JP5619668B2 - Cold stamping steel and steel belt element using the same - Google Patents

Description

  The present invention relates to a steel for cold punching and an element processed into a steel belt element used for a belt type CVT of an automobile or the like.

Belt-type continuously variable transmission for automobiles (Continuously
In Variable Transmission (CVT), power is transmitted by putting a steel belt between a pair of pulleys on the input side and output side. Such a steel belt has a structure in which a plurality of chip-like elements (steel pieces) are assembled along an annular belt. When this steel belt is fitted into the V-shaped groove of the output pulley and the groove width is changed, the steel belt moves in the radial direction of the pulley, and its rotational radius can be continuously adjusted. The rotation ratio of the pulley can be changed smoothly.

  As described above, since the steel belt element is always driven in contact with the V-shaped groove of the output pulley, steel with high hardness having excellent wear resistance is used. Generally, JIS SKS95 (mass%, C: 0.80-0.90%, Si: 0.50% or less, Mn: 0.80-1.10%, P: 0.030% or less, Steel having a relatively large carbon content such as S: 0.030% or less, Cr: 0.20 to 0.60% is used. Cold-rolled material containing carbide that has been spheroidized is cold-punched into the shape of the element and quenched and tempered from a temperature above the Acm point on the equilibrium diagram to disperse a certain amount of undissolved carbide. Given a tempered martensite structure.

  In order to cold punch the element, the productivity of the steel having high hardness is lowered. Therefore, a manufacturing method in which a steel subjected to a softening heat treatment is subjected to cold punching, and then a hardening heat treatment is considered. Here, in order to prevent the deformation of the element after the punching process, the curing heat treatment should be performed at a relatively low temperature and in a short time. On the other hand, the present inventor focused on steel near the eutectoid composition having the lowest austenite single-phase stability region temperature on the equilibrium diagram, and performed a heat treatment at a temperature near the eutectoid point to obtain a high hardness. We are studying to obtain a steel that has both high wear resistance and high toughness that can withstand contact by relative movement with the pulley.

For example, Patent Document 1 discloses a high carbon steel member that is a steel in the vicinity of a eutectoid composition and has a high impact value of 25 J / cm ² or more while maintaining a hardness of 600 to 900 Hv. Specifically, by mass%, C: 0.60 to 1.30%, Si: ≦ 1.0%, Mn: 0.2 to 1.5%, P: ≦ 0.02%, S: ≦ 0 0.02%, Mo: ≦ 0.5%, V: ≦ 0.5%, and the composition after quenching / tempering is 8.5 <15.3 × C% − It is disclosed that the coarse undissolved carbide having a particle size of 1.0 μm or more should be restricted to 2 or less per 100 μm ² of observation area with V _f <10.0 volume ratio V _f (volume%). doing. Here, it is stated that when Mo is added, hardenability and toughness are improved, Ni and a special carbide are formed, and wear resistance is also improved. It is also stated that when V is added, the austenite crystal grains can be refined during quenching to improve wear resistance.

  Patent Document 2 discloses a carbon steel which is a steel in the vicinity of a eutectoid composition and is excellent in toughness and fatigue resistance. Specifically, in mass%, C: 0.50 to 0.70%, Si: ≦ 0.5%, Mn: 1.0 to 2.0%, P: ≦ 0.02%, S: ≦ 0 0.02%, Al: 0.001 to 0.10%, V: 0.05 to 0.50%, Ti: 0.02 to 0.20%, Nb: 0.01 to 0.50% , Mo: ≦ 0.50% of component composition including one or more kinds, the spheroidization rate of undissolved carbide in the structure after annealing is 95% or more, and the particle size is 2.5 μm or more It is disclosed that no coarse undissolved carbide is produced. Here, it is stated that when Mo is added, hardenability is improved, and when V is added, carbonitrides are formed and toughness is increased.

JP 2006-63384 A JP 2009-24233 A

  Like the steel currently disclosed by patent document 1 and 2, the steel which is excellent in abrasion resistance and toughness can be obtained by adding rare metals, such as Mo and V. However, from the viewpoint of cost, it is desired that the wear resistance and toughness of the same level or higher can be obtained while reducing the amount of these rare metals added.

  The present invention has been made in view of such a situation. The object of the present invention is to provide a belt-type CVT such as an automobile excellent in wear resistance and toughness while suppressing the addition of rare metals such as Mo and V. It is an object of the present invention to provide an element for a steel belt to be used and a steel for cold punching that provides the element.

