U.S. patent application number 13/853584 was filed with the patent office on 2013-08-29 for steel plate for pipeline, having excellent hydrogen induced crack resistance, and preparation method thereof.
This patent application is currently assigned to HYUNDAI STEEL COMPANY. The applicant listed for this patent is Hyundai Steel Company. Invention is credited to Kyu-Tae Kim, Myung-Jin Lee, Kyu-Heop Park.
Application Number | 20130224063 13/853584 |
Document ID | / |
Family ID | 46136105 |
Filed Date | 2013-08-29 |
United States Patent
Application |
20130224063 |
Kind Code |
A1 |
Kim; Kyu-Tae ; et
al. |
August 29, 2013 |
STEEL PLATE FOR PIPELINE, HAVING EXCELLENT HYDROGEN INDUCED CRACK
RESISTANCE, AND PREPARATION METHOD THEREOF
Abstract
Disclosed are a steel plate for a line pipe having excellent
hydrogen induced crack resistance with a tensile strength of 450
MPa or more, and a preparation method thereof. According to the
present invention, the steel plate for a line pipe, having
excellent hydrogen induced crack resistance comprises: 0.03-0.05 wt
% of carbon (C); 0.2-0.3 wt % of silicon (Si); 0.5-1.3 wt % of
manganese (Mn); 0.010 wt % or less of phosphorus (P); 0.005 wt % or
less of sulfur (S); 0.02-0.05 wt % of aluminum (Al); 0.2-0.5 wt %
of nickel (Ni); 0.2-0.3 wt % of chromium (Cr); 0.03-0.05 wt % of
niobium (Nb); 0.02-0.05 wt % of vanadium (V); 0.01-0.02 wt % of
titanium (Ti); 0.001-0.004 wt % of calcium (Ca); and a balance of
iron (Fe) and inevitable impurities, and has a tensile strength of
450 MPa or more.
Inventors: |
Kim; Kyu-Tae; (Dangjin-gun,
KR) ; Lee; Myung-Jin; (Busan, KR) ; Park;
Kyu-Heop; (Gimhae, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hyundai Steel Company; |
|
|
US |
|
|
Assignee: |
HYUNDAI STEEL COMPANY
Incheon
KR
|
Family ID: |
46136105 |
Appl. No.: |
13/853584 |
Filed: |
March 29, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/KR2011/006373 |
Aug 29, 2011 |
|
|
|
13853584 |
|
|
|
|
Current U.S.
Class: |
420/84 ;
72/201 |
Current CPC
Class: |
C21D 8/0205 20130101;
C22C 38/06 20130101; C21D 8/0226 20130101; C21D 8/105 20130101;
C22C 38/02 20130101; C21D 2211/005 20130101; C22C 38/12 20130101;
C22C 38/002 20130101; C22C 38/14 20130101; C21D 8/10 20130101; C21D
2211/002 20130101; C22C 38/50 20130101; C22C 38/48 20130101; C22C
38/46 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/84 ;
72/201 |
International
Class: |
C21D 8/02 20060101
C21D008/02; C22C 38/48 20060101 C22C038/48; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/50 20060101
C22C038/50; C22C 38/46 20060101 C22C038/46 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 29, 2010 |
KR |
10-2010-0094599 |
Aug 29, 2011 |
KR |
10-2011-0086685 |
Claims
1. A steel plate comprising: carbon (C), 0.03.about.0.05 wt %;
silicon (Si), 0.2.about.0.3 wt %; manganese (Mn), 0.5.about.1.3 wt
%; phosphorous (P), 0.010 wt % or less; sulfur (S), 0.005 wt % or
less; aluminum (Al), 0.02.about.0.05 wt %; nickel (Ni),
0.2.about.0.5 wt %; chromium (Cr), 0.2.about.0.3 wt %; niobium
(Nb), 0.03.about.0.05 wt %; vanadium (V), 0.02.about.0.05 wt %;
titanium (Ti), 0.01.about.0.02 wt %; calcium (Ca),
0.001.about.0.004 wt %; and the balance of iron (Fe) and other
unavoidable impurities, the steel plate having a tensile strength
of 450 MPa or more.
