JPWO2010123152A1 - Cold rolled steel sheet manufacturing method and manufacturing equipment thereof - Google Patents

Abstract

The present invention provides a method and equipment for producing a cold-rolled steel sheet capable of producing a cold-rolled steel sheet having a uniform quality at a low cost and a high yield by continuously annealing a steel sheet produced by a thin slab continuous casting method. In the method of manufacturing a cold-rolled steel sheet by hot rolling, acid cleaning, cold rolling, and continuous annealing of a slab manufactured by a thin slab continuous casting method, the continuous annealing is performed on the cold-rolled steel sheet 14 by 800. A soaking step for holding at a temperature of not less than 850 ° C. and not more than 850 ° C. for a time of not less than 40 seconds and not more than 60 seconds; However, it has a cooling step of applying tensile stress and bending stress to the steel plate 14, and an overaging step of holding the cooled steel plate 14 at a temperature of 350 ° C. or higher and 400 ° C. or lower for a period of 60 seconds or more and 180 seconds or less, In the cooling step, the tensile stress applied to the steel sheet 14 is 0.5 kg / mm 2 or more and 1.5 kg / mm 2 or less, and the bending stress is 10 kg / mm 2 or more and 35 kg / mm 2 or less.

Description

  The present invention relates to a method for manufacturing a cold-rolled steel sheet and equipment for manufacturing the cold-rolled steel sheet by hot rolling, acid cleaning, cold rolling, and continuous annealing of a slab manufactured by a thin slab continuous casting method.

  Conventionally, when producing a cold-rolled steel sheet by hot rolling, acid cleaning, and cold-rolling a slab produced by a thin slab continuous casting method, the obtained cold-rolled steel sheet is hard and inferior in workability, Usually, cold-rolled steel sheets are annealed in a batch annealing furnace. For example, Patent Document 2 describes a belt-type thin slab continuous casting method, and Patent Document 3 describes a technique of continuously hot rolling a steel plate cast from thin slab continuous casting. Patent Document 1 proposes a method of manufacturing a thin steel sheet that can manufacture a cold-rolled steel sheet having excellent material stability by balancing the heat treatment and hot working of the slab to control the material.

JP 9-316533 A JP-A 61-279341 JP-A-11-77102

However, annealing of cold-rolled steel sheets using a batch annealing furnace requires a long time, so there is a problem that running costs are increased. In addition, in the annealing of cold-rolled steel sheets using a batch annealing furnace, the coiled cold-rolled steel sheet is charged into the batch annealing furnace, so that the entire coiled cold-rolled steel sheet can be uniformly annealed. It is difficult and the problem that the workability of a cold-rolled steel plate changes in a longitudinal direction arises.
On the other hand, in the manufacturing method of the thin steel plate of patent document 1, material control is aimed at balancing the heat processing and hot processing of a slab. For this purpose, the slab is held for 3 to 30 minutes in the range of Ar3 point (temperature at which transformation starts from γ iron to α iron and austenite to ferrite during cooling) to 1120 ° C, or the continuously cast slab is cooled once. After that, there is a problem that the load on the hot rolling process is increased by holding for 3 to 30 minutes in the range of Ar3 point to 1120 ° C. Moreover, although the manufacturing method of the thin steel plate of patent document 1 is applicable to any annealing process of batch annealing and continuous annealing, each condition of annealing process is not disclosed.

  The present invention has been made in view of such circumstances, and continuously anneals a steel plate formed from a slab produced by a thin slab continuous casting method to produce a cold-rolled steel plate having a uniform quality at a low cost and a high yield. An object of the present invention is to provide a manufacturing method of a possible cold-rolled steel sheet and a manufacturing facility with a simple configuration at low cost.

  As a result of intensive studies to solve such problems, the present inventors have grown crystal grains in the steel plate in the soaking process of continuous annealing, and subsequently applied tensile stress and bending stress to the steel plate in the cooling process. It has been found that residual stress is generated in the steel sheet, and further, precipitation of carbides can be promoted by the residual stress generated in the overaging process, and the present invention has been achieved. As a result, the amount of carbon dissolved in the crystal grains in the steel sheet can be reduced, and even a steel sheet formed from a slab produced by a thin slab continuous casting method can be uniformly softened. A steel plate can be obtained. The gist of the present invention is as follows.

