JP6350322B2 - Manufacturing method and processing facility for high-strength steel sheet - Google Patents

Description

  The present invention is a process for producing a high-strength steel sheet through hot rolling, pickling process, cold rolling, and heat treatment for softening the hot-rolled steel strip prior to cold rolling. The present invention relates to a manufacturing method and processing equipment, and relates to a manufacturing method and processing equipment for a high-strength steel sheet that can appropriately secure the time required for heat treatment and can efficiently soften a hot-rolled steel strip.

  In recent years, high-strength steel plates (so-called high-tensile materials) have been widely used in various fields such as automotive steel plates, and various high-tensile materials have been developed according to applications. Since such a high-tensile material contains many alloy components for increasing the strength, a hard low-temperature transformation phase (for example, bainite or martensite) is generated in the cooling process after hot rolling. For this reason, the deformation resistance when processing the high-tensile material after cooling is increased.

Especially when manufacturing high-strength cold-rolled steel sheets (hereinafter referred to as high-strength steel sheets) among high-tensile materials, the deformation resistance of the hot-rolled steel strip that is the raw material is large, and the sheet thickness is reduced to be thinner than before. Thus, the cold rolling load increases. Therefore, it is necessary not only to increase the number of cold rolling passes by passing the cold tandem rolling mill several times, but also troubles such as breakage are likely to occur during rolling, and the cold tandem rolling mill is in operation. The rate decreases, and as a result, the problem that the productivity of the high-strength steel sheet decreases occurs. Further, since the deformation resistance of the hot-rolled steel strip is large, there is a problem that the shape of the high-strength steel plate obtained by cold rolling is deteriorated and the yield is reduced.
In order to solve such a problem, a technique for reducing the deformation resistance of the hot-rolled steel strip by subjecting the hot-rolled steel strip before being cold-rolled to heat treatment to soften it has been studied.

For example, Patent Document 1 discloses a technique for uniformly softening a hot-rolled steel strip by performing a tempering process using a BAF (box annealing furnace) annealing furnace or a continuous annealing furnace prior to cold rolling. ing.
In Patent Document 1, the tempering temperature and the tempering time in the tempering process may be adjusted according to the temperature at which the hot-rolled steel strip after hot rolling is wound on a coil (hereinafter referred to as a hot-rolled coil). Have been described.

JP 2010-144243 A

  However, when a tempering treatment is performed before cold rolling in the production of a high-strength steel sheet, the following various problems occur when a BAF annealing furnace or a continuous annealing furnace is used.

  In the tempering process by the BAF annealing furnace, the hot-rolled coils are individually stored and tempered by a batch process. In this case, since the steel sheet is wound in a coil shape, it takes time to raise the temperature to a predetermined tempering temperature up to the inside of the coil, and similarly, it takes time to slowly cool the coil. For this reason, a great amount of time is required for a series of steps of heating-soaking-slow cooling the entire hot-rolled coil, and therefore the tempering efficiency cannot be improved. It is possible to increase the efficiency by installing a large number of BAF annealing furnaces and simultaneously tempering a plurality of hot-rolled coils, but it is difficult to significantly improve the efficiency, and there are restrictions on site conditions. In factories, it is extremely difficult to add a BAF annealing furnace.

  Further, if a plurality of hot-rolled coils are housed in a large container (hereinafter referred to as a bell) as shown in FIG. 6 and placed in a BAF annealing furnace, the tempering efficiency can be increased. However, when the components of the hot-rolled steel strip wound on the hot-rolled coil are different, it is necessary to adjust the tempering temperature and tempering time according to the respective components, so that tempering is performed by storing them in the same bell. I can't. For this reason, it is not always possible to operate with the storage capacity of the bell being satisfied. In other words, even if a bell is used, it is difficult to significantly improve efficiency.

  Furthermore, since batch processing using a BAF annealing furnace requires a long time for tempering, the hot-rolled steel strip may become brittle depending on the components. As a result, breakage occurs in the cold rolling, causing a problem that the operating rate of the cold rolling equipment is lowered and the yield of the high-strength steel sheet is lowered.

  On the other hand, when a continuous annealing furnace is used, it is possible to perform tempering in the region of an overaging zone, and to improve efficiency. However, the tempering time is a value determined by the relationship between the size of the continuous annealing furnace, the length of the overaging zone, the plate passing speed of the hot-rolled steel strip, etc., and is a maximum of about 3 minutes at the normal plate passing speed. That is, in a continuous annealing furnace, only a short tempering time of about 3 minutes can be obtained, so that sufficient softening cannot be achieved. In addition, in a hot-rolled steel strip having a high Mn content, the time required for Mn to concentrate in austenite cannot be secured, so there is also a problem that the mechanical properties of the high-strength steel plate deteriorate.

  The present invention is to solve such problems of the prior art, in a method for producing a high-strength steel sheet from a hot rolling to a high-strength steel sheet through a pickling process and a cold rolling, cold rolling. High-strength steel sheet that enables uniform and sufficient softening by securing an appropriate heat treatment time and also improves heat treatment efficiency when heat treatment for softening the hot-rolled steel strip is performed prior to An object of the present invention is to provide a manufacturing method and a processing facility.

As a result of intensive studies, the inventors have removed the oxide film on the surface of the steel sheet in the pickling process, and then heated the steel sheet while continuously conveying the steel sheet, and the steel sheet heated in the heating process is wound as a coil. It has been found that the above problem can be solved by having a winding step and a heat retention step of maintaining the temperature of the wound coil at 500 ° C. to 700 ° C. for 10 minutes to 10 hours.
That is, it has been found that the above object can be achieved by the following configuration.

