JP2001032034A - Hydroforming steel pipe - Google Patents

Abstract

(57) [Summary] [Problem] To provide a steel pipe which can be manufactured as it is as hot rolled or as adjusted and cooled after hot rolling, and which has excellent hydroforming properties. [Solution] C <0.06%, Si ≦ 1.0%, Mn: 0.75 to 2.50
%, Cu ≦ 2.0%, Ni ≦ 2.0%, Cr ≦ 2.0%, Mo ≦ 2.0%, V
≤0.5%, Nb≤0.5%, Ti≤0.5%, Al≤0.2%, with the balance being Fe and impurities, Cu (%) + Ni (%) +
Cr (%) + Mo (%) ≦ 5.0% and Nb (%) + V (%) + Ti
(%) ≦ 1.0%, P ≦ 0.2%, S ≦ 0.0
3%, chemical composition satisfying N ≦ 0.01%, and 95% of the tissue
The steel pipe for hydroforming in which the part exceeding is the ferrite phase. Here, the proportion of the tissue refers to the proportion of the tissue when observed with a microscope, that is, the area ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel pipe for hydroforming. More specifically, hydroforming (hereinafter referred to as “hydroforming”) is performed by applying an internal pressure of a liquid to a steel pipe to form a predetermined shape, or forming a predetermined shape by combining the internal pressure of the liquid in the steel pipe and the axial compression of the steel pipe. Forming)). Specifically, the tensile strength using a No. 5 tensile test piece or a No. 12 tensile test piece specified in JIS Z 2201 is 300 to 1
The present invention relates to a hydroforming steel pipe having an elongation of 95% or more measured at 000 MPa using a tensile test piece with an R described later.

[0002]

2. Description of the Related Art If a steel pipe is hydroformed, various large parts conventionally manufactured by a combination of press working and welding can be manufactured at low cost and high efficiency. Furthermore, in the case of hydroforming, since a closed cross-sectional structure is obtained, there is an effect that the rigidity of parts is increased. For this reason,
Hydroforming has begun to be used in various manufacturing fields.

[0003] In the automobile industry, in particular, recently, since the weight reduction of parts for the purpose of improving fuel efficiency has become an important issue due to global environmental problems, machining of parts using hydroforming has begun. It is expected that the application range of hydroforming will be further expanded in the future.

Regarding hydroforming, for example, Japanese Unexamined Patent Publication No. 10-175027 discloses a metal pipe for hydroforming in which the r value in the pipe axis direction is larger than the r value in the pipe circumferential direction. Japanese Patent Application Laid-Open No. H10-176220 discloses a molding method in which a preformed bellows tube is preformed and then subjected to hydroforming to form a main body. A method for manufacturing a forming steel pipe has been proposed.

[0005] Among the techniques disclosed in the above publications, in the case of a metal tube for hydroforming proposed in Japanese Patent Application Laid-Open No. 10-175027, concretely, as described in the specification, In addition, since a metal tube must be manufactured by removing a cold-rolled metal plate from a specific direction, there is a restriction on the plate removal.

In the case of the molding method proposed in Japanese Patent Application Laid-Open No. 10-175028, it is necessary to apply a special pre-deformation, which complicates the process and increases the manufacturing cost.

[0007] In the case of the technique proposed in Japanese Patent Application Laid-Open No. H10-176220, after a cold-worked ERW steel pipe, a mixed structure in which ferrite and a low-temperature generation phase such as martensite have a specific ratio, specifically Specifically, it is necessary to perform a special heat treatment in order to adjust the mixed structure in which the low-temperature generation phase such as martensite is 5 to 30% and the remainder is a ferrite phase, which also makes the process complicated. Manufacturing costs also increase.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object the need for cold working, special preforming, or special heat treatment after cold working. Another object of the present invention is to provide a steel pipe which can be manufactured as it is as hot rolled or as adjusted and cooled after hot rolling, and which has excellent hydroforming properties.

[0009]

The gist of the present invention resides in the following hydroforming steel pipe.

That is, "in terms of% by weight, C: less than 0.06%, Si: 2.0% or less, Mn: 0.75 to 2.50"
%, Cu: 2.0% or less, Ni: 2.0% or less, Cr:
2.0% or less, Mo: 2.0% or less, V: 0.5% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al:
0.2% or less, with the balance being Fe and unavoidable impurities, satisfying the following formulas and formulas, and having P, S, and N in the impurities of 0.2% or less, 0.03% or less, and 0%, respectively. 0.11 or less, and 9
A steel pipe for hydroforming in which a portion exceeding 5% is a ferrite phase.

