Patent Application
20200009651
This application claims priority to European Patent Application No. 18182027.5 filed Jul. 5, 2018, the disclosure of which is hereby incorporated by reference in its entirety.
The invention relates to the use of a steel as a steel powder for producing steel components by additive manufacturing processes, a method for producing a steel component using an additive manufacturing process and a steel component produced through the use of such a steel by using an additive manufacturing process.
If “%” information is given below on alloys or steel compositions, these are respectively based on the weight, unless expressly stated otherwise.
All of the mechanical properties mentioned in the present text of the steel to be used according to the invention and of the steels which may be mentioned for comparison have been determined according to DIN EN ISO 6892-1, unless stated otherwise.
The term “additive manufacturing process” here summarises all manufacturing processes in which a material is added to produce a component. This addition usually takes place in layers. “Additive manufacturing processes”, which are often referred to in the technical language as “generative processes” or generally as “3D printing”, thus stand in contrast to the classic subtractive manufacturing processes, such as the machining processes (for example, milling, drilling, and turning), in which material is removed in order to give shape to the component to be produced. Likewise, additive processes generally differ from conventional solid forming processes, such as forging and the like, in which the respective steel part is formed while retaining the mass of a starting or intermediate product.
The additive manufacturing principle makes it possible to produce geometrically complex structures that cannot be realised or can only be realised with great difficulty using conventional manufacturing processes, such as the aforementioned machining processes or primary moulding processes (casting, forging) (see VDI Status Report “Additive Fertigungsverfahren”, September 2014, published by Verein Deutscher Ingenieure e.V., Fachbereich Produktionstechnik and Fertigungsverfahren (Association of German Engineers, Department of Production Technology and Manufacturing Processes), www.vdi.de/statusadditiv).
Further definitions of the methods summarised under the generic term “additive methods” can be found, for example, in VDI Guidelines 3404 and 3405.
In practice, powdered steel alloys, which have sufficient weldability due to their carbon contents being below 0.30 wt %, are regularly used today. The steel powder produced is then melted with an electron beam or laser beam in a vacuum and, due to the small liquid quantity compared to the already produced component, rapidly cooled. Due to the high cooling rate, a critical stress state often occurs in the component, which can lead to cracks, especially in the case of elevated carbon contents.
In order to minimise the risk of stress cracks, it is common today to use a steel which forms a structural state which is as soft as possible in the case of rapid cooling. Steels which form a structure consisting of soft martensite or lancet martensite are suitable here. Examples of such steels are the “maraging steels”. Typical examples of these steels are standardised under the material numbers 1.2709, 1.6355, 1.4545 and 1.4542 in the StahlEisen-Liste. Alternatively, austenitic steels such as steel 316L (material number 1.4404) are also used as steel powder for additive manufacturing. A disadvantage of these steels is that they require high contents of alloying constituents, which are only available at high costs today.
The components produced by such additive processes are regularly subjected to an after-treatment in order to minimise the residual porosity which is usually still present in the production process. The options available for this purpose are adequately described in the prior art and summarised, for example, in DE 100 39 143 C1. The subject matter of this patent specification is a method for producing precision components by laser sintering of a powder material which comprises a mixture of at least two powder elements. The powder material is formed by the main component, this being iron powder, and other powder alloying elements which are in elemental, pre-alloyed or partially pre-alloyed form. At the same time, the laser sintering process is controlled in such a way that a powder alloy is formed from the constituents of the powder in the course of the laser sintering process. The powder alloy elements are carbon, silicon, copper, tin, nickel, molybdenum, manganese, chromium, cobalt, tungsten, vanadium, titanium, phosphorus, boron. Specifically, the content ranges indicated for these components are: C: 0.01-2 wt %, Si: up to 1 wt %, Cu: up to 10 wt %, Sn up to 2 wt %, Ni: up to 10 wt %, Mo: up to 6 wt. %, Mn: up to 2 wt. % or 10-13 wt. %, Cr: up to 5 wt. % or 12-18 wt. %, Co: up to 2 wt %, W: up to 5 wt %, V: up to 1 wt %, Ti: up to 0.5 wt %, P: up to 1 wt %, B: up to 1 wt %.
