JPH10183297A - Steel material for induction hardening, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for induction hardening, capable of securing >=Hv800 surface hardness, >=Hv300 core-part hardness, and >=50J core-part toughness by 2mm U-notched Charpy absorption energy by means of induction hardening, and its production. SOLUTION: The steel material for induction hardening has a chemical composition which consists of 0.65-1.0% C, 0.05-1.5% Si, 0.05-2.0% Mn, 0.005-0.05% Al, 0.001-0.03% N, 0.0005-0.005% B, 0-2.5% Ni, 0-0.25% Mo, <=0.08% S, and the balance Fe with inevitable impurities and in which the amount of P among the impurities is regulated to <=0.05% and also has a structure which contains graphite and in which >=50% of C exists in the form of graphite. the steel material is produced by applying heating up to 1050-1300 deg.C to perform hot working, finishing hot working at >=900 deg.C, and then carrying out softening for graphitization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for induction hardening and a method for manufacturing the same, and more particularly, to a surface hardness of 800 or more in Hv (Vickers hardness), a core hardness of 300 or more in Hv, and a 2 mm U notch by induction hardening. The present invention relates to a steel material for induction hardening capable of securing core toughness of 50 J or more with Charpy absorbed energy and a method for producing the same.

[0002]

2. Description of the Related Art Various surface hardened parts for mechanical structures, such as various gears, are provided with a large hardness to the surface to enhance wear resistance and fatigue resistance and have excellent core toughness. In order to achieve this, a so-called “case-hardened steel” having a low C content, such as SCM420, has been used as a base material steel.

[0003] The above "case hardened steel" is subjected to a carburizing and quenching treatment as a surface hardening treatment. However, the carburizing and quenching process is mainly performed in a batch process in a gas atmosphere.
Must be maintained for several hours at such high temperatures. Therefore, there is a problem that the gas atmosphere must be strictly adjusted at the time of carburizing, and the cost is increased because much time and energy are required. Furthermore, there is a problem that it is difficult to inline. In addition, recently,
There is also a request to improve the working environment of carburizing treatment.

In addition, among the surface hardened parts, for example, parts such as gears and pinions, recently, a large surface hardness of Hv 800 or more, and a central part in order to ensure the strength and toughness of the part. With a hardness of Hv300 or more and an absorption energy of 2 mm U notch Charpy
In some cases, core toughness of 0 J or more is required. However, when "case hardened steel" is carburized and quenched, the surface hardness after carburized and quenched is at most about Hv780,
A desired high surface hardness cannot be stably secured.

[0005] Among the above-mentioned problems, the surface hardening method for shortening the processing time for surface hardening to suppress energy consumption, and also for cleaning the working environment and realizing in-line processing includes the following. Induction hardening can be mentioned.

However, when case hardening steel typified by the above-mentioned SCM420 is used as a base material steel, since the C content of the steel is low, even if induction hardening is performed, a desired large surface hardness is obtained for the part. Cannot be granted. For this reason, medium carbon JIS carbon steel for machine structural use (S45C, S50C, etc.) and alloy steel (SCr440, SCM440, etc.)
It has been used as a base material steel for components to be hardened by induction hardening. However, the hardness after quenching depends very much on the C content (% by weight) of the steel. Therefore, even when the above-mentioned medium-carbon steel is used as the base material steel, the surface hardness obtained after induction hardening is such that the C content is 0.5% by weight.
In this case, the surface hardness is at most about Hv700, and a large surface hardness higher than Hv800 required for parts such as the gear and the pinion cannot be secured. Furthermore, the desired core toughness (absorbed energy of 2 mm U notch Charpy of 50 J or more) may not be ensured by induction hardening in some cases. In addition, when alloy steel is used as the base material steel among the above-mentioned medium-carbon steels, it is inevitable that the cost increases.

[0007] For this reason, some induction hardening steels have been proposed.

For example, Japanese Unexamined Patent Publication (Kokai) No. 60-169547 discloses "induction hardening steel" having a specific chemical composition. However, even if the steel described in this publication is used as the base material steel, the core toughness of the component may not always be sufficient. Furthermore, the steel proposed in this publication has a C
Since the upper limit of the content must be regulated to 0.55% by weight, a high value of Hv 800 or more cannot be secured as the surface hardness after induction hardening.

