U.S. patent application number 15/103093 was filed with the patent office on 2016-10-27 for steel plate and method for manufacturing same (as amended).
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Shigeru Endo, Kazukuni Hase, Katsuyuki Ichimiya, Shigeki Kitsuya.
Application Number | 20160312327 15/103093 |
Document ID | / |
Family ID | 53371334 |
Filed Date | 2016-10-27 |
United States Patent
Application |
20160312327 |
Kind Code |
A1 |
Ichimiya; Katsuyuki ; et
al. |
October 27, 2016 |
STEEL PLATE AND METHOD FOR MANUFACTURING SAME (AS AMENDED)
Abstract
A steel plate includes a predetermined chemical composition, in
which B is controlled to be less than 0.0003%, and the balance is
Fe and incidental impurities. The steel plate also includes
precipitates containing Ti, Nb, and Mo and having a mean particle
size of 20 nm or less, the relationship
[Nb]/([Ti]+[Nb]+[Mo]).gtoreq.0.3 being satisfied, where [Ti] is the
Ti content, [Nb] is the Nb content, and [Mo] is the Mo content,
thereby providing a thick, high tensile strength steel plate that
is suitable for use in steel structures such as marine structures,
ships, pressure vessels, and penstocks, has a yield stress (YS) of
460 MPa or greater, and has excellent low-temperature toughness of
the heat-affected zone in a multilayer weld (CTOD property) and
excellent strength and toughness after Post Weld Heat Treatment
(PWHT property).
Inventors: |
Ichimiya; Katsuyuki; (Tokyo,
JP) ; Kitsuya; Shigeki; (Tokyo, JP) ; Hase;
Kazukuni; (Tokyo, JP) ; Endo; Shigeru; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
53371334 |
Appl. No.: |
15/103093 |
Filed: |
December 10, 2014 |
PCT Filed: |
December 10, 2014 |
PCT NO: |
PCT/JP2014/083321 |
371 Date: |
June 9, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/005 20130101;
C22C 38/54 20130101; C21D 9/46 20130101; C21D 8/0226 20130101; C22C
38/02 20130101; C22C 38/42 20130101; C22C 38/16 20130101; C22C
38/48 20130101; C22C 38/04 20130101; C22C 38/58 20130101; C22C
38/001 20130101; C22C 38/44 20130101; C21D 8/02 20130101; C21D
8/0205 20130101; C22C 38/50 20130101; C21D 2211/004 20130101; C22C
38/08 20130101; C21D 8/0263 20130101; C21D 9/50 20130101; C22C
38/06 20130101; C22C 38/002 20130101; C22C 38/46 20130101; C22C
38/12 20130101; C22C 38/14 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/16 20060101 C22C038/16; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 8/02 20060101 C21D008/02; C22C 38/58 20060101
C22C038/58 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2013 |
JP |
2013-257401 |
Claims
1. A steel plate, comprising: a chemical composition including, by
mass %, C: 0.020% to 0.090%, Si: 0.01% to 0.35%, Mn: 1.40% to
2.00%, P: 0.008% or less, S: 0.0035% or less, Al: 0.010% to 0.060%,
Ni: 0.40% to 2.00%, Mo: 0.05% to 0.50%, Nb: 0.005% to 0.040%, Ti:
0.005% to 0.025%, N: 0.0020% to 0.0050%, Ca: 0.0005% to 0.0050%,
and 0: 0.0035% or less, Ceq specified by formula (1) below being in
a range of 0.420% to 0.520%, formulas (2), (3), and (4) below being
satisfied, B being controlled to be less than 0.0003%, and a
balance being Fe and incidental impurities; and precipitates
including Ti, Nb, and Mo and having a mean particle size of 20 nm
or less, wherein [Nb]/([Ti]+[Nb]+[Mo]) 0.3, where [Ti] is the Ti
content, [Nb] is the Nb content, and [Mo] is the Mo content:
Ceq=[C]+[Mn]/6 +([Cu]+[Ni])/15 +([Cr]+[Mo]+[V])/5 (1) 1.5
5.ltoreq.[Ti]/[N].ltoreq.4.0 (2)
0<{[Ca]-(0.18+130.times.[Ca]).times.[O]}/1.25/[S]<1.5 (3)
5.5[C].sup.(4/3)+15[P]+0.90[Mn]+0.12[Ni]+7.9[Nb].sup.(1/2)+0.53[Mo].ltore-
q.3.70 (4) where [M] is the content of element M by mass %.
2. The steel plate of claim 1, wherein the chemical composition
further includes, by mass %, at least one selected from the group
consisting of Cu: less than 0.7%, Cr: 0.1% to 1.0%, and V: 0.005%
to 0.05%, Mg: 0.0002% to 0.0050%, and a REM: 0.0010% to
0.0200%.
3. (canceled)
4. A method for manufacturing a steel plate, the method comprising:
heating steel having the chemical composition of claim 1 to
950.degree. C. to 1150.degree. C.; subsequently subjecting the
steel to hot rolling at a cumulative rolling reduction of 30% or
higher in a temperature range of 900.degree. C. or higher and a
cumulative rolling reduction of 30% to 70% in a temperature range
of less than 900.degree. C.; and subsequently cooling the steel at
least to 500.degree. C. with a cooling rate of 1.0.degree. C./s or
higher.
5. The method of claim 4, further comprising tempering at
450.degree. C. to 650.degree. C. after the cooling.
6. A method for manufacturing a steel plate, the method comprising:
heating steel having the chemical composition of any one of claims
2 to 950.degree. C. to 1150.degree. C.; subsequently subjecting the
steel to hot rolling at a cumulative rolling reduction of 30% or
higher in a temperature range of 900.degree. C. or higher and a
cumulative rolling reduction of 30% to 70% in a temperature range
of less than 900.degree. C.; and subsequently cooling the steel at
least to 500.degree. C. with a cooling rate of 1.0.degree. C./s or
higher.
7. The method of claim 6, further comprising tempering at
450.degree. C. to 650.degree. C. after the cooling.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a high tensile strength steel
plate, and a method for manufacturing the same, used in steel
structures such as marine structures and ships, pressure vessels,
and penstocks. In particular, this disclosure relates to a thick,
high tensile strength steel plate, and a method for manufacturing
the same, that not only has a yield stress (YS) of 460 MPa or
greater and excellent strength and toughness, but that also has
excellent low-temperature toughness in a multilayer weld (CTOD
property) and that, after Post Weld Heat Treatment (PWHT), has
excellent strength and toughness (PWHT property).
BACKGROUND
[0002] Steel plates used in ships, marine structures, pressure
vessels, and the like are subjected to welding to form structures
with desired shapes. Therefore, from the perspective of structural
safety, these steel plates are not only required to have high
strength and excellent toughness but also to have excellent
toughness in weld joints (weld metal and Heat-Affected Zone (HAZ))
upon welding.
[0003] The absorbed energy by a Charpy impact test has mainly been
used as the basis for evaluating the toughness of steel plates. In
recent years, however, a Crack Tip Opening Displacement test (CTOD
test) has often been used for greater reliability. This test
evaluates the resistance to occurrence of brittle fracture by
forming a fatigue precrack in a test piece at the location to be
evaluated for toughness, subjecting the test piece to three-point
bending, and measuring the amount of the crack opening (plastic
deformation volume) immediately before fracture.
