ENDLESS ROLLING APPARATUS AND METHOD

Abstract

Provided is an endless rolling apparatus and method, which have improved cooling conditions for producing advanced high strength steel. The endless rolling apparatus includes: a continuous casting machine for casting a slab; and a cooling bed having at least one piece of water-cooling equipment and at least one rolling mill continuously connected to the continuous casting machine, wherein, in the cooling bed, an initial position (S) at which the water-cooling equipment is provided so as to manufacture advanced high strength steel through at least one water-cooling is defined by mathematical formula 1. Here, H is the thickness (mm) of a slab, V is the casting speed (m/sec) of the slab, h is the product thickness (mm), and t is the target arrival time (sec) until entry into the cooling bed.

Description

TECHNICAL FIELD

The present disclosure relates to an endless rolling apparatus and method for manufacturing advanced high strength steel (AHSS).

BACKGROUND ART

Referring to FIG. 1, in batch rolling according to the related art, when general steel is manufactured, in order to secure a coiling temperature, after finish rolling is performed, air cooling, water cooling, and air cooling are performed, and a temperature is controlled to a target coiling temperature.

Meanwhile, when advanced high strength steel is manufactured, for cooling using a laminar flow cooling or high density cooling device, in which cooling is performed with cooling water, after finish rolling is performed, operations of air cooling, primary water cooling, air cooling, secondary water cooling, and air cooling are sequentially performed, and a temperature is controlled to a target coiling temperature, so material properties are secured.

During primary air cooling and secondary air cooling, a cooling speed and an amount of cooling water are set to be different according to a type of produced steel grade (for example, dual-phase (DP) steel, transformation induced plasticity (TRIP) steel, ferritic-bainitic (FB) steel, and the like).

On the other hand, in an endless rolling apparatus, in which a continuous casting device and a rolling mill are directly connected, according to a continuous casting speed and a slab thickness, a runout table (ROT) passing speed may be different for each final thickness. In detail, in the case of advanced high strength steel (AHSS), a speed at which the steel passes through an endless rolling apparatus is controlled to be slow. In this regard, an ROT cooling method also differs from an endless rolling process according to the related art.

However, in an endless rolling apparatus and method according to the related art, the water cooling start time required to manufacture AHSS may not be known exactly, so it may be difficult to determine a cooling position.

DISCLOSURE

Technical Problem

An aspect of the present disclosure may provide an endless rolling apparatus and method having improved cooling conditions for production of advanced high strength steel (AHSS).

Technical Solution

According to an aspect of the present disclosure, an endless rolling apparatus includes a continuous casting device casting a slab and a cooling bed having at least one rolling mill and at least one water-cooling device, continuously provided with the continuous casting device. Here, in the cooling bed, an initial position (S), at which the water-cooling device is provided to manufacture advanced high strength steel through at least one water-cooling, is defined by Formula 1,

$\begin{array}{cc}S=\frac{\left(H·V\right)}{h}·t\left(\mathrm{HERE},0<t<10\sec \right)& \mathrm{Formula}1\end{array}$

where H is the thickness (mm) of the slab, V is the casting speed (m/sec) of the slab, h is product thickness (mm), and t is target arrival time (sec) until entry into the cooling bed.

According to another aspect of the present disclosure, an endless rolling method includes: a casting operation of casting a slab using a continuous casting device; a casting speed and thickness measuring operation of measuring a casting speed of the slab produced in the casting operation and a thickness (H) of the slab; a rolling operation of rolling the slab, continuously connected to the continuous casting device, to a target thickness; a product thickness measuring operation of measuring a thickness of a product rolled in the rolling operation; a target arrival time setting operation of setting the target arrival time required for the product to enter a water cooling section of a cooling bed after completion of the rolling operation; and a water cooling start position calculating operation of setting an initial water cooling start position (S) to manufacture advanced high strength steel through at least one water-cooling in the cooling bed using a value obtained in each operation.

The water cooling start position calculating operation may be defined by Formula 1,

$\begin{array}{cc}{L}_1<\frac{\left(H\times V\right)}{h}\times t<L_2-\alpha & \mathrm{Formula}1\end{array}$

where H is the thickness (mm) of the slab, V is the casting speed (m/sec) of the slab, h is product thickness (mm), and t is target arrival time (sec) until entry into the cooling bed.

