阅读说明：本技术 中空稳定器用电阻焊钢管 (Electric resistance welded steel pipe for hollow stabilizer ) 是由 荒谷昌利 石川和俊 松井亮二 近藤友则 于 2020-05-12 设计创作，主要内容包括：本发明提供一种中空稳定器用电阻焊钢管,即使在大气中进行热处理的情况下,也可以不仅抑制铁素体脱碳层的生成而且抑制脱碳层的生成,可以得到具有优异的耐疲劳性的中空稳定器。上述中空稳定器用电阻焊钢管具有规定的成分组成,内表面和外表面的总脱碳层深度为100μm以下。(The invention provides an electric resistance welded steel pipe for a hollow stabilizer, which can inhibit not only the generation of a ferrite decarburized layer but also the generation of a decarburized layer even if the electric resistance welded steel pipe is subjected to heat treatment in the air, and can obtain a hollow stabilizer with excellent fatigue resistance. The electric resistance welded steel pipe for a hollow stabilizer has a predetermined composition, and the total depth of decarburized layers on the inner and outer surfaces is 100 μm or less.)

1. An electric resistance welded steel pipe for a hollow stabilizer, comprising the following components:

contains, in mass%, C: 0.20 to 0.40%, Si: 0.1 to 1.0%, Mn: 0.1-2.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01-0.10%, Cr: 0.01 to 0.50%, Ti: 0.010-0.050%, B: 0.0005 to 0.0050%, Ca: 0.0001-0.0050%, N: 0.0050% or less, and Sn: 0.010-0.050%, and the balance of Fe and inevitable impurities;

the total decarburized layer depth of the inner surface and the outer surface is 100 μm or less.

2. An electric resistance welded steel pipe for a hollow stabilizer according to claim 1, wherein said composition further contains, in mass%, Sb: 0.020% or less.

3. An electric resistance welded steel pipe for a hollow stabilizer according to claim 1 or 2, wherein said composition further contains, in mass%, a metal selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.05% or less, W: 0.5% or less, V: 0.5% or less, Mo: 0.2% or less, and REM: 0.02% or less of 1 or 2 or more.

Technical Field

The present invention relates to an electric resistance welded steel pipe (electric-resistance-welded pipe or tube for hollow stabilizer), and more particularly to the following electric resistance welded steel pipe for hollow stabilizer: even when the heat treatment is performed in the air in the hollow stabilizer manufacturing process, not only the generation of the ferrite decarburized layer but also the generation of the total decarburized layer can be suppressed, and a hollow stabilizer having excellent fatigue resistance can be obtained.

Background

In order to suppress the roll of the vehicle body during turning and improve the running stability during high-speed running, a stabilizer is generally mounted on the vehicle. As the stabilizer, a solid stabilizer using a steel bar has been used, but in recent years, a hollow stabilizer using a steel pipe has been generally used for the purpose of reducing the weight.

The hollow stabilizer is generally manufactured by cold-forming a steel pipe as a raw material into a desired shape and then subjecting the steel pipe to a thermal refining process such as quenching and tempering. As the steel pipe, a seamless steel pipe, an electric resistance welded steel pipe (hereinafter referred to as an electric resistance welded steel pipe), or the like is used, and among them, the electric resistance welded steel pipe is widely used because it is relatively inexpensive and has excellent dimensional accuracy.

The electric resistance welded steel pipe used as a raw material for the hollow stabilizer (electric resistance welded steel pipe for hollow stabilizer) is required to have excellent fatigue resistance after forming into a stabilizer and subjecting to heat treatment such as quenching and tempering. Therefore, various studies have been made on the influence of the surface properties after heat treatment on fatigue resistance.

In particular, surface decarburization is considered to be an important factor in surface properties. If surface decarburization occurs at the heating stage of quenching, the surface hardness cannot be increased even if quenching is performed, and as a result, sufficient fatigue resistance cannot be obtained.

