JP2007307574A - Continuous casting method of billet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method which can prevent γ-grain-boundary cracks on the surface layer of a cast billet without causing bending of the cast billet, the γ-grain-boundary cracks being easily caused at a brittle temperature range below A<SB>3</SB>transformation temperature. <P>SOLUTION: In the continuous casting method of a cast billet using a continuous casting machine having a correcting portion, a secondary cooling of the cast billet is carried out from the position just below the outlet of a casting mold in such a manner that the cooling speed is within the range from 5 to 10°C/s or less during the period in which the temperature of the portion at a depth of 5 mm from the surface of the cast billet drops from the A<SB>3</SB>transformation temperature to 800°C or lower. Then, the temperature of the portion at a depth of 5 mm from the surface of the cast billet is once returned to 950°C or higher until the cast billet is corrected. After that, the cast billet is corrected. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to continuous casting of a billet slab capable of reducing austenite grain boundary cracking that is likely to occur in the surface layer portion of the billet. More specifically, the secondary cooling rate of the billet slab is controlled within an appropriate range. The present invention relates to a continuous casting method in which after cooling, a surface layer portion of a slab is reheated, and thereafter a billet slab is straightened.

  In the process of producing seamless steel pipes from the continuously cast slab by the Mannesmann method etc. without going through the rolling or forging process, cracks often occur in the surface layer of the continuous cast slab. As a result, external flaws occur during pipe making, which becomes a product defect. Moreover, when manufacturing steel bars and wires other than seamless steel pipes, cracks in the surface layer of billet slabs may expand during rolling, resulting in 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 austenite phase (hereinafter also referred to as “γ phase”) to ferrite. It is said that it occurs in a high temperature embrittlement temperature range of about 860 ° C. to 600 ° C. accompanying transformation (hereinafter also referred to as “A ₃ transformation”) to a phase (hereinafter also referred to as “α phase”). Cracks in the slab surface layer often become a problem when casting slab slabs. It is believed that surface layer cracking occurs when strain due to bending correction of a slab and strain due to thermal stress increase in this temperature range.

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

Further, for example, Patent Document 1 discloses that when casting a slab having a width to thickness ratio of 1.8 to 10.5, cooling is started immediately after the mold outlet, and at least 1.5 m in the casting direction. after cooling to the cast slab surface temperature once more than a ₃ transformation temperature in a specific amount of water in the slab surface temperature by recuperation as 850 ° C. or higher, gamma grain boundaries at the time of correction by performing correction of the slab A continuous casting method for suppressing cracking is disclosed. Patent Document 2, by the same as Patent Document 1 method, after recuperation to A ₃ transformation temperature or higher, even continuous casting method of correcting slab is disclosed.

  Furthermore, a method of avoiding the embrittlement region by strongly cooling the slab to 600 ° C. or less is conceivable, but in the case of a billet slab, the area of the cross section is small, so the thermal stress due to strong cooling is reduced. As a result, the slab may be bent (warped).

  In addition to these, in the case of casting a billet slab, since the cross-sectional area is small as described above, generally the casting speed (drawing speed) is large. Therefore, if the cooling is too weak, the strength of the solidified shell decreases, It becomes difficult to properly maintain the cross-sectional shape of the slab. 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 the casting of the billet slab, it is necessary to suppress the surface cracking of the billet slab while ensuring a minimum cooling effect.

JP 2001-138019 A (claims and paragraph [0040]) JP 2002-86252 A (Claims and paragraph [0036])

The present invention has been made to solve the above-mentioned problems, and the problem is to generate the bending of the slab while ensuring the accuracy of the cross-sectional shape of the slab in the continuous casting of the billet slab. It is another object of the present invention to provide a continuous casting method capable of preventing the γ grain boundary cracking of the slab surface layer portion, which is likely to occur in the embrittlement temperature range below the A ₃ transformation temperature.

  In order to solve the above-mentioned problems, the present inventor has been able to prevent the γ grain boundary cracking of the slab surface layer portion without causing bending while ensuring the accuracy of the cross-sectional shape of the slab. Research and development was conducted on the casting method, and the following findings (a) to (e) were obtained to complete the present invention.

  (A) In order to prevent γ grain boundary cracking, the slab is strongly cooled by secondary cooling immediately under the mold outlet at the beginning of casting, and film-like ferrite that is likely to occur at the γ grain boundary in the slab surface layer is generated. It is effective to reduce high-temperature embrittlement that causes γ grain boundary cracking by suppressing.

