JPH08325634A - Dead soft cold rolled steel sheet excellent in fatigue characteristic and its production - Google Patents

Abstract

PURPOSE: To produce a dead soft cold rolled steel sheet for deep drawing excellent in base metal fatigue and fatigue characteristics in the spot weld zone by preparing a steel having a specified componental compsn. in which the contents of P and B are specified. CONSTITUTION: A steel having a compsn. contg., by weight, 0.0005 to 0.0026% C, <=1.2% Sr, 0.03 to 3.0% Mn, 0.015 to 0.15% P, 0.0010 to 0.020% S, 0.005 to 0.1% Al, 0.0005 to 0.0080% N and 0.0003 to 0.0030% B, contg., if necessary, one or more kinds of 0.0002 to 0.0015% Ti and 0.0002 to 0.0015% Nb, and the balance Fe with inevitable impurities is prepd. Thus, the objective dead soft cold rolled steel sheet for deep drawing excellent in base metal fatigue and fatigue characteristics in the spot weld zone can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra low carbon cold rolled steel sheet for deep drawing which is excellent in fatigue properties of a base material and spot welds, and a method for producing the same. The cold-rolled steel sheet according to the present invention is used by press forming in applications such as automobiles, household electric appliances, and buildings, and cold-rolled steel sheet in a narrow sense without surface treatment and for rust prevention, for example. It includes both surface treatment such as Zn plating and alloyed Zn plating, and further cold-rolled steel sheet having an organic film treatment thereon.

[0002]

2. Description of the Related Art With the recent progress in vacuum degassing of molten steel, it is now easy to produce ultra-low carbon steel, and the demand for ultra-low carbon steel sheet having good workability is increasing more and more.
It is well known that such an ultra-low carbon steel sheet generally contains at least one of Ti and Nb. That is, Ti and Nb have a strong attractive interaction with the interstitial solid solution elements (C, N) in steel, and easily form carbonitrides. Therefore, steel without interstitial solid solution elements (IF steel: Interstitial Free)
Steel) is easily obtained. Since the IF steel does not contain an interstitial solid solution element that causes deterioration of strain aging and workability, it has a characteristic of being non-aging and having very good workability. Furthermore, the addition of Ti or Nb also has an important role of improving the deep drawability of the cold-rolled annealed sheet by reducing the grain size of the hot-rolled sheet of ultra-low carbon steel that tends to coarsen. However, Ti
The ultra low carbon steel containing Nb and Nb has the following problems. First, the manufacturing cost is high.

That is, it is necessary to add expensive Ti and Nb in addition to the vacuum treatment cost for extremely low carbonization. Second, since solid solution C and N do not remain on the product plate, secondary processing embrittlement occurs and paint bake hardening characteristics (BH
Sex) disappears. Thirdly, the fatigue properties of the base material and spot welds are inferior. This is because the strength of the material is low because it is an extremely low carbon steel, and the structure of the heat-affected zone subjected to spot welding is easily coarsened to form a fragile portion. Fourthly, Ti and Nb are strong oxide forming elements, and these oxides deteriorate the surface quality.

A lot of research and development has been conducted for the purpose of solving such problems of IF steel. One idea to solve these problems is to base ultra low carbon steel to which Ti and Nb are not added. This is because the first, second, and fourth problems described above are naturally solved by using a steel containing no Ti or Nb as a base. For example, JP-A 63-83230 and JP-A 6-83230
3-72830, JP 59-80724, JP 60-103129, JP 1-18.
No. 4251, JP-A-58-141355, JP-A-6-93376, etc. are examples thereof, and all of them have r-values related to press formability of ultra-low carbon steel sheet containing no Ti or Nb. It focuses on characteristics such as elongation and elongation, BH characteristics, and secondary embrittlement resistance.

However, there are few studies on the third problem, fatigue characteristics. JP-A-63-317625
Japanese Patent Publication discloses a method for producing an ultra low carbon cold rolled steel sheet having excellent fatigue characteristics of a spot weld by optimizing the composite addition of Ti, Nb and B to the ultra low carbon steel and the temper rolling ratio. are doing. However, there is no description about a method for improving the fatigue properties of ultra-low carbon steel that does not contain Ti or Nb. JP-A-6-81043 and JP-A-6-810
No. 44 and Japanese Patent Laid-Open No. 6-81080,
An ultra low carbon steel sheet containing at least one of Ti and Nb, which has excellent fatigue properties and deep drawability, and a method for producing the same are disclosed.

