Technical field
[0001]
　The present invention relates to a hot forged part, more particularly, to hot forging refining and surface hardening heat treatment after hot forging is omitted.
BACKGROUND
[0002]
　Recently, refining is abbreviated hot forged part (e.g., forged crankshaft) are provided. The refining process is a quenching treatment and tempering process for improving the mechanical properties of the steel strength. Thereafter, refining is a hot forged part omitted, of non-heat treated as hot forged part.
[0003]
　Steel constituting the hot forged part of non-heat treated usually contain vanadium (V). Hot forged part of non-heat treated, the steel was hot forged, are prepared by cooling in air. Of steel constituting the hot forged part of non-heat treated tissues, a ferrite-pearlite structure. V in the steel, the cooling process after hot forging to form fine carbides in steel, to improve the fatigue strength of the steel. In short, refining is be omitted, non-heat treated hot forged part of which contains V has an excellent fatigue strength. Hot forging microalloyed steel containing V, for example, disclosed in JP-A 9-143610 (Patent Document 1). Microalloyed steels disclosed in Patent Document 1 is made of a ferrite-pearlite structure, the V, to precipitation strengthening of ferrite. Therefore, it is described as a high fatigue strength.
[0004]
　However, V is because expensive, the manufacturing cost of the hot forged part of non-heat treated is high. Thus, even without containing V, hot forged part of non-heat treated having excellent fatigue strength is required.
[0005]
　JP-10-226847 (Patent Document 2), and, JP 61-264129 (Patent Document 3), hot forging microalloyed steel having a high fatigue strength without containing V and to propose a hot-forging.
[0006]
　Microalloyed steels disclosed in Patent Document 2, in mass%, C: 0.30 ~ 0.60%, Si: 0.05 ~ 2.00%, Mn: 0.90 ~ 1.80%, cr: 0.10 ~ 1.00%, s-Al: 0.010 ~ 0.045%, N: 0.005 ~ 0.025%, and the balance Fe and impurities, the hardness after hot forging 30HRC or less, the tissue is ferrite + pearlite, pearlite lamellar spacing is 0.80μm or less, more pro-eutectoid ferrite area ratio is 30% or less. In Patent Document 2, a non-heat-treated steel having the above chemical composition to hot forging, if allowed to cool, the lamellar spacing of the pearlite becomes fine, and the area ratio of the pro-eutectoid ferrite is lowered, the fatigue strength growing, it has been described as.
[0007]
　In Patent Document 3, by mass%, C: 0.25 ~ 0.60% , Si: 0.10 ~ 1.00%, Mn: 1.00 ~ 2.00%, and, Cr: 0.30 ~ the steel containing 1.00% Ac 3 be more than the transformation point, and, after the heat was hot forged at a temperature of 1050 ° C. or less, and cooling, the pro-eutectoid ferrite content F (%) is F ≦ 85-140C% (%), lamellar spacing D of pearlite ([mu] m) is a ferrite-pearlite structure is a D ≦ 0.20 (μm). In Patent Document 3, a Mn of at least 1.00%, by containing at least 0.30% of Cr, the pro-eutectoid ferrite amount F and lamella spacing D is within the above range. Thus it is described excellent strength and toughness balance is obtained, and.
[0008]
　Incidentally, the hot forged part, not only the fatigue strength, wear resistance is also required. For example, a crank pin of the crankshaft is hot forged part is inserted into the large end of the connecting rod. When the crankshaft rotates, the crank pin is rotated via the sliding bearing and the inner surface of the large end of the connecting rod. Therefore, the surface of the crank pin, is required to have excellent wear resistance.
[0009]
　JP 2000-328193 JP JP (Patent Document 4) and Japanese Patent 2002-256384 (Patent Document 5) does not contain V, discloses a non-heat treated steel for the purpose of improving wear resistance.
[0010]
　Hot forging microalloyed steels disclosed in Patent Document 4 is a ferrite-pearlite structure. Furthermore, hot forging microalloyed steels disclosed in Patent Document 4, Si and Mn to strengthen ferrite by solid solution in the ferrite. Thus, improvement in wear resistance is achieved.
[0011]
　Non-heat treated steel for crankshafts disclosed in Patent Document 5, the pro-eutectoid ferrite fraction has a tissue pearlite principal less than 3%, and a thickness containing the following sulfide inclusions 20μm . Furthermore, Si content is less 0.60%, Al content is less than 0.005%. Accordingly, improvement of the wear resistance and machinability are achieved.
[0012]
　Usually, in order to improve the wear resistance of the hot forged part, the hot forged part, the surface hardening heat treatment is carried out. The surface hardening heat treatment for example, induction hardening treatment, or a nitriding treatment. The induction hardening process, the surface of the hot forged part is hardened layer is formed. Further, by nitriding the surface of the hot forged part is nitrided layer is formed. Hardened layer or a nitride layer has a high hardness. Therefore, the wear resistance of the surface of the hot forged part is improved.
[0013]
　However, if performing the surface hardening heat treatment, the production cost is high. Therefore, it does not contain V, and, be omitted surface hardening heat treatment, non-heat treated hot forged part of having excellent wear resistance is required.
[0014]
　The hot forged articles produced by using the non-heat treated steels disclosed in Patent Documents 2 to Patent Document 5, when the surface hardening heat treatment is omitted, there is a possibility that abrasion resistance is lowered.
[0015]
　JP 2012-1763 discloses (Patent Document 6), the refining after hot forging is not subjected and be used without also subjected surface hardening heat treatment, forging crankshafts having excellent wear resistance There has been described.
[0016]
　Forged crankshafts disclosed in Patent Document 6, 1.1C + Mn + 0.2Cr> 2.0 was filled (in each element symbol in the formula, the content of each element (mass%) is substituted), the pro-eutectoid ferrite-pearlite structure area ratio of ferrite is less than 10%, or consisting of non-heat treated steel of pearlite structure.
