Note: Descriptions are shown in the official language in which they were submitted.
~ETHOD FOR P~ODUCING S'r~EL BARS HAVING
I~ROV~D LO~ TEMPERA'I~RE TOUGH~ESS
AND STEEL :BA~S PRODUCED THEREBY
Backqround of the Invention
~ his invention relates to a method for producing steel
bars exhibitir.g high strength and improved toughness even at
ultra-low temperatures of -120 C or below. In particular,
the present invention relates to a method Eor producing
steel bars having such improved low temperature properties
and to steel bars produced by this method.
In recent years the demand for concrete reinforcing
steel bars to be used in low temperature environments (for
example, in the construction of concrete structures in cold
districts or the polar regions, concrete freezers, and tanks
for liquid gases such as LNG and LPG) has been steadily
increasing.
A 9%-L~i steel and a high manganese austenitic steel
have heretofore been developed as materials Eor producing
reinforcing steel bars for use in low temperatures such as
encountered in the above-mentioned applications of
rein~orced concre~es, but both have found only very limited
applications because of their high costs due to the high
content of expensive alloying elements.
In typical structures of reinforced concrete,
reinforcing steel bars as specified in ~IS G 3112 (those
having a yield str~ngth on the order of 42 43 Kgf/mm and
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manufactured by hot rolling at a finishing temperatu:re of
1000 - 900C fo:Llowing heating at 1100 - 1250C) are us~1.
These steel bars, however, are designed for use at or above
ambient temperatures and their mechanical properties,
particularly their toughness may become poor when they are
exposed to low temperatures as mentioned above, particularly
to ultra-low temperatures below -100 C.
Accordingly, many attempts have recently been directed
at developing steel bars which can exhibit the requisite
~ 10 high strength and high toughness even when exposed -to
; ultra-l.ow temperatures encountered in LPG tanks (-60 C or
lower) or liquid ethylene or LNG tanks (~10n C or lower).
These attempts, however, have not yet succeeded in obtaining
steel bars having satisfactory mechanical proper-ties at
ultra-low temperatures.
Objects of the Invention
As discussed above, it is expected that there will be
an increasing need for inexpensive steel bars which exhibit
- 20 improved strength and toughness at low temperatures.
Accordingly, it is an object of the present invention
to provide less expensive steel bars possessing high
strength and high toughness which are maintained at
satisfactory levels during use in ultra-low temperature
' environments of -120C or below without addition of large
amounts of expensive alloying elements, as well as a method
for their production.
~'
'
It is another object of the present invention to
provide steel bars and a method for kheir production, the
steel bars having improved lo~ temperature properties
including a fracture appearance transition temperature of
not higher than -120 C.
Other objects and advantages of the present invention
will be apparent from the following description and claims.
Th~ inventors of the present invention perormed
numerous investigations in order to achieve the above
objects and as a result made the following discoveries.
ta) When a low carbon steel comprising a controlled
amount of carbon in the range of from 0.02% to 0.10% by
weight (throughout the specification, all percen~ages
concerning the chemical composition of steel indicate weight
percentages) as well as Mn, Mo, and Nb added in specific
amount~ is subjected to hot rolling ~7ith a lower heating
temperature and a lower finishing temperature and then to
forced cooling at a cooling rate higher than 3C/sec, the
as-rolled steel has a structure of finely dispersed ferritic
and bainitic phases which has an average grain size of not
greater than 5.0 ~m and preferably contains from 30% to 70%
by volume of bainitic phases finely dispersed in ferritic
, phases. The fine bainitic phases have markedly beneficial
,J effects on enhancement of the strength of steels so that the
hot rolled steel has significantly improved strength, i.e.,
` a yield strength of greater than 40 Kgf/mm2. In addition,
since the grains are very ;re, the hot rolled steel also
'
~6~6
has Siglli ficantly irnproved low temperature toughness.
(b) When a low carbon steel comprising a controlled
amount of carbon in the range of from 0.02% to 0.10% by
weight as well as Mn, Mo, and Nb added in specific amounts
is sub~ected to hot rolling using an oval, round-type
caliber arrangement with a lower he~ting temperature and a
lower finishing temperature and then allowed air cooling,
the as-rolled steel has a structure of a fine ferritic phase
which has an average grain size of not greater than 5~0 ~m
and the formation of a texture is suppressed resulting in
steel free from anisotropy in its mechanial properties.