However, although these publications disclose a method of increasing the yield strength and improving the base material fatigue characteristics, none of the joint fatigue characteristics of the spot welded portion is examined. Moreover, only the ultra-low carbon steel to which Ti or Nb is added is targeted, and the Ti targeted by the present invention is
No ultra low carbon steel to which Nb or Nb is not substantially added has been studied. In general, ultra-low carbon steel sheet without addition of Ti or Nb is inferior in base material fatigue property due to low yield strength, and moreover abnormal grain growth is likely to occur due to heat applied during spot welding, resulting in fatigue of joints of spot welds. There is concern about the problem of insufficient characteristics. However, as described above, there is no knowledge of the technology for preventing them.

[0007]

Based on an ultra-low carbon steel that does not use expensive additive elements such as Ti and Nb, while maintaining excellent deep drawability, good base metal fatigue and spot weld fatigue. Establishing a cold-rolled steel sheet having both properties and a method for manufacturing the same is an object to be solved by the present invention.

[0008]

[Means for Solving the Problems] In a simple ultra-low carbon steel sheet that does not use expensive carbonitride forming elements such as Ti and Nb, the steel sheet is easily softened by the pressure from the electrode during spot welding because it is too soft. It was found that the contact resistance between the electrode and the steel plate or the steel plate was excessively reduced due to deformation, and the proper welding current range was narrowed and shifted to the high current side. This has the drawback of increasing the size of the welder. In addition, there is a problem that the base material fatigue property, which has a strong correlation with the yield strength of the base material, deteriorates. As a means for solving these problems, a method of adding P and B was found. This is because the addition of P and B makes it possible to inexpensively and efficiently increase the strength of a steel sheet and increase the electrical resistivity. As a result, the welding current can be maintained on the low current side. Also, fatigue of the base material can be improved.

On the other hand, in an ultra-low carbon steel sheet to which Ti or Nb is not added, abnormal grain growth is likely to occur in the HAZ portion during spot welding, which causes a problem that the joint strength and fatigue properties of the welded portion are deteriorated. As a result of earnest research to solve this problem,
We have found new findings that are extremely effective in the combined addition of P and B above a certain level. Further, in order to sufficiently exert the effect, 1) adjusting B / N> 1 so that solid solution B is present, 2) allowing a very small amount of Ti and / or Nb to be present, and 3) adjusting. Controlling the rolling rate in relation to the C content,
4) In a steel sheet having BH property, BH treatment improves joint strength and fatigue properties of spot welds, so it has been found desirable to impart BH property.

The present invention is constructed on the basis of such an idea and new knowledge, and the gist thereof is as follows. (1) By weight%, C: 0.0005 to 0.0026% Si: 1.2% or less Mn: 0.03 to 3.0% P: 0.015 to 0.15% S: 0.0010 0.020% Al: 0.005 to 0.1% N: 0.0005 to 0.0080% B: 0.0003 to 0.0030% of the base material and spot welds composed of the balance Fe and inevitable impurities Ultra low carbon cold rolled steel sheet for deep drawing with excellent fatigue properties. (2) Ti: 0.0002 to the chemical component described in (1)
0.0015%, Nb: 0.0002 to 0.0015%
An ultra low carbon cold-rolled steel sheet for deep drawing, which is excellent in fatigue properties of a base material and a spot weld, characterized by containing at least one of the above.

(3) A slab comprising the chemical components described in (1) and (2) is hot-rolled at a temperature of Ar ₃ or higher, wound at room temperature to 750 ° C., and cooled at a rolling ratio of 70% or higher. Hot rolling is performed, continuous annealing is performed at an annealing temperature of 600 to 900 ° C., and the temper rolling reduction rate is 1.5 × (1-400 × C)% or more when C is the carbon amount (wt%). 2080
X (C-0.0015)% or more, The manufacturing method of the cold-rolled steel sheet which was excellent in the fatigue characteristics of a base material and a spot welded part characterized by the above-mentioned.

