EP0699769A1 - Process for the manufacture of steel having deep-drawing properties and steel according to said process - Google Patents

Abstract

A mild steel is treated by degassing under a vacuum to reduce the carbon content to below 80 ppm, pref. below 40 ppm followed by blowing in nitrogen up to a level of 80-140 ppm.

Description

The subject of the present invention is a method for producing a steel for packaging suitable for deep drawing and little sensitive to the formation of horns during stamping, and a steel obtained from this process.

The present invention is particularly applicable to the production of very thin products obtained by deep drawing of the stamped-re-stamped boxes or stamped-ironed boxes type.

• le coefficient d'anisotropie normale moyen r encore appelé coefficient de Lankford moyen, doit être élevé afin d'assurer un écoulement correct du métal pendant l'emboutissage et éviter les plissements ;
• le coefficient d'anisotropie planaire Δr doit être proche de 0 afin de limiter la formation des cornes pendant l'emboutissage ;
• les caractéristiques mécaniques, en particulier la limite d'élasticité Re et la résistance à la traction Rm, doivent être élevées, afin d'assurer une bonne tenue mécanique des boîtes avec des parois de très faible épaisseur.

Steels for packaging, in particular steels for deep-drawn deep-drawing must, to allow a good processing, have the following characteristics:

• the average normal anisotropy coefficient r, also called the average Lankford coefficient, must be high in order to ensure correct flow of the metal during drawing and to avoid wrinkling;
• the planar anisotropy coefficient Δr must be close to 0 in order to limit the formation of the horns during stamping;
• the mechanical characteristics, in particular the elastic limit Re and the tensile strength Rm, must be high, in order to ensure good mechanical strength of the boxes with very thin walls.

During the manufacture of a metallic packaging, a sheet blank is stamped, generally made of a metal which has undergone a rolling operation, so that this metal has an anisotropy with regard to its resistance to thinning.

Therefore, for example when making cylindrical section containers by stamping circular blanks, it creates, during stamping, different thinning and thickening depending on the directions on the body of stamped objects and the flange obtained is irregular.

Indeed, this collar has a sinuous profile comprising "horns" and "hollow horns".

In the case of a stamped without collar, this irregularity is located at the end of the peripheral skirt and this irregularity creates a certain number of obstacles during subsequent operations.

This anisotropy of the metal with regard to its resistance to thinning is also responsible for the formation of folds or ripples in certain areas during stamping.

This formation of folds or ripples is due to a difference in sliding resistance of the metal depending on the areas under the blank holder which surrounds the die and the stamping punch.

This anisotropy of the metal is translated by two coefficients, well known in the technique of stamping, in particular of swaged stamping, the Lankford coefficient r and the coefficient of planar anisotropy Δr.

The Lankford coefficient r is representative of the resistance to thinning of the metal used in the direction in lacquer! The it is determined.

The direction in which the Lankford coefficient is highest corresponds to the direction in which the metal flows easily and therefore does not form folds during stamping and therefore at the position of the horns.

On the other hand, the direction in which the Lankford coefficient is the lowest corresponds to the direction in which the metal has a high resistance to sliding and therefore forms folds during drawing and therefore to the direction of the hollow of the horn.

où r(0), r(90) et r(45°) sont les valeurs des coefficients d'anisotropie normale r dans les directions longitudinale, transversale et oblique à 45° du flan de tôle.The overall anisotropy of a steel is determined by the average normal anisotropy coefficient r, also called the average Lankford coefficient: $$\text{r =}\frac{\text{r (0) + r (90) + 2r (45 °)}}{\text{4}}$$

where r (0), r (90) and r (45 °) are the values of the coefficients of normal anisotropy r in the longitudinal, transverse and oblique directions at 45 ° to the sheet blank.

dans laquelle r(0), r(90) et r(45°) sont les valeurs des coefficients d'anisotropie normale r dans les directions longitudinale, transversale et oblique à 45° du flan de tôle. Ce coefficient est exprimé en valeur absolue.The planar anisotropy coefficient Δr is defined as follows: $$\text{Δr =}\frac{\text{r (0) + r (90) - 2r (45 °)}}{\text{2}}$$

in which r (0), r (90) and r (45 °) are the values of the coefficients of normal anisotropy r in the longitudinal, transverse and oblique directions at 45 ° to the sheet blank. This coefficient is expressed in absolute value.

