EP1323837B1 - Process for manufacturing steel product made from carbon steel particularly suitable for galvanisation. - Google Patents

Abstract

The product is made from galvanized carbon steel composed the following elements by weight: carbon at 0.0005 to 0.15%; manganese at 0.08 to 2%; silicon at equal or less than 0.04%; aluminum at equal or less than 0.004%; oxygen at 0.005 to 0.05%; phosphorus at equal or less than 0.2%; sulfur at equal or less than 0.1%; and copper, chromium, nickel, molybdenum, wolfram, and cobalt each at equal or less than 1%. Other elements include titanium, niobium, vanadium, zirconium each at equal or less than 0.5%; boron at equal or less than 0.1%; nickel at equal or less than 0.04%; and tin, antimony and arsenic each at equal or less than 0.1%. The remaining composition of the product is iron and impurities than result during production. Independent claims are also included for the following: (a) a metallurgical intermediate product manufacturing method; and (b) a metallurgical product manufacturing method.

Description

The invention relates to iron and steel industry. More specifically, it relates to carbon steels of the type of those to be galvanized, that is to say a deposit of zinc on their surface by soaking the product in a liquid zinc bath. This product is usually in the form of a scrolling strip or sheet metal.

Carbon steels for galvanizing are steels containing a maximum of 0.15% carbon and 0.08 to 2% manganese, as well as the usual alloying elements and impurities in carbon steels. The different classes of galvanizing steel are distinguished mainly by their deoxidizing element contents.

"Class 3" steels have a silicon content of 0.15 to 0.25%.

"Class 2" steels have a silicon content less than or equal to 0.040%.

The so-called "class 1" steels have a silicon content less than or equal to 0.030%.

The elaboration and continuous casting of class 3 steels does not pose any particular problems, because their silicon content makes this element the deoxidation of the liquid steel by forming oxidized inclusions with dissolved oxygen (possibly in combination with manganese).

For this reason, it is not observed in the liquid steel forming CO that could cause an effervescence of the steel during casting.

• d'une part, elle provoque souvent lors de la solidification de l'acier l'apparition de « soufflures », dans la zone centrale du produit, c'est à dire de porosités correspondant à l'emplacement de poches de gaz présentes au moment de la solidification ; cet inconvénient peut cependant être annulé si l'acier subit ensuite un fort laminage à chaud qui va refermer ces porosités ;
• d'autre part, si l'effervescence devient inopinément trop importante, il y a un risque que l'acier déborde de la lingotière où a lieu sa solidification.

This is not the case for Class 1 and Class 2 steels. In their case, the silicon content is too low for this element to be involved in the deoxidation process. It is then the carbon that drives this deoxidation, and this results in a formation and release of CO, making the steel "effervescent". This effervescence has two disadvantages:

• on the one hand, it often causes during the solidification of steel the appearance of "blowholes" in the central zone of the product, ie porosity corresponding to the location of pockets of gas present at the time solidification; this disadvantage can however be canceled if the steel then undergoes a strong hot rolling which will close these porosities;
• on the other hand, if the effervescence becomes unexpectedly too great, there is a risk that the steel will overflow from the mold where its solidification takes place.

This last risk is particularly to be feared when a steel is poured continuously on a machine of the usual type to cooled bottomless mold. and oscillating, with fixed walls. If an overflow of the steel present in the mold occurs, it represents a danger for the personnel present around, and causes serious damage to the casting machine.

• soit coulés non en continu, mais en lingots dans une lingotière traditionnelle, car ce procédé tolère mieux les possibles effervescences de l'acier: le remplissage de la lingotière peut être interrompu avant son débordement si on constate une forte effervescence, et même les conséquences d'un débordement ne sont jamais graves au point de remettre en cause la marche régulière de l'aciérie ; les lingots sont ensuite laminés à chaud pour former des brames ;
• soit coulés en continu sous forme de brames sur des machines classiques à lingotière sans fond refroidie oscillante à parois fixes, mais après adjonction à l'acier d'une quantité relativement importante d'aluminium pour que ce soit cet élément qui pilote la désoxydation en formant des inclusions d'alumine solides, empêchant ainsi la formation de CO, donc l'effervescence.

