JPS6254058A - Cold-rolled steel sheet with high ductility and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet having high ductility and excellent in drawability, etc., by subjecting a steel having a specific composition reduced in the amounts of C, Al, N, Mn, etc., and increased in O quantity to hot and cold rollings under controlled conditions and then to continuous annealing at low temp. CONSTITUTION:The steel consisting of <=0.001% C, <=0.1% Mn, <=0.001% N, <=0.015% solAl, 0.003-0.020% O and the balance Fe with inevitable impurities and containing, if necessary, <=0.02% Ti and/or <=0.01% Nb is used as a stock. This steel is heated to the Ac3 point or above to undergo hot rolling. At this time, rollings are carried out so that the draft at the above hot rolling in the temp. range not exceeding the recrystallization temp. or the total draft of the above draft and the one at subsequent cold rolling is >=70%. Thereafter continuous annealing is carried out at a temp. between the recrystallization temp. and <750 deg.C. In this way, the cold-rolled steel sheet for forming working having an overall elongation of >=60% can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to cold-rolled steel sheets used for various forming processes, etc., and particularly to cold-rolled steel sheets that have high ductility of 60% or more in terms of total elongation and can be annealed at low temperatures. The present invention relates to a rolled steel plate and a method for manufacturing the same.

Conventional technology Conventionally, as a method for manufacturing cold rolled steel sheets with excellent workability such as ductility and drawability, low carbon Al quilt steel with a carbon content of about 0.03 to 0.06% is used as a raw material, and after rolling, A common method was to perform box annealing. However, the box annealing method not only requires a long time for processing and is extremely low in productivity, but also because the coil is heat treated in the coil state, the heating temperature rise rate and cooling rate vary in the radial direction of the coil. There was a fundamental problem in that it was not possible to obtain uniform characteristics throughout.

Therefore, recently, continuous annealing is increasingly being applied to overcome the drawbacks of box annealing.

However, when continuous annealing is applied, rapid heating is involved, so the recrystallization temperature of the steel sheet is 50 to 100°C higher than in case of box annealing, resulting in poor crystal growth. Due to rapid cooling, the precipitation of carbon contained in the steel does not progress sufficiently, and as a result, the steel plate produced is hard and has good ductility, drawability, and aging resistance, which cannot be obtained by box annealing. There is a problem that it is inferior to steel plate. Regarding N, in the continuous annealing method, since the heating time is short, it is difficult for N dissolved in the steel to precipitate as AIN, and even after annealing, a large amount of N dissolved in the steel remains in the steel. This is also one of the reasons why the workability of steel sheets obtained by continuous annealing is low.

One of the methods for solving the drawbacks of the continuous annealing method is the method proposed in Japanese Patent Publication No. 1341/1983. This proposed method promotes grain growth in an orientation that is advantageous for improving ductility and drawability, or the precipitation of C and N contained in the steel, by coiling at a high temperature during hot rolling. Furthermore, after rapid cooling in the continuous annealing process, overaging treatment is performed at 300 to 500°C for several seconds to several minutes to promote the precipitation of unprecipitated solid solution C, thereby improving aging resistance. This is something that aims to achieve this goal. However, winding at a high temperature during the hot rolling process as in this proposed method has the problem of reducing pickling properties, and furthermore, the cold rolled steel sheet produced by this proposed method is also similar to that obtained by box annealing. The reality is that the drawability and ductility are still inferior to cold-rolled steel sheets, and the above drawbacks have not been fundamentally solved.

On the other hand, since the main cause of deterioration of the material quality of continuously annealed materials is C dissolved in steel, the C content was reduced to 0.0.
Various methods have also been proposed for improving workability by using ultra-low C steel with a reduced carbon content of 0.05% or less. It is obvious that drastically reducing the C content in this way is advantageous in terms of ductility, drawability, and aging resistance, but simply reducing the C content will not completely prevent aging. It was difficult to reduce the weight of 1q of steel plates. Therefore, in addition to reducing Cff1, it is necessary to add carbonitride-forming elements such as Ti and Nb to improve aging resistance and ductility. However, the addition of such special elements not only increases the raw material cost but also significantly increases the recrystallization temperature, so continuous annealing must be performed at a high temperature. Similarly to the method of No.-141335, it cannot be said to be a preferable method. In addition to that)
Adding special elements as described in (e) also poses the problem of deteriorating the surface properties and chemical conversion treatment properties of the product.

