JP5207805B2 - Steel parts with excellent bending fatigue strength and manufacturing method thereof - Google Patents

Description

  The present invention relates to a technique for increasing bending fatigue strength in steel parts (rolling parts (particularly gears) such as axles, bearings, constant velocity joints, rail guides, and gears) that require bending fatigue strength and forgeability. .

  With the downsizing of structural parts aimed at improving the fuel efficiency of automobiles, there is a demand for further improvement in the bending fatigue strength of steel parts. From such a background, carburizing the case-hardened steel, which is the material of the structural component, hardens the surface of the structural component, and by applying shot peening, compressive residual stress is given to the surface of the component, and bending fatigue strength is increased. A method of improving is used.

  For example, in Patent Document 1, C: 0.1 to 0.4%, Si: 0.15% or less, Mn: 0.2 to 1.2%, P: 0.012% or less, S: 0.0. Case-hardening for manufacturing machine structural parts containing 005 to 0.07%, Cr: 0.2 to 1.2%, Al: 0.015 to 0.1%, N: 0.005 to 0.025%, respectively Using steel, the steel material is formed into a predetermined part shape, carburized, and then quenched from the austenite single-phase region, so that the residual austenite area ratio from the surface to a depth of 0.1 mm is 20% or less. Then, a method for manufacturing a machine structural part to be subjected to shot peening is described. In Patent Document 1, Si is set to 0.15% or less because Si is an element that easily forms an oxide as compared with Fe. This is because the depth of grain boundary oxidation increases and fatigue strength decreases.

  Similarly, in Patent Document 2, Si is added for deoxidation, but Si is an element that is easier to oxidize than Fe, and if added in excess of 0.15%, the depth of the grain boundary oxide layer becomes deeper. Therefore, it is said that the bending fatigue strength should be reduced to 0.15% or less.

  On the other hand, there is an example where 1% or more of Si is added (Patent Document 3), but the magnitude of compressive residual stress that can be applied to the structural component after shot peening is not sufficient. Further, as shown in Table 1 of Patent Document 3, since the amount of Mo is large, it is considered that forgeability is poor.

In addition, as a method for improving bending fatigue strength, a method such as contour induction hardening has been proposed, but a large-scale facility such as a high-frequency treatment apparatus is required. Although a method for reducing the grain boundary oxide layer has also been proposed, the prevention of the formation of the grain boundary oxide layer has already been achieved, and there is no further allowance for this point.
JP-A-8-60294 JP-A-5-59432 JP 2005-23399 A

  In the case carburizing and shot peening treatment of the conventional case-hardened steel as described above, the compressive residual stress that can be applied to the steel parts is generally 1500 MPa or less, except when other characteristics such as forgeability are considered to be sacrificed. Stay on. Therefore, an object of the present invention is to provide a steel part that improves bending fatigue strength by imparting a high compressive residual stress to the steel part and is excellent in forgeability.

  Therefore, the present inventors conducted various studies on compressive residual stress that can be applied to steel parts, including verification of conventional steel parts. Table 1 shows the steel material components of the steel parts having a low Si content, and Table 2 shows the tempering hardness after carburizing of the steel materials, the compression residual stress of the steel parts after the two-stage shot peening treatment, and the like.

As shown in Table 2, the compressive residual stress remains approximately 1500 MPa or less. In addition, as shown in Table 2, after the shot peening treatment, deformation of about 0.3 mm to 0.6 mm is confirmed on the surface of the steel part. Since the steel parts after the carburizing treatment are harder than the shot sphere used for the shot peening treatment, such deformation should normally not occur. From this fact, it was considered that the steel parts were softened for some reason during the shot peening process. Therefore, as shown in FIG. 1, the relationship between the amount of deformation of the surface of the steel part and the tempering hardness after carburizing was examined by changing the tempering temperature in various ways. Then, examine the correlation function R of tempered hardness and the deformation amount at the tempering each temperature, it summarizes the relationship between the value of R ² and tempering temperature, as in FIG. As apparent from FIG. 2, it was found that R ² was increased when the tempering temperature was 200 ° C., and it was found that the same phenomenon as temper softening occurred during the shot peening treatment. And the present inventors have found that the compressive residual stress can be increased by increasing the amount of Si and increasing the temper softening resistance, and by appropriately setting the arc height value at the time of shot peening treatment to prevent adverse effects due to the tempering phenomenon. Completed the invention.

