TW202342779A - Alloy tool steel for hot working - Google Patents

Abstract

The purpose of the present invention is to provide an alloy tool steel for hot working that has both excellent toughness and excellent softening resistance. Provided is an alloy tool steel for hot working, containing carbon, silicon, manganese, nickel, chromium, molybdenum, vanadium, niobium, and nitrogen, the remainder being iron and impurities, wherein: the metallographic structure of the alloy tool steel for hot working is martensite or bainite; the metallographic structure includes blocks with a diameter of 2.0-6.0 [mu]m; and a solid solute element on quenching parameter Q, calculated on the basis of the formula Q = (Cr1 + Mo1 + V1 + Nb1) / (Cr2 + Mo2 + V2 + Nb2) [where (Cr1 + Mo1 + V1 + Nb1) represents the total amount of chromium, molybdenum, vanadium, and niobium in solid solution in austenite at the quenching temperature and (Cr2 + Mo2 + V2 + Nb2) represents the total amount of chromium, molybdenum, vanadium, and niobium in solid solution in austenite at 800 DEG C], is at least 1.12.

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.

As the alloy steel for hot molds, JIS standard SKD61 is used. SKD61 series has excellent high temperature strength. For hot forging molds, JIS standard SKT4 is also used. SKT4 series 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 achieve both high-temperature strength and toughness.

Patent Document 2 (Japanese Patent Application Publication No. 2006-322071) discloses an alloy steel having an improved metal structure. This alloy steel has high hardness. [Prior technical literature] [Patent Document]

[Patent Document 1] Japanese Patent Application Publication No. 2011-195917 [Patent Document 2] Japanese Patent Application Publication No. 2006-322071

[Problem to be solved by the invention]

The alloys used in the past for hot forging have poor softening resistance and are not suitable for continuous use in high temperature environments.

An object of the present invention is to provide an alloy tool steel for hot working excellent in both toughness and softening resistance. [Means to solve the problem]

The alloy tool steel for hot working of the present invention (hereinafter also referred to as the "alloy tool steel of the present invention") contains: C: 0.40 mass% or more and 0.60 mass% or less, Si: 0.10 mass% or more and 0.50 mass% or less, Mn: 0.20 Mass% or more and 1.10 mass% or less, Ni: 0.20 mass% or more and 2.10 mass% or less, Cr: 0.50 mass% or more and 2.00 mass% or less, Mo: 0.10 mass% or more and 0.60 mass% or less, V and/or Nb: 0.05 mass% in total % or more and 0.30 mass% or less, and N: 0.020 mass% or less. The remainder of the alloy tool steel of the present invention is Fe and impurities. The metal structure of the alloy tool steel of the present invention is loose iron or toughened iron. The metal structure of the present invention includes blocks with a diameter of 2.0 μm or more and 6.0 μm or less. In the alloy tool steel of the present invention, the solid solution element parameter Q during quenching calculated from the following equation is 1.12 or more. [In the formula, Cr1 represents the amount of Cr that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%), Mo1 represents the amount of Mo that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%), and V1 represents the amount of Cr that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%) Temperature represents the amount of V (mass %) that is solidly dissolved in the Worthfield iron. Nb1 represents the amount of Nb that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%). Cr2 represents the amount of Nb that is solidly dissolved in the Worthfield iron at 800°C. The amount of Cr (mass %), Mo2 represents the amount of Mo (mass %) that is solidly dissolved in the Worthfield iron at 800°C, V2 represents the amount of V that is solidly dissolved in the Worthfield iron at 800°C (mass%) , Nb2 represents the amount of Nb (mass %) solidly dissolved in Worthfield iron at 800°C]. [Effects of the invention]

The alloy tool steel for hot working of the present invention has excellent toughness and softening resistance. The mold whose material is the alloy tool steel of the present invention is suitable for continuous use in high temperature environments.

[Form of carrying out the invention]

The alloy tool steel for hot working of the present invention is obtained by quenching and tempering as described in detail later. The alloy tool steel of the present invention contains: C: 0.40 mass% or more and 0.60 mass% or less, Si: 0.10 mass% or more and 0.50 mass% or less, Mn: 0.20 mass% or more and 1.10 mass% or less, Ni: 0.20 mass% or more and 2.10 mass% or less, Cr: 0.50 mass% or more and 2.00 mass% or less, Mo: 0.10 mass% or more and 0.60 mass% or less, V and/or Nb: a total of 0.05 mass% or more and 0.30 mass% or less, and N: 0.020 mass% or less. The remainder of the alloy tool steel of the present invention is Fe and impurities (inevitable impurities).