[0004] Since a fatigue precrack is used in a CTQD test, an
extremely small region is evaluated for toughness. Therefore, if a
local brittle zone exists in the steel plate, a low toughness may
in some cases be indicated by the CTOD test, even if a good
toughness is obtained with a Charpy impact test.
[0005] In a thick steel plate or the like, local brittle zones
easily occur in the heat-affected zone, which is subjected to a
complicated thermal history due to multilayer welding. In addition,
the bond (the boundary between weld metal and base material) and a
region in which the bond is formed into a dual phase region by
reheating (a region in which coarse grains are formed in the first
cycle of welding and which is heated into a ferrite and austenite
dual phase region by the subsequent welding pass, hereinafter
referred to as a dual phase reheating area) are also prone to
becoming local brittle zones.
[0006] Since the bond is exposed to a high temperature just below
the melting point at the time of welding, austenite grains are
coarsened and are likely to be transformed, by the subsequent
cooling, into an upper bainite structure that has a low toughness.
Therefore, the toughness of the matrix itself tends to reduce.
Furthermore, brittle structures such as a Widmanstatten structure
or isolated martensite (also referred to as M-A) easily occur. If
such a brittle structure occurs, the toughness of the steel plate
tends to reduce even further.
[0007] In order to improve the toughness of the heat-affected zone,
for example a technique that incorporates TiN in the steel plate by
fine particle distribution to reduce coarsening of austenite grains
and to create ferrite nucleation sites has been put to practical
use. The bond, however, may be heated to a temperature region at
which TiN dissolves. As the demand for low-temperature toughness
becomes more stringent, the heating temperature rises, thus making
it more difficult to obtain the above-described effect from fine
particle distribution of TiN.
[0008] To resolve these problems, JP H03-053367 132 (PTL 1) and JP
S60-184663 A (PTL 2) disclose techniques in which, by dispersing
fine grains in steel plates by means of combined addition of
rare-earth elements (REM) and Ti, grain growth of austenite is
suppressed, thereby improving the toughness of the weld zone.
[0009] PTL 1 and PTL 2 also disclose a technique for dispersing Ti
oxides, a technique for combining the capability of ferrite
nucleation of BN with oxide dispersion, a technique for adding Ca
and a REM to control the morphology of sulfides so as to increase
the toughness, and other such techniques.
[0010] JP 3697202 132 (PTL 3) discloses a technique for dispersing
Ti oxides into steel so as to improve the HAZ toughness.
[0011] Furthermore, in the dual phase reheating area, carbon
becomes concentrated in a region where reverse transformation to
austenite occurs due to dual phase reheating, and brittle bainite
structures including isolated martensite are generated during
cooling, resulting in reduced toughness of the steel. To avoid this
reduction in toughness, techniques have been disclosed to reduce
the contents of C and Si in the chemical composition, inhibit the
generation of isolated martensite, and improve the toughness, and
to ensure the base material strength by adding Cu (for example, JP
3045856 B2 (PTL 4) and JP 4432905 132 (PTL 5)).
[0012] The technique disclosed in PTL 4 sets the cooling rate after
rolling to 0.1.degree. C./s or less and adopts a method to
precipitate Cu particles during this process. Such a technique is
problematic, however, in terms of manufacturing stability.
[0013] With the technique disclosed in PTL 5, the N/AI ratio is set
between Q.3 and 3.0, thereby suppressing toughness degradation due
to the adverse effect of coarsened AlN and of solute N, but it is
easier to control solute N with Ti.
[0014] In thick material for which YS exceeds 460 MPa, Post Weld
Heat Treatment (PWHT) may be applied. During PWHT, the base
material is also heated at the same time. Therefore, the base
material properties need to be maintained even upon PWHT.
Conventionally, in order to control a reduction in strength due to
heat, an element that forms a precipitate at that temperature is
generally added.
CITATION LIST
Patent Literature
[0015] PTL 1: JP H03-053367 B2
[0016] PTL 2: JP S60-184663 A
[0017] PTL 3: JP 3697202 132
[0018] PTL 4: JP 3045856 B2
[0019] PTL 5: JP 4432905 132
SUMMARY
Technical Problem
[0020] The techniques disclosed in PTL 1 and PTL 2 target
relatively low strength steel material with a small amount of
alloying elements. Unfortunately, these techniques cannot be
applied to higher strength, thick material with a large amount of
alloying elements, since the HAZ structure does not include
ferrite.
[0021] With the technique disclosed in PTL 3, it is difficult to
incorporate Ti oxides stably into steel by fine particle
distribution.
[0022] Furthermore, ensuring strength by Cu precipitates often
leads to a reduction in toughness, causing problems for ensuring
the low-temperature toughness of the steel plate. In steel material
that uses strengthening by Cu precipitation as recited in PTL 5, Cu
particles also grow large during the PWHT, causing the problem of
the strength tending to reduce.
[0023] Additionally, in recent years, steel structures such as
ships, marine structures, pressure vessels, and penstocks have
increased in size, leading to a desire for even higher strength
steel.
[0024] The steel material used in these steel structures is often
thick material, for example with a plate thickness of 3.5 mm or
greater. Therefore, in order to ensure a strength such that the
yield stress is at least 460 MPa grade, a steel chemical
composition including a large amount of added alloying elements has
become advantageous.
[0025] Sufficient examination has not been made, however, of how to
improve toughness of the bonds and the dual phase reheating areas
in high tensile strength steel material with a large amount of
alloying elements. As for ensuring base metal properties after
PWHT, it is difficult to maintain strength and toughness by simply
adding precipitating elements as in conventional techniques.
[0026] Therefore, it would be helpful to provide thick, high
tensile strength steel, and a method for manufacturing the same,
that is suitable for use in steel structures such as marine
structures, ships, pressure vessels, and penstocks, has a yield
stress (YS) of 460 MPa or greater (satisfying this level of YS is
referred to as high tensile strength in this disclosure), and has
excellent low-temperature toughness of the heat-affected zone in a
multilayer weld (CTOD property) and excellent strength and
toughness after Post Weld Heat Treatment (PWHT property).
Solution to Problem
[0027] To that end, we engaged in intensive studies and made the
following discoveries.
[0028] (a) Since the CTOD property is evaluated with a test piece
having the entire thickness of the steel plate, the central
segregation area where components are concentrated becomes the
origin of fracture.
[0029] Consequently, in order to improve the CTOD property of the
heat-affected zone, it is effective to control elements that easily
concentrate as central segregation of the steel plate to a proper
amount, thereby suppressing the hardening of the central
segregation area. Furthermore, at the center of the slab, which is
the last portion to solidify when the molten steel solidifies, the
concentration of C, Mn, P, Ni, and Nb is higher than the
concentration of other elements. Hence, it is effective to control
the added amounts of these elements on the basis of the central
segregation area hardness index, thereby reducing the hardness at
the central segregation area.
[0030] (b) In order to improve the toughness of the heat-affected
zone, it is effective to use TiN efficiently to suppress coarsening
of the austenite grains in the vicinity of the weld bond. In
particular, by controlling the Ti/N ratio to an appropriate level,
TiN can be uniformly and finely dispersed in the steel.
[0031] (c) It is effective to use crystallization of the Ca
compound (CaS), which is added for morphological control of
sulfides, to improve the toughness of the heat-affected zone.
