JP5085451B2 - Billet continuous casting method - Google Patents

Description

  The present invention is a continuous casting that improves center segregation without causing internal cracks when casting a high carbon steel billet having a side length of 160 mm or less using a curved continuous casting machine. It is about the method.

  The billet is a slab having a square cross section whose length of one cross section is 160 mm or less. Conventionally, a method of continuously casting a bloom with a larger cross-sectional area, and rolling this bloom to produce a billet has been performed, but from the viewpoint of shortening the manufacturing process and promoting energy saving, the billet is directly There has been a continuous casting method.

  In continuous casting of high carbon steel billets, it is known that cracks are likely to occur inside the slab because the crack limit strain is small compared to low carbon steel and medium carbon steel due to the high carbon concentration. In the straightening position of the curved continuous casting machine, internal cracks of the slab tend to occur easily. In order to prevent cracks in the slab, the internal slab is prevented from cracking by strengthening the slab cooling to ensure the thickness of the solidified shell.

  On the other hand, high carbon steel billets are used as wire rods after the rolling process, and therefore, center segregation in the slab becomes a problem, and when primary segregation and micromartensite are generated due to center segregation in the slab, As a starting point, cracks may occur during wire drawing, causing troubles that cause disconnection.

  For this reason, in order to improve the center segregation of high carbon steel billets, in general, a slow cooling operation with a reduced amount of secondary cooling water is performed, the solidification rate inside the slab is slowed, and the concentration of segregating elements is suppressed. That is oriented.

  However, reducing the amount of cooling water by slow cooling slows the growth of the solidified shell in the slab, causing quality troubles that cause cracks inside the slab, and because the solidified shell is thin, There is a concern that it may cause operational troubles such as a break-out and a breakout in which the unsolidified molten steel in the slab is ejected.

  As described above, in the billet casting method of high carbon steel, it is necessary to suppress both internal cracks and center segregation of the slab during the casting process.

  Therefore, conventionally, as shown in Patent Document 1, in continuous billet casting of 160 mm or less, in order to prevent internal cracking of the slab before the slab is bent back, solidified shells on the upper and lower surfaces in the slab cross section In order to secure a thickness of 30 mm or more and reduce C segregation, a continuous casting method in which cooling is performed for at least 100 seconds from the start of bending back to completion of solidification, and a slab center solid phase ratio of 0.2 It has been proposed to perform light reduction under the above conditions.

JP 2000-117405 A

  However, in the method of Patent Document 1, since the thickness of the solidified shell on the upper surface and the lower surface in the cross section of the slab before bending back the slab is secured to 30 mm or more, the internal segregation of the slab can be prevented, but the center segregation is It has been found that there are cases where it does not improve.

  In other words, although cooling in the secondary cooling zone can secure a solidified shell thickness to prevent internal cracks due to bending back of the slab, thermal history after leaving the secondary cooling zone and reduction of center segregation due to cooling Was affected by changes in ambient temperature, etc., so it was found that center segregation may not satisfy the required level even if it is allowed to cool for at least 100 seconds from the start of bending back to the completion of solidification. .

  Therefore, in the present invention, when continuously casting high carbon steel in a curved billet continuous casting machine, it is possible to achieve stable production by shortening the manufacturing process by improving the center segregation while ensuring the thickness of the solidified shell. An object of the present invention is to provide a billet continuous casting method capable of achieving high efficiency and energy saving.

  The gist of the present invention is as follows.

(1) When casting a high carbon steel billet having a side length of 160 mm or less and a carbon content of 0.6 to 1.0% by mass using a curved continuous casting machine, In the continuous casting method in which the curved portion is cooled in the secondary cooling zone in two stages, first, the water density in the first stage is set to a range of 0.01 to 0.03 m ³ / m ² / sec, and then 2 The water density in the secondary cooling zone at the stage is set to less than 0.002 m ³ / m ² / sec to secure a solidified shell thickness of 30 mm or more at the curved portion correction position, and at the curved portion correction position. The continuous casting method of billet characterized by controlling the single surface temperature within a range of 1150 to 1200 ° C.

  (2) When the slab is lightly reduced following the cooling in the secondary cooling zone, the slab center solid phase ratio is lightly reduced at a portion of 0.2 or more. The continuous casting method for billets as described in 1).

  The present invention enables stable production of a high carbon steel billet in a curved billet continuous casting machine, and shortens the production process compared to the conventional process of producing a billet by rolling from a large cross section bloom. Moreover, energy saving can be promoted.

