Martensite-based free-cutting stainless steel bar and method for producing same

Abstract

Provided is a martensite-based free-cutting stainless steel bar that is characterized by: having a chemical composition of C at 0.60% exclusive to 1.00% inclusive, Si at 0.10% to 2.0%, Mn at 0.10% to 3.0%, S at 0.050% or less, P at 0.10% or less, and Cr at 11.0% to 16.0%, the balance being Fe and impurities; and having: an average Cr carbide equivalent spherical diameter of no greater than 1.20 μm, a Cr carbide number density of at least 0.10 particles / μm2, an average Cr nitride equivalent spherical diameter of no greater than 0.80 μm, and a Cr nitride number density of at least 0.05 particles / μm2.

Description

Martensitic free-cutting stainless steel bar and its manufacturing method

[0001] The present invention relates to a martensitic free-cutting stainless steel bar material that exhibits excellent machinability and suppresses tool wear, and is used in materials cut into small parts that require corrosion resistance, and to a method for producing the same.

[0002] High-hardness, highly corrosion-resistant martensitic stainless steels with a hardness of 500 Hv or more are used for industrial equipment and precision instrument parts due to their wear resistance, fatigue strength, and corrosion resistance. Precision instrument parts, particularly those cut from polished wire rods, are often used as rotating bodies, so they require particularly precise cutting workability. Specifically, tool wear during cutting must be minimal, at 200 μm or less. Tool wear is accelerated by hard precipitates and inclusions present in the workpiece.

[0003] Up until now, there has been a proposal for a high-hardness, high-corrosion-resistant martensitic stainless steel that contains S and specifies the composition and size of sulfides and carbides to improve wear resistance and machinability (Patent Document 1). ２ Only the very large carbide size mentioned above is taken into consideration, and the cutting tool life and the cut surface quality cannot be fully satisfied.

[0004] Furthermore, a cold work tool steel with high cold workability has been proposed in which the average equivalent circle diameter of carbides and the cleanliness of inclusions are specified to improve cold workability and machinability (Patent Document 2). However, this proposal aims to improve machinability and cold workability by setting the average equivalent circle diameter of carbides to 0.25 μm or more and 0.8 μm or less through low hardness. However, it does not mention that coarse carbides with an average equivalent circle diameter of carbides exceeding 0.8 μm promote tool wear and thereby deteriorate machinability. It is stated that when the average equivalent circle diameter of carbides is less than 0.25 μm, the hardness is high and the effect of improving machinability cannot be obtained. Furthermore, it does not mention the cutting surface properties.

[0005] Furthermore, in the field of S-containing martensitic stainless free-cutting steels, there have been proposed steels containing B and N to define the size of BN-based intermetallic compounds, or steels with excellent surface properties achieved by reducing the aspect ratio of inclusions in the steel to suppress the formation of built-up edge (Patent Document 3 and Patent Document 4).However, there is no mention of tool life.

[0006] JP 2015-137381 A JP 2006-193790 A JP 2013-185195 A International Publication WO2019 / 240209

[0007] The present inventors have found that the known techniques described in the background art above or combinations thereof cannot achieve an excellent tool life in which tool wear during cutting is suppressed to 200 μm or less in a high-hardness, highly corrosion-resistant martensitic free-cutting stainless steel that has been imparted with excellent machinability.

[0008] The problem to be solved by the present invention is to provide a martensitic free-cutting stainless steel bar having high hardness and high corrosion resistance and excellent tool life during cutting, which is suitable for use in precision parts made of martensitic stainless steel used in severely corrosive environments, and a method for producing the same.

[0009] In order to solve the above problems, the present inventors have conducted extensive research into martensitic free-cutting stainless steels that can be hardened to a high hardness of 500 Hv or more, and have found that controlling the average equivalent circle diameter and number density of Cr carbides and Cr nitrides results in excellent tool life during precision cutting.

