EP0896069A1 - Method of making thin ultra-low-carbon steel strip for manufacturing deep-drawn products for packages and thin strips obtained thereby - Google Patents

Abstract

Production of a thin ultra-low carbon steel sheet, for manufacturing deep drawn packaging products, comprises casting a slab of a killed and vacuum degassed steel of composition (by wt.) 0.10-0.35% Mn, less than 0.006% N, less than 0.025% P, less than 0.020% S, less than 0.020% Si, NOTGREATER 0.08% one or more of Cu, Ni and Cr as well as Al, balance Fe and impurities. The steel is cast into slab and hot rolled to strip above the Ar3 temperature, coiled, cold rolled to an intermediate sheet, continuously annealed below the Ac1 temperature and re-rolled to final thickness. The novelty is that the steel is refined to a carbon content of NOTGREATER 0.006% and an aluminium content of NOTGREATER 0.010% and that the hot rolled strip is coiled at below 620 (preferably 530-570) degrees C. Also claimed is a thin sheet of ultra-low carbon steel having the above composition and produced by cold rolling in two stages separated by a continuous anneal, the steel having NOTGREATER 0.006% C content, NOTGREATER 0.010% Al content, a homogeneous equiaxed grain structure, a Lankford coefficient (mean r value) of greater than 1.6 and a planar anisotropy coefficient ( DELTA C) of close to zero.

Description

The invention relates to a method for producing a thin sheet of ultra low carbon steel for the production of deep-drawn products for packaging such as boxes and a thin sheet obtained by the process.

For the production, by stamping, of products for packaging in steel such as boxes for food or drink, we uses blanks cut from thin sheets whose characteristics must be adapted to the stamping forming process.

The stamping processes used to make the boxes for canned food or drinks are usually processes deep drawing-deep drawing (DRD) or deep drawing-ironing (DWI).

In both cases, it is known to use thin sheets with very low carbon or ultra low carbon (ULC) whose carbon content by weight is a few thousandths of a percent and generally less than 8 thousandths of a percent.

• carbone < 0,008 %,
• manganèse compris entre 0,10 et 0,30 %,
• azote < 0,006 %,
• aluminium compris entre 0.01 et 0,06 %,
• phosphore < 0,015 %,
• soufre < 0,020 %,
• silicium < 0,020 %,
• au maximum 0,08 % d'un ou plusieurs des éléments cuivre, nickel et chrome,

le reste de la composition étant constitué par du fer et des impuretés inévitables.We know for example from FR 95-02208 a process for developing a thin sheet intended for the manufacture by stamping-ironing, of a can of the beverage can type, from a steel having the following weight composition :

• carbon <0.008%,
• manganese between 0.10 and 0.30%,
• nitrogen <0.006%,
• aluminum between 0.01 and 0.06%,
• phosphorus <0.015%,
• sulfur <0.020%,
• silicon <0.020%,
• maximum 0.08% of one or more of the copper, nickel and chromium elements,

the rest of the composition consisting of iron and unavoidable impurities.

Generally, in the case of the manufacture of boxes by deep drawing and deep drawing (DRD) or deep drawing and ironing processes (DWI), specific mechanical and drawing properties are requested for thin sheets or cut blanks in these sheets which are subjected to stamping.

In particular, thin sheets should show a slight tendency to the formation of horns during stamping and very good properties drawing by shrinking.

Good stampability is characterized by an anisotropy coefficient normal or high Lankford coefficient and anisotropy coefficient plane ΔC close to zero.

In addition, a microstructure of the steel is also sought. as homogeneous as possible along the width of the sheet and along its edges, so as to obtain a homogeneous behavior of the blanks during their stamping. In addition, we search in the sheet metal intended for stamping a microstructure as close as possible to a grain microstructure homogeneous equiaxes.

Also, because the thickness of the metal packaging in the state finish may be very small (eg less than 0.1 mm), it is necessary to have a sheet free of defects such as inclusions, that is to say a material with the best possible inclusion cleanliness.

Thin steel sheets for the production of stamped packaging are generally made from a degassed aluminum steel, degassed vacuum and generally continuously cast in the form of a slab which is then hot rolled so as to obtain a hot rolled strip which is then cold rolled in two stages separated by annealing recrystallization.

The second rolling which is generally carried out on a skin-pass rolling mill allows to obtain a sheet having the final thickness of the product on which stamping is carried out.

In the case of manufacturing ultra low carbon steels, the steel produced in the metallurgical furnace is subjected to vacuum degassing, generally with oxygen injection and calmed with aluminum, before being cast in a continuous casting plant for the development of a slab.

The slab is hot rolled at a temperature above the point Ar3 of steel to obtain a hot-rolled sheet whose thickness is generally less than 3 mm.

The hot rolled sheet is then cold rolled with a rate of reduction generally greater than 80% to obtain a sheet rolled to intermediate cold or blank which is then annealed at a lower temperature at point Ac1 of the steel before final rolling with the skin-pass, the reduction rate depends on the destination of the sheet.

Ultra low carbon steel sheets vacuum degassed and calmed to aluminum have suitable characteristics with regard to their stampability, the homogeneity of the microstructure obtained, at the end of the manufacturing cycle, and inclusive cleanliness.

However, the creation of new packaging of complex shapes with increasingly thin walls requires obtaining characteristics always higher.