The manufacturing method of the cold-rolled steel sheet according to the first invention that meets the above-described object is a method of hot rolling, acid cleaning, cold rolling, and continuous annealing of a slab manufactured by a thin slab continuous casting method, so that carbon is reduced to 0.0. In the method of manufacturing a cold-rolled steel sheet containing 5% by mass or less, silicon 0.02% by mass or more, manganese 0.15% by mass or more, and calcium 0.001% by mass or more, the continuous annealing is performed by the cold A soaking step for holding the rolled steel plate at a temperature of 800 ° C. or more and 850 ° C. or less for 40 seconds or more and 60 seconds or less, and the steel plate that has passed through the soaking step is 350 ° C. at a cooling rate of 10 ° C./sec or more. The cooling step of applying tensile stress and bending stress to the steel sheet while lowering the temperature to 400 ° C. or lower, and the steel plate that has passed through the cooling step at a temperature of 350 ° C. or higher and 400 ° C. or lower for 60 seconds or longer and 180 seconds or shorter. Overaging work to keep time Has the door, the cooling step, the tensile stress applied to the steel sheet is 0.5 kg / mm ² or more and at 1.5 kg / mm ² or less, the bending stress is 10 kg / mm ² or more and 35 kg / mm ² It is as follows.

In the method for manufacturing a cold-rolled steel sheet according to the first invention, the bending stress is oriented in an axial direction in a direction (plate width direction) perpendicular to the sheet passing direction of the steel sheet passing through the pass line of the cooling step. The path line is shifted in the direction (plate thickness direction) perpendicular to the pass line by moving both or any one of the pair of small diameter rolls disposed on both sides in the thickness direction of the steel plate with the center position shifted in the plate passing direction. It is preferable to generate by pushing in excess of.
Here, the outer diameter of the paired small-diameter rolls is 200 mm or more and 500 mm or less, the distance between the axial centers is 500 mm or more and 1000 mm or less, and the pushing distance of the small-diameter roll that is pushed beyond the pass line is 10 mm or more and It is preferable that it is 100 mm or less.

  In the method for manufacturing a cold-rolled steel sheet according to the first invention, it is preferable that the value of the bending stress is changed according to the amount of carbon contained in the steel sheet.

  The cold rolled steel sheet manufacturing facility according to the second aspect of the present invention is a cold rolled steel sheet obtained by hot rolling, acid cleaning, cold rolling, and continuous annealing of a slab manufactured by a thin slab continuous casting method. In a production facility for cold-rolled steel sheets to be produced, a continuous annealing line for performing the continuous annealing includes a heat-equalizing device for soaking the cold-rolled steel plate, and a cooling device for cooling the so-heated steel plate. An overaging device for overaging the cooled steel plate, and the cooling device is provided on a cooling means for cooling the steel plate and / or on one or both of the inlet side and the outlet side of the cooling means. The cooling is performed while directing the axial direction in a direction orthogonal to the plate passing direction of the steel plate and shifting the axial center position to the plate passing direction so as to be arranged on both sides in the thickness direction of the steel plate to support the movement of the steel plate. The pass line in a direction perpendicular to the pass line of the device It is pushed beyond and a bending stress applying means comprises a small-diameter rolls to be paired to impart bending stresses in the steel sheet.

In the manufacturing method of the cold-rolled steel sheet according to the first invention, the soaking temperature in the soaking step of continuous annealing is set to 800 to 850 ° C., which is higher than 700 to 800 ° C. set in the conventional continuous annealing furnace. Therefore, the crystal grains in the steel plate can be grown. And, by applying tensile stress and bending stress to the steel sheet in the cooling process, dislocations can be generated in the crystal grains in the steel sheet, and precipitation of carbides can be promoted by the dislocations generated in the overaging process. . As a result, the amount of carbon dissolved in the crystal grains in the steel sheet can be reduced, and the steel sheet formed from the slab manufactured by the thin slab continuous casting method is softened by applying continuous annealing. It is possible to produce a large quantity of cold-rolled steel sheets having a uniform quality within the steel sheet at low cost.
In addition, the time required for annealing to perform the annealing method of the steel sheet in a continuous annealing furnace rather than the batch annealing of the cold-rolled steel sheet in the conventional coil shape, has been shortened by several tens of minutes from the conventional about two days, The productivity of the cold-rolled steel sheet is greatly improved and the quality in the longitudinal direction of the steel sheet after annealing becomes uniform.

