JP6394319B2 - Hot forging - Google Patents

Description

  The present invention relates to a hot forged product, and more particularly to a hot forged product in which tempering and surface hardening heat treatment after hot forging are omitted.

  Recently, hot forged products (for example, forged crankshafts) in which tempering treatment is omitted have been provided. The tempering treatment is a quenching treatment and a tempering treatment for improving the mechanical properties of the steel such as strength. Hereinafter, the hot forged product in which the tempering process is omitted is referred to as a non-tempered hot forged product.

  The steel material constituting the non-tempered hot forged product usually contains vanadium (V). Non-tempered hot forged products are manufactured by hot forging steel and allowing it to cool in the atmosphere. The structure of the steel material constituting the non-tempered hot forged product is a ferrite pearlite structure. V in the steel forms fine carbides in the steel in the cooling process after hot forging and improves the fatigue strength of the steel. In short, even if the tempering treatment is omitted, the non-tempered hot forged product containing V has excellent fatigue strength. Non-heat treated steel for hot forging containing V is disclosed in, for example, Japanese Patent Application Laid-Open No. 9-143610 (Patent Document 1). The non-heat treated steel disclosed in Patent Document 1 has a ferrite / pearlite structure, and precipitates and strengthens ferrite by V. Therefore, it is described that high fatigue strength can be obtained.

  However, since V is expensive, the manufacturing cost of a non-tempered hot forged product is high. Therefore, there is a demand for a non-tempered hot forged product having excellent fatigue strength even without containing V.

  Japanese Patent Application Laid-Open No. 10-226847 (Patent Document 2) and Japanese Patent Application Laid-Open No. 61-264129 (Patent Document 3) describe a non-heat treated steel for hot forging having high fatigue strength without containing V and Propose hot forgings.

  Non-tempered steel disclosed in Patent Document 2 is mass%, C: 0.30 to 0.60%, Si: 0.05 to 2.00%, Mn: 0.90 to 1.80%, Cr: 0.10 to 1.00%, s-Al: 0.010 to 0.045%, N: 0.005 to 0.025%, remaining Fe and impurities, the hardness after hot forging Below 30 HRC, the structure is ferrite + pearlite, the pearlite lamella spacing is 0.80 μm or less, and the pro-eutectoid ferrite area ratio is 30% or less. In Patent Document 2, if a non-tempered steel having the above chemical composition is hot forged and allowed to cool, the lamella spacing of pearlite becomes fine and the area ratio of pro-eutectoid ferrite becomes low. It is described as increasing.

In Patent Document 3, by mass%, C: 0.25 to 0.60%, Si: 0.10 to 1.00%, Mn: 1.00 to 2.00%, and Cr: 0.30 The steel containing 1.00% is not lower than the _Ac3 transformation point and heated to a temperature of 1050 ° C. or lower, hot forged, and then cooled, so that the amount of proeutectoid ferrite F (%) is A ferrite-pearlite structure in which F ≦ 85-140 C% (%) and the pearlite lamellar spacing D (μm) is D ≦ 0.20 (μm) is adopted. In Patent Document 3, by containing at least 1.00% Mn and at least 0.30% Cr, the amount of pro-eutectoid ferrite F and the lamellar spacing D are within the above ranges. This describes that an excellent balance of strength and toughness can be obtained.

  By the way, not only fatigue strength but also wear resistance is required for hot forged products. For example, a crankpin of a crankshaft that is a hot forged product is inserted into the large end of the connecting rod. When the crankshaft rotates, the crankpin rotates through the inner surface of the large end of the connecting rod and the slide bearing. Therefore, excellent wear resistance is required on the surface of the crankpin.

  Japanese Patent Application Laid-Open No. 2000-328193 (Patent Document 4) and Japanese Patent Application Laid-Open No. 2002-256384 (Patent Document 5) disclose non-tempered steel that does not contain V and is intended to improve wear resistance.

  The non-heat treated steel for hot forging disclosed in Patent Document 4 has a ferrite pearlite structure. Furthermore, the non-heat treated steel for hot forging disclosed in Patent Document 4 strengthens ferrite by dissolving Si and Mn in ferrite. As a result, the wear resistance is improved.

  The non-heat treated crankshaft steel disclosed in Patent Document 5 has a pearlite-based structure with a pro-eutectoid ferrite fraction of less than 3%, and contains sulfide inclusions having a thickness of 20 μm or less. . Further, the Si content is 0.60% or less, and the Al content is less than 0.005%. Thereby, improvement of abrasion resistance and machinability is achieved.

  Usually, in order to improve the wear resistance of the hot forged product, a surface hardening heat treatment is performed on the hot forged product. The surface hardening heat treatment is, for example, induction hardening or nitriding. By induction hardening, a hardened layer is formed on the surface of the hot forged product. Further, a nitriding layer is formed on the surface of the hot forged product by nitriding. The quenching layer and the nitride layer have high hardness. Therefore, the wear resistance of the surface of the hot forged product is improved.

  However, if the surface hardening heat treatment is performed, the manufacturing cost increases. Accordingly, there is a need for a non-tempered hot forged product that does not contain V and has excellent wear resistance even if the surface hardening heat treatment is omitted.