This planar anisotropy coefficient Δr is representative of the ability of the sheet blank to present horns and horn recesses when stamped.

Thus, to guarantee the best stamping conditions, that is to say a good flow of the metal in all directions and a low level of horns, there must be a mean Lankford coefficient r as high as possible and an anisotropy coefficient planar Δr close to 0.

In addition, in the field of packaging, it is sought to limit the thickness in order to limit the weight and the current stamping methods make it possible to produce very thin boxes from blanks of thickness of the order 0.16 mm, which imposes high mechanical characteristics, in particular the elastic limit Re and the tensile strength Rm, in order to ensure the good mechanical strength of such a box.

It is known to produce steel packaging, in particular by shrinking stamping, to use mild steels which are not degassed under vacuum.

Such steels have a carbon content of the order of 200 to 800 ppm (parts per million) and exhibit, after having undergone a roll-hardening operation, known as a skin pass operation, with a skin pass reduction rate of of the order of 25%, relatively high mechanical characteristics (Rm of the order of 620 MPa).

However, this type of steel still has, after a skin pass operation with a reduction rate of around 25%, a high planar anisotropy coefficient Δr (Δr = 0.6 for a mild steel whose carbon content is equal to 800 ppm and a hot thickness of the order of 2 mm).

It is also known to use ultra low carbon steels degassed under vacuum, with low nitrogen content.

This type of steel, due to the very low carbon content, makes it possible to reduce the planar anisotropy coefficient. For a low carbon vacuum degassed mild steel with a carbon content of around 50 ppm, the planar anisotropy coefficient Δr after the skin pass operation with a reduction rate of around 25% is in the range of 0.15 to 0.20.

However, this type of steel has degraded mechanical characteristics compared to that of mild steel which is not degassed under vacuum (Rm of the order of 530 MPa after skin pass operation with a reduction rate of the order of 25%).

To obtain the mechanical characteristics obtained with mild steel which has not been degassed under vacuum, it is then necessary to carry out the skin pass operation with a higher reduction rate, of the order of 35%.

However, re-rolling with the skin pass has the effect of degrading certain properties such as for example the elongation percent after rupture and the planar anisotropy coefficient Δr.

Indeed, the planar anisotropy coefficient Δr of an ultra low carbon steel degassed under vacuum with low nitrogen content (carbon content less than 40 ppm and nitrogen content less than 40 ppm) goes from 0.16 for a rate 25% reduction in skin pass to 0.28 for a skin pass reduction rate of around 35%.

The subject of the present invention is a process for the preparation of a steel for packaging allowing, at the same reduction rate as the skin pass, to obtain mechanical characteristics substantially equal to those of mild steel which is not degassed under vacuum and properties stamping substantially identical to that of an ultra-low carbon steel degassed under vacuum with low nitrogen content.

The present invention relates more particularly to a method of producing a steel for packaging suitable for deep drawing and little sensitive to the formation of horns during drawing, in which a low carbon steel is produced by vacuum degassing in a decarburization chamber, characterized in that after degassing under vacuum, nitrogen is blown into the steel bath to add to said steel 80 to 140 ppm of nitrogen.

