JP2021021129A - Carburized steel member made of machine structural steel with excellent pitching resistance on ground surface - Google Patents

Abstract

To improve pitching resistance life of a steel member such as power transmission parts of automobiles.SOLUTION: There is provided a carburized steel member of machine structural steel which contains C: 0.10 to 0.30%, Si: 0.51 to 0.80%, Mn: 0.10 to 0.40%, P: 0.005 to 0.025%, S: 0.005 to 0.025%, Ni: 0.05 to 0.20%, Cr: 1.30 to 2.00%, Al: 0.025 to 0.050%, N: 0.0100 to 0.0250% in terms of mass%, balance Fe and unavoidable impurities. The hardness, C concentration and residual austenite amount at a depth of 0.05 mm from the surface of the steel member subjected to grinding process are hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, residual austenite amount (hereinafter, also referred to as residual γ amount): 20 to 45% by volume. In a case where the steel member subjected to grinding process has average grain size number : No. 8 or more at the depth 0.4 mm depth from the surface and formula A represents 2.1×[Si%] + [Cr%], the carburized steel member made of machine structural steel with excellent pitching resistance on ground surface satisfies 2.8 or more as the value of the formula A.SELECTED DRAWING: None

Description

This application relates to mechanical structural steels for power transmission parts used in a ground state, particularly mechanical structural steels for power transmission parts having excellent pitching resistance on a ground surface.

In power transmission parts such as automobiles, the tooth surface of a gear, which is a power transmission part, may be used in a ground state from the viewpoint of quietness and improvement of fuel efficiency. The usage environment of such a power transmission component is harsh, in which "slip" occurs due to the difference in peripheral speed between the components having high surface pressure, and it is desired to improve the pitching resistance life in the harsh environment.

By the way, there is an invention relating to a carburized member having excellent seizure resistance, which defines the surface hardness, tempering hardness, thermal conductivity, carbide area ratio, etc. of the carburized layer in order to improve the pitching resistance life (for example, Patent Document 1). reference.). As described in the problem to be solved by the present invention described later, the present invention is limited to the softening of the steel material in the usage environment, and no measures have been taken for suppressing uneven wear.
In the present invention, the surface hardness of the carburized layer is preferably 750 HV or more.

Further, an invention in which grain boundary oxidation, the maximum depth of the incompletely hardened layer, and the area ratio of the incompletely hardened layer are controlled in order to improve the pitching resistance life is disclosed (see, for example, Patent Document 2). .. This invention was invented on the premise of use in a "carburized state" surface state without grinding, based on the idea of extending the pitching life by eliminating intergranular oxidation due to wear of the incompletely hardened layer. It is a thing. Since this is intended to be carburized, it cannot be applied when grinding the surface, and the incompletely hardened layer is not used for softening the steel material in the ground surface state. It was not considered because it was outside the scope of the idea.

Japanese Patent No. 6410613 JP 2016-222982

As described in the above background technology, in steel members such as power transmission parts of automobiles, improvement of pitching resistance life is aimed at. According to a study and research conducted by the inventor to improve the pitching resistance life, for example, in the case of gears, at the stage where the initial familiarity progresses from the start of sliding of the gears, uneven contact between the gears causes It was found that minute cracks were generated on the surface of the gear and uneven wear was promoted. If such non-uniform wear of the gear is promoted, the pitching resistance life is shortened. Therefore, in order to extend the pitching resistance life of these gears, it is necessary to (1) suppress non-uniform wear of the tooth surface of the gears.

Further, when the initial familiarization between the gears is completed, a boundary lubricating film is formed on the surface of the steel member which is the gear in the next stage, and when the number of slidings increases thereafter, the surface of the steel member softens. It will happen. When such softening occurs, the pitching resistance life of power transmission parts such as gears is shortened. Therefore, in order to prevent a decrease in the pitching resistance of the power transmission component such as a gear, it is necessary to (2) suppress the softening of the steel member such as the gear under the usage environment.
As described above, in order to improve the pitching resistance life of the mechanical structural steel used for power transmission parts and the like, it is necessary to improve both the suppression of non-uniform wear and the suppression of softening.

