LU86228A1 - PROCESS FOR MANUFACTURING STEEL BARS HAVING IMPROVED LOW TEMPERATURE TENACITY AND STEEL BARS THUS MANUFACTURED - Google Patents

Description

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Method for manufacturing steel bars having better toughness at low temperatures and steel bars thus produced.

The present invention relates to a method for manufacturing steel bars having high strength and better toughness even at ultra-low temperatures of -120 ° C or less.

In particular, the present invention relates to a method of manufacturing steel bars having these best properties at low temperatures, as well as the steel bars produced by this method.

In recent years there has been an increasing demand for steel reinforcement for the reinforcement of concrete to be used in low-temperature ambient media (for example, in the construction of concrete structures in cold regions or in polar regions, concrete freezers and tanks for liquid gases such as liquefied natural gas and liquefied petroleum gas).

Hitherto, 9% Ni steel and a high manganese austenitic steel have been developed as materials for the production of reinforcing steel bars which are used at low temperatures such as those encountered in the aforementioned applications of reinforced concrete; however, these two steels have only found very limited applications due to their high cost due to the high content of expensive alloying elements.

In specific reinforced concrete structures, reinforcing steel bars of the type I specified in JIS G 3112 are used (those having an elastic limit of the order of 42-43 Kgf / mm2 and fabricating, hot-rolled at a finishing temperature of 1,000-900 ° C, followed by heating to 1,100-

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2 1.250 ° C). However, these steel bars are designed to be used at ambient or higher temperatures and their mechanical properties, in particular their toughness, can become poor when exposed to low temperatures such as those mentioned above. above, in particular, at ultra-low temperatures below -100 ° C.

As a result, many attempts have recently been directed to the development of steel bars which may have the required high strength and toughness, even when exposed to the ultra-low temperatures encountered in tanks. of liquefied petroleum gas (-60 ° C or less) or tanks of liquid ethylene or liquefied natural gas (-100 ° C or less). However, these attempts have not yet made it possible to obtain steel bars having satisfactory mechanical properties at ultra-low temperatures.

As noted above, it is expected that there will be an increasing need for inexpensive steel bars having better strength and toughness at low temperatures.

It is therefore an object of the present invention to provide less expensive steel bars having high strength and high toughness which are maintained at satisfactory levels during use in ambient environments at ultra-high temperatures. low of -120 ° C or less even without having to add large quantities of expensive alloying elements; the invention also; for subject a process for the manufacture of these i I steel bars.

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Another object of the present invention is to provide steel bars, as well as a method for their manufacture, these steel bars having better properties at low temperatures, including a transition temperature of appearance of failure. Not exceeding -120 ° C. · Other objects and advantages of the present invention will become apparent on reading the description and the claims below.

The Applicant has undertaken numerous studies in order to achieve the above objects and it has led to the discoveries described below.

(a) When subjecting a low carbon steel comprising a controlled amount of carbon in the range of 0.02-0.10% by weight (throughout this specification, all percentages relative to chemical composition of steel are percent by weight), as well as Mn, Mo and Nb added in specific amounts, to hot rolling at a lower heating temperature and at a lower finishing temperature , 'then at forced cooling at a speed higher than 3 ° C / second, the rolled steel goes to a structure made up of finely dispersed ferritic and bainitic phases having grains "of average granularity not exceeding 5 μm and containing preferably 30 to 70% by volume of finely dispersed bainitic phases in ferritic phases. The fine bainitic phases have clearly beneficial effects on improving the strength of steels, so that hot rolled steel has a markedly improved resistance, that is to say an elastic limit greater than 40 Kgf / mm2. In addition, since the grains are very fine, hot rolled steel also has a significantly better toughness at low temperatures.

(b) When subjecting a low carbon steel comprising a controlled amount of carbon in the range of 0.02 to 0.10% by weight, as well as Mn, Mo and Nb added specific quantities for hot rolling using an arrangement of round type oval gauges at a lower heating temperature and a lower finishing temperature while then allowing it to cool in air, the rolled steel has a structure consisting of a fine ferritic phase in grains with an average granularity not exceeding 5 yam, the formation of a texture being suppressed, thus giving a steel free of anisotropy 15 in its mechanical properties. Forced cooling instead of air cooling j | further improves resistance.

(c) By subjecting the steel thus obtained to tempering at a specific temperature, its elastic limit is effectively improved to a degree sufficient to give rise to an increase in strength of the order of 5-10 Kgf / mm2, all by further improving its toughness at low temperatures.

This is believed to be so because the mobile dislocations existing in the bainite phases of the rolled steels are fixed with precipitates, C or N dissolved during tempering.

(d) Therefore, steel bars having excellent properties at low temperatures (which could not be obtained in the prior art) can be made cheaper by hot rolling a steel while strictly controlling its chemical composition and the conditions of hot rolling, this treatment being optional, followed by tempering at a specific temperature.

i? j 5 j ί Based on the above findings, we have arrived at the present invention which provides a method of manufacturing steel bars having significantly improved properties at low temperatures 5 and characterized by a transition temperature. . of breaking appearance not exceeding -120 ° C.

