EP3292223B1 - Method for producing thin sheet from a stainless austenitic crmnni steel - Google Patents

Description

The present invention relates to a method for the production of sheet from a stainless, austenitic CrMnNi steel, as well as its use. Furthermore, the invention relates to a stainless austenitic steel.

State of the art

Stainless austenitic steels are used in many ways. Preferably, stretched and deep-drawn components can be produced that are exposed to elevated temperatures and corrosive aqueous media, such as heat exchangers. The efficiency of heat exchangers is determined inter alia by the heat transfer coefficient. This is proportional to the specific thermal conductivity of the material and inversely proportional to the material thickness. Although austenitic steels have poorer thermal conductivity, they are very easy to stretch and deep draw in contrast to ferritic or martensitic steels, and show greater resistance to corrosive media.

Austenitic thin sheets with material thicknesses of less than 1 mm are preferably used in the production of stretched and deep-drawn sheets. It is accepted that austenitic steels are costly to produce because of their relatively high alloy contents, especially nickel. There are numerous attempts to reduce especially the nickel content of austenitic steels while maintaining or improving the good cold workability and corrosion resistance of the austenitic steels. The development of nitrogen- and / or copper-alloyed austenitic CrMnNi steels takes into account these resource efficiency efforts.

The commercially available austenitic steels with the EN material numbers 1.4301 (X5CrNi18-10) and 1.4404 (X2CrNiMo17-12-2) are characteristic CrNi steels for the production of thin sheet from which, among other things components for deep-drawn sheets, such as heat exchangers and / or for Corrosion protection in aqueous and weakly acidic media are manufactured. The production of austenitic stainless sheet with thicknesses of less than 1 mm requires a particularly high cold workability. The cold workability of 1.4301 steel is higher than that of 1.4404 steel. However, due to its increased content of nickel and molybdenum, steel 1.4404 is characterized by a higher corrosion resistance.

A wide variety of nitrogen or copper-alloyed CrMnNi steels are already known from the prior art.

For example, this describes JP 56146862 an austenitic CrMnNi steel containing only low nitrogen contents of less than 0.03%. In addition, the reported steel has a carbon content of less than 0.03%, a silicon content of less than 0.5%, a manganese content of only 2.2 to 3.0%, a chromium content of 14 to 18%, a nickel content of 6.0 to 9.0%, a molybdenum content of 0.15 to 0.5% and a copper content of 1.0 to 3.0%. The addition of more than 0.03% of nitrogen has been dispensed with since the steel contains not only high copper contents but also relatively high nickel contents.

Another example of austenitic CrMnNi steels is in WO 2010/029012 A1 described. In particular, there is disclosed a stainless steel and a cold rolled product made therefrom which has 5 to 15% residual austenite ferrite in the cold rolled state. The steel has 0.05 to 0.14% C, 0.1 to 1.0% Si, 4.0 to 12.0% Mn,> 17.5 to 22.0% Cr, 1.0 to 4, 0% Ni, maximum 0.5% Mo, 0.03 to 0.2% N and 1.0 to 3.0% Cu. For the production of a cold strip from a hot strip, hot forming grades of up to 50% and cold forming degrees of as much as 50% are also desired. According to this document, stress cracks due to martensite formation should be avoided. Therefore, the alloy is modulated to retain 5 to 15 vol% δ-ferrite in the steel. The austenite is thus stabilized. The 0.8 mm thick cold strip produced in this way has a higher strength compared to the 1.4301 cold strip but a lower elongation at break.

From the EP1352982 B1 a steel with a very broad chemical composition is known. From this steel a stress crack-free molded part is produced by cold forming. To avoid stress cracks, martensite formation during cold forming is avoided. In addition, the steel consists of a two-phase structure with ferrite and austenite. The presence of ferrite stabilizes austenite against martensite formation. The δ-ferrite content is at least 15 to a maximum of 40% by volume.

As a rule, cold-formable, austenitic CrMnNi steels are not alloyed with molybdenum for cost reasons or their molybdenum content is less than 0.5%. Only in special applications, especially when a required increased corrosion resistance molybdenum is alloyed.

An example of such a steel is in the EP 1319091 B1 disclosed. The steel described there has the following composition: maximum 5.0% Mo, 0.01 to 0.2% C, 5.0 to 12.0 % Mn, 15.0 to 24.0% Cr, at most 3.0% Ni, 0.10 to 0.60% N and at most 2.0% Cu. In addition, the steel contains 0.30 to 3.0% aluminum and / or 0.50 to 3.00% silicon, wherein the sum of the contents of aluminum and silicon does not exceed 3.00%.

Increasing chromium and molybdenum contents increase the corrosion resistance of austenitic steels. In EP0969113 B1 Such a steel is described. This steel is alloyed with 1.0 to 4.0% copper and does not contain nitrogen as an alloying element.

Chromium and molybdenum promote ferrite formation. The disadvantage is that the Kaltumformvermögen is limited by the ferrite. For the production of austenitic steels, therefore, the sum amount of chromium and molybdenum and other ferrite-stabilizing elements must be limited.

