Drawn wire material and method for manufacturing the same

Abstract

Provided is a drawn wire material obtained by drawing a heat-treated steel material comprising: 0.38-1.05 mass% of C; 0.0-1.0 mass% of Mn; 0.0-0.50 mass% of Cr; and 0.0-1.5 mass% of Si, with the balance being Fe and unavoidable impurities, wherein the drawn wire material has a GOS value / average crystal grain diameter of at least −0.6×GAM value+1.5, at a grain boundary set angle of 2°, and a step number of 0.07 μm.

Description

[Technical Field]

[0001] The present invention relates to a drawn wire material and a method for manufacturing the same. [Background technology]

[0002] Drawn wire material, typically wire and wire rope made by twisting multiple wires together, is made from steel material called wire rod, specifically hard steel wire material (JIS G 3506) and piano wire material (JIS G 3502), which are produced by hot rolling at steel manufacturers. These wire rods, such as hard steel wire material and piano wire material, produced by steel manufacturers usually have a large variation in tensile strength in the longitudinal direction, so the wire rods are heat treated to produce high-quality wire, wire rope, etc. with stable quality in the longitudinal direction. The minimum diameter of wire rods produced by steel manufacturers is usually about 5.5 mm. To produce thinner wire, heat treatment is required. The treated wire rod is then drawn. If the diameter of the wire rod is reduced too rapidly in one drawing process, the toughness may be reduced. To avoid this, heat treatment and drawing may be repeated multiple times.

[0003] The heat treatment performed on wire rods to stabilize quality is generally called "patenting." In patenting, the wire rod is heated to a predetermined temperature and then cooled by passing it through a medium (e.g., molten lead) heated to a predetermined temperature lower than the heating temperature. Patenting allows the production of heat-treated steel (wire) with minimal variation in longitudinal tensile strength and adequate toughness. Because heat-treated steel generates iron oxide on its surface, it is sometimes removed before wiredrawing, or it is coated or plated to prevent seizure with the die before being drawn. After wiredrawing, heat-treated steel may be shipped as is, or it may be plated or coated before shipping. Several drawn, heat-treated steel strands are twisted together to make wire rope, or brass-plated to make steel cord. In either case, patenting is a critical step in the manufacturing process of high-quality wire, wire rope, steel cord, etc.

[0004] To prevent problems such as wire breakage during wire drawing, it is essential to achieve both tensile strength and toughness. For this reason, it is considered preferable for heat-treated steel (the steel before wire drawing, which is generally the target of wire drawing) to have a structure called pearlite, in which ferrite and plate-like cementite (an intermetallic compound of Fe (iron) and C (carbon)) are arranged alternately in layers. Pearlite appears when steel is heated as described above to obtain a steel whose crystal structure has been transformed from body-centered cubic to face-centered cubic (austenitized), and then rapidly cooled (see, for example, Patent Document 1).

[0005] If the heating is insufficient to obtain austenitized steel, the cementite will not go into solution during heating, resulting in a decrease in the tensile strength of the heat-treated steel and a deterioration in the toughness of the steel after wire drawing. For example, if the thickness (diameter) of the heat-treated steel is large, the surface (surface layer) of the steel may be heated sufficiently, but the center (core layer) may not be heated enough. Generally, to avoid insufficient heating (to ensure complete austenitization) (to ensure that no undissolved carbides remain and that the carbon from the cementite is uniformly dispersed within the austenite), a long heat treatment is performed with a margin of error. However, this can cause the crystal grains (austenite grains), especially in the surface area, to grow, and if the crystal grain size is large, the metal structure will become coarse and the toughness will be reduced. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 3599551 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a drawn wire rod that is excellent in both tensile strength and toughness.

[0008] Another object of the present invention is to reduce radiant heat when the cooling medium tank is kept warm, thereby reducing fuel costs.

[0009] Another object of the present invention is to create drawn wire material from steel material of the same composition (same steel type) that has a wider range of tensile strength on the higher strength side than conventional steel materials.

[0010] Another object of the present invention is to make it possible to obtain tensile strength equivalent to that of heat-treated steel to which alloy elements have been added, without adding expensive alloy elements to the heat-treated steel to increase strength.

