# US 11,111,568 B2 · Steel for cold forging and manufacturing method thereof

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- Number: US 11,111,568 B2
- Document ID: US11111568B2
- Status: Granted
- Kind: B2
- Application number: 16/334,705
- Filed: 2016-09-30
- Published: 2019-08-29
- Granted: 2021-09-07
- Earliest claimed: 2016-09-30
- Assignees: Nippon Steel Corporation
- Applicants: Nippon Steel & Sumitomo Metal Corporation
- Inventors: Akira Shiga; Manabu Kubota; Hajime Hasegawa
- Web page: https://patentgrep.com/patent/US11111568B2

## CPC

- C22C 38/44: with molybdenum or tungsten
- B22D 27/04: Influencing the temperature of the metal, e.g. by heating or cooling the mould
- C21D 8/00: Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment (hardening articles or materials formed by forging or rolling with no further heating beyond that required for the formation C21D1/02)
- C21D 9/0075: {for rods of limited length (of unlimited length C21D9/52)}
- C22C 38/00: Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00)
- C22C 38/001: {containing N}
- C22C 38/002: {containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60}
- C22C 38/02: containing silicon
- C22C 38/04: containing manganese
- C22C 38/06: containing aluminium
- C22C 38/18: containing chromium
- C22C 38/22: with molybdenum or tungsten
- C22C 38/24: with vanadium
- C22C 38/26: with niobium or tantalum
- C22C 38/28: with titanium or zirconium
- C22C 38/32: with boron
- C21D 1/32: Soft annealing, e.g. spheroidising
- C21D 8/06: during manufacturing of rods or wires
- C21D 9/28: for plain shafts
- C21D 9/32: for gear wheels, worm wheels, or the like
- C21D 9/525: {for wire, for rods (C21D9/54 takes precedence)}
- C21D 2211/004: Dispersions; Precipitations
- C22C: ALLOYS (treatment of alloys C21D, C22F)
- B22D: CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES (shaping of plastics or substances in a plastic state B29C; metallurgical processing, selection of substances to be added to metal C21, C22)
- C21D: MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING (surface treatment of metallic material involving at least one process provided for in class C23 and at least one process covered by this subclass C23F17/00; unidirectional solidification of eutectic materials or unidirectional demixing of eutectoid materials C30B)

## Abstract

A steel for cold forging has a predetermined chemical composition, satisfies d+3σ≤10.0 and SA/SB<0.30, includes 1200/mm2 or more of sulfides having an equivalent circle diameter of 1.0 to 10.0 μm in a microstructure, and has an average distance between the sulfides of less than 30.0 μm. Here, d is an average value of equivalent circle diameters of sulfides having an equivalent circle diameter of 1.0 μm or more, σ is a standard deviation of the equivalent circle diameters of the sulfides having an equivalent circle diameter of 1.0 μm or more, SA is the number of sulfides having an equivalent circle diameter of 1.0 μm or more and less than 3.0 μm, and SB is the number of the sulfides having an equivalent circle diameter of 1.0 μm or more.

## Claims

### Claim 1

A steel for cold forging comprising, as a chemical composition, by mass %,
C: 0.05% to 0.30%,
Si: 0.05% to 0.45%,
Mn: 0.40% to 2.00%,
S: 0.008% to less than 0.040%,
Cr: 0.01% to 3.00%,
Al: 0.010% to 0.100%,
Bi: 0.0001% to 0.0050%,
Mo: 0% to 1.00%,
Ni: 0% to 1.00%,
V: 0% to 0.30%,
B: 0% to 0.0200%,
Mg: 0% to 0.0035%,
Ti: 0% to 0.060%,
Nb: 0% to 0.080%, and
a remainder of Fe and impurities,
wherein N, P, and O contained in the impurities are as follows
N: 0.0250% or less,
P: 0.050% or less, and
O: 0.0020% or less,
1200/mm2 or more of sulfides having an equivalent circle diameter of 1.0 to 10.0 μm are contained in a microstructure,
an average distance between the sulfides is less than 30.0 μm, and
the sulfides in the microstructure satisfy Expression (1) and Expression (2):
d+3σ≤10.0 . . . (1)
SA/SB<0.30 . . . (2),
in Expression (1), d is an average value of equivalent circle diameters of the sulfides having an equivalent circle diameter of 1.0 μm or more, σ is a standard deviation of the equivalent circle diameters of the sulfides having the equivalent circle diameter of 1.0 μm or more, in Expression (2), SA is a number of sulfides having the equivalent circle diameter of 1.0 μm or more and less than 3.0 μm, and SB is a number of sulfides having an equivalent circle diameter of 1.0 μm or more.

### Claim 2 (depends on 1)

The steel for cold forging according to claim 1, comprising, as the chemical composition, by mass %, one or more of:
Mo: 0.02% to 1.00%,
Ni: 0.10% to 1.00%,
V: 0.03% to 0.30%,
B: 0.0005% to 0.0200%, and
Mg: 0.0001% to 0.0035%.

### Claim 3 (depends on 1)

The steel for cold forging according to claim 1, comprising, as the chemical composition, by mass %, one or two of:
Ti: 0.007% to 0.060%, and
Nb: 0.010% to 0.080%.

### Claim 4

The steel for cold forging according to claim comprising, as the chemical composition, by mass %, one or two of:
Ti: 0.002% to 0.060%, and
Nb: 0.010% to 0.080%.

### Claim 5 (depends on 1)

A manufacturing method of a steel for cold forging, comprising:
casting a cast piece having the chemical composition according to claim 1, and having a dendrite primary arm spacing of less than 600 μm in a range of 15 mm from a surface of the cast piece;
performing a hot working on the cast piece to obtain a steel material; and
annealing the steel material.

### Claim 6 (depends on 5)

The manufacturing method of a steel for cold forging according to claim 5,
wherein, in the casting, an average cooling rate in a temperature range from a liquidus temperature to a solidus temperature at a depth of 15 mm from the surface of the cast piece is set to 120° C./min to 500° C./min.