JP7060003B2 - Steel sheet cooling method, steel sheet manufacturing method, and steel sheet cooling equipment - Google Patents

Description

The present invention relates to a method for cooling a steel sheet, a method for manufacturing a steel sheet, and a cooling facility for a steel sheet, which cool the steel sheet while passing through a cooling zone of a continuous annealing line.

When manufacturing a steel sheet, the material is built in by cooling the steel sheet after heating in a continuous annealing facility to cause a phase transformation. In recent years, the application of high-strength steel sheets (HITEN) has been progressing in the automobile industry for the purpose of achieving both weight reduction of the vehicle body and collision safety, and it is advantageous for manufacturing high-strength steel sheets in order to meet such demand trends. Rapid cooling (quenching) technology is becoming more important. As a water quenching method with the fastest cooling rate, a method of immersing a heated high-temperature steel sheet in water and at the same time injecting cooling water onto the steel sheet by a quench nozzle provided in the water to quench the steel sheet is common. be. However, there is a problem that the steel sheet has a shape defect due to out-of-plane deformation such as warpage and wavy deformation during the rapid cooling.

In response to such a problem, in Patent Document 1, in order to suppress the wavy deformation of the steel sheet that occurs during quenching in a continuous annealing furnace, the tension of the steel sheet applied to the quenching and quenching step can be changed. As a means of change, a method of providing bridle rolls before and after the quenching and quenching part has been proposed.

Further, in Patent Document 2, attention is paid to the fact that thermal stress in the compression direction is generated in the width direction of the steel sheet at the quenching start point (cooling start point), and the steel sheet buckles, resulting in a shape defect. A method of suppressing out-of-plane deformation by restraining from both sides of a steel sheet in a region where compressive stress is generated in the direction or a region in the vicinity thereof has been proposed.

Further, Patent Document 3 proposes a method of suppressing out-of-plane deformation by arranging a pair of restraint rolls in a quenching and quenching apparatus and restraining a steel sheet from both sides with the restraint rolls.

Japanese Unexamined Patent Publication No. 2011-184773 Japanese Patent Application Laid-Open No. 2003-277833 WO2016 / 084283 Gazette

However, in the method described in Patent Document 1, a large tension is applied to the high-temperature steel sheet, so that the steel sheet may be broken. In addition, a large thermal crown is generated on the bridle roll before the quenching and quenching part that comes into contact with the high-temperature steel sheet, so that the bridle roll and the steel sheet contact unevenly in the width direction of the bridle roll, and as a result, buckle on the steel sheet. There was a problem that the shape of the steel sheet could not be improved due to the occurrence of flaws.

Further, in the method described in Patent Document 2, there is only one pair of pinch rolls, and it is considered that the shape correction effect of the steel sheet is small.

Further, although the quenching quenching device of Patent Document 3 can be used to prevent the steel sheet from being deformed during quenching, the cooling rate of the steel sheet temporarily decreases when the steel sheet passes through the injection device for injecting cooling water. Therefore, the characteristics of the steel sheet deteriorate. Specifically, due to the decrease in the cooling rate of the steel sheet, it may not be possible to obtain a steel sheet having desired material properties of the steel sheet, for example, a desired tensile strength.

The present invention has been completed in view of such circumstances, and is a method for cooling a steel sheet having desired material characteristics, a method for manufacturing the steel sheet, and a steel sheet while suppressing shape defects that occur in the steel sheet during rapid cooling of the high temperature steel sheet. The challenge is to provide cooling equipment for the steel sheet.

As a result of diligent studies to solve the above problems, the present inventors have obtained the following findings.

Martensitic transformation occurs from the Ms point to the Mf point, but when the cooling rate of the steel sheet in this temperature range decreases, the material properties (for example, tensile strength) of the steel sheet also decrease. The Ms point is the temperature at which martensitic transformation starts (martensite transformation start temperature), and the Mf point is the temperature at which martensitic transformation ends (martensite transformation end temperature).

In order to maintain the material properties, it is necessary to maintain a cooling rate below the Ms point, but for temperatures higher than the Ms point, a cooling rate similar to that of immersion cooling is not required, and the structure has low strength other than martensite. A cooling rate that does not generate ferrite, pearlite, or bainite is sufficient.

