JP2018165403A - Steel for carburizing having excellent low cycle fatigue strength and machinability, and carburized component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for carburizing that has excellent machinability in cutting, and improves low cycle fatigue strength of an obtained component.SOLUTION: Steel for carburizing contains predetermined chemical components, satisfying the following (1) formula, with the balance being Fe and inevitable impurities. The number of MnS with an equivalent circle diameter of 1 μm or more is 80/mmor less, the number of MnS with an equivalent circle diameter of 5 μm or more is 1/mmor more, and the MnS with an equivalent circle diameter of 5 μm or more has an aspect ratio in average of 2 or more. [Mn]/[S]≥45 (1), where [Mn] and [S] denote the contents of Mn and S (mass%).SELECTED DRAWING: None

Description

  The present invention relates to carburized steel materials and carburized parts useful as materials for power transmission parts such as gears, shafts, bearings, and CVT pulleys.

  Power transmission parts such as gears, constant velocity joint parts, CVT pulleys and the like used in automobiles and the like may be damaged at a low cycle due to an impact load when the vehicle suddenly starts and stops. Further, in recent years, higher output and smaller units have been aimed at, and the above parts are further desired to have low cycle fatigue strength (hereinafter referred to as “low cycle fatigue strength”).

  Conventionally, high cycle fatigue has been dealt with by subjecting JIS steel (SCr, SCM) to carburizing, quenching and tempering to ensure hardness at the surface and toughness at the core. However, JIS steel is not necessarily a steel material excellent in low cycle fatigue, and sufficient strength has not been obtained for low cycle fatigue which has become severe in recent years.

  Further, forging and / or cutting is used for forming the part. In cutting, from the viewpoint of reducing manufacturing costs and improving productivity, it is desired to improve machinability during cutting, particularly to reduce tool wear. Conventionally, it is known that the addition of machinability improving elements such as Pb and S is effective for improving machinability.

  However, since Pb is an environmentally hazardous substance, regulations are becoming stricter in recent years, and the addition of Pb to steel materials is being restricted. In addition, S forms MnS and the like in steel to improve machinability, but a large amount of S disperses a large amount of MnS at the grain boundary, causing a decrease in low cycle fatigue strength. Cheap.

  Under such circumstances, various methods for improving fatigue characteristics such as low cycle fatigue strength have been proposed.

  For example, Patent Document 1 discloses a case-hardened steel in which nonmetallic inclusions are finely dispersed and have excellent fatigue characteristics. In Patent Document 1, by adding REM, TiN, Al—O-based inclusions, Al—Ca—O-based inclusions, and MnS, which easily become a fatigue accumulation source and become a starting point of fracture, are rendered harmless, and fatigue characteristics are obtained. To improve.

  Patent Document 2 discloses a steel part for high-temperature carburizing that exhibits excellent low cycle fatigue strength without reducing spalling strength. In Patent Document 2, the average C concentration from the surface to a depth of 0.05 mm is set to 0.50% or more, and the size and number density of MnS and TiN are in accordance with the Vickers hardness at each part of the component. Control to be satisfied. Thereby, spalling strength and low cycle fatigue strength are improved.

  Patent Document 3 discloses a carburized steel part that is excellent in low cycle bending fatigue strength. In Patent Document 3, low cycle bending fatigue strength is improved by controlling the hardness of the surface and the hardness of the core.

  Patent Document 4 discloses a case-hardened steel having excellent rolling fatigue characteristics. In Patent Document 4, Nb is added and fine Nb carbonitride is dispersed to reinforce the parent phase and improve rolling fatigue characteristics.

Special table 2014-061784 gazette JP2015-193929A JP 2010-285689 A JP 2014-189895 A

However, in Patent Document 1, the fatigue test is performed by an ultrasonic fatigue test, and the low cycle fatigue strength of the part may not be sufficient.
Further, as in Patent Document 2, if the size and number density of MnS and TiN are controlled so as to satisfy the predetermined formula in relation to the Vickers hardness at each part of the component, low cycle fatigue of the component is achieved. The strength improvement may be insufficient.
Moreover, in patent document 3, only the hardness of the surface and the hardness of a core part are controlled, and the improvement of the low cycle fatigue strength of components may be inadequate.
In Patent Document 4, rolling fatigue is studied, and low cycle fatigue of parts may not be sufficient.
In Patent Documents 1 to 4, no consideration is given to the machinability of the steel material.

  This invention is made | formed in view of such a condition, The objective provides the carburizing steel material which improves the low cycle fatigue strength of the obtained components while having the outstanding machinability at the time of cutting.

Aspect 1 of the present invention
C: 0.15-0.30 mass%,
Si: 0.15 mass% or less (including 0 mass%),
Mn: 1.30 to 2.00% by mass,
P: 0.020% by mass or less (including 0% by mass),
S: 0.005-0.030 mass%,
Cr: 0.90 to 1.30% by mass,
Mo: 0.10 mass% or less (including 0 mass%),
Al: 0.01 to 0.05% by mass,
Ti: 0.010% by mass or less (including 0% by mass), and N: 0.0090 to 0.0250% by mass,
Satisfying the following formula (1)
The balance consists of Fe and inevitable impurities,
The number of MnS having an equivalent circle diameter of 1 μm or more is 80 / mm ² or less, the number of MnS having an equivalent circle diameter of 5 μm or more is 1 / mm ² or more, and the equivalent circle diameter is 5 μm or more. It is a carburizing steel material having an average aspect ratio of 2 or more.
[Mn] / [S] ≧ 45 (1)
However, [Mn] and [S] indicate the contents (mass%) of Mn and S, respectively.

