JPH0711025B2 - Method for manufacturing cold rolled steel sheet having high ductility - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet used for various forming processes and the like, and particularly has a high ductility of 60% or more in total elongation. The present invention relates to a method for manufacturing a cold rolled steel sheet that can be annealed at a low temperature. [0002] Conventionally, as a method for producing a cold-rolled steel sheet having excellent workability such as ductility and drawability, the carbon content is 0.03 to 0.03.
A method in which a low carbon Al killed steel of about 0.06% is used as a raw material and then subjected to box annealing after rolling is generally used. However, the box annealing method not only takes a long time for processing and is extremely low in productivity, but also the heating and heating rates and cooling rates vary in the radial direction of the coil because the heat treatment is performed in the coil state. There was a fundamental problem in that it was not possible to obtain uniform properties throughout. Therefore, recently, in order to eliminate the drawbacks of the box annealing method, the continuous annealing method is often used. However, when the continuous annealing method is applied,
Since it is accompanied by rapid heating, the recrystallization temperature of the steel sheet becomes as high as 50 to 100 ° C. as compared with the case of the box annealing method, so the crystal growth property deteriorates, and rapid cooling causes solidification in the steel. There is a problem that precipitation of dissolved C does not proceed sufficiently and the resulting steel sheet is hard and inferior in ductility, drawability, aging resistance and the like to the steel sheet obtained by the box annealing method.
Regarding N as well, in the continuous annealing method, since it is heating for a short time, it is difficult for N dissolved in steel to precipitate as AlN, and a large amount of dissolved N remains in the steel even after annealing. This is one of the causes of the low workability of the steel sheet obtained by the continuous annealing method. One of the methods for solving the drawbacks of the continuous annealing method is the method proposed in Japanese Patent Publication No. 50-1341. The method proposed by the present invention, by winding at a high temperature during hot rolling, promotes grain growth in an orientation that is advantageous for improving ductility and drawability, or promotes precipitation of C or N dissolved in steel. In the continuous annealing step, rapid cooling is performed once and then overaging treatment is performed at 300 to 500 ° C. for several seconds to several minutes to promote the precipitation of undissolved solid solution C, thereby improving the aging resistance. is there. However, winding at a high temperature in the hot rolling step as in this proposed method has a problem of lowering the pickling property,
Further, cold-rolled steel sheet produced by the method of this proposal is still inferior in drawability and ductility as compared with the cold-rolled steel sheet obtained by the box annealing method, but the above-mentioned drawbacks could not be fundamentally solved. It's a reality. On the other hand, since the main cause of deteriorating the material quality of the continuously annealed material is C which is a solid solution in the steel, the material of ultra low C steel in which the C content is reduced to 0.005% or less. Various methods of improving workability by using are proposed. As described above, it is natural that extremely reducing the C content is advantageous in terms of ductility, drawability, and aging resistance, but if the C content is simply reduced, it is completely non-aging. It was difficult to obtain a steel plate. Therefore, in addition to reducing the amount of C, T
In reality, it is unavoidable to add carbonitride forming elements such as i and Nb to improve aging resistance and ductility. However, addition of such a special element not only raises the raw material cost but also remarkably raises the recrystallization temperature. Therefore, continuous annealing must be carried out at a high temperature, which will be described later in JP-A-58. Similarly to the method of No. 141,335, it cannot be said to be a preferable method. In addition to that, addition of the special element as described above causes a problem of deteriorating the surface properties and chemical conversion treatability of the product. As a method for obtaining a cold-rolled steel sheet of an excellent material even by a continuous annealing method using an ultra-low carbon steel, there is a method proposed in JP-A-58-141335, for example. This proposed method has C ≦ 0.0030%, N ≦ 0.
0020%, Mn ≦ 0.20%, 0.020 ≦ sol.
