KR19980069885A - Brake disc material with excellent fatigue strength - Google Patents

Abstract

In addition to increasing the speed of the vehicle, improvement of the fatigue strength of the brake disc material is required.

Conventionally, 0.01-0.50 and Al: 0.01-0.10 are further added to a total of Sn + Sb + Cu to a low alloy flake graphite cast iron (NCM cast iron) of Ni, Cr, and Mo addition as a brake disc material. The base of the layer was formed, and the carbon saturation (Sc value) calculated from C% by weight + [4.23- (Si% by weight / 3.2)] was set in the range of 0.82-0.91. In the Sc value selected as the management medium as the third indicator, it is limited in the range that it can maintain at least the level of NCM cast iron or higher in thermal crack resistance and that it can exceed NCM cast iron in tensile and compressive fatigue. Thus, the ideal equilibrium relationship between the complex thermally cracked resistance, the tensile strength, and the tension and compression fatigue factor was identified.

Description

Brake disc material with excellent fatigue strength

The present invention is applied to railroad cars and the like, and more particularly relates to a brake disc material which requires a level up of fatigue strength.

As a brake disc material that is applied to a disc brake system such as a railroad car, a very strange use that repeats rapid quenching and cooling by instantaneous high-pressure sliding action with a brake shoe (e.g., Cu-based sintered alloy material or resin-based lining material). The ability to withstand the conditions is the most important requirement, and as the heat load is repeatedly applied, fine thermal cracking occurs beyond the material resistance of the sliding surface. This thermal crack usually tends to be formed in a disc shape, and gradually expands radially with respect to the circumferential direction centered on the center of the axle shaft. When the length and depth of the crack exceeds the limit, Since the risk of dividing the brake disc itself cannot be denied, the depth, length and number of cracks are measured at regular inspections, and when the predetermined inspection standard value is reached, it is decided to dispose of it and replace it with a new one.

On the other hand, the brake disc material is mounted in the form of pressurizing each wheel of the actual vehicle from both sides, and there is also a characteristic feature that the portion where the thermal stress is concentrated repeatedly is specified every time the brake operation is performed. In this case, concentration of stress on a fixed part such as a bolt fastening part, for example, cannot be physically avoided. When concentration of thermal stress is repeated, metal fatigue centered around the part accumulates and appears as a defect such as fatigue splitting. As a principle of metallurgy, it is a case that is always warned.

On the other hand, the brake disc material is also one of the prerequisites, which is also provided with heat deformation resistance, and if the permanent deformation occurs due to repeated rapid cooling and quenching, and the bending surface occurs in the sliding surface originally processed on the smooth disk shape, the uniform surface contact is prevented. Therefore, since the remarkable fall in brake characteristics starts, this viscosity is also an element which limits the selection conditions as the brake disc material. Since the properties required as brake disc materials are so complexly related to each other, it is difficult to organize them in a single condition. However, when these elements are placed on a central axis to find the best brake disc materials, they are currently in practical use. As a brake disc material, cast iron is mainly used, and the cast iron is used as a base to fix the disadvantages, and to increase the advantages, a number of developments and experiments have been repeated.

The most representative brake disc material, which has been applied to vehicles of old and new lines of old national railways, is the same JRS No. 12209-2 as common cast iron (hereinafter referred to as "FC280") based on old national railway standards and JRS No. 12209-1. And low alloy cast iron (hereinafter referred to as "NCM cast iron") in which Ni, Cr, and Mo are added in an appropriate amount. All of them are classified as flake graphite cast irons, which are suitable for braking, exhibit stable friction coefficient and low wear characteristics, and are economically advantageous because they are cast irons, mainly FC280 for conventional wires, and NCM cast iron for opening of Shinkansen. It has been applied exclusively since the beginning and has been used exclusively for a long time with high reliability without causing an accident based on material factors.

Among the flake graphite cast irons practically used as brake discs, NCM cast iron has the highest evaluation characteristics at the present time. From the material characteristics of the NCM cast iron, the superior braking characteristics of the NCM cast iron are intentionally increased in the shape of flake graphite in the structure. The reduction of mechanical properties due to graphite growth is enhanced by the addition of Ni, Cr, and Mo, which makes both thermal cracking resistance and mechanical properties superior to other flake graphite cast irons. It is admitted that there was a breakthrough in the fact that the brake discs were made in accordance with the fact that the vehicle was being speeded up and changed into the corresponding materials.

