JP2020094236A - Carburized component, shaped material for carburized component, and method for producing same - Google Patents

Abstract

To provide a carburized component, a shaped material for the carburizing component, and a method for producing the same, whereby high fatigue strength can be secured even when a normalizing treatment after cold forging is omitted, and a heating temperature in a carburizing treatment is 1030°C or higher.SOLUTION: The method for producing a carburized component of the present invention includes: a blooming process of producing a billet by heating and rolling a slab to 1300°C or higher; a rolling process of heating the billet to a temperature of 800-1075°C and rolling the billet; a spheroidizing process; a cold forging process; and a carburization treatment process. The billet comprises, in terms of mass%, C: 0.13-0.23%, Si: 0.02-0.15%, Mn: 0.55-0.95%, P: 0.020% or less, S: 0.003-0.030%, Cr: 0.85-1.23%, Mo: 0.35-0.50%, Al: 0.020-0.050%, N: 0.010-0.025%, Nb: 0.010-0.060%, and the balance of Fe and impurities.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a carburized component, a raw material for the carburized component, and a method for manufacturing the same.

Mechanical components used in a power source and a power transmission mechanism such as an engine of an automobile or an industrial machine are subjected to bending stress during use due to impact load or sliding. Therefore, high fatigue strength is required for the mechanical parts for these applications. To obtain high fatigue strength, carburized parts are often used for the mechanical parts used in these applications. The carburized parts are obtained by carburizing a steel material. The carburizing process forms a carburized layer on the surface of the carburized component. This carburized layer provides high fatigue strength. The carburized layer means a region where the C content is higher than the C content of the core of the carburized component.

In order to obtain high fatigue strength, it is effective to suppress coarsening of the former austenite crystal grains generated during the carburizing treatment.

In order to suppress coarsening of the former austenite crystal grains during the carburizing treatment, there is a method of performing normalizing treatment after cold forging and before carburizing treatment. However, in order to simplify the process of manufacturing carburized parts, there is a method for manufacturing carburized parts that can suppress coarsening of old austenite crystal grains even after normalizing after cold forging and before carburizing. It has been demanded.

Japanese Unexamined Patent Application Publication No. 2011-157579 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2012-229475 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. 2012-158827 (Patent Document 3) are after cold forging and before carburizing. We propose a technology to suppress coarsening of old austenite crystal grains during carburizing without the normalizing treatment.

The hot-rolled steel bar or wire rod described in Patent Document 1 is, in mass%, C: 0.1 to 0.3%, Si: 0.05 to 1.0%, Mn: 0.4 to 2.0. %, S: 0.005-0.05%, Cr: 0.5-2.0%, Al: 0.01-0.06%, N: 0.005-0.025% and Nb: 0. 02-0.08%, the balance consisting of Fe and impurities, P, Ti and O (oxygen) in the impurities are P: 0.025% or less, Ti: 0.003% or less and O( Oxygen): has a chemical composition of 0.002% or less. In the region from the surface of the steel bar or the wire to 1/5 of the radius and the region from the center to 1/5 of the radius, the amount of Al precipitated as AlN and AlN—Nb(CN) is 0.010% or less, The amount of Nb precipitated as Nb(CN) and AlN-Nb(CN) is 0.020% or less and the total of AlN, Nb(CN) and AlN-Nb(CN) having a diameter of 100 nm or more. It has a metal structure having a number density of 50/100 μm ² or less, an area ratio of ferrite/bainite structure of 80% or more, an area ratio of bainite of 30 to 70%, and an average ferrite grain size of 15 to 40 μm. It is described in Patent Document 1 that this can suppress coarsening of austenite grains.

The hot rolled steel bars or wire rods described in Patent Document 2 are C: 0.1 to 0.3%, Si: 0.05 to 1.0%, Mn: 0.4 to 2.0 in mass%. %, S: 0.003-0.05%, Cr: 0.5-3.0%, N: 0.010-0.025% and Al: 0.02-0.05%, the balance Is Fe and impurities, and P, Ti and O (oxygen) in the impurities are P: 0.025% or less, Ti: 0.003% or less and O (oxygen): 0.002% or less, respectively. Having a composition. It has a ferrite bainite structure or a ferrite bainite pearlite structure, has a bainite structure fraction of more than 70%, has an average grain size of ferrite of 40 μm or less, has a metal structure, and has a radius of 1 from the surface of the steel bar or wire. In the region up to /5 and in the region from the center to 1/5 of the radius, the amount of Al precipitated as AlN is 0.005% or less, and the number density of AlN having a diameter of 100 nm or more is 5/100 μm ^2. It is below. It is described in Patent Document 2 that this can suppress coarsening of austenite grains.

The hot-working steel material for surface hardening described in Patent Document 3 is, by mass%, C: 0.10 to 0.30%, Si: 0.50% or less, Mn: 0.15 to 1.5%, P: 0.04% or less, S: 0.005-0.07%, Cr: 0.7-3.0%, Al: 0.01-0.05%, N: 0.007-0.030 %, Nb: 0.02 to 0.07% and H: 0.00004% or less, and the balance has a chemical composition of Fe and impurities. Among the Nb in the steel, Nb(C, N ) Is 85% or more, the number density of Nb (C, N) with a diameter of 100 nm or more is 5/100 μm ² or less, and the standard deviation of ferrite grain size is 0.15 or less. ,. It is described in Patent Document 3 that this can suppress coarsening of austenite grains.

JP, 2011-157597, A JP2012-229475A JP2012-158827A

By the way, in recent years, from the aspect of environmental regulations, reduction of carbon dioxide emissions has been demanded. Therefore, also in the carburizing treatment, treatment at high temperature and in a short time is required. However, in the technologies of Patent Documents 1 to 3, the fatigue strength may be low in the carburizing treatment at a high temperature where the heating temperature is 1030° C. or higher.

An object of the present invention is to omit carburizing treatment after cold forging and to obtain high fatigue strength even when the heating temperature in the carburizing treatment is 1030° C. or higher. To provide materials and methods for manufacturing them.

The method for manufacturing a carburized component according to the embodiment of the present invention is, in mass %, C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 to 0.95%. , P: 0.020% or less, S: 0.003 to 0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to 0. 050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25%, Pb: 0 to 0.30. %, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to 0.20%, and Zr. A slab rolling process for producing a steel slab by heating a raw material having a chemical composition of 0 to 0.10% and a balance of Fe and impurities to 1300° C. or higher and rolling, and the produced steel slab A steel bar rolling step of producing a steel bar by heating and rolling the steel to 800 to 1075° C., and a spheroidizing annealing step of performing a spheroidizing annealing on the steel bar after the steel bar rolling step to produce a base material, Perform cold forging to produce a cold forged product by performing cold forging on the base material after the spheroidizing annealing process, and perform carburizing on the cold forged product after the cold forging process And a carburizing step.

A carburized component according to an embodiment of the present invention includes a carburized layer and a core portion inside the carburized layer, and the core portion is C:0.13 to 0.23%, Si:0. 02-0.15%, Mn: 0.55-0.95%, P: 0.020% or less, S: 0.003-0.030%, Cr: 0.85-1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30% , Ni:0 to 0.25%, Pb:0 to 0.30%, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to It contains 0.010%, V:0 to 0.20%, and Zr:0 to 0.10%, and the balance has a chemical composition of Fe and impurities. In the microstructure, the largest grain having the largest grain size has a grain size number of 4.0 or more. The difference between the average grain size number and the crystal grain size number of the largest grain is 8.0 or less. The core hardness is HV 260 or higher. The amount of Nb precipitated as Nb precipitate is 60% or more of the total Nb content. The maximum diameter of the Nb precipitate is less than 230 nm.

A manufacturing method of a base material for carburized parts according to an embodiment of the present invention is C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 in mass%. -0.95%, P: 0.020% or less, S: 0.003-0.030%, Cr: 0.85-1.23%, Mo: 0.35-0.50%, Al:0 0.020 to 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25%, Pb: 0-0.30%, Ca:0-0.005%, Bi:0-0.30%, Ti:0-0.100%, B:0-0.010%, V:0-0.20 %, and Zr: 0 to 0.10%, and the balance is a slab rolling process for producing a steel slab by heating a raw material having a chemical composition of Fe and impurities to 1300° C. or higher and rolling. A steel bar rolling step of manufacturing the steel bar by heating the manufactured steel slab to 800 to 1075° C. and rolling, and a spheroidizing annealing step of performing spheroidizing annealing on the steel bar after the steel bar rolling step.

The material for carburizing parts according to the embodiment of the present invention is, in mass %, C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 to 0. 95%, P: 0.020% or less, S: 0.003 to 0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to. 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25%, Pb: 0 to 0 .30%, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to 0.20%, and , Zr: 0 to 0.10%, and the balance has a chemical composition of Fe and impurities. The amount of Nb precipitated as Nb precipitates is 80% or more of the total Nb content, and the maximum diameter of Nb precipitates is less than 200 nm.