For example, Japanese Patent Application Laid-Open No. 1983-1999 discloses a method of obtaining a cold-rolled steel sheet with excellent material quality even by continuous annealing using ultra-low carbon steel.
There is a method proposed in J41335. This proposed method has C50,0030%, N≦0.0020%,
MnS2.20%, 0.020≦sol.

Made of steel containing A1≦0.070%, finishing temperature 85
Hot rolled at a coiling temperature of 500 to 700°C above 0°C, cold rolled at a cold rolling rate of 80% or above, and continuously annealed at a temperature of 750°C or above. It is said that cold-rolled steel sheets with excellent properties can be produced. However, in this proposed method, the F value (Lankford value) cannot reach 1.8 or higher unless the continuous annealing temperature is 750°C or higher.
It is said that deep drawability is poor at temperatures below 750°C.

Furthermore, with this proposed method, the maximum ductility of 1Δ is only 53.3%.

Continuous annealing at high temperatures of 750°C or higher as shown in the above proposal not only risks rupture of the steel plate due to excessive tension on the steel plate softened at high temperatures, but also shortens the life of the furnace and the rolls in the furnace. There are problems such as increased thermal energy costs, and especially in the case of ultra-low carbon steel such as the one proposed above, since it is extremely soft, there is a high risk of steel plate breakage, so continuous annealing at such high temperatures should be avoided. is desirable.

On the other hand, as an ultra-low carbon cold-rolled steel sheet that can be annealed at a low temperature and has excellent workability, there is one proposed in JP-A-59-166650. In other words, this proposal is C50,0
050%, MnS2.5%, P≦0.1%, N≦0.0
050%, steel containing Q 0.0016-0.035%,
and to these further B 0.0001 to 0,0050
%, Nb 0.003-0.080%, Zr0.00
The object of the present invention is to provide a cold-rolled steel sheet made of steel containing 5 to 0.1% of one or more types. However, in the case of this proposal, the annealing temperature of continuous annealing is 60% in the specification.
Although it is described as 0 to 770°C, in the examples, the continuous annealing temperature is 750°C, and it is unclear whether a continuous annealing temperature lower than 750°C is actually applicable and necessary. Furthermore, the material properties of the continuously annealed material obtained by this proposed method are: yield point strength ≧17kOf/mm'', tensile strength ≧29.9k (lf/mm2, elongation 49%, F value≦1.
73, it is not possible to obtain superior material in the long term.

Problems to be Solved by the Invention As mentioned above, in the past, the continuous annealing method was applied, and it was possible to perform continuous annealing at a low temperature without fear of breakage of the steel plate, and to achieve workability even at such low temperatures. In particular, it has been difficult to produce cold-rolled steel sheets with excellent ductility.

This invention was made against the background of the above circumstances, and improves workability, especially ductility, to a total elongation of 60% or more even during continuous annealing at low temperatures, specifically, continuous annealing at temperatures below 750°C. The purpose of the present invention is to provide rough cold-rolled steel sheets.

Means for Solving the Problems In order to achieve the above-mentioned objectives, the inventors of the present invention have carried out extensive experimental studies, and have found that the amount of components such as C, A I , N, Mn, etc. is further reduced than before, and at the same time. It was discovered that a cold-rolled steel sheet with high ductility can be produced by continuous annealing at low temperatures by increasing the amount of rope and by setting specific hot-cold rolling conditions. That's what happened.

Specifically, the cold-rolled steel sheet of the first invention has a G
0.001% or less, Mno, 1% or less, No, ooi
% or less, SOI, A 1 0.015% or less, oxygen 0.
003 to 0.020%, the remainder being Fe and unavoidable impurities, and a total elongation of 60% or more.

Further, the cold rolled steel sheet of the second invention has a C0.001 weight%.
% or less, Mn 0.1% or less, N 0.001% or less, s
ol, A1 0.015% or less, @ elementary 0.003-0.
Contains 020% and further contains lio, 02% or less and N
Contains one or two of b0.01% or less, the remainder consists of Fe and unavoidable impurities, and the total elongation is 60%
The market is characterized by the above characteristics.

