JP2016125118A - Hollow seamless steel pipe for spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hollow seamless steel pipe for high strength spring which allows a spring molded therefrom to secure sufficient fatigue strength.SOLUTION: The hollow seamless steel pipe for spring contains, by mass%, C:0.2 to 0.7%, Si:0.5 to 3%, Mn:0.1 to 2%, Cr:over 0% and 3% or less, Al:over 0% and 0.1% or less, P:over 0% and 0.02% or less, S:over 0% and 0.02% or less, N:over 0% and 0.02% or less and the balance iron with inevitable impurities and has a thickness deviation rate calculated by the following formula (1) of 7.0% or less. Thickness deviation rate=(maximum thickness-minimum thickness)/(average thickness)/2×100 (1).SELECTED DRAWING: Figure 5

Description

  TECHNICAL FIELD The present invention relates to a hollow seamless steel pipe for a spring, and more particularly to a hollow seamless steel pipe for a high-strength spring suitable for manufacturing a hollow steel suspension spring used for automobiles and the like.

  In recent years, with increasing demands for lighter and higher output vehicles for the purpose of reducing exhaust gas and improving fuel efficiency, high-stress designs have been applied to suspension springs, valve springs, clutch springs, etc. used in suspensions, engines, clutches, etc. Is oriented. Therefore, these springs are in the direction of increasing the strength and reducing the diameter, and the load stress tends to further increase. In order to respond to these trends, spring steel with higher performance in terms of fatigue resistance and sag resistance is strongly desired.

  In addition, in order to achieve weight reduction while maintaining fatigue resistance and sag resistance, it was made hollow instead of using the rod-shaped wire that has been used as a spring material, that is, a solid wire. A pipe-shaped steel material and a steel pipe without a weld seam (hereinafter referred to as a hollow seamless steel pipe) is used as a spring material. Various techniques for producing such a hollow seamless steel pipe have been proposed so far.

  For example, in Patent Document 1, a material made of a spring steel material is subjected to Mannesmann piercing, which is representative of a piercing rolling machine, and then subjected to stretching by a mandrel mill, and further reheated at 820 to 940 ° C. for 10 to 30 minutes, The technique of finish rolling after that has been proposed. In Patent Document 2, a cylindrical billet is hot isostatically extruded to produce a seamless steel pipe intermediate. After heating the seamless steel pipe intermediate, the heated seamless steel pipe intermediate is subjected to pilger mill rolling and drawing. A technique is disclosed in which at least one of the processes is performed, for example, the drawing is performed by drawing and the like, and the stretched seamless steel pipe intermediate is heated. In Patent Document 3, as in Patent Document 2, after heating the extrusion hollow billet, hot extrusion is performed, and cold working or the like is performed to manufacture a seamless steel pipe. Furthermore, in Patent Document 4, after manufacturing a bar by hot rolling, drilling is performed with a gun drill, and cold rolling or drawing (cold working) is performed to manufacture a seamless pipe. A technique for avoiding heating and reducing decarburization is disclosed.

  These prior arts attempt to improve fatigue characteristics by decarburization and reduction of soot, but at present, higher fatigue strength is required than the conventional required level. Therefore, the techniques proposed so far cannot satisfy the required fatigue strength, and are insufficient in terms of durability. In particular, in a higher stress region, the proposed techniques have limitations in improving durability, and other factors need to be examined.

JP-A-1-247532 Japanese Patent No. 4705456 JP 2012-111979 A Japanese Patent No. 5324311

  The present invention has been made under such circumstances, and an object of the present invention is to provide a hollow seamless steel pipe for a high-strength spring that can ensure sufficient fatigue strength of the spring to be molded.