As a technique for focusing on the relationship between surface decarburization and fatigue resistance, for example, the following patent documents 1 and 2 can be cited.

Patent document 1 proposes a method for producing an electric resistance welded steel pipe for a hollow stabilizer, in which the thickness of a decarburized layer on the inner surface side of the pipe is suppressed to 120 μm or less.

Patent document 2 proposes an electric resistance welded steel pipe in which at least one of Cu and Sb is added to suppress formation of a ferrite decarburized layer during quenching. Specifically, the thickness of the ferrite decarburized layer formed when the steel sheet is heated at 800 ℃ for 1 hour in the air is suppressed to less than 0.15 mm.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2018/079398

Patent document 2: japanese patent laid-open No. 2007-056283

Disclosure of Invention

As described above, in the electric resistance welded steel pipe for a hollow stabilizer proposed in patent document 1, the thickness of the decarburized layer on the inner surface side of the pipe is suppressed to 120 μm or less. However, the thickness of the decarburized layer focused in patent document 1 is a value of the steel pipe before quenching, and is not a value after quenching. In order to further improve the fatigue resistance of the stabilizer as a final product, it is considered necessary to reduce the thickness of the decarburized layer after quenching, but the thickness of the decarburized layer after quenching is affected by the quenching conditions, and therefore it cannot be said that the electric resistance welded steel pipe for a hollow stabilizer proposed in patent document 1 can sufficiently suppress surface decarburization during quenching.

The conditions that have a particularly large influence on the thickness of the decarburized layer after quenching include the atmosphere during quenching. Generally, heating in quenching is performed in the atmosphere in consideration of productivity and the like. For example, as a heating method having a short heating time and excellent productivity, electric heating can be used. In the electric heating, both ends of the stabilizer are sandwiched by electrodes, and the electrodes are energized to heat the stabilizer in the atmosphere. However, heating in the atmosphere as described above causes surface decarburization.

On the other hand, in order to suppress surface decarburization during quenching, it is conceivable to heat the steel sheet in an atmosphere containing no oxygen by using, for example, a bright heat treatment furnace (non-oxidizing heat treatment furnace). However, this method requires atmosphere control, and therefore, the equipment cost is high and the productivity is poor.

Therefore, in order to further improve fatigue resistance, a technique capable of reducing the thickness of the decarburized layer after quenching even when heating is performed in the atmosphere is required.

On the other hand, in the technique proposed in patent document 2, although attention is paid to the thickness of the decarburized layer after quenching, only the thickness of the ferrite decarburized layer (ferrite decarburized layer depth) is considered. However, the hardness of the surface layer after quenching is affected not only by the depth of the ferrite decarburized layer but also by the thickness of the total decarburized layer (total decarburized layer depth). In particular, when heating is performed in the atmosphere, the total depth of the decarburized layer increases, and as a result, fatigue resistance required for a stabilizer cannot be obtained.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an electric resistance welded steel pipe for a hollow stabilizer, which can suppress not only the generation of a ferrite decarburized layer but also the generation of a total decarburized layer even when heat treatment is performed in the atmosphere in a stabilizer manufacturing process, and can obtain a hollow stabilizer having excellent fatigue resistance.

The inventors have conducted intensive studies to solve the above problems, and as a result, have obtained the following findings (1) to (4).

(1) The surface decarburization reaction when heating the steel material proceeds by the carbon atoms in the steel diffusing outward toward the surface and reacting with oxygen. In order to suppress the outer diffusion of carbon, it is effective to increase the lattice constant of iron.

(2) In order to increase the lattice constant of iron, the most effective elements are Sb and Sn, and Cu has no effect on increasing the lattice constant. Patent document 2 proposes to add Cu for the purpose of suppressing decarburization, but this is considered because patent document 2 focuses only on ferrite decarburization and does not consider suppression of total decarburization.

(3) Patent document 2 also proposes adding Sb to suppress decarburization. As described above, Sb has an effect of increasing the lattice constant of iron, but it liquefies and attacks austenite grain boundaries upon heating, and thus reduces the toughness of the stabilizer after quenching and tempering. Therefore, the Sb addition needs to be controlled to a necessary minimum.

