BEARING PART, STEEL FOR BEARING PART AND METHOD FOR PRODUCING THEREOF

1 . A bearing part comprising, as a chemical composition, by mass %,

C: 0.95% to 1.10%;

Si: 0.10% to 0.70%;

Mn: 0.20% to 1.20%;

Cr: 0.90% to 1.60%;

Al: 0.010% to 0.100%;

N: 0.003% to 0.030%;

P: 0.025% or less;

S: 0.025% or less;

O: 0.0010% or less; and

optionally one or more selected from the group consisting of:

Mo: 0.25% or less,

B: 0.0050% or less,

Cu: 1.0% or less,

Ni: 3.0% or less, and

Ca: 0.0015% or less; and

a remainder including Fe and impurities;

wherein a metallographic structure includes a retained austenite, a spherical cementite and a martensite;

wherein an amount of the retained austenite is 18% to 25%, by volume %;

wherein an average grain size of a prior-austenite is 6.0 μm or less;

wherein an average grain size of the spherical cementite is 0.45 μm or less; and

wherein a number density of the spherical cementite is 0.45×10 6 mm −2 or more in the metallographic structure.

2 . The bearing part according to claim 1 comprising, as the chemical composition, by mass %, one or more of

Mo: 0.01% to 0.25%,

B: 0.0001% to 0.0050%,

Cu: 0.1% to 1.0%,

Ni: 0.05% to 3.0%, and

Ca: 0.0003% to 0.0015%.

3 . A steel for a bearing part comprising, as a chemical composition, by mass %,

C: 0.95% to 1.10%;

Si: 0.10% to 0.70%;

Mn: 0.20% to 1.20%;

Cr: 0.90% to 1.60%;

Al: 0.010% to 0.100%;

N: 0.003% to 0.030%;

S: 0.025% or less;

P: 0.025% or less;

O: 0.0010% or less;

optionally one or more selected from the group consisting of:

Mo: 0.25% or less,

B: 0.0050% or less,

Cu: 1.0% or less,

Ni: 3.0% or less, and

Ca: 0.0015% or less; and

a remainder including Fe and impurities;

wherein a metallographic structure includes a spherical cementite and a ferrite;

wherein a number density of the spherical cementite having a grain size of 0.5 μm to 3.0 μm is 2.0×10 6 mm −2 or more in the metallographic structure.

4 . The steel for the bearing part according to claim 3 comprising, as the chemical composition, by mass %, one or more of

Mo: 0.01% to 0.25%,

B: 0.0001% to 0.0050%,

Cu: 0.1% to 1.0%,

Ni: 0.05% to 3.0%, and

Ca: 0.0003% to 0.0015%.

5 . A method for producing a steel for a bearing part comprising,

a casting process for obtaining a steel piece having a chemical composition according to claim 3 ;

a heating process of heating the steel piece to 900° C. to 1300° C.;

a hot rolling process for obtaining a hot rolled wire rod by subjecting the steel piece to hot rolling at a finish rolling temperature of 850° C. or less after the heating process;

a winding process of winding the hot rolled wire rod after the hot rolling process at a winding temperature of 800° C. or less;

a cooling process for making a metallographic structure of the hot rolled wire rod a pearlite by cooling the hot rolled wire rod to 600° C. at a cooling rate of 3.0° C./second or less after the winding process;

a wire drawing process of subjecting the hot rolled wire rod to wire drawing in which a total reduction of an area is 50% or more after the cooling process; and

a spheroidizing annealing process for obtaining the steel for the bearing part by performing a spheroidizing annealing in which the hot rolled wire rod after the wire drawing process is held at a temperature of 650° C. to the lower of 750° C. and A 1 −5° C. for 0.5 hours to 5 hours;

wherein, A 1 represents a temperature at which A 1 transformation starts, and is a predicted value which is calculated by a following equation 1 based on the chemical composition,

and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and [B] in the equation 1 represent, by mass %, the content of C, the content of Si, the content of Mn, the content of Cu, the content of Ni, the content of Cr, the content of Mo, the content of Al and the content of B, respectively, in the hot rolled wire rod,

A 1 =750.8−26.6×[C]+17.6×[Si]−11.6×[Mn]−22.9×[Cu]−23.0×[Ni]+24.1×[Cr]+22.5×[Mo]−169.4×[Al]−894.7×[B]  (Equation 1).

