KR101552449B1 - Rolled steel bar or wire for hot forging - Google Patents

Abstract

Provided is a hot rolled steel bar or wire rod having excellent bending fatigue strength, strength in cotton, abrasion resistance and machinability even after hot forging.
The rolling bar steel or wire rod for hot forging according to the invention contains C, Si, Mn, S, Cr, Mo (may not contain), Al and N, and the balance of Fe and impurities. The chemical composition is also defined as fn1 of 1.60-2.10 as defined in equation (1). The structure of the hot rolled steel bar or wire described above is composed of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. The maximum value / minimum value of the average ferrite mean particle size is 2.0 or less when observed at 15 fields of view at random at an area of 62500 mu m ² per field of view in the cross section.
fn1 = Cr + 2 占 Mo (1)
Here, the content (mass%) of the corresponding element is substituted into each symbol of the element in the formula (1).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot rolled steel strip,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bar or a wire rod, and more particularly, to a hot rolled steel bar or wire rod.

Machine parts such as gears, pulleys and the like are used in automobiles or industrial machines. Many of these mechanical parts are manufactured in the following manner. Prepare materials made of alloy steel for machine structural use. The material has a chemical composition corresponding to, for example, JIS standard SCr420, SCM420 or SNCM420. The material is, for example, a hot-rolled steel bar or wire rod. The material is subjected to hot forging to produce an intermediate product. The intermediate product shall be called as necessary. Further, the intermediate product is subjected to cutting processing. The intermediate product to be cut is subjected to surface hardening treatment. The surface hardening treatment is, for example, carburizing quenching, carburizing nitriding, or high frequency quenching. Tempering the surface-hardened intermediate product to a tempering temperature of 200 ° C or less. The intermediate product after tempering is subjected to shot peening as necessary. The mechanical parts are manufactured by the above process.

BACKGROUND ART [0002] In recent years, in order to cope with improvement in fuel efficiency of automobiles and increase in output of engines, mechanical parts have become lightweight and downsized. The load on machine parts is increasing compared with the conventional one. For this reason, mechanical parts are required to have excellent bending fatigue strength, strength (contact fatigue strength) and wear resistance.

On the other hand, it is also required to reduce the manufacturing cost of the mechanical parts. Concretely, in order to reduce the manufacturing cost, it is required to omit additional steps such as shot peening. In addition, the ratio of the machining cost to the manufacturing cost is large. For this reason, in order to reduce the manufacturing cost, a high machinability is demanded for a hot rolled steel bar or a wire rod to be a material for mechanical parts.

Therefore, the hot rolled steel strip or wire rod to be the material of the mechanical parts is required to have superior bending fatigue strength, strength and wear resistance as well as excellent machinability.

BACKGROUND ART [0002] Techniques for improving the characteristics of steel as a material of mechanical parts are proposed in Japanese Patent Application Laid-open Nos. 60-21359, 7-242994, and 7-126803 .

In the gear steel disclosed in Japanese Patent Application Laid-Open No. 60-21359, it is specified that Si is 0.1% or less and P is 0.01% or less. With this provision, the steel for the gear wheels has high strength, is strong and has high reliability, and is disclosed in Japanese Patent Application Laid-Open No. 60-21359.

The steel for a cogwheel disclosed in Japanese Patent Application Laid-Open No. 7-242994 contains 1.50 to 5.0% of Cr, more preferably 7.5% to 2.2 x Si (%) + 2.5 x Mn (%) + Cr (%) + 5.7 x Mo (%), and Si: 0.40 to 1.0%. By having such a chemical composition, the steel for a gear wheel has excellent tooth surface strength and is described in JP-A-7-242994.

The steel for a carburizing gear wheel disclosed in JP-A-7-126803 contains 0.35 to 3.0% of Si, 0.05 to 0.5% of V, and the like. By having such a chemical composition, the steel for a cog wheel is described in Japanese Patent Application Laid-Open No. 7-126803, which has a high bending fatigue strength and a high surface strength.

However, Japanese Unexamined Patent Publication No. 60-21359 does not consider the strength of cotton fibers. Therefore, the strength of the gear steel for a gear wheel disclosed in Japanese Patent Application Laid-open No. 60-21359 may be low. Japanese Unexamined Patent Publication No. 7-242994 does not consider bending fatigue strength. For this reason, the bending fatigue strength of the gear for a gear wheel disclosed in Japanese Patent Application Laid-Open No. 7-242994 may be low. The steel for a gear wheel disclosed in JP-A-7-126803 contains V. V increases the hardness of the steel after hot forging. Therefore, the machinability of steel after hot forging may be lowered. In particular, Japanese Patent Application Laid-Open Nos. 60-21359, 7-242994, and 7-126803 have excellent bending fatigue strength, surface strength and wear resistance , And no steel having excellent machinability is disclosed.

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

The hot rolled steel strip or wire according to the present invention has a chemical composition of 0.1 to 0.25% of C, 0.30 to 0.60% of Si, 0.50 to 1.0% of Mn, 0.003 to 0.05% of S, The balance of Fe and impurities, and the content of P, Cr, and Mo in the impurities is in the range of 1.50 to 2.00% Cr, 0.10% or less (including 0%) of Mo, 0.025 to 0.05% Ti and O are 0.025% or less of P, 0.003% or less of Ti and 0.002% or less of O (oxygen), respectively, and fn1 defined in formula (1) is 1.60 to 2.10. The structure of the above-described hot rolled steel bar or wire rod is composed of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure. The maximum value / minimum value of the ferrite average particle diameter obtained by measuring the field of view at an area of 62500 mu m ² per field of view in the cross section is 2.0 or less.

fn1 = Cr + 2 占 Mo (1)

Here, the content (mass%) of the corresponding element is substituted into each symbol of the element in the formula (1).