(4) FIG. 1 is a graph showing an example of the relationship between the Sn content and the total decarburized layer depth after quenching. Specifically, hot-rolled steel sheets (sheet thickness: 4mm) having various Sn contents were held at 900 ℃ for 10 minutes in the air, and then cooled at a cooling rate of about 20 ℃/sec. Then, the total decarburized layer depth of the surface was measured. The composition of the hot-rolled steel sheet other than Sn was kept constant as follows.

C: 0.35%, Si: 0.20%, Mn: 1.22%, P: 0.018%, S: 0.0015%, Al: 0.035%, Cr: 0.15%, Ti: 0.035%, B: 0.0020%, Ca: 0.0015%, N: 0.0022%, and the balance Fe and unavoidable impurities.

From the results shown in FIG. 1, it is understood that if the Sn content is 0.010 mass% or more, the depth of the total decarburized layer can be suppressed to 150 μm or less. However, if the Sn content exceeds 0.05 mass%, the effect is saturated.

The present invention is based on the above findings, and the gist thereof is as follows.

1. An electric resistance welded steel pipe for a hollow stabilizer, comprising the following components: contains, in mass%, C: 0.20 to 0.40%, Si: 0.1 to 1.0%, Mn: 0.1-2.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01-0.10%, Cr: 0.01 to 0.50%, Ti: 0.010-0.050%, B: 0.0005 to 0.0050%, Ca: 0.0001-0.0050%, N: 0.0050% or less, and Sn: 0.010-0.050%, and the balance of Fe and inevitable impurities;

the total decarburized layer depth of the inner surface and the outer surface is 100 μm or less.

2. The electric resistance welded steel pipe for a hollow stabilizer according to the above 1, wherein the composition further comprises, in mass%, Sb: 0.020% or less.

3. The electric resistance welded steel pipe for a hollow stabilizer according to claim 1 or 2, wherein the composition further contains, in mass%, a metal selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.05% or less, W: 0.5% or less, V: 0.5% or less, Mo: 0.2% or less, and REM: 0.02% or less of 1 or 2 or more.

According to the present invention, even when heat treatment is performed in the air in the hollow stabilizer manufacturing process, it is possible to suppress not only the generation of the ferrite decarburized layer but also the generation of the total decarburized layer. Therefore, by using the electric resistance welded steel pipe of the present invention as a raw material, a hollow stabilizer having excellent fatigue resistance can be manufactured. Further, according to the present invention, it is possible to suppress surface decarburization not only in heat treatment in a non-oxidizing atmosphere at a high cost, but also in heat treatment in an atmosphere at a low cost and excellent in productivity. Therefore, the electric resistance welded steel pipe for a hollow stabilizer of the present invention can be preferably used as a raw material for producing a hollow stabilizer.

Drawings

FIG. 1 is a graph showing the relationship between the Sn content and the total decarburized layer depth after quenching.

Detailed Description

Hereinafter, embodiments of the present invention will be described.

[ composition of ingredients ]

The electric resistance welded steel pipe for a hollow stabilizer of the present invention (hereinafter, may be simply referred to as electric resistance welded steel pipe) has the above-described composition. The reasons for limiting the contents of the respective components will be described below. It should be noted that "%" means "% by mass" unless otherwise specified.

C：0.20～0.40％

C is an element that promotes the formation of martensite by increasing the hardenability and has the effect of increasing the strength (hardness) of the steel by solid solution. In order to ensure the strength (hardness) required for the hollow stabilizer, the content of the stabilizer needs to be 0.20% or more. Therefore, the C content is 0.20% or more, preferably 0.21% or more. On the other hand, if the C content exceeds 0.40%, the risk of burning cracks increases and the toughness after quenching decreases. Therefore, the C content is 0.40% or less, preferably 0.39% or less, and more preferably 0.38% or less.