Aspect 2 of the present invention
Cu: 0.2 mass% or less,
Ni: 0.2 mass% or less, and B: 0.0005-0.005 mass%
It is the steel material for carburizing of aspect 1 which further contains at least 1 or more types.

Aspect 3 of the present invention
Nb: 0.1% by mass or less,
The steel material for carburizing according to aspect 1 or 2, further containing at least one of V: 0.1% by mass or less and Hf: 0.1% by mass or less.

Aspect 4 of the present invention
Ca: 0.005 mass% or less,
Mg: 0.005 mass% or less,
It is steel for carburizing in any one of aspects 1-3 which further contains at least 1 sort (s) among Zr: 0.005 mass% or less and Te: 0.1 mass% or less.

Aspect 5 of the present invention
Pb: 0.1% by mass or less,
It is steel for carburizing in any one of aspects 1-4 which further contains at least 1 sort (s) among Bi: 0.1 mass% or less, and Sb: 0.1 mass% or less.

Aspect 6 of the present invention
It is a carburized part which consists of a steel component in any one of aspects 1-5 whose core part hardness is 323-500HV, and the average amount of retained austenite from the surface to 0.3mm depth is 20-50 volume%. .

  ADVANTAGE OF THE INVENTION According to this invention, while having the machinability outstanding at the time of cutting, the low cycle fatigue strength of the obtained components can be improved.

The figure which shows the test piece of 4 point bending test Diagram showing the appearance of a 4-point bending test Diagram for explaining flank tool wear Diagram for explaining core hardness measurement

  As a result of intensive studies, the present inventors have determined that, among the machinability when cutting carburized steel, in particular to improve tool wear (cutting tool wear), the form and number density of MnS in the steel, And found that the chemical composition of the steel material should be controlled. Further, it has been found that in order to improve the low cycle fatigue strength of carburized parts, the amount of retained austenite in the surface portion, the form and number density of MnS in the steel, and the core hardness should be controlled. Thus, it has been found that the form and number density of MnS are factors that affect both the machinability of the carburizing steel and the low cycle fatigue strength of the resulting carburized parts. This will be specifically described below.

-Carburizing steel materials First, an outline of carburizing steel materials will be described. MnS serves as a stress concentration source during the cutting of the carburizing steel material, and contributes to the suppression of tool wear because it provides a lubricating action. In the present invention, it has been found that a coarse and large aspect ratio of MnS contributes to the suppression of tool wear, and it is necessary to ensure a predetermined amount (number). In addition, in order to form a predetermined amount of coarse MnS having a large aspect ratio, it is necessary to optimize the balance between the amount of Mn and the amount of S and optimize the forging pressure ratio and the rolling heating temperature, as will be described in detail later. I found.

  Moreover, when Ti was added, tool wear increased with TiN generation, and it was found that the amount of Ti needs to be reduced.

  Moreover, in order to suppress tool wear, it discovered that it was necessary to suppress a ferrite reinforcement | strengthening element (C, Mn, Cr, Si, Mo) as much as possible.

-Carburized parts Next, an outline of carburized parts will be described. It has been found that the low cycle fatigue strength is improved by introducing a large amount of retained austenite into the part surface (region from the surface to a depth of 0.3 mm). When the oil temperature at the time of carburizing and quenching is equal to or higher than the Mf point, austenite that has not been fully martensitic transformed exists as retained austenite. Since retained austenite is unstable, when stress is applied, it induces transformation into hard martensite and simultaneously expands in volume. Thereby, it is thought that the crack opening can be suppressed.
Moreover, the control factor of the amount of retained austenite was the amount of Mn and the amount of Cr, and it was found that the influence of the amount of Mn was particularly large.

  Further, in a low cycle load region, after cracks are generated, crack propagation is suppressed, so that the improvement in toughness at grain boundaries is an important factor for improving the strength. Since MnS tends to segregate at the grain boundaries, it has been found that when there are a large number of MnS larger than a predetermined size, the toughness of the grain boundaries decreases, the crack propagation rate increases, and the low cycle fatigue strength decreases. . In order to reduce the number of MnS, it is necessary to reduce the amount of MnS or to form coarse MnS. In order to form coarse MnS, as will be described in detail later, it has been found that it is necessary to optimize the balance between the amount of Mn and the amount of S, and to optimize the solidification rate, the forging pressure ratio, and the heating temperature before the block rolling. It was.

  Further, in a low cycle load region, the carburized component itself may be plastically deformed by a high load. If plastic deformation occurs, the product will not be able to transmit power efficiently, and at the same time, it will break early and the strength will decrease. On the other hand, the higher the core hardness of the carburized component is, the more difficult it is to deform plastically, but if it is too high, the toughness is reduced. When the toughness is lowered, the crack propagation rate is increased and the low cycle fatigue strength is lowered. Therefore, it has been found that there is an optimum value for improving the low cycle fatigue strength in the core hardness of the carburized part.

1. Steel structure Details of the steel structure of the carburizing steel and carburized parts of the present invention will be described below.
In the following description of the steel structure, a mechanism that can improve various properties by having such a structure may be described. It should be noted that these are the mechanisms considered by the present inventors based on the knowledge obtained at the present time, but do not limit the technical scope of the present invention.
In the following description, for example, “20 to 50%” means “20% or more and 50% or less”.