When the rolling operation is completed, a temperature of the product may be controlled to 750° C. to 880° C.

Advantageous Effects

According to an exemplary embodiment in the present disclosure, an initial water cooling start position allowing AHSS to be manufactured using at least one water cooling process is determined, so advanced high strength steel (AHSS) may be manufactured while significantly reducing cooling after finish rolling.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating an endless rolling apparatus.

FIG. 2 is a graph illustrating a cooling pattern for production of general steel.

FIG. 3 is a graph illustrating a cooling pattern for production of AHSS in batch rolling.

FIG. 4 is a graph illustrating a cooling pattern for production of AHSS in endless rolling.

BEST MODE FOR INVENTION

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The exemplary embodiments of the present invention can be modified to have various other forms, and the scope of the present invention is not limited to the exemplary embodiments described below. In the following description, a shape and size of the components in the drawings may be exaggerated for clarity, and components having the same reference numerals illustrated in the drawings are the same components.

FIG. 1 is a view schematically illustrating an endless rolling apparatus.

Referring to FIG. 1, an endless rolling apparatus 100 of an exemplary embodiment may perform a casting process of continuously casting a slab while solidifying molten steel in a liquid phase to a solid phase without a defect, and a rolling process of rolling the slab, continuously cast as described above.

Here, in the casting process, a slab product is produced using a continuous casting device 110. The slab is manufactured as a product having a target thickness while passing through at least one rolling mill 120, continuously connected to the slab. Thereafter, the slab is manufactured as a final product while passing through a cooling bed 140 and being wound on a winding apparatus 150.

Preferably, the rolling mill 120 may include a rough rolling mill 122, an intermediate rolling mill 124, and a finish rolling mill 126. In addition, a heating device 130 for heating the slab may be provided between each of the mills included in the rolling mill 120.

Meanwhile, the cooling bed 140 is divided into an air cooling section in which cooling is performed with air, and a water cooling section in which water cooling is performed with cooling water. The water cooling section may include a water-cooling device spraying cooling water for high speed cooling. For example, the water-cooling device may be a water cooling device 142, known as a high density cooling or laminar flow cooling device and installed in a cooling bed.

Referring to FIG. 2, a graph illustrating a cooling pattern for the production of general steel, after a product, passing through the finish rolling mill 126, is air-cooled before the entry into the cooling bed 140, the product is cooled by water cooling in a process in which the product is supplied to the cooling bed 140 and passes through the water cooling device 142. Thereafter, the product is controlled to a target coiling temperature through a final air cooling process.

As described above, in general steel, ferrite and pearlite fractions may be controlled through operations of air cooling-water cooling-air cooling.

Meanwhile, referring to FIG. 3, advanced high strength steel (AHSS) is able to be manufactured using a batch rolling method. In this case, a product, passing through the finish rolling mill 126, is cooled through primary air cooling before the entry into the cooling bed 140. Thereafter, the product is cooled through primary water cooling while the product is supplied to the cooling bed 140 and passes through the water cooling device 142. In addition, the product, passing through the water cooling device 142, is cooled through secondary air cooling. Thereafter, the product passes through the water cooling device 142 again, and may be cooled through secondary water cooling.

As described above, air cooling may be performed again on the product, on which secondary air cooling and secondary water cooling have been performed. In this case, cooling may be performed according to a target coiling temperature. Depending on a temperature at which cooling is performed, ferrite and bainite and martensite fractions are controlled for each steel grade, so required steel grade properties (for example, DP steel, TRIP steel, FB steel, and the like) may be obtained.

After the advanced high strength steel (AHSS) manufactured using a batch rolling method passes through the finish rolling mill 126, an initial temperature (FDT) of 820° C. or more may be secured.

Meanwhile, in the exemplary embodiment, the advanced high strength steel (AHSS) manufactured in the endless rolling apparatus 100 may be manufactured by a cooling pattern as illustrated in FIG. 4, a graph illustrating a cooling pattern for production of AHSS in endless rolling.

In the endless rolling apparatus 100, an initial temperature (FDT), after the steel passes through the finish rolling mill 126, may be controlled to 750° C. to 880° C.

