JPH0663613A - Manufacture of seamless steel tube for automobile - Google Patents

Abstract

PURPOSE:To attain high fatigue strength by reheating a tube stock under specified conditions after rolling with a mandrel mill and next making rolling reduction of a stretch reducer the uniform state between respective stands. CONSTITUTION:At the time of manufacturing a seamless tube, by taking reheating condition after rolling with the mandrel mill 17 as the high temp. of 900 to 1050 deg.C and also the heating time as 5 to 20min, the temp. of the tube stock after rolling with the mandrel mill 17 is sufficiently raised and uniformize. In a stage in which the tube stock 22 reheated like that is screwed down with the stretch reducer 25, the rolling reduction is taken as the uniform state of <=0.5%. In this way, the out-of-roundness of the inner surface of a steel tube after rolling with the stretch reducer 25 is pertinently improved. The inner surface of the steel tube 28 of which the out-of-roundness is improved is ground by shot blasting or the like. Thus, grinding stage is relatively simplified, its cost is reduced, also decrease of wall thickness due to grinding is pertinently limited and strength property is maintained.

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 seamless steel pipes for automobiles, and relatively easily and efficiently produces seamless steel pipes as hollow shaft materials suitable for weight reduction of automobile drive shafts, stabilizers and the like. It seeks to find a way to do it.

[0002]

2. Description of the Related Art Regarding drive shafts, stabilizers and the like in automobile parts, the use of hollow materials has been studied in order to obtain a lightweight automobile body and an energy saving effect. As such a hollow material, a material in which a plug or other core metal is inserted into a steel pipe for cold drawing or the like and repeated expansion is considered. That is, this product has a smooth inner surface and is finished.

It is natural that the hollow material as described above is preferable in terms of strength, rigidity and fatigue resistance, and when heat treatment such as quenching is performed for such purpose, such heat treatment is performed. It is also possible to achieve high strength of 100 kgf / mm ² or more while maintaining good fatigue resistance.

[0004]

However, in the case where a plug or other core metal is inserted into the pipe formed as described above and the rolling is repeated, the operation is complicated and a lot of man-hours are required for the production, and the cost is low. It becomes high.

In terms of productivity, it is preferable to perform rolling after a predetermined perforation is obtained, and to perform rolling in a final step by a simple method such as a stretch reducer which does not require a mandrel. However, in a steel pipe obtained by a rolling method without using a cored bar such as a mandrel, the hollow material undergoes reduction by rolling rolls from three sides with respect to the axis during rolling, so the inner surface of the hollow material after the rolling is a perfect circle. Instead, it becomes a circle with some corners and polygons.

Further, even in the rolling using the mandrel before the stretch-ducer rolling, it is natural that the material can be rolled efficiently under high temperature conditions. Rolling at a lower rate,
In the latter half, it is common to use a low pressure reduction rate. As a result, what was largely rolled by the initial high-pressure reduction is the rolling in the latter half of the low-pressure reduction with a gap between the hollow material and the mandrel, even if the mandrel exists. The inner surface shape will be diversified. It goes without saying that this also increases the unevenness of the inner surface.

Generally, in drive shafts, stabilizers and the like made of hollow materials, the fatigue strength decreases as the strength increases due to quenching and the like. That is, the above 1
As the strength increases to over 100 kgf / mm ^2, susceptibility to stress concentration for fatigue cracking increases, and the unevenness of the surface tends to lower the fatigue strength as compared with the conventional general strength.

As described above, the steel pipe whose inner surface is diversified and whose roundness is lost is not necessarily preferable in characteristics as a drive shaft, stabilizer, etc., and its modification is not easy. Especially when trying to remove the inner surface flaws and decarburized layer, generally the flat surface of the polygonal steel pipe inner surface is preferentially removed, and the corners are cut excessively because the cutting is restricted. This means that many man-hours are required. Moreover, it is not always easy to bring the shape into a perfect circle by this work.

[0009]

Means for Solving the Problems The present invention has been studied to solve the technical problems in the conventional ones described above, and employs an efficient hot-finished steel pipe as a material such as the stretch reducer described above. However, it has succeeded in appropriately obtaining high fatigue strength by relatively simple working, and is as follows.