  The steel for cold stamping according to the present invention is 10.8 [C] +5.6 [Si] +2.7 [Mn] +0.3 [Cr] +7.8 when the mass% of the element M is [M]. It is a steel for cold stamping made of steel having a component composition satisfying [Mo] +1.4 [V] ≦ 13, in mass%, C as an essential additive element, 0.50 to 0.70%, Si 0.03 to 0.60%, Mn 0.50 to 1.00%, Cr 0.20 to 1.00%, Ti 0.01 to 0.10%, and B 0.0005% In the range of 0.0050% to 0.0050%, and as an optional additive element, P is 0.025% or less, and S is in a range of 0.015% or less, with the balance being Fe and inevitable impurities, and austenite After heating and holding in the single-phase temperature range, it is cooled at a predetermined rate, and finely mainly in the ferrite + pearlite mixed structure Characterized in that it gave 88HRB following hardness at product was dispersed tissue.

  According to this invention, as a steel for cold punching, cold punching into the shape of a steel belt element of a belt type CVT can be favorably performed. Further, it mainly has a structure in which fine carbides having B as a nucleus are dispersed in a ferrite + pearlite mixed structure. By a predetermined quenching and tempering heat treatment, coarse carbides can be suppressed and high toughness as an element can be provided while giving a high wear resistance as an element by a dispersed structure of fine carbides.

In the above-described invention, in the cross-sectional structure, coarse carbides having an equivalent circle diameter of 0.5 μm or more may be suppressed to 1.2 × 10 ⁵ or less per 1 mm square. According to this invention, a predetermined quenching and tempering heat treatment can suppress coarse carbides and can provide high toughness as an element.

  The steel belt element of the belt type CVT according to the present invention is 640 Hv or more by applying a quenching and tempering heat treatment after cold punching the steel for cold punching according to any one of the above inventions into a predetermined shape. It is characterized by giving the hardness.

According to this invention, it has high toughness as an element by the structure | tissue which suppressed the coarse carbide | carbonized_material while having high abrasion resistance as an element.
It is.

In the above-described invention, in the cross-sectional structure, coarse carbides having an equivalent circle diameter of 0.5 μm or more may be suppressed to 1.3 × 10 ⁴ or less per 1 mm square. According to this invention, it has high toughness as an element by the structure | tissue which suppressed the coarse carbide | carbonized_material while having high abrasion resistance as an element.

It is a figure which shows the manufacturing process of the element by this invention. It is a figure which shows the cross-sectional structure | tissue in softening heat processing. It is a figure which shows the component composition of an Example and a comparative example. It is a figure which shows the shape of the test piece of an impact test. It is a figure which shows the method of an abrasion test. It is the figure which put together the test result. It is a figure which shows the impact ratio with respect to the hardness after hardening heat processing. It is a figure showing composition distribution of insoluble carbide It is a photograph of a cross-sectional structure showing the progress of cracks. 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 figure which shows the observation number for every magnitude | size of the insoluble carbide | carbonized_material after softening heat processing. It is a figure which shows the observation number for every magnitude | size of the insoluble carbide | carbonized_material after hardening heat processing.

  A method of manufacturing a steel belt element of a belt type CVT as one embodiment according to the present invention will be described with reference to FIG.

  First, softening heat treatment is performed so that a thin steel plate having a predetermined component composition in the vicinity of a eutectoid composition containing a predetermined amount of B and Ti is easily punched (described later) (S1). In such softening heat treatment, the steel sheet is heated to a relatively low temperature in the austenite single-phase stable temperature range, that is, about 20 to 30 ° C. higher than the A3 and Acm lines, held for a predetermined time, and then cooled at a predetermined rate. To do. In this softening heat treatment, a steel for cold punching in which insoluble carbides are finely dispersed by B in the component composition can be obtained. This steel for cold punching has good cold punchability and can easily process the shape of the element.

  Here, as shown in FIG. 2, in the thin steel plate having a pearlite structure, B is particularly dispersed in the cementite portion in the pearlite structure (see FIG. 2 (a)). When this thin steel sheet is heated and held in the austenite single-phase stable temperature range, it changes into an austenite single-phase (see FIG. 2B). When the temperature is gradually lowered to the ferrite stable temperature range again before completely changing to the austenite single phase, carbon that cannot be dissolved in ferrite first precipitates as carbide, but some carbide precipitates dispersed B. It precipitates as a nucleus (see also FIG. 2 (c) and FIG. 8). When the temperature is continuously lowered, a structure is obtained in which carbides are finely dispersed in a mixed structure of coarse pearlite grains and ferrite grains (see FIG. 2D).