In EP-0.521.808, a method for producing sheets intended for deep drawing, for example for the manufacture of boxes by the DRD process, from steel produced by the converter, containing not more than 0.015% of carbon and less than 0.040% of aluminum. The process includes hot rolling. Hot rolled sheet is coiled at a temperature above 650 ° C, then cold rolled and finally annealed at a temperature below 700 ° C. The need to wind at a temperature above 650 ° C leads to heterogeneity of characteristics of the tape, crosswise and between the ends and the core of the coil. In addition, winding at a temperature above 650 ° C leads a hot sheet structure which is not very favorable for obtaining a sheet cold rolled with fine grains (ASTM index greater than 9).

In US-3,404,047, a method of manufacturing a sheet is described for deep drawing with very low carbon content (C ≤ 0.004%). This very low carbon content is obtained by practicing on the sheet metal a decarburizing annealing. Due to the annealing conditions (2 to 20 hours at 715 ° C), the grain index of the sheet is very low (6 to 7).

In EP-0.659.889, a method of manufacturing a sheet is described cold rolled with very low carbon content (C ≤ 0.004 %) and a very low aluminum content (between 0.005 and 0.070%). Steel contains a niobium content greater than 0.001% and up to 0.018%. Due to the presence of niobium, the recrystallization temperature of steel and therefore the temperature of the recrystallization annealing is significantly higher than in niobium-free steels.

• on élabore un acier calmé et dégazé sous vide renfermant en poids, entre 0,10 et 0,35 % de manganèse, moins de 0,006 % d'azote, moins de 0,025 % de phosphore, moins de 0,020 % de soufre, moins de 0,020 % de silicium, au plus 0,08 % d'un ou plusieurs éléments parmi le cuivre. le nickel et le chrome, ainsi que de l'aluminium,

le reste de la composition étant constitué par du fer et des impuretés inévitables,

• on coule l'acier sous forme d'une brame,
• on lamine la brame à chaud à une température supérieure à Ar3 pour obtenir une tôle laminée à chaud,
• on lamine à froid la tôle laminée à chaud, sous forme d'une tôle laminée à froid intermédiaire,
• on bobine la tôle laminée à chaud,
• on recuit la tôle laminée à froid intermédiaire en continu à une température inférieure à Ac1, et
• on relamine la tôle laminée à froid intermédiaire jusqu'à une épaisseur finale de la tôle pour emboutissage,

le procédé suivant l'invention permettant d'améliorer de manière notable l'emboutissabilité, la propreté inclusionnaire et l'homogénéité de microstructures de la tôle pour emboutissage.The object of the invention is to propose a process for producing a thin sheet of ultra low carbon steel for the production of deep-drawn packaging products in which:

• a calm and degassed steel is produced under vacuum containing by weight, between 0.10 and 0.35% of manganese, less than 0.006% of nitrogen, less than 0.025% of phosphorus, less than 0.020% of sulfur, less than 0.020 % silicon, at most 0.08% of one or more elements among copper. nickel and chromium, as well as aluminum,

the rest of the composition consisting of iron and unavoidable impurities,

• the steel is poured in the form of a slab,
• the slab is hot rolled at a temperature higher than Ar3 to obtain a hot rolled sheet,
• the hot-rolled sheet is cold rolled, in the form of an intermediate cold-rolled sheet,
• the hot rolled sheet is wound,
• the intermediate cold-rolled sheet is annealed continuously at a temperature below Ac1, and
• the intermediate cold-rolled sheet is re-rolled to a final thickness of the sheet for stamping,

the process according to the invention making it possible to significantly improve the drawability, the inclusiveness and the homogeneity of microstructures of the sheet for stamping.

For this purpose, steel is produced in such a way as to contain at most 0.006 % by weight of carbon and 0.010% by weight of aluminum and the sheet is wound up hot rolled at a temperature below 620 ° C and preferably between 530 ° C and 570 ° C.

The invention also relates to a production process in which the steel is calmed by bringing an effervescent steel obtained by production in a metallurgical furnace to a slag containing in particular aluminum and alumina Al ₂ O ₃ .

The invention also relates to a method of preparation in which steel is poured in the form of a slab in a continuous casting under inert gas.

Finally, the invention also relates to a thin sheet having a homogeneous microstructure with equiaxed grains having a low content of inclusions and having very good drawing characteristics in an ultra low carbon steel containing less than 0.010% aluminum.

In order to clearly understand the invention, we will now describe several examples of the production of thin sheets according to the invention and the microstructural and stampability characteristics of these sheets, referring to the attached figures.

Figure 1 is a diagram giving the percentage of recrystallization depending on the temperature, steels with aluminum contents different.

FIGS. 2A, 2B, 2C, 2D and 2E are microstructures, after recrystallization, of cold-rolled steel sheet having contents of different and increasing aluminum from Figure 2A to Figure 2E.

Figure 3 is a diagram giving the elastic limit as a function the aluminum content of steel sheets for deep drawing according to the invention and, comparatively, according to the prior art.

FIG. 4 is a diagram giving the mechanical strength in depending on the aluminum content of elaborate stamping steel sheets by the process according to the invention and, comparatively, of sheets steel made according to the known process of the prior art.

Figures 5A, 5B and 5C are diagrams showing the coefficient anisotropy r of a stamping sheet according to the invention, respectively in the direction of the length of the sheet, in the cross direction and at 45 °.

