EP1065282A1 - Low aluminum steel sheet for cans - Google Patents

Abstract

Annealing takes place continuously, and involves raising the temperature above that initiating perlitic transformation Ac1, maintaining the temperature above this level for more than 10s, and rapidly cooling to a temperature below 350 degrees C, at above 100 degrees C/s Production of the steel strip involves: (a) providing a hot-rolled steel strip containing (in wt.%) 0.050-0.080 C, 0.25-0.40 Mn, 0.020 Al, 0.010-0.014 N, and the remainder Fe and inevitable impurities; (b) primary cold rolling of the obtained strip; (c) annealing the cold rolled steel strip; and (d) secondary cold rolling of the annealed steel strip. An Independent claim is included for the steel strip produced by the above process. In an aged state, the ultimate elongation is in accordance with an equation provided, expressed in terms of the ultimate tensile strength.

Description

The present invention relates to the field of steels for application in the field of metal packaging, food, non food or industrial.

Steels developed for specific packaging uses metallic especially differ from thin sheets by their characteristics physical.

The thicknesses of the steel sheets for packaging vary from 0.12 mm to 0.25 mm for the majority of uses, but may achieve greater thicknesses, up to 0.49 mm for very specific applications. This is for example the case of some non-food packaging, such as certain aerosols, or certain industrial packaging. They can also go down up to 0.08 mm, for example in the case of food trays.

Steel sheets for packaging are usually coated a metallic coating (tin, remelted or not, or chrome) on which is generally deposited an organic coating (varnish, inks, films plastics).

In the case of two-piece packaging, these are produced by stamping under blank holder, or by stamping / ironing for beverage cans, and are generally axisymmetric, cylindrical cans or tapered. However, packagers are showing increasing interest more marked for steels with ever smaller thicknesses, from 0.12 mm to 0.075 mm and, in order to differentiate themselves from the competition, they seek to innovate in increasingly complex forms. So we find now boxes of original shapes, made of steel sheets small thicknesses which, although presenting greater difficulties of forming, must meet the criteria of use (mechanical strength of packaging, resistance to the axial load which they undergo during their stacking storage, resistance to internal overpressure they undergo during sterilization heat treatment and depression they undergo after cooling) and therefore present a very high mechanical resistance.

• le coefficient d'anisotropie planaire ΔC _aniso,
• le coefficient de Lankford,
• la limite d'élasticité Re,
• la résistance maximale à la rupture Rm,
• l'allongement A%,
• l'allongement réparti Ag%.

Thus, the implementation and the performance of these packages depend on a certain number of mechanical characteristics of the steel:

• the planar anisotropy coefficient ΔC _aniso ,
• the Lankford coefficient,
• the elastic limit Re,
• the maximum breaking strength Rm,
• the elongation A%,
• the distributed elongation Ag%.

To give the packaging mechanical resistance equivalent to lower steel thickness, it is essential that the steel sheet has higher maximum breaking strength.

It is known to use steels for packaging with low aluminum content, and in particular so-called "low steels" renitrided aluminum ”. Such a steel is for example described in the patent French n ° 95 11 113.

The carbon content usually targeted for this type of steel is between 0.050% and 0.080%, the manganese content included between 0.20 and 0.45%. Aluminum content is controlled to be lower at 0.020% in order to give the steel sheet a microstructure improved, good inclusiveness, and consequently high mechanical characteristics.

The nitrogen content is also controlled and is understood between 0.008 and 0.016%. This nitrogen content is ensured by adding to the bag of calcium cyanamide during steel making, or by blowing nitrogen gas in the steel bath. The known advantage of adding nitrogen is harden the steel by effect of solid solution.

These steel sheets are produced by cold rolling a hot strip, with a cold rolling rate of between 75% and more 90%, followed by continuous annealing at a temperature between 640 and 700 ° C, and a second cold rolling with an elongation rate during of this second cold rolling variable between 2% and 45% depending on the level of maximum breaking strength Rm targeted.

However, for steels with low aluminum content, high mechanical characteristics are associated with a capacity low elongation. This low ductility, in addition to the fact that it is unfavorable the shaping of the packaging, results in this shaping a thinning of the walls which will be detrimental to the performance of packaging.

