TW201730354A - Steel for carbonitriding and carbonitrided component - Google Patents

Abstract

Provided are a carbonitrided component, the surface fatigue life of which is improved by suppressing generation of pitting damage and suppressing generation of spalling damage as well by a carbonitriding treatment, a steel for carbonitriding to be used as a material for producing the carbonitrided component, and a method for producing the carbonitrided component. The steel for carbonitriding contains, in terms of % by mass, 0.15-0.3% of C, 0.5-1.5% of Si, 0.2-0.5% of Mn, more than 0% and 0.03% or less of P, more than 0% and 0.03% or less of S, 0.2-0.8% of Cr, 0.25-1% of Mo, 0.01-0.08% of Al, 0.01-0.1% of Ti, 0.0005-0.005% of B, and more than 0% and 0.01% or less of N, and the balance Fe with inevitable impurities.

Description

Steel for carburizing and nitriding and carburizing and nitriding parts

The present invention relates to a steel material for carburizing and nitriding, and a member for carburizing and nitriding using the steel material. The parts for carburizing and nitriding according to the present invention are suitably used for, for example, constant velocity joint parts such as gears and shafts, bearings, and bearings of continuously variable transmission (CVT) belts. Pass the part.

For power transmission parts, it is generally required to have a durable life for surface fatigue damage (hereinafter, referred to as surface fatigue life). The surface fatigue damage is a general term for damage (penetration damage) in which cracks occur on the sliding surfaces of the parts and peeling damage (peeling damage) which occurs in the surface layer of the parts.

In recent years, the high output of the power source and the miniaturization of the power transmission unit have been accompanied by an increase in the load load of each component. Moreover, due to the hybrid vehicle or electric vehicle of the automobile, the slip speed of the gears gradually increases. Moreover, in order to improve the transfer efficiency, the hydraulic oil is directed to a low viscosity. Based on this, the sliding environment becomes more demanding and expects Steel with excellent pitting life.

In order to prevent the occurrence of pitting damage, it is considered that the surface of the part is hardened, and as the surface hardening treatment, carburization treatment is known (for example, Patent Document 1). However, if the sliding environment in which the carburized parts are used becomes severe, the frictional heat in the slip is restored, and the surface of the parts is restored, and the surface of the parts is softened, so that pitting corrosion occurs.

Therefore, in order to improve the pitting life in a severe sliding environment, it is necessary to increase the softening resistance. As a method of increasing the softening resistance, a carburizing treatment is known. The carburizing and nitriding treatment is a process in which carbon and nitrogen are diffused on the surface of the part by heating while maintaining the temperature at a temperature of A ₃ or more, and then the surface of the part is hardened by rapid cooling. Carbonitride is formed on the surface layer of the part, whereby the carbonitride is used to increase the softening resistance. As a result, the pitting life is improved and the surface fatigue life is improved.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2010-53429

[Patent Document 2] Japanese Laid-Open Patent Publication No. 2015-127434

[Patent Document 3] Japanese Laid-Open Patent Publication No. 2006-097066

[Patent Document 4] Japanese Laid-Open Patent Publication No. 2006-307270

[Patent Document 5] Japanese Patent Laid-Open Publication No. 2005-163148

Although the carburizing and nitriding treatment is carried out to improve the pitting life of the part, the internal shear stress is increased due to the high load in the severe sliding environment, and the peeling damage occurs, resulting in surface fatigue. Reduced life.

The steel for surface hardening which is used for carburizing and nitriding treatment is disclosed in Patent Documents 2 to 5. However, the steel for surface hardening disclosed in Patent Documents 2 to 5 is not intended to improve the pitting life of the part by carburizing and nitriding treatment, and thus the peeling damage is not considered at all.

The present invention has been made in view of the above various circumstances, and an object thereof is to provide a carburizing and nitriding treatment to suppress occurrence of pitting damage and further suppress occurrence of peeling damage, thereby improving surface fatigue life. A carburized and nitrided part, a steel material for carburizing and nitriding which is a material for producing the carburized and nitrided part, and a method for producing the carburized and nitrided part.

The steel material for carburizing and nitriding according to the present invention which solves the above-described problems is mainly characterized by containing C: 0.15 to 0.3%, Si: 0.5 to 1.5%, Mn: 0.2 to 0.5%, and P: more than 0 in mass%. %, 0.03% or less, S: more than 0%, 0.03% or less, Cr: 0.2 to 0.8%, Mo: 0.25 to 1%, Al: 0.01 to 0.08%, Ti: 0.01 to 0.1%, B: 0.0005 to 0.005% And N: more than 0%, 0.01% or less, and the remainder is composed of iron and unavoidable impurities.

The steel material for carburizing and nitriding may be further contained in mass% as another element. (a) at least one selected from Nb: more than 0%, 0.1% or less, V: more than 0%, 0.5% or less, and Hf: more than 0% and 0.1% or less, and (b) from Cu: more than 0% 1% or less, and Ni: at least one selected from more than 0% and 2% or less, (c) Ca: more than 0%, 0.005% or less, Mg: more than 0%, 0.005% or less, and Zr: more than 0. %, 0.005% or less, Te: more than 0%, 0.10% or less, and REM: at least one selected from more than 0% and 0.02% or less, (d) from Pb: more than 0%, 0.10% or less, Bi: more than 0%, 0.10% or less, and Sb: at least one selected from more than 0% and 0.1% or less.

