JPH0565597A - High strength hardened steel tube for reinforcing material for automobile door - Google Patents

Abstract

PURPOSE:To improve the strength, toughness and rustproofing properties of a steel tube by subjecting a resistance welded steel tube having a compsn. in which C, Si, Mn, P, S and Al are specified to induction hardening and forming its structure into a one essentially consisting of martensite. CONSTITUTION:In this steel tube, the opening parts of both edges are closed by forming, and it is subjected to induction hardening to form its structure into a one essentially consisting of martensite. This steel tube is formed of a compsn. constituted of, by weight, 0.1 to 0.3% C, 0.05 to 0.5% Si, 0.2 to 1.5% Mn, <=0.02% P, 0.02% S, 0.01 to 0.1% Al and the balance Fe. The above steel tube has excellent strength and toughness and, furthermore, has the rustproofing properties on the surface at the inner circumferential side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hardened steel pipe used as a reinforcing component such as an impact bar attached to a side door of an automobile.

[0002]

2. Description of the Related Art Recently, regarding automobiles, especially passenger cars,
The demand for strengthening safety measures is increasing. As one of the demands, in order to absorb the impact when there is a collision from the side surface, it has been adopted to mount a reinforcing component such as an impact bar on the side door.

As the impact bar, a high-strength cold-rolled steel sheet of about 100 kg class that has been press-formed has been conventionally used. However, with the strengthening of safety measures, also aimed at weight reduction, 130 kgf / mm ² or more, preferably a high strength steel pipe having a 150 kgf / mm ² or more tensile strength is noticed as a promising material. In this connection, Japanese Patent Application No. 2-163132 proposes an induction-hardened quenched steel pipe for automobile door reinforcement.

By the way, a high-strength hardened steel pipe for automobile door reinforcement is usually cut to a product length of about 1 m for the impact bar, and brackets manufactured by forming steel plates are joined to both ends of the impact bar. It will be one part. This impact bar is installed between the inner plate and the outer plate of a vehicle door via brackets attached to both ends. Then, the impact bar is coated through the cationic electrodeposition coating process while being integrated with the door.

[0005]

A thin oxide film formed during quenching adheres to both the inner peripheral surface and the outer peripheral surface of a normal high-strength quenched steel pipe. On the surface of the outer peripheral side, a coating film effective for ensuring the rust prevention effect is formed by cationic electrodeposition coating. However, for the inner peripheral surface, although a coating film can be formed on the surface near the pipe end, most of the inner peripheral surface is due to insufficient penetration of the coating liquid into the steel pipe. Is currently barely painted.

On the other hand, rainwater penetrates inside the door of the automobile, and the constituent members are exposed to a corrosive environment. Therefore, the use of a Zn-plated steel plate or the like having an improved rust prevention effect is being promoted as a member constituting the inner plate or the like. Like the members such as the inner plate, the impact bar is also used in an atmosphere exposed to rainwater or moist air.

Therefore, there is a concern that corrosion will occur after a long period of time, and the function as a reinforcing material may not be sufficiently exerted due to the destruction starting from the corrosion occurrence portion.
Further, if a phenomenon occurs such that rust generated due to corrosion leaks out from the inside of the door, the appearance of the automobile is significantly impaired.

From such an environment of use, the high-strength quenched steel pipe forming the impact bar is also required to have excellent rust prevention. However, the inside of the steel pipe is in a severe corrosive environment compared to the outer peripheral surface where the coating film is sufficiently formed. As a result, corrosion progresses inside the steel pipe due to the entering rainwater. That is, the conventional high-strength quenched steel pipe is
Since the coating inside the steel pipe is not sufficient, it cannot be said that the rust resistance required for the impact bar is sufficiently satisfied.

In order to prevent rainwater from entering the inside of the steel pipe, when joining brackets to both ends of a high-strength quenched steel pipe, a method of welding and joining in a bracket shape that seals the opening of the steel pipe, the opening being made of resin Methods for covering with etc. are being studied. However, these methods are accompanied by an increase in cost due to the complexity of welding and an increase in the number of parts, and are not necessarily industrially effective solutions.

