Nitriding steel having excellent cold forgeability and nitriding properties

Abstract

To provide a chemical composition and a structure before cold forging capable of ensuring good cold forgeability and core hardness after nitriding and suppressing variation in core hardness required for maintaining the strength of parts in steel for nitriding.SOLUTION: Provided is a cold forging and nitriding steel that contains, in mass%, C: 0.20-0.30%, Si: 0.25-0.70%, Mn: 0.20-0.40%, S: 0.005-0.025%, Cr: 1.50-2.50%, Al: 0.015-0.035%, N: 120-170 ppm, and additional components, one or two of Mo: 0.10-0.40% and Nb: 0.030-0.050%, with the balance being Fe and unavoidable impurities, wherein the structure of the steel before cold forging is ferrite and pearlite, with an area ratio of 90% or more, and the structure of the steel before nitriding has an average lamellar spacing of 0.20 μm or more in the pearlite structure and an element segregation width of 60 μm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to cold-forged nitrided parts that are prepared by subjecting cold-forged steel parts to a surface hardening treatment such as gas nitriding or gas soft nitriding, which involves infiltrating nitrogen (N), for example, machine parts that are nitrided after cold forging, such as gears for automobiles, construction machinery, machine tools, etc., and to a cold-forged nitriding steel that is suitable as a material for these parts. [Background technology]

[0002] Steel parts such as gears and shafts are manufactured from hypoeutectoid steel with a relatively low carbon content, and when used as products, they require high strength and wear resistance on the surface. Therefore, after being machined into the part shape, a process is performed to harden the surface. However, because the carbon content is low, simply quenching the steel to form a martensite structure does not provide the hardness required for use.

[0003] To strengthen the surface, techniques such as carburizing and carbonitriding are used, which increase the hardness of the martensitic structure (Vickers hardness of 700 HV or more) by injecting carbon into the surface of the part and then quenching it. These processes can achieve a hardened layer depth of around 1 mm. On the other hand, gas carburizing, which is widely used in the production process, emits large amounts of carbon dioxide during the carburizing reaction and the combustion of excess carburizing gas, making it unsuitable for the current trend toward carbon neutrality.

[0004] Gas carburizing is disadvantageous in reducing carbon dioxide emissions during the manufacturing process of parts. Vacuum carburizing is a carburizing method that reduces carbon dioxide emissions more than gas carburizing. This process uses hydrocarbon gas as the carburizing source, so no carbon dioxide is generated during the carburizing reaction.

[0005] However, in surface hardening processes that utilize the carburizing reaction, it is essential to carry out heat treatment at a high temperature (generally 900°C or higher) that is favorable for the penetration of carbon into the steel. However, when quenching in a low-temperature refrigerant (generally 30°C or higher) after carburizing at such a high temperature, there is a problem in that heat treatment deformation occurs, which can easily promote bending of shafts and twisting or tilting of gear teeth.

[0006] Therefore, attention is being paid to heat treatments that emit less carbon dioxide and can perform surface hardening at lower temperatures. Nitriding is used as a surface hardening heat treatment method. Nitriding is a method of obtaining strength by infiltrating nitrogen from the surface and using a hard compound layer or nitrogen diffusion layer, and the heat treatment temperature is generally around 500 to 600°C. The depth of the nitrided layer is generally less than 300 μm.

[0007] As mentioned above, there is a difference in the depth of the hardened layer obtained by nitriding and carburizing, and since carburizing produces a deeper hardened layer, carburized parts can be used in applications where they are repeatedly subjected to higher stress loads. However, there are also applications where relatively small parts do not need to withstand such high stress loads. Therefore, it is expected that the application of nitriding will expand to such relatively low-stress applications.

[0008] However, one challenge with nitriding is that, because the steel is not heated to the temperature required to austenitize, the martensitic transformation cannot be used to achieve the required hardening, and a means is therefore required to achieve a core hardness that is not affected by the nitriding process.

