Steel Material In Powder Form And Process For Producing Said Steel Material

Abstract

The invention relates to a steel material in powder form for printing in additive manufacturing methods such as selective laser melting (SLM) or selective laser sintering (SLS) or for use in hot isostatic pressing methods, wherein the material has the following composition: C 0.17-0.23 Si 0.10-0.80 Mn 0.15-0.45 P ≤0.03 S ≤0.015 Cr 0.8-2.0 Mo 0.15-0.80 Ni 0.1-2.0 V 0.1-2.0 the remainder being comprised of iron, optional elements, and inevitable smelting-related impurities as well as a method for manufacturing it and a method for producing a component made thereof.

Claims

1-15. (canceled)

16. A powdered steel material, comprising in wt %: C 0.17-0.23 Si 0.10-0.80 Mn 0.15-0.45 P ≤0.03 S ≤0.015 Cr 0.8-2.0 Mo 0.15-0.80 Ni 0.1-2.0 V 0.1-2.0 and optionally one or more of the following Nb ≤0.5 W ≤1.6 Cu ≤1 Al ≤1 Co ≤1 Ti ≤0.5 Ta ≤0.5 Zr ≤0.5 N ≤0.15 B ≤1 the remainder being comprised of iron and inevitable impurities.

17. The steel material of claim 16, comprising in wt %: C 0.17-0.21 Si 0.15-0.30 Mn 0.15-0.45 P ≤0.03 S ≤0.010 Cr 0.8-1.1 Mo 0.15-0.25 Ni 1.0-1.5 V 0.1-0.2 and optionally one or more of the following Nb ≤0.5 W ≤1.6 Cu ≤1 Al ≤1 Co ≤1 Ti ≤0.5 Ta ≤0.5 Zr ≤0.5 N ≤0.15 B ≤1 the remainder being comprised of iron and inevitable impurities.

18. The powdered steel material of claim 16, wherein the powdered steel material has a particle size distribution in a range selected from the group consisting of (a) D10=15 μm and D90=63 μm, (b) D10=15 μm and D90=45 μm, and (c) D10=20 um and D90=53 μm.

19. A method of manufacturing a steel component comprising, providing a powder bed comprising the powdered steel material of claim 16, wherein the powdered steel material has a particle size distribution in a range less than 20 μm, and subjecting the powdered steel material to a procedure selected from the group consisting of (a) metal injection molding and (b) binder jetting.

20. A method of manufacturing a steel component comprising, I. providing a powder bed comprising at least one layer of the powdered steel material of claim 16, wherein the powdered steel material has a particle size distribution in a range selected from the group consisting of 1. D10=15 μm and D90=63 μm, and 2. D10=15 μm and D90=45 μm, and II. performing steps (a) and (b) in alternating fashion until the steel component is obtained: (a) melting the powdered steel material with a laser to fuse the powdered steel material together, and (b) adding at least one further layer of the powdered steel material of claim 16.

21. A method of manufacturing a steel object comprising, providing the powdered steel material of claim 16, wherein the powdered steel material has a particle size distribution in a range greater than 45 μm, and subjecting the powdered steel material to a procedure selected from the group consisting of (a) laser metal deposition, (b) direct energy deposition, and (c) electron beam melting.

22. A method of manufacturing a steel component comprising, providing a powder bed comprising the powdered steel material of claim 17, wherein the powdered steel material has a particle size distribution in a range less than 20 μm, and subjecting the powdered steel material to a procedure selected from the group consisting of (a) metal injection molding and (b) binder jetting.

23. A method of manufacturing a steel component comprising, I. providing a powder bed comprising at least one layer of the powdered steel material of claim 17, wherein the powdered steel material has a particle size distribution in a range selected from the group consisting of 1. D10=15 μm and D90=63 μm, and 2. D10=15 μm and D90=45 μm, and II. performing steps (a) and (b) in alternating fashion until the steel component is obtained: (a) melting the powdered steel material with a laser to fuse the powdered steel material together, and (b) adding at least one further layer of the powdered steel material of claim 17.

