Martensitic free-cutting stainless steel with excellent precision cutting properties

Abstract

To provide a martensite-based free-cutting stainless steel having cutting surface property of excellent flatness and tool life during precision cutting.SOLUTION: Martensitic free-cutting stainless steel of the present invention has chemical components made of, in mass%, C: 0.10 to 0.70%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, S: 0.02 or more and less than 0.15%, P: 0.10% or less, Cr: 10.5-17.0%, B: 0.001-0.01%, N: 0.02-0.15%, Al: 0.008% or less, O: 0.015% or less, and the balance of a chemical composition consisting of Fe and impurities, and the amount of solid solution N is 0.01 to 0.05%, There are 20 or more BN-based intermetallic compounds with a size of 0.5 μm or less per 100 μm2 and the hardness is 400 Hv or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a martensitic stainless steel that is excellent in precision cutting properties and suppresses the generation of built-up edge, and is used as a material to be cut into small parts that require corrosion resistance. [Background technology]

[0002] High-hardness, corrosion-resistant martensitic stainless steels with hardness of 400Hv or higher are used in industrial equipment and precision machinery components due to their wear resistance, fatigue strength, and corrosion resistance. Precision machinery components, particularly those machined from polished wire rods, are often used for rotating bodies, requiring particularly precise machinability. Specifically, they require both excellent tool life during cutting and a smooth cutting surface without built-up edge (BEE) marks. BEE is a deposit that forms and grows on the cutting tool tip due to adhesion of the base material. When it detaches from the tool tip during cutting, it is pressed onto the cutting surface, degrading surface quality. Figure 1 shows the surface quality of machined products with and without BEE marks. As can be seen from Figure 1, BEE marks significantly degrade surface quality.

[0003] To date, a martensitic free-cutting stainless steel with improved wear resistance and machinability has been proposed, in which S is added to high-hardness, high-corrosion-resistant martensitic stainless steel, and the composition and size of sulfides and carbides are specified (Patent Document 1).

[0004] In addition, a martensitic free-cutting stainless steel with high hardness and high corrosion resistance has been proposed, which is endowed with cold forgeability and hot workability by defining the relationship between the amounts of Cr, Mn, and S (Patent Document 2).

[0005] On the other hand, a martensitic stainless free-cutting steel containing S, which has excellent surface finishability, high hardness, and high corrosion resistance, and in which the amounts of Al and O and the Cr / Mn ratio in sulfides are specified, has been proposed (Patent Document 3).

[0006] Furthermore, a steel with excellent surface properties has been proposed in which B and N are added to define the size of BN-based intermetallic compounds in a high-hardness, high-corrosion-resistant, martensitic stainless free-cutting steel containing S (Patent Document 4). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6207408 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-97039 [Patent Document 3] Patent No. 3638828 [Patent Document 4] Patent No. 5907760 Summary of the Invention [Problem to be solved by the invention]

[0008] Patent Documents 1 and 2 do not describe the cutting surface properties, and do not solve the technical problem of achieving smooth cutting surface properties.

[0009] Patent Document 3 discloses a technique for suppressing coarse oxides to prevent deterioration of surface quality caused by coarse oxides, but does not disclose anything about preventing deterioration of surface quality due to built-up edge marks.

[0010] The technology of Patent Document 4 does not sufficiently suppress built-up edge marks, and has problems from the viewpoint of precise cutting surface properties.

[0011] The present inventors have found that the known techniques described in the background art above or combinations thereof cannot simultaneously achieve excellent tool life and excellent smooth surface properties with suppressed built-up edge marks in high-hardness, highly corrosion-resistant martensitic free-cutting stainless steels that have been imparted with excellent machinability.

[0012] The problem to be solved by the present invention is to provide a high-hardness, high-corrosion-resistant martensitic free-cutting stainless steel that can be used for precision martensitic stainless steel parts to be used in severely corrosive environments, and that can provide excellent tool life during cutting and cutting surface properties with excellent flatness during cutting. [Means for solving the problem]

[0013] In order to solve the above problems, the inventors conducted extensive research on S-containing martensitic free-cutting stainless steel, which can be hardened to a high hardness of 400 Hv or more. As a result, they discovered that by adding B and N to the S-containing free-cutting stainless steel and subjecting it to N solution softening treatment to cause fine BN precipitation, thereby controlling the amount of dissolved N, it is possible to suppress built-up edge marks during precision cutting, thereby achieving surface properties with excellent smoothness during cutting.

