is straightened with a pinch roll.

  This crack is a surface crack generated by distortion during slab correction. The correction distortion becomes larger as the slab thickness increases, and surface cracks are more likely to occur. In addition, this crack is a γ grain boundary crack and is presumed to be promoted by AlN precipitated at the grain boundary.

  FIG. 4 is a schematic diagram showing the relationship at this time. As shown in FIG. 4a, AlN is precipitated in the α phase constituting the grain boundary of the γ phase. As shown in Fig. 2, grain boundary cracking occurs starting from this AlN.

  As shown in FIG. 5, generally, steel has an embrittlement temperature range, and surface cracks are likely to occur when a slab is corrected within this temperature range. Conventionally, surface cracks have been prevented by avoiding the slab surface temperature during straightening to be higher or lower than the embrittlement temperature.

  Here, FIG. 5 shows the tensile temperature when C: 0.06% steel is melted at 1600 ° C. and cooled to 1250 ° C. at 10 ° C./s, and then a tensile test is performed in the temperature range of 600 to 950 ° C. It is a graph which shows the relationship with an area reduction rate (RA). A small area reduction means that embrittlement has occurred. In that temperature range, there is a high possibility that surface cracks will also occur during slab correction.

  If the slab thickness is large, trying to avoid the correction temperature to be higher than the embrittlement temperature may not be possible because the casting speed cannot be increased due to the length limit of the continuous casting machine. Although it is possible to reduce the secondary cooling water amount and raise the slab surface temperature, as described above, in the case of a round slab, roundness deteriorates when the secondary cooling water amount is reduced, Operation troubles occur in the rolling process. On the other hand, when trying to avoid a temperature lower than the embrittlement temperature, it is necessary to lower the casting speed, and the productivity is significantly deteriorated. In particular, such a tendency is conspicuous in large-diameter round slabs having a slab diameter of 300 mm or more.

  Further, when the operation is carried out by connecting the round slabs with a connecting jig, the joint cannot be corrected and the slab is warped, and the operation cannot be continued.

  Further, it is difficult to strictly define the casting conditions at the so-called unsteady part at the beginning and the end of casting, and the embrittlement region cannot be avoided both on the high temperature side and the low temperature side, and the occurrence frequency of slab surface cracks is high.

  As described above, various demerits occur when trying to prevent slab surface cracking depending on casting conditions.

  Therefore, in this invention, the solution is aimed at by prescribing the Al content and N content of the molten steel.

  FIG. 6 shows the calculation result of the precipitation amount of AlN by [Al] × [N]. It can be seen that the precipitation amount of AlN decreases as [Al] × [N] decreases.

Al + N = AlN (1)
K = aAl × aN (2)
aAl: Activity of Al in molten steel, aN: Activity of N in molten steel That is, according to the present invention, cracking is prevented by reducing the amount of precipitation of AlN at the grain boundary that promotes surface cracking. . The equilibrium reaction formula of AlN is expressed by equation (1), and the equilibrium constant K is expressed by equation (2). That is, AlN can be reduced by reducing Al and N in the molten steel. Here, in order to simplify the management index, the product of Al concentration (mass%: [Al]) and N concentration (mass%: [N]) in molten steel was adopted.

  Next, in order to ascertain the occurrence of cracks on the slab surface, we investigated the external surface defect rate of pipes other than those caused by slab and mold powder in the round billet casting-seamless steel pipe process. It was found that the defect rate was high (Fig. 7).

  FIG. 7 is a graph showing the relationship between the mold size and the surface defect rate of the pipe, and this is a backing that is caused by a slab surface crack during correction.

  When the diameter is 191 mm and the diameter is 225 mm, the slab thickness is small, so the correction distortion is small and surface cracks are unlikely to occur. Therefore, when [Al] × [N] and the surface defect rate of the pipe were investigated with a slab diameter ≧ 300mm, the surface defect rate of the pipe increased with the increase of [Al] × [N]. As a representative, the results of a survey at a diameter of 360 mm are shown in FIG.

From the above results, it was considered that the cost deterioration and slab surface cracking were suppressed by regulating the molten steel component [Al] × [N] by the slab diameter. In other words, surface cracks do not occur at slab diameters <300 mm, so [Al] × [N] is not constrained from the viewpoint of suppressing cost deterioration. Only slab diameter ≧ 300mm is restricted to [Al] × [N] ≦ 2000 × 10 ^-7 .

