HIGH FATIGUE-STRENGTH STEEL WIRE AND SPRING, AND PROCESSES FOR PRODUCING THESE

Abstract: A steel wire of pearlite structure containing 0.8-1.0 mass % of C and 0.8-1.5 mass % of Si is disclosed. In the cross section of the steel wire the average hardness in a region up to 100 µm from the surface thereof is at least 50 higher that that in a deeper region based on micro-Vickers hardness. The steel wire is manufactured by working a wire rod having the abovementioned chemical composition through shaving, patenting and drawing processes, then strain-relief annealing the resultant wire, and thereafter subjecting the thus annealed wire to a short peening process. The steel wire can be produced through a drawing process without applying a quenching and tempering process, and are superior in heat resistance and fatigue strength.

Description: TECHNICAL FIELD The present invention relates to a steel wire and spring having superior fatigue properties and to a method of manufacturing such a steel wire and spring. BACKGROUND ART Spring steel wires containing 0.6-0.8 mass % of C, 0.15-0.35 mass % of Si, and 0.3-0.9 mass % of Mn are known in the art. Such a steel wire is manufactured by being processed through steps of rolling → patenting (heating for γ-phase transition → isothermal transformation) → wire drawing → (coiling: when to be worked into springs) → strain relief annealing (at 300 ± 30 °C). EP-B-0218167 discloses a high tensile strength drawn steel wire having excellent ductility, comprising a steel containing 0.4 to 1.0 wt.-% of C, 2.0 wt.-% or less of Si, 0.2 to 2 wt.-% of Mn, 0.02 wt.-% or less of P, 0.02 wt.-% or less of S and 0.01 wt.-% or less of N, and optionally further containing: at least one ingredient selected from the group consisting of 0.05 to 3 wt.-% of Cr, 0.01 to 1 wt.-% of Mo, 0.01 to 1 wt.-% of W, 0.05 to 3 wt.-% of Cu, 0.1 to 5 wt.-% of Ni and 0.1 to 5 wt.-% of Co, and/or at least one ingredient selected from the group consisting of Al, Ti, Nb, V, Mg and Ca in an amount of 0.001 to 0.1 wt.-%, respectively, and 0.0003 to 0.05 wt.-% of B, balance Fe and unavoidable impurities, wherein said steel has a strength σ of between (0.05 σ + 23) and (0.35 σ + 28) kgf/mm 2 on the surface thereof, depending on its strength σ. However, it is rather difficult to say that such spring steel wires as mentioned above are satisfactory neither in thermal resistance nor in fatigue strength. Meanwhile, it is known in various steel wires including parallel wire that thermal resistance may be improved by increasing the Si content. In this respect, however, the purpose of using steel wires having a good thermal resistance varies with their specific uses, the thermal resistance for the case of parallel wire essentially aims at limiting the change in tensile strength (TS) of the wire small when subjected to galvanization (at 450 °C for 30 seconds). On the other hand, in the case of those springs associated with automobile engines for which the steel wire of the present invention is intended, important considerations include keeping the permanent set in the temperature range of about 100-200 °C small and at the same time providing desired fatigue properties. Thus, simply applying a chemical composition of such a parallel wire to a spring wire cannot bring forth satisfactory properties sufficient for a spring material. That is to say, while the Si addition in a parallel wire is reportedly said to be effective in improving its fatigue properties, this is mere a story of fatigue under repeated tension, which differs essentially from the fatigue properties required for a spring material. It has been shown that a decrease in surface hardness greatly affects the fatigue properties in a spring steel wire having a high Si content, although its influence on the fatigue properties is small in a parallel wire. It is also known that a steel wire superior in both thermal resistance and fatigue strength (oil-tempered wire) can be obtained by applying quenching and tempering in the final stage of the steel wire manufacture, such a quenching and tempering process adds to the cost. Accordingly, it is a primary object of the present invention to provide a steel wire and spring having a high thermal resistance and a high fatigue strength that can be produced without applying a quenching and tempering process, namely, produced through a drawing process and a method of manufacturing such a steel wire and spring. DISCLOSURE OF THE INVENTION The present invention provides a steel wire