In the alloy tool steel of the present invention, the amount of alloy elements is relatively small. Therefore, the alloy tool steel of the present invention has excellent toughness. As will be described in detail later, in the alloy tool steel of the present invention, the metal structure is appropriate, and the solid solution element parameter Q during quenching is appropriate. Therefore, the alloy tool steel system of the present invention has excellent softening resistance even though the amount of alloy elements is small.

Each element is described in detail below. In addition, "mass %" is based on the mass of the alloy tool steel of the present invention, unless otherwise specified.

[Carbon(C)] C series contributes to the hardenability, hardness and strength of alloy tool steel. Based on these viewpoints, the C content is preferably 0.40 mass% or more, more preferably 0.45 mass% or more, and particularly preferably 0.49 mass% or more. Excess C hinders the toughness of alloy tool steel. From the viewpoint of toughness, the C content is preferably 0.60 mass% or less, more preferably 0.57 mass% or less, and particularly preferably 0.55 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Silicon (Si)] Si series contributes to the hardenability and hardness of alloy tool steel. Si can further contribute to deoxidation during melting of the alloy. From these viewpoints, the Si content is preferably 0.10 mass% or more, more preferably 0.14 mass% or more, and particularly preferably 0.15 mass% or more. Excess Si hinders the toughness of alloy tool steel. From the viewpoint of toughness, the Si content is preferably 0.50 mass% or less, more preferably 0.40 mass% or less, and particularly preferably 0.34 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Manganese (Mn)] Mn system contributes to the hardenability and hardness of alloy tool steel. Mn is solidly dissolved in the matrix during quenching and promotes the precipitation of carbides during tempering. Based on these viewpoints, the Mn content is preferably 0.20 mass% or more, more preferably 0.30 mass% or more, and particularly preferably 0.50 mass% or more. Excess Mn hinders the toughness of alloy tool steel. From the viewpoint of toughness, the Mn content is preferably 1.10 mass% or less, more preferably 1.05 mass% or less, and particularly preferably 1.00 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Nickel (Ni)] Ni system replaces solid solution in the matrix during quenching heating, which contributes to the quenchability of alloy tool steel. Ni series further contributes to the toughness of alloy tool steel. Based on these viewpoints, the Ni content is preferably 0.20 mass% or more, more preferably 0.59 mass% or more, and particularly preferably 1.00 mass% or more. Excess Ni leads to twins caused by excessive reduction of Ms point, hindering the toughness of alloy tool steel. From the viewpoint of toughness, the Ni content is preferably 2.10 mass% or less, more preferably 1.94 mass% or less, and particularly preferably 1.61 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Chromium (Cr)] Cr system contributes to the hardenability of alloy tool steel. Cr is further solid-soluted in the secondary carbides (M2C, MC, etc.) of the transition metal M, promoting the precipitation of these secondary carbides. Based on these viewpoints, the Cr content is preferably 0.50 mass% or more, more preferably 1.10 mass% or more, and particularly preferably 1.50 mass% or more. Excess Cr leads to unsolved carbides after quenching, hindering the toughness of alloy tool steel. From the viewpoint of toughness, the Cr content is preferably 2.00 mass% or less, more preferably 1.95 mass% or less, and particularly preferably 1.90 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Molybdenum (Mo)] Mo system contributes to the hardenability of alloy tool steel. Mo is further solidly dissolved in the secondary carbides (M2C, MC, etc.) of the transition metal M, promoting the precipitation of these secondary carbides. Based on these viewpoints, the Mo content is preferably 0.10 mass% or more, more preferably 0.17 mass% or more, and particularly preferably 0.33 mass% or more. Excess Mo will lead to unsolidified carbides after quenching, hindering the toughness of alloy tool steel. From the viewpoint of toughness, the Mo content is preferably 0.60 mass% or less, more preferably 0.55 mass% or less, and particularly preferably 0.50 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Vanadium(V)] V can precipitate carbides. V is precipitated especially as secondary carbide VC during tempering. From this viewpoint, the V content is preferably 0.01 mass% or more, more preferably 0.05 mass% or more, and particularly preferably 0.10 mass% or more. Excess V will lead to unsolved carbides after quenching, hindering the toughness of alloy tool steel. From the viewpoint of toughness, the V content is preferably 0.30 mass% or less, more preferably 0.25 mass% or less, and particularly preferably 0.20 mass% or less. These upper limits may be combined with any of the above lower limits respectively. The alloy tool steel of the present invention contains any one or both of V and Nb. In the alloy tool steel of the present invention, V is not an essential 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 precipitate carbides. Nb precipitates especially as secondary carbide NbC during tempering. From this point of view, the Nb content is preferably 0.01 mass% or more, more preferably 0.02 mass% or more, and particularly preferably 0.03 mass% or more. Excess Nb causes unsolved carbides after quenching, hindering the toughness of alloy tool steel. From the viewpoint of toughness, the Nb content is preferably 0.30 mass% or less, more preferably 0.15 mass% or less, and particularly preferably 0.08 mass% or less. These upper limits may be combined with any of the above lower limits respectively. As mentioned above, the alloy tool steel of the present invention contains any 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 rate can be substantially zero. In other words, the Nb content may not reach the detection limit.