For this reason, Class 1 and 2 steel sheets and strips are usually obtained from semi-finished products which are:

• not cast continuously, but ingots in a traditional mold, because this process is more tolerant of possible effervescence of the steel: the filling of the mold can be interrupted before its overflow if there is a strong effervescence, and even the consequences of an overflow are never so serious as to call into question the regular progress of the steelworks; the ingots are then hot rolled to form slabs;
• is continuously cast in the form of slabs on conventional fixed-wall oscillating cooled bottomless mold machines, but after the addition of a relatively large quantity of aluminum to the steel for this element which controls deoxidation by forming inclusions of solid alumina, thus preventing the formation of CO, thus effervescence.

These two methods are not ideal, however. Ingot casting is notoriously less productive than continuous casting and then requires a greater number of hot rolling steps to obtain a product of a given thickness. As for deoxidation with aluminum, it is more expensive in alloying elements. In addition, the inclusions of alumina must be as much as possible removed before the continuous casting step so that they do not risk clogging the nozzles of the distributor of the casting machine.

These liquid alumina inclusions can be made by a calcium treatment, but this introduces an additional cost of alloying elements. It is also necessary to prevent atmospheric reoxidation as much as possible during continuous casting, to avoid the formation of new inclusions of alumina that can not be eliminated before solidification, and which will end up in the final product, they will degrade the mechanical properties. For this purpose, argon is injected into the nozzles introducing the steel into the mold, which again increases the cost of manufacture. In addition, there is a risk of entrapment of argon bubbles during solidification, which may cause defects in the product.

It would, however, be interesting to manufacture the galvanizing steels of Classes 1 and 2 as economically as possible, because these steels have the advantage of allowing higher deposition rates of the galvanizing coating than Class 3 steels. This advantage is little Sensitive when galvanizing is carried out by unwinding a steel strip in a liquid zinc bath. On the other hand, when an insulated sheet is dipped in the zinc bath, it is important for the quality of the product and the productivity of the installation that this deposit is as fast as possible.

The document EP-A-0785283 describes low-carbon steels deoxidized in two stages: first by addition of aluminum, then by addition of titanium, in order to avoid clogging of the casting nozzles. The document US Patent 4024624 discloses low deoxidized steels to which magnesium is added to improve the ductility and weldability of the hot-rolled sheets made therewith. The document EP-A-0906960 discloses steels for titanium deoxidized galvanizing and inclusions of controlled compositions. The document US Patent 4073643 describes steels with low levels of deoxidizing elements and can be coated, and cast by a conventional continuous casting process.

The object of the invention is to enable steelmakers to propose galvanizing steel strips and sheets corresponding to grades 1 and 2 previously mentioned, produced at minimum costs, that is to say produced from semi-finished products cast continuously, and containing no or very little aluminum.

• 0,0005% ≤ C ≤ 0,15% ;
• 0,08% ≤ Mn ≤ 2% ;
• Si ≤ 0,040%, de préférence ≤ 0,030% ;
• Al_total ≤ 0,010%, de préférence ≤ 0,004% ;
• Al_soluble ≤ 0,004% ;
• 0,0050% ≤ O_total ≤ 0,0500%, et de préférence ≤ 0,0300% ;
• P ≤ 0,20%, de préférence ≤ 0,03% ;
• S ≤ 0,10%, de préférence ≤ 0,03% ;
• chacun des éléments Cu, Cr, Ni, Mo, W, Co ≤ 1%, de préférence ≤ 0,5% ;
• chacun des éléments Ti, Nb, V, Zr ≤ 0,5%, de préférence ≤ 0,2% ;
• chacun des éléments Sn, Sb, As ≤ 0,1 % ;
• B ≤ 0,1%, de préférence ≤ 0,01% ;
• N ≤ 0,0400%, de préférence ≤ 0,0150% ;

le reste étant du fer et des impuretés résultant de l'élaboration.To this end, the object of the invention is to obtain a carbon steel steel product, intended to be galvanized, characterized in that it is in the form of a strip or sheet obtained from a half-cast product continuously and formed of a steel of composition by weight:

