JP2012001794A - Steel for element of belt type CVT and element using the same - Google Patents

Abstract

[PROBLEMS] To provide an element excellent in wear resistance even when the amount of undissolved carbide is reduced while improving fatigue resistance, in particular, low and medium cycle fatigue resistance, by performing predetermined quenching and tempering heat treatment. Provided element steel and element using the same
When at least C, Si, Mn, and Cr and the mass% of the element M is [M], 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr ] Belt type CVT element steel made of steel having a component composition satisfying ≦ 13. Steel is by mass%, and C: 0.50 to 0.70%, Si: 0.10 to 0.60%, Mn: 0.50 to 1.50%, Cr: 0.20 as essential additive elements It is comprised of the balance Fe and unavoidable impurities that may contain P: ≦ 0.025% and S: ≦ 0.015% as optional additional elements, and is less than 88HRB by performing a softening heat treatment. Has hardness.
[Selection figure] None

Description

  The present invention relates to an element steel for manufacturing an element incorporated in a steel belt of a belt-type continuously variable transmission (CVT) and an element using the element steel.

  A belt-type continuously variable transmission is known in which a steel belt is looped between a pair of input side and output side pulleys. By adjusting the groove width carved on the outer periphery of the output pulley, the rotation radius of the steel belt can be continuously changed, and the rotation ratio of the input pulley and the output pulley can be changed steplessly. Here, the steel belt is constructed by incorporating a plurality of coma elements in a plate-like ring. The element includes a carbon steel having a relatively large amount of carbon, for example, JIS SKS95 (mass%, C: 0.80 to 0.90%, Si: 0.50% or less, Mn: 0.80 to 1.10. %, P: 0.030% or less, S: 0.030% or less, Cr: 0.20 to 0.60%) and the like. The cold-rolled material is spheroidized with carbides contained in the cold-rolled material, then cold-punched into the shape of the element, and quenched and tempered from the temperature above the Acm point on the equilibrium diagram to disperse a certain amount of undissolved carbide. Used with a given tempered martensite structure.

  The element is particularly required to be particularly excellent in wear resistance because it slides with the groove of the output pulley. Regarding the wear resistance, it has been considered that the higher the hardness of the material of the element and the greater the amount of undissolved carbide, that is, the higher the amount of carbon, the better. On the other hand, during tempering, carbides are likely to precipitate in the form of a film along the crystal grain boundary, and impact resistance and fatigue resistance are lowered.

For example, in Patent Document 1, in the carbon steel having a relatively large amount of carbon as described above, the hardness can be increased according to the increase in the amount of carbon up to 0.6% in mass%, but beyond this, Increasing the carbon content increases soft retained austenite, resulting in martensite with low toughness and a dull increase in hardness, and disclosed carbon steel with particularly improved impact resistance. Yes. That is, in steels of C: 0.60 to 1.30%, Si: ≦ 1.0%, Mn: 0.2 to 1.5%, P: ≦ 0.02%, S: ≦ 0.02% , Hardened and tempered tissue,
8.5 <15.3 × C% −V _f <10.0
A predetermined amount of undissolved carbide having a volume ratio V _f (volume%) satisfying the above condition should remain, and coarse undissolved carbide having a particle size of 1.0 μm or more should be regulated to 2 or less per 100 μm ² of the observation area. It is disclosed.

  For example, Patent Document 2 discloses a carbon steel having a relatively large amount of carbon that is excellent not only in impact resistance but also in fatigue characteristics. That is, C: 0.50 to 0.70%, Si: ≤ 0.5%, Mn: 1.0 to 2.0%, P: ≤ 0.02%, S: ≤ 0.02%, Al: In addition to 0.001 to 0.10%, V: 0.05 to 0.50%, Ti: 0.02 to 0.20%, Nb: 0.01 to 0.50%, 1 type or 2 types The annealing steel plate which contains the above and does not produce | generate the coarse insoluble carbide | carbonized_material of the spheroidization rate of an insoluble carbide | carbonized_material 95% or more and a particle size of 2.5 micrometers or more at the time of annealing is disclosed.

