JP6752624B2 - Manufacturing method of carburized steel - Google Patents

Description

The invention according to the present application relates to a method for producing carburized steel, which makes it possible to suppress the generation of abnormal grains during carburizing treatment as much as possible.

Carburized steel has been applied to automobile parts, industrial machine parts, etc., which are conventionally required to have high fatigue characteristics. There are hot forging and cold forging as processing methods for carburized steel, but nowadays, there is a high need for cold forging from the viewpoint of cost. When this cold forging is performed, first, in order to improve the workability, the carburized steel material is softened and annealed. As this softened annealing, spheroidized annealing is mainly adopted. The softened and annealed steel material is formed by cold forging and then surface-hardened by carburizing and quenching.

In this case, strain energy due to cold forging, during carburization heating at a temperature below A _c1 point, acts as the driving force for recrystallization, tissues such as ferrite in the steel material becomes finer. Then, the crystal grains immediately after the austenite transformation also become fine, the driving force for the growth of the austenite grains increases, and abnormal grains are generated after charcoal burning. In particular, in recent years, since the treatment time can be shortened, vacuum carburizing with a high carburizing temperature is often adopted as carburizing and quenching. However, when the heating temperature at the time of carburizing is high, the problem that the austenite grains grow abnormally and become coarse becomes remarkable. The growth of the abnormal grains causes adverse effects such as a cause of strain, a decrease in toughness, and a decrease in fatigue strength.

Therefore, conventionally, if the cold-forged member is carburized as it is, abnormal grains may grow. Therefore, the normalizing treatment is performed after the cold forging to suppress the coarsening of the crystal grains after the carburizing treatment. Was there.

Further, as a technique for improving the growth of abnormal grains from the aspect of composition, for example, there is a machine structural steel for carburized parts disclosed in Patent Document 1. In Patent Document 1, C: 0.10 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.10 to 0.50%, P: 0.30% or less in mass%. , S: 0.30% or less, Cr: 1.80 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.02 to 0.10%, N: 0.0300% or less. Disclosed is a machine structural steel for carburized parts, which contains the balance Fe and unavoidable impurities, has a ferrite pearlite structure before cold working, and has an average ferrite particle size of 15 μm or more. .. In Patent Document 1, the growth of abnormal grains is suppressed by the pinning effect of a compound composed of AlN, NbC or Nb (CN).

Japanese Unexamined Patent Publication No. 2013-40376

However, it is known that Al added to steel for the purpose of deoxidation binds to N in steel and precipitates extremely fine AlN having a pinning effect, thereby suppressing the growth of abnormal grains. However, due to the spheroidizing annealing performed before cold forging, AlN aggregates and coarsens, so that a sufficient pinning effect cannot be produced, and abnormal grains are allowed to be generated during carburizing. Since the normalizing treatment after the cold forging described above is usually performed in a temperature range of about 850 ° C. to 950 ° C., the coarsened AlN in the steel does not dissolve in solid solution. Therefore, it cannot be said that a sufficient pinning effect is obtained because AlN remains coarsened and agglomerated by normalizing after cold forging.

On the other hand, as shown in Patent Document 1, Nb, Ti, V and the like are often added and stable precipitates at high temperatures are often used as pinning elements. These Nb, Ti, V and the like have been researched and developed. Since the element of is expensive, it is desired to reduce the amount of the element added.

From the above, the market has desired the development of carburizing steel that can suppress abnormal grain growth during carburizing as much as possible while ensuring sufficient cold forging property.

Therefore, as a result of diligent research, the inventors of the present invention did not adjust the steel composition and the metallographic structure before cold forging, but instead adjusted the structure of the steel material after annealing or cold forging before carburizing. By making adjustments, it has become possible to provide a method for producing steel for carburizing, which makes it possible to suppress the formation of abnormal grains during carburizing treatment.

That is, the method for producing carburized steel according to the present invention is characterized by including at least the following steps A to C.
Step A: Annealing of a steel material for machine structural use, which has an Al content of 0.01% by mass to 0.100% by mass and an N content of 0.005% by mass to 0.020% by mass. Do.
Step C: The steel material annealed in Step A is cold forged before being subjected to high-frequency heat treatment in Step B, which will be described later.
Step B: after was annealed in step A, an equivalent steel material was cold forged at step C, and pre-carburization treatment, heated to a temperature above 1100 ° C. by high-frequency induction heating method, and then cooled.

