WO2023120348A1 - Warm-forged component for carburization and method for producing same - Google Patents

Warm-forged component for carburization and method for producing same Download PDF

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WO2023120348A1
WO2023120348A1 PCT/JP2022/046148 JP2022046148W WO2023120348A1 WO 2023120348 A1 WO2023120348 A1 WO 2023120348A1 JP 2022046148 W JP2022046148 W JP 2022046148W WO 2023120348 A1 WO2023120348 A1 WO 2023120348A1
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grains
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亮太 高尾
孝佳 杉浦
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愛知製鋼株式会社
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium

Definitions

  • the present invention relates to a carburizing warm forged part and a manufacturing method thereof.
  • the structure after forging is finer than in hot forging. If the structure before carburizing is fine, the driving force for crystal grain growth during carburizing increases, and coarsening (abnormal grain growth) tends to occur. Therefore, when a conventional material is warm-forged and then carburized, grain coarsening is likely to occur.
  • a countermeasure there is a method of performing heat treatment such as normalizing after forging to roughen the structure to some extent before carburizing, but in such a case, the accuracy of the forged parts decreases due to the formation of scale. In addition, since the presence or absence of this heat treatment greatly affects the cost of parts, it is desired to avoid heat treatment after forging.
  • addition of a large amount of Ti reduces the amount of MnS due to the formation of Ti sulfides, etc., and adversely affects the machinability. Furthermore, addition of a large amount of Nb or Ti worsens the material cost.
  • the present invention has been made in view of such a background, and a warm forged part for carburizing that can suppress grain coarsening during carburizing without requiring the addition of large amounts of Nb and Ti, and a method for manufacturing the same. is intended to provide
  • One aspect of the present invention is, in mass%, C: 0.12 to 0.28%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.00%, P: 0.035% Below, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Nb: 0.020%. 004 to 0.060%, Mo: 0 to 0.60% as an optional element, and preparing a steel material having a chemical composition with the balance being Fe and unavoidable impurities, The steel material is heated to a rolling temperature of 1150 to 1350° C.
  • the area ratio of structures other than ferrite and pearlite is 5.0% or less
  • the average grain size of pearlite grains is Y ( ⁇ m) calculated from Equation 2 or more
  • the area ratio of non-recrystallized grains is 3.0% or less
  • the total number of AlN and NbCN having an equivalent circle diameter of 100 nm or more is 5 pieces/ ⁇ m 2 or less.
  • Another aspect of the present invention is, in mass %, C: 0.12 to 0.28%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.00%, P: 0.035 % or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Nb: 0 .004 to 0.060%, Mo: 0 to 0.60% as an optional element, with the balance being Fe and unavoidable impurities,
  • the area ratio of structures other than ferrite and pearlite is 5.0% or less
  • the average grain size of pearlite grains is Y ( ⁇ m) calculated from Equation 2 or more
  • the area ratio of non-recrystallized grains is 3.0% or less
  • the total number of AlN and NbCN having an equivalent circle diameter of 100 nm or more is 5 pieces/ ⁇ m 2 or less.
  • hot rolling is performed at the above specific treatment temperature.
  • warm forging is performed at the specified treatment temperature.
  • the processing temperature for warm forging must be a temperature equal to or higher than X (° C.) derived from Equation (1).
  • the cooling rate of the forged product after warm forging is also controlled under the above specific conditions. As a result, the metal structure in the obtained forged product is optimized, and in particular, it is assumed that unrecrystallized grains are not left after warm forging and cooling to room temperature, even if a small amount remains.
  • the area ratio is controlled to be 3% or less, and the average particle size of pearlite is controlled to be Y ( ⁇ m) calculated by Equation 2 or more.
  • the carburizing warm forged part with such an optimized metal structure can suppress coarsening of crystal grains during the subsequent carburizing treatment even if the heat treatment after the warm forging is omitted. That is, in the present application, it is possible to provide a warm forged part for carburizing and a method for manufacturing the same that can suppress grain coarsening during carburizing without requiring the addition of large amounts of Nb and Ti.
  • C 0.12-0.28%
  • C (carbon) is an element necessary for improving hardness after quenching treatment and obtaining internal hardness for ensuring strength.
  • C is contained in an amount of 0.12% or more.
  • the upper limit of the C content is set to 0.28% to prevent this.
  • Si 0.01-0.90%; Si (silicon) contributes to strength improvement by solid solution strengthening, so it is contained in an amount of 0.01% or more.
  • the upper limit of the Si content is made 0.90% in order to prevent it.
  • Mn 0.30-1.00%
  • Mn manganese
  • P 0.035% or less;
  • P (phosphorus) is an element contained as an impurity.
  • P is an element that easily segregates at austenite grain boundaries, and segregation causes a decrease in strength. Therefore, the upper limit of the allowable content of P is 0.035%.
  • S 0.010 to 0.040%; S (sulfur) forms MnS together with Mn to improve machinability, so it is contained in an amount of 0.010% or more. On the other hand, if a large amount of S is contained, sulfide-based non-metallic inclusions increase, which causes a decrease in strength.
  • Cr 0.40-2.00%
  • Cr chromium
  • the upper limit of the Cr content is set to 2.00% in order to prevent this.
  • Al 0.020-0.060%
  • Al (aluminum) is an element used as a deoxidizing agent during steelmaking, and exhibits the effect of suppressing abnormal grain growth during carburizing when it is combined with N and exists as fine AlN.
  • the Al content should be 0.020% or more.
  • the excessive addition of Al causes deterioration of workability and coarsening of AlN, which reduces the effect of suppressing abnormal grain growth. Therefore, the upper limit of the Al content is made 0.060%.
  • N 0.0100 to 0.0200%
  • N (nitrogen) turns into AlN and has the effect of suppressing grain coarsening due to the pinning effect, so it is contained in an amount of 0.0100% or more.
  • the N content is too high, AlN coarsens and the effect of suppressing abnormal grain growth is lowered, so the upper limit of the N content is made 0.0200%.
  • Nb 0.004 to 0.060%; Nb (niobium) is contained in an amount of 0.004% or more in order to obtain the effect of grain refinement.
  • Nb content is too high, the carburizability may deteriorate, so it is limited to 0.060% or less. Preferably, it is 0.040% or less.
  • Mo as an optional element: 0-0.60%; Mo (molybdenum) is an optional additive element and does not necessarily need to be contained, and the content may be 0%. However, in the case of manufacturing by electric furnace melting using scrap as a raw material, it may be contained as an unavoidable impurity. There is also Mo is an element effective for improving the hardenability when contained in an amount of 0.01% or more, so it can be added as necessary. On the other hand, if the Mo content is too high, it leads to an increase in cost and a decrease in machinability, so it is limited to 0.60% or less.
  • Ti is not intentionally added (it is allowed to contain about 0.01% or less as an unavoidable impurity), so that the influence of Ti addition on machinability deterioration and cost deterioration is eliminated.
  • Nb is added within a range that does not adversely affect carburization.
  • a steel material having such a chemical composition is heated to a rolling temperature of 1150 to 1350° C. and then rolled to produce a rolled material, which is then warm forged at a specific processing temperature. The significance of these manufacturing conditions will be described below.
  • AlN and Nb carbonitrides are once solid-dissolved by heating during hot forging, and fine grains are formed in the subsequent heat treatment. It was necessary to deposit on However, since warm forging requires a lower heating temperature than hot forging, AlN and Nb carbonitrides cannot be dissolved sufficiently at the heating temperature during warm forging. In addition, when AlN or Nb carbonitrides are coarsely precipitated before warm forging, they agglomerate and coarsen during warm forging and cannot be precipitated finely, and the effect of preventing grain coarsening is improved. Decrease.
  • a specific rolling heating temperature at which the effect of preventing grain coarsening is obtained is 1150°C to 1350°C.
  • the temperature is less than 1150°C, solid solution of AlN and Nb carbonitrides may not be sufficiently realized, and when it exceeds 1350°C, the energy consumption becomes too high, which is difficult. Even if the energy saving effect by omitting heat treatment after warm forging is considered, there is a problem that the energy saving effect cannot be obtained sufficiently.
  • the addition of Al and Nb leads to an increase in the recrystallization temperature of austenite.
  • Nb significantly raises the recrystallization temperature even when added in a small amount. If unrecrystallized austenite grains remain after the recrystallization temperature is raised, warm forged, and cooled to room temperature, these unrecrystallized grains are recrystallized during the subsequent carburization temperature rise to form fine crystals. As a result, the driving force for grain coarsening is greatly increased.
  • the non-recrystallized grains produced by warm forging recrystallize during cooling after forging, and the amount produced decreases.
  • Controlling the cooling rate after forging is also effective in promoting recrystallization. Specifically, in the process of cooling the forged part after warm forging, it is effective to cool the forged part to at least 700° C. under the condition of a cooling rate of 3.0° C./sec or less. By cooling under these conditions, even if unrecrystallized grains remain immediately after warm forging (before cooling to room temperature), recrystallization during subsequent cooling is promoted, and after cooling to room temperature, The proportion of non-recrystallized grains can be reduced.
  • the Nb content and forging temperature also affect the ease of recrystallization of non-recrystallized grains.
  • the crystal grains become fine after the subsequent carburizing temperature rise, and the driving force for coarsening the crystal grains increases, and the precipitation amount of AlN and Nb carbonitrides Also when the pinning effect is small due to the small amount of grains, coarsening of crystal grains may occur.
  • the crystal grain coarsening prevention force due to pinning of AlN or Nb carbonitride exceeds the driving force for crystal grain coarsening. Therefore, as a result of further investigation, the driving force of grain coarsening can be estimated from the grain size after carburizing temperature rise, and the grain size after carburizing temperature rise changes depending on the pearlite grain size after warm forging. found to do.
