WO2002024969A1 - Method for producing a steel strip or sheet consisting predominantly of mn-austenite - Google Patents

Abstract

The inventive method permits a cost-effective production of a steel strip (W) or sheet, which consists predominantly of Mn-austenite and which has increased resistance in comparison to the strips or sheets in prior art. To this end, a steel is melted, said steel containing at least the following alloy components (in wt. %): 15.00 24.00 % Cr, 5.00 12.00 % Mn, 0.10 0.60 % N, 0.01 0.2 % C, max. 3.00 % Al and/or Si, max. 0.07 % P, max. 0.05 % S, max. 0.5 % Nb, max. 0.5 % V, max. 3.0 % Ni, max. 5.0 % Mo, max. 2.0 % Cu, the remainder being made up of iron and unavoidable impurities. Said steel is cast in a casting gap formed between two rotating rollers (2,3) or cylinders into a thin strip (D) with a maximum thickness of 10 mm. The rollers or cylinders (2,3) are cooled to such an extent that the thin strip (D) in the casting gap (4) cools at a speed of at least 200 K/s.

Description

 Process for producing a steel strip or sheet consisting predominantly of Mn austenite

The invention relates to a method for producing a steel strip or sheet consisting predominantly of Mn austenite. Steels that are suitable for the manufacture of such products are assigned to the AISI 200 and have the designation S20100 to S24000. Steel materials of this type are characterized by high strength, which is retained even after welding in the area of the weld.

These good strength properties are achieved through interstitial and substitutional mixed crystal hardening. Carbon and nitrogen are particularly effective in this regard. However, higher carbon contents are avoided due to the undesirable carbide formation. Therefore, nitrogen is preferably used for interstitial mixed crystal hardening in steels of the type in question. The production of steels with an increased nitrogen content is, however, complex in relation to the alloy components or the equipment required for the production.

In a known method for the production of steels with higher nitrogen contents, the melt is melted under pressure. The pressure on the melt is so far above the partial pressure of nitrogen that the nitrogen in the respective Steel goes into solution. The advantage of this approach is that steel can be produced with higher nitrogen content without the addition of specific amounts of other alloying elements / ^'However, the disadvantage is the high cost of equipment required for this purpose.

An alternative approach to dissolving the nitrogen by pressurizing it during melting is to increase the solubility of the melt itself. This can be achieved by high levels of chromium and manganese. A description of the properties of appropriately composed steels created by M. du Toit is currently available on the Internet at "www.tecnet.co.za/mags/steel/ featurel.htm". The known steels can be melted without conventional pressure and cast conventionally, but not in continuous casting. Casting the known steels therefore involves high costs.

A further increase in the strength of steels of the conventionally castable steels explained above can be achieved by alloying aluminum and / or silicon. These two elements support mixed crystal hardening and thus lead to a further increase in strength. In addition, the addition of aluminum and silicon can influence the stacking error energy, which in turn has an influence on the deformation processes. The addition of aluminum leads to an increase in the stack error energy and favors the deformation due to the formation of twins. In contrast, silicon lowers the stack fault energy, but favors the deformation about martensite formation. The combined addition of silicon and aluminum can thus have a targeted influence on the hardening of the material during deformation. The formation of martensite leads to a firm consolidation, while the formation of twins reduces the consolidation.

The advantages of adding aluminum and silicon contents to steels of the type in question are offset by the disadvantage that they are ferrite formers and favor the primary ferritic solidification. The resulting ferrite has only a low solubility for nitrogen. The latter is consequently eliminated in the form of gas bubbles during solidification. In order to nevertheless achieve an austenitic steel of high strength while maintaining the increased nitrogen content, the austenite would have to be stabilized. However, the further increases in manganese required for this, in addition to an increase in raw material costs, lead to considerable problems in the production of such high-manganese steels in the steel mill.

The object of the invention is to provide a method for producing a steel consisting predominantly of Mn austenite, which can be produced cost-effectively and at the same time has increased strength compared to the prior art.

