JP6930296B2 - Hot-rolled steel sheets and spline bearings and their manufacturing methods - Google Patents

Description

The present invention relates to hot-rolled steel sheets and spline bearings, and methods for manufacturing them.

Abrasion resistance is one of the important properties required for steel materials used in industrial machinery or transportation equipment. In order to improve the wear resistance, it is effective to increase the hardness of the surface layer of the portion in contact with the mating material. In the fields of construction, civil engineering, mining, etc., which are used without major shape deformation, a large amount of alloying elements such as Cr and Mo are added to power shovels, bulldozers, and buckets to increase the hardness of not only the surface layer but also the entire surface. Steel material has been used.

On the other hand, in automobile transmission parts that have a very complicated shape and require wear resistance, the surface layer of the steel material is made harder by heat treatment such as carburizing and nitriding after being processed into the part shape by press molding or the like. There is a need. Therefore, steel materials that are soft and easy to process before press forming and whose surface layer is easily hardened by heat treatment such as carburizing and nitriding have been used.

For example, Patent Document 1 describes tensile strength (TS) in which a large amount of alloying elements such as Nb, V, Cu, Ni, Cr, Mo, and W are added as a hot-rolled steel sheet for abrasion resistance having excellent workability. However, the technique of high-strength steel sheet of 829 to 922 MPa is disclosed. Further, Patent Document 2 discloses a high-strength steel sheet having a tensile strength (TS) of 516 to 585 MPa to which Cr or the like is added as a hot-rolled steel sheet for wear resistance having excellent stretch flangeability.

Japanese Unexamined Patent Publication No. 2008-214736 Japanese Unexamined Patent Publication No. 9-49065

However, since the steel sheet described in Patent Document 1 has extremely high strength, it is difficult to process it into a complicated shape. In addition, there is a problem that the alloy addition cost is increased.

Further, in Patent Document 2, the amount of alloy added is reduced and the moldability is improved as compared with Patent Document 1. However, in order to obtain wear resistance, there is a problem that the heat treatment cost is high because it is necessary to increase the hardness of the steel material surface layer by heat treatment such as carburizing and nitriding after molding.

For internal gears (spline bearing tooth profiles) of spline bearings that require wear resistance in transmission parts, surface hardness increase due to costly heat treatment such as carburizing and nitriding is omitted, and only spline bearing tooth profile processing is required. It is desired that sufficient wear resistance can be obtained. The internal gear of the spline bearing may be formed by broaching.

As shown in FIG. 1, broaching is a processing method for producing a spline bearing 3 by cutting the inside of a hole of a spline bearing material 2 using a cutting tool called a broach 1 in which blades are arranged in a saw shape. It is a processing method that can finish quickly with high accuracy. That is, the steel material used for the internal gear of the spline bearing is required to have characteristics and broach formability that are good for forming by broaching (hereinafter, also referred to as broach forming). Patent Documents 1 and 2 make no mention of brooch moldability.

An object of the present invention is to solve the above problems and to provide a hot-rolled steel sheet having excellent brooch formability, a spline bearing having excellent wear resistance after brooch forming, and a method for manufacturing the same.

As a result of detailed studies on the above problems, the present inventors have omitted the surface hardness increase due to heat treatment such as carburizing and nitriding by controlling the additive elements, microstructure, etc. of the hot-rolled steel sheet which is the material. However, we have come to know a technique for exhibiting excellent wear resistance without increasing the cost while still processing the tooth profile of the spline bearing by broach forming.

The present invention has been made based on the above findings, and the gist of the present invention is the following hot-rolled steel sheet and spline bearing, and a method for manufacturing them.

(1) The chemical composition is mass%.
C: 0.025 to 0.180%,
Si: 0.05 to 1.70%,
Mn: 0.50-2.50%,
Al: 0.010-1.000%,
N: 0.0060% or less,
P: 0.050% or less,
S: 0.0050% or less,
Remaining: Fe and unavoidable impurities,
In the rolling direction cross section of the steel sheet, when the thickness of the steel sheet is t, the metallographic structure at the position of 1/4 t or 3/4 t from the surface of the steel sheet is in area%.
Martensite: 2.5-20.0%,
Bainite: less than 10.0%,
Residual austenite: less than 8.0%,
Pearlite: less than 2.5%,
Remaining: Ferrite,
The average circle equivalent diameter of ferrite is 10.0 to 100.0 μm,
The average circle-equivalent diameter of martensite is 3.0 to 18.0 μm.
The average carbon concentration of martensite is 0.80-1.20% by mass.
Hot-rolled steel sheet.

(2) The chemical composition is further increased by mass%.
Ti: 0 to 0.050%,
Nb: 0 to 0.100%,
V: 0 to 0.300%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Cr: 0-2.00%, and Mo: 0-1.00%,
Contains one or more selected from,
The hot-rolled steel sheet according to (1) above.

(3) The chemical composition is further increased by mass%.
B: 0 to 0.0100%,
Contains,
The hot-rolled steel sheet according to (1) or (2) above.

(4) The chemical composition is further increased by mass%.
Mg: 0-0.0100%,
Ca: 0 to 0.0100%, and REM: 0 to 0.1000%,
Contains one or more selected from,
The hot-rolled steel sheet according to any one of (1) to (3) above.

(5) The chemical composition is further increased by mass%.
Zr, Co, Zn, and W,
Contains 1.000% or less in total of one or more selected from
The hot-rolled steel sheet according to any one of (1) to (4) above.

(6) The method for manufacturing a hot-rolled steel sheet according to any one of (1) to (5) above.
A hot-rolled steel sheet in which a hot rolling step, a first cooling step, a winding step, and a second cooling step are sequentially performed on a steel piece having the chemical composition according to any one of (1) to (5) above. It is a manufacturing method,
The hot rolling step includes three or more steps of multi-step finish rolling.
The cumulative strain in the first to third-stage rolling in the multi-stage finish rolling is 0.003 to 0.300.
The rolling end temperature of the multi-stage finish rolling is a temperature of Ar _{3 to Ar 3} + 80 ° C. obtained by the following formula 5.
In the first cooling step, cooling is started 1.00 to 3.00 s after the multi-stage finish rolling is completed, and the temperature is 10 ° C./s or more from the rolling end temperature to the temperature range of 600 to 750 ° C. It is cooled at an average cooling rate, then held in the air for 3 to 10 s, and then further cooled to a temperature range of less than 350 ° C. at an average cooling rate of 10 ° C./s or higher.
In the winding step, the winding is performed at a winding temperature of less than 350 ° C.
In the second cooling step, cooling is performed from the winding temperature to 100 ° C. at an average cooling rate of less than 30 ° C./h.
Manufacturing method of hot-rolled steel sheet.
Ar ₃ = 970-325 x C + 33 x Si + 287 x P + 40 x Al-92 x (Mn + Mo + Cu) -46 x (Cr + Ni) ... (Equation 5)
However, the element symbol in the above formula represents the content (mass%) of each element in the hot-rolled steel sheet, and if it is not contained, 0 is substituted.

(7) A spline bearing molding that by the broaching is performed with respect to hot-rolled steel sheet according to any one of (1) to (5),
The arithmetic mean roughness (Ra) of the surface of the internal gear of the spline bearing is 2.5 μm or less, and the maximum height roughness (Rz) is 15.0 μm or less.
The micro Vickers hardness at a depth of 20 μm from the surface of the internal gear of the spline bearing is 300 Hv or more.
The average grain boundary solid solution carbon concentration at a depth of 40 μm from the surface of the internal gear of the spline bearing is 5 to 10% in atomic%.
Spline bearings.

(8) A method for manufacturing a spline bearing using the hot-rolled steel sheet according to any one of (1) to (5) above.
By the spline bearing and formed by broaching the hot-rolled steel sheet according to any one of (1) to (5),
The arithmetic mean roughness (Ra) on the surface of the internal gear of the spline bearing is 2.5 μm or less, and the maximum height roughness (Rz) is 15.0 μm or less.
The Micro Vickers hardness at a depth of 20 μm from the surface of the internal gear of the spline bearing is set to 300 Hv or more.
The average grain boundary solid solution carbon concentration at a depth of 40 μm from the surface of the internal gear of the spline bearing is 5 to 10% in atomic%.
How to manufacture spline bearings.