A steel containing Al ≤ 0.070% is used as the raw material, and the finishing temperature is 85
Hot rolling is performed at a coiling temperature of 500 to 700 ° C. at 0 ° C. or more, cold rolling is performed at a cold rolling ratio of 80% or more, and continuous annealing is performed at a temperature of 750 ° C. or more. It is said that an excellent cold rolled steel sheet can be manufactured. However, in this proposed method, the average r value (average Rankford value) does not reach 1.8 or more unless the continuous annealing temperature is 750 ° C. or higher, and the deep drawability is inferior at a temperature lower than 750 ° C. . In addition, the maximum value of ductility in this proposed method is only 53.3%. The continuous annealing at a high temperature of 750 ° C. or higher as shown in the above proposal has a risk of breaking the steel sheet due to excessive tension on the steel sheet softened at a high temperature, and shortens the life of the furnace or the rolls in the furnace. However, there is a problem that the cost of heat energy is increased. Especially, in the case of the ultra-low carbon steel such as the above-mentioned proposal, since it is extremely soft, there is a large risk of steel plate breakage, and therefore continuous annealing at such a high temperature. Should be avoided. On the other hand, as an ultra-low carbon cold-rolled steel sheet which can be annealed at a low temperature and is excellent in workability, there is one proposed in JP-A-59-166650. That is, this proposal is C ≦ 0.0050%, Mn ≦ 0.5%, P ≦ 0.1
%, N ≦ 0.0050%, O0.0016 to 0.035
% Steel and B0.0001-
0.0050%, Nb 0.003 to 0.080%, Zr
The present invention provides a cold-rolled steel sheet made of steel containing 0.005 to 0.1% of one kind or two or more kinds. However, in the case of this proposal, although the annealing temperature of continuous annealing is described as 600 to 770 ° C. in the specification, the continuous annealing temperature is 750 ° C. in the examples, and 750 ° C.
It is unclear whether lower continuous annealing temperatures are actually applicable. The material properties of the continuous annealed material obtained by the proposed method are as follows: yield point strength ≧ 17 kgf / mm ² , tensile strength ≧ 29.9 kgf / mm ² , elongation 49%, average r value ≦ 1.7.
3, it is not possible to obtain a particularly excellent material. [0010] As described above, the prior art is as follows.
It is possible to apply a continuous annealing method and enable continuous annealing at a low temperature so that there is no risk of steel plate breakage, etc., and it is possible to produce a cold-rolled steel sheet with excellent workability, especially ductility even at such a low temperature. It was difficult. The present invention has been made in view of the above circumstances. The workability, particularly the ductility, can be obtained by continuous annealing at a low temperature, specifically, a continuous annealing at a temperature of less than 750 ° C.
It is an object of the present invention to provide a method for manufacturing a cold-rolled steel sheet, which is improved to be at least%. The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, C,
By further reducing the amounts of components such as Al, N, and Mn as compared with the conventional one, the oxygen amount is set within a relatively high specific range, and further, the rolling conditions of hot-cold rolling are set to specific conditions to increase the amount of oxygen. The inventors have found that a cold-rolled steel sheet having ductility can be manufactured by low-temperature continuous annealing, and have completed the present invention. Specifically, the cold-rolled steel sheet manufacturing method according to the first aspect of the present invention uses C 0.001% or less, Mn 0.1% or less, N 0.001% or less, sol. Al 0.0020 ~
Steel containing 0.015%, oxygen more than 0.004% and 0.020% or less and the balance Fe and unavoidable impurities is heated to a temperature of Ac ₃ point or more and 1200 ° C. or less for hot rolling. Start and the reduction ratio in the temperature range below the recrystallization temperature in hot rolling, or the total reduction of the reduction ratio in the temperature range below the recrystallization temperature in hot rolling and the reduction ratio in the subsequent cold rolling. Rolling to 70% or more and 95% or less, and then the recrystallization temperature or more, 75
It is characterized by continuous annealing at a temperature of less than 0 ° C. Here, the total elongation means the total elongation at a plate thickness of 0.5 to 2.0 mm. The cold-rolled steel sheet manufacturing method according to the second aspect of the present invention uses C 0.001% or less, Mn 0.1% or less, N
0.001% or less, sol. Al 0.0020-0.0
15% or less, oxygen more than 0.004% and 0.020% or less, further containing Ti 0.02% or less and Nb0.0.
