JPH0681043A - Production of steel sheet excellent in deep drawability as well as in fatigue characteristic - Google Patents

Abstract

PURPOSE:To produce a steel sheet having superior deep drawability and fatigue characteristics by subjecting a slab of a dead-soft steel containing specific trace amounts of Ti, Nb, etc., to hot rolling and to cold rolling and then subjecting the resulting steel sheet to annealing under specific temp. conditions and, if necessary, to galvannealing. CONSTITUTION:A slab of a steel having a composition containing, by weight, <0.0050% C, <0.2% Si, 0.10-0.5% Mn, <0.03% P, <0.015% S, <0.10% Sol.Al, <0.0040% N, and further 0.005-0.10% Ti and/or 0.005-0.030% Nb is heated to 1200-1300 deg.C, hot-rolled, coiled at 450-600 deg.C, pickled to undergo the removal of surface scale, and then cold-rolled at 60-90% draft. Finally the resulting steel sheet is annealed at a temp. in the range between the recrystallization temp. and a temp. specified by [6(10Si+5Mn+100P+14)+710]. Further, if necessary, hot-dip galvanizing is applied and the resulting galvanized coating is subjected to alloying treatment, by which an Fe-Zn alloy containing >50% Fe can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having both excellent deep drawability and fatigue characteristics.

[0002]

2. Description of the Related Art Conventionally, as a method for imparting high formability to a thin steel sheet, an IF (carbon and nitride forming element such as Ti or Nb is added to an ultra-low carbon steel to precipitate and fix C and N). In
terstitial free steel is known.
Techniques relating to cold-rolled steel sheets and hot-dip galvanized steel sheets having excellent formability based on such IF steels are disclosed in, for example, JP-A-2-34722 and JP-A-1-225727. In recent years, the shape of automobile bodies has become complicated, and the number of parts such as IF steel that can only be processed by steel plates with extremely excellent workability is increasing rapidly.
F steel is widely used as a material for steel plates for automobiles.

On the other hand, low carbon aluminum killed steel has been conventionally used for parts which require fatigue strength and undergo deep drawing, but the formability of this low carbon aluminum killed steel sheet is different from that of IF steel. It does not reach. From such a background, a steel sheet having excellent fatigue properties and deep drawability is required.

[0004]

In the conventional IF steel, C and N are sufficiently precipitated as a means for achieving a high r value, as in JP-A-1-225727 and JP-A-2-34722. To contain Ti and Nb for fixing,
And high temperature annealing is performed within a range not exceeding the Ac ₃ transformation point. Certainly, high r values can be obtained by these methods. However, these techniques are not intended to improve fatigue properties at all.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a steel sheet having excellent deep drawability and fatigue characteristics comparable to those of a low carbon aluminum killed steel sheet. And

[0006]

Means and Action for Solving the Problems
%, C: 0.0050% or less, Si: 0.2
% Or less, Mn: 0.10 to 0.5%, P: 0.03% or less, S: 0.015% or less, sol. Al: 0.10%
Below, N: 0.0040% or less is contained, and further 0.00
After heating a steel material containing 5 to 0.10% of Ti and 0.005 to 0.030% of Nb, one kind or two kinds, and the balance Fe and unavoidable impurities at a temperature of 1200 ° C or higher and 1300 ° C or lower. It is hot rolled, wound around a coil at a temperature of 450 ° C or higher and 600 ° C or lower, and the obtained hot rolled steel strip is cold-rolled at a reduction rate of 60% or more and 90% or less after pickling, and at a recrystallization temperature or more and , 6 (10Si + 0.5Mn + 100P + 1
4) Provided is a method for manufacturing a steel sheet having excellent fatigue properties and deep drawability, which is characterized by annealing at a temperature of +710 or less.

Secondly, in weight%, C: 0.0050% or less, Si: 0.2% or less, Mn: 0.10 to 0.5%,
P: 0.03% or less, S: 0.015% or less, sol.
Al: 0.10% or less, N: 0.0040% or less, and one or two of Ti and Nb are -1 ≦ (12T
i ^* ) / (48C) + (12Nb) / (93C) ≦ 2
(However, Ti ^* = Ti-48 / 14 / N-48 / 32
S) is contained so that the balance is Fe and the inevitable impurities are hot-rolled. After pickling, the steel is cold-rolled at a reduction rate of 60% or more and 90% or less, and recrystallization temperature or more. And -11 {(12Ti ^* ) / (48C) +
(12Nb) / (93C)} + 800 A method for manufacturing a cold-rolled steel sheet having excellent fatigue characteristics and deep drawability, which is characterized by annealing at a temperature not higher than a prescribed temperature.

