FR2757542A1 - CONSTRUCTION STEEL LOW ALLY ACTIVE PARTICLES - Google Patents

Abstract

Steel whose chemical composition comprises, by weight: 0.005% = <C = <0.4%; 0.2% = <Mn = <2.5%; 0.05% = <Si = <0.6%; 0% = <Ni = <6%; 0% = <Cr = <3%; 0% = <Mo = <1.5%; 0% = <Cu = <1%; 0% = <V = <0.2%; 0% = <Nb = <0.1%; 0% = <B = <0.005%; 0,% = <S = <0.02%; 0.001% = <Al = <0.004%; 0.01% = <Ti = <0.03%; 0% = <N = <0.006%; optionally zirconium in amounts = <0.006%; optionally rare earths in contents of less than 0.05%; optionally calcium in contents of less than 0.005%; the remainder being iron and impurities resulting from the production. The steel contains a fine dispersion of active particles consisting of at least mixed titanium oxides and at least one element taken from aluminum, silicon and zirconium, the number of active particles per mm ** 2, counted on a micrographic section, being greater than 25. Process for the production of this steel.

Description

i

  STEEL AND PROCESS FOR PRODUCING SAID STEEL

  The present invention relates to an active particle steel which

  promote a fine ferritic grain.

  It is well known that the mechanical properties of ductility, yield strength and toughness of steels are even better than the grain is fine.

  This is particularly the case for steels with a ferritic, ferritic and

  pearlitic or ferritic-bainitic. These structures generally result from the cooling transformation of austenitic structures stable at high temperature and unstable at low temperature. The ferritic grains obtained by these transformations germinate from the austenitic grains and are

  all the more so because the size of the starting austenitic grains is small.

  Moreover, it is well known that the austenitic grain can be refined by suitable thermal or thermomechanical treatments. Also, in order to obtain steels which have high mechanical properties of ductility, yield strength and toughness, it is sought to refine the ferritic grain by thermal or thermomechanical treatments intended to refine the austenitic grain before transformation of the ferrite austenite. These treatments are, for example, normalization by reheating for a not too long time, at a temperature not too high above the austenite transformation temperature, or a thermomechanical treatment by plastic deformation of the steel in a temperature range. such that, on the one hand, the steel has an austenitic structure, and on the other hand that the hardened austenitic grains do not recrystallize in the form of coarse grains. This technique of grain refinement by thermal or thermomechanical treatments is universally used. However, it has the disadvantage of not being adapted to certain situations in which the steel is subjected to thermal cycles imposed by the particular circumstances of use, the methods of implementation or the manufacturing processes. To limit the consequences of this disadvantage, it has been proposed to add in the steel elements capable of forming a fine dispersion of high temperature stable precipitates, which block the growth of the austenitic grains. However, this technique is not always effective enough, so that it is sometimes difficult to obtain the

  desired ductility characteristics.

  The disadvantage that has just been described can be expressed quantitatively by using the transition temperature of the resilience at the level of 28 Joules after a thermal cycle consisting of heating at 1300 C followed by cooling to the temperature 4 C / s by convention, in the following, this transition temperature of resilience, measured after the thermal cycle defined above, will be called

"TK 28 J".

  In a number of circumstances, the safety of steel-built installations can only be guaranteed if the temperature "TK 28 J" is less than -45 C. However, for the steels in question here, it is impossible, in general, not guarantee that the temperature "TK 28 J" will be lower than - 45 C. This results in limitations in the use of these steels which present

other advantages elsewhere.

  The object of the present invention is to overcome this disadvantage by providing a steel having improved ability to refine the ferritic grain and to retain a fine grain, so satisfactory ductility properties, even when subjected to poorly controlled thermal cycles resulting from either the manufacturing conditions or the conditions of implementation, or finally, special circumstances of use. More specifically, the object of the invention is to propose a steel having, at the same time, a ferritic, ferritoperitic or ferritic-bainitic structure, and a temperature

"TK 28 J" less than - 45 C.