In one of its aspects, the method according to the present invention consists in: heating a bloom or a steel billet 10 to a temperature sufficient to effect the laminating

Subsequent hot inage, this temperature however not exceeding 1,000 ° C, while the steel essentially consists of the following composition on a weight basis: 15 C: 0.02-0.10%, Si: 0.5% maximum,

Mn: 1.10-2.50%, Mo: 0.15-0.50%, -

Nb: 0.010-0.100%, Al: 0.010-0.100%, and possibly one or more elements chosen from: 20 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%, the rest being iron with accidental impurities! the ; 5 | 25 hot laminate the billet or the bloom heated in j? a bar under conditions such that the finishing temperature does not exceed 850 ° C, the total reduction at a temperature in the range of 880 ° C to the finishing temperature being at least 60%; then subjecting the hot rolled bar to forced cooling at a speed of 3 ° C / second; or more up to room temperature.

In another aspect, the method of the present invention consists in: i! 35 heating a bloom or a steel billet i 6 j „1 a to a temperature sufficient to carry out the subsequent hot rolling, but not exceeding 1000 ° C., the steel essentially consisting of the following composition (on a base by weight): 5 C: 0.02-0.10%, If: 0.5% maximum,

Mn: 1.10-2.50%, Mo: 0.15-0.50%,

Nb: 0.010-0.100%, Al: 0.010-0.100%, and possibly one or more elements chosen from: 10 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%, the remainder being iron with accidental impurities; ; Hot rolling the billet or bloom heated in a bar under conditions such that the finishing temperature is 850 to 750 ° C, the total reduction occurring during hot rolling being at least 60% and the reduction per pass with an arrangement of 20 round oval calibers being 10% | or more equally for each of the last 2n passes, "n" being an integer; and subjecting the hot-rolled bar to cooling at the rate of air cooling from most to room temperature.

In a preferred embodiment of? the invention, the cooled steel bar thus obtained is further subjected to tempering at a temperature in the range of 500 to 700 ° C.

In another preferred embodiment, if the content of at least one of the elements P and S which are present in the steel as accidental impurities, is regulated as follows: | P: less than 0.010%, S: less than 0.010%.

The steel bars thus produced jqui i 35 have finely dispersed bainitic phases • I i i * x 7 in ferritic phases or a fine iron phase! tick with a granularity of 5yimou less,, preferably [2-4 yum, have an elastic limit of at | minus 40 Kgf / mm2, a value of -120 ° C or less for 5 vTrs and a value close to 30 Kgf / mm2 for vE_12o *

The reasons will now be described in detail; for which the chemical composition of steel | and the conditions of hot rolling and processing! The thermal properties of the present invention are defined as 10 above.

A. Chemical composition of the steel a) Carbon (C):

Carbon must be present to give the required strength to the steel bars. The presence of less than 0.02% of C is not sufficient to obtain the desired resistance, while the addition of C in an amount greater than 0.10% may give rise to the formation of pearlitic phases dispersing in the structure of the steel bar, which leads to a deterioration in toughness.

Therefore, the C content is between 0.02 and 0.10%, preferably between 0.04 and 0.08% in accordance with the present invention.

b) Silicon (Si): Silicon is an effective deoxidizing element and is usually added in an amount of 0.15 to 0.35%. However, the addition of Si is not always necessary in cases where Al is added in an amount sufficient to effect the deoxidation. In addition, the presence of more than 0.5% of Si can have a harmful influence on the heat treatment properties of the steel. In j; Consequently, when Si is added, the upper limit of its content is 0.5%. Preferably the Si content is from 0.20 to 0.30%.

"8 (c) Manganese (Μη):

Manganese is necessary for the desulfurization of steels. It is dissolved in the steel matrix in the form of a solid solution which serves not only to increase the strength of the steel, but also to give the steel the requisite hardenability. A quantity of at least 1.10% of Mn must be present in the steel in order to confer on the latter satisfactory properties of resistance and behavior at low temperatures via the formation of ferritic and bainitic phases. finely dispersed or of a fine ferritic phase t under the hot rolling conditions adopted according to the present invention. However, the addition of more than 2.50% of Mn can cause significant segregation altering the toughness and weldability of the steel. Consequently, the Mn content is between 1.10 and 2.50%, preferably between 1.80 and 2%.

D) Molybdenum (Mo):

Molybdenum is very effective in improving the strength of steels without causing a loss of toughness. Furthermore, in accordance with the process of the present invention, Mo is essential for regulating the hardenability and / or behavior of the steel transformation and obtaining the strength. desired structure of ferritic and bainitic phases; finely dispersed or of a fine ferritic phase i in rolled steel. These effects of Mo cannot be adequately obtained when its content i,] is less than 0.15%, however these effects are saturated and no additional benefit is obtained when Mo is present beyond 0.50%. Consequently, according to the method of the present invention, Mo is added in an amount ranging from | 9 .Μ between 0.15 and 0.50%, preferably between 0.30 and 0.40%.

e) Niobium (Nb):

Niobium is essential for obtaining the structure of finely dispersed ferritic and bainitic phases or of a fine ferritic phase; it has been found that this structure is critical to the object of the present invention. With less than 0.010% of Nb, it is difficult to prevent the austenitic grains from growing during the heating of the billet or of the steel bloom (at a temperature not exceeding 1.000 ° C.) before hot rolling, so that, ultimately, it is impossible to obtain constantly the desired structure of the finely dispersed bainitic and ferritic phases or of a fine ferritic phase. This effect of Nb on the inhibition of the magnification of the austenitic grains reaches a limit when the Nb content is 0.100%, while the addition of an excess of Nb increases the cost price of the steel. Therefore, the Nb content is between 0.010 and 0.100%, preferably between 0.03 and 0.07%.

f) Aluminum (Al):

Not only is aluminum effective in deoxidizing steels, but it also exerts an effect similar to that of Nb as mentioned above by preventing the austenitic grains from growing during heating before rolling. hot. These effects cannot be obtained when the Al content is less than 0.010%. However, the addition of more than 0.100% Al may cause * a deterioration in the heat treatment ability.