Steel containing 1 or 2 of 1% or less, with the balance being Fe and unavoidable impurities, Ac ₃ points or more, 120
The hot rolling is started by heating to a temperature of 0 ° C. or lower, and the rolling reduction in the temperature range of the recrystallization temperature or lower in the hot rolling,
Alternatively, rolling is performed so that the total reduction rate of the reduction rate in the temperature range below the recrystallization temperature in hot rolling and the reduction rate in subsequent cold rolling becomes 70% or more and 95% or less, and then recrystallization is performed. It is characterized in that continuous annealing is performed at a temperature equal to or higher than the temperature and lower than 750 ° C. Incidentally, the above-mentioned Japanese Patent Laid-Open No. 59-1666.
The cold-rolled steel sheet proposed in Japanese Patent Publication No. 50 discloses only steels having C0.0020% or more, Mn0.10% or more, N0.0017% or more, and O0.0229% or more from the description of the examples. Therefore, the cold-rolled steel sheet of the present invention further reduces C, Mn, and N as compared with the cold-rolled steel sheet proposed in JP-A-59-166650, and has a higher oxygen content, but a smaller amount than the above-mentioned proposal. It can be said that First, the experiment which is the basis of the invention will be described. C 0.0006% and Mn 0.
04%, Si 0.008%, P 0.001%, S0.0
03%, sol. Al 0.003%, N 0.0007%
Was used as a basic composition, and a slab of steel in which the amount of oxygen was changed from 0.001 to 0.030% was heated at 1050 ° C. for 30 minutes to perform hot rolling. The finishing temperature in hot rolling is 850.
℃, winding temperature is 500 ℃, and hot rolling reduction is 70%
And Furthermore, after pickling, cold rolling was performed and a reduction of 86% was applied to make the plate thickness 0.9 mm. Then, continuous annealing was carried out by heating and holding at 650 ° C. for 20 seconds, and cooled by air cooling. When a tensile test was performed on the obtained cold rolled steel sheet of each oxygen content, the results shown in FIG. 1 were obtained. For comparison, C
0.0026%, Mn 0.25%, Si 0.011%,
P0.009%, S0.007%, sol. Al0.0
Similarly, the comparative steel containing 026% and N0.0021% as the basic components was also subjected to the same hot rolling-cold rolling-continuous annealing with various changes in the oxygen content. The results of the tensile test are also shown in FIG. On the other hand, as in the above, C0.0006%, Mn
0.04%, Si 0.008%, P 0.001%, S
0.003%, sol. Al 0.003%, N 0.00
Steel having 7% as a basic component and an oxygen content of 0.009% was heated at 1000 ° C. for 30 minutes and hot-rolled. Finishing temperature in hot rolling is 600 ℃, coiling temperature is 490 ℃
And the hot rolling reduction rate was 70%. After pickling, cold rolling was performed and a reduction of 86% was applied to a plate thickness of 0.9 mm. Then, continuous annealing was carried out by heating and holding at various temperatures of 540 to 930 ° C. for 20 seconds and air cooling. When a tensile test was performed on the obtained cold rolled steel sheet at each annealing temperature, the results shown in FIG. 2 were obtained. For comparison, C0.0023%, M
n0.21%, P0.013%, S0.008%, so
l. Al0.036%, N0.0020%, O0.00
The results of the same treatment for 51% steel are also shown in FIG. As is clear from FIG. 1, C is 0.0006.
%, N in the steel of the present invention having an extremely low C and an extremely low N of 0.007%, the ductility value changes depending on the oxygen content in the steel, and in particular, the oxygen content in the steel exceeds 0.004%, It has been found that the elongation is 60% or more in the range of 0.1% or less, which is extremely high ductility. This fact is different from the conventional wisdom that ductility is improved by simply lowering C. On the other hand, FIG.
Therefore, in the steel of the present invention in which the carbon content is extremely low, the N content is extremely low, and the oxygen content is appropriate, the ductility becomes good as the annealing temperature in the continuous annealing is lowered from 900 ° C. to 650 to 70.