According to the carburized component, the base material for the carburized component, and the manufacturing method thereof according to the present invention, the normalizing treatment after cold forging is omitted, and the heating temperature in the carburizing treatment is 1030° C. or more. However, high fatigue strength can be obtained.

FIG. 1 is a flow chart showing a method of manufacturing a carburized component. FIG. 2 is a side view of the Ono-type rotary bending fatigue test piece produced in the example.

As described above, in order to obtain high fatigue strength, it is effective to suppress coarsening of the former austenite crystal grains during the carburizing treatment. However, when the heating temperature in the carburizing treatment is 1030° C. or higher, the former austenite crystal grains are more likely to be coarsened than in the case of the carburizing treatment below 1030° C. Therefore, the present inventors omitted various normalizing treatments after cold forging, and conducted various studies on a method of obtaining high fatigue strength even when the heating temperature in carburizing treatment was 1030° C. or higher, The following findings were obtained.

The inventors first manufactured carburized parts by performing the conventional manufacturing method on the steel materials of Patent Documents 1 to 3. FIG. 1 is a flow chart showing a method of manufacturing a carburized component. The conventional manufacturing method is a manufacturing method performed under the conventional manufacturing conditions using the manufacturing method of FIG. More specifically, steel materials having the chemical compositions of Patent Documents 1 to 3 were prepared. The material was heated at 1250°C for 360 minutes. The heated raw material was rolled by a reverse slab mill, and after rolling by the reverse slab mill, the raw material was rolled by a tandem rolling mill train without being heated again. A slab was manufactured by the above slab rolling (slab rolling step: S1). The produced steel slab was heated at 1100° C. for 50 minutes. A steel bar was manufactured by rolling the heated steel slab (steel bar rolling step: S2). The manufactured steel bar was held at 740° C. for 4 hours, cooled at 0.25° C./sec to 660° C., and then allowed to cool, and spheroidizing annealing was performed (spheroidizing annealing step: S3). The steel bar after the spheroidizing annealing was subjected to drawing that simulates the cold forging process as described in Examples below (cold forging process: S4). An Ono-type rotary bending fatigue test piece described below was produced from the intermediate product after drawing. The Ono-type rotary bending fatigue test piece was subjected to carburizing treatment at 1030° C. to manufacture a test piece simulating a carburized part (carburizing step: S5). The fatigue strength of these test pieces was investigated.

Among the test pieces subjected to the carburizing treatment at the heating temperature of 1030° C., the microstructure of the test piece having the low fatigue strength was observed. Then, in the microstructures of these test pieces having low fatigue strength, it was found that the crystal grain size number of the maximum grain, which is the crystal grain having the maximum prior austenite crystal grain size, was less than 4.0 in the observation visual field. .. Furthermore, in the microstructure of carburized parts with low fatigue strength, it was found that the austenite crystal grains were a mixture of coarse grains and fine grains.

The reason why the grain size number of the largest grain is less than 4.0 and the fatigue strength decreases when coarse grains and fine grains coexist is considered as follows. The fatigue strength of carburized parts correlates with the grain size of prior austenite grains. Therefore, if coarse grains and fine grains are mixed, that is, if the grain size of the former austenite crystal grains is varied, a locally low strength portion occurs, and the fracture starts from the low strength portion. Occurs. Coexistence of coarse grains and fine grains occurs particularly when the maximum grain size number is less than 4.0 and coarse old austenite grains are generated.

Therefore, the present inventors have further studied a method for suppressing the variation in the grain size of the prior austenite crystal grains (hereinafter, also referred to as grain size variation) in the structure of the carburized component.

The present inventors first focused on a precipitate having a pinning effect. The precipitates are, for example, Al nitride (AlN), Nb carbide (NbC), Nb nitride (NbN), Nb carbonitride (Nb(CN)), and NbC, NbN, NbCN, and AlN are compounded. It is a precipitate (AlN-Nb(CN)). Among these precipitates, NbC, NbN, Nb(CN), and AlN-Nb(CN) are those that can effectively obtain the pinning effect in the chemical composition of the carburized component according to the embodiment of the present invention. .. Hereinafter, NbC, NbN, Nb(CN), and AlN-Nb(CN) are referred to as Nb precipitates. In the carburized component according to the embodiment of the present invention, the proportion of Nb precipitates in all the precipitates is preferably 60 to 100% by mass.

In the prior art, the inventors of the present invention have the above-mentioned precipitates as the cause of the particle size variation when performing the carburizing treatment at a heating temperature of 1030° C. or higher, and the number of fine precipitates is insufficient. I thought it was because of it.

For example, in Patent Document 1 and Patent Document 2, the heating temperature in the slab rolling process is set low, and the heating temperature in the bar rolling process is set high. In this case, the precipitates may not be sufficiently melted in the slab rolling process, and the unmelted precipitates during the bar rolling process may grow and become coarse. These unmelted precipitates may be further coarsened during spheroidizing annealing. As a result, even if there is no problem under the conventional carburizing treatment conditions, if the 70% or more strong working is performed in the cold working process and the carburizing treatment with the heating temperature of 1030° C. or more is carried out, coarse old austenite grains May occur, resulting in variation in particle size.

In Patent Document 3, spheroidizing annealing is not performed after rolling the steel bar and before cold forging. If the spheroidizing annealing is not performed after the steel bar is rolled and before cold forging, it is considered that a sufficient number of precipitates cannot be obtained before the carburizing treatment because precipitates cannot be precipitated during the spheroidizing annealing. As a result, even if there is no problem under the conventional carburizing treatment conditions, when the carburizing treatment at a heating temperature of 1030° C. or higher is performed, coarse old austenite grains are generated and grain size variation occurs.

Therefore, the present inventors have studied a method of uniformly depositing a larger amount of deposits than the conventional ones without coarsely growing them. In order to finely precipitate more precipitates than in the past, the present inventors considered to make precipitates fine and uniform not only during spheroidizing annealing but also during hot rolling.

However, if precipitates are deposited during the slab rolling process, the precipitates grow during the steel bar rolling process and are likely to become coarse. Coarse precipitates cause the generation of coarse former austenite grains and reduce fatigue strength. Therefore, it is necessary to deposit more fine precipitates more uniformly than in the past.

Hot rolling includes a slab rolling process and a steel bar rolling process. Therefore, during hot rolling, the present inventors set the heating temperature in the slab rolling process to a high temperature to cause most of the precipitates to form a solid solution, and the heating temperature in the steel bar rolling process to be a low temperature. It was considered that a large amount of various precipitates were uniformly deposited. More specifically, by raising the heating temperature in the slab rolling process to a high temperature, most of the precipitates form a solid solution, so that fine precipitates can be deposited during the bar rolling process. By setting the heating temperature during the steel bar rolling step to a temperature not lower than the precipitation temperature of the precipitates and at a temperature at which the precipitates do not grow coarsely, a large amount of precipitates are deposited and fine precipitates are uniformly deposited. be able to.

Therefore, as an example, a carburized component was manufactured under the following manufacturing conditions by the manufacturing method shown in FIG. Specifically, a steel material having the chemical composition of test number 1 in the examples described below was prepared. The material was heated at 1355°C for 360 minutes. The heated raw material was rolled by a reverse slab mill, and after rolling by the reverse slab mill, the raw material was rolled by a tandem rolling mill train without being heated again. A slab was manufactured by the above slab rolling (slab rolling step: S1). The manufactured steel slab was heated at 1005° C. for 30 minutes. A steel bar was manufactured by rolling the heated steel slab (steel bar rolling step: S2). The manufactured steel bar was held at 760° C. for 4 hours, cooled to 660° C. at 0.25° C./sec, and then allowed to cool, and spheroidizing annealing was performed (spheroidizing annealing step: S3). The steel bar after the spheroidizing annealing was subjected to drawing that simulated the cold forging process (cold forging process: S4). The intermediate product after drawing was processed into a test piece and carburized at 1030° C. to manufacture a test piece simulating a carburized part (carburizing step: S5). The microstructure of the obtained test piece was observed by the method described below.

As a result, in the carburized part where the heating temperature during slab rolling was 1355° C., the heating temperature during bar rolling was 1005° C., and spheroidizing annealing was performed after hot rolling and before cold forging, The prior austenite grains became relatively uniform as compared to the carburized parts produced by the conventional production method. That is, the variation in particle size was reduced. Further, the average crystal number of the old austenite crystal grains was 11.2, which was finer than that of the conventional carburized parts.

Therefore, the fatigue strength was investigated by the Ono-type rotary bending fatigue test described below. As a result, the fatigue strength of the test piece having fine austenite crystal grains and small grain size variation was higher than that of the test piece having coarse old austenite grains and large grain size variation.

