Plate-shaped element of belt for belt type continuously variable transmission

Description

BACKGROUND OF THE INVENTION

This invention relates to improvements in plate-shaped elements of a belt connecting a primary pulley and a secondary pulley in a belt type continuously variable transmission, and more particularly to a composition of steel of the plate-shaped element as a component part of the belt.

A belt continuously variable transmission includes drive and driven pulleys which are separate from and drivingly connected with each other by a belt. The belt is constituted of a plurality of plate-shaped elements or blocks which are arranged in a ring form by being supported On band-shaped steel rings so that an engine torque is transmitted to a drive shaft under friction between a tapered surface or sheave surface of a frustoconical disc of the drive and driven pulleys and a contact surface or flank surface of the plate-shaped element. Therefore, the flank surface of plate-shaped element is required to have a high wear resistance, so that a special steel such as a high carbon steel is used as a material for the plate-shaped element. After the special steel is formed into a certain shape, for example, by blanking, it is subjected to a heat treatment such as quenching or tempering, thereby producing the plate-shaped element having a high hardness which provides a wear resistance.

As discussed above, since the plate-shaped element obtains the high hardness by increasing its carbon content, a metallographic structure having poor ductility and toughness due to deposition of a large quantity of high carbon martensite and carbides is unavoidably produced. For this reason, a high part strength (especially, fatigue strength and impact strength) is difficult to obtain. In view of this, a technique for solving this problem is disclosed in Japanese Patent Provisional Publication No. 2002-241834 and arranged as follows: If the deposit quantity (area percentage) of carbide is in a certain range or larger, the impact strength (toughness) of the plate-shaped element can be stabilized and improved. Also, another technique is disclosed in a technical document “Development of Metal Belt for CVT”, Honda R&D Technical Review Vol. 14, No. 1 (April 2002), page 186. According to this technique, the fatigue strength is lowered remarkably according to the shape of carbide. In other words, this technical document discloses that the fatigue strength can be improved by taking account of the shape of carbide.

SUMMARY OF THE INVENTION

The inventors of the present invention produced plate-shaped elements on an experimental basis and carried out impact and fatigue tests, on the basis of the knowledge according to the above conventional techniques. However, even in the case where the area percentage and shape of carbide is in the range of the above-described knowledge (within the range in which impact and fatigue properties can be improved), some plate-shaped elements exhibited remarkably low impact and fatigue strengths, and it seems that sufficient improvement is not achieved by the above-discussed conventional techniques.

It is, therefore, an object of the present invention to provide an improved plate-shaped element of a belt for a belt type continuously variable transmission, which can effectively overcome drawbacks encountered in conventional plate-shaped elements of the similar nature.

Another object of the present invention is to provide an plate-shaped element of a belt for a belt type continuously variable transmission, which is provided with such composition and characteristics as to obtain high part strengths (fatigue and impact strengths).

According to the present invention, a plate-shaped element of a belt for a belt type continuously variable transmission is formed of a steel which comprises at least one of martensite structure and tempered martensite structure, containing a solid-solution carbon in an amount ranging from 0.4 to 0.7% by weight. The steel has a surface hardness ranging from 55 to 65 in Rockwell hardness C-scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an embodiment of a plate-shaped element of a belt for a belt type continuously variable transmission, according to the present invention;

FIG. 2A is a schematic front view showing a manner of an Izod impact test for the plate-shaped element of FIG. 1;

FIG. 2B is a schematic side view showing the manner of the Izod impact test of FIG. 2A;

FIG. 3A is a schematic front view showing a manner of a bending fatigue (fracture) test for the plate-shaped element of FIG. 1;

FIG. 3B is a schematic side view showing the manner of the bending fatigue test of FIG. 1;

FIG. 4 is a graph showing the relationship between the content of a solid-solution carbon in martensite structure and/or tempered martensite structure and the result (absorbed energy) of the impact test;

FIG. 5 is a graph showing the relationship between the content of the solid-solution carbon in martensite structure and/or tempered martensite structure and the result (fatigue life) of the fatigue test;

FIG. 6 is a graph showing the relationship between the content of an impurity element (P) and the result (absorbed energy) of the impact test;

FIG. 7 is a graph showing the relationship between the content of the impurity element (P) and the result (fatigue life) of the fatigue test;

FIG. 8 is a graph showing the relationship between the content of an impurity element (S) and the result (absorbed energy) of the impact test;

FIG. 9 is a graph showing the relationship between the content of the impurity element (S) and the result (fatigue life) of the fatigue test;

FIG. 10 is a graph showing the relationship between the average grain size of a prior austenite and the result (absorbed energy) of the impact test;

FIG. 11 is a graph showing the relationship between the content of Ni and/or Mo and the result (absorbed energy) of the impact test; and

FIG. 12 is a graph showing the relationship between the content of Ni and/or Mo and the result (fatigue life) of the fatigue test.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 of the drawings, an embodiment of a plate-shaped element (or block) of a belt for a belt type continuously variable transmission is illustrated. A plurality of plate-shaped elements are aligned with and secured to band-shaped rings (not shown) to form a the belt through which drive and driven pulleys of the belt type continuously variable transmission is connected though not shown. The plate-shaped element includes a body 1 which laterally extends and formed at its opposite ends (side surfaces) with flank surfaces 2. The flank surfaces 2 are opposite to each other and contactable with the sheave surfaces of one of the pulleys. A neck 4 is integral with and extends vertically from the body 1 at the central portion to connect the body 1 to a head 5. The head 5 has ears 5a which extend in opposite directions to each other and parallel with the body 1. The band-shaped ring is kept between the ear 5a and the saddle surface 3 of the body 1. Additionally, a nose 6 is formed at the central portion of the head 5 and extends perpendicular to the surface of the head 5 for the purpose of positioning of the plate-shaped element relative to adjacent plate-shaped elements.

According to the present invention, the plate-shaped element is formed of a steel comprising at least one of martensite structure and tempered martensite structure, containing a solid-solution carbon in an amount ranging from 0.4 to 0.7% by weight. The steel has a surface hardness ranging from 55 to 65 in Rockwell hardness C-scale.

In order to illustrate the present invention, experiments discussed below were conducted.

Preparation of Samples

Fifteen samples of the plate-shaped element shown in FIG. 1 were prepared as sample steels, and six comparative samples of the plate-shaped element were prepared as comparative sample steels. Table 1 gives the compositions of the sample steels and comparative steels.

	TABLE 1
	Chemical composition (% by weight)

Steel	C	Si	Mn	P	S	Cu	Cr	O	Ni	Mo

Sample	0.88	0.37	0.97	0.029	0.009	0.02	0.51	0.0010	0.04	0.01
steel	0.84	0.2	0.87	0.03	0.007	0.01	0.42	0.0015	0.01	0.01
	0.84	0.2	0.82	0.007	0.004	0.01	0.45	0.0015	0.01	0.01
	0.84	0.2	0.88	0.02	0.004	0.01	0.46	0.0015	0.01	0.01
	0.75	0.35	0.6	0.018	0.005	0.02	0.46	0.0010	0.05	0.01
	0.78	0.2	0.83	0.006	0.005	0.01	0.46	0.0010	0.01	0.01
	0.84	0.2	0.88	0.005	0.008	0.01	0.44	0.0015	0.01	0.01
	0.84	0.2	0.89	0.007	0.007	0.01	0.44	0.0015	0.01	0.01
	0.84	0.2	0.68	0.012	0.006	0.01	0.44	0.0015	0.01	0.01
	0.75	0.35	0.64	0.018	0.003	0.02	0.46	0.0010	0.05	0.01
	0.75	0.35	0.62	0.006	0.005	0.02	0.5	0.0010	0.05	0.01
	0.84	0.2	0.86	0.007	0.004	0.01	0.44	0.0015	2	0.01
	0.84	0.2	0.84	0.007	0.004	0.01	0.45	0.0015	2	1
	0.84	0.2	0.85	0.007	0.004	0.01	0.44	0.0015	1	1
	0.84	0.2	0.87	0.007	0.004	0.01	0.44	0.0015	0.01	1
Compara-	0.84	0.2	0.83	0.007	0.004	0.01	0.44	0.0015	3	0.01
tive	0.84	0.2	0.82	0.007	0.004	0.01	0.45	0.0015	0.01	2
sample	0.84	0.2	0.85	0.037	0.005	0.01	0.44	0.0015	0.01	0.01
steel	0.84	0.2	0.86	0.041	0.004	0.01	0.45	0.0015	0.01	0.01
	0.84	0.2	0.86	0.018	0.016	0.01	0.44	0.0015	0.01	0.01
	0.84	0.2	0.87	0.01	0.015	0.01	0.45	0.0015	0.01	0.01