24. A method of manufacturing a steel object comprising, providing the powdered steel material of claim 17, wherein the powdered steel material has a particle size distribution in a range greater than 45 μm, and subjecting the powdered steel material to a procedure selected from the group consisting of (a) laser metal deposition, (b) direct energy deposition, and (c) electron beam melting.

25. The method of claim 20, wherein the powdered steel material has a particle size distribution of D10=15 μm and D 90=63 μm, and wherein the laser is operated at a power in the range of from 200 to 275 W and at a scanning speed of from 750 to 1000 mm/sec.

26. The method of claim 20, wherein the at least one layer and the at least one further layer of the powdered steel material have a layer thickness in the range of from 25 to 65 μm and a line spacing in the range of from 80 to 150 μm.

27. The method of claim 20, wherein the laser has a focus diameter in the range of from 80 to 120 μm

28. The method of claim 20, wherein the laser has a volume/energy density in the range of from 45 to 85 Joule/mm.sup.3.

29. The method of claim 20, further comprising hardening and tempering the steel component.

30. The method of claim 29, wherein the steel component is hardened at a temperature in the range of from 800 to 950° C. and tempered at a temperature of from 180 to 220° C.

31. The method of claim 23, wherein the powdered steel material has a particle size distribution of D10=15 μm and D90=63 μm, and wherein the laser is operated at a power in the range of from 200 to 275 W and at a scanning speed of from 750 to 1000 mm/sec.

32. The method of claim 23, wherein the at least one layer and the at least one further layer of the powdered steel material have a layer thickness in the range of from 25 to 65 μm and a line spacing in the range of from 80 to 150 μm.

33. The method of claim 23, wherein the laser has a focus diameter in the range of from 80 to 120 μm.

34. The method of claim 23, wherein the laser has a volume/energy density in the range of from 45 to 85 Joule/mm.sup.3.

35. The method of claim 23, further comprising hardening and tempering the steel component.

36. The method of claim 35, wherein the steel component is hardened at a temperature in the range of from 800 to 950° C. and tempered at a temperature of from 150 to 250° C.

37. A method of manufacturing a steel object comprising, providing the powdered steel material of claim 16, and subjecting the powdered steel material to a procedure selected from the group consisting of (a) laser metal deposition, (b) direct energy deposition, (c) electron beam melting, (d) metal injection molding, (e) binder jetting, (f) melting with a laser and (g) powder bed method.

38. A method of manufacturing a steel object comprising, providing the powdered steel material of claim 17, and subjecting the powdered steel material to a procedure selected from the group consisting of (a) laser metal deposition, (b) direct energy deposition, (c) electron beam melting, (d) metal injection molding, (e) binder jetting, (f) melting with a laser and (g) powder bed method.

Description

[0054] The invention will be explained by way of example based on the drawings. In the drawings:

[0055] FIG. 1: shows the influence of the sulfur content on the crystalline structure and the crack formation;

[0056] FIG. 2: shows the influence of the manganese and sulfur content on the structure,

[0057] FIG. 3: shows the particle size distribution;

[0058] FIG. 4: is a table showing a sample powder;

[0059] FIGS. 5a and b: show electron microscope images of the powder produced;

[0060] FIG. 6: shows the available process window for the processing of the steel powder according to the invention;

[0061] FIG. 7: shows the comparison of the structure (steel 2) with platform heating and without platform heating;

[0062] FIG. 8: is a diagram showing the possible structural directions:

[0063] FIG. 9: shows a comparison of the invention (steel 1) to a standard case-hardening (16MnCr5) and another test alloy with a higher sulfur content (steel 2);

[0064] FIG. 10: shows a comparison of the mechanical properties between steel 1 and steel 2 in the as-printed horizontal state;

[0065] FIG. 11: shows the comparison of the materials according to FIG. 10 in the as-printed vertical state;