Japanese Unexamined Patent Publication (Kokai) No. 63-100157 proposes "non-heat-treated steel for induction hardening" comprising bainite having a structure of 75% or more and having a specific chemical composition. But,
The steel proposed in this publication has a bainite structure of 75% or more, and therefore has a problem in terms of "machinability" at least at the time of cutting to obtain a desired final shape. Furthermore, the steel proposed in this publication also has C
Since the upper limit of the content has to be restricted to 0.6% by weight, a high value of Hv800 or more cannot be secured as the surface hardness after induction hardening. In this publication, Hv750 is described as the surface hardness of a steel having a C content of 0.45% by weight. However, in the case of a steel having a C content of 0.45% by weight, full martensite is used. It is a known matter that the hardness is about Hv700 as the upper limit, and the hardness of Hv750 of a structure containing 87 to 90% of bainite in addition to martensite is estimated to be, for example, “wrong” of Hv650. .

[0010] Japanese Patent Application Laid-Open No. 2-179841 discloses Al
A non-heat treated steel for induction hardening, in which N is fixed and B is contained to enhance the quenchability and secure the depth of the hardened layer, and a method for producing the same have been proposed. However, in order to fix N with Al, it is necessary to contain a relatively large amount of Al. However, when Al is excessively added, a hard Al ₂ O ₃ phase is formed and machinability is reduced. Problem. Furthermore, even if N is fixed as AlN with Al, unless the heating conditions during hot working are optimized, A
The reaction of 1N → Al + N proceeds to form solid solution N, and since the solid solution N forms B and BN, there is also a problem that desired hardenability cannot be obtained.

Japanese Patent Application Laid-Open No. 5-33101 discloses that C, M
A "non-heat treated steel for induction hardened crankshafts" in which the hardness of the central portion is secured by adjusting the contents of n and Cr is disclosed. However, as is clear from the description of the examples in this publication, the surface hardness after induction hardening is at most as high as that of the steel in which the C content must be regulated to 0.52% or less from the viewpoint of toughness. With a Rockwell hardness (HRC) 61 (about 720 in Hv), it may not be possible to impart a desired surface hardness to a part.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has a surface hardness of Hv800 or more, a core hardness of Hv300 or more, and 2 mmU
An object of the present invention is to provide a steel material for induction hardening capable of securing core toughness of 50 J or more with notch Charpy absorbed energy and a method for producing the same. More specifically, a steel material subjected to soft annealing after hot working such as hot forging is used as a starting material, and subsequent heat treatment is performed only by induction hardening to obtain a surface hardness of Hv 800 or more and Hv 3
Provided is a steel material for induction hardening, which can secure core toughness of 50 J or more with core hardness of 00 or more and absorbed energy of 2 mm U notch Charpy, and is suitable for surface hardened parts such as gears and pinions, and a method for producing the same. The purpose is to:

[0013]

The gist of the present invention resides in the following (1) a method for producing a steel material for induction hardening and (2) a method for producing a steel material for induction hardening.

(1) C: 0.65 to 1.0% by weight
%, Si: 0.05-1.5%, Mn: 0.05-2.
0%, Al: 0.005 to 0.05%, N: 0.001
0.03%, B: 0.0005 to 0.005%, N
i: 0 to 2.5%, Mo: 0 to 0.25%, S: 0.0
8% or less, with the balance being Fe and unavoidable impurities.
Has a chemical composition of 0.05% or less, the structure is a structure containing graphite, and 50% or more of C is present as graphite.

(2) The steel having the chemical composition described in the above (1) is heated to 1050 to 1300 ° C. and hot worked.
A method for producing a steel material for induction hardening, comprising performing softening annealing for graphitization after finishing hot working at a temperature of 00 ° C. or higher.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have made it possible to easily cut a part, and to obtain a part having a surface hardness of Hv800 or more, a core hardness of Hv300 or more, and a notch Various studies were made on the chemical composition and structure of the base steel of the component so that core toughness of 50 J or more could be imparted by the absorbed energy. As a result, the following findings were obtained.