According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent brooch formability and a spline bearing having excellent wear resistance after brooch forming.

It is a figure for demonstrating the brooch molding. It is a figure for demonstrating the structure of a spline shaft and a spline bearing.

FIG. 2 is a diagram for explaining the structures of the spline shaft 4 and the spline bearing 3. As shown in FIG. 2, the transmission component of an automobile is provided with an external gear 4a on a spline shaft 4 (hereinafter, also referred to as “spline shaft”), and an internal tooth on a spline bearing 3 (hereinafter, also referred to as “boss”). Some are provided with a gear 3a. Then, these gears mesh with each other and come into contact with each other to transmit power.

The spline shaft and the external gear of the spline shaft are generally carburized, and a Vickers hardness of 650 Hv or more may be used.

In such a case, if the internal gear of the spline bearing (hereinafter, also referred to as "spline bearing tooth profile") is not subjected to surface hardness treatment such as carburizing, the external gear of the spline shaft will not be worn. , Only the spline bearing tooth profile wears, and abrasive wear may occur.

In order to improve the wear resistance, it is advisable to change from the state of abrasive wear to the state of adhesive wear at an early stage. In adhesive wear, when both the external gear of the spline shaft and the tooth profile of the spline bearing are repeatedly contacted, both are worn at the initial stage of the number of repeated contacts, but the amount of wear is small thereafter. Therefore, the amount of wear is extremely reduced in the steady state.

The present inventors improve wear resistance while processing spline bearing tooth profiles by broach forming without increasing the hardness of the steel surface layer by heat treatment such as carburizing and nitriding in the manufacture of transmission parts for automobiles. We enthusiastically examined how to make it. As a result, the following findings were obtained.

(A) Even if the spline shaft in contact with the spline bearing is a carburized material having a Vickers hardness of 650 Hv or more, in order to prevent the spline bearing tooth profile portion in contact with the external gear of the spline shaft from being abrasively worn. As described above, the external gear of the spline shaft and the tooth profile of the spline bearing are in a state of adhesive wear from the initial stage when the external gear of the spline shaft and the tooth profile of the spline bearing start to transmit power. Must be.

(B) By optimizing the surface roughness of the spline bearing tooth profile in contact with the spline shaft after broaching, the Vickers hardness of the surface layer, and the average grain boundary solid-dissolved carbon concentration of the surface layer, the spline shaft The external gear and the spline bearing tooth profile are in a state of adhesive wear, and the wear resistance of the spline bearing tooth profile can be improved.

(C) In order to optimize these factors, the steel composition and microstructure of the hot-rolled steel sheet, which is the raw material, especially the grain size of ferrite, which is the main phase, the grain size of martensite, and the area ratio of martensite, And it is necessary to limit the average carbon concentration of martensite.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: 0.025 to 0.180%
In order to improve the wear resistance of the spline bearing tooth profile, when a strong process such as broach molding is applied to the spline bearing tooth profile, the solid solution C is diffused from martensite to ferrite, which is the main phase. , It is necessary to strengthen the ferrite that has undergone strong processing.

Since C is an element effective for securing martensite, if the C content is too small, the area ratio of martensite as a source of solid solution C becomes low, and strongly processed ferrite is strengthened. Can't. On the other hand, if the C content is excessive, pearlite transformation occurs during cooling after finish rolling, the desired microstructure cannot be obtained, and the moldability deteriorates.

Therefore, the C content is set to 0.025 to 0.180%. The C content is preferably 0.040% or more, and more preferably 0.050% or more. The C content is preferably 0.160% or less, more preferably 0.140% or less.

Si: 0.05 to 1.70%
Si is an element that has a deoxidizing effect, suppresses the formation of harmful carbides and is effective in forming ferrite, and further suppresses pearlite transformation during cooling after rolling and martensitic transformation after cooling with austenite. It has the effect of promoting the separation of the two phases of ferrite. On the other hand, if the Si content is excessive, the ductility is lowered, the chemical conversion treatment property is also lowered, and the corrosion resistance after painting is deteriorated. Therefore, the Si content is set to 0.05 to 1.70%.

In order to more reliably secure the above effect, the Si content is preferably 0.07% or more, and more preferably 1.00% or more. The Si content is preferably 1.50% or less, more preferably 1.40% or less.

Mn: 0.50-2.50%
Mn is an element that strengthens ferrite, enhances hardenability, and contributes to the formation of martensite. On the other hand, if the Mn content is excessive, hardenability is increased more than necessary, ferrite cannot be sufficiently secured, and slab cracking occurs during casting. Therefore, the Mn content is set to 0.50 to 2.50%. The Mn content is preferably 1.00% or more, and more preferably 1.50% or more. The Mn content is preferably 2.00% or less, and more preferably 1.80% or less.

Al: 0.010-1.000%
Al has a deoxidizing effect and an effect of forming ferrite like Si. On the other hand, if the content is excessive, embrittlement is caused and the tundish nozzle is easily blocked during casting. Therefore, the Al content is set to 0.010 to 1.000%. The Al content is preferably 0.015% or more, and more preferably 0.030% or more. The Al content is preferably 0.800% or less, and more preferably 0.700% or less.

N: 0.0060% or less N is an element effective for precipitating AlN and the like to refine crystal grains. On the other hand, if the content is excessive, not only the solid solution nitrogen remains and the ductility is lowered, but also the aging deterioration becomes severe. Therefore, the N content is set to 0.0060% or less. The N content is preferably 0.0050% or less. Further, since excessively reducing the content leads to an increase in cost during refining, the lower limit thereof is preferably 0.0010%.

P: 0.050% or less P is an impurity contained in the hot metal, and it deteriorates local ductility due to grain boundary segregation and deteriorates weldability. Therefore, it is preferable to reduce it as much as possible. Therefore, the P content is limited to 0.050% or less. The P content is preferably 0.030% or less. It is not necessary to specify the lower limit, and the lower limit is 0%. However, since reducing the content excessively increases the cost during refining, the lower limit should be 0.001%.

S: 0.0050% or less S is also an impurity contained in the hot metal and forms MnS, which deteriorates local ductility and weldability. Therefore, it is preferable to reduce the amount as much as possible. Therefore, the S content is limited to 0.0050% or less. It is not necessary to specify the lower limit, and the lower limit is 0%. However, since reducing the content excessively increases the cost during refining, the lower limit should be 0.0005%.

Ti: 0.050% or less Ti is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, if the content is excessive, C is fixed as TiC, the amount of martensite decreases and the amount of solid solution C in martensite decreases, and the solid solution C from martensite is added to the strongly processed ferrite. Diffuses and loses the effect of suppressing abrasive wear. Therefore, the Ti content is set to 0.050% or less. On the other hand, in order to sufficiently obtain the effect of precipitation strengthening, the Ti content is preferably 0.010% or more, and more preferably 0.015% or more.

Nb: 0.100% or less Nb is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, if the content is excessive, C is fixed as NbC, and the amount of martensite decreases and the amount of solid solution C in martensite decreases, so that the solid solution C from martensite is added to the strongly processed ferrite. Diffuses and loses the effect of suppressing abrasive wear. Therefore, the Nb content is set to 0.100% or less. On the other hand, in order to obtain the above effect sufficiently, the Nb content is preferably 0.010% or more.

V: 0.300% or less V is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, if the content is excessive, C is fixed as VC, and the amount of martensite decreases and the amount of solid solution C in martensite decreases, so that the solid solution C from martensite is added to the strongly processed ferrite. Diffuses and loses the effect of suppressing abrasive wear. Therefore, the V content is set to 0.300% or less. On the other hand, in order to obtain the above effect sufficiently, the V content is preferably 0.010% or more.

Cu: 2.00% or less Cu is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the Cu content is set to 2.00% or less. Further, if the Cu content exceeds 1.20%, scratches due to scale may occur on the surface of the steel sheet. Therefore, the Cu content is preferably 1.20% or less. On the other hand, in order to obtain the above effect sufficiently, the Cu content is preferably 0.01% or more.

