JP2005281780A - Heat-treated article, and heat treatment method for it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carburized or carbonitrided component having no flake defect by minimizing a martensite area rate, by keeping the component at a constant temperature in such a temperature range as to surely control the structure to ferrite plus pearlite, for a comparatively short time. <P>SOLUTION: This heat treatment method comprises: carburizing or carbonitriding a component; then, when cooling it, keeping it constant at such a temperature T[°C] as to satisfy T=(620-640)+(2.0Cr+0.3Mo+0.5Ni+0.1Mn)×5, and for such a period of time t[hr] as to satisfy t≥0.8+exp(4.0Cr+1.7Mo+0.5Ni+0.1Mn-2.9), wherein Cr, Mo, Ni and Mn represent a content ratio (wt.%) of respective elements in each alloy steel; and quench-hardening it. The heat-treated article has thus formed metallographic structure. The heat-treated article is made of an alloy steel having a composition comprising, by wt.%, 0.18-0.50% C, 0.30-3.50% Ni, 0.30-1.00% Cr, 0.20-0.50% Mo and 0.20-1.50% Mn. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a machine part requiring high fatigue strength, such as used in automobiles, railway vehicles, agricultural machines, construction machines, steel machines, etc., and in particular carburizing or carburizing used in bearings, gears, shafts, etc. The present invention relates to a heat-treated product of alloy steel subjected to nitriding treatment and a heat treatment method thereof.

  For mechanical parts that require wear resistance, fatigue strength, toughness, etc., in order to improve functionality, alloy steels with a large amount of alloy elements such as Cr, Mn, Ni, and Mo are generally used. used. These alloy elements have an effect of improving hardenability and preventing formation of an incompletely hardened structure during quenching. Further, machine parts that require particularly high wear resistance and fatigue strength, such as bearings and gears, are subjected to carburizing or carbonitriding surface hardening treatment on alloy steel.

  Prevents the occurrence of cracks caused by hydrogen, that is, white spot defects, in parts that have been carburized or carbonitrided for a long time on alloy steel to which a large amount of alloy elements such as Cr, Mn, Ni, and Mo have been added. is important. A white spot defect is an internal defect in the form of a crack, which occurs upon cooling after heat treatment or when left at room temperature, and white spots are observed when the cross section of the fracture surface is observed.

A method for preventing white spot defects is disclosed in Patent Document 1. Its feature is that the metal structure is made ferrite + pearlite and the amount of martensite is reduced by performing constant temperature holding in the temperature range from the A1 transformation point to the Ms point after carburizing or carbonitriding.
JP-A-10-168515

However, in the steel types containing a large amount of the above alloy elements, the isothermal transformation diagram (TTT diagram) becomes a double S curve (see FIG. 1), and if it is held at A1 to TPB at a constant temperature, it becomes a ferrite + pearlite structure. A bainite structure is obtained if the temperature is kept constant at the ~ Ms point. That is, there is a problem that even if a constant temperature is maintained at the A1 transformation point to the Ms point, which is the temperature range disclosed in Patent Document 1, depending on the temperature, the ferrite + pearlite structure is not necessarily obtained.
Moreover, since the constant temperature holding time shown in Patent Document 1 is very long, there are many industrial inconveniences such as longer delivery time and cost increase due to longer operating time.

  The present invention is a carburized or carbonitrided part in which the martensite area ratio is zero as much as possible by maintaining the temperature at a relatively short time in a temperature range in which a ferrite + pearlite structure can be reliably obtained, and no white spot defects are present. The purpose is to provide.

The present invention is as follows.
1) C: 0.18 to 0.50 wt%, Ni: 0.30 to 3.50 wt%, Cr: 0.30 to 1.00 wt%, Mo: 0.20 to 0.50 wt%, In steel containing Mn: 0.20 to 1.50% by weight of alloy elements,
During cooling after carburizing or carbonitriding,
T = (620 to 640) + (2.0Cr + 0.3Mo + 0.5Ni + 0.1Mn) × 5
a time t [hr] at which t ≧ 0.8 + exp (4.0Cr + 1.7Mo + 0.5Ni + 0.1Mn-2.9);
A heat-treated product characterized by having a metal structure formed by quenching and hardening by performing a heat treatment for holding at constant temperature.
In the above formula, Cr, Mo, Ni, and Mn represent numerical values of the respective contents (% by weight).

  2) The heat-treated product as set forth in (1), wherein the product is kept at a constant temperature after carburizing or carbonitriding, and then immediately heated and quenched.