• 0.0005% ≤ C ≤ 0.15%;
• 0.08% ≤ Mn ≤ 2%;
• If ≤ 0.040%, preferably ≤ 0.030%;
• Al _total ≤ 0.010%, preferably ≤ 0.004%;
• _Soluble Al ≤ 0.004%;
• 0.0050% ≤ 0 _total ≤ 0.0500%, and preferably ≤ 0.0300%;
• P ≤ 0.20%, preferably ≤ 0.03%;
• S ≤ 0.10%, preferably ≤ 0.03%;
• each of Cu, Cr, Ni, Mo, W, Co ≤ 1%, preferably ≤ 0.5%;
• each of the elements Ti, Nb, V, Zr ≤ 0.5%, preferably ≤ 0.2%;
• each of the elements Sn, Sb, As ≤ 0.1%;
• B ≤ 0.1%, preferably ≤ 0.01%;
• N ≤ 0.0400%, preferably ≤ 0.0150%;

the rest being iron and impurities resulting from the elaboration.

• on élabore en poche un acier liquide dont les teneurs en C, Mn, Si, Al, P, S, Cu, Cr, Ni, Mo, W, Co, Ti, Nb, V, Zr, Sn, Sb, As, B et N sont conformes à celles citées précédemment, et dont on maintient la teneur en oxygène dissous entre 0,0050 et 0,0500% grâce à l'établissement d'un équilibre chimique entre le métal et le laitier de poche qui le recouvre ;
• et on coule ledit acier sur une machine de coulée continue.

The subject of the invention is also a process according to claim 1, a semi-finished steel product, characterized in that:

• a liquid steel is produced in the pocket, the contents of which are C, Mn, Si, Al, P, S, Cu, Cr, Ni, Mo, W, Co, Ti, Nb, V, Zr, Sn, Sb, As, B and N are consistent with those mentioned above, and whose oxygen content is maintained dissolved between 0.0050 and 0.0500% by establishing a chemical equilibrium between the metal and the pocket slag which covers it;
• and casting said steel on a continuous casting machine.

Said continuous casting machine may be a continuous slab casting machine in a fixed-wall mold.

Said continuous casting machine may be a machine for continuous casting of thin strips in a mold with one or more moving walls accompanying the product being solidified.

Said machine can, in this case, be a continuous casting between rolls.

• on élabore et on coule un demi-produit sidérurgique, en utilisant un procédé tel que précédemment décrit
• et on lamine ledit demi-produit sous forme d'une bande.

The subject of the invention is also a process for obtaining a steel product of the above type, characterized in that:

• a steel semi-finished product is produced and cast using a process as previously described
• and laminating said semi-finished product in the form of a strip.

The subject of the invention is also a process for obtaining a steel product of the preceding type, characterized in that a steel semi-finished product is produced and cast in the form of a strip, using a continuous casting machine. thin strips.

This strip can then be rolled.

The subject of the invention is also a process for obtaining a steel product, characterized in that a strip is produced by one of the preceding processes, and in that a strip is galvanized.

• l'élaboration de l'acier liquide dans des conditions telles qu'un équilibre entre le métal liquide et le laitier de poche s'établit et impose une teneur en oxygène dissous suffisamment basse pour éviter l'apparition d'une effervescence dans la lingotière de la machine de coulée continue ; cette teneur en oxygène doit être conservée autant que possible entre la poche et la lingotière ;
• la coulée de l'acier sous forme de bandes minces (généralement de 1 à 10 mm d'épaisseur), sur une installation de coulée entre deux cylindres ou entre deux bandes en défilement, qui est plus tolérante qu'une machine de coulée continue classique à lingotière oscillante à parois fixes vis-à-vis d'une effervescence de l'acier ; on peut également utiliser à cet effet une installation de coulée sur une surface en mouvement unique, telle qu'une bande en défilement ou un cylindre en rotation.

As will be understood, according to the invention is carried out the development and continuous casting of a liquid steel whose composition characteristics meet the requirements for steels for galvanizing classes 1 or 2 without aluminum. Their casting in the form of exploitable semi-products for subsequent galvanizing is made possible under conditions of cost and safety suitable by the use of one or the other of these two methods, which can also be combined:

• the development of the liquid steel under conditions such that a balance between the liquid metal and the pocket slag is established and imposes a dissolved oxygen content sufficiently low to avoid the appearance of effervescence in the mold of the continuous casting machine; this content oxygen should be kept as much as possible between the pocket and the mold;
• the casting of steel in the form of thin strips (generally 1 to 10 mm thick), on a casting system between two cylinders or between two moving strips, which is more tolerant than a conventional continuous casting machine with oscillating mold with fixed walls vis-à-vis the effervescence of the steel; it is also possible to use for this purpose a casting installation on a single moving surface, such as a moving strip or a rotating cylinder.