  As shown in FIG. 3, when the relationship between the Si concentration of the steel part and the 200 ° C. tempering hardness is also examined, when the Si content is 0.5% or more, the 200 ° C. tempering hardness is dramatically improved. When it exceeds%, it tends to be slightly saturated. From this, in order to obtain a high compressive residual stress by shot peening, it is effective to set the Si concentration of the steel part to 0.5% or more.

In order to solve the above problems, the steel component of the present invention is
C: 0.15-0.25% (meaning “mass%”; the same shall apply hereinafter),
Si: 0.5 to 1.0%
Mn: 0.3 to 1%
Cr: 0.8 to 1.6%,
Mo: 0.45% or less (excluding 0%),
Al: 0.01 to 0.05%,
N: 0.008 to 0.03%,
Each of which has a chemical hardened layer and has a peak value of compressive residual stress in a range from the surface to a depth of 100 μm. 1600 MPa or more.

  As described above, in order to ensure the bending fatigue strength, by making the Si content 0.5 to 1.0%, which is considered to be reasonable, the steel part is subjected to shot peening treatment. The compressive residual stress that can be applied can be increased.

  Moreover, it is desirable that the steel material further contains Nb: 0.05% or less (not including 0%), and Nb carbonitride is formed.

Moreover, the manufacturing method of the steel part of the present invention includes:
C: 0.15-0.25%,
Si: 0.5 to 1.0%
Mn: 0.3 to 1%
Cr: 0.8 to 1.6%,
Mo: 0.45% or less (excluding 0%),
Al: 0.01 to 0.05%,
N: 0.008 to 0.03%,
Each of the steel materials containing Fe and unavoidable impurities is subjected to a chemical surface hardening treatment, and the steel material is subjected to a first shot peening treatment with an arc height value of 0.5 mmA or more. A second shot peening process having a value of 0.18 mmN or more and 0.5 mmN or less is performed.

  Moreover, it is desirable that the steel material further contains Nb: 0.05% or less (not including 0%), and Nb carbonitride is formed.

  The steel part of the present invention contains 0.5% or more of Si, and appropriately adjusts the arc height value of the shot peening process performed in two stages, thereby preventing softening during the shot peening process and compressing residual stress. The peak value of 1600 MPa or more. Thereby, the bending fatigue strength of steel parts can be improved. In addition, while the Si content is 0.5% or more, the forgeability can be maintained by setting the Mo content to 0.45% or less.

(I): Chemical component composition of steel part One feature of the steel part of the present embodiment is that the chemical component composition is appropriately adjusted in order to achieve both bending fatigue strength and forgeability. Therefore, first, the chemical composition of the steel material will be described.

C: 0.15-0.25%
C is an element necessary for ensuring the strength of the steel material, and is useful for increasing the bending fatigue strength. Therefore, the C content is 0.15% or more, preferably 0.18% or more. However, when C is contained excessively, the cracking susceptibility of the steel material is increased, the bending fatigue strength is decreased, and the forgeability is also decreased. Therefore, the C content is 0.25% or less, preferably 0.22% or less.

Mn: 0.3 to 1%
Mn improves the hardenability of steel and is useful for ensuring the strength of steel parts. Therefore, the Mn content is 0.3% or more, preferably 0.35% or more. However, if Mn is contained excessively, cracking may occur and the forgeability is also lowered, so the content is made 1% or less, preferably 0.8% or less, more preferably 0.6% or less.