2. The steel plate according to claim 1, wherein the steel plate
has a yield ratio (yield strength/tensile strength) of 80% or
less.
3. The steel plate according to claim 1, wherein microstructure of
the steel plate is a composite structure including acicular ferrite
and bainite structures.
4. The steel plate according to claim 3, wherein the composite
structure including acicular ferrite and bainite structures
occupies 30% or more of the entirety of the microstructure in terms
of cross-sectional area ratio.
5. The steel plate according to claim 4, wherein a composite
structure including ferrite and pearlite structures occupies 70% or
less of the entirety of the microstructure in terms of
cross-sectional area ratio.
6. A method of manufacturing a steel plate, comprising: (A)
reheating a steel slab including: carbon (C), 0.03.about.0.05 wt %;
silicon (Si), 0.2.about.0.3 wt %; manganese (Mn), 0.5.about.1.3 wt
%; phosphorous (P), 0.010 wt % or less; sulfur (S), 0.005 wt % or
less; aluminum (Al), 0.02.about.0.05 wt %; nickel (Ni),
0.2.about.0.5 wt %; chromium (Cr), 0.2.about.0.3 wt %; niobium
(Nb), 0.03.about.0.05 wt %; vanadium (V), 0.02.about.0.05 wt %;
titanium (Ti), 0.01.about.0.02 wt %; calcium (Ca),
0.001.about.0.004 wt %; and the balance of iron (Fe) and other
unavoidable impurities; (B) hot rolling the reheated steel slab;
and (C) cooling the hot-rolled steel plate.
7. The method according to claim 6, wherein the reheating (A) is
performed at a temperature of 1100.about.1250.degree. C.
8. The method according to claim 6, wherein the hot rolling (B) is
performed at a reduction rate of 50% to 70% based on the whole
reduction rate of 100 at an Ar.sub.3 temperature or less.
9. The method according to claim 6, wherein the hot rolling (B) has
a rolling finishing temperature of 750.about.850.degree. C.
10. The method according to claim 6, wherein the cooling (C) has a
cooling finishing temperature of 300.about.450.degree. C.
11. The method according to claim 6, wherein the cooling (C) is
performed at a cooling rate of 15.about.25.degree. C./sec.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of International
Application No. PCT/KR2011/006373 filed on Aug. 29, 2011, which
claims priority to Korean Application No. 10-2010-0094599 filed on
Sep. 29, 2010 and Korean Application No. 10-2011-0086685 filed on
Aug. 29, 2011, which applications are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present invention relates to a technology of
manufacturing a steel plate for pipelines having excellent
hydrogen-induced crack resistance for use as materials for oil
pipelines, and more particularly, to a steel plate for pipelines,
which does not suffer from significant reduction in impact
toughness and has excellent yield ratio and hydrogen-induced crack
resistance, and a method for manufacturing the same.
BACKGROUND ART
[0003] Recently, with recent trend of oil production from deep sea
and cryogenic sites, oil pipelines are increasing in diameter and
thus materials for oil pipelines require excellent mechanical and
chemical properties.
[0004] To fulfill such requirement, there is an increasing need for
development of a high strength/high toughness steel plate for
pipelines that have excellent hydrogen-induced crack resistance.
Such a steel plate for pipelines is generally produced through a
rolling process.
[0005] Rolling generally includes slab reheating, hot rolling,
cooling, and coiling.
[0006] In slab reheating operation, a half-steel product, that is,
a steel slab is reheated.
[0007] In hot rolling operation, the reheated slab is subjected to
hot rolling at a predetermined reduction rate using rolling
rolls.
[0008] In cooling operation, the hot-rolled steel plate is
cooled.
[0009] In coiling operation, the steel plate is coiled at a
predetermined temperature.
