JPH05255751A - Production of high strength hardened steel tube for automobile door reinforcement - Google Patents

Abstract

PURPOSE:To produce a high strength hardened steel tube for automobile door reinforcement improved in initial deformation resistance by subjecting a resistance welded tube having specific composition to induction hardening and then to a repetition of plastic deformation while keeping the shape of a straight tube. CONSTITUTION:A resistance welded tube having a composition containing, by weight, 0.10-0.30% C, 0.05-0.50% Si, 0.20-1.50% Mn, <=0.020% P, <=0.020% Si, and 0.01-0.10% Al is prepared. After the resistance welded tube is subjected to induction heating, plastic deformation is repeatedly applied while keeping the shape of a straight tube. By this method, the high strength hardened steel tube improved in initial deformation resistance and showing high absorption energy as auxiliary parts can be obtained.

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 In recent years, there has been an increasing demand for strengthening automobile safety measures. In response to these demands, a reinforcement component called an impact bar for absorbing a shock at the time of a side collision is attached to the side door. Impact bar is conventionally 100f / mm but ^secondary order of a high-strength cold-rolled steel sheet has been produced by press-forming, 130 kgf / mm ² class or 15 in terms of weight reduction
Hardened steel pipes with a high strength of 0 kgf / mm ² grade have come to be used.

When a high-strength cold-rolled steel sheet is used for an impact bar, workability is required because the steel sheet is processed into a press-formed part shape. In this respect, there is a limit to the increase in strength. On the other hand, when the hardened steel pipe is used as the impact bar, a simple welding process is performed on the attachment portion of the door, and the forming process is not particularly required. Therefore, the material strength can be increased relatively easily.

Further, the impact bar made of steel pipe having a large wall thickness has the following advantages due to the difference in shape, as compared with the conventional impact bar in which a thin plate is formed into a cross section close to a rectangle. That is, assuming a collision with a door,
The impact bar is bent and deformed by the load at the time of collision. As for the relationship between the load and the displacement at this time, as seen in FIG. 1 schematically showing, the displacement of the bending increases, and the load, that is, the deformation resistance to the bending gradually increases. Then, when the deformation progresses to a certain degree, buckling occurs in the impact bar, which causes a phenomenon in which the deformation resistance rapidly decreases. Buckling occurs in the impact bar using a thin steel plate at a relatively early stage of deformation, whereas buckling does not occur in the impact bar made of steel pipe until a larger bending deformation occurs. From this, it can be seen that the impact bar made of steel pipe exhibits greater deformation resistance.

[0005]

The impact bar made of steel pipe is advantageous in terms of buckling in bending deformation as compared with the impact bar made of thin plate as described above. However, the deformation resistance at the initial stage of deformation is smaller in the steel pipe impact bar than in the thin plate impact bar. The characteristic required for the impact bar is that the energy absorbed by bending deformation is large.

This absorbed energy is represented in FIG. 1 as a value obtained by integrating the load by displacement. When the initial deformation resistance increases together with the deformation resistance at the stage where the deformation has progressed, the energy absorbed by the bending deformation can be increased. As a result, the size of the steel pipe required to obtain the absorbed energy required for the impact bar can be reduced, and the weight of the vehicle body can be reduced accordingly.

From the viewpoint of the material properties of the impact bar, the initial deformation resistance is related to the proof stress of the material,
The larger the yield strength, the larger the initial deformation resistance. By the way, a hardened steel pipe for an impact bar is usually manufactured by induction hardening an electric resistance welded steel pipe manufactured using a steel material belonging to the category of a special steel having improved hardenability. This steel pipe is
It exhibits an as-quenched martensite structure. A steel pipe having an as-quenched martensite structure has a large tensile strength, but a large yield strength cannot be expected.

The present invention has been devised to solve such a problem, and by repeatedly performing plastic deformation processing, the initial deformation resistance is increased and a large absorbed energy is exhibited as a reinforcing component such as an impact bar. An object is to provide a high strength quenched steel pipe.

