Seamless steel pipe for hollow spring

Abstract

A seamless steel pipe for a hollow spring includes C: 0.2 to 0.7 mass %, Si: 0.5 to 3 mass %, Mn: 0.1 to 2 mass %, Cr: 3 mass % or less (excluding 0 mass %), Al: 0.1 mass % or less (excluding 0 mass %), P: 0.02 mass % or less (excluding 0 mass %), S: 0.02 mass % or less (excluding 0 mass %) and N: 0.02 mass % or less (excluding 0 mass %). A residual austenite content in an inner surface layer part of the steel pipe is 5 vol. % or less. An average grain size of a ferrite-pearlite structure in the inner surface layer part of the steel pipe is 18 m or less. A number density of a carbide having a circle equivalent diameter of 500 nm or more and being present in the inner surface layer part of the steel pipe is 1.810.sup.2 particles/m.sup.2 or less.

Claims

1. A seamless steel pipe for a hollow spring, comprising iron, 0.2 mass % to 0.7 mass % of C, 0.5 mass % to 3 mass % of Si, 0.1 mass % to 2 mass % of Mn, more than 0 mass % and 3 mass % or less of Cr, more than 0 mass % and 0.1 mass % or less of Al, more than 0 mass % and 0.02 mass % or less of P, more than 0 mass % and 0.02 mass % or less of S and more than 0 mass % and 0.02 mass % or less of N, wherein a residual austenite content in an inner surface layer part of the steel pipe is 5 vol. % or less, an average grain size of a ferrite-pearlite structure in the inner surface layer part of the steel pipe is 18 m or less and a number density of a carbide which has a circle equivalent diameter of 500 nm or more and is present in the inner surface layer part of the steel pipe is 1.810.sup.2 particles/m.sup.2 or less.

2. The seamless steel pipe for a hollow spring according to claim 1, further comprising more than 0 mass % and 0.015 mass % or less of B.

3. The seamless steel pipe for a hollow spring according to claim 2, further comprising at least one selected from the group consisting of more than 0 mass % and 1 mass % or less of V, more than 0 mass % and 0.3 mass % or less of Ti and more than 0 mass % and 0.3 mass % or less of Nb.

4. The seamless steel pipe for a hollow spring according to claim 3, further comprising at least one selected from the group consisting of more than 0 mass % and 3 mass % or less of Ni and more than 0 mass % and 3 mass % or less of Cu.

5. The seamless steel pipe for a hollow spring according to claim 1, further comprising at least one selected from the group consisting of more than 0 mass % and 1 mass % or less of V, more than 0 mass % and 0.3 mass % or less of Ti and more than 0 mass % and 0.3 mass % or less of Nb.

6. The seamless steel pipe for a hollow spring according to claim 5, further comprising at least one selected from the group consisting of more than 0 mass % and 3 mass % or less of Ni and more than 0 mass % and 3 mass % or less of Cu.

7. The seamless steel pipe for a hollow spring according to claim 1, further comprising more than 0 mass % and 2 mass % or less of Mo.

8. The seamless steel pipe for a hollow spring according to claim 1, further comprising at least one selected from the group consisting of more than 0 mass % and 0.005 mass % or less of Ca, more than 0 mass % and 0.005 mass % or less of Mg and more than 0 mass % and 0.02 mass % or less of REM.

9. The seamless steel pipe for a hollow spring according to claim 1, further comprising at least one selected from the group consisting of more than 0 mass % and 0.1 mass % or less of Zr, more than 0 mass % and 0.1 mass % or less of Ta and more than 0 mass % and 0.1 mass % or less of Hf.

Description

EXAMPLES

(1) The present invention will now be explained in more detail by reference to examples. However, the examples mentioned below should not be construed as limiting the present invention in any way, and it goes without saying that, in carrying out the present invention, various changes and modifications can be added to these examples as appropriate within the scope capable of suiting the spirits in the context described above and later. And such changes and modifications are included in the technical scope of the present invention.

(2) Various kinds of molten steels (medium carbon steels) having the chemical component compositions shown in Table 1 described below were each melted by a usual melting method. The molten steels were cooled, followed by bloom rolling to form rectangular cylinder-shaped billets having a cross-sectional shape of 155 mm155 mm. These billets were formed into round bars having a diameter of 150 mm by hot forging, followed by machine working, thereby preparing billets for extrusion. In Table 1 described below, REM was added in a form of a misch metal containing about 20% of La and about 40% to 50% of Ce. In Table 1 described below, - shows that no element was added.

(3) The billets made in the foregoing manner were heated to 1,000 C., followed by performing hot extrusion to thereby prepare an extruded pipe having an outer diameter of 54 mm and an inner diameter of 35 mm (an average cooling rate of 1.5 C./sec until the temperature achieved to 720 C. after extrusion, an average cooling rate of 0.5 C./sec from 720 C. to 600 C., and natural cooling in the air thereafter). Next, cold working (draw benching: discontinuous-type draw bench; rolling: Pilger rolling mill), annealing and pickling (kind of acid solution: 5% hydrochloric acid, pickling condition: 15 minutes) were repeated multiple times. As a result, a hollow seamless steel pipe having an outer diameter of 16 mm and an inner diameter of 8.0 mm was prepared. As to the conditions under which these operations were carried out, the atmosphere during the annealing, the annealing temperature (the highest heating temperature), the annealing time (heating time) and the average cooling rates after the annealing (heating) (cooling rate 1 and cooling rate 2) are shown in Table 2.