So for example a “low aluminum renitrided” steel having a maximum breaking strength Rm of the order of 550 MPa, will have an A% elongation rate of only 2 to 5%.

The present invention aims to provide a steel sheet to low aluminum content for packaging that presents a rate A higher elongation than% low steels state-of-the-art aluminum, at maximum resistance to equivalent break.

• on approvisionne une bande d'acier laminée à chaud comportant en poids entre 0,050 et 0,080 % de carbone, entre 0,25 et 0,40 % de manganèse, moins de 0,020 % d'aluminium, entre 0,010 et 0,014% d'azote, le reste étant du fer et des impuretés résiduelles inévitables,
• on effectue un premier laminage à froid de la bande,
• on soumet la bande laminée à froid à un recuit,
• on effectue éventuellement un laminage à froid secondaire,

caractérisé en ce que le recuit est un recuit continu dont le cycle comporte une montée en température jusqu'à une température supérieure à la température de début de transformation perlitique Ac₁, un maintien de la bande au dessus de cette température pendant une durée supérieure à 10 secondes, et un refroidissement rapide de la bande jusqu'à une température inférieure à 350°C à une vitesse de refroidissement supérieure à 100°C par seconde.To obtain these characteristics, the invention relates to a process for manufacturing a steel strip with low aluminum content for packaging, in which:

• a hot-rolled steel strip is supplied comprising by weight between 0.050 and 0.080% of carbon, between 0.25 and 0.40% of manganese, less than 0.020% of aluminum, between 0.010 and 0.014% of nitrogen, the rest being iron and unavoidable residual impurities,
• a first cold rolling of the strip is carried out,
• the cold rolled strip is subjected to annealing,
• secondary cold rolling is optionally carried out,

characterized in that the annealing is a continuous annealing, the cycle of which includes a rise in temperature up to a temperature higher than the temperature at the start of pearlitic transformation Ac ₁ , maintaining the strip above this temperature for a duration greater than 10 seconds, and rapid cooling of the strip to a temperature below 350 ° C at a cooling rate above 100 ° C per second.

• la bande est maintenue au cours du recuit à une température comprise entre Ac₁ et 800°C, pendant une durée de 10 secondes à 2 minutes ;
• la vitesse de refroidissement est comprise entre 100°C par seconde et 500°C par seconde ;
• la bande est refroidie à une vitesse supérieure à 100°C par seconde jusqu'à la température ambiante.

According to other characteristics of the process according to the invention:

• the strip is maintained during annealing at a temperature between Ac ₁ and 800 ° C, for a period of 10 seconds to 2 minutes;
• the cooling rate is between 100 ° C per second and 500 ° C per second;
• the strip is cooled at a speed greater than 100 ° C. per second to ambient temperature.

The invention also relates to a steel sheet with low aluminum content comprising by weight between 0.050 and 0.080% of carbon, between 0.25 and 0.40% of manganese, less than 0.020% of aluminum, between 0.010 and 0.014 % nitrogen, the rest being iron and unavoidable residual impurities, manufactured according to the above process, characterized in that it has, in the aged state, an elongation rate A% satisfying the relationship: (750 - Rm) / 16.5 ≤ A% ≤ (850 - Rm) / 17.5 Rm being the maximum breaking strength of the steel, expressed in MPa.

According to other characteristics of the sheet, the steel comprises carbon in the free state and / or some carbides precipitated at low temperature, and has a number of grains per mm ² greater than 30,000.

Features and benefits will become more apparent in the description which follows, given solely by way of example, made in reference to the attached figures.

Figures 1 and 2 are diagrams showing the influence of the annealing temperature on the maximum breaking strength Rm.

Figure 3 is a diagram showing the influence of speed cooling on the maximum breaking strength Rm.

Figure 4 is a diagram showing the influence of speed cooling on the maximum breaking strength Rm and the rate elongation A%.

Figure 5 is a diagram showing the influence of speed cooling on hardness HR30T.

Several tests have been carried out, first in the laboratory and then under industrial conditions, to validate the characteristics of the invention. The full results of two of these tests will now be described.

