Martensitic free-cutting stainless steel bar and producing method thereof

Abstract

To provide a high hardness / high corrosion resistant martensitic free-cutting stainless steel bar capable of obtaining cutting surface quality excellent in tool life at precision cutting work and having excellent flatness in the cutting work and a producing method thereof.SOLUTION: A martensitic free-cutting stainless steel bar has chemical composition comprising C: 0.08 to 0.60%, Si: 0.10 to 2.0%, Mn: 0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, with the remainder consisting of Fe and impurities, the martensitic free-cutting stainless steel bar having an average Cr carbide equivalent circle diameter of 0.80 μm or less, a Cr carbide number density of 0.10 pieces / μm2 or more, an average Cr nitride equivalent circle diameter of 0.80 μm or less, and a Cr nitride number density of 0.05 pieces / μm2 or more. Furthermore, preferably, the average sulfide equivalent circle diameter is 5.0 μm or less, and the sulfide number density is 0.010 pieces / μm2 or more.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] High-hardness, corrosion-resistant martensitic stainless steels with hardness of 380Hv or higher are used in industrial equipment and precision machinery components due to their wear resistance, fatigue strength, and corrosion resistance. Precision machinery components, especially those machined from polished wire rods, are often used in rotating bodies, so they require particularly precise machinability. Specifically, tool wear during cutting must be minimal (200 μm or less), and the cut surface must be smooth enough to reveal minute built-up edge (BEE) marks after cutting. Tool wear is accelerated by hard precipitates and inclusions present in the workpiece. BEE is a deposit that forms and grows on the tip of a cutting tool due to adhesion of the base material. It detaches from the tool tip during cutting and is pressed onto the cut surface, degrading the surface quality.

[0003] Up until now, there has been a proposal for a martensitic free-cutting stainless steel that contains S in a high-hardness, high-corrosion-resistant martensitic stainless steel and that improves wear resistance and machinability by specifying the sulfide composition and size and carbide size (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, there is no mention of the fact that coarse carbides with an average equivalent circle diameter of more than 0.8 μm accelerate tool wear and degrade 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, there is no mention of the cutting surface properties.

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

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-137381 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-193790 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-185195 [Patent Document 4] International Publication No. WO2019 / 240209 Summary of the Invention [Problem to be solved by the invention]

[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 high-hardness, highly corrosion-resistant martensitic free-cutting stainless steel that has been imparted with excellent machinability, and furthermore, it is not possible to achieve both an excellent tool life and an excellent smooth surface with suppressed built-up edge marks.

[0008] The problem to be solved by the present invention is to provide a high-hardness, high-corrosion-resistant martensitic free-cutting stainless steel bar material, which is suitable for use in precision parts made of martensitic stainless steel used in severely corrosive environments, and which has excellent tool life during cutting and preferably can provide a cut surface with excellent flatness during cutting, as well as a method for producing the same. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors conducted extensive research on S-containing martensitic free-cutting stainless steel, which can be hardened to a high hardness of 380 Hv or more. As a result, they discovered that controlling the average equivalent circle diameter and number density of Cr carbides and Cr nitrides can improve tool life during precision cutting, and more preferably, controlling the average equivalent circle diameter and number density of sulfides can suppress built-up edge marks and achieve surface properties with excellent smoothness.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] Chemical composition, in mass%, consisting of C: 0.08 to 0.60%, Si: 0.10 to 2.0%, Mn: 0.10 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, the balance being Fe and impurities; The average Cr carbide equivalent circle diameter is 0.80 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 particles / μm 2 A martensitic free-cutting stainless steel bar characterized by the above. [2] Chemical composition, in mass%, consisting of C: 0.08 to 0.60%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, S: 0.15 to 0.40%, P: 0.10% or less, Cr: 11.0 to 16.0%, the balance being Fe and impurities; The average Cr carbide equivalent circle diameter is 0.80 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 particles / μm 2 or more, the average sulfide equivalent circle diameter is 5.0 μm or less, and the sulfide number density is 0.010 particles / μm 2 A martensitic free-cutting stainless steel bar characterized by the above. [3] The chemical composition of the steel is, in mass%, in place of a part 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: 0.3% or less, Sb: 0.3% or less, Ag: 0.3% or less, T The martensitic free-cutting stainless steel bar according to [1] or [2], characterized in that it contains one or more elements selected from the group consisting of e: 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.