[0011] Another object of the present invention is to provide a wide range of hardness to tensile strength relationships, which can reduce sheave wear and provide wear resistance when used in ropes and the like. [Means for solving the problem]

[0012] As mentioned above, it has been thought that the preferred drawn wire material that combines tensile strength and toughness is one that is made by wiredrawing heat-treated steel material that has pearlite, in which ferrite and cementite are arranged in alternating layers.However, based on the inventor's tests and investigations, it has been discovered that drawn wire material that combines tensile strength and toughness can be provided even if it does not have pearlite, in which ferrite and cementite are arranged in alternating layers (even if the metal structure has little such pearlite).

[0013] It has also been confirmed that the wire drawn material provided by this invention has several properties different from conventional wire drawn materials. As explained below, the wire drawn material provided by this invention has several properties different from conventional wire drawn materials. As explained below, the wire drawn material provided by this invention has several properties different from conventional wire drawn materials. The drawn wire material can be specified.

[0014] When focusing on the GOS value and GAM value, the drawn wire material provided by this invention is obtained by drawing a heat-treated steel material containing, by mass%, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, with the balance being Fe and unavoidable impurities. The present invention is characterized in that when the GAM value is used as a variable at a grain boundary setting angle of 2° and a step number of 0.07 μm, the GOS value / average crystal grain size at a grain boundary setting angle of 2° is -0.6 × GAM value + 1.5 or more.

[0015] When focusing on the GOS value, the drawn wire material according to the present invention contains, by mass%, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, with the remainder being Fe and The steel is made by wiredrawing heat-treated steel material containing unavoidable impurities, and is characterized in that when the average grain size at a grain boundary setting angle of 2° is used as a variable, the GOS value / average grain size at a grain boundary setting angle of 2° is -0.18 × average grain size + 2.25 or more.

[0016] Furthermore, when paying attention to the GOS value, the drawn wire material according to the present invention contains, in mass%, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, and the remainder is It is made by wiredrawing heat-treated steel material containing Fe and unavoidable impurities, and is characterized by the fact that when the longitudinal

[0101] concentration is used as a variable, the GOS value / average crystal grain size at a grain boundary setting angle of 2° is 0.06 × concentration + 1.45 or more.

[0017] Focusing on the relationship between tensile strength and hardness, the drawn wire material provided by this invention is obtained by drawing a heat-treated steel material containing, by mass%, C: 0.38-1.05%, Mn: 0.0-1.0%, Cr: 0.0-0.50%, and Si: 0.0-1.5%, with the balance being Fe and unavoidable impurities. Therefore, in the range where the torsion fracture surface is normal in the torsion test, the tensile strength (TS) and hardness have the following relationship, and the relationship between tensile strength and hardness can be adjusted according to the heating conditions during patenting and the isothermal transformation temperature. 0.16TS+90≦Hardness≦0.16TS+290

[0018] When focusing on the cross section, the drawn wire material according to the present invention contains, in mass%, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, with the remainder being Fe and The heat-treated steel material containing unavoidable impurities is drawn into wire, and when the structure of the heat-treated steel material before the wire-drawing is observed by backscattered electron imaging (BSE), the area ratio of branched, bent, or curved iron carbides in the field of view in the two-phase structure of ferrite and iron carbide is 9% or more. The branched, bent, or curved iron carbides appear in a mottled pattern. can.

[0019] According to the present invention, a drawn wire material having high tensile strength and excellent toughness is provided.

[0020] The method for producing a drawn wire rod according to the present invention comprises, in mass%, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, with the remainder being Fe and unavoidable elements. The method includes the steps of preparing impure steel, directly heating the steel by generating heat from the steel itself, cooling the heated steel by passing it through a bath containing a cooling medium capable of isothermal transformation, and drawing the cooled steel, and is characterized in that the temperature gradient in the heating step is greatest in the final stage of heating, and by immersing the heated steel in the cooling medium immediately after the steel reaches a predetermined maximum heating temperature in the final stage of heating, cooling begins without maintaining the predetermined maximum heating temperature. The heating step may be performed using electricity or high frequency. Molten lead or the like may be used as the cooling medium.

[0021] The method for producing a wire drawn material according to the present invention can also be specified as follows: That is, the method for producing a wire drawn material according to the present invention comprises, in mass %, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, with the remainder being Fe and unavoidable impurities. The steel material is heated from room temperature to over 800°C within a few seconds, and the maximum temperature is maintained. a step of cooling the heated steel material to 620°C or less within a few seconds without heating the steel material; This includes the process of drawing the material.

