SE2350340A1 - Hot work tool steel - Google Patents

Abstract

An ESR-remelted hot work tool steel, which comprises the following main elements in weight %:C 0.32-0.38Si 0.05 - 0.29Mn 0.4 - 0.7Cr 5.4 -5.8Ni < 0.5Mo 1.9 -2.3V 0.70-0.90Al 0.0005 - 0.030balance iron and impurities.

Description

TECHNICAL EIELD The invention relates to a hot Work tool steel.

BACKGROUND OF THE INVENTION The E-mobility has led to an increased demand for large structural castings in the automotive industry, because the die cast parts contribute to a Weight reduction of the CEJIS.

The complex geometry and the large dimensions of the cast parts have led to an increased use of High Pressure Direct Casting (HPDC).

The most frequently used materials for the moulds of this HPDC is the standard H11 (1.2343) and H13 (1.23344) hot Work tool steels as Well as different modifications thereof.

H11 and H13 type of steels are matrix steels alloyed With vanadium. They have been on the market for decades and attained a considerable interest, because they combine a high Wear resistance With an excellent dimensional stability and because they have a good toughness. The steels are generally produced by conventional metallurgy and are sometimes followed by Electro Slag Remelting (ESR). Steels of this type are described in WO 99/50468 Al, EP3050986 Al, WO 03/106728 A, EP1469094 A1.

Moulds for HPDC of large structural components have to fulfil very high requirements With respect to toughness, high temperature strength and thermal fatigue resistance. HoWever, the standard H11 and H13 hot Work tool steels and the variations thereof are not optimized for HPDC of large structural components.

Accordingly, there is a need for further improvements in order to reduce the risk for hot Work tool failure of large HPDC tools. In particular, it Would be of great interest to improve the toughness, the high temperature strength and the therrnal fatigue resistance of the steel in order to reduce the risk for heat checking and gross cracking and furthermore to improve the temper softening resistance.

DISCLOSURE OF THE INVENTION The object of the present invention is to provide a hot Work tool steel With a high toughness, preferably also in large sections.

A further object is to improve the resistance against gross cracking of the moulds using high pressure die casting of large parts.

The foregoing objects, as Well as additional advantages, are achieved to a significant measure by providing a hot Work tool steel having a composition specifically adapted to HPDC of large structural parts and by ensuring that the hot Work tool steel has a very high cleanliness.

The invention is defined in the claims.

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. Although, the composition of the hot Work tool steel is defined in claim l, it is conceivable that the claimed range of one or more or the alloying elements may be eXpanded Within the limits set out below, in particular if the claimed invention is further restricted by one or more other features such as metallurgical structure, homogeneity in structure, homogeneity in hardness, cleanliness With respect to size distribution of inclusions, cleanliness With respect to impurity elements and/or With respect to one or more properties of the steel such as toughness, hot strength and hot Wear resistance.

All percentages of the chemical composition of the steel are given in Weight % (Wt. %) throughout the description. The amount of hard phases is given in volume % (vol. %).

The upper and lower limits of the elements may be freely combined within the limits set out in claim 1 and/or within the ranges defined below.

Carbon (0.28 - 0.39 %) is to be present in a minimum content of 0.28 %, preferably at least 0.29, 0.30, 0.31, 0.32, 0.33 or 0.34 %. The upper limit for carbon is 0.39 % and may be set to 0.38, 0.37, 0.36 or 0.35 %. In any case, the amount of carbon should be controlled such that the amount of primary carbides of the type M23C6, MvCg and M6C in the steel is limited, preferably the steel is free from such primary carbides.

Chromium (5.0 - 6.0 %) Chromium is to be present in a content of at least 5 % in order to provide a good hardenability in larger cross sections during heat treatment. If the chromium content is too high, this may lead to the formation of high-temperature ferrite, which reduces the hot-workability. Moreover, chromium has a negative effect on the tempering resistance, because it counteracts the formation of MX. The lower limit may be 5.1, 5.2, 5.3, 5.4 or 5.5 %. The upper limit may be 6.0, 5.9, 5.8 or 5.7 %.

Molybdenum (1.8 - 2.5 %) Mo is known to have a very favourable effect on the hardenability. Molybdenum is essential for attaining a good secondary hardening response. The minimum content is 1.8 %, and it may be set to 1.9, or 2.0 %. Molybdenum is a strong carbide forming element and also a strong ferrite former. The maximum content of molybdenum is therefore 2.5 %. Preferably Mo is limited to 2.4, 2.3 or 2.2 %.