(1) In manufacturing a seamless pipe, reheating conditions after rolling by a mandrel mill are set to 900 to 1050.
C. for 5 to 20 minutes, then the reduction amount of the stretch reducer to be performed is made uniform between the stands, and the inner surface of the seamless pipe that has undergone these treatments is cut. Manufacturing method.

(2) In manufacturing the seamless pipe, the reheating condition after rolling by the mandrel mill is set to 900 to 1050.
5 to 20 minutes at ℃, the reduction amount of the stretch reducer to be performed next is made uniform between the stands, and the internal surface cutting amount is determined by the decarburization condition of the seamless pipe that has undergone these treatments and the measurement result of the flaw depth. A method for producing a seamless steel pipe for automobiles, which comprises determining and shot blasting.

[0012]

In the production of the seamless pipe, the reheating condition after rolling by the mandrel mill is set to a high temperature of 900 to 1050 ° C., and the heating time is set to 5 to 20 minutes. The tube temperature is sufficiently raised and made uniform. In addition, the amount of reduction was determined as shown in FIG. 5 as an example in the step of reducing the temperature of the stretch reducer that has been homogenized at such a high temperature. That is, conventionally, as shown in FIG. 4, the amount of reduction in each stand fluctuates in the range of about 2% in the initial stage and in the latter half in the range of about 2%, which is not necessarily uniform. As shown in (1), the uniformity of the reduction amount was enhanced by setting the reduction amount in each stand to 1% or less, preferably 0.5% or less in both the first half and the second half. Thereby, the roundness on the inner surface of the steel pipe after being rolled by the stretch reducer is appropriately improved.

Even in the case of rolling with a mandrel as a pre-stage of rolling with a stretch reducer, conventionally, as shown in FIG. 6, the rolling reduction at the first and second stands is 20% or more. It is as high as 7% or less after the third stand, particularly 2% or less at the fifth and sixth stands, whereas it is 5% at the first to fifth stands in the present invention.
14%, especially 10 to 12% for the first to fourth stands, and 3 to 8% for the fifth and sixth stands, thereby effectively suppressing diversification in the mandrel rolling process. As a result, an even better roundness can be secured.

In consideration of rolling, it is preferable that the reheating is performed sufficiently uniformly, and the lower limit of the reheating time is 5 minutes. On the other hand, since the reheating time is 20 minutes or less, the depth of the decarburized layer during this heating is limited to 0.15 mm or less, and the decarburized layer thus limited is removed by grinding or the like. Easy and accurate. That is, the decarburization depth increases rapidly within 20 to 30 minutes.
It has been confirmed to reach 3 mm or more, but it is about 0.15 mm in about 20 minutes, and the decarburization depth is less than 0.1 mm in any case by setting the upper limit to 20 minutes. It can be easily removed.

By grinding the inner surface of the steel pipe whose roundness is increased as described above, the inner surface flaw and the decarburized layer of the seamless pipe can be removed with a relatively small grinding amount. Therefore, the wall thickness of the raw steel pipe can be effectively utilized.

The cutting amount of the inner surface of the steel pipe as described above is determined by the decarburization condition of the seamless pipe and the measurement result of the flaw depth, which is rational and accurate because the roundness is increased as described above. You can get a large amount of cutting.

By performing the above-mentioned inner surface cutting of the steel pipe by shot blast grinding, the inner surface of the steel pipe under the condition that the roundness is increased as described above can be effectively ground relatively easily with an appropriate cutting amount.

The amount of cutting processing by shot blast grinding or the like changes depending on the inner diameter of the hollow material,
In this type of hollow material that has been practically used in general, the upper limit of 500 μm makes the cutting process relatively simple and low cost, and properly limits thinning for cutting to maintain strength characteristics. To do.

The fatigue resistance is improved by cutting the inner surface of the seamless steel pipe hot-rolled as described above by 20 μm to 500 μm. In this case, if it is less than 20 μm, a preferable improvement in fatigue strength cannot be obtained, while 500
If it exceeds μm, the man-hours increase, and it is not necessary to improve the characteristics in proportion to the man-hour and cost increase.