[0009]

In order to achieve the object of the method for producing a high-strength quenched steel pipe for an automobile door reinforcing material of the present invention, C: 0.10 to 0.30% by weight, Si: 0.05
~ 0.50 wt%, Mn: 0.20 to 1.50 wt%,
After induction hardening of an electric resistance welded steel pipe containing P: 0.020% by weight or less, S: 0.020% by weight or less, and Al: 0.01 to 0.10% by weight, plastic deformation processing while maintaining a straight pipe shape. Is repeatedly applied.

The steel used is 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
Up to 1.00% by weight, Mo: 0.05 to 0.50% by weight,
V: 0.01 to 0.20% by weight, Nb: 0.01 to 0.
One or two or more selected from 20% by weight and Ca: 0.001 to 0.01% by weight may be contained.

[0011]

[Operation] The components and contents of the steel 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 of 130 kgf / mm ² or more required for the steel pipe for impact bar cannot be obtained. However, when C is contained in an amount of more than 0.30% by weight, the toughness decreases although the strength of the quenched steel pipe can be increased. Therefore, when an impact load is applied, it breaks brittlely, and exhibits an undesirable property as an impact bar. Further, even before the quenching, the ductility 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 0.10 to 0.3.
It was defined in the range of 0% by weight.

Si: An element used as a deoxidizer 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, Si is an element having a strong affinity with oxygen, and a penetrator is easily formed in the welded portion during electric resistance welding in the process of manufacturing an electric resistance welded steel pipe, which makes it difficult to obtain a sound welded portion. As a result, forming work such as flattening becomes difficult, and the toughness of the high-strength quenched steel pipe tends to be impaired. Therefore, the upper limit of the Si content is defined as 0.50% by weight.

Mn: An element effective in enhancing the hardenability of steel and strengthening it. However, excessive Mn content increases Mn-based non-metallic inclusions and further develops a striped structure. As a result, the toughness of steel decreases.
Furthermore, Mn, like Si, adversely affects the soundness of the weld. Therefore, the Mn content is 0.20 to 1.5.
It was defined in the range of 0% by weight.

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

S: Accelerates the formation of non-metallic inclusions, leading to defects such as deterioration of toughness of steel and deterioration of soundness of welds. Therefore, the S content is set to 0.020% by weight or less.

Al: An element effective as a deoxidizing agent for molten steel. However, with an Al content exceeding 0.10% by weight,
The cleanliness of the steel is impaired and surface defects are likely to occur. Therefore, the Al content is specified in the range of 0.01 to 0.10% by weight. Also, Ti, B, Ni, Cr, Mo, V, Nb, Ca, etc. added as selective components are
Each exhibits the following effects.

Ti: A carbonitride that does not easily form a solid solution during heat treatment is formed, and it has an effect of preventing the coarsening of crystal grains during quenching. In addition, N dissolved in steel is fixed as a nitride. Therefore, consumption of B due to solid solution N is suppressed, and B
The effect of improving the hardenability is effectively exhibited. To obtain these effects, it is necessary to contain 0.01% by weight or more of Ti. However, the inclusion of more than 0.10% by weight of Ti promotes the formation of coarse nitrides,
It causes deterioration of toughness. Therefore, the Ti content is 0.0
It was defined in the range of 1 to 0.10% by weight.

B: The hardenability of steel is greatly improved by the addition of B. Moreover, since N in steel is fixed by Ti, even in a very small amount of 0.0005 wt% or more,
Hardenability is improved sufficiently. However, the B content is 0.0
If it exceeds 1% by weight, a compound is formed in the steel, which not only deteriorates the hardenability, but also adversely affects the toughness. At 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 0.2
It was set in the range of 0 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, when Cr is contained in an amount of more than 1.00% by weight, a penetrator is apt to be generated in a welded portion during pipe making, and workability such as flattening is deteriorated, and
The toughness as a high strength steel pipe deteriorates. Therefore, the Cr content is specified 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 0.05% by weight or more of Mo. However, Mo is an expensive alloying element, and even if Mo is contained in an amount of more than 0.5% by weight, the effect commensurate with the increase of Mo cannot be obtained, which is economically disadvantageous. Therefore, the Mo content is specified in the range of 0.05 to 0.5% by weight.

