WO2013031587A1 - Rolled steel bar or wire for hot forging - Google Patents

Abstract

Provided is a rolled steel bar or wire for hot forging, having excellent bending fatigue strength, surface fatigue strength, abrasion resistance, and machinability even after hot forging. This rolled steel bar or wire for hot forging has a chemical composition containing C, Si, Mn, S, Cr, Mo (optional), Al, and N, with the remainder comprising Fe and impurities. The chemical composition furthermore has 1.60 to 2.10 of fn1 as defined by the formula (1). The structure of the rolled steel bar or wire for hot forging comprises a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. Upon examination of 15 fields of view randomly selected in a cross-section so that each field of view has an area of 62,500 µm², the maximum/minimum average ferrite particle diameter is 2.0 or less. fn1 = Cr + 2 × Mo (1). The corresponding element content (mass%) is substituted for each element symbol in the formula (1).

Description

Rolled steel bar or wire rod for hot forging

The present invention relates to a steel bar or wire, and more particularly to a rolled steel bar or wire for hot forging.

Mechanical parts such as gears and pulleys are used in automobiles or industrial machines. Many of these mechanical parts are manufactured in the following manner. Prepare a material made of alloy steel for machine structure. The material has a chemical composition corresponding to, for example, JIS standard SCr420, SCM420, or SNCM420. The material is, for example, hot rolled steel bar or wire. Hot forging is performed on the material to produce intermediate products. Normalize the intermediate product as necessary. Further, cutting is performed on the intermediate product. Surface hardening treatment is performed on the cut intermediate product. The surface hardening treatment is, for example, carburizing quenching, carbonitriding quenching, or induction quenching. Tempering is carried out at a tempering temperature of 200 ° C. or lower on the surface-cured intermediate product. A shot peening treatment is performed on the intermediate product after tempering as necessary. A machine part is manufactured by the above process.

In recent years, machine parts have been reduced in weight and size in order to cope with improved fuel economy of automobiles and higher engine output. The load applied to the machine parts is increased as compared with the prior art. Therefore, mechanical parts are required to have excellent bending fatigue strength, surface fatigue strength (contact fatigue strength), and wear resistance.

On the other hand, reduction of the manufacturing cost of machine parts is also required. Specifically, in order to reduce manufacturing costs, it is required to omit additional steps such as shot peening. Further, the ratio of the cutting cost to the manufacturing cost is large. Therefore, high machinability is required for hot forging rolled steel bars or wire rods as raw materials for machine parts in order to reduce manufacturing costs.

Therefore, in addition to excellent bending fatigue strength, surface fatigue strength, and wear resistance, excellent machinability is also required for the rolled steel bar or wire rod for hot forging used as a material for machine parts.

Techniques for improving the properties of steel used as a material for mechanical parts are proposed in Japanese Patent Laid-Open Nos. 60-21359, 7-242994, and 7-126803.

In the steel for gears disclosed in JP-A-60-21359, it is specified that Si: 0.1% or less and P: 0.01% or less. According to such regulations, JP-A-60-21359 describes that gear steel has high strength, is strong and has high reliability.

The gear steel disclosed in Japanese Patent Application Laid-Open No. 7-242994 contains Cr: 1.50 to 5.0%, and if necessary, 7.5%> 2.2 × Si (%) + 2 0.5 × Mn (%) + Cr (%) + 5.7 × Mo (%) is satisfied, and Si: 0.40 to 1.0% is contained. JP-A-7-242994 discloses that the gear steel has excellent tooth surface strength by having such a chemical composition.

The steel for carburized gears disclosed in JP-A-7-126803 contains Si: 0.35 to 3.0% or less, V: 0.05 to 0.5%, and the like. Japanese Patent Laid-Open No. 7-126803 discloses that gear steel has high bending fatigue strength and high surface fatigue strength by having such a chemical composition.

However, JP-A-60-21359 does not discuss surface fatigue strength. For this reason, the surface fatigue strength of the gear steel disclosed in JP-A-60-21359 may be low. Japanese Patent Laid-Open No. 7-242994 does not discuss bending fatigue strength. For this reason, the bending fatigue strength of the gear steel disclosed in JP-A-7-242994 may be low. The gear steel disclosed in JP-A-7-126803 contains V. V increases the hardness of the steel after hot forging. Therefore, the machinability of the steel after hot forging may be reduced. In short, JP-A-60-21359, JP-A-7-242994, and JP-A-7-126803 have excellent bending fatigue strength, surface fatigue strength and wear resistance, and Steel with excellent machinability is not disclosed.

An object of the present invention is to provide a rolled steel bar or wire rod for hot forging having excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability even after hot forging.

The rolled steel bar or wire rod for hot forging according to the present invention has a chemical composition of mass%, C: 0.1 to 0.25%, Si: 0.30 to 0.60%, Mn: 0.50 to 1.0%, S: 0.003 to 0.05%, Cr: 1.50 to 2.00%, Mo: 0.10% or less (including 0%), Al: 0.025 to 0.05 %, N: 0.010 to 0.025%, the balance is made of Fe and impurities, and P, Ti and O in the impurities are respectively P: 0.025% or less, Ti: 0.003% or less , O (oxygen): 0.002% or less, and fn1 defined by the formula (1) is 1.60 to 2.10. The structure of the rolled steel bar or wire rod for hot forging described above is composed of a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure. In the cross section, the maximum value / minimum value of the average ferrite particle diameter obtained by measuring 15 visual fields with an area of 62500 μm ² per visual field is 2.0 or less.
fn1 = Cr + 2 × Mo (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1).

The steel bar or wire for hot forging according to the present invention has excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability.