Figure 6 is a diagram giving the anisotropy coefficient r medium depending on the aluminum content of the stamping sheets steel produced according to the invention and, comparatively, produced according to the prior art.

In the context of a comparative study between the process for the production of sheet metal for stamping according to the invention, characterized in particular by very low carbon and aluminum contents in the thin sheets obtained and of sheet metal for stamping produced according to the known process In the prior art, these sheets containing aluminum contents greater than 0.010% by weight, various steel castings have been produced which differ only appreciably in their aluminum contents. After hot rolling, the sheet is quickly cooled and wound at a temperature below 620 ° C. Table 1 below gives the compositions of the steels used for the manufacture of stamping sheets by cold rolling of hot rolled sheets. Lab spot. Temp; end of lamin. Temp. Bob. Thick Coil (mm) VS NOT Mn P S Cu Or Cr Al. Met. Ti res. (ppm) M825 880 ° C 530 ° C 2.72 2.7 3.4 201 11 5 8 18 14 2 3 R2116A 875 ° C 570 ° C 2.96 3.5 3.5 202 13 11 8 15 15 8 1 R2115A 883 ° C 563 ° C 2.95 3.2 3.5 201 12 11 8 16 16 10 1 R1048C1 894 ° C 560 ° C 3.01 2.6 2.2 201 10 6 6 18 15 24 1 R1285 900 ° C 590 ° C 3.09 3.2 2.9 198 10 5 11 17 24 37 7 S385 881 ° C 579 ° C 2.00 2.9 3.0 197 10 11 11 19 17 56 4 R1757A 871 ° C 559 ° C 3.04 3.4 5.0 237 3 5 14 18 30 64 4

In Table 1, the weight contents of the various elements are data in thousandths of a percent, except for titanium which is indicated in ppm, that is, in tenths of a thousandths of a percent.

The chemical analyzes were carried out on the sheets rolled to hot constituting the product obtained in an intermediate stage of the process of elaboration.

In the first column, the references of the sheets have been indicated; these references will be used to designate the sheets up to their final state, that is to say in the form of thin sheets for stamping.

The first three sheets under the references M825, R2116A and R2115A are produced according to the process of the invention and include aluminum contents at most equal to 10 thousandths percent.

The following four sheets indicated in Table 1 are given for comparison and relate to sheets made according to the prior art and containing 24 thousandths percent of aluminum or more.

The second column of table 1 indicates the end temperature of rolling and the third column, the winding temperature of the rolled sheet hot.

The fourth column of the table relates to the thicknesses of the sheets hot rolled.

The following columns of the table indicate the weight contents of the different elements of sheet steel.

The steels used to make the hot-rolled sheets are produced in a metallurgical oven then poured into a pocket. The steel is degassed under vacuum and calmed before being poured into a continuous slab casting facility.

The vacuum degassing of the steel is preferably carried out in a RHOB installation, i.e. by blowing pure oxygen into the steel put circulating in a vacuum enclosure, or in a vacuum installation in tank.

Calming of steels for metallic packaging is generally made by adding aluminum to the steel.

Such a process has been used in the case of comparative steels.

Such an aluminum calming process is no longer applicable in the steel which must contain less than 0.010% aluminum.

In the case of the three steels according to the invention containing less than 0.010% aluminum, calming was carried out by reaction between the slag and steel, during brewing.

However, it is necessary to add a mixture of aluminum and alumina Al ₂ O ₃ to the slag in order to avoid reoxidation of the steel. In fact, the slag contains a large proportion of FeO and the aluminum traps the oxygen released by FeO at the time of mixing.

By adjusting the amounts of aluminum and alumina in the slag, we can adjust the final aluminum content of the steel to less than 0.010%.

Vacuum degassing, which is a common production technique ultra low carbon steels, allows to obtain a lower carbon content 0.006%.

In the case of processed steels whose composition is given on the Table 1, the carbon contents of these steels are all between 26 and 35 ppm.

In order to allow meaningful comparisons of characteristics steel mechanics, some corrections will be made to reduce the mechanical characteristics to a standard carbon content 25 ppm.

In general, the carbon content of ultra low steels carbon according to the invention is less than 0.006%.

These steels have a nitrogen content ranging from 22 to 50 ppm. In general, for steels intended for the manufacture of thin sheets for packaging, the nitrogen content is always less than 0.006% or 60 ppm.

Also in such steels, the manganese content is generally between 0.10 and 0.35%. In the case of the steels in Table 1, the manganese contents are between 0.197 and 0.237%. In the steels for thin sheets for metal packaging, phosphorus contents and sulfur should be limited to 0.025%, preferably 0.015%, and 0.020%, respectively. In the case of steels from the examples in the table 1, these contents are respectively between 0.003 and 0.013% and between 0.005 and 0.011%.

Likewise, in steels for metallic packaging in the form of thin sheet, items such as copper, nickel and chromium should not not be found as a whole in a proportion greater than 0.08%.

In the case of steels from Table 1, this total copper content, nickel and chromium is at most 0.062%.

In addition, it has been shown that low titanium or niobium contents could significantly increase the recrystallization temperature full of sheets.

To obtain suitable recrystallization conditions for the sheets, the titanium content is imperatively limited to 10 ppm and preferably to 6 ppm.

