EP1466024B1 - Method for the production of a siderurgical product made of carbon steel with a high copper content, and siderurgical product obtained according to said method - Google Patents

Abstract

The invention relates to a method for producing a siderurgical product made of carbon steel having a high copper content, according to which:-a liquid steel having the composition: 0.0005% 1%; 0.5 Cu 10%; 0 Mn 2%; 0 Si 5% 0 Ti 0.5%; 0 Nb 0.5%; 0 Ni 5%; 0 Al 2%, the remainder being iron and impurities, is produced;-said liquid steel is poured directly in the form of a thin strip having a thickness of no more than 10 mm;-the strip is subjected to forced cooling and/or is surrounded by a non-oxidizing atmosphere while having a temperature of more than 1000? C;-said thin strip is hot rolled at a reduction rate of at least 10%, the temperature at the end of the rolling process being such that all of the copper is still in a solid solution in the ferrite and/or austenite matrix;-and the strip is coiled. The invention also relates to a siderurgical product obtained according to said method.

Description

The invention relates to the field of the production of ferrous alloys, and more specifically the field of the production of steels with high levels of copper.

Copper is generally regarded as an undesirable element in carbon steels, because by favoring hot cracking, on the one hand it makes the hot working of steel difficult, and on the other hand it degrades the quality and the appearance of the surface of the products. For these reasons, it is customary to limit the copper content of high quality carbon steels to levels of less than 0.05%. Since it is not possible to remove the copper present in the molten steel, obtaining these low levels of copper is possible only by producing the steel from molten iron, which is economically viable only for production in large quantities, or by producing steel in electric furnace by melting carefully selected scrap, thus expensive.

There are, however, cases where the presence of a high copper content in the steel may be desirable. Indeed, copper can have beneficial effects for certain applications, especially for the automotive industry.

In the first place, it increases the resistance to deformation of steel by a precipitation that can be obtained by means of an income (structural hardening).

On the other hand, it improves the steel's resistance to atmospheric corrosion because it leads to the formation of a protective oxide layer.

• du fait de la formation de ladite couche d'oxyde protectrice ;
• en se substituant au manganèse, il limite la formation des inclusions de MnS autour desquelles l'hydrogène s'accumule.

Finally, it increases resistance to embrittlement by hydrogen in two ways:

• due to the formation of said protective oxide layer;
• by substituting for manganese, it limits the formation of MnS inclusions around which hydrogen accumulates.

The increase in steel strength due to structural hardening can be evaluated at about 300 MPa per 1% copper. However, it appears difficult to take advantage of this phenomenon, in that in conventional production lines of sheet by continuous casting of thick or thin slabs, hot rolling strip mill and cold rolling, copper leads to deterioration surface quality by skin cracking during hot processing in an oxidizing atmosphere. This cracking is called "crazing". A copper content of less than 1% or even 0.5% is then imperative, unless this cracking is limited by an addition of nickel or silicon, or by reheating before hot processing at a temperature below the melting point. peritectic copper (1094 ° C for a pure Fe-Cu alloy), which restricts the range of accessible thicknesses, or by a control of the reheating atmosphere incompatible with current production facilities.

In addition, the curing power of the copper by precipitation is optimal when the copper is maintained completely in solid solution before the precipitation treatment by quenching. Indeed, the contribution of the precipitation to curing is even lower than the precipitation temperature is high. Copper should not be allowed to precipitate on cooling until the tempering temperature is reached. The conventional production line does not allow the execution of such a quenching necessary to maximize the hardening power.

EP-A-1 072 689 discloses a method of manufacturing TRIP type steel thin strips by direct casting of liquid steel optionally containing between 0.5 and 2% copper. The cast strip is hot rolled and undergoes two forced cooling separated by a residence at temperatures between 55 ° and 400 ° C for there to occur a bainitic transformation.

It was proposed in the document EP-A-0 641 867 to produce carbon steel strips containing large quantities of copper (0.3 to 10%) and tin (0.03 to 0.5%) by a thin strip direct casting process of 0.1 at 15mm thickness, such as casting between rolls. The rapid solidification of the strip and the possibility of limiting, by cooling following this solidification, the residence time of the strip at more than 1000 ° C. makes it possible to solve the surface quality problems mentioned above. The strip is then cold rolled. It is thus possible to develop strips having good mechanical properties and a good surface appearance without resorting to raw materials that are poor in copper and tin. For this, we must obtain a product which, after its solidification, the primary dendrites are spaced from 5 to 100 microns. The mechanical properties sought on the thin strip are essentially good strength and good elongation to traction. This document, however, does not discuss in detail the post-casting treatments that would lead to a workable sheet for industrial application.

The object of the invention is to provide complete production processes for hot-rolled or cold-rolled carbon steel sheets having high mechanical properties, in particular high strength, good anisotropy of the deformations, as well as good welding properties, in which a high copper content is tolerated or even desired.

• on élabore un acier liquide ayant la composition, exprimée en pourcentages pondéraux :
  • * 0,0005% ≤ C ≤ 1 %
  • * 0,5 ≤ Cu ≤ 10%
  • * 0 ≤ Mn ≤2%
  • * 0 ≤ Si ≤ 5%
  • * 0 ≤ Ti ≤ 0,5%
  • * 0 ≤ Nb ≤ 0,5%
  • * 0 ≤ Ni ≤ 5%
  • * 0 ≤ Al ≤ 2%
  le reste étant du fer et des impuretés résultant de l'élaboration ;
• on coule cet acier liquide directement sous forme d'une bande mince d'épaisseur inférieure ou égale à 10 mm ;
• on refroidit rapidement la bande jusqu'à une température inférieure ou égale à 1000°C par aspersion d'eau ou d'un mélange eau-air;
• on fait subir à la bande mince un laminage à chaud à un taux de réduction d'au moins 10%, la température de fin de laminage étant telle qu'à cette température, tout le cuivre se trouve encore en solution solide dans la matrice de ferrite et/ou d'austénite ;
• on fait ensuite subir à la bande une étape de refroidissement forcé de manière à maintenir le cuivre en solution solide sursaturée dans la matrice de ferrite et/ou d'austénite ;
• et on bobine la bande ainsi refroidie.

