EP1225235B1 - Process for manufacturing a cold rolled steel strip or sheet and steel strip or sheet obtainable by the process - Google Patents

Abstract

Production of a cold rolled easily deformable strip or sheet made from steel comprises carrying out recrystallization annealing in a bell-type furnace in a bundle; and optionally leveling. The strip or sheet is heated to a temperature of more than 200 degrees C but less than the recrystallization temperature after annealing, and then cooled at a rate of at least 1 degrees C. An Independent claim is also included for the strip or sheet obtained. Preferred Features: The strip or sheet is heated to a temperature of at least 450 degrees C. The strip or sheet is cooled to at least 450 degrees C after annealing, and then re-heated to the temperature for at least 20 minutes. Annealing is carried out for 2-5 minutes.

Description

The invention relates to a method for producing a cold-rolled, readily deformable Steel strip or sheet after hot rolling, coiling and cold rolling a recrystallizing anneal in a hood furnace in the Fixed bundle with subsequent cooling to ≤ 150 ° C and possibly a skin pass is subjected to and after deformation a bake-hardening potential for having a subsequent temperature treatment.

The invention furthermore relates to a readily deformable, special cold-rolled sheet which can be produced by the process and has a bake-hardening potential after subsequent deformation and for a subsequent temperature treatment (BH ₂ potential).

For example, in the automotive industry easily deformable sheets are needed, the relatively To be thin, the weight of the vehicle should not be too high to be let. Such steel sheets are generally in the form of a Bandes produced by casting a steel slab, hot rolled and at a certain intermediate temperature is reeled. After cooling the coiled Bandes at substantially ambient temperature, the sheet is on the Final thickness cold rolled. To eliminate the resulting tensions within The material is subjected to recrystallizing annealing. Subsequently In general, the band becomes weak again with a degree of deformation between rolled about 0.5 and 2% (temper rolling).

The slight deformability of the steels is an increase in the strength values of Steel grade basically contrary, because the increased strength in principle with an impairment the easy deformability goes hand in hand. They are higher-strength steel grades have been developed (eg ZStE and ZStEi), despite higher strength values are relatively well deformable. Such steel grades are for example as ZStE steel iron material sheet SEW093 and 094 and known as isotropic steel ZStEi while the conventional "soft" steel grades as St12 to St15 (corresponding to DC01, DC03, DC04, DC05 according to DIN EN 10130) are known. The steel types differ with regard to the addition of micro-alloying elements and in terms of process management. A special steel of this kind is, for example the isotropic steel ZstEi, as described in DE 38 03 064 C2, EP 0 400 031 B1 or DD 285 298 B5 is described.

For many steel grades, one way to combine good ductility with increased yield strength after completion is to produce the steel with a so-called bake hardening potential. The effect of the bake hardening effect is that a hardening, ie an increase in the yield strength, is brought about in the case of a temperature treatment of the steel, as is the case, for example, in the case of body paneling. This is an artificial aging of the steel, which causes the additional increase in strength. The increase in strength is thus achieved after the deformation of the sheet to produce the desired component, so that the increase in strength does not interfere with the deformation of the sheet. It has been found that the previous deformation of the sheet affects the bake hardening effect. The bake-hardening effect caused only by the heat treatment without deformation is given as a BH ₀ value, while a measure of the bake-hardening effect after deformation is the BH ₂ value after deformation of the sheet 2% indicates the increase in strength due to a subsequent temperature treatment - standardized at 170 ° C for 20 min. - Indicates.

The bake-hardening effect is based on a content of dissolved carbon in the Steel that is above the state of equilibrium. To produce this supersaturation of the steel with dissolved carbon atoms, the recrystallization annealing is followed performed on the cold rolling with a continuous annealing. By the temperature increase in the continuous annealing, carbon goes into solution. Because in the continuous annealing the sheet is only heated for a short time, is a recrystallization for a well above A, lying temperature used. In conjunction with the fast Cooling of the steel strip produces the proportion of dissolved C atoms, some of them Orders of magnitude above the equilibrium state.

