JPH09272952A - Cold rolled steel sheet excellent in fatigue characteristic and deep readability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a cold rolled steel sheet excellent in fatigue characteristics and deep drawability by adding B to Al killed steel having a specified compsn. and furthermore adding a suitable amt. of O thereto and executing hit rolling, coiling, cold rolling, annealing and cooling under specified conditions. SOLUTION: A steel having a compsn. contg., by weight, <=0.005% C, <=0.1% Si, <=0.5% Mn, 0.01 to 0.05% to P, <=0.02% S, 0.005 to 0.1% Al and <=0.01% N, contg. B in the range satisfying B/P>=0.1, B⊗P>=4.0×10<-5> and also B<=0.005, contg. Ti and/or Mb in the range satisfying Ti/48>N/14 and also (Ti/48+Mb/93)>(C/12+N/14+S/32), and the balance Fe is prepd. This steel is subjected to hit rolling in the temp. range of the Ar3 point or above, is thereafter coiled at >=700 deg.C, is subjected to cold rolling at the rolling ratio of 70 to 95% and is subsequently annealed in the temp. range of the recrystallization temp. to the Ac3 point, and the cooling rate after the annealing is regulated to <=20 deg.C/sec. In this way, the objective cold rolled steel sheet having >=0.52 fatigue limit ratio and >=0.8 average (r) value can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet excellent in fatigue characteristics and deep drawability and a method for producing the same, and particularly, it requires extremely severe formability and has good fatigue characteristics. The present invention relates to a cold-rolled steel sheet having excellent fatigue properties and deep drawability suitable for required automobile parts and the like, and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, the material properties required for steel sheets for automobiles have become higher year by year. Particularly, for automobile fenders, oil pans and the like, since extremely severe press molding is performed, further improvement in deep drawability and ductility has been expected. Since then, with the development of vacuum degassing technologies such as RH and DH, ultra-low carbon steel has been developed that has reduced solid solution elements in steel and improved formability.
It has come to be used in place of the low carbon Al killed steel that has been used so far. Furthermore, recently, Ti, N
Scavenging of solid solution C and N in steel by b etc. (s
As a steel whose formability has been dramatically improved by casting, an interposer disclosed in JP-A-1-225727, JP-A-2-34722 and the like.
stial atoms free steel
(Hereinafter referred to as IF steel) has been widely used.

On the other hand, in the suspension parts such as suspension arms and road wheels, and inner plate structural members such as side members and cross members, it is natural that durability is strongly required in addition to moldability and static strength. Specially
For inner panel such as floor and dash, and outer panel parts such as hood outer and trunk lid outer,
Fatigue durability is required because a repeated load is applied to the vicinity of the joint by vibration during traveling.

To satisfy such required characteristics,
For example, JP-A-6-81080 and JP-A-6-810.
No. 44, and Japanese Patent Laid-Open No. 6-81043,
Steel sheet with excellent fatigue properties and deep drawability using IF steel,
And the manufacturing method thereof is disclosed.

[0005]

However, since the fatigue properties are generally proportional to the tensile strength and the yield strength of the material, the press formability decreases when these mechanical properties are improved. This is also a problem that applies to the steel sheet having excellent fatigue characteristics and deep drawability of the above-mentioned conventional technology. That is, in the steel sheet, the tensile strength was about 30 kgf / mm ² , the average r (Rankford) value was about 2.0, and the fatigue strength was obtained, but the yield strength was 20 kgf / mm 2.
It is not suitable for automobile parts that require a high formability of ² or more, and there is a problem with shape freezeability such as springback.

Therefore, the present invention has developed a technique for improving only the fatigue characteristics while suppressing the increase in the static strength of the material so as not to impair the press formability, and requires extremely severe formability, and It is an object of the present invention to provide a cold-rolled steel sheet having excellent fatigue properties and deep drawability that enables application to automobile parts and the like that require good fatigue properties, and a method for manufacturing the same.

