JP2007177317A - Steel for machine structure having excellent strength, ductility, toughness and abrasion resistance, its production method and metal belt using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for a machine structure having excellent strength, ductility, toughness and abrasion resistance and its production method at a low cost, and to inexpensively provide a metal belt suitable to a belt for a CVT (Continuously Variable Transmission). <P>SOLUTION: The steel for a machine structure having excellent strength, ductility, toughness and abrasion resistance has a steel composition comprising, by mass, >0.30 to 0.50% C, ≤1.0% Si, ≤1.5% Mn, 0.3 to 0.5% Mo, ≤0.1% Ti, and 0.0005 to 0.01% B, and the balance Fe with inevitable impurities, and has a hardened layer with a thickness of ≤50 μm and a Vickers hardness of ≥750 on the surface. The steel structure other than the hardened layer has an old austenite grain size of ≤10 μm, a martensite fraction of ≥90% and a Vickers hardness of 450 to <750. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention mainly relates to steel for machine structural use for automobiles and industrial machine parts, and in particular, a metal belt used for a continuously variable transmission (CVT) in which currently expensive maraging steel or the like is used. The present invention relates to a mechanical structural steel excellent in strength, ductility, toughness and wear resistance, a method for producing the same, and a metal belt using the same.

In recent years, due to increasing interest in environmental issues, further improvements in fuel efficiency and exhaust gas regulations have been demanded in the automobile field and the like. Therefore, the development direction is aimed at reducing the size and output of the drive system. As an example, development of a continuously variable transmission (CVT) generally referred to as a continuously variable transmission (transmission ratio) is active. A continuously variable transmission (hereinafter referred to as “CVT”) is a power transmission mechanism that continuously changes a gear ratio by friction using a belt or the like without using a gear. For example, a metal belt used in CVT Must have high strength, high ductility, high toughness, and wear resistance. Currently, maraging steel is used for such applications (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3). A technique using metastable austenitic stainless steel is also known (see, for example, Patent Document 4).
JP 2000-345302 A JP 2002-38251 A JP 2003-231921 A JP 2004-11683 A

  Regardless of the steel described in the above-mentioned patent documents, a method of adding an alloy element is generally used to increase the strength of the steel material. Maraging steel contains Co, Mo, Cr and the like in addition to Ni of several tens of mass%, and metastable austenitic stainless steel contains tens of mass% of Cr and Ni. Addition of such a large amount of alloy elements significantly increases the cost of steel, and in the current situation of material depletion, production itself may be intimidated.

  Therefore, the object of the present invention is to solve such problems of the prior art, and to provide a mechanical structural steel excellent in strength, ductility, toughness and wear resistance, and a method for producing the same, and further for CVT. The object is to provide a metal belt suitable for the belt at low cost.