  When the present inventors cast a high-carbon steel billet using a curved continuous casting machine, the slab at the curved portion correction position is not affected by fluctuation factors such as the heat history after the secondary cooling zone. In order to improve the center segregation of the slab without causing internal cracks, the investigation was conducted focusing on the secondary cooling conditions and the control of the slab surface temperature at the curved portion correction position.

  As a result, in the cooling of the slab in the secondary cooling zone, first, strong cooling and then slow cooling are performed, and the slab surface temperature at the correction position is controlled within a predetermined range. As a result, it was found that the center segregation of the slab can be improved while appropriately securing the thickness of the solidified shell at the curved portion correction position, and the present invention has been achieved. This will be described in detail below.

  FIG. 1 shows an outline of a curved billet continuous casting machine, and a billet continuous casting method will be described with reference to this drawing.

  Molten steel is injected and supplied by the nozzle 1 and cooled by the mold 2 to form a solidified shell at the initial stage of the slab, and then the slab is pulled out by the pinch roll 3. In the secondary cooling zone 4, the slab is spray-cooled and a solidified shell grows (here, the upper part of the secondary cooling zone 4 is 4-a, and the rest is 4-b).

  The slab is horizontally corrected (bent back) at the correction position 5 of the curved portion. At this time, since the correction strain is applied, if the thickness of the solidified shell is insufficient, an internal crack of the slab occurs. . Thereafter, the slab is reduced by a predetermined amount by the light reduction device 6, cut into a slab length of a predetermined length by the shear 7, and conveyed to produce a billet slab.

The present invention is a high carbon steel having a carbon content in the range of 0.6 to 1.0% by mass, and in continuous casting of a billet having a side length of 160 mm or less, This is a method of performing cooling in the cooling zone 4 in two stages. First, the water density in the first stage is set in the range of 0.01 to 0.03 m ³ / m ² / sec, and then in the second stage. The water density in the next cooling zone is set to less than 0.002 m ³ / m ² / sec to secure a solidified shell thickness of 30 mm or more at the curved portion correction position 3, and the slab surface temperature at the curved portion correction position is 1150 to 1200. In this method, the temperature is controlled in the range of ° C.

  In the present invention, as described above, the slab internal crack is generated at the curved portion correction position by appropriately controlling the secondary cooling condition without being affected by the fluctuation factors such as the heat history after the secondary cooling zone. The improvement of the center segregation of the slab was investigated.

  Accordingly, when considering the cooling in the secondary cooling zone 4 of the curved portion under the mold in two stages, the first stage is strongly cooled, and the securing of the solidified shell thickness is given priority first. The secondary cooling zone is a process in which the second stage is subjected to gentle cooling, and the slab surface temperature at the curved portion correction position is controlled within the range of 1150 to 1200 ° C. to improve the uniformity of the temperature distribution in the slab cross section. It was found that the center segregation can also be satisfactorily suppressed.

Here, the water density at the first stage was defined as a range of 0.01 to 0.03 m ³ / m ² / sec. The water density of 0.01 m ³ / m ² / sec or more is usually a mold (see 2 in FIG. 1) in billet continuous casting that operates at a casting speed of about 2.5 to 3.5 m / min. The density of water required to promote the growth of the solidified shell of the formed billet. If the water density is less than this value, the solidified shell will break due to insufficient growth of the solidified shell, and the unsolidified molten steel in the slab will be There is a possibility that troubles such as breakout that flows out and bulging due to insufficient solidified shell may occur.

On the other hand, the upper limit of the water density is not particularly required from the viewpoint of promoting the solidified shell growth of the slab, but is specified to be 0.03 m ³ / m ² / sec or less as a practically feasible range. Note that 0.03 m ³ / m ² / sec is a considerably large value as a general billet continuous casting machine.

The water density in the secondary cooling zone at the second stage was defined as less than 0.002 m ³ / m ² / sec. The purpose of this regulation is to reduce the amount of secondary cooling water in order to improve center segregation, to suppress the temperature gradient inside the cross section by suppressing cooling from the billet surface, and to suppress the concentration of component elements. However, when 0.002 m ³ / m ² / sec or more, the cooling strength is too strong, and it has been experimentally found that the center segregation cannot be sufficiently suppressed.

Since sufficient solidification shell growth is promoted by the secondary cooling in the first stage, there is no particular problem even in the condition of not cooling in the second stage, so the lower limit is 0 m ³ / m ² / sec. including.

  Next, the reason why the lower limit value of the solidified shell thickness at the curved portion correction position is defined as 30 mm is that it is separately confirmed that it is the minimum value for preventing the occurrence of internal cracks in the slab. The upper limit is not particularly required from the viewpoint of preventing the occurrence of internal cracks. However, the larger the solidified shell thickness, the greater the load on the straightening machine.