• l'acier est dégazé sous vide pour atteindre une teneur en carbone inférieure ou égale à 80 ppm, de préférence inférieure à 40 ppm ;
• l'acier est un acier extra doux calmé à l'aluminium sans titane ni niobium ;
• la composition de l'acier dégazé sous vide, avant insufflation de l'azote est la suivante en ppm :

  Carbone

  ≦80

  Manganèse

  de 1000 à 3000

  Aluminium

  ≦400

  Azote

  ≦40

  Phosphore

  ≦150

  Soufre

  ≦150

  Silicium

  ≦200

  Molybdène

  ≦80

  Cuivre + Nickel + Chrome ≦ 800

le reste étant du fer et des résiduels.According to other characteristics of the invention:

• the steel is degassed under vacuum to reach a carbon content of 80 ppm or less, preferably less than 40 ppm;
• the steel is an extra mild steel calmed with aluminum without titanium or niobium;
• the composition of the steel degassed under vacuum, before blowing nitrogen is as follows in ppm:

  Carbon

  ≦ 80

  Manganese

  from 1000 to 3000

  Aluminum

  ≦ 400

  Nitrogen

  ≦ 40

  Phosphorus

  ≦ 150

  Sulfur

  ≦ 150

  Silicon

  ≦ 200

  Molybdenum

  ≦ 80

  Copper + Nickel + Chrome ≦ 800

the rest being iron and residuals.

The present invention also relates to a steel for packaging suitable for deep drawing and not very sensitive to the formation of horns during stamping produced by the above process, characterized in that after its preparation, it is hot rolled and / or cold rolled, it undergoes continuous or base annealing, and it undergoes hardening by light rolling.

The present invention also relates to the use of a steel as described above for the production of very thin products obtained by deep drawing of the stamped-re-stamped boxes or stamped-ironed boxes.

The characteristics and advantages will appear during the description which follows, given only by way of example, made with reference to the appended figure representing in a reference (Rm, Δr) the position of a non-degassed mild steel, of an ultra low carbon steel degassed under vacuum with low nitrogen content and the steel of the invention.

The present invention relates to a steel for packaging suitable for deep drawing and not very sensitive to the formation of horns during stamping, particularly useful for the production of very thin products obtained by deep drawing, in particular by shrinking stamping, of the box type. stamped-stamped or stamped-ironed boxes.

This steel is a low carbon steel produced in a known manner, using a steelworks converter, then degassed under vacuum in a decarburization chamber.

The important characteristic of this steel resides in the fact that, after degassing under vacuum, nitrogen is blown into the steel bath to add to said steel from 80 to 140 ppm (parts per million) of nitrogen.

The advantage of adding nitrogen to the steelworks, after degassing under vacuum, is to harden the steel by the effect of a solid solution.

The added nitrogen content must be greater than 80 ppm, since a lower content would not be sufficiently effective and the mechanical characteristics of the steel would not be increased significantly enough.

The added nitrogen content must also be less than 140 ppm, because if more than 140 ppm of nitrogen is added, the steel becomes too effervescent and very difficult or almost impossible to flow.

Carbone

≦80 de préférence ≦40

Manganèse

de 1000 à 3000

Aluminium

≦400

Azote

≦40

Phosphore

≦150

Soufre

≦150

Silicium

≦200

Molybdène

≦80

Cuivre + Nickel + Chrome ≦800
le reste étant du fer et des résiduels.Preferably, the steel undergoes vacuum degassing until reaching a carbon content less than or equal to 80 ppm, preferably at 40 ppm, and the steel is an extra mild steel calmed with aluminum, without titanium. ni niobium, for example a steel the composition of which before insufflation of nitrogen is as follows in ppm:

Carbon

≦ 80 preferably ≦ 40

Manganese

from 1000 to 3000

Aluminum

≦ 400

Nitrogen

≦ 40

Phosphorus

≦ 150

Sulfur

≦ 150

Silicon

≦ 200

Molybdenum

≦ 80

Copper + Nickel + Chrome ≦ 800
the rest being iron and residuals.

The steel after its production according to the process described above is then cast, hot rolled and / or cold rolled, it undergoes continuous annealing or annealing on base, and it undergoes hardening by light rolling in a finishing train. , also called skin pass operation.

The Applicant has found that the addition of nitrogen after degassing under vacuum does not degrade the drawing properties of the steel, and in particular does not affect the planar anisotropy coefficient Δr.

Several tests have been carried out making it possible to show the advantage of adding nitrogen after degassing under vacuum of the steel.