In the method for producing carburized steel according to the present invention, the heating and holding time in the step B is preferably 60 seconds or less.

In the method for producing a carburized steel according to the present invention, it is preferable that after the step B, the steel material is heated to a temperature of 850 ° C. or higher and 1000 ° C. or lower as a step D, and then cooled.

In the method for producing carburized steel according to the present invention, it is preferable that the cooling rate between 800 ° C. and 600 ° C. is 10 ° C./sec or less in the step D.

In the method for producing carburized steel according to the present invention, the steel material is annealed before cold forging in consideration of workability in cold forging. After that, the steel material is heated to a temperature of 1100 ° C. or higher and 1300 ° C. or lower by a high-frequency induction heating method, so that AlN aggregated and coarsened in the steel material by annealing is solid-solved and uniformly dispersed in the steel material. be able to. Therefore, AlN dispersed in the steel material exerts a pinning effect in the subsequent carburizing treatment, and the growth of abnormal grains can be suppressed as much as possible even in the high temperature carburizing treatment. Therefore, according to the present invention, it is possible to suppress the growth of abnormal grains at the time of carburizing by the subsequent heat treatment while ensuring the workability in cold forging without using an expensive pinning element such as Nb. ..

It is a figure which shows the manufacturing process of the carburizing steel which includes the process A to process D in this invention. It is a figure which shows the heat treatment process of an Example. It is a figure which shows the heat treatment process of the comparative example 1. FIG. It is a figure which shows the heat treatment process of the comparative example 2. It is a figure which shows the heat treatment process of the comparative example 3. It is a figure which shows the metal structure photograph (500 times) and Vickers hardness before pseudo-charcoal-burning of an Example and each comparative example. Old austenite crystal grains (100 times) after pseudo-charcoal burning at 1020 ° C. in Examples and Comparative Examples. It is a figure which shows the relationship between the temperature at which abnormal particles are generated at the time of pseudo charcoal burning, an Example and each comparative example.

Hereinafter, the form of the method for producing carburized steel according to the present invention will be described in detail. The method for producing carburized steel according to the present invention is for mechanical structures having an Al content of 0.01% by mass to 0.100% by mass and an N content of 0.005% by mass to 0.020% by mass. Step A in which the steel material is annealed, step C in which the steel material annealed in step A is cold forged before high-frequency heat treatment in step B described later , and step A in which the steel material is annealed. after the person steel material was cold forged at step C, and pre-carburization treatment by high-frequency induction heating method and heated to a temperature above 1100 ° C., and at least comprising a step B of subsequent cooling. Therefore, in the embodiment described in detail below, a method for producing carburized steel including step A , step C, and step B according to the present invention will be described.

The steel material used in the method for producing the carburized steel according to the present invention may be a machine structural steel material for carburizing and quenching. The composition of the steel material is such that the content of aluminum (Al) is at least 0.01% by mass to 0.100% by mass and the content of nitrogen (N) is 0.005% by mass to 0.020% by mass. Is preferable, and other component compositions may be those having an alloy component composition usually contained as a steel material for mechanical structure for carburizing and quenching. Aluminum is a deoxidizing element and has a pinning effect of suppressing the growth of austenite grains by precipitating AlN. However, if it is less than 0.01% by mass, the pinning effect is insufficient, and if it exceeds 0.100% by mass, not only the effect is saturated, but also a large amount of hard inclusions of Al ₂ O ₃ is generated, and workability As described above, the content of aluminum is preferably 0.01% by mass to 0.100% by mass. Nitrogen has the above-mentioned pinning effect by combining with aluminum and precipitating AlN. However, although the nitrogen content depends on the balance with the aluminum content, it is preferably 0.005% by mass or more in order to sufficiently exert the pinning effect. However, if it exceeds 0.020% by mass, the cold forging property deteriorates. Therefore, as described above, the nitrogen content is preferably 0.005% by mass to 0.020% by mass.

The composition of components other than aluminum and nitrogen described above is, for example, 0.10% by mass to 0.30% by mass of carbon (C), 0.15% by mass to 0.35% by mass of silicon (Si), and 0 mass of manganese (Mn). .10% by mass to 0.90% by mass, phosphorus (P) 0.030% by mass or less, sulfur (S) 0.030% by mass or less, chromium (Cr) 1.2% by mass or less, molybdenum (Mo) 0. It is 25% by mass or less, and the balance can be iron (Fe) and unavoidable impurities. As the steel material having the chemical composition, for example, carbon steel material for machine structure, structural steel material (H steel) with guaranteed hardenability, alloy steel material for machine structure, and the like can be used.

