EP2453026A1 - Thermoformed steel product and method for producing same - Google Patents

Abstract

Producing steel product comprises: subjecting steel comprising carbon, manganese, chromium, nickel, phosphorus, and iron and steel impurities to hot forming at 900-1300[deg] C; and cooling it in air. The average austenite grain size of the steel after the final hot forming step is less than 50 mu m. The cooling of the forming heat in static or moving air is carried out such that the cooling is carried out at a temperature of 800-500[deg] C with a cooling rate of 0.1-8 K/s. The steel product comprises lower bainite, granular or upper bainite, martensite, austenite, and ferrite. Producing steel product comprises: subjecting steel comprising (in wt.%) carbon (0.03-12.2), manganese (2-4), chromium (0.05-2), nickel (0.05-1), phosphorus (upto 0.035), and iron and steel impurities (remaining amount) to hot forming at 900-1300[deg] C; and cooling it in air. The average austenite grain size of the steel after the final hot forming step is less than 50 mu m. The cooling of the forming heat in static or moving air is carried out such that the cooling is carried out at a temperature of 800-500[deg] C with a cooling rate of 0.1-8 K/s. The weight proportion (x) of the above components satisfies the condition Bs (greater than 700) is equal to 1.103-270x (x)carbon - 90x (x)manganese - 70x (x)chromium - 37x (x)nickel - 83x (x)molybdenum (less than 800). The steel product comprises (in %) lower bainite (60-95), granular or upper bainite (upto 10), martensite (upto 40), austenite (upto 20), and ferrite (upto 2). An independent claim is also included for a hot formed steel product, producible by the above method, comprising (in wt.%) carbon (0.03-0.20), manganese (2-4), chromium (0.05-2), nickel (0.05-1), phosphorus (upto 0.035), and iron and steel impurities (remaining amount).

Description

Technical area

The invention relates to a method for producing a steel product according to the preamble of claim 1 and to a hot-formed product producible therewith.

State of the art

For massive components that are stressed dynamically or suddenly, high demands are placed on the ductility of the steel used. For this reason, the final properties of the material are usually adjusted via a heat treatment. The tempering treatment aims at a tempered martensitic steel structure and allows the setting of a high level of toughness with still good strength. A broadly used tempered steel is the material 42CrMo4. At a tensile strength of about 1000 MPa, this steel still achieves a Charpy impact work (ISO-V at room temperature) of 200 J.

Annealing steels with a martensite and bainite mixed structure can also have good property combinations. The WO 2007/017161 describes such a steel for thick-walled seamless tubes (with up to 30 mm wall thickness). After quenching (cooling rate> 30 K / s) from the forming heat, a dominant martensitic microstructure with up to 40% bainite is formed. Irrespective of the original austenite grain size (primary structure), the martensitic structure has a good notched impact strength as soon as the martensite grain size is <3 μm.

A further reduction of the cooling rate (from the forming heat) leads to dominant bainitic structures. Corresponding patent publications relate in particular to forgings and rails. The resulting bainite microstructure is usually very notch sensitive, so the Charpy notched impact strength typically determined on samples with U notch.

In EP 0 845 544 describes such a microalloyed bainitic steel with C ≤.12%, which has a tensile strength of more than 1000 MPa at room temperature. In order to achieve the desired properties, the steel is austenitized again after rolling and then quenched at a cooling rate> 17 K / s. This cooling rate is still significantly higher than that of the air-cooled long products in conventional rolling mills.

EP 0 775 756 describes another bainitic-martensitic steel for the production of forgings. After forging, the tensile strength should be>1'000 MPa and the Charpy notched impact strength ISO-U is> 50 j / CM ² (or notched impact strength ISO-U> 25 J). However, the described steel composition necessarily requires an accelerated cooling from the forming heat, so that these values can be achieved. The exemplary embodiments show that the cooling rate should be> 14 K / s. This technical teaching can not be applied in conventional forging and hot rolling processes. In addition, the implementation is limited to small components in which even in the core still large cooling rates can be achieved.

GB 2 297 094 describes a carbide-free bainitic steel that can be made from the forming heat cooled in air. The steel is designed for the production of rails and is characterized by a good wear resistance and a good fatigue behavior. The notched impact strength of the material was not the focus of this development. The Charpy impact work ISO-V at room temperature is only at 20 to 40 years.