A first series of blanks was produced from mild steel of standard type with 800 ppm carbon, cold rolled, annealed and having undergone a skin pass operation with a reduction rate of 25%.

Carbone

50

Manganèse

2000

Aluminium

300

Azote

30

Phosphore

120

Soufre

130

Silicium

150

Molybdène

60

Cuivre + Nickel + Chrome 600
le reste étant du fer et des résiduels.A second series of blanks was produced from mild steel with very low carbon calmed with aluminum, without titanium or niobium, the composition of which is as follows in ppm:

Carbon

50

Manganese

2000

Aluminum

300

Nitrogen

30

Phosphorus

120

Sulfur

130

Silicon

150

Molybdenum

60

Copper + Nickel + Chrome 600
the rest being iron and residuals.

This steel was also cold rolled, annealed and subjected to a skin pass operation with a reduction rate of 25%, under conditions identical to the first series of blanks.

Finally, a third series of blanks was produced with the steel of the invention, that is to say by producing in the converter a steel identical to that of the second series of blanks and by blowing, after degassing under vacuum, 90 ppm d 'nitrogen.

This steel was then subjected to the same rolling, annealing and skin pass treatment as the other two.

The tensile strength Rm and the planar anisotropy coefficient Δr were then measured for each series of blanks.

As can be seen in the figure, the first series of standard mild steel blanks has good mechanical strength (Rm equal to 620 MPa), but poor drawing qualities (Δr = 0.6).

The second series of low carbon steel blanks degassed under vacuum with low nitrogen content has poorer mechanical strength (Rm = 530 MPa) but better stamping qualities (Δr = 0.15). This type of steel does not have sufficient mechanical strength for use in deep drawing with very small thickness.

Finally, the third series of steel blanks according to the invention has mechanical characteristics equivalent to those of the first series of blanks (Rm = 620 MPa) and stamping qualities even better than those of the second series of blanks (Δr = 0.13).

Thus, it can be seen that not only the addition of nitrogen after degassing of the steel does not degrade the drawing characteristics of the steel, but improves them slightly.

It is therefore possible, by adapting the reduction rate to the skin pass, to obtain a blank having higher characteristics (Rm of the order of 700 MPa) and with good stamping qualities (Δr <0.20) , which will facilitate the reduction of the thicknesses of products for deep drawing.

Claims (7)

1 - Process for the preparation of a steel for packaging suitable for deep drawing and not very sensitive to the formation of horns during stamping, in which a low carbon steel is produced by degassing under vacuum in a decarburization chamber , characterized in that after vacuum degassing, nitrogen is blown into the steel bath to add to said steel from 80 to 140 ppm of nitrogen. 2 - Process for developing a steel for packaging according to claim 1, characterized in that the steel is degassed under vacuum to reach a carbon content of less than 80 ppm, preferably less than 40 ppm. 3 - Process for developing a steel for packaging according to one of claims 1 or 2, characterized in that the steel is an extra mild steel calmed with aluminum without titanium or niobium. 4 - Process for the preparation of a steel for packaging according to one of the preceding claims, characterized in that the composition of the steel degassed under vacuum, before blowing nitrogen is as follows in ppm: Carbon ≦ 80, preferably ≦ 40 Manganese from 1000 to 3000 Aluminum ≦ 400 Nitrogen ≦ 40 Phosphorus ≦ 150 Sulfur ≦ 150 Silicon ≦ 200 Molybdenum ≦ 80 Copper + Nickel + Chrome ≦ 800
the rest being iron and residuals. 5 - Steel for packaging suitable for deep drawing and not very sensitive to the formation of horns during stamping, characterized in that it is produced by the process according to the preceding claims. 6 - Steel for packaging according to claim 5, characterized in that after its development, it is hot rolled and / or cold rolled, it undergoes continuous annealing or on base, and it undergoes hardening by light rolling. 7 - Use of a steel according to one of claims 5 or 6 for the production of very thin products obtained by deep drawing of the type stamped-re-stamped boxes or stamped-ironed boxes.

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