Next, the method for producing the carburized steel according to the present embodiment will be described with reference to the conceptual diagram of the heat treatment step of FIG. The method for producing the charcoal-immersed steel according to the present embodiment is a baking step as step A, a cold forging step as step C, a high-frequency heat treatment step as step B, and baking as step D. The process is carried out in sequence. Each processing step will be described in detail below.

(1) Process A
Step A in the present invention is a step of annealing a steel material. The annealing treatment of the steel material is performed in order to improve the workability in the cold forging process in the subsequent stage. For the annealing treatment, it is preferable to employ spheroidized annealing. In the present invention, the annealing treatment is carried out at 700 ° C. to 800 ° C. for 1 hour to 24 hours, and then cooled to 600 ° C. (near the nose of the S curve) at a cooling rate of 1 ° C./hour to 20 ° C./hour. After that, it is preferable to cool the air. The processing conditions for the annealing treatment can be appropriately changed depending on the type of steel material used and the degree of cold forging. In the present invention, only the annealing treatment is mentioned as the heat treatment of the steel material before the cold forging process in the subsequent stage, but the present invention is not limited to this, and prior to the annealing treatment. A homogenization treatment for adjusting the structure of the steel material or a normalizing treatment may be performed. For these homogenization treatments and normalizing treatments, conventional methods can be adopted.

(2) Process C
The step C in the present embodiment is a cold forging step performed before the steel material annealed in the above-mentioned step A is subjected to high-frequency heat treatment in the step B described later. In the cold forging step, as described above, since the steel material is softened and annealed in the step A in the previous step, the cold workability is good.

(3) Step B
Step B in the present invention is a high-frequency heat treatment step in which the steel material annealed in step A is heated by a high-frequency induction heating method before carburizing, and then cooled. When the step D performed after the step B is subjected to the baking by the high frequency induction heating method, the step B is the first step high frequency heat treatment step, and the step D is the second step high frequency. Corresponds to the heat treatment process.

In the high-frequency heat treatment step in the step B, the steel material annealed in the step A and cold-forged in the step C is heated to a temperature of 1100 ° C. or higher and 1300 ° C. or lower by a high-frequency induction heating method before the carburizing treatment. , Then cool. The heating temperature in the step B in the present invention is 1100 ° C. or higher. This is because if the temperature is lower than 1100 ° C., it becomes difficult to solid-solve AlN aggregated and coarsened in the steel material by the annealing treatment and uniformly disperse it in the steel material. From the viewpoint of solid-solving almost all AlN in the steel material, the heating temperature in the step B is preferably 1200 ° C. or higher. The upper limit of the heating temperature in the step B is not particularly limited, but is preferably 1300 ° C. or lower from the viewpoint of deformation, significant decarburization, and surface roughness. Further, the temperature rising rate in the step B is preferably 10 ° C./sec or more from the viewpoint of shortening the step time.

Further, the heating holding time in the step B is preferably 60 seconds or less. This is because by setting the heat treatment holding time of the heat treatment in step B to 60 seconds or less, it is possible to prevent significant decarburization and surface roughness.

In the present invention, in consideration of workability in cold forging, the steel material is annealed before cold forging, and after cold forging, the above-mentioned step B is performed to obtain the steel material by annealing. AlN aggregated and coarsened can be solid-solved and uniformly dispersed in the steel material. Further, the heat treatment in the step B can eliminate the strain caused by cold forging. Therefore, AlN dispersed in the steel material exerts a pinning effect in the subsequent carburizing treatment, and the growth of abnormal grains can be suppressed as much as possible even in the high temperature carburizing treatment. Therefore, in the present invention, the growth of abnormal grains during carburizing is suppressed by the subsequent heat treatment while ensuring workability in cold forging without using an expensive pinning element such as Nb which has been conventionally adopted. be able to.

(4) Step D
If martensite or lower bainite structure is partially present in the structure of the steel material obtained by cooling after the high-frequency heat treatment in the step B, the steel material is normalized as the step D after the step B. Is more preferable. If some martensite or lower bainite is present in the steel material before carburizing, the martensite or lower bainite portion becomes finer immediately after the austenite transformation during carburizing, and the driving force for grain growth increases. This is because abnormal grains may be generated in. In addition, when martensite or lower bainite occupy most of the steel material before the carburizing treatment, crystal grains having the same orientation as the old austenite grains are preferentially generated when re-austenite is formed, and the crystal grains are fine. This is because the austenite memory phenomenon that does not change occurs.

