LU87573A1 - METHOD FOR MANUFACTURING STEEL FOR STAMPING - Google Patents

Description

BL-4255

Claim to the priority of the patent application filed Belgium, September 28, 1988, under No. 08801115 Descriptive memorandum filed in support of an invention patent application for: "PROCEDE DEFABRICATION D'UN STEEL POUREMBOUTISSAGE"

filed in the name of: RESEARCH CENTER

METALLURGICAL A.S.B.L. -CENTRUM VOOR RESEARCH IN DE METALLURGIE V.Z.W. rue Montoyer 47 B-1040 BRUXELLES (Belgium)

Method of manufacturing a steel for stamping.

The present invention relates to a method for manufacturing a steel for stamping, as well as to the products produced with this steel.

The expression "steel for stamping" usually denotes a steel intended to be formed in the form of a flat product such as a strip or a sheet, and having a high ductility and a suitable texture thanks to which it can be laid shaped by deep drawing or deep drawing. The description which follows will refer more particularly to a steel strip, without the invention being limited to this single type of product.

Conventionally, the liquid steel produced at the steelworks is poured into slabs which, after solidification, are heated to the temperature required for rolling. The term "slabs" should be understood to mean both slabs obtained from individual ingots and those obtained by continuous casting and cutting, then which are either reheated in the oven after storage or equalized in temperature and sent directly to the rolling mill, according to the so-called direct rolling procedure. For simplicity, here we call the reheating temperature both the reheating temperature proper and the equalization temperature of the slabs. The slabs are hot rolled from this reheating temperature, in a process which first comprises a roughing step followed by a tandem train finishing step. The strips thus obtained are then cold rolled and annealed, either on a coil or continuously.

The reheating temperature of the slabs is typically greater than 1200 ° C. When it is desired to re-solution the aluminum nitride with a view to developing in the strips, after cold rolling and annealing in a coil, so-called structures. Favorable "pancake" with deep drawing; The presence of such structures results in high values of the Lankford coefficient r, also called the average plastic anisotropy coefficient. This effect is linked to the precipitation of aluminum nitride during the coil annealing treatment, which requires low values of the heating rate and long treatment times.

According to another well-known process, the annealing of the strip after cold rolling is carried out continuously. In this case, the winding after the hot rolling is carried out at high temperature, in order to block nitrogen in the form of aluminum nitride and thus produce a non-aging steel.

It is also well known, in the procedure including continuous annealing, that it is advisable to reduce the manganese content of steels with low carbon content (<0.01 - 0.04 XC) or ultra-low (<0.01 % C), in order to obtain a value as high as possible of the Lankford coefficient r; this condition ensures that a steel is produced having a high aptitude for deep drawing, in the cold rolled state, annealed continuously and work hardened. The reduction in the manganese contents in order to achieve this objective, however, leads to new problems encountered during hot rolling and roughing of the slabs, or even in hot rolling with the finishing train. These defects materialize in the form of superficial cracks, which appear particularly in the corners of the slabs during hot rolling. They also cause losses in ductility of cold-rolled strips.

This drawback manifests itself particularly with low carbon steels (0.010 - 0.040%). Specially economical to produce, since they do not require degassing treatment in a steelworks, these steels indeed require low manganese contents to present, after continuous annealing, a high coefficient r.

The object of the present invention is to remedy these various drawbacks by improving, by a simple and inexpensive means, the resistance to crackability of a low carbon and low manganese steel intended for stamping or stamping. deep.

According to the present invention, a method of manufacturing a steel for drawing is characterized in that a steel with low carbon and manganese contents is used, in that a limited amount of at least one element capable of forming with sulfur a stable compound.

According to the invention, said steel has a manganese content of less than 0.20%, and preferably between 0.05% and 0.15% by weight.

Also according to the invention, such an addition element is advantageously chosen from a group comprising titanium, calcium and cerium.

In this regard, it has been found to be particularly advantageous to add titanium, in an amount such that the titanium content of the steel satisfies the relationship {[Sl + 2.3 [N]} [Ti] ^ 2 {[ S] + 2.3 [N) | in which [Ti], [S] and [N] respectively represent the titanium, sulfur and nitrogen contents of the steel, expressed by weight.

It has indeed appeared that the aforementioned elements cause precipitation of sulfur at high temperature, in the form of stable sulfides.

In the case of titanium in particular, globular precipitation of TiS occurs, which is more stable than MnS.