The rolled steel bar or wire rod for hot forging according to the present invention may contain Nb: 0.08% or less in mass% instead of part of Fe.

FIG. 1 is a side view of a small roller test piece for a roller pitching test produced in the example. FIG. 2 is a side view of the Ono rotary bending fatigue test piece with a notch produced in the example. FIG. 3 is a diagram illustrating carburizing and quenching conditions in the examples. FIG. 4 is a front view of a large roller for a roller pitching test in the embodiment.

The present inventors investigated and studied the bending fatigue strength, surface fatigue strength, wear resistance, and machinability of rolled steel bars or wires for hot forging (hereinafter simply referred to as bars or wires). As a result, the present inventors obtained the following knowledge.

(A) If the Si content is high, the surface fatigue strength and wear resistance of the steel increase. Furthermore, if the Cr content and the Mo content are high, the bending fatigue strength, surface fatigue strength, and wear resistance of the steel are increased.

(B) On the other hand, if the Mo content is too high, the formation of bainite is promoted in steel after hot forging and in steel after hot forging and further normalizing. Similarly, even if Mo is not contained, the formation of bainite is promoted if the Cr content is too high. Bainite reduces the machinability of steel. Therefore, it is preferable that generation of bainite can be suppressed and deterioration of machinability of steel can be suppressed.

(C) From the above, in order to obtain excellent bending fatigue strength, surface fatigue strength, wear resistance, and excellent machinability, it is preferable to adjust the Si content, the Mo content, and the Cr content. . In particular, in order to increase the machinability while increasing the bending fatigue strength, the surface fatigue strength, and the wear resistance, it is preferable to adjust the total amount of the Cr content and the Mo content. Specifically, if the chemical composition of the steel satisfies the formula (2), excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability can be obtained.
1.60 ≦ Cr + 2 × Mo ≦ 2.10 (2)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).

(D) If the variation in the crystal grain size in the steel bar or wire is large, the bending fatigue strength decreases. If the variation in crystal grain size is large, the surface fatigue strength may further decrease. As an index indicating the degree of variation in crystal grain size, the ferrite average grain size ratio is defined as follows. From the cross section of the steel bar or the wire rod, 15 visual fields having an area of each visual field of 62500 μm ² are selected from the region excluding the surface decarburized layer. Image analysis is performed for each of the selected 15 fields of view. Specifically, the ferrite average particle diameter is measured in each visual field. The ferrite average particle diameter of each visual field is measured according to the cutting method defined in JIS G0551 (2005).

The maximum value and the minimum value are selected from the average ferrite grain sizes determined in each of the 15 visual fields. Then, the maximum value / minimum value is obtained. The obtained value is defined as the ferrite average particle size ratio. That is, the ferrite average particle size ratio is defined by the following formula (3).
Ferrite average particle size ratio = maximum value of ferrite average particle size obtained with 15 views / minimum value of ferrite average particle size obtained with 15 views (3)
When the ferrite average particle size ratio is 2.0 or less, the variation in crystal grains in the steel is small. Therefore, the bending fatigue strength and surface fatigue strength of steel are high.

The rolled steel bar or wire rod for hot forging according to the present invention was completed based on the above knowledge. Hereinafter, the rolled steel bar or wire rod for hot forging according to the present invention will be described in detail. Hereinafter, “%” of the content of elements constituting the chemical composition means “mass%”.

[Chemical composition]
The chemical composition of the steel bar or wire according to the present invention contains the following elements.

C: 0.1 to 0.25%
Carbon (C) enhances carburizing and quenching or carbonitriding and quenching. Therefore, C increases the strength of the steel. In particular, C increases the strength of the core part of the machine part after carburizing or carbonitriding. On the other hand, if C is contained excessively, the deformation amount of the machine part after carburizing and quenching or carbonitriding is significantly increased. Therefore, the C content is 0.1 to 0.25%. The lower limit of the preferable C content is higher than 0.1%, more preferably 0.15% or more, and further preferably 0.18% or more. The upper limit of the preferable C content is less than 0.25%, more preferably 0.23% or less, and still more preferably 0.20% or less.

Si: 0.30 to 0.60%
Silicon (Si) enhances the hardenability of the steel. Si further increases the temper softening resistance of the steel. Therefore, Si increases the surface fatigue strength and wear resistance of steel. On the other hand, if Si is excessively contained, the strength of the steel after hot forging becomes excessively high. As a result, the machinability of steel decreases. If Si is excessively contained, the bending fatigue strength further decreases. Therefore, the Si content is 0.30 to 0.60%. The minimum of preferable Si content is higher than 0.30%, More preferably, it is 0.40% or more, More preferably, it is 0.45% or more. The upper limit of the Si content is preferably less than 0.60%, more preferably 0.57% or less, and further preferably 0.55% or less.

Mn: 0.50 to 1.0%
Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel. Therefore, Mn increases the strength of the core of machine parts that have been carburized or carbonitrided. On the other hand, if Mn is contained excessively, the machinability of the steel after hot forging is lowered. Furthermore, if Mn is contained excessively, Mn oxide is generated on the surface of the steel. As a result, the depth of the carburizing abnormal layer after carburizing or carbonitriding is increased. The carburizing abnormal layer is, for example, a grain boundary oxide layer and an incompletely quenched layer. If the depth of the carburized abnormal layer is increased, the bending fatigue strength and the pitting strength of the steel are lowered. Pitting is one of the forms of surface fatigue failure. Therefore, if the pitching strength is low, the surface fatigue strength is also low. Therefore, the Mn content is 0.50 to 1.0%. The minimum of preferable Mn content is higher than 0.50%, More preferably, it is 0.55% or more, More preferably, it is 0.60% or more. The upper limit with preferable Mn content is less than 1.0%, More preferably, it is 0.95% or less, More preferably, it is 0.9% or less.