Copper has a similar austenite-stabilizing effect as nickel, but is less expensive. When hot rolling copper-alloyed austenitic steels, however, there is a risk that copper-rich precipitates form, which can lead to hot cracks. This has resulted in limiting the maximum copper content and, in addition, matching the hot forming conditions to the copper content present in the steel. It is known from the prior art that nitrogen-alloyed austenitic CrMnNi steels are in most cases alloyed with copper.

The exception is the AISI 201 L steel. The AISI 201 L steel has a carbon content of less than 0.03%, a silicon content of less than 0.75%, a manganese content of 5.5 to 7.5% and a chromium content of 16, 0 to 18.0%, a nickel content of 3.5 to 5.5% and a nitrogen content of less than 0.25%. The steel is not alloyed with molybdenum, aluminum and copper. The austenitic steel AISI 201 L has a very good combination of strength and toughness properties in the solution-annealed condition, which makes it easy to cold-form. Thus, the 0.2% Mindestdehngrenze are 396 MPa, the tensile strength at 785 MPa and the elongation at break at 56%.

In general, sheet and strip with a thickness of less than 3 mm are called thin sheet. Cold-rolled sheet is predominantly produced in thicknesses of 0.4 to 3.0 mm and in widths of up to 2000 mm from hot-rolled strip with thicknesses of greater than 2 mm. By cold rolling in several cold rolling stages, the hot strip is plastically deformed and thinned. In this case, the material is formed in several passes in each cold rolling stage.

During cold forming, the steel solidifies. Cold work hardening restricts the cold workability, so that after each cold rolling stage an intermediate annealing above the recrystallization temperature of the steel is required. During the intermediate annealing, the material softens and may undergo re-cold-forming.

This interplay of cold rolls and intermediate annealing can be repeated several times. The resulting increasing hardening behavior of the steel puts an end to this process. So far, the achievement of Gesamtumformgraden of up to 1.14 by cold rolling is common. In addition, according to the known method decreases with increasing number of stitches within a cold rolling stage of the degree of deformation.

For austenitic steels, cold rolling technology is specified that does not distinguish between austenitic steels with and without TRIP / TWIP properties. An effective production technology of metastable austenitic steels with TRIP / TWIP properties would be possible, but in practice it is usually not exhausted. Metastable austenitic steels with TRIP / TWIP properties have higher cold workability.

In the DE 10052745 A1 It is disclosed that regardless of the type of cold forming the highest degree of deformation is achieved when the cold forming conditions are adjusted to the chemical composition of the steel, that about 20 to 25% a'-Umformmartensit arise after maximum stress. However, if multi-stage cold forming with intermediate annealing is necessary, as it is necessary for the production of thin sheets of hot strip, so a maximum stress within a cold strip stage prohibits for reasons of premature breakage. The described method is therefore applicable only to cold forming with a cold forming stage. It is unknown what proportion of α'-transformation martensite to strive for in higher cold forming stages in order to exploit the optimum cold workability of the steel.

In addition, in the production of thin sheet of austenitic steel, the risk of edge cracking. The susceptibility to cracking is particularly pronounced in relatively unstable steels. It is additionally reinforced when edge decarburization has occurred during annealing. Glowing under protective gas reduces edge cracking. Ruptured cracks must be removed by trimming the cold strip prior to further processing.

The object of the invention is to provide a process for the production of sheet metal from a nitrogen-alloyed austenitic CrMnNi steel on an industrial scale. Furthermore, a cost-effective nitrogen-alloyed austenitic CrMnNi steel is available be used, which is used for the production of heat exchangers and corrosive stressed components.

This object is achieved with respect to the method by a method for the production of sheets and strips, in particular sheet, of austenitic stainless steel, having the following composition: Mn: 7.6 to 8.7 wt .-%; Cr: 16.50 to 16.99% by weight; Ni: 3.8 to 4.3% by weight; Mo: 0.51 to 1.0% by weight; N: 0.18 to 0.45 wt%, C: <0.04 wt%; Si: <0.5% by weight; P: <0.04 wt%; S: <0.01% by weight; The remainder being iron and unavoidable impurities, the process comprising the steps of: melting the steel by a conventional melting process, casting the molten steel in the billet or ingot, hot rolling the billet or billet into a slab, further processing the slab into one Vorband and then to a hot strip, optionally descaling and annealing of the hot strip and cold rolling to a cold strip, wherein the steel in the solution-annealed condition δ-ferrite shares <3 vol .-% and in the cold-rolled state additionally an α'-martensite portion of up to 50 vol .-% contains.

The process steps during cold rolling and intermediate annealing are specifically tailored to the steel. It has surprisingly been found that the steel with the concrete composition claimed has a higher cold workability with TRIP / TWIP properties compared to the known steels 1.4301 and 1.4404. Here, the cold forming conditions are chosen so that the required Gesamtumformgrad is achieved. The cold rolling stages, including the cold forming passes within a cold rolling stage and the intermediate anneals, may be tuned to minimize the number of cold working stages and intermediate anneals, and not require trimming of the strip, thereby making the process as a whole less expensive. The finished fine strip can be present in the annealed or in the cold-solidified state.

