JP3243589B2 - Cold rolled steel sheet having extremely excellent press formability and excellent weld joint fatigue properties, 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 fatigue strength at a weld portion and extremely excellent press formability, and a method for producing the same.

[0002]

2. Description of the Related Art Cold rolled steel sheets having improved workability, particularly deep drawability, are mainly used for press working on inner and outer plates of automobiles. Therefore, conventionally, a cold-rolled steel sheet has been manufactured by a component and a manufacturing method that are optimal for press formability. As a manufacturing technique of such a method, there is, for example, JP-A-59-193221.

However, these prior arts hardly consider the fatigue strength of spot welded joints, which is important together with formability. In automobiles, stamped parts are joined by spot welding. Therefore, the fatigue strength of spot welds is an important evaluation criterion for steel sheets used for automobiles. The spot weldability is described in
Although disclosed in Japanese Patent Publication No. 11057,
It concerns the stability of spot welding by controlling the thickness of the surface oxide and does not mention fatigue strength. JP-A-63-317647 discloses a cold rolled steel sheet in which Ti, Nb, and B are added to ultra-low carbon steel, and fine precipitates having a particle size of 0.05 μm or less are uniformly dispersed in an amount of 30 ppm or more in terms of Ti. And a method for producing the same. But,
In this method, it is necessary to finely disperse the precipitates, so that the continuous casting conditions are restricted, and the recrystallization temperature is high, high-temperature annealing is required, and even in such a case, the press working characteristics are insufficient. It is.

[0004]

From the viewpoints of workability, in particular, deep drawability during press forming and shape freezing, ultra-low carbon steels have been developed with the aim of increasing elongation and r-value and lowering the yield point. Is being developed. However, these steel sheets have a problem that the fatigue strength of the spot welded joint is inferior to the conventional steel. On the other hand, JP-A-63-317647 discloses a technique for improving spot weldability with ultra-low carbon steel, but does not disclose any means for improving press workability.

The present invention advantageously solves the above problems, and has as its object to provide a cold rolled steel sheet having extremely excellent press formability and improved spot welded joint fatigue strength, and a method for producing the same. Things.

[0006]

Means for Solving the Problems Press formability is improved by extremely low carbon, and r value and elongation are increased. Spot weld joint fatigue strength is governed by the residual stress around the spot weld. That is, the fatigue strength is improved by increasing the compressive residual stress around the weld point. That is, if a residual stress of compression can be applied around the spot welding point even in extremely low carbon steel, the press formability is good and the fatigue strength of the spot welded joint can be increased.

[0007] The present inventors have found that the application of compressive residual stress around the spot weld point causes B to be between 0.0003 and 0.004.
It has been found that adding 0% is possible. Simply B
Only by adding it, the steel sheet is hardened by increasing the recrystallization temperature, so that a steel sheet having extremely excellent press formability cannot be manufactured. However, it has been found that this problem can be overcome by controlling the size distribution of Ti precipitates such as TiN even when B is added. That is, the problem can be solved by setting the amount of the Ti precipitate having a particle size of 0.01 μm or less to 0.0050% or less. This advantageous measure can be achieved by regulating the hot rolling heating temperature and the hot rolling temperature range.

[0008] The present invention has been made based on the above findings, and the gist thereof is as follows. (1) By weight%, C: 0.0015% or less, Mn: 0.
05 to 0.50%, P: 0.015% or less, Al: 0.
010 to 0.075%, N: 0.0040% or less, T
i: 0.040 to 0.080%, B: 0.0003 to
Containing 0.0040% and a balance of Fe and unavoidable impurities, of Ti precipitates are precipitated in the steel, the grain
A cold-rolled steel sheet having extremely excellent press formability and excellent weld joint fatigue characteristics, characterized in that fine Ti precipitates having a diameter of 0.01 μm or less are dispersed in an amount of 0.0050% or less in terms of Ti amount.

