JP3391625B2 - Cold rolled steel sheet excellent in deep drawability and fatigue properties of spot welded joint and method for producing the same - Google Patents

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 having excellent deep drawability and fatigue properties of spot-welded joints, and a method for producing the same, and in particular, requires extremely severe formability and at the same time is good. The present invention relates to a cold-rolled steel sheet having excellent deep drawability and excellent fatigue properties of spot-welded joints, which are suitable for automobile parts and the like that require the fatigue properties of various spot-welded joints, and a manufacturing method thereof.

[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. At the same time, with the development of vacuum degassing technology such as RH and DH, ultra-low carbon steel with reduced solid solution elements in the steel and improved formability has been developed. It came to be used instead of steel. Furthermore, recently, Ti, Nb
As a steel in which solid solution C and N in the steel are dramatically improved by scavenging the solid solution C and N, the interstitial ato disclosed in JP-A-1-225727 and JP-A-2-34722 is disclosed.
The ms 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. In addition, spot welding is often used for joining such inner and outer plate components, and the number of spot welding spots of a single passenger car reaches several thousand. In particular, since the spot-welded joint portion is easily subjected to stress concentration due to its shape, there is a risk that it may become a starting point of fatigue fracture due to repeated load due to vibration during traveling of the automobile. In general, the fatigue properties are proportional to the tensile strength and yield strength of the material, but the fatigue properties of spot welds are difficult to sort by static strength due to their shape and the local temperature history received by welding. Therefore, special consideration must be given to the material design.

For example, in Japanese Examined Patent Publication No. 3-56301, the hardness distribution in the vicinity of the welded joint after spot welding is improved by mixing an unrecrystallized structure in the metal structure of the original plate before spot welding in an amount of 5 to 30%. A method of optimizing to improve the fatigue strength of a spot welded joint is disclosed. In addition, Japanese Patent Publication No. 7-56054 discloses that the amount of O and Al (%)
A technique is disclosed in which the value of / N (%) is specified, and particularly when Al / N ≧ 30, the structure of the welded portion is improved to improve the fatigue strength. In addition, Japanese Patent Publication No.
There is disclosed a technique for improving the fatigue strength by adding i, Nb and B in a combined manner to make the spot welds finer.

[0005]

However, in the technique disclosed in the above Japanese Patent Publication No. 3-56301, an unrecrystallized structure harmful to the formability such as deep drawability and overhanging property is an essential requirement. Therefore, it is unsuitable for application to the interior and exterior plate parts of automobiles that require high press-formability. Further, in the technique disclosed in Japanese Examined Patent Publication No. 7-56054, in order to satisfy the condition of Al / N ≧ 30, it is necessary to reduce N or increase the added amount of Al, and N is extremely limited. If it is too small, it leads to an increase in steelmaking cost, and an increase in Al leads to deterioration of the surface properties, which is not preferable.

Further, in the technique disclosed in Japanese Patent Publication No. 5-57330, although the fatigue limit can secure characteristics equal to or higher than those of the conventional low carbon steel, the time strength in a low cycle of 1,000,000 times or less is I can't improve. Therefore, it is necessary to increase the skin pass rolling rate in order to improve the time strength in a low cycle, but it is not preferable to perform excessive cold rolling after annealing because it deteriorates the material such as elongation. .

Therefore, the present invention has developed a technique capable of advantageously solving the above-mentioned problems and improving fatigue characteristics of spot welded joints without impairing press formability such as deep drawability, and forming extremely severe forms. Of cold-rolled steel sheet with excellent deep drawability and excellent fatigue characteristics of spot welded joints, which can be applied to automobile parts that require good fatigue characteristics of spot welded joints at the same time. The purpose is to do.

[0008]

The inventors have conducted many years of research in order to improve the fatigue properties of spot welded joints. By this research, low carbon A which is a conventional steel
The starting point of fatigue fracture of l-killed steel is the base metal part, whereas that of IF steel is HAZ at the tip of sheet separation.
(Heat Affected Zone: heat affected zone)
Was found (see FIG. 1). The inventors have come to think that the cause is the relative relationship between the stress field at the tip of the sheet separation and the strength of the HAZ at the stress concentration point, and as a result of intensive studies, the inventors have found that in order to strengthen the HAZ, P It was found that the combined addition of B and B is effective. That is, the combined addition of P and B strengthens the HAZ, the origin of fatigue cracks moves from the HAZ to the base metal part, and the fatigue limit of the spot welded joint is low carbon Al killed steel manufactured by batch annealing (hereinafter BAF- It has been confirmed that it is equivalent to Al killed steel).

