JP2011094169A - Case hardening steel having excellent crystal grain coarsening prevention characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a case hardening steel in which the coarsening of crystal grains can be prevented even if surface hardening treatment is performed at ≥1,050°C after working such as warm forging and cold forging. <P>SOLUTION: The case hardening steel contains, by mass, 0.1 to 0.3% C, ≤2.5% (not including 0%) Si, 0.1 to 2.0% Mn, ≤0.03% (not including 0%) P, ≤0.1% (not including 0%) S, 0.30 to 2.0% Cr, 0.05 to 1.5% Mo, ≤0.1% (not including 0%) Al, 0.055 to 0.09% Nb, 0.055 to 0.09% Ti, ≤0.008% (not including 0%) N and ≤0.003% (not including 0%) O, and the balance iron with inevitable impurities, wherein carbide, nitride and carbonitride containing Ti, Nb and Mo with a diameter of the equivalent circle of ≤100 nm satisfy inequality (1) by the average composition, and further, the above carbide, nitride and carbonitride a with diameter of the equivalent circle of ≤100 nm are present in an amount of ≥1×10<SP>8</SP>pieces/mm<SP>2</SP>: 0.05≤[Mo]/([Nb]+[Ti])≤1.0...(1). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention provides surface hardening such as carburizing (gas carburizing, plasma carburizing, vacuum carburizing, high-concentration carburizing, etc.) and carbonitriding to improve fatigue characteristics etc. in transportation equipment such as automobiles, construction machinery, and other industrial machinery. Preventing grain coarsening that is useful as a material for mechanical structural parts that need to be treated to increase the hardness of the surface layer, especially gears (shaft gears, etc.), shafts, bearings, power transmission pulleys, constant velocity joints, etc. The present invention relates to case-hardened steel having excellent characteristics and machine structural parts.

  Among machine parts used in automobiles, construction machines, and other various industrial machines, parts that require wear resistance and high fatigue strength such as gears, such as SCr, SCM, SNCM, etc. defined by JIS standards. After the case-hardened steel is formed into a desired part shape by machining such as forging and cutting, surface hardening treatment such as carburizing and carbonitriding is performed, and then it is manufactured through a finishing process (cutting, polishing, etc.).

  In recent years, as an effort to reduce the cost of manufacturing parts, we have changed from cutting to forging in order to reduce the cost of machining, and warm forging that has high dimensional accuracy even in forging and can reduce cutting costs after forging And cold forging tends to be applied. However, warm forging and cold forging have a problem that crystal grains are coarsened during surface hardening treatment due to processing strain accumulated during forging. Further, for the surface hardening treatment, carburizing at a higher temperature is desired in order to shorten the carburizing time and improve productivity, and vacuum carburizing is used instead of conventional gas carburizing. Since vacuum carburizing can be performed at a high temperature exceeding 950 ° C, it has the advantage of shortening the carburizing time and preventing the occurrence of an abnormal carburizing layer on the surface of the part. There is a problem that the properties are deteriorated and the quenching strain is increased.

  Against such a background, steel materials that are not prone to crystal grain coarsening even after performing high-temperature surface hardening treatment (vacuum carburizing treatment, etc.) after forging such as warm forging and cold forging are strongly desired. ing.

  Conventionally, as a technique for preventing coarsening of crystal grains during carburization, for example, Patent Documents 1 to 6 mainly include elements such as Al, Nb, and Ti, and finely disperse these precipitates. Proposed. Patent Documents 1 and 2 all have B (boron) as an essential component, Patent Document 1 disperses Nb precipitates, Nb and Al composite precipitates, and Patent Document 2 Ti carbides, Ti composite carbides, and Ti nitrides. It is disclosed that crystal grains can be refined by dispersing a product. In Patent Document 3, Mo or Ti is used as an element that exhibits a pinning effect, and Mo carbide, Ti carbide, or Ti—Mo carbide is precipitated. Patent Document 4 discloses that carbides and carbonitrides containing Ti and Nb at the same time and Patent Documents 5 to 6 can precipitate (Nb, Ti) (CN) to prevent coarsening of crystal grains.

