EP4161720A1

EP4161720A1 - Steel material for forming components using additive manufacturing and use of a steel material of this type

Info

Publication number: EP4161720A1
Application number: EP21729569.0A
Authority: EP
Inventors: Johannes Boes; Arne RÖTTGER; Werner Theisen; Christoph Escher; Christian MUTKE; Horst HILL; Philipp Kluge
Original assignee: Doerrenberg Edelstahl GmbH; Deutsche Edelstahlwerke Specialty Steel GmbH and Co KG
Current assignee: Doerrenberg Edelstahl GmbH; Deutsche Edelstahlwerke Specialty Steel GmbH and Co KG
Priority date: 2020-06-05
Filing date: 2021-06-02
Publication date: 2023-04-12
Also published as: WO2021245158A1; DE102020115049A1; US20230220526A1

Abstract

The invention relates to a steel material which allows for components to be formed with low residual stress via additive manufacturing without pre- or post-heating. The steel material consists of a steel with the following composition, in wt.%: C: 0.28 - 0.65 %, Co: < 10.0, Cr: 3.5 - 12.5 %, optionally Mo: 0.5 - 12.5 %, wherein the sum of the content of Cr and Mo is 4 - 16 %, the Ni equivalent Ni_eq calculated according to the formula Ni_eq [%] = 30 %C + %Ni + 0.5 %Mn from the C-content %C, the Ni-content %Ni, the Mn-content %Mn fulfills the condition (1) 10 % ≤ Ni eq ≤ 20 %, and alongside C, optionally respectively up to 9% Mn and up to 4.5% Ni are provided to fulfill condition (1), wherein the Cr equivalent Cr_eq calculated according to the formula Cr_eq [mass] = %Cr + %Mo + 1.5 %S + 0.5 %Nb + 2 %XX from the CR-content Cr%, the Mo-content Mo%, the Si-content Si%, the Nb-content %Nb and the sum %XX of the contents of at least one element of the group "Sc, Y, Ti, Zr, Hf, V, Ta" fulfills the condition (2) 4 % ≤ Cr_eq ≤ 16 %, and optionally respectively up to 2% Si, up to 2% Nb or at least one element from the group "Sc, Y, Ti, Zr, Hf, V, Ta" are provided to fulfill condition (2), wherein the total proportion of elements of this group is at most equal to the mass fraction of 2%, which Ti must not exceed if Ti is the only element selected from the group consisting of "Sc, Y, Ti, Zr, Hf, V, Ta", and wherein the rest of the steel consists of Fe and < 0.5 % impurities, including ≤ 0.025 % P and 50.025% S. The steel material is suited, in particular as a powder, for LPBF or LMD methods and as wire for the WAAM method.

Description

STEEL MATERIAL FOR SHAPING COMPONENTS BY ADDITIVE MANUFACTURING AND USE OF SUCH STEEL MATERIAL

The invention relates to a steel material for forming components by additive manufacturing.

The invention also relates to the use of such a steel material for additive manufacturing.

If in the following "%" - information on alloys or

Steel compositions are made, so these relate to the mass (specified in "Mass-"), unless expressly stated otherwise.

The proportions of certain components in the structure of a steel intermediate product or a steel component are given in volume percent in this text, unless expressly stated otherwise.

The phases and other constituents present in the structure of a component produced from steel material according to the invention can be determined by means of conventional metallographic examinations or by means of X-ray diffraction ("XRD"), whereby the analysis of the structure proportions can be carried out according to the Rietveid method.

Unless otherwise stated, all of the mechanical properties of a steel intermediate product or steel component specified in this text are determined in accordance with DIN 50125. Provided the text at hand provides information for impact energy or notched impact strength, these are determined in accordance with DIN EN 10045.

The Vickers hardness test was carried out in accordance with DIN EN ISO 6507-1: 2006-3 and the Rockwell hardness test in accordance with DIN EN ISO 6508-1: 2016-12. The conversion of hardness values given in Vickers hardness HV into hardness values given in Rockwell HRC was carried out in accordance with DIN EN ISO 18625: 2014-02.

Additive manufacturing methods are now used in many industrial and application areas. Metallic components are typically manufactured using additive manufacturing based on a metal powder. In order to form a solid body from the powder, adjacent particles of the powder are selectively and locally limitedly exposed to an energy source in order to establish a solid material bond between adjacent particles by melting or diffusion.

The term “additive manufacturing process” is used here to summarize all manufacturing processes in which a filler material, which is provided in powder form, for example, is added to produce a component. This addition is usually done in layers. "Additive manufacturing processes", which are often referred to as "generative processes" or generally as "3D printing" in technical terms, are in contrast to the classic subtractive manufacturing processes such as machining processes (e.g. milling, drilling and turning), in which material is removed in order to give the component to be manufactured its shape. Additive processes also differ fundamentally from conventional solid forming processes, such as forging and the like, in which the respective steel part is formed from a starting or intermediate product while maintaining the mass. The additive manufacturing principle makes it possible to manufacture geometrically complex structures that cannot be implemented or can only be implemented with great effort using conventional manufacturing processes, such as the machining processes or primary / forming processes (casting, forging) mentioned above (see VDI status report "Additive Manufacturing", November 2019, published by the Association of German Engineers eV, VDI Society Production and Logistics, Düsseldorf, Germany) Department of Production Technology and Manufacturing Processes, www.vdi.de/statusadditiv).