When the rupture position (crack starting point) moves to the base metal part, the time strength and fatigue life at a finite number of repetitions largely depend on the crack propagation characteristics of the base metal. The base material fatigue property of IF steel is inferior to that of BAF-Al killed steel, and it is also necessary to improve the base material fatigue property in order to improve the time strength of the spot welded joint of IF steel. The inventors investigated the fracture surface of the base material fatigue fracture of the conventional steel BAF-Al killed steel and the IF steel of the base material fatigue fracture, and found that the BAF-Al killed steel was an intragranular fracture surface. On the other hand, it was discovered that that of IF steel is a grain boundary fracture surface.

Therefore, as a result of further research, the inventors of the present invention paid attention to the grain boundary segregating elements P and B, and as a result, the grain boundary fracture surface in fatigue fracture was reduced and the fatigue characteristics were improved according to the amount of B added. The experimental results show that the fatigue properties are further improved by adding P and an appropriate amount of P after adding B, and that the properties are equivalent to those of 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. Steels having the chemical composition shown in Table 1 (excluding steel b) were heated at 1150 ° C. for 1 hour and then finished at a temperature (FT) of 9
It was hot rolled at 20 ° C and wound at 700 ° C. After pickling, cold rolling is performed at a rolling rate of 80%, and continuous annealing equipment is applied at 810 ° C.
1 minute, then cooled at a cooling rate of 20 ° C./s, and 1% skin pass rolling was performed. For steel a, after heating at 1230 ° C for 1 hour, finish temperature (FT) 87
Hot rolling at 0 ° C., winding at 550 ° C., pickling, cold rolling, and box annealing furnace (BAF: Box Anneal)
ing Furnace) at a maximum furnace temperature of 720 ° C., and 1% skin pass rolling was performed.

[0012]

[Table 1]

In the tensile test for evaluating the material quality of the thus obtained annealed sheet, the test material was first tested according to JIS Z 2
Processed into No. 5 test piece described in 201, JIS Z 224
The test method described in 1 was used. Furthermore, JIS Z
According to the fatigue test method for spot-welded joints described in 3138, a test plate having a plate thickness of 0.8 mm, a length of 140 mm and a width of 40 mm was welded under the spot welding conditions shown in Table 2,
A test body as shown in FIG. 2 was created. And also J
According to the test method described in IS Z 3138, a fatigue test was performed under the fatigue test conditions shown in Table 3.

[0014]

[Table 2]

[0015]

[Table 3]

FIG. 3 shows the hardness distribution of the spot-welded joints of the steel plates shown in Table 1. Except for the low carbon Al killed steel of Steel B, it is IF steel. Focusing on the hardness distribution of HAZ, I
Of the F steels, only the steel D in which P and B are added together has the same hardness as the low carbon Al killed steel. The steel C containing only B and the steel containing no P or B have a lower HAZ hardness than the low carbon Al-killed steel. Figure 4 shows the results of fatigue tests on spot welded joints. Among IF steels, P and B
Steel B to which B is not added has a low fatigue strength and a low time strength in a finite life region as compared with Steel B which is a low carbon Al killed steel. On the other hand, the steel D in which P and B are added in combination shows the fatigue limit and the time strength which are equal to or higher than those of the steel A which is a low carbon Al killed steel.

The present invention is constituted by the above findings, and the gist thereof is (1) C ≦ 0.005% and Si ≦ 0.1% by weight.
%, Mn ≦ 0.5%, P = 0.027 to 0.05%, S
≦ 0.02%, Al = 0.005 to 0.1%, N ≦ 0.
01% included, and further, B, B · P ≧ 4.0 × 1
0 ^-5 and incorporated within a range that satisfies B ≦ 0.005%, further, also, Ti, or Ti and Nb, Ti / 48,
> N / 14 and (Ti / 48 + Nb / 93)> (C /
12 + N / 14 + S / 32), and the balance is Fe and inevitable impurities, and the average r value is 1.8 or more and the average hardness of the heat-affected zone of the spot weld is Is a cold-rolled steel sheet with a Vickers hardness of 160 or more and excellent deep drawability and fatigue properties of spot welded joints.