The invention will be better understood on reading the description which follows.

In general, the composition of the steel that is desired to obtain has the following characteristics (the percentages are percentages by weight).

The carbon content is between 0.0005% and 0.15%.

The manganese content is between 0.08% and 2%.

The silicon content is less than or equal to 0.040% (class 2 steel), preferably less than or equal to 0.030% (class 1 steel) for, as has been said, to provide a high deposition rate during galvanization .

The "total aluminum" content is less than or equal to 0.010%, preferentially less than or equal to 0.004%. The so-called "soluble" aluminum content (that is to say soluble in an acid solution at the time of the analysis of the sample) is less than or equal to 0.004%. These two conditions amount to saying that at least during the last stages of steel making, the dissolved oxygen content has not been controlled by adding aluminum, and that this is not found in the final product as a trace. These traces are, in practice, essentially constituted by aluminum present in the form of alumina in the oxidized inclusions resulting from the contacts between the metal and the pocket slag.

The total oxygen content is between 0.0050 and 0.0500%, and preferably between 0.0050 and 0.0300%. This oxygen content results from the chemical equilibrium which has been established in the pocket during the preparation, between the liquid metal and the pocket slag, of the possible supply of atmospheric oxygen to the liquid metal which may have occurred. between the elaboration in the pocket and the casting of the metal in the mold, and the efficiency of the decantation process of the oxidized inclusions formed during and after the preparation in pocket. In general, a total oxygen content in the final product of between 0.0050 and 0.0300% is sought, since beyond 0.0300%, the mechanical properties of the product may be deteriorated.

Phosphorus and sulfur contents (less than or equal to 0.20% for sulfur, 0.10% for phosphorus, preferably less than or equal to 0.030%), copper, chromium, nickel, molybdenum, tungsten, cobalt (less than or equal to 1%, preferably less than or equal to 0.5%), titanium, niobium, vanadium, zirconium (less than or equal to 0.5%, preferably less than or equal to 0.2%), tin, antimony, arsenic (less than or equal to 0.1%), boron (less than or equal to 0.1%, preferably equal to 0.01%) and nitrogen (less than or equal to 0.0400% , preferably less than or equal to 0.015%) corresponding to the most usual requirements in galvanizing steels.

The other elements present are iron and impurities resulting from the elaboration.

According to a method for manufacturing a strip or sheet of a steel according to the invention, a steel having the contents of C, Mn, Si, P, S, Cu, Cr, is produced in the ladle. Ni, Mo, W, Co, Ti, Nb, V, Zr, Sn, Sb, As, B and N cited above. At the very beginning of the elaboration process (for example during ladle casting), aluminum can be added to capture most of the dissolved oxygen present in the liquid steel at the time of filling the ladle. casting. Alumina inclusions are thus formed which will normally decant in the pocket slag during the preparation. But in the following development, generally, we will not add more aluminum, so as to avoid finding in the final product, more than 0.010% total aluminum and more than 0.004% soluble aluminum. Under these conditions, if we do not use aluminum at all or if all the aluminum added at the beginning of processing is consumed to form alumina, which decants almost all thereafter, the dissolved oxygen content liquid steel is controlled either by carbon, silicon, manganese, or the latter two elements simultaneously. Given the very low silicon content of steel, it is in most cases carbon that should drive the deoxidation, and this would lead to the formation of CO that would make the steel "effervescent", with all the disadvantages that this entails at the moment of casting, as has already been said.

• en réglant la composition du laitier dans un domaine adéquat ;
• et en réalisant une agitation du métal liquide (par un procédé connu, tel que l'injection d'un gaz neutre et/ou l'utilisation d'un brasseur électromagnétique) de manière à réaliser un contact intime entre le laitier et le métal qui vient à son contact de manière renouvelée.

