Nitrided plate part and method for producing the same
[Object] To provide a nitrided plate part that exhibits fatigue strength equivalent to or better than that of a carburized member, and a method for producing the same. [Solution] Provided is a nitrided plate part having predetermined components and structure. Nitrogen average content in a range in which a distance from a sheared end face of the part toward an interior of the nitrided plate part in a sheared end face normal direction is equal to or greater than 0.05 mm and equal to or less than 0.10 mm is equal to or greater than 0.4000% and equal to or less than 1.2000% in mass %, and minimum nitrogen content in a range in which the distance is equal to or greater than 0.015 mm and equal to or less than 0.200 mm is 0.0600% or more. After uncoiling a steel sheet coil, stretch and compressive deformation in a range of equal to or greater than 0.03% and equal to or less than 3.00% in amount of plastic strain are alternately applied to a surface layer of a steel sheet. Then, shearing and press-forming are performed to make the steel sheet into a plate part shape, without recoiling the steel sheet again. Then, nitriding is performed under predetermined conditions.
1. A nitrided plate part having a sheared end face, a sheet-thickness central portion in a portion at least 5 mm or more away from the sheared end face having a chemical composition consisting of, in mass %,
C: equal to or greater than 0.025% and equal to or less than 0.113%,
Si: 0.10% or less,
Mn: equal to or greater than 0.71% and equal to or less than 1.49%,
P: 0.020% or less,
S: 0.0200% or less,
Ti: equal to or greater than 0.020% and equal to or less than 0.091%,
Cr: equal to or greater than 0.130% and equal to or less than 0.340%,
Al: equal to or greater than 0.10% and equal to or less than 0.35%,
N: equal to or greater than 0.0007% and equal to or less than 0.0300%,
Nb: equal to or greater than 0% and equal to or less than 0.020%,
Mo: equal to or greater than 0% and equal to or less than 0.140%,
V: equal to or greater than 0% and equal to or less than 0.100%,
B: equal to or greater than 0% and equal to or less than 0.0030%,
Cu: equal to or greater than 0% and equal to or less than 0.13%,
Ni: equal to or greater than 0% and less than 0.08%,
W: equal to or greater than 0% and equal to or less than 0.07%,
Co: equal to or greater than 0% and equal to or less than 0.07%,
Ca: equal to or greater than 0% and less than 0.007%,
Mg: equal to or greater than 0% and less than 0.005%,
REM: equal to or greater than 0% and less than 0.005%, and
the balance: Fe and impurities,
wherein nitrogen average content n a range in which a distance from the sheared end face in a sheared end face normal direction is equal to or greater than 0.05 mm and equal to or less than 0.10 mm is equal to or greater than 0.4000% and equal to or less than 1.2000% in mass %, and minimum nitrogen content in a range in which the distance is equal to or greater than 0.015 mm and equal to or less than 0.200 mm is 0.0600% or more.
2. The nitrided plate part according to claim 1 , wherein the nitrided plate part has a sheet thickness of equal to or greater than 1.0 mm and equal to or less than 8.0 mm.
3. The nitrided plate part according to claim 1 , wherein the nitrided plate part has a sheet thickness of greater than 1.2 mm and equal to or less than 6.0 mm.
4. A method for producing a nitrided plate part, comprising:
obtaining a steel sheet by performing hot rolling at hot finish rolling exit-side temperature in a range of equal to or greater than 850° C. and less than 960° C. on a slab having a chemical composition consisting of, in mass %,
C: equal to or greater than 0.025% and equal to or less than 0.113%,
Si: 0.10% or less,
Mn: equal to or greater than 0.71% and equal to or less than 1.49%,
P: 0.020% or less,
S: 0.0200% or less,
Ti: equal to or greater than 0.020% and equal to or less than 0.091%,
Cr: equal to or greater than 0.130% and equal to or less than 0.340%,
Al: equal to or greater than 0.10% and equal to or less than 0.35%,
N: equal to or greater than 0.0007% and equal to or less than 0.0100%,
Nb: equal to or greater than 0% and equal to or less than 0.020%,
Mo: equal to or greater than 0% and equal to or less than 0.140%,
V: equal to or greater than 0% and equal to or less than 0.100%,
B: equal to or greater than 0% and equal to or less than 0.0030%,
Cu: equal to or greater than 0% and equal to or less than 0.13%,
Ni: equal to or greater than 0% and less than 0.08%,
W: equal to or greater than 0% and equal to or less than 0.07%,
Co: equal to or greater than 0% and equal to or less than 0.07%,
Ca: equal to or greater than 0% and less than 0.007%,
Mg: equal to or greater than 0% and less than 0.005%,
REM: equal to or greater than 0% and less than 0.005%, and
the balance: Fe and impurities;
then starting cooling within three seconds from an end of hot finish rolling, and cooling the steel sheet to equal to or greater than 460° C. and equal to or less than 630° C. within 29 seconds from the end of hot finish rolling;
coiling the steel sheet into a steel sheet coil;
in regard to the steel sheet coil further subjected to pickling, uncoiling the steel sheet coil, and then applying bending/unbending in a range of equal to or greater than 0.03% and equal to or less than 3.00% in amount of plastic strain to the steel sheet;
performing shearing and press-forming to make the steel sheet into a plate part shape, without recoiling the steel sheet again; and
nitriding the steel sheet by causing the steel sheet to stay in a closed furnace adjusted to a temperature of equal to or greater than 500° C. and less than 62.0° C. with an atmosphere in which a volume constituent ratio of ammonia gas is greater than 30?/t, for a time of 60 minutes or more.