JP2827740B2 - Method for producing steel sheet with excellent fatigue characteristics and deep drawability - Google Patents

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 steel sheets, such as cold-rolled steel sheets, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, having both excellent deep drawability and fatigue properties.

[0002]

2. Description of the Related Art Conventionally, as a technique for imparting high formability to a thin steel plate, an IF (carbon and nitride forming element such as Ti, Nb, etc. added to ultra-low carbon steel to precipitate and fix C and N). In
Tertiary Free steel is known.
Techniques relating to such cold-rolled steel sheets and hot-dip galvanized steel sheets having excellent formability based on the IF steel are disclosed in, for example, JP-A-2-34722 and JP-A-1-225727. In recent years, since the shape of automobile bodies has become more complicated, the number of parts that cannot be machined unless steel sheets having extremely excellent workability, such as IF steel, are increasing rapidly.
F steel is widely used as a material for steel plates for automobiles.

[0003] On the other hand, low carbon aluminum killed steel is conventionally used for parts requiring fatigue strength and subjected to deep drawing, but the formability of low carbon aluminum killed steel is inferior to that of IF steel. Absent. From such a background, a steel sheet excellent in both fatigue characteristics and deep drawability has been demanded.

[0004]

In the conventional IF steel, C and N are sufficiently precipitated as a means for achieving a high r value as disclosed in Japanese Patent Application Laid-Open Nos. 1-225727 and 2-34722. Containing Ti, Nb for fixing;
And high-temperature annealing within a range not exceeding the Ac ₃ transformation point. Certainly, according to these methods, a high r value can be obtained. However, these techniques are only for achieving a high r-value and are not intended to improve fatigue properties.

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

[0006]

Means and Action for Solving the Problems The present inventors have
As a result of researching to achieve such an object, it has been found that the fatigue strength of a soft thin steel plate increases as the yield strength of the material increases. Therefore, if the yield strength can be increased while securing a high r value using IF steel, a steel sheet excellent in deep drawability and fatigue characteristics should be able to be manufactured. Generally, to increase the yield strength without impairing the deep drawability, it is effective to reduce the grain size of the crystal grains after annealing. The present inventors have assiduously studied and, when B is added to steel,
The recrystallization temperature rises, the crystal grains can be refined, and the upper limit of the annealing temperature is defined by the amount of addition of elements such as Si, Mn, P, and B, so that the yield that can be press-formed is obtained. It has been newly found that strength can be achieved and fatigue characteristics can be increased.

The fatigue characteristics and deep drawability according to the present invention are excellent.
The production method of the obtained steel sheet is C: 0.0050% by weight%.
Hereinafter, Si: 0.2% or less, Mn: 0.10 to 0.5
%, P: 0.03% or less, S: 0.015% or less, so
l. Al: 0.10% or less, N: 0.0040% or less,
B: 0.0001 to 0.0020%, and
0.005 to 0.10% Ti and 0.005 to 0.03
Hot rolling a steel containing one or two types of 0% Nb,
After pickling, cold-rolled at a draft of 60% or more and 90% or less, and re-solidified
Above the crystallization temperature and 6 (10Si + 0.5Mn + 100
P + 800B + 14) +710 or lower
AnnealingAnd the fatigue limit is 26.1 kgf / mm ^Two Or more
And the average Rankford (r) value given by
1.70 or moreWhenTo doIt is characterized by the following.r = (r ₀ + 2r ₄₅ + R ₉₀ ) / 4 Where r ₀ , R ₄₅ , R ₉₀ Is for each rolling direction
Determined using test pieces taken from 0 °, 45 °, and 90 °
Rankford value. 26.1k fatigue limit of steel plate
gf / mm ^Two Above is to obtain excellent fatigue properties
Is important. Also, the average rank Ford of steel sheet
(R) The value of 1.70 or more provides excellent deep drawability.
Is important for Steel produced by the method of the present invention
Plates (numbers 1 to 30) are fatigue-limited and average Rankford
Good balance with (r) value, both are high values
Is apparent from the results shown in Tables 5 and 6.
is there. A steel sheet with excellent fatigue characteristics and deep drawability
In order to obtain, the composition of steel and the manufacturing conditions as in the present invention
And a well-balanced combination is required.

