JP2003328082A - High impact resistance ERW steel pipe - Google Patents

Abstract

[PROBLEMS] To provide a high impact resistance ERW steel pipe which has excellent strength and toughness and has no toughness reduction in the vicinity of an ERW weld. The present invention relates to a high impact resistance ERW pipe having a round or square cross section having a tensile strength of 1700 MPa or more and a Si content in a steel of (Mn / 8) -0.0.
By controlling the range of 7 to (Mn / 8) +0.07, the toughness of the weld was improved. The composition is as follows: C: 0.22 to 0.35% in steel, Si: 0.10 to 0.30
%, Mn: 0.5 to 1.50%, P: 0.025% or less, S: 0.01% or less, Al: 0.010 to 0.05
0%, B: 2 to 35 ppm, Ti: 0.005 to 0.0
It contains 5% as an essential component. It is preferable to set the martensite structure to 95% or more by induction hardening and to set the former austenite grain size number to 6 or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high impact resistance electric resistance welded steel pipe having a round or square cross section, which is used as a shock absorbing member such as a material for a vehicle door impact beam bumper and a material for reinforcing a bumper. Is. The term “steel pipe” generally means a steel pipe having a circular cross section, but in the present specification, it means a steel pipe having a round or square cross section. Here, the round shape includes circles, ellipses, and the like, and the square shape includes not only polygons such as triangles, squares, and pentagons, but also irregular cross-sectional shapes.

[0002]

[Prior art]

[Patent Document 1] Japanese Unexamined Patent Publication No. 7-18374 SUMMARY OF THE INVENTION Regarding the high-strength electric resistance welded steel pipe used as a member subjected to impact bending such as a door impact beam,
Needless to say, toughness that affects impact energy absorption characteristics is required. Therefore, as shown in Patent Document 1, various developments have been conventionally made with the aim of improving these characteristics. However, in many cases, conventional products have strength and strength in the vicinity of an electric resistance welded portion for manufacturing an electric resistance welded steel pipe. The toughness tended to decrease.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and, in addition to being excellent in the strength and toughness of the main body portion, also has high impact resistance and electric resistance without deterioration of toughness in the vicinity of the electric resistance welded portion. It was made to provide a sewn steel pipe.

[0004]

In order to solve the above problems, the present inventor measured the Charpy absorbed energy characteristics in the vicinity of the electric resistance welded portion, and found that the Charpy absorbed energy characteristics were damaged. In the cross section, Si,
It was found that the oxide containing Mn remained, and it was found that these components were one of the causes of the decrease in toughness. Then, it was confirmed that when a specific relationship is established between Si and Mn, a decrease in toughness in the vicinity of the welded portion can be prevented.

The present invention has been made on the basis of the above findings, and has a high tensile strength of 1700 MPa or more and the Si content in steel is (Mn / 8) -0.0.
The gist is a high impact resistance electric resistance welded steel pipe characterized by being controlled in the range of 7 to (Mn / 8) +0.07. In particular, C: 0.22 to 0.35% by mass ratio in steel, Si:
0.10 to 0.30%, Mn: 0.5 to 1.50%,
P: 0.025% or less, S: 0.01% or less, Al:
0.010 to 0.050%, B: 2-35 ppm, T
It is preferable that the composition contains i: 0.005 to 0.05% as an essential component.

Further, Nb: 0.005-0.050%,
V: 0.005 to 0.070%, Cu: 0.005 to 0.5
%, Cr: 0.005-0.5%, Mo: 0.1-0.5
%, Ni: 0.1 to 0.5%, Ca: 0.01% or less, rare earth element (REM): 0.1% or less, and any optional component selected from the group may be contained. Further, it is preferable that induction hardening is performed to obtain a martensite structure of 95% or more and a prior austenite grain size number of 6 or more. The cross-sectional shape includes both round and square shapes.

As described above, S generated in the electric resistance welded portion
It is considered that the oxide containing i and Mn reduces the toughness of the welded portion, but in the present invention, the Si content is (Mn / 8).
By controlling in the range of -0.07 to (Mn / 8) +0.07, the above oxides can be discharged from the electric resistance welded portion, and as shown in the data of Examples described later, 17
It is possible to completely prevent deterioration of the toughness of the welded portion while maintaining a high tensile strength of 00 MPa or more. The present invention will be described in more detail below.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The high impact resistance ERW steel pipe of the present invention is
An electric resistance welded steel pipe was manufactured from steel to which C and Mn were added, and this was rapidly cooled to have an austenite structure as a martensite structure and a tensile strength of 1700 MPa or more was achieved. The basic composition of the steel is C: 0.22 to 0.35%, Si: 0.10.
~ 0.30%, Mn: 0.5 to 1.50%, P: 0.0
25% or less, S: 0.01% or less, Al: 0.010 to
0.050%, B: 2-35 ppm, Ti: 0.005
.About.0.05% is contained as an essential component.