More detailed definitions of the processes, which are summarized under the generic term "additive processes", can be found in VDI guidelines 3404 and 3405, for example.

The additive manufacturing processes that enable the representation of complex-shaped metallic components include in particular those under the abbreviations "L-PBF" (Laser-Powder Bed Fusion), "LMD" (Laser-Metal-Deposition) and "WAAM" (WAAM = Wire Are Additive Manufacturing) known processes.

In the L-PBF process, the material to be processed is applied as a powder in a thin layer to a base plate and remelted in the area of impact of the laser beam by means of a laser moved over the powder layer. The locally limited melt formed in this way then solidifies to form a solid volume element of the component to be shaped. In this way, a solid material layer is successively formed, which extends over the cross-sectional area and shape of the component to be formed that is assigned to it in each case. In the following step, a further powder layer is applied to the previously formed solid layer of the component, which is solidified in the same way by means of the laser beam to form a materially bonded layer to the previously formed component layer. This process is repeated until the component is completely assembled. In practice, the structure of the component is computer-aided, taking into account Volume slice data sets take place, which can be generated using computer programs known to those skilled in the art.

In the LMD process, also known as "laser deposition welding", a laser generates a locally delimited melt pool on a surface of a component and at the same time melts powder material introduced into the melt pool.

Here, too, the melt formed in this way solidifies to form a solid section of the component to be built up. In this way, material can be applied selectively and a component can be successively formed in this way.

In the WAAM process, also known as arc wire deposition welding, as in conventional arc welding, a welding torch through which a welding wire is passed is used to generate a locally limited weld pool which then solidifies to form a solid section of the component to be built. By moving the burner and the component relative to each other, material can be applied following any contours and the component to be formed can be built up successively.

In practice, there is a great need for tools with complex shapes that meet high demands on their tribo-mechanical properties. Attempts have therefore been made to use additive manufacturing processes such as L-PBF (Laser-Powder Bed Fusion) or LMD (Laser-Metal-Deposition) for the production of tools that are useful for the tool steels whose property profile meets these requirements Primary and reshaping of metals, ceramics, polymers and the like should be used. The additive manufacturing of such tools is also referred to as "rapid tooling" and allows the creation of complex tool geometries that could not be represented in this form in conventional, subtractive manufacturing methods.

For additive tool production, powders made from maraging steels ("martensitic-agina") are typically used at present. An example of this type of steel is that under DIN material number 1.2709 Standardized steel made from, all data in% by mass, <0.03% C, <0.25% Cr, <0.15% Mn, <0.1% P, <0.1% S, <0 0.05% Si, 0.8-1.20% Ti, 4.5-5.2% Mo, 17.0-19.0% Ni, the remainder Fe and technically unavoidable impurities. In maraging steels, martensitic hardening is due to the lowering of the austenite-ferrite transformation temperature ("g -» a - transformation ") due to increased Co and Ni contents and the associated formation of a high dislocation density as a result of the transformation.

The components formed from 1.27Ö9 steel can be subjected to hardening at temperatures in the range from 450 ° C to 500 ° C to further increase their strength, in which fine, strength-increasing intermetallic phases are formed as a result of the presence of elements such as Al, Ti and Ni the metal matrix form. At the same time, the soft martensitic matrix of the component is retained. This ensures a sufficiently high level of toughness, so that the level of thermal and transformation-related internal stresses in the component remains so low, despite high cooling speeds, that cracking is avoided.

Because of its consequent low tendency to form cold cracks, powder whose particles consist of the maraging steel 1.2709 could be qualified, for example, for the additive manufacturing of tools for plastic injection molding. For the production of tools for metalworking and metalworking, however, steel 1.2709 proves to be of limited use, as it has insufficient hardness for these applications and limited resistance at operating temperatures above 500 ° C.

For this reason, attempts have been made in recent years to use tool steels available on the market that are known to have the potential for greater hardness or temperature resistance as a material for powder for additive manufacturing. An example of this is the hot-work steel known under the designation "H13", which is available under the DIN Material number 1 .2344 is standardized and made of, in% by mass, 0.35 - 0.42% C, 0.80 - 1, 20% Si, 0.25 - 0.5% Mn, <0.030% P, < 0.020% S, 4.80-5.50%

Cr, 1, 20 - 1, 50% Mo, 0.85 - 1, 15% V, the remainder Fe and other technically unavoidable impurities. Another example is the high-speed steel M2, which is standardized under the DIN material number 1.3343 and made of, in% by mass, 0.86 - 0.94% C, 3.80 - 4.50% Cr, <0.40% Mn, <0.030% P, <0.030% S, <0.45% Si, 1.70 - 2.0 V, the remainder Fe and other technically unavoidable impurities. In the “cast” state, the structure of these steels consists of a carbon martensitic metal matrix in which tempered carbides are present as a result of tempering treatment after previous hardening.

