JP3017252B2 - Manufacturing method of hardened steel pipe for automobile door reinforcement - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a high-strength hardened steel pipe used as a reinforcing component such as an impact bar attached to a side door of an automobile.

[Related Art] In recent years, there has been an increasing demand for strengthening safety measures for automobiles, especially passenger cars. As one of the demands, it has been adopted to attach a reinforcing component such as an impact bar to a side door in order to absorb an impact at the time of a collision from the side.

Conventional impact bars are mainly formed by press-forming a cold-rolled high-tensile steel plate. However, on the other hand, there is a strong demand for reducing the weight of automobiles for the purpose of improving fuel efficiency, and impact bars manufactured by press forming from cold-rolled high-tensile steel sheets of the order of 100 kilograms have considerable weight. It is not suitable for weight reduction.

Therefore, in order to reduce the weight of the reinforcing component, such as the impact bar, 130 kgf / mm ² or more, preferably the use of high-strength steel pipe having a 150 kgf / mm ² or more tensile strength it has been investigated.
For example, some attempts have been made to harden carbon steel pipes such as JIS-STKM Class 19 and Class 20 carbon steel pipes for machine structures to obtain high-strength steel pipes for impact bars.

The most common quenching method for a steel material is a method in which a material is charged into a gas combustion furnace or an electric furnace, heated, and then put into a cooling medium such as water. However, when this method is applied to quenching of a steel pipe, the roundness and straightness of the steel pipe are liable to be greatly impaired, and a large amount of time and labor is required for the repair.

On the other hand, induction hardening, in which a portion heated for a short period of time is sequentially cooled and quenched, has an advantage that a shape change is suppressed and a hardened steel pipe having excellent shape characteristics can be obtained. With emphasis on this point, a method that incorporates induction hardening has been partially adopted in the manufacture of hardened steel pipes for impact bars.

[Problems to be Solved by the Invention] In the conventional induction hardening method for impact bars, as shown in FIG. The high-frequency heating coil 11 and the water cooling tube 12 are moved in the direction.
Alternatively, a method in which the short steel tube b itself is moved in the axial direction via the jig 10 and passes through the high-frequency heating coil 11 and the water cooling tube 12 is also adopted. However, both methods are batch-type methods, and there is a problem in productivity because it takes time to set the steel pipe b on the jig 10 and to reciprocate the heating coil 11 and the like.

In this regard, in order to improve productivity, a method of arranging a plurality of short steel pipes in parallel and holding each one, and simultaneously quenching using a plurality of heating coils and water cooling tubes, a length several times longer than the product dimensions A method has also been developed in which a long steel pipe having a length of is quenched and then cut into short steel pipes of a predetermined length. However, even with these methods, improvement in productivity has not yet reached a sufficiently satisfactory state.

As described above, according to the conventional quenching method, it is possible to manufacture a high-strength quenched steel pipe that satisfies high strength, light weight, quality stability including dimensional accuracy, and productivity corresponding to mass production required as a reinforcing material for automobiles. The law is not always established in terms of actual operations.

The present invention has been devised to solve the problems of the conventional quenching method, and the components
An object of the present invention is to increase the productivity of induction hardening treatment by improving the composition and the quenching method, and to manufacture a high-strength hardened steel pipe for a reinforcing material such as an impact bar which can be mass-produced at low cost.

[Means for Solving the Problems] In order to achieve the object, the present invention has a C: 0.
10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.50%, P: 0.020
%, S: 0.020% or less and Al: 0.01 to 0.10%.

Alternatively, in this component system, Ti: 0.01 to 0.10% and B: 0.000%
Steel with 5-0.010% added, Ni: 0.20-1.50%, Cr: 0.05-
1.00%, Mo: 0.05 ~ 0.50%, V: 0.01 ~ 0.20%, Nb: 0.01 ~ 0.2
Steel in which one or two or more alloy components of 0% and Ca: 0.001 to 0.01% are added to the basic component system can also be used. Further, steel in which Ti and B are combined with the above alloy component may be used.

After manufacturing an ERW steel pipe from a steel having such a composition according to a conventional method, high-frequency induction heating is performed while continuously transporting the ERW steel pipe on a conveyor, and then water-quenching is immediately performed. This has a martensitic structure and a tensile strength of 13
A hardened steel pipe for automobile door reinforcement of 0 kgf / mm ² or more is obtained. In addition, as an ERW steel pipe, not only a normal round shape but also a square shape can be used.

[Operation] The present inventors consider the strength and toughness level of a quenched steel pipe and the hardenability corresponding to induction hardening,
The quenching method suitable for improving the productivity while controlling the composition was studied. As a result, the components and compositions of the ERW steel pipe to be used are specified as described above, and the steel pipe is continuously induction-hardened while being conveyed on a line, thereby obtaining the toughness required as a reinforcing material for impact bars and the like. It has been found that high-strength steel pipes having properties such as strength can be manufactured with high productivity.