(4) A slab composed of the chemical components described in (1) and (2) is hot-rolled at a temperature of Ar ₃ or higher, and immediately thereafter, within 1.5 s at a cooling rate of 50 ° C./s or higher. Cool to 750 ° C or below and wind at room temperature to 750 ° C.
Cold rolling is performed at a rolling rate of 0% or more, and the annealing temperature is 600
Continuous annealing at ˜900 ° C. and 1.5 × (1-400 × when the temper rolling reduction rate is C is the carbon amount (wt%).
C)% or more and 2080 x (C-0.0015)% or more. A method for producing an ultra low carbon cold rolled steel sheet for deep drawability, which is excellent in fatigue properties of a base material and a spot weld. (5) In the manufacturing methods of (3) and (4), the cold rolling reduction is 84% or more, and the ultra-low carbon cold rolling for deep drawing is excellent in the strength characteristics of the base material and spot welds. Steel plate manufacturing method.

[0013]

First, the experimental results which are the basis of the present invention will be described. FIG. 1, FIG. 2 and FIG. 3 show the results of examining the effect of addition of P and B, which is particularly important in the present invention, on spot weldability and fatigue properties. In this experiment, C: about 0.0013%, Si: 0.01%,
Mn: 0.15%, P: 0.003 to 0.18%, S:
0.008%, Al: 0.075%, N: 0.0018
%, B: <0.0001 to 0.0040% of a simple extra low carbon steel plate was used. Hot rolling heating temperature is 1150
℃, finishing temperature is 920 ℃, 50 within 1.2s
It was quenched at ℃ / s and wound at 500 ℃. Plate thickness 5.0 mm
After pickling, the hot-rolled sheet was cold-rolled to 0.8 mm (reduction ratio = 8
4%), heating rate = 10 ° C./s, retention = 740 ° C. × 5
0 s, cooling = 10 ° C / s continuous annealing was performed, and the rolling reduction was 1.
0% temper rolling was performed.

The base material fatigue was evaluated by subjecting a cold-rolled, annealed and temper-rolled material to a unilateral plane bending fatigue of 25 Hz. Spot weldability is RWMA (Rcsistanc)
e welder Manufacturers' As
4.5 mm with reference to the recommended value
A φ CF type electrode is used, the pressure is 200 kgf, and the energization time is 12 Hz. The proper welding current range is
It is a range from a current (appropriate welding current lower limit value) at which the nugget diameter is 4 × t ^1/2 (t: plate thickness (mm)) or more to a current (appropriate welding current upper limit value) at which dust occurs. The joint fatigue strength was evaluated by examining the shear and cross tensile fatigue strengths of the material spot-welded at a welding current of 95% of the dust-generated welding current value among the above welding conditions.

As is apparent from FIG. 1, the base material fatigue limit of the material containing P in an amount of 0.015% or more and B in an amount of 0.0003% or more and having a repetition number of 2 × 10 ⁶ is:
Conventional Ti-added ultra-low carbon cold-rolled steel sheet used for comparison (% by weight, C: 0.0035%, Si: 0.01%, M
n: 0.15%, P: 0.01%, S: 0.01%, A
1: 0.03%, Ti: 0.045%, B: 0.000
1%, N: 0.0020%) 180 MPa, batch type box annealed low carbon Al-killed cold rolled steel sheet (% by weight, C: 0.035%, Si: 0.01).
%, Mn: 0.15%, P: 0.01%, S: 0.01
%, Al: 0.045%, N: 0.0040%) 20
It is possible to reach the same level as 8 MPa.

Further, as is clear from the result of FIG.
Based on an ultra-low carbon steel with 0.0008% added, if the amount of P added is increased, the proper welding current range widens and shifts to the low current side. If the amount of P added is 0.015% or more, the new knowledge that the proper welding current range is at the same level as the conventional material was obtained. Further, as is clear from FIGS. 3, 4 and 5, when P and B are added in a proper combination or more, H
Since the hardness of the AZ part reaches a level higher than that of the conventional material, not only the joint strength of the spot welded part but also the fatigue property of the spot welded part, which is important in the present invention, is secured and further improved by BH treatment. We have gained new knowledge that is extremely important for the industrialization of ultra-low carbon steel sheets that do not contain Nb.