[V, Nb] As mentioned above, the alloy tool steel of the present invention contains any or both of V and Nb. From the viewpoint of precipitation of secondary carbides, the total content of V and Nb is preferably 0.05 mass% or more, more preferably 0.10 mass% or more, and particularly preferably 0.12 mass% or more. From the viewpoint of toughness, the total content of V and Nb is preferably 0.30 mass% or less, more preferably 0.27 mass% or less, and particularly preferably 0.24 mass% or less. These upper limits may be combined with any of the above lower limits respectively.

[Nitrogen (N)] N can bond with V or Nb to precipitate nitride or carbonitride. These nitrides and carbonitrides respectively contribute to the toughness of the alloy tool steel. Based on these viewpoints, the N content is preferably 0.001 mass% or more, more preferably 0.002 mass% or more, and particularly preferably 0.003 mass% or more. Nitride and carbonitride are not solidly dissolved in the matrix during quenching and tend to remain. In alloy tool steels with excess nitrides and carbonitrides, the solid solution element parameter Q described below is small. In alloy tool steel 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 mass% or less, more preferably 0.015 mass% or less, and particularly preferably 0.010 mass% or less. These upper limits may be combined with any of the above lower limits respectively. In the alloy tool steel of the present invention, N is not an essential 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 by the following equation. Cr1: The amount of Cr that is solidly dissolved in the Worthfield iron at the quenching temperature (mass %) Mo1: The amount of Mo that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%) V1: The amount of Cr that is solidly dissolved in the Worthfield iron at the quenching temperature Amount of V in the iron (mass %) Nb1: Amount of Nb solidly dissolved in the iron at the quenching temperature (mass %) Cr2: Amount of Cr dissolved in the iron at 800°C (mass %) Mo2: The amount of Mo that is solidly dissolved in the Vostian iron at 800°C (mass %) V2: The amount of V that is solidly dissolved in the Vossian iron at 800°C (mass%) Nb2: The amount of Mo that is solidly dissolved in the Vossian iron at 800°C The amount of Nb in Tiantie (mass %)

In the above formula (Cr1+Mo1+V1+Nb1) represents the total amount (mass) of Cr, Mo, V and Nb dissolved in Worthfield iron (FCC_A1) at the quenching temperature applied to the alloy tool steel of the present invention. %). In the above formula, (Cr2+Mo2+V2+Nb2) represents the total amount (mass %) of Cr, Mo, V and Nb that are solidly dissolved in Worthfield iron at 800°C. For alloy tool steels with a Nb content of less than 0.01% by mass, when calculating the solid solution element parameter Q, substitute zero for Nb1 and Nb2 in the above equation. The quenching temperature applied to the alloy tool steel of the present invention is, for example, 850°C or more and 1050°C or less, preferably 900°C or more and 1020°C or less.

The solid solution element parameter Q is calculated by the software "Thermo-calc" using the database "TCFE10". Thermo-calc is an integrated thermodynamic calculation software provided by Thermo-Calc Software AB. Use this software to perform thermodynamic equilibrium calculations. The calculation conditions are as follows. Mode: Graphic Pressure: 1×10 ⁵ Pa Total size: 1 mol When selecting elements, calculations are performed taking into account all automatically selected phases.

The solid solution element parameter Q of the alloy tool steel of the present invention is preferably 1.12 or more. In alloy tool steels with a solid solution element parameter Q of 1.12 or above, the amount of secondary carbides after tempering is large. This secondary carbide inhibits the recovery of dislocation when alloy tool steel is used in a high-temperature environment. An alloy tool system with a solid solution element parameter Q of 1.12 or more has excellent softening resistance. From the viewpoint of softening resistance, the parameter Q is more preferably 1.13 or more, and particularly preferably 1.14 or more. From the viewpoint of softening resistance, the larger the parameter Q, the better. The parameter Q that can be achieved in actual alloy tool steel is 1.70 or less.

