JPH0885827A - Production of cold rolled steel sheet by continuous annealing - Google Patents

Abstract

PURPOSE: To stably produce a cold rolled steel sheet, having homogeneous characteristics in coil and excellent in deep drawability and aging characteristic at ordinary temp., from a low-carbon Al-killed steel stock by applying continuous annealing. CONSTITUTION: A steel slab, having a composition which contains 0.010-0.030% C, <=0.1% Si, 0.05-0.25% Mn, 0.004-0.10% sol.Al, 0.0005-0.015% S, and 0-0.0040% B and in which respective contents of P and N are limited, is hot-rolled, coiled at 600-750 deg.C, cold-rolled, and continuously annealed. The maximum heating temp. in the central part of the coil at annealing and the maximum heating temp. at both ends of the coil are controlled to 840-880 deg.C and 870-930 deg.C, respectively. After annealing heating, the steel sheet is cooled slowly down to 650-780 deg.C at a rate of <=10 deg.C/s, cooled rapidly down to 300-450 deg.C at a rate of >=60 deg.C/s, and then overaged at 300-450 deg.C for 90-270sec, or, after slow cooling the steel sheet is cooled rapidly down to 200-300 deg.C at >=60 deg.C/s cooling rate and then reheated to 320-450 deg.C and cooled slowly down to 200-330 deg.C in 90-240sec to undergo overaging.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is a low-cost conventional low-carbon
The present invention relates to a method for producing a cold-rolled steel sheet using Al-killed steel as a raw material, which is excellent in deep drawability and normal temperature aging by a continuous annealing method and has uniform properties in a coil.

[0002]

2. Description of the Related Art Conventionally, cold-rolled steel sheets for deep drawing used for automobiles and the like have been manufactured by a manufacturing process incorporating a box annealing method, but recently, a continuous annealing method is applied instead of the box annealing method, Its productivity is being increased. However, since the continuous annealing method is a process in which annealing and overaging are completed in a short time, the following two problems were pointed out when manufacturing cold-rolled steel sheets for deep drawing using low carbon Al killed steel as a raw material. . One is that the obtained cold-rolled steel sheet has a low r value (rank-ford value used as an index of deep drawability). That is, in order to obtain a high r value by continuous annealing in a low carbon Al-killed steel sheet, "coagulate and coarsen cementite" and "precipitate and increase AlN completely" at the stage of hot rolling. Is important, so high temperature winding is usually performed. However, in a coil that has been wound at a high temperature, the cooling rate of the inner and outer circumferences tends to be faster, so that cementite or AlN does not aggregate and coarsen sufficiently in this part, and it is difficult to obtain good deep drawability. It will be. The other is that the obtained cold-rolled steel sheet has a large amount of residual solid solution C and is inferior in room temperature aging.

Therefore, in order to solve these problems, for example, a method has been proposed in which the amounts of C and Al in the material steel are restricted to low values and a small amount of B is added (Japanese Patent Laid-Open No. 62-107025).
issue). Further, in the hot rolling process, a method has also been proposed in which cooling is controlled so that the temperature of both ends of the steel strip in the lengthwise direction is 50 to 80 ° C. higher than the temperature of the central part, and then the steel strip is wound (Japanese Patent Application Laid-Open No. Hei 4). −63232). But with these methods,
Although the formation of fine precipitates, which is not desirable for improving the r-value, is reduced to some extent, the formation cannot be sufficiently suppressed, and therefore, if the annealing temperature is raised to obtain a high r-value, There was a tendency that the temperature difference between the center and both ends of the steel strip in the lengthwise direction became large, and the difference in r value between them also became large.

That is, according to the experimental results of the present inventors who searched for a means for obtaining a high r value in low carbon Al killed steel, the C content was
When adjusted to 0.010 to 0.030% (hereinafter,% indicating the composition ratio is% by weight), the annealing temperature is higher than the conventional value.
It was found that the r-value in the central portion in the length direction of the coil was improved by increasing the temperature to 0 ° C., but even in this case, the amount of increase in the r-value at both end portions in the length direction of the coil was small, Inhomogeneities within the coil could not be avoided.