  In the hot forged product manufactured using the non-heat treated steel disclosed in Patent Documents 2 to 5, if the surface hardening heat treatment is omitted, the wear resistance may be lowered.

  Japanese Patent Application Laid-Open No. 2012-1763 (Patent Document 6) discloses a forged crankshaft having excellent wear resistance even when used without being subjected to a tempering treatment after hot forging and without being subjected to a surface hardening heat treatment. Is described.

  The forged crankshaft disclosed in Patent Document 6 satisfies 1.1C + Mn + 0.2Cr> 2.0 (the content (% by mass) of each element is substituted for each element symbol in the formula), and the initial analysis It consists of a ferrite-pearlite structure with an area ratio of ferrite of less than 10%, or a non-tempered steel material with a pearlite structure.

  However, Patent Document 6 does not discuss fatigue strength.

JP-A-9-143610 JP-A-10-226847 JP-A 61-264129 JP 2000-328193 A JP 2002-256384 A JP 2012-1763 A

  An object of the present invention is to provide a hot forged product having excellent wear resistance and fatigue strength even if the tempering treatment and surface hardening heat treatment after hot forging are omitted.

  The hot forged product according to one embodiment of the present invention has a chemical composition of mass%, C: 0.45 to 0.70%, Si: 0.01 to 0.70%, Mn: 1.0 to Contains 1.7%, S: 0.01 to 0.1%, Cr: 0.05 to 0.25%, Al: 0.003 to 0.050% and N: 0.003 to 0.02% The balance consists of Fe and impurities. The matrix having a depth of 500 μm to 5 mm from the uncut surface is composed of a ferrite / pearlite structure or a pearlite structure in which the area ratio of pro-eutectoid ferrite is 3% or less, and the pearlite having a depth of 500 μm to 5 mm from the uncut surface. The average diameter of pearlite colonies in the tissue is 5.0 μm or less.

  The chemical composition of the hot forged product according to the embodiment of the present invention may further contain Ca: 0.01% or less instead of part of Fe.

  The chemical composition of the hot forged product according to the embodiment of the present invention is further selected from the group consisting of Cu: 0.15% or less and Ni: 0.15% or less instead of part of Fe. You may contain 1 or more types.

  The hot forged product according to one embodiment of the present invention has excellent wear resistance and fatigue strength even if the tempering treatment and surface hardening heat treatment after hot forging are omitted.

FIG. 1 is a graph showing the relationship between pro-eutectoid ferrite rate and wear resistance. FIG. 2 is a graph showing the relationship between the size of pearlite colonies and fatigue strength. FIG. 3 is a view showing a main part of a crankshaft which is an example of a hot forged product. FIG. 4 is a diagram for explaining the sampling position of the microstructure in the cross section of each round bar and the observation position in the microstructure inspection. FIG. 5 is a schematic view of a rotating bending fatigue test piece taken from each round bar. FIG. 6 is a photographic image for explaining an example of a method for measuring the decarburization depth. FIG. 7 is a microstructure photograph of the sample material of the present invention example in Examples.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

[Outline of hot forged product according to this embodiment]
The present inventors investigated and examined in order to improve the wear resistance and fatigue strength of a hot forged product in which tempering treatment and surface hardening heat treatment were omitted. As a result, the present inventors obtained the following knowledge.

  (A) The hot forged product has excellent wear resistance if the matrix of the cut surface is a ferrite / pearlite structure or a pearlite structure in which the area ratio of pro-eutectoid ferrite is small. Here, “proeutectoid ferrite” means ferrite that precipitates from austenite prior to eutectoid transformation when the steel is cooled. In addition, “ferrite / pearlite structure” means a structure composed of pro-eutectoid ferrite and pearlite, and “pearlite structure” means a substantially pearlite single-phase structure in which the area ratio of pro-eutectoid ferrite is 0%. means. In the following description, the area ratio of pro-eutectoid ferrite is referred to as “pro-eutectoid ferrite ratio”.

  Proeutectoid ferrite is softer than pearlite, and proeutectoid ferrite has low wear resistance. Therefore, if the pro-eutectoid ferrite rate is less than or equal to a predetermined value, the hot forged product has excellent wear resistance.

  FIG. 1 is a graph showing the relationship between pro-eutectoid ferrite rate and wear resistance. FIG. 1 was obtained as follows. Various hot forgings having different chemical compositions and manufacturing conditions were manufactured by changing the chemical composition and cooling conditions after hot forging. Test specimens for investigating wear resistance were collected from the manufactured hot forgings. The wear resistance of the test piece was measured through a wear resistance investigation. The horizontal axis in FIG. 1 represents the pro-eutectoid ferrite rate of the structure of the hot forged product. Details of the chemical composition of the hot forged product, the cooling conditions after hot forging, the method for measuring the pro-eutectoid ferrite rate, and the wear resistance investigation will be described later.

  As shown in FIG. 1, when the pro-eutectoid ferrite ratio is 3% or less, the wear amount is 0.008 g or less.

  (B) In the above ferrite-pearlite structure or pearlite structure, if the size of the pearlite colony of the pearlite structure is small, the fatigue strength of the hot forged product is increased.

The pearlite structure has a lamellar structure in which ferrite and cementite are arranged in layers.
In the pearlite structure, a region where the crystal orientation of ferrite is almost the same is called a pearlite block. In the pearlite block, a region in which the crystal orientations of ferrite are further aligned is called a pearlite colony.

