Note: Descriptions are shown in the official language in which they were submitted.
3~
1 BACKGROUN~ OF THE INVEN~IOM
This invention relates to a method of producing
non-heat trea'ced as rolled hi.gh toughness steel having
high toughness at the base metal and the weld zone~
A controlled rolling process has been widely in
use as a method of producing line pipe material used at
low temperature or extremely low temperature. However,
novel controlled rolling methods have in recent years
been developed and have come to attract attention. They
include, in combination with the conventional controlled
rolling~ a lower temperature slab heating and controlled
rolling process in which the heating temperature is lowered
to a level immediately above Ar3 point and a rolling
process, what is generally referred to as (r-d) two
phase region rolling between Ar3 and Arl pointsO ~hese
novel processes offer the advan~ages that they cause
remarlcable grain refining (including subgral.ns) and an
increase in separation density 7 with the result that the
transition temperature from brittle to ductile fracture
in Charpy impact test and down weight tear test (DW~)
which serves as an index for stopping brittle fracture
is markedly increased and that it is possible to maintain
balance between st.rength and ductility (~racture transi-
tion temperature) when ~wo phase region rolling ls carried
out t.o increase the strength. ~he steel as rolled produced
3~
1 by these processes has the possibilities of being used
for making pressure vessels and the like, in addition to
being used as line plpe material.
United States Patent NoO 3,673~oo7 discloses a
method for manufacturing non heat treating type high
toughness steel starting from ingot making ~ollowed by
hot rolling or forging, however, it does nct disclose
properties requlred for welding particularly toughness
at heat effected zone due to weldlng.
With regard to chemical composition, ~he
U.S.P. 3673007 specifies~ in addition to the elements for
fandamental low carbon steel~ minor amount of at least
one element selected from the group consisting of
upto 0.20% Nb, 0.20~ V, upto 0.15% ~i and upto 0.30% Ta
all being by weight.
The present invention, on the other hand
critically sets forth chemical composition, especially,
allowable upper limit for S and strictly restricted ranges
for Ti and N, for the purpose of retarding formation of
MnS and to ensure formation of uni~ormly distributed
fine grains of TiN which are effective for refîning crystal
grains in the rolled steel.
With regard to the condition of slab making~
the present invention also dif~ers ~rom that of the U.S.P. 3
that is, the present invention directly performs hot
rolling of the cast slab prepared by continuous casting
in order to ensure rapid cooling necessary to obtain
sufficient amount of fine particles of ~iN~ while the
~ 3~ ~ ~
1 method of the U.S.P. relies on ordinary ingot making
method.
As to the condition of rolling reduction, the
U.S.P. merely defines thickness reduction of more than
10% and that the rolling shall be completed in the vicinity
of Ar3 point.
Differrirlg greatly from the conditions defined
by U.S.P., the present invention specifies more strict
conditions of rolling, namely~ mlnimum rolling reduction
Of 40% at t;he temperature not more than 850C and the
temperature for final rolling to be hept withln a range
between Ar3 plus 10C and not more than 800C.
These strict requirements have been established
base on the knowledge obtained by the present inventors'
finding that at least 40% of rolling reduction of
austenite at comparatively lower temperature range of
not more than 850C is indispensable to obtain refined
microstructure of the finally transformed steel stock
necessary for obtaining good mechanical property in the
direction of the thickness of the rolled steel.
Distinguishable feature of the present invention
resides in the strictly limited ranges for Ti and N and
the strictly limited controlled rolling necessary for
obtaining greatly improved toughness at heat affect;ed ~one
of the steel as well as the base metal.
However7 some disadvantages are associated with
these methods. The steel produced by them has the follow-
ing defects. (1) The steel has increased anisotropy and
t~
its mechanical properties in the directlon of plate
thickness become wrong, while energy ab~orptl~n in
Charpy and DWTT tests decreases (or ~he brittle fracture
prevention characteristic is reduced) as a reaction to
5 lncrea~ed separatlon densi~y. (2) Even i~ the base metal
may h~ve superior low temperature toughness (brlttle
rupture preventioll charac~eristic) " ~he ~oughness of a
heat a~ected zone (herelnaf'~er re~erred to as HAZ )
in weld constructlon is not compatlble with that o~ the
base metal. Thus the steel produced by these methods
i~; still limited in use and have no~ yet come to be
us ed wide ~y .