The trend of today's railway business clearly shows that there is severe competition with aircraft in long-distance train operation, and that the higher speed, more comfort, and more safety are the only means to survive and the consumption of each member of the vehicle. The superiority of driving expenses along with the renewal is an important key point at the same time as the transition to JR privatization. As for the brake operation, the speed of emergency stop brake speed is increased and the versatility of brake operation for stabilization of driving performance is developed, and the conditions of use are severe, and the economical period is called for extension of the replacement period. Is facing technical challenges.

This brake operation increases the heat load directly on the brake disc material, spurs the increase of thermal cracks generated on the sliding surface, and at the same time, the portion where the thermal stress is repeatedly concentrated from the shape factor of the brake disc, that is, the bolt that is an installation part. Fatigue splitting of fasteners, etc. is also a problem. As such, the development of brake disc properties of brake disc materials, specifically, the heat cracking resistance of brake discs placed under more severe conditions, to prevent fatigue fracture and to economically compete with competition, has been rapidly developed. It is true that significant improvements have been proposed and disclosed by including the prior art for the purpose of developing brake disc materials which further improved the NCM cast iron as a base.

As the improvement of the brake characteristics of the brake disc member belonging to the flake graphite cast iron material, two flows of strengthening the matrix and changing the graphite shape are interpreted as the main subjects of the prior art. For example, Publication No. 47-1872 is interpreted as an open technology that is a starting point of a low alloy cast iron collectively referred to as NCM cast iron, but the component range of flake graphite cast iron is Ni: 1.0-2.0, Cr: 0.3-0.6, Mo: 0.3-0.5 are added, the molten metal is heated to 1500-1600 ° C to form a white line almost disappearing graphite nuclei, and then Ca-Si and Fe-Si are added to the molten metal at the time of tapping. (Inoculation) to form a flaky graphite tissue. However, in view of recent studies and achievements, the opinion that the twisted cast iron of the flaky graphite shape, for example, sweet potato beetle shape, as the thermal crack resistance of the brake disc material is superior to the flaky graphite is not necessarily agreed.

On the other hand, development to increase the strength and toughness of cast iron bases and to improve the thermal crack resistance has taken the mainstream of the improvement of the brake disc, a number of means based on the above-mentioned NCM cast iron is disclosed. For example, as the prior art of JP-A-7-127675, bainite is formed by a component of Ni: 1.50-3.00, Mo: 1.0-2.5, Ce: 0.01-0.03, and V: 0.2-0.30 (each weight%). The brake disc material which precipitated a large amount of flake graphite at the base is proposed, and the stress crack resistance is improved by the high strength of the matrix and precipitation of the flake graphite.

On the other hand, Japanese Patent Application Laid-Open No. 59-133347 discloses heat resistance by strengthening the microstructure of the matrix structure by the components of Ni: 2.0-4.0, Mo: 1.0-3.0, V: 0.35 or less and Ce: 0.04 or less (each weight percent). The crack property was improved. In addition, by adding Ni and Mo to flake graphite cast iron, it is poured into a mold with high cooling capacity, and it is mold-degraded at high temperature or mold is cast by low-cooling mold. Japanese Patent Application Laid-Open No. Hei 6-65673 attempting to control the base structure freely, Special Publication No. 59-22780 serving as a base of martensite + bainite structure by rapid decomposition, or Japanese Patent Application Laid-Open No. 60- JP 157528 and JP-A-64-62412, which are bainite structures, are covered in various aspects.

However, if it is only as a result of the improvement of the strength of the cast iron base as a result directly connected to the heat cracking resistance, it is good to narrow the focus to strengthening the base and try more effective technical means, but the fact is not necessarily such a short-circuit relationship. However, there is no guarantee that the strengthening of the matrix directly leads to the improvement of the heat crack. Some of the above-described prior art experiments have been conducted under the premise that the fatigue improvement of the material is fully matched with the improvement of the thermal crack resistance, but the cast iron material, on the one hand, is indispensable in the presence of flake graphite, which absorbs thermal shock. Graphite is a kind of antifriction material and has excellent abrasion resistance. Instead, it has high hardness, and as it strengthens the base, it has high hardness and deteriorates impact resistance, increases brittleness and promotes the progress of cracking. Reinforcement of is limited directly to the improvement of thermal crack resistance. More carefully, taking a complex norm between the tensile strength, tensile and compressive fatigue, and thermal crack resistance as a medium with the third parameter becomes a requirement for solving the problem.