The inventors have intensively studied to solve the above problems. As a result, even in the case of a component system that does not contain a large amount of Ni or Cr, such as maraging steel or austenitic stainless steel, the steel to which C, Mo, Ti, and B are added in an appropriate range is first subjected to nitriding treatment. If a layer of high hardness is formed on the surface, then quenching is performed by heating for a very short time by high frequency, tempering is performed at a low temperature range, and the other than the surface hardened layer is made a fine martensite structure of prior austenite, Obtaining the knowledge that excellent tensile strength-elongation balance, high toughness and high hardness surpassing maraging steel were obtained, and the present invention having the following characteristics was completed.
(1) In mass%, C: more than 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Mo: 0.3% or more, 0.5% or less, Ti: 0.1% or less, B: 0.0005% As mentioned above, it is a steel composition containing 0.01% or less, the balance being Fe and inevitable impurities, having a hardened layer with a thickness of 50 μm or less and a Vickers hardness of 750 or more on the surface, and the steel structure other than the hardened layer is old A machine structural steel excellent in strength, ductility, toughness, and wear resistance, characterized by having an austenite particle size of 10 μm or less, a martensite fraction of 90% or more, and a Vickers hardness of 450 or more and less than 750.
(2) Steel composition is further selected by mass, Al: 2.0% or less, Cr: 2.5% or less, Cu: 1.0% or less, Ni: 2.0% or less, V: 0.5% or less Or the steel for machine structure excellent in the intensity | strength, ductility, toughness, and abrasion resistance as described in (1) characterized by containing 2 or more types.
(3) The steel composition further includes one or more kinds selected from Co: 2.0% or less, W: 1.0% or less, and Nb: 0.1% or less in mass%. Machine structural steel excellent in strength, ductility, toughness, and abrasion resistance as described in (1) or (2).
(4) After rolling the steel having the composition according to any one of (1) to (3) into a predetermined shape, carburizing or nitriding to form a hardened layer on the surface, and then raising the temperature A mechanical structure excellent in strength, ductility, toughness, and wear resistance, characterized by being immediately quenched after being heated to 800 ° C. to 1100 ° C. at a rate of 100 ° C./second or more and immediately tempered at a temperature of 400 ° C. or less. Steel manufacturing method.
(5) The method for producing steel for machine structural use having excellent strength, ductility, toughness, and wear resistance according to (4), wherein the heating after forming the hardened layer is performed using high-frequency heating. .
The steel having the composition according to any one of (6), (1) to (3) is rolled into a steel sheet having a thickness of 0.5 mm or less, welded after cutting to form an endless ring, and the ring Carburized or nitrided to form a hardened layer on the surface, then heated and hardened by high frequency heating, immediately tempered at a temperature of 400 ° C. or less, strength, ductility, toughness, wear resistance An excellent ring manufacturing method.
(7) A metal belt excellent in strength, ductility, toughness, and abrasion resistance, wherein a plurality of rings manufactured by the method described in (6) are stacked and integrated.

  According to the present invention, a steel for machine structure excellent in high strength, high ductility, high toughness, and wear resistance can be obtained without adding a large amount of expensive alloy elements. For this reason, a metal belt for a continuously variable transmission can be provided at low cost.

  Details of the present invention will be described below.

  First, the reason why the steel composition is limited to the above range in the present invention will be described. In the following description, the content% of component elements means mass%.

C: Over 0.30% and 0.50% or less.
C is an essential element for securing the necessary strength and toughness, and it is difficult to secure a predetermined strength at 0.30% or less. On the other hand, if it exceeds 0.50%, the ductility and toughness are reduced, and huge carbides are formed in the steel structure, so that the fatigue properties are remarkably lowered.

Si: 1.0% or less.
Since Si acts as a deoxidizer during the melting of steel, it can be contained. However, if it exceeds 1.0%, the ductility of the steel is significantly reduced, so the upper limit was made 1.0%.

Mn: 1.5% or less.
Since Mn has an action as a deoxidizing agent when melting steel, it can be contained. However, if it exceeds 1.5%, the ductility of the steel is significantly reduced, so the upper limit was made 1.5%.

Mo: 0.3% or more and 0.5% or less.
Mo is a particularly important element in the present invention. Mo improves strength and toughness without significantly impairing ductility. Addition of 0.3% or more is essential to achieve the effect. On the other hand, even if added over 0.5%, the strength and toughness are not further improved, and the cost is increased. In addition, since the ductility tends to decrease when added excessively, the upper limit was made 0.5%.

Ti: 0.1% or less.
Ti combines with N mixed as an unavoidable impurity to prevent B from forming BN and disappearing the effect of improving the hardenability of B. In order to obtain this effect, it is preferable to contain 0.005% or more, but even if added over 0.1%, a large amount of TiN is formed, leading to a decrease in strength and fatigue strength, so the upper limit is made 0.1% .

B: 0.0005% or more and 0.01% or less.
B is a useful element that is effective in improving hardenability and contributes to improving the strength of the entire steel by grain boundary strengthening. For that purpose, the content of 0.0005% or more is necessary. However, even if contained over 0.01%, the effect is saturated, so it was limited to the above range.

  The above is the basic component in the present invention, but in the present invention, other elements described below can be appropriately contained.

  You may contain 1 type, or 2 or more types selected from Al, Cr, Cu, Ni, and V.

Al: 2.0% or less.
Al is an element effective for deoxidation. Further, by suppressing the austenite grain growth during quenching, it is effective in maintaining strength and toughness, and is also an element useful for hardening the nitride layer. However, even if the content exceeds 2.0%, the effect is saturated, and a disadvantage that causes an increase in cost occurs. Therefore, the content is limited to the above range.

