Stainless steel wire rod or steel wire, method for manufacturing stainless steel wire rod or steel wire, and drilling tapping screw

Abstract

This stainless steel wire rod or steel wire contains C at 0.10-0.30%, Si at 0.10-2.0%, Mn at 0.10-3.0%, P at 0.05% or less, S at 0.010% or less, Cr at 12.0-16.0%, Mo at 1.0-3.0%, and N at 0.010-0.15%, and also contains one or more of Nb at 0.30% or less, V at 0.30% or less, Ti at 0.30% or less, and Ta at 0.30% or less. The composition is adjusted so that the (Nb + V + Ti + Ta) content is 0.03-0.30%, the DI value index is 0% or less, and the PREN index is 18.0% or more. The mass ratio (maximum segregation amount / average composition) of Mo in the central area of a transverse cross section of the stainless steel wire rod or the steel wire is 2.0 or less.

Description

[Technical Field]

[0001] This invention relates to stainless steel wire or steel wire, which are materials for high-hardness, high-corrosion-resistant stainless steel parts requiring corrosion resistance, such as screws, as well as methods for manufacturing them, and to drilling tapping screws. In particular, martensitic stainless steel is the subject of this invention. [Background technology]

[0002] To date, there has been a high demand for high corrosion resistance in high-hardness forged parts such as drilling tapping screws, particularly in the fields of building materials and automotive parts. For example, the application of high-hardness, high-corrosion-resistant martensitic stainless steel, whose composition has been adjusted to regulate the amount of δ-ferrite by adding Mo, has been proposed (Patent Document 1).

[0003] Furthermore, a high-hardness, high-corrosion-resistant martensitic stainless steel has been proposed in which the amount of δ-ferrite in the center is suppressed to less than 10%, and a retained austenite phase remains on the surface, resulting in high hardness and high toughness (Patent Document 2).

[0004] However, when these steels are processed into drilling tapping screws and other products, and then hardened or hardened and tempered to produce the final product, rust may occur on the central surface of the screw head. This rust is caused by intermetallic compounds in the positive molybdenum segregation area in the center of the material. In other words, when excessive positive molybdenum segregation in the center of the material appears on the surface of the final product, such as the central part of the drilling tapping screw head, the corrosion resistance of that area deteriorates significantly.

[0005] On the other hand, as a method to improve positive segregation in the center of the material, it has been proposed to apply pressure during the continuous casting of billets of 13%Cr stainless steel (Patent Document 3). However, this requires large-scale modifications to the continuous casting apparatus, and the effectiveness of improving the positive segregation of Mo, which is prone to segregation, remains unclear. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 3340225 [Patent Document 2] Patent No. 4252145 [Patent Document 3] Japanese Patent Application Publication No. 4-305350 [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem to be solved by the present invention is to inexpensively improve the corrosion resistance of the portion of a stainless steel wire or steel wire, which is a material for high-hardness, high-corrosion-resistant stainless steel parts, that corresponds to the central part of the material. Therefore, the objective is to provide stainless steel wire or steel wire, a method for manufacturing them, and a drilling tapping screw, in which the Mo segregation and metal structure, which are the starting points for rust formation in the center of the material, have been improved. In particular, martensitic stainless steel is the target. [Means for solving the problem]

[0008] The inventors of the present invention have arrived at the following invention as a result of various studies to solve the above problems. First, the composition of a material that can exhibit high hardness and high corrosion resistance is adjusted to suppress the formation of δ-ferrite. Then, appropriate amounts of Nb, V, Ti, and Ta, which have the effect of suppressing sensitization, are added, and in order to avoid using large-scale reduction equipment during casting, small-section casting with rapid solidification is adopted, and hot working with a high reduction rate of section is incorporated to manufacture wire rods or steel wires. As a result, the mass ratio (maximum segregation amount / average composition) based on the maximum positive segregation amount of Mo in the center of the wire rod and steel wire can be controlled to 2.0 or less. As a result, the discovery was made that the formation of Mo-based intermetallic compounds etc. in the center of the final product can be suppressed during subsequent quenching or quenching and tempering, thereby significantly improving corrosion resistance. The present invention is based on the above discovery, and its gist is as follows.