Ni: 2.00% or less Ni is an element that has the effect of improving the strength of the steel sheet by strengthening the solid solution. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the Ni content is set to 2.00% or less. Further, if the Ni content exceeds 0.60%, the ductility may deteriorate. Therefore, the Ni content is preferably 0.60% or less. On the other hand, in order to obtain the above effect sufficiently, the Ni content is preferably 0.01% or more.

Cr: 2.00% or less Cr is an element that has the effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the Cr content is set to 2.00% or less. On the other hand, in order to obtain the above effect sufficiently, the Cr content is preferably 0.01% or more.

Mo: 1.00% or less Mo is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, if the content is excessive, _{C is fixed as Mo 2} C, and the solid solution C is diffused into the strongly processed ferrite to lose the effect of suppressing abrasive wear. Therefore, the Mo content is set to 1.00% or less. On the other hand, in order to obtain the above effect sufficiently, the Mo content is preferably 0.01% or more.

B: 0.0100% or less B segregates at grain boundaries and improves low temperature toughness by increasing grain boundary strength. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the B content is set to 0.0100% or less. Further, B is a strong quenching element, and if its content exceeds 0.0020%, ferrite transformation does not proceed sufficiently during cooling, bainite is generated during cooling after rolling, and martensite is formed after cooling. Sufficient martensite may not be obtained due to insufficient promotion of separation of the two phases of austenite and ferrite that undergo site transformation. Therefore, the B content is preferably 0.0020% or less. On the other hand, in order to obtain the above effect sufficiently, the B content is preferably 0.0002% or more.

Mg: 0.0100% or less Mg is an element that improves the workability by controlling the morphology of non-metal inclusions that are the starting point of fracture and cause deterioration of workability. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the Mg content is set to 0.0100% or less. On the other hand, in order to obtain the above effect sufficiently, the Mg content is preferably 0.0005% or more.

Ca: 0.0100% or less Ca is an element that improves the workability by controlling the morphology of non-metal inclusions that are the starting point of fracture and cause deterioration of workability. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the Ca content is set to 0.0100% or less. On the other hand, in order to obtain the above effect sufficiently, the Ca content is preferably 0.0005% or more.

REM: 0.1000% or less REM (rare earth element) is an element that improves the workability by controlling the morphology of non-metal inclusions that become the starting point of fracture and cause deterioration of workability. Therefore, it may be contained if necessary. However, if the content is excessive, the effect is saturated and the economy is reduced. Therefore, the REM content is set to 0.1000% or less. On the other hand, in order to obtain the above effect sufficiently, the REM content is preferably 0.0005% or more.

Here, in the present invention, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements. Lanthanoids are industrially added in the form of misch metal.

Total of one or more selected from Zr, Co, Zn and W: 1.000% or less Even if Zr, Co, Zn and W are contained in the range of 1.000% or less in total, the effect of the present invention is obtained. Is confirmed to be intact.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and unavoidable impurities.

Here, the "unavoidable impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when the steel sheet is industrially manufactured, and is within a range that does not adversely affect the present invention. Means what is acceptable.

(B) Metallic structure of hot-rolled steel sheet The metal structure of the steel sheet of the present invention will be described. In the present invention, the metal structure is 1/4 W or 3/4 W from the end face of the steel sheet when the width and thickness of the steel sheet are W and t, respectively, in the rolling direction cross section of the steel sheet, and the steel sheet. It means the structure at the position of 1/4 t or 3/4 t from the surface of. Further, in the following description, "%" means "area%".

Martensite: 2.5-20.0%
As described above, in order to improve the wear resistance of the spline bearing, it is possible to suppress the abrasive wear of the spline bearing tooth profile that comes into contact with the spline shaft external gear and the spline shaft external gear and the spline bearing tooth profile. It is necessary to ensure that the external gear of the spline shaft and the tooth profile of the spline bearing are in a state of adhesive wear from the initial stage when the parts come into contact with each other and start transmitting power.

For that purpose, solid solution C is diffused into the ferrite of the tooth profile of the spline bearing, which has been deformed and work-hardened by strong processing such as brooch forming, to strengthen the solid solution, and the structure of the steel is hardened at the nano level. is required. As a result, the wear resistance of the spline bearing tooth profile is improved due to the combined effect of not only work hardening but also solid solution strengthening. The source for diffusing the solid solution C into ferrite is martensite.

In addition, hard martensite is compositely organized together with soft ferrite to improve press workability such as overhang molding. Therefore, the area ratio of martensite is set to 2.5% or more.

On the other hand, during broach forming, martensite has a smaller deformability than ferrite. As a result, the frequency of voids generated from the boundary between martensite and ferrite during broach forming increases, and the unevenness (arithmetic mean roughness: Ra, maximum height: Rz) formed on the formed surface increases, resulting in absorptive. Wear is likely to progress. Therefore, if the area ratio of martensite becomes excessive, the wear resistance deteriorates. Therefore, the area ratio of martensite is 20.0% or less.

Bainite: less than 10.0% Generally, bainite has a hardened structure and contains solid solution C in supersaturation and cementite, but compared to martensite, it diffuses solid solution C into processed and hardened ferrite. The capacity as a source of bainite is low. In addition, the area ratio of bainite and the area ratio of martensite are in a trade-off relationship. Therefore, the area ratio of bainite is set to less than 10.0% in order to sufficiently secure martensite having a high ability as a supply source for diffusing the solid solution C in ferrite. The area ratio of bainite is preferably 2.0% or less.

Residual austenite: less than 8.0% Residual austenite is martensitic transformed by work-induced transformation and is contained as necessary because it improves press workability such as overhang molding by transformation-induced plasticity (so-called TRIP phenomenon). You may. However, since the amount of martensite inevitably decreases as the amount of retained austenite increases, the area ratio of retained austenite should be less than 8.0% in order to secure sufficient martensite.

Furthermore, retained austenite undergoes martensitic transformation due to process-induced transformation during broach molding. Since martensite that has undergone work-induced transformation has a smaller deformability than ferrite, the frequency of voids generated from the boundary between martensite and ferrite increases during broach molding, and unevenness (arithmetic) is formed on the molded surface. Average roughness: Ra, maximum height: Rz) becomes large, and as a result, aggressive wear tends to proceed, and wear resistance deteriorates. Therefore, the area ratio of retained austenite is preferably 2.5% or less.

Pearlite: less than 2.5% Pearlite, like martensite, serves as a source for diffusing solid solution C into the ferrite of the spline bearing tooth profile, and has the effect of hardening the steel structure at the nano level. On the other hand, when pearlite is contained in an amount of 2.5% or more, the press workability including overhang molding is deteriorated. Therefore, the area ratio of pearlite is set to less than 2.5%.

Remaining part: Ferrite Ferrite has excellent ductility and easily deforms without cracking even in strong machining such as broach forming, so it is a microstructure necessary for machining spline bearing tooth profiles with high dimensional accuracy. In addition, a large amount of dislocations and atomic vacancies are introduced into the ferrite of the spline bearing tooth profile and its grain boundaries, which have been deformed and work-hardened by strong processing such as broach forming. Therefore, the solid solution C can be easily diffused into the ferrite of the spline bearing tooth profile. As a result, the wear resistance of the spline bearing tooth profile can be improved. Therefore, the structures other than martensite, bainite, retained austenite, and pearlite are ferrite.

Here, in the present invention, the area ratio of the metal structure is determined as follows. As described above, first, the sample is taken from the position of 1/4 W or 3/4 W from the end face of the steel sheet and from the position of 1/4 t or 3/4 t from the surface of the steel sheet. Then, the rolling direction cross section (so-called L direction cross section) of the sample is observed.

Specifically, the sample is night-game-etched, and after the etching, observation is performed with a field of view of 300 μm × 300 μm using an optical microscope. Then, by performing image analysis on the obtained tissue photograph, the area ratios of ferrite and pearlite, and the total area ratios of bainite, martensite, and retained austenite are obtained.

