JP6344423B2 - Case-hardened steel and method for producing case-hardened steel - Google Patents

Description

  The present invention relates to a case-hardened steel having high fatigue strength and a method for producing the case-hardened steel, which can be applied to machine structural parts used in the fields of construction machinery and automobiles.

  Machine structural parts used in construction machinery, automobiles, etc. are hardened by carburizing and tempering to improve fatigue strength after the parts are shaped by forging or cutting. . However, when coarse inclusions are present inside the steel material, sufficient fatigue strength cannot be achieved even if carburizing treatment is performed, so that it cannot be used as a part.

  Under such circumstances, there is a strong demand for case-hardened steel in which inclusions are appropriately controlled in machine structural parts used in construction machines and automobiles. For this reason, various techniques regarding an appropriate inclusion dispersion state for improving the fatigue strength of the case hardening steel have been proposed.

  For example, Patent Documents 1 to 6 propose steels with excellent fatigue life that define the type and size of inclusions and the quantity or density of inclusions of a certain size in the test area. ing.

Japanese Unexamined Patent Publication No. 2014-189895 JP 2006-161143 A JP 2003-306743 A JP 2000-297347 A JP-A-9-176784 JP-A-6-299287

  Patent Documents 1 to 5 describe that the type and size of inclusions and the quantity or density of inclusions having a certain size in the test area. All of these are inclusion control in a fixed test area, and in such inclusion control based on probability statistics, there is a possibility that coarse inclusions are also present at a fixed ratio. Therefore, there is a possibility that the component fatigue characteristics may be deteriorated at a certain rate. Since case-hardened steel is applied to important safety parts such as gears, destruction during actual use is unacceptable, and if early destruction occurs, there is a risk that claims will be generated or recalled. In addition, these conventional technologies have a very small area for the fatigue fracture risk volume of a fatigue test piece or actual part, and show the maximum predicted diameter by the extreme value statistical method. It was a practical issue.

  Patent Document 6 defines the quantity of inclusions of a certain size in the test area for sulfide inclusions. However, since sulfide inclusions are softer than oxide inclusions and nitride inclusions, they tend to stretch in the longitudinal direction during rolling, and the cross-sectional area in a section perpendicular to the longitudinal direction tends to decrease. There is a tendency. Therefore, particularly when the cross-section reduction rate during rolling is high, the sulfide inclusions have little influence on the fatigue characteristics, and thus the fatigue characteristics may not be improved sufficiently.

  This invention is developed in view of said actual condition, and it aims at proposing about the case hardening steel excellent in the fatigue strength characteristic, and its manufacturing method.

In order to achieve the above-mentioned object, the inventors have conducted intensive investigations on the relationship between the component composition of the case-hardened steel and the inclusions and the relationship between the inclusions and the fatigue characteristics, and have found a case-hardened steel having excellent fatigue strength. It was. In particular, when the cross-section reduction rate during rolling is high, the breakage starting from sulfide inclusions does not occur, and the breakage starting from oxide inclusions or nitride inclusions occurs. Therefore, it has been found that it is effective to control oxide inclusions and nitride inclusions in the control of inclusions.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. C: 0.10-0.40 mass%,
Si: 0.01-1.20 mass%,
Mn: 0.30-1.50 mass%,
P: 0.1% by mass or less,
S: 0.5 mass% or less,
Cr: 0.40 to 2.00% by mass,
Al: 0.010 to 0.080% by mass and N: 0.05% by mass or less, with the balance being a component composition of Fe and inevitable impurities, and a fisheye center obtained by observing the fracture surface after a rotating bending fatigue test with a microscope Case-hardened steel in which the area A (μm ² ) of each of the oxide inclusions and nitride inclusions located in the part satisfies the following formula (1).
√A ≦ 100 (μm) (1)

2. The component composition further includes
Mo: 1% by mass or less
Cu: 1% by mass or less
2. The case hardening steel according to 1 above, containing one or more selected from Ni: 1% by mass or less and B: 0.01% by mass or less.

