U.S. patent application number 13/806954 was filed with the patent office on 2013-08-22 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 | 20130216422 13/806954 |
Document ID | / |
Family ID | 45402264 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130216422 |
Kind Code |
A1 |
Ueda; Keiji ; et
al. |
August 22, 2013 |
ABRASION RESISTANT STEEL PLATE WHICH EXHIBITS EXCELLENT WELD
TOUGHNESS AND EXCELLENT DELAYED FRACTURE RESISTANCE
Abstract
Provided is an abrasion-resistant steel plate or sheet which
exhibits excellent weld toughness and excellent delayed fracture
resistance and is thus suitable for construction machines,
industrial machines, and so on. Specifically provided is a steel
plate or sheet which contains, in mass %, 0.20 to 0.30% of C, 0.05
to 1.0% of Si, 0.40 to 1.2% of Mn, 0.010% or less of P, 0.005% or
less of S, 0.40 to 1.5% of Cr, 0.005 to 0.025% of Nb, 0.005 to
0.03% of Ti, 0.1% or less of Al, 0.01% or less of N, and, as
necessary, one or more of Mo, W, B, Cu, Ni, V, REM, Ca and Mg, and
has a DI* of 45 to 180 while satisfying the relationship:
C+Mn/4-Cr/3+10P.ltoreq.0.47, and which has a microstructure that
comprises martensite as the matrix phase.
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)
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: |
45402264 |
Appl. No.: |
13/806954 |
Filed: |
June 29, 2011 |
PCT Filed: |
June 29, 2011 |
PCT NO: |
PCT/JP2011/065416 |
371 Date: |
February 21, 2013 |
Current U.S.
Class: |
420/83 ; 420/104;
420/106; 420/110; 420/84 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/02 20130101; C22C 38/32 20130101; C22C 38/46 20130101; C22C
38/50 20130101; C22C 38/42 20130101; C22C 38/002 20130101; C22C
38/001 20130101; C22C 38/005 20130101; C22C 38/12 20130101; C22C
38/06 20130101; C22C 38/00 20130101; C22C 38/22 20130101; C22C
38/28 20130101; C22C 38/26 20130101; C22C 38/24 20130101; C22C
38/48 20130101; C22C 38/54 20130101; C21D 9/46 20130101; C21D
2211/008 20130101; C22C 38/20 20130101; C22C 38/44 20130101; C21D
8/02 20130101 |
Class at
Publication: |
420/83 ; 420/84;
420/106; 420/104; 420/110 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C22C 38/48 20060101 C22C038/48; C22C 38/42 20060101
C22C038/42; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/24 20060101
C22C038/24; C22C 38/22 20060101 C22C038/22; C22C 38/12 20060101
C22C038/12; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/50 20060101 C22C038/50 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2010 |
JP |
2010-149649 |
Jun 28, 2011 |
JP |
2011-142506 |
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, 0.005 to 0.025% Nb, 0.005 to 0.03% Ti, 0.1% or less Al, 0.01%
or less N, 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 %
one, two or more kinds of components selected from a group
consisting of 0.05 to 1.0% Mo, 0.05 to 1.0% W, and 0.0003% to
0.0030% B.
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 0.008% 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 0.008% 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 0.008% 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 weld toughness and excellent
delayed fracture resistance.
BACKGROUND ART
[0002] When a hot-rolled steel plate is employed for making 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
obtaining 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 obtaining 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, soil 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 has been assembled, the labor for
the manufacture and a manufacturing cost are increased.
Accordingly, there has been a demand for an abrasion resistant
steel plate which can be used as a strength member of the welded
steel structure.
[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 kind 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] The most serious problem relating to the lowering of
toughness when a steel material is welded is the deterioration of
toughness at a bond area of a fusion line. In abrasion resistant
steel having martensite structure in a quenched state, the
deterioration of toughness which is referred to as low-temperature
tempering embrittlement arises as a problem also in a welded heat
affected zone (hereinafter also referred to as HAZ) reheated to a
temperature around 300.degree. C. which is away from the fusion
line. 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.
