SE542360C2 - A steel powder, a product comprised by an aggregate thereof, and use of such steel powder - Google Patents

Abstract

A steel powder having the following composition in weight %: C: 0.36 - 0.40, Si: 0.6 -1.3, Mn: 0.2 - 0.7, Cr: 13.0 - 14.0, V: 0.15 - 0.40, S: <0.05, N: 0.01 - 0.15, and, optionally, Al: 0.001-0.1, Ni: < 0.5, Mo: <0.25, Ti: <0.02, Nb: <0.03, balance Fe and impurities, and having a particle size of <500 pm.

Description

A steel powder, a product comprised by an aggregate thereof, and use of such steel powder TECHNICAL FIELD The invention relates to a steel powder. The invention also relates to a product manufactured by aggregation of said steel powder, in particular a mould for the moulding of plastics.

In particular, the invention relates to precipitation hardening steel powder suitable for the manufacturing of moulds for the moulding of plastics.

BACKGROUND OF THE INVENTION Precipitation hardening martensitic stainless steel comprise, amongst other, steel according to AISI 420 and DIN 1.2083.

Steel of this type are commonly used for products requiring a high strength and a good toughness. Typical applications are plastics moulds, i.e. steel moulds in which plastics are moulded. The steel is often delivered in a solution treated condition and is hardenable by aging. Important properties are a high strength and corrosion resistance as well as a good polishability. In addition thereto, plastic mould steel should also have a good machinability and good welding properties such that the steel can be welded without preheating and postheating.

Steel of the above-mentioned types have been used for the manufacture of products by means of powder metallurgy, including so called Additive Manufacturing, AM, which in itself comprises a plurality of different techniques, such as Laser Beam Melting (LBM), Direct Energy Deposition (DED) and Direct Metal Deposition (DMD). Alternative or supplementary powder metallurgical production methods include Hot Isostatic Pressing (HIP) and Metal Injection Moulding (MIM).

The article "Decarburization of stainless steel during selective laser melting and its influence on Young's modulus, hardness and tensile strength" by Xiao Zhao, Bo Song, Yuanjie Zhang, Xiaomeng Zhu, Qingsong Wei, Yusheng Shi, published in Materials Science & Engineering A 647 (2015) 58-61, discloses a method in which a high energy laser is applied to selectively melt metal powders layer by layer to create 3D-structured objects. Spherical AISI 420 steel powder (Changsha Hualiu Metallurgy Powder Co, Ltd., China) with an average particle size of 20 pm was used as feedstock material.

Products of steel having the following composition are known to be produced by moulding: C 0.36 - 0.40, Si 0.6 - 1.3, Mn 0.2 - 0.7, Cr 13.0 - 14.0, V 0.15 - 0.40, S < 0.05, N 0.01 - 0.15, and, optionally, Al 0.001-0.1, Ni < 0.5, Mo < 0.25, Ti <0.02, Nb <0.03, balance Fe and impurities. Examples of such steel are steel named Stavax (trade mark), produced by the present applicant and referred to as a premium grade stainless tool steel for plastic mould tooling that possess a unique combination of toughness, corrosion resistance and the ability to reach uniform hardness levels throughout large cross sections.

DISCLOSURE OF THE INVENTION An object of the present invention is to provide an alternative steel powder suitable for powder-metallurgical manufacturing of moulds for the moulding of plastics.

This object, as well as additional advantages is achieved to a significant measure by providing a steel powder as defined in the enclosed patent claims.

Accordingly, the object of the invention is achieved by means of a steel powder having the following composition in weight %: C 0.36 - 0.40 Si 0.6 - 1.3 Mn 0.2 - 0.7 Cr 13.0 - 14.0 V 0.15 - 0.40 S < 0.05 N 0.01 - 0.15 and, optionally Al 0.001-0.1 Ni < 0.5 Mo < 0.25 Ti <0.02 Nb <0.03 balance Fe and impurities, and having a particle size <500 ?m.

The powder particles are generally spherical, and size, as referred to herein, refers to diameter.

According to one embodiment, the powder is a prealloyed powder.

According to one embodiment, at least 98% of inclusions in the powder have a diameter of less than 8 ?m. According to a further embodiment, at least 98% of inclusions present in the powder have a diameter of less than 4 ?m.

According to one embodiment, at least 80%, preferably 90% of carbides present in the powder have a diameter of less than 5 ?m, preferably less than 2.5 ?m.

According to one embodiment, the powder has an isotrope microstructure. Preferably, the steel powder is the result of gas atomization of steel in nitrogen shield gas, wherein said steel has been subjected to an electro slag refinement process and has micro slag content according to ASTM E45-97.

The invention also relates to a steel product comprised by an aggregation of steel powder according to the invention, as defined hereinabove or hereinafter. An aggregation of powder is referred to as powder that, as a result of a powder metallurgical process, forms a solid body as a result of sintering together of or partial or full melting together of said powder particles.