[0009] [1] Contains, by mass%, C: 0.10%~0.30%, Si: 0.10~2.0%, Mn: 0.10~3.0%, P: 0.05% or less, S: 0.010% or less, Cr: 12.0~16.0%, Mo: 1.0~3.0%, N: 0.010~0.15%. It contains one or more of the following: Nb: 0.30% or less, V: 0.30% or less, Ti: 0.30% or less, and Ta: 0.30% or less, within a range where the (Nb+V+Ti+Ta) amount is between 0.03% and 0.30%. The remainder consists of Fe and impurities. Furthermore, the components are adjusted so that the DI value represented by equation (1) is 0% or less, and the PREN value represented by equation (2) is 18.0% or more. Stainless steel wire or steel wire characterized in that the mass ratio of Mo (maximum segregation amount / average composition) in the center of the cross-section of the wire or steel wire is 2.0 or less. DI=Cr+1.21Mo+0.48Si+2.48Al-(24.5C+18.4N+Ni+0.11Mn)-10...Equation (1) PREN=Cr+3.3Mo+16N ····(2) formula In equations (1) and (2), the element symbol represents the content (mass%) of the element. [2] The stainless steel wire rod or steel wire according to [1], characterized in that it contains one or both of the following groups (A) and (B) in place of a portion of the Fe. Group (A) At least one of the following: Ni: 2.0% or less, Cu: 2.0% or less, W: 3.0% or less, Co: 2.0% or less, B: 0.010% or less, Sn: 0.30% or less, Sb: 0.30% or less, Al: 1.00% or less. Group (B) At least one of the following: Ca: 0.006% or less, Mg: 0.006% or less, Hf: 0.010% or less, REM: 0.06% or less.

[0010] A cast slab having the components described in [3] [1] or [2] has a cross-sectional area X (cm²). 2 ) 80-350cm 2 Casting is performed by indirect cooling within the mold, or by rapid surface cooling of the cast slab using indirect cooling within the mold and direct water cooling. A method for manufacturing a stainless steel wire rod or steel wire according to [1] or [2], characterized in that hot rolling is performed with a cross-sectional reduction rate of Y (%) or more in the formula (3). Y = 0.0025X + 98.75 ··· Formula (3)

[0011] A drilling tapping screw excellent in corrosion resistance at the center of the head, characterized by using the stainless steel wire rod or steel wire according to [4], [1] or [2] as a material. A method for manufacturing a drilling tapping screw excellent in corrosion resistance at the center of the head, characterized by manufacturing using the stainless steel wire rod or steel wire according to [5], [1] or [2] as a material.

[0012] The stainless steel wire rod or steel wire of the present invention has its components adjusted and the Mo segregation at the center reduced. As a result, products such as drilling tapping screws manufactured using the stainless steel wire rod or steel wire of the present invention as a material have the effect of improving and enhancing corrosion resistance in high-hardness products where the center of the material is exposed to the surface during processing and heat treatment.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram showing the relationship between the Mo positive segregation part at the center of the wire rod or steel wire and the rusting position at the center of the head of the drilling tapping screw after processing and heat treatment.

Embodiments for Carrying Out the Invention

[0014] The present invention is directed to a stainless steel wire rod or steel wire. A wire rod means a linear material obtained by hot rolling steel. A steel wire means a linear material obtained by further cold working a wire rod. Here, the wire rod or steel wire is collectively referred to as a "linear steel material".

[0015] FIG. 1 is a diagram showing the relationship between the Mo positive segregation part 11 at the center of the wire rod or steel wire 1 and the rusting position 12 at the center of the head of the drilling tapping screw 2 after performing processing and heat treatment 13.

[0016] The following describes the chemical composition of stainless steel wire or steel wire. The "%" in the chemical composition of steel refers to mass percentage. Furthermore, numerical ranges indicated by "~" include the values ​​before and after the "~" as the lower and upper limits, respectively. However, if the values ​​before and after the "~" are preceded by "greater than" or "less than," the numerical range excludes those values ​​as the lower or upper limit.

[0017] First, the reasons for the limitations on the chemical components, etc., required for the stainless steel wire or steel wire of the present invention will be explained below.

[0018] Carbon (C) is added at a concentration of 0.10% or more along with nitrogen (N). This is to obtain a hardness of Hv≧500, which is necessary for the final product, a drilling tapping screw, to exhibit its screw-in properties and other functions as a high-hardness product. However, if the C content exceeds 0.30%, in addition to deterioration of cold workability, deterioration of toughness such as screw head breakage in the final product, and deterioration of corrosion resistance occur, so the upper limit of the C content is set to 0.30%. Preferably, it is 0.15~0.25%.