[0010] For example, such a steel plate for pipelines is disclosed
in Korean Patent Publication No. 10-2001-0060763A.
SUMMARY
[0011] An aspect of the present invention is to provide a steel
plate for pipelines, which has a low yield ratio and a tensile
strength of 450 MPa or more and exhibits excellent hydrogen-induced
crack resistance to be applied to materials for oil pipelines and
the like.
[0012] Another aspect of the present invention is to provide a
method of manufacturing a steel plate for pipelines having
excellent hydrogen-induced crack resistance by controlling process
conditions while optimizing a composition ratio of chromium (Cr)
and other alloy components excluding copper (Cu).
[0013] In accordance with one embodiment of the present invention,
a steel plate for pipelines having excellent hydrogen-induced crack
resistance includes: carbon (C): 0.03.about.0.05 wt %, silicon
(Si): 0.2.about.0.3 wt %, manganese (Mn): 0.5.about.1.3 wt %,
phosphorous (P): 0.010 wt % or less, sulfur (S): 0.005 wt % or
less, aluminum (Al): 0.02.about.0.05 wt %, nickel (Ni):
0.2.about.0.5 wt %, chromium (Cr): 0.2.about.0.3 wt %, niobium
(Nb): 0.03.about.0.05 wt %, vanadium (V): 0.02.about.0.05 wt %,
titanium (Ti): 0.01.about.0.02 wt %, calcium (Ca):
0.001.about.0.004 wt %, and the balance of iron (Fe) and other
unavoidable impurities, and has a tensile strength of 450 MPa or
more.
[0014] In accordance with another embodiment of the present
invention, a method of manufacturing a steel plate for pipelines
having excellent hydrogen-induced crack resistance includes: (A)
reheating a steel slab, the steel slab including carbon (C):
0.03.about.0.05 wt %, silicon (Si): 0.2.about.0.3 wt %, manganese
(Mn): 0.5.about.1.3 wt %, phosphorous (P): 0.010 wt % or less,
sulfur (S): 0.005 wt % or less, aluminum (Al): 0.02.about.0.05 wt
%, nickel (Ni): 0.2.about.0.5 wt %, chromium (Cr): 0.2.about.0.3 wt
%, niobium (Nb): 0.03.about.0.05 wt %, vanadium (V):
0.02.about.0.05 wt %, titanium (Ti): 0.01.about.0.02 wt %, calcium
(Ca): 0.001.about.0.004 wt % and the balance of iron (Fe) and other
unavoidable impurities; (B) hot rolling the reheated steel slab;
and (C) cooling the hot-rolled steel plate.
[0015] The steel plate for pipelines according to one embodiment
contains a suitable amount of chromium and is free from copper
(Cu), which is generally used in manufacture of steel plates,
thereby providing advantages such as insignificant reduction of
impact toughness and excellent hydrogen-induced crack
resistance.
[0016] The method of manufacturing a steel plate for pipelines
according to another embodiment provides a steel plate which
exhibits excellent hydrogen-induced crack resistance and allows
insignificant reduction in impact toughness even without containing
copper (Cu) by controlling rolling and cooling conditions while
optimizing a composition ratio of the steel plate.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a schematic flowchart of a method of manufacturing
a steel plate for pipelines in accordance with one embodiment of
the present invention.
[0018] FIG. 2 is a graph depicting yield strength and tensile
strength of specimens prepared in inventive examples and
comparative examples.
[0019] FIG. 3 is a graph depicting impact resistance according to
temperature of the specimens prepared in inventive examples and
comparative examples.
[0020] FIG. 4 depicts pictures showing occurrence of cracking of
specimens prepared in inventive examples and comparative examples
upon HIC testing.
DETAILED DESCRIPTION
[0021] The above and other aspects, features, and advantages of the
present invention will become apparent from the detailed
description of the following embodiments in conjunction with the
accompanying drawings. It should be understood that the present
invention is not limited to the following embodiments and may be
embodied in different ways, and that the embodiments are provided
for complete disclosure and a thorough understanding of the present
invention by those skilled in the art. The scope of the present
invention is defined only by the claims. Like components will be
denoted by like reference numerals throughout the
specification.