  Moreover, the slab surface temperature in a curved part correction position was prescribed | regulated as the range of 1150-1200 degreeC. This temperature range is a temperature control range necessary for improving the center segregation when the billet is cooled by the secondary cooling of the first stage and the second stage located before the correction position.

  When the temperature is lower than 1150 ° C, the slab cooling is too strong and the uniformity of the temperature distribution cannot be improved, so that the center segregation cannot be improved. This is because it has been experimentally found that a slab having a sufficient central solid phase ratio cannot be fed into a light reduction device located downstream from the correction position, and an effect of improving center segregation cannot be expected.

  Incidentally, the high carbon steel billet of the present invention is intended for high carbon steel having a carbon content in the range of 0.6 to 1.0% by mass, and is a material for wire products in which central segregation is a problem. Therefore, the carbon content, which is a general hard steel wire rod, was in the range of 0.6 to 1.0 mass%.

  In the present invention, the billet whose length of one side of the cross section is 160 mm or less was used. Conventionally, a bloom having a larger cross section than that of a billet is divided and rolled into a billet, and the maximum value of the billet that can be rolled into pieces is about 160 mm. Since the billet manufacturing is a manufacturing process that omits the bloom rolling of the bloom, the length of one side of the billet is defined as 160 mm or less.

  Here, regarding the setting of the water density at the first and second stages, the thickness of the solidified shell at the curved portion correction position (see 3, in FIG. 1) is secured at 30 mm or more, and casting at the curved portion correction position is performed. It can be obtained in advance by heat transfer calculation in consideration of the planned casting speed so that the single surface temperature is in the range of 1150 to 1200 ° C.

  That is, since the solidified shell thickness and the slab surface temperature can be obtained in advance by calculation by heat transfer calculation, appropriate water density in the first and second stages can be set.

  By the way, the first stage length of the secondary cooling zone is the cooling range necessary for promoting the solidified shell growth, but if this is extremely shortened, the solidified shell growth may be inhibited, Moreover, bulging of the slab occurs due to insufficient cooling, and there is a possibility that the shape of the slab is defective or the slab cannot be pulled out smoothly. For this reason, from the viewpoint of promoting solidified shell growth, slab shape failure, and slab extraction stability, the first stage length is preferably in the range of about 1 to 2 m.

  On the other hand, the length of the second stage of the secondary cooling zone is determined by subtracting the overall length of the secondary cooling zone. The total length of the secondary cooling zone is appropriately set within a common sense range.

  The heat transfer calculation can be obtained based on the physical property values (liquidus temperature, solidus temperature, etc.) required for the calculation and cooling conditions from the steel component values.

  Here, the physical property values (liquidus temperature, solidus temperature, etc.) necessary for the calculation can be calculated from the steel component values by, for example, Steel Handbook 3rd Edition I Basic p205.

  The cooling condition can be obtained from a heat transfer coefficient corresponding to the water density. Incidentally, the relationship between the water density and the heat transfer coefficient can be determined by, for example, Steel Handbook 3rd Edition II Steelmaking / Steelmaking p620.

  As described above, the present invention can be implemented by obtaining the solidified shell thickness and the slab surface temperature in advance by calculation based on the heat transfer calculation based on the physical property values and the cooling conditions.

  In actual operation, it is difficult to actually measure the thickness of the solidified shell, but the slab surface temperature at the curved portion correction position can be measured with a radiation thermometer, so it was obtained by heat transfer calculation. The condition can be verified, and the calculated value can be verified and confirmed.

  Further, the slab temperature can be calculated in the solidification heat transfer calculation, but since it can be used as an operational index by measuring as much as possible, it is preferable to perform temperature management while measuring.

  In addition, when the obtained billet is lightly reduced by the light reduction device 6 following the secondary cooling described above, light reduction is performed at a portion where the slab center solid phase ratio before the light reduction device is 0.2 or more. Is preferred. This is because when the slab center solid phase ratio is less than 0.2 at the time of light pressure, the liquid phase of the unsolidified portion is too much, and the center segregation suppressing effect due to light pressure is hardly exhibited.

  Incidentally, the slab center solid phase ratio can be obtained from the heat transfer calculation described above.

  In the present invention, since the slab surface temperature at the correction position is increased to 1150 ° C. or higher, in the temperature distribution of the slab cross section, the temperature difference between the surface and the inside can be reduced. Along with the improvement in uniformity, the center segregation suppressing effect due to slab pressure reduction under light pressure is exhibited well.

