Matrix high-speed tool steel having excellent softening resistance and toughness

Abstract

The purpose of the present invention is to provide a matrix high-speed tool steel having high softening resistance and a high fracture toughness value. In order to achieve the purpose, provided is a matrix high-speed tool steel comprising, in mass%, 0.45-0.60% C, 0.05-0.30% Si, 0.20-1.20% Mn, 0.01-1.00% Ni, 2.00-3.00% Cr, 0.003-0.100% Al, 0.80-1.30% V, 0.01-0.30% Nb, 2.00-4.00% Mo+0.5W, 0.003-0.040% N, and Fe and unavoidable impurities as the remainder. The unavoidable impurities have a P content of 0.050 mass% or less, an S content of 0.010 mass% or less, a Cu content of 0.30 mass% or less, and an O content of 0.050 mass% or less. The matrix high-speed tool steel has an areal primary-carbide proportion F of 0.40-0.70% and has a hardness after quenching and tempering of 55 HRC or greater.

Description

Matrix high speed steel excellent in softening resistance and toughness

[0001] The present invention relates to matrix high speed steel suitable for warm or hot dies.

[0002] "Matrix high speed steel" has the base composition of SKH51 (see JIS G 4403), which is a general-purpose high speed tool steel, as the basic component. Since the composition of the base (matrix) is comparable to that of high speed tool steel, while maintaining high strength, it is a steel material with toughness by reducing large carbides during solidification. Matrix high speed steel has been widely used as a press die material.

[0003] Regarding matrix high speed steel, the applicant has proposed matrix high speed steel suitable for nitriding treatment, which is characterized in that, in mass%, C: 0.4 to 0.7%, Mn: 0.1 to 0.6%, Cr: 3.3 to 5.0%, Mo: 3.0 to 4.6%, Al: 0.01 to 0.10%, N: 0.030% or less, O: 0.0030% or less, Si / Mo: 0.005 to 0.100, V / Mo: 0.05 to 1.00, the balance being Fe and inevitable impurities, is quenched at 1080°C or higher, tempered at 500°C or higher, and then nitrided at 400 to 600°C, with the base metal hardness being 55 HRC or higher and the crystal grain size number being 3 or more (see Patent Document 1).

[0004] Further, the applicant has, in mass%, C: 0.55 to 0.75%, Si: 0.50 to 1.50%, Mn: 0.10 to 1.00%, Cr: 3.00 to 5.00%, Mo: 2.00 to 3.00% and W: 2.00% or less, within the range of 2Mo + W: 4.00 to 6.00%, V: 0.80 to 1.30%, P: 0.030% or less, S: 0.01% or less, O: 0.0050%, N: 0.0300 or less, the balance being Fe and inevitable impurities, and the carbides remaining inside, with an equivalent circle diameter of 2.0 μm or more of the MX type or / and M

[0004] , X (X is C or N) type area ratio 1 μm 2This invention proposes a matrix high-speed steel (HSS) with excellent toughness and high strength, characterized by a matrix HSS in the form of a steel ingot with a material content of 3.0% or less per unit area, in which the steel ingot is hot-forged to a diameter of 140 mm, which is the forging ratio of 6S, the steel material is held at 1140°C for 10 minutes, quenched by immersing it in 50°C oil, and then tempered by repeating the operation of holding it at 480-620°C for 60 minutes and then air-cooling three times, resulting in a hardness of 62 HRC or higher (see Patent Document 2).

[0005] Patent No. 4738912 Patent No. 6797465

[0006] Patent Document 1 proposes a matrix high-speed steel suitable for nitriding treatment, specifically a matrix high-speed steel having sufficient toughness in both the nitrided layer and the base material after nitriding, and aims to address premature cracking caused by damage to the nitrided layer. Patent Document 2 proposes a matrix high-speed steel having high toughness and high-temperature strength by adjusting the carbide-forming elements and reducing coarse carbides.

