Patent Grant
7150800
The present invention relates to a ring-rolling method of manufacturing a continuously variable transmission belt from a metastable austenitic stainless steel strip.
Such a material with high strength as 18% Ni-maraging steel has been used so far for a continuously variable transmission belt. A metastable austenitic stainless steel is sometimes used for the purpose, as disclosed in JP 2000-63998A. The continuously variable transmission belt is conventionally manufactured by the following steps: A steel strip is formed to a ring shape by plasma- or laser-welding its front and tail ends together. The welded steel strip is heat-treated to eliminate a hardness difference between base and welded parts and smoothened at its edge by barreling. The steel strip is then ring-rolled to a predetermined thickness and stretched to a predetermined circumferential length. Thereafter, the steel strip is nitrided and aged so as to harden its surface layer.
The manufactured steel belt is subjected to a rotation-tensile fatigue test or the like for evaluation of fatigue properties. 18% Ni-maraging steel, which is strengthened by work-hardening and aging (strain-aging), has excellent fatigue properties due to a hard nitrided surface layer and effects of cold-working on mechanical properties. However, 18% Ni-maraging steel is scarcely work-hardened due to its large deformation resistance, so as not to anticipate an increase of strength derived from work-hardening even by ring-rolling with a heavy duty. The heavy-duty rolling often causes damages of a steel strip during rolling, when the steel strip lacks of ductility.
A metastable austenitic stainless steel is also a kind of steel, which is work-hardened or strain-aged by cold-rolling. Its strength is remarkably improved by formation of strain-induced martensite and work-hardening of residual austenite in comparison with 18% Ni-maraging steel, but its strengthening rate is varied in correspondence to a material temperature during rolling. Heat generation and dissipation during rolling put significant effects on mechanical properties of a rolled steel strip or belt. In this consequence, a steel belt manufactured by ring-rolling has thickness, width and cross-sectional hardness deviated in response to a manufacturing season.
In short, it is difficult to manufacture a steel belt, which has stable material strength necessary for use as a continuously variable transmission belt. The difficulty is somewhat caused by mechanical properties of the metastable austenitic stainless steel.
An object of the present invention is to manufacture a steel belt, which has stable properties necessary for a continuously variable transmission, from a metastable austenitic stainless steel strip by ring-rolling the steel strip under properly controlled conditions.
According to the present invention, a metastable austenitic stainless steel strip is used as a material of a continuously variable transmission belt. The metastable austenitic stainless steel strip preferably has a value Md(N) controlled within a range of 20–100, wherein the value Md(N) is determined by a chemical composition of the steel according to the formula of:
Md(N)=580−520C−2Si−16Mn−16Cr−23Ni−300N−10Mo.
After the steel strip is formed to a ring shape by welding its front and tail ends together, it is ring-rolled under the condition that a relationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87 is established among a material temperature T (° C.), an equivalent strain ε and the value Md(N). The equivalent strain ε is represented by the formula of ε=√{square root over (4(1n(1−R))2/3)}, wherein R is a reduction ratio. A temperature of a rolling atmosphere or a surface temperature of the steel strip at a position just before a work roll may be used as the material temperature T. Furthermore, when the steel strip is ring-rolled under the condition that a fluctuation ΔT(° C.) of the material temperature T is confined within a range of ±6.4° C., a rate of strain-induced martensite is controlled to a predetermined value with a tolerance of 5 vol. %.
When a metastable austenitic stainless steel strip is cold rolled, it is strengthened by formation of strain-induced martensite and work-hardening of residual austenite. A rate of strain-induced martensite is varied in response to a temperature and a reduction ratio R during cold-rolling as well as a value Md(N). For instance, formation of strain-induced martensite is intensified as falling of the rolling temperature with the provision that the value Md(N) and the reduction R are constant, resulting in improvement of strength. An increase of the strain-induced martensite also leads to upgrading of cross-sectional hardness.
