U.S. patent application number 13/141373 was filed with the patent office on 2013-01-03 for steel material excellent in resistance of ductile crack initiation from welded heat affected zone and base material and method for manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. Invention is credited to Satoshi Igi, Teruki Sadasue.
Application Number | 20130000798 13/141373 |
Document ID | / |
Family ID | 42287928 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130000798 |
Kind Code |
A1 |
Sadasue; Teruki ; et
al. |
January 3, 2013 |
STEEL MATERIAL EXCELLENT IN RESISTANCE OF DUCTILE CRACK INITIATION
FROM WELDED HEAT AFFECTED ZONE AND BASE MATERIAL AND METHOD FOR
MANUFACTURING THE SAME
Abstract
A steel material has a composition of C: 0.02 to 0.2%, Si: 0.01
to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al:
0.1% or lower, and N: 0.01% or lower and, as required, one or two
or more elements selected from Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr:
0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower,
Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM:
0.1% or lower in terms of % by mass, and the balance Fe with
inevitable impurities, in which the microstructure at the 1/4
position of the plate thickness contains ferrite and a hard phase,
the area fraction of the hard phase is 50 to 90%, and the average
aspect ratio of the ferrite is 1.5 or more.
Inventors: |
Sadasue; Teruki; (Tokyo,
JP) ; Igi; Satoshi; (Tokyo, JP) |
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
42287928 |
Appl. No.: |
13/141373 |
Filed: |
December 25, 2009 |
PCT Filed: |
December 25, 2009 |
PCT NO: |
PCT/JP2009/071908 |
371 Date: |
August 17, 2011 |
Current U.S.
Class: |
148/645 ;
148/320; 148/330; 148/331; 148/332; 148/333; 148/336; 148/337 |
Current CPC
Class: |
C22C 38/02 20130101;
C21D 8/0263 20130101; C22C 38/14 20130101; C21D 2211/002 20130101;
C22C 38/06 20130101; C22C 38/16 20130101; C22C 38/001 20130101;
C22C 38/18 20130101; C22C 38/005 20130101; C22C 38/12 20130101;
C21D 1/25 20130101; C22C 38/04 20130101; C21D 2211/008 20130101;
C22C 38/002 20130101; C21D 9/50 20130101; C22C 38/08 20130101; C21D
2211/005 20130101 |
Class at
Publication: |
148/645 ;
148/320; 148/332; 148/336; 148/333; 148/330; 148/337; 148/331 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C22C 38/08 20060101 C22C038/08; C22C 38/12 20060101
C22C038/12; C21D 8/00 20060101 C21D008/00; C22C 38/06 20060101
C22C038/06; C22C 38/14 20060101 C22C038/14; C22C 38/16 20060101
C22C038/16; C22C 38/18 20060101 C22C038/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2008 |
JP |
2008-333204 |
Dec 26, 2008 |
JP |
2008-333205 |
Claims
1. A steel material excellent in resistance of ductile crack
initiation from welded heat affected zone and a base material,
comprising: a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn:
0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or
lower, and N: 0.01% or lower in terms of % by mass, and the balance
Fe with inevitable impurities, having a microstructure at 1/4
position of plate thickness containing ferrite and a hard phase, an
area fraction of the hard phase of 50 to 90%, and an average aspect
ratio of the ferrite of 1.5 or more.
2. The steel material according to claim 1, further comprising, in
the chemical composition, one or more elements selected from the
group consisting of Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%,
Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or
lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1% or
lower in terms of % by mass.
3. The steel material according to claim 1, wherein the
microstructure on a surface of a steel plate contains ferrite and a
hard phase, the area fraction of the ferrite exceeds 40%, and the
average aspect ratio of the ferrite grain size exceeds 2.
4. A method for manufacturing a steel material excellent in
resistance of ductile crack initiation from welded heat affected
zone and a base material comprising: reheating a steel base
material of claim 1 to 1000.degree. C. or more, rolling the same
such that a rolling reduction rate in a temperature range of
900.degree. C. or more is 50% or more and a rolling finish
temperature is Ar.sub.3 point to Ar.sub.3-50.degree. C., starting
water cooling at Ar.sub.3-10.degree. C. to Ar.sub.3-70.degree. C.,
and terminating the water cooling at 500.degree. C. or lower.
5. The method according to claim 4 further comprising, after the
water cooling, performing tempering treatment at a temperature
lower than the highest heating temperature Ac.sub.1 point.
6. The steel material according to claim 2, wherein the
microstructure on a surface of a steel plate contains ferrite and a
hard phase, the area fraction of the ferrite exceeds 40%, and the
average aspect ratio of the ferrite grain size exceeds 2.
7. A method for manufacturing a steel material excellent in
resistance of ductile crack initiation from welded heat affected
zone and a base material comprising: reheating a steel base
material of claims 2 to 1000.degree. C. or more, rolling the same
such that a rolling reduction rate in a temperature range of
900.degree. C. or more is 50% or more and a rolling finish
temperature is Ar.sub.3 point to Ar.sub.3-50.degree. C., starting
water cooling at Ar.sub.3-10.degree. C. to Ar.sub.3-70.degree. C.,
and terminating the water cooling at 500.degree. C. or lower.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.371 of International Application No.
PCT/JP2009/071908, with an international filing date of Dec. 25,
2009 (WO 2010/074347 A1, published Jul. 1, 2010), which is based on
Japanese Patent Application Nos. 2008-333204, filed Dec. 26, 2008,
and 2008-333205, filed Dec. 26, 2008, the subject matter of which
is incorporated by reference.
TECHNICAL FIELD
[0002] This disclosure relates to steel materials suitable for use
in welded structures such as pipelines, bridges, and architectural
structures, requiring structural safety and a method for
manufacturing the same and particularly relates to one excellent in
resistance of ductile crack initiation from welded heat affected
zone and a base material. Specifically, the disclosure is targeted
to steel materials for structures having excellent resistance of
ductile crack initiation from welded heat affected zone and a base
material and having strength of Tensile strength: 490 MPa or more
in TS and high toughness of Ductile-brittle fracture transition
temperature of Charpy impact test (according to the regulation of
JIS Z 2242): vTrs of 0.degree. C. or lower.
BACKGROUND
[0003] It is known that when the welded structures such as
pipelines, bridges, and buildings, are exposed to large external
force of an earthquake or the like, ductile crack initiates in a
stress concentration zone, such as a weld toe, and the generated
ductile crack serves as a trigger to cause brittle fracture,
resulting in break and fracture of the structures in some
cases.
