Matian Santie series free-machining stainless steel bars and their manufacturing method

Abstract

The free-machining stainless steel bar of the present invention is characterized by having the following chemical composition: C: greater than 0.60% and less than 1.00%, Si: 0.10~2.0%, Mn: 0.10~3.0%, S: less than 0.050%, P: less than 0.10%, Cr: 11.0~16.0%, and the remainder is composed of Fe and impurities; the average circle equivalent diameter of Cr carbides is less than 1.20 μm, the number density of Cr carbides is more than 0.10 particles / μm2, the average circle equivalent diameter of Cr nitrides is less than 0.80 μm, and the number density of Cr nitrides is more than 0.05 particles / μm2.

Description

Technical Field

[0001] This invention relates to a material that requires corrosion resistance and can be machined into small parts, and to a free-machining stainless steel bar made of Asada-san iron series with excellent machinability and suppresses tool wear, and a method for manufacturing the same. Prior Technology

[0002] From the perspective of wear resistance, fatigue strength, and corrosion resistance, high-hardness and high-corrosion-resistant Asada steel, with a hardness of over 500 Hv, can be used in industrial machinery or precision machine parts. In particular, precision machine parts obtained from cold-drawn bars are mostly used for rotating bodies, thus requiring exceptional machinability. Specifically, tool wear during machining is required to be less than 200 µm. Hard precipitates or inclusions present in the workpiece material accelerate tool wear.

[0003] To date, a type of free-machining stainless steel based on the Asada-san iron alloy has been proposed. This type of stainless steel contains sulfur (S) and specifies the composition and size of sulfides and carbides to improve wear resistance and machinability (Reference 1 (Japanese Patent Application Laid-Open No. 2015-137381)). However, it only considers very large carbide sizes of 10 μm² or more, which cannot fully meet the requirements for tool life and cutting surface properties.

[0004] Furthermore, a steel has been proposed that improves cold workability and machinability by specifying the average circle equivalent diameter of carbides and the cleanliness of inclusions in a cold tool steel with high cold workability (Reference 2 (Japanese Patent Application Publication No. 2006-193790)). However, this proposal improves machinability and cold workability by setting the average circle equivalent diameter of carbides to be 0.25µm or more and 0.8µm or less, utilizing low hardness. It does not address the fact that coarse carbides with an average circle equivalent diameter greater than 0.8µm promote tool wear and degrade machinability, while carbides with an average circle equivalent diameter less than 0.25µm are considered to have very high hardness and cannot achieve the effect of improving machinability. Furthermore, the characteristics of the cutting surface are not addressed.

[0005] Furthermore, proposals have been made for the following types of steel: a steel containing B and N in an S-containing Asada-type stainless steel for free cutting, with specified dimensions of the BN intermetallic compound; or a steel that reduces the aspect ratio of inclusions in the steel to suppress the formation of a built-up edge, thus resulting in excellent surface properties (Document 3 (Japanese Patent Application Publication No. 2013-185195), Document 4 (International Publication No. WO2019 / 240209)). However, tool life has not been addressed. Summary of the Invention

[0006] As can be known from the prior art or combinations thereof described in the above background art, it is impossible to achieve an excellent tool life by suppressing tool wear to below 200µm during cutting in high-hardness and high-corrosion-resistant free-cutting stainless steels with excellent machinability.

[0007] The problem to be solved by the present invention is to provide a high-hardness and high-corrosion-resistant free-machining stainless steel bar and a method for manufacturing the same. The free-machining stainless steel bar exhibits excellent tool life during machining when used as a precision part of the Mada-san-based stainless steel that can be used in harsh corrosive environments.

[0008] To address the aforementioned issues, the inventors conducted various studies on free-machining stainless steels based on the Asada steel system, which can achieve a high hardness of over 500 Hv through quenching. The results yielded the following insights: by controlling the average circle equivalent diameter and number density of Cr carbides and Cr nitrides, excellent tool life can be achieved during precision machining.

[0009] This invention is made based on the above knowledge and insights, and its main purpose is as follows.