(The number of MnS with an equivalent circle diameter of 1 μm or more is 80 / mm ² or less)
When a large number of MnS with an equivalent circle diameter of 1 μm or more exists in the carburized steel, the toughness of the grain boundaries in the resulting carburized parts decreases, the crack propagation rate increases, and the low cycle fatigue strength decreases. To do. Therefore, the number of MnS having an equivalent circle diameter of 1 μm or more in the carburizing steel is 80 pieces / mm ² or less, preferably 65 pieces / mm ² or less, more preferably 50 pieces / mm ² or less. Although it does not specifically limit about a minimum, Usually, it is 10 piece / mm < ² >.
Note that MnS having an equivalent circle diameter of less than 1 μm in the carburized steel material has a small effect on low cycle fatigue strength in the obtained carburized parts. Therefore, in the present invention, attention is paid to MnS having an equivalent circle diameter of 1 μm or more. ing.

(The number of MnS having an equivalent circle diameter of 5 μm or more is 1 / mm ² or more, and the average aspect ratio of MnS having an equivalent circle diameter of 5 μm or more is 2 or more.)
MnS in the carburizing steel material becomes a stress concentration source at the time of cutting and contributes to the suppression of wear of the tool at the time of cutting because it exerts a lubricating action. The larger the equivalent circle diameter and the larger the aspect ratio, the easier it is to obtain this effect, and the larger the number, the more effective. For this reason, in MnS having an equivalent circle diameter of 5 μm or more in the carburizing steel material, the average aspect ratio must be 2 or more and the number must be 1 / mm ² or more. Preferably, the average aspect ratio is 2.5 or more and the number is 5 / mm ² or more. The upper limit is not particularly limited, but if the average equivalent circle diameter and average aspect ratio are too large, the low cycle fatigue strength of the obtained carburized parts may be lowered, so the average aspect ratio is 15 or less. Is 20 pieces / mm ² or less.
The average aspect ratio of MnS having an equivalent circle diameter of 5 μm or more is the sum of the aspect ratios of MnS having an equivalent circle diameter of 5 μm or more, and the total value is 5 μm or more of the equivalent circle diameter used for the addition. It means the value divided by the number of MnS.

(Core hardness: 323-500HV)
The core hardness of carburized parts can affect the plastic deformation and fatigue crack propagation under high load. If the core hardness of the carburized component is too low, plastic deformation occurs, so that low cycle fatigue strength can be reduced. On the other hand, if the core hardness of the carburized part is too high, the toughness of the core decreases and the propagation of fatigue cracks is promoted, and the low cycle fatigue strength can decrease, so the core hardness is preferably 323 to 500 HV. And The upper limit of the core hardness is more preferably 480 HV, and even more preferably 460 HV. Further, the lower limit of the core hardness is more preferably 360 HV, and even more preferably 370 HV.

(Average amount of retained austenite from the surface to a depth of 0.3 mm: 20 to 50% by volume)
The average amount of retained austenite from the surface of the carburized part to a depth of 0.3 mm is particularly important in improving the low cycle fatigue strength. Residual austenite on the surface of the product is induced and transformed into martensite when stress is applied, and locally expands. Therefore, the low cycle fatigue strength can be improved by suppressing the opening of the initial crack. However, if there is too much retained austenite, the surface is softened and an initial crack is generated at an early stage, so there is a possibility that strength cannot be expected. On the other hand, even if there is too little retained austenite, there is a possibility that a sufficient effect of suppressing initial cracks by induced transformation cannot be obtained. Therefore, the average amount of retained austenite from the surface to a depth of 0.3 mm is preferably 20 to 50% by volume. The upper limit of the average amount of retained austenite from the surface to a depth of 0.3 mm is more preferably 45% by volume. The lower limit of the amount of retained austenite from the surface to a depth of 0.3 mm is more preferably 25% by volume, and even more preferably 30% by volume.

2. Chemical Component Composition The chemical component composition of the carburizing steel and carburized parts according to the present invention will be described below. First, basic elements C, Si, Mn, S, Cr, Mo, Al, Ti, N, and P will be described, and elements that may be selectively added will be described.

(C: 0.15-0.30 mass%)
C is added to ensure the core hardness of the final product. However, if it is added excessively, the core hardness becomes too high and the toughness is lowered, while the low cycle fatigue strength is lowered. In addition, with the increase in hardness before carburizing, tool wear increases particularly in the machinability, so the C content is set to 0.30% by mass or less. On the other hand, if the amount is less than 0.15% by mass, the core hardness cannot be secured, so the C content is 0.15% by mass or more. The upper limit of the C amount is preferably 0.28% by mass, and more preferably 0.25% by mass. Moreover, the lower limit of the C amount is preferably 0.18% by mass, more preferably 0.20% by mass.

(Si: 0.15 mass% or less (including 0 mass%))
It is desirable to reduce Si as much as possible because it strengthens the solid solution in ferrite and increases tool wear, particularly in machinability. Therefore, the upper limit of the Si amount is set to 0.15% by mass. The upper limit of the Si amount is preferably 0.12% by mass, more preferably 0.10% by mass. However, since further reduction is accompanied by an increase in production cost, the lower limit of the Si amount is preferably 0.02% by mass, more preferably 0.04% by mass.
In the present invention, the amount of Si is made smaller than that of conventional JIS steel (SCr, SCM).