Dependency of material strength on a rate of strain-induced martensite is advantageously used as a parameter for imparting a predetermined fatigue strength to a steel belt. If a rate of strain-induced martensite, which is formed by ring-rolling, necessary for a certain fatigue strength is known beforehand, such rolling conditions as a material temperature T, an equivalent strain ε and a reduction ratio R can be preset in order to gain the forecast rate of strain-induced martensite.
The inventors have searched and examined effects of compositions, rolling temperatures and strains on a rate of strain-induced martensite for provision of a metastable austenitic stainless steel strip with fatigue strength similar or superior to 18% Ni-maraging steel, and discovered the ring-rolling conditions that properties suitable for a continuously variable transmission belt are imparted to a rolled steel strip without necessity of aging treatment or by moderate aging. That is, when a steel strip is ring-rolled under the condition that a relationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87 is established among a material temperature T (° C.), an equivalent strain ε and a value Md(N), strain-induced martensite is formed at a rate necessary for a predetermined fatigue strength. Furthermore, the rate of strain-induced martensite is controlled with a deviation of 5 vol. % by confining a fluctuation ΔT of the material temperature T within a range of ±6.4° C. during ring-rolling
A metastable austenitic stainless steel suitable for the purpose preferably has a value Md(N) within a range of 20–100.
If the value Md(N) is less than 20, strain-induced martensite is not formed at a rate enough to enhance strength, unless a steel strip is ring-rolled or cold-worked at an extremely low temperature with industrial difficulty. The low value Md(N) does not assure austenite/martensite transformation for improvement of fatigue strength, on use of the steel strip as a continuously variable transmission belt. Moreover, an austenite phase is more stable as a decrease of the value Md(N), so that a rate of strain-induced martensite does not reach 80 vol. % or more at a surface layer of the steel strip and that it is also difficult to form strain-induced martensite at a rate of 60 vol. % or more with high reliability. As a result, surface nitriding reaction does not progress to an extent necessary for improvement of wear-resistance and fatigue strength. On the other hand, a steel strip, which has a composition with a value Md(N) above 100, is transformed to martensite at a too early stage due to deformation on its use as a continuously variable transmission belt, so that fatigue strength is rather lowered.
After a steel strip is formed to a ring shape, it is ring-rolled by a rolling mill, as shown in
The steel strip 1 is sent to a gap between the work rolls 2a and 2b and gradually reduced in thickness during traveling along an endless track. During rolling, expansion of the steel strip 1 along its circumferential direction is compensated by elongation of a distance between axes of the rolls 3 and 4 in order to keep a tension, which is applied to the steel strip 1, at a constant value. Loads, which are put on the rolls 2a, 2b, 3 and 4, are controlled by a load cell 5. The circumferential length of the steel strip 1 is calculated from diameters of the rolls 3, 4 and the distance between the axes of the rolls 3 and 4 measured by a range finder 6.
A material temperature T is kept at a value within a predetermined range by a temperature control system, as shown in
When the steel strip 1 is ring-rolled under the conditions that the value Md(N) and the reduction R are held constant, a rate of strain-induced martensite to a metallurgical structure of a manufactured steel belt becomes bigger as the material temperature T falls down, as shown in
These effects of the material temperature T, the value Md(N) and the reduction R on formation of strain-induced martensite indicate that a rate of strain-induced martensite in a manufactured steel belt is adjusted to a certain value by interactions of the material temperature T, the value Md(N) and the reduction R. The inventors have arranged the relationship of
α′=−0.3913T+0.5650Md(N)+60.46ε−10.87
is established among the rate of strain-induced martensite α′, the material temperature T, the value Md(N) and an equivalent strain ε, wherein the equivalent strain ε is represented by ε=√{square root over (4(1n(1−R))2/3)} in relation with the reduction R.