(1) It is a manufacturing method of the high strength steel plate which has a hot rolling process, a pickling process, and a cold rolling process in this order, Furthermore, between a pickling process and a cold rolling process,
After removing the oxide film on the steel sheet surface in the pickling process, a heating process for heating while continuously conveying the steel sheet,
A winding process of winding the steel sheet heated in the heating process as a coil;
A method for producing a high-strength steel sheet, comprising a step of maintaining a temperature of a wound coil at 500 ° C. to 700 ° C. for 10 minutes to 10 hours.
(2) The method for producing a high-strength steel plate according to (1), wherein the steel plate is heated to a temperature of 500 ° C. to 750 ° C. in the heating step.
(3) The method for producing a high-strength steel sheet according to (1) or (2), wherein the heating method in the heating step is based on a dielectric heating method.
(4) The method for producing a high-strength steel sheet according to any one of (1) to (3), wherein the heat-retaining method in the heat-retaining step is to reduce the temperature drop of the coil by a heat-retaining cover that covers the entire coil.
(5) After the heat retention step and before the cold rolling step, the coil is inserted into an apparatus that performs the cold rolling step, and is cooled to 200 ° C. or lower while conveying the steel sheet discharged from the coil. The manufacturing method of the high strength steel plate in any one of (1)-(4) which has a cooling process.
(6) A processing equipment for a high-strength steel sheet that performs a pickling process, a heating process, and a winding process in the method for producing a high-strength steel sheet according to any one of (1) to (5),
A pickling tank that performs a pickling process to remove the oxide film on the steel sheet surface;
A heating device for heating the steel plate, disposed downstream of the pickling tank in the conveying direction of the steel plate;
The processing equipment of the high strength steel plate which has a winding device which winds up the heated steel plate continuously arranged as a coil, and is arranged downstream of the heating device.

  According to the present invention, when heat treatment for softening a hot-rolled steel strip is performed prior to cold rolling, an appropriate heat treatment time can be ensured, so that uniform and sufficient softening is possible. Moreover, the heat treatment efficiency can be improved by reducing the time required for the heat treatment.

It is a flowchart for demonstrating an example of the manufacturing method of the high strength steel plate of this invention. It is a figure which shows notionally an example of the processing equipment which implements from the pickling process of a manufacturing method of this invention to a winding process. It is a figure which shows notionally an example of the heat retention apparatus which implements the heat retention process of the manufacturing method of this invention. It is a figure which shows notionally another example of the heat retention apparatus which implements the heat retention process of the manufacturing method of this invention. It is a figure which shows notionally an example of the cold rolling equipment which implements from the cooling process of the manufacturing method of this invention to a cold rolling process. It is a figure which shows notionally an example of the conventional batch annealing furnace.

Hereinafter, the present invention will be described in detail.
The description of the constituent elements described below may be made based on typical embodiments of the present invention, but the present invention is not limited to such embodiments.
In the present specification, a numerical range expressed using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.

The method for producing a high-strength steel sheet according to the present invention is a method for producing a high-strength steel sheet having a hot rolling step, a pickling step, and a cold rolling step in this order, between the pickling step and the cold rolling step. After removing the oxide film on the surface of the steel sheet in the pickling process, the heating process of heating while continuously conveying the steel sheet, the winding process of winding the steel sheet heated in the heating process as a coil, It is a manufacturing method of the high-strength steel plate which has a heat retention process which hold | maintains temperature at 500 to 700 degreeC for 10 minutes or more and 10 hours or less.
Such a method for producing a high-strength steel sheet according to the present invention includes a step of softening a hot-rolled steel strip as a raw material of the high-strength steel sheet before cold rolling. During the cooling process of the hot-rolled coil wound with the hot-rolled steel strip on the side, the temperature is lowered to 200 ° C. or lower, and a hard low-temperature transformation phase (for example, bainite or martensite) is generated and pickled. This is effective in softening a hot-rolled steel strip that becomes difficult to be rolled down to a predetermined sheet thickness and producing a high-strength steel sheet.
In the present invention, the “high-strength steel plate” refers to a steel plate having a tensile strength of 780 MPa or more in accordance with JIS Z 2241.

[Method of manufacturing high-strength steel sheet]
A method for producing a high-strength steel sheet of the present invention (hereinafter also referred to as a production method of the present invention) will be described with reference to the flowchart shown in FIG.
FIG. 1 is a flowchart showing an example of the manufacturing method of the present invention.
The manufacturing method shown in FIG. 1 includes a hot rolling step (S100), a pickling step (S102), a heating step (S104), a winding step (S106), a heat retaining step (S108), and a cooling step. It is a manufacturing method of the high strength steel plate which has (S110) and a cold rolling process (S112) in this order.

Thus, in the production method of the present invention, after the pickling step after the hot rolling step and before the cold rolling step, the steel plate is heated while being continuously conveyed, and the heated steel plate is cooled. Without coiling, the temperature of the wound coil is maintained at 500 ° C. to 700 ° C. for a predetermined time, thereby shortening the time for raising the temperature to the heat treatment temperature when performing heat treatment prior to cold rolling. Thus, the heat treatment efficiency can be improved and an appropriate heat treatment time can be secured, so that the hot-rolled steel strip can be softened uniformly and sufficiently.
This point will be described in detail later.
Hereinafter, each step will be described in detail.