If the problem is solved only by strengthening the financial strength, the inference that the forged steel brake discs are the most superior in recent years is established. In this case, however, the fatigue strength and the thermal crack resistance can be expected to be superior to the cast iron system. On the other hand, a significant cumulative permanent deformation cannot be avoided at the same time as the instability of the friction coefficient during brake operation or repeated use. As it can be easily estimated, it is hard to make a uniformly high evaluation as a brake disc of an increasingly fast vehicle, and the contrast of the principle based on the characteristic that the presence of flake graphite buffers the thermal shock and also has a function of a kind of gamma material If so, it is hard to ignore the possibility of latent potential difficulties that are difficult to recommend.

From the history of the development of sun brake disc materials, the present invention has the origin of NCM cast iron, which has gained the most stable evaluation as a prior art, and derives an ideal material component that balances complex correlation through a medium different from all the prior arts. It is an object of the present invention to provide a novel brake disc material which solves the problem.

(Means to solve the task)

The brake disc material which is excellent in the fatigue strength concerning this invention is made into the weight%, and is C: 3.10-3.45, Si: 1.10-1.50, Mn: 0.60-1.00, Ni: 1.00-2.00, Cr: 0.30-0.60, Mo: 0.30- In addition to 0.50, P: 0.16 or less, and S: 0.12 or less, the total content of Sn + Sb + Cu contains 0.01-0.50 and Al: 0.01-0.10, with the balance being Fe, and carbon saturation (Sc value) = C The above-mentioned problems were solved as the carbon saturation calculated by weight% + [4.23- (Si weight% / 3.2)] is limited to the range of 0.82-0.91.

The reason for limitation of the upper limit and the lower limit of the content will be described in relation to the function to be provided as the brake disc material for each member.

① In case of C: 3.1% by weight or less, Al, which is a graphitization promoting element, is added, but thin parts such as pin parts of brake disc materials (brake discs are provided with a large number of radial pins on the opposite side of the sliding surface to increase heat dissipation). If it is easy to be chilled and the deformation resistance and the thermal cracking resistance are deteriorated, on the contrary, if it exceeds 3.45% by weight, the balance of carbon saturation (hereinafter referred to simply as "Sc value") is broken as described in detail, and the amount of graphite is broken down. It loses excessively and the tensile strength of a material falls clearly, and forms the cause contrary to the improvement of tension and compression fatigue.

② It exhibits the same behavior as C in relation to the Si: Sc value, and when the content is less than 1.0% by weight, it is easy to be painted, causing a decrease in deformation resistance and deterioration of thermal crack resistance. On the contrary, when it exceeds 1.50 weight%, graphite amount will become excessive and tensile strength will fall, and improvement of tensile and compression fatigue is impossible.

③ When Mn is less than 0.60% by weight, the molten metal is insufficiently deflowed and deoxidized. When it exceeds 1.00% by weight, it tends to be lacquered, resulting in deterioration of thermal cracking resistance and heat deformation as brake disc material.

④ If P: 0.16% by weight, the process of phosphorus is produced and withdrawal occurs.

⑤ If S: exceeds 0.12% by weight, S will be segregated at grain boundary and cause dropping.

The starting point of the brake disc material according to the present invention is NCM cast iron as a starting point. Therefore, the addition of alloying elements such as NCM cast iron is a basic premise.

① When Ni: less than 1.00% by weight, the pearlite of the matrix becomes coarse, and the tensile strength decreases. On the contrary, when it exceeds 2.00% by weight, the matrix becomes liable to martensite, which adversely affects the heat cracking resistance characteristic of NCM cast iron.

(2) Cr: It is effective in maintaining high-temperature strength by miniaturizing pearlite, but because of its high carbide forming ability, when it exceeds 0.60% by weight, carbides are precipitated and the tensile and compressive fatigue limit is reduced.

③ Mo: In addition to improving the high temperature strength by miniaturization of pearlite structure, even small additions increase thermal conductivity and are effective for suppressing thermal stress. However, when Mo: exceeds 0.50 wt%, graphitization is inhibited and carbides are formed to form thermal cracking resistance. Because it causes deterioration of, the upper limit is defined as the equivalent.