[0022] Now, a steel plate for pipelines in accordance with one
embodiment of the invention and a method for manufacturing the same
will be described in more detail with reference to the accompanying
drawings.
[0023] Steel Plate for Pipelines Having Excellent Hydrogen-Induced
Crack Resistance
[0024] A steel plate for pipelines having excellent
hydrogen-induced crack resistance according to one embodiment
includes: carbon (C): 0.03.about.0.05 wt %, silicon (Si):
0.2.about.0.3 wt %, manganese (Mn): 0.5.about.1.3 wt %, phosphorous
(P): 0.010 wt % or less, sulfur (S): 0.005 wt % or less, aluminum
(Al): 0.02.about.0.05 wt %, nickel (Ni): 0.2.about.0.5 wt %,
chromium (Cr): 0.2.about.0.3 wt %, niobium (Nb): 0.03.about.0.05 wt
%, vanadium (V): 0.02.about.0.05 wt %, titanium (Ti):
0.01.about.0.02 wt %, calcium (Ca): 0.001.about.0.004 wt %, and the
balance of iron (Fe) and other unavoidable impurities.
[0025] Next, functions and amounts of the respective components of
the steel plate for pipelines having excellent hydrogen-induced
crack resistance according to the embodiment will be described in
more detail.
[0026] Carbon (C)
[0027] Carbon (C) is an element for improving strength and hardness
of steel.
[0028] A higher content of carbon can provide higher strength, but
can cause deterioration in toughness of steel. In addition,
processability of the steel increase with increasing content of
carbon, causing increase in tensile strength and yield point while
decreasing elongation.
[0029] If the carbon content exceeds 0.05 wt % in the steel plate,
the steel plate can be deteriorated in hydrogen-induced crack
resistance. On the other hand, if the carbon content is less than
0.03 wt % in the steel plate, there can be difficulty in securing
strength of the steel plate.
[0030] Thus, advantageously, carbon is present in an amount of
0.03.about.0.05 wt % in the steel plate according to the present
invention.
[0031] Silicon (Si)
[0032] Silicon (Si) acts as an effective deoxidization element and
reinforces ferrite structure in steel while improving yield
strength.
[0033] Such effects of silicon can be sufficiently exhibited when
the silicon content is 0.2 wt % or more. If the silicon content
exceeds 0.3 wt % in the steel, toughness of the steel is
deteriorated, reducing formability and causing difficulty in
forging and processing.
[0034] Thus, advantageously, silicon is present in an amount of
0.2.about.0.3 wt % in the steel plate according to the present
invention.
[0035] Manganese (Mn)
[0036] Manganese (Mn) serves to improve quenching properties and
strength while increasing plasticity at high temperature to improve
casting properties. In particular, manganese is likely to bind with
an unfavorable component, that is, sulfur (S), thereby forming MnS
inclusions.
[0037] If manganese is added in an excessive amount exceeding 1.3
wt %, a steel slab can suffer from central segregation, which
promotes occurrence of hydrogen induced cracking at a segregated
portion of the steel slab. If the manganese content is less than
0.5 wt %, it is difficult to secure strength of the steel.
[0038] Thus, advantageously, manganese is present in an amount of
0.5.about.1.3 wt % in the steel plate according to the present
invention.
[0039] Phosphorous (P)
[0040] Phosphorous (P) is an element that is segregated into a
grain boundary, reducing toughness and impact resistance of steel,
and causes hydrogen-induced cracking in the steel.
[0041] Thus, advantageously, the phosphorous content is limited to
0.010 wt % or less in the steel plate according to the present
invention.
[0042] Sulfur (S)
[0043] Sulfur (S) is an essential element coupled to manganese (Mn)
to form MnS inclusions, thereby improving steel machinability.
