WO2012073485A1

WO2012073485A1 - Carburizing steel having excellent cold forgeability, and production method thereof

Info

Publication number: WO2012073485A1
Application number: PCT/JP2011/006655
Authority: WO
Inventors: 克行一宮; 三田尾　眞司
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2010-11-30
Filing date: 2011-11-29
Publication date: 2012-06-07
Also published as: CN104630634A; KR20130058069A; KR101631521B1; CN104630634B; JP2013082988A; CN103154293A; JP5927868B2; CN103154293B; US20130186522A1

Abstract

The present invention provides a carburizing steel which has a composition that contains, in terms of mass percentages, 0.1 to 0.35% of C, 0.01 to 0.22% of Si, 0.3 to 1.5% of Mn, 1.35 to 3.0% of Cr, 0.018% or less of P, 0.02% or less of S, 0.015 to 0.05% of Al, 0.008 to 0.015% of N and 0.0015% or less of O within ranges that satisfy formulae (1), (2) and (3), with the remainder comprising Fe and unavoidable impurities, and in which the total structural fraction of ferrite and pearlite in the steel structure is 85% or higher and the average ferrite particle diameter is 25 μm or lower. 3.1 ≥ {([%Si]/2) + [%Mn] + [%Cr]} ≥ 2.2 --- (1) [%C] - ([%Si]/2) + ([%Mn]/5) + 2[%Cr] ≥ 3.0 --- (2) 2.5 ≥ [%Al] / [%N] ≥ 1.7 --- (3) Here, [%M] denotes the content (mass %) of an element M.

Description

Carburizing steel excellent in cold forgeability and method for producing the same

The present invention relates to a carburizing steel excellent in cold forgeability suitable for use in automobiles, various industrial equipment, and the like, and a method for producing the same.

In recent years, gears used in automobiles and the like have been required to be reduced in size as the vehicle weight has been reduced due to energy saving, and the load on the gears has increased. Also, the load on the gears is increasing with the increase in engine output. The durability of a gear is mainly determined by the bending fatigue failure of the tooth root and the surface pressure fatigue failure of the tooth surface.

Conventionally, gears have been manufactured by preparing gear materials using case-hardened steel specified as SCM420H, SCM822H, etc. in JIS G 4053 (2003), and subjecting the gear materials to surface treatment such as carburization. However, since such gears cannot withstand use under high stress, the tooth root bending fatigue strength and pitting resistance can be reduced by changing steel materials, heat treatment methods, surface work hardening, etc. I tried to improve the sex.

For example, in Patent Document 1, while reducing Si in steel and controlling Mn, Cr, Mo and Ni, the grain boundary oxide layer on the surface after carburizing heat treatment is reduced, and the occurrence of cracks is reduced. Moreover, by suppressing the generation of an incompletely hardened layer, the reduction of surface hardness is suppressed to increase the fatigue strength, and further Ca is added to control the elongation of MnS that promotes the generation and propagation of cracks. Is disclosed.
Patent Document 2 discloses a method for increasing the temper softening resistance using a steel material to which Si is added in an amount of 0.25 to 1.50%.

Moreover, high cold forgeability is requested | required of components raw materials, such as a motor vehicle manufactured by cold-forming a bar. Therefore, spheroidizing heat treatment is performed to spheroidize carbides to improve cold forgeability.
For example, in Patent Document 3, the hardness after spheroidizing annealing is low by performing structure control as rolled and performing spheroidizing annealing after drawing wire drawing with a reduction in area of 28% or more, And the method of obtaining the steel material of homogeneous hardness is disclosed.

Japanese Patent Publication No. 07-122118 Japanese Patent No. 2945714 Japanese Patent No. 4392324

However, all of the techniques described in Patent Documents 1, 2, and 3 described above have the following problems.
That is, according to Patent Document 1, since the grain boundary oxide layer and the incompletely hardened layer are reduced by reducing Si, it is possible to suppress the occurrence of bending fatigue cracks at the tooth root. However, the temper softening resistance decreases only by simple Si reduction. As a result, temper softening due to frictional heat on the tooth surface cannot be suppressed, and the surface softens, so that pitching is likely to occur, and there is a problem that the occurrence of fracture shifts from the tooth base to the tooth surface side. .