  For this purpose, the subject of the invention is a steel whose chemical composition comprises, by weight:

0.05% <C <0.4%

  0.2% <Mn <2.5% 0.05% <Si <0.6% 0% <Ni <6% 0% <Cr <3% 0% <Mo <1, 5% 0% <Cu < 1%

0% <V <0.2%

0% <Nb <0.1%

0% <B <0.005%

0% <S <0.02%

0.001% <AI 0.004%

0.01% <Ti <0.03%

0% <N <0.006%

  optionally zirconium in contents of less than 0.006%, optionally rare earths in contents of less than 0.05%, optionally calcium in contents of less than 0.005%, the balance being iron and impurities resulting from the preparation . The steel further contains a fine dispersion of active particles consisting of at least one mixed titanium oxide and at least one of aluminum, silicon and zirconium; the number of active particles per

  mm2, counted on a micrographic section, being greater than 25.

  Preferably, the aluminum content and the titanium content satisfy the relationship (with Al and Ti expressed as% by weight): (AI - 0.0022) 2/1, 62 + (Ti - 0.021) 2/132 <10 When the steel contains zirconium, it is preferable that the content of this element be greater than 0.002%. The active particles are then

  consisting of at least one mixed oxide of zirconium and titanium.

  The active particles may also include manganese sulfide. The invention also relates to a process for the manufacture of the steel according to the invention, according to which: - a non-deoxidized liquid steel containing less than 0.005% of aluminum is produced, - silicon and manganese are added, the steel is deoxidized by the carbon under vacuum, then titanium is added,

  and the steel is cast as a half product.

  Preferably, zirconium is added and the steel is cast under

  minutes after the addition of zirconium.

  The invention will now be described in more detail, in a non

  limiting, and illustrated by examples.

  The inventors have found, in a new way, that so-called active particles, finely dispersed in steel, were germination sites for ferrite, not only by a local effect on the interfacial energy, but

  also because of the stresses generated in the metal around them.

  These stresses, which result from differences in expansion coefficient between the metal and the active particles, occur during any thermal cycle to which the steel is subjected, provided that it comprises a heating at a sufficient temperature. Such thermal cycles are encountered in

  many circumstances of use, implementation or manufacture.

  The inventors have also found in a new way that, for the stresses generated around the active particles to have a significant effect, on the one hand, it is necessary that the deformations generated by these stresses be greater than 1.5%. Finally, they found that only particles of mixed titanium oxide and at least one other element taken from aluminum, silicon and zirconium induce local deformation.

greater than 1.5%.

  More specifically, the inventors have found that particles of pure oxides of aluminum, silicon or titanium lead to deformations of less than 1.5%, that particles of mixed oxides of aluminum and titanium, or mixed particles of silicon oxides and titanium lead to deformations slightly greater than 1, 5%, and finally, the mixed oxide particles of zirconium and titanium lead to

deformations greater than 3.5%.

  Because of the large deformations generated in their vicinity, the mixed zirconium oxide and titanium oxide particles are particularly effective sites for the nucleation of ferrite. This efficiency is improved when the active particles comprise a little sulphide of

manganese associated with oxides.

  It is clear that the seeds germinated on these active particles will be finer as the active particles will be more numerous. The inventors have found that to obtain a significant effect, it is necessary that the number of active particles, counted on a micrographic section

of 1 mm2, ie greater than 25.

  To contain active particles as indicated above, the steel must contain: less than 0.004% and preferably less than 0.0035% aluminum to avoid the formation of alumina inclusions pure, but, more than 0.001% to avoid the formation of pure titanium oxides and to promote a fine dispersion of the active particles out of the segregated zones; in addition, when the aluminum content is too low, the transition temperature of resilience in the areas affected by rapid heating cycles at high temperature is degraded; - between 0.01% and 0.03% titanium so that the active particles consist partially of titanium oxide, which is imperative; optionally zirconium in contents of less than 0.006%, and preferably between 0.002% and 0.006%, in order to form zirconium oxides which will be associated with the titanium oxides, without the formation of zirconium nitrides which are unfavorable to tenacity; less than 0.006% of nitrogen to prevent the formation of large titanium or zirconium nitrides which are unfavorable to toughness; more than 0.05% silicon to obtain enough active particles, but less than 0.6% to avoid deteriorating the toughness, especially during welding operations; optionally niobium in contents of up to 0.1%; in low grade this element favors the refinement of the grain, but, beyond 0.1%, it has an adverse effect on the tenacity due to excessive precipitation of carbonitrides; sulfur in contents of less than 0.02%; in general, this element is considered to be an impurity, but, by forming manganese sulphides which associate with the active particles based on oxides, it