The structure of the steel material that has been subjected to hot working including hot forging is softened and annealed to a structure containing graphite.
If 50% or more of the contained C content forms the graphite, that is, if 50% or more of the contained C content is graphitized,
Cold workability and machinability increase. For this reason, rough die processing such as cold forging or cutting before induction hardening is relatively easy.

It should be noted that, among the C contents, C which forms graphite
, That is, the degree of graphitization can be determined by chemical analysis of extracting graphite by acid dissolution.

Graphite is sufficiently solid-dissolved in the matrix (austenite) in the austenite region even by rapid and short-time heat treatment such as induction heat treatment. Therefore, the amount of solute C in the matrix of the portion heated to the austenite region by the heating for induction hardening is higher than that of the core. Therefore, the base steel C
If the content is increased, the part that rises to the austenite region by heating for induction hardening will have a high C content in the next hardening.
, The hardness can be increased.

The heating temperature in the high-frequency heating process is set to Ac ₁
If the quenching treatment is performed at a temperature in the two-phase region between the point and the Ac ₃ point, a structure composed of ferrite and martensite phases (hereinafter referred to as a ferrite-martensite structure) is formed. This ferrite-martensite structure has the effect of suppressing the growth of cracks due to fatigue and impact fracture. For this reason, the fatigue strength and toughness of the portion quenched after heating to a temperature in the two-phase region between the Ac ₁ point and the Ac ₃ point by induction quenching are good. In the ferrite-martensite structure, undissolved graphite or the like may be present in the phase.

From the above, it can be seen that heating during induction quenching is performed in the austenitic region on the surface of the component, and at _one point Ac and A on the core.
c If the conditions are set such that the temperature is in the two-phase region between the _three points, and then quenching is performed, the surface of the component has a high C martensite structure, so that high hardness can be secured. On the other hand, since the core has a ferrite-martensite structure, good fatigue strength and toughness can be secured.

After induction hardening, the core hardness is Hv3
In order to secure a value of not less than 00, it is sufficient to rapidly cool from the temperature in the two-phase region between the Ac ₁ point and the Ac ₃ point, and mix the hard structure of martensite with ferrite.

The present invention has been completed based on the above findings.

Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the component content means “% by weight”. Further, “hardness at a position 0.5 mm from the surface layer” is referred to as “surface hardness” and “hardness of the surface portion”. Further, the central part of the steel material (part) to be subjected to induction hardening is referred to as a “core”.

(A) Chemical composition of steel material C: 0.65 to 1.0% C is an element effective for improving the hardenability and strength of steel and for ensuring a high surface hardness of Hv 800 or more after induction hardening. It is. However, if the content of C is less than 0.65%, the effect of addition is poor. On the other hand, if it exceeds 1.0%,
Since austenite remains by induction hardening, high hardness cannot be imparted to the surface. Therefore, the content of C is set to 0.65 to 1.0%. In order to secure the above-mentioned effect more stably, the content of C is set to 0.7 to 0.7%.
It is preferably set to 0.9%.

Si: 0.05-1.5% Si is an element effective for deoxidation during steelmaking. Further, Si
Has an effect of promoting graphitization. However, if the content is less than 0.05%, the above effects cannot be expected. On the other hand, when the content exceeds 1.5%, not only the above effect is saturated, but also the toughness is reduced. Therefore, the content of Si is set to 0.05 to 1.5%. In order to secure the above-mentioned effects more stably, the content of Si is set to 0.2 to
It is preferably set to 1.0%.

Mn: 0.05 to 2.0% Mn is an element effective for improving the strength and hardenability of steel. However, if the content is less than 0.05%, the desired effect cannot be obtained. On the other hand, if the content exceeds 2.0%, the diffusion of C is reduced to inhibit graphitization, so that it takes a long time for soft annealing for graphitization, which is economically disadvantageous. Further, austenite remains on the surface portion by induction hardening to cause a decrease in surface hardness. On the other hand, the amount of martensite increases in the core portion, so that the hardness increases and the core portion toughness decreases. Therefore, the content of Mn is 0.05 to
2.0%. In addition, the content of Mn is 0.07 to 0.1.
It is preferably set to 5%.

