EP0659891A2 - Method for manufacturing a thin cold rolled mild steel strip for deep drawing - Google Patents

Abstract

Process for manufacturing a cold-rolled thin mild steel strip for deep drawing, in which a mild steel is rolled hot in a temperature region where the said steel exhibits a ferritic structure with a rolling end temperature lower than 780 DEG C in order to form a hot-rolled strip. This hot-rolled strip is then rolled cold with a thickness reduction ratio higher than 75 % and preferably between 78 % and 85 %. The cold-rolled strip may next be subjected to a final annealing, either as a reel or continuously, and to a surface cold-working on a skin-pass rolling mill.

Description

The present invention relates to a method for manufacturing a thin strip of cold-rolled mild steel for stamping.

To manufacture a thin strip intended for stamping, a mild steel slab is used, continuously cast, the thickness of which is generally greater than 150 mm, for example between 150 mm and 300 mm. This slab is rolled at high temperature in a roughing rolling mill, in order to produce a blank whose thickness is generally between 20 mm and 40 mm. This blank is then rolled in a hot finishing train to the desired thickness, most often between 1.5 mm and 4 mm.

The mild steels currently used are known under the names ELC, for "Extra Low Carbon" and ULC, for "Ultra Low Carbon", whose carbon contents are less than 0.1% and 0.01% respectively.

According to the usual practice, the rolling of the blanks to the desired thickness is carried out entirely in the austenitic field, with temperatures of end of hot rolling greater than 850 ° C.

It has moreover already been proposed, in particular in patent BE-A-08801010, to practice finishing hot rolling in a temperature range where the steel has a ferritic structure. The end of rolling temperature is then less than 780 ° C, for example between 750 ° C and 300 ° C; the hot rolled strip is then wound at a temperature below 700 ° C, for example between 700 ° C and 200 ° C.

Such hot rolling in ferrite makes it possible to produce softer hot strips and more easily cold rollable.

Whatever the route chosen to operate the hot rolling, the hot strips are cooled and then cold rolled to the desired final thickness. They are then subjected to recrystallization annealing, either in coils in an oven or in a continuous annealing line comprising an overaging zone.

, augmente d'abord avec le taux de réduction du laminage à froid, jusqu'à une valeur maximale le plus souvent supérieure à 1,5 obtenue pour un taux de réduction à froid compris entre 70 % et 75 %. L'aptitude à l'emboutissage de ces bandes à froid diminue ensuite rapidement lorsque le taux de réduction du laminage à froid dépasse 75 %.It is well known that the ability to stamp thin cold strips, obtained from hot rolled strips in the austenitic field, depends on the reduction rate applied during cold rolling. This ability to stamp, expressed by the average plastic anisotropy coefficient $\overline{\text{r}}$

, increases first with the reduction rate of cold rolling, up to a maximum value more often than 1.5 obtained for a reduction rate of cold between 70% and 75%. The cold stamping ability of these strips then decreases rapidly when the reduction rate of cold rolling exceeds 75%.

de ces bandes vaut à peine 1,3 pour des taux de réduction compris entre 70 % et 75 %.Cold strips, obtained from hot strips rolled in the ferritic field, for their part have a significantly lower drawing ability than cold strips of austenitic origin, under the same cold reduction conditions. In particular, the plastic anisotropy coefficient $\overline{\text{r}}$

of these bands is hardly worth 1.3 for reduction rates between 70% and 75%.

The subject of the present invention is a process for manufacturing a thin strip of cold-rolled steel, for stamping, which makes it possible to significantly improve the ability to stamp this strip while fully taking advantage of the advantages associated with a hot finish rolling of the strip in the ferritic field, with an end of rolling temperature below 780 ° C.

According to the present invention, a method of manufacturing a thin strip of cold-rolled mild steel for stamping, in which a mild steel is hot rolled in a temperature range where said steel has a ferritic structure with a temperature end of rolling less than 780 ° C to form a hot strip, is characterized in that said hot strip is then cold rolled with a reduction in thickness giving the cold strip a degree of work hardening final corresponding to a cold reduction rate greater than 75%.

According to a particular implementation, said cold rolling is carried out with a thickness reduction rate greater than 75%, and preferably between 78% and 85%.

The thickness of the hot strip can advantageously be determined as a function of the desired final thickness of the cold strip and of the reduction rate applied to cold rolling.