The present invention that has achieved the above object is characterized in that the variation in the thickness of the steel pipe is reduced. That is, the hollow seamless steel pipe for springs of the present invention is
% By mass
C: 0.2 to 0.7%
Si: 0.5-3%,
Mn: 0.1 to 2%,
Cr: more than 0%, 3% or less,
Al: more than 0%, 0.1% or less,
P: more than 0%, 0.02% or less,
A hollow seamless steel pipe containing S: more than 0%, 0.02% or less and N: more than 0%, 0.02% or less, the balance being iron and inevitable impurities,
The gist is that the thickness deviation ratio calculated by the following equation (1) is 7.0% or less.
Uneven thickness ratio = (maximum thickness−minimum thickness) / (average thickness) / 2 × 100 (1)

It is preferable that the hollow seamless steel pipe for springs of the present invention further contains at least one of the following (a) to (f) in mass% as necessary.
(A) B: more than 0%, 0.015% or less (b) V: more than 0%, 1% or less, Ti: more than 0%, 0.3% or less, and Nb: more than 0%, 0.3% or less One or more selected from the group consisting of (c) Ni: more than 0%, 3% or less and Cu: more than 0%, 3% or less selected from the group consisting of (d) Mo: more than 0% 2% or less (e) 1 selected from the group consisting of Ca: more than 0%, 0.005% or less, Mg: more than 0%, 0.005% or less, and REM: more than 0%, 0.02% or less One or more selected from the group consisting of (f) Zr: more than 0%, 0.1% or less, Ta: more than 0%, 0.1% or less, and Hf: more than 0%, 0.1% or less.

  According to the present invention, since the uneven thickness ratio, which is an index of the variation in the thickness of the steel pipe, is highly reduced to 7.0% or less, seamless for high-strength hollow springs having high fatigue strength and excellent durability. Can provide steel pipe. The effect of the present invention can be remarkably exhibited particularly in a high stress region.

It is a graph which shows the relationship between ratio t / D with respect to the outer diameter D of the thickness t of a steel pipe, and the fluctuation | variation ratio of the internal stress by uneven thickness. It is a graph which shows the relationship between ratio t / D with respect to the outer diameter D of the thickness t of a steel pipe, and a weight reduction rate. It is the graph which plotted the thickness deviation rate at the time of thickness tolerance 0.1mm for every thickness. In the Example mentioned later, it is a figure showing the shape of the test piece used for the torsional fatigue test. In the Example mentioned later, it is the graph which showed the relationship between the thickness deviation rate and the durable frequency of a torsional fatigue test.

  In high-strength hollow springs, improvement of the fatigue strength of the inner surface that cannot be shot peened is an issue, and so far, suppression of decarburization of the inner surface and reduction of wrinkles have been studied. On the other hand, the present inventors diligently studied the influence of the wall thickness of the steel pipe as another influencing factor. As a result, it was found that the thickness deviation rate of the hollow steel pipe affects the fatigue strength.

  In the conventional techniques such as Patent Documents 1 to 4 described above, improvement of soot and decarburization is an important issue, and no consideration has been given to the uneven thickness rate. However, as a result of examination by the present inventors paying attention to the uneven thickness ratio, the influence of the uneven thickness ratio on the fatigue characteristics is great. Particularly, the fatigue strength of the seamless steel pipe is reduced by setting the uneven thickness ratio to 7.0% or less. It became clear that can be improved. The thickness deviation rate is preferably 5.0% or less, and more preferably 3.0% or less. The smaller thickness deviation ratio is better, but the lower limit is usually about 0.5%.

In the present invention, the thickness deviation rate is given by the following equation (1).
Uneven thickness ratio = (maximum thickness−minimum thickness) / (average thickness) / 2 × 100 (1)

  In fact, the above-mentioned Patent Documents 1 to 4 cannot be said to have a good thickness deviation rate. For example, in Patent Document 1, Mannesmann drilling is used to manufacture a hollow steel pipe, and Mannesmann drilling is highly productive, but compared to other hollowing methods, the material and tool used during drilling, that is, drilling, can be compared. Since the restraint is weak, deviation is likely to occur, and it is difficult to obtain a good thickness deviation rate. In particular, steel materials for high-strength springs have high deformation resistance and are difficult to process with high accuracy. In Patent Documents 2 and 3, a machined hollow billet is hot isostatically extruded. Since machining is performed, billet machining accuracy is high, and the billet is processed evenly by hydrostatic pressure. However, as shown in Examples described later, the methods disclosed in Patent Documents 2 and 3 cannot obtain a sufficient thickness deviation ratio from the viewpoint of durability. Moreover, in patent document 4, the gun drill process is employ | adopted as a hollowing method. This method should also have relatively good processing accuracy, but as shown in the examples described later, a sufficient thickness deviation rate cannot be obtained.