Consequently, the steel used according to the process of the present invention should contain 0.010 to 0 ^ 100%, preferably 0.020 to 0.060% Al. The content of 10

Al can also be between 0.010 and 0.050%.

In cases where deoxidation is carried out using aluminum and not silicon, the aluminum content is between 0.050 and 0.100%.

'. The steel to be treated by the process of the present invention may contain at least one element selected from Cu, Ni, Cr, Ti and B, these elements being effective in improving the strength of the steel obtained.

The quantities and effects of these optional items will be described below in detail.

Ig) Copper (Cu):

Copper is effective in increasing the strength of a steel without exerting a significant adverse influence on its toughness. Accordingly, according to the present invention, Cu can optionally be added when it is desired that the steel has a further improved strength. To this end, it is advisable to add at least 0.05% of Cu in order to obtain; to give satisfactory results, while the addition of more than 0) 30% of Cu may cause a deterioration in the heat treatment capacity of the steel. Consequently, when Cu is added, its content is between 0.05 and 0.30%, preferably between 0.15 and 0.25%.

h) Nickel (Ni):

Since nickel is effective for: improving the toughness at low temperatures of a steel,! In particular, when added in an amount J of at least 0.05%, the composition of the steel which is used in accordance with the present invention may optionally contain 0.05% or more Ni, preferably 0.50% or more Ni. However, the Ni content of D 35 should not exceed 1.20% since, in this case, the steel price increases and there is an increase in the tendency to scaling and other defects in the steel due to the presence of hydrogen during steel making.

5 i) Chromium (Cr):

When it is desired to further increase the strength of the steel, chromium may optionally be added because of its effectiveness in increasing the strength of the steels. When added, j 10 Cr must be present in the steel in an amount between 0.05 and 1.20%, since the addition of less than 0.05% Cr is not sufficient to exert the desired effect, while the addition of more than 1.20% of Cr may cause an alteration; 15 ration of the aptitude for cold treatment of steel.

| A preferred Cr content is between 0.30 and 0.80%.

j) Titanium (Ti):

Like Nb and Al, titanium is used to refine the austenitic grains and is effective in forming a structure of finely dispersed bainitic and ferri-tick phases or a fine ferri-tick phase. Consequently, Ti can be optionally added to the composition of the steel. However, the effect of Ti cannot be obtained with less than 0.01% Ti, while the addition of more than 0.05% Ti can cause a magnification of the titanium carbonitride particles in the steel. , while] increasing the number of these particles, which j | 30 impairs the ability to process hot. Consequently, when Ti is added, its content is situated; between 0.01 and 0.05%, preferably between 0.015 and 0.030%.

i 1 -ii 12 k) Boron (B): The addition of boron in small amounts serves to improve the hardenability of the steels, so that B can be added when it is desired to further increase the strength of the steel. The desired effect of B cannot be achieved with less than 0.0005% B, while the addition of more than 0.0030% B can result in impaired heat treatment properties of the steels. Therefore, when added, B should be present in an amount between 0.0005 and 0.0030%, preferably from 0.0005 to 0.0020%.

It is well known that in a processing steel, the toughness of the martensitic tempering phases can be improved by lowering the P and S content. However, according to the present invention, it has been found that even in bainitic phases and finely dispersed ferritics (martensitic phases not subject to tempering), by lowering at least one of the P and S contents to less than 0.010%, a significant increase in the toughness is obtained at low temperature.

Accordingly, according to the present invention, the content of P and S as accidental impurities is preferably controlled so that at least one of the P and S contents meets the requirements of the present invention. following conditions: P: less than 0.010% and S: less than 0.010%.

Of course, as long as one or the other of the contents of P or S is less than 0.010%, the hot-rolled steel bar thus obtained has the desired additional improvement in toughness at low temperature, even if the other element is present at a level found in 35 steels manufactured in a traditional way.

- 13 B. Conditions for hot rolling and heat treatment: a) Heating temperature before hot rolling: 5 It has been found that by heating the billet or bloom to a temperature exceeding 1,000 ° C before rolling to hot, magnification of the austenitic grains could occur in the steel during heating, even if the steel has a composition 10 of the type defined according to the present invention and thus, it is not possible to produce the laminated structure formed finely dispersed bainitic and ferritic phases or a fine ferritic phase and thereby achieve the desired improvement in toughness at low temperatures. Consequently, the temperature to which the billet or bloom is heated before hot rolling, i.e. the initial heating temperature, should not be higher than 1000 ° C. Lower temperatures can be chosen without resulting in a loss of low temperature properties of hot rolled steel bars. However, if the initial heating temperature is too low, the stress applied to the rolls during the hot rolling step increases to such an extent that the efficiency of the hot rolling deteriorates significantly. Therefore, as a general rule, it is preferable to heat the bloom or the billet to a temperature between about 900 and 1,000 ° C., more advantageously, between 900 and 950 ° C.

b) Pass program:

According to one of the preferred embodiments of the present invention, a pass program for hot rolling is an important factor.