[0066] FIG. 12: shows the comparison according to FIG. 10 with a tempering at 200° C.;

[0067] FIG. 13: shows the comparison according to FIG. 11 with a tempering at 200° C.;

[0068] FIG. 14: shows the comparison of the mechanical properties (steel 1, steel 2, and the comparison material 16MnCr5) after hardening at 850° C. and tempering at 200° C. in the horizontal state;

[0069] FIG. 15: shows the comparison according to FIG. 14 in the vertical state;

[0070] FIG. 16: shows the comparison according to FIG. 14, but with a hardening at 950° C.;

[0071] FIG. 17: shows the comparison according to FIG. 15, but with a hardening at 950° C.;

[0072] FIG. 18: shows the comparison of the material according to the invention to steel 2 with regard to the notched bar impact work and the Rockwell hardness in the as-printed horizontal state;

[0073] FIG. 19: shows the comparison according to FIG. 18 in the as-printed vertical state;

[0074] FIGS. 20 & 21: show the comparison according to FIGS. 18 & 19 with an additional tempering treatment at 200° C.;

[0075] FIGS. 22 & 23: show the comparison of the mechanical properties of the material according to the invention to steel 2 and the 16MnCr5 with a hardening at 850° C. and a tempering treatment at 200° C., the one in the horizontal state and the other in the vertical state;

[0076] FIGS. 24 & 25: shows the comparison according to FIGS. 22 & 23, but with a hardening at 950° C.;

[0077] FIG. 26: shows the comparison of the structure between a comparison material and the material according to the invention.

[0078] FIG. 27: shows the influence of various sulfur contents on the notched bar impact work of the printed components (without heat treatment)

[0079] The steel composition according to the invention has the following composition:

TABLE-US-00005 wt % C Si Mn P S Cr Mo Ni V Invention 0.17-0.23 0.10-0.80 0.15-0.45 <0.03 <0.015 0.8-2.0 0.15-0.80 0.1-2.0 0.1-2.0 Preferred 0.17-0.21 0.15-0.30 0.15-0.45 <0.03 <0.010 0.8-1.1 0.15-0.25 1.0-1.5 0.1-0.2

[0080] One property of this composition is that the sulfur content is below 0.015 wt % since otherwise, cracks close to the welding beads can form in the printing process.

[0081] At the top left, FIG. 1 shows the structure with 0.051% sulfur (steel 2) and as an extreme counter-example, with 0.003% sulfur next to it (steel 1). The two figures on the bottom show in measurement 1 the cracks with a sulfur content of 0.051 wt % in the printed state and in the depiction to the right of it in the printed, heat treated state. The image on the left shows a higher porosity and also a few isolated cracks. If this alloy (steel 2) is printed with a platform heating, then the cracks propagate drastically (FIG. 7).

[0082] With the invention, though, not only is the sulfur content very low, but also the manganese content is adjusted so that through the adjustment of the manganese content and low sulfur content in combination with the rapid solidification conditions of the 3D printing process, manganese sulfides that diminish the mechanical properties with regard to strength, toughness, and ductility do not form in either the printed state or the printed and heat treated state with the composition according to the invention. In FIG. 2 the structure of the composition according to the invention is shown in the left while on the right, as a comparison example, a 16MnCr5 is shown in which the manganese sulfides are visible.

[0083] The manufacture of the material will be explained below.

[0084] The steel composition according to the invention is melted in an intrinsically known way in an electric arc furnace or converter and if need be, is adjusted to the alloy composition by means of secondary metallurgy. The steel material obtained in this way is liquefied in a vacuum induction furnace and is atomized in an intrinsically known way in an atomization chamber by means of inert gas atomization (vacuum induction gas atomization). Metal powders can basically also be manufactured by means of water atomization.

