SE1651268A1

SE1651268A1 - Hot work tool steel

Info

Publication number: SE1651268A1
Application number: SE1651268A
Authority: SE
Inventors: Ejnermark Sebastian
Original assignee: Uddeholms Ab
Priority date: 2016-09-26
Filing date: 2016-09-26
Publication date: 2018-03-27
Also published as: TW201814067A; SE540108C2; WO2018056884A1

Abstract

The invention relates hot work tool steel. The steel comprises the following main components (in wt. %):C 0.26-0.38Si 0.1-0.3Mn 0.1-0.8Cr 1.4-3.9Mo 2.0-3.0W 0.8-1.5Ni 0.6-1.7V 0.1-0.4balance optional elements, iron and impurities.

Description

HOT WORK TOOL STEEL TECHNICAL FIELD The invention relates to a hot Work tool steel.

BACKGROUND OF THE INVENTION Vanadium alloyed matrix tool steels have been on market for decades and attained aconsiderable interest because of the fact, that they combine a high Wear resistance Withan excellent dimensional stability and because they also have a good toughness. Thesesteels have a Wide range of applications such as die casting and forging. The steels aregenerally produced by conventional metallurgy followed by Electro Slag Remelting(ESR).

Uddeholm DIEVAR® is a high perforrnance chromium-molybdenum-vanadium steel, containing balanced carbon and vanadium contents as described in WO995 0468 Al.

It is also known to use nitrogen and vanadium alloyed low-chromium tool steels for hotWorking, in particular for small tools, Which do not require a high hardenability butWhere the demands on tempering resistance and therrnal fatigue are high. Temperingresistance is the ability of a hot-Work tool steel to keep its hardness at an elevated temperature for prolonged time. A steel of this type is disclosed in WO20l2l 19925 Al.

Although the vanadium alloyed tool steels produced by ESR have better properties thanconventionally produced tool steels With respect to heat checking, gross cracking, hotWear and plastic deformation, there is a need for further improvements in order toreduce the risk for hot Work tool failure, such as heat checking and gross cracking inhigh pressure die casting. In addition, it Would be benef1cial to further improve the hot strength and temper resistance of hot Work tool steel.

DISCLOSURE OF THE INVENTION The object of the present invention is to provide a hot work tool steel having an improved property profile leading to an increased life of the tool.

Another object of the present invention is to provide a steel having an improvedtempering resistance in combination with a high toughness and a good hardenabilityallowing large sections to be produced with good properties. It is also desirable toimprove the heat checking, while still maintaining a good hot wear resistance and agood resistance to gross cracking. Still another object is provide a steel composition,which in powder forrn is suitable for Additive Manufacturing (AM), in particular for making or repairing injection moulding tools and dies.

The foregoing objects, as well as additional advantages are achieved to a significantmeasure by providing a hot work tool steel having a composition as set out in the claims.

The invention is defined in the claims.

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

Carbon (0.26 - 0.38 %) is to be present in a minimum content of 0.26 %, preferably at least 0.27, 0.28, 0.29,0.30, 0.3l, 0.32, 0.33 or 0.34 %. The upper limit for carbon is 0.38 % and may be set to0.37, 0.36 or 0.35%. Preferred ranges are 0.30 - 0.38 % and 0.33 - 0. 37 %. In any case,the amount of carbon should be controlled such that the amount of primary carbides ofthe type M23C6, M7C3 and MÖC in the steel is limited, preferably the steel is free from such primary carbides.

Silicon (0.1 - 0.3 %) Silicon is used for deoxidation. Si is present in the steel in a dissolved form. Si is astrong ferrite forrner and increases the carbon activity and therefore the risk for theformation of undesired carbides, which negatively affect the impact strength. Silicon isalso prone to interfacial segregation, which may result in decreased toughness andtherrnal fatigue resistance. Si is therefore limited to 0.3 %. The upper limit may 0.29,0.28, 0.27, 0.26, 0.25, 0.24, 0.23 and 0.22%. The lower limit may be 0.12, 0.14, 0.16,0.18 and 020%. Preferred ranges are 0.10 - 0.25 % and 0.15 - 024%.