In the first means, the means for solving the above problems is C: 0.10 to 0.30%, Si: 0.51 to 0.80%, Mn: 0.10 to 0 in mass%. .40%, P: 0.005 to 0.025%, S: 0.005 to 0.025%, Ni: 0.05 to 0.20%, Cr: 1.30 to 2.00%, Al: A carburized steel member of machine structural steel containing 0.025 to 0.050%, N: 0.0100 to 0.0250%, and consisting of the balance Fe and unavoidable impurities, and ground steel. The hardness, C concentration and residual austenite amount at a depth of 0.05 mm from the surface of the member are hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, and residual austenite amount (hereinafter, also referred to as residual γ amount). ): 20 to 45% by volume, average crystal grain size number from the surface of the ground steel member to a depth of 0.4 mm: No. When the value is 8 or more and the formula A is: 2.1 × [Si%] + [Cr%] (note that [element%] are all numerical values indicated by mass%), the value of formula A is 2.8. It is a carburized steel member made of mechanical structural steel having excellent pitching resistance on a ground surface, which is characterized by the above.

In the second means, in addition to the chemical components of the first means, the mechanical structural steel containing Mo: 0.10 to 0.90% by mass% and consisting of the balance Fe and unavoidable impurities is carburized. The hardness, C concentration, and residual austenite amount at a depth of 0.05 mm from the surface of the ground steel member are: hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass. %, Residual γ content: 20 to 45% by volume, and average crystal grain size number from the surface of the ground steel member to a depth of 0.4 mm: No. When it is 8 or more and the formula B: 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%] (note that [element%] are all numerical values indicated by mass%), It is a carburized steel member made of mechanical structural steel having excellent pitching resistance on a ground surface, which is characterized in that the value of the formula B is 2.8 or more.

In the third means, in addition to the chemical components of the first means, V: 0.02 to 0.10%, Ti: 0.02 to 0.10%, Nb: 0.02 to 0 in% by mass. .A carburized steel member of mechanical structural steel containing any one or more of 10% and consisting of the balance Fe and unavoidable impurities, at a depth of 0.05 mm from the surface of the ground steel member. The hardness, C concentration and residual austenite amount are hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, residual γ content: 20 to 45% by volume, and the ground steel member has a hardness. Average crystal grain size number from the surface to a depth of 0.4 mm: No. When the value is 8 or more and the formula A is 2.1 × [Si%] + [Cr%] (note that [element%] are all numerical values indicated by mass%), the value of the formula A is 2.8. It is a carburized steel member made of mechanical structural steel having excellent pitching resistance on a ground surface, which is characterized by the above.

In the fourth means, in addition to the chemical components of the second means, V: 0.02 to 0.10%, Ti: 0.02 to 0.10%, Nb: 0.02 to 0 in% by mass. .A carburized steel member of mechanical structural steel containing any one or more of 10% and consisting of the balance Fe and unavoidable impurities, at a depth of 0.05 mm from the surface of the ground steel member. The hardness, C concentration and residual austenite amount are hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, residual γ content: 20 to 45% by volume, and the ground steel member has a hardness. Average crystal grain size number from the surface to a depth of 0.4 mm: No. When it is 8 or more and the formula B is 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%] (note that [element%] are all numerical values indicated by mass%). It is a carburized steel member made of mechanical structural steel having excellent pitching resistance on a ground surface, which is characterized in that the value of the formula B is 2.8 or more.

In the roller pitching test, the carburized steel member made of mechanical structural steel in the first to fourth means according to the present invention is carburized and surface-ground as a gear or the like which is a power transmission component of an automobile or the like. _{L 50} life ratio of pitting life, in the evaluation of n = 5, and 2.0 to 3.4 times the value relative to the SCM420 equivalent steel, has greatly improved, steel for mechanical structure of the present invention The carburized steel member made of the material has excellent pitching resistance on the ground surface.

It is a conceptual diagram of a roller pitching test. The large roller side is the mating material (2), and the small roller side is the roller pitching test piece (1).