  In the method for manufacturing a cold-rolled steel sheet according to the first invention, the bending stress is oriented in the axial direction in a direction orthogonal to the sheet passing direction of the steel sheet passing through the pass line in the cooling step and the axial position is shifted in the sheet passing direction. When pushing both or one of the paired small diameter rolls arranged on both sides of the steel plate beyond the pass line in a direction perpendicular to the pass line, a bending stress is applied to the steel plate in the cooling process by a simple method. can do.

In the method for producing a cold-rolled steel sheet according to the first invention, the outer diameter of the paired small-diameter rolls is 200 mm or more and 500 mm or less, the distance between the axes is 500 mm or more and 1000 mm or less, and the small diameter is pushed beyond the pass line. When the indentation distance of the roll is 10 mm or more and 100 mm or less, the bending curvature can be adjusted to 10 kg / mm ² or more and 35 kg / mm ² or less by adjusting the processing curvature radius.

  In the method for manufacturing a cold-rolled steel sheet according to the first invention, when the value of the bending stress is changed according to the amount of carbon contained in the steel sheet, it is possible to prevent a decrease in workability due to hardening of the cold-rolled steel sheet due to residual stress. .

  In the cold rolled steel sheet manufacturing facility according to the second aspect of the present invention, tensile stress and bending stress can be applied to the steel sheet in the cooling process by a cheap and simple mechanism in the continuous annealing line, and the crystal grains of the steel sheet It is possible to reduce the amount of solid solution carbon in the crystal grains by promoting the precipitation of carbides during the overaging process by using dislocations generated in the crystal. As a result, it is possible to manufacture a cold-rolled steel sheet having a uniform quality within the steel sheet at low cost and in large quantities at a low temperature by continuously annealing the steel sheet formed from the slab manufactured by the thin slab continuous casting method.

FIG. 1 is an explanatory diagram of a continuous annealing line provided in a cold rolled steel sheet manufacturing facility according to an embodiment of the present invention.
FIG. 2 is an explanatory view of the cooling device of the continuous annealing line.
FIG. 3 is an explanatory diagram showing the relationship between the annealing temperature and softening of the steel sheet (tensile strength, yield stress, elongation).
FIG. 4 is an explanatory diagram showing the relationship between soaking time and softening of the steel sheet (tensile strength, yield stress, elongation).
FIG. 5 is an explanatory diagram showing the relationship between the cooling rate and the softening (elongation) of the steel sheet.
FIG. 6 is an explanatory diagram showing the relationship between bending stress and steel sheet softening (elongation).

Based on the Example of this invention, This invention is demonstrated, referring drawings.
A cold-rolled steel sheet manufacturing facility according to an embodiment of the present invention will be described. As shown in FIG. 1, a continuous annealing line 10 provided in a cold rolled steel sheet manufacturing facility according to an embodiment of the present invention includes, for example, an entry side facility 11, an annealing facility 12, and an exit side facility 13. ing. Here, the entry-side equipment 11 is a payoff reel that rewinds a coil 15 that has wound a steel plate 14 obtained by hot rolling, acid cleaning, and cold rolling a slab manufactured by a thin slab continuous casting method. 16, a welding machine 17 that connects the tail end of the steel sheet 14 that has been rewound earlier and the tip end of the steel sheet 14 that is rewound later, an electrolytic cleaning device 18 that cleans the rewound steel sheet 14, and cleaning And an entry-side looper 19 for gradually sending out the stored steel plate 14 while storing it. Further, the exit side equipment 13 includes an exit side looper 20 that gradually sends out the steel sheet 14 delivered from the annealing equipment 12, and elimination of yield point elongation and surface roughness of the steel sheet 14 delivered from the exit side looper 20. The temper rolling mill 36 for adjusting the winding, the winder 22 for winding the steel plate 14 that has passed through the temper rolling mill 36 to form the coil 21, and the steel plate 14 is cut in accordance with the end of winding of the coil 21. And a shearing machine 23.

