LU85636A1 - PROCESS FOR MANUFACTURING STEEL BARS HAVING IMPROVED LOW TEMPERATURE TENACITY AND PRODUCTS OF STEEL BARS THUS OBTAINED - Google Patents

Description

Process for manufacturing steel bars having better toughness at low temperature and steel bar products thus obtained.

The present invention relates to a method of manufacturing steel bars having a high

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resistance and better toughness even at very low temperatures of -120 ° C or less. In particular, the present invention relates to a process for the manufacture of steel bars having the best properties above at low temperature, as well as to the steel bars thus produced.

In recent years there has been an increasing demand for concrete reinforcing steel bars to be used in ambient environments 15 at low temperatures (for example, in the construction ί of concrete structures in cold or polar regions, concrete freezers, tanks for liquid gases, including liquefied natural gas and liquefied petroleum gas, and the like).

So far, 9% Ni steel and a high manganese austenitic steel have been developed as materials for the manufacture of reinforcing steel bars for use at the low temperatures encountered in applications. 25 mentioned above of reinforced concrete; however, it has been found that these two types of steels can only be used in very limited applications due to their high price resulting from the high content of expensive alloying elements.

30 In specific reinforced concrete structures, reinforcing steel bars of the type specified in JIS G 3112 are used (those having an elastic limit of the order of 42-43 Kgf / mm2 and manufactured by rolling to hot at a finishing temperature 35 of 1,000-900 ° C with subsequent heating to 1,100-1,250 ° C).

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However, these steel bars are intended to be used at ambient temperatures or at higher temperatures and their mechanical properties, in particular their toughness, can become poor when exposed to low temperatures. temperatures mentioned above, in particular, at very low temperatures below -100 ° C.

As a result, many attempts have recently been directed to the development of steel bars having the required high strength and toughness, even when exposed to very low temperatures such as those 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 very 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 the low temperatures mentioned above.

It is therefore an object of the present invention to provide cheaper steel bars, as well as a method for manufacturing steel bars having high strength and high toughness which are maintained at levels satisfactory during use in ambient media at very low temperatures of -120 ° C or less without adding a significant amount? of an expensive alloying element.

Another object of the present invention is to provide steel bars, as well as their manufacturing method, these steel bars having better "3

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properties at low temperature, in particular a transition temperature with the appearance of fractures' not exceeding -120 ° C.

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Other objects and advantages of this. 5 invention will emerge from the description and claims • below.

The Applicant has carried out numerous studies with a view to achieving the above objects and has made the discoveries described below.

A) When submitting a low carbon steel comprising a controlled amount of carbon in the range of 0.02% to 0.10% by weight (throughout this specification, all percentages relative to the chemical composition of steel are weight percentages), as well as Mn, Mo and Nb, added in specific amounts, to hot rolling at a lower heating temperature and at at a lower finishing temperature, the rolled steel has a structure consisting of finely dispersed ferritic and bainitic phases having an average granularity of not more than 10 μm and preferably containing 10 to 30% by volume of finely dispersed bainitic phases in ferritic phases. The fine bainitic phases have markedly beneficial effects on improving the strength of steels, so that hot-rolled steel has markedly improved strength, i.e. an elastic limit of more than 40 Kgf / mm2, which cannot be obtained with steels consisting of a single ferritic phase. In addition, since V- the grains are very fine, hot rolled steel also has a markedly improved toughness at low temperatures.

b) The toughness of the above hot-rolled steel having a structure "4" made up of finely dispersed ferritic and bainitic phases can be further improved by limiting the P content and / or the h content. S of this steel to less than 0.010%.

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Although the mechanism is not fully understood, it is believed that the above improvement takes place for the reasons set out below.

Steels usually contain about 0.02% P and 0.02% S as accidental impurities and elements such as P and S present as impurities in steels tend to concentrate in bainite phases to a considerable degree at during the austenite-to-ferrite and ferrite-to-bainite transformation. When the P and / or S content of a steel is limited to less than 0.010%, the degree of concentration of these elements in the bathing phases can however be reduced, thus making it possible to minimize the deterioration of the toughness of the bainitic phases following the concentration of impurities, so that the bainitic phases dispersed in the phase of the ferritic matrix can contribute to improving the resistance of the steel without losing its toughness.

c) The tempering of the steel thus obtained at a specific temperature is effective in improving its elastic limit, to a sufficient extent to give rise to an increase in strength of the order of 5-10 Kgf / mm2 and to further improve its toughness at low temperatures.