  That is, in order to lower the temperature to the ferrite stable temperature range while maintaining the dispersion without aggregating B, the holding temperature is changed to the austenite single phase completely at a relatively low temperature near the A3 line and the Acm line. The temperature drop is started. In addition, by including Ti together with B, Ti generates N and nitride in preference to B, suppresses the generation of B nitride, and maintains the dispersion of B.

  Referring to FIG. 1 again, an element is obtained by punching into a predetermined element shape from the steel for cold punching (S2).

  Further, a curing heat treatment is performed so as to impart wear resistance and the like to the element obtained by punching (S3). That is, quenching and tempering are performed. In order to prevent the deformation of the element made of a thin steel plate, it is preferable to perform this heat treatment at a relatively low temperature and in a short time. That is, as in the softening heat treatment (S1), the austenite single-phase stable temperature range is maintained at a relatively low temperature and quenched. In such a case, the fine carbide in which B is dispersed in the nucleus is maintained by the softening heat treatment. By the above, the steel belt element used for belt type CVTs, such as a motor vehicle excellent in abrasion resistance and toughness, can be obtained.

  Next, evaluation of the hardness required for cold punching steel for cold punching into the steel belt element, and steel when hardening heat treatment is applied to this cold punching steel The mechanical properties (toughness and wear resistance) required for belt elements were evaluated. This will be described with reference to FIGS.

First, the present inventor has obtained the following empirical formula for the relationship between component elements and hardness in adjusting the component composition such as JIS SKS95 so as to obtain hardness suitable for cold punching processing so far. .
H ₁ = 10.8 [C] +5.6 [Si] +2.7 [Mn]
+0.3 [Cr] +7.8 [Mo] +1.4 [V] +75 (Formula 1)
Therefore, when the target value of the component composition was first selected and steel was manufactured so as to obtain the predetermined hardness described later using Equation 1, the component compositions of Examples 1 to 10 and Comparative Examples 1 to 11 shown in FIG. Of steel was obtained. Here, in the component compositions of Examples 1 to 10 and Comparative Examples 1 to 11 in FIG. 3, Mo was added in Comparative Example 3 and V was added according to the target value in Comparative Example 4, but in other Examples and Comparative Examples, , Mo and V are not intended to be added as target values, and both are detected as impurities.

  A method for producing a test piece for evaluation will be described. First, 150 kg of the master alloy is melted in a vacuum induction furnace to obtain an ingot having the component composition shown in FIG.

  Next, after holding the ingot at 1200 ° C. for 3 hours, a part thereof was cut out and hot forged into a substantially cylindrical round bar having a diameter of 25 mm. The forging end temperature was 900 ° C. or higher. Subsequently, the round bar was held at 840 ° C. for 60 minutes, air-cooled and normalized. Moreover, after hot-rolling the remainder of the cut out ingot to 3.5 mm, performing the same normalization, it cold-rolled further to 1.5 mm, and obtained the rolling material.

  The rolled material was subjected to a heat treatment that was held at 760 ° C. for 1 hour, gradually cooled to 650 ° C. at 10 ° C./hr, and then air-cooled as the softening heat treatment (S1). As appropriate, polishing was performed to obtain a structure observation specimen, and the Rockwell hardness, the number of insoluble carbides, and the like were measured.

  About a round bar, after performing the heat treatment imitating the same softening heat treatment (S1), further, as the above-described curing heat treatment (S3), after holding at 800 ° C. for 30 minutes, quenching in a 70 ° C. oil bath, 180 ° C. Quenching and tempering were performed for 120 minutes. A part of the round bar after the heat treatment was cut out, and a sub-size impact test piece 1 having a shape as shown in FIG. 4 and a wear test width 15.75 mm, height 10.16 mm, and thickness 6.35 mm to be described later. And a wear test piece 13 having a substantially rectangular parallelepiped block shape. In addition, the test piece 13 performed grinding | polishing etc. suitably and measured the Vickers hardness, the number of insoluble carbides, etc. as a structure | tissue observation test piece.