  The annealing equipment 12 includes a heating device 24 that heats the cold-rolled steel sheet 14 to an annealing temperature (for example, 800 to 850 ° C.), and a steel sheet 14 that has been heated to the annealing temperature at the annealing temperature for a certain time (for example, 40 ˜60 seconds) A soaking device 25 for keeping the steel plate 14 soaked, and an aging temperature (eg, 10 ° C./second or more) of the soaked steel plate 14 at a preset cooling rate (eg, 10 ° C./second or more). A cooling device 26 for cooling to 400 to 350 ° C., an overaging device 27 for holding the cooled steel plate 14 at an aging temperature for a certain period of time (60 to 180 seconds), and an overaging treatment, and a steel plate for which the overaging treatment has been completed. 14 is further cooled (for example, cooled to 40 to 60 ° C.), and a secondary cooling device 28 for feeding to the outlet looper 20 is provided. The entrance side equipment 11, the exit side equipment 13, the heating device 24 of the annealing equipment 12, the soaking device 25, the overaging device 27, and the secondary cooling device 28 are cast pieces manufactured by a normal continuous casting method. The same equipment and equipment used in the continuous annealing line can be used when producing a cold-rolled steel sheet by hot rolling, acid cleaning, cold rolling, and continuous annealing.

As shown in FIG. 2, the cooling device 26 of the annealing equipment 12 is provided on each of the entry side and the exit side, and applies tensile stress (for example, 0.5 kg / mm ² or more and 1.5 kg / mm ² or less) to the steel plate 14. Between the first and second hot bridle rolls 29, 30 that pass the steel plate 14 while being applied, and the first and second hot bridle rolls 29, 30 are provided side by side along the plate passing direction of the steel plate 14. The first and second window boxes 31 and 32 for blowing cold air onto both surfaces of the passing steel sheet 14, the entrance side of the first window box 31, and the first window are examples of cooling means for cooling the steel sheet 14. Provided on the exit side of the box 31 (that is, the entrance side of the second window box 32) and the exit side of the second window box 32, respectively, the axial direction is perpendicular to the plate passing direction of the steel plate 14. In addition, the axial center position is shifted in the plate direction and arranged on both sides in the thickness direction of the steel plate 14 to support the movement of the steel plate 14, while exceeding the pass line PL in a direction perpendicular to the pass line PL of the cooling device 26. Bending stress applying means 35 provided with a pair of small diameter rolls 33 and 34 for applying bending stress (for example, 10 kg / mm ² or more and 35 kg / mm ² or less) to the steel sheet 14 by being pushed by δ. .

  The small-diameter rolls 33 and 34 are respectively supported by bearings (not shown) on both sides, and the bearings are fixed to the mounting base so that the axial direction of the small-diameter rolls 33 and 34 is oriented in a direction perpendicular to the sheet passing direction of the steel plate 14. ing. The mounting base is provided with a driving mechanism (for example, a fluid pressure cylinder) (not shown) that moves the mounting base so that the small-diameter rolls 33 and 34 advance and retreat in a direction perpendicular to the pass line PL of the cooling device 26. . Thereby, for example, by fixing the small diameter roll 33 and pushing the small diameter roll 34 by δ beyond the pass line PL, bending stress and tensile stress can be applied to the steel sheet 14 at the portion in contact with the small diameter roll 34. it can.

Here, the outer diameters of the first and second hot bridle rolls 29 and 30 are, for example, not less than 800 mm and not more than 1200 mm. On the other hand, the outer diameters of the small-diameter rolls 33 and 34 are 200 mm or more and 500 mm or less. As a result, a bending stress larger than the bending stress caused by bending applied to the steel plate 14 when passing through the first and second hot bridle rolls 29 and 30 can be applied by pushing the small-diameter rolls 33 and 34. If the outer diameter of the small-diameter rolls 33, 34 is less than 200 mm, the small-diameter rolls 33, 34 themselves have insufficient strength. If the outer diameter of the small-diameter rolls 33, 34 exceeds 500 mm, bending stress is effectively applied to the steel plate 14. In addition, the installation space for the small-diameter rolls 33 and 34 increases, and the cooling efficiency decreases.
The pair of small diameter rolls are provided at three locations on the entry side and the exit side of the first window box 31 (in the middle of the first and second window boxes) and the exit side of the second window box 32. Therefore, it is more effective if the bending stress applied to the steel sheet by the pair of small diameter rolls is applied more by the small diameter roll provided in a place where the temperature of the steel sheet is low.
In addition, about the optimal combination of the diameter in a pair of small diameter roll, if the said outer diameter is in the range of 200 mm-500 mm, there is no difference in both the same diameter and a different diameter. By setting the outer diameters of the pair of small-diameter rolls to the same diameter, the stock of replacement (spare parts) rolls when the rolls are damaged or worn can be reduced, and maintenance is facilitated.