V: Forming stable carbonitrides, 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, when 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, when V is contained, the range is 0.01
~ 0.20% by weight.

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

Ca: An element effective in controlling the morphology of sulfide-based inclusions to render them harmless and enhance workability before quenching and toughness after quenching. To obtain this effect, 0.
It is necessary to contain 001% by weight or more of Ca. However, when Ca is contained in excess of 0.01% by weight, the amount of nonmetallic inclusions in the steel increases, and rather the toughness deteriorates. Therefore, the Ca content is 0.001 to
It was defined in the range of 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, in terms of homogeneity of product properties, seamless seamless pipes can also be produced. But,
Considering the manufacturing cost, the method of manufacturing the electric resistance welded steel pipe by welding the plate material by high frequency induction resistance heating is most suitable for manufacturing the impact bar molten steel pipe. Further, in the case of an electric resistance welded steel pipe, it is advantageous to use an annealed material as a raw material steel sheet in order to obtain sufficient forming workability, but by adjusting the hot rolling conditions, the as-hot-rolled material can be used as a raw material. It is also possible to use as. In addition, in the electric resistance welded steel pipe, since the electric resistance welded part is hardened by hardening, the entire steel pipe may be annealed after the pipe making, or seam annealing may be performed to heat only the electric resistance welded part by high frequency heating or the like. , Is an effective means of ensuring molding processability.

The electric resistance welded steel pipe is hardened to obtain a hardened steel pipe having a martensitic structure. There are various methods for quenching at this time, but in consideration of the homogeneity of the quenching structure and the shape stability after quenching, induction hardening was adopted in the present invention.

When induction hardening a steel pipe, the length is 1 m
When using a short steel pipe of a certain length or shorter, a few meters
In some cases, a long steel pipe is used. In this respect, in the present invention, since light plastic deformation processing is repeatedly applied after quenching, it is preferable to subject the long steel pipe to the quenching treatment in consideration of the handling property during the plastic deformation processing.

After quenching the steel pipe, plastic deformation processing is repeatedly applied while maintaining the straight pipe shape to obtain a product. A quenched steel pipe having an as-quenched martensite structure has a low yield ratio of 80%, which is the ratio of proof stress to tensile strength. When a small amount of plastic deformation is performed while maintaining the straight pipe shape, the tensile strength hardly increases, but the yield ratio becomes 80% or more, and the yield strength is remarkably improved. It is presumed that, by imparting plastic deformation, dislocations introduced by quenching become entangled by imparting strain due to plastic deformation and dislocations become difficult to move, resulting in an increase in yield strength.

The plastic deformation applied at this time is such that the elastic limit is slightly exceeded, and is carried out, for example, by a slight bending deformation using a roll. By repeating this plastic deformation process, the straight pipe shape in the quenched state can be maintained. In addition, repeated application of plastic deformation processing,
It homogenizes the heat treatment strain generated by quenching and improves the straightness and roundness of the steel pipe. As a result, the steel pipe that has been subjected to the repeated plastic deformation processing has the shape such as the bend generated during quenching corrected.

The amount of strain introduced by the repeated plastic deformation processing varies depending on the position, but it is preferably 0.2 to 2% or less at the place where the maximum strain is applied. When the amount of strain is less than 0.2%, the strain remains almost within the elastic deformation and the yield strength is small. However, when the distortion amount exceeds 2%,
Due to the plastic deformation process, the roundness of the steel pipe deteriorates, and since the steel pipe is a high-strength material, defects such as cracks are likely to occur.

[0031]

EXAMPLE Steels having the components shown in Table 1 were melted in a converter,
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.

[0032]

[Table 1]

After pickling the hot-rolled steel sheet, it was slit into a predetermined width, made into an electric resistance welded steel pipe having an outer diameter of 31.8 mm by a pipe making machine, and cut into a long steel pipe having a length of 7,000 mm. Note that some hot-rolled steel sheets were further annealed after pickling.

The obtained steel pipe was subjected to high frequency induction heating to 9
After the temperature was raised to a temperature of 00 to 1000 ° C., water quenching treatment was performed. In addition, the induction hardening at this time is
Using a high-frequency heating coil, a water cooling pipe adjacent to it, and a hardening machine consisting of an inlet side and an outlet side roller table,
A steel pipe was conveyed on the roller table at 10 m / min for heating and water cooling.