Al: 0.005 to 0.05% Al is an element having a strong deoxidizing effect, and must be contained in an amount of 0.005% or more in order to secure a desired deoxidizing effect during steelmaking. It is. However, 0.05% of Al
In addition, the above effect is not only saturated, but also causes a decrease in machinability. Therefore, the content of Al is 0.00
It was 5 to 0.05%. The preferred Al content is
0.005 to 0.025%.

N: 0.001 to 0.03% N has a large affinity for B and binds to B to precipitate as BN in steel, acts as a nucleus for graphite precipitation and promotes graphitization. Having. However, its content is 0.001%
If less, the effect of addition is poor. On the other hand, if it exceeds 0.03%, the toughness is reduced. Therefore, the N content is set to 0.0
01-0.03%. The content of N is 0.00
The content is preferably set to 2 to 0.015%.

B: 0.0005% to 0.005% B combines with N to form BN and acts as a nucleus for graphite precipitation to promote graphitization. However, if the content is less than 0.0005%, the desired effect cannot be obtained. On the other hand, if the content of B exceeds 0.005%, BN coarsens, so that graphitization is hindered, and it takes a long time for soft annealing for graphitization, which is economically disadvantageous. Further, austenite remains on the surface portion by induction hardening to cause a decrease in surface hardness. On the other hand, the amount of martensite increases in the core portion, so that the hardness increases and the core portion toughness decreases. Therefore, the B content is 0.0005 to
0.005%. The content of B is 0.0005.
It is preferably set to 0.003.

Ni: 0 to 2.5%, Ni may not be added. If added, it has the effect of accelerating the decomposition of cementite and promoting graphitization. In order to ensure this effect, it is preferable that the content of Ni be 0.2% or more. However, if the content of Ni exceeds 2.5%, the effect is saturated and not only is economically disadvantageous, but also austenite remains due to induction hardening.
High hardness cannot be imparted to the surface. Therefore, the Ni content was set to 0 to 2.5%. In order to secure the above-mentioned effects more stably, the content of Ni is set to 0.5 to
Preferably it is 2.0%.

Mo: 0 to 0.25% Mo may not be added. If added, it has the effect of increasing the hardenability and ensuring the hardness of the core. To ensure this effect, it is preferable that the content of Mo be 0.05% or more. However, when Mo is contained in excess of 0.25, not only the above effect is saturated, but also austenite remains due to induction quenching, so that a high hardness cannot be imparted to the surface portion. Therefore, the Mo content is set to 0 to 0.
25%. In order to secure the above-mentioned effects more stably, the content of Mo is preferably set to 0.07 to 0.2%.

S: 0.08% or less S may not be contained. If contained, it has the effect of enhancing machinability. To ensure this effect, S should be 0.0
The content is preferably 15% or more. However, S
When a large amount of is contained, graphitization is inhibited. Especially,
If the S content exceeds 0.08%, graphitization is significantly inhibited, so that it takes a long time for soft annealing for graphitization, which is economically disadvantageous. Further, austenite remains on the surface portion by induction hardening to cause a decrease in surface hardness. On the other hand, the amount of martensite increases in the core portion, so that the hardness increases and the core portion toughness decreases. Therefore,
The content of S was set to 0.08% or less. The upper limit of the S content is preferably set to 0.05%.

In the present invention, the content of P as an impurity element is limited as follows.

P: 0.05% or less P is an impurity element that inhibits graphitization. In particular, if the content exceeds 0.05%, graphitization is significantly inhibited, and it takes a long time for soft annealing for graphitization, which is economically disadvantageous. Further, austenite remains on the surface portion by induction hardening and causes a decrease in surface hardness. On the other hand, the amount of martensite increases in the core portion, so that the hardness increases and the core portion toughness decreases. Therefore,
The content of P as an impurity element was specified to be 0.05% or less. Preferably, the content of P is reduced to 0.02% or less.