(Mn: 1.30 to 2.00% by mass)
Mn is an important element in the present invention. While enhancing the hardenability of the steel material to ensure the core hardness of the final product, it plays an important role in the formation of retained austenite after carburizing by lowering the Ms point of the steel material. Furthermore, in order to raise the crystallization temperature of MnS, it makes it easy to form coarse MnS. Therefore, the amount of Mn is 1.30% by mass or more. However, the excessive addition of Mn increases the hardness before carburizing, and in particular, the tool wear increases in the machinability, and the amount of retained austenite after carburizing becomes excessive and the low cycle fatigue strength decreases. Therefore, the amount of Mn shall be 2.00 mass% or less. The upper limit of the amount of Mn is preferably 1.90% by mass, more preferably 1.80% by mass. Moreover, the lower limit of the amount of Mn is preferably 1.40% by mass, more preferably 1.50% by mass.
In the present invention, the amount of Mn is larger than that of conventional JIS steel (SCr, SCM).

(S: 0.005-0.030 mass%)
S combines with Mn to form MnS inclusions, and contributes particularly to improvement of tool wear among machinability, but is an element that lowers low cycle fatigue strength when added in excess. In order to obtain coarse and large aspect ratio MnS that improves tool wear, the amount of S needs to be 0.005 mass% or more. On the other hand, when the amount of S increases, the number of MnS increases and the low cycle fatigue strength decreases, so the amount of S is set to 0.030% by mass or less. The upper limit of the amount of S is preferably 0.025% by mass, more preferably 0.022% by mass. Further, the lower limit of the amount of S is preferably 0.020% by mass, more preferably 0.013% by mass.
In the present invention, S is preferably an element added intentionally, but S mixed at an impurity level may be used.

(Cr: 0.90 to 1.30% by mass)
Cr is added because it is an effective element for improving the hardenability of the steel material and ensuring the core hardness of the final product and adjusting the amount of retained austenite. However, excessive addition increases the hardness before carburizing, and in particular, increases the tool wear among the machinability, so the upper limit of the Cr amount is 1.30% by mass. On the other hand, if the amount is too small, the core hardness and retained austenite cannot be obtained sufficiently, so the lower limit of the Cr content is 0.90 mass%. The upper limit of the Cr amount is preferably 1.15% by mass, more preferably 1.10% by mass. Further, the lower limit of the Cr amount is preferably 0.95% by mass, and more preferably 1.00% by mass.

(Mo: 0.10% by mass or less (including 0% by mass))
Mo tends to form a supercooled structure, and is strengthened by solid solution in ferrite to increase tool wear, and therefore it is desirable to reduce it as much as possible. Therefore, the upper limit of the Mo amount is set to 0.10% by mass. The upper limit of the amount of Mo is preferably 0.05% by mass, more preferably 0.03% by mass. On the other hand, since further reduction of Mo involves an increase in production cost, the amount of Mo is preferably 0.01% by mass or more, more preferably 0.02% by mass or more.
In the present invention, the Mo amount is set to a small amount as compared with the conventional JIS steel (SCM).

(Al: 0.01-0.05 mass%)
Al couple | bonds with N in steel materials, produces | generates AlN, and suppresses the grain coarsening (GG) at the time of carburizing. However, when Al is added excessively, Al ₂ O ₃ is generated and the workability is lowered. Therefore, the Al amount is set to 0.05% by mass or less. On the other hand, if the Al content is too small, AlN is not formed and crystal grain coarsening occurs during carburizing, so the Al content is 0.01% by mass or more. The upper limit of the amount of Al is preferably 0.04% by mass, more preferably 0.035% by mass. Moreover, the lower limit of the amount of Al is preferably 0.02% by mass, and more preferably 0.025% by mass.

(Ti: 0.010% by mass or less (including 0% by mass))
Ti combines with N in the steel material to form TiN and has the effect of increasing tool wear, so it is desirable to reduce it as much as possible. Therefore, the upper limit of the Ti amount is set to 0.010% by mass. The upper limit of the Ti amount is preferably 0.008% by mass, more preferably 0.005% by mass. However, since further reduction of the Ti amount is accompanied by an increase in production cost, the lower limit of the Ti amount is preferably 0.001% by mass, more preferably 0.002% by mass.

(N: 0.0090-0.0250 mass%)
N combines with Al and Nb in the steel material to form fine carbonitrides, and suppresses grain coarsening during carburizing. However, if N is contained excessively, cracks and / or flaws are likely to occur during production, so the N content is 0.0250% by mass or less. On the other hand, even if the amount of N is too small, AlN is not formed, GG is generated during carburizing, and the low cycle fatigue strength is lowered. Therefore, the amount of N is set to 0.0090% by mass or more. The upper limit of the N amount is preferably 0.0230% by mass, more preferably 0.0200% by mass. Moreover, the lower limit of the N amount is preferably 0.00110% by mass, more preferably 0.00120% by mass.

(P: 0.020 mass% or less (excluding 0 mass%))
P is segregated at the grain boundaries to lower the low cycle fatigue strength, so it is desirable to reduce it as much as possible, and the P content is 0.020 mass% or less. The amount of P is preferably 0.018% by mass or less, more preferably 0.015% by mass or less. On the other hand, P is an element inevitably contained in steel, and the production cost increases as the purity is increased. Therefore, the amount of P is preferably 0.001% by mass or more, more preferably 0.005% by mass. That's it.

([Mn] / [S] ≧ 45 (1)
However, [Mn] and [S] indicate the contents (mass%) of Mn and S, respectively)
As the ratio of [Mn] / [S] increases, MnS crystallizes from a high temperature, and coarse MnS is easily obtained. In carburizing steel, particularly tool wear is improved among machinability. Therefore, [Mn] / [S] ≧ 45 is set. [Mn] / [S] ≧ 60 is preferable, and [Mn] / [S] ≧ 80 is more preferable. The upper limit is not particularly limited, but is usually 400.