[0011] [4] 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 carried out by holding the temperature at 400°C or higher but lower than 700°C for 30 to 300 minutes and then cooling to 400°C at a rate of 20 to 500°C / h, or in addition to the batch annealing, strand annealing is carried out by holding the temperature at 700 to 850°C for 30 to 1000 seconds. [5] The method for producing a martensitic free-cutting stainless steel bar according to [4], further characterized in that cold working is carried out at an area reduction rate of 20.0 to 99.0%. [6] A method for producing a martensitic free-cutting stainless steel bar according to [3], characterized in that the finish rolling temperature in the hot working is 800 to 1150°C, and after the hot working, batch annealing is carried out by holding the temperature at 400°C or higher but lower than 700°C for 30 to 300 minutes and then cooling to 400°C at a rate of 20 to 500°C / h, or in addition to the batch annealing, strand annealing is carried out by holding the temperature at 700 to 850°C for 30 to 1000 seconds. [7] The method for producing a martensitic free-cutting stainless steel bar according to [6], further characterized in that cold working is carried out at an area reduction rate of 20.0 to 99.0%. [Effects of the Invention]

[0012] According to the present invention, in martensitic free-cutting stainless steel bar material, it is possible to obtain a good cutting tool life and an excellent flat cutting surface quality with suppressed built-up edge marks, and it is possible to provide a material suitable for precision parts with high hardness and excellent corrosion resistance, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0013] Each requirement of the present invention will be explained below. In the following explanation, (%) means mass (%) unless otherwise specified.

[0014] 《Essential elements》 The present invention is generally intended for parts with a hardness of 380 Hv or more, which is effective for wear resistance, and is based on a high-hardness martensitic free-cutting stainless steel that exhibits a hardness of at least 380 Hv in a quenched state. Parts with a hardness of 400 Hv or more are more preferable.

[0015] C is contained in an amount of 0.08% or more to ensure that the hardness of the base material after quenching is 380 Hv or more. A C content of 0.10% or more is more preferable because it can achieve 400 Hv or more. However, if the C content exceeds 0.60%, the average Cr carbide equivalent circle diameter exceeds 0.8 μm, shortening the cutting tool life. Therefore, the C content is limited to 0.60% or less. A preferred range is 0.20 to 0.50%.

[0016] Si is added at a content of 0.1% or more to deoxidize the steel and suppress the formation of coarse inclusions that shorten the life of the cutting tool. However, if the Si content exceeds 2.0%, the steel hardens, shortening 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. The preferred range is 0.2 to 1.0%.

[0017] Mn is contained in an amount of 0.1% or more because it deoxidizes the steel, suppresses the formation of coarse inclusions that reduce the life of cutting tools, and forms sulfides to ensure good cutting surface quality. However, if the Mn content exceeds 3.0%, the steel hardens, reducing the life of the cutting tool, and the average sulfide equivalent circle diameter exceeds 5 μm, deteriorating the cutting surface quality. Therefore, the Mn content is limited to 3.0% or less. Preferably, it is in the range of 0.2 to 2.0%.

[0018] S forms sulfides to ensure good cutting tool life and machined surface quality, so it is contained in an amount of 0.15% or more. However, if the S content exceeds 0.40%, the material becomes hard and the cutting tool life deteriorates, and the average sulfide equivalent circle diameter exceeds 5 μm, deteriorating the machined surface quality. Therefore, the S content is limited to 0.40% or less. Preferably, it is in the range of 0.17 to 0.35%.

[0019] P is introduced as an unavoidable impurity from the raw materials, but if it exceeds 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.

[0020] 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 380 Hv or more, and even more preferably 400 Hv or more, after quenching. Therefore, the Cr content is limited to 16.0% or less. Preferably, it is 11.5 to 15.0%.

[0021] {Equivalent circle diameter and number density of Cr carbides, equivalent circle diameter and number density of Cr nitrides, equivalent circle diameter and number density of sulfides} Cr carbides and Cr nitrides are extremely hard, so they cause abrasive wear of the tool during cutting, shortening the 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 Cr carbide equivalent circle diameter is set to 0.80 μm or less and the Cr carbide number density is set to 0.10 pieces / μm. 2 or more, the average Cr nitride circle equivalent 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 0.70 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 particles / μm. 2 That's all.

[0022] Sulfides act as starting points for breaking up chips during cutting, and are effective in suppressing the generation of built-up edges and obtaining good surface properties. The finer the sulfides are and the higher their number density, the more significant their effect becomes. Therefore, it is preferable to set the average sulfide equivalent circle diameter to 5.0 μm or less and the sulfide number density to 0.010 particles / μm. 2 More preferably, the average sulfide equivalent circle diameter is 3.0 μm or less, and the sulfide number density is 0.050 particles / μm 2 That's all.