Cr: 2.5% or less.
Cr is effective for improving the hardenability and is useful for securing the hardening depth. However, if contained excessively, the formation of residual carbides is promoted by the carbide stabilizing effect, resulting in a decrease in strength. Therefore, it is desirable to reduce the Cr content as much as possible, but up to 2.5% is acceptable. In addition, in order to express the effect | action which improves hardenability, it is preferable to make it contain 0.2% or more.

Cu: 1.0% or less.
Cu is effective in improving the hardenability, and improves the strength by solid solution in ferrite. However, if it exceeds 1.0%, cracking occurs during hot rolling. Therefore, it is limited to the above range. In order to develop the effect of improving hardenability and strength, it is preferable to contain 0.2% or more.

Ni: 2.0% or less.
Ni is effective in improving the hardenability and also suppresses the formation of carbides, thereby suppressing the generation of film-like carbides at the grain boundaries and increasing the grain boundary strength, thereby contributing to the improvement of strength and toughness. However, Ni is a very expensive element, and even if added over 2.0%, the effect is not only saturated, but the steel material cost is remarkably increased. Therefore, it is preferable to set it to 2.0% or less. In order to develop the effect of improving hardenability, strength, and toughness, 0.5% or more is preferably contained.

V: 0.5% or less.
V binds to C in steel and is expected to act as a strengthening element. It also has the effect of improving resistance to temper softening and contributes to strength improvement. However, even if contained over 0.5%, the effect is saturated, so it was limited to the above range. In addition, in order to express the effect | action which improves an intensity | strength, it is preferable to make it contain 0.1% or more.

  Furthermore, in this invention, 1 type, or 2 or more types selected from Co, W, and Nb component shown below can be contained.

Co: 2.0% or less.
Co is an element effective for strengthening grain boundaries, and its addition improves strength and toughness. However, it is very expensive, and if it exceeds 2.0%, the original invention significance of producing at low cost is lost. Therefore, it is limited to the range of 2.0% or less.

W: 1.0% or less.
W forms a stable carbide and is effective as a strengthening element. On the other hand, if the content exceeds 1.0%, the strength is rather lowered, so the content is limited to the above range.

Nb: 0.1% or less.
Nb contributes to the improvement of strength and toughness as a precipitation strengthening element in addition to the effect of improving hardenability. In order to exhibit this effect, it is preferable to contain 0.005% or more. However, even if the content exceeds 0.1%, the effect is saturated, so 0.1% or less is preferable.

  The balance other than the elements described above is Fe and inevitable impurities. The main inevitable impurities include S, P, N, and O. These elements are acceptable if S: 0.05% or less, P: 0.05% or less, N: 0.01% or less, and O: 0.01% or less.

  As mentioned above, although the suitable component composition range was demonstrated, in this invention, after limiting a component composition to said range, the steel structure needs to be adjusted so that it may demonstrate below.

Surface hardened layer: The surface has a hardened layer having a thickness of 50 μm or less and a hardness of Hv750 or more.
In order to use in an environment where wear resistance equal to or higher than that of maraging steel is required, in the present invention, a hardened layer is formed on the surface of the steel, and the surface hardness is set to 750 or more in terms of Vickers hardness. In this case, even if the thickness of the cured layer exceeds 50 μm, there is no change in the hardness of the cured layer itself, the treatment time for forming the cured layer becomes longer, resulting in higher costs, and rather a factor in reducing ductility. Therefore, it is limited to the above range. In order to maintain the wear resistance, the thickness of the cured layer is preferably 5 μm or more. The hardened layer needs to be formed on at least a part of the surface of the steel. In the case of a steel plate, the hardened layer is preferably formed on both surfaces which are plate surfaces.

  Next, the steel structure other than the hardened layer on the surface will be described.

Prior austenite grain size: 10 μm or less.
In the present invention, it is important to adjust the prior austenite particle size. By refining the prior austenite grain size, film-like carbides precipitated at the grain boundaries can be suppressed and the grain boundary strength can be increased. High strength, high ductility, and high toughness are exhibited by refinement of the prior austenite grain size. For this purpose, the particle size needs to be 10 μm or less.