[Hot rolling process]
A hot rolling process is a process of heating a steel slab and forming a hot-rolled steel strip with a rolling mill. As the hot-rolled steel strip forming method and hot-rolling equipment in the hot rolling process, various hot-rolled steel strip forming methods and hot-rolling equipment used in conventional high-strength steel plate manufacturing methods can be used.
In the hot rolling process, the formed hot rolled steel strip is wound around a hot rolled coil.
The hot rolled coil is cooled to a predetermined temperature or lower, for example, 200 ° C. or lower, and then subjected to a pickling process.

[Pickling process]
The pickling step is a step of removing the oxide film on the surface of the hot-rolled steel strip (steel plate) while discharging and transporting the hot-rolled steel strip from the hot-rolled coil formed in the hot rolling step.

Here, the processing apparatus which performs the pickling process in which a hot-rolled coil is provided after a hot rolling process, a heating process, and a winding process is shown in FIG. 2, About a pickling process, a heating process, and a winding process, FIG. This will be described with reference to FIG.
The processing facility 10 shown in FIG. 2 is a facility that performs a pickling process, a heating process, and a winding process while discharging the hot-rolled steel strip from the hot-rolled coil and transporting the hot-rolled steel strip in the longitudinal direction.
The processing facility 10 includes a dispenser 12, a welder 14, a pickling tank 16, a heating device 18, a cutting machine 20, a winder 22, and guide rollers 24a and 24b.

The dispenser 12 is loaded with the hot-rolled coil 26a and dispenses the hot-rolled steel strip Z. In the illustrated example, a plurality of dispensers 12 are provided, and different hot rolled coils 26a and 26b are continuously passed through. The welding machine 14 joins the terminal portion of the hot-rolled steel strip discharged from the hot-rolled coil 26a and the front end portion of the hot-rolled steel strip discharged from the hot-rolled coil 26b.
The guide rollers 24a and 24b are normal guide rollers that guide the discharged hot-rolled steel strip Z to a predetermined transport path.
The discharged hot-rolled steel strip Z is conveyed to the pickling tank 16 through a predetermined path.

The pickling tank 16 performs a pickling process for removing an oxide film (scale) generated on the surface of the hot-rolled steel strip Z by a hot rolling process.
As a pickling method in the pickling process, various pickling methods used in a conventional method for producing a high-strength steel sheet can be used. For example, a method of removing the oxide film on the surface with hydrochloric acid or sulfuric acid can be used.
Here, in this invention, the hot-rolled steel strip Z after a pickling process is wound up, after being continuously conveyed without being wound up and being used for the heating process. Therefore, as shown in FIG. 2, the processing facility 10 includes a heating device 18 and a winder 22 on the downstream side of the pickling tank 16.

[Heating process]
The heating process performed by the heating device 18 is a process of heating the hot-rolled steel strip Z from which the oxide film has been removed in the pickling process to a predetermined temperature.
The heating method of the hot-rolled steel strip Z in the heating process is not particularly limited, and a heating method (for example, a direct flame type non-oxidation method or a radiant tube heating method) heated by the combustion heat of the fuel, or an electric heating method (that is, Various heating methods used in the conventional method for producing a high-strength steel sheet, such as an electric heating method or an induction heating method, can be used. In addition, as the heating device 18, various heating devices such as a conventionally known continuous annealing furnace capable of implementing the above heating method can be used.

Especially, it is preferable to employ | adopt the heating method of an electrical heating system from a viewpoint of reducing the heating apparatus 18 in size and simplifying arrangement | positioning of the related apparatus of a processing facility. Moreover, if an electric heating system is employ | adopted, the point which also has the effect which suppresses the oxidation of the hot-rolled steel strip Z surface is preferable.
Among the electric heating methods, the current heating method allows the current to flow directly through the hot-rolled steel strip Z, which is advantageous not only for reducing the size of the heating device but also for shortening the heating time. However, the temperature of the hot-rolled steel strip Z may vary. On the other hand, the induction heating method can heat the hot-rolled steel strip Z in a short time, can easily control the temperature, and can be heated uniformly. Therefore, it is more preferable to use the induction heating method in that the hot-rolled steel strip Z can be uniformly heated in a short time. In addition, by adopting the induction heating method, it is possible to obtain an effect that the hot-rolled steel strip Z having a defective shape (for example, a wave) can be stably operated even when it is heated.

  The heating temperature of the hot-rolled steel strip Z in the heating process is the hot-rolled steel strip Z after heating (hereinafter referred to as a heat-treated steel strip), that is, the heat-treated steel when it reaches the heat-retaining step through a winding process described later What is necessary is just the temperature which the temperature of the heat processing coil 28 which wound up the belt | band | zone Z becomes a predetermined temperature range. That is, the temperature of the heat treatment coil at the start of the heat holding process depends on the time from the end of heating to the winding, or the time taken to start the heat holding process, the material of the hot rolled steel strip Z, the thickness, etc. What is necessary is just to determine heating temperature so that it may become a predetermined | prescribed temperature range.