Forced cooling instead of the air cooling results in a
further improvement in strength~
(c) Tempering of the thus obtained steel at a specific
temperature is effective in improving its yield strength
sufficiently to bring about an increase in strength on the
order of 5 - 10 Kgf/mm and to ~urther improve its low
temperature toughness~
This is believed by reason that the mobile dislocations
which are present in the bainitic phases of the as-rolled
steels are fixed with dissolved C or N or precipitates
during tempering.
td) Thus, steel bars having excellent low temperature
properties which could not be obtained in the prior art can
be produced less expensively by hot rolling of a steel with
strict control of its chemical composition and the hot
rolling conditions, optionally followed by tempering at a
~z~
specific temperature.
Summary of the_ nventio_
On the basis of the above Eindings, the present
invention has been accomplished which provides a method for
producing steel bars having markedly improved low
temperature properties characterized by a fracture
appearance transi-tion temperature of not higher than -120C.
In one aspect, the method according to the present
invention comprises:
heating a bloom or billet of steel to a temperature
sufficient to effect the subsequent hot rolling but not
exceeding 1000 C, the steel consisting essentially oE the
following composition on a weight basis:
C: 0.02 - 0.10%, Si: not greater than 0.5%,
Mn: 1.10 - 2.50~, Mo: 0.15 - 0.50%,
Nb. 0.010 - 0.100%, Al: 0.010 - 0.100%,
` and optionally one or more element,s selected from
., Cu: 0.05 - 0.30%, Ni: 0.05 - 1.20~,
. 20 Cr: 0.05 - 1.20~, Ti: 0.01 - 0.05%, and
' B: 0.0005 - 0.0030%,
with the balance being iron and incidental impurities;
. hot-rolling the heated bloom or billet into a bar under such
conditions that the finishing temperature is not higher than
850 C with the total reduction in the temperature range of
from 880C to the finishing temperature being at least 60%;
and forced cooling the hot-rolled bar at a cooling rate of
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3 C/sec or hic~her to room temperature.
In another aspect, the method of the present invention
comprises:
heating a bloom or billet of steel to a temperature
sufficient to effect the subsequent hot rolling but not
exceeding 1000C, the steel consisting essentially of the
following composition on a weight basis:
C: 0.02 - 0.10%, Si: not greater than 0.5%,
Mn: 1.10 - 2.50%, Mo: 0~15 - 0.50%,
Nb: 0.010 - 0.100%, Al: 0.010 - 0.100~,
and optionally one or more elements selected from
Cu: 0.05 - 0.30~, Ni: 0.05 - 1.20~,
Cr: 0.05 - 1.20%, Ti: 0.01 - 0.05%, and
B: 0.0005 - 0.0030~,
with the balance being iron and incidental impurities;
hot-rolling the heated bloom or billet into a bar under such
conditions that the finishing temperature is 850 - 750 C
with the total reduction duriny hot rolling being at least
60%, and the reduction per pass with an oval, round~type
caliber arrangement being 10~ or more equally for each of
the last 2n passes, wherein "n" is an integer; and
cooling the hot-rolled bar at a cooling rate of air cooliny
or higher to room temperature.
In a preferred embodiment of the invention, the
resulting cooled steel bar is further subjected to tempering
in the temperature range of from 500 C to 700 C.
In another preferred embodiment, the content of at
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~ -6-
~''
least one of P and S which are present in the steel as
incidental impurities is controlled as follows:
P: less than 0.010%, S: less than 0.010~
The thus produced steel bars which have bainitic phases
finely dispersed in the ferritic phases or a fine ferritic
phase and which have a grain size of 5 ~m or less and
preferably 2 - 4 ~m exhibit a yield strength of at least 40
Kgf/mm2, a value of -120C or lower for vTrs, and a value
close to 3Q Kgf/mm for vE 120~
Detailed Description of the Invention
The reasons why the chemi~al composition of the steel
and the conditions for ho~ rolling and heat treatment
according to the present invention are defined as above will
now be described in detail.
A. Chemical Composition of Steel
, a) Carbon (C):
Carbon should be present in order to impart the
requisite strength to steel bars. The presence of less than
0.02~ C is not sufficient to obtain the desired strength,
while the addltion of C in an amount exceeding D.10~ may
cause pearlite phases to form and intersperse in the
structure of the steel bar, which leads to deterioration in
toughness. Thus, the C content is defined as being between
0.02~ and 0.10%, preferably between 0.04 - 0.08~ in
accordance with the present invention~
~ b) Silicon (Si):
; -7-
.. . ..
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2~
Silicon is an effective deoxidiziny eleme~t and usually
added in an amount of from 0.15% to 0.35~. However, the
addition of Si is not always necessary in those cases where
A1 is added in an amount sufficient to effect deoxidation.
Moreover, the presence of more than 0.5% Si may adversely
affect the hot working properties of the steel. Therefore,
the upper limit of the Si content when it is added is
defined as being 0.5%. Preferably, the Si conten-t is 0.20%
to 0.30.
c) Manganese (Mn):
Manganese is necessary for desulfurization of steels.