First, (1) the reason for limiting the chemical composition of the mechanical structural steel in the means according to the present invention, and (2) the carburized steel member of the mechanical structural steel is 0 from the surface of the ground steel member. Reasons for limiting the numerical range of hardness, C concentration, and residual γ content at a depth of .05 mm, and (3) 0 from the surface of the ground steel member of the carburized steel member of the machine structural steel. The reason for defining the average grain size number up to a depth of .4 mm and the value of equation A or the value of equation B in (4) will be described below. Then, L ₅₀ life ratio in the roller pitting test, as well as the cracking probability when a steel member made of the machine structural steel cold working is described.

First, (1) the reason for limiting the chemical composition of the mechanical structural steel in the means according to the invention of the present application will be described. In addition,% in the description here is mass%.

C: 0.10 to 0.30%
C is an element that affects the hardenability, forgeability, and machinability of steel members. When C is less than 0.10%, the strength of the steel member becomes insufficient due to the decrease in the hardness of the core portion. On the other hand, when C is more than 0.30%, the hardness of the material of the steel member increases and the workability such as machinability and cold workability of the steel member decreases. Therefore, C is set to 0.10 to 0.30%.

Si: 0.51 to 0.80%
Si is an element necessary for deoxidation during steelmaking, and is an element effective in increasing temper softening resistance and improving pitching resistance. If the amount of Si is less than 0.51%, it is insufficient as a deoxidizing agent and the temper softening resistance is insufficient. On the other hand, if Si is more than 0.80%, the hardness of the material of the steel member increases, and the workability such as machinability and forgeability decreases. In addition, carburizing inhibition occurs, leading to deterioration of pitching resistance. Therefore, Si is set to 0.51 to 0.80%.

Mn: 0.10 to 0.40%
Mn is an element necessary for deoxidation during steelmaking and an element for improving hardenability. By the way, when Mn is less than 0.10%, it is insufficient as a deoxidizing agent and the hardenability is lowered. On the other hand, if Mn is more than 0.40%, the hardness of the material increases and the workability decreases. Therefore, Mn is set to 0.10 to 0.40%.

P: 0.005 to 0.025%
P is an unavoidable impurity in steelmaking. By the way, reducing P to less than 0.005% and reducing it excessively increases the cost. On the other hand, when P is contained in an amount of more than 0.025%, the fatigue strength decreases. Therefore, P is set to 0.005 to 0.025%.

S: 0.005 to 0.025%
S is an unavoidable impurity in steelmaking. By the way, reducing S to less than 0.005% and reducing it excessively increases the cost. On the other hand, when S is contained in an amount of more than 0.025%, Mn and MnS are formed, the cold workability is lowered, and the fatigue strength is lowered. Therefore, S is set to 0.005 to 0.025%.

Ni: 0.05 to 0.20%
Ni is an unavoidable impurity in steelmaking. By the way, reducing Ni to less than 0.05% and reducing it excessively increases the cost. On the other hand, Ni is an expensive element, and if it is contained in an amount of more than 0.20%, the cost will increase. Therefore, Ni is set to 0.05 to 0.20%.

Cr: 1.30 to 2.00%
Cr is an element necessary for ensuring the hardenability of the steel member, and is an element that enhances the temper softening resistance. By the way, if Cr is less than 1.30%, the hardenability is lowered, and the temper softening resistance is insufficient. On the other hand, if Cr is more than 2.00%, carburizing inhibition occurs and carbides are excessively generated, so that processability is lowered. Therefore, Cr is set to 1.30 to 2.00%.

Al: 0.025 to 0.050%
Al is an element that acts as a deoxidizer during steelmaking, and is an element that forms finer carbonitrides. By the way, if Al is less than 0.025%, deoxidation during steelmaking is not sufficient, and the formation of fine carbonitrides is not sufficient, resulting in coarse crystal grains and a decrease in pitching resistance. On the other hand, if Al is more than 0.050%, coarse carbonitride is formed and the workability is lowered. Therefore, it is assumed that 0.025 to 0.050% of Al is added.