(4) The thus obtained hollow seamless steel pipes were each examined for the number density of coarse carbides, structure size (average grain size) and residual austenite content in accordance with the following methods.

(5) (Number Density of Coarse Carbide Particles)

(6) As to the number density of carbides in an inner surface layer part of a steel pipe, a sample for use in observing an arbitrary traverse plane thereof (a cross section orthogonal to the axis of the pipe) was prepared by carrying out cutting, embedding with a resin, mirror polishing, and then etching through the corrosion with picral. A surface layer part ranging from the outermost surface to a depth of 100 m in the inner peripheral surface was observed by a scanning electron microscope (SEM) (magnification: 3,000 times). On a basis of SEM photographs each (number of observation spots: 3), an area occupied by carbide was determined using an image analysis software (Image-Pro), and converted into a circle equivalent diameter. And the number density of carbide particles having circle equivalent diameters of 500 nm or more was measured at each observation spot, and the average thereof was calculated.

(7) (Structure Size: Average Grain Size)

(8) As to the structure size in an inner surface layer part of a steel pipe, a sample for use in observing an arbitrary traverse plane thereof (a cross section orthogonal to the axis of thel pipe) was prepared by carrying out cutting, embedding with a resin, mirror polishing, and then etching through the corrosion with nital. A surface layer part extending from the inner surface to an inward position of 100 m was observed by an optical microscope (magnification: 100 to 400 times), and grain sizes were determined by the comparison method, followed by converting into an average grain size by the use of the expression (1) (number of measurement spots: 4).

(9) (Residual Austenite Content)

(10) As to the residual austenite content in an inner surface layer part of a steel pipe, a sample for use in observing an arbitrary traverse plane thereof (a cross section orthogonal to the axis of the pipe) was prepared by carrying out cutting, embedding with a resin, wet polishing, and then electrolytic polishing finish. The residual austenite content (unit: vol. %) in this sample was determined by X-ray diffraction analysis. The case where the residual austenite content was 5% or less was rated as o, while the case where the residual austenite content was more than 5% was rated as x.

(11) (Fatigue Strength Test: Durability)

(12) Each of the foregoing seamless steel pipes was subjected to quenching and tempering under the following conditions which were assumed to be the heat treatment to be applied to hollow springs, followed by working into a JIS test specimen (JIS Z 2274 fatigue test specimen).

(13) (Quenching and Tempering Conditions)

(14) Quenching condition: retention at 925 C. for 10 minutes and subsequent oil cooling

(15) Tempering condition: retention at 390 C. for 40 minutes and subsequent water cooling

(16) On each of the test specimens mentioned above (quenched and tempered test specimens), rotary bending fatigue test was performed at a rotation speed of 1,000 rpm under a stress of 900 MPa. The case where fracture occurred when the number of repetitions reached or exceeded 1.010.sup.5 times was rated as o, while the case where fracture occurred before the number of repetitions reached 1.010.sup.5 times was rated as x. These evaluation results are shown in Table 2 (durability test results).

(17) TABLE-US-00001 TABLE 1 Chemical Composition (mass %), Remainder: Fe and Unavoidable Impurities other than P and S Steel Ca, Mg, Zr, Ta, No. C Si Mn Cr Al P S N B V Ti Nb Ni Cu Mo REM Hf A1 0.40 2.48 1.21 1.07 0.0315 0.004 0.006 0.0028 0.0048 0.180 0.41 0.15 A2 0.41 1.72 0.17 1.01 0.0240 0.004 0.003 0.0021 0.165 0.060 0.31 0.17 A3 0.43 1.90 0.21 0.95 0.0350 0.007 0.007 0.0040 0.150 0.070 0.60 0.31 A4 0.44 1.60 0.45 0.48 0.0700 0.012 0.013 0.0050 0.050 0.040 0.13 Ca:0.0015 A5 0.45 1.75 0.70 0.75 0.0020 0.015 0.015 0.0030 0.090 0.15 0.10 REM:0.0017 Zr:0.04 A6 0.46 1.72 0.18 0.90 0.0250 0.006 0.006 0.0031 0.500 0.20 0.30 A7 0.55 1.41 0.71 0.72 0.0370 0.018 0.018 0.0049 0.200 0.6 A8 0.55 1.45 0.70 0.70 0.0280 0.015 0.015 0.0045 A9 0.60 2.10 0.60 0.17 0.0330 0.020 0.020 0.0040 0.100 0.120 0.050 A10 0.60 2.00 0.75 0.15 0.0300 0.017 0.015 0.0048 0.0050