(Remainder)
In one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. In addition, for example, like P, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.

  However, it is not limited to this embodiment. Other elements may further be included as long as the characteristics of the carburizing steel and carburized parts of the present invention can be maintained. Other elements that can be selectively contained as described above are exemplified below.

(B: 0.0005-0.005 mass%, Cu: 0.2 mass% or less and Ni: at least one or more of 0.2 mass% or less)
B, Cu, and Ni may be added to improve the hardenability of the carburizing steel and improve the toughness of the carburized component. However, excessive addition increases the cost of the carburizing steel, and forms a supercooled structure during rolling or annealing, and increases tool wear, particularly among machinability. Therefore, when adding B, the amount of B shall be 0.0005 mass% or more and 0.005 mass% or less. The upper limit of the amount of B is preferably 0.0045% by mass, more preferably 0.0040% by mass. Moreover, the lower limit of the B amount is preferably 0.0010% by mass, more preferably 0.0015% by mass. Moreover, when adding Cu, the amount of Cu shall be 0.2 mass% or less. The upper limit of the amount of Cu is preferably 0.18% by mass, more preferably 0.15% by mass. The lower limit of the amount of Cu is not particularly limited, but is preferably 0.01% by mass, more preferably 0.02% by mass. Moreover, when adding Ni, the amount of Ni shall be 0.2 mass% or less. The upper limit of the amount of Ni is preferably 0.18% by mass, more preferably 0.15% by mass. Further, the lower limit of the amount of Ni is not particularly limited, but is preferably 0.01% by mass, more preferably 0.02% by mass.

(Nb: 0.1 mass% or less, V: 0.1 mass% or less, and Hf: at least one of 0.1 mass% or less)
Nb, V and Hf may be added in order to increase the effect of suppressing the coarsening of crystal grains. However, even if added excessively, the effect is saturated and the cost of carburizing steel increases, so when adding Nb, V and Hf, the Nb content, V content and Hf content are each 0.1% by mass or less. And The Nb amount, the V amount, and the Hf amount are each preferably 0.075% by mass or less, more preferably 0.05% by mass or less. The lower limit values of the Nb amount, the V amount, and the Hf amount are not particularly limited, but are each preferably 0.01% by mass, and more preferably 0.02% by mass.

(Ca: 0.005 mass% or less, Mg: 0.005 mass% or less, Zr: 0.005 mass% or less, and Te: at least one of 0.1 mass% or less)
Ca, Mg, Zr and Te may be added to reduce the aspect ratio of the oxide and improve the toughness of the carburized component. However, when Ca, Mg, Zr and Te are added excessively, coarse oxides intervene to lower the low cycle fatigue strength. Therefore, when adding any of Ca, Mg, and Zr, it is 0.005 mass% or less, preferably 0.004 mass% or less, more preferably 0.003 mass% or less. Moreover, the lower limit of the amount of Ca, Mg and Zr is not particularly limited, but is preferably 0.001% by mass, more preferably 0.002% by mass. When adding Te, it is 0.1 mass% or less, Preferably it is 0.09 mass% or less, More preferably, it is 0.08 mass% or less. Moreover, the lower limit of the amount of Te is not particularly limited, but is preferably 0.01% by mass, and more preferably 0.02% by mass.

(Pb: 0.1% by mass or less, Bi: 0.1% by mass or less, and Sb: 0.1% by mass or less)
Pb, Bi, and Sb are elements that improve tool wear, among the machinability, and may be added. However, when Pb, Bi and Sb are added excessively, the low cycle fatigue strength is lowered. Therefore, when at least one of Pb, Bi and Sb is added, the content is 0.1% by mass or less. The Pb amount, Bi amount, and Sb amount are each preferably 0.05% by mass or less, and more preferably 0.03% by mass or less. Further, the lower limit values of the Pb amount, Bi amount, and Sb amount are not particularly limited, but are each preferably 0.01% by mass, and more preferably 0.02% by mass.

3. Manufacturing method Next, the manufacturing method of the carburizing steel materials and carburized parts according to the present invention will be described.
The present inventors have made the solidification rate when solidifying molten steel slower than before, soaking heating temperature and holding time before mass rolling, rolling heating temperature during hot working, and predetermined slab from the slab It has been found that the carburizing steel material and carburized parts of the present invention containing coarse MnS can be obtained by controlling the forging pressure ratio when rolling or forging into a size as follows.
That is, the present invention includes coarse MnS by forming coarse MnS at the stage of a slab obtained by solidifying molten steel, and controlling the coarse MnS so as not to be refined even after the subsequent steps. It is found that carburizing steel materials and carburized parts can be obtained.
Below, the manufacturing method of the steel material for carburizing of this invention is demonstrated in detail first, and also the manufacturing method of carburized components is demonstrated in detail.

・ Production method of carburizing steel (solidification rate: 0.5 ℃ / sec or less)
First, a steel ingot having the above chemical composition is melted to produce a slab (continuous casting). MnS crystallizes in the molten steel and becomes coarser as the cooling rate when the molten steel is solidified is slower. Therefore, the solidification rate is set to 0.5 ° C./second or less. The solidification rate is preferably 0.45 ° C./second or less, more preferably 0.40 ° C./second or less. The lower limit of the coagulation rate is not particularly provided, but if the coagulation rate is too slow, the productivity is remarkably lowered, and is usually 0.005 ° C./sec.
In addition, the said solidification rate means the cooling rate from the solidification start temperature measured at the surface part of molten steel to 600 degreeC.

