JPH04191325A - Manufacture of high strength steel tube excellent in straightness - Google Patents

Abstract

PURPOSE:To manufacture a high strength steel tube excellent in straightness by subjecting a steel tube contg. specified ratios of C, Si, Mn, Cr, P, S and solAl to heating under specified conditions and thereafter subjecting it to air- cooling while it is rotated around an axial center. CONSTITUTION:A steel tube having a steel compsn. contg., by weight, 0.15 to 0.40% C, 0.10 to 0.70% Si, 1.00 to 2.70% Mn, 0.50 to 2.50% Cr, <=0.025% P, <=0.015% S, 0.01 to 0.05% solAl and the balance Fe with inevitable impurities is heated at 850 to 1050 deg.C for 0.5 to 30min and is thereafter subjected to air- cooling while it is rotated around an axial center. In this way, the high strength steel tube small in bends is abundantly and stably obtd. at a low cost and is suitable as the reinforcing material for an automobile door.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing high-strength steel pipes with excellent straightness, particularly for use as structural members such as reinforcing materials for doors of transportation machines such as automobiles and various frame materials. The present invention relates to a method for inexpensively manufacturing a steel pipe with high strength and excellent straightness, which can obtain a hardened structure even when air-cooled, and is suitable for

(Conventional technology) Conventionally, regarding automobile door reinforcing materials in Japan,
The situation was as described below. In other words, ■Many automobile doors for the domestic market were not equipped with reinforcing materials.

■Corrugated plates and m-tube reinforcing materials were used in the doors of exported and foreign cars.

(2) As the corrugated sheet, a hot-rolled or cold-rolled steel sheet of 80 to 100 kgf/mm2 was used.

■The steel pipe has a weight of 80 to 150 kgf, which is thin-walled and small-diameter to reduce weight, and quenched to increase strength.
/mmz steel pipe was used, but the dimensional accuracy, especially the bending, was large.

■As a measure to prevent bending, after manufacturing seamless steel pipes and welded steel pipes, we perform dimensional correction such as bending in the soft state as rolled, and then cut them into short pipes of about 1 m and then heat treat them by water quenching and tempering. This minimized the distortion (bending) caused by heat treatment, but it was not possible to completely prevent the bending. After heat treatment, the steel pipe has a high strength and its elastic limit increases, making it difficult to plastically deform the pipe, and its toughness deteriorates and cracks occur, making dimensional correction virtually impossible.

(Problems to be Solved by the Invention) Steel pipes used as automobile door reinforcing materials are required to be made of a material as high in strength as possible, inexpensive, and with minimal bending.

For steel pipes used as automobile door reinforcing materials, the small bending means that automobiles use mass production methods, so they can be quickly and accurately installed and fixed in place inside complex door structures. is important above.

For example, in order to give a steel pipe as a reinforcing material a predetermined strength, it is essential to perform water quenching heat treatment, but it is inevitable that the steel pipe will be bent by water cooling after heating.

Therefore, it is necessary to correct such bending by mechanical means.

However, the reinforcing steel pipe after heat treatment ■It has high strength and is essentially difficult to correct.

■When heated to high temperatures, it undergoes tempering and its strength decreases significantly.

■In order to ensure strength even when heated at high temperatures, it is necessary to add large amounts of expensive alloying elements.

When trying to satisfy these problems,
This time, we are unable to provide inexpensive high-strength reinforcing steel pipes.

An object of the present invention is to solve the problems of the prior art described above and to provide a manufacturing method that can stably and inexpensively obtain a large amount of high-strength steel pipes with high strength and little bending for use as automobile door reinforcing materials.

(Means for Solving the Problem) In order to achieve the above object, the inventor has conducted extensive research,
As a result of studying various aspects of increasing the strength of automobile door reinforcing materials, dimensional correction, means thereof, and economic efficiency, the following findings were obtained and the present invention was completed.

■High-strength quenching and tempering are the easiest heat treatments to increase the strength of steel pipes for reinforcing materials. Among them, quenching and low-salt tempering provide good toughness and are advantageous for obtaining high strength at low cost. .