To this end, the subject of the invention is a process for the production of a steel product made of copper-rich carbon steel, according to which:

• a liquid steel having the composition, expressed in percentages by weight, is produced:
  • * 0.0005% ≤ C ≤ 1%
  • * 0.5 ≤ Cu ≤ 10%
  • * 0 ≤ Mn ≤2%
  • * 0 ≤ If ≤ 5%
  • * 0 ≤ Ti ≤ 0.5%
  • * 0 ≤ Nb ≤ 0.5%
  • * 0 ≤ Ni ≤ 5%
  • * 0 ≤ Al ≤ 2%
  the rest being iron and impurities resulting from the elaboration;
• this liquid steel is cast directly in the form of a thin strip of thickness less than or equal to 10 mm;
• the strip is rapidly cooled to a temperature of less than or equal to 1000 ° C. by spraying water or a water-air mixture;
• the hot strip is subjected to hot rolling at a reduction rate of at least 10%, the end-of-rolling temperature being such that this temperature, all the copper is still in solid solution in the matrix of ferrite and / or austenite;
• the strip is then subjected to a forced cooling step in order to maintain the copper in supersaturated solid solution in the ferrite and / or austenite matrix;
• and the strip thus cooled is reeled.

Preferably, the Mn / Si ratio is greater than or equal to 3.

The casting of the thin strip can be carried out on a casting installation between two internally cooled rolls rotating in opposite directions.

The hot rolling of the strip is preferably carried out in line with the casting of the strip.

avec V exprimée en °C/s et %Cu en % pondéraux.The speed V of forced cooling following hot rolling is generally such that $$\mathrm{V}\ge {\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">e</figure-callout>}}^{1,98(\%\mathrm{Cu})-0,08}$$

with V expressed in ° C / s and% Cu in% by weight.

According to a variant of the process, the carbon content of the steel is between 0.1 and 1%, and the winding of the strip is carried out at a temperature above the martensitic transformation start temperature M _s .

According to another variant of the method, the winding of the strip is carried out at less than 300 ° C., and the strip then undergoes a copper precipitation heat treatment between 400 and 700 ° C. Under these conditions, if the carbon content is between 0.1 and 1%, there is preferably no unwinding before the heat treatment.

According to another variant of the method, the winding of the strip is carried out at a temperature that is both greater than the temperature M _s of beginning of martensitic transformation and less than 300 ° C., and then a cold rolling, an annealing of recrystallization in a temperature range where the copper is in supersaturated solid solution, a forced cooling maintaining the copper in solid solution, and a precipitation income.

Said precipitation income is carried out in a continuous annealing plant between 600 and 700 ° C, or in a base annealing plant between 400 and 700 ° C.

According to another variant of the process, the winding of the strip is carried out at a temperature that is both greater than the temperature M _s of martensite transformation start and less than 300 ° C., and then a cold rolling and a base annealing are carried out. between 400 and 700 ° C serving both recrystallization annealing and precipitation income.

In cases where the strip undergoes cold rolling, the carbon content of the steel is preferably between 0.1 and 1%, or between 0.01 and 0.2%, or between 0.0005% and 0%. , 05%. In the latter case, its copper content is preferably between 0.5 and 1.8%.

Also in the latter case, prior to the precipitation income, one can cut the strip to form a sheet that is formed by stamping, and perform the precipitation income on the stamped sheet.

Finally, a final treatment of the strip can be carried out in a cold-rolling mill.

The invention also relates to a steel product obtained by one of the preceding methods.

As will have been understood, the invention essentially consists in directly casting a steel having the specified composition in a thin strip, and then imposing on it conditions which avoid the rapid cooling chipping of the strip at the outlet of the ingot mold, bringing it below 1000 ° C., and possibly keeping the strip in a non-oxidizing atmosphere at least until this temperature is reached), then hot rolling the strip, preferably in line, followed by forced cooling. now the copper in supersaturated solid solution. The tape is then wound. It can then undergo various thermal or mechanical treatments that will give it its thickness and its final properties.

• la figure 1 qui représente le diagramme de phases de l'alliage fer-cuivre pur dans son ensemble (fig.1a), et pour des teneurs en cuivre inférieures ou égales à 5% et des températures de 600 à 1000°C (fig. 1b);
• la figure 2 qui représente une portion du diagramme de phases d'un alliage fer-cuivre à 0,2% de carbone.

The invention will now be described in more detail with reference to the following appended figures:

• FIG. 1 represents the phase diagram of the pure iron-copper alloy as a whole (FIG. 1a), and for copper contents of less than or equal to 5% and temperatures of 600 to 1000.degree. C. (FIG. 1b);
• Figure 2 which shows a portion of the phase diagram of an iron-copper alloy with 0.2% carbon.

Firstly, a liquid metal is produced having the following composition (all the contents are expressed in percentages by weight).

The carbon content may range from 0.0005% to 1%, depending in particular on the applications envisaged for the final product. The lower limit of 0.0005% corresponds to practically the minimum that can be obtained by conventional methods of decarburizing the liquid metal. The upper limit of 1% is justified by the gamma-carbon effect. Indeed, beyond 1%, carbon excessively reduces the solubility of copper in ferrite. In addition, beyond 1%, the weldability of the steel is significantly degraded, which makes it unsuitable for many preferred applications of sheets obtained from the steels of the invention.

