JP2015193038A - Cooling method of casting piece of carbon steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of crack, by stably making a formation of a casting piece a fine formation, by optimizing a tertiary cooling condition corresponding to a carbon steel.SOLUTION: When a carbon steel is casted, a cooling water amount at a constant region when the casting piece is cooled by tertiary cooling satisfies (1) and (2), in which:(1) when temperature difference between maximum temperature and minimum temperature in surface temperature distribution of a casting piece width direction, is ΔT, ΔT≤160°C;(2) as cooling speed when cooling the casting piece surface temperature, from 800°C to 500°C, when temperature before cooling of the casting piece surface is 800°C-700°C, cooling speed is 25°C/min or more, when temperature before cooling is 699°C-600°C, the cooling speed is 20°C/min or more, and when temperature before cooling is 599°C-500°C, cooling speed is 15°C/min or more.

Description

  The present invention relates to a method for tertiary cooling of a slab of carbon steel cast by, for example, a continuous casting apparatus, and relates to a method for cooling a slab of carbon steel.

  Conventionally, in continuous casting equipment, the molten steel produced from the converter, secondary refining equipment, etc. is transported to the tundish using a ladle, and the molten steel in the ladle is poured into the tundish, and then this tundish is used. By supplying molten steel to the mold, molten steel (slab) is continuously cast. The slab cast by the continuous casting apparatus is cut into a predetermined length on the downstream side, and then cooled by a tertiary cooling zone before rolling or the like is performed.

In the tertiary cooling zone, there are techniques shown in Patent Documents 1 and 2 as techniques for cooling the slab.
Patent Document 1 describes a slab that is difficult to generate surface cracks during hot rolling while preventing surface cracks and bending during cooling of continuous cast slabs made of a wide range of steel types from low carbon steel to high hardenability. This is a manufacturing method, in which three or more cooling nozzle rows are arranged in the casting direction when a continuously casted bloom is cut to a predetermined length and then cooled by a bloom cooler, and the total flow rate of cooling water is obtained. Cooling at 200 to 1000 l / min.

Patent document 2 is cooling which prevents generation | occurrence | production of the surface flaw which generate | occur | produces at the time of cooling of a continuous casting bloom, Comprising: Water quantity density of an upper surface is 5 * 10 <-4> -4 * 10 < ^-3 > m < ³ > / m < ² > S, The water density is 1.5 times or more of the water density on the upper surface, the water density on the lower surface is 2.0 or more times the water density on the upper surface, and the cooling rate is increased as the bloom surface temperature approaches the Ar3 transformation point. The bloom is being cooled at the cooling rate on the left side from the start point, point B1.

  Patent Document 3 aims to prevent the progress of cracks in the slab by making the slab structure finely and stably. In Patent Document 3, in a cooling method in which a slab cast by a continuous casting machine is cooled before being charged into a heating furnace installed on the downstream side, the slab is disposed at a pitch of 200 to 1200 mm, and the cast is disposed. The slab is cooled by blowing cooling air having a spray angle of 0 to 50 ° and a wind speed of 2 to 20 m / sec against the lower surface of the piece.

Patent document 4 aims at effectively preventing the bloom cast slab surface cracking that occurs during the ingot rolling. In Patent Document 4, the heat transfer coefficient (W / m ² · K) when the surface temperature of the bloom slab is 1000 K is Hw, and the time (min) for the bloom slab to pass through the cooling zone (heat transfer coefficient Hw) is When T, the surface temperature of the bloom slab 1 exceeds the Ar3 transformation point, and the cooling strength Cs defined by Cs = Hw × T is in the range of 500 to 2500 (W · min / m ² · K). Cooling is performed under the following conditions.

  Patent Document 5 aims to prevent the billet slab from being bent when the billet slab is cooled after the steel is continuously cast and cut to a predetermined length. In Patent Document 5, when cooling the billet slab after continuously casting steel and cutting it to a predetermined length, the average cooling rate on the opposing surfaces of the slab in the range of 800 to 500 ° C. Are C1 and C2 (° C./min: C1 ≧ C2), respectively, the cooling is performed so as to satisfy (C1−C2) / C1 <0.15.

  Patent Document 6 aims to prevent the joint portion formed by temporarily stopping the injection of molten steel into the mold from being overcooled. In Patent Document 6, the secondary cooling water amount is set to the secondary cooling water amount (Q) calculated from the slab drawing speed in accordance with the slab drawing stop time so as to become relatively smaller as the drawing stop time becomes longer. Cooling is performed with the amount of secondary cooling water (α × Q) multiplied by the correction coefficient α (α <1.0).

Patent Document 7 sets the optimum cooling water amount in each cooling zone even if the casting speed becomes low and the slab existing in the cooling zone with the restriction of the minimum cooling water amount in the mold is excessively removed. The purpose is to do. In Patent Document 7, when the casting speed becomes equal to or lower than a predetermined value, the casting speed becomes equal to or lower than the predetermined value when the cast piece existing in the mold at that time passes through the secondary cooling zone. The correction coefficient for the amount of cooling water to be sprayed is calculated in consideration of the time spent and the casting speed at that time, and the amount of cooling water corrected by the correction coefficient is sprayed when the corresponding slab part passes.

JP2013-27900A JP 2003-181608 A JP 2010-5633 A JP 2008-261036 A JP 2004-344893 A JP 2013-123713 A JP-A-4-339555

  Although both Patent Documents 1 and 2 are techniques for cooling a slab cast by a continuous casting apparatus, the relationship between the composition of the slab (steel type) and cooling conditions is not shown, and these techniques are used. However, the actual situation is that surface cracks cannot be prevented in cast slabs made of carbon steel. Moreover, in patent document 3, in order to refine | miniaturize a structure | tissue, it cools by air cooling. Although this technique can be miniaturized to some extent, at this cooling rate, the crystal grains of the ferrite + pearlite structure after transformation may become coarse, and cracks may occur in the steel pieces after extraction after heating.