[0010] [1] A type of Matian Santie series free-machining stainless steel bar, characterized in that: It has the following chemical composition: In terms of mass%, C: Greater than 0.60% and less than 1.00% Si: 0.10~2.0% Mn: 0.10~3.0%, S: Below 0.050% P: below 0.10% Cr: 11.0~16.0%, and The remaining portion consists of Fe and impurities; The average circle equivalent diameter of Cr carbides is less than 1.20 μm and the number density of Cr carbides is more than 0.10 particles / μm2. The average circle equivalent diameter of Cr nitrides is less than 0.80 μm and the number density of Cr nitrides is more than 0.05 particles / μm2.

[0011] [2] As in [1], the Mada Sante is a free-machining stainless steel bar, wherein the aforementioned chemical composition of the steel contains one or more of the following groups to replace one portion of the aforementioned Fe: In terms of mass%, B: Below 0.01% N: below 0.15% Al: below 0.008% O: Below 0.015% Ni: below 1.5% Cu: less than 1.5% Co: less than 1.5% Mo: 2.5% or less, W: Below 2.5% Bi: below 0.2% Sn: below 1.0% Sb: below 1.0% Ag: less than 1.0% Te: below 0.1% V: Below 0.8% Nb: below 0.3% Ti: below 0.3% Ta: below 0.3% Mg: less than 0.01% Ca: below 0.01% Hf: below 0.01%, and REM: below 0.05%.

[0012] [3] A method for manufacturing Mada Sante series free-machining stainless steel rods, which is a method for manufacturing Mada Sante series free-machining stainless steel rods as described in [1] or [2]; The manufacturing method is characterized by: The finishing rolling temperature for hot working is 800~1150℃. After hot working, batch annealing is performed. The batch annealing is performed at a temperature above 400℃ and below 850℃ for 30~300 minutes, and then cooled to 400℃ at a rate of 20~200℃ / hour; or in addition to the aforementioned batch annealing, strand annealing is also performed at a temperature of 700~850℃ for 30~1000 seconds.

[0013] According to the present invention, the following material and manufacturing method are provided for free-cutting stainless steel bars of the Asada Santetsu series, which can achieve good cutting tool life and are suitable for precision parts with high hardness and excellent corrosion resistance. Implementation

[0014] The various elements of the present invention are described below. Furthermore, unless otherwise specified, all percentages (%) in the following description refer to mass (%). Essential Elements

[0015] In this invention, the object is a part with a hardness of 500 Hv or higher that is generally effective in terms of wear resistance, and is based on Masada-san iron-based free-machining stainless steel with a high hardness of at least 500 Hv in the quenched state.

[0016] C will give the base material a hardness of over 500 Hv after quenching, so it can contain more than 0.60% C. However, if it contains more than 1.00% C, the average circle equivalent diameter of Cr carbides will be greater than 1.20 µm, leading to a deterioration in cutting tool life. Therefore, the C content is limited to below 1.00%. The C content should preferably be in the range of 0.65% to 0.95%.

[0017] Si can deoxidize and inhibit the formation of large inclusions that degrade cutting tool life, therefore it can contain more than 0.10% Si. However, if the Si content is greater than 2.0%, it will harden, leading to a degraded cutting tool life, and will also promote the adhesion of the base material to the tool, thereby contributing to the formation of built-up edges. Therefore, the Si content is limited to 2.0% or less. The Si content should preferably be in the range of 0.20% to 1.0%.

[0018] Mn can deoxidize and inhibit the formation of large inclusions that degrade cutting tool life, and it also forms sulfides to ensure good cutting surface properties. Therefore, it can contain more than 0.10% Mn. However, if the Mn content is greater than 3.0%, it will harden and degrade cutting tool life. Therefore, the Mn content is limited to 3.0% or less. The Mn content should preferably be in the range of 0.20% to 2.0%.

[0019] The presence of sulfur (S) allows for the formation of sulfides, resulting in good tool life and surface finish. However, if the S content exceeds 0.050%, it will harden the material, leading to a decrease in tool life. Therefore, the S content is limited to 0.050% or less, and preferably 0.030% or less.

[0020] Phosphorus (P) will inevitably be introduced into the raw materials as an impurity. However, if the content of P exceeds 0.10%, it will lead to a deterioration in corrosion resistance and a significant reduction in manufacturability due to grain boundary segregation. Therefore, the P content is limited to 0.10% or less. The P content should preferably be 0.05% or less.

