JPH04263018A - Production of high strength resistance welded tube for vehicle door impact bar - Google Patents

Abstract

PURPOSE:To produce the resistance welded tube having high tensile strength and excellent in bending test characteristics with high efficiency by subjecting a cold rolled strip of a steel having a specific composition to heating and water quenching by means of continuous annealing equipment, and then to butt seam welding into tubular state. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.05-0.30% C, 0.6-2.0% Mn, 0.03-0.70% Si, and the balance iron with inevitable impurities and further containing, if necessary, one or more kinds among <=0.05% Nb, <=1.2% Cr, <=0.5% Mo, <=0.1% V, <=0.05% Ti, and <=0.005% B is prepared. After hot rolling and cold rolling, the resulting steel strip is heated to 800-900 deg.C by means of continuous annealing equipment, soaked for >=30sec, water-quenched, and successively welded while being formed into tubular state by means of butt seam welding equipment, by which the above steel tube can be produced.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for producing high-strength electric resistance welded steel pipes for vehicle door impact bars, and is a method for manufacturing high-strength electric resistance welded steel pipes, which are preferable for use in impact bars inside automobile doors, with high efficiency and at low cost. The purpose is to provide a method for manufacturing the same.

0002

[Prior Art] Regarding the doors of vehicles such as automobiles, reinforcing materials called door impact bars made of steel pipes or the like are installed inside to ensure safety in the event of a side collision and to make them lightweight. ing. In other words, one of these technologies that has recently been published is Japanese Patent Application Laid-Open No. 2-1.
No. 97525, the tensile strength is 60 to 100 kgf/mm2 as-made or after low-temperature stress relief annealing.
It is said to indicate.

0003

However, the conventional techniques described above are not necessarily preferable when used in the impact bar described above. That is, the above-mentioned tensile strength is still insufficient for the above-mentioned purposes.

[0004] In order to avoid such disadvantages, the material components are specified and the required elements are added to prepare the resulting cold-rolled plate, which is then welded into pipes and then subjected to treatments such as soaking quenching and annealing. However, the process is complicated and requires a large number of man-hours, and is inevitably expensive.

[0005]

[Means for Solving the Problems] The present invention has been devised after repeated studies to solve the technical problems of the prior art as described above, and is as follows. C: 0.05-0.30wt%, Mn: 0.6-2
．． 0 wt%, Si: 0.03 to 0.70 wt%, and if necessary, Nb: 0.05 wt% or less, C
r: 1.2wt% or less, Mo: 0.5wt% or less, V:
0.1wt% or less, Ti: 0.05wt% or less, B: 0
．． A steel strip obtained by hot rolling and cold rolling a steel containing any one or two or more of 0.005 wt% or less, the remainder consisting of Fe and unavoidable impurities, is sent to a continuous annealing facility,
Heated to 800 to 900°C in the continuous annealing equipment for 30
High-strength electric resistance welded steel pipe for vehicle door impact bars characterized by soaking for more than seconds, water quenching, sending the quenched steel strip to electric resistance welding equipment, forming it into a tubular shape and electric resistance welding to produce a product. manufacturing method.

[0006]

[Function] Regarding the present invention as described above, the reasons for limiting the composition of the steel will be explained below in terms of wt% (hereinafter simply referred to as %).

C: 0.05 to 0.30% C is an element that forms martensite and increases the hardness of martensite, so it is an essential element in order to obtain the strength targeted by the present invention. 0.05% is required as the minimum amount to obtain the desired strength as an impact bar. Moreover, if it exceeds 0.30%, cracks tend to occur in the steel slab, and impact resistance and weldability such as carbon dioxide arc welding deteriorate, so this is the upper limit.

Mn: 0.5 to 2.0% Mn is an element that improves the hardenability of austenite, and the minimum amount of Mn is 0.6 to control the amount of martensite and stably obtain a specified strength. % or more is required. In addition, the upper limit is such that when the properties of the present invention are effectively obtained using continuous annealing equipment using rapid water quenching, the effect is saturated and adversely affects bending workability during ERW welding pipe manufacturing, etc. Therefore, it was set at 2.0%.

Si: 0.03 to 0.70% Si is a deoxidizing element, contributes to the mechanical properties of the steel sheet, and is required to be present in an amount of 0.03% or more as an element that improves ductility and the like. But 0
．． If it exceeds 70%, sputtering during ERW welding will increase, and the mechanical properties of the ERW tube will deteriorate, or the impact resistance will decrease, so it is necessary to set this as the upper limit.

[0010] The device according to the present invention basically has the above-mentioned C, M
The essential elements are n and Si, and the remainder is Fe and unavoidable impurities. By doing so, the quality of the electric resistance welded part etc. can be improved.

However, in the present invention, Nb,
One or more of Cr, Mo, V, Ti, and B can be added within a limited range to utilize the characteristics of each element. In other words, Nb contributes to refinement of the martensitic structure and hardenability, and Cr, Mo, V, Ti, and B each improve hardenability, but when Nb exceeds 0.05%, sputtering during electric resistance welding occurs. This is set as the upper limit because this tends to occur and the mechanical properties of the electrical resistance welding part deteriorate. Also, Cr is 1
．． More than 2%, Mo more than 0.5%, V more than 0.1%, Ti
exceeds 0.05%, Cr2O3, MoO2
, V2 O3, TiO2, etc. will remain in the electrical resistance welded joint, degrading the mechanical properties of the electrical resistance welding part, and it is necessary to set these as upper limits in view of the impact resistance of the steel pipe. Furthermore, if B exceeds 0.005%, the workability of the steel will deteriorate, so it is necessary to set this as the upper limit.

