WO2016031169A1

WO2016031169A1 - Thick steel plate manufacturing method

Info

Publication number: WO2016031169A1
Application number: PCT/JP2015/004056
Authority: WO
Inventors: 雄太田村; 安達　健二
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2014-08-26
Filing date: 2015-08-14
Publication date: 2016-03-03
Also published as: KR101940429B1; JP6108041B2; EP3187275B1; CN106604785A; BR112017004022A2; EP3187275A1; BR112017004022B1; CN106604785B; EP3187275A4; KR20170033423A; JPWO2016031169A1

Abstract

The objective is to provide a thick steel plate manufacturing method with which a high-quality thick steel plate with minimal variation in material quality can be obtained. This thick steel plate manufacturing method, which manufactures a thick steel plate by means of a hot-rolling process, a heated correction process, and an accelerated cooling process, in this order, is characterized by having between the heated correction process and the accelerated cooling process a descaling process in which descaling water is sprayed twice, with the total energy density of the descaling water sprayed on the surface of the thick steel plate in the two sprayings in the descaling process being 0.07 J/mm² or greater, the second spraying of descaling water occurring no less than 0.5s after the first spraying of descaling water, and the surface temperature of the steel plate immediately before the second spraying of descaling water being no greater than the Ar₃ transformation point.

Description

Thick steel plate manufacturing method

The present invention relates to a method for manufacturing a thick steel plate.

In the process of manufacturing thick steel plates by hot rolling, the application of controlled cooling is expanding. For example, as shown in FIG. 1, after the thick steel plate (not shown) is reheated in the heating furnace 1, the thick steel plate is descaled in the descaling device 2. The thick steel plate is rolled by the rolling mill 3 and then corrected by the shape correcting device 4, and then controlled cooling by water cooling or air cooling is performed in the accelerated cooling device 5. In addition, the arrow in a figure is the advancing direction of a thick steel plate.

When water-cooling a thick steel plate with an accelerated cooling device, it is known that the cooling time increases as the scale on the surface of the thick steel plate increases as shown in FIG. However, if the scale thickness varies, the cooling rate becomes non-uniform, which causes a problem that materials such as strength and hardness vary.

Also, when the scale thickness is non-uniform, the cooling rate becomes non-uniform as described above. In such a case, it is known that the distribution of the steel plate surface temperature (hereinafter referred to as “cooling stop temperature”) at the time of accelerated cooling stop in the thick steel plate width direction varies as shown in FIG. Thus, since the cooling stop temperature of the thick steel plate varies, there is a problem that a uniform material cannot be obtained. When a specific example is shown, when the place where scale thickness is 40 micrometers and 20 micrometers coexists in the thickness direction of a thick steel plate, the cooling stop temperature when cooling a thick steel plate with a thickness of 25 mm from 800 ° C. to a target temperature of 500 ° C. is 40 μm. It becomes 460 degreeC in a location, and 500 degreeC in a location of 20 micrometers. At the 40 μm portion, the cooling stop temperature falls below 40 ° C. from the target temperature, and as a result, a uniform material cannot be obtained.

Therefore, Patent Document 1 discloses a method for achieving uniform cooling stop temperature by controlling the scale thickness to equalize the cooling rate. In Patent Document 1, using a descaling device provided before and after the rolling mill during rolling, when the cooling stop temperature of the tail end of the thick steel plate is lower than that of the tip, the descaling injection on the tail end side is performed. The amount of water is controlled to be larger than the amount of water jetted on the tip side. In this way, by controlling the scale removal rate and the remaining thickness in the longitudinal direction of the thick steel plate, the heat transfer coefficient of the steel plate surface during controlled cooling is changed, and the cooling stop temperature in the longitudinal direction of the thick steel plate is made uniform. .

JP-A-6-330155

In the conventional technology, the cooling stop temperature has been made uniform by adjusting the amount of cooling water and the conveyance speed. However, in this method, since the cooling rate varies due to the variation in scale thickness, it is difficult not only to make the cooling rate uniform, but also to make the cooling stop temperature uniform.

Also, in the method of Patent Document 1, since the heat transfer coefficient cannot be controlled unless the scale removal rate and the remaining thickness can be controlled online, it is not possible to achieve a highly uniform cooling rate. Further, when the scale removal rate is changed, the cooling stop temperature is different between the remaining scale portion and the peeled portion, so that the material varies.

An object of the present invention is to provide a method of manufacturing a thick steel plate that can solve the above-described problems and can secure a high-quality thick steel plate with less material variation.