(2) C ≦ 0.005% by weight%, S
i ≦ 0.1%, Mn ≦ 0.5%, P = 0.027 to 0.
05%, S ≦ 0.02%, Al = 0.005-0.1
%, N ≦ 0.01%, and further B is B · P ≧
The content is 4.0 × 10 ⁻⁵ and B ≦ 0.005%, and Ti or Ti and Nb are added.
Ti / 48> N / 14, and (Ti / 48 + Nb / 9
3)> (C / 12 + N / 14 + S / 32) is contained in a range satisfying the condition of (C / 12 + N / 14 + S / 32) and the balance is Fe and unavoidable impurities. After hot rolling in a temperature range of ₃ or more points of Ar, 70
After winding at 0 ° C or higher and cold rolling at a rolling ratio of 70% or higher and 95% or lower, annealing is performed in a temperature range of the recrystallization temperature or higher and the Ac ₃ point or lower, and the cooling rate after annealing is 20 ° C / s or lower. The average r value is 1.8 or more, and the average hardness of the heat-affected zone of the spot weld is 160 Vickers hardness.
The above is a method for producing a cold-rolled steel sheet excellent in deep drawability and fatigue properties of spot welded joints. The present invention described above is also applicable to hot-dip galvanized steel sheets.

Next, the reasons for limiting the chemical components of the present invention will be explained. 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 C 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 0.1%
If it is added excessively, not only is it not suitable for processing due to the remarkable solid solution hardening, but also the average r value is deteriorated and the adhesion of the hot-dip galvanizing is deteriorated, so it is made 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 was made 0.5% or less.

P is one of the most important elements in the present invention. As will be described later, P is a spot-welded joint HA without increasing the static strength by adding B together.
The hardness of Z can be improved and the fatigue characteristics of spot welded joints can be increased. On the other hand, P has a risk of segregating at the grain boundaries and causing intergranular cracks that are the starting point of fatigue fracture. Therefore, when the starting point of the fatigue crack moves to the base metal part instead of HAZ, The time strength may be reduced. In the present invention, B corresponding to the amount of P is added to secure the intended time strength in the finite life region. But P
Addition of more than 0.05% not only causes an increase in static strength, which is not preferable for formability, but also a large amount of B must be added to obtain a target fatigue limit ratio.
It causes deterioration of the value. From the above, in the present invention, the content of P is
It is limited to 0.027 % or more and 0.05% or less.

If S is too large, it not only causes cracks during hot rolling but also deteriorates the average r value, so it should be reduced as much as possible, but 0.02% or less is an allowable range. Al is 0.005 for deoxidation of molten steel.
%, But if added in a too large amount, not only the non-metallic inclusions increase and the ductility deteriorates, but also the cost increases, so the upper limit is made 0.1%. N is C
Similarly, since it is an element that is not preferable for the average r value,
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
At ^{x10 -5} , the desired HAZ hardness of the spot-welded joint cannot be obtained. On the other hand, if the B content exceeds 0.005%, not only the increase in the HAZ hardness of the spot-welded joint 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 B · P ≧
It is limited to 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 formability. Further, in particular, Ti is T in the high temperature region of the γ phase.
Although it precipitates as iN, if Ti / 48 ≦ N / 14, the solid solution N is not sufficiently fixed as TiN, and the remaining solid solution N precipitates as BN, so that B effective for fatigue characteristics cannot be secured. Furthermore, if Ti is added in excess of 0.03%, the adhesiveness of the hot-dip galvanizing deteriorates 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. So T
The content of i or Ti and Nb is Ti / 48> N /
14, (Ti / 48 + Nb / 93)> (C / 12 + N /
14 + S / 32). Preferably, both Ti and Nb are 0.03% or less. In the present invention, the component other than the above is Fe, but the inclusion of inevitable impurities mixed from the melting raw material such as scrap is allowed.

Next, the reasons for limiting the manufacturing method of the present invention will be described in detail below. The present invention, the additive element is added to the molten steel in the form of a simple substance or a master alloy so as to have the target component content, and the slab obtained by casting is directly sent to a hot rolling mill as a high temperature cast piece. Alternatively, hot rolling may be performed after cooling to room temperature and reheating in a heating furnace. However,
In the case of reheating, if heated above 1150 ° C, Ti ₄ C
₂ Solid solution C in which precipitates such as S ₂ are re-dissolved and inhibit formation of {111} recrystallized texture which is favorable for formability.
Therefore, the heating temperature is preferably 1150 ° C. or lower.