The in CN 1 477 226 steel described after air cooling may contain the following mixed structure: granular bainite, lower bainite, martensite, retained austenite. It achieves a tensile strength of 850 to over 1,400 MPa. In order for good toughness to result, however, the steel must be heat treated again after hot working (tempered). The carbon present then migrates into the present austenite films, and at a tensile strength of about 900 MPa, a Charpy impact toughness ISO-U greater than 110 J / cm ² (or> 55 J) can be achieved.

• ach Luftabkühlung aus der Umformhitze bei Raumtemperatur Charpy-Kerbschlagzähigkeiten an V-gekerbten Proben von > 100 J erreichbar sind
• für ein breites Spektrum an Bauteil-Abmessungen bzw. Walzabmessungen konstante Eigenschaften erreicht werden können.

Although the properties of bainitic-martensitic steels have been promising so far, so far there is no description for the production of solid components (diameter or wall thickness> 10 mm), with the

• After air cooling from the forming heat at room temperature Charpy notched impact strengths on V-notched specimens of> 100 J can be achieved
• For a wide range of component dimensions or rolling dimensions constant properties can be achieved.

Presentation of the invention

The object of the invention is to provide an improved hot-worked steel product and a method for its production, with which in particular the above disadvantages are avoided.

These objects are achieved by the method defined in claim 1 and by the defined in claim 3 hot-formed steel product.

The following percentages by weight (%) or parts per million (ppm) refer to parts by weight unless expressly stated otherwise.

• 0.03 bis 0.20 % Kohlenstoff (C),
• 2.00 % bis 4.00 % Mangan (Mn),
• 0.05 bis 2.00 % Chrom (Cr),
• 0.05 bis 1.00% Nickel (Ni),
• bis zu 0.035% Phosphor (P),

der Rest Eisen sowie stahlübliche Beimengungen,
einer Warmumformung bei 900 bis 1300°C unterzogen und danach an Luft abkühlt, wobei die mittlere Austenitkorngrösse nach dem letzten Warmumformungschritt kleiner ist als 50 11m und wobei die Abkühlung aus der Umformhitze an ruhender oder bewegter Luft so geschieht, dass der Temperaturbereich zwischen 800 und 500°C mit einer Kühlrate von 0.1 bis 8.0 K/s durchlaufen wird.In the process according to the invention for producing a steel product, a steel with a weight fraction of:

• 0.03 to 0.20% carbon (C),
• 2.00% to 4.00% manganese (Mn),
• 0.05 to 2.00% chromium (Cr),
• 0.05 to 1.00% nickel (Ni),
• up to 0.035% phosphorus (P),

the remainder iron as well as steel admixtures,
subjected to hot working at 900 to 1300 ° C and then cooled in air, the average Austenitkorngröße after the last hot forming step is less than 50 11m and wherein the cooling from the forming heat to stationary or moving air is done so that the temperature range between 800 and 500 ° C is passed through with a cooling rate of 0.1 to 8.0 K / s.

The percentages by weight x (i) of carbon, manganese, chromium, nickel and molybdenum meet the following condition: $$\mathrm{700}\mathrm{<}\mathrm{bs}\mathrm{=}\mathrm{1}\mathrm{\text{'}}\mathrm{103}\mathrm{-}\mathrm{270}\mathrm{\cdot }\mathrm{x}\left(\mathrm{C}\right)\mathrm{-}\mathrm{90}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Mn}\right)\mathrm{-}\mathrm{70}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Cr}\right)\mathrm{-}\mathrm{37}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Ni}\right)\mathrm{-}\mathrm{83}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Not\; a\; word}\right)\mathrm{<}\mathrm{800}$$

• 60 bis 95% unterer Bainit,
• bis zu 10% granularer oder oberer Bainit,
• bis zu 40% Martensit,
• bis zu 20% Restaustenit, und
• bis zu 2% Ferrit.

This ensures that after air cooling from a temperature range of 900 to 1'300 ° C the following structural components are present:

• 60 to 95% lower bainite,
• up to 10% granular or upper bainite,
• up to 40% martensite,
• up to 20% retained austenite, and
• up to 2% ferrite.

The mass number Bs used in the above condition corresponds to a known empirical approach for the bainite start temperature in Kelvin [ W. Steven, and AJ Haynes, JISI 183, pp. 349-359 (1956 )].

In the product according to the invention, the alloy components are chosen so that at usual cooling rates from the rolling heat of 0.3 to 8.0 K / s (from 800 ° C to 500 ° C) always a bainitic-martensitic microstructure with tensile strength of 900 to 1400 MPa results without having to use expensive alloying elements and / or special equipment for accelerated cooling from the rolling heat. The Charpy ISO V impact test at room temperature> 100 J.