[0009] In the case of carburizing, the core, which is not affected by the carburizing, also becomes austenitic during the carburizing heating, and then when quenched, the core also becomes martensite and hardens, so no special measures are required to strengthen the core.

[0010] However, in nitriding, martensitic transformation cannot be utilized. Therefore, in order to increase the hardness of the lower part of the surface hardened layer of a part (from the edge to the interior in the depth direction of the hardened layer) which is not affected by nitriding, it is possible to, for example, harden the part in advance by quenching so that the hardness is less likely to decrease at the nitriding temperature, or to utilize the aging precipitation of precipitates by setting the nitriding temperature close to the nitriding temperature range, and harden the core side at the same time.

[0011] However, when using these methods, there are concerns that costs will increase due to the need for pre-quenching, which is not required for carburizing and quenching, and when using age hardening, the addition of expensive alloying elements such as Ni is required, which also inevitably increases costs.

[0012] Therefore, the present applicant has previously proposed a steel for nitriding containing, by mass%, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.300%, N: 0.004-0.030%, V: 0.08-0.30%, with the balance being Fe and unavoidable impurities, with P (as unavoidable impurities): 0.030% or less and S (as unavoidable impurities): 0.030% or less, which has been softened by heat treatment at 730-760°C for 4-8 hours and then air-cooled, and which has a Rockwell hardness of 87 HRB or less (see Patent Document 1). [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 2023-142664 Summary of the Invention [Problem to be solved by the invention]

[0014] Indeed, Patent Document 1 proposes optimizing the balance of Al, V, and Cr components to provide excellent cold forgeability, and to make the structure before cold forging a structure in which carbides are densely present in ferrite, thereby increasing the hardness of the core through work hardening by cold forging prior to nitriding, and suppressing recrystallization of the work hardened portion during nitriding, thereby suppressing a decrease in the hardness of the core.

[0015] This invention is certainly effective for utilizing nitriding as a surface hardening treatment for parts while suppressing the increase in cost due to additional heat treatment or an increase in the amount of alloying elements. However, the behavior of recrystallization in the work-hardened portion is greatly affected by the state of the structure before work-hardening, and is also affected by alloying elements that delay recrystallization.

[0016] As a result of extensive research into these points, the inventors of the present application have found that the recrystallization process in the work-hardened portion does not necessarily proceed uniformly, but that there are some areas where recrystallization proceeds easily and some areas where it does not proceed easily. As a result, it has been found that the recrystallization process does not proceed uniformly, resulting in relatively large variations (unevenness) in hardness.

[0017] It is quite conceivable that such hardness variations may occur directly below the surface-hardened layer formed by nitriding, which could result in unstable mechanical response of nitrided parts incorporated into drive components. It is also conceivable that metal fatigue may easily progress below the nitrided layer. Therefore, to ensure stable operation of steel parts, it is necessary to reduce hardness variations.

[0018] When a steel is intended for use as gear or shaft parts for automobiles and the like, which are subjected to nitriding after cold forging, it is necessary for the steel to have both good cold forgeability during part processing and the core hardness necessary for part strength. It is also important to suppress variations in core hardness. Therefore, the present invention aims to provide a nitriding steel that can ensure good cold forgeability and core hardness after nitriding, as well as suppress variations in core hardness necessary for maintaining part strength, by optimizing the chemical composition of the nitriding steel and the microstructure before cold forging. [Means for solving the problem]

[0019] As a result of extensive research, the present inventors have established a method for reducing the occurrence of variations in hardness when nitriding is performed after cold forging.

[0020] The first means for solving the problems of the present invention is Contains, by mass%, C: 0.20 to 0.30%, Si: 0.25 to 0.70%, Mn: 0.20 to 0.40%, S: 0.005 to 0.025%, Cr: 1.50 to 2.50%, Al: 0.015 to 0.035%, and N: 120 to 170 ppm; Further, as optional components, at least one or two of Mo: 0.10 to 0.40% and Nb: 0.030 to 0.050% are contained, The balance is Fe and unavoidable impurities, making it a nitriding steel for cold forging.