Because a hot strip laminated in ferrite is more easily cold rollable than a hot strip laminated in austenite, the thickness of the hot strip of ferritic origin may be included in a range extended by example between 1 mm and 8 mm, without this resulting in difficulties in cold rolling.

According to another advantageous implementation of the process of the invention, said hot rolling is carried out in the ferrite by means of lubricated cylinders, with an end of rolling temperature leaving at least a degree of work hardening in the hot strip equivalent to that which corresponds to an intermediate rate of cold reduction of 10% and cold rolling is then carried out with a reduction rate equal to the difference between the desired final reduction rate and said intermediate reduction rate.

This latter implementation makes it possible to apply a total work hardening to the strip of mild steel ensuring a high drawing ability, that is to say a degree of work hardening corresponding to a cold rolling only with a thickness reduction rate of at least 75%, while performing said work hardening partially during hot rolling in the ferrite and partially during cold rolling with a thickness reduction adapted accordingly.

This accumulation of hot and cold work hardening can be carried out on any type of ULC (IF and non-IF) and ELC steel. For this purpose, hot rolling in the ferrite must be carried out with lubricated cylinders, so that the surfaces of the strip are not deformed by shearing; moreover, the temperature at the end of hot rolling in the ferrite must be lowered to an extent such as to avoid complete recrystallization steel in order to leave a determined degree of work hardening in the hot strip. This end of rolling temperature depends in particular on the type of steel considered; in addition, it will advantageously be chosen as a function of the degree of work hardening to be maintained in the hot strip.

The combination of hot rolling in ferrite and cold rolling with an increased or adapted reduction rate, according to the invention, makes it possible to substantially improve various properties of the cold strips.

For the implementation of the method of the invention, a slab of steel is continuously cast.

After pouring the slab, we can apply the so-called hot oven techniques.

In accordance with what is preferably practiced in the rolling processes comprising a phase in the ferritic domain and contrary to the prior practice, the reheating of the slab can be limited to a temperature such that the nitride AlN is not dissolved. For the type of steel envisaged here, the reheating temperature is advantageously between (A _c3 + 50 ° C) and 1150 ° C. Furthermore, this temperature is sufficient to avoid reheating in the intercritical phase, that is to say austenitic + ferritic, or in the ferritic phase, which would lead to the formation of very coarse cementite on the hot-rolled strip.

Finally, such a moderate reheating temperature, as well as a precipitation of the AlN nitrides induced during hot rolling, allows in particular that the cold strips of ELC and ULC-non-IF steels obtained by the process of the invention have excellent resistance to natural aging due to the almost total absence of soluble nitrogen in these steels.

de l'ordre de 1,5, qui permet de ranger ces bandes dans la catégorie dite DQ, c'est-à-dire Drawing Quality ou Qualité d'Emboutissage.In addition, this absence of nitrogen not precipitated on the hot strip in the ferrite causes the microstructures with elongated grains in the rolling direction, called "pancake" microstructures conventionally obtained after hot rolling in austenite, to be not developed after cold rolling and coil annealing of hot rolled strips in ferrite. Cold rolled and annealed strips, both in reel and continuous, always have microstructures with equiaxed grains. However, cold rolling with an increased reduction rate has the effect of giving these bands a plastic anisotropy coefficient $\overline{\text{r}}$

of the order of 1.5, which allows these bands to be classified in the so-called DQ category, that is to say Drawing Quality or Stamping Quality.

The process of the invention and the advantages which ensue therefrom will now be illustrated by examples of thin cold-rolled strips of ELC and ULC steels.

The first part of the strips were hot rolled in the austenitic field, cold rolled with increasing reduction rates and then continuously annealed under usual conditions. These bands serve as a reference.

A second part of the strips were hot rolled in the ferritic field, with a fully recrystallized structure, i.e. without residual work hardening, then cold rolled with reduction rates lower than 75% corresponding to the optimal rates of the austenitic bands, and finally annealed either in coil, or continuously.

A third part of the bands were hot rolled in the ferritic field, also with a fully recrystallized structure, then cold rolled with reduction rates greater than 75% according to the present invention, and finally annealed either in coil or continuously.