In the present invention, a carburized and nitrided part using the above-described steel material for carburizing and nitriding, which is focused on a carbonitride in a region having a depth of 25 to 50 μm from the surface of the part, is also included. The total area ratio is 0% or more and 5% or less.

The carburized and nitrided part can be produced by carburizing and nitriding a steel material for carburizing and nitriding which satisfies the above-described composition.

According to the present invention, since the composition of the composition is particularly focused on the amounts of Mn, Cr, and Al, it is possible to suppress the formation of carbonitrides on the surface layer of the part during the carburizing treatment. As a result, available Steel for carburizing and nitriding which is not inhibited by pitting corrosion and can be suppressed by the occurrence of peeling damage. The carburized and nitrided parts using this steel have excellent fatigue life.

1‧‧‧ test piece

2‧‧‧Load roller

3‧‧‧Sliding section

D‧‧‧ cut face

R‧‧‧Resin

Surface of the S‧‧‧ test piece

[Fig. 1] Fig. 1 is a schematic view showing the shape of a test piece.

[Fig. 2A] Fig. 2A is a schematic view for explaining the cutting direction of the test piece.

[Fig. 2B] Fig. 2B is a schematic view for explaining a procedure for observing the cut surface of the test piece.

[Fig. 3] Fig. 3 is a view in which the cut surface of the test piece is replaced by a photograph.

[Fig. 4] Fig. 4 is a graph showing the results of measuring the component composition of the precipitate at the position indicated by the arrow in Fig. 3 by energy dispersive X-ray spectroscopy.

[Fig. 5] Fig. 5 is a schematic view showing the state at the time of measuring the fatigue life of the surface.

The inventors have repeatedly tried to further improve the surface fatigue life of the carburized and nitrided parts in order to suppress the occurrence of peeling. As a result, it has been found that the carbonitride formed on the surface layer of the part during the carburizing and nitriding treatment is a cause of peeling damage, and the composition of the steel for carburizing and nitriding used as a material for producing the carburized and nitrided part is particularly For Mn, Cr When the amount of Al is appropriately adjusted, the formation of carbonitrides during the carburizing and nitriding treatment can be suppressed. Therefore, the occurrence of peeling inside the carburized and nitrided parts can be suppressed, and the surface fatigue life can be improved, thereby completing the present invention. Steel for carburizing and nitriding and carburizing and nitriding parts.

That is, C and N are diffused on the surface of the part by performing a carburizing treatment. Then, the parts after the carburizing and nitriding treatment are finely precipitated as Fe ₄ N due to the frictional heat during sliding, and the softening resistance is increased and the pitting life is improved. On the other hand, C and N diffused in the surface layer of the part are formed by bonding with alloying elements in the steel to form a hard carbonitride. In addition, when a high load is applied in a severe sliding environment, peeling occurs, and as a result of investigation, it is known that it is caused by carbonitride. The Young's modulus of the carbonitride is very high compared to the steel as the base material. Therefore, when the carbonitride-dispersed region is used as the precipitate layer, the precipitation rate of the precipitate layer is accompanied. The amount of carbonitride formed increases. Therefore, a difference in the Young's type rate occurs at the interface between the precipitate layer in which the carbonitride precipitates and the non-precipitate layer (that is, the base material) which is not precipitated by the carbonitride. Therefore, if a high load is applied in a severe sliding environment, shear stress due to the difference in the Young's type rate occurs, and as a result, internal cracks occur at the interface of the layer. It is known that this crack develops into a peeling damage.

From such a viewpoint, in order to improve the surface fatigue life under a severe sliding environment subjected to a high load, in the embodiment of the present invention, C and N which are diffused on the surface layer of the part during the carburizing treatment are suppressed. Become a carbonitride and use it as a solid solution C and a solid solution N. important. Further, it is known that the amount of carbonitride formed affects the amount of alloy elements, particularly Mn, Cr, and Al, and thus the composition of the steel is designed. Hereinafter, the chemical composition of the steel material for carburizing and nitriding according to the embodiment of the present invention will be described.

The steel material according to the embodiment of the present invention contains, as a basic component, C: 0.15 to 0.3%, Si: 0.5 to 1.5%, Mn: 0.2 to 0.5%, P: more than 0%, 0.03% or less, and S: more than 0%. , 0.03% or less, Cr: 0.2 to 0.8%, Mo: 0.25 to 1%, Al: 0.01 to 0.08%, Ti: 0.01 to 0.1%, B: 0.0005 to 0.005%, and N: more than 0%, 0.01% or less .