Hereinafter, components and compositions of the electric resistance welded steel pipe used in the present invention will be described.

C: An element necessary for obtaining the strength of the steel pipe after quenching. If it is less than 0.10%, a tensile strength of 130 kgf / mm ² or more required as a high-strength steel pipe for impact bars cannot be obtained. However, when the content of C exceeds 0.30%, the strength is increased, but the toughness is reduced. As a result, when an impact load is applied, the material breaks brittlely and exhibits an unfavorable property as an impact bar. Therefore, the C content is specified in the range of 0.10 to 0.30%.

Si: An element used as a deoxidizing agent for steel, and is also useful for enhancing hardenability. In order to obtain this effect, it is necessary to contain 0.05% or more of Si. However, since the element has a strong affinity for oxygen, a penetrator is easily formed in the welded portion during the electric resistance welding in the electric resistance welded steel pipe manufacturing process, and it is difficult to obtain a sound welded portion. Therefore, there is a tendency that the toughness of a high-strength hardened steel pipe is impaired. Therefore, the upper limit of the Si content is set to 0.50%.

Mn: An element useful for enhancing the hardenability of steel and increasing the toughness. However, an excessive content increases Mn-based nonmetallic inclusions and develops a striped structure. as a result,
The toughness decreases. Further, Mn has an adverse effect on the soundness of the weld as well as Si. Therefore, the Mn content, 0.20 ~
Defined in the range of 1.50%.

P: an element that degrades the toughness of a quenched steel pipe. Therefore,
The upper limit of the P content was set to 0.020%.

S: promotes the formation of non-metallic inclusions in steel, deteriorating toughness,
This leads to defects such as a decrease in the soundness of the weld. Therefore, the S content is set to 0.020% or less.

Al: An element useful as a deoxidizer for molten steel. But Al
If the content exceeds 0.10%, the cleanliness of the steel is impaired, and surface scratches are likely to occur. Therefore, the Al content was specified in the range of 0.01 to 0.10.

In addition, Ti, B, Ni, Cr, Mo, Nb,
V, Ca, etc. exhibit the following actions, respectively.

Ni: an element effective for improving the hardenability of steel and increasing the strength while suppressing deterioration of toughness. In order to obtain this effect, it is necessary to contain 0.20% or more of Ni. However, even if Ni is contained in excess of 1.50%, not only the effect of improving the properties becomes slow, but also the cost of the steel material increases. Therefore, the Ni content was specified in the range of 0.20 to 1.5%.

Cr: an element effective for improving the hardenability of steel, it is necessary to contain 0.05% or more of Cr. However, when Cr is contained in an amount exceeding 1.00%, a penetrator is easily generated in a welded portion at the time of pipe making, and the toughness as a high-strength steel pipe is deteriorated. Therefore, the Cr content is 0.05 to
It was specified in the range of 1.00%.

Mo: an element effective in improving the hardenability of steel, 0.05
% Of Mo must be contained. But Mo
Is an expensive alloy element. Even if it is contained in excess of 0.50%, the effect corresponding to it is not obtained, and it is economically disadvantageous. Therefore, the Mo content is specified in the range of 0.05 to 0.50%.

V: Stable carbonitride is formed, and effective effects such as suppressing crystal grain coarsening during quenching and preventing deterioration of toughness are exhibited. In order to obtain such an effect, it is necessary to contain 0.01% or more of V. However, when the V content is 0.20
%, The C concentration of the matrix is reduced by induction hardening in which the steel material is heated in a very short time due to insufficient solid solution of carbide. As a result, the required strength cannot be obtained. Therefore, the V content was set in the range of 0.01 to 0.20%.

Like Nb: V, it is an effective element in suppressing coarsening of crystal grains. However, the solid solution of the carbide in the matrix is reduced, and the strength is reduced. Therefore, the Nb content was set in the range of 0.01 to 0.20% similarly to V.

Ti: V or N by forming carbonitride which hardly forms a solid solution during heat treatment
It has the same effect as b. In addition, in the steel to which B is added, B works effectively by the denitrification of Ti, and a predetermined hardenability is ensured. In order to obtain these effects, Ti
Must be contained at 0.01% or more. But 0.
If the content of Ti exceeds 10%, coarse nitrides are formed, leading to deterioration of toughness. Therefore, the Ti content is 0.01
Specified in the range of 0.10.10%.

