EP1469954B1 - Method and installation for producing a hot rolled strip from austenitic rust-resistant steels - Google Patents

Abstract

A method for producing hot-rolled strip from austenitic stainless steels. In a first step, a cast product is subjected to a rolling operation in a rolling mill with a finishing train, and, in a second step, a heat treatment is carried out to prevent susceptibility to corrosion, especially intergranular corrosion due to chromium carbide precipitation. To establish the final rolling temperature (Twe), a run-in temperature (Tein) of the cast product into the finishing train of the rolling mill that is above 1,150° C., and preferably above 1,200° C., is established by a multistage heating process, especially a two-stage heating process, which comprises a preheating stage and an intensive heating stage, and the heat treatment is carried out by directly utilizing the rolling heat.

Description

The invention relates to a method for producing hot strip of austenitic stainless steels, wherein in a first step a cast product is subjected to a rolling operation in a rolling mill with a finishing train and in a second step a heat treatment to prevent susceptibility to corrosion, in particular with respect to an intercrystalline corrosion due to Chromium carbide precipitates, is performed. In addition, the invention relates to a plant for the production of hot strip of austenitic stainless steels, which are not susceptible to a selective, especially intergranular corrosion.

It is known that austenitic stainless steels, among which steel grades of at least 10.5% chromium and nickel are generally found, are particularly susceptible to intergranular corrosion due to chromium depletion of the near-comer regions of the structure during formation chromium-rich precipitates on the grain boundaries and the associated reduction in the corrosion resistance of these areas against microstructure areas with high content of dissolved chromium based. This happens in particular if they pass through critical Temperaurbereiche during cooling too slowly. Therefore, such austenitic Cr-Ni steels are adjusted in the solution-annealed and quenched state. In the solution annealing with subsequent quenching is a heat treatment in which at solution annealing temperatures between about 1,000 and 1,100 ° C, the chromium of the precipitated Cr carbides goes back into solution and is prevented by the subsequent quenching that form again chromium carbides by placing the C atoms in forced solution in the matrix being held. Such solution annealing followed by quenching is carried out in a separate from the rolling heat treatment process. For this purpose, the rolled products are transported to separate heat treatment plants and subjected there for the heat treatment of annealing and rapid cooling. In addition to preventing the formation of Cr carbides, solution heat treatment also improves the cold formability of austenitic Cr-Ni steels.

EP 0 415 987 B2 discloses a method for the continuous production of strip steel or sheet steel from thin slabs of approximately 50 mm thickness produced by sheet casting with horizontal discharge direction with the rolling thin slab after solidification of the strand in the arcuate guide shaft at temperatures of more as 1,100 ° C, temperature drop of the slabs by radiation or Entzundem, inductive re-heating to a temperature of about 1,100 ° C and rolling the thin slab in at least one rolling mill. By means of heating, a temperature is set in the slabs, so that adjusts a temperature gradient at the deformation means of the rolling train in such a way that when tapping into the last roll stand, the temperature is within a sufficient deformation for a good order. Here, in a third and last rolling stand of a rolling mill, the rolling stock temperature has dropped, for example, to 988 ° C and sufficient as a tapping temperature for the last rolling. The rolling stock leaves the last stand with a temperature of 953 ° C or less and is then separated at further lowered temperature in desired lengths and stacked or reeled.

In addition, systems for rolling strips and sheets from the casting heat are known, for example described in Stahl & Eisen, Vol. 2, 1993, page 37ff. Flemming et al., The CSP plant technology and its adaptation to advanced production programs. In such a plant, a thin slab is produced by means of a continuous casting machine with a specially designed mold shape, cut into individual lengths and conveyed into a roller hearth furnace for temperature compensation. Subsequently, the thin slab is accelerated to the significantly higher inlet speed of the subsequent rolling train, descaled and fed to the rolling train. In stationary production operation with a casting speed of 5.5 m / min, the thin slab reaches the roller hearth furnace with an average temperature of about 1080 ° C. The outlet temperature from the roller hearth furnace is about 1,100 ° C. The heat energy required for the rolling process is thus almost completely covered by the amount of heat contained in the cast strand. In the rolling mill, the heat losses are controlled by cooling in the rolling train and from the roller contact, so that sets a desired final rolling temperature, for example, 880 ° C. This is followed by a slow cooling in the cooling section and a subsequent coiling.

Both known methods have in common that a slab temperature is set as the inlet temperature in the finishing stand, which is just sufficient to ensure rolling in the last frame of the finishing train.

The invention has for its object to propose a method and a system with which austenitic stainless steels can be produced while saving energy and time.

This object is achieved by a method having the features of claim 1 and a system having the features of claim 11. Advantageous developments are described in the subclaims.