Another example is the high-speed steel "M2", which is also offered as "Premium 1.3343 steel" and is alloyed with high contents of W and Mo in addition to the content of alloying elements intended for steel 1.3343. According to a commercial composition, a Premium 1.3343 steel consists of, in% by mass, 0.80 - 0.88% C, <0.40% Mn, <0.45% Si, 3.80 - 4.50% Cr, 1.70 - 2.10% V, 5.90 - 6.70% W, 4.70 - 5.20% W and the remainder of Fe and other technically unavoidable impurities. The high W and Mo contents ensure the formation of eutectic carbides of the M2C and MeC types. As a result of its range of properties, steel "M2" is suitable for the production of tools for the machining of metals or tools that are exposed to high abrasive loads at elevated temperatures when in use.

Practical experience shows that cold cracking occurs when processing steel powders made from the tool steels explained above if they are processed using L-PBF or LMD.

The formation of cold cracks can be due to the formation of martensite from the supercooled residual austenite matrix and the subsequent strong increase in Residual tensile stresses as a result of the low plasticity of martensite can be reduced. In order to eliminate this problem, a preheating of at least those parts of the machine used for the respective additive manufacturing process that come into direct contact with the component to be additively manufactured has been proposed in several works. It is also known to keep the entire installation space of the machine in which the construction of the component to be formed takes place at an increased preheating temperature. If the preheating temperature is above the martensite start temperature, ie the temperature below which martensite is formed, the formation of martensite during the construction of the component to be formed can be avoided by the introduction of heat in this way and a process control at an increased temperature. However, the risk of crack formation can also be reduced with preheating temperatures below the martensite start temperature, since the material built up additively to the component cools more slowly and on the one hand has more time to relieve stresses through plastic flow at higher temperatures, and on the other hand it passes through phase fields due to the slower cooling which have a higher toughness and a higher plastic deformability.

In the L-PBF process, however, preheating leads to increased oxidation of the powder bed that has not been remelted, which limits its recyclability. In addition, the preheating can lead to a coarsening of the structure with the result that the fine-cell structure of the component that is basically achievable and the associated increase in strength cannot be achieved. For reasons of resource efficiency (powder reuse) and with a view to maintaining the good mechanical properties, the aim is to process materials of the type discussed here without additional heating.

Against the background of the prior art explained above, the object has therefore arisen to provide a for use in an additive To provide manufacturing processes with suitable steel material that allows components to be formed with low defects, residual stresses and distortion by additive manufacturing, without the need for preheating or reheating.

The invention has achieved this object by the steel material specified in claim 1.

Advantageous refinements of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

A steel material according to the invention for forming components by additive manufacturing accordingly consists of a steel with the following composition:

C: 0.28-0.65 mass%,

Co: <10.0 mass%,

Cr: 3.5 - 12.5% by mass, optional Mo: 0.5 - 12.5% by mass

- where the sum of the contents of Cr and Mo is 4 - 16% by mass,

- where that according to the formula

Ni_eq [% by mass] = 30% C +% Ni + 0.5% Mn with% C: respective C content in% by mass,

% Ni: respective Ni content in% by mass,

% Mn: respective Mn content in% by mass, calculated Ni equivalent Ni_eq fulfills the following condition (1):

(1) 10% by mass <Ni eq <20% by mass, and in steel to meet condition (1) if necessary, in addition to C, optionally up to 9% by mass Mn and / or up to 4.5% by mass

Ni are present - where that according to the formula

Cr_eq [mass-] =% Cr +% Mo + 1, 5% Si + 0.5% Nb + 2% XX with% Cr: respective Cr content in mass-%,

% Mo: respective Mo content in% by mass,

% Si: respective Si content in% by mass,

% Nb: respective Nb content in% by mass,

% XX: the respective sum of the contents of at least one element of the group "Sc, Y, Ti, Zr, Hf, V, Ta", in% by mass, calculated Cr equivalent Cr_eq fulfills the following condition (2):

(2) 4 mass-% Cr_eq ^ 16 mass-% and in steel to fulfill condition (2) in addition to Cr and the optionally available content of Mo, if necessary, optionally additionally up to 2 mass-% Si, up to 2 mass-% Nb and / or at least one mono-carbide-forming element of the group "Sc, Y, Ti, Zr, Hf, V, Ta" are present, the mass fraction of the elements of this group in total being at most equal to the maximum mass fraction of 2% that Ti is allowed to take maximally, if Ti of the elements of the group "Sc, Y, Ti, Zr, Hf, V, Ta" Ti is present alone,

- and the remainder of the steel not taken up by the contents of the elements listed above consists of iron and technically unavoidable impurities, the total content of which i is 0.5% and of which <0.025% P and <0.025% S belong.

The invention thus provides an Fe-based starting material that can be hardened by carbon martensitic, which is alloyed with molybdenum ("Mo") and chromium ("Cr") and which is built up with the component by layering compaction or in the form of a green body and is supplied with an energy source ( e.g. Laser, electron beam, arc, flame, induction, thermal radiation) can be compacted.