Further, even when a steel plate, which is a steel pipe material, is plated, the problem of corrosion remains because the plating layer is oxidized or melted by the heat input during quenching. Therefore, it is conceivable that the hardened steel pipe after the heat treatment is subjected to hot dip galvanizing, electrogalvanizing or the like to achieve sufficient rust prevention including the inner peripheral surface. Although this method is technically possible, it requires treatment for each steel pipe. Therefore, it is unavoidable that the cost becomes high due to the complicated processing, and it is not an effective measure.

As described above, in the conventional high-strength quenched steel pipe, it is possible to find a rust-preventing property, in particular, a rust-proofing property of the inner peripheral surface without increasing the cost significantly. It was difficult.

The present invention has been devised in order to solve such a problem. By closing the openings at both ends, the rust preventive property of the inner peripheral surface is improved, and by induction hardening, strength, toughness, etc. It is an object of the present invention to provide a high-strength hardened steel pipe for automobile door reinforcement, which secures the mechanical properties of

[0013]

The high-strength hardened steel pipe for automobile door reinforcement of the present invention has the following objects and advantages.
C: 0.10 to 0.30% by weight, Si: 0.05 to 0.
50% by weight, Mn: 0.20 to 1.50% by weight, P:
An electric resistance welded steel pipe containing 0.020% by weight or less, S: 0.020% by weight or less, and Al: 0.01 to 0.10% by weight, and having both end openings closed by a molding process,
It is characterized by having a structure mainly composed of martensite formed by induction hardening.

The electric resistance welded steel pipe used further has Ti: 0.01
.About.0.10% by weight, B: 0.0005 to 0.010% by weight, Ni: 0.20 to 1.50% by weight, Cr: 0.05
~ 1.00% by weight, Mo: 0.05-0.5% by weight,
V: 0.01 to 0.20% by weight, Nb: 0.01 to 0.
One or more selected from 20 wt% and Ca: 0.001 to 0.01 wt% can be contained.

As the electric resistance welded steel pipe, it is possible to use a steel pipe as-welded by electric resistance welding, or a steel pipe formed by further forming an electric resistance welded steel pipe into an irregular cross section such as a square shape. Further, as the high-strength quenched steel pipe, there is a steel pipe that is formed by processing for closing both end openings of a short steel pipe cut into a product length such as an impact bar and then quenched by high frequency induction heating or the like. Alternatively, after quenching, the long steel pipe cut into multiple lengths such as the impact bar, etc., is formed by processing the ends of the long steel pipe and multiple parts set by the gap of the product length. There is also a short steel pipe, etc., which is made by cutting the long steel pipe of to make the product length.

[0016]

[Operation] The inventors of the present invention have conducted various studies on a method of satisfying the strength and toughness of a high-strength quenched steel pipe and further improving the corrosion resistance of the inner peripheral surface of the steel pipe. As a result, they have found a high-strength quenched steel pipe suitable for use as a reinforcing member such as an impact bar. That is, considering the workability before quenching, improving the strength and toughness level of the quenched steel pipe, and further considering the quenchability corresponding to induction hardening, the components and composition in the steel are adjusted, and water is used during use as a reinforcing member. Or prevent the humid atmosphere from entering the steel pipe,
For the purpose of enhancing the rustproofing effect on the inner peripheral surface of the steel pipe, both ends of the steel pipe, which correspond to the ends of the reinforcing member products such as impact bars, are formed into a flat shape or the like by molding.

The components and composition of the steel pipe used in the present invention will be described below. C: An element necessary for obtaining the strength of the steel pipe after quenching, and if it is less than 0.10% by weight, the tensile strength required as a high strength steel pipe for impact bar is 1
A strength of 30 kgf / mm ² or more cannot be obtained. But,
When C is contained in an amount of more than 0.30% by weight, the strength of the hardened steel pipe can be increased, but the toughness is remarkably reduced, and the steel is brittle and breaks when an impact load is applied, which is preferable as an impact bar. It has the property of not presenting. Furthermore,
Even before the quenching, the toughness is deteriorated, and defects such as cracks are likely to occur during molding such as flattening, which makes sufficient molding difficult. Therefore, the C content is specified in the range of 0.10 to 0.30% by weight.

