JP2001316727A - Method for manufacturing cold rolled steel sheet excellent in processability and having reduced anisotropy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method in which cold rolled steel sheets having an extra low carbon and nitrogen content, no problems on processing, etc., and excellent processability and anisotropy can be stably obtained, and to provide a manufacturing method in which the cold rolled steel sheets having uniform quality along longitudinal direction in addition to the above characteristics can be stably obtained. SOLUTION: In manufacturing the cold rolled steel sheet by subjecting a steel slab having an extra low carbon/nitrogen type composition to heating, hot rolling, cold rolling and annealing, finish rolling is applied under the following conditions: >45-70% total draft at the passes before the final pass; 5-35% draft at the final pass; and finishing temperature between the Ar3 transformation point and (Ar3 transformation point +50 deg.C). Subsequently, within 1 s or >0.5-1 s after the completion of the finish rolling, rapid cooling is started at (200 to 2,000) deg.C/s cooling rate. The amount of temperature fall during this rapid cooling from the finishing temperature at the above finish rolling is regulated to 50-250 deg.C, and cooling-finish temperature of this rapid cooling is regulated to 650-850 deg.C. After successive slow cooling at a rate of <=100 deg.C/s or air cooling, the resultant hot rolled steel strip is coiled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet having excellent workability and small anisotropy, which is suitable for use in automobiles, electric products, cans, building materials and the like.

[0002]

2. Description of the Related Art A cold-rolled steel sheet or a surface-treated steel sheet which requires workability needs to have excellent elongation, deep drawability and mechanical properties with small anisotropy. Also, the shape of the steel sheet,
The transportability of the hot-rolled steel strip during production is also an important factor in producing such a steel sheet.

[0003] Conventionally, an extremely low-carbon / nitrogen-based component system such as Ti
By adding a carbide-forming element or a nitride-forming element such as Nb or Nb, soft high ductility has been aimed at. The basis of this idea is to remove invasive elements such as carbon and nitrogen in the steelmaking stage to the extent possible, and at that time it is economical to remove invasive elements that cannot be removed and remain at a level that remains. The improper level of interstitial elements is fixed as precipitates so that no interstitial elements are present in the steel.

However, as the requirements for workability become more severe, it is not possible to obtain a steel sheet that satisfies such requirements only by adjusting the composition, and it is necessary to further improve the material from the process point of view. Already, by making effective use of cooling technology, the grain size of the hot-rolled sheet has been reduced,
It is conceptually known to improve mechanical properties after cold rolling and annealing. The method is to shorten the time from the end of hot rolling to the start of cooling (hereinafter, referred to as cooling start time) and to increase the cooling rate as fast as possible, thereby simultaneously performing heat. The aim is to reduce the size of the rolled sheet.

[0005] The basics of this technique are as follows. After the completion of hot rolling, the strain introduced during the finish rolling is recovered and recrystallized, and the growth of γ (austenite) grains occurs quickly. Start cooling while fine
Attempt to refine by forming α (ferrite) grains from the fine γ grain boundaries, or (2) Start cooling in a shorter time to sufficiently reduce the work distortion during hot rolling. In the unreleased state, miniaturization is achieved by forming α grains with the deformation zone in γ grains as nuclei.

As for the above, when the cooling rate is low,
Since recovery and recrystallization and grain growth of γ grains occur during cooling, and grain growth of α grains occur after transformation, the cooling rate is increased to achieve finer α grains. Furthermore, by increasing the cooling rate, the γ-α transformation point is lowered, and there is an advantage that the growth after transformation tends to be suppressed as much as the temperature after transformation is lowered.

[0007] Experimentally, for example, materials and processes (v
ol. 3, (1990), p. 785: Kino et al.)
The finishing temperature was maintained at the Ar ₃ transformation point or higher, and cooling was started 0.1 seconds after the completion of hot rolling.
It is disclosed that when the grain size of a hot-rolled sheet is reduced by cooling at a rate of ° C./sec, the mechanical properties after cold rolling and annealing, particularly the r value, can be improved.

Further, the hot-rolled sheet is refined by cooling,
Various production methods have already been disclosed for improving the material quality. For example, Japanese Patent Application Laid-Open No. 7-70650 discloses a method for producing a material having an r value of 2.50 or more in an ultra-low carbon steel sheet having a C content in steel of 15 ppm or less, by finish rolling at an Ar ₃ transformation point or higher. After the completion of the rolling, the cooling start time is set within 0.5 seconds after the end of the rolling, and the temperature range from the cooling start temperature to (Ar ₃ transformation point −60 ° C.) is 50 to 4
A technique of cooling at 00 ° C./sec is disclosed. However, in this method, the cumulative rolling reduction in the three passes on the exit side of the finish rolling of hot rolling is further specified to be 50% or more. This method
By making the hot rolled sheet finer by the cooling technique and accumulating a large amount of processing strain in hot rolling, it is intended to realize an r value of 2.50 or more and deep drawability.

[0009]

However, in the technology disclosed by Kino et al. And the technology disclosed in the above publication, not all the mechanical properties such as the r value can be improved under any conditions. Depending on conditions, workability such as r-value and elongation may not be improved and may be deteriorated. In addition, when a large amount of processing strain is accumulated by hot rolling, the shape of the steel sheet is disturbed, and a problem may occur in the transportability of the steel sheet. That is, the process conditions that can stably produce a steel sheet having workability such as significantly higher r-value and elongation than before without causing any problems in the shape and transportability of the steel sheet have not yet been obtained. . Further, in this type of steel sheet, the uniformity in the longitudinal direction of the steel sheet is also important, but in the above-described conventional technology, no consideration is given to the uniformity in the longitudinal direction of the steel sheet.

The present invention has been made in view of such circumstances, and has an extremely low-carbon / nitrogen-based component system and does not cause problems in the formability and transportability of a steel sheet. An object of the present invention is to provide a manufacturing method capable of stably manufacturing a cold-rolled steel sheet having excellent anisotropy. It is another object of the present invention to provide a manufacturing method capable of stably manufacturing a cold-rolled steel sheet having a uniform material in the longitudinal direction in addition to these characteristics.

[0011]

As a result of investigations by the present inventors, the technique proposed by Kino et al. And the technique described in the above-mentioned publication have good quenching temperature drop and cooling stop temperature. If the mechanical properties (r
Value and elongation) could not be improved. That is, when the present inventors conducted experiments based on these techniques, when the rapid cooling temperature drop or the cooling stop temperature was out of the favorable range, the elongation was improved even if the average r value was high. Otherwise, the growth may be reduced,
It has been found that the average r value may also deteriorate. In other words, excessive cooling by quenching has an adverse effect on the mechanical properties, and the improvement of the material is not achieved simply by quenching a wide temperature range including a specified temperature range (temperature range extended to the low temperature side). I can't see it. Furthermore, in the case of increasing the total draft of the three passes on the exit side of finish rolling and accumulating a large amount of processing strain for grain refinement, if the reduction in these three passes is not appropriately distributed, the transportability of the steel sheet, It was found that the shape was adversely affected.

The inventors of the present invention have conducted research to solve such a problem. As a result, in a component system based on ultra-low carbon steel, the rolling condition of hot rolling was controlled, and By controlling the hot-rolled runout cooling conditions, it was found that a cold-rolled steel sheet having much better workability and anisotropy than before can be obtained without causing problems in shape and transportability. . That is, in addition to adjusting the steel composition to a specific composition of an extremely low carbon system, the following findings were obtained.

(1) Regarding the rolling reduction conditions during hot rolling, by appropriately setting the rolling reduction in the final pass of finish rolling and the rolling reduction in the two passes before the final pass, the shape of the steel sheet and the manufacturing The transferability of the hot-rolled steel sheet does not cause a problem, and the hot working distortion can be increased to the extent that there is no problem, thereby achieving fineness.