[0010] The present invention was made based on the above findings, and its gist is as follows: [1] A steel sheet having a chemical composition, in mass %, of C: more than 0.60% and not more than 1.00%, Si: 0.10 to 2.0%, Mn: 0.10 to 3.0%, S: not more than 0.050%, P: not more than 0.10%, Cr: 11.0 to 16.0%, the balance being Fe and impurities, wherein the average Cr carbide equivalent circle diameter is 1.20 μm or less, and the Cr carbide number density is 0.10 particles / μm. ２ or more, the average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 particles / μm ２[2] The chemical components of the steel are, in mass %, in place of a portion of the Fe, B: 0.01% or less, N: 0.15% or less, Al: 0.008% or less, O: 0.015% or less, Ni: 1.5% or less, Cu: 1.5% or less, Co: 1.5% or less, Mo: 2.5% or less, W: 2.5% or less, Bi: 0.2% or less, Sn: 1.0% or less, Sb: 1.0% or less, and Ag: 1.0% or less. [1] The martensitic free-cutting stainless steel bar according to [1], characterized in that it contains one or more elements selected from the group consisting of Te: 0.1% or less, V: 0.8% or less, Nb: 0.3% or less, Ti: 0.3% or less, Ta: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, Hf: 0.01% or less, and REM: 0.05% or less. [3] A method for producing a martensitic free-cutting stainless steel bar according to [1] or [2], characterized in that the finish rolling temperature of the hot working is 800 to 1150°C, and after the hot working, batch annealing is performed in which the material is held at 400°C or higher but lower than 850°C for 30 to 300 minutes and then cooled to 400°C at a rate of 20 to 200°C / h, or in addition to the batch annealing, strand annealing is performed at 700 to 850°C for 30 to 1000 seconds.

[0011] According to the present invention, a martensitic free-cutting stainless steel bar material is provided which has a good cutting tool life and is suitable for precision parts having high hardness and excellent corrosion resistance, and a method for producing the same.

[0012] Each requirement of the present invention will be explained below. In the following explanation, (%) means mass (%) unless otherwise specified.

[0013] <<Essential Elements>> The present invention is directed to parts having a hardness of 500 Hv or more, which is generally effective for wear resistance, and is based on a high-hardness martensitic free-cutting stainless steel that exhibits a hardness of at least 500 Hv in a quenched state.

[0014] C is contained in an amount exceeding 0.60% to ensure that the hardness of the base material after quenching is 500 Hv or higher. However, if the C content exceeds 1.00%, the average Cr carbide equivalent circle diameter exceeds 1.20 μm, which reduces the cutting tool life. Therefore, the C content is limited to 1.00% or less. Preferably, it is in the range of 0.65 to 0.95%.

[0015] Silicon (Si) is added at 0.10% or more to deoxidize the steel and suppress the formation of coarse inclusions that reduce the life of the cutting tool. However, if the Si content exceeds 2.0%, the steel hardens, reducing the life of the cutting tool and promoting adhesion of the base material to the tool, promoting the formation of built-up edge marks. Therefore, the Si content is limited to 2.0% or less. A preferred range is 0.20 to 1.0%.

[0016] Mn deoxidizes the steel, suppressing the formation of coarse inclusions that reduce the life of cutting tools, and forms sulfides to ensure good cutting surface properties. Therefore, Mn content is set to 0.10% or more. However, Mn content exceeding 3.0% causes hardening and reduces the life of cutting tools. Therefore, the Mn content is limited to 3.0% or less. The preferred range is 0.20 to 2.0%.

[0017] The inclusion of S forms sulfides, which can improve cutting tool life and cut surface quality, but if the S content exceeds 0.050%, the steel hardens and the cutting tool life deteriorates. Therefore, the S content is limited to 0.050% or less, preferably 0.030% or less.

[0018] P is mixed in as an unavoidable impurity from the raw materials, but if it is contained in excess of 0.10%, not only does it deteriorate corrosion resistance due to grain boundary segregation, but it also significantly reduces manufacturability. Therefore, the P content is limited to 0.10% or less, preferably 0.05% or less.

