Hot work tool steel with excellent softening resistance and hardenability.

Abstract

To provide a hot work tool steel having high levels of softening resistance, quenchability, and toughness.SOLUTION: A hot work tool steel comprises, in mass%, C: 0.32-0.50, Si: 0.2-1.2, Mn: 0.3-1.0, Cr: 3.8-5.5, Mo: 1.3-2.3, W: 0.5-2.3, V: 0.1-0.8, with the balance being Fe and inevitable impurities, and has a value of formula A (2.16-1.56C-0.02Si-0.40Mn-0.28Cr+0.02Mo-0.02W+0.26V) being 0.36 or less. After quenched and tempered, a value of formula B (39.56-11.54MC-5.13M6C-2.61M23C6) is 16.5 or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to die steel used in high-temperature environments such as hot extrusion, hot forging, casting, and die casting.

Background Art

[0002] In recent years, due to the increase in the strength of aluminum parts for the purpose of lightweighting automobiles, the mechanical and thermal loads on dies have been increasing. As a result, problems such as wear and large cracks are likely to occur in the dies. To address these problems, die materials are required to have excellent softening resistance and toughness. In addition, excellent hardenability is also required to obtain sufficient properties from the die surface to the center.

[0003] Focusing on the above problems, die materials have been proposed so far. For example, a hot die steel containing C: 0.30 to 0.50%, Si: 0.10 to 0.50%, Mn: 0.1 to 1.0%, Cr: 4.5 to 5.4%, Mo: 1.4 to 2.4%, W: 1.0% or less, and Mo + W / 2: 1.7 to 2.4%, V: 0.30 to 0.70%, and the balance being Fe and unavoidable impurities has been proposed (see Patent Document 1). This proposal aims to obtain softening resistance by controlling the carbide species and area ratio. Regarding M2C, which is a more stable carbide at high temperatures, the ratio of M2C in all carbides is 1.0% or more in terms of area ratio, which is characteristic.

[0004] In addition, a hot work steel having a composition of 0.25 < C < 0.45, 0.29 < Si < 0.40, 0.52 < Mn < 0.9, 5.95 < Cr < 6.79, 1.23 < Mo < 1.51, 0.40 < V < 0.60, and the balance being Fe and unavoidable impurities has been proposed (see Patent Document 2). This proposal has an excessive addition of Cr, and thus the toughness and softening resistance are reduced due to the excessive addition of Cr.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2017-155306 [Patent Document 2] Japanese Patent Publication No. 2014-025103 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The problem that this invention aims to solve is to provide a hot tool steel that possesses high softening resistance, hardenability, and toughness, and is applicable to molds used in high-temperature environments such as hot forging, hot extrusion, casting, and die casting. [Means for solving the problem]

[0007] As a result of their diligent development, the inventors discovered that by using the alloy components and carbide state described in the claims, a hot work tool steel possessing high softening resistance, hardenability, and toughness can be obtained.

[0008] In other words, the first means to solve the problem is a hot work tool steel with high softening resistance, high hardenability, and high toughness, in which, by mass%, C: 0.32~0.50, Si: 0.2~1.2, Mn: 0.3~1.0, Cr: 3.8~5.5, Mo: 1.3~2.3, W: 0.5~2.3, V: 0.1~0.8, the remainder being Fe and unavoidable impurities, and the value A in formula A is 0.36 or less, and furthermore, in the quenched and tempered state, the value B in formula B is 16.5 or less. Note that equation A: A = 2.16 - 1.56C - 0.02Si - 0.40Mn - 0.28Cr + 0.02Mo - 0.02W + 0.26V, equation B: B = 39.56 - 11.54MC - 5.13M6C - 2.61M 23 It is C6. Substitute the mass percentage value of the component into the element symbol in formula A. The symbols for carbides in formula B are MC, M6C, and M 23 For C6, substitute the value (%) of the area ratio of the carbide per observation area after tempering. [Effects of the Invention]