The inventors of the present invention have
invented a method of producing steel having both high
energy absorption characteristic in impact testing such
as Charpy test and low anisotropy, for which a patent
application has been filed in Japan, published as Japanese
Laid-Open Patent Publication No. 131125/80 (Japanese Post-
Exam Patent Publication No. 14848/83 dated March 22, 1983),
the applicant of this publication being Nippon Steel Corporation,
t.he inventors of this publication being Hiroo Mazuda, Hiroshi
Tamehiror Mamoru Oohashi, and Ho Nakasugi. This method,
however, suffers a disadvantage, that although anisotropy is
reduced within the plate surface, no fundamental solution has
been provided in improving the characteristics in the plate
thickness direction although they have been improved to a
certain exten-t as compared with those steels of the prior art.
~ . ~
3~
SUMMARY OF T~E INVENTION
This invention has been developed fox the purpose
of obviating the aforesaid drawbacks of the prior art.
4a-
3~13~
l Accordir~ly~ the invention has as its objec~ the provislon
of a method of producing steel of very high toughness and
strength having balanced strength and toughness, which is
low in anisotropy, high in energy absorption in Charpy
and DW~T testsg high in the toughness of the weld zone
thereby can be used as an entirely novel method of pro~
duction of steel for weld construction.
The diskinguishable feature of the in~ention
resides in that the steel having a low S content added
with small amounts of critically controlled Ti and A
is made into a slab of a thickness below 300 mm by a
continuous casting process which is heated at low
temperature to effect controlled rolling at a temperature
above Ar3 point.
The invention has been developed based on the
concept that steel of low separation and high plate
thickness direction characteristics can be produced
without lowering;the features of the inventors' prior
application9 by paying due attention to the chemical
composition of steel and to the heating and rolling
conditions.
BRIEF DESCRIPTION OF ~HE DRAWING
The sole draw~ng a graph showing the finishing
temperature in relation to the transition temperature
in the directions of plate thickness direction and the
rolling.
1 DESCRIPTION OF THE PREFERRED EMBODIMENT
~ he method according to the present invention
enables anisotropy of the steel plate to be markedly
reduced and allows e~ery absorption in Charpy impact
test and the like to be increased, while markedly increas-
ing toughness in HAZ. A reduction in energy absorption
in Charpy test and the like is accounted for the ~act
that separation occurs at the impact frackure and is
caused mainly by elongated MnS, non-recrystallized
austenite region and the formation of a (100) texture
parallel to the.plate surface created by the rolling in
(y-a) two phase region. In the present invention, the
S content of the steel is reduced and rolling is termi-
nated within the single pha~e region o. austenite to
reduce the texture9 to thereby improve the plate thickness
directîon characteristics.
It is believed that rolling at low temperature
in the vicinity o~ the Ar3 point and the (y-~) region is
lndispensable for providing improvements in low temper~-
ture toughness (transition temperat.ure). We have found,however, that by effecting thorough grain refining of t~le
austenite grains at initial stages by the combination of
low temperature heating and fine TiN particles, it is
possible to obtain enough low temperature toughness
even i~ low temperature rolling is reducted to some
exten~.
Meanwhile the end of improving the toughness
of the weld zone has been attained according to the
- 6 -
3S
l invention by forming a steel of~ low Ti and N contents
into a strand by means of a continuous castin~ process
for effecting high cooling rate and subjecting the strand
to low temperature rolling at 950-lG50C. This is because
the continuous casting process gives a higher cooling
rate and enables a formation of large amount of f'ine
TiN grains (less than 0.05 ~) in the rolled slab when the
cast strand is rolled.
The reason why the thickness of the billet
has been set at a value less than and inclusive of
300 mm is that, if~ this level is exceeded the cooling
speed is reduced and sufflcient amount of flne grains
of' TiN are unobtainable. As f'or a cooling rate, it is
most desirable that the average cooling rate at the
temperature level f'rom immediately below the liquids
line of molken steel to 1100C be kept over 60C/min
in the center of a billet. However, even if fine grains
o~ TiN are obtained in large amount in the billet, it
would be impossible to obtain a large amount of flne
grains of ~iN in the rolled product if they are coarsened
in the course of heating and rolling steps, thereby
rnaking it impossible to obtain a f`ine structure at HAZ.
