Alloy tool steel for hot working

Abstract

The purpose of this invention is to provide a hot-working alloy tool steel with excellent toughness and resistance to softening. This hot-working alloy tool steel contains C, Si, Mn, Ni, Cr, Mo, V, Nb, and N, with the remainder being Fe and impurities. The microstructure of the aforementioned hot-working alloy tool steel is either slag or modified tough iron, and the microstructure comprises blocks with a diameter of 2.0 μm to 6.0 μm. The solid solution element parameter Q, calculated using the following formula, is 1.12 or higher during quenching. [In the formula, (Cr1+Mo1+V1+Nb1) represents the total amount of Cr, Mo, V and Nb dissolved in Wostian iron at the quenching temperature, and (Cr2+Mo2+V2+Nb2) represents the total amount of Cr, Mo, V and Nb dissolved in Wostian iron at 800℃].

Description

Alloy tool steel for hot working This specification discloses an alloy steel suitable for tools used in high-temperature plastic processing such as hot forging. For hot die alloy steel, JIS standard SKD61 is used. SKD61 has excellent high-temperature strength. For hot forging dies, JIS standard SKT4 is also used. SKT4 has excellent toughness. Patent document 1 (Japanese Patent Application Publication No. 2011-195917) discloses a hot-working tool steel with an improved composition. In this tool steel, it is intended to balance high-temperature strength and toughness. Patent Document 2 (Japanese Patent Application Publication No. 2006-322071) discloses an alloy steel with an improved metallic structure. This alloy steel has high hardness. [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent Application Publication No. 2011-195917 [Patent Document 2] Japanese Patent Application Publication No. 2006-322071 [The problem that the invention aims to solve] Traditional hot forging alloys have poor resistance to softening and are not suitable for continuous use in high-temperature environments. The object of this invention is to provide a hot-working alloy tool steel with excellent toughness and resistance to softening. [Means for solving the problem] The hot-working alloy tool steel of the present invention (hereinafter also referred to as "the alloy tool steel of the present invention") contains: C: 0.40% to 0.60% by mass, Si: 0.10% to 0.50% by mass, Mn: 0.20% to 1.10% by mass, Ni: 0.20% to 2.10% by mass, Cr: 0.50% to 2.00% by mass, Mo: 0.10% to 0.60% by mass, V and / or Nb: a total of 0.05% to 0.30% by mass, and N: 0.020% by mass. The remainder of the alloy tool steel of the present invention is Fe and impurities. The microstructure of the alloy tool steel of the present invention is arsenic or ductile iron. The microstructure of the present invention comprises blocks with a diameter of 2.0 μm to 6.0 μm. The solid solution element parameter Q of the alloy tool steel of the present invention, calculated by the following formula, is 1.12 or higher during quenching. [In the formula, Cr1 represents the amount (mass%) of Cr dissolved in ferrous iron at the quenching temperature, Mo1 represents the amount (mass%) of Mo dissolved in ferrous iron at the quenching temperature, V1 represents the amount (mass%) of V dissolved in ferrous iron at the quenching temperature, Nb1 represents the amount (mass%) of Nb dissolved in ferrous iron at the quenching temperature, Cr2 represents the amount (mass%) of Cr dissolved in ferrous iron at 800℃, Mo2 represents the amount (mass%) of Mo dissolved in ferrous iron at 800℃, V2 represents the amount (mass%) of V dissolved in ferrous iron at 800℃, and Nb2 represents the amount (mass%) of Nb dissolved in ferrous iron at 800℃.] [Effects of the Invention] The hot-working alloy tool steel of this invention exhibits excellent toughness and resistance to softening. Molds made from this alloy tool steel are suitable for continuous use in high-temperature environments. [Forms of Invention] The hot-working alloy tool steel of this invention is obtained by quenching and tempering, as detailed later. The alloy tool steel of this invention contains: C: 0.40% to 0.60% by mass, Si: 0.10% to 0.50% by mass, Mn: 0.20% to 1.10% by mass, Ni: 0.20% to 2.10% by mass, Cr: 0.50% to 2.00% by mass, Mo: 0.10% to 0.60% by mass, V and / or Nb: a total of 0.05% to 0.30% by mass, and N: 0.020% by mass or less. The remainder of the alloy tool steel of this invention is Fe and impurities (unavoidable impurities). In the alloy tool steel of this invention, the amount of alloying elements is relatively small. Therefore, the alloy tool steel of this invention exhibits excellent toughness. As will be detailed later, in the alloy tool steel of this invention, the metal structure is appropriate, and the solid solution element parameter Q during quenching is appropriate. Therefore, although the amount of alloying elements is small, the alloy tool steel of this invention exhibits excellent resistance to softening. The elements are described in detail below. Furthermore, "mass%" refers to the mass of the alloy tool steel of this invention, unless otherwise specified. [Carbon (C)] The carbon