(2) C: 0.0015% or less, Mn: 0.05 to 0.50%, P: 0.015% or less, Al: 0.010 to 0.075%, N: 0.004
0% or less, Ti: 0.040 to 0.080%, B: 0.
A steel slab containing 0003 to 0.0040% and having a balance of Fe and unavoidable impurities is heated at 1180 ° C or less, and the cumulative draft at 1000 ° C or less is 90 % or more;
Hot rolling is performed at a finishing temperature of 900 ° C. or higher, then cold rolling is performed, and annealing is performed.
A method for producing a cold-rolled steel sheet having extremely excellent press formability and excellent weld joint fatigue characteristics, wherein the fine precipitate is reduced to 0.0050% or less in terms of Ti.

Hereinafter, the present invention will be described specifically. The fatigue strength of ultra low carbon steel spot welded joints is governed by the residual stress around the weld point and is not affected by the microstructure or coarseness of the weld structure, but is improved by increasing the compressive residual stress . The residual stress at the time of welding is introduced by transformation strain based on a structural change due to a heat cycle at the time of welding. That is, a compressive residual stress is generated around the nugget part due to expansion accompanying the low-temperature transformation. The low-temperature transformation organization of the nugget is obtained by adding 0.0003 to 0.0040% of B, and the coarser the structure of the nugget portion and the heat-affected portion before transformation, the more precipitates such as TiN etc. which become transformation nuclei. The lower the content, the easier it is to obtain a low-temperature transformation structure, which is advantageous from the viewpoint of fatigue strength.

That is, the addition of a small amount of Ti or Nb for making the structure finer is disadvantageous from the viewpoint of improving the weld fatigue strength. For this reason, in the present invention, the C content is specified to be 0.0015% or less, and the Ti content is specified to be 0.040% or more, so that the precipitate is made as coarse as possible. Specifically, the amount of fine Ti precipitates having a particle size of 0.01 μm or less is controlled to 0.0050% or less in terms of Ti. This advantageously acts to increase the r-value and elongation, which are indicators of the press workability of the cold-rolled steel sheet.

Hereinafter, the reason why the size distribution of the Ti precipitate affects the fatigue strength of the spot welded joint and the reason why the size distribution is specified will be described. Ti, C, N,
A steel ingot with various B contents was melted to form a steel ingot, 4.0 mm
Hot rolled to a thickness at a finishing temperature of 900 ° C.
Cold rolling was performed to a thickness of 80 mm, recrystallization annealing was performed at 800 ° C. for 1 minute, and 1.5% temper rolling was performed. The material properties of this steel sheet were investigated. The steel sheet was spot-welded, and its fatigue strength and precipitate size distribution were examined with an electron microscope. The spot welding was performed with a welding current at a pressure of 200 kg and a nugget diameter of 4.5 mmφ. The fatigue test was performed with a shear tensile type. Other conditions are JISZ3
138.

FIG. 1 shows the relationship between the amount of Ti precipitates of 0.01 μm or less converted to the amount of Ti, the spot welding fatigue strength, the r-value of the steel sheet, and the elongation of the steel containing 10 ppm of B. It is. From FIG. 1, it can be seen that as the amount of precipitates of 0.01 μm or less increases, the fatigue strength decreases, and the r value and elongation, which are indicators of deep drawability, also decrease. Based on this experimental fact, the size distribution of the precipitate was specified.

Next, the relationship between the precipitate distribution and the steel composition will be described. C needs to be 0.0015% or less in order to obtain a steel sheet having extremely excellent press workability.
In the prior art, the spot welded joint fatigue strength decreases as the C content decreases, but in the present invention, there is no concern. A preferable range for obtaining excellent press workability is 0.0010.
% Or less.