In view of the foregoing, the temperature at
which the billet is heated has been limited to the range
between 950 and 1050C. It has been ~ound that by
setting this limit it is possible to provide marked
improvements to the toughness of the HAZ as compared
with the t~oughness obtained by high temperature heating
1 ac~ording to the prior art. The upper limit of the
heating temperature range should be such that the fine
grains of TiN in the slab are prevented from being
coarsened by such heating and the lower limit thereof
is such that heating under the lower temperature limit
would not produce products acceptable for specifications
due to deterioration of the inner quality of the steel
caused by insufficient solutionizing o~ the slab in the
austenite region. Heating at a temperature over 950C
enables the inner quality of the steel to be thoroughly
imoroved because of ~ts S content having been reduced.
The fine grains of TiN which are not coarsened at the time
of heating help;refining of austenite grains at the time
of heating, recrystallized grains during rolling and of
the rolled structure as a whole~ thereby improving the
toughness o~ the base metal.
The rolling process according to the invention
will now be described. In order to obtain sufficient
strength and toughne~s~ it is essential to carry out
controlled rolling. rrO this end 7 the rolling conditions
have been set according to the invention, that is 3 rolling
reduction over 40% at a temperature below 850C~ and
the finishing temperature of rolling over Ar3 point plu~
10C but below 800C. The Ar3 point during rolling can
be empirically set forth by the following formula:
Ar3 point = 880 - 400 (C%) - 70 (Mn%) + 25 (Si%)
- 35 (Ni%) - 20 (Cu%) - 25 (Cr%) + 30 (mo%)
~3~3~
1 By carrying out rolling under the aforesaid conditions,
the steel can display greatly increased strength and
toughness. The reasons why the rolling conditions have
been limited as described hereinabove will now be
described.
When the rolling reduction is kept over 40% at
a temperatu~e below 850C, the grains of the steel are
markedly reduced in slze and the strength and koughness
o~ the steel can be greatiy increased. ~rnen the rolling
reduction is below 40%~ however, it is impossible to
obtain high strength and superb toughness. Meanwhile,
even if the rolling reduction is above 40% at a tempera-
ture below 850C, it is impossible to produce steel of
high strength and superb toughness if the finishing
temperature exceeds 800C, due to insu~ficient refining
of the grainsO By setting the finishing temperature at
a level below 800C, refining of the grains is enhanced~
thereby making it possible to increase both the strength
and toughness of steel or at least to increase its
stren~th without lowerin~ its toughness.
According to the present invention, rolling
within the ~errite-austenite region is not carried out.
~his is because i~ the lower limit o~ the finishing
temperature is below the Ar3 point~ separation occurs
in the impact ~racture, with the result that energy
ab~orption is reduced and the plate thickness direction
characteristics show deterioration (see the drawing)O
~hus the finishing temperature has been limited to the
3~
l range of over Ar3 point plus 10C and below 800C including
allowance. The desired finishing temperatxre for
achieving best propertles in the plate thickness direc-
tion lles in the range between 750 and 800Co
No particuler limitation is set to the cooling
subsequent to rolling, however~ the range of 0.2 and
10C/sec is preferred. Heating of the steel to a
temperature below the Acl ~ransformation ~oint ~or
the purpose of effecting dehydrogenation, for example~
does not impair the features of the invention.
The steel produced by the method accor~ing to
the invention offers a superb weld zone property of
base metal as compared wi~h steel produced by any prior
art methods and further has the characteristics equa] to
normallzed or quenched and tempered steel, so that the
steel produced ~y the method of the present invention is
applicable to any other practicable use ranging from
formation of line pipes for sour gas and for use in
regions of extreme coldness, pressure vessels~ marine
contruction, ship building industry, etc.
The reasons for limiting the components of the
steel according to the invention will be described. The
steel claimed in claim l of the application contains
0.01 - 0.15% C, not more than 0.6% Si, 0.8 - 2.0% Mn~
0.01 - o.08% Al~ not more khan 0.008% S~ o.oo8 - 0.025%
Ti~ and 0.001 - 0.007% N.
