JP2699261B2 - Manufacturing method of cold-rolled steel sheet for press forming with excellent fatigue properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet for automobiles, particularly, an ultra-low carbon cold-rolled steel sheet suitable for use as an original plating sheet as a surface-treated steel sheet for a fuel tank.

[0002]

2. Description of the Related Art Cold-rolled steel sheets used as steel sheets for automobiles are required to have high deep drawability, and particularly, surface-treated steel sheets for fuel tanks are required to have more severe deep drawability. Particularly recently, the press shape of the fuel tank has become more complicated along with the complexity of the vehicle body design.

[0003] Further, as four-wheel drive vehicles are used in general passenger vehicles, saddle-shaped fuel tanks have also been employed, and press working into more complicated shapes has been performed. Therefore, in order to enhance the press formability, efforts are being made to bring the steel component as close as possible to pure iron. For example, Japanese Unexamined Patent Publication (Kokai) No. 60-9830 discloses a technique for producing a cold-rolled steel sheet for deep drawing in which C is set to an extremely low C and the components and annealing conditions are specified.

One of the characteristics required for a cold rolled steel sheet for a fuel tank material is a fatigue characteristic. In the fuel tank, a steel plate pressed separately from the upper and lower sides is joined by seam welding, so that distortion remains in the entire fuel tank. Further, since volatile fuel is charged, the stress from the inside of the fuel tank changes due to a temperature change. Further, since fuel is sucked when the engine is started, a negative pressure is applied inside the fuel tank.

When the sheet is pressed into such a complicated shape, the thickness of the sheet is reduced at a certain portion, and the above-mentioned stress is concentrated at the portion, fatigue failure may occur. Therefore, in the design of the fuel tank, the thickness, design, and the like are determined in consideration of fatigue characteristics.

[0006]

It is generally known that the fatigue characteristics are proportional to the strength of a steel sheet, that is, tensile strength (TS) and yield point (YP). Increasing the tensile strength increases the fatigue strength, but this leads to a decrease in deep drawability.

[0007] From the viewpoint of deep drawability and shape freezing during pressing, the material is softened to improve the elongation (El), the YP is reduced, and the texture is improved by Ti and Nb. To improve the Rankford value (r value).

However, this results in lowering the fatigue strength. That is, the fatigue strength and the deep drawability are contradictory characteristics, and if one of them is improved, the performance of one of them is necessarily reduced. Therefore, there is a strong demand for a cold rolled steel sheet that overcomes both of them.

The present invention provides a method for producing a cold-rolled steel sheet for press forming, which has both of the above contradictory properties and is excellent in fatigue properties.

[0010]

In the present invention, the contradictory characteristics are imparted separately to the outer layer and the inner layer of the cold-rolled steel sheet. That is, the fatigue fracture causes cracks in the outer layer having the largest stress. Since this occurs due to propagation inside, it is considered effective to improve the mechanical strength of the outer layer to improve the fatigue characteristics.

In order to improve the mechanical strength of the outer layer,
Enrich the surface layer of boron (B). The ultra low carbon cold rolled steel sheet in which interstitial interpenetrating atoms are reduced to zero to the limit is clean at the grain boundaries, and in fatigue fracture, the grain boundaries of the outermost layer are cracked and propagated inside. Therefore, the grain boundaries are strengthened by B. Deep drawability is ensured by the ultra-low carbon steel used for the inner layer.

[0012] By imparting the contradictory characteristics of fatigue strength and deep drawability to each of the inner and outer layers separately, cold rolling for press forming, which has been considered to be extremely difficult so far, has excellent fatigue strength. Steel plates can now be manufactured.

FIG. 1 shows an example of a cross-sectional structure of a cold-rolled steel sheet for press forming according to the present invention. In FIG. 1, 1a and 1b are B-added ultra low carbon steels forming a surface layer, and 2 is an ultra low carbon steel forming an inner layer. The summary of the method for producing this steel sheet is as follows.