Si：0.1～1.0％

Si is an element that functions as a deoxidizer and also functions as a solid-solution strengthening element. In order to obtain the above effect, the content of the compound is required to be 0.1% or more. Therefore, the Si content is 0.1% or more, preferably 0.2% or more. On the other hand, if the content exceeds 1.0%, the electric resistance weldability decreases. Therefore, the Si content is 1.0% or less, preferably 0.8% or less, more preferably 0.5% or less, and further preferably 0.41% or less.

Mn：0.1～2.0％

Mn is an element that is solid-dissolved to contribute to the improvement of the strength of steel and the improvement of the hardenability of steel. In order to ensure the strength (hardness) required for the hollow stabilizer, the content of the stabilizer needs to be 0.1% or more. Therefore, the Mn content is 0.1% or more, preferably 0.5% or more. On the other hand, if it exceeds 2.0%, the toughness is lowered and the risk of burning cracks is increased. Therefore, the Mn content is 2.0% or less, preferably 1.8% or less, and more preferably 1.7% or less.

P: less than 0.1%

P is an element present in steel as an impurity, segregates to grain boundaries and the like, and reduces weld cracking resistance and toughness. Therefore, in order to be used as a hollow stabilizer, the P content needs to be reduced to 0.1% or less. Therefore, the P content is 0.1% or less, preferably 0.05% or less, and more preferably 0.02% or less. On the other hand, from the viewpoint of weld cracking resistance and toughness, the lower the P content, the lower limit of the P content is not limited and may be 0. However, excessively reducing the P content leads to an increase in manufacturing costs. Therefore, from the viewpoint of cost reduction, the P content is preferably 0.001% or more, more preferably 0.005% or more, and still more preferably 0.008% or more.

S: less than 0.01%

S is an element which is present as a sulfide-based inclusion in steel and which reduces hot workability, toughness, and fatigue resistance. In order to be used as a hollow stabilizer, the S content needs to be reduced to 0.01% or less. Therefore, the S content is 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less. On the other hand, from the viewpoint of hot workability, toughness and fatigue resistance, the lower the S content, the lower limit of the S content is not limited and may be 0. However, excessively reducing the S content leads to an increase in manufacturing costs. Therefore, from the viewpoint of cost reduction, the S content is preferably 0.0001% or more, more preferably 0.0005% or more, and still more preferably 0.001% or more.

Al：0.01～0.10％

Al is an element that functions as a deoxidizer and has the effect of bonding with N to secure the amount of solid-solution B effective for improving hardenability. Further, Al precipitates as AlN and serves to prevent austenite grains from becoming coarse during quenching and heating. In order to obtain the above effect, the content of the compound is required to be 0.01% or more. Therefore, the Al content is set to 0.01% or more. On the other hand, if the content exceeds 0.10%, the amount of oxide-based inclusions increases, and the fatigue life decreases. Therefore, the Al content is 0.10% or less, preferably 0.07% or less, and more preferably 0.05% or less.

Cr：0.01～0.50％，

Cr is an element having an effect of improving hardenability. In order to obtain the above effects, the Cr content is set to 0.01% or more, preferably 0.05% or more. On the other hand, if the Cr content exceeds 0.50%, oxides are easily formed, and Cr oxides remain in the electric resistance welded portion, thereby deteriorating the welding quality of electric resistance welding. Therefore, the Cr content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

Ti：0.010～0.050％

Ti is an element having an action of fixing N in steel to TiN. However, if the Ti content is less than 0.010%, the above-mentioned effects cannot be sufficiently exhibited. Therefore, the Ti content is set to 0.010% or more. On the other hand, if the Ti content exceeds 0.050%, workability and toughness of the steel are reduced. Therefore, the Ti content is 0.050% or less, and preferably 0.040% or less.