[0022] By using this manufacturing method, it is possible to produce drawn wire material with high tensile strength and excellent toughness. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a block diagram showing a schematic diagram of a patenting device. [Figure 2] FIG. 1 is a block diagram illustrating a dry wire drawing apparatus. [Figure 3] FIG. 1 is a block diagram illustrating a wet wire drawing apparatus. [Figure 4] 1 is a graph showing the temperature change of a steel material patented using a gas furnace. [Figure 5] 2 is a graph showing the temperature change of a steel material patented using the patenting device of FIG. 1. [Figure 6] The steel type and composition are shown in a table. [Figure 7] A BSE image of a conventional product is shown. [Figure 8] A BSE image of the developed product is shown. [Figure 9] This is a partially enlarged schematic diagram of a BSE image of a conventional product. [Figure 10] This is a partially enlarged schematic diagram of the BSE image of the developed product. [Figure 11] This is a BSE image of the developed product. [Figure 12] This is a BSE image of the developed product. [Figure 13] (A), (B), and (C) show the ABF image, IPF map, and LOS map of the developed heat-treated steel, respectively. [Figure 14] (A), (B), and (C) show the ABF image, IPF map, and LOS map, respectively, of the developed heat-treated steel wire drawn to φ0.76. [Figure 15] (A), (B), and (C) show the ABF image, IPF map, and LOS map, respectively, of the developed heat-treated steel wire drawn to φ0.375. [Figure 16] (A), (B), and (C) show the ABF image, IPF map, and LOS map of the conventional heat-treated steel, respectively. [Figure 17] (A), (B), and (C) show the ABF image, IPF map, and LOS map, respectively, of a wire drawn from conventional heat-treated steel to a diameter of φ0.76. [Figure 18] (A), (B), and (C) show the ABF image, IPF map, and LOS map, respectively, of a wire made from conventional heat-treated steel drawn to a diameter of 0.375 mm. [Figure 19] 1 is a graph showing the relationship between true strain and the rate of change in cross-sectional area of ​​a wire in the longitudinal direction. [Figure 20] The relationship between true strain and average grain size when the grain boundary setting angle is 15° is shown for both the developed product and the conventional product. [Figure 21] The relationship between true strain and average grain size when the grain boundary setting angle is 5° is shown for both the developed product and the conventional product. [Figure 22] The relationship between true strain and average grain size when the grain boundary setting angle is 2° is shown for both the developed product and the conventional product. [Figure 23] The relationship between true strain and average grain size when the grain boundary setting angle is 15° is shown for both the developed product and the conventional product. [Figure 24] The relationship between true strain and average grain size when the grain boundary setting angle is 2° is shown for both the developed product and the conventional product. [Figure 25] The relationship between true strain and integration density for both the developed product and the conventional product is shown. [Figure 26] The graph shows the average grain size when the grain boundary angle is 15° and the relationship between the GOS value and average grain size when the grain boundary angle is 15° for the developed product and the conventional product. [Figure 27] The graph shows the average grain size when the grain boundary angle is 2° and the relationship between the GOS value and average grain size when the grain boundary angle is 2° for the developed product and the conventional product. [Figure 28]The relationship between the degree of integration and the GOS value / average grain size when the grain boundary setting angle is 2° is shown for both the developed product and the conventional product. [Figure 29] The graph shows the relationship between the GAM value when the grain boundary setting angle is 2° and the GOS value / average crystal grain size when the grain boundary setting angle is 2° for both the developed product and the conventional product. [Figure 30] The relationship between tensile strength and hardness for conventional products is shown below. [Figure 31] The relationship between tensile strength and hardness for the developed product and the conventional product is shown below. [Figure 32] The relationship between true strain and tensile strength for the developed product and the conventional product is shown below. [Figure 33] The relationship between true strain and tensile strength for the developed product and the conventional product is shown below. [Figure 34] The relationship between true strain and tensile strength for the developed product and the conventional product is shown below. [Figure 35] The relationship between true strain and tensile strength for the developed product and the conventional product is shown below. [Figure 36] The relationship between true strain and tensile strength for the developed product and the conventional product is shown below. [Example]

[0024] Figures 1 to 3 show schematic diagrams of a manufacturing apparatus for a drawn wire material, typically wire. Figure 1 shows a patenting device that constitutes the wire manufacturing apparatus, and Figures 2 and 3 show wire drawing processing devices that constitute the wire manufacturing apparatus. In the following explanation, to distinguish between them, the steel material before patenting will simply be called "steel material 11," the steel material after patenting will be called "heat-treated steel material 12," and the heat-treated steel material 12 that has been drawn will be called "wire 13."