A fourth part of the strips were hot rolled in ferrite, with a partially recrystallized structure so as to maintain in these hot strips a degree of work hardening - estimated from the hardness of the strips - corresponding to a reduction rate d 'cold thickness greater than 10%, then cold-rolled so that the degree of total work hardening corresponds to a reduction rate in cold thickness greater than 75%.

Δr ELC ferrite 0 61 continu 1,08 0,25 0 72 continu 1,24 0,27 0 76 continu 1,41 0,22 0 80 continu 1,53 0,16 0 84 continu 1,55 0,08 0 89 continu 1,52 -0,12 ELC ferrite 0 65 bobine 1,15 0,32 0 73 bobine 1,34 0,27 0 77 bobine 1,52 0,22 0 82 bobine 1,58 0,09 0 88 bobine 1,55 -0,07 ELC austénite 0 62 continu 1,28 0,21 0 70 continu 1,48 0,29 0 74 continu 1,53 0,22 0 79 continu 1,37 0,12 0 83 continu 1,12 0,05 0 86 continu 1,05 -0,15 ULC ferrite 0 60 continu 1,12 0,25 0 71 continu 1,27 0,34 0 76 continu 1,52 0,22 0 83 continu 1,62 0,08 0 87 continu 1,67 -0,09 ULC austénite 0 64 continu 1,32 0,28 0 71 continu 1,48 0,27 0 75 continu 1,49 0,19 0 79 continu 1,28 0,05 0 82 continu 1,05 -0,08 ULC ferrite 10 50 continu 1,27 0,28 10 60 continu 1,32 0,38 20 60 continu 1,77 0,24 30 50 continu 1,84 0,32 35 40 continu 1,82 -0,12 45 35 continu 1,85 -0,22 Table 1 illustrates the improvements observed on the cold strips treated by the method of the invention by comparison with the strips obtained by conventional methods. TABLE 1. steel hot rolling hot reduction (%) * cold reduction (%) annealed $\overline{\text{r}}$

Δr EL C ferrite 0 61 continued 1.08 0.25 0 72 continued 1.24 0.27 0 76 continued 1.41 0.22 0 80 continued 1.53 0.16 0 84 continued 1.55 0.08 0 89 continued 1.52 -0.12 EL C ferrite 0 65 coil 1.15 0.32 0 73 coil 1.34 0.27 0 77 coil 1.52 0.22 0 82 coil 1.58 0.09 0 88 coil 1.55 -0.07 EL C austenite 0 62 continued 1.28 0.21 0 70 continued 1.48 0.29 0 74 continued 1.53 0.22 0 79 continued 1.37 0.12 0 83 continued 1.12 0.05 0 86 continued 1.05 -0.15 ULC ferrite 0 60 continued 1.12 0.25 0 71 continued 1.27 0.34 0 76 continued 1.52 0.22 0 83 continued 1.62 0.08 0 87 continued 1.67 -0.09 ULC austenite 0 64 continued 1.32 0.28 0 71 continued 1.48 0.27 0 75 continued 1.49 0.19 0 79 continued 1.28 0.05 0 82 continued 1.05 -0.08 ULC ferrite 10 50 continued 1.27 0.28 10 60 continued 1.32 0.38 20 60 continued 1.77 0.24 30 50 continued 1.84 0.32 35 40 continued 1.82 -0.12 45 35 continued 1.85 -0.22

In this table 1, the designation “hot reduction *” is applied to the intermediate cold reduction rate, assimilated to a hot reduction rate because it is carried out during hot rolling in ferrite, and estimated from the hardness of the hot strip.

It is found that the cold strips produced from hot strips rolled in ferrite, with a fully recrystallized structure, develop their maximum properties for cold reduction rates greater than 75%. It is also found that the cold strips produced from hot strips rolled in ferrite, but having a structure which is still partially hardened, develop their maximum properties when the rate of reduction in cumulative thickness is greater than 75%, part of the required work hardening being carried out by hot rolling and the rest being carried out by cold rolling with lower thickness reduction rates, less than 75%.

In contrast, the cold strips obtained from hot strips laminated in austenite have a maximum drawing ability for a cold reduction rate of between 70% and 75%.

et l'anisotropie planaire Δr, qui expriment de façon connue l'aptitude à l'emboutissage.As we can see in this table, the properties considered here are the plastic anisotropy coefficient $\overline{\text{r}}$

and the planar anisotropy Δr, which express in a known manner the ability to stamp.

