EP3535431B1 - Steel product with an intermediate manganese content for low temperature application and production method thereof - Google Patents

Description

The invention relates to a medium-manganese steel product for use at low temperatures and a method for its production in the form of a flat steel product or a seamless tube.

In particular, the invention relates to the production of a steel product from a medium-manganese steel with excellent low-temperature toughness and / or high strength, for use in temperature ranges up to at least minus 196 ° C, which optionally includes a TRIP (TR Transformation Induced Plasticity) and / or TWIP (TWinning Induced Plasticity) effect. In the following, steel products are understood to mean in particular flat steel products such as steel strips (hot or cold rolled) or heavy plates, as well as welded pipes made therefrom, but also seamless pipes.

From the European published application EP 2 641 987 A2 a medium-manganese high-strength steel and a method for producing this steel are known. The steel has a notched impact strength of 70 J at -196 ° C and consists of the elements (contents in% by weight and based on the molten steel): C: 0.01 to 0.06; Mn: 2.0 to 8.0; Ni: 0.01 to 6.0; Mo: 0.02 to 0.6; Si: 0.03 up to 0.5; AI: 0.003 to 0.05; N: 0.0015 to 0.01; P: up to 0.02; S: up to 0.01; as well as the remainder iron and unavoidable impurities. This steel is said to be distinguished by the fact that it is more cost-effective to manufacture than the steels containing up to 9% by weight of nickel that have been used up to now for this purpose. A method for producing a flat steel product from the above-described high-strength steel with medium manganese content comprises the following work steps: heating a steel slab to a temperature of 1000 ° C to 1250 ° C, - rolling the slab with a final rolling temperature of 950 ° C or less with a reduction rate (Degree of rolling) of 40% or less, - cooling the rolled steel to a temperature of 400 ° C or less at a cooling rate of 2 ° K / s or more, - and, following the cooling, tempering the steel for 0.5 to 4 hours at a temperature between 550 ° C and 650 ° C. The structure of the steel has martensite as the main phase and 3 to 15% by volume of retained austenite.

In the U.S. Patent 5,256,219 A medium-manganese steel for a door reinforcement tube is disclosed which, in addition to iron, contains the following elements: C: 0.15 to 0.25%; Mn: 3.4 to 6.1%; P: 0.03% or less; S: 0.03% or less; Si: 0.6% or less; Al: 0.05%; Ni, Cr, Mo: 0 to 1%; V: 0 to 0.15%. A structural composition of the steel is not described.

The U.S. Patent 5,310,431 discloses a corrosion-resistant, martensitic steel which, in addition to iron and impurities, contains the following elements: C: 0.05 to 0.15%; Cr: 2 to 15%; Co: 0.1 to 10%; Ni: 0.1 to 4%, Mo: 0.1 to 2%; Ti: 0.1 to 0.75%; B: <0.1%; N: <0.02%. In addition, the steel described can also contain, for example, <5% Mn.

The U.S. Patent 4,257,808 discloses a low-manganese steel for low-temperature applications, the composition of which does not contain any nickel.

The Chinese patent application CN 103 422 017 A also describes a steel composition for steel pipes used in the low temperature range, the composition containing (in% by weight): C: 0.02-0.13; Si: 0.15-0.4; Mn: 0.2-0.9; P: ≤ 0.012; S≤0.007; N≤0.012; Mo: 0.008-0.12; Ni: 8.5-9.6 with the balance iron including impurities.

From the published patent application US 2014/0230971 A1 a high-strength steel sheet with excellent deformation properties and a method for its production are known. In addition to iron and unavoidable impurities, the steel sheet consists of the following elements (in% by weight): C: 0.03 to 0.35; Si: 0.5 to 3; Mn: 3.5 to 10; P: <0.1; S: <0.01; N: <0.08. A microstructure is specified with more than 30% ferrite and more than 10% residual austenite.

Also the published patent application WO 2006/011503 A1 describes a steel sheet, the chemical composition of which is given in% by weight as follows: C: 0.0005 to 0.3; Si: <2.5; Mn: 2.7 to 5; P: <0.15; S: <0.015; Mo: 0.15 to 1.5; B: 0.0006 to 0.01; Al: <0.15 as well as the remainder iron and unavoidable impurities. Such a steel strip is characterized by a high modulus of elasticity of greater than 230 Gpa in the rolling direction.

The European published application EP 2 055 797 A1 relates to a ferromagnetic, iron-based alloy whose composition contains one or more of the following elements in% by weight: Al: 0.01 to 11; Si: 0.01 to 7; Cr: 0.01 to 26, the remainder being iron and unavoidable impurities. The alloy can optionally also contain 0.01 to 5% by weight of Mn and other elements.

Furthermore, in the German Offenlegungsschrift DE 10 2012 013 113 A1 So-called TRIP steels have already been described, which have a predominantly ferritic basic structure with embedded retained austenite, which can convert to martensite during forming (TRIP effect). Because of its strong work hardening, TRIP steel achieves high values of uniform elongation and tensile strength. TRIP steels are used, among other things, in structural, chassis and crash-relevant components of vehicles as sheet metal blanks and as welded blanks.

Furthermore, are from the Offenlegungsschrift WO 2005/061152 A1 Hot strips made from TRIP / TWIP steels with manganese contents of 9 to 30% by weight are known, the melt being cast into a pre-strip between 6 and 15 mm via a horizontal strip caster and then rolled out into a hot strip.

Proceeding from this, the present invention is based on the object of specifying a steel product made of a manganese-containing steel, which can be manufactured inexpensively and has an advantageous combination of strength and elongation properties at low temperatures and optionally a TRIP and / or TWIP effect. Furthermore, a method for producing such a steel product is to be specified.