Evaluation Test

As evaluation tests for the above-described samples, an Izod impact test and a bending fatigue test were conducted. FIGS. 2A and 2B are schematic illustrations showing a manner of the Izod impact test. In this test, the plate-shaped element (sample) was fixed in a sample support S in such a manner that the body 1 was hidden in the support S, in which an absorbed energy was measured upon breaking the neck 4 with impact of a hammer. FIGS. 3A and 3B are schematic illustrations showing the bending fatigue test. In this test, a load was applied to a central position of the body 1 of the plate-shaped element which position was near the neck 4 of the plate-shaped element, and simultaneously loads in the opposite direction were applied to opposite end positions of the body 1. Such application of loads was repeated until the plate-shaped element was broken so as to make a bending fatigue fracture. The number of cycles of the application of the loads at the bending fatigue fracture was measured to represent a fatigue life.

Solid-Solution Carbon Content

FIG. 4 is a graph showing the relationship between the content (% by weight) of the solid-solution carbon of martensite structure and/or tempered martensite structure and the result or absorbed energy (J) of the impact test. In FIG. 4, the absorbed energy was an average value of seven measured values of the same sample. FIG. 5 is a graph showing the relationship between the content of the solid-solution carbon of martensite structure and/or tempered martensite structure and the result or so-called B50 fatigue life (the number of cycles) of the bending fatigue test in which a stress ratio was constant while the samples had a surface hardness (in Rockwell hardness C-scale) ranging from 60 to 61. In FIG. 5, the fatigue life was an average value of seven measured values of the same sample. In both FIGS. 4 and 5, in case that the solid-solution carbon content exceeds 0.7% by weight, a remarkable decrease in strength was found. Since the quenching strength was insufficient in case that the solid-solution carbon content was lower than 0.4% by weight, the lower limit of solid-solution carbon content was set at 0.4% by weight while the upper limit thereof was set at 0.7% by weight. Also, if the surface hardness in Rockwell hardness C-scale is lower than 55, the wear resistance cannot be secured, whereas if it exceeds 65, a pulley as the mating part will be worn. Therefore, the lower limit of the surface hardness is set at 55 and the upper limit thereof is set at 65 in Rockwell hardness C-scale.

Impurity Elements

FIG. 6 is a graph showing the relationship between the content (% by weight) of P, which is an impurity element, and the result or impact energy (J) of the impact test. In FIG. 6, the absorbed energy was an average value of six measured values of the same sample. FIG. 7 is a graph showing the relationship between the content of P, which is an impurity element, and the result or so-called B50 fatigue life of the bending fatigue test in which a stress ratio was constant while the samples had a surface hardness (in Rockwell hardness C-scale) ranging from 60 to 61. In FIG. 7, the fatigue life was an average value of six measured values of the same sample. In both FIGS. 6 and 7, in case that the P content exceeds 0.03% by weight, a remarkable decrease in strength is found. This is caused by a decrease in grain boundary intensifying action due to P, exhibiting degradation of the impact strength. Therefore, the upper limit of P content is set at 0.03% by weight. Since P content is preferably as small as possible, the lower limit is not set especially

FIG. 8 is a graph showing the relationship between the content of S, which is an impurity element, and the result or absorbed energy of the impact test. In FIG. 8, the impact energy was an average value of six measured values of the same sample. FIG. 9 is a graph showing the relationship between the content of S, which is an impurity element, and the result or so-called B50 fatigue life (the number of cycles) of the bending fatigue test in which a stress ratio was constant while the samples had the surface hardness (in Rockwell hardness C-scale) ranging from 60 to 61. In FIG. 9, the fatigue life was an average value of six measured values of the same sample. In both FIGS. 8 and 9, in case that the S content exceeds 0.01% by weight, a remarkable decrease in strength is found. Therefore, the upper limit of S content was set at 0.01% by weight. Since P content is preferably as small as possible, the lower limit is not set especially.

Austenite Grain Size

FIG. 10 is a graph showing the result or absorbed energy (J) of the impact test in case that the quenching temperature of the sample steel having the following composition was changed so that the austenite grain size at high temperatures at the time of heat treatment was changed:

C: 0.84 wt %, Si: 0.2 wt %, Mn: 0.88 wt %, P: 0.012 wt %, S: 0.006 wt %, Cu: 0.01 wt %, Ni: 0.01 wt %, Cr: 0.44 wt %, and O: 0.0015 wt %.