The hot rolling process needs to 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 occurs near the surface layer of the hot-rolled sheet, and the grain size near the surface layer becomes coarse and press formability deteriorates. If the winding temperature after finish rolling is less than 700 ° C., C which is solid-soluted in the hot-rolled sheet
However, TiC, NbC and the like cannot be sufficiently coarsened and finely precipitate. In that case, the formation of a {111} recrystallized texture that is favorable for formability is hindered in the subsequent annealing step, and the average r value deteriorates.
Therefore, the winding temperature is 700 ° C. or higher. In the cold rolling process after pickling, cold rolling with a rolling ratio of 70% or more and 95% or less is performed. If the rolling ratio is less than 70%, recrystallization is not sufficiently performed in the subsequent annealing step, so that the average r value deteriorates. Further, even if cold rolling is performed at a rolling ratio of more than 95%, the average r value is rather reduced. Therefore, the rolling ratio in the cold rolling process is 70
% To 95%.

The annealing process in the present invention may be either continuous annealing or batch annealing. However, if it is annealed at a temperature of Ac ₃ point or higher, it will be annealed in the γ region or the two-phase region of α + γ, and the formation of the {111} recrystallized texture which is favorable for formability will be significantly hindered. The target average r value cannot be obtained. Further, even if annealing is performed at a temperature lower than the recrystallization temperature, the {111} recrystallization texture cannot be obtained, so that the average r value is deteriorated. Further, when the cooling rate is higher than 20 ° C./s in the continuous annealing process, B, which is a grain boundary strengthening element, does not sufficiently diffuse into the recrystallized grain boundary, and the sufficient HAZ hardness aimed at by the present invention is obtained. I can't get it. For the above reasons, the annealing condition is that the annealing is performed in the temperature range of the recrystallization temperature or higher and the Ac3 point or lower, and the cooling rate after annealing is 20 ° C./s or lower.

[0027]

EXAMPLES The present invention will be further described below with reference to examples. (Example 1) Steels A to Y having the chemical components shown in Table 4 were prepared by tapping iron ore as a raw material in a blast furnace, melting it in a converter, and continuously casting it at a heating temperature of 1150 ° C. Reheat, 910 ℃ ~ 930
After rolling to 6.0 mm at the finishing rolling temperature of ℃, 710
It was wound up at ℃. After pickling, cold rolling was performed to 0.8 mm (rolling rate 86.7%), and continuous annealing line (maximum heating temperature 810 ° C., skin pass rolling rate 0.8%) was passed. However, the chemical composition is indicated by weight% and C, B and N are ppm by weight.

[0028]

[Table 4]

According to the present invention, steels A, D, E, H,
There are 13 steel types, J, K, N, P, R, S, T, W, and Y. FIG. 5 illustrates the range of the present invention by the relationship between the B addition amount and the P addition amount. Steels other than the above have chemical compositions outside the scope of the present invention. In the tensile test for evaluating the material quality of the annealed plate thus obtained, the test material was first tested according to JIS
Processed into No. 5 test piece described in Z 2201, JIS Z
The test method described in 2241 was used. Further, spot welding was performed under the welding conditions shown in Table 2 and a fatigue test was conducted using the tensile shear fatigue test piece shown in FIG. For the tensile shear fatigue test, a 10t test star electrohydraulic servo-type fatigue tester manufactured by MTS, USA was used, and a stress ratio R = 0.05,
Repetition rate = 7 to 15 Hz, load waveform: JIS Z 2273 and JIS Z 313 under sine wave test conditions.
It carried out by the test method according to 8. 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. However, in the symbols in the table, YP: yield strength, TS: tensile strength, El: breaking elongation, Hv: Vickers hardness of HAZ, FL: fatigue limit,
rm: Average r (Rankford) value.

[0030]

[Table 5]

According to the present invention, steels A, D, E, H,
There are 13 types of steel: J, K, N, P, R, S, T, W, and Y. FIG. 6 illustrates the range of the present invention by the relationship between the average Vickers hardness of HAZ and the average r value. 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, in steels I, L, Q, U, and X , since the Si content is higher than the range of the present invention, the average r value is outside the range of the present invention. steel
Since B, C, G, L, M, U, and V 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
Since O has a B content higher than the range of the present invention, O has an average r
Values are outside the scope of the invention. Steels B, F, G, O and Q are
Since the values of B and P are lower than the range of the present invention, the Vickers hardness of HAZ is out of the range.