This object is achieved by a method for producing a steel strip or sheet consisting predominantly of Mn austenite, in which a steel is melted which contains (in% by weight) the following alloy components: 15.00 - 24.00 Cr,

5.00 - 12.00% n,

0.10 - 0.60 Ό N,

0.01 - 0.2 Ό c, max. 3.00, Ό AI and / or Si, max. 0.07 g.

Ό P, max. 0.05 g. Ό s, max. 0.5 Q. Ό Nb, max. 0.5 Q,

"o v, max. 3.0 g. Ό Ni, max. 5.0 Q. Ό Mo, max. 2.0 g." o Cu as well as Re: st iron ι and unavoidable impurities,

and in which the steel in a casting gap formed between two rotating rolls or rollers to form a thin strip with a thickness of max. 10 mm is poured, the rollers or rollers being cooled so strongly that the thin strip cools in the casting gap at a cooling rate of at least 200 K / s. The thickness of the thin strip is preferably between 1 and 5 mm. Of course, the specification of the steel composition used according to the invention also includes those alloys in which the content of those alloy elements is zero, for which only a maximum permissible upper limit of the content is specified.

According to further refinements of the invention, the chromium content of the steel can be 17.00-21.00% by weight of Cr, the manganese content can be 8.00-12.00% by weight of Mn and / or the nitrogen content limited to 0.40-0.60 wt% N his. In addition, levels of Ni, Mo and / or Cu may be present in the steel.

The contents of the alloy elements contained in the steel composition used according to the invention are each optimized with regard to the effect of these elements. Cr, Mn, Mo, V, Nb and Al increase the nitrogen solubility in the melt, while Ni, Cu as austenite formers and Si reduce the nitrogen solubility. However, as mentioned, Si also acts as a mixed crystal hardener. It is also used for grain refinement and lowers the stacking error energy. AI, on the other hand, increases the stack fault energy. Mo also acts as a mixed crystal hardener and improves the corrosion behavior. V also has a grain-refining effect and increases strength. The addition of Nb leads to an increase in strength through precipitation hardening.

The invention makes use of the basically known technology of a strip caster by casting the steel in the casting gap formed between the rolls or rolls, for example a double-roll casting apparatus ("double roller"), and thereby cooling it so much that it causes a displacement of primarily ferritic solidification comes towards primary austenitic solidification. This makes it possible to transfer the nitrogen dissolved in the melt into the steel, because the austenite has a high solubility for nitrogen. The possibility of such intensive cooling is only opened by casting a thin strip in a casting nip, the walls of which are formed by the casting rolls or rollers moving at essentially the same speed as the cast strip, so that a constant, intensive heat exchange between the walls (casting rollers / rollers) and the cast steel in the casting gap is guaranteed.

The intensive cooling, which takes place at a high cooling rate, ensures that nitrogen gas bubbles which may form in the solidifying melt remain small and the pressure directed against them is high. This prevents outgassing of the nitrogen during the solidification. In addition, such leakage of nitrogen is suppressed by the high ferrostatic pressure which occurs due to the large height of the melt pool in the casting gap. In this way it is ensured that the pressure P _N in the nitrogen gas bubbles which may arise is always lower than the sum of the ambient pressure P _A , the ferrostatic pressure P _F and twice the surface tension σ of the gas bubbles in relation to the bubble radius r (ie P _N <P _A + P _F + 2σ / r).

The rapid solidification of the cast strip during strip casting thus opens up great freedom with regard to the choice of, particularly in connection with steels of the type used according to the invention

Steel composition. As explained, larger amounts of nitrogen can be dissolved by the rapid solidification. Alloy elements that improve the material properties can therefore be added in larger quantities than in conventional manufacturing methods, regardless of their possible negative influence on nitrogen solubility. For example, if the steel contains higher amounts of Si, this is due to conventional manufacturing slow solidification and the associated increased ferrite formation, existing risk of nitrogen outgassing prevented with the procedure according to the invention. Even in the case of increased Al contents, the rapid cooling provided according to the invention avoids the formation of A1N, which occurs with slower cooling. The invention thus allows the deformation mechanism of the alloy used in each case to be adjusted in a targeted manner so that an end product with optimized properties is obtained, regardless of harmful influences caused by the slow cooling, by suitable selection of the Al and Si contents.

