Cold work tool steel and tools with excellent surface treatment properties

Abstract

To provide a cold tool steel excellent in surface treatment characteristics such as toughness and peeling resistance.SOLUTION: A cold tool steel comprises, % by mass, C: 0.7 to 1.0%, Si: 0.3 to 1.0%, Mn: 0.5% or less, Cr: 3.5 to 9.0%, Mo: 1.5 to 6.0%, W: 8.0% or less, V and Nb: 0.2 to 2.0% at V + Nb / 2, N: less than 500 ppm, and the balance consisting of Fe and unavoidable impurities, in which a value of formula S (2.0Cr + 14.0Mo + 3.3W + 6.7 (V + Nb / 2)) is 35 or more, a value of formula I (70Si-Cr + 63Mo-153V + 23W) is 150 or more, and steel material hardness is 58 HRC or larger in a quenched and tempered state.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to cold-work tool steel, and to dies and tools in which this cold-work tool steel is used as a base material and is subjected to surface treatment by nitriding and PVD (physical vapor deposition) methods. [Background technology]

[0002] In order to reduce the weight of various components and improve production efficiency, workpiece materials are becoming harder and closer to the net shape. This has led to demands for cold forming dies and tools with even greater durability.

[0003] One method for dramatically increasing the durability of dies that use cold work tool steel as the base material is to first nitride the tool steel to increase the surface hardness, and then coat it with a hard film using PVD. In this case, if the nitride layer has low hardness, the nitride layer will deform during use, causing the PVD coating to peel off early, and the expected durability will not be achieved. On the other hand, even when the nitride layer has a high hardness, if the gradient of hardness change within the nitride layer is steep, the load will be concentrated at the boundary between the nitride layer and the base tool steel, causing peeling and the entire PVD treatment film to come off, resulting in the problem of not being able to achieve the expected durability.

[0004] The applicant has invented a matrix high-speed steel suitable for nitriding treatment, which can effectively utilize the effects of nitriding by controlling the component elements, their ratios, and the grain size (see Patent Document 1).

[0005] Also, a tool steel has been proposed in which steel with a specified composition is formed into a tool, the surface is nitrided, and then a hard coating is formed by surface modification using a PVD or CVD method (see Patent Document 2). In this proposal, Al is contained to improve the adhesion of the hard coating. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 4738912 [Patent Document 2] Japanese Patent Application Publication No. 10-298710 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the hardness gradient within the nitrided layer is not taken into consideration in the aforementioned Patent Document 1. Similarly, Patent Document 2 does not fully consider the surface hardness after nitriding, the nitriding depth, or the hardness gradient within the nitrided layer, which can result in peeling of the hard coating such as PVD.

[0008] In cold working dies, in order to improve the adhesion of the PVD treatment film, it is thought to be effective to plasma-nitride the base material, form a nitride layer without a compound layer on the surface of the base material, harden it, and then apply a hardened coating. However, if the nitride layer's hardness is low, it will deform during use, causing the PVD coating to peel off early.On the other hand, even if the nitride layer has high hardness, if the hardness change within the nitride layer is sudden, when the base material deforms due to stress during cold working, the nitride layer cannot keep up with the deformation, causing peeling at the boundary between the nitride layer and the base material, and the PVD coating may be removed altogether.

[0009] Therefore, an object of the present invention is to provide a cold-working tool steel that has high adhesion to a hard coating formed on the surface of a tool or die by PVD, thereby preventing the coating from peeling off, and to provide a die or tool that uses this cold-working tool steel as a base material and has a hard coating formed on its surface by PVD. That is, an object of the present invention is to provide a cold-working tool steel that has excellent surface treatment properties such as toughness and peeling resistance. [Means for solving the problem]

[0010] As a result of intensive development, the inventors discovered that by specifying the alloy composition range, formula, hardness, and further the surface hardness when nitriding, the depth of the nitrided layer, and the change in hardness of the nitrided layer, it is possible to obtain steel, dies, and tools that have high adhesion to hard films formed by PVD treatment or the like.