As a result of research conducted to solve the above problems, the present inventors have found that the fatigue strength of a soft thin steel sheet increases as the yield strength of the material increases. That is, if the yield strength is increased after securing the r value of the steel sheet using the IF steel having excellent deep drawability, it is possible to produce a steel sheet having both excellent deep drawability and fatigue characteristics. You can do it.

In order to increase the yield strength without deteriorating the deep drawing formability, it is necessary to reduce the grain size after annealing. For that purpose, (1) high temperature heating and
Low-temperature winding is performed, and the upper limit of the annealing temperature may be defined based on the amounts of strengthening elements of Si, Mn, and P. (2) The upper limit of the annealing temperature is based on the amount of residual solid solution C,

-11 {(12Ti ^* ) / (48C) +
(12Nb) / (93C)} + 800 (however, Ti ^*
= Ti-48 / 14 / N-48 / 32S) has been newly found. The present invention having the above-described configuration has been completed based on the findings of the present inventors. Hereinafter, the present invention will be described in detail.

As described above, in the first aspect of the present invention, steel having a specific composition is heated to a high temperature, hot-rolled, wound at a low temperature, pickled and then cold-rolled to a specific temperature. , Si, Mn, P are annealed in a temperature range in which the upper limits thereof are defined based on the amounts of strengthening elements, and a steel sheet having excellent deep drawing formability and fatigue properties is manufactured.

In the second embodiment, steel having a specific composition is hot-rolled, pickled and then cold-rolled at a specific temperature.
Annealing is performed in a temperature range whose upper limit is defined based on the amount of residual solid solution C, and a steel sheet having excellent deep drawability and fatigue properties is manufactured.

In any of the embodiments, the cold-rolled steel sheet may be used as it is or may be hot dip galvanized. Further, hot dip galvanizing may be subjected to alloying treatment, and F may be applied on the alloyed plating film.
Fe-Zn alloy plating with an e content of 50% or more may be applied. The reasons for limiting the steel composition in the first aspect are as follows. C: C is preferably low in order to achieve a high r value,
The upper limit is set to 0.0050% by weight so that the effect of the present invention is not impaired in practical use.

Si: Si is a strengthening element that contributes to the increase in the yield strength and tensile strength of the steel sheet and is effective in improving the fatigue strength. Not only does it deteriorate the r-value, but it also significantly deteriorates the adhesion when hot-dip galvanizing is performed. Therefore, the upper limit is set to 0.2% by weight.

Mn: Mn, like Si, contributes to an increase in the strength of the steel sheet, but if it is less than 0.10% by weight, it has little effect, and if it is added in excess of 0.50% by weight, the steel sheet is Not only lowers the Ac ₃ transformation point, narrows the optimum annealing temperature range, but also deteriorates the r value of the steel sheet. Therefore, the Mn content is specified in the range of 0.10 to 0.5% by weight.

P: P can strengthen steel at the lowest cost, but if it exceeds 0.03% by weight, the alloying reaction of hot dip galvanization is extremely delayed, resulting in defects such as uneven alloying. Not only is it a cause, but it also deteriorates the r value of the steel sheet. Therefore, the upper limit is set to 0.03% by weight.

S: Since S deteriorates the ductility of the steel sheet,
It is desirable to reduce it as much as possible. However, it is not realistic to reduce the amount extremely, and the upper limit is 0.015% by weight, which is a range that does not impair the effects of the present invention in practice. sol. Al: Al is necessary for deoxidation, but if added in a too large amount, cost rises, so the upper limit is set to 0.10% by weight. N: N is preferably small in order to obtain a high r value, and its content is 0.0040% by weight so that a high r value can be obtained.
Defined below.