Further, when the continuous annealing method is applied to the production of cold-rolled low carbon Al-killed steel, the cold-rolled steel sheet obtained is inferior in room temperature aging because the overage time cannot be sufficiently taken, and the cold-rolled steel sheet is stored at room temperature for a long time. As a result, the yield elongation that disappeared in the skin pass occurs again and "wrinkles" tend to occur during press forming.

As a means for improving the room temperature aging of an ordinary cold-rolled low carbon Al-killed steel, which is inexpensive, for example, Japanese Patent Laid-Open No. 3-23329 discloses, "After continuous annealing following cold rolling, the temperature is temporarily lowered. A method of performing overaging treatment of rapidly cooling to (200 to 300 ° C.) and then reheating is disclosed. That is, in this method, fine carbides are precipitated in the ferrite grains by lowering the quenching end point temperature, and further reheated to grow the fine carbides to reduce the solid solution C. However, according to this method, the room-temperature aging property is significantly improved, but in order to carry out the method, a reheating device for the cold-rolled sheet or the like is required, and thus the conventional equipment must be significantly improved. was there. In addition, in recent years, the demands of customers have tended to become more and more stringent, and a cold-rolled steel sheet having improved room temperature aging more than that obtained by performing the above-mentioned "supercooling-reheating" overaging treatment is required. Is also becoming.

By the way, as described above, the precipitation condition of cementite, AlN, BN, etc. has an influence on the above-mentioned problems relating to the deep drawability and cold aging of cold-rolled steel sheets. In the case of Al-killed steel containing some C, it was considered very difficult to avoid this problem. Therefore, as a result of aiming to solve the problems of deep drawability and room temperature aging from such a viewpoint, C is reduced as much as possible at the melting stage of the material steel, and carbonitride forming elements such as Ti and Nb are added. A steelmaking method has been developed in which the residual C and N are added and fixed as precipitates, and the steel obtained by this method has attracted attention as "IF steel". However, the steel sheet obtained by applying this method contains almost no solid solution C or N that hinders deep drawability or room temperature aging, so that a "high r value" and "non-aging similar to box annealing" are continuous. Although it can be achieved by annealing, it requires a long-time vacuum degassing process to adjust the composition in the steelmaking stage, and the economical disadvantage for this has hindered its widespread use.

On the other hand, as a method for homogenizing the r-values at various points in the coil, a method of strictly controlling the maximum heating temperature of the annealing with an accuracy of ± 10 ° C. is applied to an extremely low carbon steel sheet. It is disclosed in Japanese Unexamined Patent Publication No. 62-124233.
However, the type of steel to which this method can be applied is limited to ultra-low carbon steel, which has a high production cost, and is disclosed in the above-mentioned Japanese Patent Laid-Open No.
No. 2,124,233 also reports that "the effect is small with ordinary low carbon steel".

Thus, until now, the continuous annealing method has been applied to the annealing after cold rolling from the ordinary low carbon Al killed steel material to obtain "deep drawability", "normal temperature aging" and "characteristics in the coil". It could not be said that the technology for stably manufacturing cold-rolled steel sheets excellent in "homogeneity" has been sufficiently established.

Therefore, the object of the present invention is to apply the continuous annealing method to obtain the properties in the coil from "a material of low carbon Al killed steel which is economically advantageous as compared with an ultra low carbon steel". It is an object of the present invention to provide a means for stably producing a cold-rolled steel sheet which is homogeneous and has excellent deep drawability and room temperature aging.

[0011]

[Means for Solving the Problems] In order to achieve the above object, the present inventors first carried out many experiments on the effect of continuous annealing conditions after cold rolling on the deep drawability of low carbon Al killed steel. As a result of conducting research while
In the case of adjusting to the range of 0.010 to 0.030%, the r value can be remarkably improved by increasing the annealing temperature to a considerably high temperature corresponding to the low temperature austenite region. "

Further, "the influence of the annealing temperature on the r value is different between the central portion and the both end portions in the longitudinal direction of the coil".
It became clear that the fact. That is, the r-value increases in the central part of the coil as the annealing temperature rises, whereas at both ends, the r-value rises only a little until the annealing temperature is 870 ° C., and when the annealing temperature exceeds 870 ° C., the r-value sharply increases. The improvement was seen. The reason for this is not always clear, but since the improvement of the r value correlates well with the degree of coarsening of the ferrite grain size of the annealed plate, it is generally noted that “fine grain that inhibits the growth of ferrite grains is present at both ends of the coil. "Precipitate" is generated,
It is considered that this is because when the annealing temperature is set to 880 ° C. or higher, the grains are abruptly coarsened or solid solution occurs, so that the ferrite grain growth is not hindered.