[0014] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0015] (1) A steel sheet having a chemical composition containing, by mass%, C: 0.10 to 0.70%, Si: 0.1 to 2.0%, Mn: 0.1 to 3.0%, S: 0.02 to less than 0.15%, P: 0.10% or less, Cr: 10.5 to 17.0%, B: 0.001 to 0.01%, N: 0.02 to 0.15%, Al: 0.008% or less, and O: 0.015% or less, with the balance being Fe and impurities, wherein the amount of solute N is 0.010 to 0.050%, and the amount of BN-based intermetallic compounds of 0.5 μm or less is 100 μm 2 A martensitic free-cutting stainless steel characterized by having 20 or more crystals and a hardness of 400Hv or less.

[0016] (2) The martensitic free-cutting stainless steel of (1) above, characterized in that it further contains, in mass %, one or more of Ni: 1.5% or less, Mo: 2.5% or less, Cu: 1.5% or less, Co: 1.5% or less, and W: 2.5% or less, in place of a portion of the Fe.

[0017] (3) The martensitic free-cutting stainless steel according to (1) or (2), characterized in that it further contains, in mass %, one or more of Bi: 0.20% or less, Sn: 0.30% or less, Sb: 0.30% or less, Ag: 0.30% or less, and Te: 0.10% or less, in place of a portion of the Fe.

[0018] (4) A martensitic free-cutting stainless steel according to any one of (1) to (3), characterized in that it further contains, in mass %, one or more of V: 0.8% or less, Nb: 0.3% or less, Ti: 0.3% or less, and Ta: 0.3% or less, in place of a portion of the Fe.

[0019] (5) A martensitic free-cutting stainless steel according to any one of (1) to (4), characterized in that it further contains, in mass %, one or more of Mg: 0.010% or less, Ca: 0.010% or less, Hf: 0.010% or less, and REM: 0.050% or less, in place of a portion of the Fe.

[0020] (6) A method for producing a martensitic free-cutting stainless steel of the present invention, comprising: a heating step of heating a slab having a chemical composition according to any one of (1) to (5) to 1150 to 1330°C; a processing step of hot-rolling or hot-working the heated slab; a batch annealing step of holding the slab at 750 to 900°C for 60 to 300 minutes; and a strand annealing step of holding the slab in the temperature range of 700 to 850°C and cooling it at a cooling rate of 1°C / s or more in a temperature range of 500°C or higher. [Effects of the Invention]

[0021] According to the present invention, by appropriately adjusting the components and applying heat treatment to control the metal structure and intermetallic compounds in martensitic free-cutting stainless steel, it is possible to obtain good cutting tool life and excellent flat cutting surface properties with reduced built-up cutting edge marks, thereby providing a material that is suitable for precision parts with high hardness, wear resistance, and corrosion resistance. [Brief explanation of the drawings]

[0022] [Figure 1]Figure 1 shows examples of the surface properties of cut products made from Cr-based stainless steel bars. (a) shows an example with traces of built-up cutting edge loss, and (b) shows an example without traces of built-up cutting edge loss. DETAILED DESCRIPTION OF THE INVENTION

[0023] Each requirement of the present invention will be explained below. In the following explanation, (%) means mass (%) unless otherwise specified.

[0024] <Essential composition of steel> The martensitic free-cutting stainless steel of the present invention has a hardness of 400 Hv or less, but when quenched, it becomes 400 Hv or more, which is generally effective for wear resistance. Therefore, the steel composition is based on a high-hardness martensitic stainless steel that exhibits a hardness of at least 400 Hv or more in the quenched state.

[0025] C is added at a content of 0.10% or more to ensure that the hardness of the base material after quenching is 400 Hv or more. However, if added in excess of 0.70%, coarse Cr carbides are formed, deteriorating the cutting surface properties. Therefore, the content is limited to 0.70% or less. High hardness is preferred, and to obtain a product hardness of 500 Hv or more after quenching, a content of more than 0.20% and 0.50% or less is preferred.

