Hot work tool steel

Abstract

The present invention relates to hot work tool steel for high-pressure die casting, and contains the following main components (weight %): C 0.28-0.39, Si 0.05-0.35, Mn 0.1-0.8, Cr 5.4-6.0, Ni ≤0.4, Mo 1.8-2.5, V 0.6-1.1, with the remainder being optional components, Fe and impurities.

Description

[Technical Field]

[0001] This invention relates to hot work tool steel. [Background technology]

[0002] Vanadium-alloyed matrix tool steels have been on the market for decades and have attracted considerable attention due to their combination of high wear resistance, excellent dimensional stability, and superior toughness. These steels are used in a wide range of applications, including die casting and forging. These steels are generally produced using conventional steelmaking methods, but in some cases they are subjected to electroslag remelting (ESR).

[0003] With the rise of e-mobility, the demand for large-scale structural castings in the automotive industry is increasing. Several automakers have started or are planning to start producing Bodies In White (BIW) using massive aluminum castings with high-pressure direct casting (HPDC) equipment. This technology, known as megacasting or gigacasting, involves presses operating with up to 12,000 tons of force and can even replace the entire holder block and insert with a single steel block.

[0004] The large dimensions of these blocks and the complex shapes of the molds present similar unique problems. The large dimensions can lead to segregation during casting, potentially causing zoning and structural inhomogeneity within the forged block. Several attempts have been made in the past to reduce this type of defect, particularly by altering the chemical composition of the steel, as described in WO03 / 083154A1, EP1490526A1, EP0882808A1, and US2011 / 01108169A1.

[0005] However, a decrease in segregated elements affects other properties of the steel, leading to reduced hardenability and a decrease in equilibrium carbide content at the austenitization temperature. This often results in undesirable grain growth and the imparting of undesirable mechanical properties. This effect is particularly noticeable at the austenitization temperature (T A ) This becomes more noticeable on the surface due to the longer holding time. Therefore, for large tools, molds, and dies, a lower T A This is often recommended.

[0006] Furthermore, in large HPDC molds, it can be difficult, or even impossible, to achieve sufficient rapid cooling in the center of the mold. As a result, precipitation of grain boundary carbides, formation of a coarse microstructure containing bainite, and ultimately a decrease in toughness can occur, potentially leading to serious fracture known as gross cracking. To mitigate these problems, WO2010 / 074017A1 discloses a quenching method in which the mold is rapidly cooled from 1020-1070°C to an isothermal holding temperature of 530°C, and then slowly cooled to 150°C.

[0007] The materials most frequently used in HPDC molds are standard H11 (1.2343) and H13 (1.23344) hot tool steels, and various improved versions thereof.

[0008] H11 and H13 grade steels are matrix steels alloyed with vanadium. Combining excellent wear resistance, superior dimensional stability, and high toughness, they have been on the market for decades and have attracted considerable attention.

[0009] The applicant's Uddeholm DIEVAR® is a chromium-molybdenum-vanadium matrix tool for high-performance hot working described in WO99 / 50468A1. Other examples of matrix steels include EP4095281A1, EP3050986A1, WO03 / 106728A, and EP1469094A1.

[0010] While vanadium alloy matrix tool steel produced by ESR exhibits superior properties in terms of heat checking, gross cracking, thermal wear, and plastic deformation compared to tool steel produced by conventional methods, further improvements are needed to reduce the risk of hot tool failure, particularly heat checking and gross cracking in high-pressure die casting.

[0011] Therefore, it is beneficial to further improve the steel composition to obtain higher temper resistance, particularly in combination with improvements in toughness, especially to enhance resistance to gross cracking. [Overview of the project]

[0012] A general objective of the present invention is to provide hot-working tool steel with improved temper resistance that extends tool life.

[0013] A further objective is to improve the gross cracking resistance of molds produced by high-pressure die casting. Therefore, improving the toughness, high-temperature strength, and thermal fatigue resistance of the steel is crucial in reducing the risk of heat checking and gross cracking, and further improving temper softening resistance. Consequently, to reduce the risk of catastrophic failure, a steel with high hardenability and high toughness that can form a complete martensitic structure even in large steel molds with slow cooling rates is desirable.

