Hollow seamless pipe for high-strength springs

Abstract

A hollow seamless pipe for a high-strength spring with reduced occurrence of decarburization in the inner and outer peripheral surfaces, hardened surface layers in the inner and outer peripheral surfaces during quenching, and sufficient fatigue strength is provided. The hollow seamless pipe contains a steel material, which includes 0.2 to 0.7 mass % of C, 0.5 to 3 mass % of Si, 0.1 to 2 mass % of Mn, more than 0 and 0.1 mass % or less of Al, more than 0 and 0.02 mass % or less of P, more than 0 and 0.02 mass % or less of S, and more than 0 and 0.02 mass % or less of N. The C content in the inner and outer peripheral surfaces is 0.10 mass % or more. A thickness of a whole decarburized layer in each of the inner peripheral surface and the outer peripheral surface is 200 m or less.

Claims

1. A hollow seamless pipe, comprising: a steel material comprising 0.2 to 0.7 mass % of C, 0.5 to 3 mass % of Si, 0.1 to 2 mass % of Mn, 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, more than 0 mass % and 0.02 mass % or less of N, and Fe, wherein a C content in an inner peripheral surface and an outer peripheral surface of the hollow seamless pipe is 0.10 mass % or more, a thickness of a whole decarburized layer in each of the inner peripheral surface and the outer peripheral surface of the hollow seamless pipe is 200 m or less, and an average grain size of ferrite in an inner surface layer part of the hollow seamless pipe is 11.7 m or less.

2. The hollow seamless pipe according to claim wherein a maximum depth of a flaw present in the inner peripheral surface of the hollow seamless pipe is 20 m or less.

3. The hollow seamless pipe according to claim 1 or claim 2, further comprising: at least one selected from the group consisting of groups (a) to (g): (a) more than 0 mass % and 3 mass % or less of Cr, (b) more than 0 mass % and 0.015 mass % or less of B, (c) one or more elements 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, (d) one or more elements 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, (e) more than 0 mass % and 2 mass % or less of Mo, (f) one or more elements 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, and (g) one or more elements 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.

4. The hollow seamless pipe according to claim 1, wherein the average grain size of ferrite in an inner surface layer part of the hollow seamless pipe is 10 m or less.

5. The hollow seamless pipe according to claim 4, wherein a JIS Z2274 fatigue specimen of the hollow seamless pipe has no breakage occurred up to 2.010.sup.5 cycles in a corrosion fatigue test.

Description

EXAMPLES

(1) Various kinds of molten steels having the chemical component compositions shown in Table 1 were each melted by a usual melting method. The molten steels were cooled and bloom rolled to form slabs having a cross-sectional shape of 155 mm155 mm. Thereafter, hot rolling and cooling were preformed under the conditions shown in Table 2 described below to obtain bar steels having a diameter of 25 mm. Incidentally, in Tables 1 and 2 described below, REM was added in a form of a misch metal containing about 50% of La and about 25% of Ce. In Tables 1 and 2 described below, - shows that no element was added. Incidentally, Cooling Rate 1 in Table 2 means the average cooling rate at the time when cooled to 720 C. after hot rolling, and Cooling Rate 2 means the average cooling rate at the time when cooled from the end temperature of the above-mentioned cooling to 500 C.

(2) An inside of the resulting bar steel was pierced to have an inner diameter of 12 mm by using a gun drill. Thereafter, cold rolling was performed to prepare a hollow seamless pipe having an outer diameter of 16 mm and an inner diameter of 8 mm. In the course thereof, heat treatment or annealing was performed at a stage of an outer diameter of 20 mm and an inner diameter of 10 mm in some materials (Test Nos. 2 to 4 in Table 2 described below). Incidentally, for Test Nos. 2 to 4, conditions at a stage of an outer diameter of 20 mm and an inner diameter of 10 mm and conditions at a stage of an outer diameter of 16 mm and an inner diameter of 8 mm are described separately, divided into Cold Rolling Conditions 1 and Annealing Temperature 1, and Cold Rolling Conditions 2 and Annealing Temperature 2, respectively.

(3) Further, as a comparative material, a cylindrical billet having an outer diameter of 143 mm and an inner diameter of 52 mm was prepared from a slab having a cross-sectional shape of 155 mm155 mm by hot forging and cutting, and a hollow pipe having an outer diameter of 54 mm and an inner diameter of 38 mm was also prepared by using hot hydrostatic extrusion (heating temperature: 1,150 C.) (Test No. 1 in Table 2 described below). After heat treatment or annealing and pickling, draw benching, heat treatment or annealing (700 C.20 hours) and pickling were repeated 8 times to this hollow pipe to prepare a hollow seamless pipe having an outer diameter of 16 mm and an inner diameter of 8 mm (heat treatment or annealing conditions after draw benching: 750 C.10 minutes).