In the following explanations, reference is made to the accompanying figures, of which

1 shows a diagram in which the development of the residual stress se is plotted against the temperature T, which occurs during the processing of martensitic hardening steels in the L-PBF process;

FIG. 2 shows a diagram in which the retained austenite content RA is plotted against the martensite start temperature Ms;

3 shows a diagram in which, for various steel material samples, the residual stresses sE that arise during processing in the L-PBF method are plotted against the retained austenite content;

4 shows a diagram in which the core porosity (without taking into account contour connection errors) of the model alloy processed by means of L-PBF is plotted as a function of the exposure time for samples produced and created according to the invention;

5 shows a diagram in which hardness tempering curves determined for a steel material according to the invention processed by means of L-PBF are reproduced.

The alloy of the steel of a steel material according to the invention is set in such a way that the martensite start temperature "Ms" of a steel material according to the invention and, associated therewith, the so-called "transformation plasticity" is shifted in the direction of lower temperatures. So the minimum of the curve (1) shown in Fig. 1 should in the best case at room temperature "RT" are present, as indicated in FIG. 1 by the variant of curve (1) shown in dashed lines. As the Ms temperature falls, the temperature at which the martensite formation is complete ("martensite finish") also falls below RT, so that more of the converted austenite, so-called retained austenite ("RA"), remains in the structure. The Ms temperature can be calculated according to the approach of Andrews, published in KW Andrews: Empirical Formulas for the Calculation of Some T ransformation Temperatu res. In: JISI. Vol 302, 1965, pp. 721-727, can be calculated as follows:

Ms [° C] = Ms, 0 - Ifleg (Ma -%) leg with MS, 0 = 539 ° C fleg (C) = 423 fleg (Mn) = 30.4 fleg (Cr) = 12.1 fleg (Mo ) = 7.5 fleg (Ni) = 17.7

(Ma -%) ieg: respective content of C, Mn, Cr, Mo, Ni

The relationship between RA and Ms can be found in the equation by Koistinen and Marburger, published in D. P. Koistinen, R. E. Marburger: A General Equation Prescribing the Extent of the Austenite-Martensite-Transformation in Pure Iran Carbon-Alloys and Plain Carbon Steels. In: Acta Metallurgica. 7,

1959, pp. 59-60, can be calculated as follows:

RA = exp [-B (Ms-TU)]; with B (20 ° C) = 1.1 x 10 ² x 1 / ° C;

TU: temperature below the martensite start temperature Ms to which the steel sample under consideration has cooled ("subcooling temperature") The invention has here by alloying measures the Ms-

The temperature is reduced in such a way that the plasticity of the transformation results in a stress neutrality in the component formed by L-PBF.

The correlation between the martensite start temperature Ms, the content of retained austenite RA and the internal stress se can be understood with the aid of FIGS. 2 and 3.

The relationship between RA content and Ms temperature is shown in FIG. The result is a linear approximation curve ("Fit"), in which the RA content decreases with increasing Ms temperature. From FIG. 3 it can be seen that the residual stresses se decrease with increasing RA content, so that when an RA limit content of 14% by volume is reached, on average, stress neutrality is present. 3 also shows the RA content and the level of internal stresses that arise when processing a powder whose particles consist of the austenitic steel known as 316L (Cr: 17.00 - 19.00%, Ni:

13.00 - 15.00%, Mo: 2.25 - 3.0%, C: <0.030%, Mn: <2.00%, Cu <0.50%,

P: ^ 0.025%, Si: <0.75%, S: <0.010%, N: <0.10%, remainder Fe, data in% by mass, see material data sheet EOS StainlessSteel 316L, published by EOS GmbH , Krallingen, DE, 5.2014, https://www.eos.info/de/additive- fertigung / 3d-printing-metal / eos-metal-materials-dmls / stainless steel).

The alloy of a steel material according to the invention is thus designed so that when a component is formed from it by an additive manufacturing process, a residual austenite proportion RA of at least 10% by volume, in particular at least 15% by volume, is established in the component obtained. 2 shows that such RA contents result at a martensite start temperature Ms of less than 260 ° C.

The martensite start temperature Ms and thus the RA content can be set in a targeted manner via the chemical composition. In addition, the Estimate the influence of the individual alloying elements on the residual austenite content RA using the Ni equivalent and the Cr equivalent.

The alloy of the steel material according to the invention was determined accordingly, taking into account the effect that the individual alloy elements have on the austenite (RA) and ferrite phases contained in the structure of a steel material according to the invention by additive manufacturing. The elements Ni, Co, C, Mn and N stabilize the austenite phase, whereas the carbide formers of the 3rd to 6th subgroup elements and additionally Si stabilize the ferrite phase. The stabilizing effect that emanates from each alloy element can be described using the Cr equivalent "Cr_eq" and the Ni equivalent "Ni_eq", which according to Schaeffler, published in P. Guiraldenq, OH Duparc: The genesis of the Schaeffler diagram in the history of stainless steel, In: Metal. Res. Technol. 114, 613, 2017, pp. 1-9, can be calculated.

On the basis of the findings explained above, the invention has derived the following boundary conditions for Ni_eq and Cr_eq:

- The Ni equivalent Ni_eq should be at least 10% by mass and at most 20% by mass (10% by mass <Ni_eq <20% by mass).

- The Cr equivalent Cr eq should be at least 4% by mass and at most 16% by mass (4% by mass <Cr_eq <16% by mass).