In the step D in the present invention, when martensite and / or lower bainite structure is present in 5% or more in the structure of the steel material obtained in the step B, after heating to a temperature of 850 ° C. or higher and 1000 ° C. or lower. , It is preferable to perform normalizing to cool. By normalizing the steel material at the temperature, the martensite or lower bainite structure remaining in the steel material can be converted into ferrite-pearlite to standardize the structure. Therefore, by further performing step D, the structure of the steel material is made more uniform, so that in addition to the pinning effect of uniformly dispersed AlN, the growth of abnormal grains during the carburizing treatment is more effective. It becomes possible to deter. The normalizing treatment performed after the step B may be performed by a high frequency induction heating method. In this case, as described above, the step B corresponds to the high frequency heat treatment step of the first stage, and the step D corresponds to the high frequency heat treatment step of the second stage.

Further, in the present invention, the heating rate in the step D is preferably 10 ° C./sec or more from the viewpoint of shortening the step time. Similarly, the heating holding time in the step D is more preferably 60 seconds or less from the viewpoint of shortening the step time.

Further, the cooling rate after heating in the step D is preferably at least 10 ° C./sec or less as the cooling rate between 800 ° C. and 600 ° C. By setting the cooling rate in the temperature range to 10 ° C./sec or less, the inconvenience of bainite formation in the structure can be suppressed, ferrite-pearlite can be obtained, and the structure can be standardized. .. Further, the cooling rate in the step D is preferably changed depending on the material of the steel material. For example, when an alloy steel such as SCM420 is used, the cooling rate at least between 800 ° C. and 600 ° C. is 0.5. It is preferably ° C./sec or less.

As described above, according to the method for producing carburized steel according to the present invention, AlN aggregated and coarsened in the steel material by the annealing treatment performed in consideration of workability in cold forging is produced in the steel material. It can be evenly dispersed therein. Therefore, even when the steel material is subjected to high-temperature carburizing treatment such as vacuum carburizing, the growth of abnormal grains can be suppressed as much as possible by the pinning effect of AlN uniformly dispersed in the steel material. .. In particular, the present invention can suppress the growth of abnormal grains during carburizing while ensuring workability in cold forging only by heat treatment without using an expensive pinning element such as Nb.

Next, examples and comparative examples of the method for producing carburized steel according to the present invention will be described.

In the method for producing carburized steel in the examples, a round bar having a diameter of 16 mm made of SCM420 was used as the steel material (test material). The chemical composition of the steel material is shown in Table 1 below. The balance of the chemical components of the steel material shown in Table 1 is Fe and unavoidable impurities.

In this example, a carburizing steel was produced by sequentially performing a homogenization treatment step, a normalizing treatment step, a spheroidizing annealing treatment step, a high frequency heat treatment step, and a normalizing treatment step. In addition, in order to show the abnormal grain growth behavior during the carburizing treatment depending on the presence or absence of the high-frequency heat treatment step, the cold forging corresponding to the step C in the present invention is omitted in this example and each comparative example described later. The manufacturing process of this embodiment will be described below with reference to FIG.

First, in the homogenization treatment step, the steel material as the test material described above was heated in a furnace at 1250 ° C. for 5 hours and then cooled to room temperature in the furnace.

Next, in the normalizing treatment step, a normalizing treatment of the steel material after the homogenization treatment was performed. In this embodiment, the mixture was heated in a furnace at 900 ° C. for 1 hour and then cooled to room temperature in the furnace.

Then, in the spheroidizing annealing treatment step, the spheroidizing annealing treatment of the steel material after the normalizing treatment was performed. The spheroidizing annealing treatment step corresponds to step A in the present invention. In this example, the mixture was kept at 750 ° C. for 10 hours in the furnace, cooled to 600 ° C. at a cooling rate of 10 ° C./hour, and then cooled to room temperature in the furnace.