  • the AlN and Nb carbonitrides are sufficiently dissolved in the rolling stage, and the crystal grains are coarsened when the AlN and Nb carbonitrides are precipitated by warm forging.
  • the pearlite particle size after warm forging was controlled.
  • Formula 1 is a relational expression between the forging temperature and the Nb content. If the forging temperature is less than X°C, non-recrystallized grains tend to remain during warm forging, and the pearlite grain size after forging becomes too fine, resulting in grain coarsening during subsequent carburizing. Therefore, it is necessary to set the forging treatment temperature to at least X° C. or higher.
  • Formula 2 shows that the state of the structure after forging is optimized (suppression of unrecrystallized grains), and furthermore, AlN and Nb carbonitrides are fully dissolved in the rolling stage and precipitated by warm forging. It shows the relationship between the pearlite average grain size after warm forging, which is the driving force for grain coarsening below the obtained grain coarsening prevention force.
  • the driving force for crystal grain coarsening becomes greater than the crystal grain coarsening prevention force expected from the Al and Nb contents, and crystal grain coarsening occurs. become more likely.
  • the above warm forging is performed at a processing temperature of 1100°C or less in order to enjoy the inherent energy saving effect of selecting warm forging.
  • the forged product obtained based on the above description is adjusted within the range of the chemical composition described above, and the forging temperature and cooling rate are adjusted to the structure mainly composed of ferrite and pearlite (the area ratio of other structures is 5.0% or less). Also, a forged product having a metal structure in which the pearlite grains have an average grain size of Y ⁇ m or more and the non-recrystallized grains have an area ratio of 3.0% or less can be obtained. If a structure other than ferrite/pearlite structure is mixed, or if the pearlite grain size is less than Y ⁇ m, crystal coarsening is likely to occur. can be sufficiently suppressed. Note that the pearlite particle size is most affected by the warm forging temperature, and the higher the temperature, the larger the particle size. By increasing the forging temperature, it is possible to control the particle size to a satisfactory level.
  • the area ratio of the non-recrystallized grains is more preferably 1.5% or less.
  • the non-recrystallized grains have a crushed shape due to the compression direction processing performed during warm forging if they are not recrystallized, and the aspect ratio is reduced. It can be judged whether or not the grains are non-recrystallized grains based on whether or not the grains are crushed in the direction of compression of a certain value or more.
  • Example 1 An embodiment of the warm forged part for carburizing and the method for manufacturing the same according to the present invention will be described.
  • Table 1 20 types of steel materials (Examples 1 to 14 and Comparative Examples 15 to 20) were prepared, test pieces corresponding to warm forged products were produced, and various properties were evaluated.
  • Mo which is an optional additive element, is mixed as an impurity from scrap, so it was not actively added except for Example 10, and the analysis values contained as impurities, including Table 3 below, are shown. .
  • ⁇ Simulated forging test> Using the compression test piece, it is heated to the forging temperature shown in Table 2 at a heating rate of 2 ° C./sec, and compressed under the conditions of a processing rate of 75% so that the height becomes 3 mm, and then up to 700 ° C. at 1 ° C./sec. After cooling at a cooling rate of 0.5° C./second, a simulated forging test was performed to cool to room temperature at a cooling rate of 0.5° C./second.
  • the pearlite grain size excluding non-recrystallized grains was measured by image analysis, and the average value was defined as the pearlite average grain size.
  • Table 2 the microstructures are indicated by ⁇ for ferrite and P for pearlite. ⁇ +P means a ferrite/pearlite structure.
  • ⁇ Measurement of prior ⁇ crystal grains> A simulated carburizing treatment was performed on the test piece subjected to the simulated forging test by holding at 950° C. for 4 hours and then quenching. After the simulated carburizing treatment, the cross section of the test piece was observed with an optical microscope to check whether or not the crystal grains were coarsened to form a mixed grain state. Specifically, five fields of view are observed at a magnification of 100 times, and if the area in which the grain size number is 3 or more larger than the other areas in the observed range accounts for 10% or more, it is rejected ( ⁇ ), and when it was less than 10%, it was judged to pass ( ⁇ ).
  • Table 2 shows X (°C) derived from Equation 1 and Y ( ⁇ m) derived from Equation 2 along with various evaluation results.
  • the chemical component composition is appropriate, and the microstructure is ferrite / by selecting a temperature of X ° C. or higher as the forging temperature in the range of 1100 ° C. or lower.
  • the pearlite average grain size is sufficiently large at Y ⁇ m or more, and the area ratio of unrecrystallized grains is 3.0% or less. (occurrence of mixed grains of 10% or more) did not occur.
  • Comparative Example 15 has an appropriate chemical composition, but because the forging temperature is lower than X ° C., the pearlite grain size is smaller than Y ⁇ m, and the area ratio of non-recrystallized grains is 3.0%. It was found that grain coarsening (mixed grain) occurred in the subsequent carburizing treatment.
  • Comparative Example 18 Although the chemical composition was appropriate, the temperature of hot rolling before warm forging was too low. It was found that even if the grains were changed, coarsening of crystal grains (mixed grains) occurred in the subsequent carburizing treatment.
  • Example 3 four types of steel materials (Examples 21 to 23, Comparative Example 24) were prepared in which the Nb content, which is an element that greatly affects the recrystallization temperature, was greatly changed.
  • a test piece was prepared in the same manner as in 1, and various properties were evaluated.
  • the evaluation method was also the same as in Experimental Example 1.
  • the conditions were such that the rolling temperature and the forging temperature were all the same.
  • Other conditions were the same as in Experimental Example 1.
  • the chemical composition is appropriate, and the 900 ° C. selected as the forging temperature is a temperature higher than X ° C., so the microstructure is ferrite / pearlite.
  • the pearlite grain size is sufficiently large at Y ⁇ m or more, and the area ratio of unrecrystallized grains is 3.0% or less, so that the problem of coarsening of grains in the subsequent carburizing treatment does not occur. I understand.

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Abstract

According to the present invention, a steel stock material having a specific chemical component composition is heated to a rolling temperature of 1150°C to 1350°C and is subsequently rolled, thereby producing a rolled material; and after subjecting the rolled material to warm forging at a temperature that is not less than X°C, which is calculated by formula 1, but not more than 1100°C, the resulting material is cooled at least to 700°C at a cooling rate of 3.0°C/second or less. The present invention enables the achievement of a forged component which has a metal structure that contains 5.0% by area or less of structures other than ferrite and pearlite, wherein: the average particle diameter of the pearlite particles is not less than Y µm, which is calculated by formula 2; the area ratio of the non-recrystallized particles is 3.0% or less; and the number of AlN and NbCN having a circle-equivalent diameter of 100 nm or more is 5/100 µm2 or less. Formula 1: X = 1303 × [Nb] + 857.91 Formula 2: Y = 0.43/(0.94 × [AlN] + 0.92 × [NbCN])

Description

浸炭用温間鍛造部品及びその製造方法Warm forged parts for carburizing and manufacturing method thereof
　本発明は、浸炭用温間鍛造部品及びその製造方法に関する。 The present invention relates to a carburizing warm forged part and a manufacturing method thereof.
　近年、自動車のトランスミッション等に用いられる部品の製造に際し、部品コスト低減のために、形状精度の高い鍛造部品を得ることのできる、処理温度が９００℃程度の温間鍛造が実施されることがある。また、温間鍛造は、処理温度が１２００℃程度の熱間鍛造と比較して、加熱温度が低いことにより、エネルギーコスト低減にも多大な効果を得ることができる。 In recent years, when manufacturing parts used in automobile transmissions, etc., warm forging at a processing temperature of about 900 ° C., which can obtain forged parts with high shape accuracy, is sometimes performed in order to reduce part costs. . In addition, since warm forging requires a lower heating temperature than hot forging, which uses a processing temperature of about 1200° C., it is possible to obtain a great effect in reducing energy costs.
　温間鍛造は処理温度が低いため、鍛造後の組織が熱間鍛造と比較して微細となる。浸炭前の組織が微細であると、浸炭時の結晶粒成長の駆動力が高まり、粗大化（異常粒成長）が発生しやすくなる。そのため、従来の材料を温間鍛造した後に、浸炭処理を行なった場合には、結晶粒の粗大化が発生しやすい。この対策として、鍛造後に焼準などの熱処理を実施し、浸炭前組織をある程度粗くしておく方法があるが、そうした場合にはスケールの生成などにより、鍛造部品の精度が低下してしまう。また、この熱処理の有無が部品コストに大きく影響するため、鍛造後の熱処理を避けることが望まれている。 　Because the processing temperature is low in warm forging, the structure after forging is finer than in hot forging. If the structure before carburizing is fine, the driving force for crystal grain growth during carburizing increases, and coarsening (abnormal grain growth) tends to occur. Therefore, when a conventional material is warm-forged and then carburized, grain coarsening is likely to occur. As a countermeasure, there is a method of performing heat treatment such as normalizing after forging to roughen the structure to some extent before carburizing, but in such a case, the accuracy of the forged parts decreases due to the formation of scale. In addition, since the presence or absence of this heat treatment greatly affects the cost of parts, it is desired to avoid heat treatment after forging.
