EP0849372A1 - Low alloy construction steel having active particles - Google Patents

Abstract

Steel contains by weight 0.05-0.4% carbon, 0.2-2.5% manganese, 0.05-0.6% silicon, 0-6% nickel, 0-3% chromium, 0-1.5% molybdenum, 0-1% copper, 0-0.2% vanadium, 0-0.1% niobium, 0-0.005% boron, 0-0.02% sulphur, 0.001-0.004% aluminium, 0.01-0.03% titanium, 0-0.006% nitrogen and possibly up to 0.006% zirconium, 0.05% rare earths and 0.005% calcium. Its grain structure micrograph shows a fine dispersion of more than 25 active particles per square mm., the particles comprising mixed oxides of Ti with at least one of Al, Si and Zr. Also claimed is a method of preparing the steel from non-deoxidised steel containing less than 0.005% Al, to which is added the manganese before deoxidising under vacuum by means of the C, Mn and Si to obtain an oxygen activity below 30 ppm, then adding the Ti and finally the minor ingredients and casting.

Description

The present invention relates to a steel with active particles which favor obtaining a fine ferritic grain.

It is well known that the mechanical properties of ductility, limit the elasticity and toughness of the steels are better the finer the grain. This is particularly the case for steels whose structure is ferritic, ferrito-pearlitic or ferrito-bainitic. These structures generally result from the transformation on cooling of stable austenitic structures at high temperature and unstable at low temperature. The ferritic grains obtained by these transformations germinate from the austenitic grains and are the smaller the size of the starting austenitic grains. Furthermore, it is well known that the austenitic grain can be refined by suitable thermal or thermomechanical treatments. Also, in order to get steels which have mechanical properties of ductility, yield strength and high toughness, we seek to refine the ferritic grain by thermal or thermomechanical treatments intended to refine the grain austenitic before transformation of austenite into ferrite. These treatments are, for example, normalization by reheating for a time not too long, at a temperature not too high above the temperature transformation into austenite, or thermomechanical treatment with plastic deformation of steel in a temperature range such as, on the one hand the steel has an austenitic structure, and on the other hand that the grains hardened austenitics do not recrystallize as large grains. This technique of grain refinement by heat treatments or thermomechanical is universally used. However, it presents the disadvantage of not being adapted to certain situations in which steel is subjected to thermal cycles imposed by circumstances particular use, the implementation processes or the manufacturing.

To limit the consequences of this drawback, it has been proposed add elements to the steel which may form a fine dispersion stable precipitates at high temperature, which block the growth of austenitic grains. However, this technique is not always sufficiently effective, so that it is sometimes difficult to obtain desired ductility characteristics.

The disadvantage which has just been described can be expressed in a way quantitative using the resilience transition temperature at the level 28 Joules after a thermal cycle consisting of heating to 1300 ° C followed cooling to room temperature at a rate of 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 security of facilities made of steel can only be guaranteed if the temperature "TK 28 J" is below - 45 ° C. However, for the steels in question here, one cannot, in general, not guarantee that the temperature "TK 28 J" will be lower than - 45 ° C. he this results in limitations in the use of these steels which have other advantages besides.

The object of the present invention is to remedy this drawback by providing a steel with improved grain refining ability ferritic and allowing to keep a fine grain, therefore properties of satisfactory ductility even when subjected to poor thermal cycles controlled resulting either from manufacturing conditions or from implementation, that is, finally, special circumstances of use. More specifically, the object of the invention is to provide a steel having, at the same time, a ferritic, ferrito-pearlitic or ferrito-bainitic structure, and a temperature "TK 28 J" below - 45 ° C.

• éventuellement du zirconium en des teneurs inférieures à 0,006%,
• éventuellement des terres rares en des teneurs inférieures à 0,05%,
• éventuellement du calcium en des teneurs inférieures à 0,005%,

le reste étant du fer et des impuretés résultant de l'élaboration. L'acier contient, en outre, une fine dispersion de particules actives constituées au moins d'un oxyde mixte de titane et d'au moins un élément pris parmi l'aluminium, le silicium et le zirconium ; le nombre de particules actives par mm², comptées sur une coupe micrographique, étant supérieur à 25.To this end, 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% ≤ If ≤ 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%

• possibly zirconium in contents of less than 0.006%,
• possibly rare earths in contents of less than 0.05%,
• possibly calcium in contents lower than 0.005%,

the remainder being iron and impurities resulting from processing. The steel also contains a fine dispersion of active particles consisting of at least one mixed titanium oxide and at least one element selected from aluminum, silicon and zirconium; the number of active particles per mm ² , 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 in% by weight): (Al - 0.0022) 2 / 1.6 2 + (Ti - 0.021) 2 / 13 2 ≤ 10 -6

When the steel contains zirconium, it is preferable that the content of this element is 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 contain sulfide of manganese.

