JP3319379B2 - Continuous casting method of steel billet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to carbon steel, alloy steel,
The present invention relates to a continuous casting method of a billet made of stainless steel or the like, and to a continuous casting method of a billet capable of reducing center segregation and porosity generated at and near a center of a slab.

[0002]

2. Description of the Related Art When a seamless pipe is manufactured by continuous rolling from a continuous casting to a rolling or forging process by the Eugene Sejournet method or the Mannesmann method, a billet for pipe production is manufactured by rough rolling after continuous casting. Billets are used in the pipe making process. If it is possible to omit this rough rolling and directly produce a continuously cast slab, a significant cost reduction effect can be expected.

[0003] In addition to seamless pipes, for example, in the field of products such as steel bars and wire rods, similarly, it has been aimed to omit the rough rolling of slabs after continuous casting to shorten the process.

[0004] However, segregation and porosity occur in the center of a continuously cast slab during casting, and these defects remain inside the slab. The so-called center segregation and porosity cause a problem that an inner surface flaw is generated at the time of pipe production, and a problem that a wire product breaks during wire drawing.

It is said that such center segregation and porosity are caused by solidification shrinkage at the end of solidification of a slab. For this reason, at the end of solidification in continuous casting, a reduction is applied to the slab at the end of solidification to reduce the cross-sectional area of the slab, and by removing unconsolidated molten steel enriched with impurity elements from the center of the slab, these center segregation and porosity Measures have been taken to reduce emissions.

For example, Japanese Patent Application Laid-Open No. 3-124352 proposes a method of rolling down an unsolidified portion in a slab using a roll having a diameter of 2 to 5 times the thickness of a bloom or billet slab. ing. This method reduces the center segregation and porosity with a weaker rolling force than when rolling down after complete solidification, with the center of the slab remaining unsolidified. it can.

However, according to the manufacturing method disclosed in Japanese Patent Application Laid-Open No. 3-124352, since the shape of the slab is deformed as the slab is reduced, for example, a round material having good roundness for pipe making is used. Is required, it is necessary to form a deformed cast slab. Also, in the case of material for wire rod rolling,
In the case of a greatly deformed slab, it is necessary to form the slab.

Further, when the round slab is unsolidified and reduced, the slab extends and deforms in a direction perpendicular to the rolling direction, so that an internal crack is generated due to a tensile stress acting on an internal solidification interface. There is a problem that the cracks cause internal surface flaws during pipe production.

[0009]

SUMMARY OF THE INVENTION The present invention relates to continuous casting of carbon steel, alloy steel, stainless steel, etc., in which an unsolidified reduction is applied before completely solidifying a slab, and then, after complete solidification, a predetermined cross-sectional shape is formed. It is an object of the present invention to provide a continuous casting method capable of obtaining a billet cast piece having a good shape without defects such as center segregation and porosity.

[0010]

The gist of the present invention resides in a continuous casting method shown in the following (1) and (2).

(1) Prior to complete solidification of the slab, a reduction is applied by an unsolidified rolling stand composed of at least one pair of rolls, and then, after complete solidification of the slab, a predetermined sectional shape is formed by a molding stand composed of at least one pair of rolls. A continuous casting method of a steel billet to be formed into a shape, wherein a distance from a last pair of rolls of the unsolidified rolling stand to a first pair of rolls of the forming stand is 3 to 15 m; In the stand, the center solid phase ratio of the slab is 0.1 to 0.8.
A continuous casting method of a steel billet by applying a reduction to the slab not less than the diameter of the unsolidified portion or the thickness of the unsolidified portion of the slab before rolling in the region of.

(2) The last one of the unsolidified pressing stand
Between the pair of rolls and the first pair of rolls of the forming stand, the slab surface is 5 to 5 units per unit area of the slab surface.
The method for continuous casting of a steel billet according to the above (1), wherein the steel billet is cooled with a water amount of 200 liter / m ² · min.

