EP0198024B1 - Method for producing prestressed steel - Google Patents

Abstract

In a method for producing prestressed steel having high rupture and tensile strength, and corrosion resistance properties, there is provided a fine-grained or mixed-crystal or precipitated-phase hardening, in conjunction with a thermomechanical treatment followed by a work hardening. For the hardening process, there is used a fine-grained or mixed-crystal or precipitated-phase hardening with enhanced additional effect. The mechanical treatment is carried out by means of cylinders of fine-grained alloy steel, thereby excluding the formation of martensite.

Description

The invention relates to a method for producing high-strength, weldable, corrosion-resistant and brittle fracture-proof prestressing steels.

At the moment, prestressing steels are usually made from unalloyed, high-carbon, high-grade structural steels

hot rolled, stretched and tempered bars of dimensions 15 to 40 mm round with a composition of 0.65 to 0.85 C, 0.65 to 0.85 Si, 1.10 to 1.70 Mn, 0.035 S, 0.035 P and optionally 0.10 to 0.40 V as well

patented or style-treated wire rod with dimensions of 5.5 to 14.5 mm round with a composition of 0.60 to 0.90 C, 0.10 to 0.30 Si, 0.50 to 0.80 Mn, 0.035 S and 0.035 P. This is used to make cold drawn tension wire.

In both cases, semi-finished billets of approx. 120 mm 4-kt are used as primary material, which is heat-treated according to different criteria depending on the manufacturer and existing systems, i.e. is brought to the rolling temperature and therefore also has different microstructures and properties, but in the end product it must have the mechanical properties customary for registration certificates.

These prestressing steels have the considerable disadvantage that they cannot be welded. Conventional processes are used to manufacture them, such as the well-known Siemens Martin, electric furnace or oxygen inflation process, with the steel being treated neither before nor after. If at all, steel pretreatment by desulfurization and steel post-treatment by vacuum treatment take place, if at all. Block and continuous casting are still used as the casting process.

In addition to the lack of weldability, these known prestressing steels have deficiencies with regard to their mechanical properties, susceptibility to corrosion and, in particular, insensitivity to brittle fracture, in spite of hardly any changed conceptions with regard to their chemical composition, structure and manufacturing conditions. A fact that has so far been overlooked in the assessment of prestressing steel is that the susceptibility of prestressing steel to brittle fracture can begin considerably above 0 ° C and increases rapidly at lower temperatures. The safety against brittle fracture is expressed with the so-called transition temperature for possible brittle fracture. Conventional prestressing steels usually have a door significantly above + 20 ° C !. Since temperatures of up to -40 ° C and more can occur regularly in most prestressing steel structures for months, especially in the case of bridge substructures, this fact must be taken into account when designing and developing prestressing steels. The susceptibility to brittle fracture is largely due to the internal degree of purity, oxidic and sulfidic inclusions and inclusion forms, which can today be largely improved by targeted steel aftertreatment. Then there is the susceptibility to brittle fracture and, above all, its temperature dependence in the closest connection with the pearlite (cementite) part in the steel, i.e. with the carbon content that has the greatest negative influence. To date, there are no low-pearlite, i.e. low carbon prestressing steels.

Corrosion occurs in prestressing steel in a variety of forms, be it trough, hole, gap, intercrystalline and transcrystalline corrosion. Especially. Attention should be paid to stress corrosion cracking. The corrosion-inhibiting properties of copper are known, but copper has so far not been used as an alloying element in prestressing steels.

So far, it has not been possible to manufacture high-strength, at the same time corrosion-resistant and at the same time brittle fracture-proof prestressing steels which are also suitable for welding. The inventor has set himself the task of developing such a prestressing steel and at the same time a method for its production.

• 0,05 bis 0,20 Massen-% Kohlenstoff
• 1,20 bis 1,70 Massen-% Mangan
• 0,30 bis 0,50 Massen-% Silizium
• 0,04 bis 0,06 Massen-% Niobium
• 0,035 bis 0,05 Massen-% Vanadium
• 0,30 bis 0,50 Massen-% Molybdän
• 0,30 bis 2,00 Massen-% Kupfer
• 0,04 bis 0,60 Massen-% Aluminium
• 0,015 bis 0,02 Massen-% Stickstoff
• ≤ 0,030 Massen-% Phosphor
• ≤ 0,020 Massen-% Schwefel

A method in which a steel consists of leads to the solution of this task

• 0.05 to 0.20 mass% carbon
• 1.20 to 1.70% by mass of manganese
• 0.30 to 0.50 mass% silicon
• 0.04 to 0.06% by mass of niobium
• 0.035 to 0.05 mass% vanadium
• 0.30 to 0.50 mass% molybdenum
• 0.30 to 2.00 mass% copper
• 0.04 to 0.60 mass% aluminum
• 0.015 to 0.02 mass% nitrogen
• ≤ 0.030 mass% phosphorus
• ≤ 0.020 mass% sulfur

is subjected to a thermomechanical treatment, which takes place after solidification from the melt and reheating from a second heat, the steel being kept in a first stage at the lowest possible reheating temperature (= second heat below 1150 ° C.) before the thermomechanical treatment and then a controlled rolling of the steel with a low number of passes with a high degree of deformation (10-45%) and a high deformation speed is carried out to a low forming temperature close to 850 ° C.

The reason for keeping the steel at the lowest possible reheating temperature is that vanadium and niobium dissolve at 850 ° C and 950 ° C, but are dissolved again above 1150 ° C. The latter should be avoided. A particle size of 100-200 Å and a particle quantity of 20 x 10 ⁶ per mm ^{2 should be achieved} for the intended purpose.

This is followed by controlled rolling of the Steel with a low number of stitches with a high degree of forming (10-45%) and a high forming speed down to the lowest possible forming temperature, which is close to 850 ° C. This temperature limit must be observed due to the copper present in the steel, since an effective solidifying deposition of copper can only be achieved by accelerated cooling from approx. 850 ° C to around 650/550 ° C without rolling and it is known that at one temperature at 850 ° C there is no longer any precipitation of copper when rolling.

This first stage of the thermomechanical treatment produces wire rod grades for the production of cold drawn wire, three-wire strands, seven-wire strands and tension rods, which correspond to the Euro standard 138 in their properties, however, the additional usage properties (corrosion-resistant, brittle-break proof and weldable). There is no need for costly cold forming (stretching) and subsequent tempering for tensioning rods, which already means a considerable advantage of the invention.

