JP4600436B2 - Continuous casting method for slabs - Google Patents

Description

  The present invention relates to a method for continuously casting a slab in which a slab such as billet or bloom is continuously cast, and in particular, continuous casting of a slab capable of reducing austenite grain boundary cracking that is likely to occur in the surface layer portion of the slab. Regarding the method.

  The continuously cast slab is transferred to a pipe making process through a rolling or forging process or without undergoing these processes, and becomes a seamless steel pipe by the Mannesmann method or the like. If a crack is generated in the surface layer portion of a continuously cast slab, an outer surface flaw is generated during pipe production due to this crack, which becomes a product defect. In addition, even when steel bars and wires other than seamless steel pipes are manufactured, cracks in the slab surface layer portion are enlarged during rolling and become product defects.

The cracks in the slab surface layer are cracks along the austenite grain boundaries (hereinafter also referred to as “γ grain boundary cracks”), and generally from the austenite phase (hereinafter also referred to as “γ phase”) to ferrite. It occurs in a high temperature embrittlement temperature range of about 860 ° C. to 600 ° C. due to transformation (hereinafter referred to as “A ₃ transformation”) to a phase (hereinafter also referred to as “α phase”). Γ grain boundary cracking in the slab surface layer is often a problem during the casting of slab slabs. When strain due to bending correction of a slab and strain due to thermal stress increase in the temperature range, γ grain boundary cracking occurs.

Therefore, without unduly cool the slab, than to cast in while cooling with weak cooling conditions, the embrittlement temperature range kept at a temperature higher than the temperature of the slab surface portion during bending straightening A ₃ transformation point By avoiding or reducing the generation of thermal stress due to cooling, the occurrence of γ grain boundary cracks can be suppressed.

  Various methods have been investigated for suppressing the occurrence of γ grain boundary cracks.

For example, in Patent Document 1, when casting a slab having a width to thickness ratio of 1.8 to 10.5, cooling is started immediately after the mold exit, and is specified in a casting direction up to at least 1.5 m. after once the a ₃ transformation temperature or less of the surface temperature of the cooled and slabs in water of the 850 ° C. or higher the surface temperature of the piece castings by recuperation, by performing correction of the slab, gamma intergranular cracking A continuous casting method for suppressing the above is disclosed. Patent Document 2 discloses a continuous casting method that suppresses γ grain boundary cracking by correcting the slab after reheating to a temperature equal to or higher than the A ₃ transformation temperature by the same method as Patent Document 1. Yes.

  Furthermore, a method of avoiding the embrittlement temperature range can be adopted by strongly cooling the slab to 600 ° C. or less, but the slab may be bent (warped) due to thermal stress due to strong cooling. is there. The bending of the slab is likely to occur in a billet slab having a small cross-sectional area.

  In addition to these, when casting is performed at a higher casting speed, that is, a drawing speed as in recent years, if the cooling is too weak, the strength of the solidified shell is lowered and the cross-sectional shape of the slab can be appropriately maintained. It becomes difficult. For example, when a billet with a circular cross section is cast, the cross section of the billet is deformed into an ellipse, and the roundness of the cross section of the slab is impaired. Therefore, in casting a slab, it is necessary to suppress the γ grain boundary cracking of the slab surface layer while ensuring a minimum cooling effect.

JP 2001-138019 A JP 2002-86252 A

Thus, as a problem in the continuous casting of the slab, to ensure the accuracy of the cross-sectional shape of the slab, to suppress the occurrence of bending of the slab, and, generated in the embrittlement temperature range of A ₃ transformation temperature or less For example, it is possible to prevent γ grain boundary cracking of the slab surface layer portion that is easily formed. The present applicant has proposed a continuous casting method for solving these problems in Japanese Patent Application No. 2006-137465 (hereinafter referred to as “prior application”).