If, however, the annealing of the wound steel strip in the hood furnace, ie for a comparatively long time performed and made the associated slow cooling in air, the steel strip remains in equilibrium, so that no aging potential (bake hardening potential) is formed when the content of carbon ≥ 0.02%. Only at lower carbon contents, which are adjustable only by a complex vacuum treatment, an aging potential can be produced because the C atoms in solution due to their low density and the associated longer diffusion paths only difficult to get an Eisenkarbidausscheidung (cementite) and therefore a Part remains supersaturated in solution. For C contents ≥ 0.02%, the slow cooling causes the carbon to precipitate, so that no dissolved carbon is available for the aging potential. The temperature treatment causes the carbon atoms in the solution to diffuse into dislocation regions of the matrix. The dislocations are thereby blocked, so that an increased amount of stress is required to re-create a plastic flow in the material. This effect is greatly increased by a prior deformation of the dissolved C-supersaturated steel strip. The deformation process, for example by deep drawing, leads to a significant increase in the dislocation density. In the case of temperature treatment, as is the case, for example, in stove-enamelling, the carbon atoms diffuse into the dilated regions of the dislocations. In practice, therefore, the bake-hardening effect after a previous deformation (characterized by BH ₂ ) is relevant.

The deformation of the sheets leads to a degree of deformation Work hardening. For the application of bake hardening steels is the total strength resulting from cold working through forming and Bake hardening results from the temperature treatment, relevant. The well-known bake hardening steels, which are produced with a continuous annealing, over the degree of pre-strain as a variable an approximately constant yield strength curve for the sum of work hardening and bake hardening on. The bake hardening effect is therefore at larger strains due to the vast majority Proportion of work hardening hardly relevant. It is therefore known that the application of bake-hardening steels mainly for large-area components interesting which are only weakly shaped, such as fenders, hoods, Car doors and roofs.

It is also known that the bake-hardening effect with the content of dissolved atoms rises to a saturation value. Too much content of dissolved C-atoms leads to a lack of aging resistance of the steel sheet during removal. For Bake-hardening steels therefore have a content of dissolved carbon between 5 and 10 ppm considered optimal.

Limiting the use of the bake-hardening effect on non-vacuum steels, which have been recrystallized in a continuous annealing furnace, leads to considerable restrictions for the production of suitable steel sheets. advantageous Properties of steel sheets, preferably the recrystallizing Require annealing in annealing annealing furnaces, such as the production of Steel sheets with a planar isotropy or quasi-isotropy, therefore, can be so far not produce with a bake hardening effect.

In the conference paper for the International Conference "Steel and Motor Vehicle Manufacture" XX, pages 85-94, Mizui et al. from the production of so-called "pre-batch annealed and galvannealed" steel. As for galvanizing used continuous galvanizing line has no overaging oven is the overaging treatment is replaced by pre-batch annealing the steel is heated to 690 ° C in the open bundle. Then cools the Replace steel before reopening before entering the plating bath 600 ° C is heated and heated again after passing through the plating bath becomes an alloy bond between the zinc layer and the steel to create. The cooling rate for steel tempered in an open bundle is 80 ° C / h specified. This cooling rate is about four times as high as the annealing rate of the annealed one Festbundes, from which the initially mentioned procedure emanates. By the Glow in the open collar, in which formed between the winding layers gaps become the generic properties of the band or sheet steel is not achieved in the desired manner.

The invention is therefore based on the object, the production of tapes or Sheet steel of the type mentioned above with a bake hardening potential to allow a simple temperature control to ensure the Bake-hardening effect is possible.

To achieve this object, a method of the invention mentioned above is according to the invention Art characterized in that for adjusting the bake hardening potential the band cooled down in the bundle and unwound to a temperature T with 200 ° C ≤ T ≤ A, reheated, at the temperature T for an annealing time ≤ 20 minute annealed and from the temperature T at a cooling rate ≥ 1 ° C / s is cooled.