On the other hand, carbon provides a hardening effect, as well as the precipitation of titanium and / or niobium carbides used for texture control, if titanium and / or niobium are present in significant amounts in the steel. .

• lorsque la teneur en carbone est comprise entre 0,1 et 1%, les aciers obtenus trouvent une application privilégiée dans le domaine des tôles à très haute résistance laminées à chaud, lorsqu'après la coulée ils ont été bobinés à température permettant un revenu de précipitation, ou lorsqu'ils ont été bobinés à basse température puis ont subi un revenu, ou dans le domaine des tôles laminées à froid à très haute résistance ;
• lorsque la teneur en carbone est comprise entre 0,01 et 0,2%, les aciers obtenus trouvent une application privilégiée dans le domaine des aciers soudables à haute résistance lorsqu'ils ont été laminés à chaud, ou lorsqu'ils ont été laminés à froid et traités thermiquement dans des conditions qui seront vues plus loin ;
• lorsque la teneur en carbone est comprise entre 0,0005% et 0,05%, les aciers obtenus trouvent une application privilégiée dans le domaine de l'emboutissage, lorsqu'ils ont été laminés à froid et contiennent de préférence au plus 1,8% du cuivre (les raisons en seront vues plus loin) ;

In general, we can say that:

• when the carbon content is between 0.1 and 1%, the steels obtained find a preferred application in the field of very high-strength hot-rolled plates, when after casting they have been wound at a temperature allowing precipitation, or when they were coiled at low temperature and then undergone an income, or in the field of cold-rolled sheets with very high strength;
• when the carbon content is between 0.01 and 0.2%, the steels obtained find a preferred application in the field of high strength weldable steels when they have been hot rolled, or when were cold rolled and heat treated under conditions which will be seen later;
• when the carbon content is between 0.0005% and 0.05%, the steels obtained find a preferred application in the field of stamping, when they have been cold rolled and preferably contain at most 1.8. % of the copper (the reasons will be seen later);

A carbon content of the order of 0.02% is typical of the steels of the invention, except very high strength steels hot rolled or cold.

The copper content of the steel is between 0.5 and 10%, preferably between 1 and 10%.

Below 0.5%, the copper has no precipitation hardening effect or, more accurately, the driving force of precipitation is too low to achieve precipitation hardening under reasonable time and temperature conditions in the process. perspective of an industrial application. Practically, it is better to have at least 1% copper in the steel to take advantage of its hardening effect.

When making a steel for forming hot-rolled strip, there is no metallurgical limitation to the copper content, if the conditions of cooling speed and end-of-cooling temperature of the thin strip are met. after his casting. Cooling must begin in the 100% austenitic domain (the γ-Fe domain of FIG. 1a) and be fast enough to retain all the copper in solid solution. The limitation is therefore technological. For example, the copper content (2.9%) where the temperature of appearance of the ferrite is the lowest (about 840 ° C., see FIG. 1) and for which the critical cooling rate beyond of which the copper remains in solid solution is still easily accessible (for this copper content it is about 350 ° C / s). An increase in the copper content requires an increase in the cooling rate and the end of rolling temperature. The end of rolling temperature is conditioned by the solubility limit of the copper in the austenite. But levels of the order of 4% of copper, imposing to hot roll above 1000 ° C and then cool the band to more than 2500 ° C / s, are still accessible by the technology of casting thin strips, provided to impose a low speed of scrolling of the hot product, of the order of a few m / s.

When designing a steel for forming cold-rolled strips, a recrystallization treatment of the cold-rolled sheet must be carried out. Two variants can be chosen for this purpose.

According to the first variant, it is decided to dissociate the recrystallization treatment from the precipitation treatment (case of high-strength cold-rolled sheet for stamping). At the recrystallization temperature, the copper must be completely in solid solution in the single-phase ferritic domain. The maximum copper content is then given by the solubility limit of the copper in the ferrite at the recrystallization temperature considered. It is at most 1.8% at the maximum allowable recrystallization temperature of 840 ° C (see Figure 1b).

According to the second variant, it is chosen to couple the recrystallization treatment and the precipitation treatment (case of high-strength cold-rolled sheets). Very high levels of copper, up to 10%, are tolerable by basic annealing. Nevertheless, the recrystallization optimum may not coincide with the precipitation optimum, and the treatment parameters must then be chosen so as to achieve the best compromise for the intended application.

Typically, copper contents of the order of 3% and 1.8% depending on the application may be recommended.

The manganese content must be kept below or equal to 2%. Like carbon, manganese has a hardening effect. In addition, it is gammagenic, so it decreases the solubility of copper in ferrite by reducing the extent of the ferritic domain. Typically, it is recommended a manganese content of the order of 0.3%.

The silicon content can be up to 5%, without a minimum content must imperatively impose. Its alphagenic character makes it advantageous, however, because it allows to remain in the ferritic domain even with the preferred copper contents of 1.8 or even 3% of the steels of the invention. It is recommended to adjust the ratio Mn / Si to a value preferably greater than 3, to control, during the transformation δ → γ, the transfer of roughness of the surface of the cylinders on the solidified skins and the regularity of attachment of the solidified skins, to avoid the formation of cracks on the band during solidification and cooling. For this purpose, it is also recommended (as is known) to carry out the casting using rough casting surfaces and a nitrogen-containing inert gas which is soluble in the liquid steel, so that give themselves the possibility to adjust favorably the heat transfers between the steel and the casting surfaces. The maximum Si content of 5% is imposed by the ease of production and casting of the grade at the steel mill. Typically, a content of the order of 0.05% is recommended.