S: 0.003 to 0.05%
Sulfur (S) combines with Mn to form MnS. MnS increases the machinability of steel. On the other hand, if S is contained excessively, coarse MnS is formed. Coarse MnS lowers the bending fatigue strength and surface fatigue strength of steel. Therefore, the S content is 0.003 to 0.05%. The lower limit of the preferable S content is higher than 0.003%, more preferably 0.005% or more, and still more preferably 0.01% or more. The upper limit of the preferable S content is less than 0.05%, more preferably 0.03% or less, and further preferably 0.02% or less.

Cr: 1.50 to 2.00%
Chromium (Cr) increases the hardenability of the steel and the temper softening resistance of the steel. Therefore, Cr increases the bending fatigue strength, surface fatigue strength, and wear resistance of steel. On the other hand, if Cr is excessively contained, the formation of bainite is promoted in the steel after hot forging or after normalization. Therefore, the machinability of the steel is reduced. Therefore, the Cr content is 1.50 to 2.00%. The lower limit of the preferable Cr content is higher than 1.50%, more preferably 1.70% or more, and further preferably 1.80% or more. The upper limit of preferable Cr content is less than 2.00%, More preferably, it is 1.95% or less, More preferably, it is 1.90% or less.

Mo: 0.10% or less (including 0%)
Molybdenum (Mo) may not be contained. Mo increases the hardenability and temper softening resistance of the steel. Therefore, Mo increases the bending fatigue strength, surface fatigue strength, and wear resistance of steel. On the other hand, if Mo is contained excessively, bainite generation is promoted in steel after hot forging or after normalization. Therefore, the machinability of the steel is reduced. Therefore, the Mo content is 0.10% or less (including 0%). The minimum of preferable Mo content is 0.02% or more. The upper limit of the preferable Mo content is less than 0.10%, more preferably 0.08% or less, and still more preferably 0.05% or less.

Al: 0.025 to 0.05%
Aluminum (Al) deoxidizes steel. Al further combines with N to form AlN. AlN suppresses the coarsening of austenite crystal grains due to carburizing heating. On the other hand, if Al is contained excessively, a coarse Al oxide is formed. Coarse Al oxide reduces the bending fatigue strength of steel. Therefore, the Al content is 0.025 to 0.05%. The lower limit of the preferable Al content is higher than 0.025%, more preferably 0.027% or more, and further preferably 0.030% or more. The upper limit of the preferable Al content is less than 0.05%, more preferably 0.045% or less, and further preferably 0.04% or less.

N: 0.010 to 0.025%
Nitrogen (N) combines with Al or Nb to form AlN or NbN. AlN or NbN suppresses the coarsening of austenite crystal grains due to carburizing heating. On the other hand, if N is contained excessively, it becomes difficult to produce stably in the steel making process. Therefore, the N content is 0.010 to 0.025%. The minimum of preferable N content is higher than 0.010%, More preferably, it is 0.012% or more, More preferably, it is 0.013% or more. The upper limit of the preferable N content is less than 0.025%, more preferably 0.020% or less, and still more preferably 0.018% or less.

The balance of the chemical composition of the steel bar or wire according to the present invention consists of Fe and impurities. The impurity in this specification means the element mixed from the ore and scrap utilized as a raw material of steel, or the environment of a manufacturing process. In the present invention, the contents of P, Ti, and O (oxygen) as impurities are limited as follows.

P: 0.025% or less Phosphorus (P) segregates at the grain boundaries and embrittles the grain boundaries. Therefore, P reduces the fatigue strength of steel. Therefore, the P content is preferably as low as possible. The P content is 0.025% or less. P content is preferably less than 0.025%, more preferably 0.020% or less.

Ti: 0.003% or less Titanium (Ti) combines with N to form coarse TiN. Coarse TiN reduces the fatigue strength of steel. Therefore, the Ti content is preferably as low as possible. Ti content is 0.003% or less. A preferable Ti content is less than 0.003%, more preferably 0.002% or less.

O (oxygen): 0.002% or less Oxygen (O) combines with Al to form oxide inclusions. Oxide inclusions reduce the bending fatigue strength of steel. Therefore, it is preferable that the O content is as low as possible. The O content is 0.002% or less. The preferable O content is less than 0.002%, and more preferably 0.001% or less.

The chemical composition of the steel bar or wire according to the invention further satisfies the formula (2).
1.60 ≦ Cr + 2 × Mo ≦ 2.10 (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2).

As described above, both Cr and Mo increase the hardenability and temper softening resistance of steel. Therefore, Cr and Mo increase the bending fatigue strength, surface fatigue strength, and wear resistance of steel. Compared with Mo and Cr, Mo has a half content of Cr, and has the same effect as Cr (improvement in bending fatigue strength, surface fatigue strength, and wear resistance). Therefore, it is defined as fn1 = Cr + 2Mo. The content (mass%) of the corresponding element (Cr or Mo) is substituted for each element symbol in fn1.

If fn1 is less than 1.60, at least one of the bending fatigue strength, surface fatigue strength, and wear resistance of the steel becomes low. On the other hand, if fn1 exceeds 2.10, the formation of bainite is promoted in the steel after hot forging or normalization. Therefore, the machinability of the steel is reduced. If fn1 is 1.60 to 2.10, it is possible to increase the bending fatigue strength, surface fatigue strength, and wear resistance of the steel while suppressing the deterioration of the machinability of the steel. A preferred lower limit of fn1 is 1.80 or more. A preferable upper limit of fn1 is less than 2.00.

