JPH08109435A - Steel for low strain type carburized and quenched gear - Google Patents

Abstract

PURPOSE: To stably produce a gear free from strain due to carburizing and quenching and excellent in dimensional accuracy by making the structure of a noncarburized part a dual phase structure consisting of ferrite and martensite, at the time of carburizing and quenching a gear made of a steel stock of specific composition to harden its surface. CONSTITUTION: A gear is produced by using a steel slab which has a composition consisting of, by weight, 0.10-0.35% C, 1.0-2.50% Si, 0.20-1.50% Mn, 0.10-1.50% Cr, 0.01-0.50% Ni, 0.01-1.50% Mo, 0.01-0.10% Al, and the balance Fe or further containing specific small amounts of V, Ti, Nb, Zr, etc. Then, the gear is carburized and hardened, by which the surface of the gear is hardened. By this method, the superior gear, in which the Ac3 point parameter represented by equation 1 and the ideal critical diameter D1 represented by equation 2 are regulated to 850-950 deg.C and 60-400mm, respectively, and the internal structure of a noncarburized part is composed of a dual phase structure of martensite containing 10-70% ferrite and the non-carburized-and-quenched part in the inner part of the gear is composed of a dual phase structure of martensite containing 10-70% ferrite, can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low distortion type carburized and quenched gear steel, which is suitable as a steel material for gears such as automobiles, construction machines, and industrial machines, and has an extremely small amount of strain during carburizing and quenching. is there.

[0002]

2. Description of the Related Art For example, in recent automobiles, quietness during driving has been significantly improved, and noise generated during driving is mainly due to gear noise generated from gears. Gear noises are caused by gear meshing defects.Such gear meshing defects are caused by carburizing or carbonitriding quenching to harden the surface of a gear steel material that has been formed into a prescribed shape. This is because of the distortion that occurs when the treatment (generally referred to as carburizing and quenching) is performed.

That is, when carburizing and quenching a steel for gears, transformation stress (stress due to volume expansion that occurs when transforming from an austenite structure to a martensite structure) occurs due to the formation of martensite, so that distortion occurs in the steel material. This is unavoidable, and as a result, the dimensional accuracy of the gear cannot be maintained high. In particular, in transmission gears for automobiles, despite being extremely strict with respect to noise, the structure is small and thin, and the internal structure is mainly martensite-containing bainite. Therefore, distortion is likely to occur during carburizing and quenching, which is the largest cause of gear noise.

In order to improve the dimensional accuracy of gears, it is sufficient to reduce the amount of quenching distortion by subjecting the steel material for gears that has been carburized and quenched to a tooth profile modification process in which the carburized layer is partially shaved by mechanical grinding. However, such a tooth profile correction by mechanical grinding causes a significant decrease in productivity due to an increase in the number of manufacturing steps, and also increases the manufacturing cost, and also causes unevenness in surface hardness and residual stress. There is.

From the above-mentioned point, the steel material for gears is often used without being subjected to the tooth profile correction treatment after carburizing and quenching, and therefore, the quenching strain amount is reduced in order to improve the precision of the carburized and quenched gear. Is needed. Such carburizing and quenching strain amount is greatly affected by the hardenability of the material. Further, since carburizing and quenching is usually performed at a high temperature of about 920 ° C., coarsening of austenite crystal grains during carburizing is also one of the causes of distortion.

[0006] Various studies have hitherto been made on means for reducing the amount of quenching strain of gear steel, for example, so that the quenchability is the lower limit of the Jominy band.
A method for controlling hardenability by controlling the chemical composition of steel within a specific narrow range, and JP-A-4-24
As disclosed in JP-A-7848, JP-A-59-123743, etc., a method of finely adjusting the crystal grains by incorporating a grain refinement element such as Al, Ti, Nb in steel. (Hereinafter, referred to as Prior Art 1) is known.

Further, in Japanese Unexamined Patent Publication (Kokai) No. 5-70925, a gear made of steel containing a specific range of Si, Mn, Cr, Mo and V is subjected to carbonitriding treatment, and then the tooth surface portion, that is, Cool to a temperature range below the Ar ₁ transformation point of the carbonitriding part, and then again to a temperature range above the Ar ₃ transformation point of the tooth surface, that is, the carbonitriding part, below the Ar ₁ transformation point inside the tooth (non-carburizing part). After holding, by quenching and tempering, while maintaining the austenite structure of the tooth surface,
Method of forming pearlite structure (hereinafter referred to as prior art 2)
Is disclosed.