[0025] The wire manufacturing equipment includes a patenting equipment and a wire drawing equipment.

[0026] Referring to FIG. 1, the patenting device includes a power supply 14 , a power supply roll 15 , a bath 16 and molten lead 17 stored in the bath 16 .

[0027] The steel material 11 is supplied in the form of a wire. The steel material 11 is fed from a payoff (not shown) and travels at a constant speed from left to right in Fig. 1. The steel material 11 passes through a power feed roll 15 and is immersed in molten lead 17 stored in a bath 16 for a predetermined time.

[0028] First, heat treatment is performed on the steel material 11. The power supply 14 provided in the patenting device is connected to the power feed roll 15 and the bath 16, and a closed circuit including the power supply 14, the power feed roll 15, the molten lead 17, and the bath 16 is formed. On the left side (upstream side) of the power feed roll 15, an insulating device (not shown) is provided to prevent current from flowing to the steel material 11. In the section from the power feed roll 15 to the liquid surface of the molten lead 17 stored in the bath 16, the steel material 11 is energized with current supplied from the power supply 14 and heated.

[0029] The steel material 11 is heated most at a point just before it enters the liquid surface of the molten lead 17 stored in the bath 16. The heating temperature of the steel material 11 (the maximum temperature that the steel material 11 reaches) is set to 975°C or less in order to exhibit the characteristics described below. If the heating temperature is too high, the crystal grains (austenite grains) grow, the metal structure becomes coarse, and the toughness, especially the reduction of area, decreases. However, since insufficient heating can cause the iron carbide (cementite as an example), which is an intermetallic compound of Fe and C, to become non-soluble, the heating temperature of the steel material 11 is preferably set to 800°C or higher. By adjusting the voltage or current of the power source 14, The heating temperature of the steel material 11 can be controlled. The heating time is adjusted by the path length from the power feed roll 15 to the liquid surface of the molten lead 17 and the running speed of the steel material 11.

[0030] The molten lead 17 stored in the bath 16 is heated to a constant temperature by a gas furnace (or an electric heater is also acceptable). The temperature of the molten lead 17 is lower than the heating temperature of the steel material 11 described above, and the steel material 11, which has been heated to its highest temperature just before entering the liquid surface of the molten lead 17, begins to cool as soon as it enters the molten lead 17.

[0031] The temperature of the molten lead 17 (lead furnace temperature), i.e., the isothermal transformation temperature, is set to 620°C or less. This is done by rapidly cooling the austenite to obtain the precipitation of pearlite and carbides. However, if the product is cooled too quickly, martensite and other substances that make the product brittle will appear, so the lower limit of the temperature for molten lead-17 is set at around 350°C.

[0032] The patented steel material, i.e., heat-treated steel material 12, is immersed in molten lead 17 and then pulled out of the bath 16, and then undergoes water washing, coating, and wire drawing.

[0033] Fig. 2 shows a schematic diagram of a dry wire drawing device, and Fig. 3 shows a schematic diagram of a wet wire drawing device. In general, a dry wire drawing device is used to manufacture a wire 13 with a relatively large diameter, and a wet wire drawing device is used to manufacture a wire 13 with a small diameter.

[0034] Referring to FIG. 2, the dry wire drawing apparatus includes a lubricant box that stores a dry lubricant 21, a die 22, a die holder 23, and a drawing block 24.

[0035] Dry lubricant 21 stored in the lubricant box adheres to the surface of the heat-treated steel material 12. The dry lubricant 21 is used to prevent seizure between the heat-treated steel material 12 and the die 22 described next, and to maintain a stable processed shape by making the heat-treated steel material 12 easier to slide and easier to pull out from the die 22. For the dry lubricant 21, metal soaps such as sodium-based soaps and calcium-based soaps can be used.

[0036] The heat-treated steel material 12 with the dry lubricant 21 attached to the surface is passed through holes in a die 22. The holes in the die 22 are formed so that the diameter decreases from the entrance side to the exit side, and the diameter of the heat-treated steel material 12 decreases as it passes through the holes in the die 22.

[0037] Cooling water is stored around the die 22 and the die holder 23 that fixes the die 22. The heat generated by the wire drawing process is removed by the cooling water, thereby preventing thermal damage to the heat-treated steel material 12 and the die 22.

[0038] The wire 13, whose diameter has been reduced by passing through the die 22, is wound around a drawing block 24. The drawing block 24 draws and cools the wire 13.