In addition, a product, passing through the finish rolling mill 126, is cooled through primary air cooling before entry into the cooling bed 140. Thereafter, the product is cooled through primary water cooling while the product is supplied to the cooling bed 140 and passes through the water cooling device 142. In addition, the product, passing through the water cooling device 142, is cooled through secondary air cooling. In this case, cooling may be performed according to a target coiling temperature. Depending on a temperature at which cooling is performed, ferrite, bainite and martensite fractions are controlled for each steel grade, so required steel grade properties (for example, DP steel, TRIP steel, FB steel, and the like) may be obtained.

As described above, in the endless rolling apparatus 100 of an exemplary embodiment, continuous casting-rolling processes are directly connected, and a product is continuously supplied. Thus, a speed of the product is controlled to be relatively slow, so cooling may only be sufficient when at least one high speed cooling and a desired target coiling temperature (CT) of steel may be secured. In this regard, advanced high strength steel (AHSS) may be manufactured.

In this case, in the endless rolling apparatus 100, in order to obtain a sufficient cooling effect using at least one high speed cooling, an initial position of the cooling bed 140, in which the water cooling device 142, a water-cooling device, is provided, that is an initial water cooling start position (S) is required to be accurately set.

The water cooling start position (S) may be calculated using uniform mass flow throughout an entire process of an endless rolling process. In other words, in the endless rolling apparatus 100, mass flow is uniform throughout an entire process, and the mass flow is calculated using a material cross-section and speed.

Moreover, when a thickness of a slab, manufactured in the continuous casting device 110, a casting speed, a thickness of a product after finish rolling, and a target arrival time until cooling starts are given, a position in which the water cooling device 142 is installed, that is, a water cooling start position (S), is calculated.

Thus, a position in which an initial water cooling device 142 is installed, that is, an initial water cooling start position (S) is defined by Formula 1.

$\begin{array}{cc}S=\frac{\left(H·V\right)}{h}·t\left(\mathrm{HERE},0<t<10\sec \right)& \left[\mathrm{Formula}1\right]\end{array}$

Here, H is a thickness (mm) of a slab, V is the casting speed (m/sec) of the slab, h is product thickness (mm), and t is the target arrival time (s) until entry into the cooling bed 140.

Preferably, in an exemplary embodiment, the target arrival time until entry into the cooling bed 140 is greater than 0 seconds, and is limited to being within 10 seconds (sec) to improve productivity.

For example, when a slab thickness of 90 mm and a casting speed of 6.5 m/min are given, a thickness of a product may be obtained through measurement after finish rolling. In this case, the thickness of the product is 2.0 mm, and the target arrival time until entry into the cooling bed 140 is set to be 4 seconds (sec).

In this case, a casting speed, 6.5 m/min, may be calculated while being multiplied by a conversion factor, 1/60 (min/sec), in order to be converted into movement distance per second.

When the conditions described above are input to Formula 1,

$\frac{90\left(\mathrm{mm}\right)\times 6.5\left(m\text{/}\min \right)}{2\left(\mathrm{mm}\right)}\times \frac{1}{60}\left(\min \text{/}\sec \right)\times 4\left(\sec \right)$

is represented. When Formula 1, described above, is calculated, a position in which an initial water cooling device 142, of at least one water cooling device 142, is installed, that is, an initial water cooling start position (S), may be calculated as 19.5 m or greater.

Meanwhile, an endless rolling method of an exemplary embodiment may include a casting operation of casting a slab using a continuous casting device 110. In addition, in order to obtain mass flow of an entire process, a casting speed of a slab and a thickness (H) of the slab are required to be measured. To this end, a casting speed and thickness measuring operation of measuring a casting speed of the slab, produced in the casting operation, and the thickness (H) of the slab may be performed.

Next, a rolling operation of rolling the slab, casted in the casting operation, at a target thickness, may be performed. In this case, a temperature (FDT) of the product may be controlled to 750° C. to 880° C. when the rolling operation is completely.

Thereafter, a product thickness measuring operation of measuring a thickness of the product, rolled in the rolling operation, may be performed.

Meanwhile, in a target arrival time setting operation, the target arrival time, required for a product to reach a position of the cooling bed 140, in which the product enters an initial water cooling device 142, of at least one water cooling device 142 after the rolling operation is completed, that is, an initial water cooling start position, may be set.

In addition, through a cooling start position calculating operation of setting an initial water cooling start position using a value obtained in each operation, a cooling start position may be calculated.