On the other hand, when the cooling rate is high, the heat shrinkage is large, so that the steel sheet is deformed and the product shape is deteriorated. In order to reduce the heat shrinkage, it is important to reduce the temperature gradient in the longitudinal direction of the steel sheet, and therefore, it is preferable to gradually increase the cooling rate in the longitudinal direction of the steel sheet.

When the cooling rate is changed stepwise, it is preferable to install a roll that presses the steel plate in order to divide the cooling area (cooling area). By using the roll, the movement of the refrigerant between the cooling areas on the steel sheet can be suppressed, and since the steel sheet can be pressed by the roll, the effect of suppressing the deformation of the steel sheet can be expected.

From the above, the present inventors cool the steel sheet to a temperature higher than the Ms point at a necessary and sufficient cooling rate before quenching, and quench the steel sheet below the Ms point to apply stress to the steel sheet due to thermal shrinkage. It was considered that a steel sheet having desired material properties such as tensile strength could be manufactured while suppressing the shape defect (deformation).

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] A method for cooling a steel sheet used in a cooling zone of a continuous annealing line.
The cooling rate of the steel sheet in the region upstream in the steel sheet transport direction is slowed down, and the cooling rate of the steel sheet in the region downstream in the steel sheet transport direction is rapidly cooled.
Further, the regions on the upstream side and the downstream side in the steel sheet transport direction are set as a plurality of cooling regions along the steel sheet transport direction, and the cooling rate of the cooling region on the downstream side in the steel sheet transport direction is the cooling region on the upstream side in the steel sheet transport direction. A method for cooling steel sheets that is faster than the cooling rate.
[2] A method for cooling a steel sheet used in the cooling zone of a continuous annealing line.
The cooling rate of the steel sheet in the region upstream in the steel sheet transport direction is slowed down, and the cooling rate of the steel sheet in the region downstream in the steel sheet transport direction is rapidly cooled.
Further, along the steel sheet transport direction, the region on the upstream side in the steel sheet transport direction is designated as one slow cooling cooling region, and the region on the downstream side in the steel plate transport direction is designated as one quenching cooling region.
A method for cooling a steel plate, wherein the cooling rate of the cooling region on the downstream side in the steel plate transport direction is equal to or higher than the cooling rate of the cooling region on the upstream side in the steel plate transport direction.
[3] The method for cooling a steel sheet according to [1] or [2], wherein in the slow cooling, the steel sheet is cooled to a temperature higher than the martensitic transformation start temperature of the steel sheet.
[4] Cooling of the steel sheet by the rapid cooling is started from a temperature equal to or higher than the martensite transformation start temperature of the steel sheet, and in the rapid cooling, the steel sheet is cooled to a temperature equal to or lower than the martensite transformation end temperature of the steel sheet [1] to [3]. The method for cooling a steel plate according to any one.
[5] The method for cooling a steel sheet according to any one of [1] to [4], wherein in the slow cooling, the structure is cooled so as not to proceed with a structure transformation of 10% or more from the structure before cooling.
[6] The slow cooling of the steel sheet according to any one of [1] to [5], wherein the slow cooling is performed by injecting a refrigerant of any one selected from gas, liquid, and a mixture of gas and liquid onto the steel sheet. Cooling method.
[7] The method for cooling a steel sheet according to any one of [1] to [6], wherein the quenching is a method of injecting a cooling liquid onto a steel sheet.
[8] The method for cooling a steel sheet according to any one of [1] to [7], wherein the regions on the upstream side and the downstream side in the steel sheet transport direction are divided by a roll that presses the steel sheet in the plate thickness direction.
[9] The method for cooling a steel sheet according to any one of [1] to [8], wherein the cooling region is cooled under the condition of satisfying the following formula (1).