By the method according to the invention, the additional expense of the known method can be reduced, costs and material can be saved and the cost-effectiveness can be increased.

According to a preferred embodiment, the finished sheet may have a thickness of 1.25 to 0.04 mm, i. a thickness that is commonly used in production.

Furthermore, the hot strip before the cold rolling and after descaling in the temperature range between 950 ° C to 1100 ° C are solution-annealed, the holding time at least 10 minutes. The values have proven to be particularly suitable for the process according to the invention.

According to a further embodiment of the invention, it is preferred if the hot strip is subjected to a cold forming at a forming temperature of less than 80 ° C, preferably at 40 ° C. A TRIP and / or TWIP effect is triggered in each cold rolling stage.

Advantageously, the hot strip can be subjected to cold working with an overall degree of deformation φ of up to 4.43, the cold forming being carried out in several cold rolling stages with a degree of cold working of approximately 0.75 each.

In this case, it has proved to be particularly preferred if in each case several passes are carried out with each cold rolling step with an approximately equal degree of cold work of 0.13 to 0.26, preferably 0.15 per stitch.

Furthermore, it has proved to be advantageous if, after each cold rolling stage, a recrystallization annealing under protective gas in the temperature range between 950 ° C and 1100 ° C, preferably at 1050 ° C, is carried out with subsequent cooling.

Advantageously, after the last cold rolling step, the recrystallization annealing can be omitted, which has an advantageous effect on the duration of the process as well as on the process costs.

According to a further preferred embodiment, the thin sheet after cooling may have a recrystallized structure and a passive layer. After annealing under a nitrogen atmosphere as a protective gas and subsequent cooling, the thin sheet may have a recrystallized structure with a passive layer and a passivating chromium nitride layer in the edge region up to 30 μm. Furthermore, after cooling, the thin sheet may have a work-hardened structure with a passive layer with or without a passivating chromium nitride layer. According to the invention, the desired properties of the thin sheet can thus be adjusted in a simple manner.

Here, the sheet may have a 0.2% proof stress of 326 to 390 MPa, a tensile strength of 760 to 780 MPa, an elongation at break of 60 to 70%, a passivation current density in 0.5 M sulfuric acid of 0.013 to 0.017 mA / cm ² , have a current density of 0.0025 mA / cm ² at 400 mV and a breakdown potential at pitting test in 0.5 M NaCl solution of 317 mV.

Preferably, the sheet having a degree of deformation of 0.3, a 0.2% proof stress of 940 to 1070 MPa, a tensile strength of 1187 to 1288 MPa and an elongation at break of 13 to 20%, a Passivierungsstromdichte in 0.5 M sulfuric acid of 0.005 to 0.010 mA / cm ² , a current density of 0.0024 mA / cm ² at 400 mV and a breakdown potential at pitting test in 0.5 M NaCl solution of 307 mV.

In summary, it can be stated that the object according to the invention can be achieved by determining suitable shaping conditions. This concerns in particular the setting of the forming temperature and the forming speed on the one hand and the cooling and lubrication of the rolling stock on the other hand. In each cold rolling step, a TRIP and / or TWIP effect is triggered. The TRIP effect can be demonstrated by the detection of the a'-shaped martensite and the TWIP effect by the detection of twins. The martensite fraction is measured by means of magnetic measurements. The Feritscope is used for nondestructive measurement during ongoing production. These measurement results can be specified below by destructive measuring methods. Magnetic saturation methods using MSAT or the magnetic balance are used for this purpose. For the detection of deformation twins in austenite EBSD measurements are performed.

With respect to the steel, the invention is solved by a austenitic stainless steel having the following composition: Mn: 7.6 to 8.7% by weight; Cr: 16.5 to 16.99% by weight; Ni: 3.8 to 4.3% by weight; Mo: 0.51 to 1.0% by weight; N: 0.18 to 0.45 wt%, C: <0.04 wt%; Si: <0.5% by weight; P: <0.04 wt%; S: <0.01% by weight; Remaining iron and unavoidable impurities.

In contrast to the known nitrogen-alloyed austenitic CrMnNi steels, the steel according to the invention is not alloyed with copper, aluminum, niobium, titanium or vanadium and, unlike AISI 201 L steel, has a higher manganese and molybdenum content. The inventive steel is also characterized by its low nickel content and the addition of nitrogen compared to the previously used austenitic stainless steels 1.4301 and 1.4404.

It has been shown that the inventive steel compared to the steels mentioned has both an improved cold workability and an approximately equal or higher corrosion resistance in aqueous solutions.

The present invention furthermore relates to a thin sheet produced from the steel according to the invention. In this case, the thin sheet is preferably produced in accordance with the method according to the invention.

The thin sheet produced according to the invention can be used with particular preference as a component for deep drawing sheets and stretch drawing sheets, in particular sheets and / or fins in heat exchangers.

Furthermore, the thin sheet produced according to the invention can be used particularly preferably for corrosive stressed components, in particular containers and panels.

