U.S. patent application number 13/807798 was filed with the patent office on 2013-08-15 for abrasion resistant steel plate which exhibits excellent weld toughness and excellent delayed fracture resistance.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is Shinichi Suzuki, Keiji Ueda. Invention is credited to Shinichi Suzuki, Keiji Ueda.
Application Number | 20130206286 13/807798 |
Document ID | / |
Family ID | 45402260 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130206286 |
Kind Code |
A1 |
Ueda; Keiji ; et
al. |
August 15, 2013 |
ABRASION RESISTANT STEEL PLATE WHICH EXHIBITS EXCELLENT WELD
TOUGHNESS AND EXCELLENT DELAYED FRACTURE RESISTANCE
Abstract
An abrasion resistant steel plate which is excellent in
toughness and delayed fracture resistance of a multi pass weld and
is preferably used in construction machines, industrial machines
and the like is provided. To be more specific, the composition of
the steel plate contains by mass % 0.20 to 0.30% C, 0.05 to 1.0%
Si, 0.40 to 1.2% Mn, 0.010% or less P, 0.005% or less S, 0.40 to
1.5% Cr, 0.005 to 0.025% Nb, 0.05 to 1.0% Mo, 0.005 to 0.03% Ti,
0.1% or less Al, 0.01% or less N, and 0.0003 to 0.0020% B, and
further contains one, two or more kinds of components selected from
a group consisting of W, Cu, Ni, V, REM, Ca and Mg when necessary,
wherein DI*
(DI*=33.85.times.(0.1.times.C).sup.0.5.times.(0.7.times.Si+1).times.(3.33-
.times.Mn+1).times.(0.35.times.Cu+1).times.(0.36.times.Ni+1).times.(2.16.t-
imes.Cr+1).times.(3.times.Mo+1).times.(1.75.times.V+1).times.(1.5.times.W+-
1)) is 45 to 180, C+Mn/4-Cr/3+10P.ltoreq.0.47, and a base phase of
the microstructure is formed of martensite.
Inventors: |
Ueda; Keiji; (Kanagawa,
JP) ; Suzuki; Shinichi; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ueda; Keiji
Suzuki; Shinichi |
Kanagawa
Tokyo |
|
JP
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
45402260 |
Appl. No.: |
13/807798 |
Filed: |
June 29, 2011 |
PCT Filed: |
June 29, 2011 |
PCT NO: |
PCT/JP2011/065410 |
371 Date: |
February 21, 2013 |
Current U.S.
Class: |
148/330 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/32 20130101; C22C 38/002 20130101; C22C 38/02 20130101;
C22C 38/26 20130101; C22C 38/42 20130101; C22C 38/28 20130101; C22C
38/06 20130101; C22C 38/005 20130101; C22C 38/54 20130101; C22C
38/22 20130101; C22C 38/50 20130101; C22C 38/20 20130101; C21D
8/0226 20130101; C22C 38/44 20130101; C22C 38/46 20130101; C21D
8/0263 20130101; C22C 38/48 20130101; C22C 38/001 20130101; C22C
38/24 20130101 |
Class at
Publication: |
148/330 |
International
Class: |
C22C 38/02 20060101
C22C038/02; C22C 38/26 20060101 C22C038/26; C22C 38/28 20060101
C22C038/28; C22C 38/32 20060101 C22C038/32; C22C 38/24 20060101
C22C038/24; C22C 38/20 20060101 C22C038/20; C22C 38/44 20060101
C22C038/44; C22C 38/46 20060101 C22C038/46; C22C 38/48 20060101
C22C038/48; C22C 38/50 20060101 C22C038/50; C22C 38/54 20060101
C22C038/54; C22C 38/42 20060101 C22C038/42; C22C 38/04 20060101
C22C038/04; C22C 38/00 20060101 C22C038/00; C22C 38/22 20060101
C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2010 |
JP |
2010-149650 |
Jun 28, 2011 |
JP |
2011-142507 |
Claims
1. An abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance, and having a
composition containing by mass % 0.20 to 0.30% C, 0.05 to 1.0% Si,
0.40 to 1.2% Mn, 0.010% or less P, 0.005% or less S, 0.40 to 1.5%
Cr, 0.05 to 1.0% Mo, 0.005 to 0.025% Nb, 0.005 to 0.03% Ti, 0.1% or
less Al, 0.0015 to 0.0060% N, 0.0003 to 0.0020% B, and Fe and
unavoidable impurities as a balance, wherein hardenability index
DI* expressed by a formula (1) is 45 or more, and a base phase of
the microstructure is formed of martensite.
DI*=33.85.times.(0.1.times.C).sup.0.5.times.(0.7.times.Si+1).times.(3.33.-
times.Mn+1).times.(0.35.times.Cu+1).times.(0.36.times.Ni+1).times.(2.16.ti-
mes.Cr+1).times.(3.times.Mo+1).times.(1.75.times.V+1).times.(1.5.times.W+1-
) (1), wherein the respective element symbols are contents (mass %)
of the elements.
2. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 1, wherein the steel composition further contains by mass %
0.05 to 1.0% W.
3. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 1, wherein the steel composition further contains by mass %
one or two or more kinds of components selected from a group
consisting of 1.5% or less Cu, 2.0% or less Ni, and 0.1% or less
V.
4. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 1, wherein the steel composition further contains by mass %
one, two or more kinds of components selected from a group
consisting of 0.08% or less REM, 0.005% or less Ca, and 0.005% or
less Mg.
5. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 1, wherein surface hardness of the steel plate is 400
HBW10/3000 or more in Brinell hardness.
6. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 1, wherein hardenability index DI* is 180 or less.
7. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 1, wherein the steel plate satisfies a following formula (2).
C+Mn/4-Cr/3+10P.ltoreq.0.47 (2), wherein the respective element
symbols are contents (mass %) of the elements.
8. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 2, wherein the steel composition further contains by mass %
one or two or more kinds of components selected from a group
consisting of 1.5% or less Cu, 2.0% or less Ni, and 0.1% or less
V.
9. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 2, wherein the steel composition further contains by mass %
one, two or more kinds of components selected from a group
consisting of 0.08% or less REM, 0.005% or less Ca, and 0.005% or
less Mg.
10. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 3, wherein the steel composition further contains by mass %
one, two or more kinds of components selected from a group
consisting of 0.08% or less REM, 0.005% or less Ca, and 0.005% or
less Mg.
11. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 2, wherein surface hardness of the steel plate is 400
HBW10/3000 or more in Brinell hardness.
12. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 3, wherein surface hardness of the steel plate is 400
HBW10/3000 or more in Brinell hardness.
13. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 4, wherein surface hardness of the steel plate is 400
HBW10/3000 or more in Brinell hardness.
14. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 2, wherein hardenability index DI* is 180 or less.
15. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 3, wherein hardenability index DI* is 180 or less.
16. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 4, wherein hardenability index DI* is 180 or less.
17. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 5, wherein hardenability index DI* is 180 or less.
18. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 2, wherein the steel plate satisfies a following formula (2).
C+Mn/4-Cr/3+10P.ltoreq.0.47 (2), wherein the respective element
symbols are contents (mass %) of the elements.
19. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 3, wherein the steel plate satisfies a following formula (2).
C+Mn/4-Cr/3+10P.ltoreq.0.47 (2), wherein the respective element
symbols are contents (mass %) of the elements.
20. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance according to
claim 4, wherein the steel plate satisfies a following formula (2).
C+Mn/4-Cr/3+10P.ltoreq.0.47 (2), wherein the respective element
symbols are contents (mass %) of the elements.
Description
TECHNICAL FIELD
[0001] The present invention relates to an abrasion resistant steel
plate or steel sheet having a plate thickness of 4 mm or more
preferably used in construction machines, industrial machines,
shipbuilding, steel pipes, civil engineering, architecture or the
like, and more particularly to an abrasion resistant steel plate or
steel sheet which exhibits excellent toughness and excellent
delayed fracture resistance of a multi pass weld.