B: The hardenability of steel is greatly improved by adding a small amount of B. However, if the content of B is less than 0.0005%, there is no effect in improving the hardenability. On the other hand, if the B content exceeds 0.01%, a compound is formed in the steel, which causes not only a decrease in hardenability but also an adverse effect on toughness. From this point, 0.0005-
The B content was determined within the range of 0.01%. Ca: S is an element effective in controlling the form of nonmetallic inclusions to render them harmless and increasing the toughness of steel. In order to obtain this effect, it is necessary to contain 0.001% or more of Ca. However, when 0.01% or more of Ca is contained, the amount of nonmetallic inclusions in the steel increases, and the toughness is rather deteriorated. Therefore, the Ca content is specified in the range of 0.001 to 0.01%.

Various methods can be employed for producing a steel pipe from steel containing the above alloy elements. For example, in view of the homogeneity of product characteristics, a seamless pipe can be manufactured without a seam. But,
Considering the manufacturing cost, a method of manufacturing an electric resistance welded steel pipe by welding a sheet material by high frequency induction resistance heating is the most suitable method for manufacturing an impact bar steel pipe.

This electric resistance welded steel pipe is heat-treated by, for example, an induction hardening apparatus schematically shown in FIG. That is, the long ERW steel pipes a are stocked in the inlet pipe rack 1 and are discharged one by one from the pipe rack 1 to a conveyor such as the roller conveyor 2 during the quenching operation. ERW steel pipe a
Is transported by the roller table 2 and the pinch roll 3
Reach In this transport process, the long ERW steel pipe a is transported while being rotated by the rollers of the roller table 2.

Next, the ERW steel pipe a is sent into the tunnel-shaped heating coil 4. Electric power is supplied from the high-frequency power supply 5 to the heating coil 4, and the electric resistance welded steel pipe a is heated while moving in the tunnel-shaped high-frequency coil 4, and the temperature is raised to the austenite region.

On the outlet side of the high-frequency coil 4, an annular water cooling tube 6 is arranged. The electric resistance welded steel pipe a heated to the austenite region is cooled by the cooling water injected from the water cooling pipe 6 and quenched.

The quenched ERW steel pipe a passes through a delivery pinch roller 7 and is conveyed by a delivery roller table 8. And
It is paid out from the roller table 8 to the outlet pipe rack 9.

The above steps are repeated, and a number of long steel pipes are sequentially quenched. At this time, by continuously transporting the succeeding long steel pipe to the preceding steel pipe with almost no space therebetween, it is possible to continuously process a large number of steel pipes. Moreover, since the steel pipe is fed on the roller table while rotating, the heating temperature by high-frequency heating and the cooling temperature by water cooling are made uniform in the circumferential direction of the steel pipe, thereby suppressing uneven heating and improving the roundness and straightness of the steel pipe. Will be maintained.

Further, when the quenching method according to the present invention is used, the steel pipe is always conveyed in one direction on the roller tables 2 and 8, and the heating coil 4 and the water cooling pipe 6 do not move. Therefore, the reciprocation of the heating coil 4 and the attachment / detachment step of the steel pipe holder are omitted, and the productivity can be greatly improved as compared with the conventional batch induction hardening method described with reference to FIG.

The pinch rolls 3, 7 arranged on the inlet side of the heating coil 4 and the outlet side of the water cooling tube 6, respectively, are effective in accurately controlling the transport speed of the ERW steel pipe a and stabilizing the material of the hardened steel pipe. It is.

When a normal roller table is used, a long steel pipe cut to about several meters is subjected to heat treatment. In this case, a step of cutting the quenched steel pipe to a predetermined length is added.
However, when changing the roller pitch or the like of the roller table, it is also possible to quench a short steel pipe of about 1 m corresponding to the length of the impact bar product.

It is also possible in principle to perform quenching while transporting the endless steel pipe by connecting it to a pipe-making line. However, the line speed of a normal tube making machine for producing ERW steel pipes is as high as several tens of meters per minute. Therefore, in order to perform the quenching process in synchronization with this, an extremely large-capacity high-frequency heating device is required, and the burden on equipment costs increases.

[Examples] The present invention will be specifically described by way of examples.

Melting steel with the components and compositions shown in Table 1 in a converter,
The slab was manufactured by continuous casting. This slab was hot-rolled by a hot strip mill into a hot-rolled steel sheet having a thickness of 2.3 mm. After pickling the obtained hot-rolled steel sheet, a part was further annealed, slit to a predetermined width, and an electric resistance welded steel pipe having an outer diameter of 25.4 mm was manufactured by a tube forming machine.

This ERW steel pipe was cut to a length of 5 m and quenched by the induction hardening apparatus shown in FIG. The heating temperature by high-frequency heating was set to 900 to 1000 ° C., and the line speed was set to 10 m / min.

The mechanical properties of the quenched steel pipe were investigated. For the test, a tensile test and an impact bending test were performed.
In the impact bending test, a weight was dropped on a quenched steel pipe supported at two points, and the failure mode of the steel pipe was observed. The survey results are shown in Table 2.