[0026] N is added in an amount of 0.02% to obtain BN-based intermetallic compounds in the N solution softening treatment described below, and to ensure the amount of dissolved N and suppress the formation of built-up edge marks. However, if the content exceeds 0.15%, blowholes will form, deteriorating product quality. Therefore, the N content is limited to 0.15% or less. The preferred range is 0.03 to 0.10%.

[0027] The Si content is set to 0.1% or more to suppress the formation of coarse inclusions that degrade the cutting surface quality through deoxidation. However, if the Si content exceeds 2.0%, the tool hardens, promoting adhesion of the base material to the tool and 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%.

[0028] Mn is contained in an amount of 0.1% or more to suppress the formation of coarse inclusions that deteriorate the cutting surface quality by deoxidizing and to form sulfides to ensure good tool life. However, if the Mn content exceeds 3.0%, it becomes difficult to soften the steel to 400 Hv or less by N solid solution softening treatment, and the tool life deteriorates. Therefore, the Mn content is limited to 3.0% or less. The preferred range is 0.2 to 2.0%.

[0029] S is added at 0.02% or more to form sulfides and ensure good tool life. However, if the content is 0.15% or more, the generation of built-up edge marks on the tool during precision cutting becomes significant, making it impossible to prevent the build-up edge marks on the cutting surface. Therefore, the content is limited to less than 0.15%. The preferred range is 0.03% or more and 0.12% or less.

[0030] P is an unavoidable impurity that is mixed in from the raw materials, but if it exceeds 0.1%, 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 0.1% or less, and preferably 0.05% or less.

[0031] Cr is a basic element for imparting corrosion resistance to stainless steel, and is contained in an amount of 10.5% or more. However, if the Cr content exceeds 17.0%, it becomes impossible to ensure a hardness of 400 Hv or more after quenching. Therefore, the Cr content is limited to 17.0% or less. The preferred range is 11.0 to 15.0%.

[0032] When added together with N, B forms fine BN during N solution softening treatment, suppressing adhesion of the base material to the tool surface and preventing built-up edge marks on the cutting surface. Therefore, 0.0010% or more is added. However, if added in excess of 0.0100%, coarse borides are formed, which in turn promotes the formation of built-up edge marks. Therefore, the content is limited to 0.0100% or less. The preferred range is 0.0020 to 0.0070%.

[0033] Al may be added for deoxidation, but if added in excess of 0.008%, coarse inclusions are formed, deteriorating the surface properties. Therefore, the Al content is limited to 0.008% or less, preferably 0.006% or less.

[0034] O is present as an unavoidable impurity, but if it exceeds 0.015%, coarse inclusions are formed, deteriorating the quality of the machined surface. Therefore, the O content is limited to 0.015% or less, and preferably 0.012% or less.

[0035] The martensitic free-cutting stainless steel of the present invention contains solute N in an amount of 0.01 to 0.05%.

[0036] Solute N in the annealed ferrite matrix is effective in embrittling the base material during cutting, suppressing the formation of built-up edges, and achieving good surface quality. Therefore, the amount of solute N is limited to 0.01% or more. However, attempting to dissolve N at levels exceeding 0.05% requires raising the solution temperature, which will result in the formation of a hard martensite structure and reduced tool life. Therefore, the upper limit of the amount of solute N is set at 0.05%. Note that, because typical martensitic stainless steels are annealed by batch annealing, nitrides are formed due to furnace cooling, resulting in a solute N content of less than 0.01%. In the present invention, N is dissolved in the ferrite matrix by a solution softening treatment that combines batch annealing (slow cooling) and strand annealing (rapid cooling), as described below.

[0037] The metal structure of the martensitic free-cutting stainless steel of the present invention is such that BN-based intermetallic compounds of 0.5 μm or less are mixed in a 100 μm 2 Contains 20 or more items.

[0038] BN is effective in obtaining excellent cutting surface quality by suppressing adhesion of the base material to the tool through its self-lubricating action, preventing the formation of a built-up edge. However, if coarse BN particles exceeding 0.5 μm in size are dispersed, the base material plastically deformed at the tip of the tool and the BN will be layered and accumulated, causing the formation and growth of a built-up edge. Therefore, fine dispersion of BN particles with a major axis of 0.5 μm or less will suppress the layering and accumulation. In addition, if the BN particles are 100 μm in size, the layering and accumulation will be suppressed.2 The effect becomes remarkable when BN is finely dispersed in the matrix at a number density of 20 or more. 2 There are more than 30 of them.