Maximum value near 0 ℃, total elongation 60 even at 600 ℃
It was found that a high ductility of not less than% could be obtained. As described above, the reason why a steel having an extremely low carbon content, an extremely low N content and an appropriate amount of oxygen in the steel can obtain a high elongation of 60% or more in total elongation even when continuously annealed at a low temperature of about 600 ° C. Although it is not clear yet, it can be considered as follows. That is, it is well known that recovery and recrystallization tend to be delayed when the cleanliness of the steel of the present invention is increased. However, in the case of the steel of the present invention, in addition to adding a high processing strain mainly in cold rolling, coarse precipitates are deposited due to the high oxygen content in the steel, which results in recrystallization nuclei, at low temperatures. However, recrystallization is likely to occur, and since components other than oxygen, especially C and N, are made to be extremely small, grain growth is improved and crystal grains are coarsened, and since Al is also small, fine precipitation in the grains is reduced. It is considered that the dislocations became easy to move, and these acted synergistically to remarkably improve the ductility. As described above, the extremely low carbon content and the extremely low N content, the appropriate oxygen content, and the low Al content act synergistically, and further, the rolling conditions (high working strain due to high temperature and low temperature). It has also been found that a high ductility of 60% or more in total elongation can be obtained even by continuous annealing at low temperature, and the present invention has been completed. Next, the reasons for limiting the steel components in the present invention will be explained. C: The inclusion of C is disadvantageous for obtaining a material having good ductility, drawability and aging resistance. In the present invention, it is necessary to make C 0.001% or less in order to lower the recrystallization temperature by the continuous annealing method and promote grain growth to sufficiently improve ductility. Therefore, the upper limit of C is set to 0.001%. Mn: Mn is effective for fixing S which causes red hot embrittlement, but a large amount of Mn reduces ductility. In particular, in order to obtain high ductility with an elongation of 60% or more, Mn needs to be 0.1% or less, and therefore Mn is
Was set to 0.1%. N: N, like C, if contained in a large amount, makes the crystal grains finer and impairs ductility, so it must be reduced as much as possible. Therefore, in the present invention, it is set to 0.001% or less. Sol. Al: Al is effective for fixing N, which is harmful for obtaining soft steel, as AlN, but if it is present in a large amount, it raises the recrystallization temperature and requires high-temperature annealing, which impairs the object of the present invention. . Therefore, sol. Al
Was set to 0.015%. Further, in order to properly control the amount of oxygen in steel, sol. 0.00 with Al
It is preferable that the content of sol. A
The lower limit of 1 was 0.0020%. O (oxygen): The amount of oxygen in steel is 0.004%
In the following, it becomes difficult to generate a relatively large oxide that should be the nucleus of recrystallization, and therefore the effect of promoting recrystallization cannot be obtained sufficiently and it becomes difficult to improve ductility by low temperature annealing. The lower limit of 0.004% was exceeded. However, in order to sufficiently obtain the effect of containing oxygen, the amount of oxygen is preferably 0.005% or more. On the other hand, if the amount of oxygen exceeds 0.020%, the oxide becomes too coarse and the ductility deteriorates, so the upper limit was made 0.020%. Here, the amount of oxygen in the steel is 0.004%.
If it is less than the following, the cleanliness will be improved, but on the other hand, the size of each inclusion will be very small and the grain growth will be hindered. On the other hand, when the amount of oxygen increases to some extent, the total volume of inclusions increases, but the size of each inclusion increases, and the number of inclusions decreases accordingly. Therefore, the larger the amount of oxygen is, the better the grain growth.
On the other hand, the factor that governs elongation, that is, the initiation of fracture, is always an inclusion. In this case, although the size of the inclusions is important, the number of inclusions, that is, the interval between inclusions becomes a more important factor, and the larger the interval between inclusions, the more difficult it is to break. In the range of the present invention in which the oxygen content in the steel is more than 0.004% and 0.020% or less, in addition to the above-described advantage at the time of manufacturing that recrystallization is easy, the effect of improving the characteristics of the steel structure itself is added. It is considered that very good results were obtained. In addition to the above components, other S, P, S
Inevitable impurities such as i and Fe may be used, but other impurities (S, P, Si) are also desirably reduced as much as possible in the present invention. Usually, S 0.005% or less, P
It is preferably 0.005% or less and Si 0.005% or less. Depending on the case, one or two of Nb 0.01% or less and Ti 0.02% or less may be contained. Nb and Ti form carbonitrides, have an effect of non-aging and form an texture that is advantageous for drawability, but if they are contained excessively, the recrystallization temperature rises and Since it defeats the purpose of the low temperature annealing of the invention,
It is desirable to reduce the addition amount as much as possible. Therefore, the upper limit when adding Nb and Ti is 0.0 in Ti.