[10] A method for manufacturing a steel sheet using the method for cooling a steel sheet according to any one of [1] to [9].
[11] A steel plate cooling device used in the cooling zone of a continuous annealing line.
A slow cooling device that cools the steel sheet by slowing down the cooling rate of the steel sheet in the area upstream of the steel sheet transport direction.
It is equipped with a quenching device that cools the steel sheet by quenching the cooling rate of the steel sheet in the area downstream in the steel sheet transport direction.
The regions on the upstream side and the downstream side in the steel sheet transport direction are defined as a plurality of cooling regions along the steel sheet transport direction, and the cooling rate of the cooling region on the downstream side in the steel sheet transport direction is the cooling rate of the cooling region on the upstream side in the steel sheet transport direction. The above steel sheet cooling equipment.
[12] The steel sheet cooling equipment according to [11], wherein the regions on the upstream side and the downstream side in the steel plate transport direction are divided by a roll that presses the steel plate in the plate thickness direction.
[13] The slow cooling device includes a refrigerant injection device installed on the front and back surfaces of a steel sheet and injecting a refrigerant which is one of a gas, a liquid, and a mixture of a gas and a liquid.
The steel plate cooling equipment according to [11] or [12], wherein the quenching device is installed on the front and back surfaces and includes a coolant injection device for injecting a coolant.

According to the present invention, it is possible to manufacture a steel sheet having desired material characteristics while suppressing shape defects.

FIG. 1 is a schematic view showing a cooling device of the present invention. FIG. 2 is a schematic diagram showing a CCT diagram of a steel having a tensile strength of 1500 MPa.

Hereinafter, the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments.

FIG. 1 is a schematic view showing the cooling equipment 100 of the present invention. The cooling equipment of the present invention is applied to the cooling equipment provided on the outside of the soothing tropics of the continuous annealing furnace. The arrow in FIG. 1 indicates the transport direction of the steel plate S.

The cooling equipment 100 of the present invention includes a slow cooling device 1 and a quenching device 2. The high-temperature steel plate S discharged from the soaking tropics of the continuous annealing furnace, which heats, soaks, cools, and reheats while continuously passing through the plate, is cooled by the slow cooling device 1 and then by the quenching device 2. It will be cooled. After cooling by the quenching device 2, the steel sheet S is guided to a subsequent process by changing the transport direction by the roll 6.

In the present invention, the slow cooling device 1 cools the steel sheet S by slowly cooling the steel sheet S in the region on the upstream side in the steel sheet transport direction. The quenching device 2 cools the steel plate S by slowly cooling the cooling speed of the steel plate S in the region downstream in the steel plate transport direction.

In the present invention, the regions on the upstream side and the downstream side in the steel sheet transport direction are set as a plurality of cooling regions along the steel sheet transport direction, and the cooling rate of the cooling region on the downstream side in the steel sheet transport direction is the cooling of the cooling region on the upstream side in the steel sheet transport direction. Cool the steel sheet so that it exceeds the speed. When the cooling rate is high, the heat shrinkage is large, so that the steel sheet is deformed and the product shape is deteriorated. In order to reduce the heat shrinkage, it is important to reduce the temperature gradient in the longitudinal direction of the steel sheet. In the present invention, by increasing the cooling rate on the downstream side rather than the upstream side in the longitudinal direction of the steel sheet, it is possible to suppress the shape defect that may occur in the steel sheet during rapid cooling of the high temperature steel sheet as in the conventional case.

Further, in the present invention, even when there is one cooling region for slow cooling and one cooling region for rapid cooling, the cooling rate on the downstream side is increased in the longitudinal direction of the steel sheet as in the case of a plurality of cooling regions. It is possible to suppress the shape defect that may occur in the steel sheet when the high temperature steel sheet is rapidly cooled as in the conventional case.