JP 2006-63384 A JP 2009-24233 A

By the way, recent belt-type continuously variable transmissions are also required to improve the fatigue resistance of low / medium cycles (about 10 ³ to 10 ⁵ cycles) such as an impact load when entering a pulley. . Generally, fatigue strength can be improved by adding typical grain boundary strengthening elements such as Mo and Si, but cold punching after spheroidizing treatment tends to be difficult, and the cost is increased. End up.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a steel for an element that can satisfactorily achieve the cold punching in the manufacturing process of the belt type CVT element as described above. By applying a predetermined quenching and tempering heat treatment to this, an element that is excellent in wear resistance even when the amount of undissolved carbide is reduced while improving fatigue resistance, particularly fatigue resistance of low and medium cycles. The steel for element which can be given will be provided. Another object is to provide an element using the same.

  The steel for element of the belt type CVT according to the present invention contains at least C, Si, Mn, and Cr, and when the mass% of the element M is [M], 10.8 [C] +5.6 [Si] +2.7 Belt type CVT element steel made of steel having a component composition satisfying [Mn] +0.3 [Cr] ≦ 13. The steel is in mass%, and C: 0.50 as an essential additive element. In addition to 0.70%, Si: 0.10 to 0.60%, Mn: 0.50 to 1.50%, Cr: 0.20 to 1.00%, and as an optional additive element, P: ≦ 0 0.025%, S: remaining balance Fe that may include ≦ 0.015% and inevitable impurities, and a softening heat treatment is performed to give a hardness of 88HRB or less.

  According to this invention, it is possible to satisfactorily cold-punch into the shape of the belt type CVT element, and by subjecting this to a predetermined quenching and tempering heat treatment, fatigue resistance, in particular, fatigue resistance for low and medium cycles. Even if the amount of undissolved carbide is reduced while increasing the resistance, a belt-type CVT element having excellent wear resistance can be produced.

  In the above-described invention, the optional additive element may further include Mo: ≦ 0.50%, B: ≦ 0.0050%, Ti: ≦ 0.10%, and 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 [Mo] ≦ 13 may be satisfied. According to this invention, the grain boundary strength can be further increased, and the fatigue resistance of the finally obtained belt-type CVT element, in particular, the fatigue resistance for low and medium cycles can be enhanced.

  In the above-described invention, the optional additive element may further include Ni: ≦ 0.50%, V: ≦ 0.50%, Nb: ≦ 0.20%, and 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 [Mo] +3.6 [Ni] ≦ 13 may be satisfied. According to this invention, the mechanical strength of the finally obtained belt-type CVT element can be increased.

The belt type CVT element according to the present invention is made by cold punching a belt type CVT element steel comprising any one of the above-mentioned inventions into a predetermined shape to dissolve carbides while preventing coarsening of crystal grains. It is characterized by being heated and held near the A _cm temperature and subjected to quenching and tempering heat treatment to give a hardness of 640 Hv or more.

  According to this invention, even if the amount of undissolved carbides is reduced while improving the fatigue resistance, in particular, the fatigue resistance for low and medium cycles, the wear resistance is also excellent. That is, an element of a belt type CVT having excellent mechanical strength can be obtained.

In the above-described invention, the cross-sectional structure may be characterized in that the number of insoluble carbides having an equivalent circle diameter of 0.20 μm or more is 1.6 × 10 ⁵ or less per 1 mm square. According to this invention, the wear resistance is excellent even if the amount of undissolved carbide is further reduced while improving the fatigue resistance of the low and medium cycles.

It is a figure which shows the component composition of the steel for elements in the Example and comparative example by this invention. It is a figure which shows the external appearance of the element used for the evaluation test. It is a figure which shows the method of a bending fatigue test. It is a figure which shows the method of an abrasion test. It is a figure which shows a test result. It is a figure regarding estimation of hardness. It is the enlarged photograph which squinted the crack. It is an enlarged photograph of the crack tip part. It is a graph which shows the result of an abrasion test. It is a graph which shows the result of an abrasion test. It is a graph which shows the test result of bending fatigue strength.