[0007]

DISCLOSURE OF THE INVENTION The present inventors have conducted many years of research in order to improve the fatigue property alone while suppressing an increase in the static strength of a material so as not to impair the press formability. As a result, the fracture surface of the fatigue fracture of the conventional Al killed steel is the intragranular fracture surface, whereas
It was found that that of IF steel is a grain boundary fracture surface. Therefore, as a result of advancing the research by paying attention to the grain boundary segregation elements P and B, the present inventors have found that the grain boundary fracture surface in fatigue fracture is reduced and the fatigue characteristics are improved according to the addition amount of B. It has been found that, when P and P are added in an appropriate amount after adding B and B, the fatigue properties are further improved and become about the same as an Al killed steel manufactured by batch annealing (hereinafter referred to as BAF-Al killed steel). It was also found that the contents of P and B are mutually related, have optimum values, and have optimum heat cycles.

The experimental results that form the basis of the present invention will be described below. Steel with the chemical composition shown in Table 1 was heated to 1150 ° C.
After heating for 1 hour, the product was hot rolled at a finishing temperature (FT) of 920 ° C. and wound at 700 ° C. After pickling, cold rolling was performed at a rolling ratio of 80%, and annealed at 810 ° C. for 1 minute in a continuous annealing device.
After that, cooling was performed at a cooling rate of 20 ° C./s, and 1% skin pass rolling was performed. In the tensile test for evaluating the material quality of the annealed plate thus obtained, the test material was first tested according to JIS Z
Processed into No. 5 test piece described in 2201, JIS Z 2
It carried out according to the test method of 241. Further, as shown in FIG. 1, the plate thickness is 0.8 mm, the length is 90 mm, and the width is 18 m.
The fatigue test was carried out on a uniform plane bending fatigue test piece having m, the width of the minimum cross section of 10 mm, and the radius of curvature of the notch of 30 mm. FIG. 2 is a summary of the addition amount of B, the fatigue limit ratio, the grain boundary fracture rate of the fatigue fracture surface, and the average r value. With the addition of B, the fatigue grain boundary fracture surface is reduced and the fatigue limit is improved. Therefore, if the increase in static strength is slight, the fatigue limit ratio will increase. However, excessive addition of B causes an increase in static strength, resulting in a decrease in fatigue limit ratio. Therefore, by adding P, which has a solid solution strengthening ability of the matrix phase and is inexpensive, and by adding an appropriate amount of B in order to prevent the fatigue fracture from the grain boundary caused by the addition of P, both fatigue characteristics and deep drawability are achieved. It was found that a cold rolled steel sheet was obtained.

[0009]

[Table 1]

The present invention has been made based on the above findings, and the gist thereof is as follows. (1) C: 0.005% or less, Si: 0.
1% or less, Mn: 0.5% or less, P: 0.01 to 0.0
5%, S: 0.02% or less, Al: 0.005-0.1
%, N: 0.01% or less, and further B, B / P ≧
0.1, B · P ≧ 4.0 × 10 ⁻⁵ , and B ≦ 0.005
%, One or both of Ti and Nb are Ti / 48> N / 14, and (Ti /
48 + Nb / 93)> (C / 12 + N / 14 + S / 3
2) Fatigue limit ratio of 0.52 or more and average r value of 1.8 or more, and fatigue characteristics and deep drawing characterized by being contained in a range satisfying the condition 2), with the balance being Fe and inevitable impurities. Cold rolled steel sheet with excellent properties.

(2) In% by weight, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% or less, P:
0.01-0.05%, S: 0.02% or less, Al:
0.005 to 0.1%, N: 0.01% or less, and further B is B / P ≧ 0.1, B · P ≧ 4.0 × 10 ⁻⁵ ,
In addition, it is contained in a range satisfying B ≦ 0.005%, and one or both of Ti and Nb is Ti / 48> N /
14, and (Ti / 48 + Nb / 93)> (C / 12 +
N / 14 + S / 32) contained in the range satisfying the condition,
After hot-rolling steel with the balance being Fe and unavoidable impurities in a temperature range of Ar ₃ point or higher, after winding at 700 ° C. or higher and cold rolling at a rolling ratio of 70% to 95%, The fatigue limit ratio is 0.52 or more and the average r value is 1.8, which is characterized by annealing in a temperature range of the recrystallization temperature or more and Ac ₃ point or less and the cooling rate after annealing is 20 ° C./s or less. A method for manufacturing a cold-rolled steel sheet having excellent fatigue properties and deep drawability as described above.