[0009] The hot work tool steel of the present invention exhibits high softening resistance, with a hardness reduction of 15.0 HRC or less from the initial hardness after quenching and tempering and holding at 600°C for 100 hours; exhibits excellent steel hardenability with an upper critical cooling rate of 0.4°C / s or less; and has a Charpy impact value of 15 J / cm² at room temperature after quenching and tempering. 2 A material exhibiting the above-mentioned high toughness can be obtained. [Modes for carrying out the invention]

[0010] First, prior to describing embodiments of the present invention, the reasons for specifying the chemical components to be added to the hot work tool steel by the means of the present invention, and the reasons for specifying formulas A and B, will be explained. Note that % in the chemical components refers to mass %.

[0011] C: 0.32~0.50% Carbon (C) is a component that maintains hardenability, temper hardness, and high-temperature hardness, and also combines with carbide-forming elements such as Cr, Mo, W, and V to form carbides, thereby providing grain refinement, wear resistance, and softening resistance. If the C content is less than 0.32%, sufficient of the above properties cannot be obtained. On the other hand, if the C content exceeds 0.50%, segregation is promoted, making it easier for coarse carbonitrides to crystallize, leading to a decrease in toughness. Therefore, the C content should be between 0.32% and 0.50%. Preferably, the C content is between 0.34% and 0.47%.

[0012] Si 0.2~1.2% Si is an essential component for deoxidation and ensuring hardenability in steelmaking. If Si is less than 0.2%, it does not exhibit sufficient effects. If Si is more than 1.2%, it leads to a decrease in hot workability and toughness. Therefore, Si should be between 0.2% and 1.2%. Preferably, Si is between 0.3% and 1.0%.

[0013] Mn: 0.3~1.0% Mn is an essential component for deoxidation and ensuring hardenability in steelmaking. If the Mn content is less than 0.3%, it will not exhibit sufficient effects. If the Mn content is greater than 1.0%, it will lead to a decrease in toughness and machinability. Therefore, the Mn content should be between 0.3% and 1.0%. Preferably, the Mn content is between 0.4% and 0.8%.

[0014] Cr: 3.8 - 5.5% Cr is a component necessary for improving hardenability and wear resistance. If Cr is less than 3.8%, sufficient hardenability cannot be obtained. On the other hand, if more than 5.5% of Cr is added, excessive addition will cause an excessive amount of Cr-based carbides to form, leading to a decrease in toughness and softening resistance. Therefore, Cr is set to 3.8 - 5.5%. Preferably, Cr is 3.8 - 5.0%.

[0015] Mo: 1.3 - 2.3% Mo is a useful component for obtaining precipitation carbides that contribute to secondary hardening and wear resistance. Also, fine carbides that did not dissolve during quenching suppress the coarsening of crystal grains. If Mo is less than 1.3%, sufficient effects cannot be obtained. If Mo is more than 2.3%, not only does the effect saturate even with excessive addition, but the toughness decreases due to the coarsening and aggregation of carbides. Also, the cost increases. Therefore, Mo is set to 1.3 - 2.3%. Preferably, Mo is 1.4 - 2.2%.

[0016] W: 0.5 - 2.3% W is a useful component for obtaining precipitation carbides that contribute to secondary hardening and wear resistance. Also, fine carbides that did not dissolve during quenching suppress the coarsening of crystal grains. If W is less than 0.5%, sufficient effects cannot be obtained. If W is more than 2.3%, not only does the effect saturate even with excessive addition, but the toughness decreases due to the coarsening and aggregation of carbides. Also, the cost increases. Therefore, W is 0.5 - 2.3%. Preferably, W is 0.7 - 2.1%.