6 . A method for producing a bearing part comprising,

a casting process for obtaining a steel piece having a chemical composition according to claim 1 ;

a heating process of heating the steel piece to 900° C. to 1300° C.;

a hot rolling process for obtaining a hot rolled wire rod by subjecting the steel piece to hot rolling at a finish rolling temperature of 850° C. or less after the heating process;

a winding process of winding the hot rolled wire rod after the hot rolling process at a winding temperature of 800° C. or less;

a cooling process for making a metallographic structure of the hot rolled wire rod a pearlite by cooling the hot rolled wire rod to 600° C. at a cooling rate of 3.0° C./second or less after the winding process;

a wire drawing process of subjecting the hot rolled wire rod to wire drawing in which a total reduction of an area is 50% or more after the cooling process;

a spheroidizing annealing process for obtaining a steel for a bearing part by performing spheroidizing annealing in which the hot rolled wire rod after the wire drawing process is held at a temperature of 650° C. to the lower of 750° C. and A 1 −5° C. for 0.5 hours to 5 hours;

a forming process of rough forming the steel for the bearing part after the spheroidizing annealing process;

a quenching process of performing quenching by heating the steel for the bearing part after the forming process to 800° C. to 890° C.;

a tempering process of subjecting the steel for the bearing part to tempering at 250° C. or less after the quenching process; and

a finish machining process for obtaining the bearing part by subjecting the steel for the bearing part to finish machining after the tempering process;

wherein, A 1 represents a temperature at which A 1 transformation starts, and is a predicted value which is calculated by a following equation 2 based on the chemical composition,

and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and [B] in the equation 2 represent, by mass %, the content of C, the content of Si, the content of Mn, the content of Cu, the content of Ni, the content of Cr, the content of Mo, the content of Al and the content of B, respectively, in the hot rolled wire rod,

A 1 =750.8−26.6×[C]+17.6×[Si]−11.6×[Mn]−22.9×[Cu]−23.0×[Ni]+24.1×[Cr]+22.5×[Mo]−169.4×[Al]−894.7×[B]  (Equation 2).

7 . A method for producing a steel for a bearing part comprising,

a casting process for obtaining a steel piece having a chemical composition according to claim 4 ;

a heating process of heating the steel piece to 900° C. to 1300° C.;

a hot rolling process for obtaining a hot rolled wire rod by subjecting the steel piece to hot rolling at a finish rolling temperature of 850° C. or less after the heating process;

a winding process of winding the hot rolled wire rod after the hot rolling process at a winding temperature of 800° C. or less;

a cooling process for making a metallographic structure of the hot rolled wire rod a pearlite by cooling the hot rolled wire rod to 600° C. at a cooling rate of 3.0° C./second or less after the winding process;

a wire drawing process of subjecting the hot rolled wire rod to wire drawing in which a total reduction of an area is 50% or more after the cooling process; and

a spheroidizing annealing process for obtaining the steel for the bearing part by performing a spheroidizing annealing in which the hot rolled wire rod after the wire drawing process is held at a temperature of 650° C. to the lower of 750° C. and A 1 −5° C. for 0.5 hours to 5 hours;

wherein, A 1 represents a temperature at which A 1 transformation starts, and is a predicted value which is calculated by a following equation 1 based on the chemical composition,

and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and [B] in the equation 1 represent, by mass %, the content of C, the content of Si, the content of Mn, the content of Cu, the content of Ni, the content of Cr, the content of Mo, the content of Al and the content of B, respectively, in the hot rolled wire rod,

A 1 =750.8−26.6×[C]+17.6×[Si]−11.6×[Mn]−22.9×[Cu]−23.0×[Ni]+24.1×[Cr]+22.5×[Mo]−169.4×[Al]−894.7×[B]  (Equation 1).

8 . A method for producing a bearing part comprising,

a casting process for obtaining a steel piece having a chemical composition according to claim 2 ;

a heating process of heating the steel piece to 900° C. to 1300° C.;

a hot rolling process for obtaining a hot rolled wire rod by subjecting the steel piece to hot rolling at a finish rolling temperature of 850° C. or less after the heating process;

a winding process of winding the hot rolled wire rod after the hot rolling process at a winding temperature of 800° C. or less;

a cooling process for making a metallographic structure of the hot rolled wire rod a pearlite by cooling the hot rolled wire rod to 600° C. at a cooling rate of 3.0° C./second or less after the winding process;

a wire drawing process of subjecting the hot rolled wire rod to wire drawing in which a total reduction of an area is 50% or more after the cooling process;

a spheroidizing annealing process for obtaining a steel for a bearing part by performing spheroidizing annealing in which the hot rolled wire rod after the wire drawing process is held at a temperature of 650° C. to the lower of 750° C. and A 1 −5° C. for 0.5 hours to 5 hours;

a forming process of rough forming the steel for the bearing part after the spheroidizing annealing process;

a quenching process of performing quenching by heating the steel for the bearing part after the forming process to 800° C. to 890° C.;

a tempering process of subjecting the steel for the bearing part to tempering at 250° C. or less after the quenching process; and

a finish machining process for obtaining the bearing part by subjecting the steel for the bearing part to finish machining after the tempering process;

wherein, A 1 represents a temperature at which A 1 transformation starts, and is a predicted value which is calculated by a following equation 2 based on the chemical composition,

and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and [B] in the equation 2 represent, by mass %, the content of C, the content of Si, the content of Mn, the content of Cu, the content of Ni, the content of Cr, the content of Mo, the content of Al and the content of B, respectively, in the hot rolled wire rod,

A 1 =750.8−26.6×[C]+17.6×[Si]−11.6×[Mn]−22.9×[Cu]−23.0×[Ni]+24.1×[Cr]+22.5×[Mo]−169.4×[Al]−894.7×[B]  (Equation 2).