JP2007270349A - Steel tube for hollow part, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel tube for hollow parts improved in fatigue strength by suppressing the occurrence of a ferrite decarburization layer, and a manufacturing method for the same. <P>SOLUTION: The steel tube for the hollow parts is characterized in that an electric resistance welded tube is subjected to diameter reduction rolling and that a wall thickness is ≥5 mm, t/D, i.e. ratio of the wall thickness t, and an external diameter D is ≥0.2 and the depth of the decarburization layer on the inner surface of the tube is ≤20 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel pipe for hollow parts with improved fatigue strength and a method for producing the same.

  As one of the measures to improve the fuel efficiency of automobiles, weight reduction of the car body is being promoted. Stabilizers and drive shafts that reduce the rolling of the car body when cornering an automobile and ensure the stability of the car body at high speeds are also cited as countermeasures. Conventionally, stabilizers and drive shafts were manufactured by processing solid materials such as steel bars into the required shape, but in order to reduce weight, the use of steel pipes for hollow parts such as ERW welded pipes has been promoted. Yes.

  As an electric resistance welded steel pipe of a stabilizer, Patent Document 1 specifies a composition so that the metal structure of the electric resistance welded part and the base metal is uniform, the hardness difference between the electric resistance welded part and the base metal part is small, and processing An electric resistance welded steel pipe for a hollow stabilizer is disclosed, and Patent Document 2 discloses an electric resistance welded steel pipe for a hollow stabilizer that secures hardenability by defining the contents of Ti and N Has been.

  For example, the stabilizer is manufactured using a thick-walled ERW welded steel pipe having a required thickness by further reducing the diameter of the ERW welded steel pipe. A ferrite decarburized layer is likely to be formed on the surface of the thick-walled ERW welded steel pipe when passing through the α-γ2 phase region that is being cooled to room temperature after being hot-rolled. Since this ferrite decarburized layer is weak, if it is formed on the surface of a thick ERW welded steel pipe, the ferrite decarburized layer will be the starting point for fatigue failure. As a result, the fatigue strength of the thick ERW welded pipe will be reduced. It causes a decrease.

As a countermeasure when the above-described ferrite decarburized layer is formed on the surface of a steel pipe, there are the following techniques.
For example, the ferrite decarburized layer is ground from 25 to 500 μm and removed from the steel pipe surface. Thereby, the fall of the fatigue strength by a ferrite decarburization layer can be suppressed (for example, refer patent document 3).

  Further, by performing shot peening on the surface of the steel pipe, it is possible to improve the strength of the steel pipe and suppress a decrease in fatigue strength due to the ferrite decarburized layer (see, for example, Patent Document 4). Moreover, the strength of the steel pipe can be improved by forming a carburized layer on the surface of the steel pipe, and a decrease in fatigue strength due to the ferrite decarburized layer can be suppressed (see, for example, Patent Document 5).

WO2002 / 070767 JP 2004-011009 A JP-A-7-215038 JP 2002-331326 A JP 2000-118224 A

  As described above, in conventional steel pipes for hollow parts, when the steel pipe is hot-reduced and cooled to room temperature, a ferrite decarburized layer is easily formed on the surface of the thick ERW welded steel pipe. The ferrite decarburized layer is a cause of reducing the fatigue strength of thick-walled ERW welded steel pipe. For this reason, it is desirable to manufacture the steel pipe for hollow parts which suppressed generation | occurrence | production of the ferrite decarburization layer.

  Further, the conventional countermeasure when the ferrite decarburized layer is formed on the surface of the steel pipe is not preferable because an additional process such as a grinding process is required, which increases the manufacturing cost.

  The present invention has been made in consideration of the above-described circumstances, and an object thereof is to provide a steel pipe for hollow parts with improved fatigue strength by suppressing the occurrence of a ferrite decarburized layer and a method for manufacturing the same. It is in.

In order to solve the above problems, a steel pipe for a hollow part according to the present invention is a steel pipe for a hollow part in which an electric resistance welded steel pipe is hot reduced in diameter,
The wall thickness is 5 mm or more, and the ratio of wall thickness t to outer diameter D is t / D is 0.2 or more,
The depth of the decarburized layer on the inner surface of the pipe is 20 μm or less.

  According to the steel pipe for hollow parts according to the present invention, the fatigue strength close to that of a steel bar having the same outer diameter can be obtained by setting t / D, which is the ratio of the wall thickness t and the outer diameter D, to 0.2 or more. it can. Further, the fatigue strength can be improved by setting the depth of the decarburized layer on the inner surface of the pipe to 20 μm or less.