Likewise, niobium should be limited to 10 ppm.

In ultra low carbon steels known from the prior art, the content in metallic aluminum after the sheet production is generally greater than 0.010% by weight or 10 thousandths, this content being generally between 10 and 60 thousandths of a percent.

The particular mode of production of the steels of the invention and research with an aluminum content of at most 0.010% allow to obtain, as will be shown below, sheets having a microstructure improved, greater microstructure homogeneity, greater inclusive cleanliness and better drawing properties.

We were able to show in particular that the improvement of the microstructure sheets, better microstructure homogeneity and good properties Stampability was due to the low residual aluminum content.

The calmed steel is degassed under vacuum and poured into a continuous casting of slabs under an inert atmosphere.

Inerting the casting prevents re-oxygenation of the steel during continuous casting and therefore phenomena of effervescence and breakthrough in casting.

The slab poured into the continuous casting plant is rolled to hot at a temperature higher than the Ar3 temperature of the steel.

In the case of the sheets mentioned in Table 1, the second column the end of rolling temperature of the sheets rolled to hot.

The hot rolled sheets are then wound at a temperature below the recrystallization temperature of the steel and always below at 620 ° C.

In table 2 below, the microstructural characteristics of the hot-rolled sheets are indicated, the compositions and rolling conditions of which are given in table 1. Landmark IG EI Rp0.2 T
(MPa) Rm t
(MPa) AT % rT M825 8.5 1.0 216 316 40.0 1.01 R21 16A 8.7 1.0 292 349 29.4 0.81 R2115A 8.2 1.0 281 333 33.5 0.99 R1048 C1 8.2 1.0 276 333 35.0 0.95 R1285 7.0 1.0 238 317 36.3 0.96 S 385 8.0 1.0 226 318 36.2 0.90 R1757A 10 1.0 255 342 34.7 0.84

In the first column of the table, the references of the hot rolled sheets; in the second column, we have marked the index of grains of the hot rolled sheet and in the third column, the elongation seeds.

The microstructural characteristics correspond to the central part in the heart of hot plates.

It appears that the microstructure at the heart of the different laminated sheets hot does not seem to be dependent on the aluminum content.

A finer grain (GI = 10.0) in the case of sample R1757A seems mainly due to the presence of quantities of nitrogen, manganese, more copper and chromium in the alloy. Conversely, larger grains (lG = 7.0) for sample R1285 seem to be linked to the production rolling at higher temperature (900 ° C) resulting in magnification austenitic grain.

Columns 4, 5, 6 and 7 are also shown in Table 2 respectively, the elastic limit of the sheets at 0.2% in the cross direction, the mechanical resistance in the cross direction, elongation at break and coefficient of normal anisotropy rT in the cross direction.

There is an increase in mechanical characteristics and a decrease in elongation with an increase in the aluminum content steel as well as a decrease (except in the case of sheet R2116A) the normal anisotropy coefficient rT.

The hot rolled sheets were subjected, after cooling, cold rolling with a reduction rate of 85 to 95%. We obtain then intermediate sheets whose thickness is of the order of 0.2 to 0.3 mm.

These sheets are then subjected to annealing in an installation of continuous annealing at a temperature below the steel temperature Ac1.

The blank of cold rolled sheet is then re-rolled to the thickness end of the sheet for stamping.

Continuous annealing is carried out at a temperature which is higher generally from 20 ° C to 30 ° C at the recrystallization temperature of the steel; in the case of the process according to the invention, the annealing temperature is at more equal to 700 ° C; the sheet metal heating speed is around 27 ° per second. The steel is maintained at the annealing temperature above the recrystallization temperature for a period of less than 3 minutes and which is usually, for practical reasons, 20 or 30 seconds about. The sheet is cooled, after continuous annealing, is carried out, initially, at a speed of the order of 8 ° per second and. in a second time, at a speed of the order of 10 ° per second.

Depending on the destination of the sheets for stamping, the rolling stages cold and annealing hot sheets produced according to the invention are performed differently.

In the case of sheets intended for the formation of boxes by stamping-re-stamping (DRD), hot rolled sheet with a thickness of 2.3 mm is cold rolled with a cold rolling rate of 85 at 89%.

The cold-rolled intermediate sheet is then continuously annealed at a temperature of approximately 650 ° C. for a period of the order of 20 seconds.

The second cold rolling or finishing rolling is carried out at skin-pass with a reduction rate of between 23 and 31%.

In the case of sheets intended for the manufacture of drink cans by stamping-ironing (DWI), hot rolled sheet with a thickness of about 3 mm is cold rolled with a reduction rate of 90 to 93%.

Annealing is carried out at a temperature of the order of 670 ° C. for a duration of approximately 30 seconds.

The final skin-pass lamination is carried out with a reduction rate from 2.5 to 17%.

The significant reduction rate during the final rolling in the case DRD sheets can develop strong mechanical characteristics in cold rolled sheets.

In Table 3 below, we have entered in the first column, the references of the sheets which correspond to the references of Tables 1 and 2, the different sheets differing, in terms of their composition, mainly by their aluminum content.

TABLE 3 (see next page)

The first three sheets have compositions according to the invention while the following four sheets are comparative sheets.

In the second column of Table 3, the reduction rate is plotted hot-rolled sheets during a first cold rolling. This first cold rolling is followed by a second cold rolling with a skin pass with an identical elongation for all sheets, of 2.5%.