It is dissolved in the steel matrix as a solid solution,
which serves not only to increase the strength of the steel
but to impart the requisite hardenability to the steel. At
least 1.10% Mn should be present in the steel in order to
provide the steel with satisfactory strength and low
temperature properties through the formation of finely
dispersed ferri-tic and bainitic phases or a fine Eerritic
~`phase under the hot rolllng conditions employed in the
present invention. However, the addition of more than 2.50%
Mn may cause significant segregation resulting in
deterioration in the toughness and weldabili-ty of the steel.
Accordingly, the Mn content is defined herein as being
between l.lO~ and 2.50%, preferably between 1.80% and 2.00%.
d) Molybdenum (Mo):
-Molybdenum is quite effective for enhancing the
strength of steels without a loss of their toughness. In
:
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addition, according to the method of the present invention,
Mo is essential to control the hardenability and/or
transformation behavior oL the steel and develop the desired
structure of finely dispersed ferritic and bainitic phases
or a Eine ferri-tic phase in the as-rolled steel. These
effects of Mo cannot be adequately achieved when the Mo
content is less than 0.15%, but they are sa-turated and no
additional benefits are obtained when Mo is present in
excess of 0.50%. Therefore, according to the method of the
present invenkion, Mo is added in an amount of from 0.15% to
0.50~, preferably 0.30% to 0.40~.
e) Niobium (Nb):
~ iobium is essential to develop the structure of finely
dispersed ferritic and bainitic phases or of a fine ferritic
phase which has been found critical for the purpose of this
invention. With less than 0.010% Nb, it is difficult to
prevent the austenitic grains from coarsening during heating
of the steel bloom or billet tat a temperature of not higher
~ than I000C) prior to hot rolling, which ultimately makes it
; 20 impossible to steadily produce the desired structure of
finely dispersed ferritic and bainitic phases or of a fi~e
ferritic phase. This effect of Nb on inhibition of
coarsening of austenitic grains reaches a limit when the Nb
' content is 0.100%, and the addition of an excess amount of
-~ Nb adds to the cost of the steel. Thus, the Nb content is
defined herein as being between 0.010% and 0.100%,
preferably between 0.03~ and 0.07%.
''`
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,
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2~
~) Al~minum (Al):
Alumunurn is effective not only for deoxidation oE
steels but also has an effect like Nb as mentioned above on
prevention of the austenitic grains Erom coarsening during
heating prior to hot rolling. These ef~ects cannot be
obtained when the A1 content is less than 0.010%. The
addition of more than 0.100% Al, however, may cause a
deterioratlon in hot workability. Therefore, the steel
employed according to the me-thod of -the present invention
should contain from 0.010% to 0.100% Al, preferably from
0.020~ to 0.060~ Al. The Al content may also be from 0.010%
to 0.050~.
In cases where the deoxidation is carried out using
aluminum, not silicon, the aluminum content is defined as
0.050 - 0~100~.
` The steel to be treated by the method of the present
invention may contain at least one of Cu, ~i, Cr, Ti, and ~,
these elements being ef~ective to improve the strength of
the resulting steel.
The amounts and eEfects of these optional elements will
be detailed hereinafter.
g) Copper (Cu):
Copper is effective for increasing the strength of a
steel without an appreciable adverse influence on its
toughness. According to the present invention, therefore,
Cu may optionally be added when it is desired -that the steel
have ~urther improved strength. For this purpose at least
-10-
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0.05~ Cu should be added to achieve satisfactory results,
while the addition of more than 0.30% Cu may cause a
deterioration in the hot workability of the steel.
Therefore, the Cu content ~hen it is added is defined herein
as being from 0.05~ to 0.30%, preferab]y 0.15% to 0.25%.
h) Nickel (Ni):
Since nickel is effective in improving the low
temperature toughness of a steel particularly when added in
an amount of at least 0.05%, the steel composition employed
according to the present invention may optionally contain
0.05% or more of Ni, preferably 0.50% or more Ni. The Ni
; content, however, should not exceed 1.20% because the
addition of more than 1.20% Ni adds to the cost of the steel
,~ and tends to increase the susceptibility of the steel to
flaking and other defects caused by the presence of hydrogen
during manufacturing of the steel.
~ i) Chromium tCr):
;', When it is desired that -the steel have further
increased strength, chromium may optionally be added because
Of its effectiveness in increasiny the strength of steels.
When added, Cr should be presen-t in the steel in an amount
ranging from 0.05% to 1.20% since the addition of less than
, 0.05% Cr is not sufficient to exert the desired effect and
;~ the addition of more than 1.20% Cr may cause the steel to
','7, have deteriorated cold workability. A preferred Cr content
. ,
ls from 0.30% to 0.80%.
j) Titanium (Ti)~
~' .