The actual hardening processes of the hardening mechanisms used here take place especially during the range between 850 ° C. and a dwell time which should be close to the Ar ₃ limit. In a further process step according to the invention, there is an accelerated cooling to about 650/550 ° C. without rolling, which results in a lowering of the ole-a conversion with a simultaneous recrystallization delay.

When using stages 1 and 2 of the method according to the invention, strength classes of tempered wire or parlor in accordance with Euro standard 138 are achieved, and indeed without the costly tempering and tempering, a further advantage of the method according to the invention based on the chemical composition according to the invention. The tensile strength of the tension rods and wire rod is increased by at least 20% compared to conventional grades, from which cold-drawn wires and strands can be produced with correspondingly increased strength properties.

According to the invention, a third stage of the treatment can also be provided, in which, starting at about 650/550 ° C., rolling is again carried out in a controlled manner with one or a few passes, that is to say with a high degree of deformation at high speed. In the following, a dwell time and a delayed cooling down, for example in still air, are considered. As a result, an increased precipitation hardening process results in an increase in strength of over (at least) 40% compared to conventional prestressing steels. The accompanying diagram serves to illustrate this process sequence.

After the thermomechanical treatment and the hardening mechanisms that take place with it, the steel can also be strain hardened, provided that higher strength classes are aimed for or are necessary.

In the course of the thermomechanical treatment in accordance with the present invention, the mechanisms of increasing the strength due to the chemical composition and the targeted metering of the microalloying elements interact in an additive manner. These mechanisms are in particular fine grain hardening, mixed crystal hardening and very particularly precipitation hardening, in which the alloying element copper is particularly effective. This means that the thermomechanical treatment, along with the chemical composition for melting and hardening fine grains, is the most important step in achieving the desired goal, namely for the production of high-strength, corrosion-resistant, brittle-breakable and weldable prestressing steels. The dosage of the alloy elements is designed so that not only the strength is increased considerably, but also the toughness is increased at the same time, in particular through the fine grain hardening. The targeted metering of the alloying elements also ensures that the highest degree of solidification takes place via precipitation hardening. Elimination in ferrite is the most effective for increasing strength.

Because precipitation hardening, in particular, achieves the highest effect of strengthening due to the accelerated cooling and a low final rolling temperature with a high degree of deformation and high deformation speed with subsequent dwell time after the final deformation and delayed cooling, this phase of the thermomechanical treatment is also the highest Importance, because the targeted dosing of the alloying elements also achieves the highest safety against brittle fracture, in particular through the interaction of the elements manganese and molybdenum.

Precondition for an effective increase in strength in the sense of the invention is furthermore fine-grain hardening, fine-grain melting being necessary to achieve it optimally, which simultaneously increases toughness. The grain size to be achieved according to ASTM 112 should be at least 9, but if possible at least 12, to which an increased manganese content of 1.45% on average contributes.

For this purpose, an austenite grain that is as fine as possible should already be sought, since this also determines the size of the ferrite grain. For this purpose, it is necessary that the microalloying elements provided in the directional analysis, in particular aluminum, nitrogen, niobium and vanadium, are used to inhibit grain growth and to form strengthening obstacles to the dislocations through fine precipitations in the austenite structure. A particle size of 100 to 200 Å is most effective for this, the particle quantity per mm ² approx. 20. 10 ⁶ should be.

• 1. eine Stahl-Vorbehandlung wobei eine weitgehende Entschwefelung angestrebt wird. Dies geschieht z.B. durch Calcium-Behandlung CAB, beispielsweise durch das TN-Verfahren.
• 2. Eine Stahl-Nachbehandlung, wobei insbesondere an ein Inertgasspülen, ein Vakuumbehandeln, ein Desoxydieren, Vollberuhigung, sowie nach Möglichkeit und Maßgebe an ein Einschlussmodifizieren und/oder eine Pfannenbehandlung mit metallischem Calcium oder Calciumhalogenid-Schlacken gedacht ist.

According to the invention, the fine-grain melting should include the following stages:

• 1. a steel pretreatment, where extensive desulfurization is aimed for. This is done, for example, by calcium treatment CAB, for example by the TN method.
• 2. A steel aftertreatment, in particular with inert gas purging, vacuum treatment, deoxidizing, full sedation, as well as, if possible and relevant, modification of inclusion and / or pan treatment with metallic calcium or calcium halide slags.

Continuous casting should be a suitable type of casting. Continuous casting is the most economical and, at the same time, the best quality of casting and solidifying the molten steel into the primary material used to manufacture prestressing steel: billets. To ensure a high * level of quality required for prestressing steel, special measures must be taken to prevent such defects, such as e.g. Protection against reoxidation, concealed casting, electromagnetic stirring.

The low carbon content of 0.05-0.20% provided in the directional analysis largely prevents the occurrence of the abovementioned faults and at the same time favors the economy of continuous casting for the production of prestressing steel grades by the costly measures to a greater extent, such as for the conventional high-carbon prestressing steel grades are not required, while at the same time ensuring a high degree of purity, homogeneity and quality.

• nulldimensionale. Dies sind punktförmige Hindernisse wie Fremdatome im Mischkristall. Steigerung der Festigkeit durch Mischkristallhärtung.
• eindimensionale. Dies sind linienförmige Hindernisse als Versetzungen. Verfestigung durch Kaltverformen.
• zweidimensionale. Dies sind flächenförmige Hindernisse als Korngrenzen. Verfestigung durch Kornverfeinerung.
• dreidimensionale. Dies sind räumliche Hindernisse als Ausscheidungen. Verfestigung durch Teilchenhärtung oder Dispersionshärtung.

In order to take account of the first sub-task, namely the increase in the strength of prestressing steel, the most important factors influencing the strength must be taken into account. This includes in particular obstacles to dislocation movements. The structure has a particularly high influence on the strength properties of prestressing steels, since the presence or formation of obstacles to the displacement movement must be present in order to achieve any kind of strength increases. These obstacles can be divided into their dimensions

• zero dimensional. These are punctiform obstacles like foreign atoms in the mixed crystal. Increased strength through mixed crystal hardening.
• one-dimensional. These are line-shaped obstacles as dislocations. Strain hardening by cold working.
• two-dimensional. These are surface obstacles as grain boundaries. Solidification through grain refinement.
• three-dimensional. These are spatial obstacles as excretions. Solidification through particle hardening or dispersion hardening.