Here, the billet used in the present invention means that the shape of the slab is JISG of Japanese Industrial Standard.
Bloom and billet described in the steel term (product and quality) defined in No. 0203, which is a slab with a substantially square cross section, a rectangular shape with a long side of about twice or less the short side, or a circular cross section. is there. Next, the slab is subjected to product rolling from the product in the same cross-sectional shape as it is without performing rough rolling for forming a material for product rolling. After rolling,
Product rolling may be performed.

Incidentally, the diameter of the unsolidified portion described in the above (1) is
The slab having a circular cross section is used, and the thickness of the unsolidified portion is used for a slab having a square or rectangular cross section. The unsolidified thickness in the case of a rectangular cross section means the unsolidified thickness between the long-side solidified shells.

The diameter of the unsolidified portion or the thickness of the unsolidified portion is defined as
The diameter or thickness of the unsolidified portion having a solid phase ratio of 0.99 or less.

A carbon steel having a C (carbon) content of 0.5% by weight or more, an alloy steel containing an alloy component, a stainless steel, or the like, is subjected to a predetermined solid phase by a non-solidification rolling stand before completely solidifying the slab. Uncoagulation reduction by a predetermined amount at a rate, further downstream,
When applying the method of forming a slab shape deformed under unsolidified pressure by a molding stand after complete solidification of a slab, after the center segregation and porosity disappear once due to the unsolidified pressure of the slab, the molding stand When molding slabs by
New porosity (hereinafter referred to as forming porosity) may occur, and this forming porosity may be a harmful flaw in a subsequent product rolling process.

In the case of a bearing steel having a C content of about 1% by weight including an alloy component, eutectic cementite generated by a eutectic reaction during the solidification process exists around a typical forming porosity generated after forming. . On the other hand, in an as-cast slab which is not subjected to unsolidification reduction and molding, eutectic cementite is crystallized also in the micro-segregated portion and the central segregated portion between dendrite trees.

FIG. 2 is a Fe-C pseudo-binary phase diagram of an Fe-1.5% Cr-C ternary system corresponding to the chemical composition of the bearing steel. It is understood that in a bearing steel having a C content of about 1% by weight, eutectic cementite does not crystallize in the base metal composition, but in fact, solute elements such as C segregate with solidification, and as shown in FIG. Eutectic cementite crystallizes because C moves to the high C content side on the right.

The eutectic temperature of this bearing steel is 1150 ° C., while the forming temperature of the slab after complete solidification is 1150 ° C.
Because of the temperature of ℃ or more, the eutectic cementite in a molten state is processed and becomes a molded porosity.

FIG. 3 is a diagram showing the results of an investigation on the relationship between the slab center temperature during molding and the presence or absence of molding porosity, obtained by heat transfer solidification analysis, for this bearing steel. The occurrence of molding porosity was investigated by counting the condition of the central portion 30 mm with a loupe 20 times as large as that of the slab longitudinal section sample. It can be seen that there is a clear correlation between the slab center temperature during molding and the state of occurrence of molding porosity, and molding porosity occurs when molding is performed under conditions higher than 1150 ° C.

In ordinary carbon steel, the C
A eutectic structure is formed due to the increased content. That is,
In a carbon steel having a C content of 0.5% by weight or more, a eutectic cementite having a low melting point is crystallized during the solidification process, or the melting point of the central segregated portion where the C content is increased without crystallization decreases,
As a result of the remaining molten portion during molding, molding porosity may occur.

Since various alloying components are also added to stainless steel, an alloying component that causes microsegregation, for example, an austenitic stainless steel containing a large amount of B, forms a low melting point eutectic structure in the microsegregated portion. However, molding porosity may occur during molding.

As described above, the molding porosity generated at the time of molding is such that the formed micro-segregated portion has a low melting point eutectic structure, or even if it does not have a eutectic structure, has a low melting point due to component concentration due to component segregation. In this case, the molded porosity is obtained because the molten porosity is subjected to molding.

