SE540108C2

SE540108C2 - Hot work tool steel

Info

Publication number: SE540108C2
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; WO2018056884A1; SE1651268A1

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

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 resistancewith an excellent dimensional stability and because they also have a good toughness.These steels have a wide range of applications such as die casting and forging. Thesteels are generally produced by conventional metallurgy followed by Electro Slag Remelting (ESR).

Uddeholm D|EVAR® is a high performance chromium-molybdenum-vanadium steel, containing balanced carbon and vanadium contents as described in WO9950468 A1. lt is also known to use nitrogen and vanadium alloyed low-chromium tool steels forhot working, in particular for small tools, which do not require a high hardenability butwhere the demands on tempering resistance and thermal 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 WO2012119925 A1.

Although the vanadium alloyed tool steels produced by ESR have better propertiesthan conventionally produced tool steels with respect to heat checking, gross cracking,hot wear 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. ln addition, it would be beneficial to further improve the hot strength and temper resistance of hot work tool steel.

DISCLOSURE OF THE INVENTION The object ofthe present invention is to provide a hot work tool steel having an improved property profile leading to an increased life ofthe 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 form is suitable for Additive I\/Ianufacturing (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.

BRIEF DESCRIPTION OF THE DRAWINGSFig 1 is a diagram depicting tempering resistance at 600 °C. The hardness is given in HRC on the vertical axis as function of tempering time in hours on the horizontal axis.

DETAILED DESCRIPTION The importance ofthe separate elements and their interaction with each other as wellas the limitations of the chemical ingredients of the claimed alloy are briefly explainedin the 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, 0.31, 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 %. ln any case,the amount of carbon should be controlled such that the amount of primary carbidesof the type IVIBCG, IV|7C3 and IVIGC 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 former 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 andthermal 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 0.20%. Preferred ranges are 0.10 - 0.25 % and 0.15 - 0.24%.

Manganese (0.1 - 0.8 %) I\/langanese contributes to improving the hardenability of the steel and together withsulphur manganese contributes to improving the machinability by forming manganesesulphides. I\/langanese 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. I\/ln 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 good hardenability in larger cross sections during heat treatment. lf the chromium content is too high, this may lead to the formation of high-temperature ferrite, which reducesthe hot-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. I\/|olybdenum 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 %. I\/|olybdenum is a strong carbide formingelement and also a strong ferrite former. The maximum content of molybdenum 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 IVIZCduring the secondary hardening. lt is believed that the large radius ofthe tungstenatom make 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. ln addition, it would appear that Ni in combination withMo reduces the amount of retained austenite in the claimed type of steel. However,because ofthe expense, the nickel content ofthe steel should be limited. The upperlimit may therefore be set to 1.6, 1.5, 1.5, 1.4 or 1.3 %. The lower limit may be set to0.7, 0.8, 0.9, 1.0 or 1.1 %.

Vanadium (0.1 - 0.4 %)Vanadium forms evenly distributed primary precipitated carbides and carbonitrides of the type V(N,C) in the matrix of the steel. This hard phase may also be denoted I\/IX, 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 I\/|n. 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 0.06% for avoiding precipitation of undesired phases such as AIN.

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

Nitrogen (0.01 - 0.12 %) Nitrogen is an optional element. N may be restricted to 0.01 - 0.12 % in order toobtain the desired type and amount of hard phases, in particular V(C,N). When thenitrogen content is properly balanced against the vanadium content, vanadium richcarbonitrides V(C,N) will form. These will partly be dissolved during the austenitizingstep and then precipitated during the tempering step as particles of nanometer size.The thermal stability of vanadium carbonitrides is considered to be better than that ofvanadium carbides, hence the tempering resistance of the tool steel may be improvedand the resistance against grain growth at high austenitizing temperatures isenhanced. The lower limit may be 0.011, 0.012, 0.013, 0.014, 0.015, 0.016, 0.017,0.018, 0.019 or 0.02%. 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 ofthe steel. lf used, the preferred range is 0.02 - 1%. However, itis not possible to extract copper from the steel once it has been added. This drasticallymakes the scrap handling more difficult. For this reason, copper is normally not deliberately added.