  The hollow seamless steel pipe targeted by the present invention has an outer diameter D of about 8 to 22 mm, a wall thickness t of about 0.8 to 7.7 mm, and a ratio t / D of the wall thickness t to the outer diameter D of about 0.10 to 0.35.

FIG. 1 is a graph in which the relationship between the ratio t / D of the wall thickness t to the outer diameter D and the fluctuation ratio of the internal stress due to uneven thickness is plotted for each uneven thickness ratio of 3%, 7%, and 10%. Wherein the variation ratio of inner surface stresses, ₁ an internal surface stress in the absence of uneven thickness sigma, when the inner surface stress when there is uneven thickness and sigma _2, is a value given by σ ₂ / σ _1. As can be seen from FIG. 1, the inner surface stress fluctuation ratio when the uneven thickness is generated increases as t / D increases. Also, the variation ratio of the internal stress is small even if the thickness deviation rate changes when t / D is low, but the effect of the thickness deviation rate on the internal stress variation ratio when t / D is high. Becomes prominent. As in the prior art, when the uneven thickness ratio exceeds 7.0%, especially when t / D is 0.15 or more, the uneven thickness ratio has a great influence on the fluctuation ratio of the internal stress, that is, The present invention is particularly effective when t / D is 0.15 or more.

  FIG. 2 is a graph showing the relationship between t / D and the weight reduction rate. As can be seen from FIG. 2, the weight reduction rate decreases as t / D increases, and a high-strength hollow spring may require a weight reduction of 25% or more. Therefore, t / D is preferably 0.25 or less.

  FIG. 3 is a graph in which the wall thickness tolerance, that is, the thickness deviation rate when the difference between the maximum wall thickness and the minimum wall thickness is 0.1 mm is plotted for each wall thickness. As can be seen from FIG. 3, for example, when the thickness is 0.5 mm, even if the tolerance is only 0.1 mm, it is equivalent to 10% in terms of the thickness deviation rate. Since the uneven thickness ratio is over 7.0%, it is very difficult to improve the uneven thickness ratio with a small thickness.

The inventors of the present invention manufactured a hollow shell by the following method (1) or (2), particularly as a manufacturing method for setting the uneven thickness ratio of the hollow seamless steel pipe to 7.0% or less. A method for obtaining a hollow seamless steel pipe by further cold rolling, drawing, annealing, etc. on the base pipe was examined.
(1) A method of obtaining a hollow billet from a raw billet by machining and hot-extrusion using the hollow billet.

  In the hot extrusion method of (1) above, the thickness deviation rate is changed by changing the dimensions of the hollow billet, and the hollow steel billet inner diameter is 38 mm, so that the thickness deviation rate of the seamless steel pipe finally obtained is An element tube of 7.0% or less was realized. On the other hand, in Patent Documents 2 and 3 described above, the inner diameter of the hollow billet is 40 mm or 52 mm, and an uneven thickness ratio of 7.0% or less cannot be achieved. In addition, in the method using the gun drill of (2) above, the uneven thickness ratio varies depending on the dimensions of the bar steel and the gun drilling dimensions, and the seamless steel pipe finally obtained by subjecting the 40 mm diameter steel bar to a 20 mm diameter gun drilling process An element tube with an uneven thickness ratio of 7.0% or less was realized. On the other hand, in Patent Document 4 described above, a steel drill having a diameter of 12 mm is applied to a steel bar having a diameter of 25 mm, and an uneven thickness ratio of 7.0% or less cannot be realized.

  In the method (1), the heating temperature before hot extrusion may be set to 1000 to 1100 ° C., for example. In the method (2), the heating temperature at the time of hot rolling may be, for example, about 950 to 1100 ° C., and the minimum rolling temperature may be 800 to 900 ° C., and the average from 650 to 750 ° C. after the hot rolling. The cooling rate may be about 1.5 to 5 ° C./second, and then the average cooling rate up to 500 ° C. or less may be 0.3 to 1.0 ° C./second. In any of the above methods (1) and (2), the obtained tube is annealed at, for example, 900 to 1000 ° C. for 5 to 30 minutes, subjected to cold rolling and drawing, and further about 600 to 1000 ° C. Can be annealed.