(2) It is effective to start predetermined rapid cooling within as short a time as possible after the finish rolling in order to refine the hot rolled sheet and improve the mechanical properties. However, if the cooling time is too short, the degree of grain refinement often differs due to a slight difference in the cooling start time, so that the cooling start time has an appropriate range in order to make the material uniform in the longitudinal direction of the plate.

(3) By appropriately setting the range of the amount of temperature drop by rapid cooling, excessive cooling by rapid cooling can be suppressed, and workability such as elongation and deep drawability and anisotropy can be improved.

(4) By appropriately setting the cooling stop temperature of the rapid cooling, a desired fine structure can be obtained.

(5) By making the cooling after the rapid cooling an appropriate slow cooling, it is possible to form appropriate polygonal ferrite grains.

The present invention has been made based on the above-mentioned findings, and the first invention is based on the following.
03% or more and 0.01% or less, Si: 0.05% or less, M
n: 0.05% or more and 2.5% or less, P: 0.003%
0.1% or less, S: 0.0003% or more and 0.02 or less
% Or less, Sol. A slab made of steel containing Al: 0.005% or more and 0.1% or less and N: 0.0003% or more and 0.004% or less is heated, hot-rolled, and cold-rolled.
In producing the cold-rolled steel sheet by annealing, in the finish rolling, the total rolling reduction in the two passes before the final pass is more than 45% and 70% or less, and the rolling reduction in the final pass is 5% in the finish rolling.
At least 35% or less, and further finish the finish rolling by setting the finishing temperature to the Ar ₃ transformation point or more (Ar ₃ transformation point + 50 ° C.) or less, and then within 200 seconds / 200 ° C.
Rapid cooling is started at a cooling rate of not less than 2000 sec / sec and not more than 50 ° C to not more than 250 ° C from the finishing temperature of the finish rolling in the rapid cooling. 650 ° C or higher 8
Cold-rolling with excellent workability, characterized by winding the resulting hot-rolled steel strip after slow cooling or air-cooling at 50 ° C or lower and subsequently at 100 ° C / sec or lower. Provided is a method for manufacturing a steel sheet.

In the second invention, C: 0.000% by weight.
3% or more and 0.01% or less, Si: 0.05% or less, M
n: 0.05% or more and 2.5% or less, P: 0.003%
0.1% or less, S: 0.0003% or more and 0.02 or less
% Or less, Sol. A slab made of steel containing Al: 0.005% or more and 0.1% or less and N: 0.0003% or more and 0.004% or less is heated, hot-rolled, and cold-rolled.
In producing the cold-rolled steel sheet by annealing, in the finish rolling, the total rolling reduction in the two passes before the final pass is more than 45% and 70% or less, and the rolling reduction in the final pass is 5% in the finish rolling.
Not less than 35% and the finishing temperature is not less than Ar ₃ transformation point (Ar ₃ transformation point + 50 ° C.) and the finish rolling is completed.
Rapid cooling is started at a cooling rate of 200 ° C./sec or more and 2000 ° C./sec or less, and the amount of temperature drop from the finishing temperature of the finish rolling in the rapid cooling is 50 ° C. or more to 250 ° C.
° C or less, and the cooling stop temperature of the rapid cooling is 650.
To 850 ° C or lower and subsequently 100 ° C / sec.
A method for manufacturing a cold-rolled steel sheet having excellent workability and low anisotropy, characterized in that the obtained hot-rolled steel strip is wound after performing slow cooling or air cooling of c or less.

According to a third invention, in the method for producing a cold-rolled steel sheet according to the first invention or the second invention, the steel further comprises one or more of Ti, Nb, V, and Zr in total by weight. In a content of 0.005% or more and 0.1% or less.

According to a fourth invention, in the method for producing a cold-rolled steel sheet according to any one of the first to third inventions, the steel further contains 0.015% or more and 0.08% or less by weight of Cu. It is characterized by doing.

According to a fifth aspect, in the method for producing a cold-rolled steel sheet according to any one of the first to fourth aspects, the steel further contains B in an amount of 0.0001% to 0.001% by weight. It is characterized by doing.

In the prior art, for example, Japanese Unexamined Patent Publication No.
JP-A-70650, JP-A-6-212354 and JP-A-6-17141 disclose a case where the temperature itself is expressed as "finishing temperature: Ar ₃ temperature or higher" as a rule using the Ar ₃ transformation point. , from the "··· (Ar ₃ -
The 50 ° C.), so that both the case of using the prescribed temperature during cooling so that rapid cooling ... "are present, the higher the Ar ₃ transformation point to rapid cooling drops, Ar ₃ transformation point in the latter is not the same temperature as the Ar ₃ transformation point in the former, always shows the temperature Ar ₃ transformation point is lower in the former. However, in the prior art, the transformation point in the latter context and the transformation point in the former context are often interpreted as the same temperature, which is not academically correct. Further, since the Ar ₃ transformation point as the cooling rate is fast drops, to say categorically Ar ₃ transformation point in the latter context, if you can not actually understand what indicates what value is large. For this reason, in the present invention, the temperature in the case of rapid cooling is defined not by the ambiguous expression of the Ar ₃ transformation point but by a numerical value.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing a cold-rolled steel sheet according to the present invention will be specifically described with respect to steel composition and process conditions. 1. Steel composition The steel composition in the present invention is expressed by weight% and C: 0.0003.
% To 0.01%, Si: 0.05% or less, Mn:
0.05% to 2.5%, P: 0.003% to 0.1%, S: 0.0003% to 0.02%, Sol. Al: 0.005% or more and 0.1% or less,
N: 0.0003% or more and 0.004% or less. Further, from the viewpoint of improving the stretch flangeability, in addition to the steel composition, Ti, Nb,
One or more of V and Zr are added in a total range of 0.005% or more and 0.1% or less. Further, from the viewpoint of reducing the adverse effect of solid solution S, Cu is added in an amount of 0.015% or more and 0.08% or less as necessary in addition to any one of the steel compositions. Furthermore, from the viewpoint of improving the longitudinal crack resistance of the steel, B is added in an amount of 0.0001% or more and 0.001% or less, if necessary, in addition to any one of the above steel compositions.

C: 0.0003% or more and 0.01% or less C realizes extremely low carbon steel or IF steel (Interstitial-Free steel) based steel in which solid solution interstitial elements do not exist in the steel. To a level that can do it. IF here
BH steel (Bake-Hard stee) based on IF steel
l) Including. Although the ductility and deep drawability are improved as the C content is smaller, the lower limit of the C content is set to 0.0003% in consideration of the current steelmaking conditions. On the other hand, when the C content is 0.1.
If it is 01% or less, carbide forming elements (Ti, Nb, etc.)
By fixing with C, the ductility and deep drawability of the IF steel can be improved.
01% or less. When the C content is 0.002% or less, the elongation and deep drawability can be made higher, and the amount of the carbide-forming element added can be reduced. Therefore, the C content is 0.002%. It is preferable to set the following. On the other hand, even when the C content is 0.002% or more and 0.01% or less, the winding temperature can be set higher, or Ti, Nb, or the like can be added as a carbide-forming element.
Elongation and deep drawability can be made higher, and anisotropy can be kept low.

Si: 0.05% or less Si is an element that has an adverse effect on the properties of soft and high ductility, and has an adverse effect on surface treatment such as Zn plating, but is also used as a deoxidizing element. You. Si content is 0.
If it exceeds 0.05%, the adverse effect on the material and the surface treatment becomes remarkable, so the content is made 0.05% or less.

Mn: 0.05% or more and 2.5% or less Mn is an element that improves the toughness of steel and can be effectively used for solid solution strengthening. Adversely affect On the other hand, Mn converts S to Mn.
It can be effectively used for precipitation as S. In the present invention, the Mn content is set to 2.5% or less to give priority to the expression of elongation and deep drawability and to use the steel for strengthening. On the other hand, the lower limit of the Mn content is set to 0.05% in consideration of the cost of removing S in steelmaking.