The present invention has been made to solve the above-mentioned conventional problems, and the gist thereof is as follows.
[1] In a method of manufacturing a thick steel plate in the order of a hot rolling step, a hot straightening step, and an accelerated cooling step, descaled water is injected twice between the hot straightening step and the accelerated cooling step. A scaling step, and in the descaling step, the energy density of descaling water sprayed on the surface of the thick steel plate is 0.07 J / mm ² or more in total of the ^two sprays, and the first descaling water is Production of a thick steel plate, characterized in that the second descaling water is jetted 0.5 s or more after the jetting, and the steel sheet surface temperature immediately before the second descaling water jet is set to the Ar ₃ transformation point or less. Method.
[2] In the method of manufacturing a thick steel plate in the order of the hot rolling step, the hot straightening step, and the accelerated cooling step, the descaling water is injected twice or more between the hot straightening step and the accelerated cooling step. A descaling step, wherein in the descaling step, the energy density of descaling water sprayed on the surface of the thick steel plate is 0.07 J / mm ² or more in total of ^two or more sprays, A thick steel plate characterized in that the final descaling water is jetted 0.5 s or more after the scaling water is jetted, and the steel sheet surface temperature immediately before the final descaling water jet is set to the Ar ₃ transformation point or less. Production method.
[3] In the method for producing a thick steel plate according to [1] or [2], if the thick steel plate temperature before cooling is T [K], the time from the end of the descaling step to the start of the accelerated cooling step t [s] satisfies the formula t ≦ 5 × 10 ⁻⁹ × exp (25000 / T).

According to the present invention, the cooling rate and the cooling stop temperature can be made uniform. As a result, it is possible to manufacture a high-quality thick steel plate with little material variation.

FIG. 1 is a schematic view showing a conventional equipment for producing thick steel plates. FIG. 2 is a diagram showing the relationship among scale thickness, cooling time, and thick steel plate surface temperature during accelerated cooling. FIG. 3 is a diagram showing the relationship between the position in the width direction of the thick steel plate and the cooling stop temperature after accelerated cooling. FIG. 4 is a schematic view showing a thick steel plate manufacturing facility according to an embodiment of the present invention. FIG. 5 is a schematic diagram showing the arrangement relationship of the spray nozzles of the descaling device, (a) is a schematic diagram showing the positional relationship of the spray nozzles, and (b) is a schematic diagram showing a spray pattern. FIG. 6 is a diagram illustrating the relationship between the energy density of descaling water and the scale peeling rate. FIG. 7 is a diagram showing the temperature history of the thick steel plate at each time of the descaling process. FIG. 8 is a transformation diagram of the thick steel plate from the first descaling to the second descaling. FIG. 9 is a side view of the accelerated cooling apparatus according to the embodiment of the present invention. FIG. 10 is a side view of another accelerated cooling apparatus according to an embodiment of the present invention. FIG. 11 is a diagram for explaining an example of the nozzle arrangement of the partition wall according to the embodiment of the present invention. FIG. 12 is a diagram for explaining the flow of the cooling drainage on the partition wall. FIG. 13 is a diagram for explaining another flow of the cooling drainage on the partition wall. FIG. 14 is a diagram for explaining a temperature distribution in the width direction of a thick steel plate of a conventional example. FIG. 15 is a diagram illustrating the flow of cooling water in the acceleration cooling device. FIG. 16 is a diagram for explaining non-interference with cooling water on the partition wall in the accelerated cooling device.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 4 is a schematic view showing a thick steel plate manufacturing facility according to an embodiment of the present invention. In FIG. 4, an arrow is a conveyance direction of a thick steel plate. From the upstream side in the conveying direction of the thick steel plate, the heating furnace 1, the descaling device 2, the rolling mill 3, the shape correcting device 4, the descaling device 6, the descaling device 7, and the accelerated cooling device 5 are arranged in this order. In FIG. 4, after the thick steel plate (not shown) is reheated in the heating furnace 1, the thick steel plate is descaled in the descaling device 2 to remove the primary scale. The thick steel plate is hot-rolled by the rolling mill 3 and corrected by the shape correcting device 4, and then descaling is performed to completely remove the scale in the descaling device 6 and the descaling device 7. Then, controlled cooling by water cooling or air cooling is performed in the acceleration cooling device 5.

In the present invention, two descaling devices, that is, a descaling device 6 and a descaling device 7 are arranged between the shape correction device 4 and the acceleration cooling device 5. The descaling device shown in FIG. 4 has only two columns. In addition, you may comprise in 3 or more rows. As shown in FIG. 4, when the descaling device has two rows, the energy density of descaling water sprayed from the descaling device 6 and the descaling device 7 onto the surface of the steel plate is the sum of the two rows of spray nozzles. 0.07 J / mm ² or more, and after descaling water injection from the descaling device 6, descaling water is jetted from the descaling device 7 after 0.5 s or more, and immediately before descaling water injection from the descaling device 7. The steel sheet surface temperature is not more than the Ar ₃ transformation point. When the descaling device has three or more rows, the total of the spray nozzles of all the descaling device rows to be configured is 0.07 J / mm ² or more, and after the descaling water jet from the last descaling device immediately before After 0.5 s or more, the final descaling water is injected, and the surface temperature of the steel plate immediately before the final descaling water injection is set to the Ar ₃ transformation point or less. By doing so, the scale can be completely removed and uniform cooling can be realized.