Japanese Patent No. 3480630 Japanese Patent Laid-Open No. 10-81938 JP 2006-9150 A JP 2006-28568 A JP 2006-307270 A JP 2006-307271 A

  The present invention has been made in view of the above circumstances, and the purpose thereof is to make crystal grains coarse even if surface hardening is performed at a temperature higher than 1050 ° C., which is higher than before, after processing such as warm forging and cold forging. An object of the present invention is to provide a case-hardened steel that can be prevented.

  Conventionally, as a precipitate having a pinning effect, Al, Nb, and Ti-based precipitates as shown in Patent Documents 1 to 6 described above have been used. However, as a result of repeated studies by the inventor, the conventionally proposed precipitate is not dissolved when the surface hardening treatment is performed at a high temperature of 1050 ° C. or higher, and a large precipitate is a small precipitate. As a result of so-called Ostwald growth that takes in and coarsens, it has been found that a sufficient effect of preventing coarsening of crystal grains cannot be exhibited.

  In recent years, however, high-precision near-net forming has been desired, and thus there is a strong need for cold strong working and high-temperature surface hardening treatment, and more advanced crystal grain coarsening prevention characteristics are required. Therefore, as a result of repeated studies by the present inventors, fine Ti, Nb, and Mo-containing carbides, nitrides, and carbonitrides, and the contents of Ti, Nb, and Mo in the precipitates are determined. If appropriately controlled ones (hereinafter referred to as “Ti—Nb—Mo-based precipitates”) are deposited, the precipitates themselves even when subjected to surface hardening treatment at a high temperature after warm forging or cold forging. It has been found that the effect of preventing the coarsening of crystal grains can be sufficiently exerted without melting or causing Ostwald growth. In addition, in order to form Ti—Nb—Mo based precipitates, the Ti, Nb, and Mo contents in the steel are appropriately controlled, and the cooling rate in the casting stage is slowed to reduce the Ti—Nb—Mo type. It was also revealed that it is effective to shorten the heating time without depositing the precipitate and heating it after that (heating before mass rolling, heating before hot rolling, etc.).

  In Patent Documents 1 and 4 to 6, when all of Ti, Nb, and Mo are added, the amount of Ti is small and 0.049% at the maximum. In Patent Document 4, the cooling rate during casting is particularly high. It is fast and the reheating temperature of the slab is high. In Patent Document 2, Ti is used in excess of 0.1 wt%, and workability (warm forgeability, cold forgeability, machinability, etc.) deteriorates, and all of Ti, Nb, and Mo are added. In the case, the amount of Nb is small. In Patent Document 3, to which Ti, Nb, and Mo are added, the amount of Nb is small, and the heating temperature before hot working is increased to 1100 ° C. or more to sufficiently dissolve the carbide, and the subsequent cooling process With this, fine precipitates are deposited.

The case-hardened steel excellent in crystal grain coarsening prevention characteristics of the present invention that has achieved the above-mentioned problems is C: 0.1 to 0.3% (meaning mass%, hereinafter the same), Si: 2.5%. Or less (excluding 0%), Mn: 0.1 to 2.0%, P: 0.03% or less (not including 0%), S: 0.1% or less (not including 0%), Cr: 0.30 to 2.0%, Mo: 0.05 to 1.5%, Al: 0.1% or less (excluding 0%), Nb: 0.055 to 0.09%, Ti: 0.055-0.09%, N: 0.008% or less (not including 0%), O: 0.003% or less (not including 0%), the balance being iron and inevitable impurities The carbide, nitride, and carbonitride containing Ti, Nb, and Mo having an equivalent circle diameter of 100 nm or less satisfy the following formula (1) with an average composition, and the equivalent circle diameter of 10 nm below the carbide, the number of nitrides, and carbonitrides are characterized by the presence 1 × 10 ⁸ pieces / mm ² or more.
0.05 ≦ [Mo] / ([Nb] + [Ti]) ≦ 1.0 (1)
(However, [Mo], [Nb], and [Ti] indicate the contents (mass%) of Mo, Nb, and Ti in the carbide, nitride, and carbonitride, respectively.)