Tungsten (í 1 %) ln principle, molybdenum may be replaced by twice as much with tungsten. However, tungsten is expensive and it also complicates the handling of scrap metal. The maximum amount is therefore limited to 1 %, preferably 0.5 %, more preferably 0.3 % and most preferably no deliberate addition is made. W is then accepted in an amount of up to 0.1 %.

Vanadíum (0.6 - 1.1 %) 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 MX, wherein M is mainly V but Cr and Mo may be present and X is one or more of C, N and B. Vanadium shall therefore be present in an amount of 0.6 - 1.1 %. The upper limit may be set to 1.05, 1.0, 0.95, 0.9 or 0.85%. The lower limit may be 0.65, 0.7 or 0.75 %.

Nítrogen (0.0010 - 0.080 %) Nitrogen may optionally be added in order to obtain the desired type and amount of hard phases, in particular V(C,N). Nitrogen is restricted to 0.0010 - 0.080 %. The lower limit may be 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007% 0.008 %, 0.009 %, 0.01 %, 0.011 %, 0.012 %, 0.013%, 0.014 %, 0.015 %, 0.016 % 0.017 %, 0.018 %, 0.019 %, 0.020 %, 0.022 %, 0.024 %, 0.024 %, 0.026 %, 0.028 %, 0.030 %, 0.035 % or 0.040 %. The upper limit may be 0.07 %, 0.06 %, 0.05 %, 0.04 %, 0.03 %, 0.02 % or 0.01 %. When the nitrogen content is properly balanced against the Vanadium content, Vanadium rich carbonitrides V(C,N) will form. These will be partly dissolved during the austenitizing step 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 of Vanadium carbides, hence the tempering resistance of the tool steel may be improved. Further, by tempering at least twice, the tempering curve will have a higher secondary peak. However, the dissolution of the M(C,N) particles during the austenitizing will be more difficult at higher nitro gen contents. A preferred range of N may therefore be set to 0.012 - 0.06 %, 0.015 - 0.05 % or 0.02 - 0.04 %.

Níobíum (í 0.03 %) 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). The maximum amount is therefore 0.03 %. The upper limit may be 0.02 %, 0.01 %, 0.005 % or 0.003 %.

Preferably, niobium is not deliberately added.

Silicon (0.05 - 0.35 %) Silicon is used for deoXidation. Si is present in the steel in a dissolved form. Si is a strong ferrite former and increases the carbon activity and therefore the risk for the forrnation of undesired carbides, Which negatively affect the impact strength. Si is therefore limited to 0.35%. The upper limit may be 0.30 %, 0.29 % or 0.28 %.

Manganese (0.1 - 0.8 %) Manganese contributes to improving the hardenability of the steel and together With sulphur manganese contributes to improving the machinability by forrning manganese sulphides. Manganese shall therefore be present in a minimum content of 0.1 %, preferably at least 0.2 %. The steel shall contain maximum 0.8 %. The upper limit may be 0.7 %, 0.65 % or 0.6 %.

Nickel (S 1 %) Nickel may be present in an amount of up to 1 %. It gives the steel a good hardenability and toughness. The presence of nickel may also result in an improved machinability, possibly by reducing the amount of carbon in the martensite. HoWever, because of the eXpense, the nickel content of the steel is limited to 1.0 %. The upper limit may be 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.25 or 0.2 %.

Aluminium (i 0.05%) Al may be used in combination With Si and Mn for deoXidation of the steel. In addition, it may be deliberately added during the re-melting in the ESR-unit. The lower limit may be set to 0.001, 0.002, 0.003 or 0.004 %. The upper limit is restricted to 0.05 % in order to avoid precipitation of undesired phases such as A1N. The upper limit may be 0.04, 0.03 0.02 or 0.015%.

Copper (S 0.5 %) Cu is considered as an impurity element. It is not possible to eXtract copper from the steel. This drastically makes the scrap handling more difficult. For this reason, the maximum content of Cu is set to 0.5 %. The upper limit may be set to 0.4, 0.3, 0.2, 0.15, 0.12, 0.10 or 0.08 %.