[0020]

Embodiments of the present invention as described above,
To explain further, the seamless steel pipe according to the present invention is generally manufactured by a process as shown in FIG. That is, the billet 10 is charged into the heating furnace 12, heated and soaked, extracted, sent to the Mannesmann mill 13, and perforated by the roll 14 and the plug 15.

The raw pipe 16 perforated by the Mannesmann mill 13 as described above is then sent to the mandrel mill 17.
After inserting the mandrel bar 18, four or more sets of rolls 20 and the mandrel bar 18 were used to perform reduction as in the example of the present invention shown in FIG. That is, in the case of the present invention as shown by the solid mark in FIG. 6, the mandrel bar and the steel pipe were rolled in a state of being in close contact with each other and had good inner surface properties.

The material rolled by the mandrel mill 17 as described above is cooled by the mandrel bar 18 located in the hole and the roll 20 on the outer surface of the raw pipe 16 is cooled.
Since it is also cooled by, the temperature drops considerably.

Therefore, the raw pipe 22 passed through the mandrel mill 17
Then, it is charged into the reheating furnace 24 and reheated. The heating condition in the reheating furnace 24 is 100 in the present invention.
The temperature is 0 to 1050 ° C., and the time is 5 to 20 minutes. That is, while such reheating is generally performed at around 900 ° C. in the conventional method, in the present invention, it is higher than 1000 ° C. and the heating time is 5 minutes or more and 20 minutes or less. , Do not exceed 20 minutes. That is, it is obvious that the temperature and time are adopted so that they are heated at high temperature and uniformly.
The decarburization can be controlled appropriately and the decarburized layer can be limited to 100 μm or less.

The raw tube 22 that has been uniformly and reheated at high temperature as described above is then subjected to rolling by the stretch reducer 25 by the multi-stage three-set roll 26, which generally has more than 20 sets. That is, in this way, the multi-saddle type 3
When rolling with the twin rolls 26, rolling is performed with a large number of rolls 26 as described above without using a mandrel bar or other core metal in the raw tube 22. In this case, in the present invention, the former stage and the latter stage are used. As an example, the variation in the amount of reduction between the roll sets was set to a uniform state of 0.5% or less as shown in FIG.

That is, in the conventional stretch reducer, the reduction of each stand from the inlet side roll set to the outlet side roll set was not necessarily uniform as shown in FIG. 4 described above, but in the present invention, it is in a uniform state. Divided into two groups.

The roundness of the inner surface of the steel pipe 28 after the stretch reducer treatment obtained as described above, that is, the difference between the major axis and the minor axis on the inner hole surface is about 0.5 mm by the conventional method. On the other hand, in the case of the present invention, the diameter can generally be set to about 0.2 mm, and the diversification in the steel pipe inner hole becomes extremely small.

With respect to the seamless steel pipe hot-finished as described above, the inner surface is ground by shot blasting or the like. The grinding amount varies depending on the inner diameter of the steel pipe, but generally 250 μm or less, particularly 150 μm or less is sufficient, and the target drive shaft and stabilizer with which fatigue resistance is improved easily and efficiently. It is possible to obtain a hollow material for business use.

A concrete production example of the product according to the present invention will be described. As shown in FIG. 1, the product rolled by the mandrel mill 17 by the above-mentioned manufacturing equipment has an inner diameter of 68.
It is obtained as a raw tube 22 having a wall thickness of 16 mm and a thickness of 16 mm. The element tube 2
2 is heated in a reheating furnace 24 at 1020 ° C. for 10 minutes, and then subjected to reduction by a stretch reducer 26 in which three sets of saddle type rolls 24 are sequentially arranged, and for drive shaft, (a) inner diameter 15 mm and outer diameter 35 mm, (b) Inner diameter 20 mm, outer diameter 40 mm, for stabilizers (c)
Inner diameter 28.7 mm, outer diameter 42.7 mm, (d) inner diameter 25.1
A steel pipe having an outer diameter of 38.1 mm in mm was manufactured.

The inner surface of each hot-finished steel pipe obtained as described above was ground to (a) 10 to 150 μm,
(B) is 100 to 500 μm, and (c) is 10 to 1
50 μm, (d) is 20 to 200 μm, and all of these grindings were performed by the shot blast method.

