Method for producing hot work tool steel, and hot work tool steel

Abstract

Provided is a method for producing a hot work tool steel making it possible to reduce the grain size of prior austenite in a structure when quenching and tempering have been performed. The method for producing a hot work tool steel comprises a finish hot forging step for: readying a material for finish hot forging comprising, by mass, 0.25-0.50% of C, 0.1-1.2% of Si, 0.2-0.9% of Mn, over 0% to 0.6% of Ni, 3.8-5.5% of Cr, Mo and W standalone or in combination such that (Mo + 1 / 2W) is 1.1-2.7%, and 0.3-1.2% of V, with the balance being Fe and impurities; heating the material for finish hot forging to 1000-1060°C; and then performing hot free forging of a plurality of passes by solid forging. The method further comprises a recrystallization promotion step in which, after the total amount of strain imparted to the central part of the material during the finish hot forging step reaches 0.6, 300 seconds or more is allowed to elapse while the temperature of the central part is kept within the range of 1000-1060°C.

Description

Method for producing hot work tool steel and hot work tool

[0001] The present invention relates to a method for producing hot work tool steel and a hot work tool.

[0002] Hot work tools are used while being in contact with high-temperature workpieces or hard workpieces, so they need to have toughness that can withstand impact. As materials (hot work tool steel) applied to hot work tools, for example, SKD61 series alloy tool steels which are JIS steel grades are known. Such hot work tool steel is usually produced by using a steel ingot or a raw material obtained by blooming a steel ingot as a starting material, subjecting the starting material to various hot working processes and heat treatments to obtain a hot worked material having a predetermined square shape or round bar shape, and then annealing the hot worked material. The produced hot work tool steel in an annealed state with low hardness is then transferred to a step of machining into desired die or tool shapes. The above-mentioned hot working is carried out by open forging using a press or rolling using rolls. In particular, when producing large-sized steel materials, production is mainly carried out by open forging using a press. Hot work tool steel machined into the shape of a hot work tool is generally adjusted to a predetermined working hardness by quenching and tempering, and then subjected to finish machining. Quenching is an operation in which the hot work tool material is heated to an austenite temperature range and then rapidly cooled to transform the structure into martensite. Therefore, the component composition of the hot work tool material is such that it can be adjusted to a martensite structure by quenching. However, for large-sized hot work tools, the cooling rate during quenching becomes slow, and the structure may sometimes contain bainite.

[0003] By the way, it is known that the toughness of hot work tools can be improved by refining the martensite or bainite. Specifically, this is achieved by refining the prior austenite grain size observed in the martensite or bainite structure. And methods for refining the prior austenite grain size have been proposed (Patent Documents 1 and 2).

[0004] Japanese Patent Application Laid-Open No. 2000-129349, Japanese Patent Application Laid-Open No. 2005-163123

[0005] The technologies disclosed in the aforementioned Patent Documents 1 and 2 are techniques for obtaining fine crystal grains from tool steel after hot working or subsequent heat treatment. However, the inventors have found that when hot working material is compressed during the hot working process, particularly in hot free forging, the prior austenite grain size becomes non-uniform with respect to the thickness, width, and length directly below the contact surface between the press anvil and the material. This problem of non-uniform prior austenite grain size in the material affects mechanical properties such as toughness, and poses a significant problem in the practical application of hot tool steel as tools and molds. Furthermore, the inventors have found that after the hot forging material is removed from the heating and holding furnace for hot forging, it cools from the surface, while the work done by plastic deformation is converted not only into deformation of the material but also into thermal energy, and the temperature of the central part of the material can rise due to processing heat. Therefore, in the central part of the material, the accumulation of strain necessary for recrystallization and the subsequent progress of recrystallization may be insufficient compared to the surface, which may further increase the non-uniformity of the prior austenite grain size.

[0006] The object of the present invention is to provide a hot work tool steel that can finely adjust the prior austenite grain size in the central part of the hot work tool after quenching and tempering.

[0007] The inventors of the present invention have arrived at this invention by optimizing the reduction amount, timing, and temperature range when performing the free forging described above in order to obtain hot work tool steel having a fine structure, thereby finding conditions under which a fine prior austenite grain size can be obtained even in the central part of the hot work tool product.

[0008] In other words, one aspect of the present invention involves preparing a material for finishing hot forging, comprising, by mass%, C: 0.25-0.50%, Si: 0.1-1.2%, Mn: 0.2-0.9%, Ni: greater than 0% and 0.6% or less, Cr: 3.8-5.5%, Mo and W individually or in combination (Mo + 1 / 2W): 1.1-2.7%, V: 0.3-1.2%, with the remainder being Fe and impurities, and then applying 1000 to the finishing hot forging material. A method for manufacturing hot tool steel, comprising a finish hot forging step in which the material is heated to 1060°C and then subjected to one or more hot free forging passes by physical forging, and a recrystallization acceleration step in which, after the total amount of strain applied to the central part of the material during the finish hot forging step reaches 0.6, the central part temperature is kept within the range of 1000 to 1060°C for 300 seconds or more.