[0017]
　However, Patent Document 6, have not been studied fatigue strength.
CITATION
Patent Document
[0018]
Patent Document 1: JP-A-9-143610 JP
Patent Document 2: JP-A-10-226847
Patent Document 3: JP 61-264129 Patent Publication
Patent Document 4: JP 2000-328193 Patent Publication
Patent Document 5: JP open 2002-256384 JP
Patent Document 6: JP 2012-1763 JP
Summary of the Invention
[0019]
　The object of the present invention is also refining and surface hardening heat treatment after hot forging is omitted, it is to provide a hot forged part having excellent abrasion resistance and fatigue strength.
[0020]
　Hot forged part according to an embodiment of the present invention, the chemical composition, in mass%, C: 0.45 ~ 0.70%, Si: 0.01 ~ 0.70%, Mn: 1.0 ~ 1.7%, S: 0.01 ~ 0.1%, Cr: 0.05 ~ 0.25%, Al: 0.003 ~ 0.050%, N: 0.003 ~ 0.02%, Ca : 0 ~ 0.01%, Cu: 0 ~ 0.15%, and, Ni: 0 contains ~ 0.15%, the balance being Fe and impurities. Matrix of the cutting has been depth 500 [mu] m ~ 5 mm from the surface not is made of a ferrite-pearlite structure or pearlite structure area ratio of pro-eutectoid ferrite is 3% or less, the cutting is depth 500 [mu] m from the not not surface ~ 5 mm pearlitic the average diameter of pearlite colonies in tissue is less than 5.0 .mu.m.
[0021]
　Hot forged part according to an embodiment of the present invention also refining and surface hardening heat treatment after hot forging is omitted, having an excellent abrasion resistance and fatigue strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022]
[1] Figure 1 is a graph showing the pro-eutectoid ferrite ratio and the abrasion of the relationship.
FIG. 2 is a graph showing the relationship between the size and the fatigue strength of the pearlite colonies.
FIG. 3 is a drawing showing the essential portion of the crank shaft is an example of a hot forged part.
[4] FIG. 4 is a diagram for explaining an observation position in the sampling position, and the microstructure investigation of the microstructure in the cross-section of the round bar.
FIG. 5 is a schematic diagram of a rotating bending fatigue test pieces taken from each round bar.
[6] Figure 6 is a photographic image illustrating an example of a method of measuring the decarburization depth.
[7] FIG. 7 is a microstructure photograph of a test material of the present invention examples in the examples.
DESCRIPTION OF THE INVENTION
[0023]
　Hereinafter, with reference to the drawings, an embodiment of the present invention in detail. Its description will not be repeated the same reference numerals designate like or corresponding parts in FIG.
[0024]
　[Present hot forged part overview in the form of]
　to the present invention have found that the tempering treatment and surface hardening heat treatment to improve the wear resistance and fatigue strength of the hot forged part omitted, investigated and studied It was carried out. As a result, the present inventors have obtained the following findings.
[0025]
　(A) hot forging, the matrix of the cutting surface is, if the area ratio of the pro-eutectoid ferrite is less ferrite-pearlite structure, or pearlite structure, has excellent wear resistance. Bainite and martensite, ferrite-pearlite structure, or the wear resistance is inferior to the pearlite structure. Here, the "pro-eutectoid ferrite", when cooling the steel refers to ferrite precipitated from austenite prior to eutectoid transformation. Further, the "ferrite-pearlite structure" means a structure comprising a pro-eutectoid ferrite and pearlite, the "pearlite structure", the area ratio of the pro-eutectoid ferrite is 0% substantially pearlite single phase structure It means. In the following description, the area ratio of the pro-eutectoid ferrite, referred to as "pro-eutectoid ferrite ratio".
[0026]
　Pro-eutectoid ferrite, softer compared to perlite, the wear resistance of the pro-eutectoid ferrite is low. Therefore, if the pro-eutectoid ferrite ratio is less than a predetermined value, the hot forged part has excellent wear resistance.
[0027]
　1, a ferrite-pearlite structure, or the hot forged part having a pearlite structure is a graph showing the relationship between the pro-eutectoid ferrite ratio and wear resistance. Figure 1 was obtained by the following method. By changing the cooling condition after the chemical composition and the hot forging, the chemical composition and manufacturing conditions to produce different various hot forged part. It was taken test pieces for wear resistance investigation from hot forged part produced. It was measured the abrasion loss of the specimen by performing the abrasion resistance investigation. The horizontal axis of FIG. 1 is a pro-eutectoid ferrite of the tissue of the hot forged part. Cooling conditions after chemical composition and hot forging of the hot forged part, the measuring method of the pro-eutectoid ferrite ratio, and the details of the abrasion resistance investigation later.
[0028]
　As shown in FIG. 1, if the following pro-eutectoid ferrite ratio is 3%, the wear amount becomes less 0.008Og.
[0029]
　(B) In the above ferrite-pearlite structure or pearlite structure, the smaller the size of pearlite colonies pearlite structure, the fatigue strength of the hot forged part is high.
[0030]
　Pearlite structure includes a ferrite and cementite having a lamellar structure aligned in layers. In the pearlite structure, the crystal orientation of the ferrite is referred to as substantially the same area as pearlite blocks. Further, in the pearlite block, called a region where the crystal orientation of the ferrite is further aligned with the pearlite colonies.
[0031]
　In this specification, the pearlite structure, the difference in crystal orientation of the ferrite is surrounded by more than 15 ° of the boundary region is defined as a pearlite block. In other words, the same in the pearlite block, the difference in crystal orientation of the ferrite is less than 15 °. Further, in the pearlite structure, misorientation ferrite a region surrounded by the boundaries of the less 15 ° or 2 ° is defined as pearlite colonies. In other words, in the same pearlite colonies, the difference in crystal orientation of the ferrite is less than 2 °.