Cobalt (5 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 IVIStemperature. 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 %, 0.2% 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 I\/|(N,C) and may reduce thehardenability at high contents. The maximum amount is therefore 0.1 %, preferably0.05 %. Nb precipitates are more stable than V precipitates and may therefore be usedfor grain refinement, since the fine dispersion of NbC plays the role of pinning thegrain boundaries leading to grain refinement and improved toughness and improvedresistance to softening at high temperatures. For this reason Nb may be present in an amount of S 0.1 %, preferably in an amount of 0.01 - 0.05 %.

Ti, Zr and TaThese elements are carbide formers and may be present in the alloy in the claimedranges for altering the composition ofthe hard phases. However, normally none of these elements are added.

Boron (S 0.01%) B may be used in order to further increase the hardness ofthe 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 I\/|etals)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. lmpurity 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 0.03%, preferably to0.01%. S may be limited to 0.0015, 0.0010, 0.0008, 0.0005 or even 0.0001%. O may belimited to 0.0015, 0.0012, 0.0010, 0.0008, 0.0006 or 0.0005 %. However, S mayoptionally be used in the range of 0.005 - 0.5 %, in particular 0.05 - 0.3 % 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 Furnace (EAF) and furtherrefining in a ladle, optionally followed by a vacuum treatment 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,a more preferred processing route for the claimed steel is gas atomizing followed byhot isostatic pressing (HIP). The steel thus produced can be used as H|Ped or be subjected to further working such as forging and rolling.

Normally the steel is subjected to hardening and tempering before being used.

Austenitizing may be performed at an austenitizing temperature (TA) in the range of1000-1070 °C, preferably 1030 - 1050 °C. A typical TA is 1040 °C with a holding time of30 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.

EXAl\/IPLE ln this example two inventive alloys are compared to according to the premium steel Uddeholm 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 0.35 0.37 0.55 Ni 1.26 1.56 - balance iron and impurities.

All steels were subjected to austenitization at 1020 °C in a vacuum furnace followed bygas quenching with a time of 100 s in the interval 800-500 °C (t8/_f,= 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 ofthe alloys was examined at a temperature of °C. Although the inventive steels had a lower initial hardness at the beginning of the test it is apparent from 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 toproduce HlPed products having superior structural uniformity. Powder ofthe alloy canbe used for producing or repairing dies, in particular by additive manufacturing methods.

Claims

1. A steel for hot working consisting of in weight % (wt.%): CSiI\/|nCrMoWNiV 0.26 - 0.380.1 - 0.30.1 - 0.81.4- 3.92.0-3.00.8- 1.50.6 - 1.70.1 - 0.4 optionally one or more of AlNCuCoNbTiZrTaBCaMgREMS 0.001 - 0.060.01 - 0.120.02 - 2 S 8 S 0.1 S 0.05 S 0.05 S 0.05 S 0.010.00005 - 0.009S 0.01 S 0.2 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 11 microstructure is less than 5 um, preferably less than 2.5 um, wherein the ECD =2\/A/rt where A is the surface of the carbide particle in the studied section.

3. A steel according to claim 1 or 2 fulfilling at least one of the following requirements: C 0.26 - 0.37l\/ln 0.2 - 0.8 Cr 1.6 - 3.7l\/lo 2.0 - 3.0 W 0.8 - 1.5 Ni 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 - 0.35l\/ln 0.2 - 0.7 Cr 1.7 - 3.5l\/lo 2.1 - 2.9 W 0.8 - 1.3 Ni 0.8 - 1.5 V 0.20 - 0.35Al 0.005 - 0.05N 0.01 - 0.07 12

5. A steel according to any of the preceding claims fulfilling the following requirements: C 0.28 - 0.35Cr 1.7 - 3.5l\/lo 2.1 - 2.9W 0.8 - 1.3Ni 0.8 - 1.5N 0.01 - 0.07

6. A steel powder having a composition according to any of claims 1 and 3-5.

7. A steel powder according to claim 6, wherein the powder has a size of S 500 pm.

8. Use of a steel powder according to claims 6 or 7 for laser cladding, additive manufacturing or metal injection moulding.

9. A mould comprising at least one part comprising an alloy as defined in any of claims 1 and 3-5.