Mixed crystal hardening works through the type of chemical composition, whereby the influence of the foreign atoms in substitution mixed crystals and the interstitially dissolved foreign atoms is of particular importance. There are numerous diagrams and tables from which the individual alloy elements and their effect on increasing the yield point can be read. The influence of the various alloy elements can be explained by the distortion that these elements cause in the lattice. The greater the distortion, the higher the increase in strength.

Of all four types of hardening, fine grain hardening has to be given the most consideration because the hardening mechanism resulting from it is characterized not only by an increase in strength but also a simultaneous increase in toughness. Furthermore, the two-dimensional obstacles to moving dislocations are such strong obstacles that they cannot be overcome by them. The dislocation has then become impossible and numerous dislocations form a build-up at the grain boundary, which results in a significant stress concentration and therefore an influence on the strength. However, the average grain size influences the lower yield strength.

In the case of particle hardening by precipitation, it must be emphasized that the highest degree of solidification is given when the particle size and the particle spacing are just large enough that cutting does not occur. The precipitation processes for particle hardening are strongly influenced by the degree of supersaturation, the deformation, the conversion and ultimately the recrystallization, which must be given special attention below in the thermomechanical treatment to increase the strength. When developing high-strength prestressing steels, the precipitation of carbides, nitrides or carbonitrides by particle hardening must also be taken into account. It must also be noted that the precipitation of special carbides or carbonitrides of the micro-alloy elements niobium and vanadium has a specifically higher hardening effect than, for example, copper deposits. According to the invention, these individual hardening mechanisms can be combined with one another and with a specific work hardening, their effect being additive, but their respective proportions can change considerably depending on the specified conditions. According to the invention, however, it was found that the basic mechanisms of the individual hardenings can only be achieved by a further, the most important treatment _- step, become optimal, namely through the so-called thermomechanical treatment.

• Verformungs-Temperatur,
• Verformungs-Grad,
• Verformungs-Geschwindigkeit,
• Verformungs-Zeitpunkt,
• Endverformungs-Temperatur,
• Abkühlungsgeschwindigkeit,
• Umwandlung ö-a,
• Verweilzeit nach der Verformung sowie anschliessende Abkühlung
  jede für sich eine bedeutende Rolle spielen im Hinblick auf die optimale Verbesserung der Stahleigenschaften. Durch eine thermomechanische Behandlung können praktisch alle Kennwerte der mechanischen Eigenschaften beeinflusst werden, insbesondere aber Festigkeits- und Zähigkeits- Eigenschaften sowie die Uebergangs-Temperatur und damit die Sprödbruch-Unempfindlichkeit.

A number of specially controlled shaping processes are to be subsumed under the term thermomechanical treatment, in which the influencing variables

• Deformation temperature,
• Degree of deformation,
• Deformation speed,
• Deformation time,
• Final deformation temperature,
• Cooling rate,
• Conversion ö-a,
• Dwell time after deformation and subsequent cooling
  each plays an important role in terms of the optimal improvement of the steel properties. A thermomechanical treatment can influence practically all characteristic values of the mechanical properties, but in particular strength and toughness properties as well as the transition temperature and thus the insensitivity to brittle fracture.

The thermomechanical treatment within the scope of the invention is carried out by a very specific sequence of controlled rolling of the microalloyed and fine-grained steel specifically developed for this purpose, in particular a low final rolling temperature, rapid cooling before the last rolling pass and a high degree of final deformation, so that recrystallization occurs leads to the finest possible austenite grain before the ferrite-pearlite transformation. In the controlled rolling of the steel according to the invention after directional analysis in the sequence according to the invention, in the case of this microalloyed steel, the rolling process additionally results in precipitation of carbides, nitrides or carbonitrides, as well as solid-crystal and fine-grain and particle hardening. In addition, the temperature control is controlled by alloying and rolling technology in such a way that the Ö-a conversion takes place shortly before and / or after the lowest possible final rolling temperature which comes to be just before A, ₃ . In any case, the formation of martensite should be excluded.

It is important for low-pearlite, micro-alloyed steels that the carbides and nitrides of the micro-alloying elements niobium and vanadium have face-centered cubic lattices, as well as being isomorphic and therefore completely miscible. However, the highest strength-increasing effect through the aforementioned hardening mechanisms is effective in the body-centered cubic lattice. The shape and size of the carbonitride precipitate must also be taken into account. The particle size and quantity, or the particle spacing, as well as the shape and arrangement of the precipitates and their strength itself are decisive for influencing the mechanical properties.

These variables are influenced by the chemical composition and above all by the temperature-time conditions under which the precipitates form. Depending on the temperature, they can precipitate the carbonitrides in austenite, during the o-a transformation or in ferrite. Elimination in ferrite is the most effective for increasing strength. The kinetics, the extent and the temperature of the precipitations depend not only on the thermodynamic conditions, but also on the diffusibility of the alloy atoms, the degree of hypothermia and the germination conditions of the precipitates.

The practical measures that have to be used to optimize the proportion of fine grain hardening in connection with the thermomechanical treatment required for this development are in the case of the alloy according to the invention:

Low, fine-grain pusher furnace temperature, in particular to prevent or limit the redissolution of carbide, nitride and / or carbonitride precipitates,

High degree of deformation with few sequences,

Low forming temperature,

Lowering the b-α conversion temperature by accelerated cooling and / or by alloy and / or

Recrystallization delay.

In the thermomechanically treated state, the optimally cheapest (finest) grain size is achieved with all the advantages in terms of strength increase and at the same time the most favorable effect on toughness properties and transition temperature.

• eine hohe Umformgeschwindigkeit und -grade,
• eine schnelle und gesteuerte Abkühlung vor dem letzten Walzstich und
• eine anschliessende verzögerte Abkühlung,
• abgestimmt auf die Herstellung von thermomechanische behandelten. Stäben oder Walzdraht für die Herstellung von kaltgereckten Spanndrähten und Litzen daraus.

The proportions of fine grain hardening and hardening and thus a significant proportion of the possible increase in strength are very much determined in the present invention by the manufacturing conditions, ie the thermomechanical treatment. This requires for this purpose

• high forming speed and grade,
• a quick and controlled cooling before the last roll pass and
• a subsequent delayed cooling,
• matched to the manufacture of thermomechanical treated. Rods or wire rod for the production of cold-drawn tension wires and strands made from them.

The final rolling temperature and the degree of deformation, particularly in the last pass, are decisive for the mechanical properties that can be achieved. As the final rolling temperature drops, the pearlite content decreases, which means that low-carbon, micro-alloyed microstructures have only a small, often no pearlite content in the structure in a controlled final-rolled state. The mechanical properties experience an additional favorable influence.