One of the technical features of the present invention is to obtain a brake characteristic that exceeds the level of the brake disc material of the prior art. Specifically, at least the excellent thermal crack resistance possessed by the conventional NCM cast iron is maintained, and the tensile strength and tension Compression fatigue is well balanced and improved, and it is intended to cope with the high speed of railroad cars and frequent brake disc braking. Therefore, although the relative management method between the new additive component and the element by a new index which were not recognized by the prior art was introduce | transduced, it demonstrates first from an additive component.

(1) One or two or more components selected from Sn, Sb, and Cu, and their total amount contains from 0.01 to 0.50% by weight. It is already known that Sb and Sn exhibit a remarkable effect on the stabilization of fine layered pearlite even when added alone at about 0.01%. Moreover, although Cu is known to be a graphitization promoter, it is also known to act as a stabilizing element of pearlite when coexisted with Mn. However, when a large amount of these components were added, brittle compounds were formed, and toughness such as impact value was lowered, so the upper limit of the compound addition was limited to 0.50% by weight.

(2) Al: It is well known that this element has a remarkable effect as a graphitization accelerator, but it must be fully considered the relation with Sc value determined by other components, especially C, Si and its content. That is, after suppressing the Sc value at a relatively low level and adding the 0.01% or more to maintain the shape of the desired flaky graphite, 0.10% is set as the upper limit because the addition of a large amount causes dropping.

As described above, the development of the brake characteristics of the brake disc material used for the high speed vehicle is not determined by simply presenting the upper limit and the lower limit of each element. Brake characteristics should be derived from the complex correlation between tensile strength, tensile and compressive fatigue resistance and thermal cracking resistance, and if evaluated only by simple component limitations and simple physical tests, it is necessary to suffer from the difference between the tests used in the actual vehicle. Applicants made use of the long experience of knowing the relationship between the development of abundant brake disc materials and field tests, paying attention to the Sc value at the origin of the casting material as the third indicator, and after repeating many simulated amplification tests described below, Although the trends have been identified, this perception is a completely new technical finding that has not yet been anticipated by some people. The principle of gathering highly original test results as a means of solving the task is as follows. In addition, the data on which this principle is reached are specified in the embodiments of the following paragraphs.

The basic objective of the present invention is the development of a new material having a brake characteristic exceeding NCM cast iron, which is the most stable evaluation as the prior art. That is, the evaluation of all new materials is based on comparison with Sc value. Fig. 2 is a relation diagram of plotting Sc teeth on the horizontal axis and tensile strength on the vertical axis, and the present invention is superior to NCM cast iron, which is representative of the prior art, in any Sc tooth, and above all, almost linearly with increasing Sc value. It is found that the tensile strength continues to fall proportionally.

FIG. 1 is a relational diagram of tensile strength on the horizontal axis and tensile / compression fatigue on the vertical axis, and the present invention is superior to NCM cast iron in any tensile strength, indicating that the tensile and compression fatigue is excellent even at the same tensile strength. have. From this point of view, in the three-stage method, the present invention necessarily surpasses the NCM cast iron within the range of the limited component, but even if it is present, the lower the Sc value, the lower the tensile and compression fatigue limit can be derived.

On the other hand, Fig. 3 is a relationship diagram in which the thermal shock repetition number is plotted on the horizontal axis and the thermal crack length on the vertical axis, and the thermal crack resistance is shown in relation to the range of Sc values. According to this figure, the present invention hardly changes in NCM cast iron and the thermal crack resistance itself, and furthermore, in more detail, the higher the Sc value, the better the thermal crack resistance and superior to the conventional NCM cast iron. It is also recognized, but it is understood that the superiority reverses when the Sc value is lost and this superiority is lost, and when a certain range is exceeded.

