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  • C21D7/00—Modifying the physical properties of iron or steel by deformation
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a material machinable into a small component that requires corrosion resistance, more specifically relates to a martensitic free-machining stainless bar-shaped steel product that exhibits reduced tool abrasion and built-up edge and is excellent in machinability, and a manufacturing method thereof.
• Highly corrosion-resistant martensitic stainless steel with a high hardness of 400 HV or more is, in view of its wear resistance, fatigue strength, and corrosion resistance, used for components of industrial machinery and precision equipment.
• components of precision equipment produced by machining cold-drawn bar made of a bar steel, which are often used for rotors, are required to be highly accurately machined.
• the components of precision equipment are required to have tool abrasion as small as 200 ⁇ m or less during the machining process and to have flat and smooth surface with minute trace of built-up edge sparsely observed after being machined.
• the tool abrasion is promoted by hard precipitates and inclusions present in the machined material.
• the built-up edge which is a deposit of a base material generated/grown by adhering on an edge of a cutting tool, falls off from the edge of the tool in the process of machining to be pressed and attached on the machined surface, thereby deteriorating the texture of the machined surface.
• a martensitic free-machining stainless steel which is an S-containing highly corrosion-resistant martensitic stainless steel with high hardness, has so far been proposed, where the composition and size of sulfide and the size of carbide have been specifically defined to improve wear resistance and machinability (see Patent Literature 1).
• the proposal which focuses only on carbide of a very large size that is 10 ⁇ m 2 or more, cannot achieve satisfactory machining tool's life span and satisfactory machined surface texture.
• a steel with improved cold workability and machinability by specifying an average equivalent circle diameter of carbide and cleanliness of inclusions (see Patent Literature 2).
• the average equivalent circle diameter of carbide is defined to be in a range from 0.25 ⁇ m to 0.8 ⁇ m, thereby improving machinability and cold workability with low hardness. It is not described that coarse carbide with the average equivalent circle diameter of more than 0.8 ⁇ m promotes tool abrasion to lower machinability. In contrast, it is taught that the average equivalent circle diameter of carbide of less than 0.25 ⁇ m results in high hardness and no improvement in machinability. Further, nothing is disclosed on the machined surface texture.
• Patent Literatures 3, 4 Although nothing is disclosed on the tool's life span.
• the present inventors have found that the known techniques disclosed in the above "BACKGROUND ART" section or a combination thereof cannot achieve a highly corrosion-resistant martensitic free-machining stainless steel with high hardness, which is provided with excellent machinability and can restrain the tool abrasion in a machining process at 200 ⁇ m or less to achieve excellent tool's life span, and, further, cannot simultaneously achieve the tool's life span and a flat and smooth surface texture with the trace of built-up edge being restrained.
• An object to be solved by the invention is to provide a highly corrosion-resistant martensitic free-machining stainless bar-shaped steel product with high hardness, which is usable for precision components made of a martensitic stainless steel to be used under a severely corrosive environment, is excellent in tool's life span in a machining process, and can provide a machined surface texture suitably with excellent flatness by a machining process, and a manufacturing method thereof.
• a martensitic free-machining stainless bar-shaped steel product capable of providing excellent machining tool's life span and an excellently flat machined surface texture with restrained trace of built-up edge, which is usable as a material suitable for precision components with high hardness and excellent corrosion resistance; and a manufacturing method thereof.
• Components with 400 HV or more hardness which are generally effective in ensuring wear resistance and are targets to be achieved by the invention, are based on a martensitic free-machining stainless steel exhibiting high hardness of at least 400 HV or more after being quenched.
• a C content is 0.10% or more in order to achieve the hardness of 400 HV or more after a base material is quenched.
• the C content exceeds 0.60%, an average equivalent circle diameter of Cr carbide exceeds 0.8 ⁇ m to lower a machining tool's life span. Accordingly, the C content is limited to 0.60% or less.
• the C content is in a range from 0.20 to 0.50%.
• Si which is a deoxidizing element for restraining generation of coarse inclusions that decrease the machining tool's life span, is contained at a content of 0.1% or more.
• the Si content exceeding 2.0% hardens the steel and decreases the machining tool's life span and, in addition, promotes adhesion of the base material on the tool to assist generation of the trace of built-up edge. Accordingly, the Si content is limited to 2.0% or less.
• the Si content is in a range from 0.2 to 1.0%.