[0021] The second means is a steel for nitriding for cold forging according to the first means, in which 90% or more of the area ratio of the structure of the steel before cold forging is ferrite and pearlite.

[0022] The third means is a steel for nitriding for cold forging according to either the first or second means, in which the lamellar spacing in the pearlite structure of the steel before nitriding is 0.20 μm or more on average.

[0023] The fourth means is a steel for nitriding for cold forging according to any one of the first to third means, wherein the width of component segregation in the structure of the steel before nitriding is 60 μm or less.

[0024] The fifth means is a nitriding steel for cold forging according to any one of the first to fourth means, which has a core hardness of an average Vickers hardness of 270 HV or more when cold forged at a compression ratio of 50% to 70%.

[0025] The sixth means is a cold-forged nitriding steel having a core hardness of 230 to 350 HV after cold forging, which is made using the cold-forging nitriding steel according to any one of the first to fourth means. That is, the nitriding steel is characterized in that it uses any one of the first to fourth nitriding steels for cold forging and has a core hardness of 230 to 350 HV in the cold forged state before nitriding treatment.

[0026] The seventh means is a steel part made using the cold forging nitriding steel according to any one of the first to fifth means, which is a cold forged nitriding steel part in a nitrided state, having an average core hardness of 250 HV or more. That is, the steel part cold forged using any one of the first to fifth nitriding steels for cold forging is characterized in that the core hardness after nitriding treatment is 250 HV or more on average.

[0027] The eighth means is a steel part using the cold forging nitriding steel according to any one of the first to fifth means, which is a cold forged nitriding steel part in a nitrided state with a core hardness of 230 to 350 HV. That is, the steel part is characterized in that the core hardness after nitriding treatment of the steel part cold forged using any one of the first to fifth nitriding steels for cold forging is 230 to 350 HV. [Effects of the Invention]

[0028] According to the present invention, a steel for nitriding having good cold forging properties can be obtained, that is, the core hardness before cold forging is 270 HV or less, and the steel has excellent workability.

[0029] Furthermore, according to the present invention, a nitriding steel in a cold-forged state with high hardness can be obtained. That is, the core hardness after cold forging at a compression ratio of 50 to 70% is an average Vickers hardness of 270 HV or more, and high hardness is obtained by work hardening during cold forging.

[0030] Furthermore, according to the present invention, a cold-forged steel for nitriding can be obtained that has a uniform internal hardness. That is, the core hardness after cold forging at a compression ratio of 50 to 70% is 230 to 350 HV in Vickers hardness, so that a uniform internal hardness is obtained.

[0031] Furthermore, the present invention provides cold-forged nitrided steel parts with ensured core hardness. That is, parts that are cold-forged at a compression ratio of 50 to 70% and then tempered at a simulated high temperature (600°C) for nitriding have a core hardness of 250 HV or more in Vickers hardness, meaning that the core hardness is maintained even after nitriding.

[0032] Furthermore, the present invention provides cold-forged nitrided steel parts with uniform internal hardness, i.e., cold-forged at a compression ratio of 50-70% and then subjected to simulated high-temperature tempering at 600°C to obtain a core hardness of 230-350 HV, achieving uniform internal hardness.

[0033] Furthermore, according to the present invention, a nitrided steel part for cold forging that is resistant to torsion can be obtained. That is, since a uniform internal hardness is obtained, areas where stress is extremely concentrated are unlikely to appear, and the part is resistant to torsion. [Brief explanation of the drawings]