[0009] Another aspect of the present invention provides a finishing hot forging material comprising, by mass%, C: 0.25-0.50%, Si: 0.1-1.2%, Mn: 0.2-0.9%, Ni: greater than 0% and 0.6% or less, Cr: 3.8-5.5%, Mo and W individually or in combination (Mo + 1 / 2W): 1.1-2.7%, V: 0.3-1.2%, with the remainder being Fe and impurities; a finishing hot forging step comprising, heating the finishing hot forging material to 1000-1060°C, and then performing hot free forging in one or more forging passes by physical forging; and in the finishing hot forging step, finishing hot forging in one or more forging passes. A method for manufacturing hot tool steel, comprising: an intermediate recrystallization acceleration step in which, when the total strain applied to the central part of the material during forging is 0.3 or more and less than 0.6, the temperature of the central part is kept in the range of 1000 to 1060°C for 120 seconds or more before the forging pass in which the next strain is introduced; and a recrystallization acceleration step in which, after the intermediate recrystallization acceleration step, the temperature of the central part is kept in the range of 1000 to 1060°C for 120 seconds or more after the total strain applied to the central part of the material during the finishing hot forging step reaches 0.6, and the total elapsed time of the intermediate recrystallization acceleration step and the recrystallization acceleration step is 300 seconds or more. Preferably, the cross-sectional area of ​​the material for finishing hot forging is 120,000 mm². ２The present invention is preferable for the manufacture of hot work tool steel having a length of 1000 mm or more. Another aspect of the present invention is a hot work tool comprising, by mass%, C: 0.25 to 0.50%, Si: 0.1 to 1.2%, Mn: 0.2 to 0.9%, Ni: greater than 0% and 0.6% or less, Cr: 3.8 to 5.5%, Mo and W individually or in combination (Mo + 1 / 2W): 1.1 to 2.7%, V: 0.3 to 1.2%, the remainder being Fe and impurities, with a cross-sectional area of ​​120,000 mm². ２ A hot work tool that is 1000 mm or longer, has a maximum grain size number of 5.5 or higher for the prior austenite grain size.

[0010] According to the present invention, it is possible to obtain a hot work tool steel in which the prior austenite grain size is fine in the central part of the hot work tool after quenching and tempering.

[0011] This is a schematic diagram illustrating the forging path of the present invention. This is a diagram showing the strain distribution of the material after the finish hot forging process in a forging simulation. This is a graph showing the strain and temperature changes in the center of the material during the finish hot forging process of the present invention example. This is a graph showing the strain and temperature changes in the center of the material during the finish hot forging process of the comparative example. This is a graph showing the conditions for the finish hot forging process of the present invention example (No. 1). This is a graph showing the conditions for the finish hot forging process of another present invention example (No. 2). This is a schematic diagram illustrating the crystal grain size measurement position.

[0012] A key feature of the present invention is that by performing finish hot forging on a material for finish hot forging obtained by split forging within a predetermined temperature and time range, the prior austenite grain size can be made fine even in the central part of the hot tool steel. The constituent elements of the present invention will be described below. The hot tool steel obtained by the manufacturing method of the present invention is a hot tool steel having an annealed structure and being used after quenching and tempering, and having a component composition that can be adjusted to a martensitic structure by the above quenching. Here, prior austenite grain size refers to the grain size observed in the hot tool steel product after quenching or quenching and tempering, that is, the grain size that is formed above the A3 transformation point during the quenching process and observed after quenching and cooling or after tempering. In the manufacturing process of hot tool steel in the present invention, the size of the crystal grains formed after heating above the A3 transformation point and by recrystallization during hot working or holding is referred to as the austenite grain size.

[0013] The effect of refining the microstructure of hot tool steel obtained by the manufacturing method of the present invention can be achieved by applying the manufacturing method of the present invention, described below, to a material having a component composition of C: 0.25-0.50%, Si: 0.1-1.2%, Mn: 0.2-0.9%, Ni: greater than 0% and less than or equal to 0.6%, Cr: 3.8-5.5%, Mo and W (either individually or in combination, Mo + 1 / 2W): 1.1-2.7%, V: 0.3-1.2%, with the remainder being Fe and impurities. The reasons for limiting the above component ranges are described below.

[0014] C: 0.25 to 0.50 mass% (hereinafter simply referred to as "%") Carbon (C) is a fundamental element of hot work tool steel. Part of it dissolves into the matrix to impart strength, and part of it forms carbides to enhance wear resistance and seizure resistance. Furthermore, when C dissolved as interstitial atoms is added together with substitutional atoms that have a high affinity for C, such as Cr, it is expected to have the effect of increasing the strength of the hot work tool through the I (interstitial atom) - S (substitutional atom) effect; acting as drag resistance of the solute atoms. However, excessive addition leads to a decrease in toughness and hot strength. Therefore, C should be 0.25 to 0.50%. In this invention, it is preferable to add 0.30% or more C in order to further refine the prior austenite grain size. A more preferable lower limit for C is 0.33%, and a preferable upper limit for C is 0.45%.