Against this background, the problem has arisen of identifying a steel which is particularly suitable for use as a steel powder in producing components by means of an additive manufacturing process and which can be alloyed cost-effectively.
Furthermore, a method based on such a steel powder is to be identified which allows components to be produced reliably using an additive manufacturing process.
Finally, a component is also to be specified which consists of steel and can be produced cost-effectively using an additive process.
The invention is based on the recognition that a modification of a steel, which is generally already known from EP 3 168 312 A1 for the production of components using forging technology, is also especially suitable as a material for the production of steel components by using an additive manufacturing process. The soft martensitic steels commonly used today for additive manufacturing require high proportions of expensive alloy elements such as Co, Ni and Mo and a subsequent heat treatment to set the degree of hardness. The solutions known from practical experience which are based on steel with an austenitic structure are severely limited in terms of the maximum strength that can be achieved.
The invention proposes, in contrast, a material composition for use as a powder for the production of a steel component by additive manufacturing, in which, by a plurality of repetitions of the steps “applying a layer of steel powder to a previously produced steel layer”, “melting the applied steel powder layer”, “cooling the steel layer produced by melting”, the steel component is successively built from a corresponding number of steel layers. This material composition opens a large window for the cooling rate to set a bainitic structure which is less solid than martensite.
According to the invention, a steel in the form of steel powder having a mean grain diameter of 5-150 μm is used for the production of steel components in an additive process, the steel comprising (in wt %): 0.08-0.35% C, up to 0.80% Si, 0.20-2.00% Mn, up to 4.00% Cr, 0.3-3.0% Mo, 0.004-0.020% N, 0.004-0.050% Al, up to 0.0025% B, at least one element from the group “Nb, Ti, V, S”, with the proviso that the Nb content is 0.003-0.20 wt %, the Ti content is 0.001-0.02 wt %, the V content is 0.02-0.40 wt % and/or the S content is 0.001-0.4 wt %, up to 1.5% Ni, up to 0.3% Cu, up to 2.0% Co, and the remainder being iron and unavoidable impurities, the Al content % Al, the Nb content % Nb, the Ti content % Ti, the V content % V and the N content % N of the steel satisfying the following condition: % Al/27+% Nb/45+% Ti/48+% V/25>% N/3.5. In this case, the unavoidable impurities due to production include all elements which are present in quantities which are ineffective in terms of alloying in respect of the properties of interest here and which enter the steel because of the selected route for producing the steel powder or the selected starting material (scrap). In particular, the unavoidable impurities also include P contents of up to 0.0035 wt %.
The steel alloy used according to the invention as a powder for additive manufacturing is composed in such a way that there are yield strength-enhancing precipitates of carbides, such as special carbides, carbonitrides or nitrides, as a result of the intrinsic heat treatment required by the additive manufacturing due to heat conduction, said heat treatment resulting from the melting in layers of successively applied and melted powder layers. Thus, in the case of the steel alloy provided according to the invention for use as steel powder for an additive manufacturing process, taking advantage of the process-inherent heat input, an increase in strength occurs through precipitation formation, without the need for a separate heat treatment. In this case, the steel to be used according to the invention forms a homogeneous bainitic structure which, despite the omission of a separate heat treatment, ensures uniform properties over the volume of the additive-manufactured component. The homogeneous structural state made possible by the steel alloy to be used according to the invention ensures low residual stresses and thus a low risk of cracking.