Further, a hot-dip galvanized layer may be formed on the surface. In this case, the content of Fe in the hot-dip galvanized layer is preferably 5 to 15%. Further, a Fe—Zn alloy plating layer having an Fe content of 50% or more may be further formed on the surface of such a zinc plating layer. next,
The reason for limiting the steel composition of the present invention as described above will be described. C: C should be low to achieve a high r value,
The upper limit is set to 0.0050% as long as the effect of the present invention is not practically impaired.

Si: Si is a strengthening element that contributes to the increase in the yield strength and tensile strength of a steel sheet and is effective for improving the fatigue strength. Not only the value deteriorates, but also the adhesion of hot-dip galvanized coating deteriorates remarkably, so the upper limit is made 0.2%.

Mn: Like Mn, Mn also contributes to an increase in the strength of the steel sheet. However, if it is less than 0.10%, there is almost no effect, and if it exceeds 0.50%, the Ac ₃ transformation of the steel sheet occurs. In order to lower the point and not only narrow the optimum annealing temperature range, but also lower the r-value of the steel sheet,
The range was 0.50%.

P: P can strengthen steel at the lowest cost, but if it is contained in excess of 0.03%, the alloying reaction of hot-dip galvanizing is extremely slowed down, and it only causes defects such as uneven alloying. Alternatively, the upper limit was limited to 0.03% in order to lower the r value of the steel sheet.

S: S deteriorates the ductility of the steel sheet.
It is desirable to reduce as much as possible. However, as a range that does not impair the effect of the present invention practically, the upper limit is 0.0
15%. sol. Al: Al is necessary for deoxidation, but adding too much increases the cost, so the upper limit was made 0.10%. N: N is preferably as small as possible to obtain a high r value. Therefore, the content is set to 0.0040% or less.

Ti: Ti is added in order to fix solid solution C and N in steel as precipitates and obtain a high r value. That is, if the content is less than 0.005%, the effect is not obtained.
%, The effect is saturated and the cost is increased. Therefore, the Ti content is set in the range of 0.005 to 0.100%.

Nb: Nb is also added to obtain a high r value like Ti. That is, if the content is less than 0.005%, the effect is not obtained. If the content exceeds 0.030%, the ductility of the steel is significantly reduced.
The range was 0.030%.

B: B is one of the most important additive elements in the present invention. That is, the recrystallization temperature is increased by the addition of B, and fine grains are obtained. That is, if B is less than 0.0001%, the effect cannot be obtained, and
When the content exceeds 20%, the r value is significantly reduced. Therefore, the B content is set in the range of 0.0001 to 0.0020%. next,
The reason why the manufacturing conditions of the present invention are limited as described above will be described.

The hot rolling step may be performed by a conventional method. That is, a method of performing hot rolling after subjecting a slab obtained by continuous casting to a heat treatment, or a method of directly hot rolling a slab directly sent to a rolling mill as a high-temperature slab may be used. The hot-rolled steel strip thus obtained was pickled by a conventional method, and the rolling reduction was 60%.
Cold rolling under the condition of not less than 90% and not more than 90%. Reduction rate 60
%, A high r value cannot be obtained.
%. Further, even if the rolling is performed at a rolling reduction of 90% or more, r
Not only is there no effect on the increase in the value, but the load on the rolling mill during rolling increases, so the upper limit is 90%.
Determined.

It is important that annealing after cold rolling be performed at a temperature higher than the recrystallization temperature. The upper limit of the annealing temperature is set to 6 (10
Si + 0.5Mn + 100P + 800B + 14) +71
It is important to use a value obtained by the equation of 0. The present inventors have found that by annealing a steel sheet in such a temperature range, a thin steel sheet having excellent deep drawability and excellent fatigue properties can be manufactured. Hereinafter, the reason for setting the annealing temperature as described above in the present invention will be described.

C: 0.0020%, Si: tr to 0.3
%, Mn: 0.1 to 0.8%, P: 0.003 to 0.0
3%, S: 0.010%, sol. Al: 0.055
%, N: 0.0025%, Ti: 0.005 to 0.10
%, Nb: 0.005 to 0.035%, B: tr to 0.
0030% of steel was melted and the resulting slab was
After heating at 50 ° C., the sheet was hot-rolled to a thickness of 4.0 mm.
And wound up at 640 ° C. on a coil. 0.8 after pickling
mm, and continuously annealed in the range of 700 to 900 ° C. The annealed plate was subjected to a temper rolling of 0.5%, and a test piece was taken therefrom and subjected to a mechanical test and an axial tensile fatigue test. The fatigue test was performed under partial pulsating conditions, and the repetition rate was 20 Hz. Hereinafter, the tensile fatigue test in the present invention was performed under the above conditions.