Specifically, in the heating step, heating to a temperature in the range of 500 ° C to 750 ° C is preferable, and heating to 550 ° C to 650 ° C is more preferable.
In the heat retention step, it is necessary to keep the temperature of the heat treatment coil 28 at 500 ° C. or higher. Therefore, in the heating step, it is preferable to heat to 500 ° C. or higher.
On the other hand, when the temperature of the hot-rolled steel strip Z is heated to 750 ° C. or higher, not only the energy consumption increases (that is, the operation cost increases), but also the necking in the width direction of the hot-rolled steel strip Z occurs and is stable. Transport becomes difficult, and as a result, there is a risk of shape defects and deterioration of dimensional accuracy. In addition, when the heating temperature is too high, a lot of hard low-temperature transformation phases such as martensite and bainite are generated in the cooling process after the heat-holding process, which may make it difficult to soften the hot-rolled steel strip Z. There is. Therefore, the heating temperature of the hot-rolled steel strip Z in the heating step is preferably in the range of 500 ° C to 750 ° C.

Moreover, it is good also as a system which passes the heating apparatus 18, arrange | positioning a bridle roll etc. before and behind the heating apparatus 18, and providing predetermined tension | tensile_strength.
Moreover, you may have multiple heating apparatus at the point which adjustment of heating temperature etc. become easy.

It is also preferable that the inside of the heating device 18 be an inert gas atmosphere. Thereby, the oxidation of a hot-rolled steel strip can be suppressed. By suppressing oxidation of the surface of the hot-rolled steel strip, it is possible to form a good chemical conversion film or plating film when chemical conversion treatment or hot dip galvanization treatment is performed on a high-strength steel sheet obtained in the cold rolling process described later. it can.
The heat-treated steel strip Z heated in the heating process (heating device 18) is continuously conveyed and supplied to the winding process (winding machine 22).

[Winding process]
The winding process performed by the winder 22 is a process of winding the heat-treated steel strip Z heated in the heating process as a coil.
In the example shown in FIG. 2, a cutting machine 20 for cutting a steel strip is provided upstream of the winding machine 22, and a plurality of winding machines 22 are provided to wind a predetermined length around the heat treatment coil 28b. If it takes, the steel strip cut | disconnected and wound further is wound up to the next heat processing coil 28a, and it has the structure which winds up a coil continuously.
The winding method of the heat-treated steel strip Z and the winder 22 in the winding process can be used in a conventional method for producing a high-strength steel sheet as long as the hot-rolled steel strip heated to a high temperature can be wound up. Various winding machines are available. The winder 22 is preferably one that can be fitted with a heating sleeve or a heat insulating sleeve on the inner periphery of the coil.
The heat treatment coil 28 wound up in the winding process is subjected to a heat retaining process, and is held in a range of 500 ° C. to 700 ° C. for a predetermined time.

As described above, in order to reduce the cold rolling load when manufacturing a high-strength steel sheet, a technique for performing a heat treatment for softening the hot-rolled steel strip has been studied.
However, when tempering is performed before cold rolling, when a hot rolled coil is individually stored using a BAF annealing furnace or the like and tempered by batch processing, it is wound in a coil shape. Because it takes a long time to raise the temperature to a predetermined tempering temperature up to the inside of the coil, and a series of steps for heating, soaking, and slow cooling the entire hot rolled coil, the tempering efficiency is poor. was there.
On the other hand, it is conceivable to shorten the heating time by performing heat treatment while continuously conveying the hot-rolled steel strip at the pickling step, but securing sufficient time for softening is not possible. There was a problem that it was difficult. In particular, in the production of high-strength steel sheets, in order to improve the material, it is necessary to diffuse the elements in the structure, so only by heat treatment during continuous conveyance, the elements cannot be sufficiently diffused, There was a problem that the material could not be improved.

On the other hand, in the present invention, after the pickling step and before the cold rolling step, the steel plate is heated while being continuously conveyed, and the heated steel plate is wound around the coil without being cooled. The temperature of the taken coil is kept for a predetermined time.
Thus, in the manufacturing method of a high-strength steel sheet, when performing heat treatment prior to cold rolling, heating is performed before winding the hot-rolled steel strip after the pickling process as a coil, and then as a coil. By winding up, the coil temperature can be raised to the heat treatment temperature without heating in the coil state, so the temperature raising time can be shortened. Therefore, the heat treatment efficiency can be improved.
Further, as described later, the coiled coil is kept warm for a predetermined time, so that a sufficient time for softening can be ensured, and the hot rolled steel strip is uniformly and sufficiently stretched over the entire length and width of the hot rolled steel strip. Can be softened.
That is, according to the present invention, both shortening of heat treatment time and uniform softening can be achieved. Therefore, by cold rolling the softened hot-rolled steel strip, it is possible to obtain a shape of a high-strength steel plate having a favorable shape, thereby improving the yield.
Moreover, it can easily cope with the case where the heat treatment conditions (that is, the heating temperature in the heating step, the heat holding temperature and the heat holding time in the heat holding step, etc.) are changed according to the components of the hot rolled steel strip. Is possible.

  Moreover, since it can suppress that a hot-rolled steel strip is cooled by winding the heated hot-rolled steel strip in a coil in a winding process, the heating temperature in a heating process and the heat retention temperature in a heat retention process And the difference can be reduced. That is, the heating temperature in the heating process can be further lowered. Therefore, energy consumption can be reduced, and defects (necking, inability to sufficiently soften, etc.) due to the heating temperature being too high can be suitably suppressed.