This inventive method thus allows the production of a bake hardening steel strip or sheet recrystallizing in a hood furnace in a tight coil has been annealed, even if the C content in the Steel is ≥ 0.02%.

Surprisingly, it is possible by the short-term annealing according to the invention after cooling of the recrystallizing annealed strip or sheet to ≤ 150 ° C, preferably to about room temperature, possible to bring C precipitated as carbides back into solution. Incidentally, since the temperature of the short-time annealing is lower than the A ₁ temperature of the steel, this annealing does not substantially change the technological properties of the steel, particularly its texture. Due to the short-term annealing and the subsequent cooling, which can be carried out in a conventional manner with air, but also with water, a part of the dissolved C remains in solution and leads to the aging potential for the subsequent temperature treatment, for example during a stoving.

The short-term annealing is preferably effected in a continuous annealing furnace. For the generation of a sufficient bake-hardening effect must be at a low level Annealing temperature T a relatively long annealing time are met while higher annealing temperatures significantly reduce the required annealing time. It is Therefore, it is preferable to use a temperature T of short-time annealing of ≥450 ° C. Furthermore, it is preferred that the annealing time of the short-term annealing between 2 Min. And 5 min.

It will generally make sense to tape or sheet after the short-term To dress glow, so to deform in the usual way weak. It can also be useful if the band or sheet already trained before the short-term glow although this does not always seem necessary.

For the production of galvanized sheets or strips, it is particularly expedient a hot-dip galvanizing the sheet or strip at least as part of the short-term To use glowing. However, the inventive method also not at all or electrolytically, i. without heat, to be galvanized Sheets are used.

The strip or sheet produced by the process according to the invention differs from conventional tapes or sheets with a bake hardening potential in that the total hardening of the steel (work-hardening + bake-hardening) increases with greater previous deformation of the sheet. Further The steel according to the invention contains cementite precipitates in the matrix and at the grain boundaries. Conventional, continuously annealed bake hardening steels are virtually free of cementites. If these steels are subjected to an overaging treatment, Although cementite forms, but with the loss of the bake-hardening effect. In contrast, the steel according to the invention has cementite precipitates and a bake-hardening effect. This is true even if the steel has a C content ≥ 0.02%. After baking, the sheet has a through the bake-hardening effect clearly, i. around at least 30 MPa, increased yield strength.

0,02 bis 0,12 % C

max. 0,50 % Si

0,1 bis 1,2 % Mn

max. 0,1 % P

max. 0,025 % S

max. 0,009 % N

0,01 bis 0,08 % Al

0,01 bis 0,04 % Ti

Rest Eisen und nicht vermeidbare Verunreinigungen

und ist mit dem erfindungsgemäßen Verfahren herstellbar sowie mit einem Bake-Hardening-Potential nach einer Verformung und für eine anschließende Temperaturbehandlung und mit Zementitausscheidungen in der Matrix und an den Korngrenzen versehen. A readily deformable cold-rolled strip or sheet according to the invention has the following composition according to the invention

0.02 to 0.12% C

Max. 0.50% Si

0.1 to 1.2% Mn

Max. 0.1% P

Max. 0.025% S

Max. 0.009% N

0.01 to 0.08% Al

0.01 to 0.04% Ti

Remaining iron and unavoidable impurities

and can be produced by the process according to the invention and provided with a bake-hardening potential after deformation and for a subsequent temperature treatment and with cementite precipitates in the matrix and at the grain boundaries.

As far as lower limits for the o.a. Components have not been specified, result these are unavoidable contaminants with these elements.

The steel according to the invention may have a hot-dip galvanized surface and have been trained after hot dip galvanizing.

The invention will be explained in more detail below with reference to some examples.

Corresponding tests have been carried out with steels of the grades St15, St14, two variants of the grade ZStE220i and the grade ZStE340, whose chemical compositions can be found in the attached Table 1 .

Thus, steel grades have been used for the tests, all with a C content of ≥ 0.02%. In the case of ZStE340 steel, the C content is even 0.075%.