With a higher stitch decrease and a smaller number of stitches, smaller austenite grain sizes are achieved, which over a correspondingly smaller one Ferrite grain gives more favorable mechanical properties. Increasing stitch decreases of 10 to 45% have a particularly favorable effect on a finer ferrite grain size and then on a noticeable improvement in the transition temperature or the resistance to brittle fracture. Stitch acceptance and final rolling temperature as well as any holding times must be matched to the desired properties and dimensions of the end products prestressing bars for the production of cold-drawn bars and wire rod for the production of cold-drawn wires, in order to guarantee the desired metallurgical effect on the one hand and a rolling process economically on the other. The decisive factors influencing the mechanical properties that can be achieved are rapid rolling and cooling after finish rolling. A low temperature affects the ferrite grain size on the one hand as a result of the above-mentioned conversion shifted by accelerated cooling to lower temperatures, and on the other hand the precipitation processes taking place in the subsequent slow cooling are considerably supported.

The recrystallization, the Öa conversion and the excretion of microalloying elements are decisive for the structure formation that results in prestressing steel. These processes can run side by side in a very short time of a few minutes and also influence each other. For these reasons, it is necessary for the development of high-strength, corrosion-resistant and brittle-fracture prestressing steels, based on mechanical properties and dimensional ranges, to record and optimize the processes taking place and their assignment to the resulting microstructure and the properties caused by them.

The last stage of the process following the thermomechanical treatment is work hardening, which consists in particular of stretching or drawing. Through this subsequent cold working, which is used to manufacture all prestressing steels and for which the steels of the new design are particularly well suited, a considerable increase in strength is achieved compared to today's prestressing steel grades by means of the degree of deformation to be used.

In order to further improve the properties of the prestressing steels according to the invention in connection with the thermomechanical treatment, the additions of microalloying elements can be ascribed. Of possible microalloying elements, niobium has the most effective influence on fine grain hardening and hardening through thermomechanical treatment, i.e. the strength increase, followed by vanadium. The same applies to the improvement of the transition temperature.

Microalloying with niobium and vanadium increases the strengthening proportion of the manganese and silicon content with increasing levels while at the same time reducing pearlite.

An increase in the nitrogen content in the presence of vanadium causes an additional increase in the yield strength. This also increases the tensile strength, so that an increase in the yield strength ratio, which is particularly important for prestressing steels, is brought about from around 70% to 90%.

With steel, niobium alloys result in a much larger proportion of fine-grain hardening than hardening and therefore not only a higher yield strength than with a titanium or vanadium alloy, but above all, as already mentioned, a very favorable low transition temperature. The high ratio of fine grain hardening to hardening due to the addition of niobium is therefore a major reason why niobium must be used here, since niobium also brings about the greatest reduction in the transition temperature.

With regard to the improvement of the transition temperature or the insensitivity to brittle fracture, it must be stated that by adding niobium and vanadium there is a connection between the increase in the yield point and the improvement in the transition temperature regardless of the microalloying elements. With the same yield point but different niobium or vanadium contents, almost the same insensitivity to brittle fracture or transition temperature is achieved.

Manganese and silicon at levels below about 0.5% also shift the transition temperature to lower temperatures.

In addition to solidification, grain refinement also leads to a significant improvement in toughness, which is manifested in a sharp reduction in the transition temperature. In addition, the desired influence is reinforced by a decreasing percentage of pearlite. Low-pearlite steels are therefore generally particularly insensitive to brittle fracture with fine ferrite grains.

Particular attention must also be paid to the chemical composition of the prestressing steels with regard to their cold forming properties. The sulfur content plays a decisive role in the anisotropy of toughness, the most important factor influencing cold formability. A desired lower sulfur content, ie a reduced number of sulfide inclusions, significantly improves the toughness with regard to constriction of the fracture, a property that is particularly important for prestressing steels. In addition, the reduction in the sulfide length is particularly effective for a more favorable constriction of the fracture. Strong desulphurization can be achieved by adding calcium, which is common in ladle metallurgy, whereby the high vapor pressure of calcium, which is 1.86 bar at a melt temperature of 1600 ° C, and its high pressure Special attention must be paid to oxygen affinity, ie measures must be taken to prevent the evaporation of calcium. Even with sulfur contents of 0.008%, no more manganese sulfides are found in aluminum-calmed steels, but spherical inclusions made of calcium and aluminum oxides, which contain small amounts of sulfur in solution on their surface. The favorable conditions of calcium aluminates with regard to separation from the melt also improve the degree of oxidic purity. The mechanical properties that can be achieved with calcium treatment show a significantly reduced spatial anisotropy of the toughness properties. The constriction of the fracture, which is so important for ensuring the quality values for prestressing steels, improves considerably as a result of the calcium treatment and as the sulfur content decreases. Desulphurization should be carried out to below 0.020% by mass if possible.

With regard to the most suitable combined use of micro-alloy elements, molybdenum-niobium-alloyed structural structures give the best properties. An additional improvement in the properties is achieved by the combination of niobium-vanadium-molybdenum-copper with simultaneous thermomechanical treatment according to the invention, the best results being achieved by using a low final rolling temperature and the highest possible degree of final deformation.

In addition to the consequences of thermal treatment, the speed of rolling and cooling and cooling in bed also have an effect on the production of prestressing steels. Both strength and toughness improvements are found down to 750 ° C. The effectiveness of the mechanisms which are responsible for the increase in strength is increased considerably by alloying with molybdenum and by regulating the rolling speed with the purpose of reducing the 6-a conversion as far as possible in the range between 650 and 550 ° C Area in which the strengthening mechanisms are most effective, especially through precipitation hardening.

However, the most effective way to achieve optimal mechanical properties is to produce a fine grain. The refinement of the grain size leads to an increase in the yield strength with a simultaneous improvement in the transition temperature. In practice, an austenite grain that is as fine as possible is sought, since this also determines the size of the ferrite grain. As a general empirical value, a reduction in the austenite grain size has a factor of around 0.3 that affects the reduction in the ferrite grain size. The essential process in the growth of the austenite grain is not the dissolution of the excretions, but their aggregation into large and thus effective particles.

