EP0754770A1 - Method of producing a thin steel strip having improved deep-drawing properties - Google Patents

Abstract

A ductile steel band is made from an alloy containing the following proportions of elements indicated in thousandths of percent by weight: 0-20, pref. 2-20 of C, 0-500, pref. 100-500 Si, 0-1000 Mn, 0-100, pref. 50-100 P, 0-50 S, 0-100 Al, 0-10, pref. 2-8 N and one or more of 0-150 Ti, 0-150, pref. 10-50 Nb and 0-5, pref. 0-2 B, the remainder being Fe. The alloy is first hot-rolled, then cold-rolled to reduce its thickness by over 20%, annealed at up to 920 degrees C, cold rolled again, also reducing the thickness by over 20% while impressing a degree of rugosity on the surface, and finally annealed at above recrystallisation temp. A thin steel band made the process is also claimed.

Description

The present invention relates to a method of manufacturing a strip of thin sheet metal with improved drawability having good capacity for expanding deformation and for shrinking deformation, as well as a thin sheet metal with improved stampability obtained by this method.

Steels for stamping, either shrinking in the context of steels for packaging, or expanding in the context of steels for example for cars, must have a certain number of particular characteristics to allow their shaping.

Their elastic limit Re must be as low as possible for a given level of breaking strength Rm, in order to facilitate deformation.

The work hardening coefficient n must be as high as possible. Indeed, this work hardening coefficient defines the ability of the material to deform in expansion and, the higher this coefficient, the better the material deforms in expansion.

doivent également être élevés. En effet, les écrouissages à froid importants que subit la bande de tôle lors du laminage se traduisent, après recuit, par la formation de textures cristallographiques conduisant à une anisotropie des propriétés mécaniques. Son influence sur la limite d'élasticité Re, la résistance à la rupture Rm et l'allongement pour cent à la rupture A% est relativement faible, mais il n'en est pas de même pour l'amincissement de la tôle lors des contraintes subies par la tôle au cours de la mise en forme.The anisotropy coefficient r _ϕ in the direction where it is lowest and the average anisotropy coefficient $\overline{\text{r}}$

must also be raised. Indeed, the significant cold work hardening which the sheet metal strip undergoes during rolling results, after annealing, in the formation of crystallographic textures leading to an anisotropy of the mechanical properties. Its influence on the elastic limit Re, the tensile strength Rm and the elongation percent at break A% is relatively weak, but it is not the same for the thinning of the sheet during stresses suffered by the sheet during shaping.

To measure this anisotropy, we use the ratio r _ϕ between the rational deformation in width of a test piece, during a tensile test, and its rational deformation in thickness, where _ϕ represents the angle between the direction of traction of l 'specimen and the direction of rolling of the sheet.

, encore appelé coefficient de Lankford $$\overline{\text{r}}\text{=}\frac{{\text{r}}_{\text{0}}{\text{+ 2r}}_{\text{45}}{\text{+ r}}_{\text{90}}}{\text{4}}\text{\hspace{1em} (ϕ= 0°, 45°, 90°)}$$

où r₀, r₉₀ et r₄₅ sont les valeurs des coefficients d'anisotropie r dans les directions longitudinale, transversale et oblique à 45° par rapport à la direction de laminage du flan de tôle.The average anisotropy of a steel is determined by the average anisotropy coefficient $\overline{\text{r}}$

, also called Lankford coefficient $$\overline{\text{r}}\text{=}\frac{{\text{r}}_{\text{0}}{\text{+ 2r}}_{\text{45}}{\text{+ r}}_{\text{90}}}{\text{4}}\text{(ϕ = 0 °, 45 °, 90 °)}$$

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

représente la capacité d'amincissement de la tôle et pour garantir les meilleures conditions d'emboutissage, c'est à dire un bon écoulement du métal dans toutes les directions, il faut un coefficient de Lankford $\overline{\text{r}}$

le plus élevé possible.This coefficient $\overline{\text{r}}$

represents the thinning capacity of the sheet and to guarantee the best drawing conditions, that is to say a good flow of the metal in all directions, a Lankford coefficient is required $\overline{\text{r}}$

as high as possible.