In Patent Document 2, the amount of Si is increased in order to increase the temper softening resistance. However, this increases the deformation resistance during cold working, and is unsuitable for use in cold forging.

Further, in Patent Document 3, an extra step of performing wire drawing before spheroidizing annealing is required, resulting in an increase in cost.

Furthermore, the form of the microstructure as it is rolled affects the structure and hardness after the spheroidizing heat treatment. In particular, in the case of a relatively coarse ferrite + pearlite structure, the control range for obtaining an appropriate spheroidized structure is narrow, so that it is difficult to obtain a stable structure.

The present invention has been developed in view of the above situation, and is suitable as a material for high-strength gears having higher bending fatigue strength at the root than conventional gears and excellent in surface fatigue properties, It is an object of the present invention to propose a carburizing steel that can obtain a spheroidized annealed structure relatively easily at low cost, has excellent cold forgeability, and can be mass-produced, and an advantageous manufacturing method thereof.

As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
a) By optimizing the amounts of Si, Mn, and Cr in the steel material, the resistance to temper softening is increased, and if this optimization prevents softening due to heat generation on the gear contact surface, cracks in the tooth surfaces that occur during gear drive Occurrence can be suppressed.
b) For grain boundary oxide layers that can be the origin of bending fatigue and fatigue cracks, the growth direction of the grain boundary oxide layer is changed from the depth direction to the surface density increasing direction by adding a certain amount of Si, Mn and Cr. change. Therefore, since there is no oxide layer grown in the depth direction to be a starting point, it becomes difficult to be a starting point for bending fatigue and fatigue cracks.
c) As described in the above a and b, Si, Mn and Cr are effective in improving the temper softening resistance and controlling the grain boundary oxide layer. In order to achieve both of these effects, Si, Mn and Cr It is necessary to strictly control the content of Cr.
d) In order to promote spheroidization of carbide and improve cold forgeability, it is necessary to strictly control the contents of C, Si, Mn and Cr. In particular, a large amount of Cr is effective.

e) In order to stably obtain spheroidization of carbides, it is important to make the microstructure as-rolled into a fine ferrite-pearlite structure. Therefore, the spheroidizing heat treatment conditions shown in FIG. 1 are applied to a high temperature heated rolled material (1140 ° C., coarse ferrite-pearlite structure) and a low temperature heated rolled material (950 ° C. heated, fine ferrite-pearlite structure). I evaluated it. About this evaluation result, FIG. 2 shows the influence of the annealing holding temperature on the hardness after spheroidizing annealing. In the case of a ferrite-pearlite structure with a high heating temperature and a coarse structure, compared to a fine ferrite-pearlite structure with a low heating temperature, the overall hardness is high and the region where the Vickers hardness is HV130 or less is a very narrow temperature. It turns out that it is realizable only in the range. In particular, when the annealing holding temperature is low, a low-temperature heated rolled material is advantageous.
The steel subjected to the experiment contains basic components that satisfy the requirements and suitable conditions described later.

f) Further, the microstructure is influenced by the cold forgeability, but this microstructure is strongly influenced by the structure before annealing in addition to the above spheroidizing annealing conditions. That is, the pre-annealing structure was investigated with respect to the ferrite-pearlite structure fraction and the ferrite grain size.
FIG. 3 shows the control of the structure before spheroidizing annealing on the cold forgeability after spheroidizing treatment (765 ° C.-8 hours), specifically, the ferrite and pearlite. It was found that a steel material having excellent cold forgeability can be obtained by setting the total structural fraction to 85% or more and the average grain size of ferrite to 25 μm or less.
In the experiment shown in FIG. 3, the limit upsetting rate is the upsetting rate when the column is upset by a press machine and a crack occurs at the end. The steel composition is the same as in the experiment of FIG.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.1 to 0.35%
Si: 0.01-0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
N: 0.008 to 0.015% and O: 0.0015% or less are contained within the range satisfying the following formulas (1), (2) and (3), the balance is the composition of Fe and inevitable impurities, and the steel structure Steel for carburization in which the total structural fraction of ferrite and pearlite is 85% or more and the average grain size of ferrite is 25 μm or less.
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C]-([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (mass%)

The above carburizing steel is subjected to cold forging which is processed into various parts shapes after carburizing treatment. Prior to this cold forging, it is preferable to perform spheroidizing annealing, but depending on the required amount of processing, etc., it may be subjected to cold forging without performing spheroidizing annealing.