[0032] Since CaS crystalizes at a low temperature as compared to
oxides, CaS can be distributed uniformly as fine particles.
Furthermore, by controlling the amount of Ca added and the amount
of dissolved oxygen in the molten steel at the time of addition to
be within appropriate ranges, solute S can also be guaranteed after
CaS crystallization. Hence, MnS precipitates on the surface of the
CaS to form a complex sulfide. Since a Mn dilute zone is formed
around the MnS, ferrite transformation is further promoted.
[0033] (d) In addition to Nb, which forms precipitates, adding Ti
and Mo as essential elements allows formation, at the stage of
manufacturing a steel plate, of fine precipitates of a complex
carbonitride of Mo, Ti, and Nb, which does not coarsen even when
subjected to heating by PWHT (performed in a range of approximately
550.degree. C. to 650.degree. C. for 2 h to 4 h).
[0034] Conventionally, the reduction in strength after PWHT in a
steel plate exceeding YS: 460 MPa grade is significant, but in a
developmental steel plate, we discovered that by the stable
existence of fine Mo, Ti, Nb complex precipitates (carbides,
nitrides, or carbonitrides), strengthening by precipitation can be
maintained, and the reduction in strength of the steel plate can be
suppressed. Furthermore, due to the existence of fine Mo, Ti, Nb
complex precipitates, the toughness of the steel plate can also be
maintained.
[0035] This disclosure was completed based on the above
discoveries, and the primary features thereof are as follows.
[0036] 1. A steel plate, comprising:
[0037] a chemical composition including, by mass %,
[0038] C: 0.020% to 0.090%,
[0039] Si: 0.01% to 0.35%,
[0040] Mn: 1.40% to 2.00%,
[0041] P: 0.008% or less,
[0042] S: 0.0035% or less,
[0043] Al: 0.010% to 0.060%,
[0044] Ni: 0.40% to 2.00%,
[0045] Mo: 0.05% to 0.50%,
[0046] Nb: 0.005% to 0.040%,
[0047] Ti: 0.005% to 0.025%,
[0048] N: 0.0020% to 0.0050%,
[0049] Ca: 0.0005% to 0.0050%, and
[0050] O: 0.0035% or less,
[0051] Ceq specified by formula (1) below being in a range of
0.420% to 0.520%, formulas (2), (3), and (4) below being satisfied,
B being controlled to be less than 0.0003%, and a balance being Fe
and incidental impurities; and
[0052] precipitates including Ti, Nb, and Mo and having a mean
particle size of 20 nm or less, wherein [Nb]/([Ti]+[Nb]+[Mo]) 0.3,
where [Ti] is the Ti content, [Nb] is the Nb content, and [Mo] is
the Mo content:
Ceq=[C]+[Mn]/6 +([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 (1)
1.5 5[Ti]/[N]4.0 (2)
0<{[Ca]-(0.18+130.times.[Ca])x[0]}/1.25/[S]<1.5 (3)
5.5[C].sup.(4/3)+15[P]+0.90[Mn]+0.12[Ni]+7.9[Nb].sup.(1/2)+0.53[Mo].ltor-
eq.3.70 (4)
[0053] where [M] is the content of element M by mass %.
[0054] 2. The steel plate of 1., wherein the chemical composition
further includes, by mass %, at least one selected from the group
consisting of Cu: less than 0.7%, Cr: 0.1% to 1.0%, and V: 0.005%
to 0.05%.
[0055] 3. The steel plate of 1. or 2., wherein the chemical
composition further includes, by mass %, at least one selected from
the group consisting of Mg: 0.0002% to 0.0050% and a REM: 0.0010%
to 0.0200%.
[0056] 4. A method for manufacturing a steel plate, the method
comprising:
[0057] heating steel having the chemical composition of any one of
1. to 3. to 950.degree. C. to 1150.degree. C.;
[0058] subsequently subjecting the steel to hot rolling at a
cumulative rolling reduction of 30% or higher in a temperature
range of 900.degree. C. or higher and a cumulative rolling
reduction of 30% to 70% in a temperature range of less than
900.degree. C.; and
[0059] subsequently cooling the steel at least to 500.degree. C.
with a cooling rate of 1.0.degree. C./s or higher.
[0060] 5. The method of 4., further comprising tempering at
450.degree. C. to 650.degree. C. after the cooling.
Advantageous Effect
[0061] According to this disclosure, a thick, high tensile strength
steel plate, and a method for manufacturing the same, that is
suitable for use in large steel structures such as marine
structures, has a yield stress (YS) of 460 MPa or greater, and has
an excellent CTOD property in a multilayer weld and an excellent
PWHT property can be obtained. Therefore, this disclosure is
extremely useful in industrial terms.
BRIEF DESCRIPTION OF THE DRAWINGS
[0062] In the accompanying drawings:
[0063] FIG. 1 illustrates the relationship between change in
strength/toughness and the precipitate size/composition during
PWHT; and
[0064] FIG. 2 illustrates TEM replica observation and EDX analysis
results for precipitates in a steel plate.
DETAILED DESCRIPTION
[0065] Our methods and products will be described in detail
below.
[0066] First, reasons why the chemical composition (steel
composition) of the steel plate (also referred to as thick
material) has been restricted to the aforementioned ranges will be
described in detail for each element. The % representations below
indicating the chemical composition of the steel plate are in mass
% unless stated otherwise. C: 0.020% to 0.090%
[0067] C is an essential element for ensuring the strength of a
high tensile strength steel plate. When the C content is less than
0.020%, quench hardenability is degraded, and it becomes necessary
to add a large amount of quench hardenability-improving elements,
such as Cu, Ni, Cr, or Mo, in order to ensure strength, resulting
in a rise in costs. Conversely, when the amount of C added exceeds
0.090%, the toughness of the weld zone degrades. Therefore, the C
content is set in the range of 0.020% to 0.090%, and preferably
0.020% to 0.080%.
[0068] Si: 0.01% to 0.35%
[0069] Si is added as a deoxidizing element and in order to obtain
the steel plate strength. To obtain these effects, the Si content
needs to be at least 0.01%. On the other hand, a large Si content
exceeding 0.35% leads to deterioration in weldability and toughness
of the weld joint. Therefore, the Si content needs to be set in the
range of 0.01% to 0.35%, preferably 0.01% to 0.23%.
[0070] Mn: 1.40% to 2.00%
[0071] In order to ensure the steel plate strength and the weld
joint strength, the Mn content needs to be at least 1.40%.
Conversely, when the amount of Mn content exceeds 2.00%, the
weldability deteriorates, quench hardenability becomes excessive,
and the toughness of the steel plate and the toughness of the weld
joint deteriorate. Therefore, the Mn content is set in the range of
1.40% to 2.00%, more preferably 1.40% to 1.95%.
[0072] P: 0.008% or less
[0073] P, which is an impurity element, degrades the toughness of
the steel plate and the toughness of the weld zone. In particular,
when the P content in the weld zone exceeds 0.008%, the CTOD
property markedly degrades. Therefore, the P content is set to
0.008% or less, preferably 0.006% or less. The P content is
preferably as small as possible, but considering factors such as
refining cost, the lower limit may be approximately 0.002%.