3) C: 0.18 to 0.50 wt%, Ni: 0.30 to 3.50 wt%, Cr: 0.30 to 1.00 wt%, Mo: 0.20 to 0.50 wt%, Mn: at the time of cooling after carburizing or carbonitriding of steel containing 0.20 to 1.50% by weight of alloy elements,
T = (620 to 640) + (2.0Cr + 0.3Mo + 0.5Ni + 0.1Mn) × 5
a time t [hr] at which t ≧ 0.8 + exp (4.0Cr + 1.7Mo + 0.5Ni + 0.1Mn-2.9);
A method for heat treatment of steel, characterized by maintaining a constant temperature.
In the above formula, Cr, Mo, Ni, and Mn represent numerical values of the respective contents (% by weight).

  According to the heat-treated product and the heat-treatment method of the present invention, in the heat-treated product manufactured by applying the surface hardening method by carburizing or carbonitriding, the occurrence of white spot defects, which has been a problem in the past, has occurred in a short time. This can be surely prevented, and the toughness can be improved by improving the structure, so that the life of the machine part can be extended.

Long-term carburized products have a large amount of hydrogen intruding, increasing the risk of white spot defects. Hydrogen is an interstitial element that diffuses very quickly and is discharged to some extent during cooling after heat treatment. However, if the cooling rate is extremely high, hydrogen cannot be discharged in time, and high-concentration hydrogen remains in the steel. Resulting in.
The occurrence of white spot defects is mainly due to the penetration of hydrogen into the steel. However, white spot defects do not necessarily occur even when high concentration hydrogen is charged in the steel, but in the steel. It does not occur unless an amount of hydrogen is charged and external stress is applied, for example, internal stress such as martensitic transformation is not applied.

  The white spot defect depends on the structure, and when martensite is mixed in the ferrite + pearlite structure, the susceptibility to cracking is most enhanced. (Refer to the supplementary edition, steel and alloy elements (above), Japan Society for the Promotion of Science, 19th Committee, edited by Seikodo Shinkosha, p. 452). In particular, the presence of untransformed austenite (residual austenite) due to segregation of alloying elements makes this austenite a barrier to hydrogen diffusion, hindering the discharge of hydrogen to the outside and facilitating the trapping of hydrogen. The hydrogen discharged during the transformation is concentrated in the austenite. This austenite undergoes martensitic transformation by cooling, but hydrogen is difficult to be purged to the outside in martensite, so that the diffusion of hydrogen is slow, and combined with the action of transformation stress, the sensitivity to white spot defects becomes high.

Therefore, in order to prevent white spot defects in carburized and carbonitrided parts, hydrogen that has entered the steel during carburizing or carbonitriding may be discharged to the outside before cooling to room temperature.
The hydrogen concentration in steel, which increases the risk of white spot defects, is 4 ppm or more (see supplementary edition, steel and alloying elements (above), Japan Society for the Promotion of Science, 19th Committee of Steel Making, Sebundo Shinkosha, page 448) Therefore, the amount of hydrogen needs to be 4 ppm or less, and is 2 ppm or less (more preferably 1 ppm or less) in anticipation of safety.

  Furthermore, in order to prevent white spot defects, it is necessary to eliminate the transformation stress by eliminating martensite. In other words, eliminating martensite means eliminating hydrogen trap sites, that is, reducing the amount of hydrogen in the steel.

  Next, further details will be described with reference to the drawings. FIG. 2 shows the relationship between the solubility of hydrogen in steel and the temperature. As is clear from FIG. 2, the solubility of hydrogen is high at 5 ppm or more in the austenite temperature range, but the solubility is significantly reduced to 3.5 ppm or less in the ferrite (non-carburized part) temperature range. That is, maintaining the temperature in the ferrite temperature range after the carburization or carbonitriding at a constant temperature means that the hydrogen charged in the steel can be discharged in a shorter time.

  FIG. 3 shows the relationship between the temperature and the permeation amount of hydrogen in steel (corresponding to the permeation rate). As apparent from FIG. 3, the permeation rate at a temperature below the A1 transformation point is higher than the temperature above the A1 transformation point, which is the carburizing temperature. This is because the diffusion speed of hydrogen depends on the structure of the steel, the diffusion is faster in the ferrite + pearlite structure than in the austenite structure, and is charged in the steel by holding it below the A1 transformation point. This means that hydrogen can be discharged efficiently.