Moreover, the steel pipe for hollow parts which concerns on this invention WHEREIN: The said ERW welded steel pipe is the mass%. C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
It is preferable that N: 0.001 to 0.006% is contained, the balance is Fe and inevitable impurities, and the following formula is satisfied.
923-513C-101Mn ≦ 700

Moreover, the steel pipe for hollow parts which concerns on this invention WHEREIN: The said ERW welded steel pipe is the mass%. C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006%, Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%, the balance being Fe and inevitable impurities And satisfying the following formula.
923-513C-101Mn-204Cr-1515Nb ≦ 700

Moreover, the steel pipe for hollow parts which concerns on this invention WHEREIN: The said ERW welded steel pipe is the mass%. C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% is contained,
Further, it is preferable that S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070% are contained, the balance is Fe and inevitable impurities, and the following formula is satisfied.
923-513C-101Mn ≦ 700

Moreover, the steel pipe for hollow parts which concerns on this invention WHEREIN: The said ERW welded steel pipe is the mass%. C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% is contained,
And at least one of Cr: 0.05-1.0% and Nb: 0.005-0.1%,
Further, it is preferable that S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070% are contained, the balance is Fe and inevitable impurities, and the following formula is satisfied.
923-513C-101Mn-204Cr-1515Nb ≦ 700

The method for manufacturing a steel pipe for hollow parts according to the present invention includes a first step of hot rolling a diameter-welded welded steel pipe,
A second step of cooling the steel pipe reduced in diameter in the first step to a temperature lower than the α-γ2 phase temperature range;
A method for producing a steel pipe for hollow parts comprising:
The steel pipe reduced in diameter in the first step has a thickness of 5 mm or more and a ratio t / D which is a ratio of the thickness t to the outer diameter D is 0.2 or more.
The cooling rate when passing through the α-γ2 phase temperature range in the process of cooling the inner surface of the steel pipe by the second step is 5 ° C / second or more,
The depth of the decarburized layer generated on the inner surface of the steel pipe cooled in the second step is 20 μm or less.

  According to the method for manufacturing a steel pipe for hollow parts according to the present invention, the cooling rate when passing through the α-γ2 phase temperature range in the process of cooling the inner surface of the steel pipe in the second step is 5 ° C / second or more. By doing, the depth of the ferrite decarburization layer which generate | occur | produces on the inner surface of the said steel pipe can be suppressed to 20 micrometers or less.

  As described above, according to the present invention, by setting t / D, which is the ratio of the wall thickness t and the outer diameter D, to 0.2 or more, fatigue strength close to that of a steel bar having the same outer diameter can be obtained. Moreover, fatigue strength can be improved by suppressing the depth of the decarburized layer on the inner surface of the pipe to 20 μm or less. Therefore, the steel pipe for hollow parts which improved fatigue strength by suppressing generation | occurrence | production of a ferrite decarburization layer, and its manufacturing method can be provided.

Embodiments of the present invention will be described below with reference to the drawings.
The steel pipe for hollow parts according to the embodiment of the present invention is a steel pipe that has been hot-reduced and cooled to room temperature using an ERW welded steel pipe as a mother pipe, and has a wall thickness of 5 mm or more (preferably 7 mm or more). And it is a steel pipe whose t / D which is the ratio of the wall thickness t (mm) and the outer diameter D (mm) is 0.2 or more (preferably 0.23 or more). The depth of the ferrite decarburized layer on the inner surface of this steel pipe is 20 μm or less, more preferably 10 μm or less.

The reason why the thickness / outer diameter ratio t / D is 0.2 or more is as follows.
The starting point of fatigue failure of a steel pipe is from the inner surface side of the steel pipe when the thickness / outer diameter ratio t / D is generally less than 0.2, and the outer surface of the steel pipe when the ratio is 0.2 or more. From the side. Therefore, in order to obtain the same fatigue strength as that of a steel bar (solid material), the thickness / outer diameter ratio t / D needs to be 0.2 or more.

  Moreover, it is difficult to manufacture a steel pipe having a high thickness / outer diameter ratio of t / D in a normal electric resistance welded steel pipe manufacturing process. For this reason, the steel pipe for hollow parts by this Embodiment is manufactured by carrying out hot diameter reduction rolling of the electric resistance welded steel pipe which is a mother pipe.