One measure for controlling the austenite grain size is the incorporation of fine precipitates in the austenite structure, which inhibits grain growth. In addition to aluminum, which produces this effect via aluminum nitride, it is primarily the microalloying elements niobium and vanadium in particle sizes from 100 to 200 Å that have a comparable effect via their carbides, nitrides or carbonitrides. The most favorable conditions for preventing the sharp increase in grain growth when reheating in the pusher furnace for rolling show higher aluminum contents (up to 0.050%) and nitrogen contents (up to 0.020%). With increasing niobium content, the start of the sudden grain growth is also shifted to higher temperatures.

Another measure for preventing or restricting the re-dissolution of such precipitates when heating before rolling is the lowest possible pusher furnace temperature. Furthermore, the austenite grain can also be refined by higher degrees of deformation. The grain refinement effect is most pronounced at low final deformation temperatures.

If the transformation 5-a is shifted to lower temperatures by an accelerated cooling, the lower transformation temperature causes a higher nucleation frequency and a lower mobility of the grain boundaries, which results in a reduction in the ferrite grain size.

In addition to grain refinement, there is the possibility of delaying the recrystallization of the austenite. Portions of non-recrystallized austenite are then deformed during the final rolling temperature, which results in elongated grains and thus greatly enlarged austenite grain surfaces. The transformation of this structure in the ferrite-pearlite stage results in a strong grain refinement due to the increased germ density and the inhibited growth of the grains formed from these germs.

In addition to controlling the cooling rate, alloying small amounts of molybdenum to the micro-alloyed, low-pearlite structure can also favor the delay in austenite recrystallization, which shifts the--a conversion to lower temperatures. It is precisely this possibility that is used in thermomechanical treatment, as a result of which an even more fine-grained structure is achieved with a simultaneous additional improvement in the transition temperature.

The fact that the favorable transition temperature for niobium and vanadium or niobium plus vanadium-alloyed structural structures remains unchanged or even improves can be explained by a larger proportion of the grain refinement. The grain refinement therefore brings about in addition to solidification, the significant improvement in toughness likewise sought in the present invention, which at the same time manifests itself in a sharp reduction in the transition temperature. In addition, this desired influence in this development is reinforced by a decreasing percentage of pearlite. Low-pearlite structures are therefore generally particularly insensitive to brittle fracture in the case of fine ferrite grains.

In the connection between the microalloy components and the fine grain hardening, it must be taken into account that incoherent niobium and vanadium carbonitrides act in different particle sizes and quantities on the ferrite grain size. In the thermomechanically treated state, vanadium only causes a slight grain refinement. The basic composition plays a role in that higher carbon and nitrogen contents result in a finer secondary structure via a stronger or faster excretion before or during the Ö-a conversion. It should also be noted that the optimal grain refinement due to niobium contents between 0.04 and 0.10% is equally effective, but that of vanadium is also increasingly effective with increasing contents.

The carbon and nitrogen content of the steel influences the ferrite grain size significantly less in steels with niobium than in those with vanadium. With decreasing carbon contents, the influence of nucleation due to separated particles on the grain size decreases in favor of a very pronounced and, in the present case, desirable recrystallization inhibition due to dissolved niobium. Low-pearlite steels therefore have smaller ferrite grain sizes in the thermomechanically treated state than steels with a higher carbon content.

Dissolved vanadium or niobium or titanium cause a further fine grain effect by delaying the austenite transformation desired here. Rising manganese contents also lower the transition temperature, ensure optimal particle separation and thus the optimal effect of particle hardening.

In addition to the time-shifted austenite transformation, there is usually a delay in recrystallization, i.e. the recrystallization takes place later at lower temperatures, which is due to the requirement

Lowering the Ö-a conversion,

• der Einstellung einer möglichst niedrigen Endwalz-Temperatur
  entgegenkommt und gleichzeitig die optimale Ausscheidung, beispielsweise von Kupfer, ermöglicht, wobei zusammenwirkend eine maximal mögliche Festigkeitssteigerung stattfindet. Eine weitgehende Gefügeverfeinerung tritt dabei infolge erhöhter Keimdichte und Wachstumsbehinderung der neugebildeten Ferritkörner ein.

Recrystallization delay and thus

• the setting of the lowest possible final rolling temperature
  accommodates and at the same time enables the optimal elimination, for example of copper, interacting to achieve the maximum possible increase in strength. Extensive microstructure refinement occurs due to the increased germ density and growth hindrance of the newly formed ferrite grains.

With regard to precipitation hardening, it must be taken into account in connection with thermomechanical treatment that the hardening maxima occur in the temperature range between 550 and 650 ° C. This can be explained by the effect of the chemically non-detectable coherent precipitates (clusters) of niobium, carbon and nitrogen atoms that precede the incoherent precipitate. After the curing maximum has been reached, the waste of the stretching unit is important. This drop is caused by rising temperatures or exceeding the holding time and is due to the reduction of the coherent stresses when the coherent particles transition into incoherent and the subsequent increase in the particle diameter and quantity.

• Kohlenstoff 0,05 bis 0,20 Massen-%
• Mangan 1,20 bis 1,70 Massen-%
• Silizium 0,30 bis 0,50 Massen-%
• Niobium 0,04 bis 0,06 Massen-%
• Vanadium 0,035 bis 0,05 Massen-%
• Molybdän 0,30 bis 0,50 Massen-%
• Kupfer 0,30 bis 2,00 Massen-%
• Aluminium 0,04 bis 0,06 Massen-%
• Stickstoff 0,015 bis 0,02 Massen-%
• Phosphor ^:5 0,030 Massen-%
• Schwefel S 0,020 Massen-%

According to the invention, a steel which has the following alloying elements in its directional analysis is to be used as the starting material for carrying out the method according to the invention:

• Carbon 0.05 to 0.20 mass%
• Manganese 1.20 to 1.70 mass%
• Silicon 0.30 to 0.50 mass%
• Niobium 0.04 to 0.06 mass%
• Vanadium 0.035 to 0.05 mass%
• Molybdenum 0.30 to 0.50 mass%
• Copper 0.30 to 2.00 mass%
• Aluminum 0.04 to 0.06 mass%
• Nitrogen 0.015 to 0.02 mass%
• Phosphorus ^: 5 0.030 mass%
• Sulfur S 0.020 mass%

About the individual elements:

The carbon content causes a substantial solidification via the cementite (pearlite) and plays an important role in this connection. However, since the carbon content via the pearlite component has the most significant negative influence on the brittle fracture safety (transition temperature) also specified in this development and on the weldability, and increasingly with increasing pearlite component, the carbon content on components increases limit, which allow both an increase in strength and improvement of corrosion resistance, but also improve the brittle fracture safety down to around -40 ° C and the weldability. With regard to the optimal fine grain formation to be aimed at, it should also be taken into account that the carbon content has a considerable influence on this. With decreasing carbon content, the influence of nucleation due to separated particles on the grain size decreases in favor of a very pronounced and, in the present case, desirable recrystallization inhibition due to dissolved niobium. In the thermomechanically treated state, low-pearlite microstructures have smaller ferrite grain sizes than microstructures with a higher carbon content.