series contributes to the hardenability, hardness, and strength of alloy tool steels. Based on this viewpoint, the C content is preferably 0.40% by mass or more, more preferably 0.45% by mass or more, and particularly preferably 0.49% by mass or more. Excessive carbon series hinders the toughness of alloy tool steels. From a toughness perspective, the C content is preferably 0.60% by mass or less, more preferably 0.57% by mass or less, and particularly preferably 0.55% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. [Silicon (Si)] Si contributes to the hardenability and hardness of alloy tool steels. Si further aids in deoxidation during alloy melting. Based on these considerations, the Si content is preferably 0.10% by mass or more, more preferably 0.14% by mass or more, and particularly preferably 0.15% by mass or more. Excess Si hinders the toughness of alloy tool steels. From a toughness perspective, the Si content is preferably 0.50% by mass or less, more preferably 0.40% by mass or less, and particularly preferably 0.34% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. [Manganese (Mn)] Mn compounds contribute to the hardenability and hardness of alloy tool steels. During quenching, Mn compounds dissolve in the matrix and promote carbide precipitation during tempering. Based on these considerations, the Mn content is preferably 0.20% by mass or more, more preferably 0.30% by mass or more, and particularly preferably 0.50% by mass or more. Excess Mn compounds hinder the toughness of alloy tool steels. From a toughness perspective, the Mn content is preferably 1.10% by mass or less, more preferably 1.05% by mass or less, and particularly preferably 1.00% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. [Ni] Ni compounds displace and dissolve into the matrix during quenching heating, contributing to the hardenability of alloy tool steels. Ni compounds further contribute to the toughness of alloy tool steels. Based on these considerations, the Ni content is preferably 0.20% by mass or more, more preferably 0.59% by mass or more, and particularly preferably 1.00% by mass or more. Excessive Ni compounds lead to twinning due to an excessive decrease in the Ms point, hindering the toughness of the alloy tool steel. From a toughness perspective, the Ni content is preferably 2.10% by mass or less, more preferably 1.94% by mass or less, and particularly preferably 1.61% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. [Chromium (Cr)] Cr compounds contribute to the hardenability of alloy tool steels. Cr compounds further dissolve in secondary carbides (M2C, MC, etc.) of transition metals M, promoting the precipitation of these secondary carbides. Based on this viewpoint, the Cr content is preferably 0.50% by mass or more, more preferably 1.10% by mass or more, and particularly preferably 1.50% by mass or more. Excess Cr compounds result in undissolved carbides after quenching, hindering the toughness of alloy tool steels. From a toughness perspective, the Cr content is preferably 2.00% by mass or less, more preferably 1.95% by mass or less, and particularly preferably 1.90% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. [Mo] The Mo group contributes to the hardenability of alloy tool steels. The Mo group further dissolves in secondary carbides (M2C, MC, etc.) of the transition metal M, promoting the precipitation of these secondary carbides. Based on this viewpoint, the Mo content is preferably 0.10% by mass or more, more preferably 0.17% by mass or more, and particularly preferably 0.33% by mass or more. Excess Mo leads to undissolved carbides after quenching, hindering the toughness of the alloy tool steel. From a toughness perspective, the Mo content is preferably 0.60% by mass or less, more preferably 0.55% by mass or less, and particularly preferably 0.50% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. Vanadium (V) can cause carbide precipitation. V precipitates as secondary carbides (VC), especially during tempering. Based on this viewpoint, the V content is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and particularly preferably 0.10% by mass or more. Excess V leads to undissolved carbides after quenching, hindering the toughness of the alloy tool steel. From a toughness perspective, the V content is preferably 0.30% by mass or less, more preferably 0.25% by mass or less, and particularly preferably 0.20% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. The alloy tool steel of the present invention contains either or both of V and Nb. In the alloy tool steel of the present invention, V is not a necessary element. Therefore, the V content can be substantially zero. In other words, the V content may not reach the detection limit value. [Niobium (Nb)] Nb can cause carbide precipitation. Nb precipitates as secondary carbides, specifically NbC, during tempering. Based on this viewpoint, the Nb content is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and particularly preferably 0.03% by mass or more. Excess Nb leads to undissolved carbides after quenching, hindering the