Ti, Nb: Ti, Nb is added to fix the solid solution C, N in steel as a precipitate and obtain a high r value. However, if the content of Ti is less than 0.005% by weight, the effect is not exerted, and if it exceeds 0.10% by weight, the effect is saturated and only the cost is increased. Further, if the content of Nb is less than 0.005% by weight, there is no effect, and if it exceeds 0.03% by weight, the ductility of the steel is remarkably reduced. Therefore, Ti and Nb are 0.0
It is specified in the range of 05 to 0.10% by weight and 0.005 to 0.03% by weight. And since these have the same action, it is sufficient that at least one of them is contained. Next, the reasons for limiting the manufacturing conditions of the first aspect will be described.

In the hot rolling process, first, the slab 1
Heating is performed at a temperature of 200 ° C. or higher and 1300 ° C. or lower. 120
By heating at a temperature of 0 ° C. or higher, coarse precipitates such as TiN in the slab stage can be redissolved during heating, and these precipitate finely at the end of hot rolling and grain growth during annealing. And contributes to an increase in yield strength. However, if the heating temperature exceeds 1300 ° C., the scale formed on the slab surface becomes thick and the scale releasability during hot rolling deteriorates. Therefore, the heating temperature during hot rolling is 120
It is specified at 0 to 1300 ° C.

After heating to this temperature, hot rolling is performed. The hot rolling at this time is preferably completed at a temperature not lower than the Ar ₃ transformation point according to a conventional method. This is because the r value after annealing deteriorates at a temperature below the Ar ₃ point. However, hot rolling may be performed in the ferrite region under the condition that hot lubrication is sufficiently performed.

The coiling temperature after hot rolling is 450 ° C. or higher and 600 ° C.
It is specified below. The reason why the temperature is specified to be 600 ° C. or lower is to suppress coarsening of precipitates such as TiC finely precipitated during hot rolling and to suppress grain growth. In order to increase the yield strength, the lower the coiling temperature is, the better. However, if the temperature is less than 450 ° C, the r value after annealing deteriorates.
Specify in ° C.

The hot-rolled steel strip thus obtained is pickled by a conventional method and then cold-rolled at a rolling reduction of 60% or more and 90% or less. At this time, if the rolling reduction is less than 60%, a high r value cannot be obtained, and even if rolling is performed at a rolling reduction of 90% or more, not only is there no effect on the increase of the r value, Since the load increases, the lower limit is set to 60% and the upper limit is set to 90%.

In the subsequent annealing step, the recrystallization temperature or higher and 6 (10Si + 0.5Mn + 100P + 14) +7
It is performed at a temperature not higher than 10 The present inventors have newly found that by limiting the annealing temperature to this range, it is possible to produce a thin steel sheet having excellent formability and fatigue characteristics. Such knowledge is obtained through the following experiments by the present inventors.

First, C: 0.0020%, Si: tr
0.3%, Mn: 0.1% to 0.8%, P: 0.003
~ 0.03%, S: 0.010%, sol. Al: 0.
055%, N: 0.0025%, Ti: 0.005-
0.10%, Nb: 0.005 to 0.035% of steel is melted, the obtained slab is heated at 1250 ° C., and then hot rolled to a plate thickness of 4.0 mm, 580 ° C. I wound it up in a coil. After pickling, cold rolling was performed to 0.8 mm, and continuous annealing was performed in the range of 700 ° C to 900 ° C. After adjusting the pressure of 0.5% on the annealed plate, a mechanical test and an axial tensile fatigue test were performed. The fatigue test was performed under partial swaying conditions, and the repetition rate was 20 Hz. All the subsequent tensile fatigue tests were conducted under such conditions.
Figure 1 shows the relationship between yield strength and fatigue limit. As shown in FIG. 1, the fatigue limit can be arranged by the yield strength, and it was confirmed that the higher the yield strength of the material, the higher the fatigue limit. From this result, it is understood that in order to obtain a material having excellent fatigue properties and high formability, it is necessary to manufacture a material having as high a yield strength as possible within the range where the formability is allowed.

Next, an example of the relationship between the annealing temperature and the yield strength and r value is shown in FIG. Here, as the r value, a value obtained from a test piece taken from 0 °, 45 °, 90 ° with respect to the rolling direction, an average r value represented by the following formula using r ₀ , r ₄₅ , and r ₉₀ It was adopted.

[0026]

[Equation 1] Table 1 shows the chemical composition of the steel sheet shown in FIG.