It is also important that the anisotropy coefficient r _ϕ in the direction where it is the lowest r _mini is as high as possible, which ensures that in all directions of stress, the metal does not go too thin.

traduisent une bonne aptitude de la tôle à la déformation en rétreint.The high character of these two coefficients r _mini and $\overline{\text{r}}$

reflect a good ability of the sheet to deformation in necking.

It is also important that the elongation percent at break A% be high, which translates into significant ductility of the material.

Finally, the Young's modulus must also be as high as possible, and a steel is all the more stampable as it develops these properties.

It is known for this type of application to use standard steels, for example a steel having the following composition in thousandths of a percent by weight: carbon <5 for example = 3 silicon <200 for example = 9 manganese <500 for example = 140 phosphorus <30 for example = 8 sulfur <50 for example = 5 aluminum <100 for example = 35 nitrogen <10 for example = 3 titanium <150 for example = 56 the rest being iron and residuals from the production process, having undergone hot rolling, then cold rolling with a reduction rate of between 75 and 85%, for example equal to 80%, followed by a continuous annealing at 800 ° C and a work hardening operation by light rolling with a reduction rate of 1%.

This type of steel has good drawing characteristics.

.It is also known to produce steels in a very specific way to further increase the drawing properties, in particular the Lankford coefficient $\overline{\text{r}}$

.

est égal à 3, obtenu en élaborant un acier contenant, en millièmes de pour cent : carbone 2 titane 41 manganèse 120 phosphore 11 soufre 11 aluminium 45 azote 2 le reste étant du fer, et des résiduels.For example, there is a steel whose Lankford coefficient $\overline{\text{r}}$

is equal to 3, obtained by developing a steel containing, in thousandths of a percent: carbon 2 titanium 41 manganese 120 phosphorus 11 sulfur 11 aluminum 45 nitrogen 2 the rest being iron, and residuals.

The steel thus produced is hot rolled, cold rolled with a reduction rate of 50%, then annealed at 750 ° C for 20 seconds, cold rolled with a reduction rate of 77% and annealed a second time at 870 ° C for 20 seconds.

élevé mais ne présente pas forcément les autres propriétés nécessaires à un bon emboutissage, et il est extrêmement coûteux à réaliser essenciellement du fait de la faible teneur en carbone necessaire pour obtenir ce coefficient de Lankford, ainsi que des températures élevées des recuitsThis type of steel sheet has a Lankford coefficient $\overline{\text{r}}$

high but does not necessarily have the other properties necessary for good stamping, and it is extremely expensive to carry out essentially because of the low carbon content necessary to obtain this Lankford coefficient, as well as the high temperatures of the annealing

The object of the present invention is to propose a method for manufacturing a strip of thin sheet metal with improved stampability having a good ability both in expanding deformation and in shrinking deformation, as well as improved elongation at break. , and which is economical to produce.

• élaborer un acier comprenant en millième de pour cent poids une teneur en carbone inférieure à 20, une teneur en silicium inférieure à 500, une teneur en manganèse inférieure à 1000, une teneur en phosphore inférieure à 100, une teneur en soufre inférieure à 50, une teneur en aluminium inférieure à 100, une teneur en azote inférieure à 10 et aucun, un ou plusieurs éléments parmi le titane, avec une teneur inférieure à 150, le niobium, avec une teneur inférieure à 150, le bore avec une teneur inférieure à 5, le reste étant du fer et des résiduels,
• réaliser un laminage à chaud.
• réaliser un laminage à froid avec un taux de réduction supérieur à 20%,
• réaliser un recuit à une température comprise entre la température de restauration de l'acier et 920°C,
• réaliser un second laminage à froid avec un taux de réduction supérieur à 20% en imprimant à la bande de tôle une rugosité déterminée,
• effectuer un second recuit à une température supérieure à la température de recristallisation de l'acier.