The present invention is also applicable to hot-dip galvanized steel sheets. Next, the reasons for limiting the chemical components of the present invention will be described. If C is present in the steel in a solid solution state during cold rolling, an orientation unfavorable for deep drawability is generated during subsequent annealing, so the content is made 0.005% or less. When stricter workability is required, it is desirable to set it to 0.003% or less.

Si is a solid solution strengthening element, and if added in excess of 0.1%, solid solution hardening becomes remarkable and not only becomes unsuitable for processing, but also the average r value deteriorates and the adhesion of hot dip galvanization In order to deteriorate the property, it is set to 0.1% or less. When stricter workability is required, it is desirable to set it to 0.05% or less. If Mn is added in excess of 0.5%, the average r value deteriorates and the ductility also decreases, so the content is made 0.5% or less.

P is one of the most important elements in the present invention. Addition of a small amount of P can increase only the fatigue characteristics without increasing the static strength, but 0.01
If it is less than%, the target fatigue limit ratio cannot be obtained. On the other hand, P has the risk of segregating at the grain boundaries and causing intergranular cracking that is the starting point of fatigue fracture. Therefore, in the present invention, B corresponding to the amount of P is added to secure the target fatigue limit ratio. To do. However, if P exceeds 0.05%, not only the static strength unfavorably increased in formability but also a large amount of B must be added in order to obtain the target fatigue limit ratio. It also causes deterioration of the r value. From the above,
In the present invention, the P content is limited to 0.01 to 0.05%.

If the content of S is too large, not only will cracking occur during hot rolling, but also the average r value will deteriorate, so it should be reduced as much as possible, but if it is 0.02% or less, it is within the allowable range. Al is added in an amount of 0.005% or more for deoxidation of molten steel. However, if added in too large an amount, not only increases non-metallic inclusions and deteriorates ductility, but also causes an increase in cost. 1%

Since N is an element which is not preferable for the average r value like C, the content of N is set to 0.01% or less. B is one of the most important elements in the present invention together with P, and the content thereof is defined by the relationship with the P content as described above. That is, B · P <4.0 × 10 ⁻⁵ or B / P
When <0.1, the target fatigue limit ratio cannot be obtained. On the other hand, if the B content exceeds 0.005%, not only the increase in the fatigue limit ratio becomes saturated, but also the recrystallization temperature rises and the average r value also deteriorates. From the above, in the present invention, the content of B is limited to B / P ≧ 0.1, B · P ≧ 4.0 × 10 ⁻⁵ , and B ≦ 0.005%.

Ti and Nb are (Ti / 48 + Nb / 9
3)> (C / 12 + N / 14 + S / 32) Within a range satisfying the relationship, C, N, and S are fixed as precipitates to secure moldability. Further, particularly Ti is Ti in the high temperature region of the γ phase.
However, when Ti / 48 ≦ N / 14, the solid solution N is not sufficiently fixed as TiN, and the remaining solid solution N is B.
B, which precipitates as N and is effective for fatigue properties, cannot be secured.
Furthermore, if Ti is added in excess of 0.03%, the adhesiveness of the hot dip galvanizing is deteriorated and powdering occurs during press forming. Further, if Nb is added in excess of 0.03%, the recrystallization temperature rises, so that recrystallization cannot be completed sufficiently during annealing, resulting in deterioration of the average r value. Therefore, Ti, Nb
Content of Ti / 48> N / 14 and (Ti / 48
+ Nb / 93)> (C / 12 + N / 14 + S / 32) is satisfied. Preferably, both Ti and Nb are 0.03% or less.