This characteristic is believed to be due to the fact that the mobile dislocations present in the bainitic phases of rolled steels are fixed 3 with dissolved C or N or precipitates during tempering.

d) Consequently, steel bars having excellent low temperature properties, which cannot be obtained according to the prior art, can be manufactured by hot rolling of a steel whose composition chemical is strictly controlled in the same way as the hot rolling conditions, this operation being optionally followed by a. returned to a specific temperature.

On the basis of the above findings, the present invention will be arrived at, which provides a process for the manufacture of steel bars having markedly improved properties at low temperatures and characterized by a transition temperature with the appearance of non-fractures. not exceeding -120 ° C. The process according to the present invention comprises the steps consisting in: heating a bloom or a steel billet 5 to a temperature sufficient to carry out the subsequent hot rolling, but not exceeding 1,000 ° C., this steel essentially consisting of the following composition (on a weight basis): 20 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 of the elements chosen from, 25 Or: 0.05 - 0.30% Ni: 0.05 - 1.20%

Cr: 0.05 - 1.20% Ti: 0.01 - 0.05%, and f B: 0.0005-0.0030%, the remainder consisting of iron and accidental impurities; ^ 30 hot laminate the billet or bloom heated in a bar under conditions such that * the finishing temperature does not exceed 850 ° C, the total reduction in the temperature range being between 880 ° C and the temperature of finishing 35 being at least 50%, then air-cool the hot rolled bar 6 "i to bring it to room temperature.

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In a preferred embodiment of the invention, the re-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, the content of at least one of the elements that constitute P and S, present in the steel as accidental impurities, is adjusted as follows: P: less than 0.010% S: less than 0.010%.

The steel bars thus obtained comprising finely dispersed bainitic phases in the ferritic phases and whose grains of the bainitic phases have a granularity of 10 µm or less, preferably S of 3-7 µm, have an elastic limit d '' at least 40 Kgf / mm2, a value of -120 ° C or less ~ for vTrs and a value close to 30 Kgf / mm2 for vE-120 * 20 In the accompanying drawings, Figures 1 to 5 are graphs illustrating the tensile properties and the granularity of the bainitic phase of steels 1 and 12 which are obtained in the examples of implementation of the present invention.

Now we will describe in detail the reason why the chemical composition of the steel and the conditions of hot rolling and heat treatment according to the invention are defined as indicated above.

30 A. Chemical composition of steel.

-r ~ a) Carbon (C): - Carbon must be present in order to give the required resistance to the steel bars. The presence of less than 0.02% of C is not sufficient to obtain the desired resistance, while adding carbon in an amount greater than 0.10%, Ί it is possible to bring about phases of perlite. to form and î to disperse in the structure of the steel bar, which leads to a deterioration in toughness. Therefore, the C content must be 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, it is not always necessary to add Si when the Al is added in an amount sufficient to effect the deoxidation. In addition, the presence of more than 0.5% of Si 15 can have a detrimental influence on the hot forming properties of the steel. Therefore, when adding Si, the upper limit of its content must be 0.5%. Preferably, the Si content is between 0.20 and 0.30%.

2o c) Manganese (Mn):

Manganese is a necessary element for the desulfurization of steels. It is dissolved in the steel matrix in the form of a solid solution serving not only to increase the strength of the steel, but also to give it the requisite quenchability. The steel must contain at least 1.10% * Mn in order to give it satisfactory resistance and low temperature properties as a result of the formation of finely dispersed ferritic and bainitic phases under the rolling conditions at: · · Hot adopted according to the present invention. However, the addition of more than 2.50% of Mn may give rise to significant segregation, leading to an alteration in the toughness and weldability of the steel. Consequently, the Mn content should be between 1.10% and 2.50%, preferably between 1.80% and 2%. t d) Molybdenum (Mo): λ Molybdenum is an element which is very effective in improving the strength of steels without causing a loss of their toughness. Furthermore, in accordance with the process of the present invention, the Mo is essential for controlling the quenchability of the steel and for obtaining, in the rolled steel, the desired structure made up of finely dispersed ferritic and bainitic phases. These effects of

