EP0699769B1 - 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 preparing a steel for packaging suitable for deep drawing and not very sensitive to formation of horns during stamping, and a steel obtained from this process.

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

• 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 boítes 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 properties, 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.

When manufacturing a metal package, a sheet blank, generally made of a metal which has undergone an operation of rolling, so that this metal has an anisotropy with regard to its resistance to thinning.

Therefore, for example when making containers of cylindrical section by stamping circular blanks, it is created during deep drawing, thinning and thickening depending on the directions on the body of the 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 flange, this irregularity located at the end of the peripheral skirt and this irregularity creates a number of obstacles in subsequent operations.

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

This formation of folds or ripples is due to a difference resistance to sliding of the metal according to 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 art of stamping, in particular of deep-drawn stamping, the Lankford coefficient r and the coefficient planar anisotropy Δr.

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

The direction in which the Lankford coefficient is most high 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 coefficient of Lankford is the lowest corresponds to the direction in which the metal has a high resistance to sliding and therefore forms folds during stamping and therefore to the direction of the hollow of the horn.

The overall anisotropy of a steel is determined by the average normal anisotropy coefficient r, also called the average Lankford coefficient: r = r (0) + r (90) + 2r (45 °) 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.

The planar anisotropy coefficient Δr is defined as follows: Δr = r (0) + r (90) - 2r (45 °) 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 metal blank to present horns and horn recesses to stamping.

So, to guarantee the best stamping conditions, that is to say a good flow of the metal in all directions and a low horn level, you need a medium Lankford coefficient r the most high possible and a planar anisotropy coefficient Δr close to 0.

In addition, in the field of packaging, we seek to limit thickness to limit weight and current stamping processes allow to make very thin boxes from blanks 0.16 mm thick, which imposes characteristics high mechanical strengths, in particular the yield strength Re and the resistance to traction Rm, to ensure good mechanical strength of such a box.

It is known to produce steel packaging, in particular by shrinking stamping, to use mild steel not degassed under empty.

Such steels have a carbon content of the order of 200 to 800 ppm (parts per million) and present, after having undergone an operation work hardening by rolling, called skin pass operation, with a rate of 25% reduction in skin pass, mechanical characteristics relatively high (Rm of the order of 620 MPa).

But this type of steel is present, always after skinning pass with a reduction rate of around 25%, a coefficient planar anisotropy Δr high (Δr = 0.6 for mild steel with a content of carbon 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 vacuum degassed, low nitrogen content.

This type of steel, due to the very low carbon content, reduces the planar anisotropy coefficient. For mild steel low carbon degassed under vacuum 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 around 0.15 to 0.20.

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

To obtain the mechanical characteristics obtained with mild steel not 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, the re-rolling on 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 steel low carbon 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 skin pass reduction rate of around 25% to 0.28 for a rate 35% reduction in skin pass.

The subject of the present invention is a method for preparing a steel for packaging allowing the same skin pass reduction rate, obtain mechanical characteristics substantially equal to those of a mild steel not degassed under vacuum and deep drawing properties 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 not very 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 the composition of the steel degassed under vacuum, before insufflation of the 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 remainder being iron and residuals, and in that, after degassing under vacuum, nitrogen is blown into the steel bath to add to said steel from 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.

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;
• steel is an extra mild steel calmed with aluminum without titanium or niobium.

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 developed by the above process which, after its preparation, is hot rolled and / or cold rolled, undergoes continuous or base annealing, and undergoes rolling hardening light, so as to have a planar anisotropy coefficient Δr <0.20.

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

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

The present invention relates to a steel for packaging suitable for deep drawing and little sensitive to the formation of horns during deep drawing, particularly useful for the production of very thin obtained by deep drawing, in particular by deep drawing in shrinkage, of the stamped-re-stamped boxes or stamped-ironed boxes.

This steel is a low carbon steel produced from known manner, to the steelworks converter, then degassed under vacuum in a decarburization enclosure.

The important characteristic of this steel is that that, after vacuum degassing, nitrogen is blown into the steel bath to add 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 effect of solid solution.

The added nitrogen content must be greater than 80 ppm, because a lower content would not be effective enough and we would not significantly increase the characteristics steel mechanics.

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

Preferably, the steel undergoes vacuum degassing until reaching a carbon content of 80 ppm or less, preferably at 40 ppm, and the steel is an extra mild steel, calmed with aluminum, free of titanium and niobium.

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

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

Several tests have been carried out to show the interest adding nitrogen after degassing the steel under vacuum.

A first series of blanks was made of mild steel type standard at 800 ppm carbon, cold rolled, annealed and subjected to skin pass operation with a 25% reduction rate.

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 skin pass operation with a 25% reduction rate, in conditions identical to the first series of blanks.

Finally a third series of blanks was made with steel 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 nitrogen.

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

We then measured for each series of blanks the resistance to traction Rm as well as the planar anisotropy coefficient Δr.

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

The second series of low carbon steel blanks degassed under low nitrogen vacuum has poorer strength mechanical (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).

So we can see that not only the addition of nitrogen after degassing of steel does not degrade the drawing characteristics of 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 700 MPa) and with good drawing qualities (Δr <0.20), which will facilitate the reduction of the thicknesses of the products for deep drawing.

Claims (4)

1. Process for the manufacture of a steel for packaging capable of being deep-drawn and not very susceptible to earing during the drawing operation, in which process a low-carbon steel is manufactured by vacuum degassing in a decarburizing chamber, characterized in that the composition of the vacuum-degassed steel, before nitrogen is injected, is as follows, in ppm: Carbon ≤ 80, preferably ≤ 40 Manganese from 1000 to 3000 Aluminium ≤ 400 Nitrogen ≤ 40 Phosphorus ≤ 150 Sulphur ≤ 150 Silicon ≤ 200 Molybdenum ≤ 80 Copper + Nickel + Chrome ≤ 800 the balance being iron and residuals,
   and in that, after vacuum degassing, nitrogen is injected into the bath of steel so as to add from 80 to 140 ppm of nitrogen to the said steel.
2. Process for the manufacture of a steel for packaging according to Claim 1, characterized in that the steel is an aluminium-killed ultra-mild steel containing no titanium and no niobium.
3. Steel for packaging capable of being deep-drawn and not very susceptible to earing during the drawing operation, manufactured by the process according to the preceding claims and which, after it has been manufactured, is hot rolled and/or cold rolled, undergoes continuous or box annealing, and undergoes work hardening by light rolling so that it exhibits a planar anisotropy coefficient Δr < 0.20.
4. Use of a steel according to Claim 3 for making very thin products obtained by deep-drawing of the drawn-and-redrawn can or drawn-and-ironed can type.

Images