Here, the steel alloy selected according to the invention for use as steel powder for an additive manufacturing process is also particularly suitable insofar as it minimises the critical cooling rate for the formation of the bainitic structure. This means that the “bainitic nose B” obtained in the time-temperature diagram (see
As can be understood by way of example with reference to
The steel used according to the invention as steel powder for additive manufacturing may contain up to 0.35 wt % carbon (“C”) to contribute to increasing the strength of the material by carbide formation. Thus, by adding respectively 0.01 wt %, a respective increase in strength of approximately 70 MPa can be effected. This effect succeeds from a C content of 0.08 wt %, in particular from a content of at least 0.09 wt % C. Limiting the C content to at most 0.35 wt % ensures that a steel to be used according to the invention has good elongation and toughness properties despite its maximised strength. At the same time, the comparatively low C content in a steel to be used according to the invention also contributes to the acceleration of the bainite transformation, so that the formation of undesired structural constituents is avoided. An optimised effect of the presence of C in the steel to be used according to the invention can be achieved by setting the C content to 0.08-0.25 wt %, in particular 0.09-0.25 wt %.
Silicon (“Si”) suppresses cementite formation and shifts ferrite formation to shorter times. The Si content of a steel to be used according to the invention is therefore limited to at most 0.80 wt %, in particular to at most 0.45 wt %, in order to allow the bainite transformation to take place as early as possible. At the same time, Si contents up to this upper limit increase the strength due to solid solution strengthening.
Molybdenum (“Mo”) is present in the steel to be used according to the invention at contents of 0.3-3.0 wt % to delay the transformation of the structure into ferrite or perlite. This effect occurs in particular when at least 0.6 wt %, in particular more than 0.70 wt % Mo, are present in the steel. At contents of more than 3.0 wt %, in the steel to be used according to the invention there is no economically justifiable further increase in the positive effect of Mo. In addition, above 3.0 wt % Mo, there is a risk of forming a molybdenum-rich carbide phase which may adversely affect the toughness properties. Optimum effects of Mo in the steel to be used according to the invention can be expected when the Mo content is at least 0.7 wt %. Mo contents of at most 2.0 wt %, in particular at most 1.5 wt % or at most 1.0 wt %, have proved to be particularly effective.
Manganese (“Mn”) is present at contents of 0.20-2.00 wt % in the steel to be used according to the invention to adjust the tensile strength and yield strength by mixed crystal formation. A minimum content of 0.20 wt % of Mn is required in order to increase the strength. If this effect is to be achieved particularly reliably, then a Mn content of at least 0.35 wt % can be provided. Excessively high Mn contents, however, would delay the bainite transformation and thus lead to a predominantly martensitic transformation. Therefore, the Mn content is limited to at most 2.00 wt %, in particular at most 1.5 wt %. Negative influences of the presence of Mn can be avoided particularly reliably by limiting the Mn content in the steel to be used according to the invention to a maximum of 1.1 wt %.
The content of sulphur (“S”) in the steel to be used according to the invention can be up to 0.4 wt %, in particular at most 0.1 wt %, in order to promote the machinability of the steel. At the same time, the presence of S reduces the interfacial tension of the droplets during atomisation, whereby a finer steel powder is obtained. For this purpose, an S-content of at least 0.001 wt % may be provided. At S-contents above 0.4 wt % there is a risk of red-shortness occurring. Optimum effects of the presence of S in the steel to be used according to the invention can be achieved at contents of 0.003-0.1 wt %.
The fine adjustment of alloying techniques with regard to the mechanical properties and the structure quality of a steel to be used according to the invention is carried out in accordance with the alloying concept to be used according to the invention through a combined microalloying of the elements boron (“B”) in optional contents of up to 0.0025 wt %, in particular at least 0.0005 wt % B, nitrogen (“N”) in contents of 0.004-0.020 wt %, in particular at least 0.006 wt % N or up to 0.0150 wt % N, aluminium (“Al”) in contents of 0.004-0.050 wt % and niobium (“Nb”) in optional contents of up to 0.20 wt %, in particular at least 0.003 wt % or at least 0.005 wt % Nb, wherein the Nb content may in particular also be limited to at most 0.05 wt % Nb, titanium (“Ti”) in optional contents of up to 0.02 wt %, in particular at least 0.001 wt % or at least 0.005 wt % Ti, and vanadium (“V”) in optional contents of up to 0.40 wt %, in particular at least 0.01 wt % or at least 0.02 wt % V.
