EP1070148B1 - Method for making a hot-rolled steel strip for swaging - Google Patents

Description

Technical area

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

State of the art

At present, the steel strips for stamping operations are generally cold rolled steel strips, which have very favorable properties in this respect. The manufacture of these cold bands, however, includes various thickness reduction and heat treatment operations that increase the cost.

The use of hot-rolled steel strips for stamping operations, instead of traditional cold-rolled stripes, is attracting increasing interest, both in manufacturing and in users.

It is well known that steels for stamping are mild steels, that is to say steels whose carbon content is less than 0.2% by weight, and preferably less than 0.1%. in weight.

According to the usual practice, the mild steels are hot-rolled in the austenitic range and the end-of-rolling temperature is higher than the transformation temperature Ar 3. However, the possibilities of using these conventional hot runners are very limited because of their random texture and poor stamping ability. In addition, it is impossible in practice to manufacture hot-rolled thin strips by this conventional method. Indeed, the small thickness of the strips leads to rapid cooling thereof, even during hot rolling, so that it is not possible to perform finishing rolling in the austenitic field. For these various reasons, there is currently a growing interest in so-called ferritic rolling.

According to this rolling method, known in particular from the patent EP-A-0 524 162, as well as from BE-A-1010164 or EP-A-376733, the rough rolling is carried out in the austenitic field, while the Finishing rolling is done in the ferritic field, ie at lower temperature. According to DE-A-196 00 990 a finishing lamination is possible for ULC, ELC or IF steels in the austenitic and ferritic domains.

In hot production lines with cold casting of continuous casting slabs, this technique is particularly interesting because it allows for substantial energy savings. Indeed, the reheating temperature is lower than in conventional processes. Finishing rolling at lower temperatures thus makes it possible to manufacture hot-rolled strips of small thickness, having good drawability.

The implementation of this technique is however subject to various conditions and therefore has some disadvantages.

In particular, the amount of carbon in solution during ferritic rolling must be minimized. As a result, it is only possible to manufacture sheets for deep drawing from steels containing no interstitial atoms, called IF steels (Interstitial Free). It has been proposed to improve the situation by additions of titanium and sulfur, but such a solution may cause surface defects and is not always economically attractive.

In addition, the ferritic rolling must be carried out at a sufficiently low temperature to prevent recrystallization in the finishing mill. Higher rolling forces in a low temperature rolling mill can therefore be a limiting factor for rolling hot thin strips.

Hot-rolled thin tapes, made of ferrite-lined Ti + S-IF steel and recrystallized, with increased titanium and sulfur contents, have a high Lankford coefficient, with r ≈ 1.6-1.7 , but their planar anisotropy remains too important (Δ r ≈ 1) to allow deep drawing.

Presentation of the invention

To avoid the aforementioned drawbacks, the present invention provides a method of manufacturing a hot rolled steel strip having a reduced titanium content and an improved drawing ability compared to the steel strips discussed in the introduction. . A hot-rolled steel strip produced by the process of the invention is distinguished in particular by an improved anisotropy coefficient (r _average ) and a planar anisotropy (Δ r) significantly reduced compared to the state of the art.

According to the invention according to claim 1, a method of manufacturing a hot-rolled steel strip for stamping, in which a steel slab is subjected to a coating at a temperature greater than Ac 3 in the area of austenitic and subsequently to finishing rolling, is characterized in that the steel is a Ti-IF type steel containing less than 0.05% by weight of titanium and 0.015% to 0.075% by weight of niobium, in that said finishing rolling is carried out at least partly in the ferritic range of the steel, with a ferritic rolling start temperature of between 875 ° C and 800 ° C, with lubricated rolls and with a reduction ratio of at least 80% thickness during said finishing rolling in the ferritic range, and in that the steel strip is reeled at a temperature of between 750 ° C and 500 ° C.

The winding in the temperature range between 680 ° C and 750 ° C makes it possible to produce a recrystallized hot strip and for direct application in stamping. Winding between 500 ° C and 680 ° C leads to the formation of a non-recrystallized strip which requires a recrystallization treatment in a continuous annealing or galvanizing line.

According to the invention, said finishing rolling is carried out partly in the low-temperature region of the austenitic domain of the steel, without lubrication of the rolls and with a thickness reduction ratio greater than or equal to 30%, preferably included between 30% and 80%, and partly in the ferritic field of steel, with lubricated cylinders and with a ferritic rolling start temperature of between 860 ° C and 800 ° C and a ferritic end-of-rolling temperature included between 750 ° C and 600 ° C, said strip is reeled at a temperature between 650 ° C and 500 ° C and annealed continuously at a temperature between 800 ° C and 850 ° C for a time between 30 seconds and 2 minutes.

For the lubrication of the rolls during rolling in the ferritic range, an ester-based oil is preferably used.

As mentioned above, the process of the invention is directed to Ti-IF steels, in which the titanium is partially replaced by niobium. It has indeed been observed that the precipitation of niobium carbide (NbC) began at increasing temperatures, in the austenitic domain, when the niobium content increases in the steel. As a result, the carbon in solution is almost completely fixed before the beginning of the deformation in the ferritic domain. It is thus possible to obtain a final product, that is to say a hot rolled strip, which has a substantially continuous tensile curve and which therefore lends itself remarkably to deep drawing.