In the present invention, the components other than the above are F
However, the inclusion of inevitable impurities such as scrap mixed in from the smelting raw material is acceptable. Next, the reasons for limiting the manufacturing method of the present invention will be described in detail below. The present invention
Addition elements to molten steel in the form of a simple substance or in the form of a master alloy so that the target component content is obtained, and the slab obtained by casting may be sent directly to the hot rolling mill as it is as a high-temperature slab.
After cooling to room temperature, it may be reheated in a heating furnace and then hot rolled. However, in the case of reheating, when heated to a temperature higher than 1150 ° C., precipitates such as Ti ₄ C ₂ S ₂ re-dissolve and inhibit the formation of {111} recrystallized texture which is favorable for formability. Since solid solution C is generated, the heating temperature is preferably 1150 ° C. or lower.

The hot rolling step must be completed in a temperature range where the final rolling final pass temperature (FT) is Ar ₃ point or higher. If the rolling temperature falls below the Ar ₃ point during hot rolling, α zone rolling will occur in the vicinity of the surface layer of the hot-rolled sheet, and the grain size in the vicinity of the surface layer will become coarse and press formability will deteriorate. When the coiling temperature after finish rolling is less than 700 ° C., C which is solid-soluted in the hot rolled sheet
However, they cannot be sufficiently coarsened as precipitates of TiC, NbC, etc., and are finely precipitated. In that case, in the subsequent annealing step, the formation of {111} recrystallized texture, which is favorable for formability, is hindered, and the average r value deteriorates. Therefore, the winding temperature is 700 ° C. or higher.

In the cold rolling process after pickling, the rolling ratio is 70% or more 9
Perform cold rolling at 5% or less. Rolling rate is less than 70%
And because it does not recrystallize sufficiently in the subsequent annealing step,
The average r value deteriorates. In addition, cold rolling at a rolling ratio of over 95%
Even if the extension is performed, the average r value is rather decreased. Therefore, cold
The rolling rate in the rolling step is 70% or more and 95% or less. Departure
The annealing process in Ming is either continuous annealing or batch annealing.
May be. However, Ac_ThreeAnneal at temperatures above the point
Then, it becomes an annealing in the γ region or the two-phase region of α + γ.
Formation of {111} recrystallized texture favorable for moldability
Therefore, the average r value, which is the object of the present invention, is
I can't get it. Also, even if annealed below the recrystallization temperature,
Since no {111} recrystallized texture can be obtained, the average r value
Deteriorates. Furthermore, in the continuous annealing process, the cooling
When the speed is higher than 20 ° C / s, the grain boundary strengthening element B is
Does not sufficiently diffuse into the recrystallized grain boundaries and is sufficient for the purpose of the present invention.
Fatigue characteristics cannot be obtained. For the above reasons, annealing conditions
Is above the recrystallization temperature Ac _ThreeAnneal in the temperature range below
The cooling rate after blunting is 20 ° C./s or less.

[0021]

The present invention will be further described with reference to the following examples. [Example 1] Tables 2 and 3 (continued from Table 2-1), Table 4
A to AL having chemical components shown in (Continued-2 of Table 2)
Steel is iron ore as a raw material, tapped in a blast furnace, melted in a converter, continuously cast, then reheated at a heating temperature of 1150 ° C., and a finish rolling temperature of 910 to 930 ° C. of 6.0 mm. After being rolled into a sheet, it was wound at 710 ° C. After pickling, cold rolling was performed to 0.8 mm (rolling rate 86.7%), continuous annealing line (maximum heating temperature 810 ° C, skin pass rolling rate 0.8%).
Passed through. However, for J, K, L and T steels, after cold rolling, hot dip galvanizing line (maximum heating temperature 82
0 ℃, hot dip galvanized 460 ℃, alloying treatment 520 ℃ ×
The plate was passed for 20 seconds with a skin pass rolling rate of 0.8%). However, the chemical composition is indicated by weight, and C, B and N are by weight ppm.