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

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

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

Fig. 1 is a side view of a small roller test piece for roller pitching test produced in the example. Fig.
Fig. 2 is a side view of the Ono type rotary bending fatigue test piece having the notch produced in the embodiment. Fig.
Fig. 3 is a chart showing the carburization quenching conditions in the examples. Fig.
4 is a front view of a roller for a roller pitching test in the embodiment.

The inventors of the present invention have studied and studied the bending fatigue strength, the strength, the abrasion resistance and the machinability of a hot rolled steel bar or a wire rod (hereinafter simply referred to as a bar steel or a wire rod). As a result, the present inventors have obtained the following findings.

(a) When the Si content is high, strength and abrasion resistance are enhanced by the surface of the steel. When the Cr content and the Mo content are high, the bending fatigue strength of the steel, the strength and the wear resistance are improved by the cotton.

(b) On the other hand, if the Mo content is too high, generation of bainite is promoted in the steel after hot forging and in the steel after hot forging and soaking. Similarly, when Mo is not contained, if the Cr content is too high, the production of bainite is promoted. Bainite reduces the machinability of the steel. Therefore, it is preferable that the generation of bainite is suppressed and the lowering of the machinability of the steel can be suppressed.

(c) From the above, it is preferable to adjust the Si content, the Mo content and the Cr content in order to obtain excellent bending fatigue strength, strength and abrasion resistance in cotton, and excellent machinability. Particularly, in order to increase the bending fatigue strength, the strength and the abrasion resistance with the cotton, and the machinability, it is preferable to adjust the total amount of the Cr content and the Mo content. Specifically, when the chemical composition of the steel satisfies the formula (2), excellent bending fatigue strength, surface strength, abrasion resistance and machinability can be obtained.

1.60? Cr + 2 占 Mo? 2.10 (2)

Here, the content (mass%) of the corresponding element is substituted into each symbol of the element in the formula (2).

(d) If the deviation of the crystal grain size in the bar or wire is large, the bending fatigue strength is lowered. If the deviation of the crystal grain size is large, the strength may also be lowered by the cotton fiber. As an index showing the degree of deviation of the crystal grain size, the average ferrite ratio of ferrite is defined as follows. From the cross-sectional surface of the bar or wire, the 15 field of view with an area of 62500 mu m ² is selected from the area where the decarbonization layer of the surface layer is to be removed. Image analysis is performed for each of the 15 fields selected. Specifically, the average particle size of ferrite is measured in each field of view. The ferrite average grain size of each field of view is measured in accordance with the cutting method specified in JIS G0551 (2005).

The maximum and minimum values of the ferrite average particle diameters determined in each of the 15 fields of view are selected. Then, the maximum value / minimum value is obtained. The value obtained is defined as the average ferrite opening ratio. That is, the average ferrite ratio of ferrite is defined by the following formula (3).

The average value of the ferrite average particle size = the maximum value among the average particle diameters of ferrite obtained in the field of view of 15 / the minimum value of the average particle diameters of ferrite particles obtained in the field of view of 15 (3)

When the ferrite average grain size ratio is 2.0 or less, the deviation of crystal grains in the steel is small. Therefore, the strength is high due to the bending fatigue strength and surface tension of the steel.

The hot rolled steel bar or wire rod according to the present invention was completed on the basis of the above-described findings. Hereinafter, the hot rolled steel bar or wire according to the present invention will be described in detail. Hereinafter, "%" of the content of the elements constituting the chemical composition means "% by mass".

[Chemical Composition]

The chemical composition of the steel strip or wire according to the present invention contains the following elements.

C: 0.1 to 0.25%

Carbon (C) improves carburization, quenching, or carburizing. Therefore, C improves the strength of the steel. Particularly, C improves the strength of the deep portion of the mechanical parts after carburizing quenching or carburizing and nitriding quenching. On the other hand, if C is contained excessively, the deformation amount of the mechanical parts after the carburizing quenching or carbo-nitriding quenching is remarkably 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 higher than or equal to 0.15%, and still more preferably higher than or equal to 0.18%. The upper limit of the preferable C content is less than 0.25%, more preferably not more than 0.23%, and still more preferably not more than 0.20%.

Si: 0.30 to 0.60%

Silicon (Si) improves the hardenability of steel. Si also increases the temper softening resistance of the steel. Therefore, Si increases the strength and wear resistance at the surface of steel. On the other hand, if Si is contained excessively, the strength of the steel after hot forging becomes excessively high. As a result, the machinability of the steel decreases. If Si is contained excessively, the bending fatigue strength also decreases. Therefore, the Si content is 0.30 to 0.60%. The lower limit of the Si content is preferably higher than 0.30%, more preferably 0.40% or higher, and still more preferably 0.45% or higher. The upper limit of the preferable Si content is less than 0.60%, more preferably not more than 0.57%, still more preferably not more than 0.55%.