The lower limit of the carbon content to 0.03 wt .-% is ensured in combination with manganese, chromium and molybdenum that there are no ferrite in the structure. Ferrite levels affect both the strength level and the impact strength of the product.

The upper limit of the carbon to 0.20 wt .-% ensures that the tensile strength does not rise above 1400 MPa. Higher strength values degrade machinability in the downstream drawing or machining process. Higher carbon contents also promote the formation of carbides, which adversely affects ductility.

The lower limit of 2.00% by weight in manganese ensures that a bainite start temperature below 800 K can be achieved without expensive alloying additions. This deep bainite start temperature ensures a fine steel structure, which consists predominantly of lower bainite.

Another addition of manganese reduces the bainite start temperature again. The bainite area narrows and the resulting bainite decreases in favor of retained austenite and martensite. If the upper limit in manganese is exceeded, the desired bainitic structure is no longer present. The microstructure will then be dominant martensitic.

Molybdenum suppresses the grain boundary segregation of embrittling elements such as phosphorus. An addition of at least 0.15 wt .-% molybdenum thus improves the tempering resistance of the steel. If no downstream heat treatment takes place, the addition of molybdenum is not mandatory.

A molybdenum content above 0.5% by weight promotes the formation of carbon-rich martensite islands. These lead to a marked deterioration of the toughness of the steel. For this reason, the molybdenum content should be at most 0.5% by weight.

Chromium may be alloyed in place of manganese to adjust the bainite start temperature. However, the use of chromium is more expensive than the use of manganese. Since manganese segregates strongly, it may nevertheless make sense for certain applications to replace part of the manganese with chromium. Since chromium increases the risk for the formation of chromium-rich nitrides and carbides, which can lead to a deterioration of the toughness, the chromium content is limited to 2.0% by weight.

The addition of silicon is not necessary to achieve the desired properties. A metered addition of silicon suppresses carbide formation. A preferred embodiment of the product according to the invention therefore contains 0.40 to 0.80% by weight of silicon.

Nickel improves Charpy impact strength at low temperatures. In general, the properties are sufficient without addition of nickel. For cost reasons, the nickel content is limited to 1.0 wt .-%.

The addition of aluminum is not mandatory for the production of the product according to the invention. If a subsequent heat treatment of the product, e.g. case hardening is necessary to set a wear-resistant surface, then austenite grain stability can be ensured by adding aluminum. In this case, aluminum contents of 0.02 to 0.04 wt.% Are common.

Phosphorus is a steel pest. It goes to the Austenitkorngrenzen and weakens the structure. For this reason, the phosphorus content was limited to 0.035 wt .-%.

Since even small amounts of ferrite negative impact on the notched impact strength If possible, ferrite formation should be avoided as far as possible. This can be ensured by a sufficiently rapid cooling of the hot-formed product. If the cooling rate is insufficient, addition of boron may additionally be provided. Boron goes to the austenite grain boundaries and suppresses ferrite formation. In this case, a boron content of 10 to 50 ppm is sufficient.

An addition of 0.03 to 0.10% by weight of titanium ensures that the nitrogen dissolved in the liquid steel of up to 0.02% by weight is precipitated during the solidification of the steel in the form of titanium carbonitrides. This is the prerequisite for the fact that elemental boron can reach the austenite grain boundaries and is not present in the form of boron nitrides. If no boron is alloyed, no titanium addition must be provided.

The chemical composition of the steel should be chosen such that, after cooling in air, a structure is created which is predominantly composed of lower bainite. This is preferably achieved by setting the bainite start temperature Bs low enough. For this reason, the Bs temperature should not be more than 800 K. The low transformation temperature ensures a very fine microstructure, which is decisive for achieving the high notched impact strength.

A sufficiently fine microstructure is achieved if the mean grain size of the dominant bainitic secondary microstructure is less than 5 μm. The grain size is defined by the linear distance between grain boundaries. The crystallographic orientation at the grain boundary should change by more than 15 °.

A too low selected Bs temperature slows the kinetics of bainite formation. It produces significantly less bainite and the structure becomes dominant martensitic. The Bs temperature should therefore be above 700 K. To set as much bainite as possible, the bainite start temperature should preferably be between 750 and 800 K.