According to the manufacturing method according to the invention, the steelmaker responsible for the preparation ensures that despite its low content, silicon (possibly in combination with manganese), is the element that drives deoxidation. For this purpose, a chemical equilibrium is achieved between the metal and the slag covering the liquid steel in the pocket:

• by regulating the composition of the slag in a suitable field;
• and by agitating the liquid metal (by a known method, such as injecting a neutral gas and / or using an electromagnetic stirrer) so as to make an intimate contact between the slag and the metal which comes to his contact in a renewed way.

The steelmaker can determine, using theoretical models available in the literature, which slag compositions can enable him to obtain a given dissolved oxygen content, for given Si and Mn contents. He can adjust the composition of his pocket slag by adding lime, silica, alumina and / or magnesia to form a "synthetic slag". For this purpose, it can proceed to chemical analyzes of slag being developed, so as to determine which oxides must be added to obtain the desired composition. The result of this practice can be controlled by measurements of the dissolved oxygen content of the molten steel made using known electrochemical cells. At the end of the preparation, a steel is obtained whose dissolved oxygen content must be within the limits prescribed for the total oxygen content of the steel according to the invention, and the ladle is sent to the continuous casting plant. .

By way of example, it can be said that a steel containing 0.02% of Si and 0.8% of Mn and equilibrated with a slag of composition 40% CaO, 35% SiO ₂ , 10% of MnO, 10% MgO, 5% various oxides contains 70 ppm of dissolved oxygen.

Similarly, a steel containing 0.01% Si and 0.6% Mn and equilibrated with a slurry of composition 35% CaO, 35% SiO ₂ , 20% MnO, 10% MgO and more. Various oxides contain 100 ppm of dissolved oxygen.

• continuer le brassage de l'acier liquide en poche pendant la coulée, de manière à assurer la conservation de l'équilibre métal-laitier dans la poche pendant toute la durée de la coulée ;
• conférer à la poudre de couverture recouvrant l'acier présent dans le répartiteur de la machine de coulée une composition procurant un équilibre métal-laitier permettant de conserver la teneur en oxygène dissous obtenue dans la poche dans les limites recherchées ;
• protéger autant que possible le métal liquide des réoxydations atmosphériques en l'exposant à un gaz non oxydant (argon, hélium, voire azote si on accepte une teneur en azote relativement élevée dans le métal final) jusqu'à son introduction dans la lingotière ; à cet effet on peut réaliser une injection de gaz non oxydant dans les tubes en réfractaire protégeant les jets de coulée entre poche et répartiteur et répartiteur et lingotière, et/ou réaliser un capotage intégral du répartiteur et injecter du gaz non oxydant sous le capot.

During continuous casting, it must be ensured that the dissolved oxygen content obtained at the end of the bagging process is not increased too much subsequently because of the reoxidation that may occur in contact with the atmosphere. To preserve the dissolved oxygen content, one can propose several procedures that can be cumulated:

• continue the stirring of the liquid steel in the ladle during casting, so as to ensure the preservation of the metal-milk balance in the pocket during the entire duration of the casting;
• imparting to the covering powder covering the steel present in the tundish of the casting machine a composition providing a metal-milk balance that makes it possible to keep the dissolved oxygen content obtained in the pocket within the desired limits;
• protect the liquid metal as much as possible from atmospheric reoxidation by exposing it to a non-oxidizing gas (argon, helium or even nitrogen if a relatively high nitrogen content is accepted in the final metal) until it is introduced into the mold; for this purpose it is possible to inject non-oxidizing gas into the refractory tubes protecting the pouring jets between the ladle and tundish and tundish and mold, and / or to realize an integral rollover of the tundish and to inject non-oxidizing gas under the bonnet.

Under these conditions, the liquid steel present in the ingot mold at the time of casting contains a dissolved oxygen content insufficient to cause a reaction with carbon which would lead to a significant release of CO, potentially causing a dangerous effervescence. This avoids a risk of overflow of the liquid metal out of the mold.

This procedure is applicable to steels continuously cast in the form of slabs on machines using oscillating molds with fixed walls. They may be of the conventional type used to cast slabs of the order of 20 cm thick which are then hot-rolled to obtain hot stripes. These can then be galvanized and used as is, or can be cold rolled and other heat or thermomechanical treatments before galvanizing.

