Seamless steel pipe and method of manufacture thereof

Patent metadata

Field	Value
Publication number	US-12338942-B2
Application number	US-202118020458-A
Country	US
Kind code	B2
Filing date	Jul 21, 2021
Priority date	Aug 19, 2020
Publication date	Jun 24, 2025
Grant date	Jun 24, 2025

Abstract

A seamless steel pipe has a t/D out of 0.05 to 0.40, where t is a wall thickness (mm) of the seamless steel pipe, and D out is an outside diameter (mm) of the seamless steel pipe. The seamless steel pipe has a maximum depth d max (mm)≤0.350 mm for defects in an inner surface of the steel pipe on a cross section perpendicular to a pipe axis. The seamless steel pipe has an average defect depth d ave (mm)≤0.200 mm for defects having a depth of 0.050 mm or more in the inner surface of the steel pipe. The seamless steel pipe, per millimeter of an inner circumferential length of the pipe, has at most 30 defects having a depth of 0.050 mm or more in the inner surface of the steel pipe.

First claim

The invention claimed is:

1. A seamless steel pipe having:
a t/D out of 0.05 to 0.40, where t is a wall thickness (mm) of the seamless steel pipe, and D out is an outside diameter (mm) of the seamless steel pipe,
a maximum depth d max (mm)≤0.350 for defects in an inner surface of the seamless steel pipe in a cross section of the seamless steel pipe perpendicular to a pipe axis,
an average defect depth d ave (mm)≤0.200 for defects having a depth of 0.050 mm or more in the inner surface of the seamless steel pipe,
per millimeter of an inner circumferential length of the seamless steel pipe, at most 30 defects having a depth of 0.050 mm or more in the inner surface of the seamless steel pipe,
a chemical composition comprising, in mass %:
C: 0.05 to 0.45%,
Si: 0.05 to 0.45%,
Mn: 0.05 to 1.2%, and
Cr: less than 4.5%, and

a microstructure comprising ferrite grains having an average grain size of 15 μm or less.

2. The seamless steel pipe according to claim 1 , having a microstructure comprising:
prior austenite grains having an average grain size of 15 μm or more.

3. The seamless steel pipe according to claim 1 , wherein the chemical composition further comprises, in mass %, one or more selected from the group consisting of:
Ni: 0.5% or less,
Mo: 1.5% or less,
W: 2.5% or less,
N: 0.10% or less, and
B: 0.010% or less.

4. A method for manufacturing the seamless steel pipe of claim 1 , the method comprising:
subjecting a steel billet to pierce rolling, wall-thickness reduction and stretch rolling to form a steel pipe; and
sizing the steep pipe according to the following formula (1):
( D ini −D out ) 2 ×t 0 2 ×GD 2 ≤9980  (1),

where:
GD is an average grain size (mm) of austenite before sizing,
D ini is a steel pipe outside diameter (mm) before sizing,
D out is a steel pipe outside diameter (mm) after sizing, and
t 0 is a wall thickness (mm) before sizing,

thereby producing the seamless steel pipe of claim 1 .

5. A method for manufacturing the seamless steel pipe of claim 2 , the method comprising:
subjecting a steel billet to pierce rolling, wall-thickness reduction and stretch rolling to form a steel pipe; and
sizing the steep pipe according to the following formula (1):
( D ini −D out ) 2 ×t 0 2 ×GD 2 ≤9980  (1),

where:
GD is an average grain size (mm) of austenite before sizing,
D ini is a steel pipe outside diameter (mm) before sizing,
D out is a steel pipe outside diameter (mm) after sizing, and
t 0 is a wall thickness (mm) before sizing,

to produce the seamless steel pipe of claim 2 .

6. A method for manufacturing the seamless steel pipe of claim 3 , the method comprising:
subjecting a steel billet to pierce rolling, wall-thickness reduction and stretch rolling to form a steel pipe; and
sizing the steep pipe according to the following formula (1):
( D ini −D out ) 2 ×t 0 2 ×GD 2 ≤9980  (1),

where:
GD is an average grain size (mm) of austenite before sizing,
D ini is a steel pipe outside diameter (mm) before sizing,
D out is a steel pipe outside diameter (mm) after sizing, and
t 0 is a wall thickness (mm) before sizing,

to produce the seamless steel pipe of claim 3 .

7. The method according to claim 4 , further comprising:
subjecting the sized steel pipe to a heat treatment at a heating temperature in a range of 850 to 1,150° C. with at least 10 minutes of soaking retention time.

8. The method according to claim 5 , further comprising:
subjecting the sized steel pipe to a heat treatment at a heating temperature in a range of 850 to 1,150° C. with at least 10 minutes of soaking retention time.

9. The method according to claim 6 , further comprising:
subjecting the sized steel pipe to a heat treatment at a heating temperature in a range of 850 to 1,150° C. with at least 10 minutes of soaking retention time.

Assignees

• Jfe Steel Corp

Inventors

• Sasaki Shunsuke
• Tachi Ryosuke
• Yamasaki Hiroyuki
• Fukuda Hiroyuki

Classifications

• C21D8/10 Primary

  during manufacturing of tubular bodies · CPC title

• B21B19/04

  Rolling basic material of solid, i.e. non-hollow, structure; Piercing {, e.g. rotary piercing mills} · CPC title

• C22C38/22

  with molybdenum or tungsten · CPC title

• C22C38/001

  containing N · CPC title

• C22C38/44

  with molybdenum or tungsten · CPC title