[0039] Referring to Figure 3, the wet wire drawing device includes two drawing capstans 32, 33 spaced apart, and a plurality of dies 31 (three in Figure 3) provided between the two drawing capstans 32, 33. The drawing capstans 32, 33 include one or more coaxial capstans, and the drawing capstans 32, 33 shown in Figure 3 include three capstans with small, medium, and large diameters. The heat-treated steel material 12 is hung in this order around the small diameter capstan of one drawing capstan 32, the small diameter capstan of the other drawing capstan 33, the medium diameter capstan of one drawing capstan 32, the medium diameter capstan of the other drawing capstan 33, the large diameter capstan of one drawing capstan 32, and the large diameter capstan of the other drawing capstan 33. The heat-treated steel material 12 is passed through the holes of a die 31 provided between two drawing capstans 32, 33. The holes of the die 31 are also formed so that the diameter decreases from the entrance side to the exit side, and the diameter of the heat-treated steel material 12 decreases each time it passes through the holes of the die 31.

[0040] The drawing capstans 32, 33 and the die 31 are all immersed in a lubricating liquid, which prevents seizure between the heat-treated steel material 12 and the die 31. In addition, in the wet wire drawing device, the lubricating liquid also serves to cool the heat-treated steel material 12 and the die 31.

[0041] The wire 13 thinned by the above-mentioned dry wire drawing device, wet wire drawing device, or both is then wound onto a winding drum (not shown).

[0042] Figure 4 shows the temperature change (heat-rise curve) of steel material 11 (heat-treated steel material 12) patented using a gas furnace, and Figure 5 shows the temperature change (heat-rise curve) of steel material 11 (heat-treated steel material 12) patented using the patenting device shown in Figure 1. In both the graphs of Figure 4 and Figure 5, the temperature drops sharply at the moment steel material 11 enters molten lead 17. Please note that the scales of the time axes (horizontal axes) are different between Figure 4 and Figure 5.

[0043] Referring to Figure 4, when a gas furnace is used, the steel material 11 is heated gradually. In an atmosphere heating furnace, such as a gas furnace, the time required for heating is proportional to the wire diameter of the steel material 11; the thinner the wire diameter, the shorter the heating time, and the thicker the wire diameter, the longer the heating time. Figures 4 and 5 are graphs for steel material 11 with a wire diameter of φ2.11, and when a gas furnace is used, it takes about 40 seconds to reach the maximum temperature (target heating temperature). On the other hand, referring to Figure 5, when the patenting device shown in Figure 1 is used, the steel material 11 reaches the maximum temperature (target heating temperature) in a few seconds. The patenting device shown in Figure 1 can maintain a constant heating rate regardless of the wire diameter.

[0044] Comparing the graphs in Figure 4 and Figure 5, the shapes of the temperature rise curves are significantly different. In the graph in Figure 4, the temperature rise rate slows down from around 723°C, where austenitization begins, The ratio of the time required for austenitization increases, while the graph in Figure 5 shows that the time required for austenitization increases above 723°C. In Figure 4, the temperature rise rate is fast and the proportion of the time required for austenitization is short. Also, in Figure 4, the maximum temperature is reached and held for about 20 seconds, whereas in Figure 5, cooling begins immediately after the maximum temperature is reached.

[0045] The starting wire rod 11 and the patented heat-treated steel 12 are carbon steels containing iron (Fe) and carbon (C). By setting the carbon content (carbon concentration) to 0.38% (meaning mass %, the same applies hereinafter) or more, sufficient strength is easily obtained, and by setting it to 1.05% or less, deterioration in workability and fatigue limit, etc. are suppressed.

[0046] In addition to Fe and C, the heat-treated steel material 12 may contain manganese (Mn), chromium (Cr), and silicon (Si).

[0047] Manganese (Mn) is contained as a deoxidizer, and its content is limited to 1.0% or less to prevent deterioration of workability.

[0048] Chromium (Cr) generally refines pearlite and is effective in improving toughness. Adding a large amount of Cr can actually reduce toughness, so the content should be kept below 0.50%.

[0049] Silicon (Si) is used as a deoxidizer. The content of about 1.5% is used to avoid deterioration of ductility. It can be kept in a limited quantity.

[0050] Others include vanadium (V) (0.50% or less), molybdenum (Mo) (0.25% or less), boron (B) (0.005% or less), titanium (Ti) (0.050%), nickel (Ni) (0.50% or less), aluminum (0.10% or less), zirconium (Zr) (0.050% or less), etc. Other elements may be added to the steel material 11 (heat-treated steel material 12) depending on the application.