35 precise control of the pass program can ensure.] *> ♦ 14 a markedly improved toughness at low temperature, that is to say properties of resistance to breakage by brittleness at an ultra-low temperature of -120 ° C. , which cannot be achieved by conventional hot rolling of sheets. Therefore, according to this embodiment of the present invention, the pass program is as follows: (i) The total reduction in thickness during hot rolling is limited to at least 60%.

(Ii) The reduction in thickness per pass has a constant value of 10% or more for each of the last 2n passes, "n" being an integer.

The hot rolling for the last 2n passes is carried out using an arrangement of 15 oval round-type calibers.

It is necessary to obtain a fine ferritic structure in grains with an average granularity, of 5 jim or less in order to obtain a better tenacity at low temperatures. For this purpose, the total reduction in thickness during hot rolling should be defined at 60% or more, while the reduction for each of the last 2n passes ("n" = whole number) has the same value. 10% or more.

In addition, in order to prevent the deterioration of the toughness at low temperature following the formation of a texture, it is advisable to carry out the hot rolling using an arrangement of oval calibers of round type for each of the last 2n passes, "n" being an integer. It is also stipulated that the reduction has the same value for each of these 2n passes.

The reasons why hot rolling for each of the last 2n passes ("n" is an integer) is carried out using ~ 35 oval and round sizes, resides first of all in the fact that the hot rolling carried out with an even number of passes can effectively prevent the formation; ’Of a texture which sometimes forms as a result of an uneven thickness in the direction of rolling, thus preventing a deterioration in toughness. Secondly, even if the last two 2n passes are made using an oval and round gauge, a texture will increasingly form, causing a marked decrease in toughness if the reduction in thickness for each pass is not equal. This characteristic is due to the fact that the hot rolling is carried out practically in one direction just like the rolling of sheets. As indicated above, this texture, the formation of which deteriorates the toughness, is that in which a direction <011> of the crystals conforms to the direction of rolling, ji while a plane £ l00 ^ of the crystals is in accordance with the vertical plane in the direction of the final reduction.

| According to the discoveries of the Applicant, the formation of such a texture alters the toughness of the steel and the Applicant has also found a program of passes making it possible to prevent the formation of the texture.

I * In this specification, the expression i i 25 "arrangement of oval round-type calibers" denotes j V a sequence of calibres in which a caliber i | oval and a round caliber are arranged alternately.

i

An oval caliber of the round type is well known in the art.

J 30 c) Hot rolling temperature and degree of deformation:

In order to give the steel a predetermined level of strength and toughness, it is necessary to subject the steel to deformation ej; to a repeated recrystallization following the reduction 2 16 undergone during hot rolling, in particular, at a temperature below 880 ° C., so as to refine the austenitic grains.

It has been found that the desired refining of the austenitic grains cannot be achieved when the total thickness reduction to less than 880 ° C is less than 60% under the usual conditions. Therefore, according to the process of the present invention, it is desirable to carry out hot rolling under conditions such as total reduction in the temperature range between 880 ° C. and the finishing temperature, or at least 60%, preferably 90% or more.

When hot rolling is carried out by following the pass program mentioned above, the total reduction can be that measured during hot rolling.

The upper limit of the degree of deformation is not critical and it can be appropriately chosen according to various factors, in particular the capacity of the hot cylinder, the dimensions of the billet or of the bloom, as well as the dimension of the final product, again that the higher the upper limit *, the better the results.

25 d) Finishing temperature:? It has been found that by carrying out the finishing rolling at a temperature above 850 ° C., the desired fine structure of the grains could not be obtained and that the steel did not have the desired good toughness. Therefore, according to the process of the present invention, hot rolling must be carried out at a finishing temperature of 850 ° C or less.

However, when the finishing temperature 35 is too low, a steel having a chemical composition 17 of the type defined in this specification will be hot rolled under conditions such that the austenitic phases do not undergo recrystallization, thus establishing anisotropy in 5 mechanical properties as a result of texture growth. For this reason, the finishing temperature is preferably between 850 and 750 ° C, more advantageously between 825 and 775 ° C.

Furthermore, the finishing temperature is then less than 750 ° C. and the steel then consists of a structure with double austenitic and ferritic phases in which the ferritic phase causes a deterioration of the toughness during rolling, e) Conditions Cooling: According to one of the preferred embodiments of the present invention, the steel is cooled at a rate of 3 ° C / second or more after hot rolling. Forced cooling at a rate of 3 ° C / second or more results in the formation of finely divided grains with a diameter of 5 µm or less. A volumetric proportion of bainite is in the range of 30 to 70%, thus giving rise to an improvement in strength and toughness.