It has been shown that the forming properties as well as the passivation behavior and the pitting resistance of the fine strip are similar to or higher than those of the commercially available CrNi steels 1.4301 and 1.4404. The use of the inventive thin sheet thus represents a cost effective alternative to the use of thin sheets of steels 1.4404, 1.4301 and AISI 201 L.

The invention provides a stainless, nitrogen and molybdenum alloyed austenitic CrMnNi steel having higher levels of manganese and lower levels of nickel over the previously used 1.4301 (X5CrNi18-10) and 1.4404 (X2CrNiMo17-12-2) steels. Of the steel AISI 201 L, the innovative steel differs by its increased manganese content and its addition of molybdenum. The inventive steel is not alloyed with copper, such as a variety of new nitrogen-alloyed austenitic CrMnNi steels.

The steel according to the invention is an austenitic steel with a δ-ferrite content of not more than 3% by volume. The alloy components of the inventive steel are chosen so that the structure has a δ- ferrite content less than 3% after the solution annealing. The metastable δ-ferrite reduces with each recrystallization annealing, so that the steel has a δ-ferrite content of less than 1% after about three intermediate anneals. In the cold-rolled state, the structure also contains an a'-martensite content of up to 50%, preferably about 20%. This α'-martensite is a consequence of the α'-TRIP effect induced during cold working. The Martensitanteil is dimensioned so that thereby a high cold workability is possible. This is the prerequisite for reducing the work steps to the finished sheet with respect to the necessary cold rolling stages and intermediate annealing, and producing a thin sheet with adjustable high strength and / or high toughness in the solution annealed and / or cold rolled condition. The inventive steel has a better formability compared to the steels 1.4404, 1.4301 and AISI 201 L.

With its high strength and a nickel content of 3.8-4.3%, the inventive steel consequently represents a cost-effective alternative material in comparison with known steels, which can advantageously be processed by cold rolling to form thin sheet with thicknesses of up to 0.04 mm at present.

Chromium or molybdenum are added as ferrite stabilizing elements, which are added to the inventive steel to maintain and improve corrosion resistance at levels of 16.50 to 16.99% and 0.51 to 1.0%, respectively. If the chromium and molydengths are at the upper tolerance limit, the best corrosion properties are achieved.

In addition, chromium and molybdenum as well as all other alloying elements increase the austenite stability against the formation of α'-deformation martensite. The strain-induced a'-martensite formation during cold rolling is made more difficult. For this reason, the alloying elements of ferrite and austenite stabilizing elements in the invention are coordinated so that under the given cold forming conditions during a cold rolling stage up to 50 vol .-% α'-deformation martensite, preferably about 20 vol -.% Arises.

Carbon and nitrogen are strong austenite formers that make it difficult to form a'-deformation martensite. Therefore, the carbon content is set at <0.04 mass% and the nitrogen content at 0.18-0.45 mass%. In this case, nitrogen contents of up to 0.22% by mass can be added to the steel without special metallurgical measures; otherwise, pressure nitriding is necessary. It is desirable that carbon and nitrogen are dissolved in austenite. Above all, chromium carbide formation during cooling of annealing temperatures is to be suppressed in order to ensure intergranular corrosion resistance.

It should also be noted that carbon and nitrogen atoms in the dissolved state lead to solid solution hardening of the austenite and at the same time to a strain of the martensite. As a result, the strength of the steel increases, while the toughness properties deteriorate.

Silicon supports the formation of ferrite. The inventive steel has silicon contents less than 0.5 mass%.

Manganese as austenite stabilizing element is alloyed in the inventive steel at contents of 7.6 to 8.7 mass%. The manganese contents are raised compared to the manganese content in the steel AISI 201 L. The manganese levels are in a range that is not considered by the metallurgist is viewed critically. Due to the manganese content, the nickel content can be reduced compared to steels 1.4301 and 1.4404.

Since it is possible to dispense with the addition of copper as an alloying element, savings in alloying costs are achieved both with respect to the conventional austenitic CrNi steels and to the embossed copper-alloyed austenitic steels. In addition, the recycling capacity is improved.

Nickel is alloyed to the inventive steel as Austenitbildner. The nickel content is determined only by a narrow concentration range of 3.8 to 4.3% by mass. If these nickel contents are not reached, the formation of a'-cooling martensite can be expected. At the same time, the δ-ferrite content increases above 5% by volume. Both effects lead to a deterioration of the cold workability of the steel. If the nickel contents are exceeded, the austenite is relatively stable. The formation of a'-deformation martensite remains, which also reduces cold workability.

With regard to the strength and toughness properties of the thin sheet, a corresponding quotient m of ferrite-stabilizing to austenite-stabilizing elements must be observed in accordance with the relationship given. The value m should be in the range of 1.4 to 1.5. According to the invention, the properties can be adjusted particularly reliably when m is in the vicinity of 1.46. $$\mathrm{m}=\left(\%\mathrm{Cr}+2\%\mathrm{Mo}+1.5\%\mathrm{Si}\right)/\left(0.3\%\mathrm{Mn}+\%\mathrm{Ni}+15\left(\%\mathrm{C}+\%\mathrm{N}\right)\right)$$

The thin sheet according to the invention with a thickness of 1.25 to 0.04 mm is produced in the following working steps: "smelting of the steel", "pouring of the melt in the strand or cast and production of a strand or ingot", "hot rolling to the slab, to pre-and hot strip "," preparation (descaling / pickling, solution annealing) and cold rolling of the hot strip ".