Si: An element used as a deoxidizing agent for steel, which is also effective in improving hardenability. To obtain this effect, it is necessary to contain Si in an amount of 0.05% by weight or more. However, since it is an element having a strong affinity with oxygen, the penetrator is likely to be formed in the welded portion during the electric resistance welding in the process of manufacturing the electric resistance welded steel pipe, and it becomes difficult to obtain a sound welded portion. For this reason, forming such as flattening is difficult, and it tends to impair the toughness of a high-strength quenched steel pipe. Therefore, the upper limit of the Si content is set to 0.50% by weight.

Mn: an element useful for enhancing the hardenability of steel and strengthening it. However, the excess Mn content is
It increases Mn-based non-metallic inclusions and develops a striped structure. As a result, toughness is reduced. Furthermore, Mn is S
As with i, the soundness of the welded portion is adversely affected. From these facts, the Mn content is defined in the range of 0.20 to 1.50% by weight.

P: A harmful element that deteriorates the toughness of a quenched steel pipe. Therefore, the upper limit of the P content is restricted to 0.020% by weight.

S: Accelerates the formation of non-metallic inclusions in the steel, causing defects such as deterioration of toughness and deterioration of soundness of the welded portion. Therefore, the S content is restricted to 0.020% by weight or less.

Al: an element useful as a deoxidizing agent for molten steel, and needs to be added in an amount of 0.01% by weight or more. However, if the Al content exceeds 0.10% by weight, the cleanliness of steel is impaired and surface defects are likely to occur. Therefore, the Al content is defined in the range of 0.01 to 0.10% by weight.

Further, Ti added as a selective component,
B, Ni, Cr, Mo, V, Nb, Ca, etc. exhibit the following actions, respectively.

Ti: A carbonitride that does not easily form a solid solution during heat treatment is formed, and it has an action of preventing crystal grains from becoming coarse during quenching. Further, it is also effective in fixing N dissolved in steel as a nitride. Therefore, the consumption of B by the solid solution N is suppressed, and the hardenability improving action of B is efficiently exhibited. In order to obtain these effects, Ti is set to 0.
It is necessary that the content be 01 wt% or more. But,
When Ti is contained in excess of 0.10% by weight, coarse nitrides are formed, resulting in deterioration of toughness. Therefore, Ti
The content was in the range of 0.01 to 0.10% by weight.

B: The hardenability of steel is greatly improved by the addition of B. Further, since N in the steel is fixed by Ti, even if the B content is a very small amount of 0.0005% by weight or more, the hardenability can be sufficiently improved. However, if the B content exceeds 0.01% by weight, a compound is formed in the steel, which adversely affects the hardenability and adversely affects the toughness. From this point, the B content was set in the range of 0.0005 to 0.01% by weight.

Ni: An element effective in improving the hardenability of steel and suppressing the deterioration of toughness and strengthening the steel. In order to obtain this effect, it is necessary to contain 0.20% by weight or more of Ni. However, even if Ni is contained in an amount of more than 1.50% by weight, the property improving effect is saturated and the cost of the steel material is increased. Therefore, the Ni content is 0.
It is specified in the range of 20 to 1.50% by weight.

Cr: An element effective for improving the hardenability of steel, and it is necessary to contain 0.05% by weight or more of Cr. However, if Cr is contained in excess of 1.00% by weight, a penetrator is likely to be generated in the welded portion during pipe making, workability such as flattening is deteriorated, and toughness as a high strength steel pipe is also deteriorated. .. for that reason,
The Cr content was set in the range of 0.05 to 1.00% by weight.

Mo: An element effective in improving the hardenability of steel, and it is necessary to contain Mo in an amount of 0.05% by weight or more. However, Mo is an expensive alloying element, and even if it is contained in an amount of more than 0.5% by weight, an effect commensurate with it cannot be obtained, which is economically disadvantageous. So Mo
The content was specified in the range of 0.05 to 0.5% by weight.

V: Forming a stable carbonitride, suppressing coarsening of crystal grains during quenching, and exhibiting an effective action of preventing deterioration of toughness. To obtain this effect,
It is necessary to contain 0.01% by weight or more of V. However, if the V content exceeds 0.20% by weight, in the induction hardening in which the steel material is heated to the hardening temperature in a very short time, the C concentration of the matrix decreases due to the insufficient solid solution of carbide. As a result, the required strength cannot be obtained. Therefore, the V content is defined in the range of 0.01 to 0.20% by weight.