2. In the above 1, the steel is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
Carburizing steel containing one or more selected from V: 0.5% or less and Nb: 0.06% or less.

3. % By mass
C: 0.1 to 0.35%
Si: 0.01-0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
A steel material containing N: 0.008 to 0.015% and O: 0.0015% or less in a range satisfying the following formulas (1), (2) and (3), with the balance being a composition of Fe and inevitable impurities, Heated to 1160 ° C or higher and lower than 1220 ° C to perform hot working, once finished hot working in a temperature range of ₃ or more points of Ar, cooled to 450 ° C or lower, then over 900 ° C to 970 ° C or lower After reheating, the hot working is resumed, and after the hot working is completed under the condition that the total reduction ratio after reheating is 70% or more, the temperature range of 800-500 ° C is 0.1-1.0 ° C / s. A method for producing carburizing steel that is cooled at a low temperature.
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] － ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)

4). In 3 above, the steel material is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
A manufacturing method of carburizing steel containing one or more selected from V: 0.5% or less and Nb: 0.06% or less.

According to the present invention, for example, when processed into a gear, carburizing steel excellent in not only the bending fatigue property of the tooth root but also the surface pressure fatigue property of the tooth surface is mass-produced in a process involving cold forging. Can be obtained at

It is a figure which shows the heat processing conditions in spheroidization heat processing. It is a figure which shows the influence of the annealing holding temperature which acts on the hardness after spheroidization heat processing. It is a graph which shows the influence of the structure | tissue before spheroidizing annealing on the cold forgeability after a spheroidizing process. It is a figure which shows the heat processing conditions in spheroidization heat processing.

The present invention will be specifically described below.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.1-0.35%
In order to increase the hardness of the core part by quenching after carburizing treatment, 0.1% or more of C is required. However, if the content exceeds 0.35%, the toughness of the core part decreases, so the amount of C is 0.1 to Limited to a range of 0.35%. Preferably it is 0.1 to 0.3% of range.

Si: 0.01-0.22%
Si is an element that increases the softening resistance in the temperature range of 200 to 300 ° C., which is expected to be reached during the rolling of gears and the like, and at least 0.01% addition is indispensable for exerting the effect. Preferably 0.03% or more is added. However, on the other hand, since Si is a ferrite stabilizing element, excessive addition raises the Ac ₃ transformation point, and ferrite tends to appear in the core portion having a low carbon content in the normal quenching temperature range. , Leading to a decrease in strength. Excessive addition also has the disadvantage of hardening the steel material before carburizing and degrading the cold forgeability. In this respect, when the Si content is 0.22% or less, the above-described adverse effects do not occur, so the Si content is limited to a range of 0.01 to 0.22%. Preferably it is 0.03 to 0.22% of range.

Mn: 0.3-1.5%
Mn is an element effective for hardenability, and requires addition of at least 0.3%. However, Mn tends to form an abnormal carburizing layer, and excessive addition causes an excessive amount of retained austenite and leads to a decrease in hardness, so the upper limit was made 1.5%. Preferably it is 0.4 to 1.2% of range. More preferably, it is in the range of 0.6 to 1.2%.

Cr: 1.35 to 3.0%
Cr is an element effective for improving not only hardenability but also temper softening resistance. However, if its content is less than 1.35%, its addition effect is poor. On the other hand, if it exceeds 3.0%, the effect of increasing the softening resistance is saturated, and rather, it becomes easier to form a carburized abnormal layer, so the Cr content is limited to the range of 1.35 to 3.0%. Preferably it is 1.35 to 2.6% of range.

P: 0.018% or less P is segregated at the grain boundary and lowers the toughness of the carburized layer and the core. Therefore, the lower the content, the better, but 0.018% is acceptable. Preferably it is 0.016% or less. Usually, it is difficult to make the content 0%, but if possible, the content may be 0%.