[0032]
　2, satisfies the chemical composition described below, a ferrite-pearlite structure, or the hot forged part having a pearlite structure is a graph showing the relationship between the size and the fatigue strength of the pearlite colonies. Figure 2, was obtained in the following manner. Similar to FIG. 1, to produce various hot forged part. It was taken fatigue test piece bending rotation from the hot forged part produced. Performing fatigue test to measure the fatigue strength of the rotary bending fatigue test piece. The horizontal axis of FIG. 2, the average diameter of the tissue pearlite colonies hot forged part. The diameter of pearlite colony is of a circle equal to the area of ​​the pearlite colony diameter (equivalent circle diameter). Hereinafter, the average diameter of pearlite colony as colonies diameter. Method for measuring the area of ​​the pearlite colonies, and details of the fatigue test to be described later.
[0033]
　As shown in FIG. 2, when the colony diameter decreases, the fatigue strength is increased. More colonies diameter is small, the boundary between the pearlite colonies increased. Increase of the boundary are believed to inhibit the extension of the fatigue crack.
[0034]
　As shown in FIG. 2, colonies diameter is equal to or less than 5.0 .mu.m, the fatigue strength is more than 400 MPa.
[0035]
　(C) Colony diameter can be controlled and the chemical composition, by the cooling rate after hot forging. By increasing the cooling rate after hot forging, colony size is reduced, the fatigue strength of the hot forged part is increased. On the other hand, when the cooling rate after hot forging is too large, the martensite and bainite is produced on the surface structure of the hot forged product, the hardness of the surface of the hot forged part is excessively high. Hot forged part may be machined. The higher surface hardness by the formation of martensite or bainite, machinability of the hot forged part is reduced.
[0036]
　Hot forged part according to the present embodiment has been completed based on the above findings, the chemical composition, in mass%, C: 0.45 ~ 0.70%, Si: 0.01 ~ 0.70%, Mn : 1.0 ~ 1.7%, S: 0.01 ~ 0.1%, Cr: 0.05 ~ 0.25%, Al: 0.003 ~ 0.050%, N: 0.003 ~ 0 .02%, Ca: 0 ~ 0.01%, Cu: 0 ~ 0.15%, and, Ni: 0 contains ~ 0.15%, the balance being Fe and impurities. Matrix of the cutting has been depth 500 [mu] m ~ 5 mm from the surface not is made of a ferrite-pearlite structure or pearlite structure area ratio of pro-eutectoid ferrite is 3% or less, the cutting is depth 500 [mu] m from the not not surface ~ 5 mm pearlitic the average diameter of pearlite colonies in tissue is less than 5.0 .mu.m.
[0037]
　The chemical composition, Ca: may contain from 0.0005 to 0.01%.
[0038]
　The chemical composition, Cu: 0.02 to 0.15%, and, Ni: may contain one or more selected from the group consisting of 0.02 to 0.15%.
[0039]
　Hot forged part according to the present embodiment is a crank shaft, for example.
[0040]
　Described in detail below hot forged part according to the present embodiment.
[0041]
　Configuration of the hot forged part]
　FIG. 3 is a diagram illustrating the main portion of the crank shaft 1 which is an example of a hot forged part according to the present embodiment. Crankshaft 1 has a crank pin 2, the crank journal 3, the crank arm 4, and a counterweight 6. Crank arm 4 is disposed between the crank pin 2 and the crank journal 3, is connected to the crank pin 2 and the crank journal 3. Counterweight 6 is connected to the crank arm 4. Kurankuyafuto 1 further includes a fillet portion 5. Fillet 5 corresponds to the joint portion between the crank pin 2 and the crank arm 4.
[0042]
　Crank pin 2 is rotatably mounted with respect to the connecting rod, not shown. Crank pin 2 is disposed offset from the crankshaft rotation shaft. Crank journal 3 is disposed coaxially with the rotary axis of the crankshaft 1.
[0043]
　Crank pin 2 is inserted into the large end of the connecting rod. When the crankshaft rotates, the crank pin 2 is rotated via the sliding bearing and the inner surface of the large end of the connecting rod. Therefore, the surface of the crank pin 2, the wear resistance is required.
[0044]
　In the surface of the crank shaft 1, and a portion to be cut, the portion which is not cut (portion where cutting is omitted) are present. For example, the side portions 41 of the crank arm 4, may not be cut. Surface of the counterweight 6 also, may not be cut.
[0045]
　As described above, in the conventional hot-forging, surface hardening heat treatment is carried out. Surface hardening heat treatment, for example, a high-frequency quenching treatment or nitriding treatment. The surface hardening heat treatment, and curing the surface of the crank pin, the wear resistance is improved.
[0046]
　However, the crankshaft 1 of the present embodiment, with respect to the crank pin 2, the surface hardening heat treatment is not performed. Thus, the manufacturing cost can be reduced. Incidentally, with the crank pin 2, with respect to the crank journal 3, to the surface hardening heat treatment may be omitted, for the entire crankshaft 1, the surface hardening heat treatment may be omitted.
[0047]
　Hot forged part according to the present embodiment, cutting the previous so-called intermediate product with (hot forgings entire surface is not cut), a portion of the hot forged part (surface which is the final product after cutting not been cut, the balance includes a hot forged part) that is cut.
[0048]
　[Chemical composition]
　hot forged part according to the present embodiment, consists of the following chemical composition. % Related elements, unless otherwise specified, it means mass%.