B：0.0005～0.0050％

B is an element which can improve the hardenability of the steel by adding a trace amount. In addition, B has an effect of strengthening grain boundaries, and suppresses grain boundary embrittlement caused by P segregation. In order to obtain the above effect, it is necessary to contain 0.0005% or more. Therefore, the B content is 0.0005% or more, preferably 0.0010% or more. On the other hand, if the content exceeds 0.0050%, the effect is saturated, and this is economically disadvantageous. Therefore, the B content is set to 0.0050% or less, preferably 0.0030% or less.

Ca：0.0001～0.0050％

Ca is an element having an action of controlling the form of sulfide-based inclusions to fine substantially spherical inclusions. By adding Ca, the number of coarse MnS particles having a particle size of 10 μm or more and coarse TiS particles having a particle size of 10 μm or more, which become starting points of corrosion pits, can be reduced. In order to obtain the above effects, the Ca content is set to 0.0001% or more. On the other hand, if the content exceeds 0.0050%, the coarse CaS-based clusters become too large and rather become the starting point of fatigue crack, and the corrosion resistance and fatigue resistance deteriorate. Therefore, the Ca content is 0.0050% or less, preferably 0.0030% or less, and more preferably 0.0015% or less.

N: 0.0050% or less

N is an element inevitably contained as an impurity, and bonds with a nitride-forming element in the steel to suppress coarsening of crystal grains, thereby contributing to an increase in strength after tempering. However, if the content exceeds 0.0050%, toughness of the weld portion may be reduced. Therefore, the N content is set to 0.0050% or less, preferably 0.0040% or less. On the other hand, the lower limit of the N content is not limited and may be 0, but the above-described effect may be obtained by adding N in a certain amount. In addition, excessively reducing the N content leads to an increase in manufacturing cost. From these viewpoints, the N content is preferably 0.001% or more, and more preferably 0.0015% or more.

Sn：0.010～0.050％

Sn is one of the most important elements in the present invention. By adding Sn, the lattice constant of iron increases, whereby the outward diffusion of carbon in steel is suppressed, and thus the surface decarburization reaction is suppressed. In order to obtain the above effect, it is necessary to add 0.010% or more. Therefore, the Sn content is 0.010% or more, preferably 0.020% or more. On the other hand, even if the amount exceeds 0.050%, the effect is saturated. Accordingly, the Sn content is 0.050% or less, and preferably 0.045% or less.

An electric resistance welded steel pipe according to an embodiment of the present invention has the following composition: contains the above elements, and the balance is made up of Fe and unavoidable impurities.

In another embodiment of the present invention, the above-mentioned composition may optionally contain Sb in the following amount.

Sb: 0.020% or less

Sb is an element having an action of increasing the lattice constant of iron and suppressing the outer diffusion of carbon in steel, similarly to Sn. Therefore, by adding Sb in addition to Sn, surface decarburization can be further suppressed. However, Sb liquefies and attacks austenite grain boundaries during heating, and therefore reduces the toughness of the stabilizer after quenching and tempering. Therefore, the addition of Sb needs to be controlled to the minimum necessary. Therefore, when Sb is added, the Sb content is 0.020% or less, preferably less than 0.010%, and more preferably 0.008% or less.

In still another embodiment of the present invention, the above-mentioned composition may further contain 1 or 2 or more kinds selected from Cu, Ni, Nb, W, V, Mo and REM in the following amounts.

Cu: 1.0% or less

Cu is an element having an effect of improving hardenability and corrosion resistance. However, Cu is an expensive alloying element, and therefore if the Cu content exceeds 1.0%, a rise in material cost is caused. Therefore, the Cu content is 1.0% or less, preferably 0.50% or less. The lower limit of the Cu content is not particularly limited, but in the case of adding Cu, the Cu content is preferably 0.05% or more from the viewpoint of enhancing the effect of adding Cu.

Ni: 1.0% or less

Like Cu, Ni is an element having an action of improving hardenability and corrosion resistance. However, Ni is an expensive alloying element, and therefore if the Ni content exceeds 1.0%, a rise in material cost results. Therefore, the Ni content is 1.0% or less, preferably 0.50% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but in the case where Ni is added, the Ni content is preferably 0.05% or more from the viewpoint of enhancing the effect of adding Ni.