comprising a pearlite structure containing 0.8-1.0 mass % of C and 0.8-1.5 mass % of Si, wherein in the cross section of the steel wire the average hardness in an outer region up to 100 µm from the surface thereof is at least 50 higher than that of a deeper region based on micro-Vickers hardness. This steel wire has a high thermal resistance and fatigue strength, and is particularly suited for spring steel wire. Particularly, it is preferable that the deeper region have an average hardness of 500 or above with the outer region having an average hardness at least 150 higher than that of the deeper region based on micro-Vickers hardness. Preferably, the steel wire may further contain 0.03-0.1 mass % of Mo. Further, it may contain 0.3-0.9 mass % Mn and/or 0.2 mass % or less Cr. For providing a sufficient fatigue strength, this steel wire preferably has a tensile strength above 1,900 N/mm 2 . In addition, it is preferable the steel wire have a residual surface compression stress of 300 MPa or above. Further, a method of manufacturing the steel wire according to the present invention is characterized by comprising the steps of: shaving a steel wire of pearlite structure containing 0.8-1.0 mass % of C and 0.8-1.5 mass % of Si; patenting the resultant steel wire, and drawing the patented steel wire; processing the resultant drawn steel wire through a strain relief annealing at 350-450 °C; subsequently subjecting the thus processed steel wire to a shot peening process. This method of manufacture can produce the steel wire of the present invention without resorting to a quenching and tempering process, and can produce a steel wire having a high thermal resistance and fatigue strength at low cost. For working the steel wire into a spring according to the present invention, a coiling process may be interposed between the drawing and strain relief annealing processes mentioned above. It may also be preferred to provide a nitriding process subsequent to the strain relief annealing. Further, it may be preferable to provide a secondary strain relief annealing at around 250 °C after the above-described shot peening or following the nitriding and the succeeding shot peening processes. Hereinafter, the aforementioned features of the present invention will be discussed further in detail. Chemical composition C: The lower limit of the C content was determined based on the fatigue strength, while its upper limit was determined based on the wire drawability. Si: Si is a chemical element essentially required for improvement of thermal resistance. With its content lower than the previously mentioned lower limit no sufficient thermal resistance will be achieved, while the resultant steel wire becomes susceptible to surface flaws if the Si content is higher than its upper limit. Mo: With an Mo content lower than its lower limit described above it will have a smaller effect on the improvement in the thermal resistance and fatigue strength of the steel wire, while its content exceeding the upper limit will elongate the time required for patenting, resulting in a lowered productivity. Mn: Mn is added for improving the quench hardenability of steel wire. Mn content exceeding the upper limit tends to increase segregation and lowers wire drawability. Cr: The aforementioned upper limit is determined, because a longer patenting time becomes required with a Cr content exceeding that level, thus resulting in a lowered productivity. Shaving A purpose of the shaving process is to remove a low hardness layer on the surface of steel wire. The fatigue properties are improved by removing those outer layers having a micro-Vickers hardness at least 50 lower than that of the inner portion of steel wire. Strain relief annealing The strain relief annealing process is applied at 350-450 °C for improving the fatigue properties of resulting springs. By annealing at temperatures in this range, strains of the steel wire caused in the course of its drawing and coiling processes can be effectively removed. Such high temperatures to which the steel wire is exposed during its strain relief annealing does not lower the strength of the resultant steel wire because of its Si content. An annealing temperature below the lower limit has only a little effect on fatigue properties improvement, while the strength and fatigue strength of wire both decrease if the annealing temperature exceeds its upper limit. A preferable annealing time may be about 20 minutes in view of effects and productivity. Shot peening To