  As described above, the surface of the steel pipe when passing through the α-γ2 phase temperature range in the course of being cooled to room temperature after hot-reducing the diameter-welded steel pipe as the mother pipe. A ferrite decarburized layer is easily formed. If a ferritic decarburized layer is formed on the inner surface of the steel pipe, even if the steel pipe has a t / D of 0.2 or more and the starting point of fatigue failure is from the outer surface side, the ferritic decarburized layer on the inner surface is weakened. The starting point of destruction shifts to the inner surface side. Therefore, it is necessary to suppress the occurrence of a ferrite decarburized layer on the inner surface of the steel pipe.

  Since the ferrite decarburized layer is generated in the α-γ2 phase temperature range, the generation of the ferrite decarburized layer can be suppressed by shortening the transit time in the α-γ2 phase temperature range. As a method of shortening the passage time in the α-γ two-phase temperature region, it is preferable to supply water to the outer surface of the steel pipe obtained by reducing the diameter of the ERW welded steel pipe and cool it at a high speed. Thereby, the cooling rate at the time of passing through the α-γ2 phase temperature region in the process of cooling the inner surface of the steel pipe can be set to 5 ° C / second or more. As a result, the depth of the ferrite decarburized layer generated on the inner surface of the steel pipe can be suppressed to 20 μm or less (more preferably 10 μm or less). Thereby, it can suppress that the origin of fatigue failure transfers to an inner surface side by a ferrite decarburization layer. Therefore, the fall of the fatigue strength of the steel pipe by a ferrite decarburization layer can be suppressed.

  The reason why the ferrite decarburized layer generated on the outer surface of the steel pipe is not a problem is that the steel pipe is usually cooled by supplying water to the outer surface, so the cooling speed of the outer surface of the steel pipe is the cooling speed of the inner surface. As a result, a ferrite decarburized layer is hardly generated on the outer surface of the steel pipe. In addition, even when the steel pipe is cooled from the outer surface, the difference in cooling rate between the inner surface and the outer surface is small in the thin steel pipe. In many cases, a ferrite decarburized layer is hardly formed. However, in the case of a thick steel pipe having a thickness of 5 mm or more as in the present embodiment, the inner surface side is difficult to be cooled when cooled from the outer surface, so there is a possibility that a ferrite decarburized layer is generated only on the inner surface side. is there. Therefore, in a steel pipe having a wall thickness of 5 mm or more, it is important to suppress generation of a ferrite decarburized layer on the inner surface.

  Moreover, in this Embodiment, although water is supplied and cooled to the outer surface of a steel pipe, it is also possible to supply and cool not only an outer surface but the inner surface of a steel pipe. Thereby, the cooling rate of the inner surface of the steel pipe can be further increased, and the depth of the ferrite decarburized layer generated on the inner surface of the pipe can be further reduced.

Next, chemical components of the electric resistance welded steel pipe will be described.
(First chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% is contained, the balance consists of Fe and inevitable impurities, and satisfies the following formula (1).
923-513C-101Mn ≦ 700 (1)

(Second chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006%, Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%, the balance being Fe and inevitable impurities And satisfies the following formula (2).
923-513C-101Mn-204Cr-1515Nb ≦ 700 (2)

(Third chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% is contained,
Furthermore, it contains S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070%, the balance is made of Fe and inevitable impurities, and satisfies the following formula (1).
923-513C-101Mn ≦ 700 (1)

(Fourth chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.15-0.5%
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% and Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%
Furthermore, it contains S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070%, the balance is made of Fe and inevitable impurities, and satisfies the following formula (2).
923-513C-101Mn-204Cr-1515Nb ≦ 700 (2)

The first chemical component has no added Cr and Nb, and the second chemical component has at least one of Cr and Nb added.
The third and fourth chemical components are obtained by adding S and Ca to the first or second chemical component.

(The more desirable range of the first chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.2 to 0.4%
Si: 0.2-0.3%
Mn: 0.5 to 1.5%
Ti: 0.01-0.03%
Al: 0.01 to 0.05%
B: 0.0010 to 0.0030%
N: 0.002 to 0.004% is contained, the balance consists of Fe and inevitable impurities, and satisfies the following formula (1).
923-513C-101Mn ≦ 700 (1)

(The more desirable range of the second chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.2 to 0.4%
Si: 0.2-0.3%
Mn: 0.5 to 1.5%
Ti: 0.01-0.03%
Al: 0.01 to 0.05%
B: 0.0010 to 0.0030%
N: 0.002 to 0.004%, Cr: 0.1 to 0.5% and Nb: 0.01 to 0.05%, the balance being Fe and inevitable impurities And satisfies the following formula (2).
923-513C-101Mn-204Cr-1515Nb ≦ 700 (2)