[0019] Si is added at a concentration of 0.10% or more as a deoxidizing agent to reduce coarse inclusions and ensure toughness such as cold workability and resistance to screw head breakage in the final product. However, adding Si in excess of 2.0% will conversely deteriorate cold workability and toughness, so the upper limit of Si content is limited to 2.0%. Preferably, it is 1.0% or less.

[0020] Mn is added at a concentration of 0.10% or more to reduce coarse inclusions as a deoxidizing agent, ensuring toughness such as the screw head's resistance to breakage in the final product, and to fix S by forming MnS to ensure hot workability. However, adding Mn in excess of 3.0% deteriorates the cold workability of screws, etc., so the upper limit of the Mn content is limited to 3.0%. Preferably, it is 2.0% or less.

[0021] P is limited to 0.05% or less in order to ensure toughness such as the screw head's resistance to breakage in the final product. Preferably, it is 0.035% or less.

[0022] S is limited to 0.010% or less to ensure cold workability and corrosion resistance and head breakage resistance of the final product screw. Preferably, it is 0.005% or less.

[0023] Cr is added at a concentration of 12.0% or more to ensure the corrosion resistance of the final screw product. However, if Cr is added at a concentration exceeding 16.0%, δ-ferrite is formed in the final product, degrading its corrosion resistance and preventing the final product from achieving a hardness Hv ≥ 500, thus degrading its screw-in properties. Therefore, the upper limit of the Cr content is limited to 16.0%. Preferably, it is between 12.5% ​​and 15.0%.

[0024] Mo is added at a concentration of 1.0% or more to ensure the corrosion resistance of the final screw product. However, if more than 3.0% of Mo is added, even with small cross-section casting and high-reduction-ratio hot rolling, excessive positive segregation of Mo occurs in the center of the wire and steel wire, leading to the formation of intermetallic compounds and other factors that degrade the corrosion resistance of the center and the toughness of the screw product. Therefore, the upper limit for Mo is set at 3.0%. Preferably, it is between 1.3% and 2.5%.

[0025] N is added along with C at a concentration of 0.010% or more. This is to ensure the corrosion resistance of the final product, such as screws, and to obtain a hardness of Hv≧500, which is necessary for high-hardness products to perform their functions, such as the screw-in properties of drilling tapping screws. However, if the N content exceeds 0.15%, not only will the toughness of the final product, such as screw head breakage, deteriorate, but defects due to blowholes will occur, degrading cold workability and corrosion resistance. Therefore, the upper limit for the N content is set at 0.15%. Preferably, it is between 0.05% and 0.13%.

[0026] Nb, V, Ti, and Ta are necessary to prevent sensitization by suppressing the precipitation of Cr-based carbonitrides during heat treatment, thereby ensuring the corrosion resistance of the final screw product. Therefore, one or more of Nb, V, Ti, and Ta are added so that the (Nb+V+Ti+Ta) amount is 0.03% or more. However, if any of Nb, V, Ti, or Ta, or the (Nb+V+Ti+Ta) amount, exceeds 0.30%, not only will the cold workability of the screw deteriorate, but the toughness, such as the screw head's resistance to chipping, will also deteriorate. Therefore, the upper limit for each is set at 0.30%. Preferably, the (Nb+V+Ti+Ta) amount is 0.05% or more and 0.25% or less.

[0027] The stainless steel wire or steel wire of the present invention contains the above essential components, with the remainder being Fe and impurities. The following components may be included in place of a portion of the above Fe.

[0028] Ni and Co are added as needed to improve the toughness of the final product, such as the resistance to screw head breakage. However, adding more than 2.0% will degrade the cold workability of the screws, so the upper limit is limited to 2.0%. Preferably, it is 1.5% or less.

[0029] Cu, W, Sn, Sb, and Al are added as needed to improve the corrosion resistance of the final screw product. However, if these are added in excess of Cu:2.0%, W:3.0%, Sn:0.30%, Sb:0.30%, and Al:1.00%, respectively, the cold workability of the screw and the toughness of the final screw product will deteriorate. Therefore, the upper limits are limited to Cu:2.0%, W:3.0%, Sn:0.30%, Sb:0.30%, and Al:1.00%. Preferably, the values ​​are Cu:1.0% or less, W:2.0% or less, Sn:0.10% or less, Sb:0.20% or less, and Al:0.10% or less.