■Necessity of straightening to improve dimensional accuracy To ensure the dimensional accuracy required for reinforcing materials, especially bending, it is necessary to use some kind of straightening machine. However, as mentioned above, when the strength is high (the elastic limit becomes high) and the toughness decreases, straightening becomes extremely difficult, and cracking may occur when straightening at room temperature.

Therefore, by using a steel material that can be hardened even by air cooling without water cooling or oil cooling, and can sufficiently obtain a predetermined strength and toughness, the occurrence of bending due to hardening can be suppressed.

Further, if a steel material having a hardened structure by air cooling is used, dimensional correction can be easily performed during air cooling when even higher dimensional accuracy is required.

■Economy: If the process can be carried out on long pipes, the heat treatment cost will be dramatically improved compared to short pipes.

Here, the present invention, which was made based on the above knowledge, is based on the above findings.
So, C: 0.15~0.40%, Si: 0.10~
0.70%, Mn + 1.00-2.70%, C
r: 0.50-2.50%, P: 0.025%
Hereinafter, S: 0.015% or less, sol. Al:
0.01-0.05%, or further, Mo: 0.05-1.00%, V: 0.02-0.10
%, Ni: 0.20-2.50%, Ti:
0.02-0.10%, Nb: 0.01-0.10
%, and B: 0.0005 to 0.0050%, and a steel pipe having a steel composition consisting of one or more of 0.0005 to 0.0050%, the balance being Fe and unavoidable impurities is heated at 850 to 1050°C for 0.5 to 30 minutes. This method of manufacturing high-strength steel pipes with excellent straightness is characterized in that the pipes are cooled in air while being rotated around the axis.

According to another feature of the invention, 850-10
The steel pipe may be straightened at a temperature of 300°C or higher during air cooling after heating at 50°C for 0.5 to 30 minutes.

From a further point of view, as mentioned above, 850 to 1050℃
After heating for 0.5 to 30 minutes at
Alternatively, the mixture may be heated at ℃ for 1 to 30 minutes and then allowed to cool.

Note that the high-strength steel pipe according to the present invention includes specific ones,
Generally, but not limited to, seamless steel pipes or welded steel pipes.

(Function) Next, the reason why the steel composition and treatment conditions are limited as described above in the present invention will be explained in further detail.

C; This is an essential element in order to obtain high strength at low cost.

If it is less than 0.15%, the required 120 kgf/
Strength greater than arm2 cannot be obtained by heat treatment. 0
．． If it exceeds 40%, the strength will be too high if quenched and cracking will occur.

Si: A necessary component for deoxidation during steel manufacturing. If it is less than 0.10%, deoxidation is insufficient and toughness cannot be ensured. When it exceeds 0.70%, welding defects are likely to occur during welded pipe manufacturing.

Mn: A component effective in improving hardenability. If it is less than 1.0%, the effect is insufficient, and if it exceeds 2.70%, defects will easily occur during vilent production during steel manufacturing, and the toughness will deteriorate after heat treatment.

PXS: These are typical impurities in steel. Prevents cracking,
0.025 each to prevent toughness deterioration after heat treatment
It is essential to limit it to below the upper limit of Z and 0.015χ. In particular, when T and S≧150 kgf/mm2, in order to ensure toughness (vTrs≦-20°C), P≦0.
015%, S≦o, and oos% are desirable.

sol,^l: Like Si, it is added as a deoxidizing component, and the amount added is limited to 0.01 to 0.052 for the same reason as in the case of Si.

Cr: Effective for improving hardenability, improving toughness, and increasing resistance to temper softening. In particular, long tubes are air-cooled and quenched.
Like Mn, it is an essential component in order to ensure a certain level of strength and prevent bending due to air cooling. If the upper limit of 2.5χ is exceeded, the cost becomes high and defects in the weld cannot be prevented. If it is less than the lower limit of 0.5χ, hardenability, toughness, and softening resistance will not be improved.

In the present invention, in order to further improve the strength and toughness, a steel further containing at least one or more of Mo, V, Ni, Ti, Nb, and B may be used. Preferably Ti-BXN
It is a combination of b-Ti-B.

Therefore, the reasons for limiting these additive elements will be explained below.

MO: Provides the same effect as Cr. It has an auxiliary role to Cr addition. 0.05-1. Outside the addition range of OOχ, Cr
have the same disadvantages.