Further, at a temperature of 870 ° C. or higher, the temperature range is considerably high in austenite, but the C content is 0.010 to 0.030.
%, The annealing temperature is 9 in the case of low carbon Al killed steel.
As long as the temperature did not exceed 30 ° C., the “r-value decrease due to randomization of texture” was not observed. In the case of a low carbon Al killed steel having a C content higher than 0.030%, the austenitizing temperature is low, so that even in a relatively low temperature region where fine precipitates do not form a solid solution or coarsen sufficiently, It is considered that randomization occurs, and as a result, the r value is not improved due to grain growth at both ends of the coil, and the inhomogeneity in the coil is not improved.

By the way, as described above, it is also important for cold-rolled steel sheets for deep drawing to have excellent room temperature aging. In order to improve this room temperature aging property, it is necessary to reduce solid solution C. An effective means for reducing the solid solution C is to finely precipitate cementite in ferrite grains to shorten the diffusion distance of C required for the growth of cementite. Therefore, the above-mentioned Japanese Patent Laid-Open No.
According to the method disclosed in Japanese Patent No. 2329, cementite is finely precipitated in ferrite grains by rapid cooling to a low temperature range of 200 to 300 ° C. after annealing, and cementite is grown by reheating to reduce solid solution C. I am letting you. This method has a facility limitation because a reheating device is required, but it can be said that this method is an effective method for reducing the solid solution C.

However, the inventors of the present invention have found, through further research, that it is extremely effective to coarsen the ferrite grains to be formed in order to finely precipitate cementite in the grains. I got it. It should be noted that annealing at a high temperature exceeding 870 ° C. is extremely effective for coarsening the ferrite grains at both ends of the coil, and such high-temperature annealing causes sufficient coarsening of the ferrite grains. Even if the "means for performing reheating treatment after annealing and quenching" disclosed in Japanese Patent Laid-Open No. 3-2323 is not applied, intragranular cementite fine precipitation is effectively promoted. However,
Reheating treatment is preferably performed for the purpose of sufficiently precipitating the intra-grain cementite. From this, high temperature annealing of both ends of the coil, which was effective in improving the deep drawability, also improves the room temperature aging of both ends of the coil through grain size coarsening. It was concluded that this is an effective way to promote the transformation.

The present invention has been completed based on the above findings and the like. "C: 0.010 to 0.030%, Si: 0.1% or less, Mn: 0.
05 to 0.25%, sol.Al: 0.004 to 0.10%, S: 0.0005 to
Steel pieces with a chemical composition of 0.015%, B: 0 to 0.0040%, the balance consisting of Fe and inevitable impurities, and the contents of P and N, which are inevitable impurities, are P: 0.05% or less and N: 50 ppm or less, respectively. Is hot-rolled at or above the Ar ₃ transformation point and then wound at 600 to 750 ° C., followed by pickling, cold rolling and continuous annealing. As shown in FIG. The maximum heating temperature of the central part is 840-880 ℃
The maximum heating temperature of both ends of the coil is 870-930 ° C.
Controlled to 650-78 after annealing heating
After gradually cooling to a temperature range of 0 ° C at a cooling rate of 10 ° C / s or less,
This time, it is rapidly cooled to a temperature range of 300 to 450 ° C at a cooling rate of 60 ° C / s or more, and then overaged for 90 to 240 seconds in a temperature range of 300 to 450 ° C, or as shown in Fig. 2. After slowly cooling, the temperature range of 200-300 ℃ is 60 ℃.
quenching at a cooling rate of / s or more, then reheating to 320 to 450 ° C, and then 90 to a temperature range of 200 to 330 ° C.
By performing over-aging by gradually cooling it for 240 seconds, it is possible to stably manufacture cold-rolled steel sheet with excellent deep drawability and room temperature aging and homogeneous coil internal characteristics. " are doing.