[0014] In particular, it is desirable that the above objectives also be achieved in HPDC molds for pressure die casting of large parts such as tools used in megacasting and gigacasting. For this reason, it is desirable to provide large blocks made of hot work tool steel and large molds suitable for megacasting and gigacasting. Here, the block preferably has a thickness of at least 200 mm, a length of at least 1000 mm, and a width of at least 400 mm, and / or the sum of the thickness, length, and width of the block and mold is at least 2000 mm, 2500 mm, 3000 mm, or 3500 mm.

[0015] The aforementioned objectives and additional advantages are largely achieved by providing hot work tool steels with compositions particularly suited to general HPDCs and HPDCs for large structural components. The present invention is defined by the claims. [Modes for carrying out the invention]

[0016] Detailed explanation The importance of each element, their interactions, and limitations on the chemical composition of the claimed alloy are briefly explained below. Throughout this specification, the proportions of the chemical composition of steel are all expressed in weight percent (wt%). The amount of hard phase is expressed in volume percent (vol%). The upper and lower limits of chemical elements can be freely combined within the ranges defined in claim 1 and / or the ranges defined in the specification.

[0017] Carbon (0.28-0.39%) The carbon content is at least 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%, but may be set to 0.38, 0.37, 0.36, or 0.35%. In any case, the amount of carbon is M in the steel. 23 The amounts of primary carbides of type C6, M7C3, and M6C must be controlled to be limited, and preferably, these primary carbides are not present in the steel.

[0018] Silicon (0.05 - 0.35%) Silicon is used for deoxidation. Si exists in a dissolved state in steel. Si is a strong ferrite - forming element and, in order to enhance the activity of carbon, the risk of forming undesirable carbides increases, which has an adverse effect on the impact strength. When the Si content is low, carbides are likely to be refined, which is beneficial to the ductility and toughness of the steel. Therefore, the Si content is limited to 0.35%. The upper limit may be 0.30, 0.29 or 0.28%. The lower limit may be 0.10, 0.15 or 0.20%.

[0019] Manganese (0.1 - 0.65 %) Manganese contributes to improving the hardenability of steel and improves the machinability by forming manganese sulfide in co - existence with sulfur. Therefore, the content of manganese should be at least 0.1%, preferably 0.2% or more. The content of manganese in the steel is at most 0.65 %. The upper limit is 、0 .6%.

[0020] Chromium (5.4 - 6.0%) Chromium needs to have a content of at least 5.4% in order to obtain good hardenability in large cross - sections during heat treatment. If the chromium content is too high, it may lead to the formation of high - temperature ferrite and there is a risk of reducing the hot - workability. Furthermore, chromium inhibits the formation of MX and also has an adverse effect on the tempering resistance. The lower limit can be 5.42, 5.44, 5.46, 5.48 or 5.50, 5.51 or 5.53%. The upper limit can be 6.0, 5.9, 5.8 or 5.7%.

[0021] Nickel (≤ 0.3 %) Nickel is at most 0.3It can be contained up to %. Nickel imparts excellent hardenability and toughness to steel. The presence of nickel also has the effect of improving machinability by reducing the amount of carbon in martensite. However, from a cost perspective, the nickel content in steel is 0.3 limited to %. The upper limit may be 、0 .25, 0.20 or 0.15%.

[0022] Molybdenum (1.8 - 2.5%) Mo is known to have a very favorable effect on hardenability. Molybdenum is essential for achieving a good secondary hardening reaction through the formation of dispersed nano-sized Mo2C. This suppresses the rearrangement of dislocations, suppresses recrystallization, and improves the resistance to tempering softening. The minimum content is 1.8%, but it may be set to 1.9 or 2.0%. Molybdenum is a strong carbide-forming element and also a strong ferrite-forming element. Therefore, the maximum content of molybdenum is 2.5%. It is preferable to limit Mo to 2.4, 2.3 or 2.2%.

[0023] Vanadium (0.6 - 1.1%) Vanadium forms primary precipitation vanadium carbides (VC) and V(N,C)-type carbonitrides that are uniformly distributed in the steel matrix. This hard phase is also called MX, where M is mainly V, but may contain a small amount of Cr and Mo, and X is one or more of C and N. Therefore, the vanadium content is 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%.