Rockwell hardness, using a commercially available Rockwell hardness testing apparatus, and the average value of measurements at arbitrary 5 points and the measured value H _2. Since the empirically ease cold punching are different relative to the hardness of less than 88HRB, 6, when the measurement value of H ₂ less than 88HRB good cold punching property (○), no more The cold punching property was evaluated as poor (x).

Vickers hardness, using a commercially available Vickers hardness meter, was measured five points at a depth of about 25μm from the surface in the cross section of the wear test piece 13 was the average value and the measured value H _3.

  The impact test was performed using a commercially available Charpy impact test apparatus. In addition, the impact ratio of FIG. 6 is a ratio with respect to the measured value in the test piece of Comparative Example 3. When this impact ratio is 1 or more, it is evaluated that the toughness is good (◯), and less than 1, the toughness is poor (x).

  The wear test was performed by a block-on-ring method using a wear test apparatus 10 as shown in FIG. Specifically, a wear test piece 13 is brought into contact with a rotating ring 11 immersed in a tank 14 storing 110 ° C. oil 12 at a load of 1200 N, and the wear amount at a relatively sliding distance of 3000 m is measured. did. The sliding speed of the wear test piece 13 with respect to the ring 11 is 0.05 m / sec. 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 to a hardness of about 750 Hv. The wear ratio in FIG. 6 is a ratio to the wear area of Comparative Example 3 by measuring the cross-sectional area of the wear portion of the wear test piece 13. When this wear ratio was less than 1, the wear resistance was good (◯), and when it was 1 or more, the wear resistance was poor (x).

  The number of insoluble carbides was measured by image analysis of the cross-sectional structure. The number of undissolved carbides having an equivalent circle diameter of 0.50 μm or more present per 100 μm square was converted to the number present per 1 mm square.

  FIG. 6 summarizes the results.

First, consistent well above the predicted value H ₁ hardness was calculated from the composition of Fig. 3 using Equation 1 and the measurement value H _2. This shows that the effect on the hardness can be predicted by Formula 1 even in the present example in which B and Ti are added.

In Examples 1 to 10, the hardness after the softening heat treatment (S1) (hereinafter referred to as “hardness after the softening heat treatment”) H ₂ is smaller than 88HRB and has excellent punchability. On the other hand, the hardness after hardening heat treatment (S3) (hereinafter referred to as “hardness after hardening heat treatment”) H ₃ is approximately the same as or lower than that of Comparative Example 3, but the impact ratio is 1 or more, the wear ratio. Is less than 1. That is, the toughness and the wear resistance are equal to or higher than those of conventional materials.

As a reference, in Comparative Example 1 in which the content of C was increased without adding Mo to Comparative Example 3, the hardness H ₂ after the softening heat treatment was as high as 89.0HRB, and the punchability was poor. Further, while the hardness H ₃ after the curing heat treatment is high, the impact ratio is smaller than 1, and the wear ratio is larger than 1. That is, it is inferior to Comparative Example 3 in toughness and wear resistance. The number of undissolved carbides after the curing heat treatment is much larger than that in Comparative Example 3, and thus it is considered that the wear resistance is lowered.

In Comparative Example 2 in which Mo was not added and the C content was reduced with respect to the conventional material, the hardness H ₃ after curing heat treatment was as low as 537 Hv, the impact ratio was larger than 1, but the wear ratio was 5 .58 and it was very large. That is, it is greatly inferior to conventional materials in wear resistance.

  In Comparative Example 4 in which V is added instead of Mo with respect to the conventional material, the toughness and wear resistance are substantially the same as those in Comparative Example 3.

In Comparative Examples 5 and 7 in which the content of Si and Mn was increased without adding Mo to the conventional material, the hardness H ₂ after the softening heat treatment exceeded 88 HRB, and steel for cold punching Tends to be inferior in cold punchability.

On the other hand, in Comparative Example 8 in which Mo is not added and the Mn content is reduced compared to the conventional material, the hardness H ₂ after the softening heat treatment is low, and the cold punching steel is good as a steel for cold punching. is there. However, the wear ratio is much larger than 1, which is much inferior to conventional materials in wear resistance.

  In Comparative Example 9 in which the Mo content was not added and the Cr content was increased with respect to the conventional material, the cold punchability was good as a steel for cold punching. However, the impact ratio is smaller than 1, the wear ratio is larger than 1, and the toughness and wear resistance are inferior to the conventional materials.