According to the present invention, the surface component is C ≦ 0.002.
%, Mn: ≦ 0.20%, P ≦ 0.008%, Al:
0.005 to 0.05 %, N ≦ 0.004 %, Ti: T
i / (C + N) = 5-20 and 0.02-0.
07%, B: 0.0007 to 0.0020%,
The balance consists of Fe and inevitable impurities, and the inner layer component is C ≦ 0.002%, Mn: ≦ 0.20%,
P ≦ 0.008%, Al: 0.005 to 0.05%, N
≦ 0.004 %, Ti: Ti / (C + N) = 5 to 20 and 0.02 to 0.07%, B ≦ 0.0003%
, And the balance consisting of Fe and unavoidable impurities, the ratio of which to the total thickness of the surface layer is 3 to 25% on one side is hot-rolled, subsequently pickled, and cold-rolled at a reduction of 75 to 90%. And a method for producing a cold-rolled steel sheet for press forming having excellent fatigue characteristics, wherein annealing is performed at 750 to 850 ° C. for 5 minutes or less.

[0015]

The present invention will be described below in detail. First, the reasons for defining the steel components will be described.

C has an upper limit of 0.002% or less (20 pp).
m). Although there is a good correlation between the deep drawability and the C content in steel, it is more preferable that the C content is low in order to obtain good deep drawability, and the amount is made 0.002% or less. 0.0
In the case of more than 02%, since the grain boundaries are strengthened by C, the feature of the present invention of enriching B in the surface layer is lost.

N is 0.004% or less (40 ppm or less)
To be specified. N penetrates into the lattice of Fe like C, and E
Since the mechanical properties such as 1 are degraded, it is necessary to reduce the amount of N as much as possible and fix it in the form of TiN with Ti.

P is set to 0.008% or less. Since P lowers El and also raises the recrystallization temperature, its upper limit is set to 0.1.
008%.

Since Mn is also an element used for hardening the mechanical strength of steel, its amount must be kept low, and the recrystallization temperature is also increased, so that it is specified to be 0.2% or less. .

Al is required to be at least 0.005% in order to improve the yield of Ti. On the other hand, if the content exceeds 0.05%, the manufacturing cost also increases.
5%.

[0021] Ti is specified to be 0.02% or more and 0.07% or less. Even if Ti is added in an amount larger than necessary for fixing C and N, the influence on the texture is small, and the Rankford value (r
Value) is known not to decrease. But El
Is slightly affected, and the manufacturing cost is also increased. Therefore, the upper limit of the amount of Ti added is set to 0.07% or more. The reason for setting the lower limit of the amount of Ti added to 0.02% is that
If the content is 0.02% or less, the r-value significantly decreases, and the deep drawability deteriorates.

Further, it is assumed that Ti / (C + N) = 5 to 20.
Here, Ti, C, and N are calculated in terms of% by weight. However, Ti, which is stoichiometrically required to fix C and N, is required to be 1.85 times the content of C and N. is there. But Ti is C
It has a strong affinity with elements other than N and N, and for example, fixes S and the like to improve the texture during recrystallization. Then Ti / (C +
The lower limit of N) was set to 5. The reason why the upper limit of Ti / (C + N) is set to 20 is that, in addition to having almost no influence on the texture, the manufacturing cost is increased and the influence on El is also exhibited.

B separately defines the contents of the inner layer and the outer layer. Since B is not fixed by Ti, the material of the steel plate can be easily controlled. Since B penetrates between the lattices of Fe like C and N and lowers the material of the steel sheet, it is better not to include B from the viewpoint of deep drawability. Therefore, it is preferable not to actively add B to the steel serving as the inner layer, and the amount is specified to be 0.0003% or less.

B precipitates at the grain boundaries after press working or by aging to improve the grain boundary strength. Therefore, not only the fatigue strength is improved, but also the secondary work cracking resistance generated in extremely cold regions (which means that cracking occurs when subjected to impact at cryogenic temperature after press working) is improved. It is added within the range that does not affect.