• Mn which is a deoxidizing element for restraining generation of coarse inclusions that decrease the machining tool's life span and forms a sulfide to ensure an excellent machined surface texture
• Mn content exceeding 3.0% hardens the steel and decreases the machining tool's life span and, in addition, exhibits an average sulfide equivalent circle diameter exceeding 5 ⁇ m to deteriorate the machined surface texture.
• the Mn content is limited to 3.0% or less.
• the Mn content is in a range from 0.2 to 2.0%.
• S which forms a sulfide to ensure excellent machining tool's life span and machined surface texture, is contained at a content of 0.15% or more.
• the S content exceeding 0.40% hardens the steel and decreases the machining tool's life span and, in addition, exhibits an average equivalent circle diameter exceeding 5 ⁇ m to deteriorate the machined surface texture.
• the S content is limited to 0.40% or less.
• the S content is in a range from 0.17 to 0.35%.
• P is an element mixed in a form of inevitable impurities from a source material.
• the P content exceeding 0.10% not only lowers the corrosion resistance by grain boundary segregation but also significantly lowers the productivity. Accordingly, the P content is limited to 0.10% or less.
• the P content is 0.05% or less.
• Cr which is a fundamental element for providing the corrosion resistance to a stainless steel, is contained at a content of 11.0% or more.
• the hardness of 400 HV or more cannot be achieved after quenching at a Cr content of more than 16.0%. Accordingly, the Cr content is limited to 16.0% or less.
• the Cr content is in a range from 11.5 to 15.0%.
• the average equivalent circle diameter of the Cr carbide is 0.80 ⁇ m or less and the number density of the Cr carbide is 0.10 pieces/ ⁇ m 2 or more.
• the average equivalent circle diameter of the Cr carbide is 0.70 ⁇ m or less and the number density of the Cr carbide is 0.20 pieces/ ⁇ m 2 or more.
• the average equivalent circle diameter of the sulfide is preferably limited to 5.0 ⁇ m or less and the number density of the sulfide is preferably limited to 0.010 pieces/ ⁇ m 2 or more. More preferably, the average equivalent circle diameter of the sulfide is 3.0 ⁇ m or less and the number density of the sulfide is 0.050 pieces/ ⁇ m 2 or more.
• the stainless steel of the invention contains chemical components consisting of Fe and impurities in addition to the above-described elements.
• element(s) is optionally contained in addition to the above components in place of a part of Fe.
• B which forms minute BN when being contained together with N to restrain the base material from being attached on the surface of the tool and to prevent the trace of built-up edge on the machined surface, is optionally contained as necessary.
• the B content is limited to 0.01% or less.
• the B content is preferably in a range from 0.002% to 0.007%.
• N which not only forms minute BN to prevent the trace of built-up edge on the machined surface but also increases the hardness after the base material is quenched, is optionally contained as necessary. However, at the N content exceeding 0.15%, coarse nitride is formed to lower the machining tool's life span. Accordingly, the N content is 0.15% or less. In order to securely achieve the above advantages, the N content is preferably in a range from 0.03% to 0.10%.
• Al is optionally contained for deoxidation.
• the Al content exceeds 0.008%, coarse inclusions are formed to lower the machining tool's life span. Accordingly, the Al content is limited to 0.008% or less.
• the Al content is 0.006% or less.
• the O content is limited to 0.015% or less.
• the O content is 0.012% or less.
• Ni, Cu, and Co which improve corrosion resistance and toughness of products, are optionally contained as necessary.
• a content of each of the above elements exceeds 1.5%, the machining tool's life span is lowered. Accordingly, the content of each of the elements is limited to 1.5% or less.
• the content of each of the elements is preferably in a range from 0.01% to 1.0%.
• Mo and W which improve corrosion resistance of products, are optionally contained as necessary.
• a content of each of the above elements exceeds 2.5%, the effect is saturated and the machining tool's life span is lowered. Accordingly, the content of each of the elements is limited to 2.5% or less.
• the content of each of the elements is preferably in a range from 0.01% to 2.0%.
• Bi which exhibits self-lubricity during a machining process to restrain generation of the built-up edge to improve the machined surface texture, is optionally contained as necessary. However, at a content exceeding 0.2%, hot workability is too lowered to produce any products. Accordingly, the Bi content is limited to 0.2% or less. In order to securely achieve the above advantages, the Bi content is preferably in a range from 0.005% to 0.10%.
• each of the above elements which exhibit self-lubricity during a machining process to restrain generation of the built-up edge to improve the machined surface texture, are optionally contained as necessary.
• a content of each of the above elements exceeds 0.3%, hot workability is too lowered to produce any products. Accordingly, the content of each of the elements is limited to 0.3% or less.