N: 0.0100 to 0.0250%
N is an element that forms a fine carbonitride. By the way, if N is less than 0.0100%, the formation of fine carbonitrides is not sufficient, and the crystal grains become coarse, resulting in a decrease in the pitching resistance life. On the other hand, if N is more than 0.0250%, the nitride is excessively produced and the workability is lowered. Therefore, it is assumed that N is added in an amount of 0.0100 to 0.0250%.

Mo: 0.10 to 0.90%
Mo is an expensive element that greatly increases the material cost, but is an element that enhances hardenability, contributes to hardness improvement, and improves temper softening resistance. By the way, if Mo is less than 0.10%, improvement in hardenability and improvement in temper softening resistance cannot be obtained. On the other hand, if Mo is more than 0.90%, the cost increases, the material hardness of the steel member increases, and the workability decreases. Therefore, Mo is set to 0.10 to 0.90%.

V: 0.02 to 0.10%
V is an element effective for forming carbides during carburizing and refining crystal grains. By the way, if V is less than 0.02%, the effect of refining the crystal grains cannot be obtained. On the other hand, if V is contained in an amount of more than 0.10%, carbides are excessively generated, the processability is lowered, and the cost is increased. Therefore, V is set to 0.02 to 0.10%.

Nb: 0.02 to 0.10%
Nb is an element effective for forming carbides during carburizing and refining crystal grains. By the way, if Nb is less than 0.02%, the effect of refining the crystal grains cannot be obtained. On the other hand, if Nb is contained in an amount of more than 0.10%, carbides are excessively generated, the processability is lowered, and the cost is increased. Therefore, Nb is set to 0.02 to 0.10%.

Ti: 0.02 to 0.10%
Ti is an element effective for forming carbides during carburizing and refining crystal grains. By the way, if Ti is less than 0.02%, the effect of refining the crystal grains cannot be obtained. On the other hand, if Ti is contained in an amount of more than 0.10%, carbides are excessively generated, the processability is lowered, and the cost is increased. Therefore, Ti is set to 0.02 to 0.10%.

In addition, V, Nb, and Ti are selective addition elements, and when they are added, any one or more of them can be added.

Subsequently, the carburized steel member such as a power transmission component of an automobile or the like made of mechanical structural steel in the means according to the invention of the present application (2) has a depth of 0.05 mm from the surface of the ground steel member. The hardness, C concentration, and residual γ amount of the above will be described.

Hardness of 0.05 mm depth of steel member ground after carburizing: 680-750 HV
When 0.05mm depth hardness of grinding steel member after the carburization is less than 680HV, the result wear is accelerated carburizing steel member, L ₅₀ life ratio of pitting life of the roller pitching test It is lower than 2.0 times the value based on the SCM420 equivalent steel of the comparative steel 13. That is, if it is less than 680 HV, non-uniform wear is promoted and the load surface pressure cannot be withstood, and the pitching resistance life is shortened. On the other hand, when the hardness is larger than 750 HV, the tangential force (force generated in the tangential direction due to sliding) increases, and as a result, the pitching resistance life decreases. Therefore, the hardness at a depth of 0.05 mm from the surface of the steel member ground after carburizing is set to 680 to 750 HV.

C concentration at a depth of 0.05 mm for steel members ground after carburizing: 0.5 to 0.9%
If the C concentration at a depth of 0.05 mm of the steel member ground after carburizing is less than 0.5%, hard martensite cannot be obtained due to the insufficient amount of solid solution C, and the desired hardness can be obtained. As a result, the pitching resistance life is reduced. On the other hand, when the C concentration is more than 0.9%, the residual γ is excessively increased due to the excessive amount of C, and coarser carbides are generated, so that the pitching resistance life is lowered. Therefore, the C concentration at a depth of 0.05 mm of the steel member ground after carburizing is set to 0.5 to 0.9%.