[0007] In applications involving high thermal and mechanical loads, such as hot-working molds, steel materials must be able to withstand softening due to heat from the workpiece, as well as plastic deformation and wear of the surface due to friction with the workpiece. Furthermore, heat can cause cracks and heat checks, and the propagation of these cracks can lead to large cracks in the mold, posing a significant problem.

[0008] Important properties of mold materials include resistance to softening and toughness against crack propagation, which are crucial for preventing large cracks.

[0009] However, conventional methods are still insufficient to achieve both resistance to softening of mold materials and toughness against crack propagation, and there is a problem in that they cannot achieve a sufficient lifespan in harsh applications such as hot and cold molds.

[0010] Therefore, the present invention aims to provide matrix high-speed steel with higher softening resistance and fracture toughness than conventional materials by optimizing the amount of Cr to suppress the coarsening of secondary carbides and optimizing the prior ausnite particle size, thereby addressing the challenge of achieving both softening resistance and toughness against crack propagation.

[0011] As a result of diligent research, the inventors have found that by controlling the Cr content to 2-3%, the coarsening of secondary carbides due to heat is suppressed, thereby increasing softening resistance and improving toughness. Furthermore, the inventors have found that if there is an excess of primary carbides, the grain size becomes excessively fine, and some of the secondary carbides precipitate at the grain boundaries, resulting in a decrease in both toughness and softening resistance. Therefore, they concluded that it is useful to define an appropriate range for the area ratio of primary carbides.

[0012] To solve the above problems, the present invention provides a matrix high-speed steel comprising, by mass%, C: 0.45 to 0.60%, Si: 0.05 to 0.30%, Mn: 0.20 to 1.20%, Ni: 0.01 to 1.00%, Cr: 2.00 to 3.00%, Al: 0.003 to 0.100%, V: 0.80 to 1.30%, Nb: 0.01 to 0.30%, Mo + 0.5W: 2.00 to 4.00%, N: 0.003 to 0.040%, and the remainder being Fe and unavoidable impurities, wherein the P, S, Cu, and O content in the unavoidable impurities is P: 0.050% or less, S: 0.010% or less, Cu: 0.30% or less, and O: 0.050% or less, respectively. The present invention provides a matrix high-speed steel having a primary carbide area ratio F of 0.40 to 0.70%, and a quenched and tempered hardness of 55 HRC or higher.

[0013] In this invention, "quenched and tempered hardness" refers to the hardness obtained after repeating a tempering treatment three times, in which the steel material is austenitized at 1130°C, oil-cooled, and then held at a temperature of 550°C or higher for at least one hour before air-cooling.

[0014] The matrix high-speed steel of the present invention has a hardened hardness of 55.0 HRC or higher, a Rockwell hardness of 36.5 HRC or higher as measured by a softening resistance test, demonstrating excellent resistance to softening due to heat, and a fracture toughness value of 24 MPa·√m or higher as measured by a fracture toughness test, demonstrating excellent toughness. Therefore, it is a steel material that combines both softening resistance and toughness against crack propagation, making it suitable for hot or warm molds.

[0015] <Component Composition> The component composition of the Matrix High Speed ​​Steel of the present invention is described below. Note that "%" used as the unit for the content of each element means mass percent.

[0016] The matrix high-speed steel of the present invention contains C, Si, Mn, Ni, Cr, Al, V, Nb, Mo, W, and N as essential elements. In the matrix high-speed steel of the present invention, the remainder other than the essential elements is Fe and unavoidable impurities.

[0017] C: 0.45-0.60% C is a component necessary to obtain room temperature hardness. If the amount of C is insufficient, sufficient hardness cannot be obtained. Therefore, the lower limit of the C content is set to 0.45% or more. Preferably, the lower limit of the C content is 0.48% or more. On the other hand, if the amount of C is excessive, toughness decreases. Therefore, the upper limit of the C content is set to 0.60% or less. Preferably, the upper limit of the C content is 0.58% or less. Regarding the C content, each of the above lower limits may be combined with any of the above upper limits, and the range defined by these combinations is included in the present invention.