  The case-hardened steel according to the present invention has, as necessary, (a) Cu: 0.3% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%), (b) Zr: 0.20% or less (not including 0%) and / or V: 0.20% or less (not including 0%), (c) Pb: 0.10% or less (not including 0%), Bi: 0.10% or less (excluding 0%) and Ca: at least one selected from the group consisting of 0.010% or less (not including 0%) may be included.

  The present invention also includes a machine structural component obtained from the above case-hardened steel.

  According to the present invention, fine Ti—Nb—Mo-based precipitates whose composition is appropriately controlled are deposited, and thus surface hardening treatment was performed at a high temperature of 1050 ° C. or higher after warm forging or cold forging. Even in this case, dissolution of the precipitate itself and Ostwald growth can be suppressed, and crystal grain coarsening during the surface hardening treatment can be prevented.

FIG. 1 is a transmission electron micrograph of a 900 ° C. normalized material in an example described later. FIG. 2 is a graph in which the intensity of characteristic X-rays is measured by EDX (energy dispersive X-ray analyzer) for the precipitates of 100 nm or less observed in FIG.

  The present invention is characterized in that fine Ti—Nb—Mo based precipitates are deposited. Hereinafter, the composition, size, and number of Ti—Nb—Mo based precipitates will be described.

Ti-Nb-Mo-based precipitates in the present invention are carbides, nitrides, and carbonitrides containing Ti, Nb, and Mo, and the contents of Ti, Nb, and Mo in the precipitates are as follows. (1) The expression is satisfied. More specifically, as will be described in the examples described later, when the composition of the precipitate is analyzed, at least one of C and N and all of Ti, Nb, and Mo are detected, and the precipitate is The Ti, Nb, and Mo contents satisfy the following formula (1).
0.05 ≦ [Mo] / ([Nb] + [Ti]) ≦ 1.0 (1)
(However, [Mo], [Nb], and [Ti] indicate the contents (mass%) of Mo, Nb, and Ti in the carbide, nitride, and carbonitride, respectively.)

Since the Ti—Nb—Mo based precipitate in the present invention is less likely to cause Ostwald growth of the precipitate at a high temperature than the conventionally proposed Ti—Nb based precipitate, The effect of preventing grain coarsening is exhibited effectively. This is considered to be because the diffusion rate of Mo in steel is slower than that of Ti and Nb. The diffusion rate in steel is Ti on the order of 10 ⁻⁵ m ² / s and Nb on the order of 10 ⁻⁴ m ² / s, whereas Mo is on the order of 10 ⁻⁶ m ² / s. Since the diffusion rate of Mo is slow, a drag effect due to Mo is caused. The drag effect is that when Ti or Nb is about to diffuse, Mo also diffuses to restore the equilibrium state. However, when Ti or Nb diffuses by dragging Mo, Mo having a low diffusion rate is resistant to the movement of Ti or Nb. Say that. It is considered that the Ostwald growth of Ti—Nb—Mo based precipitates can be suppressed by such a drag effect by Mo.

  When the value of [Mo] / ([Nb] + [Ti]) in the above formula (1) is less than 0.05, the above-described drag effect of Mo is not sufficient, so Ti—Nb at high temperatures. -Ostwald growth of Mo-based precipitates cannot be prevented. On the other hand, when the value of [Mo] / ([Nb] + [Ti]) exceeds 1.0, the melting point of the Ti—Nb—Mo based precipitate becomes low, and the Ti—Nb—Mo based at a high temperature. It becomes easy for the precipitate to dissolve into the substrate. The preferable lower limit of [Mo] / ([Nb] + [Ti]) is 0.10, the more preferable lower limit is 0.15, and the preferable upper limit is 0.90.

  In the present invention, the size of the precipitate is targeted for an equivalent circle diameter of 100 nm or less. The equivalent circle diameter means the diameter of a perfect circle having the same area for each precipitate. This is because precipitates exceeding the equivalent circle diameter of 100 nm (hereinafter simply referred to as “100 nm”) are insufficient in preventing the grain coarsening. The lower limit of the size of the Ti—Nb—Mo-based precipitate targeted in the present invention is not particularly limited, but is about 5 nm from the viewpoint of the transmission electron microscope (TEM) resolution and the magnification of the TEM photograph.