[0027]

[Table 1]

As is clear from FIG. 2, the r value increases with the increase of the annealing temperature, but the yield strength decreases. Therefore, in order to increase the r value while maintaining the yield strength in the range where the fatigue characteristics are good, the annealing temperature has an upper limit value. The upper limit values are Si, M of steels with various chemical compositions.
As a result of multiple regression analysis based on the addition amounts of n and P, FIG.
The relationship shown in is obtained. That is, the upper limit of the annealing temperature is 6 (10Si + 0.5Mn + 100P +
14) +710 was found to be given.

If the above-mentioned annealing process is carried out in a hot dip galvanizing line and hot dip galvanizing is performed without impairing the effect of the present invention, and the product is required to have rust-preventing properties, such hot dip galvanizing is performed. Is preferably applied. When properties such as weldability are required, 450 to 550 ° C
Alloying treatment of hot dip galvanizing is performed at about the temperature. Further, when press formability is particularly required, the friction coefficient is lowered and the formability is greatly improved by applying the Fe—Zn alloy plating with the Fe content of 50% or more to the upper layer of the plating film. The obtained steel sheet has 0.5 to 1.
A temper rolling of about 5% is performed to obtain a product. Further, even if Zn-based plating is applied to the surface of the cold-rolled steel sheet by electroplating, the effect of the present invention is not impaired. Next, the second aspect will be described.

The steel composition in this embodiment is basically the same as that in the first embodiment except the amounts of Ti and Nb. As described above, Ti and Nb are added to fix C and N in steel and improve the r value. To increase the r value, the more Ti and Nb, the better. When added to Ti, C and N in the steel are all fixed and TiC and NbC
Precipitates, etc., become coarse and the grain growth property increases, leading to a decrease in strength. In this aspect, focusing on this point, (12Ti ^* ) /
(48C) + (12Nb) / (93C) (however, Ti
^* = Ti-48 / 14 / N-48 / 32S), and the amounts of Ti and Nb are defined. That is, if this value exceeds 2, the precipitates of TiC, NbC, etc. become coarse and the grain growth property increases, leading to a decrease in strength. Therefore, the upper limit was defined as 2. On the other hand, when the addition amount of Ti and Nb is too small, a large amount of C and N remains, a high r value cannot be obtained, and the room temperature non-aging property cannot be maintained.
And Next, the reason for limiting the manufacturing conditions of the second aspect will be described.

The conditions for hot rolling in this embodiment are not particularly limited and may be carried out by a conventional method. That is, a method in which a slab obtained by continuous casting is subjected to a heat treatment and then hot rolling may be performed, or a method in which a slab directly sent to a rolling mill as a high temperature slab is directly hot rolled may be used. The obtained hot-rolled steel strip is pickled by a conventional method, and then cold-rolled at a rolling reduction of 60% or more and 90% or less, as in the first embodiment. The reason for this limitation is the same as in the first mode.

In the subsequent annealing step, the recrystallization temperature or higher and -11 {(12Ti ^* ) / (48C) + (12N
b) / (93C)} + 810 or less. The present inventors have newly found that by limiting the annealing temperature to this range, it is possible to produce a thin steel sheet having excellent formability and fatigue characteristics. Such knowledge is obtained through the following experiments by the present inventors.

First, C: 0.0020%, Si: 0.0
2%, Mn: 0.2%, P: 0.010%, S: 0.0
10%, sol. Al: 0.055%, N: 0.002
5%, Ti: tr to 0.050%, Nb: tr to 0.0
1250 of the slab obtained by melting 30% of the steel composition
After heating at ℃, hot rolling to a plate thickness of 4.0 mm,
It was wound into a coil at 580 ° C. After pickling, cold rolling was performed to 0.8 mm, and continuous annealing was performed in the range of 600 ° C to 880 ° C. After adjusting the pressure of 0.5% on the annealed plate, a mechanical test and an axial tensile fatigue test were performed. As for the relationship between the yield strength and the fatigue limit at that time, it was confirmed that the higher the yield strength of the material, the higher the fatigue limit, as in FIG. 1 of the first mode. In order to obtain a material that is excellent in moldability and has high moldability,
It was confirmed that a material with high yield strength should be manufactured as much as possible. Next, the annealing temperature and yield strength and r
An example of the relationship with the value is shown in FIG. The chemical composition of the steel shown in FIG. 4 is shown in Table 2.