3. The component composition further includes
Nb: 0.1% by mass or less,
Hf: 0.1 mass% or less,
Ta: 0.1% by mass or less
Se: The case hardening steel of said 1 or 2 containing 1 type, or 2 or more types chosen from 0.3 mass% or less.

4). The component composition further includes
The case-hardened steel according to any one of 1 to 3 above, containing one or two selected from Ti: 0.1% by mass or less and V: 0.1% by mass or less.

5. The component composition further includes
Sb: The case hardening steel in any one of said 1 thru | or 4 containing 0.5 mass% or less.

6). C: 0.10-0.40 mass%,
Si: 0.01-1.20 mass%,
Mn: 0.30-1.50 mass%,
P: 0.1% by mass or less,
S: 0.5 mass% or less,
Cr: 0.40 to 2.00% by mass,
Al: 0.010 to 0.080% by mass and N: 0.05% by mass or less, the balance being hot with a steel slab having a component composition of Fe and inevitable impurities at a cross-sectional reduction rate satisfying the following formula (2) A method for producing case-hardened steel that is processed into bar steel or wire.
(S _i -S _f ) / S _i ≧ 0.985 (2)
However, S _i : slab cross-sectional area (mm ² )
S _f : Cross section of steel bar or wire after hot working (mm ² )

7). The component composition further includes:
Mo: 1% by mass or less
Cu: 1% by mass or less
The manufacturing method of the case hardening steel of said 6 containing 1 type (s) or 2 or more types chosen from Ni: 1 mass% or less and B: 0.01 mass% or less.

8). The component composition further includes
Nb: 0.1% by mass or less,
Hf: 0.1 mass% or less,
Ta: 0.1% by mass or less
Se: The method for producing a case-hardened steel as described in 6 or 7 above, containing one or more selected from 0.3% by mass or less.

9. The component composition further includes
9. The method for producing a case hardening steel according to any one of 6 to 8 above, containing one or two selected from Ti: 0.1% by mass or less and V: 0.1% by mass or less.

10. The component composition further includes
Sb: The manufacturing method of the case hardening steel in any one of said 6 thru | or 9 containing 0.5 mass% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the case hardening steel excellent in fatigue strength can be provided.

It is explanatory drawing which shows carburizing quenching and tempering conditions. It is a graph which shows the correlation with the inclusion maximum predicted diameter and the shortest fatigue life of the rotating bending fatigue test. Is a graph showing the correlation between the values of reduction of area _{(S i -S f) / S} i values and √A (μm). It is a graph which shows correlation with (square root) A and the rotational bending fatigue test shortest life.

First, the component composition of steel in the present invention will be described in detail.
C: 0.10-0.40 mass%
In order to increase the hardness of the center portion of the case-hardened steel by quenching after carburizing and heat-treating the case-hardened steel, C of 0.10% by mass or more is required. On the other hand, when the content of C exceeds 0.40 mass%, the toughness of the core portion decreases, so the C content is limited to a range of 0.10 to 0.40 mass%. Preferably, it is the range of 0.13-0.27 mass%. More preferably, it is the range of 0.15-0.25 mass%.

Si: 0.01-1.20% by mass
Si is necessary as a deoxidizing agent, and at least 0.01% by mass or more must be added. However, Si is an element that preferentially oxidizes in the carburized surface layer and promotes grain boundary oxidation. In addition, the upper limit is set to 1.20% by mass in order to increase deformation resistance by solid solution strengthening and degrade forgeability. Preferably it is 0.02-0.35 mass%. More preferably, it is 0.03-0.15 mass%. In the case of cold forging, the most preferable range is 0.03 to 0.09% by mass.

Mn: 0.30-1.50 mass%
Mn is an element effective for improving the hardenability, and requires addition of at least 0.30% by mass. However, excessive addition of Mn causes an increase in deformation resistance due to solid solution strengthening, so the upper limit was made 1.50% by mass. Preferably it is 0.40-1.0 mass%, More preferably, it is 0.40-0.90 mass%.

