JP2745823B2 - Manufacturing method of as-roll type ultra-high tensile ERW steel pipe for vehicle door impact bar excellent in flattening test characteristics - 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 manufacturing an as-roll type ultra-high tensile electric resistance welded steel pipe for a vehicle door impact bar. It is an object of the present invention to provide a method capable of stably and at low cost producing an electric resistance welded steel pipe excellent in quality.

[0002]

2. Description of the Related Art A door for a vehicle such as an automobile is provided with a reinforcing member called a door impact bar made of a steel pipe or the like in order to secure safety in a side collision or the like and to obtain a light weight. ing. That is, a 70-80 kg class impact bar has been generally used as such an impact bar. Recently, Japanese Unexamined Patent Publication (Kokai) No. 2-197525 has been disclosed, Tensile strength after strain relief annealing is 60-100 kgf / mm
² is indicated.

[0003]

However, the above-mentioned prior art is not always preferable in the case of the above-mentioned impact bar. That is, the above-mentioned degree of tensile strength is still insufficient for the above-described purpose, and the thickness is inevitably increased, and the weight and safety are not sufficiently enhanced.

[0004] In order to avoid such disadvantages, the raw material components are specified, and the required elements are added. The obtained cold rolled sheet is used to form a welded pipe and then soaked and tempered in another line. It is conceivable to perform processing such as
Since the process is performed on a steel pipe that has been formed and cut to a fixed size, the process is complicated, requires many man-hours, and is inevitably expensive. In addition, there is a disadvantage that the surface roughness is inferior in quality.

[0005]

SUMMARY OF THE INVENTION The present invention is studying on to solve the technical problems in the prior art Do you I above, is as follows A was conceived. By weight%, C: 0.1-0.2%, Mn: 1.2-
1.8%, Si: 0.2-0.5%, S: 0.010%
Or less, P: 0.030% or less, sol. Al: 0.05
The steel containing 0% or less hot rolling, cold rolling and continuous heat treatment
To the treatment equipment, and 800 to 910 in the continuous annealing equipment.
℃, soak for more than 1 minute, water quench, and continuously
Rapid cooling at 600-900 ° C / sec.
An ultra-high tensile steel strip obtained by overaging at 300 ° C. is subjected to ERW welding at an ERW pipe manufacturing facility and the welded seam is locally heat-treated at 650 to 850 ° C. An excellent method for manufacturing an as-roll type ultra-high tensile ERW steel pipe for vehicle door impact bars.

[0006]

In the present invention as described above, the reasons for limiting the composition of steel will first be described in terms of wt% (hereinafter simply referred to as%) as follows.

C: 0.1 to 0.2 % C is a martensite-generating element and an element that increases the hardness of martensite, and is therefore an indispensable element for obtaining the intended strength of the present invention. Yes, about 0.1 % is necessary to obtain a preferable strength as an impact bar. If the content exceeds 0.2 %, the steel slab is likely to crack.

Mn: 1.2 to 1.8% Mn is an element for improving the hardenability of austenite. To control the amount of martensite and stably obtain a predetermined strength, Mn is required to be at least 1.2%. Further, in order to effectively obtain the characteristics of the present invention, the above effect is saturated in the case of performing the continuous annealing equipment by rapid water quenching, which adversely affects the characteristics.

Si: 0.2-0.5% Si is a deoxidizing element and requires 0.2% or more as an element contributing to the mechanical properties of the steel sheet. However, if it exceeds 0.5%, an unfavorable tendency is observed in ERW, and the mechanical properties of the ERW pipe deteriorate or the impact resistance decreases, so the upper limit is set.

According to the present invention, in principle, the above C, M
n and Si as essential elements, and the remainder is Fe
And unavoidable impurities, S: 0.010% or less, P:
The content is set to 0.030% or less, and sol. Al: 0.050% or less. By doing so, the quality in an electric resistance welded portion or the like is improved.

In the present invention, if necessary, N
One or more of b: 0.05% or less, Cr: 1.0% or less, Mo: 0.6% or less, V: 0.12% or less, Ti: 0.1% or less, B: 0.01% or less added within a limited range. And
The characteristics of each element can be used.

The steel having the above-mentioned composition is hot-rolled and cold-rolled into a steel strip. The steel strip is sent to a continuous annealing facility and heated to 800 to 910 ° C. It is. 800 ° C. is a temperature condition necessary for obtaining an austenite structure, while an upper limit of 910 ° C. avoids coarsening of austenite grains and adversely affects workability and impact properties. The soaking time is generally set to 1 minute or more so that the variation in the temperature distribution in the steel sheet is taken into account, and the austenite structure can be uniformly obtained in the width direction of the steel sheet.

The soaked material is water-quenched and continuously heated at 200 to 1200 ° C./sec (preferably 6 to 1200 ° C./sec).
(00-900 ° C / sec), but can be overaged at 100-300 ° C as needed.
In other words, if the temperature is soaked, the solid solution C is large and thermally unstable, and the tensile strength changes during the coating of the impact bar, so that the overaging treatment is performed in the above temperature range. If the temperature exceeds 300 ° C., martensite is softened and the strength is greatly reduced.