The C system is an element necessary for ensuring the hardness of the core portion of the carburized and nitrided part. If the amount of C is less than 0.15%, the core hardness cannot be ensured, and the surface fatigue life is lowered. Therefore, in the embodiment of the present invention, the C amount is set to 0.15% or more. The amount of C is preferably 0.17% or more, more preferably 0.18% or more. However, if C is excessively contained, the ferrite fraction increases before the shape of the part is processed, and the workability of the shape of the part deteriorates. Therefore, in the embodiment of the present invention, the C amount is set to 0.3% or less. The amount of C is preferably 0.27% or less, more preferably 0.25% or less.

The Si system increases the element of the softening resistance to sliding heat. If the amount of Si is less than 0.5%, the softening resistance to sliding heat is lowered, and the surface fatigue life cannot be improved. Therefore, in the embodiment of the present invention, the amount of Si is set to 0.5% or more. The amount of Si is preferably 0.6% or more, more preferably 0.65% or more. However, if Si is excessively contained, the workability of the shape of the part is deteriorated. Moreover, the activity of the carbon atom of the steel material is lowered to cause poor carburization. Therefore, in the embodiment of the present invention, the amount of Si is set to 1.5% or less. The amount of Si is preferably 1.3% or less, more preferably 1.2% or less.

Mn is bonded to S to form MnS, and suppresses the formation of FeS which deteriorates the workability of the shape of the part. In order to exhibit such an effect, the amount of Mn is set to 0.2% or more. The amount of Mn is preferably 0.3% or more, more preferably 0.35% or more. However, if Mn is excessively contained, carbonitride is formed during the carburizing and nitriding treatment, and the surface fatigue life is lowered. Therefore, in the embodiment of the present invention, it is important that the amount of Mn is 0.5% or less. The amount of Mn is preferably 0.47% or less, more preferably 0.45% or less.

Since the element which is inevitably contained in the P system is segregated in the grain boundary, the surface fatigue life is lowered, so it is necessary to reduce it as much as possible. From this point of view, the P system is set to 0.03% or less. The amount of P is preferably 0.025% or less, more preferably 0.020% or less. Although the amount of P is preferably as low as possible, the manufacturing cost is increased in order to increase the purity. From such a viewpoint, the amount of P is preferably 0.003% or more, more preferably 0.005% or more.

The element that is inevitably contained in the S system and the MnS system formed by bonding with Mn reduce the surface fatigue life. Therefore, in the embodiment of the present invention, the S amount is set to 0.03% or less. The amount of S is preferably 0.025% or less, more preferably 0.020% or less. However, a small number of S systems have the effect of improving the machinability. Moreover, in order to increase the purity, the manufacturing cost increases. From such a viewpoint, the amount of S is preferably 0.003% or more, more preferably 0.005% or more.

Cr is an element which forms a carbonitride during the carburizing and nitriding treatment and reduces the surface fatigue life. Thus, in an embodiment of the invention, Cr It is important that the amount is set to 0.8% or less. The amount of Cr is preferably 0.75% or less, more preferably 0.60% or less. However, when the amount of Cr is less than 0.2%, the hardenability is lowered and the surface fatigue life is lowered. Therefore, the amount of Cr is 0.2% or more. The amount of Cr is preferably 0.3% or more, more preferably 0.35% or more.

Mo is an element which suppresses the formation of a soft incompletely hardened structure during the carburizing treatment, improves the softening resistance, and improves the surface fatigue life. Therefore, in the embodiment of the present invention, the Mo amount is set to 0.25% or more. The amount of Mo is preferably 0.30% or more, more preferably 0.35% or more. However, when Mo is excessively contained, the workability of the shape of the part is deteriorated. Moreover, it becomes a high cost. From this point of view, the Mo amount is set to 1% or less. The amount of Mo is preferably 0.9% or less, more preferably 0.8% or less.

Al is an element that is inevitably contained, but functions as a deacidifying agent, and forms AlN to suppress an element which is coarsened in crystallizing during the carburizing treatment. In order to exhibit such an effect, it is important that the amount of Al is set to 0.01% or more. The amount of Al is preferably 0.015% or more, more preferably 0.020% or more. However, when Al is excessively contained, hot workability is deteriorated. Further, carbonitride is formed during the carburizing and nitriding treatment, and the surface fatigue life is lowered. Therefore, in the embodiment of the present invention, it is important that the amount of Al is 0.08% or less. The amount of Al is preferably 0.06% or less, more preferably 0.05% or less.

Ti is an element which forms TiN by bonding with N in steel to form a solid solution of B, thereby improving the hardenability of steel and increasing the strength. In this regard, in the embodiment of the present invention, the amount of Ti is set to 0.01% or more. The amount of Ti is preferably 0.02% or more, more preferably 0.03% or more. but, When Ti is contained in a large amount, the cost is high. Therefore, the amount of Ti is set to be 0.1% or less. The amount of Ti is preferably 0.09% or less, more preferably 0.08% or less.

B is an element which improves the hardenability and improves the strength, and increases the grain boundary strength to improve the surface fatigue life. In order to achieve such an effect, in the embodiment of the present invention, the amount of B is 0.0005% or more. The amount of B is preferably 0.0010% or more, more preferably 0.0012% or more. However, even if B is excessively contained, the effect is saturated, and BN is formed, which in turn deteriorates hot workability. Therefore, in the embodiment of the present invention, the amount of B is set to 0.005% or less. The amount of B is preferably 0.004% or less, more preferably 0.003% or less.