However, if sulfur is present in an excessive amount in the steel,
sulfur deteriorates hot processability of the steel, causes
fracture, and forms coarse inclusions, causing defects upon surface
treatment.
[0044] Thus, advantageously, sulfur is present in an amount of
0.005 wt % or less in the steel plate according to the present
invention.
[0045] Aluminum (Al)
[0046] Aluminum (Al) is a strong deoxidization element and is
coupled to nitrogen for grain refinement. However, if the aluminum
content exceeds 0.05 wt % in the steel, there can be problems of
deterioration in impact toughness and hydrogen-induced crack
resistance. Further, if the aluminum content is less than 0.02 wt
%, insufficient deoxidization can be obtained. Thus,
advantageously, aluminum is present in an amount of 0.02.about.0.05
wt % in the steel plate according to the present invention.
[0047] Nickel (Ni)
[0048] In this invention, the content of nickel (Ni) is suitably
adjusted to obtain desired yield strength and a yield ratio of 80%
or less even in the absence of copper (Cu). If the nickel content
is less than 0.2 wt %, it is difficult for the steel to have a
yield strength of 450 MPa or more. If the nickel content exceeds
0.5 wt %, the steel has a yield ratio exceeding 80%. Thus,
advantageously, nickel is present in an amount of 0.2.about.0.5 wt
% in the steel plate according to the present invention.
[0049] Chromium (Cr)
[0050] According to the present invention, the steel plate includes
chromium and is free from copper (Cu), which is generally used in
manufacture of existing steel plates. Copper can cause
deterioration of weldability and surface quality of the steel
plate. Thus, the steel plate of the invention does not contain
copper and contains an optimal amount of chromium.
[0051] Through addition of chromium, it is possible to manufacture
a steel plate, which does not suffer from significant reduction in
impact toughness and has a low yield ratio and excellent
hydrogen-induced crack resistance. Here, if the chromium content
exceeds 0.3 wt %, the steel plate can suffer from deterioration in
hydrogen-induced crack resistance. If the chromium content is less
than 0.2 wt %, the steel plate cannot obtain desired strength.
Thus, advantageously, chromium is present in an amount of
0.2.about.0.3 wt % in the steel plate according to the present
invention.
[0052] Niobium (Nb)
[0053] Niobium (Nb) prevents grains of steel from being coarsened
at high temperature and promotes refinement of the grains to
improve ductility and toughness of the steel.
[0054] To obtain strength improvement, niobium is desirably added
in an amount of 0.03 wt % or more. Since secondary phases
containing niobium can act as sites for initiation of
hydrogen-induced cracking, an upper niobium limit is set to 0.05 wt
%.
[0055] Thus, advantageously, niobium is present in an amount of
0.03.about.0.05 wt % in the steel plate according to the present
invention.
[0056] Vanadium (V)
[0057] Vanadium (V) serves to improve resistance to
hydrogen-induced cracking.
[0058] Advantageously, vanadium is present in an amount of
0.02.about.0.05 wt % in steel. If the vanadium content is less than
0.02 wt %, the effect of vanadium is not sufficiently exhibited. On
the contrary, if the vanadium content exceeds 0.05 wt %, the steel
can suffer from deterioration in toughness and hydrogen-induced
crack resistance.
[0059] Titanium (Ti)
[0060] Titanium is an element which forms carbide or nitride in
steel, and serves to improve both strength and low temperature
toughness through grain refinement.
[0061] Titanium precipitates reduces a diffusion coefficient of
hydrogen and increases hydrogen-induced crack resistance. If the
titanium content exceeds 0.02 wt %, the steel can be deteriorated
in hydrogen-induced crack resistance, and if the titanium content
is less than 0.01 wt %, it is difficult to obtain desired strength.
Thus, advantageously, titanium is present in an amount of
0.01.about.0.02 wt % in the steel plate according to the present
invention.