The step of preparing (descaling / pickling, solution annealing) and cold rolling the hot strip comprises solution annealing and then cold rolling the hot strip to produce cold rolled and / or annealed sheet having a thickness of 1.25 to 0.04 mm. Particularly suitable conditions are defined in the subclaims.

If mechanical descaling has occurred, then martensite may have formed. In this case, it has proved to be useful when the hot strip is solution annealed.

At the same time the setting of a homogeneous austenite with a maximum Gesamtumformgrad of about 4.43 for cold strip is connected.

It is necessary to adjust the forming conditions to the steel. This concerns above all the choice of the forming temperature, the cold rolling stages to be carried out and the necessary intermediate annealing after the cold rolling step. This ensures that in each cold rolling stage, the formation of a'-Umformmartensit and / or deformation twins is recorded. That is, it triggers a TRIP and / or TWIP effect. For the inventive steel the most favorable forming temperature is in a temperature range of 20 ° C to 80 ° C, preferably at 40 ° C. In order to comply with this temperature range, a cooling of the rolling stock is usually required. The forming a'-Umformmartensit is a maximum of about 50%.

In addition, the inventive steel is exposed to several cold rolling stages. The total degree of deformation within a cold rolling stage is 0.75 to 1.00. The required final thickness of the thin sheet determines the number of cold rolling stages. Sheet with a thickness of e.g. 0.1 mm was prepared in 5 cold rolling stages. Thus, the number of cold rolling stages is lower than commercially available austenitic CrNi or CrNiMn steels with the same sheet thickness, so that the production of the thin sheet can be carried out more cheaply. Within a cold rolling stage, the material is cold-formed in several passes. Conveniently, a degree of deformation of 0.15 to 0.26 per stitch should be selected.

After each cold rolling stage is usually an intermediate or recrystallization annealing in the temperature range between 950 ° C and 1050 ° C, preferably at 1000 ° C. This annealing can be carried out under protective gas in a continuous furnace or hood furnace with subsequent cooling in lead bath, water or in air. If the annealing is carried out as a final annealing of the fine strip, then the finished sheet is in the annealed, that is, in the recrystallized state. The surface of the thin sheet has a passive layer.

On the other hand, the intermediate or recrystallization annealing can be carried out after each cold rolling stage in the temperature range between 950 ° C. and 1000 ° C., preferably at 980 ° C., under a nitrogen atmosphere in a continuous furnace with subsequent cooling in lead bath, water or in air. If this annealing takes place as an intermediate and final annealing of the fine strip, then the finished thin sheet is in the annealed, that is, in the recrystallized state, the near-surface region of the thin sheet has passivating chromium nitride precipitates to a depth of 30 μm.

Regardless of the chosen furnace atmosphere during the intermediate or recrystallization annealing, the austenite softens and becomes finer. The austenite is thereby cold formable again. In contrast to the first cold rolling stage with a degree of deformation of 0.71, in which up to 15 vol .-% α'-Umformmartensit are formed, the formation of α'-Umformmartensit is difficult for each additional cold rolling stage. After 5 cold rolling stages and a degree of deformation of 0.73, the fine strip had a Umformmartensitanteil of 3 vol .-%. The carriers of the plastic deformation of the austenite are then preferably induced twins and sliding processes. In these cases, the TRIP effect is replaced by the TWIP effect. The interplay between the TRIP and TWIP effect is the prerequisite for achieving a higher overall degree of deformation unlike the previous procedure.

Annealing under nitrogen atmosphere in the temperature range below 1000 ° C causes the formation of chromium nitride precipitates, which brings both advantages and disadvantages. Thus, on the one hand, the resistance to intergranular corrosion can be increased and, on the other hand, an increase in the strength properties and a decrease in the toughness properties are associated with it. At the same time, the TRIP effect is attenuated by stabilizing the austenite.

The intermediate or recrystallization annealing under protective gas or under nitrogen atmosphere after the last cold rolling stage can also be omitted. In this case, there is a cold consolidated thin sheet with appropriate passive layer. The passive layer of the inventive thin sheet determines decisively the corrosion properties.

The passivating chromium nitride precipitates in the near-surface region also influence the gloss of the surface as well as the mechanical properties of the thin sheet. Compared to thin sheet with an unaffected passive layer, a higher corrosion resistance and a matt, dark gray gloss are registered.

The Passivierungsstromdichte of the steel according to the invention in the solution-annealed state in 0.05 M sulfuric acid 0.013-0.017 mA / cm ² , at a voltage of +400 mV 0.0025 mA / cm ² . The breakdown potential of the pitting test in 0.5 M NaCl solution is 317 mV. The sheet annealed under inert gas has a 0.2% proof stress of 330 to 390 MPa, a tensile strength of 760 to 830 MPa and an elongation at break of 60 to 83%.