[0021] Cr is a fundamental element used to obtain the corrosion resistance of stainless steel, and it can contain more than 11.0%. However, if the Cr content exceeds 16.0%, it becomes impossible to guarantee a hardness of 500 Hv or higher during quenching. Therefore, the Cr content is limited to 16.0% or less. The Cr content should preferably be between 11.5% and 15.0%. Equivalent diameter and number density of Cr carbides and Cr nitrides

[0022] Cr carbides and Cr nitrides have very high hardness, which causes tool wear and abrasion during cutting, thus reducing tool life. The larger the Cr carbides and Cr nitrides and the lower their number density, the more significant the effect. Therefore, the average circle equivalent diameter of Cr carbides is limited to below 1.20 μm, and the number density of Cr carbides is limited to above 0.10 particles / μm². The average circle equivalent diameter of Cr nitrides is limited to below 0.80 μm, and the number density of Cr nitrides is limited to above 0.05 particles / μm². Ideally, the average circle equivalent diameter of Cr carbides should be below 1.00 μm, the number density of Cr carbides should ideally be above 0.20 particles / μm², and the average circle equivalent diameter of Cr nitrides should ideally be below 0.70 μm, and the number density of Cr nitrides should ideally be above 0.10 particles / μm². "Selection of Elements"

[0023] In the stainless steel of the present invention, apart from the elements previously described, the composition consists of a chemical composition of Fe and impurities. Furthermore, in addition to containing the aforementioned components, elements as shown below may be selectively included to replace a portion of the Fe.

[0024] By containing both nitrogen (N) and boron (B), fine boron oxides (BN) are formed, inhibiting the adhesion of the base material to the tool surface and thus preventing chip build-up marks on the cutting edge. Therefore, B can also be included as needed. However, if the B content exceeds 0.01%, coarse borides will form, leading to a decrease in cutting tool life and promoting chip build-up. Therefore, the B content is limited to 0.01% or less. To reliably achieve the above effects, the B content should be set between 0.002% and 0.007%.

[0025] Nitrogen (N) not only forms fine BN (Brown Nitride) to prevent chip buildup on the cutting edge, but also increases the hardness of the base material after quenching. Therefore, N can be included as needed. However, if the N content exceeds 0.15%, the average circle equivalent diameter of Cr nitride will exceed 0.80 μm, leading to a deterioration in tool life. Therefore, the N content should be set below 0.15%. To reliably achieve the above effects, the N content should be set between 0.03% and 0.10%.

[0026] Al can be included for deoxidation, but if the Al content exceeds 0.008%, it will generate large inclusions, leading to a deterioration in the cutting tool's life. Therefore, the Al content is limited to 0.008% or less. The Al content should preferably be 0.006% or less.

[0027] O (O) will inevitably be introduced as an impurity; however, if the O content exceeds 0.015%, it will generate large inclusions, leading to a deterioration in cutting tool life. Therefore, the O content is limited to 0.015% or less. Ideally, the O content should be 0.012% or less.

[0028] Ni, Cu, and Co can improve the corrosion resistance and toughness of products, and therefore may be included as needed. However, if each contains more than 1.5%, it will degrade the life of cutting tools. Therefore, the content of each element is limited to 1.5% or less. To ensure the above-mentioned effects, it is advisable to set the content of each element to be more than 0.01% and less than 1.0%.

[0029] Mo and W can improve the corrosion resistance of products, and therefore may be included as needed. However, if each contains more than 2.5%, the effect will saturate and the cutting tool life will be reduced. Therefore, the content of each element is limited to 2.5% or less. To ensure the above-mentioned effects, the content of each element should be set between 0.01% and 2.0%.

[0030] Bi (Bi) functions as a self-lubricant during machining, inhibiting the formation of chip build-up edges and thus improving the surface finish. Therefore, Bi can be included as needed. However, if the Bi content exceeds 0.2%, the thermal manufacturability will significantly deteriorate, making manufacturing impossible. Therefore, the Bi content is limited to 0.2% or less. To reliably achieve the above effects, the Bi content should be set between 0.005% and 0.10%.

[0031] Sn, Sb, and Ag function as self-lubricants during machining, inhibiting the formation of chip build-up edges and thus improving the surface finish. Therefore, they may be included as needed. However, if each element contains more than 1.0%, thermal manufacturability will significantly deteriorate, rendering the material unmanufacturable. Therefore, the content of each element is limited to 1.0% or less. To reliably achieve the aforementioned effects, the content of each element should be set between 0.005% and 0.50%.