According to the method of the present invention, since the slab is completely solidified including micro-segregation by the time of molding, and then molded under appropriate rolling conditions, the occurrence of molding porosity can be suppressed.

[0025]

FIG. 1 is a view showing an example of a continuous casting apparatus for carrying out the method of the present invention.

Here, two sets of a pair of upper and lower horizontal roll stands 21 are provided as an example of the unsolidified pressing stand 2, and a pair of vertical rolls 31 and a downstream thereof are provided as an example of the forming stand 3 after the solidification pressing. An example is shown in which a pair of upper and lower horizontal rolls 32 is provided on the side.

If the slab is formed in a separate step between the continuous casting step and the product rolling step, the cost will increase.
It is desirable to carry out in-line by a molding stand in a continuous casting apparatus installed downstream of the unsolidified rolling stand. In other words, it is desirable that the slab 1 whose center is not solidified be reduced by the unsolidified rolling stand 2 and then formed into the desired shape by the forming stand 3.

The unsolidified pressure is set at a center solid fraction of 0.1 to 0.8.
Need to be done between. Here, the central solid fraction is the volume fraction of the solid portion of the slab with respect to the total amount of the liquid phase and the solid phase in a state where the liquid phase and the solid phase coexist in the process of solidification. . The center solidus fraction can be determined from the temperature of the slab central part calculated by heat transfer solidification analysis and the liquidus temperature and solidus temperature specific to the steel.

In order to reduce center segregation and porosity during unsolidification reduction, it is necessary to reduce the solidification interface until it is completely pressed. When the central solid phase ratio is reduced at a time smaller than 0.1, the unsolidified portion has a large diameter, so that a large amount of reduction is required to press the solidified interfaces together. Therefore, since the slab is greatly deformed,
It is disadvantageous for later molding. Further, unevenness occurs in the reduction at a low solid phase ratio and the penetration of the reduction into the center of the slab, which causes center segregation.

On the other hand, if the reduction is performed at the time when the central solid phase ratio exceeds 0.8, the molten steel in which the segregated components are concentrated cannot be squeezed out, and the central segregation remains. When the central solid phase ratio is high, in addition to the low liquidity of the molten steel, the ratio of the residual molten steel between the solid phases is small, so that even when the molten steel is lowered, the residual molten steel flows between the solid phases. This is because it becomes difficult.

As described above, in order to reduce the deformation of the slab during rolling and to more completely eliminate center segregation and porosity, the center solid fraction is preferably 0.1 to 0.8, more preferably It is desirable to reduce the pressure in the range of 0.2 to 0.7.

The distance between the last pair of rolls of the unsolidified rolling stand and the last pair of rolls of the forming stand is 3 to 3 mm.
It must be 15m. In the method of the present invention, the formation of the molding porosity is prevented by completely solidifying the central portion of the slab that has been unsolidified and reduced to microsegregation containing eutectic cementite by the time of molding. Hereinafter, conditions for completely solidifying the central portion of the slab will be described.

When the unsolidified pressure is reduced in the range of the center solid fraction of 0.1 to 0.8, most of the unsolidified molten steel at the center in the cross section of the slab is washed away to the upstream side. For this reason, the release of the latent heat of solidification stops, and the temperature at the center decreases rapidly.

FIG. 4 shows the results of calculating the transition of the slab center temperature after unsolidification reduction by heat transfer solidification analysis. Here, the cast pieces were round cast pieces having a diameter of 360 mm and 190 mm, and the casting speed was 1.1 m / min and 2.8 m / min, respectively.
And After the unsolidification pressure, cooling was performed until molding.

At the end of the non-solidification reduction, the center temperature becomes about 1350 ° C. with the solidification interface pressed, and the temperature gradually decreases thereafter. In any case, the center temperature was less than 1150 ° C. 3 m after the unsolidification reduction, and the eutectic temperature was 1
Below 150 ° C. Therefore, in order to prevent generation of molding porosity, it is necessary to separate the unsolidified pressing stand and the molding stand downstream thereof by 3 m or more.