[0015] Si: 0.1–1.2% Si is mainly added as a deoxidizing agent during steelmaking, but too much can lead to the formation of ferrite in the tool structure after quenching and tempering. Therefore, the upper limit of Si is set at 1.2%. On the other hand, Si has the effect of improving the machinability of hot work tool steel. To obtain this effect, the lower limit of Si is set at 0.1%. The preferred lower limit of Si is 0.2%, the preferred upper limit of Si is 1.0%, and more preferably 0.8%.

[0016] Mn: 0.2–0.9% Too much Mn increases the viscosity of the base material, reducing the machinability of hot work tool steel. Therefore, the upper limit of Mn should be 0.9% or less. On the other hand, Mn has the effect of improving hardenability, suppressing the formation of ferrite in the tool structure, and obtaining appropriate quenching and tempering hardness. In addition, its presence as nonmetallic inclusions (MnS) has a significant effect on improving machinability. To obtain these effects, the lower limit of Mn should be 0.2%. Note that to obtain the aforementioned effect of MnS, S may be added in the range of 0.01–0.05% rather than as an impurity.

[0017] Ni: Greater than 0% and less than or equal to 0.6%. Ni is an element that increases the toughness of the matrix and reduces machinability. On the other hand, Ni is an element that suppresses the formation of ferrite in the structure of hot tool steel. It is also an effective element that, along with C, Cr, Mn, Mo, W, etc., imparts excellent hardenability to hot tool steel and forms a martensite-dominant structure even when the cooling rate during quenching is slow, thereby preventing a decrease in toughness. Furthermore, it also improves the intrinsic toughness of the matrix, so in this invention, it can be added up to an upper limit of 0.6%. To obtain the effects of Ni addition described above, an addition of 0.15% or more is preferable. If the effects of Ni can be obtained with other elements, the Ni content can be low, and in that case, it is acceptable for it to be present in amounts greater than 0% and less than 0.15%.

[0018] Cr: 3.8-5.5% Cr is an element that enhances hardenability and forms carbides, which are effective in strengthening the base material and improving wear resistance. It is also a basic element of hot work tool steel that contributes to improved tempering softening resistance and high-temperature strength. However, excessive addition can lead to a decrease in high-temperature strength and a decrease in toughness due to the formation of coarse carbides. Therefore, the amount of Cr should be 3.8-5.5%. To enhance the aforementioned effect of improving hardenability, the lower limit of Cr is preferably 4.8%, and preferably 5.0%.

[0019] Mo and W can be added individually or in combination at (Mo + 1 / 2W): 1.1–2.7%. Mo and W can be added individually or in combination to impart strength and improve softening resistance by precipitating or agglomerating fine carbides through tempering. The amount added can be specified together in terms of the Mo equivalent of (Mo + 1 / 2W), since W has approximately twice the atomic weight of Mo (of course, either one or both can be added). To obtain the above-mentioned effects, the addition amount should be 1.1% or more in terms of (Mo + 1 / 2W). A more preferable lower limit is 1.2%. However, too much will lead to a decrease in machinability and toughness, so the addition amount should be 2.7% or less in terms of (Mo + 1 / 2W). A preferable upper limit is 2.3%, and a more preferable upper limit is 1.6% or less. Also, MC type and M containing W ２ Since C-type carbides are formed during the solidification process in the manufacturing process and tend to remain in the final product, reducing toughness, it is more preferable to add Mo alone.

[0020] V: 0.3-1.2% V forms carbides, which have the effect of strengthening the matrix, improving wear resistance, and tempering softening resistance. The carbides distributed in the annealed structure act as pinning particles that suppress the coarsening of austenite crystal grains during quenching, contributing to improved toughness. Furthermore, in this invention, it is expected that the grain growth of austenite grains after recrystallization will be suppressed not only during quenching but also during hot working. To obtain these effects, the lower limit of V is set to 0.3% or more. To further refine the structure, a preferable lower limit of V is 0.6%. However, if it is too high, it will lead to a decrease in machinability and toughness due to an increase in the carbides themselves, so the upper limit of V is set to 1.2%. A more preferable upper limit of V is 1.0%.

[0021] In addition to the above-mentioned elemental species, the following elemental species may also be included: Co: 0-1.0% Since Co reduces toughness, it is preferable to keep the amount below 1.0%. On the other hand, when using a hot work tool, Co forms an extremely dense and highly adhesive protective oxide film on its surface as it heats up. This oxide film prevents metal-to-metal contact with the mating material, suppresses the temperature rise of the tool surface, and provides excellent wear resistance. Therefore, Co may be added as needed, such as when it is necessary to reliably obtain the effects described above. When added, the amount may be greater than 0% but less than 0.3%, and an addition of 0.3% or more is preferable. If the effects of Co described above are not important, or if the effects of Co can be compensated for by other added elements, it is acceptable to omit Co (0%).