According to one embodiment said steel product is a mould for the moulding of plastics.

The present invention also relates to a method of producing a steel product as defined hereinabove or hereinafter, said method being characterised in that it comprises the step of gas atomizing an alloy into a powder as defined hereinabove, and that the powder is aggregated through a hot isostatic pressing (HIP) process, and that, after the hot isostatic pressing, the resulting product is subjected to a heat treatment at a temperature in the range of 975°C to 1050°C in such a way that the resulting product after said heat treatment has a microstructure in which at least 98% of the carbides therein has a diameter of less than 4 pm. The alloy from which the powder has been formed may be an alloy which has been subjected to an electro slag refinement process before being subjected to said gas atomisation. The heat treatment applied after HIP typically has a durance of approximately 30 minutes and is controlled in order to obtain said microstructure.

The invention also refers to use of a steel powder according to the invention, as defined hereinabove or hereinafter, as powder in any of the methods of hot isostatic pressing, powder extrusion, metal injection moulding and additive manufacturing or for providing a surface layer on a substrate by thermal spraying or overlay welding for manufacturing a steel product, as defined hereinabove or hereinafter.

The invention also relates to the use of a steel powder according to the invention, as defined hereinabove or hereinafter, as feedstock in an additive manufacturing method for manufacturing of a steel product according to the invention, as defined hereinabove or hereinafter. According to the ASTM standard F2792-10, additive manufacturing, referred to as AM, is the « process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining.

DETAILED DESCRIPTION The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. The amount of hard phases is given in volume % (vol. %). Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims.

Carbon (0.36 - 0.40 %) Carbon is effective for improving the strength and the hardness of the steel. However, if the content is too high the steel may be difficult to machine after cooling from hot working. C should be present in a minimum content of 0.36 %, preferably at least 0.037 %. The upper limit for carbon is 0.40%, preferably 0.39 %. The nominal content is 0.38%.

Silicon (0.6 - 1.3%) Silicon is used for deoxidation. Si is also a strong ferrite former. Si is therefore limited to 1.3%. The upper limit may be 1.25, 1.20, 1.15, 1.10 or 1.00%. The lower limit may be 0.65, 0.70, 0.75, 0.80 or 0.85%. Nominal content is 0.95%.

Manganese (0.20 - 0.70 %) Manganese contributes to improving the hardenability of the steel. If the content is too low then the hardenability may be too low. At higher sulphur contents manganese prevents red brittleness in the steel. Manganese shall therefore be present in a minimum content of 0.20%, preferably at least 0.25, 0.30, 0.35 or 0.40%. The steel shall contain maximum 0.70% Mn, preferably maximum 0.65, 0.60 or 0.55%. A preferred range is 0.40 - 0.50%. Nominal content is 0.45%.

Chromium (13.0 - 14.0 %) Chromium is to be present in a content of at least 13.0 % in order to make the steel stainless and to provide a good hardenability in larger cross sections during the heat treatment. However, high amounts of Cr may lead to the formation of hightemperature ferrite, which reduces the hot-workability. The lower limit may be 13.2 or 13.4 %. The upper limit of Cr is 14.0 % and the amount of Cr may be limited to 13.8 %. A preferred range is 13.4 - 13.8 %. Nominal content is 13.6 %.

Vanadium (0.15 - 0.40 %) Vanadium forms precipitations of vanadium nitride together with nitrogen, which adds to the hardening of the steel. The lower limit to Vanadium may be 0.20 or 0.25%. The upper limit may be 0.35 or 0.30%. A preferred range is 0.25-0.30 %. Nominal content is 0.27%.

Sulphur (<0.05%) S contributes to improving the machinability of the steel and may therefore be deliberately added to the steel in an amount of <0.05%. At higher sulphur contents there is a risk for red brittleness. Moreover, high sulphur contents may have a negative effect on the fatigue properties of the steel. The upper limit shall therefore be 0.05%. According to one embodiment the upper limit of Sulphur is 0.03%.

Aluminium (0.001 - 0.1%) Aluminium is used for precipitation hardening, possibly in combination with Ni. The upper limit is restricted to 0.1 % for avoiding the formation of aluminium oxide during any powder metallurgical process in which the powder will be used and for avoiding the formation of delta ferrite.

Nitrogen (0.01 - 0.15 %) Nitrogen is a strong austenite former and also a strong nitride former. The inventors of the present invention have surprisingly found that nitrogen can be deliberately added in order to the steel without impairing the polishability of steel, provided that the size of at least 80 vol. %, preferably at least 90% of the nitride particles present in the matrix has a diameter which is limited to not more than 8 pm, preferably not more than 4 ?m. Preferably, the said size may restricted to 3 ?m, 2 ?m or even 1 ?m. The small size of the nitrides also result in grain refining effect. According to a first embodiment, the upper limit of N in the powder is 0.15%. According to alternative embodiments it is 0.12% or 0.05%.