However, if the interval of 5 m or more is provided, the center temperature decreases to about 1100 ° C., so that various steel types,
It is more desirable to provide an interval of 5 m or more from the viewpoint of more reliably preventing the formation of molding porosity in response to a change in manufacturing conditions such as casting conditions.

On the other hand, if the unsolidified pressing stand and the molding stand on the downstream side are separated by more than 15 m, the temperature of the slab decreases and the strength of the slab increases, so that excessive equipment is required for molding. Becomes Although it is possible to heat the slab, a heating furnace is required, which greatly increases equipment costs and manufacturing costs. Therefore, the upper limit was set to 15 m. More preferably, it is within 10 m. In this case, molding can be performed at a relatively high temperature, and a large-sized molding device is not required, so that equipment costs can be reduced.

From the last pair of rolls of the unsolidified rolling stand to the first pair of rolls of the forming stand,
It is desirable to cool the slab with a water amount of 5 to ²⁰⁰ l / m ² · min. By cooling the slab by water spray or the like between the unsolidified pressing stand and the molding stand, the temperature of the slab is reduced.

As described above with reference to FIG. 4, even when the slab is allowed to cool, the temperature at the center of the slab is 1150 by the time of molding.
Lower than ° C. However, when it is expected that the temperature of the central portion of the slab does not reach less than 1150 ° C. by the time of molding due to fluctuations in manufacturing conditions and the like, the slab is reliably cooled by water cooling to 115 ° C.
It is possible to control the temperature to less than 0 ° C., and it is possible to reliably prevent the formation of molding porosity.

In this cooling, if the amount of water is less than 5 liter / m ² · min per unit area of the slab surface, a substantial slab cooling effect cannot be obtained. In addition, 200 liters / m ²
If the cooling is performed in excess of min, supercooling occurs, and the temperature of the slab is remarkably lowered to increase the strength of the slab, so that an excessive load is required during molding. Further, thermal stress cracking due to rapid cooling may occur in the slab.

The method of the present invention is based on Japanese Industrial Standard JISG
The invention can be applied to any of the cast pieces having a round section shape, a square shape, or a rectangular shape, which is a bloom or billet shape defined in No. 0203.

The unsolidification pressure and the molding pressure of the slab are as follows:
It is preferable from the viewpoint of production efficiency to perform the operation with one-to-two upper and lower rolls or two-to-four upper and lower rolls, but it is also possible to use another mechanism such as a forging device.

[0043]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, an unsolidified pressing stand 2 comprising a pair of upper and lower rolls 21 and a forming stand 3 comprising a pair of vertical rolls 31 and a pair of upper and lower horizontal rolls 32 are provided. Using a curved continuous casting machine, a round-section billet was manufactured. The center of segregation, porosity, and roundness of the obtained slab were evaluated.

A pair of upper and lower rolls 2 of the unsolidified pressing stand
1 is at a position 23 m from the molten steel meniscus, and the first pair of vertical rolls 31 of the forming stand is also 25 m to 3 m
The test was carried out by setting the position of the first pair of vertical rolls 31 of the forming stand at 9 m and changing the position.

The roll 21 for performing unsolidification rolling has a diameter of 60
A pair of vertical rolls 31 and a pair of upper and lower horizontal rolls 32 are formed by applying a load of a rolling force of 100 ton by a pair of upper and lower rolls of 0 mm and grinding the roll surface into a round shape with a diameter of 600 mm. With a pressing force of 15
The test was performed with a load of 0 ton.