Fatigue fracture usually occurs when a crack in the outermost layer having the highest stress propagates inside. Therefore, B is added only to the upper layer (surface layer) in order to improve the mechanical strength of the outermost layer. The upper limit of the amount is 0.003% (30 ppm). If the amount exceeds this amount, the mechanical properties of the entire steel sheet deteriorate and the deep drawability decreases. The lower limit is 0.0007%. If the amount is less than this, improvement in fatigue strength is not expected.

Referring to the above-described cross-sectional structure of the cold-rolled steel sheet for press forming according to the present invention shown in FIG. 1, the clad ratio is 3 to 25% in this structure. Here, the cladding ratio is obtained by multiplying the thickness of one surface 1a or 1b (upper layer) with respect to the total plate thickness by 100.

The reason why the lower limit is set to 3% is that below this, the effect of improving the fatigue characteristics is small, and B is an element having a small atomic radius and is scattered to the inner layer portion 2 by diffusion. The upper limit is set to 25%. If the upper limit is exceeded, the ratio of the upper layers 1a and 1b to the entire plate thickness exceeds 50%.
This is because the mechanical properties of the entire steel sheet deteriorate and the deep drawability decreases.

The steel having the above-described steel components is melted by a conventional method, and further decarbonized to a predetermined carbon content by vacuum degassing. The present invention discloses a method for producing a multi-layer steel sheet having a three-layer structure as shown in FIG. 1, but the method for producing a multi-layer steel sheet having such a non-uniform component is not particularly limited. The following are typical examples of the production method.

(A) Cast-in method, (B) Two immersion nozzle method, (C) Hot rolling and pressure bonding, (D) Explosion, (E) Wire addition method.

Of these, it is industrially suitable to form a multilayer at the casting stage after the smelting of steel, and it is most economical to manufacture by continuous casting in particular. Therefore, in order to manufacture the steel sheet according to the present invention, the wire addition method is desirable.

When a wire is used, a hollow wire made of iron is used. This is filled with B powder, chips or ferroboron, and this wire is inserted into molten steel during continuous casting.

This wire addition method is described in, for example,
As disclosed in JP-A-108947, the B content of the B-added ultra-low carbon steel as the upper layer and the cladding ratio are determined by the wire insertion speed, the casting pulling speed, and the position of the electromagnetic brake provided around the turn dish. And its magnetic force.

That is, a magnetic field causes a flow in a certain direction on the outer periphery of the slab during casting. B added by the wire will be uniformly dispersed in the outer layer of the slab along this flow.

The slab thus manufactured by continuous casting is manufactured in the same process as that of a normal deep-drawn cold-rolled steel sheet. That is, after the slab is heated and hot rolling is performed, pickling of the scale is performed. In the subsequent cold rolling, the rolling reduction is set to 75% to 90%. 75
%, The deep drawability is low, and if it exceeds 90%, the in-plane anisotropy is too high.

The cold-rolled coil is recrystallized by a continuous annealing method. Annealing conditions are 750 ° C. to 850 ° C. in a temperature range of 5 minutes or less. If the temperature exceeds 850 ° C., the crystal grains grow abnormally, and roughening called orange peel occurs during press working. If the temperature is lower than 750 ° C., recrystallization is insufficient, and sufficient deep drawability cannot be obtained. After annealing, temper rolling may be performed as necessary.

[0036]

EXAMPLES Table 1 shows the chemical composition of the steel, and Table 2 shows the production conditions for the steel sheet and the results of evaluation of various properties. All of the products of the present invention were made into slabs by the wire addition method and 1200
After the slab was heated to ℃, hot rolling was performed to obtain a hot rolled sheet at a finishing temperature of 900 ° C and a winding temperature of 600 ° C. After pickling this hot-rolled sheet, cold rolling and annealing were performed to prepare a 0.8 mm cold-rolled steel sheet, and various evaluation tests were performed.