Nb: less than 0.05%

Nb is an element that forms fine carbides to contribute to increase strength (hardness). However, if the Nb content exceeds 0.05%, the addition effect is saturated and the effect commensurate with the content cannot be obtained, so that it is economically disadvantageous. Therefore, the Nb content is 0.05% or less, preferably 0.03% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but in the case of adding Nb, it is preferable to set the Nb content to 0.001% or more from the viewpoint of enhancing the effect of adding Nb.

W: less than 0.5%

W is an element that forms fine carbides to contribute to increase strength (hardness) as in Nb. However, if the W content exceeds 0.5%, the addition effect is saturated and the effect commensurate with the content cannot be obtained, so that it is economically disadvantageous. Therefore, the W content is 0.5% or less, preferably 0.3% or less. On the other hand, the lower limit of the W content is not particularly limited, but in the case of adding W, it is preferable to set the W content to 0.01% or more from the viewpoint of enhancing the effect of adding W.

V: less than 0.5%

Like Nb and W, V is an element that forms fine carbides to contribute to increase strength (hardness). However, if the V content exceeds 0.5%, the addition effect is saturated and the effect commensurate with the content cannot be obtained, so that it is economically disadvantageous. Therefore, the V content is 0.5% or less, preferably 0.3% or less. On the other hand, the lower limit of the V content is not particularly limited, but in the case of adding V, the V content is preferably 0.01% or more from the viewpoint of enhancing the effect of adding V.

Mo: less than 0.2%

Mo is an element having an effect of improving hardenability. However, Mo is a very expensive element, and thus excessive addition leads to an increase in material cost. Therefore, the Mo content is 0.2% or less, preferably 0.15% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of enhancing the effect of adding Mo, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

REM: less than 0.02%

Like Ca, REM (rare earth metal) is an element having an action of controlling the form of sulfide-based inclusions to fine, substantially spherical inclusions. REM may be optionally added to supplement the effect of Ca. However, if the REM content exceeds 0.02%, the amount of inclusions that become the starting points of fatigue cracks becomes excessive, and therefore the corrosion and fatigue resistance is rather reduced. Therefore, the REM content is 0.02% or less, preferably 0.01% or less. On the other hand, the lower limit of the REM content is not particularly limited, but in the case of adding REM, it is preferable to set the REM content to 0.001% or more from the viewpoint of enhancing the effect of adding REM.

[ Total decarburized layer depth ]

Total decarburized layer depth: less than 100 μm

The total depth of decarburized layer on the inner surface and the total depth of decarburized layer on the outer surface of the electric resistance welded steel pipe for a hollow stabilizer of the present invention are both 100 μm or less. The total decarburized layer depth referred to herein is the total decarburized layer depth of the electric resistance welded steel pipe (blank pipe) for a hollow stabilizer as a raw material before the stabilizer is supplied to the manufacturing process. In other words, the total decarburization depth is the total decarburization depth before the heat treatment such as quenching is performed. The total decarburized layer depth can be measured by the method described in examples.

When the total decarburized layer depth exceeds 100 μm, the total decarburized layer depth is further increased in the heat treatment in the subsequent stabilizer manufacturing process, and as a result, the fatigue strength required for the stabilizer cannot be secured. This increase in the total decarburized layer depth is particularly significant in the case of heat treatment in the atmosphere. Therefore, in order to obtain a hollow stabilizer having excellent fatigue resistance even when heat treatment is performed in the atmosphere in the stabilizer production process, it is necessary to set the total decarburized layer depth of each of the inner surface and the outer surface of the electric resistance welded steel pipe for a hollow stabilizer to 100 μm or less. The total decarburized layer depth is preferably 50 μm or less, more preferably 20 μm or less.