toughness of the alloy tool steel. From a toughness perspective, the Nb content is preferably 0.30% by mass or less, more preferably 0.15% by mass or less, and particularly preferably 0.08% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. As mentioned above, the alloy tool steel of the present invention contains either or both of V and Nb. In the alloy tool steel of the present invention, Nb is not an essential element. Therefore, the Nb content can be substantially zero. In other words, the Nb content may not reach the detection limit value. [V, Nb] As mentioned above, the alloy tool steel of the present invention comprises either or both of V and Nb. From the viewpoint of secondary carbide precipitation, the total content of V and Nb is preferably 0.05% by mass or more, more preferably 0.10% by mass or more, and particularly preferably 0.12% by mass or more. From the viewpoint of toughness, the total content of V and Nb is preferably 0.30% by mass or less, more preferably 0.27% by mass or less, and particularly preferably 0.24% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. [Nitrogen (N)] N can bond with V or Nb, causing nitrides or carbonitrides to precipitate. These nitrides and carbonitrides contribute to the toughness of alloy tool steels. Based on this viewpoint, the N content is preferably 0.001% by mass or more, more preferably 0.002% by mass or more, and particularly preferably 0.003% by mass or more. Nitrides and carbonitrides tend to remain in the matrix during quenching as they do not dissolve completely. In alloy tool steels with an excess of nitrides and carbonitrides, the solid solution element parameter Q (described later) is small. In alloy tool steels with a small solid solution element parameter Q, the amount of secondary carbides after tempering is insufficient. In alloy tool steels with a small solid solution element parameter Q, the softening resistance is poor. From the viewpoint of softening resistance, the N content is preferably 0.020% by mass or less, more preferably 0.015% by mass or less, and particularly preferably 0.010% by mass or less. These upper limits can be combined with any of the aforementioned lower limits. In the alloy tool steel of this invention, N is not a necessary element. Therefore, the N content can be substantially zero. In other words, the N content may not reach the detection limit. [Solid solution element parameter Q during quenching] In this specification, the solid solution element parameter Q during quenching is calculated using the following formula. Cr1: Amount of Cr dissolved in ferrous iron at the quenching temperature (mass %) Mo1: Amount of Mo dissolved in ferrous iron at the quenching temperature (mass %) V1: Amount of V dissolved in ferrous iron at the quenching temperature (mass %) Nb1: Amount of Nb dissolved in ferrous iron at the quenching temperature (mass %) Cr2: Amount of Cr dissolved in ferrous iron at 800℃ (mass %) Mo2: Amount of Mo dissolved in ferrous iron at 800℃ (mass %) V2: Amount of V dissolved in ferrous iron at 800℃ (mass %) Nb2: Amount of Nb dissolved in ferrous iron at 800℃ (mass %) In the above formulas, (Cr1+Mo1+V1+Nb1) represents the total amount (mass%) of Cr, Mo, V, and Nb dissolved in Wostian iron (FCC_Al) at the quenching temperature of the alloy tool steel of the present invention. In the above formulas, (Cr2+Mo2+V2+Nb2) represents the total amount (mass%) of Cr, Mo, V, and Nb dissolved in Wostian iron at 800°C. For alloy tool steels with an Nb content of less than 0.01% by mass, Nb1 and Nb2 in the above formulas are substituted with zero in the calculation of the solid solution element parameter Q. The quenching temperature of the alloy tool steel of the present invention is, for example, 850°C to 1050°C, preferably 900°C to 1020°C. The solid solution element parameter Q was calculated using the database "TCFE10" and the software "Thermo-calc". Thermo-calc is an integrated thermodynamic calculation software provided by Thermo-Calc Software AB. Thermodynamic equilibrium calculations were performed using this software. The calculation conditions are as follows: Mode: Graphical; Pressure: 1×10⁻⁶ 5 Pa Total size: 1 mol When selecting elements, all phases are automatically selected for calculation. The solid solution element parameter Q of the alloy tool steel of the present invention is preferably 1.12 or higher. In alloy tool steels with a solid solution element parameter Q of 1.12 or higher, the amount of secondary carbides after tempering is high. These secondary carbides suppress the recovery of dislocation when the alloy tool steel is used at high temperatures. Alloy tools with a solid solution element parameter Q of 1.12 or higher exhibit excellent softening resistance. From the viewpoint of softening resistance, this parameter Q is preferably 1.13 or higher, and particularly preferably 1.14 or higher. From the viewpoint of softening resistance, the larger the parameter Q, the better. In practical alloy tool steels, a parameter Q of 1.70 or lower is achievable. As a means to obtain alloy tool steel with a solid