[0032] Te can form spherical sulfides, inhibiting the buildup and growth of chips on the cutting edge, thereby improving the surface properties of the cut. Therefore, Te can be included as needed. However, if the Te content exceeds 0.1%, the heat manufacturability will deteriorate significantly, making it unmanufacturable. Therefore, the Te content is limited to 0.1% or less. To ensure the above effects are achieved, the Te content should be set between 0.005% and 0.05%.

[0033] Vitamin V can improve the corrosion resistance of products, and therefore may be included as needed. However, if the content of V exceeds 0.8%, it will form coarse carbonitrides, which will degrade the life of cutting tools. Therefore, the V content is limited to 0.8% or less. To ensure the above-mentioned effects, the V content should be set between 0.05% and 0.5%.

[0034] Nb, Ti, and Ta can improve the corrosion resistance of products, and therefore may be included as needed. However, if each contains more than 0.3%, coarse carbonitrides will form, degrading the life of cutting tools. Therefore, the content of each element is limited to 0.3% or less. To ensure the above-mentioned effects, the content of each element should be set at 0.01% or more and 0.2% or less.

[0035] Mg, Ca, and Hf have the effect of improving thermal manufacturability, and therefore may be included as needed. However, even if each contains more than 0.01%, the effect will saturate, and coarse oxides will be formed, which will degrade the cutting tool life. Therefore, the content of each element is limited to 0.01% or less. To reliably obtain the above-mentioned effects, it is advisable to set the content of each element to 0.001% or more and 0.005% or less.

[0036] REM (Rare Earth Elements) improves thermal performance and can therefore be included as needed. However, even with a REM content greater than 0.05%, this effect will saturate, and coarse oxides will form, degrading tool life. Therefore, the REM content is limited to 0.05% or less. To reliably achieve the aforementioned effects, the REM content should be set between 0.001% and 0.005%. By general definition, REM refers to scandium (Sc) and yttrium (Y), and 15 elements from lanthanum (La) to ilmenite (Lu) (lanthanides). These elements can be present individually or in mixtures.

[0037] Regarding the impurities contained in the stainless steel of this invention, representative impurities include Zn, Pb, Ge, Se, etc., and are usually mixed in as impurities in the range of about 0.1% during the steelmaking process.

[0038] Oxygen, as an impurity, mainly exists in steel in the form of inclusions, and the oxygen content of stainless steel produced by general refining is 0.001~0.015%.

[0039] Furthermore, regarding the selection of elements, the representative elements have been specified in the aforementioned [2], but even elements not recorded in this specification may still be included within the scope that does not impair the effect of the present invention. Methods for controlling Cr carbides and Cr nitrides

[0040] This invention describes a method for controlling Cr carbides and Cr nitrides.

[0041] To achieve a fine dispersion of Cr carbides with an average circumferential equivalent diameter of 1.20 μm or less, a Cr carbide number density of 0.10 particles / μm² or more, an average circumferential equivalent diameter of Cr nitrides of 0.80 μm or less, and a Cr nitride number density of 0.05 particles / μm² or more, hot working such as hot rolling at a finishing rolling temperature of 800–1150 °C is recommended. Following hot working, batch annealing is performed at 400 °C or higher but below 850 °C for 30–300 minutes, followed by cooling to 400 °C at a rate of 20–200 °C / hour. In addition to batch annealing under the same conditions as described above, strand annealing at 700–850 °C for 30–1000 seconds is also recommended. This range of manufacturing conditions is referred to as the suitable range.

[0042] When the finishing rolling temperature during hot working is below 800°C, the rolling load increases, thus reducing manufacturability. On the other hand, if the finishing rolling temperature is above 1150°C, the Cr carbides and Cr nitrides will coarsen, thus failing to meet the aforementioned conditions regarding the average circle equivalent diameter of Cr carbides and Cr nitrides. Therefore, the finishing rolling temperature during hot working is specified as 800~1150°C.