[0022] Ti, Zr, Hf, Nb, Ta: 0-0.1% of one or more of the above-mentioned V and lanthanide elements. The group 3 and 4 elements, excluding V and lanthanide elements, form carbides and nitrides, which have the effect of strengthening the matrix and improving wear resistance. They also increase tempering softening resistance and, like V, suppress grain coarsening, contributing to improved toughness. Therefore, these elements may be added as needed. To obtain the above effects, elements may be added individually or in combination of two or more elements. However, since coarse M(C,N) type carbides are formed by combining with V added to the steel during the solidification process in the manufacturing process, and these tend to remain in the final product and reduce toughness, it is preferable to add 0.1% or less. When added, it is also acceptable to add less than 0.01%, but since these elements are unavoidable elements in the steel and may be present in amounts between 0% and less than 0.01%, it is preferable to add 0.01% or more to obtain sufficient effects.

[0023] In this embodiment, components other than those mentioned above are Fe and impurities. Examples of impurity elements include unavoidable impurities as shown below. The main elements that may remain in the steel as unavoidable impurities are Cu, Al, Ca, Mg, O (oxygen), N (nitrogen), etc. In the present invention, it is preferable that these elements be as low as possible. However, on the other hand, it is acceptable to include small amounts in order to obtain additional effects such as control of the morphology of inclusions, other mechanical properties, and improvement of manufacturing efficiency. In this case, a range of Cu ≤ 0.25%, Al ≤ 0.05%, Ca ≤ 0.01%, Mg ≤ 0.01%, O ≤ 0.005%, and N ≤ 0.03% is sufficient and is a preferred regulatory upper limit in the present invention.

[0024] In addition to the unavoidable impurity elements mentioned above, S and P are also elements that remain in steel. Of these, S is an impurity element that is usually unavoidably present in various hot work tool materials, and if it remains in excess, it deteriorates toughness. Therefore, in order to prevent deterioration of the toughness of hot work tools, it is preferable to limit its content to 0.05% or less. On the other hand, as mentioned above, S combines with Mn and exists as the nonmetallic inclusion MnS, which has the effect of improving machinability. When machinability is required for hot work tool steel, it is preferable to add 0.01% or more.

[0025] The impurity phosphorus (P) is usually unavoidably present in various hot tool materials. During heat treatments such as tempering, it segregates at the prior austenite grain boundaries, causing them to become brittle. In particular, when obtaining large hot tools from the hot tool steel of the present invention, it is desirable to minimize the influence of P. To prevent deterioration of the toughness of the hot tool steel obtained in the present invention, it is preferable to limit the amount to 0.05% or less.

[0026] In addition to the unavoidable impurities mentioned above, other elements may be included as impurities, as long as they do not hinder the effects of the present invention. The elements included as impurities vary greatly depending on the raw materials used in steel production, such as iron ore and iron scrap, so examples of elements are not given here. However, for example, the total content of elements included as impurities can be set to 1% or less.

[0027] Next, the manufacturing method of the present invention will be described using embodiments. <Preparation of material for finishing hot forging> First, in this embodiment, a material for finishing hot forging having the composition of the hot tool steel described above is prepared. Hot tool steel having an annealed structure is usually made by using a steel ingot or a steel billet (slab, bloom, billet) obtained by splitting a steel ingot as the material for finishing hot forging, performing various hot working and heat treatments on it to obtain a predetermined forging material, and then performing an annealing treatment on this forging material to finish it into a desired shape such as a block shape. The aforementioned splitting can be done by hot free forging by pressing or hot rolling by rolling. On the other hand, in the case of hot rolling, strain distribution tends to occur in the thickness and width directions at the position where the rolls contact, but because the rolls contact the material uniformly in the rolling direction, strain distribution in the length direction is less likely to occur. For this reason, it is more preferable to apply the manufacturing method of the present invention to hot working by free forging, in which strain distribution tends to occur in the forging direction, i.e., the length direction as well. The steel ingot in this invention can be manufactured through at least one melting process, and may be a product that has undergone a remelting process such as electroslag remelting (ESR) or vacuum arc remelting (VAR).

[0028] In this embodiment, for example, when obtaining a material for finishing hot forging using bloc forging, the bloc should be heated to 1100 to 1250°C, and then a bloc forging process should be carried out. Normally, as the size of the bloc increases, the solidification rate slows down and coarse dendrites grow in the bloc obtained by casting. In addition, there are many voids due to solidification shrinkage in the final solidified portion. In order to break down this dendritic structure and compress the voids, it is preferable to perform bloc forging at a high temperature, so the heating temperature in the bloc forging process of the present invention is set to 1100 to 1250°C. Furthermore, depending on the cross-sectional area of ​​the bloc and the cross-sectional area of ​​the hot tool steel to be ultimately obtained, upsetting may be carried out during the bloc forging process.

[0029] In this embodiment, annealing may be performed following the block forging process. By incorporating annealing into the block forging process, residual stress can be removed from the material for finish hot forging after block forging. The annealing temperature at this time is preferably 600°C or higher. More preferably, the annealing temperature is above the austenite transformation point and below 900°C. By transforming the structure from ferrite to austenite once, it is expected that the austenite grain size in the next process will be adjusted.