Next, the night-game-etched portion is subjected to repera-etching, and observation is performed with a field of view of 300 μm × 300 μm using an optical microscope. Then, the total area ratio of retained austenite and martensite is calculated by performing image analysis on the obtained tissue photograph. Further, using a sample surface-cut from the direction normal to the rolled surface to a depth of 1/4 of the plate thickness, the volume fraction of retained austenite is obtained by X-ray diffraction measurement, and the value is taken as the area fraction of retained austenite.

Then, by subtracting the area ratio of retained austenite obtained by X-ray diffraction measurement from the total area ratio of retained austenite and martensite obtained by repera etching and image analysis, the value is taken as the area ratio of martensite. do. Furthermore, by subtracting the total area ratio of retained austenite and martensite obtained by repera etching and image analysis from the total area ratio of bainite, martensite and retained austenite obtained by nighttal etching and image analysis. Find the area ratio of bainite.

By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite can be obtained.

Further, in the present invention, the average circle-equivalent diameter of ferrite, the average circle-equivalent diameter of martensite, and the average carbon concentration of martensite are also defined as follows.

Average circle equivalent diameter of ferrite: 10.0-100.0 μm
As described above, in order to improve the wear resistance of the spline bearing tooth profile, solid solution C is diffused from martensite into ferrite, which is the main phase, when strong processing such as broach molding is performed. , Spline bearing tooth profile needs to be hardened at the nano level by solid solution strengthening of C. The ferrite grain boundary is important as a path for diffusing the solid solution C from martensite to ferrite in a short time. For that purpose, it is effective to make ferrite into fine grains to increase the area of grain boundaries. Therefore, the average circle-equivalent diameter of ferrite is set to 100.0 μm or less.

On the other hand, when the ferrite becomes fine grains and the area of the grain boundaries becomes large to some extent, not only the effect is saturated, but also the solid solution C at the ferrite grain boundaries decreases and the grain boundary strength decreases. Further, since the unevenness (arithmetic mean roughness: Ra, maximum height: Rz) formed on the surface of the spline bearing tooth profile becomes large, aggressive wear tends to proceed, and the wear resistance deteriorates. Therefore, the average circle-equivalent diameter of ferrite is set to 10.0 μm or more.

The average circle-equivalent diameter (diameter) of ferrite is obtained by individually measuring the area of ferrite from the sample, calculating the circle-equivalent diameter (diameter), and averaging the circle-equivalent diameter (diameter).

Average circle equivalent diameter of martensite: 3.0-18.0 μm
As described above, when the spline bearing tooth profile is subjected to strong processing such as broaching, solid solution C is diffused from martensite into ferrite, which is the main phase, to strengthen the solid solution of C. It is necessary to harden the structure of steel at the nano level. As a result, the wear resistance of the spline bearing tooth profile is improved due to the combined effect of not only work hardening but also solid solution strengthening.

If the average circle equivalent diameter of martensite is less than 3.0 μm, the strain due to broach molding will be dispersed, and a sufficient amount of solid solution C cannot be diffused from martensite to ferrite, resulting in soft ferrite. It is insufficient to diffuse the solid solution C to obtain the effect of strengthening the solid solution of C. Therefore, the average circle-equivalent diameter of martensite should be 3.0 μm or more.

On the other hand, when the average circle-equivalent diameter of martensite becomes large, the unevenness (arithmetic mean roughness: Ra, maximum height: Rz) formed on the molded surface during brooch molding becomes large, so that abrasive wear easily progresses. , Abrasion resistance deteriorates. Therefore, the average circle-equivalent diameter of martensite is set to 18.0 μm or less.

It is known that martensite exists in an island shape at the ferrite grain boundary. Therefore, in the present invention, the average circle-equivalent diameter (diameter) of martensite is obtained by individually measuring the area of the martensite grains from the sample, calculating the circle-equivalent diameter (diameter), and averaging them.

Average carbon concentration of martensite: 0.80-1.20% by mass
As described above, martensite is a source for diffusing solid solution C into work-hardened ferrite. Therefore, the average carbon concentration of martensite is 0.80% or more in mass%. On the other hand, when the average carbon concentration of martensite increases, the hardness of martensite increases and the deformability decreases. As a result, the frequency of voids generated from the boundary between martensite and ferrite increases, and the unevenness (arithmetic mean roughness: Ra, maximum height: Rz) formed on the molded surface increases, resulting in abrasive wear. It becomes easy to proceed and the wear resistance deteriorates. Therefore, the average carbon concentration of martensite is 1.20% or less.

The average carbon concentration of martensite is a field emission type with an acceleration voltage of 200 kV, which is obtained by collecting a transmission electron microscope sample from the sample and adding a composition analysis function of electron energy loss spectroscopy (EELS). Measurement is performed by a transmission electron microscope equipped with an electron gun (Field Emission Gun: FEG). Twenty arbitrary martensites are selected, the measurement beam diameter of the above EELS is expanded to about 3 μm, and the average carbon concentration of martensite is measured.

Here, in the above analysis, the measurement target by the transmission electron microscope is the portion of martensite and retained austenite previously revealed by the repera etching. Strictly speaking, the measurement of the average carbon concentration of martensite in the present invention includes the case of measuring retained austenite other than martensite. However, in the present invention, since the area ratio of martensite is larger than that of retained austenite, the measurement result obtained by the above method is regarded as the average carbon concentration of martensite.

(C) Mechanical Properties of Hot-Rolled Steel Sheet The present invention effectively utilizes the press workability, which is a function of conventional general hot-rolled steel sheets, and the part that has undergone severe processing such as brooch forming. It is characterized by improved wear resistance.

The steel sheet according to the present invention has a tensile strength (TS) of 540 to 780 MPa, which is equivalent to that of a conventional general hot-rolled steel sheet.

Further, if the total elongation is small, the plate thickness tends to decrease due to necking during press molding, which causes press cracking. In order to ensure press moldability, it is desirable that the product of total elongation (t-EL) and tensile strength (TS) satisfies (TS) × (t-EL) ≧ 17,000 MPa%. However, the tensile strength (TS) indicates the tensile strength of JIS Z 2241 2011. The total elongation (t-EL) indicates the total elongation at break of JIS Z 2241 2011.

(D) Method for manufacturing hot-rolled steel sheet The manufacturing conditions for the steel sheet according to the present invention are not particularly limited, but can be manufactured by, for example, the following method. In the following manufacturing method, the following steps (a) to (e) are performed in order. Each process will be described in detail.

(A) Melting process The manufacturing method of the melting process prior to hot rolling is not particularly limited. That is, after melting in a blast furnace or an electric furnace, various secondary smelting is performed to adjust the composition so as to have the above-mentioned composition. Then, it may be cast by a method such as ordinary continuous casting or thin slab casting. At that time, scrap may be used as the raw material as long as it can be controlled within the component range of the present invention.

(B) Hot-rolling step The cast ingot (slab) is heated and hot-rolled to obtain a hot-rolled steel sheet. The heating conditions of the slab in the hot rolling step are not particularly limited, but for example, the heating temperature before hot rolling is preferably 1050 to 1260 ° C. In the case of continuous casting, it may be cooled to a low temperature once and then heated again and then hot-rolled, or it may be heated and hot-rolled following continuous casting without any particular cooling.

However, if the slab heating temperature is less than 1050 ° C., rolling resistance may increase in hot rolling, making rolling difficult. On the other hand, if the temperature exceeds 1260 ° C., not only the yield decreases due to scale-off, but also the austenite grain size during slab heating and the recrystallized austenite grain size during subsequent heat become coarse, and the average circle of ferrite after transformation becomes coarse. If the equivalent diameter becomes too large, the ferrite grain boundaries as a path for diffusing the solid solution C into the work-hardened ferrite in a short time may decrease, and the wear resistance may deteriorate.