[0023] Selective Element 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 chemical composition, the stainless steel may selectively contain the following elements in place of part of the Fe.

[0024] B may be added as needed because, when added together with N, it forms fine BN, which inhibits 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 marks. Therefore, the B content is limited to 0.01% or less. To ensure the above effects, it is preferable to limit the B content to 0.002% or more and 0.007% or less.

[0025] N 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, so it may be added as needed. However, if the N content exceeds 0.15%, the average Cr nitride equivalent circle diameter exceeds 0.8 μm, deteriorating the cutting tool life. Therefore, the N content is set to 0.15% or less. To ensure the above effects, the N content is preferably set to 0.03% or more and 0.10% or less.

[0026] Although Al may be added for deoxidation, if the content exceeds 0.008%, coarse inclusions are formed, shortening the life of the cutting tool. Therefore, the Al content is limited to 0.008% or less, and preferably 0.006% or less.

[0027] O is present as an unavoidable impurity, but if its content exceeds 0.015%, coarse inclusions are formed, shortening the life of cutting tools. Therefore, the O content is limited to 0.015% or less, preferably 0.012% or less.

[0028] Ni, Cu, and Co may be added as needed to improve the corrosion resistance and toughness of the product. However, if the content of each exceeds 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 to limit the content of each element to 0.01% or more and 1.0% or less.

[0029] 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.

[0030] Bi acts as a self-lubricant during cutting, suppressing the formation of built-up edges and improving the quality of the cut surface, so it may be added as needed. However, if the content exceeds 0.2%, hot workability is significantly deteriorated and manufacturing becomes impossible. Therefore, the Bi content is limited to 0.2% or less. To ensure the above effects, it is preferable that the Bi content be 0.005% or more and 0.10% or less.

[0031] 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 the content of each exceeds 0.3%, hot workability is significantly impaired, making production impossible. 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 0.005% or more and 0.20% or less.

[0032] Te may be added as needed because it generates spherical sulfides, suppresses the accumulation and growth of built-up edges, and improves cutting surface quality. However, if the content exceeds 0.1%, hot workability deteriorates significantly, making it impossible to manufacture. Therefore, the Te content is limited to 0.1% or less. To ensure the above effects, it is preferable to limit the Te content to 0.005% or more and 0.05% or less.

[0033] V may be added as needed to improve the corrosion resistance of the product. However, if the V content exceeds 0.8%, coarse carbonitrides are formed, shortening the cutting tool life. Therefore, the V content is limited to 0.8% or less. To ensure the above effects, it is preferable that the V content be 0.05% or more and 0.5% or less.

[0034] Nb, Ti, and Ta may be added as needed to improve the corrosion resistance of the product. However, if the content of each exceeds 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.

[0035] 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.

[0036] REMs have 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 keep the REM content between 0.001% and 0.005%. According to the general definition, REM (rare earth elements) refers to the two elements scandium (Sc) and yttrium (Y) and the 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). REMs may be added alone or in mixtures.

[0037] 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.

[0038] The impurity oxygen exists mainly as inclusions in steel, and the oxygen content of stainless steel produced by conventional refining is 0.001 to 0.015%.

[0039] Furthermore, while representative elements for the selective inclusion are specified in [3] above, elements not listed in this specification may also be included within a range that does not impair the effects of the present invention.

[0040] <<Method for controlling Cr carbides and Cr nitrides>> The method for controlling Cr carbides and Cr nitrides according to the present invention will now be described.

[0041] The average Cr carbide equivalent circle diameter is 0.80 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 particles / μm 2 To achieve this fine dispersion, it is preferable to perform hot processing such as hot rolling at a finish rolling temperature of 800 to 1150°C, and then hold the hot processing at 400°C or higher but lower than 700°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 500°C / h. In addition to the 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.

[0042] 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 coarsen, and the above-mentioned average Cr carbide circle equivalent diameter and average Cr nitride circle equivalent diameter conditions will no longer be met. Therefore, the finish rolling temperature of the hot working is specified to be 800 to 1150°C.

[0043] In batch annealing performed after hot working, if the batch annealing temperature is lower than 400°C, the batch annealing holding time is shorter than 30 minutes, and the cooling rate to 400°C after holding is faster than 500°C / h, annealing will be insufficient and manufacturability will be reduced. On the other hand, if the batch annealing temperature is 700°C or higher, Cr nitrides will coarsen and the above-mentioned circle equivalent diameter will not be satisfied. Furthermore, if 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. The cooling rate below 400°C does not need to be particularly limited.