(The more desirable range of the third chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.2 to 0.4%
Si: 0.2-0.3%
Mn: 0.5 to 1.5%
Ti: 0.01-0.03%
Al: 0.01 to 0.05%
B: 0.0010 to 0.0030%
N: 0.002 to 0.004% is contained,
Furthermore, it contains S: 0.006 to 0.008% and Ca: 0.0020 to 0.0050%, the balance is made of Fe and inevitable impurities, and satisfies the following formula (1).
923-513C-101Mn ≦ 700 (1)

(The more desirable range of the fourth chemical component)
The electric resistance welded steel pipe is in mass%,
C: 0.2 to 0.4%
Si: 0.2-0.3%
Mn: 0.5 to 1.5%
Ti: 0.01-0.03%
Al: 0.01 to 0.05%
B: 0.0010 to 0.0030%
N: 0.002 to 0.004%, and Cr: 0.1 to 0.5% and Nb: 0.01 to 0.05%,
Furthermore, it contains S: 0.006 to 0.008% and Ca: 0.0020 to 0.0050%, the balance is made of Fe and inevitable impurities, and satisfies the following formula (2).
923-513C-101Mn-204Cr-1515Nb ≦ 700 (2)

  Since the generation of the ferrite decarburized layer is limited by the diffusion rate of carbon in the γ phase, the lower the temperature in the α-γ2 phase region, the lower the carbon diffusion rate, so that the ferrite decarburization can be suppressed. Therefore, the inventors repeated experiments to adjust the elements that enhance the hardenability so that the Ar3 point at 5 ° C./s becomes 700 ° C. or less. As a result, the present inventors have found that the Ar3 point can be set to 700 ° C. or less by satisfying the chemical component and the formula (1) or the formula (2). In addition, the said Formula (1) or Formula (2) is a regression type obtained by repeating experiment.

Below, each component contained in the said ERW welded steel pipe is demonstrated. In addition, content of each component is described in the mass%.
C is an element that increases the strength of steel by being precipitated as solid solution or carbide in the matrix. A general automobile structural member needs to have a strength of at least 100 kg / mm ² , and a corresponding hardness of around Hv320 is obtained with a 90% martensite structure when the C content is 0.15%. Therefore, it is necessary that C is contained in an amount of 0.15% or more. However, if it exceeds 0.5%, the workability and weldability deteriorate, so the content is 0.15 to 0.5. % Range. In addition, Preferably, it is 0.2 to 0.4%.

  Si is an alloying element that contributes to solid solution strengthening, and in order to obtain its effect, it is necessary to contain 0.1% or more. Si-Mn-based inclusions that become welding defects are easily generated, which adversely affects the soundness of the ERW weld. For this reason, Si content is taken as 0.1 to 0.4% of range. In addition, Preferably, it is 0.2 to 0.3%.

  Mn is an element that improves the hardenability. If the content is less than 0.3%, the effect of improving the hardenability cannot be sufficiently ensured. In order to adversely affect the properties, the Mn content is set to a range of 0.3 to 2%. In addition, Preferably, it is 0.5 to 1.5%.

  Since Al is an element necessary as a deoxidizer for molten steel and also an element for fixing N, its amount has a great influence on the crystal grain size and mechanical properties. If the content exceeds 0.05%, the crystal grain size becomes coarse and the toughness is lowered, or nonmetallic inclusions increase and surface defects are likely to occur in the product, so the Al content is 0.005. -0.05%. In addition, Preferably, it is 0.01 to 0.05%.

  B is an element that greatly improves the hardenability of the steel material by adding a small amount, and also has an effect of strengthening the grain boundary. If the content is less than 0.0005%, the effect of improving the hardenability cannot be expected. On the other hand, if the content exceeds 0.0050%, a coarse B-containing phase tends to be generated, and embrittlement tends to occur. For this reason, B content shall be 0.0005 to 0.0050%. In addition, Preferably, it is 0.0010 to 0.0030%.

  Ti acts to stably and effectively improve the hardenability by adding B by fixing N in steel and suppressing precipitation of BN, but if the content is less than 0.005%, its effect On the other hand, if it exceeds 0.05%, the toughness tends to deteriorate, so the Ti content is in the range of 0.005 to 0.05%. In addition, Preferably, it is 0.01 to 0.03%.