Like Nb: V, it is an effective element for suppressing the coarsening of crystal grains. However, the solid solution of carbide in the matrix is reduced, which causes a decrease in strength. Therefore, the Nb content is 0.01 to 0.01 as in V.
The range was set to 0.20% by weight.

Ca: S is an element effective in controlling the form of non-metallic inclusions to render them harmless and enhance workability before quenching and toughness after quenching. To get this effect,
It is necessary to contain 0.001% by weight or more of Ca by weight. However, when Ca is contained in an amount of more than 0.01% by weight, the amount of nonmetallic inclusions in the steel increases, and rather the toughness deteriorates. Therefore, the Ca content is 0.
It was defined in the range of 001 to 0.01% by weight.

In order to manufacture a steel pipe from the steel having the above composition system, various methods can be adopted. For example, when considering the homogeneity of product properties, it is also possible to produce seamless seamless pipes. However, in consideration of the manufacturing cost, the method of manufacturing an electric resistance welded steel pipe by welding a plate material by high frequency induction resistance heating is most suitable for manufacturing a steel pipe for automobile door reinforcement such as an impact bar.

When producing an electric resistance welded steel pipe by high frequency welding, it is advantageous to use an annealed material as a steel sheet as a raw material for the purpose of obtaining sufficient formability. However, by adjusting the hot rolling conditions, it is possible to use the steel sheet as hot rolled as a raw material.

In the electric resistance welded steel pipe, the electric resistance welded portion is hardened by hardening. Therefore, it is also an effective means to secure the formability by annealing the entire steel pipe after pipe making, or by performing seam annealing in which only the electric resistance welded portion is heated by high frequency induction heating or the like.

Regarding the shape of the steel pipe, it is advantageous to use the steel pipe in a circular cross-section as it is, from the viewpoints of cost of the steel pipe material and easiness of heat treatment. However, it is also possible to use it as a square steel pipe having a rectangular cross section. The rectangular cross section does not necessarily have to be square, and a flat rectangular cross section having different vertical and horizontal lengths can be adopted. When mounted on a vehicle as a reinforcement such as an impact bar, this square steel pipe has a large deformation resistance in the initial stage of deformation when a bending load is applied, and when mounted in the gap between the outer and inner plates of a door. It has advantages such as easy design.

The electric resistance welded steel pipe of the method of the present invention is cut to a length when it is used as a reinforcing material for an impact bar, etc.
As shown in FIG. 5, the openings 11 and 12 at both ends are formed into a flat shape in which they are in close contact with each other. In the molding at this time, it is not always necessary that the openings at both ends are brought into close contact with each other. Further, as long as the tube end opening is closed, the shape is not limited to the flat shape, and it may be formed into a cross shape shown in FIG. 2, for example. However, if both end portions 11 and 12 are formed into a too complicated shape, not only the forming process becomes difficult, but also the work of filling the gap created by the forming process by arc welding or the like becomes troublesome, which is not preferable.

When the flattening process or the like is performed, the width of the processed part is widened, which may hinder the heat treatment work. In such a case, as shown in FIG. 3, both ends 11 and 12 of the short steel pipe 10 after forming are trimmed to have a predetermined width.

The lengths of both ends 11, 12 of the short steel pipe 10 to be formed are not particularly limited. However, if the length is excessively long, the flexural rigidity required as a reinforcing material cannot be obtained. Therefore, the molding length is appropriately set in consideration of the dimension of the mounting portion to the door.

The high-strength quenched steel pipe of the present invention is several meters long.
It is also possible to manufacture a reinforcement material such as an impact bar by quenching a long steel pipe having a length and then cutting it into a short steel pipe having a product length.