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~ .ike ~b and Al, titaniurn serves to refine -the
austenitic grains and is effective in producing a structure
of finely dispersed ferritic and bainitic phases or a fine
ferritic phase. Therefore, Ti may optionally be added to
the steel composition. The effect of Ti, however, canno-t be
obtained with less than 0.01% Ti, while the addition of more
than 0.05~6 Ti may cause coarsening of -the titanium
carbonitride particles in the steel and increase the number
of these particles, resulting in a deterioraion in hot
].0 workability. Therefore, the Ti content when it is added is
defined as being from 0.01% to 0.05%, preferably from 0.015%
to 0.030%.
k) Boron (s):
The addition of boron in small amounts serves to
improve the hardenability of steels so that B rnay be added
when i-t is desired that the steel have further enhanced
strength. The desired effect of B is not attainable with
less than 0.0005% B, while the addition of more than 0.0030%
B may bring about a deterioration in the hot workability of
steels. Therefore, when added, B should be present in an
amount of from 0.0005% to 0.0030%, preferably from 0.0005%
to 0.0020%.
It is well known that in a quenched and tempered steel,
the toughness of the tempered martensitic phases can be
improved by lowering its P and S content. According to the
present invention, however, it has been found that even in
finely dispersed ferritic and bainitic phases, not tempered
-12-
martensit:ic phases, lowering of at least one oE the P and S
contents to less than 0.010% brings abou-t a significant
increase in low temperature toughness.
Therefore, according to the present invention, the
content of P and S as incidental impurities is preferably
controlled such that at least one of the P and S contents
meets the following requirements:
P: less than 0.010%, and S: less than 0.010%.
Of course, as long as either the P or S content is less
than 0.010%, the resulting hot rolled steel bar exhibits the
desired further improvement in low temperature toughness
even if the other is present at a level found in steels
produced in a conventional manner.
:;
Bo Conditions for Hot ~olling and Heat Treatment:
a) Heating temperature prior to hot rolling:
It has been found that if the bloom or billet is heated
to a temperature exceeding 1000C prior to hot rolling,
coarsening of the austenitic grains in the steel may occur
¦during the heating even -thouyh the steel has a cornposition
~20 as defined accorcliny to the present invention, as a result
:;
of which it is not possible to produce the as-rolled
structure of finely dispersed ferritic and bainitic phases
or a fine ferritic phase and thereby achieve the desired
improvement in low temperature toughness. Therefore~ the
temperature at which the bloom or billet is heated prior to
hot rolling, i.e., the initial heating temperature, should
` not be higher than 1000 C. Lower temperatures rnay be
, . .
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selected without loss of low temperature properties of the
hot rolled steel bars. However, if the initial heating
temperatuxe is too low, the load applied to the rolls in the
hot rolling step will increase to such an extent that the
efficiency of the hot rolling deteriorates significantly.
Thus, in general~ it is preferred that the bloom or billet
be heated to a temperature ranging from approximately 900C
to 1000C, and more advantageously from 900C to 950C.
b~ Pass Schedule:
According to one of the preferred embodiments of the
present invention, a pass schedule for hot rolling is an
important factor. Precise contxol of the pass schedule can
achieve a markedly improved low temperature toughness, i.e.
properties to resist brittle failures at an ultra-low
temperature of -1~0C, which is not attainable by the
conventional hot rolling of plates. Thus, according to this
embodiment of the present invention the pass schedule is
defined as follows:
~i) The total reduction in thickness during hot rolling
; 20 is restricted to not lower than 60~.
(ii) The reduction in thickness per pass has a constant
value of 10% or more for each of the last 2n passes, wherein
"n" is an integer.
; The hot rolling for the last 2n passes is carried out
using an oval, round-type caliber arrangement.
It is necessary to obtain a fine ferritic structure
having an average grain Si;e of 5 ~m or less in order to
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achieve improved toughness at low temperatures. For this
purpose it is required to define the total reduction in
thickness during hot rolling as 60~ or more and the
reduction for each of the last 2n passes ("n" is an integer)
each having the sarne value of 10~ or more.
In addition, in order to prevent deterioration in low
temperature toughness caused by the formation of texture, it
is advisable to carry out the hot rolling using an oval,
round-type caliber arrangement for each of the last 2n
passes, wherein "n" is an integer. The reduction is also
defined as having the same value for each of these 2n
passes.