Mn: 0.50 to 1.0%

Manganese (Mn) improves the hardenability of steel and increases the strength of steel. Therefore, Mn enhances the strength of the deep portion of the carburizing quenched or carburized nitrided machine parts. On the other hand, when Mn is contained excessively, the machinability of steel after hot forging is lowered. Further, when Mn is contained excessively, Mn oxide is generated on the surface of the steel. As a result, the depth of the carburization abnormal layer after carburization quenching or carburizing nitriding becomes large. The abnormal carburization layer is, for example, a grain boundary oxide layer and an incomplete quench-hardened layer. When the depth of the carburization abnormal layer is increased, the bending fatigue strength and the pitching strength of the steel decrease. Pitching is one of the forms of failure of surface fatigue. Therefore, if the pitching strength is low, the strength is also lowered by cotton. Therefore, the Mn content is 0.50 to 1.0%. The lower limit of the preferable Mn content is higher than 0.50%, more preferably 0.55% or higher, and still more preferably 0.60% or higher. The upper limit of the Mn content is preferably less than 1.0%, more preferably 0.95% or less, and still more preferably 0.9% or less.

S: 0.003 to 0.05%

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

Cr: 1.50 to 2.00%

Chromium (Cr) improves the hardenability of the steel and the softening resistance of the steel. Therefore, Cr improves the bending fatigue strength of the steel, the strength and the wear resistance of the steel. On the other hand, if Cr is contained excessively, generation of bainite is promoted in the steel after hot forging or after quenching. As a result, the machinability of the steel decreases. 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 higher, and still more preferably 1.80% or higher. The upper limit of the preferable Cr content is less than 2.00%, more preferably not more than 1.95%, and further preferably not more than 1.90%.

Mo: 0.10% or less (including 0%)

Molybdenum (Mo) may or may not be contained. Mo improves the hardenability of the steel and the softening resistance of the steel. Therefore, Mo improves the bending fatigue strength of the steel, the strength and the wear resistance by the cotton. On the other hand, when Mo is contained excessively, bainite formation is promoted in the steel after hot forging, or after quenching. As a result, the machinability of the steel decreases. Therefore, the Mo content is 0.10% or less (including 0%). The lower limit of the preferable Mo content is 0.02% or more. The upper limit of the preferable Mo content is less than 0.10%, more preferably not more than 0.08%, still more preferably not more than 0.05%.

Al: 0.025 to 0.05%

Aluminum (Al) deoxidizes the steel. Al also combines with N to form AlN. AlN suppresses coarsening of austenite grains by carburizing heating. On the other hand, if Al is contained excessively, a coarse Al oxide is formed. The coarse Al oxide lowers the bending fatigue strength of the 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 higher, and still more preferably 0.030% or higher. 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 grains by carburization heating. On the other hand, if N is contained excessively, it becomes difficult to stably produce steel in the steelmaking process. Therefore, the N content is 0.010 to 0.025%. The lower limit of the preferable N content is higher than 0.010%, more preferably 0.012% or higher, and still more preferably 0.013% or higher. The upper limit of the preferable N content is less than 0.025%, more preferably 0.020% or less, and more preferably 0.018% or less.

The balance of the chemical composition of the bar or wire according to the present invention is composed of Fe and impurities. The impurities in the present specification mean elements incorporated from ore or scrap used as a raw material for steel or from the environment of the manufacturing process. In the present invention, the content of P, Ti and O (oxygen) as impurities is limited as follows.

P: not more than 0.025%

Phosphorus (P) segregates at grain boundaries and brittle grain boundaries. Therefore, P lowers the fatigue strength of the steel. Therefore, it is preferable that the P content is as low as possible. The P content is 0.025% or less. The preferable P content is less than 0.025%, more preferably 0.020% or less.

Ti: not more than 0.003%

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

O (oxygen): not more than 0.002%

Oxygen (O) combines with Al to form oxide inclusions. The oxide inclusions lower the bending fatigue strength of the steel. Therefore, the O content is preferably as low as possible. O content is 0.002% or less. The preferred O content is less than 0.002%, more preferably 0.001% or less.

The chemical composition of the bar or wire according to the present invention also satisfies the formula (2).

1.60? Cr + 2 占 Mo? 2.10 (2)

Here, the content (mass%) of the corresponding element is substituted into the symbol of the element in the formula (2).

As described above, Cr and Mo together increase the hardenability of the steel and the softening resistance of the steel. Therefore, Cr and Mo increase the bending fatigue strength, the strength and the wear resistance of the steel. Comparing Mo and Cr, Mo shows an effect equivalent to that of Cr (improvement in bending fatigue strength, strength in strength and wear resistance) in a content of half of Cr. Therefore, fn1 = Cr + 2Mo is defined. 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 or more of the bending fatigue strength of the steel, the strength and the wear resistance of the steel are lowered. On the other hand, if fn1 exceeds 2.10, the generation of bainite is promoted in the steel after hot forging or after hot rolling. As a result, the machinability of the steel decreases. When fn1 is in the range of 1.60 to 2.10, the bending fatigue strength of the steel, the strength and the wear resistance of the steel can be increased while suppressing the lowering of the machinability of the steel. The preferable lower limit of fn1 is 1.80 or more. The preferable upper limit of fn1 is less than 2.00.