[0034] [Figure 1] FIG. 1 is a diagram illustrating the annealing heat treatment procedure of the present invention. [Figure 2] FIG. 2 is an SEM image of the structure of Invention Steel No. 1 after annealing according to the procedure of the present invention, and indicates ferrite and pearlite. [Figure 3] FIG. 3 is an SEM image of the structure of steel No. 1 that was annealed using a comparative procedure. [Figure 4] FIG. 4 is an SEM image of the structure of the comparative steel No. 3, which was annealed according to the procedure of the present invention. [Figure 5] FIG. 5 is an SEM image showing the lamellar spacing of the steel No. 1 that was annealed according to the procedure of the present invention. [Figure 6]FIG. 6 is a diagram illustrating a method for identifying the width of component segregation. DETAILED DESCRIPTION OF THE INVENTION

[0035] Before describing the embodiments of the present invention, the reasons for specifying the chemical composition of the steel for nitriding for cold forging will be described. Note that % of the chemical composition is mass %.

[0036] C: 0.20 to 0.30% C is a useful component for ensuring core strength. If the C content is too low, the core hardness after nitriding will decrease, resulting in insufficient strength, so the C content is set to 0.20% or more. On the other hand, if the C content is too high, the material hardness will increase too much, reducing workability (machinability and cold workability), and it will also inhibit the diffusion of nitrogen, reducing the hardened layer depth, so the C content is set to 0.30% or less. Therefore, the C content is set to 0.20 to 0.30%.

[0037] Si: 0.25 to 0.70% Si is a useful component for deoxidation during manufacturing. If the Si content is too low, it is likely to result in insufficient deoxidation, which will reduce the inclusion quality, so the Si content is set to 0.25% or more. On the other hand, if the Si content is too high, the material hardness increases and workability decreases, so the Si content is set to 0.70% or less. Therefore, the Si content is set to 0.25 to 0.70%.

[0038] Mn: 0.20 to 0.40% Mn is an element that improves core hardness (material hardness). If the Mn content is too low, the core hardness will be insufficient, so the Mn content should be 0.20% or more. On the other hand, if there is an excess of Mn, during heat treatment to soften the steel for cold forging, significant scattered striped Mn segregation bands are formed, and C, which has the same tendency to gather in the same places as Mn, also segregates there, making the structure after the softening heat treatment likely to be non-uniform. In this case, the areas where alloying elements segregate will contain a large amount of pearlite structure, and areas with few alloying elements will contain a large amount of ferrite structure. If steel with such a heterogeneous structure is work-hardened by cold forging and then nitrided, recrystallization will proceed quickly in areas with a high ferrite content, reducing hardness, while recrystallization will be delayed in areas with a high pearlite content, further increasing hardness variation. The resulting scattered areas of particularly low hardness can destabilize the mechanical response of the part and make it impossible to prevent rapid fatigue progression. To prevent this, the Mn content must be limited to 0.40% or less. Therefore, the Mn content is set to 0.20 to 0.40%.

[0039] S: 0.005 to 0.025% S is effective for deoxidation. If there is too little S, deoxidation will be insufficient. From these viewpoints, the S content is set to 0.005% or more. On the other hand, if there is too much S, defects will easily occur in the hot working of the steel. From these viewpoints, the S content is set to 0.025% or less. Therefore, the S content is set to 0.005 to 0.025%. Note that S is also an element that reduces cold forgeability, so if cold forgeability is important, it is desirable that the S content be 0.020% or less. More preferably, the S content is 0.015% or less.

[0040] Cr: 1.50~2.50% Cr is an element that improves the core hardness of steel and also improves the hardness when the steel is nitrided. If the Cr content is too low, the hardness after nitriding will be insufficient, resulting in insufficient core hardness. Therefore, the Cr content is set to 1.50% or more, and more preferably 1.70% or more. On the other hand, if the Cr content is too high, the material hardness will increase, reducing workability. Furthermore, the diffusion of nitrogen will be hindered, reducing the hardened layer depth and resulting in shallow nitriding. Therefore, the Cr content is set to 2.50% or less. Since Cr is also an element that slightly reduces cold forgeability, when cold forging is important, the upper limit is desirably set to 1.90% or less. Therefore, the preferred Cr content is 1.70 to 1.90%.