[0012] Steel having the above-mentioned composition is hot-rolled and cold-rolled into a steel strip as shown in FIG. Heat and soak until heated. The lower limit of this heating temperature, 800°C, is the temperature condition necessary to obtain an austenitic structure; on the other hand, if the heating temperature exceeds the upper limit of 900°C, it will lead to coarsening of austenite grains, which will have a negative impact on workability and impact properties. It was decided to influence the situation. The reason for the lower limit of the soaking time is that if the soaking time is less than this, the temperature distribution in the steel sheet will vary and the austenite structure described above will not be uniformly obtained in the width direction of the steel sheet.

The soaked material as described above is water quenched and then continuously rapidly cooled at 200 to 1500°C/sec, preferably 500 to 1000°C/sec.
Overaging treatment at 100 to 300°C can be performed as necessary. That is, if soaked as described above, there will be a large amount of solid solute C and it will be thermally unstable, and the tensile strength will change when the impact bar is coated, so over-aging treatment in the above temperature range is performed.
If the temperature is lower than 00°C, the effect cannot be obtained, and if the temperature exceeds 300°C, the martensite will soften and the strength will decrease significantly, so the upper temperature range is set.

[0014] Regarding the above-mentioned rapid cooling, 200°C/
If the cooling rate is less than 1,500°C/sec, martensite will not be formed properly, while if it exceeds 1,500°C/sec, it will be extremely disadvantageous industrially. This can be easily carried out using jet water in the cooling zone of a continuous annealing facility for steel strips fed at ~120 m/min.

In the present invention, the steel strip quenched as described above is subsequently sent to an electric resistance welding facility, where it is curved into a circular shape by forming rolls, and the opposing edges are joined between electrode rolls to form an electric resistance welding pipe. be done. That is, in the present invention, the process up to the manufacture of the electric resistance welded tube is carried out continuously, and the electric resistance welded tube is sequentially cut into predetermined dimensions and made into the desired impact bar, thereby achieving highly efficient and continuous manufacturing. It becomes possible.

The results of measuring the hardness in the vicinity of the ERW weld in the obtained product are as shown in FIG. As a result, the high strength characteristics of the steel pipe as a whole are sufficiently increased even when ERW welding is performed, and the hardness is equal to or higher than that of the other parts.
Effectively secure 170kgf/mm2.

[0017]

[Example] To explain specific examples of the invention as described above, the present inventors have prepared examples of the invention (Nos. 1 to 8) and comparative examples (Nos. 1 to 8) as shown in Table 1 below.
．． Steels having the compositions 9 and 10) were prepared.

[0018]

[Table 1]

Each of the steels listed in Table 1 above is hot-rolled at a finishing temperature of Ar3 or above to form a hot-rolled sheet with a uniform structure and a thickness of 5.2 mm, and then generally rolled at a temperature of 500 to 650°C. It will be done. This hot-rolled sheet was pickled in an on-line pickling tank and then cold-rolled into a cold-rolled sheet having a thickness of 2.1 mm.

[0020] The above-mentioned cold-rolled sheet is heated to 790 to 1075°C in the continuous annealing equipment, then subjected to soaking treatment in the soaking zone for 5 minutes, and then sent to the cooling zone where jet water is used. It was cooled at 780° C./sec to form a cold-rolled steel strip with a thickness of 2.1 mm at room temperature.

[0021] The cold-rolled steel strip subjected to the above-described steps was sent to an electric resistance welding facility, and was subjected to electric resistance welding by passing through a forming roll and a squeeze roll at a speed of 35 m/min.

The mechanical properties of the obtained as-welded steel pipe were tested and measured, and the results are shown in Table 2 below.

[0023]

[Table 2]

That is, in the example No. of the present invention. Nos. 1 to 8 showed a high tensile strength of 125 to 168 kgf/mm2, and the maximum load during three-point bending was 1202 kgf or more, whereas Comparative Example No. Since the tensile strength of Nos. 9 to 14 was less than 120 kgf/mm2, the maximum load during three-point bending was as low as 1098 kgf or less, confirming that the products according to the present invention have favorable characteristics as impact bars for automobile doors. It was done.

[0025]

[Effects of the Invention] According to the present invention as described above, steel pipes for door impact bars with high tensile strength, excellent yield strength and bending test characteristics can be produced as products with uniform characteristics at high efficiency and at low cost. Since it can be manufactured, it is an invention that has great industrial effects.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the present invention.

FIG. 2 is a graph showing an example of hardness distribution in the vicinity of the electric resistance welding part of the electric resistance welding impact bar according to the present invention.

Claims (1)

[Claims] [Claim 1] C: 0.05 to 0.30 wt%,
Mn: 0.6-2.0 wt%, Si: 0.03-0.
Contains 70 wt%, and if necessary, Nb: 0.0
5wt% or less, Cr: 1.2wt% or less, Mo: 0.5
wt% or less, V: 0.1wt% or less, Ti: 0.05w
Continuous annealing of a steel strip obtained by hot rolling and cold rolling a steel containing one or more of B: 0.005 wt % or less, and the remainder consisting of Fe and unavoidable impurities. 800 to 90 in the continuous annealing equipment.
A vehicle door impact bar characterized by heating to 0°C, soaking for 30 seconds or more, water quenching, sending the quenched steel strip to an ERW welding equipment, forming it into a tubular shape and ERW welding it into a product. Manufacturing method for high-strength ERW steel pipes.