P: 0.1% by mass or less P is segregated at the grain boundary and lowers the toughness. Therefore, the lower the mixing thereof, the better, but 0.1 mass% is acceptable. Preferably, it is 0.02 mass% or less. Further, there is no problem even if the lower limit is not particularly limited, but wasteful reduction in P increases the refining time and increases the refining cost.

S: 0.5% by mass or less S is an element that exists as a sulfide inclusion and is effective for improving machinability. However, excessive addition causes a decrease in cold forgeability, so the upper limit is 0.5% by mass. It was. Moreover, although it does not specifically limit about a minimum, since excessively low S will raise refining cost, it is good to set it as 0.003 mass% or more. Preferably it is 0.004-0.3 mass%, More preferably, it is 0.005-0.09 mass%.

Cr: 0.40 to 2.00% by mass
Cr contributes to improving hardenability and temper softening resistance, and is also an element useful for promoting the spheroidization of carbides. However, if the content is less than 0.40% by mass, its addition effect is poor. If it exceeds 2.00 mass%, excessive carburization and the formation of retained austenite are promoted, and the fatigue strength is adversely affected. Therefore, the Cr content is limited to the range of 0.40 to 2.00% by mass. Preferably it is the range of 0.7-1.9 mass%. More preferably, it is 0.8-1.8 mass%.

Al: 0.010 to 0.080 mass%
Al is an element effective for deoxidation, and is effective in preventing the coarsening of crystal grains by forming a nitride, but if the content is less than 0.010% by mass, the addition effect is poor. In addition, excessive addition causes an increase in inclusions, increases the starting point of fatigue fracture, and causes low fatigue strength, so the upper limit was made 0.080% by mass. Preferably, it is 0.015-0.080 mass%, More preferably, it is 0.015-0.060 mass%. The hardenability improvement by solid solution B in combination with B is also effective for improving the fatigue strength. In that case, the range of 0.035 to 0.070 mass% is preferable.

N: 0.05% by mass or less N is mixed from the atmosphere during refining of steel. If it exceeds 0.05% by mass, cracking during solidification becomes prominent and remains as wrinkles even after rolling or forging, making it unusable as a product. If forging is carried out with the wrinkles remaining, the wrinkles will open and cracks will easily occur. Therefore, the upper limit of N is set to 0.05% by mass. Moreover, although it does not specifically limit about a minimum, since excessively low N raises refining cost, it is good to set it as 0.001% or more. Preferably it is 0.0015-0.03 mass%, More preferably, it is 0.002-0.025 mass%. Moreover, when the crack at the time of casting is completely suppressed and a yield improvement is anticipated, it is good also considering an upper limit as 0.015 mass%. Furthermore, when reducing the amount of solute N in steel, suppressing the increase of cold forging load due to dynamic strain aging during cold forging, and reducing the forging load, the range of 0.002 to 0.0060 mass% is suitable. It is.

As mentioned above, although the basic component of this invention was demonstrated, in this invention, it is possible to add suitably each component shown below further as needed.
Mo: 1% by mass or less, Cu: 1% by mass or less, Ni: 1% by mass or less, and B: 0.01% by mass or less

Mo: 1% by mass or less
Mo is a useful element that contributes to the improvement of hardenability and temper softening resistance, and further exhibits the effect of reducing the carburizing abnormal layer. However, if the content exceeds 1% by mass, the hardenability becomes excessive, and there is a concern that wrinkles or cracks may occur during handling after rolling. Therefore, the Mo content is limited to a range of 1% by mass or less. In order to exhibit the effects of improving the hardenability and temper softening resistance and reducing the carburizing abnormal layer by Mo, Mo is preferably contained in an amount of 0.01% by mass or more. Furthermore, it is preferably in the range of 0.03-0.5% by mass. More preferably, it is 0.05-0.3 mass%.

Cu: 1% by mass or less
Cu is an element that contributes to improving hardenability, and is a useful element that, when added together with Se, binds to Se in steel and exhibits an effect of preventing crystal grain coarsening. In order to obtain these effects, Cu is preferably contained at 0.01% by mass or more. On the other hand, if the Cu content exceeds 1% by mass, the surface of the rolled material becomes rough and there is a concern that it remains as soot. Therefore, the amount of Cu is limited to a range of 1% by mass or less. Preferably it is the range of 0.015-0.5 mass%. More preferably, it is 0.03-0.3 mass%.