Manganese (0.1 - 0.8 %) Manganese contributes to improving the hardenability of the steel and together withsulphur manganese contributes to improving the machinability by forrning manganesesulphides. Manganese shall therefore be present in a minimum content of 0.1 %,preferably at least 0.2, 0.3, 0.35, 0.4, 0.45 or 0.5 %. At higher sulphur contentsmanganese prevents red brittleness in the steel. Mn may also cause undesirable micro-segregation resulting in a banded structure. The steel shall contain maximum 0.8 %, preferably maximum 0.7, 0.6, 0.55 or 0.5 %.

Chromium (1.4 - 3.9 %) Chromium is to be present in a content of at least 1.6 % in order to provide a goodhardenability in larger cross sections during heat treatment. If the chromium content istoo high, this may lead to the formation of high-temperature ferrite, which reduces thehot-workability. The lower limit may be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5,or 2.6 %. The upper limit may be 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7 or 3.8%.

Molybdenum (2.0 - 3.0 %) Mo is known to have a very favourable effect on the hardenability. Molybdenum isessential for attaining a good secondary hardening response. The minimum content is2.0 %, and may be set to 2.1, 2.2 or 2.3 %. Molybdenum is a strong carbide forrningelement and also a strong ferrite former. The maximum content of mo lybdenum is therefore 3.0 %. Mo may be limited to 2.9, 2.8, 2.7, 2.6, 2.5 or 2.4 %.

Tungsten (0.8 - 1.5 %) Tungsten is an essential element in the present invention. W contributes to thesecondary hardening and does not react with nitrogen to form nitrides if the steel issubjected to nitrogen gas atomizing. Tungsten may form carbides of the type MgCduring the secondary hardening. It is believed that the large radius of the tungsten atommake its diffusion slow and therefore positively contributes to the improved tempering resistance.

Nickel (0.6 -1.7 %) Nickel shall be present in an amount of 0.6 -1.7 % in order to give the steel a goodhardenability and toughness. In addition, it would appear that Ni in combination withMo reduces the amount of retained austenite in the claimed type of steel. However,because of the expense, the nickel content of the steel should be limited. The upper limitmay therefore be set to 1.6, 1.5, 1.5, 1.4 or 1.3 %. The lower limit may be set to 0.7, 0.8,0.9,1.0 or 1.1 %.

Vanadium (0.1 - 0.4 %) Vanadium forms eVenly distributed primary precipitated carbides and carbonitrides ofthe type V(N,C) in the matrix of the steel. This hard phase may also be denoted MX,wherein M is mainly V but Cr and Mo may be present and X is one or more of C, N andB. Vanadium shall therefore be present in an amount of 0.1 - 0.4 %. The upper limitmay be set to 0.39, 0.38, 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31 or 0.30 %. The lowerlimit may be 0.15, 0.16, 0.17, 0.18, 0.19 or 0.20 %.

Aluminium (0.001 - 0.06 %) Aluminium is used for deoxidation in combination with Si and Mn. The lower limit isset to 0.001, 0.003, 0.005 or 0.007% in order to ensure a good deoxidation. The upperlimit is restricted to 006% for avoiding precipitation of undesired phases such as AlN.

The upper limit may be 0.05, 0.04, 0.03, 0.02 or 0.015%.

Nitrogen (0.01 - 0.12 %) Nitro gen is an optional element. N may be restricted to 0.01 - 0.12 % in order to obtainthe desired type and amount of hard phases, in particular V(C,N). When the nitrogencontent is properly balanced against the vanadium content, vanadium rich carbonitridesV(C,N) Will forrn. These Will partly be dissolved during the austenitizing step and thenprecipitated during the tempering step as particles of nanometer size. The therrnalstability of vanadium carbonitrides is considered to be better than that of vanadiumcarbides, hence the tempering resistance of the tool steel may be improved and theresistance against grain growth at high austenitizing temperatures is enhanced. Thelower limit may be 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017, 0.018, 0.019 or0.02%. The upper limit may be 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04 or 0.03 %.