The chemical composition of the rolled steel bar or wire rod for hot forging according to the present invention may contain Nb instead of a part of Fe.

Nb: 0.08% or less Niobium (Nb) is a selective element. Nb combines with C and N to form Nb carbide, Nb nitride or Nb carbonitride. Nb carbide, Nb nitride, and Nb carbonitride suppress the coarsening of austenite crystal grains during carburizing heating, as with Al nitride. If Nb is contained even a little, the above effect can be obtained. On the other hand, if Nb is contained excessively, Nb carbonitride, Nb nitride and Nb carbonitride become coarse. Therefore, coarsening of austenite crystal grains cannot be suppressed during carburizing heating. Therefore, the Nb content is 0.08% or less. The minimum with preferable Nb content is 0.01% or more. The upper limit of the preferable Nb content is less than 0.08%, and more preferably 0.05% or less.

[Microstructure]
The microstructure of the steel bar or wire according to the present invention comprises a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure. Here, the “ferrite / pearlite structure” means a two-phase structure in which a matrix (matrix) is composed of ferrite and pearlite. The “ferrite / pearlite / bainite structure” means a three-phase structure in which the matrix is composed of ferrite, pearlite, and bainite. The “ferrite bainite structure” means a two-phase structure in which the matrix is composed of ferrite and bainite.

In short, the microstructure of the steel bar or wire according to the present invention does not contain martensite. Martensite is hard and reduces the ductility of the steel. Accordingly, when a steel bar or wire containing martensite is transported or corrected, cracks are likely to occur in the steel bar or wire. Since the microstructure of the steel bar or wire according to the present invention does not contain martensite, cracking is unlikely to occur during correction or conveyance.

Each phase described above is identified by the following method. A sample including the center of a cross section (cross section) perpendicular to the longitudinal direction of the steel bar or wire is cut out. The surface (including the center part) of the cut sample is mirror-polished. Corrodes the polished surface with nital. The corroded surface is observed with a microstructure using an optical microscope with a magnification of 400 times. Specifically, among the corroded surface, 15 visual fields are arbitrarily selected from the region excluding the steel bar or the surface decarburized layer of the wire rod. Each field of view is observed to identify the microstructure. If bainite is included in any of the 15 visual fields, it is determined that bainite is included in the microstructure of the steel. The same judgment is made for ferrite and pearlite. The size of each field is 250 microns (μm) × 250 microns (μm) = 62500 μm ² .

In the above microstructure, the ferrite average particle size ratio defined by the formula (3) is 2.0 or less in the cross section.

Execute image analysis for each of the above 15 fields of view. Specifically, the ferrite phase is identified in each visual field. Measure the ferrite grain size in the identified ferrite phase. The average ferrite grain size in each field of view is measured according to the cutting method defined in JIS G0551 (2005).

¡Select the maximum and minimum values from the average ferrite grain size (15 particles in total) determined for each of the 15 fields of view. And based on said Formula (3), a ferrite average particle diameter ratio = (the maximum value of a ferrite average particle diameter / the minimum value of a ferrite average particle diameter) is calculated | required.

If the crystal grain size is non-uniform in the steel material after hot rolling (that is, as-rolled steel), the crystal grain size remains non-uniform even after hot forging, which is a subsequent process, or after carburizing and quenching. It is. If the crystal grain size is not uniform, bending fatigue strength and surface fatigue strength are reduced. Therefore, it is preferable that the crystal grain size in the hot-rolled material is as uniform as possible. In order to evaluate the degree of uniformity of the crystal grain size, it is preferable to evaluate the ferrite average grain size ratio. The ferrite particle diameter can be easily observed by etching as compared with pearlite or bainite. Therefore, if the degree of uniformity of the average ferrite grain size (that is, the ratio of average ferrite grain diameter) is examined, it is easy to evaluate the degree of crystal grain uniformity in the structure. Furthermore, fatigue fracture occurs starting from the lowest strength part. Therefore, using the maximum value / minimum value of the ferrite average particle size as an index is more suitable for evaluating the bending fatigue strength and the surface fatigue strength than using the standard deviation of the ferrite average particle size as an index.

If the microstructure is composed of various mixed structures including the above-mentioned ferrite and the ferrite average particle size ratio is 2.0 or less, the variation in crystal particle size in the steel bar or wire is small. Therefore, the bending fatigue strength and surface fatigue strength of the steel after hot forging or after carburizing and quenching are increased. The ferrite average particle size ratio is preferably 1.6 or less.

On the other hand, if the ferrite average particle size ratio exceeds 2.0, one or more of the bending fatigue strength and surface fatigue strength of the steel will be low.

[Production method]
An example of a method for manufacturing a steel bar or wire according to the present invention and an example of a method for manufacturing a machine part represented by a gear and a pulley will be described. In addition, a manufacturing method is not limited to the following.

A molten steel having the above chemical composition and satisfying the formula (2) is manufactured. A slab (slab or bloom) is produced by continuous casting using molten steel. In the continuous casting method, a reduction is applied to the slab in the middle of solidification. Next, the slab is heated. The heating temperature at this time is 1250 to 1300 ° C., and the heating time is 10 hours or more. The heated cast slab is subjected to block rolling with a block mill to produce a steel slab (billet).