When changing the cooling rate so that the downstream side in the steel plate transport direction is larger than the upstream side in the steel plate transport direction, a roll 3 for pressing the steel plate S in the plate thickness direction is used to divide a plurality of cooling regions (cooling areas). It is preferable to install it. For example, as shown in FIG. 1, by installing the rolls 3 on the front and back surfaces of the steel plate S, it is possible to suppress the movement of the refrigerant between the cooling areas Z (Z1, Z2, Z3, Z4) on the steel plate S. Further, since the steel plate S can be pressed by the roll 3, the effect of suppressing the deformation of the steel plate can be expected. There is no particular limitation on the number of rolls 3 installed (that is, the number of cooling areas Z). It is preferable to install at least one pair of rolls 3 on the front and back surfaces of the steel plate S, and the rolls 3 may be offset. When offsetting, it is preferable to offset the steel plate S in the longitudinal direction so as to be equal to or smaller than the diameter of the roll 3. Regarding the length of the cooling area (= roll pitch), the faster the cooling speed, the easier it is to deform, and the shorter the roll pitch, the easier it is to suppress deformation. On the side), even if the cooling area is long, the steel plate is less likely to be deformed. Therefore, there is no particular problem even if the area length of the cooling area on the slow cooling side (upstream side in the steel sheet transport direction) is lengthened.

As an example, in FIG. 1, a pair of rolls 3 is installed at the boundary between the slow cooling device 1 and the quenching device 2, and in both the slow cooling device 1 and the quenching device 2, two pairs of rolls 3 are installed. is set up. As a result, four cooling areas Z1 to Z4 are created, and the cooling rate of Z3 or Z4 is set to be higher than the cooling rate of Z1 or Z2.

When slowing down in the slow cooling device 1, it is preferable to inject one of a refrigerant selected from a gas, a liquid, and a mixture of gas and liquid into the steel sheet S. The specific type of the refrigerant may be selected according to the chemical composition contained in the steel sheet S. If the steel sheet S is not to be oxidized more than necessary, an inert gas such as nitrogen may be used as a gas.

In the case of slow cooling in the slow cooling device 1, for example, as shown in FIG. 1, the refrigerant injection device 4 installed in each cooling area Z may be used to inject the refrigerant onto the front and back surfaces of the steel plate S. The refrigerant injection device 4 has a nozzle for injecting the refrigerant. FIG. 1 is an example in which a plurality of refrigerant injection devices 4 are installed on the front and back surfaces of the steel plate S and in the width direction, but the arrangement is not limited to this.
If the cooling capacities on the front and back surfaces of the steel sheet are the same, it is not necessary to install the refrigerant injection devices 4 facing each other between the rolls 3.

Next, the steel sheet S after being slowly cooled by the slow cooling device 1 is conveyed to the quenching device 2 and rapidly cooled. When quenching with the quenching device 2, for example, as shown in FIG. 1, a plurality of coolant injection devices 5 installed in each cooling area Z may be used to inject the coolant onto the front and back surfaces of the steel sheet S. .. The coolant injection device 5 has a nozzle for injecting the coolant. A plurality of coolant spraying devices 5 may be installed on the front and back surfaces of the steel plate S and in the width direction, and the coolant may be sprayed on both surfaces of the steel plate S. As the coolant, for example, cooling water is preferable. The steel sheet cooled to, for example, the temperature of the coolant by the injection of the coolant is conveyed to the next step.

The steel sheet S is thermally shrunk by being rapidly cooled by the slow cooling device 1 and the quenching device 2. In particular, in the case of a material in which the steel sheet S causes a martensitic transformation, when the temperature of the steel sheet S changes from the Ms point, which is the martensitic transformation start temperature, to the Mf point, which is the temperature at which the martensitic transformation ends. Rapid thermal contraction and transformational expansion occur simultaneously in the steel sheet S, the stress acting on the steel sheet S increases, and the shape tends to collapse.

As described in Patent Document 3, in the prior art, the steel sheet from the annealing furnace is allowed to cool in the atmosphere inside the furnace and then cooled by a quenching device. In that case, since the temperature is rapidly cooled from the annealing temperature to the water temperature, a very large heat shrinkage occurs.

In order to suppress the deformation of the steel sheet, in the present invention, the temperature higher than the Ms point is cooled by the slow cooling device 1 at a cooling rate slower than that of the quenching device 2, and the quenching device is cooled from the temperature above the Ms point. It is preferable to perform quenching in step 2. That is, it is preferable that the steel plate S is cooled by the slow cooling in the slow cooling device 1 at a temperature higher than the Ms point. Further, in the cooling of the steel sheet S by quenching in the quenching device 2, the cooling of the steel sheet by quenching is started from the temperature above the Ms point of the steel sheet S, and the steel sheet is cooled by quenching to the temperature below the Mf point of the steel sheet. preferable.