[0016] In a region which is reheated at a low-temperature tempering
embrittlement temperature, hydrogen which invades a weld from a
shielding gas at the time of welding and a residual stress
generated by welding heat overlap with each other so that delayed
fracture (cracks which occur in the weld are referred to as
low-temperature cracks in general) is liable to occur and,
particularly, delayed fracture is liable to occur in an abrasion
resistant steel having high strength.
[0017] Accordingly, in applying an abrasion resistant steel plate
to a strength member of a welded structure, it is necessary to
enhance toughness of the bond area and the welded heat affected
zone reheated to a temperature around 300.degree. C. which is away
from a fusion line. However, in the conventional abrasion resistant
steel plate, cold cracking susceptibility of the weld is high and
hence, to prevent cold cracks, it is necessary to discharge
hydrogen in the steel plate and to lower a residual stress in the
steel plate by performing treatments such as preheating and post
heating before and after welding.
[0018] Patent documents 1 and 2 fail to describe the enhancement of
weld toughness in the 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.
[0019] Accordingly, it is an object of the present invention to
provide an abrasion resistant steel plate which exhibits excellent
weld toughness and excellent delayed fracture resistance without
inducing lowering of productivity and the increase in a
manufacturing cost. In the present invention, weld toughness means
toughness of a welded heat affected zone, and the excellent weld
toughness means particularly that the toughness is excellent in a
bond area and a low-temperature tempering embrittlement temperature
area.
Means for Solving the Problem
[0020] To achieve the above-mentioned object, inventors of the
present invention have made extensive studies on various factors
which determine chemical components of a steel plate, a method of
manufacturing the steel plate and the microstructure of the steel
plate so as to secure weld toughness and delayed fracture
resistance with respect to an abrasion resistant steel plate, and
have made following findings.
[0021] 1. To secure excellent abrasion resistant property, it is
indispensable to form the base microstructure or the main
microstructure 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.
[0022] 2. To achieve the excellent weld toughness, it is necessary
to suppress grain particles in the bond area 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.
[0023] 3. 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, 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, 0.005 to 0.025% Nb, 0.005 to 0.03% Ti, 0.1% or less Al, 0.01%
or less N, 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 % one, two
or more kinds of components selected from a group consisting of
0.05 to 1.0% Mo, 0.05 to 1.0% W, and 0.0003% to 0.0030% B.
[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
any one 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 weld
toughness and excellent delayed fracture resistance. 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 and toughness of a
low-temperature tempering region are deteriorated. 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 in a tempering
embrittlement area of a welded heat affected zone. It is necessary
for the steel plate to contain 0.05% or more Si to acquire such an
effect. On the other hand, when the content of Si exceeds 1.0%,
toughness of the 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 welded heat
affected zone. 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 in the
tempering embrittlement area of the welded heat affected zone. 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%.
Nb: 0.005 to 0.025%
[0043] Nb is an important element having both an effect of
improving toughness of the welded heat affected zone and an effect
of suppressing the occurrence of delayed fracture by making the
microstructure of the base material and the welded heat affected
zone finer by causing the precipitation of carbonitride and also by
fixing solid solution N. It is necessary for the steel plate to
contain 0.0050% 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%
[0044] Ti has an effect of suppressing grains in the bond area 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
[0045] 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 a bond area from becoming coarse and an
effect of suppressing the deterioration of toughness and the
occurrence of delayed fracture in a 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: 0.01% or Less
[0046] N forms a nitride with Nb or Ti, and has an effect of
suppressing grains of welded heat affected zone from becoming
coarse. However, when the content of N exceeds 0.01%, toughness of
a base material and weld toughness is remarkably lowered and hence,
the content of N is limited to 0.01% or less. The content of N is
preferably limited to a value which falls within a range from
0.0010 to 0.0070%. A balance is Fe and unavoidable impurities.
[0047] 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 Mo, W,
B, Cu, Ni, V, REM, Ca and Mg.
Mo: 0.05 to 1.0%
[0048] Mo is an element effective for remarkably increasing
hardenability thus increasing hardness of a base material. The
content of Mo may preferably be 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. Accordingly, the content of Mo is set to 1.0% or
less.