The steel slab is hot-rolled to produce hot forging bar or wire. Specifically, the steel piece is heated. The heating temperature at this time is 1150 to 1200 ° C., and the heating time is 1.5 hours or more. The heated steel slab is hot-rolled to produce a steel bar or wire. The finishing temperature in hot rolling is set to 900 to 1000 ° C. Water cooling is not performed before finish rolling. After the finish rolling, the steel bar or wire is cooled at a cooling rate equal to or lower than that in the air (hereinafter simply referred to as “cooling”) until the surface temperature reaches 600 ° C. or lower. In hot rolling, the cross-section reduction rate (%) defined by the formula (4) is set to 87.5% or more.
Cross-section reduction rate = {1− (section area of bar, wire rod / section area of steel slab)} × 100 (4)

It is not necessary to cool the steel bar or wire rod after finish rolling to room temperature at a cooling rate equal to or lower than that of cooling. After the surface temperature of the bar or wire becomes 600 ° C. or lower, the bar or wire may be cooled at a higher cooling rate than that of cooling, such as air cooling, mist cooling, or water cooling.

The above-mentioned heating temperature means an average value of the furnace temperature of the heating furnace. The above heating time means the in-furnace time at the above heating temperature. The finishing temperature means the surface temperature of the steel bar and wire immediately after finish rolling. For example, finish rolling means rolling at the last stand among a plurality of stands used for rolling in a continuous mill. The cooling rate after finishing means the surface cooling rate of steel bars and wire rods.

An example of a method of manufacturing a machine part using hot-rolled steel bar or wire rod for hot forging is as follows.

熱 Hot forging is performed on rolled steel bars or wire rods for hot forging to produce coarse intermediate products. A tempering treatment may be performed on the intermediate product. The tempering process is, for example, normalizing. The intermediate product is machined into a predetermined shape. Machining is, for example, cutting or drilling.

Surface hardening treatment may be performed on the intermediate product after machining. The surface hardening treatment is, for example, carburizing treatment, nitriding treatment or induction hardening treatment. Finishing is performed on the intermediate product that has been subjected to the surface hardening treatment to produce a machine part.

The steel bar or wire manufactured by the above process has excellent bending fatigue strength, surface fatigue strength, wear resistance and excellent machinability even after hot forging.

Steels A to C having chemical components shown in Table 1 were melted in a 70-ton converter.

A 400 mm × 300 mm slab (bloom) was manufactured by continuous casting using molten steel of steels A to C. The produced bloom was allowed to cool to 600 ° C. in the atmosphere. In the continuous casting process, the slab in the middle of solidification was reduced.

Next, the manufacturing conditions shown in Table 2 were set.

Specifically, the “heating temperature” column in the “slab” column of Table 2 indicates the heating temperature (° C.) of the slab under each condition. The “heating time” column in the “slab” column of Table 2 shows the heating time (minutes) of the slab in each condition. Similarly, the “heating temperature” column in the “steel” column of Table 2 shows the heating temperature (° C.) of the steel slab under each condition. The “heating time” column in the “steel” column shows the heating time (minutes) of the steel slab under each condition. The “water cooling before finish rolling” column in the “rolling conditions” column indicates whether or not the steel slabs are water cooled before finish rolling in each condition. “Yes” in the column indicates that water cooling was performed. “None” indicates that water cooling was not performed. The “finishing temperature” column in the “rolling conditions” column indicates the finishing temperature (° C.) under each condition. The “cooling condition” column in the “rolling condition” column shows the cooling condition after finish rolling in each condition.

Based on the steel shown in Table 1 and the manufacturing conditions shown in Table 2, steel bars with test numbers 1 to 10 shown in Table 3 were produced.

Specifically, in each test number, the steel slab shown in Table 3 was heated under the manufacturing conditions shown in Table 3 (heating temperature and heating time of the slab). The heated slab was rolled into blocks to produce a steel slab of 180 mm × 180 mm. The produced steel slab was cooled to room temperature (25 ° C.).

Next, the steel slab was heated under the manufacturing conditions shown in Table 3 (heating temperature and heating time of the steel slab). The heated steel slab was hot-rolled under the production conditions shown in Table 3 (water cooling before finish rolling, finishing temperature, cooling conditions) to produce steel bars having a diameter of 50 mm and a diameter of 70 mm. The rolled steel bar was allowed to cool to room temperature in the atmosphere. In other words, the steel bar was a hot rolled material.

[Microstructure observation test]
A steel bar having a diameter of 50 mm was cut perpendicular to the longitudinal direction. A sample including the center portion of the cut surface was cut out. Of the surface of the sample, the surface corresponding to the above-described central portion was polished into a mirror surface. The polished surface was corroded with nital. The corroded surface was observed in 15 visual fields with an optical microscope having a magnification of 400 times. Fifteen visual fields were arbitrarily selected from the area excluding the surface decarburized layer. The size of each visual field was 250 μm × 250 μm. The microstructure was observed in each field.

As a result of the microstructure observation test, the microstructure of any test number did not contain martensite. The microstructure of each test number was either a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure. In the “Microstructure” column of Table 3, the microstructure observation results are shown. “F + P” in the table indicates that the microstructure of the corresponding test number is a ferrite pearlite structure. “F + P + B” indicates a ferrite / pearlite / bainite structure. “F + B” indicates a ferrite bainite structure.

[Ferrite average particle size measurement]
The ferrite average particle diameter of the 15 fields of view described above was measured according to the cutting method defined in JIS G0551 (2005).

The maximum value and the minimum value among the average ferrite particle diameter (15 particles in total) in each field of view were specified. And based on Formula (3), the ferrite average particle diameter ratio (= maximum value / minimum value) was calculated | required. Table 3 shows the average ferrite particle size ratio.