Furthermore, the method for producing a cold rolled steel sheet of the third invention has a G
0.001% or less, 1Vln 0.1% or less, N 0.
001% or less, sol, A I 0.015% or less,
A steel containing 0.003 to 0.020% oxygen and the balance consisting of Fe and unavoidable impurities is used as a material, and this material is heated to a temperature of Ac 3 or higher to start hot rolling, and the hot rolling The total reduction ratio of the reduction ratio in the temperature range below the recrystallization temperature in rolling, or the reduction ratio in the temperature range below the recrystallization temperature during hot rolling and the reduction ratio in subsequent cold rolling is 70% or more. It is characterized in that it is rolled as described above and then continuously annealed at a temperature higher than the recrystallization temperature and lower than 750''C.

The method for producing a cold-rolled steel sheet according to the fourth invention is characterized in that C
0.001% or less, Mrl 0.1% or less, N 0.00
1% or less, sol, A I 0.015% or less, oxygen 0.003 to 0.020%, and T i 0
．． The material is steel containing one or two of 0.02% or less and 0.01% or less Nb, with the remainder consisting of Fe and unavoidable impurities, and this material is heated to a temperature of Ac 3 or higher and hot rolled. The rolling reduction in the temperature range below the recrystallization temperature during hot rolling, or the total reduction of the rolling reduction in the temperature range below the recrystallization temperature during hot rolling and the subsequent cold rolling. Rolled so that the ratio is 70% or more, and then rolled at 750°C at a temperature higher than the recrystallization temperature.
It is characterized by continuous annealing at a temperature below.

Note that the cold-rolled steel sheet proposed in JP-A-59-166650 mentioned above is C.
0.0020% or more, Mno, 10% or more, N 0.0
0.017% or more, Q 0.0229% or more, and therefore, the cold rolled steel sheet of this invention has more C, Mn,
Although the amount of N is further reduced and the amount of oxygen is increased, it can be said that the amount is smaller than the above proposal.

Function First, the experiments that formed the basis of this invention will be explained.

Weight%: co, oooe%, MnQ, Q4%, S
i0.008%, P 0.001%, 30.003%,
A steel slab with a basic composition of SOl, AIO, 003%, N 0.0007% and varying oxygen content from 0.001 to 0.030% was heated at 1050°C for 30 minutes and hot rolled. was applied.

The finishing temperature during hot rolling is 850℃, and the coiling temperature is 50℃.
The temperature was 0° C., and the hot rolling reduction was 70%.

Furthermore, after pickling, cold rolling was applied to apply a reduction of 86%, and the plate thickness was made to be 0.9111 m. Next, continuous annealing was performed by heating and holding at 650° C. for 20 seconds, followed by air cooling.

A tensile test was conducted on the obtained cold-rolled steel sheets with each oxygen content, and the results shown in FIG. 1 were obtained. For comparison, C0.0026%, Mn0.25%, 3io, oii
%, P 0.009%, S 0.007%, SOI, A
For comparative steels whose basic components are Io, 0.0026% and N 0.0021%, the oxygen content was similarly varied,
Hot rolling-cold rolling and continuous annealing were performed in the same manner as described above.

The tensile test results are also shown in FIG.

On the other hand, as above, G 0.0006%, Mn 0.04%
, 3i0.008%, F) o, oo1%, S 0.0
03%, SOI, A I 0.003%, N 0.0
Steel with 0.07% as a basic component and 0.009% of solen content, and 0.0040% and 0.012% of oxygen ω.
For Ti-added steel, those slabs were
Hot rolling was performed by heating at ℃ for 30 minutes. The finishing temperature in hot rolling was 600°C, the coiling temperature was 490°C, and the hot rolling reduction was 70%. After pickling and cold rolling, 86
% reduction was applied to make the plate thickness 0.9 mm. then 540
Continuous annealing was performed by heating and holding at various temperatures of ~930''C for 20 seconds and cooling in air.Tensile tests were conducted on cold rolled steel sheets at each annealing temperature, and the results shown in Figure 2 were obtained. .For comparison, C0.0023%, M
n0.21%, P 0.013%, S 0.008%,
sol, AI 0.036%, N 0.0020%
, Q 0.0051, the results of the same treatment are also shown in FIG.