　また、結晶粒の粗大化防止のために、ＡｌＮやＮｂ、Ｔｉなどの微細な析出物を活用する方法が挙げられる。従来の高温での熱間圧延や熱間鍛造などを採用する場合には、その加熱時にＡｌＮやＮｂ、Ｔｉを一度固溶させ、その後更に低い温度での圧延や熱処理を施すことにより、その工程で微細に析出させることで、浸炭処理時の結晶粒の粗大化防止の効果を得ることができる（例えば、特許文献１参照）。 In addition, in order to prevent coarsening of crystal grains, there is a method of utilizing fine precipitates such as AlN, Nb, and Ti. When adopting conventional hot rolling or hot forging at high temperature, AlN, Nb, and Ti are dissolved once during heating, and then rolling or heat treatment at a lower temperature is performed to reduce the temperature of the process. By finely precipitating at , it is possible to obtain the effect of preventing coarsening of crystal grains during carburizing treatment (see, for example, Patent Document 1).
　しかし、粗大化防止効果を得るためにＮｂやＴｉを添加したうえで、温間鍛造を実施しようとする場合には、温間鍛造時の再結晶温度が上昇し、加工されたままの組織である未再結晶粒が残存しやすくなる。この未再結晶粒は浸炭昇温時に再結晶化し、非常に微細な結晶粒が生成するため、結晶粒粗大化の駆動力が大幅に増加する。このような場合において、浸炭処理時の結晶粒の粗大化を防止するためには、ピン止めによる結晶粒粗大化防止効果を高めるために、ＮｂやＴｉを多量に添加する必要があった。Ｎｂの多量添加は浸炭処理後の浸炭品質（表面炭素濃度や有効硬化深さ）に悪影響を及ぼすおそれがある。また、Ｔｉの多量添加はＴｉ硫化物などの生成によりＭｎＳ量が減少し、切削性へ悪影響を及ぼす。更に、ＮｂやＴｉの多量添加は材料コストを悪化させる。 However, when trying to perform warm forging after adding Nb or Ti to obtain the effect of preventing coarsening, the recrystallization temperature during warm forging rises, and the structure as processed Some non-recrystallized grains tend to remain. These non-recrystallized grains are recrystallized when the carburizing temperature is raised, and extremely fine crystal grains are generated, so that the driving force for coarsening the crystal grains is greatly increased. In such a case, in order to prevent grain coarsening during carburizing, it has been necessary to add a large amount of Nb or Ti in order to enhance the effect of pinning to prevent grain coarsening. Addition of a large amount of Nb may adversely affect the carburizing quality (surface carbon concentration and effective hardening depth) after carburizing. Also, the addition of a large amount of Ti reduces the amount of MnS due to the formation of Ti sulfides, etc., and adversely affects the machinability. Furthermore, addition of a large amount of Nb or Ti worsens the material cost.
特開２００７－３２１２１１号公報JP 2007-321211 A
　上記のように、温間鍛造後の熱処理を省略し、浸炭処理時の結晶粒粗大化を抑制するためには、未再結晶粒が温間鍛造後に残存する影響を考慮すると、ピン止めによる結晶粒粗大化防止効果を高めるために、Ｎｂ、Ｔｉを多量に添加する必要があるが、浸炭性や切削性、コストの観点からＮｂ、Ｔｉの添加量は極力抑えたいという相反する要求があった。 As described above, in order to omit the heat treatment after warm forging and suppress grain coarsening during carburizing treatment, considering the influence of non-recrystallized grains remaining after warm forging, grains due to pinning In order to increase the effect of preventing grain coarsening, it is necessary to add large amounts of Nb and Ti, but from the viewpoint of carburization, machinability, and cost, there was a conflicting demand to suppress the amount of Nb and Ti added as much as possible. .
　本発明は、かかる背景に鑑みてなされたものであり、Ｎｂ及びＴｉの多量添加を必要とすることなく、浸炭処理時の結晶粒粗大化を抑制可能な浸炭用温間鍛造部品及びその製造方法を提供しようとするものである。 The present invention has been made in view of such a background, and a warm forged part for carburizing that can suppress grain coarsening during carburizing without requiring the addition of large amounts of Nb and Ti, and a method for manufacturing the same. is intended to provide
　本発明の一態様は、質量％において、Ｃ：０．１２～０．２８％、Ｓｉ：０．０１～０．９０％、Ｍｎ：０．３０～１．００％、Ｐ：０．０３５％以下、Ｓ：０．０１０～０．０４０％、Ｃｒ：０．４０～２．００％、Ａｌ：０．０２０～０．０６０％、Ｎ：０．０１００～０．０２００％、Ｎｂ：０．００４～０．０６０％、任意元素として、Ｍｏ：０～０．６０％を含有し、残部がＦｅ及び不可避的不純物からなる化学成分組成を有する鋼素材を準備し、
　該鋼素材を、１１５０～１３５０℃の圧延温度に加熱してから圧延を行って圧延材を作製し、
　１１００℃以下であるとともに、下記式１から算出されるＸ（℃）以上の温度で温間鍛造を行ない、その後、少なくとも７００℃までは、冷却速度３．０℃／秒以下の条件で冷却し、
　金属組織が、フェライト及びパーライト以外の組織の面積率が５．０％以下であり、パーライト粒の平均粒子径が式２から算出されるＹ（μｍ）以上であり、未再結晶粒の面積率が３．０％以下であり、円相当径１００ｎｍ以上のＡｌＮ及びＮｂＣＮの合計個数が、５個／μｍ2以下である鍛造部品を得る、浸炭用温間鍛造部品の製造方法にある。
（式１）：Ｘ＝１３０３×［Ｎｂ］＋８５７．９１、
（式２）：Ｙ＝０．４３／（０．９４×［ＡｌＮ］＋０．９２×［ＮｂＣＮ］）、
（ここで、［ＡｌＮ］は、［Ａｌ］×４１／２７の値と［Ｎ］×４１／１４の値のうち小さい方の値であり、［ＮｂＣＮ］は、［Ｎｂ］×１０４．９／９２．９の値と（［Ｃ］＋（［Ｎ］×１２／１４））×１０４．９／１２の値のうち小さい方の値であり、上記式１、式２及びこれらの関係式において、［Ｎｂ］、［Ａｌ］、［Ｎ］及び［Ｃ］は、それぞれ、Ｎｂ、Ａｌ、Ｎ及びＣの含有率（％）の値を意味する。） One aspect of the present invention is, in mass%, C: 0.12 to 0.28%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.00%, P: 0.035% Below, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Nb: 0.020%. 004 to 0.060%, Mo: 0 to 0.60% as an optional element, and preparing a steel material having a chemical composition with the balance being Fe and unavoidable impurities,
The steel material is heated to a rolling temperature of 1150 to 1350° C. and then rolled to produce a rolled material,
Warm forging is performed at a temperature not higher than 1100°C and not lower than X (°C) calculated from Equation 1 below, and then cooled to at least 700°C at a cooling rate of 3.0°C/second or less. ,
In the metal structure, the area ratio of structures other than ferrite and pearlite is 5.0% or less, the average grain size of pearlite grains is Y (μm) calculated from Equation 2 or more, and the area ratio of non-recrystallized grains is 3.0% or less, and the total number of AlN and NbCN having an equivalent circle diameter of 100 nm or more is 5 pieces/μm 2 or less.
(Formula 1): X = 1303 x [Nb] + 857.91,
(Formula 2): Y = 0.43/(0.94 x [AlN] + 0.92 x [NbCN]),
(where [AlN] is the smaller of [Al] x 41/27 and [N] x 41/14, and [NbCN] is [Nb] x 104.9/ 92.9 and ([C] + ([N] x 12/14)) x 104.9/12, whichever is smaller, and in the above formulas 1 and 2 and these relational expressions , [Nb], [Al], [N] and [C] mean the contents (%) of Nb, Al, N and C, respectively.)
　本発明の他の態様は、質量％において、Ｃ：０．１２～０．２８％、Ｓｉ：０．０１～０．９０％、Ｍｎ：０．３０～１．００％、Ｐ：０．０３５％以下、Ｓ：０．０１０～０．０４０％、Ｃｒ：０．４０～２．００％、Ａｌ：０．０２０～０．０６０％、Ｎ：０．０１００～０．０２００％、Ｎｂ：０．００４～０．０６０％、任意元素として、Ｍｏ：０～０．６０％を含有し、残部がＦｅ及び不可避的不純物からなる化学成分組成を有し、
　金属組織が、フェライト及びパーライト以外の組織の面積率が５．０％以下であり、パーライト粒の平均粒子径が式２から算出されるＹ（μｍ）以上であり、未再結晶粒の面積率が３．０％以下であり、円相当径１００ｎｍ以上のＡｌＮ及びＮｂＣＮの合計個数が、５個／μｍ2以下である、浸炭用温間鍛造部品にある。
（式２）：Ｙ＝０．４３／（０．９４×［ＡｌＮ］＋０．９２×［ＮｂＣＮ］）、
（ここで、［ＡｌＮ］は、［Ａｌ］×４１／２７の値と［Ｎ］×４１／１４の値のうち小さい方の値であり、［ＮｂＣＮ］は、［Ｎｂ］×１０４．９／９２．９の値と（［Ｃ］＋（［Ｎ］×１２／１４））×１０４．９／１２の値のうち小さい方の値であり、上記式１、式２及びこれらの関係式において、［Ｎｂ］、［Ａｌ］、［Ｎ］及び［Ｃ］は、それぞれ、Ｎｂ、Ａｌ、Ｎ及びＣの含有率（％）の値を意味する。） Another aspect of the present invention is, in mass %, C: 0.12 to 0.28%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.00%, P: 0.035 % or less, S: 0.010 to 0.040%, Cr: 0.40 to 2.00%, Al: 0.020 to 0.060%, N: 0.0100 to 0.0200%, Nb: 0 .004 to 0.060%, Mo: 0 to 0.60% as an optional element, with the balance being Fe and unavoidable impurities,
In the metal structure, the area ratio of structures other than ferrite and pearlite is 5.0% or less, the average grain size of pearlite grains is Y (μm) calculated from Equation 2 or more, and the area ratio of non-recrystallized grains is 3.0% or less, and the total number of AlN and NbCN having an equivalent circle diameter of 100 nm or more is 5 pieces/μm 2 or less.