On the other hand, the lower limit is not particularly limited, and may be, for example, 0 μm, since the smaller the total decarburized layer depth is, the better. However, since high management of production conditions is required to completely prevent the total decarburization, the total depth of the decarburized layer on the inner surface and the outer surface is preferably 1 μm or more, more preferably 5 μm or more, from the viewpoint of easiness of production.

Since the total decarburized layer depth is always larger than the ferrite decarburized layer depth, if the total decarburized layer depth is 100 μm or less, the ferrite decarburized layer depth is inevitably 100 μm or less. Therefore, the depth of the ferrite decarburized layer on the inner surface and the outer surface of the electric resistance welded steel pipe for a hollow stabilizer of the present invention is 100 μm or less.

[t/D]

The size of the electric resistance welded steel pipe for a hollow stabilizer is not particularly limited, and may be any size, but it is preferable that the ratio t/D of the wall thickness t (mm) to the outer diameter D (mm) of the steel pipe is 10 to 30%.

[ production method ]

The electric resistance welded steel pipe for a hollow stabilizer of the present invention is not particularly limited, and can be produced by any method. That is, a steel billet having the above-described composition can be used and manufactured according to a conventional method. Hereinafter, a preferred method for producing an electric resistance welded steel pipe for a hollow stabilizer in one embodiment of the present invention will be described.

The electric resistance welded steel pipe for a hollow stabilizer may be produced as follows: electric resistance welded steel pipes are produced by electric resistance welding steel sheets, and the electric resistance welded steel pipes are reheated and then hot reduced in diameter. As the steel sheet, any steel sheet may be used as long as it has the above-described composition. The steel sheet is preferably a hot-rolled steel sheet.

The resistance welding pipe is not particularly limited, and may be produced by any method. For example, an electric resistance welded steel pipe can be produced as follows: the steel sheet is continuously cold-formed by a plurality of rolls to form a substantially cylindrical open pipe, and then the ends in the width direction of the open pipe are butted against each other by squeeze rolls to perform electric resistance welding. The electric resistance welding may be performed by, for example, high-frequency resistance welding, induction heating, or the like.

It should be noted that the surface decarburization proceeds particularly remarkably at a high temperature exceeding 1000 ℃. In the production process of the electric resistance welded steel pipe, the heating to such a high temperature is generally performed only in the reheating step after the electric resistance welding pipe production and before the hot reducing rolling. Therefore, the conditions such as the heating temperature and the heating time in the reheating step can be adjusted so that the total decarburized layer depth of the finally obtained electric resistance welded steel pipe for a stabilizer satisfies the above conditions.

In particular, the heating temperature (reheating temperature) at the reheating is preferably 850 to 1000 ℃. If the reheating temperature is less than 850 ℃, desired weld toughness may not be ensured in some cases. On the other hand, when the reheating temperature exceeds 1000 ℃, surface decarburization is remarkable.

The rolling temperature in the hot reduction rolling is preferably 650 ℃ or higher. If the rolling temperature is less than 650 ℃, the workability is lowered and it is sometimes difficult to form into a desired stabilizer shape. The cumulative reduction ratio in the hot reduction rolling is preferably 30 to 90%. If the cumulative reduction ratio is 30 to 90%, an electric resistance welded steel pipe for a hollow stabilizer excellent in workability can be obtained.

Examples

Next, the present invention will be further specifically explained based on examples.

A hot-rolled steel sheet (thickness: 4.5mm) having a composition shown in Table 1 was continuously cold-formed by a plurality of rolls to form a substantially cylindrical open pipe. Then, the circumferential ends of the open pipe were butted and pressure-bonded to each other, and resistance welding was performed by a high-frequency resistance welding method to produce a resistance welded steel pipe (outer diameter 89.1 mm. phi. times.wall thickness 4.5 mm). Then, the obtained electric resistance welded steel pipe was further heated to 980 ℃ by induction heating, and then subjected to reducing rolling to obtain an electric resistance welded steel pipe for a hollow stabilizer. The conditions of the reducing rolling are as follows: 800 ℃ and cumulative reduction ratio: 71 percent. Here, the rolling temperature is a temperature measured on the exit side of the final rolling stand using a radiation thermometer. The obtained electric resistance welded steel pipe for a hollow stabilizer had dimensions of 25.4mm in diameter and 4.0mm in wall thickness.