4 and 5, 2P-3B and 2P-
18B, 8P-3B and 8P-18B are steels of the present invention, and of the above components, 2P and 8P each have a P content of 0.0.
2% P and 0.08% P, and 3B and 18B have B amounts of 0.0003% and 0.0018%. Further, Ti-IF of the comparative steel is an ultra low carbon cold-rolled steel sheet containing the above-mentioned components and containing general Ti and B which are commonly used at present. As described above, the metallurgical reason why the combined addition of P and B improves the base metal fatigue and the spot weldability (including the proper welding current range, joint strength, and fatigue characteristics of the weld) is considered as follows.

In the ultra low carbon steel to which Ti or Nb is not added, C is in a solid solution state and contributes to the strength increase. P
Is an element whose atomic radius is significantly smaller than that of Fe among substitutional solid solution elements, and B is also an interstitial solid solution element.
These effectively increase the yield strength. Further, at the same time, the electric resistance is increased. As a result, it has excellent base material fatigue characteristics. Further, the proper welding current range shifts to the low current side. Further, P is well known as a grain boundary segregation element and has a large interaction with the grain boundary, so that it has an effect of suppressing grain boundary movement and refining the structure. Furthermore, since B has an attractive interaction with C, it suppresses the γ → α transformation in the cooling process after spot welding, and contributes to the refinement of the structure of the HAZ part and the increase in hardness.

In the ultra-low carbon steel sheet to which Ti or Nb is not added, such a PZ and B HAZ part structure refining effect appears synergistically when both coexist. Although the reason is not always clear, γ in the cooling process after spot welding
→ P and B are segregated at the α transformation interface, P reduces the moving speed of the interface as described above, and B suppresses the diffusion of C due to the interaction with C. It is thought that it was suppressed to. As a result, it was speculated that the hardenability of the HAZ part was improved, the hardness was significantly increased, and the spot weldability, joint strength, and fatigue properties were improved. In addition, controlling the amount of C and the rolling reduction of temper rolling within an appropriate range is a problem of ultra-low carbon steel sheet that does not add Ti or Nb, and lowers the appropriate welding current lower limit value during non-aging and spot welding. We gained new knowledge that it is extremely effective in suppressing.

First, the experimental results that form the basis of the present invention will be described. FIG. 6 shows the relationship between the amount of C and the temper rolling conditions that affect the aging property and the lower limit of the appropriate spot welding current.
In this experiment, the amount of C was 0.0003 to 0.0030.
%, Si: 0.01%, Mn: 0.1
5%, P: 0.03%, S: 0.008%, Al: 0.
A simple ultra low carbon steel sheet containing 075%, N: 0.0018%, B: 0.0010% was used. The above sample melted in the laboratory was hot rolled. Hot rolling heating temperature is 1150
The finishing temperature was 920 ° C, and the film was wound at 500 ° C. A hot-rolled sheet having a sheet thickness of 6.0 mm was pickled and cold-rolled to 0.8 mm (reduction rate = 87%), heating rate 10 ° C / s, retention = 740 ° C x 50 s, cooling = 10 ° C / s. Continuous annealing was performed and temper rolling was performed while changing the rolling reduction.

FIG. 6 shows 100 ° C. as an index of aging.
Yield point elongation in a tensile test after accelerated aging for 1 h (YP
-El)) was used. Further, the lower limit value of the appropriate spot welding current was used as an index of spot weldability. The welding conditions are the same as those already described. As is clear from the figure, in order to secure the non-aging property, the rolling reduction is set to 0.3% or more, the C content is 0.0026% or less, and the relationship with the C content is 2080 × (C-0.0015). ) It is necessary to control to the range surrounded by the area of% or more. Further, the lower limit value of the spot welding proper current is 1.5 × (1-400 ×
It can be kept low by controlling C)% or more. Since the amount of solid solution C also increases as the total amount of C increases, the reduction rate required for non-aging is considered to increase. Further, the lower limit value of the proper current for spot welding is related to the yield strength (YP) of the material and shifts to the low current side as YP increases. It is thought that it is preferable to allow it.

Here, the reason why the steel composition and manufacturing conditions are limited as described above in the present invention will be further explained. (1) C: C is an extremely important element that determines the material properties of the product. If the amount of C exceeds the upper limit of 0.0026%, even if the rolling reduction of the temper rolling is controlled, it is no longer non-aged at room temperature, and the aging deterioration of ductility is remarkable, so the upper limit of 0.0
It is set to 026%. On the other hand, if the C content is less than 0.0005%, the fatigue properties of the base material and the spot welds deteriorate. Further, secondary working embrittlement occurs. In addition, it is an extremely difficult area to reach in terms of steelmaking technology, and the cost increases significantly. Therefore, the lower limit is made 0.0005%.