Austenite is not completely transformed into bainite during structural transformation. In order to dominate the properties of the lower bainite, however, should be at least 60% of the structure of lower bainite.

Austenite, which does not convert to bainite during hot-dip cooling, is either stabilized to a sufficient carbon content or converts to martensite at lower temperatures. At an average carbon content in the steel of 0.05 wt .-% is expected to be present in the structure no retained austenite and the resulting martensite may be up to 40%.

At a carbon content of 0.2% by weight, a part of the austenite is stabilized during the cooling from the hot working. There may still be 20% retained austenite in the final product.

Since the manganese content of the steel is more than 2.0% by weight, a microscopically uneven manganese distribution in the industrially produced product is to be expected (segregation zones). For this reason, the transformation behavior of austenite during cooling from hot working may vary locally. Thus, isolated grains of ferrite, granular bainite or upper bainite can not be completely excluded. As long as their ingredients are small, they will not affect the good properties of the product. Therefore, up to 10% granular or upper bainite and up to 2% ferrite are permissible for the product produced according to the invention.

Brief description of the drawings

Fig. 1

die Abhängigkeit der ISO-V-Kerbschlagarbeit von der Bainit-Starttemperatur;

Fig. 2

ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 1;

Fig. 3

ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 2;

Fig. 4

ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 6;

Fig. 5

ein Gefügebild (Ätzmittel: LePera) für einen 18 mm Stabstahl aus Stahl 7;

Fig. 6

Gefügebilder bei 3000X Vergrösserung im hochauflösenden Rasterelektronenmikroskop: oben Stahl 6 und unten Stahl 7, und

Fig. 7

das Ergebnis einer EBSD-Untersuchung am Beispiel von Stahl 7; die Sekundär-Korngrössen wurden entlang den Linien d1, d2 und d3 ausgewertet; die mittlere Korngrösse beträgt 4.51 µm (± 1.09 µm).

Exemplary embodiments of the invention will be described in greater detail below with reference to the drawings, in which:

Fig. 1

the dependence of the ISO-V impact energy on the bainite start temperature;

Fig. 2

a microstructure (etchant: LePera) for a 18 mm steel bar 1;

Fig. 3

a microstructure (etchant: LePera) for a 18 mm steel bar steel 2;

Fig. 4

a microstructure (etchant: LePera) for a steel 18 mm steel bar 6;

Fig. 5

a microstructure (etchant: LePera) for a steel 18 mm steel bar 7;

Fig. 6

Micrographs at 3000X Magnification in the high-resolution scanning electron microscope: steel 6 at the top and steel 7 at the bottom and steel 7 at the bottom

Fig. 7

the result of an EBSD study using the example of Stahl 7; the secondary grain sizes were evaluated along lines d1, d2 and d3; the average particle size is 4.51 μm (± 1.09 μm).

Ways to carry out the invention

The crucial connections were discovered during laboratory melt tests. Steel blocks of Ø00 x 400 mm were manufactured and forged to Ø0 mm round billets. To simulate rolling in a conventional hot rolling mill, the billets were rolled in 8 passes to Ø18 mm: the final rolling temperature at the last hot forming was 1,040 to 1,060 ° C. To simulate the cooling of thin wires, an accelerated air cooling with a cooling rate of approx. 5.5 K / s was set between 900 and 600 ° C. During structural transformation below 600 ° C, the rods were cooled in still air.

The room temperature Charpy impact values determined at nine melts are in Fig. 1 as a function of the bainite start temperature Bs (determined according to Steven & Hayns). It has been discovered that the melts with Bs <800 K always have a good notched impact strength.

Carbon, manganese, molybdenum were used in the trial melts to adjust the Bs temperature. No chromium and nickel were alloyed. The measured chromium and nickel contents (as accompanying elements or impurities in the steel) were between 0.05 and 0.09 wt .-%.

The three steels according to the invention are compared with the six non-inventive steels in Tables 1 and 2. The essential difference between the four steels is the fineness and morphology of the microstructure. This is set by the chemical composition of the steel, according to Bs (in K) = 1103-270C-90Mn-70Cr-37Ni-83Mo <800K. Selected micrographs are in Fig. 2 to 5 shown.

Steel 1 is dominantly a coarse granular bainite ( Fig. 2 ). Occasionally ferrite grains were also found. The tensile strength Rm is correspondingly low. The ISO-V impact work at room temperature falls below 100 years.