[0004] It is important that steel materials constituting the same
are excellent in resistance of ductile crack initiation to avoid
such break and fracture of the welded structures.
[0005] Japanese Unexamined Patent Application Publication No.
2008-202119 discloses a high tensile-strength steel material
excellent in resistance of ductile crack initiation in which, in
the microstructure a steel material surface zone, the ferrite area
fraction is 10 to 40%, the bainite area fraction is 50% or more,
and the average grain size is 5 .mu.m or lower.
[0006] Japanese Unexamined Patent Application Publication No.
2000-3281777 discloses a steel plate excellent in arrestrability
and resistance of ductile fracture in which the microstructure is
substantially constituted by a ferrite structure, a pearlite
structure, and a bainite structure and, when divided into three
layers of both surface zones and the central zone in the plate
thickness direction of the steel plate, each zone has a specific
microstructure.
[0007] Both the surface zones of the steel plate are constituted by
a layer which has 50% or more of a ferrite structure containing
ferrite grains in which the equivalent (circle) diameter is 7 .mu.m
or lower and the aspect ratio is 2 to 4 over 5% or more of the
plate thickness of each of the structure zones and in which the
bainite area fraction of the portion is 5 to 25% or lower. The
central zone in the plate thickness direction of the steel plate is
constituted by a layer which contains ferrite grains in which the
equivalent (circle) diameter is 4 to 10 .mu.m and the aspect ratio
is 2 or lower over 50% or more of the plate thickness and in which
the bainite area fraction of the zones is 10% or lower.
[0008] More specifically, the technique of JP '177 is directed to a
steel plate in which three layers having a ferrite/pearlite
structure containing ferrite grains different in the aspect ratio
are present in the plate thickness direction from the plate surface
of the steel plate and further in which a bainite structure which
is a hard phase is appropriately dispersed in a soft phase which is
the ferrite/pearlite structure. The technique increases the
arrestrability by positively forming processed ferrite grains
having a high aspect ratio and also appropriately dispersing a
bainite structure on each of both the surface zones of the three
zones of the steel plate and, in contrast, increases extension
characteristics, which are important to ductile fracture at room
temperature, by controlling the central zone of the steel plate to
have a uniform equiaxed ferrite grain structure and also
suppressing a bainite structure, and thus satisfies both opposite
characteristics of "arrestrability" and "ductile fracture
characteristics" by controlling both the surface zones and the
central zone of the steel plate to the three-layer structure.
[0009] Also the technique of Japanese Unexamined Patent Application
Publication No. 2003-221619 is directed to a technique of obtaining
deformed ferrite grains on the steel plate surface zone of
ferrite/pearlite steel and also controlling the microstructure of
the central zone to a uniform equiaxed ferrite grain structure
similarly as the technique of JP '177.
[0010] More specifically, JP '619 discloses a method for
manufacturing a thick steel plate excellent in arrestrability and
ductile fracture characteristics in which the rolling conditions
are strictly controlled so that the steel plate surface zone has a
specific microstructure.
[0011] Specifically, when the thickness during plate rolling is
defined as t, an equivalent plastic strain .epsilon. of
.epsilon..gtoreq.0.5 in a non-recrystallization temperature zone of
Ar.sub.3 transformation point or more and 900.degree. C. or lower
is given to a surface layer zone of 0.05 t or more and 0.15 t or
lower from both the surfaces in the plate thickness direction.
[0012] Thereafter, the surface layer zone is cooled to a
temperature range of 450 to 650.degree. C. at a cooling rate of 2
to 15.degree. C./s while maintaining the temperature of the central
zone defined as t/4 to 3t/4 of the plate thickness at the Ar.sub.3
transformation point or more within a period of time when the
residual and cumulative equivalent plastic strain .epsilon.r of the
surface layer zone satisfies .epsilon.r.gtoreq.0.5, and
subsequently rolling is restarted.
[0013] In the restarted rolling, the residual and cumulative
equivalent plastic strain .epsilon.r of
0.35.ltoreq..epsilon.r<0.55 is given to the central zone to
complete the rolling at the Ar.sub.3 transformation point or more
and also the surface layer is recuperated to the Ar.sub.3
transformation point or lower by processing heat and internal
sensible heat, and thereafter cooling is performed in such a manner
that the average cooling rate is 1 to 10.degree. C./s.
[0014] The techniques of JP '119, JP '177 and JP '619 all relate to
techniques of forming fine subgrains in austenite to miniaturize
the structure after transformation by performing rolling in a non
recrystallization zone (fine grain temperature zone) of austenite
or performing rolling at a rolling finish temperature Ar.sub.3 or
more.
[0015] However, according to the techniques of JP '119, JP '177 and
JP '619, when the surface layer structure changes to the welded
heat affected zone by welding or the like, there is a concern that
the effect of resistance of ductile crack initiation is lost.
[0016] Moreover, in all of a scale breaker for use in treatment of
the surface of a slab extracted from a heating furnace described in
Examples of JP '119, two-stage rolling of rolling in a
pulverization temperature range and rolling in a set temperature
zone described in Examples of JP '177, and various kinds of rolling
or temperature control for separately creating the structure of a
surface layer and the structure inside a steel plate described in
JP '619, the manufacturing process is complicated.
[0017] Then, in view of the problems of such former techniques, it
could be helpful to provide steel materials excellent in resistance
of ductile crack initiation from the welded heat affected zone and
a base material by a simple method and a method for manufacturing
the same.
SUMMARY
[0018] We conducted extensive research on the microstructure of
base material excellent in resistance of ductile crack initiation
of welded heat affected zones and found that, when the
microstructure of a base material has a ferrite and a hard phase in
which the average aspect ratio of the ferrite and the area fraction
of the hard phase are at the 1/4 position of the plate thickness
exhibiting an average structure in the plate thickness direction of
a steel plate, the resistance of ductile crack initiation is
excellent also in the welded heat affected zone and such a steel
material is excellent also in the resistance of ductile crack
initiation of the base material, and further manufacturing
conditions of a steel plate having the microstructure.
[0019] We thus provide: [0020] (1) A steel material excellent in
resistance of ductile crack initiation from welded heat affected
zone and a base material has a composition of C: 0.02 to 0.2%, Si:
0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or
lower, Al: 0.1% or lower, and N: 0.01% or lower in terms of % by
mass, and the balance Fe with inevitable impurities, in which the
microstructure at the 1/4 position of the plate thickness contains
ferrite and a hard phase, the area fraction of the hard phase is 50
to 90%, and the average aspect ratio of the ferrite is 1.5 or more.