[Preparation of surface fatigue strength test piece and bending fatigue strength test piece]
The steel bars of each test number were heated at 1200 ° C. for 30 minutes. Next, hot forging was performed at a finishing temperature of 950 ° C. or higher to produce a round bar having a diameter of 35 mm. A round bar having a diameter of 35 mm is machined, and a roller pitching small roller test piece (hereinafter simply referred to as a small roller test piece) shown in FIG. 1 and an Ono-type rotary bending fatigue test piece with a notch shown in FIG. 2 (FIG. 1). 2 and FIG. 2, the unit of the dimension in the drawing was mm). The small roller test piece shown in FIG. 1 was provided with a test part (a cylindrical part having a diameter of 26 mm and a width of 28 mm) at the center.

Carburizing and quenching was performed on the prepared test pieces under the conditions shown in FIG. 3 using a gas carburizing furnace. After quenching, tempering was performed at 150 ° C. for 1.5 hours. For the small roller test piece and the Ono type rotating bending fatigue test piece, the gripping part was finished for the purpose of removing heat treatment strain.

[Surface fatigue strength test]
In the roller pitching test, the small roller test piece was combined with a large roller having the shape shown in FIG. 4 (the unit of dimensions in the drawing was mm). The large roller shown in FIG. 4 is made of steel that satisfies the standard of JIS standard SCM420H, and is a general manufacturing process, that is, normalization, test piece processing, eutectoid carburization with a gas carburizing furnace, low temperature tempering and polishing. Made by.

A roller pitching test using a small roller test piece and a large roller was performed under the conditions shown in Table 4.

As shown in Table 4, the rotation speed of the small roller test piece was 1000 rpm, the slip rate was −40%, the contact surface pressure between the large roller and the small roller test piece under test was 4000 MPa, and the number of repetitions was 2.0 × 10. ^{It was set to 7} cycles. When the rotation speed of the large roller was V1 m / sec and the rotation speed of the small roller test piece was V2 m / sec, the slip ratio (%) was obtained by the following equation.
Slip rate = (V2−V1) / V2 × 100

During the test, a lubricant (commercial oil for automatic transmission) was sprayed from the direction opposite to the rotation direction to the contact portion (surface of the test part) between the large roller and the small roller test piece at an oil temperature of 90 ° C. A roller pitching test was performed under the above conditions to evaluate the surface fatigue strength.

For each test number, the number of tests in the roller pitching test was six. After the test, an SN diagram was prepared with the surface pressure on the vertical axis and the number of repetitions until the occurrence of pitching on the horizontal axis. Among those in which pitching did not occur until the number of repetitions of 2.0 × 10 ⁷ times, the highest surface pressure was defined as the surface fatigue strength of the test number. In addition, when the area of the largest thing became 1 mm < ² > or more among the places where the surface of a small roller test piece was damaged, it defined as generating pitting.

Table 3 shows the surface fatigue strength obtained by the test. With respect to the surface fatigue strength in Table 3, the surface fatigue strength of Test No. 1 was set as a reference value (100%). And the surface fatigue strength of each test number was shown by ratio (%) with respect to a reference value. If the surface fatigue strength was 120% or more, it was judged that excellent surface fatigue strength was obtained.

[Abrasion resistance evaluation]
In the roller pitching test, the wear amount of the test portion of the small roller test piece having a repetition number of 1.0 × 10 ⁶ was measured. Specifically, the maximum height roughness (Rz) was determined according to JIS B0601 (2001). It shows that abrasion resistance is so high that Rz value is small. A roughness meter was used to measure the amount of wear. Table 3 shows the amount of wear. For the amount of wear in Table 3, the amount of wear for test number 1 was taken as the reference value (100%). And the abrasion loss of each test number was shown by ratio (%) with respect to a reference value. If the amount of wear was 80% or less, it was judged that excellent wear resistance was obtained.

[Bending fatigue strength test]
The bending fatigue strength was determined by an Ono type rotating bending fatigue test. The number of tests in the Ono rotary bending fatigue test was 8 for each test number. The rotational speed at the time of the test was 3000 rpm, and the others were tested by ordinary methods. Among those that did not break until the number of repetitions of 1.0 × 10 ⁴ and 1.0 × 10 ⁷ , the highest stress was defined as medium cycle and high cycle rotational bending fatigue strength, respectively.

Table 3 shows the bending fatigue strength of medium and high cycles. In the middle cycle and high cycle bending fatigue strength, the bending fatigue strength of test number 1 in the middle cycle and high cycle was defined as a reference value (100%). And the bending fatigue strength of the middle cycle and the high cycle of each test number was shown by ratio (%) with respect to a reference value. It was judged that an excellent bending fatigue strength was obtained when the bending fatigue strength was 115% or more in both the middle cycle and the high cycle.

[Cutting test]
A cutting test was conducted to evaluate machinability. A cutting specimen was obtained by the following method. A steel bar having a diameter of 70 mm for each test number was heated at a heating temperature of 1250 ° C. for 30 minutes. The heated steel bar was hot forged at a finishing temperature of 950 ° C. or higher to obtain a round bar having a diameter of 60 mm. A cutting test piece having a diameter of 55 mm and a length of 450 mm was obtained from this round bar by machining. A cutting test was performed using the cutting test piece under the following conditions.

Cutting test (turning)
Insert: Base material: Carbide P20 grade, coating None Conditions: peripheral speed 200m / min, feed 0.30mm / rev, cutting 1.5mm, water-soluble cutting oil used Measurement item: flank after 10 minutes of cutting time Main cutting edge wear amount

Table 3 shows the amount of main cutting edge wear obtained. In Table 3, the main cutting edge wear amount of test number 2 (using steel B) was set as a reference value (100%). And the amount of main cutting edge abrasion of each test number was shown by ratio (%) with respect to a reference value. If the main cutting edge wear amount was 80% or less, it was judged that excellent machinability was obtained.