As is clear from Figure 1, C is 0.0006% and N is 0.
．． In the steel of the present invention, which has extremely low C and N of 0.007%, the ductility value changes depending on the amount of oxygen in the steel, especially when the amount of oxygen in the steel is 0.007%.
It has been found that within the range of 0.003% to 0.020%, the elongation becomes 60% or more, resulting in remarkable high ductility. This fact is different from the conventional wisdom that simply lowering C improves ductility. -5 From Fig. 2, 4aj In the steel of the present invention with low C, extremely low N, and appropriate oxygen content, as the annealing temperature in continuous annealing is lowered from 900°C, the ductility becomes good, reaching 650-700°C. It was found that the maximum value was reached near 600°C, and high ductility with a total elongation of 60% or more could be obtained even at about 600°C.

As mentioned above, the reason why high ductility with a total elongation of 60% or more can be obtained even with continuous annealing at a low temperature of about 600 °C in steel with extremely low C, extremely low N, and appropriate oxygen content is still unclear. No, but it can be thought of as follows.

That is, it is well known that recovery and recrystallization generally tend to be delayed when the cleanliness is increased as in the steel of the present invention. However, in the case of the steel of the present invention, in addition to applying high working strain mainly during cold rolling, coarse precipitates precipitate and become recrystallization nuclei due to the high oxygen content in the steel. However, recrystallization is more likely to occur, and components other than oxygen,
First of all, since the amount of C1N is kept to a minimum, grain growth is improved and the crystal grains become coarse, and since there is also less A1, there are fewer fine precipitates within the grains, making it easier for dislocations to move, and these factors act synergistically. This is thought to be due to a marked improvement in ductility.

As mentioned above, extremely low C and N, appropriate oxygen content, and low AI work synergistically, and rolling conditions (high processing strain due to low temperature and high pressure) also work synergistically. They discovered that high ductility with a total elongation of 60% or more can be obtained even with continuous annealing at low temperatures, leading to the creation of this invention.

Next, the reasons for limiting the steel components in this invention will be explained.

C: Containing C is disadvantageous in obtaining a material with good ductility, drawability, and aging resistance. In the present invention, in order to lower the recrystallization temperature in the continuous annealing method and promote grain growth to sufficiently improve ductility, it is necessary to reduce the C@o,ooi% or less. Therefore, the upper limit of C was set to 0.001%.

Mn: Although Mn is effective for fixing S, which causes red-hot brittleness, a large amount of Mn decreases ductility. In particular, in order to obtain high ductility with an elongation rate of 6°0% or more, it is necessary to reduce Mn to 0.1% or less, and the upper limit of tMn is 0.1%.
And so.

N: If a large amount of N is contained in the same sphere as C, it will make the crystal grains finer.
Since it causes loss of ductility, it is necessary to reduce it as much as possible. Therefore, in this invention, the content is set to 0.001% or less.

SOl, Al: AI is effective in fixing N, which is harmful to soft steel, as AIN, but if it exists in a large amount, it increases the recrystallization temperature and requires high-temperature annealing, which is the purpose of this invention. damage. Therefore sol. The upper limit of Al was set to 0.015%.

0 (oxygen): If the amount of oxygen in the steel is less than 0.003%, it becomes difficult to generate relatively large oxides that should serve as nuclei for recrystallization, and therefore the effect of promoting recrystallization cannot be obtained sufficiently. Since it is difficult to improve ductility with low-temperature annealing, the lower limit of the oxygen content was set at 0.003%. However, in order to fully obtain the effect of oxygen inclusion, it is desirable that the oxygen content be 0.005% or more. On the other hand, if oxygen ω exceeds 0.020%, the oxide becomes too coarse and the ductility deteriorates, so the upper limit was set at 0.020%.

Here, when the amount of oxygen in the steel is less than 0.003%, the cleanliness improves, but on the other hand, the size of each inclusion becomes very small, which inhibits grain growth. On the other hand, if there is a certain amount of oxygen, the total volume of inclusions increases, but the size of each inclusion increases, and the number of inclusions decreases. On the other hand, the factor that controls elongation, that is, the initiation of fracture, is always inclusions.In this case, although the size of inclusions is important, the number of inclusions, that is, the spacing between inclusions, is a more important factor. ,
The larger the interval between inclusions, the more difficult it is to break. In the range of this invention, where the amount of oxygen in the steel is 0.003 to 0.020%, in addition to the above-mentioned advantage of easy recrystallization during manufacturing, the effect of improving the properties of the steel structure itself is very good. It is thought that this result was obtained.