Residual γ content at a depth of 0.05 mm of steel members ground after carburizing: 20 to 45% by volume
If the residual γ content at a depth of 0.05 mm of the steel member ground after carburizing is less than 20% by volume, the hardness becomes excessive and the tangential force generated in the tangential direction due to slippage increases, resulting in a long pitching resistance. descend. On the other hand, if the amount of residual γ is more than 45% by volume, the hardness is insufficient and wear is promoted, so that the pitching resistance life is shortened. Therefore, the residual γ content at a depth of 0.05 mm of the steel member ground after carburizing is set to 20 to 45% by volume.

Further, regarding the carburized steel member of the mechanical structural steel in the means according to the invention of the present application (3), the average particle size number from the surface of the ground steel member to a depth of 0.4 mm will be described.

Crystal grain size number from the surface of the steel member ground after carburizing to a depth of 0.4 mm: No. The crystal grain size numbers from the surface of the steel member ground after carburizing to a depth of 0.4 mm are No. If it is less than 8 and the crystal grains are large, the non-uniform deformation promotes non-uniform wear, resulting in a decrease in the pitching resistance life. Therefore, the crystal grain size numbers from the surface of the steel member ground after carburizing to a depth of 0.4 mm are No. 8 or more.

Further, the value of the formula A or the value of the formula B of the above (4) will be described.

Value of formula A = 2.1 × [Si%] + [Cr%]: 2.8 or more,
When the value of the formula A = 2.1 × [Si%] + [Cr%] ([element%] is a numerical value of the content of the element represented by mass%) is less than 2.8, the roller pitching Tempering of the steel member is likely to proceed during the test, softening is promoted, and the pitching resistance life is shortened.
Therefore, the value of the formula A = 2.1 × [Si%] + [Cr%] is set to 2.8 or more.

Value of formula B = 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%]: 2.8 or more,
The value of the formula B = 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%] ([element%] is a numerical value of the content of the element represented by mass%) is 2. If it is less than 8, easily proceeds tempered steel member in the roller pitting test, softening is facilitated, pitting life is reduced, L ₅₀ life ratio in the roller pitting test, as shown in Table 4 below , Less than 2.0.
Therefore, the value of the formula B = 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%] is set to 2.8 or more.

Here, the manufacturing process of steel for machine structure in the means according to the invention of the present application will be described.
Table 1 shows No. 1 in the invention steel. It shows the composition of the mechanical structural steel in which the total of 1 to 22 of the chemical components represented by mass% and the remaining Fe and unavoidable impurities is 100%. First, 100 kg of each invention steel having these component compositions was melted in a VIM (vacuum induction melting furnace) to produce an ingot. Next, each of these ingots was heated to 1250 ° C., and further, the invention steel No. It was forged to 1 to 22 φ60 mm steel bars.

Table 2 shows No. in comparative steel. It shows the composition of the mechanical structural steel in which the total of 1 to 23 of the chemical components represented by mass% and the remaining Fe and unavoidable impurities is 100%. An ingot was produced by melting 100 kg of each comparative steel having these component compositions in a VIM (vacuum induction melting furnace). Next, each of these ingots was heated to 1250 ° C., and further, the comparative steel No. It was forged into 1 to 23 φ60 mm steel bars.

Further, after normalizing each of the invention steel and the comparative steel having a diameter of 60 mm at 900 ° C. for 1 hour, φ14 mm from the material in the vicinity of D / 4 (D represents the diameter) of each of these steel bars. Each rod-shaped test piece having a length of 21 mm was prepared and used as a test piece for evaluation of cold workability.

Further, as other test pieces, each of the above-mentioned invention steel and comparative steel having a diameter of 60 mm was further forged to φ30 mm, tempered at 900 ° C. for 1 hour, and then roughly processed into each test piece. Each of the rough-processed test pieces was carburized at a carburizing temperature of 930 ° C. at a target Cp = 0.90%, and after quenching and tempering, each test piece was finished. ~ 7. Each test was carried out for each of the invention steel and the comparative steel.

