JPH04289147A - High yield ratio and high strength resistance welded steel tube for vehicle door impact bar - Google Patents

Abstract

PURPOSE:To obtain a high yield. ratio and high strength resistance welded steel tube for a vehicle door impact bar by subjecting a steel having a specified compsn. constituted of C, Mn, Si, Nb, Cr, Mo, V, Ti, B and Fe to hot and cold rolling and making a tube. CONSTITUTION:A steel contg., by weight, 0.05 to 0.3O% C, 0.6 to 2.0% Mn and 0.03 to 0.70% Si, furthermore contg., at need, one or more kinds among <=0.05% Nb, <=1.2% Cr, <=0.5% Mo, <=0.1% V, <=0.05% Ti and <=0.005% B and the balance Fe with inevitable impurities is subjected to hot rolling and cold rolling. This cold rolled steel sheet is heated to about 800 to 900 deg.C and is soaked for about >=30sec. Next, the steel sheet is subjected to water hardening and is rapidly cooled at about 200 to 1500 deg.C/sec. After that, the steel sheet is subjected to overaging treatment at about 100 to 300 deg.C according to necessary to make a tube by a resistance butt seam welding equipment. In this way, the high yield ratio and high strength resistance welded steel tube for a vehicle door impact bar having >=85% yield ration and 120 to 170kgf/mm<2> tensile strength can be obtd.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a high-yield-ratio, high-strength electric resistance welded steel pipe for vehicle door impact bars, which has high strength and is suitable for use in impact bars installed in limited spaces such as inside automobile doors. The purpose is to provide an electric resistance welded steel pipe with a high yield ratio.

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 purpose, and even if only this tensile strength is improved, bending is likely to occur, and at least the yield is reduced, making it difficult to use as the door impact bar. Effective characteristics have not yet been obtained.

[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, inevitably becomes expensive, and a high yield ratio cannot be obtained appropriately.

[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 pipe made from a hot-rolled and cold-rolled steel plate containing any one or more of 0.005wt% or less, with the remainder consisting of Fe and unavoidable impurities,
Yield ratio is 85% or more and tensile strength is 120-170
A high-yield ratio, high-strength electric resistance welded steel pipe for a vehicle door impact bar, characterized in that the resistance is 1 kgf/mm2.

[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 the refinement of the martensitic structure and hardenability, and Cr, Mo, V, Ti, and B each improve hardenability, but when Nb exceeds 0.05%, sputtering occurs during electric resistance welding. 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, deteriorating 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 to 30 seconds or more. The lower limit of this heating temperature is 80
0°C is the temperature condition necessary to obtain an austenitic structure, while the upper limit of 900°C was determined because increasing the heating temperature beyond this will lead to coarsening of austenite grains and adversely affect workability and impact properties. 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.

[0013] 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 is a large amount of solid solution C, which is thermally unstable, and the tensile strength changes when the impact bar is coated, so over-aging treatment is performed in the above temperature range.
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 equipment, 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.

[0016] The obtained product has a power of 120 to 170 kgf/m.
m2 effectively and has a yield ratio of 85% or more, making it an effective reinforcement for steel pipes for door impact bars.

[0017]

[Example] To explain the specific examples of the invention as described above, the present inventors have developed various products having the component compositions of the invention examples (Nos. 1 to 8) as shown in Table 1 below. Prepared the steel.

[0018]

[Table 1]

Each of the steels in Table 1 above is hot-rolled at a finishing temperature of Ar3 or higher, and after being made into a hot-rolled sheet, it is heated to a temperature of 500 to 650
It is generally rolled up at a temperature of °C. This hot-rolled sheet was pickled in an on-line pickling tank and then cold-rolled to obtain a cold-rolled sheet having a thickness of 2.1 mm.

[0020] The above-mentioned cold-rolled sheet is heated to 870 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.

The cold-rolled steel strip that has gone through the above steps is sent to an electric resistance welding facility, passed through a forming roll and a squeeze roll at a speed of 35 m/min, and subjected to electric resistance welding to produce an example product of the present invention. And so.

As a comparative example, steel No. 1 in Table 1 above was used.
A steel slab according to No. 2 was hot-rolled, and a steel plate obtained by cold-rolling the hot-rolled steel plate was made into pipes according to the conventional method, cut into pieces, soaked at 870°C, and then water-quenched (cooled). speed 6
00 to 800℃/sec) and then heated to 100 to 150℃
It was obtained by annealing for 10 minutes.

[0023] The obtained examples of the present invention (Nos. 1 to 8)
) and Comparative Examples (Nos. 9 to 11), the results of measuring mechanical properties are shown in Table 2 below.

[0024]

[Table 2]

That is, the present invention example No. Nos. 1 to 8 showed a high tensile strength of 126 to 159 kgf/mm2 and a yield ratio (YR) of 90% or more, whereas comparative example No. 9 to 11 have a tensile strength of 135 kgf/mm2
Since the yield ratio is 81% or less, the displacement indicating the maximum load during three-point bending is 53 mm or less for the present invention, which is significantly lower than the comparative example of 68 to 85 mm. It was confirmed that this product has favorable characteristics as an impact bar for doors.

[0026]

[Effects of the Invention] According to the present invention as described above, steel pipes for door impact bars with high tensile strength and excellent yield ratio 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 a manufacturing process for obtaining examples of the present invention and comparative examples.

FIG. 2 is a schematic explanatory diagram of a three-point bending test of a steel pipe according to the present invention.

FIG. 3 is an explanatory diagram of displacement during the three-point bending test in FIG. 2;

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
t% or less, B: 0.005wt% or less, and the remainder is Fe and unavoidable impurities. A high-yield ratio, high-strength electric resistance welded steel pipe for a vehicle door impact bar, characterized in that the yield ratio is 85% or more and the tensile strength is 120 to 170 kgf/mm2.