The distance between the shaft centers of the small diameter rolls 33 and 34 is 500 mm or more and 1000 mm or less. When the distance between the shaft centers of the small-diameter rolls 33 and 34 exceeds 1000 mm, the processing curvature radius due to the indentation of the small-diameter rolls 33 and 34 becomes large. It becomes difficult and the length of the pass line PL in the cooling device 26 becomes long, and the cooling efficiency is lowered.
On the other hand, when the distance between the shaft centers of the small diameter rolls 33 and 34 is less than 500 mm, when the small diameter rolls 33 and 34 are pushed in, the distance between the shaft centers of the small diameter rolls 33 and 34 is the diameter of the small diameter rolls 33 and 34. There is a risk that the steel sheet 14 may be rolled down by the two small-diameter rolls 33 and 34. The pushing distance δ of the small diameter roll 34 pushed beyond the pass line PL is preferably 10 mm or more and 100 mm or less. When the pushing distance is less than 10 mm, when the small-diameter rolls 33 and 34 come into contact with the steel plate 14, a slip occurs in the steel plate 14, causing a problem of generating wrinkles in the steel plate 14. On the other hand, if the pushing distance exceeds 100 mm, the steel plate 14 may interfere with the first and second window boxes 31 and 32.
The surface roughness of the small-diameter rolls 33 and 34 was Ra = 2 to 3. This is because the steel sheet is pushed in by 10 mm to 100 mm with the small diameter roll, so that the steel sheet is wound around a part of the circumference of the small diameter roll, and the slip of the steel sheet with the small diameter roll is eliminated. Therefore, the surface roughness of the small diameter roll can be lowered as described above. Thereby, the cost reduction at the time of manufacture of a small diameter roll is attained. In general, the surface roughness Ra of the roll used in the furnace is 4-5.
The wall thickness of the small diameter rolls 33 and 34 is 10 to 20 mm. Since this requires strength because the steel sheet is pushed in with a small-diameter roll, it is preferable that the wall thickness be slightly thicker than the wall thickness (about 15 mm) generally used in the furnace.

Then, the manufacturing method of the cold rolled steel plate which concerns on one embodiment of this invention is demonstrated.
The method for producing a cold-rolled steel sheet according to the present embodiment includes hot-rolling, acid cleaning, cold-rolling, and continuous annealing of a slab produced by a thin slab continuous casting method, so that carbon is 0.5% by mass. Hereinafter, it is a method for producing a cold-rolled steel sheet made of a low carbon steel containing 0.02 mass% or more of silicon, 0.15 mass% or more of manganese, and 0.001 mass% or more of calcium. In addition, the thickness of the steel sheet to which the present invention can be applied is to produce a cold-rolled steel sheet by hot-rolling, pickling, and cold-rolling a slab produced by a conventionally known thin slab continuous casting method. In this case, a steel sheet of 0.15 mm or more and 3.2 mm or less is suitable. In the continuous annealing, first, the cold-rolled steel sheet 14 is heated by a heating device 24 to an annealing temperature of 800 ° C. or more and 850 ° C. or less (heating process), and the steel plate 14 heated to the annealing temperature is heated by a soaking device 25. And the steel sheet 14 is held for 40 seconds to 60 seconds at the annealing temperature (soaking step). As a result, dislocations disappear in crystal grains of the steel sheet 14 that has been cold-rolled and hardened, and crystal grains having a size corresponding to the annealing temperature and holding time are generated, and precipitates in the crystal grains are decomposed and crystallized. It dissolves in the grains.

Here, the soaking process will be described. The cold-rolled steel sheet 14 was annealed for 40 seconds in a temperature range of 730 to 850 ° C., then cooled to 400 ° C. at a cooling rate of 10 ° C./second and held at 400 ° C. for 180 seconds. Then, after cooling to 20 ° C., 1.0% temper rolling was performed, and then a tensile test was performed to determine the tensile strength, yield stress, and elongation to break of the steel sheet 14. The result is shown in FIG.
As shown in FIG. 3, it was confirmed that the tensile strength and the yield stress were lowered and the elongation was increased as the annealing temperature was raised. That is, it can be seen that the steel plate 14 becomes softer as the annealing temperature is higher. However, if the temperature is too high, the steel plate 14 becomes too soft, and the plateability of the steel plate 14 in the heating device 24 and the heat equalizing device 25 is likely to be affected (for example, a heat buckle is generated). For this reason, the upper limit of the annealing temperature (soaking temperature) was set to 850 ° C., and the lower limit of the annealing temperature (soaking temperature) was set to 800 ° C. at which the increase in elongation starts.