(Soaking heating temperature: 1260 ° C or less, holding time: 1 hour or less)
Subsequently, partial rolling is performed on the obtained slab. When the soaking (soaking) heating temperature before the block rolling is high and the holding time is long, a part of the crystallized MnS is dissolved and precipitated in the subsequent cooling step, so that the MnS is refined. Therefore, the upper limit of the heating temperature needs to be 1260 ° C. The heating temperature is preferably 1250 ° C. or lower, more preferably 1240 ° C. or lower. However, if the heating temperature is too low, the manufacturability decreases due to an increase in load during the block rolling, so the lower limit temperature is preferably 1100 ° C, more preferably 1150 ° C. Moreover, when holding time exceeds 1 hour, it will heat to the inside of a billet, MnS will dissolve, and coarse MnS (5 micrometers or more) will not be obtained. Therefore, the holding time is 1 hour or less. The holding time is preferably 30 minutes or less, more preferably 20 minutes or less.

(Rolling heating temperature: 900 ° C to 1100 ° C)
Subsequently, the steel pieces for carburization according to the present invention are obtained by reheating and hot-working the steel pieces after the block rolling (for example, hot rolling such as bar rolling). The higher the rolling heating temperature during reheating, the more MnS extends and the aspect ratio increases. Therefore, rolling heating temperature shall be 900 to 1100 degreeC. The upper limit of the rolling heating temperature is preferably 1050 ° C, more preferably 1030 ° C. The lower limit of the rolling heating temperature is preferably 950 ° C, more preferably 970 ° C.

(Forging ratio: 5 to 200)
As described above, when rolling or forging from a slab to a predetermined diameter, the larger the final diameter, that is, the smaller the forging pressure ratio, the harder the MnS becomes and the larger it becomes. However, if the forging ratio is too small, MnS during rolling does not sufficiently extend, and the required aspect ratio cannot be obtained. Therefore, the forging pressure ratio is set to 5 to 200. The upper limit of the forging pressure ratio is preferably 150, more preferably 40. Further, the lower limit of the forging pressure ratio is preferably 7, more preferably 20.
In the present invention, the forging pressure ratio means “cross-sectional area perpendicular to the casting direction of the slab / cross-sectional area perpendicular to the processing direction of the rolled material or forged material after rolling or forging”.

-Manufacturing method of carburized parts The carburized parts of the present invention are obtained by processing the carburized steel material by one or more methods selected from the group consisting of cutting and forging according to a conventional method to obtain an intermediate product. It can be produced by quenching and tempering or carbonitriding and quenching and tempering. In the present specification, carburizing quenching and tempering and carbonitriding quenching and tempering may be simply referred to as carburizing treatment.

  The steel material may be annealed according to a conventional method as necessary before being processed into an intermediate product. Further, the intermediate product may be subjected to a normalizing treatment according to a conventional method as needed before carburizing treatment.

  As the carburizing gas, for example, a mixed gas of metamorphic gas and propane gas may be used.

  Carburizing and quenching, for example, has an average C concentration of 0.60 to 0.90 mass% from the surface to a depth of 0.05 mm, and an effective hardened layer depth ECD (Effective Case. Depth) of 0.4 to 1.5 mm. As described above, the carbon potential CP in the atmosphere was adjusted to 0.60 to 0.9, held at 800 to 1000 ° C. for 1 to 3 hours, and then held at 800 to 900 ° C. for 0.3 to 2 hours. You can quench it.

  After carburizing and quenching, it may be kept at 150 to 200 ° C. for 0.5 to 3 hours and then allowed to cool and tempered.

  Further, after the carburizing treatment, polishing, lubricating coating treatment, shot peening treatment, or the like may be performed according to a conventional method as necessary.

  If it is those skilled in the art who contacted the manufacturing method of the carburizing steel materials and carburized parts which concern on embodiment of this invention demonstrated above, the steel materials for carburizing which concern on this invention by the manufacturing method different from the manufacturing method mentioned above by trial and error, and There is a possibility that carburized parts can be obtained.

1. Sample Production Steel ingots having the chemical composition shown in Table 1 were melted in a converter and a small melting furnace at a solidification rate of 0.08 ° C./second to 0.8 ° C./second shown in Table 2. The obtained slab was soaked in a heating furnace at 1200 ° C. to 1300 ° C. and a holding time of 0.5 to 1.5 hours, and then subjected to block rolling. Furthermore, hot rolling was performed at 850 ° C. to 1050 ° C., and hot rolling was performed to a predetermined diameter of φ25 to 90 mm to obtain a carburized steel material. Table 2 also shows the forging pressure ratio of each sample.
Sample No. Reference numeral 1 is a sample simulating a conventional JIS steel (SCM420H) as a base.

  After the carburizing steel was processed into a predetermined shape, carburizing and quenching was performed in a gas carburizing furnace (carburizing gas: RX gas + propane gas). The atmosphere so that the average C concentration from the surface to 0.05 mm depth is 0.6 mass% to 0.9 mass%, and the effective hardened layer depth ECD (Effective Case. Depth) is 0.4 mm to 1.5 mm. The inner carbon potential CP was kept in the range of 0.6 to 0.9 at 920 ° C. for 2 hours, and then quenched immediately after being held at 870 ° C. for 0.5 hour. Furthermore, after hold | maintaining at 160 degreeC for 2 hours, it stood to cool and tempered.