[0085] Because of the manganese content of the composition according to the invention, the melting preferably takes place in a shielding gas atmosphere and particularly in an argon atmosphere or argon shielding gas atmosphere in order to prevent a vaporization of the manganese. Then the actual atomization process takes place with the aid of a very high gas pressure. In this case, the fireproof crucible is tilted as a result of which the liquid melt flows into a tundish (distributing trough, distributing vessel) and the liquid metal flows out an opening at the bottom of the tundish into a nozzle. The nozzle atomizes the molten metal into fine metal particles that are smaller than 1 mm in size. The metal particles undergo an abrupt cooling and after the atomization, are in a powdered form.

[0086] For example, argon or nitrogen can be used as the atomization gas.

[0087] The powder obtained in this way then requires processing.

[0088] For a corresponding classification of this powder, it is possible to pass the powder through a screen and it is also possible for the powder to be correspondingly classified by means of air classifying in a deflector wheel classifier. Preferably, the size of the powder particles and/or the particle size distribution corresponds to the requirements of the respective additive manufacturing method. For the powder bed method, for example, the desired particle size distribution is for example 15-63 μm (for special applications, narrower limits can also be set), 15-45 μm or 20-53 μm. The lower value here is the D10 value and the upper one is the D90 value.

[0089] This size of the powder particles and the desired particle size distributions, as has already been explained above, can be achieved by means of screening; the screening can ensure the classification of the powder by particle size into different powder fractions. The different screening fractions can be combined into a desired particle range if necessary.

[0090] In air classifying, the classification is carried out by using different settling rates of different-sized particles in a gas flow. This method is particularly suitable for large quantities of powder; it can also be preceded by a screening.

[0091] In air classifying, the cut can be influenced by the gas quantity that is conveyed through the classifier and by the deflector wheel rotation speed.

[0092] FIG. 3 shows an example of a possible particle size distribution in the material according to the invention. This particle size distribution is shown in the table according to FIG. 4.

[0093] In order to characterize powders of this kind, the particle size, sphericity, and pourability are determined. In particular, an optical analysis and an inspection of the powder by means of SEM images are performed. With particle sizes <20 μm, the powder is particularly suitable for the so-called metal injection molding sintering method and the so-called binder jetting method.

[0094] Particle size distributions of 15-63 μm, in particular 15-45 μm, are particularly used in laser powder bed methods (e.g. selective laser melting) or electron beam melting, whereas powders with a size >45 μm are used in the laser metal deposition method and in the direct energy deposition method.

[0095] Naturally, such powders can also be used for hot isostatic pressing methods.

[0096] FIGS. 5a and 5b show images with different enlargements of a typical powder produced from the material according to the invention.

[0097] The powder obtained in this way is then ready for processing.

[0098] The processing in the powder bed method will be explained in greater detail below.

[0099] FIG. 6 shows the process window of the material in powder form according to the invention; it is clear that a very wide range of laser energy is possible and also a very large range of laser advancing speed so that this demonstrates in a very striking way that the steel material in powder form according to the invention can be printed in a particularly favorable way so that a broad spectrum of conventional AM or 3D printers can be used without going beyond the range in which very good results are achieved.

[0100] Because of the low carbon content of around 0.19 wt %, with the material according to the invention, a powder bed preheating is not necessary, which further simplifies the printing process to a considerable degree.

[0101] In the above-mentioned process window according to FIG. 6, there is a very high stability of the porosity of 0.01-0.03%, which likewise demonstrates how well and simply the material according to the invention can be printed.

[0102] With the use of a conventional EOS M290 printer, laser powers of for example 200-275 W with a scanning speed of 775-1000 mm/sec can be used. Usable layer thicknesses are between 30 and 60 μm with a line spacing of 110 μm and a laser focus diameter of 100 μm. The volume/energy density in this case is between 50 and 75 Joule/mm.sup.3 so that the process has very large tolerances, which in turn ensure a simple printability.

[0103] With the material according to the invention, no preheating is required. The preheating exhibits a negative effect with the alloy steel 2 because in this case, the high sulfur content results in increased crack formation due to the additional energy input. The additional energy input due to the increase in the laser power can result in the vaporization of the material, weld spatters, and instability of the weld pool (steel 1 and 2).