P: 0.003% or more and 0.1% or less P is a solid solution strengthening element, and the ductility is deteriorated as the content increases. Therefore, the P content is set to 0.1% or less. On the other hand, although the ductility is improved as P is removed, the lower limit of the P content is set to 0.003% from the viewpoint of the balance between the removal cost in steelmaking and workability. In order to obtain better processability, the content is preferably 0.015% or less. However, in this case, the grain growth becomes active and it becomes difficult to reduce the grain size of the hot-rolled sheet. It is better to set the temperature lower.

S: 0.0003% or more and 0.02% or less Since S is an element causing red-hot embrittlement, its upper limit is generally defined according to the amount of Mn added having the function of fixing S. When the S content is large, the precipitation of sulfide increases, and elongation and deep drawability deteriorate. Therefore, in the present invention, the S content is set to 0.02% or less in consideration of such a point. on the other hand,
The lower the S content is, the better the workability is. However, the lower limit of the S content is set to 0.000 in consideration of the cost of removing steel.
3%. When the S content is 0.012% or less, the elongation and the deep drawability can be made higher, and the amount of the sulfide forming element can be reduced.
Preferably, the content is 0.012% or less. However, in this case, the grain growth becomes active and it becomes difficult to reduce the grain size of the hot-rolled sheet. Therefore, it is preferable to set the winding temperature after hot rolling to a lower temperature. On the other hand, even when S is 0.012% or more and 0.02% or less, elongation and deep drawability can be increased to a higher level by setting the winding temperature after hot rolling higher. , Anisotropy can also be kept low.

Sol. Al: 0.005% or more and 0.1%
Hereinafter, Al effectively acts as a deoxidizing element of molten steel.
If too much is added, workability is adversely affected.
l. The Al content was set to 0.1% or less. On the other hand, even when the amount of Al added is the minimum necessary for deoxidation, 0.005% or more of Sol. Since Al remains,
Considering that point, Sol. The lower limit of Al was made 0.005%.

N: 0.0003% or more and 0.004% or less N decreases the amount of N to improve ductility and deep drawability. However, the lower limit of N is set to 0.0003% in consideration of the current steelmaking conditions. And On the other hand, the N content is 0.004%
In the following cases, by fixing with a nitride-forming element (Ti, Nb, etc.), it becomes possible to improve the ductility and deep drawability as an IF steel in which the interstitial element dissolved in the steel is not present in the steel. , N content is set to 0.004% or less.
When the N content is 0.002% or less, the elongation and deep drawability can be made higher, and the amount of the nitride-forming element added can be reduced.
02% or less is preferable. However, in this case, since the grain growth becomes active and it becomes difficult to reduce the grain size of the hot-rolled sheet, it is preferable to set the winding temperature lower. On the other hand, N is 0.002
% Or more, the elongation and deep drawability can be made higher and the anisotropy can be kept low by setting the winding temperature higher.

Ti, Nb, V, Zr: 0.005% or more and 0.1% or less in total of at least one kind Ti, Nb, V, and Zr are formed by forming carbides, nitrides, and sulfides. It is an element that fixes C, N, and S dissolved therein as precipitates, improves elongation, and improves deep drawability. In particular, when these properties are required, one of these elements is used.
It is preferred to add more than one species. Ti, Nb, V, Z
If the total amount of r is less than 0.005%, elongation occurs and the effect of improving the deep drawability cannot be obtained. Conversely, if the total amount exceeds 0.1%, the workability deteriorates. From these, Ti, Nb, V, Z
The total amount of r is set to 0.005% or more and 0.1% or less.

Cu: 0.015% or more and 0.08% or less Cu works effectively as a sulfide-forming element.
Is an element that reduces the adverse effect on the material, and is preferably added particularly when such an effect is required. Such an effect can be obtained when 0.005% or more of Cu is added.
Since the content is less than 01%, the Cu content is set to 0.015% or more. On the other hand, if the Cu content exceeds 0.08%, the steel becomes hard, so the content is set to 0.08% or less.

B: 0.0001% or more and 0.001% or less B is an element which improves the vertical cracking resistance of steel, and is preferably added when such an action is required. If B is less than 0.0001%, the effect of improving longitudinal crack resistance cannot be obtained, and if B exceeds 0.001%, the effect is saturated.
Is added, the amount of addition is 0.0001% or more and 0.001% or less.

2. In the process conditions of the present invention, a slab made of steel having the above composition is heated, hot-rolled, cold-rolled, and then subjected to annealing to produce a cold-rolled steel sheet. The total draft of the two passes before the final pass is more than 45% and 70%
In the following, the rolling reduction of the final pass is set to 5% or more and 35% or less, and the finishing temperature is set to the Ar ₃ transformation point or more (Ar ₃ transformation point + 50 ° C.) or less, and finish rolling is completed. Within two seconds or within one second and more than 0.5 seconds, rapid cooling is started at a cooling rate of 200 ° C./sec or more and 2000 ° C./sec or less, and a temperature drop from the finishing temperature of the finish rolling in the rapid cooling is performed. More than 50 ℃ 2
50 ° C. or less, and the cooling stop temperature of the rapid cooling is 6
50 ° C. or higher and 850 ° C. or lower, and subsequently 100 ° C. /
After slow cooling or air cooling for not more than sec, the obtained hot rolled steel strip is wound up. Hereinafter, these conditions will be described.

(1) Total rolling reduction of two passes before the final pass of finish rolling: more than 45% and 70% or less, rolling reduction of the final pass of finish rolling: 5% or more and 35% or less This is because, while ensuring the shape of the hot-rolled steel strip and the transportability of the hot-rolled steel strip during production, a sufficient amount of strain is accumulated to make the hot-rolled sheet finer. The rolling reduction in the two passes before the final pass here means the sheet thickness (L2) before the steel strip enters the two passes before the final pass of the finishing mill and the one before the final pass. From the plate thickness (L1) after passing through the path (L2) / L2 × 100.

In order to reduce the grain size of the hot rolled sheet, it is desirable to accumulate strain by hot working near the transformation point. However, in the hot rolling process, the sheet temperature decreases as it moves from the entry side to the exit side, and the steel strip gradually hardens and the processing resistance increases, so there is a limit to performing large reduction in the final pass. . That is, if a large reduction is performed in the final pass, the shape of the steel sheet will be disturbed, and problems will occur in the transportability of the steel strip. For this reason, in order to secure the shape and transportability of the steel sheet and to accumulate the processing strain and perform the grain refinement, the rolling reduction in the final pass of the finish rolling and the two passes before the final pass is set as described above. , It is necessary to introduce an appropriate amount of distortion at an appropriate timing. In other words, the total rolling reduction in the two passes before the final pass is increased to accumulate a large amount of strain, and the strain is also accumulated in the final pass. Lower.

More specifically, 2 before the final pass in finish rolling
The reason why the total rolling reduction in the pass is 70% or less is as follows.
This is for ensuring the transportability and shape of the steel sheet in these passes after accumulating the processing strain. On the other hand, the reason why the total draft is more than 45% is to sufficiently perform strain accumulation during hot working and to ensure the soft high ductility and high workability of the steel sheet. Although the final pass rolling reduction is not problematic from the viewpoint of introducing processing strain, it is not problematic. However, in order to ensure the transportability and shape of the steel sheet at a level without any problem, the rolling reduction is set to 35% or less. 5% or more, which is the minimum level required for correction and the like. As long as the conditions of the hot rolling as described above are satisfied, the rough rolling process of the hot rolling, and the rolling reduction of the three passes before the final pass during the finish rolling are not particularly problematic, and conventionally performed. Is enough.