[0030] When performing the aforementioned block forging process, soaking, which involves heating the steel ingot to 1250°C or higher, can also be performed before the block forging process. Normally, when steel solidifies, the initial solidified portion has a dilute composition, while the most solidified portion has a concentrated composition and forms coarse eutectic carbides. Furthermore, as the size of the steel ingot increases and the cooling rate of solidification slows down, the eutectic carbides tend to become coarser and the segregation of components worsens. Heating to 1250°C or higher can be expected to solidify the eutectic carbides in the matrix and diffuse the components. Since a similar effect can be expected during the heating and holding process of the steel ingot for the block forging process described above, the soaking process can also be combined with this process. However, since the effect of solidification and segregation diffusion of eutectic carbides is small at lower temperatures, it is preferable to perform this process at a temperature higher than the heating temperature for the block forging process.

[0031] <Finishing Hot Forging Process> Next, the material for finishing hot forging is heated to 1000-1060°C, and a finishing hot forging process is performed in which hot free forging is carried out in multiple passes (multiple forging passes) by physical forging to obtain hot tool steel. As will be described later, "heating the material for finishing hot forging to 1000-1060°C" means heating the material (material during finishing hot forging) so that the central part reaches 1000-1060°C. In the hot free forging process of this invention, a forging pass refers to the process from when the material for finishing hot forging is forged with a press, as shown in Figure 1, starting from when the reduction of the end of the material for finishing hot forging is started, until the reduction of the entire length to the opposite end in the length direction is completed by repeatedly forging. Hereafter, "thickness" refers to the length of the shorter side when the cross-section of the forged material (finished hot-forged material) after finish hot forging is approximated as a rectangle, and "width" refers to the length of the longer side. However, when the cross-section of the finished hot-forged material can be approximated as a square, there is no need to make a particular distinction. In the finishing hot forging process of this embodiment, an appropriate heating temperature can be selected so that the temperature of the central part of the material undergoing finish hot forging, after 300 seconds or more have elapsed since the necessary total strain was applied to the central part of the material undergoing finish hot forging (described later), is in the range of 1000°C to 1060°C. If the heating temperature exceeds 1060°C, there is a risk that the V-type carbides, which are pinning particles, will solid dissolve and crystal grain growth will occur, so it is preferable to set the upper limit of heating to 1060°C. Furthermore, the inventors' studies have revealed that if the heating temperature of the material for finish hot forging and the temperature of the central part of the material undergoing finish hot forging fall below 1000°C, the progress of recrystallization becomes extremely slow. This is thought to be because the amount of heat removed from the surface is greater than the amount of heat generated by plastic deformation, causing the temperature to drop before sufficient strain is applied and enough time has passed. Furthermore, if the heating temperature of the material for finishing hot forging and the temperature of the central part of the material during finishing hot forging fall below 1000°C, there is a concern that a coarse austenite structure will remain if sufficient recrystallization does not occur even when strain is applied. In order to obtain a fine structure at any point in the finishing hot forged material without requiring a heat retention or heating process, it is preferable to set the lower limit of the heating temperature to 1000°C and provide sufficient holding time so that the central part of the material during finishing hot forging reaches 1000°C. More preferably, the upper limit of the heating temperature should be set to 1030°C.This allows for greater flexibility in selecting the reduction amount and forging pass timing, as described later, in addressing factors that are difficult to control during finish hot forging, such as heat generation and cooling in the central part of the material.

[0032] In the finishing hot forging process of the present invention, hot free forging is performed in multiple passes by physical forging so that the total strain in the central part of the material to be finished hot forged is 0.6 or more. In this invention, the "central part of the material to be finished hot forged" refers to the center of the thickness T and width W (T / 2 - W / 2). Therefore, since it is difficult to directly measure the temperature, the strain due to forging in the central part of the material to be finished hot forged, and the temperature changes due to heat dissipation from the material surface and processing heat generation are calculated using simulation analysis. Any commercially available plastic deformation simulation software can be used for the simulation software. In this embodiment, FORGE manufactured by Transvalor is used. Figure 2 shows an example of a forging simulation in which finishing hot forging is performed on a material for finishing hot forging with a thickness T: 400 mm x width W: 620 mm. It can be seen that there is a region where the strain value is small along the length direction L at the center of the thickness T and width W (T / 2 - W / 2). The forging pass is designed to introduce a strain of 0.6 or more in the region where this strain distribution is minimized (the center of the thickness T and width W). This forging pass may be calculated by performing regression analysis on the forging analysis results of multiple different dimensions of finish hot forged materials to determine the amount of reduction required to introduce the target strain and the time between forging passes to process the central temperature in the temperature range of 1000 to 1060°C. In this embodiment, the amount of reduction is adjusted while also considering the cross-sectional area of ​​the material for finish hot forging before finish hot forging, so that the total strain is 0.6 or more in the region where the strain distribution formed by reduction due to finish hot forging is minimized. At this time, both the strain introduced by reduction in the thickness direction and the strain introduced by reduction in the width direction of the material for finish hot forging may be added together as the total strain. Normally, during casting, a portion called a riser exists at the top of the ingot. The riser is the final solidification part of the ingot and is unsuitable as a product due to severe concentration of components and the presence of voids. Therefore, it is usually cut off before becoming the final product and is not used as a hot work tool or mold. There is no need to particularly control the amount of strain at that location. Here, the strain in this invention refers to the equivalent strain that is generally used in plastic deformation simulations. The "total strain" can be evaluated by the cumulative value of the equivalent strain.