  Further, from the viewpoint of preventing cracks by forming a fine structure, in any of Patent Documents 4 to 7, detailed cooling conditions, for example, the difference between the maximum temperature and the minimum temperature in the surface temperature distribution, the surface temperature distribution There is no indication of the corresponding cooling method, cooling of the unsteady part of the slab, etc. Therefore, even if these techniques are used, surface cracks cannot be prevented in a slab made of carbon steel.

  Therefore, in the present invention, there is provided a cooling method for a slab of carbon steel that can stably prevent a crack by making the slab structure finer by optimizing the tertiary cooling conditions corresponding to the carbon steel. The purpose is to provide.

In order to achieve the above object, the present invention has taken the following measures.
That is, the method for cooling a slab of carbon steel according to the present invention includes, as components, C = 0.13% to 0.18% (mass%, hereinafter the same), Si = 0.15% to 0.35%, Mn = 0.30% to 0.60%, P ≦ 0.030%, S ≦ 0.035%, Cr ≦ 0.15%, the balance of Fe and unavoidable impurities, and the following conditions It is characterized by the third cooling so as to satisfy.

(1) ΔT ≦ 160 ° C., where ΔT is the temperature difference that is the difference between the maximum temperature and the minimum temperature in the surface temperature distribution in the slab width direction.
(2) As a cooling rate for cooling the slab surface temperature from 800 ° C. to 500 ° C., when the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more, 699 ° C. to 600 ° C. Then, the cooling rate is set to 15 ° C./min or more at a cooling rate of 20 ° C./min or more at 599 ° C. to 500 ° C.

Moreover, the cooling method of the slab of carbon steel is as components C = 0.13% to 0.18% (mass%, the same applies hereinafter), Si = 0.15% to 0.35%, Mn = 0. 30% to 0.60%, P ≦ 0.030%, S ≦ 0.035%, Cr ≦ 0.15%, the balance of Fe and unavoidable impurities in the slab of carbon steel, the following ( Under the conditions of 1) and (2), the amount of tertiary cooling water Q in the steady portion of the slab is obtained,
(1) ΔT ≦ 160 ° C., where ΔT is the temperature difference that is the difference between the maximum temperature and the minimum temperature in the surface temperature distribution in the slab width direction.

(2) As a cooling rate for cooling the slab surface temperature from 800 ° C. to 500 ° C., when the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more, 699 ° C. to 600 ° C. Then, the cooling rate is 20 ° C / min or more, and the cooling rate is 15 ° C / min at 599 ° C to 500 ° C.
It is set to min or more.
According to the following (3) to (6), the amount of tertiary cooling water Q ′ in the unsteady part of the slab is obtained,
(3) A stop correction coefficient a based on the slab drawing stop time is set.

(4) The conveyance correction coefficient b based on the slab conveyance speed is set.
(5) A casting speed correction coefficient c based on the average casting speed in the continuous casting apparatus is set.
(6) Q ′ = Q × a × b × c
The stationary part is subjected to tertiary cooling with a tertiary cooling water amount Q, and the unsteady part is subjected to tertiary cooling with a tertiary cooling water amount Q ′.

  According to the present invention, by optimizing the tertiary cooling conditions corresponding to the slab of carbon steel, the structure of the slab can be stably made into a fine structure and cracking can be prevented.

It is a conceptual diagram of a continuous casting device, a cutting device, and a tertiary cooling device. It is the layout which has arrange | positioned the cooling nozzle around a slab. It is a flowchart which shows the procedure of the tertiary cooling of slab. It is a related figure of the temperature difference in an upper surface, and the water quantity density of a nozzle. It is a related figure of the temperature difference in a narrow surface, and the water quantity density of a nozzle. It is a related figure of the temperature difference in a lower surface, and the water quantity density of a nozzle. It is a figure which shows the characteristic of an upper cooling nozzle. It is a figure which shows the characteristic of a lower cooling nozzle. It is a figure which shows the characteristic of a narrow surface cooling nozzle. It is a flowchart which shows the procedure of the tertiary cooling of the slab in 2nd Embodiment.

An embodiment of the present invention will be described with reference to the drawings.
[First Embodiment]
The method for cooling a slab of carbon steel according to the present invention is a method of performing tertiary cooling of a slab cast by a continuous casting apparatus, that is, a slab cut after cutting a bloom. In this embodiment, the bloom may be referred to as “slab”.

FIG. 1 shows a continuous casting apparatus, a cutting apparatus disposed downstream of the continuous casting apparatus and cut into a predetermined length of a slab cast by the continuous casting apparatus, and the slab cut by the cutting apparatus is cooled. 3 shows a tertiary cooling device.
First, a continuous casting device, a cutting device, and a tertiary cooling device will be described.
As shown in FIG. 1, the continuous casting apparatus 1 continuously casts bloom, and a ladle 3 into which a refined molten steel 2 is charged and a molten steel in the ladle 3 are injected. The tundish 4 for temporarily storing the molten steel 2, the mold 5 to which the molten steel 2 from the tundish 4 is supplied, and the slab S formed by the mold 5 are drawn out and the slab S is supported. And a plurality of support rolls 6.

The tundish 4 has a bottomed box shape, and an inlet for injecting molten steel is provided at the bottom of the tundish 4, and an immersion nozzle is connected to the inlet. The immersion nozzle can be opened and closed by a slide valve, and molten steel can be injected or stopped from the tundish 4 to the mold 5 by opening and closing the slide valve.
In the continuous casting apparatus 1, the molten steel 2 produced from a converter, secondary refining apparatus, etc. is transported to the tundish 4 by the ladle 3, and the molten steel 2 in the transported ladle 3 is injected into the tundish 4. The molten steel 2 in the mold 5 can be continuously cast by opening the slide valve. The molten steel 2 (slab S) is subjected to primary cooling by the mold 5 and secondary cooling by a cooling nozzle (not shown) disposed on the downstream side of the mold 5.