By the way, the slab center solid phase ratio is
Slab center solid phase ratio = (T1 / T3) / (T1 / T2)
T1: Liquidus temperature of cast slab (° C)
T2: Solidus temperature of cast slab (° C)
T3: Center temperature (° C) of the slab cross section
The temperatures T1 and T2 can be calculated mainly from the component values of the slab, and T3 can be calculated from the heat transfer calculation described above.

  Using the curved billet continuous casting machine shown in FIG. 1, the steel having a carbon content of 0.6% by mass was cast by setting the secondary cooling conditions of the present invention and the conventional conditions.

  The equipment conditions in this example were a curved part arc radius of the curved billet continuous caster of 5 m, a correction position of 7 m from the meniscus 8, and a light reduction device at a position of 12 m from the meniscus 8.

  Also, the operating conditions in this example are as follows: a billet of 122 mm square is cast on one piece, the casting speed is a constant condition of 3 m / min, and the degree of superheated molten steel during casting (temperature from the liquidus in the component system) Was performed under the conditions of 20 to 34 ° C. Details of the secondary cooling conditions are shown in Table 1.

  Table 2 shows the test results when cast under these conditions. The slab surface temperature at the correction position described in the test results of Table 2 is a value actually measured using a radiation thermometer, and the solidified shell thickness and the pre-slab center slab solid fraction under light pressure are calculated by the above heat transfer calculation. It is a value calculated from

  Further, regarding the slab internal quality (slab slab internal crack, center segregation), a sample was taken from the cast slab cast in this example, and the slab internal crack presence (in Table 2) , “With no internal cracks” and “X” with internal cracks), and the maximum particle size of the center segregation (From the results obtained from the final product of the wire, the center segregation maximum particle size should be 4.5 mm or less. Therefore, the maximum segregation of 4.5 mm or less is regarded as acceptable.In this case, the acceptance is determined as “◯”, and the rejection is indicated as “×”.

In this embodiment, the secondary cooling water density density length of the first stage under the mold is 1.7 m, the water density at the upper part of the secondary cooling zone is 0.009 to 0.013 m ³ / m ² / sec, and thereafter Casting was performed with the secondary cooling zone water density in the second stage being 0.0013 to 0.0025 m ³ / m ² / sec.

As a result, regarding the water density in the first stage of the secondary cooling zone, it is possible to secure a solidified shell thickness of 30 mm or more at the correction position by setting the water density to 0.01 m ³ / m ² / sec or more. It can be seen that there is no internal cracking in the slab.

Next, regarding the secondary cooling condition in the second stage after the upper part of the curved secondary cooling zone is 1.7 m, the slab is formed when the water density is 0.002 m ³ / m ² / sec or more. Although the central solid phase ratio was 0.2 or more, the central segregation was not improved and the maximum segregated particle size could not be satisfied.

For this reason, when the water density is less than 0.002 m ³ / m ² / sec, when the slab center solid phase ratio is 0.2 or more, the center segregation becomes an acceptable level, and the slab quality improving effect is achieved. I was able to confirm. The slab surface temperature at the correction position in this case was 1150 ° C. or higher.

From this example, when casting high carbon steel with a curved billet continuous caster, the water density of the secondary cooling zone in the first stage under the mold is set to 0.01 m ³ / m ² / sec or more. By making the water density in the subsequent secondary cooling zone less than 0.002 m ³ / m ² / sec, the slab shell thickness at the curved portion correction position is secured to 30 mm or more to prevent the occurrence of internal slab cracks, Confirm that a slab with good center segregation can be produced by setting the slab surface temperature at the curved portion correction position to 1150 ° C or higher and the slab center solid phase ratio before light reduction to 0.2 or higher. I was able to.

It is a figure which shows the outline of the curved billet continuous casting machine which implements this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nozzle 2 Mold 3 Pinch roll 4 Secondary cooling zone 5 Curved part correction position 6 Light reduction device 7 Shear

Claims (2)

When casting a high carbon steel billet having a side length of 160 mm or less and a carbon content of 0.6 to 1.0% by mass using a curved continuous casting machine, In the continuous casting method in which cooling in the secondary cooling zone is performed in two stages, first, the water density in the first stage is set in the range of 0.01 to 0.03 m ³ / m ² / sec, and then in the second stage. The water density in the secondary cooling zone is less than 0.002 m ³ / m ² / sec, the solidified shell thickness of the slab at the curved portion correction position is secured to 30 mm or more, and the slab surface temperature at the curved portion correction position Is controlled in the range of 1150 to 1200 ° C., the billet continuous casting method.   2. The method according to claim 1, wherein, when the slab is lightly reduced following the cooling in the secondary cooling zone, the slab center solid phase ratio is lightly reduced at a portion of 0.2 or more. Billet continuous casting method.

Images