  The continuous casting method of the prior application is a continuous casting method of a slab using a continuous casting machine having a correction part, and at the time of secondary cooling performed immediately below the mold outlet, the temperature at a part 5 mm deep from the surface of the slab The cooling rate is specified, and then the temperature of the same part is reheated to 950 ° C. or higher, and the slab is corrected, thereby controlling the structure of the slab surface layer portion and reducing γ grain boundary cracking. .

  However, as a result of repeated tests by the present inventors, depending on the difference in transformation characteristics depending on the steel type, even if continuous casting is performed according to the secondary cooling conditions in the continuous casting method of the prior application, there is a significant difference in the effect. It has been found.

  The present invention has been made in view of the above problems, and its purpose is to suppress the occurrence of bending of the slab while ensuring the accuracy of the cross-sectional shape of the slab in continuous casting of the slab, An object of the present invention is to provide a continuous casting method of a slab that can accurately prevent γ grain boundary cracking of the slab surface layer portion according to the transformation characteristics of the steel type.

  In order to achieve the above-mentioned object, the inventors strongly cooled the slab by secondary cooling immediately below the mold outlet at the beginning of casting, which is the basis of the continuous casting method of the prior application, and the γ grain boundary in the slab surface layer portion. As a result of intensive investigations on the premise that high temperature embrittlement, which causes γ grain boundary cracking, is reduced by suppressing the formation of film-like ferrite that tends to occur in Completed the invention.

The present invention is a continuous casting method of a slab using a continuous casting machine having a straightening portion, and when continuously casting a slab containing 0.05 mass% or more of C with a steel component composition, the secondary of the slab For the cooling, the ratio H / M of the specific water amount H [L (liter) / kg-steel] of the cooling water and the C content M [mass%] in the steel component composition of the slab was set to 5.0 or more. in condition, it goes from just under the mold exit, once cooled below a ₃ transformation temperature of the surface temperature of the slab, then, before correcting the cast slab, by recuperation of the surface temperature of the slab above 950 ° C., Then the slab is straightened.

In cooling conditions specified from such specific water H and C content M, and secondary cooling of the cast piece from just below the mold outlet, by the surface temperature of the slab below A ₃ transformation temperature, the cast slab surface A steel structure in which the appearance of film-like ferrite is suppressed is effectively generated in the part, and γ grain boundary cracking in the slab surface layer part can be prevented. In addition, since the secondary cooling just below the mold outlet of the slab is performed by strong cooling, the strength of the solidified shell is maintained, so that the accuracy of the cross-sectional shape of the slab can be ensured. Furthermore, since the surface temperature of the slab is reheated to 950 ° C or higher before the slab is straightened, the straightening force of the slab is small and the residual stress in the slab is reduced. Can be suppressed.

Here, the specific water amount H is defined by the following equation (1).
H = Q / (ρ × A × Vc) (1) where Q: amount of cooling water [L / min (min)], ρ: density of slab [kg / m ³ ], A; The cross-sectional area of the slab [m ² ], Vc; casting speed [m / min]. The density ρ of the slab is 7000 kg / m ³ .

  The slab means a billet slab or a bloom slab, and is a square slab having a thickness of about 150 mm to 500 mm or a round slab having a diameter of about 150 mm to 500 mm.

  The surface temperature of the slab is a surface temperature that can be measured by, for example, a radiation thermometer, and means a temperature from the surface of the slab to just below the skin. The surface temperature of the slab is determined according to the distance from the molten steel meniscus by solidification heat transfer analysis if conditions such as the type of steel to be cast, the size of the slab, the casting speed, and the secondary cooling conditions of the slab are determined. The surface temperature of the piece can also be determined.

  According to the continuous casting method of the slab of the present invention, while ensuring the accuracy of the cross-sectional shape of the slab, the occurrence of bending of the slab is suppressed, and the surface of the slab surface layer portion is accurately determined according to the transformation characteristics of the steel type. It is possible to produce a slab in which γ grain boundary cracks are significantly reduced. In particular, by using the obtained slab as a raw material for pipe making, it becomes possible to effectively suppress the occurrence of outer surface flaws during the production of seamless steel pipes.