In order to obtain more excellent elongation, deep drawability and anisotropy in a cold-rolled annealed sheet, the total rolling reduction in two passes before the final pass in finish rolling should be 55% to 70%. Either one or both of accumulating a large amount of processing strain to reduce the grain size of the hot rolled sheet and / or reducing the final pass reduction rate to 15% or more and 35% or less to reduce the grain size of the hot rolled sheet. It is preferable that one of the two is satisfied. The shape of the steel sheet,
When importance is placed on the transportability of the hot-rolled steel strip during manufacturing, the final pass draft should be 5% or more and 15% or less to correct the shape, ensure transportability, and introduce processing strain. Is preferred.

When the rolling reduction in the finish rolling is large as in the present invention, in general, a shape abnormality occurs, transportability cannot be ensured (meandering), and thus, when the product is wound on a coiler, it can be wound neatly. Without protruding outside,
It may be dented inward. Further, an abnormality may occur in the properties of the material in the width direction. During hot rolling, these phenomena are caused by slight temperature unevenness in the hot-rolled steel strip,
This occurs because the elongation changes between the center and the edge of the sheet width during rolling.

In the present invention, the last pass and the two
The reduction ratio with the pass is divided and specified, and the shape of the hot rolled steel strip,
Although transportability is ensured, in order to further improve the shape and transportability, the hot-rolled steel strip is further heated offline or online, and the temperature distribution in the sheet width direction is made uniform. Is preferred. As the method, a device for heating a rough bar (hot-rolled steel strip after the completion of rough rolling) by an induction heating device online, a method for winding the rough bar, and then a coil Box
The temperature distribution in the sheet width direction is made uniform by using a device for heating in the above, an induction heating device installed in a finish rolling device, or the like.

The thickness at the coarse bar stage before the finish rolling is desirably 20 mm or more. This is because by setting the thickness of the rough bar in this manner, the absolute amount of reduction can be increased, and the material can be easily formed by rolling. However, it is not essential to have such a rough bar thickness.For example, even with a hot rolling device in which a continuous casting machine for thin slabs and a hot rolling device are directly connected, the predetermined pass of finish rolling is as described above. If the conditions are satisfied, it is possible to realize a material superior to the material produced by the conventional method (material after cold rolling annealing), provided that the process is controlled to satisfy the following conditions. .

(2) Finishing temperature: Ar ₃ transformation point or more (Ar
₍₃ transformation point + 50 ° C.) or less The reason for defining the finishing temperature in this way is that finish rolling is completed in the γ region, the working strain is accumulated in the γ region, and the hot rolled sheet is formed by utilizing fine γ grains. This is for sufficiently reducing the particle size. Finishing temperature is A
If the transformation point is less than the r ₃ transformation point, the rolling becomes α region, and the crystal grains become coarse. On the other hand, if the finishing temperature exceeds (Ar ₃ transformation point + 50 ° C.), γ-grain growth occurs after the end of rolling, which is disadvantageous for grain refinement of the hot-rolled sheet.
r ₃ transformation point + 50 ° C) or less.

(3) Cooling rate: 200 ° C./sec or more 2
2,000 ° C./sec or less The reason why the cooling rate after the finish rolling is 200 ° C./sec or more is to reduce the grain size of the hot-rolled sheet and to improve the mechanical properties of the obtained cold-rolled steel sheet. In the present invention, the cooling is performed while breaking up the steam film formed on the surface of the steel sheet during cooling, instead of the method of cooling while raising the water vapor (cooling in the film boiling mode) as seen in the cooling by the laminar method. The cooling is intended mainly for the method (cooling in the nucleate boiling mode), and in such a cooling method, the cooling rate is necessarily 200 ° C./sec or more. In addition, the upper limit of the cooling rate is set to 2000 ° C./sec from the approximate theoretical limit in the cooling in the nucleate boiling mode. As a device capable of realizing such a cooling rate, any system capable of performing nucleate boiling mode cooling, such as a perforated jet system, a super-proximity nozzle + high pressure + large water volume system, etc. May be used.

Since the cooling rate varies depending on the sheet thickness, in order to more accurately define the cooling rate, for example, “sheet thickness 2.
The steel sheet having a size of 5 mm or more and 3.5 mm or less may be cooled at a rate of 200 ° C./sec or more and 2000 ° C./sec or less. For this reason, it is preferable to use a device having a cooling capacity capable of cooling at such a cooling rate regardless of the thickness of a normal hot-rolled steel sheet. A more preferred range of the cooling rate is 40
0 ° C./sec or more and 2000 ° C./sec or less. By cooling in this range, the elongation and the deep drawability of the cold-rolled annealed sheet are further improved, and the anisotropy can be suppressed lower.

In the present invention, the cooling rate after finish rolling is defined as 200 / Δt using the time (Δt) required for cooling 200 ° C. from 900 ° C. to 700 ° C. The rapid cooling in the present invention is started “within the Ar ₃ transformation point or more (Ar ₃ transformation point + 50 ° C.) or less and within one second after the completion of finish rolling”, and actually starts cooling depending on the steel composition of the slab. The cooling temperature may be lower than 900 ° C., but also in this case, the cooling rate follows this definition. That is, the cooling rate is a value determined when the steel strip is temporarily cooled from 900 ° C. to 700 ° C. The temperature at which cooling is actually started may be 900 ° C. or less, and the temperature at which rapid cooling is stopped may be 700 ° C. or less.

(4) Cooling start time: Within 1 second after finishing rolling or within 0.5 second and within 1 second The cooling start time is defined in this way after increasing the cooling rate as described above. This is because the particle size of the hot-rolled sheet is sufficiently reduced by shortening the cooling start time.
This has the effect of increasing elongation, deep drawability, and reducing anisotropy. If the cooling start time exceeds 1 second, the particle diameter of the hot rolled sheet is almost the same as that of ordinary laminar cooling or air cooling in a laboratory experiment, and the particle diameter of the hot rolled sheet cannot be sufficiently reduced.

In the present invention, the lower limit of the cooling start time is not particularly defined. However, even if the rolling speed is increased and the cooling is started immediately near the exit side of the finish rolling, the cooling device housing or the rolling roll radius is not required. In consideration of the protrusion of the cooling, 0.01 second is substantially the lower limit of the cooling start time.

Even if the cooling start time is less than 1 second, the characteristics developed depending on the cooling start time are different. When the cooling start time is set within 0.5 second, the deep drawability and the anisotropy are particularly preferred. When the cooling start time is set to 0.5 seconds or more and 1 second or less, elongation is particularly preferentially improved.
The reason for the difference in the properties developed in this way is probably due to the slight difference in ferrite grain size at the stage of the cold-rolled annealing plate, but the mechanism is not clear.

In order to make the material uniform in the longitudinal direction of the steel sheet, it is preferable that the cooling start time is more than 0.5 seconds and less than 1 second. Even when the cooling start time is set to 0.5 seconds or less, the improvement of the material by refining the hot-rolled sheet can be expected. Non-uniformity may occur.

In order to make the cooling start time within 1 second,
For example, the rolling speed (conveying speed of the hot-rolled steel strip during rolling) is 1
In the case of 300 m / min or less, a cooling device (for example, a cooling device capable of performing the cooling in the nucleate boiling mode described above) is set at 15 m from the nearest side from the final pass exit side of the finish rolling device according to the rolling speed. Install in the vicinity of That is,
If the rolling speed is high, it may be installed behind this range. If the rolling speed is low, it may be installed before this range to realize a cooling start time of 1 second or less. Also,
When high-speed rolling at a rolling speed exceeding 1300 m / min becomes possible, it can be predicted that the installation position of the cooling device will be farther from the exit side of the final pass.