  In addition, in steel in which band-shaped martensite is formed when it is allowed to cool without being kept at a constant temperature after carburizing, this martensite inhibits hydrogen diffusion, and hydrogen concentration is likely to be trapped. In addition, the highest risk of white spot defects due to internal stress due to martensitic transformation. In such steel, when heat treatment is performed to keep the temperature constant after carburizing or carbonitriding, martensite, which is a site where hydrogen is trapped, can be reduced, the hydrogen concentration can be reduced, and stress can be prevented from being generated. It is particularly suitable for preventing any point defects.

  FIG. 1 schematically shows an isothermal transformation diagram (TTT diagram). The TTT diagram shows the structural change when supercooled austenite is held at a constant temperature. There are two types of alloy steel TTT diagrams: (a) one having a pair of noses (nose) and bays (one-stage transformation type), and (b) one having two pairs of noses and bays (two Stage transformation). (A) causes pearlite transformation at a temperature higher than the nose and bainite transformation at a temperature lower than the nose. In (b), the nose and bay in a relatively high temperature zone are due to pearlite transformation, and the other nose and bay exhibit bainite transformation.

  The standard for dividing the shape of the TTT diagram into (a) and (b) depends on the type of alloy element contained in the steel. That is, (b) is shown for steel containing Cr, Mo, V, W, Ti, Nb, B. Note that these alloy elements need to be dissolved in heated austenite. When the austenitizing temperature is low, the alloy elements are not so much dissolved and are released as carbides or carbonitrides. No effect on the figure (handbook of heat treatment technology, edited by Japan Heat Treatment Technology Association, Nikkan Kogyo Shimbun).

  That is, according to the present invention, the steel containing the carbide and carbonitride-forming element is austenitized at a temperature equal to or higher than A3 (carburizing temperature) or Acm (carbonitriding temperature), and immediately maintained at A1 to TPB. Thus, a ferrite + pearlite structure is formed, and hydrogen in steel is discharged to the outside while maintaining a constant temperature, and the martensite volume ratio, which is a hydrogen trap site, is made zero as much as possible, thereby eliminating white spot defects.

In order to achieve such an object, the heat-treated product according to the above 1) becomes T = (620 to 640) + (2.0Cr + 0.3Mo + 0.5Ni + 0.1Mn) × 5 upon cooling after carburizing or carbonitriding. At a temperature T [° C.], a time t [hr] at which t ≧ 0.8 + exp (4.0Cr + 1.7Mo + 0.5Ni + 0.1Mn-2.9) is applied, and a metal structure formed by quenching and hardening is subjected to heat treatment for holding at a constant temperature. It is characterized by having. In the above formula, Cr, Mo, Ni, and Mn represent numerical values of the content (% by weight) in each alloy steel.
In this case, the heat-treated product has a weight percentage of C: 0.18 to 0.50%, Ni: 0.30 to 3.50%, Cr: 0.30 to 1.00%, Mo: 0 Alloy steel containing 20 to 0.50% and Mn: 0.20 to 1.50%.

  Also, after carburizing or carbonitriding, the martensite area ratio of the heat-treated product maintained at a constant temperature is preferably 0.1% or less and the hydrogen concentration is preferably 2 ppm or less, more preferably the martensite area ratio of the heat-treated product is 0%, hydrogen The concentration is preferably 1 ppm or less.

The present invention will be described below with reference to the drawings.
First, a preferred heat treatment condition range will be described.

[Constant temperature holding temperature after carburizing or carbonitriding]
The constant temperature holding temperature was examined from the viewpoint of preventing white spot defects. From the relationship between the temperature shown in FIG. 3 and the hydrogen permeation amount (permeation rate), the hydrogen permeation rate takes the largest value immediately below the A1 transformation point. From the viewpoint of the efficiency of hydrogen discharge, the constant temperature holding temperature is A1. Directly below the transformation point is most suitable. When the holding temperature is 600 ° C. or lower, the diffusion rate of hydrogen is slower than the carburizing temperature of 900 to 950 ° C. and hydrogen is not discharged effectively.

  On the other hand, when viewed from the relationship between the temperature and the holding time shown in the TTT diagram of FIG. 1, the temperature range (TPB to A1) in which pearlite transformation can occur, which is indicated by hatching in the figure, can pass through the Pf line. A pearlite structure is obtained in the time domain. In particular, the pearlite structure can be obtained in the shortest time at the nose temperature (TPP), and the required holding time increases as the temperature becomes higher or lower than the nose temperature.