Niobium and titanium may, preferably but not necessarily, be present at levels up to 0.5% each. They produce carbides favorable to texture control, and when they are over-stoichiometric with respect to carbon, they increase the temperature A _C1 of the steel, thus the solubility of copper in ferrite. Typically, each of these elements may be present at a level of about 0.05%.

The nickel content can be up to 5%, this element being only optional. Nickel is often added to copper steels to combat hot cracking. His action is double. On the one hand, by increasing the solubility of copper in austenite, nickel delays the segregation of copper at the metal-oxide interface. On the other hand, since it is miscible with copper in any proportion, nickel increases the melting point of the segregating phase. It is usually considered that a nickel addition of the order of copper is sufficient to prevent hot cracking. The rapid cooling and possibly the inerting after casting of the process according to the invention prevents hot cracking, which reduces the interest of adding nickel with this objective in view. However, it is possible to add nickel to facilitate hot rolling.

The aluminum content can be up to 2% without damaging the properties of the steel, but this element is not necessarily present. However, it is advantageous for its alphagenic role comparable to that of silicon. Typically, aluminum is present at a level of about 0.05%.

The other chemical elements are present as residual elements at levels resulting from the production of steel according to conventional methods. In particular, the tin content is less than 0.03%, the nitrogen content is less than 0.02%, the sulfur content is less than 0.05%, the phosphorus content is less than 0.05%.

The liquid steel whose composition has just been exposed is then continuously cast directly as a thin strip of thickness less than or equal to 10 mm. For this purpose, the steel is typically poured into a bottomless mold, the casting space of which is limited by the internally cooled side walls of two cylinders rotated in opposite directions, and by two refractory side walls pressed against the ends. planes of cylinders. This process is now well known in the literature (it is described in EP-A-0 641 867 in particular), and we will not talk about it further. It would also be conceivable to use a method of casting by solidification of the steel on a single cylinder, which would give access to thinner strips than the casting between two rolls.

• refroidir rapidement la bande venant d'être coulée, par exemple par aspersion d'eau ou d'un mélange eau/air, de manière à la porter en dessous de 1000°C avant qu'un enrichissement en cuivre ne se produise à l'interface métal-calamine ; on considère que cet objectif est atteint pour une vitesse de refroidissement de 25°C/s lorsque la bande a une teneur de 3% en cuivre ;
• éventuellement empêcher l'oxydation du fer en maintenant la bande dans une atmosphère non oxydante, au moins jusqu'à ce qu'elle atteigne une température inférieure à 1000°C; cela peut être réalisé classiquement en faisant passer la bande dans une enceinte dont l'atmosphère est pauvre en oxygène (moins de 5%) et est constituée essentiellement par un gaz neutre, argon ou azote ; la présence d'un gaz réducteur tel que l'hydrogène est également envisageable.

In order to avoid the problems of cracking of the surface of the strip related to the intergranular infiltration of liquid copper in the steel under the scale when the temperature of the strip exceeds the melting temperature of the copper-rich phase, ie 1000 ° C approximately, then:

• rapidly cool the strip that has just been cast, for example by spraying water or a water / air mixture, so as to bring it below 1000 ° C before copper enrichment occurs at the metal-calamine interface; it is considered that this objective is achieved for a cooling rate of 25 ° C / s when the strip has a content of 3% of copper;
• possibly prevent the oxidation of iron by keeping the band in a non-oxidizing atmosphere, at least until it reaches a temperature below 1000 ° C; this can be done conventionally by passing the band in a chamber whose atmosphere is low in oxygen (less than 5%) and consists essentially of a neutral gas, argon or nitrogen; the presence of a reducing gas such as hydrogen is also conceivable.

These two solutions can be combined, being used simultaneously or in succession.

The strip then undergoes hot rolling. This can be carried out on a separate installation of the casting installation, after reheating of the strip at a temperature not exceeding 1000 ° C to avoid cracking (unless this reheating is performed in a non-heated atmosphere. oxidant). But it is preferable, for economic reasons, to carry out this hot rolling in line, that is to say on the same installation as the casting of the strip, by placing one or more roll stands on the path of the bandaged. In-line rolling also makes it possible to dispense with a sequence of winding / uncoiling / reheating operations between casting and hot rolling, which may present metallurgical risks: surface cracking, and incrustation of scale on winding in particular.

This hot rolling is carried out, with a reduction rate of at least 10%, in one pass or more. It basically has three functions.

In the first place, the recrystallization it causes suppresses the solidification structure, which is unfavorable to the shaping of the sheet. Moreover, this recrystallization leads to a refinement of the grain which is necessary for the simultaneous improvement of the strength and tenacity properties of the strip, if it is intended to be used in the state of hot-rolled sheet.

Second, it closes the pores that could be formed in the heart of the band during solidification, and which would also be harmful during shaping.

In addition, it guarantees the respect of the dimensional specifications of the band concerning its flatness, its curvature, its symmetry.

Finally, it improves the surface appearance of the band.

The end of rolling temperature must be such that the copper is still at this stage in solid solution in ferrite and / or austenite. Indeed, the precipitation of the copper before the end of the rolling would not allow to draw the maximum of hardening. This maximum is of the order of 300 MPa per 1% copper, when the precipitation conditions are well controlled. This end of rolling temperature to be respected therefore depends on the composition of the steel, in particular its copper and carbon contents.