  Further, from the above viewpoint, the difference between the heating temperature in the heating step and the temperature at the time of winding in the winding step (hereinafter also referred to as winding temperature) is preferably within 100 ° C, and more preferably within 50 ° C. By setting the heating temperature and the coiling temperature within 100 ° C., the difference between the heating temperature and the heat retention temperature can be further reduced.

In order to reduce the difference between the heating temperature and the coiling temperature, the hot-rolled steel strip may be kept warm in the conveying process from the heating process to the coiling process or in the coiling process. As a heat retaining method, a method such as covering a part or all of the conveyance path with a heat-insulating cover (hereinafter referred to as a gentle cooling cover), and covering the periphery of the winder with a gentle cooling cover can be used.
Moreover, you may heat a hot-rolled steel strip in a winding process. As a heating method in the winding process, a method similar to the heating method in the heating process can be used.
Such a configuration is suitable when the conveying speed of the hot-rolled steel strip Z is low, or when the distance between the heating device 18 and the winder 22 is long.
Moreover, it is also preferable to make the conveyance path to the winder 22 and the periphery of the winder 22 an inert gas atmosphere. Thereby, the oxidation of a hot-rolled steel strip can be suppressed.

[Heat retention process]
The heat retention step is a step of maintaining the temperature of the heat treatment coil 28 that is heated and wound in the range of 500 ° C. to 700 ° C. for 10 minutes to 10 hours.
As described above, the hot-rolled steel strip can be uniformly and sufficiently softened by maintaining the temperature of the heat treatment coil 28 within a range of 500 ° C. to 700 ° C. for a predetermined time.

The heat retaining temperature of the heat treatment coil 28 in the heat retaining step may be set as appropriate according to the composition of the hot rolled steel strip, the temperature history, the use of the high strength steel plate, etc., but the effect of softening the hot rolled steel strip is obtained. At 500 ° C. or higher. Further, in the cooling process after the heat retention step, a large amount of hard low-temperature transformation phase such as martensite and bainite is generated, and the temperature is set to 700 ° C. or less in order to prevent the softening of the hot-rolled steel strip from becoming difficult. .
From the above viewpoint, the heat retention temperature is preferably 500 ° C to 700 ° C, and more preferably 550 ° C to 650 ° C.
Note that the temperature of the heat treatment coil 28 in the heat insulation step is the temperature of the outer peripheral surface of the heat treatment coil 28 or the ambient temperature in the heat insulation cover.

  In addition, the time for maintaining the temperature of the heat treatment coil 28 in the heat retention step (heat retention time) may be appropriately set according to the composition of the hot rolled steel strip, the temperature history, the use of the high strength steel plate, and the like. It is set to 10 minutes or more and 10 hours or less in that the effect of softening the steel strip is obtained.

In addition, when producing a hot-rolled steel strip that can be softened in a short time with a heat retention time of less than 10 minutes, the hot-rolled steel strip is sufficient even if a conventional heat treatment furnace (for example, a continuous annealing furnace) is used. Although it may be softened, in the case of a hot-rolled steel strip that requires a heat retention time of 10 minutes or more, it cannot be sufficiently softened.
On the other hand, when manufacturing a hot-rolled steel strip that requires a heat retention time exceeding 10 hours, the ratio of the time required for heating to the entire time required for heat treatment is reduced. Therefore, the difference between the overall heat treatment time when a conventional batch heat treatment furnace (for example, a BAF annealing furnace) is used and the overall heat treatment time when the present invention is applied is reduced.
Also from such a point, it is preferable to apply the production method of the present invention to a hot-rolled steel strip that is softened within a heat retention time of 10 minutes to 10 hours.
Moreover, in a hot-rolled steel strip having a large Mn content, the heat retention time is preferably 1 hour to 3 hours from the viewpoint of securing the time required for Mn to concentrate in austenite.

There is no particular limitation on the method of maintaining the temperature in the heat retaining step, and for example, a method of housing in a container having heat resistance and heat insulation, covering with a cover having heat resistance and heat insulation, and the like can be used.
For example, the heat retention device 30 shown in FIG. 3 includes a bell-shaped cover 32 having an open surface that can accommodate the heat treatment coil 28 therein. As the cover 32, a material having heat resistance and heat insulation, for example, a steel cover whose inside is covered with a fibrous heat insulating material containing alumina, silica or the like can be used. That is, the cover 32 is a heat insulating cover in the present invention.
The heat retaining device 30 accommodates the heat treatment coil 28 in the cover 32, suppresses the dissipation of heat energy from the heat treatment coil 28, and retains heat while suppressing the temperature drop of the heat treatment coil 28.

In the example shown in FIG. 3, the heat retaining device 30 includes auxiliary heating means 36. The auxiliary heating means 36 is a known heating means, and heats the inside of the heat retaining device 30.
By having the auxiliary heating means 36, the heat treatment coil 28 can be kept at a temperature of 500 ° C. or more for a long time. The heat retaining device 30 may have a configuration without the auxiliary heating means 36 as long as the heat treatment coil 28 can be kept at a predetermined temperature range for a predetermined time.

Moreover, the heat retention apparatus 30 is provided with the atmosphere control apparatus 38 as a preferable aspect.
The atmosphere control device 38 supplies an inert gas into the cover 32 and controls the gas concentration and the like. By making the inside of the cover 32 an inert gas atmosphere, the oxidation of the surface of the heat treatment coil 28 (hot-rolled steel strip) can be suppressed.

  In this way, a decrease in productivity can be suppressed by performing the heat retention process using a separate apparatus (heat retention apparatus 30) from the processing facility 10 that performs the heating process and the winding process.