The casting speed was 1.5 to 2.3 m / min, and a slab having a circular cross section of 240 mm in diameter was cast. Secondary cooling in the continuous casting machine was performed at a specific water volume of 0.2 L / kg · steel between 6 m from the outlet of the mold. At the time of casting, electromagnetic stirring with a current value of 300 A was applied to a position 200 mm from the meniscus in the mold. The purpose of electromagnetic stirring is to increase the unsolidified rolling effect by changing the solidification structure from columnar crystals to equiaxed crystals. Further, the casting speed was changed to change the center solid fraction at the unsolidified rolling reduction position.

Table 1 shows the chemical composition of the steel used for the test. Steel A is a bearing steel with a C content of 1.0% by weight, steel B
Is a carbon steel having a C content of 0.8% by weight.

[0048]

[Table 1]

The diameter of the unsolidified portion and the ratio of the solid phase in the center were determined by one-dimensional unsteady heat transfer solidification analysis in the radial direction, and further confirmed by the investigation of the thickness of the solidified shell by adding Fe-S to the slab during casting.

The degree of center segregation is 5 m in diameter from the center of the slab.
The ratio C ₁ / C ₀ of the ladle analysis value C ₀ of the slab to the C content C ₁ of the cut obtained by drilling with a drill blade of m.
Was evaluated.

The roundness is determined by calculating the center of gravity of the cross section of the slab, measuring the distance from the center of gravity to the outer surface at a 30 ° pitch in the circumferential direction, and calculating the difference from the radius of the perfect circle to be obtained. The value obtained by dividing by the radius of was defined as the true circular deviation rate (%) and evaluated. The permissible roundness deviation rate for the pipe making process is usually 3
%.

The number of porosity produced was determined by a method of examining a center width of 30 mm of a slab longitudinal section sample with a 20-fold loupe.

Table 2 shows the test results of the bearing steel having a C content of 1.0% by weight. Here, the molding position represents a distance from the last pair of rolls of the unsolidified drafting stand to the first pair of rolls of the molding stand (the following description and the table mean the same).

[0054]

[Table 2]

In Examples 1 to 4 of the present invention, the center solid fraction under unsolidified pressure is 0.1 to 0.5 within the range specified by the present invention, and the molding position is also 4.0. ~ 12.0
m. In addition, between the unsolidified pressing stand and the molding stand, the slab was not cooled with water but allowed to cool.

In the tests of Examples 1 to 4, the degree of center segregation,
Good slabs were obtained in both roundness and central porosity. However, in Example 2, which was within the range specified in the present invention but the solid phase ratio at the time of unsolidification reduction was as low as 0.1, the unsolidification reduction amount was large, and the roundness deviation rate after molding was slightly large. .

In Example 3 where the molding position was 4 m, since the slab temperature was slightly high, very slight molding porosity was observed during molding. Example 4 in which the molding position is 12 m
In this case, the temperature of the cast slab slightly decreased, a large force was required for molding, and the roundness deviation rate was slightly increased.

In Comparative Example 5, since the molding position was set at 2 m and molding was performed in a short time after the non-solidification reduction, the central segregated portion was molded without solidification and a large number of porosity was generated. In Comparative Example 6, the molding position was set to 16 m and molding was performed after a certain period of time after the unsolidification reduction, so that the slab temperature was lowered, and the molding stand having a reduction force of 150 ton could not perform sufficient molding, and the roundness deviation rate was extremely poor. . The slab of Comparative Example 6 could not be formed.

Comparative Example 7 is a case where the casting speed was increased and the unsolidified pressure was reduced at the center solid fraction of 0.05. Although the center segregation was not accompanied by crystallization of eutectic cementite, the degree of segregation was large. In addition, a large reduction was performed in order to press the unsolidified interface by pressure, and molding was possible only to a roundness that was difficult to apply to pipe making.

In Comparative Example 8, the casting speed was reduced, and the unsolidified pressure was reduced at a center solid fraction of 0.9. Due to insufficient flow of the molten steel in which the segregated components were concentrated under unsolidified pressure, the degree of central segregation was large and porosity occurred.