The results appearing in this table 1 show that indeed the "ferritic" ELC steel can be produced in DQ quality (r> 1.4) when the cold reduction rate is sufficiently high (> 75%). This quality of stamping DQ is moreover very constant for these high cold rates and there is associated a low planar anisotropy (Δr) (reduction of the drawing horns).

Likewise, the process of the invention makes it possible to produce DDQ (Deep Drawing Quality) grades from ULC steel.

Table 2 shows the improvement in the resistance to natural aging of continuously annealed cold strips obtained by the process of the invention (I), compared with reference strips (R) hot rolled in austenite. .

ΔRe (MPa) palier (%) ELC I ferritique 1,53 3 0,8 I ferritique 1,55 4 1,2 R austénitique 1,48 18 5,3 R austénitique 1,53 24 5,8 ULC I ferritique 1,52 5 1,3 I ferritique 1,62 6 1,5 R austénitique 1,49 36 6,7 This improvement is expressed by the increase in the elastic limit ΔRe, in MPa and by the relative length, expressed in%, of the level of the elastic limit after aging for 1 hour at 100 ° C. cold having undergone a slight surface hardening with the skin-pass rolling mill. TABLE 2 steel hot rolling $\overline{\text{r}}$

ΔRe (MPa) level (%) EL C I ferritic 1.53 3 0.8 I ferritic 1.55 4 1.2 R austenitic 1.48 18 5.3 R austenitic 1.53 24 5.8 ULC I ferritic 1.52 5 1.3 I ferritic 1.62 6 1.5 R austenitic 1.49 36 6.7

It can be seen that the increases in the elastic limit (ΔRe) and in the length of the plateau of the elastic limit after the aging test are small, since they do not exceed 6 MPa and 1 respectively, 5% for the ELC and ULC steel strips produced according to the invention. These bands can be considered as not aging, unlike the reference bands which, with aging, show significant increases in the elastic limit (ΔRe = 18 - 36 MPa) and in the length of the stage of the elastic limit ( 5.3 - 6.7%).

In a particular embodiment of the process of the invention, to produce a thin strip of quality steel for deep drawing, a slab of ELC steel is heated to a temperature between A₃ and 1150 ° C, performs the rough lamination of this slab in the austenitic domain, the finish lamination of this preform is carried out to a thickness of 1 mm to 8 mm with a finish lamination temperature below 780 ° C., to form a hot strip and this hot strip is subjected to at least one cold rolling step to the desired final thickness, with a total reduction rate greater than 75%.

Claims (8)

Method for manufacturing a thin strip of cold rolled mild steel for stamping, in which a mild steel is hot rolled in a temperature range where said steel has a ferritic structure with an end of rolling temperature below 780 ° C. to form a hot strip, characterized in that said hot strip is then cold rolled with a thickness reduction rate giving the cold strip a final degree of work hardening corresponding to a higher cold reduction rate at 75%. Process according to Claim 1, characterized in that the said cold rolling is carried out with a thickness reduction rate greater than 75%. Process according to either of Claims 1 and 2, characterized in that the said cold rolling is carried out with a thickness reduction rate of between 78% and 85%. Process according to Claim 1, characterized in that the said hot rolling is carried out in the ferrite by means of lubricated cylinders, with an end of rolling temperature leaving a degree of work hardening remaining at least equivalent to that which corresponds to an intermediate rate of cold reduction of 10% and in that the cold rolling is then carried out with a reduction rate equal to the difference between the desired final reduction rate and said intermediate reduction rate. Process according to either of Claims 1 to 4, characterized in that the cold-rolled strip is subjected to a final annealing. Process according to either of Claims 1 to 5, characterized in that the cold strip has, after annealing, a microstructure with equiaxed grains. Method according to claim 6, characterized in that the annealed strip is subjected to surface hardening with the skin-pass rolling mill. Method for manufacturing a thin strip of quality steel for deep drawing, in which a slab of ELC steel is heated to a temperature between A₃ and 1150 ° C., the rough-rolling of this slab is carried out in the austenitic field , and the finishing rolling of this blank is carried out to a thickness of 1 mm to 8 mm with an end finishing rolling temperature of less than 780 ° C., to form a hot strip, characterized in that the this strip is subjected to hot at least one cold rolling step to the desired final thickness, with a total reduction rate greater than 75%.