[0030] B is added as needed to improve the toughness of the final product screw, such as its resistance to head breakage. However, adding more than 0.010% of B actually degrades the toughness due to the formation of boride. Therefore, the upper limit of the B content is limited to 0.010%. Preferably, it is 0.006% or less.

[0031] Ca, Mg, Hf, and REM are added as needed to improve hot workability. However, adding more than 0.006%, 0.006%, 0.010%, and 0.06% of each will degrade the cold workability of screws and the toughness of the final screw product due to coarse inclusions. Therefore, the upper limits are limited to 0.01%, 0.01%, 0.010%, and 0.06% of each. Preferably, these are 0.004% or less for Ca, 0.004% or less for Mg, 0.004% or less for Hf, and 0.03% or less for REM.

[0032] Typical impurities contained in the stainless steel of the present invention include Zn, Bi, Pb, Ge, Se, Ag, Se, Te, etc., which are usually present as impurities in the steel manufacturing process in a range of about 0.1%. Furthermore, the oxygen content of the stainless steel of the present invention is in the normal range of 0.001 to 0.02%.

[0033] The DI formula ((1)) was obtained as a result of investigating the influence of various elements on the amount of δ-ferrite in the base material, and shows the elements that are effective in suppressing the amount of δ-ferrite and the degree of their influence. Cr, Mo, Si, Al, C, N, Ni, and Mn have an influence. If the value of DI exceeds 0 (%), δ-ferrite is clearly present, causing carbonitrides to precipitate at the δ-ferrite interface during quenching, and significantly degrading the overall corrosion resistance of the product, so it should be limited to 0 (%) or less. Preferably, it is -0.5 (%) or less.

[0034] The PREN formula ((2)) was obtained as a result of investigating the effects of various elements on the corrosion resistance of the base material, and it shows the elements that are effective in improving corrosion resistance and the degree of their influence. Cr, Mo, and N have an effect. If the PREN value falls below 18(%), the screw product will rust. Therefore, it is limited to 18.0(%) or higher. Preferably, it is 18.5(%) or higher.

[0035] For wire rods or steel wires containing 1.0-3.0% Mo, if the mass ratio of Mo (maximum segregation amount / average composition) exceeds 2.0, rust will occur on the central surface of the screw head, which is the final product, due to the formation of intermetallic compounds and other factors. Therefore, the maximum segregation amount of Mo in the central part of the cross-section of the wire rod or steel wire used as the screw material should be limited to a ratio of Mo (maximum segregation amount / average composition) of 2.0 or less. Preferably, it should be 1.5 or less. In general, cold forging is often used as a processing method for screw products, and in that case, the metal flow during processing fluctuates arbitrarily, making it impossible to accurately identify the location of positive Mo segregation in the product. Therefore, it is important to control the segregation value in the central part of the cross-section of the wire rod or steel wire, which is the raw material and has a nearly circularly symmetrical cross-sectional shape.

[0036] Next, the method for manufacturing stainless steel wire or steel wire according to the present invention will be described. To suppress positive Mo segregation in the center of wire rods or steel wires, it is important to reduce segregation at the raw material stage, such as in cast slabs or billets, before wire rod rolling, and then to perform hot rolling with a high processing rate. In particular, controlling the relationship between the size of the cast slab and the reduction rate of the cross-sectional area during hot rolling is effective.

[0037] The size of the cast slab during casting and the indirect or direct water cooling of the slab surface during casting affect the degree of macrosegregation of Mo in the center of the slab during solidification, i.e., the segregation ratio. When the slab surface is indirectly water-cooled, the cross-sectional area of ​​the slab is 350 cm². 2If this value is exceeded, the solidification and cooling rate in the center becomes slow, less than 0.2°C / s. As a result, the amount of macrosegregation of Mo increases, and even if a hot rolling process with a high processing rate is used in the subsequent hot rolling, the mass ratio of (maximum segregation amount / average composition) of the maximum segregated portion of Mo in the center of the wire rod exceeds 2.0. On the other hand, if the cross-section of the cast slab is 80 cm 2 If the value falls below a certain level, the processing rate of hot wire rolling decreases, and conversely, the diffusion and homogenization of Mo does not progress, causing the mass ratio of (maximum segregation amount / average composition) of the maximum positive segregation of Mo in the center of the wire or steel wire to exceed 2.0. Therefore, when combined with indirect water cooling of the slab surface, the cross-sectional area of ​​the slab is 80-350 cm². 2 A small cross-section is preferred. Furthermore, 100-300 cm 2 This is a small cross-section.

