JPH11236644A - Steel for induction hardening excellent in high strength characteristic and low heat treating strain characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the strength characteristics and low heat treating strain characteristics of a steel by containing a specified compsn. of C, Si, Mn, S, Cr, B, Al, N, Ti, P and O, furthermore forming its microstructure into ferritic- pearlitic one, regulating the ferrite crystal grain size to a specified value or below and regulating the score of a ferrite band in the structure of a cross-section in parallel with the hot rolling direction to a specified value. SOLUTION: This steel consists of a compsn. contg. by weight, >0.38 to 0.58% C, 0.01 to 0.15% Si, 0.95 to 1.7% Mn, 0.005 to 0.15% S, <=0.35% Cr, 0.0005 to 0.005% B, 0.015 to 0.05% Al, <0.007% N, 0.015 to 3.4N+0.02% Ti, <=0.025% P, <=0.0025% O and the balance Fe with inevitable impurities. Then, its microstructure is substantially formed on the ferritic-pearlitic one. The ferrite crystal grain size is regulated to <=25 μm and the score of the ferrite band in the structure of a cross-section in parallel with the hot rolling direction is regulated to 1 to 5.

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 more particularly, to various gears and various shafts such as drive shafts and outer rings manufactured by induction hardening at a frequency of 5 to 40 kHz. TECHNICAL FIELD The present invention relates to a steel material for induction hardening excellent in high strength characteristics and low heat treatment distortion characteristics, which is suitable as a kind of material, and a method for producing the same. The strength characteristics referred to in the present invention are mainly torsional strength,
They are torsional fatigue strength and bending fatigue strength. Forming process of parts to which the steel material is applied is a process of directly performing cold forging without performing annealing, a process of performing annealing before or during cold forging, a process including a cutting process in these processes, Alternatively, a process of forming a part mainly by cutting, a step of forming a part by cutting including a partial annealing step, a step including rolling in any of these, or warm forging in any of these These are combined steps. In addition, since the components targeted by the present invention are manufactured by cold working such as cutting and cold forging, the cold workability is also noted.

[0002]

2. Description of the Related Art Various types of gears and shafts manufactured in an induction hardening process have a strong tendency to be strengthened in accordance with the recent increase in output of automobile engines or compliance with environmental regulations. There is also a strong demand for reduction of heat treatment distortion generated during induction hardening in order to omit the polishing step and the correction step.

The present invention is directed to a product which is induction hardened at a frequency of 5 to 40 kHz. The products that are induction hardened at a frequency of around 100 kHz, which is the conventional mainstream, have a shallow hardened layer depth (for example, the hardened layer depth is about one-fourth of the radius), and mainly ensure wear resistance and the like. Therefore, securing the outermost surface hardness was an important issue. On the other hand, in the products subject to the present invention that are subjected to induction hardening at a frequency of 5 to 40 kHz, the main aim is to secure the torsional strength and the torsional fatigue strength. Preventing unevenness in hardness is the key to high strength. Further, by preventing the unevenness of the hardness of the cured layer, the heat treatment distortion is reduced. For the above reasons, it is essential to ensure a net induction hardening property.

On the other hand, JP-A-3-17737 discloses that C: 0.38 to 0.45% and Si: 0.35%.
% Or less, Mn: 0.3 to 1.0%, B: 0.0005 to
0.0035%, Ti: 0.01 to 0.05%, Al:
0.01 to 0.06%, N: 0.01% or less, ferrite crystal grain size number: 6 or more, microstructure: ferrite and pearlite, hardness HRB80 to 90, decarburization depth specified by JIS0558: DM-T0 A steel material for direct cutting and induction hardening having a diameter of 0.2 mm or less is shown. The invention steel material is characterized in that B steel is applied and the decarburization depth is defined.
Although the gazette describes the characteristics when induction hardening is performed at a frequency of 100 kHz, it is intended that the present invention be applied to 5
The characteristics when induction hardening is performed at a frequency of ４０40 kHz are not described. In addition, the publication does not mention at all the static torsional strength, torsional fatigue strength, and low heat treatment strain characteristics which are focused on in the present invention. That is,
Because the steel material described in the publication lacks induction hardening properties,
Insufficient hardness of the induction hardened part and unevenness in hardness may cause problems, and it is considered that strength characteristics such as torsional strength may not be sufficient.