[0062] Calcium (Ca)
[0063] Calcium is an element for spheroidizing MnS inclusions. MnS
inclusions have a low melting point and are elongated upon rolling
to act as starting point of hydrogen-induced cracking. The added
calcium reacts with MnS to surround the MnS inclusions, thereby
obstructing elongation of the MnS inclusions.
[0064] For efficient spheroidization of the MnS inclusions, calcium
is advantageously present in an amount of 0.001 wt % or more. On
the other hand, if the calcium content is excessive, an excess of
oxide inclusions acting as starting points of hydrogen-induced
cracking can be created. Thus, advantageously, an upper limit of
the calcium content is set to 0.004 wt %.
[0065] Advantageously, the steel plate for pipelines according to
the present invention has a yield ratio (YS)/(TS) of 80% or
less.
[0066] In addition, advantageously, the microstructure of the steel
plate comprises a composite structure consisting of acicular
ferrite and bainite structures. Here, advantageously, a composite
structure consisting of acicular ferrite and bainite structures
occupies 30% or more of the entirety of the microstructure in terms
of cross-sectional area ratio, and a composite structure consisting
of ferrite and pearlite structures occupies 70% or less of the
entirety of the microstructure in terms of cross-sectional area
ratio.
[0067] If the composite structure consisting of acicular ferrite
and bainite structures occupies 30% or less of the entirety of the
microstructure in terms of cross-sectional area ratio, it is
difficult to achieved desired strength.
[0068] Method of Manufacturing a Steel Plate for Pipelines Haying
Excellent Hydrogen-Induced Crack Resistance
[0069] FIG. 1 is a schematic flowchart of a method of manufacturing
a steel plate for pipelines in accordance with one embodiment of
the present invention.
[0070] Referring to FIG. 1, the method of manufacturing the steel
plate for pipelines includes: (A) reheating a steel slab, the steel
slab including carbon (C): 0.03.about.0.05 wt %, silicon (Si):
0.2.about.0.3 wt %, manganese (Mn): 0.5.about.1.3 wt %, phosphorous
(P): 0.010 wt % or less, sulfur (S): 0.005 wt % or less, aluminum
(Al): 0.02.about.0.05 wt %, nickel (Ni): 0.2.about.0.5 wt %,
chromium (Cr): 0.2.about.0.3 wt %, niobium (Nb): 0.03.about.0.05 wt
%, vanadium (V): 0.050.about.0.095 wt %, titanium (Ti):
0.01.about.0.02 wt %, calcium (Ca): 0.001.about.0.004 wt %, and the
balance of iron (Fe) and other unavoidable impurities; (B) hot
rolling the reheated steel slab; and (C) cooling the hot-rolled
steel plate.
[0071] The method of manufacturing the steel plate for pipelines
according to the invention reduces fractions of polygonal ferrite
and band structures relatively vulnerable to hydrogen-induced
cracking, and includes finish-rolling to be performed at an
Ar.sub.3 transformation temperature or less to induce generation of
mobile dislocations, which are advantageous for reduction of yield
ratio. As the generation of mobile dislocations is induced by this
method, the steel plate is reduced in yield strength, thereby
lowering the yield ratio. That is, the steel plate according to the
present invention has a low yield ratio, thereby providing
excellent plastic deformation and anti-vibration effects. Further,
in the manufacturing method of the steel plate according to the
invention, the cooling rate is controlled to form acicular ferrite
and bainite structures in a fraction of 30% or more.
[0072] The manufacturing method of the steel plate according to the
present invention will be described in more detail.
[0073] (A) Slab Reheating (S110)
[0074] In continuous casting of a steel slab, elements such as Mn,
P, S, and the like are likely to be segregated in the steel slab,
so that the steel slab has a higher concentration in a central
region than in peripheral regions. Since such central segregation
provides a propagation passage of hydrogen-induced cracking, it is
desirable to suppress central segregation. During slab reheating,
such elements causing central segregation diffuse into peripheral
regions, thereby relieving central segregation.