[0033] As described above, in this embodiment, recrystallization is promoted and a crystalline grain structure can be obtained by keeping the temperature of the central part of the material undergoing finish hot forging within the range of 1000 to 1060°C for 300 seconds or more. At this point, a strain of 0.6 or more is introduced into the central part of the material undergoing finish hot forging in the finish hot forging process of this embodiment, and even after 300 seconds or more have passed and the temperature of the central part has fallen below 1000°C, further reduction may be applied to adjust the dimensions to those to be used as a mold or tool. However, since a sufficiently fine structure has already been obtained, the energy efficiency of the hot working is poor. Therefore, the cross-sectional area of ​​the material undergoing finish hot forging may be changed with the guideline that the total strain in the central part after all hot working is completed should be 1.0 or less.

[0034] In this embodiment, the "300 seconds" in the recrystallization acceleration step (and the intermediate recrystallization acceleration step described later) can include the time from the moment when strain is introduced into the material for finishing hot forging due to reduction until the reduction in one pass is completed along its entire length. This time varies depending on the stroke speed of the press used for hot free forging, the length of contact between the material for finishing hot forging and the anvil, and the total length of the material for finishing hot forging. Furthermore, the more the cross-sectional area of ​​the material for finishing hot forging decreases due to reduction in thickness and width, and the more it is forged in the length direction, the longer the time until the next forging pass is completed. For example, when hot free forging a material for finishing hot forging weighing 10 tons, it takes approximately 20 to 180 seconds to complete one forging pass. Furthermore, light reduction may be applied to correct the shape of the corners and end faces of the material undergoing finish hot forging. However, if it is predicted that the temperature will exceed 1060°C due to processing heat, it is preferable to allow the material undergoing finish hot forging or the finished hot forged material to cool by standing still. The surface of the material undergoing finish hot forging falls below 1000°C earlier than the center, but it is under greater strain than the center. Therefore, recrystallization is completed earlier, and the temperature drops, preventing grain growth. As a result, when quenched and tempered, the prior austenite grain size in the center of the hot tool can be made finer, similar to the center.

[0035] Another embodiment of the present invention is a method for manufacturing hot tool steel, comprising: an intermediate recrystallization acceleration step in which, when the total amount of strain applied to the central part of the material during the finishing hot forging process by one or more forging passes is 0.3 or more and less than 0.6, the temperature of the central part is kept in the range of 1000 to 1060°C for 120 seconds or more before the forging pass that introduces the next strain; and a recrystallization acceleration step after the intermediate recrystallization acceleration step, when the total amount of strain applied to the central part of the material during the finishing hot forging process reaches 0.6, the temperature of the central part is kept in the range of 1000 to 1060°C for 120 seconds or more, wherein the total elapsed time of the intermediate recrystallization acceleration step and the recrystallization acceleration step is 300 seconds or more. By performing the aforementioned intermediate recrystallization acceleration process and recrystallization acceleration process, the recrystallization acceleration effect can be achieved in the same way as when only the recrystallization acceleration process is performed, resulting in finer grains of prior austenite in the center of the hot tool after quenching and tempering. There is no particular upper limit on the time from the intermediate recrystallization acceleration process to the next forging pass that introduces strain, but it is necessary to perform the recrystallization acceleration process that introduces a total strain of 0.6 or more before the temperature of the center of the material undergoing finish hot forging falls below 1000°C due to cooling from the surface. Therefore, it is preferable to adjust the timing of the next pass from the aforementioned simulation and regression equation. Figure 3 shows the forging pass, strain in the center of the material undergoing finish hot forging, and temperature changes during an example of the present invention. The temperature is below 1000°C 390 seconds after the strain reaches 0.6, and the present invention is satisfied under the conditions described in Figure 3. In the intermediate recrystallization acceleration process, the forging time is 167 seconds and the standing time is 223 seconds out of the elapsed time of 390 seconds. Figure 4 shows the forging path, strain in the center of the material during finish hot forging, and temperature changes in a comparative example that deviates from the present invention. In Figure 4, the temperature falls below 1000°C 23 seconds after a strain of 0.6 has been introduced, so the conditions of the present invention are not met.

[0036] The present invention relates to a method for manufacturing hot tool steel, wherein the cross-sectional area of ​​the material for finishing hot forging is 120,000 mm². ２ Preferably, the cross-sectional area of ​​the material for finishing hot forging is 400,000 mm².２ The present invention is particularly useful in the above-mentioned situations. Even with large materials as described above, the manufacturing method of the present invention makes it possible to stably obtain a fine structure. Furthermore, the upper limits of the cross-sectional area and length of the material for finishing hot forging in the manufacturing method of the present invention are determined by the weight of the steel ingot. As a guideline, it is preferable to set the upper limits to the cross-sectional area and length obtained from a 30-ton steel ingot.