• the content of each of the elements is preferably in a range from 0.005% to 0.20%.
• Te which generates spherical sulfide to restrain lamination and growth of the built-up edge to improve the machined surface texture, is optionally contained as necessary.
• the Te content is limited to 0.1% or less.
• the Te content is preferably in a range from 0.005% to 0.05%.
• V which improves corrosion resistance of products, are optionally contained as necessary.
• the V content exceed 0.8%, coarse carbonitride is formed to lower the machining tool's life span. Accordingly, the V content is limited to 0.8% or less.
• the V content is preferably in a range from 0.05% to 0.5%.
• Nb, Ti, and Ta which improve corrosion resistance of products, are optionally contained as necessary.
• a content of each of the above elements exceeds 0.3%, coarse carbonitride is formed to lower the machining tool's life span. Accordingly, the content of each of the elements is limited to 0.3% or less.
• the content of each of the elements is preferably in a range from 0.01% to 0.2%.
• Mg, Ca, and Hf which are effective in improving hot workability, are optionally contained as necessary.
• a content of each of the above elements exceeds 0.01%, its advantageous effect is saturated and coarse oxide is formed to lower the machining tool's life span. Accordingly, the content of each of the elements is limited to 0.01% or less.
• the content of each of the elements is preferably in a range from 0.001 % to 0.005%.
• REM which is effective in improving hot workability, is optionally contained as necessary.
• a content of REM exceeds 0.05%, its advantageous effect is saturated and coarse oxide is formed to lower the machining tool's life span. Accordingly, the REM content is limited to 0.05% or less.
• the REM content is preferably in a range from 0.001 % to 0.005%.
• REM Radar Metal
• Sc scandium
• Y yttrium
• Lu lanthanoids
• One of the REM may be solely contained or a mixture of two or more of the REM may be contained.
• Oxygen (one of the impurities), which is present in steel mainly in a form of inclusions, is contained in a stainless steel produced through a typical smelting process at a content ranging from 0.001 to 0.015%.
• the steel In order to obtain the average equivalent circle diameter of Cr carbide to 0.80 ⁇ m or less and to finely disperse the Cr carbide so that the number density becomes 0.10 pieces/ ⁇ m 2 or more, it is preferable to subject the steel to hot working (e.g. hot rolling) at a finish rolling temperature ranging from 800 to 1150 degrees C, and, after the hot working, subject the steel to batch annealing for retaining the steel at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and cooling the steel to 600 degrees C at a rate ranging from 20 to 200 degrees C/h or more.
• hot working e.g. hot rolling
• the finish rolling temperature during hot working is defined to range from 800 to 1150 degrees C.
• the conditions for the batch annealing are the same as the above-described conditions for subjecting the steel only to the batch annealing.
• the strand annealing temperature is lower than 700 degrees C and a retention time of the strand annealing is shorter than 30 seconds, productivity is lowered due to insufficient annealing.
• the Cr carbide is coarsened to fail to satisfy the above-described conditions for the equivalent circle diameter and, in addition, martensitic transformation occurs to harden the steel when the steel is cooled at a temperature exceeding the Ac1 temperature, whereby delayed fracture and the like easily occur.
• the finish rolling temperature during the hot working ranges from 800 to 1150 degrees C
• the batch annealing is performed under the conditions that the retention is performed at a temperature ranging from 700 to 900 degrees C for 60 to 300 minutes and the cooling is performed at the cooling rate ranging from 20 to 200 degrees C/h until the temperature reaches 600 degrees C
• the strand annealing is performed under the conditions that the retention is performed at a temperature ranging from 700 to 850 degrees C for 30 to 1000 seconds.
• the average equivalent circle diameter of the Cr carbide tends to decrease and the number density of the Cr carbide tends to increase with the decreasing finish rolling temperature, the decreasing temperature for the batch annealing, the decreasing retention time for the batch annealing, the increasing cooling rate for the batch annealing after the retention, the decreasing temperature for the strand annealing, and the decreasing retention time for the strand annealing, within the above suitable range. Accordingly, the average equivalent circle diameter of the Cr carbide can be reliably reduced to 0.80 ⁇ m or less and the Cr carbide can be reliably finely dispersed so that the number density of the Cr carbide is 0.10 pieces/ ⁇ m 2 or more by controlling the manufacture conditions.
• the finish rolling temperature is preferably less than 1000 degrees C, more preferably less than 900 degrees C in order to finely disperse the Cr carbide.