(Formula 2): Y = 0.43/(0.94 x [AlN] + 0.92 x [NbCN]),
(where [AlN] is the smaller of [Al] x 41/27 and [N] x 41/14, and [NbCN] is [Nb] x 104.9/ 92.9 and ([C] + ([N] x 12/14)) x 104.9/12, whichever is smaller, and in the above formulas 1 and 2 and these relational expressions , [Nb], [Al], [N] and [C] mean the contents (%) of Nb, Al, N and C, respectively.)
　上記浸炭用温間鍛造部品の製造方法においては、Ｔｉは積極添加せず、Ｎｂの多量添加を避けた上記特定の化学成分組成を有することを前提として、上記特定の処理温度で熱間圧延を施した後、上記特定の処理温度で温間鍛造を行う。そして、温間鍛造の処理温度は、式１により導かれるＸ（℃）以上の温度で行うことを厳守する。さらに、温間鍛造後の鍛造品の冷却速度も上記特定の条件で制御する。これにより、得られる鍛造品における金属組織を適正化し、特に、未再結晶粒については、温間鍛造し、室温まで冷却した時点で原則として残さないことを前提にし、仮に少量残存する場合でも、面積率で３％以下となるように制御するとともに、さらに、パーライトの平均粒子径を式２で計算されるＹ（μｍ）以上となるように制御する。このような金属組織の適正化を図った上記浸炭用温間鍛造部品は、温間鍛造後の熱処理を省略しても、その後の浸炭処理時の結晶粒粗大化を抑制することができる。すなわち、本願においては、Ｎｂ及びＴｉの多量添加を必要とすることなく、浸炭処理時の結晶粒粗大化を抑制可能な浸炭用温間鍛造部品及びその製造方法を提供することができる。 In the method for manufacturing carburizing warm forged parts, on the premise that Ti is not actively added and Nb is avoided from being added in a large amount, hot rolling is performed at the above specific treatment temperature. After application, warm forging is performed at the specified treatment temperature. The processing temperature for warm forging must be a temperature equal to or higher than X (° C.) derived from Equation (1). Furthermore, the cooling rate of the forged product after warm forging is also controlled under the above specific conditions. As a result, the metal structure in the obtained forged product is optimized, and in particular, it is assumed that unrecrystallized grains are not left after warm forging and cooling to room temperature, even if a small amount remains. The area ratio is controlled to be 3% or less, and the average particle size of pearlite is controlled to be Y (μm) calculated by Equation 2 or more. The carburizing warm forged part with such an optimized metal structure can suppress coarsening of crystal grains during the subsequent carburizing treatment even if the heat treatment after the warm forging is omitted. That is, in the present application, it is possible to provide a warm forged part for carburizing and a method for manufacturing the same that can suppress grain coarsening during carburizing without requiring the addition of large amounts of Nb and Ti.
　まず、上記浸炭用温間鍛造部品の化学成分組成の限定理由を説明する。 First, the reason for limiting the chemical composition of the warm forged parts for carburizing will be explained.
Ｃ：０．１２～０．２８％；
　Ｃ（炭素）は、焼入れ処理後の硬さを向上させ、強度確保のための内部硬さを得るために必要な元素である。この効果を得るために、Ｃは０．１２％以上含有させる。一方、Ｃの過剰添加は、硬さ上昇による切削性低下につながるため、それを防止すべくＣ含有率の上限は０．２８％とする。 C: 0.12-0.28%;
C (carbon) is an element necessary for improving hardness after quenching treatment and obtaining internal hardness for ensuring strength. To obtain this effect, C is contained in an amount of 0.12% or more. On the other hand, excessive addition of C leads to a decrease in machinability due to an increase in hardness, so the upper limit of the C content is set to 0.28% to prevent this.
Ｓｉ：０．０１～０．９０％；
　Ｓｉ（ケイ素）は、固溶強化により強度向上に寄与するため、０．０１％以上含有させる。一方、Ｓｉの過剰添加は切削性低下につながるため、それを防止すべくＳｉ含有率の上限は０．９０％とする。 Si: 0.01-0.90%;
Si (silicon) contributes to strength improvement by solid solution strengthening, so it is contained in an amount of 0.01% or more. On the other hand, since excessive addition of Si leads to deterioration of machinability, the upper limit of the Si content is made 0.90% in order to prevent it.
Ｍｎ：０．３０～１．００％；
　Ｍｎ（マンガン）は、焼入れ性向上効果を得るために、０．３０％以上含有させる。一方、Ｍｎの過剰添加は、鍛造後の硬さが上昇による切削性低下と残留オーステナイの増加による浸炭層の硬さ低下につながるため、それを防止すべくＭｎ含有率の上限は１．００％とする。 Mn: 0.30-1.00%;
Mn (manganese) is contained in an amount of 0.30% or more in order to obtain an effect of improving hardenability. On the other hand, excessive addition of Mn leads to a decrease in machinability due to an increase in hardness after forging and a decrease in the hardness of the carburized layer due to an increase in residual austenite. and
Ｐ：０．０３５％以下；
　Ｐ（リン）は、不純物として含まれる元素である。オーステナイ卜粒界に偏析しやすい元素であり、偏析すると強度低下の原因となるため、Ｐの許容含有率の上限は０．０３５％とする。 P: 0.035% or less;
P (phosphorus) is an element contained as an impurity. P is an element that easily segregates at austenite grain boundaries, and segregation causes a decrease in strength. Therefore, the upper limit of the allowable content of P is 0.035%.
Ｓ：０．０１０～０．０４０％；
　Ｓ（硫黄）は、ＭｎとともにＭｎＳを生成して切削性を向上させるため、０．０１０％以上含有させる。一方、Ｓを多量に含有すると硫化物系の非金属介在物が増加し、これが強度の低下の原因となるため、Ｓの許容含有率の上限は０．０４０％とする。 S: 0.010 to 0.040%;
S (sulfur) forms MnS together with Mn to improve machinability, so it is contained in an amount of 0.010% or more. On the other hand, if a large amount of S is contained, sulfide-based non-metallic inclusions increase, which causes a decrease in strength.
Ｃｒ：０．４０～２．００％；
　Ｃｒ（クロム）は、焼入れ性を高める効果を得るために、０．４０％以上含有させる。一方、Ｃｒの過剰添加は、鍛造後の硬さ上昇による切削性低下につながるため、それを防止すべくＣｒ含有率の上限は２．００％とする。 Cr: 0.40-2.00%;
Cr (chromium) is contained in an amount of 0.40% or more in order to obtain the effect of improving hardenability. On the other hand, since excessive addition of Cr leads to deterioration in machinability due to increased hardness after forging, the upper limit of the Cr content is set to 2.00% in order to prevent this.
Ａｌ：０．０２０～０．０６０％；
　Ａｌ（アルミニウム）は、製鋼時の脱酸剤として使用される元素であるとともに、Ｎと結合して微細なＡｌＮとして存在する場合に、浸炭時の異常粒成長を抑制する効果を発揮する。これらの効果を得るために、Ａｌ含有率は０．０２０％以上とする。一方、Ａｌの過剰添加は加工性の低下を招くとともに、ＡｌＮの粗大化を招いて異常粒成長抑制効果が低下するため、Ａｌ含有率の上限は０．０６０％とする。 Al: 0.020-0.060%;
Al (aluminum) is an element used as a deoxidizing agent during steelmaking, and exhibits the effect of suppressing abnormal grain growth during carburizing when it is combined with N and exists as fine AlN. In order to obtain these effects, the Al content should be 0.020% or more. On the other hand, the excessive addition of Al causes deterioration of workability and coarsening of AlN, which reduces the effect of suppressing abnormal grain growth. Therefore, the upper limit of the Al content is made 0.060%.
Ｎ：０．０１００～０．０２００％；
　Ｎ（窒素）は、ＡｌＮとなって、ピン止め効果により結晶粒粗大化を抑制する効果があるため、０．０１００％以上含有させる。一方、Ｎ含有率が高すぎると、ＡｌＮの粗大化を招いて異常粒成長抑制効果が低下するため、Ｎ含有率の上限は０．０２００％とする。 N: 0.0100 to 0.0200%;
N (nitrogen) turns into AlN and has the effect of suppressing grain coarsening due to the pinning effect, so it is contained in an amount of 0.0100% or more. On the other hand, if the N content is too high, AlN coarsens and the effect of suppressing abnormal grain growth is lowered, so the upper limit of the N content is made 0.0200%.
Ｎｂ：０．００４～０．０６０％；
　Ｎｂ（ニオブ）は、結晶粒微細化の効果を得るため、０．００４％以上含有させる。一方、Ｎｂ含有率が高すぎると、浸炭性が低下するおそれがあるため、０．０６０％以下に制限する。好ましくは、０．０４０％以下である。 Nb: 0.004 to 0.060%;
Nb (niobium) is contained in an amount of 0.004% or more in order to obtain the effect of grain refinement. On the other hand, if the Nb content is too high, the carburizability may deteriorate, so it is limited to 0.060% or less. Preferably, it is 0.040% or less.