[0019] Cr is a basic element for imparting corrosion resistance to stainless steel, and is contained in an amount of 11.0% or more. However, if the Cr content exceeds 16.0%, it becomes impossible to ensure a hardness of 500 Hv or more after quenching. Therefore, the Cr content is limited to 16.0% or less. Preferably, it is 11.5 to 15.0%.

[0020] <<Equivalent circle diameter and number density of Cr carbides, equivalent circle diameter and number density of Cr nitrides>> Cr carbides and Cr nitrides are extremely hard, so they cause abrasive wear of tools during cutting, shortening tool life. The larger the Cr carbides and Cr nitrides are and the lower their number density, the more significant the effect becomes. Therefore, the average equivalent circle diameter of Cr carbides is set to 1.20 μm or less, and the number density of Cr carbides is set to 0.10 pieces / μm. ２ or more, the average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 pieces / μm ２ Preferably, the average equivalent circle diameter of Cr carbides is 1.00 μm or less, and the number density of Cr carbides is 0.20 particles / μm. ２ The average equivalent circle diameter of Cr nitrides is 0.70 μm or less, and the number density of Cr nitrides is 0.10 pieces / μm ２ That's all.

[0021] <<Selectively Contained Elements>> The stainless steel of the present invention is composed of chemical components consisting of Fe and impurities, in addition to the elements described above. Furthermore, in addition to the above-mentioned component composition, the stainless steel may selectively contain the following elements in place of a portion of Fe.

[0022] B may be added as needed because, when added together with N, it forms fine BN, suppresses adhesion of the base material to the tool surface, and prevents built-up edge marks on the cutting surface. However, if the B content exceeds 0.01%, coarse borides are formed, shortening the cutting tool life and promoting the formation of built-up edge. Therefore, the B content is limited to 0.01% or less. To reliably obtain the above effects, it is preferable that the B content be 0.002% or more and 0.007% or less.

[0023] N may be added as needed, since it not only forms fine BN to prevent built-up edge marks on the cutting surface but also increases the hardness of the base material after quenching. However, if the N content exceeds 0.15%, the average Cr nitride equivalent circle diameter exceeds 0.80 μm, deteriorating the cutting tool life. Therefore, the N content is set to 0.15% or less. To reliably obtain the above effects, it is preferable that the N content be 0.03% or more and 0.10% or less.

[0024] Although Al may be contained for deoxidation, if the content exceeds 0.008%, coarse inclusions are formed, which reduces the life of the cutting tool. Therefore, the Al content is limited to 0.008% or less, and preferably 0.006% or less.

[0025] O is present as an unavoidable impurity, but if the O content exceeds 0.015%, coarse inclusions are formed, shortening the life of the cutting tool. Therefore, the O content is limited to 0.015% or less, and preferably 0.012% or less.

[0026] Ni, Cu, and Co may be added as needed to improve the corrosion resistance and toughness of the product. However, if each of them is added in an amount exceeding 1.5%, it will shorten the cutting tool life. Therefore, the content of each element is limited to 1.5% or less. To ensure the above effects, it is preferable that the content of each element be 0.01% or more and 1.0% or less.

[0027] Mo and W may be added as needed to improve the corrosion resistance of the product. However, if the content of each exceeds 2.5%, the effect saturates and the cutting tool life deteriorates. Therefore, the content of each element is limited to 2.5% or less. To ensure the above effects, it is preferable to limit the content of each element to 0.01% or more and 2.0% or less.

[0028] Bi acts as a self-lubricant during cutting, suppressing the formation of built-up edges and improving the cutting surface quality, so it may be added as needed. However, if it is added in excess of 0.2%, hot workability is significantly deteriorated and manufacturing becomes impossible. Therefore, the Bi content is limited to 0.2% or less. To reliably obtain the above effects, it is preferable that the Bi content be 0.005% or more and 0.10% or less.

[0029] Sn, Sb, and Ag act as self-lubricants during cutting, suppressing the formation of built-up edges and improving the quality of the cut surface, so they may be added as needed. However, if each of them is contained in an amount exceeding 1.0%, hot workability is significantly deteriorated and manufacturing becomes impossible. Therefore, the content of each element is limited to 1.0% or less. To reliably obtain the above effects, it is preferable that the content of each element be 0.005% or more and 0.50% or less.