  From the above, the constant temperature holding temperature for preventing white spot defects is preferably the TPB to A1 transformation point, and the nose temperature TPP is the best. In the steel containing the above alloy elements, it is desirable to hold at a temperature in the range of (620 to 640) + (2.0Cr + 0.3Mo + 0.5Ni + 0.1Mn) × 5.

[Constant temperature retention time]
The time for completing the isothermal transformation of steel is usually delayed by the addition of alloying elements. Here, as a result of investigations by paying attention to four elements of Cr, Mo, Ni, and Mn that delay pearlite transformation as additive elements affecting the isothermal transformation characteristics, the present inventors have found that the degree of transformation delay is Cr> Mo. >Ni> Mn It is experimentally found that the transformation completion time when the temperature is maintained in the constant temperature holding temperature range can be determined by a calculation formula using the contents of these elements as parameters, and the following formula (1) Shown in
t [hr] ≧ 0.8 + exp (4.0Cr + 1.7Mo + 0.5Ni + 0.1Mn-2.9) (1)

  From the previous formula, the minimum required constant temperature holding time can be determined from the Cr, Mo, Ni, and Mn contents. In addition, if heat treatment that satisfies the minimum constant temperature holding time t (Hr) determined by the equation (1) is performed, even a mechanical part using a material in which crystal grains are not refined in the case of secondary quenching can be refined. It is possible to improve mechanical properties, for example, when used for a rolling bearing, the life of the rolling bearing can be extended.

[Cooling method, cooling rate]
If the cooling rate is too slow during cooling from the carburizing or carbonitriding temperature to the constant temperature holding temperature, carbon is spouted from the austenite base, and cementite precipitates in a network form along the austenite grain boundary, and austenite in the vicinity of the network carbide. The carbon concentration in it decreases. When the temperature falls below the A1 transformation point in this state, pearlite is generated from austenite by eutectoid transformation, but the carbides do not precipitate in the layer because of the carbide nuclei, and the network carbides become coarse. It progresses preferentially, and since the carbon concentration is reduced in the vicinity of the mesh-like carbide, it becomes ferrite. Even if reheating is performed to the austenite temperature range by secondary quenching treatment from this state, a part of the reticulated carbide is re-dissolved in the matrix, but because the carbon concentration in the austenite near the reticulated carbide is low, it is sufficiently spherical. Precipitation of cementitized cementite does not occur, and a deficient region of spheroidized cementite is formed, resulting in a decrease in hardness. In order to prevent this, it is preferable to cool to a predetermined constant temperature holding temperature at a rate of at least 2 to 3 ° C./min or more so as to suppress grain boundary pro-eutectoid cementite precipitation.

  In consideration of productivity, it is preferable to quickly cool at a cooling rate of about 20 ° C./min. Therefore, the cooling rate is 2 to 20 ° C./min, preferably 10 to 20 ° C./min.

  In addition, when the temperature falls below a predetermined constant temperature holding temperature before entering the constant temperature holding stage and falls below the Ms point of the core (non-carburized part), martensite is formed and a large amount of hydrogen is generated. Since it is in the remaining state, there is an increased risk that both cool-down cracks and white spot defects will occur. Therefore, it is necessary to avoid a temperature drop below the Ms point of the core before performing the constant temperature maintenance after carburizing or carbonitriding. In addition, since the pearlite transformation has already been completed by holding the constant temperature after the constant temperature holding, the cooling rate at the time of cooling to room temperature can be cooled at an arbitrary cooling rate.

Next, the limited range of the chemical composition of steel will be described. In addition,% is weight%.
C: When the content is less than 0.18%, the carburization or carbonitriding time is increased and the heat treatment productivity is deteriorated. On the other hand, when the content exceeds 0.50%, the toughness is greatly reduced. 18% to 0.50%.

  Cr: Cr improves hardenability and solid-solution strengthens the base, and carbide, nitride and carbonitride are deposited on the surface layer by carburizing or carbonitriding to improve mechanical properties, for example rolling in rolling bearings Since it is useful for improving the fatigue life, the lower limit of the Cr content is set to 0.30%. If it is 0.30% or less, the effect of addition is small. Further, when added in a large amount, Cr oxide is formed on the surface, which inhibits the intrusion of carbon and nitrogen from the surface during carburizing or carbonitriding and lowers the heat treatment productivity, so the Cr content is 0.30. -1.00%.

  Mn: Since Mn is an element that improves hardenability and forms retained austenite (effective for rolling life in the presence of foreign matter), the content is set to 0.20% or more. However, Mn is an element that reinforces the ferrite of the material, and if the content is too large, the cold workability is remarkably lowered, so the Mn content is 0.20 to 1.50%, and 0.40 to 0 70% is preferable.