Japanese Patent Application Laid-Open No. Hei 5-179400 discloses that C: 0.38 to 0.45%, Si: 0.35% or less, Mn: more than 1.0% to 1.5%, B: 0 .0005-
0.0035%, Ti: 0.01 to 0.05%, Al:
Direct cutting with a fine grain structure of 0.01 to 0.06%, N: 0.01% or less, ferrite grain size number: 6 or more
A steel for induction hardening is shown. The invention steel material is a steel material in which the amount of Mn is increased from that of JP-A-3-17737. The publication does not mention low heat treatment strain characteristics. In this publication, torsional strength is described, but torsional fatigue strength is not described.
Again, it is considered that the steel material also lacks the net induction hardening property, and the hardness of the induction hardened portion is insufficient, and the occurrence of hardness unevenness is a problem, and the case where the torsional fatigue strength or the like is not sufficient may occur. Further, it is considered that the cold workability is not good because the N amount or the Si amount is high.

[0006] From the above circumstances, at present, the above steel materials are not always widely applied.

[0007]

SUMMARY OF THE INVENTION It is considered that the steel materials disclosed above have insufficient induction hardening properties, and insufficient strength characteristics after induction hardening and low heat treatment distortion characteristics. The present invention solves such a problem and provides a steel material for induction hardening excellent in high strength characteristics and low heat treatment distortion characteristics, and a method of manufacturing the same.

[0008]

The inventor has solved the above-mentioned problem by using the following means. That is, in the invention of claims 1 to 3 of the present invention, C: more than 0.38 to 0.58%,
i: 0.01 to 0.15%, Mn: 0.85 to 1.7
%, S: 0.005 to 0.15%, Cr: 0.35% or less (including 0%), B: 0.0005 to 0.005%,
Al: 0.015 to 0.05%, N: less than 0.007% (including 0%), and Ti according to the N content,
Containing 0.015 to 3.4N + 0.02%;
0.025% or less (including 0%), O: 0.0025%
Below (including 0%), or if necessary, further, Mo: 0.02 to 0.3%, Ni: 0.02 to
One or two of 1.0%, or if necessary, Nb: 0.002 to 0.035%, V:
One or two of 0.03 to 0.4%, the balance being iron and unavoidable impurities, and the microstructure is substantially a ferrite-pearlite structure, and the ferrite grain size is 25 μm. This is a steel material for induction hardening excellent in high strength characteristics and low heat treatment distortion characteristics, characterized in that a ferrite band having a cross section parallel to the hot rolling direction has a rating of 1 to 5.

According to a fourth aspect of the present invention, a steel having the above-mentioned composition is prepared by heating a steel at a heating temperature of 1050 ° C. or more, a finishing temperature of hot rolling at 750 to 1000 ° C. The steel is hot-worked into a wire or a steel bar under the condition that the temperature is slowly cooled at a cooling rate of 1 ° C./second or less in a temperature range of 1 ° C., and the microstructure after the hot working is substantially a ferrite-pearlite structure, and a ferrite crystal grain size. Is a high-frequency steel excellent in high-strength characteristics and low heat-treating distortion characteristics, characterized in that the steel material has a ferrite band rating of 1 to 5 having a structure of a cross section parallel to the hot rolling direction of 25 μm or less. This is a method for producing hardened steel.

[0010] By using the steel material and the manufacturing method of the present invention, it is possible to obtain a product excellent in high strength characteristics and low heat treatment distortion characteristics after induction hardening.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the manufacture of induction hardened parts, the present inventors have conducted intensive studies to realize excellent high strength characteristics and low heat treatment distortion characteristics after induction hardening, and have clarified the following points. .

