Method of producing a high-strength coil spring

Abstract: A high-strength coil spring with high fatigue resist­ ance comprising a clean steel wire, such as chromium-­ vanadium steel wire or chromium-silicon steel wire, formed in the shape of a spring, which is quenched and tempered at lower temperatures to heighten the tensile strength and subjected to a shot peening treatment followed by an elec­ trolytic polishing treatment, which does not exert a bad influence on fatigue resistance, to remove surface defects, and a method or producing the same.

Description: The present invention relates to a high-strength coil spring and a method of producing the same. The coil spring according to the present invention may be effectively used as a high-strength spring for an engine or as other high-strength springs requiring high fatigue resistance. In general, a higher tensile strength is desired for spring materials but it is known that if tensile strength exceeds a certain limit, toughness and fatigue resistance are contrarily reduced. In addition, coil springs have been used after forming and then being subjected to a quenching treatment followed by being subjected to a shot peening treatment to add a compressive residual stress to a surface thereof.But an effective shot peening treatment gives a surface roughness Rmax of 6 to 20µm, so that not only has it been impossible to remove surface defects having a surface roughness of 6 to 20µm or less,but also impressions due to the shot peening have covered the surface defects which may be turned into damage sites and fatigue nuclei in many cases. It goes without saying that the Rmax can be reduced by various subsequent polishing treatments but since a surface layer is removed, portions of the outer layer to which a compressive residual stress has been applied with much trouble are lost, whereby the fatigue resistance is on the contrary reduced. Various spring steels, their treatment and their properties are disclosed in Stahlschlüssel 1983 13th edition pp 42-45 and in GB-A-2112810. It is expected that if 'clean' steels, ie those in which the concentration of nonmetallic inclusions has been reduced (such as chromium-vanadium steel and chromium-silicon steel, disclosed for example in GB-A-2210299) are used, then the conditions for drawing forth the highest fatigue resistance as a spring are different from the conventional ones. That is to say, the tensile strength of the present chromium-vanadium steel and chromium-silicon steel is set so that the best fatigue properties may be obtained with a level of inclusions and surface defects in the conventional materials as the base but it can be expected that if merely the problems of surface defects are solved for the clean steels, the fatigue resistance can be improved by still further heightening the tensile strength. In view of the above description, the present inventors have produced a high-strength coil spring with high fatigue resistance using a clean steel wire, such as chromium-vanadium steel wire and chromium-silicon steel wire, by forming it in the shape of a spring, quenching and tempering it at lower temperatures to heighten the tensile strength, and subjecting it to a shot peening treatment followed by an electrolytic polishing treatment, which does not adversely affect fatigue resistance, to remove surface defects. That is to say, the present invention provides a method of producing a high-strength coil spring from a steel wire comprising 0.4 to 1.0% by weight of C, 0.1 to 2.0% by weight of Si, 0.4 to 1.2% by weight of Mn, 0.3 to 1.5% by weight of Cr, 0.001 to 0.3% by weight of V and the remainder of Fe and inevitable impurities, characterised in that the cleanliness is adjusted to 0.01% or less and the steel is subjected to coiling to form it into an appointed spring shape, followed by a quenching and tempering treatment to adjust the tensile strength, and then to a shot peening treatment followed by a polishing treatment to give a surface roughness Rmax of 5µm or less. The coiling of the steel wire can be carried out by cold forming or by hot forming. In one preferred method the steel wire is coiled at a temperature of 820°C or more and then subjected to the quenching treatment. In another, the wire is heated at 820°C or more then formed into a coil at 400 to 600°C and subjected to the quenching treatment as it is. Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein: FIG. 1(A) to (D) are graphs showing the relationship between tempering temperature and mechanical properties of a chromium-silicon steel wire quenched in oil, in which FIG. 1(A) shows the relationship between tempering temperature and hardness; Fig 1(b) shows the relationship between tempering temperature and tensile strength; Fig 1(C) shows the relationship between tempering temperature and reduction of area; and Fig 1(D) shows the relationship between tempering temperature and fatigue strength; Fig 2 is a graph showing a distribution of residual stress in the direction of depth of a steel wire after a quenching treatment and a tempering treatment in terms of the relationship between the distance from a surface and longitudinal residual stress. Fig 3(A) and (B) are graphs showing a distribution of a residual stress on the inner side of a coil spring in a process (F-1) of the present invention and the conventional process (F-7). The reasons for adjusting the cleanliness to 0.01% or less is that fatigue fracture due to non-metallic inclusions contained in the steel wire having the above described chemical composition is thus rendered less likely. This can be achieved by devising a deoxidation method, such as by optimising the conditions of vacuum degassing and refining slag. In addition, the reason why the quenching treatment and the tempering treatment are carried out after the coiling is that if the quenching and tempering treatment is carried out before the coiling, the resulting high-strength material is apt to be insufficient in toughness, and also it s sensitivity to surface defects is strong, so that the probability of breakage during coiling increases. Furthermore, the reason why the tensile strength of the chromium-vanadium steel wire quenched in oil made by the present invention for use for example in a valve-spring, is increased by 10% in comparison with the value provided in Table 5 of JIS G-3565, and the tensile strength of the chromium-silicon steel wire is increased by 10% in comparison with the value provided in Table 6 of JIS G-3566, is that if the surface defects and inclusions are removed, the matrix itself has sufficient toughness and also the fatigue strength can be enhanced even though the strength is enhanced beyond the conventional value. Fig. 1(A) to (D) are graphs showing the influence of lowering the tempering temperature for a chromium-silicon steel wire quenched in oil having a diameter of 4.0 mm, compared with that for the conventional material (tempered at 400°C for obtaining the tensile strength corresponding to JIS G-3566) upon mechanical properties such as hardness, tensile strength, reduction in area and fatigue strength. It is normal that if the tempering temperature is lowered, as shown in Fig. 1(A), the hardness is increased. The tensile strength and the fatigue strength (by the rotating bending test) are contrarily reduced, as shown by (b) in Fig. 1(B) and (D). However, in the case where the surface is subjected to the electrolytic polishing, they are contrarily increased up to a certain temperature (250°C as for the tensile strength and 350 o C as for the fatigue strength) with a reduction of the tempering temperature, as shown by (a) in Fig. 1(B) and (D). That is to say, it is found that according to the conventional method, the strength of the matrix itself is not sufficiently exhibited due to the surface defects. It can be found from the above description that even though the tensile strength after the quenching and the tempering treatment is increased over that of the conventional materials, superior performances can be obtained by reducing the surface defects. Fig. 1(C) is a graph showing a comparison of the steel wire (b) as heat treated with the steel wire (a) electrolytically polished after heat treatment, regarding the reduction of area. The reason why the polishing treatment is carried out after the shot peening treatment is that a zone having the largest compressive residual stress exists at a depth of 100 to 150 µm from the surface, as shown by Fig. 2 which is a graph showing the distribution of the residual stress in the direction of depth of a steel elementary wire after the quenching treatment and the tempering treatment. Accordingly, it can be thought that ifthethickness of a portion to be removed by the polishing treatment after the shot peening treatment is 100 µm or less, the compressive residual stress of the uppermost surface is rather increased, so that no bad influence is exerted on the fatigue characteristics. The steel wire used in the present invention comprises C, Si, Mn, Cr, V, Fe and inevitable impurities,but it is for the following reasons that the content of C is limited within a range of 0.4 to 1.0 % by weight, Si 0.1 to 2.0 % by weight, Mn 0.4 to 1.2 % by weight, Cr 0.3 to 1.5 % by weight and V 0.001 to 0.3 % by weight. That is to say, if the content of C is less than 0.4 % by weight, a sufficient strength is not obtained and if the content of C exceeds 1.0 % by weight, shrink cracking is apt to be brought about during the quenching treatment. If the content of Si is less than 0.1 % by weight, the heat resistance is deteriorated and if the content of Si exceeds 2.0 % by weight, cracks are apt to be