[0038] Indirect and direct water cooling during casting accelerates the cooling rate of the center of the slab, where the cooling rate is particularly low, and reduces the concentration of solute elements in the center due to the compressive stress of the surface solidification shell, for a cross-sectional area of ​​80-350 cm². 2 This contributes to suppressing central segregation of Mo within the specified range. Therefore, in the present invention, indirect water cooling during casting, or indirect water cooling followed by direct water cooling, is effective. In actual casting processes, the scope of the invention includes indirect water cooling of molten steel by water cooling the mold during continuous casting, and direct water cooling of the surface of the cast slab immediately after its surface solidification and withdrawal from the mold.

[0039] On the other hand, regarding the reduction rate of cross-sectional area in hot rolling, the slab size (cross-sectional area X (cm)) 2 )) requires hot rolling with a cross-sectional reduction rate of Y(%) or greater, as expressed by equation (3). This makes it possible to reduce Mo segregation that occurs in the center of the cast slab during solidification by hot rolling. If the cross-sectional reduction rate is less than Y(%), a large amount of Mo segregation remains, and the mass ratio of (maximum segregation amount / average composition) of the maximum Mo segregation part in the center of the wire rod exceeds 2.0. Preferably, the cross-sectional reduction rate is even larger, Y+0.25(%) or more. Y=0.0025X+98.75 ···(3) formula

[0040] Note that the cross-sectional reduction rate during hot working may be carried out in two or more steps, such as hot forging, block rolling, wire rolling, etc. Also, the heating during hot working of the material is preferably carried out at 1050 to 1300 °C.

[0041] Next, the reasons for limiting the drilling tapping screw of the present invention will be explained. The drilling tapping screw manufactured using the wire or steel wire with suppressed Mo positive segregation in the central part of the material as the material of the present invention has no rusting at the center of the screw head due to excessive positive segregation of Mo in the central part of the wire or steel wire, and the whole drilling tapping screw will have excellent corrosion resistance. Therefore, it is limited to the drilling tapping screw using the stainless steel wire or steel wire of the present invention. By applying the manufacturing method of the drilling tapping screw using the stainless steel wire or steel wire of the present invention, a drilling tapping screw with excellent corrosion resistance at the center of the head can be manufactured.

[0042] According to the present invention described above, in high-hardness and high-corrosion-resistance stainless steel parts, the corrosion resistance of the part corresponding to the vicinity of the central part of the material of the part can be improved. In particular, martensitic stainless steel is targeted.

Example

[0043] (Example 1) Steel with the chemical compositions shown in Tables 1 to 3 was melted at about 1600 °C in a 150 kg vacuum melting furnace, and then cast in an indirect water-cooled mold with water-cooling pipes arranged on the outer periphery of the mold. The mold was a magnesia-based mold with a cross-sectional area X of 200 cm 2 and the Y in formula (3) at this time was 99.3 (%).

[0044] After the casting in the mold was completed, the cast slab was heated to 1150 °C, and then hot-worked by hot wire rolling with a cross-sectional reduction rate of 99.8% (not less than the above Y (%)) and air-cooled to room temperature to obtain a wire with a diameter of 6.0 mm. Then, full annealing and pickling were carried out at 880 °C, and cold wire drawing, BA (bright annealing) at 800 °C, oxalic acid film, and skin pass wire drawing were carried out to obtain a steel wire for forging with a diameter of φ5.2 mm.

[0045] [Table 1]

[0046] [Table 2]

[0047] [Table 3]

[0048] Subsequently, the steel wire was formed into a hexagonal-headed drilling tapping screw 2 as shown in Figure 1 using the standard method of cold double-heading (oil lubrication), coating removal, cutting edge processing, and rolling. Then, the drilling tapping screw prototype was fabricated by vacuum hardening at 1100°C, tempering at 250°C, barrel polishing, and immersion in 15% nitric acid for 30 minutes.

[0049] The evaluation included assessing the Mo segregation ratio in the center of the cross-section of the wire and steel wire, cold workability, corrosion resistance of the head of the drilling tapping screw, hardness, head toughness, and microstructure. The results are shown in Tables 4 to 6.