Meanwhile, the endless rolling method may be used in the apparatus according to the related art, and the endless rolling method is applied to the apparatus according to the related art using Formula 2 to determine whether the apparatus according to the related art is operated as the endless rolling apparatus 100 of an exemplary embodiment.

The apparatus according to the related art may be provided with a plurality of water cooling devices 142 in the cooling bed 140 so as to allow cooling-air cooling operations to be performed several times.

In this case, mass flow is uniform in an entire process in the case of endless rolling, so a distance from the finish rolling mill 126 to a water cooling device 142 in which first cooling is performed, and a distance from the finish rolling mill 126 to a water cooling device 142 in which final cooling is performed should be satisfied with Formula 2.

$\begin{array}{cc}{L}_1<\frac{\left(H\times V\right)}{h}\times t<L_2-\alpha & \left[\mathrm{Formula}2\right]\end{array}$

Here, L₁ is a distance (m) from the finish rolling mill 126 to a first ROT cooling device, and L₂ is a distance (m) from the finish rolling mill 126 to a final ROT cooling device.

In addition, H is a thickness (mm) of a slab, V is a casting speed (m/s) of the slab, h is a thickness (mm) of a product, and t is the target arrival time (sec) until the entry into the cooling bed 140. Moreover, a is a constant of a length of a cooling device required to secure a target coiling temperature (CT).

For example, using Formula 2, it is determined whether the endless rolling method of an exemplary embodiment is applied to the endless rolling apparatus 100 in which a distance L₁ from the finish rolling mill 126 to a first ROT cooling device is 10 m, and a distance L₂ from the finish rolling mill 126 to a final ROT cooling device is 48 m.

In the endless rolling method of an exemplary embodiment, when a slab thickness is 90 mm, a casting speed is 6.5 m/min, a product thickness is 2.0 mm, and the target arrival time until entry into the cooling bed 140 is set to be 4 seconds (sec), a position of the cooling bed 140, in which an initial water cooling device 142, of at least one water cooling device 142, is installed, that is, an initial water cooling start position (S), is calculated using Formula 1 by unit conversion, so the initial water cooling start position from the finish rolling mill 126 is 19.5 m.

In this case, a position in which the water cooling device 142 is installed, that is, a water cooling start position (S), 19.5 m, is greater than a distance L₁ from the finish rolling mill 126 to a first ROT cooling device, 10 m.

Moreover, a position of the cooling bed 140, in which initial water cooling, of at least one water-cooling, starts, that is, an initial water cooling start position (S), 19.5 m is smaller than 41.6 m, required when a distance L₂ from the finish rolling mill 126 to a final ROT cooling device is 48 m, and a constant of a length of a cooling device required to secure a target coiling temperature (CT), a is 7.4 m.

Thus, the endless rolling method of an exemplary embodiment can be applied through the apparatus according to the related art, and advanced high strength steel (AHSS) may be produced by applying the endless rolling method to the apparatus according to the related art.

While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

1. An endless rolling apparatus comprising a continuous casting device casting a slab and a cooling bed having at least one rolling mill and at least one water-cooling device, continuously provided with the continuous casting device, wherein, in the cooling bed, an initial position (S), at which the water-cooling device is provided to manufacture advanced high strength steel through at least one water-cooling, is defined by Formula 1,

2. An endless rolling method comprising: a casting operation of casting a slab using a continuous casting device;a casting speed and thickness measuring operation of measuring a casting speed of the slab produced in the casting operation and a thickness (H) of the slab;a rolling operation of rolling the slab, continuously connected to the continuous casting device, to a target thickness;a product thickness measuring operation of measuring a thickness of a product rolled in the rolling operation;a target arrival time setting operation of setting the target arrival time required for the product to enter a water cooling section of a cooling bed after completion of the rolling operation; anda water cooling start position calculating operation of setting an initial cooling start position (S) to manufacture advanced high strength steel through at least one water-cooling in the cooling bed using a value obtained in each operation.

3. The endless rolling method of claim 2, wherein the water cooling start position calculating operation is defined by Formula 1,

4. The endless rolling method of claim 2, wherein, when the rolling operation is completed, a temperature of the product is controlled to 750° C. to 880° C.

5. The endless rolling method of claim 3, wherein, when the rolling operation is completed, a temperature of the product is controlled to 750° C. to 880° C.