brought about on the surface during the hot rolling. If the content of Mn is less than 0.4 % by weight, the quenchability is deteriorated to lead to an insufficient strength and if the content of Mn exceeds 1.2 % by weight, the workability is deteriorated. The content of Cr within the range of 0.3 to 1.5 % by weight is effective for the achievement of the superior hardenability and heat resistance. The content of V within the range of 0.001 to 0.3 % by weight is preferable in view of the preservation of a superior micronization of crystalline particles and hardenability. The present invention will be below described in detail with reference to the preferred embodiments. EXAMPLE 1 A steel wire with a diameter of 4.0 mm and a chemical composition and cleanliness shown in Table 1 was produced, and springs of the dimensions shown in Table 3 were produced by the manufacturing processes shown in Table 2 from this steel wire. The mechanical properties after the quenching treatment and the tempering treatment, and the number of cycles to fracture when a fatigue test was carried out at a mean clamping stress τ m of 588.4 MPa (60 kg/mm 2 ) and an amplitude stress τ a of 441.3 MPa (45 kg/mm 2 ), are shown in Table 4. In addition, the mechanical properties of a sample obtained by coiling followed by being subjected to the quenching treatment and the tempering treatment in the manufacturing process shown in Table 2 are difficult to measure, so that the mechanical properties of this sample were substituted by characteristic values for a sample obtained by subjecting an elementary wire, which had not been subjected to the coiling, to the same subsequent treatments. In addition, the result of the fatigue test is an average value for n = 4 to 11. Chemical Composition and cleanliness of Steel Wires to be Tested C (wt%) Si (wt%) Mn (wt%) P (wt%) S (wt%) Cr (wt%) V (wt%) Fe (wt%) cleanliness (%) A 0.51 0.25 0.78 0.009 0.008 1.02 0.22 Rest 0.003 B 0.46 0.34 0.50 0.008 0.010 1.2 0.25 Rest 0.005 C 0.64 0.13 0.94 0.010 0.005 0.81 0.16 Rest 0.003 D 0.59 0.20 0.48 0.007 0.006 1.10 0.20 Rest 0.042 E 0.58 0.22 0.70 0.006 0.007 0.96 0.23 Rest 0.078 Dimensions of Coil Spring Diameter of elementary wire 4 mm Average coil diameter 24 mm Free height 55 mm Total number of turns 6.5 Effective number of turns 4.5 EXAMPLE 2 A steel wire with a diameter of 4.0 mm and a chemical composition and cleanliness shown in Table 5 was produced,and springs having the same dimensions as those shown in Table 3 of EXAMPLE 1 were produced by the manufacturing processes shown in Table 6 from this steel wire. The mechanical properties after the quenching treatment and the tempering treatment, and the number of cycles to fracture when a fatigue test was carried out at a mean clamping stress τ m of 588.4 MPa (60 kg/mm 2 ) and an amplitude stress τ a of 490.3 MPa (50 kg/mm 2 ), are shown in Table 7. In addition, the mechanical properties of a sample obtained by coiling followed by being subjected to the quenching treatment and the tempering treatment in the manufacturing process shown in Table 6 are difficult to measure, so that the mechanical properties of this sample were substituted by characteristic values for a sample obtained by subjecting an elementary wire, which had not been subjected to the coiling, to the same subsequent treatments. In addition, the result of the fatigue test is an average value for n = 4 to 11. Chemical Compositions and cleanliness of Steel Wires to be Tested C (wt%) Si (wt%) Mn (wt%) P (wt%) S (wt%) Cr (wt%) V (wt%) Fe (wt%) cleanliness (%) F 0.64 1.43 0.68 0.007 0.013 0.70 0.002 Rest 0.004 G 0.50 1.21 0.52 0.006 0.009 0.54 0.002 Rest 0.003 H 0.77 1.64 0.80 0.010 0.010 1.02 0.003 Rest 0.008 I 0.62 1.47 0.65 0.009 0.015 0.69 0.002 Rest 0.026 J 0.62 1.44 0.68 0.007 0.012 0.68 0.004 Rest 0.089 It is found from the above described Table 4 of EXAMPLE 1 and Table 7 of EXAMPLE 2 that springs obtained by A-1, A-2, B-1, B-2, B-3, C-1, C-2, F-1, F-2, G-1, G-2, G-3, H-1, H-2 and H-3, which are the preferred embodiments of the present invention, are remarkably superior in fatigue useful life time. Springs of D, E, I and J types of inferior cleanliness, that is D-1, D-2, D-3, D-4, D-5, E-1, I-1, I-2, I-3, I-4, I-5 and J-1 are inferior in fatigue resistance. In addition, even in the case where steel wires containing the chemical compositions of A and F types are used, springs obtained by the manufacturing processes, in which the electrolytic polishing is not or insufficiently carried out, that is springs obtained by the processes of A-3, A-7, F-3 and F-7, are inferior in fatigue resistance. Besides, also springs obtained by A-8 and F-8, which are the conventional manufacturing processes of A-7 and F-7 plus the electrolytic polishing process, are inferior to those obtained according to the preferred embodiments of