Manganese has a particularly fine grain refinement and, at the same time, due to solidification of the solid solution and increased hardening, so that the manganese-- The content should preferably be arranged at the upper limit because the increase in strength due to manganese is very strongly dependent on the pearlite content and, thanks to a suitably low pearlite content, also ensures a favorable transition temperature and thus also safety against brittle fracture. Rising manganese contents make a considerable contribution to delaying the austenite transformation desired here and thus cause optimal fine grain formation. With the simultaneous presence of niobium and vanadium as microalloying elements, the increasing solidifying proportion of manganese becomes effective in the case of low-pearlite structures with increasing manganese content.

The latter for manganese also applies to the silicon content. If the silicon content is below about 0.5%, the transition temperature is also shifted to lower temperatures. However, silicon also has a strengthening effect above 0.5%, but at the same time is becoming increasingly brittle, which is to be avoided here for prestressing steels.

Niobium has the most effective influence on fine grain hardening and hardening through thermomechanical treatment, i.e. on the achievable strength increase, followed by vanadium. It causes the greatest reduction in the transition temperature. The niobium-containing structure results in a much larger proportion of fine-grain hardening than in hardening and therefore not only a higher yield strength than is achieved by vanadium alloy structure structures, but above all a very favorable, low transition temperature. Niobium reduces the ferrite grain size to a particularly large extent.

The high ratio of fine grain hardening to hardening in the structure with the addition of niobium is therefore an essential reason for the preference for niobium. Niobium has the additional strengthening effect of increasing manganese contents even when pearlite is low.

Like niobium, vanadium forms precipitates of special carbides, which on the one hand contribute to fine grain formation and hardening and on the other hand to precipitation hardening and thus significantly increase strength. Like niobium, vanadium therefore helps to control the austenite grain size by embedding fine precipitates in the austenite structure, which inhibits grain growth. Like niobium, vanadium contributes to solid-solution strengthening, but both are insoluble in ferrite. Their elimination in ferrite is therefore the most effective for increasing strength. The carbides and nitrides of vanadium and niobium have face-centered cubic lattices, are isomorphic and therefore completely miscible. In contrast to titanium, they do not contribute to the formation of sulfide. With an increased nitrogen content, vanadium has the greatest influence on the formation of a fine ferrite grain size and causes an additional increase in the yield strength. Like niobium, dissolved vanadium affects this fine grain effect and hardening by delaying the austenite transformation.

The delay in austenite recrystallization is greatly promoted by adding small amounts of molybdenum to the microalloyed, low-pearlite structure, which shifts the o-a conversion to lower temperatures. This possibility is used in the thermomechanical treatment by an even lower final forest temperature, whereby an even more fine-grained structure is achieved with a simultaneous improvement in the transition temperature. In addition, by alloying with molybdenum and the resulting possibility of shifting the ole-a conversion to lower temperatures, it is also additionally possible to take full advantage of the considerable strengthening properties of copper. With micro-alloyed structures of the type described here and at the same time low pearlite content and high copper content, both hardening mechanisms work both through the precipitation of mixed crystals and through the formation of carbonitrides, especially at temperatures between 650 and 550 ° C.

With additionally high manganese and molybdenum contents, such as here for high-strength prestressing steels, an additional increase in strength can be achieved in copper-alloyed structure structures in addition to particle hardening through a high dislocation density and fine grain hardening.

Copper is used for the purpose envisaged here because of its two advantages. First, because of its strong hardening effect through hardening. Second, because of its strong anti-corrosive effects. The corrosion-inhibiting effect of copper can be used particularly well in high-strength microstructures created with thermomechanical treatment, because at the low final rolling temperatures, which also lead to the highest strength increases, the element copper simultaneously with the precipitation-hardening elements used here between 650 and 550 ° C, in addition to its corrosion-inhibiting effect, also acts as a precipitation-hardening element. By rapid cooling from the 6 region at approx. 840 ° C, around 2% copper can be dissolved in low-pearlite microstructures and the thermomechanical treatment already provided here. A copper-rich, cubic surface-centered mixed crystal is then separated out in the form of incoherent, spherical particles, which, from a certain particle size, leads to a considerable precipitation hardness effect by the bypass mechanism. If niobium is present, micro-alloyed structures with a low pearlite content and a high copper content both cause hardening mechanisms due to the precipitation of mixed crystals and carbonitrides. At high copper contents, however, a nickel content of up to 1% must be added to the copper-alloyed microstructures in order to increase the solder fragility caused by copper prevent. With additional high manganese and molybdenum contents, as also provided here, an additional increase in strength can be achieved with a copper alloy structure apart from particle hardening through a high dislocation density and fine grain hardening. The corrosion-inhibiting effect of copper is already very effective with a very low copper content (0.25 to 0.40%). It is therefore necessary to adjust the copper content in order to be able to optimally use the corrosion-inhibiting effect and the hardening mechanisms on the one hand, but on the other hand not to let the solder fragility, which would be unsustainable for prestressing steels, come into effect and, if possible, to avoid adding nickel to this prevention .

Due to the aluminum content, the sudden grain growth is increased to around 1150 ° C when the primary material is heated, whereby the holding time is also important. In addition to aluminum, which produces this effect via aluminum nitride, it is primarily the microalloying elements niobium and vanadium that have a comparable effect via their carbides, nitrides or carbonitrides. To prevent or restrict the re-dissolution of such precipitates when heating before rolling, the lowest possible pusher furnace temperature is essential. The most favorable conditions for preventing the sharp increase in grain growth when reheating for rolling show higher aluminum contents. Aluminum also contributes to solidification of the solid solution.

In addition to aluminum, the sudden grain growth before heating for rolling is also increased by nitrogen to higher temperatures of around 1150 ° C. An increased nitrogen content also makes a significant contribution to increasing the strength by increasing the nitride content. Particularly when vanadium is present, the yield strength increases significantly. This also increases the tensile strength, so that an increase in the yield strength ratio from 70% to 90%, which is particularly important for prestressing steels, is brought about.