  (B) However, since the billet slab has a small cross-sectional area, the shrinkage in the mold is large during the casting process, the adhesion between the mold and the surface of the slab is poor, and the high-temperature state continues even after the mold exits. The internal temperature is less likely to drop than the single surface.

  (C) In order to effectively prevent γ grain boundary cracking, the structure in which the formation of the film-like ferrite (a) is suppressed over the entire surface of the slab with a thickness of at least 5 mm from the surface of the slab. For this purpose, it is necessary to set the cooling rate at a part 5 mm deep from the slab surface to 5 ° C./s or more in the secondary cooling.

  (D) In order to prevent the billet slab from being bent, the cooling rate at a part having a depth of 5 mm from the surface of the slab of (d) is set to less than 10 ° C./s, and further, It is necessary to reduce the residual stress in the slab by once reheating the temperature of the part to 950 ° C. or higher.

  (E) The secondary cooling just below the mold is performed within the range from the position immediately below the mold outlet to 4 m at the longest in the casting direction, and the specific water amount of the cooling water is 1.1 to 1.8 liters / kg-steel (hereinafter referred to as “the cooling water”). And “L / kg-steel”).

  The present invention has been completed based on the above findings, and the gist thereof is the following continuous casting method for billet cast pieces.

“Continuous casting method of billet slab using continuous casting machine having straightening part, and cooling rate while temperature at portion of depth of 5 mm from surface of billet slab is lowered from A ₃ transformation temperature to 800 ° C. or less The secondary cooling of the billet slab is 5 ° C./s or more and less than 10 ° C./s from directly under the mold outlet, and then the part 5 mm deep from the surface of the billet slab until the slab is corrected. The billet slab is continuously reheated to 950 ° C. or higher and then the billet slab is straightened. ”
In the above continuous casting method, the secondary cooling is preferably performed within the range of at most 4 m in the casting direction immediately after the mold outlet, and the specific water amount is preferably 1.1 to 1.8 L / kg-steel.

  In the present invention, the “billet slab” means a square slab having a cross-sectional shape of about 150 mm square to 400 mm square or a round slab having a diameter of about 150 mm to 400 mm.

According to the continuous casting method of the billet slab of the present invention, while securing the accuracy of the cross-sectional shape of the slab, without causing bending of the slab, and, at the embrittlement temperature range of A ₃ transformation temperature or less It is possible to produce a billet slab in which the γ grain boundary cracking of the slab surface layer portion that is likely to occur is significantly reduced. Therefore, by using the billet slab obtained by the casting method of the present invention as a raw material for pipe making, it is possible to effectively suppress the occurrence of outer surface defects during the production of seamless pipes.

(1) Basic configuration of the invention As described above, the present invention is a continuous casting method of a billet cast using a continuous casting machine having a correction portion, and the temperature at a part having a depth of 5 mm from the surface of the billet cast is The secondary cooling of the billet slab where the cooling rate is 5 ° C./s or more and less than 10 ° C./s while decreasing from the A ₃ transformation temperature to 800 ° C. or less is performed immediately below the mold outlet, and then the slab is straightened. This is a billet slab continuous casting method in which the temperature at a part 5 mm deep from the surface of the billet slab is once reheated to 950 ° C. or higher and then the billet slab is corrected.

The present invention has been completed in consideration of the problems and problems of the billet casting. As described above, in Patent Document 1 or Patent Document 2, when casting a slab having a cross-sectional flatness ratio of 1.8 to 10.5, cooling is started immediately after the mold outlet and 1.5 m in the casting direction. After the slab surface temperature is once lowered to the A ₃ transformation temperature or less by strong cooling until the slab surface temperature is reheated to a temperature of 850 ° C. or more or a temperature of the A ₃ transformation temperature or more, γ is obtained. A continuous casting method for suppressing grain boundary cracking is disclosed. Similarly to the method disclosed in the above-mentioned patent document, the continuous casting method of the present invention is a film-like ferrite generated at the γ grain boundary in the slab surface layer portion by strongly cooling the slab directly under the casting mold. It suppresses the formation and utilizes the action and effect of reducing high temperature embrittlement that causes γ grain boundary cracking.

  However, the methods disclosed in Patent Documents 1 and 2 are intended for slabs having a flatness ratio of 1.8 to 10.5, whereas the present invention is a billet, that is, the flatness ratio is 1.0. Basically, it is different in that the slab is targeted. Generally, in the case of billet casting, due to the small cross section of the slab, the shrinkage of the slab in the mold is large, the adhesion between the mold and the surface of the slab is poor, and the high temperature after exiting the mold. The condition continues and the slab surface temperature does not drop so much.