Ni: 1% by mass or less
Ni contributes to improving hardenability and is an element useful for improving toughness. In order to obtain these effects, Ni is preferably contained at 0.01% by mass or more. On the other hand, even if it contains exceeding 1 mass%, said effect will be saturated. Therefore, the Ni content is limited to a range of 1% by mass or less. Preferably it is the range of 0.015-0.5 mass%. More preferably, it is 0.03-0.3 mass%.

B: 0.01% by mass or less B is effective in improving hardenability by segregating at grain boundaries and suppressing diffusion type transformation. In addition, it strengthens grain boundaries and causes the occurrence and progress of fatigue cracks. It also has the effect of suppressing fatigue strength. In order to acquire this effect by B, it is preferable to contain B at 0.0003 mass% or more. On the other hand, if it exceeds 0.01%, the toughness decreases, so the B content is limited to a range of 0.01% by mass or less. Preferably, it is the range of 0.0005-0.005 mass%. More preferably, it is 0.0007-0.002 mass%.

Nb: 0.1% by mass or less, Hf: 0.1% by mass or less, Ta: 0.1% by mass or less and Se: 0.3% by mass or less
Nb: 0.1% by mass or less
Nb forms NbC in the steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment by the pinning effect. In order to obtain this effect due to Nb, it is preferable to add Nb at least 0.003% by mass or more. On the other hand, if added in excess of 0.1% by mass, there is a risk of reducing the coarsening suppression ability and deterioration of fatigue strength due to coarse precipitation of NbC, so the Nb content is made 0.1% by mass or less. Preferably, it is 0.005-0.08 mass%. More preferably, it is 0.01-0.06 mass%.

Hf: 0.1% by mass or less
Hf forms carbides in the steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment by the pinning effect. In order to obtain this effect by Hf, it is preferable to add Hf at least 0.003 mass% or more. On the other hand, if added over 0.1% by mass, coarse precipitates are formed during casting solidification, which may lead to a decrease in coarsening suppression ability and fatigue strength, so the Hf content is 0.1% by mass or less. And Preferably, it is 0.005-0.06 mass%. More preferably, it is 0.01-0.05 mass%.

Ta: 0.1% by mass or less
Ta forms carbides in the steel and suppresses the coarsening of austenite grains during the carburizing heat treatment due to the pinning effect. In order to obtain this effect by Ta, it is preferable to add Ta at least 0.003% by mass or more. On the other hand, if added over 0.1% by mass, cracks are likely to occur during casting solidification, and soot may remain even after rolling and forging, so the Ta content is 0.1% by mass or less. Preferably, it is 0.005-0.06 mass%. More preferably, it is 0.01-0.05 mass%.

Se: 0.3% by mass or less
Se combines with Mn and Cu and disperses as precipitates in the steel. Se precipitates exist stably in the carburizing heat treatment temperature range with little precipitate growth, and have a high pinning effect on the austenite grain size. For this reason, the addition of Se is effective for preventing coarsening of crystal grains, but in order to obtain this effect, it is preferable to add Se at least 0.001% by mass or more. On the other hand, even if added over 0.3% by mass, the effect of preventing coarsening of crystal grains is saturated. For this reason, Se content shall be 0.3 mass%. Preferably, it is 0.005-0.1 mass%. More preferably, it is 0.008-0.09 mass%.