[0039] The martensitic free-cutting stainless steel of the present invention is set to have a hardness of 400 Hv or less, taking into consideration the tool life during cutting.

[0040] As mentioned above, the formation of a hard martensite structure significantly reduces tool life. Therefore, the hardness of the material for cutting is limited to 400 Hv. Preferably, it is 200 to 350 Hv. The lower limit of Hv hardness that can be achieved by softening annealing is 150 Hv.

[0041] Selectively contained ingredients 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.

[0042] The elements Ni, Cu, and Co do not need to be added. Addition has the effect of improving the corrosion resistance and toughness of the product. However, if added in excess of 1.5%, the steel will be difficult to soften during annealing, and the hardness after annealing will exceed 400 Hv, shortening the tool life. Therefore, the content is set 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.

[0043] The elements Mo and W do not need to be added. Addition has the effect of improving the corrosion resistance of the product. However, if added in excess of 2.5%, the effect saturates and, conversely, tool life during cutting processing is reduced. Therefore, the content is set to 2.0% or less. To ensure the above effects, it is preferable to set the content of each element to 0.01% or more and 2.0% or less.

[0044] The element Bi does not need to be added. If added, it acts as a self-lubricant during cutting, suppressing the formation of built-up edges and improving the quality of the cut surface. However, if added in excess of 0.20%, hot workability will be significantly degraded, making it impossible to manufacture. Therefore, the content should be 0.20% or less. To ensure the above effects, the content should preferably be 0.005% or more and 0.10% or less.

[0045] The elements Sn, Sb, and Ag do not necessarily need to be added. If added, they act as a self-lubricant during cutting, suppressing the formation of built-up edges and improving the quality of the cut surface. However, if added in excess of 0.30%, hot workability deteriorates significantly, making manufacturing impossible. Therefore, the content is set to 0.30% or less. To ensure the above effects, it is preferable to set the content to 0.005% or more and 0.20% or less.

[0046] The element Te does not need to be added. If added, it generates spherical sulfides, which have the effect of suppressing the accumulation and growth of built-up edges and improving the cutting surface quality. However, if added in excess of 0.10%, hot workability deteriorates significantly, making it impossible to manufacture. Therefore, the content should be 0.10% or less. To ensure the above effect, it is preferable to keep the content between 0.005% and 0.05%.

[0047] The element V does not need to be added. Addition has the effect of improving the corrosion resistance of the product. However, if added in excess of 0.8%, coarse carbonitrides are formed, promoting the stacking and growth of built-up edges, deteriorating the cutting surface quality and shortening the tool life. Therefore, the upper limit is set to 0.8%. To ensure the above effect, it is preferable to keep the content between 0.05% and 0.5%.

[0048] The elements Nb, Ti, and Ta do not need to be added. Addition has the effect of improving the corrosion resistance of the product. However, if added in excess of 0.3%, coarse carbonitrides are formed, promoting the stacking and growth of built-up edges, deteriorating the cutting surface quality and shortening the tool life. Therefore, the upper limit is set to 0.3%. To ensure the above effects, it is preferable to keep the content between 0.01% and 0.2%.

[0049] The elements Mg, Ca, and Hf do not need to be added. Addition has the effect of improving hot workability. However, if added in excess of 0.010% of each, the effect saturates and, conversely, coarse oxides are generated, degrading the cutting surface quality. Therefore, the content is set to 0.010% or less. To ensure the above effects, it is preferable to set the content to 0.001% or more and 0.005% or less.

[0050] The addition of REM elements is not necessary. Addition improves hot workability. However, if added in excess of 0.050%, the effect saturates and, conversely, coarse oxides are generated, degrading the cutting surface quality. Therefore, the content should be 0.050% or less. To ensure the above effect, it is preferable to keep the content between 0.001% and 0.005%. REM (rare earth elements), as generally defined, refers collectively to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) ranging from lanthanum (La) to lutetium (Lu). They may be added alone or in mixtures.