Mn is an element effective for improving the fatigue strength of spot welded joints. To obtain this effect, 0.05
% Must be added at a minimum. However, when the amount of addition increases, press formability deteriorates, and it becomes impossible to obtain extremely excellent press formability, which is one of the objects of the present invention.
Was specified as 0.50%. P is an impurity element, and the workability is degraded when the added amount is large, so that it is preferable to reduce P as much as possible. For this reason, the amount of P is set to 0.0
It was specified to be 15% or less. A preferable range is 0.010% or less for the same reason.

S is an element causing hot embrittlement,
In the present invention, Mn and Ti, which are sulfide-forming elements, are added, so there is no need to worry about this. However, as the amount added increases,
Since it is necessary to increase the amount of these additions and the alloy cost increases, the upper limit of S is preferably set to 0.015%. Al has a large deoxidizing effect, and at least 0.0100% is required to increase the yield of Ti addition. on the other hand,
If the addition amount is too large, the steel sheet is hardened, which has an adverse effect on press formability. Therefore, it is necessary to suppress the content to 0.080% or less.

N, like C, softens the steel sheet and adversely affects the r value and elongation. Therefore, it is preferable to reduce N as much as possible. If N: exceeds 0.0040%, press formability starts to significantly deteriorate, so the upper limit was specified to 0.0040%. The preferred range is 0.0025% for the same reason.
It is as follows. Ti is one of the important components of the present invention.
It is well known that Ti is an element that fixes C, N, and S in steel and enhances workability of a steel sheet. In addition, when the addition amount is small, the fatigue strength of the spot welded joint is reduced as well as the workability. For these two reasons, a minimum addition of 0.040% is necessary. However, if the addition amount is large, not only the effect is saturated, but also the alloy cost becomes high,
Was specified as 0.080%.

B, together with the precipitate size distribution, is an important requirement of the present invention. Experimental facts in which the amount of B added is specified will be described below. C: 0.0010%, Mn: 0.15
%, P: 0.010%, S: 0.008%, Al: 0.
025%, N: 0.0023%, Ti: 0.050%,
B: The steel changed to ~ 0.0040% is melted in a vacuum melting furnace to form a steel ingot, a heating temperature of 1100 ° C, a finishing temperature of 900 ° C,
Hot-rolling at a thickness of 4.0 mm and a winding temperature of 600 ° C.
After pickling, cold rolled to a thickness of 0.80 mm, 800 ° C x 1
min recrystallization annealing and 1.5% temper rolling. FIG. 2 shows the material properties of this steel sheet. In addition, the steel plate was spot-welded to investigate its fatigue characteristics. For spot welding, pressing force: 200 kg, nugget diameter 4.5 φm
m was selected. The fatigue test was a shear tensile type, and the others were performed according to JISZ3138. Figure 2 shows the result.
Are shown together.

As can be seen from FIG.
When the amount of precipitates of μm or less is less than 0.0040% in terms of Ti, the fatigue properties are improved from the B content of 3 ppm.
However, those with many fine precipitates begin to improve the fatigue strength only by adding 15 ppm of B. On the other hand, the elongation, which is an index of the press workability, shows a small decrease in elongation even when the amount of B added increases, when the amount of fine precipitates is small. However, it can be seen that, when the amount of B added is large, the elongation sharply decreases with an increase in the amount of added B, and there is no region where both the press formability and the fatigue strength of the spot welded joint are excellent.

Based on this experimental fact, a lower limit of B of 0.0003% was specified as a condition for obtaining a steel sheet having good press formability and spot welded joint fatigue strength. Further, as the B content increases, the r-value decreases in the cold rolling reduction at which a normal cold-rolled steel sheet is manufactured. Therefore, the upper limit is specified to 0.0040%. Preferred conditions for the amount of B added to obtain extremely excellent press formability and excellent spot welded joint fatigue properties are 0.0010 to 0.0030%.

Elements such as Si do not need to be particularly added.
As low as possible is desirable to obtain press formability. The molten steel having such a composition is melted in a normal melting furnace such as a converter or an electric furnace, and further subjected to vacuum degassing. It is common to make a slab by continuous casting. The hot rolling heating temperature and hot rolling conditions are effective means for effectively controlling the Ti precipitate size distribution within the scope of the present invention.