Copper (0.02 - 2.0%) Cu is an optional element, Which may contribute to increase the hardness and thecorrosion resistance of the steel. If used, the preferred range is 0.02 - 1%. HoWever, it isnot possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is norrnally not deliberately added.

Cobalt (í 8 %) Co is an optional element. Co causes the solidus temperature to increase and thereforeprovides an opportunity to raise the hardening temperature, Which may be 15 - 30 °Chigher than Without Co. During austenitization it is therefore possible to dissolve largerfraction of carbides and thereby enhance the hardenability. Co also increases the MStemperature. HoWever, a large amount of Co may result in a decreased toughness andWear resistance. The maximum amount is 8 % and, if added, an effective amount maybe 2 - 6 %, in particular 4 to 5 %. HoWever, for practical reasons, such as scraphandling, deliberate addition of Co is generally not made. The maximum impurity content may then be set to 1 %, 0.5%, 0.3 %, 02% or 01%.

Niobium (S 0.1 %)Niobium is similar to vanadium in that it forms carbonitrides of the type M(N,C).

HoWever, Nb results in a more angular shape of the M(N,C) and may reduce the hardenability at high contents. The maximum amount is therefore 0.1 %, preferably 0.05%. Nb precipitates are more stable than V precipitates and may therefore be used forgrain refinement, since the ﬁne dispersion of NbC plays the role of pinning the grainboundaries leading to grain refinement and improved toughness and improvedresistance to softening at high temperatures. For this reason Nb may be present in an amount ofS 0.1 %, preferably in an amount of 0.01 - 0.05 %.

Ti, Zr and TaThese elements are carbide forrners and may be present in the alloy in the claimedranges for altering the composition of the hard phases. HoWever, norrnally none of these elements are added.

Boron (S 0.01%) B may be used in order to further increase the hardness of the steel. The amount islimited to 0.01%, preferably S 0.005%. A preferred range for the addition of B is 0.001- 0.004 %.

Ca, Mg and REM (Rare Earth Metals)These elements may be added to the steel in the claimed amounts for modifying thenon-metallic inclusion and/or in order to further improve the machinability, hot Workability and/or Weldability.

Impurity elements P, S and O are the main impurities, Which generally have a negative effect on themechanical properties of the steel. P may therefore be limited to 003%, preferably to0.0l%. S may be limited to 0.00l5, 0.00l0, 00008, 00005 or even 0.000l%. O may belimited to 0.00l5, 00012, 0.00l0, 00008, 00006 or 00005 %. HoWever, S mayoptionally be used in the range of 0.005 - 0.5 %, in particular 0.05 - 03 % for improving the machinability of the steel.

Steel production The tool steel having the claimed chemical composition can be produced byconventional metallurgy including melting in an Electric Arc Fumace (EAF) and furtherrefining in a ladle, optionally followed by a vacuum treatrnent before casting. Theingots may also be subjected to Electro Slag Remelting (ESR) in order to furtherimprove the cleanliness and the microstructural homogeneity of the ingots. However, amore preferred processing route for the claimed steel is gas atomizing followed by hotisostatic pressing (HIP). The steel thus produced can be used as HIPed or be subjected to further Working such as forging and rolling.

Norrnally the steel is subj ected to hardening and tempering before being used.Austenitizing may be performed at an austenitizing temperature (T A) in the range of1000-1070 °C, preferably 1030 - 1050 °C. A typical TA is 1040 °C with a holding timeof 30 minutes followed by rapid quenching. The tempering temperature is chosenaccording to the hardness requirement and is performed at least twice at 600 - 650 °C for 2 hours (2X2h) followed by cooling in air.

