JPH05117748A - Production of high-strength electric resistance welded tube - Google Patents

Abstract

PURPOSE:To produce the steel tube having high strength and decreased bends in a large amt. by hot rolling a steel having a specific compsn. and subjecting the steel to a softening treatment in the form of an as-hot rolled coil, then forming and welding the steel. CONSTITUTION:The compsn. of the steel is composed, by weight %, of 0.15 to 0.40% C, 0.1 to 0.7% Si, 1.0 to 2.7% Mn, 0.5 to 2.5% Cr, <=0.025% P, <=0.015% S, 0.01 to 0.05% SolAl, and the balance Fe and unavoidable impurities. Such steel is hot rolled. The resulted hot coil 10 is subjected to a softening treatment by a high-frequency heater 12 and is adjusted to about <=80kgf/mm<2> tensile strength. The steel is then formed to a circular shape by means of forming rolls 14 and both ends of a coil 10 are butt welded by high-frequency welding in a joining section 16.

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 high-strength electric resistance welded steel pipe, and in particular, a high-strength electric resistance suitable for use as a structural material such as a reinforcing material for doors of transportation machines such as automobiles and various frame materials. The present invention relates to a method for manufacturing a sewn steel pipe. The high-strength electric resistance welded steel pipe manufactured according to the present invention is particularly suitable as a vehicle door reinforcing material.

[0002]

2. Description of the Related Art The present invention will be described below by taking a high strength steel pipe for a door reinforcing material as an example. Conventionally, automobile door reinforcement materials in Japan have been in the following situations. In other words, many domestic automobile doors did not have reinforcements installed. Corrugated sheets and steel pipe reinforcements were used for doors of export cars and foreign cars. As the corrugated sheet, hot rolling with a tensile strength of 80 to 100 kgf / mm ²
Cold rolled steel was used. As the steel pipe, it is thin and has a small diameter for weight reduction, and has a tensile strength of 80-150k that has been quenched for higher strength.
A gf / mm ² steel pipe was used, but the dimensional accuracy, especially bending, was large.

As a measure for preventing the bending, after manufacturing a seamless steel pipe or a welded steel pipe, the softness of the rolled product is subjected to dimensional correction such as bending, and then cut into short pipes of about 1 m before water quenching-baking. By performing the heat treatment for returning, the strain (bending) due to the heat treatment was minimized, but the bending could not be completely prevented. After the heat treatment, the steel pipe becomes high in strength, the elastic limit is increased and it becomes difficult to plastically deform it, and the toughness is deteriorated to cause cracking, so that the dimensional correction cannot be practically performed.

[0004]

A steel pipe as such a vehicle door reinforcing material is required to be as strong as possible, inexpensive, and small in bending. Since the automobile adopts the mass production method, it can be quickly and accurately installed and fixed in a predetermined place inside the complicated door structure.

Incidentally, the electric resistance welded steel pipe is known as an inexpensive steel pipe manufacturing method. So far, heat-treated steel pipes with a tensile strength of 150 kgf / mm ² class that have been tempered with ERW steel pipe or tempered at 50 to 350 ℃ as needed have been proposed for automobiles. However, tempering is costly and difficult to put into practical use for automobiles.

In addition, as an electric resistance welded steel pipe, Japanese Patent Laid-Open No. 2-19
No. 7525 discloses an electric resistance welded steel pipe having a tensile strength of almost 100 kgf / mm ² class, and Japanese Laid-Open Patent Publication No. 1-205032 discloses a tensile strength as-built.
An electric resistance welded steel pipe showing 120 kgf / mm ² or more is disclosed in JP-A-59-25.
Japanese Patent Publication No. 931 discloses electric resistance welded steel pipes which have been subjected to normalizing and quenching after pipe making.

However, these materials cannot achieve a tensile strength of 150 kgf / mm ² unless they are quenched and tempered, and are used as a reinforcing material for automobile doors not only in terms of cost but also in terms of bending during heat treatment. It was difficult.