[0104] Through selective printing process parameter variations, it has been possible to demonstrate that based on the parameter window that is already very broadly defined anyway, the increased energy input increases the diffusion of sulfur and thus increases the tendency for sulfur segregation and the crack formation that results from this. The specimens prepared with the above-mentioned “EOS M290” system without a heating of the build chamber were produced as specimens in the as-built or as-printed state, without subsequent heat treatment and in the hardened and tempered state; the hardening temperature was 850° C. in the one instance and 950° C. in the other. Quenching was performed with water. These hardening and tempering treatments comprise a hardening procedure (850° C. or 950°) with a holding time of approx. 30 min. The duration of the subsequent tempering process (200° C.) is 2 hours, followed by cooling in air.

[0105] The specimens were also tempered at 200° C. immediately after printing, without a prior hardening. According to FIG. 8, in this case, corresponding mechanical tests were performed in the Z structural direction and in the XY structural direction, which means that the mechanical sampling was performed once in accordance with the advancing progression of the welding bead (XY-structural direction) and once in the welding direction of the successive layers (Z-structural direction).

[0106] The prototype steel powder material used—as a low-alloyed steel alloy with the potential for case-hardening—was as follows (steel 1): 0.18% C, 0.29% Si, 0.23% Mn, 0.005% P, 0.0031% S, 0.97% Cr, 0.20% Mo, 1.27% Ni, 0.13% V, as well as residual iron and impurities. A steel with a higher sulfur content and the same composition otherwise was used as a reference (steel 2 with 0.051% S). A comparison to a standard 16MnCr5 was also included.

[0107] The mechanical properties were compared to two other materials according to FIG. 9. The tensile test was performed according to DIN EN ISO 6892-1 with the specimen body B02 and method B. The notched bar impact work was determined according to the ASTM E23 notched bar impact test at room temperature and with Charpy V specimens. The hardness in Rockwell C was determined according to ASTM E18-17.

[0108] FIGS. 10 and 11 first show the strength values, measured in terms of tensile strength (R.sub.m in MPa) in the printed, but not heat-treated state. In this case, FIG. 10 shows the printed horizontal state, i.e. in the XY direction, and FIG. 11 shows the printed vertical state.

[0109] A comparison between steel 1 and steel 2 shows that the tensile strengths R.sub.m do not exhibit any great differences in either the printed horizontal state or the printed vertical state. In both cases, a very high tensile strength of around 1200 MPa can be achieved. In addition, the 0.2% yield strength (R.sub.p0.2 in MPa) is comparatively high in both steels.

[0110] But if the elongation at break (A5 in %) and the contraction at break (Z in %) are taken into account, it is clear that steel 1 is far superior to steel 2. Among other things, this is due to the negative influence of sulfur.

[0111] FIGS. 12 and 13 show the above-mentioned examples, but with a subsequent tempering process in addition to the as-printed state. This does not really result in a change to the image; here, too, steel 1 is far superior to steel 2 when it comes to the elongation at break and the contraction at break.

[0112] FIGS. 14 and 15 show the comparison of three materials with a hardening after the printing at 850° C. and a subsequent tempering treatment at 200° C. The strength values (R.sub.m) in the material according to the invention are in the same range as those of 16MnCr5. With vertically built specimens, the material according to the invention exhibits higher strengths (R.sub.m), namely of greater than 1400 MPa. Also with regard to the contraction at break (Z), the material according to the invention outperforms the 16MnCr5 by approx. 20% in vertical specimens. Consequently, with a higher strength, the material according to the invention also has a higher ductility in comparison to the known material. With regard to steel 2 with the higher sulfur content, the material according to the invention exhibits a strength (R.sub.m) that is 200 MPa higher. Here, too, the material according to the invention is far superior to the comparison material from the preceding figures in terms of the elongation at break and contraction at break.