[0018] Si: 0.05-0.30% Si is a useful component for deoxidation. If the Si content is too low, the deoxidation effect may not be sufficiently obtained. Therefore, the lower limit of the Si content is set to 0.05% or more. Preferably, the lower limit of the Si content is 0.07% or more. On the other hand, if the Si content is too high, the toughness decreases. Therefore, the upper limit of the Si content is set to 0.30% or less. Preferably, the upper limit of the Si content is 0.20% or less. The lower limits of the Si content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is included in the present invention.

[0019] Mn: 0.20 to 1.20% Mn is a component that improves hardenability. To obtain hardenability, the lower limit of the Mn content is 0.20% or more. Preferably, the lower limit of the Mn content is 0.30% or more, and more preferably 0.40% or more. On the other hand, if there is too much Mn, toughness decreases. Therefore, the upper limit of the Mn content is 1.20% or less. Preferably, the upper limit of the Mn content is 1.10% or less. The lower limit of the Mn content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is included in the present invention.

[0020] Ni: 0.01 to 1.00% Ni is a component that improves hardenability. To obtain hardenability, the lower limit of the Ni content is 0.01% or more. Preferably, the lower limit of the Ni content is 0.02% or more, more preferably 0.04% or more. On the other hand, if there is too much Ni, the resistance to softening decreases. Therefore, the upper limit of the Ni content is 1.00% or less. Preferably, the upper limit of the Ni content is 0.80% or less, more preferably 0.70% or less. The lower limit of the Ni content may be combined with any of the upper limits above, and the range defined by these combinations is included in the present invention.

[0021] Cr: 2.00-3.00% Cr is a component that improves hardenability. Cr is also a component that forms primary and secondary carbides. By controlling the Cr content to 2.00-3.00%, the coarsening of secondary carbides due to heat can be suppressed, improving softening resistance and toughness. If Cr is too low, hardenability and hardness decrease. Therefore, the lower limit of the Cr content is set to 2.00% or more. Preferably, the lower limit of the Cr content is 2.10% or more. On the other hand, if Cr is too high, softening resistance decreases. Therefore, the upper limit of the Cr content is set to 3.00% or less. Preferably, the upper limit of the Cr content is 2.90% or less. The lower limits of the Cr content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is encompassed within the present invention.

[0022] Al: 0.003 to 0.100% Al is a useful component for deoxidation. If the amount of Al is too low, the deoxidation effect may not be sufficient. Therefore, the lower limit of the Al content is set to 0.003% or more. Preferably, the lower limit of the Al content is 0.005% or more, and more preferably 0.008% or more. On the other hand, if there is too much Al, coarse Al oxides are formed, and toughness decreases. Therefore, the upper limit of the Al content is set to 0.100% or less. Preferably, the upper limit of the Al content is 0.080% or less. The lower limits of the Al content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is included in the present invention.

[0023] V: 0.80-1.30% V is a component that forms primary and secondary carbides. If V is low, the resistance to softening decreases. Therefore, the lower limit of the V content is set to 0.80% or more. Preferably, the lower limit of the V content is 0.90% or more. On the other hand, if V is excessive, coarse carbides are formed, and toughness decreases. Therefore, the upper limit of the V content is set to 1.30% or less. Preferably, the upper limit of the V content is 1.20% or less, more preferably 1.10% or less. The lower limits of the V content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is included in the present invention.

[0024] Nb: 0.01 to 0.30% Nb is a component that forms primary and secondary carbides. If Nb is low, the resistance to softening decreases. Therefore, the lower limit of the Nb content is set to 0.01% or more. Preferably, the lower limit of the Nb content is 0.02% or more, and more preferably 0.05% or more. On the other hand, if Nb is excessive, coarse carbides are formed, and toughness decreases. Therefore, the upper limit of the Nb content is set to 0.30% or less. Preferably, the upper limit of the Nb content is 0.20% or less. The lower limit of the Nb content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is included in the present invention.