[0043] In batch annealing following hot working, if the batch annealing temperature is below 400°C and the dwell time is less than 30 minutes, the annealing will be insufficient and manufacturability will decrease. On the other hand, if the batch annealing temperature is above 850°C, the dwell time is longer than 300 minutes, and the cooling rate from the end of the dwell time to 400°C is slower than 20°C / hour, Cr carbides and Cr nitrides will coarsen and no longer meet the aforementioned equivalent diameter. Furthermore, if the cooling rate is faster than 200°C / hour, hardening due to the formation of ferromagnetic precipitates will occur, making the material more prone to delayed fracture. Cooling rates below 400°C do not need to be specifically limited.

[0044] When performing strand annealing in addition to batch annealing after hot working, the batch annealing conditions are the same as those for individual batch annealing. Regarding strand annealing, if the strand annealing temperature is below 700°C and the strand annealing dwell time is less than 30 seconds, the annealing will be insufficient and the manufacturability will decrease. On the other hand, if the strand annealing temperature is above 850°C and the strand annealing dwell time is longer than 1000 seconds, the Cr carbides and Cr nitrides will coarsen, not only failing to meet the above-mentioned equivalent diameter, but also exceeding the Ac1 point, resulting in hardening and hardening of the rust-like aggregate during cooling, making it prone to delayed fracture, etc.

[0045] As mentioned above, the finishing rolling temperature for hot working is limited to 800~1150℃; the batch annealing conditions are limited to a holding time of 30~300 minutes at a temperature above 400℃ and below 850℃, and a cooling rate of 20~200℃ / hour to 400℃; and the strand annealing conditions are limited to a holding time of 30~1000 seconds at a temperature of 700~850℃.

[0046] Within the aforementioned suitable range, the lower the finishing rolling temperature, the lower the batch annealing temperature, the shorter the dwell time, the faster the cooling rate after dwell, and the lower the strand annealing temperature and the shorter the dwell time, the more likely the average circumferential diameter of Cr carbides and Cr nitrides will decrease, and the more likely the number density of Cr carbides and Cr nitrides will increase. Therefore, based on this tendency, by adjusting the manufacturing conditions within the aforementioned suitable range, it is possible to reliably achieve a fine dispersion of Cr carbides with an average circumferential diameter of 1.20 µm or less, a Cr carbide number density of 0.10 particles / µm² or more, an average circumferential diameter of Cr nitrides of 0.80 µm or less, and a Cr nitride number density of 0.05 particles / µm² or more.

[0047] Furthermore, to achieve fine dispersion of Cr carbides and Cr nitrides, the finishing rolling temperature should preferably be below 1000°C, and more preferably below 900°C. In a preferred range, the finishing rolling temperature is 850°C or higher and below 900°C; the batch annealing conditions are a holding time of 450–800°C for 60–200 minutes, followed by a cooling rate of 30–100°C / hour to 400°C; and the strand annealing conditions are a holding time of 750–825°C for 40–800 seconds. By manufacturing within these conditions, it is possible to reliably achieve a fine dispersion of Cr carbides with an average circle equivalent diameter of 1.20 μm or less, a Cr carbide number density of 0.10 particles / μm² or higher, an average circle equivalent diameter of Cr nitrides of 0.80 μm or less, and a Cr nitride number density of 0.05 particles / μm² or higher.

[0048] Furthermore, the so-called stranded annealing refers to the following annealing method: unwinding the coiled wire or steel coil into a straight line, performing a short-time heat treatment in a straight line state (in a gaseous environment such as nitrogen, Ar, or ammonia decomposition gas), and then gas cooling or indirect water cooling. Compared to batch annealing of the entire coil, stranded annealing can significantly increase the cooling rate.

[0049] According to the present invention described above, a free-cutting stainless steel of the Asahi-San-Tetsu series can be provided as a stainless steel rod for high-hardness parts that can be precision machined, and its tool life during cutting is excellent.

[0050] Furthermore, in this invention, the so-called Asada-san-teki stainless steel refers to steel that hardens through the metamorphic transformation of Asada-san-teki during quenching. In this invention, it means, for example, steel that exhibits an Asada-san-teki microstructure in more than 50% of its metal structure and hardens to more than 500 Hv when quenched by air cooling starting from 1050°C.