  In the present invention, an uneven thickness ratio of 7.0% or less can be realized by the above-described method, but the method for manufacturing the hollow seamless steel pipe of the present invention is not limited to the above-described method.

  Next, chemical components of the hollow seamless steel pipe for high-strength springs of the present invention will be described. In addition, all the chemical component composition shown in this specification means the mass%.

C: 0.2-0.7%
C is an element necessary for ensuring strength, and the C content needs to be 0.2% or more. The amount of C is preferably 0.30% or more, and more preferably 0.35% or more. However, when the amount of C becomes excessive, it becomes difficult to ensure ductility. Therefore, the C content is set to 0.7% or less. C content is preferably 0.65% or less, more preferably 0.60% or less.

Si: 0.5 to 3%
Si is an element effective for improving the sag resistance necessary for the spring, and in order to obtain the sag resistance necessary for the spring of the strength level targeted in the present invention, the Si amount is 0.5% or more. It is necessary to. The amount of Si is preferably 1.0% or more, and more preferably 1.5% or more. However, since Si is also an element that promotes decarburization, if Si is excessively contained, formation of a decarburized layer on the steel surface is promoted. As a result, a peeling process for removing the decarburized layer is required, which is inconvenient in terms of manufacturing cost. For these reasons, the amount of Si was determined to be 3% or less. The amount of Si is preferably 2.5% or less, and more preferably 2.2% or less.

Mn: 0.1 to 2%
Mn is a useful element that can be used as a deoxidizing element and can be detoxified by forming S and MnS, which are harmful elements in steel. In order to exhibit such an effect effectively, the amount of Mn needs to be 0.1% or more. The amount of Mn is preferably 0.15% or more, more preferably 0.20% or more. However, when the amount of Mn becomes excessive, a segregation zone is formed and the material varies. Therefore, the amount of Mn is set to 2% or less. The amount of Mn is preferably 1.5% or less, more preferably 1.0% or less.

Cr: more than 0%, 3% or less Cr is an element effective for securing the strength after tempering and improving the corrosion resistance, and is particularly important for a suspension spring that requires a high level of corrosion resistance. Such an effect increases as the amount of Cr increases. In order to effectively exhibit such an effect, Cr is preferably contained in an amount of 0.2% or more, and more preferably 0.5% or more. However, when the amount of Cr is excessive, an overcooled structure is likely to be generated, and it is concentrated to cementite to lower the plastic deformability, resulting in deterioration of cold workability. Moreover, when the amount of Cr becomes excessive, Cr carbide different from cementite is likely to be formed, and the balance between strength and ductility is deteriorated. For these reasons, the Cr content was determined to be 3% or less. The amount of Cr is preferably 2.0% or less, and more preferably 1.7% or less.

Al: more than 0%, 0.1% or less Al is mainly added as a deoxidizing element. In addition, N and AlN are formed to render solute N harmless and contribute to the refinement of the structure. In particular, in order to fix the solid solution N as AlN, it is preferable to contain Al so as to exceed twice the N content. The amount of Al is preferably 0.001% or more, more preferably 0.01% or more, and still more preferably 0.025% or more. However, since Al is an element that promotes decarburization in the same way as Si, it is necessary to suppress the amount of Al added in a steel containing a large amount of Si. Therefore, the Al content is determined to be 0.1% or less. The amount of Al is preferably 0.07% or less, more preferably 0.05% or less.

P: more than 0% and 0.02% or less P is a harmful element that deteriorates the toughness and ductility of steel materials, so it is important to reduce it as much as possible. Therefore, the P amount is set to 0.02% or less. The amount of P is preferably 0.010% or less, more preferably 0.008% or less. In addition, since P is an impurity inevitably contained in the steel material, it is difficult for industrial production to make its amount 0%, and usually about 0.001% is contained.