S: 0.02% or less S is an element that exists as sulfide inclusions and is effective in improving machinability. However, excessive addition causes a decrease in fatigue strength, so the upper limit was made 0.02%. From the viewpoint of machinability, 0.004% or more may be contained.

Al: 0.015-0.05%
Al is an element which combines with N to form AlN and contributes to the refinement of austenite crystal grains. To obtain this effect, 0.015% or more, preferably 0.018% or more is required to be added. On the other hand, if the content exceeds 0.05%, the formation of Al ₂ O ₃ inclusions harmful to fatigue strength is promoted, so the Al content is limited to a range of 0.015 to 0.05%. Preferably it is 0.015 to 0.037% of range.

N: 0.008 to 0.015%
N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains. Therefore, the appropriate addition amount is determined by the quantitative balance with Al, but 0.008% or more of addition is necessary to exert the effect. However, if added in excess, bubbles are generated in the steel ingot during solidification and deterioration of forgeability is caused, so the upper limit is made 0.015%. Preferably it is 0.010 to 0.015% of range.

O: 0.0015% or less O is present as an oxide inclusion in steel and is an element that impairs fatigue strength. Therefore, the lower the content, the better, but 0.0015% is acceptable. Usually, it is difficult to make the content 0%, but if possible, the content may be 0%.

In the above, the proper composition range of the basic component of the present invention has been described. However, in the present invention, it is not sufficient that each element simply satisfies the above range. For C, Si, Mn, Cr, Al and N, It is important to satisfy the relationships of the following expressions (1), (2) and (3).
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] － ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)

The above formula (1) is a factor that affects the hardenability and temper softening resistance.If the formula (1) is less than 2.2, the effect of improving the hardenability and temper softening resistance is not sufficient, and the fatigue strength is low. It becomes insufficient. On the other hand, if it exceeds 3.1, not only the above-mentioned improvement effect is saturated, but also cold workability is deteriorated.
In addition, the above formula (2) is a factor that affects the ease of spheroidizing of the carbide, and when the formula (2) satisfies 3.0 or more, the spheroidization becomes easy. By combining this composition with the knowledge of e and f, extremely excellent cold forgeability after spheroidizing annealing can be obtained.
Furthermore, the above equation (3) is a factor that affects the refinement of austenite crystal grains. If the value of the equation (3) is less than 1.7, the refinement effect is poor and the fatigue strength is insufficient. On the other hand, if it exceeds 2.5, the crystal grains easily become coarse and fatigue strength becomes insufficient, and workability is reduced due to solute Al and solute N.

The basic components of the present invention have been described above. However, in the present invention, the following components can be appropriately contained as needed.
Cu: 1.0% or less Cu is effective in improving the strength of the base metal. However, if the content exceeds 1.0%, hot brittleness occurs and the surface properties of the steel deteriorate, so the content is made 1.0% or less. A suitable addition amount is 0.01% or more.

Ni: 0.5% or less Ni is effective in improving the strength and toughness of the base metal, but it is expensive. A suitable addition amount is 0.01% or more.

Mo: 0.5% or less Mo, like Ni, is effective in improving the strength and toughness of the base metal, but is included at 0.5% or less because it is expensive. The content may be 0.2% or less. A suitable addition amount is 0.05% or more.

V: 0.5% or less V, like Si, is an element useful for increasing the temper softening resistance. However, if the content exceeds 0.5%, the effect is saturated. A suitable addition amount is 0.01% or more.

Nb: 0.06% or less Nb, like V and Si, is an element useful for increasing the temper softening resistance. However, if the content exceeds 0.06%, the effect is saturated, so 0.06% or less. A suitable addition amount is 0.007% or more.
The balance composition of the steel material is Fe and inevitable impurities. For example, B is not particularly added, but may be contained as an impurity as long as it is less than about 0.0003%.