(2): Addition of 0.02% or more provides high softening resistance against tempering at high temperatures. Additions above the upper limit of 0.10χ are expensive.

Ni: Addition of 0.20% or more is effective in improving hardenability and toughness. Even if the upper limit of 2,50χ is exceeded, the effect is effective, but it becomes expensive.

T1, Nb: These elements are effective in preventing coarsening of steel grains during quenching and improving the toughness of welded joints when added in amounts of 0.02X and 0.01% or more, respectively. However, if each exceeds the upper limit of 0.102, the toughness deteriorates.

B: Addition of B is effective in improving hardenability. 0.0005χ
Below the lower limit of , the effect is insufficient; on the other hand, 0,00
If the upper limit of 50χ is exceeded, the toughness deteriorates.

Next, the reasons for limiting the processing conditions for the manufacturing method of the present invention will be explained.

First, a steel having the above-mentioned composition is prepared and then a steel pipe is manufactured by appropriate means, but the present invention is not particularly limited to such a pipe-making process.

As a preferred example, in the case of a seamless steel pipe, it may be manufactured by the Mannesmann method, hot extrusion method, or the like. Also, as a welded steel pipe,
Typical examples include ERW steel pipes (electric resistance welded steel pipes), forge welded steel pipes, TIG welded steel pipes, submerged arc welded steel pipes, laser welded steel pipes, and composite welded steel pipes that combine ERW, TIG, and laser.

The steel pipe prepared in this way has a diameter of 850 to 1050'
After heating for CX 0.5 to 30 minutes, air cooling is performed to obtain a quenched structure.

850" to austenitize steel for hardening
Heating above C is required. If it exceeds 1050"C, the grains of the steel will become coarser and the toughness will deteriorate. On the other hand, if the heating time is less than 0.5 minutes, the entire steel pipe cannot be uniformly heated, and the final It becomes difficult to make the mechanical properties of the steel pipe uniform.If the heating temperature is close to 1050℃ for more than 30 minutes, the crystal grains of the steel will become coarser, the toughness will deteriorate, and quench cracks will occur. It becomes easier.

The easiest and cheapest way to harden and make it 111ilIi is to heat it and then water-cool it (making most of the martensitic m weave), but such a method results in large quenching distortion and high In high-strength steel, quench cracking becomes a serious problem.

Therefore, in the present invention, by using a component that can be sufficiently hardened by air cooling as described above, it is possible to manufacture an inexpensive and stable steel pipe.

Note that in the present invention, air cooling includes all types of cooling that utilize air as a cooling medium without using water, oil, or the like. For example, it includes not only air cooling but also forced air cooling.

However, such air cooling alone may cause bending, so it is necessary to cool the pipe while rotating it around the axis. Specifically, for example, the curve can be corrected by allowing it to cool on a cooling bed configured to rotate around the axis of the tube. Other possible methods include air cooling while horizontally mounting and rotating on a row of turning rollers.

In the case of the present invention, the tempering treatment described below is not necessarily required, but it is preferable to perform the tempering treatment to improve toughness.

Tempering is performed by heating at 50 to 3,500°C for 1 to 30 minutes and then cooling, but sufficient toughness cannot be ensured at higher temperatures and for shorter times than this. On the other hand, sufficient strength cannot be obtained at higher temperatures and for longer periods of time.

As already mentioned, according to the present invention, straightening is not necessarily necessary since bending can be prevented by air cooling while rotating around the axis during the air cooling stage, but if necessary, mechanical straightening can be performed. Straightening may be performed, and in that case, the straightening may be performed during the air cooling step, instead of or following the air cooling performed while rotating around the tube axis.

Such mechanical deformation is carried out using a roll straightening machine, an inclined roll straightening machine, a press straightening machine, or the like. Straightening during air cooling can eliminate the above-mentioned difficulties and easily obtain the desired dimensional accuracy. However, at low temperatures below 300°C, the strength of the steel pipe increases, making dimensional straightening difficult due to plastic deformation, and furthermore, machining becomes difficult. A crack forms inside.

