Two-phase stainless steel wire rope having high fatigue resistance and corrosion resistance

Abstract: The invention relates to a two-phase stainless steel wire rope having a high fatigue resistance and a high corrosion resistance comprising two-phase stainless steel wires of 0.1 % by weight or less of C, 1.0 % by weight or less of Si, 1.5 % by weight or less of Mn, 0.04 % by weight or less of P, 0.03 % by weight or less of S, 18.0 to 30.0 % by weight of Cr, 3.0 to 8.0 % by weight of Ni, 0.1 to 3.0 % by weight of Mo and the balance of Fe, wherein the volume ratio of ferrite is 30.0 to 80.0 % and the wires are controlled to have a mean slenderness ratio (M R value) of 4 to 20 by wire drawing. The wire rope of the invention is suitable for dynamic uses such as a rope for an elevator or a skilift.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a two-phase stainless steel wire rope having a high fatigue strength and a high corrosion resistance. 2. Description of the Prior Art: In the field of wire ropes, hitherto wire ropes made of stainless steel such as SUS 304 and SUS 316 have been used in a very limited application field for static uses such as simply hanging an article, etc., as they are thought to be inappropriate for so-called dynamic use , since a characteristic of high corrosion resistance cannot be sufficiently taken advantage of due to a low fatigue resistance, which shortens the durability and causes a wire breakage in a short time when it is frequently exposed to repetitive bending. On the other hand, a high carbon steel wire rope, in contrast with the stainless steel wire rope, is used as wire rope for dynamic use as well as that for static use, because it has a high fatigue strength and provides a long durability against repetitive bending as well, and exclusive use of the high carbon steel wire rope is legally specified even for important security members such as an elevator rope which human life relies upon. However, the high carbon steel wire rope, in contrast with the stainless steel wire rope, has a disadvantage of inferior corrosion resistance, and thereby, the fatigue strength may be significantly lowered due to occurrence of corrosion pits even in the atmospheric air, if the corrosion prevention is not sufficient. SUMMARY OF THE INVENTION As described above, it is widely known that the stainless steel wire rope is superior in corrosion resistance but shorter in life, while the high carbon steel wire rope is longer in life but inferior in corrosion resistance, hence, in the light of such actual conditions, the invention has been achieved, and it is an object thereof to double the safety and quality assurance capability for dynamic use by providing a durable stainless steel wire rope which is considerably superior in both fatigue durability and corrosion resistance. In order to achieve the above object, the invention is constituted as follows. The invention presents a two-phase stainless steel wire rope having a high fatigue resistance and a high corrosion resistance comprising two-phase stainless steel wires of 0.1 % by weight or less of C, 1.0 % by weight or less of Si, 1.5 % by weight or less of Mn, 0.04 % by weight or less of P, 0.03 % by weight or less of S, 18.0 to 30.0 % by weight of Cr, 3.0 to 8.0 % by weight of Ni, 0.1 to 3.0 % by weight of Mo and the balance of Fe, wherein the volume ratio of ferrite is 30.0 to 80.0 % and the wires are controlled to have a mean slenderness ratio (M R value) of 4 to 20 by wire drawing with a reduction of area between 40 and 97 %. In order to achieve higher yield strength and fatigue strength, the said wire rope is further subjected to aging treatment at the temperature of 150 to 600°C for a minute to an hour. The present invention has been completed based on a conventionally unknown novel finding that repetitive bending fatigue strength of a wire rope fabricated by stranding two phase stainless steel wires of the above range in chemical composition, which are drawn and finished in a predetermined diameter, has a close relation with the phase balance indicated by a content ratio of ferrite phase to austenite phase of the two-phase stainless steel wire as well as with the reduction of area by drawing indicated by the slenderness ratio of the individual phase, and further that yield strength at 0.2 % and repetitive bending fatigue strength of the wire rope have a close relations with the aging treatment. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a magnified view showing structure of a two-phase stainless steel wire. Fig. 2 shows a relation between the reduction of area by drawing (%) and mean slenderness ratio M R of the two-phase stainless steel wire. Fig.3 shows a relation between 0.2 % yield strength of a two-phase stainless steel wire with the volume ratio of ferrite (α) at 50 % and the aging temperature , with a reduction cf area as a parameter. Fig. 4 shows a relation between the mean slenderness ratio M R and the number of bending repeated until the wire breakage ratio comes to be 10%, with the volume ratio of ferrite in a stainless steel wire rope taken as a parameter, and also with comparison between those with aging treatment and without aging treatment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with respect to the accompanying drawings. Fig. 1 is a magnified view showing the structure of two-phase stainless steel wire. Numeral 1 shows