[0050] The Mo segregation ratio in the center of the cross-section of the wire rod and steel wire was determined by embedding and polishing the cross-sections of the wire rod and steel wire as inspection surfaces, measuring the distribution of Mo in the diametrical direction by EPMA line analysis, and determining the maximum segregation amount by the Mo content at the point with the most Mo near the center of the cross-section, and calculating the ratio of this to the average Mo composition obtained from the chemical analysis of the wire rod and steel wire (maximum segregation amount / average composition).

[0051] Cold workability was evaluated by cold forging 100 drilling tapping screws using a cold double-header and checking for any cracks in the screw heads. If 10 or more screws had cracks, the cold workability was rated as X (fail), and if fewer than 10 screws had cracks, it was rated as A (pass).

[0052] Regarding the corrosion resistance of the heads of the drilling tapping screws, a JIS salt spray test was conducted for 240 hours using 10 drilling tapping screws. If rust formed on two or more screw heads, it was evaluated as X (fail), and if it formed on fewer than two screw heads, it was evaluated as A (pass).

[0053] For the hardness of screw products, the cutting edge of a drilling tapping screw was embedded and ground, and the center was measured using the Vickers hardness test under a load of 1 kgf. A score of Hv ≥ 500 was rated as A (pass), and a score of Hv < 500 was rated as X (fail).

[0054] The toughness of the screw head was evaluated by inserting a tapping screw into a jig with a seating surface inclined at 10° to the screw, and then striking the head with a hammer until the head seating surface contacted the jig. The evaluation was based on whether the head broke off or cracked in the rounded part below the shank. If head breakage or cracking was observed, it was evaluated as X (fail), and if none was observed, it was evaluated as A (pass).

[0055] The microstructure of screw products was evaluated by embedding and polishing the longitudinal cross-section of a prototype screw as the inspection surface, and then etching with aqua regia. If δ-ferrite was present throughout, including near the surface (1 vol.%), it was evaluated as X (fail), and if it was not present, it was evaluated as A (pass).

[0056] [Table 4]

[0057] [Table 5]

[0058] [Table 6]

[0059] As shown in Tables 1-3 and 4-6, the advantages of the components of the present invention are clear.

[0060] (Example 2) Next, to investigate the effects of the cross-sectional area of ​​the cast slab during casting, indirect water cooling, and the reduction in cross-sectional area during hot rolling, steels with the chemical compositions of steels A and J shown in Table 1 were melted at approximately 1600°C in a 150 kg vacuum melting furnace, and then cast into magnesia molds of various sizes with and without indirect water cooling. Indirect cooling was used when water cooling pipes were placed around the outer circumference of the mold, and indirect cooling was not used when water cooling pipes were not placed. As shown in Table 7, the average cross-sectional area size (cross-sectional area X) of the cylindrical mold ranged from 50 to 400 cm². 2 By changing this, the solidification and cooling rate of the center of the cast slab during solidification was altered. The cooling rate of the center of the cast slab during solidification was determined by measuring the secondary dendrite arm spacing: λ (μm) at the center of the cross-section of a SUS304 sample cast slab melted and solidified under the same conditions, with λ = 82 × R -0.3 The cooling rate was estimated using the following formula: R (°C / s).

[0061] After casting, as shown in Table 7, wire rods with diameters ranging from 6.0 mm to 13.0 mm were prototyped by hot rolling after heating the cast slab at 1150°C. Further prototypes of 6.0 mm diameter steel wire were then produced using a combination of complete annealing at 880°C, pickling, and cold drawing. Subsequently, steel wires and drilling tapping screws were prototyped using the same method as in Example 1. For wire rods with a diameter of 6.0 mm, the cold drawing process was performed using a skin pass (cross-sectional reduction rate: 1%).

[0062] [Table 7]

[0063] After prototyping, the Mo segregation ratio, cold workability, corrosion resistance, hardness, toughness, and microstructure of the wire and steel wire cores were evaluated using the method described in Example 1. The results are shown in Table 7.

[0064] As shown in Table 7, the superior quality of drilling tapping screws produced using wire rods or steel wires manufactured by applying the casting and hot rolling conditions of the present invention is evident.