Mo cannot be obtained adequately when the Mo content is less than 0.15%, however they are saturated and no additional advantage is obtained when the Mo is present in an amount greater than 0 , 50%. Accordingly, in accordance with the process of the present invention, the Mo is added in an amount of 0.15% to 0.50%, preferably - 0.30% to 0.40%.

e) Niobium (Nb): Niobium is an essential element for obtaining the structure of finely dispersed ferritic and bainitic phases, a structure which, as has been observed, is critical for the subject of the present invention. With less than 0.010% Nb, it is difficult to prevent the enlargement of the austenitic grains during the heating of the bloom or the steel plate (at a temperature not exceeding 1000 ° C. ) before hot rolling, which, in the end, does not make it possible to constantly obtain the desired structure of the finely dispersed ferritic and bainitic phases. This effect of Nb on the inhibition of the magnification of austenitic grains reaches a limit when the Nb content is 0.100% and when the addition of an excess quantity of Nb ipso facto increases the cost price of the steel .

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9 Therefore, the Nb content must be between 0.010% f ‘and 0.100%, preferably between 0.03% and 0.07%.

f) Aluminum (Al): l ·.

Not only is aluminum effective for the deoxidation of steels, but it also has an effect similar to that of Nb as mentioned above on the inhibition of the magnification of austenitic grains during heating. preceding hot rolling. These effects cannot be exerted when the Al content is less than 0.010%. However, the addition of more than 0.100% Al may give rise to a deterioration in the ability to be hot formed. Accordingly, the steel used in accordance with the process of the present invention should contain 0.010% to 0.100% Al, preferably 0.020% to 0.060% Al. The Al content can also be between 0.010% and 0.050%.

~ g) Copper (Cu):

Copper is effective in increasing the strength of a steel without exerting a significant detrimental influence on its toughness. Accordingly, according to the present invention, Cu can optionally be added when it is desirable that the steel has a further improved strength. To this end, * 25 at least 0.05% Cu should be added to obtain satisfactory results, while the addition of more than 0.30% Cu may affect the hot-forming ability of steel. Consequently, when adding Cu, its content should be between 0.05% and 0.30%, preferably between 0.15% and 0.25%.

h) Nickel (Ni): Since nickel effectively improves the toughness of a steel at low temperatures, in particular when added in an amount of at least 0.05%, the composition d The steel used> 10 according to the present invention may optionally contain 0.05% or more of Ni, preferably 0.50% or more of Ni. However, the content of Ni should not exceed 1.20%, since the addition of more than 1.20% of Ni increases the cost price of the steel and tends to increase the tendency of the latter to undergo flaking and other defects resulting from the presence of hydrogen during the manufacture of steel.

^ 0 i) Chrome (Cr):

Where it is desirable for the steel to have increased strength, chromium may optionally be added because of its effectiveness in increasing the strength of the steels. When added, the Cr must be present in the steel in an amount between 0.05 and 1.20%, since the addition of less than 0.05% of Cr n ' is · .- not sufficient to exert the desired effect, while the addition of more than 1.20% of Cr may affect the cold forming ability of the steel. A preferred Cr content is between 0.30 and 0.80%.

j) Titanium (Ti):

Like Nb and Al, titanium is used to refine austenitic grains and is effective for. 25 obtain a structure made up of finely dispersed ferritic and bainitic phases. As a result, Ti can optionally be added to the steel composition. However, the effect of Ti cannot be exerted with less than 0.01% of Ti, while the addition of more than 0.05% of Ti can give rise to an enlargement of the carbonitride particles of titanium in steel, while increasing the number of these particlesi so that the ability to hot work is impaired. Consequently, when Ti is added, its content should be between 0.01% and 0.05%, preferably between 0.015% and 0.030%.

7 k) Boron (B): The addition of boron in small amounts serves to improve the hardenability of steels, 5 so that B can be added when it is desirable that the steel has a more improved strength. The desired effect of B cannot be obtained with less than 0.0005% of B, while the addition of more than 0.0030% of B may cause a deterioration in the hot-forming ability of steels. Therefore, when added, B should be present in an amount from 0.0005% to 0.0030%, preferably from 0.0005% to 0.0020%.

1) Phosphorus (P) and sulfur (S): The content of phosphorus and sulfur is important for the subject of the present invention.