Martensite structure fraction: 90% or more.
Martensite is an essential structure for obtaining strength. In the case of the present invention, the martensitic structure having a fraction of 90% or more exhibits excellent characteristics. Therefore, it was limited to the above range. If the martensite fraction is 90% or less, the amount of precipitates such as untransformed phases such as retained austenite phase and carbides that do not contribute to the increase in strength is excessive, and it is as high as maraging steel. Achievement of strength becomes difficult. The fraction of the martensite structure can be measured from the area ratio by, for example, observing a certain area with an optical microscope after corroding the steel surface.

Hardness: Vickers hardness is 450 or more and less than 750.
In order to replace the maraging steel, which is currently expensive, in order for the steel of the present invention to have a strength level equal to or higher than that of the maraging steel, the hardness needs to be 450 or more in terms of Vickers hardness. Moreover, in order to maintain a ductility and toughness level, it is necessary to prescribe | regulate a hardness upper limit. Therefore, the Vickers hardness is limited to a range of 450 or more and less than 750. If the parent phase other than the hardened layer is in this hardness range, it is superposed on the fine grains and exhibits high strength, high ductility, and high toughness.

  By satisfying the above component composition and steel structure, it has the same characteristics as maraging steel, has a tensile strength of 2000 MPa or more, satisfies a total elongation of 10% or more, maintains high toughness, and has excellent wear resistance. Steel for machine structural use.

  Next, the manufacturing method of the steel for machine structure excellent in the strength, ductility, toughness, and wear resistance of the present invention will be described. The steel for machine structure of the present invention is manufactured by using a steel having the above-described composition, forming a hardened layer on the surface of a material having a predetermined shape, and performing quenching and tempering.

  As the steel containing the above-described components, either steel manufactured by melting in a converter or steel manufactured by vacuum melting can be used. The ingot or continuous cast slab is heated and hot-rolled, pickled and scaled, and then cold-rolled to a predetermined thickness to obtain a predetermined shape.

  Thereafter, carburization or nitriding is first performed to form a hardened layer on the surface. The carburizing process can be performed using ordinary gas carburizing. Nitriding can be performed by gas nitriding or salt bath nitriding.

  After forming the hardened layer on the surface, quenching and tempering are performed to make the steel structure a martensite structure.

Quenching treatment: It is preferable to perform induction hardening.
One of the important conditions in the production method of the present invention is that quenching is performed by heating for a very short time after the surface is cured. This is to prevent the hardened layer generated previously from disappearing, and to avoid unnecessary coarsening of crystal grains and to obtain a fine crystal grain structure. For this purpose, it is heated to a maximum temperature of 800 ° C. to 1100 ° C. at a temperature rising rate of 100 ° C./s or more, and immediately quenched after reaching the maximum temperature. In order to achieve such extremely short heating, it is preferable to use high-frequency heating and water quenching.

Tempering temperature: 400 ° C. or less.
The tempering temperature is also an important condition in the present invention. Such a low temperature is a temperature range not normally used for tempering. However, in the case of the present invention, by using this temperature range, the contained B is concentrated at the grain boundary without appropriately diffusing or unnecessary precipitation, and appropriately contributing to strengthening of the grain boundary. And since the tempering temperature is not high, the strength level, toughness, and surface hardness above a certain level are maintained by superimposing the surface hardened layer and the fine grain effect.

  In the production method of the present invention, it is also an important condition that quenching and tempering is performed after the hardened layer is formed. When the surface hardening treatment is performed after quenching, the strength is lowered and it is difficult to achieve both high strength and wear resistance.

  Although the steel material thus obtained can be manufactured at a low cost, it has a balance of strength and ductility comparable to maraging steel and can be used for automobile parts that require high strength, high ductility, high toughness, and wear resistance. Can be used.

  Next, the metal belt manufactured using the steel for machine structure of the present invention will be described.

  The metal belt can be manufactured as follows. A steel having a component composition in the range of the present invention is manufactured as a steel plate having a thickness of 0.5 mm or less, and both ends cut into a belt shape are welded to form a loop shape to form a ring (endless ring). For welding, for example, laser welding or plasma welding can be used. Carrying or nitriding the formed endless ring and forming a hardened layer on the surface, followed by quenching and tempering as described above, and endless ring having high strength, high ductility, high toughness, and wear resistance Manufacturing. A plurality of such endless rings, for example, 10 are stacked to form a metal belt.