W: 0.05 to 1.0%
[0049] W is an element effective for remarkably increasing
hardenability thus increasing hardness of a base material. The
content of W may preferably be 0.05% or more for acquiring such an
effect. However, when the content of W 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.
B: 0.0003 to 0.0030%
[0050] B is an element effective for remarkably increasing
hardenability with addition of a trace amount of B thus increasing
hardness of a base material. The content of B may preferably be
0.0003% or more for acquiring such an effect. However, when the
content of B exceeds 0.0030%, B adversely influences toughness,
ductility and weld crack resistance of the base material.
Accordingly, the content of B is set to 0.0030% 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 DI* 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 DI* 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 a main phase 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] 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, 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 .mu.m, 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.
[0064] 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.
[0065] 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 obtaining 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
[0066] 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.
[0067] 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]
[0068] 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 to 3000 kgf.
[Base-Material Toughness 1]
[0069] 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 a test temperature of
0.degree. C. was 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.
[0070] 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]
[0071] 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 sands made of 100%
SiO.sub.2 as an abrasive material.
[0072] 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]
[0073] 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.
[0074] 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
welding, 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]
[0075] In a synthetic heat-affected zone test, a bond area and a
low-temperature tempering embrittlement area when one pass CO.sub.2
gas shielded arc welding with a welding heat input of 17 kJ/cm is
performed were simulated. In the simulation of the bond area, the
bond area was held at 1400.degree. C. for 1 second and was cooled
at a cooling rate of 30.degree. C./s from 800 to 200.degree. C. On
the other hand, in the simulation of the low-temperature tempering
embrittlement area, the low-temperature tempering embrittlement
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.
[0076] 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.
[0077] The steel plate where an average value of three absorbed
energies (vE.sub.0) in the bond area and the low-temperature
tempering embrittlement area was 30 J or more was determined as the
steel plate having excellent weld toughness (within the scope of
the present invention).
[Weld Toughness 1-2]
[0078] Further, to confirm toughness of an actual weld joint, bead
on plate welding was applied to a steel plate by shielded metal arc
welding (heat input: 17 kJ/cm, preheating: 150.degree. C., welding
material: LB52UL (4.0 mm.PHI.)). A Charpy impact specimen was
sampled from a position 1 mm below a surface of the steel plate,
and a V notch Charpy impact test was carried out in accordance with
the stipulation of JIS Z 2242 (1998) using a notch location as the
bond area. FIG. 2 shows a sampling position of the Charpy impact
specimen and the notch location.
[0079] 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. 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 bond area
toughness (within the scope of the present invention).
[0080] 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 and hence, it
was confirmed that the present invention examples exhibited
excellent weld toughness.
[0081] On the other hand, with respect to comparison examples
(steels No. 6 to 14) 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
[0082] 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.
[0083] 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]
[0084] 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]
[0085] 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.
[0086] 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]
[0087] 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 mL when the plate thickness was less than 10 mmt), and
by using abrasive sand made of 100% SiO.sub.2 as an abrasive
material.
[0088] 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]
[0089] 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.
[0090] 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 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]
[0091] In a synthetic heat-affected zone test, a bond area and a
low-temperature tempering embrittlement area when one pass CO.sub.2
gas shielded arc welding with a welding heat input of 17 kJ/cm is
performed were simulated. In the simulation of the bond area, the
bond area was heated at 1400.degree. C. for 1 second and was cooled
at a cooling rate of 30.degree. C./s from 800 to 200.degree. C.
Further, in the simulation of the low-temperature tempering
embrittlement area, the low-temperature tempering embrittlement
area was heated at a temperature of 300.degree. C. for 1 second and
was performed at a cooling rate of 5.degree. C./s from 300 to
100.degree. C.
[0092] 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 test
temperatures at 0.degree. C. and -40.degree. C. at respective
temperatures.