Next 1. ~ 7. Shows the evaluation items of each test and the evaluation results of each test for each of the above-mentioned invention steels and comparative steels.
1. 1. Hardness measurement at a depth of 0.05 mm from the surface Using the test piece prepared above, cut at the T-plane, which is a cross section perpendicular to the length direction, and use a Micro Vickers hardness tester to measure the hardness at a depth of 0.05 mm from the surface. The hardness of the position was measured. The average value of n = 5 tests was defined as a hardness of 0.05 mm depth from the surface.
2. 2. C concentration measurement at a depth of 0.05 mm from the surface Using the test piece prepared above, cut at the T-plane, which is a cross section perpendicular to the length direction, and use an electron probe microanalyzer (EPMA) to a depth of 0.05 mm from the surface. The C concentration at the vertical position was measured. The average value of n = 3 tests was taken as the value of C concentration at a depth of 0.05 mm from the surface.
3. 3. Measurement of residual γ content at a depth of 0.05 mm from the surface Using the test piece prepared above, electrolytic polishing is performed to a depth of 0.05 mm from the surface, and residual at a position of 0.05 mm from the surface by XRD (X-ray diffraction). The amount of γ was measured. The average value of n = 3 tests was taken as the value of the residual γ amount at a position 0.05 mm from the surface.
4. Determining the grain size number from the surface to a depth of 0.4 mm Using the test piece prepared above, cut at the T-plane, which is a cross section perpendicular to the length direction, and observe by revealing the old γ grain boundaries with saturated picric acid. The crystal grain size number up to a depth of 0.4 mm was determined, and the average value of n = 5 determinations was taken as the value of the crystal grain size number.
5. Value of formula A or B Formula A is 2.1 × [Si%] + [Cr%] (note that [element%] are all numerical values indicated by mass%), and formula B is 2.1 × [Si%]. When + [Cr%] + 3.3 x [Mo%] ([element%] are all numerical values indicated by mass%), the value of the formula A or the value of the formula A depends on the chemical composition contained in the invention steel or the comparative steel. The value of the formula B was used.
6. Evaluation of pitting life (L ₅₀ life ratio in the roller pitting test)
The test piece prepared above was used as a roller pitching test piece for evaluation of pitching resistance, and the mating material: SCM420 steel carburized abrasive, slip ratio: -40%, surface pressure: 3.3 GPa, lubricating oil temperature: 80. A roller pitching test as shown in FIG. 1 was carried out at ° C. _{L 50} life of the specimen, SCM420 comparison of corresponding steel Steel No. When the value of L ₅₀ life of 13 is set to 1.0 of the reference value, it is evaluated by how many times the reference value is equivalent, and the average value of the evaluation values of n = 5 tests is L _50. The life ratio was used. In the present invention, if has excellent resistance to pitting life was assumed L ₅₀ life ratio is 2.0 times or more.
7. Probability of cracking during cold working A 70% cold installation was performed on a rod-shaped test piece with a diameter of 14 mm and a length of 21 mm, and the presence or absence of cracks in the length direction was investigated on the surface of the test piece. .. The number of cracks generated during cold working was determined, and the ratio of the number of cracks generated during cold working n = 5 times was defined as the crack occurrence probability. In the present invention, a crack occurrence probability of less than 80% is defined as a case where crack generation during cold working is excellent.

Invention Steel No. The above evaluation results for 1 to 22 are shown in Table 3. In addition, the comparative steel No. Table 4 shows the evaluation results for 1 to 21.

No. of invention steels shown in Table 1. The chemical components 1 to 22 are all within the range described in the claims of the present application, and the characteristics of the steel material are described in No. 1 of Table 3. As shown in 1 to 22, 0.5 mm hardness is 680 to 750 HV, 0.5 mm C concentration is 0.5 to 0.9%, 0.5 mm residual γ amount is 24 to 45 volume%, up to 0.4 mm. the value of the grain size number is 8-11, wherein Aor type B is 2.82 to 5.88, No. _{L 50} life ratio in the roller pitting test is the reference comparison steel The value of 13 is 2.0 to 3.4 times, and the probability of cracking during cold working is 0 to 60%. All of these characteristics in 1 to 22 are excellent. In the invention steels shown in Table 3, L ₅₀ life ratio in the roller pitting test are to be superior to more than 2 times. Further, in Table 3 and Table 4 below, "at a depth of 0.05 mm from the surface of the ground steel member" is abbreviated as "0.05 mm", and further, "the surface of the ground steel member" is described as "0.05 mm". "Average from to 0.4 mm depth" is abbreviated as "up to 0.4 mm", and "value of formula A or value of formula B" is abbreviated as "formula A or formula B". There is.