In addition to the adjustment of the component composition described above, it is necessary to control the steel structure of the material before spheroidizing annealing.
Total structure fraction of ferrite and pearlite: 85% or more When the bainite fraction in the structure before spheroidizing annealing increases, the deformation resistance increases and the cold forgeability deteriorates, so the total structure of ferrite and pearlite It is necessary to lower the bainite fraction by setting the fraction to 85% or more. The upper limit may be 100%.
In the present invention, since steel with high hardenability that satisfies the above-mentioned formula (1) is used, it is difficult to secure the amount of ferrite + pearlite in the normal production method, but the heating temperature, total rolling reduction rate and cooling during rolling are difficult. By adjusting the speed, ferrite + pearlite: 85% or more can be realized.
Average ferrite particle diameter: 25 μm or less The structure before spheroidizing annealing greatly affects the characteristics after spheroidizing annealing. That is, if the ferrite grain size in the microstructure before spheroidizing annealing exceeds 25 μm, the cold forgeability after spheroidizing treatment deteriorates. In particular, since the influence on the limit upsetting rate is large, the average particle diameter of ferrite is set to 25 μm or less. In terms of technical thought, it is not necessary to specify a lower limit, but a practical lower limit is about 5 μm.

Next, the manufacturing conditions of the present invention will be described.
In the present invention, the steel material having the above-mentioned preferred component composition is heated to 1160 ° C. or more and less than 1220 ° C., then rolled in a temperature range of Ar ₃ points or more, temporarily cooled to 450 ° C. or less, and then 900 ° C. After re-heating to a temperature below 970 ° C and hot rolling under the condition of a total reduction of 70% or more after reheating, the temperature range of 800-500 ° C is at a rate of 0.1-1.0 ° C / s. It is necessary to cool.
Hereinafter, the reason why each processing condition is limited as described above will be described.

[Steel material heating temperature (1st stage): 1160 ℃ or more and less than 1220 ℃]
In the present invention, since it is necessary to dissolve AlN sufficiently once from the solidified state, the steel material is heated to a temperature of 1160 ° C. or higher. However, if the heating temperature is too high, the first stage heating temperature is set to less than 1220 ° C. because scale loss, surface property deterioration, fuel cost increase, and the like.

[After the hot working is completed in the temperature range of Ar ₃ points or higher, it is once cooled to 450 ° C or lower]
In this hot working step, preferably the hot rolling step, in order to break the cast structure and obtain a ferrite-pearlite structure, the working is finished at Ar ₃ or higher and cooled to 450 ° C. or lower. Moreover, it is advantageous from the viewpoint of obtaining a ferrite-pearlite structure that the hot working is performed at a reduction rate of 50% or more. There is no particular need to set a lower limit for the cooling end temperature, and a realistic value may be selected in consideration of the reheating cost. It is not necessary to provide an upper limit for the reduction ratio in hot working, and a realistic value may be selected in consideration of equipment load.

[Steel material reheating temperature (second stage): Over 900 ° C and below 970 ° C]
In order to obtain a spheroidized annealed structure and low hardness, it is necessary to re-heat to a temperature of 970 ° C. or lower because the structure needs to be a fine ferrite-pearlite structure as it is rolled. When the temperature exceeds 970 ° C., AlN is coarsely precipitated, whereas when it is 970 ° C. or lower, it is effective for suppressing coarsening during carburization by fine precipitation. However, since the AlN is not sufficiently precipitated by heating at 900 ° C. or lower, the second stage heating temperature is set to over 900 ° C. Preferably it is 920 degreeC or more.

[Total rolling reduction in hot working: 70% or more]
If the total rolling reduction ratio in hot working after reheating, that is, the total rolling reduction ratio in the processing step after reheating is small, the crystal grains are coarse and the ferrite fraction after cooling is reduced. Not only is it easy to occur, but the hardness of the workpiece increases, so 70% or more. The upper limit of the rolling reduction is not particularly required, and a realistic value may be selected in consideration of the equipment load.
In addition, this rolling reduction means the reduction rate of thickness when the steel material obtained by hot working is a plate, and the area reduction rate when it is a steel bar or wire.

[Cooling rate in the temperature range of 500 to 800 ° C: 0.1 to 1.0 ° C / s]
In the cooling process after hot working, if the cooling rate in the temperature range of 800 to 500 ° C. is less than 0.1 ° C./s, the ferrite grain size becomes large and a coarse ferrite-pearlite structure is formed. On the other hand, when it exceeds 1.0 ° C./s, the ferrite fraction after cooling decreases, and a mixed structure of bainite and ferrite-pearlite is obtained. Therefore, the cooling rate in this temperature range is limited to the range of 0.1 to 1.0 ° C./s.

