JPH0756054B2 - Method for manufacturing cold-rolled steel sheet for processing with excellent fatigue resistance of spot welded joints - Google Patents

Description

TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet suitable for use in applications such as bending, press forming, drawing, deep drawing and ultra deep drawing. In particular, the present invention is intended to improve the fatigue resistance of spot welded joints.

(Prior Art) Cooled steel sheets are widely used as steel sheets for working such as press forming and deep drawing due to their excellent workability. However, because of its usefulness, it is required to have a good material for many properties in addition to workability.
Cold-rolled steel sheets are very often spot-welded, and the automobile assembly process is a good example. The number of spot welding spots for a single passenger car reaches to several thousand. Since stress is concentrated on the welded joint part when a load is applied from the outside, fatigue failure due to repeated stress during running of a vehicle may cause a serious accident. Therefore, the fatigue strength of the spot-welded joint is a very important property in the cold-rolled steel sheet for working.

On the other hand, as the manufacturing process of cold-rolled steel sheets has changed from conventional box annealing to continuous annealing, ultra-low carbon steel has come to be widely used. Extra-low carbon steel may have low fatigue strength in spot welded joints depending on the conditions.

Here, improving the safety of automobiles and the like is now a global theme, and further improving the fatigue strength of spot-welded joints has become an important issue compared to conventional steel sheets.

Techniques similar to the steel sheet for processing of the present invention include, for example, Japanese Patent Laid-Open Nos. 54-135616, 53-52222, and 6-.
1-246344, JP-A-58-25436, JP-A-53-137
021, JP-A-58-110659 and JP-A-57-454
No. 55, etc., but all of them are characterized by the improvement of the mechanical properties of the cold rolled steel sheet, and there is no description about the fatigue strength of spot welded joints.

(Problems to be Solved by the Invention) It is an object of the present invention to obtain a steel sheet which is excellent not only in workability such as press forming but also particularly in fatigue resistance of spot welded joints.

(Means for Solving the Problems) As a result of intensive studies to improve the fatigue resistance of spot welded joints of steel sheets, the inventors have regulated the composition of O, Al, and N within an appropriate range. Find that it is extremely effective in achieving the intended purpose,
It came to complete this invention.

That is, the present invention is C: 0.0028 wt% (hereinafter simply referred to as%), Si: 1.0% or less, Mn: 1.0% or less, P: 0.15% or less, S: 0.0200% or less, O: 0.0045% or less, N : 0.0020% or less and Al: (30 x% N) to 0.15%, and sometimes at least one selected from Nb: 0.001 to 0.025% and B: 0.0002 to 0.0020%, with the balance being Fe and unavoidable. A steel slab consisting of mechanical impurities is hot-rolled at a finishing temperature of 600 ° C or more, then cold-rolled at a rolling reduction of 60% or more, and then subjected to recrystallization annealing at a temperature below the Ac ₃ transformation point. This is a method for manufacturing a cold-rolled steel sheet for processing, which has excellent fatigue resistance of spot-welded joints.

(Function) Although the fatigue characteristics of spot welded joints of cold-rolled steel sheets are extremely important characteristics in products made of steel sheets, there have been few studies on the influence of steel components.

The inventors have conducted extensive research on the effect of steel components on the fatigue properties of spot-welded joints of cold-rolled steel sheets, and have obtained the following findings.

FIG. 1 shows the results of an examination of the relationship between the fatigue limit value (10 million times) in shear tension of a 0.8 mm thick cold-rolled steel sheet, 0 content and the Al / N ratio. Other steel composition and manufacturing conditions are shown in Table 1.
As shown in. Table 2 shows spot welding conditions. The welding current was adjusted so that the nugget diameter was about 5 mm.

In FIG. 1, a region having a higher fatigue limit value than that of the conventional low carbon Al killed box annealed material is shown by hatching.
0.0045% or less and Al / N of 30 or more correspond to the region.

To elucidate the reason for this, an examination of the hardness distribution in the cross section of the welded portion of a sample with a high fatigue limit showed that the difference in hardness from the fusion zone to the heat-affected zone was smaller than that of conventional steel sheets, and the distribution was smooth. It has been found. This is considered to have an advantageous effect on the occurrence and propagation of fatigue cracks due to stress concentration in the joint portion when stress is applied.

Further, it was also found that the fatigue limit value becomes higher when Nb and / or B is added in an appropriate amount.

The reason why the component composition of the raw material is limited to the above range in the present invention will be described below.

C: C must be a very low carbon type which is lower than that of conventional low carbon materials in order to obtain a steel sheet having good elongation and r value.
Further, the lower the C content, the more advantageous the fatigue resistance. Therefore, it is preferable to reduce C as much as possible, but 0.0028%
The following is acceptable. A particularly preferred range is 0.0015% or less.

Si: If Si is present in an amount of more than 1.0%, the elongation and drawability of the steel sheet deteriorate, so the content is made 1.0% or less.

Similar to Si, Mn: Mn is 1.0% or less because excessive addition deteriorates elongation and drawability of the steel sheet.

