High-strength steel, method for manufacturing high-strength steel, steel pipe, and method for manufacturing steel pipe

Patent metadata

Field	Value
Publication number	US-10570477-B2
Application number	US-201615561236-A
Country	US
Kind code	B2
Filing date	Mar 25, 2016
Priority date	Mar 27, 2015
Publication date	Feb 25, 2020
Grant date	Feb 25, 2020

Abstract

High-strength steel having a specified chemical composition, wherein X (%) calculated by using equation (1):

X =0.35Cr+0.9Mo+12.5Nb+8V  (1)

is 0.75% or more, wherein the symbols of chemical elements in equation (1) respectively denote the contents (mass %) of the corresponding chemical elements and wherein the symbol of a chemical element which is not included is assigned a value of 0, a microstructure having a bainite phase fraction of 50% or more, a dislocation density of 1.0×10 15 /m 2 or more after aging has been performed under the condition of a Larson-Miller parameter (LMP) of 15000, and a yield strength of 550 MPa or more before and after the aging is performed, as well as methods of manufacturing such high-strength steel, is disclosed. The high-strength steel can be used as a raw material for manufacturing large diameter steel pipe having the strength properties required for its use in steam transportation.

First claim

The invention claimed is:

1. High-strength steel having
a chemical composition containing, by mass %, C: 0.04% to 0.09%, Si: 0.05% to 0.20%, Mn: 1.5% to 2.0%, P: 0.020% or less, S: 0.002% or less, Cr: 0.50% or less, Mo: 0.10% to 0.50%, Nb: 0.010% to 0.050%, V: 0.070% or less, Ti: 0.005% to 0.02%, Al: 0.01% to 0.04%, N: 0.006% or less, and the balance being Fe and inevitable impurities, wherein X (%), as calculated by equation (1):
X (%)=0.35Cr+0.9Mo+12.5Nb+8V  (1),

is 0.75% or more, wherein the symbols of chemical elements in equation (1) respectively denote the contents (mass %) of the corresponding chemical elements, and the symbol of a chemical element which is not included is assigned a value of 0
a microstructure having a bainite phase fraction of 50% or more,
a dislocation density of 1.0 ×10 15 /m 2 or more after aging has been performed under the condition of a Larson-Miller parameter (LMP) of 15000, and
a yield strength of 550 MPa or more before and after the aging is performed.

2. The high-strength steel according to claim 1 , wherein the chemical composition of the high-strength steel further contains, by mass %, one or both of Cu: 0.50% or less and Ni: 0.50% or less.

3. A steel pipe composed of the high-strength steel according to claim 1 .

4. A method for manufacturing the high-strength steel according to claim 1 , the method comprising:
a heating process wherein a steel raw material is heated to a temperature of 1100° C. to 1200° C.;
a hot rolling process wherein the steel raw material, which has been heated in the heating process, is hot-rolled under the conditions of an accumulated rolling reduction ratio in a temperature range of 900° C. or lower of 50% or more and a rolling finish temperature of 850° C. or lower; and
an accelerated cooling process wherein the hot-rolled steel plate, which has been obtained in the hot rolling process, is subjected to accelerated cooling under the conditions of a cooling rate of 5° C/s or more and a cooling stop temperature of 300° C. to 550° C.

5. A method for manufacturing a steel pipe, the method comprising:
a cold forming process wherein a steel plate composed of the high-strength steel according to claim 1 is subjected to cold forming so as to be formed into a pipe shape; and
a welding process wherein the butt portions of the steel plate, which has been formed into a pipe shape in the cold forming process, are welded.

6. A steel pipe composed of the high-strength steel according to claim 2 .

7. A method for manufacturing the high-strength steel according to claim 2 , the method comprising:
a heating process wherein a steel raw material is heated to a temperature of 1100° C. to 1200° C.;
a hot rolling process wherein the steel raw material, which has been heated in the heating process, is hot-rolled under the conditions of an accumulated rolling reduction ratio in a temperature range of 900° C. or lower of 50% or more and a rolling finish temperature of 850° C. or lower; and
an accelerated cooling process wherein the hot-rolled steel plate, which has been obtained in the hot rolling process, is subjected to accelerated cooling under the conditions of a cooling rate of 5° C/s or more and a cooling stop temperature of 300° C. to 550° C.

8. A method for manufacturing a steel pipe, the method comprising:
a cold forming process wherein a steel plate composed of the high-strength steel according to claim 2 is subjected to cold forming so as to be formed into a pipe shape; and
a welding process wherein the butt portions of the steel plate, which has been formed into a pipe shape in the cold forming process, are welded.

9. The high-strength steel according to claim 1 , wherein the total amount of the Cu content, the Ni content, the Cr content, and the Mo content (Cu+Ni+Cr+Mo) is 0.90% or less.

Assignees

• Jfe Steel Corp

Inventors

• Ota Shusaku
• Shimamura Junji
• Ishikawa Nobuyuki
• Endo Shigeru

Classifications

• C21D6/005

  containing Mn · CPC title

• C22C38/001

  containing N · CPC title

• C21D8/0226

  Hot rolling · CPC title

• C21D2211/002

  Bainite · CPC title

• C22C38/24

  with vanadium · CPC title