Steel for bolt use, bolt, and method for manufacturing bolt

Patent metadata

Field	Value
Publication number	US-9695488-B2
Application number	US-201214370814-A
Country	US
Kind code	B2
Filing date	Nov 26, 2012
Priority date	Jan 11, 2012
Publication date	Jul 4, 2017
Grant date	Jul 4, 2017

Abstract

A steel is used for providing a bolt that has a high strength and still exhibits excellent hydrogen embrittlement resistance. The steel contains C of 0.30% to 0.50%, Si of 1.0% to 2.5%, Mn of 0.1% to 1.5%, P of greater than 0% to 0.015%, S of greater than 0% to 0.015%, Cr of 0.15% to 2.4%, Al of 0.010% to 0.10%, N of 0.001% to 0.10%, Cu of 0.1% to 0.50%, Ni of 0.1% to 1.0%, Ti of 0.05% to 0.2%, and V of 0% to 0.2%, with the remainder including iron and inevitable impurities, in which a ratio [Ni]/[Cu] is 0.5 or more, and a total content [Ti]+[V] is 0.085% to 0.30%.

First claim

The invention claimed is:

1. A bolt, comprising: a steel comprising
C in a mass percentage of 0.30% or more and less than 0.45%;
Si in a mass percentage of 1.0% to 2.5%;
Mn in a mass percentage of 0.1% to 1.5%;
P in a mass percentage of greater than 0% to 0.015%;
S in a mass percentage of greater than 0% to 0.015%;
Cr in a mass percentage of 0.15% to 2.4%;
Al in a mass percentage of 0.010% to 0.10%;
N in a mass percentage of 0.001% to 0.10%;
Cu in a mass percentage of 0.1% to 0.50%, and Ni in a mass percentage of 0.1% to 1.0%, so that [Ni]/[Cu]≧0.5 is met, where [Ni] and [Cu] denote the mass percentage of Ni and Cu in the steel, respectively; and
Ti in a mass percentage of 0.05% to 0.2%, and V in a mass percentage of 0% to 0.2%, so that [Ti]+[V] is 0.085% to 0.30%, where [Ti] and [V] denote the mass percentage of Ti and V in the steel respectively; and
iron,
wherein
the bolt has an austenitic grain size number of 9.0 or more in a shank thereof;
the bolt has a G-value meeting a condition as specified by Expression (1) as follows:
G -value=( L/L 0)×100≦60  (1)

where:
the G-value indicates a percentage of carbide particles formed at austenite grain boundaries in the shank,
L denotes a total length of carbide particles formed at the austenite grain boundaries and having a thickness of 50 nm or more; and
L0 denotes a length of the austenite grain boundaries.

2. The bolt according to claim 1 , comprising an iron oxide layer containing Si and Cu on a surface of the shank and having a thickness of 2.0 to 100 nm.

3. The bolt according to claim 1 , having a tensile strength of 1400 MPa or more.

4. A method for manufacturing the bolt according to claim 1 , the method comprising:
heating the steel to a temperature of 1050° C. or higher;
hot-rolling the steel after the heating and performing finish rolling at a temperature of 1000° C. or lower;
forming the bolt; and
treating the bolt in a quenching-tempering process after bolt forming comprising a quenching at a quenching temperature of 860° C. to 930° C. and a tempering at a tempering temperature of 400° C. to T° C., where T is specified by Expression (2) as follows:
T (° C.)=68.2 Ln [Si]+480  (2)

where Ln denotes a natural logarithm; and [Si] denotes the mass percentage of Si in the steel.

5. The method according to claim 4 , wherein:
before tempering, the bolt has an iron oxide layer on a shank surface of 0 to 100 nm thick; and
the tempering is performed in an inert gas atmosphere having an oxygen content of 10 ppm by volume or less.

6. The bolt according to claim 1 , wherein the steel further comprises:
Mo in a mass percentage of greater than 0% to 0.1%.

7. The bolt according to claim 1 , having a delayed fracture strength ratio of 0.70 or more.

8. The bolt according to claim 1 , wherein the G-value is 45% or less.

9. The bolt according to claim 1 , wherein the G-value is 35% or less.

10. The bolt according to claim 1 , wherein the G-value is 5% or more.

11. The bolt according to claim 1 , wherein the austenitic grain size number is 9.5 or more.

12. The bolt according to claim 1 , wherein the austenitic grain size number is 10.0 or more.

13. The bolt according to claim 1 , wherein the austenitic grain size number is 15 or less.

14. The method according to claim 4 , wherein the quenching temperature is 880° C. or higher.

15. The method according to claim 4 , wherein the quenching temperature is 890° C. or higher.

16. The method according to claim 4 , wherein the quenching temperature is 920° C. or lower.

17. The method according to claim 4 , wherein the quenching temperature is 910° C. or lower.

18. The bolt according to claim 1 , wherein
the steel does not comprise V and
Ti is present in a mass percentage of 0.085% to 0.2%.

Assignees

• Kobe Steel Ltd

Inventors

• Matsumoto Yosuke
• Inada Atsushi
• Chiba Masamichi

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

• C22C38/001

  containing N · CPC title

• C21D9/0093 Primary

  for screws; for bolts · CPC title

• C21D1/18

  Hardening (C21D1/02 takes precedence); Quenching with or without subsequent tempering (quenching devices C21D1/62) · CPC title

• C22C38/00

  Ferrous alloys, e.g. steel alloys (cast-iron alloys C22C37/00) · CPC title