The lower limit 0.01% of C is a minimum
essential level for ensuring that the base metal and
-- 10 --
1 weld zone have satisfactory strength and that carbide
forming ele~ents, such as Nb and V, can satisfactorily
their effects. When ~he amount o~ C is too largeg
however, coarse grains of fainite or island-like
martensite might be rormed in large amounts in the base
metal and HAZ, which unfavorably affecting -toughness and
considerably reducing weldability. Thus the upper limit
is set at 0.15%.
Si which is inevitably contained in steel due
to its addition for deoxidizing purposes is not desirable
for improving weldability and increasing the toughness
of the HAZ. Thus the upper limit of Si is set at o.6%.
Steel can be deoxidized by Al alone, so that the content
of Si can pre~erably be main-tained not more than 0.2%.
Mn is an important element which lowers the
transformation point of the steel and enables the effects
of improving the quality of steel by controlled rolling
according to the invention, thereby enabling strength
and toughness to be simultaneously increased. When the
Mn content is less than 0.8~ the strength and toughness
of the steel are lowered, so that its lower limit is
set at 0.8%. However5 when Mn is too large in amount~
the hardenability of the steel increases and coarse
grains o~ bainite or islant-like martensite are formed in
large quantities, thereby reducing the toughness of both
the base metal an~ HAZ. Thus the upper limit is set at
2.0%.
Since Al is used as a deoxidizing agent~ it is
~ .
l inevitably contained in this type of killed s'ceel.
However, when it is less than 0.01% in amount, deoxidiz~
ing cannot be satisfactorily effected and the base metal
has its toughness reduced, so that the lower limit is
set at 0.01%. Meanwhile when Al exceeds o.o8%9 the
cleanliness of the steel and the HAZ toughness are
reduced, so that the upper limit is set at 0.08%.
The reason why S as an impurity is limited not
more than o.008% is that the anisotropy of the base metal
should be reduced and energy absorption should be increas
ed. In the method according to the invention, rolling
is carried out at a temperature below Ar3 point.
Howeverg the steel heated at low temperature wouid have
increased isotropy and energy absorption in Charpy impact
testing would be reduced as compared with ordinary cold
rolled material even when rolling is carried out at a
temperature above Ar3 point.
This is accounted for by the fact that, as set
forth hereinabove, the presence of MnS in the steel and
rolling of austenite ln the non-recrystallized region
form texture. Limitations are placed on the amount of
S in order to reduce the absolute amount of MnS. 3y
setting S at a level not more than o.oo8%, it is possible
to markedly increase the toughness of the steel. In
this case~ the lower the S content of steel is, the higher
is the troughness thereof. By setting the S content at
a level not more than 0.0015%9 it is possible to greatly
increase the toughness of the steel.
12 -
1 However, it would be impossible to entirely
eliminate MnS no matter how small the content of S in
the steel may be made. Thus the present invention
controlls the formation of textured structure by finishing
rolling at a temperature above Ar3 pointO
Ti and N are added for the purpose of increasing
the toughness of the HAZ by dispersing minute grains of
TiN in the steel, as set forth hereinabove. ~o this end 7
it is effective to have fine grains of ~iN distributed
as many as possible in the slab~ However, when the amounts
of Ti and N are too large~ ~iN could be coarsened while
the molten steel is being cooled and solidified even if
a continuous ca~ting process is used. Thus the upper
limits of Ti and N are set at 0.025% and 0.007%, respec-
tively. Meanwhile when the amounts of Ti and N are toosmall, no marked effects can be achieved in improving
the toughness of HAZ, so that the lower limits of Ti
and N are set at 0.008% and 0.001%, respectively.
The steel according to the invention comprises
P as an impurity. ~he amount of this element is usually
not more than 0.030~, and the lower the P content is,
the higher the toughness of the weld zone and the more
improved the weldability. ~he amount of this element is
preferably not more than 0.015~ to improve welding
characteristics. Oxygen content of the steel aceording
to the invention is not more than o.oo8%. In order to
- achieve improved cleanliness and toughness of the steel,
the amount of this element is preferably as small as
- 13 ~
33~i
1 possible.