BACKGROUND ART
[0002] When a hot-roiled steel plate is employed for forming steel
structural products, machines, devices or the like in construction
machines, industrial machines, shipbuilding, steel pipes, civil
engineering, architecture or the like, there may be a case where
the steel plates are required to possess abrasion resistant
property. Conventionally, to impart excellent abrasion resistant
property to a steel material, hardness is increased in general, and
hardness of the steel material can be remarkably enhanced by
forming the steel material into the martensite single phase
microstructure. The increase of an amount of solid solution carbon
is also effective for enhancing hardness of martensite
microstructure per se.
[0003] Accordingly, the abrasion resistant steel plate exhibits
high cold cracking susceptibility so that the steel plate exhibits
inferior weld toughness in general whereby when the abrasion
resistant steel plate is used in forming the welded steel
structure, in general, the abrasion resistant steel plate is
laminated to a surface of a steel member which is brought into
contact with rock, earth and sand or the like as a liner. For
example, with respect to a vessel of a damped motor lorry, there
has been known a case where the vessel is assembled by welding
using mild steel and, thereafter, an abrasion resistant steel plate
is laminated to only a front surface of the vessel which is brought
into contact with earth and sand.
[0004] However, in the manufacturing method in which the abrasion
resistant steel plate is laminated to the welded steel structure
after the welded steel structure is assembled, labor for the
manufacture and a manufacturing cost are increased. Accordingly,
there has been a demand for an abrasion resistant steel plate which
is excellent in weldability and weld toughness and can be used as a
strength member of the welded steel structure, and such an abrasion
resistant steel plate has been proposed in patent documents 1 to 5,
for example.
[0005] Patent document 1 relates to an abrasion resistant steel
plate which exhibits excellent delayed fracture resistance and a
method of manufacturing the abrasion resistant steel plate. In
patent document 1, there is the description that, to improve the
delayed fracture resistance, steel which further contains one, two
or more kinds of components selected from a group consisting of Cu,
V, Ti, B and Ca in the composition of a type containing low-Si,
low-P, low-S, Cr, Mo and Nb is subjected to direct quenching
(hereinafter also referred to as DQ), and tempering is performed
when necessary.
[0006] Patent document 2 relates to steel having high abrasion
resistant property and a method of manufacturing a steel product.
In patent document 2, there is described steel which has the
composition composed of a 0.24 to 0.3C--Ni, Cr, Mo, B system,
satisfies a parameter formula constituted of contents of these
elements, and includes martensite containing 5 to 15 volume % of
austenite or martensitic structure and bainitic structure thus
enhancing abrasion resistant property. Patent document 2 also
describes that the steel having the above-mentioned components is
cooled at a cooling rate of 1.degree. C./sec or more at a
temperature between an austenitizing temperature and 450.degree.
C.
[0007] Patent document 3 relates to an abrasion resistant steel
material which exhibits excellent toughness and excellent delayed
fracture resistance and a method of manufacturing the abrasion
resistant steel material. In patent document 3, there is described
a steel material which has the composition containing Cr, Ti, and B
as indispensable components, wherein a surface layer is formed of
tempered martensite, an internal part is formed of tempered
martensite and tempered lower bainitic structure, and an aspect
ratio of prior austenite grain diameter between the wall thickness
direction and the rolling direction is defined. Patent document 3
also describes that the steel having the content composition is
subject to hot rolling at a temperature of 900.degree. C. or below
and at a cumulative reduction ratio of 50% or more and, thereafter,
is directly quenched and tempered.
[0008] Patent document 4 relates to an abrasion resistant steel
material which exhibits excellent toughness and excellent delayed
fracture resistance and a method of manufacturing the abrasion
resistant steel material. In patent document 4, there is described
a steel material which has the composition containing Cr, Ti and B
as indispensable components, wherein a surface layer is formed of
martensite, and an internal part is formed of the mixed structure
of martensite and lower bainitic structure or lower bainitic
single-phase structure, and an elongation rate of prior austenite
grains expressed by an aspect ratio between prior austenite grain
diameter at a plate thickness center portion and prior austenite
grain diameter in the rolling direction is defined. Patent document
4 also describes that the steel having the composition is subjected
to hot rolling at a temperature of 900.degree. C. or below and at a
cumulative reduction ratio of 50% or more and, thereafter, is
directly quenched.
[0009] Patent document 5 relates to abrasion resistant steel which
exhibits excellent weldability, excellent abrasion resistant
property and excellent corrosion resistance, and a method of
manufacturing the abrasion resistant steel. In patent document 5,
there is described steel which contains 4 to 9 mass % of Cr as an
indispensable element, contains one or two kinds of Cu and Ni and
satisfies a parameter formula constituted of contents of specific
components. Patent document 5 also describes that the steel having
the composition is subjected to hot rolling at a temperature of
950.degree. C. or below and at a cumulative reduction ratio of 30%
or more and, thereafter, the steel is reheated at a temperature of
Ac3 or more and is quenched.
PRIOR ART LITERATURE
Patent Document
[0010] [Patent Document 1] JP-A-5-51691 [0011] [Patent Document 2]
JP-A-8-295990 [0012] [Patent Document 3] JP-A-2002-115024 [0013]
[Patent Document 4] JP-A-2002-80930 [0014] [Patent Document 5]
JP-A-2004-162120
SUMMARY OF THE INVENTION
Task to be Solved by the Invention
[0015] In case of a weld joint using a steel plate having a plate
thickness of 4 mm or more, it is often the case that the weld joint
is formed by multi pass welding. However, in a weld, a bond area
formed by a preceding weld pass is reheated by succeeding welding
so that a region where toughness is remarkably deteriorated
appears. Particularly, in an abrasion resistant steel plate, when a
bond area formed by welding for forming a first layer is reheated
to a temperature around 300.degree. C. due to succeeding welding,
toughness is remarkably deteriorated due to low-temperature
tempering embrittlement.
[0016] It is thought that low-temperature tempering embrittlement
is brought about by a synergistic action between a morphology
change of carbide in martensite and the intergranular segregation
of impurity elements or the like. In a bond area which has coarse
grain particles and contains a large quantity of solid solution N,
the low-temperature tempering embrittlement becomes conspicuous. It
has been pointed out that delayed fracture is liable to occur in a
region which is reheated to such a low-temperature tempering
embrittlement temperature.
[0017] Patent documents 1 and 2 fail to describe the enhancement of
weld toughness in an abrasion resistant steel, and patent documents
3 and 4 also define the microstructure aiming at the enhancement of
toughness of a base material. Although patent document 5 studies
weldability and abrasion resistant property of a weld, the study
does not aim at the enhancement of weld toughness. That is, the
abrasion resistant steels proposed in patent documents 1 to 5 and
the like are less than optimal with respect to the improvement of
both weld toughness and delayed fracture resistance of a multi pass
weld.
[0018] Accordingly, it is an object of the present invention to
provide an abrasion resistant steel plate which exhibits excellent
toughness of a multi pass weld and excellent delayed fracture
resistance without inducing lowering of productivity and the
increase in a manufacturing cost.
Means for Solving the Problem
[0019] To achieve the above-mentioned object, inventors of the
present invention have made extensive studies on various factors
which decide chemical components of a steel plate, a method of
manufacturing the steel plate and the microstructure of the steel
plate so as to secure toughness of a multi pass weld and delayed
fracture resistance with respect to an abrasion resistant steel
plate, and have made following findings.
[0020] 1. To secure excellent abrasion resistant property, it is
indispensable to form the base microstructure or the main
microstructure (also referred to as a base phase or a main phase)
of the steel plate into martensite. For this end, it is important
to strictly control the chemical composition of the steel plate
thus securing quenching property.
[0021] 2. To achieve the excellent toughness of a multi pass weld,
it is necessary to suppress grain particles in a welded heat
affected zone from becoming coarse, and for this end, it is
effective to make use of a pinning effect by dispersing fine
precipitates in the steel plate. Accordingly, a control of Ti, N is
important.
[0022] 3. The reduction of solid solution N in a bond area forming
an initial layer is effective for suppressing low-temperature
tempering embrittlement caused by succeeding welding. For this end,
it is important to strictly control B for fixing solid solution N
as BN.