In order to realize excellent high strength characteristics and low heat treatment distortion characteristics, the point is to reduce the hardness unevenness of the hardened layer as described above, and for that purpose, the net induction hardenability is increased. This is very important. Compared with the hardenability in the case of normal furnace heating, the induction hardenability is greatly affected by the prestructure due to rapid heating. If coarse ferrite exists in a row as a ferrite band, the solution of the carbide is insufficient, resulting in insufficient hardness or uneven hardness after induction hardening. FIG. 1 is a diagram schematically showing what effect is exerted after induction hardening when a ferrite band is present in the structure before induction hardening. When the ferrite band is remarkable, as shown in FIG. 1, after induction hardening, the part of the pearlite structure originally becomes high carbon martensite, the part of the ferrite band originally becomes low carbon martensite, and the hard layer and the soft layer form an axis. It will exist in layers along the direction. When a torsional stress is applied to such a steel material, the axial direction becomes the direction of the maximum shear stress, so that shear cracks are generated and propagate along the soft low-carbon martensite layer, and fracture at low strength is prevented. Invite. The same applies when bending stress acts like a gear.
Cracks due to bending stress are generated and propagate along the low carbon martensite layer. Furthermore, the larger the ferrite grain size is, the longer the carbon diffusion distance becomes, and the above hardness unevenness becomes remarkable. Therefore, in order to prevent the hardness unevenness and increase the depth of the hardened layer, it is necessary to use a fine ferrite grain size. Is also important. From the above, it is essential to limit the crystal grain size of ferrite to a certain value or less and suppress the ferrite band. JP-A-5-179400 and JP-A-3-179400 described in the section of the prior art.
Since there is no provision regarding the ferrite band as described above in JP-A No. 177537, insufficient hardness of the induction hardened portion and occurrence of uneven hardness are problematic, and strength characteristics such as torsional fatigue strength are not sufficient. Is thought to occur. Here, as shown in FIG. 2, the degree of the ferrite band is 1 to 7 in “The Journal of the Japan Institute of Metals, Vol. 34, No. 9, page 961” published in 1970.
It is graded on a scale. That is, in the above-mentioned Journal of the Japan Institute of Metals, Vol. 34, No. 9, pp. 957 to 962, as described in the title, "Influence of austenite grain size and forging ratio on ferrite stripe structure" is described. On page 961, the left column, lines 7 to 8, it states that "a reference photograph of Photo.4 was prepared in order to quantitatively display the degree of the striped structure."
4Classifications of ferri
In "te bands (x50x2 / 3x5 / 6)", reference photos 1 to 7 are published. In the rating, the smaller the rating number, the lighter the ferrite band, and the larger the rating number, the more prominent the ferrite band. In order to suppress unevenness in hardness after induction hardening, the ferrite band having a structure having a cross section parallel to the hot rolling direction as defined in the above-mentioned Journal of the Japan Institute of Metals, Vol. It is necessary to be.

[0013] In order to reduce the degree of the ferrite band of the steel material after hot working, the heating temperature at the time of hot working is set at 105 ° C.
What is necessary is just to set it as high as 0 degreeC or more, and to optimize the finishing temperature and cooling conditions after rolling.

In order to prevent the above-mentioned poor melting of carbides and to reduce unevenness in hardness, the content of Cr is reduced and M
It is more effective to increase the content of n to ensure hardenability.

Further, in order to improve the strength characteristics of the induction hardened material, it is important to strengthen the former austenite grain boundary. For strengthening the grain boundary of the former austenite grain boundary,
It is effective to lower P and add B.

Next, since the parts targeted by the present invention are manufactured by cold working such as cutting or cold forging, securing cold workability is also an important issue. In order to suppress the improvement of hardness at the stage of material and improve the induction hardening,
And the addition of B are effective. In order to make B effective for hardenability, it is necessary to fix solid solution N. For this purpose, Ti is usually added. Here, TiN and Ti (CN) are:
Deteriorates cold workability. That is, TiN, Ti (C
The precipitation hardening by N) increases the cold deformation resistance and causes cold forging cracking. Therefore, the amount of N is set to 0.
007% or less. Further, the amount of Ti added to fix the solute N must be minimized in accordance with the amount of N. Further, since the oxide-based inclusions deteriorate the cold forgeability, it is necessary to limit the oxygen amount to a specific amount or less.