S: More than 0% and 0.02% or less Since S is a harmful element that deteriorates the toughness and ductility of steel materials like P, it is important to reduce it as much as possible. Therefore, the S amount is set to 0.02% or less. The amount of S is preferably 0.010% or less, and more preferably 0.008% or less. In addition, since S is an impurity inevitably contained in the steel material, it is difficult for industrial production to make the amount 0%, and it is usually contained about 0.001%.

N: more than 0%, 0.02% or less N, when Al, Ti, etc. are present, has the effect of forming a nitride to refine the structure, but if present in a solid solution state, N has the effect of toughness and resistance to steel. Degradation of hydrogen embrittlement characteristics. Therefore, the N amount is set to 0.02% or less. The amount of N is preferably 0.010% or less, and more preferably 0.005% or less. In addition, since N is an element inevitably contained in the steel material, it is difficult for industrial production to make the amount 0%, and it is usually contained about 0.001%.

  The basic components of the seamless steel pipe of the present invention are as described above, and the balance is substantially iron. However, it is naturally allowed that inevitable impurities brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc. are contained in the steel. Furthermore, in this invention, you may contain the following arbitrary elements as needed.

B: More than 0% and 0.015% or less B has an effect of suppressing breakage from the prior austenite grain boundaries after quenching and tempering of the steel material. In order to exhibit such an effect, the amount of B is preferably 0.001% or more, and more preferably 0.0015% or more. However, when the amount of B is excessive, coarse carbon borides are formed, the characteristics of the steel material are damaged, and the generation of wrinkles of the rolled material is also caused. For these reasons, the B content is preferably 0.015% or less. The amount of B is more preferably 0.010% or less, still more preferably 0.005% or less.

One or more selected from the group consisting of V: more than 0%, 1% or less, Ti: more than 0%, 0.3% or less and Nb: more than 0%, 0.3% or less V, Ti and Nb are C, N, S and carbides, nitrides and carbonitrides (hereinafter referred to as carbon / nitrides), or sulfides are formed to have the effect of detoxifying these C, N, and S. In addition, the carbon / nitride exhibits the effect of refining the structure. Furthermore, V, Ti and Nb also have the effect of improving the delayed fracture resistance. The V amount is preferably 0.05% or more, more preferably 0.1% or more, and further preferably 0.13% or more. Both Ti amount and Nb amount are preferably 0.03% or more, more preferably 0.04% or more, and still more preferably 0.05% or more.

  However, when the amounts of V, Ti, and Nb are excessive, coarse charcoal / nitride is formed, and the toughness and ductility may be deteriorated. Therefore, it is preferable that the V amount is 1% or less, the Ti amount is 0.3% or less, and the Nb amount is 0.3% or less. More preferably, the V amount is 0.5% or less, the Ti amount is 0.1% or less, and the Nb amount is 0.1% or less. Furthermore, from the viewpoint of cost reduction, the V content is preferably 0.3% or less, the Ti content is 0.05% or less, and the Nb content is preferably 0.05% or less.

One or more selected from the group consisting of Ni: more than 0%, 3% or less and Cu: more than 0%, 3% or less Ni has a lower limit in particular to refrain from addition when considering cost reduction However, when suppressing surface layer decarburization or improving corrosion resistance, it is preferable to contain 0.1% or more. However, when the amount of Ni becomes excessive, the properties of the steel material may deteriorate due to the occurrence of a supercooled structure or the presence of retained austenite after quenching in the rolled material. For these reasons, when Ni is contained, the upper limit is preferably made 3% or less. From the viewpoint of cost reduction, the Ni content is preferably 2.0% or less, more preferably 1.0% or less.

  Cu, like Ni, is an element effective for suppressing surface decarburization and improving corrosion resistance. In order to exhibit such an effect effectively, it is preferable to contain 0.1% or more of Cu, more preferably 0.15% or more, and still more preferably 0.20% or more. However, when the amount of Cu is excessive, a subcooled structure or cracking during hot working may occur. For these reasons, when Cu is contained, the Cu content is preferably 3% or less. From the viewpoint of cost reduction, the Cu content is preferably 2.0% or less, and more preferably 1.0% or less.