First, the reason for limiting the numerical values of each component in the basic composition will be described. C is an essential component for strengthening martensite itself and improving hardness, and is 1700M.
At least 0.22% is required to obtain TS of Pa or more. However, if C is excessive, the martensite structure becomes brittle and causes quenching cracks that break during quenching.
It should be 0.35% or less.

Si, Mn, and Ti are components that promote martensitic transformation from austenite during quenching, and Si: 0.10 to 0.30%, Mn: 0.
If it is less than the respective numerically limited ranges of 5 to 1.50% and Ti: 0.005 to 0.05%, the hardenability deteriorates and residual austenite and residual ferrite are generated, and desired material properties cannot be obtained. On the contrary, if the value exceeds the above-mentioned numerical range, it may cause quench cracking or segregation, which is not preferable. Note that Ti is N
By fixing, it has the effect of improving hardenability.

B is a component which suppresses the precipitation of ferrite, but is combined with N contained as a gas component in steel to form BN.
In that case, the effect is lost, so the content should be 2 ppm or more. However, when it exceeds 35 ppm, it becomes a segregated inclusion. P
Since S and S become segregation inclusions and make the martensite structure brittle, it is necessary to set P: 0.025% or less and S: 0.02% or less. Al is a deoxidizing agent, and if it is less than 0.010%, the deoxidizing effect becomes insufficient, and if it exceeds 0.050%, its oxide becomes an intercrystalline inclusion, which is not preferable.

In addition to the essential components described above, Nb:
0.005-0.050%, V: 0.005-0.070
%, Cu: 0.005 to 0.5%, Cr: 0.005 to 0.5.
5%, Mo: 0.1 to 0.5%, Ni: 0.1 to 0.5%,
Ca: 0.01% or less, rare earth element (REM): 0.1%
It is also possible to include any optional component selected from the following groups.

Nb and V are precipitation strengthening components that improve the strength by forming precipitates in the martensite structure and preventing passage of dislocations. Cu, Cr, Mo and Ni are solid solution strengthening components that improve the strength by being dissolved in the martensite crystal to prevent dislocation passage. Note that Cr and Mo also act as precipitation strengthening components. Although these components contribute to the increase in strength, they cause a cost increase and excessive addition causes segregation inclusions. Therefore, Nb: 0.0
05-0.050%, V: 0.005-0.070%, C
u: 0.005 to 0.5%, Cr: 0.005 to 0.5%,
Mo: 0.1 to 0.5% and Ni: 0.1 to 0.5% are suitable.

Ca and rare earth elements (REM) are components that contribute to the morphology control of inclusions, but excessive addition causes harmful segregation leading to destruction of the martensite structure, so C
It is suitable that a: 0.01% or less and REM: 0.1% or less. Examples of rare earth elements (REM) are Y, La, C
e and Sm can be used.

As described above, in the high impact resistance electric resistance welded steel pipe of the present invention, in order to promote the martensitic transformation from austenite, Si: 0.10 to 0.30%, Mn: 0.
Including 5 to 1.50% as essential, tensile strength 1700
A high strength of at least MPa is achieved. Further, Si and Mn are (Mn / 8) -0.07 ≦ Si ≦ (M
The greatest feature is that the toughness of the electric resistance welded portion is prevented from being lowered by controlling so as to satisfy the formula of (n / 8) +0.07. FIG. 1 is a graph showing the results of toughness measurement of the upset welded portion, and it was confirmed that the relative Charpy absorbed energy becomes maximum when the Si amount is within the above range. The relative Charpy absorbed energy is -4
Si = (Mn / 8) + α (α = − when the Charpy absorbed energy of the component of Si = Mn / 8 at 0 ° C. is set to 1
It is the relative value of the Charpy absorbed energy of the components of 0.30 to +0.30).

As in the present invention, a steel pipe in which C and Mn are added to steel to attain a high strength of 1700 MPa or more has a lower melting point than a low-strength steel pipe, and is melted during electric resistance welding. The viscosity of the oxide formed on the surface of the metal becomes relatively low. Therefore, it is considered that it is particularly important to control the amount of Si that affects the oxide residue in the weld zone as described above to eliminate the oxide, in order to prevent the deterioration of the toughness of the weld zone.