[0021] (2) The steel material excellent in resistance of ductile
crack initiation from welded heat affected zone and a base material
according to (1), further contains, in the chemical composition,
one or two or more elements selected from Cu: 0.01 to 2%, Ni: 0.01
to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1%
or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower,
and REM: 0.1% or lower in terms of % by mass. [0022] (3) In the
steel material excellent in resistance of ductile crack initiation
from welded heat affected zone and a base material according to (1)
or (2) above, the structure on the surface of a steel plate
contains ferrite and a hard phase, the area fraction of the ferrite
exceeds 40%, and the average aspect ratio of the ferrite grain size
exceeds 2. [0023] (4) A method for manufacturing a steel material
excellent in resistance of ductile crack initiation from welded
heat affected zone and a base material includes reheating a steel
base material having the chemical compositions of (1) or (2) to
1000.degree. C. or more, rolling the same in such a manner that the
rolling reduction rate in a temperature range of 900.degree. C. or
more is 50% or more and the rolling finish temperature is Ar.sub.3
point to Ar.sub.3-50.degree. C., starting water cooling at
Ar.sub.3-10.degree. C. to Ar.sub.3-70.degree. C., and terminating
the water cooling at 500.degree. C. or lower. [0024] (5) The method
for manufacturing a steel material excellent in resistance of
ductile crack initiation from welded heat affected zone and a base
material according to (4) further includes, after the water
cooling, performing tempering treatment at a temperature of lower
than the highest heating temperature Ac.sub.1 point.
[0025] A steel material capable of suppressing ductile crack
initiation from welded heat affected zone and a base material that
can suppress ductile crack initiation from a stress concentration
zone, such as a weld toe, and prevent collapse or break of steel
structures even when the steel structures greatly deform due to an
earthquake or the like, for example, can be easily and stably
mass-produced and industrially remarkable effects are
demonstrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a view illustrating a ductile crack initiation
test method of a welded heat affected zone.
[0027] FIG. 2 is a view illustrating influence of the area fraction
of a hard phase and the average aspect ratio of ferrite on ductile
crack initiation of a 1400.degree. C. simulated heat cycle
material.
[0028] FIG. 3 is a view illustrating a ductile crack initiation
test method of a base material.
[0029] FIG. 4 is a view illustrating influence of the area fraction
of a hard phase and the average aspect ratio of ferrite on ductile
crack initiation of a base material.
DETAILED DESCRIPTION
[0030] The chemical composition and the microstructure are
specified. In the description of the chemical composition, % by
mass is simply represented by % unless otherwise specified.
Chemical Composition
C: 0.02 to 0.2%
[0031] C is an element having an action of increasing the strength
of steel and, particularly, contributes to the generation of a hard
phase. A C content of 0.02% or more is required to obtain such an
effect. In contrast, when the C content exceeds 0.2%, the ductility
or the bending workability are reduced and also the weldability
decreases. Therefore, the C content is limited in the range of 0.02
to 0.2%. More preferably, the C content is 0.02 to 0.18%.
Si: 0.01 to 0.5%
[0032] Si acts as a deoxidizing agent and has an action of forming
a solid solution to increase the strength of steel. An Si content
of 0.01% or more is required to obtain such an effect. In contrast,
when the Si content exceeds 0.5%, the toughness is reduced and also
the weldability is reduced. Therefor, Si is limited in the range of
0.01 to 0.5%. More preferably, the Si content is 0.01 to 0.4%.
Mn: 0.1 to 2.5%
[0033] Mn has an action of increasing the strength of steel and
also increasing the toughness through an increase in hardenability.
An Mn content of 0.1% or more is required to obtain such an effect.
In contrast, when the Mn content exceeds 2.5%, the weldability is
reduced. Therefore, Mn is limited in the range of 0.1 to 2.5%. More
preferably, the content is 0.5 to 2.0%.
P: 0.05% or lower
[0034] Since P causes degradation of toughness, the P content is
preferably reduced as much as possible, but the content up to 0.05%
is permissible. Therefore, the P content is limited to 0.05% or
lower. More preferably, the content is 0.04% or lower.
S: 0.05% or lower
[0035] Since S is present as an inclusion in steel and degrades the
ductility and the toughness, the S content is preferably reduced as
much as possible. However, the content up to 0.05% is permissible.
Therefore, the S content is limited to 0.05% or lower. More
preferably, the content is 0.04% or lower.
Al: 0.1% or lower
[0036] Al is an element that acts as a deoxidizing agent and also
contributes to pulverization of crystal grains. However, an
excessive content of Al in a proportion exceeding 0.1% causes a
reduction in toughness. Therefore, the Al content is limited to
0.1% or lower. More preferably, the content is 0.05% or lower.
N: 0.01% or lower
[0037] N is an element that increases the strength of steel by
solid solution strengthening similarly as C. However, an excessive
content of N causes a reduction in toughness. Therefore, the N
content is limited to 0.01% or lower. More preferably, the content
is 0.005% or lower.
[0038] The chemical compositions described above are basic chemical
compositions but one or two or more elements selected from Cu: 0.01
to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or
lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca:
0.01% or lower, and REM: 0.1% or lower may be further contained
according to the desired properties.
Cu: 0.01 to 2%
[0039] Cu is an element that has an action of increasing the
strength of steel through an increase in hardenability or solid
solution. The content of 0.01% or more is required to secure such
an effect. In contrast, when the content exceeds 2%, the
weldability decreases and also cracks are likely to generate during
manufacturing of steel materials. Therefore, when Cu is added, the
content is in the range of 0.01 to 2%. More preferably, the content
is 0.01 to 1%.
Ni: 0.01 to 5%
[0040] Ni is added as required, because Ni contributes to an
increase in low temperature toughness, an increase in
hardenability, and prevention of hot ductility of Cu when Cu is
contained. Such an effect is recognized when Ni is contained in the
proportion of 0.01% or more. However, the addition of 5% or more
causes a reduction in steel material cost and also a reduction in
weldability. Therefore, when Ni is added, the content is in the
range of 0.01 to 5%. More preferably, the content is 0.01 to
4.5%.
Cr: 0.01 to 3%
[0041] Cr is added as required to increase the strength of steel
materials through improvement of hardenability or an increase in
tempering softening resistance. Such an effect is recognized when
Cr is contained in the proportion of 0.01% or more. In contrast,
the addition exceeding 3% reduces weldability and toughness.