• the finish rolling temperature is 850 degrees C or more and less than 900 degrees C
• the batch annealing is performed under the conditions that the retention is performed at a temperature ranging from 750 to 850 degrees C for 90 to 200 minutes and the cooling is performed at the cooling rate ranging from 30 to 100 degrees C/h until the temperature reaches 600 degrees C after the retention
• the strand annealing is performed under the conditions that the retention is performed at a temperature ranging from 750 to 825 degrees C for 40 to 800 seconds.
• the Cr carbide can be more reliably finely dispersed so that the average equivalent circle diameter of the Cr carbide is 0.80 ⁇ m or less and the number density of the Cr carbide is 0.10 pieces/ ⁇ m 2 or more.
• the strand annealing refers to a process of annealing where wire rods or a coil of steel wire wound in a ring-shape are linearly stretched to provide a single linear wire rod /steel wire and the stretched single linear wire rod /steel wire are subjected to a short-time thermal treatment (in an atmosphere of nitrogen, Ar, ammonia decomposition gas, or the like) to be air-cooled or indirectly water-cooled.
• the cooling rate can be significantly increased as compared with the batch annealing on the entirety of the ring-shaped coil.
• the steel is subjected to cold working (e.g. cold drawing) at a reduction rate ranging from 20.0 to 99.0% after the hot working (e.g. hot rolling) and annealing as described in the above "Cr Carbide Control Method.”
• the sulfide which has been stretched through the hot rolling and annealing, is fragmented to be finely dispersed by the cold working (e.g. cold drawing).
• the reduction rate is defined to be 20.0% or more.
• the reduction rate is defined to be 99.0% or less.
• the reduction rate is in a range from 30.0 to 90.0%.
• defects such as shrinkage cracks are sometimes caused due to work-hardening at a reduction rate of approximately 70% or more during the cold working (e.g. cold drawing).
• the cold working (e.g. cold drawing) and annealing are optionally repeated for plural times.
• the annealing performed between the cold working processes has to be done under the annealing conditions described in the above "Cr Carbide Control Method" in order to control the Cr carbide.
• the reduction rate in the cold working (e.g. cold drawing) in the above case refers a sum of reduction rates of the cold working processes performed for plural times.
• the above-described aspect of the invention can provide a martensitic free-machining stainless steel that provides excellent tool's life span during a machining process and excellent machined surface texture, as a stainless bar-shaped steel product for high-hardness components to be precisely machined.
• the martensitic stainless steel of the invention refers to a steel hardened by martensitic transformation during a quenching process.
• the martensitic stainless steel refers to, for instance, a steel whose 70% or more of micro structure exhibits a martensitic structure when being quenched by air-cooling from 1050 degrees C to be hardened to 400 HV or more.
• the bar-shaped steel product herein includes “rod steel,” “bar steel,” “wire rod,” “steel wire,” “deformed wire,” “deformed rod steel” and the like.
• steels of chemical compositions shown in Tables 1 to 3 were melted in a vacuum smelting furnace of 45 kg at 1600 degrees C and then were each casted in a casting die. Subsequently, the steels were each heated to 1200 degrees C, followed by hot-rolling at a finish rolling temperature of 880 degrees C to be formed into a wire rod with a 5.5-mm diameter, and cooled to a room temperature. Subsequently, each of the steels was subjected to batch annealing under the conditions of the annealing temperature of 850 degrees C, the retention time of 180 minutes, and the cooling rate of 50 degrees C/h until the temperature reaches 600 degrees C after the retention.
• each of the steels was subjected to cold working (cold drawing) at a reduction rate of 66.1% to reduce the diameter to 3.2 mm.
• the wire rod was subjected to straightening and centerless polishing processes to prepare a cold-drawn bar (bar steel) with a 3.0-mm diameter, which was used as a material to be machined.
• Quench hardness was evaluated after quenching by air-cooling from 1050 degrees C, where 400 HV or more was evaluated to be satisfactory. Corrosion resistance was evaluated through an acetic acid salt spray test for 48 hours, where an instance without flow rust was evaluated to be satisfactory. When one of the quench hardness and corrosion resistance was out of the satisfactory range, a comment "poor quench hardness” or “poor corrosion resistance” was made in Remarks and no further evaluation was made. Further, when the steel was not producible for some reason(s), a comment "non-producible" was made and no evaluation was made on quality.
• a test sample was embedded in a resin so that a vertical cross section of the bar steel was positioned to be an inspection surface. Then, after the inspection surface was mirror-polished and etched with aqua regia, five visual fields each with an area of 100 ⁇ m2 were observed using an SEM. Among the inclusions in each of the visual fields, the inclusions in which much Cr and C were detected by EDX analysis were identified as Cr carbide.