  In this specification, in the pearlite structure, a region surrounded by a boundary where the difference in the crystal orientation of ferrite is 15 ° or more is defined as a pearlite block. In other words, in the same pearlite block, the difference in ferrite crystal orientation is less than 15 °. Further, in the pearlite structure, a region surrounded by a boundary where the orientation difference of ferrite is 2 ° to 15 ° is defined as a pearlite colony. In other words, within the same pearlite colony, the difference in the crystal orientation of the ferrite is less than 2 °.

  FIG. 2 is a graph showing the relationship between the size of pearlite colonies and fatigue strength. FIG. 2 was obtained as follows. Similar to FIG. 1, various hot forgings were produced. A rotating bending fatigue test piece was collected from the manufactured hot forged product. A fatigue test was performed to measure the fatigue strength of the rotating bending fatigue test piece. The horizontal axis in FIG. 2 represents the average diameter of pearlite colonies in the structure of the hot forged product. The diameter of a pearlite colony is the diameter of a circle (equivalent circle diameter) that is equal to the area of the pearlite colony. Hereinafter, the average diameter of the pearlite colony is referred to as the colony diameter. Details of the method for measuring the area of the pearlite colony and the fatigue test will be described later.

  As shown in FIG. 2, the fatigue strength increases as the colony diameter decreases. The smaller the colony diameter, the greater the boundary between pearlite colonies. It is thought that the increase in the boundary suppresses the extension of fatigue cracks.

  As shown in FIG. 2, if the colony diameter is 5.0 μm or less, the fatigue strength is 400 MPa or more.

  (C) Colony diameter can be controlled by the chemical composition and the cooling rate after hot forging. If the cooling rate after hot forging is increased, the colony diameter is reduced and the fatigue strength of the hot forged product is increased. On the other hand, when the cooling rate after hot forging is too high, martensite and bainite are generated in the surface structure of the hot forged product, and the surface hardness of the hot forged product becomes excessively high. The hot forged product may be cut. If the surface hardness is increased by the formation of martensite or bainite, the machinability of the hot forged product is lowered.

  Based on the above knowledge, the present inventors completed the hot forging product by this Embodiment. Hereinafter, the hot forged product according to the present embodiment will be described in detail.

[Composition of hot forged products]
FIG. 3 is a view showing a main part of the crankshaft 1 which is an example of a hot forged product according to the present embodiment. The crankshaft 1 includes a crankpin 2, a crank journal 3, a crank arm 4, and a counterweight 6. The crank arm 4 is disposed between the crankpin 2 and the crank journal 3 and is connected to the crankpin 2 and the crank journal 3. The counterweight 6 is connected to the crank arm 4. The crankshaft 1 further includes a fillet portion 5. The fillet portion 5 corresponds to a joint portion between the crankpin 2 and the crank arm 4.

  The crankpin 2 is rotatably attached to a connecting rod (not shown). The crank pin 2 is arranged so as to be shifted from the rotation axis of the crank shaft 1. The crank journal 3 is disposed coaxially with the rotation axis of the crankshaft 1.

  The crankpin 2 rotates through the inner surface of the large end portion of the connecting rod and a slide bearing. Therefore, wear resistance is required on the surface of the crankpin 2.

  Note that, on the surface of the crankshaft 1, there are a portion to be cut and a portion not to be cut (a portion where cutting is omitted). For example, the side surface portion 41 of the crank arm 4 may not be cut. The surface of the counterweight 6 may not be cut.

  As described above, surface hardening heat treatment is performed in a normal hot forged product. The surface hardening heat treatment is, for example, induction hardening or nitriding. By the surface hardening heat treatment, the surface of the crankpin is hardened and the wear resistance is improved.

  However, in the crankshaft 1 according to the present embodiment, the surface hardening heat treatment is not performed on the crankpin 2. This reduces the manufacturing cost. The surface hardening heat treatment may be omitted for the crank journal 2 as well as the crankpin 2, or the surface hardening heat treatment may be omitted for the entire crankshaft 1.

  The hot forged product according to the present embodiment is a so-called intermediate product before cutting (a hot forged product in which the entire surface is not cut) and a hot forged product (part of the surface is a final product after the cutting). A hot forged product) that is not cut and the remainder is cut.

[Chemical composition]
The hot forged product according to the present embodiment has the following chemical composition. Hereinafter, “%” related to elements means “% by mass”.

C: 0.45-0.70%
Carbon (C) reduces the pro-eutectoid ferrite rate in the steel and increases the area ratio of pearlite in the steel. This increases the strength and hardness of the steel and increases the wear resistance. If the C content is too small, the pro-eutectoid ferrite rate is too high in the steel structure. On the other hand, if there is too much C content, steel will harden excessively and the machinability of steel will fall. Therefore, the C content is 0.45 to 0.70%. The minimum with preferable C content is 0.48%, More preferably, it is 0.50%. The upper limit with preferable C content is 0.60%, More preferably, it is 0.58%.

