EP1469954B2 - Method 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 corrosion susceptibility, in particular with respect to an intercrystalline corrosion due to Chromium carbide precipitates, is performed.

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 keeping the C atoms in forced solution in the matrix. 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.

From the EP 0 415 987 B2 is a process for the continuous production of strip steel or steel sheet from thin strip of about 50 mm thickness produced by sheet casting with horizontal discharge direction with the process steps rolling the thin slab after solidification of the strand in the arcuate guide shaft at temperatures of more than 1100 ° C, temperature drop of Slabs by radiation or Entzundem, inductive reheating to a temperature of about 1,100 ° C and rolling the thin slab in at least one rolling mill known. 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 Steel & Iron, Vol. 2, 1993, page 37ff. Flemming et al., The CSP Plant Engineering and its Adaptation to Advanced Production Programs. In such a system, a thin slab is produced by means of a continuous casting machine with specially designed mold, 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 by 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. Advantageous further developments are described in the subclaims.

According to the basic idea of the invention, for the production of hot strip or hot strip made of austenitic stainless steels, the heat treatment to prevent susceptibility to corrosion is carried out directly from the rolling heat, ie immediately after the rolling process by utilizing the The fact that the temperatures in the belt are so high that no Cr carbides are eliminated 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.

The final rolling temperature of the rolling stock is set at temperatures above 1000 ° C, preferably above 1050 ° C, i.e., at temperatures above 1000 ° C. at temperatures at which the carbide precipitate 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, there is a rolled, already heat treated product that has the advantage of saving energy and time in its manufacture as compared to a product subjected to separate solution annealing and quenching.

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.

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 without cooling to room temperature 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 temperature set in the intensive heating zone 10 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 For comparison, the prior art casting-heat rolling apparatus is to be solution-annealed in a separate process. Of 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.

In particular, the invention relates to austenitic stainless steels, ie steels having a mass fraction of at least 10.5% Cr and at most 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 help of the proposed method is achieved that stainless steel already after the passage of an in-line casting and rolling plant in the solution-annealed condition and thus corrosion-resistant. This saves energy and time and therefore costs. The process chain for the production of stainless corrosion-resistant steels is shortened.

Claims (10)

1. Method for 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 heat treatment is carried out for preventing susceptibility to corrosion, particularly with respect to intercrystalline corrosion due to chromium carbide precipitation, characterised in that for setting of the final rolling temperature (T_we) above 1000°C, preferably above 1050°C, an entry temperature (T_in) of the cast product into the finishing train of the rolling mill is set to be above 1150°C, preferably above 1200°C, by a multi-stage, preferably two-stage, heating, which comprises a preheating stage and an intensive heating stage, and the heat treatment is undertaken directly from the rolling heat.
2. The method according to claim 1, characterised in that the final rolling temperature (T_we) of the rolling stock (15) is set to values at 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 precipitation of the chromium carbide is suppressed.
3. The method according to claim 2, characterised in that the final rolling temperature (T_we) of the rolling stock (15) is set to temperatures above 1000°C, preferably above 1050°C, and that subsequently the rolling stock is quenched to temperatures (T_a) below 600°C, preferably below 450°C, within 20 seconds.
4. The 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 1000°C and 1150°C and that in the subsequent intensive heating zone the temperature is increased to values above 1200°C.
5. The 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. The method according to one of claims 1 to 5, characterised in that a descaling is carried out between the preheating stage and the intensive heating stage.
7. The method according to one of claims 1 to 6, characterised in that an additional heating of the rolling stock in the last section of the finishing train (13), preferably by induction, takes place so that the temperature is maintained in the dynamic recrystallisation range during the rolling operation.
8. The method according to one of claims 1 to 7, characterised in that the rolling stock is conveyed at the defined final rolling temperature (T_we) through a heating zone that follows the finishing train in order to continue maintaining it at temperatures at which complete recrystallisation of the rolling stock occurs, and that it is quenched only subsequently.
9. The method according to one of claims 1 to 8, characterised in that the heat treatment for preventing susceptibility to corrosion is carried out directly using the rolling heat coming from the casting heat on a cast product (6) having near-final dimensions.
10. The method according to one of claims 1 to 9, characterised in that the heat treatment for preventing susceptibility to corrosion is carried out directly using the rolling heat on a rolled product that has been continuously cast and rolled on a hot wide-strip rolling train.

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