[0025] Mo + 0.5W: 2.00-4.00% Mo and W are components that form primary and secondary carbides. However, the effect of Mo is twice as strong as that of W, and twice the amount of W is needed to obtain the same effect. The effect of both components can be expressed in terms of Mo equivalent (Mo + 0.5W). Mo + 0.5W means (Mo content) + 0.5 × (W content). If the amount of Mo + 0.5W is low, the softening resistance decreases. Therefore, the lower limit of Mo + 0.5W is set to 2.00% or more. Preferably, the lower limit of Mo + 0.5W is 2.50% or more, and more preferably 2.80% or more. If there is too much Mo + 0.5W, coarse carbides are formed, and the toughness decreases. Therefore, the upper limit of Mo + 0.5W is set to 4.00% or less. The upper limit of Mo + 0.5W is preferably 3.80% or less. The lower limit of Mo + 0.5W may be combined with any of the upper limits, and the ranges defined by these combinations are all included in the present invention.

[0026] N: 0.003 to 0.040% N dissolves in a portion of the carbide and becomes a primary carbide as carbonitride. This carbonitride suppresses extreme grain coarsening. Therefore, the lower limit of the N content is set to 0.003% or more. Preferably, the lower limit of the N content is 0.005% or more, and more preferably 0.007% or more. On the other hand, if there is too much N, coarse carbonitride is formed, and toughness decreases. Therefore, the upper limit of the N content is set to 0.040% or less. Preferably, the upper limit of the N content is 0.030% or less, and more preferably 0.025% or less. The lower limits of the N content mentioned above may be combined with any of the upper limits mentioned above, and the range defined by these combinations is included in the present invention.

[0027] Remainder: Fe and unavoidable impurities. The remainder of the matrix high-speed metal is Fe and unavoidable impurities. Examples of unavoidable impurities include P, S, Cu, and O. The unavoidable impurities may contain one or more selected from P, S, Cu, and O. The content of P, S, Cu, and O in the unavoidable impurities is as follows:

[0028] P: 0.050% or less. P is an unavoidable impurity. Excessive P reduces toughness. Therefore, the upper limit of the P content is set to 0.050% or less. Preferably, the upper limit of the P content is 0.040% or less, and more preferably 0.030% or less.

[0029] S: 0.010% or less. S is an unavoidable impurity. Excessive S reduces toughness. Therefore, the upper limit of the S content is set to 0.010% or less. Preferably, the upper limit of the S content is 0.008% or less, and more preferably 0.005% or less.

[0030] Cu: 0.30% or less. Cu is an unavoidable impurity component that is mixed in as an impurity in the raw material. If there is too much Cu, the hot workability will decrease. Therefore, the upper limit of the Cu content is set to 0.30% or less. Preferably, the upper limit of the Cu content is 0.20% or less, and more preferably 0.15% or less.

[0031] O: 0.050% or less. O is an unavoidable impurity. Too much O reduces toughness. Therefore, the upper limit of the O content is 0.050% or less. Preferably, the upper limit of the O content is 0.020% or less, and more preferably 0.010% or less.

[0032] <Primary Carbide Area Ratio F> The primary carbide area ratio F of the matrix high-speed material of the present invention will be described below.

[0033] Primary carbide area ratio F: 0.40-0.70% "Primary carbides" are carbides that do not dissolve in austenite during austenitization and remain after quenching. Primary carbides are mainly expressed in stoichiometric ratios of MX. "M" is a transition metal, containing large amounts of V and Nb, and also containing Fe, Cr, Mo and Wo. "X" is C or N. Primary carbides mainly function as pinning particles against grain coarsening. If there are too few primary carbides, the pinning effect is not obtained, and the grain size becomes excessively coarse. If there are too many primary carbides, the grain size becomes excessively fine, and some of the secondary carbides precipitate at the grain boundaries, reducing both toughness and softening resistance. "Secondary carbides" are carbides that precipitate during the tempering treatment after quenching. Secondary carbides are mainly MX or M ２It is represented by X. The secondary carbide is a nano-order carbide or carbonitride. The secondary carbide mainly has the effect of increasing softening resistance. From these viewpoints, it is useful to set the area ratio F of the primary carbide within an appropriate range. Therefore, the area ratio F of the primary carbide is set to 0.40 to 0.70%. The area ratio F of the primary carbide is preferably 0.45 to 0.70%, more preferably 0.50 to 0.70%.