The carburizing steel obtained by the above manufacturing method is preferably subjected to spheroidizing annealing and then subjected to cold forging. The spheroidizing annealing is preferably performed at 760 to 820 ° C. for about 2 to 15 hours. However, the present invention can obtain excellent cold forgeability even at a relatively low temperature spheroidizing annealing of about 740 to 760 ° C. it can. The structure after spheroidizing annealing is a structure obtained by dividing and spheroidizing plate-like cementite in the layered pearlite of the previous structure. Although the ground structure is ferrite, it retains the two-phase region of austenite and ferrite in the heating stage, and therefore generally inherits the previous structure.
The steel that has been cold forged into a predetermined part shape is subjected to carburizing heat treatment by a conventional method. The surface of the member after the carburizing heat treatment has a structure mainly composed of martensite (tempered martensite when tempered).

Steels having various component compositions shown in Table 1 were melted in a 100 kg vacuum melting furnace, and the slab was rolled under the hot working conditions and cooling conditions shown in Table 2 to obtain bar steel. That is, the first stage hot working is performed by heating at the heating temperature shown in Table 2, and after cooling to 450 ° C. or lower, the heating is performed at the heating temperature, the total reduction rate, and the cooling rate conditions shown in Table 2. The second stage hot working of rolling and cooling was performed to obtain a steel bar. The obtained steel bar was evaluated under the following conditions for the structural fraction, ferrite average particle size, cold workability, spheroidizing heat treatment property, carburized portion property, and fatigue property.

(1) Structure fraction and ferrite average particle size After 1 / 4D position of the cross section in the L direction of the steel bar is mirror-polished, it is corroded with nital, and the photograph taken at 400 times is analyzed by image analysis. The fraction (area fraction) and the average particle diameter of the ferrite were determined.

(2) Evaluation method of cold workability (cold forgeability) Cold workability was evaluated by two items of a deformation resistance value and a limit upsetting rate.
That is, the deformation resistance value was obtained by collecting a test piece having a diameter of 10 mm and a height of 15 mm from a position 1 / 4D from the surface of a rolled steel bar (diameter D), and using a 300 t (3000 kN) press machine. The compressive load at 70% upsetting was measured, and the deformation resistance measurement method by end face constrained compression proposed by the Japan Society for Technology of Plasticity was used.
The limit upsetting rate was defined as the upsetting rate when compression processing was performed by a method of measuring deformation resistance and a crack occurred at the end.
If the deformation resistance value is 918 MPa or less and the limit upsetting rate is 76% or more, it can be said that the cold workability is good.

(3) Evaluation method of spheroidizing heat treatment property The spheroidizing heat treatment property was evaluated by three items: hardness after spheroidizing heat treatment, deformation resistance value, and limit upsetting rate.
That is, in the same manner as the evaluation of cold workability in (2) above, a test piece having a diameter of 10 mm and a height of 15 mm was taken from a position 1 / 4D from the surface of the rolled steel bar (diameter D). After subjecting this test piece to spheroidizing heat treatment, the deformation resistance value and the limit upsetting rate were determined. The spheroidizing heat treatment was carried out under the two conditions (A) and (B) shown in FIG. 4, and 9 points were measured by a Vickers hardness test [load: 98 N (10 kgf)] to obtain an average value and a maximum value. If the average hardness after spheroidizing heat treatment is less than HV130 and the maximum value is HV135 or less, it can be said that the cold forgeability is very excellent and the stability is also excellent.
Further, if the deformation resistance value after spheroidizing heat treatment (condition (A)) is 890 MPa or less and the limit upsetting rate is 80% or more, it can be said that the cold workability is good.