[0075] Advantageously, reheating is performed at temperatures of
1000.degree. C. or more in order to relieve central
segregation.
[0076] Further, Nb and V can be sufficiently dissolved in the steel
during reheating of the steel slab and can be finely precipitated
to increase strength of the steel during rolling. Thus,
advantageously, the steel slab may be reheated at a temperature
from 1100.degree. C. to 1250.degree. C. to achieve sufficient
dissolution of Nb and V in the steel slab.
[0077] (B) Hot Rolling (S120)
[0078] (Finish-rolling finishing temperature: 750.about.850.degree.
C.)
[0079] As mentioned above, in the method of manufacturing the steel
plate according to the invention, finish-rolling is performed at an
Ar.sub.3 transformation temperature or less to induce generation of
mobile dislocations which are favorable to reduction of the yield
ratio.
[0080] The finish-rolling finishing temperature may be set to
750.degree. C. or more to ensure that the steel plate has excellent
hydrogen-induced crack resistance and the acicular ferrite and
bainite structures are formed in a fraction of 30% or more.
[0081] Although the pearlite fraction is decreased with increasing
finish-rolling finishing temperature before initiation of
quenching, the strength of the steel is also decreased. Thus,
advantageously, the finish-rolling finishing temperature is set to
850.degree. C. or less to prevent the decrease in strength of the
steel plate.
[0082] (Finish-rolling reduction rate: 50% to 70% based on a
reduction rate of 100 at an Ar.sub.3 transformation temperature or
less)
[0083] Advantageously, the reduction rate of hot rolling is set in
the range of 50% to 70% based on a reduction rate of 100 at an
Ar.sub.3 transformation temperature or less in order to restrict an
average grain size of acicular ferrite microstructure in a final
product of the steel plate according to the present invention.
[0084] (C) Cooling (S130)
[0085] (Cooling finishing temperature: 300.about.450.degree.
C.)
[0086] In order to improve hydrogen-induced crack resistance, the
cooling finishing temperature may be restricted in the cooling
stage.
[0087] An excess of ferrite and pearlite microstructures can cause
deterioration not only in hydrogen-induced crack resistance but
also in low temperature toughness. Thus, the cooling finishing
temperature may be set to 300.degree. C.
[0088] However, since a cooling finishing temperature exceeding
450.degree. C. can cause increase in pearlite fraction in the
microstructure of the steel plate, the cooling finishing
temperature may be sent to 450.degree. C. or less.
[0089] (Cooling rate: 15.about.25.degree. C./sec)
[0090] By controlling the cooling rate in the cooling stage, it is
possible to control central microstructure and hardness of the
steel plate according to the invention.
[0091] A cooling rate of less than 15.degree. C./sec makes it
difficult for the steel plate to obtain sufficient hardness. In
addition, a cooling rate exceeding 25.degree. C./sec can cause
deterioration in hydrogen-induced crack resistance.
[0092] Advantageously, the cooling rate is set in the range of
15.about.25.degree. C./sec.
Example
[0093] Next, constitution and operation of the present invention
will be described in more detail with reference to some inventive
examples. It should be noted that the following examples are
provided for illustration only and should not be construed in any
way as limiting the scope of the present invention.
[0094] Descriptions of details apparent to those skilled in the art
will be omitted.
[0095] 1. Preparation of Specimens
[0096] Table 1 shows compositions of steel specimens prepared in
examples and comparative examples.
[0097] In Table 1, steel specimens prepared in Comparative Examples
1 to 3 are conventional steel plates for pipelines, and steel
specimens prepared in Examples 1 to 3 are inventive steel plates
for pipelines in which chromium and other alloy components are
present in a suitable composition ratio without adding copper.