Further, the slab S cast by the continuous casting apparatus 1 is sent to the cutting apparatus 10 and is cut into a predetermined length by the cutting apparatus 10. This cutting device 10 is installed on the downstream side of the continuous casting device 1 (downstream side of the pinch roll 9 installed on the most downstream side of the continuous casting device 10). For example, the slab S is cut with gas. It consists of a gas cutter.
The tertiary cooling device (tertiary cooling zone) 11 is a facility (bloom cooler) for cooling the surface of the cast bloom, and is installed on the downstream side of the cutting device 10 and is cast after being cut by the cutting device 10. The piece S is cooled (third cooling of the cast piece S). That is, the tertiary cooling zone 11 cools the slab cut to a predetermined length before charging it into the heating furnace. The tertiary cooling zone 11 performs cooling for each of the cut slabs S.

  As shown in FIGS. 1 and 2, the tertiary cooling zone 11 includes a transport device 12 that transports the slab S, and cooling nozzles 13 that are arranged at predetermined intervals along the transport direction of the slab S. Yes. The conveying device 12 is configured by a roller conveying device that conveys the placed slab S with a roller. The cooling nozzle 13 cools the narrow surface by an upper cooling nozzle 13a that cools the upper surface (the upper surface on the wide surface side) of the slab S, a lower cooling nozzle 13b that cools the lower surface (the lower surface on the wide surface side) of the slab S, and the like. A pair of narrow surface cooling nozzles 13c are provided. Accordingly, the four surfaces of the rectangular slab S can be cooled by the upper cooling nozzle 13a, the lower cooling nozzle 13b, and the narrow surface cooling nozzle 13c.

Now, in the tertiary cooling zone 11, cracks may occur on the surface of the slab S or the like if not properly cooled. In particular, even under the same cooling conditions, if the steel type is different, cracking may occur on the surface of the slab S due to supercooling or the like, and it is necessary to appropriately control the steel type and the cooling conditions.
In the present invention, when the slab S is subjected to the tertiary cooling, by performing the tertiary cooling suitable for the steel type, cracks progress stably on the slab surface of the slab (bloom) with the microstructure of the slab stably being a fine structure. To prevent it.

Hereinafter, the tertiary cooling of the slab S will be described in detail.
The slab S cast by the continuous casting apparatus 1, that is, the slab S to be subjected to tertiary cooling is carbon steel, and C = 0.13% to 0.18% (mass%, the same applies hereinafter) as a component, Si = 0.15% to 0.35%, Mn = 0.30% to 0.60%, P ≦ 0.030%, S ≦ 0.035%, Cr ≦ 0.15%, balance Fe and inevitable Contains impurities.

  Such carbon steel, ie, S15C phase steel, has a high temperature for transformation to austenite after peritectic reaction, so the austenite grain size becomes coarse, and if heated and rolled as it is, surface cracks are likely to occur at the prior austenite grain boundaries. . In order to prevent this, it is necessary to make the austenite ferrite-palite once below the A1 transformation point, thereby refining the crystal grains when the austenite is newly transformed in a heating furnace, thereby eliminating the prior austenite grain boundaries. In detail, when cooling the slab S of carbon steel by tertiary cooling, it is necessary to cool so that the conditions (1) and (2) may be satisfied.

  Specifically, as shown in FIG. 3, in the condition (1), when the temperature difference that is the difference between the maximum temperature and the minimum temperature in the surface temperature distribution in the slab width direction is ΔT, the temperature difference is the temperature difference. ΔT ≦ 160 ° C. When the temperature distribution on the straight line L1 along the width direction (on the straight line L1 orthogonal to the casting direction) is observed on each of the upper surface, the lower surface, and the narrow surface of the slab S, the maximum temperature and the minimum temperature on the straight line The difference (ΔT) is 160 ° C. or less. In other words, the difference between the highest temperature and the lowest temperature of the surface temperature distribution on the upper surface is 160 ° C. or less, and the difference between the highest temperature and the lowest temperature of the surface temperature distribution on the lower surface is 160 ° C. or less. The difference between the maximum temperature and the minimum temperature of the surface temperature distribution is also 160 ° C. or less.

Further, the difference between the maximum temperature and the minimum temperature indicated in the condition (1) is satisfied in each management section indicated in the condition (2) described later. For example, ΔT ≦ 160 ° C. is satisfied in each of a first management section, a second management section, and a third management section described later.
FIG. 4 shows the relationship between the temperature difference ΔT on the upper surface and the nozzle water density in the slab S composed of carbon steel, and FIG. 5 shows the relationship between the temperature difference ΔT on the narrow surface and the nozzle water density. FIG. 6 shows the relationship between the temperature difference ΔT on the lower surface and the water density of the nozzle.
FIGS. 4 to 6 summarize the presence or absence of stress cracking by visually checking the surface of the slab S after increasing / decreasing the water density of the tertiary cooling of the actual bloom cooler 11 and performing cooling. is there.

As shown in FIGS. 4 to 6, when the temperature difference ΔT between the maximum temperature and the minimum temperature exceeds 160 ° C. on any of the upper surface, the narrow surface, and the lower surface, thermal stress cracking occurs, and the temperature difference ΔT is 160 ° C. or less. In this case, stress cracking did not occur. That is, in the slab S of carbon steel, during the third cooling,
By setting the temperature difference ΔT1 to 160 ° C. or less, stress cracking can be prevented.
Now, in the present invention, for the slab S (carbon steel slab S) after the third cooling, by setting the cooling rate appropriately, the structure within 5 mm from the surface layer of the slab is obtained as ferrite + pearlite. Have an organization. In addition, although bainite may be formed by C equivalent, in this case, crystal grain refinement can be expected.