In order to suppress the improvement in hardness at the stage of the raw material and to secure excellent cold workability, it is necessary to have a ferrite-pearlite structure which is as-rolled and substantially does not contain a bainite structure. . In order to suppress the formation of a bainite structure in a steel material after hot working and obtain a ferrite-pearlite structure substantially, it is necessary to optimize the finishing temperature and cooling conditions after rolling.

The present invention has been made based on the above new findings.

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the components will be described.

C is an element effective for imparting necessary strength to steel, but if it is less than 0.38%, the required strength cannot be secured. If it exceeds 0.58%, it becomes hard and becomes cold. Since the interworkability deteriorates, it is necessary to be within the range of more than 0.38 to 0.58%. The preferred range is 0.4-0.56%.

Si is an element effective for deoxidizing steel, and is an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. The effect is inadequate. On the other hand, when it exceeds 0.15%, the hardness is increased and the cold workability is deteriorated. For the above reasons, its content needs to be within the range of 0.01 to 0.15%. The preferred range is 0.03-0.1%.

Mn is an element effective for improving induction hardening. Focusing on high strength characteristics and low heat treatment distortion characteristics,
In order to ensure sufficient hardenability, the effect is insufficient if less than 0.85%. On the other hand, when the content exceeds 1.7%, the hardness is significantly increased and the cold workability is deteriorated. Therefore, the content needs to be in the range of 0.85% to 1.7%.
The preferred range is 0.85 to 1.4%.

S forms MnS in the steel and is added for the purpose of improving the machinability. However, if the content is less than 0.005%, the effect is insufficient. On the other hand, if the content exceeds 0.15%, the effect is saturated, and rather, grain boundary segregation is caused to cause grain boundary embrittlement. For the above reasons, the content of S is 0.005 to
It must be within the range of 0.15%. The preferred range is 0.
005 to 0.04%.

Cr is an element effective for improving hardenability. However, Cr stabilizes cementite by forming a solid solution in cementite. For this reason, poor penetration of cementite is likely to occur during short-time heating in induction quenching, which causes uneven hardness. This behavior is particularly noticeable at 0.35
%, It becomes remarkable. For the above reasons, it is necessary to limit the content to 0.35% or less (including 0%). A preferred range is 0.15% or less.

B is added aiming at the following three points. In steel bar and wire rod rolling, boron ferrous carbide is generated in a cooling process after rolling, thereby increasing the growth rate of ferrite and promoting softening as-rolled. At the time of induction hardening, steel is given hardenability. By improving the grain boundary strength of the induction hardened material, the fatigue strength and impact strength as mechanical parts are improved. If the addition is less than 0.0005%, the above effect is insufficient, and 0.005%
%, The effect saturates, so the content must be within the range of 0.0005 to 0.005%. A preferred range is 0.001 to 0.003%.

Al is added as a deoxidizing agent. 0.015
%, The effect is insufficient. On the other hand, 0.05%
Is exceeded, AlN remains without solution during rolling and heating, and becomes a precipitation site of Ti precipitates, deteriorating cold workability. For the above reasons, the content is 0.015
Must be within the range of 0.05%. The preferred range is 0.
02 to 0.04%.

It is desirable to limit N as much as possible for the following two reasons. B is added for the purpose of improving hardenability and strengthening the grain boundary as described above. However, since the effect of B is manifested only in the state of solid solution B in steel, the amount of N is reduced to reduce BN. It is essential to suppress the generation of. In addition, when N combines with Ti in steel, it generates coarse TiN and increases hardness, and at the same time, TiN causes cold forging cracking, so that cold workability is significantly deteriorated. The above adverse effects are particularly significant when the N content is 0.007% or more.
For the above reasons, the content is less than 0.007% (0%
). The preferred range is 0.005
% Or less.