secure a high fatigue strength, a spring wire requires a high surface hardness and a large compression stress. Since the strain relief annealing substantially removes strains from the steel wire, it becomes easier for a shot peening process to impart a stress to the wire in process, and thus the resulting steel wires and springs can have excellent fatigue strength. Nitriding When subjected to nitriding for imparting a residual stress, the prior art piano wires will have a decreased strength in its matrix structure and therefore such piano wires cannot have a sufficient residual stress even when treated through nitriding and shot peening. Since the steel wire with an increased Si content according to the present invention has an improved heat resistance and undergoes only a small reduction in matrix strength, the compression stress imparted can effectively contribute to the improvement of fatigue strength. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing a relation between strain relief annealing temperature and fatigue limit amplitude stress. Figure 2 is a graph showing a hardness distribution across the cross section of each steel wire; Figure 3 is a graph showing a relation between Si content and fatigue limit amplitude stress; Figure 4 is a graph showing a relation between hardness distribution over wire cross section and varied shooting conditions in shot peening; Figure 5 is a graph is a graph showing a hardness distribution across the cross sections of steel wires worked through nitriding and/or shot peening; Figure 6 is a graph showing a relation between Si contents and fatigue limit amplitude stresses of steel wires worked through nitriding and/or shot peening. THE BEST MODE FOR CARRYING OUT THE INVENTION Experimental example 1 Ingots weighing 100kg having chemical compositions shown in Table 1 were melt-cast in a vacuum melting equipment, respectively, and the resultant cast products were worked through hot-forging and rolling into wire rods of 11 mm, respectively. Chemical composition (mass%) C Si Mn Cr Shot peening Preferred example 1 0.82 1.05 0.51 0.09 With Comparative example 1 0.82 1.05 0.51 0.09 Without Comparative example 2 0.82 0.21 0.50 0.09 With The resultant wire rods were shaved to remove surface layers to 10 mm and then the shaved wire rods were subjected, under the conditions given below, to patenting, drawing, and strain relief annealing to be worked into steel wires of pearlite structure. Patenting: 950 °C → lead bath at 580 °C Drawing: 10 mm → 4 mm Strain relief annealing: at 300 °C, 350 °C, 400 °C, 450 °C, 500 °C for 20 min. each In this experiment, three types of steel wire specimens were prepared, among which the steel wire specimens of the preferred example 1 and comparative example 1 were made from a material having the same chemical composition, but the latter example specimens were not worked through shot peening, while the comparative example 2 specimens from a material containing a significantly smaller amount of Si were prepared by being worked through shot peening as the preferred example 1, as shown in Table 1 above. These three types of steel wire specimens were further subjected to a secondary strain relief annealing (at 250 °C for 20 minutes). The shot peening was performed for 20 minutes using 0.3 mm equi-sized steel balls. Then, the resultant specimens were subjected to a fatigue test on a Nakamura's rotating bending fatigue tester with the withstanding minimum fatigue threshold being set at 10 7 times of bending stress application. The test results of which are given in Figure 1. As can be seen in Figure 1, the steel wires of the preferred example 1 worked through shot peening exhibit a superior fatigue strength with their highest fatigue limit amplitude stresses among others. The comparative example 1 specimens prepared without shot peening and the comparative example 2 specimens with a low Si content prepared with shot peening both show inferiority in fatigue strength. Meanwhile, strain relief annealing yields a satisfiable result in the temperature range of 350 to 450 °C. Then, hardness distribution across the cross section was also determined for each of these three types of steel wires. Regarding the hardness measurement, the strain relief annealing temperature was set at 400 °C for specimens of the preferred example 1 and comparative example 1, and at 300 °C for the comparative example 2 specimens. The test results of which are given in Figure 2. As shown in Figure 2, the comparative example 