  N is an important element for precipitating nitride or carbonitride and increasing the strength. The effect is exhibited by adding 0.001% or more, but if the content exceeds 0.006%, the toughness deteriorates due to the decrease in hardenability due to precipitation of BN, the coarsening of nitride, and age hardening. The tendency to do is seen. For this reason, N content is taken as 0.001 to 0.006% of range. In addition, Preferably, it is 0.002 to 0.004%.

  Cr is an element that improves hardenability and also has an action of suppressing softening due to tempering. If the content exceeds 1%, defects tend to occur during ERW welding. For this reason, Cr content is taken as 0.05 to 1% of range. In addition, Preferably, it is 0.1 to 0.5%.

In addition to the effect of precipitation strengthening by Nb carbonitride, Nb has the effect of greatly reducing the Ar3 point due to the combined addition effect with B.
If the Nb content exceeds 0.1%, carbides increase and the toughness decreases. For this reason, Nb content shall be 0.005-0.1%. In addition, Preferably, it is 0.01 to 0.05%.

  Since S impairs the electroweldability, it is desirable to reduce it as much as possible without causing an increase in cost from the viewpoint of manufacturing. On the other hand, S has an effect of suppressing the ferrite decarburized layer. Therefore, if special attention can be paid to ERW welding, it is desirable from the viewpoint of suppressing ferrite decarburization to contain 0.0040% or more, preferably 0.0060% or more of S. However, in that case, it is necessary to add Ca simultaneously with S for the purpose of suppressing the precipitation of MnS in order to precipitate a large amount of MnS in the steel and impair the mechanical properties. On the other hand, if S exceeds 0.0100%, no matter how much care is taken, it is no longer possible to perform sound electric resistance welding, and the addition of Mn cannot suppress the precipitation of Mn. For this reason, S content is 0.0040 to 0.0100%.

  Ca needs to be added to suppress precipitation of MnS when S is added in the range of 0.0040 to 0.0100% for the purpose of suppressing ferrite decarburization. In order to obtain the effect, 0.0005% or more of addition is necessary, and if added over 0.0070%, Ca inclusions accumulate in the ERW welding abutment and the soundness of the weld Is damaged. For this reason, Ca content is 0.0005 to 0.0070%, More preferably, it is 0.0020 to 0.0050.

Next, the manufacturing method of the steel pipe for hollow parts of this Embodiment is demonstrated.
Molten steel melted to have the required chemical composition is cast into a slab, or once made into a steel ingot, then hot rolled into a steel slab, and this slab or steel slab is hot rolled Thus, a hot rolled steel sheet is obtained.

  This hot-rolled steel sheet is made into an ERW welded steel pipe by a conventional method for producing ERW welded pipe, for example, hot or cold electric resistance welding. That is, an electric resistance welded steel pipe having a ratio t / D, which is a ratio of the wall thickness t to the outer diameter D, of 0.15 or less (which is also referred to as a mother pipe) is manufactured by an ordinary electric resistance welded steel pipe making machine. Further, this is subjected to hot reduction rolling, and the thickness t / D which is the ratio of the thickness t to the outer diameter D is 0.2 or more (preferably 0.23) when the thickness is 5 mm or more (preferably 7 mm or more). In the process of supplying the water to the outer surface of the steel pipe to cool it to a temperature lower than the α-γ2 phase temperature range, and cooling the inner surface of the steel pipe. The cooling rate when passing through the γ2 phase temperature range is set to 5 ° C./second or more.

Reduction rolling can be performed using a stretch reducer or the like.
The stretch reducer is a rolling device provided with a plurality of rolling stands having three or four rolls around the rolling shaft and in series with the rolling shaft, and adjusts the roll rotation speed and the rolling force of each rolling stand of this rolling device. By doing this, it is possible to perform reduction rolling that controls the tension in the tube axis direction (rolling direction) and the compressive force in the circumferential direction of the steel pipe, thereby increasing the thickness / outer diameter ratio.

  That is, in reduced diameter rolling, the outer diameter is reduced by the rolling force of the outer diameter of the steel pipe while the wall thickness is increased, but on the other hand, the wall thickness is decreased by the tension acting in the tube axis direction of the steel pipe. The final wall thickness is determined by the balance. Since the thickness of the steel pipe subjected to diameter reduction rolling is mainly determined by the tension between the rolling stands, the tension between the rolling stands for obtaining the target thickness is obtained from the rolling theory, and the tension works. Thus, it is necessary to set the number of roll rotations of each rolling stand.