The presence of N in the contents provided according to the invention allows formation of nitrides and carbonitrides to increase the strength and raise the fine grain resistance, without causing embrittlement. Al forms aluminium nitride with N, which contributes to fine grain stability.
The optional presence of B delays the formation of ferrite or perlite and thus ensures the formation of the desired bainitic structure in the steel to be used according to the invention. B contents above 0.0025 wt % would entail the risk of embrittlement. The respective micro-alloying elements Nb, Ti and V, which are also optionally present, form carbonitrides and can thus make a significant contribution to optimising the fine grain stability and strength of the steel to be used according to the invention.
In order to safely use the advantages of the presence of the micro-alloying elements and of aluminium, it may be expedient to adjust the Al content to at least 0.005 wt %, the Ti content to at least 0.001 wt %, the V content to at least 0.02 wt % or the Nb content to at least 0.003 wt %. In this case, the micro-alloying elements V, Ti, Nb on the one hand and Al on the other hand may be present respectively in combination with one or more elements from the group “Al, V, Ti, Nb” or alone in amounts above said minimum contents. At contents of up to 0.01 wt % Ti, up to 0.025 wt % Nb, up to 0.075 wt % V or up to 0.040 wt % Al, the effects of these elements can be utilised particularly effectively in a steel to be used according to the invention. Again, the stated upper limits of the contents of Ti, Nb, V or Al can be maintained alone or in combination with each other in order to achieve the optimal effect of the respective alloying element. The contents % Al, % Nb, % Ti, % V and % N of Al, Nb, Ti, V and N are combined together in the steel to be used according to the invention under the condition
%Al/27+%Nb/45+%Ti/48+%V/25>%N/3.5
such that the nitrogen contained in the steel to be used according to the invention is bound completely via the respectively present contents of Al as well as any additionally added contents of Nb, Ti and V, and boron can thus delay transformation. In addition, the binding of N according to the invention makes it possible for the optionally present boron to act as a dissolved element in the matrix of the steel and to suppress the formation of ferrite and/or perlite.
Optional contents of chromium (“Cr”) of up to 4.00 wt %, in particular up to 3 wt % or up to 2.5 wt %, contribute to the hardenability and corrosion resistance of the steel used according to the invention due to the formation of special carbides and chromium nitrides in an optional nitriding treatment. For example, at least 0.5 wt % or at least 0.8 wt % of Cr may be provided for this purpose. Cr contents above 4.00 wt % would promote undesirable martensite formation in the structure of the steel to be used according to the invention.
Contents of Ni of up to 1.5 wt % which are also optionally present improve the toughness of the steel to be used according to the invention. If this effect is to be utilised, it occurs from a Ni content of at least 0.1 wt %, in particular at least 0.15 wt %.
The alloying elements occurring or deliberately added via the starting material into the steel to be used according to the invention include Cu, whose content is limited to max. 0.3 wt %, in particular less than 0.3 wt %, to avoid negative influences in the steel to be used according to the invention.
At contents of up to 2.0 wt %, cobalt (“Co”) optionally present in the steel to be used according to the invention causes a shift in bainite formation to shorter times. The positive influence of Co can be utilised in particular at Co contents of at least 0.1 wt %, in particular at least 0.5 wt %.
In accordance with the above explanations, a method according to the invention for producing a steel component comprises the following steps:
%Al/27+%Nb/45+%Ti/48+%V/25>%N/3.5
The manufacture of the steel powder consisting of steel to be used according to the invention can be carried out in a conventional manner, for example by gas atomisation or by any other suitable method. For this purpose, a steel melt melted according to the invention can be atomised into the steel powder, for example by gas or water atomisation or a combination of these two atomisation methods. If necessary, the powder particles obtained in this manner can be sieved in order to select those having a suitable grain size for further processing according to the invention. Here, grains having an average diameter of 5-150 μm have proven to be suitable for the purposes of the invention.