[0034] The primary carbide area ratio F can be measured by the method described in the examples.

[0035] <Hardness after quenching and tempering> The hardness after quenching and tempering of the matrix high-speed steel of the present invention will be described below.

[0036] Quenching and tempering hardness: 55 HRC or higher. Considering the hardness required when used as a warm or hot mold, the quenching and tempering hardness shall be 55 HRC or higher. Preferably, the quenching and tempering hardness shall be 56 HRC or higher, more preferably 57 HRC or higher. There is no particular upper limit to the quenching and tempering hardness. For example, the upper limit of the quenching and tempering hardness may be 65 HRC or lower.

[0037] The hardness after quenching and tempering can be measured by the method described in the examples.

[0038] <Grain Size> The grain size of the matrix high-speed steel of the present invention will be described below.

[0039] The grain size is a guideline for evaluating whether the amount of primary carbides is within an appropriate range, as too little primary carbides can easily lead to coarsening, while too much primary carbides can easily lead to excessive refinement. The lower limit of the grain size of the matrix high-speed steel of the present invention is preferably 30 μm or more, more preferably 35 μm or more, and even more preferably 40 μm or more. The upper limit is preferably 60 μm or less, more preferably 55 μm or less, and even more preferably 50 μm or less. The lower limit of the grain size may be combined with any of the upper limits, and any range defined by these combinations is included in the present invention.

[0040] The grain size can be measured by the method described in the examples.

[0041] Hereinafter, examples of the present invention will be described.

[0042] <Method for Producing Matrix High-Speed Steel> For each of Examples 1 to 7 and Comparative Examples 1 to 4, 100 kg of steel having the chemical components shown in Table 1 (the balance being Fe and inevitable impurities) was melted using a vacuum melting furnace to produce a 100-kg steel ingot.

[0043] Thereafter, the melted ingot was subjected to a homogenization treatment of holding at 1230°C for 6 h, then heated to 1100°C and forged with a forging ratio of 5. Thereafter, spheroidizing annealing was performed at 870°C, and the obtained forged material was used as a steel material.

[0044] For the steel material, the following heat treatment was performed as hardening and tempering. Hardening: After holding at 1130°C for 15 min, it was oil-cooled. Tempering: After holding at 560°C for 1 h, the tempering treatment of air-cooling was repeated 3 times.

[0045]

[0046] <Method for Measuring Area Ratio F of Primary Carbide> The method for measuring the area ratio F of primary carbide is as follows. After holding at 1130°C for 15 min and then oil-cooling, the steel material after hardening and before tempering was mirror-polished, and five SEM images were obtained at a magnification of 2000 using a scanning electron microscope (SEM). The area ratio F of primary carbide in the present invention is calculated by the point counting method. That is, equally spaced grids are overlaid on each SEM image, and the percentage ((total number of primary carbides located at grid points (cross positions of meshes)) / (total number of grid points (measurement points)) × 100) of the total number of primary carbides located at grid points to the total number of grid points is obtained, and this is taken as the area ratio (%) of primary carbide in each SEM image. Specifically, for each SEM image obtained at a magnification of 2000, equally spaced grids with a mesh area of 5 μm ２ and a total number of grid points of 609 are overlaid to obtain the area ratio (%) of primary carbide in each SEM image. The average value of the area ratios of primary carbide in the five SEM images is taken as the primary carbide area ratio F (%).