The inventive coordination of the Ni and Cr equivalents of a steel material according to the invention ensures that components made from the steel material according to the invention are free of cracks and have only minimal thermally induced residual stresses, without additional technical measures such as preheating or post-heat treatment, must be carried out. The Ni equivalent of materials according to the invention is in the range of 10-20 mass%. Materials alloyed according to the invention, the Ni equivalents of which are 12-20% by mass, in particular at least 12.00% by mass or more than 12% by mass, have proven to be particularly suitable with regard to the property profile aimed at according to the invention the Ni equivalent is preferably limited to a maximum of 20.00% by mass, in particular <20% by mass.

The Cr equivalent of materials according to the invention is 4-16% by mass, Cr equivalents of at least 5.50% by mass, in particular more than 5.5% by mass or at least 5.70% by mass, proving to be particularly advantageous to have.

Optimized properties of a steel material according to the invention can be achieved by adding the sum of the Cr and Ni equivalents

22.5 - 30% by mass, in particular at least 23.00% by mass. The effects aimed for by the invention can be achieved in particular when the sum of the Cr and Ni equivalents is at least

23.5 mass% or at least 24.00 mass%, such as 25 mass%, or more than 25 mass%.

The contents of the individual alloy elements are determined as follows:

Carbon ("C") is contained in the steel material according to the invention in contents of 0.28% by mass to 0.65% by mass in order to achieve the carbon-martensitic transformation during the material processing. For this, at least 0.28% by mass of C is required, whereby this effect can be achieved particularly reliably with C contents of at least 0.45% by mass. C contents of more than 0.65% by mass would lead to the formation of too high a residual austenite content with which the targeted tribomechanical properties would not be able to be realized. In addition, would be If the C content is too high, the martensite start temperature Ms is lowered in such a way that the internal stress-reducing effect due to the transformation plasticity only occurs at temperatures below room temperature and the effects used by the invention would not be effective. Such unfavorable effects of excessively high C contents can, if necessary, be avoided particularly reliably in the material according to the invention by limiting the C content to a maximum of 0.60% by mass. An embodiment of the invention that is particularly advantageous in practice therefore provides that the C content of a steel material according to the invention is 0.40-0.60% by mass.

Chromium (“Cr”) is present in a steel material according to the invention in contents of 3.5% by mass to 12% by mass.

Molybdenum (“Mo”) can optionally be present in the steel material according to the invention in contents of 0.5% by mass to 12.5% by mass.

With a Cr content of at least 3.5% by mass, molybdenum can substitute chromium in a ratio of 1: 1. In the event that Cr in a content of at least 3.5% and Mo in a content of at least 0.5 mass% are present at the same time, Mo and Cr thus make the same contribution to setting the Cr equivalent Cr eq.

The Cr content and the optional Mo content of the steel of a steel material according to the invention are set so that the Cr equivalent is stabilized in the range specified according to the invention and in this way, taking into account the stipulations prescribed according to the invention for the Ni equivalent, the martensite start temperature Ms des Steel is shifted in a temperature range ranging from 125 ° C to 260 ° C, in particular up to 200 ° C, a residual austenite content of at least 10% by volume, in particular at least 15% by volume, is stabilized in the structure of the component produced in each case and the effect of the transformation plasticity at room temperature is maximal. As explained above, according to the invention, the requirement that the sum of the contents of Cr and Mo should be 4% by mass to 16% by mass can be met by adding 4.0% by mass to 12.5% by mass of Cr are present in the steel of the steel material according to the invention if Mo is absent in it, or in that at least 3.5% by mass of Cr are present and at the same time at least 0.5% by mass of Mo are contained in the steel, the Cr and Mo present in each case -Contents in this case are adapted to each other so that their total does not exceed 16% by mass.

The advantageous influences of the presence of Cr in the steel of the steel material according to the invention can be used particularly reliably if the Cr contents are at least 4.5% by mass, with Cr contents of at least 5.0% by mass, in particular at least

5.5% by mass, which allows the effects achieved by the presence of Cr to be used in a particularly operationally reliable manner. With Cr contents of more than 12.5 mass%, on the other hand, there could be excessive segregation of Cr in the residual melt, which is associated with increased carbide formation. The Cr content set in carbides does not contribute to the adjustment of the transition temperatures of the metal matrix. The positive effect of the presence of Cr in the steel material according to the invention can be used particularly reliably with contents of at least 6% by mass of Cr. Cr contents of up to 10% by mass, in particular up to 10.00% by mass or less than 10% by mass, in the steel material according to the invention have proven to be particularly practical.

Contents of at least 0.75% by mass of Mo also contribute to the advantageous properties of the steel material according to the invention. Contents of more than

12.5 mass% Mo would lead to a minimum content of

3.5 mass% Cr, the sum of the contents of Cr and Mo would exceed the upper limit of 16.0 mass%. With such high total contents of Cr and Mo, no further increases in the effect of these elements would be achieved. The presence of Mo im Steel material according to the invention achieved effects with Mo contents of up to 8% by mass, in particular up to 4% by mass, use.