This forced cooling can be carried out "with forced circulation of air, mist or water, this cooling being carried out immediately after the hot rolling. The temperature at which the forced cooling is stopped is not defined in the present invention, however, it is desirable to effect forced cooling at a temperature in the range of 350 ° C to room temperature.

When the cooling rate is less than 3 ° C / second, the structure obtained then comprises 18 large grains with an average diameter of 5 μm or more, which gives rise to a deterioration of the toughness at low temperature.

When the pass program for hot rolling 5 is controlled as indicated above, air cooling can be adopted, f) Tempering temperature:

As mentioned above, a steel bar having a composition of the type defined in this specification and obtained by hot rolling under the conditions of the process of the present invention has a structure made up of ferri-tic and bainitic phases finely dispersed or of a fine ferritic phase, even after rolling. However, if necessary, the rolled steel bar can be subjected to tempering treatment at a temperature in the range of 500 to 700 ° C in order to raise the yield strength and also further improve the toughness of the steel bar.

When the hot-rolled steel bar is subjected to tempering, the tempering thereof should be in the range of 500 to 700 ° C as mentioned above. At an annealing temperature below 500 ° C, favorable results cannot be fully achieved while at an annealing temperature above 700 ° C, recrystallization of the bainitic and ferritic phases or d 'a fine ferritic phase, thereby destroying the finely dispersed phase (s) with consequent deterioration in toughness, preferably the tempering temperature is in the range of 575 to 625 ° C.

/ /! * -. ί * 19 i I u i i; Examples The invention will be further illustrated by the following nonlimiting examples. It is understood that these examples are given solely by way of illustration, while modifications may be made thereto without departing from the scope of the invention.

EXAMPLE 1 i Various molten steels were prepared | 10 having the chemical compositions indicated in Table 1 by a conventional melting process and on | poured them into blooms each having a square cross section I measuring 160 mm on each side.

Each bloom was then heated to 950 ° C, and then subjected to hot rolling to form a round bar 25 mm in diameter under conditions such as total reduction at a temperature below 880 ° C or 90% with a finishing rolling at a temperature of 800 ° C.

20 After the finishing rolling, the round bar obtained was subjected to cooling! forced at a speed of 10 ° C / second up to room temperature.

! ~ The rolled round bars thus obtained! 25 were subjected to microscopic examination, i? ’Tensile and strength tests | to shocks.

During the microscopic examination, the microstructure of each ij 30 laminated test piece was observed under the microscope in order to distinguish the ferritic, bainitic and pearlitic phases and also in order to; determine the granularities.

i

The tensile tests were carried out with j; JIS n ° 4 test pieces each having a jj 35 calibrated part 14 mm in diameter and obtained by machining the rolled bars. The test specimens were subjected to tests in order to determine the elastic limit at a total elongation of 0.5%, the tensile strength, the elongation (calculated with a calibrated length of 50 mm) and the reduction in area.

The impact resistance tests were carried out with Charpy JIS n ° 4 test pieces each comprising a V-cut of 2 mm, so that the low temperature toughness of each steel bar 10 was evaluated by determining the energy absorbed at -120 ° C (vE_12q) fracture appearance transition temperature (temperature at which the transition takes place between ductile fractures and brittleness fractures) (vTrs).

The results are summarized in Table 2 below.

As can be seen from the results shown in Table 2, steels A to P whose compositions fall within the range of the present invention and which were treated in accordance with the process of the present invention had a microstructure combination of fine ferrite + bainite with an average grain diameter of 5 yam or less with an elastic limit of 40 kgf / mm2 and a value VE_12Q of about 30 kgf-m. Therefore, strength and toughness have been advantageously improved by the present invention. Furthermore, the value of vTrs was less than -120 ° C for each steel and a brittleness fracture did not occur even at a temperature of -120 ° C.

On the other hand, steels Q to W whose compositions lie outside the range of the present invention, but which have undergone the treatment according to the process of the invention, had a low value for vE_ ^ 2q and a higher vTrs value

^ ‘N

21 at -120 ° C: A fragility fracture occurred at a temperature of -120 ° C. These steels exhibited unsatisfactory tenacity properties. Furthermore, some of these steels had an elastic limit of less than 40 kgf / mm 2 and their strength properties were not satisfactory. Example 2

V V

Following the process described in Example 1, blooms of steel A of Example 10 1 were prepared having a square cross section measuring 160 mm on each side and used to form 25 mm round bars. in diameter under different hot rolling conditions.

After the finishing rolling, the round bars obtained were subjected to forced cooling with air at a cooling rate of 10 ° C / second to room temperature.

! The rolled steel bars thus obtained were subjected to tests to determine the | 20 microstructure, tensile properties and toughness at low temperature in the same manner as that described in Example 1. The results of these tests are summarized in Table 3.

As can be seen from table 3, when the hot rolling i 'has been carried out under conditions outside the inter | As defined in the present specification, even the use of a steel having a composition according to the present invention and cooled after hot rolling in accordance with the present invention, gave a steel bar of 'insufficient toughness and / or resistance and the envisaged values of 40 Kgf / mm2 or more for the elastic limit and of -120 ° C or less for the value 35 vTrs have not been obtained.