[0044] 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 will coarsen, failing to satisfy the above-mentioned circle equivalent diameter. Furthermore, the steel will exceed the Ac1 point and undergo martensitic transformation upon cooling, resulting in hardening and making delayed fracture more likely to occur.

[0045] As described above, the hot working finish rolling temperature is limited to 800 to 1150°C, the batch annealing conditions are limited to holding at 400°C or higher but lower than 700°C for 30 to 300 minutes with a cooling rate to 400°C of 20 to 500°C / h, and the strand annealing conditions are limited to holding at 700 to 850°C for 30 to 1000 seconds.

[0046] 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 0.80 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 particles / μm 2 It is possible to disperse the particles more finely than above.

[0047] 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 450-650°C with a holding time of 60-200 minutes, followed by a cooling rate of 30-200°C / h to 400°C, and strand annealing conditions of 750-825°C with a holding time of 40-800 seconds. By manufacturing within these conditions, it is possible to reliably achieve an average Cr carbide equivalent circle diameter of 0.80μm 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 particles / μm 2 The fine dispersion can be achieved as described above.

[0048] Strand annealing is an annealing method in which a wire rod or steel wire coil wound in a ring shape is unrolled into a straight line and heat-treated for a short time in a straight line (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.

[0049] Sulfide Control Method Next, a sulfide control method will be described.

[0050] The average sulfide equivalent circle diameter is 5.0 μm or less, and the sulfide number density is 0.010 particles / μm 2 To achieve the above-mentioned fine dispersion, hot working such as hot rolling and annealing described in the above "Method for Controlling Cr Carbide and Cr Nitride" is followed by cold working such as cold wire drawing with an area reduction of 20.0 to 99.0%. Although the sulfides are elongated after hot rolling and annealing, the sulfides are broken down and finely dispersed by cold working such as cold wire drawing. Therefore, the area reduction is set to 20.0% or more. However, if the area reduction exceeds 99.0%, the base material becomes embrittled, reducing manufacturability, so the area reduction is limited to 99.0% or less. Preferably, the area reduction is 30.0 to 90.0%.

[0051] If the area reduction rate of cold working, such as cold wire drawing, is higher than approximately 70%, defects such as cracks may occur due to work hardening. To prevent this, cold working, such as cold wire drawing, and annealing may be repeated multiple times. In this case, the annealing performed between cold working steps must be performed under the annealing conditions described in the above-mentioned "Method for Controlling Cr Carbide and Cr Nitride" in order to control Cr carbides and Cr nitrides. Furthermore, the above-mentioned area reduction rate of cold working, such as cold wire drawing, is the sum of the area reduction rates of each of the multiple cold working steps.

[0052] According to the present invention as described above, it is possible to provide a martensitic free-cutting stainless steel bar that is excellent in tool life during cutting and in machined surface properties and can be used as a stainless steel bar for high-hardness parts that are precision cut.

[0053] 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, 70% or more of the metal structure exhibits a martensitic structure when quenched by air cooling from 1050°C, and which hardens to 380 Hv or more.

[0054] 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." [Example]

[0055] Example 1 To investigate the influence of the components, steels with 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. After heating to 1200°C, they were hot-rolled at a finish rolling temperature of 880°C to obtain wire rods with a diameter of 5.5 mm, which were then cooled to room temperature. Subsequently, batch annealing was performed under the following conditions: 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, the wire rods were cold-worked with a reduction in area of ​​66.1% to obtain a diameter of 3.2 mm. Subsequently, they were straightened and centerless ground to obtain polished bars (wire rods) with a diameter of 3.0 mm, which were used as raw materials for cutting.

[0056] [Table 1]

[0057] [Table 2]

[0058] [Table 3]

[0059] 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, average sulfide circle-equivalent diameter, sulfide number density, tool life after peripheral cutting, and surface properties 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.

[0060] Hardness after quenching was evaluated by air-cooling from 1050°C, with 380Hv or higher being considered good and 400Hv or higher being considered particularly good. If the hardness after quenching was particularly good (400Hv or higher), no note was entered in the remarks column; if it was good (380Hv or higher but less than 400Hv), "good hardness after quenching" was entered in the remarks column. If the hardness after quenching did not meet the "good" standard, "poor hardness after quenching" was entered in the remarks, and no other evaluation was performed. Corrosion resistance was evaluated by performing an acetic acid salt spray test for 48 hours, and if there was no rust, it was considered good. If the corrosion resistance did not meet the "good" standard, "poor corrosion resistance" was entered in the remarks, and no other evaluation was performed. Furthermore, if the product could not be manufactured for some reason, "not manufacturable" was entered, and no quality evaluation was performed.