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

Nb: not more than 0.08%

Niobium (Nb) is a selective element. Nb combines with C, N to form Nb carbide, Nb nitride or Nb carbonitride. Nb carbide, Nb nitride and Nb carbonitride suppress the coarsening of the austenite grains during the carburization heating, like Al nitride. When a small amount of Nb is contained, the above effect can be obtained. On the other hand, when Nb is contained excessively, Nb carbonitride, Nb nitride and Nb carbonitride are coarsened. Therefore, coarsening of the austenite grains can not be suppressed at the time of carburization heating. Therefore, the Nb content is 0.08% or less. The lower limit of the preferable Nb content is 0.01% or more. The upper limit of the preferable Nb content is less than 0.08%, more preferably not more than 0.05%.

[Micro-organization]

The microstructure of the bar or wire according to the present invention is composed of 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 the matrix (parent phase) is composed of ferrite and pearlite. "Ferrite / pearlite bainite structure" means a three-phase structure in which the matrix is composed of ferrite, pearlite and bainite. "Ferrite bainite structure" means a two-phase structure in which the matrix is composed of ferrite and bainite.

In short, the microstructure of the bar or wire according to the present invention does not contain martensite. Martensite is hard and lowers the ductility of steel. Therefore, when the bar or wire containing martensite is conveyed or when it is calibrated, cracks tend to occur in the bar or wire. Since the microstructure of the bar steel or wire rod according to the present invention does not contain martensite, cracking is less likely to occur at the time of calibration or transportation.

Each of the images described above is identified by the following method. A sample containing the center of a section perpendicular to the longitudinal direction of the bar or wire (cross section) is cut out. The surface (including the center portion) of the cut sample is mirror-polished. The polished surface is etched away. The corroded surface is observed under a microscope with an optical microscope at a magnification of 400 times. Specifically, the 15 field of view is arbitrarily selected from the area where the decarburized layer of the surface layer of the bar or wire rod is to be removed from the corroded surface. Then, each view is observed to identify a microstructure. If any one of the 15 field of view includes bainite, it is determined that bainite is included in the microstructure of the steel. The ferrite and pearlite are similarly judged. The size of each field of view is 250 microns (μm) × 250 microns (μm) = 62500 μm ² .

In the microstructure, on the cross section, the average ferrite ratio of ferrite defined by formula (3) is 2.0 or less.

Image analysis is carried out for each of the 15 fields described above. Specifically, the ferrite phase is identified in each field of view. The ferrite grain size in the identified ferrite phase is measured. The ferrite average grain size of each field of view is measured in accordance with the cutting method specified in JIS G0551 (2005).

15 Select the maximum value and the minimum value among the ferrite average particle diameters determined in each of the fields of view (15 in total). Then, based on the above formula (3), the ferrite average particle size ratio is calculated as (maximum value of ferrite average particle size / minimum value of ferrite average particle size).

In the case where the crystal grain size is uneven in the steel material after hot rolling (that is, hot rolled material), the crystal grain size remains uneven even after the hot forging or carburizing quenching in the subsequent step. If the crystal grain size is uneven, the strength is lowered by the bending fatigue strength and the surface texture. Therefore, it is preferable that the crystal grain size in the hot-rolled material be as uniform as possible. In order to evaluate the uniformity of the crystal grain size, it is preferable to evaluate the ferrite average grain size ratio. The ferrite grain size can be easily observed by etching as compared with pearlite or bainite. Therefore, it is easy to evaluate the degree of uniformity of the crystal grain size in the structure by examining the degree of uniformity of the ferrite mean grain size (i.e., the ferrite mean grain size ratio). Also, fatigue failure occurs with the portion having the lowest strength as a starting point. Therefore, it is appropriate to use the maximum value / minimum value of the average ferrite mean particle size as indexes to evaluate the bending fatigue strength and the strength by the surface texture, rather than using the standard deviation of the ferrite average particle size as an index.

When the microstructure is composed of various mixed structures including the ferrite described above and the ferrite average grain size ratio is 2.0 or less, the deviation of crystal grain size in the bar or wire is small. Therefore, the strength is increased by the bending fatigue strength and the surface roughness of the steel after hot forging or quenching. The ferrite average particle size ratio is preferably 1.6 or less.

On the other hand, when the average ferrite ratio exceeds 2.0, the bending fatigue strength and the strength of the steel are lowered by at least one of the strengths.

[Manufacturing method]

An example of a method of manufacturing a bar or a wire rod of the present invention and an example of a method of manufacturing a machine part represented by a toothed wheel and a pulley will be described. The production method is not limited to the following.

Molten steel having the above-described chemical composition and satisfying the formula (2) is produced. A cast (slab or bloom) is produced by continuous casting using molten steel. In the continuous casting method, the cast steel is subjected to a pressure drop during solidification. Next, the cast steel is heated. The heating temperature at this time is 1250 to 1300 占 폚, and the heating time is 10 hours or more. The heated cast slab is crushed by a crushing mill to produce billets.

The steel strip is hot-rolled to produce hot-forged steel bars or wire rods. Specifically, the steel strip is heated. The heating temperature at this time is 1150 to 1200 占 폚 and the heating time is 1.5 hours or more. Hot rolled steel strip is heated to produce bars or wire rods. The finishing temperature in hot rolling is 900 to 1000 占 폚. Water cooling is not performed before finish rolling. After the finish rolling, the bar or the wire rod is cooled at a cooling rate equal to or lower than the air cooling in the atmosphere (hereinafter simply referred to as " air cooling ") until the surface temperature becomes 600 deg. In the hot rolling, the section reduction ratio (%) defined by the formula (4) is set to 87.5% or more.