Comparative Steel No. 1 to 21 are the chemical components shown in Table 2, and the characteristics of each steel material are as shown in Table 4. In the comparative steels of Table 2, the chemical components shown by the underline are out of the range of the invention steels of Table 1. Then, in Table 4, the characteristics of the underlined parts are out of the range of the characteristics of the present invention.

Comparative steel No. In 1, are those the C content 0.08% and lower, the value of the formula A is as low as _{2.67, L 50} life ratio becomes as 1.8 times as low.
No. In No. 2, the C content was as high as 0.33%, and the probability of cracking during cold working was as high as 80%.
No. In 3 are those content of Si 0.42% of a low, low value of the expression A is _2.33, low and _{L 50} life ratio is 1.1.
No. In No. 4, the Si content is as high as 0.89%, the hardness at a depth of 0.05 mm is as low as 664 HV, the C concentration is as low as 0.42%, and the residual γ content is as low as 18% by volume. , Carburizing inhibition occurred.
No. In No. 5, the Mn content is as high as 0.41%, and the probability of cracking during cold working is as high as 100%.
No. In 6 are those of Cr content 1.15 percent lower, the value of the formula A is as low as _{2.20, L 50} life ratio is 1.3 times as low.
No. In No. 7, the Cr content is as high as 2.24%, the hardness at a depth of 0.05 mm is as low as 650 HV, and the C concentration is as low as 0.39 mass%. In addition, carburizing inhibition occurred.
No. In 8 are those Al content 0.020% or as low as the grain is large because the grain size number of up to 0.4mm is small and _{6, L 50} life ratio is 1.7 times as low.
No. In No. 9, the Al content is as high as 0.054%, and the probability of cracking during cold working is as high as 80%.
No. In No. 10, the N content is as low as 0.0097%, and the crystal grain size number up to 0.4 mm is as small as 6, so the crystal grains are large and the probability of cracking during cold working is as high as 80%.
No. In No. 11, the Si content is as low as 0.37%, the Cr content is as low as 1.02%, and the value of the formula A is as low as 1.80%.
No. In No. 12, the Mn content is as high as 0.78%, the Al content is as high as 0.094%, the N content is as low as 0.0080%, and the grain size numbers up to 0.4 mm. Since the value is as small as 7, the crystal grains are large, and the probability of cracking during cold working is as high as 100%.

Further, the comparative steel No. Reference numeral 13 denotes a steel equivalent to SCM420, which has a low Si content of 0.32%, a high Mn content of 0.84%, a low Cr content of 1.10%, and a value of Formula B. Is as low as 2.43. L ₅₀ life ratio is 1.0, less than 2.0 times of the invention steels.
No. In No. 14, the Mo content is as high as 1.01%, N is not contained, and the probability of cracking during cold working is as high as 80%.
No. In No. 15, the C content is as high as 0.35%, the Mo content is as high as 1.12%, and the probability of cracking during cold working is as high as 100%.

Furthermore, No. In 16, the value of the formula B is as low as 2.78, because the softened is likely to _{accelerate, L 50} life ratio was as low as 1.4 times.
No. In No. 17, the V content was as high as 0.15%, and the probability of cracking during cold working was as high as 80%.
No. In No. 18, the Ti content was as high as 0.20%, and the probability of cracking during cold working was as high as 100%.
No. In No. 19, the Nb content was as high as 0.14%, and the probability of cracking during cold working was as high as 80%.
No. In No. 20, Si is 1.07% and Mn is 0.82%, both of which have more chemical components than the specified amounts, the hardness is as low as 661 HV, and the C concentration is as low as 0.41 mass%. In addition, carburizing inhibition occurred.
No. In No. 21, Si is 0.24%, which is lower than the specified amount, Cr is 2.41%, which is more than the specified amount, and N is more than specified, and the hardness is as low as 638 HV. , C concentration is as low as 0.40% by mass, and residual γ amount is as low as 17% by mass. In addition, carburizing inhibition occurred.