It is thus considered that for high copper contents of about 7% and higher, the end-of-lamination temperature must be greater than 1094 ° C, this temperature being approximately the temperature of the peritectic bearing that the Fe-Cu phase diagram exhibits. shown in Figure 1a, for very low carbon contents. This also implies that the hot rolling is carried out in a non-oxidizing atmosphere, and that if the strip is cooled immediately after its solidification, this cooling is stopped at a sufficiently high temperature to then allow hot rolling of the strip. strip under conditions resulting in a rolling end temperature greater than 1094 ° C.

Between 2.9 and 7% copper, the end-of-lamination temperature must be higher than the solubility limit of the copper in the austenite, as given by the Fe-Cu phase diagram, for the carbon content considered. As an indication, for a very low carbon content, this temperature T would be given by $$\mathrm{T}\left(\mathrm{K}\right)\mathrm{=}\frac{\mathrm{3093}}{\mathrm{3}\mathrm{,}\mathrm{186}\mathrm{-}{\mathrm{<figure-callout\; id="10"\; label="log"\; filenames="imgf0001.png"\; state="\{\{state\}\}">log</figure-callout>}}_{\mathrm{10}}\mathrm{Cu}\left(\mathrm{\%}\right)}$$

Between 2,9 and 1,8% of copper, the end-of-lamination temperature must be higher than 840 ° C for the very low carbon contents, this temperature corresponding to the eutectoid plateau (see Fig. 1b).

pour le fer α paramagnétique (entre 840°C et la température de Curie de 759°C, pour une teneur en cuivre de 1,08 à 1,8%), et par $$\mathrm{T}\left(\mathrm{K}\right)\mathrm{=}\frac{4627}{\mathrm{4}\mathrm{,}\mathrm{495}\mathrm{-}{\log }_{\mathrm{10}}\mathrm{Cu}\left(\mathrm{\%}\right)}$$

pour le fer α ferromagnétique (entre 690°C et 759°C, pour une teneur en cuivre de 0,5 à 1,08%).Below 1.8% copper, the end-of-lamination temperature must be higher than the solubility limit of the copper in the ferrite, as given by the Fe-Cu phase diagram for the carbon content considered. As an indication, for a very low carbon content, this temperature T would be given by $$\mathrm{T}\left(\mathrm{K}\right)\mathrm{=}\frac{\mathrm{3351}}{\mathrm{3}\mathrm{,}\mathrm{279}\mathrm{-}{\mathrm{<figure-callout\; id="10"\; label="log"\; filenames="imgf0001.png"\; state="\{\{state\}\}">log</figure-callout>}}_{\mathrm{10}}\mathrm{Cu}\left(\mathrm{\%}\right)}$$

for paramagnetic α-iron (between 840 ° C and Curie temperature of 759 ° C, for a copper content of 1.08 to 1.8%), and $$\mathrm{T}\left(\mathrm{K}\right)\mathrm{=}\frac{4627}{\mathrm{4}\mathrm{,}\mathrm{495}\mathrm{-}{\mathrm{<figure-callout\; id="10"\; label="log"\; filenames="imgf0001.png"\; state="\{\{state\}\}">log</figure-callout>}}_{\mathrm{10}}\mathrm{Cu}\left(\mathrm{\%}\right)}$$

for ferromagnetic α (between 690 ° C and 759 ° C, for a copper content of 0.5 to 1.08%).

It should be noted, however, that the numerical values above are only indicative, as they vary slightly according to the authors.

As the carbon content of the steel increases, the above figures are also modified because the carbon has a gamma-effect, as seen in the Fe-Cu phase diagram extract of Figure 2, established for a carbon content of 0.2%. The temperature of the eutectoid bearing is lowered compared to the case of very low carbon contents, and is often below 800 ° C. We can then afford to lower the end of rolling temperature compared to previously described cases. For these relatively carbon-rich steels, structural hardening is also achieved by the action of quenching precipitating constituents, such as bainite or martensite, in addition to hardening due to copper precipitation.

In view of what has just been said, it appears that it is not possible to define quantitatively in a simple and very precise way the value of the minimum end-of-rolling temperature of the process according to the invention. What is certain is that this end-of-lamination temperature must not be lower than the temperature for which, given the composition of the steel, precipitation of the copper would be observed. The determination of this temperature for a given steel composition can be made at of current experiments by metallurgists, in case a measurement of this temperature would not be available in the literature.

• si la température de fin de laminage est supérieure à 1000°C (ce qui, on l'a vu, est souhaitable principalement pour les aciers à teneur en cuivre très élevée), ce refroidissement garantit qu'entre la température de fin de laminage et 1000°C il n'y aura pas d'oxydation significative du fer, et qu'on ne constatera pas de faïençage sur la bande ;
• et surtout, il permet de maintenir le cuivre en solution solide sursaturée dans l'austénite et/ou la ferrite ; cette condition est importante pour profiter au maximum de l'effet de durcissement par précipitation du cuivre.

After hot rolling, the strip undergoes a new forced cooling. This cooling has several functions:

• if the end-of-rolling temperature is above 1000 ° C (which, as we have seen, is desirable mainly for steels with a very high copper content), this cooling ensures that between the end of rolling temperature and 1000 ° C there will be no significant oxidation of iron, and we will not notice cracking on the band;
• and above all, it makes it possible to maintain the copper in solid solution supersaturated in austenite and / or ferrite; this condition is important to take full advantage of the precipitation hardening effect of copper.

avec V en °C/s et %Cu en % pondéraux.For copper contents of 3% and less, it is assumed that the maintenance of the copper in solid solution is generally achieved if, during the whole time that the band passes in scrolling, without being wound, the cooling rate V of the band is as $$\mathrm{V}\mathrm{\ge }{\mathrm{<figure-callout\; id="1"\; label="e"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">e</figure-callout>}}^{\mathrm{1}\mathrm{,}\mathrm{98}\left(\mathrm{\%}\mathrm{Cu}\right)\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{08}}$$

with V in ° C / s and% Cu in% by weight.