(2) Si: Si is an element that increases strength at a low cost, but if it exceeds 1.2%, problems such as deterioration of chemical conversion treatment property and deterioration of plating property occur, so the upper limit is 1 0.2%. (3) Mn: Mn is an element effective for increasing the strength like Si. Further, in the steel of the present invention to which Ti or the like is not added, Mn fixes S, so Mn has a role of preventing cracking during hot rolling. It has been conventionally said that lowering Mn is preferable for improving the r value, but if the Mn content is less than 0.03%, cracking occurs during hot rolling. Therefore, Mn
The lower limit of the amount is 0.03%. On the other hand, it was found that Mn is effective for refining the crystal grains of the hot-rolled plate of the ultra-low carbon steel containing P as in the present invention. This is because both elements act thermodynamically in the direction of canceling the Ar ₃ temperature, and both elements delay the transformation from γ to α kinetically. It is also effective for making the structure of the spot-welded HAZ portion finer. However, if the content exceeds 3%, the r value, that is, the deep drawability deteriorates. For the above reasons, the upper limit of the amount of Mn is 3%.

(4) P: P is also known as an element that increases strength like Si and Mn, and the amount of addition thereof changes depending on the target strength level. In addition, Ti and Nb
The grain size of the ultra-low carbon steel hot-rolled sheet without addition of Cr generally becomes coarse, but the addition of 0.015% or more of P makes it significantly finer, and the product sheet after cold rolling / annealing Has the effect of improving the deep drawability of. Further, as described above, the addition of P is effective for securing the spot weldability, and the necessary addition amount is 0.015% or more as shown in FIG. On the other hand, if the addition amount exceeds 0.15%, deterioration of cold rollability and secondary work embrittlement occur, so the upper limit of the P amount is made 0.15%.

(5) S: The lower the S content, the better,
If it is less than 0.001%, the manufacturing cost increases significantly, so this is made the lower limit. On the other hand, if it exceeds 0.020%, a large amount of MnS is excessively precipitated and the workability deteriorates.
This is the upper limit. (6) Al: Al is used for deoxidation adjustment, but 0.005
If it is less than%, stable deoxidation becomes difficult. on the other hand,
If it exceeds 0.1%, the cost increases. Therefore, these values are set as the lower limit and the upper limit. (7) N: N is preferably low. But 0.000
If it is less than 5%, the cost is significantly increased, so this is made the lower limit. On the other hand, if it is 0.0080% or more, the workability is significantly deteriorated, so 0.0080% is made the upper limit of the N content.

(8) B: B is an essential element for ensuring the joint strength and fatigue characteristics of the spot welded portion.
In order to exert its effect, addition of 0.0003% or more is necessary. If it is less than 0.0003%, it is not sufficient for the refinement of the structure of the HAZ part. Further, if it exceeds 0.0030%, the addition cost increases and slab cracking is caused, so this is made the upper limit. Furthermore, the addition amount of B is B / N>
1 is preferred. This is due to B
This is because B in a solid solution state that does not form N is effective.

(9) Ti, Nb: In the present invention, basically, these expensive elements are not added, but as a result of intensive studies by the present inventors, at least 1 of Ti and Nb
It was also found that the presence of a very small amount of 0.0002 to 0.0015% of each kind of element improves the material properties of the product plate represented by the r value and the strength and fatigue properties of the spot welds.
The improvement effect is not seen at less than 0.0002%, while on the other hand, in order to stably add more than 0.0015%, the addition cost increases in industrial production, so this is the upper limit.