After heating, the ingot extracted from the heating furnace is subjected to rough rolling and subsequent finish rolling. The conditions for rough rolling are not particularly limited, but if the rolling temperature becomes too low, the rolling load may increase and sufficient rolling may not be possible. Therefore, it is desirable that the rough rolling end temperature is 1000 ° C. or higher. As described above, the finish rolling is a multi-step finish rolling performed by continuous rolling of three or more steps (for example, 6 steps or 7 steps). Then, the final finish rolling is performed so that the cumulative strain (effective cumulative strain) in the rolling of the first to third stages is 0.003 to 0.300.

As described above, the effective cumulative strain is the change in crystal grain size due to the temperature during rolling and the rolling reduction of the steel sheet, and the change in crystal grain size in which the crystal grains statically recover over time after rolling. It is an index considered. The effective cumulative strain (εeff) can be calculated by the following equation.

Effective cumulative strain (εeff) = Σεi (ti, Ti) ・ ・ ・ (Equation 1)
Σ in the above equation 1 indicates the sum for i = 1-3.
However, i = 1 indicates the first-stage rolling in multi-stage finish rolling, i = 2 indicates the second-stage rolling, and i = 3 indicates the third-stage rolling.

Here, in each rolling represented by i, εi is represented by the following formula.
εi (ti, Ti) = ei / exp ((ti / τR) ^2/3 ) ・ ・ ・ (Equation 2)
ti: Time from the i-th stage rolling to the start of primary cooling after the final stage rolling (s)
Ti: Rolling temperature (K) of i-th stage rolling
ei: Logarithmic strain when rolling in the i-th stage ei = | ln {1- (inside plate thickness of the i-stage-outside plate thickness of the i-stage) / (inlet-side plate thickness of the i-stage)} |
= | Ln {(thickness of the exit side of the i-th stage) / (thickness of the entrance side of the i-th stage)} |
τR = τ0 ・ exp (Q / (R ・ Ti)) ・ ・ ・ (Equation 4)
τ0 = 8.46 × ^10-9 (s)
Q: Constant of activation energy related to dislocation movement of Fe = 183200 (J / mol)
R: Gas constant = 8.314 (J / (K · mol))

If the effective cumulative strain is less than 0.003, the driving force for recrystallization of austenite is small and the austenite is not sufficiently finely divided, so that the ferrite grains after transformation become coarse. On the other hand, if it exceeds 0.300, the austenite grains become excessively finely divided, and the transformed ferrite grains become excessively finely divided.

By setting the effective cumulative strain to 0.003 to 0.300, the diameter equivalent to the average circle of ferrite is limited, and the ferrite of the tooth profile of the spline bearing, which has been deformed and work-hardened by strong processing such as broach forming, can be used. A ferrite grain boundary can be secured as a path for diffusing the solid solution C from martensite in a short time. Then, when a strong process such as broach forming is performed, the frequency of voids generated at the ferrite grain boundaries can be suppressed, and a hot-rolled steel sheet having excellent broach formability and subsequent wear resistance can be obtained. ..

The rolling end temperature of the finish rolling, that is, the rolling end temperature of the continuous hot rolling process should be Ar ₃ (° C.) or higher and Ar ₃ (° C.) + 80 ° C. or lower. As a result, the desired microstructure can be obtained while optimizing the amount of martensite by controlling the ferrite transformation.

If the rolling end temperature is _{less than Ar 3} (° C), two-phase rolling occurs, and work-hardened ferrite remains, which deteriorates moldability and increases plastic anisotropy due to the formation of a strong texture. Anisotropy occurs in the component shape and plate thickness distribution. On the other hand, when the rolling end temperature exceeds Ar ₃ (° C) + 80 ° C, recovery / recrystallization and grain growth of austenite are promoted, ferrite transformation is delayed, and the desired microstructure cannot be obtained, and the amount of martensite is not obtained. Reduces and wear resistance deteriorates.

The value of Ar ₃ can be calculated by the following formula 5.
Ar ₃ = 970-325 x C + 33 x Si + 287 x P + 40 x Al-92 x (Mn + Mo + Cu) -46 x (Cr + Ni) ... (Equation 5)
However, the element symbol in the above formula represents the content (mass%) of each element in the hot-rolled steel sheet, and if it is not contained, 0 is substituted.

(C) First Cooling Step The hot-rolled steel sheet after the multi-stage finish rolling is started to be cooled after 1.00 to 3.00 s, and the temperature is in the range of 600 to 750 ° C. from the rolling end temperature. (Hereinafter referred to as “holding temperature”), the mixture is cooled at an average cooling rate of 10 ° C./s or more (hereinafter referred to as “pre-holding average cooling rate”), and then held in the air for 3 to 10 s. Further, the mixture is cooled to a temperature within the range of less than 350 ° C. (hereinafter, referred to as "cooling stop temperature") at an average cooling rate of 10 ° C./s or more (hereinafter, referred to as "average cooling rate after holding").

If the time from the end of finish rolling to the start of cooling is less than 1.00 s, the nucleation of ferrite becomes excessive, the crystal grain size of ferrite becomes fine, and the target diameter equivalent to the average ferrite circle cannot be obtained. .. On the other hand, if it exceeds 3.00 s, the grain growth of austenite progresses and the ferrite transformation does not proceed sufficiently in the first cooling step, and the desired microstructure cannot be obtained.

Further, when the average cooling rate before holding is less than 10 ° C./s, the grain growth of the transformed ferrite progresses and the crystal grain size of the ferrite becomes coarse, and the target diameter equivalent to the average ferrite circle cannot be obtained. On the other hand, the upper limit of the average cooling rate before holding is not particularly limited, but it is desirable to set the temperature to 100 ° C./s or less in order to avoid excessive formation of bainite and martensite due to supercooling.

Then, it is held in the air for 3 to 10 s. If this retention time is less than 3 s, the ferrite transformation does not proceed sufficiently, and the desired microstructure cannot be finally obtained. On the other hand, if it exceeds 10 s, pearlite transformation occurs, and the desired microstructure cannot be finally obtained. It should be noted that this intermediate retention is particularly significantly related to the surface integral of martensite. In addition, this retention may be allowed to cool in the atmosphere. The actual cooling rate of this cooling is about 2 to 7 ° C./sec, although it depends on the plate thickness.

Then, it is further cooled to a temperature range of less than 350 ° C. at an average cooling rate of 10 ° C./s or more. If the average cooling rate after holding is less than 10 ° C./s, pearlite transformation or bainite transformation occurs during cooling, and the desired microstructure cannot be finally obtained. On the other hand, the upper limit of the cooling rate is not particularly limited, but it is desirable to set the cooling rate to 100 ° C./s or less in order to avoid excessive formation of bainite and martensite due to supercooling.

(D) Winding step After the first cooling, the hot-rolled steel sheet is wound at a winding temperature of less than 350 ° C. Even if the temperature rises due to reheating or transformation latent heat after the first cooling and the winding temperature becomes higher than the first cooling end temperature, if the winding temperature is in the temperature range of less than 350 ° C. There is no problem.

(E) Second Cooling Step After winding, the product is cooled from the winding temperature to 100 ° C. at an average cooling rate of less than 30 ° C./h. If this cooling rate is 30 ° C./h or higher, the diffusion of C from ferrite to martensite becomes insufficient, and the average carbon concentration of martensite may not increase sufficiently. On the other hand, the lower limit of this cooling rate is not particularly limited, but if it is less than 10 ° C./h, not only a heat insulating facility is required, but also the coil can be safely recalled and rewound. , It is not preferable in terms of delivery date management because it is necessary to wait for a long time until the temperature of the coil is cooled to 100 ° C. or lower. Therefore, it is desirable to set the temperature to 10 ° C./h or higher.

The cooling rate after winding is measured on the center side of 8 or more turns from the inner diameter of the coil after winding, and 8 or more turns on the center side of the outer circumference of the coil after winding. It is a position 20 mm or more in the width direction of the coil from the end (edge) of the rear coil to the center side in the width direction. The average cooling rate will be obtained by attaching a thermocouple to the site and measuring the relationship between time and temperature.

(E) Spline bearing The hot-rolled steel sheet obtained as described above has excellent wear resistance without heat treatment and with the spline bearing tooth profile formed by broach forming, which has not been possible in the past. It is possible to obtain a spline bearing having excellent wear resistance at low cost.