As a result of various investigations by changing the heating temperature at the time of slab rolling and the heating temperature at the time of bar steel rolling, the present inventors have set the heating temperature at the time of slab rolling to 1300° C. or higher during hot rolling, It has been found that by setting the heating temperature at the time of bar wire rolling to 800 to 1075° C., a large amount of fine precipitates can be deposited, and as a result, variation in grain size of carburized parts can be suppressed. More specifically, if the heating temperature at the time of slab rolling is a high temperature of 1300° C. or higher, it is possible to produce a steel slab in which precipitates are sufficiently dissolved. By performing bar steel rolling on the manufactured steel billet at a low heating temperature of 800 to 1075° C., fine precipitates are uniformly deposited. Further, if the temperature during rolling of the steel bar is 800 to 1075° C., the precipitates deposited during heating do not grow coarsely.

Further, if spheroidizing annealing is performed after hot rolling and before cold forging, a large amount of Nb in the steel can be finely precipitated as Nb precipitates. A sufficient pinning effect can be obtained by these precipitates. As a result, it is possible to suppress the generation of coarse former austenite grains and suppress the variation in grain size. This can increase the fatigue strength of the carburized component.

In the embodiment of the present invention, the large variation of the prior austenite crystal grain size means that the difference between the average grain size number and the grain size number of the maximum grain exceeds 8.0.

The method for manufacturing a carburized component according to the embodiment of the present invention completed based on the above findings is C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn:% by mass. 0.55 to 0.95%, P: 0.020% or less, S: 0.003 to 0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25% , Pb:0 to 0.30%, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to An agglomerate containing 0.20% and Zr: 0 to 0.10%, with the balance being a material having a chemical composition consisting of Fe and impurities, heated to 1300° C. or higher and rolled to produce a steel slab. A rolling process, a steel bar rolling process for manufacturing a steel bar by heating and rolling the manufactured steel slab to 800 to 1075° C., and spheroidizing annealing is performed on the steel bar after the steel bar rolling process to form a raw material. The spheroidizing annealing process to be manufactured, a cold forging process in which cold forging is performed by performing cold forging on the raw material after the spheroidizing annealing process, and a cold forging product after the cold forging process. And a carburizing process for carrying out carburizing treatment.

With the above-described manufacturing method, in the carburized component according to the embodiment of the present invention, it is possible to finely and uniformly deposit more precipitates than in the conventional case. As a result, the normalizing treatment after cold forging is omitted, and even when the heating temperature in the carburizing treatment is 1030° C. or higher, generation of coarse former austenite grains is suppressed, variation in grain size is suppressed, and high. Fatigue strength can be obtained.

The chemical composition of the material of the carburized parts is Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, Pb: 0.01 to 0.30%, Ca: 0.0003 to. 0.005%, Bi: 0.01 to 0.30%, Ti: 0.005 to 0.100%, B: 0.0001 to 0.010%, V: 0.005 to 0.20%, and , Zr: 0.0003 to 0.10%, and may contain one or more selected from the group consisting of.

A carburized component according to an embodiment of the present invention includes a carburized layer and a core portion inside the carburized layer, and the core portion is C:0.13 to 0.23%, Si:0. 02-0.15%, Mn: 0.55-0.95%, P: 0.020% or less, S: 0.003-0.030%, Cr: 0.85-1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30% , Ni:0 to 0.25%, Pb:0 to 0.30%, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to It contains 0.010%, V:0 to 0.20%, and Zr:0 to 0.10%, and the balance has a chemical composition of Fe and impurities. In the microstructure, the largest grain having the largest grain size has a grain size number of 4.0 or more. The difference between the average grain size number and the crystal grain size number of the largest grain is 8.0 or less. The core hardness is HV 260 or higher. The amount of Nb precipitated as Nb precipitate is 60% or more of the total Nb content. The maximum diameter of the Nb precipitate is less than 230 nm.

The chemical composition of the core of the carburized component is as follows: Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, Pb: 0.01 to 0.30%, Ca: 0.0003. -0.005%, Bi: 0.01-0.30%, Ti: 0.005-0.100%, B: 0.0001-0.010%, V: 0.005-0.20%, And, one or more selected from the group consisting of Zr: 0.0003 to 0.10% may be contained.

A manufacturing method of a base material for carburized parts according to an embodiment of the present invention is C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 in mass%. -0.95%, P: 0.020% or less, S: 0.003-0.030%, Cr: 0.85-1.23%, Mo: 0.35-0.50%, Al:0 0.020 to 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25%, Pb: 0-0.30%, Ca:0-0.005%, Bi:0-0.30%, Ti:0-0.100%, B:0-0.010%, V:0-0.20 %, and Zr: 0 to 0.10%, and the balance is a slab rolling process for producing a steel slab by heating a raw material having a chemical composition of Fe and impurities to 1300° C. or higher and rolling. A steel bar rolling step of manufacturing the steel bar by heating the manufactured steel slab to 800 to 1075° C. and rolling, and a spheroidizing annealing step of performing spheroidizing annealing on the steel bar after the steel bar rolling step.

Here, the raw material for carburized parts is a material that has undergone a slabbing rolling process, a steel bar rolling process, and a spheroidizing annealing process, and is a cold forging process and a carburizing process. There is nothing.

The chemical composition of the raw material for the carburized parts is Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, Pb: 0.01 to 0.30%, Ca. : 0.0003 to 0.005%, Bi: 0.01 to 0.30%, Ti: 0.005 to 0.100%, B: 0.0001 to 0.010%, V: 0.005 to 0 20% and Zr: 0.0003 to 0.10%, and may contain one kind or two or more kinds selected from the group consisting of.

The material for carburizing parts according to the embodiment of the present invention is, in mass %, C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 to 0. 95%, P: 0.020% or less, S: 0.003 to 0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to. 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25%, Pb: 0 to 0 .30%, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to 0.20%, and , Zr: 0 to 0.10%, and the balance has a chemical composition of Fe and impurities. The amount of Nb precipitated as Nb precipitates is 80% or more of the total Nb content, and the maximum diameter of Nb precipitates is less than 200 nm.

The chemical composition of the material for the carburized parts is as follows: Cu: 0.02-0.30%, Ni: 0.02-0.25%, Pb: 0.01-0.30%, Ca:0. 0.0003 to 0.005%, Bi: 0.01 to 0.30%, Ti: 0.005 to 0.100%, B: 0.0001 to 0.010%, V: 0.005 to 0.20. %, and Zr: 0.0003 to 0.10%, and may contain one kind or two or more kinds selected from the group consisting of.

Hereinafter, the material for carburizing parts and the carburizing parts of the present invention will be described in detail. In the following description, “%” of the content of each element means “mass %” unless otherwise specified.

[Material for carburizing parts]
A cast material for carburized parts is a material that has undergone a slabbing rolling process, a steel bar rolling process and a spheroidizing annealing process, but has not undergone a cold forging process and a carburizing process. is there.

[Chemical composition]
The cast material for carburized parts of the present invention has a chemical composition containing the following elements.

C: 0.13 to 0.23%
Carbon (C) secures the hardness of the core of the component when carburized and quenched, and enhances the fatigue strength. If the C content is less than 0.13%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.23%, the deformation resistance increases and the cold forgeability of the steel material remarkably decreases. Therefore, the C content is 0.13 to 0.23%. The preferable lower limit of the C content is 0.14%. The preferable upper limit of the C content is 0.22%.

Si: 0.02-0.15%
Silicon (Si) enhances hardenability. If the Si content is less than 0.02%, this effect cannot be obtained. On the other hand, if the content exceeds 0.15%, the deformation resistance becomes high and the cold forgeability of the steel material deteriorates. Therefore, the Si content is 0.02 to 0.15%. When enhancing hardenability, the preferable lower limit of the Si content is 0.03%. The preferable upper limit of the Si content is 0.14%.

Mn: 0.55 to 0.95%
Manganese (Mn) enhances hardenability and temper softening resistance. Mn further enhances fatigue strength. If the Mn content is less than 0.55%, these effects cannot be obtained. On the other hand, if the Mn content exceeds 0.95%, not only the effect of increasing the fatigue strength is saturated, but also the deformation resistance increases and the cold forgeability of the steel material deteriorates. Therefore, the Mn content is 0.55 to 0.95%. The preferable lower limit of the Mn content is 0.60%. The preferable upper limit of the Mn content is 0.90%.

P: 0.020% or less Phosphorus (P) is an impurity and is unavoidably contained in the cast material. That is, the lower limit of the P content is more than 0%. P easily segregates at the grain boundaries and makes the grain boundaries brittle. If the P content exceeds 0.020%, the fatigue strength decreases. Therefore, the P content is 0.020% or less. The preferable upper limit of the P content is 0.015%. It is preferable that the P content is as low as possible. Therefore, the lower limit of the P content is not particularly limited. However, in actual operation, the manufacturing cost becomes excessively high to reduce the P content to less than 0.002%. Therefore, the preferable lower limit of the P content is 0.002%.

S: 0.003 to 0.030%
Sulfur (S) combines with Mn to form MnS and enhances the machinability of the steel material. However, if the S content exceeds 0.030%, coarse MnS is produced. Coarse MnS becomes a starting point of cracking, so that the fatigue strength of carburized parts is reduced. Therefore, the S content is 0.003 to 0.030%. Further, if the S content is too low, the machinability is deteriorated. Therefore, the preferable lower limit of the S content is 0.005%. The preferable upper limit of the S content is 0.025%.