Ti combines with N in the steel to form TiN, which prevents the precipitation of BN by fixing solid solution N,
That is, it is added for the purpose of securing solid solution B. However, if less than 0.015%, the effect is insufficient. On the other hand, when Ti is added in excess of 3.4N + 0.02% according to the N content, precipitation hardening due to TiC becomes remarkable, and the cold workability deteriorates remarkably. For the above reasons, the content is adjusted to 0.015 to 3.4 N + in accordance with the N content.
It must be within the range of 0.02%. The preferred range is
0.02 to 3.4N + 0.015%.

Since P is an element that increases the deformation resistance during cold forging and deteriorates toughness, the cold workability deteriorates. In addition, the fatigue strength of the final product is degraded by embrittlement of the crystal grain boundaries of the component after induction hardening and tempering, so that it is desirable to reduce the fatigue strength as much as possible. Therefore, it is necessary to limit the content to 0.025% (including 0%) or less. A preferred range is 0.015% or less.

O forms oxide inclusions such as Al ₂ O _{3 in} steel. When a large amount of oxide-based inclusions is present in steel, the cold workability deteriorates. O content is 0.00
If it exceeds 25%, the tendency becomes particularly remarkable. For the above reasons, it is necessary to limit the content to 0.0025% or less (including 0%). A preferred range is 0.002% or less.

Next, in the second claim of the present invention, Mo, Ni
1 or 2 types.

Mo is an element effective for imparting strength and hardenability to steel and for improving the grain boundary strength after induction hardening to increase the strength characteristics. However, if less than 0.02%, the effect is ineffective. Sufficiently, if it exceeds 0.3%, the hardness is increased and the cold workability is deteriorated. For the above reasons, the content needs to be within the range of 0.02 to 0.3%.

Ni is also an effective element for imparting strength and hardenability to steel, but its effect is insufficient if it is less than 0.02%, and if it exceeds 1.0%, the hardness increases. Invited cold forgeability deteriorates. For the above reasons, the content needs to be within the range of 0.02 to 1.0%.

The third aspect of the present invention contains one or two of Nb and V.

Nb combines with C and N in the steel to form Nb (C
N) is an element effective for reducing the crystal grain size and increasing the core hardness by precipitation hardening. If it is less than 0.002%, the effect is insufficient. On the other hand, if the content exceeds 0.035%, the hardness of the material becomes hard and the cold workability deteriorates, and it becomes difficult to form a solution at the time of heating the bar and wire rod rolling.
For the above reasons, the content is 0.002 to 0.035
% Must be within the range. The preferred range is 0.005
~ 0.03%.

V is also added for the same effect as Nb. If it is less than 0.03%, the effect is insufficient. On the other hand, when the content exceeds 0.4%, the hardness of the material becomes hard and the cold workability is deteriorated, and it becomes difficult to form a solution at the time of heating the bar and wire rod rolling. For the above reasons, the content is 0.03
It must be within the range of ~ 0.4%. The preferred range is
0.05-0.3%.

Next, in the present invention, the microstructure after hot working is substantially a ferrite / pearlite structure, the ferrite crystal grain size is limited to 25 μm or less, and the structure of a cross section parallel to the hot rolling direction is provided. Of ferrite band of 1
Limited to the range of ~ 5. The score of the ferrite band is a score defined in the Journal of the Japan Institute of Metals, Vol. 34, page 961, as described above. In the present invention, the reason why the tissue factor is limited as described above will be described below.

First, the reason why the microstructure is a ferrite-pearlite structure is that if a hard structure such as bainite or martensite is mixed in the microstructure, the cold workability is remarkably deteriorated, and cold forging or cutting is difficult. Because it becomes difficult.