[0049]
　C: 0.45 ~ 0.70%
　carbon (C) reduces the pro-eutectoid ferrite ratio in the steel, increasing the area ratio of pearlite in the steel. This increases the strength and hardness of the steel, increased wear resistance. If the C content is too small, the steel structure, pro-eutectoid ferrite ratio becomes too high. On the other hand, if the C content is too high, the steel is excessively cured, the machinability of the steel is lowered. Therefore, C content is 0.45 to 0.70%. The preferable lower limit of C content is 0.48%, more preferably 0.50%. The preferable upper limit of C content is 0.60%, more preferably 0.58%.
[0050]
　Si: 0.01 ~ 0.70%
　silicon (Si) is, to strengthen the ferrite in solid solution in the ferrite in the pearlite. Thus, Si enhances the strength and hardness of the steel. Si also deoxidized steel. If Si content is too small, the strength and hardness of the steel is lowered. On the other hand, if Si content is too large, the steel is decarburization during hot forging. In this case, cutting allowance after hot forging is increased. Therefore, Si content is 0.01 to .70%. A preferable lower limit of Si content is 0.20%. The preferable upper limit of the Si content is 0.65%.
[0051]
　Mn: 1.0 ~ 1.7%
　manganese (Mn) increases the strength and hardness of the steel by solid solution in the steel. Mn further, suppressing the generation of pro-eutectoid ferrite. If the Mn content is too small, pro-eutectoid ferrite ratio is too high. Further, if the Mn content is too small, it is impossible to increase the strength and hardness. On the other hand, if the Mn content is too high, martensite and bainite is generated. Martensite and bainite lowers the wear resistance and machinability of the steel. Therefore, it is not preferable that martensite or bainite are produced. Therefore, Mn content is from 1.0 to 1.7%. The preferable lower limit of the Mn content is 1.2%, more preferably from 1.3%. The preferable upper limit of the Mn content is 1.65%, more preferably 1.6%.
[0052]
　S: 0.01 ~ 0.1%
　of sulfur (S) generates a sulfide such as MnS, enhance the machinability of the steel. On the other hand, if the S content is too large, decrease the hot workability of the steel. Thus, S content is 0.01 to 0.1%. The preferable lower limit of the S content is 0.03%, more preferably 0.04%. The preferable upper limit of the S content is 0.07%, more preferably 0.06%.
[0053]
　Cr: 0.05 ~ 0.25%
　chromium (Cr) increases the strength and hardness of the steel. Cr further suppresses the generation of pro-eutectoid ferrite in the steel. If the Cr content is too small, pro-eutectoid ferrite ratio is too high. On the other hand, if the Cr content is too high, martensite and bainite is generated. Therefore, Cr content is 0.05 to 0.25%. A preferable lower limit of Cr content is 0.08% and a preferable upper limit is 0.20%.
[0054]
　Al: 0.003 ~ 0.050%
　of aluminum (Al) is deoxidized steel. Al is further generated to suppress the coarsening of the crystal grains of the nitride. Therefore, strength of the steel, a significant reduction in the hardness and toughness is suppressed. On the other hand, if the Al content is too large, Al 2 O 3 inclusions are produced. Al 2 O 3 inclusions lowers the machinability of the steel. Therefore, Al content is from 0.003 to 0.050%. A preferable lower limit of Al content is 0.010%, and the preferable upper limit is 0.040%. Al content in the present embodiment is a content of acid-soluble Al (Sol. Al).
[0055]
　N: 0.003 ~ 0.02%
　nitrogen (N) produces a nitride or carbo-nitride. Nitrides or carbonitrides suppresses coarsening of crystal grains, to prevent the strength of the steel, a significant reduction in hardness and toughness. On the other hand, if the N content is too large, defects such as voids are likely to occur in the steel. Therefore, N content is 0.003 to 0.02%. The preferable lower limit of the N content is 0.005%, more preferably 0.008%, still more preferably 0.012%. The preferable upper limit of the N content is 0.018%.
[0056]
　The remainder of the chemical composition of the hot forged part is composed of Fe and impurities. Impurities as referred to herein, refers to an element being mixed from the environment, such as ores and scrap or manufacturing process, which is used as a raw material of steel. Impurities are, for example, phosphorus (P) and oxygen (O) and the like.
[0057]
　The chemical composition of the hot forged product of the present embodiment further includes, in place of part of Fe, and may contain Ca.
[0058]
　Ca: 0 ~ 0.01%
　of calcium (Ca) is an arbitrary element, it may not be contained. If contained, Ca enhances the machinability of the steel. Specifically, Ca is included in the Al-based oxides, which lower the melting point. Therefore, the steel machinability increases at high temperature cutting. However, if the Ca content is too high, toughness of the steel is lowered. Therefore, Ca content is from 0 to 0.01%. The preferable lower limit of the Ca content is 0.0005%.
[0059]
　The chemical composition of the hot forged product of the present embodiment further includes, in place of part of Fe, may contain one or more selected from the group consisting of Cu and Ni. Both of these elements, the solid-solution strengthening of the steel.
[0060]
　Cu:
　0 ~ 0.15%, Ni: 0 ~ 0.15%
　of copper (Cu), nickel (Ni) is an optional element and may not be contained. If contained, both Cu and Ni contributes to strengthening steel by solid solution in the steel. However, if the Cu content is too high, improves hardenability, bainite or martensite structure is likely to occur. Even Ni content is too high, improves hardenability, bainite or martensite structure is likely to occur. Therefore, Cu content is 0 to 0.15%, Ni content is 0 to 0.15%. The preferable lower limit of Cu content is 0.02%. A preferable lower limit of Ni content is 0.02%.
[0061]
　Organization
　of the surface of the hot forged part, the matrix of depth 500 [mu] m ~ 5 mm from the surface that is not cutting made of ferrite-pearlite structure or pearlite structure pro-eutectoid ferrite of not more than 3%. Hereinafter, of the surface of the hot forged part, the range of depth 500 [mu] m ~ 5 mm from the surface that has not been cut, referred to as "surface area".