• on élabore un acier liquide non désoxydé contenant moins de 0,005% d'aluminium,
• on ajoute du silicium et du manganèse,
• on désoxyde l'acier par le carbone sous vide,
• puis on ajoute du titane,

et on coule l'acier sous forme d'un demi produit.The invention also relates to a method for manufacturing the steel according to the invention, according to which:

• a non-deoxidized liquid steel containing less than 0.005% aluminum is produced,
• we add silicon and manganese,
• the steel is deoxidized by carbon under vacuum,
• then add titanium,

and the steel is poured in the form of a semi-finished product.

Preferably, zirconium is added and the steel is poured less than 15 minutes after the addition of zirconium.

The invention will now be described in more detail, in a manner not limiting, and illustrated by examples.

The inventors have found, in a new way, that so-called particles active, 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 constraints which result from differences in the coefficient of expansion between the metal and active particles, appear during any thermal cycle to which steel is subjected, provided that it involves reheating to a sufficient temperature. Such thermal cycles are encountered in numerous circumstances of use, implementation or manufacture.

The inventors have also found in a new way that, in order for the stresses generated around the active particles have an effect significant, on the one hand, it is necessary that the deformations generated by these constraints are 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 the particles pure aluminum, silicon or titanium oxides lead to deformations of less than 1.5%, than mixed oxide particles aluminum and titanium, or mixed particles of silicon oxides and titanium lead to deformations slightly greater than 1.5%, and, finally, that the particles of mixed zirconium and titanium oxide lead to deformations greater than 3.5%.

Due to the significant deformations generated in their vicinity, the particles of mixed zirconium and titanium oxides are sites of particularly effective ferrite germination. This efficiency is improved when the active particles contain a little sulfide of manganese associated with oxides.

It is clear that the grains which have germinated on these active particles will be all the finer as the active particles will be more numerous. The inventors have found that to obtain a significant effect, the number of active particles, counted on a micrographic section of 1 mm ² , must be greater than 25.

• moins de 0,004 % et, de préférence, moins de 0,0035 % d'aluminium pour éviter la formation d'inclusions d'alumine pure, mais, plus de 0,001% pour éviter la formation d'oxydes de titane purs et pour favoriser une fine dispersion des particules actives hors des zones ségrégées ; de plus, lorsque la teneur en aluminium est trop faible, la température de transition de résilience dans les zones affectées par des cycles de chauffage rapide à haute température est dégradée ;
• entre 0,01% et 0,03% de titane pour que les particules actives soient constituées partiellement d'oxyde de titane, ce qui est impératif ;
• éventuellement du zirconium en des teneurs inférieures à 0,006%, et, de préférence, entre 0,002% et 0,006%, afin de former des oxydes de zirconium qui seront associés aux oxydes de titane, sans qu'il se forme de nitrures de zirconium défavorables à la ténacité ;
• moins de 0,006% d'azote pour éviter la formation de gros nitrures de titane ou de zirconium défavorables à la ténacité ;
• plus de 0,05% de silicium pour obtenir suffisamment de particules actives, mais, moins de 0,6% pour éviter de détériorer la ténacité notamment lors d'opérations de soudage ;
• éventuellement du niobium en des teneurs pouvant aller jusqu'à 0,1% ; en faible teneur cet élément favorise l'affinement du grain, mais, au delà de 0,1% il a un effet défavorable sur la ténacité du fait d'une précipitation trop importante de carbonitrures ;
• du soufre en des teneurs inférieures à 0,02% ; en général, cet élément est considéré comme étant une impureté, mais, en formant des sulfures de manganèse qui s'associent aux particules actives à base d'oxydes, il augmente l'efficacité de ces particules actives.