[0041] Al: 0.015 to 0.035% Al is a useful component for deoxidation during manufacturing and is effective in improving surface hardness after nitriding. Since Al does not tend to shallow the nitriding depth, it can be added proactively. If the Al content is too low, it is likely to result in insufficient deoxidation during manufacturing, reducing the cleanliness of inclusions. Furthermore, the surface hardness and hardened layer depth of the steel part after nitriding will also be insufficient. From these perspectives, the Al content is set to 0.015% or more. On the other hand, if the Al content is too high, coarse nitrides (AlN) are formed, which reduces fatigue properties and workability, thereby worsening manufacturability. From these perspectives, the Al content is set to 0.035% or less. Therefore, the Al content is set to 0.015 to 0.035%.

[0042] N: 130-170 ppm N is a component that forms carbonitrides. If the N content is too low, the carbonitrides will be insufficient and the crystal grains will become coarse, which will tend to reduce toughness and fatigue properties. Therefore, the N content is set to 130 ppm or more. On the other hand, if the N content is too high, coarse carbonitrides will be formed, which will reduce fatigue properties and workability, and the pinning effect will not be exerted as the fine carbonitrides that have a pinning effect will decrease, resulting in coarsening of the crystal grains. Therefore, the N content is set to 170 ppm or less. Therefore, the N content is set to 130 to 170 ppm.

[0043] Next, the selectively added elements of the steel of the present invention will be explained. The steel of the present invention further contains one or both of Mo and Nb in the following ranges.

[0044] Mo: 0.10 to 0.40% Mo is an element effective in enhancing the effect of suppressing ferrite recrystallization, and adding 0.10% or more is recommended when a particularly high recrystallization suppression effect is desired. One such case is when nitriding is performed at 560°C or higher after cold forging. Mo dissolves in carbides in pearlite, thereby retarding the progress of spheroidization of carbides in pearlite during the nitriding process after cold forging, thereby facilitating the utilization of the carbides' function as a barrier to retard the progress of ferrite recrystallization. Too much Mo impairs cold forgeability and machinability after cold forging, so the addition amount is set to 0.40% or less. Therefore, the Mo content is preferably 0.10 to 0.40%. More preferably, the Mo content is 0.15 to 0.40%.

[0045] Nb: 0.030 to 0.050% Nb is an element that forms carbonitrides in steel and inhibits ferrite recrystallization, and is effective in preventing a decrease in core hardness during nitriding after cold forging. To achieve this effect, 0.030% or more of Nb should be added. Furthermore, by forming carbonitrides, the pinning effect makes it difficult for the recrystallized grains in the core to become coarse during nitriding after cold forging. On the other hand, excessive Nb reduces workability. From this perspective, the Nb content should be 0.050% or less. Therefore, the Nb content should be 0.030 to 0.050%.

[0046] Remainder: Fe and unavoidable impurities The balance is Fe and unavoidable impurities. Note that P and Ni may be contained as unavoidable impurities, and it is preferable to specify the upper limits as follows.

[0047] P (unavoidable impurities): 0.025% or less Since P promotes grain boundary segregation and reduces toughness, it is desirable to reduce the content of P, an unavoidable impurity, to 0.025% or less, and more desirably, to 0.020% or less.

[0048] Ni (unavoidable impurity): 0.025% or less If Ni is too much, cold forgeability is impaired, so Ni content is preferably 0.025% or less.

[0049] Next, the reasons for specifying the structure, hardness, etc. of the steel of the present invention will be described. First, the nitriding steel of the present invention has a structure of ferrite and pearlite before processing, with the area ratio of ferrite and pearlite being 90% or more. By making the structure ferrite and pearlite have an area ratio of 90% or more, carbides are sufficiently dispersed in the steel, making it easier to obtain the hardness required for the core of a nitrided part.

[0050] Furthermore, by making the carbide morphology lamellar rather than spherical, recovery and recrystallization can be delayed when the carbide is held at high temperatures during nitriding, which is useful for maintaining core hardness.