This object is achieved by a steel product according to the invention having the features of claims 1 to 3. Advantageous refinements of the invention are specified in the subclaims. A method according to the invention for producing such a steel product is specified with the features of claim 9 or 13 and its subclaims.

In addition, the production of this manganese-containing steel product according to the invention with a medium manganese content based on the alloying elements C, Mn, Al, Mo and Si is cost-effective, since it relies on an increased addition of nickel of up to 9% by weight to achieve the low-temperature toughness can generally be dispensed with. The steel product according to the invention has a stable austenite component even at low temperatures down to at least −196 ° C., which converts at the earliest when deformed at low temperatures, but is otherwise metastable to stable. This austenite content of at least 2% by volume, which is present at low temperatures, improves the low-temperature toughness and thus the elongation properties.

The steel product according to the invention can advantageously be used as a substitute for steels with a high Ni content in low-temperature applications, such as in the areas of shipbuilding, boiler construction / container construction, construction machinery, transport vehicles, crane construction, mining, mechanical and plant engineering, power plant industry, oil field pipes, Petrochemicals, wind turbines, pressure pipelines, precision pipes, pipes in general and for the substitution of high-alloy steels, in particular Cr, CrN, CrMnN, CrNi, CrMnNi steels.

The optionally alloyed elements advantageously have the following contents in% by weight: Ti: 0.002 to 0.5; V: 0.006 to 0.1; Cr: 0.05 to 4; Cu: 0.05 to 2; Nb: 0.003 to 0.1; B: 0.0005 to 0.014; Co: 0.003 to 3; W: 0.03 to 2; Zr: 0.03 to 1; Ca: <0.004 and Sn: <0.5

The steel product according to the invention, in particular in the form of a seamless tube, has a multiphase structure consisting of 2 to 90% by volume, preferably up to 80% by volume or up to 70% by volume of austenite, less than 40% by volume, preferably less than 20% by volume ferrite and / or bainite and the remainder martensite or tempered martensite and optionally a TRIP and / or TWIP effect. Some of the martensite is in the form of tempered martensite and some of the austenite of up to 90% can be in the form of annealing or deformation twins. The steel can optionally have both a TRIP and a TWIP effect, with part of the austenite being able to convert into martensite during a subsequent deformation / forming / processing of the steel strip, whereby at least 20% of the original austenite must be retained in order to maintain the low-temperature properties to guarantee.

The steel product according to the invention is also characterized by an increased resistance to delayed crack formation (delayed fracture) and to hydrogen embrittlement. This is mainly achieved by precipitating molybdenum carbide, which acts as a hydrogen trap. In addition, the steel has a high resistance to liquid metal embrittlement (LME) during welding.

The use of the term "to" in the definition of the content ranges, such as 0.01 to 1% by weight, means that the benchmarks - in the example 0.01 and 1 - are included.

The steel according to the invention is particularly suitable for producing heavy plate or hot and cold strip as well as welded and seamless tubes which can be provided with metallic or non-metallic, organic or other inorganic coatings.

The steel product advantageously has a yield strength Rp0.2 of 450 to 1150 MPa, a tensile strength Rm of 500 to 2100 MPa and an elongation at break A50 of more than 6% to 45% at room temperature, with higher tensile strengths tending to be associated with lower elongation at break and vice versa are. For the elongation at break investigations with tensile tests, a flat specimen with an initial measurement length of A50 was used in accordance with DIN 50 125.

Alloy elements are usually added to steel in order to specifically influence certain properties. An alloy element can influence different properties in different steels. The effect and interaction generally depends considerably on the amount, the presence of other alloying elements and the state of solution in the material. The relationships are varied and complex. In the following, the effect of the alloying elements in the alloy according to the invention will be discussed in more detail. The positive effects of the alloying elements used according to the invention are described below:
Carbon C: C is required for the formation of carbides, stabilizes the austenite and increases the strength. Higher contents of C worsen the welding properties and lead to a deterioration in the elongation and toughness properties, which is why a maximum content of less than 0.3% by weight is specified. In order to achieve a fine precipitation of carbides, a minimum addition of 0.01% by weight is required. For an optimal combination of mechanical properties and weldability, the C content is advantageously set at 0.03 to 0.15% by weight.

Manganese Mn: Mn stabilizes the austenite, increases the strength and the toughness and optionally enables a deformation-induced martensite and / or twin formation in the alloy according to the invention. Contents of less than 4% by weight are not sufficient to stabilize the austenite and thus worsen the elongation properties, while contents of 10% by weight and more, the austenite is stabilized too strongly, so that the deformation-induced mechanisms TRIP and TWIP effect are not sufficiently effective and thereby the strength properties, in particular the 0.2% yield strength, are reduced. For the manganese steel according to the invention with medium manganese contents, a range of 4 to <8% by weight is preferred.

Aluminum Al: Al is used to deoxidize the melt. An Al content of 0.003% by weight and more is used to deoxidize the melt. This results in a higher effort when potting. Al contents of more than 0.03% by weight completely deoxidize the melt, influence the transformation behavior and improve the strength and elongation properties. Al contents of more than 2.9% by weight deteriorate the elongation properties. Higher Al contents also significantly worsen the casting behavior in continuous casting. Therefore, a maximum content of 2.9% by weight and a minimum content of more than 0.003% by weight are specified. However, the steel preferably has an Al content of 0.03 to 0.4% by weight.