The general purpose of the present invention is to solve the above-mentioned problems of the prior art and to obtain a large amount of high-strength, low-bending high-strength steel pipe useful as a vehicle door reinforcing material at low cost. It is to provide a manufacturing method. A specific object of the present invention is to provide a method for manufacturing an electric resistance welded steel pipe having a tensile strength of 150 kgf / mm ² class as it is in order to form an automobile door reinforcing material with the electric resistance welded steel pipe.

[0009]

The applicant of the present patent has proposed, as Japanese Patent Application No. 2-323776, a method for manufacturing a reinforcing material for an automobile door by using a component system which is completely cooled by air cooling. As a result of subsequent research and development, when an electric resistance welded steel pipe was manufactured using such a hot-rolled steel sheet having a composition, it was sometimes impossible to form and weld the electric resistance welded steel pipe.

Then, various investigations have been made on the cause of the problem, and in the production line of the factory, even the as-rolled hot-rolled coil is burned, and the tensile strength is 120 kgf / mm ²
It was found that the strength was higher than the above. For this reason,
When the tensile strength of the coil was 80 kgf / mm ² or more, the following drawbacks occurred in the production of ERW steel pipe.

Formability of the forming roll group is deteriorated. Since it is too hard, the spring back is too tight, a perfect circle cannot be obtained, and the waist is bent, resulting in a polygonal shape. In addition, the welded portion, that is, the end of the steel strip is sharpened like a chestnut tip.

[0012] Defects are generated in the welded portion due to poor forming in forming. Impurities in the weld remain because the bead molding is not good. Therefore, the present inventor performed spheroidizing annealing (softening treatment) on the hot-rolled coil prior to forming, and found that the tensile strength of the hot-rolled coil was 80 kgf / mm.
^The present invention has been completed, knowing that it can be reduced to the level of ² and there is no problem in molding.

Therefore, the gist of the present invention is, by weight%, C: 0.15 to 0.40%, Si: 0.1 to 0.7%, Mn: 1.
0 to 2.7%, Cr: 0.5 to 2.5%, P ≦ 0.025%, S ≦ 0.
015%, sol.Al: 0.01-0.05%, hot-rolled coil is manufactured by hot rolling using steel with composition consisting of balance Fe and unavoidable impurities. A method for producing a high-strength electric resistance welded steel pipe, which is characterized in that the strength level is set to a level of 80 kgf / mm ² or less, and then forming and welding are performed for producing an electric resistance welded steel pipe.

Further, the steel having the above composition further has Mo: 0.05.
~ 1.00%, V: 0.02 ~ 0.10, Ni: 0.2 ~ 2.5%, Ti: 0.01
~ 0.10%, Nb: 0.02-0.10%, and B: 0.0005-0.00
It may contain at least one selected from the group consisting of 50%. In the present invention, hot rolling and forming and welding for the production of electric resistance welded steel pipe may be conventional ones,
You are not limited to a particular one.

According to a preferred embodiment of the present invention, the above-mentioned softening treatment is only required to secure a tensile strength of about 80 kgf / mm ² or less, and it is generally 750 ° C. × 20 h, but preferably 680 to It may be carried out by heating at 720 ° C. for 0.5 to 2 hours and then air cooling, which also makes it possible to obtain a tensile strength of about 80 kgf / mm ² or less.

[0016]

Next, the method of manufacturing the electric resistance welded steel pipe of the present invention is preferably performed as follows. A coil that has been softened after hot rolling is used, and conventional forming and welding for producing an electric resistance welded steel pipe are used. As a special case, the softening treatment may be performed in a special line having the following induction heating device. FIG. 1 shows an example of the electric resistance welded steel pipe production line of the present invention. In the illustrated example, the coil strength is 80 kgf by soaking the coil immediately before forming to 400 to 720 ° C. by induction heating. / mm ² or less.