Cr: 0.85 to 1.23%
Chromium (Cr) enhances hardenability and temper softening resistance. As a result, the fatigue strength of carburized parts is increased. If the Cr content is less than 0.85%, this effect cannot be obtained. On the other hand, if the Cr content exceeds 1.23%, not only the effect of increasing fatigue strength saturates but also the deformation resistance increases. As a result, the cold forgeability of the steel material deteriorates. Therefore, the Cr content is 0.85 to 1.23%. The preferable lower limit of the Cr content is 0.90%. The preferable upper limit of the Cr content is 1.22%.

Mo: 0.35-0.50%
Molybdenum (Mo) enhances hardenability. Mo further increases the temper softening resistance. As a result, the fatigue strength of carburized parts is increased. If the Mo content is less than 0.35%, this effect cannot be obtained. On the other hand, when the Mo content exceeds 0.50%, not only the effect of increasing fatigue strength saturates but also the deformation resistance increases. As a result, the cold forgeability of the steel material is significantly reduced. Therefore, the Mo content is 0.35 to 0.50%. The preferable lower limit of the Mo content is 0.36%. The preferable upper limit of the Mo content is 0.45%.

Al: 0.020 to 0.050%
Aluminum (Al) deoxidizes steel. Al further combines with N to form AlN. As a result, coarsening of the former austenite crystal grains is suppressed during the carburizing process. If the Al content is less than 0.020%, these effects cannot be obtained. On the other hand, if the Al content exceeds 0.050%, coarse AlN is generated in the steel material, and the old austenite crystal grains are rather coarsened. Therefore, the Al content is 0.020 to 0.050%. The preferable lower limit of the Al content is 0.021%, more preferably 0.022%. The preferable upper limit of the Al content is 0.045%, more preferably 0.040%. The Al content in the present embodiment means the total Al content in the steel.

N: 0.010 to 0.025%
Nitrogen (N) combines with Al, Nb, V and Ti to form AlN, NbN, VN and TiN. These precipitates suppress coarsening of the former austenite crystal grains. If the N content is less than 0.010%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.025%, a flaw is generated on the surface of the rolled material, especially in the hot rolling step, and stable mass production becomes difficult. Therefore, the N content is 0.010 to 0.025%. The preferable lower limit of the N content is more than 0.011%, and more preferably 0.012%. The preferable upper limit of the N content is 0.024%, more preferably 0.023%.

Nb: 0.010 to 0.060%
Niobium (Nb) combines with C and N to form precipitates NbC, NbN and Nb(CN). These precipitates prevent coarsening of prior austenite grains. If the Nb content is less than 0.010%, this effect cannot be obtained. On the other hand, when the Nb content exceeds 0.060%, the precipitate becomes coarse. As a result, the austenite crystal grains become coarse. Therefore, the Nb content is 0.010 to 0.060%. The preferable lower limit of the Nb content is 0.015%, more preferably 0.020%. The preferable upper limit of the Nb content is 0.058%, and more preferably 0.055%.

The balance of the chemical composition of the casting for carburized parts according to the invention consists of Fe and impurities. Here, the impurities, when industrially manufacturing the raw material for carburized parts according to the embodiment of the present invention, ore as a raw material, scrap, or those that are mixed from the manufacturing environment, It means a material that is allowed within a range that does not have a significant adverse effect on cold forgeability and fatigue strength when a base material for carburized parts according to an embodiment of the present invention is manufactured.

[About arbitrary elements]
The chemical composition of the base material for carburized parts according to the embodiment of the present invention is further selected from the group consisting of Cu, Ni, Pb, Ca, Bi, Ti, B, V and Zr in place of part of Fe. One or two or more of the above may be contained. These elements are arbitrary elements.

Cu: 0 to 0.30%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances hardenability and fatigue strength. This effect can be obtained if Cu is contained at least. However, if the Cu content exceeds 0.30%, not only the above effect is saturated, but also the deformation resistance increases. As a result, the cold forgeability of the steel material deteriorates. Therefore, the Cu content is 0 to 0.30%. The preferable lower limit of the Cu content for obtaining the above effect stably is 0.02%, and more preferably 0.05%. The preferable upper limit of the Cu content is 0.20%.

Ni: 0 to 0.25%
Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When included, Ni enhances hardenability and fatigue strength. This effect can be obtained if Ni is contained in any amount. However, when the Ni content exceeds 0.25%, not only the above effects are saturated, but also the deformation resistance increases. As a result, the cold forgeability of the steel material deteriorates. Therefore, the Ni content is 0 to 0.25%. A preferable lower limit of the Ni content for obtaining the above effect stably is 0.02%, and more preferably 0.05%. The preferable upper limit of the Ni content is 0.20%.

Pb: 0 to 0.30%
Lead (Pb) is an optional element and may not be contained. That is, the Pb content may be 0%. When contained, Pb enhances machinability. For this reason, when the machinability is more important than the fatigue strength when it is desired to finish the inner surface or the like of the cold-formed part by more precise cutting, the content may be included as necessary. This effect can be obtained if Pb is contained even a little. However, if the Pb content exceeds 0.30%, the fatigue strength is significantly reduced. Therefore, the Pb content is 0 to 0.30%. A preferable lower limit of the Pb content for stably obtaining the above effect is 0.01%, and more preferably 0.02%. The preferable upper limit of the Pb content is 0.25%.

Ca: 0 to 0.005%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca enhances machinability like Pb. For this reason, like Pb, if the machinability is more important than the fatigue strength, such as when it is desired to finish the inner surface of a cold-formed part by more precise cutting, it should be contained as necessary. Good. This effect can be obtained if even a small amount of Ca is contained. However, if the Ca content exceeds 0.005%, coarse oxides are formed and the fatigue strength is reduced. Therefore, the Ca content is 0 to 0.005%. The preferable lower limit of the Ca content for obtaining the above effect stably is 0.0003%, and more preferably 0.0005%. The preferable upper limit of the Ca content is 0.004%.

Bi: 0 to 0.30%
Bismuth (Bi) is an optional element and may not be contained. That is, the Bi content may be 0%. When contained, Bi enhances machinability like Pb and Ca. For this reason, like Pb and Ca, if necessary, if the machinability is more important than the fatigue strength, such as when finishing the inner surface of a cold-formed component by further precision cutting, etc. You may let me. This effect can be obtained if Bi is contained at all. However, if the Bi content exceeds 0.30%, the fatigue strength of the carburized parts is reduced. Therefore, the Bi content is 0 to 0.30%. A preferable lower limit of the Bi content for stably obtaining the above effect is 0.01%, and more preferably 0.02%. The preferable upper limit of the Bi content is 0.25%.

Ti: 0 to 0.100%
Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When included, Ti forms carbonitrides with C and N. The pinning action of carbonitride suppresses coarsening of former austenite crystal grains. This effect can be obtained if Ti is contained at all. However, if the Ti content exceeds 0.100%, the above effect is saturated and the manufacturing cost increases. If the Ti content exceeds 0.100%, the hardenability is further reduced, and the fatigue strength of the carburized component is reduced. Therefore, the Ti content is 0 to 0.100%. A preferable lower limit of the Ti content for obtaining the above effect stably is 0.005%, and more preferably 0.010%. The preferable upper limit of the Ti content is 0.09%.

B: 0 to 0.010%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B enhances the hardenability of steel. This effect can be obtained if B is contained in any amount. However, if the B content exceeds 0.010%, the effect is saturated and the cost increases. If the B content exceeds 0.010%, the hardenability further deteriorates. As a result, fatigue strength decreases. A preferable lower limit of the B content for obtaining the above effects stably is 0.0001%, and more preferably 0.0003%. The preferable upper limit of the B content is 0.008%.

V: 0 to 0.20%
Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When included, V enhances hardenability and temper softening resistance. This effect can be obtained if V is contained in any amount. However, if the V content exceeds 0.20%, the strength of the steel material before cold forging becomes too high, and the cold forgeability of the steel material deteriorates. Therefore, the V content is 0 to 0.20%. The preferable lower limit of the V content for stably obtaining the above effect is 0.005%, and more preferably 0.010%. The preferable upper limit of the V content is 0.15%.

Zr: 0 to 0.10%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When included, Zr combines with C and/or N to form fine carbides, nitrides and carbonitrides. As a result, the austenite crystal grains are refined. This effect can be obtained if Zr is contained even in a small amount. However, if the Zr content exceeds 0.10%, the strength of the steel material before cold forging becomes too high, and the cold forgeability of the steel material deteriorates. Therefore, the Zr content is 0 to 0.10%. The preferable lower limit of the V content for stably obtaining the above effect is 0.0003%, and more preferably 0.0005%. The preferable upper limit of the Zr content is 0.07%.