The properties and the microstructure of the non-inventive steel 2 are similar to steel 1. The bainite start temperature is slightly lower and the microstructure is accordingly somewhat finer ( Fig. 3 ). The structure consists predominantly of a carbide-free granular bainite. The quantitative microstructure analysis (by means of X-ray diffraction for austenite and quantitative SEM analysis for ferrite, bainite and M / A phase fractions) revealed the following structural composition: 80% bainite (dominant granular), 17% martensite and 3% retained austenite.

Upon further lowering of the bainite start temperature, the microstructure changes from granular bainite to pale lower bainite. Already in the micrograph (LOM) of the non-inventive steel 6, the much finer microstructure can be seen ( Fig. 5 ). The strength increases markedly compared to steels 1 and 2, but the impact value remains low. Reason for the unsatisfactory toughness are coarse grains of granular, upper bainite, which are embedded in a matrix of fine lower bainite. The structure consists of 87% bainite, 10% martensite and 3% retained austenite.

Due to the grains of granular bainite present, the microstructure is not as fine as it appears in the LOM or in the scanning electron microscope. To visualize the fineness of the structure, EBSD studies were performed. This method measures the crystallographic orientations in the microstructure. A grain boundary exists when the crystallographic orientation changes by more than 15 °. The average linear extent of the grains can thus be determined. For steel 6, the mean grain size is 10.1 μm (± 0.93 μm).

Fig. 7 shows the structure of the inventive steel 7 compared to the steel 6. The structure has become even finer in steel 7. The quantitative analysis gives the following structure composition: 70 to 72% lower bainite and 28 to 30% self-tempered martensite. Rough structural components such as ferrite or granular upper bainite are missing. The mean grain size determined by EBSD is correspondingly small. It is 4.51 μm (± 1.09 μm) and thus only half the size of steel 6.

• Er kann mit konventioneller Walz- und Schmiedetechnologie hergestellt werden kann. Eine Feinung des Primärgefüges durch Absenkung der Temperatur im letzten Umformschritt unterhalb der Rekristallisations-Stopptemperatur ist nicht zwingend gefordert. Im Ausführungsbeispiel liegt eine Austenitkorngrösse von ca. 30 µm vor.
• Die Eigenschaften werden bei tiefen Abkühlraten erreicht (hier an ruhender Luft), so dass massive Bauteile daraus gefertigt werden können (im Beispiel 18 mm Walzstahl). Die Abkühlrate entspricht der Herstellung von Stabstahl im Durchmesserbereich von 40 bis 50 mm.
• Die Eigenschaften werden direkt aus der Warmumformung erreicht. Eine nachgelagerte Wärmebehandlung (z.B. ein Anlassen) ist nicht zwingend notwendig.
• Der Einsatz von teuren Legierungsmitteln (wie Mikrolegierungsmittel oder Nickel und Molybdän) ist nicht zwingend notwendig.

The steel according to the invention represents a steel analysis with high strength (about 1000 MPa) and toughness (ISO-V at room temperature is 150 to 200 J):

• It can be manufactured using conventional rolling and forging technology. A refining of the primary structure by lowering the temperature in the last forming step below the Recrystallization stop temperature is not mandatory. In the exemplary embodiment, an austenite grain size of approximately 30 μm is present.
• The properties are achieved at low cooling rates (here in still air), so that massive components can be made from them (in the example, 18 mm rolled steel). The cooling rate corresponds to the production of steel bars in the diameter range from 40 to 50 mm.
• The properties are achieved directly from hot forming. A subsequent heat treatment (eg tempering) is not absolutely necessary.
• The use of expensive alloying agents (such as microalloying agent or nickel and molybdenum) is not mandatory.

Claims (15)

A process for producing a steel product, wherein a steel having a weight fraction of 0.03 to 0.20% carbon (C), 2.00% to 4.00% manganese (Mn), 0.05 to 2.00% chromium (Cr), 0.05 to 1.00% nickel (Ni), up to 0.035% phosphorus (P), the remainder iron as well as steel admixtures, subjected to hot working at 900 to 1300 ° C, and then cooled in air, wherein the average austenite grain size after the last hot forming step is less than 50 microns and wherein the cooling from the forming heat to static or moving air is such that the temperature range between 800 and 500 ° C is passed through with a cooling rate of 0.1 to 8.0 K / s, characterized in that the percentage by weight x (i) of carbon, manganese, chromium, nickel and molybdenum satisfy the following condition: $$\mathrm{700}\mathrm{<}\mathrm{bs}\mathrm{=}\mathrm{1}\mathrm{\text{'}}\mathrm{103}\mathrm{-}\mathrm{270}\mathrm{\cdot }\mathrm{x}\left(\mathrm{C}\right)\mathrm{-}\mathrm{90}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Mn}\right)\mathrm{-}\mathrm{70}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Cr}\right)\mathrm{-}\mathrm{37}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Ni}\right)\mathrm{-}\mathrm{83}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Not\; a\; word}\right)\mathrm{<}\mathrm{800}$$