These tests relate to two cold coils made of steel with low aluminum content, the characteristics of which are reproduced in Table 1 below. Content (10 ^-3 %) Hot rolling Cold rolling VS Mn Al NOT TFL (° C) Tbob (° C) Ep (mm) T red. (%) Ep (mm) AT 59 345 15 10.5 842 598 2.06 91.2 0.18 B 66 309 17 12 841 587 2.00 87 0.28

In the first column, we identified the coil; in the second to fifth columns, the contents of the main constituents of importance are indicated in 10 ^-3 % by weight. The sixth to eighth columns relate to the hot rolling conditions: in the sixth column, the end of hot rolling temperature is indicated; in the seventh column, the winding temperature; in the eighth column, the thickness of the hot strip. Finally, columns nine and ten relate to the cold rolling conditions: in the ninth column, the reduction rate of the cold rolling is indicated and in the tenth column, the final thickness of the cold strip.

These two standard bands were subjected to differentiated annealing followed by second cold rollings also differentiated.

The annealing holding temperatures varied from 650 ° C to 800 ° C, the cooling rates varied from 40 ° C / s to 400 ° C / s, and the rates of elongation on second rolling varied from 1% to 42%.

In addition to micrographic examinations, the characterization of the metal resulting from these various tests consisted on the one hand in doing pull-ups on ISO 12.5x50 test pieces in the rolling direction and in the transverse direction, fresh and aged after aging at 200 ° C for 20 minutes, on the other hand to determine the hardness HR30T also in the fresh state and in the state aged.

These tests have demonstrated that it is possible considerably increase the maximum breaking strength Rm for the same steel with low aluminum content, at elongation rate on the second identical cold rolling, if one practices between the two cold rolling a continuous annealing according to the conditions of the invention.

In other words, these tests have demonstrated that it is possible to considerably increase the ductility A% for the same steel low aluminum content, with identical maximum breaking strength Rm, if between the two cold rolling operations a continuous annealing according to the conditions of the invention, because the same level of Rm is reached with a rate lower elongation during the second rolling. So it becomes possible to produce qualities of low aluminum content steel with a level of Rm of the order of 380 MPa without requiring a second rolling after annealing, except maybe a light work hardening operation called skin-pass which eliminates the elastic limit level present on the metal after annealing.

Impact of steel composition

As indicated above, the invention does not lie in the composition of steel, which is a low aluminum steel standard.

• l'aluminium est utilisé pour calmer l'acier. Il est limité à 0,020% dans le but de conférer à la tôle d'acier une microstructure améliorée, une bonne propreté inclusionnaire, et part voie de conséquence des caractéristiques mécaniques élevées ;
• la teneur en azote est également contrôlée et est comprise entre 0,008 et 0,016 %. Cette teneur en azote est assurée par ajout en poche de cyanamide calcique lors de l'élaboration de l'acier, ou par soufflage d'azote gazeux dans le bain d'acier. L'intérêt connu de l'ajout d'azote est de durcir l'acier par effet de solution solide.

Like all steels with low aluminum content renitrided, it is essentially the aluminum and nitrogen contents which are important:

• aluminum is used to calm steel. It is limited to 0.020% in order to give the steel sheet an improved microstructure, good inclusiveness, and consequently high mechanical characteristics;
• the nitrogen content is also controlled and is between 0.008 and 0.016%. This nitrogen content is ensured by adding a pocket of calcium cyanamide during the production of the steel, or by blowing nitrogen gas into the steel bath. The known advantage of adding nitrogen is to harden the steel by the effect of a solid solution.

• la teneur en carbone visée habituellement pour ce type d'acier est comprise entre 0,050% et 0,080% ;
• la teneur en manganèse est comprise entre 0,25 et 0,40%.

Carbon and manganese are also two elements that should be controlled.

• the carbon content usually targeted for this type of steel is between 0.050% and 0.080%;
• the manganese content is between 0.25 and 0.40%.

Impact of hot denaturing conditions

Continuously annealed renitrided low aluminum steels are generally rolled at a temperature above Ar ₃ .

