CA1262226A - Method for producing steel bars having improved low temperature toughness and steel bars produced thereby - Google Patents

Abstract

METHOD FOR PRODUCING STEEL BARS HAVING IMPROVED LOW
TEMPERATURE TOUGHNESS AND STEEL BARS PRODUCED THEREBY

Abstract of the Disclosure A method for producing steel bars is disclosed, which comprises the steps of:
heating a steel to a temperature not exceeding 1000°C, the steel consisting essentially of :
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%, Cu: 0 - 0.30%, Ni: 0 - 1.20%, Cr: 0 - 1.20%, Ti: 0 - 0.05%, and B: 0 - 0.0030%, with the balance being iron and incidental impurities;
hot-rolling the heated steel into a bar under such conditions that the finishing temperature is not higher than 850°C or 850 - 750°C with the total reduction during hot rolling, preferably in the temperature range between 880°C
and the finishing temperature, being at least 60%, preferably the hot rolling for the last 2n passes being carried out using oval-, and round-type calibers with an equal reduction in thickness for each of the 2n passes; and cooling the hot-rolled bar to room temperature.

Description

~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 , .
~ -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):

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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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~) 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 ,~ , Z~

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 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.

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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 J .
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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 J

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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 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 ., ,:
'~' 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.
, ::, ., ' Example 1 :, Various molten steels having the chemical compositions shown in Table 1 were prepared by a conventional melting ... ..

,., --19--., `,''' ','' ,,' ~6~;~2~i 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 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%
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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 ( .,;

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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 ,~"
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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 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 , 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 , .
' ' 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:

.

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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 ., .

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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.

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Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing steel bars having improved low temperature toughness, which comprises the steps of:
heating a bloom or billet of steel to a temperature sufficient to effect the subsequent hot rolling but not exceeding 1000°C, said 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%, Cu: 0 - 0.30%, Ni: 0 - 1.20%, Cr: 0 - 1.20%, Ti: 0 - 0.05%, and B: 0 - 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 between 880°C and the finishing temperature being at least 60%; and force-cooling the hot-rolled bar at a cooling rate of 3°C/sec or higher to room temperature.

2. The method according to Claim 1 wherein the content of Cu, Ni, Cr, Ti, and/or B of the steel when intentionally added is in the following range:
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%.

3. The method according to Claim 1 or 2 wherein the content of at least one of P and S which are present in the steel as incidental impurities is restricted to:
P: less than 0.010%, S: less than 0.010%.

4. The method according to Claim 1 which further comprises the step of tempering the force-cooled steel bar in the temperature range of from 500°C to 700°C.

5. The method according to Claim 4, in which the force-cooled steel bar is tempered in the temperature range of from 575°C to 625°C.

6. The method according to Claim 1, which comprises the steps of:
heating a bloom or billet of steel to a temperature of from 900°C to 950°C, said steel consisting essentially of the following composition on a weight basis:
C: 0.04 - 0.08%, Si: 0.20 - 0.30%, Mn: 1.80 - 2.00%, Mo: 0.30 - 0.40%, Nb: 0.030 - 0.07%, Al: 0.020 - 0.060%, Cu: 0 - 0.25%, Ni: 0 - 1.20%, Cr: 0 - 0.80%, Ti: 0 - 0.030%, and B: 0 - 0.0020%, with the balance being iron and incidental impuri-ties;
hot rolling the heated bloom or billet into a bar under such conditions that the finishing temperature is from 775°C to 825°C with the total reduction in the tem-perature range between 880°C and the finishing tempera-ture being at least 90%, and force-cooling the hot-rolled bar at a cooling rate of 3°C/sec or higher to room temperature.

7. The method according to Claim 1, in which sili-con is not incorporated in the steel and the aluminum content is 0.05 - 0.100%.

8. A steel bar produced according to the method of Claim 1, which has a yield strength of at least 40 Kgf/mm2 and which shows a value of -120°C or lower for vTr and a value close to 30 kgf-m for vE-120.

9. A method for producing steel bars having improved low temperature toughness, which comprises the steps of:
heating a bloom or billet of steel to a temperature sufficient to effect the subsequent hot rolling but not exceeding 1000°C, said 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%, Cu: 0 - 0.30%, Ni: 0 - 1.20%, Cr: 0 - 1.20%, Ti: 0 - 0.05%, and B: 0 - 0.0030%, with the balance being iron and incidental impuri-ties;
hot-rolling the heated bloom or billet into a bar under such conditions that the finishing temperature is 850°C - 750°C with the total reduction during hot rolling being at least 60%, the reduction per pass with an oval, round-type caliber arrangement having a constant value of 10% or more for each of the last 2n passes, wherein "n"
is an integer; and cooling the hot-rolled bar at a cooling rate of air cooling or higher to room temperature.

10. The method according to Claim 9 wherein the con-tent of Cu, Ni, Cr, Ti, and/or B of the steel when inten-tionally added is in the following range:
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%.

11. The method according to Claim 9 or 10 wherein the content of at least one of P and S which are present in the steel as incidental impurities is restricted to:
P: less than 0.010%, S: less than 0.010%.

12. The method according to Claim 9 which further comprises the step of tempering the air-cooled steel bar in the temperature range of from 500°C to 700°C.

13. The method according to Claim 12, in which the air-cooled steel bar is tempered in the temperature range of from 575°C to 625°C.

14. The method according to Claim 9, which comprises the steps of:
heating a bloom or billet of steel to a temperature of from 900°C to 950°C, said steel consisting essentially of the following composition on a weight basis;
C: 0.04 - 0.08%, Si: 0.20 - 0.30%, Mn: 1.80 - 2.00%, Mo: 0.30 - 0.40%, Nb: 0.030 - 0.07%, Al: 0.020 - 0.060%, Cu: 0 - 0.25%, Ni: 0 - 1.20%, Cr: 0 - 0.80%, Ti: 0 - 0.030%, and B: 0 - 0.0020%, with the balance being iron and incidental impuri-ties;
hot-rolling the heated bloom or billet into a bar under such conditions that the finishing temperature is from 775°C to 825°C with the total reduction during hot rolling being at least 90%; and cooling the hot-rolled bar at a cooling rate of air cooling or higher to room temperature.

15. The method according to Claim 9, in which sili-con is not incorporated in the steel and the aluminum content is 0.05 - 0.100%.

16. A steel bar produced according to the method of Claim 9, which has a yield strength of at least 40 Kgf/
mm2 and which shows a value of -120°C or lower for vTr and a value close to 30 kgf-m for vE-120.