EP1323837A1 - Steel product made from carbon steel particularly for galvanisation and process for manufacturing the same - 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 the steel industry. More specifically, it concerns carbon steels of the type to be galvanized, i.e. a deposit of zinc on their surface by soaking the product in a zinc bath liquid. This product is then generally in the form of a strip in scrolling or sheet metal.

Carbon steels intended 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 steel for galvanizing are mainly distinguished by their contents of deoxidizing elements.

The so-called “class 3” steels have a silicon content of 0.15 to 0.25%.

So-called “class 2” steels have a lower silicon content or equal to 0.040%.

So-called “class 1” steels have a lower silicon content or equal to 0.030%.

The continuous production and casting of class 3 steels does not pose no particular problems, because their silicon content makes this element controls the deoxidation of liquid steel by forming dissolved oxygen with oxidized inclusions (possibly in combination with manganese).

For this reason, we do not observe within the liquid steel formation of CO which is likely to cause effervescence of the steel during its 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.

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

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

This latter risk is particularly to be feared when a steel is continuously cast on a machine of the usual type with an ingot mold without a cooled bottom and oscillating, with fixed walls. If an overflow of the steel present in the ingot 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, steel sheets and strips of classes 1 and 2 are usually obtained from semi-finished products which are:

• either poured not continuously, but into ingots in a traditional ingot mold, because this process better tolerates the possible effervescence of steel: the filling of the ingot mold can be interrupted before its overflow if there is a strong effervescence, and even the consequences of 'an overflow is never so serious as to call into question the regular operation of the steelworks; the ingots are then hot rolled to form slabs;
• either continuously cast in the form of slabs on conventional ingot molds without a cooled oscillating bottom with fixed walls, but after adding a relatively large amount of aluminum to the steel so that it is this element which controls the deoxidation by forming solid alumina inclusions, thus preventing the formation of CO, and therefore effervescence.

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

These alumina inclusions can be made liquid by treatment with calcium, but this introduces an additional cost in alloying elements. It is also necessary to prevent reoxidation as much as possible atmospheric during continuous casting, to avoid the formation of new alumina inclusions which cannot be removed before solidification, and which will be found in the final product, whose properties they will degrade mechanical. To this end, argon is injected into the nozzles introducing the steel in the mold, which again increases the manufacturing cost. Of more, there is a risk of trapping argon bubbles at the time of solidification, likely to cause defects in the product.

It would however be interesting to manufacture steels for galvanization of classes 1 and 2 by a process as economical as possible, because these steels have the advantage of allowing coating deposition rates of higher galvanization than class 3 steels. sensitive when galvanizing is carried out by unwinding a strip of steel in a bath of liquid zinc. On the other hand, when an insulated sheet is soaked in the zinc bath, it is important for the quality of the product and the productivity of the installation that this deposit be as quick as possible.

The object of the invention is to put the steelmakers in a position to offer galvanized steel strips and sheets corresponding to shades of classes 1 and 2 previously mentioned, produced at cost minimal, i.e. made from semi-finished products, containing little or no 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 invention relates to a steel product in carbon steel, intended to be galvanized, characterized in that it is in the form of a strip or a sheet obtained from a half product continuously cast and formed from 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%;
• Al _soluble ≤ 0.004%;
• 0.0050% ≤ O _total ≤ 0.0500%, and preferably ≤ 0.0300%;
• 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 ≤ 0.0400%, preferably ≤ 0.0150%;

the remainder being iron and impurities resulting from processing.

The invention also relates to a steel product resulting from galvanizing the previous product.

• 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 invention also relates to a process for obtaining a semi-finished steel product, characterized in that:

• a liquid steel is produced in the ladle, the contents of C, Mn, Si, Al, P, S, Cu, Cr, Ni, Mo, W, Co, Ti, Nb, V, Zr, Sn, Sb, As, B and N are in accordance with those cited above, and the dissolved oxygen content of which is maintained between 0.0050 and 0.0500% by virtue of the establishment of 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 can be a casting machine continuous slabs in a mold with fixed walls.

Said continuous casting machine can be a casting machine continuous thin strips in a mold with one or more movable walls accompanying the product being solidified.

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

• 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 preceding type, characterized in that:

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

The invention also relates to a process for obtaining a product. steelmaking of the previous type, characterized in that one elaborates and one runs steel semi-finished product in the form of a strip, using a continuous casting of thin strips.

The said strip can then be laminated.

The invention also relates to a process for obtaining a product. steel, characterized in that a strip is produced by one of the processes previous, and in that one performs a galvanization of said strip.