  increases the efficiency of these active particles.

  Preferably, in the case of titanium-killed steels (that is to say whose silicon content is low, of the order of less than 0.15%), and in order to obtain optimal active particles consisting of In the case of mixed titanium and aluminum oxides, the aluminum and titanium contents must satisfy the relationship: (AI - 0.0022) 2 / 1.62 + (Ti - 0.021) 2/132 <10-6 it makes it possible, in fact, to define in a plane "titanium content / aluminum content" the composition range most favorable to the formation of mixed oxides of titanium and aluminum in liquid steel or in progress solidification. In addition to the elements which have just been indicated and necessary for controlling the formation of the active particles, the steel contains the elements which give it its general use properties, for example its mechanical characteristics. The content domains for each of these elements define the family of steels to which the active particle technique applies. These are low or medium alloy steels which may have austenite to ferrite conversion and

  have a ferritic or ferritic-pearlitic or ferritic structure

  bainitic, the composition of which comprises by weight (in addition to the elements indicated above):

0.005% <C <0.4%

  0.2% <Mn <2.5% 0% <Ni <6% 0% <Cr <3% 0% <Mo <1.5% 0% <Cu <1%

0% <V <0.2%

0% <B <0.005%

  optionally rare earths in contents of less than 0.05%, and possibly calcium in contents of less than 0.005%,

  the rest being iron and impurities resulting from the elaboration.

  To obtain a fine dispersion of active particles, the steel must be produced according to one or other of the following methods of production: according to a first embodiment, a non-deoxidized liquid steel containing less than 0.005% is produced; of aluminum, to which manganese is added before deoxidizing it under vacuum with carbon, manganese and silicon, so as to obtain an oxygen activity of approximately 30 ppm, and then the titanium is added in the form of ferro -titanium, or ferro-silico-titanium, and finally it is nuanced by adjusting the contents of alloying elements; when the steel must contain zirconium, this element is added at the end of the preparation less than 15 minutes before pouring, whether this is done continuously or ingots; according to a second embodiment, a liquid steel is developed which is deoxidized by silicon and manganese in order to fix the oxygen activity at approximately 40 ppm, then the larger inclusions are allowed to decant, the content of carbon and the titanium is added before final shading; optionally zirconium is added less than 15 minutes

before the casting.

  By way of example and comparison, Steels 1 to 6 according to the prior art, and 7 to 9 according to the invention, were manufactured and their temperatures were measured "TK 28 J" (as defined above , ie measured after heating to

1300 C and rapid cooling).

  The chemical compositions (in thousandths by weight) and the temperatures "TK 28 J" (in C) were: ## STR2 ## 0.6 6 -25 ant-2 82 1620 250 1 12 3 0 0 1.8 3.7 -10 3 82 1590 252 2.2 10 5 18 20 1.6 4.3 -25 4 73 1585 5 3 9 0.7 3 13 7 - 5.2 nd -8 69 1555 101 4 9 1 3 11 4 - 2.5 nd O0 I 6 82 1590 252 2 10 5.2 - - 18 20 1.6 nd -25 inven 7 77 1485 234 na na 3 - 18 0 1,4 1,7 -50 -tion 8 77 1,600 230 2 11 4 - 10 3 2,7 2,5 -

  9 * 68 1488 84 6 13 2 2 14 20 - 2.4 4.3 -

  The steel 9 also contains Ni = 405, Cr = 20, Cu = 191 (in thousandths of a% by weight). The N 7 steel according to the invention contains active particles of 1 to 5 μm consisting of oxides of titanium and aluminum partially associated with manganese sulphide. The refinement of the microstructure is attested by the

  transition temperature "TK 28 J" which is less than - 45 C.