[0023] 4. To secure the excellent toughness and to suppress delayed
fracture in a low-temperature tempering embrittlement temperature
area of the welded heat affected zone, it is important to properly
control quantities of alloy elements such as C, Mn, Cr, Mo, P.
[0024] The present invention has been made by further studying the
above-mentioned findings. That is, the present invention is
directed to:
[0025] 1. An abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance, and having a
composition containing by mass % 0.20 to 0.30% C, 0.05 to 1.0% Si,
0.40 to 1.2% Mn, 0.010% or less P, 0.005% or less S, 0.40 to 1.5%
Cr, 0.05 to 1.0% Mo, 0.005 to 0.025% Nb, 0.005 to 0.03% Ti, 0.1% or
less Al, 0.0015 to 0.0060% N, 0.0003 to 0.0020% B, and Fe and
unavoidable impurities as a balance, wherein hardenability index
DI* expressed by a formula (1) is 45 or more, and a base phase of
the microstructure is formed of martensite.
DI*=33.85.times.(0.1.times.C).sup.0.5.times.(0.7.times.Si+1).times.(3.33-
.times.Mn+1).times.(0.35.times.Cu+1).times.(0.36.times.Ni+1).times.(2.16.t-
imes.Cr+1).times.(3.times.Mo+1).times.(1.75.times.V+1).times.(1.5.times.W+-
1) (1),
wherein the respective element symbols are contents (mass %) of the
elements.
[0026] 2. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance described in 1,
wherein the steel composition further contains by mass % 0.05 to
1.0% W.
[0027] 3. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance described in 1
or 2, wherein the steel composition further contains by mass % one,
two or more kinds of components selected from a group consisting of
1.5% or less Cu, 2.0% or less Ni, and 0.1% or less V.
[0028] 4. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance described in
any one of 1 to 3, wherein the steel composition further contains
by mass % one, two or more kinds of components selected from a
group consisting of 0.008% or less REM, 0.005% or less Ca, and
0.005% or less Mg.
[0029] 5. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance described in
any one of 1 to 4, wherein surface hardness of the steel plate is
400 HBW10/3000 or more in Brinell hardness.
[0030] 6. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance described in
any one of 1 to 5, wherein hardenability index DI* is 180 or
less.
[0031] 7. The abrasion resistant steel plate having excellent weld
toughness and excellent delayed fracture resistance described in
anyone of 1 to 6, wherein the steel plate satisfies a following
formula (2).
C+Mn/4-Cr/3+10P.ltoreq.0.47 (2),
wherein the respective element symbols are contents (mass %) of the
elements.
Advantage of the Invention
[0032] According to the present invention, it is possible to
acquire the abrasion resistant steel plate having excellent
toughness and excellent delayed fracture resistance at a multi pass
weld. The present invention largely contributes to the enhancement
of manufacturing efficiency and safety at the time of manufacturing
a steel structure thus acquiring an industrially remarkable
effect.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a view for explaining a T shape fillet weld
cracking test.
[0034] FIG. 2 is a view showing a position where a Charpy impact
test piece is taken from a weld.
MODE FOR CARRYING OUT THE INVENTION
[0035] The present invention defines the composition and the
microstructure.
[Composition]
[0036] In the explanation made hereinafter, % indicates mass %. C,
0.20 to 0.30%
[0037] C is an important element for increasing hardness of
martensite and for allowing the steel plate to secure the excellent
abrasion resistant property. It is necessary for the steel plate to
contain 0.20% or more C to acquire such effects. On the other hand,
when the content of C exceeds 0.30%, not only weldability is
deteriorated but also toughness of a bond area of a multi pass weld
is deteriorated due to low-temperature tempering embrittlement.
Accordingly, content of C is limited to a value which falls within
a range from 0.20 to 0.30%. The content of C is preferably limited
to a value which falls within a range from 0.20 to 0.28%.
Si: 0.05 to 1.0%
[0038] Si acts as a deoxidizing agent, and not only Si is necessary
for steel making but also Si has an effect of increasing hardness
of a steel plate by solid solution strengthening where Si is
present in steel in a solid solution state. Further, Si has an
effect of suppressing the deterioration of toughness of a bond area
of a multi pass weld due to low-temperature tempering
embrittlement. It is necessary for the steel plate to contain 0.03%
or more Si to acquire such an effect. On the other hand, when the
content of Si exceeds 1.0%, toughness of the multi pass welded heat
affected zone is remarkably deteriorated. Accordingly, the content
of Si is limited to a value which falls within a range from 0.05 to
1.0%. The content of Si is preferably limited to a value which
falls within a range from 0.07 to 0.5%.
Mn: 0.40 to 1.2%
[0039] Mn has an effect of increasing hardenability of steel, and
it is necessary for the steel plate to contain 0.40% or more Mn to
secure hardness of a base material. On the other hand, when the
content of Mn exceeds 1.2%, not only toughness, ductility and
weldability of the base material are deteriorated, but also
intergranular segregation of P is accelerated thus accelerating the
generation of delayed fracture. Accordingly, the content of Mn is
limited to a value which falls within a range from 0.40 to 1.2%.
The content of Mn is preferably limited to a value which falls
within a range from 0.40 to 1.1%.
P: 0.010% or Less
[0040] When the content of P exceeds 0.010%, P is segregated in a
grain boundary, the segregated P becomes an initiation point of
delayed fracture, and deteriorates toughness of a multi pass weld.
Accordingly, an upper limit of the content of P is set to 0.010%
and it is desirable that the content of P is set as small as
possible. Since the excessive reduction of P pushes up a refining
cost and becomes economically disadvantageous, the content of P is
desirably set to 0.002% or more.
S: 0.005% or Less
[0041] S deteriorates low-temperature toughness and ductility of a
base material and hence, the content of S is desirably set small
with an allowable upper limit of 0.005%.
Cr: 0.40 to 1.5%
[0042] Cr is an important alloy element in the present invention,
and has an effect of increasing hardenability of steel and also has
an effect of suppressing the deterioration of toughness of the bond
area of the multi pass weld due to low-temperature tempering. This
is because the inclusion of Cr delays the diffusion of C in the
steel plate and hence, when the steel plate is reheated to a
temperature region where the low-temperature tempering
embrittlement occurs, morphology change of carbide in martensite
can be suppressed. It is necessary for the steel plate to contain
0.40% or more of Cr to acquire such an effect. On the other hand,
when the content of Cr exceeds 1.5%, the effect is saturated so
that not only does it become economically disadvantageous but also
weldability is lowered. Accordingly, the content of Cr is limited
to a value which falls within a range from 0.40 to 1.5%. The
content of Cr is preferably limited to a value which falls within a
range from 0.40 to 1.2%.
Mo: 0.05 to 1.0%
[0043] Mo is an element effective for remarkably increasing
hardenability thus increasing hardness of a base material. Further,
Mo has an effect of suppressing the deterioration of toughness of
the bond area of the multi pass weld due to low-temperature
tempering. The content of Mo is set to 0.05% or more for acquiring
such an effect. However, when the content of Mo exceeds 1.0%, Mo
adversely influences toughness, ductility and weld crack resistance
of the base material and hence, the content of Mo is set to 1.0% or
less. Accordingly, the content of Mo is limited to a value which
falls within a range from 0.05 to 1.0%. The content of Mo is
preferably limited to a value which falls within a range from 0.1
to 0.8%.
Nb: 0.005 to 0.025%
[0044] Nb is an important element having both an effect of
improving toughness of the multi pass weld and an effect of
suppressing the occurrence of delayed fracture by making the
microstructure of the base material and the multi pass weld finer
by causing the precipitation of carbonitride and also by fixing
solid solution N. It is necessary for the steel plate to contain
0.005% or more Nb to acquire such effects. On the other hand, when
the content of Nb exceeds 0.025%, coarse carbonitride precipitates
and there may be a case where the coarse carbonitride becomes an
initiation point of fracture. Accordingly, the content of Nb is
limited to a value which falls within a range from 0.005 to 0.025%.
The content of Nb is preferably limited to a value which falls
within a range from 0.007 to 0.023%.