EXAMPLE In this example two inventive alloys are compared to according to the premium steelUddeholm Dievar®The alloys had the following compositions (in wt. %): Steel 1 Steel 2 Uddeholm Dievar® C 0.31 0.33 0.36 Si 0.27 0.24 0.20 Mn 0.1 0.4 0.5 Cr 3.0 1.4 5.0 Mo 2.0 2.4 2.3 W 0.92 1.04 - V 0.35Ni 1.26 0.37 0.551.56 - balance iron and impurities.

All steels were subj ected to austenitization at 1020 °C in a Vacuum fumace followed bygas quenching With a time of 100 s in the interval 800-500 °C (tg/g = 100s). Steel 1 andUddeholm Dievar® were subjected to tempering twice for two hours (2x2) at 615 °C.Steel 2 was subjected to tempering three times for two hours (3x2) at 615 °C.

The tempering resistance of the alloys was examined at a temperature of °C. Althoughthe inVentiVe steels had a lower initial hardness at the beginning of the test it is apparentfrom Fig. 1 that the inVentiVe steels had a significant better tempering resistance than the comparative steel Uddeholm Dievar®.

INDUSTRIAL APPLICABILITY The tool steel of the present inVention is useful in dies requiring a good hardenabilityand a good tempering resistance. Atomized powder of the alloy can be used to produceHIPed products having superior structural uniforrnity. Powder of the alloy can be used for producing or repairing dies, in particular by additiVe manufacturing methods.

Claims

1. A steel for hot Working consisting of in Weight % (Wt.%): C 0.26 - 0.38Si 0.1 - 0.3Mn 0.1 - 0.8 Cr 1.4 - 3.9Mo 2.0 - 3.0 W 0.8 - 1.5Ni 0.6 - 1.7 V 0.1 - 0.4optionally one or more ofAl 0.001 - 0.06N 0.01 - 0.12Cu 0.02 - 2 Co S 8 Nb S 0.1 Ti S 0.05 Zr S 0.05 Ta S 0.05 B S 0.01 Ca 0.00005 - 0.009Mg S 0.01REM S 0.2 S 0.005 - 0.5 balance Fe apart from impurities.

2. A steel according to claim 1, Wherein the steel is produced by gas atomizationfollowed by hot isostatic pressing and Wherein the Equivalent Circle Diameter (ECD) of at least 80 % of the carbides, nitrides and/or carbonitrides in the microstructure is less than 5 um, preferably less than 2.5 um, wherein the ECD =2\/A/1r where A is the surface of the carbide particle in the studied section.

3. A steel according to claim 1 or 2 ﬁalfilling at least one of the following requirements : C 0.26 - 0.37Mn 0.2 - 0.8Cr 1.6 - 3.7Mo 2.0 - 3.0W 0.8 - 1.5Ni 0.6 - 1.6 V 0.15 - 0.40Al 0.001 - 0.06N 0.01 - 0.10Nb 0.01 - 0.05

4. A steel according to any of the preceding claims fulfilling at least one of the following requirements: C 0.28 - 0.35Mn 0.2 - 0.7Cr 1.7 - 3.5Mo 2.1 - 2.9W 0.8 - 1.3Ni 0.8 - 1.5 V 0.20 - 0.35Al 0.005 - 0.05N 0.01 - 0.07

5. A steel according to any of the preceding claims fulfilling the following requirements : 11 C 0.28 - 0.35 Cr 1.7 - 3.5M0 2.1 -2.9W 0.8 - 1.3Ni 0.8 - 1.5N 0.01 - 0.07 . A steel powder having a composition according to any of claims 1 and 3-5. . A steel powder according to claim 6, wherein the powder has a size of 5 500 nrn. . Use ofa steel powder according to claims 6 or 7 for laser cladding, additive manufacturing or metal injection moulding. . A mould comprising at least one part comprising an alloy as defined in any of claims 1 and 3-5.