That is, in the present invention, by performing the tertiary cooling, a ferrite + pearlite structure or a bainite structure is generated within 5 mm of the slab, and as a result, the surface crack is prevented from spreading while preventing the subcutaneous crack. .
Specifically, in order to form a ferrite + pearlite structure or bainite (Zw) over a range of 5 mm or more from the surface layer of the slab S, the cooling rate of the tertiary cooling is set as shown in the condition (2). It is set. Condition (2) is a condition of the cooling rate when the slab surface temperature is cooled from 800 ° C. to 500 ° C., and when the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min. As described above, the cooling rate is 20 ° C./min or more at 699 ° C. to 600 ° C., and the cooling rate is 15 ° C./min or more at 599 ° C. to 500 ° C. In other words, the upper cooling nozzle 13a, the lower cooling nozzle 13b, and the narrow cooling nozzle 13c are cooled so that the cooling rate when the slab S is cooled satisfies the condition (2). Cooling is performed by setting the amount of water (water density) of the surface cooling nozzle 13c.

  As shown in this condition (2), when the slab surface temperature is cooled to 800 to 500 ° C., the cooling rate is controlled to 800 ° C. to 700 ° C. (first management interval), 699 to There are three sections of 600 ° C. (second management section), 599 to 500 ° C. (third management section), and the temperature width (temperature range from the highest temperature to the lowest temperature) of each management section is 100 ° C. or less. Thus, since the temperature width in each management section is set to 100 ° C. or less, it is easy to satisfy the temperature difference condition shown in the condition (1), and conversely, the temperature width of the management section is made larger than 100 ° C. Therefore, not only is it difficult to satisfy the condition of the temperature difference shown in the condition (1), but when trying to satisfy the condition (1), the cooling rate in each management section must be extremely slow, It becomes difficult to form a ferrite + pearlite structure or a bainite structure.

The relationship between the water density of the tertiary cooling and the cooling rate is preferably obtained by an external experiment or the like. For example, in an out-of-machine experiment, a thermocouple is embedded in the test piece, the test piece is cooled at various cooling densities, and the relationship between the water density and the cooling rate is obtained.
In actual operation, the water density is obtained from the water quantity and the nozzle characteristics as the tertiary cooling conditions, and the cooling with respect to the water density is obtained from the relationship between the water density and the cooling rate obtained in the external experiment. Search for speed and conduct operations. Here, in condition (2), the upper limit value of the cooling rate is not defined, but it is desirable to set it based on the capability of the actual machine. In other words, the cooling rate is obtained with a capability adapted to the actual machine without departing from the operation range of ordinary methods by those skilled in the art. For example, when the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min to 200 ° C./min, and when 699 ° C. to 600 ° C., the cooling rate is 20 ° C./min to 200 ° C./min. Hereinafter, at 599 ° C. to 500 ° C., the cooling rate is 15 ° C./min to 200 ° C./min. As a matter of course, if the cooling rate is too high, the martensite structure can be formed into a slab, so the cooling rate is set so that the martensite structure does not occur.

That is, in the actual operation, the water amount density (cooling water amount Q) in the stationary part is set so as to satisfy the conditions (1) and (2). For example, the cooling of the upper cooling nozzle 13a, the lower cooling nozzle 13b, and the narrow surface cooling nozzle 13c so as to satisfy the condition (1) and the condition (2) on each cooling surface (upper surface, lower surface, narrow surface) of the slab. Set the water quantity Q.
And after setting each cooling water quantity Q of each nozzle 13a, 13b, 13c, the presence or absence of the slab on the entrance side of the tertiary cooling zone 11 is detected, and when the slab S is present, that is, the slab When S exists, the slab S is cooled with the set amount of cooling water. For detecting the slab S, a sensor or the like is installed in advance on the inlet side of the tertiary cooling zone 11, and whether or not the slab S has entered the tertiary cooling zone 11 by this sensor (presence or absence of the slab S) is determined. In addition, you may confirm the presence or absence of the slab S visually.

Next, when casting the slab S of carbon steel having the above-described components, the amount of cooling water in the stationary part when the slab S is cooled by tertiary cooling is set in the conditions (1) and (2). Then, the presence or absence of the slab S was detected at the inlet of the tertiary cooling zone, and the example in which the slab was cooled with the cooling water amount set when the slab S was present was compared with the cooling performed by a method different from the example An example will be described.
In this example and the comparative example, the continuous casting apparatus 1 used “Kobe Steel Works, vertical bending type continuous casting machine”. The mold size was 430 × 300 mm. The mold height was 1200 mm. The casting speed was 0.70 to 1.05 m / min, and the specific water amount of secondary cooling in the continuous casting apparatus 1 was 0.20 to 0.55 L / kg · s. In the tertiary cooling method in the tertiary cooling zone 11, mist-like cooling water (water + air) was sprayed onto the four surfaces of the slab S, the upper surface, the lower surface, and the narrow surface. The number of cooling nozzles in the tertiary cooling zone 11 was 24 per surface. The length of the tertiary cooling zone 11 was 7.23 m.

Further, the upper cooling nozzle 13a having the characteristics shown in FIG. 7 was used, the lower cooling nozzle 13b having the characteristics shown in FIG. 8 was used, and the narrow surface cooling nozzle 13c having the characteristics shown in FIG. 9 was used. The air / water volume ratio of the cooling nozzle was set to 31, and the nozzle angle was set to 95 ° with respect to the upper and lower surfaces of the slab S and 111 ° to the narrow surface.
The slab (steel ingot) obtained by the continuous casting apparatus 1 was subjected to tertiary cooling, charged in a heating furnace and heated to 1200 to 1300 ° C., and then rolled into a billet (cross section 155 mm × 155 mm). Split rolling and the like were carried out in the same manner as those skilled in the art.

  The surface defect of the slab was observed at the stage of rolling into billets (155 mm square) and examined for cracks. That is, based on JISZ2323, a magnetic particle flaw inspection was performed for one billet (size: 155 mm × 155 mm × 10 m). In the magnetic particle flaw detection test, the peak of luminance per field of view (2 mm × 2 mm) was measured, and the one in which the peak was detected was taken as a bowl.