Cu (%) + Ni (%) + Cr (%) + Mo (%) ≦ 5.0% Nb (%) + V (%) + Ti (%) ≦ 1.0% The proportion of the tissue is the proportion of the tissue when observed under a microscope,
In other words, it refers to the area ratio. "

The present inventors first studied the relationship between the structure of the steel pipe and the hydroforming property in order to solve the above-mentioned problems. As a result, the following findings were obtained.

(A) A steel pipe having a large proportion of a ferrite phase in a structure and a small proportion of a so-called “hard second phase” such as martensite, pearlite, and cementite has a good hydroforming property.

Then, a basic study on the plastic deformation behavior of the steel pipe during hydroforming was conducted, and the following findings were obtained.

(B) The bulging deformation in hydroforming is a plane distortion deformation in which deformation in the longitudinal direction of the steel pipe is restricted.

(C) In the plane strain deformation, a so-called "triaxial stress" is generated from the initial stage of the deformation. For this reason, if the “hard second phase” such as martensite occupies a large proportion in the structure, cracks occur relatively early in the deformation from the interface between these and the ferrite phase.

Further, in order to easily evaluate the hydroforming properties of the steel pipe, tensile test pieces of various shapes are sampled from various directions of the steel pipe, or from the steel pipe by cutting open and flattening. did. As a result, the following findings were obtained.

(D) A steel pipe is cut open and flattened, and a tensile test piece of a specific shape taken from a direction corresponding to the pipe circumferential direction of the original steel pipe, that is, a "tensile test piece with R" shown in FIG. Elongation is in good positive correlation with the hydroforming properties of the steel pipe.

The present invention has been completed based on the above findings.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each requirement of the present invention will be described in detail below. In addition, "%" of the content of each element means "% by weight".

(A) Chemical composition of steel pipe C: less than 0.06% C is a preferable element to increase the strength economically, but when its content is 0.06% or more, martensite occupying the structure, The ratio of the “hard second phase” such as pearlite and cementite increases, and the elongation in the plane strain state decreases, and the hydroforming property deteriorates remarkably. Therefore, the content of C is set to less than 0.06%. The C content is preferably set to 0.04% or less. From the viewpoint of elongation in the plane strain state, that is, the hydroforming property, the lower the C content, the better. Therefore, the lower limit of the C content need not be particularly defined.

Si: 2.0% or less Si has an effect of improving the balance between strength and elongation in a plane strain state through a solid solution strengthening effect. Si also has a deoxidizing effect. To ensure these effects, Si
Is preferably 0.01% or more. However, when Si is excessively contained, the weldability and surface properties of the steel pipe are impaired. In particular, when the content exceeds 2.0%,
Deterioration of the weldability and surface properties of the steel pipe becomes remarkable. Therefore, the content of Si is set to 2.0% or less. When extremely excellent surface properties are required or when hot-dip galvanizing is performed, the lower the Si content, the better. Therefore, the lower limit of the Si content need not be particularly specified. When the weldability and the surface properties are particularly important, the Si content is set to 0.1.
It is preferable to set it to 30% or less.

Mn: 0.75 to 2.50% Mn is also an element that improves the balance between strength and elongation in a plane strain state through a solid solution strengthening action. In the case of the steel pipe according to the present invention, since the content of C is kept low in order to enhance the hydroforming property, the content of Mn is 0.75.
%, The desired tensile strength (5% specified in JIS Z 2201)
No. 30 when using the No. 10 tensile test piece or the No. 12 tensile test piece
(Tensile strength of 0 to 1000 MPa). On the other hand, Mn
Is contained in excess in cementite to increase the proportion of pearlite as a hard second phase in the structure due to solid solution, or to enhance hardenability to increase hard second phase such as martensite.
By increasing the proportion of the phase, the hydroforming property is reduced. In particular, if the content exceeds 2.50%, the hydroforming property is significantly reduced. Therefore, the content of Mn is set to 0.75 to 2.50%. When the Mn content is high, it is necessary to use expensive metallic Mn in addition to ferromanganese or other alloyed iron, so the upper limit of the Mn content is preferably set to 1.80%.

Cu: 2.0% or less Cu may not be added. When added, it has an effect of improving the balance between strength and elongation in a plane strain state through a solid solution strengthening effect. In order to ensure this effect, it is preferable that the content of Cu be 0.2% or more. But,
If Cu is contained in excess of 2.0%, the transformation from austenite to ferrite is slowed down.
It is difficult to make the portion exceeding the value of ferrite into a ferrite phase.
Therefore, the content of Cu is set to 2.0% or less.