That is, the hot-rolled coil 10 having the above-mentioned steel composition is rewound and softened by the high-frequency heating device 12 to adjust the tensile strength to about 80 kgf / mm ² or less, and then the forming roll 14 Then, the both edges of the coil are subjected to high-frequency heating and butt-welded at the joint 16 at the joining portion 16. Finally, the obtained electric resistance welded steel pipe is heated at 850 to 1050 ° C for 5 to 30 minutes for hardening treatment and then air-cooled, as it is or at 50 to further low temperature tempering.
By heating at 350 ° C for 1 to 30 minutes and allowing to cool, the tensile strength is 12
A steel pipe for door reinforcement of 0 to 200 kgf / mm ² can be obtained. Here, the reason why the steel composition is limited as described above in the present invention is as follows.

C: This is an essential element for obtaining high strength at low cost. If it is less than 0.15%, a tensile strength of 120 kgf / mm ² or more required as a reinforcing material cannot be obtained. 0.
If it exceeds 40%, the strength will be too high when quenching and quench cracking will occur.

Si: A component required for deoxidation during steelmaking. 0.1
If it is less than 0%, the deoxidation is insufficient and the toughness cannot be secured. 0.70
If it exceeds%, welding defects are likely to occur during welded pipe manufacturing. Mn: A component effective in improving hardenability. If it is less than 1.0%, its effect is insufficient, and if it exceeds 2.7%, defects are likely to occur during billet production during steelmaking, and toughness deteriorates after heat treatment.

P, S: These are typical impurities in steel. In order to prevent quenching cracks and toughness deterioration after heat treatment, the upper limit is limited to 0.025% and 0.015%, respectively. In particular, at tensile strength ≧ 150 kgf / mm ² , toughness (vTrs ≦ −20
℃), P ≤ 0.015%, S ≤ 0.00
A 5% limit is desirable. Like sol.Al:Si, it is added as a deoxidizing component, and for the same reason as in the case of Si, the addition amount is limited to 0.01 to 0.05%.

Cr: Effective in improving hardenability and toughness and increasing temper softening resistance. In particular, it is an essential component like Mn in order to prevent bending of long tubes as they are due to air cooling. If the upper limit of 2.5% is exceeded, the cost becomes high and defects in the weld cannot be prevented. If it is less than the lower limit of 0.5%, the hardenability, toughness, and softening resistance are not improved.

In the present invention, in order to further improve strength and toughness, steel containing at least one or more of Mo, V, Ni, Ti, Nb and B may be used. Are more preferably Ti-B and Nb-Ti-B.
Is a combination of. Therefore, the reasons for limiting these additional elements will be described below.

It has the same effect as Mo: Cr. It has an auxiliary role of Cr addition. Outside the addition range of 0.05-1.00%
Has the same disadvantages as Cr. V: Addition of 0.02% or more has high softening resistance against tempering at high temperature. Addition exceeding the upper limit of 0.10% is expensive.

Addition of Ni: 0.20% or more is effective for improving hardenability and toughness. The effect is effective even if the upper limit of 2.50% is exceeded, but it becomes expensive. Ti and Nb: Additions of 0.01% and 0.02% or more, respectively, are effective elements for preventing coarsening of steel crystal grains during quenching and improving toughness of welds. However, if the respective upper limits of 0.10% are exceeded, the toughness deteriorates. B: Addition of B is effective for improving hardenability. Below the lower limit of 0.0005%, the effect is insufficient, while at 0.0050%
Above the upper limit of, the toughness deteriorates.

[0025]

EXAMPLES The present invention will be described in more detail by way of examples. Example 1 Molten steel (C = 0.26%, Si: 0.18%, Mn = 1.60%, Cr = 2.00
%, P: 0.020%, S: 0.011%, sol.Al: 0.03%, Mo = 0.
54%) is melted and made into a slab by continuous casting, and hot rolled coil (876 mm width × 2.0 mm) by hot rolling in the normal process.
Thick) was manufactured. The tensile strength of the as-rolled coil is 152kgf
It is as high as / mm ^2, and as shown in the comparative example in Table 1, it is considered that forming and welding of electric resistance welded steel pipe in a normal production line is almost impossible. Next, softening treatment was performed under the coil heat treatment conditions shown in Table 1 as described below. Tensile strength, roundness of forming, and welding defects were evaluated, and the results are also summarized in Table 1.