One or two selected from Ti: 0.1% by mass or less and V: 0.1% by mass or less
Ti: 0.1% by mass or less
Ti forms carbides and nitrides in the steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment due to the pinning effect. In order to acquire this effect by Ti, it is preferable to contain Ti at least 0.003 mass% or more. On the other hand, if added in excess of 0.1% by mass, there is a risk of reducing the coarsening suppression ability due to coarse precipitates, so the Ti content is 0.1% by mass or less.
Ti has a very strong bonding force with N and forms a nitride even in a small amount. Since this Ti nitride is coarsely formed from the solidification stage and remains after rolling, the contact fatigue strength may be significantly reduced. In particular, in parts where contact fatigue failure such as pitting and surface peeling preferentially occurs or gear parts with very high load stress in the usage environment, it is desirable to avoid mixing as much as possible without adding Ti rather than 0.003 mass % Or less.

V: 0.1% by mass or less V forms VC in the steel and suppresses the coarsening of the austenite grain size during the carburizing heat treatment by the pinning effect. In order to acquire this effect by V, it is preferable to contain V at least 0.003 mass% or more. On the other hand, adding more than 0.1% by mass only increases the cost of the alloy, and the effect of preventing the coarsening of the crystal grains is saturated. Therefore, the V content is 0.1% by mass or less. Preferably, it is 0.005-0.08 mass%. More preferably, it is 0.01-0.06 mass%.

Sb: 0.5% by mass or less
Sb is an effective element for suppressing the decarburization of the steel surface and preventing the decrease in surface hardness. In order to exhibit this effect, it is preferable to contain 0.0003 mass% or more of Sb. On the other hand, excessive addition degrades forgeability, so the Sb content is 0.5 mass% or less. Preferably, it is 0.001-0.05 mass%, More preferably, it is 0.0015-0.035 mass%.

The balance other than the elements described above is Fe and inevitable impurities.
Here, among the inevitable impurities described above, Sn, in particular, deteriorates forgeability, so it is preferable to reduce it as much as possible. That is, the Sn content is preferably limited to 0.004% by mass or less. More preferably, it is 0.003% by mass or less, and most preferably 0.0015% by mass or less which is achieved by adjusting the blending of raw materials such as scrap.

Next, the prescription | regulation regarding the inclusion in this invention is demonstrated. In the case-hardened steel of the present invention, the area A (μm ² ) of each of the oxide inclusions and nitride inclusions located in the center of the fish eye obtained by observing the fracture surface after the rotating bending fatigue test with a microscope. However, it is necessary to satisfy the following formula (1).
√A ≦ 100 (μm) (1)

The left side of the above equation (1) is an index indicating the maximum size of oxide inclusions and nitride inclusions that are the starting points of fatigue fracture. Here, A is the direction of stretching by hot working, that is, the rolling direction in the case of hot rolling, and the direction of stretching by forging in the case of hot forging. It is obtained by observing the fracture surface after the fatigue test with a scanning electron microscope.
That is, the area of the oxide-based and nitride-based inclusions located at the center of the fish eye, which is characteristic of fatigue fracture due to oxide-based and nitride-based inclusions, is quantified by image analysis. This rotating bending fatigue test piece is a test piece having a parallel part diameter of 8 mm and a parallel part length of 16 mm. Then, a rotating bending fatigue test was performed on seven test pieces having this size, causing fish eye breakage, and the fracture surface of the seven test pieces having the lowest fatigue life was observed with a microscope. Measure the area A of oxide-based and nitride-based inclusions located in Prepared from the steel in the component composition range described above, by carburizing heat treatment on the test piece, polishing the surface with a thickness of 0.1 mm after carburizing, and performing a rotating bending fatigue test under conditions of a load stress of 600 MPa or more, Causes eye destruction. The square root of the area A of oxide-based and nitride-based inclusions present in the center of the fish eye thus induced is the size of the largest oxide-based and nitride-based inclusions that affect fatigue life. It can be evaluated as. According to the method for determining the size of the inclusion according to the present invention, the size of the maximum inclusion in a volume of (7.9 mm ÷ 2) ² × 16 mm × 7 = 343 mm ³ can be evaluated. The conventional method for measuring the size, quantity or density of inclusions present in the test area cannot measure the state of inclusions in such a large volume, and the evaluation of inclusions affecting fatigue life Can not do.