Next, in the high-frequency heat treatment step, high-frequency heat treatment of the steel material that had undergone the spheroidizing annealing treatment step was performed. The high frequency heat treatment step corresponds to step B in the present invention. In this high-frequency heat treatment step, the steel material was heated to 1250 ° C. in 10 seconds by high-frequency induction heating at a frequency of 10 kHz, the temperature was maintained for 10 seconds, and then air-cooled to room temperature. In the subsequent normalizing step by the second high-frequency heat treatment, the steel material is heated to 950 ° C. in 10 seconds by high-frequency induction heating at a frequency of 10 kHz, the temperature is maintained for 10 seconds, and then from 950 ° C. to 600 ° C. During the period, cooling was performed at a cooling rate of 0.1 ° C./sec, and then air cooling was performed to room temperature. The carburized steel of this example was obtained by sequentially carrying out the above homogenization treatment step, normalizing treatment step, spheroidizing annealing treatment step, high frequency heat treatment step, and normalizing treatment step.

Comparative example

In Comparative Examples 1 to 3, carburizing steel was produced using the same steel material as in the above-mentioned Examples. As shown in FIG. 3, Comparative Example 1 was subjected to a normalizing treatment step and then a normalizing treatment step in the same manner as in the above-mentioned Examples. The conditions of the homogenization treatment and the conditions of the normalizing treatment in Comparative Example 1 were the same as those in the Examples. As a result, the carburizing steel of Comparative Example 1 was obtained.

In Comparative Example 2, as shown in FIG. 4, a homogenization treatment step, a normalizing treatment step, and a spheroidizing annealing treatment step were performed in the same manner as in the above-mentioned Examples. The conditions for the homogenization treatment, the normalizing treatment, and the spheroidizing annealing treatment in Comparative Example 2 were the same as those in the Examples. As a result, the carburizing steel of Comparative Example 2 was obtained.

In Comparative Example 3, as shown in FIG. 5, after performing the homogenization treatment step, the normalizing treatment step, and the spheroidizing annealing treatment step in the same manner as in the above-described embodiment, the normalizing is performed again. A processing step was performed. The conditions for the homogenization treatment, the normalizing treatment, and the spheroidizing annealing treatment in Comparative Example 3 were the same as those in the Examples. In the normalizing treatment step performed after the spheroidizing annealing treatment, the mixture was heated at 900 ° C. for 1 hour in the furnace and then cooled to room temperature in the furnace. As a result, the carburizing steel of Comparative Example 3 was obtained.

[Evaluation]
The carburized steels of the above-mentioned Examples and Comparative Examples were observed in structure and Vickers hardness was measured using an optical microscope. The structure observation and the Vickers hardness measurement were both performed at a position of about 4 mm (1/2 of the radius) from the surface of the circular cross section of each steel material. Further, as shown in FIG. 5, the carburized steels of Examples and Comparative Examples were subjected to pseudo carburizing quenching simulating the thermal cycle of carburizing treatment, and the behavior of abnormal grain growth was confirmed. In the pseudo carburizing and quenching, the temperature was raised to 900 ° C. to 1060 ° C. at a heating rate of 300 ° C./hour in a vacuum atmosphere, held for 3 hours, and then water-cooled. Foremost austenite crystal grains were observed using an optical microscope in the examples after the simulated charcoal burning and each comparative example. The measurement position of the old austenite crystal grain observation was the same as that of the structure observation of the steel material before the simulated charcoal burning. The results of each evaluation will be described below.

Structure observation and Vickers hardness before pseudo-charcoal-burning: Fig. 6 shows a photograph of the metallographic structure and Vickers hardness of this example and each comparative example before pseudo-charcoal-burning. Examples (homogenization treatment / normalizing treatment / spheroidizing annealing treatment / high frequency heat treatment / normalizing treatment), Comparative Example 1 (homogenization treatment / normalizing treatment), and Comparative Example 3 (homogenization treatment). -The normalizing treatment, the spheroidizing normalizing treatment, and the re-normalizing treatment) all had a metallographic structure composed of ferrite and pearlite. On the other hand, Comparative Example 2 (homogenization treatment, normalizing treatment, spheroidizing annealing treatment) has a metallographic structure in which spherical carbides are dispersed, and the Vickers hardness is also the lowest value (128HV5) among the four. Indicated.

