JPH106769A - Bracket integral type door impact beam and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce costs for manufacture by setting a slit width formed when a belt-like steel is formed to be cylindrical by roll-forming to a prescribed value or lower in a final product. SOLUTION: A steel containing various chemical components is melted in a converter and continuously casted to a slab. This slab is hot-rolled in a hot strip mill to form a hot-rolled steel plate having a thickness of 1.8mm. The obtained hot rolled steel plate is subjected to acid pickling and slit to form a belt-like billet and then a cylindrical shape having a diameter of 31.8mm is formed. For forming the cylindrical shape by roll forming, the width of a slit formed between both edges of the belt steel is set to 3mm or lower. Further, this is cut to a length of 7m and, by subjecting this to heating of 900 to 1000 deg.C and water hardening, a structure mainly of martensite is obtained. Then, the steel is cut again to a part length of 750mm and a bracket part is formed in a pipe end part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impact beam mounted on a side door of an automobile and a method of manufacturing the same.

[0002]

2. Description of the Related Art As part of safety measures for automobiles, an impact beam for absorbing an impact is mounted on a side door to protect an occupant in a side collision.
This impact beam generally has a 1200 N / mm ²
A quenched steel pipe having a strength of 1,600 N / mm ² is used, and a steel plate bracket is welded and joined to the quenched steel pipe, and then attached to an automobile.

[0003] The impact beam based on the quenched steel pipe is excellent in toughness, component weight and mountability, but has a drawback of high cost.

[0004]

Current impact beams based on quenched steel pipes are tough, lightweight and excellent in mountability, but suffer from high cost. An object of the present invention is to propose a bracket-integrated door impact beam and a method for manufacturing the same, which achieve cost reduction by means such as reduction in the number of parts, omission of welding, etc., while preserving their excellent properties. is there.

[0005]

According to the present invention, a strip-shaped steel is formed into a cylindrical shape, but is not welded or joined like an electric resistance welded steel pipe, leaving a slit formed between both edges of the strip. Cut to length. Next, a quenching treatment is performed to increase the strength as a structure such as martensite. Thereafter, the tube is cut again to a predetermined length, and both ends of the tube are softened by heating. Further, it is characterized in that the softened slits at both ends of the pipe are pushed and expanded so as to be flattened and processed into a bracket. By adopting such a processing method, the central part has the same toughness as the conventional hardened steel pipe, and since the bracket part is not separately formed and processed, the number of parts, welding processing, etc. can be omitted, An inexpensive bracket-integrated door impact beam can be manufactured.

Although the bracket-integrated door impact beam of the present invention does not particularly specify a steel component, it is desirable to secure roll forming properties, shape freezing properties during roll forming, and strength and toughness during quenching. , C: 0.15 to 0.35 mass%, Si: 0.05 to
0.50 mass%, Mn: 0.20 to 1.50 mass%, P:
A belt-shaped steel containing 0.020 mass% or less, S: 0.020 mass% or less, and Al: 0.01 to 0.10 mass% is used, or Ti: 0.01 to 0.10 mass%.
B: 0.0005 to 0.010 mass%, Ni: 0.20
１．５1.50 mass%, Cr: 0.05〜1.00 mass%,
And Ca: 1 selected from 0.001 to 0.01 mass%
It is preferable to use a strip-shaped steel containing one or more kinds.

[0007]

The present inventors have studied various measures for cost reduction while retaining the features of a door impact beam using a high-strength hardened steel pipe. As a result, performance characteristics required for door impact beams, such as strength, toughness, static bending strength, and absorbed energy, are equivalent to those of quenched steel pipes, and inexpensive brackets that can reduce the number of parts and omit welding processing They have found an integrated door impact beam and a method of manufacturing it.

Hereinafter, the configuration of the present invention will be described in detail. In the present invention, a cylindrical shape is formed by roll forming, but a first feature is that a slit formed between both edges of the strip is left as it is without welding. This means that there is no formation of a white band, a high-carbon internal melting bead, or the like on the welded joint surface seen in the ERW steel pipe. Therefore, during quenching, factors that cause unstable destruction, such as quenching of the inner surface molten bead portion or formation of a fragile lens-like martensite structure, are released, and the stability of product quality is improved.