  From the experiments and considerations of the present inventors, in order to effectively prevent the γ grain boundary cracking, the steel structure in which the formation of the film-like ferrite is suppressed over a thickness of at least 5 mm from the surface of the slab is cast. It was found that it was necessary to form the entire surface. If temperature control is performed focusing only on the temperature of the slab surface, the thickness of the target slab structure becomes extremely insufficient, and γ grain boundary cracking occurs in a very shallow region of the slab surface layer. Became clear.

  Therefore, it was found that it is necessary to perform temperature management so as to control the steel structure in the part in consideration of the required thickness of the surface layer portion, not focusing on the temperature of the surface layer portion alone. . Further, in the case of billet slab, since the cross section of the slab is small, excessively strong cooling may cause the entire slab to bend, so it is necessary to prevent the occurrence of bending. . Also from this viewpoint, it has been found that it is effective to grasp the slab temperature and manage it with a thickness of at least 5 mm from the slab surface.

(2) Reason for limitation and preferred range of the invention The reason and preferred range for defining the scope of the present invention as described above will be described below.

1) Cooling rate of cast slab and target part for regulating cooling rate in slab In the present invention, the cooling rate of a part having a depth of 5 mm from the slab surface is controlled to 5 ° C./s or more and less than 10 ° C./s.

The steel structure targeted by the present invention is a steel structure in which the appearance of a film-like ferrite phase is suppressed, and in order to obtain such a steel structure, it is necessary to cool at a high speed of a predetermined speed or higher. The reason is that when the α phase appears from the γ phase, if the cooling rate is slow, the α phase tends to preferentially appear at the γ grain boundary where the energy required for transformation is low, and the film-like ferrite is present at the γ grain boundary. (Α) phase is formed. In order to suppress the appearance of the film-like α phase, it is effective to precipitate the α phase in the γ grains. As a result of various experiments and examinations, the temperature from the A ₃ transformation temperature to 800 ° C. or less. It has been found that it is necessary to set the cooling rate for cooling to 5 ° C./s or more.

  Here, it is necessary to consider the difference in cooling effect depending on the radial position in the slab. The cooling effect from the outer surface of the slab is large at a portion shallow from the surface of the slab, and therefore the cooling rate at the surface layer portion less than 5 mm deep from the surface of the slab is higher than the cooling rate at the portion 5 mm deep from the slab surface. Is also big. The steel structure targeted by the present invention is a steel structure in which the appearance of a film-like α phase is suppressed, and in order to obtain this steel structure, as described above, an area at least 5 mm deep from the surface of the slab is used. It is necessary to cool at a cooling rate of 5 ° C./s or more. For this reason, the steel structure in the region within 5 mm depth from the slab surface is controlled by controlling the steel structure in the region 5 mm deep from the slab surface to the target steel structure in the present invention. Can be controlled by the organization.

  However, if the cooling rate becomes excessively fast, it reaches the low temperature martensitic transformation temperature range, and the transformation rate becomes fast, so that rapid volume expansion occurs due to martensitic transformation, which causes the billet slab to bend. Cause. Further, strong cooling itself tends to cause uneven cooling of the slab surface layer, and uneven temperature due to strong cooling also causes bending of the slab. In the casting process, since the slab is supported by a support roll or the like, it is difficult to bend, but after leaving the continuous casting machine, it is cut to a fixed size by a cutter or the like, and if the support is lost, The slab is bent due to the residual stress. In order to prevent this bending from occurring, it has been found that the cooling rate needs to be 10 ° C./s or less.

2) define the target temperature range of the cooling rate, the cooling rate in the temperature range up to temperature of a portion of a depth of 5mm from the secondary cooling position and the specific amount of water slab surface is reduced to a temperature below 800 ° C. from A ₃ transformation temperature The reason defined as described above is that the cooling rate in the range from the A ₃ transformation temperature to about 800 ° C. is most likely to be the fastest, and therefore the cooling rate in the temperature region where the cooling rate is maximum is specified. In contrast, in the low temperature region than the temperature range, since the temperature difference between the cooling medium slab is reduced, the cooling rate of the slab is slower, therefore, since the A ₃ transformation rate also decreases, steel It is not appropriate as a temperature range that defines the cooling rate when performing tissue control.

  The reason for performing secondary cooling of the slab in the spray zone directly under the mold outlet is as follows. That is, at the position immediately below the mold, there is still enough unsolidified molten steel inside the slab, so that the solidified molten steel inside the slab solidifies even after the slab surface layer has been strongly cooled. This is because it is possible to sufficiently recuperate (heat up) the temperature of the slab surface layer portion only by the latent heat released to.