From the viewpoint of equalizing the material in the longitudinal direction of the steel sheet, it is more preferable that the cooling start time is constant in the longitudinal direction of the coil. However, a cooling device for performing rapid cooling in the existing hot rolling mill is required. If one control unit is used, the cooling start time may change in the longitudinal direction of the coil. The reason is that hot rolling is not always performed at a constant speed. In other words, rolling is performed at a low rolling speed until the head of the steel strip winds around the coiler, and thereafter the steel strip winds around the coiler, and after tension is applied to the steel strip, the rolling speed is gradually increased to a constant speed. Since the rolling is performed to the rear end of the coil in this state, the cooling start time changes due to the change in the rolling speed.

In order to avoid such grain refinement and variation in the material, it is preferable to divide the cooling device into small units and perform ON / OFF control in conjunction with each unit in association with the rolling speed. In this case, at the tip of the coil where the rolling speed is slow, cooling is performed using the unit on the final pass side,
Then, according to the rolling speed that is gradually accelerated, by shifting the unit of cooling to the unit installed on the coiler side, the cooling start time in the longitudinal direction of the coil is made uniform, and fine-graining and The material can be homogenized.

(5) Temperature drop of rapid cooling: not less than 50 ° C. and not more than 250 ° C. Such rapid cooling is performed by optimally refining the hot-rolled sheet, elongating the cold-rolled annealed sheet, This is because the drawability is improved and the anisotropy is suppressed low. As described above, when the two conditions of “the cooling rate is set to 200 ° C./sec or more and 2000 ° C./sec or less” and “the cooling start time is set to 1 second or less” are satisfied, the temperature drop after the final pass is Since the cooling start temperature and the finishing temperature can be regarded as substantially the same temperature, the “temperature drop from the finishing temperature” is defined in this way.

In order to perform hot-rolled sheet refining optimally, it is not only necessary to rapidly cool the specified temperature range as described above. It is necessary to If the amount of temperature drop due to this quenching exceeds an appropriate range, polygonal ferrite grains cannot be realized, resulting in grains extending in the rolling direction and grains having a quenched structure, resulting in excellent workability and anisotropy. Can not be obtained. For this reason, in the present invention, the amount of temperature drop by rapid cooling is specified as described above.

The reason why the temperature drop due to the rapid cooling is set to 50 ° C. or more is that at least the temperature drop of 50 ° C. is required for cooling across the γ-α transformation point at the above-mentioned cooling rate. is there. Further, the reason why the temperature drop amount is set to 250 ° C. or less is that when the temperature drop amount exceeds 250 ° C., an adverse effect due to excessive cooling becomes remarkable. In particular, when it is desired to improve the elongation of the cold-rolled annealed sheet, it is preferable that the temperature drop is 150 ° C. or less.

In order to control the amount of temperature drop by rapid cooling within the above range, the cooling device for performing rapid cooling in the nucleate boiling mode is divided into small units in the rolling direction, and each unit is linked to the rolling speed. It is effective to perform ON / OFF control of the cooling in. The amount of temperature drop due to rapid cooling is determined by the cooling speed of the cooling device that performs rapid cooling, the length of the portion of the cooling device that performs rapid cooling, and the rolling speed (transport speed of the steel strip). It is difficult to keep the amount of temperature drop due to quenching within the above range,
The amount of temperature drop cannot be made constant over the entire length in the longitudinal direction of the coil, so that the characteristics of the cold-rolled annealed sheet vary.

More specifically, the rapid cooling rate in the nucleate boiling mode changes according to the thickness of the sheet, and is slower for a thicker sheet and faster for a thinner sheet. In addition, the rolling speed is rarely constant over the entire length of the coil, so the speed until the steel strip is wound around the coiler is slowed down, and then the steel strip is accelerated under tension so that the speed becomes constant. Often, the rolling speed is taken. Therefore, the cooling device is divided into small units, and the number of units to be cooled and the positions of the units are determined according to the rolling speed that fluctuates as described above, and ON / OFF control of each unit is performed. Accordingly, the amount of temperature drop due to rapid cooling can be appropriately controlled.

It is additionally important that the water used for rapid cooling be quickly removed. For example, if water flows out after the outlet side of the cooling device, the cooling of the steel sheet continues according to the amount of remaining water. If more water than necessary remains on the steel sheet at the exit side of the cooling device, the cooling mode in that area differs depending on the water pressure and rolling speed of the steel sheet, etc., but a mode in which the nucleate boiling mode and the film boiling mode are mixed. Or, it is a mode of a process of shifting to the cooling of the film boiling mode. In either mode, cooling at a higher cooling rate than in the simple film boiling mode is continued. This is directly related to the variation in the effect of improving the properties of the steel sheet, which is manifested by rapid cooling, and if the temperature is excessively cooled, polygonal ferrite grains cannot be realized, leading to deterioration of the material. In order to prevent this, a draining device, a draining roll, an air curtain, or the like may be installed on the outlet side of the cooling device.

(6) Cooling stop temperature of rapid cooling: 650 ° C.
Not less than 850 ° C. The cooling stop temperature of the rapid cooling is defined in this manner in combination with the above-mentioned conditions of “cooling rate”, “cooling start time”, and “amount of temperature drop by rapid cooling”. This is for appropriately reducing the size of the plate. If the cooling stop temperature exceeds 850 ° C., the grain growth after the cooling stop may not be ignored, which is not preferable from the viewpoint of making the hot-rolled sheet finer. On the other hand, if the cooling stop temperature is lower than 650 ° C., a quenched structure may be formed even if the above-described conditions of “cooling rate”, “cooling start time”, and “amount of temperature drop due to rapid cooling” are satisfied. In such a case, the properties of the cold-rolled annealed sheet cannot be improved. The quenching stop temperature is the plate temperature at the time of exiting the rapid cooling device, and is (finish temperature) −
(Temperature drop due to rapid cooling). In addition, the quenching stop temperature must be set to be equal to or higher than the winding temperature. The quenching stop temperature is substantially the plate temperature at the time of exiting the rapid cooling device.For example, when the cooling device has a multi-bank configuration, the steel band is used for cooling. The temperature at the time of passing may be controlled in the above-mentioned appropriate range. In order to control the cooling stop temperature within the above range, a draining device, a draining roll, an air curtain, or the like may be installed on the outlet side of the cooling device, and the cooling stop temperature may be controlled by these.

(7) Cooling after rapid cooling: 100 ° C./sec
After the rapid cooling in the hot rolled runout performed as described above, the slow cooling or the air cooling of 100 ° C./sec or less to the winding temperature is performed by the polygonal as described above. In addition, it is for improving the characteristics of the cold-rolled annealed sheet by producing fine ferrite grains. If cooling to the winding temperature is performed only by rapid cooling, adverse effects due to excessive cooling are observed and a desired structure cannot be obtained. Therefore, slow cooling at 100 ° C./sec or less or air cooling is essential. Cooling rate is 100 ° C / sec
If it exceeds 2,000, it will be difficult to produce polygonal ferrite grains.

(8) Winding Temperature The winding temperature is not particularly limited, but is 550 ° C. or more and 750 ° C.
It is desirable to make the following. If the winding temperature is lower than 550 ° C., the steel hardens. In addition, as described above, when quenching is performed, the winding temperature is inevitably lower than 750 ° C., and even if the winding temperature is higher than 750 ° C., no improvement in characteristics is observed.

When the C, S and N contents in the steel are large, ie, C: 0.002% or more and 0.01% or less;
012% or more and 0.02% or less, or N: 0.002
% Or more and 0.004% or less, the winding temperature is 6
It is preferable that the temperature be 30 ° C. or more and 750 ° C. or less. By setting the content within this range, a factor (fine precipitate) that promotes the formation and growth of precipitates and inhibits ferrite grain growth of the cold-rolled annealed sheet can be removed.

On the other hand, when the amounts of C, S, P, and N in the steel are small, that is, C: 0.0003% or more and 0.002% or less, S: 0.0003% or more and 0.012% or less, and P:
0.003 or more and 0.015% or less, or N: 0.0
When it is 003% or more and 0.002% or less, the winding temperature is preferably 550 ° C or more and 680 ° C or less.
By setting the content within this range, extremely active grain growth can be suppressed because these elements are small, and the grain size of the hot-rolled sheet can be effectively reduced.