[0024] Aluminum (0.001 - 0.03%) Al is used in combination with Si and Mn for deoxidation of steel. It may also be intentionally added during remelting in an ESR apparatus. The lower limit may be set to 0.001, 0.002, 0.003, or 0.004%. The upper limit is restricted to 0.03% to avoid precipitation of undesirable phases such as spinel phases like AlN or Al2O3·MgO. The upper limit may be 0.02 or 0.015%.

[0025] Nitrogen (0.0010~0.01%) Nitrogen is added as needed to obtain the desired type and amount of hard phases, particularly V(C,N). The nitrogen content is limited to 0.0010–0.01%. The lower limit may be 0.002% or 0.003%. The upper limit may be 0.009%, 0.008% or 0.007%. When the nitrogen content is properly balanced with the vanadium content, vanadium-rich carbonitrides V(C,N) are formed. These partially dissolve in the austenitization process and then precipitate as nanometer-sized particles in the tempering process. Since the thermal stability of vanadium carbonitrides is considered to be better than that of vanadium carbides, the tempering resistance of tool steels may be improved.

[0026] Sulfur (≤0.004%) S is an impurity in steel that adversely affects its mechanical properties. The S content may be limited to 0.003, 0.001, 0.0008, 0.0007, or even 0.0005%.

[0027] Phosphorus (≤0.05%) P is an impurity element that adversely affects the mechanical properties of steel. Therefore, the P content is limited to 0.05, 0.04, 0.03, 0.02, 0.01, or 0.008%.

[0028] Copper (≦0.5%) Cu is considered an impurity element. Copper cannot be extracted from steel. Therefore, handling the scrap becomes extremely difficult. For this reason, the upper limit for Cu content is set at 0.5%. The upper limit may be set at 0.4, 0.3, 0.2, 0.15, 0.12, 0.10, or 0.08%.

[0029] Cobalt (≤0.5%) Co may be included in any amount up to a maximum of 0.5%. Because Co increases the solidus temperature, it is possible to raise the quenching temperature, allowing it to be set 15-30°C higher than when Co is not included. Therefore, it becomes possible to dissolve more carbides during the austenitizing process, improving hardenability. Co also increases the Ms temperature. However, adding large amounts of Co may reduce toughness and wear resistance. Furthermore, for practical reasons such as scrap processing, there is no need to intentionally add Co. The maximum content may be set to 0.45, 0.40, 0.35, 0.30, 0.25, 0.20, 0.15, or 0.10%.

[0030] Tungsten (≤1%) In principle, molybdenum can be substituted with twice the amount of tungsten. However, tungsten is expensive, and handling the scrap metal becomes complicated. Therefore, the maximum amount is limited to 1%, preferably 0.5%, more preferably 0.3%, and most preferably not added at all. W may be present up to a maximum of 0.1%.

[0031] Niobium (≤0.03%) Niobium, like vanadium, forms M(N,C) type carbonitrides. However, niobium gives the M(N,C) a more angular shape. Therefore, the maximum amount to be added is 0.03%. The upper limit can be 0.02%, 0.01%, 0.005%, or 0.003%. It is preferable not to intentionally add niobium.

[0032] Ti, Zr, Ta These elements are carbide-forming elements and may be present as impurities in the alloy. The upper limit of the impurity content of these elements may preferably be set to 0.1%, 0.05%, 0.01%, or 0.005%.

[0033] hydrogen Hydrogen is an undesirable impurity element in steel. Therefore, it is desirable to reduce the hydrogen content in molten steel as much as possible by vacuum degassing. The impurity content is preferably limited to 0.0004% (4 ppm), 0.003%, 0.00025%, or 0.0002%.

[0034] Mo / V The Mo / V ratio is preferably in the range of 2.3 to 3.5, more preferably 2.5 to 2.7. To limit the amount of Mo2C, promote VC, and obtain the desired precipitation order and precipitation potential of secondary carbides, the upper limit can be restricted to 3.2, 3.1, 3.0, or 2.9.