The reasons why the hot rolling for each of the last 2n
passes ("n" stands for an integer) is carried out using
oval, and round calibers are that, first, hot rolling
through an even number of passes may successfully prevent
the formation of texture which is sometimes formed by an
uneven thickness in the rolling directions, thereby
preventing a deterloration in toughness. Secondly, even if
the last 2n passes are carried out using oval, and round
calibers, a texture will form increasingly to cause a marked
decrease in toughness, when the reduction in thickness for
each pass is not equal. This is because the hot rolling is
carried out substantially in one direction just like plate
rolling. Such texture as in the above, the formation of
which impairs toughness, is the one in which a crystal
direction <011> conforms to the rolling direction and a
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,~
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crystal plane {lOO}conforms to the plane vertical to the
final reduction direction.
According to the findings oE the inventors of the
present invention, de~elopment of such a texture will
deteriorate the toughness of the s-teel material and the
inventors found a pass schedule which can prevent th
formation of the texture.
In this specification, the term "oval, round-type
caliber arrangement" means a sequence of calibers in which
an oval caliber and 2 round one are positioned
al-ternatively. An oval, round-type caliber arrangemen-t is
well known in the art.
c) Hot rolling temperature and amount of deformation:
In order to provide the steel with a predetermined
level of strength and toughness, it is necessary to subject
the steel to repeated deformation and recrystallization
caused by the reduction incurred in hot rolling,
particularly in the temperature range below 880 C so as to
refine the austenitic grains.
It has been found that the desired refining of
austenitic grains cannot be attained when the total
reduction in thickness in the temperature range below 88Q C
is less than 60~ under usual conditions. Thus, according to
the method of the present invention r it is desirable that
the hot rolling be conducted under such conditions that the
~r~ total reduction in the temperature range between 880 C and
the finishing temperature is at least 60~l and preferably
-16-
90% or more.
When the hot rolling is carried out following the pass
schedule mentioned above, the total reduction may be defined
as that measured during hot rolling.
The upper limit of the amount of deformation is not
critical and can be selected appropriately depending on
various factors including the capacity of the hot roll, the
size of the bloom or billet, and the size of the final
product, although the higher the upper limit the better.
d) Finishing temperature:
It has been found that if the finish ro]ling is
conducted at a temperature higher than 850C, then the
desired fine grain structure cannot be developed and the
steel does not have the desired good toughness. According
` to the method of the present invention, therefore, the hot
rolling should be conducted with a finishing temperature of
850C or below.
When the finishlng temperature is too Low, however, a
steel having a chemical composition as defined herein will
be hot rolled under such conditions that the austenitic
phases do not undergo recrystallization, thereby producing
anisotropy in mechanical properties due to the growth of the
texture. For this reason, the finishing temperature
preferably ranges from 850C to 750C, and more
advantageously from 825C to 775 C. In addition, then the
finishing temperature is lower than 750C, the steel turns
to that or an austenitic and ferritic dual phase structure
: .
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,
in which the fe~ritic one results in a deterioration in
toughness upon being rolled.
e) Cooling conditions:
According to one of the preferred embodiments of the
present invention the steel is cooled at a cooling rate of
3 C/sec or higher following the hot rolling~ Forced cooling
at a rate of 3C/sec or higher results in the formation of
finely divided grains of 5.0 ~m or less in diameter. A
volumeric proportion of bainite falls within 30 - 70%,
resulting in an improvement in strength and toughness.
Such forced cooling may be carried out by means of
forced air cooling, mist cooling, or water cooling, which
axe applied immediately after the hot rolling. The
temperature at which the forced cooling is stopped is not
defined in the present invention. ~owever~ it is desirable
to carry out the forced cooling in the temperature range of
350C to room temperature.
When the cooling rate is lower than 3C/sec, the
obtained structure becomes coarse with an average grain
diameter of 5.0 ~m or more, resulting in a deterioration in
low temperature toughness.
When the pass schedule of the hot rolling is controlled
:
as in the above, the air cooling may be obtained~
f) Tempering temperature:
As previously mentioned, a steel bar having a
composition as defined herein and produced by hot rolling
under the conditions according to the method of the present
.,
,:
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2~i
invention has a structure oE finely dispersed ferritic and
bainitic phases or of a fine ferritic phase even in the
as-rolled condition. If necessary, however, the as-rolled
I steel bar may be subjected to tempering in the temperature
range of from 500C to 700C to increase the yield strength
and also to further improve the toughness o the steel bar.
When the hot rolled steel bar is tempered, the
tempering temperature should be in the range of Erom 500 C
to 700C as mentioned above. At a tempering temperature
lower than 500C the favorable results cannot be fully
achieved, while at a tempering temperature exceeding 700C
recrystallization o the ferritic and bainitic phases or a
, fine ferritic phase may occur resul-ting in destruction of
the finely dispersed phase(s), which in turn results in a
, deterioration in toughness. A preferred range of the
tempering temperature is from 575 C to 625 C.