[0074] S: 0.0035% or less
[0075] S is an impurity element, and when the content thereof
exceeds 0.0035%, the toughness of the steel plate and the weld zone
deteriorate. Therefore, the S content is set to 0.0035% or less,
preferably 0.0030% or less. The S content is preferably as small as
possible, but considering factors such as refining cost, the lower
limit may be approximately 0.0004%.
[0076] Al: 0.010% to 0.060%
[0077] Al is an element to be added in order to deoxidize molten
steel, and the Al content needs to be set to 0.010% or more. When
the Al content exceeds 0.060%, however, the toughness of the steel
plate and the toughness of the weld zone are degraded, and Al is
mixed into the weld metal by dilution due to welding, which
degrades toughness. Therefore, the Al content is limited to 0.060%
or less, preferably 0.017% to 0.055%. In this disclosure, the Al
content is specified in terms of acid-soluble Al (also referred to
as "Sol. Al" or the like).
[0078] Ni: 0.40% to 2.00%
[0079] Ni is an element useful for improving the strength and
toughness of the steel plate and is also useful for improving the
CTOD property of the weld zone. In order to obtain these effects,
the added content of Ni needs to be 0.40% or more. Ni is an
expensive element, however, and excessive addition thereof also
increases the likelihood of damage to the surface of the slab at
the time of casting. Therefore, the upper limit of the Ni content
is set to 2.00%.
[0080] Mo: 0.05% to 0.50%
[0081] Mo is an element that fulfills an important function in this
disclosure, and adding an appropriate amount thereof is useful for
increasing the strength of the steel plate. This effect is obtained
by improving the quench hardenability and the temper softening
resistance at the time of tempering. Furthermore, Mo maintains the
complex precipitate formed by Mo, Ti, and Nb in a fine state,
thereby strengthening thick material and controlling a reduction in
toughness. In order to obtain these effects, the Mo content needs
to be at least 0.05%. An excessive Mo content, however, adversely
affects the toughness of the thick material. Hence, the upper limit
on the Mo content is set to 0.50%. The Mo content is more
preferably in the range of 0.08% to 0.40%, and even more preferably
in the range of 0.16% to 0.30%.
[0082] Nb: 0.005% to 0.040%
[0083] Nb forms an unrecrystallized zone of austenite in the low
temperature region. Therefore, by performing rolling in such a
temperature region, the structure of the steel plate can be refined
and the toughness of the steel plate can be increased. Furthermore,
Nb has the effect of improving the quench hardenability, and by
being added in combination with Mo and Ti, Nb has the effect of
improving the softening resistance at the time of tempering. Nb is
also a useful element for improving the strength of the steel
plate. In order to obtain these effects, the Nb content needs to be
at least 0.005%. When the Nb content exceeds 0.040%, however, the
toughness deteriorates. Hence, the upper limit on the Nb content is
set to 0.040%, preferably 0.035%.
[0084] Ti: 0.005% to 0.025%
[0085] Ti is precipitated as TiN when molten steel solidifies,
which suppresses coarsening of austenite in the weld zone, thus
contributing to improvement in the toughness of the weld zone.
Furthermore, by being added in combination with Mo and Nb, Ti has
the effect of improving the softening resistance at the time of
tempering. When the Ti content is less than 0.005%, however, such
an effect is small. On the other hand, when the Ti content exceeds
0.025%, TiN coarsens, and it is not possible to obtain the effect
of improving the toughness of the steel plate and the weld zone.
Therefore, the Ti content is set in the range of 0.005% to
0.025%.
[0086] N: 0.0020% to 0.0050%
[0087] N reacts with Ti and Al to form precipitates. Crystal grains
are thereby refined, and the toughness of the steel plate is
improved. Furthermore, N is a necessary element for forming TiN
which suppresses coarsening of the structure of the weld zone. In
order to obtain such effects, the N content needs to be set to
0.0020% or more. On the other hand, when the N content exceeds
0.0050%, solute N markedly degrades the toughness of the steel
plate and the weld zone and leads to a deterioration in strength
due to a reduction in solute Nb caused by generation of complex
precipitates of Ti and Nb. Therefore, the upper limit on the N
content is set to 0.0050%.
[0088] Ca: 0.0005% to 0.0050%
[0089] Ca is an element that improves toughness by fixing S. In
order to obtain this effect, the Ca content needs to be at least
0.0005%. On the other hand, Ca content exceeding 0.0050% causes
saturation of the effect. Therefore, Ca is added in the range of
0.0005% to 0.0050%.
[0090] 0: 0.0035% or less
[0091] If the O content exceeds 0.0035%, the toughness of the steel
plate deteriorates. Hence, the O content is set to 0.0035% or less,
preferably 0.0028% or less. The O content is preferably as small as
possible, but considering factors such as refining cost, the lower
limit may be approximately 0.0010%.
[0092] Ceq: 0.420% to 0.520%
[0093] When Ceq specified by the formula below is less than 0.420%,
a thick material strength of 460 MPa grade cannot be obtained. On
the other hand, if Ceq exceeds 0.520%, the weldability of the thick
material and the toughness of the weld zone deteriorate. Hence, Ceq
is set to 0.520% or less. Ceq is preferably in the range of 0.440%
to 0.520%. In the formula below, [M] represents the content (mass
%) of element "M" in steel. Elements that are not included are
calculated as zero.
Ceq=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5
[0094] [Ti]/[N]: 1.5 to 4.0
[0095] When the value of [Ti]/[N] is less than 1.5, the amount of
TiN formed decreases, and solute N not forming TiN degrades the
toughness of the weld zone. On the other hand, when the value of
[Ti]/[N] exceeds 4.0, TiN is coarsened to degrade the toughness of
the weld zone. Therefore, the value of [Ti]/[N] is set in the range
of 1.5 to 4.0, and preferably 1.8 to 3.5.
0<{[Ca]-(0.18+130.times.[Ca]).times.[O]}/1.25/[S]<1.5
[0096] The expression
{[Ca]-(0.18+130.times.[Ca]).times.[O]}/1.25/[S] is a value
representing the Atomic Concentration Ratio (ACR) of Ca and S,
which are effective for sulfide morphological control, and can be
adjusted by controlling the amount of Ca added and the amount of
dissolved oxygen in the molten steel at the time of addition to be
within appropriate ranges. The sulfide morphology can be estimated
by this ACR value, but in this disclosure, the ACR value is
specified as an index in order to finely disperse CaS which does
not dissolve even at high temperatures and which acts as nuclei for
ferrite transformation.
[0097] When the ACR value is 0 or less, CaS is not crystallized.
Consequently, S is precipitated in the form of MnS only, which
easily dissolves in the heat-affected zone, making it impossible to
obtain ferrite product nuclei. Furthermore, the MnS precipitated
alone is elongated during rolling and causes degradation in the
toughness of the steel plate. Therefore, in this disclosure, the
ACR value needs to exceed zero.
[0098] On the other hand, when the ACR value is 1.5 or more, the
proportion of oxides among Ca-based inclusions increases, and the
proportion of sulfides that function as nucleation sites decreases,
making it impossible to obtain the effect of improving toughness.
Therefore, in this disclosure, the ACR value needs to be less than
1.5.
[0099] Accordingly, by controlling the ACR value to exceed 0 and be
less than 1.5, complex sulfides mainly composed of CaS can be
formed effectively and caused to function effectively as ferrite
nucleation sites. The ACR value is preferably in the range of 0.15
to 1.30, and more preferably in the range of 0.20 to 1.00.