the present invention in fatigue resistance. Furthermore, springs obtained by A-4, A-5, A-6, F-4, F-5 and F-6, of which conditions are similar to those in the preferred embodiments of the present invention but the tempering conditions are not suitable, do not exhibit the sufficient fatigue resistance when they are too hard or soft. Springs obtained by A-9 and F-9, of which treatment conditions in each process are the same as those in the preferred embodiments of the present invention but the sequence of the processes are different, show problems in that they are inferior in fatigue resistance and difficult to be formed into springs. Springs obtained by B-2 and G-2, in which the hot coiling is carried out, and springs obtained by B-3 and G-3, in which the hot coiling is carried out and then the quenching is carried out at that temperature, all exhibit superior fatigue resistance if the same low-temperature tempering process and subsequent processes as those in the preferred embodiments of the present invention are adopted. It has been found from the above described EXAMPLE 1 and EXAMPLE 2 that a long useful life time of almost 10 8 as tested by the fatigue test at τ = 588.4 ± 441.3 MPa (60 ± 45 kg/mm 2 ) (the fatigue test at τ = 588.4 ± 490.3 MPa 60 ± 50 kg/mm 2 , for chromium-silicon steel wire) is obtained if a chromium-vanadium steel wire or a chromium-silicon steel wire is subjected to the cold or hot coiling and then quenching and tempering treatment to adjust its tensile strength to be greater than that of a chromium-vanadium steel oil-tempered wire, for use in a valve spring according to JIS G-3565, by about 10 %,or to be greater than the tensile strength of a chromium-silicon steel oil-tempered wire,for use in a valve spring according to JIS G-3566 ,by about 10 %,and the subsequent shot peening followed by the polishing treatment to give the surface roughness Rmax of 5µm or less. In addition, graphs showing the distribution of residual stress inside the coil after each process of F-1, which is the preferred embodiment of the present invention, and F-7, which is the conventional example, are shown in Fig. 3. In Fig. 3, a full line shows a longitudinal direction and a dotted line shows a tangential direction. It is found from Fig. 3 that in F-1 the residual stress before the shot peening is about ± 0 but in F-7 a residual tensile stress is remained in the longitudinal direction. Accordingly, it seems that a compressive residual stress in the longitudinal direction after the shot peening in F-7 is reduced as much as that and the fatigue resistance is deteriorated. On the other hand, it is found that in both F-1 and F-7 the compressive residual stress in a zone up to a depth of 20µm from the surface after the shot peening is smaller than that in a zone deeper than 20µm. Accordingly, it is found that the removal of the surfaces having the surface roughness of 20µm or less by the polishing treatment has no bad influence upon the fatigue resistances on the whole. In F-1 and H-1 in EXAMPLE 2 the thickness of the surface layer removed by the polishing treatment was 15µm and that in H-2 was 12µm. As above described, the spring obtained by the present invention exhibits remarkably superior fatigue resistance, so that it is very useful for purposes, such as valve springsfor use in car engines requiring reliability.

Claims: A method of producing a high-strength coil spring from a steel wire comprising 0.4 to 1.0 % by weight of C, 0.1 to 2.0 % by weight of Si, 0.4 to 1.2% by weight of Mn, 0.3 to 1.5 % by weight of Cr, 0.001 to 0.3 % by weight of V and the remainder of Fe and inevitable impurities, characterised in that the cleanliness of the steel is adjusted to 0.01 % or less and the steel is subjected to coiling to form it into an appointed spring shape, followed by a quenching and tempering treatment to adjust the tensile strength, and then to a shot peening treatment followed by a polishing treatment to give a surface roughness Rmax of 5µm or less. A method according to claim 1 characterised in that the coiling of the steel wire is carried out by cold forming. A method according to claim 1 characterised in that the coiling of the steel wire is carried out by hot forming. A method according to claim 1 characterised in that the steel wire is coiled at a temperature of 820°C or more and then subjected to the quenching treatment. A method according to claim 1, characterised in that the steel wire is heated to 820°C or more and then subjected to the coil forming at temperatures of 400 to 600°C followed by the quenching treatment.

Inventors: SHIBATA TAKESHI, YAMAMOTO SUSUMU, SHIBATA, TAKESHI, YAMAMOTO, SUSUMU, SHIBATA, TAKESHI C/O ITAMI WORKS OF SUMITOMO, Yamamoto, Susumu c/o Itami Works of Sumitomo
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: C21D9
IPC codes: C21D9