In the third column, the temperature of continuous annealing (Rc) is indicated.

We then measure a set of mechanical characteristics of the sheets after final rolling with the skin-pass, as will be indicated below.

The reduction rate during the first cold rolling which is around 90% or slightly higher and the reduction rate on the second rolling cold which is of the order of 2.5% are characteristic of the preparation DWI sheets.

Samples were taken from the sheets obtained at the end of the final skin-pass rolling, the direction of specimen removal being indicated in the fourth column of Table 3 (L: in the long direction of the sheet, T: crosswise, 45: at 45 °).

The following columns of Table 3 indicate the values measured of the elastic limit Re, the mechanical resistance Rm, the elongation At%, the Lankford coefficient rd and the parameter nd for each test pieces taken from the sheets.

The following columns indicate the Lankford coefficient medium r medium and the parameter n medium, for the whole sheet.

In the next column, the coefficient of anisotropy has been indicated plane ΔC measured which, as we can see, is close to zero.

In the last column, the characteristics of the grains have been indicated in the form of the grain index IG and the elongation of the grains El.

The measurement results shown in Table 3 will be commented on by the continued with reference to Figures 3 to 6 on which the results have been reported graphically.

A first goal of the study carried out on sheets whose references are shown in Table 3 was to determine the influence of the content of aluminum sheets on the recrystallization temperature and on the microstructure recrystallization obtained from the sheets after cold rolling final.

Between the two cold rolling of the sheets, different continuous annealing simulations on sheet samples to determine the percentage of recrystallization as a function of the holding temperature continuous annealing, for the various sheets whose compositions are indicated in table 1.

The results are shown in Figure 1 on which we have plotted the recrystallization curves for each of the sheet compositions, the three first sheets having compositions corresponding to the following process the invention and the following four being comparative sheets.

The holding time at the annealing temperature is in all cases 30 seconds.

The three sheets according to the invention have practically the same recrystallization curve plotted in solid lines in FIG. 1.

A complete recrystallization annealing is obtained at 640 ° C.

R1285 sheet with 37 thousandths of aluminum including the recrystallization curve is shown in dashed lines shows a recrystallization temperature complete of the order of 660 ° C.

S 385 sheet with 56 thousandths of aluminum has a temperature of complete recrystallization of the order of 680 ° C. and the sheet R1757 at 64 thousandths of aluminum a complete recrystallization temperature of 710 ° C.

We therefore observe a 40 ° shift in the recrystallization temperature sheets, when the aluminum content changes from the corresponding contents to the process for the production of sheets according to the invention with a sheet of 64 thousandths of aluminum. In the case of sheets containing 37 and 56 thousandths of aluminum, respectively, the offset is approximately 20 and 40 ° C respectively.

With regard to sheet R1048 with 24 thousandths of aluminum, the offset recrystallization temperature is less than 20 ° C.

Figures 2A, 2B, 2C, 2D and 2E are micrographs at magnification of 290 showing the grains of sheets according to the invention at the end of the annealed.

In FIG. 2A, we see the microstructure of a cold-rolled sheet whose aluminum content is 2 thousandths, this sheet corresponding to the sheet M825 in Tables 1, 2 and 3. The grains of the sheet are regular in shape and equiaxial and the grain index is 10.5 with an elongation of grains of 1.

FIG. 2B is a micrograph showing the grain of a sheet containing 8 thousandths of aluminum which corresponds to sheet R2116A in the tables. The grains of the sheet are equiaxed, of uniform structure and size. The grain index and the grain elongation are identical in the case of Figure 2A.

Figure 2C is a micrograph of a sheet of 24 thousandths of aluminum which corresponds to the sheet R1048 C1 mentioned in the tables.

The grains of the sheet are no longer of uniform size and structure purely equiaxial.

The GI grain index is 10.8 and the grain elongation 1.0.

Figures 2D and 2 E are micrographs of sheets containing respectively 37 and 64 thousandths of aluminum, these sheets corresponding to sheets R1285 and R1757A of the tables.

The grains no longer have an equiaxed structure but an irregular structure and elongated known as "pancake".

The grain indices are 11 and 11.5 respectively and the elongations grains of 1.4 and 2.

It therefore appears that for aluminum contents of 2 and 8, that is to say for sheets produced according to the process of the invention, the grains are homogeneous and equiaxial in shape, which suggests behavior consistent during stamping and reduced risk of defects such as stamping horns.

On the other hand, for the sheets produced according to the process of the prior art with an aluminum content of more than 10 thousandths percent, grains are no longer homogeneous and equiaxial, which suggests behavior less good at stamping.

In addition, a low aluminum content, less than 10 thousandths percent, allows to obtain a good homogeneity of the microstructure in the longitudinal and transverse directions.

In FIGS. 3 and 4, the mechanical characteristics have been reported indicated in table 3, in the form of diagrams giving the limit elastic Re and mechanical resistance Rm in MPa as a function of the content in aluminium.

Most of the points relating to elastic limit and resistance measurements mechanical in the long direction and in the cross direction are aligned according to lines which have been drawn in dotted lines in FIGS. 3 and 4. From in general, the elastic limit Re and the mechanical resistance Rm increase with aluminum content.

In the case of steels produced according to the process of the invention, the elastic limit and mechanical strength reduced to a carbon content 25 ppm are slightly higher than 250 and 345 MPa respectively.