  The cooling rate was measured in advance in an out-of-machine experiment by embedding a thermocouple at a depth of 5 mm from the surface of the slab, cooling the slab at a predetermined water density for a predetermined time, and measuring the cooling rate for each water density. In this out-of-machine experiment, the same cooling nozzle as that of the actual machine was used, and the arrangement of the cooling nozzles was the same as that of the actual machine. Also, in the out-of-machine experiment, the slab size (sample size) and the initial slab temperature (sample initial temperature) were the same as in the actual machine, so that there was no difference in the measurement conditions between the out-of-machine experiment and the actual machine. .

  Table 1 shows that the casting speed (target casting speed) is 1.05 m / min, the drawing stop time is 0 minute, the average casting speed is 1.05 m / min, and the third cooling is performed on the upper surface of the slab S. The results are summarized.

In Examples (Sequential Numbers) 1 to 5 and 15, in the first control section (surface temperature: 700 ° C. to 800 ° C.) where the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more. And ΔT ≦ 160 ° C. In the second control section (surface temperature: 600 ° C. to 699 ° C.) in which the temperature before cooling of the slab surface is 699 ° C. to 600 ° C., the cooling rate is set to 20 ° C./min or more, and ΔT ≦ 160 ° C. Furthermore, the temperature before cooling of the slab surface is 599 ° C. to 500 ° C.
In the control section (surface temperature: 500 ° C. to 599 ° C.), the cooling rate is 15 ° C./min or more, and ΔT ≦ 160 ° C. Therefore, there was no thermal stress cracking of the slab and no surface cracking occurred on the upper surface of the slab (flaw detection test “◯”).

  On the other hand, in Comparative Examples (Sequential Numbers) 6, 7, and 14, the cooling rate in the first management section was less than 25 ° C./min, so surface cracking occurred (flaw detection test “×”). In Comparative Examples 8, 9, and 13, since the cooling rate in the second management section was less than 20 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 10 to 12, since the cooling rate in the third management section was less than 15 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 21 and 22, since the cooling rate in the first management section was less than 25 ° C./min and the cooling rate in the second management section was less than 20 ° C./min, surface cracking occurred (flaw detection test "X"). In Comparative Examples 16 to 20, since the temperature difference ΔT in the third management section exceeds 160 ° C., surface cracks (thermal stress cracks) occurred.

  Table 2 shows the tertiary cooling of the narrow surface of the slab S with a casting speed (target casting speed) of 1.05 m / min, a drawing stop time of 0 minutes, and an average casting speed of 1.05 m / min. This is a summary of the results.

In Examples (Sequential Numbers) 1 to 4, in the first control section (surface temperature: 700 ° C. to 800 ° C.) where the temperature before cooling of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more, ΔT ≦ 160 ° C. In the second control section (surface temperature: 600 ° C. to 699 ° C.) in which the temperature before cooling of the slab surface is 699 ° C. to 600 ° C., the cooling rate is set to 20 ° C./min or more, and ΔT ≦ 160 ° C. Further, in the third management section (surface temperature: 500 ° C. to 599 ° C.) where the temperature before cooling of the slab surface is 599 ° C. to 500 ° C., the cooling rate is set to 15 ° C./min or more, and ΔT ≦ 160 ° C. Therefore, there was no thermal stress cracking of the slab and no surface cracking occurred on the upper surface of the slab (flaw detection test “◯”).

  On the other hand, in Comparative Examples (Sequential Numbers) 5, 6, and 20, since the cooling rate in the first management section was less than 25 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 7, 8, and 12, since the cooling rate in the second management section was less than 20 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 9, 10, 13, and 23, since the cooling rate in the third management section was less than 15 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 11 and 21, since the cooling rate in the first management section was less than 25 ° C./min and the cooling rate in the third management section was less than 15 ° C./min, surface cracking occurred (flaw detection test "X"). In Comparative Example 22, since the cooling rate in the first management section was less than 25 ° C./min and the cooling rate in the second management section was less than 20 ° C./min, surface cracks occurred (flaw detection test “× "). In Comparative Examples 14 to 19, since the temperature difference ΔT in the third management section exceeded 160 ° C., surface cracks (thermal stress cracks) occurred.

  Table 3 shows that the casting speed (target casting speed) is 1.05 m / min, the drawing stop time is 0 minute, the average casting speed is 1.05 m / min, and the third cooling is performed on the lower surface of the slab S. The results are summarized.

In Examples (Sequential Numbers) 1 to 4, in the first control section (surface temperature: 700 ° C. to 800 ° C.) where the temperature before cooling of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more, ΔT ≦ 160 ° C. In the second control section (surface temperature: 600 ° C. to 699 ° C.) in which the temperature before cooling of the slab surface is 699 ° C. to 600 ° C., the cooling rate is set to 20 ° C./min or more, and ΔT ≦ 160 ° C. Further, in the third management section (surface temperature: 500 ° C. to 599 ° C.) where the temperature before cooling of the slab surface is 599 ° C. to 500 ° C., the cooling rate is set to 15 ° C./min or more, and ΔT ≦ 160 ° C. Therefore, there was no thermal stress cracking of the slab and no surface cracking occurred on the upper surface of the slab (flaw detection test “◯”).