  As described above, in the present embodiment, the ERW welded steel pipe (host pipe) is heated to 800 to 1200 ° C. and subjected to hot diameter reduction rolling at a cross-section reduction rate of 40 to 80%. Water is supplied to the outer surface of the steel pipe to cool it to a temperature lower than the α-γ2 phase temperature range, the cooling rate at that time is 5 ° C./second or more, and the ratio of the wall thickness t to the outer diameter D is t / A thick steel pipe having a D of 0.2 or more, preferably 0.23 or more is manufactured. Here, the reduction rate of the cross section is (outer diameter of steel pipe before diameter reduction-outer diameter of steel pipe after diameter reduction) / outer diameter of steel pipe before diameter reduction × 100 (%).

  When the heating temperature of the ERW welded steel pipe at the time of diameter reduction rolling is less than 800 ° C., the deformation resistance is large. For this reason, the heating temperature is preferably in the range of 800 to 1200 ° C.

  Further, if the reduction rate of the cross section during the diameter reduction rolling is less than 40%, the compressive force is insufficient, and the thickness / outer diameter from an electric resistance welded steel pipe (master pipe) having a wall thickness / outer diameter ratio of 0.15 or less. It is difficult to obtain a thick steel pipe having a ratio of 0.2 or more, preferably 0.23 or more. On the other hand, when the cross-sectional reduction rate exceeds 80%, the generation of surface flaws in the steel pipe due to reduced diameter rolling becomes significant, and it becomes difficult to ensure a uniform shape. For this reason, it is preferable that the reduction rate of the cross section in diameter reduction rolling shall be 40 to 80%.

  Whether or not the hollow part steel pipe of the present embodiment is manufactured by diameter reduction rolling is determined by observing the angled state of the inner surface of the cross section (C cross section) perpendicular to the pipe axis direction or by measuring the wall thickness. It can be judged by measurement.

  For example, as described above, a stretch reducer used for diameter reduction rolling is a rolling device provided with a plurality of rolling stands having three or four rolls around a rolling axis in series with the rolling axis, and is usually adjacent to each other. The rolls of the rolling stands (for example, N and N + 1 rolling stands) are out of phase, and are arranged so as to be out of phase by 60 ° for a 3 roll rolling stand and 45 ° for a 4 roll rolling stand. .

  Therefore, the inner surface shape of the cross section (C cross section) perpendicular to the tube axis direction of the hollow part steel pipe manufactured by the reduced diameter rolling is hexagonal, four roll rolling stand when the stretch reducer is equipped with a three roll rolling stand. When it is provided, it becomes an octagon.

  Thus, when the inner surface shape of the cross section perpendicular to the pipe axis direction of the steel pipe for hollow parts forms the polygonal shape as described above, the steel pipe for hollow parts is manufactured by reduction rolling. I know that there is.

As described above, in the present embodiment, it is possible to obtain a steel pipe for a hollow part and a method for manufacturing the same for which fatigue strength is improved by suppressing generation of a ferrite decarburized layer.
Further, in this embodiment, in order to suppress the occurrence of the ferrite decarburized layer, additional processes such as a grinding process, a shot peening process, a carburizing process, etc., as a countermeasure of the prior art when the ferrite decarburized layer is formed. Becomes unnecessary, and an increase in manufacturing cost can be suppressed.

Hereinafter, the present invention will be described more specifically with reference to examples.
(Production conditions of materials in Table 1 and Table 2, measurement of ferrite decarburized layer, fatigue test method)
Various steels having the compositions shown in Table 1 were melted and cast into slabs. This slab was heated to 1150 ° C. and hot-rolled at a rolling finishing temperature of 890 ° C. and a winding temperature of 630 ° C. to obtain a steel plate having a thickness of 6 mm. Test pieces of 20 × 20 × 4 mm were taken from these plates, and the thickness of the decarburized layer on the surface when heated to 800 ° C. and cooled at 5 ° C./s was observed and measured with an optical microscope. The results are shown in Table 1.

  No. 1 shown in Table 1 For steels 3, 4, 7, 11 and 13, this hot-rolled steel sheet was slit to a predetermined width, roll-formed, and then made into an electric resistance welded steel pipe (master pipe) having an outer diameter of 90 mm by high-frequency electric resistance welding. . Subsequently, these steel pipes were heated to 980 ° C. by high-frequency induction heating and subjected to reduction rolling to produce thick steel pipes having an outer diameter of 30 mm and a wall thickness of 4.0 to 6.5 mm. In that case, the cooling rate between 700 degreeC-600 degreeC of an inner surface was changed in the range of 1-8 degreeC / s by water-cooling from the outer surface side immediately after diameter reduction rolling. The thick-walled steel pipe thus obtained was subsequently quenched at 960 ° C. and tempered at 350 ° C. × 1 hr.