Furthermore, with contents of Ni> 0.01% by weight, a minimum content (in% by weight) of more than 0.11 and less than 3 should be observed for the sum of C and Al, which increases the strength of the austenite in particular C is increased, but the precipitation of undesirably coarse carbides is suppressed by Al. A C + Al content of 3% by weight and more deteriorates the strength properties and makes production difficult. With total C + Al contents of 0.11% by weight or less, tensile strengths of> 1200 MPa cannot be achieved with the specified alloy after the final heat treatment.

Silicon Si: The addition of Si in contents of more than 0.02% by weight hinders the carbon diffusion, reduces the specific density and increases the strength and the elongation and toughness properties. Furthermore, an improvement in cold rollability could be observed through the addition of Si. Contents of more than 0.8% by weight lead to embrittlement of the material and have a negative impact on hot and cold rollability and coatability, for example by galvanizing. Therefore, a maximum content of 0.8% by weight and a minimum content of 0.02% by weight are specified. Contents of 0.08 to 0.3% by weight have proven to be optimal.

Molybdenum Mo: Mo acts as a carbide former, increases strength and increases resistance to hydrogen-induced delayed cracking and hydrogen embrittlement. Contents of Mo of more than 0.8% by weight deteriorate the elongation properties, which is why a maximum content of 0.8% by weight and a minimum content of 0.01% by weight, which is necessary for sufficient effectiveness, are specified. A Mo content of 0.1 to 0.5% by weight has proven to be advantageous in terms of increasing strength in combination with the lowest possible cost.

Phosphorus P: P is a trace or accompanying element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. As a rule, however, attempts are made to lower the phosphorus content as much as possible, since it is, among other things, highly susceptible to segregation due to its low diffusion rate and to a great extent reduces the toughness. The accumulation of phosphorus at the grain boundaries can cause cracks to appear along the grain boundaries during hot rolling. In addition, phosphorus increases the transition temperature from tough to brittle behavior by up to 300 ° C. For the reasons mentioned above, the phosphorus content is limited to values less than 0.04% by weight.

Like phosphorus, sulfur S: S is bound as a trace or accompanying element in iron ore or is introduced through coke during production via the blast furnace route. It is generally undesirable in steel because it tends to segregate strongly and has a strong embrittling effect, as a result of which the elongation and toughness properties are impaired. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt (e.g. through deep desulphurisation). For the reasons mentioned above, the sulfur content is limited to values less than 0.02% by weight.

Nitrogen N: N is also an accompanying element in steel production. He improves in the dissolved state for steels with a higher manganese content with greater than or equal to 4% weight% Mn, the strength and toughness properties. Lower Mn-alloyed steels with less than 4% by weight tend to have a strong aging effect in the presence of free nitrogen. The nitrogen diffuses at dislocations even at low temperatures and blocks them. It thus causes an increase in strength combined with a rapid loss of toughness. A setting of the nitrogen in the form of nitrides is possible, for example, by adding aluminum and / or titanium as well as Nb, V, B, aluminum nitrides in particular having a negative effect on the forming properties of the alloy according to the invention. For the reasons mentioned above, the nitrogen content is limited to less than 0.02% by weight.

Titanium Ti: When optionally added, Ti acts as a carbide former to refine the grain, which at the same time improves strength, toughness and elongation properties. Furthermore, Ti reduces intergranular corrosion. Contents of Ti of more than 0.5% by weight deteriorate the elongation properties, which is why a maximum Ti content of 0.5% by weight is specified. A minimum content of 0.002 is optionally specified in order to advantageously eliminate nitrogen with Ti.

Vanadium V: When optionally added, V acts as a carbide former to refine the grain, which at the same time improves strength, toughness and elongation properties. Contents of V of more than 0.1% by weight give no further advantages, which is why a maximum content of 0.1% by weight is specified. A minimum content of 0.006% by weight is optionally specified, which is necessary for the separation of the finest carbides.

Chromium Cr: With the optional addition, Cr increases the strength and reduces the corrosion rate, delays the formation of ferrite and pearlite and forms carbides. The maximum content is set at 4% by weight, since higher contents result in a deterioration in the elongation properties. A minimum Cr content for effectiveness is set at 0.05% by weight.

Nickel Ni: The addition of at least 0.01% by weight of nickel stabilizes the austenite, especially at lower temperatures, and improves the strength and toughness properties and reduces carbide formation. The For reasons of cost, the maximum content is set at 3% by weight. A maximum Ni content of 1% by weight has proven to be particularly economical.

A particularly cost-effective alloy system can be achieved if the following condition is met in combination with manganese: 6 <1.5 Mn + Ni <8.

Copper Cu: Cu reduces the rate of corrosion and increases strength. Contents of more than 2% by weight worsen the producibility due to the formation of low-melting phases during casting and hot rolling, which is why a maximum content of 2% by weight is specified. In order to achieve a strength-increasing effect through Cu, a minimum of 0.05% by weight is specified.

Niobium Nb: When optionally added, Nb acts as a carbide former to refine the grain, which at the same time improves strength, toughness and elongation properties. Contents of Nb of more than 0.1% by weight give no further advantages, which is why a maximum content of 0.1% by weight is specified. Optionally, a minimum content of 0.003% by weight is specified, which is necessary for the separation of the finest carbides.

Boron B: B retards the austenite transformation, improves the hot forming properties of steels and increases the strength at room temperature. It develops its effect even with very low alloy contents. Contents above 0.008% by weight increasingly deteriorate the elongation and toughness properties, which is why the maximum content is set at 0.014% by weight. A minimum content of 0.0005% by weight is optionally specified in order to take advantage of the strength-increasing effect of boron.