Next, since the induction hardening is rapid heating, if the ferrite in the structure before the induction hardening is coarse, the ferrite portion has insufficient carbon diffusion after austenitization and the carbon concentration is low. It becomes lower than the concentration of added carbon, and after quenching, the hardness at that position decreases. Here, a striped structure called a ferrite band is generally observed in a cross section parallel to the rolling direction of the steel material after hot rolling. When coarse ferrite exists continuously in a row as a ferrite band, as shown in FIG. 1, the hardness unevenness after quenching becomes particularly remarkable, and the hardness of the ferrite band in the longitudinal direction corresponds to the original ferrite band. Form a soft band. Therefore, when a torsional moment is applied to the final component, a shear crack force is generated along the soft band, and the component is broken with low strength. The above phenomenon is caused when the ferrite grain size is 25 μm
And when the rating of the ferrite band exceeds 5, it becomes particularly remarkable. For the above reasons, the ferrite grain size is 25
The rating of the ferrite band was restricted to 1 μm or less and the structure of the cross section parallel to the hot rolling direction was rated 1 to 5. The preferred range is a ferrite crystal grain size of 20 μm or less, and a score of a ferrite band having a structure of a cross section parallel to the hot rolling direction is 1 to 4
Range. The ferrite grain size referred to in the present invention corresponds to the circle equivalent diameter when the ferrite form is granular, and corresponds to the width when the ferrite form is plate-like.

Next, a fourth aspect of the present invention is to set the heating temperature to 1
050 ° C or higher, hot rolling finish temperature 750-100
Subsequent to hot rolling at 0 ° C., the steel sheet is hot-worked into a wire or a bar under the condition of gradually cooling a temperature range of 750 to 500 ° C. at a cooling rate of 1 ° C./sec or less.

First, the heating temperature is set to 1050 ° C. or higher for the following reason. If the heating temperature is less than 1050 ° C,
As the rating of the ferrite band exceeds 5, the ferrite band becomes remarkable, and the hardness unevenness after the induction hardening increases. If the heating temperature is lower than 1050 ° C.,
At the time of heating, TiC cannot form a solution and coarsens as Ti (CN), deteriorating cold workability. Therefore, it is necessary to heat at a temperature of 1050 ° C. or more during hot working. A preferred range is 1100 ° C. or higher. Particularly, in the case of Nb-added steel, the heating temperature is desirably set to 1100 ° C. or higher in order to increase the austenitic grain size after induction hardening to 10 or more to achieve high strength.

Next, the finishing temperature of the hot rolling is set to 750 to 1
The reason why the temperature is set to 000 ° C. is as follows. Finishing temperature is 75
If the temperature is lower than 0 ° C., the ferrite band becomes more remarkable as the rating exceeds 5, and the hardness unevenness after the induction hardening increases. On the other hand, when the finishing temperature exceeds 1000 ° C., the hardness of the rolled material becomes hard and the cold workability deteriorates. For the above reasons, the finishing temperature of hot rolling is set to 750 to 1000 ° C.
And The preferred range is 800-960 ° C.

Next, following the hot rolling, 750 to 50
The reason why the temperature range of 0 ° C. is gradually cooled at a cooling rate of 1 ° C./sec or less is as follows. When the cooling rate exceeds 1 ° C./s, the increase in hardness as it is rolled becomes remarkable, and the cold workability deteriorates. Therefore, the cooling rate is limited to 1 ° C./second or less. A preferred range is 0.7 ° C./s or less. In addition, as a method of reducing the cooling rate, there is a method of installing a heat insulating cover or a heat insulating cover with a heat source behind the rolling line, thereby performing slow cooling.

In the present invention, the size of the slab, the cooling rate at the time of solidification, and the conditions of the bulk rolling are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied.

[0046]

EXAMPLES The effects of the present invention will be more specifically described below with reference to examples.

Converter steelmaking compositions having the composition shown in Table 1 were continuously cast and, if necessary, were subjected to a slab rolling process to obtain a rolled material of 162 mm square. Comparative steel No. k is JIS S40C
No. 1 is JIS S53C. continue,
Bars having a diameter of 36 to 45 mm were manufactured by hot working. For cooling after hot rolling, some materials were air-cooled, and some materials were cooled at a lower cooling rate than air-cooling by using a heat insulating cover installed on a cooling floor.