  The present inventor has conceived that the amount of C in steel is reduced in obtaining an element particularly excellent in fatigue resistance as compared with an element made of JIS SKS95 which is a conventional material. In other words, it is only necessary to reduce the amount of carbide precipitated in the form of a film along the grain boundary to improve fatigue resistance, particularly low / medium cycle fatigue resistance, and to make the wear resistance equal to or higher than that of conventional materials. I thought. On the other hand, the element is provided with a strength adjusted by heat treatment after the shape is processed by cold punching. That is, it is preferable that the element steel before heat treatment has a punchability (hardness) comparable to that of the conventional material. Therefore, a steel with the content of C and the amount of a predetermined element such as Si, Mn, Cr adjusted is manufactured. (1) Evaluation of hardness required as element steel, (2) After heat treatment The mechanical properties (fatigue resistance and wear resistance) required for the element were evaluated.

  First, the test method for the above evaluation will be described with reference to FIGS.

  Test pieces were prepared for the steels having the component compositions of Examples 1 to 10 and Comparative Examples 1 to 13 shown in FIG. First, 150 kg of a master alloy was melted in a vacuum induction furnace to form an ingot, which was held at 1200 ° C. for 3 hours, and then hot forged into a substantially cylindrical round bar having a diameter of 25 mm. The forging completion temperature at this time was 900 degreeC or more. Subsequently, this round bar was kept at 840 ° C. for 60 minutes and air-cooled and heat treated. Further, it was held at 760 ° C. for 1 hour, cooled to 650 ° C. at 10 ° C./hr, and then air-cooled for the first spheroidizing annealing and heat treatment was performed.

After these softening heat treatments, a part of the round bar is kept in the vicinity of the A _cm temperature, that is, at 800 ° C. for 30 minutes so as to dissolve the carbide while preventing the coarsening of crystal grains, and then quenched in a 70 ° C. oil bath, 180 A quenching and tempering heat treatment was carried out at 120C for 120 minutes. This was processed into a wear test piece 13 (see FIG. 4) described later. About this test piece 13, it used for the measurement of the hardness after quenching and tempering mentioned later, and the measurement of the number of undissolved carbides.

  Further, a part of the round bar is cold-rolled to a thickness of 1.5 mm, subjected to the same spheroidizing annealing heat treatment as in the first time, and a part of the hardness test for evaluation of punchability described later. It was a piece.

  Further, a part of the rolled material was cold punched into a predetermined shape, and subjected to quenching and tempering heat treatment to obtain a bending fatigue test piece 1 (see FIG. 2).

  Here, the punchability was evaluated by measuring the hardness of the above-described hardness test piece with a Rockwell hardness tester. The hardness was measured at five arbitrary points and the average value was adopted (spheroidizing annealing and hardness).

Evaluation of fatigue strength was performed on the above-described bending fatigue test piece 1 using a fatigue tester 4 as shown in FIG. Referring to FIGS. 2 and 3, the fatigue testing machine 4 is attached to the fixing portion 5 that holds the bending fatigue test piece 1, the rod 6 that contacts the leg portion 3 of the bending fatigue test piece 1, and the upper end of the rod 6. And the repeated stress applying portion 7. The rod 6 is vibrated up and down at a frequency of 50 Hz by the repetitive stress applying portion 7, and a repetitive bending load is applied to the R portion between the leg portion 3 and the connecting portion 8 of the bending fatigue test piece 1. The load for breaking the leg portion 3 when the rod 6 was vibrated 10 ⁵ times was defined as bending fatigue strength, and the ratio to the bending fatigue strength of JIS SKS95 (Comparative Example 1) was defined as the bending fatigue strength ratio.