Mechanical test, cold-rolled steel sheet having a thickness of 0.5 mm was processed into a JIS No. 5 specimen,
YP, TS, El, and r values were measured.

Evaluation of forming processability, using a flat-headed punch having a diameter of 50 mm, wrinkle holding pressure 700
The cylinder was deep drawn using a commercial rust preventive oil as a lubricant. The thickness of the test material was kept constant at 0.8 mm, and the limit drawing ratio (LDR) was determined by changing the blank size from 100 mm to 120 mm in 5 mm increments. Here LD
R is defined as in Equation 1 below.

[0039]

LDR = blank diameter (mm) / punch diameter (mm)

The state of the surface (the degree of skin roughness) after deep drawing was observed and evaluated in five steps (、 ５, ○, Δ, ×, XX).

Fatigue Test, No. 1 fatigue test piece (sheet thickness 0.8 mm) based on JIS-Z-2275 was prepared and subjected to a fatigue test. The test was performed using a Schenk double swing bending tester with a repetition rate of 180.
0 times / minute. The fatigue strength was defined as a strength that did not cause breakage up to 1,000,000 times.

[0042]

[Table 1]

[0043]

[Table 2]

FIG. 2 shows the deep drawability (LD) obtained from the embodiment.
R) and the fatigue limit were shown. The conventional example is a characteristic value of a steel sheet obtained by a conventional cold-rolled steel sheet manufacturing method having a uniform composition in a cross-sectional direction.

In the conventional material, the fatigue properties are lowered with the improvement of the press formability. On the contrary, in the present invention, the pressability is secured without impairing the fatigue properties, that is, according to the present invention, a cold-rolled steel sheet having both excellent properties and excellent deep drawability without impairing the fatigue durability is provided. Manufacturing becomes possible.

[0046]

As described above, in the steel sheet manufactured by the method of the present invention, the deep drawability is ensured by the ultra-low carbon steel used for the inner layer, and the ultra-low carbon cold-rolled steel sheet forming the outer layer has a mechanical strength. By enriching boron for the purpose of improving the strength, the properties of contradictory fatigue strength and deep drawability can be improved by improving the inner layer,
By separately holding each outer layer, it is possible to manufacture a cold-rolled steel sheet for press forming with excellent fatigue strength, which has been extremely difficult so far.For example, this can be performed with high deep drawability and further repeated When applied to an automobile fuel tank or the like that is required to have fatigue resistance against stress, the characteristics can be sufficiently exhibited.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of the structure of a cold-rolled steel sheet for press forming according to the present invention.

FIG. 2 is a drawing showing the relationship between the deep drawability and the fatigue limit of a conventional example and an example of the present invention.

[Explanation of symbols]

 1a, 1b B-added ultra low carbon steel forming surface layer 2 Ultra low carbon steel forming inner layer

Claims (1)

(57) [Claims] Claims: 1. A surface layer component comprising: C ≦ 0.002%, Mn:
≦ 0.20%, P ≦ 0.008%, Al: 0.005 to
0.05%, N ≦ 0.004% , Ti: Ti / (C +
N) = 5-20 and 0.02-0.07%,
B: 0.0007 to 0.0020%, with the balance being Fe
And unavoidable impurities, the inner layer component is C ≦ 0.002%, Mn: ≦ 0.20%,
P ≦ 0.008%, Al: 0.005 to 0.05%, N
≦ 0.004 %, Ti: Ti / (C + N) = 5 to 20 and 0.02 to 0.07%, B ≦ 0.0003%
, The balance consisting of Fe and unavoidable impurities, the ratio of which to the total thickness of the surface layer is 3 to 25% on one side is hot-rolled, then pickled and cold-rolled at a reduction of 75 to 90% And a method for producing a cold-rolled steel sheet for press forming having excellent fatigue characteristics, wherein annealing is performed at 750 to 850 ° C. for 5 minutes or less.