In addition, such an addition of niobium delays recrystallization between the rolling mill stands in the ferritic range. The inlet temperature in the finishing mill may therefore be higher than in prior practice and thus reach a value of 850 ° C to 875 ° C as mentioned above.
The rolling forces can thus be lower in the finishing mill; likewise, the waiting time between the roughing mill and the finishing mill can be shortened, in favor of an increase in productivity. This waiting time can also be completely eliminated by applying accelerated cooling of the product to the output of the roughing mill.

Brief description of Figure 1

FIG. 1 shows the comparative evolution of the value of the coefficient r for a conventional Ti + S-IF steel and two Ti + Nb-IF steels according to the invention.

Modes of implementing the invention

Slabs of steels having the compositions shown in Table 1 below were made. <i> TABLE 1 - Chemical composition of steels (thousandths </ i>% <i> weight) </ i> Steel VS NOT S al mn Ti Nb high Ti + S + IF 2.9 2.2 10.0 38 161 90 - NbTi-IF1 2.2 2.5 14.0 35 107 35 38 NbTi-IF2 2.0 2.8 9.0 34 94 27 52

The products were heated to 1050 ° C and then hot rolled in six passes to a final thickness of 1.3 mm. During this finishing lamination, the first three passes were made in the region of low temperatures of the austenitic domain, that is to say at temperatures slightly higher than Ac3, with a thickness reduction ratio of 73. % and without lubrication of the cylinders.

The other three passes were made in the ferritic field, with cylinders lubricated with an ester-based oil. The product temperature at the inlet of the first ferritic rolling pass was about 840 ° C and the outlet temperature of the last pass was about 620 ° C.
After hot rolling, the strip was coiled at 500 ° C and then continuously annealed at 820 ° C for 60 seconds.

The mechanical properties of these samples were measured, their texture was determined at mid-thickness, and the evolution of the coefficient of anisotropy r was calculated.

The single FIG. 1 illustrates this evolution for the three steels of Table 1, as a function of the angle α with respect to the rolling direction.

Curves 1 and 2, respectively corresponding to Nb + Ti-IF1 and Nb + Ti-IF2 steels, show a clear improvement in the _average anisotropy coefficient r, as well as planar anisotropy (Δr = r _max - r _min ). compared to conventional Ti + S-IF steel (curve 3).

In another example (curve 4), an Nb + Ti-IF1 steel slab was manufactured, which was heated at high temperature, ie 1250 ° C., and then the rough-rolling was carried out at high temperature in the field. austenitic. The product, having a thickness of 20 mm, was cooled from its roughing end-milling temperature of 1040 ° C. to a temperature of 900 ° C., with a cooling rate of about 40 ° C. C / s. The finishing rolling was carried out in four passes in the ferritic range, with an inlet temperature of 870 ° C, with lubricated cylinders and with a total reduction of thickness, in the ferrite, of 80%.
The strip was cooled in calm air and then wound at 730 ° C.

Curve 4 of FIG. 1 shows the evolution of the coefficient of anisotropy r; there is still a slight improvement in the _average coefficient r, as well as in the planar anisotropy Δ r, with respect to curve 3.

The method of the invention makes hot rolled steel strip for stamping, which have little risk of surface defects, due to a low titanium content. In addition, the presence of niobium substantially raises the temperature of non-recrystallization in the austenite. This results in a finer grain structure after allotropic austenite-ferrite transformation and, hence, an improvement in planar anisotropy.

Finally, the higher rolling temperatures in the finishing mill reduce the rolling forces.

Claims (3)

1. Method for manufacturing a hot-rolled steel strip for drawing, said steel being of the Ti-IF type comprising less than 0.05% titanium by weight and 0.015% to 0.075% niobium by weight, in which a slab of steel at a temperature higher than Ac3 is first subjected to rough rolling in the austenitic phase and then to finish rolling, characterised by the following successive stages:

   - after the rough rolling of the steel in the austenitic phase, the steel is subjected to accelerated cooling upon exit from the roughing stand until a temperature that is not lower than the temperature at the start of the finish rolling;

   - said finish rolling is carried out in part in the low temperature range of the steel austenitic phase without lubrication of the cylinders, and in part in the steel ferritic phase with lubricated cylinders, a temperature at the start of ferritic rolling between 875°C and 800°C, and a total thickness reduction rate of at least 80%, and

   - the steel strip is coiled at a temperature between 750°C and 500°C.
2. Method according to Claim 1, characterised in that:

   - said finish rolling is carried out in part in the low temperature range of the steel austenitic phase with a thickness reduction rate between 30% and 80%;

   - said finish rolling is carried out in part in the steel ferritic phase with a temperature at the start of the ferritic rolling between 860°C and 800°C and a temperature at the end of the ferritic rolling between 750°C and 600°C;

   - said strip is then coiled at a temperature between 650°C and 500°C and

   - the strip is continuously annealed at a temperature between 800°C and 850°C for a period between 30 seconds and 2 minutes.
3. Method according to Claim 1 or 2, characterised in that the lubrication of the cylinders during rolling in the steel ferritic phase is achieved by an ester-based oil.

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