22

Example 3

After hot rolling under the following conditions, blooms of steel A were cooled as indicated in table 1, having a square transverse section of 160 mm on a side in order to determine the influence of the speed. cooling: Initial bloom heating temperature: 950 ° C Total reduction below 880 ° C: 90%

Finishing temperature: 800 ° C

In this way, round bars with a diameter of 25 mm were obtained which were then subjected to forced cooling at a speed lying between that of air cooling (0.8 ° C / second ) and that of water cooling (100 ° C / second).

In the same manner as that described in Example 1, the round bars obtained were subjected to tests for determining the microstructure, the resistance and the toughness.

As can be seen from the re-results of these tests (shown in Table 4), the cooling rate exerts a strong influence on the toughness at low temperature. The shock energy absorbed at a temperature of -120 ° C is about 30 kgf-m at a cooling rate of 3 ° C / second or more. However, when the cooling rate is less than 3 ° C / second, the structure obtained comprises crystalline grains having an average diameter of more than 5 µm and the impact energy absorbed at -120 ° C decreases considerably to give rise to to a fragility fracture at a temperature of -120 ° C.

Example 4

Following the process described in Example 1, blooms of steel A and steel 35 L of Example 1 were each prepared having a square cross section * 23 measuring 160 mm on a side and were used for form round bars with a diameter of 25 mm. After the finishing rolling, the round bars obtained were subjected to forced cooling at a speed of 10 ° C / second to room temperature.

As shown in Table 5, the steel round bars thus obtained were then tempered at 480-720 ° C for one hour, after which they were subjected to air cooling. The steel bars thus obtained were subjected to tests with a view to determining the microstructure, the tensile properties and the toughness at low temperature in the same manner as that described in Example 1.

As can be seen from the results of these tests (indicated in Table 5), when the tempering is carried out at 480 ° C., the steel bars obtained have an elastic limit and a value vTrs lying practically at the same level as that of the rolled bar, so that there is no income effect.

On the other hand, when the tempering is carried out at 500-700 ° C., not only is the elastic limit significantly improved, but the value vTrs is also greatly reduced. Therefore, it is obvious that the method of the present invention allows to significantly improve the strength and toughness of the round steel bar in question. On the other hand, when the tempering was carried out at a temperature above 700 ° C, the microstructure grew and not only did the strength decrease, but the toughness was also impaired.

24

Example 5

Various molten steels having the chemical compositions shown in Table 6 (steels 1-38) were prepared by a conventional melting process and were cast into bloots each having a square cross section measuring 160 mm on each side. Each bloom was then heated to 950 ° C, then subjected to hot rolling to obtain a 25 mm diameter round bar under conditions such that the total reduction was 98%, the rolling finishing being carried out at a temperature of 800 ° C. The hot rolling consisted of 16 passes and, for the successive passes to the 6th pass from the finishing pass, an arrangement of round-type oval gauges was used with an equal thickness reduction of 10% for each pass . After the finish rolling, the round bar obtained was allowed to cool in air to room temperature. The 20 rolled round bars thus obtained were subjected to microscopic examination, tensile tests and impact resistance tests.

During microscopic examination, the microstructure of each laminated test piece was observed under a microscope in order to determine the granularity of a ferric phase.

The tensile tests and the impact resistance tests were carried out in the same manner as that 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 perpendicular to the direction of | 35 rolling and a complete pole figure / · 25 was prepared using not only the Shultz reflection method, but also the Decker permeability method with CoK0 radii (for the same specimens.

5 As can be seen from the results in Table 7, in all steel bars having a chemical composition of the type defined according to the present invention (steels No. 1-27) and obtained under the conditions according to the process of In the present invention, significantly improved strength and toughness have been observed. In each of these steel bars, the microstructure exhibited a fine-grained iron phase with a granularity of 5 yum or less, the elastic limit was at least 40 kgf / mm2 • 15 and the value vE_ ^ 2q was close to 30 kgf-m. In addition, each of these steel bars had a vTrs value of less than -120 ° C, which clearly indicates that these bars did not undergo any fragility fracture even at a temperature of -120 ° C.

It should be noted that a textured structure did not form which would have affected the toughness.

In contrast, steel bars which were manufactured under the hot rolling conditions defined in this specification, but having a chemical composition outside the defined range (steels # 28-38), exhibited lower vE values, while their vTrs values were all greater than -120 ° C, indicating that they had poor toughness and suffered a fragility fracture at -120 ° C.

Likewise, it can be seen that these comparative steel bars do not always have a satisfactory resistance, since some of them have an elastic limit of less than 40 Kgf / mm 2.

35; 26 Example 6

In this example, the heating temperature, the total thickness reduction and the finishing temperature are varied to determine the effects on mechanical properties, including toughness.

After the blooms of steel n ° 1 of table 6 were machined in order to form test pieces having the dimensions indicated in cross section, the test pieces thus obtained were subjected to hot rolling with a total reduction of 57-98%. . The heating temperature and the finishing temperature were also varied. The test specimens were subjected to finishing in steel bars 25 mm in diameter. The hot rolling was carried out under the following conditions: for the successive passes to the 6th pass from the finishing pass, an oval size of round type was used with, for each pass, an equal reduction of 10% . After the hot rolling, the steel bars obtained were subjected to air cooling to room temperature.