FIG. 1 shows the relationship between yield strength and fatigue limit.
As is clear from the figure, the fatigue limit can be arranged by the yield strength,
It was found that the higher the yield strength, the higher the fatigue limit. From these results, in order to obtain a material with excellent fatigue characteristics and high deep drawability, within the allowable range of formability,
It is understood that a material having as high a yield strength as possible must be manufactured.

FIG. 2 is a plot showing the relationship between the annealing temperature on the horizontal axis and the yield strength and r value of the steel sheet on the vertical axis. Here, as the r value, a value obtained from test pieces taken from 0 °, 45 °, and 90 ° with respect to the rolling direction, r ₀ , r ₄₅ , and r ₉₀ are used. The value was adopted. In the figures, circles are plots of the results of yield strength,
The triangle is a plot of the result of the average r value.

[0021]

(Equation 1) Table 1 shows the chemical components of the steel sheets shown in FIGS.

[0022]

[Table 1]

As is apparent from FIG. 2, the average r value increases as the annealing temperature increases, but on the other hand, the yield strength decreases. Therefore, the average r value is maintained while maintaining the yield strength within a good range of fatigue characteristics. In order to increase the r value, there is an upper limit of the annealing temperature. As a result of performing multiple regression analysis on the upper limit value based on the addition amounts of Si, Mn, P, and B of steels of various chemical components, a relationship as shown in FIG. 3 was obtained. That is, the upper limit of the annealing temperature is 6 (10Si +
0.5Mn + 100P + 800B + 14) +710.

The effect of the present invention is not impaired even if the above annealing step is performed in a hot-dip galvanizing line and hot-dip galvanizing is performed. If the product is required to have rustproofing properties, hot-dip galvanizing is applied. When characteristics such as weldability are required, alloying is performed at a temperature of about 450 to 550 ° C. When press formability is particularly required,
Fe-Zn with Fe content of 50% or more on the plating film
By applying the alloy plating, the coefficient of friction is reduced, and the formability is greatly improved. In the obtained steel sheet, if necessary, 0.5
Perform temper rolling of about 1.5% to obtain a product. Also,
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 at all. Thus, the steel sheet of the present invention has excellent fatigue characteristics and deep drawability, and can be manufactured stably at low cost.

[0025]

EXAMPLES Examples of the present invention will be described below in more detail. (Example 1)

A steel having the composition shown in Table 2 was melted,
And then hot-rolled to a thickness of 4.0 mm.
And wound up at 620 ° C. on a coil. After pickling, the obtained hot-rolled steel strip was cold-rolled to 0.8 mm, continuously annealed at 780 ° C., and then subjected to hot-dip galvanizing. Then 50
After alloying at 0 ° C., temper rolling of 0.5% was performed to obtain a product. From the obtained steel plate, a tensile test piece and a fatigue test piece were sampled and subjected to a test.

FIG. 4 shows the result. In the figure, circles indicate the yield strength, and triangles indicate the average r value. As is clear from the figure, the addition of B increases the yield strength and at the same time increases the fatigue limit. However, the amount of B added is 0.0020.
%, It has been found that the yield strength increases but the average r-value significantly decreases.

[0028]

[Table 2] (Example 2)

The steels having the components shown in Tables 3 and 4 were melted, and 1
After heating at a temperature of 150-1250 ° C, hot rolling
The plate thickness is set to 4.0 mm, and in the temperature range of 500 to 680 ° C.
Wound on a coil. Cold rolling to 0.8mm after pickling
After performing continuous annealing in the range of 650 to 900 ° C.,
Product by applying pressure, hot-dip galvanized after annealing
After the application, the alloying treatment is performed at a temperature of 450 to 550 ° C.
Fe-Zn alloy on the zinc plating film
Mechanical test and axial tensile fatigue
A labor test was performed. The amount of hot-dip galvanized coating was 60
/ 60g / m^Two And The upper layer plating adhesion amount is 3g.
/ M^Two And In addition, to hot-dip galvanized
In order to determine the powdering resistance,
Test. In addition, the peeling amount per one side is 5 g / m.^Two
The above was regarded as defective.