5.5[C].sup.(4/3)+15[P]0.90[Mn]+0.12[Ni]+7.9[Nb].sup.(1/2)+0.53[Mo].ltore-
q.3.70
[0100] The value of the left-hand side of the formula above
(5.5[C].sup.(4/3)+15[P]+0.90[Mn]+0.12[Ni]+7.9[Nb].sup.(1/2)+0.53[Mo])
is the hardness index of the central segregation area formed by
components that are likely to be concentrated in the central
segregation area and is referred to below as the Ceq* value.
[0101] Since the CTOD test is carried out over the entire thickness
of a steel plate, test pieces include central segregation. In the
case where the composition concentration in the central segregation
is significant, a hardened region occurs in the heat-affected zone.
Therefore, a good value cannot be obtained in the CTOD test.
[0102] Therefore, in this disclosure, by controlling the Ceq* value
to be in an appropriate range, an excessive increase in hardness in
the central segregation area can be suppressed, and an excellent
CTOD property can be obtained even in the weld zone of a thick
steel plate.
[0103] The appropriate range of the Ceq* value has been
experimentally obtained. When the Ceq* value exceeds 3.70, the CTOD
property is degraded. Therefore, the Ceq* value is set to be 3.70
or less, and preferably 3.50 or less. No lower limit is placed on
the Ceq* value, but considering factors such as productivity,
preferably the lower limit is approximately 2.2.
[0104] In this disclosure, in addition to the above-described
essential elements, at least one selected from the group consisting
of Cu: less than 0.7%, Cr: 0.1% to 1.0%, and V: 0.005% to 0.05% may
be added to increase quench hardenability.
[0105] Cu: less than 0.7%
[0106] Adding Cu improves the strength of the steel plate. If the
content of Cu exceeds 0.7%, however, the hot ductility
deteriorates. Therefore, the Cu content is limited to 0.7% or less.
The Cu content is preferably 0.1% to 0.6%.
[0107] Cr: 0.1% to 1.0%
[0108] Cr is an element effective in increasing the strength of the
steel plate. In order to obtain this effect, the Cr content is set
to 0.1% or more. An excessively high Cr content, however, adversely
affects toughness. Therefore, the Cr content is preferably 0.1% to
1.0%, and more preferably 0.2% to 0.8%, when included.
[0109] V: 0.005% to 0.05%
[0110] V is an element that is effective in improving the strength
and toughness of the steel plate at a content of 0.005% or more.
Setting the V content to exceed 0.05%, however, leads to
deterioration of toughness. Therefore, the V content is preferably
0.005% to 0.05% when included.
[0111] In this disclosure, in addition to the above-described
essential elements, at least one selected from the group consisting
of Mg: 0.0002% to 0.0050% and a REM: 0.0010% to 0.0200% may be
added to increase the HAZ toughness.
[0112] Mg and REM are elements having the effect of improving the
toughness of steel via the dispersion of oxides. In order to bring
out such an effect, the Mg content is set to 0.0002% or more, and
the REM content to 0.0010% or more. On the other hand, Mg content
exceeding 0.0050% and REM content exceeding 0.0200% merely causes
saturation of this effect. Accordingly, when adding these elements,
the content is preferably set in the above-mentioned ranges. The Mg
content is more preferably 0.0005% to 0.0020%, and the REM content
is more preferably 0.0020% to 0.0150%.
[0113] Components other than the above-mentioned chemical
composition are Fe and incidental impurities. In particular, when
the steel plate is cooled from the austenite region, 13 exists in a
segregated manner at austenite grain boundaries, suppresses ferrite
transformation, and generates bainite structures that include a
large amount of M-A. Hence, B has the disadvantage of making the
structure brittle particularly in the heat-affected zone.
Accordingly, in this disclosure, the content of B in the steel
plate needs to be kept below 0.0003%.
[0114] In the steel plate, the size of precipitates needs to change
little before and after PWHT, and the strength and toughness of the
steel plate need to be maintained. FIG. 1 illustrates the
relationship between (i) the precipitate size and precipitate
composition after PWHT and (ii) the change in strength and
toughness before and after PWHT (.DELTA.TS, .DELTA.vTrs), and FIG.
2 illustrates TEM replica observation and EDX analysis results for
precipitates in steel.
[0115] From the perspective of stability, the change in strength
and in toughness before and after PWHT respectively need to satisfy
the following ranges: .DELTA.TS of 5 MPa to -15 MPA, and
.DELTA.vTrs of 10.degree. C. to -5.degree. C. In order to satisfy
these ranges, it is clear from FIG. 1 that the mean size of the
precipitates needs to be kept to 20 nm or less, and that the Ti
content (represented as [Ti]), the Nb content (represented as
[Nb]), and the Mo content (represented as [Mo]) in the precipitates
need to satisfy the relationship
[Nb]/([Ti]+[Nb]+[Mo]).gtoreq.0.3.
[0116] As is clear from Table 1, which lists the EDX analysis
results for precipitates in steel in FIG. 2, the above-mentioned
precipitates are Ti, Nb, and Mo precipitates. Since it suffices for
the relationship [Nb]/([Ti]+[Nb]+[Mo]).gtoreq.0.3 to be satisfied,
the precipitates need to be at least Nb precipitates, whereas Ti
and Mo precipitates may be included in any range that satisfies
this relationship. In this disclosure, having an excellent PWHT
property refers to ATS being in the range of 5 MPa to -15 MPa, and
AvTrs being in the range of 10.degree. C. to -5.degree. C. The
precipitates in this disclosure (complex precipitates) are Mo, Ti,
and Nb precipitates. Specifically, these precipitates are carbides,
nitrides, or carbonitrides of Mo, Ti, and Nb, or a mixture
thereof.
[0117] [Table 1]
TABLE-US-00001 TABLE 1 Measuring Point Ti Nb Mo [Nb]/ No. (mass %)
(mass %) (mass %) ([Ti] + [Nb] + [Mo]) 5 15.51 82.1 2.39 0.821 6
27.63 72.37 -- 0.724 7 20.52 79.48 -- 0.795 8 20.92 65.6 13.48
0.656
[0118] [Method of Determining Precipitate Particle Size]
[0119] The method of determining the precipitate particle size in
this disclosure is in accordance with the TEM replica method.
Specifically, after appropriately collecting the precipitate zone
of Ti, Nb, and Mo carbides from the steel, the average equivalent
circular diameter was determined using image processing on an
observation made at 100,000X over four fields of view and taken as
the particle size of the precipitates. In this disclosure, the
lower limit on the measurement target for precipitate size was set
to 2 nm. The reason is that precipitates with a precipitate size
smaller than 2 nm are hard to measure.
[0120] Next, our method for manufacturing steel will be described.
Our steel is preferably manufactured with the method of
manufacturing described below.
[0121] Molten steel adjusted to have a chemical composition within
the above-described ranges is prepared by steelmaking with an
ordinary method using a converter, an electric heating furnace, a
vacuum melting furnace, or the like. Next, after forming the molten
steel into a slab by continuous casting, the slab is hot rolled to
a desired plate thickness. The result is then cooled, and as
necessary, tempered. During the hot rolling of this disclosure, the
slab heating temperature and rolling reduction are prescribed.