Therefore, when Cr is added, the content is in the range of 0.01 to
3%. More preferably, the content is in the range of 0.01 to
2.5%.
Mo: 0.01 to 2%
[0042] Mo is added as required to increase the strength of steel
materials through improvement of hardenability or an increase in
tempering softening resistance. Such an effect is recognized when
Mo is contained in the proportion of 0.01% or more. In contrast,
the addition exceeding 2% reduces weldability or toughness.
Therefore, when Mo is added, the content is in the range of 0.01 to
2%. More preferably, the content is in the range of 0.01 to 1%.
Nb: 0.1% or lower
[0043] Nb is an element that precipitates as a carbide or a
carbonitride in tempering and increases the strength of steel
through precipitation strengthening. Moreover, Nb also has an
effect of pulverizing austenite grains during rolling to increase
toughness. The content of 0.001% or more is preferable to obtain
the effects. However, the content exceeding 0.1% reduces toughness.
Therefore, when Nb is added, the content is 0.1% or lower. More
preferably, the content is 0.05% or lower.
V: 0.1% or lower
[0044] V is an element that precipitates as a carbide or a
carbonitride in tempering and increases the strength of steel
through precipitation strengthening. Moreover, V also has an effect
of pulverizing austenite grains during rolling to increase
toughness. The content of 0.001% or more is preferable to obtain
the effects. However, the content exceeding 0.1% reduces toughness.
Therefore, when Nb is added, the content is 0.1% or lower. More
preferably, the content is 0.05% or lower.
Ti: 0.1% or lower
[0045] Ti is added as required because Ti has an effect of
pulverizing austenite in a welded heat affected zone to increase
toughness. The content of 0.001% or more is preferable to obtain
the effect. However, the addition exceeding 0.1% reduces toughness
and also causes a sudden rise of steel material cost. Therefore,
when Ti is added, the content is 0.1% or lower. More preferably,
the content is 0.05% or lower.
B: 0.01% or lower
[0046] B is added as required because B has an effect of increasing
hardenability and increasing the strength of steel with a small
content thereof. The content is preferably 0.0001% or more to
obtain the effect. However, the addition exceeding 0.01% reduces
weldability. Therefore, when B is added, the content is 0.01% or
lower. More preferably, the content is 0.005% or lower.
Ca: 0.01% or lower
[0047] Ca is added as required because Ca increases the base
material toughness by controlling the shape of a CaS inclusion and
further increase the toughness of a welded heat affected zone. The
content of 0.0001% or more is preferable to obtain the effects.
However, the addition exceeding 0.01% reduces toughness due to an
increase in the amount of the CaS inclusion. Therefore, when Ca is
added, the content is 0.01% or lower. More preferably, the content
is 0.009% or lower.
REM: 0.1% or lower
[0048] REM is an element that increases the toughness of a welded
heat affected zone and is added as required. The content is
preferably 0.0001% or more to obtain the effect. However, the
addition exceeding 0.1% causes a reduction in toughness. Therefore,
when REM is added, the content is 0.1% or lower. More preferably,
the content is 0.05% or lower.
[0049] REM is a general term of Y, Ce and the like that are rare
earth elements and the addition amount as used herein refers to the
total amount of these rare earth elements.
Microstructure
[0050] The steel material has a microstructure in which the
structure at the 1/4 position of the plate thickness contains
ferrite and a hard phase, the area fraction of the hard phase is 50
to 90%, and the average aspect ratio of the ferrite grain size is
1.5 or more. When the area fraction of the hard phase is lower than
50% and exceeds 90% or the aspect ratio of the ferrite grain size
is lower than 1.5, there is a possibility that ductile crack
initiation occurs.
[0051] The upper limit of the average aspect ratio of the ferrite
grain size does not need to particularly specify and is 5 or lower
in view of the capability and the like of a rolling mill. The area
fraction of the hard phase is more preferably 52 to 90% and the
average aspect ratio of the ferrite grain size is more preferably
1.6 or more. The average aspect ratio is more preferably 1.7 or
more.
[0052] In a two phase mixed structure containing ferrite and a hard
phase, the yield ratio (or Y/T ratio) of a base material decreases,
and the strain concentration in a stress concentration zone is
eased even in the base material as it is or even after a simulated
heat cycle of simulating the welded heat affected zone. Such an
effect is not obtained in the case of a single phase of ferrite or
a single phase of a hard phase.
[0053] In the steel material, the structure of the surface of a
steel plate (1 mm position from the plate surface) contains ferrite
and a hard phase, in which the area fraction of the ferrite exceeds
40% and is more preferably 50% or more. The average aspect ratio of
the ferrite grain size exceeds 2. When the area fraction of the
ferrite is lower than 40% or the average aspect ratio of the
ferrite grain size is 2 or lower, the resistance of ductile crack
initiation in a welded heat affected zone is poor.
[0054] The hard phase is bainite, martensite, or a
bainite/martensite mixed structure and contains 5% or lower, in
terms of area fraction, of an island martensite (M-A constituent)
(MA).
[0055] FIG. 2 illustrates the results of examining the resistance
of ductile crack initiation using a simulated heat cycle specimen
of a welded zone (highest heating temperature of 1400.degree. C.).
As illustrated in FIG. 2, when the area fraction of the hard phase
of the base material is 50 to 90% and the average aspect ratio of
the ferrite thereof is 1.5 or more, ductile crack initiation is not
observed also after the simulated heat cycle.
[0056] The results illustrated in FIG. 2 were obtained by specimens
of 12 mm thickness (=plate thickness direction).times.12 mm
width.times.200 length from the 1/4 center of the plate thickness
(1/2 center of the plate thickness in the case of a plate thickness
of 25 mm or lower) from the steel materials obtained by producing
steel having a composition in our range by various manufacturing
methods and changing the microstructure, and then giving a
simulated heat cycle (time for reaching the highest heating
temperature: 6 s, cooling rate from the highest heating temperature
to room temperature: 40.degree. C./s) of a welded zone thereto by a
Gleeble tester to obtain sample materials.