[0093] The steel plate where an average value of three absorbed
energies (vE.sub.0) in the bond area and the low-temperature
tempering embrittlement area was 30 J or more and an average value
of three absorbed energies (vE.sub.-40) in the bond area and the
low-temperature tempering embrittlement area was 27 J or more was
determined as the steel plate having excellent weld toughness
(within the scope of the present invention).
[0094] 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 in the bond area and the
low-temperature tempering embrittlement area and an average value
of three absorbed energies (vE.sub.-40) was 13 J or more in the
bond area and the low-temperature tempering embrittlement area was
determined as the steel plate having excellent weld toughness
(within the scope of the present invention).
[Weld Toughness 2-2]
[0095] Further, to confirm toughness of an actual weld joint, bead
on plate welding was applied to a steel plate by shielded metal arc
welding (heat input: 17 kJ/cm, preheating: 150.degree. C., welding
material: LB52UL (4.0 mm.PHI.)). A Charpy impact specimen was
sampled from a position 1 mm below a surface of the steel plate,
and a V notch Charpy impact test was carried out in accordance with
the stipulation of JIS Z 2242 (1998) using a notch location as the
bond area. FIG. 2 shows a sampling position of the Charpy impact
specimen and the notch location.
[0096] 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 temperatures 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 bond area toughness (within the scope of the
present invention).
[0097] 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 bond area toughness (within the
scope of the present invention).
[0098] 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. 15 to
17 (steel No. 17 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 and hence, it was confirmed that the
present invention examples exhibited excellent weld toughness.
[0099] On the other hand, it was confirmed that although the steel
No. 18 where the composition falls within the scope of the present
invention but DI* exceeds 180 exhibited favorable results in
surface hardness, abrasion resistant property, base-material
toughness and a T shape fillet weld cracking test, the results of a
reproduced heat cycle Charpy impact test corresponding to the
low-temperature tempering embrittlement area and an actual weld
joint Charpy impact test were close to lower limit values of
targeted performances and hence, the steel No. 18 was inferior to
other present invention examples with respect to low-temperature
weld toughness.
[0100] The steel No. 19 fell outside the range of the present
invention with respect to Si in composition. Accordingly, although
the steel No. 19 exhibited the favorable results in surface
hardness, abrasion resistant property and base-material toughness,
toughness in the tempering embrittlement area of the welded heat
affected zone were deteriorated and hence, the steel No. 19 could
not satisfy the targeted performances with respect to a T shape
fillet weld cracking test, a synthetic heat-affected zone Charpy
impact test corresponding to the low-temperature tempering
embrittlement area and an actual weld joint Charpy impact test.
[0101] Although the steel No. 20 falls within the scope of the
present invention in composition, a value obtained by the formula
(2) exceeded 0.47. Accordingly, it was confirmed that vE.sub.-40 is
close to a lower limit of the performance of the present invention
in both a synthetic heat-affected zone Charpy impact test and an
actual weld joint Charpy impact test so that the steel No. 20 is
inferior to other present invention examples. In the description of
Tables 4, 5 and 6, although the steels No. 18 and 20 fall within
the scope of the present invention called for in claim 3 in
composition, the value of DI* and the value of the formula (2) fall
outside the scope of the present invention called for in claims 6,
7 and hence, these steels are set as comparison examples.