(depth of decarburized layer before heat treatment)

Test pieces for texture observation were collected from the obtained electric resistance welded steel pipe for a hollow stabilizer so that the observation plane became a cross section parallel to the pipe axial direction, and the ferrite decarburized layer depth and the total decarburized layer depth of the inner surface and the outer surface were measured according to the method specified in JIS G0558.

(decarburized layer depth after heat treatment)

Next, in order to evaluate the depth of the decarburized layer after the heat treatment, the obtained electric resistance welded steel pipe for a hollow stabilizer was subjected to heat treatment. Specifically, first, the electric resistance welded steel pipe for a hollow stabilizer was heated in an atmospheric furnace, held at 900 ℃ for 10 minutes, and then cooled at a cooling rate of 80 ± 10 ℃/sec, thereby being quenched. Then, tempering treatment was performed in an atmosphere in a furnace at a tempering temperature of 350 ℃ for a holding time of 20 minutes. Then, test pieces for texture observation were taken from the electric resistance welded steel pipe for a hollow stabilizer after the heat treatment so that a cross section perpendicular to the pipe axial direction became an observation plane, and the ferrite decarburization depth and the total decarburization depth were measured according to the method of JIS G0558. The temperature of the steel pipe during the heat treatment was measured using a K thermocouple attached to the steel pipe.

(fatigue resistance)

Next, in order to confirm the effect of the present invention, the decrease in fatigue strength when heat-treated in the air was evaluated in the following procedure.

Step 1

First, the fatigue strength was evaluated in the case of heat treatment in the air according to the following procedure. A tubular test piece having a length of 400mm was sampled from the obtained electric resistance welded steel pipe for a hollow stabilizer, and the tubular test piece was subjected to quenching and tempering. The quenching is performed as follows: the tubular test piece was held at 900 ℃ for 10 minutes in an atmospheric furnace, and then put into a quenching tank (water) and rapidly cooled at a cooling rate of 80. + -. 10 ℃ per second. The tempering is carried out at a tempering temperature of 350 ℃ for a holding time of 20 minutes. The tempering temperature was measured by a thermocouple attached to the test piece.

The tubular test piece after the quenching and tempering was subjected to a torsional fatigue test in the air, and the number of repetitions until the occurrence of cracks (fatigue life) was determined. The conditions of the torsional fatigue test are: 400MPa (symmetrical alternation), duty cycle: 1 Hz.

The average fatigue life was determined by performing the above test on 10 samples.

Step 2

Next, a reference sample was produced without surface decarburization by performing heat treatment under the same conditions as in the above-described step 1, except that the electric resistance welded steel pipe for a hollow stabilizer produced under the same conditions was heated during quenching in a non-oxidizing atmosphere furnace (bright heat treatment furnace). Using the above reference samples, a torsional fatigue test was performed under the same conditions as in the above step 1, and the average fatigue life of 10 samples was determined.

Step 3

The reduction rate of the average fatigue life obtained in the above step 1 with respect to the average fatigue life of the reference sample obtained in the above step 2 was calculated as the fatigue strength reduction rate. The case where the fatigue strength reduction rate is less than 10% was judged as a good result.

The obtained results are shown in table 2. The electric resistance welded steel pipe for a hollow stabilizer satisfying the conditions of the present invention has both the depth of ferrite decarburized layer and the total decarburized layer of 70 μm or less on the inner surface and the outer surface even after heat treatment at 900 ℃ for 10 minutes in the atmosphere.

Further, the electric resistance welded steel pipe for a hollow stabilizer satisfying the conditions of the present invention has a fatigue strength reduction rate of less than 10% when heat-treated in the atmosphere.