In the present invention, the number of Ti—Nb—Mo based precipitates having an equivalent circle diameter of 100 nm or less is 1 × 10 ⁸ pieces / mm ² or more. When the number of the precipitates is less than 1 × 10 ⁸ pieces / mm ² , the effect of preventing the coarsening of crystal grains is not sufficiently exhibited. The number of the precipitates is preferably 5 × 10 ⁸ pieces / mm ² or more, more preferably 2 × 10 ⁹ pieces / mm ² or more. The upper limit of the number of precipitates is not particularly limited, but is usually about 1 × 10 ¹⁰ pieces / mm ² .

  Below, the chemical component composition of the case hardening steel which concerns on this invention is described.

C: 0.1 to 0.3%
Since C is an element necessary for ensuring the core hardness of the mechanical structural component, the C content is set to 0.1% or more. The amount of C is preferably 0.13% or more, more preferably 0.15% or more. On the other hand, when the amount of C is excessive, the hardness of the steel material becomes higher than necessary, and the machinability and cold forgeability are lowered. Therefore, the amount of C is determined to be 0.3% or less. The amount of C is preferably 0.25% or less, more preferably 0.22% or less.

Si: 2.5% or less (excluding 0%)
Si is an element that greatly contributes to improving the softening resistance of the surface hardened layer. In order to effectively exhibit such action, the Si content is preferably 0.03% or more, more preferably 0.1% or more. On the other hand, if the amount of Si becomes excessive, the machinability and cold forgeability at the time of machining are remarkably deteriorated, so the Si amount is determined to be 2.5% or less. The amount of Si is preferably 2% or less, and more preferably 1.5% or less.

Mn: 0.1 to 2.0%
Mn acts as a deoxidizer during melting, reduces oxide inclusions and improves the internal quality of steel parts, and improves hardenability and increases the core hardness and hardened layer depth of steel parts. It is an effective element for ensuring the strength of parts. In order to exhibit such an action effectively, Mn was determined to be 0.1% or more. The amount of Mn is preferably 0.3% or more, more preferably 0.5% or more. On the other hand, when the amount of Mn becomes excessive, the amount of Mn is set to 2.0% or less in order to promote the segregation of P to the grain boundary to lower the grain boundary strength and consequently lower the low cycle fatigue strength. The amount of Mn is preferably 1.5% or less, more preferably 1.0% or less.

P: 0.03% or less (excluding 0%)
Since P is an element that promotes cracking during hot working, the amount of P is determined to be 0.03% or less. The amount of P is preferably 0.02% or less, and more preferably 0.015% or less. The smaller the amount of P, the better. However, since it causes an increase in the manufacturing cost of the steel material, it is difficult to make it 0%, and a residual of about 0.001% is allowed.

S: 0.1% or less (excluding 0%)
S induces anisotropy of impact strength by forming MnS which is an inclusion in steel. Therefore, the S amount is set to 0.1% or less. The amount of S is preferably 0.05% or less, more preferably 0.03% or less. The smaller the amount of S, the better. However, since it causes an increase in the manufacturing cost of the steel material, it is difficult to make it 0%, and a residual of about 0.001% is allowed.

Cr: 0.30 to 2.0%
Cr has the effect of enhancing the hardenability of the steel material and sufficiently securing a stable hardened layer depth and core hardness, and is important for ensuring static strength and fatigue strength as a structural member such as gears. It is an element. In order to effectively exhibit such an action, the Cr content is determined to be 0.30% or more. The amount of Cr is preferably 0.60% or more, and more preferably 0.80% or more. On the other hand, when the amount of Cr becomes excessive, segregation occurs at the prior austenite grain boundaries to form carbides, so that the low cycle fatigue strength decreases. Therefore, the Cr amount is set to 2.0% or less. The amount of Cr is preferably 1.8% or less, and more preferably 1.3% or less.