Cobalt Co: Co increases the strength of the steel and stabilizes the austenite. Contents of more than 3% by weight worsen the elongation properties, which is why a maximum content of 3% by weight is optionally specified. An optional minimum content of 0.003% by weight is preferably provided, which, in addition to the strength properties, particularly advantageously influences the austenite stability.

Tungsten W: W acts as a carbide former and increases strength. W contents of more than 2% by weight deteriorate the elongation properties, which is why a maximum W content of 2% by weight is specified. An optional minimum content of 0.03% by weight is specified for the effective elimination of carbides.

Zirconium Zr: Zr acts as a carbide former and improves strength. Contents of Zr of more than 1% by weight deteriorate the elongation properties, which is why a maximum content of 1% by weight is specified. In order to enable the precipitation of carbides, an optional minimum content of 0.03% by weight is specified.

Calcium Ca: Ca is used to modify non-metallic oxidic inclusions, which otherwise could lead to undesired failure of the alloy due to inclusions in the structure, which act as stress concentration points and weaken the metal bond. Furthermore, Ca improves the homogeneity of the alloy according to the invention. Contents above 0.004% by weight Ca do not result in any further advantage in the inclusion modification, impair the producibility and are to be avoided due to the high vapor pressure of Ca in steel melts. Therefore, an optional maximum content of 0.004% by weight is provided.

Tin Sn: Sn increases the strength, but, like copper, accumulates under the scale and at the grain boundaries at higher temperatures. By penetrating into the grain boundaries, it leads to the formation of low-melting phases and the associated cracks in the structure and to solder brittleness, which is why a maximum content of less than 0.5% by weight is optionally provided.

• Erschmelzen einer Stahlschmelze nach einem der Ansprüche 1 bis 3 über die Prozessroute Hochofen-Stahlwerk oder Elektrolichtbogenofen-Stahlwerk jeweils mit optionaler Vakuumbehandlung der Schmelze;
• Vergießen der Stahlschmelze zu einem Vorband mittels eines endabmessungsnahen horizontalen oder vertikalen Bandgießverfahrens oder Vergießen der Stahlschmelze zu einer Bramme oder Dünnbramme mittels eines horizontalen oder vertikalen Brammen- oder Dünnbrammengießverfahrens,
• Erwärmen auf eine Walztemperatur von 1050 °C bis 1250 °C oder Inlinewalzen aus der Gießhitze heraus,
• Warmwalzen des Vorbandes oder der Bramme oder der Dünnbramme zu einem Grobblech mit einer Dicke von über 3 bis 200 mm oder einem Warmband mit einer Dicke von 0,8 bis 28 mm, mit einer Walzendtemperatur von 650 °C bis 1050 °C,
• Aufhaspeln des Warmbandes bei einer Temperatur von mehr als 100 °C bis 600 °C,
• optional Beizen des Warmbandes,
• optional Glühen des Grobbleches oder des Warmbandes in einer Glühanlage bei einer Glühzeit von 0,3 bis 24 h und Temperaturen von 500 °C bis 840 °C, bevorzugt 520 °C bis 600 °C bei einer Glühzeit von 0,5 bis 6 h,
• optional Kaltwalzen des Warmbandes bei Raumtemperatur oder erhöhter Temperatur von 60 °C bis 450 °C vor dem ersten Walzstich in einem oder mehreren Walzstichen auf eine Dicke von ≤ 3 mm mit einem Abwalzgrad von 10 bis 90%, vorzugsweise 30 bis 60%,
• optional Glühen des Kaltbandes in einer Glühanlage bei einer Glühzeit von 0,3 bis 24 h und Temperaturen von 500 °C bis 840 °C, bevorzugt 520 °C bis 600 °C bei einer Glühzeit von 0,5 bis 6 h,
• optional Dressieren des Warm- oder Kaltbandes,
• optional elektrolytisches Verzinken, Feuerverzinken oder Beschichten mit einer organischen oder anorganischen Beschichtung, wobei das Stahlflachprodukt eine hervorragende Tieftemperaturzähigkeit bei Temperaturen von unter -196 °C und eine gute Kombination von Festigkeits-, Dehnungs- und Umformeigenschaften aufweist.

A steel product in the form of a flat steel product, such as hot strip, cold strip or heavy plate, is supplied according to the invention by a method comprising the steps:

• Melting a steel melt according to one of Claims 1 to 3 via the process route blast furnace-steelworks or electric-arc furnace-steelworks, each with optional vacuum treatment of the melt;
• Pouring the steel melt into a pre-strip by means of a near-net-shape horizontal or vertical strip casting process or casting the steel melt into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,
• Heating to a rolling temperature of 1050 ° C to 1250 ° C or inline rolling from the casting heat,
• Hot rolling of the pre-strip or the slab or the thin slab into a heavy plate with a thickness of more than 3 to 200 mm or a hot strip with a thickness of 0.8 to 28 mm, with a final rolling temperature of 650 ° C to 1050 ° C,
• Coiling of the hot strip at a temperature of more than 100 ° C to 600 ° C,
• optional pickling of the hot strip,
• optional annealing of the heavy plate or the hot strip in an annealing plant with an annealing time of 0.3 to 24 h and temperatures of 500 ° C to 840 ° C, preferably 520 ° C to 600 ° C with an annealing time of 0.5 to 6 h,
• optional cold rolling of the hot strip at room temperature or at an elevated temperature of 60 ° C to 450 ° C before the first rolling pass in one or more rolling passes to a thickness of ≤ 3 mm with a degree of rolling of 10 to 90%, preferably 30 to 60%,
• optional annealing of the cold strip in an annealing plant with an annealing time of 0.3 to 24 h and temperatures of 500 ° C to 840 ° C, preferably 520 ° C to 600 ° C with an annealing time of 0.5 to 6 h,
• optional skin-passing of the hot or cold strip,
• optionally electrolytic galvanizing, hot-dip galvanizing or coating with an organic or inorganic coating, the flat steel product having excellent low-temperature toughness at temperatures below -196 ° C and a good combination of strength, elongation and deformation properties.