[0030] Te may be added as needed because it generates spherical sulfides, suppresses the accumulation and growth of built-up cutting edges, and improves cutting surface quality. However, if the content exceeds 0.1%, hot workability is significantly deteriorated and manufacturing becomes impossible. Therefore, the Te content is limited to 0.1% or less. To reliably obtain the above effects, it is preferable that the Te content be 0.005% or more and 0.05% or less.

[0031] V may be added as needed to improve the corrosion resistance of the product. However, if it is added in excess of 0.8%, coarse carbonitrides are formed, shortening the cutting tool life. Therefore, the V content is limited to 0.8% or less. To reliably obtain the above effects, it is preferable that the V content be 0.05% or more and 0.5% or less.

[0032] Nb, Ti, and Ta may be added as needed to improve the corrosion resistance of the product. However, if each of these elements is added in an amount exceeding 0.3%, coarse carbonitrides are formed, shortening the cutting tool life. Therefore, the content of each element is limited to 0.3% or less. To ensure the above effects, it is preferable to limit the content of each element to 0.01% or more and 0.2% or less.

[0033] Mg, Ca, and Hf may be added as needed because they have the effect of improving hot workability. However, if each of these elements is added in excess of 0.01%, the effect saturates and coarse oxides are generated, shortening the cutting tool life. Therefore, the content of each element is limited to 0.01% or less. To ensure the above effects, it is preferable that the content of each element be 0.001% or more and 0.005% or less.

[0034] REM has the effect of improving hot workability and may be added as needed. However, if the REM content exceeds 0.05%, the effect saturates and coarse oxides are generated, shortening the cutting tool life. Therefore, the REM content is limited to 0.05% or less. To ensure the above effect, it is preferable to limit the REM content to 0.001% or more and 0.005% or less. According to the general definition, REM (rare earth element) refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu). REM may be added alone or in a mixture.

[0035] Typical impurities contained in the stainless steel of the present invention include Zn, Pb, Ge, Se, etc., which are usually mixed in at a level of about 0.1% as impurities during the steel manufacturing process.

[0036] Oxygen, an impurity, exists mainly as inclusions in steel, and the oxygen content of stainless steel produced by conventional refining is 0.001 to 0.015%.

[0037] Furthermore, while representative elements for the selectively contained elements are specified in [2] above, elements not described in this specification can also be contained within a range that does not impair the effects of the present invention.

[0038] <<Method for Controlling Cr Carbide and Cr Nitride>> The method for controlling Cr carbide and Cr nitride of the present invention will be described.

[0039] The average Cr carbide equivalent circle diameter is 1.20 μm or less, and the Cr carbide number density is 0.10 particles / μm ２ The average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 pieces / μm ２To achieve this fine dispersion, it is preferable to perform hot working such as hot rolling at a finish rolling temperature of 800 to 1150°C, and then hold the hot working at 400°C or higher but lower than 850°C for 30 to 300 minutes, followed by batch annealing in which the steel is cooled to 400°C at a rate of 20 to 200°C / h. In addition to batch annealing under the same conditions as above, it is also preferable to perform strand annealing at 700 to 850°C and hold for 30 to 1000 seconds. This range of manufacturing conditions is called the preferred range.

[0040] If the finish rolling temperature of the hot working is less than 800°C, the rolling load will increase, resulting in reduced manufacturability. On the other hand, if the finish rolling temperature exceeds 1150°C, the Cr carbides and Cr nitrides will become coarse, and the above-mentioned average Cr carbide circle equivalent diameter and average Cr nitride circle equivalent diameter conditions will no longer be satisfied. Therefore, the finish rolling temperature of the hot working is specified to be 800 to 1150°C.