[0062]
　Matrix of the surface layer region may be a ferrite-pearlite structure pro-eutectoid ferrite of not more than 3%, pro-eutectoid ferrite ratio may be 0% of the pearlite structure. Bainite and martensite, ferrite-pearlite structure, or the wear resistance is inferior to the pearlite structure.
[0063]
　Here, the area ratio of pro-eutectoid ferrite (pro-eutectoid ferrite ratio), is defined in the following manner. First, a sample is taken for microstructure observation including surface area of the hot forged part in the observation plane. The observation surface of the sample was mirror-polished, corroded with nital etchant. And within the observation plane, 20 fields, each (150μm × 200μm / field) 0.03 mm 2 to observe the region. The photomicrograph image processing, measuring the area ratio of the pro-eutectoid ferrite in each field, and the average value and the area ratio of the pro-eutectoid ferrite.
[0064]
　Matrix of the surface layer region is, if the ferrite-pearlite structure or pearlite structure area ratio of pro-eutectoid ferrite is 3% or less, increases the wear resistance of the hot forged product. Area ratio of preferred pro-eutectoid ferrite is less than 3%.
[0065]
　Hot forging further, the average diameter of the hot forged part pearlite colonies ferrite-pearlite structure or pearlite structure of the surface area of ​​(colony diameter) is less than or equal to 5.0 .mu.m.
[0066]
　Here, the colony diameter, is defined as follows. Collecting a specimen containing the viewing surface of the surface layer region of the hot forged part. Using this test piece, measuring the electron beam diffraction image by FEI Co. electron microscope Quanta (trade name) and Oxford Co. EBSD electron backscatter diffraction (EBSD) device HKL (trade name). From the electron beam diffraction image, to determine the boundaries of the organization of pearlite colonies. Calculating the area of ​​pearlite colonies from the boundary of pearlite colony. Calculated area from pearlite colony diameter Request (equivalent circle diameter). Each seek diameter of pearlite colonies from four places of the test piece corresponding to the surface area of ​​the hot forged part, and the average value and colony diameter. Incidentally, the misorientation of the ferrite in the pearlite structure of the region surrounded by the boundaries of the less 15 ° or 2 ° to pearlite colonies.
[0067]
　Smaller colonies diameter, boundary pearlite colonies increased. Increased boundary suppresses the propagation of fatigue cracks, increase the fatigue strength of the hot forged part.
[0068]
　Hot forged part according to the present embodiment has the above structure in the surface region, even if the surface hardening heat treatment is omitted, and a fatigue strength and excellent excellent wear resistance.
[0069]
　[Production Method]
　illustrating an example of the manufacturing method of the hot forged part.
[0070]
　Producing molten steel of said chemical composition. To cast slab by the molten steel continuous casting method. Molten steel may be ingot (steel ingot) by the ingot casting method. The slab or ingot by hot working, the billet may be (slab) and steel bars.
[0071]
　Slab, heating the ingot, billet or steel bar in the heating furnace. The heating temperature is preferably 1200 ° C. or higher. Heated slab, an ingot, billet or steel bar by hot forging, for manufacturing the intermediate product. Finishing temperature for hot forging is preferably 900 ° C. or higher.
[0072]
　The intermediate product after hot forging, controlled cooling at a predetermined rate. Specifically, the surface temperature of the intermediate product is the cooling rate in the 800 ~ 500 ° C., to 100 ~ 300 ° C. / min. When the cooling rate is too low, pearlite colonies is increased, not high fatigue strength. In addition, the cooling rate is too small, pro-eutectoid ferrite rate increases. On the other hand, if the cooling rate is too high, martensite and bainite is generated. Therefore, the cooling rate the surface temperature of the intermediate products in the 800 ~ 500 ° C. is 100 ~ 300 ° C. / min.
[0073]
　The cooling may, for example, it is possible to realize mist cooling by fluid mixture of air and water, strong cooling with compressed air or by strong air cooling by a blower. Incidentally, temperature range higher than 800 ° C., and the cooling rate in the temperature region lower than 500 ° C. is optional.
[0074]
　In this way, the hot forged part is an intermediate product is produced. The steel of the chemical composition and hot forging, by cooling in the above cooling rate, the matrix of the surface layer region of the hot forged part is, a ferrite-pearlite structure or pearlite area ratio of the pro-eutectoid ferrite is 3% or less the organization. Furthermore, colony size in pearlite structure in the surface region is less than 5.0 .mu.m. The hot forged part is not tempering treatment is performed, a non-heat treated.
[0075]
　A portion of the hot forged part of the surface by cutting by machining, to produce the crankshaft 1 is a hot forged part as a final product. The thickness to be removed by cutting (cutting margin) is the depth 500 [mu] m ~ about 5mm from the surface of the hot forged part as the intermediate product. Thus, for example, from the crankshaft 1 of the surface after machining to a depth of several mm to the above such tissues, in cutting the previous hot forged part (intermediate product), 500 [mu] m from the surface ~ 5 mm matrix in depth position may be a ferrite-pearlite structure or pearlite structure pro-eutectoid ferrite of not more than 3%. Similarly, in the hot forged part before cutting, colony diameter of pearlite structure in the 500 [mu] m ~ 5 mm depth position from the surface can fall within the following 5.0 .mu.m.
[0076]
　The production surface of the crank shaft 1, there are also surfaces which are not cut. Matrix at a depth 500 [mu] m ~ 5 mm position from this surface is pro-eutectoid ferrite ratio is less than 3% of a ferrite-pearlite structure or pearlite structure, colony diameter of pearlite structure at the position of depth 500 [mu] m ~ 5 mm from the surface 5.0μm is less than or equal to.