The results are shown in Tables 5 and 6. All of the steels of the present invention have a high fatigue limit and a high average r.
Values have been obtained. For example, in steel plate of steel number 1-30
Has a fatigue limit of 26.1 kgf / mm ² or more
And have excellent fatigue characteristics. Especially steel material number
No. 1,5-8,11,17,22,28,29
Has a high fatigue limit of 28.0 kgf / mm ² or more.
Was. Further, the average r value is not large in the steel sheets of steel numbers 1 to 30.
The displacement is as high as 1.70 or more, and it is excellent in deep drawability.
It that are me. Especially steel material number 1,3,12,14,1
1.85 for steel plates 7 to 21, 23 to 27, 29, and 30
As a result, a high average r value was obtained. On the other hand, the comparative steel 31 has a low fatigue limit because no B is added, and the comparative steel 32 has a low average r value because the B is excessively added.
The deep drawability has deteriorated. Further, the comparative steels 33 and 41 have a low annealing limit because of the high annealing temperature, and the comparative steel 42 has a low average r value because the annealing temperature is lower than the recrystallization temperature. Comparative steels 34, 35 and 36 are C, Si and Mn, respectively.
, The average r value decreases. In addition, comparative steel 37
Thus, not only the average r-value is lowered due to the high P, but also the plating adhesion is deteriorated. Further, the comparative steel 38 has a high Nb, so that the average r-value is reduced and the deep drawability is deteriorated.
In No. 9, the average r-value was reduced due to the absence of Ti and Nb, and deep drawing was performed.
Has deteriorated . Furthermore, since the comparative steel 40 has a large amount of N, the average r value is reduced and the deep drawability is deteriorated .

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

[0034]

[Table 6]

[0035]

According to the present invention, excellent deep drawability is obtained,
The production of cold-rolled steel sheets and hot-dip galvanized steel sheets having excellent fatigue properties is possible for the first time, and their industrial value is great.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a plot showing the results of an examination of the relationship between the yield strength of a steel sheet on the horizontal axis and the fatigue limit of the steel sheet on the vertical axis.

FIG. 2 is a plot showing the results of examining the respective relationships, with the horizontal axis representing the annealing temperature of the steel sheet and the vertical axis representing the yield strength and the average r value of the steel sheet.

FIG. 3 is a plot showing the result of examining the relationship between an index obtained by putting each component numerical value into a mathematical formula defining the annealing temperature of a steel sheet on the horizontal axis and an upper limit of the annealing temperature on the vertical axis. .

FIG. 4 is a plot diagram showing the results of examining the relationship between the B addition amount on the horizontal axis and the yield strength and the average r value on the vertical axis.

──────────────────────────────────────────────────続 き Continued from the front page (72) Inventor Akihide Yoshitake 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-63-310924 (JP, A) JP-A Sho 63-317625 (JP, A) JP 4-5732 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 9/48 C21D 8/04

Claims (4)

(57) [Claims] 1. The method according to claim 1, wherein C: 0.0050% or less,
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, B:
0.0001 to 0.0020%, and further 0.0%
0.5-0.10% Ti and 0.005-0.030%
After hot rolling steel containing one or two types of Nb and pickling it
Cold rolling at a rolling reduction of 60% or more and 90% or less, recrystallization temperature
6 (10Si + 0.5Mn + 100P + 8
00B + 14) + annealing at a temperature below 710
And the fatigue limit is 26.1 kgf / mm ^Two And above, and
The average Rankford (r) value given by the following equation is 1.7
0 or moreWhenTo doFatigue characteristics and deep drawing
Method for manufacturing steel sheets with excellent heat resistance.r = (r ₀ + 2r ₄₅ + R ₉₀ ) / 4 Where r ₀ , R ₄₅ , R ₉₀ Is for each rolling direction
Determined using test pieces taken from 0 °, 45 °, and 90 °
Rankford value. 2. The method for producing a steel sheet having excellent fatigue characteristics and deep drawability according to claim 1, further comprising hot-dip galvanizing. 3. The method for producing a steel sheet according to claim 2, further comprising an alloying treatment. 4. The fatigue characteristics and deep drawability according to claim 3, wherein an Fe—Zn alloy plating layer having an Fe content of 50% or more is formed on the zinc plating layer. Excellent steel plate manufacturing method.