Next, the reasons for limiting the manufacturing conditions will be described. (9) Hot rolling conditions: Finishing is performed at a temperature of Ar ₃ or higher in order to secure the workability of the product sheet. If the temperature is less than Ar _{3, the} grain size of the hot-rolled sheet will be significantly coarsened, and the deep drawability of the product sheet will be deteriorated. In addition, surface irregularities called ridging occur. In the ultra-low carbon steel to which Ti and Nb are not added, the crystal grain size of the hot-rolled sheet becomes fine when it is rapidly cooled to a temperature of 750 ° C or less at a cooling rate of 50 ° C / s or more within 1.5s after finishing, This is preferable because the deep drawability of the final product plate is improved. Particularly, rapid cooling within 0.5 s is preferable. If the winding temperature exceeds 750 ° C., the pickling property deteriorates and the material becomes non-uniform in the longitudinal direction of the coil, and abnormal grain growth occurs during winding, so 750 ° C. is the upper limit. Further, since the workability of the product plate does not deteriorate even if the winding temperature is lowered to room temperature, this is set as the lower limit value.

(10) Cold rolling condition: The rolling reduction is 70% or more for the purpose of securing the r value of the product sheet. In the case of the ultra-low carbon steel sheet targeted by the present invention, r ₄₅ is remarkably improved and the in-plane anisotropy of r value is reduced when the rolling reduction is 84% or more. Further, this condition is particularly preferable because the structure becomes finer and the spot weldability is improved. (11) Continuous annealing conditions: An annealing temperature of 600 to 900 ° C. is used for continuous annealing. If the annealing temperature is less than 600 ° C, recrystallization is insufficient and the workability of the product sheet becomes a problem. Although the workability improves as the annealing temperature rises, if it exceeds 900 ° C, the temperature will be too high and the plate fracture and the flatness of the plate will deteriorate. In addition, workability and fatigue characteristics are also deteriorated.

(12) Temper rolling conditions: In order to simultaneously secure the non-aging property and spot weldability of an ultra-low carbon steel sheet to which Ti or Nb is not added, the reduction ratio and the C content of the temper rolling should be within proper ranges. The point is to control. The non-aging property has a reduction rate of 0.3% or more and 2080 x (C-0.0015)%.
This can be ensured by controlling the above to a range surrounded by a region where the C content is 0.0026% or less. In addition, the lower limit of the appropriate spot welding current is the reduction ratio of 1.5 × (1-400 ×
It can be kept low by controlling C)% or more and increasing YP. Thus, the present invention is constructed on the basis of a new idea and a new finding, and according to the present invention, it is possible to improve the base material fatigue and the fatigue characteristics of spot welds without adding expensive elements such as Ti and Nb. It is possible to obtain a cold-rolled steel sheet for deep drawing which has excellent BH property at room temperature and is not aged.

[0031]

【Example】

Example 1 A continuous cast slab having the composition shown in Table 1 was heated to 1150 ° C., hot rolling was finished at 920 ° C. to form a 5.5 mm hot rolled sheet, and then 50 ° C./s within 1.0 s. Cool, 60
Winded at 0 ° C. Then, after cold rolling with a reduction rate of 85% to a thickness of 0.8 mm, continuous annealing was performed at 740 ° C.,
The temper rolling was performed with a rolling reduction of 1.2%. Table 2 shows the results obtained by examining the mechanical properties, the base metal fatigue strength, the minimum welding current, and the shear and cross fatigue strength of the spot welds of the steel sheets thus obtained. The spot welding conditions were the same as those described above, and the strength of the spot welded portion was evaluated at a value of 95% of the current value at which the welding current caused dust. Table 1
As is clear from 2 and 2, the steel of the present invention is a cold-rolled steel sheet for non-aging deep drawability which is excellent in base material fatigue and fatigue strength of spot welds. Further, BH property can be imparted by controlling the C content. When a steel plate having BH property is subjected to BH treatment,
The base material fatigue strength and the spot weld joint fatigue strength are further improved. Contrary to this, in the steel which deviates from the scope of the present invention, the base material fatigue strength, the fatigue strength of the spot welds (steel G, H) and r ₄₅ (steel F, G), and 100 ° C-
There is a problem with YP-El (steel F) after 1 h.

[0032]

[Table 1]

[0033]

[Table 2]

Example 2 Using Steel A in Table 1, the same process as in Example 1 was performed until continuous annealing, and then the rolling reduction of temper rolling was 0.5 to 3.
After various changes to 0%, the yield point elongation after artificial aging of each steel sheet at 100 ° C. × 1 h, the lower limit of the appropriate welding current for spot welding and the base material fatigue strength were examined. Table 3 shows the results. The spot welding conditions are the same as those already mentioned.
The welding strength was evaluated by the value of 95% of the current value at which the welding current generated dust. As is clear from Table 3, by controlling the rolling reduction of the temper rolling within the appropriate range of the present invention, both non-aging property and spot weldability and fatigue properties can be achieved.