[0113] The heat treatment or hardening at 950° C. and a subsequent tempering treatment at 200° C. produces an image in the horizontal and vertical state in FIGS. 16 & 17 that is similar to the ones in FIGS. 14 & 15. It is possible, however, to determine that the maximum strength of steel 1 is achieved with a heat treatment that has a hardening temperature of 850° C.

[0114] If the toughness is compared, it is clear that although the strength values of steel 2 in the printed state are virtually the same in comparison to steel 1, steel 1 is nevertheless far superior to steel 2 when it comes to toughness. The high notched bar impact work in the printed state is clearly due to the fine grain structure, which is produced by the printing process with its very rapid solidification and on the other hand by the adapted alloy composition, which is optimized for the printing process. In particular, the addition of vanadium to the alloy according to the invention here also has a noticeable effect because it shifts the pearlite region toward longer times such that a more bainitic structure is produced, which promotes toughness. In FIGS. 18 and 19, it is clear that the hardnesses measured in HRc are otherwise the same, whereas the toughness values are strikingly far apart.

[0115] FIGS. 20 and 21 show a comparison of the two materials, which have been tempered immediately after the printing. The image is similar to the merely printed state, but the notched bar impact work has decreased somewhat in comparison to the merely printed state. This also strikingly demonstrates that with steel 1, outstanding properties are achieved in a simple way already with the printed material without curing.

[0116] FIGS. 22 and 23 show the material comparison, but also include 16MnCr5 in the state in which it has been hardened at 850° C. and tempered at 200° C. In this case, it is clear that the notched bar impact work values (K.sub.v in J) of steel 1 are considerably higher than those of 16MnCr5 and, as has also been already demonstrated above, naturally also in comparison to steel 2.

[0117] As is clear from FIGS. 24 and 25, in the 16MnCr5, the notched bar impact work increases when the hardening temperature is increased, which is due to a grain coarsening. The hardness is virtually the same in all of the materials and even the increase in the notched bar impact work in the 16MnCr5 never achieves the outstanding toughness values of steel 1.

[0118] FIG. 26 shows an overview of the structure of 16MnCr5 in comparison to the invention (steel 1). Steel 1 (FIG. 26 left pictures) exhibits a martensitic/bainitic structure, which occurs with the addition of vanadium. The grain size is approximately 10 μm. FIG. 26 pictures in the middle shows the structure of the 16MnCr5, which is purely martensitic and the grain size is approx. 20 μm. If the hardening temperature of the 16MnCr5 is increased to 970° C., then a grain coarsening occurs (FIG. 26 right pictures).

[0119] FIG. 27 shows the notched bar impact work Kv of the printed component without heat treatment as a function of the sulfur content. The rest of the alloy elements are analogous to those of steel 1 and steel 2. Three specimens of each were tested with a standard deviation of ±10%. By reducing the sulfur content, it was possible to improve the notched bar impact work considerably. At 0.003% S, it was 140 J.

[0120] In steel 1, it is advantageous that even without subsequent heat treatment, it already exhibits superior mechanical properties, which are also achievable even in a very wide process window so that this material can be printed with great success by more or less “anyone.” It is thus possible to produce not only prototypes, but also near-series quality components or small series of them in a simple way with great success, which is necessary for achieving widespread use of the 3D printing process and also minimizes the costs of such printing processes. With the invention, it is also advantageous that the adjusted alloying state does not cause any change in the component geometry since residual austenite after the printing process is avoided. The unwanted transformation of residual austenite into martensite would lead to a volume increase of 3%. The resulting stresses could lead to component damage.

[0121] Due to the adjusted chemical composition, the material can undergo further processing after the printing process and also after reaching the heat-treated state. Other processing methods include, for example, surface treatment methods such as case-hardening, nitriding, and carburizing. Repair welding processes such as the laser deposition method (LMD) or the direct energy deposition method (DED) can also be carried out. The material is also suitable for surface-hardening methods using mechanical impact such as shot peening or deep rolling.