In FIGS. 5A, 5B and 5C, variations of the coefficients have been shown. from Lankford in the long direction, in the cross direction, and at 45 °.

A high Lankford coefficient indicates a strong anisotropy normal favorable for stamping.

As it appears on the curves of Figures 5A, 5B and 5C, whatever either the direction of sampling of the test pieces, the Lankford coefficient r is high for aluminum contents close to zero and then decreases for stabilize at a minimum value for the highest aluminum contents high.

In Figure 6, the average Lankford coefficient is shown for the whole sheet, r average, depending on the aluminum content.

By plotting the curve passing through the measurement points, we can see that the value of the average r coefficient extrapolated for 0% aluminum is around 1.9 and that for an aluminum content of 10 thousandths, the value of the Lankford coefficient is slightly greater than 1.60 (1.63).

It is assumed that a value of the upper average Lankford coefficient at 1.6 improves the drawability on shrinking.

Above 10 thousandths of aluminum in the steel sheets, the average Lankford coefficient very quickly drops below 1.6 to stabilize around 1.45 for the highest aluminum contents of the samples of sheet metal on which the tests were carried out. Steel VS Mn Al NOT Tfin LAC T winding LAF rate T Annealed r ΔC IG AT 7 188 15 4.7 870 620 90.1 650 ° C
30s 1.40 -0.35 11.6 B 8 199 13 4.3 870 715 89.7 650 ° C
30s 1.60 -0.20 10 VS 3.2 201 10 3.5 883 563 90.3 670 ° C
30s 1.68 0.12 10.5 D 5.3 200 12 5.6 865 670 89.5 670 ° C
30s 1.65 -0.02 9 E 5.8 209 12 4.9 865 540 90.0 670 ° C
30s 1.63 -0.07 10.7 F 12 204 12 5.5 872 590 90.2 650 ° C
30s 1.30 -0.38 11.3 G 13 187 6 4.8 869 595 89.9 650 ° C
30s 1.35 -0.36 10.8 H 12 204 12 5.5 874 700 90.1 650 ° C
30s 1.50 -0.20 10.3 I 13 187 6 4.8 872 695 90.0 650 ° C
30s 1.55 -0.20 9.1 J 3.5 202 8 3.5 875 698 89.8 670 ° C
30s 1.69 0.04 9 K 2.7 204 33 2.3 868 555 89.9 650 ° C
8h 1.88 0.24 7 L 2.7 204 33 2.3 868 555 91.1 670 ° C
30s 1.66 0.06 11

In Table 4, the compositions, the temperatures of rolling, winding and annealing as well as the characteristics r, ΔC and IG relating to stampability, for sheets constituting comparative examples, with respect to the sheets according to the invention appearing in the first part of Table 3 above.

The steels of the comparative examples whose references are given in the first column of table 4, with the exception of steel C, which corresponds R2115A steel according to the invention shown in Table 3, have compositions which differ from the composition of a steel according to the invention, either by their carbon content (G and I steels), i.e. by their aluminum content (steels D, E, J, K and L), either again by their carbon content and their aluminum content (steels A, B, F and H).

In addition, the sheets having compositions B, D, H, I and J were wound, after hot rolling, at a temperature above 620 ° C. which is the upper limit of the winding temperature in the case of the invention.

The sheets of the comparative examples given in Table 4 have characteristics which are generally inferior to the characteristics of the steels of the steels of the invention. In addition, these steels, when they contain aluminum contents greater than 10 thousandths percent have structural consistency and cleanliness inclusive lower than the steels of the invention.

By comparing the characteristics of the sheet of example C according to the invention and of example J, which presents a composition according to the invention and which was obtained by a process in which the hot-rolled sheet has been wound at a temperature above 620 ° C (698 ° C), we realize that the sheets obtained have Lankford coefficients r very close and largely greater than 1.60 and values of ΔC close to 0. However, the ASTM IG grain index of the sheet according to Example J is lower than the index of grains of the sheet according to Example C and less than 10. The final grain of the sheet is therefore less thin in the case where the sheet has been wound higher temperature.

In the case of the sheet of Example A, the steel contains a content of carbon (70 ppm) greater than the limit of 60 ppm of the sheets produced according to the invention and the hot-rolled sheet is wound at 620 ° C., that is to say at the upper limit of the winding temperature interval according to the invention. The Lankford coefficient r is low (only 1.40). The coefficient anisotropy ΔC is very different from 0 (-0.35). The size index of grains, on the other hand, is entirely satisfactory (11.6).

In the case of the sheet of example B, the composition of which is close from that of steel according to Example A, the winding temperature is 715 ° C, that is to say a temperature substantially higher than the limit of 620 ° C. Sheet B has a relatively satisfactory Lankford coefficient (1.60), an anisotropy coefficient quite far from 0 (-0.20) and an index of grains lower than the grain index in the case of sheet A.

In the case of the sheet of example D, compared with example E, the steels D and E being steels with 53 and 58 ppm carbon, respectively, the rise in the winding temperature above 620 ° (670 ° C) has practically no influence on the Lankford coefficient and on the coefficient anisotropy. On the other hand, the grain index goes from 10.7 to 9, when the winding temperature goes from 540 (example E) to 670 ° C (example D).