Ni: 2.0% or less Ni may not be added. When added, it has an effect of improving the balance between strength and elongation in a plane strain state through a solid solution strengthening effect. In order to ensure this effect, it is preferable that the content of Ni be 0.2% or more. But,
If Ni is contained in excess of 2.0%, the transformation from austenite to ferrite becomes slow.
It is difficult to make the portion exceeding the value of ferrite into a ferrite phase.
Therefore, the content of Ni is set to 2.0% or less.

Cr: 2.0% or less Cr may not be added. When added, it has an effect of improving the balance between strength and elongation in a plane strain state through a solid solution strengthening effect. To ensure this effect, the content of Cr is preferably set to 0.2% or more. But,
If Cr is contained in excess of 2.0%, the transformation from austenite to ferrite is slowed down.
It is difficult to make the portion exceeding the value of ferrite into a ferrite phase.
Therefore, the content of Cr is set to 2.0% or less.

Mo: 2.0% or less Mo need not be added. When added, it has an effect of improving the balance between strength and elongation in a plane strain state through a solid solution strengthening effect. To ensure this effect, it is preferable that the content of Mo be 0.2% or more. But,
When Mo is contained in excess of 2.0%, the transformation from austenite to ferrite is slowed down.
It is difficult to make the portion exceeding the value of ferrite into a ferrite phase.
Therefore, the content of Mo is set to 2.0% or less.

Cu (%) + Ni (%) + Cr (%) + M
o (%): 5.0% or less Even when the upper limits of the contents of Cu, Ni, Cr, and Mo are each set to the above values, if the sum of the contents of these elements exceeds 5.0%, austenite Transformation from ferrite to ferrite is delayed, so that it is difficult to make a portion exceeding 95% of the structure into a ferrite phase. Therefore, C
Cu (%), which is the sum of the contents of u, Ni, Cr and Mo
+ Ni (%) + Cr (%) + Mo (%) = 5.0%
It was as follows.

V: 0.5% or less V may not be added. If added, a carbonitride is formed to cause a precipitation strengthening action, and through this strengthening action, it has an action of improving the balance between strength and elongation in a plane strain state. To ensure this effect, it is preferable that the content of V be 0.01% or more. However, if the content exceeds 0.5%, the elongation in the plane strain state is rather lowered and the hydroforming property is deteriorated. Further, the carbonitrides are coarsened when the slab or billet is heated, so that the above-mentioned effects are greatly reduced and the cost is increased. Therefore, V
Was made 0.5% or less.

Nb: 0.5% or less Nb may not be added. If added, a carbonitride is formed to cause a precipitation strengthening action, and through this strengthening action, it has an action of improving the balance between strength and elongation in a plane strain state. In order to surely obtain this effect, the content of Nb is preferably set to 0.01% or more. However, if the content exceeds 0.5%, the elongation in the plane strain state is rather lowered and the hydroforming property is deteriorated. Further, the carbonitrides are coarsened when the slab or billet is heated, so that the above-mentioned effects are greatly reduced and the cost is increased. Therefore,
The content of Nb was set to 0.5% or less.

Ti: 0.5% or less Ti may not be added. If added, a carbonitride is formed to cause a precipitation strengthening action, and through this strengthening action, it has an action of improving the balance between strength and elongation in a plane strain state. To ensure this effect, it is preferable that the content of Ti be 0.01% or more. However, if the content exceeds 0.5%, the elongation in the plane strain state is rather lowered and the hydroforming property is deteriorated. Further, the carbonitrides are coarsened when the slab or billet is heated, so that the above-mentioned effects are greatly reduced and the cost is increased. Therefore,
The content of Ti was set to 0.5% or less.

Nb (%) + V (%) + Ti (%): 1.0% or less Even when the upper limits of the contents of Nb, V, and Ti are set to the above-mentioned values, respectively, the sum of the contents of these elements does not increase. If it exceeds 1.0%, the elongation in the plane strain state is rather lowered and the hydroforming property is deteriorated. Therefore, Nb, V,
Nb (%) + V (%) + Ti which is the sum of Ti contents
(%) Was set to 1.0% or less.

Al: 0.2% or less Al may not be added. If added, it has a deoxidizing effect. To ensure this effect, the content of Al is preferably set to 0.01% or more. However, Al was added to 0.1.
Even if the content exceeds 2%, the above effect is saturated.
The cost is only increased, and the elongation in the state of plane distortion is reduced, so that the hydroforming property is deteriorated. Therefore, the content of Al is set to 0.2% or less. When extremely excellent surface properties are required or when hot dip galvanization is performed, it is better not to add Si.
It is preferable to contain at least about 0.02% of Al for deoxidation.