[0017]

Next, the reason why the chemical composition of the raw material steel and the conditions for producing the steel sheet in the present invention are limited as described above will be described in detail. (A) Chemical composition of raw steel

A) In order to improve the room temperature aging of the C cold-rolled steel sheet, it is necessary to precipitate cementite in the grains in the continuous annealing process (during quenching after annealing), thereby reducing the solid solution C as much as possible. Is. Then, in order to effectively precipitate cementite in the grains during the quenching process after annealing, it is necessary to add C to the ferrite in a supersaturation degree.
Must be in solid solution. In the present invention,
When the C content is less than 0.010%, the supersaturation degree of C is small, so that it cannot be precipitated as cementite, and the aging property of the steel sheet obtained by increasing the amount of solute C deteriorates. on the other hand,
If C is contained in excess of 0.030%, cementite will be precipitated at the grain boundary. Therefore, the supersaturation degree of C is insufficient near the grain boundary and it cannot be precipitated as cementite. The time required for the vacuum degassing treatment for reducing the C content becomes long, which causes a cost increase. Further, if the C content exceeds 0.030%, even if the annealing temperature is raised to a high temperature, the austenitization causes the texture to be randomized and a high r value cannot be obtained, so the annealing temperature should be raised. In order to increase the r-value, the C content needs to be 0.030% or less. Therefore, the C content is set to 0.010 to 0.030%, but it is preferably adjusted to 0.013 to 0.027% if possible.

B) Si Si is a solid solution strengthening element, and when the content is large, the effect of increasing the strength of the steel sheet and deteriorating the workability becomes remarkable, so the content was determined to be 0.1% or less.

C) Mn Mn can combine with S to form cementite precipitation nuclei.
And has the effect of promoting the precipitation of cementite. However, in order to precipitate cementite finely, Mn
It is preferable to generate S minutely, and for that purpose, the Mn content is preferably low. However, when the Mn content becomes too small and the amount of solid solution S increases, FeS having a low melting point is formed, which causes ear cracks during rolling. From these perspectives
The Mn content was set to 0.05 to 0.25%.

D) sol.Al Al is added as a deoxidizing agent for steel, but also has a function of fixing solid solution N to improve the aging property. However,
Excessive addition leads to the formation of non-metallic inclusions and deterioration of workability. Therefore, from these viewpoints, the Al content was determined to be 0.004 to 0.10% in terms of sol.Al content.

E) S S can combine with Mn to form cementite precipitation nuclei.
It has the function of forming MnS and promoting the precipitation of cementite. However, excessive addition leads to too much non-metallic inclusions of MnS leading to deterioration of workability, and a large amount of S forms FeS having a low melting point, which also causes edge cracking during rolling. Therefore, from these viewpoints, the S content is
It was set as 0.0005 to 0.015%.

F) BB Since B has the effect of improving the room temperature aging of the steel sheet, it is a component added as necessary when the room temperature aging of the steel sheet needs to be further improved. That is, B precipitates as BN and reduces the solid solution N, and exhibits the action of promoting precipitation of cementite with BN as a nucleus. Also, solid solution B
As a result, it also segregates at the grain boundaries, reduces the amount of solid solution C at the grain boundaries, and suppresses the precipitation of cementite at the grain boundaries. Furthermore, since it also has the effect of making the ferrite grains after annealing easier to coarsen, it has the effect of softening the steel sheet through this action and improving the workability as well as facilitating the precipitation of cementite in the grains. However, the effects due to these actions are saturated when the content exceeds 0.0040%, so the upper limit of the B content was set to 0.0040%. It should be noted that the effect of B addition is recognized even with a very small amount of content, but from the standpoint of the effect, it can be said that when B is added, it is preferable to adjust to the range of 0.0003 to 0.0040%.

G) P P is a solid solution strengthening element and is effective in increasing the strength of the steel sheet. However, the P content was limited to 0.05% or less because the effect of deteriorating the workability by increasing the strength of the steel sheet too much becomes remarkable as the content increases, but preferably 0.
It is good to adjust it to 02% or less.