However, the width of the cylindrical slit is 3.0.
mm, and when a vertical stress is applied to the slit, only the vicinity of the slit is deformed and buckled preferentially, and the static bending absorbed energy is lower than that of the quenched steel pipe. .

[0010] The quenching heating method employed in the present invention may be any of radiation heating, direct current heating, etc., and need not be particularly specified. It is not necessary to particularly define the type of the quenching refrigerant, the stirring method, and the like.

A second feature of the present invention is that both ends are tempered after the whole is quenched and cut to a predetermined component length. Recently, there has been a demand for improving the safety of automobiles, and there is a trend toward strengthening the structure of a vehicle body. Door impact beams are also safety components required to protect occupants during a side collision, and improvements are being made to improve their performance. As a recent improvement, not only the strength improvement of the door alone, but also the strengthening of the vehicle body structure including a center pillar for fixing the door is being promoted. With these activities, as the stiffness of the components surrounding the door impact beam has been improved, the local lack of strength of the door impact beam itself has become unacceptable. That is, sufficient performance cannot be expected with a partially quenched door impact beam, a door impact beam with one pipe end obliquely cut, or the like. The present invention has been made based on such a background.

In the present invention, the end of the door impact beam is constituted only by the bracket. Therefore, in order to secure the strength of the bracket portion, the tube end portion is subjected to quenching and tempering to increase rigidity. If the tempering temperature is lower than 350 ° C., it is difficult to open the slit portion and form a bracket, so the lower limit tempering temperature is 350 ° C. Further, when tempering at a temperature exceeding 650 ° C., the strength of the bracket portion is significantly reduced, so the upper limit tempering temperature was set to 650 ° C. This process of strengthening the quenching and tempering of the bracket portion reduces the plate width of the bracket portion to the developed width of the steel pipe, which is basically different from the bracket forming of the unquenched portion.

The degree of bending back for forming the bracket portion is the largest at the pipe end, and the degree of bending back becomes smaller as it enters the inside. Therefore, in order to form the bracket, it is not necessary to temper the entire area to be bent back at a high temperature, and the rigidity of the door impact beam is secured as the reheating length for tempering becomes shorter. However, the length of the reheating section at both ends is 20 mm
If it is less than 10 mm, it is difficult to bend the bracket portion back to a flat shape, so the minimum length was set to 20 mm. Further, since the length of the bracket portion is generally within 150 mm, the maximum value of the reheating length of both pipe ends to be tempered was set to 150 mm.

[0014] The temperature at which the bending back after reheating is performed does not necessarily have to be cooled to room temperature. When performing a series of processes from re-cutting to the part length, reheating for tempering, and bending back processing for bracket formation, performing bending back processing in the cooling process makes the cooling equipment simpler, In addition, the processing energy and processing cycle are shortened, which is advantageous in terms of equipment and economy.

Next, desirable steel components in the present invention will be described. C: An element necessary for obtaining strength after quenching, and if less than 0.15 mass%, the strength required as a door impact beam cannot be obtained. Further, in the present invention, since welding is not performed unlike the ERW steel pipe, there is no formation of a high-carbon inner surface molten bead which leads to the introduction of defects, so that more carbon can be contained and economical high strength can be obtained. Can be. However, if it exceeds 0.35% by mass, the toughness is remarkably reduced, not only exhibiting unfavorable properties as a door impact beam such as breaking under bending stress, but also making the forming of the bracket part difficult. Therefore, the C content is specified in the range of 0.15 to 0.35 mass%.

Si: an element used as a deoxidizing agent for steel, which is also effective in improving hardenability.
If it is less than 05 mass%, the effect cannot be secured. When the amount of addition increases, the material becomes brittle, so the upper limit is 0.50.
mass%.

Mn: Mn is an element that is useful in suppressing the adverse effects of S, increasing the hardenability of steel, and achieving toughness. However, an excessive content lowers the toughness. From these facts, the content was specified in the range of 0.20 mass%, to 1.50 mass%.