[0077]
　Of the crankshaft 1 to be manufactured, at least the crank pin 2, the surface hardening heat treatment is omitted. In other words, at least the crank pin 2 surface, high-frequency quenching treatment and the nitriding treatment is not performed. Incidentally, the fillet portion 5 is carried out fillet rolling may be further enhanced surface hardness of the fillet portion 5 by work hardening. The fillet rolling, while rotating the hot forged part 1, presses the roller to the surface of the fillet portion 5. Thus, the surface of the fillet portion 5 is plastically processed, to work hardening. Fillet 5 may not be carried out fillet rolling.
[0078]
　Be an intermediate product in the hot forged part produced by the above process, even in the final product (crankshaft 1), a matrix of depth 500 [mu] m ~ 5 mm from the surface that is not cutting, pro-eutectoid ferrite ratio There consisting of ferrite-pearlite structure or pearlite structure is less than or equal to 3%. Furthermore, colony diameter of pearlite structure of depth 500 [mu] m ~ 5 mm from the surface is less than 5.0 .mu.m.
[0079]
　Of the surface of the hot forged part as a final product, the matrix of the cutting surface is made a ferrite-pearlite structure or pearlite structure pro-eutectoid ferrite of not more than 3%, colony diameter of pearlite structure in the surface 5. 0μm is less than or equal to.
[0080]
　Since having the above structure does not contain V, also refining and surface hardening heat treatment is omitted, hot forged product of the present embodiment includes a fatigue strength and excellent excellent wear resistance. Furthermore, Si content of hot forged product of the present embodiment, since a proper amount, it is possible to suppress the decarburized layer depth formed on the surface of the hot forged part is an intermediate product. Therefore, it is possible to suppress the cutting allowance of the hot forging after hot forging.
Example
[0081]
　Steel chemical compositions shown in Table 1 (Test Nos. 1 to 7 and a ~ i) was melted in a vacuum induction furnace, and molten steel. And ingot casting the molten steel, to produce a columnar ingot. Each ingot is manufactured, the weight is 25 kg, the outer diameter was 75 mm.
[0082]
[Table 1]

[0083]
　The column of each element symbol in Table 1, the content of the corresponding element (mass%) is described. In Table 1, "-" indicates that the corresponding element is a impurity levels. The remainder of each steel was Fe and impurities.
[0084]
　The ingots produced from the steel to produce a forging with hot forging. Specifically, it was heated to 1250 ° C. Each ingot heating furnace. The heated ingots hot forged to round bars forgings having an outer diameter of 15 mm (hereinafter, simply referred to as round bars) was prepared. Finishing temperature during hot forging was 950 ℃.
[0085]
　After hot forging, to cool the rod down to room temperature (23 ° C.) at a cooling rate shown in Table 1. Cooling rate surface temperature at 800 ℃ ~ 500 ℃ (℃ / min) are as shown in Table 1. Specifically, the test No. 1 ~ 7, b, c, d, e, g, in h, and i, were performed mist cooling at 800 ℃ ~ 500 ℃. In Test No. a, was carried out air cooling using a blower at 800 ℃ ~ 500 ℃. In Test No. f, it was carried out allowed to cool at 800 ℃ ~ 500 ℃.
[0086]
　[Microstructure investigation]
　the micro samples were taken from each round bar, to observe the organization. Figure 4 is a diagram for explaining an observation position in the sampling position, and the microstructure investigation of the microstructure in the cross-section of the round bar. As it is shown by the chain line in FIG. 4, from the round bar, and the micro sample containing the surface of each round bar and four collected every 90 °.
[0087]
　The surface of each micro-sample was mirror-polished, and the polished surface was corroded with nital etchant. The corroded surface was observed at 400 times optical microscope.
[0088]
　As shown in FIG. 4, each micro-sample, 5 mm depth position from 500μm depth position and surface from the surface of the round bar, i.e. at a position that is circled, 5 fields per each one position, a total of 20 fields, each (150μm × 200μm / field) 0.03 mm 2 was observed region of. Photomicrographs of each region and the image processing to determine the area ratio of the pro-eutectoid ferrite occupied in each region. The average value of the area ratio of the pro-eutectoid ferrite in 20 fields of view observed at 500μm depth position from the surface, and the surface of the micro-sample and pro-eutectoid ferrite ratio of 500μm depth position. The average value of the area ratio of the pro-eutectoid ferrite in 20 fields of view observed at 5mm depth position from the surface, and the surface of the micro-sample and eutectoid ferrite ratio of at 5mm depth position.
[0089]
　[Pearlite colonies Survey
　using EBSD device was measured colony diameters of the pearlite structure at the observation position of each micro-sample. More specifically, FEI Co. electron microscope Quanta (trade name), and by Oxford Co. EBSD analyzer HKL (trade name), was subject to electron beam diffraction image. By analyzing the crystal orientation and the like from the electron beam diffraction image to determine the boundaries of the pearlite colonies was calculated the area of each pearlite colony therefrom. Analysis was carried out by HKL (trade name).
[0090]
　As with the microstructure investigation, each micro-sample, in 5mm depth position from 500μm depth position and surface from the surface, they were respectively measured colony diameters. The beam diameter of the electron beam is 1 [mu] m, 1 single mapping area is 100 [mu] m × 200 [mu] m, an average value of 4 points mapping area and colony size.
[0091]
　[Surface Hardness Survey
　the hardness of the cross section of the round bar with each micro-sample, was measured by Vickers hardness test according to JIS Z2244 (2009). Test force was 98.07N (10kgf). For each micro-sample, from the surface of the round bar to the inside rod, to measure the hardness of a total of five points at 1mm pitch, those obtained by averaging was defined as the average hardness of the micro-sample.