(4) Evaluation method of carburized zone characteristics Carburized zone characteristics are 2 of the presence of coarse grains in the carburized zone and the grain boundary oxidation depth after carburizing at 930 ° C for 7 hours and carbon potential: 0.8%. We evaluated by item.
That is, in the carburized portion, the case where there was no generation of coarse particles was marked as ◯, and the case where coarse particles were generated was marked as x.
Grain boundary oxidation behavior was evaluated by observing the surface of the test piece after carburizing treatment with an optical microscope and measuring the grain boundary oxidation depth. That is, an optical microscope observation was performed at a magnification of 400 times, the maximum grain boundary oxidation depth in each field of view was obtained, and the average value of 10 fields of view was defined as the grain boundary oxidation depth.
If there is no generation of coarse grains in the carburized part and the grain boundary oxidation depth is 10 μm or less, it can be said that the carburized part characteristics are excellent.

(5) Evaluation Method of Fatigue Properties Fatigue properties were evaluated by two items: a rotating bending fatigue test piece and surface fatigue strength.
That is, a rotating bending fatigue test piece and a roller pitching test piece for evaluating surface fatigue strength were processed from a rolled steel bar and subjected to the test. These test pieces were carburized under conditions of 930 ° C., 7 hours, carbon potential: 0.8%, and then subjected to heat tempering treatment at 180 ° C. for 1 hour.
The rotating bending fatigue test was carried out at a rotational speed of 1800 rpm and evaluated with a strength of 10 ⁷ times.
The roller pitching test was evaluated with a strength of 10 ⁷ times under the conditions of slip ratio: 40% and oil temperature: 80 ° C.
If the rotational bending fatigue strength is 806 MPa or more and the surface fatigue strength is 3250 MPa or more, it can be said that the fatigue strength is good.
The obtained results are shown in Table 3.

As shown in Table 3, all of the inventive examples obtained according to the present invention are excellent in cold workability after rolling and after spheroidizing heat treatment, have a shallow grain boundary oxidation depth, and have coarse grains in the carburized portion. It can be seen that there is no occurrence and that the rotating bending fatigue strength and the contact pressure fatigue strength are superior to those of the comparative example.

According to the present invention, it is possible to provide a carburizing steel that is excellent in cold workability and excellent in rotational bending fatigue strength and surface pressure fatigue strength. Therefore, for example, when machined into gears, it is possible to obtain carburizing steel excellent in not only the bending fatigue characteristics of the tooth root but also the surface pressure fatigue characteristics of the tooth surface under mass production in the process involving cold forging. it can.

Claims

% By mass
C: 0.1 to 0.35%
Si: 0.01-0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
N: 0.008 to 0.015% and O: 0.0015% or less are contained within the range satisfying the following formulas (1), (2) and (3), the balance is the composition of Fe and inevitable impurities, and the steel structure Steel for carburization in which the total structural fraction of ferrite and pearlite is 85% or more and the average grain size of ferrite is 25 μm or less.
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C]-([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (mass%)
The steel according to claim 1, wherein the steel is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
Carburizing steel containing one or more selected from V: 0.5% or less and Nb: 0.06% or less.
% By mass
C: 0.1 to 0.35%
Si: 0.01-0.22%,
Mn: 0.3-1.5%
Cr: 1.35 to 3.0%
P: 0.018% or less,
S: 0.02% or less,
Al: 0.015-0.05%
A steel material containing N: 0.008 to 0.015% and O: 0.0015% or less in a range satisfying the following formulas (1), (2) and (3), with the balance being a composition of Fe and inevitable impurities, Heated to 1160 ° C or higher and lower than 1220 ° C to perform hot working, once finished hot working in a temperature range of 3 or more points of Ar, cooled to 450 ° C or lower, then over 900 ° C to 970 ° C or lower After reheating, the hot working is resumed, and after the hot working is completed under the condition that the total reduction ratio after reheating is 70% or more, the temperature range of 800-500 ° C is 0.1-1.0 ° C / s. A method for producing carburizing steel that is cooled at a low temperature.
3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2 --- (1)
[% C] － ([% Si] / 2) + ([% Mn] / 5) +2 [% Cr] ≧ 3.0 --- (2)
2.5 ≧ [% Al] / [% N] ≧ 1.7 --- (3)
However, [% M] is the content of element M (% by mass)
In Claim 3, the steel material is further in mass%,
Cu: 1.0% or less,
Ni: 0.5% or less,
Mo: 0.5% or less,
A manufacturing method of carburizing steel containing one or more selected from V: 0.5% or less and Nb: 0.06% or less.