According to the principles of the invention, for the production of hot strip or hot strip made of austenitic stainless steels, the heat treatment to prevent corrosion susceptibility is carried out directly from the rolling heat, ie immediately after the rolling process, taking advantage of the fact that the temperatures in the strip are so high that no Cr carbides have precipitated or that, starting from the rolling temperatures, only very small temperature differences have to be overcome in order to set temperatures for dissolution of the chromium. Overall, the rolled product is no longer solution heat treated in a separate heat treatment step, which includes annealing from room temperature to solution annealing temperature, but using the rolling heat and thus saving the high energy annealing process. The steels can therefore be produced without subsequent separately performed heat treatment, consisting of solution annealing and quenching treatment, while saving energy and time.

According to the invention, this desired relatively high final rolling temperature at the end of the finishing train is achieved by setting a higher inlet temperature of the cast product in the finishing mill of the rolling mill, which is above 1,150 ° C., preferably above 1,200 ° C. Then, the temperature level of the rolling stock despite the temperature gradient during the rolling process is always above the temperature at which Cr carbides could excrete. In order to achieve such inlet temperatures, the cast product is subjected to a multi-stage, in particular two-stage, heating, which comprises a preheating stage and an intensive heating stage.

Preferably, the final rolling temperature of the rolling stock is set at temperatures above 1000 ° C, preferably above 1050 ° C, ie at temperatures at which the carbide precipitating chromium of the chromium-containing stainless steels is in solution. The final rolling temperature should be at a level at which no precipitate Cr carbides, but in which the structure still recrystallized. The term final rolling temperature refers to the temperature of the rolling stock in the last or last stands of the finishing train. Subsequently, preferably in the immediate connection, the rolling stock is quenched to temperatures below 600 ° C, preferably below 450 ° C, with a precipitation of particular Cr carbides is suppressed. Overall, a rolled, already heat treated product is available, compared to a product that is a separate one Solution heat treatment and a quenching process has the advantage of energy and time savings in its production.

Advantageously, in the preheating stage, the temperature of the cast product is set to values between 1000 and 1150 ° C., wherein the temperature is raised to values of above 1200 ° C. only in the subsequent intensive heating zone. The pre-heat stage is preferably carried out in a gas- or oil-heated oven and the subsequent intensive heating stage in an induction oven or in an induction heating zone. This has the particular advantage that the preheating can take place in a roller hearth furnace, while the heating step is shifted to temperatures above 1,200 ° C to an inductive heating zone. This prevents that the roller hearth furnace is charged too much, which could possibly lead to its thermal destruction. In the gas- or oil-heated preheating furnace, the slab temperature is increased to temperatures between 1,000 and 1,150 ° C without exceeding the load capacity of the furnace elements.

In order to avoid adverse effects of a strongly heated primary scale layer on the surface quality of the rolling stock, the casting product surface, in particular the slab surface, is descaled before the setting of the inlet temperature. For this purpose, a Entzunderungs device is provided between the Vorwämstufe and the Intensivheizstufe. The inlet temperature is then set in the inductive intensive heating zone. It is also proposed, in addition or alone, to carry out a descaling even before the roller hearth of the preheating to protect the roles of the furnace from scale and thus the surfaces of the slabs against unwanted scale marks and to improve the heat transfer into the slab.

As a further embodiment for setting the desired high final rolling temperature is proposed that in addition a heating of the rolling stock in the last section of the finishing train, preferably inductively takes place.

This ensures that, at the end of the rolling process, the temperatures of the rolling stock are reliably maintained at temperatures at which recrystallization processes take place.

It is proposed as a further development that the rolling stock with the defined final rolling temperature is passed through a - preferably inductive heating section adjoining the finishing line for further holding at temperatures at which accelerated recrystallization processes take place and only then is quenched. This has the advantage that longer times are provided for desirable recrystallization processes because of the associated strength reduction. This heating section can be used when it is determined that the desired Endwalztemperatur despite high inlet temperatures could not be achieved, for example by an unwanted unfavorable rolling process.

A plant according to the invention for carrying out the proposed method is characterized in that the temperature adjustment means for preheating of the cast product and a device for intensive heating for setting the inlet temperature above 1150 ° C, preferably (T _a) of the cast product into the finishing train of the rolling mill above 1200 ° C to set a desired final rolling temperature (T _we ) to make a heat treatment directly from the rolling heat can.

Here, the means for setting the desired high Endwalztemperatur part of the temperature-adjustment system, ie by setting a high inlet temperature and a high final rolling temperature is set taking into account the temperature gradient during the rolling process. In order to preserve the preheating furnace, which is in particular a roller hearth furnace, such a temperature adjustment system is composed of the preheating device and a subsequent inductive intensive heating zone.