Mo: 0.05-1.5%
Mo is one of the most important elements in the present invention, and forms carbides, nitrides, carbonitrides together with Ti and Nb to act as pinning particles, and also ensures the hardenability of the steel material and an incompletely quenched structure. It has the effect | action which suppresses the production | generation of. In order to effectively exhibit such an action, the Mo amount is determined to be 0.05% or more. The amount of Mo is preferably 0.1% or more, and more preferably 0.15% or more. On the other hand, if the Mo amount becomes excessive, the core hardness of the steel part becomes higher than necessary, and the machinability and cold forgeability during machining deteriorate, so the Mo amount is set to 1.5% or less. . The amount of Mo is preferably 1% or less, and more preferably 0.5% or less.

Al: 0.1% or less (excluding 0%)
Al effectively acts as a deoxidizer during melting, and also has a function of preventing the coarsening of crystal grains during carburizing and quenching by forming fine nitrides. In order to effectively exhibit such an action, the Al amount is preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, when the amount of Al becomes excessive, non-metallic inclusions such as oxides increase and deteriorate toughness and machinability. Therefore, the Al content is set to 0.1% or less. The amount of Al is preferably 0.08% or less, and more preferably 0.05% or less.

Nb: 0.055-0.09%
Nb has the effect of preventing crystal grain coarsening during surface hardening at high temperatures by forming carbides, nitrides, and carbonitrides alone or together with Ti and Mo. In order to effectively exhibit such an action, the Nb amount is set to 0.055% or more. The Nb amount is preferably 0.060% or more, more preferably 0.065% or more. On the other hand, when the amount of Nb is excessive, coarse Nb carbonitride is generated during casting, which causes casting cracks and rolling cracks. Therefore, the Nb amount is set to 0.09% or less. The amount of Nb is preferably 0.080% or less, more preferably 0.075% or less.

Ti: 0.055-0.09%
Ti forms a carbide, nitride, and carbonitride alone or together with Nb and Mo, thereby preventing the grain coarsening during the surface hardening treatment at a high temperature. In order to effectively exhibit such an action, the Ti content is determined to be 0.055% or more. The amount of Ti is preferably 0.060% or more, and more preferably 0.065% or more. On the other hand, when the amount of Ti is excessive, coarse TiN inclusions are generated, and rolling fatigue characteristics and machinability are deteriorated. Therefore, the Ti amount is determined to be 0.09% or less. The amount of Ti is preferably 0.080% or less, and more preferably 0.075% or less.

N: 0.008% or less (excluding 0%)
N is an element that forms nitrides with other elements (Al, Ti, Nb, Mo, etc.) and contributes to the refinement of crystal grains, and the N amount is preferably 0.003% or more. On the other hand, if the amount of N is excessive, the hot workability and ductility are adversely affected, so the amount of N is set to 0.008% or less. The amount of N is preferably 0.006% or less.

O: 0.003% or less (excluding 0%)
O forms a hard oxide and greatly degrades the machinability, so it was determined to be 0.003% or less. The amount of O is preferably 0.002% or less, and more preferably 0.001% or less.

  The basic components of the case-hardened steel according to the present invention are as described above, and the balance is substantially iron. However, inevitable impurities (Mg, As, Sb, Sn, Te, Se, W, Ta, Co, rare earth elements, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. As a matter of course, it is allowed to be contained in steel as long as the properties are not impaired. Furthermore, the case hardening steel of this invention may contain the following arbitrary elements as needed.

Cu: 0.3% or less (excluding 0%)
Ni: 1.0% or less (excluding 0%)
Both Cu and Ni are effective elements for improving the weather resistance, and Ni also has an action of improving the toughness by solid solution in the matrix. In order to effectively exhibit such an action, both the Cu content and the Ni content are preferably 0.1% or more. On the other hand, if the amount of Cu becomes excessive, cracks and wrinkles occur on the steel surface during hot working (rolling, etc.), so the amount of Cu is preferably 0.3% or less, more preferably 0.25% or less. Further, when the amount of Ni becomes excessive, a bainite or martensite structure is generated during rolling and the toughness deteriorates, so the amount of Ni is preferably 1.0% or less, more preferably 0.8% or less. Cu and Ni may be added alone or in combination.