• 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, the continuous production and casting of a liquid steel is carried out, the composition characteristics of which meet the conditions required for steels intended for galvanizing of classes 1 or 2 without aluminum. Their casting in the form of semi-finished products which can be used for subsequent galvanizing is made possible under suitable cost and safety conditions by the use of one or the other of these two methods, which can moreover be combined:

• the production of liquid steel under conditions such that a balance between the liquid metal and the pocket slag is established and imposes a sufficiently low dissolved oxygen content to avoid the appearance of effervescence in the ingot mold. the continuous casting machine; this oxygen content must be kept as far as possible between the pocket and the ingot mold;
• casting steel in the form of thin strips (generally from 1 to 10 mm thick), on a casting installation between two rolls or between two running strips, which is more tolerant than a conventional continuous casting machine with oscillating ingot mold with fixed walls vis-à-vis an effervescence of the steel; a casting installation can also be used for this purpose 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 one wishes to obtain has the following characteristics (percentages are percentages 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 steel 2), preferably less than or equal to 0.030% (class 1 steel) for, as mentioned, provide a high deposition rate during galvanizing.

The “total aluminum” content is less than or equal to 0.010%, preferably less than or equal to 0.004%. Aluminum content says "Soluble" (ie soluble in an acid solution at the time of analysis of the sample) is less than or equal to 0.004%. These two conditions come back to say that at least during the last stages of steel making, the content dissolved oxygen was not controlled by an addition of aluminum, and that this one is only found in the final product in trace amounts. These traces are, in the practical, essentially constituted by aluminum present in the form alumina in oxidized inclusions resulting from contact between the metal and the pocket dairy.

The total oxygen content is between 0.0050 and 0.0500%, and preferably between 0.0050 and 0.0300%. This oxygen content results from chemical equilibria that were established in the pocket, during the development, between the liquid metal and the pocket slag, the possible supply of oxygen atmospheric liquid metal that may have occurred between the preparation in the pocket and pouring the metal into the mold, and the efficiency of the decanting of the oxidized inclusions formed during and after the preparation in poached. In general, we aim for a total oxygen content in the final product between 0.0050 and 0.0300%, because above 0.0300%, the properties the mechanical properties of the product may be damaged.

Phosphorus and sulfur contents (less than or equal to 0.20% for sulfur, at 0.10% for phosphorus, preferably less than or equal to 0.030%), copper, chromium, nickel, molybdenum, tungsten, cobalt (lower or equal to 1%, preferably less than or equal to 0.5%), made of 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 steels for galvanizing.

The other elements present are iron and impurities resulting from development.

According to a method of manufacturing a strip or a sheet of steel according to the invention, a steel having the contents is produced in the ladle in C, Mn, Si, P, S, Cu, Cr, Ni, Mo, W, Co, Ti, Nb, V, Zr, Sn, Sb, As, B and N mentioned above. At the very beginning of the elaboration (for example during the pouring in pocket), aluminum can be added to capture most of the dissolved oxygen present in the liquid steel at the time of filling the ladle. This forms alumina inclusions which normally go decant in the pocket slag during the elaboration. But in the following elaboration, generally, no more aluminum will be added, so as to avoid to find, in the final product, more than 0.010% of total aluminum and more than 0.004% soluble aluminum. Under these conditions, if you do not use at all aluminum or if all the aluminum added at the start of production is consumed to form alumina which decant almost entirely thereafter, the content of dissolved oxygen in liquid steel is controlled either by carbon or by the silicon, either by manganese, or by these last two elements simultaneously. Given the very low silicon contents of the steel, it is in most cases the carbon which should drive deoxidation, and this would lead to the formation of CO which would make the steel "effervescent", with all the disadvantages that this entails at the time 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 process 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 which controls deoxidation. To this end, a chemical balance is achieved between the metal and the slag covering the liquid steel in the ladle:

• by adjusting the composition of the slag in a suitable area;
• and by carrying out a stirring of the liquid metal (by a known process, such as the injection of a neutral gas and / or the use of an electromagnetic stirrer) so as to make an intimate contact between the slag and the metal which comes into contact with him in a renewed way.

The steelmaker can determine, using available theoretical models in the literature, what slag compositions can allow it to obtain a given dissolved oxygen content, for given Si and Mn contents. he can adjust the composition of their pocket slag by adding lime, silica, alumina and / or magnesia so as to form a slag synthetic ”. To this end, it can carry out chemical analyzes of the slag in being developed, in order to determine which oxides should be added to it to obtain the desired composition. The result of this practice can be controlled by measurements of the dissolved oxygen content of the liquid steel, made using known electrochemical cells. At the end of development, we obtains a steel whose dissolved oxygen content must be located within prescribed limits 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% Si and 0.8% Mn and balanced with a slag of composition 40% CaO, 35% SiO ₂ , 10% of MnO, 10% MgO, 5% of various oxides contains 70 ppm of dissolved oxygen.