[About precipitate]
In the base material for carburized parts according to the embodiment of the present invention, the amount of Nb precipitated as Nb precipitates is 80% or more of the total Nb content, and the maximum diameter of Nb precipitates is less than 200 nm.

The ratio of the amount of Nb precipitated as Nb precipitate to the total Nb content is measured as follows. The ratio of the amount of Nb precipitated as Nb precipitates to the total Nb content is the Nb precipitation rate. The amount of Nb deposited is determined by the following extraction residue analysis method. A sample of 10 mm×10 mm×10 mm is cut out from the raw material for carburized parts and used as an extraction residue analysis sample.

The sample for extraction residue analysis is electrolyzed in solution using common conditions. For example, the conditions of electrolysis are: current density: 250 to 350 A/m ² , time: 120 minutes, room temperature (25° C.). The solution is a 10% AA-based (liquid in which tetramethylammonium chloride, acetylacetone, and methanol are mixed at 1:10:100) solution. The electrolyzed extraction residue analysis sample is taken out and suction filtered with a filter. The mesh size of the filter is 0.2 μm. This collects the residue. Even if a 0.2 μm filter is used, the filter may be clogged with precipitates during the filtration process, so that it is actually possible to extract fine precipitates of 0.2 μm or less.

The residue collected on the above filter is analyzed by general chemical analysis to determine the Nb deposition amount (mass %). For example, the amount of Nb precipitation is determined by the ICP emission spectroscopy analysis method defined in JIS G 1258-4 (2007).

The obtained Nb deposition amount is divided by the Nb content according to the chemical composition of the raw material for carburized parts, and the percentage conversion is taken as the Nb deposition rate.

In the base material for carburized parts according to the embodiment of the present invention, the amount of Nb precipitated as Nb precipitate is 80% or more of the total Nb content. That is, the Nb precipitation rate is 80% or more. Therefore, a sufficient number of precipitates are deposited to obtain the pinning effect.

The maximum diameter of the Nb precipitate is measured by the following method. An extraction replica sample is prepared by an extraction replica method in which a cross section of a raw material for carburizing parts is polished and then corroded, and carbon vapor deposition is performed, and 10 fields of view are observed for each sample using a transmission electron microscope (TEM). The magnification is 20000 times, and the area per visual field is 10 μm ² . The maximum length in the longitudinal direction of each Nb precipitate was defined as the major axis, the length in the direction perpendicular to the major axis was defined as the minor axis, and the length was measured. The arithmetic mean of the major axis and the minor axis is calculated, and the maximum value in 10 fields of view is set as the maximum diameter of the Nb precipitate. If the maximum diameter of the Nb precipitate is less than 200 nm, it can be evaluated as a fine precipitate.

In the base material for carburized parts according to the embodiment of the present invention, the amount of Nb precipitated as Nb precipitates is 80% or more of the total Nb content, and the maximum diameter of Nb precipitates is less than 200 nm. .. Therefore, in the carburized component, coarse old austenite grains are not generated, and the variation in grain size is small, so that excellent fatigue strength is obtained.

[Carburized parts]
A carburized part can be obtained by performing a cold forging step and a carburizing step on a base material for a carburized part.

The carburized component according to the embodiment of the present invention includes a carburized layer and a core portion inside the carburized layer. The "carburized layer" of a carburized part is the part that is affected by the carburizing process. The “core portion” of the carburized component means a portion where the carbon concentration does not change even after the carburizing treatment, that is, the inside of the carburized layer of the carburized component. In the present specification, the core portion of the carburized component means an internal region having a depth of 2 mm or more from all the surfaces of the carburized component. The area less than 2 mm from the surface of the carburized part is defined as the "carburized layer".

[Chemical composition]
The core of the carburized part is not affected by the carburizing process. Therefore, the chemical composition of the core of the carburized component according to the embodiment of the present invention is the same as the chemical composition of the base material for the carburized component. That is, the core portion of the carburized component according to the embodiment of the present invention is, in mass %, C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 to 0. 95%, P: 0.020% or less, S: 0.003 to 0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to. 0.050%, N: 0.010 to 0.025%, Nb: 0.010 to 0.060%, Cu: 0 to 0.30%, Ni: 0 to 0.25%, Pb: 0 to 0 .30%, Ca:0 to 0.005%, Bi:0 to 0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to 0.20%, and , Zr: 0 to 0.10%, and the balance has a chemical composition of Fe and impurities.

The action of each element of the chemical composition of the core of the carburized component according to the embodiment of the present invention is the same as the action of the corresponding element of the above-mentioned raw material for carburized component.

[Microstructure]
[About former austenite grains]
In the microstructure of the carburized component according to the embodiment of the present invention, the crystal grain size number of the former austenite crystal grains is 4.0 or more, and the difference between the average grain size number and the crystal grain size number of the maximum grain is 8.0 or less. .. Therefore, the carburized component according to the embodiment of the present invention does not include coarse former austenite grains and has a small grain size variation. As a result, it has high fatigue strength.

The grain size number of the former austenite grains is measured according to the following method. A specimen for microscopic observation is taken from a cross section including a portion of the carburized component according to the embodiment of the present invention having the highest degree of cold work. The portion with the highest cold workability can be identified by, for example, a simulation using FEM (finite element method) analysis. Using the collected test piece, a microscope test method of crystal grain size is carried out according to JIS G 0551 (2013) to evaluate the austenite crystal grain size number. Specifically, the surface of the test piece is mirror-polished and corroded using a saturated aqueous solution of picric acid to which a surfactant is added to expose the crystal grain boundaries of the former austenite on the surface. The corroded surface is observed with an optical microscope at a magnification of 100 times, and the grain size number is evaluated by comparison with the grain size standard chart defined in 7.2 of JIS G 0551 (2013). .. The entire test surface is observed and the particle size number of the largest particle having the largest particle size is determined. If the grain size number of the largest grain is 4.0 or more, it can be evaluated as a carburized component that does not contain coarse former austenite crystal grains.

The difference between the average grain size number and the grain size number of the largest grain is measured as follows. Similar to the measurement of the former austenite grain size number, it is observed with an optical microscope at a magnification of 100 times. Observe 10 fields of view on the test surface and determine the particle size number as described above. The average particle size number in 10 fields of view is defined as the average particle size number. If the difference between the average grain size number and the crystal grain size number of the largest grain is 8.0 or less, it can be evaluated as a carburized part with a small grain size variation.

If the grain size number of the largest grain is less than 4.0, the fatigue strength will decrease. Further, even if the crystal grain size number of the maximum grain is 4.0 or more, if the difference between the average grain size number and the crystal grain size number of the maximum grain exceeds 8.0, the fatigue strength decreases. In the carburized component according to the embodiment of the present invention, the grain size number of the largest grain is 4.0 or more, and the difference between the average grain size number and the grain size number of the largest grain is 8.0 or less. Therefore, the carburized component according to the embodiment of the present invention has excellent fatigue strength.

[Core hardness]
In the core hardness of the carburized component according to the embodiment of the present invention, the Vickers hardness is HV260 or higher. If the Vickers hardness is less than HV260, the fatigue strength required for carburized parts cannot be obtained. A preferable lower limit of the core hardness is HV280. The upper limit of the core hardness is not particularly limited, but is HV500, for example.

The core hardness of carburized parts is evaluated as follows. The core hardness of a carburized component is the hardness of any portion at a depth of more than 2 mm from all surfaces. A sample for hardness evaluation is taken from an arbitrary portion of the core of the carburized component according to the embodiment of the present invention, and the Vickers hardness is determined by a method according to JIS Z2244 (2009). The test force is 2.94N. When the obtained Vickers hardness is 260 or more, it has the strength required for a carburized part. In the carburized component according to the embodiment of the present invention, the core hardness is HV260 or higher. Therefore, the carburized component according to the embodiment of the present invention has excellent fatigue strength.

[About precipitate]
In the core portion of the carburized component according to the embodiment of the present invention, the amount of Nb precipitated as Nb precipitate is 60% or more of the total Nb content, and the maximum diameter of Nb precipitate is less than 230 nm.

The ratio of the amount of Nb precipitated as Nb precipitates to the total Nb content, and the method for measuring the maximum diameter of Nb precipitates are as follows. Of the total content of Nb and the maximum diameter of the Nb precipitate are the same as the measuring method.

[Production method]
A method of manufacturing a base material for a carburized part and a method of manufacturing a carburized part according to an embodiment of the present invention will be described. A flow chart of a method for manufacturing a carburized component according to an embodiment of the present invention is shown in FIG.

[Method of manufacturing base material for carburized parts]
A method for manufacturing a base material for carburized parts according to an embodiment of the present invention is a step of manufacturing a steel slab by heating a material having the above chemical composition to 1300° C. or higher and rolling it (agglomerating step: S1). ), a step of manufacturing a steel bar by heating and rolling the manufactured steel slab to 800 to 1075° C. (steel bar rolling step: S2), and performing spheroidizing annealing on the steel bar after the steel bar rolling step. And a step of producing a blank (spheroidizing annealing step: S3). Hereinafter, each step will be described in detail.