  Inclusions according to the steel composition of the present invention mainly include sulfides, oxides, and nitrides, and can be selected as follows on the fracture surface after the rotary bending fatigue test described above. That is, the composition analysis of inclusions is possible by using an SEM (scanning electron microscope) equipped with EDX (energy dispersive X-ray spectroscopy apparatus). In the present invention, as a result of analysis by EDX, inclusions containing oxygen are treated as oxide inclusions, and inclusions containing nitrogen are treated as nitride inclusions.

In the above-described evaluation method for oxide-based and nitride-based inclusions in the present invention, the size of oxide-based and nitride-based inclusions that actually became the starting point of fatigue fracture of steel in a large volume of 343 mm ^3. Therefore, the fatigue life prediction accuracy is further improved.

  Then, the oxide and nitride inclusion sizes are reduced to a range that does not exist in the conventional steel so as to satisfy the above formula (1) by using the index of the maximum inclusion size by the method with improved prediction accuracy. It was found that a case-hardened steel having a high fatigue life can be obtained. That is, when the left side of the above formula (1) exceeds 100, early fatigue failure is indicated and the fatigue strength is reduced. More preferably, the left side of the formula (1) is 80 or less, and more preferably 50 or less.

Next, the manufacturing method of the case hardening steel of this invention is demonstrated.
In order to satisfy the above formula (1), in addition to the adjustment of the component composition to the above range, the slab is subjected to hot forging and / or heat at a cross-sectional reduction rate satisfying the following formula (2). It is necessary to perform hot working by hot rolling to form a steel bar or wire.
(S _i -S _f ) / S _i ≧ 0.985 (2)
However, S _i : slab cross-sectional area (mm ² )
S _f : Cross section of steel bar or wire after hot working (mm ² )

The left side of the above equation (2) is an index indicating the cross-sectional reduction rate when hot working is performed on the slab. Here, the hot working may be hot forging or hot rolling. Furthermore, it may be a case where both hot forging and hot rolling are performed. Each S _i, it is S _f, the billet cross-sectional area in a cross section perpendicular to the stretching direction at the time of hot working (mm ^2), the cross-sectional area of the steel bar or wire rod after hot working (mm ^2). If the index shown on the left side of the above equation (2) is less than 0.985, it indicates early fatigue failure due to coarse inclusions in the fatigue test, and the fatigue strength decreases. More preferably, the left side of the formula (2) is 0.990 or more, and more preferably 0.995 or more. The optimum is above 0.9993.

  The case-hardened steel of the present invention manufactured as described above is subjected to processing such as hot forging, cold forging, cutting and the like to be finished into a part shape, and then subjected to a known carburizing treatment to obtain a component and Become. In processing, when hot forging or cold forging is performed, the size of oxide-based and nitride-based inclusions changes, but the side that deteriorates fatigue life, that is, the side where the value of A increases Therefore, it is effective to use the case-hardened steel of the present invention even when these parts are processed to form a part.

  Hereinafter, according to an Example, the structure and effect of this invention are demonstrated more concretely. However, the present invention is not limited by the following examples, and can be appropriately changed within the scope that can meet the gist of the present invention, and these are all included in the technical scope of the present invention. It is.

  Steels having the component compositions shown in Tables 1 and 2 were melted and hot-rolled at various cross-section reduction rates to form round bars having various diameters. The obtained round bar was evaluated for various properties required for machine structural steel. That is, optical microstructure observation of as-rolled steel, hardness measurement, inclusion distribution measurement, forgeability (limit crack test), further, hardness distribution measurement after carburizing steel, post-carburizing rotational bending fatigue test, after carburizing Old austenite grain observation was carried out, respectively.

Here, as-rolled steel is observed with an optical microscope by etching a 1/4 depth position (hereinafter referred to as a 1/4 diameter position) from the surface of the cross section of the round bar with mirror polishing and then etching with 3% Nital solution. And then observed. Magnification: After 400 times in the 10 field of view shooting, to quantify the ferrite area ratio by image analysis software (Media Cybernetics, Inc. Image-Pro _{_} PLUS), were evaluated.