In the invention as claimed in claim 2, the
steel as claimed ~urther conta~n at least one element
selected ~rom the group consisting of not more than
o.o8% Nb, not more than 0.10% V, not more than 2.0% Ni,
not more than 1.0% Cu, not more than 1.0% Cr and not more
than 0.4% MOg in addition to the elements contained in
the steel claimed in claim 1.
The reason why these elemen~s are additionally
corltained in the steel according to the invention
is that it is desired to increase strength3 toughness
and to expand a thickness range of a steel to be produced~
so that the amounts of the elements should be naturally
limited.
Nb is contained in the steel o~ the invention
to achieve both grain refining and precipitation hardening
o~ the rolled structure. It is an important element
for increasing both strength and toughness. However,
when the amount o~ Nb exceeds o.o8%. It has harmful effects
in weldability and in increasing the toughness of the HAZ.
~hus the upper limit is set at 0.08%.
V achleves substantially the same effects as Nb.
Its upper lim~t can be as hl~h as 0.10%.
Ni increases the strength and toughness of the
b~se m~tal without adversely affecting the hardenability
and toughness of the HAZ. However, when the amount
exceeds 2.0%, the hardenability and toughness of the HAZ
are ad~rersely affected, so that the upper limit is set
- 14 -
l a~ 2.0%.
Like Ni, Cu has the effect of increasing the
corrosion resistance of steelO However, when the amount
exceeds 1.0%, Cu cracks might initiate druing rolling
when rolling is carried out at low temperature heating
as done in the present invention, and making production
difficult. Thus ~he upper limit is set at 1.0%.
Cr increases the strengkh of the base metal
and weld zone and has the effect of preventing h~dro~en
induced cracking. However~ if the amount of this element
is too greatg the hardenability of the HAZ would be
increased and the toughness and weldability thereof
would be reduced. Thus the upper limit is set at 1.0~.
Mo is an element effective for inc~easing both strength
and tou~hness of the base metal, however~ excessive
amount of MOg like excessive Cr in the steel, liable
to excessively increase hardenability of the steel
which gives rise to lower toughness at ~AZ and weldability,
Accordingly~ upper limit of Mo is speci~ied as 0.4%.
The lower limits of these additive elements are
desired to be essential minimum value for achieving excel-
lent results in improving the quality of the steel. The
lower limits of Nb, V, Ni, Cu, Cr and Mo are 0.01%,
0.01%, 0~1%g 0.1%, 0.1% and 0.05%, respectively.
The steel and the production method caIimed
in claims 3 and 4 respectively contain 0.0005 - 0.005%
Ca in addition to the components of the steel as claimed
in claims l and 2, respectively.
- 15 -
`~`
l Ca is added for the purpose of increasing
energy absorption and improving the characteristics in
the plate thickness direction by effecting morphological
control of sulfides (MnS). The reason why the amount o~
Ca is limited to the range ~etween 0.0005 and 0.005%
is as follows. When it is less than 0.0005% 3 the addi-
tion of this element can achieve no practical e~ect;
when it exceeds 0.005%, it has harmful effects in
increasing the toughness of steel and its cleanliness
because of the production of large amounts of non~- -
metallic inclusion, such as Ca-O~S. AlSOg an increase
in the amount of this element gives rise to a problem
with regard to operability of welding principally in
carrying out C02 gas arc welding.
An embodiment of the invention will now be
described. Strands o~ a variety o~ chemical composition
were produced by an oxygen converter-continuous casting
processg and then they were rolled down as plates within
the thickness range of 18 - 35 mm under varying heating
and rolling conditions. The mechanical properties of
the base metal and the weld zone are shown in Tables l
and 2.
- 16 -
~`~``!
`"I
3~
Table l
. _ . . . . _ . _ .