  In addition, what described the line | wire (-) in Table 1 means that the chemical component composition was not detected. In Tables 1 to 3, underlined numerical values indicate that they are out of the scope of the embodiment of the present invention. However, it should be noted that “-” is not underlined even if it falls outside the scope of the embodiment of the present invention.

2. Evaluation of Steel Structure / MnS A longitudinal section of each sample of the carburizing steel was investigated. From the 1/4 position of the diameter D of each sample, a substantially cubic sample of 10 mm (longitudinal direction (corresponding to the rolling direction)) × 10 mm (radial direction) × 10 mm (direction perpendicular to the radial direction) was cut out and the cross section was cut. Polished. Then, a cross section perpendicular to the radial direction and parallel to the rolling direction was observed. Compositions were analyzed by EPMA for inclusions having a minor axis of 1 μm or more present on the observation surface (one visual field) to identify MnS. Further, the aspect ratio and equivalent circle diameter of each MnS were measured by image analysis. The number density of MnS having an equivalent circle diameter of 1 μm or more (per 1 mm ² ) was calculated from (the number of detected MnS having an equivalent circle diameter of 1 μm or more) ÷ (measurement visual field area). The number density of MnS having an equivalent circle diameter of 5 μm or more (per 1 mm ² ) was calculated from (the number of detected MnS having an equivalent circle diameter of 5 μm or more) ÷ (measurement visual field area). Further, the average aspect ratio of MnS having an equivalent circle diameter of 5 μm or more was calculated. Table 3 shows the calculation results.
In addition, MnS made MnS made into object by this invention that the mixing ratio of Mn and S is 5: 3. Moreover, although MnS with an equivalent circle diameter of less than 1 μm was observed depending on the sample, the number was small.
The analysis target elements are Al, Si, Ti, Mn, S, O, Cr, Mg, Ca, Nb, N, and Fe, and using a known substance, the relationship between the X-ray intensity and the element concentration of each element is determined. A calibration curve was obtained in advance, and the amount of elements contained in each sample was quantified from the X-ray intensity obtained from the inclusions to be analyzed and the calibration curve.
The EPMA measurement conditions are as follows.
EPMA device: JXA 8500F (manufactured by JEOL)
EDS Analysis: Thermo Fisher Scientific system six
Acceleration voltage: 15 kV
Scanning current: 1.7 nA
Measurement visual field area: 10 mm ²

・ Measurement of the average amount of retained austenite from the surface to a depth of 0.3 mm The average amount of retained austenite at the center of the notch bottom of the test piece after carburizing treatment was measured using an X-ray residual stress measuring device (manufactured by Rigaku Corporation). It measured using the line diffraction method (this measurement result is set as the result 1). Subsequently, the amount of retained austenite in a region where the central part of the notch bottom of the test piece was corroded by 25 μm by electropolishing was measured with an X-ray residual stress measuring device (manufactured by Rigaku Corporation) (this measurement result is referred to as result 2). . The same measurement was performed by changing the corrosion amount to 50 μm, 100 μm, 200 μm, and 300 μm (respective measurement results are referred to as result 3, result 4, result 5, and result 6). And the average value of the results 1-6 was made into the retained austenite average amount to 0.3 mm depth from the surface. Table 3 shows the measurement results.

3. Mechanical properties and low cycle fatigue strength For test specimens after carburization, the life until the specimen breaks due to various stresses using a hydraulic servo tester (manufactured by Shimadzu Corporation) and a jig that supports 4-point bending. Was measured, and the strength at which breakage occurred in 100 cycles by single regression was calculated as low cycle fatigue strength (GPa). The test piece was manufactured by performing the carburizing and quenching process described above after processing into the shape shown in FIG. FIG. 2 shows the appearance of a four-point bending test. A single swing load is applied from the back while two horizontal parts on the notch side are supported. The results are shown in Table 3. Table 3 also shows the life ratio with the base (sample No. 1). A sample having a life ratio with respect to the base of 1.01 or less, in other words, a sample having a low cycle fatigue strength equal to or inferior to that of the base was rejected. The test conditions were as follows.
Frequency: 1Hz

-Machinability For each sample of the above carburized steel, a cutting test was carried out using super steel (P10) at a cutting speed of 200 m / min, a feed speed of 0.25 mm / rev, and a cutting depth of 1.0 mm. The amount of wear on the flank tool after parting (FIG. 3) was measured. Table 3 shows the measurement results. Table 3 also shows the reduction rate from the base (sample No. 1). And the sample whose reduction rate from a base is 0.0 or less, in other words, the sample whose machinability is inferior to a base was rejected.

-Core part hardness The above-mentioned test piece after carburizing treatment was cut so as to include the center of the notch as shown in FIG. The point that bisects the length of the line segment from the notch bottom to the bottom surface in the normal direction was defined as the core. Five Vickers hardnesses on a straight line passing through the core portion and parallel to the bottom surface were measured in accordance with JIS Z 2244 at a load of 300 gf, and the average was taken as a representative value of the core portion hardness. The carburized layer (the layer in which the amount of C has increased compared to that before carburizing) is 2 mm at most, and if measured by the above method, the core hardness of the non-carburized layer can be measured.
The results of the evaluation described above are shown in Table 3.

Consider the results in Table 3.
Sample No. Reference numerals 2 to 13, 23 to 24, 30, 32, 35 to 36, 40 and 41 are examples which satisfy all the requirements of the present invention. Sample No. whose machinability and low cycle fatigue strength are both base. It was better than 1.