The microstructure, strength, toughness and texture of the steel bars obtained were examined in the same manner as that described in Example 1.

The results are summarized in Table 8.

As can be seen from Table 8, when the hot rolling was carried out under conditions outside the range defined in this specification for either factor such as the heating temperature, total reduction and finishing temperature, even the use of a steel having a composition according to the present invention gave a steel bar having insufficient tenacity, while the desired values of -120 ° C or even less for vTrs have not been reached.

Example 7 This example is given to illustrate the effect of the pass program according to the present invention.

Example 1 was repeated, except for the pass program. In this example, the number of passes in which an arrangement of round-type oval gauges was used, as well as the reduction in thickness, were varied in order to assess the critical nature of the pass program defined according to the pre- - invention, as shown in Table 9.

The rolled steel bars obtained were tested to determine the microstructure, tensile properties, low temperature toughness and texture in the same manner as that described in Example 1; the results are summarized in Table 10.

As can be seen from Table 10, when hot rolling has been carried out under conditions outside the range defined in this specification for the pass program, even the use of steel .

- Having a composition according to the present invention gave a steel bar having a clearly insufficient tenacity.

The toughness at low temperature is notably altered unless a round oval gauge is used for each of the last 2n cages with one; equal reduction in thickness of 10% or more for each of these cages. When an arrangement of oval round gauges was used for an odd number 35 of cages or when the reduction in thickness was not equal for all these cages, the formation of a textured structure and the alteration of the tenacity were clearly marked.

Example 8 5 This example is given in order to evaluate the effect of the cooling rate after hot rolling.

Blooms of steel n ° 1 (table 6) each having a square cross section of 160 mm 10 on a side were subjected to hot rolling in 16 passes under the following conditions:

Initial heating temperature: 950 ° C

Total reduction during hot rolling: 90%

15 Finishing temperature: 800 ° C

Round bars 25 mm in diameter were thus obtained which, immediately after passing through the finishing cage, were subjected to cooling in various ways. The 20 pass program was designed so that in the last six passes, an arrangement of round-type oval gauges was used with a thickness reduction of 10% for each of these six passes.

The cooling was carried out using one or the other of the following three types of cooling:

Air cooling (the cooling rate is approximately 0.8 ° C / second).

Fog cooling (the cooling rate is around 3 ° C / second).

Water cooling (the cooling rate is around 10-100 ° C / second).

Water cooling was applied while controlling the flow rate as well as the pressure to change the cooling rate from 29 ° C / second to 100 ° C / second.

The round steel bars obtained were subjected to tests for determination of microstructure, strength, toughness and texture. The results are shown in Table 11.

As can be seen from Table 11, according to the present invention, satisfactory toughness can be obtained at low temperatures even if the cooling after hot rolling is air cooling. No deterioration in toughness was observed at low temperatures even at a higher cooling rate.

It should therefore be noted that it is advantageous to switch from air cooling to cooling (by mist or to water cooling when it is desired to increase the resistance while maintaining toughness at low temperatures.

Example 9 This example is given in order to assess the effect of the tempering temperature according to the present invention.

Example 5 was repeated for steels 1, 1, 12 and 24 of Table 6. Blooms of these steels; 25 each having a square cross section of 160 - mm side were subjected to hot rolling as indicated in Example 1 in order to obtain bars! 25 mm diameter rounds which were then subjected to air cooling.

The bars obtained were subjected to tempering comprising heating at 480-720 ° C for one hour and air cooling to temperature! ambient. The income conditions are indicated I in Table 12.

i 30

The round bars obtained were subjected to tests for determining the microstructure, the resistance and the toughness in the same manner as that described in Example 1. The results are shown in Table 12.

As can be seen from Table 12, at a tempering temperature of 480 ° C, the tempered steel bars did not exhibit significant changes in the yield strength. 10 and the vTrs value compared to rolled steel bars, so that the income did not have its effect.

In contrast, at tempering temperatures between 500 and 700 ° C, the steel bars 15 subjected to tempering exhibited a significantly increased yield strength, as well as significantly lower vTrs values. Therefore, the heat treatment according to the process of the present invention is clearly effective in improving significantly both the strength and the toughness of the rolled steel bars.

However, when the tempering was carried out at a temperature above 700 ° C, the microstructure of the steel grew during the tempering, so that the tempered steel bars had lower strength and toughness altered.

As indicated above, according to the process of the present invention, steel bars having high strength and high toughness can be obtained at low cost which are maintained at satisfactory levels. even at ultra-low temperatures of -120 ° C or lower even by adjusting only the chemical composition of the steel and the conditions of hot rolling without having to add expensive alloying elements in large proportions or without performing , complicated operations. Therefore, the process according to the present invention is of great commercial value.