Ti: 0.005 to 0.03%
[0045] Ti has an effect of suppressing grains in the bond area of
the multi pass weld from becoming coarse by forming TiN due to
fixing of solid solution N, and also has an effect of suppressing
the deterioration of toughness and the occurrence of delayed
fracture in the low-temperature tempering temperature region due to
the decrease of solid solution N. It is necessary for the steel
plate to contain 0.005% or more Ti to acquire such effects. On the
other hand, when the content of Ti exceeds 0.03%, TiC precipitates
so that toughness of the base material is deteriorated.
Accordingly, the content of Ti is limited to a value which falls
within a range from 0.005 to 0.03%. The content of Ti is preferably
limited to a value which falls within a range from 0.007 to
0.025%.
Al: 0.1% or Less
[0046] Al acts as a deoxidizing agent and is most popularly used in
a molten steel deoxidizing process of a steel plate. Further, by
forming AlN by fixing solid solution N in steel, Al has an effect
of suppressing grains in the bond area of the multi pass weld from
becoming coarse and an effect of suppressing the deterioration of
toughness and the occurrence of delayed fracture in the
low-temperature tempering temperature region due to the reduction
of solid solution N. On the other hand, when the content of Al
exceeds 0.1%, Al is mixed into weld metal at the time of welding
thus deteriorating toughness of weld metal. Accordingly, the
content of Al is limited to 0.1% or less. The content of Al is
preferably limited to a value which falls within a range from 0.01
to 0.07%.
N, 0.0015 to 0.0060%
[0047] N is bonded with Ti thus precipitating TiN, and has an
effect of contributing to the enhancement of toughness by
suppressing austenite particles from becoming coarse in HAZ. It is
necessary for the steel plate to contain 0.0015% or more N to
secure a required quantity of TiN having such an effect. On the
other hand, when the content of N exceeds 0.0060%, in a region
which is heated to a temperature at which TiN is melted at the time
of welding, a quantity of solid solution N is increased so that the
deterioration of toughness in the low-temperature tempering
temperature region becomes conspicuous. Accordingly, the content of
N is limited to a value which falls within a range from 0.0015 to
0.0060%. The content of N is preferably limited to a value which
falls within a range from 0.0020 to 0.0055%.
B: 0.0003 to 0.0020%
[0048] B is an element effective for remarkably increasing
hardenability with addition of a trace amount of B thus increasing
hardness of a base material. Further, in a region which is heated
to a temperature at which TiN is melted at the time of welding,
solid solution N is fixed as BN so that B has an effect of
suppressing the deterioration of toughness in the low-temperature
tempering temperature region due to succeeding welding. The content
of B may preferably be 0.0003% or more for acquiring such an
effect. However, when the content of B exceeds 0.0020%, B adversely
influences toughness, ductility and weld crack resistance of the
base material. Accordingly, the content of B is set to 0.0020% or
less. The content of B is preferably limited to a value which falls
within a range from 0.0005 to 0.0018%. The balance of the steel
plate is Fe and unavoidable impurities.
[0049] According to the present invention, to further enhance
properties of the steel plate, in addition to the above-mentioned
basic component system, the steel plate may contain one, two or
more kinds of components selected from a group consisting of W, Cu,
Ni, V, REM, Ca and Mg.
W: 0.05 to 1.0%
[0050] N is an element effective for remarkably increasing
hardenability thus increasing hardness of a base material. The
content of N may preferably be 0.05% or more for acquiring such an
effect. However, when the content of N exceeds 1.0%, W adversely
influences toughness, ductility and weld crack resistance of the
base material. Accordingly, the content of W is set to 1.0% or
less.
[0051] All of Cu, Ni and V are elements which contribute to the
enhancement of strength of steel, and the steel plate may contain
proper amounts of Cu, Ni, V depending on strength which the steel
plate requires.
Cu: 1.5% or Less
[0052] Cu is an element effective for increasing hardenability thus
increasing hardness of the base material. The content of Cu may
preferably be 0.1% or more for acquiring such an effect. However,
when the content of Cu exceeds 1.5%, the effect is saturated and Cu
causes hot brittleness thus deteriorating surface property of a
steel plate. Accordingly, the content of Cu is set to 1.5% or
less.
Ni: 2.0% or Less
[0053] Ni is an element effective for increasing hardenability thus
increasing hardness of the base material. The content of Ni may
preferably be 0.1% or more for acquiring such an effect. However,
when the content of Ni exceeds 2.0%, the effect is saturated so
that it becomes economically disadvantageous. Accordingly, the
content of Ni is set to 2.0% or less.
V: 0.1% or Less
[0054] V is an element effective for increasing hardenability thus
increasing hardness of the base material. The content of V may
preferably be 0.01% or more for acquiring such an effect. However,
when the content of V exceeds 0.1%, toughness and ductility of the
base material is deteriorated. Accordingly, the content of V is set
to 0.1% or less.
[0055] All of REM, Ca and Mg contribute to the enhancement of
toughness, and these elements are selectively added corresponding
to properties which the steel plate desires. When REM is added, the
content of REM may preferably be 0.002% or more. On the other hand,
when the content of REM exceeds 0.008%, the effect is saturated.
Accordingly, an upper limit of REM is set to 0.008%.
[0056] When Ca is added, the content of Ca may preferably be
0.0005% or more. On the other hand, when the content of Ca exceeds
0.005%, the effect is saturated. Accordingly, an upper limit of Ca
is set to 0.005%.
[0057] When Mg is added, the content of Mg may preferably be 0.001%
or more. On the other hand, when the content of Mg exceeds 0.005%,
the effect is saturated. Accordingly, an upper limit of Mg is set
to 0.005%.
DI*=33.85.times.(0.1.times.C).sup.0.5.times.(0.7.times.Si+1).times.(3.33-
.times.Mn+1).times.(0.35.times.Cu+1).times.(0.36.times.Ni+1).times.(2.16.t-
imes.Cr+1).times.(3.times.Mo+1).times.(1.75.times.V+1).times.(1.5.times.W+-
1) (1),
wherein the respective element symbols are contents (mass %) of the
elements.
[0058] This parameter: DI*(hardenability index) is defined to form
the base structure of the base material into martensite thus
imparting excellent abrasion resistant property to the base
structure within the range of the above-mentioned composition, and
a value of the parameter is set to 45 or more. When the value of
the parameter is set to less than 45, a quenching depth from a
surface layer in the plate thickness direction becomes less than 10
mm and hence, a lifetime of the steel plate as the abrasion
resistant steel plate is shortened.
[0059] When the value of the parameter exceeds 180, the base
structure of the base material is martensite and hence, the base
structure exhibits favorable abrasion resistant property. However,
low-temperature crack property at the time of welding and
low-temperature weld toughness are deteriorated. Accordingly, the
value of the parameter DI* is preferably set to 180 or less. The
value of the parameter 51* is more preferably set to a value which
falls within a range from 50 to 160.
C+Mn/4-Cr/3+10P.ltoreq.0.47 (2),
wherein the respective element symbols are contents (mass %) of the
elements.
[0060] When the basic structure of the base material of the steel
plate is formed of martensite and has the composition which
exhibits excellent toughness in both the bond area and the
low-temperature tempering embrittlement area when welding is
performed, a value of the parameter: C+Mn/4-Cr/3+10P is set to 0.47
or less within a range of the above-mentioned composition. Although
the base structure of the base material is held in martensite and
exhibits favorable abrasion resistant property even when the value
of the parameter exceeds 0.47, weld toughness is remarkably
deteriorated. The value of parameter may preferably be 0.45 or
less.
[Microstructure]
[0061] According to the present invention, to enhance abrasion
resistant property, a base phase or of the microstructure of a
steel plate is defined to martensite. The structure such as bainite
or ferrite other than martensite lowers abrasion resistant property
and hence, it is preferable not to mix such structure into
martensite as much as possible. However, when a total area ratio of
these structures is less than 10%, the influence exerted by these
structures can be ignored. Further, when surface hardness of the
steel plate is less than 400 HBW10/3000 in Brinell hardness, a
lifetime of the steel plate as abrasion resistant steel is
shortened. Accordingly, it is desirable to set the surface hardness
to 400 HBW10/3000 or more in Brinell hardness.