  In Comparative Example 10 in which Mo was not added to the conventional material and the Cr content was reduced, the wear ratio was larger than 1 and the wear resistance was inferior to that of the conventional material.

  On the other hand, in Example 10 in which B and Ti were added instead of Mo as compared with the conventional material, the steel had good cold punchability as a steel for cold punching, the impact ratio was as large as 1.22, and wear The ratio is as small as 0.65. That is, it has good toughness and wear resistance.

  Even in Example 1, where B and Ti are added instead of Mo and Mn is added while reducing the C content, the steel has good punchability as a steel for cold punching, The ratio is very large at 1.40 and has particularly excellent toughness.

  Even in Example 2 in which B and Ti are added instead of Mo and the contents of Cr and P are increased with respect to the conventional material, the steel has good punchability as a steel for cold punching. Moreover, both toughness and wear resistance are equal to or higher than those of conventional materials.

  In Example 3 in which B and Ti were added instead of Mo and the Mn content was further reduced compared to the conventional material, the steel had good punchability as a steel for cold punching, and the impact ratio was 1. The wear ratio is as small as 0.68, and it has good toughness and wear resistance.

  In Example 4 in which B and Ti were added instead of Mo to the conventional material and the Si content was increased, the steel had good punchability as a steel for cold punching, and the impact ratio was 1 .20, wear ratio is as small as 0.65, and has good toughness and wear resistance.

  Even in Example 5 in which B and Ti are added instead of Mo, and the content of C is increased while reducing the Si content, it has good punchability as a steel for cold punching. Have. Moreover, both toughness and wear resistance are equal to or higher than those of conventional materials.

  In Examples 6 and 7 in which B and Ti are added instead of Mo to the conventional material and the contents of B and Cr are reduced, the steel has good punchability as a cold punching steel. Further, the toughness of the steel belt element is equal to or higher than that of the conventional material, and the wear resistance is good.

  In Examples 8 and 9 in which B and Ti are added instead of Mo and the contents of S and Ti are increased with respect to the conventional material, the steel has good punchability as a steel for cold punching. It has excellent toughness and wear resistance as a steel belt element.

  By the way, with respect to Example 10, Comparative Example 6 in which B and Ti were not added has good punchability as a steel for cold punching. However, the impact ratio is smaller than 1, the wear ratio is larger than 1, and both toughness and wear resistance are inferior to those of conventional materials. In particular, the toughness and wear resistance are greatly deteriorated with respect to Example 10.

  Further, in Comparative Example 11 in which Ti was not added to Example 10, the steel had good punchability as a cold punching steel. However, the impact ratio is less than 1, the wear ratio is greater than 1, and both the toughness and wear resistance of the steel belt element are inferior to those of conventional materials. In particular, the toughness and wear resistance are greatly deteriorated with respect to Example 10.

  The tendency obtained from each result of the above examples and comparative examples will be described.

As shown in FIG. 7, in the graph of the impact ratio with respect to the hardness H ₃ after curing heat treatment, Examples 1 to 10 to which B and Ti were added are Comparative Example 3 to which Mo was added and Comparative Example 4 to which V was added. It is in almost the same position. At least in the upper right position of Comparative Examples 1 and 2 and Comparative Examples 5 to 10 in which B and Ti were not added. That is, even if the hardness H ₃ after the curing heat treatment is high, the impact resistance tends to be excellent. This is thought to be because B increases the grain boundary strength and suppresses the precipitation of coarse undissolved carbide that becomes the starting point of fracture. As described above, B becomes a precipitation nucleus of undissolved carbide during the softening heat treatment, and as a result, suppresses the precipitation of coarse undissolved carbide that becomes a starting point of fracture, but as shown in FIG. The concentration of B is high at the center of the insoluble carbide 15.

Further, as compared with Comparative Example 11 was not added Ti added B, Example 1 to 10 have a high impact ratio to after hardening heat treatment hardness H _3. In Examples 1 to 10, it is considered that Ti is bonded to N before B, thereby reducing N bonded to B and further enhancing the effect of B described above.