  For the hardness measurement, the Vickers hardness was measured at 300 gf at the 1/4 position of the diameter of the round bar cross section. Ten points were measured, and the average value was calculated and evaluated. HV180 or less is desirable for use in cold forging. On the other hand, in the case of hot forging, there is no problem even if the hardness is as it is rolled.

Inclusion distribution measurement to determine the maximum predicted inclusion inclusion diameter is based on the extreme value statistical method, assuming that the maximum size of oxide and nitride inclusions in the as-rolled material follows the extreme value distribution (Weibull distribution). Calculated. That is, a 1/4 position of the diameter of the cross section of the round bar was mirror-polished, and a magnification of 100 × 20 fields (15 mm ² per field, total field area 300 mm ² ) was observed using an SEM equipped with EDX. The areas of the maximum oxide and nitride inclusions in each field of view are obtained by image analysis, and the square root of the area is calculated. Using the square root of the area of the maximum oxide-based and nitride-based inclusions measured in 20 fields of view, using an extreme probability paper, the predicted value at an area of 40,000 mm ² was defined as the maximum size. The method is a general method, and the measurement conditions other than those described above may follow a conventional method. References include, for example, “Problems of JIS point calculation method and inclusion evaluation by extreme value statistical method and its Application Iron and Steel Vol. 79 (1993) No. 12 ”.

  The forgeability was evaluated by the limit upsetting rate obtained by the cold upsetting test (established by the Japan Society for Technology of Plasticity Cold Forging). A cylindrical test piece having a diameter of 14 mm and a height of 21 mm was taken from the position (center) of the diameter of the steel bar, and a V-groove was formed on the peripheral surface of the cylindrical test piece. The upsetting rate until cracking occurred from the groove bottom of the V groove was evaluated. The V groove had a groove bottom R of 0.15, an opening angle of 30 °, and a depth of 0.8 mm. In both hot and cold forging, cracks are unlikely to occur if the limit upsetting rate is 30% or more.

  After carburizing, the hardness distribution was measured by carburizing and tempering a 30 mmφ round bar under the heat history conditions shown in FIG. As the effective hardened layer depth (ECD), the layer depth from the surface layer to the position of HV550 mm was evaluated. In order to ensure fatigue strength, it is desirable that ECD is 0.5 mm or more.

In the rotating bending fatigue test after carburizing, a test piece with a parallel part diameter of 8 mm x parallel part length of 16 mm was taken from the 1/2 position of the diameter of the round bar and carburized and tempered under the conditions shown in FIG. Then, it was subjected to a double swing Ono type rotary bending fatigue test. The load stress was 1000 MPa, the rotation speed was 3500 rpm, and the test piece with the lowest fatigue life was evaluated among the seven test pieces that caused fisheye fracture. In other words, the specimen with the lowest fatigue life was observed, and the fracture surface was observed with an SEM equipped with EDX, and the area of oxide and nitride inclusions in the center of fish eye was analyzed (Media Cybernetics). determined in manufactured by Image-Pro _{_} PLUS), it was the area a. In this test, if the fatigue life is 100,000 times or more until the fracture, it can be regarded as high fatigue strength.

After carburizing, the old austenite grains were observed by taking a cylindrical test piece with a diameter of 14 mm and a height of 21 mm from the position (center) of the diameter 1/2 of the steel, and forming a V-groove on the peripheral surface of the cylindrical test piece. Then, using a 300-ton press, cold forging with an upsetting rate of 0% (no forging) and 70%, carburizing and tempering under the heat history conditions shown in FIG. organization, of JIS _{_} G0551 - in accordance with the "steel microscope test method of grain size", was carried out to evaluate the grain size. In addition, for the cross-sectional structure of the same specimen, after taking 10 fields of view with an optical microscope at a magnification of 400 times, the area ratio of crystal grains of 62 μm or less and the area ratio of crystal grains of over 177 μm were calculated using image analysis software quantified at Cybernetics Inc. Image-Pro _{_} PLUS), it was evaluated.