Section reduction rate = {1- (cross-sectional area of bar steel, wire rod / cross-sectional area of the billet)} 100 (4)

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

The heating temperature described above means the average value of the furnace temperature of the furnace. The heating time described above means the time of the furnace at the heating temperature described above. Finishing temperature means the surface temperature of bars and rods immediately after finish rolling. Finishing rolling means, for example, rolling in the end of a plurality of stands used for rolling in a continuous mill. The cooling rate after finishing means the surface cooling rate of the bar and wire.

An example of a method of manufacturing a mechanical part using a hot rolled steel bar or wire rod is as follows.

The hot rolled strip steel or wire rod is hot-forged to produce a coarse intermediate product. The intermediate product may be subjected to a tempering treatment. The tempering treatment is, for example, the so-called. The intermediate product is machined into a predetermined shape. Machining is, for example, cutting or drilling.

The intermediate product after machining may be subjected to surface hardening treatment. The surface hardening treatment is, for example, carburizing treatment, nitriding treatment, or high frequency quenching treatment. The intermediate product subjected to the surface hardening treatment is subjected to finishing to produce a mechanical part.

The steel bars or rods produced by the above process have excellent bending fatigue strength, strength in the face, wear resistance and excellent machinability even after hot forging.

≪ Example 1 >

Strengths A to C having the chemical compositions shown in Table 1 were dissolved in a 70-ton converter furnace.

A cast steel (bloom) of 400 mm x 300 mm was produced by continuous casting using molten steel of A to C. The prepared bloom was allowed to stand in air to 600 캜. Further, in the continuous casting step, the cast steel during the solidification was pressed down.

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

Specifically, the "heating temperature" in the column of "cast steel" in Table 2 shows the heating temperature (° C.) of the cast steel under each condition. The "heating time" column in the "casting" column of Table 2 shows heating time (minutes) for casting in each condition. Similarly, the "heating temperature" column in the "billet" column of Table 2 shows the heating temperature (° C.) of the billet in each condition. The "heating time" column in the "piece" indicates the heating time (minutes) of the steel piece under each condition. The term " water-cooling before finish rolling " in the " rolling conditions " column indicates the presence or absence of water cooling of the billet before finish rolling under each condition. &Quot; Present " in the column indicates water-cooling. &Quot; No " indicates that water cooling was not performed. The term "finishing temperature" in the "rolling conditions" column indicates the finishing temperature (° C.) in each condition. The "cooling conditions" column in the "rolling conditions" column shows cooling conditions after finish rolling in each condition.

Bars 1 to 10 shown in Table 3 were produced on the basis of the steel shown in Table 1 and the production conditions shown in Table 2.

Specifically, for each test number, the steel slabs shown in Table 3 were heated under the production conditions shown in Table 3 (heating temperature and heating time for casting). The heated cast slab was crushed and rolled to prepare a 180 mm x 180 mm steel strip. The produced billet was cooled to room temperature (25 캜).

Next, the slab was heated under the manufacturing conditions shown in Table 3 (heating temperature of the slab, heating time). The heated billet was hot-rolled under the manufacturing conditions shown in Table 3 (water-cooling, finishing temperature and cooling conditions before finish rolling) to produce bars having a diameter of 50 mm and a diameter of 70 mm. The bars after rolling were left to cool to room temperature in the air. That is, the bars were hot rolled.

[Microstructure observation test]

A bar having a diameter of 50 mm was cut perpendicular to the longitudinal direction. A sample containing the center of the section was cut out. Of the surface of the sample, the surface corresponding to the central portion described above was polished to a mirror surface. The abrasive surface was etched away. The corrosion surface was observed with an optical microscope at a magnification of 400 times at 15 o'clock. The 15 field of view was arbitrarily selected from the area from which the surface decarbonization layer was to be removed. The size of each field of view was 250 μm × 250 μm. Microstructures were observed in each field of view.

As a result of the microstructure observation test, no microstructure of any test number and martensite were included. The microstructure of each test number was either a ferrite / pearlite structure, a ferrite / pearlite / bainite structure, or a ferrite / bainite structure. &Quot; Microstructure " column in Table 3 shows the result of microstructure observation. "F + P" in the table indicates that the microstructure of the corresponding test number is a ferrite / pearlite structure. &Quot; F + P + B " indicates a ferrite / pearlite / bainite structure. &Quot; F + B " indicates a ferrite-bainite structure.

[Measurement of Ferrite Average Particle Size]

The ferrite mean grain size of the 15 field of view described above was measured in accordance with the cutting method specified in JIS G0551 (2005).

The maximum and minimum values of the ferrite mean particle diameters (15 in total) of each field of view are specified. Then, based on the formula (3), the average ferrite ratio (= maximum value / minimum value) was obtained. Table 3 shows the average perforation ratio of ferrite.

[Preparation of Cotton Strength Specimen and Bending Fatigue Strength Specimen]

The bars of each test number were heated at 1200 ° C for 30 minutes. Next, a round bar having a diameter of 35 mm was produced by hot forging at a finishing temperature of 950 캜 or higher. A round bar having a diameter of 35 mm was machined to obtain 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 rotational bending fatigue test piece having a notch shown in Fig. 2 , And the unit of dimension in the drawing is mm). The small roller test piece shown in Fig. 1 was provided with a test portion (a circumferential portion having a diameter of 26 mm and a width of 28 mm) in the center.