By the way, a steel belt, which is manufactured by ring-rolling a steel strip at a material temperature T of 0° C., 25° C. or 50° C. with a constant value Md(N) and a constant reduction ratio R, has the metallurgical structure that a rate of strain-induced martensite α′ is varied in relation with the material temperature T, as shown in
In fact, a fatigue test was performed, using a bending-stretching fatigue testing machine, wherein a test piece 12 was fixed to a subsidiary belt 13 with a snap pin 11 and disposed between a driving pulley 14 of 70 mm in diameter and a testing pulley 15 with a diameter D (mm), as shown in
Under these conditions, a maximum stress σmax is calculated according to the formula of σmax=T+E·t/2ρ, wherein E is Young's modulus, t is thickness (mm) of the test piece 12 and ρ is a bend radius [ρ=(D+t)/2]. Calculation results in
−0.3913T+0.5650Md(N)+60.46ε≧65.87
The rate of strain-induced martensite α′ is also variable in relation with an atmospheric temperature during ring-rolling. For instance, dissipation of processing heat is varied in correspondence to an atmospheric temperature different between winter and summer seasons. Variation of the heat dissipation leads to seasonal fluctuations in a rate of strain-induced martensite α′, even when a metastable austenitic stainless steel strip is ring-rolled under the same conditions. Fluctuations in the rate of strain-induced martensite α′ cause change of deformation-resistance of the steel strip 1, and finally induce deviations of thickness, width and hardness in a manufactured steel belt.
Parameters, i.e. the value Md(N) and the equivalent strain ε, in the formula of α′=−0.3913T+0.5650Md(N)+60.46ε−10.87 can be regarded as constants, which are determined by a reduction ratio R calculated from an original thickness of a steel strip 1 and a target thickness of a manufactured steel belt. The remaining parameter, the material temperature T, is variance, which is influenced by heat generation and heat dissipation during ring-rolling as well as seasonal change of an atmospheric temperature. In this sense, the formula of α′=−0.3913T+0.5650Md(N)+60.46ε−10.87 for determination of a rate of strain-induced martensite α′ is rewritten to the formula of α′=−0.3913T+A+B (A and B are constants) involving the material temperature T as only one parameter. The constants A, B are deleted from the formula by handling a variation ΔT of the material temperature T during ring-rolling and a variation Δα′ as indices, and the formula is rewritten to Δα′=−0.3913ΔT.
Even when a material temperature T is kept at a constant value, a rate of strain-induced martensite α′ is fluctuated, as noted in
−6.4≦ΔT≦6.4
The formula of −6.4≦ΔT≦6.4 means tolerance of the material temperature T for production of a steel belt with stable quality characteristics, wherein a variation Δα′ of strain-induced martensite α′ is controlled with fluctuations within a range of 5 vol. % when a steel strip 1 is ring-rolled at a constant material temperature T with a constant value Md(N) and a constant reduction ratio R. In short, a variation Δα′ of strain-induced martensite α′ is confined within a range of 5 vol. % by controlling a material temperature T with a variation within a range of ±6.4° C. during ring-rolling, resulting in production of a steel belt, which has a stable profile with stable quality.
The other features of the present invention will be clearly understood from the following Examples.
Example 1 used a ring-rolling mill, which had a tension roll 3 and a return roll 4 each of 75 mm in diameter with a couple of work rolls 2a, 2b of 70 mm in diameter located between the rolls 3 and 4.
A steel strip 1 of 0.35 mm in thickness and 15 mm in width was prepared from a metastable austenitic stainless steel, which had a composition consisting of 0.086 mass % C, 2.63 mass % Si, 0.31 mass % Mn, 8.25 mass % Ni, 13.73 mass % Cr, 0.175 mass % Cu, 2.24 mass % Mo, 0.064 mass % N and the balance being Fe except inevitable impurities with a value Md(N) of 74.03. The specified composition allows formation of a dual phase structure of strain-induced martensite/austenite during aging.
The steel strip 1 was formed to a ring shape with a circumferential length of 611 mm by laser-welding its front and tail ends together.
After the steel strip 1 was disposed between the tension roll 3 and the return roll 4, it was continuously sent to a gap between the work rolls 2a and 2b along an endless track with a tension of approximately 5 kgf. The steel strip 1 was ring-rolled to a steel belt of 0.20 mm in thickness with a circumferential length of 1070 mm, while controlling a rolling load and a tension applied to the steel strip 1 under the conditions that a maximum rolling load, a circumferential speed of the work rolls 2a, 2b and a tension of the tension roll 3 were adjusted to 3 ton, 2 m/minute and 200 kgf, respectively. Herein, a reduction ratio R was 42.9%, and an equivalent strain ε was 0.647.