Therefore, according to the present invention, the structure consisting of finely dispersed ferritic and bainitic phases can be obtained by using a specific chemical steel composition in combination with specific hot rolling conditions so as to obtain a matrix of steel with good toughness. The toughness of these finely dispersed ferritic and bainitic phases * 25 can be considerably improved by limiting the content of the steel in at least one and, preferably, in two of the elements which constitute P and S; less than 0.010%.

In the art, it is well known that, in a steel subjected to a quenching and tempering treatment, the toughness of the martensitic phases subjected to tempering can be improved by reducing their P and S contents. However, according to the present invention, it has been found that even in finely dispersed ferritic and bainitic phases, martensitic phases not subject to tempering, * t 12 a reduction in the content of at least one of the elements: P and S; at less than 0.010% gave Λ a significant increase in tenacity to u? * low temperature.

5 As previously mentioned, P

and S are concentrated in the bainitic phases of the mixed-phase structure during the transformation from austenite, thus exerting a detrimental influence on the toughness of the steel obtained.

However, when at least the P content or the S content of the steel is reduced to less than 0.010%, the P and S enrichment of the bainite phase is simplified and the presence of bainite can contribute effectively to improve the strength of steel 15 without causing a loss of its toughness. In other words, the presence of the bainitic phases is effective in improving the strength and toughness in the case of steel bars, only when the bainitic phases are practically free of an S-concentration and / or in P.

Based on the above finding, in accordance with the present invention, the content of P and the content of S as accidental impurities are preferably controlled so that at least the content of P or the S content meets the following conditions: P = less than 0.010% and S = less than 0.010%.

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

35 13 B. Conditions for hot rolling and heat treatment: a) Heating temperature before hot rolling: V "

It has been found that if the bloom 5 or the billet is heated to a temperature above 1,000 ° C before hot rolling, austenitic grains of the steel may grow during this heating, even if the steel has a composition of the type defined in accordance with the present invention, so that it is not possible to obtain the laminated structure consisting of finely dispersed ferritic and bainitic phases and thus to obtain improvement desired toughness at low temperatures. Therefore, the temperature to which the billet or bloom is heated before hot rolling, i.e. the initial heating temperature, should not exceed 1,000 ° C. Lower temperatures can be chosen without loss of properties (at low temperatures) of the 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 is significantly impaired. 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) Hot rolling temperature and degree of deformation:

In order to impart to the steel a predetermined level of strength and toughness, it is necessary to subject it to repeated deformation and recrystallization resulting from the reduction occurring during hot rolling, in particular 14 to a temperature below 880 ° C, in order to refine the austenitic grains.

Λ We found that the desired ripening of the grains ”*

austenitics could not be obtained when the total reduction-5 at a temperature below 880 ° C

was less than 60%. Therefore, according to the process of the present invention, the hot rolling must be carried out under conditions such that the total reduction at a temperature in the range between 880 ° C and the finishing temperature, ie at least 60%, preferably 90% or more.

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 bloom or the billet and the dimensions of the pro-c final product.

c) Finishing temperature: It has been found that, if the finishing rolling was carried out at a temperature above 850 ° C., the desired fine grain structure could not be obtained and that the steel did not have the desired good toughness. . Accordingly, 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 is too low, a steel having a chemical composition of the type defined in this specification will be hot rolled under conditions such that a. The austenitic phases do not undergo recrystallization, establishing thus anisotropy in mechanical properties as a result of texture growth, for this reason the finishing temperature is preferably between 850 and 750 ° C, more preferably between 825 and 775 ° C.

In order to obtain a steel having, after rolling, a structure consisting of finely dispersed ferritic and bainitic phases, it is important to • hot-roll a steel bloom or billet having the composition defined in this specification and in the aforementioned conditions, d) Tempering temperature: As mentioned previously, 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 even after rolling, it has a structure i made up of finely dispersed ferritic and bainitic phases. However, if necessary, the rolled steel bar can be subjected to tempering at a temperature of between 500 and 700 ° C. in order to increase the elastic limit and also further improve the toughness of this steel bar. .

When the hot-rolled steel bar is subjected to tempering, the temperature of this operation should be between 500 and 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 ferritic and bainitic phases may occur, thereby giving rise to a des-30 truction of finely dispersed phases which, in turn, leads to an alteration in tenacity.

Preferably, the tempering temperature is between 57Γ and é25 ° C.

16

Examples The invention will be illustrated below in more detail by the following nonlimiting examples.

It is understood that these examples are given solely by way of illustration and that variants and modifications of these examples can be envisaged without departing from the scope of the invention.