For each of the prepared test specimens, the carburizing furnace was used to carry out carburization quenching under the conditions shown in Fig. After quenching, tempering was carried out at 150 DEG C for 1.5 hours. The small roller test piece and the Ono type rotational bending fatigue test piece were subjected to finishing of the grip portion for the purpose of removing heat treatment deformation.

[Strength test with cotton]

In the roller pitching test, the small roller test piece described above is combined with a large roller (the unit of dimension in the drawing is mm) in the shape shown in Fig. The roller shown in Fig. 4 is made of steel satisfying the JIS standard SCM420H, and is manufactured by a general manufacturing process, that is, a step of vacancy carburization, low-temperature tempering and polishing, .

Roller pitching tests using a small roller test piece and a large roller were conducted under the conditions shown in Table 4.

As shown in Table 4, the number of revolutions of the small roller test piece was set to 1000 rpm, the sliding ratio was -40%, the contact surface pressure of the large roller and the small roller test piece during the test was 4000 MPa, and the number of repetitions was 2.0 x 10 ⁷ cycles. The slip ratio (%) was obtained from the following equation when the rotating speed of the large roller was V1 m / sec and the rotating speed of the small roller test piece was V2 m / sec.

Sliding ratio = (V2 - V1) / V2 100

During the test, a lubricant (commercially available automatic transmission oil) was sprayed from the direction opposite to the rotating direction to the contact portion (the surface of the test portion) of the large roller and the small roller test piece under the condition of the oil temperature of 90 캜. The roller pitching test was carried out under the above conditions, and the strength was evaluated by cotton.

For each test number, the number of tests in the roller pitching test was 6. After the test, an SN diagram was drawn by taking the number of repetitions until the occurrence of pitching on the abscissa and the surface pressure on the ordinate. The highest surface pressure among those in which pitching did not occur until the number of repetitions of 2.0 x 10 ⁷ was defined as the intensity in terms of the surface area of the test number. In addition, a case in which the surface area of the small roller test piece is 1 mm ² or more among the damaged portions is defined as pitching occurrence.

Table 3 shows the strength of the cotton fabric obtained by the test. In Table 3, the strength was determined as the reference value (100%) at the surface of Test No. 1 in terms of strength. The intensity of each test number was expressed by the ratio (%) with respect to the reference value. When the strength was 120% or more by cotton, it was judged that the strength was obtained with a good cotton texture.

[Abrasion Resistance Evaluation]

In the roller pitching test, the wear amount of the test portion of the small roller test piece having the number of repetitions of 1.0 x 10 ⁶ was measured. Specifically, the maximum height roughness (Rz) was obtained in accordance with JIS B0601 (2001). The smaller the Rz value, the higher the abrasion resistance. A roughness meter was used to measure the amount of wear. Table 3 shows the amount of wear. In the amount of wear in Table 3, the wear amount of test No. 1 was set as the reference value (100%). Then, the wear amount of each test number was expressed as a ratio (%) to the reference value. When the wear amount was 80% or less, it was judged that excellent abrasion resistance was obtained.

[Bending fatigue strength test]

The bending fatigue strength was obtained by Ono type rotational bending fatigue test. The number of tests in the Ono type rotary bending fatigue test was 8 for each test number. The number of revolutions at the time of the test was 3000 rpm, and otherwise, the test was carried out by a usual method. The highest stress among those that did not break up to 1.0 x 10 ⁴ times and 1.0 x 10 ⁷ times of repetition number were defined as an internal jerk and a high cycle rotational bending fatigue strength, respectively.

Table 3 shows the bending fatigue strength of the medium cycle and high cycle. The bending fatigue strength of the intermediate cycle and the high cycle of Test No. 1 was defined as the reference value (100%) at the intermediate cyclic cycle and the high cycle bending fatigue strength. Then, the bending fatigue strength of the test cycle in the intermediate cycle and the high cycle was expressed as a ratio (%) to the reference value. It was judged that excellent bending fatigue strength was obtained when the bending fatigue strength was 115% or more in both the double-cycle and high cycle.

[Cutting test]

A cutting test was conducted to evaluate the machinability. A cutting test piece was obtained by the following method. Each bar having a diameter of 70 mm in each test number was heated at a heating temperature of 1250 캜 for 30 minutes. The heated bar steel was hot-forged at a finishing temperature of 950 占 폚 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 the round bar by machining. Using a cutting test piece, a cutting test was carried out under the following conditions.

Cutting test (turning)

Tip: Base Material Carbide P20 grade grade, no coating

Condition: Main speed 200m / min, feed 0.30mm / rev, infeed 1.5mm, use water-soluble coolant

Measurement item: Main cutting edge wear amount of clearance surface after 10 minutes of cutting time

Table 3 shows the obtained main cutting edge wear amount. In Table 3, the abrasion loss of the main cutting edge of Test No. 2 (using steel B) was set as a reference value (100%). The abrasion loss of the main cutting edge of each test number was expressed as a ratio (%) to the reference value. When 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 composition (steel C) of the bars of Test Nos. 4 and 9 was within the scope of the present invention, and fn1 satisfied the formula (2). The ferrite average particle size ratios of Test Nos. 4 and 9 were all 2.0 or less. Therefore, the flexural fatigue strength of the test cycles No. 4 and 9 was higher than 115%, and the strength was 120% or more. The amount of wear was 80% or less. The main cutting edge wear amount was 80% or less. Therefore, the bars of Test Nos. 4 and 9 had excellent bending fatigue strength, strength to cotton, abrasion resistance and machinability.