[Slump rolling step (S1)]
First, a material having the above chemical composition is prepared. For example, the material is manufactured by the following method. Molten steel having the above-described chemical composition is manufactured using a converter, an electric furnace, and the like. A slab is manufactured by a continuous casting method using molten steel. Alternatively, an ingot is manufactured using molten steel by the ingot making method.

The prepared material (cast piece, ingot) is heated to 1300°C or higher. The heated material is rolled by a reverse slab mill. A material rolled by a reverse slab mill is rolled by a tandem rolling mill train to produce a billet. Rolling with a reverse slab mill may be omitted.

In the method for manufacturing the base material for carburized parts according to the embodiment of the present invention, since the raw material is heated at a high heating temperature of 1300° C. or higher, the precipitate is sufficiently solid-dissolved. Precipitates precipitate finely and uniformly in the steel during cooling of the billet after slab rolling or subsequent bar rolling. If the conditions of the steel bar rolling process described later are satisfied, a large amount of fine precipitates can be uniformly deposited. If the heating temperature is lower than 1300°C, the precipitates cannot be sufficiently solid-dissolved. Therefore, undissolved precipitates become coarse in the steel bar rolling step and coarse old austenite grains are generated in the subsequent carburizing step, and the grain size variation becomes large, so that the fatigue strength of the carburized parts decreases. The "heating temperature" means the average value of the temperature inside the heating furnace.

The preferable lower limit of the heating temperature of the material is 1310°C, more preferably more than 1350°C. On the other hand, even when the heating temperature of the material exceeds 1400°C, the above effect is saturated. Therefore, the upper limit of the heating temperature of the raw material is preferably 1400°C.

The preferred heating time is more than 10 hours.

[Steel rolling process (S2)]
Hot rolling is further performed on the steel slab produced by the slab rolling process to produce a steel bar. The rolling here is, for example, continuous rolling using a tandem type rolling mill train in which horizontal roll stands and vertical roll stands are alternately arranged in a row.

First, the billet is placed in a heating furnace and heated. The heating temperature is 800 to 1075°C. The "heating temperature" refers to the average value of the temperature inside the heating furnace.

If the heating temperature in the slabbing process is 1300°C or higher and the heating temperature in the steel bar rolling is 800 to 1075°C, the amount of Nb precipitated as Nb precipitates in the raw material for carburizing parts is reduced. The Nb content may be 80% or more and the maximum diameter of Nb precipitates may be less than 200 nm.

If the heating temperature during bar rolling exceeds 1075°C, the precipitate may grow coarsely. In this case, the pinning effect cannot be sufficiently obtained. Therefore, the former austenite crystal grains become coarse during the carburizing treatment described later, and the grain size variation occurs. As a result, fatigue strength decreases.

When the heating temperature is lower than 800° C., the deformation resistance is large, the load is applied to the rolling mill, and industrial mass production is difficult.

Therefore, the heating temperature during bar rolling is 800 to 1075°C. The lower limit of the heating temperature during bar rolling is preferably 825°C. The upper limit of the heating temperature during bar rolling is preferably 1050°C.

The heated steel slab is used to hot-roll (finish-roll) in a row of finish rolling mills to produce steel bars having a predetermined diameter. The finish rolling mill train includes a plurality of stands arranged in a line. Each stand includes a plurality of rolls arranged around the pass line.

[Spheroidizing annealing step (S3)]
Spheroidizing annealing is performed on the manufactured steel bar. By performing the spheroidizing annealing, a large number of precipitates can be finely precipitated in the steel. As a result, the pinning effect of the precipitate can be sufficiently exerted. The method of spheroidizing annealing is not particularly limited and may be a general method. The spheroidizing annealing is, for example, a long-time heating method, a repeated heating/cooling method, a slow cooling method and an isothermal transformation method.

The manufacturing conditions of the spheroidizing annealing process may be general ones. The heating temperature in the spheroidizing annealing step is, for example, 680 to 780°C. The heating time in the spheroidizing annealing step is, for example, 150 to 1800 minutes.

Through the above manufacturing process, a raw material for carburized parts is manufactured. That is, the raw material for carburized parts according to the embodiment of the present invention is a material subjected to a slab rolling process, a steel bar rolling process and a spheroidizing annealing process.

[Method of manufacturing carburized parts]
A method of manufacturing a carburized component using the above-described base material for carburized component will be described. The method of manufacturing a carburized component is a step of performing cold forging on the raw material after the spheroidizing annealing step to manufacture a cold forged product (cold forging step: S4), and a step after the cold forging step. And a step of performing a carburizing treatment on the cold forged product (carburizing step: S5). Each step will be described below.

[Cold forging step (S4)]
Cold forging is carried out by a well-known method using the material for carburizing parts to manufacture an intermediate product. A wire drawing process may be performed before the cold forging process. The wire drawing process may be performed only by the primary wire drawing, or may be performed by a plurality of times such as secondary wire drawing.

In the method for manufacturing a carburized component according to the embodiment of the present invention, since a sufficient number of fine precipitates are obtained, coarsening of old austenite crystal grains can be suppressed. Therefore, normalizing treatment after cold forging can be omitted.

[Carburizing step (S5)]
Carburize the manufactured intermediate product. Tempering may be performed on the intermediate product after the carburizing treatment. Machining (cutting, etc.) may be further performed on the intermediate product after tempering. A carburized component is manufactured by the above manufacturing process.

The conditions for the carburizing treatment may be general ones. For example, the vacuum carburizing step is a step of introducing a carburizing gas into a depressurized atmosphere to heat the steel material in the range of 900 to 1050°C.

As the type of carburizing gas used in the carburizing treatment, known ones used in the vacuum carburizing treatment can be used. The carburizing gas is, for example, a hydrocarbon gas such as acetylene, propane and ethylene.

The preferable carburizing temperature is 980 to 1075°C. The more preferable lower limit of the carburizing temperature is 1030°C. When the carburizing temperature is 1030° C. or higher, a carburized component having a predetermined carbon concentration can be obtained in a shorter time. The more preferable upper limit of the carburizing temperature is 1050°C. If the carburizing temperature is 1050° C. or lower, the crystal grains are unlikely to become coarse.

The treatment time in the carburizing treatment is appropriately adjusted according to the chemical composition of the intermediate product (steel material), the target hardness of the core portion and the target surface carbon concentration.

In the cooling step after heating in the carburizing process, the intermediate product is rapidly cooled and quenched to manufacture a carburized component. Quenching may be water quenching or oil quenching. By quenching the intermediate product, it is possible to obtain a core hardness of HV260 or higher in the carburized component.

Through the above steps, the carburized component according to the present embodiment is manufactured. Note that tempering may be performed on the intermediate product after the carburizing treatment. Machining (cutting, etc.) may be further performed on the intermediate product after tempering.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Various steels for cold forging were manufactured, and cold forgeability, fatigue strength, and availability of mass production were evaluated.

Components of the molten steels of Test Nos. 1 to 63 having the chemical compositions shown in Table 1 and Table 2 were adjusted in a 70-ton converter. After adjusting the components, continuous casting was performed to produce a slab of 400 mm×300 mm square. The obtained slab was cooled to 600°C.

The produced slab was heated to the heating temperature (agglomeration heating temperature) shown in Table 3 and Table 4, respectively. After heating, slab rolling was performed to produce a steel piece of 180 mm×180 mm square. The obtained steel slab was left to cool to room temperature.

The steel pieces were heated to the heating temperatures (bar steel heating temperatures) shown in Tables 3 and 4, respectively, and then bar steel rolling was performed to obtain steel bars having a diameter of 50 mm and a length of 1000 mm.

The produced steel bar was held at 760° C. for 4 hours, cooled to 660° C. at 0.25° C./sec, then allowed to cool, and subjected to spheroidizing annealing to obtain a raw material for carburized parts. A round bar test piece having a diameter of 30 mm and a length of 1000 mm was sampled from the central portion of the raw material for carburizing parts. The round bar test piece was pickled and lubricated. After that, the round bar test piece was subjected to drawing for simulating cold forging. The final diameter was 16.5 mm in order to obtain a surface reduction rate of 70%. From the center of the drawn material obtained, a plurality of Ono-type rotary bending fatigue test pieces according to JIS Z 2274 (2011) shown in FIG. 2 were collected. The Ono-type rotary bending fatigue test piece had a parallel part diameter of 10 mm, a parallel part length of 20.67 mm, a shoulder radius of 24 mm, and a depth of 1 mm at the center of the parallel part in the longitudinal direction. The notch had a semicircular notch with a bottom radius of 1.0 mm. The central axis of the Ono-type rotary bending fatigue test piece was coaxial with the central axis of the drawn material. The above Ono-type rotary bending fatigue test piece was carburized at 1030°C.