[0047] <Method for Measuring Grain Size> The method for measuring grain size is as follows. Grain size is determined in accordance with JIS G 0551. After holding the steel material at 1130°C for 15 min and then oil-cooling it, the average grain size is determined from the average line segment length using the cutting method. Grain size is a guideline for evaluating whether the amount of primary carbides is within an appropriate range, as too little primary carbides can easily lead to coarsening, and too much primary carbides can easily lead to excessive fineness.

[0048] <Method for Measuring Hardness After Quenching and Tempering> The method for measuring hardness after quenching and tempering is as follows: Hardness after quenching and tempering is measured by using a 15 mm square steel material (test piece) after quenching and tempering treatment, and measuring the Rockwell hardness of the surface that has been ground down by 0.5 mm or more according to JIS Z 2245. If the Rockwell hardness is 55 HRC or higher, it is evaluated as having excellent hardness after quenching and tempering.

[0049] <Plane Strain Fracture Toughness Test> The method for the plane strain fracture toughness test is as follows: The plane strain fracture toughness test is performed in accordance with ASTEM E399. First, a CT test specimen is cut from the T-L direction of the steel material, and after quenching and tempering, it is finished. The "T-L direction" refers to the direction in which the CT test specimen is taken, in which a load is applied in the width direction (T direction) of the steel material and the crack propagates in the longitudinal direction (L direction). The fracture toughness value at room temperature is measured using the obtained test specimen. If the fracture toughness value is 24 MPa·√m or higher, it is evaluated as having excellent fracture toughness.

[0050] <Softening Resistance Test> The softening resistance test is performed as follows: A 15 mm square steel specimen (test piece) that has undergone quenching and tempering treatment is held at 650°C for 30 hours in an atmospheric furnace and then air-cooled. Using the air-cooled specimen, the Rockwell hardness at room temperature is determined for the surface that has been ground to a thickness of 0.5 mm or more, in accordance with JIS Z 2245. If the Rockwell hardness is 36.5 HRC or higher, it is evaluated as having excellent softening resistance.

[0051] For the steel materials of Examples 1 to 7 and Comparative Examples 1 to 4, the primary carbide area ratio F (%), grain size (μ), quenched and tempered hardness (HRC), fracture toughness (MPa·√m), and softening resistance (HRC) were measured using the procedure described above. The results are shown in Table 2. Underlined values ​​indicate deviations from the provisions of the present invention.

[0052]

[0053] Examples 1 to 7 satisfy the requirements of the present invention in terms of component composition, primary carbide area ratio F, and quenched and tempered hardness, resulting in excellent fracture toughness of 24 MPa·√m or higher and excellent softening resistance of 36.5 HRC or higher.

[0054] Comparative Example 1 had excess Cr and V, a large amount of primary carbides, excessively fine grain size, low fracture toughness, and poor toughness. Comparative Example 2 had insufficient Cr, a small amount of primary carbides, coarse grain size, and poor quench and temper hardness. Comparative Example 3 had excess Cr and poor softening resistance. Comparative Example 4 had excess Si and Mn, low fracture toughness, and poor toughness.

Claims

1. A matrix high-speed steel comprising, by mass%, C: 0.45-0.60%, Si: 0.05-0.30%, Mn: 0.20-1.20%, Ni: 0.01-1.00%, Cr: 2.00-3.00%, Al: 0.003-0.100%, V: 0.80-1.30%, Nb: 0.01-0.30%, Mo+0.5W: 2.00-4.00%, N: 0.003-0.040%, and the remainder being Fe and unavoidable impurities, wherein the P, S, Cu, and O content in the unavoidable impurities is P: 0.050% or less, S: 0.010% or less, Cu: 0.30% or less, and O: 0.050% or less, respectively. The matrix high-speed steel having a primary carbide area ratio F of 0.40 to 0.70%, and a quenched and tempered hardness of 55 HRC or higher.

Images