1 specimens for which the shot peening was omitted had a lower hardness at a region close to the surface, while the specimens of the preferred example 1 and comparative example 2 had a higher hardness at their corresponding surface regions. In addition, the preferred example 1 specimens showed a generally higher hardness as compared with the comparative example 2 specimens across their cross sections. In particular, as can be seen in Figure 2, the preferred example 1 specimens had an average hardness of 675 Hmv (micro-Vickers hardness) in a region up to 100 µm from the surface, with their more inner regions keeping an hardness of 620 Hmv, which was relatively high as compared with the prior art similar steel wires represented by the comparative example 1. Besides, the steel wire specimens of the respective examples above had the following tensile strengths: Preferred example 1: 2,140 N/mm 2 Comparative example 1: 2,130 N/mm 2 Comparative example 2: 1,960 N/mm 2 Experimental example 2 In the next place, the same procedure as in the aforementioned preferred example 1 was repeated by using varied C and Si contents, and the resultant steel wires were subjected to a fatigue test in the same manner as in the experimental example 1 above. In this experiment, the strain relief annealing was performed at 300 °C for 20 minutes for specimens with a 0.2 mass % Si content, and at 400 °C for 20 minutes for other specimens. The test results are given in Figure 3. In Figure 3, the curve marked with (×) indicates a failure of experiments, in which the process experienced so frequent occurrence of flaws in process that substantially no specimens could be prepared and thus no fatigue test performed. As can be understood from the graph of Figure 3, preferable C and Si contents range from 0.7 to 1.0 mass % and 0.8 to 1.5 mass %, respectively. Experimental example 3 Further, in the same manner as in the preferred example 1 of the aforementioned experimental example 1 (except for the strain relief annealing conditions being limited only to 400 °C for 20 minutes), 4 types of steel wire specimens were prepared under 4 varied shot peening conditions, respectively, and the resultant specimens were tested for hardness distribution across their cross section. The shot peening conditions were varied by changing the shot peening material used and/or shot peening time. The test results are shown in Figure 4. As can be seen in the graph of Figure 4, the shot peening applied could produce steel wire specimens in which in their cross sections the average hardness in an outer region up to 100 µm from the surface thereof was at least 50 higher than that of an inner region based on micro-Vickers hardness. The specimens tested had the following fatigue limit amplitude stresses, respectively: Specimen A: 575 N/mm 2 Specimen B: 590 N/mm 2 Specimen C: 660 N/mm 2 Specimen D: 690 N/mm 2 Experimental example 4 Materials having chemical compositions given below were subjected to the same processes as in the previously described experimental example 1 up to strain relief annealing step (except for the strain relief annealing conditions being limited only to 400 °C for 20 minutes), respectively, and then worked through the respective corresponding processes into steel wire specimens (of the preferred examples 2, 3 and comparative example 3), the resultant specimens being tested for hardness distribution across their cross sections. Preferred example 2 Shot peening followed by secondary strain relief annealing Chemical composition: C: 0.82, Si: 1.35, Mn: 0.51, Cr: 0.09 mass % Preferred example3 Nitriding followed by shot peening and secondary strain relief annealing Chemical composition: C: 0.82, Si: 1.35, Mn: 0.51, Cr: 0.09 mass % Comparative example 3 Nitriding followed by shot peening and secondary strain relief annealing Chemical composition: C: 0.82, Si: 0.21, Mn: 0.50, Cr: 0.09 mass % The shot peening and the secondary strain relief annealing were performed under the same conditions as in the experimental example 1 above, with nitriding being conducted at 450 °C for 2 hours. The test results are shown in Figure 5. As can be seen in the graph of Figure 5, the preferred example 2 specimens had in a region within 100 µm from the surface a surface hardness approximately 55 Hmv higher than that in an inner region across the cross section, while in the preferred example 3 specimens the surface hardness was approximately 150 Hmv higher than the hardness in its inner region. In addition, the preferred