The other alloy components manganese (“Mn”), nickel (“Ni”), silicon (“Si”), niobium (“Nb”), as well as titanium (“Ti”), scandium (“Sc”), yttrium (“Y "), Zirconium (" Zr "), hafnium (" Hf "), vanadium (" V ") or tantalum (" Ta ") can each optionally be present in the steel of a steel material according to the invention in order to achieve the nickel equivalent Ni_eq and the Set Cr equivalent Cr_eq according to the requirements of the invention. If necessary, Mn and Ni will serve to adjust the Ni equivalent Ni_eq to the steel. Si, Nb, Ti, Sc, Y, Zr, Hf, V, Ta, on the other hand, can be provided in the steel of the steel material according to the invention in order to bring the Cr equivalent into the range according to the invention.

Since the C content is included in the calculation of the Ni equivalent Ni_eq with a factor of 30, a Ni equivalent Ni_eq of at least 10.0% by mass results for C contents of more than 0.33% by mass. The requirement set according to the invention for the value of the nickel equivalent Ni_eq can therefore already be met if C alone is present in sufficiently high contents. However, regardless of the setting of the Ni equivalent Ni_eq, the presence of Ni in the steel can also have positive influences on the properties of a steel material according to the invention, such as an increase in toughness. If this effect is to be used, at least 0.25% by mass of Ni, in particular at least 0.5% by mass of Ni, can be provided for this purpose. However, the Ni content should not exceed 4.5% by mass, in particular 3.0% by mass, in order to avoid an excessive increase in the Ni equivalent. Ni contents of 0.75 to 1.25 mass% in the steel material according to the invention have proven to be particularly practical.

Because of its comparable effect, Mn contents in the steel alloyed according to the invention can substitute Ni contents in a ratio of 2: 1, if necessary. For example, 1% by mass of Ni can be replaced by 2% by mass Mn to be replaced. However, the Mn content should remain limited to a maximum of 9 mass%, in particular a maximum of 7 mass%, in order to avoid an excessive increase in the Ni equivalent. The effect of Mn can be used already at levels of at least 0.25% by mass of Mn, wherein Mn contents of at least 0.5 mass% or at least 1.0 mass%, especially at least 2 _wt:% advantageous have turned out. According to a particularly practical embodiment, a steel material according to the invention accordingly contains 2-3% by mass of Mn.

Silicon "Si" has a comparatively strong effect on the value of the Cr equivalent and can be added to the steel of a steel material according to the invention if it is required for deoxidation during steel production. In the event that the steel material according to the invention is to be provided in powder form, Si contents of at least 0.15% by mass, in particular 0.75% by mass, can be used to set a melt viscosity that is favorable for atomizing the melt into powder particles. However, excessively high Si contents can, among other things, impair the mechanical properties of a component made from steel according to the invention. Therefore, the Si content is limited to 2 mass% or less. The positive influences of Si can be used particularly effectively with contents of at most 1.25% by mass.

To set the Cr equivalent in accordance with the invention, it is optionally also possible to use a mono-carbide-forming element or several mono-carbide-forming elements from the group “Sc, Y, Ti, Zr, Hf, V, Ta”. Of these elements particularly suitable for the purposes of the invention is Ti, which is preferably used as the only one of the mono-carbide-forming elements of this group in the steel according to the invention and can then be present in contents of up to 2% by mass.

The other elements of the group “Sc, Y, Ti, Zr, Hf, V, Ta” can be added to the steel in combination or as a substitute for Ti. However, will the content of the respective elements is set so that the total content of the elements of the group "Sc, Y, Ti, Zr, Hf, V, Ta" does not exceed the upper limit of the content applicable to Ti alone. This means that the mass fraction of the contents of the elements of the group “Sc, Y, Ti, Zr, Hf, V, Ta” present in the steel should not be greater than the mass fraction of 2%, which is the maximum permitted for Ti is when only Ti is present in the steel from the elements of the group "Sc, Y, Ti, Zr, Hf, V, Ta". Accordingly, in the event that the respective element is to be added alone, the maximum permissible Sc content is calculated as 47.867 / 44.956 x 2% by mass = 2.13% by mass, the maximum permissible content of Y at 47.867 / 88.906 x 2 % By mass = 1.08% by mass, the maximum permissible content of Zr at 47.867 / 91.224 x 2% by mass = 1.05% by mass, the maximum permissible content of Hf at 47.867 / 178.49 x 2 % By mass = 0.54% by mass, the maximum permissible content of V at 47.867 / 50.942 x 2% by mass - 1.88% by mass and the maximum permissible content of Ta at 47.867 / 180.95 x 2% by mass % = 0.53 mass%. In the event that two or more elements of the group “Sc, Y, Ti, Zr, Hf, V, Ta” of the mono-carbide-forming elements are to be added to the steel of a steel material according to the invention, their contents are to be adjusted accordingly so that their mass fraction in total is at most equal to the mass fraction occupied by 2% by mass of Ti if Ti were added on its own. For example, instead of 2% Ti, up to 0.94% by mass of V in combination with up to 0.5% by mass of Zr could be added to the steel.

Cobalt (“Co”) can optionally be added to the steel of a steel material according to the invention in order to promote the formation of the secondary hardness maximum in the direction of higher tempering temperatures as well as the increase in the solidus temperature and the associated increase in the solution state through higher hardening temperatures.

