Steel for cold forging and manufacturing method thereof

Patent metadata

Field	Value
Publication number	US-11111568-B2
Application number	US-201616334705-A
Country	US
Kind code	B2
Filing date	Sep 30, 2016
Priority date	Sep 30, 2016
Publication date	Sep 7, 2021
Grant date	Sep 7, 2021

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.

First claim

What is claimed is:

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/mm 2 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.

2. 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%.

3. 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%.

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%.

5. 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.

6. 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.

Assignees

• Nippon Steel & Sumitomo Metal Corp
• Nippon Steel Corp

Inventors

• Shiga Akira
• Kubota Manabu
• Hasegawa Hajime

Classifications

• C21D8/00

  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) · CPC title

• C21D8/06

  during manufacturing of rods or wires · CPC title

• C22C38/02 Primary

  containing silicon · CPC title

• C22C38/04

  containing manganese · CPC title

• C22C38/18

  containing chromium · CPC title