In the future, the phosphorus content must remain limited, although a higher content would increase the yield strength, but at the same time the steel becomes very brittle. Combined oxygen blowing / inert gas flushing makes it possible to lower the phosphorus content and largely prevent its embrittling effect. A corresponding reduction in the phosphorus content is also possible with ladle metallurgy.

According to the invention, the lowest possible phosphorus content is of particular importance and should therefore be aimed for.

The sulfur content plays a decisive role in the anisotropy of toughness, the most important factor influencing their cold formability for prestressing steels.

A lower sulfur content, i.e. a reduced number of sulfide inclusions improves the toughness significantly with regard to fracture constriction, a property that is particularly important for prestressing steels. In addition, the reduction in the sulfide length is particularly effective for a more favorable constriction of the fracture. A strong desulfurization can be achieved by the calcium additions that are common in ladle metallurgy.

Regarding the titanium not involved in the invention, it should be noted that, in contrast to niobium and vanadium, it participates in the formation of sulfide. On the other hand, it first binds all nitrogen to nitrides, TiN, and then sulfur to a titanium carbosulfide, Ti ₄ C ₂ S ₂ . For both reasons, titanium is not taken into account here, since, among other things, the effect of one of the austenite grain growth and that of an increase in strength in interaction with the other microalloying elements would be offset by an increased nitrogen content.

In the production of high-strength, corrosion-resistant and brittle fracture-proof prestressing steels according to the invention, all those difficulties which have to be taken into account when producing conventional, high-carbon prestressing steel grades are eliminated. Above all, there are no major concerns about production using the continuous casting process, which results primarily from the resulting central increases and surface defects that affect the pullability. The economic viability of continuous casting compared to ingot casting then comes into play, both in terms of effort and in terms of quality. For one thing, the previously possible accumulation of carbon in the middle of the strand is largely eliminated, which leads to eutectoid deposits of cementite networks and thus to a considerable deterioration not only of the structure and the properties thereof, but also of the safety against brittle fracture.

The measures which must be taken during the entire production process due to the high affinity for carbon for oxygen are also largely eliminated, both during melting (e.g. during the build-up or remelting process), freshening and the subsequent steel aftertreatment, but in particular also expensive reoxidation protection . The achievement of a high microscopic degree of purity, largely avoiding oxidic and sulfidic inclusions, is favored. Continuous casting largely eliminates the high effort required for electromagnetic stirring in the manufacture of high-carbon wire grades, which also largely prevents the very disadvantageous central increases, solidification bridges, directional solidification structures, internal and surface defects.

• wesentlich höhere Festigkeitswerte,
• wesentlich herabgesetzte Eigenspannungen,
• wesentlich erhöhte Sprödbruch-Sicherheiten,
• wesentlich erhöhte Verschleiss-Festigkeiten,
• wesentlich verbesserte Einsatzmöglichkeiten wegen ihrer Schweisseignung und
• wesenttic.i verbesserte Korrosionsbeständigkeit.

The presently produced according to the invention Possess prestressing steel

• much higher strength values,
• significantly reduced internal stresses,
• significantly increased safety against brittle fracture,
• significantly increased wear resistance,
• significantly improved application possibilities because of their sweat suitability and
• wesenttic.i improved corrosion resistance.

Regarding the latter two advantages, it should also be pointed out that, with a view to improving the corrosion resistance according to the present invention, elements which can be used economically are considered and which act in a similar manner to chromium in the case of stainless steels. In addition, such corrosion-inhibiting elements can be used particularly well in high-strength steels produced with thermomechanical treatment because, in addition to the corrosion-inhibiting effect due to precipitation hardening, they also contribute to the increase in strength due to the low final rolling temperatures, which also result in the highest strength increases. If, in addition to increasing the strength of high-strength prestressing steels, it is also possible to achieve welding suitability, this results in considerable and significant possibilities for constructively simplifying and improving the clamping system currently in use. As is known, e.g. In bridge construction, the coupling links are the most sensitive weak points for the occurrence of damage caused by the penetration of corrosive media up to the steel. According to today's technical possibilities, such coupling elements are usually arranged at too short intervals from one another. The resulting high number of coupling joints results in a high number of weak points.

When using the high-strength, corrosion-resistant and brittle fracture-proof prestressing steels according to the invention, it becomes possible to produce longer tendons by means of which the number of coupling members and thus the weak points is reduced. In addition, if the tensioning systems are also simplified and improved due to the weldability of these prestressing steels, this also results in a significant reduction in susceptibility to damage.

• geringere und damit leichter zu beherrschende Durchmesser von Spanndrähten, -stäben oder -litzen,
• durch die höheren Festigkeits-Eigenschaften wird auch die Konstruktion von geringeren Beton-Dicken möglich, wodurch sich
• eine Einsparung von Konstruktions-Gewicht insgesamt, einerseits, und
• erheblich gesteigerte Möglichkeiten in der Konstruktions-Gestaltung, andererseits, ergeben, also

Other advantages are

• smaller and therefore easier to control diameter of tension wires, rods or strands,
• Due to the higher strength properties, the construction of smaller concrete thicknesses is possible, which makes
• a saving in overall construction weight, on the one hand, and
• considerably increased possibilities in the construction design, on the other hand, result

• eine Verringerung der Totallast von bewegten Konstruktionen (Brückenbau, Elementbau z.B.) und
• Verringerung der Transportkosten bei bewegten Konstruktionen und beim Spannstahl.

Execution of constructions which cannot be realized with conventional prestressing steel with lower strength for technical or economic reasons, as well

• a reduction in the total load of moving structures (bridge construction, element construction, for example) and
• Reduction of transport costs for moving structures and prestressing steel.

Despite the use of microalloy elements and improved steel aftertreatment to increase strength, increase corrosion resistance and brittle fracture safety, the current price level of prestressing steels can be roughly maintained or even improved thanks to the considerable advantages in their manufacture and use. The additional design options, which result from the welding suitability of clamping systems, greatly increase cost-effectiveness. In their entirety, however, economic advantages would outweigh the disadvantage of price increases.