(B) Structure of Steel Material Even when a steel having the chemical composition described in the item (A) is used as the base material steel, the desired surface hardness of Hv 800 or more and the core hardness of Hv 300 or more are obtained by induction hardening. In order to ensure core toughness of 50 J or more with absorbed energy of 2 mm U notch Charpy, the structure of steel material is a structure containing graphite,
In addition, it is necessary that 50% or more of the C content of the base steel be present as graphite, that is, 50% or more of the C content of the base steel be graphitized.

As described above, graphite is sufficiently dissolved in the matrix in the austenite region even when heat treatment is performed for a short time such as high-frequency heat treatment. Therefore, if 50% or more of the C content of the base steel is graphitized, the amount of solid solution C in the portion heated to the austenite region by heating for induction hardening can be sufficiently ensured, and high in the next hardening. A martensitic structure of C can be obtained. Therefore, high surface hardness can be ensured. On the other hand, since the solid solution of graphite does not proceed to the matrix in the portion where the temperature is not raised to the austenite region, if 50% or more of the C content of the base steel is present as graphite, the hardenability of the above portion is reduced. Keep low. For this reason, even if it is quenched, a ferrite / martensite structure is obtained, so that good fatigue strength and toughness can be secured.

For the above reasons, the structure of the steel material is a structure containing graphite, and 50% or more of the C content of the base steel is present as graphite.

It is to be noted that the upper limit of the ratio of the C content to be present as graphite is not particularly specified. Almost 100% of the C content may be present as graphite.

(C) Softening annealing for hot working and graphitization In the present invention, the steel having the chemical composition of (A) is heated to 1050 to 1300 ° C. Heating temperature is 1050
If the temperature is lower than ℃, the hot workability of the steel material is reduced, and a work crack may occur. On the other hand, if the heating temperature exceeds 1300 ° C., the crystal grains become extremely coarse, resulting in a decrease in toughness. Therefore, in the present invention, the heating is 1050 to 1050.
The temperature range was limited to 1300 ° C.

After the steel is heated to the above temperature range, hot working such as hot forging and hot rolling must be completed at a temperature of 900 ° C. or more. Hot working end temperature is 90
If the temperature is lower than 0 ° C., the hot ductility of the steel decreases, the hot workability deteriorates, and a work crack may occur.

After the steel having the chemical composition described in the above (A) is subjected to hot working under the above conditions to obtain a steel material, the structure is a structure containing graphite, and 50% or more of C is present as graphite. In order to achieve this, it is necessary to perform soft annealing for graphitization. As this softening annealing, it is sufficient to perform softening annealing for graphitization which is usually performed, that is, treatment for 3 to 50 hours in a temperature range between 600 ° C. and _one point of Ac.

Under the manufacturing conditions described above, the "steel material for induction hardening" of the present invention is obtained. This steel material
It is formed into a predetermined component shape by a forming process such as cold working or mechanical working described below, and then subjected to induction hardening to obtain various surface-hardened components as final products.

(D) Forming The steel material which has been subjected to softening annealing for graphitization to have a predetermined structure is subjected to cold working such as cold forging or mechanical working to be formed into a desired surface hardened part. You. The forming method is not particularly limited, and may be performed by a usual method.

(E) Induction quenching The induction quenching is an indispensable treatment for imparting the characteristics required as a product to the molded surface-hardened component of (D). The surface has a high C martensite structure to secure a high surface hardness of Hv800 or more, a core hardness of Hv300 or more is secured, and the core has a ferrite-martensite structure of 2 m.
In order to secure core toughness of 50 J or more and good fatigue strength with the absorbed energy of mU notch Charpy, heat quenching treatment is performed under conditions that heat is applied not only to the so-called "surface layer" but also to the "core". good. As a specific process, for example, the Ac ₁ point and the Ac ₃ point of the steel material described above are obtained by an experiment, and the temperature in the temperature range between the Ac ₁ point and the Ac ₃ point is set to 5 to 5.
After heating at a heating rate of 60 ° C./sec, the temperature is maintained for 1 to 60 seconds, and then the temperature in the temperature range of 950 to 1100 ° C.
Heating at a heating rate of ６０60 ° C./sec and holding for 1 to 60 seconds;
Thereafter, there is a process of quenching.