In the present invention, the contents of P, S and N as impurity elements are regulated as follows.

P: 0.2% or less P decreases the toughness of the steel material. In particular, since the hot-rolled steel strip is gradually cooled after winding, P segregates at the interface between the hard second phase and the ferrite phase, causing so-called “tempering embrittlement”, and at the interface between the hard phase and the ferrite phase. Since cracking is promoted, hydroforming properties are reduced. In particular, when the P content exceeds 0.2%, the toughness is significantly reduced, and the hydroforming property is greatly reduced. Therefore, the content of P as an impurity element is set to 0.
2% or less. Note that the content of P as an impurity element is preferably set to 0.025% or less, and 0.020% or less.
% Is more preferable.

S: 0.03% or less S forms sulfides with Fe and causes not only surface cracking of the steel material during hot working, but also lowers the toughness of the steel material. In particular, when the S content exceeds 0.03%, the toughness is significantly reduced. Further, since S combines with Mn added for solid solution strengthening to form MnS, the content of Mn effective for solid solution strengthening is reduced, and therefore, in order to obtain a desired Mn effect, , It is necessary to contain more Mn, so that the cost increases. Therefore, the content of S as an impurity element is set to 0.03% or less.

N: 0.01% or less N decreases the toughness of the steel material. In particular, when the N content exceeds 0.01%, the toughness significantly decreases. Therefore, the content of N as an impurity element is set to 0.01% or less.

(B) Structure of Steel Pipe A steel pipe having the above-described chemical composition is provided with a desired strength and elongation in the circumferential direction of the pipe (No. 5 tensile test piece specified in JIS Z 2201 or 12).
No. 300-1000MP in tensile test using tensile test piece
In order to secure the tensile strength of a and the elongation of 95% or more in the “tensile test specimen with R” shown in FIG. 2 described later), it is necessary that a portion of the structure of the steel pipe exceeding 95% be a ferrite phase. . As a method of manufacturing steel pipes for that purpose,
For example, after heating a slab to 1000 ° C. or higher, normal hot drilling is performed, and finished at a temperature of 850 ° C. or higher, and at a cooling rate of 30 ° C./second or lower, at least 600 ° C.
There is a method of manufacturing a seamless steel pipe that is allowed to cool to room temperature. Also, after heating the billet to 1000 ° C. or more, normal hot rolling is performed, and after finishing at a temperature of 850 ° C. or more, it is wound at about 600 ° C. to produce a hot-rolled steel strip. There is a method of manufacturing an electric resistance welded steel pipe in which a steel strip is formed into a tubular shape and a butt portion is welded.

Since the ferrite phase occupies only 95% of the structure, the upper limit is 100%.
It may be a value close to%.

The chemical composition of the above (A) is designed so that a desired structure is formed by producing a steel pipe by at least the above method.

It is to be noted that the structure and thickness of the hot-rolled steel sheet or the hot-rolled steel sheet or the hot-rolled steel strip can be easily adjusted as compared with the case of the seamless steel pipe. Therefore, the steel pipe according to the present invention is preferably an electric resistance welded steel pipe.

As shown in FIG. 1, as an example, the critical expansion ratio at the time of hydroforming of a steel pipe has a good positive correlation with the elongation in a tensile test using the “tensile test piece with R” in FIG. . That is, the hydroforming property of the steel pipe can be easily evaluated using the pipe circumferential elongation using the tensile test piece with R as an index. The above growth is 9
When it is 5% or more, the hydroforming property of the steel pipe is good, and a critical pipe expansion rate of 20% or more can be secured. Here, the “tensile test piece with R” shown in FIG. 2 is obtained by cutting a steel pipe, flattening it, and collecting it from a direction corresponding to the pipe circumferential direction of the original steel pipe.

FIG. 1 shows the results of investigations on ERW steel pipes having an outer diameter of 60.5 mm, a wall thickness of 2.5 mm, a tensile strength of 370 to 420 MPa, and various structures. The expansion ratio is defined by the following equation.

Critical expansion ratio = {(perimeter of fracture portion−perimeter of base tube) / (perimeter of base tube)} × 100 The present invention will be described in more detail with reference to the following examples.

[0045]

EXAMPLE Steel having the chemical composition shown in Table 1 was melted and made into a slab. Incidentally, steels 1 to 16 in Table 1 are steels of Examples of the present invention whose steels have chemical compositions within the range specified by the present invention, and Steels A to F
Is a steel of a comparative example in which one of the components is out of the range of the content specified in the present invention.