grain boundary. In a two-phase structure of austenite phase 3 and ferrite phase 2 coexisting as shown in Fig. 1, regarding the slenderness ratio of the phases, the slenderness ratio γ R of austenite and slenderness ratio α R of ferrite are expressed as γ R =γ L /γ W and α R =α L /α W respectively. As the phases are mutually mixed up to present a two-phase structure, it is considered that a characteristic observed as a whole material is obviously related to the mean value of them, thus, the mean slenderness ratio M R can be expressed as M R =V r ·γ R + V a ·α R . Where V r is the volume ratio of austenite and V a is the volume ratio of ferrite. In Fig. 2, a relation between the reduction of area by drawing (%) and the mean slenderness ratio M R of the two-phase stainless steel wire is graphically shown. As shown in the figure, although the mean slenderness ratio M R is valued at 1 due to isometric crystals before wire drawing, it increases approximately in linear function upon wire drawing because each phase is slenderly stretched in the drawing direction. Fig.3 is a graph showing the characteristic of age-hardening of two-phase stainless steel wire with the volume ratio of ferrite (α) at 50 %. This graph shows that the 0.2 % yield strength increases considerably at the temperature of 150 to 600 deg. C. , and also shows that 40 % or more of the reduction of area is necessary to obtain yield strength for practical use. This tendency is the same irrespective of the volume ratio of ferrite . It was thus found by the inventors, as a result of repeated experiments, that the repetitive bending fatigue strength has an obvious relation with the M R and volume ratio of ferrite. It was also found out that the said fatigue strength is affected by the aging treatment. In Fig. 4, a relation between the mean slenderness ratio M R of stainless steel wire rope and the number of bending repeated until the breakage ratio comes to 10% is shown graphically with the volume ratio of ferrite taken as a parameter. Curves 1 to 6 show the products with the volume ratios of ferrite of 10%, 20%, 30%, 50%, 80% and 85% respectively. Curves 1' to 6' show the products with the volume ratios of ferrite of 10%, 20%, 30%, 50%, 80% and 85% respectively and with aging treatment at the temperature of 400 deg. C. for each of them. Lines 10 and 20 show the longevity level of stainless steel wire rope and high carbon steel wire respectively. In other words, although an SUS304 austenite stainless steel rope and a high carbon steel rope are compared with regard to the longevity level in Fig. 4, it is recognized that the stainless steel wire rope having an M R value of 4 to 20 and a structure of 30 to 80% in ferrite amount and the wire rope further subjected to aging treatment show a higher values than high carbon steel wire rope which is said to have a long life. This is a novel finding that has never been recognized before. Additionally, as understood clearly from the figure, under the conditions that M R is less than 4 or more than 20 and the ferrite amount is less than 30% or more than 80%, the life is shortened. Moreover, Fig. 3 shows that the enforcement of agehardening is preferable at the temperature of 150 to 600 deg. C., because below 150 deg. C. the increase of yield strength is slight, and above 600 deg. C. softening occurs. And the time of aging treatment from one minute to 1hr.is preferable, because the long aging treatment will increase costs in view of economy. Hence, from Fig. 2, the fact that a longer fatigue life is obtained at M R of 4 to 20 means that it is required to limit the reduction of area by drawing at 40 to 97%. Moreover, as this two-phase stainless steel wire rope contains 18 to 30% Cr and 0.1 to 3.0% Mo, the superior corrosion resistance is obvious, thereby enabling a completion of wire rope having a uniquely high corrosion resistance that has never been found in the prior art. Succeedingly, each element contained is described below: C: As a large amount of C facilitates an inter-granular precipitation of carbide in the process of rapid cooling down from 1050 deg. C., and deteriorates the corrosion resistance, it is required to be limited at 0.1 % or less, preferably between 0.01 and 0.1% by weight. Si: Although Si is a deoxidizing element and an appropriate content is required, as a large amount renders the steel structure brittle, it is required to be limited at 1% or less, preferably between 0.2 and 1.0% by weight. Mn: Although Mn is a desulfurizing element and an appropriate content is required, as a large amount causes a significant hardening of the material in process and sacrifices workability, it should be 1.5% or less, preferably between 0.5 and 1.5% by weight. P: For normal melting, it should be reduced to the economically attainable level of 0.04% or less, preferably between 0.01 and 0.04% by weight. S: For the same reason as above, it should be 0.03% or less, preferably between 0.002 and 0.03% by weight. Cr: The corrosion resistance is inferior at 18% or less of Cr, while with the content of Cr exceeding 30% the hot workability is deteriorated and it is not economical. When the Cr content is excessively high in forming the two-phase composition, an increased amount of Ni is required to be added for balancing of the phases, which is another disadvantage. Thus, it should be limited at 18 to 30% by weight. Ni: In order to achieve