For a copper content of 1%, V must therefore be greater than or equal to 7 ° C / s, which is easily accessible. For a copper content of 3%, V must be greater than or equal to 350 ° C / s. This high speed is however accessible on a thin strip casting installation.

For copper contents above 3%, the above formula is no longer valid, and an experimental control of the results of the The cooling must be performed to verify that it has been sufficient to maintain the copper in supersaturated solid solution.

The winding of the band then takes place. We can take advantage of the period when the band stays in the coil state to make a copper precipitation income that causes the hardening of the steel. The hardness of the HV steel obtained depends on the composition of the steel, but also on the duration of the stay of the strip in coil form and the winding temperature, knowing that, in practice, a coil remains about 1 hour at its winding temperature before cooling at a rate of about 10 to 20 ° C / h. It can be seen that the curve HV = f (t) has a maximum HV _max for a given duration t _HVmax , beyond which the hardness decreases. It is therefore advisable to cool the wound strip (or unwind) as soon as t _Hvmax has been reached.

avec t_HVmax en h, %Cu en % pondéraux et T en K.Experience shows that t _HVmax is given by the equation: $${\mathrm{t}}_{\mathrm{HV\; max}}\mathrm{=}\frac{{\mathrm{8.10}}^{\mathrm{-}\mathrm{8}}}{\left(\mathrm{\%}\mathrm{Cu}\right)}{\mathrm{e}}^{\frac{\mathrm{14343}}{\mathrm{T}}}$$

with t _HVmax in h,% Cu in% by weight and T in K.

It is thus possible to choose, for a given copper content, the preferred combinations (t _HV , T) compatible with the industrial tool used. In the case where one chooses to make an income during winding, t _HV is imposed (greater than 1 h); we can only play on the winding temperature.

On the other hand, the value of the maximum hardness that can be obtained increases as the temperature of the precipitation of precipitation decreases, provided that the band is allowed enough time to reach this maximum hardness.

Moreover, the choice of the winding temperature of the strip and the choice of subsequent operations depend on the type of product that it is desired to manufacture.

As has been said, it is possible to manufacture hot-rolled sheets according to the method of the invention. Two modes of operation are conceivable.

According to a first operating mode, the winding of the strip is carried out after hot rolling at an elevated temperature, for example that (calculated as a function of the copper content according to formula (2) above) which makes it possible to reach the maximum hardness in 1h (time from which, as said, the temperature of the coil usually starts to decrease). The period during which the strip undergoes a stay at high temperature is therefore the initial phase of its stay in the form of coil following rapid cooling.

où les teneurs en les divers éléments sont exprimées en % pondéraux.In the case of steels with carbon content between 0.1 and 1%, an additional condition on the winding temperature is that it is above the martensitic transformation start temperature M _S. Indeed, the formation of martensite could cause the appearance of cracks during unwinding. M _S is given by the classical formula called "Andrews formula": $${\mathrm{M}}_{\mathrm{s}}\left(\mathrm{°\; C}\right)\mathrm{=}\mathrm{539}\mathrm{-}\mathrm{423}\mathrm{VS}\mathrm{\%}\mathrm{-}\mathrm{30}\mathrm{,}\mathrm{4}\mathrm{mn}\mathrm{\%}\mathrm{-}\mathrm{17}\mathrm{,}\mathrm{7}\mathrm{Or}\mathrm{\%}\mathrm{-}\mathrm{12}\mathrm{,}\mathrm{1}\mathrm{Cr}\mathrm{\%}\mathrm{-}\mathrm{11}\mathrm{Yes}\mathrm{\%}\mathrm{-}\mathrm{7}\mathrm{MB}\mathrm{\%}$$

where the contents of the various elements are expressed in% by weight.

• soit, au cours du refroidissement, on aura formé de la bainite (les aciers à faible teneur en carbone ne sont pas « trempants »), ce qui empêche la formation de martensite ;
• soit on forme effectivement de la martensite ; mais comme la teneur en carbone est faible, la quantité de martensite formée est réduite et ne provoque pas d'incidents au débobinage.

For steels with carbon content between 0.0005 and 0.1%, it is not necessary to take M _S into account. In their case M _S is of the order of 400 to 500 ° C, which is high and, most often, above the winding temperature which would be easily accessible on the installation. But here there is no problem winding below M _S because:

• either, during cooling, bainite will have been formed (the low carbon steels are not "hardening"), which prevents the formation of martensite;
• either one actually forms martensite; but as the carbon content is low, the amount of martensite formed is reduced and does not cause incidents uncoiling.

After complete cooling of the coil (which, depending on requirements, can be carried out in a completely natural way or be carried out in a forced manner after the lapse of the time required to obtain the desired hardness), the hot-rolled sheet is ready to use.

However, it should be known that the germination rate of copper precipitates is an increasing exponential function of the degree of cooling of the band. Under these conditions, it is advisable, in order to obtain a maximum precipitation hardening effect, to complete the germination phase at a temperature lower than that at which the growth of the grains will take place. It is therefore possible to propose a second operating mode for the manufacture of hot-rolled strips. According to this second operating mode, the strip is wound at a sufficiently low temperature so that, during the natural cooling of the coil, there is no precipitation of the copper, the latter remaining in supersaturated solid solution. It is estimated that a winding temperature of less than 300 ° C is sufficient for this purpose. There is, in this case, no problem in winding the band in the martensitic transformation field. Indeed, the band (always wound, at least in the case where the winding took place below M _s ) then undergoes a heat treatment of income between 400 and 700 ° C which makes it possible to remove the martensite. But the main role of this income is to precipitate the copper, so as to obtain the desired properties for the hot sheet. The parameters of this treatment (temperature and duration) can be determined using equation (2) previously given.