(4) TABLE-US-00001 TABLE 1 Steel Chemical Component Composition (mass %) Species C Si Mn P S Cu Ni Cr Mo V Nb A 0.42 1.90 0.20 0.005 0.005 0.20 0.32 1.01 0.17 B 0.59 2.06 0.94 0.005 0.005 0.45 0.47 0.15 C 0.42 1.69 0.60 0.005 0.005 1.00 0.15 D 0.40 1.86 0.60 0.005 0.005 0.99 0.080 E 0.40 1.95 0.31 0.005 0.005 0.30 0.050 F 0.38 1.64 0.54 0.005 0.005 1.00 0.050 G 0.37 1.76 0.25 0.005 0.005 0.95 0.93 0.21 H 0.24 1.13 0.86 0.005 0.005 0.95 0.93 0.15 0.045 I 0.43 1.89 0.19 0.005 0.005 0.21 0.35 0.99 0.15 J 0.42 1.92 0.21 0.005 0.005 0.20 0.36 1.00 0.14 K 0.42 1.88 0.20 0.005 0.005 0.21 0.35 0.99 0.16 L 0.43 1.90 0.21 0.005 0.005 0.20 0.34 1.01 0.17 M 0.42 1.92 0.20 0.005 0.005 0.20 0.33 0.99 0.15 N 0.42 1.90 0.20 0.005 0.005 0.21 0.35 1.01 0.17 Steel Chemical Component Composition (mass %) Species Ti Al B Ca Mg REM Zr, Hf, Ta N A 0.068 0.030 0.0045 B 0.029 0.0042 C 0.050 0.025 0.0049 D 0.023 0.0026 0.0055 E 0.051 0.025 0.0034 F 0.049 0.022 0.0020 0.0042 G 0.026 0.0041 H 0.076 0.021 0.0032 0.0033 I 0.065 0.025 Zr: 0.019 0.0043 J 0.068 0.027 Hf: 0.045 0.0042 K 0.070 0.031 Ta: 0.032 0.0044 L 0.071 0.028 0.0021 0.0041 M 0.069 0.030 0.0010 0.0045 N 0.070 0.031 0.0025 0.0046 Remainder: iron and unavoidable impurities other than P and S

(5) TABLE-US-00002 TABLE 2 Hot Rolling Conditions Heating Minimum Rolling Cooling Conditions Test Steel Temperature Temperature Cooling Rate 1 Cooling Rate 2 No. Species Hollowing Method ( C.) ( C.) ( C./s) ( C./s) 1 A Hydrostatic extrusion + draw benching 2 A Hot rolling + gun drill 1300 900 0.5 0.2 3 A Hot rolling + gun drill 1300 900 2 0.5 4 A Hot rolling + gun drill 1030 900 2 0.5 5 A Hot rolling + gun drill 1030 900 2 0.5 6 A Hot rolling + gun drill 1000 850 2 0.5 7 B Hot rolling + gun drill 1000 850 2 0.5 8 C Hot rolling + gun drill 1030 900 2 0.5 9 D Hot rolling + gun drill 1030 900 2 0.5 10 E Hot rolling + gun drill 1030 900 2 0.5 11 F Hot rolling + gun drill 1030 900 2 0.5 12 G Hot rolling + gun drill 1000 850 2 0.5 13 H Hot rolling + gun drill 1000 850 2 0.5 14 I Hot rolling + gun drill 1000 850 2 0.5 15 J Hot rolling + gun drill 1000 850 2 0.5 16 K Hot rolling + gun drill 1000 850 2 0.5 17 L Hot rolling + gun drill 1000 850 2 0.5 18 M Hot rolling + gun drill 1000 850 2 0.5 19 N Hot rolling + gun drill 1000 850 2 0.5 Cold Rolling Conditions 1 Cold Rolling Conditions 2 Finish Finish Outer Inner Annealing Outer Inner Annealing Test Diameter Diameter Reduction Temperature 1 Diameter Diameter Reduction Temperature 2 No. (mm) (mm) of Area (%) ( C.) (mm) (mm) of Area (%) ( C.) 1 750 2 20 10 38 750 16.0 8.0 36 750 3 20 10 38 750 16.0 8.0 36 750 4 20 10 38 750 16.0 8.0 36 750 5 16 8 60 750 6 16 8 60 650 7 16 8 60 650 8 16 8 60 700 9 16 8 60 700 10 16 8 60 750 11 16 8 60 650 12 16 8 60 700 13 16 8 60 700 14 16 8 60 650 15 16 8 60 650 16 16 8 60 650 17 16 8 60 650 18 16 8 60 650 19 16 8 60 650

(6) A center part of the resulting hollow seamless pipe was cut in an axis direction thereof, and the C content was measured using an EPMA, thereby measuring the thickness of decarburized layers (ferrite decarburized layer and whole decarburized layer) and measuring the average grain size of ferrite in the vicinity of an inner peripheral surface (a region from a surface to a depth of 500 m) with an EBSP. Respective detailed measuring conditions are as follows.

(7) (Measuring Conditions of EPMA)

(8) Acceleration voltage: 15 kV

(9) Irradiation current: 1 A

(10) Line analysis direction: from the outside of the pipe to the inside thereof.