[0017] V: 0.1 - 0.8% V is a component that precipitates fine and hard carbides and carbonitrides during tempering and contributes to high-temperature strength and wear resistance. Also, V is a component that suppresses the coarsening of crystal grains and the decrease in toughness by fine carbides and carbonitrides during quenching. If V is less than 0.1%, these effects cannot be fully obtained. If V is more than 0.8%, coarse precipitates are generated during solidification, inhibiting toughness. Therefore, V is 0.1 - 0.8%. Preferably, V is 0.15 - 0.75%.

[0018] Formula A: A = 2.16 - 1.56C - 0.02Si - 0.40Mn - 0.28Cr + 0.02Mo - 0.02W + 0.26V A: Below 0.36 Formula A is an index of hardenability based on the components of C, Si, Mn, Cr, Mo, W, and V from the perspective of hardenability. In Formula A, the elemental symbols are substituted with the values of mass % of the corresponding components. It is necessary for the value of Formula A to be below 0.36 to ensure hardenability. If it exceeds 0.36, the required hardenability cannot be ensured.

[0019] Formula B: B = 39.56 - 11.54MC - 5.13M6C - 2.61M 23 C6 B: 16.5 or below This Formula B serves as an index of tempering resistance. To obtain tempering resistance, it is necessary for the value of Formula B to be 16.5 or below. If it exceeds 16.5, the required tempering resistance cannot be ensured. In Formula B, MC, M6C, and M 23 C6 refers to the area ratio of secondary carbides during tempering, so substitute the value (%) of the area ratio of each corresponding carbide per observation area.

[0020] (Example) First, steels composed of the chemical components described in Invention Steels 1 to 14 and Comparative Steels 15 to 30 in Table 1, with the balance being Fe and inevitable impurities, were melted in a vacuum induction melting furnace to obtain 100 kg steel ingots, and then these were hot forged and stretched into blocks with a width of 65 mm and a height of 30 mm.

[0021]

Table 1

[0022] Note that the balance of the chemical components in Table 1 is Fe and inevitable impurities. Also, the "-" in Comparative Steel No. 27 of V in Table 1 means that it contains less than 0.01% as an inevitable impurity. The underlines shown in the chemical components in Table 1 mean that they are outside the scope defined by the present invention.

[0023] Next, after annealing these forged materials at 870 °C, round bars with a diameter of 20 mm and a length of 160 mm were sampled from the intermediate position between the surface and the center. After holding this round bar at 1030 °C, quenching was carried out by air cooling. After tempering twice at 570 - 670 °C, identification of the precipitated carbide species, investigation of toughness, and investigation of softening resistance were carried out. For the evaluation of hardenability, a test piece for measuring the transformation point with a diameter of 4 mm × 10 mm was fabricated from the remaining part of the forged material, and an investigation was carried out.

[0024] (Regarding the area ratio of carbides) For the test piece for carbide observation, a surface parallel to the forging direction of the sample after quenching and tempering was polished and prepared by the extraction replica method. Using the bright-field image of a transmission electron microscope (TEM), this test piece was observed in a region with a total area of 500 μm 2 . Carbide species such as MC, M6C, M 23 C6 were determined from the results of electron beam diffraction and the shape of the carbides. The photographed pictures were subjected to image analysis, and the area ratio of each carbide per observation area was calculated as a percentage. The area ratios of various carbides are shown in Table 1 in %.

[0025] (Regarding toughness, Charpy impact value: 15 J / cm 2 or more) Toughness was evaluated by a Charpy impact test at room temperature. The test piece was fabricated from the sample after quenching and tempering. The test piece shape was a 2 mm U-notch Charpy test piece, and the notch direction was perpendicular to the forging direction. In Table 1, if the impact value is 15 J / cm 2 or more, the toughness is shown as ○ indicating good toughness, and if it is less than 15 J / cm [[ID=二十一]] 2 less, it is shown as × indicating poor toughness.