H) N N, which is an unavoidable impurity, combines with Al and B to partially form fine precipitates, which hinders grain growth. Further, solid solution N that does not form a precipitate promotes the formation of a texture structure that deteriorates the r value during recrystallization during annealing, or deteriorates the room temperature aging property. Therefore, N content is 50ppm
It was limited to below (0.0050% or less).

(B) Steel Sheet Manufacturing Conditions Hot rolling must be completed at the Ar ₃ transformation point or higher.
This is because when rolling is completed at a temperature below the Ar ₃ transformation point, the ferrite phase is processed and the texture is changed, which causes a decrease in r value. Further, the winding after the hot rolling is performed at 600 to 750 ° C., but the winding temperature is 60.
If the temperature is lower than 0 ° C, the carbon nitride will not be sufficiently aggregated to reduce the r-value of the cold rolled steel sheet after annealing, and if it is wound at a temperature higher than 750 ° C, seizure of the coil or the grain of the hot rolled steel sheet will occur. This is because the diameter becomes coarse and problems such as deterioration of the r value after cold rolling and annealing may occur.

When the coil is wound after hot rolling, it is preferable that the central part of the coil be wound at 600 to 700 ° C. and the both ends should be wound at a temperature higher by 40 ° C. or more. Here, the “both ends of the hot-rolled coil” in which the characteristic deterioration is a problem is 20 to 6 from both ends.
Says the range of 0m. By the way, the heating temperature at the time of hot rolling is not particularly specified, but it is preferably set to 1050 to 1250 ° C. in order to suppress segregation of impurities. Also,
The slab charged into the heating furnace may be a slab that remains hot after casting or may be a slab that has been cooled to room temperature. The coil after hot rolling and winding is pickled as usual and then cold rolled. The reduction ratio of the cold rolling is not particularly specified, but it is preferably 70 to 94% in order to secure a good r value.

In carrying out continuous annealing after cold rolling, in the present invention, the maximum heating temperature of the central portion in the coil longitudinal direction during annealing is set to 840 to 880 ° C., and the maximum heating temperature of both longitudinal end portions of the coil is set. It is important to control to 870-930 degreeC. That is, good r in the center of the coil
To obtain the value, an annealing temperature of 840 ° C or higher is required. However, in the central part of the coil, the cooling after the hot rolling and winding is slow, so that the coarsening of the precipitation part is achieved.
When the annealing temperature is higher than 0 ° C, the ferrite grains are excessively coarsened, so that the surface defects are likely to occur due to roughening of the surface after processing.
Therefore, the maximum heating temperature of the coil center is 840 to 88.
Controlled to 0 ° C. On the other hand, at both ends of the coil, as described above, since there are many fine precipitates and the grain growth is poor,
It is recognized that the r value is sharply improved from the annealing temperature of 870 ° C. or higher at which the fine precipitates are likely to be coarsened or to form a solid solution significantly. Therefore, in order to obtain a good r value, an annealing temperature of 870 ° C. or higher is necessary. However, when the annealing temperature is higher than 930 ° C., the ferrite grains are excessively coarsened, and surface defects are likely to occur. Therefore, the maximum heating temperature of both ends of the coil is controlled to 870 to 930 ° C.

Although the amount of transformation into austenite is considerably large in the above-mentioned maximum heating temperature range of annealing, in this temperature range, the r value, especially rolling 4
It has been found that the r value in the 5 ° direction is improved. Also,
The term “both ends of the coil” as used herein means portions corresponding to 20 to 60 m from both ends of the hot-rolled coil, respectively, but as long as the both ends are annealed at a higher temperature than the center of the coil, The "length of the cold-rolled coil" is preferably within 300 m from both ends. Furthermore, the soaking time in annealing is not particularly specified, but 120 seconds or less as usual is sufficient.

After the annealing heating (soaking), the material is gradually cooled at a cooling rate of 10 ° C./s or less up to a quenching start temperature of 650 to 780 ° C. This is because it promotes the growth of ferrite which is preferable for the r value, and when the cooling rate at this time exceeds 10 ° C / s,
The amount of ferrite generated before the start of quenching is small, so that ferrite nucleates newly and the texture becomes random, and the amount of solid solution C in the remaining ferrite becomes too low and precipitation of cementite in the grains occurs. Is less likely to occur.