Cr: 0.8 to 1.6%
Cr improves the strength and hardenability of steel parts, and its content is 0.8% or more, preferably 1.0% or more, and more preferably 1.2% or more. However, if Cr is excessively contained, the steel material becomes too hard and the forgeability is lowered. Therefore, the upper limit of the Cr content is 1.6%, preferably 1.5%.

Al: 0.01 to 0.05%
When the Al content increases, the amount of oxide inclusions increases and the toughness of the steel parts deteriorates. Therefore, the upper limit of the Al content is 0.05%, preferably 0.04%, and more preferably 0.03%. On the other hand, Al acts as a deoxidizer and may be actively added for refinement of steel crystal grains. In this case, the lower limit of the Al content is, for example, 0.01%, preferably 0.02%.

N: 0.008 to 0.03%
N forms nitrides with Al, Nb, etc., and contributes to refinement of the structure of steel parts. In order to exhibit such an effect effectively, the N content is set to 0.008% or more, preferably 0.01% or more. However, if N is excessively contained, the hot workability and ductility are adversely affected, so 0.03% or less, preferably 0.02% or less.

Si: 0.5 to 1.0%
Si is an element necessary for improving the bending fatigue strength under the condition of applying the shot peening treatment in order to improve the hardenability and increase the softening resistance during the shot peening treatment as described above. The content of Si that effectively exhibits such an effect is 0.5% or more, preferably 0.55% or more, and more preferably 0.6% or more. However, if Si is excessively contained, deformation resistance is increased, so that the forgeability is deteriorated and also the brittleness of the steel part is caused. Therefore, the Si content is 1.0% or less, preferably 0.8% or less.

Mo: 0.45% or less (excluding 0%)
Mo is an element that effectively acts to improve the hardenability, strength, and toughness, but at the same time causes a reduction in forgeability. In the present invention, since a large amount of Si is contained as described above, there is a concern that the forgeability of the steel part is lowered, and the Mo content is set to 0.45% or less. Thereby, the deformation resistance value of the steel material can be less than 90 MPa. This is a 20% increase in the deformation resistance value (74 MPa) of the existing SCM420 steel. The Mo content is more preferably 0.35% or less, further preferably 0.25% or less, and further preferably 0.15% or less.

Here, the addition amounts of Si and Mo will be described in more detail. FIG. 4 shows the influence of the Si content and the Mo content on the forgeability and bending fatigue strength of steel parts. In FIG. 4, the horizontal axis indicates the Si content, and the vertical axis indicates the Mo content. Moreover, the meaning of each symbol in FIG. 4 is as follows.
◯: Bending fatigue strength is 2100 MPa or more and deformation resistance is less than 90 MPa Δ: Bending fatigue strength is 2100 MPa or more and deformation resistance is 90 MPa or more ▲: Bending fatigue strength is less than 2100 MPa and deformation resistance is less than 90 MPa ×: Bending fatigue strength is less than 2100 MPa and deformation resistance is 90 MPa or more

  As is apparent from FIG. 4, in order to prevent the deformation resistance from increasing while improving the bending fatigue strength, the Si content is set to 0.50 to 1.00%, and the Mo content is set to 0.45. It can be seen that it should be less than%. In addition, about bending fatigue strength, the deformation | transformation was repeated using the test piece of the steel component shown in FIG. The strength imparted to the condition test piece was determined to be a strength at which the sample was broken by repeated bending tests of 20,000 times. For the forgeability test, a steel part test piece having a diameter of 8 mm and a length of 12 mm was heated to 1200 ° C., and a 60% compression test was performed with the end face of the test piece being unconstrained.

  The balance of the steel part according to the present embodiment contains iron and unavoidable impurities, but as unavoidable impurities, in addition to the elements described above, for example, a small amount of S, P, O, and the effects of the present invention are adversely affected. A small amount of alloying elements that do not affect, for example, Mg, Ca, Na, Ba, Cu, Ni, As, Sb, Sn, Ti, Zr, V, Ta, Co, W, B, or rare earth elements. It may be. Of these, S and P are preferably set to 0.03% or less as described below.