As described above, the present invention is a method for continuously casting a slab using a continuous casting machine having a straightening portion, and the secondary cooling of the slab is performed by using a specific amount of cooling water H [L / kg-steel], the ratio H / M of the C content of the steel chemical composition of the slab M [wt%] in the conditions of 5.0 or more, conducted from immediately below the mold outlet, once a ₃ transformation temperature of the surface temperature of the slab This is a continuous casting method of a slab in which the surface temperature of the slab is reheated to 950 ° C. or higher before the slab is corrected, and then the slab is corrected.

  The continuous casting method of the present invention, like the continuous casting method of the prior application, is a film-like shape generated at the γ grain boundary in the steel structure of the slab surface layer portion by strongly cooling the slab immediately under the mold outlet at the initial stage of casting. The effect | action and effect which suppress the production | generation of the ferrite of this and reduce the high temperature embrittlement which causes a gamma grain-boundary crack are utilized. However, in order to obtain the effect of reducing the occurrence of γ grain boundary cracks, the prior application is used to generate a steel structure (hereinafter also referred to as “SSC structure”) in which the appearance of film-like ferrite is suppressed in the surface portion of the slab. In the continuous casting method, the cooling rate and temperature are controlled at a portion 5 mm below the slab surface layer, whereas in the continuous casting method of the present invention, cooling conditions are set in accordance with the transformation characteristics of various steel types to be cast. Thus, the secondary cooling of the slab is performed.

In the continuous casting method of the present invention, as will be described later, the secondary cooling performed in the spray zone immediately below the mold outlet is performed under the condition that the specific water content / C content is 5.0 or more, and the surface temperature of the slab is adjusted. once cooled to a ₃ transformation temperature or less, by correcting the slab from the surface temperature of the subsequent slab is recuperation above 950 ° C., SSC organizations to effectively produce the cast slab surface portion, bending strain on correction Further, it is based on the knowledge that the occurrence of γ grain boundary cracking due to strain due to thermal stress can be reduced and thus prevented.

  According to the continuous casting method of a slab of the present invention, γ grain boundary cracking of the slab surface layer portion can be accurately prevented according to the transformation characteristics of the steel type. Moreover, since the specific water content of the cooling water and the C content of the slab component composition, which are simple operation parameters in the casting process, are used for setting the cooling conditions, management is easy.

  In addition, since the secondary cooling just below the mold outlet of the slab is performed by strong cooling, the strength of the solidified shell is maintained, so that the accuracy of the cross-sectional shape of the slab can be ensured. Furthermore, since the surface temperature of the slab is reheated to 950 ° C or higher before the slab is straightened, the slab straightening force is small and the residual stress in the slab is reduced, so that the slab is bent. Can be suppressed.

  The reason why the continuous casting method of the slab of the present invention is defined as described above and the preferred range will be described.

(1) Cooling condition of slab The steel structure targeted in the present invention is an SSC structure in which the appearance of a film-like ferrite phase is suppressed. In order to form this SSC structure in the slab surface layer part, it is necessary to exert an effect of cooling down to the inside of the surface layer due to the thermal resistance of the solidified shell of the slab surface layer. Is desirable.

  Here, when the test which casts the slab from which a steel type differs on the same cooling conditions was performed, it turned out that the formation thickness of the SSC structure | tissue from a slab surface changes greatly for every steel type. This can be assumed to be due to the following factors.

  In the case of steel grades with high C content and easy stabilization of the γ phase, the amount of ferrite (α) phase precipitated in the cooling process itself is small, and the α phase is preferred at the γ grain boundary where the energy required for transformation in the precipitation process is low. On the other hand, in the case of a steel type having a low C content and a large amount of α-phase precipitation, the α-phase is likely to precipitate in the γ grains. That is, a steel type having a lower C content is more likely to obtain a stabilized thick SSC structure.

  Table 1 shows test conditions and test results performed to determine the prescribed range of cooling conditions in the continuous casting method of the present invention.