Example 1

Various molten steels having the chemical compositions indicated in Table 1 were prepared by a conventional melting process without checking the P and S contents and they were poured into blooms 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 ΐ to transform it into a round bar 25 mm in diameter under conditions in which the total reduction at a temperature below 880 ° C was 90% with a finish rolling at a temperature of 800 ° C.

After the finishing rolling, the round bar obtained was allowed to cool to room temperature.

The rolled round bars were subjected to microscopic examination, tensile strength tests and impact strength tests.

During the microscopic examination, the microstructure of each laminated test piece was observed under the microscope in order to distinguish between the ferritic, bainitic and perlitic phases and in order to determine the granularities, i The tensile tests were carried out with test pieces according to Japanese standard JIS n ° 4 each having a calibrated part of 14 mm in diameter, these test pieces being formed by machining the laminated bars' / 17 Ί. The test pieces were subjected to tests to determine the elastic limit at a total elongation of 0.5%, the tensile strength, the elongation (calculated with a calibrated length 5 of 50 mm) and the reduction in area.

The impact resistance tests were carried out with Charpy specimens according to the Japanese standard JIS n ° 4 each comprising a V-notch of 2 mm, so that the toughness at low temperature-rature of each steel bar was evaluated by determining the energy absorbed at -120 ° C (vE the transition temperature with the appearance of fractures (temperature at which a transition occurs between fractures due to ductility and 15 fractures due to brittleness) (vTrs ).

The results are shown in Table 2 below.

Example 2

The process of Example 1 was repeated, with the exception that the molten steels were prepared by a conventional process making it possible to reduce the P and / or S contents. The chemical compositions of the steels and the results tests are shown in Tables 3 and 4 respectively.

As can be seen from the results indicated in Tables 2 and 4, in all the steel bars having a chemical composition of the type defined according to the present invention (steels Nos. 1-16 and 24-39 ) and obtained under the conditions according to the process of the present invention, clearly improved strength and toughness were observed. In each of these steel bars, the microstructure had finely dispersed / ferritic + bainitic phases having a granularity of 10 µm or less, the elastic limit was at least * * 18 40 Kgf / mm2 and the value vE_12Q 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 do not suffer fracture due to brittleness even at a temperature of -120 ° C.

On the other hand, steel bars which were obtained under the hot rolling conditions defined in this specification, but having a chemical composition outside the defined range (steels Nos. 17-23 and 40- 48) had lower vE_12q values, while their vTrs values were all greater than -120 ° C, indicating that they had poor toughness and were susceptible to fracture due to fragility at -120 ° C. Similarly, 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 / mm2.

20 Example 3

Following the process described in Example 1 or in Example 2, blooms of steel n ° 1 of example 1 and steel n ° 24 of example 2 were prepared, each having a square cross section measuring 160 mm on a side and they were used to form round bars 25 mm in diameter under different hot rolling conditions.

After the finish rolling, the resulting round bars were allowed to cool to room temperature.

The rolled steel bars i and thus obtained were subjected to tests for determining the microstructure, the tensile properties and the toughness at low temperature in the same manner as that described in Example 1; the results obtained è 19

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are shown in Tables 5 and 6 below.

As can be seen from the results of Tables 5 and 6, when hot rolling has been carried out under conditions outside the range defined in this specification, even the use of steel having a composition according to the present invention gave a steel bar of insufficient strength and / or toughness and the target values of 40 Kgf / mm2 or more were not reached in the elastic limit and -120 ° C or less in the vTrs value.

The results of tests n ° 49-52 are summarized by a graph in FIG. 1, which brings out the critical meanings of the initial heating temperature et and of the granularity of the bainitic phases with respect to the mechanical properties.

The results of tests no. 53-55 are also summarized by a graph in FIG. 2, which brings out the critical meanings of the total reduction below 880 ° C., as well as of the granularity of the bainitic phases with respect to mechanical properties.

The results of tests no. 56-59 are also summarized by a graph in FIG. 3 bringing out the critical meanings of the finishing temperature and of the granularity of the bainitic phases with regard to the mechanical properties. EXAMPLE 4 Under the following conditions, blooms of steels 1, 12, 24 and 35, shown in Table 1 or 2, were hot rolled, each having a square cross section of 160 mm. side:

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20

Initial bloom heating temperature: 950 ° C

Total reduction below 880 ° C: 90%

Finishing temperature: 800 ° C., to obtain round bars of 25 mm diameter 5 which were then subjected to tempering by keeping them for one hour at a temperature indicated in Tables 7 and 8 below and lying in the range 480 to 720 ° C, after which air cooling was carried out.