The thin sheet annealed under a nitrogen atmosphere has a thickness of 0.27 mm after 4 rolling stages. The 0.2% proof strength is 500 MPa, the average tensile strength is 843 MPa and the breaking elongation 25%. There is a recrystallized austenitic ground structure with chromium nitride precipitates on the edge.

The cold-formed sheet of a thickness of 0.2 mm to 0.6 mm cold formed with a degree of deformation of 0.3, which was not subjected to a final annealing and depending on the number of intermediate anneals under nitrogen has chromium nitrides in the near-surface region between 15 and 30 microns depth, is characterized by an increasing with increasing proportion of chromium nitrite 0.2% proof strength of 940 to 1153 MPa and a tensile strength of 1187 MPa to 1288 MPa. The elongation at break decreases with increasing proportion of chromium nitride from 20% to 14%. According to metallographic investigations, the steel has a cold-strengthened austenitic matrix. The proportion of Umformmartensit is in the work-hardened 0.6 mm thick band 5%, in the cold-worked 0.2 mm thick band about 1%.

The Passivierungsstromdichte in 0.05M sulfuric acid decreases by the Cr nitride formation to 0.0036 to 0.0038 mA / cm ² , at a voltage of +400 mV, the passive current density is 0.0024 mA / cm ² . The breakdown potential of the pitting test in 0.5 M NaCl solution drops to 149 mV compared to the annealed material.

Components that are subject to deep drawing and stretching stresses, such as plates and / or fins in heat exchangers, can be produced economically from the thin sheet produced according to the invention. In addition, the steel according to the invention is suitable for corrosive stressed components that are exposed to aqueous solutions, such as containers and linings.

At the end of the production of the thin sheet, a finishing of the cold strip (dressing, straightening, trimming) takes place. Each of these steps can contain optional work steps according to the system technology and customer requirements.

The present invention will be explained in more detail below with reference to an embodiment, this embodiment is to be understood only by way of example and the invention is not limited to the specific example.

example

To produce the steel according to the invention, firstly alloyed and unalloyed scrap and ferroalloys were melted in an electric arc furnace and a master alloy was made. In a VOD (Vacuum Oxygen Decarburisation) plant, the melt was subjected to a vacuum-oxygen decarburization process to achieve the required allow relatively low carbon content at the same time good chromium application. The VOD process offers optimum conditions for improving the homogeneity and the degree of purity of the melt and, by further alloying, makes it possible to adapt to the steel composition according to the invention.

The molten steel thus obtained had a chemical composition of 0.031 wt% C, 0.211 wt% Si, 8.306 wt% Mn, 16.91 wt% Cr, 3.88 wt% Ni, 0.599 Wt% Mo and 0.186 wt% N, remainder iron and unavoidable impurities.

Subsequently, the melt was poured into a mold rising to a cast block. The dimensions of the mold were 450 mm * 450 mm * 1700 mm. The solidification was due to the chemical composition of the steel primarily ferritic. During cooling to room temperature, the ferrite transforms into austenite. However, it may be possible for residual amounts of ferrite to be present in the austenitic structure. In total, in the context of this example, two blocks with a total tonnage of 5400 kg were poured industrially.

For subsequent hot working, each ingot was heated to 1200 ° C over a period of 8 hours and held at this temperature for 6 hours. This was followed by 19-23 (approximately 20) stitches, i. Rolling, the transformation into a slab with a width of 350-370 mm and 100-110 mm thickness. The forming temperature should not fall below 1000 ° C.

After descaling the slab in a continuous production process in a rolling stand at temperatures of 1200 ° C to 1050 ° C, first a 35 mm thick Vorband and then after reheating in eight hot rolling passes a 3.5 mm thick continuously rolled hot strip produced. The degree of deformation of the individual passes of hot rolling should not exceed 0.35. The Gesamtumformgrad of the hot strip was starting from the pre-strip 2.2. The ferrite content of the hot strip was up to 17 vol .-%.

The subsequent descaling of the hot strip was used to remove the oxide layer formed on the strip surfaces during the hot rolling process. The tape was, if necessary, ground and pickled in a hydrochloric acid bath.

After descaling, austenitizing was followed at 1100 ° C for 10 minutes under inert gas (75% H ₂ /25% N ₂ ) in the flash annealing furnace. The steel was homogenized and the δ-Ferittanteil in steel reduced to a maximum of 3 vol .-%. In addition, this is shaped by this Procedure the residual ferrite content largely, thereby improving the cold workability.

In this case, according to the method of the invention, it is important to avoid subsequent mechanical processing of the hot strip after austenitizing in order to prevent premature microstructural transformation of the metastable austenite into a'-shaped martensite.

The subsequent cold rolling of the hot strip was carried out by the following method steps, according to the inventive method for producing cold-rolled and / or annealed sheet having a thickness of 1.25-0.04 mm.

To obtain a 0.04 mm thick thin sheet, the hot strip undergoes cold forming with a Gesamtumformgrad of 4.43. Depending on the required sheet thickness up to seven cold rolling stages with a degree of deformation of not more than 0.75 were carried out. Within each cold rolling stage, several passes were made with a degree of deformation of 0.15-0.26 per stitch.