[0051] In the following explanation, the heat-treated steel material 12 obtained by heating as shown in Figure 4 and ensuring a maximum temperature holding time of about 20 seconds, and the wire 13 produced by wiredrawing such heat-treated steel material 12, are referred to as "conventional products," while the heat-treated steel material 12 obtained by heating as shown in Figure 5 and starting cooling immediately after reaching the maximum temperature, and the wire 13 produced by wiredrawing such heat-treated steel material 12, are referred to as "developed products." Figure 6 summarizes the steel type names and components of the multiple steel materials 11 (heat-treated steel material 12, wire 13) described below.

[0052] (Backscattered electron image) In Figures 7 and 8, the steel material 11 before heat treatment is the same (both are SWRH62A with a diameter of 2.11 mm), but the patenting methods are different as shown in Figures 4 and 5, respectively. These are backscattered electron (BSE) images of heat-treated steel 12 obtained by milling, with Figure 7 being the BSE image of the conventional product and Figure 8 being the BSE image of the developed product. The backscattered electron images were taken in the longitudinal direction of the heat-treated steel 12 after polishing the heat-treated steel 12 and milling it using argon gas. The length of the white horizontal rectangle shown at the bottom of the BSE images in Figures 7 and 8 corresponds to 1 μm (magnification: 10,000x). Figure 9 is a partially enlarged schematic diagram of the BSE image of the conventional product shown in Figure 7, and Figure 10 is a partially enlarged schematic diagram of the BSE image of the developed product shown in Figure 8.

[0053] Figure 7 shows the BSE image of conventional heat-treated steel 12 obtained at a temperature of 565°C for molten lead 17. Figure 8 shows the BSE image of the developed heat-treated steel12 obtained by setting the temperature of the molten lead17 at 450°C. be.

[0054] As shown in Figures 7 and 9, the BSE image of the conventional product shows a layered structure in which ferrite and cementite (Fe3C) are arranged alternately in layers within the prior austenite grain boundaries. In the BSE image of the conventional product, the cementite appears as multiple thin, parallel stripes.

[0055] The white areas in Figure 8 contain cementite (Fe3C), but they are also iron carbides (e.g., Fe 2~2.5 C,Fe 2~3 As can be seen by comparing Figures 7 and 8, or Figures 9 and 10, the thickness (layer thickness) of the multiple iron carbides in the developed product is uneven and thick (approximately 60 nm) compared to the conventional product.

[0056] In the following explanation, to distinguish it from the "cementite" (Fe3C) that is the iron carbide that constitutes the layered structure confirmed in the developed product, the iron carbide (Fe3C, Fe 2~2.5 C,Fe 2~3 C, etc.) is called "special cementite."

[0057] As shown in Figures 8 and 10, the layered structure of ferrite and special cementite can be confirmed in the BSE images of the developed product. However, there are very few special cementites arranged in layers (thin, parallel lines in the BSE image), and the layer thickness (thickness of the lines in the BSE image) is uneven, with many of them being refracted, branched, or curved (area ratio within the field of view is 9% or more). In the BSE images of the developed product, there is very little special cementite arranged in layers (thin, parallel lines in the BSE image), and the layer thickness (thickness of the lines in the BSE image) is uneven, and many of them are bent, branched, or curved (area ratio within the field of view is 9% or more). appears to be a patchy pattern.

[0058] Figures 11 and 12 show the results of using SWRS92A steel, and Figure 11 shows the temperature of molten lead 17. Figure 11 is a BSE image of the developed heat-treated steel 12 obtained at a temperature of 565°C, and Figure 12 is a BSE image of the developed heat-treated steel 12 obtained at a temperature of 450°C. In the BSE images of the developed product shown in Figures 11 and 12, there is little special cementite arranged in layers, and it appears mottled.

[0059] Various measurements were carried out to determine the properties of wire 13, which was produced by wiredrawing heat-treated steel material 12, a developed product with a structure different from that of conventional products. Measurements were also carried out on wire produced by wiredrawing heat-treated steel material of conventional products. The measurement results are explained below.