In addition, there is no limitation in particular in the conveyance method of the heat processing coil 28 from the processing equipment 10 which performs a winding process to the heat retention apparatus 30 which performs a heat retention process. For example, a coil carriage used in normal heat treatment equipment may be used. At that time, from the viewpoint of suppressing the temperature drop of the heat treatment coil 28 during conveyance, it is preferable to convey the heat treatment coil 28 while covering it with a heat insulating material or the like.
Further, when the distance from the winder 22 to the heat retaining device 30 is short (that is, when the transport time is short), it is possible to transport heavy objects such as cranes and forklifts as the transport means instead of the coil transport cart. Transportation equipment may be used. Although it is difficult to improve the heat insulating property when transporting using such transporting equipment, a decrease in temperature of the heat treatment coil 28 can be suppressed if transporting for a short time.

Further, the heat retaining device 30 shown in FIG. 3 is configured to accommodate one heat treatment coil 28, but is not limited thereto, and may be configured to accommodate a plurality of heat treatment coils 28.
For example, like a heat retaining device 40 shown in FIG. 4, a box-shaped pit 42 having one surface opened and a cover 44 covering the opening of the pit, a plurality of heat treatment coils 28 are accommodated in the pit 42, The opening may be covered with the cover 44.
The heat treatment coil 28 (hereinafter referred to as the heat retaining coil 29) that has undergone the heat retaining process is subjected to a cooling process.

[Cooling process]
The cooling step is a step of cooling the heat-retaining coil 29 that has been kept warm for a predetermined time before the cold rolling step.
There is no particular limitation on the cooling method of the heat retaining coil 29 in the cooling step, and after the heat retaining device keeps heat for a predetermined time, the cover may be removed to promote air cooling, or may be cooled. Once stored in the coil yard, it may be cooled.

Alternatively, forced cooling may be performed by positively spraying a cooling medium (for example, liquid, gas, etc.) on the heat retaining coil or the steel strip discharged from the heat retaining coil.
Especially, it is preferable to cool, conveying the steel strip paid out from the heat retention coil, and it is preferable to perform a cooling process and the cold rolling process mentioned later continuously.
If the coil remains as it is, it takes time to cool down to the inside. However, by cooling the steel strip from the heat retaining coil, the cooling time can be shortened and the productivity of the high-strength steel sheet can be further improved.

In FIG. 5, an example of the cold rolling equipment which performs a cooling process and a cold rolling process continuously is shown.
The cold rolling facility 50 includes a payer 52, a welding machine 54, a cooling device 56, a looper 58, a cold rolling machine 60, a cutting machine 62, a winder 64, and guide rollers 66a to 66d. Have

The cold rolling equipment 50 pays out the steel strip Z from the heat-retaining coil 29a by the dispenser 52, passes the cooling device 56 and the cold rolling mill 60, and winds the cold rolled steel strip Z with the winder 64. It is wound around a coil.
The dispenser 52 is loaded with the heat retaining coil 29a and dispenses the steel strip Z. In the illustrated example, a plurality of dispensers 52 are provided, and different heat retaining coils 29a and 29b are continuously passed through. The welding machine 54 joins the end portion of the steel strip discharged from the heat retaining coil 29a and the tip of the steel strip discharged from the heat retaining coil 29b.
In addition, a cutting machine 62 for cutting the steel strip is provided on the upstream side of the winding machine 64, and the winding machine 64 includes a plurality of winding shafts 68, and the cold rolling coil 70 has a predetermined length. When the steel strip is wound, the steel strip that is cut and further conveyed is wound around the next cold rolled coil and continuously wound around the coil.
The guide rollers 66a to 66d are ordinary guide rollers that guide the steel strip Z that has been paid out to a predetermined transport path.
The steel strip Z paid out by the payer 52 is conveyed to the cooling device 56 through a predetermined path.

  The cooling device 56 cools the steel strip Z to be conveyed. As the cooling device 56, various known cooling devices used in the manufacture of conventional high-strength steel sheets such as a water spray type cooling device and a gas injection type cooling device can be used.

  In addition, there is no limitation in particular about the temperature of the heat retention coil 29 at the time of charging the dispenser 52. Charging at a higher temperature is advantageous in terms of production efficiency because the waiting time until the coil temperature decreases can be shortened. However, since a heat load corresponding to the temperature at the time of charging is applied to the equipment, it is necessary to make the heat resistance specification of the dispenser 52 corresponding to that, and the equipment cost increases as the equipment inserts a high-temperature coil. . Therefore, it may be determined in consideration of productivity, economy and the like.

Moreover, although there is no limitation in particular in the cooling temperature of the heat retention coil (steel strip) in a cooling process, when it reaches a cold rolling process (cold rolling mill 60), it cools so that it may become 200 degrees C or less. Is preferred.
In the cold rolling process, the higher the steel plate temperature, the lower the deformation resistance during cold rolling, and the reduction of the rolling load can be achieved, but the steel plate exceeding 200 ° C. is continuously rolled. Then, the thermal crown of the rolling roll develops, and the shape of the steel plate is disturbed. In some cases, the steel plate breaks, and operational troubles easily occur.
From the above viewpoint, the cooling temperature in the cooling step is preferably 200 ° C. or lower, and more preferably 50 ° C. or lower.