A part of the quenched steel pipe was passed through a multi-roll type straightening machine to repeatedly give a slight plastic deformation work. Then, it cut | disconnected by the grindstone cutting machine to the length of 1200 mm.

The steel pipe after quenching was subjected to a tensile test, an impact bending test and a static bending test to investigate the mechanical properties. In the impact bending test, the quenched steel pipe is supported at two points spaced at a distance of 500 mm, and the tip portion has a radius of 5 with a mass of 100 kg.
A 0 mm arc-shaped weight was dropped from a height of 2 m and the state of deformation and destruction of the steel pipe was observed. In the static bending test, the hardened steel pipe was supported at two points with an interval of 900 mm,
A circular arc punch with a radius of 150 mm is pressed against the center between the fulcrums, the load at that time and the displacement of the punch are measured, and the load-
A displacement curve was created.

As is clear from Table 2 showing the results of the tensile test and the impact bending test, the tests No. 3, 5 in which the components and the content were within the ranges specified in the present invention and the plastic deformation processing was repeated after quenching In each of Examples 7 and 7 of the present invention, the tensile strength shows a high tensile strength of 150 kgf / mm ² or more, and the yield ratio is as high as 82% or more.

On the other hand, in the comparative example of the test No. 1 in which the content of the component specified in the present invention was deviated, 130 kgf / mm ²
Since the above tensile strength was not obtained and the toughness was insufficient in the comparative example of No. 8, brittle fracture occurred when an impact load was applied. In addition, even if the components satisfy the requirements specified in the present invention, in Comparative Examples of Test Nos. 2, 4 and 6 in which the shape is not changed by quenching, it is a test of the present invention. The yield strength is lower than Nos. 3, 5 and 7.

[0039]

[Table 2]

The load-displacement curves in the static bending tests of tests No. 2 and 3 are shown in FIG. As is clear from FIG. 2, in the example of the present invention of test No. 3 which was passed through the shape straightening machine, the initial deformation resistance, that is, the load was large, in bending deformation, as compared with test No. 2 which was a comparative example. It can be seen that the absorbed energy is increasing.

[0041]

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 repeatedly applying plastic deformation while maintaining a straight pipe shape after quenching, a product having both strength and toughness required as a high-strength steel pipe for automobile door reinforcing members is obtained. A product that has been repeatedly subjected to plastic deformation processing has a higher yield strength than the as-quenched material, and the deformation resistance during initial deformation when bending deformation is applied increases. As a result, the absorbed energy in bending deformation becomes large, and it can be used as a reinforcing material such as an impact bar having excellent characteristics.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a load-displacement relationship in a bending test of a thin steel plate impact bar and a steel pipe impact bar.

FIG. 2 is a graph showing the effect of cyclic plastic deformation on the load-displacement curve in a static bending test of a hardened steel pipe.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C21D 9/08 F // C22C 38/00 301 Z 38/06

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
After induction hardening of an electric resistance welded steel pipe containing Pt: 0.020 wt% or less, P: 0.020 wt% or less, S: 0.020 wt% or less, and Al: 0.01 to 0.10 wt%, while maintaining a straight pipe shape. A method for manufacturing a high-strength quenched steel pipe for automobile door reinforcement, which is characterized by repeatedly applying plastic deformation processing. 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 and Al: 0.01-0.10% by weight, further Ti: 0.01-0.10% by weight, B : 0.0
005 to 0.010% by weight, Ni: 0.20 to 1.50
% By weight, Cr: 0.05 to 1.00% by weight, Mo: 0.
05-0.50% by weight, V: 0.01-0.20% by weight, Nb: 0.01-0.20% by weight and Ca: 0.0
Automotive door reinforcement characterized by being subjected to induction hardening of an electric resistance welded steel pipe containing one or more selected from 01 to 0.01% by weight, and then repeatedly subjecting it to plastic deformation while maintaining a straight pipe shape. Manufacturing method of high strength quenched steel pipe for steel.