S: 0.03% or less (excluding 0%)
S significantly reduces the strength of the steel material. Therefore, the S content is 0.03% or less, preferably 0.025% or less, and more preferably 0.02% or less. The smaller the S content, the better. However, it is technically difficult to completely remove S, and it is usually more than 0%.

P: 0.03% or less (excluding 0%)
P causes segregation at the grain boundaries to lower the grain boundary strength and causes embrittlement of the steel material. Therefore, the content is preferably as low as possible. Therefore, it is recommended to suppress the P content to 0.03% or less, preferably 0.02% or less, more preferably 0.01% or less.

  The steel part of the present embodiment further contains Nb: 0.05% or less (not including 0%), preferably 0.03% or less (not including 0%), if necessary. It is also effective to improve the high temperature carburizability by forming Nb carbonitride and further increase the bending fatigue strength of the steel part.

(II) Chemical surface hardening treatment The chemical surface hardening treatment is not particularly limited as long as it is a treatment for impregnating carbon into the surface of the carburizing steel. For example, carburizing treatment (solid carburizing method, liquid carburizing method, gas Any of a carburizing method, a vacuum carburizing method) and a carbonitriding process may be used. By these treatments, the amount of retained austenite on the outermost surface (chemical surface hardened layer: carburized layer or carbonitrided layer) after the chemical surface hardening treatment can be secured, and a large amount of compressive residual stress is generated during the subsequent shot peening treatment. Can be granted. The amount of retained austenite on the steel surface before shot peening is suitably about 10 to 70% by volume (preferably 20 to 60% by volume).

  In the chemical surface hardening treatment, it is recommended to quench (for example, oil cooling) at the final stage. The core strength can be increased by quenching.

(III) Two-stage shot peening treatment In order to increase the compressive residual stress on the surface of the steel part, at least two-stage shot peening treatment is performed on the steel material after the chemical surface hardening treatment. First, a first shot peening process is performed with an arc height value of 0.5 mmA or more, preferably 0.6 mmA or more, and more preferably 0.7 mmA or more. Although there is no restriction | limiting in particular about the upper limit of the arc height value of a 1st shot peening process, In order to make a crack difficult to enter into the steel component surface, it is preferable to set it as 1.3 mmA or less.

  Thereafter, a second shot peening process is performed with an arc height value of 0.18 mmN or more and 0.5 mmN or less. Thereby, the peak value of the compressive residual stress of the steel part can be set to 1600 MPa or more.

  The reason why the arc height value in the second shot peening process is set to 0.18 mmN or more is that if it is less than 0.18 mmN, sufficient compressive residual stress cannot be applied. A preferable arc height value is 0.20 mmN or more, more preferably 0.21 mmN or more. On the other hand, the upper limit of the arc height value in the second shot peening process is set to 0.5 mmN or less. When shot peening is performed exceeding 0.5 mmN, for example, by increasing the size of the shot sphere, This is because it is considered that the amount of heat generated on the surface becomes too large and the compressive residual stress is lost. By using a shot sphere having a smaller size than the shot sphere used in the first shot peening process in the second shot peening process, the arc height value can be lowered. The arc height value is preferably 0.4 mmN or less, and more preferably 0.3 mmN or less.

  The preferred size of the shot sphere in the first shot peening treatment is 0.4 mm to 0.8 mm in diameter, more preferably 0.5 mm to 0.7 mm, and the preferred hardness of the shot sphere is 600 HV to 900 HV, more preferably. 700HV to 800HV. A preferable shot pressure is 0.3 MPa to 0.6 MPa, more preferably 0.4 MPa to 0.5 MPa, and a preferable projection time is 10 seconds to 100 seconds, more preferably 20 seconds to 50 seconds.