  N 8 steel, also in accordance with the invention, differs from the previous by the presence of a small zirconium addition which leads to the formation of active particles consisting of mixed oxides of zirconium and titanium particularly effective for refining the microstructure. This favorable effect results in a temperature "TK 28 J" 20 K lower than 0 ° compared to the temperature "TK 28 J" of the steel N 7 and, consequently, very

less than - 45 C.

  N 9 steel, according to the invention, has contents of AI and Ti satifaising the relation (AI - 0,0022) 2 / 1,62 + (Ti - 0,021) 2/132 <10-6, has been calmed with titanium

  and has an excellent transition temperature "TK 28 J".

  On the other hand: the steel N 1 killed with aluminum, without titanium contains inclusions of alumina which are not very active with respect to ferritic germination; its temperature "TK 28 J" is only -25 C, which is too high; Silicon and manganese-killed N 2 steel, without titanium, contains inclusions of manganese silicate which are not very active with respect to the germination of ferrite, and the microstructure is very coarse; its temperature "TK 28 J" is only -10 C; However, the low aluminum N 3 steel with titanium and zirconium does not have good toughness because the zirconium content is too high and the steel contains large zirconium nitrides which degrade this property; its temperature "TK 28 J" is -25 C; - Steel N 4, has an aluminum content too low, so that the precipitation of mixed oxides of titanium and aluminum is insufficient; the temperature "TK 28 J" is -8 C, which is much too high; - Steel N 5, has a satisfactory aluminum content but a titanium content too low; the formation of mixed oxides is insufficient and the temperature "TK 28 J" is 0 C, which is even worse than in the previous case; N 6 steel has an excessively high aluminum content and an excessive zirconium content which gives rise to coarse nitrides which are unfavorable to the

  tenacity; the temperature "TK 28 J" is -25 C.

Claims (4)

  1 - Steel characterized in that its chemical composition comprises, by weight: 0.05% <C <0.4%   0.2% <Mn <2.5% 0.05% <Si <0.6% 0% <Ni <6% 0% <Cr <3% 0% <Mo <1, 5% 0% <Cu < 1% 0% <V <0.2% 0% <Nb <0.1% 0% <B <0.005% 0% <S <0.02% 0.001% <AI <0.004% 0.01% <Ti <0.03% 0% <N <0.006%   optionally zirconium in contents of less than 0.006%, optionally rare earths in contents of less than 0.05%, optionally calcium in contents of less than 0.005%, the balance being iron and impurities resulting from the preparation , The steel containing a fine dispersion of active particles consisting of at least one mixed oxide of titanium and at least one element selected from aluminum, silicon and zirconium, the number of active particles per mm 2, counted sure   a micrographic section, being greater than 25.   2 - Steel according to claim 1 characterized in that the aluminum content and the titanium content satisfy the relationship: (AI - 0.0022) 2/1, 62 + (Ti - 0.021) 2/132 <10-6 3 - Steel according to claim 1 characterized in that it contains more than 0.002% of zirconium and in that the active particles are constituted   at least one mixed oxide of zirconium and titanium.   4 - Steel according to any one of claims 1 to 3 characterized in   the active particles further comprise manganese sulfide.   5 - Process for the manufacture of a steel according to any one of   Claims 1 to 4 characterized in that:   a non-deoxidized liquid steel containing less than 0.005% of aluminum is prepared, manganese is added, the steel is deoxidized under vacuum, with carbon, manganese and silicon, so as to obtain an oxygen activity. less than 30 ppm, titanium is added, then it is shaded, and the steel is cast in the form of a half product. 6 Process according to claim 5 characterized in that, after the addition of titanium, zirconium is added and in that the steel is cast less   15 minutes after the addition of zirconium.