[0062] In the developed steel according to the present invention,
the microstructure of the bond area is the mixed structure of
martensite and bainite. The structure such as ferrite other than
martensite and bainite lowers abrasion resistant property and
hence, it is preferable not to mix such structure as much as
possible. However, when a total area ratio of these structures is
less than 20%, the influence exerted by these structures can be
ignored.
[0063] Further, in the developed steel according to the present
invention, to secure toughness of the bond area, it is preferable
that carbonitride particles of Nb and Ti having an average particle
size of 1 .mu.m or less are present at a rate of 1000
pieces/mm.sup.2 or more, an average particle size of prior
austenite is less than 200 and an average particle size of lower
microstructure surrounded by a large tilt grain boundary having a
radial hook of 15.degree. or more is less than 70 .mu.m. The
abrasion resistant steel according to the present invention can be
manufactured under the following manufacturing conditions. In the
explanation made hereinafter, the indication ".degree. C." relating
to temperature means temperature at 1/2 position of a plate
thickness. It is preferable that a molten steel having the
above-mentioned composition is produced by a known molten steel
producing method, and the molten steel is formed into a raw steel
material such as a slab having a predetermined size by a continuous
casting process or an ingot-making/blooming method.
[0064] Next, the obtained raw steel material is immediately
subjected to hot rolling without cooling or is subjected to hot
rolling following heating at a temperature of 950 to 1250.degree.
C. after cooling thus forming a steel plate having a desired plate
thickness. Immediately after hot rolling, water cooling is
performed or quenching is performed after reheating. Thereafter,
when necessary, tempering is performed at a temperature of
300.degree. C. or below.
EMBODIMENT 1
[0065] Steel slabs which were prepared with various compositions
shown in Table 1 by way of a steel converter, ladle refining and a
continuous casting method were heated at a temperature of 1000 to
1250.degree. C. and, thereafter, the steel slabs were subjected to
hot rolling under manufacturing conditions shown in Table 2. Water
cooling (quenching (DQ)) was applied to some steel plates after
rolling. With respect to other steel plates, air cooling was
performed after rolling, and water cooling (quenching (RQ)) was
performed after reheating.
[0066] On the obtained steel plates, the surface hardness
measurement, the evaluation of abrasion resistant property, the
base material toughness measurement, a T shape fillet weld cracking
test (evaluation of delayed fracture resistant property), a
synthetic heat-affected zone test and a toughness test of a weld of
an actual weld joint were carried out in accordance with following
manners. The acquired result is shown in Table 3.
[Surface Hardness 1]
[0067] The surface hardness measurement was carried out on each
steel plate in accordance with the stipulation of JIS Z 2243 (1998)
for measuring surface hardness below a surface layer (hardness of a
surface measured after removing scales on the surface layer). In
the measurement, tungsten hard balls having a diameter of 10 mm
were used, and a load was set 3000 kgf.
[Base-Material Toughness 1]
[0068] A V notch test specimen was sampled from each steel plate in
the direction perpendicular to the rolling direction at a position
away from a surface of the steel plate by 1/4 of a plate thickness
in accordance with the stipulation of JIS Z 2202 (1998), and a
Charpy impact test was carried out at three respective temperatures
with respect to each steel plate in accordance with the stipulation
of JIS Z 2242 (1998), absorbed energies at a test temperature of
0.degree. C. and -40.degree. C. were obtained, and base-material
toughness is evaluated. The test temperature of 0.degree. C. was
selected by taking the use of the steel plate in a warm area into
consideration.
[0069] The steel plate where an average of three absorbed energies
(also referred to as vE.sub.0) at the test temperature of 0.degree.
C. was 30 J or more was determined as the steel plate having
excellent base-material toughness (within the scope of the present
invention).
[Abrasion Resistant Property 1]
[0070] With respect to abrasion resistant property, a rubber wheel
abrasion test was carried out on each steel plate in accordance
with the stipulation of ASTM G65. The test was carried out by using
specimens each having a size of 10 mmt (t: plate
thickness).times.75 mmw (w: width).times.20 mL (L: length) (t
(plate thickness).times.75 mmw.times.20 mL when the plate thickness
is less than 10 mmt), and by using abrasive sand made of 100%
SiO.sub.2 as an abrasive material.
[0071] A weight of the specimen was measured before and after the
test, and wear of the specimen was measured. The test result was
evaluated based on an abrasion resistance rate: (wear of soft steel
plate)/(wear of each steel plate) using the wear of soft steel
plate (SS400) as the reference (1.0). This means that the larger
the abrasion resistance rate, the more excellent the abrasion
resistant property becomes, and with respect to the scope of the
present invention, the steel plate which exhibited the abrasion
resistance rate of 4.0 or more was determined excellent.
[Delayed Fracture 1]
[0072] In a T shape fillet weld cracking test, restriction welding
was carried out on specimens each of which was assembled in a T
shape as shown in FIG. 1 by shielded metal arc welding and,
thereafter, test welding was carried out at a room temperature
(25.degree. C..times.humidity 60%) or after preheating to
100.degree. C.
[0073] The welding method was shielded metal arc welding (welding
material: LB52UL (4.0 mm.PHI.), wherein a heat input was 17 kJ/cm,
and welding of 3 layers and 6 passes was carried out. After the
test, the specimen was left at a room temperature for 48 hours and,
thereafter, 5 pieces of weld cross-sectional observation samples
(bead length 200 mm being equally divided by 5) were sampled from
the test plate, and the presence or non-presence of occurrence of
cracks in a welded heat affected zone was investigated by a
projector and an optical microscope. In both the specimens prepared
without preheating and the specimens prepared with preheating at a
temperature of 100.degree. C., in 5 respective sampled
cross-sectional samples, the samples where the occurrence of cracks
in the welded heat affected zone was not found at all were
evaluated as being excellent in delayed fracture resistance.
[Weld Toughness 1-1]
[0074] In a synthetic heat-affected zone test, the low-temperature
tempering of the bond area when two pass CO.sub.2 gas shielded arc
welding with a welding heat input of 17 kJ/cm is performed was
simulated. A heat cycle is applied to the bond area in such a
manner that the bond area in first pass welding (initial pass) was
held at a temperature of 1400.degree. C. for 1 second and was
cooled at a cooling rate of 30.degree. C./s from 800 to 200.degree.
C., and next, as the low-temperature tempering by second pass
welding (succeeding welding), the bond area was held at a
temperature of 300.degree. C. for 1 second and was cooled at a
cooling rate of 5.degree. C./s from 300 to 100.degree. C.
[0075] A square bar test specimen sampled in the rolling direction
was subjected to the above-mentioned heat cycle by a high-frequency
induction heating device and, thereafter, a V notch Charpy impact
test was carried out in accordance with the stipulation of JIS Z
2242 (1998). The V notch Charpy impact test was carried out with
respect to three specimens for each steel plate while setting a
test temperature at 0.degree. C.
[0076] The steel plate where an average value of three absorbed
energies (vE.sub.0) was 30 J or more was determined as the steel
plate having excellent HAZ toughness (within the scope of the
present invention).
[0077] With respect to the steel plates having a plate thickness of
less than 10 mm, V notch Charpy specimens having a sub size (5
mm.times.10 mm) were sampled and were subjected to a Charpy impact
test. The steel plate where an average value of three absorbed
energies (vE.sub.0) was 15 J or more was determined as the steel
plate having excellent HAZ toughness (within the scope of the
present invention).
[Weld Toughness 1-2]
[0078] Further, to confirm toughness of an actual weld joint, a
multi-pass weld joint (V-shape groove) was prepared by shielded
metal arc welding (heat input: 17 kJ/cm, preheating: 150.degree.
C., interpass temperature: 150.degree. C., welding material: LB52UL
(4.0 mm.PHI.)).
[0079] A Charpy impact specimen was sampled from the weld joint at
a position 1 mm below a surface of the weld joint, and a notch
location was the V-shape groove and is set at a bond on a groove
side perpendicular to the surface of the steel plate. A V notch
Charpy impact test was carried out in accordance with the
stipulation of JIS Z 2242 (1998) using the specimens sampled in
this manner. FIG. 2 shows a sampling position of the Charpy impact
specimen and the notch location.