Since the internal gear (spline bearing tooth profile) of the spline bearing after broach molding is subjected to extremely strong processing, it becomes difficult to distinguish the metal structure. Therefore, it is impossible to specify the metal structure of the spline bearing tooth profile.

(F) State of surface and surface layer of spline bearing The state of surface and surface layer of spline bearing according to the present invention will be described. In the present invention, the surface and surface layer of the spline bearing after broaching are the module m in Table 1 of JIS B 1603 1995 after broaching using the involute spline broach described in B1002445 of JIS B 4239 2009. The surface and surface layer of the spline bearing tooth profile according to the definition of the involute spline described in 1.

Surface roughness: Arithmetic mean roughness (Ra) is 2.5 μm or less and maximum height (Rz) is 15.0 μm or less The surface roughness of the spline bearing is 2.5 μm or less in Ra and 15 in Rz. If it is 0.0 μm or less, the amount of wear (wear height of the tooth profile) at 4 million repetitions in the wear test described later is 0.4 mm or less, and good wear resistance is exhibited. This is because if the surface roughness of the spline bearing is less than this value, chipping such as chipping or peeling does not occur on the surface of the spline bearing, so that aggressive wear is suppressed, and at the initial stage of the wear test described later. This is because it changes to adhesive wear.

The surface roughness after broach molding is determined according to JIS B 0601 2013, using "Surftest SV-3200" manufactured by Mitutoyo Co., Ltd. as a measuring device, and filter: PC50 (Gaussian filter PC50 (S (x) = exp (S (x) = exp). −π (x / (αλc)) ² ) / (αλc), where α = ( ^{ln2 / π) 0.5} = 0.4697 removes the waviness wavelength component)), cutoff: 0.8 mm (λc) = 0.8 mm), and measure for a length of 10 mm. The measurement direction is parallel to the axial direction of the spline shaft, and the surface roughness after broaching is the internal gear (spline bearing tooth profile) of the spline bearing after broaching, and the external gear of the spline shaft. The roughness of the contact surface to be contacted.

Micro Vickers hardness on the surface layer: 300 Hv or more If the Micro Vickers hardness on the surface layer of the spline bearing (at a depth of 20 μm from the tooth profile of the spline bearing) is 300 Hv or more, the wear test described later can be repeated 4 million times. The amount of wear (wear height of the tooth profile) is 0.4 mm or less, showing good wear resistance. This is because if the micro Vickers hardness on the surface layer is equal to or higher than this value, even if the spline shaft, which is the mating component, is a carburized material with a Vickers hardness of 650 Hv or higher, it is abrasive at the initial stage of the wear test described later. This is because wear is suppressed and adhesion wear occurs at an early stage.

Micro Vickers hardness on the surface is measured by the method described in JIS Z 2244 2009 under a load of 0.49 N. Specifically, since the spline bearing tooth profile portion is composed of a plurality of teeth having the same shape, these tooth profile portions are cut out from the spline bearing tooth profile portion and used as a sample, and at least two tooth profile portions of the spline shaft. At positions where the external gear and the spline bearing tooth profile are in contact with each other and slide, 10 points are measured at each location and the average value is calculated.

Average grain boundary solid solution carbon concentration on the surface: 5-10 at%
If the average grain boundary solid solution carbon concentration at the surface layer of the spline bearing (40 μm depth from the surface of the spline bearing tooth profile) is 5 to 10% in atomic%, the wear test described later can be repeated 4 million times. The amount of wear (wear height of the tooth profile) is 0.4 mm or less, showing good wear resistance.

This is because if the average grain boundary solid solution carbon concentration on the surface layer is within this value range, ferrite is caused by repeated contact between the external gear of the spline shaft and the tooth profile of the spline bearing at the earliest stage of the wear test described later. This is because the carbon dissolved in the grain boundaries diffuses into the grains. As a result of carbon diffusing into ferrite and strengthening of ferrite by solid solution strengthening, even if the spline shaft, which is the mating component, is a carburized material with a Vickers hardness of 650 Hv or more, the number of repetitions is initial in the wear test described later. Abrasive wear at the stage is suppressed, and adhesion wear progresses at an early stage.

The average grain boundary solid solution carbon concentration on the surface layer is the contact between the internal gear of the spline bearing (spline bearing tooth profile) and the external gear of the spline shaft using the above sample used for the Micro Vickers hardness. The cross section of the surface is measured by the three-dimensional atom probe method.

It is appropriate to use the three-dimensional atom probe method to measure the concentration of solute carbon present at the grain boundaries and within the grains. In 1988, A.M. of Oxford University. The position sensitive atom probe (PoSAP) developed by Cerezo et al. Incorporates a position sensitive atom probe into the detector of the atom probe, and the microchannel plate and fluorescent plate are used for analysis. It is a device that can simultaneously measure the flight time and position of an atom that has reached the detector without using a certain aperture (probe hole).

Using this device, not only can all the constituent elements in the alloy existing on the sample surface be displayed as a two-dimensional map with spatial resolution at the atomic level, but also the sample surface can be displayed one atomic layer at a time using the electric field evaporation phenomenon. By evaporating, the 2D map can be expanded in the depth direction and displayed / analyzed as a 3D map.

For grain boundary observation, in order to prepare a needle-shaped sample for atom probe (AP) including the grain boundary, a FIB (focused ion beam) device / FB2000A manufactured by Hitachi, Ltd. was used, and the cut out sample was arbitrarily polished by electropolishing. The grain boundary is set to the tip of the needle with the shape scanning beam. The sample processed as described above is placed at a position 40 μm in the vertical direction from the surface of the tooth profile of the spline bearing, and the crystal orientation is different due to the channeling phenomenon of SIM (scanning ion microscope). Utilizing what occurs, the grain boundaries are identified while observing, and cut with an ion beam.

The channeling phenomenon is a phenomenon in which charged particles are bound between crystal axes or crystal planes by a local crystal field in the crystal and proceed only in a specific direction. When the channeling phenomenon occurs, the charged particle beam is bound between the crystal axes and crystal planes, so that the beam intensity scattered in completely different directions can be suppressed to a considerably low level.

The device used as the three-dimensional atom probe is OTAP manufactured by CAMECA, and the measurement conditions are a sample position temperature of about 70 K, a total probe voltage of 10 to 15 kV, and a pulse ratio of 25%. The number of atoms of all elements contained in the steel of the present invention is measured three times at the grain boundaries and inside the grains of each sample. Then, the denominator is the number of atoms of all the elements contained in the steel, the numerator is the number of carbon atoms, and the atomic density expressed in% by multiplying by 100 is defined as the grain boundary solid solution carbon concentration (at%). The average grain boundary solid solution carbon concentration (at%) is defined as a representative value of the values measured three times each at the grain boundary and the grain, that is, the average value of the grain boundary solid solution carbon concentrations measured a total of six times.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steel having the chemical composition shown in Table 1 is forged to prepare a slab, and the slab is subjected to a hot rolling step, a first cooling step, a winding step and a second cooling step under the conditions shown in Table 2. It was applied in order to produce a hot-rolled steel sheet. The finish rolling in the hot rolling step was performed by continuous rolling in a 7-step system. The thickness of the obtained hot-rolled steel sheet is also shown in Table 2.

[Metal structure]
The metallographic structure of the obtained hot-rolled steel sheet was observed, and the area ratio of each structure was measured. Specifically, first, in the rolling direction cross section of the steel sheet, when the width and thickness of the steel sheet are W and t, respectively, it is 1/4 W from the end face of the steel sheet and 1/4 t from the surface of the steel sheet. A test piece for observing the metallographic structure was cut out from the position.

Then, the rolling direction cross section (so-called L direction cross section) of the test piece was subjected to nightl etching, and after etching, observation was performed with a visual field of 300 μm × 300 μm using an optical microscope. Image analysis was performed on the obtained microstructure photographs to determine the area ratios of ferrite and pearlite, and the total area ratios of bainite, martensite, and retained austenite.