In the case of Examples H and I, a substantially carbon content greater than 6 thousandths of a percent (12 and 13 thousandths of a percent) results, on the sheet obtained, by a low Lankford coefficient and a anisotropy coefficient ΔC far from the zero value. In the case of examples F and G, the compositions of steels F and G being identical, respectively the compositions of alloys H and I, the coiling temperatures hot rolled sheet are less than 620 ° C, the characteristics r and ΔC are very poor but the grain index is satisfactory and more favorable than in the case of examples H and I where the rolled sheet hot was coiled at temperatures of the order of 700 ° C.

In the case of all the sheets of the examples mentioned above, a recrystallization annealing is carried out continuously, for a duration of the order 30 seconds at a temperature of about 650 ° C or slightly higher.

These comparative examples show that, on the one hand, a content of carbon less than 6 thousandths of a percent (or 60 ppm) is required, in the composition of steel, to obtain a sheet having characteristics r and ΔC satisfactory. On the other hand, these examples also show that, in the case of a carbon content of less than 6 thousandths percent, a moderate winding temperature, generally below 620 ° C, provides a satisfactory grain index, generally greater than 10, that is to say a sheet having a fine grain.

Generally, the winding will be carried out at a temperature between 450 ° C and 620 ° C and preferably between 530 and 570 ° C, as shown in particular in Table 1, if we consider the three first examples in the table which are examples of steels following the invention.

In the case where a steel having a higher carbon content is used at 60 ppm, Example B in Table 4 shows that we can obtain relatively satisfactory characteristics r and ΔC when winding hot-rolled strip at a temperature of the order of 715 ° C. However, the grain index is then only 10 when it was 11.6 in the case of the steel of example A.

In the context of the method according to the invention, to obtain a sheet metal fine grains with good drawability characteristics, a steel with a carbon content of less than 60 ppm and a winding temperature of hot-rolled sheet within a range between 450 ° C and 620 ° C, after rapid cooling of the rolled sheet to hot.

It has been shown above that, in the case of a steel having a content carbon less than 60 ppm, lowering the aluminum content, below 10 thousandths percent, gave very good drawing properties, in addition to a high structural homogeneity and very good inclusiveness.

By comparing the examples in Table 4, F and G on the one hand, and H and I on the other hand, we see that the lowering of the aluminum content from 12 to 6 thousandths percent has practically no effect on the parameters r and ΔC, whose values remain quite poor in the case of steels with 12 and 13 thousandths percent of carbon. This effect is practically identical, whatever the winding temperature of the hot sheet.

However, in the context of the invention, when the carbon content is less than 6 thousandths of a percent in steel, the lowering of aluminum content below 10 thousandths percent allows to significantly improve the parameters r and ΔC.

The sheets for stamping according to the invention must have sufficiently fine grains (grain index at least equal to 9) and a structure homogeneous.

To obtain this result, the sheet is rapidly cooled between the temperature end of hot rolling and the winding temperature, which must be less than 620 ° C. This rapid cooling and winding to a temperature relatively low limit grain growth in the hot rolled sheet, and get a good grain index in the final sheet obtained after cold rolling.

As shown in Table 4 (example K), an ultra low carbon steel and ultra low aluminum obtained by vacuum degassing at the steelworks, which is annealed at 650 ° C for eight hours, has a Lankford coefficient r high (1.88), an anisotropy coefficient significantly different from 0 (0.24) and a very low grain index (GI = 7).

When the same steel is used for the manufacture of a sheet which is annealed continuously at 670 ° C for 30 seconds (example L), the coefficients r and ΔC as well as the grain index have satisfactory values, although the aluminum content of steel is significantly higher than the given limit in the case of the invention. In this case, however, we cannot guarantee very good structural homogeneity and very good inclusiveness.

In the case of ultra low carbon steels, as shown by examples K and L, it is preferable to perform continuous annealing at a temperature slightly higher than 650 ° C, for example 670 ° C, during a duration of 30 seconds. Long-term annealing at these temperatures, in addition to the increase in the cost of production of sheets, leads to a deterioration the anisotropy coefficient ΔC and the grain index.

As can be seen in Table 1, the steels used in the context of the invention contains very small amounts of titanium, of the order of 1 to a few ppm. It was also stated that, in order to avoid an increase the recrystallization temperature of the steel, the content is limited in titanium at 10 ppm and preferably at 6 ppm in steel.

It has been shown that, for a titanium content of 10 ppm, the temperature recrystallization is 670 ° C, instead of 640 ° C for a content of substantially zero titanium. Because annealing must be carried out at a temperature higher than the recrystallization temperature of steel by 20 ° C or 30 ° C, the titanium content must not exceed 10 ppm to have a temperature annealing up to 700 ° C. In addition, titanium, in a proportion greater than 10 ppm, causes a deviation of the anisotropy coefficient ΔC with respect to the zero value.

Similarly, niobium increases the recrystallization temperature by steel in proportions substantially analogous to titanium. For a niobium content of 3 ppm, the recrystallization temperature of the steel is equal to 640 ° C whereas, for 10 ppm of niobium, it is 680 ° C. For limit the annealing temperature of the steel to a value close to 700 ° C., while ensuring complete recrystallization throughout the sheet metal coil, we therefore limits the niobium content to 10 ppm.

The steels used in the context of the invention are therefore steels substantially free from titanium and niobium, the contents of these elements being limited to 10 ppm, or 0.001% by weight.