In addition to the above components, other unavoidable impurities such as s, p, sr, etc. and Fe may be used, but other impurities (S, P, Sl) should also be reduced as much as possible in this invention. is desirable, usually S 0.005% or less, p
It is preferable that the Si content be 0.005% or less, and the Si content be 0.005% or less.

In some cases, Nb0.01% or less and Ti
One or two of these may be contained in an amount of 0.02% or less. NbSTi forms carbonitrides, which has the effect of deaging and forming a grain structure that is advantageous for drawability, but if they are contained in excess, the recrystallization temperature increases. Therefore, it is desirable to reduce the number of addition ports as much as possible, since this would defeat the purpose of low-temperature annealing of the present invention.
2% and ＼b-C・0.01%.

Next, process conditions in the manufacturing method of the present invention will be explained.

First, the heating temperature for hot rolling, that is, the slab heating temperature, needs to be at least Ac 3 point. This is because it is necessary to remelt the precipitates that have precipitated during cooling after the molten steel is cast, and to homogenize the structure before heat treatment.

Next, in hot rolling and cold rolling, it is necessary to perform rolling at a reduction rate of 70% or more in a temperature range below the recrystallization temperature of the steel. That is, as mentioned above, high working strain is applied to highly purified steel to lower the recrystallization temperature during annealing, and finally high ductility with an elongation rate of 60% or more is obtained even when continuously annealing at a temperature lower than 750 ° C. 7 in the temperature range below the recrystallization temperature.
It is necessary to give a rolling reduction rate of 0% or more. Note that here, a rolling zone with a reduction of 70% or more below the recrystallization temperature means a reduction zone below the recrystallization temperature in hot rolling, even if the reduction rate below the recrystallization temperature in hot rolling is 70% or more. The total rolling reduction ratio of the rolling reduction ratio and the rolling reduction ratio in cold rolling may be 7Q% or more. In the latter case, it is sufficient if the rolling reduction in cold rolling is 70% or more.

It goes without saying that the annealing temperature (soaking temperature) in continuous annealing after rolling must be higher than the recrystallization temperature in order to ultimately obtain a soft and highly ductile steel sheet, but the upper limit of the annealing temperature shall be less than 750°C. In the case of the steel of this invention,
As mentioned above, even continuous annealing can lower the recrystallization temperature sufficiently, and therefore, sufficiently high ductility can be obtained at a low temperature of less than 750°C. When annealing is performed at a high temperature of 750°C or higher, steel with extremely low C and extremely low N as in the present invention becomes extremely soft, so there is a risk that the steel plate will break due to the tension due to continuous annealing. In addition, in order to shorten the life of the furnace rolls and the furnace body, the upper limit of the continuous annealing temperature was set to less than 750°C. In the case of the steel of this invention, the recrystallization temperature is 55%, although it varies depending on the composition.
It can be heated at about 0 to 650°C, so in reality it is 550 to 72
Continuous annealing may be performed in a temperature range of 0° C. culm.

Example A slab made of steel A-H having the composition shown in Table 1 ((auA-E is steel within the composition range of the present invention, F-H is steel outside the composition range of the present invention) was heated to 940 to 1000°C. After that, the sheet bar was rough-rolled in 3 baths to obtain a sheet bar with a thickness of 30 mm.Subsequently, hot rolling was completed at a finishing temperature in the temperature range of 510 to 890°C in a 6-stand finish rolling machine, and the sheet bar was Thickness: 5.0m
It was set as m. After pickling, cold rolling is performed with a rolling ratio of 81% or more,
Next, continuous annealing was performed. Continuous annealing conditions are heating rate of 20℃/sec, soaking temperature of 550 to 750''G.
, soaking time 10~40SeC1 cooling rate 20℃/s
I'm at ec. After continuous annealing, skin pass rolling with a rolling reduction of 1.0% was performed to determine the mechanical properties, i.e., yield strength (YS), tensile strength (TS), total elongation (El), and yield point elongation (YE).
l>, and the Lankford value (“value”).

Detailed processing conditions for fine tone are shown in Table 2, and investigation results of mechanical characteristics are shown in Table 3.