The rapid cooling described above is performed at 200 ° C. /
If it is less than sec, the generation of martensite will not be performed properly,
On the other hand, if it exceeds 1200 ° C / sec, it is industrially disadvantageous,
The cooling rate in the above range is continuously heated and soaked to heat the steel strip, and the steel strip sent at 50 to 120 m / min is used in the cooling zone in the continuous annealing equipment, such as jet water,
Can be properly implemented.

In the present invention, the steel strip quenched as described above is continuously sent to an electric resistance welding machine, is formed into a curved shape by a forming roll, and is joined to the electric resistance welding pipe by joining the opposite edges between the electrode rolls. Is done. The electric resistance welding speed is 20 to 100 m / min, preferably 30 to 60 m / min.
/ Min.

FIG. 1 shows an outline of the above-mentioned electric resistance welded pipe equipment after welding.
A is as, steel P those welded by the welding roll 1 is corrected with a sizing roll 5 through air cooling zone 3, the water cooling zone 4, is cleaved machine 6 cut seam immediately following the welding rolls Heat treatment is performed by the heat treatment mechanism 2.

The heat treatment mechanism 2 for the ERW steel pipe fed by ERW welding as described above has a width of about 10 mm and a length of 3 to 10 m, preferably 4 to 6 m, with respect to the ERW seam.
A high-frequency induction heating mechanism of about m is provided to locally heat the welded portion of the running ERW steel pipe. That is, by such local heating, the structure hardened by rapid heating and rapid cooling at the time of electric resistance welding is appropriately improved, and generation of cracks at the welded portion at the time of flattening or other processing is avoided.

[0018]

The present invention as described above will be described in detail with reference to specific examples. The present inventors prepared each steel having a component composition of 130 kgf / mm ² grade as shown in Table 1 below.

[0019]

[Table 1]

Each of the steels in Table 1 is hot-rolled at a finishing temperature of at least _three points Ar and formed into a hot-rolled sheet having a uniform structure and a thickness of 5.2 mm, and then generally rolled at a temperature of 500 to 650 ° C. Taken. This hot-rolled sheet was pickled in a pickling tank on the line and then cold-rolled to form a cold-rolled sheet having a thickness of 2.1 mm.

[0021] The cold-rolled sheet in a continuous annealing equipment receives the soaking in the soaking zone of 5 minutes after being heated to seven hundred ninety to eight hundred and ninety ° C. at which pressurized hot zone, then sent using jet water cooling zone, Cooled at about 800 ° C / sec, thickness 2.1
mm Cold-rolled steel strip at room temperature.

The cold rolled steel strip having undergone the above-described steps was sent to an electric resistance welding equipment, and was subjected to electric resistance welding at a speed of 45 m / min through a forming roll and a squeeze roll.

The obtained welded steel pipe was then passed through the heat treatment mechanism 2 having a length of 5 m to obtain a product. FIG. 2 shows the results of test measurement of the mechanical properties of the obtained ERW steel pipe.

The height H when the product obtained as described above was flattened as shown in FIG. 3 and cracks occurred in the weld, that is, the flattened height was obtained. As shown in the figure, it was confirmed that the flat height at which cracks were generated was reduced to one third or less according to the present invention, and that the height was remarkably improved.

[0025]

[Table 2]

[0026]

According to the present invention as described above, a steel pipe for door impact bars having high tensile strength and excellent flattening test characteristics is provided with uniform characteristics at high performance and at low cost. Therefore, the invention is industrially highly effective.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an outline of an apparatus according to the present invention.

FIG. 2 is a graph showing a tensile test result of one example of the electric resistance welded impact bar according to the present invention.

FIG. 3 is an explanatory diagram of a procedure for obtaining a flat height at which a crack occurs in a flattening test.

[Explanation of symbols]

 Reference Signs List 1 welding roll 2 seam heat treatment device 3 air cooling zone 4 water cooling zone 5 sizing roll 6 cut-off

──────────────────────────────────────────────────の Continuation of front page (51) Int.Cl. ⁶ Identification code FI C21D 9/50 101 C21D 9/50 101A C22C 38/00 301 C22C 38/00 301U 38/06 38/06 (72) Inventor Sugimoto Yuji 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Mitoshi Morimoto 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. 1-2-1 Kumarunouchi Nippon Kokan Co., Ltd. (56) References JP-A-60-224717 (JP, A) JP-A-50-86442 (JP, A)

Claims (1)

(57) [Claims] C .: 0.1 to 0.2% by weight, M in weight%
n: 1.2 to 1.8%, Si: 0.2 to 0.5%, S:
0.010% or less, P: 0.030% or less, sol. A
l: Steel hot rolling containing 0.050% or less, Hiyakan圧
And then sent to the continuous heat treatment equipment, where the continuous annealing equipment
Heat to 00-910 ° C, soak for at least 1 minute, then water quench
And rapid cooling at 600 to 900 ° C / sec continuously.
Flattening, wherein an ultra-high tensile steel strip obtained by overaging at 100 to 300 ° C. is subjected to ERW welding at an ERW pipe manufacturing facility, and the welded seam portion is locally heat-treated at 650 to 850 ° C. Manufacturing method of as-roll type ultra-high tensile electric resistance welded steel pipe for vehicle door impact bar which has excellent aging test characteristics.