When the heating temperature is high, a large Ti precipitate can be easily produced, but a small number of precipitates increase, making it difficult to obtain a precipitate distribution within the range of the present invention. For this reason, the hot rolling heating temperature needs to be 1180 ° C. or less. The hot rolling temperature range affects the precipitate size distribution. As a result of examining the relationship between the rolling conditions and the precipitate size distribution, it was found that at a heating temperature of 1180 ° C or less, the precipitate falls within the scope of the present invention when the rolling reduction at 1000 ° C or less becomes 90 % or more. Based on this fact, the cumulative rolling reduction below 1000 ° C is 90 %
I specified above.

The finishing temperature needs to be 900 ° C. or higher. When the hot-rolling finishing temperature is lower than 900 ° C., the in-plane anisotropy of the material becomes large, so that extremely excellent press workability cannot be obtained, and at the same time, the plate shape, the plate thickness accuracy and the like are deteriorated and the operability is impaired. The winding temperature is not particularly limited in the present invention, but is preferably 650 ° C. or lower in order to reduce acidity and material fluctuation in the coil.

The steel sheet produced in this manner is subjected to cold rolling after descaling, and the cold rolling rate is desirably 70% or more in order to improve press formability. Subsequent annealing, which does not detract from the features of the present invention, whether in a continuous or batch type manner. Annealing must be performed at a temperature higher than the recrystallization temperature, but in the present invention, the temperature is preferably 750 ° C. or higher for continuous annealing, and 6 ° C. for box annealing.
A temperature of 80 ° C. or higher is desirable.

[0025] The annealed steel sheet is temper-rolled if necessary and provided to the product. Annealed steel sheets are Zn, Al,
The features of the present invention can be exerted even when hot-dip plating of an alloy such as Pb or Sn or electroplating is performed.

[0026]

【Example】

(Example 1) A slab manufactured by continuous casting with a converter having a steel composition shown in Table 1 and a vacuum degassing apparatus was manufactured by using 115
Heating to 0 ° C, cumulative rolling reduction below 1000 ° C: 90 ~
92%, finishing temperature; 900 to 910 ° C, winding temperature: 60
A 4.0 mm-thick hot rolled coil was manufactured at 0 ° C. The coil was pickled and then cold rolled to a thickness of 0.80 mm,
A recrystallization annealing of × 60 seconds was performed, and a temper rolling of 1.5% was performed. The material properties of this steel sheet are shown in Table 2 (continued from Table 1). Furthermore, this steel plate is pressed and the distortion is 9 to
A sample is taken from a range of 12%, and the applied pressure: 200 k
g, retention time 14 cycles, nugget diameter 4.5mmφ
Was selected and a shear tensile type fatigue test was performed. The fatigue test was performed according to JISZ3138. The fatigue limit is also shown in Table 2. In Table 2, ≦ 0.01μ
The amount of the precipitate m is a Ti-converted amount of the precipitate of 0.01 μm or less.

[0027]

[Table 1]

[0028]

[Table 2]

Coil No. Nos. 1 to 5 are examples within the scope of the present invention. 6 to 8 are comparative examples outside the scope of the present invention. As can be seen from Tables 1 and 2, those within the range of the present invention have extremely excellent r-value and elongation, which are indicators of press formability, and also have good spot welded joint fatigue strength. On the other hand, in the comparative examples in which the steel composition and the precipitate size distribution are out of the range of the present invention, the press formability is inferior or the spot weld joint fatigue strength is inferior, and both the press formability and the spot weld joint fatigue strength are satisfied. Can not do. That is, it is understood that the present invention is completed only when the steel composition and the precipitate are closely controlled.