In Table 2, condition number 7 is the cold rolling reduction rate (less than 70%) for steel B within the composition range of the present invention.
The total reduction ratio of the reduction ratio below the recrystallization temperature in hot rolling and the reduction ratio in cold rolling is 75%, which exceeds 70%. Further, in condition number 11, although the composition of the steel is within the range of the present invention (steel B), the reduction ratio below the recrystallization temperature is 62%, which is less than 70%.

As is clear from Table 3, the cold-rolled steel sheet obtained according to the production conditions of this invention using steel within the composition range of this invention is different from the cold-rolled steel sheet whose composition is outside the range of this invention. Compared to cold-rolled steel sheets whose rolled steel sheets and manufacturing conditions, especially rolling reduction conditions, are outside the scope of the present invention, they have significantly higher ductility than 750.
Despite being continuously annealed at a low temperature below ℃, it shows high ductility with a total elongation of 60% or more. Also, Y other than ductility (
It can be seen that the directivity properties are also equal to or better than those of the steel sheet of the comparative example.

Effects of the Invention As is clear from the examples above, the cold-rolled steel sheet of this invention exhibits extremely high ductility with a total elongation of 60% or more even during continuous annealing at low temperatures, and also has excellent drawability. Therefore, it is ideal as a cold-rolled steel sheet for various forming processes that undergo particularly severe processing. In addition, since the method of this invention enables continuous annealing at low temperatures as described above, there is less risk of steel plate breakage during continuous annealing, which enables high-efficiency production through low-temperature, high-speed threading. - Extending the lifespan of the furnace and furnace body, and reducing energy consumption, etc.
Various effects can be obtained.

[Brief explanation of drawings]

Figure 1 is a correlation diagram showing the relationship between the amount of oxygen in the steel and total elongation El, and Figure 2 is a correlation diagram showing the relationship between the annealing temperature and total elongation El in continuous annealing.

Claims (4)

[Claims] (1) C0.001% or less, Mn 0.1% by weight
Below, N0.001% or less, sol. Al0.015%
The following describes a highly ductile cold-rolled steel sheet containing 0.003 to 0.020% of oxygen, the remainder consisting of Fe and unavoidable impurities, and having a total elongation of 60% or more. (2) C0.001% or less, Mn 0.1% by weight
Below, N0.001% or less, sol. Al0.015%
The following contains 0.003 to 0.020% of oxygen, and further 1 of 0.02% or less of Ti and 0.01% or less of Nb.
1. A highly ductile cold-rolled steel sheet, characterized in that it contains at least one type of steel, the remainder consisting of Fe and unavoidable impurities, and has a total elongation of 60% or more. (3) C0.001% or less, Mn 0.1% by weight
Below, N0.001% or less, sol. Al0.015%
Hereinafter, a steel containing 0.003 to 0.020% oxygen and the balance consisting of Fe and unavoidable impurities is used as a raw material, and this material is heated to a temperature of Ac_3 or higher to start hot rolling. The rolling reduction ratio in the temperature range below the recrystallization temperature in hot rolling, or the total reduction ratio of the rolling reduction ratio in the temperature range below the recrystallization temperature in hot rolling and the rolling reduction ratio in subsequent cold rolling, is 70% or more A method for producing a highly ductile cold-rolled steel sheet having a total elongation of 60% or more, comprising rolling the steel sheet so that the steel sheet has a total elongation of 60% or more, and then continuously annealing at a temperature higher than the recrystallization temperature and lower than 750°C. (4) C0.001% or less, Mn 0.1% by weight
Below, N0.001% or less, sol. Al0.015%
The following contains 0.003 to 0.020% of oxygen, and further 1 of 0.02% or less of Ti and 0.01% or less of Nb.
A steel containing one or two species and the remainder consisting of Fe and unavoidable impurities is used as a material, and this material is heated to a temperature of Ac_3 or higher to start hot rolling, and the recrystallization temperature in the hot rolling is The total reduction ratio of the reduction ratio in the following temperature range or the reduction ratio in the temperature range below the recrystallization temperature in hot rolling and the reduction ratio in subsequent cold rolling is 70%
After rolling to a temperature higher than the recrystallization temperature,
A method for producing a highly ductile cold-rolled steel sheet having a total elongation of 60% or more, the method comprising continuous annealing at a temperature of less than 750°C.