(11) For the line analysis, measurement was made by giving the minimum beam diameter (about 3 m) and swing by the beam in a width of 30 m. At this time, when a part having a C content of less than 0.10% was present in a surface layer part, the ferrite decarburized layer was considered to be present, which was evaluated as B. When no part having a C content of less than 0.10% was present, no ferrite decarburized layer was considered to be present, which was evaluated as A. Further, a part having a carbon concentration of less than 95% in a center part of the pipe thickness was considered as the whole decarburized layer, and the thickness thereof was measured. When the thickness of the decarburized layer was 200 m or less, it was evaluated as A. In the case of exceeding 200 m, it was evaluated as B.

(12) (Measuring Conditions of EBSP)

(13) Region: 300300 (m)

(14) Number of frames: 2

(15) Measuring pitch: 0.4 m

(16) The average grain size was calculated, taking an orientation difference of 15 C. or more as a grain boundary and neglecting 3 m or less.

(17) Further, the center part of the resulting hollow seamless pipe was cut in a circumferential direction thereof, and the whole circumference was observed with an optical microscope (400 magnification). The maximum flaw depth at that time was determined. At this time, three cross-sections were observed, and the maximum one was evaluated as the maximum inner peripheral surface flaw depth.

(18) Each of the above-mentioned hollow seamless pipes was quenched and tempered under the following conditions, followed by working to a JIS specimen (JIS Z2274 fatigue specimen)

(19) (Quenching and Tempering Conditions)

(20) Quenching conditions: maintaining at 930 C. for 20 minutes.fwdarw.thereafter, water cooling

(21) Tempering conditions: maintaining at 430 C. for 60 minutes

(22) (Corrosion Fatigue Test)

(23) The above-mentioned specimen (quenched and tempered specimen) was sprayed with a 5% NaCl aqueous solution at 35 C., and subjected to a rotary bending corrosion fatigue test at a stress of 784 MPa and a rotation rate of 100 rpm. The presence or absence of breakage up to the number of repeated cycles of 2.010.sup.5 was examined. The case of 1.010.sup.5 cycles or more was evaluated as B, and the case where no breakage occurred up to 2.010.sup.5 cycles was evaluated as A (the case where breakage occurred up to less than that was evaluated as C).

(24) These results are shown together in Table 3 described below. As apparent from these results, the hollow seamless pipes obtained under the proper production conditions (Test Nos. 5 to 19, examples of the invention) satisfy the requirements specified in the invention, and it is revealed that the ones having good fatigue strength for springs are obtained.

(25) On the other hand, the ones of Test Nos. 1 to 3 (comparative examples) does not satisfy the requirements specified in the invention because of the improper production methods, and it is revealed that the fatigue strength for springs is deteriorated. Incidentally, in Test No. 4, the average grain size of ferrite which is the preferred requirement is coarsened, so that the fatigue strength for springs is somewhat decreased.

(26) TABLE-US-00003 TABLE 3 Ferrite Decarburization Total Decarburization Evaluation Evaluation Grain Size in Vicinity of Test Steel Outer Peripheral Inner Peripheral Outer Peripheral Inner Peripheral Inner Peripheral Surface No. Species Surface Surface Surface Surface (m) 1 A B B B B 2 A B A B A 3 A A A B A 4 A A A A A 15.7 5 A A A A A 8.3 6 A A A A A 6.1 7 B A A A A 11.7 8 C A A A A 8.9 9 D A A A A 8.5 10 E A A A A 6.8 11 F A A A A 7.1 12 G A A A A 6.1 13 H A A A A 5.5 14 I A A A A 5.8 15 J A A A A 5.7 16 K A A A A 5.2 17 L A A A A 6.5 18 M A A A A 6.6 19 N A A A A 6.7 Maximum Flaw Depth of Test Inner Peripheral Surface Corrosion Fatigue No. (m) Property Total Evaluation Note 1 22 C C Due to hydrostatic extrusion, much decarburization: x 2 C C Due to high heating temperature and slow cooling rate, ferrite decarburization and total decarburization: x 3 C C Due to high heating temperature, total decarburization: x 4 6 B B Due to low reduction of area, large grain size 5 5.3 A A 6 4.3 A A 7 7.1 B B 8 5.4 A A 9 6.1 A A 10 7.2 A A 11 6.3 A A 12 5.9 A A 13 5.2 A A 14 6.2 A A 15 6.5 A A 16 5.7 A A 17 6.1 A A 18 6.6 A A 19 6.7 A A

(27) While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

(28) This application is based on Japanese Patent Application No. 2009-119030 filed on May 15, 2009, and the entire subject matter of which is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

(29) In the invention, a chemical component composition of a steel as a material is properly adjusted, and production conditions thereof are strictly defined, thereby being able to realize a hollow seamless pipe, in which no ferrite decarburization is occurred in an inner peripheral surface and outer peripheral surface and a thickness of a decarburized layer is reduced as much as possible. It becomes possible to secure sufficient fatigue strength for a spring formed from such a hollow seamless pipe.