Run No .: A softening treatment by spheroidizing annealing of carbide was performed at 750 ° C. × 20 h for the purpose of softening treatment of the hot rolled coil. The tensile strength of the annealed material was 58 kgf / mm ² , and subsequently a 96 mm hot rolled coil with a narrow width was cut to an outer diameter of 31.8 mm,
Although it was manufactured into an electric resistance welded steel pipe with a wall thickness of 2.0 mm, no problems occurred in the pipe manufacturing process.

Run No .: As a method for improving the above-mentioned softening treatment, a softening treatment by tempering at 700 ° C. × 2 h was performed. The tensile strength after tempering was 65 kgf / mm ² , and it was easy to make pipes in the same way as above in the subsequent steps.

Run No .: In order to soften the coil before pipe making in the electric resistance welded steel pipe production line, the coil narrowly cut just before forming was heated at various temperatures by induction heating. The heating temperature was changed from 350 ° C to 720 ° C as shown in Table 1, but at 400 ° C or lower, the strength of the coil was large and the forming and welding were not stable.

The obtained electric resistance welded steel pipe (diameter 31.8 mm × thickness 2.0 mm) was heated at 920 ° C. for 10 minutes and then air-cooled. In all of the coil heat-treated products, the tensile strength of the product pipe was 158 kgf / mm ² . The tensile strength after additional tempering at 170 ° C for 20 minutes is
It was 153 kgf / mm ² .

[0030]

[Table 1]

Example 2 In this example, a steel having the steel composition shown in Tables 2 and 3 was hot-rolled, and then a heat-rolled coil was heat-treated and molded / welded to obtain an electric resistance welded steel pipe. Tables 4 and 5 collectively show the steel composition, heat treatment conditions, tensile strength after heat treatment, forming properties, and welding properties at this time. In the table, "Run No." in the column of coil heat treatment means Run No. 1 to 5 in Table 1 and indicates the contents of the softening treatment for the hot rolled coil. However, the temperature and time conditions were changed as shown in Tables 4 and 5.

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

According to the present invention, a steel pipe for an automobile door reinforcing material can be manufactured from an electric resistance welded steel pipe, which makes a great contribution to the automobile industry as a high-strength and inexpensive material.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an example of a production line for electric resistance welded steel pipe according to the present invention.

[Explanation of symbols]

 10: Hot rolled coil 12: High frequency heating device 14: Forming roll 16: Joined part

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/38

Claims (3)

[Claims] 1. By weight%, C: 0.15 to 0.40%, Si: 0.1 to 0.7%, Mn: 1.0 to 2.7.
%, Cr: 0.5 to 2.5%, P ≤ 0.025%, S ≤ 0.015%, sol.Al: 0.01 to 0.05%, hot-rolled coil is manufactured by hot rolling using steel with composition consisting of balance Fe and unavoidable impurities. Then, the tensile strength is
A method for producing a high-strength electric resistance welded steel pipe, which comprises subjecting the strength level to a level of 80 kgf / mm ² or less, and then performing forming and welding for producing an electric resistance welded steel pipe. 2. The steel of the composition further has Mo: 0.05-1.
00%, V: 0.02 to 0.10, Ni: 0.2 to 2.5%, Ti: 0.01 to 0.
The method according to claim 1, containing at least one selected from the group consisting of 10%, Nb: 0.02 to 0.10%, B: 0.0005 to 0.0050%. 3. The softening treatment for reducing the tensile strength to about 80 kgf / mm ² by soaking the hot rolled coil according to claim 1 or 2 by induction heating to 400 to 720 ° C., and then forming. A method for producing a high-strength electric resistance welded steel pipe, in which a coil is formed into a circular shape through a roll, and then both edges of the coil are subjected to high frequency heating and butt welding.