[0051] In the nitriding steel of the present invention, the pearlite structure of the steel before nitriding has an average lamellar spacing of 0.20 μm or more. The coarse lamellar spacing, i.e., the thick lamellar carbides, makes it difficult for the carbides to dissolve, and the carbides also have the effect of suppressing ferrite recrystallization, making it easier to maintain hardness.

[0052] In the steel for nitriding of the present invention, the width of element segregation in the microstructure of the steel before nitriding is preferably 60 μm or less. By restricting the width of element segregation (the width of the region where the concentration of C or Mn is relatively high) and thereby reducing the region where the concentration of C or Mn is relatively low formed around it, it is possible to suppress the variation in hardness because it is difficult to form a localized area where hardness is reduced due to the recovery and recrystallization phenomenon at the nitriding temperature.

[0053] The core hardness of the steel for nitriding of the present invention after cold forging at a compression ratio of 50% to 70% is set to an average Vickers hardness of 270 HV or more. Since hardness is obtained by work hardening after cold forging, it is easy to maintain hardness even after nitriding treatment.

[0054] The core hardness of the steel for nitriding of the present invention after being compressed by cold forging at a compression ratio of 50% to 70% is set to 230 to 350 HV in Vickers hardness. Since a uniform hardness is obtained after cold forging, it is easy to maintain the uniform hardness even after nitriding treatment.

[0055] The core hardness of parts that have been nitrided after cold forging using the nitriding steel of the present invention should be 250 HV or more on average. If the core hardness is less than this, the core hardness after nitriding will be insufficient, and the overall strength of parts such as gears will be insufficient.

[0056] The core hardness of parts made from the nitriding steel of the present invention, which have been cold forged and then nitrided, is set to 230 to 350 HV. If the core hardness after nitriding is not uniform, the mechanical response of the nitrided parts incorporated into drive components may become unstable. It is also expected that metal fatigue may easily progress below the nitrided layer.

[0057] Next, an embodiment of the present invention will be described using the steel of the present invention as an example. As an example of the present invention, Invention Steel No. 1 is shown, which has the chemical compositions shown in Table 1, with the balance being Fe and unavoidable impurities. If it is desired to further harden Invention Steel No. 1, 0.030 to 0.050% Nb may be added. For any steel with the invention composition, the variation in core hardness can be suppressed by further controlling the lamellar spacing and segregation width. Therefore, the following explanation will be given using Invention Steel No. 1 as an example.

[0058] As a steel having other inventive components, Inventive Steel No. 2 is shown, which has the chemical components shown in Table 2 with the balance being Fe and unavoidable impurities. Also, as a comparative example, Comparative Steel No. 3 is shown, which has the chemical components shown in Table 3 with the balance being Fe and unavoidable impurities.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] Steel bars having a diameter of 65 mm were produced by hot forging for the inventive steel No. 1 and the comparative steel No. 3. Steel bars having a diameter of 40 mm were produced by hot forging for the inventive steel No. 2. Thereafter, the specimen was subjected to a normalizing treatment in which it was held at a temperature of 900°C for 1.5 hours and then air-cooled.Furthermore, a softening heat treatment was performed.

[0063] (About softening heat treatment) The softening heat treatment was carried out using a Kanthal furnace, and the steel bars of invention steel No. 1, invention steel No. 2, and comparison steel No. 3 were subjected to two types of heat treatment: the procedure for implementing the present invention and a comparative procedure, and the results were compared. (A) Annealing procedure of the present invention: Fig. 1 shows the heat treatment pattern for annealing. This pattern is an example, and the present invention is not limited to these conditions. (a) Comparative spheroidizing annealing: The heat treatment pattern for spheroidizing annealing was set to a maximum heating temperature of 800°C.

[0064] (Observation of the structure of nitriding steel) To observe the structure of the nitriding steel before cold forging, the specimen was cut parallel to the rolling direction through the center, the cut surface was polished, and the polished surface was etched with nital solution, after which it was observed with a scanning electron microscope (SEM).