Next, the night-game-etched portion was subjected to repera-etching, and observation was performed using an optical microscope in a field of view of 300 μm × 300 μm. Then, the total area ratio of retained austenite and martensite was calculated by performing image analysis on the obtained tissue photograph. Further, using a sample surface-cut from the direction normal to the rolled surface to a depth of 1/4 of the plate thickness, the volume fraction of retained austenite was determined by X-ray diffraction measurement, and the value was taken as the area fraction of retained austenite.

Then, by subtracting the area ratio of retained austenite obtained by X-ray diffraction measurement from the total area ratio of retained austenite and martensite obtained by repera etching and image analysis, the value is taken as the area ratio of martensite. bottom. Furthermore, by subtracting the total area ratio of retained austenite and martensite obtained by repera etching and image analysis from the total area ratio of bainite, martensite and retained austenite obtained by nighttal etching and image analysis. The area ratio of bainite was calculated.

By this method, the area ratios of ferrite, bainite, martensite, retained austenite, and pearlite were determined.

In addition, the average circle-equivalent diameter (diameter) of ferrite and martensite was determined by the following procedure. That is, the number of ferrites and martensites in the sample was measured by a cutting method based on the evaluation by the Annex C cutting method according to the JIS G 0551 2013 steel-grain size microscopic test method, and measured by image analysis. The equivalent circle diameter (diameter) was determined from the areas of ferrite and martensite, respectively.

Furthermore, the average carbon concentration of martensite is equipped with a field emission electron gun (Field Emission Gun: FEG) with an acceleration voltage of 200 kV, which is obtained by taking a transmission electron microscope sample from each sample and adding an EELS composition analysis function. It was measured by a transmission electron microscope. Twenty arbitrary martensites were selected, the measurement beam diameter of EELS was expanded to about 3 μm, and the average carbon concentration of martensite was measured.

Here, as described above, the measurement target by the transmission electron microscope is the portion of martensite and retained austenite that was previously revealed by repera etching, and the measured value is regarded as the average carbon concentration of martensite. bottom.

[mechanical nature]
Of the mechanical properties, the tensile strength characteristics (tensile strength (TS), total elongation (t-EL)) are either 1/4 W or 3/4 W in the plate width direction from one end of the plate when the plate width is W. At this position, the evaluation was performed in accordance with JIS Z 2241 2011 using the No. 5 test piece of JIS Z 2241 2011 collected with the direction perpendicular to the rolling direction (width direction) as the longitudinal direction.

[Brooch moldability]
Subsequently, as shown in FIG. 1, each hot-rolled steel sheet was formed by using broaching. A hard broach tool HW-5008 manufactured by Nachi-Fujikoshi Co., Ltd. was used for broach forming. Using the involute spline broach described in B1002445 of JIS B 4239 2009, the involute-shaped spline bearing tooth profile portion shown in module m = 1 in Table 1 of JIS B 1603 1995 was processed.

The broach steel tool used was SKH55 steel which had been subjected to nitriding treatment. The brooch forming conditions were a cutting length of 16 mm, an inner diameter of 18 mm, a tooth height of 1 mm, a tooth upper side length of 1.2 mm, a tooth lower side length of 2.4 mm, a cutting speed of 7 m / min, and Idemitsu Kosan Daphne Marg Plus was used as the cutting oil. Those in which obvious cracks and chips were generated by this brooch molding were evaluated as x (failed), and those in which cracks and chips were not generated in appearance were evaluated as ◯ (passed).

[Roughness of the surface of the spline bearing tooth profile]
The roughness of the tooth profile, which is the contact surface of the spline bearing internal gear (spline bearing tooth profile) that comes into contact with the external gear of the spline shaft, is measured by Mitsutoyo Co., Ltd. in accordance with JIS B 0601 2013. Filter: PC50 (Gaussian filter PC50 (S (x) = exp (-π (x / (αλc)) ² ) / (αλc), but α = (ln2 /) π) ^0.5 = 0.4697 to remove the waviness wavelength component)), cutoff: 0.8 mm (λc = 0.8 mm), measured for a length of 10 mm. The measurement direction was parallel to the spline shaft axis, and the surface roughness after broach molding was the roughness of the tooth profile that is the contact surface of the spline bearing teeth.

[Micro Vickers hardness at 20 μm from the surface of the spline bearing tooth profile]
The Micro Vickers hardness at a position 20 μm perpendicular to the surface of the spline bearing tooth profile was measured by the method described in JIS Z 2244 2009 with a load of 0.49 N and a pressing time of 15 s. Specifically, since the spline bearing tooth profile portion is composed of a plurality of teeth having the same shape, these tooth profile portions are cut out from the spline bearing tooth profile portion and used as a sample, and at least two tooth profile portions of the spline shaft. At positions where the external gear and the spline bearing tooth profile are in contact with each other and sliding, 10 points were measured at each location, and the average value of these was calculated.

[Average grain boundary solid solution carbon concentration at 40 μm from the surface of the spline bearing tooth profile]
Spline bearing tooth profile with respect to the cross section of the contact surface of the spline bearing internal gear (spline bearing tooth profile) that is broached using the above sample used for Micro Vickers hardness and in contact with the external gear of the spline shaft. The average grain boundary solid-dissolved carbon concentration at a position 40 μm in the vertical direction from the surface was measured by a three-dimensional atom probe method. For grain boundary observation, a FIB (focused ion beam) device / FB2000A manufactured by Hitachi, Ltd. was used to prepare a needle-shaped sample for AP including the grain boundary, and the cut out sample was electropolished and grained with an arbitrary shape scanning beam. The boundary part is the tip of the needle.

The sample processed as described above is placed at a position 40 μm in the vertical direction from the surface of the tooth profile of the spline bearing, and the crystal orientation is different due to the channeling phenomenon of SIM (scanning ion microscope). Utilizing the occurrence, the grain boundaries were identified while observing, and the grains were cut with an ion beam.

The device used as the three-dimensional atom probe is OTAP manufactured by CAMECA, and the measurement conditions are a sample position temperature of about 70 K, a total probe voltage of 10 to 15 kV, and a pulse ratio of 25%. The number of atoms of all elements contained in the steel was measured three times at the grain boundaries and inside the grains of each sample. Then, the denominator was the number of atoms of all the elements contained in the steel, the numerator was the number of carbon atoms, and the atomic density expressed in% by multiplying by 100 was defined as the grain boundary solid solution carbon concentration (at%). The average grain boundary solid solution carbon concentration (at%) was defined as a representative value of the values measured three times each at the grain boundary and the grain, that is, the average value of the grain boundary solid solution carbon concentrations measured a total of six times.

[Abrasion test]
The wear resistance was evaluated by the wear test described below. SCr420 carburized material (Hv ≧ 650) is brought into contact with a hot-rolled steel sheet at a surface pressure of 150 MPa (contact area 2 mm × 10 mm), and a so-called ATF (automatic transmission) is used in the longitudinal direction of the contact area (10 mm direction) with a stroke of ± 1 mm. Wear was generated by sliding in fluid (automatic transmission fluid), and the durability of wear up to 0.4 mm was evaluated.

In order to exhibit practical wear resistance in actual vehicle running, the durability by the above method needs to be 4 million times or more, preferably 5 million times or more. In this embodiment, those in which the number of sliding times is 4 million and the amount of wear of the internal gear 3a (spline bearing tooth profile) of the spline bearing 3 satisfies the standard of 0.4 mm or less are marked with ○ (pass). Those that did not are marked with x (failed).

The results of these measurements are shown in Tables 3 and 4.

As can be seen from Tables 3 and 4, the examples of the present invention satisfying the provisions of the present invention show excellent brooch moldability and excellent wear resistance after processing.

On the other hand, in the comparative example, at least one of the brooch moldability and the wear resistance after processing was inferior.

Specifically, the test No. In No. 6, since the slab heating temperature is too high, the ferrite grain size is large and the wear resistance is deteriorated. Test No. Since the slab heating temperature of No. 7 was too low, the load of rough rolling exceeded the upper limit and finish rolling could not be performed, so that no finished product could be obtained.

