Nitrided plate part and method for producing the same

• US 10,808,311 B2
• Filed: 12/02/2016
• Issued: 10/20/2020
• Est. Priority Date: 12/04/2015
• Status: Active Grant

• Current Assignee: Nippon Steel Corporation
• Original Assignee: Nippon Steel Corporation
• Inventors: Sakurada, Eisaku, Saitoh, Shinya, Hyodo, Yoshinori, Miura, Kazuya, Wakatuki, Michiko
• Primary Examiner(s): Roe, Jessee R
• Application Number:
  US15/781,396
• Publication Number:
  US 20180363122A1
• Time in Patent Office: 1,418 Days
• CPC Class Codes: C21D 1/06 Surface hardening
• CPC Class Codes: C21D 6/004 containing Cr and Ni
• CPC Class Codes: C21D 6/005 containing Mn
• CPC Class Codes: C21D 6/007 containing Co
• CPC Class Codes: C21D 6/008 containing Si
• CPC Class Codes: C21D 8/0205 of ferrous alloys
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0263 following hot rolling
• CPC Class Codes: C21D 8/0278 involving a particular surface treatment C21D8/0294 takes precedence
• CPC Class Codes: C21D 9/46 for sheet metals
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/002 containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/005 containing rare earths, i.e. Sc, Y, Lanthanides
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/42 with copper
• CPC Class Codes: C22C 38/44 with molybdenum or tungsten
• CPC Class Codes: C22C 38/46 with vanadium
• CPC Class Codes: C22C 38/48 with niobium or tantalum
• CPC Class Codes: C22C 38/50 : with titanium or zirconium
• CPC Class Codes: C22C 38/52 : with cobalt
• CPC Class Codes: C22C 38/54 : with boron
• CPC Class Codes: C23C 8/26 : of ferrous surfaces

Abstract

[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.

Claims

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.