  From these steel pipes, Spring Papers, 28 (1983) p. 46, a fatigue test piece bent at a bending radius of 60 mm as shown in FIG. 1 is collected, one side is fixed, and the stress condition is such that the first principal stress amplitude is 600 MPa with a solid material having the same diameter. A double-side fatigue test was carried out to determine the number of repeated fractures. The results are shown in Table 2.

  Further, a sample for optical microscope observation in which the L section was mirror-polished from each tube was collected, etched with nital, and then the structure near the inner surface was observed to measure the thickness of the decarburized layer. The results are shown in Table 2.

(Description of comparative example)
No. 1 having chemical components of the present invention shown in Table 1. It can be seen that the steels 1 to 10 are excellent in decarburization resistance at a cooling rate of 5 ° C./s. No. It can be seen that steels 9 and 10 are particularly excellent in decarburization resistance at a cooling rate of 5 ° C./s because they contain S having an effect of suppressing the ferrite decarburization layer. On the other hand, No. of the comparative example. Steels 11 to 13 are inferior in decarburization resistance because they do not satisfy the formula (1).
923-513C-101Mn-204Cr-1515Nb ≦ 700 (1)

  Comparative Examples H and I shown in Table 2 are steel pipes having chemical components in the range of the present invention. However, since the wall thickness is thinner than the range of the present invention, the outer surface side is easily cooled on the inner surface side. This is an example showing that it is not difficult to suppress decarburization because a cooling rate exceeding ℃ / s can be achieved. Further, the steel pipes of Comparative Examples J and K are steel pipes having chemical components and wall thicknesses within the scope of the present invention. However, since the cooling rate of the inner surface did not reach 5 ° C./s, decarburization was not suppressed and fatigue was reduced. This is an example of inferior characteristics. Moreover, since the steel pipes of Comparative Examples L, M, and N are steel pipes that do not satisfy the formula (1), the decarburized layer is thick and the fatigue strength is low.

  FIG. 2 is a diagram showing the relationship between the thickness of the ferrite decarburized layer on the inner surface of the steel pipe and the fatigue strength. This figure shows the ferrite decarburized layer thicknesses and the number of repetitions of breakage of each of the inventive examples a, b, c, d, g and comparative examples J, K, L, M, N shown in Table 2. . According to FIG. 2, it can be seen that the thickness of the ferrite decarburized layer on the inner surface of the steel pipe is 20 μm or less as the scope of the present invention.

  FIG. 3 is a diagram showing the relationship between the cooling rate (° C./second) in the α-γ two-phase temperature region on the inner surface of the steel pipe and the thickness of the ferrite decarburized layer on the inner surface of the steel pipe. In this figure, the cooling rates and ferrite decarburized layer thicknesses of the inventive examples a, b, c, d, g and comparative examples H, I, J, K shown in Table 2 are shown. Also in FIG. 3, it is understood that it is appropriate that the thickness of the ferrite decarburized layer on the inner surface of the steel pipe is 20 μm or less as a range of the present invention.

  FIG. 4 is a diagram showing the relationship between the Ar3 transformation point and the thickness of the ferrite decarburized layer when the crystal grain size is 30 μm and cooling from the γ region at 5 ° C./second. This figure shows an example No. of the present invention shown in Table 1. 1-10 and Comparative Example No. 11 shows the value of the left side of the above formula (1) and the thickness of the ferrite decarburized layer. According to FIG. 4, it is understood that it is appropriate to satisfy the above formula (1) as the scope of the present invention.

  FIG. 5 is a graph showing the relationship between the fatigue strength and t / D, which is the ratio of the wall thickness t to the outer diameter D. FIG. 2 shows t / D and the number of repetitions of breakage of each of the inventive examples a, b, c, d, g and comparative examples H, I shown in Table 2. According to FIG. 5, it can be seen that it is appropriate to set t / D, which is the ratio of the wall thickness t and the outer diameter D, to 0.2 or more as a range of the present invention.

  FIG. 6A shows an optical microscope observation sample obtained by mirror-polishing a cross section at a 90 ° position from the welded portion of the sample tube of Comparative Example K, etched with nital, and then showing a structure near the inner surface. It is a photograph. FIG. 6 (b) shows a structure near the inner surface after a sample for optical microscope observation in which the cross section at 90 degrees is mirror-polished from the welded portion of the sample tube of Example g of the present invention and etched with nital. It is a microphotograph.