  Sample No. Reference numeral 1 is a sample simulating a conventional JIS steel (SCM420H) as a base. The amount of Si, Mn and Mo was high, [Mn] / [S] was low, the number density of MnS with an equivalent circle diameter of 1 μm or more was high, and the number density of MnS with an equivalent circle diameter of 5 μm or more was low.

Sample No. In No. 14, the machinability decreased because the amount of Si deviated higher.
Sample No. In No. 15, the amount of Cr deviated higher, so the machinability decreased.
Sample No. No. 16 has a low Cr fatigue strength because the Cr content is low, the average retained austenite is small, and the core hardness is small.

Sample No. In No. 17, the amount of Mn deviated higher, the hardness before carburizing increased, and the average amount of retained austenite was high, so the machinability and low cycle fatigue strength decreased.
Sample No. In No. 18, since the amount of Mn was slightly lowered and the average amount of retained austenite was low, the low cycle fatigue strength was lowered.
Sample No. In No. 19, the amount of Mo deviated higher, and the machinability decreased.

Sample No. In No. 20, since the amount of C was increased and the core hardness was high, machinability and low cycle fatigue strength were lowered.
Sample No. No. 21 had a lower C content, a higher Cr content, and a lower core hardness, resulting in lower low cycle fatigue strength.
Sample No. In No. 22, the low cycle fatigue strength decreased because the N content was slightly lower.

Sample No. In No. 25, the amount of S deviated higher, and the number density of MnS with an equivalent circle diameter of 1 μm or more was high, so the low cycle fatigue strength was comparable to that of the base.
Sample No. In No. 26, the amount of S was slightly lowered and the number density of MnS having an equivalent circle diameter of 5 μm or more was low, so that machinability was lowered.
Sample No. In No. 27, the machinability decreased because the amount of Ti deviated higher.

Sample No. In No. 28, since the amount of P deviated higher, the low cycle fatigue strength decreased.
Sample No. No. 29 has low [Mn] / [S], a high number density of MnS with an equivalent circle diameter of 1 μm or more, and a low number density of MnS with an equivalent circle diameter of 5 μm or more. Therefore, machinability and low cycle fatigue strength Decreased.
Sample No. In No. 31, the solidification rate deviated so high that the number density of MnS having an equivalent circle diameter of 1 μm or more was high and the number density of MnS having an equivalent circle diameter of 5 μm or more was low, so that machinability and low cycle fatigue strength were lowered.

Sample No. In No. 33, since the heating temperature of the block rolling was high, the number density of MnS with an equivalent circle diameter of 1 μm or more was high, and the number density of MnS with an equivalent circle diameter of 5 μm or more was low, so machinability and low cycle fatigue strength were high. Declined.
Sample No. No. 34 has a long rolling holding time, a high number density of MnS with an equivalent circle diameter of 1 μm or more, and a low number density of MnS with an equivalent circle diameter of 5 μm or more. The fatigue strength was comparable to the base.
Sample No. In No. 37, the rolling heating temperature was lowered and the average aspect ratio of MnS having an equivalent circle diameter of 5 μm or more was lowered so that the machinability was lowered.

Sample No. In No. 38, the forging pressure ratio was too high, the number density of MnS having an equivalent circle diameter of 1 μm or more was high, and the number density of MnS having an equivalent circle diameter of 5 μm or more was low, so that machinability and low cycle fatigue strength were lowered.
Sample No. In No. 39, the forging pressure ratio was lowered, and the average aspect ratio of MnS having an equivalent circle diameter of 5 μm or more was lowered so that the machinability was lowered.

Claims (6)

C: 0.15-0.30 mass%,
Si: 0.15 mass% or less (including 0 mass%),
Mn: 1.30 to 2.00% by mass,
P: 0.020% by mass or less (including 0% by mass),
S: 0.005-0.030 mass%,
Cr: 0.90 to 1.30% by mass,
Mo: 0.10 mass% or less (including 0 mass%),
Al: 0.01 to 0.05% by mass,
Ti: 0.010% by mass or less (including 0% by mass), and N: 0.0090 to 0.0250% by mass,
Satisfying the following formula (1)
The balance consists of Fe and inevitable impurities,
The number of MnS having an equivalent circle diameter of 1 μm or more is 80 / mm ² or less, the number of MnS having an equivalent circle diameter of 5 μm or more is 1 / mm ² or more, and the equivalent circle diameter is 5 μm or more. A carburizing steel material having an average aspect ratio of 2 or more.
[Mn] / [S] ≧ 45 (1)
However, [Mn] and [S] indicate the contents (mass%) of Mn and S, respectively. Cu: 0.2 mass% or less,
Ni: 0.2 mass% or less, and B: 0.0005-0.005 mass%
The steel material for carburizing according to claim 1, further comprising at least one or more of them. Nb: 0.1% by mass or less,
The steel for carburizing according to claim 1 or 2, further comprising at least one of V: 0.1% by mass or less and Hf: 0.1% by mass or less. Ca: 0.005 mass% or less,
Mg: 0.005 mass% or less,
The steel for carburizing according to any one of claims 1 to 3, further comprising at least one of Zr: 0.005 mass% or less and Te: 0.1 mass% or less. Pb: 0.1% by mass or less,
The steel for carburizing according to any one of claims 1 to 4, further comprising at least one of Bi: 0.1 mass% or less and Sb: 0.1 mass% or less.   The carburized part which consists of a steel component in any one of Claims 1-5 whose core part hardness is 323-500HV and whose residual austenite average amount from the surface to 0.3 mm depth is 20-50 volume%.

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