[0065] The results are shown in Figures 2 to 5, which are SEM secondary electron images. Figure 2 is an image of steel No. 1 that was annealed according to the present invention. Figure 3 is an image of steel No. 1 that was annealed according to the present invention for comparison. Figure 4 is an image of steel No. 3 that was annealed according to the present invention for comparison. Figure 5 is an image of steel No. 1 that was annealed according to the present invention for comparison, observed at a higher magnification to observe the lamellar spacing.

[0066] (Measuring lamellar spacing) The lamellar spacing in the pearlite structure was measured as follows. First, three fields of view in which the lamellar spacing in the pearlite structure could be easily measured were photographed using an SEM. Next, as shown in Fig. 5, the lamellar spacing was measured from the photograph using the line segment method, and the procedure of calculating the average value at nine locations was repeated multiple times. The multiple average values ​​were further averaged, and the resulting value was used as the average lamellar spacing in the present invention.

[0067] In the present invention, by limiting the width of the elemental segregation to 60 μm or less, it is possible to make it difficult for low-hardness regions to form. Such a structure can be obtained by isothermal heat treatment, in which the material is cooled from the austenite temperature range to a temperature at which pearlite transformation occurs relatively quickly, and then held for a predetermined time. The holding time in this case must be determined taking into account the influence of the chemical composition. The time required for the isothermal transformation to be completed can be measured using an automatic transformation point measuring device or the like, and the holding time can be appropriately set based on the measurement results.

[0068] (Measuring the width of component segregation) Micro-corrosion can be used to measure the width of elemental segregation in the pre-nitriding state. For example, corrosion using a 5% Nital etching solution reveals bands of thickly corroded areas (positive segregation areas with a high pearlite structure) and bands of lightly corroded areas (negative segregation areas with a high ferrite structure), allowing the width of corrosion to be observed. In this case, the distance from the center of a darkly corroded area to the center of an adjacent darkly corroded area (or both of these can be used as lightly corroded areas) can be measured at five locations on an optical microscope image at a magnification of 150x or higher, and the average value can be used to determine the width of elemental segregation in the steel before nitriding.

[0069] Figure 6 shows how to identify and measure this element segregation width. The element segregation width is determined by focusing on the band-like areas that appear as dark corroded bands, and measuring the distance from the center of each band as the element segregation width, as shown in Figure 6(a). Also, even when dark corroded bands are scattered like islands, the element segregation width can be measured based on the parallel distance between the bands, as shown in Figure 6(b).

[0070] In addition to using micro-corrosion, measurements can also be made by using an EPMA (wavelength dispersive elemental analyzer) to determine the width of component segregation in a similar manner as shown in Figure 6.

[0071] As shown in the SEM images in Figures 2 and 5, Developmental Steel No. 1, which was heat-treated using the annealing pattern of the present invention, has a structure consisting of ferrite and pearlite, with the total area ratio of ferrite and pearlite being 90% or more. Furthermore, the lamellar spacing is an average of 0.20 μm or more, and the width of the elemental segregation is 60 μm or less.

[0072] Next, a cylindrical test piece with a diameter of 14 mm and a length of 21 mm was cold forged from a steel bar, compressed to a compression ratio of 70% at a compression rate of 10 mm / min. After cold forging at a compression ratio of 70%, the test piece underwent a simulated high-temperature tempering process in which it was held at 600°C for two hours in a Kanthal furnace and then air-cooled. Although this heat treatment does not involve nitriding, it is effective as a method for confirming the hardness characteristics of the core at the nitriding temperature.

[0073] The core hardness (HV) after nitriding simulated high-temperature tempering was measured by the following procedure. The results are shown in Table 4. The hardness after cold forging (Table 5) and after nitriding (Table 4) was measured by cutting cylindrical test pieces after cold forging and after nitriding simulated high-temperature tempering into four pieces and measuring the hardness with a Vickers hardness tester in a range of 5 mm from the center in the compression direction toward the periphery at 0.5 mm intervals. Note that the "position" in Tables 4 and 5 refers to the distance from the center in the compression direction.