,:
, Exampl_s
The invention will be further illustrated by the
' 20 followlng non-limiting examples. It should be understood
tha-t they are given only for the purpose of illustration,
and modification of these examples can be made without
departing from the scope of the invention.
,
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.,
' Example 1
:,
Various molten steels having the chemical compositions
shown in Table 1 were prepared by a conventional melting
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method and cast into blooms each having a square cross
section measuring 160 mm on each side. Each bloom was then
heated to 950C and then su~jected to hot rolling into a
round bar 25 ~n in diameter under such conditions that the
total reduction in the temperature range below a80c was 90
with finish rolling at a temperature of 800C.
Af-ter the Einish rolling, the resulting round bar was
force cooled at a cooling rate o~ 10C/sec to room
temperature.
The as-rolled round bars obtained were subjected to
microscopic examination, tensile tests, and impact tests.
In the .nicroscopic examination, ~he microstructure of
each as-rolled specimen was observed microscopically -to
distinguish ferritic, bainitic, and pearlitic phases and to
determine the grain sizes.
The tensile tests were carried out with JIS No. ~ test
specimens each having a gauge portion 14 mm in diameter made
by machining the as-rolled bars. The specimens were tes-ted
to determine the yield strength at 0.5~ total eLongation,
tensile strength, elongation (calculated with a gage length
of 50 mm), and reduction in area.
The impact tests were conducted with JIS No. 4 Charpy
specimens each having a 2-mm V-notch whereby the low
temperature toughness o~ each steel bar was evaluated by
determining the absorbed energy at -l2ooc (vE 120) and the
fracture appearance transition temperature (the temperature
at which transition from ductile to brittle ~ractures
-20-
occurs) (vTrs).
The results are summarized in Table 2 below,
As is apparent from the data shown in Table 2, Steels
through P, the compositions of which fell within the range
of the present invention and which were processed in
accordance with the method of the present invention had a
cor~ined microstructure of fine ferrite ~ bainite, the
average grain diameter of which was 5.0 ~m or less, and
exhibited a yield strength of ~0 kgf/rnm2, and vE-120 of
abou~ 30 kgf-m. Thus, the strength and toughness were
advantageously improved by the present invention. In
addition, the value of vTrs was lower than -120C for each
steel and tough failure did not occur even at a temperature
of -120 C.
. In contrast, Steels Q through W, the steel compositions
of which fell outside the range of the present invention,
,~ although they were treated in accordance with the method of
the present invention, exhibited.a low value for vE-120 and
a value of vTxs higher than -120 C. Tough failure occuxred
at a temperature of -120C. These steels exhibited
unsatisfactory toughness properties. In addit.ion, some of
q, these steels exhibited a yield strength of lower than 40
i "
kgf/mm~ and the strength properties thereof were not
satisfactory.
. Example 2
i
~ Following the procedure described in Example 1, steel
, .
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blooms of Steel A of Example 1 having a square cross section
measuring 160 mm on a side were prepared and used to produce
round bars 25 mm in diameter under various hot rolling
conditions.
After the finish rolling, the resulting round bars were
forced cooled with air at a cooling rate of 10C/sec to room
temperature.
The thus obtained as-rolled steel bars were tested for
; microstructure~ tensile properties, and low temperature
toughness in the same way as described in Example 1. The
results of the tests are summarized in Table 3.
As can be seen from Table 3, when the ho-t rolling was
conducted under conditions outside the range defined herein,
even -the use of a steel having a composition according to
~ the present invention and cooled after hot rolling according
: to the presen-t invention produced a steel bar with
insufficient streng-th and/or toughness, and the target
values of 40.0 Kgf/mm or more in yield streng-th and -120 C
or lower in vTrs were not achieved.
Example 3
Steel blooms of Steel A shown in Table 1 having a
square cross section of 160 mm on a side were cooled after
hot rolling under the following conditions to determine the
influence of the cooling rate:
Initial heating temperature of bloom: 950 C
Total reduction below 880 C: 90%
'' '
-22-
-
:
Yinishing temperature: 800C
Ro~d bars 25 mm in diameter were thus obtained, and
these were then subjected to forced cooling at a cooling
rate ranging from air cooling (0.8C~sec) to water cooling
(100C/sec).
The resulting round bars were tested for
microstructure, strength, and toughness in the same way as
' described in Example 1.
As can be seen from the results of these tests as shown
in Table 4, the cooling rate has a great influence on the
low temperature toughness. The absorbed impact energy at a
i temperature of -120C is about 30 kgf-m at a cooling rate of
3 C/sec or higher. However, when the cooling rate is lower
than 3C/sec, the resulting structure comprises crystal
grains having an average diameter of over 5.0 ~m and the
absorbed impact energy at -120C will remarkably decrease
resulting in tough failure at a temperature of -120C.