As a means to obtain alloy tool steel with a solid solution element parameter Q of 1.12 or more, examples can be given: (1) Add a relatively large amount of Cr, Mo, V or Nb, (2) Comparatively reduce the N content rate, and (3) Increase the quenching temperature relatively.

As mentioned above, the alloy tool steel of the present invention is obtained by quenching and tempering. The metal structure of the alloy tool steel of the present invention is loose iron or toughened iron. Alloy tool steel with this metal structure has excellent toughness and softening resistance. For example, compared with alloy steel having a fat-grained iron structure obtained by annealing, the alloy tool steel of the present invention can achieve both toughness and softening resistance at a higher level. From the viewpoint that the diameter of the block described below is large and therefore the softening resistance is more excellent, the preferred metal structure is an alloy tool steel of toughened iron.

The loose iron and toughened iron in the old Worthfield iron grains are the lower structure and have bundles, blocks and laths. A bundle is a group of laths with the same crystal surface. The azimuth difference of this beam is more than 15°. Blocks are structures of separated bundles, which are groups of laths with the same crystallographic orientation. The azimuth difference of this block is more than 15°. The lath is considered to be the smallest lower structure of Asada Santetsu. The azimuth difference between the slats is about 2°.

The present inventors found that the recovery of misalignment is suppressed in alloy tool steels with large block diameters. Alloy tool steel systems with large block diameters have excellent softening resistance. From the viewpoint of softening resistance, the diameter of the block 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 diameter of the block 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 may be combined with any of the above lower limits respectively.

The diameter of the block was measured from the orientation map of αFe obtained by the FESEM-EBSP method. The measurement conditions are as follows. Field of view: 50μm×50μm Pitch: 0.05μm Software: OIM-Analysis (TSL Solusions) The area surrounded by an azimuth difference of 15° or more from adjacent crystals is treated as one block, and the equivalent circle diameter is calculated from the area of the block. The average value of the area load of the calculated equivalent diameter of the circle is 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) Increase the quenching temperature relatively, (2) Comparatively increase the holding time during quenching, and (3) Make the addition amounts of V and Nb appropriate.

[Manufacturing method] Hereinafter, an example of the manufacturing method of the alloy tool steel of this invention is demonstrated. In this manufacturing method, first, a base material having the aforementioned composition is obtained by melting. This base material is subjected to plastic processing to obtain an intermediate. The intermediate was quenched. The quenching temperature is usually above 850℃ and below 1050℃. The quenching temperature is preferably between 900°C and 1020°C. During quenching, the intermediate is rapidly cooled. By rapid cooling, metamorphosis occurs in the intermediate. This intermediate is tempered. The tempering temperature is usually above 550℃ and below 700℃. Through this tempering, a product whose material is alloy tool steel is obtained. The metal structure of this alloy tool steel is loose iron or toughened iron.

[Applications of alloy tool steel] Typical uses of the alloy tool steel of the present invention are tools for high-temperature plastic processing. Tools used for high-temperature plastic processing are also called thermal processing tools. In particular, the alloy tool steel of the present invention is suitable for molds used in hot forging. Examples of dies for hot forging include hammer dies and press dies. Hot working tools such as dies for hot forging, for example, for the purpose of improving workability or for controlling the structure to obtain desired characteristics after hot working, are in contact with the workpiece heated to a high temperature. Therefore, the vicinity of the surface is exposed to a corresponding temperature (for example, 180 to 1300°C) due to heat transfer from the workpiece.

[Mold for hot forging] As mentioned above, this manual is also directed to dies for hot forging. The material of the hot forging mold of the present invention is the alloy tool steel of the present invention. The alloy tool steel of the present invention contains: C: 0.40 mass% or more and 0.60 mass% or less, Si: 0.10 mass% or more and 0.50 mass% or less, Mn: 0.20 mass% or more and 1.10 mass% or less, Ni: 0.20 mass% or more and 2.10 mass% or less, Cr: 0.50 mass% or more and 2.00 mass% or less, Mo: 0.10 mass% or more and 0.60 mass% or less, V and/or Nb: a total of 0.05 mass% or more and 0.30 mass% or less, and N: 0.020 mass% or less. The remainder of the alloy tool steel of the present invention is Fe and impurities. The metal structure of the alloy tool steel of the present invention is loose iron or toughened iron. The metal structure includes blocks having a diameter of not less than 2.0 μm and not more than 6.0 μm. The solid solution element parameter Q during quenching of the alloy tool steel of the present invention is 1.12 or more. The mold system for hot forging of the present invention is excellent in toughness and softening resistance. The hot forging mold of the present invention has a long service life. [Example]

Hereinafter, although the effects of the alloy tool steel for hot working in the Examples have been clarified, the scope disclosed in this specification should not be interpreted in a limited manner based on the description of the Examples.