solution element parameter Q of 1.12 or higher, examples include: (1) adding more Cr, Mo, V or Nb, (2) reducing the N content, and (3) increasing the quenching temperature. As mentioned above, the alloy tool steel of this invention is obtained through quenching and tempering. The metallic structure of the alloy tool steel of this invention is ferrite or ferroalloy. Alloy tool steel with this metallic structure exhibits excellent toughness and softening resistance. For example, compared with alloy steel with granular iron structure obtained by annealing, the alloy tool steel of this invention can achieve a higher degree of balance between toughness and softening resistance. From the viewpoint that a larger block diameter results in better softening resistance, the preferred metallic structure is ferroalloy. Asada-san iron and toughened iron form the lower structure within the old Wostian iron grains, consisting of packets, blocks, and laths. Packets are groups of laths with the same crystal orientation. The azimuth difference between these packets is 15° or more. Blocks are groups of laths that separate the packets, also with the same crystal orientation. The azimuth difference between these blocks is 15° or more. Laths are considered the smallest lower structure of Asada-san iron. The azimuth difference between the laths is approximately 2°. The inventors have discovered a way to suppress the recovery of differential packing in alloy tool steels with large block diameters. Alloy tool steels with large block diameters exhibit excellent softening resistance. From the viewpoint of softening resistance, the block diameter is preferably 2.0 μm or more, more preferably 2.2 μm or more, and particularly preferably 2.4 μm or more. From the viewpoint of toughness, the block diameter is preferably 6.0 μm or less, more preferably 5.7 μm or less, and particularly preferably 5.5 μm or less. These upper limits can be combined with any of the aforementioned lower limits. The diameter of the block was determined using an azimuth map of αFe obtained by the FESEM-EBSP method. The measurement conditions were as follows: Field of view: 50 μm × 50 μm; Spacing: 0.05 μm; Software: OIM-Analysis (TSL Solusions). The region enclosed by azimuth differences of 15° or more from adjacent crystals was treated as a block, and the equivalent diameter of the circle was calculated from the area of ​​the obtained block. The average area load of the calculated equivalent diameter of the circle was defined as the block diameter. As a means of obtaining alloy tool steel with a block diameter of 2.0 μm or more and 6.0 μm or less, examples include: (1) increasing the quenching temperature, (2) increasing the holding time during quenching, and (3) making the amount of V and Nb added appropriate. [Manufacturing Method] Hereinafter, an example of a method for manufacturing the alloy tool steel of the present invention will be described. In this manufacturing method, firstly, a base material having the aforementioned composition is obtained by melting. The base material is then subjected to plastic processing to obtain an intermediate. The intermediate is then quenched. The quenching temperature is typically 850°C to 1050°C. Preferably, the quenching temperature is 900°C to 1020°C. During quenching, the intermediate is rapidly cooled. Due to rapid cooling, the intermediate undergoes a transformation. The intermediate is then tempered. The tempering temperature is typically 550°C to 700°C. By tempering, an article of alloy tool steel is obtained. The metallic structure of this alloy tool steel is slag or ductile iron. [Applications of Alloy Tool Steel] A typical application of the alloy tool steel of the present invention is for tools used in high-temperature plastic processing. Tools used in high-temperature plastic processing are also called hot working tools. In particular, the alloy tool steel of the present invention is suitable for dies used in hot forging. Examples of dies used in hot forging include hammer dies and pressing dies. Hot working tools such as dies used in hot forging are used, for example, for the purpose of improving machinability or for controlling the microstructure to obtain desired properties after hot working. Because they are in contact with the workpiece heated to a high temperature, their surface area is exposed to a considerable temperature (e.g., 180–1300°C) through heat transfer from the workpiece. [Hot Forging Die] As mentioned above, this specification also pertains to hot forging dies. The material of the hot forging die of this invention is the alloy tool steel of this invention. The alloy tool steel of this invention contains: C: 0.40% to 0.60% by mass, Si: 0.10% to 0.50% by mass, Mn: 0.20% to 1.10% by mass, Ni: 0.20% to 2.10% by mass, Cr: 0.50% to 2.00% by mass, Mo: 0.10% to 0.60% by mass, V and / or Nb: a total of 0.05% to 0.30% by mass, and N: 0.020% by mass. The remainder of the alloy tool steel of this invention is Fe and impurities. The metallic structure of the alloy tool steel of this invention is slag or ductile iron. This