[0061] [Table 4]

[0062] [Table 5]

[0063] [Table 6]

[0064] "Average Cr carbide and Cr nitride equivalent circle diameter, Cr carbide and Cr nitride number density" The specimen was embedded in resin so that the longitudinal cross section of the wire rod was the test surface, and the test surface was mirror polished, then etched with aqua regia and measured at 100 μm using an SEM. 2 The area of ​​5 fields of view was observed. Among the inclusions in the field of view, 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 and Cr nitride circle equivalent diameter was calculated by averaging the circle equivalent diameters of 100 or more Cr carbides or Cr nitrides. The Cr carbide and Cr nitride number density was calculated by counting the total number of Cr carbides or Cr nitrides in the 5 fields of view and dividing them by 1 μm 2The number of Cr carbides and Cr nitrides per unit area was calculated.

[0065] "Average sulfide circle equivalent diameter and sulfide number density" The specimen was embedded in resin so that the longitudinal cross section of the wire rod was the test surface, and the test surface was mirror polished. 2 The area of ​​5 fields was observed. Among the inclusions in the field, those in which a large amount of S was detected by EDX analysis were identified as sulfides. The average equivalent circle diameter of sulfides was calculated by averaging the equivalent circle diameters of 100 or more sulfides. The sulfide number density was calculated by measuring the total number of sulfides in the 5 fields and dividing them by 1 μm 2 The number of sulfides per unit area was calculated by converting the number of sulfides per unit area.

[0066] "Cutting tool life" The outer periphery of the wire rod was cut in the circumferential direction for one 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, and 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 "◎", if it was more than 100 μm but less than 200 μm, it was evaluated as "○", and if it was more than 200 μm, it was evaluated as "×".

[0067] "Cutting surface properties" The outer periphery of the wire rod was cut in the circumferential direction for one hour under the precision cutting conditions of: tool: P-type carbide, cutting edge R 0.03 mm, cutting speed: 20 m / min, feed rate: 0.01 mm / rev, depth of cut: 0.1 mm, and cutting oil (mineral oil): present. After cutting, the surface of the wire rod was observed under an optical microscope at 100x magnification. The evaluation was given as "×" if clear built-up edge marks were observed on the surface, "○" if minute built-up edge marks were observed scatteredly, and "◎" if no clear marks were observed.

[0068] Inventive Examples 1 to 58 in Tables 4 and 5, the average Cr carbide circle equivalent diameter was 0.80 μm or less, and the Cr carbide number density was 0.10 particles / μm 2 or more, the average Cr nitride circle equivalent diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 particles / μm 2or more, the average sulfide circle equivalent diameter is 5.0 μm or less, and the sulfide number density is 0.010 particles / μm 2 All of these results demonstrate excellent cutting tool life and cutting surface quality.

[0069] On the other hand, Comparative Examples 1 to 32 in Table 6 are outside the composition ranges of the present invention and are unable to satisfy both excellent cutting tool life and machined surface quality. Even when the average sulfide equivalent circle diameter and sulfide number density are within the preferred ranges, the machined surface quality deteriorated in accordance with the deterioration of cutting tool life.

[0070] <Example 2> To evaluate the influence of the manufacturing process, steel with the chemical composition shown in Table 1 (Steel C) 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. For hot processing, the steel was heated to 1200°C and then hot-rolled at finish rolling temperatures of 760 to 1170°C to produce wire rods with diameters of 5.5 to 40.0 mm. The wires were then cooled to room temperature. Subsequently, batch annealing was performed alone or in addition to batch annealing. For batch annealing, the annealing temperature was 350 to 750°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 550°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, for Inventive Examples 60 to 68 and Comparative Examples 33 to 44 in Table 7, cold drawing was performed with an area reduction of 19.0 to 99.4%, followed by straightening and centerless grinding to produce polished bars (wire rods) with a diameter of 3.0 mm, which were then used as cutting materials. When the area reduction rate was high and work hardening was significant, annealing and wiredrawing were repeated. For Inventive Example 59, a φ5.5 wire rod was used without cold drawing, and centerless grinding was performed to produce polished bars (wire rods) with a diameter of 3.0 mm, which were then used as cutting materials. The average Cr carbide circle equivalent diameter, Cr carbide number density, average Cr nitride circle equivalent diameter, Cr nitride number density, average sulfide circle equivalent diameter, sulfide number density, tool life after peripheral cutting, and surface properties were then evaluated using the same evaluation methods as in Example 1. The results are shown in Table 7.