Si: 0.01 to 0.70%
Silicon (Si) strengthens the ferrite by dissolving in the ferrite in the pearlite. Therefore, Si increases the strength and hardness of the steel. Si also deoxidizes the steel. If there is too little Si content, the intensity | strength and hardness of steel will become low. On the other hand, if the Si content is too high, the steel is decarburized during hot forging. In this case, the machining allowance after hot forging becomes large. Therefore, the Si content is 0.01 to 0.70%. A preferable lower limit of the Si content is 0.20%. The upper limit with preferable Si content is 0.65%.

Mn: 1.0 to 1.7%
Manganese (Mn) is dissolved in steel to increase the strength and hardness of the steel. Mn further suppresses the formation of proeutectoid ferrite. If the Mn content is too small, the pro-eutectoid ferrite rate becomes too high. On the other hand, if the Mn content is too large, martensite and bainite are generated. Martensite and bainite reduce the wear resistance and machinability of steel. Therefore, it is not preferable that martensite and bainite are generated. Therefore, the Mn content is 1.0 to 1.7%. The minimum with preferable Mn content is 1.2%, More preferably, it is 1.3%. The upper limit with preferable Mn content is 1.65%, More preferably, it is 1.6%.

S: 0.01 to 0.1%
Sulfur (S) generates sulfides such as MnS and improves the machinability of steel. On the other hand, if there is too much S content, the hot workability of steel will fall. Therefore, the S content is 0.01 to 0.1%. The minimum with preferable S content is 0.03%, More preferably, it is 0.04%. The upper limit with preferable S content is 0.07%, More preferably, it is 0.06%.

Cr: 0.05-0.25%
Chromium (Cr) increases the strength and hardness of the steel. Cr further suppresses the formation of proeutectoid ferrite in the steel. If the Cr content is too small, the pro-eutectoid ferrite rate will be too high. On the other hand, if the Cr content is too large, martensite and bainite are generated. Therefore, the Cr content is 0.05 to 0.30%. The minimum with preferable Cr content is 0.08%, and a preferable upper limit is 0.20%.

Al: 0.003 to 0.050%
Aluminum (Al) deoxidizes steel. Further, Al generates nitrides and suppresses coarsening of crystal grains. Therefore, the remarkable fall of the intensity | strength of steel, hardness, and toughness is suppressed. On the other hand, if the Al content is too high, Al ₂ O ₃ inclusions are generated. Al ₂ O ₃ inclusions reduce the machinability of the steel. Therefore, the Al content is 0.003 to 0.050%. The minimum with preferable Al content is 0.010%, and a preferable upper limit is 0.040%. The Al content in the present embodiment is the content of acid-soluble Al (Sol. Al).

N: 0.003 to 0.02%
Nitrogen (N) produces nitrides and carbonitrides. Nitride and carbonitride suppress the coarsening of crystal grains and prevent a remarkable decrease in the strength, hardness and toughness of steel. On the other hand, if the N content is too large, defects such as voids are likely to occur in the steel. Therefore, the N content is 0.003 to 0.02%. The minimum with preferable N content is 0.005%, More preferably, it is 0.008%, More preferably, it is 0.012%. The upper limit with preferable N content is 0.018%.

  The balance of the chemical composition of the hot forged product consists of Fe and impurities. The impurities referred to here are ores and scraps used as raw materials for steel, or elements mixed in from the environment of the manufacturing process. Impurities are, for example, phosphorus (P) and oxygen (O).

  The hot forged product does not substantially contain V. V contributes to the formation of pro-eutectoid ferrite in the steel. In the hot forged product, V is an impurity, and preferably V is not contained.

  The chemical composition of the hot forged product of the present embodiment may further contain Ca instead of a part of Fe.

Ca: 0.01% or less Calcium (Ca) is an optional element and may not be contained. When contained, Ca increases the machinability of steel. Specifically, Ca is contained in the Al-based oxide and the melting point is lowered. Therefore, the machinability of steel is increased during high temperature cutting. However, if the Ca content is too high, the toughness of the steel decreases. Therefore, the Ca content is 0.01% or less. A preferable lower limit of the Ca content is 0.0005%.

  The chemical composition of the hot forged product of the present embodiment may further include one or more selected from the group consisting of Cu and Ni instead of a part of Fe. All of these elements strengthen the steel in solution.

Cu: 0.15% or less,
Ni: 0.15% or less Copper (Cu) and nickel (Ni) are optional elements and may not be contained. When contained, both Cu and Ni are dissolved in the steel and contribute to the strengthening of the steel. However, if the Cu content is too high, the hardenability is improved and a bainite structure or a martensite structure is likely to occur. If the Ni content is too high, the hardenability is improved and a bainite structure or a martensite structure is likely to be generated. Therefore, the Cu content is 0.15% or less, and the Ni content is 0.15% or less. The minimum with preferable Cu content is 0.02%. A preferable lower limit of the Ni content is 0.02%.

[Organization]
Of the surface of the hot forged product, the matrix having a depth of 500 μm to 5 mm from the uncut surface is composed of a ferrite / pearlite structure or a pearlite structure having a pro-eutectoid ferrite ratio of 3% or less. Hereinafter, the range of 500 μm to 5 mm in depth from the uncut surface among the surfaces of the hot forged product is referred to as “surface layer region”.

  The matrix in the surface layer region may have a ferrite / pearlite structure with a pro-eutectoid ferrite ratio of 3% or less, or a pearlite structure with a pro-eutectoid ferrite ratio of 0%.