[0080] The V notch Charpy impact test of the actual weld joint was
carried out using three specimens while setting the test
temperature at 0.degree. C. The steel plate where an average value
of three absorbed energies (vE.sub.0) is 30 J or more was
determined as the steel plate having excellent toughness at a multi
pass weld (within the scope of the present invention).
[0081] With respect to the steel plates having a plate thickness of
less than 10 mm, V notch Charpy specimens having a sub size (5
mm.times.10 mm) were sampled and were subjected to a Charpy impact
test. The steel plate where an average value of three absorbed
energies (vE.sub.0) was 15 J or more was determined as the steel
plate having excellent bond area toughness (within the scope of the
present invention).
[0082] Table 2 shows manufacturing conditions of steel plates used
in the test, and Table 3 shows the results of the above-mentioned
respective tests. The present invention examples (steels No. 1 to
5) had the surface hardness of 400 HBW10/3000 or more, exhibited
excellent abrasion resistant property, and had base-material
toughness of 30 J or more at 0.degree. C. Further, no cracks
occurred in the T shape fillet weld cracking test, and the present
invention examples had excellent toughness also with respect to the
synthetic heat-affected zone test and the actual weld joint
toughness and hence, it was confirmed that the present invention
examples exhibited excellent weld toughness.
[0083] On the other hand, with respect to comparison examples
(steels No. 6 to 19) whose compositions were outside the scope of
the present invention, it was confirmed that the comparison
examples could not satisfy targeted performances with respect to
any one or a plurality of properties and tests among surface
hardness, abrasion resistant property, the T shape fillet weld
cracking test, base-material toughness, the reproduced heat cycle
Charpy impact test, the Charpy impact test of the actual weld
joint.
EMBODIMENT 2
[0084] Steel slabs which were prepared with various compositions
shown in Table 4 by way of a steel converter, ladle refining and a
continuous casting method were heated at a temperature of 1000 to
1250.degree. C. and, thereafter, the steel slabs were subjected to
hot rolling under manufacturing conditions shown in Table 5. Water
cooling (quenching (DQ)) is applied to some steel plates
immediately after rolling. With respect to other steel plates, air
cooling was applied to other steel plates after rolling, and water
cooling (quenching (RQ)) was performed after reheating.
[0085] On the obtained steel plates, the surface hardness
measurement, the evaluation of abrasion resistant property, the
base material toughness measurement, a T shape fillet weld cracking
test (evaluation of delayed fracture resistant property), a
synthetic heat-affected zone test and a toughness test of a weld of
an actual weld joint were carried out in accordance with following
manners. The acquired result is shown in Table 6.
[Surface Hardness 2]
[0086] The surface hardness measurement was carried out in
accordance with the stipulation of JIS Z 2243 (1998) thus measuring
surface hardness below a surface layer (hardness of a surface
measured after removing scales on the surface layer). In the
measurement, tungsten hard balls having a diameter of 10 mm were
used, and a load was set to 3000 kgf.
[Base-Material Toughness 2]
[0087] A V notch test specimen was sampled from each steel plate in
the direction perpendicular to the rolling direction at a position
away from a surface of the steel plate by 1/4 of a plate thickness
in accordance with the stipulation of JIS Z 2202 (1998), and a
Charpy impact test was carried out at three respective temperatures
with respect to each steel plate in accordance with the stipulation
of JIS Z 2242 (1998), and absorbed energy at test temperatures of
0.degree. C. and -40.degree. C. were obtained, and base-material
toughness was evaluated. The test temperature of 0.degree. C. was
selected by taking the use of the steel plate in a warm region into
consideration, and the test temperature of -40.degree. C. was
selected by taking the use of the steel plate in a cold region into
consideration.
[0088] The steel plate where an average value of three absorbed
energies (also referred to as vE.sub.0) at the test temperature of
0.degree. C. was 30 J or more and an average value of three
absorbed energies (also referred to as vE.sub.-40) at the test
temperature of -40.degree. C. was 27 J or more was determined as
the steel plate having excellent base-material toughness (within
the scope of the present invention). With respect to the steel
plates having a plate thickness of less than 10 mm, V notch Charpy
specimens having a sub size (5 mm.times.10 mm) were sampled and
were subjected to a Charpy impact test. The steel plate where an
average value of three absorbed energies (vE.sub.0) was 15 J or
more and an average value of three absorbed energies (vE.sub.-40)
was 13 J or more was determined as the steel plate having excellent
base-material toughness (within the scope of the present
invention).
[Abrasion Resistant Property 2]
[0089] With respect to abrasion resistant property, a rubber wheel
abrasion test was carried out in accordance with the stipulation of
ASTM G65. The test was carried out by using a specimen having a
size of 10 mmt (t: plate thickness).times.75 mmw (w:
width).times.20 mL (L: length) (t (plate thickness).times.75
mmw.times.20 mmL when the plate thickness was less than 10 mmt),
and by using abrasive sand made of 100% SiO.sub.2 as an abrasive
material.
[0090] A weight of the specimen was measured before and after the
test and wear of the specimen was measured. The test result was
evaluated based on an abrasion resistance rate: (wear of soft steel
plate)/(wear of each steel plate) using wear of soft steel plate
(SS400) as the reference (1.0). This means that the larger the
abrasion resistance rate, the more excellent the abrasion resistant
property becomes, and with respect to the scope of the present
invention, the steel plate which exhibits the abrasion resistance
rate of 4.0 or more was determined excellent.
[Delayed Fracture 2]
[0091] In a T shape fillet weld cracking test, restriction welding
was carried out on a specimen which was assembled in a T shape as
shown in FIG. 1 by shielded metal arc welding and, thereafter, test
welding was carried out at a room temperature (25.degree.
C..times.humidity 60%) or after preheating to 100.degree. C.
[0092] The welding method was shielded metal arc welding (welding
material: LB52UL (4.0 mm.PHI.)), wherein a welding heat input was
17 kJ/cm, and welding of 3 layers and 6 passes was carried out.
After the test, the specimen was left at a room temperature for 48
hours and, thereafter, 5 pieces of weld cross-sectional observation
samples (bead length 200 mm being equally divided by 5) were
sampled from a test plate, and the presence or non-presence of
occurrence of cracks in a welded heat affected zone was
investigated by a projector and an optical microscope. In both the
specimens prepared without preheating and the specimens prepared
with preheating at a temperature of 100.degree. C., among 5
respective sampled cross-sectional samples, the samples where the
occurrence of cracks in the welded heat affected zone was not found
at all were evaluated as being excellent in delayed fracture
resistance.
[Weld Toughness 2-1]
[0093] In a synthetic heat-affected zone test, the low-temperature
tempering of the bond area of the welded heat affected zone when
two layer CO.sub.2 gas shielded arc welding with a welding heat
input of 17 kJ/cm is performed was simulated. A heat cycle is
applied to the bond area in such a manner that the bond portion in
first pass welding (initial pass) was held at a temperature of
1400.degree. C. for 1 second and was cooled at a cooling rate of
30.degree. C./s from 800 to 200.degree. C., and next, as the
low-temperature tempering by second pass welding (succeeding
welding), the bond area was held at a temperature of 300.degree. C.
for 1 second and was cooled at a cooling rate of 5.degree. C./s
from 300 to 100.degree. C.
[0094] A square bar test specimen sampled in the rolling direction
was subjected to the above-mentioned heat cycle by a high-frequency
induction heating device and, thereafter, a V notch Charpy impact
test was carried out in accordance with the stipulation of JIS Z
2242 (1998). The Charpy impact test was carried out with respect to
three specimens for each steel plate while setting test
temperatures at 0.degree. C. and -40.degree. C. at respective
temperatures.
[0095] The steel plate where an average value of three absorbed
energies (vE.sub.0) was 30 J or more and an average value of three
absorbed energies (vE.sub.-40) was 27 J or more was determined as
the steel plate having excellent HAZ toughness (within the scope of
the present invention).