任意元素としてのＭｏ：０～０．６０％；
　Ｍｏ（モリブデン）は、任意添加元素であり、必ずしも含有させる必要はなく、含有率０％でもよいが、スクラップを原料とする電気炉溶解により製造する場合には、不可避的不純物として含有される場合もある。そして、Ｍｏは、０．０１％以上含有させることにより、焼入れ性向上に有効な元素であるので、必要に応じ添加することもできる。一方、Ｍｏ含有率が高すぎると、コストアップ及び切削性低下につながるため、０．６０％以下に制限する。 Mo as an optional element: 0-0.60%;
Mo (molybdenum) is an optional additive element and does not necessarily need to be contained, and the content may be 0%. However, in the case of manufacturing by electric furnace melting using scrap as a raw material, it may be contained as an unavoidable impurity. There is also Mo is an element effective for improving the hardenability when contained in an amount of 0.01% or more, so it can be added as necessary. On the other hand, if the Mo content is too high, it leads to an increase in cost and a decrease in machinability, so it is limited to 0.60% or less.
　そして、上記化学成分組成においては、Ｔｉの積極添加は行わず（０．０１％程度以下の不可避的不純物としての含有は許容する。）、Ｔｉ添加による切削性低下及びコスト悪化への影響を無くす一方、上記のごとく、浸炭性へ悪影響を及ぼさない範囲でＮｂを添加する。このような化学成分組成の鋼素材を、１１５０～１３５０℃の圧延温度に加熱してから圧延を行って圧延材を作製し、その後、特定の処理温度で温間鍛造を行う。以下、これらの製造条件の意義について説明する。 In addition, in the above chemical composition, Ti is not intentionally added (it is allowed to contain about 0.01% or less as an unavoidable impurity), so that the influence of Ti addition on machinability deterioration and cost deterioration is eliminated. On the other hand, as described above, Nb is added within a range that does not adversely affect carburization. A steel material having such a chemical composition is heated to a rolling temperature of 1150 to 1350° C. and then rolled to produce a rolled material, which is then warm forged at a specific processing temperature. The significance of these manufacturing conditions will be described below.
　多くの実験の結果、Ｔｉの意図的な添加をせず、かつ、Ｎｂ含有率を浸炭性へ悪影響を及ぼさない範囲とした場合に、浸炭時の結晶粒粗大化を抑制するためには、ＡｌＮやＮｂ炭化物等のピンニング粒子の析出状態は、必要最低限で制御した上で、結晶粒粗大化の駆動力を低減させることが重要であることが分かった。具体的な方策として、鍛造後の金属組織の状態を最適化、つまり、未再結晶粒の抑制及びパーライト粒子径の制御を行う必要があることが分かった。 As a result of many experiments, it was found that AlN It has been found that it is important to reduce the driving force for grain coarsening while controlling the precipitation state of pinning particles such as Nb and Nb carbides to the minimum necessary. As a specific measure, it was found that it is necessary to optimize the state of the metal structure after forging, that is, to suppress non-recrystallized grains and control the pearlite particle size.
　従来、ＡｌＮやＮｂ炭窒化物を用いて浸炭時の結晶粒粗大化を抑制するためには、ＡｌＮ、Ｎｂ炭窒化物を熱間鍛造時の加熱で一度固溶させ、その後の熱処理において、微細に析出させる必要があった。しかしながら、温間鍛造は熱間鍛造と比較して加熱温度が低いために、温間鍛造時の加熱温度では、ＡｌＮ、Ｎｂ炭窒化物を十分に固溶させることができない。また、温間鍛造前にＡｌＮやＮｂ炭窒化物が粗大に析出していた場合には、温間鍛造時に凝集、粗大化し、微細に析出させることができず、結晶粒の粗大化防止効果が減少する。 Conventionally, in order to suppress grain coarsening during carburizing using AlN and Nb carbonitrides, AlN and Nb carbonitrides are once solid-dissolved by heating during hot forging, and fine grains are formed in the subsequent heat treatment. It was necessary to deposit on However, since warm forging requires a lower heating temperature than hot forging, AlN and Nb carbonitrides cannot be dissolved sufficiently at the heating temperature during warm forging. In addition, when AlN or Nb carbonitrides are coarsely precipitated before warm forging, they agglomerate and coarsen during warm forging and cannot be precipitated finely, and the effect of preventing grain coarsening is improved. Decrease.
　そこで、鍛造前の工程である圧延時の加熱において、ＡｌＮやＮｂ炭窒化物のほとんど（８割以上）を一度固溶させることで、温間鍛造時にＡｌＮやＮｂ炭窒化物を微細析出させ、結晶粒の粗大化防止効果が得られるようにする。結晶粒の粗大化防止効果が得られる具体的な圧延加熱温度としては、１１５０℃～１３５０℃とする。この温度範囲とすることにより、圧延工程においてＡｌＮやＮｂ炭窒化物のほとんど（８割以上）を固溶させることが可能となる。圧延加熱温度が１１５０℃未満の場合には、ＡｌＮやＮｂ炭窒化物の固溶を十分に実現できない場合があり、また、１３５０℃を超える場合には、エネルギーの消費が高くなりすぎ、折角の温間鍛造後の熱処理省略によるエネルギー節約効果を考慮しても、省エネ効果が十分に得られなくなるという問題がある。 Therefore, most of the AlN and Nb carbonitrides (80% or more) are dissolved once in the heating during rolling, which is a process before forging, so that the AlN and Nb carbonitrides are finely precipitated during warm forging. To obtain an effect of preventing coarsening of crystal grains. A specific rolling heating temperature at which the effect of preventing grain coarsening is obtained is 1150°C to 1350°C. By setting the temperature within this range, most of the AlN and Nb carbonitrides (80% or more) can be dissolved in the rolling process. When the rolling heating temperature is less than 1150°C, solid solution of AlN and Nb carbonitrides may not be sufficiently realized, and when it exceeds 1350°C, the energy consumption becomes too high, which is difficult. Even if the energy saving effect by omitting heat treatment after warm forging is considered, there is a problem that the energy saving effect cannot be obtained sufficiently.
　上記化学成分組成において、Ａｌ及びＮｂの添加は、オーステナイトの再結晶温度を上昇させることにつながる。特にＮｂはわずかな添加量であっても、大幅に再結晶温度を上昇させてしまう。再結晶温度が上昇し、温間鍛造し、室温まで冷却した状態で、未再結晶のオーステナイト粒が残存した場合、この未再結晶粒は、その後の浸炭昇温時に再結晶し、微細な結晶粒となり結晶粒粗大化の駆動力が大幅に増加してしまう。なお、温間鍛造後に未再結晶粒が残っていたとしても、従来のように鍛造後に熱処理を行えば未再結晶粒を再結晶させ、浸炭時に未再結晶粒が存在しない状態を得ることができるが、それでは、熱処理省略によるエネルギー削減効果が得られなくなる。 In the above chemical composition, the addition of Al and Nb leads to an increase in the recrystallization temperature of austenite. In particular, Nb significantly raises the recrystallization temperature even when added in a small amount. If unrecrystallized austenite grains remain after the recrystallization temperature is raised, warm forged, and cooled to room temperature, these unrecrystallized grains are recrystallized during the subsequent carburization temperature rise to form fine crystals. As a result, the driving force for grain coarsening is greatly increased. Even if unrecrystallized grains remain after warm forging, if heat treatment is performed after forging as in the past, the unrecrystallized grains can be recrystallized, and a state in which no unrecrystallized grains exist during carburization can be obtained. Although it can be done, the energy saving effect by omitting the heat treatment cannot be obtained.
　温間鍛造で生成した未再結晶粒は、鍛造後の冷却中に再結晶し、生成量が減少していく。再結晶を促進させるためには、鍛造後の冷却速度を制御することも有効である。具体的には、温間鍛造後の鍛造部品の冷却過程において、少なくとも７００℃までは、冷却速度３．０℃／秒以下の条件で冷却することが有効である。この条件で冷却することにより、温間鍛造直後（室温への冷却前の時点）に未再結晶粒が残存していたとしても、その後の冷却途中における再結晶を促進させ、室温まで冷却後の未再結晶粒の比率を減少させることができる。 The non-recrystallized grains produced by warm forging recrystallize during cooling after forging, and the amount produced decreases. Controlling the cooling rate after forging is also effective in promoting recrystallization. Specifically, in the process of cooling the forged part after warm forging, it is effective to cool the forged part to at least 700° C. under the condition of a cooling rate of 3.0° C./sec or less. By cooling under these conditions, even if unrecrystallized grains remain immediately after warm forging (before cooling to room temperature), recrystallization during subsequent cooling is promoted, and after cooling to room temperature, The proportion of non-recrystallized grains can be reduced.
　また、Ｎｂの含有率及び鍛造温度についても、未再結晶粒の再結晶のしやすさに影響を与えることが判明した。また、温間鍛造後のパーライト粒子径が微細な場合でも、その後の浸炭昇温後は微細な結晶粒となり、結晶粒粗大化の駆動力が増加するとともに、ＡｌＮやＮｂ炭窒化物の析出量が少なくピン止め効果が小さい場合も結晶粒粗大化が起きるおそれがある。 In addition, it was found that the Nb content and forging temperature also affect the ease of recrystallization of non-recrystallized grains. In addition, even if the pearlite particle size after warm forging is fine, the crystal grains become fine after the subsequent carburizing temperature rise, and the driving force for coarsening the crystal grains increases, and the precipitation amount of AlN and Nb carbonitrides Also when the pinning effect is small due to the small amount of grains, coarsening of crystal grains may occur.