[0057] FIG. 1 illustrates the specimen shape and the test
conditions. The sample material (specimen 1), to which the
simulated heat cycle was given, in which a single through-thickness
edge notch is introduced with the length of 3 mm in the plate
thickness direction into the center of a simulated heat cycle zone
2 of the sample material (specimen 1) was fixed with clamps 5, then
a tensile load (arrow 6) was applied to 0.6 mm in terms of
displacement of a clip gage 3 between knife-edges 4 that are
screwed, the load was removed, and then the specimen was ground to
the central zone and mirror polished. Then, the presence of crack
initiation at the notch tip was evaluated. The case where the
ductile crack from the notch bottom was 50 .mu.m or more was
defined as crack initiation.
[0058] It is considered that the results illustrated in FIG. 2 are
obtained due to the fact that the yield ratio (or Y/T ratio) (0.2%
proof stress/tensile strength) decreased also in the structure
after the simulated heat cycle and the degree of distortion
concentration at the notch tip zone decreased by the use of the
base material having a complex structure of ferrite and a hard
phase.
[0059] Such outstanding characteristics were observed in common
also in a base material to which the simulated heat cycle was not
given.
[0060] More specifically, FIG. 4 illustrates the results of
examining the influence of the microstructure of the base material
exerted on the resistance of ductile crack initiation. As
illustrated in FIG. 4, when the area fraction of the hard phase of
the base material is 50 to 90% and the average aspect ratio of the
ferrite is 1.5 or more, ductile crack initiation is not
accepted.
[0061] The results of the base material illustrated in FIG. 4 were
obtained by specimens of 12 mm thickness (=plate thickness
direction).times.12 mm width.times.200 length from the 1/4 center
of the plate thickness (1/2 center of the plate thickness in the
case of a plate thickness of 25 mm or lower) from steel materials
obtained by producing steel having a composition in our range by
various manufacturing methods and changing the microstructure.
[0062] FIG. 3 illustrates the specimen shape and the test
conditions. The sample material (specimen 1) in which a single
through-thickness edge notch is introduced into the center was
fixed with clamps 5, then a tensile load (arrow 6) was applied to
0.8 mm in terms of displacement of a clip gage 3 between
knife-edges 4 that are screwed, the load was removed, and then the
specimen was ground to the central zone and mirror polished. Then,
the presence of crack initiation at the notch tip was evaluated.
The case where the ductile crack from the notch bottom was 50 .mu.m
or more was defined as crack initiation.
[0063] We believe that the results illustrated in FIG. 4 are
obtained due to the fact that the yield ratio (or Y/T ratio) (0.2%
proof stress/tensile strength) decreased and the degree of
distortion concentration at the notch tip zone decreased by the use
of a base material having a complex structure of ferrite and a hard
phase.
[0064] Moreover, it is also considered to be one of the factors
that the slip plane greatly leaned to the crack initiation
direction in the base material as it is and also after the
simulated heat cycle by increasing the average aspect ratio of the
ferrite, i.e., the development of the specific aggregate structure.
The aspect ratio refers to the ferrite grain size in the rolling
direction (major axis)/the ferrite grain size in the plate
thickness direction (minor axis) in a cross section parallel to the
rolling direction.
[0065] The same results as those of FIG. 2 were obtained also when
the highest heating temperature of the simulated heat cycle was
760.degree. C., 900.degree. C., and 1200.degree. C.
[0066] The steel material is obtained by successively subjecting
the steel material of the above-described chemical compositions to
a hot rolling process, a water cooling process, or further a
tempering process.
[0067] The hot rolling includes reheating to 1000.degree. C. or
more and performing rolling in such a manner that the rolling
reduction rate in a temperature range of 900.degree. C. or more is
50% or more and the rolling finish temperature becomes Ar.sub.3 to
Ar.sub.3-50.degree. C. A more preferable rolling finish temperature
is lower than Ar.sub.3 to Ar.sub.3-40.degree. C. By setting the
rolling finish temperature in the invention range, processing
strain (or residual strain) can be added to ferrite generated
during rolling to thereby increase the aspect ratio of the ferrite.
When the reheating temperature is lower than 1000.degree. C., hot
rolling that gives a desired cumulative rolling reduction rate
cannot be performed to the steel material.
[0068] When the cumulative rolling reduction rate at 900.degree. C.
or more is lower than 50%, desired strength and toughness cannot be
secured. When the rolling finish temperature exceeds Ar.sub.3, the
aspect ratio of ferrite does not reach 1.5 or more. When the
rolling finish temperature is lower than Ar.sub.3-50.degree. C.,
the area fraction of the hard phase obtained by the subsequent
water cooling does not reach 50% or more.
[0069] In the water cooling process, the water cooling is started
at Ar.sub.3-10.degree. C. to Ar.sub.3-70.degree. C. immediately
after hot rolling, and then the water cooling is terminated at
500.degree. C. or lower. When the water cooling start temperature
exceeds Ar.sub.3-10.degree. C., ferrite of lower than 10% in terms
of area fraction (hard phase exceeding 90% in terms of area
fraction) precipitates. When the water cooling start temperature is
lower than Ar.sub.3-70.degree. C. or water cooling is not started
immediately after (within 300 seconds) hot rolling, ferrite
exceeding 50% in terms of area fraction (hard phase not reaching
50% in terms of area fraction) or pearlite, which is not intended
to precipitate, precipitates. Thus, desired characteristics cannot
be satisfied.
[0070] After performing the cooling, tempering treatment can be
further performed at a temperature of lower than the Ac.sub.1
point. By performing tempering treatment, toughness and ductility
increase, and desired strength and toughness can be achieved. When
the tempering temperature exceeds the Ac.sub.1 point, a large
amount of island martensite generates to reduce the toughness.
[0071] The Ar.sub.3 point and the Ac.sub.1 point can be calculated
by the following equation based on the content (% by mass) of each
chemical composition:
Ar.sub.3(.degree. C.)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
Ac.sub.1(.degree. C.)=723-14Mn+22Si-14.4Ni+23.3Cr.
[0072] The disclosure will be described in more detail based on
Examples.
EXAMPLES
[0073] Steel materials containing the chemical compositions shown
in Table 1 were subjected to hot rolling at the conditions shown in
Table 2 to thereby obtain steel plates having a plate thickness of
12 to 100 mm.
[0074] The obtained steel plates were subjected to microstructure
observation, a tensile test, a toughness test, a ductile crack
initiation test after a simulated heat cycle, and a ductile crack
initiation test of base materials. The test methods were performed
as described in the following items (1) to (5).
(1) Microstructure Observation
[0075] From the obtained steel plates, specimens were extracted in
the cross section parallel to the rolling direction. Then, the
specimens were mirror polished, and then etched with nital.