After the gradual cooling, the rapid cooling is started from 650 to 780 ° C. When the rapid cooling start temperature is lower than 650 ° C., the supersaturation degree of C which is the driving force for the cementite precipitation in the ferrite grains after the rapid cooling is insufficient. As a result, the amount of cementite precipitated in the grains is reduced, the amount of solid solution C in the manufactured cold rolled steel sheet is increased, and the room temperature aging is deteriorated. On the other hand, if the quenching is started from a temperature range exceeding 780 ° C, problems such as heat buckle on the strip running occur, and the residual ferrite ratio becomes too small, and new nucleation of ferrite occurs, and the texture becomes random. Alternatively, the amount of solid solution C in the remaining ferrite becomes too low, and the precipitation of cementite in the grains becomes difficult to occur. The preferable quenching start temperature is 700 to 780 ° C (even more preferably 720 to 720 ° C).
760 ° C). That is, 700 to 780 ° C. is a region in which austenite remains, and when rapidly cooled from this temperature range, there is no time margin for C to diffuse into grain boundaries because austenite rapidly transforms to ferrite, and therefore cementite is present in the grains. Is easily deposited and easily grown.

The quenching at this time is 60 ° C from the quenching start temperature.
It is carried out at a cooling rate of / s or more. When the cooling rate is less than 60 ° C./s, the amount of cementite precipitated at the grain boundaries becomes too large and the supersaturation degree of solid solution C in the ferrite grains becomes insufficient, so that cementite does not precipitate in the grains, resulting in The amount of solute C increases and the aging property at room temperature deteriorates.

After the rapid cooling, the overaging treatment is carried out. However, if the reheating is not performed before the overaging, the quenching stop temperature is set to 300-4.
The temperature is 50 ° C., and the overaging temperature is 300 to 450 ° C. This is the temperature at which cementite can precipitate and grow in the grains. If the quenching stop temperature or overaging temperature is out of this range, the precipitation of cementite will be delayed, and a steel sheet with good aging characteristics cannot be obtained. The overaging time is 90 to 240 seconds, but if the time is less than 90 seconds, the cementite growth is insufficient and the amount of dissolved C is not sufficiently reduced, while if the overaging time exceeds 240 seconds. Since the effect is saturated, it causes productivity deterioration. In addition, in the overaging zone, the effect does not change even if "gradient overaging" is performed to lower the overaging temperature within the above temperature range.

On the other hand, when reheating is performed before overaging, the quenching stop temperature is set to 200 to 300 ° C., and after quenching is stopped, the temperature is heated to 320 to 450 ° C. and then to the temperature range of 200 to 330 ° C. Slowly cool over 240 seconds. In this case, quenching to 200 to 300 ° C. is a supersaturated solid solution C
Is to be precipitated as intragranular cementite, and if the quenching stop temperature is higher than 300 ° C, the effect of improving aging due to subsequent reheating cannot be expected, and if it is cooled to a temperature lower than 200 ° C, the precipitated cementite is fine. If it passes, the yield stress increases and the workability deteriorates.

Reheating after quenching accelerates the diffusion rate of C and promotes the growth of precipitated cementite, so that solid solution C is formed.
This is to reduce In this case, the reheating temperature is 320
Below ℃, the diffusion of C is slow and its effect is small.
When reheated to a temperature exceeding 50 ° C., the precipitated cementite re-dissolves, the number of cementite decreases, and the aging property deteriorates. In addition, in order not to impair the productivity, it is preferable that the time required for reheating after quenching is 10 seconds or less and the heating rate at the time of reheating is 10 ° C./s or more.

After the reheating, the reason for gradually cooling to a temperature range of 200 to 330 ° C. over 90 to 240 seconds is to accelerate the precipitation of C. At this time, if the temperature is gradually cooled to a temperature higher than 330 ° C or lower than 200 ° C,
Since the precipitation of cementite is delayed, a steel sheet with good aging characteristics cannot be obtained. If the slow cooling time is less than 90 seconds, the precipitation of C is insufficient, while if it exceeds 240 seconds, the effect is saturated.