Test Example 1 shows that C, which has the greatest effect on the quenching strength,
Low content. Further, in Test Example 6, since the content of the element for improving the hardenability is small, during cooling in induction hardening according to the present invention, part of ferrite deformation occurs and the amount of martensite decreases. As a result, as shown in Table 2, in both Comparative Examples 1 and 6, the tensile strength required as an impact bar of 130 kgf / mm ² or more was not obtained.

Further, in Test Example 7 using a steel material containing a large amount of C, although a very high tensile strength of 192 kgf / mm ² was obtained, the ductility was reduced. Therefore, the toughness required for the impact bar is insufficient.

On the other hand, in Test Examples 2 to 5 according to the present invention, a high strength of 130 kgf / mm ² or more, which is required as an impact bar, is obtained, and the toughness is excellent.

In addition, in the above example, the case where the round-shaped ERW steel pipe was used was described. However, the present invention is not restricted to this, and is similarly applied to, for example, a steel pipe formed by electric resistance welding and formed into a square shape. In this case, the rectangular steel pipe does not roll on the roller table. However, the high frequency applied to the long steel pipe is homogenized due to the long length, acts on each part, and does not generate uneven heat. Therefore, the structure after quenching becomes uniform.

[Effects of the Invention] As described above, in the present invention, the components and compositions of the electric resistance welded steel pipe to be used are adjusted, and induction hardening is performed while being continuously conveyed by a roller table. As a result, it is possible to manufacture hardened steel pipes with good shape characteristics, with properties that are significantly improved compared to conventional manufacturing methods, and have properties corresponding to the strength and toughness required for vehicle reinforcement parts such as impact bars. It is possible to do. Further, in consideration of excellent strength and toughness, the thickness and thickness of the reinforcing component can be reduced accordingly.

[Brief description of the drawings]

FIG. 1 shows an outline of an induction hardening apparatus according to the present invention, and FIG. 2 shows an outline of a conventional batch type induction hardening apparatus. 1: Inlet pipe rack, 2: Inlet roller table 3: Inlet pinch roller, 4: High frequency heating coil 5: High frequency power supply, 6: Ring-shaped water cooling tube 7: Outlet pinch roll, 8: Outlet roller table 9: Outgoing pipe rack, a: ERW steel pipe

────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI C22C 38/00 301 C22C 38/00 301A 38/06 38/06 (56) References JP-A-60-67623 (JP, A) JP-A-62-139848 (JP, A) JP-A-60-215719 (JP, A) JP-A-3-122219 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9 / 08,1 / 42,1 / 18 B60J 5/04 C22C 38/00-38/60

Claims (4)

(57) [Claims] (1) C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.50%, P: 0.020% or less, S: 0.020% or less, and Al: 0.01 to 0.10% by weight. A method for producing a quenched steel pipe for an automobile door reinforcing material having excellent tensile strength, wherein high-frequency induction heating is performed while continuously transporting an ERW steel pipe on a conveyor, followed by water-cooling quenching immediately. 2. The weight ratio of C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.50%, P: 0.020% or less, S: 0.020% or less, Al: 0.01 to 0.10% Ti: 0.01 A steel door reinforcement material with excellent tensile strength, characterized in that an electric resistance welded steel tube containing up to 0.10% and B: 0.0005 to 0.010% is subjected to high frequency induction heating while being continuously conveyed on a conveyor, and then water-quenched immediately. For manufacturing hardened steel pipes. 3. The weight ratio of C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.50%, P: 0.020% or less, S: 0.020% or less, Al: 0.01 to 0.10%, and Ni: 0.20 to 1.50%, Cr: 0.05 to 1.00%, Mo: 0.05 to 0.50%, V:
High-frequency induction heating of ERW steel pipes containing one or more of 0.01 to 0.20%, Nb: 0.01 to 0.20%, and Ca: 0.001 to 0.01% while continuously transporting them on a conveyor, followed by water cooling and quenching immediately A method for producing a hardened steel pipe for an automobile door reinforcing material having excellent tensile strength. 4. The weight ratio of C: 0.10 to 0.30%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.50%, P: 0.020% or less, S: 0.020% or less, Al: 0.01 to 0.10%, Ti: 0.01 to 0.10%, B: 0.0005 to 0.010% and Ni: 0.20 to 1.50%, Cr: 0.05 to 1.00%, Mo: 0.05 to 0.50%, V:
High-frequency induction heating of ERW steel pipes containing one or more of 0.01 to 0.20%, Nb: 0.01 to 0.20%, and Ca: 0.001 to 0.01% while continuously transporting them on a conveyor, followed by water cooling and quenching immediately A method for producing a hardened steel pipe for an automobile door reinforcing material having excellent tensile strength.