  The wear resistance evaluation was performed on the wear test piece 13 (see FIG. 4) having a substantially rectangular parallelepiped shape having a width of 15.75 mm, a height of 10.16 mm, and a thickness of 6.35 mm, as shown in FIG. 10 was used. The abrasion test apparatus 10 includes a tank 14 in which oil 12 at 110 ° C. is stored, and a ring 11 that is rotated by being partially immersed in the oil 12. The ring 11 is an annular body having an outer diameter of 35 mm and a thickness of 8.74 mm, and is made of steel obtained by tempering a carburizing and tempering material of SCM420 simulating a pulley to a hardness of about 750 Hv. While rotating the ring 11 so that the sliding speed is 0.05 m / sec, the wear test piece 13 is brought into contact with the load with a load of 1200 N, and the wear amount of the wear test piece 13 at a relatively slid distance of 3000 m is measured. (Block-on-ring method). The amount of wear was determined by measuring the cross-sectional area of the worn portion of the wear test piece 13, and the ratio with the amount of wear in Comparative Example 1 was taken as the wear ratio.

  The hardness after quenching and tempering was measured using a general Vickers hardness tester (hardness after quenching and tempering). Five points of Vickers hardness at a position of a depth of about 25 μm from the surface in the cross section of the abrasion test piece 13 were measured, and an average value thereof was adopted. The number of undissolved carbides is measured by image analysis of the number of undissolved carbides having an equivalent circle diameter of 0.20 μm or more per 100 μm square from the cross-sectional structure observation of the wear test piece 13. It converted into the number which exists per 1mm square.

  FIG. 5 shows the results of each test. In addition, the comparative example 1 is a result of JIS SKS95 which is the conventional material of an element.

  In Examples 1 to 4 in which the contents of the four elements C, Si, Mn, and Cr are different, the spheroidizing annealing hardness is smaller than that of 88HRB of Comparative Example 1, and is predicted to be better than the conventional material. Is done. On the other hand, the hardness after quenching and tempering is in the range of 640 to 714 Hv, which is almost the same as that of Comparative Example 1, while the bending fatigue strength ratio exceeds 1 and the wear ratio is smaller than 1, which is lower than that of the conventional material. It can also be seen that it is excellent in fatigue strength and wear resistance.

By the way, the regression calculation was performed about several steel including Examples 1-4 about the spheroidizing annealing hardness with respect to content of 4 elements, C, Si, Mn, and Cr, and the prediction formula was made. That is, the mass% of the element M is [M], and the spheroidizing annealing hardness value H1 is
H1 = 75 + 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] (Formula 1)
It can be expressed as FIG. 6 shows the predicted value H1 and the measured values of spheroidizing annealing hardness in Examples and Comparative Examples.

  For reference, in Comparative Examples 2 and 3 in which the content of C is decreased and the Cr content is increased with respect to the conventional material, the hardness after quenching and tempering is low as 538 Hv and 584 Hv, in particular, the wear ratio is large and the wear resistance is high. It turns out that it is inferior.

  In Comparative Example 4 in which the content of C is increased with respect to the conventional material, the spheroidizing annealing and the hardness are as high as 90 HRB, and it is predicted that the punchability is inferior. Further, although the hardness after quenching and tempering was high, the bending fatigue strength ratio was smaller than 1, and the wear ratio was larger than 1. That is, it turns out that it is inferior to fatigue strength and abrasion resistance. Here, it is considered that the amount of undissolved carbide is much larger than that of Comparative Example 1 and the wear resistance is thereby lowered.

  In Comparative Examples 5 and 6 in which the content of Si or Mn was increased with respect to the conventional material, the spheroidizing annealing and the hardness were about Comparative Example 1, but the predicted value H1 calculated as described above. Then, since it becomes higher than an actual measurement value, it is predicted that it tends to be inferior in punchability.

  On the other hand, in Comparative Example 7 in which the content of Mn is reduced with respect to the conventional material, spheroidizing annealing is performed, the hardness is low, and the punchability is predicted to be good. However, it can be seen that the wear ratio is much greater than 1 and is greatly inferior in wear resistance.

  In Comparative Example 8 in which the Cr content was increased with respect to the conventional material, the punchability was similarly predicted to be good, but the wear ratio was larger than 1, indicating that the wear resistance was inferior.

  On the other hand, it can be seen that even in Comparative Example 9 in which the Cr content was reduced compared to the conventional material, the wear ratio was larger than 1 and the wear resistance was inferior.

  In Comparative Example 13 in which the C content is reduced and the Si and Mn contents are increased compared to the conventional material, the spheroidizing annealing and the hardness are as high as 90 HRB, and it is predicted that the punchability is inferior.

  In addition to the above, the influence of elements other than the four elements of C, Si, Mn, and Cr was investigated.

  In Comparative Example 10 in which the P content is increased and Comparative Example 11 in which the S content is increased with respect to the conventional material, the bending fatigue strength ratios are both lower than 1 and inferior in fatigue resistance strength. I understand that. This is considered that the fatigue strength decreased due to an increase in non-metallic inclusions.

  In Comparative Example 12 in which Mo was added to the conventional material, it was predicted that spheroidizing annealing was performed and the hardness was high and the punchability was poor.

  On the other hand, Example 5 having a smaller Mo content than Comparative Example 12 is predicted to be excellent in punchability, and it can be seen that both fatigue strength and wear resistance are good.

  Further, in Example 6 in which Ti and B are added instead of lowering the Mo content, it is predicted that the punchability is excellent, and it is understood that both the fatigue resistance and the wear resistance are good.

  In addition, in Examples 7 and 8 where Mo and Ti and B are added, it is predicted that the punching property is excellent, and it is understood that both the fatigue strength and the wear resistance are good.

  Further, in Example 9 in which Nb is added in addition to Ti and B, it is predicted that the punching property is excellent, and it is understood that both the fatigue strength and the wear resistance are good.

  In particular, in Examples 6 to 9 to which Ti and B are added, the wear ratio is as very small as 0.68 to 0.76, and it can be seen that the wear resistance is very excellent.

  In Example 10 to which Ni and V are added, it is predicted that the punching property is excellent, and it is understood that both the fatigue resistance and the wear resistance are good.

By the way, as described above, when the predicted value H1 is 88 or less, it can be predicted that the punching property is superior to that of the conventional material. In other words,
10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] ≦ 13 (Formula 2)
If it satisfies, it can be predicted that the punching performance is superior to that of the conventional material.

In addition to the predicted value H1 described above, the influence of [Mo] is similarly spheroidized and the predicted value of hardness is obtained.
H2 = 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 [Mo] +75 (Formula 3)
Met. In other words, if this value is 88 or less, that is,
10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 [Mo] ≦ 13 (Formula 4)
If it satisfies, it can be predicted that the punching performance is superior to that of the conventional material.

Further, regarding the influence of [Ni], when the spheroidizing annealing and the predicted hardness value H3 are obtained,
H3 = 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 [Mo] +3.6 [Ni] +75 (Formula 5)
It became. In other words,
10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 [Mo] +3.6 [Ni] ≦ 13 (Formula 6)
If so, it can be predicted that the punching property is superior to that of the conventional material.

  By the way, as shown in FIG. 7, it is considered that the wear proceeds by the growth of the microcracks 23 from the surface and the peeling of the surface. Referring also to FIG. 8, it can be seen that the microcracks 23 preferentially propagate through the interface between the undissolved carbide 27 and the parent phase 28. That is, it is considered that the wear resistance is improved when the number of insoluble carbides 27 is reduced. FIG. 9 is a graph showing the wear ratio with respect to the number of insoluble carbides. It shows that the wear ratio decreases as the number of insoluble carbides decreases, that is, the wear resistance is improved.

  On the other hand, the number of undissolved carbides depends on the C content, and if the C content is reduced so as to reduce the insoluble carbides, it is predicted that the hardness also decreases and the wear resistance decreases. FIG. 10 is a graph showing the relationship between the hardness after quenching and tempering and the wear ratio. At a hardness of 640 Hv or less, the wear ratio increases rapidly, that is, the wear resistance is greatly degraded. From the above, there is a suitable range of the C content for wear resistance.