Mo: more than 0%, 2% or less Mo is an element effective for securing strength after tempering and improving toughness. In order to exhibit such an effect, the Mo amount is preferably 0.1% or more, more preferably 0.2% or more, and further preferably 0.3% or more. However, when the amount of Mo becomes excessive, toughness deteriorates. For these reasons, the Mo content is preferably 2% or less. The amount of Mo is more preferably 1% or less, still more preferably 0.5% or less.

One or more selected from the group consisting of Ca: more than 0%, 0.005% or less, Mg: more than 0%, 0.005% or less, and REM: more than 0%, 0.02% or less Ca, Mg and REM (Rare Earth Metal, rare earth elements) all have an effect of improving toughness by forming sulfides and preventing elongation of MnS, and can be added according to required characteristics. In order to effectively exhibit such effects, the Ca content and the Mg content are both preferably 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. The REM amount is preferably 0.0005% or more, more preferably 0.0010% or more, and further preferably 0.0012% or more. However, when the amount of Ca, Mg and REM becomes excessive, the toughness is deteriorated. Therefore, both the Ca content and the Mg content are preferably 0.005% or less, more preferably 0.004% or less, and still more preferably 0.003% or less. The amount of REM is preferably 0.02% or less, more preferably 0.01% or less, and still more preferably 0.005% or less. In the present invention, REM means 15 lanthanoid elements from La to Ln, and Sc and Y.

One or more selected from the group consisting of Zr: more than 0%, 0.1% or less, Ta: more than 0%, 0.1% or less, and Hf: more than 0%, 0.1% or less Zr, Ta and Hf Has the effect of forming a nitride in combination with N, thereby suppressing the growth of the austenite grain size during heating, refining the final structure, and improving toughness. In order to exhibit such an effect effectively, the amount of Zr is preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more. Both Ta amount and Hf amount are preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.03% or more. However, excessive amounts of Zr, Ta, and Hf are not preferable because nitrides are coarsened and fatigue characteristics are deteriorated. For these reasons, the amount of Zr is preferably 0.1% or less, more preferably 0.09% or less, still more preferably 0.05% or less, and particularly preferably 0.025% or less. Both Ta amount and Hf amount are preferably 0.1% or less, more preferably 0.08% or less, still more preferably 0.05% or less, and particularly preferably 0.025% or less.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

  Molten steel having the chemical composition shown in Table 1 was melted by an ordinary melting method, and this molten steel was cast and divided and rolled into a billet having a cross-sectional shape of 155 mm × 155 mm. The REM in Table 1 was added in the form of a misch metal containing about 50% La and about 25% Ce.

  In the method of hot extrusion using a hollow billet, a cylindrical hollow billet was prepared from the raw billet by machining, and hot extrusion was performed to obtain a raw tube. Thereafter, cold rolling and drawing were performed to produce a hollow seamless steel pipe having an outer diameter of 16 mm, an inner diameter of 8 mm, and a length of 3000 mm. A detailed manufacturing method is as shown to AD of Table 2.

  In the method of manufacturing a steel bar by hot rolling and then hollowing it by gun drilling, the steel billet is hot-rolled under the conditions described in E and F of Table 2 to obtain a bar steel, and then hollowed by gun drilling. The raw tube was obtained. Thereafter, cold rolling and drawing were performed to produce a hollow seamless steel pipe having an outer diameter of 16 mm, an inner diameter of 8 mm, and a length of 3000 mm.

  Note that C in Table 2 is the manufacturing method disclosed in Patent Document 3, D is the method disclosed in Patent Document 2, and E is the method disclosed in Patent Document 4.

  The hollow seamless steel pipe thus obtained was measured and evaluated by the following method.

(1) Measurement of thickness deviation rate The thickness of the pipe end portion of the hollow seamless steel pipe was measured at four locations every 90 ° with a micrometer, and the thickness deviation rate was calculated by the following equation (1).
Uneven thickness ratio = (maximum thickness−minimum thickness) / (average thickness) / 2 × 100 (1)

(2) Evaluation of fatigue characteristics The hollow seamless steel pipe was quenched and tempered under the following conditions.
Quenching condition: Hold at 925 ° C for 10 minutes, then oil-cooled Tempering condition: Hold at 390 ° C for 40 minutes, then water-cooled

The hollow seamless steel pipe after quenching and tempering was processed into a cylindrical test piece 1 shown in FIG. FIG. 4A is a front view, and FIG. 4B is a side view showing the end face of the test piece. A torsional fatigue test was performed using the cylindrical test piece 1. The inner diameter of the test piece was about 8.0 mm, the outer diameter of the restraining portion 1a was 16 mm, the outer diameter of the central portion 1b was 12 mm, and the load stress expressed by the outer surface stress of the central portion was 550 ± 375 MPa. The number of times until rupture was measured as the number of endurance, and the test was stopped for those that did not rupture even 10 ⁶ times.