Further, as described in claim 4, it is preferable that induction hardening is performed to obtain a martensite structure of 95% or more and a prior austenite grain size number of 6 or more in order to secure low temperature impact bending characteristics. Figure 2 shows high impact resistance ERW steel pipes with different austenite grain sizes (tensile strength 1
(700 MPa), an impact bending test was performed, and the result of observing the presence or absence of cracks was observed, and by using fine crystals of former austenite grain size number 6 or more, a steel pipe having excellent low temperature impact bending properties can be obtained. I understand. Note that grain refinement can be achieved by, for example, lowering the quenching temperature, refining the structure before quenching, and the effect of additive elements such as Nb, V, and Ti. The former austenite grain size measurement may be performed by a cutting method or image analysis after revealing the former austenite grain boundary of the base material with a commonly used austenite grain boundary developing solution.

As a method of manufacturing a square steel pipe having a square cross section, a method of continuously bending and forming a steel strip to perform square forming during electric resistance welding, and a forming roll after the electric resistance welding step are used. There is a method of performing corner forming. In the former case, if the plate thickness is designed to be an ultra-high strength material, it becomes difficult to secure the amount of upset during welding and the risk of oxide residue increases, but the present invention affects oxide residue. Controlling the amount of Si as described above is particularly effective because oxides can be eliminated and welding quality can be stabilized.

[0019]

[Examples] ERW steel pipes having various compositions shown in Tables 1 and 2 were produced, and the tensile strength, the ratio of weld strength / main body tensile strength,
The presence or absence of cracks in low temperature impact bending was measured and recorded in the table. The value of α = Si-Mn / 8 and the former austenite grain size number are also shown in the table. Examples 1-5 and Comparative Example 1
3 to 3 are round type electric resistance welded steel pipes, and Examples 6 and 7 and Comparative Example 4 are examples of rectangular electric resistance welded steel pipes.

[0020]

[Table 1]

[0021]

[Table 2]

The composition in the table is% by mass (however, only B is ppm by mass), the balance is Fe and unavoidable impurities, and ○ in the low temperature impact bending column indicates the value when the impact bending test was performed under the low temperature condition of −60 ° C. No cracking occurred, and x means cracking occurred. The strength is tensile strength and its unit is MPa. The blank of the component content means no addition.

[0023]

As described above, the high impact resistance electric resistance welded steel pipe of the present invention is not only excellent in strength and toughness, but also has no deterioration in strength and toughness in the vicinity of the electric resistance welded portion and is suitable for door impact beam bumpers. It is suitable for use as a shock absorbing member such as a material or a bumper reinforcing material.

[Brief description of drawings]

FIG. 1 is a graph showing the results of toughness measurement of an upset weld.

FIG. 2 is a graph showing the relationship between the prior austenite grain size and the presence or absence of cracking during the impact bending test.

Continued front page    (72) Inventor Hiroto Tanabe             5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation             Ceremony Company Nagoya Steel Works

Claims (5)

[Claims] 1. A steel having a high tensile strength of 1700 MPa or more and an amount of Si present in steel is (Mn / 8) -0.
A high impact resistance ERW steel pipe characterized by being controlled in the range of 07 to (Mn / 8) + 0.07. 2. A mass ratio of C: 0.22 to 0.35 in steel.
%, Si: 0.10 to 0.30%, Mn: 0.5 to 1.
50%, P: 0.025% or less, S: 0.01% or less,
Al: 0.010-0.050%, B: 2-35pp
m, Ti: 0.005-0.05% is contained as an essential component, The high impact resistance electric resistance welded steel pipe according to claim 1. 3. Nb: 0.005 to 0.005 in mass ratio in steel.
050%, V: 0.005 to 0.070%, Cu: 0.0
05-0.5%, Cr: 0.005-0.5%, Mo: 0.0.
1-0.5%, Ni: 0.1-0.5%, Ca: 0.01%
The high impact resistance electric resistance welded steel pipe according to claim 2, further comprising an optional component selected from the group of rare earth element (REM): 0.1% or less. 4. The high impact resistance electric resistance welded steel pipe according to claim 2 or 3, wherein the martensite structure is 95% or more by induction hardening and the former austenite grain size number is 6 or more. 5. The high impact resistance electric resistance welded steel pipe according to any one of claims 1 to 4, wherein the cross-sectional shape is round or square.