Test No. In No. 9, the finish rolling end temperature was too low, so that the moldability deteriorated and cracks occurred during broach forming. Test No. In No. 10, the finish rolling end temperature is too high, so that the area ratio of martensite is small and the wear resistance is deteriorated.

Test No. In No. 11, in addition to the finish rolling end temperature being too low, the effective cumulative strain is too high, so that the average circle-equivalent diameter of ferrite becomes excessively fine and the wear resistance deteriorates. Test No. In No. 12, since the cumulative strain of the three stages in the front stage is too low, the diameter corresponding to the average circle of ferrite becomes coarse and the wear resistance deteriorates.

Test No. In No. 13, since the time until the start of cooling is too short, the diameter corresponding to the average circle of ferrite is excessively finely divided, and the wear resistance is deteriorated. Test No. In No. 14, since the time until the start of cooling is too long, the diameter corresponding to the average circle of ferrite becomes coarse, and the wear resistance deteriorates.

Test No. In No. 15, since the average cooling rate before holding in the first cooling step is too low, the average circle-equivalent diameter of ferrite becomes coarse and the wear resistance deteriorates. Test No. In No. 16, since the holding temperature in the first cooling step is too high, the diameter corresponding to the average circle of ferrite becomes coarse, and the wear resistance deteriorates. Test No. Since the holding temperature of No. 17 in the first cooling step is too low, the desired microstructure cannot be obtained, and the wear resistance is deteriorated.

Test No. No. 18 has a short holding time in the first cooling step, while the test No. 18 has a short holding time. Since the holding time of 19 is long, the desired microstructure cannot be obtained in any of them, and the wear resistance is deteriorated. Test No. In No. 20, since the average cooling rate after holding in the first cooling step is too low, the desired microstructure cannot be obtained, and the wear resistance is deteriorated.

Test No. Since the winding temperature of No. 21 is too high, the desired microstructure cannot be obtained, and the wear resistance is deteriorated. Test No. In No. 22, the average cooling rate from the winding temperature in the second cooling step is too high, so that the average carbon concentration of martensite is low and the wear resistance is deteriorated.

Test No. Since the C content of No. 40 is too high, the desired microstructure cannot be obtained, and the wear resistance is deteriorated. Test No. Since the C content of 41 is too low, the area ratio of martensite is small and the wear resistance is deteriorated. Test No. Since the Si content of 42 is too high, the ductility is deteriorated. Test No. Since the Si content of No. 43 is too low, the area ratio of martensite is small and the wear resistance is deteriorated.

Test No. Since the Mn content of 44 is too low, the area ratio of martensite is small and the wear resistance is deteriorated. Test No. Since the Mn content of No. 45 was too high, slab cracking occurred and no finished product could be obtained.

From the above examples, the effect of the present invention (both excellent wear resistance and moldability) was confirmed.

According to the present invention, it is possible to obtain a hot-rolled steel sheet having excellent brooch formability and a spline bearing having excellent wear resistance after brooch forming. Therefore, we can stably supply hot-rolled steel sheets that can manufacture transmission parts for automobiles with excellent wear resistance without heat treatment while the spline bearing tooth profile by brooch molding is processed. can. In addition, it is possible to manufacture transmission parts for automobiles having excellent wear resistance without increasing costs.

1. 1. Brooch 2. Spline bearing material 3. Spline bearing 3a. Internal gear 4. Spline axis 4a. External gear

Claims (8)

The chemical composition is mass%,
C: 0.025 to 0.180%,
Si: 0.05 to 1.70%,
Mn: 0.50-2.50%,
Al: 0.010-1.000%,
N: 0.0060% or less,
P: 0.050% or less,
S: 0.0050% or less,
Remaining: Fe and unavoidable impurities,
In the rolling direction cross section of the steel sheet, when the thickness of the steel sheet is t, the metallographic structure at the position of 1/4 t or 3/4 t from the surface of the steel sheet is in area%.
Martensite: 2.5-20.0%,
Bainite: less than 10.0%,
Residual austenite: less than 8.0%,
Pearlite: less than 2.5%,
Remaining: Ferrite,
The average circle equivalent diameter of ferrite is 10.0 to 100.0 μm,
The average circle-equivalent diameter of martensite is 3.0 to 18.0 μm.
The average carbon concentration of martensite is 0.80-1.20% by mass.
Hot-rolled steel sheet. The chemical composition is further increased by mass%.
Ti: 0 to 0.050%,
Nb: 0 to 0.100%,
V: 0 to 0.300%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Cr: 0-2.00%, and Mo: 0-1.00%,
Contains one or more selected from,
The hot-rolled steel sheet according to claim 1. The chemical composition is further increased by mass%.
B: 0 to 0.0100%,
Contains,
The hot-rolled steel sheet according to claim 1 or 2. The chemical composition is further increased by mass%.
Mg: 0-0.0100%,
Ca: 0 to 0.0100%, and REM: 0 to 0.1000%,
Contains one or more selected from,
The hot-rolled steel sheet according to any one of claims 1 to 3. The chemical composition is further increased by mass%.
Zr, Co, Zn, and W,
Contains 1.000% or less in total of one or more selected from
The hot-rolled steel sheet according to any one of claims 1 to 4. The method for manufacturing a hot-rolled steel sheet according to any one of claims 1 to 5.
Manufacture of a hot-rolled steel sheet in which a hot rolling step, a first cooling step, a winding step, and a second cooling step are sequentially performed on a steel piece having the chemical composition according to any one of claims 1 to 5. Is the way
The hot rolling step includes three or more steps of multi-step finish rolling.
The cumulative strain in the first to third-stage rolling in the multi-stage finish rolling is 0.003 to 0.300.
The rolling end temperature of the multi-stage finish rolling is a temperature of Ar _{3 to Ar 3} + 80 ° C. obtained by the following formula 5.
In the first cooling step, cooling is started 1.00 to 3.00 s after the multi-stage finish rolling is completed, and the temperature is 10 ° C./s or more from the rolling end temperature to the temperature range of 600 to 750 ° C. It is cooled at an average cooling rate, then held in the air for 3 to 10 s, and then further cooled to a temperature range of less than 350 ° C. at an average cooling rate of 10 ° C./s or higher.
In the winding step, the winding is performed at a winding temperature of less than 350 ° C.
In the second cooling step, cooling is performed from the winding temperature to 100 ° C. at an average cooling rate of less than 30 ° C./h.
Manufacturing method of hot-rolled steel sheet.
Ar ₃ = 970-325 x C + 33 x Si + 287 x P + 40 x Al-92 x (Mn + Mo + Cu) -46 x (Cr + Ni) ... (Equation 5)
However, the element symbol in the above formula represents the content (mass%) of each element in the hot-rolled steel sheet, and if it is not contained, 0 is substituted. A spline bearing molding that by the broaching against hot-rolled steel sheet is performed according to any of claims 1 to 5,
The arithmetic mean roughness (Ra) of the surface of the internal gear of the spline bearing is 2.5 μm or less, and the maximum height roughness (Rz) is 15.0 μm or less.
The micro Vickers hardness at a depth of 20 μm from the surface of the internal gear of the spline bearing is 300 Hv or more.
The average grain boundary solid solution carbon concentration at a depth of 40 μm from the surface of the internal gear of the spline bearing is 5 to 10% in atomic%.
Spline bearings. The method for manufacturing a spline bearing using the hot-rolled steel sheet according to any one of claims 1 to 5.
By the spline bearing and formed by broaching the hot-rolled steel sheet according to any one of claims 1 to 5,
The arithmetic mean roughness (Ra) on the surface of the internal gear of the spline bearing is 2.5 μm or less, and the maximum height roughness (Rz) is 15.0 μm or less.
The Micro Vickers hardness at a depth of 20 μm from the surface of the internal gear of the spline bearing is set to 300 Hv or more.
The average grain boundary solid solution carbon concentration at a depth of 40 μm from the surface of the internal gear of the spline bearing is 5 to 10% in atomic%.
How to manufacture spline bearings.

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