• the average equivalent circle diameter of Cr carbide was calculated by calculating the equivalent circle diameter of each of 100 or more Cr carbides and averaging the calculated equivalent circle diameters. The number density of Cr carbide was calculated by counting a total of the number of Cr carbide in the five visual fields and converting the number into the number of Cr carbide per 1 ⁇ m 2 .
• a test sample was embedded in a resin so that a vertical cross section of the bar steel was positioned to be an inspection surface. Then, after the inspection surface was mirror-polished, five visual fields each with an area of 3 mm 2 were observed using an SEM. Among the inclusions in each of the visual fields, the inclusions in which much S was detected by EDX analysis were identified as sulfide.
• the average equivalent circle diameter of the sulfide was calculated by calculating the equivalent circle diameters of 100 or more sulfides and averaging the calculated equivalent circle diameters.
• the number density of the sulfide was calculated by counting a total of the number of sulfides in the five visual fields and converting the number into the number of sulfide per 1 ⁇ m 2 .
• An outer circumferential portion of the bar steel was machined in a circumferential direction for an hour under the following precision machining conditions: Used tool: super hard P type, edge R: 0.03 mm, machining speed: 20 m/min, feed: 0.01 mm/rev, cutting depth: 0.1 mm, cutting oil (mineral oil): yes.
• the tool after the machining process was observed using an optical microscope of 200 times magnification.
• the tool's life span was evaluated to be "S" when a boundary wear width of the tool after the machining process was 100 ⁇ m or less, evaluated to be “G” when the boundary wear width was more than 100 ⁇ m and 200 ⁇ m or less, and evaluated to be "X" when the boundary wear width was more than 200 ⁇ m.
• An outer circumferential portion of the bar steel was machined in a circumferential direction for an hour under the following precision machining conditions: Used tool: super hard P type, edge R: 0.03 mm, machining speed: 20 m/min, feed: 0.01 mm/rev, cutting depth: 0.1 mm, cutting oil (mineral oil): yes.
• a surface of the machined bar steel was observed using an optical microscope of 100 times magnification. The machined surface texture was evaluated to be "X" when a clear trace of built-up edge was observed on the surface, evaluated to be "G” when a minute trace of built-up edge was sparsely observed, and was evaluated to be "S” when no clear trace of built-up edge was observed.
• the steel of the chemical composition shown as "Steel C" in Table 1 was melted in a vacuum smelting furnace of 45 kg at 1600 degrees C and was casted in a casting die.
• the manufacture conditions are shown in Table 7.
• the steel was hot-worked (hot-rolled) at a finish rolling temperature ranging from 760 to 1170 degrees C to be formed into a wire rod whose diameter ranges from 5.5 to 40.0 mm and then cooled to a room temperature.
• the steel was subjected solely to batch annealing or subjected to strand annealing in addition to batch annealing.
• the annealing temperature was in a range from 650 to 950 degrees C
• the retention time was in a range from 40 to 360 minutes
• the cooling rate after the retention until the temperature reaches 600 degrees C was in a range from 10 to 250 degrees C/h.
• the annealing temperature was in a range from 680 to 870 degrees C and the retention time was in a range from 20 to 1200 seconds.
• a cold-drawn bar (bar steel) with a diameter of 3.0 mm was produced by subjecting the steel to cold drawing at a reduction rate ranging from 19.0 to 99.4% and then subjecting the steel to straightening and centerless polishing processes, thereby preparing a material to be machined.
• the steel showed great work-hardening due to high reduction rate, the material was prepared through repetition of annealing and drawing.
• Example 58 the wire rod with a 5.5-diameter was not subjected to cold drawing but was subjected to centerless polishing into a cold-drawn bar (bar steel) with a 3.0-mm diameter, which was used as the material to be machined. Subsequently, the average equivalent circle diameter of Cr carbide, the number density of the Cr carbide, the average equivalent circle diameter of sulfide, the number density of the sulfide, the tool's life span after machining an outer circumferential portion, and a machined surface texture were evaluated according to the same evaluation methods as in the above Example 1. The results are shown in Table 7.
• the invention can provide a martensitic free-machining stainless steel with excellent tool's life span and, preferably, an excellent machined surface texture during a precision machining process, which can significantly improve durability of high-hardness components to be used in a highly corrosive environment requiring fatigue resistance and wear resistance, and thus is extremely industrially useful.

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