  In addition, a preferable lower limit of the size of the shot sphere in the second shot peening process is 0.01 mm in diameter, more preferably 0.03 mm, and a preferable upper limit is 0.1 mm. The hardness of the shot sphere is preferably 600 HV to 800 HV, more preferably 650 HV to 750 HV, the preferable lower limit value of the shot pressure is 0.5 MPa, more preferably 0.7 MPa, and the preferable upper limit value is 0.9 MPa. A preferable projection time is 100 seconds to 400 seconds, more preferably 200 seconds to 300 seconds.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and may be appropriately modified within a range that can be adapted to the purpose described below. It is also possible to carry out and they are all within the technical scope of the present invention.

  Steels having the components shown in Table 3 below were melted and cast in an experimental furnace. The obtained slab was formed into a 17 mm square material by forging and normalized (temperature 900 ° C. × 60 minutes: air-cooled). In addition, the process so far imitates the partial rolling in an actual machine, steel bar rolling, and hot forging. As shown in FIG. 5, the rod-shaped material was a 13 mm square and 100 mm long square bar piece processed by cutting, cutting and polishing into a test piece shape having a cut portion at the center. The polished portion surface roughness of the cut portion is a center line average roughness (Ra) of 0.4a.

  Each test piece thus obtained was subjected to a carburizing treatment by a gas carburizing method so that the carbon concentration on the test piece surface was 0.6% to 0.85%, and then at 200 ° C. for 180 minutes. Was tempered. Table 4 shows the carburizing conditions for each test piece, the surface hardness immediately after carburizing, and the surface hardness after tempering (measured at a depth of 50 μm from the steel surface). In the column of “Carburizing conditions” in Table 4, “A”, “B”, and “C” are described, but the types of gases used for carburizing are “A” and “B”, RX A mixed gas of gas and CO gas, “C” is propane gas.

  FIG. 6 (a) shows the thermal history and equilibrium carbon concentration during carburizing under “condition A”, and FIG. 6 (b) shows the thermal history and equilibrium carbon concentration during carburizing under “condition B”. FIG. 6C shows thermal histories according to “Condition C”. “Cp” in FIG. 6 indicates the equilibrium carbon concentration (%), and the gas partial pressure, gas flow rate, etc. during the carburizing process are adjusted according to the target equilibrium carbon concentration.

Next, a two-step shot peening process was performed on each test piece after the carburizing process. The conditions of shot peening at each stage are as shown below. Table 4 shows the peak values of compressive residual stress and bending fatigue strength in the range from the test piece surface after the shot peening treatment to a depth of 100 μm. Shown respectively.
(First stage)
Shot sphere size: Diameter 0.6mm
Shot sphere hardness: 800HV
Shot pressure: 0.45 MPa
Projection time: 33 seconds Arc height value: 0.7 mmA
(Second stage)
Shot sphere size: 0.05mm diameter
Shot sphere hardness: 700HV
Shot pressure: 0.8 MPa
Projection time: 300 seconds Arc height value: 0.22 mmN

  Sample numbers 17 to 19 are not subjected to the second stage shot peening process. As apparent from Table 4, for sample numbers 1 to 8 that satisfy Si: 0.5 to 1.0% and 0.45% or less, the peak value of the compressive residual stress is 1600 MPa or more, and represents forgeability. It can be seen that the deformation resistance value is less than 90 MPa, the bending fatigue strength of the steel part is improved, and the forgeability is maintained within a practical range.

  On the other hand, Sample Nos. 9 to 12 have an insufficient Si content, so that the peak value of compressive residual stress is below 1600 MPa. Since sample number 13 has too much Si content, its deformation resistance is high. Since sample number 14 has too much Mo content, its deformation resistance is high. In Sample Nos. 15 and 16, since the Si content is insufficient and the Mo content is too large, the peak value of compressive residual stress is less than 1600 MPa, the deformation resistance is high, and the forgeability is poor. Since sample numbers 17 to 19 are not subjected to the second stage shot peening process, the magnitude of the compressive residual stress is completely insufficient.