[Example 1] The raw materials were put into a vacuum melting furnace to be melted, and poured into a mold with a diameter of 190 mm to obtain an ingot (ingot). The ingot was heated to 1100° C. and subjected to extend forging to obtain a square material with a size of 15 mm × 15 mm. After maintaining the square material at a temperature of 900°C for 30 minutes, it was quenched by oil cooling. As shown in Table 2, the quenching temperature is adjusted to 900°C. The square material was heated to a specified temperature, and air-cooled tempering was performed to obtain the alloy tool steel for hot working of Example 1. Adjust the tempering temperature so that the hardness of the tempered square material becomes 39HRC to 41HRC. The composition of this alloy tool steel is shown in Table 1 below.

[Examples 2-6 and Comparative Examples 1-10] Examples 2-6 and Comparative Examples were obtained in the same manner as in Example 1, except that the composition was as shown in Table 1 and the quenching temperature was adjusted in the range of 850°C or more to 1250°C so that the quenching temperature was as shown in Table 2. Example 1-10 Alloy tool steel for hot working.

[Softening resistance test] The aforementioned square material (after tempering) was kept at a temperature of 620°C for 100 hours. Air cools the square. The hardness of the square material is measured and rated according to the following standards. A: The hardness is 26HRC or above. B: The hardness does not reach 26HRC. The results are shown in Table 2 below.

[Difference Density] X-ray diffraction measurement was performed on the square material after the aforementioned softening resistance test under the following conditions. X-ray source: Cu-Kα ray Counting time: 2 seconds Step size: 0.02° Measuring range 2θ: 35° to 140° Using the obtained diffraction peak of Asada powder, the Modified Williamson-Hall/Modified Warren-Averbach method was implemented Quantification of differential density. The broadening of the wave peak caused by the device is corrected using LaB6. The differential density is rated according to the following benchmarks. A: The differential density is 1.0×10 ¹⁴ or more. B: The differential density does not reach 1.0×10 ¹⁴ . The results are shown in Table 2 below.

[Toughness] For the above-mentioned square material (after tempering), a Charpy impact test was carried out in accordance with the provisions of "JIS Z 2242:2005" to measure the impact value. The conditions are as follows. Test piece: JIS-3 length: 10mm, width: 10mm, length: 50mm Notch: U notch Temperature: Normal temperature The impact value is rated according to the following standards. A: The impact value is 50J/cm2 or ^more . B: The impact value is 50J/ ^cm2 or less. The results are shown in Table 2 below.

The remainder of each alloy shown in Table 1 is Fe and unavoidable impurities.

As shown in Table 2, the alloy tool steel for hot working of each example is excellent in toughness and softening resistance. From these evaluation results, the advantages of the alloy tool steel of the present invention can be clearly understood. [Industrial utilization possibility]

The alloy tool steel system for hot working described above is suitable for various metal products used in high temperature environments.

Claims (1)

An alloy tool steel for hot working, containing: C: 0.40 mass% or more and 0.60 mass% or less, Si: 0.10 mass% or more and 0.50 mass% or less, Mn: 0.20 mass% or more and 1.10 mass% or less, Ni: 0.20 mass% or more 2.10 mass% or less, Cr: 0.50 mass% or more and 2.00 mass% or less, Mo: 0.10 mass% or more and 0.60 mass% or less, V and/or Nb: a total of 0.05 mass% or more and 0.30 mass% or less, and N: 0.020 mass% or less , and the remaining part is Fe and impurities. The metal structure of the alloy tool steel for hot working is Asada loose iron or toughened iron. The metal structure includes blocks with a diameter of 2.0 μm or more and 6.0 μm or less, with the following number The solid solution element parameter Q during quenching calculated by the formula is above 1.12, [In the formula, Cr1 represents the amount of Cr that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%), Mo1 represents the amount of Mo that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%), and V1 represents the amount of Cr that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%) Temperature represents the amount of V (mass %) that is solidly dissolved in the Worthfield iron. Nb1 represents the amount of Nb that is solidly dissolved in the Worthfield iron at the quenching temperature (mass%). Cr2 represents the amount of Nb that is solidly dissolved in the Worthfield iron at 800°C. The amount of Cr (mass %), Mo2 represents the amount of Mo (mass %) that is solidly dissolved in the Worthfield iron at 800°C, V2 represents the amount of V that is solidly dissolved in the Worthfield iron at 800°C (mass%) , Nb2 represents the amount of Nb (mass %) solidly dissolved in Worthfield iron at 800°C].