In the present invention, for example, as shown in FIG. 5A, two rows of deske headers 6-1 of the descaling device 6 and deske headers 7-1 of the descaling device 7 are arranged in the longitudinal direction of the thick steel plate. . Descaling water is sprayed onto the thick steel plate 1 from a plurality of spray nozzles 6-2 and 7-2 provided on the desk header, and a spray pattern 22 as shown in FIG. 6B is formed. In order to prevent the splash water of the descaling water of the second row descaling device 7 from interfering with the descaling water of the first row descaling device 6, the arrangement of the injection nozzles 6-2 and 7-2 is arranged. As a relationship, it is desirable that the distance is 500 mm or more in the longitudinal direction of the thick steel plate, that is, the conveying direction of the steel plate. Further, it is preferable that the ejection pattern in the width direction is a staggered arrangement in which the ejection nozzle 6-2 and the ejection nozzle 7-2 are shifted in the width direction. There are two columns of the descaling apparatus shown in FIG. The same effect can be obtained with three or more rows. Even in the case where there are three or more descaling devices, it is preferable that the nozzle rows are spaced apart by 500 mm or more in the longitudinal direction to form a staggered arrangement, as in the case of two descaling devices. Here, since the above-mentioned effect is saturated when it exceeds 3 rows, the upper limit is preferably 3 rows.

At the time of descaling, the scale surface is cooled by the descaling water, so that thermal stress is generated on the scale and the impact force by the descaling water acts. As a result, the scale is removed by peeling or breaking. When the present inventors diligently studied, the effect of the thermal stress generated at the time of descaling is obtained twice or more by performing descaling twice or more between the hot shape correction process and the accelerated cooling process. Can do. The relationship between the energy density and the scale peeling rate (the ratio of the area where the scale peels and the steel sheet area) is specifically as “no transformation” in FIG.

Furthermore, as shown in “with transformation” in FIG. 6, the energy density of descaling water sprayed on the surface of the thick steel plate is set to 0.07 J / mm ² or more in total of ^two sprays, and the descaling device 6 After the descaling water is injected on the surface of the thick steel plate, the descaling water is injected from the descaling device 7 onto the surface of the thick steel plate after 0.5 s or more, and the steel plate surface temperature at the start of the descaling water injection from the descaling device is By setting the Ar ₃ transformation point or less, the scale can be removed more efficiently. The effect that the scale can be removed more efficiently by setting the steel sheet surface temperature at the start of descaling water injection below the Ar ₃ transformation point is confirmed even when the descaling water injection number is 3 times or more. It was done. Here, the total energy density of the two times of descaling can be calculated by summing up the energy density of each time of descaling calculated by the formula described later. The Ar ₃ transformation point can be calculated by the following formula (*).
Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo (*)
However, the element symbol indicates the content (mass%) of each element in steel, and 0 if not contained.

When the present inventors investigated, the energy density of descaling water sprayed on the surface of a thick steel plate was set to 0.07 J / mm ² or more in total of ^two or more sprays, and immediately before the final descaling water spray The steel plate surface temperature can be lowered below the Ar ₃ transformation point, and the steel plate surface can be transformed. Due to the transformation of the base iron, a deviation occurs at the interface between the scale and the base iron, reducing the scale adhesion. Descaling by scaling becomes easy, and descaling is possible with descaling water with a smaller energy density.

The temperature history of the

descaling devices

6 and 7 during the descaling water injection is as shown in FIG. Since the outermost layer part of the iron base is supercooled and the transformation is promoted, even if the holding time below the Ar ₃ transformation point is a very short time of 1 s or less, only several tens of μm of the outermost surface layer of the iron iron undergoes ferrite transformation. Occur. In addition, when the present inventors investigated whether or not the ferrite transformation of the outermost layer portion of the ground iron was changed by variously changing the time of the descaling water injection of the first descaling and the second descaling, as shown in FIG. I understood it. The steel plate surface temperature at the start of the descaling water injection in the second descaling is below the Ar ₃ transformation point, and the time from the first descaling to the second descaling being 0.5 s or more For example, the ferrite transformation occurs in the outermost layer of the steel. Since the transformation occurs only in the tens of μm of the outermost surface layer of the ground iron, the scale can be easily peeled off by scaling without affecting the material such as strength.

Therefore, the time from the first descaling water injection to the second descaling water injection is 0.5 s or more, and the steel sheet surface temperature immediately before the descaling water injection in the second descaling is below the Ar ₃ transformation point. If it exists, the scale peeling effect in the second descaling is improved, and the energy of the descaling water at the time of descaling necessary for the scale peeling is reduced.

Similarly, when the number of times of descaling water injection is 3 times or more, the time from the last descaling water injection immediately before the final time to the final descaling water injection time is 0.5 s or more. If the steel sheet surface temperature is below the Ar ₃ transformation point, the scale peeling effect in final descaling is improved, and the energy of descaling water at the time of descaling necessary for scale peeling is reduced.

The inventors also examined the energy density of the first descaling by the descaling device 6 and the energy density of the second descaling by the descaling device 7. As described above, in the case where the iron surface layer undergoes ferrite transformation before the second descaling water is jetted by the first descaling, the scale peeling effect by the second descaling is improved. Therefore, the scale can be peeled off more efficiently by supplying the energy necessary for the transformation of the surface layer of the ground metal at the first time and descaling at a larger energy density at the second time. Specifically, it is preferable to set the energy density of the first descaling to 0.02 J / mm ² or more. If it is smaller than this, in order to transform the surface layer by the first descaling water cooling, it is necessary to cool the steel plate before descaling, for example, by lowering the steel plate temperature before starting descaling. There is no upper limit of the energy density of descaling water as descaling capability. However, if the sum of the two times is 0.7 J / mm ² or more, the discharge pressure of the pump becomes enormous, and therefore it is preferably 0.7 J / mm ² or less.