  When manufacturing an endless ring, it is important to form a hardened layer and quench and temper after forming a loop shape. When a steel sheet for machine structural use according to the present invention is manufactured by subjecting a steel sheet having a thickness of 0.5 mm or less to a predetermined heat treatment, cutting and welding to produce an endless ring, the strength and surface hardness of the welded portion cannot be obtained. Therefore, it is not desirable.

  The endless ring produced as described above has a hardened layer with a thickness of 50 μm or less and a Vickers hardness of 750 or more on the surface, and the prior austenite grain size of steel other than the hardened layer is 10 μm or less and has a martensite fraction. It is 90% or more, and is made of the steel for machine structure of the present invention having a Vickers hardness of 450 or more and less than 750.

  Since the metal belt needs to have wear resistance on the surface portion, it is effective if only the two outer layers, which are the surface layer portions stacked on each other, are endless rings using the steel of the present invention. In that case, for the endless ring other than the surface layer portion, it is preferable to use, for example, the endless ring manufactured by omitting the formation of the hardened layer.

  Metal belts manufactured using the steel for machine structural use according to the present invention have strength, ductility, toughness, and wear resistance equivalent to or higher than those manufactured using maraging steel, and are manufactured at low cost. It can be used suitably for a metal belt for CVT or the like.

  Steel No. 1 having chemical components shown in Table 1. 2 to 19 steels were produced by vacuum melting. These steels were heated to 1100 ° C. and hot-rolled to give a plate having a thickness of 3 mm. Thereafter, pickling was performed to remove the surface scale, and then cold rolling was performed. Rolling was performed many times, and when the thickness was 0.8 mm, annealing was performed once to remove processing strain, and further cold rolling was performed. The final thickness is 0.4 mm and the material is subjected to the following heat treatment to produce test pieces of symbols 1-2 to 1-19, tensile test, toughness evaluation, structure evaluation, surface hardness It was used for the measurement.

A test piece was cut out from the material by electric discharge machining into a shape of a tensile test piece (JIS No. 5). The test piece was subjected to tuftride treatment at 570 ° C. for 120 minutes, and a nitrided hard layer was provided on the surface. Next, after heating to 920 ° C. at a heating rate of 300 ° C./s by high-frequency heating, water quenching was performed immediately. Thereafter, tempering was carried out at 170 ° C. for 20 minutes and subjected to a tensile test.

Steel No. shown in Table 1 No. 1 is maraging steel (Fe-18 mass% Ni-8 mass% Co-5 mass% Mo-0.4 mass% Ti). The maraging steel is also made into a raw material having a thickness of 0.4 mm by cold rolling. After cutting out a test piece having the same shape as in Example 1, the sample was heated at 820 ° C., then quenched by air cooling, and subjected to an aging treatment by heating at 520 ° C. Thereafter, gas nitriding (treatment at 550 ° C. for 5 hours) was performed. (Symbol 1-1)
Only the evaluation of toughness was different from the above, and the thickness was 15 mm by hot rolling. A U-notch Charpy test was cut out so as to coincide with the C direction of the rolled material. The test piece was subjected to nitriding under the same conditions as described above, then heated to 920 ° C. at 300 ° C./s with a temperature increase rate by induction hardening, and then immediately quenched with water . Tempering was performed at 170 ° C. for 30 minutes and then subjected to a Charpy test. The maraging steel was heat-treated under the same conditions as described above. The test was performed under two conditions of 40 ° C. and −40 ° C., and the absorbed energy was compared.

  The tissue evaluation was performed by the following method. First, regarding the depth of the hardened layer, the hardness was measured at a pitch of 10 μm with 10 g of micro Vickers from the cross section, and the depth of the portion of Hv750 or higher was obtained as the hardened layer depth from the hardness profile. The hardness of the hardened layer was an average value of hardness measurement values of Hv750 or higher. For the hardness of the tissue part other than the hardened layer, the hardness measurement value at the center in the thickness direction was adopted. Regarding the prior austenite particle size, the austenite particle size was revealed by corrosion, then observed and photographed at 1000 times, and obtained from the obtained image by a cutting method. As for the martensite fraction, observation and photographing were carried out at 5 times with 400% after corrosion with 3% nitric acid alcohol, and the area ratio of martensite was obtained from the obtained image to obtain the fraction.