To contain active particles in accordance with what has been indicated above, the steel must contain:

• less than 0.004% and preferably less than 0.0035% aluminum to avoid the formation of inclusions of pure alumina, but more than 0.001% to avoid the formation of pure titanium oxides and to promote fine dispersion of the active particles outside the segregated areas; moreover, when the aluminum content is too low, the impact toughness transition temperature in the areas affected by rapid heating cycles at high temperature is degraded;
• between 0.01% and 0.03% of 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 unfavorable to tenacity ;
• less than 0.006% nitrogen to avoid the formation of large titanium or zirconium nitrides unfavorable to toughness;
• more than 0.05% of silicon to obtain sufficient 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 content this element promotes the refinement of the grain, but, beyond 0.1% it has an unfavorable effect on the toughness due to too great a 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-quenched 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 '' mixed titanium and aluminum oxides, the aluminum and titanium contents must satisfy the relationship: (Al - 0.0022) 2 / 1.6 2 + (Ti - 0.021) 2 / 13 2 ≤ 10 -6 This makes it possible, in fact, to define in a “titanium content / aluminum content” plan the composition area most favorable to the formation of mixed titanium and aluminum oxides in liquid steel or in solidification courses.

• éventuellement des terres rares en des teneurs inférieures à 0,05%,
• éventuellement du calcium en des teneurs inférieures à 0,005%, le reste étant du fer et des impuretés résultant de l'élaboration.

In addition to the elements which have just been indicated and which are necessary for controlling the formation of active particles, steel contains the elements which give it its general properties of use, for example its mechanical characteristics. The content ranges for each of these elements define the family of steels to which the active particle technique applies. These are low or medium alloyed steels capable of exhibiting a transformation from austenite to ferrite and having a structure of ferritic or ferrito-pearlitic or ferrito-bainitic type, the composition of which comprises by weight (in addition to the elements indicated below). 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%

• possibly 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.

• selon un premier mode de réalisation, on élabore un acier liquide non désoxydé contenant moins de 0,005% d'aluminium, au quel on ajoute du manganèse avant de le désoxyder sous vide par le carbone, le manganèse et le silicium, de façon à obtenir une activité en oxygène d'environ 30 ppm, puis on ajoute le titane soit sous forme de ferro-titane, soit de ferro-silico-titane, et, enfin on met à nuance en ajustant les teneurs en éléments d'alliage ; lorsque l'acier doit contenir du zirconium, cet élément est ajouté en fin d'élaboration moins de 15 minutes avant la coulée, que celle-ci soit effectuée en continu ou en lingots ;
• selon un deuxième mode de réalisation, on élabore un acier liquide qu'on désoxyde par le silicium et le manganèse pour fixer l'activité en oxygène à 40 ppm environ, puis on laisse décanter les plus grosses inclusions, on règle alors la teneur en carbone et on ajoute le titane avant d'effectuer la mise à nuance finale; éventuellement on ajoute du zirconium moins de 15 minutes avant la coulée.

To obtain a fine dispersion of active particles, the steel must be produced according to one or other of the following production methods:

• according to a first embodiment, a non-deoxidized liquid steel containing less than 0.005% aluminum is produced, to which manganese is added before deoxidizing it under vacuum with carbon, manganese and silicon, so as to obtain a oxygen activity of approximately 30 ppm, then the titanium is added either in the form of ferro-titanium or of 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 production less than 15 minutes before casting, whether this is carried out continuously or in ingots;
• according to a second embodiment, a liquid steel is produced which is deoxidized by silicon and manganese to fix the oxygen activity at around 40 ppm, then the larger inclusions are allowed to settle, the carbon content is then adjusted and adding the titanium before effecting the final shade; possibly zirconium is added less than 15 minutes before pouring.

By way of example and comparison, steels 1 to 6 were manufactured according to the prior art, and 7 to 9 according to the invention, 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 of% by weight) and the temperatures "TK 28 J" (in ° C) were: No. VS Mn Yes S P Al V Nb Ti Zr O NOT TK28J prior art 1 80 1580 235 1 11 26 0 0 0.6 6 - 25 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 5 69 1555 101 4 9 1 3 11 4 - 2.5 nd 0 6 82 1590 252 2 10 5.2 - - 18 20 1.6 nd -25 invention 7 77 1485 234 nd nd 3 - - 18 0 1.4 1.7 - 50 8 77 1600 230 2 11 4 - - 10 3 2.7 2.5 - 70 9 68 1488 84 6 13 2 2 14 20 - 2.4 4.3 - 60

Steel No. 7, according to the invention, contains active particles of 1 at 5 µm consisting of titanium and aluminum oxides partially associated with manganese sulfide. The refinement of the microstructure is attested by the transition temperature "TK 28 J" which is less than - 45 ° C.