[0037] In the method for manufacturing hot work tool steel according to the present invention, it is preferable to perform annealing on the finished hot forged material after the finish hot forging process. By combining annealing with the finish hot forging process, residual stress is removed from the forged material (hot work tool steel) after finish hot forging, and the hardness is sufficiently reduced, making it easier to machine it into the shape of a hot work tool such as a mold, as described later. The annealing temperature at this time is preferably 600°C or higher. Furthermore, it is preferable that the annealing temperature be above the austenite transformation point and below 900°C.

[0038] The hot work tool steel obtained by the manufacturing method of the present invention is prepared into a martensitic or bainite structure with a predetermined hardness by quenching and tempering, and then prepared into a hot work tool product. The hot work tool steel is then shaped into a hot work tool by various machining processes such as cutting and drilling. The timing of the machining is preferably when the material is in a low hardness state (annealed state) before quenching and tempering. In this case, finishing machining may be performed after quenching and tempering. In some cases, machining may be performed in a pre-hardened state after quenching and tempering.

[0039] The aforementioned quenching and tempering temperatures vary depending on the composition of the hot work tool steel and the target hardness, but it is preferable that the quenching temperature is approximately 1000 to 1100°C and the tempering temperature is approximately 500 to 650°C. If the quenching temperature is too high, the pinned carbides mentioned above will dissolve into the matrix and grain growth will occur, so it is preferable to set the temperature so that the pinned carbides remain sufficiently intact. For example, in the case of SKD61, a representative type of hot work tool steel, the quenching temperature is approximately 1000 to 1030°C and the tempering temperature is approximately 550 to 650°C. The quenched and tempered hardness is preferably 50 HRC or less, more preferably 48 HRC or less.

[0040] A hot-working tool obtained by quenching and tempering the hot-working tool steel produced by the production method of the present invention comprises, by mass%: 0.25 to 0.50% of C, 0.1 to 1.2% of Si, 0.2 to 0.9% of Mn, more than 0% and 0.6% or less of Ni, 3.8 to 5.5% of Cr, Mo and W alone or in combination satisfying (Mo + 1 / 2W): 1.1 to 2.7%, 0.3 to 1.2% of V, with the balance being Fe and impurities, and has a cross-sectional area of 120000 mm ２ or more and a length of 1000 mm or more, and the maximum grain size number of prior austenite grains at the longitudinal center portion of the hot-working tool is 5.5 or more. Through the production method of the present invention described above, even for large hot-working tools ２ having a cross-sectional area of 120000 mm<2> or more and a length of 1000 mm or more, the maximum grain size of prior austenite grains at the center portion of the hot-working tool can be refined to obtain a more uniform structure. Here, the maximum grain size in the present invention may be measured by a method in accordance with ASTM-E930. More specifically, unnecessary portions are cut from both ends of the hot-working tool steel obtained by implementing the production method of the present invention, the steel is sliced at a position about 15 mm from both end faces of the sample to include the obtained cross-section to collect two plate-shaped test pieces, the two test pieces that have been quenched or quenched and tempered are measured, and if the maximum grain size of both test pieces is 5.5 or more, the product falls within the scope of the present invention. Preferably, a plate-shaped test piece is further collected from near the center of the steel material, and the maximum grain size of all three plate-shaped test pieces is 5.5 or more.

[0041] In addition to the aforementioned specification of the maximum grain size number, the hot-working tool of the present invention preferably has an average grain size number of 7.0 or more. A more preferable average grain size is 7.5 or more. The average grain size in the present invention can be measured by a method in accordance with JIS-G0551 (ASTM-E112).

[0042] Using a 10-ton arc melting furnace, an ingot of hot work tool steel having the component composition shown in Table 1 was melted. After subjecting this ingot to soaking held at a temperature of 1250°C or higher, it was bloomed at 1180°C, and then annealed at 870°C to obtain a material for finish hot forging. The chemical composition can be analyzed in accordance with the analytical methods specified in JIS and ASTM. For C, S, N, and O, analysis can be performed by combustion-infrared absorption method and inert gas fusion-infrared absorptiometry in accordance with ASTM-E1019, and for other elements, component analysis can be performed by emission spectrometry and X-ray fluorescence spectrometry in accordance with JIS-G0320 or G0321. The forged material obtained by subjecting the material for finish hot forging to the steps shown in Table 2 was annealed at 870°C to obtain a hot work tool steel as an example of the present invention. The conditions for the finish hot forging step by hot open die forging in the example of the present invention were implemented in accordance with an approximate expression obtained in advance for conditions that allow finish hot forging to be performed while keeping the temperature at the center of the material within the range of 1000 to 1060°C during finish hot forging. In the comparative example, the condition was set such that the total elapsed time of the intermediate recrystallization promotion step and the recrystallization promotion step is less than 300 seconds. The elapsed time of the intermediate recrystallization promotion step and the recrystallization promotion step is the sum of the finish hot forging time and the standing time. For example, in the intermediate recrystallization promotion step of Inventive Example 1, among the 510 seconds of elapsed time, the finish hot forging time is 60 seconds and the standing time is 450 seconds. Further, in the intermediate recrystallization promotion step of Inventive Example 2, among the 213 seconds of elapsed time, the finish hot forging time is 98 seconds and the standing time is 115 seconds. Regarding the relationship between the equivalent strain at the center of the material of Inventive Example 1 and the forging time, details of the finish hot forging conditions are shown in FIG. 5, and the relationship for Inventive Example 2 is shown in FIG. 6. The equivalent strain shown in FIG. 5 and FIG. 6 is the equivalent strain cumulatively applied to the center of the material during the finish hot forging step, and is a value for evaluating the total strain amount.