In the case where it is desired to produce cold-rolled sheets according to the method of the invention, the winding temperature must be greater than M _s for steels whose carbon content is between 0.1 and 1%, since there is no heat treatment that would eliminate the martensite between winding and unwinding before cold rolling. But the In any case, the coiling temperature must also be less than 300 ° C. so that the cold rolling and subsequent recrystallization annealing take place on a steel in which the copper is in a supersaturated solid solution.

In the case where it is desired to manufacture cold-rolled sheets with very high strength which may contain high copper and carbon contents (0.1 to 1% of C), or cold-rolled sheets with high strength and easily weldable, for which a relatively low carbon content is required (0.01 to 0.2%), it is possible to propose different operating modes, depending on whether it is desired to use a continuous annealing plant or a base annealing plant to achieve heat treatment of precipitation income.

In all cases, the cold rolling (typically at a reduction rate of 40 to 80% and at room temperature) is first carried out on the strip whose copper is in supersaturated solid solution and then on a recrystallization annealing carried out in the high temperature range where copper is also in solid solution in ferrite and / or austenite. It has already been seen in connection with the choice of the end temperature of hot rolling what could be the conditions adapted for this purpose, depending on the copper content of the strip.

The duration of this recrystallization annealing depends on the ability to have previously preserved the copper in solid solution. In fact, at the recrystallization temperature of 840 ° C., where up to 1.8% of copper can be converted into solid solution, the growth of the grains may be excessive. If the copper is already in solid solution before recrystallization, the annealing time is fixed not by the kinetics of dissolution of the copper precipitates, but by the kinetics of growth of the grains. The dissolution of the copper before recrystallization thus facilitates the optimization of the texture, and this situation is the most advantageous for the metallurgist. Depending on the state in which the copper is (completely in solution or partially precipitated), the recrystallization annealing, if carried out at 840 ° C, has a duration that can vary from 20s to 5mn. It can advantageously be executed in an installation of "Compact annealing" giving access in a short time to high temperatures that allow to resubmit large amounts of copper.

After the recrystallization annealing, the precipitation income is made. These two operations are separated by a rapid cooling step, intended to keep the copper in solid solution. This cooling must therefore obey the equation (1) previously mentioned.

If for the precipitation income is used a continuous annealing plant (preferably chained directly with the compact annealing system that was used to achieve the recrystallization annealing), for which there is only a short time to reach the hardness maximum HV _max of the band (see equation (2) for its calculation), it is necessary to perform this income at a relatively high temperature (600-700 ° C). This limits the extent of hardening obtained by precipitation, since this hardening, as has been said, is all the more important as the income is made at a lower temperature.

Therefore, when very high levels of resistance are sought, it is preferable to achieve the precipitation income at relatively low temperature (400 to 700 ° C), but for a prolonged period determined, preferably, by the equation (2) above, in a base annealing installation where the band stays in the coil state. In this case, the rapid cooling following treatment should bring the band below 300 ° C to keep the copper in supersaturated solids.

The use of a compact annealing die followed by a very rapid cooling (easily accessible on this type of installation) - annealing base "is therefore particularly advantageous for obtaining steels with a high copper content, thus having a high capacity to be hardened by precipitation and, consequently, a very high final strength. This die is however relatively long because of the presence of the annealing base.

Alternatively, as has been said, it is possible to couple the two recrystallization and precipitation operations during a base anneal performed at 400-700 ° C for a time that can be determined by equation (2). previous, without prior recrystallization annealing, so directly after the cold rolling. This procedure is particularly applicable to steels with the highest copper content (up to 10%). In some cases, the treatment parameters will need to be chosen in order to achieve the best possible compromise between recrystallization requirements and copper precipitation requirements.

In the case where it is desired to manufacture a cold rolled sheet made of low carbon steel (less than 0.05%) and with good drawability, a procedure is proposed which comprises, as previously, a cold rolling (typically at a rate of reduction of 40 to 80% and at ambient temperature) carried out on the strip where the copper is in supersaturated solid solution, a recrystallization annealing and a precipitation income.

In order for the sheet to retain good stamping properties, the recrystallization must take place in the ferritic field and must not allow the copper to precipitate. The recrystallization temperature is therefore determined by the solubility limit of the copper in the ferrite as seen above. Practically, it is advisable to carry out the recrystallization annealing at the eutectoid temperature (of the order of 840 ° C for low-carbon copper steels), where the solubility of copper in ferrite is maximum (1.8% ).

It is necessary to avoid excessive growth of the ferritic grain during the recrystallization annealing. It may also be necessary to raise the temperature A _C1 of the steel so that the complete dissolution of the copper can be carried out in the ferritic phase in the case where the cooling after hot rolling has not made it possible to retain it in its entirety. supersaturation. The addition of titanium or niobium makes it possible to satisfy these two requirements. These elements also have a favorable effect on the recrystallization texture by trapping carbon and nitrogen in particular.

As is conventional, the hot or cold rolled strip can undergo a final treatment in a skin-pass mill to give it its final surface and flatness and adjust its mechanical properties.

Finally, if the implementation of the sheet obtained from the belts according to the invention requires a very high drawability, it is possible to achieve it before the precipitation income, which is therefore performed either on the raw strip but on the stamped product.