The drive shaft (a) and (b) steel pipes for inner surface grinding as described above were obtained by processing test pieces as shown in FIG. That is, parallel test portions 1 having an appropriate length range on the outer surface were carved and formed, and grip portions 2 were formed on both end sides thereof. The test pieces thus obtained were quenched and heat treated. After that, the load torque was variously changed and a torsional fatigue test was conducted.

That is, in the case of (a) having a relatively small diameter inner hole, even if the inner surface grinding amount is about 10 μm, an appropriate increase in the torsional load torque is shown in comparison with the comparative material having no inner surface grinding. It is further improved by increasing the amount.

The above relationship is the same in the case of (b), and in the case of (b), the grinding amount is 5
About 0 μm, 500 N ・ compared to the comparative material (without internal grinding)
A load torque increase of not less than m can be obtained, and a torsion load torque of 1000 N · m or more can be obtained with a grinding amount of about 300 μm. Table 1 shows the results of the torsional fatigue test performed on the production examples (a) and (b).

[0033]

[Table 1]

Steel pipes for stabilizers (c) and (d)
Was bent as shown in FIG. The test piece thus obtained was subjected to a quenching and tempering heat treatment, and then subjected to a durability test in which one side of the U-shape was fixed and the other side of the other side was given an amplitude of ± 80 mm. The results are shown in Table 2.

[0035]

[Table 2]

With respect to the above-mentioned test body, the effect began to appear when the inner surface grinding amount was 20 μm for the comparative material without inner surface grinding,
By increasing this grinding amount, it is further improved,
At μm, a considerable improvement in durability was recognized. That is, the comparative test body without inner surface grinding breaks after about 40,000 to 60,000 times, while the test body with inner surface grinding amount of about 100 μm is broken.
Durability of over 10,000 times was obtained.

[0037]

As described above, according to the present invention, it is possible to appropriately improve the fatigue strength of the hollow drive shaft steel pipe and the hollow stabilizer steel pipe by a relatively small amount of inner surface cutting, which is effective at low cost. Since it is possible to obtain advantageous steel pipes for hollow drive shafts, steel pipes for automobile hollow stabilizers, etc., it is an invention that is industrially highly effective.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a process of manufacturing a steel pipe of the present invention step by step.

FIG. 2 is an explanatory view of a shape configuration of a torsion fatigue test piece.

FIG. 3 is an explanatory diagram of the shape configuration of a stabilizer test body.

FIG. 4 is a chart showing a rolling reduction rate of each stand in a stretch reducer according to a conventional method.

FIG. 5 is a chart showing a rolling reduction rate of each stand in a stretchducer according to an example of the present invention.

FIG. 6 is a chart showing the rolling reduction in the mandrel mill in comparison between the conventional method and the method of the present invention, in which the open mark is the conventional method and the solid mark is the case of the present invention.

[Explanation of symbols]

 1 Test Part 2 Gripping Part 10 Billet 11 Inlet / Outlet 12 Heating Furnace 13 Mannesmann Mill 14 Its Roll 15 Piercer 16 Element Pipe 17 Mandrel Mill 18 Mandrel 19 Guide Roll 20 Rolling Roll 22 Element Pipe after Mandrel Mill 24 Reheating Furnace 25 Stretch Re Juicer 26 3 rolls 28 Hot finished steel pipe

Claims (2)

[Claims] 1. In producing a seamless pipe, the reheating condition after rolling by a mandrel mill is 900 to 1050 ° C.
A method for producing a seamless steel pipe for automobiles, characterized in that the stretch reduction is performed for about 20 minutes, and then the reduction amount of the stretch reducer is made uniform between the stands, and the inner surface of the seamless pipe subjected to these treatments is cut. 2. In the production of a seamless pipe, the reheating condition after rolling with a mandrel mill is 900 to 1050 ° C.
~ 20 minutes, then the reduction amount of the stretch reducer is performed in a uniform state between the stands, determine the internal cutting amount by the decarburization situation of seamless pipes after these treatments and the measurement result of the flaw depth, A method for producing seamless steel pipes for automobiles, characterized by performing shot blast grinding.