[0096] With respect to the steel plates having a plate thickness of
less than 10 mm, V notch Charpy specimens having a sub size (5
mm.times.10 mm) were sampled and were subjected to a Charpy impact
test. The steel plate where an average value of three absorbed
energies (vE.sub.0) was 15 J or more and an average value of three
absorbed energies (vE.sub.-40) was 13 J or more was determined as
the steel plate having excellent HAZ toughness (within the scope of
the present invention).
[Weld Toughness 2-2]
[0097] Further, to confirm toughness of an actual weld joint, a
multi-pass weld joint (V-shape groove) was prepared by shielded
metal arc welding (heat input: 17 kJ/cm, preheating: 150.degree.
C., interpass temperature: 150.degree. C., welding material:
LB52UL, (4.0 mm.PHI.)).
[0098] A Charpy impact specimen was sampled from the weld joint at
a position 1 mm below a surface of the weld joint, and a notch
location was the V-shape groove and is set at a bond on a groove
side perpendicular to the surface of the steel plate. A V notch
Charpy impact test was carried out in accordance with the
stipulation of JIS Z 2242 (1998) using the specimens sampled in
this manner. FIG. 2 shows a sampling position of the Charpy impact
specimen and the notch location.
[0099] The V notch Charpy impact test of the actual weld joint was
carried out using three specimens for each test temperature while
setting the test temperature at 0.degree. C. and -40.degree. C. The
steel plate where an average value of three absorbed energies
(vE.sub.0) is 30 J or more and an average value of three absorbed
energies (vE.sub.-40) is 27 J or more was determined as the steel
plate having excellent multi pass weld toughness (within the scope
of the present invention).
[0100] With respect to the steel plates having a plate thickness of
less than 10 mm, V notch Charpy specimens having a sub size (5
mm.times.10 mm) were sampled and were subjected to a Charpy impact
test. The steel plate where an average value of three absorbed
energies (vE.sub.0) was 15 J or more and an average value of three
absorbed energies (vE.sub.-40) was 13 J or more was determined as
the steel plate having excellent multi pass weld toughness (within
the scope of the present invention).
[0101] Table 5 shows manufacturing conditions of steel plates used
in the test, and Table 6 shows the results of the above-mentioned
respective tests. The present invention examples (steels No. 20 to
22 (steel No. 22 having a plate thickness of 8 mm)) had the surface
hardness of 400 HBW10/3000 or more, exhibited excellent abrasion
resistant property, and had base-material toughness of 30 J or more
at 0.degree. C. and base-material toughness of 27 J or more at
-40.degree. C. Further, no cracks occurred in the T shape fillet
weld cracking test, and the present invention examples also had
excellent toughness with respect to the synthetic heat-affected
zone test and the actual weld joint and hence, it was confirmed
that the present invention examples exhibited excellent multi pass
weld toughness.
[0102] On the other hand, it was confirmed that although the steel
No. 23 where the composition falls within the scope of the present
invention but DI* exceeds 180 exhibited favorable results in
surface hardness, abrasion resistant property and base-material
toughness, the results of a T shape fillet weld cracking test, a
synthetic heat-affected zone test and an actual weld joint
toughness were close to lower limit values of targeted performances
and hence, the steel No. 23 was inferior to other present invention
examples. With respect to the composition of the steel No. 24, the
content of Si falls outside the scope of the present invention.
Accordingly, although steel No. 24 exhibited favorable results in
surface hardness, abrasion resistant property and base-material
toughness, steel No. 24 could not satisfy targeted performances in
a T shape fillet weld cracking test, a synthetic heat-affected zone
test and an actual weld joint toughness.
[0103] Although the composition of the steel No. 25 fell within the
scope of the present invention, the value of a parameter
C+Mn/4-Cr/3+10? on a left side of the formula (2) exceeded 0.47.
Accordingly, the result of the synthetic heat-affected zone test
and the actual weld joint toughness were close to lower limits of
the targeted performances and hence, steel No. 25 was inferior to
other present invention examples. In the description of Tables 4, 5
and 6, although the steel No. 23 falls within the scope of the
present invention called for in claim 3 in composition, the value
of DI* falls outside the scope of the present invention called for
in claim 6 and hence, the steel No. 23 is set as the comparison
example. Although the steel No. 25 falls within the scope of the
present invention called for in claim 1 in composition, the steel
No. 25 does not satisfy the formula (2) and falls outside the scope
of the present invention called for in claim 7 and hence, the steel
No. 25 is set as the comparison example.
TABLE-US-00001 TABLE 1 Composition (mass %) No. C Si Mn P S Al Cr
Mo Nb Ti W Cu Ni 1 0.241 0.29 0.75 0.007 0.0014 0.031 0.48 0.11
0.016 0.014 2 0.215 0.12 0.46 0.008 0.0012 0.023 0.67 0.35 0.023
0.025 0.08 3 0.279 0.10 0.62 0.004 0.0010 0.037 1.01 0.09 0.021
0.009 0.13 0.15 0.12 4 0.221 0.47 1.05 0.009 0.0015 0.047 0.52 0.18
0.007 0.018 5 0.251 0.16 0.41 0.005 0.0009 0.026 0.43 0.42 0.013
0.011 0.12 6 0.178 0.25 0.94 0.007 0.0019 0.031 0.59 0.18 0.020
0.019 7 0.329 0.38 0.53 0.006 0.0012 0.026 0.47 0.21 0.018 0.015
0.22 0.20 8 0.260 0.15 1.47 0.005 0.0010 0.039 0.44 0.08 0.012
0.012 9 0.273 0.29 0.84 0.015 0.0021 0.025 0.53 0.17 0.020 0.015
0.07 0.11 10 0.217 0.45 0.79 0.008 0.0012 0.028 0.18 0.31 0.018
0.008 11 0.238 0.27 0.65 0.006 0.0009 0.044 0.95 0.03 0.021 0.020
0.32 12 0.254 0.38 0.57 0.007 0.0011 0.031 0.71 0.28 0.001 0.015 13
0.228 0.24 0.64 0.008 0.0012 0.037 1.01 0.11 0.039 0.021 0.41 14
0.264 0.15 0.59 0.006 0.0010 0.030 0.77 0.34 0.021 0.001 15 0.271
0.23 1.04 0.008 0.0009 0.027 0.43 0.22 0.011 0.033 16 0.229 0.17
0.61 0.004 0.0010 0.033 0.82 0.30 0.018 0.017 17 0.245 0.23 0.80
0.007 0.0011 0.025 0.63 0.12 0.012 0.015 0.10 18 0.230 0.38 0.96
0.005 0.0019 0.047 0.55 0.21 0.009 0.021 19 0.286 0.12 0.45 0.009
0.0012 0.045 0.47 0.10 0.017 0.011 Composition (mass %) Formula No.
V N B REM Ca Mg DI* (2) Remarks 1 34 8 59.9 0.34 Present invention
example 2 56 5 21 76.5 0.19 Present invention example 3 24 17 16
98.3 0.14 Present invention example 4 0.04 44 10 53 105.2 0.40
Present invention example 5 0.05 54 9 78.9 0.26 Present invention
example 6 45 10 18 76.8 0.29 Comparison example 7 27 15 81.5 0.36
Comparison example 8 0.04 38 8 50 92.0 0.53 Comparison example 9 40
10 27 95.1 0.46 Comparison example 10 0.05 23 12 69.4 0.43
Comparison example 11 56 9 22 72.9 0.14 Comparison example 12 0.04
40 10 98.7 0.23 Comparison example 13 57 16 90.8 0.13 Comparison
example 14 0.04 37 9 39 103.7 0.21 Comparison example 15 31 8 92.5
0.47 Comparison example 16 11 12 91.5 0.15 Comparison example 17
0.03 68 11 20 87.6 0.31 Comparison example 18 31 1 97.3 0.34
Comparison example 19 53 8 19 40.6 0.33 Comparison example Note 1:
Underlined values being outside the scope of the present invention
Note 2: Contents of N, B, REM, Ca, Mg indicated by ppm in chemical
components Note 3: DI* = 33.85 .times. (0.1 .times. C).sup.0.5
.times. (0.7 .times. Si + 1) .times. (3.33 .times. Mn + 1) .times.