Here, the area ratio of pro-eutectoid ferrite (pro-eutectoid ferrite ratio) is defined as follows. First, a sample for microstructural observation including a surface layer region of a hot forged product on the observation surface is collected. The observation surface of this sample is mirror-polished and corroded with a nital etchant. Then, within the observation plane, 20 visual fields, each (150 μm × 200 μm / visual field), 0.03 mm ² area are observed. This micrograph is subjected to image processing to determine the area ratio of pro-eutectoid ferrite in each visual field, and the average value is defined as the area ratio of pro-eutectoid ferrite.

  If the matrix in the surface layer region is a ferrite / pearlite structure or a pearlite structure in which the area ratio of pro-eutectoid ferrite is 3% or less, the wear resistance of the hot forged product is increased. The area ratio of preferable pro-eutectoid ferrite is less than 3%.

  The hot forged product further has an average diameter (colony diameter) of a ferrite / pearlite structure in the surface region of the hot forged product or a pearlite colony of the pearlite structure of 5.0 μm or less.

  Here, the colony diameter is defined as follows. A specimen including the surface layer region of the hot forged product on the observation surface is collected. Using this test piece, an electron beam diffraction image is measured by an electron microscope Quanta (trade name) manufactured by FEI and an EBSD electron beam backscatter diffraction (EBSD) apparatus HKL (trade name) manufactured by Oxford. The boundary of the pearlite colony of the tissue is determined from the electron diffraction image. The area of the pearlite colony is calculated from the boundary of the pearlite colony. The diameter of the pearlite colony (equivalent circle diameter) is determined from the calculated area. The diameter of a pearlite colony is calculated | required from four places of the test piece corresponded to the surface layer area | region of a hot forging, respectively, and let the average value be a colony diameter. In the pearlite structure, a region surrounded by a boundary where the orientation difference of ferrite is 2 ° to 15 ° is defined as a pearlite colony.

  If the colony diameter is small, the boundary of the pearlite colony increases. The increase in the boundary suppresses the propagation of fatigue cracks and increases the fatigue strength of the hot forged product.

  The hot forged product according to the present embodiment has the above structure in the surface layer region, and therefore has excellent wear resistance and excellent fatigue strength even if the surface hardening heat treatment is omitted.

[Production method]
An example of a method for producing a hot forged product will be described.

  A molten steel having the above chemical composition is produced. The molten steel is made into a slab by a continuous casting method. You may make molten steel into an ingot (steel ingot) by the ingot-making method. The slab or ingot may be hot-worked into a billet (steel piece) or a steel bar.

  A slab, ingot, billet or steel bar is heated in a heating furnace. The heating temperature is preferably 1200 ° C. or higher. Hot slabs, ingots, billets or steel bars are hot forged to produce intermediate products. The finishing temperature of hot forging is preferably 900 ° C. or higher.

  The intermediate product after hot forging is controlled and cooled at a predetermined speed. Specifically, the cooling rate when the surface temperature of the intermediate product is 800 to 500 ° C. is set to 100 to 300 ° C./min. If this cooling rate is too low, pearlite colonies will become large and high fatigue strength will not be obtained. If the cooling rate is too low, the pro-eutectoid ferrite rate increases. On the other hand, if the cooling rate is too high, martensite and bainite are generated. Therefore, the cooling rate when the surface temperature of the intermediate product is 800 to 500 ° C. is 100 to 300 ° C./min.

  This cooling can be realized by, for example, mist cooling using a mixed fluid of air and water, strong air cooling using compressed air, or strong air cooling using a blower. In addition, the cooling rate in a temperature range higher than 800 ° C. and a temperature range lower than 500 ° C. is arbitrary.

  In this way, a hot forged product that is an intermediate product is manufactured. By hot forging the steel having the above chemical composition and cooling at the above cooling rate, the matrix in the surface layer region of the hot forged product is a ferrite / pearlite structure or pearlite in which the area ratio of pro-eutectoid ferrite is 3% or less. Become an organization. Furthermore, the colony diameter in the pearlite structure in the surface layer area is 5.0 μm or less. The hot forged product is not tempered and is not tempered.

  A part of the surface of the hot forged product is cut by machining to produce a crankshaft 1 that is a hot forged product as a final product. The thickness (cutting allowance) removed by the cutting is about 500 μm to 5 mm from the surface of the hot forged product as the intermediate product. Therefore, for example, in order to make the above structure from the surface of the crankshaft 1 after cutting to a depth of about several millimeters, in a hot forged product (intermediate product) before cutting, 500 μm to 5 mm from the surface. The matrix at the depth position may be a ferrite / pearlite structure or a pearlite structure having a pro-eutectoid ferrite ratio of 3% or less. Similarly, in the hot forged product before cutting, the colony diameter of the pearlite structure at a depth position of 500 μm to 5 mm from the surface may be 5.0 μm or less.

  The surface of the manufactured crankshaft 1 includes a surface that is not cut. The matrix at a depth of 500 μm to 5 mm from the surface is a ferrite pearlite structure or a pearlite structure having a proeutectoid ferrite ratio of 3% or less, and the colony diameter of the pearlite structure at a depth of 500 μm to 5 mm from the surface is 5.0 μm or less.