Thereafter, the microstructure at the 1/4 position of the plate
thickness and the microstructure 1 mm below the surface were
observed. The observation of each of the positions was performed
with Field number: 20 fields of view. The area fraction was
determined by binarizing the ferrite and the hard phase and
observing at a magnification of 200.times.. The average aspect
ratio of the ferrite was determined by determining the length in
the rolling direction and the length in the plate thickness
direction of each ferrite present in the field of view at a
magnification of 400.times., determining the length in the rolling
direction/the length in the plate thickness direction, and then
determining the average value thereof
(2) Tensile Test
[0076] From the obtained steel plates, full thickness JIS No. 5
specimens were extracted so that the tensile direction was
perpendicular to the rolling direction of the steel plate according
to the regulation of JIS Z 2201 (1998). The tensile test was
performed according to JIS Z 2241 (1998), and then the 0.2% proof
(.sigma..sub.0.2) and the tensile strength (TS) were determined to
evaluate the static tensile properties.
(3) Toughness Test
[0077] From the obtained steel plates, V notch specimens were
extracted so that the longitudinal direction was in parallel to the
rolling direction according to the regulation of JIS Z 2242 (2005),
and then the ductile-brittle fracture transition temperature was
determined to evaluate the toughness. The specimens were extracted
in such a manner that the 1/4 position of the plate thickness when
the plate thickness was 20 mm or more or the 1/2 position of the
plate thickness when the plate thickness was lower than 20 mm was
the center.
(4) Ductile Crack Initiation Test after Simulated Heat Cycle
[0078] From the obtained steel plates, specimens of 12 mm thickness
(=plate thickness direction=t).times.12 mm width and 200 mm in full
length were extracted at the 1/4 center of the plate thickness (1/2
center of the plate thickness when the plate thickness was 25 mm or
lower). The specimens were subjected to a simulated heat cycle of a
welded heat affected zone in which the highest heating temperature
was 760.degree. C., 900.degree. C., 1200.degree. C., and
1400.degree. C. (time for reaching the highest heating temperature:
6 s, Cooling rate from the highest heating temperature to room
temperature: 40.degree. C./s) using a Gleeble tester.
[0079] Thereafter, as illustrated in FIG. 1, a single
through-thickness edge notch was introduced with the length of 3 mm
in the plate thickness direction into the center of the simulated
heat cycle zone. The notch processing was carried out by electrical
discharge machining, and the notch tip radius was 0.1 mm.
[0080] In the test, a tensile load was applied while gripping the
specimens with both right and left ends thereof with a constraint
length of 50 mm. During the test, the displacement between the
knife-edges screwed near the notch was measured with the clip gage.
A tensile load was applied to 0.6 mm in terms of clip gage
displacement, and then the load was removed. Thereafter, the
specimen was ground to the width center and mirror polished. Then,
the crack initiation state at the notch bottom was analyzed under a
microscope with a magnification of 50.times.. It was defined that
the ductile crack initiation occurred when a ductile crack extended
in the length of 50 .mu.m or more from the notch bottom.
(5) Ductile Crack Initiation Test of Base Material
[0081] From the obtained steel plates, specimens of 12 mm thickness
(=plate thickness direction=t).times.12 mm width and 200 mm in full
length were extracted at the 1/4 center of the plate thickness (1/2
center of the plate thickness when the plate thickness was 25 mm or
lower).
[0082] To the obtained specimens, a single through-thickness edge
notch was introduced with the length of 3 mm in the plate thickness
direction into the center of the specimens as illustrated in FIG.
3. The notch processing was carried out by electrical discharge
machining, and the notch tip radius was 0.1 mm.
[0083] In the test, a tensile load was applied while gripping the
specimens with both right and left ends thereof with a constraint
length of 50 mm. During the test, the displacement between the
knife-edges screwed near the notch was measured with the clip gage.
A tensile load was applied to 0.8 mm in terms of clip gage
displacement, and then the load was removed. Thereafter, the test
was ground to the width center and mirror polished. Then, the crack
initiation state at the notch bottom was analyzed under a
microscope with a magnification of 50.times.. It was defined that
the ductile crack initiation occurred when a ductile crack extended
in the length of 50 .mu.m or more from the notch bottom.
[0084] With respect to the specimens that were subjected to the
simulated heat cycle, the obtained experimental results are shown
in Table 3. All of the steel plates of Nos. 1 to 10 produced using
the chemical compositions and our manufacturing method have our
structure. We found that the steel plates have excellent strength
and toughness and have excellent resistance of ductile crack
initiation of a welded heat affected zone.
[0085] In contrast, the steel plate (Steel type K*) of No. 11 in
which the C content does not satisfy the lower limit of our range
has low tensile strength. The steel plate (Steel type L*) of No. 12
in which the content of each of C, P, and S exceeds the upper limit
of our range has low toughness and has poor ductile crack
initiation characteristics of a welded heat affected zone.
[0086] The steel plate of No. 13 in which the reheating temperature
of slab is lower than our and the cumulative rolling reduction rate
at 900.degree. C. or more is outside our range has low toughness.
In the steel plate of No. 14 in which the rolling finish
temperature and the water cooling start temperature exceed our
range, ferrite is not generated, our microstructure is not
obtained, and the resistance of ductile crack initiation of a
welded heat affected zone is poor.
[0087] In the steel plate of No. 15 in which the cooling start
temperature is lower than our range and the steel plate of No. 16
in which the water cooling stop temperature exceeds our range, the
hard phase area fraction and the average aspect ratio of ferrite do
not satisfy our values and both the steel plates have low tensile
strength and poor resistance of ductile crack initiation of welded
heat affected zones. In the steel plate of No. 17 in which the
tempering temperature exceeds our range, since a large amount of
island martensite is generated, the toughness is low and the
resistance of ductile crack initiation of a welded heat affected
zone is poor.
[0088] The obtained experimental results of the base material are
shown in Table 4. All of the steel plates of Nos. 18 to 27 produced
using the chemical compositions and our manufacturing method have
our structure. We found that the steel plates have excellent
strength and toughness and have excellent resistance of ductile
crack initiation of a welded heat affected zone.
[0089] In contrast, the steel plate (Steel type W*) of No. 28 in
which the C content does not satisfy the lower limit of our range
has low tensile strength. The steel plate (Steel type X*) of No. 29
in which the content of each of C, P, and S exceeds the upper limit
of our range has low toughness. The steel plate of No. 30 in which
the reheating temperature of slab is lower than our range and the
cumulative rolling reduction rate at 900.degree. C. or more does
not satisfy our range has low toughness.