  In the sample shown in FIG. 6 (a), since quenching is not performed, the thickness of the ferrite decarburized layer is formed thick, whereas in the sample shown in FIG. 6 (b), quenching is performed. The thickness of the ferrite decarburized layer can be kept thin.

  Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the steel pipe for hollow parts of the above embodiment can be applied to various parts, and can be applied to, for example, a stabilizer.

It is a figure for demonstrating the method of a fatigue test. It is a figure which shows the relationship between the ferrite decarburization layer thickness of a steel pipe inner surface, and fatigue strength. It is a figure which shows the relationship between the cooling rate (degreeC / sec) in the alpha-gamma 2-phase temperature range in the steel pipe inner surface, and the thickness of the ferrite decarburization layer of the steel pipe inner surface. It is a figure which shows the relationship between the Ar3 transformation point and the thickness of a ferrite decarburization layer at the time of cooling at 5 degree-C / sec from a (gamma) area | region when a crystal grain diameter is 30 micrometers. It is a figure which shows the relationship between t / D which is ratio of thickness t and the outer diameter D, and fatigue strength. (A) is a microphotograph showing the structure near the inner surface of Comparative Example K, and (b) is a microphotograph showing the structure near the inner surface of Invention Example g.

Claims (10)

ERW welded steel pipe is a steel pipe for hollow parts that has been reduced in diameter hot,
The wall thickness is 5 mm or more, and the ratio of wall thickness t to outer diameter D is t / D is 0.2 or more,
A steel pipe for hollow parts, wherein the depth of the decarburized layer on the inner surface of the pipe is 20 μm or less. In Claim 1, The said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
A steel pipe for hollow parts, containing N: 0.001 to 0.006%, the balance being Fe and inevitable impurities, and satisfying the following formula.
923-513C-101Mn ≦ 700 In Claim 1, The said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006%, Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%, the balance being Fe and inevitable impurities A steel pipe for hollow parts, characterized by satisfying the following formula:
923-513C-101Mn-204Cr-1515Nb ≦ 700 In Claim 1, The said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% is contained,
Further, a hollow part containing S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070%, the balance being Fe and inevitable impurities, and satisfying the following formula Steel pipe.
923-513C-101Mn ≦ 700 In Claim 1, The said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% and Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%
Further, a hollow part containing S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070%, the balance being Fe and inevitable impurities, and satisfying the following formula Steel pipe.
923-513C-101Mn-204Cr-1515Nb ≦ 700 A first step of hot rolling the ERW welded steel pipe;
A second step of cooling the steel pipe reduced in diameter in the first step to a temperature lower than the α-γ2 phase temperature range;
A method for producing a steel pipe for hollow parts comprising:
The steel pipe reduced in diameter in the first step has a thickness of 5 mm or more and a ratio t / D which is a ratio of the thickness t to the outer diameter D is 0.2 or more.
The cooling rate when passing through the α-γ2 phase temperature range in the process of cooling the inner surface of the steel pipe by the second step is 5 ° C / second or more,
The depth of the decarburized layer which generate | occur | produced in the inner surface of the said steel pipe cooled by the said 2nd process is 20 micrometers or less, The manufacturing method of the steel pipe for hollow parts characterized by the above-mentioned. In Claim 6, the said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
A method for producing a steel pipe for hollow parts, comprising N: 0.001 to 0.006%, the balance being Fe and inevitable impurities, and satisfying the following formula.
923-513C-101Mn ≦ 700 In Claim 6, the said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006%, Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%, the balance being Fe and inevitable impurities A method of manufacturing a steel pipe for hollow parts, characterized by satisfying the following formula:
923-513C-101Mn-204Cr-1515Nb ≦ 700 In Claim 6, the said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% is contained,
Further, a hollow part containing S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070%, the balance being Fe and inevitable impurities, and satisfying the following formula Steel pipe.
923-513C-101Mn ≦ 700 In Claim 6, the said ERW welded steel pipe is C: 0.15-0.5% in the mass%.
Si: 0.1 to 0.4%
Mn: 0.3 to 2.0%
Ti: 0.005 to 0.05%
Al: 0.005 to 0.05%
B: 0.0005 to 0.0050%
N: 0.001 to 0.006% and Cr: 0.05 to 1.0% and Nb: 0.005 to 0.1%
Further, a hollow part containing S: 0.004 to 0.010% and Ca: 0.0005 to 0.0070%, the balance being Fe and inevitable impurities, and satisfying the following formula Steel pipe.
923-513C-101Mn-204Cr-1515Nb ≦ 700