[0035] Steel manufacturing Tool steel having the chemical composition described in the claims can be manufactured by conventional metallurgical methods, including melting in an electric arc furnace (EAF) and further refining in a ladle. If necessary, the steel may also be vacuum-treated before casting into an ingot. The ingot is subjected to pressurized electroslag remelting (PESR) to further improve cleanliness and microstructural uniformity. The remelted ingot is then subjected to conventional forging or upsetting forging, and subsequently machined to the desired block size.

[0036] Hardening and tempering Typically, steel undergoes quenching and tempering before use. The austenitization process involves an austenitization temperature (T A This can be done in the range of 1020-1070°C, preferably 1040-1060°C. Typical T AThe material is heated to 1050°C and held for 30 minutes before quenching. The tempering temperature is selected according to the required hardness and is performed at 600-650°C at least twice for 2 hours each time (2 x 2 hours), followed by air cooling.

[0037] Microstructure after quenching and tempering Even after relatively slow cooling, the microstructure becomes completely martensite, resulting in steel with high toughness even in thick-walled sections subjected to slow cooling. At a temperature range of 800°C to 500°C with a cooling time of 1000 seconds (t8 / 5), no bainite is formed, resulting in a complete martensite structure.

[0038] In this application, this means that the amount of tempered martensite is at least 97 volume%, preferably 98% to 100 volume%. Small amounts of hard phase particles such as carbides and / or nitrides and / or carbonitrides may be present. Small amounts of retained austenite may also be present. The content of hard phase particles and retained austenite is preferably 3 volume% or less, preferably less than 2 volume%, and more preferably less than 1 volume%. Hard phase particles and tempered martensite can be measured using a scanning electron microscope (SEM) at a magnification of 1500x. Retained austenite can be measured using an X-ray diffractometer with ASTM E975-13. [Examples]

[0039] Example 1 In this example, the steel according to the present invention is compared with the high-grade hot-worked steel DIEVAR®. The composition of steel is as follows (by weight %). The steel of the present invention, DIEVAR (registered trademark). C 0.35 0.37 Si 0.25 0.2 Mn 0.5 0.5 Cr 5.6 5.0 Mo 2.1 2.3 V 0.80 0.53 Mo / V 2.6 4.2 The remainder is Fe and impurities.

[0040] The steel of the present invention has an austenitizing temperature (T A The steel was heated to 1050°C and held for 30 minutes before quenching. Subsequently, the steel of the present invention was tempered twice at 620°C for 2 hours each (2 x 2 hours). Due to the balanced composition of the steel, it retains a martensitic structure even in large cross-sections, even when the cooling time (t8 / 5) in the temperature range of 800°C to 500°C reaches or exceeds 1000 seconds. Therefore, the steel of the present invention is less susceptible to hardness reduction at high temperatures, and higher tempering temperatures can be used to remove retained austenite without compromising hardness.

[0041] The comparative steel is subjected to the recommended heat treatment, and the austenitizing temperature (T A The material was heated to 1010°C and held for 30 minutes, then hardened, followed by two tempering cycles (2 x 2h) at 615°C for 2 hours each.

[0042] Both the steel of the present invention and the comparative steel were cooled for 1248 seconds, and the phase transformation was examined using a dilatometer. The curves obtained with the dilatometer clearly showed that the steel of the present invention underwent only a transformation to martensite, while the comparative steel underwent a transformation to bainite in addition to martensite.

[0043] The tempering resistance of the two types of steel was evaluated by heating the samples to 600°C and measuring their hardness after holding for 70 and 100 hours. The hardness of the steel of the present invention was 33 HRC after 70 hours and 30 HRC after 100 hours. The hardness of the comparison steel was 31 HRC and 29 HRC, respectively. Therefore, it can be concluded that the steel of the present invention has excellent resistance to softening at high temperatures.

[0044] Example 2 In this embodiment, the cleanliness of hot work tool steel remelted using ESR according to the present invention was investigated. The steel was manufactured on an industrial scale in a 65-ton EAF, then cast into ingots through conventional secondary metallurgical treatment including vacuum degassing, and remelted in a PESR apparatus. The composition of the obtained steel was as follows (by weight %). C 0.35 Si 0.25 Mn 0.5 Cr 5.6 Mo 2.1 V 0.80 Al 0.005 N 0.004 S 0.0001 P 0.006 O 0.0004 H 0.00005 The remainder is Fe and impurities.