P: P is limited to 0.15% or less because if it is contained in a large amount exceeding 0.15%, the segregation amount at grain boundaries increases and embrittlement occurs.

S: S is limited to 0.0200% or less because if added in excess of 0.0200%, it causes rapid deterioration of corrosion resistance. However, if it is too low, the descaling property is deteriorated and the surface properties are deteriorated, so 0.0035% or more is preferable.

The range of O: O is particularly important in the present invention, and unless the O content is suppressed to 0.0045% or less, the fatigue resistance characteristics higher than those of conventional steel sheets cannot be obtained. It is considered that this is because the amount of O in the solid solution state or the amount of O in the oxide state affects the occurrence and propagation of fatigue cracks. Particularly preferred range is 0.0035%
It is the following.

If N: N increases, not only the material deteriorates, but also the required amount of Al becomes excessive and the surface quality deteriorates as described later, so the content is made 0.0020% or less. Particularly preferably 0.0017%
It is the following.

The range of Al: Al is also particularly important in this invention,
Al / N in relation to the amount of 30 or more, particularly preferably 35
It is necessary to be above.

It is considered that the distribution state of Al or AlN in the form of a solid solution as a precipitate affects the structure of the weld heat affected zone to improve the fatigue resistance.

However, the addition of too much causes deterioration of the surface properties as described above, so it is necessary to set the content to 0.15% or less.

In addition to what has been described above, in the present invention, for the purpose of further improving the fatigue resistance, Nb: 0.001 to 0.025
%, B: 0.0002 to 0.0020% of 1 type or 2 types can be added.

Next, the manufacturing method according to the present invention will be described.

First, the steel-making method may be carried out according to a conventional method, and the conditions are not particularly limited in the present invention. In hot rolling, if the finishing temperature is less than 600 ° C, the deep drawability deteriorates, so the temperature is set to 600 ° C or higher. In cold rolling, sufficient workability cannot be obtained unless the rolling reduction is at least 60%. The annealing temperature at the time of continuous annealing after cold rolling may be a recrystallization temperature or higher as usual, but it is preferably an annealing temperature of (primary recrystallization temperature + 30 ° C.) or higher. Also, if the annealing temperature exceeds Ac ₃ , coarse grains are formed, so A
It is necessary to anneal at a temperature below c ₃ . Box annealing may be used as the annealing method here.

Note that the temper rolling after annealing may be performed within a normal range (about plate thickness (mm)%) for the purpose of straightening the plate shape.

(Example) Steel slabs having various compositions (No. 1 to 18) shown in Table 3 were prepared.
Finishing temperature: After hot rolling at 850-900 ℃, rolling reduction: 71
It was cold rolled to a thickness of 0.8 mm at ~ 78%, and then continuously annealed in the temperature range of 790 to 830 ° C. No. 18 is a conventional low carbon Al killed steel, which is manufactured by box annealing.

Table 4 shows the results obtained by examining the mechanical properties of the cold-rolled sheet thus obtained and the fatigue limit (10 million cycles) during tensile shearing.
Shown in. The tensile test was performed using JIS No. 5 test pieces. The welding conditions are as shown in Table 2 above.

As is clear from Table 4, all of the cold-rolled steel sheets obtained according to the present invention have not only mechanical properties but also excellent fatigue resistance characteristics as compared with Comparative Examples and Conventional Examples.

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain a cold-rolled steel sheet for working which is excellent not only in formability but particularly in fatigue resistance of spot welded joints.

[Brief description of drawings]

FIG. 1 is a graph showing the effects of O content and Al / N ratio on the tensile shear fatigue limit of spot welded joints.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Inventor Keiji Nishimura 1 Kawasaki-cho, Chiba-shi Kawasaki Steel Co., Ltd. Chiba Works

Claims (2)

[Claims] 1. C: 0.0028 wt% or less, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.0200 wt% or less, O: 0.0045 wt% or less, N: 0.0020 A steel slab containing less than wt% and Al: (30 x% N) to 0.15 wt% with the balance Fe and unavoidable impurities is hot-rolled at a finishing temperature of 600 ° C or more, and then a rolling reduction of 60 % Of cold rolling and then recrystallization annealing at a temperature of Ac ₃ transformation point or lower. A method for producing a cold-rolled steel sheet having excellent fatigue resistance of a spot-welded joint. 2. C: 0.0028 wt% or less, Si: 1.0 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.0200 wt% or less, O: 0.0045 wt% or less, N: 0.0020 wt% or less and Al: (30 x% N) to 0.15 wt%, and at least one selected from Nb: 0.001 to 0.025 wt% and B: 0.0002 to 0.0020 wt% with the balance being Fe and A steel slab consisting of unavoidable impurities is hot-rolled at a finishing temperature of 600 ° C or more, then cold-rolled at a rolling reduction of 60% or more, and then subjected to recrystallization annealing at a temperature not higher than the Ac ₃ transformation point. A method for producing a cold-rolled steel sheet for working, which is excellent in fatigue resistance of a spot welded joint.