Likewise, a steel containing 0.01% Si and 0.6% Mn and balanced with a slag of composition 35% CaO, 35% SiO ₂ , 20% MnO, 10% MgO and d '' various oxides contains 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, care must be taken that the dissolved oxygen content obtained at the end of ladle preparation is not increased too appreciably as a result of the reoxidation that may occur on contact with the atmosphere. To keep the dissolved oxygen content, several procedures can be proposed which can be combined:

• continue stirring the liquid steel in the ladle during pouring, so as to ensure the conservation of the metal-slag balance in the ladle throughout the duration of the pouring;
• give the covering powder covering the steel present in the distributor of the casting machine a composition providing a metal-slag balance allowing the dissolved oxygen content obtained in the ladle to be kept 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 ladle and distributor and distributor and ingot mold, and / or carry out an integral covering of the distributor and inject non-oxidizing gas under the hood.

Under these conditions, the liquid steel present in the ingot mold at casting time contains insufficient dissolved oxygen content to cause a reaction with the carbon which would give off CO important, risking dangerous effervescence. We thus avoid a risk of liquid metal overflowing out of the mold.

This procedure is applicable to steels continuously cast under slab shape on machines using ingot molds without oscillating bottom with fixed walls. They can be of the conventional type used for casting slabs about 20cm thick which are then hot rolled to obtain hot strips. These can then be galvanized and used as which, or may undergo cold rolling and other heat treatments or thermomechanical before galvanizing.

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

According to another variant of the invention, a steel is cast liquid prepared as above on a continuous casting installation of the type having a bottomless ingot mold including two large movable walls the product being solidified. The two main known processes meeting this characteristic are the casting between two scrolling bands cooled and pouring between two cylinders with cooled horizontal axes internally and rotated in opposite directions. The casting space where takes place the solidification of the product is closed laterally by fixed lateral faces. Products are thus obtained directly in the form of strips, generally 1 to 10mm thick, which can then be hot rolled (possibly on a cage arranged in line with the casting installation). The strip can then be used directly, or undergo cold rolling and various other usual thermomechanical treatments.

In the case of the casting of steels according to the invention, intended especially in galvanizing, the use of such a direct casting installation of strips is advantageous in that the liquid well present in the mold has shallower depth than in a conventional continuous casting mold. The CO bubbles that form in the lower part of the liquid well therefore have less possibility of growth before reaching the surface of the well liquid, and the effervescence is significantly reduced compared to what we would observe when casting the same steel on a conventional continuous casting. In addition, the flared shape towards the top of the mold is more suitable than the practically constant cross section of conventional fixed molds attenuation level variations due to effervescence. Finally, if an overflow liquid metal occurs, its consequences are usually less gravity than in the case of a conventional continuous slab casting, because the organs present under the mold and likely to be affected by steel liquid are less numerous and more easily protectable. If porosities at center of the strip appear on solidification, it is possible to close them by hot rolling.

Alternatively, the strip can be cast on an installation the mold of which has only one movable wall, such as a strip of scrolling or rotating cylinder. We can thus have access to thicknesses tape less than 1mm.

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

Claims (10)

Carbon steel steel product, intended to be galvanized, characterized in that it is in the form of a strip or a sheet obtained from a semi-finished product continuously cast 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%; Al _soluble ≤ 0.004%; 0.0050% ≤ O _total ≤ 0.0500%, and preferably ≤ 0.0300%; 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 ≤ 0.0400%, preferably ≤ 0.0150%; the remainder being iron and impurities resulting from processing. Steel product, characterized in that it results from the galvanization of the product according to claim 1. Process for obtaining a steel semi-finished product, characterized in that : a liquid steel is produced in the ladle, the contents of C, Mn, Si, Al, P, S, Cu, Cr, Ni, Mo, W, Co, Ti, Nb, V, Zr, Sn, Sb, As, B and N are in accordance with those cited in claim 1, and the dissolved oxygen content of which is maintained between 0.0050 and 0.0500% by virtue of the establishment of a chemical equilibrium between the metal and the pocket slag which it covers; and casting said steel on a continuous casting machine. A method according to claim 3, characterized in that said continuous casting machine is a continuous slab casting machine in a mold with fixed walls. A method according to claim 3, characterized in that said continuous casting machine is a continuous casting machine for thin strips in a mold with one or more movable walls accompanying the product being solidified. Method according to claim 5, characterized in that said machine is a continuous casting between cylinders. Process for obtaining a steel product according to claim 1, characterized in that : a steel semi-finished product is produced and poured, using a process according to claim 3 or 4 and laminating said semi-finished product in the form of a strip. Process for obtaining a steel product according to claim 1, characterized in that a semi-steel product is produced and poured in the form of a strip, using a process according to claim 5 or 6. Method according to claim 8, characterized in that said strip is laminated. Process for obtaining a steel product, characterized in that a strip is produced by the process according to one of claims 7 to 9, and in that said strip is galvanized.