[0034]

[Table 2]

As is apparent from FIG. 4, as in FIG. 2, the r-value increases with the increase of the annealing temperature, but the yield strength decreases, but the yield strength in a good fatigue property range is maintained. In order to increase the r value, the annealing temperature has an upper limit value. The upper limit is T of various chemical components
As a result of performing the multiple regression analysis by the addition amounts of i and Nb, the relationship shown in FIG. 5 was obtained. That is, the upper limit of the annealing temperature is -11 {(12Ti ^* ) / (48C)
It was found to be given by + (12Nb) / (93C)} + 810.

As in the case of the first embodiment, even if the above-mentioned annealing process is performed in a hot dip galvanizing line and hot dip galvanizing is performed, the effect of the present invention is not impaired and the product is required to have rust preventive properties. In such a case, it is preferable to apply such hot dip galvanizing. When properties such as weldability are required, alloying treatment is performed at a temperature of about 450 to 550 ° C. Also, when press formability is particularly required,
Fe-Zn with an Fe content of 50% or more in the upper layer of the plating film
By applying alloy plating, the coefficient of friction is lowered and the formability is greatly improved. The obtained steel plate has 0.5 if necessary.
Approximately 1.5% is temper rolled to obtain a product. Also,
Similar to the first aspect, the Zn-based plating on the surface of the cold-rolled steel sheet by electroplating does not impair the effects of the present invention.

Thus, the present invention makes it possible for the first time to inexpensively and stably manufacture cold-rolled steel sheets and hot-dip galvanized steel sheets having excellent formability and fatigue characteristics.

[0038]

EXAMPLES Examples of the present invention will be described below. The following Examples 1 to 3 correspond to the first aspect,
Examples 4 and 5 correspond to the second mode. (Example 1)

Steels having the components shown in Table 3 were melted at 1100 ° C.
After heating at ~ 1300 ° C, hot-rolling to a plate thickness of 4.0 mm
And was wound on a coil at 580 ° C. 0.8 after pickling
After cold rolling to mm and continuous annealing at 760 ° C., hot dip galvanizing was performed. Then, after alloying at 500 ° C., 0.5% pressure adjustment was performed to obtain a product. Tensile test pieces and fatigue test pieces were sampled from the obtained steel sheet and subjected to a tensile test and a fatigue test. The result is shown in FIG.
FIG. 6 is a diagram showing the relationship between the slab heating temperature on the horizontal axis and the tensile strength and fatigue limit on the vertical axis. As is clear from this figure, the slab heating temperature is 1200 ° C.
It was confirmed that the yield strength and fatigue limit were high values by the above.

[0040]

[Table 3] (Example 2)

Steels having the components shown in Table 4 were melted, and the temperature was 1250 ° C.
After that, it is hot-rolled to a plate thickness of 4.0 mm, 4
The coil was wound in the range of 00 ° C to 700 ° C. After pickling, cold rolling was performed to 0.7 mm, continuous annealing was performed at 770 ° C., and then 0.5% pressure adjustment was performed to obtain a product. Tensile test pieces and fatigue test pieces were sampled from the obtained steel sheet and subjected to the test. The results are shown in Fig. 7. FIG. 7 is a diagram showing the relationship between the winding temperature on the horizontal axis and the yield strength, the fatigue limit, and the average r value on the vertical axis. As is clear from this figure, when the winding temperature exceeds 600 ° C, a high r value is obtained, but the yield strength decreases and the fatigue limit decreases, and in the temperature range below 450 ° C, the yield strength and fatigue limit decrease. It was confirmed that a high value was obtained but the r value decreased.
That is, it was confirmed that 450 to 600 ° C. is suitable as the winding temperature.

[0042]

[Table 4] (Example 3)

Steels having the components shown in Tables 5 and 6 were melted and
After heating at a temperature of 00 ° C. to 1300 ° C., it was hot-rolled to a plate thickness of 4.0 mm and wound on a coil in a temperature range of 400 ° C. to 650 ° C. After pickling, cold rolling was performed to 0.8 mm, and continuous annealing was performed in the range of 650 ° C to 900 ° C. After that, a part of these steel materials is subjected to pressure regulation to obtain a product, and the other is annealed and then hot-dip galvanized, and then 45
The alloying treatment was performed at a temperature of 0 to 550 ° C. Then, a part of the alloyed material was further subjected to Fe—Zn alloy plating on the upper layer of the zinc plating film. Steel numbers 1 to 30 are steels of the present invention, and steel numbers 31 to 42 are comparative steels.