The N system is bonded to Al, Ti, and Nb in the steel to form fine carbonitrides, and an element that coarsens crystal grains during the carburizing treatment by the pinning effect is suppressed. In order to effectively exhibit such an effect, the amount of N is preferably 0.001% or more, more preferably 0.003% or more. However, when N is excessively contained, BN is formed to lower the hardenability of steel, and the surface fatigue life cannot be improved. Therefore, in the embodiment of the present invention, the amount of N is set to 0.01% or less. The amount of N is preferably 0.009% or less, more preferably 0.008% or less.

The basic composition of the above steel material is as described above, and the remainder is actually iron. However, it is of course allowed to include inevitable impurities mixed in raw materials, materials, manufacturing equipment, and the like in the steel.

In addition to the above elements, the steel material for carbonitriding according to the embodiment of the present invention may further contain, as a further element, (a) Nb: more than 0%, 0.1% or less, and V: more than 0%. , 0.5% or less, and Hf: at least one selected from the group consisting of more than 0% and 0.1% or less, and (b) at least one selected from the group consisting of Cu: more than 0%, 1% or less, and Ni: more than 0% and 2% or less. (c) Ca: more than 0%, 0.005% or less, Mg: more than 0%, 0.005% or less, Zr: more than 0%, 0.005% or less, Te: more than 0%, 0.10% or less, and REM: At least one selected from 0% and 0.02% or less, (d) selected from Pb: more than 0%, 0.10% or less, Bi: more than 0%, 0.10% or less, and Sb: more than 0% and 0.1% or less. At least one, etc.

(a) Nb, V, and Hf are elements which form a carbonitride bond with C and N in steel. The coarse carbonitride formed during the carburizing and nitriding treatment adversely affects the surface fatigue life, but the fine carbonitride formed before the carburizing and nitriding treatment has a pinching effect to prevent carburization. The effect of coarsening of crystal grains during nitriding treatment. Therefore, Nb, V, and Hf may be used alone or in combination of two or more.

In order to effectively exhibit such an effect, the amount of Nb is preferably 0.01% or more, more preferably 0.015% or more, still more preferably 0.020% or more. The amount of V is preferably 0.01% or more, more preferably 0.05% or more, still more preferably 0.10% or more. The amount of Hf is preferably 0.01% or more, more preferably 0.02% or more, still more preferably 0.03% or more.

However, even if Nb, V and Hf are excessively contained, the crystal grains are coarse. The prevention effect is also saturated, and the surface fatigue life is deteriorated. Moreover, it becomes a high cost. Therefore, in the embodiment of the present invention, the amount of Nb is preferably 0.1% or less, more preferably 0.09% or less, still more preferably 0.08% or less. The amount of V is preferably 0.5% or less, more preferably 0.45% or less, still more preferably 0.40% or less. The amount of Hf is preferably 0.1% or less, more preferably 0.09% or less, still more preferably 0.08% or less.

(b) Cu, and Ni are elements that improve hardenability and improve surface fatigue life. Cu, and Ni may be used alone or in combination.

In order to effectively exhibit such an effect, the amount of Cu is preferably 0.01% or more, more preferably 0.05% or more, still more preferably 0.10% or more. The amount of Ni is preferably 0.01% or more, more preferably 0.1% or more, still more preferably 0.5% or more. However, if Cu and Ni are excessively contained, it becomes a high cost. From such a viewpoint, the amount of Cu is preferably 1% or less, more preferably 0.7% or less, still more preferably 0.5% or less. The amount of Ni is preferably 2% or less, more preferably 1.9% or less, still more preferably 1.8% or less.

(c) Ca, Mg, Zr, Te, and REM (Rare Earth Metal; rare earth elements) are all elements that improve the machinability. In particular, Te and REM contribute to the improvement of the element to be cut by suppressing the elongation of MnS. Ca, Mg, Zr, Te, and REM may be used alone or in combination of two or more.

From such a viewpoint, the amount of Ca is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more. The amount of Mg is preferably 0.0001% or more, more preferably 0.0005% or more, still more preferably 0.0010% or more. The amount of Zr is preferably 0.0001% or more, more preferably 0.0005. More than %, more preferably 0.0010% or more. The amount of Te is preferably 0.001% or more, more preferably 0.01% or more, still more preferably 0.03% or more. The amount of REM is preferably 0.0001% or more, more preferably 0.001% or more, still more preferably 0.005% or more.

However, if it is contained in excess, the surface fatigue life will decrease. Therefore, in the embodiment of the present invention, the amount of Ca is preferably 0.005% or less, more preferably 0.004% or less, still more preferably 0.003% or less. The amount of Mg is preferably 0.005% or less, more preferably 0.0045% or less, still more preferably 0.0040% or less. The amount of Zr is preferably 0.005% or less, more preferably 0.004% or less, still more preferably 0.003% or less. The amount of Te is preferably 0.10% or less, more preferably 0.07% or less, still more preferably 0.05% or less. The amount of REM is preferably 0.02% or less, more preferably 0.019% or less, still more preferably 0.018% or less.