[0071] [Table 7]

[0072] Inventive Examples 59 and 60 in Table 7, the average Cr carbide equivalent circle diameter was 0.80 μm or less, and the Cr carbide number density was 0.10 particles / μm 2 or more, the average Cr nitride circle equivalent diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 particles / μm 2 In addition, in Example 59 of the present invention, cold drawing was not performed, and in Example 60 of the present invention, the area reduction rate of the cold drawing was outside the lower limit of the preferred range of the present invention, and as a result, the average sulfide circle equivalent diameter and sulfide number density were outside the preferred ranges, and it was not possible to achieve excellent cutting surface properties.

[0073] Inventive Examples 61 to 68 in Table 7, the average Cr carbide circle equivalent diameter was 0.80 μm or less, and the Cr carbide number density was 0.10 particles / μm 2 or more, the average Cr nitride circle equivalent diameter is 0.80 μm or less, and the Cr nitride number density is 0.05 particles / μm 2 or more, the average sulfide circle equivalent diameter is 5.0 μm or less, and the sulfide number density is 0.010 particles / μm 2 As described above, all of the above exhibit excellent cutting tool life and excellent cutting surface properties.

[0074] On the other hand, in Comparative Examples 33 to 43 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 satisfy both excellent cutting tool life and machined surface properties. Furthermore, in Comparative Example 44, the wire drawing area reduction rate was outside the upper limit of the preferred range of the present invention, and manufacturing was impossible.

[0075] As is clear from the above examples, the present invention can provide a martensitic free-cutting stainless steel having an excellent tool life during precision cutting and, more preferably, excellent machined surface properties. This can significantly improve the durability of high-hardness parts used in environments where corrosion is severe and fatigue strength and wear resistance are required, and is therefore extremely useful in industry.

Claims

1. In mass%, C: 0.08-0.60%, Si: 0.10-2.0%, Mn: 0.10-3.0%, S: 0.15-0.40%, P: 0.10% or less, Cr: 11.0-16.0%, The balance has a chemical composition consisting of Fe and impurities, The average Cr carbide equivalent circle diameter is 0.80 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 pieces / μm ２ A martensitic free-cutting stainless steel bar characterized by the above.

2. In mass%, C: 0.08-0.60%, Si: 0.10-2.0%, Mn: 0.10-3.0%, S: 0.15-0.40%, P: 0.10% or less, Cr: 11.0-16.0%, The balance has a chemical composition consisting of Fe and impurities, The average Cr carbide equivalent circle diameter is 0.80 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 pieces / μm ２ or more, the average sulfide equivalent circle diameter is 5.0 μm or less, and the sulfide number density is 0.010 particles / μm ２ A martensitic free-cutting stainless steel bar characterized by the above.

3. The chemical composition of the steel is, in mass%, replacing a part 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: 0.3% or less, Sb: 0.3% or less, Ag: 0.3% 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. The martensitic free-cutting stainless steel bar according to claim 1, further comprising one or more elements selected from the group consisting of: Cr, Ni, Ni, Al, Ni, and Fe;

4. 3. A method for producing a martensitic free-cutting stainless steel bar according to 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 carried out by holding the temperature at 400°C or higher but lower than 700°C for 30 to 300 minutes and then cooling to 400°C at a rate of 20 to 500°C / h, or in addition to the batch annealing, strand annealing is carried out by holding the temperature at 700 to 850°C for 30 to 1000 seconds.

5. The method for producing a martensitic free-cutting stainless steel bar according to claim 4, further comprising cold working at an area reduction rate of 20.0 to 99.0%.

6. 4. A method for producing a martensitic free-cutting stainless steel bar according to claim 3, characterized in that the finish rolling temperature of the hot working is 800 to 1150°C, and after the hot working, batch annealing is carried out by holding the temperature at 400°C or higher but lower than 700°C for 30 to 300 minutes and then cooling to 400°C at a rate of 20 to 500°C / h, or in addition to the batch annealing, strand annealing is carried out by holding the temperature at 700 to 850°C for 30 to 1000 seconds.

7. The method for producing a martensitic free-cutting stainless steel bar according to claim 6, further comprising cold working at an area reduction rate of 20.0 to 99.0%.

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