The "soft" grades St15 and St14 have no relevant amounts of micro-alloying elements (Ti, V, Nb, Mo). In contrast, the isotropic steel grade ZSt220 characterized by a titanium content of between 0.01 and 0.04% can be and is set to about 0.02% in the experimental examples. The higher strength Goodness ZSt340 has a similar titanium content and beyond a significant niobium content.

The study of steel grades St14 and St15 have for those interested here Parameters show no relevant differences. The same applies to the experiments with the two cold bands of the ZStE220i variety. The following is therefore each of the Result of only one representative of these grades indicated and discussed.

Since the steel grades used in the market and therefore familiar to the expert are known, the expert knows the for the production of steel grades required process steps and their characteristics to achieve the desired Steel grades. A detailed description can therefore be omitted here become. For the isotropic steel grades, reference is made to DE 38 03 064 C2, EP 0 400 031 B1 and DD 285 298 B5 described production method.

All steel grades used are in the usual way at the required temperatures poured into the slab and then hot rolled. After one Coiling at a suitable intermediate temperature is carried out a cooling in air Service. Subsequently, the cold rolling steps have been carried out. Thereafter, the steel strip has been annealed recrystallizing in the hood furnace, wherein the usual annealing time is between 20 and 70 hours.

The steel strip cooled to about room temperature has been partially dressed and partially undressed for the tests carried out here before the short-term annealing according to the invention is carried out, preferably in a continuous furnace. In order to be able to determine the BH ₂ effect, which alone is of importance in practice, the material has been pre-stretched.

In all cases, after the brief annealing, the cooled material is dressed Service.

FIG. 1 shows the measurement results for the BH ₂ effect for the steel St15 as a function of the annealing temperature and the annealing time, each of which has been set at 0.5 min, 2 min and 5 min. The non-annealed samples were said to be " 1 x " due to the post-anneal annealing, the pre-stressed samples were termed " 2 x dressed ".

It turns out that even at the annealing temperature of 200 ° C and a short annealing time an increased BH ₂ potential is present, which increases for all samples with increasing annealing temperature and increasing annealing time, wherein at the annealing temperature of 700 ° C by extending the annealing time No or no substantial increase in the BH ₂ potential is achieved over 2 min.

For all specimens, the application of the material before short-term annealing does not significantly increase the BH ₂ effect, and in some cases even a noticeable reduction is noted.

Figure 2 shows the results for the same tests on steel ZStE220i. A very large BH ₂ effect is achieved at an annealing temperature of 700 ° C and an annealing time of 2 min. An extension of the annealing time at this temperature leads to a reduction of the BH ₂ effect. Again, the tempering before the premature annealing for the size of the BH ₂ effect is rather harmful.

The results for the steel grade ZStE340 shown in FIG. 3 make it clear that, in this case, the tempering before the short-term annealing, at least for medium annealing temperatures, is favorable. At the low annealing temperature of 200 ° C, a maximum at the annealing temperature of 2 min. Forms for the 1 x dressed steel. For shorter and longer firing times, the BH ₂ effect even goes back to zero.

Figures 4 to 6 illustrate the dependence of the BH value on the degree of prior stretching of the material. In all cases, a more or less pronounced maximum sets in at about 2% degree of stretching, whereas conventional bake-hardening steels have a decreasing BH value with increasing degree of stretching.

Figure 4 shows the results for undisturbed ZSt220i, St14 and ZSt340 grades, annealed at 500 ° C for 5 min and deformed between 0.5 and 1% depending on the steel grade of the skin pass. The bake hardening annealing has taken place according to the test specifications at 170 ° for 20 min.

The results shown in FIG. 5 relate to the same steels with the same degree of temper rolling, but the short-term annealing has been carried out at 500 ° C. for an annealing time of 15 minutes.

The results shown in Figure 6 relate to the equally treated steel grades which have been annealed at 700 ° C for 5 min. Striking is the high bake hardening potential for the isotropic steel grade ZStE220i, which has been pre-stretched with a degree of deformation of between 2 and 3%.