The cold-rolled steel sheet 14 is annealed (soaking) at an annealing temperature of 850 ° C. for 20 to 60 seconds and then cooled to 400 ° C. at a cooling rate of 10 ° C./second to 400 ° C. For 180 seconds. Thereafter, it was cooled to 20 ° C., subjected to temper rolling at 1.0%, and then a tensile test was conducted to determine the tensile strength, yield stress, and elongation to break of the steel sheet 14. The result is shown in FIG.
As shown in FIG. 4, it can be seen that the steel plate 14 becomes softer because the tensile strength and the yield stress decrease as the soaking time becomes longer. On the other hand, the elongation shows a maximum value when the soaking time is 40 seconds, and shows a tendency to decrease when it exceeds 40 seconds. For this reason, taking into account that the tensile strength and yield stress decrease with increasing soaking time and that the elongation shows a maximum value when the soaking time is 40 seconds, the holding time (soaking time) is It was 40 seconds or more and 60 seconds or less.

Next, the cooling process will be described. After performing the annealing process which hold | maintains the temperature of the steel plate 14 which passed the soaking process at 850 degreeC, for example for 40 second, for example, the steel plate 14 is cooled to 400 degreeC with the cooling rate of 10-70 degreeC / second, and is 180 degreeC at 400 degreeC. Held for 2 seconds and then cooled to 20 ° C. Then, after performing temper rolling of 1.0%, a tensile test was performed to determine the elongation until the steel sheet 14 was broken. The result is shown in FIG.
As shown in FIG. 5, if the range of the cooling rate is 10 ° C./second or more and 40 ° C./second or less which is a general cooling rate in normal continuous annealing, the cooling rate is increased (the steel plate 14 is softened). It is understood that there is no effect on Therefore, the lower limit of the cooling rate can be set to 10 ° C./second without any problem. On the other hand, considering that the elongation value is slightly reduced at a cooling rate of 50 ° C./second and that the equipment cost is increased by increasing the cooling rate, the upper limit of the cooling rate is 40 ° C./second. It is preferable to set.

In order to generate dislocations in the steel sheet 14 in the cooling process, stress may be applied to the steel sheet 14 in the cooling process. For example, the tension applied to the steel plate 14 in the cooling process may be made larger than the tension applied to the steel sheet 14 in the soaking process and the overaging process. However, in order to cover the stress necessary to generate dislocations only with the tension applied to the steel plate 14, it is necessary to maintain a large tension with respect to the steel plate 14 in the cooling process. For this purpose, it is necessary to increase the number of bridle rolls installed in the cooling device 26. For this reason, only the first and second hot bridle rolls 29 and 30 are installed in the cooling device 26, and the tensile stress due to the tension applied to the steel sheet 14 in the cooling process is within a practical range, for example, 0.5 kg / mm. ² or more (preferably 1 kg / mm ² or more) and 1.5 kg / mm ² or less.
The stress that is insufficient for the occurrence of dislocation is applied to the steel sheet 14 by pushing one of the small-diameter rolls 33 and 34, for example, the small-diameter roll 34 beyond the pass line PL in a direction orthogonal to the pass line PL. A tensile stress generated by a bending stress that can be applied is applied to the steel sheet 14. Here, as described above, the small-diameter rolls 33 and 34 have their roll axis directions oriented in a direction orthogonal to the sheet passing direction of the steel sheet 14 passing through the pass line PL in the cooling process, and their roll axis positions. The steel plates 14 are arranged so as to oppose both sides of the steel plate 14 while being shifted in the through plate direction.