Three values, i.e. 0° C., 25° C. and 50° C., were preset as a material temperature T. A surface temperature of the steel strip 1 was measured by the noncontact radiation thermometer 9 at a position where the steel strip 1 was just sent to the gap between the work rolls 2a and 2b. The material temperature T of the steel strip 1 was feed-back controlled by changing a volume of hot air, which was supplied from the generator 8 to the heating box 10, in response to the measured value.
The rolling conditions are summarized in Table 1.
A rate of strain-induced martensite in the steel belt manufactured by ring-rolling was measured. Results are shown in
It is noted in
It is confirmed from the above-mentioned results that a rate of strain-induced martensite α′ is forecast according to the formula of α′=−0.3913T+0.5650Md(N)+60.46ε−10.87 and adjusted to 55 vol. % or more by controlling a material temperature T, an equivalent strain ε and a value Md(N) so as to satisfy the condition of −0.3913T+0.5650Md(N)+60.46ε≧65.87. As a result, a stainless steel belt excellent in fatigue property and mechanical strength useful for continuously variable transmission is offered.
A steel strip 1 was formed to a ring shape with a circumferential length of 611 mm from the same metastable austenitic stainless steel as Example 1, by laser-welding its front and tail ends together. The welded steel strip was ring-rolled to a steel belt of 0.20 mm in thickness with a circumferential length of 1070 mm under the same conditions as Example 1 except for controlling a material temperature T to 10±0.5° C. or 30±0.5° C. at the atmospheric temperature of 10° C. or 30° C., respectively.
For comparison, the same steel strip 1 was ring-rolled at an atmospheric temperature of 10° C. or 30° C. without controlling a material temperature T. In this case, the material temperature T was elevated by approximately 10° C. at a position in the vicinity of an exit of the work rolls 2a, 2b, due to generation of processing heat at any atmospheric temperature of 10° C. or 30° C.
Thickness, width and cross-sectional hardness of each manufactured steel belt were measured at several points along its circumferential direction. Deviations were calculated from the measured values. Calculation results in Table 2 prove that steel belts, which were manufactured at a controlled material temperature T, had substantially uniform thickness, width and cross-sectional hardness with deviations smaller than halves of steel belts, which were manufactured without controlling the material temperature T.
According to the present invention as mentioned above, a rate of strain-induced martensite α′, which is formed by ring-rolling a metastable austenitic stainless steel strip, is forecast by the formula of α′=−0.3913T+0.5650Md(N)+60.46ε−10.87. When a rate of strain-induced martensite α′ is adjusted to a value of 55 vol. % or more by controlling a material temperature T, an equivalent strain ε and a value Md(N) so as to satisfy the relationship of −0.3913T+0.5650Md(N)+60.46ε≧65.87, a steel belt manufactured by ring-rolling is bestowed with fatigue strength similar or superior to a conventional continuously variable transmission belt made of a 18%-Ni maraging steel. A rolling load is also alleviated by lowering a material temperature T to a lowest possible level and a rolling reduction R. Moreover, a rate of strain-induced martensite α′ is controlled to a predetermined value with a tolerance of ±2.5 vol. %, by properly confining a variation ΔT of the material temperature T during ring-rolling. Consequently, a steel belt excellent in quality and dimensional accuracy useful for a continuously variable transmission is manufactured from a metastable austenitic stainless steel.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2001-117699 | Apr 2001 | JP | national |
| 2001-117700 | Apr 2001 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP02/02742 | 3/22/2002 | WO | 00 | 10/16/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/085548 | 10/31/2002 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 5640868 | Lauener | Jun 1997 | A |
| Number | Date | Country |
|---|---|---|
| 62-199214 | Sep 1987 | JP |
| 2000-063998 | Feb 2000 | JP |
| 2002-053936 | Feb 2002 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20040112481 A1 | Jun 2004 | US |