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

The chemical composition (steel B) of the bar of Test No. 2 corresponded to SCM420H of JIS standard. Therefore, the Si content and the Cr content in Test No. 2 were less than the lower limit of the Si content and Cr content of the present invention. The Mo content of Test No. 2 exceeded the upper limit of the Mo content of the present invention. Also, fn1 in Test No. 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 workability was low.

The chemical composition (steel C) of Test No. 3 was within the range of the chemical composition of the present invention. Also, fn1 satisfies the expression (2). However, since the heating time of the cast steel was too short (see manufacturing condition 1 in Table 2), the average ferrite ratio exceeded 2.0. Therefore, the bending fatigue strength of the test cycle No. 3 and the high cycle was as low as less than 115%.

The chemical composition of Test No. 5 was within the range of the chemical composition of the present invention, and fn1 satisfied the formula (2). However, in Test No. 5, water cooling was carried out before finish rolling (see Production Condition 3 in Table 2). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength of the test cycle No. 5 in the intermediate cycle and the high cycle was as low as less than 115%.

The chemical composition of Test No. 6 was within the range of the chemical composition of the present invention, and fn1 satisfied the formula (2). However, in Test No. 6, the bars after finish rolling were water-cooled to 800 ° C (see Production Conditions 4 in Table 2). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength of the test cycle No. 6 and the high cycle was all less than 115%. In addition, the strength was as low as less than 120%. In addition, the abrasion resistance was lower than 80%.

The chemical composition of Test No. 7 is within the range of the chemical composition of the present invention, and fn1 also satisfies Formula (2). However, in Test No. 7, the heating time of the cast steel was too short, and the heating time of the billet was too short (see Production Condition 5). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength of the test cycle No. 7 in the intermediate cycle and the high cycle was as low as less than 115%.

The chemical composition of Test No. 8 was within the range of the chemical composition of the present invention, and fn1 satisfied the formula (2). However, in Test No. 8, the heating temperature of the billet was too high and the finishing temperature was too high (see Production Condition 6). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength of the test cycle No. 8 in the intermediate cycle and the high cycle was as low as less than 115%. In addition, the strength was as low as less than 120%. In addition, the wear amount exceeded 80%, and the abrasion resistance was low.

The chemical composition of Test No. 10 was within the range of the chemical composition of the present invention, and fn1 satisfied the formula (2). However, in Test No. 10, the heating temperature of the cast steel was too low (see Production Condition 8). Therefore, the average ferrite opening ratio exceeded 2.0. For this reason, the bending fatigue strength of the double-faced cycle was as low as less than 115%.

≪ Example 2 >

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

Bars of Test Nos. 11 to 42 shown in Table 6 were produced under the same production conditions as in Example 1. The diameter of the bar was 50 mm and 70 mm. The same test as in Example 1 was carried out using the produced steel bar. Then, the bending fatigue strength, the strength, the abrasion resistance, and the main cutting edge wear amount of the medium cycle and high cycle were respectively determined.

Table 6 shows the obtained results. Referring to Table 6, the chemical compositions of Test Nos. 17, 19, 21, 23, 31, 33, 41 and 42 were within the range of the chemical composition of the present invention, and fn1 satisfied Formula (2). In addition, the average ferrite ratio of these test numbers was 2.0 or less. For this reason, the bending fatigue strength of the test cycle in the intermediate cycle and the high cycle was 115% or more, and the strength was 120% or more in cotton. The amount of wear was 80% or less. The main cutting edge wear amount was 80% or less.

On the other hand, the Si content and Cr content of the chemical composition (steel D) of Test No. 11 were lower than the lower limit of the Si content and Cr content of the present invention. Therefore, the strength of the test piece No. 11 was less than 120%, and the wear amount was higher than 80%. Test No. 12 used the same steel D as Test No. 11. As a result, the strength and abrasion resistance of the cotton fabric were low. In Test No. 12, the heating time of the cast steel was too short (manufacturing condition 1). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the mid cycle and high cycle was as low as less than 115%.

The chemical composition (steel E) of Test No. 13 was within the range of the chemical composition of the present invention, but fn1 was less than the lower limit of the formula (2). Therefore, the bending fatigue strength of the high cycle was as low as less than 115%. Test No. 14 used the same steel E as Test No. 13. Therefore, the bending fatigue strength in the high cycle was low. In Test No. 14, water cooling was also performed before the finish rolling (manufacturing condition 3). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was lower than that of Test No. 13.

The Si content of the chemical composition (steel F) of Test No. 15 exceeded the upper limit of the Si content of the present invention. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was as low as less than 115%. In addition, the main cutting edge wear amount was higher than 80%, and the machinability was low.

Test No. 16 used the same steel F as Test No. 15. Therefore, the bending fatigue strength and machinability were low. In Test No. 16, the bars after finish rolling were water-cooled to 800 ° C (Manufacturing Condition 4). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength was lower than that of Test No. 15. Also, the strength of cotton was less than 120%, and the amount of wear was higher than 80%.