(18) TABLE-US-00002 TABLE 2 Annealing Condition Cooling Condition Number Highest Heating Cooling rate Cooling rate Density heating time 1 <900 to 2 <750 to of Coarse Structure Durability Test Steel temperature <900 C. or 750 C.> 600 C.> Carbides Size Residual Test Result No. No. Atmosphere ( C.) more> (min) ( C./sec) ( C./sec) (particles/m.sup.2) (m) Austenite 900 MPa 1 A1 Ar gas 920 4 1.7 0.2 0.8 10.sup.2 12 2 A2 Ar gas 920 5 1.8 0.3 0.7 10.sup.2 10 3 A2 Ar gas 920 5 3.2 0.3 0.5 10.sup.2 6 4 A2 Ar gas 920 5 0.4 0.4 0.3 10.sup.2 20 x 5 A2 Ar gas 920 5 3.2 3.1 0 3 x x 6 A2 Ar gas 900 2 2.1 0.3 0.6 10.sup.2 6 7 A2 Ar gas 950 8 1.9 0.3 0.6 10.sup.2 8 8 A2 Ar gas 1,000 5 1.7 0.3 0.3 10.sup.2 27 x 9 A3 Ar gas 920 1 0.7 0.9 1.1 10.sup.2 7 10 A3 Ar gas 920 1 0.7 0.5 1.1 10.sup.2 8 11 A3 Ar gas 920 5 1.7 0.4 1.1 10.sup.2 11 12 A3 Ar gas 920 20 1.8 0.4 0.5 10.sup.2 19 x 13 A3 Ar gas 920 60 1.8 0.4 0.3 10.sup.2 21 x 14 A3 Ar gas 905 2 2.2 0.4 0.4 10.sup.2 5 15 A3 Ar gas 950 9 1.5 0.4 0.1 10.sup.2 15 16 A3 Ar gas 1,000 5 1.6 0.4 0.1 10.sup.2 25 x 17 A4 Ar gas 920 5 1.4 0.4 1.8 10.sup.2 17 18 A4 Air 680 60 *1 0.3 2.8 10.sup.2 8 x 19 A4 Air 750 60 *1 0.3 4.2 10.sup.2 9 x 20 A5 Ar gas 920 5 1.4 0.4 0.3 10.sup.2 13 21 A6 Ar gas 920 3 1.3 0.3 0.6 10.sup.2 16 22 A7 Ar gas 920 3 1.5 0.3 0.7 10.sup.2 15 23 A7 Ar gas 920 3 1.8 1.5 0.7 10.sup.2 5 x x 24 A8 Ar gas 930 1 1.2 0.3 0.2 10.sup.2 15 25 A9 Ar gas 920 3 1.9 0.4 0 8 26 A10 Ar gas 930 1 1.5 0.3 0.2 10.sup.2 13 *1: Heating time (staying time) in each of No. 18 and No. 19 was under temperatures of 650 C. or more.

(19) As can be seen from these results, the hollow seamless steel pipes produced from steel materials having appropriate chemical compositions under appropriate conditions (Test. Nos. 1 to 3, 6, 7, 9 to 11, 14, 15, 17, 20 to 22 and 24 to 26) were good in fatigue strength of the springs made therewith.

(20) On the other hand, it can be seen that deterioration in fatigue strength occurred in Test Nos. 4, 5, 8, 12, 13, 16, 18, 19 and 23 because the production processes were inappropriate, and hence the requirements specified by the present invention were not satisfied.

(21) More specifically, the Test No. 4 is an example that the cooling rate 1 was slow, and thus, the average grain size (structure size) of the ferrite-pearlite structure was large, namely coarse, resulting in decrease of fatigue strength (durability).

(22) The Test Nos. 5 and 23 are examples that the cooling rate 2 was too fast, and thus, the residual austenite content was large, resulting in decrease of fatigue strength (durability).

(23) The Test Nos. 8 and 16 are examples that the highest heating temperature during the annealing was high, and thus, the average grain size (structure size) of the ferrite-pearlite structure was large, resulting in decrease of the fatigue strength (durability).

(24) The Test Nos. 12 and 13 are examples that the heating time at a temperature of 900 C. or more was too long, and thus, the fatigue strength (durability) was decreased.

(25) The Test Nos. 18 and 19 are examples that the annealing was carried out in the air at low temperatures. In these examples, the number density of coarse carbides was large and the fatigue strength (durability) was decreased.

(26) The present patent application has been illustrated above in detail or by reference to the specified embodiments. It will, however, be apparent to persons skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

(27) This application is based on Japanese Patent Application No. 2012-132104, filed on Jun. 11, 2012, the contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

(28) In producing the present seamless steel pipe for a hollow spring, not only the chemical composition of a steel material as raw material was appropriately adjusted, but also various structures (residual austenite, an average grain size of a ferrite-pearlite structure, and coarse carbides) in an inner surface layer part of the steel pipe are controlled appropriately. Thus, springs made from the seamless steel pipe for a hollow spring are able to secure sufficient fatigue strength.