  The results are shown in Table 3 and FIG. FIG. 5 is a graph showing the relationship between the wall thickness ratio and the number of times of endurance of the torsional fatigue test for the inventive examples and comparative examples of the present invention.

No. of Table 3 where the uneven thickness ratio was 7.0% or less. 1, 6-9 and 14-20 corresponded to the circles in FIG. 5, and the torsional fatigue test had a durability of 10 ⁵ times or more, indicating good durability. In particular, no. In Nos. 1, 6-9, and 15-20, the number of endurance was 5 × 10 ⁵ times or more, and the thickness deviation rate was 3.0% or less. In 1, 7 to 9, 15 to 17, and 19, the durability was 10 ⁶ times or more. On the other hand, No. with an uneven thickness ratio exceeding 7.0%. 2～5,10～13, as indicated by the × mark in FIG. 5, the endurance was less than 10 ⁵ times. Among these, No. 3 to 5, 11 to 13 are examples manufactured under the manufacturing conditions C to E corresponding to the above Patent Documents 2 to 4, and the thickness deviation rate exceeds 7.0%.

  By using the hollow seamless steel pipe of the present invention, a high-strength hollow spring having high fatigue strength and excellent durability can be produced. For example, the present invention is suitable for a spring having a strength of 1100 MPa or more, preferably 1200 MPa or more, more preferably 1300 MPa or more. Can be used. Therefore, according to the present invention, the hollowing of parts such as a suspension spring, a valve spring, and a clutch spring can be promoted, and further weight reduction of a vehicle such as an automobile can be achieved, which is industrially useful. .

DESCRIPTION OF SYMBOLS 1 Cylindrical test piece 1a Restraint part 1b Center part 1c Cavity

Claims (7)

% By mass
C: 0.2 to 0.7%
Si: 0.5-3%,
Mn: 0.1 to 2%,
Cr: more than 0%, 3% or less,
Al: more than 0%, 0.1% or less,
P: more than 0%, 0.02% or less,
A hollow seamless steel pipe containing S: more than 0%, 0.02% or less and N: more than 0%, 0.02% or less, the balance being iron and inevitable impurities,
A hollow seamless steel pipe for a spring having an uneven thickness ratio calculated by the following formula (1) of 7.0% or less.
Uneven thickness ratio = (maximum thickness−minimum thickness) / (average thickness) / 2 × 100 (1)   The hollow seamless steel pipe according to claim 1, further comprising, by mass%, B: more than 0% and 0.015% or less.   Further, it contains at least one selected from the group consisting of V: more than 0%, 1% or less, Ti: more than 0%, 0.3% or less, and Nb: more than 0%, 0.3% or less by mass%. The hollow seamless steel pipe according to claim 1 or 2.   The hollow seamless according to any one of claims 1 to 3, further comprising at least one selected from the group consisting of Ni: more than 0%, 3% or less and Cu: more than 0%, 3% or less in mass%. Steel pipe.   The hollow seamless steel pipe according to any one of claims 1 to 4, further comprising, by mass%, Mo: more than 0% and 2% or less.   Further, one or more selected from the group consisting of Ca: more than 0%, 0.005% or less, Mg: more than 0%, 0.005% or less, and REM: more than 0%, 0.02% or less in mass%. The hollow seamless steel pipe according to any one of claims 1 to 5, comprising:   Further, one or more selected from the group consisting of Zr: more than 0%, 0.1% or less, Ta: more than 0%, 0.1% or less, and Hf: more than 0%, 0.1% or less by mass%. The hollow seamless steel pipe according to any one of claims 1 to 6, comprising:

Images