　結晶粒粗大化を効果的に抑制するためには、ＡｌＮやＮｂ炭窒化物のピン止めによる結晶粒粗大化防止力が、結晶粒粗大化の駆動力を上回ることが必要である。そこで、さらに検討した結果、結晶粒粗大化の駆動力は、浸炭昇温後の結晶粒径から見積もることができ、浸炭昇温後の結晶粒径は、温間鍛造後のパーライト粒径によって変化することがわかった。 In order to effectively suppress crystal grain coarsening, it is necessary that the crystal grain coarsening prevention force due to pinning of AlN or Nb carbonitride exceeds the driving force for crystal grain coarsening. Therefore, as a result of further investigation, the driving force of grain coarsening can be estimated from the grain size after carburizing temperature rise, and the grain size after carburizing temperature rise changes depending on the pearlite grain size after warm forging. found to do.
　そこで、本発明では、ピン止め効果を強化するため、ＡｌＮやＮｂ炭窒化物を圧延段階で十分に固溶させるとともに、ＡｌＮやＮｂ炭窒化物を温間鍛造で析出させた際の結晶粒粗大化防止力に対して、結晶粒粗大化の駆動力をそれ以下とするために、温間鍛造後のパーライト粒子径を制御することとした。 Therefore, in the present invention, in order to strengthen the pinning effect, the AlN and Nb carbonitrides are sufficiently dissolved in the rolling stage, and the crystal grains are coarsened when the AlN and Nb carbonitrides are precipitated by warm forging. In order to keep the driving force for crystal grain coarsening below the anti-hardening force, the pearlite particle size after warm forging was controlled.
　以上の知見を総合し、下記式１及び式２を導いた。 By summarizing the above knowledge, the following formulas 1 and 2 were derived.
（式１）：Ｘ＝１３０３×［Ｎｂ］＋８５７．９１、
（式２）：Ｙ＝０．４３／（０．９４×［ＡｌＮ］＋０．９２×［ＮｂＣＮ］）、
（ここで、［ＡｌＮ］は、［Ａｌ］×４１／２７の値と［Ｎ］×４１／１４の値のうち小さい方の値であり、［ＮｂＣＮ］は、［Ｎｂ］×１０４．９／９２．９の値と（［Ｃ］＋（［Ｎ］×１２／１４））×１０４．９／１２の値のうち小さい方の値であり、上記式１、式２及びこれらの関係式において、［Ｎｂ］、［Ａｌ］、［Ｎ］及び［Ｃ］は、それぞれ、Ｎｂ、Ａｌ、Ｎ及びＣの含有率（％）の値を意味する。） (Formula 1): X = 1303 x [Nb] + 857.91,
(Formula 2): Y = 0.43/(0.94 x [AlN] + 0.92 x [NbCN]),
(where [AlN] is the smaller of [Al] x 41/27 and [N] x 41/14, and [NbCN] is [Nb] x 104.9/ 92.9 and ([C] + ([N] x 12/14)) x 104.9/12, whichever is smaller, and in the above formulas 1 and 2 and these relational expressions , [Nb], [Al], [N] and [C] mean the contents (%) of Nb, Al, N and C, respectively.)
　式１は、鍛造温度とＮｂ含有率との関係式である。鍛造の処理温度をＸ℃未満とした場合には、温間鍛造時に未再結晶粒が残存しやすく、また、鍛造後のパーライト粒子径が微細になりすぎ、その後の浸炭処理時に結晶粒粗大化が起きやすくなるため、鍛造の処理温度は、少なくともＸ℃以上の温度に設定する必要がある。 Formula 1 is a relational expression between the forging temperature and the Nb content. If the forging temperature is less than X°C, non-recrystallized grains tend to remain during warm forging, and the pearlite grain size after forging becomes too fine, resulting in grain coarsening during subsequent carburizing. Therefore, it is necessary to set the forging treatment temperature to at least X° C. or higher.
　式２は、鍛造後の組織の状態が最適化（未再結晶粒の抑制）され、さらに、ＡｌＮやＮｂ炭窒化物を圧延段階で十分に固溶させ、温間鍛造で析出させた際に得られる結晶粒粗大化防止力以下の結晶粒粗大化の駆動力となる、温間鍛造後のパーライト平均粒子径との関係について、表したものである。パーライト平均粒子径をＹμｍ未満とした場合には、Ａｌ、Ｎｂ含有率から予想される結晶粒粗大化防止力以上に、結晶粒粗大化の駆動力が大きくなり、結晶粒の粗大化が発生するおそれが高くなる。 Formula 2 shows that the state of the structure after forging is optimized (suppression of unrecrystallized grains), and furthermore, AlN and Nb carbonitrides are fully dissolved in the rolling stage and precipitated by warm forging. It shows the relationship between the pearlite average grain size after warm forging, which is the driving force for grain coarsening below the obtained grain coarsening prevention force. When the pearlite average particle size is less than Y μm, the driving force for crystal grain coarsening becomes greater than the crystal grain coarsening prevention force expected from the Al and Nb contents, and crystal grain coarsening occurs. become more likely.
　上記式１及び式２の両方を具備するよう鍛造温度と鍛造後のパーライト粒子径を制御することで、結晶粒粗大化の駆動力を小さくし、浸炭時の結晶粒の粗大化防止効果が得られる。 By controlling the forging temperature and the pearlite particle size after forging so that both the above formulas 1 and 2 are satisfied, the driving force for grain coarsening is reduced, and the effect of preventing grain coarsening during carburization is obtained. be done.
　なお、上記温間鍛造は、温間鍛造を選択することによる本来の省エネ効果等を享受するために、１１００℃以下の処理温度で行う。 In addition, the above warm forging is performed at a processing temperature of 1100°C or less in order to enjoy the inherent energy saving effect of selecting warm forging.
　以上の説明に基づき得られる鍛造品は、上記した化学成分組成の範囲内に調整し、前記の鍛造温度、冷却速度に調整することによって、フェライト及びパーライト主体の組織（他の組織の面積率が５．０％以下）とすることができる。また、パーライト粒の平均粒子径がＹμｍ以上であり、未再結晶粒の面積率が３．０％以下である金属組織からなる鍛造品を得ることができる。フェライト・パーライト組織以外の組織が混在したり、パーライト粒の大きさがＹμｍ未満になると、結晶粗大化が起きやすくなるため、この金属組織を有することにより、その後の浸炭加熱時の結晶粒粗大化を十分に抑制することが可能となる。なお、パーライト粒子径は、温間鍛造温度が最も大きく影響し、その温度が高いほど粒子径を大きくすることができる。鍛造温度を高くすることで、満足する粒子径に制御することができる。 The forged product obtained based on the above description is adjusted within the range of the chemical composition described above, and the forging temperature and cooling rate are adjusted to the structure mainly composed of ferrite and pearlite (the area ratio of other structures is 5.0% or less). Also, a forged product having a metal structure in which the pearlite grains have an average grain size of Y μm or more and the non-recrystallized grains have an area ratio of 3.0% or less can be obtained. If a structure other than ferrite/pearlite structure is mixed, or if the pearlite grain size is less than Y μm, crystal coarsening is likely to occur. can be sufficiently suppressed. Note that the pearlite particle size is most affected by the warm forging temperature, and the higher the temperature, the larger the particle size. By increasing the forging temperature, it is possible to control the particle size to a satisfactory level.
　上記未再結晶粒の面積率はより好ましくは１．５％以下がよい。また、未再結晶粒は、温間鍛造時に与える歪の大きさにもよるが、再結晶していなければ、温間鍛造時に行う圧縮方向の加工によりつぶれた形状となっており、アスペクト比が一定値以上の圧縮方向につぶれた形状となっているかどうかで未再結晶粒であるかどうかを判断することができる。 The area ratio of the non-recrystallized grains is more preferably 1.5% or less. In addition, depending on the magnitude of the strain applied during warm forging, the non-recrystallized grains have a crushed shape due to the compression direction processing performed during warm forging if they are not recrystallized, and the aspect ratio is reduced. It can be judged whether or not the grains are non-recrystallized grains based on whether or not the grains are crushed in the direction of compression of a certain value or more.
（実験例１）
　本発明の浸炭用温間鍛造部品およびその製造方法に係る実施例について説明する。本例では、表１に示すように、２０種類の鋼材（実施例１～１４、比較例１５～２０）を準備し、温間鍛造品に相当する試験片を作製し、種々の特性について評価を行った。ここで、任意添加元素であるＭｏは、スクラップから不純物として混入するので、実施例１０を除き、積極添加しておらず、後述の表３を含め、不純物として含有していた分析値を示した。 (Experimental example 1)
An embodiment of the warm forged part for carburizing and the method for manufacturing the same according to the present invention will be described. In this example, as shown in Table 1, 20 types of steel materials (Examples 1 to 14 and Comparative Examples 15 to 20) were prepared, test pieces corresponding to warm forged products were produced, and various properties were evaluated. did Here, Mo, which is an optional additive element, is mixed as an impurity from scrap, so it was not actively added except for Example 10, and the analysis values contained as impurities, including Table 3 below, are shown. .
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
＜試験片の作製＞
　各鋼材を電気炉にて溶解・鋳造を行って鋼塊を得て、当該鋼塊に鍛伸加工を加えてΦ３２ｍｍの丸棒を得た。この丸棒を、表２に示す圧延温度まで加熱して１時間保持した後放冷するという、模擬圧延工程を施した。その後、模擬圧延工程終了後の丸棒から、外径Ｄに対して外表面から１／４Ｄ深さの位置から、試験片用母材を採取し、Φ８ｍｍ×１２ｍｍ高さの圧縮試験片を機械加工により準備した。 <Preparation of test piece>
Each steel material was melted and cast in an electric furnace to obtain a steel ingot, and the steel ingot was forged and drawn to obtain a φ32 mm round bar. This round bar was subjected to a simulated rolling process in which it was heated to the rolling temperature shown in Table 2, held for 1 hour, and then allowed to cool. After that, from the round bar after the simulated rolling process, the base material for the test piece was sampled from the position of 1/4D depth from the outer surface with respect to the outer diameter D, and a compression test piece of Φ8 mm × 12 mm height was machined. Prepared by processing.