The temperature at the Ms point and the Mf point can be estimated from the component composition of the steel sheet. For example, in the case of steel having a tensile strength of 1500 MPa class, it is preferable that the cooling by the slow cooling in the slow cooling device 1 is stopped at a temperature higher than 420 ° C. from the CCT diagram as shown in FIG. The quenching by the quenching device 2 is preferably started from 420 ° C. or higher and cooled to 50 ° C. or lower. Further, the temperature of the steel sheet can be measured by a method such as attaching a thermocouple to the steel sheet.

In order to suppress deformation and obtain a good steel plate shape, it is sufficient to control the cooling area and the cooling rate separated by rolls as described above, but the following determinations obtained by the present inventors from analysis and experiment The judgment may be made using the equation (1).

In equation (1), V is the plate passing speed (m / s), ν is the Poisson's ratio (= 0.3), α is the linear expansion coefficient (= 12 × 10 ^-6 / ° C), and C _vt is the cooling rate. Product of plate thickness (unit is ° C.m / s), k is deformation coefficient (= 840), π is pi, L is cooling area length separated by roll (unit is m), t is plate thickness (unit: m) The unit is m). It is possible to suppress the deformation of the steel sheet by making a judgment by the above formula in each cooling area and satisfying the above formula in all the cooling areas.

The derivation of the equation is as follows.
The stress applied to the steel sheet by cooling is expressed by the following equation (2).

Here, σ is the stress and ΔT is the temperature difference before and after cooling. Further, E is Young's modulus (Pa).
Since the temperature difference between the rolls is ΔT, it is expressed by the following equation (3).

The following equation (4) can be derived from the above two equations.

In addition, the following formula (5) is known from the literature: Buckling Design Guideline Development Subcommittee, Steel Structure Series 2 Buckling Design Guideline, Japan Society of Civil Engineers, 1987.

Here, σ _cr is the critical stress at which buckling occurs, and π is the pi.
Since buckling occurs when σ> σ _cr , the condition for avoiding it is the following equation (1).

In the present invention, it is desirable to control the cooling rate of the cooling by slow cooling in the slow cooling device 1 so that the tissue transformation of 10% or more does not occur before quenching including martensite. In the present invention, it is desirable that the structure in the quenching state (quenching in the quenching device 2) is martensite in which 100% transformation has occurred. Therefore, in slow cooling, it is preferable to cool the tissue before cooling so as not to promote tissue transformation of 10% or more. As shown in the CCT diagram of a steel having a tensile strength of 1500 MPa shown in FIG. 2, for example, in this steel, ferrite transformation and bainite transformation occur about 10 seconds after the start of cooling, and the Ms point is 420 ° C. As shown in FIG. 2, ferrite transformation and bainite transformation occur after a long period of time even at the Ms point or higher. Therefore, it is preferable that the transformation rate is less than 10%. In FIG. 2, when the annealing temperature is 850 ° C., the average cooling rate is preferably about 45 ° C./s or more in order to cool to the Ms point of 420 ° C. in about 10 seconds.

The time until the start of transformation can be measured by an apparatus such as Formaster, and the transformation described in JP-A-8-62181 between the slow cooling device 1 and the quenching device 2. By installing a rate sensor, the cooling rate before quenching may be controlled while confirming that the structure is desirable.

Further, the cooling rate at the time of slow cooling in the slow cooling device 1 may be controlled by a method such as changing the flow rate of gas or liquid, changing the flow velocity, or changing the mixing ratio.

Further, the faster the cooling rate, the greater the stress applied to the steel sheet S due to heat shrinkage. Therefore, in slow cooling, it is preferable to cool at a cooling rate close to the lower limit, that is, at the minimum cooling rate at which slow cooling does not cause transformation. .. Regarding the slow cooling in the slow cooling device 1 and the quenching in the quenching device 2, the slow cooling is preferably 10 ° C./s or more and less than 500 ° C./s, and the quenching is 500 to 2000 ° C./s. preferable.