2% and 0.01% for Nb. Next, the process conditions in the present invention will be described. First, the heating temperature for hot rolling, that is, the slab heating temperature must be Ac ₃ points or higher. This is because it is necessary to remelt the precipitates that have been precipitated during the cooling after casting the molten steel to homogenize the structure before heat treatment. In order to dissolve fine precipitates, the minimum temperature must be Ac ₃ or higher. On the other hand, if the slab heating temperature is too high, it melts even the coarse precipitates that precipitate during cooling after casting the molten steel, and makes fine precipitates during cooling to make the crystal grains finer and reduce the total elongation. Since it deteriorates, it is necessary to set the temperature to 1200 ° C. or lower. Next, in hot rolling and cold rolling,
In the temperature range below the recrystallization temperature of steel, the rolling reduction is 70% or more, 95
% Or less rolling is required. That is, as described above, the high-purity steel is subjected to high work strain to lower the recrystallization temperature during annealing, and finally, even in continuous annealing at a temperature lower than 750 ° C., a high ductility with an elongation of 60% or more is obtained. In order to achieve this, it is necessary to give a rolling reduction of 70% or more in the temperature range below the recrystallization temperature. Here, the reduction ratio of 70% or more at the recrystallization temperature or less means that the reduction ratio at the recrystallization temperature or less in hot rolling is 70% or less.
% Or more, or 70% as the total reduction ratio of the reduction ratio below the recrystallization temperature in hot rolling and the reduction ratio in cold rolling.
It may be more. In the latter case, it is sufficient if the reduction ratio in cold rolling is 70% or more. The higher the total rolling reduction, the better. However, 95%
In order to achieve a higher reduction, the upper limit of the total reduction is 95% because the load on the equipment, especially the rolling mill, increases.
And Needless to say, the annealing temperature (soaking holding temperature) in the continuous annealing after rolling needs to be higher than the recrystallization temperature in order to finally obtain a soft and highly ductile steel sheet.
The upper limit of the annealing temperature is less than 750 ° C. In the case of the steel of the present invention, the recrystallization temperature can be sufficiently lowered even by continuous annealing as described above, and therefore, sufficiently high ductility can be obtained at a low temperature of less than 750 ° C. 750 ° C
When annealed at a high temperature as described above, the steel with extremely low carbon content and extremely low N content as in the present invention is significantly softened, so that the steel sheet may be broken by the tension for continuous annealing,
In addition, the upper limit of the continuous annealing temperature is set to less than 750 ° C. in order to shorten the life of the furnace roll and the furnace body. In the case of the steel of the present invention, the recrystallization temperature is about 550 to 650 ° C, although it varies depending on the composition, so in practice 550 to 550 ° C.
Continuous annealing may be performed in a temperature range of about 720 ° C. EXAMPLES Steels A to H having the composition shown in Table 1 (provided that A to E were used)
Is a steel within the composition range of the present invention, and F to H are steels outside the composition of the present invention)
After heating the slab consisting of
Rough rolling was performed for 3 passes to obtain a sheet bar having a plate thickness of 30 mm. Then, 510-890 with a 6-stand finishing mill.
The hot rolling was completed at the finishing temperature in the temperature range of ° C, and the plate thickness was 5.0 mm. After pickling, cold rolling was performed at a reduction rate of 55% or more, and then continuous annealing was performed. The continuous annealing conditions are: heating rate 20 ° C./sec, soaking and holding temperature 550 to 750.