[0051] In this invention, the term "bar steel" includes the concepts of "bar steel", "bar wire", "wire", "steel wire", "shaped wire", and "shaped bar steel". Example <Example 1>

[0052] To investigate the influence of composition, steels with the chemical compositions shown in Tables 1-3 were melted at 1600°C in a 45kg vacuum melting furnace and then cast in a mold. Subsequently, as a hot working process, the steel was heated to 1200°C and then hot-rolled to a finishing rolling temperature of 880°C to produce φ5.5mm wire, which was then cooled to room temperature. Afterward, batch annealing was performed at an annealing temperature of 550°C, a dwell time of 180 minutes, and a cooling rate of 50°C / hour to 400°C after the dwell time. Following batch annealing, as a cold working process, the steel was cold-drawn to a φ3.2mm wire with a reduction of area of ​​66.1%. Then, it was finished by straightening and centerless grinding to produce φ3.0mm cold-drawn bars (wires) as cutting blanks. [Table 1] [Table 2] [Table 3]

[0053] For the bar wires obtained in the above manner, the following evaluation method was used to evaluate the average circle equivalent diameter of Cr carbides, the number density of Cr carbides (number per unit area), the average circle equivalent diameter of Cr nitrides, the number density of Cr nitrides (number per unit area), and the tool life after peripheral cutting. The results are shown in Tables 4 to 6. Tables 4 and 5 show the evaluation results of the present invention example, and Table 6 shows the evaluation results of the comparative example.

[0054] The quenching hardness is evaluated based on the above-obtained bars and wires quenched by air cooling starting at 1050°C, with a value of 500 Hv or higher being rated as good. Corrosion resistance is evaluated based on the above-obtained bars and wires subjected to an acetic acid-salt water spray test for 48 hours, with no rust formation being rated as good. If either the quenching hardness or corrosion resistance falls outside the good standard, "Poor quenching hardness" or "Poor corrosion resistance" will be noted in the remarks, and no further evaluation will be conducted. Furthermore, if manufacturing is impossible for any reason, "Unable to manufacture" will be noted, and no quality evaluation will be performed. [Table 4] [Table 5] [Table 6] "The mean circle equivalent diameter of Cr carbides and the mean circle equivalent diameter of Cr nitrides, the number density of Cr carbides and the number density of Cr nitrides"

[0055] The sample was embedded in resin using a longitudinal section passing through the central axis of the rod as the inspection surface. After mirror polishing of the inspection surface, it was etched with aqua regia, and a 100 μm² area was observed using SEM with five fields of view. Among the inclusions in the fields of view, EDX analysis identified inclusions with a large amount of Cr and C as Cr carbides, and inclusions with a large amount of Cr and N as Cr nitrides. The average circular equivalent diameter of Cr carbides and Cr nitrides was calculated by averaging the circular equivalent diameters of more than 100 Cr carbides or Cr nitrides. The number density of Cr carbides and Cr nitrides was determined by measuring the total number of Cr carbides or Cr nitrides in the five fields of view and converting it to the number of Cr carbides and Cr nitrides per 1 μm². "Cutting tool life"

[0056] The outer circumference of the bar / wire was machined circumferentially for 1 hour under the following precision machining conditions: tool: superhard P type, tool tip radius: 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 was observed after machining using a 200x optical microscope. A tool with a boundary wear width of less than 100 µm was rated "A"; a boundary wear width greater than 100 µm but less than 200 µm was rated "B"; and a boundary wear width greater than 200 µm was rated "X".

[0057] In Examples 1 to 57 of the present invention in Tables 4 and 5, the average circle equivalent diameter of Cr carbides is 1.20 μm or less, the number density of Cr carbides is 0.10 particles / μm² or more, the average circle equivalent diameter of Cr nitrides is 0.80 μm or less, and the number density of Cr nitrides is 0.05 particles / μm² or more, all of which exhibit excellent cutting tool life.