TABLE-US-00001 TABLE 1 Chemical components (mass %) No. C Si Mn P S
Al Cr Nb Ti Mo W Cu Ni 1 0.237 0.30 0.91 0.008 0.0015 0.032 0.58
0.016 0.014 2 0.215 0.20 0.49 0.009 0.0011 0.021 1.21 0.024 0.025
0.21 3 0.283 0.14 0.61 0.005 0.0009 0.038 0.78 0.021 0.009 0.10
0.15 0.12 4 0.223 0.41 1.14 0.007 0.0016 0.044 0.44 0.008 0.019 5
0.254 0.26 0.55 0.004 0.0008 0.028 0.49 0.012 0.011 0.10 0.05 6
0.16 0.32 1.05 0.008 0.0021 0.031 0.59 0.020 0.019 0.18 7 0.321
0.40 0.51 0.007 0.0014 0.025 0.71 0.015 0.012 0.15 0.21 0.18 8
0.263 0.19 1.43 0.007 0.0007 0.040 0.43 0.019 0.010 0.08 9 0.274
0.24 0.95 0.013 0.0022 0.030 0.71 0.020 0.011 0.06 0.21 10 0.226
0.43 0.87 0.008 0.0014 0.023 0.14 0.015 0.007 0.23 11 0.241 0.30
1.05 0.006 0.0023 0.042 0.60 0.001 0.014 0.11 12 0.230 0.27 0.69
0.005 0.0010 0.028 1.01 0.039 0.008 0.05 0.41 13 0.255 0.21 0.77
0.009 0.0014 0.031 0.47 0.018 0.001 0.14 14 0.284 0.13 0.46 0.007
0.0013 0.051 0.51 0.021 0.010 Chemical components (mass %) P in No.
V N B REM Ca Mg DI* formula (2) Remarks 1 30 57.3 0.35 Present
invention example 2 14 12 87.7 0.02 Present invention example 3 61
23 64.3 0.23 Present invention example 4 0.04 27 5 32 65.1 0.43
Present invention example 5 62 22 19 51.9 0.27 Present invention
example 6 45 10 82.5 0.31 Comparison example 7 28 20 74.3 0.28
Comparison example 8 0.05 38 36 50 93.2 0.55 Comparison example 9
35 14 80.9 0.40 Comparison example 10 59 11 20 56.8 0.48 Comparison
example 11 0.03 31 7 91.9 0.36 Comparison example 12 53 84.5 0.12
Comparison example 13 31 8 63.2 0.38 Comparison example 14 55 33.1
0.30 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: P in formula (2): left side of formula (2) = C + Mn/4
- Cr/3 + 10P Respective element symbols being contents (mass %)
TABLE-US-00002 TABLE 2 Hot rolling Heat treatment Raw material
Plate Heating Hot rolling finish Heating Steel thickness thickness
temperature temperature temperature Cooling No. (mm) (mm) ((C.)
((C.) Cooling method ((C.) method Remarks 1 200 12 1150 900 air 900
water Present cooling cooling invention example 2 200 32 1050 880
air 900 water Present cooling cooling invention example 3 200 25
1200 920 air 930 water Present cooling cooling invention example 4
200 25 1150 890 water no heat Present cooling treatment invention
example 5 200 20 1150 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 900 water cooling no heat
treatment Comparison example 8 250 32 1200 950 air cooling 900
water cooling Comparison example 9 180 20 1100 880 air cooling 930
water cooling Comparison example 10 300 25 1150 920 water cooling
no heat treatment Comparison example 11 200 32 1050 870 air cooling
900 water cooling Comparison example 12 250 16 1200 900 water
cooling no heat treatment Comparison example 13 200 12 1150 860 air
cooling 930 water cooling Comparison example 14 250 25 1150 900 air
cooling 900 water cooling Comparison example Note: Underlined
values being outside the scope of the present invention
TABLE-US-00003 TABLE 3 Abrasion T shape weld cracking test
Synthetic heat-affected zone test Shielded resistant Base
Preheating to Corresponding to metal Surface property material No
preheating 100.degree. C. low-temperature arc welding hardness
Abrasion toughness (presence or (presence or Corresponding
tempering Toughness Steel HBW resistance vE0 non-presence
non-presence to bond area embrittlement region of weld joint No.