  The secondary cooling just below the mold is preferably performed within the range of at most 4 m in the casting direction from directly below the mold outlet, and the specific water amount of the cooling water is preferably 1.1 to 1.8 L / kg-steel. The reason why it is preferable to perform the secondary cooling immediately below the mold at a position within the above range is that the billet slab is less likely to exhibit a cooling effect by preferably shortening the cooling time and increasing the cooling rate throughout the cooling period. This is to prevent the formation of a film-like ferrite phase in the surface layer portion of the steel sheet, and to make it easy to reheat by latent heat at the time of solidification of the unsolidified molten steel existing inside the slab.

  The reason why the specific water amount is preferably within the above range is that the specific water amount is disclosed in, for example, 0.4 to 0.75 L / kg-steel described in Patent Document 1 or Patent Document 2. It is because it is easy to improve the cooling effect of billet slab by increasing it compared with about 1.3 L / kg-steel.

  Furthermore, as long as the recuperation temperature after secondary cooling of the mold to be described later is ensured, secondary cooling may be further appropriately performed on the downstream side in the casting direction after the secondary cooling immediately below the mold, It is not necessary to carry out.

3) Recuperation temperature and recuperation temperature regulation target part of the slab The reason for reheating the temperature of the part 5 mm deep from the surface of the slab to 950 ° C. or higher is that the temperature during straightening of the slab is set high. This is because the straightening operation is performed in a state where the reaction force from the slab is low. In addition, it is preferable that the temperature of the slab at the time of correction is 800 degreeC or more.

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 has the property of following 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. The 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 method of the present invention, the low straightening force is used. The continuous casting method for correcting the slab with a large effect in preventing the 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 slab.

(1) Casting test method FIG. 1 shows a longitudinal section of a continuous casting apparatus used for carrying out the continuous casting method of the present invention. As the continuous casting apparatus, a curved continuous casting machine for round billet casting with an 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 the lower mold spray (hereinafter referred to as “MD” over the range of 0.8 to 1.1 m from the meniscus 4. The spray is also referred to as “spray”) over the range of 1.1 to 3.1 m, and the top zone secondary cooling device (hereinafter also referred to as “TZ spray”) 70 is the first over the range of 3.1 to 6.1 m. A zone secondary cooling device (hereinafter also referred to as “1st zone spray”) 71 and a second zone secondary cooling device (hereinafter also referred to as “2nd zone spray”) 72 over a range of 6.1 to 9.3 m. Are installed.

  These sprays all employed an air mist spray system, and the air-water ratio was about 50 (NL / min-air / (L / min-water)), and the test was performed under a constant condition regardless of the amount of water. The MD spray was for preventing bulging under the mold by cooling the slab at the lower part of the mold, and the amount of water was 130 L / min. Also, the amount of water in the 1st zone spray was constant at 130 L / min. In order to confirm the effect of the present invention, the water flow rate of the TZ spray, which is the secondary cooling directly under the mold, was greatly changed in the range of 200 to 1400 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 TZ spray 70 installed immediately below the mold 2 and then appropriately cooled by the 1st zone spray 71 and the 2nd zone spray 72. Then, the slab 6 is pulled out by the pinch roll 9. The slab 6 is reheated 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 is corrected to a linear shape.

  The temperature distribution in the slab 6 was obtained by unsteady heat transfer calculation. Regarding the calculation accuracy, the solidification shell thickness obtained by comparing the calculation result with the temperature measurement result of the slab surface and the result of the hammering test, and the artificial crack generation test of the internal crack is calculated as the temperature distribution. By confirming with the solidified shell thickness obtained from the above, it was confirmed in advance that it has high accuracy. The slab surface temperature at the start of cooling and the temperature transition of the part at a depth of 5 mm from the slab surface in the cooling process can be estimated with high accuracy by this heat transfer calculation. In the 2nd zone spray, water is not usually supplied, but for comparative tests, water was supplied in some tests.

  In the casting test, the steel component composition was mass%, C: 0.05 to 0.18%, Si: 0.10 to 0.3%, Mn: 0.5 to 1.5%, P: 0.00. A round billet slab having a diameter of 310 mm was cast at a casting speed of 1.5 m / min using steel of 01 to 0.02% and S: 0.005 to 0.01%.