Nickel (optional, <0.5%) Nickel is austenite stabilizer and supress the formation of delta ferrite. Nickel gives the steel a good hardenability and toughness. Nickel is also beneficial for the machinability and polishability of the steel. Nickel may contribute to precipitation hardening as it together with Al may form minute intermetallic NiAI-particles during aging. However, excess Ni additions results in too high amount of retained austenite.

Molybdenum (optional, <0,25 %) Mo in solid solution is known to have a very favourable effect on the hardenability. Molybdenum is a strong carbide forming element and also a strong ferrite former. A controlled amount of Mo in the matrix counteracts the formation of reverted austenite during aging.

Niobium (optional, <0.03%), Titanium (optional, <0.02%) Nb and Ti are strong carbide and nitride formers. The content of these elements should therefore be limited in order to avoid the formation of undesired carbides and nitrides. The maximum amount of these elements is therefore 0.03 and 0.02% respectively.

Impurity elements P, S and O are the main impurities, which may have a negative effect on the mechanical properties of the steel. P may therefore be limited to 0.05, 0.04, 0.03 0.02 or 0.01 %.

If sulphur is not deliberately added, then the impurity content of S may be limited to 0.05, 0.04, 0.003, 0.001, 0.0008, 0.0005 or even 0.0001%.

EXAMPLE According to one exemplifying embodiment, the prealloyed powder according to the present invention is produced by gas atomizing of a steel having the following composition in weight%: C: 0.38, Si: 0.95, Mn: 0.45, Cr: 13.6, V: 0.27, Al: 0.050, S: 0.02. As a result of the gas atomisation taking place in an atmosphere that contains nitrogen (N2 as shield gas), the resulting powder will obtain a nitrogen content in the range of 0.01-0.05%. Contact with oxygen is avoided, in order to prevent the formation of oxides in an on the powder particles that are produced.

Preferably, at least 98% of the inclusions in the powder (mainly oxides) have a particle size (diameter) of less than 8 ?m.

In order to make the powder suitable for used as feedstock in AM, a gas atomisation process that produces powder particles having a high degree of roundness and a low amount of satellites is applied, such as close-coupled atomization. The size of the powder should be limited to 500 ?m and depending on the intended use specific powder fractions may be used. For AM the maximum size range is 5 - 150 ?m, and the preferred size range is 10 - 100 ?m with a means size of about 25 - 45 ?m. For MIM the preferred maximum size is 50 ?m, for thermal spraying preferred ranges are 32-125 ?m and 20 - 90 ?m and for overlay welding the preferred range is 45 - 250 ?m.

INDUSTRIAL APPLICABILITY The steel powder of the present invention may be used as feedstock in an AM process or as a powder for manufacture of steel products by means of other PM processes, such as MIM or HIP.

Claims (10)

1. A steel powder having the following composition in weight %: C 0.36 - 0.40 Si 0.6 - 1.3 Mn 0.2 - 0.7 Cr 13.0 - 14.0 V 0.15 - 0.40 S < 0.05 N 0.01 - 0.15 and, optionally Al 0.001-0.1 Ni < 0.5 Mo < 0.25 Ti <0.02 Nb <0.03 balance Fe and impurities, and having a particle size of <500 ?m.

2. A steel powder according to claim 1, characterised in that the particles of the powder have a size of <150 ?m.

3. A steel powder according to claim 1 or 2, characterised in that the powder is a prealloyed powder.

4. A steel powder according to any one of claim 1-3, characterised in that at least 98% of inclusions therein have diameter of less than 8 ?m.

5. A steel powder according to any one of claims 1-4, characterised in that at least 80%, preferably 90% of carbides present therein has a diameter of less than 5 ?m, preferably less than 2.5 ?m.

6. A steel product comprised by an aggregation of steel powder according to any one of claims 1-5.

7. A steel product according to claim 6, wherein said steel product is a mould for the moulding of plastics.

8. A method of producing a steel product according to claim 6 or 7, characterised in that it comprises the step of gas atomizing an alloy into a powder according to any one of claims 1-5, and that the powder is aggregated through a hot isostatic pressing (HIP) process, and that, after the hot isostatic pressing, the resulting product is subjected to a heat treatment at a temperature in the range of 975°C to 1050°C in such a way that the resulting product after said heat treatment has a microstructure in which at least 98% of the carbides therein has a diameter of less than 4 ?m.

9. Use of a steel powder according to any one of claims 1-5 as powder in any of the methods of hot isostatic pressing, powder extrusion, metal injection moulding and additive manufacturing or for providing a surface layer on a substrate by thermal spraying or overlay welding for manufacturing a product according to claim 6 or 7.

10. Use of a steel powder according to any one of claims 1-5 as feedstock in an additive manufacturing method for manufacturing of a steel product according to any one o of claims 6-8.