  Ni: Ni improves the hardenability of steel and increases toughness. The effect becomes remarkable when 0.30% or more is added. However, even if the content exceeds 3.50%, the effect is not improved, and excessive addition increases the cost. Therefore, the Ni content is set to 0.30 to 3.50%, and 1.50 to 2%. .50% is preferable.

  Mo: Mo increases the hardenability and temper softening resistance of steel, and precipitates carbides, nitrides, and carbonitrides on the surface layer by carbonitriding to improve the hardness of the material. Further, there is an effect of delaying the pearlite transformation and dividing the transformation region into two stages in the TTT diagram. The effect becomes significant when the content is 0.20% or more. However, the delay of Ar ′ transformation is strongest until the content is up to 0.50%, and since the TTT diagram does not change much beyond this, the Mo content is 0.20 to 0.50%. And preferably 0.20 to 0.40%.

Hereinafter, the present invention will be described by way of examples.
(Examples and Comparative Examples)
Heat treatment was performed under the following conditions A to E for 200 NU240-equivalent ring materials made of steel types 1 to 6 having the compositions shown in Table 1. Table 2 shows the heat treatment conditions.

[Heat Treatment A]
Carburizing in an atmosphere of RX gas + enriched gas for 20 hours at 950 ± 10 ° C., then furnace cooling to 550 ° C. in a nitrogen gas atmosphere in the furnace, air cooling to room temperature outside the furnace, and again in the furnace Is heated to 840 ± 10 ° C. and quenched, and further tempered at 180 ± 10 ° C. for 2 hours.

[Heat treatment B]
Carburizing was performed at 950 ± 10 ° C. for 20 hours in an RX gas + enriched gas atmosphere, then kept at 640 ± 10 ° C. for 10 hours in a nitrogen gas atmosphere, air-cooled to room temperature, and then again 840 ± 10 in the furnace. It is heated to 0 ° C. and quenched, and then tempered at 180 ± 10 ° C. for 2 hours.

[Heat treatment C]
Carburizing was performed at 950 ± 10 ° C. for 20 hours in an RX gas + enriched gas atmosphere, kept at 640 ± 10 ° C. for 2 hours in a nitrogen gas atmosphere, air-cooled to room temperature, and then again 840 ± 10 in the furnace. It is heated to 0 ° C. and quenched, and then tempered at 180 ± 10 ° C. for 2 hours.

[Heat treatment D]
After carburizing in an atmosphere of RX gas + enriched gas for 20 hours at 950 ± 10 ° C., holding at 640 ± 10 ° C. for 10 hours in a nitrogen gas atmosphere, heating to 840 ± 10 ° C. again, quenching is performed, Next, tempering is performed at 180 ± 10 ° C. for 2 hours.

[Heat Treatment E]
After carbonitriding in an atmosphere of RX gas + enrich gas + ammonia gas 7% at 950 ± 10 ° C. for 20 hours, kept constant at 640 ± 10 ° C. for 10 hours in a nitrogen gas atmosphere, air cooled to room temperature, and again It is heated to 840 ± 10 ° C. in a furnace, quenched, and then tempered at 180 ± 10 ° C. for 2 hours.
Here, heat treatments A and C are comparative examples, and heat treatments B, D, and E are examples.

About the ring material which heat-processed AE, while measuring the hydrogen concentration in steel by the method mentioned later, the martensite area ratio was investigated from structure | tissue observation. Microscopic observation was also performed on the sample before quenching after completion of constant temperature holding. In addition, fatigue strength was investigated by a fatigue test. As shown in FIG. 4, the fatigue strength tester includes a load roll 2 and a drive roll 3 that rotatably hold a ring material (test piece) 1, and a support roll 4 that supports the outer periphery of the ring material 1. Prepare. In the fatigue test, the rotational speed of the drive roll 3 was N = 1000 rpm, and 1 × 10 ⁶ times was set as the fatigue limit. Table 3 summarizes the results of various measurements.

  A method for measuring the hydrogen concentration in steel will be described. 100 ring materials (φ30 × 10 mm) having been maintained at B to E were prepared, and 50 of them were immediately stored in a dewar bottle containing dry ice. These test pieces were surface-polished, degreased and dried with cold air before hydrogen measurement was performed, and the measurement was started within 1 hour after the test pieces were taken out from the dewar. For the remaining 50 test pieces, a cross-section including the central axis was cut out, and the martensite area ratio was examined by structure observation using an optical microscope (the ratio of the area occupied by martensite to the total cross-sectional area was determined).