In this test, casting was performed by changing the specific water amount in each secondary cooling for a steel type having a C content of 3 ranks. It was confirmed by measurement with a radiation thermometer that the surface temperature of each slab was once cooled below the A ₃ transformation temperature by secondary cooling, and that it was reheated to 950 ° C. or higher before correction. The continuous casting apparatus used for the test and the method for investigating the properties of the cast slab will be described in detail in Examples described later.

  FIG. 1 is a diagram showing the correlation between the specific water amount / C content and the thickness of the SSC structure based on the test results. In this figure, the horizontal axis shows the reciprocal of the C content [mass%] as the ferrite potential and the product of the strength of the spray specific water volume [L / kg-steel] directly under the mold outlet as a parameter. The vertical axis shows the average thickness [mm] of the SSC structure.

  As shown in the figure, it is understood that the average thickness of the SSC structure can be secured to 3 mm or more by setting the specific water content / C content value to 5.0 or more. Moreover, if the average thickness of the SSC structure can be secured 3 mm or more, the occurrence of grain boundary cracks in the slab can be reduced, and if 4 mm or more can be secured, the occurrence of γ grain boundary cracks in the slab can be further reduced.

Based on such results, in the present invention, the secondary cooling of the slab that is performed immediately below the mold outlet is performed by the specific water amount H [L / kg-steel] of the cooling water and the inclusion of C in the steel composition of the slab. The ratio H / M with the rate M [mass%] was set to 5.0 or more, and the surface temperature of the slab was controlled to be once cooled to the A ₃ transformation temperature or less.

  In the present invention, the secondary cooling of the slab is carried out in the spray zone directly under the mold outlet. Under the mold, there is still enough unsolidified molten steel inside the slab. This is because even after strong cooling, the surface temperature of the slab surface can be sufficiently raised (recovered) by the latent heat released by the unsolidified molten steel inside the slab.

  In addition, this secondary cooling directly under the mold can exert a sufficient effect if it is performed within a range of about 3 m from the position immediately below the mold outlet to the casting direction in order to effectively reform the steel structure of the slab surface layer to the SSC structure. . For example, in a continuous casting apparatus that will be described in detail in the examples to be described later, a lower mold spray apparatus and a top zone spray apparatus can be adopted as the secondary cooling spray apparatus directly under the mold, and the total water amount of both spray apparatuses is set as described above. The amount of water used for calculating the cooling conditions.

  However, as long as the recuperation temperature after secondary cooling of the slab can be ensured to be 950 ° C. or more, in order to prevent bulging, further cooling is performed on the downstream side in the casting direction after secondary cooling immediately below the mold outlet. May be implemented as appropriate. For example, in a continuous casting apparatus that will be described in detail in the examples described later, the first zone spray apparatus and the second zone spray apparatus that follow the downstream side of the top zone spray apparatus correspond to the downstream spray apparatus. Cooling by the first zone spray device and the second zone spray device does not affect the modification of the steel structure of the slab surface layer.

(2) Recuperation temperature of the slab In the present invention, the surface temperature of the slab is reheated to 950 ° C or higher. The temperature during straightening of the slab is high and the reaction force from the slab is low. This is to perform the operation. The surface temperature of the slab during straightening is preferably 800 ° C. or higher.

This recuperation causes the A ₃ transformation temperature to once exceed the A ₃ transformation temperature, so that the steel structure once loses the α phase, but due to the temperature drop of the slab again, the surface layer becomes below the A ₃ transformation temperature. The α phase begins to precipitate. Since this α phase follows the precipitation history of the α phase precipitated by the previous strong cooling, a film-like ferrite phase is not formed at the γ grain boundary. An increase in the straightening force causes a large residual stress in the slab, and this residual stress may cause the slab to bend. Therefore, as in the present invention, the casting is performed under a low straightening force. The continuous casting method for correcting the slab exhibits a great effect in preventing bending of the slab.

  In order to confirm the effect of the continuous casting method of the present invention, the following continuous casting test was conducted to evaluate the occurrence of γ grain boundary cracking and bending of the cast piece.