• il génère des hétérogénéités de caractéristiques mécaniques en liaison avec les différences de vitesses de refroidissement entre le coeur et les extrémités de la bande ;
• il induit un risque de croissance anormale des grains, laquelle peut se produire pour certains couples (température de fin de laminage, température de bobinage) et peut constituer un défaut rédhibitoire aussi bien en tôle à chaud qu'en tôle à froid.

The essential parameter is the winding temperature, and a cold winding is preferred, between 500 and 620 ° C. Indeed, hot winding, at a temperature above 650 ° C has two drawbacks:

• it generates heterogeneities of mechanical characteristics in connection with the differences in cooling rates between the core and the ends of the strip;
• it induces a risk of abnormal grain growth, which can occur for certain pairs (end of rolling temperature, winding temperature) and can constitute a crippling defect in both hot and cold sheets.

Nevertheless a hot winding can be carried out by practicing for example a selective winding: the temperature is higher in ends of the strip.

Impact of cold rolling conditions

Due to the small final thicknesses to be achieved, the field of Cold reduction rate ranges from 75% to over 90%.

The main factors involved in defining the cold reduction rates are obviously the final thickness of the product, and on this point we can play on the thickness of the hot product, as well as metallurgical considerations.

Metallurgical considerations are based on the incidence of the rate of cold reduction on the microstructural state, and by way of consequence on the mechanical characteristics after recrystallization and annealed. Thus the more the cold reduction rate increases, the more the temperature of the lower the recrystallization, the weaker the grains and the more Re and Rm are high. In particular, the reduction rate has a very strong impact on the Lankford coefficient.

In the case of requirements in terms of drawing horns, it for example, optimize the steel grade and especially the content of carbon, and the reduction rate of cold rolling with hardness or mechanical characteristics desired to obtain a metal known as “metal without horns ".

Incidence of annealing

An important characteristic of the invention lies in the annealing temperature. It is important that the annealing temperature is higher than the point of start of pearlitic transformation Ac ₁ (of the order of 720 ° C. for this type of steel).

Another important characteristic of the invention resides in the cooling rate which must be greater than 100 ° C / s.

During the maintenance of the strip at a temperature above Ac ₁ , austenite, rich in carbon, is formed. The rapid cooling of this austenite makes it possible to maintain a certain amount of carbon in the free state and / or a precipitation of carbides at low temperature, fine and dispersed. This carbon in the free state and / or these carbides formed at low temperature promote the blocking of dislocations, which makes it possible to achieve high levels of mechanical characteristics without requiring a significant reduction during the second cold rolling which follows.

It is therefore important to achieve rapid cooling, between 100 and 500 ° C / s at least up to a temperature below 350 ° C. If rapid cooling is stopped before 350 ° C, the atoms of free carbon will be able to combine and the desired effect will not be achieved. It is obvious that rapid cooling down to the temperature ambient is possible.

It is also possible to cool to a speed greater than 500 ° C / s, but the Applicant has found that beyond 500 ° C / s, the influence of an increase in the cooling rate is no longer very significant.

Figures 1 and 2 show the influence of the annealing at constant cooling rate (Aimed at 100 ° C / s and performed 73 to 102 ° C / s in Figure 1; Aimed at 300 ° C / s and performed 228 to 331 ° C / s on the Figure 2) on the maximum breaking strength Rm.

These figures show a marked increase in Rm to identical elongation rate of the second rolling for steels annealed at 750 ° C and 800 ° C compared to the same steel annealed at 650 ° C.

However, this influence of the annealing temperature on the maximum breaking strength Rm is not very noticeable for rates elongation at the second cold rolling less than 3%. She only becomes really significant that from 5% elongation on the second rolling at cold.

A too high temperature, above 800 ° C, causes a precipitation, at least partial, of nitrogen in the form of nitrides aluminum. This precipitated nitrogen no longer participates in the hardening of the steel, which has the effect of reducing the maximum breaking strength Rm. This phenomenon is glimpsed in Figure 2 on which we notice, for elongation rates greater than 10%, a decrease in the increase in maximum tensile strength Rm between the annealed sample at 750 ° C and the sample annealed at 800 ° C.