[Microstructure observation]
[Former austenite grain size measurement]
The cross section including the notch bottom of the Ono-type rotary bending fatigue test piece obtained above was cut out. The cut surface was mirror-polished and corroded with a saturated aqueous picric acid solution containing a surfactant. Observation with an optical microscope at a magnification of 100 times was carried out, and according to JIS G 0551 (2005), the state of coarsening of old austenite crystal grains was investigated by an evaluation method by comparison with a crystal grain standard diagram. The entire test surface was observed and the particle size of the largest particles was measured. The results are shown in the "maximum grain" column of Tables 3 and 4. When old austenite crystal grains with a grain size number of less than 4 were observed, it was determined that coarse grains had occurred. When no particles having a particle size number less than 4 were observed, it was determined that coarse particles did not occur.

Furthermore, the test surface was randomly observed in 10 fields of view to determine the average particle size number. The difference between the average particle size number and the above-mentioned maximum particle size number is shown in Table 3 and Table 4 in the "Average-maximum particle" column. If the difference between the average grain size number and the maximum grain size number was 8.0 or less, it was evaluated that the variation in the former austenite crystal grain size was small.

[Core hardness measurement]
The Ono-type rotary bending fatigue test piece obtained as described above was cut out in a cross section including a notch bottom. The cut surface was mirror-polished, and the hardness at a position of 2 mm from the surface was measured by a method according to JIS Z 2244 (2009). The test force was 2.94N. The results are shown in the "core hardness HV" column of Tables 3 and 4.

[Measurement of precipitates]
A 10 mm cubic extraction residue test piece was sampled from the core of the Ono-type rotary bending fatigue test piece. Extraction (electrolysis) was performed at a current density of 250 to 350 A/m ² using a 10% AA solution, which is a general condition. The extracted solution was filtered with a filter having a mesh size of 0.2 μm, and the filtered product was subjected to general chemical analysis. The amount of Nb deposited as a deposit was determined by chemical analysis. The Nb precipitation rate (%) with respect to the Nb content of the test piece was determined by the above method. The results are shown in the "Nb deposition rate" column of Tables 3 and 4.

An extracted replica sample was prepared from the core of the Ono-type rotating bending fatigue test piece by a general method. Using a transmission electron microscope, 10 fields of view were randomly observed with a magnification of 20000 times and an area of 10 μm ² per field of view. The maximum length in the longitudinal direction of each Nb precipitate was defined as the major axis, the length in the direction perpendicular to the major axis was defined as the minor axis, and the length was measured. The arithmetic mean of the long axis and the short axis was calculated, and the maximum value in 10 fields of view was set as the maximum diameter of the Nb precipitate. The results are shown in the "maximum precipitate" column of Tables 3 and 4.

The Nb precipitation rate and the maximum precipitate were also determined in the same manner in the above-obtained base materials for carburized parts. The results are shown in the "form/Nb precipitation rate" column and the "form/maximum precipitate" column in Tables 3 and 4, respectively.

[Fatigue strength evaluation test]
The Ono-type rotary bending fatigue test piece obtained as described above was subjected to a fatigue test. The number of test pieces in the Ono-type rotary bending fatigue test was three per one stress condition. An Ono-type rotary bending fatigue test was carried out using an Ono-type rotary bending fatigue test piece at room temperature in the air atmosphere in accordance with JIS Z 2274 (2011). The rotation speed was 3000 rpm. The highest stress was defined as the fatigue strength (MPa) before the fracture occurred in all three specimens up to the repetition number of 1×10 ⁷ . Generally, the fatigue strength required for carburized gears and shafts is about 450 MPa, so 450 to 475 MPa is C, 475 to 500 MPa is B, and 500 MPa is A, and 450 MPa or less is evaluated as x. The results are shown in the "fatigue strength" column of Tables 3 and 4.

[Cold forgeability evaluation test]
A simulated blank with a diameter of 14 mm and a length of 21 mm was sampled from the blank after the spheroidizing annealing. In addition, instead of the above-mentioned simulated raw material, a normalizing material was also prepared in which it was further held at 920° C. for 60 minutes, and then allowed to cool to room temperature. Then, the cold forgeability was investigated. For each steel, the relative value of the deformation resistance value of the simulated raw material when the deformation resistance value of the normalized material was standardized as "100" was obtained. The results are shown in the "Cold forging" column of Tables 3 and 4. When the above relative value was 90 or less, it was judged that the deformation resistance after spheroidizing annealing was low and the cold forgeability was excellent. The results are shown in the "Cold forgeability" column of Tables 3 and 4.

[Mass production availability]
Since the carburized component according to the embodiment of the present invention is a product for use as a mechanical component, the possibility of mass production was also examined. It was judged that the ones that can be manufactured without any particular problem in the steel making process and the hot rolling process are the most suitable for mass production, and are shown by "○" in the "Mass production availability" column of Tables 3 and 4. It is judged that mass production is possible, though not optimal, if there are problems such as cost and productivity in the steel making process and hot rolling process, and the yield may decrease. It is shown by "△" in the column.

[Test results]
With reference to Tables 1 to 4, the chemical compositions of test numbers 1 to 17, 20 to 25, 28 to 33, 36 to 49 and 51 to 54 are within the scope of the present invention. The grain size number of the grains is 4.0 or more, the difference between the average grain size number and the grain size number of the largest grain is 8.0 or less, the core hardness is HV260 or more, and Nb precipitates are formed. The amount of Nb contained was 60% or more of the total Nb content, and the maximum diameter of Nb precipitates was less than 230 nm. As a result, the variation in grain size was suppressed, and the fatigue strength was high. In the cast material for carburized parts, the amount of Nb precipitated as Nb precipitate was 80% or more of the total Nb content, and the maximum diameter of Nb precipitate was less than 200 nm. Then, in the cold forgeability evaluation test, the relative value was 90 or less, which showed excellent cold forgeability.

On the other hand, in Test Nos. 18, 19, 26, 27, 34, 35, 50, and 55 to 63, it was determined that the desired fatigue strength could not be obtained, or that mass production would require a large cost.

In test number 18, the Al content exceeded the upper limit of the Al content specified in the present invention. Therefore, the Nb deposition rate was less than 80% for the base material and less than 60% for the carburized parts. In the carburized parts, the grain size number of the largest grain was less than 4.0, and the difference between the average grain size number and the grain size number of the largest grain also exceeded 8.0. As a result, the target fatigue strength was not obtained. Therefore, we determined that it was not suitable for mass production.

In test number 19, the Al content was less than the lower limit of the Al content specified in the present invention. Therefore, the crystal grain size number of the largest grain was less than 4.0, and the difference between the average grain size number and the crystal grain number of the largest grain also exceeded 8.0. As a result, the target fatigue strength was not obtained.

In the test number 26, the N content exceeded the upper limit of the N content specified in the present invention. Therefore, it was judged that the surface of the steel would be flawed and that it would be costly to stably mass-produce.

In test number 27, the N content was less than the lower limit of the N content specified in the present invention. Therefore, the Nb deposition rate was less than 80% for the base material and less than 60% for the carburized parts. In the carburized parts, the grain size number of the largest grain was less than 4.0, and the difference between the average grain size number and the grain size number of the largest grain also exceeded 8.0. As a result, the target fatigue strength was not obtained.

In the test number 34, the Nb content exceeded the upper limit of the Nb content specified in the present invention. Therefore, the maximum deposit size of the cast material exceeded 200 nm, and the maximum deposit size of the carburized parts also exceeded 230 nm. As a result, the grain size number of the largest grain was less than 4.0. As a result, the target fatigue strength was not obtained.

In test number 35, the Nb content was less than the lower limit of the Nb content specified in the present invention. Therefore, the Nb deposition rate was less than 80% for the base material and less than 60% for the carburized parts. The crystal grain size number of the largest grain was less than 4.0. As a result, the target fatigue strength was not obtained.

In test number 50, the maximum deposit size of the cast material exceeded 200 nm, and the maximum deposit size of the carburized parts also exceeded 230 nm. Furthermore, the crystal grain size number of the maximum grain was less than 4.0, and the difference between the average grain size number and the crystal grain size number of the maximum grain also exceeded 8.0. As a result, the target fatigue strength was not obtained. It is considered that this is because the heating temperature during slab rolling was less than 1300°C.

In test number 55, the maximum deposit size of the cast material exceeded 200 nm, and the maximum deposit size of the carburized parts also exceeded 230 nm. Furthermore, the crystal grain size number of the maximum grain was less than 4.0, and the difference between the average grain size number and the crystal grain size number of the maximum grain also exceeded 8.0. As a result, the target fatigue strength was not obtained. This is probably because the heating temperature during bar rolling was too high.

In test number 56, the heating temperature during bar rolling was too low, and the load on the roll was too large. For this reason, it was determined that stable mass production could be costly.

In test number 57, the Mo content was less than the lower limit of the Mo content specified in the present invention. Furthermore, the heating temperature during slab rolling was too low, and the heating temperature during bar rolling was too high. Therefore, the maximum deposit size of the cast material exceeded 200 nm, and the maximum deposit size of the carburized parts also exceeded 230 nm. Further, the crystal grain size number of the largest grain was less than 4.0. As a result, the target fatigue strength was not obtained.