Further, in the embodiment of the present invention, the REM system includes a bismuth element (15 elements from La to Lu) and Sc (钪) and Y (钇).

(d) Pb, Bi, and Sb are all elements that improve the machinability. Pb, Bi, and Sb may be used alone or in combination of two or more.

From such a viewpoint, the amount of Pb is preferably 0.001% or more, more preferably 0.002% or more, still more preferably 0.003% or more. The amount of Bi is preferably 0.001% or more, more preferably 0.002% or more, still more preferably 0.003% or more. The amount of Sb is preferably 0.001% or more, more preferably 0.0015% or more, still more preferably 0.0020% or more. However, if it is contained in excess, the surface fatigue life will decrease. Therefore, in the embodiment of the present invention, the amount of Pb It is preferably 0.10% or less, more preferably 0.08% or less, still more preferably 0.06% or less. The amount of Bi is preferably 0.10% or less, more preferably 0.08% or less, still more preferably 0.05% or less. The amount of Sb is preferably 0.1% or less, more preferably 0.08% or less, still more preferably 0.05% or less.

The component composition of the steel material for carburizing and nitriding according to the embodiment of the present invention will be described.

In the embodiment of the present invention, a carburized and nitrided member using the above-described steel material for carburization is also included. In the carburized and nitrided part, the total area ratio of the carbonitrides present in the surface layer of the part is 0% or more and 5% or less. If a large amount of carbonitride is present in the surface layer of the part, when subjected to a high load in a severe environment and the internal shear stress becomes large, the carbonitride becomes a starting point of cracking, and the peeling damage is promoted. Reduced fatigue life. Therefore, in the embodiment of the present invention, the total area ratio of the carbonitrides present in the surface layer of the component is 5% or less. The total area ratio of the carbonitride is preferably 4% or less, more preferably 3% or less. The total area ratio of carbonitrides is preferably 0%.

The above-mentioned part surface layer means a region from a depth of 25 μm to a depth of 50 μm when the surface of the part is used as a reference.

The total area ratio of the carbonitrides is a region of 25 to 50 μm in depth from the surface of the component by using a scanning electron microscope, and the area of the carbonitrides present in the observation field is measured and totaled. The total area ratio of the carbonitrides to the observation field of view may be calculated. The number of fields of view is observed, for example, as long as it is 5 fields or more.

In the embodiment of the present invention, it will be observed under a microscope It was observed that the granular precipitate was judged to be carbonitride. When the precipitate of such a shape is analyzed by energy dispersive X-ray spectroscopy (EDX) or the like, at least one selected from the group consisting of Mn, Cr, and Al can be usually detected. . In other words, the carbonitride of the embodiment of the present invention usually contains at least one selected from the group consisting of Mn, Cr, and Al.

The carburized nitride component according to the embodiment of the present invention is a region from the surface of the part to a depth of 25 μm by an electron beam microanalyzer (EPMA) at a distance of 5 μm from the surface toward the depth. When the amount of C and the amount of N are measured and the average value is determined, the average amount of C is preferably 0.4 to 1%, and the average amount of N is 0.2 to 0.6%. By carburizing and nitriding, C atoms and N atoms are introduced into the surface of the part, whereby the hardness of the surface of the part is increased and the softening resistance is increased. As a result, the surface fatigue life is improved. When the average C amount is less than 0.4%, the hardness of the granulated iron is not sufficiently increased, and the improvement in the surface fatigue life is insufficient. Therefore, in the embodiment of the present invention, the average C amount of the surface of the component is preferably set to 0.4% or more. The average C amount is preferably 0.45% or more, more preferably 0.50% or more. However, if C is excessively contained, the amount of residual Worthite iron is increased, or coarse carbides are precipitated, and the surface fatigue life is lowered. Therefore, in the embodiment of the present invention, the average C amount is preferably 1% or less. The average C amount is preferably 0.9% or less, more preferably 0.8% or less.

When the average N amount is less than 0.2%, the amount of solid solution N decreases, and Fe ₄ N does not precipitate during sliding, and the improvement in the surface fatigue life is insufficient. Therefore, in the embodiment of the present invention, the average N amount of the surface of the component is preferably 0.2% or more. The average amount of N is more preferably 0.25% or more, still more preferably 0.30% or more. However, if N is excessively contained, the nitride of the alloy component is precipitated, and the improvement in the surface fatigue life is insufficient. Therefore, in the embodiment of the present invention, the average amount of N is preferably 0.6% or less. The average amount of N is preferably 0.55% or less, more preferably 0.50% or less.

The above C amount and N amount can be adjusted by controlling the conditions of the carburizing treatment.

Next, a method for producing a steel material for carbonitriding according to an embodiment of the present invention will be described.