＜模擬鍛造試験＞
　上記圧縮試験片を用い、表２に示す鍛造温度まで昇温速度２℃／秒で加熱して、高さが３ｍｍになる加工率７５％の条件で圧縮し、その後、７００℃まで１℃／秒の冷却速度で冷却した後、０．５℃／秒の冷却速度で室温まで冷却する模擬鍛造試験を行った。 <Simulated forging test>
Using the compression test piece, it is heated to the forging temperature shown in Table 2 at a heating rate of 2 ° C./sec, and compressed under the conditions of a processing rate of 75% so that the height becomes 3 mm, and then up to 700 ° C. at 1 ° C./sec. After cooling at a cooling rate of 0.5° C./second, a simulated forging test was performed to cool to room temperature at a cooling rate of 0.5° C./second.
＜ミクロ組織観察＞
　模擬鍛造試験を施した試験片の軸方向中心線を含む断面を、光学顕微鏡を用いて観察してミクロ組織写真を撮影した。そして、この撮影画像を用いて、画像解析により、パーライト粒子径及び未再結晶粒の面積率を測定した。圧縮後の歪は、試験片の部位より異なるが、試験片の断面中心付近が最も歪が高くなるので、この部位における歪の大きさを考慮して、アスペクト比が３以上となる扁平状のパーライト粒を未再結晶粒と判断し、その面積率を画像解析により測定した。未再結晶粒を除いたパーライト粒径を画像解析にて測定し、その平均値をパーライト平均粒子径とした。表２において、ミクロ組織については、フェライトをα、パーライトをＰで示した。α＋Ｐはフェライト・パーライト組織を意味する。 <Microstructure Observation>
A cross section including the axial centerline of the test piece subjected to the simulated forging test was observed using an optical microscope to take a microstructure photograph. Then, the pearlite particle diameter and the area ratio of non-recrystallized grains were measured by image analysis using the photographed image. The strain after compression differs depending on the part of the test piece, but since the strain is highest near the center of the cross section of the test piece, considering the magnitude of the strain in this part, a flat shape with an aspect ratio of 3 or more was used. Perlite grains were determined to be non-recrystallized grains, and their area ratio was measured by image analysis. The pearlite grain size excluding non-recrystallized grains was measured by image analysis, and the average value was defined as the pearlite average grain size. In Table 2, the microstructures are indicated by α for ferrite and P for pearlite. α+P means a ferrite/pearlite structure.
＜析出物観察＞
　前記した模擬鍛造試験片の断面をＳＥＭ観察した。観察は、倍率２万倍で、１０視野で行い、各視野のＳＥＭ像を撮影した。その後画像解析を行い、析出物であるＡｌＮとＮｂＣＮの円相当径を算出した。なお、析出物がＡｌＮまたはＮｂＣＮであることの確認は、ＥＤＸ（エネルギー分散型Ｘ線分析）により行った。 <Observation of deposits>
The cross section of the simulated forging test piece was observed by SEM. Observation was performed in 10 fields of view at a magnification of 20,000 times, and SEM images of each field of view were taken. After that, image analysis was performed to calculate the equivalent circle diameters of the AlN and NbCN precipitates. Confirmation that the precipitates were AlN or NbCN was performed by EDX (energy dispersive X-ray analysis).
＜旧γ結晶粒の測定＞
　模擬鍛造試験を施した試験片に対し、９５０℃にて４時間保持後急冷する模擬浸炭処理を施した。模擬浸炭処理後の試験片の断面を、光学顕微鏡を用いて観察して結晶粒が粗大化し、混粒状態となっていないかどうかを調べた。具体的には、１００倍の倍率で５視野を観察し、観察した範囲内でほかの領域に比べ、粒度番号で３以上大きく粒成長した領域が１０％以上を占める場合は、不合格（×）と判断し、１０％未満の場合は、合格（○）と判断した。 <Measurement of prior γ crystal grains>
A simulated carburizing treatment was performed on the test piece subjected to the simulated forging test by holding at 950° C. for 4 hours and then quenching. After the simulated carburizing treatment, the cross section of the test piece was observed with an optical microscope to check whether or not the crystal grains were coarsened to form a mixed grain state. Specifically, five fields of view are observed at a magnification of 100 times, and if the area in which the grain size number is 3 or more larger than the other areas in the observed range accounts for 10% or more, it is rejected (× ), and when it was less than 10%, it was judged to pass (○).
　表２には、各種評価結果とともに、式１により導かれるＸ（℃）と式２により導かれるＹ（μｍ）を示した。 Table 2 shows X (°C) derived from Equation 1 and Y (μm) derived from Equation 2 along with various evaluation results.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　表２からわかるように、実施例１～１４は、化学成分組成が適切であるとともに、鍛造温度として、１１００℃以下の範囲で、Ｘ℃以上の温度を選択したことにより、ミクロ組織がフェライト・パーライト組織であるとともに、パーライト平均粒子径がＹμｍ以上で十分に大きく、未再結晶粒の面積率は３．０％以下となり、これにより、その後の浸炭処理において、問題となる結晶粒の粗大化（１０％以上の混粒の発生）が生じないことがわかった。 As can be seen from Table 2, in Examples 1 to 14, the chemical component composition is appropriate, and the microstructure is ferrite / by selecting a temperature of X ° C. or higher as the forging temperature in the range of 1100 ° C. or lower. In addition to having a pearlite structure, the pearlite average grain size is sufficiently large at Y μm or more, and the area ratio of unrecrystallized grains is 3.0% or less. (occurrence of mixed grains of 10% or more) did not occur.
　比較例１５は、化学成分組成が適切であるものの、Ｘ℃よりも低い温度を鍛造温度としたことにより、パーライト粒子径がＹμｍ未満と小さくなり、未再結晶粒の面積率が３．０％を超え、その後の浸炭処理において結晶粒の粗大化（混粒）が生じることがわかった。 Comparative Example 15 has an appropriate chemical composition, but because the forging temperature is lower than X ° C., the pearlite grain size is smaller than Y μm, and the area ratio of non-recrystallized grains is 3.0%. It was found that grain coarsening (mixed grain) occurred in the subsequent carburizing treatment.
　比較例１６及び１７は、化学成分組成が適切であるものの、Ｘ℃よりも低い温度を鍛造温度としたことにより、未再結晶粒の面積率が３．０％を超え、その後の浸炭処理において結晶粒の粗大化（混粒）が生じることがわかった。 In Comparative Examples 16 and 17, although the chemical composition was appropriate, the area ratio of unrecrystallized grains exceeded 3.0% due to the forging temperature being lower than X ° C., and in the subsequent carburizing treatment It was found that coarsening of crystal grains (mixed grains) occurred.
　比較例１８は、化学成分組成が適切であるものの、温間鍛造前の熱間圧延の温度が低すぎたことにより、１００ｎｍ以上の析出物が５個／１００μｍ2以上となり、鍛造処理条件を適正化したとしても、その後の浸炭処理において結晶粒の粗大化（混粒）が生じることがわかった。 In Comparative Example 18, although the chemical composition was appropriate, the temperature of hot rolling before warm forging was too low. It was found that even if the grains were changed, coarsening of crystal grains (mixed grains) occurred in the subsequent carburizing treatment.
　比較例１９及び２０は、化学成分組成が適切であるものの、パーライト平均粒子径がＹμｍ未満であるために、結晶粒粗大化の駆動力が大きくなりすぎ、その後の浸炭処理において結晶粒の粗大化（混粒）が生じることがわかった。 In Comparative Examples 19 and 20, although the chemical composition was appropriate, the average pearlite particle size was less than Y μm, so the driving force for coarsening the crystal grains was too large, and the crystal grains coarsened in the subsequent carburizing treatment. (Mixed grains) was found to occur.
（実験例２）
　次に、未再結晶粒の制御及びそのための適切な温度で鍛造することが、本発明にとって非常に重要なことを確認するための別の実施例を示す。 (Experimental example 2)
Next, another example is given to confirm that the control of non-recrystallized grains and forging at an appropriate temperature for it are very important to the present invention.
　本例では、表３に示すように、再結晶温度に大きく影響する元素であるＮｂ含有率を大きく変化させた４種類の鋼材（実施例２１～２３、比較例２４）を準備し、実験例１と同様にして試験片を作製し、種々の特性について評価を行った。評価方法も実験例１と同じにした。そして、本例では、表４に示すように、圧延温度及び鍛造温度を全て同じとする条件とした。その他は、実験例１と同様の条件とした。再結晶温度に大きく影響するＮｂ含有率が高いほど、式１の温度が高くなるため、それによる結晶粒粗大化への影響を正確に評価することができる。 In this example, as shown in Table 3, four types of steel materials (Examples 21 to 23, Comparative Example 24) were prepared in which the Nb content, which is an element that greatly affects the recrystallization temperature, was greatly changed. A test piece was prepared in the same manner as in 1, and various properties were evaluated. The evaluation method was also the same as in Experimental Example 1. In this example, as shown in Table 4, the conditions were such that the rolling temperature and the forging temperature were all the same. Other conditions were the same as in Experimental Example 1. The higher the Nb content, which greatly affects the recrystallization temperature, the higher the temperature in Formula 1, so that the effect of this on grain coarsening can be accurately evaluated.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　表４からわかるように、実施例２１～２３は、化学成分組成が適切であるとともに、鍛造温度として選択した９００℃が、Ｘ℃よりも高い温度となっているため、ミクロ組織がフェライト・パーライト組織であるとともに、パーライト粒子径がＹμｍ以上で十分に大きく、未再結晶粒の面積率は３．０％以下となり、これにより、その後の浸炭処理において問題となる結晶粒の粗大化が生じないことがわかった。 As can be seen from Table 4, in Examples 21 to 23, the chemical composition is appropriate, and the 900 ° C. selected as the forging temperature is a temperature higher than X ° C., so the microstructure is ferrite / pearlite. In addition to the structure, the pearlite grain size is sufficiently large at Y μm or more, and the area ratio of unrecrystallized grains is 3.0% or less, so that the problem of coarsening of grains in the subsequent carburizing treatment does not occur. I understand.