In the test number 58, the Si content and the Cr content exceeded the upper limits of the Si content and the Cr content specified in the present invention. Furthermore, the Mo content was less than the lower limit of the Mo content specified in the present invention. Furthermore, it did not contain Nb. Furthermore, the heating temperature during slab rolling was too low, and the heating temperature during bar rolling was too high. Therefore, Nb precipitates did not precipitate, and the maximum grain size number was less than 4.0. As a result, the target fatigue strength was not obtained.

In test number 59, the Si content, P content, and Cr content exceeded the upper limits of the Si content, P content, and Cr content specified in the present invention. Further, the Mn content and the Mo content were less than the lower limits of the Mn content and the Mo content specified in the present invention. Therefore, the crystal grain size number of the largest grain was less than 4.0, and the difference between the average grain size number and the crystal grain number of the largest grain also exceeded 8.0. As a result, the target fatigue strength was not obtained.

In the test number 60, the Si content and the Cr content exceeded the upper limits of the Si content and the Cr content specified in the present invention. Furthermore, the Mo content was less than the lower limit of the Mo content specified in the present invention. Therefore, the crystal grain size number of the largest grain was less than 4.0. As a result, the target fatigue strength was not obtained. Furthermore, cold forgeability was low.

In test number 61, the Si content and the Cr content exceeded the upper limits of the Si content and the Cr content specified in the present invention. Furthermore, the Mo content was less than the lower limit of the Mo content specified in the present invention. Furthermore, it did not contain Nb. Furthermore, the heating temperature during slab rolling was too low, and the heating temperature during bar rolling was too high. Therefore, Nb precipitates did not precipitate, and the maximum grain size number was less than 4.0. As a result, the target fatigue strength was not obtained.

In test number 62, the Si content exceeded the upper limit of the Si content specified in the present invention. Further, the Mn content and the Mo content were less than the lower limits of the Mn content and the Mo content specified in the present invention. Furthermore, it did not contain Nb. Furthermore, the heating temperature during slab rolling was too low, and the heating temperature during bar rolling was too high. Therefore, Nb precipitates did not precipitate, and the maximum grain size number was less than 4.0. As a result, the target fatigue strength was not obtained.

In test number 63, the Si content exceeded the upper limit of the Si content specified in the present invention. Therefore, cold forgeability was low.

As described above, the steel material for cold forging according to the present invention is excellent in cold forgeability and further has high fatigue strength. Therefore, it can be widely applied as a material for machine structural parts such as automobile parts, industrial machine parts, construction machine parts, etc., which have been manufactured in the "hot forging-cutting" process up to now, and the parts can be made into a near net shape. Can contribute to.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit thereof.

Claims (8)

% By mass, C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 to 0.95%, P: 0.020% or less, S: 0.003 ~0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to 0.050%, N: 0.010 to 0.025%, Nb:0.10 to 0.060%, Cu:0 to 0.30%, Ni:0 to 0.25%, Pb:0 to 0.30%, Ca:0 to 0.005%, Bi:0 .About.0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to 0.20%, and Zr:0 to 0.10%, and the balance Fe. And a slab rolling process for producing a steel slab by heating and rolling a raw material having a chemical composition of impurities to 1300° C. or higher,
A steel bar rolling step of manufacturing a steel bar by heating the produced steel billet to 800 to 1075° C. and rolling it;
A spheroidizing annealing step of producing a blank by performing spheroidizing annealing on the steel bar after the steel bar rolling step,
A cold forging step of manufacturing a cold forged product by performing cold forging on the raw material after the spheroidizing annealing step,
And a carburizing step of performing a carburizing treatment on the cold forged product after the cold forging step. The method for manufacturing a carburized component according to claim 1, wherein the chemical composition of the material is Cu: 0.02 to 0.30%, Ni: 0.02 to 0.25%, Pb: 0.01. ~ 0.30%, Ca: 0.0003 to 0.005%, Bi: 0.01 to 0.30%, Ti: 0.005 to 0.100%, B: 0.0001 to 0.010%, V: 0.005-0.20% and Zr: 0.0003-0.10% The manufacturing method of the carburized component containing 1 type or 2 types or more selected from the group which consists of 0%. A carburized layer and a core portion inside the carburized layer,
The core portion is mass%,
C: 0.13 to 0.23%,
Si: 0.02-0.15%,
Mn: 0.55 to 0.95%,
P: 0.020% or less,
S: 0.003 to 0.030%,
Cr: 0.85 to 1.23%,
Mo: 0.35-0.50%,
Al: 0.020 to 0.050%,
N: 0.010 to 0.025%,
Nb: 0.010 to 0.060%,
Cu: 0 to 0.30%,
Ni: 0 to 0.25%,
Pb: 0 to 0.30%,
Ca: 0 to 0.005%,
Bi: 0 to 0.30%,
Ti: 0 to 0.100%,
B: 0 to 0.010%,
V: 0 to 0.20%, and
Zr: 0 to 0.10%, the balance has a chemical composition of Fe and impurities,
In the microstructure, the largest grain having the largest grain size has a grain size number of 4.0 or more,
The difference between the average grain size number and the grain size number of the largest grain is 8.0 or less,
The core hardness is HV260 or higher,
The amount of Nb precipitated as Nb precipitate is 60% or more of the total Nb content,
A carburized part in which the maximum diameter of the Nb precipitate is less than 230 nm. The carburized component according to claim 3,
The chemical composition of the core is
Cu: 0.02 to 0.30%,
Ni: 0.02-0.25%,
Pb: 0.01 to 0.30%,
Ca: 0.0003 to 0.005%,
Bi: 0.01 to 0.30%,
Ti: 0.005 to 0.100%,
B: 0.0001 to 0.010%,
V: 0.005 to 0.20%, and
Zr: A carburized part containing one or more selected from the group consisting of 0.0003 to 0.10%. % By mass, C: 0.13 to 0.23%, Si: 0.02 to 0.15%, Mn: 0.55 to 0.95%, P: 0.020% or less, S: 0.003 ~0.030%, Cr: 0.85 to 1.23%, Mo: 0.35 to 0.50%, Al: 0.020 to 0.050%, N: 0.010 to 0.025%, Nb:0.10 to 0.060%, Cu:0 to 0.30%, Ni:0 to 0.25%, Pb:0 to 0.30%, Ca:0 to 0.005%, Bi:0 .About.0.30%, Ti:0 to 0.100%, B:0 to 0.010%, V:0 to 0.20%, and Zr:0 to 0.10%, and the balance Fe. And a slab rolling process for producing a steel slab by heating and rolling a raw material having a chemical composition of impurities to 1300° C. or higher,
A steel bar rolling step of manufacturing a steel bar by heating the produced steel billet to 800 to 1075° C. and rolling it;
And a spheroidizing annealing step of performing spheroidizing annealing on the steel bar after the steel bar rolling step. It is a manufacturing method of the raw material for carburizing parts of Claim 5, Comprising: The said chemical composition of the said raw material is Cu: 0.02-0.30%, Ni: 0.02-0.25%, Pb: 0.01 to 0.30%, Ca: 0.0003 to 0.005%, Bi: 0.01 to 0.30%, Ti: 0.005 to 0.100%, B: 0.0001 to 0.010%, V: 0.005-0.20%, and Zr: 0.0003-0.10%, and one or more selected from the group consisting of elements for carburized parts. Manufacturing method of profile. In mass %,
C: 0.13 to 0.23%,
Si: 0.02-0.15%,
Mn: 0.55 to 0.95%,
P: 0.020% or less,
S: 0.003 to 0.030%,
Cr: 0.85 to 1.23%,
Mo: 0.35-0.50%,
Al: 0.020 to 0.050%,
N: 0.010 to 0.025%,
Nb: 0.010 to 0.060%,
Cu: 0 to 0.30%,
Ni: 0 to 0.25%,
Pb: 0 to 0.30%,
Ca: 0 to 0.005%,
Bi: 0 to 0.30%,
Ti: 0 to 0.100%,
B: 0 to 0.010%,
V: 0 to 0.20%, and
Zr: 0 to 0.10%, the balance has a chemical composition of Fe and impurities,
The amount of Nb precipitated as Nb precipitate is 80% or more of the total Nb content,
A raw material for carburizing parts, wherein the maximum diameter of the Nb precipitate is less than 200 nm. A base material for carburized parts according to claim 7,
The chemical composition is
Cu: 0.02 to 0.30%,
Ni: 0.02-0.25%,
Pb: 0.01 to 0.30%,
Ca: 0.0003 to 0.005%,
Bi: 0.01 to 0.30%,
Ti: 0.005 to 0.100%,
B: 0.0001 to 0.010%,
V: 0.005 to 0.20%, and
Zr: A cast material for carburized parts, containing one or more selected from the group consisting of 0.0003 to 0.10%.

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