Example 4
Following the procedure described in Example 1, steel
blooms of Steel A and Steel L of Example 1 each having a
square cross section measuring 160 mm on a side were
prepared and used to produce round bars 25 mm in diameter.
After the finish rolling, the resulting round bars were
forced to cool at a cooling rate of 10C/sec to room
temperature.
As shown in Table 5, the resulting round steel bars
-23-
(
.,;
$~
were then subjected to temperiny at ~80 - 720C for one hour
and were chen air-cooled.
The thus obtained steel bars were tested Eor
microstructure, tensile properties, and low temperature
toughness in the same way as described in Example 1.
^ As can be seen from the results of these tests shown in
Table 5, when the tempering was conducted at 480C, the
resulting steel bars had a yield strength and vTrs
substantially on the same level as that of -the as-rolled
bar, exhibiting no effect of tempering.
In contrast, when the tempering was carried out at 500
- 700C, not only was the yield strength markedly improved,
but also vTrs greatly decreased. Thus, it is apparent that
the process of the present invention can markedly improve
strength as well as toughness of the objective round steel
bar. On the other hand, when the temepring was carried out
at a temperature higher than 700 C, the microstructure grew
coarse, and not only did the strength decrease, but also the
toughness deteriorated.
E~ample 5
Various molten steels having the chemical compositions
shown in Table 6 as Steels 1 - 38 were prepared by a
conventional melting method and cast into blooms each having
a square cross section measuring 160 mm on each side. Each
bloom was then heated to 950C and then subjected to hot
i rolling into a round bar 25 mm in diameter under such
-24-
,~"
.
,....
.,
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cc)nditions that the total reduction was 98% with finish
rolling at a temperature of 800C. The hot rolling
comprisPd 16 passes, and for the passes after the 6th pass
from the finishing pass, an oval, round-type caliber
arrangement was employed with an equal reduction of 10% in
thickness for each pass.
After the finish rolling, the resulting round bar was
allowed to cool in the air to room temperature.
The as-rolled round bars obtained were subjected to
microscopic examination, tensile tests, and impact tests.
In the microscopic examination, the mucrostructure of
each as-rolled specimen was observed microscopically to
determine the grain size of a ferritic phase.
The tensile tests and impact tests were carried out in
the same manner as described in Example 1.
In determining the crystal structure of a texture, thin
film specimens were prepared from a portion lying in a
direction parallel to the section which was cut
perpendicularly with respect to the rolling direction, and a
whole pole figure was prepared using not only Shultz's
reflection method but also Decker's permeability method with
CoRa rays for the same specimens.
As is apparent from the results shown in Table 7, all
the steel bars having a chemical composition as defined in
the present invention (Steel Nos~ 1 - 27) which were
produced under the conditions according to the method of the
present invention had significantly improved strength and
-25-
toughness. In each of these steel bars, the microstructureshowed a f inely grained ferritic phase having a grain size
of 5 ~m or less, the yield strength was at least 40 Kgf/mm2,
and the value of vE 120 was close to 30 kgf-m. In addition,
each of these steel bars showed a value of vTrs lower than
-120C, which clearly indicates that these bars did not
undergo brittle fracture even at a temperature of -120C.
It is noted that a texture structure which would impair
toughness was not formed.
In contrast, steel bars which were produced under the
hot rolling conditions defined herein but which had a
chemical composition outside the defined range ~Steel Nos.
28 - 38) showed lower values of vE_120 and their values of
vTrs were all higher than -120~C, indicating that they had
poor toughness and would undergo brittle fracture at ~120Cr
Also it can be seen that these comparative steel bars did
not always show satisfactory strength because some of them
had a yield strength of less than 40 Kgf/mm2.
Bxample 6
In this example, the heating temperature, total
reduction in thickness, and finishing temperature were
vari~d to determine their effects on mechanical properties
including toughness.
After machining steel blooms of Steel No.l of Table 6
, to produce specimens with the indicted cross-sectional
dimensions, the thus obtained specimens were subjected to
-26-
,
hot rolliny with a total reduction of 57 9~. The heating
temperature and finishing temperature were also varied. The
specimens were finished into steel bars 25 mm in diameter.
The hot rolling was carried out under condi-tions such that
for passes after the 6th pass from the Einishing pass an
oval, round-type caliber was used with an equal reduction
for each pass of 10%. After the hot rolling, the resulting
steel bars were allowed to air-cool to room temperature.
The microstructure, strength, toughness, and fixture of
the resulting steel bars were examined in the same manner as
described in Example 1.
The results thereof are summarized in Table 8.