  Tables 3 and 4 show the measurement and evaluation results thus obtained. 2 shows the relationship between the maximum predicted inclusion diameter by microscopic observation and the rotational bending fatigue life, FIG. 3 shows the relationship between the area reduction ratio and √A, and FIG. 4 shows the relationship between √A and the rotational bending fatigue test results. Each relationship is shown. 2 to 4 exclude the influence of the steel composition on the rotational fatigue life as much as possible so that the influence of the predicted maximum diameter of inclusions and √A on the rotational fatigue life can be simply seen. The data is only for the component composition within the scope of the present invention. From these, the predicted maximum inclusion diameter by microscopic observation has little correlation with the fatigue life, but √A obtained by the above-described method of the present invention has a high correlation with the fatigue life. Therefore, it turns out that the case hardening steel according to this invention has high fatigue strength.

Claims (10)

C: 0.10-0.40 mass%,
Si: 0.01-1.20 mass%,
Mn: 0.30-1.50 mass%,
P: 0.1% by mass or less,
S: 0.5 mass% or less,
Cr: 0.40 to 2.00% by mass,
Al: 0.010 to 0.080 mass%,
N: 0.05% by mass or less, with the balance being Fe and inevitable impurities, with oxide-based intervention located at the center of the fish eye obtained by observing the fracture surface after a rotating bending fatigue test with a microscope Case-hardened steel in which the area A (μm ² ) of each of the inclusions and nitride inclusions satisfies the following formula (1).
√A ≦ 100 (μm) (1) The component composition further includes
Mo: 1% by mass or less
Cu: 1% by mass or less
The case hardening steel of Claim 1 containing 1 type (s) or 2 or more types chosen from Ni: 1 mass% or less and B: 0.01 mass% or less. The component composition further includes
Nb: 0.1% by mass or less,
Hf: 0.1 mass% or less,
Ta: 0.1% by mass or less
Se: The case hardening steel of Claim 1 or 2 containing 1 type, or 2 or more types chosen from 0.3 mass% or less. The component composition further includes
The case hardening steel according to any one of claims 1 to 3, comprising one or two selected from Ti: 0.1 mass% or less and V: 0.1 mass% or less. The component composition further includes
The case-hardened steel according to any one of claims 1 to 4, comprising Sb: 0.5% by mass or less. C: 0.10-0.40 mass%,
Si: 0.01-1.20 mass%,
Mn: 0.30-1.50 mass%,
P: 0.1% by mass or less,
S: 0.5 mass% or less,
Cr: 0.40 to 2.00% by mass,
Al: 0.010 to 0.080% by mass and N: 0.05% by mass or less, the balance being hot with a steel slab having a component composition of Fe and inevitable impurities at a cross-sectional reduction rate satisfying the following formula (2) The area A (μm) of oxide inclusions and nitride inclusions located at the center of the fish eye obtained by observing the fracture surface after the rotating bending fatigue test with a microscope by processing into a steel bar or wire rod ² ) is a method for producing case-hardened steel that satisfies the following formula (1) .
Record
√A ≦ 100 (μm) (1)
(Si−Sf) /Si≧0.985 (2)
Si: slab cross-sectional area (mm ² )
Sf: Sectional area of steel bar or wire after hot working (mm ² ) The component composition further includes:
Mo: 1% by mass or less
Cu: 1% by mass or less
The manufacturing method of the case hardening steel of Claim 6 containing 1 type (s) or 2 or more types chosen from Ni: 1 mass% or less and B: 0.01 mass% or less. The component composition further includes
Nb: 0.1% by mass or less,
Hf: 0.1 mass% or less,
Ta: 0.1% by mass or less
Se: The manufacturing method of the case hardening steel of Claim 6 or 7 containing 1 type, or 2 or more types chosen from 0.3 mass% or less. The component composition further includes
The manufacturing method of the case hardening steel in any one of Claim 6 thru | or 8 containing 1 type or 2 types chosen from Ti: 0.1 mass% or less and V: 0.1 mass% or less. The component composition further includes
The manufacturing method of the case hardening steel in any one of Claim 6 thru | or 9 containing 0.5 mass% or less of Sb.

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