It is also possible to use for this purpose thin slab casting installations, on which the thickness of the product at the output of the machine is of the order of 3 to 15 cm, possibly after the product leaving the mold has undergone an operation of compression on liquid heart. The slabs thus cast are then hot-rolled.

According to another variant of the invention, the casting of a liquid steel produced as above is carried out on a continuous casting plant of the type having a bottomless mold with two large moving walls accompanying the product being solidified. The two main known processes corresponding to this characteristic are the casting between two cooled scrolling strips and the casting between two cylinders with horizontal axes internally cooled and rotated in opposite directions. The casting space where the solidification of the product takes place is closed off laterally by fixed lateral faces. Thus products are obtained directly in the form of strips, generally from 1 to 10 mm in thickness, which can then undergo hot rolling (possibly on a cage arranged in line with the casting installation). The strip can then be used directly, or cold rolled, and various other conventional thermomechanical treatments.

In the case of the casting of steels according to the invention, intended in particular for galvanizing, the use of such a direct strip casting plant is advantageous in that the liquid well present in the mold has less depth than in a conventional continuous casting mold. The bubbles of CO that form in the lower part of the liquid well therefore have a lower possibility of growth before reaching the surface of the liquid well, and the effervescence is substantially attenuated compared to what would be observed during the casting of the same steel on a conventional continuous casting. In addition, the flared up shape of the mold is more suitable than the substantially constant section of conventional fixed molds to attenuate the level variations due to effervescence. Finally, if an overflow of liquid metal occurs, its consequences are generally of a lesser gravity than in the case of a continuous casting of conventional slabs, because the bodies present under the mold and likely to be reached by the steel less liquid and more easily protectable. If porosities in the center of the band appear on solidification, it is possible to close them by hot rolling.

In a variant, the casting of the strip can be carried out on an installation whose mold has only one moving wall, such as a moving strip or a rotating cylinder. It is thus possible to have access to band thicknesses of less than 1 mm.

It goes without saying that the products according to the invention can find applications outside the strict field of galvanization.

Claims (4)

1. A process for obtaining a semi-finished steel product, characterised in that:

   - in a ladle a liquid steel is processed which has the following composition_by weight:

   - 0.0005% ≤ C ≤ 0.15%;

   - 0.08% ≤ Mn ≤ 2%:

   - Si ≤ 0.040%, preferably ≤ 0.030%;

   - Al_total ≤ 0.010%, preferably ≤ 0.004%;

   - Al_soluble ≤ 0.004%;

   - P ≤ 0.20%, Preferably ≤ 0.03%;

   - S ≤ 0.10%, preferably ≤ 0.03%;

   - each of the elements Cu, Cr, Ni, Mo, W, Co ≤ 1%, preferably ≤ 0.5%;

   - each of the elements Ti, Nb, V, Zr ≤ 0.5%, preferably ≤ 0.2%;

   - each of the elements Sn, Sb, As ≤ 0.1%;

   - B ≤ 0.1%, preferably ≤ 0,01%;

   - N s 0.0400%, preferably ≤ 0,0150%;

   the rest comprising iron and impurities resulting from the processing
   and the dissolved oxygen content of which is kept at between 0.0050 and 0.0500% by establishing a chemical equilibrium between the metal and the ladle slag that covers it;
   and said steel is cast onto a continuous caster in thin strips in an ingot mould with one or more fixed or moveable walis accompanying the product during solidification, limiting reoxidation between ladle processing and casting the steel into the ingot mould such as to obtain a total oxygen content in the semi-finished product of between 0.0050 and 0.0500%, preferably between 0.0050 and 0,0300%, and to prevent the appearance of effervescence in the ingot mould.
2. The process according to Claim 1, characterised in that said machine provides continuous casting between cylinders.
3. A process for obtaining a steel product, characterised in that:

   - a semi-finished steel product is processed and cast using a process according to Claim 1 or 2

   - and said semi-finished product is laminated in the form of a strip.
4. A process for obtaining a steel product, characterised in that a strip is processed by the process according to any of Claims 1 to 3, and in that galvanisation of said strip is carried out.