Zr: 0.20% or less (excluding 0%)
V: 0.20% or less (excluding 0%)
Both Zr and V are active elements such as carbon and nitrogen, and the crystal grain coarsening preventing property can be improved by forming fine precipitates. In order to effectively exhibit such an action, both the Zr amount and the V amount are preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Zr amount and V amount are excessive, cracks and wrinkles occur on the steel surface during hot working (for example, hot rolling), so both the Zr amount and V amount should be 0.20% or less. Is more preferable, and 0.15% or less is more preferable. Zr and V may be added alone or in combination.

Pb: 0.10% or less (excluding 0%)
Bi: 0.10% or less (excluding 0%)
Ca: 0.010% or less (excluding 0%)
Pb, Bi, and Ca are all elements that improve the machinability of the steel material. Furthermore, about Ca, it suppresses the expansion | extension of the sulfide in steel materials, improves an impact characteristic, and has the effect | action which suppresses the production | generation of coarse Ti sulfide and improves forgeability. In order to effectively exhibit such effects, the Pb amount and Bi amount are both preferably 0.02% or more, and more preferably 0.05% or more. The Ca content is preferably 0.0005% or more, and more preferably 0.001% or more. On the other hand, when the amount of Pb and Bi is excessive, the strength is reduced, and when the amount of Ca is excessive, coarse Ca oxide is generated, thereby reducing the strength. The Pb content and Bi content are both preferably 0.10% or less, more preferably 0.09% or less, and still more preferably 0.05% or less. The Ca content is preferably 0.010% or less, and preferably 0.005% or less. Pb, Bi, and Ca may be added alone or in combination of two or more.

  In order to control the amount of Ti-Nb-Mo-based precipitate within the scope of the present invention, in the series of manufacturing processes of melting, casting, block rolling, and hot rolling, particularly the cooling rate during casting, and thereafter It is preferable to control the heating conditions. More specifically, the cooling rate between 1000 to 700 ° C. (the surface center temperature of the slab) during casting is slowed down, and in the subsequent heating process (for example, heating before mass rolling, heating before hot rolling) It is effective to lower the heating temperature and shorten the holding time. Preferred conditions for each step are as follows.

Cooling rate between 1000 and 700 ° C. during casting (surface center temperature of slab): 15 ° C./min or less The cooling rate between 1000 and 700 ° C. is controlled by Ti—Nb—Mo in this temperature range. This is because system precipitates are deposited. When the cooling rate exceeds 15 ° C./min, the value of [Mo] / ([Nb] + [Ti]) in the Ti—Nb—Mo system precipitate becomes less than 0.05, and the Ti—Nb—Mo system The crystal grain coarsening preventing property due to the precipitate cannot be effectively exhibited. The cooling rate is preferably 13 ° C./min or less, more preferably 10 ° C./min or less. The lower limit of the cooling rate is not particularly limited, but is about 5 ° C./min in actual operation.

Heating temperature before split rolling 1200 ° C or less, holding time: 1 hour or less When heating temperature before split rolling exceeds 1200 ° C, and heating time exceeds 1 hour, it was generated during cooling during casting. As a result of the dissolution of fine Ti-Nb-Mo-based precipitates and the Ostwald growth, the number of fine Ti-Nb-Mo-based precipitates is insufficient, preventing grain coarsening during surface hardening treatment. The effect cannot be exhibited effectively. The lower limit of the heating temperature before the partial rolling is about 1100 ° C. from the viewpoint of deformability during the partial rolling.