  In the present invention, the chemical composition of the steel is not particularly limited. However, in the preferred embodiment, application of the present invention to a steel type having the following chemical composition more reliably exhibits its effect. Preferred above.

C: 0.05-0.20%
Mn: 0.3-1.5%
Si: 0.15-0.35%
P, S: 0.03% or less N: 0.015% or less
Al: 0.05% or less In addition, alloy elements such as Cr, Ni, Mo, Nb, and V can be appropriately blended as necessary.

  Here, the operation of the continuous casting method according to the present invention will be specifically described as follows.

  Although the molten steel whose Al and N contents are prescribed in advance according to the present invention is continuously poured into the mold, the continuous casting apparatus used in the present invention may be either a curved type or a vertical bending type, It is desirable that the cross-section be cooled uniformly. The mold used in the present invention has a round cross-section and is limited to a round cast slab diameter of 300 mm or more. This is because there are particularly few problems when the diameter is smaller than that, and a smaller diameter than that. It is also possible to use the present invention for the production of round slabs.

  The molten steel cast into the mold is primarily cooled through the mold and then withdrawn from the mold and further water-cooled. This is called secondary cooling. The secondary cooling region usually refers to the region immediately below the mold to the roller apron exit side, but in the case of the present invention, it refers to the region immediately below the mold to the correction belt entry side. This refers to the total amount of cooling water in the region immediately below to the correction belt entry side.

  Thus, according to the present invention, it is necessary to provide a large number of expensive forming rolls because good roundness can be obtained in an as-cast state without being reduced by a forming roll, and surface cracks can be effectively prevented. In addition, it is possible to obtain an effect that the equipment itself is simplified.

  Next, the effects of the present invention will be described more specifically with reference to examples.

  Carbon steel and low alloy steel having the components shown in Table 1 were continuously cast by a curved continuous casting machine having a round mold in cross section and a diameter of 310 mm and a diameter of 360 mm and a radius of curvature of 10.6 meters.

  The roundness at this time was evaluated by the method described above, and the slab surface crack was evaluated by a penetrant flaw test after removing the scale on the slab top side with a grinder.

As a result, no roundness defect occurred at a specific water amount of ≧ 0.40 L / kg. However, when the specific water volume is 1.OL / kg, warpage occurs at the slab joint, so it is desirable that the specific water volume is <l.OL / kg. Surface cracking could be prevented when [Al] × [N] ≦ 2000 × 10 ⁻⁷ .

  Table 2 summarizes these results.

  FIG. 9 is a graph showing the relationship between the roundness and surface cracks obtained at this time. The roundness is good and no surface cracks are observed within the range defined by the present invention. In the figure, it is indicated by a white circle.

  As can be seen from these results, a good roundness can be secured when the secondary cooling specific water amount is 0.4 L / kg or more. However, when the specific water amount at this time is 1.OL / kg, the slab is warped. Preferably, the specific water amount is less than l.OL / kg.

Further, when [Al] × [N] ≦ 2000 × 10 ⁻⁷ is satisfied, surface cracking is prevented, but as shown in Comparative Examples 7 to 9, [Al] × [N] ≦ 2000 × 10 ^{If -7} is deviated, surface cracks will occur if the round slab diameter is ≥300 mm because of the large straightening distortion.

It is a figure which shows the definition and required specification of roundness. It is a graph which shows the relationship between secondary cooling specific water amount and roundness It is typical explanatory drawing which shows the slab surface crack generation | occurrence | production situation. 4A and 4B are schematic explanatory views of a surface crack generation mechanism. It is a graph which shows the high temperature tension test result of steel. It is a graph which shows the AlN precipitation amount calculation result. It is a graph which shows the relationship between mold size and the surface defect rate of a pipe. It is a graph which shows the relationship between [Al] x [N] and the surface defect rate of a pipe. It is a graph which shows the relationship between roundness and a surface crack.

Claims (2)

When producing a round slab of carbon steel and low alloy steel by a curved type or vertical curved type continuous casting machine, [Al%] and [N%] of the molten steel component injected into the continuous casting mold are set to [Al%] × [N%] ≦ 2 × 10 ^{−4 While} adjusting the casting speed to 1.4 m / min or less and casting the round slab drawn from the mold, the secondary cooling specific water amount is 0.4 L / kg or more , A continuous casting method characterized by secondary cooling at less than 1.0 L / kg .   The continuous casting method according to claim 1, wherein a slab diameter of the round slab is 300 mm or more.

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