In FIG. 10, the sample steels were same in surface hardness and generally same in composition, in which the absorbed energy was an average value of six measured values of the same sample.

For reference, “Heat Treatment of Steel” edited by The Iron and Steel Institute of Japan, Tokyo, (1966), p. 213 describes that an impact value (absorbed energy) decreases as the crystal grains of austenite structure become coarse, and decreases remarkably in case that the crystal grains are about ASTM grain size number 7 or smaller (about 35 μm or larger). The decreasing tendency is also recognized on the plate-shaped element; however, as shown in FIG. 10, the impact value decreased suddenly in case that the average grain size of austenite structure is about 20 μm or larger. That is to say, it was found that the impact value decreases in case that the austenite grain size is at a value smaller than the value described in the above literature. Therefore, the average grain size of so-called prior austenite (i.e., austenite at the time of quenching heating) is preferably 20 μm or smaller.

Although FIG. 10 shows the test result or absorbed energy (J) for only one of the sample steels, the above-described tendency is recognized on other sample steels in the same way. Therefore, even when any composition is selected, the average grain size of the prior austenite is preferably 20 μm or smaller.

Addition of Ni and/or Mo

FIG. 11 shows the relationship between the contents or added amounts of Ni and/or Mo and the result or absorbed energy (J) of the impact test. In FIG. 11, the absorbed energy was an average value of six measured values of the same sample. FIG. 12 shows the relationship between the contents or added amounts of Ni and/or Mo and the result or so-called B50 fatigue life (the number of cycles) of the bending fatigue test. In FIG. 12, the fatigue life was an average value of six measured values of the same sample. Ni and/or Mo are elements that are effective in enhancing the quenching properties, so that in order to secure the quenching properties, the lower limit of content of each of Ni and Mo were set at 0.3% by weight. Also, although the impact strength increases along with the addition of Ni and/or Mo, an excessive addition leads to a heat treatment distortion. Therefore, the upper limit of Ni content was set at 2% by weight, and that of Mo content was set at 1% by weight.

Carbide Grain Size

The grain size of carbide deposited in a remainder of the sample steels and comparative sample steels was measured. The reminder is other than the martensite structure and/or tempered martensite structure. As a result, it was found that smaller carbide grain size is preferable. Specifically, a carbide grain size is preferably 10 μm or smaller. If the grain size exceeds 10 μm, the start point of fatigue fracture may be formed.

Addition of Ti and Nb

Either Ti or Nb is preferably added in the range of 0.03 to 0.2% by weight. If either Ti or Nb is added, an effect of preventing the austenite grains from coarsening is obtained. However, even if the content of Ti or Nb exceeds 0.2% by weight, the effect does not so increase, so that the upper limit of addition of Ti or Nb was set at 0.2% by weight.

As discussed above, it was found that, in this embodiment, the impact properties and fatigue properties of the plate-shaped element depended mainly on the content of carbon forming a solid solution in martensite or tempered martensite and the content of impurity elements such as P and S, in which these properties were improved with a decrease in the contents. Also, it was found that the impact value depended on the grain size of austenite structure, which is a structure at the time of quenching heating, in which the impact properties were improved as the grains of the austenite structure become fine. Also, it was found that the properties were further improved by the addition of alloy elements such as Ni, Mo, Ti, Nb, etc. Also, it was found that the properties were improved by making carbide grains fine.

As appreciated from the above, it was found that if the content of solid-solution carbon is lower than 0.4% by weight, the quenching strength is insufficient, and if it exceeds 0.7% by weight, the fatigue strength and impact strength were deteriorated. Also, if the surface hardness was lower than 55 in Rockwell hardness C-scale, the wear resistance could not be secured, and if it exceeded 65 in Rockwell hardness C-scale, a pulley to be mated with the plate-shaped element will be worn. By finding these conditions, the plate-shaped element of a belt for a belt type continuously variable transmission, capable of attaining a high part strength, was obtained.

The entire contents of Japanese Patent Application P2004-106697 (filed Mar. 31, 2004) are incorporated herein by reference.

Although the invention has been described above by reference to certain embodiments and examples of the invention, the invention is not limited to the embodiments and examples described above. Modifications and variations of the embodiments and examples described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.