In Comparative Example 9, the amount of reduction during unsolidification reduction was reduced. Since the solidification interfaces were not pressed together, the concentrated steel in the center was not discharged to the upstream side, and the center segregation was remarkable and a large number of porosity was generated. Incidentally, Comparative Examples 5 to 9 also between the unsolidified pressing stand and the molding stand,
The slab was allowed to cool without water cooling.

Table 3 shows the test results of carbon steel having a C content of 0.8% by weight. Example 10 of the present invention and Comparative Example 11
In the test, the casting speed and the unsolidification rolling conditions were kept constant within the range specified in the present invention, and only the molding position was changed.
In both Example 10 and Comparative Cool 11, the cast slab was allowed to cool without water cooling between the unsolidified pressing stand and the forming stand. In Example 10 of the present invention, good cast pieces were obtained. On the other hand, in Comparative Example 11 in which molding was performed only at an interval of 2 m after the unsolidification reduction, a large number of molding porosity occurred.

[0063]

[Table 3]

Next, using a bearing steel, under the same casting speed and unsolidified rolling conditions and molding conditions as in Example 3 in Table 2,
Table 4 shows the test results obtained by changing only the water cooling conditions between the unsolidified pressing stand and the molding stand.

[0065]

[Table 4]

Example 13 was a case where the material was allowed to cool after unsolidified pressure reduction, which was within the range specified by the present invention, but the rolling position was 4 m.
The temperature of the slab was rather high due to the short length, and as a result, negligible slight forming porosity was generated by forming.

On the other hand, the amount of water was set to 20 liters / m ² · min.
In Example 12, the porosity was completely prevented. In addition, the amount of water is 250 liters / m ²
In the case of Example 14 in which the molding porosity was prevented, the temperature of the slab was reduced, the stress required for molding was increased, and the rolling force of 150 ton was insufficient, and the rolling force was insufficient, and the roundness deviation was reduced. The rate was bad.

[0068]

By applying the method of the present invention, it is possible to reduce the center segregation and porosity generated at and near the center of a continuous cast slab of billets such as carbon steel, alloy steel, and stainless steel. Furthermore, it is possible to prevent the occurrence of molding porosity due to molding after unsolidification reduction.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing one example of a molding facility after unsolidification rolling and solidification of a slab suitable for carrying out the method of the present invention.

FIG. 2 Fe-1.5% by weight Cr—C ternary Fe—
It is a C pseudo binary system phase diagram.

FIG. 3 is a diagram showing the relationship between the slab center temperature during molding and the number of formed porosity.

FIG. 4 is a view showing a relationship between a distance from an unsolidified rolling stand and a slab center temperature obtained by a heat transfer solidification analysis.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-11-179509 (JP, A) JP-A-11-129060 (JP, A) JP-A-10-128512 (JP, A) JP-A-11-179 188468 (JP, A) JP-A-11-216547 (JP, A) JP-A-3-124352 (JP, A) JP-A-10-52744 (JP, A) JP-A-3-35855 (JP, A) JP-A-2-160151 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/20 B22D 11/124 B22D 11/128 350

Claims (2)

(57) [Claims] An unsolidified rolling stand consisting of at least one pair of rolls applies a pressure reduction before completely solidifying a slab, and after a slab is completely solidified, it is formed into a predetermined sectional shape by a molding stand consisting of at least one pair of rolls. A continuous casting method of a steel billet to be formed, comprising:
The distance from the pair of rolls to the first pair of rolls of the forming stand is 3 to 15 m, and in the unsolidified pressing stand, the central solid phase ratio of the slab is in the range of 0.1 to 0.8, A continuous casting method for a steel billet, wherein a reduction of more than the diameter of the unsolidified portion or the thickness of the unsolidified portion of the slab before rolling is applied to the slab. 2. The slab surface between 5 and 200 units per unit area of the slab surface from the last pair of rolls of the unsolidified pressing stand to the first pair of rolls of the forming stand.
The method according to claim 1, wherein the steel billet is cooled with a water amount of liter / m ² · min.