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

According to the present invention, D, J, N, P,
R, S, T, V, X, Y, Z, AA, AB, AF, A
16 steels of G and AJ. Steels other than the above have chemical compositions outside the scope of the present invention. FIG. 3 illustrates the range of the present invention by the relationship between the B addition amount and the P addition amount. The tensile test as the material evaluation of the annealed plate thus obtained,
The test material was first processed into a No. 5 test piece described in JIS Z 2201, and the test method was described in JIS Z 2241. Further, a fatigue test was conducted using the uniform plane bending fatigue test piece shown in FIG. For the uniform plane bending fatigue test, a PWOG type plane bending fatigue tester manufactured by Tokyo Koki Co., Ltd. is used, and the stress ratio is −1, the repetition rate is 25 Hz, and the load waveform is a sine wave. The test conditions are JIS Z 2273 and JIS. The test method according to Z 2275 was performed. In order to confirm whether or not a predetermined stress was applied to the test piece, strain gauges were attached to the front and back of the test piece and the load waveform was monitored during the test. The test results are shown in Table 5 and Table 6.
(Continuation-1 of Table 5) and Table 7 (Continuation-2 of Table 5). However, in the symbols in the table, YP: Yield strength, T
S: Tensile strength, El: Elongation at break, FL: Fatigue limit, FL /
TS: fatigue limit ratio, rm: average r value.

[0026]

[Table 5]

[0027]

[Table 6]

[0028]

[Table 7]

In accordance with the present invention, steel D, J, N, P,
R, S, T, V, X, Y, Z, AA, AB, AF, A
16 steels of G and AJ. FIG. 4 illustrates the range of the present invention by the relationship between the average r value and the fatigue limit ratio. The comparative steels are out of the scope of the present invention with respect to the fatigue limit ratio, the average r value, or both characteristics, for the reasons described below.
That is, since the steels H and O have a P content higher than the range of the present invention, the average r value is outside the range of the present invention. Steel Q, AI
Has a P content higher than the range of the present invention, and therefore the static strength increases too much, so that the fatigue limit ratio is outside the range of the present invention. Steels K, U, AC, and AK have a P content lower than the range of the present invention and a low fatigue limit increase, so the fatigue limit ratio is outside the range of the present invention. Steel AD has a B content higher than the range of the present invention, and thus the average r value is outside the range of the present invention.
Steel A, B, C, E, F, I, L, M, W, AE, AH,
AL has a fatigue limit ratio outside the scope of the present invention because one or both of the values of B / P and BP are lower than the scope of the present invention. Steel G has both the values of B / P and B · P lower than the range of the present invention, and thus the average r value is outside the range of the present invention.

Example 2 Among the slabs having the chemical compositions shown in Tables 2 to 4, steels D, J, N, P, R, S, T, V,
For X, Y, and Z, cold-rolled-annealed sheets were manufactured under the conditions different from the manufacturing conditions performed in Example 1. First, it was reheated to various temperatures in a heating furnace, rolled to 2.0 to 10.0 mm and wound. After pickling, cold rolling to 0.8 mm, continuous annealing line (skin pass rolling rate 0.8%)
Passed through. However, J-3, J-4, T-3, T-4
And for the steel of T-5, after cold rolling, hot dip galvanizing line (hot dip galvanizing 460 ° C., alloying treatment 520 ° C.
× 20 seconds, skin pass rolling rate 0.8%). For details of manufacturing conditions, see Tables 8 and 9 (continued from Table 8).
Shown in However, in the symbols in the table, SRT: slab heating temperature, FT: finish rolling end temperature, CT: winding temperature,
CRR: cold rolling rate, ST: annealing temperature, SCR: cooling rate after annealing.