Observation of old austenite grain boundaries after pseudo-carburizing and quenching: Fig. 7 shows a metallographic photograph showing the former austenite grain boundaries after pseudo-carburizing and quenching at 1020 ° C. for this example and each comparative example. Examples of simulated carburizing and quenching at 980 ° C and 1020 ° C (homogenization treatment / normalizing treatment / spheroidizing annealing treatment / high frequency heat treatment / normalizing treatment) and Comparative Example 1 (homogenization treatment / normalizing treatment). In the normalizing treatment), it was confirmed that the distribution of the crystal grains of the old austenite grains was uniform, and no abnormal grains were formed in other fields of view. On the other hand, Comparative Example 2 (homogenization treatment / normalizing treatment / spheroidizing annealing treatment) and Comparative Example 3 (homogenizing treatment / normalizing treatment / spheroidizing treatment) in which pseudo carburizing and quenching was performed at 980 ° C. and 1020 ° C. In the annealing treatment / re-normalizing treatment), very coarse crystal grains with a maximum grain size of about 1.5 (nominal grain size of about 240 μm) were observed in a wide range, and abnormal grains were generated. It was confirmed that there was.

FIG. 8 shows the relationship between the temperature at which abnormal grains are generated during simulated charcoal burning and the examples and each comparative example. In FIG. 8, circles show no abnormal grains, white triangles show abnormal grains, but the occupied area ratio of abnormal grains is less than 20%, and crosses indicate the occupied area ratio of abnormal grains. Is 20% or more. Here, the abnormal grains are coarse crystal grains having a particle size number different from the crystal grains having the highest frequency in the structure by 3 or more, or coarse crystal grains having a grain size number of less than 5.

In Comparative Example 3 in which the spheroidizing annealing treatment was followed by the normalizing treatment, abnormal grains were generated when the carburizing temperature was 940 ° C. or higher, as in Comparative Example 2 in which the spheroidizing annealing treatment was performed. Was. Therefore, it can be inferred that the AlN aggregated and coarsened by the spheroidizing annealing treatment could not be uniformly dispersed in the subsequent normalizing treatment, and the pinning effect of the AlN was insufficient. it can.

On the other hand, in the example in which the spheroidizing annealing treatment was followed by the high frequency heat treatment and the normalizing treatment, even if the temperature at the time of carburizing was raised to 980 ° C. or 1020 ° C. as described above, the abnormal grains were found. No outbreak was observed. This is because AlN in the structure is solid-solved by heating the steel material to 1250 ° C. by high-frequency heat treatment after the spheroidizing annealing treatment, and by the subsequent normalizing treatment and heating during pseudo carburizing, it is uniform and It is considered that the AlN was finely dispersed in the steel material and the pinning effect of AlN was sufficiently exhibited. In the examples, the generation of abnormal grains is observed when the temperature at the time of carburizing is raised to 1060 ° C. However, usually, high-temperature carburizing such as vacuum carburizing is carried out in the range of 980 ° C to 1050 ° C, and in this example in which no abnormal grains are observed at the carburizing temperatures of 980 ° C and 1020 ° C, the high temperature is observed. It can be said that it is particularly effective when carburizing.

In Comparative Example 1 in which the homogenization treatment and the normalizing treatment were performed, no abnormal grains were observed even when the carburizing temperature was raised to 980 ° C. or 1020 ° C. as in the example, but cold. Considering forgeability, it is necessary to soften by spheroidizing annealing treatment. From the above, it can be said that the present invention in which high-frequency heat treatment is performed after spheroidizing annealing treatment is useful as a method for producing a steel material to be cold forged and charcoal-burned. In particular, by performing high-frequency heat treatment at a high temperature after cold forging, the strain generated by cold forging is eliminated, and in this respect as well, the growth of abnormal grains can be suppressed.

The method for producing carburized steel according to the present invention is particularly useful for carburized steel that is processed by cold forging.

Claims (4)

A method for producing carburized steel, which comprises at least the following steps A to C.
Step A: Annealing of a steel material for machine structural use, which has an Al content of 0.01% by mass to 0.100% by mass and an N content of 0.005% by mass to 0.020% by mass. Do.
Step C: The steel material annealed in Step A is cold forged before being subjected to high-frequency heat treatment in Step B, which will be described later.
Step B: after was annealed in step A, an equivalent steel material was cold forged at step C, and pre-carburization treatment, heated to a temperature above 1100 ° C. by high-frequency induction heating method, and then cooled. The method for producing carburized steel according to claim 1, wherein the heating and holding time in the step B is 60 seconds or less. The method for producing a carburized steel according to claim 1 or 2, wherein after the step B, the steel material is heated to a temperature of 850 ° C. or higher and 1000 ° C. or lower as a step D, and then cooled. The method for producing carburized steel according to claim 3 , wherein in the step D, the cooling rate between 800 ° C. and 600 ° C. is 10 ° C./sec or less.