Further, in FIG. 1, four nozzles 4 and 5 are provided on both sides of the steel plate passing line, but it is sufficient that the steel plate S can be cooled from both sides, and at least two nozzles are provided on both sides of the steel plate S passing line. A plurality of each (at least one for the slow cooling device 1 and at least one for the quenching device 2) may be provided.

From the above, according to the present invention, it is possible to obtain a steel sheet having desired material characteristics while suppressing shape defects. That is, by applying the cooling method of the present invention to the steel sheet manufacturing equipment, it is possible to manufacture a steel sheet having desired material characteristics while suppressing shape defects.

Hereinafter, examples will be described for further understanding of the present invention. The examples do not limit the present invention in any way.

Using the cooling equipment shown in FIG. 1, a cold-rolled steel sheet having a plate thickness of 1.0 to 2.0 mm, a plate width of 1000 mm, and a tensile strength of 1500 MPa was manufactured. Cooling was performed under the conditions that the temperature immediately after the annealing furnace of the steel sheet was 850 ° C. and the cooling completion temperature was 30 ° C. (water temperature). The Ms point of the steel sheet is 420 ° C., and the Mf point is 200 ° C.

In Invention Example 1, the plate thickness was 1.2 mm, the plate passing speed was 167 mmp, and cooling was performed under the conditions shown in Table 1.

In Invention Example 2, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 2.

In Invention Example 3, the plate thickness was 1.2 mm, the plate passing speed was 167 mmp, and cooling was performed under the conditions shown in Table 3.

In Invention Example 4, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 4.

In Invention Example 5, the plate thickness was 1.2 mm, the plate passing speed was 167 mmp, and cooling was performed under the conditions shown in Table 5.

In Invention Example 6, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 6.

In Invention Example 7, the plate thickness was 1.2 mm, the plate passing speed was 167 mpm, and cooling was performed under the conditions shown in Table 7.

In Invention Example 8, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 8.

In Invention Example 9, the plate thickness was 1.2 mm, the plate passing speed was 167 mpm, and cooling was performed under the conditions shown in Table 9.

In Invention Example 10, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 10.

In Comparative Example 1, the cooling device before quenching was not used, and only quenching was performed at 167 mpm (cooling rate 1250 ° C./s) under the condition of a plate thickness of 1.2 mm.

In Comparative Example 2, the plate thickness was 1.2 mm, the plate passing speed was 167 mmp, and cooling was performed under the conditions shown in Table 11.

In Comparative Example 3, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 12.

In Comparative Example 4, the plate thickness was 1.2 mm, the plate passing speed was 167 mmp, and cooling was performed under the conditions shown in Table 13.

In Comparative Example 5, the plate thickness was 1.8 mm, the plate passing speed was 111 mpm, and cooling was performed under the conditions shown in Table 14.

The tensile strength and the shape of the steel sheet were evaluated for the obtained steel sheet. The tensile strength was determined by collecting JIS No. 5 tensile test pieces and conducting a tensile test in accordance with the provisions of JIS Z 2241 so that the tensile direction is the rolling direction. When the tensile strength was less than 1470 MPa, it was evaluated as x, when it was 1470 MPa or more and less than 1500 MPa, it was evaluated as ◯, and when it was 1500 MPa or more, it was evaluated as ⊚.

Further, regarding the shape of the steel plate, it was determined based on the following criteria whether or not the formula (1) was satisfied.
⊚: Both slow cooling and quenching satisfy the formula (1), and as a result, the warp of the steel sheet is 0 mm or more and less than 5 mm. More than 10 mm ×: The steel sheet did not satisfy the formula (1) in both slow cooling and quenching, and the warpage of the steel sheet was 10 mm or more. The results are shown in Table 15.

All of the examples of the present invention satisfied the tensile strength and the shape of the steel plate. On the other hand, in Comparative Example 1, the quenching temperature was high, the heat shrinkage due to the quenching was large, and the warp of the steel sheet was large. Further, in Comparative Examples 2 and 3, the quenching temperature was low, the martensitic transformation was insufficient, and the tensile strength of the steel sheet was lowered. Further, even in the slow cooling before quenching, the cooling rate is reversed in a part of the cooling region, so that the shape is also deteriorated.