Hydrogen was measured by a vacuum heating method. The measurement conditions are shown below.
Heating furnace: Infrared image furnace Heating temperature: Room temperature to 700 ° C
Temperature increase rate: 10 ° C / min

The results of Table 3 will be described for each heat treatment condition.
In Table 3, the martensite area ratio indicates the ratio of martensite to the entire metal structure after completion of the isothermal transformation.

  In heat treatment A (comparative example), there is no constant temperature holding process, so hydrogen is not sufficiently discharged, and the entire structure is martensite by cooling after carburization, and white spot defects and cooling The risk of both cracking is extremely high. In addition, since no constant temperature is maintained, the martensite structure is not refined during the secondary quenching, so that the fatigue strength is low for all steel types.

  In the heat treatment B (Example), since the minimum constant temperature holding time obtained from the above-described equation (1) is in the range of 1.3 to 9.3 [hr], the actual constant temperature holding time is less than 10 hours. In any of the steel types 1 to 6, the hydrogen concentration is as low as 1 ppm or less, the martensite area ratio is 0%, and there is no possibility of occurrence of white spot defects. In addition, high fatigue strength is shown by the effect of refining the martensite structure during secondary quenching.

  In the heat treatment C (comparative example), the minimum isothermal holding time calculated from the formula (1) is in the range of 4.2 to 9.3 [hr], and is larger than the actual isothermal holding time of 2 hours. In the holding time of 2 hours, the high-temperature transformation is not completed, so a large amount of martensite is formed, and the hydrogen concentration shows a high value, so that there is a high risk of white spot defects. Further, although the fatigue strength is low, this is because the high-temperature transformation has not been completed, and the refinement of the martensite structure in the secondary quenching process has not sufficiently progressed.

  In the heat treatment D (Example), both steel types have a low hydrogen concentration level and the martensite area ratio is 0% for the same reason as the heat treatment B described above, and white spot defects can occur. There is no sex.

  In the heat treatment E (Example), for the same reason as the heat treatment condition B described above, no cracking was observed in any of the steel types even in the case of carbonitriding treatment, indicating high fatigue strength. The heat treatment method according to the present invention is extremely effective in preventing cracking and improving the structure not only in carburizing but also in carbonitriding.

It is a schematic diagram of an isothermal transformation diagram (TTT diagram). It is drawing which shows the relationship between the hydrogen solubility in steel, and temperature. It is drawing which shows the relationship between temperature and the permeation | transmission amount of hydrogen in steel. It is drawing which shows the outline | summary of a fatigue testing machine.

Explanation of symbols

1 Test piece 2 Load roll 3 Drive roll 4 Support roll

Claims (3)

C: 0.18 to 0.50 wt%, Ni: 0.30 to 3.50 wt%, Cr: 0.30 to 1.00 wt%, Mo: 0.20 to 0.50 wt%, Mn: In steels containing 0.20 to 1.50% by weight of alloying elements,
During cooling after carburizing or carbonitriding,
T = (620 to 640) + (2.0Cr + 0.3Mo + 0.5Ni + 0.1Mn) × 5
a time t [hr] at which t ≧ 0.8 + exp (4.0Cr + 1.7Mo + 0.5Ni + 0.1Mn-2.9);
A heat-treated product characterized by having a metal structure formed by quenching and hardening by performing a heat treatment for holding at a constant temperature.
In the above formula, Cr, Mo, Ni, and Mn represent numerical values of the respective contents (% by weight).   The heat-treated product according to claim 1, wherein after the carburizing or carbonitriding treatment, the temperature is maintained, and then the temperature is immediately raised and quenching is performed. C: 0.18 to 0.50 wt%, Ni: 0.30 to 3.50 wt%, Cr: 0.30 to 1.00 wt%, Mo: 0.20 to 0.50 wt%, Mn: During cooling after carburizing or carbonitriding of steel containing 0.20 to 1.50% by weight of alloy elements,
T = (620 to 640) + (2.0Cr + 0.3Mo + 0.5Ni + 0.1Mn) × 5
a time t [hr] at which t ≧ 0.8 + exp (4.0Cr + 1.7Mo + 0.5Ni + 0.1Mn-2.9);
A method for heat treatment of steel, characterized by maintaining a constant temperature.
In the above formula, Cr, Mo, Ni, and Mn represent numerical values of the respective contents (% by weight).

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