[0044]

[Table 5]

[0045]

[Table 6]

Mechanical tests and axial tensile fatigue tests were performed on these steel materials. The amount of hot-dip galvanized is 6
0/60 g / m ² And In addition, the coating weight of the upper layer is 3
g / m ² And Furthermore, the hot-dip galvanized product was subjected to a draw bead test in order to check the powdering resistance. At this time, the peeling amount per one side is 5 g /
m ² The above was considered defective. The results are shown in Tables 7 and 8.

[0047]

[Table 7]

[0048]

[Table 8]

As is clear from these tables, it was confirmed that all of the steels of the present invention have a high fatigue limit and a high r value. On the other hand, the comparative steel 31 had a low fatigue limit because the heating temperature was low, and the comparative steels 32 and 33 had a low fatigue limit because the coiling temperature and the annealing temperature were high.
Further, in the comparative steel 41, the r-value is remarkably deteriorated because the coiling temperature is low, and in the comparative steel 42, the r-value is low because the annealing temperature is lower than the recrystallization temperature. Further, in the comparative steels 34 and 40, since the C and N contents are large, the r value is low.
In No. 7, since the amounts of Si and P were large, the adhesion of the hot dip galvanizing was poor, and in Comparative Steel 36, the amount of Mn was large and the r value was low. Further, Comparative Steel 39 has an extremely low r value because Ti and Nb are not added, and Comparative Steel 38 has Nb.
As a result, the r value was low. (Example 4)

Steels having the components shown in Table 9 were melted at 1250 ° C.
And then hot-rolling to a plate thickness of 4.0 mm, 600
It was wound into a coil at ° C. After pickling, it was cold-rolled to 0.8 mm, continuously annealed at 770 ° C., and then hot-dip galvanized. Then, after alloying at 500 ° C,
The pressure was adjusted to 0.5% to obtain a product. Tensile test pieces and fatigue test pieces were sampled from the obtained steel sheet and subjected to the test. The results are shown in Fig. 8. In Fig. 8, the horizontal axis is (12T
i ^* ) / (48C) + (12Nb) / (93C), and the yield strength and the average r value are plotted on the vertical axis to show these relationships. As is clear from this figure, (1
2Ti ^* ) / (48C) + (12Nb) / (93C) value is less than -1, a high yield strength can be obtained, but the r value is remarkably reduced, and when the value exceeds 2, r
Although the value was high, it was confirmed that the yield strength decreased and sufficient fatigue strength could not be obtained. That is, (12Ti ^* ) / (4
It was confirmed that the value of 8C) + (12Nb) / (93C) is -1 or more and 2 or less.

[0051]

[Table 9] (Example 5)

Steels having the components shown in Tables 10 and 11 were melted and
After heating at a temperature of 150 ° C. to 1250 ° C., it was hot-rolled to a plate thickness of 4.0 mm and wound on a coil in a temperature range of 500 ° C. to 600 ° C. After pickling, cold rolling was performed to 0.8 mm, and continuous annealing was performed in the range of 650 ° C to 880 ° C. After that, a part of these steel materials is subjected to pressure regulation to obtain a product, and the other is annealed and then hot-dip galvanized, and then 45
The alloying treatment was performed at a temperature of 0 to 550 ° C. Then, a part of the alloyed material was further subjected to Fe—Zn alloy plating on the upper layer of the zinc plating film. Steel numbers 51 to 80 are steels of the present invention, and steel numbers 81 to 91 are comparative steels.

[0053]

[Table 10]

[0054]

[Table 11]

Mechanical tests and axial tensile fatigue tests were performed on these steel materials. The amount of hot dip galvanized is 60/60 g / m ² And In addition, the amount of adhesion of the upper layer is 3g
/ M ² And Furthermore, the hot-dip galvanized product was subjected to a draw bead test in order to check the powdering resistance. At this time, peeling amount per side 5 g / m
² The above was considered defective. These results are shown in Table 12 and Table 13.
Shown in.