　比較例２４は、化学成分組成が適切であるもののＮｂ含有率が上限値に近い場合には、再結晶温度が上昇して式１から導かれるＸ（℃）が９００℃を超える温度となり、その結果、未再結晶粒の面積率が３．０％を超え、その後の浸炭処理において結晶粒の粗大化（１０％以上の混粒の発生）が生じることがわかった。 In Comparative Example 24, when the chemical component composition is appropriate but the Nb content is close to the upper limit, the recrystallization temperature rises and X (° C.) derived from Equation 1 exceeds 900° C. As a result, it was found that the area ratio of non-recrystallized grains exceeded 3.0%, and grain coarsening (occurrence of mixed grains of 10% or more) occurred in the subsequent carburizing treatment.

Claims (2)

  1. 　質量％において、Ｃ：０．１２～０．２８％、Ｓｉ：０．０１～０．９０％、Ｍｎ：０．３０～１．００％、Ｐ：０．０３５％以下、Ｓ：０．０１０～０．０４０％、Ｃｒ：０．４０～２．００％、Ａｌ：０．０２０～０．０６０％、Ｎ：０．０１００～０．０２００％、Ｎｂ：０．００４～０．０６０％、任意元素として、Ｍｏ：０～０．６０％を含有し、残部がＦｅ及び不可避的不純物からなる化学成分組成を有する鋼素材を準備し、
    　該鋼素材を、１１５０～１３５０℃の圧延温度に加熱してから圧延を行って圧延材を作製し、
    　１１００℃以下であるとともに、下記式１から算出されるＸ（℃）以上で温間鍛造を行ない、その後、少なくとも７００℃までは、冷却速度３．０℃／秒以下の条件で冷却し、
    　金属組織が、フェライト及びパーライト以外の組織の面積率が５．０％以下であり、パーライト粒の平均粒子径が式２から算出されるＹ（μｍ）以上であり、未再結晶粒の面積率が３．０％以下であり、円相当径１００ｎｍ以上のＡｌＮ及びＮｂＣＮの合計個数が、５個／μｍ2以下である鍛造部品を得る、浸炭用温間鍛造部品の製造方法。
    （式１）：Ｘ＝１３０３×［Ｎｂ］＋８５７．９１、
    （式２）：Ｙ＝０．４３／（０．９４×［ＡｌＮ］＋０．９２×［ＮｂＣＮ］）、
    （ここで、［ＡｌＮ］は、［Ａｌ］×４１／２７の値と［Ｎ］×４１／１４の値のうち小さい方の値であり、［ＮｂＣＮ］は、［Ｎｂ］×１０４．９／９２．９の値と（［Ｃ］＋（［Ｎ］×１２／１４））×１０４．９／１２の値のうち小さい方の値であり、上記式１、式２及びこれらの関係式において、［Ｎｂ］、［Ａｌ］、［Ｎ］及び［Ｃ］は、それぞれ、Ｎｂ、Ａｌ、Ｎ及びＣの含有率（％）の値を意味する。）     In mass %, C: 0.12 to 0.28%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 ~0.040%, Cr: 0.40-2.00%, Al: 0.020-0.060%, N: 0.0100-0.0200%, Nb: 0.004-0.060%, Prepare a steel material having a chemical composition containing Mo: 0 to 0.60% as an optional element, with the balance being Fe and unavoidable impurities,
    The steel material is heated to a rolling temperature of 1150 to 1350° C. and then rolled to produce a rolled material,
    Warm forging is performed at a temperature of 1100° C. or less and at least X (° C.) calculated from the following formula 1, and then cooled to at least 700° C. at a cooling rate of 3.0° C./sec or less,
    In the metal structure, the area ratio of structures other than ferrite and pearlite is 5.0% or less, the average grain size of pearlite grains is equal to or larger than Y (μm) calculated from Equation 2, and the area ratio of non-recrystallized grains is 3.0% or less, and the total number of AlN and NbCN having an equivalent circle diameter of 100 nm or more is 5 pieces/μm 2 or less.
    (Formula 1): X = 1303 x [Nb] + 857.91,
    (Formula 2): Y = 0.43/(0.94 x [AlN] + 0.92 x [NbCN]),
    (where [AlN] is the smaller of [Al] x 41/27 and [N] x 41/14, and [NbCN] is [Nb] x 104.9/ 92.9 and ([C] + ([N] x 12/14)) x 104.9/12, whichever is smaller, and in the above formulas 1 and 2 and these relational expressions , [Nb], [Al], [N] and [C] mean the contents (%) of Nb, Al, N and C, respectively.)
  2. 　質量％において、Ｃ：０．１２～０．２８％、Ｓｉ：０．０１～０．９０％、Ｍｎ：０．３０～１．００％、Ｐ：０．０３５％以下、Ｓ：０．０１０～０．０４０％、Ｃｒ：０．４０～２．００％、Ａｌ：０．０２０～０．０６０％、Ｎ：０．０１００～０．０２００％、Ｎｂ：０．００４～０．０６０％、任意元素として、Ｍｏ：０～０．６０％を含有し、残部がＦｅ及び不可避的不純物からなる化学成分組成を有し、
    　金属組織が、フェライト及びパーライト以外の組織の面積率が５．０％以下であり、パーライト粒の平均粒子径が式２から算出されるＹ（μｍ）以上であり、未再結晶粒の面積率が３．０％以下であり、円相当径１００ｎｍ以上のＡｌＮ及びＮｂＣＮの合計個数が、５個／μｍ2である、浸炭用温間鍛造部品。
    （式２）：Ｙ＝０．４３／（０．９４×［ＡｌＮ］＋０．９２×［ＮｂＣＮ］）、
    （ここで、［ＡｌＮ］は、［Ａｌ］×４１／２７の値と［Ｎ］×４１／１４の値のうち小さい方の値であり、［ＮｂＣＮ］は、［Ｎｂ］×１０４．９／９２．９の値と（［Ｃ］＋（［Ｎ］×１２／１４））×１０４．９／１２の値のうち小さい方の値であり、上記式１、式２及びこれらの関係式において、［Ｎｂ］、［Ａｌ］、［Ｎ］及び［Ｃ］は、それぞれ、Ｎｂ、Ａｌ、Ｎ及びＣの含有率（％）の値を意味する。）     In mass %, C: 0.12 to 0.28%, Si: 0.01 to 0.90%, Mn: 0.30 to 1.00%, P: 0.035% or less, S: 0.010 ~0.040%, Cr: 0.40-2.00%, Al: 0.020-0.060%, N: 0.0100-0.0200%, Nb: 0.004-0.060%, As an optional element, it contains Mo: 0 to 0.60%, and the balance has a chemical composition consisting of Fe and unavoidable impurities,
    In the metal structure, the area ratio of structures other than ferrite and pearlite is 5.0% or less, the average grain size of pearlite grains is Y (μm) calculated from Equation 2 or more, and the area ratio of non-recrystallized grains is 3.0% or less, and the total number of AlN and NbCN having an equivalent circle diameter of 100 nm or more is 5/μm 2 .
    (Formula 2): Y = 0.43/(0.94 x [AlN] + 0.92 x [NbCN]),
    (where [AlN] is the smaller of [Al] x 41/27 and [N] x 41/14, and [NbCN] is [Nb] x 104.9/ 92.9 and ([C] + ([N] x 12/14)) x 104.9/12, whichever is smaller, and in the above formulas 1 and 2 and these relational expressions , [Nb], [Al], [N] and [C] mean the contents (%) of Nb, Al, N and C, respectively.)
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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS60262941A (en) * 1984-06-08 1985-12-26 Daido Steel Co Ltd Steel for warm forging
JP2001303174A (en) * 2000-04-26 2001-10-31 Nippon Steel Corp Base shape stock for high temperature carburized parts excellent in crystal grain coarsening preventing characteristic and its producing method
JP2007321211A (en) * 2006-06-01 2007-12-13 Kobe Steel Ltd Hot-rolled material having superior properties of preventing crystal grain from coarsening when carburized at high temperature
JP2017078193A (en) * 2015-10-20 2017-04-27 トヨタ自動車株式会社 Method of producing forged material for carburization
JP2021154387A (en) * 2020-03-25 2021-10-07 愛知製鋼株式会社 Manufacturing method for forging material for carburization

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS60262941A (en) * 1984-06-08 1985-12-26 Daido Steel Co Ltd Steel for warm forging
JP2001303174A (en) * 2000-04-26 2001-10-31 Nippon Steel Corp Base shape stock for high temperature carburized parts excellent in crystal grain coarsening preventing characteristic and its producing method
JP2007321211A (en) * 2006-06-01 2007-12-13 Kobe Steel Ltd Hot-rolled material having superior properties of preventing crystal grain from coarsening when carburized at high temperature
JP2017078193A (en) * 2015-10-20 2017-04-27 トヨタ自動車株式会社 Method of producing forged material for carburization
JP2021154387A (en) * 2020-03-25 2021-10-07 愛知製鋼株式会社 Manufacturing method for forging material for carburization

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