In addition, according to the present invention, even if a long tube with a length of 5 m or more is heat-treated, the dimensional rounding can be suppressed to 1111 m or less per piece while maintaining high strength. Productivity can be improved by cutting short tubes to length.

EXAMPLES The present invention will be specifically explained by examples, but the present invention is not limited thereto.

Table 1 shows the components of the test steel in the present invention.

(Hereinafter, blank spaces) The steels shown in Table 1 above were made into pipes under the following conditions and heat treated.

An example of A-series steel is melted in a converter, hot-rolled using a Q trowel, and made into hot coils of various plate thicknesses, which were made into electrical resistance welded steel pipes with an outer diameter of 25I using the ERW method.

Examples of B-series steel are seamless steel pipes with an outer diameter of 30 mm and various wall thicknesses manufactured by electric furnace melting → blooming → Mannesmann pipe manufacturing method.

In each case, the raw tube was heated in a Hutch furnace or a continuous high-frequency heating furnace, and then air-cooled and quenched. Thereafter, dimensional correction during air cooling was carried out while rotating the tube shaft on a cooling bed, or using a 2-1-21 type inclined roll straightening machine with an interval of 400 mm between stands.

In addition, the heating using the Hatsuchi furnace described above is a large amount of heating in which multiple steel pipes are usually supported and mounted at multiple locations in the longitudinal direction and heated in a stationary state. This is a heating means that increases the amount of bending.

In addition, high-frequency heating continuous furnace heating involves rotating the steel pipes one by one around their axis and moving them in the axial direction; Equipped with a ring-shaped induction heating coil.

This method heats the steel pipe by passing it through the coil, and the amount of bending of the steel pipe after heating is extremely small compared to the hatch furnace heating described above.

These processing conditions and results are summarized in Table 2.

(hereinafter F margin) The bending of the product was expressed as a value per meter (mm).

Straightening of the bend using an inclined roll straightening machine was performed in one process. In the table, long tubes are 5 m or more, and short tubes are tubes cut to about 1 m.

If the specified heat treatment is performed within the specified range of ingredients, the required straightness and strength and toughness (vTr
A product satisfying the following conditions (s≦-20°C) was obtained. In particular, with regard to bending, it can be seen that even long tubes have bends of 11,811 or less after treatment, and that precision can be further improved if dimension correction is performed using a straightening machine during cooling.

On the other hand, in the case of comparative examples and conventional examples that do not meet the conditions of the present invention, one or more of straightness, strength, and toughness do not satisfy the target.

(Effects of the Invention) Since the present invention is configured as described above, it is possible to obtain a reinforcing material for automobile doors that has strength and toughness exceeding a predetermined level and excellent dimensional accuracy by using the steel composition, heat treatment, and straightening method according to the present invention. Steel and steel pipes are economically available and extremely useful in industry.

Claims (4)

[Claims] (1) In weight%, C: 0.15-0.40%, Si: 0.10-0.70
%, Mn: 1.00-2.70%, Cr: 0.50-2
．． 50%, P: 0.025% or less, S: 0.015% or less, sol. A steel pipe having a steel composition consisting of Al: 0.01-0.05%, balance Fe and unavoidable impurities is heated at 850-1050°C for 0.5-30 minutes, and then air-cooled while rotating around the axis. A method for manufacturing high-strength steel pipes with excellent straightness. (2) In addition to straightening the bending by rotating the steel pipe around its axis during air cooling after heating at 850 to 1050°C for 0.5 to 30 minutes, or alternatively, mechanically straightening the bending of the steel pipe in a temperature range of 300°C or higher. The method for manufacturing a high-strength steel pipe with excellent straightness according to claim 1. (3) The method for producing a high-strength steel pipe with excellent straightness according to claim 1, wherein the method comprises heating at 850-1050°C for 0.5-30 minutes, cooling in air, then heating at 50-3500°C for 1-30 minutes, and then allowing to cool. (4) The steel composition, in weight%, further includes Mo: 0.05 to 1.00%, V: 0.02 to 0.10.
%, Ni: 0.20-2.50%, Ti: 0.02-0
．． 10%, Nb: 0.01 to 0.10%, and B: 0.0005 to 0.0050%. A method for manufacturing high-strength steel pipes with excellent strength.