[0122] In this disclosure, unless otherwise noted, the temperature
conditions on the steel plate are prescribed by the temperature at
the central portion in the plate thickness direction of the steel
plate. The temperature at the central portion in the plate
thickness direction is determined from the plate thickness, the
surface temperature, the cooling conditions, and the like by
simulation calculation or the like. For example, the temperature at
the central portion in the plate thickness direction may be
determined by calculating the temperature distribution in the plate
thickness direction using the finite difference method.
[0123] Slab reheating temperature: 950.degree. C. to 1150.degree.
C.
[0124] The slab reheating temperature is set to 950.degree. C. or
higher in order to remove casting defects in the slab reliably with
hot rolling. If the slab is reheated to a temperature exceeding
1150.degree. C., however, the austenite crystallized grains
coarsen, causing the toughness of the steel plate to degrade.
Hence, the upper limit on the slab reheating temperature is set to
1150.degree. C.
[0125] Cumulative rolling reduction of hot rolling in a temperature
range of 900.degree. C. or higher: 30% or higher
[0126] In order to render casting defects harmless by pressure
bonding and to provide austenite grains with a fine microstructure
by recrystallization, the cumulative rolling reduction of hot
rolling in a temperature range of 900.degree. C. or higher is set
to 30% or higher. The reason is that if the cumulative rolling
reduction is less than 30%, coarse grains formed during reheating
remain and adversely affect the toughness of the steel plate. No
upper limit is placed on the cumulative rolling reduction of hot
rolling in a temperature range of 900.degree. C. or higher, but in
industrial terms, the upper limit is approximately 95%.
[0127] Cumulative rolling reduction of hot rolling in a temperature
range of less than 900.degree. C.: 30% to 70%
[0128] In this temperature range, the rolled austenite grains do
not sufficiently recrystallize. Therefore, austenite grains that
remain flattened after rolling constitute a state of high internal
distortion that includes numerous defects, such as an internal
distortion zone. These austenite grains act as the driving force
for ferrite transformation and encourage phase transformation.
[0129] If the cumulative rolling reduction is less than 30%,
however, accumulation of internal energy in the distortion zone is
insufficient, making it difficult for ferrite transformation to
occur and reducing the toughness of the steel plate. Conversely, if
the cumulative rolling reduction exceeds 70%, generation of
polygonal ferrite is encouraged, making high strength and high
toughness incompatible. Accordingly, in this disclosure, the
cumulative rolling reduction of hot rolling in a temperature range
of less than 900.degree. C. is set in the range of 30% to 70%.
[0130] Cooling rate at least to 500.degree. C.: 1.0.degree. C./s or
higher
[0131] After hot rolling, accelerated cooling is performed at least
to 500.degree. C. at a cooling rate of 1.0.degree. C./s or higher.
The reason is that if the cooling rate is less than 1.0.degree.
C./s, sufficient strength of the steel plate is not obtained.
Furthermore, if cooling is stopped at a higher temperature than
500.degree. C., the proportion of ferrite and pearlite structure
increases, making high strength and high toughness of thick
material incompatible. While no lower limit is placed on the stop
temperature of accelerated cooling, the steel is preferably cooled
to room temperature.
[0132] Tempering temperature: 450.degree. C. to 650.degree. C.
[0133] In this disclosure, when tempering the steel, a sufficient
tempering effect is not obtained if the tempering temperature is
less than 450.degree. C. On the other hand, when tempering at a
temperature exceeding 650.degree. C., the precipitates may coarsen,
lowering the toughness and the strength of the steel. Hence, a
temperature exceeding 650.degree. C. is not preferable.
[0134] The tempering in this disclosure more preferably uses
induction heating, which suppresses coarsening of carbides during
tempering. In this case, the tempering is preferably performed so
that the temperature at the center of the steel plate calculated by
a simulation using the finite difference method or the like is from
450.degree. C. to 650.degree. C.
[0135] In this disclosure, when desired properties of the steel
plate have been obtained, as for a TMCP steel plate or the like,
the above-described tempering need not be performed.
[0136] Thick material in this disclosure has a thickness of 15 mm
or greater. Accordingly, the thickness in this disclosure refers to
a steel thickness of 15 mm or greater, but the effects of this
disclosure are best obtained when the steel thickness is in a range
of 40 mm to 100 mm. Manufacturing conditions other than the
above-described manufacturing conditions on thick, high tensile
strength steel may be in accordance with conventional methods.
[0137] In the thick, high tensile strength steel of this
disclosure, coarsening of austenite grains in the heat-affected
zone is suppressed while finely dispersing nuclei for ferrite
transformation that do not dissolve even at high temperatures,
thereby refining the structure of the heat-affected zone. High
toughness is thus obtained. Also in an area reheated to a dual
phase by the thermal cycle at the time of multilayer welding, the
structure of the heat-affected zone due to initial welding is
refined. Therefore, in the dual phase reheating area, the toughness
of the non-transformed area can be improved, the austenite grains
that undergo retransformation can be refined, and the extent of
reduction in toughness can be reduced. Additionally, by generating
fine complex precipitates of Ti, Nb, and Mo, a thick, high tensile
strength steel plate is provided with an excellent CTOD property
and PWHT property.
EXAMPLES
[0138] Examples according to this disclosure are now described.
[0139] Using continuously-cast slabs with steel codes A to Z having
the chemical composition listed in Table 2 as raw material, the hot
rolling and heating listed in Table 3 were performed to produce
thick steel plates with a thickness of 50 mm to 150 mm. The steel
plates were evaluated by a tensile test in which JIS No. 4 test
pieces were collected from the 1/2 position along the thickness of
the steel plates, so that the longitudinal direction of each test
piece was perpendicular to the rolling direction of the steel
plate. The yield stress (YS) and tensile strength (TS) were then
measured.
[0140] A Charpy impact test was also performed by collecting JIS
No. 4 V-notch test pieces measuring 2 mm from the 1/2 position
along the thickness of the steel plates, so that the longitudinal
direction of each test piece was perpendicular to the rolling
direction of the steel plate. The absorbed energy
vE.sub.-40.degree. C. at -40.degree. C. was then measured. In these
Examples, the base metal properties were evaluated as being good
when all of the following relationships were satisfied:
YS.gtoreq.460 MPa, TS.gtoreq.570 MPa, and vE.sub.-40.degree.
C..gtoreq.200 J.
[0141] The toughness of the weld zone was evaluated by producing a
multilayer fill weld joint, using a single bevel groove, by
submerged arc welding having a welding heat input of 35 kJ/cm and
measuring the absorbed energy vE.sub.-40.degree. C. at -40.degree.
C. with a Charpy impact test, using the weld bond on the straight
side at the 1/2 position along the thickness of the steel plates as
the notch position for the test. The toughness of the weld zone was
determined to be good when the mean for three tests satisfied the
relationship vE.sub.-40.degree. C..gtoreq.150 J.
[0142] Using the weld bond at the straight side as the notch
position for the CTOD test pieces, the CTOD value at -10.degree.
C., i.e. 8-10.degree. C., was measured. The CTOD property of the
weld joint was determined to be good when the minimum of the CTOD
value (.delta.-10.degree. C.) over three tests was 0.5 mm or
greater.
[0143] Furthermore, the precipitate zone in the steel was collected
by the TEM replica method, and the average equivalent circular
diameter was determined using image processing on an observation
made at 100,000.times. over four fields of view. Precipitates with
a particle size near the mean were selected by EDX, the precipitate
composition thereof was determined, and [Nb]/([Ti]+[Nb]+[Mo]) was
determined as the mean of three precipitates.