[0008]

The prior art 1 has a limit in suppressing the occurrence of strain associated with martensitic transformation, and has a problem that the strain cannot be sufficiently reduced. Further, in Prior Art 2, since the inside of the tooth (non-carburized portion) has a ferrite-pearlite structure, it is difficult to secure sufficient toughness, and the heat treatment operation is complicated,
There is a problem that not only productivity is hindered but also cost is increased.

Therefore, an object of the present invention is to solve the above-mentioned problems, to obtain a gear with extremely small distortion due to carburizing and quenching treatment, high dimensional accuracy, no gear noise during use, and easy heat treatment. It is an object of the present invention to provide a low distortion type carburized and quenched gear steel that can be economically manufactured by, and is suitable as a material for gears of automobiles, construction machinery, industrial machinery, and the like.

[0010]

From the above-mentioned viewpoints, the present inventors have developed a low strain type carburized and quenched gear steel that produces a highly accurate dimensional precision gear train with extremely little distortion due to carburizing and quenching treatment. As a result, he conducted extensive research.

Since the main cause of affecting the amount of quenching strain of the steel for gears is the amount of strain resulting from the volume expansion that occurs when the austenite structure transforms into the martensite structure, the present inventors have It was found that when 10 to 70% of ferrite is mixed in the austenite structure during heating, and the structure after carburizing and quenching is made into a ferrite-martensite two-phase structure, the quenching strain amount is significantly reduced.

Under normal carburizing conditions, it is necessary to raise the Ac ₃ transformation temperature in order to mix ferrite in the austenite structure. Therefore, Si, Mn, Cr, Mo in steel
As a result of detailed examination of the effects of V and V on the Ac ₃ transformation temperature, by adjusting the contents of these elements, a ferrite-martensite two-phase structure can be easily obtained under normal carburizing conditions, and It was discovered that the ferrite strengthening element strengthens the inside of the tooth (non-carburized portion), and the amount of quenching strain can be significantly reduced without lowering the fatigue strength.

The present invention has been made based on the above findings, and the low strain type carburized and quenched gear steel of the present invention has carbon (C): 0.10 to 0.35 wt.%, Silicon (Si): 1.0 ~ 2.50wt.%, Manganese (Mn): 0.20-1.50wt.%, Chromium (Cr): 0.10-1.50wt.%, Nickel (Ni): 0.01-0.50wt.%, Molybdenum (Mo): 0.01-1.50 wt.%, aluminum (Al): 0.01 to 0.10 wt.%, and the rest: iron (Fe) and unavoidable impurities, and, if necessary, vanadium (V): 0.01 to 0.50 wt.% , Titanium (Ti): 0.01 to 0.10 wt.%, Niobium (Nb): 0.01 to 0.10 wt.%, Zirconium (Zr): 0.01 to 0.10 wt.%, Containing at least one element selected from the group consisting of The Ac ₃ point parameter calculated by the following equation (1) is in the range of 850 to 940 ℃, Ac ₃ = 920 −203 √C ＋ 44.7Si ＋ 31.5Mo−30Mn−15.2Ni−11Cr + 400Al ── (1) Below The ideal critical diameter (D ₁ ) calculated by equation (2) is 60
Within the range of ~ 400 mm, D ₁ = 7.95√C (1 + 0.70Si) (1 + 3.3Mn) (1 + 2.16Cr) (1 + 0.36Ni) (1 + 3.00Mo) ─── (2 ) And, the internal structure of the non-carburized part after carburizing and quenching is
It is characterized by having a two-phase structure composed of martensite containing 10 to 70% of ferrite.

[0014]

According to the steel of the present invention, by increasing the contents of Si and Mo, which are elements that raise the Ac ₃ transformation temperature and improve hardenability, the ferrite-martensite dual-phase structure can be easily obtained by carburizing and quenching treatment. Can be
Since the ferrite absorbs the expansion strain of martensite, the amount of quenching strain is significantly reduced, and the core hardness at the time of quenching can be sufficiently secured, so that fatigue strength comparable to conventional steel can be obtained.

Further, in the gears of automobiles, shot peening treatment is often performed for the purpose of improving fatigue strength. According to the steel of the present invention, the intergranular oxide layer is reduced and a structure with poor quenching is formed. Since it does not occur, the surface roughness does not deteriorate even if shot peening is applied,
Therefore, the surface fatigue strength is improved.

Next, the reason why the chemical composition of the steel for carburizing and quenching gears of the present invention is limited to the above-mentioned range will be described below. (1) Carbon (C): Carbon is a basic element necessary for guaranteeing core strength by carburizing and quenching, and in order to exert its effect, it must be contained at 0.10 wt.% Or more. is necessary. However, when the carbon content exceeds 0.35 wt.%, Toughness is deteriorated and machinability is deteriorated. Therefore, the carbon content should be limited to the range of 0.10 to 0.35 wt.%.

(2) Silicon (Si): Silicon is a relatively inexpensive element effective for increasing the Ac ₃ transformation point. However, when the silicon content is less than 1.00 wt.%, The silicon concentration in the vicinity of the surface layer that binds to the trace oxygen in the carburizing gas during carburization is low, so the trace oxygen penetrates deep into the steel and the grain boundary As a result of the oxygen amount becoming extremely deep, fatigue strength is lowered. On the other hand, if the silicon content exceeds 2.50 wt.% And becomes excessive, the SiO ₂ -based inclusions increase, resulting in a decrease in fatigue strength. Therefore, the silicon content is 1.
It should be limited to the range of 00-2.50 wt.%.

(3) Manganese (Mn): Manganese is an element effective for improving hardenability and ensuring core strength, and in order to exert its action, it is contained at 0.20 wt.% Or more. It is necessary. However, since manganese has an action of lowering the Ac ₃ transformation point, when the content thereof exceeds 1.50 wt.% And becomes large, not only the two-phase structure cannot be obtained, but also the hardness becomes too high, This leads to deterioration of machinability. Therefore, the manganese content is 0.20 to 1.50 wt.%
Should be limited to within the range.

(4) Chromium (Cr): Chromium, like manganese, is an element effective for improving hardenability, and in order to exert its action, it is necessary to contain 0.10 wt.% Or more. Is. However, chromium, like manganese, has the effect of lowering the Ac ₃ transformation point, so
If the content exceeds 1.50 wt.% And becomes large, not only the two-phase structure cannot be obtained, but also the hardness becomes too high and the machinability deteriorates. Therefore, the manganese content is 0.10 ~
It should be limited to the range of 1.50 wt.%.

(5) Nickel (Ni): Nickel is an element effective for improving hardenability and toughness, and it is necessary to contain 0.01 wt.% Or more in order to exert its action. . However, the nickel content is 0.50
If the amount exceeds 0.5% by weight, the hardness becomes too high, the machinability deteriorates, and nickel is expensive, which causes an economical disadvantage. Therefore, the nickel content should be limited to the range of 0.01 to 0.50 wt.%.

(6) Molybdenum (Mo): Molybdenum is Ac
₃ Higher transformation point, and, like nickel, hardenability,
It is an element effective in improving toughness and fatigue strength, and in order to exert its action, it is necessary to contain 0.01 wt.% Or more. However, if the molybdenum content exceeds 1.50 wt.%, The effect is saturated and an economic disadvantage is brought about. Therefore, the molybdenum content is 0.01-1.50.
It should be limited to the range of wt.%.

(7) Aluminum (Al): Aluminum is an element effective in improving the toughness and fatigue strength in addition to forming AlN by combining with nitrogen, refining crystal grains to reduce quenching strain. Therefore, in order to exert its effect, it is necessary to contain 0.01 wt.% Or more.
However, if the aluminum content exceeds 0.10 wt.% And becomes large, there arises a problem that the alumina-based inclusions increase. Therefore, the aluminum content should be limited to the range of 0.01-0.10 wt.%.

(8) Vanadium (V): Vanadium is an element that is effective for enhancing hardenability and fatigue strength, and also produces carbonitrides to make crystal grains finer and to reduce quenching strain. It has a suppressing effect, and in order to exert its effect, it is necessary to contain 0.01 wt.% Or more. However, if the vanadium content exceeds 0.50 wt.%, The effect is saturated and an economic disadvantage is brought about. Therefore, the vanadium content should be limited to the range of 0.01 to 0.50 wt.%.

(9) Titanium (Ti), niobium (Nb), zirconium (Zr): Titanium, niobium and zirconium are
It is an element effective for refining austenite crystal grains, and also has an effect of increasing the yield strength of the carburized part and the inside and contributing to the improvement of fatigue strength. Therefore, if necessary, at least one of the above elements is contained. However, if the content of at least one of titanium, niobium and zirconium is less than 0.01 wt.%, The above-mentioned action cannot be obtained. On the other hand, when the content of at least one of the above-mentioned elements exceeds 0.10 wt.%, The effect is saturated and an economic disadvantage is brought about. Therefore, the content of at least one of titanium, niobium and zirconium should be limited to the range of 0.01 to 0.10 wt.%.

In addition to the above-mentioned elements, the steel of the present invention may contain P, S, Cu, etc. as impurities. In addition, in order to improve machinability, S, Pb,
Free-cutting elements such as Ca and Se may be included.

[0026] (10) Ac ₃ point parameter: The Ac ₃ point parameter is calculated by the following equation (1) is less than 850 ° C.,
Even if kept at the carburizing temperature, ferrite cannot be secured in austenite. On the other hand, when the Ac ₃ point parameter exceeds 940 ° C, the amount of ferrite becomes excessive and the core strength becomes insufficient. Therefore, the Ac _3- point parameter calculated by the following equation (1) should be limited to the range of 850 to 940 ° C. _{Ac 3 = 920 -203 √C + 44.7Si} + 31.5Mo-30Mn-15.2Ni-11Cr + 400Al ── (1)

(11) Ideal critical diameter (D ₁ ): Ideal critical diameter (D ₁ )
Is a value representing the hardenability of steel. In order to secure the desired fatigue strength, the austenite grain size of 8 is given below.
The ideal critical diameter (D ₁ ) value calculated by equation (2) must be 60 mm or more. On the other hand, when the above (D ₁ ) value exceeds 400 mm, the effect of the ferrite mixed in the austenite structure disappears, and the amount of quenching strain increases. Therefore, the ideal critical diameter (D ₁ ) calculated by the following equation (2) for the austenite grain size No. 8 should be limited to the range of 60 to 400 mm. D ₁ = 7.95√C (1 + 0.70Si) (1 + 3.3Mn) (1 + 2.16Cr) (1 + 0.36Ni) (1 + 3.00Mo) ─── (2)

(12) Ferrite amount in internal structure (non-carburized part): When the ferrite amount in internal structure (non-carburized part) is less than 10%, transformation strain of martensite cannot be sufficiently absorbed and quenching strain The amount cannot be kept small.
On the other hand, if the ferrite content exceeds 70%, it becomes difficult to maintain desired strength and toughness. Therefore, the amount of ferrite in the internal structure (non-carburized portion) should be limited to the range of 10 to 70%.

[0029]

EXAMPLES Next, the present invention will be described by way of Examples in comparison with Comparative Examples. Table 1 shows the chemical composition, Ac ₃ point parameter and ideal critical diameter (D) within the scope of the present invention.
Inventive steel Nos. 1 to 3 having ₁ ) and chemical composition,
At least one of the Ac ₃ point parameter and the ideal critical diameter (D ₁₎ was prepared out of range of conventional steel No.1~3 and slabs of comparative steel No.4~5 of the present invention.

[0030]

[Table 1]

Conventional steel Nos. 1 to 3 are conventional JIS steel types, conventional steel No. 1 is JIS SMnC420, and conventional steel No. 2
Is JIS SCr420, conventional steel No. 3 is JIS SCM420, and both have a small Si content and Ac ₃ point parameters outside the scope of the present invention. Is D
_One value is small outside the range of the present invention. In addition, comparative steel No. 4
Is a steel having a D ₁ value exceeding the range of the present invention, and Comparative Steel No. 5 is a steel having an Ac ₃ point parameter exceeding the range of the present invention.

The slabs of the steel of the present invention, the conventional steel and the comparative steel were hot rolled and then forged to prepare a round steel bar having a diameter of 20 to 90 mm, and the obtained round steel bar was subjected to normalizing treatment. ,
Hardened strain test pieces and fatigue test pieces were processed. Then, after carburizing and quenching and tempering the test piece obtained in this way, the following amount of carburizing and quenching strain,
The rotating bending fatigue characteristics and gear fatigue characteristics were investigated.

(1) Carburizing Quenching Strain: A disc-shaped Navy C test piece 1 having a diameter of 65 mm from a round bar steel and having an opening 2 and a circular space 3 shown in a front view in FIG. 1 and a side view in FIG.
Was prepared. The dimensions of each part of the test piece 1 are as follows. Test piece diameter (a): 60 mm, thickness (b): 12 mm, circular space diameter (c): 34.8 mm, opening dimension (d): 6 mm.

A Navy C test piece 1 having the above-mentioned shape was applied to each steel.
Ten pieces were prepared, and after carburizing on this test piece 1 under the condition of 900 ℃ × 3 Hr, oil quenching was performed from the temperature of 840 ℃, and then
The rate of change of the opening 2 produced when tempered under the condition of 160 ° C. × 2 Hr was measured and taken as the amount of carburizing and quenching strain. Table 2 shows the grain boundary oxide layer depth, quenching failure layer depth, effective hardened layer depth, core hardness, ferrite area ratio and quenching strain amount of each steel.

[0035]

[Table 2]

(2) Rotating Bending Fatigue Properties: A rotating bending fatigue test piece (stress concentration factor α = 1.8) having a notch with a radius of 1 mm in the parallel part was prepared from a round steel bar with a diameter of 20 mm, and this test piece was prepared. After carburizing and quenching treatment, shot peening treatment (arc height: 0.6 mmA, coverage: 300%) was performed 10 ⁷ times using the Ono-type rotary bending fatigue tester on the test piece subjected to such treatment. A rotary bending fatigue test was performed and the rotary bending fatigue strength was measured. Table 3 shows the measurement results of the rotating bending fatigue strength.

(3) Gear Fatigue Characteristics: Outer diameter 75 mm, module 2.5, number of teeth from round bar steel with a diameter of 90 mm by cutting
After preparing 28 test gears and subjecting them to carburizing and shot peening under the same conditions as the above-mentioned rotational bending fatigue characteristics, using a power circulation type gear fatigue tester for the obtained test pieces, Fatigue test was performed at 3000 rpm and the number of repetitions
The torque value that was not damaged after 10 ⁷ times was determined as the tooth root strength of the gear. Table 3 also shows the gear fatigue durability torque and the presence or absence of chipping.

[0038]

[Table 3]

As is clear from Tables 1 and 2, in the conventional steel Nos. 1 to 3, the ferrite area ratio, that is, the amount of ferrite was 5
It was ˜7%, which was outside the range of the present invention, was small, had a large grain boundary oxidation depth and a poorly hardened layer depth, and had a large amount of hardening strain. Since Comparative Steel No. 4 has a high ideal critical diameter (D ₁ ) value, the amount of quenching strain was large even if ferrite was mixed in the austenite structure. Comparative steel No. 5 is
Since the amount of ferrite was 76%, which was more than the range of the present invention, the core hardness was low.

On the other hand, the steel Nos. 1 to 6 of the present invention have a carburizing and quenching property in which the grain boundary oxide layer is significantly reduced as compared with the conventional steel, a poor quenching layer is not recognized at all, and the carburizing and quenching characteristics are exhibited. The effective hardened layer depth and core hardness of the steel are equal to or higher than those of conventional steel, and since they have a ferrite-martensite dual-phase structure with 16 to 67% ferrite, the amount of quenching strain Was about half that of conventional steel, and there was little variation within the lot.

As is clear from Tables 1 and 3, the conventional steel Nos. 1 to 3 and the comparative steel No. 5 have chipping on the tooth surface in the low torque region, and the comparative steel No. 5 is Due to the low core hardness, the rotary bending fatigue strength and gear fatigue characteristics were poor. On the other hand, the steel Nos. 1 to 6 of the present invention have fatigue strength and tooth root strength superior to those of conventional steels,
Moreover, since there is no hardened layer and the Si content increases,
The temper softening resistance was increased, chipping did not occur, and the surface pressure strength was also strengthened.

[0042]

As described above, according to the present invention,
About 50% more strain due to carburizing and quenching than conventional steel
It is possible to obtain a small amount of gear steel with excellent tooth root strength by a normal carburizing and quenching treatment, and is suitable as a gear for automobiles that does not undergo tooth profile modification, and after carburizing and quenching construction machinery, industrial machinery, etc. Even in gears that require tooth profile modification, the amount of carburizing and quenching strain can be reduced, so the amount of tooth profile modification is small, and therefore, it is possible to reduce machining costs and improve productivity. Excellent effect is brought about.

[Brief description of drawings]

FIG. 1 is a front view showing an example of a test piece for measuring the amount of carburizing and quenching strain.

FIG. 2 is a side view of the test piece shown in FIG.

[Explanation of symbols]

 1 test piece, 2 openings, 3 circular space.

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[Procedure amendment]

[Submission date] November 16, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

Further, Japanese Patent Laid-Open No. 5-70925 discloses that
After carbonitriding a gear made of steel containing a specific range of Si, Mn, Cr, Mo, V, etc., it is cooled to a temperature range below the Ar ₁ transformation point of the tooth surface part, ie carbonitriding part. , And then again, the tooth surface portion, that is, the carbonitrided portion of Ar ₃
By holding in the temperature range below the Ar ₁ transformation point in the tooth (non-carburized portion) above the transformation point and below, quenching and tempering
There is disclosed a method (hereinafter referred to as prior art 2) in which the tooth interior has a fine ferrite-pearlite structure while maintaining the tooth surface portion in a martensite structure.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction content]

[0010]

From the above viewpoints, the inventors of the present invention develop a low distortion type carburized and quenched gear steel that produces a gear with high dimensional accuracy with very little distortion due to carburizing and quenching treatment. As much as possible, repeated research.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

(2) Silicon (Si): Silicon is A
c ₃ A relatively inexpensive element effective for increasing the transformation point. However, the silicon content is 1.00 wt. %
If less than the above, since the silicon concentration in the vicinity of the surface layer that binds to trace oxygen in the carburizing gas during carburizing is low, the trace oxygen penetrates deep into the steel and the intergranular oxide layer becomes extremely deep, resulting in fatigue strength. Cause a decrease in On the other hand, the silicon content is 2.50 wt. %, If it becomes excessive, the amount of SiO ₂ -based inclusions increases, and conversely, the fatigue strength decreases.
Therefore, the silicon content is 1.00 to 2.50 wt.
It should be limited to the range of%.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

(4) Chromium (Cr): Chromium, like manganese, is an element effective for improving hardenability, and in order to exert its action, 0.10 wt. %
It is necessary to contain the above. However, chromium, like manganese, has the effect of lowering the Ac ₃ transformation point, so the content thereof is 1.50 wt. If the amount exceeds 50%, not only the two-phase structure cannot be obtained, but also the hardness becomes too high and the machinability deteriorates. Therefore, the chromium content is 0.10 to 1.50 wt. It should be limited to the range of%.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Name of item to be corrected] 0029

[Correction method] Change

[Correction content]

[0029]

EXAMPLES Next, the present invention will be described by way of Examples in comparison with Comparative Examples. Inventive Steel No. ₁ having chemical composition, Ac ₃ point parameter and ideal critical diameter (D ₁ ) within the scope of the present invention shown in Table 1. Conventional steel Nos. 1 to 6 , and at least one of the chemical composition, the Ac ₃ point parameter, and the ideal critical diameter (D ₁ ) are outside the scope of the present invention. 1-3 and comparative steel Nos. Four to five slabs were prepared.

Claims (2)

[Claims] 1. Carbon (C): 0.10 to 0.35 wt.%, Silicon (Si): 1.0 to 2.50 wt.%, Manganese (Mn): 0.20 to 1.50 wt.%, Chromium (Cr): 0.10 to 1.50 wt. .%, Nickel (Ni): 0.01 to 0.50 wt.%, Molybdenum (Mo): 0.01 to 1.50 wt.%, Aluminum (Al): 0.01 to 0.10 wt.%, And the rest: iron (Fe) and inevitable impurities The Ac _3- point parameter calculated by the following equation (1) is
Within the range of 850 to 940 ℃, Ac ₃ = 920 −203 √C ＋ 44.7Si ＋ 31.5Mo -30Mn −15.2Ni -11Cr ＋ 400Al ── (1) The ideal critical diameter (D ₁ ) Is 60
Within the range of ~ 400 mm, D ₁ = 7.95√C (1 + 0.70Si) (1 + 3.3Mn) (1 + 2.16Cr) (1 + 0.36Ni) (1 + 3.00Mo) ─── (2 ) And, the internal structure of the non-carburized part after carburizing and quenching is
A low strain type carburized and quenched gear steel characterized by having a two-phase structure consisting of martensite containing 10 to 70% of ferrite. 2. At least one selected from the group consisting of:
The low strain carburized and hardened gear steel according to claim 1, further containing two elements. Vanadium (V): 0.01 to 0.50 wt.%, Titanium (Ti): 0.01 to 0.10 wt.%, Niobium (Nb): 0.01 to 0.10 wt.%, And zirconium (Zr): 0.01 to 0.10 wt.%.