(2) Method of investigating steel structure, γ grain boundary cracking and slab bend The sample was cut out over a length of about 2 m after casting, and the surface scale was lightly dropped by glider polishing for a total length of 2 m. Thereafter, the presence or absence of γ grain boundary cracks was 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 out from one end of the 2 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 macroscopic structure observation, it is clear that the structure in which the appearance of film-like ferrite is suppressed, that is, the target steel structure in the present invention, is generated from the surface layer portion around the circular slab cross section to the inside. I was able to determine. At 16 points divided at equal intervals in the circumferential direction of the cross section, the thickness of the steel structure in which the appearance of the film-like ferrite was suppressed was measured, and these were arithmetically averaged to obtain the structure in which the film-like ferrite was suppressed. And the thickness.

  Regarding the presence or absence of bending of the slab, in the above-mentioned sample having a length of 2 m, when the distance between the arc formed in the longitudinal direction by the slab and the string corresponding to the arc is 20 mm or more, “being bent” It was judged. The temperature and cooling rate of the part 5 mm deep from the slab surface corresponding to each TZ spray cooling condition were determined by heat transfer analysis under each condition.

(3) Evaluation of test results Table 1 shows the test conditions and test results performed to confirm the effects of the present invention.

  Test Nos. 1 to 4 are tests for examples of the present invention that satisfy the conditions specified in the present invention, and Test Nos. 5 to 11 are tests for comparative examples that do not satisfy at least one of the conditions specified by the present invention. It is.

  In Test Nos. 1 to 4 of the present invention example, the steel structure in which the formation of film-like ferrite is suppressed is formed over a thickness of 5 mm or more from the slab surface, γ grain boundary cracking is prevented, and No bending has occurred. In particular, in the test numbers 1 to 3 where the secondary cooling is performed within 4 m in the casting direction immediately after the mold outlet and the specific water amount is in the range of 1.1 to 1.8 L / kg-steel, γ Both the prevention of grain boundary cracking and the prevention of bending of the slab are very good.

  In the test numbers 5 to 7 of the comparative examples, by reducing the flow rate of the TZ spray, the cooling rate at the part having a depth of 5 mm from the slab surface was less than 5 ° C./s, and the formation of film ferrite was suppressed. As a result of the formation thickness of the steel structure being thinner than 5 mm, γ grain boundary cracking occurred. In test number 8 of the comparative example, the flow rate of the TZ spray was particularly small, so the cooling of the slab in the top zone was too weak, and the temperature rose at a part 5 mm deep from the slab surface due to heat conduction from the inside of the slab. Then, recuperation occurred. As a result, the minimum temperature reached at the above site was not 800 ° C. or lower, and γ grain boundary cracking occurred.

  In the test number 9 of the comparative example, since the flow rate of the TZ spray is extremely large, the cooling rate at the part 5 mm deep from the slab surface is extremely high at 11.6 ° C./s, and the minimum temperature reached 433 ° C. As a result, the slab was bent.

  The test numbers 10 and 11 of the comparative examples are tests in which the slab was cooled by operating the 2nd zone spray under the conditions of the test example 2 of the present invention. As a result, it did not reheat to 950 ° C. or higher, the slab straightening force increased, the residual stress increased, and the slab was bent.

  From the results of the above examples, the excellent effect of the continuous casting method of the present invention was confirmed. The present embodiment is an example in which a round billet slab is used, but the same effect can be obtained in the case of using a square billet slab.

According to the continuous casting method of the present invention, while securing the accuracy of the cross-sectional shape of the slab, without causing bending of the slab, and tends to occur in the A ₃ transformation temperature below the embrittlement temperature range slab A billet cast slab in which γ grain boundary cracking in the surface layer part is remarkably reduced can be produced. Therefore, the method of the present invention is a technique that can be widely applied throughout the casting and pipe making processes as a continuous casting method of billet cast pieces that can supply high-quality materials capable of suppressing the occurrence of external flaws in the seamless pipe manufacturing process. is there.

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

1: immersion nozzle, 2: mold, 3: molten steel, 4: molten steel meniscus,
5: Solidified shell, 6: Slab, 70: Top zone secondary cooling device (TZ spray),
71: 1st zone secondary cooling device (1st zone spray), 72: 2nd zone secondary cooling device (2nd zone spray), 8: Support roll, 9: Pinch roll,
10: Tundish, 11: Unsolidified molten steel

Claims (1)

A continuous casting method for billets slab using a continuous casting machine having a straightening portion, the cooling rate during the temperature at the site of a depth of 5mm from the surface of the billet slab decreases from A ₃ transformation temperature to 800 ° C. or less Secondary cooling of the billet slab at 5 ° C./s or more and less than 10 ° C./s is performed immediately below the mold outlet, and then the part of the 5 mm depth from the surface of the billet slab is corrected until the slab is corrected. A continuous casting method for a billet slab, characterized in that the temperature is once reheated to 950 ° C. or higher and then the billet slab is corrected.

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