In the case of further processing of the flat steel product into a longitudinally or spiral-welded pipe, the annealing required to achieve the required low-temperature toughness and thus the setting of the final structure can not be carried out on the hot or cold strip, but optionally only after the Tube production take place, the annealing of the tube in an annealing plant with an annealing time of 0.3 to 24 h and temperatures of 500 ° C to 840 ° C, preferably 520 ° C to 600 ° C with an annealing time of 0.5 to 6 h he follows. If necessary, the tube can be given an organic or inorganic coating on one or both sides after annealing.

With regard to further advantages, reference is made to the statements made above on the steel according to the invention.

The usual thickness ranges for pre-strip are 1 mm to 35 mm and for slabs and thin slabs 35 mm to 450 mm. It is preferably provided that the slab or thin slab is hot-rolled into a heavy plate with a thickness of more than 3 mm to 200 mm or a hot strip with a thickness of 0.8 mm to 28 mm, or the pre-strip, cast close to its final dimensions, is hot-rolled into a hot strip with a thickness of 0.8 mm to 3 mm is hot rolled. The cold strip according to the invention has a thickness of at most 3 mm, preferably 0.1 mm to 1.4 mm.

In connection with the above method according to the invention, a pre-strip produced near net dimensions using the two-roll casting method with a thickness of less than or equal to 3 mm, preferably 1 mm to 3 mm, is already understood as hot strip. The pre-strip produced in this way as hot strip does not have an original cast structure due to the reshaping of the two counter-rotating rolls. Hot rolling thus already takes place inline during the two-roller casting process, so that separate hot rolling can optionally be omitted.

The cold rolling of the hot strip can take place at room temperature or advantageously at an elevated temperature before the first rolling pass, in one or more rolling passes.

Cold rolling at elevated temperatures is advantageous in order to reduce the rolling forces and to promote the formation of deformation twins (TWIP effect). Advantageous temperatures of the rolling stock before the first rolling pass are 60 ° C to 450 ° C.

If necessary, the steel strip can be skin-pass after cold rolling, whereby the surface structure (topography) required for the end application is set. Passing can be done using the Pretex® process, for example.

In an advantageous further development, the flat steel product produced in this way receives a surface refinement, for example by electrolytic galvanizing or hot-dip galvanizing and, instead of galvanizing or in addition, a coating on an organic or inorganic basis. The coating systems can be, for example, organic coatings, plastic coatings or lacquers or other inorganic coatings such as iron oxide layers.

The flat steel product produced according to the invention can be used both as sheet metal, sheet metal section or blank or further processed into a pipe that is welded longitudinally or helically.

• Erschmelzen einer Stahlschmelze enthaltend (in Gewichts-%): C: 0,1 bis < 0,3; Mn: 4 bis < 10; Al: 0,003 bis 2,9; Mo: 0,01 bis 0,8; Si: 0,02 bis 0,8; Ni: 0.01 bis 3; P: < 0,04; S: < 0,02; N < 0,02; Rest Eisen einschließlich unvermeidbarer stahlbegleitender Elemente, wobei
• für die Legierungszusammensetzung die Gleichung
  6 < 1,5 Mn + Ni < 8 erfüllt ist, mit optionaler Zulegierung von einem oder mehreren der folgenden Elemente: Ti: 0,002 bis 0,07; V: 0,006 bis 0,1; Cr: 0,05 bis 4; Cu: 0,05 bis 2; Nb: 0,003 bis 0,1; B: 0,0005 bis 0,014; Co: 0,003 bis 3; W: 0,03 bis 2; Zr: 0,03 bis 1; Ca: weniger als 0,004; Sn: weniger als 0,5,
• oder für die Legierungszusammensetzung die Gleichung 0,11 < C + Al < 3 erfüllt ist, mit optionaler Zulegierung von einem oder mehreren der folgenden Elemente: Ti: 0,002 bis 0,07; V: 0,006 bis 0,1; Cr: 0,05 bis 4; Cu: 0,05 bis 2; Nb: 0,003 bis 0,1; B: 0,0005 bis 0,014; Co: 0,003 bis 3; W: 0,03 bis 2; Zr: 0,03 bis 1; Ca: weniger als 0,004; Sn: weniger als 0,5,
• oder die Legierungszusammensetzung neben Ni mindestens eines oder mehrere der Elemente B, V, Nb, Co, W oder Zr enthält, mit optionaler Zulegierung von einem oder mehreren der folgenden Elemente: Ti: 0,002 bis 0,07; Cr: 0,05 bis 4; Cu: 0,05 bis 2; Ca: weniger als 0,004; Sn: weniger als 0,5
  über die Prozessroute Hochofen-Stahlwerk oder Elektrolichtbogenofen-Stahlwerk jeweils mit optionaler Vakuumbehandlung der Schmelze;
• Vergießen des Stahls in einem Stranggießverfahren zu einem Strang und Teilen des Strangs in einen Stranggussabschnitt, insbesondere einen massiven Block,
• Erwärmen des Blocks auf eine Umformtemperatur von 700 °C bis 1250 °C,
• Lochen des auf Umformtemperatur befindlichen Blocks zu einem Hohlblock
• Optional Wieder-Erwärmen des Hohlblocks vor einem Warmwalzen auf 700 °C bis 1250 °C
• Warmwalzen zu einem nahtlosen Rohr, beispielsweise in einem Stopfenwalzwerk, Schrägwalzwerk, Lösewalzwerk, Diescherwalzwerk, Asselwalzwerk, Kontiwalzwerk, Pilgerwalzwerk oder einer Stoßbankanlage mit beispielsweise folgendem Ablauf: Fertigung eines Hohlblocks aus einem Vorblock, anschließendes Elongieren (Strecken) des Hohlblocks zu einer Luppe (dickwandiges Rohr) und Fertigwalzen der Luppe zum Rohr
• Optional Zwischenerwärmen zwischen den Walzschritten auf eine Temperatur von 60 °C bis 1250 °C
• Optional Fertigwalzen des nahtlosen Rohres bei einer Temperatur von Raumtemperatur bis unterhalb Ac3-Temperatur, bevorzugt 60 °C bis 450°C unter bevorzugter Ausnutzung des TWIP-Effekts
• Optionales Beizen des Rohres
• Optional Nachwalzen oder Kalibrierwalzen oder sonstige anschließendes Umformen des Rohres beispielsweise Ziehen mittels Reduzierring, Aufweiten oder Innenhochdruckumformen, optional bei einer Temperatur von Raumtemperatur bis unterhalb Ac3-Temperatur, bevorzugt 60 °C bis 450 °C
• Optional Ausnutzen des TRIP-Effektes bei Umformen von Raumtemperatur bis 60 °C zur Erzielung einer höheren Festigkeit
• Optional Ausnutzen des TWIP-Effektes bei Umformen in einem Temperaturbereich von 60 °C bis 450 °C zur Erzielung einer höheren Restbruchdehnung und höheren Streckgrenze
• Optional abschließendes Wärmebehandeln bei 400 °C bis 900 °C für 1 min bis 24 h in einer kontinuierlichen oder diskontinuierlich arbeitenden Glüheinrichtung, wobei kürzere Zeiten tendenziell höheren Temperaturen zugeordnet werden und umgekehrt
• Optional Weiterverarbeiten des nahtlosen Rohres zu einem Bauteil mittels Innenhochdruckumformung, Halbwarmumformung oder Halbwarm-Innenhochdruckumformung.