[Evaluation results]
Referring to Table 3, the chemical compositions (steel C) of the steel bars of test numbers 4 and 9 were within the scope of the present invention, and fn1 satisfied the formula (2). Furthermore, the ferrite average particle diameter ratios of Test Nos. 4 and 9 were both 2.0 or less. Therefore, the middle and high cycle bending fatigue strengths of test numbers 4 and 9 were 115% or more, and the surface fatigue strength was 120% or more. Furthermore, the amount of wear was 80% or less. Further, the main cutting edge wear amount was 80% or less. Therefore, the steel bars of test numbers 4 and 9 had excellent bending fatigue strength, surface fatigue strength, wear resistance and machinability.

On the other hand, the chemical composition (steel A) of the steel bar of test number 1 corresponded to JIS standard SCr420H. Therefore, the Si content and the Cr content of Test No. 1 were less than the lower limits of the Si content and the Cr content of the present invention. Furthermore, fn1 of test number 1 was less than the lower limit of formula (2). Therefore, the bending fatigue strength, surface fatigue strength, and wear resistance of Test No. 1 were low.

The chemical composition (steel B) of the steel bar of test number 2 corresponded to JIS standard SCM420H. Therefore, the Si content and the Cr content of Test No. 2 were less than the lower limits of the Si content and the Cr content of the present invention. Furthermore, the Mo content of Test No. 2 exceeded the upper limit of the Mo content of the present invention. Furthermore, fn1 of test number 2 was less than the lower limit of formula (2). Therefore, the bending fatigue strength of Test No. 2 was as low as less than 115%, and the machinability was also low.

The chemical composition of test number 3 (steel C) was within the range of the chemical composition of the present invention. Further, fn1 also satisfied the formula (2). However, since the heating time of the slab was too short (see production condition 1 in Table 2), the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and high cycle of test number 3 was less than 115% and was low.

The chemical composition of test number 5 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2). However, in test number 5, water cooling was performed before finish rolling (see production condition 3 in Table 2). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and the high cycle of test number 5 was less than 115% and was low.

The chemical composition of test number 6 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2). However, in test number 6, the steel bar after finish rolling was water-cooled to 800 ° C. (see production condition 4 in Table 2). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and the high cycle of test number 6 was both less than 115% and low. Furthermore, the surface fatigue strength was less than 120% and was low. Further, the wear amount exceeded 80%, and the wear resistance was low.

The chemical composition of test number 7 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2). However, in test number 7, the heating time of the slab was too short, and the heating time of the steel slab was too short (see manufacturing condition 5). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength of the middle cycle and the high cycle of test number 7 was both less than 115% and low.

The chemical composition of test number 8 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2). However, in test number 8, the heating temperature of the steel slab was too high, and the finishing temperature was too high (see production condition 6). Therefore, the ferrite average particle size ratio exceeded 2.0. For this reason, the bending fatigue strength of the middle and high cycles of test number 8 was both less than 115% and low. Furthermore, the surface fatigue strength was less than 120% and was low. Further, the wear amount exceeded 80%, and the wear resistance was low.

The chemical composition of test number 10 is within the range of the chemical composition of the present invention, and fn1 also satisfies the formula (2). However, in test number 10, the heating temperature of the slab was too low (see production condition 8). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.

Molten steel having chemical compositions of D to S shown in Table 5 was produced in the same manner as in Example 1.

Under the same production conditions as in Example 1, steel bars with test numbers 11 to 42 shown in Table 6 were produced. The diameters of the steel bars were 50 mm and 70 mm. The test similar to Example 1 was implemented using the manufactured steel bar. Then, the bending fatigue strength, surface fatigue strength, wear resistance, and main cutting edge wear amount of medium cycle and high cycle were determined.

Table 6 shows the results obtained. Referring to Table 6, the chemical compositions of test numbers 17, 19, 21, 23, 31, 33, 41 and 42 were within the range of the chemical composition of the present invention, and fn1 satisfied the formula (2). Furthermore, the ferrite average particle size ratios of these test numbers were all 2.0 or less. Therefore, the middle and high cycle bending fatigue strengths of these test numbers were 115% or more, and the surface fatigue strength was 120% or more. Furthermore, the amount of wear was 80% or less. Further, the main cutting edge wear amount was 80% or less.

On the other hand, the Si content and the Cr content of the chemical composition of test number 11 (steel D) were less than the lower limits of the Si content and the Cr content of the present invention. Therefore, the surface fatigue strength of Test No. 11 was less than 120%, and the wear amount was higher than 80%. Test number 12 used the same steel D as test number 11. Therefore, the surface fatigue strength and wear resistance were low. In test number 12, the slab heating time was too short (manufacturing condition 1). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low.

Although the chemical composition of test number 13 (steel E) was within the range of the chemical composition of the present invention, fn1 was less than the lower limit of formula (2). Therefore, the high cycle bending fatigue strength was less than 115%, which was low. Test number 14 used the same steel E as test number 13. Therefore, the bending fatigue strength at a high cycle was low. In test No. 14, water cooling was further performed before finish rolling (manufacturing condition 3). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was lower than the test number 13.

The Si content of the chemical composition of test number 15 (steel F) exceeded the upper limit of the Si content of the present invention. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low. Further, the amount of wear of the main cutting edge was higher than 80%, and the machinability was low.

Test number 16 used the same steel F as test number 15. Therefore, bending fatigue strength and machinability were low. In test number 16, the steel bar after finish rolling was further water-cooled to 800 ° C. (production condition 4). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength was lower than test number 15. Furthermore, the surface fatigue strength was less than 120%, and the wear amount was higher than 80%.

The chemical composition of test number 18 (steel G) was within the scope of the present invention, and fn1 satisfied the formula (2). However, the heating time of the slab was too short (manufacturing condition 1). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low. Furthermore, the surface fatigue strength was less than 120% and was low.