metallic structure comprises blocks with a diameter of 2.0 μm to 6.0 μm. The alloy tool steel of this invention has a solid solution element parameter Q of 1.12 or higher during quenching. The hot forging die system of this invention exhibits excellent toughness and softening resistance. The hot forging die system of this invention has a long service life. [Example] While the effects of the hot-working alloy tool steel in the embodiments have been explained below, the scope disclosed in this specification should not be interpreted in a limited way based on the description of the embodiments. [Example 1] The raw material was melted in a vacuum melting furnace and poured into a 190mm diameter mold to obtain an ingot. The ingot was heated to 1100°C and extended forging was performed to obtain a square bar with dimensions of 15mm × 15mm. The square bar was held at 900°C for 30 minutes and then subjected to oil-cooled quenching. As shown in Table 2, the quenching temperature was adjusted to 900°C. The square bar was heated to a specified temperature and subjected to air-cooled tempering to obtain the hot-working alloy tool steel of Example 1. The tempering temperature was adjusted so that the hardness of the tempered square bar was between 39HRC and 41HRC. The composition of this alloy tool steel is shown in Table 1 below. [Examples 2-6 and Comparative Examples 1-10] Except that the composition was set as shown in Table 1 and the quenching temperature was adjusted in the range of 850°C to 1250°C as shown in Table 2, the hot working alloy tool steels of Examples 2-6 and Comparative Examples 1-10 were obtained in the same manner as in Example 1. [Softening Resistance Test] The aforementioned square material (after tempering) was held at 620°C for 100 hours. The square material was then air-cooled. The hardness of the square material was measured and rated according to the following criteria: A: Hardness above 26 HRC. B: Hardness below 26 HRC. The results are shown in Table 2 below. [Differential Packing Density] For the square material after the aforementioned softening resistance test, X-ray diffraction measurements were performed under the following conditions: X-ray source: Cu-Kα line; counting time: 2 seconds; step size: 0.02°; measurement range 2θ: 35° to 140°. The differential packing density was quantified using the diffraction peaks of the obtained Asada iron using the Modified Williamson-Hall / Modified Warren-Averbach method. Due to peak broadening caused by the apparatus, LaB6 correction was used. The differential packing density was rated according to the following criteria: A: Differential packing density is 1.0 × 10⁻⁶. 14 That's all. B: The differential packing density did not reach 1.0 × 10⁻⁶. 14 The results are shown in Table 2 below. [Toughness] The aforementioned square material (after tempering) was subjected to a Charpy impact test according to "JIS Z 2242:2005" to determine the impact value. The conditions are as follows: Test piece: JIS-3, longitudinal: 10mm, transverse: 10mm, length: 50mm; Notch: U-notch; Temperature: room temperature. The impact value was rated according to the following criteria: A: Impact value 50J / cm². 2 That's all. B: Impact value is 50 J / cm. 2 The results are shown in Table 2 below. The remaining portion of each alloy shown in Table 1 consists of Fe and unavoidable impurities. As shown in Table 2, the hot-working alloy tool steels of each embodiment exhibit excellent toughness and softening resistance. These evaluation results clearly demonstrate the advantages of the alloy tool steel of the present invention. [Industrial Applicability] The hot-working alloy tool steels described above are suitable for various metal products used in high-temperature environments.

Claims

1. A hot-working alloy tool steel, comprising: C: 0.40% to 0.60% by mass, Si: 0.10% to 0.50% by mass, Mn: 0.20% to 1.10% by mass, Ni: 0.20% to 2.10% by mass, Cr: 0.50% to 2.00% by mass, Mo: 0.10% to 0.60% by mass, V and / or Nb: a total of 0.05% to 0.30% by mass, and N: 0.020% by mass, with the remainder being Fe and impurities. The aforementioned hot-working alloy tool steel has a microstructure of slag or ductile iron, and the aforementioned microstructure comprises blocks with a diameter of 2.0 μm to 6.0 μm. The solid solution element parameter Q, calculated by the following formula, is 1.12 or higher during quenching. [Where,] Cr1 represents the amount (mass%) of Cr dissolved in ferrous iron at the quenching temperature, Mo1 represents the amount (mass%) of Mo dissolved in ferrous iron at the quenching temperature, V1 represents the amount (mass%) of V dissolved in ferrous iron at the quenching temperature, Nb1 represents the amount (mass%) of Nb dissolved in ferrous iron at the quenching temperature, Cr2 represents the amount (mass%) of Cr dissolved in ferrous iron at 800℃, Mo2 represents the amount (mass%) of Mo dissolved in ferrous iron at 800℃, V2 represents the amount (mass%) of V dissolved in ferrous iron at 800℃, and Nb2 represents the amount (mass%) of Nb dissolved in ferrous iron at 800℃.