  Next, although it is a form of abrasion, abrasion advances by generation | occurrence | production and growth of the micro crack 21 from a surface, and peeling of a surface. As shown in FIG. 9, the microcracks 21 preferentially propagate through the interface between the undissolved carbide 22 and the parent phase 23. That is, it is considered that as the insoluble carbide 22 is larger, stress concentration is more likely to occur at the interface between the insoluble carbide 22 and the parent phase 23 and the microcracks 21 are easily propagated. Therefore, as shown in FIG. 10, when the relationship between the number of coarse undissolved carbides having an equivalent circle diameter of 0.5 μm or more and the wear ratio is summarized, the wear ratio decreases as the number decreases. That is, it can be seen that the wear resistance can be improved by reducing the number of coarse insoluble carbides 22.

  On the other hand, the number of coarse undissolved carbides depends on the C content, but if the C content is reduced, the hardness is lowered and the wear resistance is lowered. As shown in FIG. 11, when the hardness is 640 Hv or less after the curing heat treatment, the wear ratio increases rapidly, that is, the wear resistance greatly decreases. That is, there is a lower limit value of the C content necessary to give good wear resistance as a steel belt element. On the other hand, Comparative Example 1 and the like having a high C content have poor cold punchability as a steel for cold punching, and there is an upper limit for the C content.

The range of the C content that gives both the wear resistance as the steel belt element and the cold punchability as the steel for cold punching is 0.50 to 0.70% in mass%, The number of coarse undissolved carbides having a hardness of 640 Hv or more and an equivalent circle diameter of 0.5 μm or more after curing heat treatment is 130 or less per 100 μm square, that is, about 1.3 × 10 ⁴ or less per 1 mm square. It is said.

  Next, with respect to the addition of B and coarse undissolved carbide, as shown in FIG. 12, after softening heat treatment, Example 10 to which B was added was compared to Comparative Example 6 to which B was not added. There are more undissolved carbides having an equivalent circle diameter of 0.30 μm or less, and fewer undissolved carbides larger than this. In other words, it can be seen that the addition of B refines the insoluble carbide after the softening heat treatment.

  Further, as shown in FIG. 13, after curing heat treatment, Example 10 to which B was added had more undissolved carbide having an equivalent circle diameter of 0.30 μm or less than Comparative Example 6 to which B was not added. There are fewer undissolved carbides larger than this. That is, the effect of making carbide fine by adding B is effective even after the heat treatment for curing.

  As described above, without adding Mo or V, the predetermined component composition and the predetermined heat treatment to which B and Ti are added, and the toughness is increased, while the insoluble carbide is finely precipitated and wear resistance is increased. In addition, it is possible to obtain a steel for cold punching that can provide an excellent steel belt element.

  Based on the above examples and comparative examples, the composition range of the steel for cold stamping was determined by the following guidelines. First, the essential additive elements C, Si, Mn, Cr, B, and Ti will be described.

  As described above, C is the most important element for ensuring the wear resistance required as a steel belt element. If the amount of C added is too small, the hardness cannot be ensured after the curing heat treatment and the wear resistance is lowered. On the other hand, if the amount of addition of C is too large, coarse undissolved carbides remain after the heat treatment for curing, and the wear resistance is also lowered. Moreover, the carbide | carbonized_material precipitates in a film form at a grain boundary, grain boundary strength is reduced, and toughness will fall. Therefore, as described above, C is in the range of 0.50 to 0.70% by mass%.

  Si is an effective element as a deoxidizing element for steel. If the amount of Si added is too small, the steel cannot be sufficiently deoxidized. On the other hand, when there is too much addition amount of Si, the hardness after softening heat processing will raise and the cold punching property required as steel for cold punching will be deteriorated. Therefore, the Si content is in the range of 0.03 to 0.60% by mass%.

  Mn improves the hardenability of steel and is effective in ensuring the mechanical strength required as a steel belt element. If the amount of Mn added is too small, hardenability cannot be ensured, and the wear resistance required as a steel belt element is reduced. On the other hand, when there is too much addition amount of Mn, the cold punching property required as cold punching steel will be deteriorated. Therefore, in terms of mass%, Mn is in the range of 0.50 to 1.00%.

  Cr, like Mn, improves the hardenability of steel and is effective in ensuring the mechanical strength required as a steel belt element. However, if the amount of Cr added is too large, it easily dissolves in iron carbide, stabilizes insoluble carbides, and increases coarse insoluble carbides. That is, the wear resistance required as a steel belt element is reduced. Therefore, Cr is in the range of 0.20 to 1.00% by mass%.