  On the other hand, in Comparative Examples (Sequential Numbers) 5, 6, 11, and 20, the cooling rate in the first management section was less than 25 ° C./min, so surface cracking occurred (flaw detection test “×”). In Comparative Examples 7, 8, and 12, since the cooling rate in the second management section was less than 20 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 9, 10, and 13, since the cooling rate in the third management section was less than 15 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Examples 18 and 19, the cooling rate in the first management section is less than 25 ° C./min, the cooling rate in the second management section is less than 20 ° C./min, and the cooling rate in the third management section is Since it was less than 15 ° C./min, surface cracking occurred (flaw detection test “×”). In Comparative Example 21, since the cooling rate in the first management section was less than 25 ° C./min and the cooling rate in the second management section was less than 20 ° C./min, surface cracks occurred (flaw detection test “× "). In Comparative Example 22, since the cooling rate in the second management section was less than 20 ° C./min and the cooling rate in the third management section was less than 15 ° C./min, surface cracks occurred (flaw detection test “× "). In Comparative Examples 14 to 17, since the temperature difference ΔT in the third management section exceeded 160 ° C., surface cracks (thermal stress cracks) occurred.

  Tables 4 to 6 summarize the comparative examples when the temperature range of the tertiary cooling control section is 100 ° C. or higher and each surface is cooled. In this comparative example, the casting speed (target casting speed) is summarized. 1.05 m / min, the drawing stop time was 0 minutes, and the average casting speed was 1.05 m / min.

In Comparative Examples 1 to 9 in Table 4, although the temperature difference ΔT in the fourth management section (650 to 800 ° C.) and the fifth management section (500 to 649 ° C.) was 160 ° C. or less, surface cracking occurred. (Flaw detection test “×”). Moreover, in Comparative Examples 10-20 of Table 4, although the temperature difference (DELTA) T in a 4th management area was 160 degrees C or less, the thermal stress crack generate | occur | produced.
In Comparative Examples 1 to 7 in Table 5, although the temperature difference ΔT in the fourth management section (650 to 800 ° C.) and the fifth management section (500 to 649 ° C.) was 160 ° C. or less, surface cracking occurred. (Flaw detection test “×”). Moreover, in Comparative Examples 8-20 of Table 5, although the temperature difference (DELTA) T in a 4th management area was 160 degrees C or less, the thermal stress crack generate | occur | produced.

  In Comparative Examples 1 to 6 in Table 6, although the temperature difference ΔT in the fourth management section (650 to 800 ° C.) and the fifth management section (500 to 649 ° C.) was 160 ° C. or less, surface cracking occurred. (Flaw detection test “×”). Moreover, in Comparative Examples 7-20 of Table 6, although the temperature difference (DELTA) T in a 4th management area was 160 degrees C or less, the thermal stress crack generate | occur | produced. Moreover, thermal stress cracking also occurred in Comparative Example 21 in Table 6.

As described above, according to the present invention, when carbon steel is cast, the amount of cooling water in a stationary part when the slab S is cooled by tertiary cooling is set under the conditions (1) and (2). Before the slab S is inserted into the heating furnace, the slab S is detected by detecting the presence or absence of the slab at the tertiary cooling zone inlet and cooling the slab with the cooling water amount set when the slab S is present. The surface layer can be a ferrite + pearlite structure or a bainite structure. Therefore, even if a magnetic particle flaw detection test is performed on the slab S, no defects appear on the surface, and a slab free of defects can be manufactured. That is, on the surface of the slab during continuous casting, the structure of the slab is stably made into a fine structure and cracks are prevented from progressing. In addition, thermal stress cracking of the slab can be prevented.
[Second Embodiment]
In the second embodiment, in addition to the steady portion of the slab shown in the first embodiment, the non-steady portion of the slab is also subjected to tertiary cooling.

  Specifically, when tertiary cooling of the slab of carbon steel is performed, the amount of tertiary cooling water Q at the steady portion of the slab is determined under conditions (1) and (2). That is, the water amount density (tertiary cooling water amount Q) in the stationary part is set so as to satisfy the conditions (1) and (2). For example, the tertiary of the upper cooling nozzle 13a, the lower cooling nozzle 13b, and the narrow surface cooling nozzle 13c so as to satisfy the condition (1) and the condition (2) on each of the cooling surfaces (upper surface, lower surface, narrow surface) of the slab. Set the cooling water quantity Q. In addition, the method of calculating | requiring the component of carbon steel and the amount Q of tertiary cooling water is the same as 1st Embodiment.

In accordance with the following steps (3) to (6), the tertiary cooling water amount Q ′ at the unsteady portion of the slab is obtained. In addition, the order of the steps (3) to (5) may be changed. The tertiary cooling water amount Q ′ is calculated on each of the upper surface, the lower surface, and the narrow surface as in the first embodiment.
(3) A stop correction coefficient a based on the slab drawing stop time is set.
(4) The conveyance correction coefficient b based on the slab conveyance speed is set.
(5) A casting speed correction coefficient c based on the average casting speed in the continuous casting apparatus is set.

(6) Q ′ = Q × a × b × c
Next, the steps (3) to (6) will be described in detail.
The drawing stop shown in the step (3) is “stopping casting while drawing the slab S” in the continuous casting apparatus 1, and the drawing stop time is, for example, a pre-charge and a post-charge. This is the time when drawing is stopped to load the sequence block in the mold 5 when switching to the charge.

  While the drawing of the slab S is stopped, the slab S remaining in the continuous casting apparatus 1 is cooled excessively. Therefore, after setting the stop correction coefficient a based on the length of the drawing stop time, the tertiary cooling water quantity Q ′ at the unsteady part is obtained by multiplying the tertiary cooling water quantity Q by the stop correction coefficient a. Table 7 shows a stop correction table (list) showing the relationship between the drawing stop time and the stop correction coefficient a. This stop correction table is a numerical value obtained through experiments and operations.

  The stop correction coefficient a is expressed as a predetermined value (percentage) according to the length of the pull-out stop time. That is, the stop correction coefficient a allows the ferrite + pearlite structure or bainite (Zw) to be formed over the range of 5 mm or more from the surface layer of the slab S even if the drawing stop time is long, and further the supercooling crack This is a parameter for setting the tertiary cooling water amount Q ′ so as not to occur. The stop correction coefficient “a” of “100%” indicates that it is not necessary to reduce the amount of water even if there is a drawing stop.