Speci- Type Chemical Composition (~)
men of
No. Steel
C Sl Mn A1 S Ca Tl
__ __ ~ . _
1 A O .14 O . 20 1. 37 O . 022 O . o()4 _ O . 009
._ ____ _ _ ........... .. ... _ _ _ _
2 B ,. ,. " ,. ,. _ "
__ ___ __ __ _ _ _
_ 3 A 0 . 10 0 . 17 1 . 58 0 ~ TO 35 . 002 _ 0 . 015 _
4 B ~ ~ ~t II II = ~l
_ _ _ ~ ___._ . T _ _ _ _ _
A 0 . 04 0 . 24 1. 66 0 . 027 0 . 006 _ 0 . 022
~ . . . __ _ _ ~ _
6 B ~ .. n I n ~- _
____ ~ ._L_ T _ _ __ _
7 A 0 .13 0 . 25 1. 06 l O . 027 0 . 005 _ 0 . 017
__ . . . ,~ _ _ _
8 B " ,. " I ,. ,. _ ..
.__ _ _ . _ ~ _ .
9 A 0 . 08 0 . 28 1 . 46 0 . 015 0 . 001 0 . 0019 0 . 011
_ __ . . __ __ __ _ _ __ , _
B ,. ,. ., ,. ,. " "
. ~ - - - -- _ _ _ .. .__ ~ _
11 A 0 . 13 0 . 28 1 . 50 0 . 035 0 . 002 0 . 0016 0 . 014
_ _ .. ., . _
12 B ,. ,. ,. ,- ,. ,. ,.
_ .. ,. ._ ~ "",, ~_._ __ _ _
13 A 0 ~ o6 0 . o6 1. 05 0 . 033 0 . 003 0 . 0042 0 . 016
_ __ . , _ . . . _ _
14 B ,. ,. ,~ ,. ,. .l "
. . _ . . ~ .... . .... _ _
A 0 . 07 0 . 16 1 . 10 0 . 0 32 0 . 001 0 . 0017 0 . 013
1 6 B __ _ _~ ~ ~ ____
- Cont'd -
Note: A: Inventive Steel,
B: Steel ~or Comparison
- 17 ~
- Table 1 (Cont'd)
Production Conditions
_. ,_ ___ _ . _ . _
Slab Heating Rolling
Other Thick- Tempera- Reduction
N Elements Ar3 ness ture below 850C
(mm) (C) (%)
_ _ _
0.0026 _ 733 250 95 65
... . ...... __ __.__ _ _
ll ~ " ,. 1150 "
. . . ., ~ . ___ ~
o.oo48 _ 734 210 1000 70
.. ~ . _ _ _ _ _ _
,. _, " .. ,. ,.
_ . _. . . _ .. _
0.0065 Nl 0 32743 ,- 950 55
,- _ _ . _ . _
__ __ _
0.0046 Nb- 0 030 756 " 950 7o
_ ... ,.. ~ _ ..
,~ _" ~ " " 30
0.003~ V : 0.020 753 .. 1050 60
. .... ~ .. . _
" " " ,. " 30
. _ _
0.0042 Nb. 0.010 73 1000 50 _
ll ,. ,- ,. ,. "
_ ~ ., ._ _ .. _
0.0050 Cu 0 25 720 ,- 950 5o
_ _ _ _ . ~ .. ~ _
_ _ _ _ . ~ . ~, _ 1~50 _
0.0044 Mo~ 0;10 730 160 1000 65
_ .. . __ . . . .. __ __ . _
____ _ ~t ~I . ~ . .__.. . . __
- Cont~d r
18
~'
Table 1 (Cont 'd~
Finishing Plate
TemperaThickness Remarks
t ure
( C ) (n~n)
.. ..... _. ~ _. .
750 23
_
. . . _ _ _ ~__
760 30
... _ _ . . .. _.
695 "
_ ____._. .. ,
765 25
_.
~ 1~
........ . _~
775 35
. . . _ _ _ - - -~
~ ~1
_ _
Air Cooling for
I u~ LU 20 Min. at 600Co
_ .._. _
_. .