TABLE-US-00001 TABLE 1 No. C Si Mn P S Cr Ni Al Cu Ti Nb V Ca Ca/S
Ceq CE 1 0.04 0.25 1.20 0.005 0.0012 -- -- 0.021 -- 0.013 0.04
0.027 0.0018 1.4 0.245 CE 2 0.04 0.25 1.20 0.005 0.0012 -- 0.23
0.020 0.16 0.014 0.039 0.031 0.0019 1.6 0.272 CE 3 0.04 0.25 1.21
0.005 0.0011 0.24 0.24 0.022 0.20 0.013 0.038 0.028 0.0017 1.5
0.325 Ex. 1 0.04 0.25 1.20 0.005 0.0012 0.23 0.21 0.021 -- 0.014
0.040 0.029 0.0019 1.6 0.302 Ex. 2 0.04 0.24 1.21 0.006 0.0011 0.24
0.40 0.021 -- 0.014 0.040 0.030 0.0018 1.6 0.332 Ex. 3 0.04 0.26
1.20 0.006 0.0012 0.25 0.23 0.020 -- 0.014 0.039 0.027 0.0018 1.5
0.311
[0098] The steel specimens prepared in Comparative Examples 1 to 3
are conventional steel plates for pipelines, and the steel
specimens prepared in Examples 1 to 3 are inventive steel plates
for pipelines in which chromium and other alloy components are
present in a suitable composition ratio without adding copper.
[0099] 2. Measurement and Evaluation of Physical Properties
[0100] Each of the specimens prepared in the comparative examples
and the inventive examples was subjected to tensile testing, impact
testing and HIC (hydrogen-induced cracking) testing to observe
occurrence of cracking.
[0101] FIG. 2 is a graph depicting yield strength and tensile
strength of each of the specimens prepared in the inventive
examples and the comparative examples. In the bar graph, left-side
bars indicate yield strength (YS) and right-side bars indicate
tensile strength (TS).
[0102] The specimens of Examples 1 to 3 did not contain copper
(Cu), which is included in conventional steel plates for pipelines.
It can be confirmed that these steel specimens have a tensile
strength of 450 MPa or more even without containing Cu.
[0103] Since the specimens of the inventive examples did not
undergo significant reduction in impact toughness even without
containing Cu, the specimens of the inventive examples had similar
results to the specimens of the comparative examples in impact
testing. FIG. 3 is a graph depicting results of impact testing with
respect to the respective specimens of the inventive examples and
the comparative examples.
[0104] Results of tensile testing and impact testing are summarized
in Table 2.
TABLE-US-00002 TABLE 2 Tensile testing Impact testing Specimen YS
(MPa) TS (MPa) EL (%) 0.degree. C. -80.degree. C. Comparative 452
531 35 357 302 Example 1 Comparative 484 560 30 375 302 Example 2
Comparative 512 610 25 324 305 Example 3 Example 1 455 580 27 337
304 Example 2 533 651 29 362 327 Example 3 480 640 27 359 292
[0105] FIG. 4 shows results of hydrogen-induced cracking
testing.
[0106] Pictures of the specimens before and after HIC testing are
provided in this drawing.
[0107] It can be seen that the specimens of Examples 1 to 3 undergo
no cracking and have excellent hydrogen-induced crack
resistance.
[0108] In Table 3, results of HIC testing with respect to the
respective specimens are summarized.
TABLE-US-00003 TABLE 3 Total Total length of thickness of Specimen
crack (mm) crack (mm) CLR CTR CSR Comparative 3.4 0.07 5.7% 0.23%
0.04% Example 1 Comparative 1.8 0.2 3% 0.67% 0.06% Example 2
Comparative 0 0 0 0 0 Example 3 Example 1, 2, 3 0 0 0 0 0 CLR:
Crack length ratio, CTR: Crack thickness ratio, CSR: Crack
sensitivity ratio
[0109] Although some embodiments have been described herein, it
will be understood by those skilled in the art that these
embodiments are provided for illustration only, and various
modifications, changes, alterations and equivalent embodiments can
be made without departing from the scope of the present invention.
Therefore, the scope and spirit of the present invention should be
defined only by the accompanying claims and equivalents
thereof.
* * * * *