With the method according to the invention, it is possible to manufacture very high strength sheets not necessarily produced from molten iron, which makes them economical.

Another advantage of these sheets is that the presence of copper in significant proportion makes them less sensitive to atmospheric corrosion, and may therefore be able to do without anticorrosive coating.

• les tôles laminées à chaud ou à froid contenant jusqu'à 10% de cuivre et de 0,1 à 1% de carbone peuvent avoir des résistances très supérieures à 1000 MPa ; les tôles laminées à chaud ou à froid ayant des teneurs en carbone moindres ont des résistances moins élevées, mais qui sont toujours supérieures à 1000 MPa, et elles présentent une bonne soudabilité qui rend leur emploi possible notamment dans l'industrie automobile ;
• les tôles laminées à froid contenant jusqu'à 1,8% de cuivre et 0,05% de carbone présentent une résistance de l'ordre de 700 à 900 MPa et un allongement à la rupture de 15 à 30%, donc une très bonne emboutissabilité.

Regarding the mechanical properties accessible by the process according to the invention:

• hot-rolled or cold-rolled sheets containing up to 10% copper and 0.1 to 1% carbon may have strengths well above 1000 MPa; the hot-rolled or cold-rolled sheets having lower carbon contents have lower strengths, but which are still greater than 1000 MPa, and they have a good weldability which makes their use possible particularly in the automotive industry;
• cold rolled sheets containing up to 1.8% copper and 0.05% carbon have a strength of the order of 700 to 900 MPa and an elongation at break of 15 to 30%, so a very good drawability.

Claims (18)

1. Process for manufacturing a steel product made of copper-rich carbon steel, in which:

   - a liquid steel is melted that has the composition, expressed in percentages by weight:

   0.0005% ≤ C ≤ 1%

   0.5 ≤ Cu ≤ 10%

   0 ≤ Mn ≤ 2%

   0 ≤ Si ≤ 5%

   0 ≤ Ti ≤ 0.5%

   0 ≤ Nb ≤ 0.5%

   0 ≤ Ni ≤ 5%

   0 ≤ Al ≤ 2%

   the balance being iron and impurities resulting from this melting;

   - this liquid steel is cast directly in the form of a thin strip with a thickness of 10 mm or less;

   - the strip is rapidly cooled down to a temperature of 1000°C or below by spraying with water or a water/air mixture;

   - the thin strip undergoes hot rolling with a reduction ratio of at least 10%, the end-of-rolling temperature being such that, at this temperature, all the copper is still in solid solution in the ferrite and/or austenite matrix;

   - the strip then undergoes a forced cooling step so as to keep the copper in supersaturated solid solution in the ferrite and/or austenite matrix; and

   - the strip thus cooled is coiled.
2. Process according to Claim 1, characterized in that the Mn/Si ratio is equal to 3 or higher.
3. Process according to Claim 1 or 2, characterized in that the thin strip is cast in a casting installation between two internally-cooled counterrotating rolls.
4. Process according to one of Claims 1 to 3, characterized in that the hot rolling of the strip is carried out in line with the casting of the strip.
5. Process according to one of Claims 1 to 4, characterized in that the forced cooling rate V after the hot rolling is such that: $$\mathrm{V}\ge {\mathrm{e}}^{1.98(\%\mathrm{Cu})-0.08}$$

   

   where V is expressed in °C/s and %Cu in % by weight.
6. Process according to one of Claims 1 to 5, characterized in that the carbon content of the steel is between 0.1 and 1% and in that the coiling of the strip is carried out at a temperature above the martensite-transformation-start temperature M_S.
7. Process according to one of Claims 1 to 5, characterized in that the coiling of the strip is carried out at below 300°C and in that the strip then undergoes a copper precipitation heat treatment between 400 and 700°C.
8. Process according to Claim 7, characterized in that the carbon content of the steel is between 0.1 and 1% and in that the strip undergoes the precipitation heat treatment without prior uncoiling.
9. Process according to one of Claims 1 to 5, characterized in that the coiling of the strip is carried out at a temperature both above the martensite-transformation-start temperature M_S and below 300°C and in that the strip then undergoes cold rolling, recrystallization annealing, in a temperature range in which the copper is in supersaturated solid solution, forced cooling, keeping the copper in solid solution, and tempering for precipitation hardening.
10. Process according to Claim 9, characterized in that said tempering for precipitation hardening is carried out between 600 and 700°C in a continuous annealing installation.
11. Process according to Claim 9, characterized in that said tempering for precipitation hardening is carried out between 400 and 700°C in a box annealing installation.
12. Process according to one of Claims 1 to 5, characterized in that the coiling of the strip is carried out at a temperature both above the martensite-transformation-start temperature M_S and below 300°C and in that the strip then undergoes cold rolling and box annealing between 400 and 700°C serving both for recrystallization annealing and tempering for precipitation hardening.
13. Process according to one of Claims 9 to 12, characterized in that the carbon content of the steel is between 0.1 and 1%.
14. Process according to one of Claims 9 to 12, characterized in that the carbon content of the steel is between 0.01 and 0.2%.
15. Process according to one of Claims 9 to 12, characterized in that the carbon content of the steel is between 0.0005% and 0.05% and in that its copper content is between 0.5 and 1.8%.
16. Process according to Claim 15, characterized in that, prior to the tempering for precipitation hardening, the strip is cut so as to form a sheet that is formed by drawing and in that the tempering for precipitation hardening is carried out on the drawn sheet.
17. Process according to one of Claims 1 to 15, characterized in that the strip undergoes a final treatment in a skin-pass rolling mill.
18. Steel product, characterized in that it is obtained by a process according to one of Claims 1 to 17.

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