As explained, by setting the alloying of the steel used according to the invention as a steel powder, the structure of the steel component produced according to the invention can be determined. Here, a component produced according to the invention is characterised in that it has a structure comprising at least 80 vol % bainite, in particular a completely bainitic structure in the technical sense.
The steel powder to be used according to the invention is suitable for any additive manufacturing process of the type known from the prior art and explained at the outset, in which the adjoining particles collected by the heat supplied are fused by local heat input, and thus the steel powder portion treated in this way is solidified during the subsequent cooling in the respective section to be formed of the component to be produced. In particular, the known laser melting and laser sintering processes which permit a precisely limited, intensive heating of the steel powder particles and a correspondingly accurate formation of the component to be produced, are suitable for the production of a component according to the invention.
Optionally, the strength and ductility of a steel component produced according to the invention can be set via a conventional heat treatment. This heat treatment may comprise a fine adjustment of the bainitic structure of the steel component produced according to the invention, in which the steel component is heated to an austenitising temperature above the Ac3 temperature and then cooled from the austenitising temperature in air or in oil, the cooling being carried out at a cooling rate of at least 0.5 K/s and less than 300 K/s, in particular 3.0-300 K/s, which is sufficient for setting the bainitic structure, which should be as complete as possible. In this heat treatment, it proves to be particularly advantageous if the steel to be used according to the invention has a B content of 0.0005-0.0025 wt %. The presence of such B contents is particularly effective here in preventing the formation of undesirable ferrite or perlite in the structure of the steel component in the course of the cooling.
Optionally, as an alternative or in addition to the fine adjustment of the structure, which is also optionally carried out as explained above, a tempering treatment may be carried out, in which the steel component is held at a temperature of 450-600° C. over a period of 0.5-6 hours, wherein the specifically provided duration of the tempering treatment can be chosen depending on the size and volume of the steel component, in order to again support, in a targeted manner, the previously mentioned strength increase due to special carbide formation, said strength increase already being obtained in the course of the additive manufacturing.
Steel components produced according to the invention, after completion of the additive manufacturing (step d), i.e. before step e), already have a tensile strength of at least 900 MPa, in particular at least 1145 MPa, a yield strength of at least 560 MPa, in particular at least 675 MPa, and an elongation at break A5.65 of at least 8%.
By virtue of the respective optional heat treatment steps (step e), the mechanical properties of a steel component according to the invention can be improved to such an extent that its tensile strength is at least 1050 MPa, in particular at least 1230 MPa, its yield strength is at least 615 MPa, in particular at least 750 MPa, and its elongation at break A5.65 is at least 8%.
Three melts S1, S2, S3 according to the invention were melted and respectively atomised in a conventional manner in the gas flow into steel powder having a grain size of 5-150 μm. The composition of melts S1-S3 is given in Table 1.
From the steel powders obtained, steel components were produced using “Selective Laser Melting” (“3D printing/SLM method”), which were test bodies for determining the optimal parameters for additive manufacturing, namely cubes with approx. 20 mm edge length, and blanks for tensile and impact tests.
The tensile strength Rm_V of the steel components obtained after the additive manufacturing is shown in Table 2.
The steel components thus obtained were subjected to a machining operation in order to optimally adapt them to their respectively required final shape.
Subsequently, the components each underwent a heat treatment in which they were aged for a period tA at a temperature TA. The respective ageing time tA and ageing temperature TA are also listed in Table 2.
Likewise, Table 2 indicates the tensile strength Rm_N which the components exhibited after mechanical processing and ageing.
In addition, Table 2 indicates the structure of the components.
It can be seen from the TTT diagrams displayed in
| Number | Date | Country | Kind |
|---|---|---|---|
| 18182027.5 | Jul 2018 | EP | regional |