Hereinafter, the present invention will be described with reference to examples.

[0047]

EXAMPLES Steel having the chemical composition shown in Tables 1 and 2 was vacuum-melted in an amount of 150 kg by a conventional method. Steels 1 to 7 in Table 1 and Steels 17 to 23 in Table 2 are steels of the present invention, and are Steels 8 to 16 in Table 1 and Steels 24 to 24 in Table 2.
35 is a steel of a comparative example in which any of the components is out of the range of the content specified in the present invention. In addition, among the steels of the comparative example, steel 35 is case hardening steel corresponding to SCM420 of JIS and used for carburizing and quenching.

[0048]

[Table 1]

[0049]

[Table 2]

Next, hot forging for heating these steels to 1250 ° C. and finishing at 1000 ° C. was repeated to obtain a steel having a diameter of 2 mm.
A 5 mm round bar was used. After the final hot forging, the air was cooled to room temperature (room temperature).

The forged steel rods 1 to 34 are heated to 680 to 720 ° C. at a heating rate of 40 ° C./hour and maintained for 5 to 15 hours depending on the steel composition, and then maintained at 40 ° C. to 600 ° C. After cooling at a cooling rate of / h, soft annealing was performed for graphitization by air cooling to room temperature.

From the round bar of steels 1-34 after soft annealing, the diameter was 3 mm.
Cut out a test piece for transformation point measurement of 10 mm in length and 10 mm in length,
Ac ₁ point and A when heated at a heating rate of 5 to 60 ° C / sec.
c _Three points were measured. Tables 1 and 2 show Ac ₁ point and Ac ₃ point of each steel actually measured.

The round bars of the softened and annealed steels 1 to 34 have a diameter of 20.
mm, and a 15 mm long test piece is cut out from a round bar having a diameter of 20 mm, and the Vickers hardness at a position 0.5 mm from the surface layer, R / 2 part (R = 10 mm) and the center part. (Hv) was measured. The structure was observed using an optical microscope.

The hardness as softened and annealed for graphitization was almost constant regardless of the measurement position for each steel type.
The structure was composed of ferrite, graphite and cementite regardless of the steel type and the measurement position.

In the soft annealed material, the ratio of C forming graphite in the C content, that is, the degree of graphitization was measured by chemically analyzing graphite extracted by acid dissolution.

The round bar having a diameter of 20 mm of each of the cut steels 1 to 34 is heated at a heating rate of 5 to 60 ° C./sec to a temperature in a temperature range between the Ac ₁ point and the Ac ₃ point depending on the steel composition. Then 1-6
Induction quenching treatment was carried out by holding for 0 second, heating to a temperature in a temperature range of 950 to 1100 ° C. at a heating rate of 30 to 60 ° C./second, holding for 1 to 60 seconds, and oil-cooling.

A JIS No. 3 Charpy impact test piece was collected from the center of the induction hardened round bar having a diameter of 20 mm.
The impact characteristics at room temperature were investigated. Further, a test piece having a length of 15 mm was cut out from a round bar having a diameter of 20 mm subjected to induction hardening, and the Vickers hardness (Hv) at a position 0.5 mm from the surface layer and at the center was measured. The structure of the center was also observed with an optical microscope.

Of the steels of the comparative examples, case 35, which is case-hardened steel, was hot forged to a diameter of 25 mm, heated at 940 ° C. for 30 minutes, then cooled to 600 ° C. and held at 600 ° C. for 1 hour. After the air-cooled heat treatment, it was cut to a diameter of 20 mm. The thus obtained round bar having a diameter of 20 mm is 930 ° C. × 10 hours (carbon potential: 0.9%) by a usual method.
Oil quenching after carburizing, and then 170 ° C x 2
Tempered for hours. Carburized and tempered diameter 2
A JIS No. 3 Charpy impact test specimen was sampled from the center of a 0 mm round bar, and the impact characteristics at room temperature were investigated. In addition, a carburized and tempered round bar with a diameter of 20 mm
A 5 mm test piece was cut out, and the Vickers hardness (Hv) at a position 0.5 mm from the surface layer and at the center was measured.