[0045] The cleanliness of the steel was tested for microslag according to Method A of ASTM E45-97. The results are shown below.

[0046] [Table 1] Furthermore, cleanliness was measured using Oxford Instruments' INCA feature automated feature detection software, which can measure the number, size, shape, and chemical analysis of inclusions, and an FEI Quanta 600F SEM. The survey area was 6000 mm². 2 The size of the inclusion is expressed by the equivalent circle diameter (ECD). Here, ECD = 2√(A / π), where A is the surface area of ​​the particles in the cross-section being investigated. The test results revealed the following: The ECD of 80% of the total oxide particles was 10 μm or less, and the ECD of all inclusions was 50 μm or less.

[0047] Example 3 In this embodiment, the homogeneity of the steel block according to the present invention was investigated. The scrap was melted in an EAF, cast into an ingot through VD treatment, and remelted in a PESR apparatus. The remelted ingot was then upset forged and subsequently machined to a predetermined block size. The composition of the steel block is as follows (wt%). C 0.35 Si 0.25 Mn 0.5 Cr 5.6 Mo 2.1 V 0.80 The balance is iron and impurities.

[0048] The size of the steel block was 305 mm in thickness, 712 mm in width, and 3500 mm in length. The block was heated to 860 °C and held for 4 hours, then cooled to 750 °C at a cooling rate of 10 °C / h, subsequently cooled to 700 °C at a cooling rate of 7 °C / h, and then naturally cooled in air for stress relieving annealing, and the Brinell hardness was 169 HBW 10 / 3000 resulted. The Brinell hardness HBW 10 / 3000 is measured with a 10 mm diameter tungsten carbide ball under a load of 3000 kgf (29400 N). The maximum deviation from the average Brinell hardness value in the thickness direction is measured in accordance with ASTM E10-01, where the minimum distance from the end of the specimen or the end of another indentation to the center of the indentation is at least 2.5 times the indentation diameter, and the maximum distance does not exceed 4 times the indentation diameter. It was confirmed that the maximum deviation from the average Brinell hardness value in the thickness direction was less than 1o%.

[0049] Example 4 In this example, the toughness of the hot work tool steel remelted by ESR according to the present invention was investigated. The steel was produced on an industrial scale in a 65-ton EAF, then cast into an ingot through a conventional secondary metallurgical treatment including vacuum degassing, and remelted in a PESR apparatus. The composition of the obtained steel was as follows (wt%). C 0.36 Si 0.21 Mn 0.48 Cr 5.54 Mo 2.12 V 0.77 Al 0.006 N 0.005 The remainder is iron and impurities.

[0050] The steel was softened and annealed in the same manner as in Example 3, forged to a cross-section of 799 × 273 mm, then quenched by holding at an austenitizing temperature of 1050°C for 30 minutes, followed by gas quenching with a cooling time (t8 / 5) of 1000 seconds in the temperature range of 800°C to 500°C. Subsequently, it was tempered twice at 620°C for 2 hours each (2 × 2 hours), resulting in a hardness of 44.4 HRC. Surprisingly, the microstructure was entirely martensite. Using a standard Charpy-V test in accordance with SS-EN ISO148-1 / ASTM E23, the average impact energy in the LT direction of six samples was measured, resulting in an average value of 18 J. Therefore, the steel of the claims exhibits high hardenability and high toughness even in large cross-sections, making it suitable for demanding HPDC applications such as molds used in megacasting and gigacasting.

[0051] Industrial applicability The tool steel of the present invention is particularly useful for large molds for HPDCs that require good toughness, good hardenability, and good temper resistance.