Steel No. 8, also in accordance with the invention, differs from preceded by the presence of a small addition of zirconium which leads to the formation of active particles made up of mixed oxides of zirconium and titanium particularly effective in refining the microstructure. This effect favorable results in a temperature "TK 28 J" 20 K lower by compared to the temperature "TK 28 J" of steel N ° 7 and, therefore, very below - 45 ° C.

Steel No. 9, according to the invention, has contents of Al and Ti satifaisant relation (Al - 0.0022) ² / 1.6 ² + (Ti - 0.021) ^{2/13 2} ≤ 10- ⁶ , has been calmed with titanium and has an excellent transition temperature "TK 28 J".

• l'acier N°1 calmé à l'aluminium, sans titane contient des inclusions d'alumine peu actives vis à vis de la germination ferritique ; sa température "TK 28 J" n'est que de -25°C, ce qui est trop élevé ;
• l'acier N°2 calmé au silicium et au manganèse, sans titane, contient des inclusions de silicate de manganèse peu actives vis à vis de la germination de la ferrite, et la microstructure est très grossière ; sa température "TK 28 J" n'est que de -10°C ;
• l'acier N°3 à bas aluminium, avec du titane et du zirconium n'a cependant pas une bonne ténacité car la teneur en zirconium est trop élevée et l'acier contient de gros nitrures de zirconium qui dégradent cette propriété ; sa température "TK 28 J" est de - 25°C ;
• l'acier N°4, a une teneur en aluminium trop basse, si bien que la précipitation d'oxydes mixtes de titane et aluminium est insuffisante ; la température "TK 28 J" est de - 8°C, ce qui est beaucoup trop élevé ;
• l'acier N°5, a une teneur en aluminium satisfaisante mais une teneur en titane trop basse ; la formation d'oxydes mixtes est insuffisante et la température "TK 28 J" est de 0°C, ce qui est encore moins bon que dans le cas précédent;
• l'acier N°6 a une teneur en aluminium trop forte et une teneur en zirconium excessive qui donne naissance à des nitrures grossiers défavorables à la ténacité ; la température "TK 28 J" est de -25°C.

Conversely :

• steel N ° 1 calmed with aluminum, without titanium contains inclusions of alumina not very active with respect to ferritic germination; its temperature "TK 28 J" is only -25 ° C, which is too high;
• steel N ° 2 calmed with silicon and manganese, without titanium, contains inclusions of manganese silicate little active with respect to the germination of ferrite, and the microstructure is very coarse; its temperature "TK 28 J" is only -10 ° C;
• No. 3 steel with low aluminum, with titanium and zirconium, however, 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 No. 4 has too low an aluminum content, 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 No. 5, has a satisfactory aluminum content but too low a titanium content; 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;
• steel No. 6 has too high an aluminum content and an excessive zirconium content which gives rise to coarse nitrides which are unfavorable to toughness; the temperature "TK 28 J" is -25 ° C.

Claims (6)

Steel characterized in that its chemical composition comprises, by weight: 0.05% ≤ C ≤ 0.4% 0.2% ≤ Mn ≤ 2.5% 0.05% ≤ If ≤ 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% possibly zirconium in contents of less than 0.006%, possibly rare earths in contents of less than 0.05%, possibly calcium in contents lower than 0.005%, the remainder being iron and impurities resulting from the production, the steel containing a fine dispersion of active particles consisting of at least one mixed titanium oxide 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. Steel according to claim 1 characterized in that the aluminum content and the titanium content satisfy the relationship: (Al - 0.0022) 2 / 1.6 2 + (Ti - 0.021) 2 / 13 2 ≤ 10 -6 Steel according to claim 1 characterized in that it contains more 0.002% zirconium and in that the active particles consist at least one mixed oxide of zirconium and titanium. Steel according to any one of Claims 1 to 3, characterized in that the active particles also contain manganese sulfide. 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% aluminum is produced, we add manganese, the steel is deoxidized under vacuum, with carbon, manganese and silicon, so as to obtain an oxygen activity of less than 30 ppm, we add titanium, then we put nuances, and the steel is poured in the form of a semi-finished product. Method according to claim 5 characterized in that, after the addition of titanium, we add zirconium and in that we pour the steel less 15 minutes after the addition of zirconium.