1 Slow cooling device 2 Quench cooling device 3 roll 4 Refrigerant injection device 5 Coolant injection device 6 roll 100 Cooling equipment S Steel plate Z (Z1 to Z4) Cooling area

Claims (7)

It is a cooling method for steel sheets used in the cooling zone of continuous annealing lines.
In the region on the upstream side in the steel sheet transport direction, the steel sheet is slowly cooled to a temperature higher than the martensitic transformation start temperature at a cooling rate of 10 ° C./s or more and less than 500 ° C./s.
In the region downstream of the steel sheet transport direction, the steel sheet is rapidly cooled at a cooling rate of 500 ° C./s to 2000 ° C./s from the temperature above the start of martensitic transformation of the steel sheet to the temperature below the end temperature of martensitic transformation of the steel sheet.
Further, the regions on the upstream side and the downstream side in the steel plate transport direction are further divided into one or more cooling areas along the steel plate transport direction by a roll that presses the steel plate in the plate thickness direction, and the cooling rate of one cooling area is set. It is higher than the cooling rate of all the cooling areas in the previous stage,
A method for cooling a steel sheet that is cooled so as to satisfy the formula (1) in each cooling area.

In equation (1), V is the plate passing speed (m / s), ν is the Poisson's ratio (= 0.3), α is the coefficient of linear expansion (= 12 × 10 ^-6 / ° C), and C _vt is the cooling rate. Product of plate thickness (unit is ° C.m / s), k is coefficient of deformation (= 840), π is pi, L is cooling area length separated by roll (unit is m), t is plate thickness (unit: m) The unit is m) The method for cooling a steel sheet according to claim 1, wherein in the slow cooling, the structure before cooling is cooled so that the structure transformation having a transformation rate of 10% or more does not proceed. The method for cooling a steel sheet according to claim 1 or 2, wherein the slow cooling is performed by injecting one of a refrigerant selected from a gas, a liquid, and a mixture of a gas and a liquid onto the steel sheet. The method for cooling a steel sheet according to any one of claims 1 to 3, wherein the quenching is a method of injecting a cooling liquid onto a steel sheet. A method for manufacturing a steel sheet using the method for cooling a steel sheet according to any one of claims 1 to 4. A steel plate cooling device used in the cooling zone of a continuous annealing line.
In the region on the upstream side in the steel sheet transport direction, a slow cooling device that slowly cools the steel sheet to a temperature higher than the martensitic transformation start temperature at a cooling rate of 10 ° C./s or more and less than 500 ° C./s.
Quenching device that quenches the steel sheet at a cooling rate of 500 ° C / s to 2000 ° C / s from the temperature above the martensitic transformation start temperature of the steel sheet to the temperature below the martensitic transformation end temperature of the steel sheet in the region downstream in the steel sheet transport direction. And with
A roll for pressing the steel plate in the plate thickness direction, which further divides the upstream and downstream regions in the steel plate transport direction into one or more cooling areas along the steel plate transport direction, is provided.
The cooling rate of one cooling area is higher than the cooling rate of all the cooling areas in the previous stage.
A steel plate cooling facility that cools each cooling area so as to satisfy the formula (1).

In equation (1), V is the plate passing speed (m / s), ν is the Poisson's ratio (= 0.3), α is the coefficient of linear expansion (= 12 × 10 ^-6 / ° C), and C _vt is the cooling rate. Product of plate thickness (unit is ° C.m / s), k is coefficient of deformation (= 840), π is pi, L is cooling area length separated by roll (unit is m), t is plate thickness (unit: m) The unit is m) The slow cooling device includes a refrigerant injection device installed on the front and back surfaces of a steel sheet and injecting a refrigerant which is one of a gas, a liquid, and a mixture of a gas and a liquid.
The steel plate cooling equipment according to claim 6, wherein the quenching device is installed on the front and back surfaces and includes a coolant injection device for injecting a coolant.

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