1 Roller pitching test piece (small roller)
2 Opposite material (large roller)

Claims (4)

By mass%, C: 0.10 to 0.30%, Si: 0.51 to 0.80%, Mn: 0.10 to 0.40%, P: 0.005 to 0.025%, S: 0.005 to 0.025%, Ni: 0.05 to 0.20%, Cr: 1.30 to 2.00%, Al: 0.025 to 0.050%, N: 0.0100 to 0. A carburized steel member of mechanical structural steel containing 0250% and consisting of the balance Fe and unavoidable impurities. The hardness, C concentration and retained austenite at a depth of 0.05 mm from the surface of the ground steel member. The amounts were: hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, residual austenite amount (hereinafter, also referred to as residual γ amount): 20 to 45% by volume, and were ground. Average grain size number from the surface of the steel member to a depth of 0.4 mm: No. When the value is 8 or more and the formula A is: 2.1 × [Si%] + [Cr%] (note that [element%] are all numerical values indicated by mass%), the value of formula A is 2.8. A carburized steel member made of machine structural steel having excellent pitching resistance on a ground surface, which is characterized by satisfying the above. A carburized steel member of mechanical structural steel containing Mo: 0.10 to 0.90% in mass% in addition to the chemical component of claim 1 and composed of the balance Fe and unavoidable impurities. The hardness, C concentration and residual austenite amount at a depth of 0.05 mm from the surface of the ground steel member are hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, residual γ content: 20. Average crystal grain size number from the surface of the ground steel member to a depth of 0.4 mm, which is ~ 45% by volume: No. When it is 8 or more and the formula B: 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%] (note that [element%] are all numerical values indicated by mass%), A carburized steel member made of machine structural steel having excellent pitching resistance on a ground surface, which satisfies the value of the formula B of 2.8 or more. In addition to the chemical component of claim 1, in mass%, any one of V: 0.02 to 0.10%, Ti: 0.02 to 0.10%, and Nb: 0.02 to 0.10%. A carburized steel member of machine structural steel containing seeds or more and consisting of a balance Fe and unavoidable impurities, such as hardness, C concentration and retained austenite at a depth of 0.05 mm from the surface of the ground steel member. The amount is hardness: 680 to 750 HV, C concentration: 0.5 to 0.9% by mass, residual γ amount: 20 to 45% by volume, and up to a depth of 0.4 mm from the surface of the ground steel member. Average grain size number: No. When the value is 8 or more and the formula A is 2.1 × [Si%] + [Cr%] (note that [element%] are all numerical values indicated by mass%), the value of the formula A is 2.8. A carburized steel member made of machine structural steel having excellent pitching resistance on a ground surface, which is characterized by satisfying the above. In addition to the chemical component of claim 2, in mass%, any one of V: 0.02 to 0.10%, Ti: 0.02 to 0.10%, and Nb: 0.02 to 0.10%. A carburized steel member of machine structural steel containing seeds or more and consisting of a balance Fe and unavoidable impurities, such as hardness, C concentration and retained austenite at a depth of 0.05 mm from the surface of the ground steel member. The amount is hardness: 680 to 750 HV, C concentration: 0.5 to 0.9 mass%, residual γ amount: 20 to 45% by volume, and up to a depth of 0.4 mm from the surface of the ground steel member. Average grain size number: No. When it is 8 or more and the formula B is 2.1 × [Si%] + [Cr%] + 3.3 × [Mo%] (note that [element%] are all numerical values indicated by mass%). A carburized steel member made of machine structural steel having excellent pitching resistance on a ground surface, which satisfies the value of the formula B of 2.8 or more.

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