example 3 as well as 2 had in the inner region an average hardness above approximately 520 Hmv, which was relatively high as compared with similar steel wires of the prior art. In contrast, the comparative example 3 specimens underwent a substantial reduction in strength by being exposed to high temperatures in the nitriding step, consequently having a low inner region hardness of approximately 470 Hmv with substantially lower surface hardness as compared with any of above two preferred examples. Further, the same procedure as in the aforementioned preferred example 3 was repeated by using varied C and Si contents, and the resultant steel wires were subjected to a fatigue test to determine their fatigue limit amplitude stresses. The test results are given in Figure 6. As can be seen in the graph of Figure 6, the fatigue limit amplitude stress increases with C and Si contents. However, Si content of 2.0 mass % resulted in an experiment failure because of frequent occurrence of flaws in process. It can be also seen that the fatigue limit amplitude stress decreases significantly if Si content goes down below 0.5 mass %. In the next place, the specimens of the aforementioned preferred examples 2, 3 and the comparative examples were tested for their residual surface compression stresses. The test results are shown given in Table 2 below along with hardness in their surface and inner regions. Residual stresses (MPa) Central hardness of the examples (Hmv) Surface hardness of the examples (Hmv) Structure Preferred example 2 -620 618 660 Pearlite Preferred example 3 -780 522 720 Pearlite Comparative example 3 -430 470 565 Pearlite As shown in Table 2 above, both the preferred examples 2 and 3 have a high residual surface compression stress to achieve superior fatigue strength and thus are best suited for spring steel wires. INDUSTRIAL APPLICABILITY OF THE INVENTION As fully described hereinbefore, the steel wire according to the present invention provided with a high heat resistance and a high fatigue resistance may be effectively used for spring wires. Particularly, the steel wire of the present invention is best suited for springs associated with automobile engines. Besides above, the steel wire of the present invention may be used for stranded PC steel wires, control cables, steel cords, and parallel wires, etc.

Claims: A highly fatigue-resistant steel wire comprising a pearlite structure containing 0.8 - 1.0 mass % of C and 0.8 - 1.5 mass % of Si, wherein in the cross section of the steel wire an average hardness in a region up to 100 µm from the surface thereof is at least 50 higher than that of an inner region based on micro-Vickers hardness. The highly fatigue-resistant steel wire according to claim 1, wherein in the cross section of the steel wire an average hardness in said region up to 100 µm from the surface thereof is at least 150 higher than that of said inner region based on micro-Vickers hardness. The highly fatigue-resistant steel wire according to claim 1, wherein a tensile strength is approximately 1,900 N/mm 2 or above. The highly fatigue-resistant steel wire according to claim 1, wherein a residual surface compression stress is approximately 300 MPa or above. The highly fatigue-resistant steel wire according to claim 1, wherein the average hardness in said inner region is approximately 500 or above based on micro-Vickers hardness. A spring manufactured from a steel wire as set forth in any one of the preceding claims 1 through 5. A method of manufacturing a highly fatigue-resistant steel wire comprising the steps of: shaving a steel wire of pearlite structure containing 0.8 - 1.0 mass % of C and 0.8 - 1.5 mass % of Si; patenting the resultant steel wire, and drawing the patented steel wire; processing the resultant drawn steel wire through a strain relief annealing at 350 - 450°C; and subsequently subjecting the thus processed steel wire to a shot peening process. The method according to claim 7, wherein there is further provided a step of nitriding succeeding to said strain relief annealing. A method of manufacturing a spring comprising the steps set forth in claim 7, wherein there is further provided a step of coiling as interposed between said steps of drawing and strain relief annealing.

Inventors: KAWABE NOZOMU, MURAI TERUYUKI, KAWABE, NOZOMU, MURAI, TERUYUKI, KAWABE, NOZOMU ITAMI WORKS, MURAI, TERUYUKI ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1998
Main classification: C22C38
IPC codes: C21D9, C22C38, B21F35, C21D7