  Next, in order to clearly show the improvement in characteristics of the steel part in this example, the peak values of the compressive residual stress and the bending fatigue strength shown in Table 4 are shown in FIG. As is apparent from FIG. 7, it can be seen that the peak value of the compressive residual stress of the steel part in the example of the present invention is improved from the conventional low silicon steel subjected to the two-stage shot peening treatment.

  FIG. 8 shows the relationship between the arc height value in the second-stage shot peening process and the peak value of compressive residual stress for two types of steel parts (sample numbers 1 and 12) having different Si contents. is there. As is apparent from FIG. 8, when the arc height value is increased, the peak value of the compressive residual stress generally increases, but the low Si content (0.19%) is the second stage shot peening treatment. It can be seen that the peak value of the compressive residual stress does not increase even when the arc height value is increased from 0.18 mmN to 0.22 mmN. In contrast, when the Si content is 0.60%, the peak value of the compressive residual stress is further improved when the arc height value is increased from 0.18 mmN to 0.22 mmN.

It is a figure which shows the relationship between the surface deformation amount after the shot peening process of the conventional steel components, and tempering hardness. It is a figure which shows the relationship between the square of the correlation function R of the tempering hardness of the conventional steel components, and a deformation amount, and each tempering temperature. It is a figure which shows the relationship between Si density | concentration of the conventional steel components, and 200 degreeC tempering hardness. It is a figure which shows the influence which Si content and Mo content of the steel components in embodiment of this invention have on the forgeability and bending fatigue strength of steel components. It is a figure which shows the test piece for measuring the bending fatigue strength of the steel components in embodiment of this invention. (A) It is a figure which shows the heat history by the carburizing condition A of the steel components in embodiment of this invention. (B) It is a figure which shows the heat history by the carburizing condition B of the steel components in embodiment of this invention. (C) It is a figure which shows the heat history by the carburizing condition C of the steel components in embodiment of this invention. It is a figure which shows the peak value of the compressive residual stress in a steel component in embodiment of this invention, and the conventional steel component, and bending fatigue strength. The steel part in embodiment of this invention and the conventional steel part show the relationship between the arc height value in the second shot peening process, and the peak value of compressive residual stress.

Claims (5)

C: 0.15-0.25% (meaning “mass%”; the same shall apply hereinafter),
Si: 0.5 to 1.0%
Mn: 0.3 to 1%
Cr: 0.8 to 1.6%,
Mo: 0.45% or less (excluding 0%),
Al: 0.01 to 0.05%,
N: 0.008 to 0.03%,
Each of which is a steel part consisting of Fe and inevitable impurities,
The steel part has a chemically hardened layer, and the peak value of compressive residual stress in the range from the surface to a depth of 100 μm is 1600 MPa or more.   The steel part according to claim 1, further comprising Nb: 0.05% or less (not including 0%). C: 0.15-0.25%,
Si: 0.5 to 1.0%
Mn: 0.3 to 1%
Cr: 0.8 to 1.6%,
Mo: 0.45% or less (excluding 0%),
Al: 0.01 to 0.05%,
N: 0.008 to 0.03%,
Each of the steel materials containing Fe and unavoidable impurities is subjected to a chemical surface hardening treatment, and the steel material is subjected to a first shot peening treatment with an arc height value of 0.5 mmA or more. A method for producing a steel part, comprising performing a second shot peening treatment having a value of 0.18 mmN or more and 0.5 mmN or less. The method for manufacturing a steel part according to claim 3, wherein the second shot peening treatment is performed with an arc height value of 0.18 mmN or more and 0.3 mmN or less. The method for manufacturing a steel part according to claim 3 or 4 , wherein the steel material further contains Nb: 0.05% or less (not including 0%).

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