wherein a steel product is formed with the following structural constituents: 60 to 95% lower bainite, up to 10% granular or upper bainite, up to 40% martensite, up to 20% retained austenite, and up to 2% ferrite. The method of claim 1, wherein the weight percentages x of carbon, manganese, chromium, nickel and molybdenum satisfy the condition: 750 <Bs = 1'103 - 270 x (C) - 90 x x (Mn) - 70 x x ( Cr) - 37 × (Ni) - 83 × (Mo) <800. Hot-formed steel product producible by a process according to any one of the preceding claims, having a weight fraction of 0.03 to 0.20% carbon (C), 2.00% to 4.00% manganese (Mn), 0.05 to 2.00% chromium (Cr), 0.05 to 1.00% nickel (Ni), up to 0.035% phosphorus (P), the remainder iron as well as steel admixtures, wherein the percentages by weight x of carbon, manganese, chromium, nickel and molybdenum satisfy the following condition: $$\mathrm{700}\mathrm{<}\mathrm{bs}\mathrm{=}\mathrm{1}\mathrm{\text{'}}\mathrm{103}\mathrm{-}\mathrm{270}\mathrm{\cdot }\mathrm{x}\left(\mathrm{C}\right)\mathrm{-}\mathrm{90}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Mn}\right)\mathrm{-}\mathrm{70}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Cr}\right)\mathrm{-}\mathrm{37}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Ni}\right)\mathrm{-}\mathrm{83}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Not\; a\; word}\right)\mathrm{<}\mathrm{800}$$

the steel product having the following structural constituents: 60 to 95% lower bainite, up to 10% granular or upper bainite, up to 40% martensite, up to 20% retained austenite, and up to 2% ferrite. Hot-formed steel product according to claim 3, wherein the mean grain size of the dominant bainitic secondary structure is smaller than 5 μm. A hot-worked steel product according to claim 3 or 4, wherein the weight percentages x of carbon, manganese, chromium, nickel and molybdenum satisfy the condition: $$750\mathrm{<}\mathrm{bs}\mathrm{=}\mathrm{1}\mathrm{\text{'}}\mathrm{103}\mathrm{-}\mathrm{270}\mathrm{\cdot }\mathrm{x}\left(\mathrm{C}\right)\mathrm{-}\mathrm{90}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Mn}\right)\mathrm{-}\mathrm{70}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Cr}\right)\mathrm{-}\mathrm{37}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Ni}\right)\mathrm{-}\mathrm{83}\mathrm{\cdot }\mathrm{x}\left(\mathrm{Not\; a\; word}\right)\mathrm{<}\mathrm{800th}$$

Hot-formed steel product according to one of claims 3 to 5, having the following structural constituents: 60 to 95% lower bainite, up to 10% granular or upper bainite, up to 30% martensite, up to 5% retained austenite, and up to 2% ferrite. A hot-worked steel product according to any one of claims 3 to 6, wherein the molybdenum content is 0.15 to 0.50 wt%. A hot-worked steel product according to any one of claims 3 to 7, wherein the silicon content is 0.40 to 0.80 wt%. A hot-worked steel product according to any one of claims 3 to 8, wherein the boron content is 10 to 50 ppm. A hot-worked steel product according to any one of claims 3 to 9, wherein the aluminum content is 0.02 to 0.04 wt%. A hot-worked steel product according to any one of claims 3 to 10, wherein the titanium content is 0.03 to 0.10% by weight. Hot-formed steel product according to one of claims 3 to 11, with a minimum wall thickness or minimum diameter of 10 mm. Hot-formed steel product according to one of claims 3 to 12, having a Charpy notched impact strength ISO-V at room temperature of more than 100 years. Hot-formed steel product according to one of claims 3 to 13, having a tensile strength Rm at room temperature of 800 to 1400 MPa. Use of a hot-worked product according to any one of claims 3 to 14 for processing in a press for the production of cold extrusion or cold upsetting parts.

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