Cobalt (5 1 %) Co may be optionally added in amounts of up to 1 %, because Co causes the solidus temperature to increase and therefore provides an opportunity to raise the hardening temperature, Which may be 15 - 30 °C higher than Without Co. During austenitization it is therefore possible to dissolve larger fraction of carbides and thereby enhance the hardenability. Co also increases the MS temperature. HoWever, a large amount of Co may result in a decreased toughness and Wear resistance. HoWever, for practical reasons, such as scrap handling, deliberate additions of Co need not to be performed.

The maximum content may then be set to 1, 0.5, 0.3, 0.2 or 0.1 %. suiphur <5 0.0o4%> S is an impurity in the steel and negatively affects the mechanical properties of the steel.

The content of S may be limited to 0.003, 0.001, 0.0008, 0.0007 or even 0.0005 %.

Phosphorous (S 0.05%) P is an impurity element, Which has negative effects on the mechanical properties of the steel. P may therefore be limited to 0.05, 0.04, 0.03, 0.02, 0.01 or 0.008 %.

Ti, Zr and Ta These elements are carbide forrners and may be present in the alloy as impurities. The upper impurity limit of these elements may preferably be set to 0.1 %, 0.05 %, 0.01 % or 0.005 %.

Hydrogen Hydrogen is an undesirable impurity element in the steel. It is therefore desirable to decrease the hydrogen content in the liquid steel as much as possible by vacuum degassing. Preferably, the impurity content is limited to 0.0004 % (4ppm), 0.0002 % or 0.0001 %.

Steel production The tool steel having the claimed chemical composition can be produced by conventional metallurgy, including melting in an Electric Arc Furnace (EAF) and further refining in a ladle. Optionally, the steel may be subjected to vacuum treatment before casting into ingots. The ingots are subjected to Pressurized Electro Slag Remelting (PESR) in order to further improve the cleanliness and the microstructural homogeneity. The remelted ingots are thereafter subjected to upset forging followed by machining to the desired block size.

Normally the steel is subjected to hardening and tempering before being used. Austenitizing may be performed at an austenitizing temperature (TA) in the range of 1020 - 1070 °C, preferably 1040 - 1060 °C. A typical TA is 1050 °C with a holding time of 30 minutes followed by rapid quenching. The tempering temperature is chosen according to the hardness requirement and is performed at least twice at 600 - 650 °C for 2 hours (2X2h) followed by Cooling in air.

EXAMPLE 1 In this example, the cleanliness of the ESR-remelted hot work tool steel according to the present invention was investigated. The steel was produced in an industrial scale in a 65 ton EAF followed by conventional secondary metallurgy involving vacuum degassing and cast into ingots, which were remelted in a PES R-unit.

The steel thus obtained had the following composition (in wt. %): C 0.35 Si 0.25 Mn 0.5 Cr 5.6 Mo 2.1 V 0.80 Al 0.005 N 0.004 0.000l 0.006 0.0004 0.00005 EOWVJ balance iron and impurities.

The cleanliness of steel Was examined With respect to micro-slag according to ASTM E45-97, Method A. The result is shown below A A B B C C D D T H T H T H T H 0 0 l.0 0.5 0 0 l.0 0.5 The cleanliness was also measured by an automatic feature detecting software, INCA feature of Oxford Instruments, using a FEI Quanta 600F SEM, which allows the number, size, shape and chemical analysis of the inclusions to be determined. The investigated area was 6000 mmz. The size of the inclusions is given as Equivalent Circle Diameter (ECD), wherein the ECD =2\/A/1I, where A is the surface of the particles in the studied section.

The examination revealed the following results.: The ECD of 80 % of the number of all oxide particles was i l0 um and all inclusions had an ECD of S 50 um.

EXAMPLE 2 ln this example, the tempering resistance of a steel composition, which may be used in the present invention, was compared to the tempering resistance of the premium hot work steel DIEVAR®.

The steels comprised the following main elements (in wt. %): Inventive steel DIEVAR® C 0.35 0.37 Si 0.25 0.2 Mn 0.5 0.5 Cr 5.6 5.0 M0 2.1 2.3 V 0.80 0.53 MO/V 2.6 4.2 balance iron and impurities.

The inventive steel was heated to an austenitizing temperature (TA) 1050 °C with a holding time of 30 minutes followed by quenching. The inventive steel was thereafter subjected to tempering twice for two hours at a temperature of 620 °C (2X2h). Because of the balanced composition of the steel, it will have a martensitic matrix even in large sections, i.e. when the cooling time in the temperature interval of 800 °C- 500 °C (t8/5) is up to or even longer than 1000 s. The inventive steel is therefore less sensitive to hardness decrease at high temperatures such that higher tempering temperatures can be used for removing retained austenite without impairing the hardness.