(9) Cold Rolling The conditions for the cold rolling are not particularly limited, but the rolling reduction (cold rolling ratio) at that time is preferably 50% or more and 90% or less. By setting the cooling pressure ratio in this range, the effect of improving the characteristics from the finely grained hot-rolled sheet obtained as described above is large.

(10) Annealing The conditions for annealing the cold-rolled sheet are not particularly limited. However, from the viewpoint of improving properties and preventing roughening of the surface, the temperature is 700 ° C. or more and 850 ° C.
It is preferable to anneal at a temperature of not more than ℃. Annealing may be performed by any method such as continuous annealing or batch annealing.

In the present invention, a method of hot rolling a continuously cast slab without heating it in a heating furnace, a method in which the temperature of the continuously cast slab cannot be reduced to room temperature,
A method of hot rolling after heating to a predetermined temperature in a heating furnace, a method of hot rolling to a predetermined temperature in a heating furnace after the temperature of the slab has dropped to room temperature, a continuous thin slab casting device And hot rolling with a device connected to a hot rolling device, the ingot manufactured slabs, after maintenance, such as a method of hot rolling by heating in a heating furnace, such as when using any method. Also, by applying the above process conditions to steel having the above composition, a preferable material can be produced.

The cold rolled steel sheet of the present invention can be suitably used for applications requiring workability, such as steel plates for automobiles, steel plates for electric products, steel plates for cans, and steel plates for building materials. The characteristics can be sufficiently exhibited even when used in the above. Further, the cold-rolled steel sheet according to the present invention includes those subjected to a surface treatment such as Zn plating or alloyed Zn plating.

[0069]

Embodiments of the present invention will be described below. [Example 1] Steel having the components shown in Table 1 was continuously cast into a slab having a thickness of 200 to 300 mm, and
After heating to 1250 ° C., a hot-rolled sheet having a sheet thickness of 2.8 mm was formed by hot rolling under hot-rolling conditions including the cooling conditions shown in Table 2, and cold-rolled to a sheet thickness of 0.8 mm. Temperature speed 6
The temperature was raised at a temperature of not less than 20 ° C./sec and not more than 20 ° C./sec. 1 to 18 cold-rolled steel sheets were obtained. At the time of hot rolling, in order to ensure the transportability and shape of the hot-rolled steel strip to a level that does not cause any problems, a rough bar (hot-rolled steel strip that has undergone rough rolling) is introduced immediately before introduction into the finish rolling device. The steel strip was heated by an induction heating device to make the temperature distribution in the width direction of the steel strip constant. In the case of "conventional laminar cooling" shown in Table 2, laminar cooling was performed on the hot-rolled steel strip that passed through the final pass of finish rolling while cooling while raising steam. On the other hand, after finish rolling, 200 ° C / sec.
In the case of rapid cooling of c or more, steam is generated at the time of cooling in the cooling in the film boiling mode, and the steam film envelops the steel sheet and cannot be rapidly cooled. In order to perform cooling while destroying the vapor film, fresh water always hits the steel plate, realizing cooling in the nucleate boiling mode that allows rapid cooling, and changing the amount of water and water pressure. The quenching was carried out at various cooling rates shown in FIG.

For these steel sheets, 0.8% of the cold-rolled steel sheets
The total elongation was measured using a mm material, and r0, the r value in the L direction (0 ° direction to the rolling direction), r45, the r value in the D direction (45 ° direction to the rolling direction), and C R90, which is the r value in the direction (90 ° with respect to the rolling direction), was measured. Table 2 shows the total elongation and the average r value as indices for evaluating the workability of the steel sheet, and r0, r45, and r9 as indices for evaluating anisotropy.
Among steel sheets, r45 shows the lowest tendency among the steel sheets, in which r45 indicates Δr, and steel sheets in which r45 takes an intermediate value between r0 and r90 indicate the maximum value-minimum value of the r value. Here, the average r value is a value defined by the average r value = (r0 + 2 × r45 + r90) / 4. Δr is expressed as Δr = (r0 +
r90−2 × r45) / 2.

[0071]

[Table 1]

[0072]

[Table 2]

As shown in Table 2, No. 1 was manufactured under the process conditions of the present invention in which rapid cooling was performed. 2,4,6
Each of the steel sheets of 8, 10, 12, 14, 16, and 18 has an extremely high elongation and an average r value, and has a Δr or r
The maximum value-minimum value was extremely low, and workability and anisotropy were extremely excellent. On the other hand, laminar cooling was performed from the top and bottom of the steel plate on the runout table after the final pass. 1,3,5,7,9,11,1
The steel sheets 3, 15, and 17 were inferior in any of the properties.

As described above, if a cold-rolled steel sheet is manufactured using the steel having the composition in the range specified in the present invention under the process conditions specified in the present invention, the shape and transportability are excellent, and It was confirmed that a cold-rolled steel sheet having much better workability and anisotropy than the conventional one could be manufactured.

Example 2 Steel having the components shown in Table 3 was cast into a slab having a thickness of 220 mm by continuous casting. The slab was cleaned, heated to 1200 ° C., and heated under the conditions shown in Table 4. After cold rolling and cold rolling, the heating rate is 1
After continuously annealing for 90 seconds at an annealing temperature of 840 ° C. at a temperature of 0 ° C./sec or more and 20 ° C./sec or less, 19 to 44 cold-rolled steel sheets were obtained. In hot rolling, the transportability of hot rolled steel strip,
In order to secure the shape to a level that does not cause any problem, the rough bar (hot-rolled steel strip after rough rolling) is heated by an induction heating device immediately before being introduced into the finish rolling device, and the width direction of the steel strip is measured. Was made constant. At this time, No. For No. 30, the thickness of the hot-rolled sheet was 1.5 mm and the thickness of the cold-rolled annealed sheet was 0.75 mm. 19-29,3
Regarding 1 to 44, the thickness of the hot-rolled sheet was 2.
8 ± 0.2 mm, and the thickness of the cold-rolled annealed sheet was 0.8 mm. In addition, No. shown in Table 4 The cooling rate of 30 is a value when the thickness of the hot-rolled sheet is 1.5 mm, and is 2.8 to 3.5.
When the cooling rate was checked with a thick material of
° C / sec. Table 4 shows the results of evaluating the characteristics of the cold-rolled steel sheet obtained as described above in the same manner as in Example 1. Note that, in Table 4, No. For a total elongation of 30, the value measured on a 0.75 mm thick cold rolled steel sheet was
The value converted into elongation of 0.8 mm material by the er rule is shown.

[0076]

[Table 3]

[0077]

[Table 4]

As shown in Table 4, No. 1 manufactured under the process conditions of the present invention. 20, 25-30, 33-3
The steel sheets of Nos. 6, 38 to 40, and 44 each have the shape and transportability of the steel sheets at a level at which there is no problem, and furthermore, the elongation and the average r value are extremely high, and the Δr is extremely low. And the workability and the anisotropy were extremely excellent. On the other hand, in the case of No. 1 in which any condition is out of the range of the present invention. 19, 21-24, 31, 32, 3
In the steel sheets of 7, 41 to 43, any of the characteristics was inferior.

More specifically, No. 19 before the final pass 2
Since the total rolling reduction of the pass exceeded the range of the present invention, no.
In No. 21, since the rolling reduction of the final pass exceeded the range of the present invention, in both cases, meandering occurred during production, and the shape and transportability of the steel sheet were inferior, and it was difficult to produce stably. Table 4 shows the best data among the material properties of the sample of the cold rolled annealed sheet obtained from a part of the hot rolled coil that could be manufactured. As shown in Table 4, 19 and 21 exhibited excellent material properties in some cases, but the production itself was difficult and the dispersion of the material properties was large.