The steel material for carburizing and nitriding according to the embodiment of the present invention can be produced by casting, rolling, and finally rolling a steel which is melted according to a usual method according to a usual method. Specifically, the cast piece obtained by casting may be heated at 1100 to 1300 ° C for 30 minutes to 5 hours, and then block rolling may be performed. The steel piece after the block rolling is cooled to a temperature equal to or lower than A ₁ point by setting the average cooling rate to 0.01 to 5 ° C / sec, and further, the heating is carried out at 800 to 1100 ° C for the final rolling, and further. The steel material of the embodiment of the present invention was obtained by cooling the average cooling rate to 0.01 to 5 ° C / sec to room temperature.

The shape of the steel material according to the embodiment of the present invention is, for example, a steel bar, and the diameter is, for example, 20 to 50 mm.

The intermediate product is processed into an intermediate product by one or more methods selected from the group consisting of cutting, cold forging, and hot forging according to a conventional method, and the intermediate product is subjected to carbonitriding treatment. A carbonitrided component of an embodiment of the present invention is produced.

It is also possible to perform annealing treatment, solution treatment, and normalization treatment according to a usual method before performing the above-described carburizing and nitriding treatment.

The conditions of the carburizing and nitriding treatment are not particularly limited, and well-known conditions can be applied. Specifically, as long as the carbon potential CP is 0.5 to 1.0% by mass, it may be carried out at 800 to 1000 ° C for 30 minutes to 6 hours in a propane gas atmosphere containing 2 to 15% of NH ₃ by volume fraction. After the carburizing and nitriding treatment, the hardening may be carried out according to a usual method, and further heated to 100 to 300 ° C for 30 minutes to 3 hours to perform tempering.

The carburizing and nitriding treatment described above may also be subjected to carburizing and nitriding treatment after carburizing treatment. For example, as the carburizing treatment, the carbon potential CP is set to 0.5 to 1.0% by mass, and after holding at 850 to 1000 ° C for 30 minutes to 3 hours, the carbon potential CP is set to 0.5 as a carbonitriding treatment. 1.0% by mass, maintained at 800 to 900 ° C for 30 minutes to 3 hours in a propane gas atmosphere containing 2 to 15% of NH ₃ by volume fraction. Further, the carburization treatment may be carried out in two or more steps. The environment in the case of heating to the temperature of the carburizing treatment may be a carbonitriding environment.

The carburizing and nitriding method is not particularly limited, and for example, a known method such as gas carburizing or vacuum carburizing or nitriding can be employed. The degree of vacuum at the time of vacuum carburizing and nitriding may be, for example, about 0.01 MPa or less.

After the carburizing and nitriding treatment, grinding, lubrication coating treatment, or beading treatment may be performed according to a usual method as needed.

Carburizing and nitriding parts obtained by carburizing carbonitriding treatment, It can be suitably used for power transmission parts such as gears, bearings, shafts, CVT belts, and the like.

[Examples]

In the following, the present invention is not limited by the following examples, but the present invention is not limited by the following examples, and it is of course possible to carry out the modifications within the scope of the above-mentioned and the following description. Any of them is included in the technical scope of the present invention.

The steel (the remaining part of the iron and the unavoidable impurities) having the composition shown in Tables 1 and 2 below was dissolved in a small dissolution furnace to produce an ingot. In Tables 1 and 2 below, "-" means that it is not detected.

After the obtained ingot is heated at 1100 to 1300 ° C for 30 to 120 minutes, hot forging is performed. The 32 mm bar steel was further subjected to a solution treatment, and was heated and held at 1250 ° C for 60 minutes. After cooling, it was further heated and maintained at 900 ° C for 60 minutes as a normalization treatment. The solution treatment simulates the block rolling of the real machine, and the normalization process simulates the final rolling of the real machine.

Will be normalized The 32 mm steel material was processed into a test piece of the shape shown in Fig. 1. In addition, the test piece has a cylindrical shape.

Next, the obtained test piece was subjected to a carbonitriding treatment in a gas carburizing furnace. Specifically, first, as the carburization treatment, the carbon potential CP was set to 0.9% by mass, and the carbon potential CP was held at 930 ° C for 90 minutes, and then the carbon potential CP was set to 0.75% by mass, and held at 930 ° C for 60 minutes. Then, after maintaining for 60 minutes, the temperature was lowered to 850 ° C, and a carbonitriding treatment was performed. The carbon potential CP was 0.75% by mass, and after maintaining for 2 hours in an RX gas atmosphere containing 12% of NH ₃ gas by volume fraction. Oil hardened immediately. After the oil was hardened, it was tempered by further heating to 170 ° C for 2 hours and then allowing to cool. In order to remove the heat treatment strain of hardening and tempering after carburizing and nitriding treatment, The handle of the 24mm is ground.

The test piece obtained by hardening and tempering after the carbonitriding treatment was measured for the total area ratio of the carbonitrides present in the surface layer of the part. The measurement procedure will be described using FIG. 2A and FIG. 2B.