[0074] [Table 4]

[0075] Inventive Steel No. 1 and Inventive Steel No. 2 of Table 1, which were heat treated according to the procedure pattern shown in Figure 1, had an average core hardness of 250 HV or more after 70% cold forging and nitriding treatment, with a hardness range of 230 to 350 HV.

[0076] Furthermore, for Invention Steel No. 1 and Invention Steel No. 2 of the present invention, which were heat treated according to the procedure pattern shown in FIG. 1 , the core hardness (within a range of 5 mm from the center to the outer periphery at 0.5 mm intervals) was measured. The results are shown in Table 5.

[0077] [Table 5]

[0078] In order to prevent the formation of areas with locally reduced hardness during nitriding, which is performed at 600°C or less after cold forging, it is necessary to control the width of the component segregation (the width of the area with a relatively high concentration of C or Mn) and thereby reduce the areas that form around it where the concentration of C or Mn is relatively low. For this purpose, for example, annealing can be used, in which the steel is slowly cooled from the austenite temperature range to 600°C or less at a rate of 25°C / hr or less. A cooling rate of 20°C / hr or less is preferable.

[0079] Regarding the invention steel No. 1 and invention steel No. 2 of the present invention in Table 1, the steels that were subjected to the heat treatment pattern of FIG. 1 had an average core hardness of 250 HV or more after nitriding simulated high-temperature tempering at 600°C, as shown in Table 4, with a hardness range of 230 to 350 HV.

[0080] As described above, the nitriding steel for cold forging having the chemical composition of the present invention can be further subjected to heat treatment that controls the pre-nitriding structure, thereby obtaining good cold forgeability and core hardness due to delayed recovery and recrystallization during nitriding, and can also suppress the variation in hardness necessary to maintain strength. [Industrial Applicability]

[0081] The steel for nitriding for cold forging of the present invention is suitable for cold forged nitrided parts that are subjected to a surface hardening treatment such as gas nitriding or gas soft nitriding, which allows nitrogen (N) to penetrate after cold forging. Therefore, it is suitable for machine parts that are nitrided after cold forging, such as gears and shafts for automobiles, construction machinery, machine tools, etc.

Claims

1. In mass%, C: 0.20-0.30%, Si: 0.25-0.70%, Mn: 0.20-0.40%, S: 0.005-0.025%, Cr: 1.50-2.50%, Al: 0.015-0.035%, N: Contains 120 to 170 ppm, Further, as optional components, at least one or two of Mo: 0.10 to 0.40% and Nb: 0.030 to 0.050% are contained, The balance consists of Fe and unavoidable impurities. Nitriding steel for cold forging.

2. 2. The nitriding steel for cold forging according to claim 1, wherein ferrite and pearlite account for 90% or more of the area ratio of the structure of the steel before cold forging.

3. 2. A steel for nitriding and cold forging according to claim 1, wherein the lamellar spacing in the pearlite structure of the steel before nitriding is 0.20 μm or more on average.

4. 2. The steel for nitriding used in cold forging according to claim 1, wherein the width of the component segregation in the structure of the steel before nitriding is 60 μm or less.

5. 2. The nitriding steel for cold forging according to claim 1, wherein the core hardness when cold forged at a compression ratio of 50% to 70% is an average of 270 HV or more.

6. A cold-forged nitriding steel having a core hardness of 230 to 350 HV after cold forging, which is made using the cold-forged nitriding steel according to claim 1.

7. A cold-forged, nitrided steel part made using the steel for nitriding for cold forging according to claim 1, wherein the core hardness after nitriding is 250 HV or more on average.

8. A steel part made using the cold forging nitriding steel according to claim 1, wherein the cold forged nitriding steel part has a core hardness of 230 to 350 HV after nitriding.

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