To maintain the final rolling temperature (T _we ) after rolling, a heating zone is arranged downstream of the rolling mill. This heating zone is preferably heated inductively; and temperatures above 1000 ° C can be set. It can also be a tunnel oven.

Fig. 1

eine Anlage zur Durchführung des vorgeschlagenen Verfahrens nach der ersten Ausführungsform;

Fig. 2

eine Anlage nach dem Stand der Technik.

Further details and advantages of the invention will become apparent from the dependent claims and from the following description in which the embodiments of the invention shown in the figures are explained in more detail. In addition to the combinations of features listed above, features alone or in other combinations are essential to the invention. Show it:

Fig. 1

a plant for carrying out the proposed method according to the first embodiment;

Fig. 2

a plant according to the prior art.

Fig. 1 shows a plant for the production of sheets or strips of chromium and nickel alloyed steel grades, which are rolled to room temperature without cooling and heat treated, so that the final product is already solution-annealed and quenched available.

Such a plant 1 comprises a continuous casting plant 2, which is shown here schematically with the aid of a ladle 3 for the molten steel, a distributor 4 and a mold 5. The near-final cast strand or cast product 6 is cut into slabs in front of the roller hearth furnace or preheating furnace 7 with the aid of scissors 8, and these then enter the furnace 7 in order to be heated to temperatures of between 1,000 and 1,150 ° C. to experience a temperature compensation. The heated slabs pass through a descaling device 9, in order then to run into an inductive intensive heating zone 10. Here, the slabs are raised to temperatures in an interval of 1,000-1,300 ° C, preferably above 1,200 ° C, in a short, rapid heating process. The set in the intensive heating zone 10 Temperature must be sufficient to set the desired final rolling temperatures above 1000 ° C. Possibly. It may also be sufficient to heat to temperatures around 1000 ° C., provided that only a very small temperature loss occurs during the rolling process. The preheating furnace 7 and the intensive heating zone 10 form the temperature adjusting system 11. The means for performing the heat treatment are the preheating furnace 7 and the intensive heating zone 10 and the cooling section for rapid cooling.

After passing through the intensive heating zone 10, the hot slabs are again descaled (second descaling device 12) and introduced into the finishing train 13, which here consists of six stands 13a-f. The inlet temperatures are in the temperature range of 1,050-1,250 ° C, preferably at temperatures above 1,200 °. Temperatures of 1050 ° C are also adjustable, provided that the temperature loss in the rolling mill is low and the desired Endwalztemperaturen be achieved. Before the second descaling device 12, an emergency shear 14 is provided for incidents.

During the rolling process, the temperatures of the slabs decrease by radiation and cooling, but do not sink to the end of the rolling train 13 to temperatures below 1000 to 1100 ° C, so that the chromium always remains in solution and no Cr carbides on the grain boundaries can excrete the structure and a complete recrystallization takes place. Subsequently, the rolling stock 15 enters the means for cooling 16 or in a cooling section, the cooling parameters are set so that the rolling stock is cooled rapidly to temperatures in an interval of 400-650 ° C, preferably below 600 ° C, to keep the dissolved Cr atoms in forced solution. The cooling section shown here is cooling bar 17 with water cooling, other types of cooling are also conceivable. Subsequently, the thus rolled and already heat-treated and thus corrosion-resistant tape is reeled in a coiler 18.

Fig. 2 illustrates, by way of comparison, a prior art casting heat rolling apparatus in which the strip must be solution annealed in a separate process. The Fig. 1 corresponding system parts are provided with corresponding reference numerals. In addition, usual slab or strip temperatures which prevail or are set in the individual system parts are mentioned. In such a plant, the cast product 106 is cut and then fed into an equalizing furnace 107 to be subsequently rolled. The held in a separate part of the plant with annealing solution annealing with subsequent quenching process is not shown.

The invention particularly relates to austenitic stainless steels, i. Steels with a mass fraction of at least 10.5% Cr and not more than 1.2% C The invention is particularly directed to stainless steels in which intercrystalline corrosion by Cr depletion upon precipitation of Cr carbides is to be prevented. With the aid of the proposed method it is achieved that stainless steels already present in the solution-annealed state and thus corrosion-resistant after passing through an in-line casting and rolling plant. This saves energy and time and therefore costs. The process chain for the production of stainless corrosion-resistant steels is shortened.