  The conveyance speed shown in the step (4) is the speed of the slab S that moves after the completion of the cutting of the slab S. The conveyance speed is, for example, the average speed of the slab S when the slab S moves between the exit side of the gas cutter 10 and the entrance side of the tertiary cooling zone 11. When the moving speed of the slab S moving in the tertiary cooling zone 11 is set to be constant, the moving speed of the slab S from the outlet side of the gas cutter 10 to the outlet side of the tertiary cooling 11 is set to the conveyance speed. It is good.

  When the conveyance speed of the slab S is low, the slab S is cooled until the tertiary cooling zone 11 is entered after the slab S is cut. Therefore, after setting the conveyance correction coefficient b based on the conveyance speed, the tertiary cooling water amount Q ′ at the unsteady part is obtained by multiplying the tertiary cooling water amount Q by the conveyance correction coefficient b. Table 8 shows a transport correction table (list) showing the relationship between the transport speed and the transport correction coefficient b. This conveyance correction table is a numerical value obtained through experiments and operations.

The conveyance correction coefficient b is represented by a predetermined value (percentage) according to the conveyance speed. That is, even if the conveyance speed is low, the conveyance correction coefficient b causes ferrite + pearlite structure or bainite (Zw) to be formed over a range of 5 mm or more from the surface layer of the slab S. This is a parameter for setting the tertiary cooling water amount Q ′ so as not to occur. A conveyance correction coefficient b of “100%” indicates that there is no need to reduce the amount of water.
The average casting speed shown in the step (5) is the speed of the slab until the predetermined slab S is cast by the continuous casting apparatus 1 and cut. For example, the average casting speed refers to the elapsed time until the slab (molten steel) S located at the meniscus in the mold 5 enters the gas cutter 10, the length of the machine body of the continuous casting apparatus 1 (from the upper end of the mold 5 to the gas cutter 10. It is a value obtained by dividing by (distance to). In other words, the molten steel located in the upper part of the mold 5 moves while being gradually cooled to become a slab S, and a value obtained by dividing the elapsed time until reaching the gas cutter 10 by the length of the machine body is defined as the average casting speed.

  When the average casting speed of the slab S is slow, the slab S is excessively cooled in the continuous casting apparatus 1. Therefore, after setting the casting speed correction coefficient c based on the average casting speed, the tertiary cooling water quantity Q 'at the unsteady part is obtained by multiplying the tertiary cooling water quantity Q by the casting speed correction coefficient c. Table 9 shows a casting correction table (list) showing the relationship between the average casting speed and the casting speed correction coefficient c. It is a numerical value obtained by this casting correction table experiment and operation.

  The casting speed correction coefficient c is represented by a predetermined value (percentage) according to the average casting speed. That is, the casting speed correction coefficient c allows the ferrite + pearlite structure or bainite (Zw) to be formed over the range of 5 mm or more from the surface layer of the slab S even if the average casting speed is slow, and further supercooling. This is a parameter for setting the amount of tertiary cooling water Q ′ so that cracking does not occur. A casting speed correction coefficient c of “100%” indicates that there is no need to reduce the amount of water.

  As described above, after setting the stop correction coefficient a, the conveyance correction coefficient b, and the casting speed correction coefficient c, in the step (6), “Q ′ = Q × a × b × c” The tertiary cooling water amount Q ′ at the steady portion is obtained. In the step (6), the tertiary cooling water amount Q corresponding to the steady part is multiplied by a correction coefficient (stop correction coefficient a, conveyance correction coefficient b, casting speed correction coefficient c), so that the tertiary cooling water amount in the unsteady part is obtained. Q 'is being sought.

Thus, after calculating | requiring the tertiary cooling water amount Q 'of an unsteady site | part, tertiary cooling is performed by the tertiary cooling water amount Q' with respect to an unsteady site | part.
Tables 10 to 45 summarize the results of the tertiary cooling by the method for cooling a slab of carbon steel of the present invention and the results of the tertiary cooling by a method different from the method of the present invention. In addition, implementation conditions, such as a continuous casting apparatus and a cooling nozzle, are the same as the conditions (conditions shown in Examples and Comparative Examples) shown in the first embodiment. Tables 10 to 22 show the results of the tertiary cooling on the upper surface of the slab. Tables 23 to 33 show the results of the tertiary cooling in the narrow surface of the slab. Tables 34 to 45 show the results of tertiary cooling on the lower surface of the slab.

  As shown in FIG. 10, in S1 to S3, the flow rate of the slab (the amount of tertiary cooling water Q) in the steady part is obtained. In S4, the stop correction coefficient a is set for each surface. In S5, the conveyance correction coefficient b is set for each surface. In S6, the casting speed correction coefficient c is set for each surface. In S7, the flow rate of the slab (the amount of tertiary cooling water Q ') in the unsteady part is obtained. In S8, the presence or absence of a slab is detected at the entrance of the tertiary cooling zone, and when the slab S is present, tertiary cooling is performed with the tertiary cooling water amount Q in the steady portion, and tertiary cooling is performed with the tertiary cooling water amount Q ′ in the unsteady portion. I do.

  The slab drawing stop time was the time between the pre-charge and the post-charge. As the conveyance speed, the actual value from the slab S to the tertiary cooling zone 11 from the gas cutter 10 was used. As the average casting speed, the actual value during casting of the slab S was used. The conveyance speed and the average casting speed were performed every predetermined time (for example, 1 second). Specifically, the conveyance speed was calculated from the rotation speed of the conveyance roll 8 installed on the downstream side of the gas cutter 10, and the average casting speed was calculated from the rotation speed of the pinch roll 9 installed on the upstream side of the gas cutter 10.