[0144] Regarding the change in base metal properties after PWHT,
.DELTA.TS (TS(after PWHT)-TS(before PWHT)) and .DELTA.vTrs
(vTrs(after PWHT)-vTrs(before PWHT)) were determined. PWHT was
performed by maintaining the steel at 580.degree. C. for four
hours, with temperature raising and lowering rates of 70.degree.
C./h.
[0145] Table 3 illustrates the hot rolling conditions, heat
treatment conditions, base metal properties, the results of the
above-described Charpy impact test and CTOD test on the weld zone,
the precipitate size/composition, and the change in base metal
properties after PWHT.
TABLE-US-00002 TABLE 2 Steel No. C Si Mn P S Al Ni Mo Nb Ti B N A
0.054 0.09 1.75 0.003 0.0026 0.034 0.63 0.41 0.021 0.012 0.0001
0.0037 B 0.065 0.23 1.52 0.005 0.0009 0.030 1.45 0.26 0.026 0.009
0.0001 0.0030 C 0.072 0.20 1.54 0.004 0.0017 0.025 1.29 0.16 0.009
0.012 0.0001 0.0036 D 0.076 0.07 1.53 0.003 0.0020 0.024 0.51 0.40
0.016 0.012 0.0002 0.0034 E 0.078 0.09 1.45 0.005 0.0014 0.017 1.31
0.24 0.028 0.010 0.0002 0.0038 F 0.105 0.10 1.60 0.005 0.0014 0.035
0.52 0.40 0.016 0.010 0.0001 0.0031 G 0.078 0.42 1.65 0.004 0.0016
0.027 1.36 0.22 0.029 0.009 0.0002 0.0032 H 0.067 0.08 1.18 0.004
0.0026 0.013 1.14 0.19 0.012 0.008 0.0001 0.0025 I 0.064 0.14 2.37
0.005 0.0022 0.018 0.77 0.20 0.017 0.009 0.0001 0.0027 J 0.060 0.20
1.53 0.014 0.0011 0.039 0.58 0.42 0.026 0.012 0.0002 0.0034 K 0.064
0.19 1.47 0.006 0.0007 0.079 1.05 0.24 0.016 0.011 0.0001 0.0032 L
0.076 0.10 1.69 0.004 0.0013 0.024 1.45 0.27 0.030 0.013 0.0011
0.0038 M 0.052 0.06 1.54 0.006 0.0016 0.018 0.84 0.35 0.029 0.010
0.0001 0.0034 N 0.041 0.16 1.46 0.005 0.0017 0.039 1.30 0.26 0.056
0.007 0.0002 0.0025 P 0.079 0.23 1.66 0.005 0.0008 0.017 0.51 0.33
0.025 0.037 0.0002 0.0047 Q 0.050 0.21 1.65 0.003 0.0010 0.035 0.66
0.36 0.027 0.009 0.0001 0.0078 R 0.043 0.14 1.46 0.003 0.0008 0.040
1.04 0.28 0.023 0.009 0.0001 0.0030 S 0.026 0.23 1.45 0.004 0.0023
0.020 0.88 0.84 0.018 0.009 0.0002 0.0034 T 0.057 0.24 1.58 0.005
0.0026 0.031 0.67 0.10 0.012 0.009 0.0002 0.0038 U 0.079 0.09 1.41
0.006 0.0006 0.021 0.71 0.16 0.020 0.011 0.0002 0.0021 V 0.072 0.21
1.42 0.004 0.0007 0.033 1.13 0.26 0.021 0.011 0.0001 0.0025 W 0.079
0.14 1.88 0.006 0.0009 0.040 0.78 0.45 0.025 0.008 0.0002 0.0025 X
0.071 0.15 1.43 0.003 0.0019 0.017 1.31 0.37 0.014 0.006 0.0001
0.0048 Y 0.068 0.21 1.45 0.006 0.0021 0.034 1.29 0.23 0.019 0.014
0.0002 0.0025 Z 0.087 0.08 1.97 0.006 0.0018 0.024 1.47 0.45 0.038
0.007 0.0002 0.0023 Formula Formula Steel [Ti]/ (3) (4) No. Ca Cu
Cr V O Mg REM Ceq [N] ACR Ceq* A 0.0035 0.28 -- -- 0.0012 -- --
0.488 3.24 0.84 3.17 B 0.0015 -- -- -- 0.0026 -- -- 0.468 3.00 0.47
3.17 C 0.0021 -- 0.12 0.012 0.0012 0.0008 -- 0.473 3.33 0.73 2.60 D
0.0019 0.14 -- -- 0.0011 -- 0.0030 0.454 3.53 0.57 2.87 E 0.0023 --
0.14 -- 0.0022 -- -- 0.483 2.63 0.71 3.17 F 0.0020 0.24 -- 0.012
0.0024 -- -- 0.505 3.23 0.54 3.06 G 0.0012 0.33 -- -- 0.0015 -- --
0.510 2.81 0.35 3.35 H 0.0016 -- -- -- 0.0017 -- -- 0.377 3.20 0.29
2.37 I 0.0014 -- 0.36 -- 0.0016 0.0007 -- 0.623 3.33 0.30 3.58 J
0.0012 0.18 -- -- 0.0012 -- -- 0.450 3.53 0.58 3.28 K 0.0011 --
0.24 -- 0.0018 -- -- 0.475 3.44 0.59 2.81 L 0.0015 0.30 -- --
0.0020 -- -- 0.529 3.42 0.46 3.45 M 0.0023 1.24 0.42 -- 0.0015 --
-- 0.600 2.94 0.79 3.21 N 0.0012 0.22 -- -- 0.0013 -- 0.0064 0.437
2.80 0.36 3.63 P 0.0011 -- 0.22 0.012 0.0021 -- -- 0.501 7.87 0.42
3.24 Q 0.0027 0.15 0.19 -- 0.0010 -- -- 0.490 1.15 1.74 3.20 R
0.0012 -- 1.34 0.018 0.0021 0.0013 -- 0.683 3.00 0.49 2.91 S 0.0024
-- -- -- 0.0011 -- -- 0.494 2.65 0.65 3.02 T 0.0011 0.14 0.49 0.068
0.0010 -- -- 0.506 2.37 0.24 2.62 U 0.0006 0.22 -- -- 0.0028 -- --
0.408 5.24 -0.16 2.83 V 0.0032 0.36 0.22 -- 0.0025 -- -- 0.505 4.40
1.95 2.92 W 0.0029 0.45 0.36 0.015 0.0016 -- -- 0.640 3.20 1.79
3.55 X 0.0019 0.36 -- -- 0.0020 -- -- 0.495 1.25 0.44 2.78 Y 0.0017
-- -- 0.010 0.0017 -- -- 0.444 5.60 0.39 2.91 Z 0.0023 -- -- --
0.0013 -- -- 0.603 3.04 0.75 4.03 Underlined values are outside the
range of this disclosure Formula (3) ACR: {[Ca] - (0.18 + 130
.times. [Ca]) .times. [O]}/1.25/[S] Formula (4) Ceq*: 5.5
[C].sup.4/3 + 15[P] + 0.90[Mn] + 0.12[Ni] + 7.9[Nb].sup.1/2 +
0.53[Mo]
TABLE-US-00003 TABLE 3 Rolling Conditions Cumulative Cumulative
Rolling Rolling Cooling Conditions Base Metal Reheating Reduction
at Reduction at Final Finish Rolling Cooling Cooling Stop Tempering
Properties Sample Steel Temperature 900.degree. C. or Less Than
Thickness Temperature Rate Temperature Temperature YP TS No. No.