[0092]
　[Fatigue Strength Survey
　from each round bar was collected rotary bending fatigue test piece. Figure 5 is a schematic diagram of a rotating bending fatigue test pieces taken from each round bar. Rotary bending test piece, the diameter of the parallel portion is 8 mm, the diameter of the gripping portion was 12 mm. The central axis of the rotary bending fatigue test piece so as to coincide with the central axis of the round bar, to produce a rotary bending fatigue strength test piece. Specifically, by turning, by cutting from the surface of the round bar to a depth of 3.5 mm, to prepare a parallel portion. Thus, the surface of the parallel portion at least, was equivalent to the range of a depth of 5mm from the surface of the round bar. That is, rotation bending fatigue strength test pieces were assumed crankshaft 1 after cutting the intermediate product.
[0093]
　The parallel portion of the rotating bending fatigue strength test pieces were conducted final polishing was adjusted surface roughness. Specifically, the center line average surface roughness (Ra) of not exceed a 3.0 [mu] m, and the maximum height (Rmax) within 9.0 .mu.m.
[0094]
　Using a rotating bending fatigue strength test pieces was carried out final polishing, room temperature (23 ° C.), at ambient atmosphere was Ono type rotating bending fatigue test under the conditions of both swing speed 3600 rpm. The fatigue test was conducted changing the applied stress to the plurality of test strips, 10 7 to the highest stresses were not broken after cycles and fatigue strength (MPa).
[0095]
　[Abrasion resistance Survey
　at a depth of 500 [mu] m ~ 1000 .mu.m from the surface of the round bar, such that the center below the main surface, 1.5 mm × 2.0 mm × 3.7 mm wear resistance survey test strip of It was collected. Surface of 2.0 mm × 3.7 mm of each specimen (hereinafter, referred to as the main surface) was parallel to the cross section of the round bar. That is, the normal of the main surface of each specimen was parallel to the central axis of the round bar.
[0096]
　For each test piece was subjected to pin-on-disk wear test using an automatic polishing machine. More specifically, the surface of the rotating disk of an automatic polishing machine was attached emery paper number (grit) 800. Then, the main surface of 26gf / mm specimens on emery paper 2 while pressing at a surface pressure of the rotary disc and rotated for 50 minutes at a peripheral speed of 39.6m / min. After rotation for 50 minutes, the weight difference of the test piece before and after the test was defined as the wear amount (g).
[0097]
　[Decarburization depth investigation]
　decarburization depth of the round bar of each test number was determined by the following method. Perpendicularly against the axial direction of the round bar, a round bar was cut, and the cut surface was taken micro sample to the test surface. The surface of each micro-sample was mirror-polished, corroded to the polishing surface nital etchant. The corroded surface was observed at 400 times optical microscope. Then, to produce a photographic image of any one field of view of the surface layer portion including the surface of the round bar (800μm × 550μm). Figure 6 is an example of the generated image.
[0098]
　Using the generated photographic image, obtained decarburization depth ([mu] m) in the following manner. A line segment (550 .mu.m) connecting both ends 50 of the surface of the round bar in the photographic image was defined as the reference surface 60. Has two sides parallel to the reference surface 60, the width is provided a measurement area 100 of 10 [mu] m. The measurement region 100, and the depth direction from the reference surface 60 is moved at a 1μm units. Each time to 1μm movement was calculated pro-eutectoid ferrite ratio of the measurement area 100. Pro-eutectoid ferrite ratio depth no longer become a value of 4% or more (the distance from the reference surface 60 to a width center of the measurement region 100) was defined as the decarburized depth ([mu] m). The "pro-eutectoid ferrite ratio depth no longer become a value of more than 4%", both in a position deeper than its depth, means a depth at which the pro-eutectoid ferrite ratio is less than 4%.
[0099]
　[Survey]
　shows the survey results in Table 2.
[0100]
[Table 2]

[0101]
　Table 2, a round bar made from the steel, in 5mm depth position from 500μm depth position and surface from the surface, the tissue, pro-eutectoid ferrite ratio, colony diameter is described.
[0102]
　The column of "tissue" tissue obtained by microstructure investigation is described. In Table 2, "F + P" represents a ferrite-pearlite structure, "P" represents a pearlite structure, "M" represents a martensitic structure, "B + P" represents a bainite-pearlite structure, "M + B + P "indicates a martensite-bainite-pearlite structure. In the column of "pro-eutectoid ferrite ratio (%)" in the microstructure investigation, four positions taken every 90 °, the average value of pro-eutectoid ferrite percentage of microscopic samples of a total of 20 fields of view are described. In the column "Colony diameter ([mu] m)" in microstructure investigation, have been described average colony size of four places of micro samples taken every 90 °. Table 2 in the "-" indicates that colonies diameter of unmeasured.
[0103]
　In the column of "average hardness (HV)", in surface hardness survey, the average hardness of the average value of the four micro samples taken every 90 ° (i.e., the average value of the total of 20 points) have been described there. The average hardness is can not be obtained a high fatigue strength is less than 300 HV. On the other hand, the average hardness is difficult cutting exceeds 400 HV.
[0104]
　In the column of "fatigue strength (MPa)" is the fatigue strength obtained by the fatigue strength survey is described. Fatigue strength is preferably at least 400 MPa.
[0105]
　In the column "amount of wear (g)" is the wear amount obtained by the abrasion test are described. Wear amount is preferably less 0.008Og.
[0106]
　In the column "decarburization depth ([mu] m)" is described decarburization depth to pro-eutectoid ferrite ratio obtained by decarburization depth investigation is less than 4% ([mu] m) is. Less than 4% decarburization depth is preferably less than 500 [mu] m. Table 2 in the "-" indicates that decarburization depth is not measured.