Heating temperature before hot rolling: 1000 ° C. or less When the heating temperature before hot rolling exceeds 1000 ° C., Ti—Nb—Mo-based precipitates generated during cooling during casting grow Ostwald and are coarse. Precipitates cause steel material cracking during rolling and cracking during forging, and as a result of insufficient number of fine Ti-Nb-Mo-based precipitates, effectively preventing grain coarsening during surface hardening treatment I can't demonstrate it. The lower limit of the heating temperature before hot rolling is preferably about 850 ° C. from the viewpoints of rollability and surface quality of the rolled material.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  The steel of chemical composition shown in Table 1 was melted in accordance with a normal melting method, and after cooling at 1000 to 700 ° C. (surface center temperature of the slab) at the time of casting at the cooling rate shown in Table 2, the slab was It was reheated to the temperature equivalent to the partial rolling heating shown in Table 2, held for 1 hour, and forged into a 30 mm diameter steel bar. Next, in order to simulate the heating before hot rolling, after normalizing at a temperature of 900 ° C. and spheroidizing annealing at 760 ° C., a cylindrical compression test piece of φ12 mm × 18 mm was cut out from the steel bar.

(1) Measurement of crystal grain size The cylindrical compression test piece was compressed in the height direction at room temperature (compression rate: 60%, height: 7.2 mm), and then subjected to vacuum carburization under the carburizing conditions shown below. The particle size was measured. The crystal grain size is measured by etching with the carburized layer of the cross section of the test piece subjected to vacuum carburizing treatment at the location where the equivalent strain is 1.2, and observing with an optical microscope (magnification: 200 times), The particle size number of the prior austenite grains was determined according to JIS G0551. A grain size number of 7 or more was considered acceptable.

Carburizing conditions Carburizing period Temperature: 1050 ° C
Time: 10 minutes
Gas: Acetylene Diffusion period Temperature: 1050 ° C
Time: 10 minutes
Gas: Acetylene

(2) Measurement of Ti—Nb—Mo precipitate The measurement of the Ti—Nb—Mo precipitate will be described with reference to FIGS. FIG. 1 is a transmission electron micrograph of the above-mentioned normalizing material at 900 ° C., and FIG. 2 is an EDX (energy dispersive X-ray analyzer) for precipitates of 100 nm or less observed in the transmission electron micrograph. ) It is the graph which analyzed. A sample for a transmission electron microscope was prepared from a D / 4 position (D is a diameter) in a cross section perpendicular to the axis of the above-mentioned 900 ° C. normalizing material, and observed at an observation magnification of 100,000 times (FIG. 1). . Arbitrary five of the precipitates of 100 nm or less are selected and the characteristic X-ray intensity is measured with an EDX (energy dispersive X-ray analyzer) (FIG. 2). Was semi-quantified to mass% with the alloying elements contained, and the contents of C, N, Ti, Nb, and Mo in the precipitate were calculated. As a result, the experiment No. In any of the experimental examples except 17, at least one of C and N was detected from all the five selected precipitates. Furthermore, [Mo] / ([Nb] + [Ti]) was calculated to determine the average value of the five precipitates. The average composition of precipitates of 100 nm or less in the present invention can be empirically represented by the average composition of any five precipitates of 100 nm or less. Therefore, for the value of [Mo] / ([Nb] + [Ti]), the average value of the five precipitates is the same as that of the Ti—Nb—Mo based precipitate having an equivalent circle diameter of 100 nm or less in the present invention. The average value of Mo] / ([Nb] + [Ti]) was used.

Next, the number of precipitates having a circle-equivalent diameter of 100 nm or less was measured by observing three fields of view with a measurement field of view 0.81 μm ² at an observation magnification of 100,000, and the average of the three fields was defined as the number of precipitates of 100 nm or less. Here, as described above, since the average composition of precipitates of 100 nm or less in the present invention can be represented by the composition of any five precipitates of 100 nm or less, the number of precipitates of 100 nm or less measured here is , The number of precipitates having an average composition calculated from any of the above five.

  The results are shown in Table 2.

  Experiment No. 2 in Table 2 1 to 16, since the chemical composition and production conditions of steel are appropriately controlled, the value of [Mo] / ([Nb] + [Ti]) of a Ti—Nb—Mo based precipitate of 100 nm or less, and The number satisfies the requirements defined in the present invention, and the crystal grains after high-temperature carburization can be made fine.

  On the other hand, Experiment No. Examples 17 to 25 are examples in which the crystal grains after high-temperature carburization are coarsened because either the chemical components of steel or the production conditions are inappropriate.