The present invention relates more particularly to a process for manufacturing a strip of thin sheet metal with improved stampability having good aptitude for expanding deformation and for shrinking deformation, characterized in that it consists in:

• develop a steel comprising in thousandths of a weight percent a carbon content of less than 20, a silicon content of less than 500, a manganese content of less than 1000, a phosphorus content of less than 100, a sulfur content of less than 50, an aluminum content of less than 100, a nitrogen content of less than 10 and none, one or more elements from titanium, with a content of less than 150, niobium, with a content of less than 150, boron with a content of less than 5, the remainder being iron and residuals,
• perform hot rolling.
• cold rolling with a reduction rate of more than 20%,
• anneal at a temperature between the steel restoration temperature and 920 ° C,
• perform a second cold rolling with a reduction rate of more than 20% by imparting a defined roughness to the sheet metal strip,
• perform a second annealing at a temperature higher than the recrystallization temperature of the steel.

• l'acier élaboré dans la première étape comprend en millième de pour cent poids une teneur en carbone comprise entre 2 et 20, une teneur en silicium comprise entre 100 et 500, une teneur en manganèse comprise entre 0 et 1000, en teneur en phosphore comprise entre 50 et 100, une teneur en soufre comprise entre 0 et 50, une teneur en aluminium comprise entre 0 et 100, une teneur en azote comprise entre 2 et 8, et un ou plusieurs éléments parmi le titane, avec une teneur comprise entre 0 et 150, le niobium, avec une teneur comprise entre 10 et 50, le bore, avec une teneur comprise entre 0 et 2, le reste étant du fer et des résiduels.
• le premier recuit est effectué à une température comprise entre la température de restauration de l'acier et sa température de recristallisation.
• le taux de réduction du premier laminage à froid est important et le taux de réduction du second laminage à froid est faible.
• le taux de réduction du premier laminage à froid est faible et le taux de réduction de second laminage à froid est important.
• le taux de réduction du premier laminage à froid est compris entre 35 et 50 % et le taux de réduction du second laminage à froid est compris entre 65 et 75 %.
• le taux de réduction du premier laminage à froid est compris entre 65 et 75 % et le taux de réduction du second laminage à froid est compris entre 35 et 50 %.
• Pendant le second laminage à froid, on imprime à la bande de tôle une rugosité moyenne Ra comprise entre 0,9 et 1,7 microns.

According to other characteristics of the invention:

• the steel produced in the first stage comprises, in thousandths of a weight percent, a carbon content of between 2 and 20, a silicon content of between 100 and 500, a manganese content of between 0 and 1000, of phosphorus content between 50 and 100, a sulfur content between 0 and 50, an aluminum content between 0 and 100, a nitrogen content between 2 and 8, and one or more elements among titanium, with a content between 0 and 150, niobium, with a content of between 10 and 50, boron, with a content of between 0 and 2, the remainder being iron and residuals.
• the first annealing is carried out at a temperature between the temperature for restoring the steel and its recrystallization temperature.
• the reduction rate of the first cold rolling is high and the reduction rate of the second cold rolling is low.
• the reduction rate of the first cold rolling is low and the reduction rate of the second cold rolling is high.
• the reduction rate of the first cold rolling is between 35 and 50% and the reduction rate of the second cold rolling is between 65 and 75%.
• the reduction rate of the first cold rolling is between 65 and 75% and the reduction rate of the second cold rolling is between 35 and 50%.
• During the second cold rolling, the sheet strip is printed with an average roughness Ra of between 0.9 and 1.7 microns.

The present invention also relates to a strip of thin sheet metal having a good drawing ability obtained by the process according to the above characteristics.

The characteristics and advantages will appear during the description which follows, given only by way of example.

The invention relates to a strip of thin sheet steel of improved stampability, that is to say having a low yield strength, a high Lankford coefficient greater than 2, preferably 2.4 in all directions of the sheet, significant consolidation, significant ductility and a high Young's modulus greater than 23,000 kg / mm ² .