the two-phase composition, 3 to 8% by weight of Ni corresponding to the Cr content as specified above is required. Mo: At 0.1% by weight, the corrosion resistance is improved, and, although the effect is enhanced significantly as the content is increased, 3% by weight is sufficient because it is an expensive element. Summarizing the above points, a two-phase stainless steel wire containing 0.1% by weight or less of C, 1.0% by weight or less of Si, 1.5% by weight or less of Mn, 0.04% by weight or less of P, 0.03% by weight or less of S, 18.0 to 30.0% by weight of Cr, 3.0 to 8.0% by weight of Ni, 0.1 to 3.0% by weight of Mo and the balance of Fe, and a volume ratio 30.0 to 80.0% of ferrite, which is controlled to have a mean slenderness ratio (M R value) of 4 to 20 with wire drawing rate between 40 and 97% reduction of the cross-sectional area, represents the essential requirements for the invention. Moreover after stranding and closing the above two-phase stainless steel wire, enforcing the aging treatment at the temperature at 150 to 600 deg. C. is the essential requirement for the invention. In order to clarify specific effects of two-phase stainless steel wire rope according to the invention, a property comparison was performed with reference ropes. In other words, five types of two-phase stainless steel having different volume ratio of ferrite ranging from 20 to 85% were rolled to 5.5 mm diameter wire materials and finished to a final wire diameter of 0.33 mm by repetitive intermediate drawings and intermediate annealings, then stranded finally into wire ropes having a structure of 7 x 19 and an outer diameter of 5 mm. In this case, the temperatures of intermediate annealing and annealing before the final wire drawing were both set at 1050 deg. C. The M R values were also changed by changing the reduction of area by drawing in each steel type to 30, 50, 70, 90 and 98.5%. Therefore, the intermediate wire diameter before final drawing is different in each process. The wire drawing was performed by using a conical type cone pulley wire drawing machine, drawing 3 to 20 times depending on the reduction of area by drawing, at the drawing speed of 100 to 350 m/min. And moreover the above rope with an outer diameter of 5 mm is subjected to aging treatment at the temperature of 100, 400, 650 deg. C. respectively. Conventional SUS304 rope materials for comparison were also processed by the same method to obtain a final wire diameter of 0.33 mm, and stranded to form a wire rope having a structure of 7 x 19 and an outer diameter of 5 mm. The annealing temperature of SUS304 is 1150 deg. C. On the other hand, a conventional high carbon steel wire rope was fabricated by repetitive intermediate wire drawings and salt patentings to obtain a final wire diameter of 0.33 mm as described above and stranding to form a wire rope having a structure of 7 x 19 and an outer diameter of 5 mm. The composition, mean slenderness ratios (M R value) and the load at breakage of these wire ropes are shown in Table 1 below. These wire ropes were further exposed to a repetitive bending fatigue test. In this repetitive bending fatigue test, a load (P) applied to a sample wire was set at 20% of the load at breakage of wire rope to obtain a relation between the number of repetitive passages along half the circumference of a test sheave portion with D/d at 40 (wherein, D: diameter of the sheave groove and d: diameter of the rope) and the number of wire breakages, and the life of the rope is defined as the number of repetitions when the number of wire breakages observed came to be 10% of the total number of wires in the rope. The result is shown in Table 2 below. In Table 2, fatigue durabilities corresponding to the ropes shown in Table 1 and the time to rust occurrence by 3% NaCl salt water spray test are shown respectively. As seen from Table 2, it is recognized that, with the volume ratio of ferrite at 30 to 80%, the wire drawing work limited at 40 to 97% , M R value controlled to be 4 to 20 and the aging treatment at the temperature between 150 and 600 deg. C., a two-phase stainless steel wire rope of the present invention is obtained, wherein not only the fatigue life at 10% wire breakage exceeds that of a high carbon steel wire rope which is said to be presently the longest in said fatigue life and superior in reliability, but also the time to rust occurrence is longer than SUS304, showing a very superior corrosion resistance. On the other hand, in the cases of rope A of less than 30% in volume ratio of ferrite and rope E of 85% or more, although the corrosion resistance shows a value equal to or more than that of SUS304, the fatigue life is inferior to the high carbon steel wire rope even when M R value is between 4 and 20. Obviously, this is an example that cannot be included in the invention. As described herein, since the rope according to the invention shows a very long fatigue life and a high corrosion resistance, it can be sufficiently used as the wire rope for dynamic use as in an elevator or in a skilift to which application of a conventional stainless steel rope has been prohibited. Thus, needs for such two-phase stainless steel rope will undoubtedly increase in a very wide range including application fields of both conventional stainless steel rope and high carbon steel rope, and the invention, thus, has an outstandingly superior effectiveness.