That is, even if it falls within the category of the above-mentioned limited component, the larger the Sc value, in other words, the higher the C and Si, the lower the tensile strength. However, the change that greatly exceeds and degrades the NCM cast iron in absolute value is larger than that of the NCM cast iron. On the other hand, the higher the tensile strength, the higher the tensile / compression fatigue limit, which also exceeds the NCM cast iron in absolute value. On the contrary, the higher the Sc value, the higher the resistance, and the lower the Sc value, the lower the NCM cast iron. This is because the crack cracking resistance is almost uniformly distributed over the sliding surface of the brake disk and is controlled by the thermal stress of the load, and the tensile cracking fatigue can be a part of the cause of the crack cracking resistance. It is reasonable to think that it accumulates in the area where the thermal shock is repeatedly concentrated, such as the installation part of the disk, and eventually appears to be a phenomenon that can be recognized, such as breaking. Therefore, the conventional technique of simply shortening and improving tensile and compressive fatigue resistance and thermal cracking resistance or tensile strength and thermal cracking resistance is not sufficient in analyzing the external force acting when driving a real vehicle. There is no way. As a result of summarizing this causal relationship, in the present invention,

① It is a range that can maintain at least the level of NCM cast iron or more in thermal crack resistance.

② Although it is limited to 0.82-0.91 in two points that it can exceed NCM cast iron in tension and compression fatigue limit, the basis is described in the following paragraph.

1 is a diagram showing the tensile and compressive fatigue limit of the present invention and the comparative material.

Figure 2 is a diagram showing the tensile strength of the present invention and the comparative material.

3 is a diagram showing the heat crack resistance of the present invention and the comparative material.

4 is a comparison of the component ranges (C, Si, Mn) of the present invention and representative prior art.

5 is a comparison of the component ranges (Ni, Mo, Cr, V) of the present invention and representative prior art.

Table 1 is a list of components and Sc values of the comparative example based on the numerical limits for comparison with the embodiment of the present invention.

Chemical Composition of Inventive Materials (%) C Si Mn Ni Cr Mo Sn + Sb + Cu Sc teeth Comparative material 1 3.10 1.10 0.64 1.70 0.42 0.35 0.17 0.798 Inventive Material 1 3.17 1.23 0.79 1.33 0.30 0.37 0.15 0.824 Inventive Material 2 3.24 1.21 0.73 1.68 0.37 0.30 0.28 0.841 Inventive Material 3 3.27 1.47 0.60 2.00 0.53 0.41 0.22 0.867 Inventive Material 4 3.35 1.35 0.84 1.49 0.41 0.39 0.41 0.879 Inventive Material 5 3.43 1.37 0.65 1.86 0.39 0.50 0.36 0.902 Comparative material 2 3.45 1.50 0.98 1.02 0.60 0.45 0.49 0.917 In total, P is less than 16% and S is less than 0%. 0.91≥Sc≥0.82 with 12% or less

In Table 1, Comparative Material 1 has a Sc value of 0.798 and is slightly lower than that of the prior art NCM cast iron as shown in FIG. Maintaining at least equivalent to or more than conventional materials requires a Sc value of 0.82 or more, which is the lower limit of the Sc value. On the other hand, the comparative material 2 has a Sc value of 0.917, and if the Sc value is increased, the thermal crack resistance is improved, but the tensile strength related to the fatigue strength is the tensile strength required by JRS No. 12209-1 as shown in Fig. 2 28 Kgf / mm 2 ≒ 275 N / Since the upper limit value was 0.91, the upper limit value was 0.91, and the range of the Sc value was finally limited to 0.82≤ Sc value≤ 0.91.

Fig. 4 and Fig. 5 show a comparison of the NCM cast iron, which is typical of the prior art, and the main component range of the presently disclosed main technology and the present invention. In Fig. 1, reference numeral 1 denotes a conventional technique that becomes a prototype of NCM cast iron in the Japanese Patent Application No. 47-1872, ② Japanese Patent Application Laid-Open No. 6-65673, ③ Japanese Patent Application Laid-Open No. 7-127675, and ④ Japanese Patent Application Laid-Open No. 59-. No. 133347 is shown, and it enumerates for each component with the limited component of this invention.

As can be seen from the drawing as a remarkable tendency, C and Si are lower than those of the prior art, and the deterioration of the graphite shape based on the reduction of the Sc value is compensated by the addition of Al, while the tensile and compressive fatigue limit is improved. It is an ingredient effective in achieving the main object of this invention. For the alloying elements to be added, the original ① NCM cast iron is not changed especially, and the excellent performance of the past is kept as it is. Doing. In the case of materials such as cast iron, which have a very high C content, the addition of a strong carbide forming element, V, is most effective for improving general wear resistance, but it has to be very careful for improvement of metal fatigue by repeated thermal shock, and for improvement of thermal crack resistance. The question is not without it.