[0011] That is, the first means for solving the problems of the present invention is: A cold work tool steel consisting of, by mass%, C: 0.7 to 1.0%, Si: 0.3 to 1.0%, Mn: 0.5% or less, Cr: 3.5 to 9.0%, Mo: 1.5 to 6.0%, W: 8.0% or less, V and Nb: V + Nb / 2 = 0.2 to 2.0%, N: less than 500 ppm, the balance being Fe and unavoidable impurities, in which the S value of formula S is 35 or more and the I value of formula I is 150 or more, and which has a hardness of 58 HRC or more after quenching and tempering. Formula S:S=2.0Cr+14.0Mo+3.3W+6.7(V+Nb / 2) Formula I: I = 70Si-Cr + 63Mo-153V + 23W However, the percentage value of the component is substituted for the element symbol in the formula.

[0012] The second means is a cold work tool steel with a nitride layer formed on the surface of the cold work tool steel described in the first means, in which the hardness at a depth of 30 μm from the outermost surface of the nitride layer is 800 HV or more, the depth of the nitride layer is 60 μm or more, and the change in hardness between the position 30 μm and the position 60 μm from the outermost surface of the nitride layer is within 75 HV.

[0013] The third means is a mold having the cold work tool steel of the first or second means as a base material, and further having a hard coating formed on the surface by a PVD method.

[0014] The fourth means is a tool having the cold work tool steel of the first or second means as a base material, and further having a hard coating formed on the surface by a PVD method. [Effects of the Invention]

[0015] When the cold work tool steel of the present invention is used as a base material, it has the properties of the base material, such as toughness and hardness, of a cold work tool steel. Furthermore, when a hard coating is formed on the surface by PVD, surface hardness is ensured and a sudden change in hardness at the surface is suppressed, so peeling and chipping of the PVD hard coating are suppressed. Molds and tools in which a nitride layer is formed on the surface of a base material and then a PVD hard coating are formed have excellent adhesion to the hard coating, a sudden change in hardness is suppressed, and peeling and chipping are further suppressed. In other words, a tool in which a PVD hard coating is formed on the surface of a cold work tool steel of the present invention as a base material has a Charpy impact value of 20 J / cm in a Charpy impact test. 2 As a result, the critical peel load in a scratch test is 80N or more, and the film is excellent in both toughness and peel resistance. DETAILED DESCRIPTION OF THE INVENTION

[0016] Before describing the embodiments of the present invention, the reasons for specifying the chemical composition of the steel in the present invention, the reasons for specifying the values ​​of formula S and formula I, the reasons for specifying the hardness of the steel material after quenching and tempering, the hardness of the nitrided layer, and the hardness change of the nitrided layer will be explained. In the following chemical compositions, % means mass %.

[0017] C: 0.7 to 1.0% Carbon is an essential element for imparting the high hardness required for tool steel by dissolving in steel and forming carbides. To obtain the hardness required for tools, at least 0.7% carbon is required. However, if the carbon content exceeds 1.0%, a large amount of coarse carbides tends to form, reducing the toughness of the steel itself and the adhesion between the substrate and the coating. Therefore, carbon content is 0.7 to 1.0%, and preferably 0.7 to 0.9%.

[0018] Si: 0.3 to 1.0% Si contributes to the deoxidation effect, hardenability, and solid solution strengthening during steelmaking. To achieve this, at least 0.3% Si is required. However, if Si exceeds 1.0%, toughness decreases. Therefore, Si is set to 0.3 to 1.0%. Preferably, Si is 0.4 to 1.0%.

[0019] Mn: 0.5% or less If Mn exceeds 0.5%, it increases the amount of retained austenite and promotes the formation of segregation bands, thereby deteriorating toughness and dimensional anisotropy. Therefore, the Mn content is set to 0.5% or less.

[0020] Cr: 3.5 to 9.0% Cr is an element necessary for improving hardenability and ensuring tempered hardness. However, if the Cr content is less than 3.5%, these effects are insufficient. On the other hand, if the Cr content exceeds 9.0%, a large amount of coarse carbides tends to form, reducing the toughness of the steel itself and the adhesion between the substrate and the film. Therefore, the Cr content is set to 3.5 to 9.0%. Preferably, the Cr content is 4.5 to 8.5%.

[0021] Mo: 1.5 to 6.0% Mo contributes to improving hardenability and tempered hardness. To obtain these effects, at least 1.5% Mo is required. On the other hand, if Mo exceeds 6.0%, these effects become saturated, and excessive addition promotes the formation of coarse carbides, reducing adhesion to the film. Therefore, Mo is set to 1.5 to 6.0%. Preferably, Mo is 1.5 to 5.0%.