The sheets for stamping obtained by the production process according to the invention, which in particular have a carbon content at more than 6 thousandths of a percent and aluminum not more than 10 thousandths of a percent, have, after the final cold rolling, a homogeneous microstructure with equiaxed grains and very good characteristics stampability. In particular, the microstructure of the sheet presents good homogeneity in the cross direction and the edges of the sheet present a homogeneous equiaxed grain whose size is slightly larger than the grain size in the part of the band adjacent to the axis. In addition, studies have shown that the sheets obtained by the process of the invention have very good inclusiveness, when deoxidation is carried out by the slag and that inerting of the continuous casting is carried out.

In particular, the reduction in slag affects the average standard deviation of sizes of inclusions and the number of inclusions in the steel. In addition, the very low aluminum reduces the average density of inclusions in steel.

Very good inclusiveness is of great interest. in particularly in the case of very thin sheets used for manufacturing by stamping of metallic packaging such as boxes for beverages.

The preparation process according to the invention makes it possible to reduce the percentage of waste due to heterogeneous microstructures or the presence unacceptable inclusions in sheet metal for stamping and particularly in sheet metal for stamping-ironing type DWI.

In addition, the method according to the invention, which uses very low quantities of aluminum for the calming of steel, allows to achieve a saving on the purchase of aluminum, in the context of sheet metal production for stamping.

The invention is not limited to the embodiment which has been described.

This is how we can achieve the calming of steel other than by slag reduction and that the presence of aluminum in the sheet steel for stamping with a content of less than 10 thousandths of a percent in itself provides substantial benefits with regard to the microstructure, uniformity and drawability of the steel sheet.

The invention is equally applicable to sheets for DRD stamping than sheet metal for DWI stamping. Rolling rates during first and second cold rolling can be adapted to the use of the sheet metal for the production of stamped specific packaging products.

Claims (7)

Process for the production of a thin sheet of ultra low carbon steel for the production of stamped products for packaging in which: a calm and degassed steel is produced under vacuum containing, by weight. between 0.10 and 0.35% manganese, less than 0.006% nitrogen, less than 0.025% phosphorus, less than 0.020% sulfur, less than 0.020% silicon, at most 0.08% of a or more of the elements copper, nickel and chromium as well as aluminum, the rest of the composition consisting of iron and unavoidable impurities, the steel is poured in the form of a slab, the slab is hot rolled at a temperature higher than Ar3 to obtain a strip of hot rolled sheet, the hot rolled sheet is wound, the hot-rolled sheet is cold rolled, in the form of an intermediate cold-rolled sheet, the intermediate cold-rolled sheet is annealed continuously at a temperature below Ac1, the intermediate cold-rolled sheet is re-rolled to a final thickness of the sheet for stamping, characterized in that the steel is produced so as to contain at most 0.006% by weight of carbon and 0.010% by weight of aluminum and that the hot-rolled sheet is coiled at a temperature below 620 ° C and preferably between 530 and 570 ° C. Method according to claim 1, characterized in that the steel contains at most 0.001% by weight of titanium and 0.001% by weight of niobium, and that the cold rolled sheet is annealed at a temperature below 700 ° C for a period of less than 3 minutes and preferably close to 30 seconds. Process according to claim 2, for the manufacture of a sheet thin for deep drawing by the deep drawing and deep drawing process DRD, characterized in that the hot-rolled sheet has a similar thickness 2.3 mm, roll the hot rolled sheet with a reduction rate between 85 and 89%, that the intermediate rolled sheet is annealed to cold by continuous annealing at a temperature of about 650 °, for twenty seconds or so, and re-roll the cold rolled intermediate sheet on a skin-pass rolling mill with a reduction rate of between 23 and 31%. Method according to claim 2 for the production of a sheet for deep drawing by the DWI deep drawing and deep drawing process, characterized in that the hot rolled sheet with a thickness close to 3 mm, cold rolled hot rolled sheet with a reduction rate 90 to 93%, that the intermediate cold-rolled sheet is annealed continuously at a temperature in the region of 670 ° C for a period of the order of thirty seconds and re-laminate the intermediate sheet after annealing in a skin-pass rolling mill, with a reduction rate of between 2.5 and 17%. Method according to any one of claims 1 to 4, characterized by the fact that the steel is calm in contact with a slag having a adjusted aluminum and alumina content. Method according to any one of claims 1 to 5, characterized by the fact that the steel is poured in the form of a slab in a continuous casting installation in an inert atmosphere. Thin sheet of ultra-low carbon steel for making products deep drawn packaging containing by weight, between 0.10 and 0.35% of manganese, less than 0.006% nitrogen, less than 0.025% phosphorus, less than 0.020% sulfur, less than 0.020% silicon, at most 0.08% one or more of the elements copper, nickel and chromium as well as aluminum, the rest of the composition consisting of iron and impurities unavoidable, the thin sheet being obtained by cold rolling of a rolled sheet hot by first rolling and by second separate rolling by continuous annealing, characterized in that the steel of the sheet contains not more than 0.006% by weight of carbon and 0.010% by weight of aluminum, and that it has a homogeneous structure with equiaxed grains, that it has a Lankford coefficient (mean r) greater than 1.6 and a coefficient plane anisotropy (ΔC) close to 0.

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