The cold rolling equipment 50 shown in FIG. 5 has a looper 58 as a preferred embodiment.
The looper 58 is for adjusting the tension applied to the steel strip Z to be conveyed, and various known loopers used in the manufacture of conventional high-strength steel plates can be used.
The arrangement of the welding machine 54, the cooling device 56, and the looper 58 is not limited to the order shown in FIG. 5 as long as each device can exhibit a predetermined function. For example, the cooling device 56, the welder 54, and the looper 58 may be arranged in this order.
The steel strip (heat-retaining coil) cooled in the cooling process is supplied to the cold rolling process (cold rolling mill 60).

[Cold rolling process]
The cold rolling step is a step of forming a high-strength steel sheet by cold rolling the steel strip cooled to a predetermined temperature or lower in the cooling step.
As the cold rolling method and the cold rolling mill 60 in the cold rolling process, various cold rolling methods and cold rolling mills that are used in a conventional method for producing a high-strength steel sheet can be used.

  Here, in the present invention, as described above, the hot-rolled steel strip can be softened because it is kept at a temperature of 500 ° C. to 700 ° C. for 10 minutes to 10 hours after hot rolling. The deformation resistance during cold rolling can be reduced.

  As mentioned above, although the manufacturing method and processing equipment of the high strength steel plate of this invention were demonstrated in detail, this invention is not limited to the above-mentioned example, In the range which does not deviate from the summary of this invention, various improvement and change are performed. Of course, you may.

  Hereinafter, the present invention will be described in more detail based on examples. The materials, amounts used, ratios, processing details, processing procedures, and the like shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the following examples.

[Examples 1 to 4]
As Examples 1 to 4, steels each having the components shown in Table 1 and the balance being Fe and inevitable impurities were melted, and high-strength steel plates were produced using steel slabs obtained by continuous casting.
Table 1 also shows the heat treatment conditions necessary for softening the hot-rolled steel strip having each component composition.

[Hot rolling process]
The steel slab is fed to a hot rolling process and heated to 1250 ° C. in a heating furnace, and subsequently as a hot rolled steel strip (sheet thickness 2.0 mm) in a finish rolling mill (finish rolling temperature 870 ° C.), It was wound up on a hot rolled coil. And the hot-rolled coil was air-cooled to 100 degrees C or less in the coil yard.

[Pickling process, heating process, winding process]
Next, the hot-rolled coil was charged into the processing facility 10 shown in FIG. The pickling tank used a normal pickling solution mainly composed of hydrochloric acid, and removed the scale while discharging the hot-rolled steel strip Z of the hot-rolled coil 26 and continuously conveying it.
The pickled hot-rolled steel strip Z was heated by a heating device 18 disposed on the downstream side thereof. An induction heating device was used as the heating device. The heating temperature was the heat retention temperature + 40 ° C. in the heat retention step.
Thereafter, the heated hot-rolled steel strip Z was wound around the heat treatment coil 28 by the winder 22.

[Heat retention process]
Next, the wound heat treatment coil 28 was housed in a heat retaining device 30 shown in FIG. The material of the heat retaining device 30 was isowool (Isolite Kogyo Co., Ltd.) having a thickness of 100 mm.
In addition, as the heat retaining device 30, a device without the auxiliary heating means 36 and the atmosphere control device 38 was used.
In the heat retention step, the heat retention temperature (the temperature in the cover 32) was maintained in the range from the temperature shown in Table 1 to the temperature in Table 1 + 40 ° C.

[Cooling process / Cold rolling process]
After the heat insulation process, the heat insulation coil 29 was inserted into the cold rolling equipment 50 shown in FIG.
While the steel strip Z of the heat-retaining coil 29 is discharged and continuously conveyed, the cooling device 56 cools the steel strip Z to 100 ° C. or less using a water spray, followed by cold rolling with a cold rolling mill 60. Rolling was performed to produce a high-strength steel plate.
The cold rolling mill is a complete continuous rolling mill equipped with a welding machine on the entry side, and is a quadruple type 5 stand mill. With such a cold rolling mill, cold rolling was performed to a plate thickness of 1.0 mm by rolling at a total reduction ratio of 50%.

[Comparative Example 1]
As Comparative Example 1, after performing a pickling process and a winding process using a processing facility that does not have a heating device on the downstream side of the pickling tank, a batch-type annealing facility 200 as shown in FIG. 6 is used. Except for performing the heating step, the heat retaining step and the cooling step, and performing the cold rolling step using a cold rolling facility which does not have a cooling device on the upstream side of the cold rolling mill, it is the same as in Example 1. A strength steel plate was produced.
The batch type annealing equipment 200 shown in FIG. 6 is for heat treatment by storing hot rolled coils 206 stacked in four stages with a spacer 204 in a bell 202.

[Comparative Example 2]
As Comparative Example 2, the heating apparatus 18 in the treatment facility 10 is installed on the upstream side of the pickling tank 16, except that the pickling process is performed after the heating process and the heat retaining process is not performed. Similarly, a high-strength steel plate was produced.

<Evaluation>
〔processing time〕
In Table 2, in each example and comparative example, the time taken for the heating process, the time taken for storage in the heat retaining device after the heating process (including the winding process) ("conveying process 1"), The time taken for the heat retention process, the time taken for the start of the cooling process after the completion of the heat retention process ("transport process 2"), and the time taken for the cooling process are shown.