To discuss the tensile strength and the fatigue failure test associated therewith, it is likely that ties will be far from the intended material development unless the effect is confirmed by a test piece that can reproduce the actual strength of the brake disc running in the field. In general, brake discs used in railway vehicles have a sliding surface plate part of approximately 20 mm thickness corresponding to both left and right sides, and a plurality of pins radially arranged side by side in order to maintain strength as a structure and to have a cooling effect on the sliding surface. Is done. Therefore, the modulus of the actual product is calculated or measured by the cooling rate near the solidification time, and the shape of the specimen similar to the actual product is determined experimentally by the confirmation of tensile strength and microscopic structure. The shape of the specimen was determined.

The components of the specimens are as shown in Table 1. First, as shown in Fig. 2, the relationship between the tensile strength and the Sc value was confirmed by comparison with the NCM cast iron, and then, as shown in Fig. 1, the samples having different tensile strengths. (I.e., samples having different Sc values) were subjected to both types of tensile-compression fatigue tests. Test condition

Tester: Hydraulic Sub-Fatigue Tester

Test piece shape: 10 mm circumferential test piece

Control method: Load control-both tension / compression

Repetition Rate: 20 Hz

The correlation which can be summarized by the tensile strength was obtained, and the identification of the improved new material of the fatigue strength superior to the NCM cast iron was realized like the tensile strength. In the thermal crack resistance, as shown in Fig. 3, the thermal fatigue was evaluated by Schirret in a repeated thermal shock test by the applicant's circumferential test specimen. Test condition

Test piece shape: circumferential test piece of 25 ømm x 25 H

Thermal Shock Method: Rapidly heats up to the highest heating temperature by high frequency induction heating, and after that, it is quenched with water shower immediately. Temperature increase 15 seconds, maximum heating temperature 780 ℃, repeated water 300 times.

Judgment method: Count the thermal cracks on the circumferential surface of the most water-cooled (10 long cracks), and cumulatively examine the occurrence.

As a result, the results indicate that the thermal crack resistance is at least equal to or superior to that of NCM cast iron in the limited Sc value in the limited component range as described above, and greatly exceeds the tensile strength and the tensile and compression fatigue.

In order to confirm the actual brake characteristics, the weight percentage is used and C: 3.3, Si: 1.3, Mn 0.6, Ni: 1.5, Cr: 0.4, Mo: 0.4, (Sn + Sb + Cu): 0.40 are actually used in conventional lines. Brake discs were melted and bench tests (17 random tests, maximum initial speed of test 135 km / hr) at the thickness (20 mm) and limit thickness (10 mm) assuming actual driving were carried out to change the brake characteristics. Confirmed. As a result, for the two types of current non-asbestos-based lining materials, both of the average friction coefficients fms and fmt were equivalent to the current NCM cast iron from low speed to high speed, and a guarantee that the braking property did not change was recognized.

As described above, the present invention is given as a condition in which a further improvement in high speed and comfort is survived in response to contention with an aircraft or the like for a conventional ship and a Shinkansen. The effect of solving the double problem which requires a period is remarkable. The principle of the development is well known in the past, and it is based on NCM cast iron, which has been an exemplary member of safe driving, and limited the active ingredient by adding unique additive elements that no one has ever thought of before. At the same time, the NCM cast iron, which has high evaluations, has established an ideal equilibrium relationship between the elements of heat cracking resistance, tensile strength, and tension and compression fatigue, which are complexly selected by selecting Sc value as a management medium. The effect of achieving the characteristics as a brake disc material while greatly improving and harshening the tensile and compression fatigue limit while ensuring the equivalent crack resistance and thermal cracking resistance is excellent.

Claims (1)

By weight%, other than C: 3.10-3.45, Si: 1.10-1.50, Mn: 0.60-1.00, Ni: 1.00-2.00, Cr: 0.30-0.60, Mo: 0.30-0.50, P: 0.16 or less, and S: 0.12 or less , Sum + Sn + Sb + Cu. 50, Al: 0.01-0.10, the balance is made of Fe, and Carbon saturation (Sc value) = C wt% + [4.23-(Si wt% / 3.2)] Brake disk material excellent in fatigue strength, characterized in that the carbon saturation is calculated to be limited to the range of 0.82-0.91.