If seamless tubes are to be produced as steel products, these can advantageously be produced according to the invention using the following process steps:

• Melting a steel melt containing (in% by weight): C: 0.1 to <0.3; Mn: 4 to <10; Al: 0.003 to 2.9; Mo: 0.01 to 0.8; Si: 0.02 to 0.8; Ni: 0.01 to 3; P: <0.04; S: <0.02; N <0.02; Remainder iron including unavoidable steel-accompanying elements, whereby
• for the alloy composition the equation
  6 <1.5 Mn + Ni <8 is fulfilled, with the optional addition of one or more of the following elements: Ti: 0.002 to 0.07; V: 0.006 to 0.1; Cr: 0.05 to 4; Cu: 0.05 to 2; Nb: 0.003 to 0.1; B: 0.0005 to 0.014; Co: 0.003 to 3; W: 0.03 to 2; Zr: 0.03 to 1; Ca: less than 0.004; Sn: less than 0.5,
• or for the alloy composition the equation 0.11 <C + Al <3 is fulfilled, with the optional addition of one or more of the following elements: Ti: 0.002 to 0.07; V: 0.006 to 0.1; Cr: 0.05 to 4; Cu: 0.05 to 2; Nb: 0.003 to 0.1; B: 0.0005 to 0.014; Co: 0.003 to 3; W: 0.03 to 2; Zr: 0.03 to 1; Ca: less than 0.004; Sn: less than 0.5,
• or the alloy composition contains at least one or more of the elements B, V, Nb, Co, W or Zr in addition to Ni, with the optional addition of one or more of the following elements: Ti: 0.002 to 0.07; Cr: 0.05 to 4; Cu: 0.05 to 2; Ca: less than 0.004; Sn: less than 0.5
  via the process route blast furnace-steelworks or electric-arc furnace-steelworks, each with optional vacuum treatment of the melt;
• Casting the steel in a continuous casting process to form a strand and dividing the strand into a continuously cast section, in particular a solid block,
• Heating the block to a forming temperature of 700 ° C to 1250 ° C,
• Punch the block, which is at the forming temperature, into a hollow block
• Optionally reheating the hollow block to 700 ° C to 1250 ° C before hot rolling
• Hot rolling to a seamless tube, for example in a plug rolling mill, cross rolling mill, loosening mill, diescher rolling mill, Assel rolling mill, continuous rolling mill, pilger rolling mill or a push bench system with, for example, the following sequence: production of a hollow block from a bloom, subsequent elongation (stretching) of the hollow block to form a hollow (thick-walled tube) ) and finish rolling the billet to the pipe
• Optional intermediate heating between the rolling steps to a temperature of 60 ° C to 1250 ° C
• Optionally, finish rolling the seamless tube at a temperature from room temperature to below Ac3 temperature, preferably 60 ° C. to 450 ° C. with preferred utilization of the TWIP effect
• Optional pickling of the pipe
• Optional re-rolling or calibrating rolling or other subsequent forming of the tube, for example drawing by means of a reducing ring, expanding or internal high-pressure forming, optionally at a temperature from room temperature to below Ac3 temperature, preferably 60 ° C to 450 ° C
• Optional use of the TRIP effect when forming from room temperature to 60 ° C to achieve higher strength
• Optional use of the TWIP effect when forming in a temperature range of 60 ° C to 450 ° C to achieve a higher residual elongation at break and a higher yield point
• Optional final heat treatment at 400 ° C to 900 ° C for 1 min to 24 h in a continuous or discontinuous annealing device, with shorter times tending to be associated with higher temperatures and vice versa
• Optional further processing of the seamless tube into a component by means of hydroforming, warm forming or warm hydroforming.