[0041] In batch annealing performed after hot working, if the batch annealing temperature is lower than 400°C and the batch annealing holding time is shorter than 30 minutes, annealing will be insufficient and manufacturability will be reduced. On the other hand, if the batch annealing temperature is 850°C or higher, the batch annealing holding time is longer than 300 minutes, and the cooling rate to 400°C after holding is slower than 20°C / h, Cr carbides and Cr nitrides will coarsen, and the above-mentioned circle equivalent diameter will not be satisfied. Note that if the cooling rate is faster than 200°C / h, martensitic transformation will harden the steel, making delayed fracture and other problems more likely to occur. The cooling rate below 400°C does not need to be particularly limited.

[0042] When strand annealing is performed in addition to batch annealing after hot working, the batch annealing conditions are the same as those for batch annealing alone. Regarding strand annealing, if the strand annealing temperature is lower than 700°C and the strand annealing holding time is shorter than 30 seconds, annealing is insufficient and manufacturability is reduced. On the other hand, if the strand annealing temperature is higher than 850°C and the strand annealing holding time is longer than 1000 seconds, the Cr carbides and Cr nitrides become coarse and do not satisfy the above-mentioned circle equivalent diameter. Furthermore, if the temperature exceeds the Ac1 point and the steel undergoes martensitic transformation during cooling, the steel becomes hard, and delayed fracture and other problems become more likely to occur.

[0043] As described above, the finish rolling temperature of the hot working is limited to 800 to 1150°C, the batch annealing conditions are limited to holding at 400°C or higher but lower than 850°C for 30 to 300 minutes and cooling to 400°C at a rate of 20 to 200°C / h, and the strand annealing conditions are limited to holding at 700 to 850°C for 30 to 1000 seconds.

[0044] Within the above-mentioned preferred range, the lower the finish rolling temperature, the lower the batch annealing temperature and the shorter the holding time, and the faster the cooling rate after holding, and the lower the strand annealing temperature and the shorter the holding time, the smaller the average Cr carbide and Cr nitride equivalent circle diameter and the larger the number density of Cr carbides and Cr nitrides. Therefore, by adjusting the manufacturing conditions within the above-mentioned preferred range based on this tendency, it is possible to reliably achieve an average Cr carbide equivalent circle diameter of 1.20 μm or less and a Cr carbide number density of 0.10 particles / μm. ２ The average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 pieces / μm ２ It is possible to disperse the particles more finely than above.

[0045] In order to finely disperse Cr carbides and Cr nitrides, the finish rolling temperature is preferably less than 1000°C, and more preferably less than 900°C. Preferred ranges are a finish rolling temperature of 850°C or higher and less than 900°C, batch annealing conditions of 450 to 800°C held for 60 to 200 minutes, followed by a cooling rate of 30 to 100°C / h to 400°C after holding, and strand annealing conditions of 750 to 825°C held for 40 to 800 seconds. By manufacturing within these conditions, it is possible to reliably achieve an average Cr carbide equivalent circle diameter of 1.20 μm or less and a Cr carbide number density of 0.10 particles / μm. ２ The average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 pieces / μm ２ The fine dispersion can be achieved as described above.

[0046] Strand annealing is an annealing method in which a wire rod or a steel wire coil wound in a ring shape is unrolled into a straight line and heat-treated in a single straight line for a short time (in an atmosphere of nitrogen, Ar, ammonia decomposition gas, etc.), followed by air cooling or indirect water cooling. This method allows for a significantly faster cooling rate than batch annealing of the entire ring-shaped coil.

[0047] According to the present invention as described above, it is possible to provide a martensitic free-cutting stainless steel bar having excellent tool life during cutting, which can be used as a stainless steel bar for high-hardness parts to be precision-cut.

[0048] In the present invention, martensitic stainless steel refers to a steel that hardens due to martensitic transformation during quenching. In the present invention, this means a steel in which, for example, 50% or more of the metal structure exhibits a martensitic structure when quenched by air cooling from 1,050°C, and which hardens to 500 Hv or more.

[0049] In the present invention, the term "steel bar" is a concept that encompasses "steel bar," "wire rod," "wire rod," "steel wire," "deformed wire," and "deformed steel bar."