Example 2 Of the slabs having the chemical compositions shown in Table 4, steels A, D, E,
Cold rolled-annealed sheets were manufactured under the conditions different from the manufacturing conditions performed in Example 1 for the four steel types H. 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. Details of manufacturing conditions are shown in Table 6.
However, in the symbols in the table, SRT: slab heating temperature, FT: finish rolling end temperature, CT: winding temperature, CR
R: cold rolling rate, ST: annealing temperature, SCR: cooling rate after annealing. Further, as in Example 1, a JIS 5 test piece was subjected to a tensile test, spot welding was performed under the welding conditions shown in Table 2, and a fatigue test was performed using the tensile shear fatigue test piece shown in FIG. It was The test results are shown in Table 7. The symbols in the table are the same as those in Table 5 of Example 1.

[0033]

[Table 6]

[0034]

[Table 7]

The steels of the present invention are steels A-2, D-1, E-5,
H-1, H-2, H-3, and H-4 . 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 heating temperatures of Steels A-4, D-2, E-1, and E-2 are higher than the range of the present invention, the average r value is outside the range of the present invention. In Steel A-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 A-3, E-3, and E-4 were below the lower limit of the present invention, the average r value was outside the range of the present invention. Steels A-4, E-2, and E-3 have rolling ratios in cold rolling less than the lower limit of the present invention, and thus the average r value is outside the range of the present invention. Steel A-1, D-2, E-1,
Since the cooling rate after annealing of E-2 exceeds the range of the present invention, the fatigue limit ratio is outside the range of the present invention.

[0036]

As described above, the present invention provides a cold-rolled steel sheet excellent in deep drawability and fatigue properties of a spot-welded joint and a method for producing the same. By using these cold-rolled steel sheet, It is expected that the fatigue durability of parts will be greatly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a crack initiation position in a tensile shear joint fatigue test piece.

FIG. 2 is a drawing showing a tensile shear fatigue test piece.

FIG. 3 is a diagram showing a Vickers hardness distribution near a spot weld.

FIG. 4 is an L-N showing the fatigue characteristics of the present invention together with comparative examples.
It is a curve.

FIG. 5 is a diagram showing the range of the present invention by the relationship between the added amount of B and the added amount of P.

FIG. 6 is a diagram showing the range of the present invention by the relationship between the average Vickers hardness of HAZ and the average r value.

Continuation of front page (56) Reference JP-A-5-117758 (JP, A) JP-A-2-163323 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38 / 00-38/60

Claims (2)

(57) [Claims] 1. In weight%, C ≦ 0.005%, Si ≦
0.1%, Mn ≦ 0.5%, P = 0.027 to 0.05
%, S ≦ 0.02%, Al = 0.005 to 0.1%, N
≦ 0.01%, and further, B is B · P ≧ 4.0 ×
10 ⁻⁵ and B ≦ 0.005% is contained in the range,
Furthermore, Ti or Ti and Nb are replaced with Ti / 4.
8> N / 14 and (Ti / 48 + Nb / 93)> (C
/ 12 + N / 14 + S / 32) is contained in a range satisfying the condition of (12 + N / 14 + S / 32), and the balance is Fe and unavoidable impurities. A cold-rolled steel sheet with a Vickers hardness of 160 or more and excellent deep drawability and fatigue properties of spot welded joints. 2. In% by weight, C ≦ 0.005%, Si ≦
0.1%, Mn ≦ 0.5%, P = 0.027 to 0.05
%, S ≦ 0.02%, Al = 0.005 to 0.1%, N
≦ 0.01%, and further, B is B · P ≧ 4.0 ×
10 ⁻⁵ and B ≦ 0.005% is contained in the range,
Furthermore, Ti or Ti and Nb are replaced with Ti / 4.
8> N / 14 and (Ti / 48 + Nb / 93)> (C
/ 12 + N / 14 + S / 32) is contained in a range satisfying the condition, and the balance is Fe and unavoidable impurities.
After hot rolling in a temperature range of r ₃ points or higher, coiling at 700 ° C. or higher and cold rolling at a rolling ratio of 70% or higher and 95% or lower, in a temperature range of recrystallization temperature or higher and Ac ₃ point or lower. Annealed,
A deep drawability having an average r value of 1.8 or more and an average hardness of a heat-affected zone of a spot welded portion of 160 or more in Vickers hardness, characterized in that a cooling rate after annealing is set to 20 ° C./s or less. And method for producing cold-rolled steel sheet with excellent fatigue characteristics of spot welded joints.