The structure of the bar after rolling was observed, and the ferrite crystal grain size and the score of the ferrite band in a cross section parallel to the rolling direction were determined.

Further, the Vickers hardness of the bar after rolling was measured. Since machinability is proportional to hardness, hardness was used as an index of machinability. Further, an upsetting test piece was prepared from the as-rolled steel bar, and cold deformation resistance and a limit upsetting ratio were obtained as indices of cold workability. The cold deformation resistance was represented by the deformation resistance at an equivalent strain of 1.0.

The hardness and cold forgeability of the material obtained by spheroidizing an as-rolled steel bar were also evaluated in the manner described above.

Further, a static torsional test piece and a torsional fatigue test piece having a parallel portion diameter of 20 mm were sampled from the rolled material. The frequency of the static torsional test piece and the torsional fatigue test piece is 8.5 kHz,
Induction hardening was performed at a maximum heating temperature of 950 ° C., and then tempering was performed at 170 ° C. × 1 hour. In addition,
For Comparative Examples 37 and 38, the maximum heating temperature was 1030 ° C.
Induction hardening was performed under the following conditions. Thereafter, a static torsional test and a torsional fatigue test were performed. The torsional fatigue property is 1 × 10 ⁵
Evaluated by the time intensity in the cycle.

Further, from the rolled material, a diameter 22 mm × length 2
A 00 mm test piece was prepared. This test piece was subjected to induction hardening under the same conditions as the above-mentioned torsion test piece, and then tempered at 170 ° C. × 1 hour. Then, the amount of distortion in the heat treatment due to induction hardening was measured by measuring the amount of deflection at the center of the test piece.

Tables 2 and 3 show the results of these investigations together with the hot working conditions. The hardened layer depth of the induction hardened material is H
It is indicated by the ratio of the depth t to the radius r of V450.

Comparative Example 37 shows the characteristics of JIS S40C, and Comparative Example 38 shows the characteristics of JIS S53C. Comparative Example 37 for the 0.4 to 0.42% C steel of the present invention example,
As for the 0.47 to 0.54% C steel of the present invention, as compared with Comparative Example 38, the cold deformation resistance of the present invention example is generally small, the critical upsetting ratio is generally excellent, and the hardness is generally high. small. That is, the cold workability of the example of the present invention was higher than that of the comparative example
It is superior to 7, 38. The same can be said for the spheroidized annealing material. On the other hand, the present invention example is comparative example 3
Although the maximum heating temperature during induction hardening was set lower than in Examples 7 and 38, the static torsional strength and torsional fatigue strength of the induction hardened material of the present invention were nevertheless comparative examples 37 and 38. Are better than both. Heat treatment distortion is small.

Next, in Table 3, Comparative Example 27 is a case where the content of C is lower than the range specified in the present invention, and the hardened layer hardness of the induction hardened material is low and the strength characteristics are insufficient. Comparative Example 28 was a case where the content of C exceeded the range specified by the present invention, and Comparative Example 29 was a case where the content of Si exceeded the range specified by the present invention. Hard,
Poor cold workability. Comparative Example 30 is a case where the Mn content was below the range specified in the present invention, and the hardened layer depth of the induction hardened material was small, the hardness unevenness was large, and the strength characteristics were insufficient. Comparative Example 31 is a case where the Mn content exceeds the range specified in the present invention, and is harder and inferior in cold workability as compared with the present invention. Comparative Example 32 is a case where the content of Cr exceeds the range specified in the present invention, and is harder than that of the present invention.
In addition, hardness unevenness after induction hardening is large, and heat treatment distortion is large.

In Comparative Example 33, the N content exceeded the range specified in the present invention, and the critical compressibility of cold forgeability was remarkably inferior. Comparative Example 31 is a case where the content of Ti exceeds the range specified in the present invention, and is hard, has a high cold deformation resistance, and is remarkably inferior in the critical compressibility.