If the steel plate surface temperature at the second descaling is higher than the Ar ₃ transformation point, or if the time from the first descaling to the second descaling is less than 0.5 s, before the second descaling No ferrite transformation occurs and no improvement in scale peelability can be expected due to the transformation.

From this relationship, descaling is performed twice or more, and even if the total energy density is 0.07 J / mm ² or more, if transformation has not occurred before the second descaling water injection, Some scale remains, the cooling stop temperature varies, and the material becomes non-uniform.

Even when the number of descaling is 3 times or more, it is preferable that the energy density of the last descaling is 0.02 J / mm ² or more, as in the case of 2 times of descaling. The total energy density of descaling water in the total number of times is preferably 0.7 J / mm ² or less.

Here, the energy density E (J / mm ² ) of descaling water sprayed on the thick steel plate is an index of the ability to remove the scale by descaling and is defined as the following equation (1). .
E = Qρv ² t ÷ ( ² dW) (1)
However, Q: Descaling water injection flow rate [m ³ / s], d: Flat nozzle spray injection thickness [mm], W: Flat nozzle spray injection width [mm], Fluid density ρ [kg / m ³ ] The fluid velocity v [m / s] at the time of collision of the thick steel plate and the collision time t [s] (t = d / 1000 V, transport velocity V [m / s]).

However, since it is not always easy to measure the fluid velocity v at the time of collision with a thick steel plate, a great deal of labor is required to accurately determine the energy density E defined by the equation (1).

Therefore, as a result of further studies, the present inventors adopted water density × injection pressure × collision time as a simple definition of the energy density E (J / mm ² ) of descaling water injected into the thick steel plate. I found out that I should do. The water density (m ³ / (mm ² · min)) is a value calculated by “descaled water injection flow rate ÷ descaling water collision area”. The injection pressure (N / m ² = Pa) is defined by the discharge pressure of descaling water. The collision time (s) is a value calculated by “descaled water collision thickness ÷ thick steel plate conveyance speed”. In addition, the relationship between the energy density of the high pressure water of this invention calculated by this simple definition and a scale peeling rate is the same as that of FIG.

By the way, regarding the scale of the surface of the thick steel plate that affects the stability during cooling of the thick steel plate by the accelerated cooling device 5, the growth of the scale of the thick steel plate can be generally organized by diffusion rate control. It is known that
ξ ² = a × exp (−Q / RT) × t (2)
Where ξ: scale thickness, a: constant, Q: activation energy, R: constant, T: thick steel plate temperature [K] before cooling, t: time.

Therefore, in consideration of the scale growth after descaling by the

descaling devices

6 and 7, a scale growth simulation experiment is performed at various temperatures and times, and the constant of the above equation (2) is experimentally derived. The thickness and cooling stability were studied earnestly. As a result, it was found that cooling was stable when the scale thickness was 15 μm or less, more stable when the scale thickness was 10 μm or less, and very stable when the scale thickness was 5 μm or less.

When the scale thickness is 15 μm or less, the following formula (3) can be derived based on the above formula (2). That is, when the time t [s] from the end of descaling of the thick steel plate by the

descaling devices

6 and 7 to the start of cooling the thick steel plate by the accelerated cooling device 5 satisfies the following equation (3): Cooling by the acceleration cooling device 5 is stabilized.
t ≦ 5 × 10 ⁻⁹ × exp (25000 / T) (3)
However, T: Thick steel plate temperature [K] before cooling.

When the scale thickness is 10 μm or less, the following formula (4) can be derived based on the above formula (2). That is, when the time t [s] from the end of removal of the scale of the thick steel plate by the

descaling devices

6 and 7 to the start of cooling of the thick steel plate by the accelerated cooling device 5 satisfies the following equation (4): The cooling by the acceleration cooling device 5 is more stable.
t ≦ 2.2 × 10 ⁻⁹ × exp (25000 / T) (4)
Furthermore, when the scale thickness is 5 μm or less, the following formula (5) can be derived based on the above formula (2). That is, when the time t [s] from the end of descaling of the thick steel plate by the

descaling devices

6 and 7 to the start of cooling the thick steel plate by the accelerated cooling device 5 satisfies the following equation (5): Cooling by the acceleration cooling device 5 is very stable.
t ≦ 5.6 × 10 ⁻¹⁰ × exp (25000 / T) (5)
The accelerated cooling device 5 of the present invention will be described. As shown in FIG. 9, the upper surface cooling facility of the accelerated cooling device 5 of the present invention includes an upper header 11 that supplies cooling water to the upper surface of the thick steel plate 10, and cooling that jets rod-shaped cooling water suspended from the upper header 11. The water injection nozzle 13 and the partition 15 installed between the thick steel plate 10 and the upper header 11 are provided. The partition wall 15 is provided with a plurality of water supply ports 16 for inserting the lower end portion of the cooling water injection nozzle 13 and drain ports 17 for draining the cooling water supplied to the upper surface of the thick steel plate 10 onto the partition wall 15. Preferably it is.

Specifically, the upper surface cooling facility includes an upper header 11 for supplying cooling water to the upper surface of the thick steel plate 10, a cooling water injection nozzle 13 suspended from the upper header 11, and the upper header 11 and the thick steel plate 10. And a partition wall 15 having a large number of through-holes (water supply port 16 and drain port 17) installed horizontally in the width direction of the thick steel plate. The cooling water injection nozzle 13 is a circular pipe nozzle that injects rod-shaped cooling water, and its tip is inserted into a through-hole (water supply port 16) provided in the partition wall 15 and above the lower end portion of the partition wall 15. It is installed to become. The cooling water injection nozzle 13 may be inserted into the upper header 11 so that the upper end of the cooling water injection nozzle 13 protrudes into the upper header 11 in order to prevent the foreign matter at the bottom in the upper header 11 from being sucked and clogged. preferable.

Here, the rod-shaped cooling water in the present invention is cooling water injected in a state of being pressurized to some extent from a circular (including elliptical or polygonal) nozzle outlet, and is cooled from the nozzle outlet. The water injection speed is 6 m / s or more, preferably 8 m / s or more, and the water flow jetted from the nozzle outlet has a continuous circular shape, and the water flow has a continuous and straight flow. Say. That is, it is different from a free fall flow from a circular tube laminar nozzle or a liquid ejected in a droplet state such as a spray.

The reason why the tip of the cooling water spray nozzle 13 is inserted into the through hole and is located above the lower end of the partition wall 15 is that the partition wall 15 is inserted even when a thick steel plate whose tip is warped upward enters. This is to prevent the cooling water injection nozzle 13 from being damaged. As a result, the cooling water injection nozzle 13 can be cooled for a long period of time in a good state, so that it is possible to prevent the occurrence of temperature unevenness in the thick steel plate without repairing the equipment.

Further, since the tip of the circular tube nozzle 13 is inserted into the through hole, as shown in FIG. 16, there is no interference with the flow in the width direction of the drained water indicated by the dotted arrow flowing through the upper surface of the partition wall 15. Therefore, the cooling water jetted from the cooling water jet nozzle 13 can reach the upper surface of the thick steel plate equally regardless of the position in the width direction, and uniform cooling in the width direction can be performed.

As an example of the partition wall 15, as shown in FIG. 11, a large number of through-holes having a diameter of 10 mm are opened in a grid pattern at a pitch of 80 mm in the thick steel plate width direction and 80 mm in the transport direction. A cooling water injection nozzle 13 having an outer diameter of 8 mm, an inner diameter of 3 mm, and a length of 140 mm is inserted into the water supply port 16. The cooling water injection nozzles 13 are arranged in a staggered pattern, and the through holes through which the cooling water injection nozzles 13 do not pass serve as cooling water drains 17. As described above, the large number of through holes provided in the partition wall 15 of the accelerated cooling device of the present invention are composed of the substantially same number of water supply ports 16 and drain ports 17, and each share a role and function.

At this time, the total cross-sectional area of the drain port 17 is sufficiently larger than the total cross-sectional area of the inner diameter of the circular pipe nozzle 13 of the cooling water injection nozzle 13, and about 11 times the total cross-sectional area of the inner diameter of the circular pipe nozzle 13 is ensured. As shown in FIG. 9, the cooling water supplied to the upper surface of the thick steel plate is filled between the thick steel plate surface and the partition wall 15, led to the upper side of the partition wall 15 through the drain port 17, and quickly discharged. The FIG. 12 is a front view for explaining the flow of cooling drainage near the end in the width direction of the thick steel plate on the partition wall. The drainage direction of the drainage port 17 is upward opposite to the cooling water injection direction, and the cooling drainage drained upward from the partition wall 15 changes the direction outward in the thick steel plate width direction, between the upper header 11 and the partition wall 15. It drains through the drainage channel.

On the other hand, in the example shown in FIG. 13, the drain port 17 is inclined in the thick steel plate width direction, and the slant direction is directed outward in the width direction so that the drain direction is directed outward in the thick steel plate width direction. By doing in this way, the flow of the discharged water 19 on the partition wall 15 in the thickness direction of the steel plate becomes smooth and drainage is promoted, which is preferable.

Here, as shown in FIG. 14, when the drainage port and the water supply port are installed in the same through hole, the cooling water does not easily escape above the partition wall 15 after colliding with the steel plate, and the steel plate 10. And the partition wall 15 flow toward the end in the width direction of the thick steel plate. Then, since the flow rate of the cooling drainage between the thick steel plate 10 and the partition wall 15 increases as it approaches the end in the plate width direction, the force that the jet cooling water 18 penetrates the staying water film and reaches the thick steel plate is the plate width. The direction end portion is inhibited.