Claims: A two-phase stainless steel wire rope having a high fatigue resistance and a high corrosion resistance comprising two-phase stainless steel wires of 0.1 % by weight or less of C, 1.0 % by weight or less of Si, 1.5 % by weight or less of Mn, 0.04 % by weight or less of P, 0.03 % by weight or less of S, 18.0 to 30.0 % by weight of Cr, 3.0 to 8.0 % by weight of Ni, 0.1 to 3.0 % by weight of Mo and the balance of Fe, wherein the volume ratio of ferrite is 30.0 to 80.0 % and the wires are controlled to have a mean slenderness ratio (M R value) of 4 to 20 by wire drawing. The wire rope as set forth in claim 1 which is further subjected to aging treatment at the temperature of 150 to 600°C. A method of fabricating a wire for two-phase stainless steel wire rope having a high fatigue resistance and a high corrosion resistance, wherein the two-phase stainless steel wire of 0.1 % by weight or less of C, 1.0 % by weight or less of Si, 1.5 % by weight or less of Mn, 0.04 % by weight or less of P, 0.03 % by weight or less of S, 18.0 to 30.0 % by weight of Cr, 3.0 to 8.0 % by weight of Ni, 0.1 to 3.0 % by weight of Mo and the balance of Fe, having a volume ratio of ferrite of 30.0 to 80.0 % is drawn at a rate of 40 to 97 % reduction of cross-sectional area after annealing to obtain a mean slenderness ratio (M R value) of 4 to 20. A method of fabricating the wire as set forth in claim 3, wherein said wire is further subjected to an aging treatment at the temperature of 150 to 600°C. The wire rope as set forth in claim 1 or 2, wherein C is present in an amount of 0.01 to 0.1 % by weight, Si is present in an amount of 0.2 to 1.0 % by weight, Mn is present in an amount of 0.5 to 1.5 % by weight, P is present in an amount of 0.01 to 0.04 % by weight, and S is present in an amount of 0.002 to 0.03 % by weight. The use of two-phase stainless steel wires of 0.01 to 0.1 % by weight of C, 0.2 to 1.0 % by weight of Si, 0.5 to 1.5 % by weight of Mn, 0.01 to 0.04 % by weight of P, 0.002 to 0.03 % by weight of S, 18.0 to 30.0 % by weight of Cr, 3.0 to 8.0 % by weight of Ni, 0.1 to 3.0 % by weight of Mo and the balance of Fe, wherein the volume ratio of ferrite is 30.0 to 80.0 % which are controlled to have a mean slenderness ratio (M R value) of 4 to 20 by wire drawing in the manufacture of a steel wire rope for dynamic applications such as a rope for an elevator and a rope for a ski lift.

Inventors: MASUTANI HIROSHI, TAMAI KISHIO, YAMAOKA YUKIO, MASUTANI, HIROSHI, TAMAI, KISHIO, YAMAOKA, YUKIO
Applicants: SHINKO WIRE COMPANY LTD ALSO K, SHINKO WIRE COMPANY, LTD., ALSO KNOWN AS SHINKO KOSEN KOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: C22C38
IPC codes: C22C38