The chemical composition of test number 20 (steel H) was within the scope of the present invention, and fn1 satisfied the formula (2). However, water cooling was performed before the finish rolling (production condition 3). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low.

The chemical composition (steel I) of test number 22 is within the scope of the present invention, and fn1 satisfies the formula (2). However, the steel bar after finish rolling was water-cooled to 800 ° C. (Production condition 4). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low.

The chemical composition of test number 24 (steel J) was within the scope of the present invention, and fn1 satisfied the formula (2). However, the slab heating time and the steel slab heating time were too short (manufacturing condition 5). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.

The Cr content of the chemical composition of test number 25 (steel K) exceeded the upper limit of the Cr content of the present invention. Therefore, the main cutting edge wear amount was higher than 80% and the machinability was low. This is probably because the Cr content was too high and bainite was excessively generated in the steel.

Test No. 26 used the same steel K as Test No. 25. Therefore, machinability was low. In test number 26, the heating time of the slab and the heating time of the steel slab were too short (manufacturing condition 5). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115%, which was low.

The Cr content of the chemical composition of test number 27 (steel L) was less than the lower limit of the Cr content of the present invention. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115% and was low. Furthermore, the surface fatigue strength was also low, less than 120%.

Test number 28 used the same steel L as test number 27. Therefore, the bending fatigue strength was low. Furthermore, in the test number 28, the heating temperature of the steel slab was too high, and the finishing temperature was too high (manufacturing condition 6). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115%, which was low. Furthermore, the surface fatigue strength was also low, less than 120%.

The Mo content of the chemical composition of test number 29 (steel M) exceeded the upper limit of the Mo content of the present invention. Therefore, the main cutting edge wear amount of test number 29 exceeded 80%, and the machinability was low. This is probably because the Mo content was too high and bainite was excessively produced in the steel.

Test No. 30 used the same steel M as the test No. 29. Therefore, machinability was low. In test number 30, the heating temperature of the slab was too low (manufacturing condition 8). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle and the high cycle was less than 115%, which was low.

The chemical composition of test number 32 (steel N) is within the scope of the present invention, and fn1 satisfies the formula (2). However, the heating temperature and finishing temperature of the steel slab were too high (manufacturing condition 6). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.

The chemical composition of test number 34 (steel O) was within the scope of the present invention, and fn1 satisfied the formula (2). However, the heating temperature of the slab was too low (manufacturing condition 8). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low.

The Mn content and Al content of the chemical composition of test number 35 (steel P) were less than the lower limits of the Mn content and Al content of the present invention. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low. Furthermore, the surface fatigue strength was less than 120% and was low.

The test No. 36 used the same steel P as the test No. 35. Therefore, the bending fatigue strength and surface fatigue strength in the middle cycle were low. In Test No. 35, the steel bar after finish rolling was further water-cooled to 800 ° C. (Production condition 4). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength at a high cycle was less than 115% and was low. Furthermore, the bending fatigue strength in the middle cycle was lower than the test number 35.

The Mn content and Al content of the chemical composition of test number 37 (steel Q) exceeded the upper limits of the Mn content and Al content of the present invention. Therefore, the bending fatigue strength at a high cycle was less than 115% and was low. Furthermore, the main cutting edge wear amount exceeded 80%, and the machinability was low.

The same steel Q as the test number 37 was used for the test number 38. Therefore, the bending fatigue strength at a high cycle was low and the machinability was also low. Furthermore, in the test number 38, the heating temperature and finishing temperature of the steel slab were too high. Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength in the middle cycle was less than 115% and was low. Further, the high cycle bending fatigue strength was lower than the test number 37.

Although the chemical composition of the test number 39 (steel R) is within the range of the chemical composition of the present invention, fn1 exceeded the upper limit of the formula (2). Therefore, the machinability of the steel of test number 39 was low. The test No. 40 used the same steel R as the test No. 39. Therefore, the machinability of the steel of test number 40 was low. In Test No. 40, water cooling was further performed before rolling (Production Condition 3). Therefore, the ferrite average particle size ratio exceeded 2.0. Therefore, the bending fatigue strength at the middle cycle and the high cycle was lower than the test number 39.

As mentioned above, although embodiment of this invention was described, embodiment mentioned above is only the illustration for implementing this invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (3)

Chemical composition is mass%,
C: 0.1 to 0.25%
Si: 0.30 to 0.60%,
Mn: 0.50 to 1.0%,
S: 0.003 to 0.05%,
Cr: 1.50 to 2.00%,
Mo: 0.10% or less (including 0%),
Al: 0.025 to 0.05%,
N: 0.010 to 0.025% is contained, with the balance being Fe and impurities,
P, Ti and O in the impurities are each
P: 0.025% or less,
Ti: 0.003% or less,
O (oxygen): 0.002% or less,
Fn1 defined by the formula (1) is 1.60 to 2.10,
The structure is composed of ferrite pearlite structure, ferrite pearlite bainite structure, or ferrite bainite structure,
A rolled steel bar or wire rod for hot forging in which the maximum value / minimum value of the average ferrite grain diameter obtained by observing and measuring 15 fields of view with an area of 62500 μm ² per field of view is 2.0 or less.
fn1 = Cr + 2 × Mo (1)
Here, the content (mass%) of the corresponding element is substituted for each element symbol in the formula (1). The rolled steel bar or wire rod for hot forging according to claim 1, wherein fn1 is 1.80 or more. The rolled steel bar or wire rod for hot forging according to claim 1 or 2, which contains Nb: 0.08% or less in mass% instead of part of Fe.

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