The cold-rolled steel sheet produced according to the above conditions has uniform properties in the coil and exhibits excellent deep drawability and room temperature aging. The cold-rolled steel sheet according to the present invention is Further, there is no problem even if it is used after being subjected to surface treatment such as electroplating. Next, the present invention will be described with reference to examples.

[0038]

【Example】

Example 1 First, steel having the chemical composition shown in Table 1 was melted in a converter, and then a slab was manufactured by continuous casting.

[0039]

[Table 1]

Next, this slab is heated to 1150 ° C. and immediately hot rolling is started, and the finishing temperature is set to 900 ° C.
After producing a hot rolled steel sheet of mm, the average cooling rate is 20 ° C / s, 680 ° C at both ends in the longitudinal direction, and 630 ° C at the center.
I wound it up. After that, pickling this hot rolled steel sheet 0.8
Cold rolled to mm.

Subsequently, the obtained cold rolled coil was continuously annealed by an actual machine under the conditions shown in Tables 2 and 3 and further temper-rolled at an elongation of 1.2%. Then, a JIS No. 5 tensile test was sampled from the obtained cold-rolled steel sheet from the portions corresponding to the "top portion", "middle portion" and "bottom portion" of hot rolling, and the tensile test was performed. The results of this tensile test are also shown in Tables 2 and 3.

The "yield strength", "tensile strength" and "elongation" are in the rolling direction, and the "r value" is 0 °, 45 ° of rolling.
And the average value in the 90 ° direction were evaluated. Also, JIS No. 5 tensile test pieces taken in the rolling direction from the cold-rolled steel sheet were subjected to a heat treatment at 170 ° C. for 20 minutes after a pre-strain of 2% and then subjected to a tensile test again. The value obtained by subtracting the deformation stress was evaluated as "BH amount". This BH amount is an index that the lower the value, the better the room temperature aging property.

[0043]

[Table 2]

[0044]

[Table 3]

From the results shown in Tables 2 and 3, the following can be understood. That is, by applying the manufacturing conditions specified in the present invention to steels A, B or C whose C content satisfies the conditions specified in the present invention, good room temperature aging and a high r value can be obtained from the top to the bottom of the coil. The cold rolled steel sheet shown is obtained. However, in the steel D whose C content deviates from the conditions specified in the present invention, the r value and BH at both ends are not improved even if the manufacturing method specified in the present invention is applied, and the heterogeneity in the coil Is not improved.

Samples were taken from the cold rolled coils of Steel A and Steel D from the portions corresponding to the top and middle portions of hot rolling, and the annealing temperature was changed under the conditions of FIG. A simulation of continuous annealing and 1.2% temper rolling were performed. Then, a JIS No. 5 tensile test piece was cut out from this sample, and a tensile test and measurement of the BH amount were carried out in the same manner as described above. The "influence of annealing temperature on r value and BH amount" investigated in this way was arranged and shown in FIG.

The following is clear from FIG. C
In the steel sheet according to Steel A, the content of which satisfies the conditions specified in the present invention, the annealing temperature at the hot rolling middle equivalent portion is 840 to 880 ° C.
By doing so, the annealing temperature at the hot rolled top equivalent part is 87
By setting the temperature to 0 to 930 ° C, good r value and BH can be obtained. On the other hand, in steel D in which the C content deviates from the conditions specified in the present invention, the r value and BH accompanying the increase in the annealing temperature
Improvement is small, especially in the hot rolled top equivalent part.

Example 2 In this example, the effect of annealing conditions was investigated in the laboratory on the steel sheets of Steel A shown in Table 1 above. That is, in the same manner as in Example 1, the slab of Steel A in Table 1 obtained by melting in a converter and continuous casting was heated to 1150 ° C. and then hot-rolled at a finishing temperature of 900 ° C. to obtain a heat of 4.0 mm. The rolled steel sheet was wound at an average cooling rate of 20 ° C./s at 680 ° C. at both ends in the longitudinal direction and at 630 ° C. at the center. Then, the hot rolled steel sheet was pickled and cold rolled to 0.8 mm. Subsequently, samples were taken from the "top equivalent portion in hot rolling" and the "corresponding middle portion in hot rolling" of the obtained cold rolled coil and subjected to continuous annealing equivalent treatment under the conditions shown in Tables 4 and 5. Was applied.