  Of the manufactured crankshaft 1, at least the crankpin 2 is omitted from the surface hardening heat treatment. That is, at least the surface of the crankpin 2 is not subjected to induction hardening or nitriding. The fillet portion 5 may be subjected to fillet roll processing, and the surface hardness of the fillet portion 5 may be further increased by work hardening. In the fillet roll processing, a roller is pressed against the surface of the fillet portion 5 while rotating the hot forged product 1. Thereby, the surface of the fillet part 5 is plastic-worked and work-hardened. The fillet portion 5 may not be subjected to fillet roll processing.

  In the hot forged product manufactured by the above process, whether it is an intermediate product or a final product (crankshaft 1), a matrix having a depth of 500 μm to 5 mm from the uncut surface has a proeutectoid ferrite ratio. Is made of a ferrite / pearlite structure or a pearlite structure with 3% or less. Furthermore, the colony diameter of a pearlite structure having a depth of 500 μm to 5 mm from the surface is 5.0 μm or less.

  Of the surface of the hot forged product as the final product, the matrix of the cut surface is composed of a ferrite / pearlite structure or a pearlite structure having a pro-eutectoid ferrite ratio of 3% or less, and the colony diameter of the surface pearlite structure is 5. 0 μm or less.

  Since it has the above structure, even if it does not contain V and the tempering treatment and the surface hardening heat treatment are omitted, the hot forged product has excellent wear resistance and excellent fatigue strength. Furthermore, since the Si content of the hot forged product of the present embodiment is an appropriate amount, the depth of the decarburized layer formed on the surface of the hot forged product that is an intermediate product can be suppressed. Therefore, the cutting allowance of the hot forging product after hot forging can be suppressed.

  Steels having the chemical compositions shown in Table 1 (test numbers 1 to 7 and a to g) were melted in a vacuum induction heating furnace to obtain molten steel. Molten steel was ingoted to produce a columnar ingot. Each manufactured ingot had a weight of 25 kg and an outer diameter of 75 mm.

  In the column of each element symbol in Table 1, the content (% by mass) of the corresponding element is described. In Table 1, “-” indicates that the corresponding element was at the impurity level. The balance of each steel was Fe and impurities.

  Ingots made from each steel were hot forged to produce forged products. Specifically, each ingot was heated to 1250 ° C. in a heating furnace. The heated ingot was hot forged to produce a round bar forged product (hereinafter simply referred to as a round bar) having an outer diameter of 15 mm. The finishing temperature during hot forging was 950 ° C.

  After hot forging, each round bar was cooled to room temperature (23 ° C.) at the cooling rate shown in Table 1. The cooling rate (° C./min) when the surface temperature was 800 ° C. to 500 ° C. was as shown in Table 1. Specifically, in test numbers 1 to 7, b, c, d, e, and g, mist cooling was performed at 800 ° C to 500 ° C. In test number a, air cooling using a blower was performed at 800 ° C to 500 ° C. In test number f, cooling was performed at 800 ° C to 500 ° C.

[Microstructure investigation]
Micro samples were taken from each round bar and the tissue was observed. FIG. 4 is a diagram for explaining the sampling position of the microstructure in the cross section of each round bar and the observation position in the microstructure inspection. As shown by the alternate long and short dash line in FIG. 4, four micro samples including the surface of each round bar were collected from each round bar at 90 ° intervals.

  The surface of each micro sample was mirror-polished, and the polished surface was corroded with a nital etchant. The corroded surface was observed with a 400 × optical microscope.

As shown in circles in FIG. 4, for each micro sample, at a depth position of 500 μm from the surface of the round bar and at a depth position of 5 mm from the surface, 5 fields per site, 20 fields in total, each (150 μm × The area of 0.03 mm ² was observed. The micrographs in each region were subjected to image processing, and the area ratio of pro-eutectoid ferrite in each region was determined. The average value of the area ratio of pro-eutectoid ferrite in 20 visual fields observed at a depth of 500 μm from the surface was defined as the pro-eutectoid ferrite ratio at a depth of 500 μm from the surface of the micro sample. The average value of the pro-eutectoid ferrite area ratio in 20 fields of view observed at a depth of 5 mm from the surface was defined as the pro-eutectoid ferrite ratio at a depth of 5 mm from the surface of the micro sample.

[Perlite colony survey]
The colony diameter of the pearlite structure | tissue in the observation position of each micro sample was measured using the EBSD apparatus. More specifically, an electron beam diffraction image was measured with an electron microscope Quanta (trade name) manufactured by FEI and an EBSD analyzer HKL (trade name) manufactured by Oxford. The crystal orientation and the like were analyzed from the electron diffraction image to determine the boundary of the pearlite colony, and the area of each pearlite colony was calculated therefrom. The analysis was performed by HKL (trade name).

  As in the case of the microstructure investigation, the colony diameter was measured for each micro sample at a depth position of 500 μm from the surface and a depth position of 5 mm from the surface. The beam diameter of the electron beam was 1 μm, one mapping area was 100 μm × 200 μm, and the average value of the four mapping areas was defined as the colony diameter.