In addition, No. In No. 22, since the finishing temperature was lower than the range of the present invention and the α-region rolling was performed, the total elongation was particularly deteriorated. On the other hand, No. In No. 23, since the finishing temperature exceeded the range of the present invention, the characteristics were deteriorated.
This is presumably because the growth of γ grains progressed before rapid cooling. No. In No. 24, since the cooling rate was lower than the range of the present invention, rapid cooling was not sufficient, the hot-rolled sheet could not be refined, and the effect of improving the γ value was not sufficiently obtained. N
o. 31 and No. 31. In No. 32, since the cooling start time exceeded the range of the present invention, it was considered that the grains had grown, and the hot rolled sheet was not sufficiently refined, and the effects of improving workability and anisotropy were sufficiently obtained. I couldn't. No. 37,
Since the temperature drop during quenching was smaller than the range of the present invention and the quenching stop temperature was higher than the range of the present invention, the hot-rolled sheet was not sufficiently refined, and the effect of improving the r-value was not sufficiently obtained. No. In No. 41, the rapid cooling temperature drop was larger than the range of the present invention, the quenching stop temperature was lower than the range of the present invention, and the winding temperature was lower than the preferred range of the present invention. The structure became hardened structure-like grains, and the characteristic value was significantly deteriorated. No. 42
In, because the quenching stop temperature was lower than the range of the present invention,
The structure of the hot-rolled sheet did not become polygonal fine grains, and the characteristic values deteriorated. No. In No. 43, since the cooling rate after quenching was higher than the range of the present invention, polygonal fine particles could not be obtained at the stage of hot rolling, and all the characteristic values were inferior.

As described above, a cold-rolled steel sheet having much better workability and anisotropy than the conventional one can be obtained only by a manufacturing method that satisfies all the conditions specified in the present invention. It has been confirmed that it can be manufactured without any problems.

Example 3 In order to investigate the effect of the cooling start time, steel having the components shown in Table 5 was continuously cast into
A slab with a thickness of 00 to 300 mm, 1180 ° C to 12
The sheet was heated to 50 ° C., and hot-rolled at a total draft of the two passes before the final pass, a final pass draft, a finishing temperature, a cooling condition, and a winding temperature in the range shown in Table 6 to obtain a sheet thickness of 2. 8 mm hot rolled sheet, cold-rolled to a sheet thickness of 0.8 mm, heated at a heating rate of 6 ° C./sec or more and 20 ° C./sec or less, and continuously annealed at an annealing temperature of 850 ° C. for 90 seconds for various kinds. A cold rolled steel sheet was obtained. At this time, in Table 6, "Conventional laminar cooling" indicates that the hot-rolled steel strip passed through the final pass of finish rolling was:
Laminar cooling was performed to cool while raising the steam. On the other hand, in the case of quenching at 200 ° C./sec or more after finish rolling, in the film boiling mode cooling, steam is generated at the time of cooling, and the steam film envelops the steel sheet and cannot be quenched. Using a multi-jet cooling system, fresh water always hits the steel plate to perform cooling while destroying the vapor film, realizing nucleate boiling mode cooling that enables rapid cooling.

For these steel sheets, as in Example 1, the total elongation was measured using a 0.8 mm cold-rolled steel sheet, and the r values r0, r45, and r90 in each direction were measured. FIG. 1 shows the relationship between the measured total elongation value and the cooling start time, and the average r obtained from the measured r values.
FIG. 2 shows the relationship between the value and the cooling start time. In addition, Δr is used as an index for evaluating anisotropy, and FIG. 2 also shows the value of Δr for some steel sheets.

As shown in FIGS. 1 and 2, when the cooling start time is set to 0.5 seconds or less, the behavior of the total elongation is not uniform and greatly fluctuates. Is increasing at a higher rate as the is shorter. For this reason, if the cooling start time is 0.5 seconds or less, the material changes greatly even if the cooling start time is slightly shifted, and it is difficult to industrially manufacture a steel sheet having a uniform material in the longitudinal direction.

On the other hand, when the cooling start time is in the range of more than 0.5 seconds to 1 second, both the total elongation and the r value are almost constant. Therefore, even if the cooling start time is slightly shifted, a steel sheet having a uniform material in the longitudinal direction can be manufactured.
Further, both the total elongation and the average r value are improved as compared with the case where laminar cooling in the film boiling mode is performed.

On the other hand, when the cooling start time exceeds 1 second, both the total elongation and the r value decrease monotonically, and the material level is not much different from the case where the laminar cooling in the film boiling mode is performed.

Although Δr is not shown for all the cooling start times, as shown in FIG. 2, Δr is small in a steel sheet whose r value has been improved by cooling. From this, if the r value is at the same level, Δr may be considered to be at substantially the same level. Therefore, when the cooling start time is in the range of more than 0.5 seconds to 1 second or less, it is considered that the value of Δr is stably improved as compared with the case where the conventional laminar cooling is performed.

As described above, when the cooling start time is in the range of more than 0.5 seconds to 1 second, it is possible to produce a cold-rolled steel sheet having excellent workability, small anisotropy and uniform material in the longitudinal direction. confirmed.

[0089]

[Table 5]

[0090]

[Table 6]

Example 4 A steel having the components shown in Table 7 was cast into a slab having a thickness of 200 to 300 mm by continuous casting, heated to 1180 to 1250 ° C., and cooled under the conditions shown in Table 8. A hot-rolled sheet having a thickness of 2.8 mm was formed by hot rolling under hot rolling conditions, cold-rolled to a thickness of 0.8 mm, and then heated at a heating rate of 6 ° C./sec or more and 20 ° C./sec or less. 8 for 90 seconds at the annealing temperature shown in FIG.
o. 45 to 64 cold-rolled steel sheets were obtained. In hot rolling, rough bars (hot-rolled steel strips that have finished rolling) are used to ensure the transportability and shape of the hot-rolled steel strips at a level that does not cause any problems.
Was heated by an induction heating device immediately before being introduced into the finish rolling device, so that the temperature distribution in the width direction of the steel strip was made uniform. Also, in Table 8, "Conventional laminar cooling" indicates:
The hot rolled steel strip that passed through the final pass of finish rolling was subjected to laminar cooling in which the steam was raised while cooling. On the other hand, in the case of quenching at 200 ° C./sec or more after the finish rolling, steam is generated at the time of cooling in the cooling in the film boiling mode, and the steam film wraps around the steel sheet and cannot be quenched. Using a multi-jet type cooling device, fresh water always hits the steel plate to perform cooling while destroying the vapor film, realizing nucleate boiling mode cooling that can perform rapid cooling, the amount of water and water pressure Table 8
The cooling was carried out at various cooling rates shown in FIG. The properties of the cold-rolled steel sheet obtained as described above were evaluated in the same manner as in Example 1. Table 8 shows the results.

[0092]

[Table 7]

[0093]

[Table 8]

[0094] As shown in Table 8, no. 46,48,5
Each of the steel sheets 0, 52, 54, 56, 58, 62, and 64 has extremely high elongation and r value, and the maximum value-minimum value of Δr or r value is extremely low. The anisotropy was extremely excellent. On the contrary,
The laminar cooling was performed from the top and bottom of the steel plate on the runout table after the final pass. 45, 47, 49, 51, 5
The steel sheets of 3, 55, 57, 59, 61 and 63 were inferior in any of the properties.

As described above, if a cold-rolled steel sheet is manufactured using the steel having the composition in the range specified in the present invention under the process conditions specified in the present invention, the shape and transportability are excellent, and It was confirmed that a cold-rolled steel sheet having much better workability and anisotropy than the conventional one could be manufactured.