To obtain such a strip of thin sheet metal, a steel is first produced, in a known manner, for example using a steelworks converter, the composition of which in thousandths of a percent is as follows: carbon <20 silicon <500 manganese <1000 phosphorus <100 sulfur <50 aluminum <100 nitrogen <10 and none, one or more of the following: titanium <150 niobium <150 boron <5 the rest being iron and residuals from the production process.

Preferably, the steel has the following composition, in thousandths of a percent: carbon from 2 to 20 silicon from 100 to 500 manganese from 0 to 1000 phosphorus from 50 to 100 sulfur from 0 to 50 aluminum from 0 to 100 nitrogen from 2 to 8 and none, one or more of the following: titanium from 0 to 150 niobium from 10 to 50 boron from 0 to 2 the rest being iron and residuals from the production process.

Given the carbon content of this steel, which can range up to 20, it is possible to develop it by argon blowing, which is less costly than the vacuum degassing technique.

The steel thus produced is then poured into slabs and then hot rolled.

The hot rolled strip is then cold rolled with a reduction rate greater than 20%.

The strip thus cold-rolled is then subjected to annealing at a temperature between the temperature for restoring the steel and 920 ° C., preferably between the temperature for restoring the steel and its recrystallization temperature.

Annealing can either be basic annealing or continuous annealing.

A second cold rolling is then carried out with a reduction rate greater than 20% by imparting a determined roughness to the sheet metal strip. Finally, a second annealing is carried out at a temperature higher than the recrystallization temperature of the steel.

de la tôle afin que celui-ci soit supérieur à 2, de préférence 2,4 lorsque la température du premier recuit s'effectue à une température comprise entre la température de restauration de l'acier et sa température de recristallisation, de préférence à une température égale à la température de restauration de l'acier plus 40 à 60°C.The Applicant has found that it is possible to increase the value of the Lankford coefficient $\overline{\text{r}}$

of the sheet so that it is greater than 2, preferably 2.4 when the temperature of the first annealing takes place at a temperature between the temperature for restoring the steel and its recrystallization temperature, preferably at a temperature equal to the steel restoration temperature plus 40 to 60 ° C.

To obtain a Young's modulus greater than 23,000 kg / mm ² and a Lankford coefficient greater than 2, preferably 2.4, it is necessary that either the reduction is significant during the second cold rolling while the reduction was low during the first cold rolling, either the reduction or small during the first cold rolling while the reduction was significant during the first cold rolling.

Two cases can therefore be envisaged.

The first scenario, in which the reduction rate of the first cold rolling is low, while being greater than 20%, preferably of the order of 35 to 50%, and the reduction rate of the second rolling is high , preferably of the order of 65 to 75%.

The second scenario, in which the reduction rate of the first cold rolling is high, preferably of the order of 65 to 75% and the reduction rate of the second cold rolling is low, while being greater than 20 %, preferably of the order of 35 to 50%.

To obtain the required stamping qualities, it is also advantageous during the second cold rolling of the sheet, to print on it by means of the rolling rolls a required roughness.

For example, the printed roughness is such that the average roughness Ra is between 0.9 and 1.7 microns.

Preferably, the average roughness Ra is between 1.2 and 1.7 to promote stamping. But if one wishes to obtain a more shiny appearance to the sheet, while retaining good stampability, the sheet is printed with an average roughness Ra of between 0.9 and 1.4 microns.

. De plus, le fait de s'affranchir de l'opération de skin-pass permet de supprimer une étape dans la fabrication de la bande de tôle et par conséquent en diminue le coût.This operation makes it possible, by eliminating the skin-pass operation, to guarantee a high Lankford coefficient greater than 2, preferably 2.4 and a low elastic limit because the skin-pass operation has the consequence of increasing the yield strength of the metal, and degrading the work hardening coefficient $\overline{\text{not}}$

. In addition, the fact of being free from the skin-pass operation makes it possible to omit a step in the manufacture of the sheet metal strip and consequently reduces the cost thereof.