(Example 2) A steel composition of the present invention shown in Table 3 and a steel composition outside the scope of the present invention were melted in a converter and a vacuum degassing furnace to produce a 250 mm thick slab. A hot-rolled coil having a thickness of 4.0 mm was produced under the indicated hot-rolling conditions, then pickled, cold-rolled to a thickness of 0.80 mm, and maintained at a temperature of 775.
C., retention time: continuous annealing for 45 seconds, and temper rolling of 1.0%. The precipitate size distribution of this steel sheet was examined with an electron microscope, and a material test and a spot weld joint fatigue test were performed. The spot welding conditions and the fatigue test are the same as in Example 1. The results of the investigation are shown in Table 5. Table 5
In the formula, the amount of precipitates of 0.01 μm or less is the amount in terms of Ti.

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

Coil No. Examples 10 and 11 are examples within the scope of the present invention in terms of components, but the hot rolling conditions are changed. When the hot rolling conditions change, the amount of fine precipitates of 0.01 μm or less changes, and the material characteristics and the fatigue limit of spot welded joints change depending on the amount of precipitates. That is, the coil No. which is an embodiment within the scope of the present invention in terms of both components and manufacturing conditions. 10
Has extremely high El and r values, which are indicators of press formability, and has a high fatigue limit for spot welded joints. On the other hand, the coil No. which is within the range of the present invention in terms of components but whose heating temperature is outside the range of the present invention. In No. 11, the press formability is lower than that of the examples within the scope of the present invention, and the spot welding joint fatigue limit is also low.

Coil No. Examples 15 to 17 are components outside the scope of the present invention. Coil No. 15 is 0.01μ
The amount of precipitates less than m is as small as 0.0045% and the press formability is good, but the spot welded joint fatigue strength is low. In addition, the coil No. Nos. 16 and 17 have insufficient El and r values, which are indicators of press formability, and have poor spot welding fatigue strength.

As described in detail in the above examples, it is only when the steel component and the precipitate size distribution are satisfied that a steel sheet excellent in press formability and excellent in spot welded joint fatigue strength can be manufactured. .

[0037]

As described above, according to the present invention, a cold-rolled steel sheet having extremely excellent press formability and excellent spot welded joint fatigue strength can be obtained.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing a relationship between a Ti-converted amount of a fine precipitate of 0.01 μm or less, an elongation, an r value, and a fatigue limit of a spot welded joint.

FIG. 2 is a graph showing the relationship between the B content, elongation, r value, and spot weld joint fatigue limit.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-7-197187 (JP, A) JP-A-3-199342 (JP, A) JP-A-63-317647 (JP, A) (58) Investigation Field (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. In% by weight, C: 0.0015% or less, Mn: 0.05 to 0.50%, P: 0.015% or less, Al: 0.010 to 0.075%, N: 0 .0040% or less, Ti: from 0.040 to 0.080%, B: containing 0.0003 to 0.0040%, and a balance of Fe and unavoidable impurities, the steel
Among the Ti precipitates precipitated, the particle size is 0.01 μm
A cold-rolled steel sheet having extremely excellent press formability and excellent weld joint fatigue characteristics, wherein the following fine Ti precipitates are dispersed in an amount of 0.0050% or less in terms of Ti amount. 2. In% by weight, C: 0.0015% or less, Mn: 0.05 to 0.50%, P: 0.015% or less, Al: 0.010 to 0.075%, N: 0 A steel slab containing 0.0040% or less, Ti: 0.040 to 0.080%, B: 0.0003 to 0.0040%, and the balance consisting of Fe and unavoidable impurities is heated at 1180 ° C or less. , 1000 ° C. cumulative reduction of 90% or more of the following, finishing temperature and hot rolling as 900 ° C. or higher, subsequently cold rolled, annealed grain size 0.01μm following Ti fine precipitates in the steel and A method for producing a cold-rolled steel sheet having extremely excellent press formability and excellent weld joint fatigue characteristics, characterized in that the content is 0.0050% or less in terms of Ti.