10/3000 rate (J) of cracks) of cracks) vE0(J) vE0(J) vE0(J) Remarks
1 442 4.7 68 no cracks no cracks 60 48 119 Present invention
example 2 410 4.2 95 no cracks no cracks 83 70 151 Present
invention example 3 519 5.6 42 no cracks no cracks 39 33 77 Present
invention example 4 428 4.5 85 no cracks no cracks 76 66 128
Present invention example 5 490 5.0 57 no cracks no cracks 50 47 94
Present invention example 6 328 3.0 168 no cracks no cracks 140 155
182 Comparison example 7 598 6.0 14 cracks occurred cracks occurred
6 5 23 Comparison example 8 501 5.1 42 cracks occurred cracks
occurred 35 32 70 Comparison example 9 522 5.4 37 cracks occurred
cracks occurred 28 8 40 Comparison example 10 435 4.6 66 no cracks
no cracks 46 15 29 Comparison example 11 456 4.7 25 cracks occurred
cracks occurred 20 11 32 Comparison example 12 432 4.5 21 no cracks
no cracks 17 9 21 Comparison example 13 486 4.8 23 no cracks no
cracks 14 10 19 Comparison example 14 369 3.5 42 no cracks no
cracks 43 47 65 Comparison example Note: Underlined values being
outside the scope of the present invention
TABLE-US-00004 TABLE 4 Chemical components (mass %) No. C Si Mn P S
Al Cr Nb Ti Mo W Cu Ni 15 0.209 0.32 0.73 0.006 0.0018 0.032 1.05
0.023 0.017 0.17 16 0.227 0.27 0.63 0.005 0.0020 0.025 1.31 0.014
0.012 0.29 0.44 0.21 17 0.216 0.18 0.60 0.007 0.0025 0.017 0.60
0.023 0.028 0.12 18 0.245 0.37 0.52 0.007 0.0018 0.038 1.09 0.017
0.011 0.57 0.24 0.14 19 0.276 0.03 0.93 0.009 0.0027 0.051 0.47
0.023 0.016 20 0.290 0.27 1.08 0.009 0.0021 0.034 0.51 0.011 0.009
0.14 0.10 Chemical components (mass %) No. V N B REM Ca Mg DI*
Formula (2) Remarks 15 31 12 101.4 0.10 Present invention example
16 0.04 27 10 15 178.7 0.00 Present invention example 17 0.05 30
57.0 0.24 Present invention example 18 30 12 21 188.6 0.08
Comparison example 19 0.04 30 50.7 0.44 Comparison example 20 25
72.0 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 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 + 10P
Respective element symbols being contents (mass %)
TABLE-US-00005 TABLE 5 Hot rolling Heat treatment Raw material
Plate Heating Hot rolling finish Heating Steel thickness thickness
temperature temperature temperature Cooling No. (mm) (mm) (.degree.
C.) (.degree. C.) Cooling method (.degree. C.) method Remarks 15
250 40 1150 900 air cooling 900 water cooling Present invention
example 16 300 60 1120 880 air cooling 870 water cooling Present
invention example 17 200 8 1150 830 air cooling 900 water cooling
Present invention example 18 250 32 1100 870 air cooling 900 water
cooling Comparison example 19 250 25 1100 900 water cooling no heat
treatment Comparison example 20 300 40 1150 900 air cooling 900
water cooling Comparison example Note: Underlined values being
outside the scope of the present invention
TABLE-US-00006 TABLE 6 Synthetic heat-affected zone test Abrasion
Base T shape weld cracking test Corresponding to Surface resistant
material No Preheating to low-temperature Shielded metal arc hard-
property toughness preheating 100.degree. C. tempering welding ness
Abrasion vE- (presence or (presence or Corresponding to
embrittlement Toughness of weld Steel HBW resistance vE0 40
non-presence non-presence bond area region joint No. 10/3000 rate
(J) (J) of cracks) of cracks) vE0(J) vE-40(J) vE0(J) vE-40(J)
vE0(J) vE-40(J) Remarks 15 411 4.2 83 66 no cracks no cracks 90 61
72 46 133 105 Present invention example 16 435 4.7 70 49 no cracks
no cracks 65 40 59 38 83 50 Present invention example 17 415 4.3 48
33 no cracks no cracks 43 28 38 30 65 39 Present invention example
18 482 4.7 50 36 no cracks no cracks 36 28 30 27 35 27 Comparison
example 19 528 5.5 35 24 cracks cracks 31 23 21 9 28 14 Comparison
occurred occurred example 20 546 5.8 38 34 no cracks no cracks 33
28 30 27 35 27 Comparison example Note: Underlined values being
outside the scope of the present invention
* * * * *