A solid block (round cast bar) is essentially understood to mean a continuously cast section produced by round casting, which section already has a desired length.

In connection with the aforementioned method, it is expressly pointed out that the method steps specified as optional can all or every sub-combination thereof also be provided in the method.

Warm forming or warm internal high pressure forming is the term used here for forming and internal high pressure forming processes in which at least the first forming step takes place at a temperature above room temperature to below the Ac3 temperature, preferably at 60 ° C to 450 ° C.

Tests were carried out to investigate the mechanical properties of alloys 1 and 2 not according to the invention with regard to the Ni content and with a standard alloy. The standard alloy and alloys 1 and 2 contain the following elements in the listed contents in% by weight: alloy C. Ni Mn Si P. S. Mon V. B. X8Ni9 / 1.5662 (standard) Max 0.1 8.5-10.0 0.3-0.8 Max. 0.35 Max. 0.02 Max. 0.01 Max 0.1 0.05 max - Leg. 1 0.03 0.004 6.4 0.12 0.023 0.006 0.43 - 0.001 Leg. 2 0.06 0.004 6.3 0.12 0.022 0.006 0.43 - -

The steel products made from the aforementioned alloys were subjected to different heat treatments and the impact energy was measured in accordance with the Charpy impact test with V-notch: alloy Status Heat treatment Impact work at -196 ° C [J / cm ² ] in the transverse direction X8Ni9 / 1.5662 (standard) hardened and tempered According to standard 1.5662 ≥ 50 Leg. 1 annealed 600 ° C, 2.5 h ≥ 64 Leg. 2 annealed 580 ° C, 4 h ≥ 58

Properties of the steel strips produced from the aforementioned alloys were also determined under the same treatment condition. Characteristic values for hot strip / heavy plate are shown below: alloy Re (upper yield point) [MPa] Rm [MPa] Elongation at break (A50) [%] X8Ni9 / 1.5662 (standard) > 585 680-820 > 13.7 Leg. 1 790 820 17.6 Leg. 2 855 867 11.5

The elongation at break A50 of the X8Ni9 was converted in accordance with DIN ISO 2566/1 from the elongation at break A5.65 in accordance with the standard to a sample cross-section of 20 mm. The elongation parameters represent the elongation in the rolling direction.

Claims (13)

1. Steel product for low temperature use with a minimum notch impact energy according to the Charpy V-notch impact test at -196°C in a transverse direction of ≥50 J/cm², having the following chemical composition in wt.%: C: 0.01 to <0.3, preferably 0.03 to 0.15; Mn: 4 to <10, preferably 4 to <8; Al: 0.003 to 2.9, preferably 0.03 to 0.4; Mo: 0.01 to 0.8, preferably 0.1 to 0.5; Si: 0.02 to 0.8, preferably 0.08 to 0.3; Ni: 0.01 to 3; P: <0.04; S: <0.02; N: <0.02; with the remainder being iron including unavoidable steel-accompanying elements, wherein for the alloy composition the equation 6 <1.5 Mn + Ni <8 is satisfied, with optional addition by alloying of one or more of the following elements in wt.%; Ti: 0.002 to 0.5; V: 0.006 to 0.1; Cr: 0.05 to 4; Cu; 0.05 to 2; Nb: 0.003 to 0.1; B: 0.0005 to 0.014; Co: 0.003 to 3; W: 0.03 to 2; Zr: 0.03 to 1; Ca: <0.004 and Sn: <0.5;
   comprising a microstructure consisting of 2 to 90 vol.% austenite, less than 40 vol.% ferrite and/or bainite, with the remainder being martensite.
2. Steel product for low temperature use with a minimum notch impact energy according to the Charpy V-notch impact test at -196°C in a transverse direction of ≥50 J/cm², having the following chemical composition in wt.%: C: 0.01 to <0.3, preferably 0.03 to 0.15; Mn: 4 to <10, preferably 4 to <8; Al: 0.003 to 2.9, preferably 0.03 to 0.4; Mo: 0.01 to 0.8, preferably 0.1 to 0.5; Si: 0.02 to 0.8, preferably 0.08 to 0.3; Ni: 0.01 to 3; P: <0.04; S: <0.02; N: <0.02; with the remainder being iron including unavoidable steel-accompanying elements, wherein for the alloy composition the equation 0.11 < C + Al <3 is satisfied, with optional addition by alloying of one or more of the following elements in wt.%; Ti: 0.002 to 0.5; V: 0.006 to 0.1; Cr: 0.05 to 4; Cu; 0.05 to 2; Nb: 0.003 to 0.1; B: 0.0005 to 0.014; Co: 0.003 to 3; W: 0.03 to 2; Zr: 0.03 to 1; Ca: <0.004 and Sn: <0.5;
   comprising a microstructure consisting of 2 to 90 vol.% austenite, less than 40 vol.% ferrite and/or bainite, with the remainder being martensite.
3. Steel product for low temperature use with a minimum notch impact energy according to the Charpy V-notch impact test at -196°C in a transverse direction of ≥50 J/cm², having the following chemical composition in wt.%: C: 0.01 to <0.3, preferably 0.03 to 0.15; Mn: 4 to <10, preferably 4 to <8; Al: 0.003 to 2.9, preferably 0.03 to 0.4; Mo: 0.01 to 0.8, preferably 0.1 to 0.5; Si: 0.02 to 0.8, preferably 0.08 to 0.3; Ni: 0.01 to 3; P: <0.04; S: <0.02; N: <0.02; with the remainder being iron including unavoidable steel-accompanying elements, wherein, in addition to Ni, the alloy composition contains at least one or more of the following elements in wt.%; B: 0.0005 to 0.014; V: 0.006 to 0.1; Nb: 0.003 to 0.1; Co: 0.003 to 3; W: 0.03 to 2 or Zr: 0.03 to 1, with optional addition by alloying of one or more of the following elements in wt.%: Ti: 0.002 to 0.5; Cr: 0.05 to 4; Cu: 0.05 to 2; Ca: <0.004 and Sn: <0.5;
   comprising a microstructure consisting of 2 to 90 vol.% austenite, less than 40 vol.% ferrite and/or bainite, with the remainder being martensite.
4. Steel product as claimed in at least one of claims 1 to 3, characterised in that the microstructure of the steel product, in particular of a seamless pipe, comprises an austenite proportion of 2 to 80 vol.%, preferably 2-70 vol.%, a ferrite and/or bainite proportion of less than 20 vol.% with the remainder being martensite.
5. Steel product as claimed in at least one of claims 1 to 4, characterised in that a proportion of at least 20% of the martensite is present as tempered martensite.
6. Steel product as claimed in at least one of claims 1 to 5, characterised in that a proportion of up to 90% of the austenite is present in the form of annealing or deformation twins.
7. Steel product as claimed in at least one of claims 1 to 6, characterised in that the steel has a yield strength Rp0.2 of 450 to 1050 MPa, a tensile strength Rm of 500 to 1500 MPa and an elongation at break A50 of more than 6 to 45%.
8. Steel product as claimed in at least one of claims 1 to 7, characterised in that the steel product is metallically, inorganically or organically coated and optionally one or more further metal, other inorganic or organic coatings are optionally applied to the coating.
9. Method for producing a steel product in the form of a flat steel product, comprising the following steps:

   - melting a steel melt as claimed in any one of claims 1 to 3 by means of the blast furnace-steel mill processing route or the arc furnace process in each case with optional vacuum treatment of the melt;

   - casting the steel melt to form a pre-strip by means of a horizontal or vertical strip casting process approximating the final dimensions or casting the steel melt to form a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,

   - heating to a rolling temperature of 1050°C to 1250°C or in-line rolling from the casting heat,

   - hot-rolling the pre-strip or the slab or the thin slab to form a heavy plate with a thickness of above 3 to 200 mm or to form a hot strip with a thickness of 0.8 to 22 mm at a final rolling temperature of 650 to 1050°C,

   - reeling the hot strip at a temperature of above 100°C to 600°C,

   - optionally pickling the hot strip,

   - optionally annealing the heavy plate or the hot strip in an annealing plant for an annealing time of 0.3 to 24 hours and at temperatures of 500°C to 840°C, preferably 520°C to 600°C for an annealing time of 0.5 to 6 hours,

   - optionally cold-rolling the hot strip at ambient temperature or at a raised temperature upstream of the first reduction stage in one or more reduction stages to a thickness of ≤3 mm with a degree of thinning by rolling of 10 to 90%, preferably 30 to 60%,

   - optionally annealing the cold strip in an annealing plant for an annealing time of 0.3 to 24 hours and at temperatures of 500°C to 840°C, preferably 520°C to 600°C for an annealing time of 0.5 to 6 hours,

   - optionally skin pass rolling the hot or cold strip,

   - optionally electrolytically galvanising, hot-dip galvanising or coating with an organic or inorganic coating.
10. Method as claimed in claim 9, characterised in that the first reduction stage takes place during cold-rolling of the hot strip at a temperature of 60°C to 450°C.
11. Method as claimed in claim 9 and 10, characterised in that the flat steel product is further processed to form a component.
12. Method as claimed in claim 11, characterised in that the component is a pipe with a longitudinal welded seam or a helical welded seam.
13. Method for producing a steel product in the form of a seamless pipe, comprising the following steps:

    - melting a steel melt as claimed in any one of claims 1 to 3 by means of the blast furnace-steel mill or electric arc furnace steel mill processing route in each case with optional vacuum treatment of the melt;

    - casting the steel in a continuous casting process to form a strand and dividing the strand into a solid block,

    - heating the block to a forming temperature of 700°C to 1250°C,

    - piercing the block at forming temperature to form a hollow block,

    - optionally reheating the hollow block prior to hot-rolling to 700°C to 1250°C,

    - hot-rolling by means of elongation of the hollow block to form a hollow and finish-rolling the hollow to form the pipe,

    - optionally intermediately heating between the rolling steps to a temperature of 60°C to 1250°C,

    - optionally finish-rolling the seamless pipe at a temperature of ambient temperature to below Ac3 temperature, preferably 60°C to 450°C, preferably exploiting the TWIP effect,

    - optionally pickling the pipe,

    - optionally temper-rolling or calibration-rolling,

    - optionally subsequently forming or drawing the pipe,

    - optionally widening or internal high-pressure forming, optionally at a temperature of ambient temperature to below Ac3 temperature, preferably 60°C to 450°C,

    - optionally exploiting the TRIP effect during forming from ambient temperature to 60°C in order to achieve an increased level of strength,

    - optionally exploiting the TWIP effect during forming in a temperature range of 60°C to 450°C in order to achieve an increased residual elongation at break and raised elastic limit,

    - optionally finally heat-treating at 400°C to 900°C for 1 minute to 24 hours in a continuous or stationary annealing apparatus, wherein shorter times tend to be associated with increased temperatures and vice versa,

    - optionally further processing the seamless pipe to form a component by means of internal high-pressure forming, warm-forming or warm-internal high-pressure forming.