The rest of the steel of a steel material according to the invention, in each case not by the contents of the invention in the manner explained above to alloyed alloy elements is ingested, is filled with iron and technically unavoidable impurities, the total content of which may amount to a maximum of 0.5% by mass and up to 0.025% by mass of phosphorus ("P") and up to 0.025% by mass Count sulfur ("S"). The impurities include, in particular, all the elements of the periodic table not listed here that are not specifically added to the steel, but can inevitably get into the steel due to the processing of recycling material or the processes used in steel production and processing. The contents of these elements in the steel of a steel material according to the invention are in any case set so low that they are not considered to be present in the technical sense because they have no influence on the properties of the steel material according to the invention. Typically, this also includes, for example, N contents of less than 0.1% N. The contents of impurities are preferably limited in such a way that their total is <0.3% by mass, in particular <0.15% by mass , with impurities, the total content of which is at most 0.05% by mass, have proven to be particularly advantageous with regard to the desired work result.

According to a variant of the invention that is particularly important for practice, the steel material according to the invention is provided as steel powder which is produced in a conventional manner, for example by atomizing a melt alloyed according to the invention. The grain sizes of the steel particles of a powder alloyed according to the invention are typically 15-180 μm.

Due to its special properties, steel powder alloyed according to the invention is particularly suitable for processing by means of the additive manufacturing processes "L-PBF" or "LMD". Steel powders with a particle size of 15-63 μm are particularly suitable for the L-PBF process, while powder particles with a grain size of 63-180 μm are suitable for the LMD process. If necessary, the particles of the corresponding grain size from the commercially produced powder particles in conventionally selected by sieving and / or sifting. Steel powder with a bulk density determined according to DIN EN ISO 3923-1 of 3.75 g / cm ³ to 5.75 g / cm ³ , a tap density determined according to DIN EN ISO 3953 of 4.25 g / cm ³ to 6.25 g / cm ³ and a flow behavior of less than 30 sec / 50 g, determined in accordance with DIN EN ISO 4490, prove to be particularly suitable for processing in such processes.

As an alternative to processing as a powder, the steel material according to the invention can also be provided in wire form. In this form, the steel material according to the invention is particularly suitable for processing in the WAAM process or comparable additive manufacturing processes based on the principle of build-up welding.

It is also possible to provide the steel material in the form of a hollow body which is filled with a steel powder designed according to the invention. Such a hollow body can typically be a filler wire or the like. It is conceivable to fill the respective hollow body, such as a filler wire or a tube, with the individual elements of the alloy of a steel material according to the invention in pure form, the mass fractions of the elements in question in the filling taking into account the alloy and the mass of the material from which the hollow body is made, their contents correspond to the alloy of a steel material according to the invention. From the hollow body filled in this way, the alloy of the steel material according to the invention is formed in situ in the course of the respective additive manufacturing process in the active area of the respective heat source used.

Regardless of which of the above-explained dosage forms (powder, wire, hollow body with filling) the steel material according to the invention is used, it provides a starting material that is ideally suited for the production of components by additive manufacturing. In practice, the invention thus by using a steel material that consists of steel alloyed according to the invention for the additive manufacturing of components. The steel material according to the invention is particularly suitable for use in an L-PBF or LMD or WAAM process.

Due to its combination of properties, the steel material according to the invention can be used to produce mechanically or tribologically highly stressed components or tools, in particular through powder- or wire-based additive manufacturing, which have an optimal quality without the use of preheating strategies and the like.

The components produced from steel material according to the invention are characterized by residual austenite contents of typically at least 10% by volume, in particular at least 15% by volume.

The invention is explained in more detail below on the basis of exemplary embodiments.

To test the invention, a melt is melted, the composition of which and its Cr equivalent Cr_eq and Ni equivalent Ni eq calculated according to the formulas explained above are given in Table 1.

The melt was atomized in the conventional way by means of gas atomization to a steel powder, from which the particles were then selected by sieving and sifting, which had a grain size of 10-63 μm suitable for processing in the L-PBF process.

The steel powder obtained in this way was processed into test pieces with an L-PBF system offered by Realizer under the name “SLM 100” using the process parameters listed in Table 2. When processing the powder, no building board preheating was used, as is usually used in the processing of martensitic hardenable tool steels in order to counteract the formation of cold cracks. In this way it was possible to test whether the alloy can be processed with few defects without using preheating of the substrate plate.

To assess the general workability of the alloy using L-PBF, metallographic sections of the manufactured samples were made and examined with regard to porosity and crack density.

4 shows the core porosity values of the L-PBF-manufactured samples determined by means of quantitative image analysis (porosity in the hatch area without taking into account the contour area). It can be seen that with all selected process parameters, high sample densities can be achieved in the hatch area. This indicates a comparatively broad process window with which the alloy can be processed with little pores. In addition, only a very small number of cold cracks could be detected in the structure of the L-PBF-manufactured alloy. The samples were to be assessed as largely free of cracks. An exposure time of 110 ps with a point spacing of 30 pm was evaluated as the optimal parameter. Overall, it can be stated that the model alloy derived from preliminary investigations can be processed with low defects using L-PBF without preheating the substrate plate.