P: an element that lowers the toughness of steel. In particular, since the effect is remarkable in a quenched structure such as martensite, the upper limit of the content is set to 0.020 mass% or less.

S: Since sulfides such as MnS are formed and the toughness of the steel is reduced, the upper limit of the content is specified to be 0.020 mass% or less.

Al: an element effective as a deoxidizing agent,
If the content is less than 0.01 mass%, the effect is not sufficient, and if it exceeds 0.10 mass%, the formation amount of alumina-based inclusions becomes remarkable and the toughness of the steel is deteriorated. Specified within the range of 10 mass%.

Also, Ti, B, Ni, C as selective components
r and Ca exhibit the following actions, respectively.

Ti: a strong deoxidizing element, and nitrogen is replaced by Ti
It is an element that is fixed as N and is also useful in securing the effect of improving the hardenability of B. However, if the content is less than 0.01 mass%, the effect is not sufficient. If the content exceeds 0.10 mass%, a large amount of carbonitride is formed in large quantities, and the toughness of the steel is impaired. Therefore, the content is 0.01 mass% to 0.10 mass
Specified in the range of%.

B: An element that improves the hardenability of steel and improves the toughness after quenching. By coexistence of Ti etc.
Even at 0.0005 mass%, the effect appears. However, if it exceeds 0.010 mass%, borides are formed and the toughness of the steel is reduced. Therefore, its content is 0.000
It was specified in the range of 5 mass% to 0.010 mass%.

Ni: a useful element that improves the hardenability of steel and improves toughness and strength at the same time. However, 0.
If it is less than 20 mass%, the effect is not clear. 1.5
Exceeding 0 mass% not only saturates the effect, but also increases the cost. Therefore, its content is 0.20 to 1.50 ma
ss%.

Cr: an element effective for improving the hardenability of steel. However, if added excessively, the material becomes brittle, so its content is specified in the range of 0.05 to 1.00 mass%.

Ca: Spheroidal inclusions are made spherical,
It is an element effective for improving workability and toughness. But,
If the content exceeds 0.01 mass%, the amount of inclusions increases, and on the contrary, the toughness of the steel decreases. Therefore, its content is defined as 0.001 to 0.01 mass%.

The steel of the above-mentioned composition is worked into a strip to obtain a raw material. The strip may be worked by hot rolling or cold rolling.

The method of forming the strip-shaped steel into a cylindrical shape by roll forming is not particularly limited, but a tube forming machine is most effective, and a high-frequency tube forming machine is capable of forming at a high speed. It is a target.

After forming into a cylindrical shape and cutting and quenching, it is cut again to a predetermined component length and reheated for tempering.
As a reheating method, induction heating or the like that can be heated for a short time and has high temperature control accuracy is desirable.

Further, although there is no particular limitation on the processing step of bending back the slits at both ends of the pipe and forming it into a bracket shape,
The non-bent part is restrained and fixed, the conical jig is pressed in from both ends, and both ends are pushed and expanded simultaneously, and the bracket is formed in two steps by flattening the ends by pressing. It is efficient to do so.

[0031]

The present invention will be specifically described below with reference to examples. Steels of various chemical components were melted in a converter and continuously cast into slabs. This slab was hot-rolled by a hot strip mill to produce a hot-rolled steel sheet having a thickness of 1.8 mm. The resulting hot-rolled steel sheet was pickled, slit into strip-shaped materials, formed into a cylindrical shape having a diameter of 31.8 mm, cut into 7 m lengths,
Heating and water quenching at 00 to 1,000 ° C. was performed to obtain a structure mainly composed of a martensite structure. Next, 750mm
The length of the pipe was cut again, and the as-quenched pipe and both pipe ends tempered at a temperature of 300 to 650 ° C. were bent back to form a bracket part. Table 1 shows the chemical components of each sample, heat treatment conditions, processing conditions, average slit width of the final product, and the like.