In Comparative Example 32, the content of P exceeds the range specified in the present invention, and the critical compressibility of cold forging deteriorates and the strength characteristics after induction hardening are insufficient. Comparative Example 36 is a case where the O content exceeds the range specified in the present invention, and the critical compressibility of cold forgeability is remarkably inferior.

Next, in Comparative Example 39, the hot rolling heating temperature was lower than the range specified in the present invention, and the score of the ferrite band having a cross section parallel to the rolling direction exceeded the range specified in the present invention. In Comparative Example 40, the finishing temperature during hot rolling was lower than the range specified in the present invention, and the rating of the ferrite band having a cross section parallel to the rolling direction was higher than the range specified in the present invention. The hardness unevenness of the hardened layer is large, and both the static torsional strength and the torsional fatigue strength are remarkably inferior. Comparative Example 41 is a case where the hot rolling finish temperature exceeds the range specified in the present invention, and Comparative Example 42 is a case where the cooling rate following hot rolling exceeds the range specified in the present invention. And the cold workability is remarkably inferior. Comparative Example 43 was a case where the ferrite crystal grain size exceeded the range specified in the present invention, and the hardness unevenness of the hardened layer of the induction hardened material was large, and both the static torsional strength and the torsional fatigue strength were remarkably inferior.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

[0062]

The use of the steel material for induction hardening excellent in high strength characteristics and low heat treatment strain of the present invention and the method of manufacturing the same can reduce the hardness unevenness after induction hardening in the production of high strength induction hardened parts. A product having excellent strength characteristics and low heat treatment distortion characteristics can be obtained. By using the steel of the present invention, it is possible to increase the strength of gears and shafts manufactured by induction hardening at a frequency of 5 to 40 kHz. As described above, the industrial effects of the present invention are extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of a ferrite band structure before induction hardening on after induction hardening.

FIG. 2 is a micrograph of a metal structure that quantitatively indicates the degree of a striped structure (magnification: 28 times).

Claims (4)

[Claims] 1. C .: More than 0.38 to 0.58 by weight%
%, Si: 0.01-0.15%, Mn: 0.85-
1.7%, S: 0.005 to 0.15%, Cr: 0.3
5% or less (including 0%), B: 0.0005 to 0.00
5%, Al: 0.015 to 0.05%, N: 0.007
% (Including 0%), and contains Ti in a range of 0.015 to 3.4N + 0.02% depending on the N content;
P: 0.025% or less (including 0%), O: 0.002
5% or less (including 0%), the balance being iron and unavoidable impurities, and the microstructure is substantially a ferrite-pearlite structure, the ferrite crystal grain size is 25 µm or less, A ferrite band having a cross-section parallel to the hot rolling direction, wherein the ferrite band has a rating of 1 to 5, wherein the steel material for induction hardening is excellent in high strength characteristics and low heat treatment distortion characteristics. 2. In addition, Mo: 0.02 to 2% by weight.
The steel material for induction hardening excellent in high strength characteristics and low heat treatment distortion characteristics according to claim 1, characterized in that it contains one or two of 0.3% and Ni: 0.02 to 1.0%. . 3. Nb: 0.002 to 2% by weight.
3. High strength characteristics and low heat treatment distortion characteristics according to claim 1 or 2, characterized by containing one or two of 0.035% and V: 0.03 to 0.4%. Steel material for induction hardening. 4. The steel according to claim 1, wherein the steel is heated at a temperature of at least 1050 ° C., a finishing temperature of the hot rolling is 750 to 1000 ° C., and 750 to 750 following the hot rolling. Hot working into a wire or steel bar under the condition of gradually cooling the temperature range of 500 ° C. at a cooling rate of 1 ° C./sec or less,
A steel material whose microstructure after hot working is substantially a ferrite-pearlite structure, a ferrite crystal grain size is 25 μm or less, and a ferrite band rating of a structure having a cross section parallel to the hot rolling direction is 1 to 5. A method for producing a steel material for induction hardening excellent in high strength characteristics and low heat treatment distortion characteristics, characterized in that: