JP6265048B2 - Case-hardened steel - Google Patents

Description

  The present invention relates to case-hardened steel. In particular, the present invention relates to a case hardening steel excellent in cold workability suitable for use as a material for carburized parts such as automobile drive system parts formed by cold forging.

Carburized parts such as automobile drive system parts are mainly manufactured by hot forging, carburizing and quenching, and cutting processes. Carburizing and quenching, after infested-diffusing C in the austenite region of temperatures higher than _C3 points A, it is a process of hardening. Conventionally, low-carbon case-hardened steel has been used as material steel (material steel) for carburized parts.

  In recent years, automobiles are required to be low in cost, light in weight, and high in torque, and carburized parts such as drive train parts are required to have high hardenability. For this reason, nickel-hardened molybdenum steel such as SNCM220H limited to JIS G 4052 (2003) and chromium-molybdenum steel such as SCM420H are often used for case-hardened steel that is a material for carburized parts.

  Both Ni and Mo are useful elements that increase the depth of the carburized layer and the hardness of the core. Moreover, since both Ni and Mo are non-oxidizing elements, they have the effect of improving the hardenability of the carburized layer without increasing the depth of the grain boundary oxide layer formed on the surface during gas carburizing. ing. However, Ni and Mo are expensive alloy elements, and it is required to reduce the component cost by suppressing the content as much as possible.

  In order to meet such a demand, Patent Document 1 discloses that the contents of Si, Mn, Cr, and S are 30 in terms of fn1 and fn2 represented by the following formulas (1) and (2), respectively. ≦ fn1 ≦ 150 and 0.7 ≦ fn2 ≦ 1.1 are satisfied (fn1 = Mn / S (1), fn2 = Cr / (Si + 2Mn) (2), provided that the formula (1) and (2) The element symbol in the formula represents the content in mass% of the element.) Case-hardened steel has been proposed. The case-hardened steel proposed in Patent Document 1 is suitable for the production of carburized parts by hot forging.

By the way, in order to improve the productivity of carburized parts, it is desirable to employ cold forging, which has higher productivity, better dimensional accuracy, and better steel yield than hot forging. However, in cold forging, there is a problem that work hardening occurs in the steel material and ductility is lowered. In particular, MnS elongated by hot rolling causes anisotropy and the ductility in a specific direction becomes extremely low.
In order to maintain the spherical shape of Mn sulfide against such a problem, Ca is dissolved in Mn sulfide, and the dissolution of Ca into Mn sulfide by Te is promoted, and Ca, Te, S Further, free-cutting steel for cold forging in which the content and balance of O and O are made appropriate has been proposed (see, for example, Patent Document 2).

JP 2009-249865 A JP 2012-1117098 A

The case-hardened steel proposed by Patent Document 1 is premised on the production of parts by hot forging, and no consideration is given to the anisotropy of ductility due to MnS extended in the rolling direction. For this reason, the cold forgeability may not be satisfied.
On the other hand, in the steel for cold forging proposed by patent document 2, the effect of Te at the time of increasing Cr content is not examined, and it is not necessarily to make carburizing hardenability and cold forgeability compatible. May not be possible.

  The cold forgeability of case-hardened steel has anisotropy, and during cold forging, if MnS stretched in the rolling direction is subjected to a tensile stress from a direction perpendicular to the rolling direction, cracking is likely to occur. In order to ensure hardenability without containing Ni and Mo as much as possible, it is effective to increase the Cr content. However, when the Cr content is increased, carburizing and quenching may cause a grain boundary oxidation layer and an incomplete quenching layer that are carburizing abnormal layers. The carburized abnormal layer adversely affects bending fatigue strength and pitting strength.

  The present invention does not contain expensive elements Ni and Mo as much as possible, has a component composition in which the Cr content is increased, has a low anisotropy in cold forgeability, and has a depth of carburizing abnormal layers due to carburizing and quenching. The object of the present invention is to provide a case-hardened steel that is shallow and has excellent cold forgeability and hardenability.

  The present inventors have studied the contents of Mn, S, Te, Cr, and Si that affect the hardenability, the shape of MnS, and the carburized abnormal layer, and obtained the following knowledge (a) to (c). It was.

(A) In order to obtain excellent cold forgeability, it is effective to reduce coarse MnS as much as possible. In order to suppress the formation of coarse MnS, it is necessary not only to control the individual contents of Mn and S but also to optimize the balance of the contents of Mn and S. Specifically, the value of fn1 represented by the formula [fn1 = Mn / S] is controlled to 30 or more and 150 or less, with the element symbol in the formula as the content in mass% of the element. This can suppress the formation of coarse MnS. Therefore, in order to obtain excellent cold forgeability, it is necessary to control the individual contents of Mn and S and to satisfy the above relational expression.

(B) In order to make the shape of MnS as close to a sphere as possible, the addition of Te is effective. Furthermore, the present inventors have clarified for the first time that it is necessary to optimize the content balance of Cr and Te in order to control the shape of MnS. Specifically, the value of fn2 represented by the formula [fn2 = Cr × Te] is controlled to 0.010 or less with the element symbol in the formula as the content in mass% of the element. By this, the spheroidization of the shape of MnS at the time of manufacture can be promoted. Therefore, in order to reduce the anisotropy of cold forgeability and obtain an excellent cold forgeability, the individual contents of Cr and Te are controlled, and they should satisfy the above relational expression. There is a need.

(C) The depth of the grain boundary oxide layer and the incompletely quenched layer, which are carburized abnormal layers, can be reduced by optimizing the content balance of oxidizing elements, particularly Cr, Si and Mn. Specifically, the value of fn3 represented by the formula [fn3 = Cr / (Si + 2Mn)] is 0.70 or more and 1.10 or less, where the element symbol in the formula is the content in mass% of the element. To. As a result, even if the Cr content is increased, the depth of the carburized abnormal layer due to carburizing and quenching can be reduced, and excellent hardenability can be ensured.

  The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] By mass%
C: 0.10 to 0.30%,
Si: 0.02 to 1.00%,
Mn: 0.30 to 1.00%
S: 0.002 to 0.015%,
Cr: 1.50 to 3.00%,
Al: 0.010 to 0.050%,
N: 0.0040 to 0.0250%,
Te: 0.0003 to 0.0050%
Containing
P: 0.020% or less,
Ti: less than 0.005%,
O: limited to 0.0015% or less, the balance is made of Fe and inevitable impurities,
The contents of Si, Mn, Cr, S and Te are 30 ≦ fn1 ≦ 150 and fn2 in the values of fn1, fn2 and fn3 represented by the following formulas (1), (2) and (3), respectively. Case-hardened steel satisfying ≦ 0.010 and 0.70 ≦ fn3 ≦ 1.10.
fn1: Mn / S (1)
fn2: Cr × Te (2)
fn3: Cr / (Si + 2Mn) (3)
However, the element symbol in the formulas (1), (2) and (3) represents the content in mass% of the element.
[2] Furthermore, in mass%,
Mo: 0.10% or less,
Cu: 0.20% or less,
Ni: Case-hardened steel according to [1] above, containing one or more of 0.20% or less.
[3] Furthermore, in mass%,
V: 0.20% or less,
Nb: The case-hardened steel according to [1] or [2] above, containing one or both of 0.050% or less.
[4] Furthermore, in mass%,
Ca: 0.0050% or less,
One or both of Zr: 0.010% or less are contained, The case hardening steel of any one of said [1]-[3] characterized by the above-mentioned.

  According to the present invention, case hardening steel excellent in cold forgeability and hardenability can be provided. In particular, according to the present invention, there is provided a case-hardened steel which has no anisotropy in cold forgeability and suppresses the formation of a carburized abnormal layer by carburizing and quenching without containing Ni and Mo as much as possible. It becomes possible. Therefore, the industrial contribution of the present invention is extremely remarkable.

It is a figure explaining the cutting-out direction of the raw material cut out from the steel bar in order to create an annular notch test piece. It is a figure which shows a R3 annular notch tensile test piece. It is a figure which shows a R20 annular notch tensile test piece. It is a figure explaining the load-displacement curve and ductile crack generation | occurrence | production point of a R3 annular notch tensile test piece and a R20 annular notch tensile test piece. It is a figure explaining the relationship between the addition amount of Cr and Te, and (epsilon) _{L- (} epsilon _{) C.} It is a figure which shows the heat pattern of carburizing quenching and tempering. It is a figure which shows the relationship between the value of [Cr / (Si + 2Mn)] and the grain boundary oxide layer depth. It is a figure which shows the relationship between the value of [Cr / (Si + 2Mn)], and an incomplete quenching layer depth.

  The inventors of the present invention particularly examined the hardenability, the shape of MnS, and the contents of Mn, S, Te, Cr, and Si that affect the carburized abnormal layer. Specifically, steel materials having various component compositions were melted, made into steel pieces after melting, and formed into round bars having a diameter of 55 mm by hot rolling as shown in FIG. Using the obtained round bars of various component compositions, the cold forgeability, the grain boundary oxide layer depth after carburizing and quenching, and the incomplete quenching layer depth were investigated.

<1> Investigation of cold forgeability:
There is a correlation between the equivalent plastic strain at which a crack is generated by a compression test and the equivalent plastic strain at which a crack is generated by an annular notch tensile test. From this, the crack resistance during cold forging was evaluated by an annular notch tensile test.
FIG. 1 is a diagram for explaining a cutting direction of a material cut out from a steel bar (round bar) in order to create an annular notch test piece. As shown in FIG. 1, a material parallel to the rolling direction (parallel material) and a material perpendicular to the rolling direction (vertical material) were cut out from the steel bars having various component compositions. Particles made of MnS in the steel bar are stretched in the rolling direction as shown in FIG.

  Each of the parallel material and the vertical material cut out from the steel bar is subjected to spheroidizing annealing, and then machined to make an R3 annular notch tensile test piece having a notch radius of curvature of 3 mm, as shown in FIG. An R20 annular notch tensile test piece shown in FIG. 3 having a radius of curvature of 20 mm was produced, and a tensile test was performed. The R3 annular notch tensile specimen is a specimen with severe stress concentration. The R20 annular notch tensile test piece is a test piece with a low stress concentration.

In each material, a unique load-displacement curve was obtained as shown in FIG. 4 from the result of the tensile test for each test piece, and the ductile crack initiation point was determined from the shape of each load-displacement curve.
A finite element analysis is performed based on the shape of the annular notch tensile test piece and the tensile test result (that is, the load-displacement curve), and the crack in the cross section of the notch at the ductile crack initiation point is obtained. The equivalent plastic strain of the element corresponding to the generating part was obtained.

Cold forgeability was evaluated by the equivalent plastic strain ε _{L at} the ductile crack initiation point of a test piece obtained from a material cut out in parallel to the rolling direction. Steels with an R3 annular notch tensile specimen ε _L of 0.75 or more and / or an R20 annular notch tensile specimen ε _L of 1.05 or more are less susceptible to cracking during cold forging and have a cold forgeability. It is good.

The anisotropy of the cold forgeability of the steel was cut out perpendicular to the rolling direction and the equivalent plastic strain ε _{L at} the ductile crack initiation point of the test piece obtained from the material cut out parallel to the rolling direction. It can be evaluated by the difference (ε _L −ε _C ) from the equivalent plastic strain ε _{C at} the ductile crack initiation point of the test piece obtained from the material. In the present invention, the steel having ε _L -ε _C of 0.3 or less was evaluated as having low anisotropy and excellent cold forgeability regardless of the radius of curvature of the annular notch tensile test piece.

The relationship between the cold forgeability of steel and the component composition was examined. As a result, it was found that there was a correlation between the anisotropy of cold forgeability (ε _L −ε _C ) and the added amounts of Cr and Te. It was also found that the cold forgeability anisotropy (ε _L −ε _C ) can be organized by the addition amount of Cr and Te and the product Cr × Te.
FIG. 5 is a diagram for explaining the relationship between the added amounts of Cr and Te and ε _L -ε _C in test pieces having various component compositions. In FIG. 5, ◯ represents a test piece having ε _L -ε _C of 0.3 or less, and x represents a test piece having ε _L -ε _C of more than 0.3. Further, the curve shown in FIG. 5 is represented by Cr × Te = 0.010.
As shown in FIG. 5, within the range of the component composition satisfying Cr: 1.50 to 3.0% by mass, Te: 0.0003 to 0.0050% by mass, and Cr × Te ≦ 0.010, ε _{L-epsilon C} becomes 0.3 or less, the anisotropy is small, it was found that excellent cold forgeability can be obtained.

<2> Investigation of grain boundary oxide layer depth and incomplete quenching layer depth:
Carburization quenching-tempering by the heat pattern shown in FIG. 6 was performed on the R3 annular notch tensile test piece shown in FIG. 2 obtained from materials having various component compositions cut out in parallel with the rolling direction. In addition, Cp in FIG. 6 represents a carbon potential. In FIG. 6, 120 ° C. oil quenching indicates quenching in oil having an oil temperature of 120 ° C., and AC indicates air cooling. About oil quenching, the tensile test piece was thrown into the stirring oil which is stirring so that it might quench uniformly.
Next, the R3 annular notch tensile specimen subjected to carburizing and tempering was cut so as to cross the notch having a diameter of 6 mm. Thereafter, the test piece was embedded in a resin so that the cut surface, which is a cross section perpendicular to the rolling direction, was the test surface, and the test surface was mirror-finished by mechanical polishing.

The surface of the test piece was arbitrarily observed in 10 fields with an optical microscope at a magnification of 1000 times without corroding the test surface of the test piece after mechanical polishing. And the oxide layer observed along a grain boundary in a surface part was made into the grain boundary oxide layer, and those depths were arithmetically averaged and the grain boundary oxide layer depth was calculated | required.
Further, the test surface of the test piece after mechanical polishing was corroded with nital for 0.2 to 2 seconds, and the surface portion of the test piece was arbitrarily observed in 10 fields with an optical microscope at a magnification of 1000 times. Then, in the surface portion, the portion where the degree of corrosion was more prominent than the surrounding was defined as an incompletely quenched layer, and the depth of the incompletely quenched layer was obtained by arithmetically averaging the depths.

FIG. 7 shows the relationship between the value of the formula [Cr / (Si + 2Mn)] and the grain boundary oxide layer depth in test pieces having various component compositions. In FIG. 7, ◯ indicates a test piece having a grain boundary oxide layer depth of 10 μm or less, and x indicates a test piece having a grain boundary oxide layer depth of more than 10 μm.
FIG. 8 shows the relationship between the value of the [Cr / (Si + 2Mn)] formula and the depth of the incompletely hardened layer in test pieces having various component compositions. In FIG. 8, ◯ indicates a test piece having an incomplete quenching layer depth of 15 μm or less, and x indicates a test piece having an incomplete quenching layer depth of more than 15 μm.
As shown in FIGS. 7 and 8, when [Cr / (Si + 2Mn)] is in the range of 0.7 to 1.1, the grain boundary oxide layer depth and the incomplete quenching layer depth become shallow, and carburization abnormality occurs. It was found that the formation of layers can be suppressed.

  The present invention will be described in detail below. In addition, “%” of the content of each element means “mass%”.

C: 0.10 to 0.30%
C is an essential element for securing the strength of the component, and a content of 0.10% or more is necessary. However, if the C content is too large, the hardness increases and machinability is reduced. In particular, when the content of C exceeds 0.30%, the machinability is significantly lowered with an increase in hardness. Therefore, the content of C is set to 0.10 to 0.30%. In addition, when much better machinability is required, the C content is preferably 0.15 to 0.20%.

Si: 0.02 to 1.00%
Si has an action of improving hardenability and a deoxidizing action. Moreover, Si has a temper softening resistance and has an effect of preventing the softening of steel under high temperature conditions. In order to obtain these effects, it is necessary to contain 0.02% or more of Si. On the other hand, since Si is an oxidizing element, when its content increases, Si is selectively oxidized by a small amount of H ₂ O or CO ₂ contained in the carburizing gas, and Si oxide is generated on the steel surface. . For this reason, the depth of the grain boundary oxide layer and the incompletely quenched layer, which are carburized abnormal layers, increases. And if the depth of a carburizing abnormal layer becomes large, it will cause the fall of bending fatigue strength and pitching strength. Further, when the Si content increases, the effect of temper softening resistance is saturated, and the machinability also decreases. In particular, when the Si content exceeds 1.00%, the bending fatigue strength and the pitching strength are significantly decreased due to the increased depth of the carburized abnormal layer, and the machinability is also significantly decreased. Therefore, the Si content is set to 0.02 to 1.00%. When much better machinability is required, the Si content is preferably 0.70% or less. In order to obtain the above-described effect by containing Si, the Si content is preferably 0.05% or more.

  In addition, Si content needs to satisfy | fill the value of fn3 represented by said Formula (3) also satisfy | fills 0.70 <= fn3 <= 1.10 in said range.

Mn: 0.30 to 1.00%
Mn has an action of improving hardenability and a deoxidizing action. In order to obtain these effects, a Mn content of 0.30% or more is necessary. However, when the Mn content increases, the hardness increases and the machinability decreases. In particular, when the content of Mn exceeds 1.00%, the machinability is significantly lowered due to the increase in hardness. Moreover, since Mn is an oxidizing element like Si, when its content increases, Mn oxide is generated on the steel surface, and the grain boundary oxidation and the depth of the incompletely quenched layer, which are carburizing abnormal layers, Becomes larger. And if the depth of a carburizing abnormal layer becomes large, it will cause the fall of bending fatigue strength and pitching strength. In particular, when the Mn content exceeds 1.0%, the bending fatigue strength and the pitching strength are significantly reduced due to the increased depth of the carburized abnormal layer. Therefore, the content of Mn is set to 0.30 to 1.00%. In addition, the minimum with preferable Mn content is 0.60%. Moreover, the upper limit with preferable Mn content is 0.90%.

  Note that the content of Mn is within the above range, the value of fn1 represented by the above formula (1) satisfies 30 ≦ fn1 ≦ 150, and the value of fn3 represented by the above formula (3) is 0.00. It is necessary to satisfy 70 ≦ fn3 ≦ 1.10.

S: 0.002 to 0.015%
S combines with Mn to form MnS and has the effect of improving machinability. However, if the S content exceeds 0.015%, coarse MnS is formed, and hot workability, cold forgeability, bending fatigue strength, and pitching strength are reduced. Therefore, the S content is limited to 0.015% or less. In order to obtain the above-described machinability improving effect, the S content is set to 0.002% or more, and preferably 0.010% or more.

  In addition, S content needs to satisfy | fill 30 <= fn1 <= 150 also in the value of fn1 represented by said Formula (1) in said range.

Cr: 1.50 to 3.00%
Cr has an effect of improving hardenability, an effect of promoting temper softening resistance and spheroidizing of MnS, an effect of preventing softening of steel under a high temperature condition, and anisotropy of cold forgeability effective. In order to obtain these effects, a content of 1.50% or more is required. However, as the Cr content increases, the hardness increases and the machinability decreases. In particular, when the Cr content exceeds 3.00%, the machinability is significantly lowered as the hardness increases. In addition, since Cr is an oxidizing element like Si and Mn, when its content increases, Cr oxide is generated on the steel surface, and the grain boundary oxide layer and the incompletely hardened layer are carburized abnormal layers. The depth of will increase. And if the depth of a carburizing abnormal layer becomes large, it will cause the fall of bending fatigue strength and pitching strength. In particular, when the Cr content exceeds 3.00%, the bending fatigue strength and the pitching strength are significantly reduced due to the increased depth of the carburized abnormal layer. Therefore, the Cr content is set to 1.50 to 3.00%. If even better machinability is required, the Cr content is preferably 2.00% or less. In order to acquire the said effect by containing Cr, it is preferable to make content of Cr into 1.80% or more.

  The Cr content is within the above range, and the value of fn2 represented by the above formula (2) satisfies fn2 ≦ 0.010, and the value of fn3 represented by the above formula (3) is 0.00. It is necessary to satisfy 70 ≦ fn3 ≦ 1.10.

Al: 0.010 to 0.050%
Al has a deoxidizing action. Moreover, Al also has the effect | action which combines with N, forms AlN, refines | miniaturizes a crystal grain, and strengthens steel. However, when the Al content is less than 0.010%, it is difficult to obtain the above effect. On the other hand, when the Al content is excessive, the machinability is lowered due to the formation of hard and coarse Al ₂ O ₃ , and the bending fatigue strength and the pitching strength are also lowered. In particular, when the Al content exceeds 0.050%, the machinability, bending fatigue strength, and pitching strength are significantly reduced. Therefore, the content of Al is set to 0.010 to 0.050%. In addition, the minimum with preferable content of Al is 0.020%. Moreover, the upper limit with preferable content of Al is 0.040%.

N: 0.0040 to 0.0250%
N has the effect of making the crystal grains finer by forming nitrides and improving the bending fatigue strength. In order to acquire this effect, it is necessary to contain N 0.0040% or more. However, when the N content is excessive, coarse nitrides are formed, leading to a reduction in toughness. In particular, when the N content exceeds 0.0250%, the toughness is significantly lowered. Therefore, the content of N is set to 0.0040 to 0.0250%. In addition, the minimum with preferable N content is 0.0130%. Moreover, the upper limit with preferable N content is 0.0200%.

Te: 0.0003 to 0.0050%
Te is an extremely important element that promotes the spheroidization of MnS, and 0.0003% or more is added to reduce the anisotropy of cold forgeability. On the other hand, when the Te content exceeds 0.0050%, the hot workability of the steel decreases. Therefore, the Te content is set to 0.0003 to 0.0050%. The Te content is preferably 0.0005% or more in order to further reduce the anisotropy of cold forgeability. The Te content is preferably 0.0030% or less in order to suppress a decrease in hot workability.

  The Te content is within the above range, and the value of fn2 represented by the above formula (2) is 0.00. It is necessary to satisfy fn2 ≦ 0.010.

fn1 value: 30 or more and 150 or less Coarse MnS is a starting point of cracking during hot working such as hot rolling and hot forging and cracking during cold forging, so cracking and cold during hot working In order to suppress cracking during forging, it is necessary to reduce coarse MnS as much as possible.
When the value of fn1 represented by the above formula (1) is less than 30, the content of S becomes excessive, and generation of coarse MnS is inevitable. On the other hand, when the value of fn1 exceeds 150, the Mn content becomes excessive and coarse MnS is generated in the central segregation part. Therefore, in any case, the cold forgeability is adversely affected. Therefore, the value of fn1 represented by the above formula (1), that is, [fn1 = Mn / S] is set to satisfy 30 ≦ fn1 ≦ 150. The preferred lower limit of the value of fn1 is 50. The preferred upper limit of fn1 is 100.

fn2 value: 0.010 or less MnS elongated in the rolling direction causes ductile anisotropy. For this reason, it is necessary to make the shape of MnS as close to a sphere as possible. In order to promote the spheroidization of MnS, the addition of Te is effective, but when the Cr content is high, the value of fn2 represented by the above formula (2) exceeds 0.010, the temperature is high. Ductility decreases. Although this reason is not necessarily clear, as a result, it has a bad influence on cold forgeability. Therefore, the value of fn2 expressed by the above equation (2), that is, [fn2 = Cr × Te] is determined to satisfy fn2 ≦ 0.010. A preferable upper limit of fn2 is 0.008.

fn3 value: 0.70 or more and 1.10 or less In order to provide high bending fatigue strength and high pitching strength without containing Ni and Mo as much as possible, it is a carburizing abnormal layer while ensuring hardenability. It is necessary to reduce the depth of the grain boundary oxide layer and incomplete hardenability. For that purpose, the content of Cr, Si, Mn among the oxidizing elements is set within the above range, and the content of fn3 expressed by the above formula (3) as the balance of the content of these elements. The value needs to be 0.70 or more and 1.10 or less. That is, when the value of fn3 represented by the above formula (3) is less than 0.70 and exceeds 1.10, the depth of the carburized abnormal layer increases, so that the bending fatigue strength and the pitching are increased. Strength will fall. Therefore, the value of fn3 expressed by the above equation (3), that is, [fn3 = Cr / (Si + 2Mn)] is set to satisfy 0.70 ≦ fn3 ≦ 1.10. In addition, the preferable minimum of fn3 is 0.80 or more. The preferable upper limit of fn3 is 1.08.

  In the present invention, P, Ti and O (oxygen) are impurities, and their contents are limited to P: 0.020% or less, Ti: less than 0.005% and O: 0.0015% or less, respectively. There is a need to.

P: 0.020% or less P is an impurity contained in the steel and segregates at the grain boundaries to embrittle the steel. In particular, when the P content exceeds 0.020%, the degree of embrittlement becomes significant. Therefore, in the present invention, the content of P in the impurities is set to 0.020% or less. In addition, it is preferable that content of P in an impurity shall be 0.010% or less. Moreover, although the one where content of P is small is preferable, since removal P cost increases, it is preferable that it is 0.003% or more.

Ti: Less than 0.005% Ti has a high affinity with N, so it combines with N in steel to form TiN, which is a hard, coarse non-metallic inclusion, and lowers bending fatigue strength and pitting strength. Furthermore, machinability is also reduced. Therefore, in the present invention, the content of Ti in the impurities is less than 0.005%. The Ti content is preferably 0.003% or less.

O (oxygen): 0.0015% or less O (oxygen) combines with Si or Al in steel to generate an oxide. Among the oxides, in particular, Al ₂ O ₃ is hard, so that machinability is reduced, and further, bending fatigue strength and pitching strength are reduced. Therefore, in the present invention, the content of impurity O is set to 0.0015% or less. The O (oxygen) content is preferably 0.0013% or less. Moreover, although the one where content of O (oxygen) is small is preferable, since de-O (oxygen) cost increases, it is preferable that it is 0.0002% or more.

The case-hardened steel of the present invention can further selectively contain one or more elements of each group of the following first group to third group as required.
First group: Mo: 0.10% or less, Cu: 0.20% or less, Ni: one or more of 0.20% or less, second group: V: 0.20% or less, Nb : One or both of 0.050% or less, 3rd group: Ca: 0.0050% or less, Zr: One or both of 0.010% or less.

First group: one or more of Mo: 0.10% or less, Cu: 0.20% or less, and Ni: 0.20% or less Mo, Cu and Ni all have an effect of improving hardenability. Have. For this reason, when it is desired to obtain greater hardenability, it may be contained in the following range. However, these alloy elements are expensive, and the content is preferably limited in order to reduce the cost.

Mo: 0.10% or less Mo has an effect of improving hardenability, and improves the surface hardness, the hardened layer depth and the core hardness after carburizing and quenching to ensure the strength of the carburized component. effective. Moreover, since Mo is a non-oxidizing element, the steel surface can be toughened without increasing the depth of the grain boundary oxide layer during carburizing. For this reason, in order to acquire these effects, you may contain Mo. However, Mo is an expensive element, and excessive addition leads to an increase in component cost. In particular, when the Mo content exceeds 0.10%, the cost increases. Therefore, the Mo content is set to 0.10% or less. In order to reliably obtain the above-described effect of improving Mo characteristics, the Mo content is preferably 0.04% or more. The Mo content is preferably 0.08% or less in order to suppress an increase in cost.

Cu: 0.20% or less Cu has an effect of improving hardenability, and may be contained for further improving hardenability. However, Cu is an expensive element, and as the content increases, hot workability is reduced. In particular, when the Cu content exceeds 0.20%, the hot workability is significantly reduced. Therefore, the Cu content is preferably 0.20% or less and preferably 0.18% or less. Moreover, in order to acquire the hardenability improvement effect by containing Cu reliably, it is preferable that content of Cu shall be 0.05% or more.

Ni: 0.20% or less Ni has an effect of improving hardenability. Ni has the effect of improving toughness and is a non-oxidizing element, so that the steel surface can be toughened without increasing the depth of the grain boundary oxide layer during carburizing. For this reason, in order to acquire these effects, you may contain Ni. However, Ni is an expensive element, and excessive addition leads to an increase in component cost. In particular, when the Ni content exceeds 0.20%, the cost increases. Therefore, the Ni content is set to 0.20% or less. In order to surely obtain the above-described effect of improving Ni characteristics, the Ni content is preferably 0.05% or more. Further, the Ni content is preferably 0.18% or less in order to suppress an increase in cost.

  In addition, said Mo, Cu, and Ni can contain only any 1 type in them, or 2 or more types in combination.

Group 2: V: 0.20% or less and Nb: 0.05% or less, one or two of them V and Nb all combine with C and N to form fine carbides, nitrides and charcoal It has the effect of forming nitrides to refine crystal grains and improving bending fatigue strength and pitching strength. For this reason, V and Nb may be contained in the following ranges in order to further improve the bending fatigue strength and the pitching strength.

V: 0.20% or less V has the effect of combining with C and N to form fine carbides, nitrides, and carbonitrides to refine crystal grains and improve bending fatigue strength and pitching strength. However, when the V content is excessive, hot ductility is reduced. In particular, when the content exceeds 0.20%, the hot ductility is remarkably lowered, and surface scratches are likely to occur during hot rolling. Therefore, the content of V is set to 0.20% or less. In order to surely obtain the effect of improving the characteristics of V described above, the content is preferably 0.05% or more. For this reason, the more desirable V content is 0.05 to 0.20%. Note that the more desirable lower limit of the V content is 0.08%. A more desirable upper limit of the V content is 0.10%.

Nb: 0.050% or less Nb combines with C and N to form fine carbides, nitrides, and carbonitrides to refine crystal grains, and has an effect of improving bending fatigue strength and pitching strength. However, when the Nb content is excessive, hot ductility is reduced. In particular, when the content exceeds 0.050%, the hot ductility is significantly lowered, and surface scratches are likely to occur during hot rolling. Therefore, the Nb content is set to 0.050% or less. In order to reliably obtain the above-described Nb characteristic improvement effect, the content is preferably set to 0.005% or more. For this reason, the more desirable Nb content is 0.005 to 0.050%. Note that a more preferable lower limit of the Nb content is 0.020%. A more preferred upper limit is 0.040%.

  In addition, said V and Nb can contain only any 1 type among them, or 2 types in combination.

Group 3: Ca: 0.0050% or less and Zr: 0.010% or less One or two of Ca and Zr are formed by spheroidizing MnS and reducing anisotropy, thereby reducing cold forgeability. Improve. For this reason, you may make it contain in the following ranges for the further improvement of cold forgeability.

Ca: 0.0050% or less Ca is a deoxidizing element, and not only generates soft oxides and improves machinability, but also dissolves in MnS to lower its deformability, thereby rolling and hot forging. Even so, there is a function of suppressing distraction of the MnS shape. Therefore, it is an effective element for reducing anisotropy. However, even if Ca is added in excess of 0.0050%, a large amount of hard CaO is generated, and mechanical properties such as machinability and fatigue properties are deteriorated. Therefore, the Ca content is specified to be 0.0050% or less, and preferably 0.0030% or less. Further, the Ca content is preferably 0.0003% or more in order to stably exhibit the machinability improving effect.

Zr: 0.010% or less Zr is a deoxidizing element and generates an oxide. The Zr oxide is considered to be ZrO _2, and since ZrO ₂ serves as a precipitation nucleus of MnS, the MnS precipitation sites are increased and MnS is uniformly dispersed. Further, Zr dissolves in MnS to form a composite sulfide to reduce its deformability, and has the function of suppressing the elongation of the MnS shape even when rolled or hot forged. Therefore, Zr is an element effective for reducing anisotropy. However, even if Zr is added in excess of 0.010%, a large amount of hard ZrO ₂ or ZrS is generated, and mechanical properties such as machinability and fatigue properties are deteriorated. Therefore, the Zr content is specified to be 0.010% or less, and preferably 0.008% or less. Further, the Zr content is preferably 0.002% or more in order to stably exhibit the machinability improving effect.

The case hardening steel of this embodiment can be manufactured by a conventionally well-known method. For example, the steel material having the above-described composition is melted by a normal method such as a converter, an electric furnace, etc., and is manufactured by hot rolling into a wire rod or a steel bar through a casting process and, if necessary, a batch rolling process. it can.
The carburizing and quenching conditions of the case hardening steel of the present embodiment are not particularly limited. It can be determined according to the application of the parts processed using the case-hardened steel of the present embodiment.

  Steel materials having the composition shown in Table 1 were melted in a 150 kg vacuum melting furnace, then rolled into billets, and further hot-rolled into round bars with a diameter of 55 mm. Manufactured. Steels 1 to 9 in Table 1 are steels of the present invention examples whose chemical compositions are within the range specified by the present invention, while Steels 10 to 17 shown in Table 1 have chemical compositions of the present invention. This is a comparative steel that deviates from the specified conditions.

  Among the steels of the above comparative examples, Steel 10 and Steel 11 are steels corresponding to SNCM220H of nickel chromium molybdenum steel and SCM420H of chromium molybdenum steel respectively defined in JIS G 4052 (2003).

Using the method described in the above <1> cold forgeability investigation, the R3 annular notch tensile test piece shown in FIG. 2 and the R20 annular cut shown in FIG. A notch tensile test piece was collected and ε _L −ε _C (equivalent plastic strain ε _{L at} the ductile crack initiation point of the test piece obtained from the material cut in parallel to the rolling direction and perpendicular to the rolling direction. The difference from the equivalent plastic strain ε _{C at} the ductile crack initiation point of the specimen obtained from the cut material and ε _L were determined. In addition, the heat processing for 4.5 hours was performed at the temperature of 760 degreeC as spheroidization annealing.

When epsilon _L is 1.05 or more R3 cyclic notched tensile specimens of epsilon _L is 0.75 or more and R20 annular notched tensile specimens, difficult to break during cold forging, cold forgeability is good Evaluated that there was. Moreover, when ε _L -ε _C was 0.3 or less, it was evaluated that the anisotropy was small and the cold forgeability was excellent.

  Using the method described in the above-mentioned investigation of <2> grain boundary oxide layer depth and incomplete quenching layer depth, the following test pieces were obtained. That is, carburizing and quenching with the heat pattern shown in FIG. 6 is performed on the R3 annular notch tensile test piece shown in FIG. -Tempered. Subsequently, it cut | disconnected so that the notch part of 6 mm in diameter of the R3 cyclic | annular notch tensile test piece which carburized and quenched and tempered could be crossed. Thereafter, the test piece was embedded in a resin and mechanically polished so that a cut surface having a cross section in a direction perpendicular to the rolling direction became the test surface, and a test piece having a mirror-finished test surface was obtained.

"Effective hardened layer depth"
Then, in accordance with JIS Z 2244, using a micro Vickers hardness tester, the test force is set to 2.94 N in the direction from the surface of the test piece after polishing toward the center (direction perpendicular to the rolling direction), and the Vickers hardness (Hv hardness) was measured. The depth from the surface when the Hv hardness was 550 was measured at any 10 locations, and the minimum value was taken as the effective hardened layer depth.

"Grain boundary oxide layer depth"
As the test piece, the test surface of the test piece used for the measurement of the effective hardened layer depth was polished again, and a mirror-finished surface was used. The grain boundary oxide layer depth was determined by using the method described in the investigation of the penetration depth. The case where the grain boundary oxide layer depth was 10 μm or less was evaluated as good.

"Incomplete hardened layer depth"
The test surface of the test piece used for the measurement of the grain boundary oxide layer depth was corroded with nital for 0.2 to 2 seconds, and the above-mentioned <2> grain boundary oxide layer depth and incomplete quenching layer depth Using the method described in the survey, the incompletely hardened layer depth was determined. The incomplete quenching layer depth was evaluated as good when it was 15 μm or less.

"Average aspect ratio of MnS particles"
The specimen used for the measurement of the grain boundary oxide layer depth was cut along the rolling direction. Then, it embedded in resin so that the cut surface which is a cross section along a rolling direction might turn into a test surface, and performed mechanical polishing, and the test piece which has a test surface which carried out mirror surface finish was obtained. The test surface was observed using a scanning electron microscope (trade name: JSM-5700, manufactured by JEOL Ltd.) at a magnification of 500 times. And about 300 MnS particle | grains, the vertical and horizontal length are measured by making the longitudinal direction of MnS particle | grains into the vertical direction, and average aspect ratio (vertical length / horizontal length) is used using each average value. Calculated. The average aspect ratio of MnS particles was evaluated as good when it was 6 or less. The aspect ratio of the MnS particles is not different before and after carburizing and quenching.

Table 2 summarizes the results of ε _L , ε _L -ε _C, effective hardened layer depth, grain boundary oxidized layer depth, incompletely quenched layer depth, and average aspect ratio of MnS particles.

As shown in Table 2, in the case of test numbers 1 to 9 using steels 1 to 9 satisfying the conditions specified in the present invention as materials, ε _L -ε _C is 0.3 or less, and cold forgeability The anisotropy of was small. Further, steel with test Nos. 1 to 9, comprising the epsilon _L of R3 annular notch tensile specimen 0.75 or more and epsilon _L of R20 annular notch tensile specimen 1.05 or more, cold It was difficult to break during forging. Therefore, the steels with test numbers 1 to 9 have good cold forgeability.
Moreover, in the test numbers 1-9 using the steel which satisfy | fills the conditions prescribed | regulated by this invention, the average aspect-ratio of MnS particle | grains is 6 or less, and the extension to the rolling direction of MnS particle | grains was suppressed.

Furthermore, in test numbers 1 to 9 using steels 1 to 9, the grain boundary oxide layer depth is 10 μm or less and the incomplete quenching layer depth is 15 μm or less, and the formation of an abnormal carburizing layer due to carburizing and quenching is suppressed. It had been.
Test Nos. 1 to 9 are effective hardened layer depths equivalent to those of Test Nos. 10 and 11 using Steel 10 (corresponding to SNCM220H of nickel chromium molybdenum steel) and Steel 11 (corresponding to SCM420H of chromium molybdenum steel). Compared with steel 10 and steel 11, the grain boundary oxide layer depth and the incompletely hardened layer depth were shallow.

In the case of test number 10, since Te is not added to the steel 10, the value of ε _L -ε _C is high, and the anisotropy of cold forgeability is large. Furthermore, test number 10 has a high average aspect ratio. In Test No. 10, ε _L of the R3 and R20 annular notch tensile test pieces is small, and the cold forgeability is insufficient. In Test No. 10, since the value of [Cr / (Si + 2Mn)] is below the range specified in the present invention, the intergranular oxide layer depth and the incomplete quenching layer depth adversely affecting the bending fatigue strength and the pitting strength. However, it is deeper than test numbers 1-9.

In the case of test number 11, since Te is not added to the steel 11, the value of ε _L -ε _C is high, and the anisotropy of cold forgeability is large. Furthermore, test number 11 has a high average aspect ratio. In Test No. 11, ε _L of the R3 and R20 annular notch tensile test pieces is small, and the cold forgeability is insufficient. In Test No. 11, the value of [Cr / (Si + 2Mn)] is below the range defined in the present invention, so that the grain boundary oxide layer depth and the incomplete quenching layer depth are compared with Test Nos. 1 to 9. deep.

In the case of test number 12, since the addition amount of Cr in steel 12 is below the range defined in the present invention, the average aspect ratio is larger than those in test numbers 1-9.
In the case of test number 13, ε _{L of} the R3 and R20 annular notch tensile test pieces is small, and the cold forgeability is poor as compared with test numbers 1 to 9. Test number 13 has a high average aspect ratio. These results are presumed to be due to the generation of coarse Mn / S because the value of [fn1 = Mn / S] of steel 13 is below the range defined in the present invention.

In the case of test number 14, since the value of [fn2 = Cr × Te] of steel 14 exceeds the range specified in the present invention, the value of ε _L -ε _C is high, which is anisotropic compared to test numbers 1-9. The nature is great. Furthermore, test number 14 has a high average aspect ratio. In Test No. 14, ε _L of the R3 and R20 annular notch tensile test pieces is small, and the cold forgeability is insufficient.
In the case of test number 15, since the value of [fn3 = Cr / (Si + 2Mn)] of steel 15 exceeds the range specified in the present invention, the grain boundary oxide layer depth and the incomplete quenching layer depth are test numbers 1 to 1. Deeper than 9.

In the case of test number 16, ε _{L of} the R3 and R20 annular notch tensile test pieces is small, and the cold forgeability is poor as compared with test numbers 1-9. This is presumably because coarse [MnS] was generated because the value of [fn1 = Mn / S] of the steel 16 exceeded the range defined in the present invention.
In the case of test number 17, since the value of [fn3 = Cr / (Si + 2Mn)] of steel 17 is below the range specified in the present invention, the grain boundary oxide layer depth and the incomplete quenching layer depth are the test numbers 1 to 1. Deeper than 9.

  The case-hardened steel of the present invention is low in component cost because it contains less Ni and Mo than nickel-molybdenum steel SNCM220H and chromium-molybdenum steel SCM420H specified in JIS G 4052 (2003). Small forging anisotropy and good forgeability. In addition, the carburized parts made from this case-hardened steel have the same effective hardened layer depth as carburized parts made from SNCM220H and SCM420H, and have a grain boundary oxide layer that adversely affects bending fatigue strength and pitting strength. Shallow depth and incompletely hardened layer depth. For this reason, the case-hardened steel of the present invention is suitable for use as a material for carburized parts that require excellent cold forgeability and hardenability for cost reduction, weight reduction, and torque increase.

Claims (4)

% By mass
C: 0.10 to 0.30%,
Si: 0.02 to 1.00%,
Mn: 0.30 to 1.00%
S: 0.002 to 0.015%,
Cr: 1.50 to 3.00%,
Al: 0.010 to 0.050%,
N: 0.0040 to 0.0250%,
Te: 0.0003 to 0.0050%
Containing
P: 0.020% or less,
Ti: less than 0.005%,
O: limited to 0.0015% or less, the balance is made of Fe and inevitable impurities,
The contents of Si, Mn, Cr, S and Te are 30 ≦ fn1 ≦ 150 and fn2 in the values of fn1, fn2 and fn3 represented by the following formulas (1), (2) and (3), respectively. Case-hardened steel satisfying ≦ 0.010 and 0.70 ≦ fn3 ≦ 1.10.
fn1: Mn / S (1)
fn2: Cr × Te (2)
fn3: Cr / (Si + 2Mn) (3)
However, the element symbol in the formulas (1), (2) and (3) represents the content in mass% of the element. Furthermore, in mass%,
Mo: 0.10% or less,
Cu: 0.20% or less,
The case-hardened steel according to claim 1, comprising Ni: 0.20% or less. Furthermore, in mass%,
V: 0.20% or less,
One or both of Nb: 0.050% or less are contained, The case hardening steel of Claim 1 or 2 characterized by the above-mentioned. Furthermore, in mass%,
Ca: 0.0050% or less,
One or both of Zr: 0.010% or less are contained, The case hardening steel of any one of Claims 1-3 characterized by the above-mentioned.

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