℃, soaking time 10-40sec, cooling rate 20 ℃ / se
c. After continuous annealing, temper rolling with a reduction rate of 1.0% was performed to obtain various mechanical properties, that is, yield strength (YS), tensile strength (TS), total elongation (El), yield point elongation (YE).
1) and the rank field value (mean r value) were examined.
Table 2 shows the detailed processing conditions for each steel, and Table 3 shows the results of investigation of mechanical properties. In Table 2, condition No. 7 is steel B within the composition range of the present invention, and the cold rolling reduction is less than 70%, but the reduction and cold rolling below the recrystallization temperature in hot rolling are used. The rolling reduction and the total rolling reduction are 75%, which exceeds 70%. In Condition No. 11, the composition of the steel is within the range of the present invention (steel B), but the reduction rate at the recrystallization temperature or lower is 62%, which is less than 70%. [Table 1] [Table 2] [Table 3] As is apparent from Table 3, the cold-rolled steel sheet obtained by using the steel within the composition range of the present invention as a raw material according to the production conditions of the present invention has a chemical composition outside the scope of the present invention, or manufactured. Conditions In particular, rolling reduction conditions have markedly higher ductility than cold-rolled steel sheets out of the range of the present invention, and all elongation is 60% despite continuous annealing at a low temperature of less than 750 ° C. The above high ductility is exhibited. Further, it is understood that the characteristics such as the average r value other than the ductility are equal to or higher than those of the steel sheets of the comparative examples. As is clear from the above examples, the cold-rolled steel sheet obtained by the method of the present invention exhibits remarkably high ductility of 60% or more in total elongation even when continuously annealed at a low temperature. ,
Further, it has excellent drawability and the like, and is therefore most suitable as a cold-rolled steel sheet for various forming processes in which particularly severe processing is performed. Further, in the method of the present invention, since it becomes possible to continuously anneal at a low temperature as described above, there is little risk of breakage of the steel sheet in the continuous annealing, and therefore, it becomes possible to perform high-efficiency production by the low-temperature high-speed rolling, and the roll in the furnace It is possible to obtain various effects such as extending the life of the furnace, extending the life of the furnace body, and reducing the energy consumption rate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a correlation diagram showing the relationship between the oxygen content in steel and the total elongation El. FIG. 2 is a correlation diagram showing the relationship between the annealing temperature and the total elongation El in continuous annealing.

Continued front page    (72) Inventor Kozo Kadoyama             1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.             Corporate Technology Research Division

Claims (1)

(Claims) (1) C 0.001% or less by weight%, Mn 0.1
% Or less, N 0.001% or less, sol. Al 0.002
0 to 0.015%, oxygen more than 0.004% and 0.020
% Or less, the balance of which is Fe and unavoidable impurities, is heated to a temperature of Ac ₃ point or more and 1200 ° C. or less to start hot rolling, and a temperature not higher than the recrystallization temperature in the hot rolling. Reduction ratio in the temperature range or the total reduction ratio of the reduction ratio in the temperature range below the recrystallization temperature in hot rolling and the reduction ratio in the subsequent cold rolling is 70% or more and 95%.
Roll to the following, then, above the recrystallization temperature,
Characterized by continuous annealing at a temperature of less than 750 ° C.,
A method for producing a high ductility cold rolled steel sheet having a total elongation of 60% or more. (2) C 0.001% or less by weight% and Mn 0.1
% Or less, N 0.001% or less, sol. Al 0.002
0 to 0.015%, oxygen more than 0.004% and 0.020
% Or less, Ti 0.02% or less and Nb
A steel containing 1 or 2 of 0.01% or less, the balance being Fe and inevitable impurities, has an Ac of ₃ or more,
The hot rolling is started by heating to a temperature of 1200 ° C. or lower, and the rolling reduction in the temperature range lower than the recrystallization temperature in the hot rolling or the rolling reduction in the temperature range lower than the recrystallization temperature in the hot rolling. And the rolling reduction in the subsequent cold rolling so that the total rolling reduction is 70% or more and 95% or less, followed by continuous annealing at a temperature of not less than the recrystallization temperature and less than 750 ° C. A method for producing a high-ductility cold-rolled steel sheet having a total elongation of 60% or more.