The strip holding time between Ac ₁ and 800 ° C must be sufficient to put all the carbon corresponding to equilibrium back into solution. Maintaining for 10 seconds is sufficient to ensure this redissolution of the amount of carbon corresponding to equilibrium for steels whose carbon content is between 0.020 and 0.035%, and maintaining beyond 2 minutes, although possible, is unnecessary and expensive.

Figures 3 and 4 show the influence of the speed of constant annealing temperature (750 ° C) maintained for 20 seconds.

As can be seen in Figure 3, at 10% elongation at second cold rolling, the maximum tensile strength Rm of the steel is equal to around 560 MPa if the cooling rate is equal to 100 ° C / s, when it only reaches 505 MPa if the cooling speed is equal to 50 ° C / s.

We can therefore produce a steel with low aluminum content whose Rm value is 560 MPa with only 10% elongation at the second cold rolling if the cooling rate is equal to 100 ° C / s, whereas a second cold rolling must be carried out with a 17% elongation rate if the cooling rate is only 50 ° C / s.

This lower elongation rate on the second cold rolling allows less deterioration of the ductility of steel. We thus see in Figure 4 that the steel whose Rm is equal to 560 MPa has a ductility A% equal to 12.5 when the cooling rate is 100 ° C / s, while it is 5.5 when the cooling rate is 50 ° C / s.

This observation also applies to the hardness of the steel. As seen in Figure 5, for the same elongation rate at second cold rolling, the hardness of the steel increases if the speed of cooling is equal to 100 ° C / s. This increase in hardness is due at a higher free carbon content and / or the presence of precipitates fine and scattered.

Micrographic analyzes of the samples have shown that the number of grains per mm ² is greater (greater than 30,000), and that the carbides, when formed, are intergranular cementite.

Thus this manufacturing process makes it possible to produce a steel with low aluminum content for packaging, comprising by weight between 0.050 and 0.080% of carbon, between 0.25 and 0.40% of manganese, less than 0.020% of aluminum, between 0.010 and 0.014% nitrogen, the rest being iron and inevitable residual impurities, which in the aged state has an elongation rate A% satisfying the relationship: (750 - Rm) / 16.5 ≤ A% ≤ 850 - Rm) / 17.5 Rm being the maximum breaking strength of the steel, expressed in MPa.

Claims (6)

Method for manufacturing a low aluminum content steel strip for packaging, in which: a hot-rolled steel strip is supplied comprising by weight between 0.050 and 0.080% of carbon, between 0.25 and 0.40% of manganese, less than 0.020% of aluminum, between 0.010 and 0.014% of nitrogen, the rest being iron and unavoidable residual impurities, a first cold rolling of the strip is carried out, the cold rolled strip is subjected to annealing, secondary cold rolling is optionally carried out, characterized in that the annealing is a continuous annealing, the cycle of which includes a rise in temperature up to a temperature higher than the temperature at the start of pearlitic transformation Ac ₁ , maintaining the strip above this temperature for a duration greater than 10 seconds, and rapid cooling of the strip to a temperature below 350 ° C at a cooling rate above 100 ° C per second. Method according to claim 1, characterized in that the strip is maintained during annealing at a temperature between Ac ₁ and 800 ° C, for a period of 10 seconds to 2 minutes. Method according to claim 1, characterized in that the cooling rate is between 100 ° C per second and 500 ° C per second. Method according to claim 1, characterized in that the strip is cooled at a speed greater than 100 ° C per second until the ambient temperature. Low aluminum sheet steel for packaging, containing by weight between 0.050 and 0.080% of carbon, between 0.25 and 0.40% of manganese, less than 0.020% of aluminum, between 0.010 and 0.014% of nitrogen, the remainder being iron and unavoidable residual impurities, produced according to the method of claims 1 to 4, characterized in that it has, in the aged state, an elongation rate A% satisfying the relationship: (750 - Rm) / 16.5 ≤ A% ≤ 850 - Rm) / 17.5 Rm being the maximum breaking strength of the steel, expressed in MPa. Sheet steel according to claim 5, characterized in that the steel comprises carbon in the free state and / or some carbides precipitated at low temperature, and has a number of grains per mm ² greater than 30000.

Images