  Table 1 also shows the measurement results of the fraction of martensite structure, prior austenite grain size, hardened layer (nitriding layer) depth and surface hardness, tensile strength, total elongation, and toughness. According to Table 1, a steel having a structure including a component and a nitrided layer within the scope of the present invention has a tensile strength of 2000 MPa or more, a total elongation of 10% or more, and a Charpy absorbed energy of 40 J or more at both low and high temperatures. The results were good as the maraging steel, and the strength ductility balance exceeded that of the maraging steel.

  In this example, the effects of martensite fraction and prior austenite grain size were examined. Steel No. 1 in Table 1 In order to observe the effects of martensite fraction and prior austenite grain size on No. 4, test pieces were produced under various conditions by changing the frequency (quenching temperature) and frequency of high-frequency heating. All other experimental methods were the same as in Example 1. The measurement results are shown in Table 2. According to Table 2, it can be seen that when the martensite fraction is lower than 90%, the strength is significantly reduced. It can also be seen that when the prior austenite grain size is larger than 10 μm, the strength and ductility are reduced, but the toughness is also significantly reduced.

  In this example, the effect of adding other components (Al, Cr, Cu, Ni, V, Co, W, Nb) was examined. Steels having chemical components as shown in Table 3 (steel Nos. 21 to 34) were manufactured by vacuum melting. All other experimental methods were the same as in Example 1, and for the symbols 3-1 to 3-14, a tensile test, toughness evaluation, structure evaluation, and surface hardness measurement were performed. The results are also shown in Table 3. Co addition further increases the strength of the steel, and if Cr and W are contained excessively, the strength and toughness are reduced. Al, Cu, Ni, V, and Nb are also effective even if added excessively. It turns out to be saturated.

  In this example, steel No. 1 in Table 1 was used. For No. 4, the effect of quenching and tempering conditions was examined. The test piece of the symbol 4-1 was performed in the same manner as the symbol 1-4 (high-frequency heating quenching) in Example 1 until the nitriding treatment, and the subsequent quenching was performed by infrared induction heating (temperature increase rate 20 ° C./s). . The time to reach the maximum temperature was 1 hour. After reaching the maximum temperature, it was immediately removed and subjected to water quenching. Thereafter, tempering was performed at 170 ° C. for 30 minutes.

  Moreover, about the influence of tempering temperature, after performing to quenching similarly to the code | symbol 1-4 in Example 1 (tempering temperature 170 degreeC) about the test piece of symbols 4-2 to 4-4, tempering temperature is set to 225,450. The measurement was carried out at 550 ° C.

  The results are shown in Table 4. In the atmosphere furnace heating, the prior austenite grain size becomes large and the necessary characteristics are not exhibited. It can also be seen that the nitride layer is dissipated and the effect disappears.

  Regarding the tempering temperature, the strength is significantly reduced at 400 ° C. or higher. Moreover, it turns out that a surface hardening layer is lost when it tempers at 550 degreeC or more.

  In this example, the fatigue strength when actually manufacturing an endless ring was evaluated. Steel No. For 1, 4 and 11, a 0.4 mm-thick material was produced in the same manner as in Example 1, cut to a width of 20 mm, welded at both ends, and connected in a ring shape, and then symbols 1-1, 1- Heat treatment similar to 4, 1-11, 2-5 and 4-3 was performed to produce an endless ring. The fatigue strength is determined by applying a constant tensile load (P = 3500 N) by applying an endless ring 2 to a pulley 1 made of SUJ2 as shown in FIG. 1 (the outer diameter of the belt wheel 1a is 120 mm and the outer diameter of the belt wheel 1b is 200 mm). The number of rotations until breakage when rotating at 2000 rpm while being applied was measured and evaluated. The materials used for the experiment are maraging steel (symbol 1-1), steel of the present invention (symbol 1-4), and comparative steels (symbols 1-11, 2-5, 4-3). Each test was performed three times (n = 3).