The C content is also expected to affect the fatigue strength. FIG. 11 is a graph showing the relationship of the C content to the bending fatigue strength ratio. When the C content is reduced from 0.85% which is a conventional material, the fatigue strength tends to increase. This is because undissolved carbides are reduced and carbides precipitated in the form of film at the grain boundaries are also reduced. For medium and low cycle fatigue of at least about 10 ³ to 10 ⁵ cycles, the growth of cracks at the grain boundaries is prevented. We think that it is thing to suppress. However, when the C content is further reduced from about 0.60%, the fatigue strength tends to decrease. This is considered to be due to a decrease in yield strength accompanying a decrease in the C content.

The range of the content of C that gives both wear resistance and fatigue strength, particularly fatigue strength against medium / low cycle fatigue, is 0.50 to 0.70% in terms of mass%. Is required to be 1.524 × 10 ⁵ or less per 1 mm square, that is, approximately 1.6 × 10 ⁵ or less per 1 mm square. The

  As described above, even if the content of C is reduced as compared with JIS SKS95 that has been used conventionally, the amount of undissolved carbides can be reduced while increasing fatigue resistance by a predetermined additive element and heat treatment. It is possible to obtain steel for an element that can provide an element that is reduced but also has excellent wear resistance.

  As described above, with respect to C, Si, Mn, Cr, Mo, and Ni, by satisfying Equation 2, Equation 4, and Equation 6, the hardness can be reduced to 88 HRB or less by a specific softening heat treatment. That is, good punchability can be obtained, but in addition, the composition range as elemental steel was determined by the following guidelines. First, the essential additive elements C, Si, Mn, and Cr will be described.

  As described above, C is the most important element for ensuring fatigue resistance and wear resistance required as an element. If the amount of C added is too small, the hardness cannot be ensured after quenching and tempering, resulting in a decrease in wear resistance required as an element. However, excessive addition of C causes a large amount of undissolved carbide to remain even after quenching, resulting in deterioration of wear resistance required as an element. Further, carbides are precipitated in the form of a film at the grain boundaries during tempering, leading to a decrease in grain boundary strength, and the fatigue strength required for the elements cannot be ensured. As described above, in mass%, C is in the range of 0.50 to 0.70%.

  Si is an effective element as a deoxidizing element for steel. Furthermore, it is hardly dissolved in iron carbide, and it is essential to add the iron carbide to ensure the fatigue strength required for the element in order to suppress the formation of grain boundaries of iron carbide and to suppress the decrease in grain boundary strength. . However, if added excessively, the hardness after spheroidizing annealing is increased and the punchability required for element steel is deteriorated. Therefore, in mass%, Si is in the range of 0.10 to 0.60%.

  Mn improves the hardenability of steel and is effective in ensuring the strength and wear resistance required as an element. However, excessive addition of Mn leads to deterioration of punchability required as elemental steel. Therefore, in terms of mass%, Mn is in the range of 0.50 to 1.50%.

  Cr improves the hardenability of steel and is effective in securing the strength, hardness and wear resistance required as an element. However, excessive addition of Cr stabilizes undissolved carbide by easily dissolving in iron carbide and promotes the increase, resulting in a decrease in wear resistance required as an element. Therefore, Cr is in the range of 0.20 to 1.00% by mass%.

  Next, the optional additive element will be described. For the optional additive element, the upper limit value was determined within a range that does not impair the characteristics of the element by the above essential additive element.

  P decreases the strength of the grain boundaries. Therefore, in mass%, P is in the range of 0.025% or less.

  Since S combines with Mn to generate MnS inclusions, if excessively contained, the amount of inclusions that are the starting point of stress concentration is increased, leading to a decrease in fatigue strength required as an element. Therefore, in mass%, S is within a range of 0.015% or less.