In the case of a thin steel plate, the influence is limited because the plate width is about 2 m at most. However, the influence cannot be ignored especially in the case of a thick steel plate having a plate width of 3 m or more. Accordingly, the cooling at the end in the width direction of the thick steel plate is weakened, and the temperature distribution in the width direction of the thick steel plate in this case becomes a non-uniform temperature distribution.

On the other hand, in the accelerated cooling device 5 of the present invention, as shown in FIG. 15, the water supply port 16 and the water discharge port 17 are provided separately and share the roles of water supply and water discharge. 15 flows smoothly through the drainage port 17 and above the partition wall 15. Accordingly, since the drainage after cooling is quickly removed from the upper surface of the thick steel plate, the cooling water supplied subsequently can easily penetrate the staying water film, and a sufficient cooling capacity can be obtained. In this case, the temperature distribution in the width direction of the thick steel plate is a uniform temperature distribution, and a uniform temperature distribution in the width direction can be obtained.

Incidentally, if the total cross-sectional area of the drain port 17 is 1.5 times or more the total cross-sectional area of the inner diameter of the circular tube nozzle 13, the cooling water is discharged quickly. This can be realized, for example, by making holes larger than the outer diameter of the circular tube nozzle 13 in the partition wall 15 and making the number of drain ports equal to or greater than the number of water supply ports.

If the total cross-sectional area of the drain port 17 is smaller than 1.5 times the total cross-sectional area of the inner diameter portion of the circular tube nozzle 13, the flow resistance of the drain port increases and the stagnant water becomes difficult to drain. The amount of cooling water that can penetrate and reach the surface of the thick steel plate is greatly reduced, and the cooling ability is lowered, which is not preferable. More preferably, it is 4 times or more. On the other hand, if there are too many drain openings or the sectional diameter of the drain openings becomes too large, the rigidity of the partition wall 15 becomes small, and it becomes easy to be damaged when a thick steel plate collides. Therefore, the ratio of the total cross-sectional area of the drain outlet and the total cross-sectional area of the inner diameter of the circular tube nozzle 13 is preferably in the range of 1.5 to 20.

Further, it is desirable that the gap between the outer peripheral surface of the circular tube nozzle 13 inserted in the water supply port 16 of the partition wall 15 and the inner surface of the water supply port 16 be 3 mm or less. If this gap is large, the cooling drainage discharged to the upper surface of the partition wall 15 is drawn into the gap between the outer peripheral surface of the circular pipe nozzle 13 of the water supply port 16 due to the influence of the accompanying flow of the cooling water injected from the circular pipe nozzle 13. As a result, the steel sheet is again supplied onto the thick steel plate, resulting in poor cooling efficiency. In order to prevent this, it is more preferable that the outer diameter of the circular tube nozzle 13 is substantially the same as the size of the water supply port 16. However, in consideration of machining accuracy and mounting errors, a gap of up to 3 mm that has substantially little influence is allowed. More preferably, it is 2 mm or less.

Furthermore, in order to allow the cooling water to penetrate the staying water film and reach the thick steel plate, it is necessary to optimize the inner diameter and length of the circular tube nozzle 13, the injection speed of the cooling water and the nozzle distance.

That is, the nozzle inner diameter is preferably 3 to 8 mm. If it is smaller than 3 mm, the bundle of water sprayed from the nozzle becomes thin and the momentum becomes weak. On the other hand, when the nozzle diameter exceeds 8 mm, the flow rate becomes slow, and the force penetrating the staying water film becomes weak.

The length of the circular tube nozzle 13 is preferably 120 to 240 mm. The length of the circular tube nozzle 13 here means the length from the inlet at the upper end of the nozzle that penetrates into the header to some extent to the lower end of the nozzle inserted into the water supply port of the partition wall. When the circular tube nozzle 13 is shorter than 120 mm, the distance between the lower surface of the header and the upper surface of the partition wall becomes too short (for example, the header thickness is 20 mm, the protrusion amount of the nozzle upper end into the header is 20 mm, and the insertion amount of the nozzle lower end into the partition wall is 10 mm. Therefore, the drainage space above the partition wall becomes small, and the cooling drainage cannot be discharged smoothly. On the other hand, if it is longer than 240 mm, the pressure loss of the circular tube nozzle 13 becomes large, and the force penetrating the staying water film becomes weak.

The jet speed of cooling water from the nozzle is required to be 6 m / s or more, preferably 8 m / s or more. This is because if it is less than 6 m / s, the force of the cooling water penetrating through the staying water film becomes extremely weak. If it is 8 m / s or more, a larger cooling capacity can be secured, which is preferable. The distance from the lower end of the cooling water spray nozzle 13 for upper surface cooling to the surface of the thick steel plate 10 is preferably 30 to 120 mm. If it is less than 30 mm, the frequency with which the thick steel plate 10 collides with the partition wall 15 becomes extremely high, and equipment maintenance becomes difficult. If it exceeds 120 mm, the force through which the cooling water penetrates the staying water film becomes extremely weak.