[0060] As explained in detail below, EBSD (Electron Backscattered Diffraction) analysis is used to measure the characteristics of the developed and conventional products. In EBSD analysis, the measurement area of ​​a polished sample cross section (in this example, the longitudinal cross section (longitudinal cross section) of wire 13) is divided into measurement points (commonly called "pixels"), an electron beam is incident on each of the divided pixels, and the crystal orientation at each pixel is measured based on the backscattered electrons obtained when the incident electron beam is reflected at the pixel. The obtained crystal orientation data is analyzed using the above-mentioned EBSD analysis software, and various parameters are calculated. In this example, an EBSD analyzer made by TSL Solutions Co., Ltd. was used, and a regular hexagonal pixel shape was adopted.

[0061] A grain boundary setting angle (grain boundary setting value) is set in the EBSD analysis software. In EBSD analysis, the crystal orientation obtained for each pixel is used, and boundaries where the difference in crystal orientation between adjacent pixels is equal to or greater than the grain boundary setting angle are treated as "grain boundaries," and the area surrounded by the grain boundaries is treated as a "crystal grain." If the grain boundary setting angle (grain boundary setting value) is reduced, the crystal grain size becomes smaller and the number of crystals in the observation area increases. Conversely, if the grain boundary setting angle is increased, the crystal grain size becomes larger and the number of crystals in the observation area decreases. In EBSD analysis, carbides are too small to be measured, so the crystal orientation of ferrite is evaluated.

[0062] When performing EBSD analysis on wire 13, the object to be measured that has undergone the wire drawing process (plastic processing) described above, the crystal lattice of the object to be measured is distorted by the plastic processing, which can result in inaccurate crystal orientation measurement results in some areas. The crystal lattice is particularly disrupted at grain boundaries, making the crystal orientation measurement more likely to be inaccurate, leading to a high likelihood of incorrect analysis. While the method for handling inaccurately measured areas differs depending on the manufacturer of the EBSD analyzer, the EBSD analyzer used here, manufactured by TSL Solutions, Inc., uses a CI (Confidence Index) value, which indicates the probability that the crystal orientation analyzed for each pixel is accurate, and only uses areas where the crystal orientation is measured correctly with a probability of 95% or higher, i.e., areas with a CI value of 0.1 or higher.

[0063] When performing EBSB analysis, it is verified that there is no problem in excluding the parts where the crystal orientation measurement is inaccurate. For the verification, the developed heat-treated steel material 12, SWRH62A, φ2.11, and its heat Two types of wires 13 are used, which are made by drawing the processed steel material 12 to φ0.76 and φ0.375. For this purpose, we conducted structural observation using s-TEM (Scanning Transmission Electron Microscopy). We use t-EBSD (Transmission Electron Backscattered Diffraction) (transmission EBSD), which has higher resolution than conventional EBSD, for structural observation.

[0064] Figure 13(A) shows an annular bright-field (ABF) image taken by s-TEM, Figure 13(B) shows an IPF map taken by t-EBSD, and Figure 13(C) shows an LOS map taken by t-EBSD. All of these are observation results for heat-treated steel 12 (i.e., before wiredrawing).

[0065] The s-TEM ABF image shown in Figure 13(A) combines an image of the entire field of view with a partially enlarged image for clarity. In the ABF image of the developed heat-treated steel 12, grain boundaries and special cementite can be observed.

[0066] The t-EBSD shown in Figures 13(B) and 13(C) was analyzed with a grain boundary setting angle of 15° (same below). The t-EBSD IPF (Inverse Pole Figure) map shown in Figure 13(B) shows the measurement points color-coded by crystal orientation. For ease of illustration, the IPF map in Figure 13(B) is not color-coded, and the crystal orientation is indicated only by the shading (brightness) of the image (the same applies below). The LOS (Local Orientation Spread) map shown in Figure 13(C) indicates the difference in crystal orientation between adjacent pixels by color. In Figure 13(C) as well, for ease of illustration, the LOS map is not color-coded, and the difference in crystal orientation is indicated only by the shading (brightness) of the image (the same applies below).

[0067] The CI value was 0.1 in the IPF map in Figure 13(B) and the LOS map in Figure 13(C). The observation results in Fig. 13(A) to Fig. 13(C) show that the parts of the developed heat-treated steel 12 where the CI value is less than 0.1 are concentrated at the grain boundaries. Therefore, it is found that it is better to exclude the grain boundary part where the CI value is less than 0.1 from the analysis. Furthermore, the LOS map in Figure 13(C) shows that the developed heat-treated steel 12 has almost no subgrain boundaries within the crystal grains.

[0068] Figures 14(A), (B) and (C) show the ABF image, IPF map and LOS map, respectively, of the cross section of wire 13 drawn from heat-treated steel material 12 of the developed product from φ2.11 to φ0.76.