(0.35 .times. Cu + 1) .times. (0.36 .times. Ni + 1) .times. (2.16
.times. Cr + 1) .times. (3 .times. Mo + 1) .times. (1.75 .times. V
+ 1) .times. (1.5 .times. W + 1) Note 4: Formula (2) = C + Mn/4 -
Cr/3 - Mo/6 + 10P Respective element symbols being contents (mass
%)
TABLE-US-00002 TABLE 2 Hot rolling Raw Hot rolling Heat treatment
material Plate Heating finish Heating Steel thickness thickness
temperature temperature Cooling temperature Cooling No. (mm) (mm)
(.degree. C.) (.degree. C.) method (.degree. C.) method Remarks 1
200 12 1150 890 air cooling 900 water cooling Present invention
example 2 200 25 1080 900 air cooling 930 water cooling Present
invention example 3 200 32 1150 910 air cooling 900 water cooling
Present invention example 4 200 25 1200 880 water cooling no heat
treatment Present invention example 5 200 16 1100 900 water cooling
200 air cooling Present invention example 6 200 25 1150 900 air
cooling 900 water cooling Comparison example 7 200 20 1150 890
water cooling no heat treatment Comparison example 8 250 32 1200
930 air cooling 910 water cooling Comparison example 9 200 25 1100
900 air cooling 900 water cooling Comparison example 10 300 20 1150
920 water cooling no heat treatment Comparison example 11 250 16
1150 880 air cooling 900 water cooling Comparison example 12 300 32
1050 870 air cooling 900 water cooling Comparison example 13 200 16
1200 900 water cooling no heat treatment Comparison example 14 250
25 1150 860 air cooling 930 water cooling Comparison example 15 250
25 1150 890 air cooling 900 water cooling Comparison example 16 300
30 1050 870 air cooling 930 water cooling Comparison example 17 250
20 1150 900 water cooling no heat treatment Comparison example 18
300 32 1100 900 air cooling 900 water cooling Comparison example 19
250 25 1150 900 air cooling 900 water cooling Comparison example
Note 1: Underlined values being outside the scope of the present
invention
TABLE-US-00003 TABLE 3 Synthetic heat-affected T shape fillet weld
zone test Abrasion cracking test Corresponding Shielded resistant
Base Preheating to to multi pass metal arc Surface property
material No preheating 100.degree. C. weld (bonding + welding
hardness Abrasion toughness (Presence or (Presence or
low-temperature Toughness of Steel HBW resistance vE0 non-presence
non-presence tempering) multi pass No. 10/3000 rate (J) of cracks)
of cracks) vE0(J) weld vE0(J) Remarks 1 462 4.8 77 no cracks no
cracks 57 92 Present invention example 2 410 4.3 102 no cracks no
cracks 71 121 Present invention example 3 526 5.5 44 no cracks no
cracks 38 60 Present invention example 4 428 4.2 90 no cracks no
cracks 71 112 Present invention example 5 470 4.9 60 no cracks no
cracks 44 73 Present invention example 6 342 3.6 153 no cracks no
cracks 112 159 Comparison example 7 609 6.5 18 cracks cracks 5 22
Comparison example occurred occurred 8 493 5.2 44 cracks cracks 16
34 Comparison example occurred occurred 9 531 5.5 35 cracks cracks
13 29 Comparison example occurred occurred 10 427 4.3 70 no cracks
no cracks 19 39 Comparison example 11 441 4.6 59 no cracks no
cracks 17 35 Comparison example 12 457 4.9 23 cracks cracks 15 31
Comparison example occurred occurred 13 448 4.5 27 no cracks no
cracks 13 27 Comparison example 14 505 5.3 18 no cracks no cracks 8
18 Comparison example 15 506 5.3 10 cracks cracks 5 15 Comparison
example occurred occurred 16 433 4.6 40 no cracks no cracks 14 29
Comparison example 17 469 4.9 11 cracks cracks 6 20 Comparison
example occurred occurred 18 451 4.7 50 cracks cracks 19 35
Comparison example occurred occurred 19 372 3.7 47 no cracks no
cracks 33 61 Comparison example Note 1: Underlined values being
outside the scope of the present invention
TABLE-US-00004 TABLE 4 Composition (mass %) No. C Si Mn P S Al Cr
Mo Nb Ti W Cu Ni 20 0.214 0.35 0.59 0.007 0.0018 0.031 1.05 0.17
0.019 0.014 21 0.227 0.12 0.70 0.005 0.0015 0.020 0.96 0.48 0.023
0.017 0.25 0.24 22 0.251 0.22 0.62 0.006 0.0011 0.035 0.68 0.15
0.017 0.012 23 0.265 0.39 1.02 0.009 0.0021 0.022 0.71 0.21 0.015
0.011 0.52 0.42 24 0.282 0.01 0.89 0.008 0.0025 0.035 0.49 0.14
0.024 0.018 0.12 25 0.278 0.35 1.12 0.009 0.0021 0.031 0.45 0.12
0.020 0.015 Composition (mass %) Formula No. V N B REM Ca Mg DI*
(2) Remarks 20 0.04 41 10 96.5 0.05 Present invention example 21
0.04 30 12 174.6 0.05 Present invention example 22 29 7 67.9 0.21
Present invention example 23 0.05 27 11 188.5 0.34 Comparison
example 24 37 14 40 78.3 0.40 Comparison example 25 40 10 89.1 0.48
Comparison example Note 1: Underlined values being outside the
scope of the present invention Note 2: Contents of N, B, REM, Ca,
Mg indicated by mass ppm in chemical components Note 3: DI* = 33.85
.times. (0.1 .times. C).sup.0.5 .times. (0.7 .times. Si + 1)
.times. (3.33 .times. Mn + 1) .times. (0.35 .times. Cu + 1) .times.
(0.36 .times. Ni + 1) .times. (2.16 .times. Cr + 1) .times. (3
.times. Mo + 1) .times. (1.75 .times. V + 1) .times. (1.5 .times. W
+ 1) Note 4: P in formula (2): C + Mn/4 - Cr/3 - Mo/6 + 10P
Respective element symbols being contents (mass %)
TABLE-US-00005 TABLE 5 Hot rolling Raw Hot rolling Heat treatment
material Plate Heating finish Heating Steel thickness thickness
temperature temperature Cooling temperature Cooling No. (mm) (mm)
(.degree. C.) (.degree. C.) method (.degree. C.) method Remarks 20
300 45 1150 900 air cooling 900 water cooling Present invention
example 21 300 60 1100 880 air cooling 870 water cooling Present
invention example 22 200 8 1150 820 air cooling 900 water cooling
Present invention example 23 250 32 1100 870 air cooling 900 water
cooling Comparison example 24 250 25 1100 900 water cooling no heat
treatment comparison example 25 300 40 1150 900 air cooling 900
water cooling comparison example Note 1: Underlined values being
outside the scope of the present invention
TABLE-US-00006 TABLE 6 Synthetic heat-affected T shape fillet weld
zone test Shielded Abrasion cracking test Corresponding metal arc
resistant Preheating to multi pass welding Surface property Base
material No preheating to 100.degree. C. weld (bonding + Toughness
hardness Abrasion toughness presence or presence or low-temperature
of multi Steel HBW resistance vE0 vE-40 non-presence non-presence
tempering) pass weld No. 10/3000 rate (J) (J) of cracks of cracks
vE0(J) vE-40(J) vE0(J) vE-40(J) Remarks 20 412 4.3 96 74 no cracks
no cracks 69 40 110 72 Present invention example 21 436 4.5 71 50
no cracks no cracks 59 40 83 50 Present invention example 22 473
5.0 40 32 no cracks no cracks 35 28 50 31 Present invention example
23 499 5.3 32 32 no cracks no cracks 30 27 40 27 Comparison example
24 530 5.5 32 30 cracks cracks 20 8 50 15 Comparison example
occurred occurred 25 524 5.4 33 32 no cracks no cracks 31 27 46 27
Comparison example Note 1: Underlined values being outside the
scope of the present invention
* * * * *