When a small-diameter roll 34 is pushed into a steel sheet 14 having a thickness t and the steel sheet 14 is subjected to a bending process in which the processing radius of curvature is R, the strain ε generated in the steel sheet 14 is t / 2R, and the longitudinal elastic modulus of the steel sheet 14 at this time Is E, the bending stress applied to the steel sheet 14 is Eε, that is, Et / 2R.
Further, since the processing radius of curvature R is related to the pushing distance δ of the small diameter roll 34 pushed beyond the pass line PL, the value of the bending stress applied to the steel sheet 14 is determined by adjusting the pushing distance δ of the small diameter roll 34. be able to. The outer diameters of the paired small-diameter rolls 33 and 34 are 200 mm or more and 500 mm or less and smaller than the outer diameters of the first and second hot bridle rolls 29 and 30 (for example, 800 mm or more and 1200 mm or less). A bending stress larger than the bending stress applied to the steel sheet 14 when passing through the second hot bridle rolls 29 and 30 can be applied by pushing the small diameter roll 34. Here, the value of the bending stress applied to the steel plate 14 is increased as the amount of carbon contained in the steel plate 14 is increased, and as the amount of carbon contained in the steel plate 14 is increased, more dislocations are generated in the crystal grains. Good.

For example, when the steel plate 14 passing through the cooling device 26 is conveyed by a hearth roll, a bending stress of about 10 kg / mm ² is applied. For this reason, it is not effective to apply a bending stress of less than 10 kg / mm ² to the steel sheet 14, but as shown in FIG. 6, it is confirmed that the elongation increases when the bending stress exceeds 10 kg / mm ^2. It was. Therefore, the lower limit value of the bending stress applied to the steel plate 14 by pushing the small diameter roll 34 is set to 10 kg / mm ² . On the other hand, after performing an annealing treatment (soaking) for 40 seconds at 850 ° C., the cold-rolled steel sheet 14 was cooled to 400 ° C. at a cooling rate of 10 ° C./second and held at 400 ° C. for 180 seconds, Then, after cooling to 20 ° C., 1.0% temper rolling was performed, and then a tensile test was performed to determine the elongation of the steel sheet 14 until breakage. The result is shown in FIG.
As shown in FIG. 6, the elongation shows a maximum value when the bending stress is 23 kg / mm ² , and shows a tendency to slightly decrease when the bending stress exceeds 23 kg / mm ² . The elongation is greatly reduced when the bending stress exceeds 35 kg / mm ² . If the bending stress is set large, the pushing distance δ of the small-diameter roll 34 becomes too large in order to reduce the processing curvature radius R, so the upper limit of the bending stress is set to 35 kg / mm ² . As described above, the value of the bending stress is desirably 23 kg / mm ² or more and 35 kg / mm ² or less.

Next, the overaging process will be described. The steel plate 14 that has passed through the cooling step and has dislocations introduced into the crystal grains is introduced into the overaging device 27 and held at a temperature of 350 ° C. or higher and 400 ° C. or lower for a period of 60 seconds or longer and 180 seconds or shorter (overaging step). ). At this time, carbon that is dissolved in the crystal grains is precipitated while generating carbide around the dislocation. As a result, the amount of carbon dissolved in the crystal grains is reduced, and the steel plate 14 is softened. When the amount of carbon contained in the steel plate 14 is large, the value of the bending stress applied to the steel plate 14 is increased to introduce a large amount of dislocations into the crystal grains. Therefore, the amount of carbon dissolved in the crystal grains is reduced. It can reduce efficiently and the softening of the steel plate 14 is attained.
On the other hand, when the amount of carbon contained in the steel sheet 14 is small, the amount of carbon dissolved in the crystal grains is small, so that the dislocation introduced into the crystal grains by reducing the value of the bending stress applied to the steel sheet 14. Therefore, the steel plate 14 can be softened while suppressing an increase in residual stress due to the occurrence of dislocations.
The steel plate 14 that has been over-aged is delivered to the exit looper 20. After the overaging treatment is completed, it may be conveyed to the secondary cooling device 28 and further cooled (for example, cooled to 40 to 60 ° C.). (Secondary cooling step).

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above-described embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included.
For example, both of the paired small-diameter rolls may be pushed over the pass line in a direction orthogonal to the pass line to apply bending stress to the steel sheet.
Further, in the above-described embodiment, there are a pair of (two) small-diameter rolls that apply bending stress to the steel sheet. However, three small-diameter rolls are provided in one place in the vertical direction, and the small-diameter roll provided at the center is passed. It is also possible to push in beyond the pass line PL in a direction orthogonal to the line PL and apply bending stress to the latter half.
In addition, since the cooling apparatus of the annealing equipment of the continuous annealing line used when manufacturing a normal steel plate may be provided with a support roll that abuts on both sides of the steel plate and supports the through plate, this support roll Can also be used as the small-diameter roll of the present invention. By doing so, the cooling device which can be used for the manufacturing method of the cold-rolled steel plate of this invention can be manufactured cheaply.