[0090] In the steel plate of No. 31 in which the rolling finish
temperature and the water cooling start temperature exceed our
range, ferrite is not generated, our microstructure is not
obtained, and the resistance of ductile crack initiation is
poor.
[0091] In the steel plate of No. 32 in which the cooling start
temperature is lower than our range and the steel plate of No. 33
in which the water cooling stop temperature exceeds our range, the
hard phase area fraction and the average aspect ratio of ferrite do
not satisfy our values and both the steel plates have low tensile
strength and poor resistance of ductile crack initiation. In the
steel plate of No. 34 in which the tempering temperature exceeds
our value, a large amount of island martensite (M-A constituent) is
generated. Thus, the toughness is low and the resistance of ductile
crack initiation is poor.
REFERENCE SIGNS LIST
[0092] 1. Specimen [0093] 2. Simulated heat cycle zone [0094] 3.
Clip gage [0095] 4. Knife-edge [0096] 5. Clamp [0097] 6. Tensile
load
TABLE-US-00001 [0097] TABLE 1 Steel Chemical composition (mass %)
type C Si Mn P S Cu Ni Cr Mo Nb V A 0.14 0.33 1.59 0.005 0.002 --
-- -- -- -- -- B 0.06 0.11 1.96 0.009 0.001 0.25 0.14 0.04 0.21
0.046 0.005 C 0.18 0.04 1.16 0.006 0.005 -- -- -- -- -- 0.048 D
0.12 0.39 0.54 0.008 0.003 -- -- 0.22 0.35 -- -- E 0.03 0.26 0.52
0.003 0.005 -- 4.49 -- -- 0.012 -- F 0.09 0.32 1.32 0.042 0.007
0.98 1.33 -- -- 0.022 0.015 G 0.08 0.24 1.18 0.002 0.009 -- -- 2.48
0.98 0 0.018 H 0.11 0.18 1.22 0.001 0.004 -- 0.52 0.16 0.21 0.045
0.033 I 0.05 0.32 1.38 0.002 0.043 -- 2.43 -- 0.25 0.033 0.019 J
0.09 0.25 1.44 0.005 0.002 -- -- 0.08 0.11 0.022 0.038 K* 0.01*
0.21 1.56 0.004 0.003 -- -- -- -- -- -- L* 0.32* 0.18 0.55 0.193*
0.183* 0.25 0.15 0.23 0.14 -- -- M 0.15 0.32 1.58 0.006 0.001 -- --
-- -- -- -- N 0.06 0.12 1.95 0.008 0.004 0.22 0.15 0.02 0.21 0.045
0.004 O 0.19 0.02 1.15 0.005 0.005 -- -- -- -- -- 0.047 P 0.11 0.38
0.52 0.007 0.003 -- -- 0.21 0.33 -- -- Q 0.03 0.25 0.51 0.003 0.004
-- 4.51 -- -- 0.018 -- R 0.08 0.31 1.33 0.041 0.006 0.95 1.26 -- --
0.025 0.011 S 0.09 0.20 1.14 0.003 0.004 -- -- 2.45 0.97 -- 0.009 T
0.12 0.16 1.25 0.004 0.003 -- 0.55 0.15 0.22 0.043 0.032 U 0.05
0.27 1.37 0.001 0.042 -- 2.42 -- 0.28 0.032 0.019 V 0.09 0.22 1.44
0.003 0.003 -- -- 0.07 0.15 0.028 0.042 W* 0.01* 0.25 1.55 0.005
0.004 -- -- -- -- -- -- X* 0.31* 0.15 0.51 0.180* 0.173* 0.23 0.11
0.21 0.15 -- -- Chemical composition Ar.sub.3 Ac.sub.1 Steel (mass
%) [.degree. C.] [.degree. C.] type Ti B Ca REM Al N Note (1) Note
(2) A -- -- -- -- 0.033 0.0048 739 729 B 0.011 0.0003 -- -- 0.032
0.0042 705 697 C -- -- -- -- 0.044 0.0029 761 708 D -- -- -- 0.097
0.029 0.0041 798 729 E 0.049 -- -- -- 0.018 0.0029 612 657 F -- --
0.0089 0.031 0.022 0.0031 684 692 G -- -- -- -- 0.025 0.0028 675
770 H 0.017 0.0046 -- -- 0.018 0.0035 731 706 I -- -- 0.0033 0.044
0.022 0.0028 630 676 J 0.013 0.0011 -- -- 0.032 0.0041 757 710 K*
-- -- -- -- 0.028 0.0039 782 706 L* -- -- 0.0022 0.008 0.022 0.0038
739 722 M -- -- -- -- 0.032 0.0048 737 729 N 0.012 0.0002 -- --
0.031 0.0041 706 697 O -- -- -- -- 0.048 0.0029 759 707 P -- -- --
0.098 0.028 0.0042 805 729 Q 0.048 -- -- -- 0.016 0.0028 612 656 R
-- -- 0.0091 0.032 0.017 0.0041 691 693 S -- -- -- -- 0.022 0.0032
677 769 T 0.018 0.0048 -- -- 0.011 0.0035 723 705 U -- -- 0.0032
0.048 0.023 0.0028 629 675 V 0.012 0.0015 -- -- 0.032 0.0041 754
709 W* -- -- -- -- 0.031 0.0039 783 707 X* -- -- 0.0021 0.011 0.028
0.0037 747 722 Note: The cells marked by * are outside our range
and the steel types K, L, W, and X are comparative steels. Note
(1): Ar3(.degree. C.) = 910-310 C--80 Mn--20 Cu--15 Cr--55 Ni--80
Mo Each alloy element amount indicates the content (%). Note (2):
Ar1(.degree. C.) = 723-14 Mn + 22 Si--14.4 Ni + 23.3 Cr Each alloy
element amount indicates the content (%).
TABLE-US-00002 TABLE 2 Cumulative rolling Water Water Slab
reduction Rolling cooling cooling Plate reheating rate at finish
start stop Tempering Steel thickness temperature 900.degree. C. or
temperature temperature temperature temperature No. type (mm)
[.degree. C.] more [%] [.degree. C.] [.degree. C.] [.degree. C.]