(1) Casting test method FIG. 2 is a diagram schematically showing a longitudinal section of a continuous casting apparatus used for carrying out the continuous casting method of the present invention. As the continuous casting device, a curved continuous casting machine for round billet casting having a circular arc radius of 10.5 m and a three-point correction type was used.

  The casting direction length of the mold 2 is 0.9 m, the length from the molten steel meniscus 4 to the lower end of the mold is 0.8 m, and a mold lower spray device (not shown) over the range of 0.8 to 1.1 m from the meniscus 4. (Hereinafter referred to as “MD spray”) is also applied to the top zone spray device (hereinafter referred to as “TZ spray”) 70 over the range of 1.1 to 3.1 m, and further over the range of 3.1 to 6.1 m. A 1-zone spray device (hereinafter referred to as “1st zone spray”) 71 and a second-zone spray device (hereinafter referred to as “2nd zone spray”) 72 were installed over a range of 6.1 to 9.3 m, respectively. .

  All of these spray devices employ an air mist spray system, the air / water ratio is about 50 (NL / min-air) / (L / min-water), and the test is performed under a constant condition regardless of the amount of water. The MD spray is for preventing bulging under the mold by cooling the slab at the lower part of the mold, and the amount of water was 30 L / min. The amount of water in the 1st zone spray was fixed at 130 L / min. No water was passed through the 2nd zone spray 72. In order to confirm the effect of the present invention, the amount of water of the TZ spray, which is the secondary cooling just below the mold, was greatly changed in the range of 120 to 970 L / min.

The molten steel 3 injected into the mold 2 from the tundish 10 through the immersion nozzle 1 is strongly cooled by the MD spray and TZ spray 70 installed immediately below the mold 2, and then by the 1st zone spray 71, 2nd zone spray 72. It was cooled appropriately and pulled out by a pinch roll 9 to form a slab 6. At that time, the surface temperature of the slab 6 is once cooled to A ₃ transformation temperature or less. The slab 6 is reheated to 950 ° C. or more by the solidification latent heat of the unsolidified molten steel 11 existing in the slab after the secondary cooling is finished and before the curved shape is corrected, and then into a linear shape. It was corrected.

  The surface temperature of the slab 6 was measured with a radiation thermometer.

  In the test, the steel component composition is mass%, C: 0.05 to 0.23%, Si: 0.10 to 0.3%, Mn: 0.5 to 1.5%, P: 0.01 The steel used was -0.02%, S: 0.005-0.01%. In particular, in order to confirm the effect of the present invention, casting was performed with 3 rank steel types having C content of 0.05, 0.18 and 0.23%.

  As billets, round billet cast pieces having diameters of 310 mm and 225 mm were employed, and casting was performed at casting speeds of 1.4 m / min and 2.4 m / min according to each diameter.

(2) Method of investigating steel structure, γ grain boundary cracking and slab bend The sample was cut out over a length of about 10 m after casting and the surface scale was lightly dropped by glider polishing for a total length of 10 m. Thereafter, the presence or absence of γ grain boundary cracks and the size of the cracks were confirmed by a die check method, and the number of γ grain boundary cracks was measured. After the treatment, a 15 mm thick cross section sample was cut from one end of a 10 m long sample, the cross section was polished, the cross section was corroded with a nitric acid aqueous solution having a nitric acid concentration of 5 mass%, and the steel structure was Observed.

  By macro observation of the steel structure, it is confirmed that the SSC structure in which the appearance of film-like ferrite is suppressed, that is, the steel structure targeted in the present invention, is generated from the surface layer portion around the circular slab cross section to the inside. It was possible to distinguish clearly. The thickness of the SSC structure was measured at 16 points divided at equal intervals in the circumferential direction of the cross section, and these were arithmetically averaged to obtain the thickness of the SSC structure.

  Regarding the presence or absence of bending of the slab, in the sample having a length of 10 m, the case where the maximum distance between the arc formed in the longitudinal direction by the slab and the string corresponding to the arc is 20 mm or more is referred to as “with bending”. It was judged.