Several tests have been carried out with several types of steel, the compositions of which are as follows in thousandths of a percent, the balance being iron: TYPE OF STEEL VS Yes Mn P S Al NOT Ti Nb B AT 3.9 4 142 13 5 40 3.2 65 <1 / B 8 6 187 4 9 30 3.6 109 / / VS 3 9 140 8 5 35 3 56 / /

As can be seen, steels A and B are steels according to the invention and steel C corresponds to a composition according to the state of the art.

The restoration temperature of steel A, which was produced with a first base annealing, is equal to 450 ° C. and its recrystallization temperature is equal to 680 ° C.

The restoration temperature of steel B, which was produced with a first continuous annealing, is equal to 450 ° C. and its recrystallization temperature is equal to 630 ° C.

Each of the three types of steel was then subjected to different rolling and annealing treatments and several sheet metal strips less than 1 mm thick were thus produced.

In each sheet, a series of test specimens was taken and various tests were carried out to determine the characteristic parameters of this steel.

As can be seen in this table, the steel of the invention makes it possible to obtain, compared with steel C of the prior art, a significant gain in terms of ductility and in terms of drawability.

The advantage of printing a controlled roughness during the second cold rolling makes it possible to obtain an elastic limit Re appreciably lower than if one abstained from printing this roughness.

Claims (9)

Method for manufacturing a strip of thin sheet metal with improved stampability having good suitability for expanding deformation and for shrinking deformation, as well as improved elongation at break, characterized in that it consists in: - develop a steel comprising in thousandths of a weight percent a carbon content less than 20, a silicon content less than 500, a manganese content less than 1000, a phosphorus content less than 100, a sulfur content less than 50 , an aluminum content less than 100, a nitrogen content less than 10 and none, one or more element among titanium, with a content less than 150, niobium, with a content less than 150, boron, with a content less than 5, the rest being iron and residuals, - carry out hot rolling, - cold rolling with a reduction rate of more than 20%, - annealing at a temperature between the steel restoration temperature and 920 ° C, - carry out a second cold rolling with a reduction rate greater than 20% by imparting a defined roughness to the sheet metal strip, - perform a second annealing at a temperature higher than the recrystallization temperature of the steel. A method of manufacturing a strip of improved sheet metal stamping according to claim 1, characterized in that the steel produced in the first step comprises in thousandths of a weight percent a carbon content of between 2 and 20, a content of silicon between 100 and 500, a manganese content between 0 and 1000, a phosphorus content between 50 and 100, a sulfur content between 0 and 50, an aluminum content between 0 and 100, a content in nitrogen between 2 and 8, and one or more elements among titanium, with a content between 0 and 150, niobium, with a content between 10 and 50, boron, with a content between 0 and 2, the rest being iron and residuals. Method for manufacturing a strip of thin sheet metal with improved stampability according to one of claims 1 and 2, characterized in that the first annealing is carried out at a temperature between the temperature for restoring the steel and its recrystallization temperature . Method of manufacturing a strip of thin sheet metal with improved stampability according to one of claims 1 and 2, characterized in that the reduction rate of the first cold rolling is high and the reduction rate of the second cold rolling is low . Method for manufacturing a strip of thin sheet metal with improved stampability according to one of claims 1 and 2, characterized in that the reduction rate of the first cold rolling is low and the reduction rate of the second cold rolling is high . A method of manufacturing a strip of improved sheet metal stamping according to claim. 4, characterized in that the reduction rate of the first cold rolling is between 35 and 50% and the reduction rate of the second cold rolling is between 65 and 75%. A method of manufacturing a strip of improved sheet metal stamping according to claim 5, characterized in that the reduction rate of the first cold rolling is between 65 and 75% and the reduction rate of the second cold rolling is included between 35 and 45%. Method for manufacturing a strip of thin sheet metal with improved stampability according to claim 1, characterized in that during the second cold rolling, the sheet strip is printed with an average roughness Ra of between 0.9 and 1.7 microns . Strip of thin sheet metal with improved drawability having good suitability for expanding deformation and where the deformation shrinks, characterized in that it is obtained by the method according to claims 1 to 8.