The chemical composition (strength G) of Test No. 18 was within the scope of the present invention, and fn1 satisfied Formula (2). However, the heating time of the casting was too short (manufacturing condition 1). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle was as low as less than 115%. Also, the strength was less than 120% in cotton fiber.

The chemical composition (steel H) of Test No. 20 was within the range of the present invention, and fn1 satisfied the formula (2). However, water cooling was performed before finishing rolling (manufacturing condition 3). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was as low as less than 115%.

The chemical composition (strength I) of Test No. 22 was within the scope of the present invention, and fn1 satisfied the formula (2). However, the bars after finishing rolling were water-cooled to 800 ° C (production condition 4). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was as low as less than 115%.

The chemical composition (J) of Test No. 24 was within the range of the present invention, and fn1 satisfied the formula (2). However, the heating time of the cast steel and the heating time of the billet were too short (manufacturing condition 5). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle was as low as less than 115%.

The Cr content of the chemical composition (K) of Test No. 25 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. It is considered that the Cr content is too large and bainite is excessively generated in the steel.

Test No. 26 used the same K as Test No. 25. Therefore, the machinability was low. In Test No. 26, the heating time of the cast steel and the heating time of the billet were too short (manufacturing condition 5). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was as low as less than 115%.

The Cr content of the chemical composition (steel L) of Test No. 27 was less than the lower limit of the Cr content of the present invention. Therefore, the bending fatigue strength at the mid cycle and high cycle was as low as less than 115%. In addition, the strength was also low, less than 120%.

Test No. 28 used the same steel L as Test No. 27. Therefore, the bending fatigue strength was low. In Test No. 28, the heating temperature of the billet was too high and the finishing temperature was too high (Production Condition 6). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was as low as less than 115%. In addition, the strength was also low, less than 120%.

The Mo content of the chemical composition (steel M) of Test No. 29 exceeded the upper limit of the Mo content of the present invention. Therefore, the main cutting edge wear amount of Test No. 29 exceeded 80% and the machinability was low. The Mo content is so large that bainite is excessively formed in the steel.

Test No. 30 used the same steel M as Test No. 29. Therefore, the machinability was low. In Test No. 30, the heating temperature of the cast steel was too low (Production Condition 8). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was as low as less than 115%.

The chemical composition (steel N) of Test No. 32 was within the range of the present invention, and fn1 satisfied the formula (2). However, the heating temperature and finishing temperature of the billet were too high (manufacturing condition 6). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle was as low as less than 115%.

The chemical composition (steel O) of Test No. 34 was within the range of the present invention, and fn1 satisfied the formula (2). However, the heating temperature of the cast steel was too low (production condition 8). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle was as low as less than 115%.

The Mn content and the Al content of the chemical composition (steel P) of Test No. 35 were less than the lower limit of the Mn content and Al content of the present invention. Therefore, the bending fatigue strength at the intermediate cycle was as low as less than 115%. In addition, the strength was lower than 120%.

Test No. 36 used the same steel P as Test No. 35. Therefore, the strength was low due to the flexural fatigue strength and the surface roughness at the mid-cycle. In Test No. 35, the bars after finish rolling were water-cooled to 800 ° C (Manufacturing Condition 4). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength in the high cycle was as low as less than 115%. Also, the bending fatigue strength at the intermediate cycle was lower than that of Test No. 35.

The Mn content and the Al content in the chemical composition (steel Q) of Test No. 37 exceeded the Mn content and the Al content upper limit of the present invention. Therefore, the bending fatigue strength in the high cycle was as low as less than 115%. Also, the main cutting edge wear exceeds 80%. The machinability was low.

Test No. 38 used the same steel Q as Test No. 37. Therefore, the bending fatigue strength in the high cycle was low and the workability was low. In Test No. 38, the heating temperature and finishing temperature of the billet were also too high. Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle was as low as less than 115%. The bending fatigue strength of the high cycle was lower than that of Test No. 37.

The chemical composition (steel R) of Test No. 39 is within the range of the chemical composition of the present invention, but fn1 exceeds the upper limit of the formula (2). Therefore, the machinability of the steel of Test No. 39 was low. Test No. 40 used the same steel R as Test No. 39. For this reason, the machinability of the steel of Test No. 40 was low. In Test No. 40, water cooling was also performed before rolling (Manufacturing Condition 3). Therefore, the average ferrite opening ratio exceeded 2.0. Therefore, the bending fatigue strength at the intermediate cycle and high cycle was lower than that of Test No. 39.

Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for practicing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and it is possible to appropriately modify and carry out the above-described embodiment within the scope not departing from the gist of the present invention.

Claims (3)

Chemical composition, in% by 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 being Fe and impurities,
P, Ti, and O in the impurity are, respectively,
P: 0.025% or less,
Ti: 0.003% or less,
O (oxygen): 0.002% or less,
The fn1 defined in the formula (1) is 1.80 to 2.10,
The structure is composed of a ferrite-pearlite structure, a ferrite-pearlite-bainite structure, or a ferrite-bainite structure,
And the maximum value / minimum value of the average ferrite mean particle size obtained by observation and measurement at an area of 62500 mu m ² per field of view in cross section is 2.0 or less.
fn1 = Cr + 2 占 Mo (1)
Here, the content (mass%) of the corresponding element is substituted into each symbol of the element in the formula (1). delete The method according to claim 1,
A steel strip or wire rod for hot forging containing, in mass%, 0.08% or less of Nb, instead of a part of Fe.

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