In the range of contents corresponding to the relationship given above, the amount of titanium is suitable for fixing the nitrogen (TiN) and the sulfur (TiS) remaining in the steel after the deoxidation and desulfurization treatments of the steel. Liquid. This presupposes the prior addition of one or more suitable elements, such as aluminum, so as to produce carbonated steels or steels with reduced aluminum content of the pseudo-effervescent type. This quantity is however insufficient to give rise to the formation of TiC carbide, which would have the effect of hardening the steel and would therefore be unfavorable for stamping.

The addition of the aforementioned elements, in accordance with the invention, not only markedly reduces the high degree of steel deformation when hot, but also preserves the excellent drawing properties of the steel after cold rolling and continuous annealing.

Finally, the addition of the abovementioned elements, and in particular titanium, in accordance with the invention, makes it possible to improve the general characteristics of weldability of the steel with low carbon and manganese content. It appears that this effect can be attributed to better control of the austenitic grain in the heat affected area during welding cycles.

The following examples show the effects of different additions of titanium to low carbon and low manganese steels for deep drawing.

Table I indicates the composition, expressed in 10 ^% by weight, of 7 steels of the aforementioned type. Steels 1 and 2 do not contain titanium; Steels 3 to 5 have titanium contents between the limits set by the above-mentioned relationship; Steels ó and 7 contain titanium in a content lower, respectively higher, than these Limits.

TABLE I .: Composition of steels (10 ^ ‘4 weights).

Two crackability indices (a) and (b) were determined for these steels. These indices, established on blanks, that is to say in roughing rolling, express Ca) The number of cracks per meter and (b) the depth of the cracks, in mm.

The values obtained are shown in Table II. They show that steels 3 to 5, in accordance with the invention, are particularly insensitive to the formation of cracks. Steel 7 is also interesting in this respect, but it contains too much titanium and therefore does not have sufficient ductility, as will be seen below.

TABLE II .: Crickiness indices of steels.

Finally, the properties of cold rolled and continuously annealed strips were determined.

These strips were hot-rolled, with an end-of-rolling temperature of between 890 ° C and 910 ° C, then they were wound at 700 ° C - 720 ° C. They were then cold rolled with a hardening rate of 70 - 71 X; it will be recalled that the rate of work hardening 6 is defined by the relation

in which R represents the rate of reduction in thickness of the strip during the operation under consideration. The strips were continuously annealed to 830 ° C, cooled in-line and subjected to survival at 400 ° C, the hardening or skin-pass rolling being carried out subsequently with a reduction rate of 0.6 %.

We measured, by a tensile test, the Elasticity Limit (Re), the breaking load (Rr), the L elongation CA), the average plastic anisotropy coefficient (r) and the modul 'hardening (n). We also determined an internal defect index (IDI), which expresses the number of internal microcravities per cm2 of the microstructure in the steel deformed by 20% in tension during a simulation of an operation deformation by stamping. Table III below collates the results obtained.

TABLE III .: Properties of annealed steels.

Steels 1, 2 and 6 have interesting drawing properties, but their IDI index is not very favorable. Steel 7 has a good IDI index, but its drawing properties are poor. Steels 3 to 5, produced in accordance with the invention, have excellent drawing properties as well as a very good IDI index.

Claims (9)

1. A method of manufacturing a steel for stamping, characterized in that a steel with low carbon and manganese contents is used, in that a limited quantity of at least one capable element is added to said steel to form with sulfur a stable compound. 2. Method according to claim 1, characterized in that said steel has a manganese content of less than 0.20% by weight. 3. Method according to either of claims 1 and 2, characterized in that said steel has a manganese content of between 0.05% and 0.15% by weight. 4. Method according to either of claims 1 to 3, characterized in that said steel has a carbon content of between 0.010% and 0.040% by weight. 5. Method according to either of claims 1 to 3, characterized in that said steel has a carbon content of less than 0.010% by weight. 6. Method according to either of claims 1 to 5, characterized in that said element is chosen from a group comprising titanium, calcium and cerium. 7. Process according to either of Claims 1 to 6, characterized in that the said element is titanium and in that it is added to the steel in an amount such that the titanium content of the steel satisfies the relationship

in which [Ti], [S] and [N] respectively represent the titanium, sulfur and nitrogen contents of the steel, expressed by weight. 8. A method of manufacturing a flat product such as a strip or a sheet intended for stamping, characterized in that a hot rolled steel conforms to either of claims 1 to 7. 9. Method of manufacturing a flat product such as a strip or a sheet intended for stamping, characterized in that it is hot rolled, that it is cold rolled and that it is annealed continuously a steel according to either of claims 1 to 7.