[.degree. C.] 1 A 14 1160 87 702 683 431 -- 2 B 22 1190 75 679 665
378 -- 3 C 12 1210 92 731 687 298 -- 4 D 100 1150 56 782 779 421 --
5 E 75 1240 62 583 578 72 620 6 F 35 1190 73 672 634 388 -- 7 G 24
1150 81 641 623 28 580 8 H 68 1240 55 701 687 426 -- 9 I 34 1170 72
607 598 426 -- 10 J 18 1160 82 718 704 388 -- 11 K* 22 1120 78 748
726 315 -- 12 L* 45 1180 68 725 706 248 -- 13 C 73 970* 34* 741 732
42 650 14 A 14 1160 87 785* 777* 413 -- 15 B 28 1230 72 695 600*
388 -- 16 J 19 1240 78 712 699 638* -- 17 D 75 1090 64 768 749 62
760* 18 M 15 1150 86 695 681 401 -- 19 N 20 1180 76 686 664 308 --
20 O 12 1200 91 721 697 498 -- 21 P 100 1130 55 792 777 418 -- 22 Q
75 1250 61 602 589 25 600 23 R 35 1200 72 668 643 418 -- 24 S 25
1160 73 647 625 72 500 25 T 72 1250 58 708 697 457 -- 26 U 37 1170
71 618 605 412 -- 27 V 15 1150 83 723 703 378 -- 28 W* 25 1100 84
758 748 258 -- 29 X* 48 1200 69 721 710 243 -- 30 O 75 930* 33* 737
717 23 600 31 M 15 1150 88 755* 747* 428 -- 32 N 25 1220 71 679
605* 352 -- 33 V 18 1250 77 715 703 658* -- 34 P 77 1080 62 776 748
245 750* Note: The steel types marked by * are outside our
range.
TABLE-US-00003 TABLE 3 Microstructure of Microstructure 1 mm 1/4
Plate thickness/4 below the surface Hard Hard phase Hard Ferrite
phase Ferrite Ferrite struc- phase average struc- phase average
Ductile crack initiation ture fraction aspect ture fraction aspect
.sigma.0.2 TS vTrs characteristics Note (2) Classi- No. Note (1)
[%] ratio Note (1) [%] ratio [MPa] [MPa] [.degree. C.] 760.degree.
C. 900.degree. C. 1200.degree. C. 1400.degree. C. fication 1 B 59
1.9 B 55 2.4 428 548 -57 .smallcircle. .smallcircle. .smallcircle.
.smallcircle. Example 2 B 75 2.2 B 68 3.1 563 728 -105
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Example 3
B, M 54 2.3 B, M 77 4.8 521 689 -33 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. Example 4 B 64 1.6 B 48 2.2 408 521 -29
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Example 5
TB 90 1.7 TM 41 2.6 555 667 -98 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. Example 6 B 62 1.8 B 59 2.3 473 621 -47
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Example 7
TM 55 2.1 TM 72 2.4 481 582 -92 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. Example 8 B 83 1.8 B 42 2.1 529 683 -64
.smallcircle. .smallcircle. .smallcircle. .smallcircle. Example 9 B
87 1.7 B 40 2.2 433 538 -41 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. Example 10 B 75 2.5 M 58 3.3 428 548
-72 .smallcircle. .smallcircle. .smallcircle. .smallcircle. Example
11 B 73 2.2 B 53 3.0 325 421* -18 .smallcircle. .smallcircle.
.smallcircle. .smallcircle. Comparative Example 12 B, M 72 1.7 B, M
43 2.3 677 991 15* .smallcircle. x x x Comparative Example 13 TM 72
2.3 TM 40 2.5 521 609 8* .smallcircle. .smallcircle. .smallcircle.
.smallcircle. Comparative Example 14 B 100* -- B 0* --* 548 678 -21
x x x x Comparative Example 15 P 14* 1.1* P 87 1.4* 344 472* -11 x
x x x Comparative Example 16 P 21* 1.3* P 81 1.4* 388 488* -18 x x
x x Comparative Example 17 B, MA 63 1.8 M, MA 48 2.8 521 622 6* x x
x x Comparative Example Note: The cells marked by * are outside our
range. Note (1): B: Bainite., M: Martensite, P: Pearlite, TB:
Tempered bainite, TM: Tempered martensite, MA: Island martensite
Note (2): .smallcircle.: No ductile crack initiation x: Ductile
crack initiation
TABLE-US-00004 TABLE 4 Microstructure of Microstructure 1 mm 1/4
Plate thickness below the surface Hard Hard Ferrite Hard Ferrite
Ferrite Ductile crack phase phase average phase phase average
initiation structure fraction aspect structure fraction aspect
.sigma.0.2 TS vTrs characteristics No. Note (1) [%] ratio Note (1)
[%] ratio [MPa] [MPa] [.degree. C.] Note (2) Classification 18 B 55
1.8 B 57 3.1 436 528 -48 .smallcircle. Example 19 B 72 2.1 B 42 4.9
573 726 -121 .smallcircle. Example 20 B, M 52 2.2 B, M 66 3.9 511
698 -21 .smallcircle. Example 21 B 62 1.6 B 48 2.2 359 515 -28
.smallcircle. Example 22 TB 89 1.8 TM 41 4.1 552 628 -111
.smallcircle. Example 23 B 68 1.9 B 49 2.8 487 615 -35
.smallcircle. Example 24 TM 59 2.0 TM 57 3.1 472 577 -98
.smallcircle. Example 25 B 84 1.6 B 42 2.7 507 641 -63
.smallcircle. Example 26 B 88 1.7 B 43 2.9 402 513 -34
.smallcircle. Example 27 B 77 2.4 B 53 3.3 425 538 -66
.smallcircle. Example 28 B 74 2.1 M 58 3.7 368 411* -38
.smallcircle. Comparative Example 29 B, M 78 1.8 M 55 2.2 687 983
10* .smallcircle. Comparative Example 30 TM 69 2.4 TM 42 2.4 513
618 7* .smallcircle. Comparative Example 31 B 100* -- B 0* --:* 558
688 -18 x Comparative Example 32 P 12* 1.2* P 89 1.4* 358 451* -13
x Comparative Example 33 P 18* 1.4* P 83 1.4* 398 473* -21 x
Comparative Example 34 B, MA 68 1.7 M, MA 44 2.8 535 637 5* x
Comparative Example Note: The cells marked by * are outside our
range. Note (1): B: Bainite., M: Martensite, P: Pearlite, TB:
Tempered bainite, TM: Tempered martensite, MA: Island martensite
Note (2): .smallcircle.: No ductile crack initiation x: Ductile
crack initiation
* * * * *