When cooling the upper surface of the thick steel plate, it is preferable to install draining rolls 20 before and after the upper header 11 so that the cooling water does not spread in the longitudinal direction of the thick steel plate. Thereby, the cooling zone length becomes constant and the temperature control becomes easy. Here, since the flow of the cooling water in the direction of transporting the thick steel plate is blocked by the draining roll 20, the cooling drainage flows outward in the width direction of the thick steel plate. However, the cooling water tends to stay in the vicinity of the draining roll 20.

Therefore, as shown in FIG. 10, among the rows of circular tube nozzles 13 aligned in the thick steel plate width direction, the cooling water jet nozzle in the uppermost stream side row in the thick steel plate transport direction is 15 to upstream in the thick steel plate transport direction. It is preferable that the cooling water jet nozzles at the most downstream side in the thick steel plate conveyance direction are inclined 15 to 60 degrees in the downstream direction in the thick steel plate conveyance direction. By carrying out like this, a cooling water can be supplied also to the position near the draining roll 20, a cooling water does not stay in the draining roll 20 vicinity, and it is suitable for a cooling efficiency to rise.

The distance between the lower surface of the upper header 11 and the upper surface of the partition wall 15 is such that the cross-sectional area in the width direction of the thick steel plate in the space surrounded by the lower surface of the header and the upper surface of the partition wall is 1.5 times the total cross-sectional area of the cooling water spray nozzle inner diameter. It is preferable to be provided, for example, about 100 mm or more is preferable. When the cross-sectional area of the thick steel plate in the width direction is not 1.5 times or more than the total cross-sectional area of the cooling water jet nozzle inner diameter, the cooling drainage discharged from the drain port 17 provided on the partition wall to the top surface of the partition wall 15 is smoothly thick. There is a possibility that it cannot be discharged in the width direction of the steel sheet.

In the accelerated cooling device of the present invention, the range of the water density that exhibits the most effect is 1.5 m ³ / (m ² · min) or more. When the water density is lower than this, the accumulated water film does not become so thick, and even if a known technique for cooling the thick steel plate by dropping the rod-shaped cooling water freely is applied, the temperature unevenness in the width direction does not become so large. In some cases. On the other hand, even when the water density is higher than 4.0 m ³ / (m ² · min), it is effective to use the technique of the present invention, but there are problems in practical use such as an increase in equipment cost. Therefore, 1.5 to 4.0 m ³ / (m ² · min) is the most practical water density.

The application of the cooling technique of the present invention is particularly effective when a draining roll is arranged before and after the cooling header. However, it is possible to apply even when there is no draining roll. For example, the header is relatively long in the longitudinal direction (when it is about 2 to 4 m), and it is applied to cooling equipment that sprays water spray for purging before and after the header to prevent water leakage to the non-water cooling zone. Is also possible.

In the present invention, the cooling device on the lower surface side of the thick steel plate is not particularly limited. In the embodiment shown in FIGS. 9 and 10, an example of the cooled header 12 including the circular tube nozzle 14 similar to the cooling device on the upper surface side is shown. However, in the cooling on the lower surface side of the thick steel plate, the injected cooling water naturally falls after colliding with the thick steel plate, so that there is no need for the partition wall 15 for discharging cooling drainage as in the upper surface side cooling in the thick steel plate width direction. Moreover, you may use the well-known technique which supplies film-form cooling water, spray-like spray cooling water, etc.

Note that the heating furnace 1 and the descaling device 2 of the present invention are not particularly limited, and conventional devices can be used. The descaling device 2 need not have the same configuration as the

descaling devices

6 and 7 of the present invention.

Hereinafter, examples of the present invention will be described. In the following description, the steel plate temperature is the temperature of the steel plate surface.

The thick steel plate of the present invention was manufactured using a thick steel plate manufacturing facility as shown in FIG. After the slab was reheated in the heating furnace 1, the primary scale was removed in the descaling device 2, hot rolled by the rolling mill 3, and the shape was corrected by the shape correcting device 4. After shape correction, descaling was performed. In the case of descaling after hot straightening, in the case of two times, two descaling devices of a descaling device 6 and a descaling device 7 were arranged, and descaling was performed twice on the surface of the thick steel plate. When the descaling is performed three times or more, the descaling devices are arranged in three or more rows, and the nozzle rows are spaced apart by 500 mm or more in the longitudinal direction to form a staggered arrangement. After the descaling was completed, controlled cooling of the thick steel plate was performed using the accelerated cooling device 5.

In both the descaling device 6 and the descaling device 7, the spray distance (the surface distance between the spray nozzle of the descaling device and the thick steel plate) was 130 mm, the nozzle spray angle was 66 °, and the angle of attack was 15 °. After descaling by the descaling device 7, it was cooled to 500 ° C. by the acceleration cooling device 5. Further, the nozzles of the descaling device 6 and the descaling device 7 are arranged in the width direction so that the ejection regions of adjacent nozzles overlap to some extent. The distance between the descaling device 6 and the descaling 7 was arranged at a distance of 1.1 m in the longitudinal direction. The nozzle was a flat spray nozzle. Here, the spray pressure of the descaling nozzle after hot rolling and the spray flow rate per nozzle were the same for both the descaling device 6 and the descaling device 7 and were performed under the conditions shown in Table 1. Further, the Ar ₃ transformation point of the used steel sheet was 780 ° C. The sheet thickness after rolling in the rolling mill 3 was 30 mm, and the steel plate temperature was 830 ° C. or 840 ° C.
The cooling conditions calculated from the above-described equations (3), (4), and (5) are as follows. After removing the scale of the thick steel plate by the descaling device, the cooling of the thick steel plate is started by the acceleration cooling device. The time t until it is 42 s or less, preferably 19 s or less, more preferably 5 s or less.