[0050] Example 1 To investigate the influence of the components, steels having the chemical compositions shown in Tables 1 to 3 were melted at 1600°C in a 45 kg vacuum melting furnace and then cast into a mold. Subsequently, as hot working, the steel was heated to 1200°C and then hot rolled at a finish rolling temperature of 880°C to obtain a wire rod having a diameter of 5.5 mm, which was then cooled to room temperature. Subsequently, batch annealing was performed at an annealing temperature of 550°C, a holding time of 180 minutes, and a cooling rate of 50°C / h to 400°C after holding. After batch annealing, cold working was performed by cold wiredrawing at an area reduction rate of 66.1% to obtain a wire rod having a diameter of 3.2 mm. Subsequently, the wire rod was straightened and centerless ground to obtain a polished bar having a diameter of 3.0 mm, which was used as a cutting material.

[0051]

[0052]

[0053]

[0054] The wire rods thus obtained were evaluated for the average Cr carbide circle equivalent diameter, Cr carbide number density (number per unit area), average Cr nitride circle equivalent diameter, Cr nitride number density (number per unit area), and tool life after peripheral cutting using the evaluation methods described below. The results are shown in Tables 4 to 6. Tables 4 and 5 show the evaluation results for the inventive examples, and Table 6 shows the evaluation results for the comparative examples.

[0055] The quenched hardness of the obtained wire rods was evaluated after quenching by air cooling from 1050°C, and a value of 500 Hv or more was considered good. The corrosion resistance of the obtained wire rods was evaluated by conducting an acetic acid salt spray test for 48 hours, and a value of good was determined when there was no rust flow. If either the quenched hardness or corrosion resistance did not meet the good standard, the remarks were entered as "poor quenched hardness" or "poor corrosion resistance," and no other evaluation was performed. Furthermore, if the product could not be manufactured for some reason, the remark was entered as "unmanufacturable," and no quality evaluation was performed.

[0056]

[0057]

[0058]

[0059] "Average Cr carbide circle equivalent diameter and average Cr nitride circle equivalent diameter, Cr carbide number density and Cr nitride number density" The sample was embedded in resin so that the longitudinal cross section passing through the central axis of the wire rod was the test surface, and the test surface was mirror-polished and then etched with aqua regia. ２ The area of ​​5 visual fields was observed. Among the inclusions in the visual fields, those in which a large amount of Cr and C was detected by EDX analysis were identified as Cr carbides, and those in which a large amount of Cr and N was detected were identified as Cr nitrides. The average Cr carbide circle equivalent diameter and the average Cr nitride circle equivalent diameter were determined by calculating the circle equivalent diameter for 100 or more Cr carbides or Cr nitrides and averaging the circle equivalent diameters. The Cr carbide number density and Cr nitride number density were determined by measuring the total number of Cr carbides or Cr nitrides in the 5 visual fields and calculating the average of the circle equivalent diameters. ２ The number of Cr carbides and Cr nitrides per unit area was calculated.

[0060] "Cutting Tool Life" The outer periphery of a wire rod was cut in the circumferential direction for 1 hour under the precision cutting conditions of: Tool used: Carbide P type, Cutting edge R: 0.03 mm, Cutting speed: 20 m / min, Feed rate: 0.01 mm / rev, Depth of cut: 0.1 mm, Cutting oil (mineral oil): present. The tool after cutting was observed under an optical microscope at 200x magnification. If the notch wear width of the tool after cutting was 100 μm or less, it was evaluated as "A", if it was more than 100 μm but not more than 200 μm, it was evaluated as "B", and if it was more than 200 μm, it was evaluated as "X".

[0061] Inventive Examples 1 to 57 in Tables 4 and 5, the average Cr carbide equivalent circle diameter was 1.20 μm or less, and the Cr carbide number density was 0.10 particles / μm ２ or more, the average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 pieces / μm ２ All of these results indicate excellent cutting tool life.

[0062] On the other hand, Comparative Examples 1 to 32 in Table 6 are outside the range of components of the present invention and are unable to achieve excellent cutting tool life.