[0032]

[Table 1]

Regarding the product characteristics of the bracket-integrated door impact beam,
A tensile test, a static bending test, and an impact bending test were performed. Regarding the workability of the bracket, the effects of the chemical composition and the tempering conditions were examined. The static bending test conditions were such that the slit portion was on the lower side, the distance between the fulcrums was 800 mm, and one point at the center was loaded with an arc-shaped jig having a radius of 150 mm. The impact bending test conditions were the same as in the static bending test, with the slit portion being on the lower side and two points supported at a distance between the fulcrum points of 430 mm.
Weight of 14 kg, 2.1 m weight with a tip radius of 25 mm
The steel pipe was dropped from its height and the deformation and breakage of the steel pipe were observed. The deformation amount here indicates a deformation state of the door impact beam. The larger the numerical value, the sharper the door impact beam is bent, and the smaller the cabin space. In addition, evaluation of the workability of the bracket was performed by pressing a conical jig from both pipe ends and bending it back, then breaking the product (×) and deforming the formed shape in a combined process of flattening the pipe end by pressing. (△) was evaluated, and the conditions for forming the bracket of the target shape were good (○)
And Table 2 shows the evaluation results.

[0034]

[Table 2]

As apparent from Table 2, the sample N0.1
However, since the carbon content in steel is low, the strength after quenching is low, and the strength performance as a door impact beam has not been obtained.

Sample No. 2 to 5 are the results of evaluating the same chemical components with different slit widths of the products,
Sample No. having a slit width exceeding 3 mm. 5 shows that the static bending test causes early buckling, and the impact bending test has a large amount of deformation and has a problem in securing a space necessary for ensuring the safety of the occupant.

For samples N0.6 to N9, the workability of the bracket portion was examined by changing the tempering temperature of the sample having the chemical composition range of the present invention. All of the samples have satisfactory mechanical properties as a door impact beam, but have a lower tempering temperature than the range of the present invention. In the case of No. 6, cracks occur during the formation of the bracket portion, and a good product cannot be obtained.

Sample 10 is a sample in the range of the chemical components of the present invention to which Ni is added, but has good mechanical properties. In addition, when an attempt was made to form a bracket by setting the tempering range of the pipe end to 50 mm, a product which did not hinder the function although some distortion remained was obtained.

Samples N0.11 to N14 are the results of evaluation of a sample in the chemical composition range of the present invention which has been subjected to calcium treatment. Sample NO. In No. 11, when the jig was pressed in, the material was broken and could not be processed. Sample N with wide product slit width
O. No. 13 shows that the specimen did not buckle early in the static bending test and broke in the impact bending test and could not obtain the performance characteristics as a door impact beam.
O. Nos. 12 and 14 showed good characteristics.

Samples N0.15 to N18 investigate the effect of chemical components in the high carbon region. N0.
Nos. 15 and 17 have good mechanical properties even with high carbon, but the sample N0.16 with a high phosphorus content and the N0.18 with a carbon content outside the range of the present invention are static. In both the bending test and the impact bending test, the test piece breaks, and the mechanical properties required for the door impact beam cannot be obtained.

FIG. 1 shows the hardness distribution of the product of the present invention obtained by tempering a pipe end of 30 mm at 600 ° C. in comparison with that of a door impact beam whose pipe end is not quenched. It is clear that the product of the present invention has a high overall hardness, a gradual change, and a structure in which stress concentration does not occur.

[0042]

As described above, according to the present invention, a bracket-integrated door impact can be achieved at low cost by reducing the number of parts, omitting welding work, etc. while preserving the excellent properties of the current door impact beam. Could be a beam.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a distance from a pipe end and hardness.

Claims (3)

[Claims] 1. A bracket-integrated door impact beam, wherein a slit width formed when a belt-shaped steel is formed into a cylindrical shape by roll forming is 3.0 mm or less in a final product. 2. A belt-shaped steel is formed into a cylindrical shape by roll forming, cut into a predetermined length, and reinforced by quenching.
Then, after re-cutting to a predetermined component length, heating and softening both pipe ends, and further opening both softened pipe ends through slits and processing them into a bracket shape, a bracket-integrated door impact characterized by the following features: Beam manufacturing method. 3. The length of the reheating portions at both ends is 20 mm to 15 mm.
3. The method of manufacturing a bracket-integrated door impact beam according to claim 2, wherein the re-heating temperature is 0 to 350 [deg.] C.