[0051] Typical impurities contained in the stainless steel of the present invention include Zn, Pb, Ge, Se, Ag, Se, etc., and these impurities are usually mixed in at a concentration of about 0.1% during the steel manufacturing process.

[0052] 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%.

[0053] Furthermore, while typical optional elements are specified in (2) to (5) above, elements not listed in this specification may also be included within a range that does not impair the effects of the present invention.

[0054] <Method for manufacturing martensitic stainless steel> A method for producing the martensitic stainless steel of the present invention will now be described.

[0055] BN of 0.5μm or less size is 100μm 2 In order to finely disperse BN at a number density of 20 or more in the steel, a cast slab having the above-mentioned components is first heated to a high temperature of 1150 to 1330°C, and then hot-rolled or hot-worked to dissolve the BN. The resulting steel material (wire material, steel wire, etc.) is then batch-annealed at 700 to 900°C for 60 to 300 minutes.

[0056] If the heating temperature of the slab is less than 1150°C, BN does not dissolve and the above number density is not satisfied, and if it exceeds 1330°C, manufacturability deteriorates. Therefore, the heating temperature is set to 1150 to 1330°C, and from the viewpoint of dissolving BN, it is preferably set to more than 1250°C and not more than 1320°C.

[0057] Furthermore, if the batch annealing temperature is lower than 700°C and the time is shorter than 60 minutes, the amount of BN precipitates decreases, and the above-mentioned number density is not met. On the other hand, if the batch annealing temperature is higher than 900°C and the time is longer than 300 minutes, the BN size becomes coarse, and the above-mentioned number density is not met. Therefore, the batch annealing conditions are limited to 700 to 900°C and holding time of 60 to 300 minutes.

[0058] In order to ensure the amount of dissolved N in ferrite while maintaining the above-mentioned dispersed state of BN, strand annealing is performed after the batch annealing, in which the temperature is held in the range of 700 to 850°C and cooled at a cooling rate of 1°C / s or more in a temperature range of 500°C or higher.

[0059] In the softening annealing of ordinary martensitic stainless steel, the entire coil of wire rod or steel wire is held at a temperature range of approximately 650 to 950°C by batch annealing and then furnace cooled, resulting in slow cooling at a cooling rate of the order of 0.02°C / s, for example. As a result, N that was in solid solution at high temperatures changes its state to nitride during slow cooling, so the amount of solute N in ordinary batch annealing is less than 0.01%.

[0060] In the present invention, after the above-mentioned annealing, strand annealing is performed by holding the temperature in the 700 to 850°C range where the N solid solubility limit in the ferrite phase is 0.01 to 0.05% and BN does not form a solid solution, and quenching at 1°C / s or more in the temperature range of 500°C or higher where nitrides form. This makes it possible to maintain the N solid solution in the ferrite phase at 0.01 to 0.05% while maintaining the finely dispersed state of BN.

[0061] Strand annealing is an annealing method in which a wire rod or steel wire coil wound into 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 makes it possible to achieve a significantly faster cooling rate than batch annealing of the entire ring-shaped coil.

[0062] At this time, if the holding temperature is less than 700°C, the amount of solute N will be less than 0.010%, and if it is 850°C or higher, a hard martensite structure will form and BN will also dissolve, making it impossible to obtain the above-mentioned sufficient machinability (cut surface properties, tool life). Therefore, the strand annealing temperature is limited to the range of 700 to 850°C, preferably 750 to 820°C. The storage time in the furnace is optional, but a range of 30 to 1000 seconds is preferable because it prevents thermal deformation of the material and allows for uniform heating.

[0063] Furthermore, processing such as cold wire drawing may be performed between the batch annealing and strand annealing steps. Batch annealing and strand annealing may be performed multiple times, but the final heat treatment must be strand annealing.

[0064] According to the present invention as described above, it is possible to provide a martensitic free-cutting stainless steel that can suppress the formation and growth of built-up edges that deteriorate the quality of the machined surface during cutting, and is suitable for use in high-hardness parts that are precision-cut.