First, the test piece shown in Fig. 1 above is The 26 mm portion is cut in a direction perpendicular to the axial direction as shown by a broken line in Fig. 2A. Then, as shown in FIG. 2B, the resin R is embedded so that the cut surface D can be observed, the cut surface D is polished, and then corroded with picric acid to perform Au vapor deposition. The arrow shown in Fig. 2B shows the direction of observation.

With respect to the surface S of the test piece of the cut surface D (that is, the circumferential surface of the test piece), a region having a depth of 25 μm to a depth of 50 μm was observed by a scanning electron microscope at an observation magnification of 4000 times, and the observation field was 200 μm × 150 μm, observed for any five fields of view. Image analysis was performed on the photographed photographed, and the total area ratio of the carbonitrides observed in each visual field was calculated, and the average value was calculated|required. The results are shown in Tables 1 and 2 below. Further, in an example of the present invention, the particulate precipitates confirmed in the observation field are judged to be carbonitrides.

For reference, the component composition of the granular precipitates confirmed in the observation field was measured by energy dispersive X-ray spectroscopy (EDX). In the case of No. 31 shown in Table 2 below, the photographed surface of the cut surface is replaced by a photograph, and is shown in Fig. 3.

The spectrum measured by EDX is shown in Fig. 4 for the granular precipitate shown by the arrow in Fig. 3 .

As is apparent from Fig. 4, the granular precipitates contain carbon nitrides of Cr. Further, in the EDX spectrum shown in Fig. 4, Au was also detected, but this Au was caused by deposition of Au on the cut surface for easy observation of precipitates.

Next, for the test piece obtained by hardening and tempering after the carbonitriding treatment, the amount of C and the amount of N from the surface of the part to the position (surface of the part) at a position of 25 μm in depth were measured.

The amount of C and the amount of N are measured by polishing the surface of the test piece (that is, the cut surface) after the precipitate is observed, and then using an electron beam micro analyzer (EPMA) on the surface of the test piece (that is, The circumferential surface of the test piece) was oriented in the depth direction (that is, the center direction of the axis) to a depth of 25 μm at intervals of 5 μm. The average value of the measurement results was calculated, and the average C amount and the average N amount were calculated. The results are shown in Tables 1 and 2 below.

Then, the test piece obtained by the hardening and tempering after the carburizing treatment was subjected to measurement of the surface fatigue life using a "RP-201 roll pitting tester" manufactured by KOMATSU Co., Ltd.

In the fifth figure, as the appearance of the test, the test piece 1 is displayed. It is in contact with the load roller 2 and is rotated while sliding. 3 of Fig. 5 shows a sliding portion.

For the load roller 2, a high carbon chromium steel SUJ2 specified in JIS G4805 was used, and a commercially available automatic oil was used for the test oil system. The measurement conditions were set to test surface pressure: 3.5 GPa, slip ratio: -40%, and number of revolutions: 1000 rpm. The test surface pressure is 3.5 GPa, which simulates a harsh sliding environment.

The number of rotations until the test machine was stopped due to the peeling damage was measured, and the number of rotations was taken as the surface fatigue life. In the case where the number of rotations reaches 20 million times, the test is terminated at this point in time. Each of the two steel grades was tested and the average value was determined. The results are shown in Tables 1 and 2 below. In Tables 1 and 2 below, α E+β means α × 10 ^β .

In one example of the present invention, the case where the number of rotations is 10 million or more is regarded as acceptable, and it is evaluated that the surface fatigue life is excellent.

According to the following Tables 1 and 2, it can be examined as follows.

No. 1 to 23 are examples of the requirements of the present invention, and the number of revolutions in the surface fatigue life evaluation test is 10 million or more, and it is known that the surface fatigue life is excellent.

No. 24 to 45, which is an example that does not satisfy any of the requirements of the present invention, fails to improve the surface fatigue life. The details are as follows.

No. 24, which is an example in which the amount of C is too small, and the surface fatigue life cannot be improved.

No. 25 is an example in which the amount of Si is too small, and the surface fatigue life cannot be improved.

Nos. 26 and 27 are examples in which the amount of Mn is too large, and since the carbonitride is excessively formed during the carburizing treatment, the surface fatigue life cannot be improved.

No. 28, which is an example in which the amount of P is too large, the surface fatigue life cannot be improved.

No. 29, which is an example in which the amount of S is too large, the surface fatigue life cannot be improved.

Nos. 30 and 31 are examples in which the amount of Cr is excessive, and since the carbonitride is excessively formed during the carburizing treatment, the surface fatigue life cannot be improved.

No. 32 is an example in which the amount of Mo is too small, and the surface fatigue life cannot be improved.

No. 33 and 34 are examples in which the amount of Al is too large, and since the carbonitride is excessively formed during the carburizing treatment, the surface fatigue life cannot be improved.

No. 35, which does not contain Ti, cannot improve the surface fatigue life.

No. 36, which does not contain B, does not improve the surface fatigue life.

No. 37, an example in which the amount of N is too large, the surface fatigue life cannot be improved.

No. 38 is an example in which the amount of Ca is excessive, and the surface fatigue life cannot be improved.

No.39, an example in which the amount of Mg is too large, and the surface fatigue cannot be improved. life.