  Experiment No. No. 17 is an example in which Ti and Nb are not contained, and Ti—Nb—Mo-based precipitates are not generated, so that the crystal grains after carburization become coarse. Experiment No. In No. 17, since Ti and Nb are not contained, Ti—Nb—Mo-based precipitates cannot exist, so the number of precipitates is not counted.

  Experiment No. 18 is an example in which the amount of Mo was small, and the value of [Mo] / ([Nb] + [Ti]) in a Ti—Nb—Mo based precipitate of 100 nm or less was small, and the drag effect by Mo was sufficient. As a result, the crystal grains after carburization became coarse.

  Experiment No. No. 19 is an example in which the amount of Nb and Ti was small, and the number of Ti—Nb—Mo-based precipitates of 100 nm or less was insufficient, so that the crystal grains after carburization became coarse.

  Experiment No. No. 20 is an example in which the amount of Ti was small, and the value of [Mo] / ([Nb] + [Ti]) was increased, and the Ti—Nb—Mo based precipitates melted during carburizing, so the number of precipitates was insufficient. Later crystal grains became coarse.

  Experiment No. No. 21 is an example in which the amount of Mo was large, and the value of [Mo] / ([Nb] + [Ti]) in the Ti—Nb—Mo-based precipitates of 100 nm or less was increased, and Ti—Nb—Mo during carburization. Since the system precipitates were dissolved in the substrate, the crystal grains after carburization became coarse.

  Experiment No. No. 22 is an example in which the cooling rate of the slab was high, and the value of [Mo] / ([Nb] + [Ti]) in the Ti—Nb—Mo-based precipitates of 100 nm or less became small, and after carburizing The crystal grains became coarse.

  Experiment No. No. 23 is an example in which the temperature corresponding to cracking and heating was high, and the number of Ti—Nb—Mo-based precipitates of 100 nm or less was insufficient, so that the crystal grains after carburization became coarse.

  Experiment No. 24 and 25 are examples in which the amount of Nb or Ti was large, and the value of [Mo] / ([Nb] + [Ti]) in the Ti—Nb—Mo-based precipitates of 100 nm or less became small, and after carburizing The crystal grains became coarse.

Claims (5)

C: 0.1 to 0.3% (meaning mass%; hereinafter the same),
Si: 2.5% or less (excluding 0%),
Mn: 0.1 to 2.0%,
P: 0.03% or less (excluding 0%),
S: 0.1% or less (excluding 0%),
Cr: 0.30 to 2.0%,
Mo: 0.05-1.5%,
Al: 0.1% or less (excluding 0%),
Nb: 0.055 to 0.09%,
Ti: 0.055-0.09%,
N: 0.008% or less (excluding 0%),
O: 0.003% or less (excluding 0%), the balance being iron and inevitable impurities,
The carbide, nitride, and carbonitride containing Ti, Nb, and Mo having an equivalent circle diameter of 100 nm or less satisfy the following formula (1) with an average composition,
A case hardening steel excellent in crystal grain coarsening prevention characteristics, wherein the number of carbides, nitrides, and carbonitrides having an equivalent circle diameter of 100 nm or less is 1 × 10 ⁸ pieces / mm ² or more.
0.05 ≦ [Mo] / ([Nb] + [Ti]) ≦ 1.0 (1)
(However, [Mo], [Nb], and [Ti] indicate the contents (mass%) of Mo, Nb, and Ti in the carbide, nitride, and carbonitride, respectively.) Furthermore,
The case hardening steel according to claim 1, containing Cu: 0.3% or less (not including 0%) and / or Ni: 1.0% or less (not including 0%). Furthermore,
The case hardening steel according to claim 1 or 2, containing Zr: 0.20% or less (not including 0%) and / or V: 0.20% or less (not including 0%). Furthermore,
Pb: 0.10% or less (excluding 0%),
Bi: 0.10% or less (not including 0%), and Ca: 0.010% or less (not including 0%)
The case hardening steel in any one of Claims 1-3 containing the at least 1 sort (s) chosen from the group which consists of.   A machine structural component obtained from the case-hardened steel according to any one of claims 1 to 4.