[0063] Example 2 To evaluate the influence of the manufacturing process, steel having the chemical composition shown in Steel C in Table 1 was melted at 1600°C in a 45 kg vacuum melting furnace and cast into a mold. The manufacturing conditions are shown in Table 7. Hot rolling was performed as hot working, and the hot rolling conditions were heating to 1200°C followed by a finish rolling temperature of 760 to 1170°C to produce a φ5.5 mm wire rod, which was then cooled to room temperature. Subsequently, batch annealing was performed alone, or in addition to batch annealing, strand annealing was performed. For batch annealing, the annealing temperature was 350 to 900°C, the holding time was 20 to 360 minutes, and the cooling rate to 400°C after holding was in the range of 10 to 250°C / h. For strand annealing (in ammonia decomposition gas), the annealing temperature was 680 to 870°C, and the holding time was 20 to 1200 seconds. After annealing, the wire was cold drawn as cold working, with a reduction in area of ​​66.1% to a diameter of 3.2 mm. The wire was then straightened and centerless ground to form a polished bar (wire rod) with a diameter of 3.0 mm, which was used as a cutting material. The average Cr carbide circle equivalent diameter, Cr carbide number density, average Cr nitride circle equivalent diameter, Cr nitride number density, and tool life after peripheral cutting were evaluated using the same evaluation methods as in Example 1. The results are shown in Table 7.

[0064]

[0065] Inventive Examples 58 to 65 in Table 7, the average Cr carbide equivalent circle diameter was 1.20 μm or less, and the Cr carbide number density was 0.10 particles / μm ２ or more, the average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 pieces / μm ２ All of these results indicate excellent cutting tool life.

[0066] On the other hand, in Comparative Examples 33 to 44 in Table 7, the manufacturing conditions were outside the preferred conditions of the present invention, and manufacturing was impossible, or even if manufacturing was possible, the average Cr carbide circle equivalent diameter, Cr carbide number density, average Cr nitride circle equivalent diameter, and Cr nitride number density were outside the ranges of the present invention. As a result, it was not possible to achieve excellent cutting tool life.

[0067] As is clear from the above examples, the present invention can provide a martensitic free-cutting stainless steel that has excellent tool life during precision cutting, and can significantly improve the durability of high-hardness parts used in environments where corrosion is severe and fatigue strength and wear resistance are required, making it extremely useful in industry.

Claims

1. A steel having a chemical composition consisting of, in mass%, C: more than 0.60% and 1.00% or less, Si: 0.10 to 2.0%, Mn: 0.10 to 3.0%, S: 0.050% or less, P: 0.10% or less, Cr: 11.0 to 16.0%, the balance being Fe and impurities, wherein the average Cr carbide circle equivalent diameter is 1.20 μm or less, and the Cr carbide number density is 0.10 particles / μm ２ or more, the average Cr nitride equivalent circle diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 particles / μm ２ A martensitic free-cutting stainless steel bar characterized by the above.

2. The chemical composition of the steel, in place of a portion of the Fe, is, in mass%, B: 0.01% or less, N: 0.15% or less, Al: 0.008% or less, O: 0.015% or less, Ni: 1.5% or less, Cu: 1.5% or less, Co: 1.5% or less, Mo: 2.5% or less, W: 2.5% or less, Bi: 0.2% or less, Sn: 1.0% or less, Sb: 1.0% or less, Ag: 1.0% or less, Te: 0.1% or less, V: 0.8% or less, Nb: 0.3% or less, Ti: 0.3% or less, Ta: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, Hf: 0.01% or less, and REM: 0.05% or less.

3. A method for producing martensitic free-cutting stainless steel bar as set forth in claim 1 or 2, characterized in that the finish rolling temperature of the hot working is 800 to 1150°C, and after the hot working, batch annealing is performed by holding the temperature at 400°C or higher but lower than 850°C for 30 to 300 minutes and then cooling to 400°C at a rate of 20 to 200°C / h, or in addition to the batch annealing, strand annealing is performed by holding the temperature at 700 to 850°C for 30 to 1000 seconds.

Images