  The results are shown in Table 5. The endless ring (endless ring No. 5-2) using the steel of the present invention has the same number of times to break as the maraging steel (endless ring No. 5-1), but the endless ring using comparative steel ( In endless rings No. 5-3 to 5-5), it can be seen that the number of times until breakage is significantly reduced.

  In this example, the steel No. 1 shown in Table 1 was used. 4 was used. A tensile test piece was produced in the same manner as in Example 1, and after performing nitriding treatment, quenching and tempering were performed, exactly the same as symbol 1-4 in Example 1, and nitriding treatment was performed after quenching as shown below. A comparison was made on the symbol 6-1 performed.

  That is, for the symbol 6-1, steel no. From the material manufactured in the same manner as in Example 1 using 4, a test piece was cut out in the shape of a tensile test piece (JIS No. 5) by electric discharge machining. Next, after heating to 920 ° C. by high frequency heating, quenching was performed immediately. Thereafter, tempering was performed at 170 ° C. for 20 minutes. Further, this test piece was subjected to tuftride treatment at 570 ° C. for 120 minutes, and a nitrided hardened layer was provided on the surface. It used for evaluation after that.

  The test piece evaluation method was the same as in Example 1, and a tensile test, toughness evaluation, structure evaluation, and surface hardness measurement were performed.

  The evaluation results are shown in Table 6. The sign 6-1 that has been subjected to nitriding after quenching and tempering has a sufficient hardness with a good hardened layer formed on the surface, but the hardness (internal hardness) of the structure other than the hardened layer decreases. Thus, it was outside the scope of the present invention, the strength as a whole was lowered, and the tensile strength was lowered to the same level (symbol 4-4 in Example 4) as high temperature tempering.

Schematic which shows the fatigue strength test method of an endless ring.

Explanation of symbols

1 Pulley 1a, 1b Belt wheel 2 Endless ring

Claims (7)

  In mass%, C: more than 0.30%, 0.50% or less, Si: 1.0% or less, Mn: 1.5% or less, Mo: 0.3% or more, 0.5% or less, Ti: 0.1% or less, B: 0.0005% or more, 0.01% The steel composition comprising the following, the balance being Fe and inevitable impurities, having a hardened layer with a thickness of 50 μm or less and a Vickers hardness of 750 or more on the surface, and the steel structure other than the hardened layer has a prior austenite grain size A machine structural steel excellent in strength, ductility, toughness, and wear resistance characterized by having a martensite fraction of 10% or less, a martensite fraction of 90% or more, and a Vickers hardness of 450 or more and less than 750.   Steel composition is one or more selected from mass%, Al: 2.0% or less, Cr: 2.5% or less, Cu: 1.0% or less, Ni: 2.0% or less, V: 0.5% or less The steel for machine structural use according to claim 1, which is excellent in strength, ductility, toughness, and wear resistance.   The steel composition further comprises one or more selected from Co: 2.0% or less, W: 1.0% or less, and Nb: 0.1% or less in mass%. The steel for machine structure excellent in strength, ductility, toughness, and wear resistance according to claim 2.   The steel having the composition according to any one of claims 1 to 3 is rolled into a predetermined shape, and then a carburizing process or a nitriding process is performed to form a hardened layer on the surface. Production of steel for mechanical structures with excellent strength, ductility, toughness, and wear resistance, characterized by being immediately quenched after being heated to 800 ° C to 1100 ° C for at least 2 seconds and then immediately tempered at a temperature of 400 ° C or lower. Method.   Heating after forming a hardened layer is performed using high frequency heating, The manufacturing method of the steel for machine structures excellent in the intensity | strength, ductility, toughness, and abrasion resistance of Claim 4 characterized by the above-mentioned.   The steel having the composition according to any one of claims 1 to 3 is rolled into a steel sheet having a thickness of 0.5 mm or less, welded after cutting to form an endless ring, and carburizing treatment or A ring with excellent strength, ductility, toughness and wear resistance, characterized by forming a hardened layer on the surface by nitriding, then heating and quenching by high-frequency heating, and immediately tempering at a temperature of 400 ° C. or lower Manufacturing method.   A metal belt excellent in strength, ductility, toughness, and abrasion resistance, wherein a plurality of rings manufactured by the method according to claim 6 are stacked and integrated.

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