[0056]

[Table 12]

[0057]

[Table 13]

As is clear from these tables, it was confirmed that each of the steels of the present invention had a high fatigue limit and a high r value. On the other hand, the comparative steel 81 is Ti, Nb.
Since the amount is small, the r value is low.
The fatigue limit was lowered because the amount of Nb was too large. Further, the comparative steels 84, 85, and 86 each have a large r, so that the r value is low, and the comparative steel 87 has a large r, so that the r value is low and the plating adhesion is poor. Also, comparative steel 8
No. 8 has a low r-value because Ti and Nb are not added at all, and Comparative Steel 89 has a large r-value due to a large amount of N.
Further, although Comparative Steel 90 has a high annealing temperature and thus a high r value, the fatigue limit is low, and Comparative Steel 91 has an extremely low r value because the annealing temperature is below the recrystallization temperature.

[0059]

According to the present invention, there is provided a method for manufacturing a steel sheet having high formability and excellent fatigue characteristics. By the method of the present invention, a cold-rolled steel sheet and a hot-dip galvanized steel sheet having such excellent properties can be manufactured for the first time, and their industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between yield strength and fatigue limit.

FIG. 2 is a diagram showing an example of a relationship between an annealing temperature, a yield strength, and an r value.

FIG. 3 is a diagram showing the relationship between 10Si + 0.5Mn + 100P + 14 and the upper limit of annealing temperature.

FIG. 4 is a diagram showing another example of the relationship between the annealing temperature and the yield strength and r value.

FIG. 5 (12Ti ^* ) / (48C) + (12Nb) /
The figure which shows the relationship between (93C) and the upper limit of annealing temperature.

FIG. 6 is a diagram showing changes in tensile strength and fatigue limit in the first embodiment depending on the quenching heating temperature of the slab.

FIG. 7 is a diagram showing changes in yield strength, fatigue limit, and average r value depending on the winding temperature in the first mode.

FIG. 8 shows (12Ti ^* in the second embodiment ^. ) / (48
The figure which shows the relationship of the value of (C) + (12Nb) / (93C), a yield strength, and an average r value.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Akihide Yoshitake 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (5)

[Claims] 1. C, 0.0050% or less by weight%, S
i: 0.2% or less, Mn: 0.10 to 0.5%, P:
0.03% or less, S: 0.015% or less, sol. A
L: 0.10% or less, N: 0.0040% or less, and 0.005 to 0.10% Ti and 0.005.
To 0.030% of Nb, one or two of which is contained, and the balance Fe and unavoidable impurities are heated to a temperature of 1200 ° C. or higher and 1300 ° C. or lower, and then hot-rolled.
The hot-rolled steel strip was wound around a coil at a temperature of ℃ or more and 600 ° C or less, and cold-rolled at a rolling reduction of 60% or more and 90% or less after pickling, and at a recrystallization temperature or more and 6 (10Si + 0.5
Mn + 100P + 14) +710, The manufacturing method of the steel plate excellent in the fatigue characteristics and deep drawability characterized by annealing at the temperature below. 2. C: 0.0050% or less by weight%, S
i: 0.2% or less, Mn: 0.10 to 0.5%, P:
0.03% or less, S: 0.015% or less, sol. A
1: 0.10% or less, N: 0.0040% or less, and T
One or two of i and Nb are −1 ≦ (12Ti ^* )
/ (48C) + (12Nb) / (93C) ≦ 2 (however, Ti ^* = Ti-48 / 14 / N-48 / 32S), hot rolling is performed on a steel material containing the balance Fe and unavoidable impurities, and the steel sheet is pickled, and the rolling reduction is 60% or more and 90% or more. Cold-rolled below, above the recrystallization temperature, and −11 {(12Ti ^* ) / (48C) + (1
2Nb) / (93C)} + 800, and a method for producing a cold-rolled steel sheet having excellent fatigue properties and deep drawability, characterized by annealing at a temperature not higher than that specified. 3. The method for producing a steel sheet having excellent fatigue properties and deep drawability according to claim 1 or 2, further comprising hot dip galvanizing. 4. The method for producing a steel sheet having excellent fatigue properties and deep drawability according to claim 3, further comprising an alloying treatment to form a galvannealed alloy. 5. The method for producing a steel sheet excellent in fatigue properties and deep drawability according to claim 4, further comprising: applying Fe—Zn alloy plating having an Fe content of 50% or more to the upper layer of the plating film. .