Tables 3 and 4 show the results of various investigations.

[0060]

[Table 3]

[0061]

[Table 4]

Tables 3 and 4 show that steels 1 to 7, which are steels of the present invention.
And steels 17 to 23 have a surface hardness of Hv 800 or more higher than the surface hardness of the carburized steel 35, and have a core hardness of Hv 300 or more and a core toughness of 50 J or more with a 2 mm U notch Charpy absorbed energy. It is clear that you are.

On the other hand, in the case of steels 8 to 16 and steels 24 to 34, which are comparative examples, even if the treatment is carried out by the method of the present invention, a desired value is obtained in either the surface hardness or the core toughness. Not obtained.

Since steel 8 and steel 24 have a C content lower than the value specified in the present invention, they have a low surface hardness and a desired Hv8.
A value of 00 or more is not obtained.

Steel 9 and steel 25 have a low surface hardness because the C content exceeds the value specified in the present invention and austenite remains on the surface by induction hardening.

Steels 10 and 26 have a low core toughness because the Si content exceeds the value specified in the present invention.

Since the Mn content of the steels 11 and 27 exceeds the value specified in the present invention, the graphitization has not sufficiently progressed.
For this reason, austenite remains on the surface portion by induction hardening and the surface hardness is low. On the other hand, the amount of martensite increases in the core portion, so that the hardness increases and the core portion toughness is low.

Since the contents of P in steels 12 and 28 and the contents of S in steels 13 and 29 exceed the values specified in the present invention, graphitization has not sufficiently progressed. Austenite remains on the surface by induction hardening and the surface hardness is low. On the other hand, the amount of martensite increases in the core, so that the hardness is high and the core toughness is low.

Since the steel 14 and steel 32 have a B content lower than the value specified in the present invention, graphitization has not sufficiently occurred.
For this reason, austenite remains on the surface portion by induction hardening and the surface hardness is low. On the other hand, the amount of martensite increases in the core portion, so that the hardness increases and the core portion toughness decreases.

In steels 15 and 33, since the B content exceeds the value specified in the present invention, BN coarsens and graphitization does not sufficiently occur, so that austenite remains on the surface by induction hardening and the surface hardness is increased. On the other hand, since the amount of martensite in the core increases, the hardness increases and the core toughness decreases.

Steel 16 and steel 34 have low core toughness because the N content exceeds the value specified in the present invention.

Since the Ni content of the steel 30 exceeds the value specified in the present invention, austenite remains due to induction hardening and the surface hardness is low.

Since the content of Mo in the steel 31 exceeds the value specified in the present invention, austenite remains by induction hardening and the surface hardness is low.

[0074]

The steel material for induction hardening according to the present invention can obtain a surface hardness of not less than Hv800, a core hardness of not less than Hv300 and a core toughness of not less than 50 J with an absorption energy of 2 mm U notch Charpy simply by induction hardening. be able to. Therefore, if this induction hardening steel is used as a material for various surface hardened parts for machine structures, especially gears and pinions, the processing time for surface hardening is shortened and energy consumption is reduced. In addition, it is possible to clean the working environment and realize in-line processing. The steel material for induction hardening can be manufactured relatively easily by the method of the present invention.

Claims (2)

[Claims] C. 0.65 to 1.0% by weight, S
i: 0.05 to 1.5%, Mn: 0.05 to 2.0%,
Al: 0.005 to 0.05%, N: 0.001 to 0.
03%, B: 0.0005 to 0.005%, Ni: 0 to 0%
2.5%, Mo: 0 to 0.25%, S: 0.08% or less, with the balance being Fe and unavoidable impurities with P in the impurities of 0.
A steel material for induction hardening, having a chemical composition of not more than 05%, a structure containing graphite, and wherein not less than 50% of C is present as graphite. 2. A steel having the chemical composition according to claim 1,
A method for producing a steel material for induction hardening, comprising: performing hot working by heating to 050 to 1300 ° C .; performing hot working at a temperature of 900 ° C. or higher; and then performing softening annealing for graphitization.