  Mo has the effect of suppressing the formation of film-like carbides at the crystal grain boundaries, and can be expected to further improve the bending fatigue strength required as an element when added. Further, Mo has an effect of greatly increasing the hardenability, and addition is recommended. However, excessive addition of Mo causes a significant deterioration in punchability and an increase in cost required as element steel. Therefore, in mass%, Mo is in the range of 0.50% or less.

  Since B has the effect of suppressing grain boundary segregation of impurities such as P and increasing the grain boundary strength, addition of B is recommended to further improve the bending fatigue strength required as an element. However, excessive addition of B causes an increase in cost. Therefore, in mass%, B is in the range of 0.0050% or less.

  Ti can be Ti nitride, suppresses formation of B nitride, and contributes to improvement of grain boundary strength by B. However, excessive addition of Ti causes an increase in cost. Therefore, by mass%, Ti is in the range of 0.20% or less.

  Ni has the effect of increasing the intragranular strength of steel, and is effective in improving the fatigue strength required as an element. However, excessive addition of Ni deteriorates manufacturability in order to reduce the workability of the material, and causes an increase in cost. Therefore, in mass%, Ni is in the range of 0.50% or less.

  V forms fine V carbides in the steel and makes the crystal grains fine, so it is effective for further improving the fatigue strength and wear resistance required for the element. However, excessive addition of V causes excessive generation of V carbides, consumes carbon, and decreases hardenability. Therefore, in mass%, V is in the range of 0.50% or less.

  Nb forms fine Nb carbides in the steel and makes the crystal grains fine, so it is effective for further improving the fatigue strength and wear resistance required for the element. However, excessive addition of Nb causes excessive production of Nb carbides, consumes carbon, and decreases hardenability. Therefore, Nb is in the range of 0.20% or less in terms of mass%.

  Up to this point, the representative embodiments according to the present invention and the modifications based thereon have been described, but the present invention is not necessarily limited thereto. Those skilled in the art will recognize a variety of alternative embodiments and modifications without departing from the scope of the appended claims.

DESCRIPTION OF SYMBOLS 1 Bending fatigue strength test piece 4 Fatigue testing machine 10 Wear test apparatus 13 Wear test piece

Claims (5)

When containing at least C, Si, Mn, Cr, and the mass% of the element M is [M],
10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] ≦ 13
A steel for element of belt type CVT made of a steel having a component composition satisfying
The steel is in mass% as an essential additive element,
C: 0.50 to 0.70%,
Si: 0.10 to 0.60%,
Mn: 0.50 to 1.50%,
Cr: 0.20 to 1.00%
As an optional additive element,
P: ≦ 0.025%,
S: ≦ 0.015%
It is composed of the remaining Fe and unavoidable impurities, and is softened by heat treatment to give a hardness of 88 HRB or less. Steel for belt type CVT elements to produce excellent belt type CVT elements. As the optional additive element,
Mo: ≦ 0.50%,
B: ≦ 0.0050%,
Ti: ≦ 0.10%,
Can include
10.8 [C] +5.6 [Si] +2.7 [Mn]
+0.3 [Cr] +7.8 [Mo] ≦ 13
The steel for element of belt type CVT according to claim 1, wherein: As the optional additive element,
Ni: ≦ 0.50%,
V: ≦ 0.50%
Nb: ≦ 0.20%
Can include
10.8 [C] +5.6 [Si] +2.7 [Mn]
+0.3 [Cr] +7.8 [Mo] +3.6 [Ni] ≦ 13
The steel for element of belt type CVT according to claim 1 or 2, characterized by satisfying. A belt type CVT element steel comprising any one of claims 1 to 3 is cold punched into a predetermined shape and heated to a temperature close to an A _cm temperature so as to dissolve carbides while preventing coarsening of crystal grains. A belt-type CVT element characterized by being subjected to holding and quenching and tempering heat treatment to give a hardness of 640 Hv or more. ^5. The belt-type CVT element according to claim 4, wherein in the cross-sectional structure, the number of insoluble carbides having an equivalent circle diameter of 0.20 μm or more is 1.6 × 10 ⁵ or less per 1 mm square.