[0026] (Regarding softening resistance, Rockwell hardness: the difference in hardness between the initial and after quenching and tempering is 15 HRC or less) Softening resistance was evaluated by holding the quenched and tempered sample at 600°C for 100 hours, air-cooling it, and then measuring its hardness at room temperature using a Rockwell hardness tester. If the difference between the initial temper hardness and the hardness after testing was 15 HRC or less, it was indicated with a circle (○) in Table 1 as having excellent softening resistance; if the difference was 15 HRC or more, it was indicated with a cross (×) as having poor softening resistance.

[0027] (Hardenability) Hardenability was evaluated by measuring the critical cooling rate (Bs) from the microstructure observation results after experiments in which the cooling rate from 1030°C was varied. A critical cooling rate of 0.4°C / s or less was indicated as having excellent hardenability (marked with ○). A critical cooling rate exceeding 0.4°C / s was indicated as having poor hardenability (marked with ×).

[0028] As shown in Table 1, steels No. 1 to 14 of the present invention all satisfy the specified range of chemical composition and the values ​​of formula A and formula B. They all receive a "○" rating for toughness, hardenability, and softening resistance, resulting in excellent evaluations. Thus, hot work tool steels with high softening resistance, hardenability, and toughness were obtained.

[0029] Comparative steel No. 15 had an insufficient carbon content and exceeded the value of formula B, resulting in insufficient softening resistance. Comparative steel No. 16 had an excessive amount of carbon, resulting in reduced toughness. Comparative steel No. 17 had an insufficient Si content and exceeded the value of formula A, resulting in insufficient hardenability. Comparative steel No. 18 had an excessive amount of Si, resulting in reduced toughness. Comparative steel No. 19 had an insufficient Mn content and exceeded the value of formula A, resulting in insufficient hardenability. Comparative steel No. 20 had an excessive amount of Mn, resulting in reduced toughness. Comparative steel No. 21 had an insufficient Cr content and exceeded the value of formula A, resulting in insufficient hardenability. Comparative steel No. 22 had an excessive amount of Cr, and the value of equation B was exceeded, so sufficient softening resistance could not be obtained. Comparative steel No. 23 had an insufficient Mo content and exceeded the value of equation B, resulting in insufficient softening resistance. Comparative steel No. 24 had an excessive amount of Mo, resulting in reduced toughness. Comparative steel No. 25 had an insufficient amount of W and exceeded the value of formula B, so it did not exhibit sufficient softening resistance. Comparative steel No. 26 had an excessive amount of W, resulting in reduced toughness. Comparative steel No. 27 did not exhibit sufficient softening resistance because its V content was insufficient and the value of formula B was exceeded. Comparative steel No. 28 had an excessive amount of V, resulting in reduced toughness. Although comparative steel No. 29 met the requirements of the present invention in terms of its component composition, it did not achieve sufficient hardenability because the value of formula A exceeded the limit. Although comparative steel No. 30 met the requirements of the present invention in terms of its component composition, it did not exhibit sufficient softening resistance because the value of formula B exceeded the limit.

Claims

[Claim 1] A hot work tool steel with high softening resistance, high hardenability, and high toughness, having mass%, C: 0.32-0.50, Si: 0.2-1.2, Mn: 0.3-1.0, Cr: 3.8-5.5, Mo: 1.3-2.3, W: 0.5-2.3, V: 0.1-0.8, with the remainder being Fe and unavoidable impurities, and having a value A in formula A of 0.36 or less, and further having a value B in formula B of 16.5 or less after quenching and tempering. Note that Equation A: A = 2.16 - 1.56C - 0.02Si - 0.40Mn - 0.28Cr + 0.02Mo - 0.02W + 0.26V, Equation B: B = 39.56 - 11.54MC - 5.13M 6 C-2.61M 23 C 6 In addition, the elemental symbols in formula A are replaced with the mass percentage values ​​of the respective components. Also, the symbols for carbides in formula B are MC and M. 6 C, M 23 C 6 Substitute the area percentage (%) of the carbide per unit area observed after tempering into the given field.

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