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 obtained are also shown in Tables 7 and 8.

As can be seen from the results given in these tables, at an tempering temperature of 480 ° C., the steel bars subjected to tempering did not undergo any significant change in the elastic limit and 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 subjected to tempering exhibited a markedly increased yield strength, as well as much 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 laminated steel bars.

However, when the tempering was carried out at a temperature above 700 ° C, the microstructure of the steel increased during the tempering, so that the tempered steel bars exhibited a lower resistance and altered tenacity.

/ 21

The results of tests n ° 71-76 for steel n ° 1 are summarized by a graph in figure 4 highlighting the critical meanings of the tempering temperature and the granularity of the pha-5 its bainitics with respect to the mechanical properties.

The results of tests n ° 77-82 for steel n ° 12 are also summarized by a graph in figure 5 highlighting the critical meanings of the tempering temperature and the granularity of the bainitic phases vis-à-vis mechanical properties.

As described above, according to the method of the present invention, steel bars having high strength and high toughness, which are maintained at satisfactory levels even at very low temperatures of -120 ° C or less can be obtained at low cost by controlling only the chemical composition of the steel and the hot rolling conditions without having to add large proportions of expensive alloying elements or without having to resort to complicated operations. Consequently, the process according to the present invention offers great commercial interest.

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Claims (10)

1. A method of manufacturing bars of steel having better toughness at low temperature, characterized in that it comprises the steps 5 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, 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% 15 Cr: 0 - 1.20% Ti: 0 - 0.05%, and B: 0 - 0.0030%, the remainder consisting of iron and accidental impurities; hot rolling the billet or bloom 20 heated into a bar under conditions such that the finishing temperature does not exceed 850 ° C, the total reduction to a temperature in the range between 880 ° C and the finishing temperature , being at least 60%; and air-cooling the hot-rolled bar to bring it to room temperature. 2. Method according to claim 1, characterized in that, when they are intentionally added, the contents of Cu, Ni, Cr, w 30 Ti and / or B of the steel lie in the inter- following values: - 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%. 33 3. Method according to claim 1 or 2, characterized in that the content of at least one of the elements that constitute P and S, which are present <>. in steel as accidental impurities, is bound to: P: less than 0.010%, S: less than 0.010%. 4. Method according to any one of claims 1 to 3, characterized in that it comprises the additional step of subjecting the air-cooled steel bar 10 to tempering at a temperature in the range from 500 to 700 ° C. 5. Method according to claim 4, characterized in that the air-cooled steel bar i 15 is subjected to tempering at a temperature ranging from 575 to 625 ° C. 6. Method according to any one of claims 1 to 5, characterized in that it comprises' .V the steps which consist in: heating a bloom or a steel billet to a temperature of 900 to 950 ° C, this steel essentially consisting 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% '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 rest being made up of iron and dental impurities; ^ hot laminate the billet or bloom heated in a bar under conditions such that the finishing temperature is between 775 and 35,825 ° C, the total reduction at a temperature being. i 34 in the range between 880 ° C and the finishing temperature, being at least 90%; and f- · air-cooling the hot-rolled bar to bring it to room temperature. * '. 5 7. Steel bar obtained according to the method according to any one of claims 1 to 6, characterized in that it has an elastic limit of at least 40 Kgf / mm2, while it has a value vTrs of - 120 ° C or less and a value 10 v ^ -i20 Close to 30 8. Steel bar having better toughness at low temperature, characterized in that the steel composition comprises: C: 0.02 - 0.10% Si: 0.5% maximum 15 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%, 20 the remainder consisting of iron and accidental impurities; the steel composition comprising a bainitic phase having an average granularity not exceeding 10 jiva. 9. Steel bar according to claim 25 8, characterized in that it has an average granularity of 3-7y ^ m. 10. Steel bar obtained in accordance with the method according to any one of claims 8 and 9, characterized in that it has an elastic limit - 30 tick of at least 40 Kgf / mm2, a value vTrs of -120 ° C or less and a value VE_ ^ 20 Close to kgf-m. * Γ t f t! : \ - I