[0022] W: 8.0% or less W has an effect similar to that of Mo, but if it exceeds 8.0%, it will have the opposite effect, just like Mo. Therefore, W content is set to 8.0% or less.

[0023] V+Nb / 2:0.2~2.0% Both V and Nb form fine, hard precipitates during tempering, contributing to secondary hardening. To achieve these effects, the total amount of V and Nb (V + Nb / 2) must be at least 0.2%. However, if V + Nb / 2 exceeds 2.0%, the excess V and Nb will promote the formation of coarse carbides, resulting in a decrease in adhesion to the coating. Therefore, the content of V and Nb is set to 0.2 to 2.0% (V+Nb / 2), and preferably 0.2 to 1.8% (V+Nb / 2).

[0024] N: 500ppm or less N has an effect similar to that of C, but if it exceeds 500 ppm, it forms nitrides that are hard and difficult to dissolve, significantly reducing the toughness of the steel. Therefore, N is set to 500 ppm or less, and preferably 400 ppm or less.

[0025] S=2.0Cr+14.0Mo+3.3W+6.7(V+Nb / 2)≧35 The value S in the formula S is an index used to predict the surface hardness after nitriding based on the type and amount of each added element (the element symbols in the formula are substituted with percentage values). The larger the value of S, the greater the surface hardness. If S is less than 35, the surface hardness will be insufficient and adhesion will decrease. Therefore, the value of S should be 35 or more.

[0026] I=70Si-1Cr+63Mo-153V+23W≧150 The value I in formula I is an index that predicts the hardness distribution in the nitrided layer from the type and amount of each added element (substitute percentage values ​​for the element symbols in the formula). The larger the value of I, the smaller the hardness change in the nitrided layer. If the value of I is less than 150, the film is more likely to peel or chip. Therefore, the value of I is set to 150 or more.

[0027] Hardness of steel after quenching and tempering: 58HRC or more If the hardness after quenching and tempering is less than 58HRC, the adhesion to the hard coating will deteriorate, so the hardness of the base steel material is set to 58HRC or more.

[0028] Surface hardness after nitriding of the base material surface: 800HV or more at a depth of 30μm from the outermost surface of the nitrided layer When the cold work tool steel of the present invention, which serves as the base material, is nitrided, the adhesion between the PVD hard coating and the base material is particularly improved if the hardness of the surface (at a depth of 30 μm from the outermost surface) is set to 800 HV or more. Therefore, the surface hardness of the nitrided layer at a depth of 30 μm from the outermost surface is set to 800 HV or more. The nitriding treatment is preferably a plasma nitriding treatment, and the following description will be given taking the plasma nitriding treatment as an example.

[0029] Depth of nitride layer after nitriding: 60 μm or more The nitride layer formed by plasma nitriding treatment has a depth of 60 μm or more, and if the nitride layer depth is less than 60 μm, the adhesion between the PVD hard coating and the base material decreases. For this reason, the nitride layer depth is set to 60 μm or more.

[0030] Hardness change of nitride layer: within 75HV If the degree of change in hardness between the 30 μm and 60 μm positions from the outermost surface of the nitrided layer exceeds 75 HV, the large difference in hardness between the film directly below and the interior will promote peeling and reduce adhesion to the base material. Therefore, the above hardness change was set to 75 HV or less.

[0031] (About the Examples) First, 100 kg of steel having the chemical composition shown in Table 1 with the balance being Fe was melted in a vacuum induction melting furnace, and each steel was forged at a forging temperature of 1100 to 1200°C into a 30 mm square, cut, and quenched and tempered. The hardness of the steel material at the central periphery of the cross section of the squared bar was then measured using a Rockwell hardness tester. Next, Charpy impact test specimens (10R-C notch) were prepared parallel to the longitudinal direction of the squared bar, and the toughness was evaluated by a Charpy impact test.

[0032] [Table 1]

[0033] (Rating I) Charpy impact test value: 20J / cm 2 Those having the above grade were evaluated as excellent in toughness and marked with ◯, and those having less than this grade were evaluated as poor in toughness and marked with ×.