[0065] (Example 3) Next, regarding steel A shown in Table 1, in the normal stainless steel manufacturing process, after refining, a diameter of φ195 mm and a cross-sectional area of ​​300 cm² is obtained. 2 Continuous casting was performed using the specified mold size, and the effects of indirect water cooling within the mold during casting and direct water cooling of the slab surface immediately after withdrawal from the mold were investigated. Example 32 of the present invention in Table 8 performed both indirect water cooling within the mold and water cooling of the slab surface immediately afterward. Example 33 of the present invention performed indirect water cooling within the mold, but did not perform direct water cooling of the slab surface immediately afterward. Comparative Example 47 did not perform either indirect water cooling within the mold or direct water cooling of the slab surface immediately afterward. After casting, steel wire was prototyped under the conditions shown in Table 8 and the same conditions as in Example 2, and drilling tapping screws were prototyped.

[0066] After prototyping, the Mo segregation ratio in the center of the cross-section of the wire and steel wire, cold workability, corrosion resistance of the head of the drilling tapping screw, hardness, toughness of the head, and microstructure were evaluated using the method performed in Example 1. The results are shown in Table 8.

[0067] [Table 8]

[0068] As shown in Table 8, by applying indirect and direct water cooling conditions during the solidification of the cast slab according to the present invention, wire rods or steel wires with suppressed central segregation of Mo are manufactured, and the drilling tapping screws manufactured using said wire rods or steel wires clearly demonstrate superior quality. [Industrial applicability]

[0069] As is clear from the above embodiments, the present invention can stably provide wire rods or steel wires for high-hardness stainless steel parts with improved corrosion resistance in the core of the product, and can obtain stable corrosion resistance including in the core of high-corrosion-resistant, high-hardness parts, making it extremely useful in industry. In particular, it is applicable to martensitic stainless steel. [Explanation of symbols]

[0070] 1. Wire rod or steel wire 2 Drilling Tapping Screws 11. Central Mo positive segregation region 12. Rust formation location at the center of the screw head 13 Processing and Heat Treatment

Claims

1. In mass percent, C: 0.10% to 0.30%, Si: 0.10-2.0%, Mn: 0.10-3.0%, P: 0.05% or less, S: 0.010% or less, Cr: 12.0-16.0%, Mo: 1.0-3.0%, N: Contains 0.010 to 0.15%, Nb: 0.30% or less, V: 0.30% or less, Ti: 0.30% or less, Ta: Contains one or more of the following in an amount of 0.30% or less (Nb + V + Ti + Ta) within the range of 0.03 to 0.30%. The remainder consists of Fe and impurities. Furthermore, the components are adjusted so that the DI value represented by formula (1) is 0.0% or less, and the PREN value represented by formula (2) is 18.0% or more. Stainless steel wire or steel wire characterized in that the Mo content determined from chemical analysis of the wire or steel wire is used as the average composition, and the mass ratio of (maximum segregation amount / average composition) of Mo in the center of the cross-section of the wire or steel wire is 2.0 or less. DI=Cr+1.21Mo+0.48Si+2.48Al-(24.5C+18.4N+Ni+0.11Mn)-10...Formula (1) PREN=Cr+3.3Mo+16N...Equation (2) In equations (1) and (2), the element symbol represents the content (mass %) of the element.

2. The stainless steel wire or steel wire according to claim 1, characterized in that it contains one or both of the following groups (A) and (B) in place of a portion of the Fe. Group (A) Ni: 2.0% or less, Cu: 2.0% or less, W: 3.0% or less, Co: 2.0% or less, B: 0.010% or less, Sn: 0.30% or less, Sb: 0.30% or less, Al: 1.00% or less, one or more of the above. (B) group At least one of the following: Ca: 0.006% or less, Mg: 0.006% or less, Hf: 0.010% or less, REM: 0.06% or less.

3. A cast slab having the components described in claim 1 or claim 2 has a cross-sectional area X (cm²). ２ ) 80-350cm ２ Casting is performed by indirect cooling within the mold, or by rapid surface cooling of the cast slab using indirect cooling within the mold and direct water cooling. The method for manufacturing stainless steel wire or steel wire according to claim 1 or 2, characterized in that hot rolling is then performed with a cross-sectional reduction rate of Y (%) or more in equation (3). Y=0.0025X+98.75...Equation (3)

4. A drilling tapping screw having excellent corrosion resistance in the center of the head, characterized by being made of stainless steel wire or steel wire as described in claim 1 or claim 2.

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