[Surface hardness survey]
The hardness of the cross section of the round bar was measured by the Vickers hardness test based on JIS Z2244 (2009) using each micro sample. The test force was 98.07 N (10 kgf). About each micro sample, the hardness of a total of five places was measured by 1 mm pitch toward the inside of a round bar from the surface of a round bar, and what averaged was made into the average hardness of the micro sample.

[Fatigue strength survey]
A rotating bending fatigue test piece was collected from each round bar. FIG. 5 is a schematic view of a rotating bending fatigue test piece taken from each round bar. The rotating bending test piece had a parallel part diameter of 8 mm and a grip part diameter of 12 mm. A rotating bending fatigue strength test piece was prepared so that the central axis of the rotating bending fatigue test piece coincided with the central axis of the round bar. Specifically, the parallel part was produced by cutting to a depth of 3.5 mm from the surface of the round bar by lathe processing. Therefore, the surface of the parallel part corresponded at least within the range of 5 mm in depth from the surface of the round bar. That is, the rotational bending fatigue strength test piece assumed the crankshaft 1 after cutting the intermediate product.

  The parallel portion of the rotating bending fatigue strength test piece was finish-polished to adjust the surface roughness. Specifically, the centerline average roughness (Ra) of the surface was set to 3.0 μm or less, and the maximum height (Rmax) was set to 9.0 μm or less.

Using the rotating bending fatigue strength test piece subjected to finish polishing, an Ono-type rotating bending fatigue test was performed at room temperature (23 ° C.) in an air atmosphere under the conditions of both swings at a rotational speed of 3600 rpm. The fatigue test was carried out by changing the stress applied to the plurality of test pieces, and the highest stress that did not break after 10 ⁷ cycles was defined as the fatigue strength (MPa).

[Abrasion resistance survey]
A 1.5 mm × 2.0 mm × 3.7 mm test specimen for wear resistance was collected so that the position of a depth of 500 μm to 1000 μm from the surface of each round bar was the center of the following main surface. The surface of each test piece of 2.0 mm × 3.7 mm (hereinafter referred to as the main surface) was parallel to the cross section of the round bar. That is, the normal line of the main surface of each test piece was parallel to the central axis of the round bar.

Each test piece was subjected to a pin-on-disk wear test using an automatic polishing machine. Specifically, emery paper with a number (grit) 800 was attached to the surface of a rotating disk of an automatic polishing machine. Then, the rotating disk was rotated at a peripheral speed of 39.6 m / min for 50 minutes while pressing the main surface of the test piece on the emery paper with a surface pressure of 26 gf / mm ² . After rotating for 50 minutes, the difference in the weight of the test piece before and after the test was defined as the amount of wear (g).

[Decarburization depth investigation]
The decarburization depth of the round bar of each test number was determined by the following method. A round sample was cut perpendicular to the axial direction of the round bar, and a micro sample having a cut surface as a test surface was collected. The surface of each micro sample was mirror-polished, and the polished surface was corroded with a nital etchant. The corroded surface was observed with a 400 × optical microscope. And the photographic image of arbitrary 1 visual fields (800 micrometers x 550 micrometers) of the surface layer part including the surface of a round bar was generated. FIG. 6 is an example of the generated photographic image.

  The decarburization depth (μm) was determined by the following method using the generated photographic image. A line segment (550 μm) connecting both ends 50 of the surface of the round bar in the photographic image was defined as the reference surface 60. A measurement region 100 having two sides parallel to the reference surface 60 and having a width of 10 μm was provided. The measurement region 100 was moved from the reference surface 60 in the depth direction in units of 1 μm. For each 1 μm movement, the pro-eutectoid ferrite ratio in the measurement region 100 was calculated. The depth at which the pro-eutectoid ferrite ratio became no more than 4% (the distance from the reference surface 60 to the center of the width of the measurement region 100) was defined as the decarburization depth (μm). “The depth at which the pro-eutectoid ferrite ratio is no longer 4% or more” means the depth at which the pro-eutectoid ferrite ratio is less than 4% at positions deeper than the depth.

[Investigation result]
Table 2 shows the survey results.

  Table 2 shows the structure, ferrite ratio, and colony diameter of a round bar manufactured from each steel at a depth position of 500 μm from the surface and a depth position of 5 mm from the surface.

  In the “structure” column, the structure obtained by the microstructural examination is described. In Table 2, “F + P” represents a ferrite pearlite structure, “P” represents a pearlite structure, “M” represents a martensite structure, and “M + B + P” represents a martensite bainite pearlite structure. . In the column of “ferrite ratio (%)”, the average value of pro-eutectoid ferrite ratios of micro-samples with a total of 20 fields of view taken at 90 ° intervals in the micro structure investigation is described. In the column of “colony diameter (μm)”, the average value of the colony diameters of four micro samples collected every 90 ° in the microstructural survey is described. “-” In Table 2 indicates that the colony diameter has not been measured.

  In the column of “average hardness (HV)”, the average value of the average hardness of four micro samples taken every 90 ° in the surface hardness survey (that is, the average value of a total of 20 points) is described. Yes. Note that high fatigue strength cannot be obtained when the average hardness is less than 300 HV. On the other hand, when the average hardness exceeds 400 HV, cutting becomes difficult.

  In the column “Fatigue Strength (MPa)”, the fatigue strength obtained by the fatigue strength investigation is described. The fatigue strength is preferably 400 MPa or more.