The forming speed was between 50 and 100 m / min during rolling in the 20-roll rolling mill and is matched to the steel so that the forming temperature of 60 ° C was not exceeded. This ensures the formation of a'-Umformmartensit and / or deformation twins in each performed cold rolling stage and achieved the desired TRIP / TWIP effect. The cold workability of the steel is improved, which manifests itself in the reduction of cold rolling stages.

After each cold rolling stage an intermediate annealing was carried out under a nitrogen atmosphere in a continuous furnace in a temperature range of 950-980 ° C with subsequent cooling in the lead bath. The final-formed and final-annealed sheet was in the recrystallized state after the annealing treatment. As the number of intermediate anneals increased, chromium nitride precipitations smaller than 1 μm were produced on the surface of the strip down to a depth of 30 μm at the grain and twin boundaries. The resulting passivation layer was positively influenced by chromium nitride precipitations. The expression for this was a matt shiny surface or a gray tint and an increase in corrosion resistance and hardness.

Cold-formed, 0.6 mm thick sheet with a degree of deformation of 0.3 and chromium nitrides in the passivated edge region that was not subjected to a final annealing, was characterized by a 0.2% proof strength of 1030 MPa and a tensile strength of 1187 MPa. The elongation at break was 20%. In the steel 5 vol .-% α'-Umformmartensit was formed.

Similar solidification effects of chromium nitride and reduction of the TRIP / TWIP effect were observed for 0.2 mm thick sheet with a degree of deformation of 0.3 and chromium nitrides in the passivated edge region. The 0.2% proof strength for this sheet was 940 MPa, the tensile strength 1288 MPa and the elongation at break dropped to 14%. The reduction in elongation at break was due to the tendency of some chromium nitrides, which grew under nitrogen during the intermediate anneals, to crack. The steel in this state has work hardened austenite, with a magnetically determined proportion of a 'transformation martensite of <1%.

When the sheet was subjected to a 15 minute final annealing in the temperature range of 1050-1100 ° C, the chromium nitrite precipitates were largely dissolved. The sheet recrystallized in this way had a passive layer which is not affected by chromium nitrides. Sheets having a thickness of 0.6 mm in this state had a 0.2% proof stress of 335 MPa and a tensile strength of 830 MPa. The elongation at break was 83%.

For 0.2 mm thick sheets after annealing under the same conditions, the 0.2% proof stress was 385 MPa, the tensile strength was 760 MPa and the elongation at break was 61%. After metallographic and magnetic measurements using MSAT, the steel exhibited an austenitic, ferrite-free structure. In a subsequent transformation, the TRIP / TWIP effect is triggered again. In the work-hardened microstructure, up to 10% a'-shaped martensite can be detected after the tensile test.

A 0.1 mm thick austenitic fine strip finally annealed at 1100 ° C for 20 minutes had a 0.2% proof stress of 520 MPa and a tensile strength of 866 MPa. The elongation at break was 36%.

The corrosion behavior of the steel according to the invention was tested electrolytically in comparison with the conventional steels 1.4301 and 1.4404. The passivation in 0.05 M sulfuric acid at room temperature, both in the annealed state and in the work-hardened state (φ = 0.3) of the strip thicknesses of 1.25 mm, 0.6 mm and 0.2 mm compared to compact samples of comparative steels faster. The passivation current density of the steel according to the invention in the solution-annealed state in 0.05 M sulfuric acid was 0.013-0.017 mA / cm ² . The passive area was enlarged. The current density in the passivation region was 0.0025 mA / cm ² at +400 mV. The passive layer was thus formed more rapidly and is more durable than the passive layers of steels 1.4301 and 1.4404 with current densities at +400 mV of 0.0030 mA / cm ² and 0.0035 mA / cm ² . The Repassivation region of the steel according to the invention was more pronounced than the repassivation region of the two comparative steels. With chromium nitride precipitates in the edge region, the passivation current density dropped to 0.005-0.010 mA / cm ² despite the work hardened state. The current density in the passivation region was almost unchanged at 0.0024 mA / cm ² at +400 mV.

The breakdown potential of the pitting test in 0.5 M NaCl solution was 317 mV in annealed chromium nitride-free steel strip. Cold-strengthened chromium nitride-containing sheet with a degree of deformation of 0.3 had a breakdown potential of 307 mV. In both states the steel according to the invention was more resistant than the conventional comparison steel 1.4301 with 159 mV in the annealed state and comparable to the properties of the steel 1.4404 with 318 mV in the annealed state. The pitting resistance fell after cold working with a degree of deformation of 0.3 due to the Chromnitridausscheidungen only slightly compared to the annealed state. The steel according to the invention was more resistant than the comparison steel 1.4301, and in the precipitation-free state has a pitting resistance comparable to that of steel 1.4404.

The use of the thin sheets according to the invention can be carried out as a less expensive component wherever, for example, sheets of the material 1.4301 and after examination of the corrosion conditions of the steel 1.4404 are used. These are, for example, containers and panels that are exposed to aqueous solutions.