In FIG. 7 , the sum of deformation hardening (work hardening WH) and bake hardening strengthening (BH) as a function of the degree of stretching is indicated for the three steel grades. While conventional bake hardening grades show a substantially constant sum of yield strength increase across the different degrees of stretch, the grades of the invention exhibit a yield strength increase that increases with the degree of stretch. The steels treated according to the invention therefore differ in their mechanical properties from the conventionally produced bake hardening steels.

FIGS. 8 to 10 illustrate the course of the work hardening curve and the bake hardening curve as a function of the degree of pre-strain for the steel grades St 15 (FIG. 8), ZStE 220i (FIG. 9) and ZStE 340 (FIG. 10). While the pure bake-hardening effect tends to decrease with increasing pre-strain, the work-hardening effect increases disproportionately, resulting in the increasing cumulative curve for the steel according to the invention.

FIG. 11 illustrates the dependence of the sum of the yield strength increase on the annealing temperatures and the annealing times. For all steel grades, the highest yield strength increase is achieved at the highest (permissible) annealing temperature of approx. 700 ° C with long annealing time (5 min.). A further increase in the annealing temperature is not possible because the A ₁ value (about 720 ° C) must not be exceeded during the annealing process. Exceeding the A ₁ temperature would cause transformations that would adversely affect the properties of the steel.

In Table 2 , the significant mechanical values for steels treated with the present invention having a BH ₂ effect are compared with the mechanical properties of steel grades as described in Euronorm EN 10 130, Applicant's W5 / 94 or SEW 093 and SEW steels 094 are shown.

All percentages are% by weight.

Claims (13)

1. Process for producing a cold-rolled strip or sheet of steel with good deforming properties, which is subjected to recrystallizing annealing in a bell-type furnace while firmly coiled with subsequent cooling to ≤ 150°C and, if appropriate, a dressing operation after hot rolling, reeling and cold rolling and after a deformation has a bake-hardening potential for a subsequent temperature treatment, the strip cooled while coiled being unreeled and reheated to a temperature T with 200°C ≤ T ≤ A₁, annealed at the temperature T for an annealing period of ≤ 20 minutes and cooled from the temperature T at a cooling rate of ≥ 1°C/s for setting the bake-hardening potential.
2. process according to Claim 1, characterized in that the temperature is T ≥ 450°C.
3. Process according to Claim 1 or 2, characterized in that the annealing period of the brief annealing is chosen between 2 minutes and 5 minutes.
4. process according to one of Claims 1 to 3, characterized in that the cooling from the temperature T is performed at a cooling rate of ≥ 2°C/s.
5. Process according to one of Claims 1 to 4, characterized in that the strip or sheet is dressed before the brief annealing.
6. Process according to one of Claims 1 to 5, characterized in that the strip or sheet is dressed after the brief annealing.
7. Process according to one of Claims 1 to 6, characterized in that hot galvanizing of the sheet or strip is used as part of the brief annealing.
8. Process according to one of Claims 1 to 7, characterized in that a steel with a C content of ≥ 0.02% is used.
9. Process according to one of Claims 1 to 8, characterized by the use of a steel grade which has been selected from the steel grades St12 to St15, ZStE and ZStEi.
10. Cold-rolled strip or sheet with good deforming properties, with the composition

    0.02 to 0.12% C

    max. 0.50% Si

    0.1 to 1.2% Mn

    max. 0.1% P

    max. 0.009% N

    0.01 to 0.08% Al

    0.01 to 0.04% Ti

    remainder iron and unavoidable impurities;

    can be produced by the process according to one of Claims 1 to 9, with a bake-hardening potential of ≥ 30 MPa after a deformation and for a subsequent temperature treatment and with cementite precipitations in the matrix and at the grain boundaries.
11. Strip or sheet according to Claim 10, characterized in that it has a hot-galvanized surface.
12. Strip or sheet according to Claim 11, characterized in that it is dressed after the hot galvanizing of the surface.
13. Stove-enamelled sheet, produced from a strip or sheet according to Claim 10 or 11, with a yield strength significantly increased by the stove-enamelling.

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