Claims (16)

1. Method of producing hot strip from austenitic stainless steels, wherein in a first step a cast product (6) is subjected to a rolling process in a rolling mill with a finishing train and in a second step a thermal treatment is carried out for preventing susceptibility to corrosion, particularly with respect to an intercrystalline corrosion due to chromium carbide dissociations, characterised in that for setting of the final rolling temperature (T_we) an entry temperature (T_ein) of the cast product into the finishing train of the rolling mill is set to be above 1,150° C, preferably above 1,200° C, by a multi-stage, particularly two-stage, heating, which comprises a preheating stage and an intensive heating stage, and the thermal treatment is undertaken directly from the rolling heat.
2. Method according to claim 1, characterised in that the final rolling temperature (T_we) of the rolling stock (15) is set to a value in which a complete dynamic recrystallisation of the steel still takes place and that the rolling stock (15) after the last pass in the finishing train is quenched from the final rolling temperature (T_we) to a temperature (T_a) such that dissociation of the chromium-carbide is suppressed.
3. Method according to claim 2, characterised in that the final rolling temperature (T_we) of the rolling stock is set to temperatures above 1,000° C, preferably above 1,050° C, and that subsequently the rolling stock is quenched to temperatures (T_a) below 600° C, preferably below 450° C, within 20 seconds.
4. Method according to one of claims 1 to 3, characterised in that in the preheating stage the temperature of the cast product is set to values between 1,000 to 1,150° C and that in the subsequent intensive heating zone the temperature is increased to values above 1,200° C.
5. Method according to one of claims 1 to 4, characterised in that the preheating stage is carried out in a gas-heated or oil-heated furnace (7) and the subsequent intensive heating stage is carried out in an induction heating zone (10).
6. Method according to one of claims 1 to 5, characterised in that a descaling is undertaken between the preheating stage and the intensive heating stage.
7. Method according to one of claims 1 to 6, characterised in that an additional heating of the rolling stock takes place, preferably inductively, in the last section of the finishing train (13) so that during the rolling process the temperature is kept in the region of the dynamic recrystallisation.
8. Method according to one of claims 1 to 7, characterised in that the rolling stock with the defined final rolling temperature (T_we) is led through a heating zone, which adjoins the rolling train, for further maintenance at temperatures at which complete recrystallisation of the rolling stock takes place and is quenched only subsequently.
9. Method according to one of claims 1 to 8, characterised in that the thermal treatment for prevention of susceptibility to corrosion is undertaken coming directly out of the rolling heat at a cast product (6), which is cast close to final size, coming out of the casing heat.
10. Method according to one of claims 1 to 8, characterised in that the thermal treatment for prevention of susceptibility to corrosion is undertaken directly from the rolling heat to a continuously cast rolling product rolled on a hot wide strip rolling train.
11. Plant (1) for producing hot strip from austenitic stainless steels, for carrying out the method according to one of claims 1 to 10, wherein in a first step a cast product (6) is subjected to a rolling process in a rolling mill with a finishing train and in a second step a thermal treatment for prevention of susceptibility to corrosion, particularly with respect to an intercrystalline corrosion due to chromium-carbide disassociations, is undertaken, comprising a continuous casting plant (2) for producing a cast product (6) as well as a rolling mill (13) with an upstream temperature setting system (11) and downstream device (16) for cooling down the rolling stock (15), characterised in that the temperature setting system (11) comprises a device (7) for preheating of the cast product and a device (10) for intensive heating, for setting the entry temperature (T_ein) of the cast product in the finishing train of the rolling mill above 1,150° C, preferably above 1,200° C, for setting a final rolling temperature (T_we) for undertaking the thermal treatment directly from the rolling heat.
12. Plant according to claim 11, characterised in that the device (16) for cooling the rolling stock comprises means (16) for quenching the rolling stock to temperatures below 600° C, preferably below 450° C, for suppression of dissociation of chromium-carbides during the cooling down.
13. Plant according to claim 12, characterised in that this comprises, after the rolling mill, a heating zone for keeping the final rolling temperature (T_we) of the rolling stock to temperatures above 1,000° C, preferably above 1,050° C.
14. Plant according to claim 12 or 13, characterised in that this comprises means for further heating of the rolling stock, during the rolling, for setting a final rolling temperature (T_we).
15. Plant according to one of claims 11 to 14, comprising a continuous casting machine (2) for casting cast products (6) close to final size, a device (8) for cutting off the cast product (6) prior to the temperature setting system (11), selectably a first descaling device (9) between the device (7) for preheating and the device (10) for intensive heating, a second descaling device (12) between the temperature setting system (11) and the finishing train (13), a device (16), which directly adjoins the finishing train (13) or a temperature maintaining furnace, for rapid cooling down, and a device (18) for reeling up the strip or a device for separating and stacking the thermally treated rolled stock.
16. Plant according to claim 11, comprising a temperature setting system for heating continuously cast slabs or billets, optionally a preliminary rolling stand as well as an adjoining hot wide strip rolling train or wire rolling train, a device, which directly adjoins the hot wide strip rolling train or wire rolling train or a temperature maintaining furnace, for rapid cooling down and a device for reeling up the strip or a device for separating and stacking or coiling the thermally treated rolled product.

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