(3) Evaluation of test results Table 2 shows test conditions and test results performed to confirm the effects of the present invention.

  In test numbers 1 to 6 of the present invention example, the specific water amount / C content is 5.0 or more as defined in the present invention, the SSC structure is formed over a thickness of 3 mm or more from the slab surface, and the γ grain boundary crack is There was no outbreak or it was only minor. Among them, in Test Nos. 1 and 2 in which the specific water content / C content rate is much higher than the specified value of 5.0 or more, the SSC structure became thick and the state of preventing γ grain boundary cracking was extremely good.

  On the other hand, in the test numbers 7 to 9 of the comparative examples, the specific water content / C content does not satisfy the prescribed 5.0 or more, and the formation thickness of the SSC structure becomes thinner than 3 mm. Was noticeable. In addition, when the cross-sectional sample was observed in detail, there was a discontinuous portion at which no SSC structure was formed, and a large grain boundary crack occurred in that portion.

Further, in Test No. 7-9 Test Nos 1-6, and Comparative Examples of the present invention embodiment, both the surface temperature of the slab by the secondary cooling once it becomes A ₃ transformation temperature or less, further surface temperature of the slab Was reheated to 980-1050 ° C., which exceeded 950 ° C., and no macro bending of the slab occurred.

  In the test shown in Table 2, water was not passed through the 2nd zone spray at all. However, on a trial basis, the water volume of 100 L / min and 200 L / min was applied to the 2nd zone spray under the same conditions as in test numbers 1 and 2. As a result of conducting additional tests in which water was passed through, the surface temperature of the slab was not reheated to 950 ° C. or higher, but was 920 ° C. and 930 ° C., respectively. Bending occurred in the slab.

  The SSC structure of the slab surface layer and the γ grain boundary cracking in this additional test were as good as the results of Test Nos. 1 and 2 shown in Table 2. That is, the cooling condition in which the surface temperature of the slab cannot be reheated to 950 ° C. or more causes bending of the slab, but the cooling by the 2nd zone spray does not affect the modification of the steel structure of the slab surface layer. I understood.

  From the results of the above examples, the excellent effect of the continuous casting method of the present invention was confirmed. In this example, a round billet slab was used, but the same effect was obtained when a square billet slab was used. Moreover, the same effect was acquired not only in billet slabs but also in bloom slabs.

  According to the continuous casting method of the slab of the present invention, while ensuring the accuracy of the cross-sectional shape of the slab, the occurrence of bending of the slab is suppressed, and the surface of the slab surface layer portion is accurately determined according to the transformation characteristics of the steel type. It is possible to produce a slab in which γ grain boundary cracks are significantly reduced. Therefore, the present invention is extremely useful as a continuous casting method capable of supplying a high-quality material capable of suppressing the occurrence of outer surface defects particularly in the production process of seamless steel pipes.

It is a figure which shows correlation with the amount of specific water / C content rate, and the thickness of a SSC structure | tissue. It is a figure which shows typically the longitudinal cross-section of the continuous casting apparatus used in order to implement the continuous casting method of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Immersion nozzle 2 Mold 3 Molten steel 4 Molten steel meniscus 5 Solidified shell 6 Cast slab 70 Top zone spray device (TZ spray)
71 1st zone spray device (1st zone spray)
72 Second zone spray device (2nd zone spray)
8 Support roll 9 Pinch roll 10 Tundish 11 Unsolidified molten steel

Claims (1)

A continuous casting method of a slab using a continuous casting machine having a correction part,
When continuously casting a slab containing 0.05% by mass or more of C with a steel component composition, the secondary cooling of the slab is performed using a specific amount of cooling water H [L (liter) / kg-steel] and a slab. Under the condition that the ratio H / M with the C content M [mass%] in the steel composition of the steel is 5.0 or more, the surface temperature of the slab is once lowered to the A ₃ transformation temperature or less, immediately under the mold outlet. Cool,
Then, before the slab is corrected, the surface temperature of the slab is reheated to 950 ° C. or higher, and then the slab is corrected.

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