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Claims (19)

47 1. A method of manufacturing steel bars having better toughness at low temperatures, characterized in that it comprises the steps which consist in: heating a bloom or a steel billet to a temperature sufficient to carry out the subsequent hot rolling, but not exceeding 1,000 ° C, V this steel essentially consisting of the following composition (on a weight basis): C: 0.02-0.10%, Si: 0 , 5% maximum, 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%, j Cr: 0-1.20%, Ti: 0-0.05%, and; B: 0-0.0030%, the remainder being iron and accidental impurities; hot rolling the billet or bloom 20 heated in a bar under conditions such that the finishing temperature does not exceed 850 ° C, the total reduction obtained in the temperature range being between 880 ° C and the temperature finishing being at least 60%; and c 25 subjecting the hot-rolled bar to forced cooling at a cooling rate of 3 ° C / second or more to room temperature. 2. Method according to claim 1, charac-30 terized in that the contents of Cu, Ni, Cr, Ti and / or B of the steel (when added intentionally) are in the following intervals:! 'Cu: 0.05-0.30%, Ni: 0.05-1.20%, ICr: 0.05-1.20%, Ti: 0.01-0.05%, and- 35 B: 0.0005-0.0030%. ri i: 48 3. Method according to claim 1 or 2, characterized in that the content of at least one of the elements P and S which are present in the steel as accidental impurities, is limited to: 5 P: less than 0.010%, S: less than 0.010%. A .. Method according to any one of claims 1 to 3, characterized in that it ** still includes the step of subjecting the steel bar * having undergone forced cooling to an income to / V- 10 a temperature in the range of 500 to 700 ° C. 5. Method according to claim 4, characterized in that the steel bar having been subjected to forced cooling is subjected to tempering at a temperature ranging from 575 to 625 ° C. t 6. Method according to any one of claims 1 to 5, characterized in that it comprises the steps which consist in: heating a bloom or a steel billet to a temperature of 900 to 950 ° C, this steel being consisting essentially of the following composition (on a weight basis): ^ C: 0.04-0.08%, Si: 0.20-0.30%, = 25 Mn: 1.80-2.00%, Mo : 0.30-0.40%, i 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%, the remainder being iron and accidental impurities; hot rolling the billet or bloom heated in a bar under conditions such that the finishing temperature is between 775 and 825 ° C, the total reduction obtained in the temperature range between 880 ° C and the temperature / / "49 finishing being at least 90%; and subjecting the hot-rolled bar to forced cooling at a cooling rate of 3 ° C / second or more to room temperature.: 7. Process according to any one of claims 1 to 6, characterized in that the silicon is not incorporated in the steel, while the aluminum content * is 0.05-0.100%. 8. Steel bar manufactured according to the method of either of claims 1 to 7, this steel having an elastic limit of at least 40 Kgf / mm2, while it has a value of -120 ° C or even less for vTrs and a value close to 15 30 kgf-m for vE_12o * 9. A method of manufacturing steel bars having better toughness at low temperatures, characterized in that it comprises the steps which consist in: heating a bloom or a steel billet to a temperature sufficient to carry out rolling at subsequent heating, but not exceeding 1,000 ° C, this steel essentially consisting of the following composition (on a weight basis): »25 C: 0.02-0.10%, Si: 0.5% maximum , Ç 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%, the remainder being iron and accidental impurities; Hot rolling the billet or the bloom heated in a bar under conditions such that the finishing temperature is 850-750 ° C. The total reduction obtained during hot rolling / w ^ 50 being at least minus 60%, while the reduction by i passes with an arrangement of oval calibers of type i ~ i round has a constant value of 10% or more for each of the last 2n passes, "n" being an integer 5; and cooling the hot rolled bar at air cooling speed or more to below room temperature. 10. Process according to claim 9, characterized in that the contents of Cu, Ni, Cr, Ti and / or B of the steel, when they are intentionally added, lie within the following ranges: - ij vants: | Cu: 0.05-0.30%, Ni: 0.05-1.20%, J 15 Cr: 0.05-1.20%, Ti: 0.01-0.05%, and | B: 0.0005-0.0030%. 11. Method according to claim 9 or 3 i 10, characterized in that the content of at least one of the elements P and S which are present in the steel as accidental impurities is limited to: P: less than 0.010%, S: less than 0.010%. 12. Method according to any one of claims 9 to 11, characterized in that it also includes the step consisting in subjecting the steel bar having undergone air cooling to an income temperature in the range of 500 to 700 ° C. 13. The method of claim 12, characterized in that the air-cooled steel bar is subjected to tempering at a temperature in the range of 575 to 625 ° C. 14. Method according to any one of '1 (claims 9 to 13, characterized in that it comprises the steps which consist in: i I' fi / ί 51 J i ί. Heating a bloom or a steel billet at a temperature of 900 to 950 ° C, this steel essentially consisting of the following composition (on a weight basis): 5 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 10 B: 0-0.0020%, the remainder being iron and accidental impurities; hot laminate the billet or bloom heated in a bar under conditions such that the finishing temperature is between 775 and! 15,825 ° C, the total reduction occurring during; hot rolling being at least 90%; and; cool the hot rolled bar at air cooling speed or more to room temperature. 15. Method according to any one of I claims 9 to 14, characterized in that the silicon is not incorporated in the steel, while the aluminum content is between 0.05 and 0.100%. 16. Steel bar manufactured according to the pro-ceded method of either of claims 9 to 15, this steel having an elastic limit of at least 40 Kgf / mm2, while it has a value of -120 ° C or less for vTrs and a value close to! 30 kgf-m for vE_12Q. ; \ · R \! , \! Ί v v. . · 1 I,! • Λ f] 'i -'. i I - f