The comparative steel was subjected to the recommended heat treatment, i.e. it was heated to an austenitizing temperature (TA) 1010 °C with a holding time of 30 minutes followed by quenching followed by tempering twice for two hours at a temperature of 615 °C (2X2h).

The tempering resistance of the two steels was examined by measuring the hardness of the samples after heating to 600 °C and holding times of 70 h and 100 h. The inventive steel had a hardness of 33 HRC after 70 h and a hardness of 30 HRC after 100 h. The corresponding values for the comparative steel were 31 HRC and 29 HRC, respectively.

It can thus be concluded, that the inventive steel has a better resistance against softening at high temperatures.

INDUSTRIAL APPLICABILITY The tool steel of the present invention is particular useful in large dies for HPDC requiring a good toughness, a good hardenability and a good tempering resistance.

Claims (2)

1. weight %:C 0.32 - 0.Si 0.05 - 0.Mn 0.4 - 0.Cr 5.4 - 5.10 Ni S 0.Mo 1.9 - 2.V 0.70 - 0.Al 0.0005 - 0.N S 0.0l 15 S S 0.00l P S 0.0l O S 0.00lH S 0.Cu S 0.l20 Co S l balance Fe apart from impurities, wherein the cleanliness measured by light optical microscopy fulfils the following requirements with respect to micro-slag according to ASTM E45-97, Method A: A A B B C C D D T H T H T H T H l.0 0 l.5 l.0 0 0 l.5 l.and wherein the cleanliness measured in by scanning electron microscopy on an area of 6000 mmz fulfils the following requirements:the ECD of at least 50 % of the number of all oXide particles is S 10 um and at least 50 % of the number of all oXide particles have an ECD of S 50 um. . An ESR-remelted hot Work tool steel as defined in claim 1, Wherein the hot Work tool steel fulfils one or more of the following requirements in Weight %: C 0.33 - 0.37 Si 0.15 - 0.3 Mn 0.4 - 0.6 Cr 5.5 - 5.7 Ni S 0.Mo 2.0 - 2.

2. 2 V 0.75 - 0.85 Al 0.001 - 0.025 N S 0.S S 0.P S 0.O S 0.0012 H S 0.0003 Cu S 0.Co S 0.. An ESR-remelted hot Work tool steel as defined in claim 1 or 2, Wherein the hot Work tool steel fulfils one or more of the following requirements in Weight %: C 0.34 - 0.36 Si 0.20 - 0.28 Mn 0.45 - 0.55 Mo 2.05 - 2.15 Ni S 0.V 0.77 - 0.83 N 0.001 - 0.P S 0.008 S 0.O S 0.Cu S 0.. An ESR-remelted hot Work tool steel as defined in any of the preceding claims, Wherein the content of Mo and V in the hot Work tool steel is adjusted to fulfil the requirement Mo/V = 2.3-3.0, preferably 2.5-2. . An ESR-remelted hot Work tool steel as defined in any of the preceding claims, Wherein the cleanliness fulfils the following maximum requirements With respect to n1icro-slag according to ASTM E45-97, Method A: A B B C C D D H T H T H T H 0 l.0 0.5 0 0 l.0 0.. An ESR-remelted hot Work tool steel as defined in any of the preceding claims, Wherein the steel is in the soft annealed condition, has a mean hardness of S 230 HBWio/sooo, has a thickness of at least l00 mm and Wherein the maximum deviation from the mean Brinell hardness value in the thickness direction measured in accordance With ASTM El0-0l is less than l0 %, preferably less than 5 %, and Wherein the minimum distance of the centre of the indentation from the edge of the specimen or the edge of another indentation shall be at least two and a half times the diameter of the indentation and the maximum distance shall be no more than 4 times the diameter of the indentation. . An ESR-remelted hot Work tool steel as defined in any of claims l-5, Wherein the mean impact energy measured in the LT direction of 6 samples using a standard Charpy-V test in accordance With SS-EN lSOl48-l/ASTM E23 is at least l5 J,Wherein the Cooling time in the temperature interval of 800 °C- 500 °C (t8/5) is 1000 s.