Claims

1. A hot work tool steel for high-pressure die casting, characterized in that the steel is obtained by ESR and contains the following elements in weight percent. C 0.28-0.39 Si 0.05-0.35 Mn 0.1-0.8 Cr 5.4-6.0 Ni ≤ 0.4 Mo 1.8-2.5 V 0.6-1.1 Al 0.001-0.03 N 0.001-0.01 S ≤ 0.004 P ≤ 0.05 Cu ≤ 0.5 Co ≤ 0.5 W ≤ 1 Nb ≤ 0.03 Remaining portion: Fe with impurities removed

2. The hot work tool steel for high-pressure die casting according to claim 1, characterized in that the hot work tool steel contains one or more of the following elements in weight percent. C 0.33-0.37 Si 0.15-0.3 Mn 0.4-0.6 Cr 5.5-5.7 Ni ≤ 0.25 Mo 2.0-2.2 V 0.75-0.85 Al 0.001-0.02 N 0.001-0.009 S ≤ 0.001 P ≤ 0.010 Cu ≤ 0.12 Co ≤ 0.1 W ≤ 0.1 Nb ≤ 0.005

3. The hot work tool steel for high-pressure die casting according to claim 1 or 2, characterized in that the hot work tool steel contains one or more of the following elements in weight percent. C 0.34-0.36 Si 0.20-0.28 Mn 0.45-0.55 Cr 5.53-5.68 Mo2.05-2.15 Ni ≤ 0.15 V 0.77-0.83 N 0.001-0.05 P ≤ 0.008 S ≤ 0.0007 Cu ≤ 0.10

4. The hot work tool steel for high-pressure die casting according to any one of the claims, characterized in that the content of Mo and V in the hot work tool steel is adjusted to satisfy the requirement of Mo / V = 2.3 to 3.0, preferably 2.5 to 2.

7.

5. Regarding microslag prepared by ASTM E45-97, Method A, the cleanliness meets the following maximum requirements: Table 1 and / or 6000 mm ２ The cleanliness measured by a scanning electron microscope over an area satisfies the following requirements: The ECD of at least 50% of the total number of oxide particles is 10 μm or less, and the ECD of at least 50% of the total number of oxide particles is 50 μm or less. A hot work tool steel for high-pressure die casting according to any of the above claims, characterized in that

6. Regarding microslag prepared by ASTM E45-97, Method A, the cleanliness meets the following maximum requirements: Table 2 and / or 6000 mm ２ The cleanliness measured by a scanning electron microscope over an area satisfies the following requirements: The ECD of at least 50% of the total number of oxide particles is 10 μm or less, and the ECD of at least 50% of the total number of oxide particles is 50 μm or less. A hot work tool steel for high-pressure die casting according to any of the above claims, characterized in that

7. A hot work tool steel for high-pressure die casting according to any one of the claims, characterized in that the average impact energy measured in the LT direction of six samples using a standard Charpy-V test in accordance with SS-EN ISO148-1 / ASTM E23 is at least 15 J, and the cooling time (t8 / 5) in the temperature range of 800°C to 500°C is 1000 seconds.

8. The steel is in a softened annealed state, 230 HBW １０／３０００ A hot work tool steel for high-pressure die casting according to any of the claims, having the following average hardness, having a thickness of at least 100 mm, the maximum deviation from the average value of Brinell hardness in the thickness direction measured according to ASTM E10-01 being less than 10%, preferably less than 5%, and further characterized in that the minimum distance from the center of the indentation to the edge of the specimen or the edge of another indentation is at least 2.5 times the diameter of the indentation, and the maximum distance does not exceed 4 times the diameter of the indentation.

9. The hot work tool steel is provided as a large block and / or large mold for casting large structural components, wherein the block and / or mold has a thickness of at least 200 mm, a length of at least 1000 mm, and a width of at least 400 mm, and / or the sum of the thickness, length, and width is at least 2000 mm, and / or the weight of the block or mold is at least 5 tons, at least 10 tons, at least 15 tons, at least 20 tons, or at least 25 tons, and / or the block or mold has an ASTM grain size of at least 7, preferably at least 8, more preferably at least 9, according to any one of the claims, for use as a hot work tool steel for high-pressure die casting.

10. The quenched and tempered hot work tool steel according to any one of claims 1 to 7 and 8, characterized in that the structure contains at least 98 volume percent of tempered martensite, is preferably quenched after austenitization in the range of 1020 to 1070°C, and has a cooling time (t8 / 5) in the temperature range of 800°C to 500°C of at least 1000 seconds, preferably at least 1248 seconds.

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