[0058] On the other hand, Comparative Examples 1 to 32 in Table 6 fall outside the composition range of the present invention and none of them can meet the requirement of excellent cutting tool life. <Example 2>

[0059] To assess the impact of the process, steel with the chemical composition shown in Table 1 (Steel C) was melted at 1600°C in a 45kg vacuum melting furnace and then cast in a mold. Manufacturing conditions are listed in Table 7. Hot rolling was performed as a hot working process, with hot rolling conditions set at 1200°C followed by a finishing rolling temperature of 760–1170°C to produce φ5.5mm wire, which was then cooled to room temperature. Subsequently, batch annealing was performed individually, or in addition to batch annealing, strand annealing was also performed. For batch annealing, the annealing temperature was set at 350–900°C, the residence time at 20–360 minutes, and the cooling rate to 400°C after residence was set at a range of 10–250°C / hour. For strand annealing (in ammonia decomposition gas), the annealing temperature was set at 680–870°C, and the residence time at 20–1200 seconds. After annealing, cold drawing was performed as a cold working process, with the shrinkage rate of the cold-drawn wire set at 66.1%, resulting in a diameter of φ3.2mm. Subsequently, straightening and centerless grinding were used to finish the cold drawing into a φ3.0mm cold-drawn bar (wire) to produce a cutting blank. Then, using the same evaluation method as in Example 1, evaluations were conducted on the mean circle equivalent diameter of Cr carbides, the number density of Cr carbides, the mean circle equivalent diameter of Cr nitrides, the number density of Cr nitrides, and the tool life after peripheral cutting. The results are shown in Table 7. [Table 7]

[0060] In the present invention examples 58-65 in Table 7, the average circle equivalent diameter of Cr carbides is 1.20µm or less, the number density of Cr carbides is 0.10 particles / μm2 or more, the average circle equivalent diameter of Cr nitrides is 0.80µm or less, and the number density of Cr nitrides is 0.05 particles / μm2 or more. These invention examples all demonstrate excellent cutting tool life.

[0061] On the other hand, in Comparative Examples 33-44 of Table 7, the manufacturing conditions were outside the suitable conditions of the present invention and could not be manufactured, or even if successfully manufactured, the average circle equivalent diameter of Cr carbides, the number density of Cr carbides, the average circle equivalent diameter of Cr nitrides, and the number density of Cr nitrides still fell outside the scope of the present invention. As a result, excellent cutting tool life could not be achieved.

[0062] As can be clearly seen from the above embodiments, according to the present invention, a free-machining stainless steel based on the Mada Sante series can be provided, which has excellent tool life during precision cutting; for high-hardness parts used in environments with severe corrosion and requiring fatigue strength and wear resistance, its durability can be greatly improved, which is extremely useful in industry.

Claims

1. A type of free-machining stainless steel bar made of Matian Santetsu, characterized in that it has the following chemical composition: by mass%, C: greater than 0.60% and less than 1.00%, Si: 0.10~2.0%, Mn: 0.10~3.0%, S: less than 0.050%, P: less than 0.10%, Cr: 11.0~16.0%, N: less than 0.15%, and the remainder is composed of Fe and impurities; the average circle equivalent diameter of Cr carbides is less than 1.20 μm, the number density of Cr carbides is more than 0.10 particles / μm2, the average circle equivalent diameter of Cr nitrides is less than 0.80 μm, and the number density of Cr nitrides is more than 0.05 particles / μm2.

2. As claimed in item 1, the Mada Santetsu is a free-cutting stainless steel bar, wherein the aforementioned chemical composition of the steel contains one or more of the following elements that replace one portion of the aforementioned Fe: by mass % B: less than 0.01%, Al: less than 0.008%, O: less than 0.015%, Ni: less than 1.5%, Cu: less than 1.5%, Co: less than 1.5%, Mo: less than 2.5%, W: less than 2.5%, Bi: less than 0.2%, Sn: less than 1.0%, Sb: less than 1.0%, Ag: less than 1.0%, Te: less than 0.1%, V: less than 0.8%, Nb: less than 0.3%, Ti: less than 0.3%, Ta: less than 0.3%, Mg: less than 0.01%, Ca: less than 0.01%, Hf: less than 0.01%, and REM: less than 0.05%.

3. A method for manufacturing free-machining stainless steel bars of the Mada-san-teki series, comprising the method for manufacturing free-machining stainless steel bars of the Mada-san-teki series as claimed in claim 1 or claim 2; the method is characterized in that: the finishing rolling temperature for hot working is 800~1150°C, and batch annealing is performed after hot working, wherein the batch annealing is performed at a temperature above 400°C and below 850°C for 30~300 minutes, and cooled to 400°C at a rate of 20~200°C / hour; or in addition to performing the aforementioned batch annealing, strand annealing is also performed at a temperature of 700~850°C for 30~1000 seconds.