In this case, as shown in FIG. 4, with respect to the long steel pipe 20, in addition to the end portions 21 and 22, a plurality of portions 23, 24, which are set at intervals corresponding to the final product length,
At 25, a forming process is performed so that the inner surfaces of the steel pipe come into contact with each other. The long steel pipe 20 after the forming process is subjected to induction hardening as it is, and then cut from the formed parts 23, 24, 25. As a result, the two ends of the steel pipe are closed by contacting the inner surface of the steel pipe, and a plurality of short steel pipes 26 to 29 having a predetermined product length are manufactured.

The forming process for the steel pipe before quenching is not particularly limited, and a forming process such as a shrink pipe process can be adopted. However, in consideration of the simplicity of the process, a normal press forming process is performed. Is preferred. Further, when adopting a simple flattening process, it is possible to perform press molding in parallel in a cutting step after pipe making.

Various methods can be used for quenching the steel pipe, but in consideration of the homogeneity of the quenching structure and the shape stability after quenching, induction hardening is an effective means.

The steel pipe after quenching has a tensile strength of 130 k.
A strength of gf / mm ² or more is required. Tensile strength 13
If the strength is less than 0 kgf / mm ² , there is no great difference in strength from the high-strength cold-rolled steel sheet, and the merit of weight reduction cannot be obtained.

The quenched structure has a tensile strength of 130 kgf /
As long as it has a strength of mm ² or more, it basically does not necessarily have to have a martensite structure. However, in induction hardening, it is difficult to strictly control the cooling rate. Therefore, for example, when induction hardening is applied to a steel material having a structure in which bainite and the like are mixed, the mechanical properties are likely to largely change depending on the cooling rate. Therefore, it is effective to stabilize the mechanical properties by making the structure mainly composed of martensite whose strength does not depend on the cooling rate.

Further, since the heating condition, the cooling condition, and the like of the portion that has been subjected to forming such as flattening are different from those of other portions that have not been formed, the quenching state fluctuates, and the tensile strength 13
It may happen that a structure having a strength of 0 kgf / mm ² or more cannot be obtained. However, when an impact load is applied to the door of the automobile, a bending moment is hardly applied to both ends of the steel pipe, so that there is no substantial problem.

As described above, in the case of a hardened steel pipe in which both end openings of the steel pipe are closed, when mounted on the door as an impact bar, rainwater or a wet atmosphere invading the interior of the steel pipe causes Do not invade. for that reason,
Even if the coating film by the cationic coating is not sufficiently formed on the inner peripheral surface, the generation of rust can be suppressed.

If the openings at both ends cannot be sufficiently adhered by the molding process, or if the adhesion is deteriorated due to thermal strain in the heat treatment after the molding process, the arc as shown in FIG. It is effective to bury the beads 13 and 14 on the end surface of the steel pipe by welding or the like to fill the gap between the pipe ends. The welding at this time is extremely easy because the welding beads 13 and 14 are linearly arranged as compared with the case where the entire circumference of the steel pipe that is still open is welded and the opening is closed. It becomes a welding process.

[0048]

EXAMPLES The present invention will be specifically described below with reference to examples. Steel having the components and compositions shown in Table 1 was melted in a converter and continuously cast into a slab. This slab was hot-rolled by a normal hot strip mill to produce a hot-rolled steel plate having a plate thickness of 2.3 mm. After pickling the obtained hot-rolled steel sheet, further annealing some of it, and then slitting it into a predetermined width to make an electric resistance welded steel pipe having an outer diameter of 25.4 mm with a pipe making machine, and seam welding at the electric resistance welded portion. Annealed. Next, ERW steel pipe is 700mm
Was cut to a length of 1, and a part of the steel pipe was flattened in a close contact state using a press molding machine so that both ends of the steel pipe were closed within a range of 80 mm.

[0049]

[Table 1]

These steel pipes were heated by high frequency induction heating to 9
After the temperature was raised to 00 to 1000 ° C., it was quenched by cooling with water. In induction hardening at this time,
The steel pipe is held by the upper and lower chuck parts, and a method is adopted in which it is passed between the high frequency induction heat coil and the water cooling ring while being rotated.

The steel pipe after quenching was subjected to a tensile test and an impact bending test to investigate its mechanical properties. In the impact bending test, a quenched steel pipe is supported at two points with a gap of 500 mm, and a mass of 100 kg is applied to the quenched steel pipe with a radius of 50 mm at the tip.
The weight having an arc shape was dropped from a height of 2 m, and the deformed and broken state of the steel pipe was observed.