The cost advantage achieved by the invention when processing steels of the type used according to the invention which are difficult to form per se is considerable. This applies both to those steels containing up to 7.5% by weight of Mn, which can be cast in conventional continuous casting, and to those steels containing more than 7.5% by weight of Mn, which conventionally cast only in ingot casting and can then be rolled to the desired final thickness in several passes with any necessary reheating.

Hot strip made of continuously cast alloy can currently only be produced on a conventional hot strip mill with a thickness of at least 3.5 mm. The production of cold strip in the typical target thicknesses of 0.8 - 1.2 mm can only be achieved by intermediate annealing. In contrast, in the procedure according to the invention using strip casting, intermediate annealing is no longer necessary due to the smaller thickness of the hot strip obtained. There can be produced by the strip casting according to the invention, a thin strip whose final thickness is between 1 and 3 mm, it is in many cases also possible to adjust the final thickness ^"of the strip produced so that cold rolling can be omitted entirely. In this way, the the low formability of Mn-austenite-related problems in conventional manufacturing can be avoided.

The procedure according to the invention makes it possible to produce steel strips and sheets which have particularly high nitrogen contents of 0.4 to 0.6% by weight and to which up to 3% of aluminum and / or silicon are alloyed at the same time, without the steel production having to do so must be carried out under excess pressure or particularly high levels of manganese are required. The steel products produced in this way have a fine-grained, isotropic structure with a small macro-segregation or a small number of coarse inclusions. Due to its Al and / or Si content, they also have increased strength and ductility compared to the prior art. In the case of a steel strip or sheet produced according to the invention, the hardening and thus the energy absorption in the event of deformation can also be set in a targeted manner by the choice of alloy.

The thin strip is preferably cast under a protective gas atmosphere. By casting under protective gas, a thin strip with a modified surface can be produced in a simple manner, the degree of oxidation of which can be influenced in a targeted manner. In this way, scaling can be avoided. The strip obtained in this way can then be hot-rolled "inline" in a roll stand without the risk of the rollers sticking together. In this context, it is particularly advantageous if the thin strip is heated to an initial rolling temperature before hot rolling. This increase in temperature enables higher degrees of forming to be achieved during hot rolling.

By subjecting the hot strip to a heat treatment after hot rolling, its structure can be specifically optimized. The heat treatment can include annealing and subsequent controlled cooling.

Due to their range of properties, steel sheets produced according to the invention are particularly suitable for the production of body sheet metal parts, of stiffening structural components used in particular in general vehicle construction and especially in automobile construction, of chassis parts, of vehicle wheels and of fuel tanks. In all of these uses, the particularly good strength properties of steel sheets produced by the method according to the invention have an advantageous effect. In addition, the good corrosion resistance of steel sheets and strips according to the invention proves to be advantageous in those applications in which they come into contact with aggressive media, such as fuels. The invention is explained in more detail below on the basis of a drawing illustrating an exemplary embodiment.

The single figure shows schematically a strip casting installation 1. In this installation, for example, a steel is processed which, in addition to the usual unavoidable impurities (in% by weight), contains 0.08% C, 0.5% Si, 10% Mn, 19% Cr, 0.5% N , 0.3% AI and the rest contains iron.

The strip caster 1 comprises a “double roller” two-roll casting apparatus, of which the rolls 2, 3, which rotate in opposite directions about an axis of rotation, are shown in the figure. A casting gap 4 is formed between the rollers 2, 3 and is continuously filled with melt, so that a melt pool S is formed above the casting gap 4.

Via cooling devices, not shown, the rollers 2, 3 are intensively cooled during the casting process, so that the melt entering the casting gap 4 primarily solidifies austenitically at cooling rates of more than 200 K / s and the casting gap 4 as thin strip D with a thickness of 1 to 5 mm leaves. The thin strip D produced in this way then passes through a furnace 5, in which it is heated to an initial rolling temperature.

Both the two-roll casting device with the rolls 2, 3 and the furnace 5 are accommodated in a housing 6 in which a protective gas atmosphere is contained. By casting the thin strip D and reheating it in the furnace 5 under protective gas, the The formation of scale on the surface of the thin strip D is largely avoided.

The thin strip D heated to the initial rolling temperature enters a rolling mill 7, in which it is hot-rolled to a final dimension. Due to the high initial rolling temperature, large degrees of forming are possible. The hot strip W rolled out of the thin strip D, which reaches the rolling mill essentially without scale, has a particularly high-quality surface after hot rolling.

After hot rolling in the rolling mill 7, the hot strip W is annealed in a continuous annealing furnace 8 and then cooled in a controlled manner under a cooling device 9 in order to specifically improve its structure. The hot strip W heat-treated in this way is finally wound into a reel 10.

Steel strip produced in the manner explained above has particularly high strength compared to conventionally assembled and produced steel strips due to its high nitrogen content achieved by the rapid cooling between the rolls 2, 3 of the two-roll casting apparatus, and at the same time good deformability and good energy absorption capacity.

The table below compares the superior strength values of the hot strip W produced according to the invention in the casting and rolling mill 1 with the strength values of Mn austenite steels conventionally produced by continuous casting.

REFERENCE NUMBERS

1 casting and rolling system

2.3 rollers

4 casting gap

5 oven

6 housing

7 rolling mill

8 continuous annealing furnace

9 cooling device

10 reels

D thin band

W hot strip

S melt pool

Claims

PATENT TO SPEECH 1. Method for producing a steel strip (W) or sheet consisting predominantly of Mn austenite, - in which a steel is melted, which (in% by weight) contains the following alloy components: 15.00 - 24.00 g. Cr, 5.00 - 12.00, "o Mn, 0.10 - 0.60% N, 0.01 - 0.2 Ό c, max. 3.00 g. AI and / or Si, max. 0.07 Q. Ό P, max. 0.05 s, max. 0.5 Nb, max. 0.5 g, Ό V, max. 3.0 g, "o Ni, max. 5.0 g. Mo, max. 2.0 g. Cu as well as iron, and unavoidable impurities, and in which the steel in a casting gap formed between two rotating rollers (2, 3) or rollers to form a thin strip (D) with a thickness of Max. 10 mm is poured, the rollers (2, 3) or rollers being cooled to such an extent that the thin strip (D) cools in the casting gap (4) at a cooling rate of at least 200 K / s. 2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t, that the thickness of the thin strip (D) is 1 to 5 mm. 3. The method according to any one of the preceding claims, that the steel contains 17.00-21.00 wt.% Cr. 4. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, d a ß the steel contains 8.00 - 12.00 wt .-% Mn. 5. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, d a ß the steel contains 0.40 - 0.60 wt .-% N. 6. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the steel additionally contains contents of Ni, Mo and / or Cu. 7. The method according to any one of the preceding claims, characterized in that ß the thin strip (D) is poured under a protective gas atmosphere. 8. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that the thin strip (D) is continuously hot-rolled to form a hot strip (W) following the casting. 9. The method according to claim 8, d a d u r c h g e k e n n z e i c h n e t, that the thin strip (D) is heated to a rolling start temperature before hot rolling. 10. The method according to claim 9, d a d u r c h g e k e n n z e i c h n e t, that the heating takes place under protective gas. 11. The method according to any one of claims 8 to 10, d a d u r c h g e k e n n e z e c h n e t, that the hot strip (W) is subjected to a heat treatment after hot rolling. 12. Use of a steel strip produced according to one of claims 1 to 11 as a material for body panel parts. 13, Use of a steel strip produced according to one of claims 1 to 11 as a material for stiffening structural components. 14. Use of a steel strip produced according to one of claims 1 to 11 as a material for chassis parts. 15. Use of a steel strip produced according to one of claims 1 to 11 as a material for vehicle wheels. 16. Use of a steel strip produced according to one of claims 1 to 11 as a material for fuel tanks.