In the same manner as in Example 1, a JIS 5 test piece was subjected to a tensile test, and a uniform plane bending fatigue test piece shown in FIG. 1 was subjected to a fatigue test. Table 1 shows the test results.
0 and shown in Table 11 (continued from Table 10). The symbols in the table are the same as those in Tables 5 to 7 of Example 1.

[0032]

[Table 8]

[0033]

[Table 9]

[0034]

[Table 10]

[0035]

[Table 11]

The steels of the present invention are steel D-2, J-1, J-4,
J-6, N-1, N-3, N-4, P-1, P-2, P
-3, R-1, R-2, S-1, T-5, X-1, X-
2, Z-1, Z-2, Z-3 and Z-4. The comparative steels have a fatigue limit ratio and an average r for the following reasons.
Values or both properties are outside the scope of the invention. That is, steel D-3, J-2, J-3, V-2, Y-1,
For Y-2 and Y-3, the finish rolling finish temperature is below the lower limit of the present invention, so the average r value is outside the range of the present invention. Since the winding temperatures of Steels N-2, T-3, and T-4 are below the lower limit of the present invention, the average r value is outside the range of the present invention. The rolling ratios of the steels D-4 and R-3 in the cold rolling are below the lower limit of the present invention, and thus the average r value is outside the range of the present invention. Steel D-1, J-5, S-2, T-1, T
-2 and V-1, since the cooling rate after annealing exceeds the range of the present invention, the fatigue limit ratio is outside the range of the present invention.

[0037]

As described above, the present invention provides a cold-rolled steel sheet having excellent fatigue properties and deep drawability, and a method for producing the same. By using these cold-rolled steel sheets, automobile parts, etc. It can be expected that the fatigue durability will be greatly improved.

[Brief description of drawings]

FIG. 1 is a plan view of a uniform plane bending fatigue test piece subjected to a fatigue test.

FIG. 2 is a diagram showing the influence of the addition amount of B on the fatigue limit ratio, the grain boundary fracture surface ratio of the fatigue fracture surface, and the average r value.

FIG. 3 is a diagram showing the range of the present invention by the relationship between the addition amount of B and the addition amount of P.

FIG. 4 is a diagram showing a range of the present invention by a relationship between an average r value and a fatigue limit ratio.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuo Koyama 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technical Development Division

Claims (2)

[Claims] 1. By weight%, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% or less, P: 0.01 to 0.05%, S: 0.02 % Or less, Al: 0.005 to 0.1%, N: 0.01% or less, and further B is B / P ≧ 0.1, B · P ≧ 4.0 × 10 ⁻⁵ , and B Content of ≦ 0.005%, Ti, Nb, or both, Ti / 48> N / 14, and (Ti / 48 + Nb / 93)> (C / 12 + N / 14 +
S / 32) The fatigue limit ratio is characterized in that it is contained in a range satisfying the following condition and the balance is Fe and inevitable impurities.
A cold-rolled steel sheet having a fatigue property and a deep drawability of 52 or more and an average r value of 1.8 or more. 2. In% by weight, C: 0.005% or less, Si: 0.1% or less, Mn: 0.5% or less, P: 0.01 to 0.05%, S: 0.02 % Or less, Al: 0.005 to 0.1%, N: 0.01% or less, and further B is B / P ≧ 0.1, B · P ≧ 4.0 × 10 ⁻⁵ , and B Content of ≦ 0.005%, Ti, Nb, or both, Ti / 48> N / 14, and (Ti / 48 + Nb / 93)> (C / 12 + N / 14 +
S / 32) Steel containing a content satisfying the following condition, with the balance being Fe and unavoidable impurities, is hot-rolled in a temperature range of Ar ₃ points or higher, and then wound at 700 ° C. or higher, with a rolling ratio of 70%. 9 above
After cold rolling at 5% or less, Ac ₃
Annealing in the temperature range below the point, the cooling rate after annealing is 20 ° C /
A method for producing a cold-rolled steel sheet having a fatigue limit ratio of 0.52 or more and an average r value of 1.8 or more, which is excellent in deep drawability and fatigue characteristics.