[0046]

[Table 1]

Next, the slab was hot-rolled and wound by a usual method to produce a hot-rolled steel strip having a thickness of 2.5 mm. In addition, the heating temperature of the slab is 1200 ° C, the hot rolling finish temperature is about 900 ° C, and the winding temperature is about 600 ° C.
And

The hot-rolled steel strip obtained as described above is used as a raw material and subjected to electric resistance welding by a usual method to obtain a diameter d of 60.
An electric resistance welded steel pipe of 5 mm was produced.

When steel C was used as the material steel, defects (penetrators) occurred at the welded portions. In addition, when steel D and steel E were used as the material steel, cracks occurred at the welded portions during electric resistance welding.

A test piece for observing the structure was collected from the welded portion of the ERW steel tube and a portion of the tube in the circumferential direction opposite to the welded portion. Of the ferrite phase (area ratio) was investigated.

Further, a No. 12B tensile test piece specified in JIS Z 2201 was sampled from the tube axis direction of the steel pipe, and a tensile test was performed at room temperature to measure the tensile strength. Further, the steel pipe was cut open and flattened, and a tensile test piece with R shown in FIG. 2 was taken from a direction corresponding to the circumferential direction of the original steel pipe, and a tensile test was performed at room temperature to measure elongation.

On the other hand, FIG.
A hydroforming test using upper and lower molds as shown in (a) was performed. That is, the upper and lower dies which form a space having a length of 4d (that is, four times the diameter of the test steel pipe) are formed on both sides from the space by 5d (that is, 5 times the diameter of the test steel pipe).
Times) the steel pipe part, and apply internal pressure with water inside the steel pipe,
The steel pipe was swelled in the mold space, and a hydroforming test in which a pipe expansion ratio represented by the following equation was 20% was performed.
FIG. 3B shows that a crack was generated in the hydroforming test at the pipe expansion ratio of 20%.

Expansion ratio = {(perimeter after deformation−perimeter of base tube) / (perimeter of base tube)} × 100 Table 2 shows the results of microstructure observation, the results of the two types of tensile tests, and the expansion ratio. The results of hydroforming at 20% are shown together.

[0054]

[Table 2]

From Table 2, it can be seen that the steel pipes according to the present invention of Test Nos. 1 to 16 have the desired tensile strength of 300 to 1000 MPa, and have no crack even when hydroformed at a pipe expansion ratio of 20%. It is clear that it has excellent properties.

On the other hand, the steel pipes according to the comparative examples of Test Nos. 17 to 22 were cracked by hydroforming with an expansion ratio of 20%, and were inferior in hydroforming properties.

[0057]

The steel pipe of the present invention has a tensile strength of 300 to 1
It has a strength of 000 MPa and an elongation of 95% or more in the "tensile test piece with R", so that it has excellent hydroforming properties and a critical expansion ratio of 20% or more.
For this reason, it can be used as a material for various parts including automobile parts.

[Brief description of the drawings]

FIG. 1 is a diagram showing that a critical expansion ratio at the time of hydroforming of a steel pipe has a good positive correlation with elongation in a tensile test using a “tensile test piece with R”.

FIG. 2 is a diagram illustrating “tensile test specimen with R”.

FIG. 3 is a diagram illustrating a hydroforming test;
(A) is a test mold, and (b) is a steel pipe that has cracked in a hydroforming test.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Keisuke Ichiiri 1850 Minato, Wakayama-shi, Wakayama Prefecture Sumitomo Metal Industries, Ltd. Inside Wakayama Works (72) Inventor Masayasu Kojima 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries Co., Ltd. (72) Inventor Saburo Inoue 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries Co., Ltd.

Claims (1)

[Claims] C. Less than 0.06% by weight, Si:
2.0% or less, Mn: 0.75 to 2.50%, Cu:
2.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Mo: 2.0% or less, V: 0.5% or less, Nb:
0.5% or less, Ti: 0.5% or less, Al: 0.2% or less, with the balance being Fe and unavoidable impurities, satisfying the following formula and the formula,
S and N are 0.2% or less, 0.03% or less, respectively.
A hydroforming steel pipe having a chemical composition that satisfies 01 or less and further having a ferrite phase in more than 95% of the structure. Cu (%) + Ni (%) + Cr (%) + Mo (%) ≦ 5.0% Nb (%) + V (%) + Ti (%) ≦ 1.0% Here, the proportion of the structure Is the tissue ratio when observed under a microscope,
That is, it indicates the area ratio.