755 26
_ _
710 I~
~ ~ ~ __ ~
750 26
._ _
I~ 1 26
,, I
755 1 20
__ _ ~ _
700 1 20
- 19 -
1~
83~i
Table 2
~ . . _ .. ~__
Speci- Type Base Metal Characteristics
men of
No. Steel Yield Tensile 2vE-60C vTrS _
(kg/mm2) Strength (kg-m) (C)
_ _ _ ___ _
1 A 40,4 52.7 20.6 - 95
_ _ __ _~
2 B 36.5 50.4 11.4 - 60
. . . _ ~
3 A 33.7 5.5 30.1 - 95 _
4 B 45.3 55~6 6.3 -105
_ . . __
A 37.2 48.1 34.6 -120 _
B 35.8 47.8 22.5 -105 _
7 A 39.4 48.9 25.8 -115
_ . . _ _ _ _ , ~_ . . .__ _ _
8 B 34 . 2 45.8 20.4 - 75
__ ~ . .. _ ~ . . _ __
9 A 32.6 45.2 25.6-105
... ___ _ _ _ ~ , _ _
B 30.2 44.6 9.0 - 75
~ _ ~ . . _., _
11 A 43.2 56.o 33.3 -115
_ . . __ . . ~ _
12 48.6 57.3 6.5 -120 _
3 37.6 48.3 35.8 -160 _
14 B 34.7 47~6 30.7 130
__ . ~ _ _ ~ _ _
A 42.4 51.3 31.9 ~150
- .~ _ . . _,.~. _. . . ~_, _
16 B 48.9 53.4 10.4 -160
- Cont'd
;~ - 20 -
,
~i j
~able 2 (Cont'd)
Toughness of
Weld Zone
_ _ _ . . . __ . ___
Thickness 2vE-600C
Direc~ion (kg-m)
_~
_ 60 14.3
- 40 4~7
_
- 85 17.7
-
-~ 20 15.9
- 65 25.0
- 55 3.1
_ 65 14.6
__ . .. ..
- 65 ~3.5
- 95 16.3
.
- 60 802
~ . . . _ . _
-100 15.3
~ . _ _ __ _
~ 30 14.6
.. .. __ _ . .
-120 29.7
~ 95 lg.3
., ..., . _ .
-1~0 25.6
~ .. .~_ _ _ . .
O ~
-
3~i
l The slabs of steel pro~uced by the method accord-
ing to the invention have high ~oughness at a base metal
and at weld zone. However, steel slabs produced by a
method of the prior do not have satisfactory toughness
eigher at base metal or at a weld zone. Thus the steel
slabs of the prior art lack necessary balance that makes
them suitable for use as weld constructions.
The steel slabs presented for testing will now
be reviewed. Comparison of specimens l and 2 of the same
composltion shows that spechmen 2 is inferior in the
toughness at a base metal and at a weld zone because its
heating temperature is high.
Specimens 3 and 4 are of the same composition,
but specimen 4 is inferior in the toughness at a base
metal because o~ ~ts finishing temperature being low.
Particularly~ specimen 4 is markedly low in toughness in
the directlon o~ thickness.
Specimens 5 and 6 a.re substantially o~ the same
composition~ but specimen 6 is infericr to specimen 5 in
the toughness at the base metal and at the weld zone
because no Ti is added, in spite o~ the ~act that other
production conditions are similar.
Specimens 7 and 8 are of the same composition,
but specimen 8 is inferior to specimen 7 in the toughness
at the base metal due to low rolling reduction rate at a
temperatxre below 8500C.
Speclmens 9 and lO are of the same composition7
but specimen lO is inferior to specimen 9 due to low
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3~;
1 rolling reduction rate at a temperature below 850C.
Specimen 9 is su~vected to hea~ treatment after rolling
by heating at 600C ~or 20 minutes followed by air cooling.
Specimen 9 has good toughness at the base rnetal and at
the weld zone and this indicates that the characteristics
of the steel according to the invention are not impaired
by such heat treatment.
Specimens 11 and 12 are of the same composition
but specimen 12 is inferior to specimen 11 ln the tough-
ness of at the base metal due to its 10W finishing tempera~ture o~ rolling. Particularly its ~hickness direction
toughness is extremely low.
Specimens 13 and 14 are of the same cornposition,
but specimen 14 is inferior to specimen 13 in the toughness
at the base metal because of its high heating temperature.
Specimens 15 and 16 are o~ the same composition
but specimen 16 is inferior in -the toughness of a base
metal due to the low finishing temperatxre. Particularly
its thickness direction toughness is remarkably low.
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