Example 5 Steel having the components shown in Table 9 was continuously cast into a slab having a thickness of 220 mm. The slab was cleaned, heated to 1200 ° C., and heated under the conditions shown in Table 10. After cold rolling and cold rolling, the steel sheet was continuously annealed at an annealing temperature of 840 ° C. for 90 seconds at a heating rate of 10 ° C./sec or more and 20 ° C./sec or less, and No. 65 to 82 cold-rolled steel sheets were obtained. At the time of hot rolling, in order to ensure the transportability and shape of the hot-rolled steel strip at a level that does not cause any problems, a rough bar (hot-rolled steel strip after the end of rolling) is immediately introduced into the finishing mill. The sheet was heated by an induction heating device to make the temperature distribution in the sheet width direction of the steel sheet uniform. At this time, No. For No. 74, the thickness of the hot-rolled sheet was 1.5 mm and the thickness of the cold-rolled annealed sheet was 0.75 mm. 65-73,7
Regarding 5 to 82, in each case, the thickness of the hot-rolled sheet was 2.8 ± 0.2 mm, and the thickness of the cold-rolled annealed sheet was 0.8 mm. In addition, No. shown in Table 10 The cooling rate of 74 is a value when the thickness of the hot-rolled sheet is 1.5 mm and is 2.8 to
The cooling rate was confirmed to be 270 for a 3.5 mm thick material.
It was ± 70 ° C / sec. Table 10 shows the results of evaluating the properties of the cold-rolled steel sheets obtained as described above in the same manner as in Example 1. Note that in Table 10, No. Regarding the total elongation of 74, a value obtained by converting a value measured for a cold-rolled steel sheet having a thickness of 0.75 mm into an elongation of a 0.8 mm material according to Oliver's law is shown.

[0097]

[Table 9]

[0098]

[Table 10]

As shown in Table 10, No. 1 manufactured under the process conditions of the present invention. 66,69-74,76-7
Each of the steel sheets No. 8, 82 has a shape and transportability of the steel sheet at a level where there is no problem, and furthermore, the elongation and the average r value are extremely high, and the Δr is extremely low. And the anisotropy was extremely excellent. On the other hand, if any of the conditions is out of the scope of the present invention, N
o. In the steel plates of 65, 67, 68, 75, 79-81,
Either property was inferior.

More specifically, No. 65 before the last pass 2
Because the total rolling reduction of the pass was higher than the scope of the present invention,
No. In 67, since the rolling reduction of the final pass was higher than the range of the present invention, in either case, the meandering during manufacturing was
The shape and transportability of the steel sheet were inferior, and it was difficult to manufacture stably. Table 10 shows the best data among the materials shown in the sample of the cold-rolled annealed sheet obtained from a part of the hot-rolled coil that could be manufactured. As shown in Table 10, although No. 65 and 67 sometimes showed excellent material properties, the production itself was difficult and the dispersion of the material properties was large.

No. In No. 68, since the cooling rate was lower than the range of the present invention, rapid cooling was not sufficient, the grain size of the hot rolled sheet could not be reduced, and the effect of improving the γ value could not be sufficiently obtained. N
o. At 75, the rapid cooling temperature drop was smaller than the range of the present invention, and the rapid cooling stop temperature was higher than the range of the present invention.
The grain refinement of the hot rolled sheet was insufficient, and the effect of improving the r value was not sufficiently obtained. No. In No. 79, the rapid cooling temperature drop was larger than the range of the present invention, the quenching stop temperature was lower than the range of the present invention, and the winding temperature was lower than the preferred range of the present invention. The resulting grains were in a textured structure, and the characteristic values were significantly deteriorated. No. 8
At 0, the quenching stop temperature was lower than the range of the present invention, so that the structure of the hot-rolled sheet was not polygonal fine grains, and the characteristic values were degraded. No. In No. 81, since the cooling rate after quenching was higher than the range of the present invention, polygonal fine particles could not be obtained at the stage of hot-rolled sheet, and all the characteristic values were inferior.

As described above, a cold-rolled steel sheet having much better workability and anisotropy than the conventional one can be obtained only by a manufacturing method that satisfies all the conditions specified in the present invention. It has been confirmed that it can be manufactured without any problems.

[0103]

According to the present invention, a cold-rolled steel sheet having remarkably excellent workability and anisotropy as compared with that of the same component system produced by a conventional method can be obtained. It can be manufactured without causing a problem in transportability.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a value of total elongation and a cooling start time in Example 3.

FIG. 2 is a graph showing a relationship between an average r value and a cooling start time in Example 3.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toru Inazumi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Within Nihon Kokan Co., Ltd. (72) Inventor Yoichi Honashiki 1-1-2, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Sadanori Imada 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Japan Inside Nippon Kokan Co., Ltd. (72) Yoshiro Tsuchiya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Kokan Co., Ltd. (72) Inventor Kenichi Mitsuka 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (5)

[Claims] C .: 0.0003% to 0.01%, Si: 0.05% or less, Mn: 0.0% by weight.
5% or more and 2.5% or less, P: 0.003% or more and 0.1
%, S: 0.0003% to 0.02%, S
ol. Al: 0.005% or more and 0.1% or less, N:
In producing a cold-rolled steel sheet by heating, hot rolling, cold rolling, and annealing a steel slab containing 0.0003% or more and 0.004% or less, the hot rolling is performed in finish rolling. the total reduction ratio in the final pass prior to 2-pass than 45% to 70%, and the reduction ratio of the final pass to 35% or less than 5%, further, the finishing temperature than the Ar ₃ transformation point (Ar ₃ transformation point + 50 ° C or less) and finish rolling is completed. Then, within 1 second after finishing rolling, 200 ° C / sec.
Rapid cooling is started at a cooling rate of 2000 ° C./sec or less, a temperature drop from the finishing temperature of the finish rolling in the rapid cooling is set to 50 ° C. or more and 250 ° C. or less, and a cooling stop temperature of the rapid cooling is set to 650 ° C or higher and 850 ° C or lower, followed by slow cooling or air cooling at a rate of 100 ° C / sec or lower, and then winding the obtained hot rolled steel strip. Manufacturing method of small cold rolled steel sheet. 2. In% by weight, C: 0.0003% or more and 0.01% or less, Si: 0.05% or less, Mn: 0.0% or less.
5% or more and 2.5% or less, P: 0.003% or more and 0.1
%, S: 0.0003% to 0.02%, S
ol. Al: 0.005% or more and 0.1% or less, N:
In producing a cold-rolled steel sheet by heating, hot rolling, cold rolling, and annealing a steel slab containing 0.0003% or more and 0.004% or less, the hot rolling is performed in finish rolling. the total reduction ratio in the final pass prior to 2-pass than 45% to 70%, and the reduction ratio of the final pass to 35% or less than 5%, further, the finishing temperature than the Ar ₃ transformation point (Ar ₃ transformation point + 50 ° C or less) and finish rolling is completed.
The rapid cooling is started at a cooling rate of not less than 2000 ° C./sec and not more than 50 ° C. and not more than 250 ° C. in the rapid cooling. The stop temperature is set to 650 ° C. or higher and 850 ° C. or lower. Subsequently, after slow cooling or air cooling at 100 ° C./sec or lower, the obtained hot-rolled steel strip is wound up. Manufacturing method of cold-rolled steel sheet with small anisotropy. 3. The steel according to claim 1, further comprising:
at least one of b, V, and Zr is 0.005% in total
The method for producing a cold-rolled steel sheet having excellent workability and low anisotropy according to claim 1 or 2, wherein the content is 0.1% or less. 4. The steel according to claim 1, further comprising:
The method for producing a cold-rolled steel sheet having excellent workability and low anisotropy according to any one of claims 1 to 3, characterized by containing 015% or more and 0.08% or less. 5. The steel according to claim 1, further comprising:
The method for producing a cold-rolled steel sheet having excellent workability and low anisotropy according to any one of claims 1 to 4, wherein the cold-rolled steel sheet has a content of 001% or more and 0.001% or less.