[0107]
　Referring to Table 1, the chemical composition of the material under test with the test number 1-7 are within the scope of the present invention, the cooling rate after hot forging was also appropriate. Referring to Table 2, in Test No. 1-7, tissue 5mm depth position from the surface from 500μm depth position and the surface was pro-eutectoid ferrite of 3% or less of ferrite-pearlite structure or pearlite structure. Figure 7 is a microstructure photograph of the test material at 5mm from the surface of the test number 2. 7, most of the microstructure is pearlite P, pro-eutectoid ferrite F was 2% in the area ratio. Note that in the structure photograph of FIG. 7, which extends in the transverse direction is MnS.
[0108]
　Further, in Test No. 1-7, colony diameter tissue 5mm depth position from the surface from 500μm depth position and the surface was not more than 5.0 .mu.m. As a result, the fatigue strength of the test numbers 1 to 7 is at least 400 MPa, the wear amount was less than 0.008Og. The average hardness of the test numbers 1-7 was at least 300 HV. Further, the average hardness of the test numbers 1-7, were less than 400HV excellent machinability can be obtained. Further, decarburization depth of Test No. 2 and 3 were less than 500 [mu] m.
[0109]
　In Test No. a, small Mn content, also, V is contained. Mn is an element for suppressing formation of ferrite, V is an element which contributes to the formation of ferrite. Therefore, in Test No. a, tissue 5mm depth position from the surface from 500μm depth position and the surface was ferrite-pearlite structure of pro-eutectoid ferrite ratio exceeds 3%. As a result, the wear amount of the test number a is exceeded 0.008Og. The average hardness of the test number a is less than 300 HV.
[0110]
　In Test No. b, it was less C content. C is an element for suppressing formation of ferrite. Therefore, Test No. b, the tissue of 5mm depth position from the surface from 500μm depth position and the surface was ferrite-pearlite structure of pro-eutectoid ferrite ratio exceeds 3%. As a result, the wear amount of the test number b is exceeded 0.008Og. The average hardness of the test number b was less than 300 HV.
[0111]
　In Test No. c, small C content, also small Mn content, further, in many cases the Cr content. Cr is an element which contributes to the formation of martensite. Therefore, Test No. c, the tissue of 5mm depth position from the surface from 500μm depth position and the surface was martensitic. Martensite and bainite are prone to wear than pearlite, as a result, the wear amount of the test number c is exceeded 0.008Og. The average hardness of the test numbers c was greater than 400 HV.
[0112]
　Si content of Test No. d was higher. Therefore, deep decarburization depth was measured to 600μm depth of field that can be observed and terminated. Decarburization depth was deeper than 600μm.
[0113]
　Although the chemical composition of Test No. e was appropriate, the cooling rate after hot forging was too large. Therefore, the organization of 5mm depth position from the surface from 500μm depth position and the surface, as well as perlite, contained about 30% of martensite and bainite area ratio. Therefore, the average hardness of the test number i exceeds 400 HV.
[0114]
　Although the chemical composition of Test No. f was appropriate, the cooling rate after hot forging is too small. Therefore, colonies diameter of pearlite structure of 5mm depth position from the surface from 500μm depth position and surface exceeds 5.0 .mu.m. As a result, the fatigue strength of the test numbers e was less than 400 MPa.
[0115]
　Cr content of the test number g was too high. Therefore, the organization of 5mm depth position from the surface from 500μm depth position and the surface, as well as perlite, including martensite and bainite. Therefore, the average hardness of the test number i exceeds 400 HV.
[0116]
　In the test number h, it was less Mn content. Mn is an element for suppressing formation of ferrite. Therefore, Test No. h, the tissue of 5mm depth position from the surface from 500μm depth position and the surface was ferrite-pearlite structure of pro-eutectoid ferrite ratio exceeds 3%. As a result, the wear amount of the test number h is exceeded 0.008Og. The average hardness of the test number h is less than 300 HV, the fatigue strength was less than 400 MPa.
[0117]
　In Test No. i, Mn content was too high. Mn is an element which contributes to the formation of bainite. Therefore, test number i, tissue 5mm depth position from the surface from 500μm depth position and the surface was bainite-pearlite structure. Martensite and bainite are prone to wear than pearlite, as a result, the wear amount of the test number i exceeds 0.008Og. The average hardness of the test number i exceeds 400 HV.
[0118]
　In the above embodiment, the hot forged part is explained a case where the crankshaft. However, the present invention can be used as a hot forged part other than the crankshaft.
[0119]
　Having described the embodiments of the present invention, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by modifying the above-described embodiments without departing from the scope and spirit thereof as appropriate.

claims
[Requested item 1]
　Chemical composition, in
　mass%,
　C: 0.45
　~ 0.70%, Si: 0.01 ~ 0.70%,
　Mn: 1.0 ~ 1.7%, S: 0.01 ~ 0.1
　%,
　Cr:
　0.05 ~ 0.25%, Al: 0.003 ~
　0.050%, N: 0.003 ~ 0.02%, Ca: 0 ~
　0.01%, Cu: 0 ~ 0. 15%, and,
　Ni: 0 contains ~ 0.15%, the balance being Fe and impurities,
　the matrix of depth 500 [mu] m-5 mm from a surface that has not been cut, the area ratio of the pro-eutectoid ferrite is 3% consists ferrite-pearlite structure or pearlite structure is,
　the average diameter of the cutting by depth 500μm from have not surface ~ 5 mm pearlite structure of pearlite colonies is less than 5.0 .mu.m, the hot forged part.
[Requested item 2]
　A hot forged part according to claim 1,
　wherein the chemical
　composition, Ca: 0.0005 contains ~ 0.01%, the hot forged part.
[Requested item 3]
　A claim 1 or hot forged part according to claim 2,
　wherein the chemical
　composition, Cu: 0.02 ~ 0.15%,
　and, Ni: 0.02 ~ 0.15%,
　the group consisting of containing one or more selected from, the hot forged part.
[Requested item 4]
　A hot forged part according to any one of claims 1 to 3,
　wherein the hot forging is the crankshaft, the hot forged part.