[0035]

[Table 3]

[0036]

As described above in detail, according to the present invention, T
Even if an expensive element such as i or Nb is not added, a cold-rolled steel sheet for deep drawability excellent in base material fatigue and fatigue characteristics of spot welds can be obtained. Further, non-aging property and BH property can be imparted. These fatigue properties are further improved after BH treatment. Further, the present invention can be applied to a surface-treated steel sheet subjected to electroplating, hot dip plating, etc., and its production. As described above, the present invention provides a steel sheet that is cheaper than the prior art and has excellent utilization characteristics for the user, and the production thereof. Since expensive Ti and Nb are not used,
Contribute to the saving of global resources. Further, since the present invention can also provide a high-strength steel sheet, it is considered that it contributes to global environment conservation by weight reduction, and the effect of the present invention is remarkable.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a base material fatigue limit (2 × 10 ⁶ times) and P and B contents,

FIG. 2 is a diagram showing a relationship between an appropriate current range for spot welding and a P amount;

FIG. 3 is a diagram showing the effect of P and B on the hardness distribution near the HAZ portion after spot welding,

FIG. 4 is a diagram showing the relationship between shear strength and cross tensile strength of spot welded joints and P and B contents.

FIG. 5 is a diagram showing a relationship between joint fatigue characteristics of spot welds and BH properties and P and B contents;

FIG. 6 is a diagram showing the influence of the total C amount and the rolling reduction of temper rolling on the spot weldability (appropriate lower limit of welding current) and aging (YP-El after 100 ° C.-1 h).

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location C22C 38/14 C22C 38/14 (72) Inventor Makoto Makoto 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel shares Company Kimitsu Works

Claims (5)

[Claims] 1. By weight%, C: 0.0005 to 0.0026% Si: 1.2% or less Mn: 0.03 to 3.0% P: 0.015 to 0.15% S: 0.0. 0010 to 0.020% Al: 0.005 to 0.1% N: 0.0005 to 0.0080% B: 0.0003 to 0.0030% and a base metal and a spot weld consisting of the balance Fe and unavoidable impurities Ultra-low carbon cold-rolled steel sheet for deep drawing with excellent fatigue properties of the welded part. 2. The chemical composition according to claim 1, wherein Ti: 0.
0002-0.0015%, Nb: 0.0002-0.
An ultra low carbon cold-rolled steel sheet for deep drawing, which is excellent in fatigue properties of a base material and a spot weld, characterized by containing at least one of 0015%. 3. A slab composed of the chemical components according to claim 1 and 2 is hot-rolled at a temperature of Ar ₃ or higher, wound at room temperature to 750 ° C., and cold-rolled at a rolling ratio of 70% or more. If the annealing temperature is 600 to 900 ° C. and the temper rolling reduction ratio is C (carbon amount (wt%)), 1.5 × (1-400 × C)% or more and 2080 ×
(C-0.0015)% or more, The manufacturing method of the cold-rolled steel sheet which was excellent in the fatigue characteristics of the base material and the spot welds, which is characterized by being above (C-0.0015)%. 4. A slab made of the chemical composition according to claim 1 or 2 is hot-rolled at a temperature of Ar ₃ or higher, and immediately thereafter, within 1.5 s, at a cooling rate of 50 ° C./s or higher, 75
It is cooled to 0 ° C. or lower, wound at room temperature to 750 ° C., cold rolled at a rolling ratio of 70% or more, and an annealing temperature of 600 to 90.
1.5 × (1-400 × C)% when the temper rolling reduction rate is 0 ° C. continuous annealing and C is the carbon amount (weight%).
Above and 2080 x (C-0.0015)% or more, a method for producing an ultra low carbon cold rolled steel sheet for deep drawability excellent in fatigue properties of a base material and a spot weld. 5. The manufacturing method according to claim 3 or 4,
A method for producing an ultra-low carbon cold-rolled steel sheet for deep drawing, which is excellent in strength characteristics of a base material and a spot weld, characterized by having a cold rolling reduction of 84% or more.