[0065] In the present invention, martensitic stainless steel refers to a steel that hardens due to martensitic transformation during quenching. The martensitic free-cutting stainless steel of the present invention is a steel in which, for example, when quenched by air cooling from 1,050°C, 70% or more of the metal structure exhibits a martensitic structure and hardens to 400 Hv or more. [Example]

[0066] Example 1 Steels with the chemical compositions shown in Tables 1 and 2 were melted at 1600°C in a 65 kg vacuum melting furnace and then cast into a mold. After heating to 1200°C, the wire was hot-worked to a diameter of 9 mm and batch-annealed at 700-900°C for 85-120 minutes. The wire was then cold-drawn to a diameter of 6.3 mm and strand-annealed (in ammonia decomposition gas) at 800°C for 200 seconds. The wire was then drawn to a diameter of 6.0 mm using a drawing machine to create a polished bar, which was then used as a material for cutting.

[0067] [Table 1]

[0068] [Table 2]

[0069] The bars thus obtained were evaluated for the number density of BN compounds (number per unit area), material hardness, surface properties and tool life after peripheral cutting, and quenched hardness using the evaluation methods described below. The results are shown in Tables 3 and 4. Table 3 shows the evaluation results for the steels of the present invention, and Table 4 shows the evaluation results for the comparative steels.

[0070] [Table 3]

[0071] [Table 4]

[0072] "Intermetallic compounds (BN compounds)" The longitudinal section of the rod wire was embedded in resin and mirror-polished, and the precipitates were electrolytically extracted using an alcohol-based etching solution. A replica sample was then made by carbon deposition, and the size and distribution of the precipitates were then investigated using a transmission electron microscope. Five fields of view were observed over an area of 100 μm2, and the number density of BN compounds with a major axis of 0.5 μm or less per 100 μm2 was determined.

[0073] "Material hardness" The specimen was embedded in resin and mirror-polished, and the center of the longitudinal section was measured for Hv hardness under a 1 kg load according to JIS Z2244.

[0074] "Cutting surface properties" The outer periphery of the wire rod was cut in the circumferential direction under the following precision cutting conditions: tool used: carbide P type, cutting edge R 0.4 mm, cutting speed: 50 m / min, feed rate: 0.02 mm / rev, depth of cut: 0.1 mm, cutting oil (mineral oil): present, feed rate: 0.05 mm / rev. The surface after cutting was observed with a 100x magnifying glass and evaluated for the presence or absence of clear BEE marks. If clear BEE marks were observed, they were evaluated as ×, if tiny BEE marks were observed, they were evaluated as ◯, and if they were not clearly observed, they were evaluated as ◎.

[0075] "Tool life" Tool used: Carbide P type, cutting edge R 0.4 mm, cutting speed: 200 m / min, feed rate: 0.02 mm / rev, depth of cut: 0.11 mm, cutting oil (mineral oil): with feed rate 0.05 mm / rev or less, depth of cut ≦ 0.3 mm. Under these precision cutting conditions, the outer periphery of the wire rod was cut in the circumferential direction, and the condition of the tool after 30 minutes of cutting was examined. If the amount of flank wear of the tool after use, including notch wear and knot wear, was 20 μm or less, it was evaluated as ◎; if it was more than 20 μm but less than 50 μm, it was evaluated as ○; if it was more than 50 μm, it was evaluated as ×.

[0076] "Quenched hardness" After cutting into 10 mm lengths, the specimens were quenched by air cooling from 1100°C, embedded in resin, polished, and the Hv hardness of the center of the longitudinal section was measured under a 1 kg load according to JIS Z2244.

[0077] "Solute N amount" 1 g of the material was electrolyzed in maleic anhydride, and the precipitate was extracted using 0.2 μm mesh filter paper. The extract was then dissolved in an acid solution and the amount of N precipitated as nitrides in the precipitate was determined by atomic absorption spectrometry. The amount of dissolved N was calculated by subtracting the amount of N in the precipitate from the total amount of nitrogen in the steel.

[0078] Examples 1 to 45 of the present invention in Table 3 have a number density of BN with a major axis of 0.5 μm or less of 20 or more per 100 μm2, a solute N content of 0.01% or more, and a material hardness of 400 Hv or less, all of which exhibit excellent cutting surface properties and tool life.