[0043]

[0044]

[0045] Next, the cross-sectional microstructure of the manufactured hot tool steel (samples) No. 1 to No. 2 and No. 11 was observed. The cross-sectional observation locations are shown in Figure 7. The cross-sections observed were obtained by cutting off the unnecessary portions from both ends of the forged hot tool steel. Then, two plate-shaped test pieces were taken by slicing the sample approximately 15 mm from both end faces, including the obtained cross-sections. Furthermore, for No. 2 and No. 11, plate-shaped test pieces were taken from near the center in the L direction. In addition, test pieces of approximately 10 mm to 20 mm were cut from the center W / 2-T / 2 and W / 8-T / 8 of the plate-shaped test pieces. The test pieces cut above were subjected to quenching heat treatment at 1030°C, and the prior austenite grain size was evaluated using the average grain size number and the maximum grain size number. The plane parallel to the forging direction (i.e., the length direction of the sample) was observed. The average grain size number was measured according to JIS-G0551 (ASTM-E112), and the maximum grain size number was measured according to ASTM-E930. The grain size number results are shown in Table 3.

[0046]

[0047] Hot tool steels No. 1 and No. 2, manufactured under the finishing hot forging conditions of the present invention, were confirmed to have a maximum grain size number of 5.5 or higher even in the center of the forged material. On the other hand, No. 11, manufactured under the finishing hot forging conditions of the comparative example, was found to have a maximum grain size number of 4.5 to 5.0 in the center of the forged material depending on the location. Furthermore, the difference between the average grain size number and the maximum grain size number is smaller in the examples of the present invention than in the comparative example, confirming that a fine structure is uniformly generated. It should be noted that the grain size does not change before and after tempering, as confirmed in prior studies.

Claims

1. A finishing hot forging material is prepared, comprising, by mass%, C: 0.25-0.50%, Si: 0.1-1.2%, Mn: 0.2-0.9%, Ni: greater than 0% and less than or equal to 0.6%, Cr: 3.8-5.5%, Mo and W individually or in combination (Mo + 1 / 2W): 1.1-2.7%, V: 0.3-1.2%, with the remainder being Fe and impurities; and a finishing hot forging process is provided, in which the finishing hot forging material is heated to 1000-1060°C, and then hot free forging is performed in one or more forging passes by physical forging. A method for manufacturing hot tool steel, comprising a recrystallization acceleration step in which, after the total strain applied to the central part of the material during the finishing hot forging process reaches 0.6, the central part temperature is kept within the range of 1000 to 1060°C for 300 seconds or more.

2. A finishing hot forging material is prepared, comprising, by mass%, C: 0.25-0.50%, Si: 0.1-1.2%, Mn: 0.2-0.9%, Ni: greater than 0% and less than or equal to 0.6%, Cr: 3.8-5.5%, Mo and W individually or in combination (Mo + 1 / 2W): 1.1-2.7%, V: 0.3-1.2%, with the remainder being Fe and impurities; a finishing hot forging process is provided, in which the finishing hot forging material is heated to 1000-1060°C, and then hot free forging is performed in one or more forging passes by physical forging; and in the finishing hot forging process, when the total strain applied to the central part of the material during finishing hot forging by one or more forging passes is 0.3 or more and less than 0.6, A method for manufacturing hot tool steel, comprising: an intermediate recrystallization acceleration step of allowing 120 seconds or more while keeping the central temperature within the range of 1000 to 1060°C until the next forging pass to introduce strain; and a recrystallization acceleration step after the intermediate recrystallization acceleration step, where, after the total amount of strain applied to the central part of the material during the finishing hot forging step reaches 0.6, the central temperature is kept within the range of 1000 to 1060°C for 120 seconds or more, wherein the total elapsed time of the intermediate recrystallization acceleration step and the recrystallization acceleration step is 300 seconds or more.

3. The cross-sectional area of ​​the material for hot forging is 120,000 mm². ２ The method for manufacturing hot work tool steel according to claim 1 or 2, wherein the length is 1000 mm or more.

4. A hot work tool comprising, by mass%, C: 0.25-0.50%, Si: 0.1-1.2%, Mn: 0.2-0.9%, Ni: greater than 0% and less than or equal to 0.6%, Cr: 3.8-5.5%, Mo and W individually or in combination (Mo + 1 / 2W): 1.1-2.7%, V: 0.3-1.2%, with the remainder being Fe and impurities, having a cross-sectional area of ​​120,000 mm². ２ A hot work tool having a length of 1000 mm or more and a maximum grain size number of 5.5 or higher for the prior austenite grain size.

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