No. 40, which is an example in which the amount of Zr is too large, the surface fatigue life cannot be improved.

No. 41, which is an example in which the amount of Te is too large, the surface fatigue life cannot be improved.

No. 42 is an example in which the amount of REM is too large, and the surface fatigue life cannot be improved.

No. 43 is an example in which the amount of Pb is too large, and the surface fatigue life cannot be improved.

No. 44 is an example in which the amount of Bi is too large, and the surface fatigue life cannot be improved.

No. 45, which is an example in which the amount of Sb is too large, the surface fatigue life cannot be improved.

The invention of the present specification includes the following aspects.

Pattern 1:

A steel material for carburizing and nitriding, characterized by containing C: 0.15 to 0.3%, Si: 0.5 to 1.5%, Mn: 0.2 to 0.5%, P: more than 0%, and 0.03% or less, and S: More than 0%, 0.03% or less, Cr: 0.2 to 0.8%, Mo: 0.25 to 1%, Al: 0.01 to 0.08%, Ti: 0.01 to 0.1%, B: 0.0005 to 0.005%, and N: more than 0%, 0.01% or less, and the remainder is composed of iron and unavoidable impurities.

Pattern 2:

The steel material for carburizing and nitriding according to the aspect 1 further contains, by mass%, Nb: more than 0%, 0.1% or less, V: more than 0%, 0.5% or less, and Hf: more than 0. At least one selected from the group consisting of % and 0.1% or less.

Figure 3:

The steel material for carbonitriding according to the aspect 1 or 2 further contains, as other elements, by mass% Cu: at least one selected from the group consisting of more than 0%, 1% or less, and Ni: more than 0% and 2% or less.

Figure 4:

The steel material for carburizing and nitriding according to any one of the above aspects, which further contains, by mass%, Ca: more than 0%, 0.005% or less, and Mg: more than 0%, 0.005% or less. Zr: more than 0%, 0.005% or less, Te: more than 0%, 0.10% or less, and at least one selected from the group consisting of REM: more than 0% and 0.02% or less.

Figure 5:

The steel material for carbonitriding according to any one of the above aspects, which further contains, by mass%, Pb: more than 0%, 0.10% or less, and Bi: more than 0%, 0.10% or less. And Sb: at least one selected from the group consisting of more than 0% and 0.1% or less.

Figure 6:

A carburized and nitrided part, which is a carburized and nitrided part of a steel material for carburizing and nitriding according to any one of the above aspects 1 to 5, characterized in that the surface has a depth of 25 to 50 μm from the surface of the part. The total area ratio of the carbonitrides in the case is 0% or more and 5% or less.

Aspect 7:

A method for producing a carburized and nitrided part, characterized in that the steel material for carburizing and nitriding according to any one of the aspects 1 to 5 is subjected to a carburizing and nitriding treatment.

The present application claims priority on the basis of Japanese Patent Application No. Hei. No. 2016-004567, filed on Jan. 13, 2016. Japanese Patent Application No. 2016-004567 is incorporated herein by reference.

Claims (7)

A steel material for carburizing and nitriding, characterized by containing C: 0.15 to 0.3%, Si: 0.5 to 1.5%, Mn: 0.2 to 0.5%, P: more than 0%, and 0.03% or less, and S: More than 0%, 0.03% or less, Cr: 0.2 to 0.8%, Mo: 0.25 to 1%, Al: 0.01 to 0.08%, Ti: 0.01 to 0.1%, B: 0.0005 to 0.005%, and N: more than 0%, 0.01% or less, and the remainder is composed of iron and unavoidable impurities. The steel material for carbonitriding according to claim 1 further contains, by mass%, Nb: more than 0%, 0.1% or less, V: more than 0%, 0.5% or less, and Hf: more than 0. At least one selected from the group consisting of % and 0.1% or less. The steel material for carbonitriding according to claim 1 further contains, as other elements, at least 1 selected from the group consisting of Cu: more than 0%, 1% or less, and Ni: more than 0% and 2% or less by mass%. Kind. The steel material for carburizing and nitriding according to claim 1 further contains, by mass%, Ca: more than 0%, 0.005% or less, Mg: more than 0%, 0.005% or less, and Zr: more than 0%. : 0.005% or less, Te: more than 0%, 0.10% or less, and at least one selected from the group consisting of REM: more than 0% and 0.02% or less. The steel material for carbonitriding according to claim 1 further contains, by mass%, Pb: more than 0%, 0.10% or less, Bi: more than 0%, 0.10% or less, and Sb: more than 0. At least one selected from the group consisting of % and 0.1% or less. A carburized and nitrided part, which is a carburized and nitrided part of a steel material for carburizing and nitriding according to any one of claims 1 to 5, characterized in that the surface has a depth of 25 to 50 μm from the surface of the part. The total area ratio of the carbonitrides in the case is 0% or more and 5% or less. A method for producing a carburized and nitrided part, characterized in that the steel material for carburizing and nitriding according to any one of claims 1 to 5 is subjected to a carburizing and nitriding treatment.