[0079] On the other hand, Comparative Examples 1 to 9 and 11 to 35 in Table 4 were outside the ranges of the composition, BN number density, material hardness, and solute N content of the present invention, and were unable to satisfy all of the requirements for excellent cut surface quality and tool life. Comparative Example 10 had a low Cr content, and the ranges of the BN number density, material hardness, and solute N content were within the ranges of the present invention, and the cut surface quality and tool life were excellent, but the corrosion resistance was insufficient.

[0080] Example 2 Steel B was melted at 1600°C in a 65 kg vacuum melting furnace and cast into a mold. After heating to 1100-1350°C, it was hot-worked into a φ9 mm wire rod, softened at 650-1000°C for 30-500 minutes, cold-drawn to a φ6.3 mm rod, and strand-annealed (in ammonia decomposition gas) at 650-900°C for 20-1500 seconds. It was then drawn into a φ6.0 mm polished bar using a drawing machine to prepare the material for finish cutting. The number density (number per unit area) of BN compounds, the material hardness, the surface quality and tool life after peripheral cutting, and the quenched hardness were evaluated. The results are shown in Table 5.

[0081] [Table 5]

[0082] In Table 2, Examples 46 to 53 of the present invention have a number density of BN with a major axis of 0.5 μm or less and a number density of 100 μm 2 The particles are 20 or more, the amount of solute N is 0.01% or more, and the material hardness is 400Hv or less, all of which result in excellent cutting surface properties and tool life.

[0083] On the other hand, Comparative Examples 36 to 42 in Table 5 are outside the component ranges, BN number density range, material hardness range, and solute N content range of the present invention, and are unable to satisfy all of the requirements of excellent cutting surface properties and tool life. [Industrial Applicability]

[0084] As is clear from the above examples, the present invention can provide a martensitic free-cutting stainless steel that exhibits excellent flatness in cut surface properties during precision cutting and long tool life, 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. In mass%, C: 0.10-0.70%, Si: 0.1-2.0%, Mn: 0.2-3.0%, S: 0.02 or more and less than 0.15%; P: 0.10% or less, Cr: 10.5-17.0%, B: 0.0010 to 0.010%, N: 0.02 to 0.15%, Al: 0.008% or less, and O: 0.015% or less and the balance being Fe and impurities, The amount of solute N is 0.010 to 0.050%; BN-based intermetallic compounds of 0.5 μm or less are 100 μm 2 There are more than 20 of them, Hardness is 400Hv or less A martensitic free-cutting stainless steel wire rod characterized by:

2. Part of the Fe is replaced with further mass% Ni: 1.5% or less, Mo: 2.5% or less, Cu: 1.5% or less, Co: 1.5% or less, and W: 2.5% or less Contains one or more of the following:

2. The martensitic free-cutting stainless steel wire rod according to claim 1.

3. Part of the Fe is replaced with further mass% Bi: 0.20% or less, Sn: 0.30% or less, Sb: 0.30% or less, Ag: 0.30% or less, and Te: 0.10% or less Contains one or more of the following:

3. The martensitic free-cutting stainless steel wire rod according to claim 1 or 2.

4. Part of the Fe is replaced with further mass% V: 0.8% or less, Nb: 0.3% or less, Ti: 0.3% or less, and Ta: 0.3% or less Contains one or more of the following:

4. The martensitic free-cutting stainless steel wire rod according to claim 1.

5. Part of the Fe is replaced with further mass% Mg: 0.010% or less, Ca: 0.010% or less, Hf: 0.010% or less, and REM: 0.050% or less Contains one or more of the following:

5. The martensitic free-cutting stainless steel wire rod according to claim 1.

6. A method for producing a martensitic free-cutting stainless steel wire rod according to any one of claims 1 to 5, comprising: A heating step of heating a cast having the chemical composition according to any one of claims 1 to 5 to 1150 to 1330 ° C.; a processing step of hot rolling or hot working the heated slab; Batch annealing process at 750 to 900°C for 60 to 300 minutes; A strand annealing process in which the temperature is maintained in the range of 700 to 850°C and cooled at a rate of 1°C / s or more in the temperature range of 500°C or more. The method for producing a martensitic free-cutting stainless steel wire rod of the present invention, comprising:

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