For the obtained thick steel plate, in order to obtain a thick steel plate with less material variation, a steel plate having a cooling stop temperature variation of 25 ° C. or less was accepted.

Manufacturing conditions and results are shown in Table 1. In Table 1, T is the thick steel plate temperature (K) before cooling.

In Invention Example 1, since the second descaling was performed after the surface of the thick steel plate was transformed from austenite to ferrite, the scale could be completely removed. The variation in the cooling stop temperature of Invention Example 1 (hereinafter simply referred to as temperature unevenness) was 15 ° C.

Also in Invention Example 2, since the second descaling was performed after the thick steel plate surface was transformed from austenite to ferrite, the scale could be completely removed. In particular, in Invention Example 2, since the time from the end of descaling to control cooling is as short as 3 seconds, the scale that grows between the end of scale removal and the start of cooling becomes thin. As a result, the cooling was more stable and the temperature unevenness was 10 ° C.

In Invention Example 3, since the third descaling was performed after the thick steel plate surface was transformed from austenite to ferrite, the scale could be completely removed. Since the time from the end of descaling to the start of control cooling is as short as 3 s, the scale that grows from the end of descaling to the start of cooling becomes thinner, resulting in more stable cooling and temperature unevenness of 10 ° C. It was.

In Invention Example 4, since the second descaling was performed after the thick steel plate surface was transformed from austenite to ferrite, the scale could be completely removed. The time from the end of descaling to the start of control cooling was 19 s, the scale grew from the end of scale removal to the start of cooling, and the temperature unevenness slightly increased to 18 ° C.

In Comparative Example 1, the time from the first to the second descaling is 0.52 s, the steel plate surface temperature at the second descaling is 779 ° C., and the second time after the thick steel plate surface is transformed from austenite to ferrite. Descaling is performed. However, since the total energy density was as small as 0.06 J / mm ² , the scale remained in a part of the steel sheet, and the temperature unevenness was 40 ° C.

In Comparative Example 2, the energy density was 0.07 J / mm ² . However, the steel plate surface temperature at the second descaling was 785 ° C. Since the second descaling was performed in a state where the thick steel plate surface was not transformed from austenite to ferrite, the scale remained in a part of the steel plate, and the temperature unevenness became 40 ° C.

In Comparative Example 3, the energy density was 0.07 J / mm ² . However, the time from the first to the second descaling was 0.48 s. Since the second descaling was performed in a state where the thick steel plate surface was not transformed from austenite to ferrite, the scale remained in a part of the steel plate, and the temperature unevenness became 40 ° C.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Descaling apparatus 3 Rolling mill 4 Shape correction apparatus 5 Acceleration cooling apparatus 6 Descaling apparatus 6-1 Deske header 6-2 Injection nozzle 7 Descaling apparatus 7-1 Deske header 7-2 Injection nozzle 10 Thick steel plate 11 Upper header 12 Lower header 13 Upper cooling water injection nozzle (circular tube nozzle)
14 Lower cooling water injection nozzle (circular tube nozzle)
15 Partition 16 Water supply port 17 Drain port 18 Injection cooling water 19 Drained water 20 Draining roll 21 Draining roll 22 Spray pattern

Claims

In the method of manufacturing a thick steel plate in the order of the hot rolling step, the hot straightening step, and the accelerated cooling step, a descaling step in which injection of descaling water is performed twice between the hot straightening step and the accelerated cooling step. In the descaling step, the energy density of descaling water sprayed on the surface of the thick steel plate is 0.07 J / mm 2 or more in total of the two sprays, and the first descaling water is sprayed. A method for producing a thick steel sheet, characterized in that the second descaling water is injected 0.5 s or more after the time and the steel sheet surface temperature immediately before the second descaling water injection is set to the Ar 3 transformation point or less.
In the method of manufacturing a thick steel plate in the order of a hot rolling step, a hot straightening step, and an accelerated cooling step, a descaling step in which injection of descaling water is performed twice or more between the hot straightening step and the accelerated cooling step. In the descaling step, the energy density of descaling water sprayed on the surface of the thick steel plate is set to 0.07 J / mm 2 or more in total of two or more sprays, and the descaling water immediately before the final is used. A method for producing a thick steel plate, characterized in that a final descaling water is jetted 0.5 s or more after jetting, and a steel sheet surface temperature immediately before the final descaling water jet is set to an Ar 3 transformation point or less.
In the manufacturing method of the thick steel plate according to claim 1 or 2, when the thick steel plate temperature before cooling is T [K], the time t [s] from the end of the descaling step to the start of the accelerated cooling step is , T ≦ 5 × 10 −9 × exp (25000 / T).