As can be seen from Table 8, when the hot rolling was
conducted under conditions outside the range defined herein
regarding any one of the heating temperature, total
reduction, and finishing temperature, even the use of a
steel having a composition accordiny to the present
invention gave a steel bar with insuf~icien-t touyhness, and
the target values of -120 C or lower in vTrs were not
1.
achieved.
Example 7
This example is presented to show the effect of the
pass schedule of the present invention.
Example 1 was repeated except for the pass schedule.
~n this example, the number of passes in which the oval,
round-type caliber arrangement was used and the reduction in
-27-
, .
'
'
2~
thickness were varied to evaluate the criticality of the
pass schedule defined in the present invention, as shown in
Table 9.
The obtained, as-rolled steel bars were tested for
microstructure, tensile properties, low temperature
toughness, and texture in the same way as described in
Example 1 and the results are summarized in Table 10.
As can be seen from Table 10, when the hot rolling was
conducted under conditions outside the range defined herein
regarding pass schedule, even the use of a steel having a
composition according to the present invention gave a steel
bar with markedly insufficient toughness.
Namely, the low temperature toughness will deteriorate
unless an oval, round caliber is used for each of the last
2n stands with an equal reduction in thickness of 10~ or
more for each of these stands. When the oval, round-type
caliber arrangement was used for an odd number of stands, or
when the reduction in thickness was not equal for all these
stands, the formation of a textured structure and
deterioration in toughness were marked.
Example 8
This example is presented to evaluate the effect of the
cooling rate after hot rolling.
Steel blooms of Steel No~ 1 shown in Table 6 each
having a square cross section of 160 mm on a side were hot
rolled through 16 passes under the following conditions:
-28-
.
~6~
Initial heating temperature: 950C
Total reduction during hot rolling: 90%
Finishing temperat.ure: 800C
to give round bars 25 mm in di~meter, which immediately
after passing through the finishing stand were subjected to
cooling in a variety of manners.
The pass schedule was such that for the last 6 passes,
an oval, round-type caliber arrangement was used with a
reduction in thickness of 10% for each of these six passes.
The cooling was carried out using either of the
following three types of cooling:
Air-Cooling ( cooling rate is about 0.8 C/sec)
Mist-Cooling (cooling rate is about 3.0 C/sec)
Water-Cooling (cooling ra~e is about 10-100C/sec)
The water cooling was applied while adjusting the flow
rate as well as its pressure to change the cooling rate from
10C/sec to 100C/sec.
; The resulting round steel bars were tested for
microstructure, strength, toughness and texture. The
results thereof are shown in Table 11.
As is apparent from the Table 11, according to th~
present invention satsifactory low temperature toughness can
be obtained even when the cooling after hot rolling is air
cooling. No deterioration in low temperature toughness is
observed even for a higher cooling rate. Thus, it is noted
that it i5 advantageous to change the type of cooling from
air cooling to mist cooling or to water cooling when it is
., .
-29-
,
,~
.
2~
desired to increase strength while maintaininy low
temperature toughness.
Example 9
This example is presented to evaluate the effect of the
tempering temperature of the present invention.
Example 5 was repeated for Steels Nos. 1, 12, and 24 of
Table 6. Steel blooms of these steel each having a square
cross section of 160 mm on a side were subjected to hot
rolling as shown ln Example 1 to give round bars 25 mm in
diameter and then were air-cooled.
The resulting bars were subjected to tempering
including heating at 480 - 720C for one hour and ~ir
cooling to room temperature. The tempering conditions are
, shown in Table 12.
The resulting round bars were tested for
microstructure, strength, and toughness in the same way as
described in Example 1. The results are shown in Table 12.
As can be seen from the Table 12, at a tempering
temperature of ~80C, the tempered s~eel bars did not show
appreciable changes in yield strength and vTrs as compared
with the as-rolled steel bars 50 that the tempering did not
exert its effect.
In contrast, at tempering temperatures ranging from
t, 500C to 700C, the tempered steel bars had signifioantly
increased yield strength as well as greatly lo~er values of
vTrs. Thus, the heat treatment according to the method of
l~ -30-
:
,
,,,
the present invention is clearLy effective in significantly
improving both the strength and the toughness of the
as-roll.ed steel bars.
When the tempering was conducted at a temperature
e~ceeding 700C, however, the microstruc-ture of the steel
coarsened during the tempering, which caused the tempered
steel bars to have decreased strength and deteriorated
toughness.
As discussed above, according to the method of the
present invention, steel bars having high strength and high
toughness which are maintained at satisfactory levels even
in ultra-low temperatures of -120C or below can be provided
at a low cost b~ controlling only the chemical composition
of the steel and the hot rolling conditions without the need
for the addition of expensive alloying elements in large
proportions or the use of complicated operations. Thus, the
method according to the present invention is of great
commercial value.
-31-
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