  In Tables 1 to 45, the tertiary cooling of the stationary part was performed by setting the tertiary cooling water amount Q so as to satisfy the condition (1) and the condition (2), similarly to the first embodiment, and performing the tertiary cooling ( (Cooling speed judgment column “◯”). Further, in the tertiary cooling of the unsteady part, when the stop correction coefficient a, the conveyance correction coefficient b and the casting speed correction coefficient c are appropriately set and the tertiary cooling is performed with the tertiary cooling water amount Q ′ of the unsteady part, There was no stress cracking and surface cracking did not occur (flaw detection test “◯”).

For example, in the experiment (additional number) 1, the stop correction coefficient a is set to 100% (correction rate column) with respect to the drawing stop time of 1.0 min, and the stop correction coefficient a is shown in the stop correction table. It is set appropriately according to the value. The conveyance correction coefficient b is set to 100% (correction rate column) for an average conveyance speed of 1.0 m / min, and the conveyance correction coefficient b is appropriate according to the value shown in the conveyance correction table. Is set. Further, the casting speed correction coefficient c is set to 100% (correction rate column) for an average casting speed of 1.0 m / min, and the casting speed correction coefficient c is set to a value indicated by a casting correction table. Appropriately set accordingly. Then, using these values, the tertiary cooling water amount Q ′ of the unsteady part is obtained, and the tertiary cooling is performed with the tertiary cooling water amount Q ′ (column of corrected water amount). As a result, the temperature difference ΔT could be reduced to 130 ° C. or less in all the sections, and there was no thermal stress cracking of the slab and no surface cracking occurred.

  On the other hand, when the tertiary cooling water amount Q ′ was not properly set in the unsteady portion, thermal stress cracking of the slab or surface cracking occurred. For example, in Experiment (Additional Number) 2, the stop correction coefficient a is set to 100% regardless of the drawing stop time of 6.0 min. That is, the amount of tertiary cooling water is not corrected in spite of a long drawing stop time. In the experiment (additional number) 11, the conveyance correction coefficient b is set to 100% with respect to the average conveyance speed of 0.85 m / min. That is, the tertiary cooling water amount is not corrected although the average transport speed is low. In the experiment (additional number) 20, the casting speed correction coefficient c is set to 100% with respect to the average casting speed of 0.90 m / min. That is, although the average casting speed is low, the amount of tertiary cooling water is not corrected.

  As described above, according to the second embodiment, the stop correction coefficient a based on the slab drawing stop time is set, the transport correction coefficient b based on the slab transport speed is set, and the average casting speed in the continuous casting apparatus is set. A casting speed correction coefficient c based on the above is set. After that, the tertiary cooling water amount Q is corrected by the stop correction coefficient a, the conveyance correction coefficient b, and the casting speed correction coefficient c to obtain the tertiary cooling water amount Q ′ in the unsteady part, and the tertiary cooling water quantity Q for the unsteady part. Because of the third cooling, the structure of the slab was made fine and the cracks could be prevented.

  It should be noted that matters not explicitly disclosed in the embodiment disclosed this time, such as operating conditions and operating conditions, various parameters, dimensions, weights, volumes, and the like of a component, deviate from the range normally practiced by those skilled in the art. However, matters that can be easily assumed by those skilled in the art are employed.

DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 2 Molten steel 3 Ladle 4 Tundish 5 Mold 6 Support roll 8 Conveyance roll 9 Pinch roll 10 Cutting apparatus 11 Tertiary cooling apparatus 12 Conveyance apparatus 13 Cooling nozzle 13a Upper cooling nozzle 13b Lower cooling nozzle 13c Narrow surface cooling nozzle S Slab

Claims (2)

As components, C = 0.13% to 0.18% (mass%, the same applies hereinafter), Si = 0.15% to 0.35%, Mn = 0.30% to 0.60%, P ≦ 0. A carbon steel slab comprising 030%, S ≦ 0.035%, Cr ≦ 0.15%, the balance Fe and unavoidable impurities is subjected to tertiary cooling so as to satisfy the following conditions. A method for cooling the slab.
(1) ΔT ≦ 160 ° C., where ΔT is the temperature difference that is the difference between the maximum temperature and the minimum temperature in the surface temperature distribution in the slab width direction.
(2) As a cooling rate for cooling the slab surface temperature from 800 ° C. to 500 ° C., when the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more, 699 ° C. to 600 ° C. Then, the cooling rate is set to 15 ° C./min or more at a cooling rate of 20 ° C./min or more at 599 ° C. to 500 ° C. As components, C = 0.13% to 0.18% (mass%, the same applies hereinafter), Si = 0.15% to 0.35%, Mn = 0.30% to 0.60%, P ≦ 0. In the third cooling of the slab of carbon steel composed of 030%, S ≦ 0.035%, Cr ≦ 0.15%, the balance Fe and inevitable impurities,
Under the conditions of (1) and (2) below, the amount of tertiary cooling water Q in the steady part of the slab is obtained,
(1) ΔT ≦ 160 ° C., where ΔT is the temperature difference that is the difference between the maximum temperature and the minimum temperature in the surface temperature distribution in the slab width direction.
(2) As a cooling rate for cooling the slab surface temperature from 800 ° C. to 500 ° C., when the pre-cooling temperature of the slab surface is 800 ° C. to 700 ° C., the cooling rate is 25 ° C./min or more, 699 ° C. to 600 ° C. Then, the cooling rate is set to 15 ° C./min or more at a cooling rate of 20 ° C./min or more at 599 ° C. to 500 ° C.
According to the following (3) to (6), the amount of tertiary cooling water Q ′ in the unsteady part of the slab is obtained,
(3) A stop correction coefficient a based on the slab drawing stop time is set.
(4) The conveyance correction coefficient b based on the slab conveyance speed is set.
(5) A casting speed correction coefficient c based on the average casting speed in the continuous casting apparatus is set.
(6) Q ′ = Q × a × b × c
A cooling method for a slab of carbon steel, wherein the stationary part is subjected to tertiary cooling with a tertiary cooling water amount Q, and the unsteady part is subjected to tertiary cooling with a tertiary cooling water amount Q ′. .