As a rust resistance test of the steel pipe, after applying seal paint to the outer peripheral surface of the hardened steel pipe, a salt spray test is conducted for 24 hours. After the test, the steel pipe is cut to check the rust condition on the inner peripheral surface. Examined. At this time, with respect to the flattened steel pipe, the flattened steel pipe and the arc-welded steel pipe along the gap at the end were tested. The test results are shown in Table 2.

[0053]

[Table 2]

As is clear from Table 2, in the comparative examples of Test Nos. 1 and 8 having the components and compositions outside the range defined by the present invention, the tensile strength was 130 kgf, although the openings at both ends of the steel pipe were closed. / Mm ² or more is not obtained, or the toughness is insufficient and brittle fracture occurs when an impact load is applied. On the other hand, even when the steel materials having the components and compositions within the ranges specified in the present invention are used, in the comparative examples of the test numbers 2, 3 and 6 in which the steel pipes in which the openings at both ends are left open are used, Red rust was generated over almost the entire area of the peripheral surface.

On the other hand, in the examples of test Nos. 4, 5 and 7 in which the components / compositions and the shapes of both ends satisfy the conditions specified in the present invention, the test numbers 4, 5 and 7 are sufficiently satisfied as the high strength steel pipe for the door reinforcing member. In addition to having mechanical properties, there was no or very little rust on the inner peripheral surface. That is, in the as-flattened steel pipe, red rust was found at the portions with gaps at both ends, but when the gap was sealed with weld beads, there was no occurrence at the inner peripheral surface.
From this, it is understood that the high-strength quenched steel pipe according to the present invention is also excellent in rust prevention.

[0056]

As described above, in the present invention, the composition and composition of the electric resistance welded steel pipe to be used are adjusted, and
By performing the forming process to close the openings on both ends of the steel pipe, it has the strength and toughness required for a high-strength quenched steel pipe for automobile door reinforcement members, and also has the rust prevention property on the inner peripheral side surface, so it is excellent. It can be used as a reinforcing material for impact bars.

[Brief description of drawings]

[Fig.1] Short steel pipe with both ends flattened

[Fig. 2] Short steel pipe with both ends formed into a cross shape

[Fig. 3] Short steel pipe with flattened both ends trimmed

FIG. 4 Long steel pipe with both ends and a plurality of central parts flattened

[Fig. 5] Short steel pipe with weld beads placed in the end gap after flattening

[Explanation of symbols]

10 Short Steel Pipe 11, 12 Formed Pipe End 13, 14 Weld Bead 20 Long Steel Pipe 21, 22 Flattened Pipe End 23-25 Flattened Central Part 26-29 Cut from Long Steel Pipe Short steel pipe L Length equivalent to the product length of the reinforcing material such as impact bar

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C22C 38/06 38/54

Claims (2)

[Claims] 1. C: 0.10 to 0.30% by weight, Si:
0.05 to 0.50% by weight, Mn: 0.20 to 1.50
%, P: 0.020% by weight or less, S: 0.020% by weight or less, and Al: 0.01 to 0.10% by weight, and an electric resistance welded steel pipe whose both ends are closed by molding. A high-strength quenched steel pipe for automobile door reinforcement, which has a structure mainly composed of martensite formed by induction hardening. 2. C: 0.10 to 0.30% by weight, Si:
0.05 to 0.50% by weight, Mn: 0.20 to 1.50
% By weight, P: 0.020% by weight or less, S: 0.020% by weight or less, Al: 0.01 to 0.10% by weight and Ti:
0.01-0.10% by weight, B: 0.0005-0.0
10% by weight, Ni: 0.20 to 1.50% by weight, Cr:
0.05-1.00% by weight, Mo: 0.05-0.5% by weight, V: 0.01-0.20% by weight, Nb: 0.01
~ 0.20% by weight, Ca: 0.001 to 0.01% by weight
Is an electric resistance welded steel pipe containing one or more selected from, and having openings at both ends closed by molding, and having a structure mainly composed of martensite formed by induction hardening. High-strength quenched steel pipe for automobile door reinforcement.