(.degree. C.) Higher (%) 900.degree. C. (%) (mm) (.degree. C.)
(.degree. C./s) (.degree. C.) (.degree. C.) (MPa) (MPa) 1 A 1108 39
59 75 700 5 210 520 524 635 2 A 1128 45 58 70 720 5 220 530 531 638
3 A 1230 50 67 50 760 10 330 590 541 651 4 B 1149 18 80 50 730 10
330 510 551 651 5 B 998 35 49 100 740 2 300 530 533 619 6 C 1104 44
58 70 750 5 180 580 515 638 7 C 1064 30 31 150 740 0.8 250 590 445
588 8 D 984 55 48 70 770 5 230 560 527 638 9 D 1044 55 48 70 680 5
580 560 405 638 10 E 1025 61 57 50 750 10 200 680 432 651 11 E 1079
48 68 50 680 10 190 570 510 651 12 F 995 42 60 70 690 5 230 570 528
638 13 G 993 43 59 70 720 5 270 590 580 638 14 H 1023 48 55 70 720
5 290 550 412 638 15 I 1104 48 55 70 690 5 170 560 541 638 16 J
1128 55 48 70 770 5 250 540 501 638 17 K 1121 47 69 50 700 10 320
590 546 651 18 L 976 54 64 50 730 10 260 540 598 651 19 M 1132 48
68 50 770 10 330 560 588 651 20 N 1043 51 66 50 770 10 340 500 497
651 21 P 987 49 35 100 770 2 340 510 503 619 22 Q 971 41 44 100 760
2 270 560 522 619 23 R 1106 45 39 100 740 2 260 540 546 619 24 S
1010 38 62 70 740 5 210 550 593 638 25 T 1064 45 58 70 740 5 220
500 589 638 26 U 976 51 52 70 680 5 300 510 534 638 27 V 1018 35 64
70 760 5 150 560 506 638 28 W 1088 50 53 70 680 5 260 580 574 638
29 X 1025 59 43 70 730 5 160 500 452 638 30 Y 1131 45 58 70 760 5
320 550 471 638 31 Z 1021 41 60 70 770 5 290 530 531 638 32 C 1120
60 55 40 710 12 420 -- 592 702 Heat Affected Zone Base Metal
Properties Precipitates Properties CTOD Mean [Nb]/([Ti] + Base
Metal Properties After PWHT Sample vE -40.degree. C. vE -40.degree.
C. .delta.-10.degree. C. Particle [Nb] + YP TS .DELTA.TS
.DELTA.vTrs No. (J) (J) (mm) Size (nm) [Mo]) (MPa) (MPa) (MPa)
(.degree. C.) Notes 1 239 238 1.13 8.7 0.55 529 617 -18 5 Example 2
208 230 1.28 5.3 0.64 530 621 -17 6 Example 3 98 216 1.15 12.0 0.55
531 605 -46 19 Comparative Example 4 55 241 0.82 8.6 0.45 515 655 4
23 Comparative Example 5 211 211 0.87 8.1 0.36 538 626 7 4 Example
6 222 209 1.13 9.7 0.59 587 605 -33 6 Example 7 46 221 1.73 13.5
0.25 459 521 -67 16 Comparative Example 8 284 181 1.18 6.5 0.60 531
615 -23 8 Example 9 105 229 0.52 -- -- -- -- -- -- Comparative
Example 10 297 217 0.55 -- -- -- -- -- -- Comparative Example 11
213 163 1.48 8.2 0.44 516 655 4 7 Example 12 106 56 0.16 -- -- --
-- -- -- Comparative Example 13 54 64 0.09 -- -- -- -- -- --
Comparative Example 14 248 -- -- -- -- -- -- -- -- Comparative
Example 15 46 37 0.11 -- -- -- -- -- -- Comparative Example 16 257
56 0.10 -- -- -- -- -- -- Comparative Example 17 87 -- -- -- -- --
-- -- -- Comparative Example 18 35 56 0.12 -- -- -- -- -- --
Comparative Example 19 54 -- -- -- -- -- -- -- -- Comparative
Example 20 35 66 0.31 -- -- -- -- -- -- Comparative Example 21 56
-- -- -- -- -- -- -- -- Comparative Example 22 45 74 0.25 -- -- --
-- -- -- Comparative Example 23 65 49 0.16 -- -- -- -- -- --
Comparative Example 24 68 -- -- -- -- -- -- -- -- Comparative
Example 25 51 -- -- -- -- -- -- -- -- Comparative Example 26 245 52
0.41 -- -- -- -- -- -- Comparative Example 27 206 19 0.43 -- -- --
-- -- -- Comparative Example 28 65 -- -- -- -- -- -- -- --
Comparative Example 29 200 87 0.34 -- -- -- -- -- -- Comparative
Example 30 229 49 0.25 -- -- -- -- -- -- Comparative Example 31 273
71 0.14 -- -- -- -- -- -- Comparative Example 32 215 197 1.54 8.8
0.55 578 674 -28 7 Example Underlined values are outside the range
of this disclosure .DELTA.TS = TS(after PWHT) - TS(before PWHT)
.DELTA.vTrs = vTrs(after PWHT) - vTrs(before PWHT)
[0146] As listed in Table 2, steel codes A to E represent steel
conforming to this disclosure, whereas steel codes F to Z represent
comparative steel in which one of the steel components is outside
of the range of this disclosure. Sample Numbers 1, 2, 5, 6, 8, and
11 in Table 3 are all Examples for which the results of the Charpy
impact test on the weld bond, the results of the three-point
bending CTQD test on the weld bond, the precipitate
size/composition in the steel plate, and the PWHT property all
satisfy the targets.
[0147] Conversely, in Sample Numbers 3, 4, 7, 9, 10, and 12 to 31,
at least one of the steel plate components, the manufacturing
conditions, and the precipitate size/composition fall outside of
the ranges of this disclosure, and one of the base metal
properties, the results of the Charpy impact test on the weld bond,
the results of the three-point bending CTOD test on the weld bond,
and the PWHT property does not satisfy the target. In Table 3,
entries marked with a horizontal line are entries for which
measurement was not possible.
[0148] The steel of the Examples according to this disclosure has
both excellent strength and toughness of the steel plate, as the
yield stress (YS) of the steel plate is 460 MPa or higher, and the
Charpy absorbed energy (vE.sub.-40.degree. C.) is 200 J or higher.
Furthermore, in the weld joint bond, vE.sub.-40.degree. C. is 150 J
or higher, and the CTOD value is 0.5 mm or higher, thus also
providing excellent toughness in the heat-affected zone. When the
mean particle size of the precipitates is 20 .mu.m or less and
[Nb]/([Ti]+[Nb]+[Mo]).gtoreq.0.3, then the base metal property
after PWHT is also excellent. By contrast, in the Comparative
Examples outside of the ranges of this disclosure, only steel
plates for which one of the above-described properties was inferior
could be achieved.
* * * * *