[0034] After evaluation I, for each steel material, a small piece measuring 25 mm wide x 7 mm thick x 50 mm long was cut out from the 30 mm square material after quenching and tempering, and its surface was subjected to plasma nitriding treatment (heated to 500°C) to form a nitride film layer, after which a TiAlN film was formed on top of the nitride film layer using PVD treatment.

[0035] A piece of approximately 5 mm in length was cut out from the small piece after the PVD treatment, and the depth of the nitrided layer and the change in hardness of the nitrided layer were measured on the cut surface.

[0036] (Regarding nitride layer depth measurement) Using a micro Vickers hardness tester, the hardness distribution in the nitride layer was measured at 10 μm intervals from a position immediately below the coating on the cut surface, that is, 30 μm deep from the top surface of the nitride layer, to a depth of 200 μm.

[0037] In addition, starting from a position 1100 μm deep from the outermost surface of the nitride layer, hardness was measured at 100 μm intervals up to a depth of 2000 μm, and the average value of the hardness values ​​at these 10 points was taken as the "internal Vickers hardness."

[0038] The nitrided layer depth was determined as the deepest position among the positions showing a value 25 points or more higher than the internal Vickers hardness in the hardness distribution of 30 to 200 μm in the nitrided layer measured at 10 μm intervals.

[0039] (Hardness change of nitride layer) The difference in hardness measured at the depth of the nitrided layer of 30 μm and 60 μm from the outermost surface of the nitrided layer using a micro Vickers hardness tester was taken as the hardness difference.

[0040] (Measurement of critical load for peeling by scratch test) Only for those that were rated ○ in Evaluation I, i.e., those that had sufficient toughness, small pieces of the remaining material measuring 25 mm wide x 7 mm thick x 50 mm long were used to measure the critical load for peeling in a scratch test. Therefore, scratch tests were not performed on comparative steels No. 19 to 31, as they lack toughness and do not possess the properties required of cold work tool steels.

[0041] The test conditions were: minimum load: 1 N, load speed: 30 N / min, scratch speed: 1.51 mm / min, indenter: diamond, indenter curvature radius: 200 μm.

[0042] (Rating II) Those with a peel critical load of 100 N or more in the scratch test were evaluated as excellent peel resistance, ◯; those with a peel critical load of 80 N or more were evaluated as fair, Δ; and those with a peel critical load of less than 80 N were evaluated as poor peel resistance, ×.

[0043] Table 2 shows the results of Evaluation I of the Charpy impact test and Evaluation II of the scratch test.

[0044] [Table 2]

[0045] It was confirmed that the invention steels No. 1 to 18 have a PVD hard coating formed on top of the nitride layer formed by plasma nitriding treatment, which gives them excellent toughness while making the hard coating less likely to peel off from the base material.

[0046] Comparative steels No. 19 to 31 have a Charpy impact value of 20 J / cm 2 Comparative steels Nos. 32 to 34 had a critical peel load of 74 N or less, below 80 N, resulting in poor peel resistance.

Claims

1. A cold work tool steel with a nitride layer, the steel consisting of, by mass%, C: 0.7 to 1.0%, Si: 0.3 to 1.0%, Mn: 0.5% or less, Cr: 3.5 to 9.0%, Mo: 1.5 to 6.0%, W: 8.0% or less, V and Nb: V+Nb / 2 = 0.2 to 2.0%, N: less than 500 ppm, the balance being Fe and unavoidable impurities, the S value of formula S being 35 or more and the I value of formula I being 150 or more, which has been quenched and tempered, and further having a nitride layer formed on the surface layer, the base material hardness of the central portion being 58 HRC or more, the hardness at a depth of 30 μm from the outermost surface of the nitride layer being 800 HV or more, the nitride layer depth being 70 μm or more and less than 100 μm, and the change in hardness between a position 30 μm from the outermost surface of the nitride layer and a position 60 μm from the outermost surface of the nitride layer being 75 HV or less. Formula S: S=2.0Cr+14.0Mo+3.3W+6.7(V+Nb / 2) Formula I: I=70Si-Cr+63Mo-153V+23W However, the percentage value of the component is substituted for the element symbol in the formula.

2. A mold using the cold work tool steel according to claim 1 as a base material and further comprising a hard coating formed on the surface by a PVD method.

3. A tool using the cold work tool steel according to claim 1 as a base material and further comprising a hard coating formed on the surface thereof by a PVD method.

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