Then, a JIS No. 5 tensile test was taken from this sample to perform a tensile test, and the "BH amount" was measured for each sample. The "yield strength", "tensile strength", "elongation", "r value" and "BH amount" were evaluated in the same manner as in Example 1. The results are also shown in Tables 4 and 5.

From the results shown in Tables 4 and 5, the following can be confirmed. That is, if the continuous annealing conditions satisfy the conditions specified in the present invention, an r value of more than 1.7 and a BH of 30 N / mm ² or less can be obtained regardless of the hot rolled top portion and the hot rolled middle portion. It is possible to manufacture a cold-rolled steel sheet having excellent deep drawability and room temperature aging.

[0051]

[Summary of Effects] As described above, according to the present invention, a cold-rolled steel sheet which is excellent in deep drawability and room temperature aging and has little characteristic variation in the coil is suitable for, for example, interior and exterior of automobiles. Industrially useful effects such as low cost and low cost can be provided.

[Brief description of drawings]

FIG. 1 is an explanatory view of a continuous annealing heat pattern according to the present invention.

FIG. 2 is an explanatory view of a continuous annealing heat pattern according to another example of the present invention.

FIG. 3 is an explanatory view of continuous annealing conditions performed in a laboratory in Example 1.

FIG. 4 is a graph showing the effect of annealing temperature on the r value and BH at the hot-rolled top-corresponding portion and middle-corresponding portion of the cold rolled steel sheets of Steels A and D.

[Table 4]

[Table 5]

Claims (2)

[Claims] 1. By weight ratio, C: 0.010 to 0.030%, Si: 0.1% or less, Mn: 0.05
~ 0.25%, sol.Al: 0.004 ~ 0.10%, S: 0.0005 ~
Steel pieces with a chemical composition of 0.015%, B: 0 to 0.0040%, the balance being Fe and unavoidable impurities, and the contents of P and N, which are unavoidable impurities, are P: 0.05% or less and N: 50 ppm or less, respectively. Is hot-rolled above the Ar ₃ transformation point and then wound at 600 to 750 ° C., followed by pickling, followed by cold rolling and continuous annealing, the maximum heating temperature of the coil central part during annealing is Control the maximum heating temperature of both ends of the coil to 840 to 880 ° C. and 870 to 930 ° C., respectively, and keep the temperature range of 650 to 780 ° C. 1 after annealing heating.
After gradually cooling at a cooling rate of 0 ° C / s or less, this time 300-4
Quench to a temperature range of 50 ° C at a cooling rate of 60 ° C / s or more,
A method for producing a cold-rolled steel sheet, which is further excellent in deep drawability and normal temperature aging and has uniform coil internal characteristics, characterized by further performing overaging for 90 to 240 seconds in a temperature range of 300 to 450 ° C. 2. C: 0.010 to 0.030% by weight, Si: 0.1% or less, Mn: 0.05
~ 0.25%, sol.Al: 0.004 ~ 0.10%, S: 0.0005 ~
Steel pieces with a chemical composition of 0.015%, B: 0 to 0.0040%, the balance being Fe and unavoidable impurities, and the contents of P and N, which are unavoidable impurities, are P: 0.05% or less and N: 50 ppm or less, respectively. Is hot-rolled above the Ar ₃ transformation point and then wound at 600 to 750 ° C., followed by pickling, followed by cold rolling and continuous annealing, the maximum heating temperature of the coil central part during annealing is Control the maximum heating temperature of both ends of the coil to 840 to 880 ° C. and 870 to 930 ° C., respectively, and keep the temperature range of 650 to 780 ° C. 1 after annealing heating.
After slowly cooling at a cooling rate of 0 ° C / s or less, this time 200 to 3
Rapid cooling to a temperature range of 00 ° C at a cooling rate of 60 ° C / s or more,
Then reheat to 320-450 ° C, then 200-3
A method for producing a cold-rolled steel sheet, which is excellent in deep drawability and normal temperature aging and has uniform coil internal characteristics, which is characterized by performing over-aging by gradually cooling to a temperature range of 30 ° C over 90 to 240 seconds.