  The present invention can be used in the manufacture of steel sheets. In particular, the present invention can be applied to a cooling device for a continuous annealing line that can be used when a cold-rolled steel sheet is manufactured from a slab manufactured by a slab continuous casting method.

10: Continuous annealing line 11: Incoming equipment 12: Annealing equipment 13: Outgoing equipment 14: Steel plate 15: Coil 16: Payoff reel 17: Welding machine 18: Electrolytic cleaning device 19: Incoming looper 20: Outgoing looper 21: Coil 22: Winding machine 23: Shearing machine 24: Heating device 25: Soaking device 26: Cooling device 27: Overaging device 28: Secondary cooling device 29: First hot bridle roll 30: Second hot bridle roll 31: First window box 32: Second window box 33, 34: Small diameter roll 35: Bending stress applying means 36: Temper rolling mill

Claims (5)

The slab manufactured by the thin slab continuous casting method is hot-rolled, pickled, cold-rolled, and continuously annealed, carbon is 0.5 mass% or less, silicon is 0.02 mass% or more, and manganese is 0.0. In the method of producing a cold-rolled steel sheet containing 15% by mass or more and 0.001% by mass or more of calcium, the continuous annealing is performed by using the cold-rolled steel sheet at a temperature of 800 ° C. or higher and 850 ° C. or lower for 40 seconds or more 60 Soaking the steel sheet that has passed through the soaking process at a cooling rate of 10 ° C./second or more to a temperature of 350 ° C. to 400 ° C. A cooling step for imparting a heat treatment, and an overaging step for holding the steel sheet that has passed through the cooling step at a temperature of 350 ° C. or higher and 400 ° C. or lower for a time period of 60 seconds or longer and 180 seconds or shorter. The pull to be given to Ri stress 0.5 kg / mm ² or more and at 1.5 kg / mm ² or less, the bending stress method for producing a cold-rolled steel sheet, characterized in that 10 kg / mm ² or more and is 35 kg / mm ² or less.   The bending stress is arranged on both sides in the thickness direction of the steel sheet with the axial direction oriented in the direction orthogonal to the plate passing direction of the steel sheet passing through the pass line of the cooling step and the axial center position shifted in the plate passing direction. The production of the cold-rolled steel sheet according to claim 1, wherein both or any one of the paired small-diameter rolls is generated by being pushed beyond the pass line in a direction orthogonal to the pass line. Method.   The outer diameter of the paired small-diameter rolls is 200 mm or more and 500 mm or less, the distance between the axes is 500 mm or more and 1000 mm or less, and the pushing distance of the small-diameter rolls that are pushed beyond the pass line is 10 mm. The method for producing a cold-rolled steel sheet according to claim 2, wherein the manufacturing method is 100 mm or less.   The method for producing a cold-rolled steel sheet according to any one of claims 1 to 3, wherein the value of the bending stress is changed according to the amount of carbon contained in the steel sheet. In the cold rolled steel sheet manufacturing equipment for producing cold rolled steel sheet by hot rolling, acid cleaning, cold rolling, and continuous annealing of the slab manufactured by the thin slab continuous casting method,
The continuous annealing line for performing the continuous annealing includes a soaking device for soaking the cold-rolled steel plate, a cooling device for cooling the soaked steel plate, and an overaging treatment for the cooled steel plate. The cooling device is provided on a cooling means for cooling the steel plate and / or on either the entry side or the exit side of the cooling means, and is orthogonal to the sheet passing direction of the steel plate. The axial direction is directed to the direction and the axial center position is shifted in the plate passing direction, and the steel plate is disposed on both sides in the thickness direction of the steel plate and pushed beyond the pass line in a direction orthogonal to the pass line of the cooling device. Bending provided with a pair of small-diameter rolls that applies a tensile stress of 0.5 kg / mm ^{2 to} 1.5 kg / mm ² and a bending stress of 10 kg / mm ^{2 to} 35 kg / mm ² to the steel sheet Stress application means And a cold-rolled steel sheet manufacturing facility.

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