(Break)
In the cold rolling process, it was evaluated whether cold rolling was possible without breaking the steel sheet. Steel sheets containing many alloy components as high-strength steel sheets may be embrittled by heat-treating hot-rolled steel sheets. In such cases, it is necessary to use a looper in the cold rolling process or a leveler for shape correction. It broke by bending deformation and did not reach the rolling mill.
The case where breakage did not occur was evaluated as “A”, and the case where breakage occurred was evaluated as “B”.

[Rolling load]
For steel sheets that could be rolled by a cold rolling mill without causing breakage, whether or not softening by heat treatment was sufficient was evaluated by a rolling load. The upper limit of the maximum load per single stand of the rolling mill was set to 23000 kN, and it was evaluated whether or not rolling up to a sheet thickness of 1.0 mm was possible with a rolling load less than that. The reason why such a maximum load is set is that if a rolling load exceeding this is generated, the shape control ability of the rolling mill is insufficient, and operation troubles such as fracture due to a defective shape of the steel sheet occur.
The case where rolling to a plate thickness of 1.0 mm was possible was evaluated as “A”, and the case where rolling to a plate thickness of 1.0 mm was not possible was evaluated as “B”.
The measurement results are shown in Table 2.

As shown in Table 2, after the pickling process, before the cold rolling process, the steel sheet was heated while being continuously conveyed, and the heated steel sheet was wound around the coil without being cooled, and the coil was wound up. Examples 1-4 which are the manufacturing methods of the high-strength steel plate of this invention which hold | maintain the temperature of 500 degreeC-700 degreeC for predetermined time are compared with the comparative example 1 which heat-processes with a batch-type annealing equipment. It can be seen that the treatment time can be greatly reduced and the heat treatment efficiency can be improved. In particular, in Comparative Example 1, it takes about 20 times as long as the necessary heat retention time, whereas in Examples 1 to 4, the necessary heat retention time is about 1.1 to 1.4 times longer. It turns out that it becomes processing time and can reduce unnecessary processing time.
Moreover, in the comparative example 1, since the time of a heating process is long, it turns out that a steel plate becomes embrittled and a fracture | rupture arises in a cold rolling process.
On the other hand, in Examples 1-4, since the time of a heating process is short, it turns out that a fracture does not arise in a cold rolling process, without a steel plate becoming brittle.

Moreover, in the batch-type annealing equipment used in Comparative Example 1, steel strips having different components as in Examples 1 to 4, that is, steel strips having different heat treatment conditions cannot be accommodated at one time.
Therefore, when continuously heat-treating steel strips having different components, it is necessary to sequentially perform the heat treatment or prepare a plurality of annealing facilities to perform the heat treatment. Also in this point, the comparative example 1 is significantly inferior in processing efficiency than the example.

Moreover, it turns out that the comparative example 2 which does not perform a heat retention process cannot perform sufficient softening, Therefore, the rolling load in a cold rolling process becomes large and cannot be rolled to desired thickness.
In contrast, the high-strength steel plates of Examples 1 to 4 have a small rolling load in the cold rolling process and are sufficiently softened.
From the above, the effects of the present invention are clear.

DESCRIPTION OF SYMBOLS 10 Processing equipment 12,52 Discharge machine 14,54 Welding machine 16 Pickling tank 18 Heating device 20,62 Cutting machine 22,64 Winding machine 24,66 Guide roller 26 Hot rolling coil 28 Heat treatment coil 29 Heat retention coil 30,40 Heat retaining device 32, 44 Cover 36 Auxiliary heating device 38 Atmosphere control device 42 Pit 50 Cold rolling equipment 56 Cooling device 58 Looper 60 Cold rolling mill 68 Winding shaft 70 Cold rolling coil 200 Patch annealing equipment 202 Bell 204 Spacer 206 coil

Claims (6)

A method for producing a high-strength steel sheet having a hot rolling step, a pickling step and a cold rolling step in this order, and further, between the pickling step and the cold rolling step,
After removing the oxide film on the surface of the steel plate in the pickling step, a heating step of heating while continuously conveying the steel plate,
A winding step of winding the steel sheet heated in the heating step as a coil;
A method for producing a high-strength steel sheet, comprising a step of maintaining a temperature of the coiled coil at 500 ° C. to 700 ° C. for 10 minutes to 10 hours.   The manufacturing method of the high strength steel plate of Claim 1 which heats the said steel plate to the temperature of 500 to 750 degreeC in the said heating process. The method for producing a high-strength steel sheet according to claim 1 or 2, wherein a heating method in the heating step is an induction heating method.   The method for producing a high-strength steel sheet according to any one of claims 1 to 3, wherein the heat-retaining method in the heat-retaining step is to reduce a temperature drop of the coil by a heat-retaining cover that covers the entire coil.   After the heat retention step and before the cold rolling step, the coil is inserted into an apparatus for performing the cold rolling step, and the steel sheet discharged from the coil is conveyed at 200 ° C. or lower. The manufacturing method of the high-strength steel plate of any one of Claims 1-4 which has a cooling process which cools to 5 nm. It is the processing equipment of the high strength steel plate which performs the pickling process in the manufacturing method of the high strength steel plate according to any one of claims 1 to 5, the heating process, and the winding process,
A pickling tank for performing the pickling step to remove the oxide film on the steel sheet surface;
A heating device that heats the steel sheet, disposed on the downstream side of the pickling tank in the conveying direction of the steel sheet,
The processing equipment of the high strength steel plate which has a winding device which winds up the heated steel plate as a coil, and is arranged downstream of the heating device.