In particular, the CrMnNi steel according to the invention is particularly suitable as a thin sheet for deep-drawn sheets, for example in heat exchangers, in particular for plates and / or fins in heat exchangers.

Claims (19)

1. Method for producing sheets and strips, in particular thin sheets, from a stainless austenitic CrMnNi steel, having the following composition:

   Mn: 7.6 to 8.7 wt.%;

   Cr: 16.5 to 16.99 wt.%;

   Ni: 3.8 to 4.3 wt.%;

   Mo: 0.51 to 1.0 wt.%;

   N: 0.18 to 0.45 wt.%;

   C: < 0.04 wt.%;

   Si: < 0.5 wt.%;

   P: < 0.04 wt.%;

   S: < 0.01 wt.%;

   rest iron and unavoidable impurities,

   the method comprising the following steps:

   melting the steel by a conventional casting method,

   casting the steel melt in a strand or to an ingot,

   hot rolling of the strand or ingot into a slab, further processing of the slab into a pre-strip and then into a hot strip,

   descaling and annealing of the hot strip and

   cold rolling into a cold strip,

   wherein the steel in the solution-annealed condition has σ-ferrite proportions < 3% by volume and in the cold-rolled condition additionally contains an a'-martensite proportion of up to 50% by volume.
2. Method according to claim 1, characterized in that the finished thin sheet has a thickness of 1.25 to 0.04 mm.
3. Method according to claim 1 or 2, characterized in that the hot strip is solution-annealed before cold rolling and after descaling in the temperature range between 950°C and 1100°C, the holding time being at least 10 minutes.
4. Method according to one of claims 1 to 3, characterized in that the hot strip is subjected to cold forming at a forming temperature of less than 80°C, preferably 40°C.
5. Method according to claim 4, characterized in that the hot strip is subjected to cold forming with a total degree of forming ϕ of up to 4.43, the cold forming being carried out in a plurality of cold rolling stages each with a degree of cold forming of about 0.75.
6. Method according to claim 5, characterized in that a plurality of passes with an approximately equally high cold forming degree of 0.13 to 0.26, preferably 0.15 per pass are carried out in each cold rolling stage.
7. Method according to one of claims 5 to 6, characterized in that a TRIP and/or TWIP effect is triggered at least during each cold rolling stage.
8. Method according to one of claims 5 to 7, characterized in that after each cold rolling stage a recrystallization annealing under protective gas in the temperature range between 950°C and 1100°C, preferably at 1050°C, is carried out with subsequent cooling.
9. Method according to claim 1 to 8, characterized in that no recrystallization annealing is carried out after the last cold rolling stage.
10. Method according to claim 8, characterized in that, after cooling, the thin sheet has a recrystallized structure and a passive layer.
11. Method according to claim 8, characterized in that after annealing under a nitrogen atmosphere as protective gas and subsequent cooling, the thin sheet has a recrystallized structure with a passive layer and a passivating chromium nitride layer in the edge region of up to 30 µm.
12. Method according to claim 9, characterized in that the thin sheet after cooling has a work-hardened structure with a passive layer with or without a passivating chromium nitride layer.
13. A method according to claims 8 and 10, characterized in that the thin sheet has a 0.2 % yield strength of 326 to 390 MPa, a tensile strength of 760 to 780 MPa, an elongation at break of 60 to 70 %, a passivation current density in 0.5 M sulfuric acid of 0.013 to 0.017 mA/cm2, a current density of 0.0025 mA/cm² at 400 mV, and a breakdown potential on pitting corrosion testing in 0.5 M NaCl solution of 317 mV.
14. M method according to claims 9 and 12, characterized in that the thin sheet having a degree of deformation of 0.3 has a 0.2% yield strength of 940 to 1030 MPa, a tensile strength of 1187 to 1288 MPa and an elongation at break of 13 to 20%, a passivation current density in 0.5 M sulfuric acid of 0.005 to 0.010 mA/cm², a current density of 0.0024 mA/cm² at 400 mV, and a breakdown potential on potting corrosion testing in 0.5 M NaCl solution of 307 mV.
15. Stainless austenitic CrMnNi steel having the following composition:

    Mn: 7.6 to 8.7 wt%;

    Cr: 16.5 to 16.99 wt%;

    Ni: 3.8 to 4.3 wt%;

    Mo: 0.51 to 1.0 wt%;

    N: 0.18 to 0.45 wt%;

    C: < 0.04 wt%;

    Si: < 0.5 wt.%;

    P: < 0.04 wt.%;

    S: < 0.01 wt.%;

    rest iron and unavoidable impurities.
16. Stainless austenitic steel according to claim 15, characterized in that the steel in the solution-annealed condition has delta ferrite contents of < 3 vol% and in the cold rolled condition additionally contains an α'-martensite content of up to 50 vol%.
17. Thin sheet produced from the steel according to claims 15 and 16.
18. Use of the thin sheet as a structural element produced according to the method of claims 1 to 14 for deep-drawn and stretch-drawn sheets, in particular plates and/or fins in heat exchangers.
19. Use of the thin sheet produced according to the method of claims 1 to 14 for components subject to corrosive stress, in particular containers and claddings.