  B suppresses the grain boundary segregation of impurities such as P and increases the grain boundary strength, and is therefore effective in improving the toughness required as a steel belt element. Further, as described above, since it is dispersed in the old cementite portion in the pearlite phase, it becomes a precipitation nucleus of insoluble carbide precipitated during the softening heat treatment, and the insoluble carbide is finely dispersed and precipitated. Thereby, it has the effect which improves the abrasion resistance required as an element for steel belts. However, increasing the amount of B added increases the cost. Therefore, in mass%, B is in the range of 0.0005 to 0.0050%.

  Ti bonds preferentially to N over B and becomes Ti nitride, so that formation of B nitride is suppressed and contributes to improvement of grain boundary strength and wear resistance by B. If the addition amount of Ti is too small, formation of B nitride cannot be sufficiently suppressed, and improvement in grain boundary strength and wear resistance due to B cannot be obtained. On the other hand, increasing the amount of Ti added increases the cost. Therefore, in mass%, Ti is in the range of 0.01 to 0.10%.

  Next, the optional additive element will be described. For the optional additive element, the upper limit value was determined within a range not impairing the characteristics of the above-described essential additive element as a steel belt element.

  P decreases the strength of the crystal grain boundaries, but if the content is below a certain content, the decrease in the grain boundary strength is slight. In addition, the suppression of the addition amount can prolong the refining process and cause an increase in cost. Therefore, in mass%, P is in the range of 0.025% or less.

Since S combines with Mn to produce MnS inclusions, if contained excessively, the amount of inclusions that become the starting point of stress concentration is increased, leading to a decrease in fatigue strength required as a steel belt element. However, if the content is below a certain level, the decrease in fatigue strength is very slight. Therefore, in mass%, S is within a range of 0.015% or less.

  Furthermore, Mo and V which can be contained as inevitable impurities without actively adding them will be described.

  Mo has the effect of suppressing the formation of film-like cementite at the grain boundaries, and by adding it, further improvement in toughness can be expected. Mo also has the effect of significantly increasing the hardenability. However, the addition of Mo causes a significant deterioration in punchability and an increase in cost required as steel for cold punching. Further, according to the above-described embodiment, the addition of Mo is not necessarily required in order to obtain characteristics required as a steel belt element.

  V forms fine V carbides in the steel and makes the crystal grains fine, so that toughness and wear resistance can be improved. However, the addition of V increases costs. In addition, according to the above-described embodiment, the addition of V is not always necessary in order to obtain the characteristics required as a steel belt element.

  In addition, since the Ac1 transformation point of steel in an above-described composition range is 714 degreeC-753 degreeC, it is preferable that the retention temperature of softening heat processing is 700 degreeC-780 degreeC.

  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 10 Wear test apparatus 11 Ring 13 Wear test piece 21 Micro crack 22 Undissolved carbide 23 Mother phase

Claims (4)

If the mass% of the element M is [M],
10.8 [C] +5.6 [Si] +2.7 [Mn]
+0.3 [Cr] +7.8 [Mo] +1.4 [V] ≦ 13
It is a steel for cold punching made of steel having a component composition satisfying
In mass%, as an essential additive element,
C from 0.50 to 0.70%,
0.03 to 0.60% Si,
Mn from 0.50 to 1.00%,
0.20 to 1.00% of Cr,
Ti from 0.01 to 0.10%, and
B within the range of 0.0005 to 0.0050%,
As an optional additive element,
P is 0.025% or less, and
S within the range of 0.015% or less,
It has a component composition with the balance Fe and inevitable impurities,
Cold-punching characterized by giving a hardness of 88HRB or less in a structure in which fine carbides are dispersed in a ferrite + pearlite mixed structure after cooling at a predetermined rate after heating and holding in the austenite single phase temperature range Steel. The steel for cold punching according to claim 1, wherein in the cross-sectional structure, coarse carbides having an equivalent circle diameter of 0.5 µm or more are suppressed to 1.2 × 10 ⁵ or less per 1 mm square.   A steel belt element, wherein the steel for cold punching according to claim 1 or 2 is cold punched into a predetermined shape and then subjected to quenching and tempering heat treatment to give a hardness of 640 Hv or more. The steel belt element according to claim 3, wherein in the cross-sectional structure, coarse carbides having an equivalent circle diameter of 0.5 μm or more are suppressed to 1.3 × 10 ⁴ or less per 1 mm square.