Claims (25)

1. Process for the manufacture of producing high strength, weldable, more corrosion resistant and more brittle fracture resistant prestressed steels, characterised in that steel consisting of

0.05 to 0.20% weight-carbon

1.20 to 1.70% weight-manganese

0.30 to 0.50 weight-silicon

0.04 to 0.06% weight-niobium

0.035 to 0.05% weight-vanadium

0.30 to 0.50% weight-molybdenum

0.30 to 2% weight-copper

0.04 to 0.06% weight-aluminium

0.015 to 0.02% weight-nitrogen

50.030% weight-phosphorus

sO.20% weight-sulphur
is subjected to a thermomechanical treatment which, after solidification from the melt and reheating takes place from the second heat, wherein in a first step steel is held before the thermomechanical treatment at a reheating temperature as low as possible (= second heat below 1150°C) and subsequently a controlled rolling of the steel is carried out with a small number of passes at a high degree of deformation (10-45%) and a high deformation speed down to a lower deformation temperature closely above 850°C.

2. Process according to Claim 1, characterised in that the thermomechanical treatment comprises a second stage in which from around 850°C an accelerated cooling without rolling takes place to around 650/550°C, whereby a reduction of the 6-a transformation and a delay of recrystallisation are effected.

3. Process according to Claim 2, characterised in that the thermomechanical treatment comprises a third stage in which from around 650/ 550°C, again controlled in one or a few passes, that is with a high degree of deformation at high speed, rolling takes place to a lower final rolling temperature closely above the Ar₃ boundary and then, after a dwell time, a delayed cooling takes place.

4. Process according to at least one of Claims 1 to 3, characterised in that, following on the thermomechanical treatment, a cold hardening of the steel by stretching (for pre-stressed steels) or drawing (for cold drawn wires) take place.

5. Process according to at least one of Claims 1 to 4, characterised in that the steel before and/or after refining is substantially desulphurised.

6. Process according to Claim 5, characterised in that the molten steel before and/or after refining is subjected to a calcium treatment.

7. Process according to Claim 5 or 6, characterised in that additionally a steel post-treatment, for example inert gas flushing, vacuum treatment, deoxidisation, inclusion modification or a ladle treatment with metallic calcium or calcium halide slags takes place.

8. Process according to one of Claims 1 to 7, characterised in that, as hardening mechanism during the thermomechanical treatment, both a mixed crystal fine grain and particle or separation hardening with substantially additive effect takes place.

9. Process according to one of Claims 1 to 8, characterised in that the thermomechanical treatment takes place by the controlled rolling of microalloyed fine grain molten steels and excludes martensite formation.

10. Process according to Claim 9, characterised in that the recrystallisation of these fine grain molten microalloyed steels lead to the most fine possible austenite grains before the ferrite perlite transformation.

11. Process according to Claim 10, characterised in that, in the microalloyed steels, the rolling process is additionally enhanced by the separation of carbides, nitrides and/or carbonitrides, whereby both mixed crystallisation and also a fine grain and particularly intensified particle hardening is effected.

12. Process according to one of Claims 2 to 11, characterised in that the temperature sequence is so controlled that a 6-a transformation takes place shortly before and/or during the lowest possible final rolling temperature, which comes to lie shortly before A,3.

13. Process according to Claim 1, characterised in that by means of the first stage of the thermomechanical treatment, rolled wire goods for the manufacture of cold drawn wires, three and seven strand cables as well as tensioning bars are achieved, which correspond in their properties to Euro-Norm 138, but which have the additional useful properties of corrosion resistance, security against brittle fracture and weldability.

14. Process according to one of Claims 1 to 13, characterised in that, by the alloying in of microalloying elements niobium and/or vanadium and/or molybdenum, an optimum possible particle hardening in the form of carbides, nitrides and/or carbonitrides is effected by precipitation during the thermomechanical treatment additionally to the fine grain and mixed crystal hardening.

15. Process according to one of Claims 1 to 14, characterised in that a rendering finer of the austenite grains by the alloying in of fine precipitates such as aluminium nitrides as well as carbides, nitrides and/or carbonitrides, particularly of the microalloying elements niobium and vanadium takes place, and this is a particle quantity of from 20 x 10⁶ per mm² and of particle diameter of 100 to 200 A, wherein a greatest possible degree of deformation and speeds are used together with the lowest possible final rolling temperature.

16. Process according to one of Claims 1 to 15 characterised in that by a recrystallization-delaying, portions of not recrystallized austenite are deformed during the lower final rolling temperatures, from which longitudinally extended and accordingly strongly increased austenite grain surfaces are generated, on the conversion of which in the ferrite perlite stage, because of an increased nucleus density and the restricted growth of the grains formed on these nuclei, a stronger fine grain effect and because of this an optimal increase in strength emerges, both by fine grain and also by particle hardening.

17. Process according to one of Claims 14 to 16, characterised in that the austenite recrystallisation, as well as the control of the cooling speed, is displaced by the alloying in of molybdenum and thereby the is-a transformation displaced to lower temperatures.

18. Process according to one of Claims 14 to 17, characterised in that by alloying in an increased manganese content in the scope of the above mentioned standard analysis the fine grainness striven for and then simultaneously an optimum increase in strength from mixed crystral strengthening and increased hardening is guaranteed.

19. Process according to Claim 18, characterised in that by increasing manganese content simultaneously, an optimum delay of the austenite transformation striven for and thereby the optimum fine grain formation is guaranteed.

20. Process according to Claim 18 or 19, characterised in that, by increasing manganese content simultaneously, the delay of recrystallisation which is striven for is guaranteed, and this by shifting the δ­α transformation to lower temperatures and adjusting the lowest possible final rolling temperature as well as simultaneous application of the thermomechanical treatment.

21. Process according to one of Claims 18 to 20, characterised in that, by an increased manganese content, the optimum precipitation of particles and thereby optimum action of the particle hardening is guaranteed to the maximum possible increase in strength.

22. Process according to one of Claims 18 to 21, characterised in that by increased manganese content and simultaneous presence of niobium and vanadium, as well as perlite deficiency also the strengthening and thereby strength increasing proportion of the manganese is increased.

23. Process according to one of Claims 1 to 22, characterised in that the perlite proportion is reduced.

24. Process according to one of Claims 1 to 23, characterised in that, for hindering rapid grain growth on preheating in the preheating furnace or the like, increased aluminium nitrogen contents are set within the scope of the above given standard analysis, wherein, in this process for this purpose, one strives for a particle quantity of from 20 x 10⁶ per mm² and a particle diameter of 100 to 200 A.

25. Process according to one of Claims 1 to 24. characterised in that by an increasing niobium content within the above given standard analysis, the beginning of the grain growth which is to be prevented is displaced to higher temperatures.

Images