To evaluate the basic suitability of the alloy as a material for tool applications or components subject to other wear and tear, the hardness-tempering behavior of the alloy processed using L-PBF was investigated. The samples examined were produced with the set of parameters identified as being well suited (exposure time 110 ps, point spacing 30 pm). Scanning electron microscopic

Recordings of the etched structure of the samples were made in the respective heat treatment conditions and the Vickers hardness was determined. The tests were carried out on samples in the heat treatment conditions shown in Table 3.

The results of the hardness tempering tests are shown in FIG. A hardening and tempering behavior that is common for secondary hardenable, martensitic tool steels can be seen. A high initial hardness and a pronounced secondary hardness maximum can be set by suitable heat treatment and prove that steel materials according to the invention are particularly suitable for the production of tools by additive manufacturing.

Table 1 Data in% by mass, remainder iron and unavoidable impurities

Table 2 Overview of the process parameters used to compact the model alloy. Effective laser power = 77.4 W; Layer thickness = 30 μm; no building board preheating.

Table 3

Claims

PATENT CLAIMS

1 . Steel material for forming components by additive manufacturing, consisting of a steel with the following composition:

C: 0.28-0.65 mass%,

Co: <10.0 mass%,

Cr: 3.5 - 12.5% by mass, optional Mo: 0.5 - 12.5% by mass

- where the sum of the contents of Cr and Mo is 4 - 16% by mass,

- where that according to the formula

% Ni: respective Ni content in% by mass,

(1) 10 mass% <Ni_eq 20 mass%, and in the steel to fulfill condition (1), if necessary, up to 9 mass% Mn and / or up to 4.5 mass% Ni are optionally present in addition to C ,

- where that according to the formula

% Mo: respective Mo content in% by mass,

% Si: respective Si content in% by mass,

% Nb: respective Nb content in% by mass, % XX: the respective sum of the contents of at least one element of the group "Sc, Y, Ti, Zr, Hf, V, Ta", in% by mass, calculated Cr equivalent Cr_eq fulfills the following condition (2):

(2) 4 mass% <Cr_eq <16 mass% and in the steel to fulfill condition (2) in addition to Cr and the optionally present content of Mo, if necessary, optionally up to 2 mass% Si, up to 2 mass% % Nb and / or at least one mono-carbide-forming element of the group "Sc, Y, Ti, Zr, Hf, V, Ta" are present, whereby the mass fraction of the elements of this group in total is at most equal to the maximum mass fraction of 2%, the maximum Ti may occupy if Ti is present alone from the elements of the group "Sc, Y, Ti, Zr, Hf, V, Ta" Ti,

- and the remainder of the steel not taken up by the contents of the elements listed above consists of iron and technically unavoidable impurities, the total content of which is s 0.5% and of which s 0.025% P and <0.025% S belong.

2. Steel material according to claim 1, characterized in that d a s s applies to the sum Cr_eq + Ni eq formed from the Cr equivalent and Ni equivalent

22.5 mass% <Cr_eq + Ni_eq <30 mass%.

3. Steel material according to claim 1, characterized in that its C content is 0.40-0.60% by mass.

4. Steel material according to one of the preceding claims, characterized in that its Cr content is 5.50-10% by mass.

5. Steel material according to one of the preceding claims, characterized in that its Mo content is 0.75-4% by mass.

6. Steel material according to one of the preceding claims, characterized in that its Ni content is 0.75-1.25% by mass.

7. Steel material according to one of the preceding claims, characterized in that its Mn content is 2-3% by mass.

8. Steel material according to one of the preceding claims, characterized in that its Si content is 0.75-1, 25% by mass.

9. Steel material according to one of the preceding claims, characterized in that of the elements of the group "Sc, Y, Ti, Zr, Hf, V, Ta", if necessary, Ti alone with a content of up to 2% by mass im Steel of the steel material is available.

10. Steel material according to one of the preceding claims, characterized in that its martensite start temperature Ms is 125-260 ° C.

11. Steel material according to one of the preceding claims, characterized in that it is a steel powder.

12. Steel material according to claim 11, characterized in that the particles of the steel powder have an average grain size of 15-180 μm.

13. Steel material according to one of claims 11 or 12, characterized in that the steel powder has a bulk density of 3.75 g / cm ³ to 5.75 g / cm ³ (determined in accordance with DIN EN ISO 3923-1).

14. Steel material according to one of claims 11-13, characterized in that the steel powder has a tap density of 4.25 g / cm ³ to 6.25 g / cm ³ (determined in accordance with DIN EN ISO 3953).

15. Steel material according to one of claims 11-14, characterized in that the steel powder has a flow behavior of less than 30 sec / 50 g, determined in accordance with DIN EN ISO 4490.

16. Steel material according to one of claims 1-10, characterized in that it is a steel wire.

17. Steel material according to one of claims 1 to 10, characterized in that it is a hollow body which is filled with a steel powder formed according to one of claims 11 to 15.

18. Use of a steel material designed according to one of the preceding claims for the production of components by an additive manufacturing process.

19. Use according to claim 18, characterized in that the additive manufacturing process is a laser powder bed fusion process, a laser metal deposition process or a wire arc additive manufacturing process.

20. Use according to one of claims 18 or 19, characterized in that the residual austenite content in the structure of the component, which is produced from the steel material provided according to one of claims 1 to 17, is at least 10% by volume.