  In the column of “amount of wear (g)”, the amount of wear obtained by the wear resistance test is described. The amount of wear is preferably 0.0080 g or less.

  In the column of “Decarburization depth (μm)”, the decarburization depth (μm) until the pro-eutectoid ferrite ratio obtained by the decarburization depth investigation is less than 4% is described. The decarburization depth of less than 4% is preferably less than 500 μm. “-” In Table 2 indicates that the decarburization depth is not measured.

  With reference to Table 1, the chemical composition of the test materials of test numbers 1 to 7 was within the scope of the present invention, and the cooling rate after hot forging was also appropriate. Referring to Table 2, in Test Nos. 1 to 7, the structure at a depth position of 500 μm from the surface and a depth position of 5 mm from the surface was a ferrite / pearlite structure or a pearlite structure having a proeutectoid ferrite ratio of 3% or less. FIG. 7 is a microstructure photograph of the specimen at a position of 5 mm from the surface of test number 2. Referring to FIG. 7, most of the microstructure was pearlite P, and pro-eutectoid ferrite F was 2% in area ratio.

  Furthermore, in the test numbers 1 to 7, the colony diameter of the tissue at a depth position of 500 μm from the surface and a depth position of 5 mm from the surface was 5.0 μm or less. As a result, the fatigue strengths of Test Nos. 1 to 7 were 400 MPa or more, and the wear amount was 0.0080 g or less. Moreover, the average hardness of the test numbers 1-7 was 300HV or more. Furthermore, the average hardness of Test Nos. 1 to 7 was 400 HV or less at which excellent machinability was obtained. Furthermore, the decarburization depths of test numbers 1 to 7 were less than 500 μm.

  In test number a, the Mn content was low and V was contained. Mn is an element that suppresses the formation of ferrite, and V is an element that contributes to the formation of ferrite. Therefore, in the test number a, the structure at a depth position of 500 μm from the surface and a depth position of 5 mm from the surface was a ferrite pearlite structure having a ferrite ratio exceeding 3%. As a result, the wear amount of test number a exceeded 0.0080 g. Moreover, the average hardness of the test number a was less than 300HV.

  In test number b, the C content was low. C is an element that suppresses the formation of ferrite. Therefore, in test number b, the structure at a depth of 500 μm from the surface and a position at a depth of 5 mm from the surface was a ferrite pearlite structure with a ferrite ratio exceeding 3%. As a result, the wear amount of test number b exceeded 0.0080 g. Moreover, the average hardness of the test number b was less than 300HV.

  In test number c, the C content was low, the Mn content was low, and the Cr content was high. Cr is an element that contributes to the formation of martensite. Therefore, in test number c, the structure at a depth position of 500 μm from the surface and a position at a depth of 5 mm from the surface was a martensite structure. Martensite and bainite were more easily worn than pearlite, and as a result, the wear amount of test number c exceeded 0.0080 g. Moreover, the average hardness of the test number c exceeded 400HV.

  The Si content of test number d was too high. Therefore, the decarburization depth was deep, and the measurement was completed up to a depth of 600 μm, which is an observable visual field. The decarburization depth was deeper than 600 μm.

  Although the chemical composition of test number e was appropriate, the cooling rate after hot forging was too high. Therefore, the structure at a depth of 500 μm from the surface and a depth of 5 mm from the surface contained not only pearlite but also martensite and bainite having an area ratio of about 30%. Therefore, the average hardness of test number i exceeded 400 HV.

  Although the chemical composition of test number f was appropriate, the cooling rate after hot forging was too small. Therefore, the colony diameter of the pearlite structure at a depth position of 500 μm from the surface and a depth position of 5 mm from the surface exceeded 5.0 μm. As a result, the fatigue strength of test number e was less than 400 MPa.

  The Cr content of test number g was too high. Therefore, the structure having a depth of 500 μm from the surface and a depth of 5 mm from the surface contained not only pearlite but also martensite and bainite. Therefore, the average hardness of test number i exceeded 400 HV.

  In the above-described embodiment, the case where the hot forged product is a crankshaft has been described. However, the present invention can also be used as a hot forged product other than the crankshaft.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (4)

Chemical composition is mass%,
C: 0.45-0.70%,
Si: 0.01-0.70%,
Mn: 1.0-1.7%,
S: 0.01 to 0.1%
Cr: 0.05 to 0.25%,
Al: 0.003 to 0.050%, and N: 0.003 to 0.02%, the balance consists of Fe and impurities,
The matrix having a depth of 500 μm to 5 mm from the uncut surface consists of a ferrite / pearlite structure or a pearlite structure in which the area ratio of pro-eutectoid ferrite is 3% or less,
A hot forged product in which an average diameter of a pearlite colony having a pearlite structure having a depth of 500 μm to 5 mm from the uncut surface is 5.0 μm or less. The hot forged product according to claim 1,
The chemical composition is further replaced with a part of Fe,
Ca: Hot forged product containing 0.01% or less. The hot forged product according to claim 1 or 2,
The chemical composition is further replaced with a part of Fe,
Cu: 0.15% or less, and
Ni: 0.15% or less,
A hot forged product containing one or more selected from the group consisting of: The hot forged product according to any one of claims 1 to 3,
The hot forged product is a crankshaft, which is a hot forged product.