[0069] In the ABF image shown in Figure 14(A), the special cementite is unclear. From the IPF map and LOS map shown in Figure 14(B) and Figure 14(C), the area with a CI value of less than 0.1 is It is clear that the grain boundaries are concentrated at the grain boundaries, and it is best to exclude the grain boundary areas from the analysis. Furthermore, the LOS map shown in Figure 14(C) shows that lines with a crystal orientation difference of less than 15° between adjacent pixels are observed within the grains, which are not seen in the heat-treated steel 12 before wiredrawing (Figure 13(C)), and it is clear that subgrain boundaries are generated by the wiredrawing process.

[0070] Figures 15(A), (B), and (C) show the results of further stretching the developed heat-treated steel 12 to φ0.375. ABF image, IPF map, and LOS map of the cross section of the processed wire 13 are shown, respectively.

[0071] Similar to the φ0.76 wire 13 described with reference to Figures 14(A) to (C), the wire is drawn to φ0.375. For the processed wire 13, the IPF map shown in FIG. 15(B) and the LOS map shown in FIG. 15(C) show that the parts with a CI value of less than 0.1 are concentrated at the grain boundaries. It is clear that it is better to exclude from the analysis. In addition, the LOS map of wire 13 drawn to φ0.76 (Fig. 14(C)) and the LOS map of wire 13 drawn to φ0.375 (Fig. 14(D)) are 5(C)), the LOS map of wire 13 drawn to φ0.375 shows that the crystalline There are some areas where wires with a misorientation of less than 15° and areas with a CI value of less than 0.1 are connected, and after wire drawing, It can be seen that the sub-boundaries generated by the drawing process become grain boundaries through further wire drawing.

[0072] 16(A), (B) and (C) show the ABF image, IPF map and LOS map of the conventional heat-treated steel material 12, respectively.

[0073] The CI value is less than 0.1 at the bottom of the IPF map (Fig. 16(B)) and LOS map (Fig. 16(C)). The reason for the large number of "less than" regions is that in t-EBSD, the sample is thinned, an electron beam is irradiated, and the transmitted analysis image is used, but when creating the thin film, the film becomes thicker the further away from the edge of the sample, and the analysis image becomes unclear.Unlike the LOS map of the developed heat-treated steel 12 shown in Figure 13(C), the LOS map in Figure 16(C) shows that subgrain boundaries with a crystal orientation difference of less than 15° are present within the crystal grains of the conventional heat-treated steel 12.

[0074] 17(A), (B) and (C) show the ABF image, IPF map and LOS map, respectively, of the cross section of wire 13 obtained by drawing conventional heat-treated steel material 12 to φ0.76.

[0075] In the ABF image shown in Figure 17(A), cementite is more clearly visible than in the wire 13 (Figure 14(A)) made by drawing the heat-treated steel material 12 of the developed product to a diameter of 0.76 mm. The LOS map in Figure 17(C) shows that many subgrain boundaries have occurred, just like in the developed product (Figure 14(C)).

[0076] Figures 18(A), (B) and (C) show the conventional heat-treated steel material 12 wire-drawn to φ0.375. 10 shows an ABF image, an IPF map, and an LOS map of the cross section of the wire 13, respectively.

[0077] Similar to the developed product shown in Figure 15(C), the LOS map of the conventional product in Figure 18(C) shows that there are more subgrain boundaries.

[0078] For both the developed product and the conventional product, the crystal grain size does not become smaller by the area reduction rate of the wire drawing process. When heat-treated steel 12 is drawn, subgrain boundaries are generated, and when the wire is drawn further, the subgrain boundaries become grain boundaries. As a result, it can be seen that the larger the area reduction rate (the more thin the wire is made), the smaller the crystal grain size becomes.

[0079] When comparing the developed product with the conventional product, it was found that in t-EBSD, the sample is made into a thin film, so the sample is very small and the observation range is narrow, making it difficult to secure an observation area to observe a statistically sufficient number of grain boundaries. Also, in the cross section, the proportion of grain boundaries in the observation range is high, and the proportion where crystal orientation can be measured accurately is low. For this reason, it is better to use a conventional EBSD that can widen the measurement range and perform observations in the longitudinal section (longitudinal section), where the proportion of grain boundaries is smaller than in the cross section and the proportion where accurate measurement is high. Also, when heat-treated steel 12 is subjected to wet wire drawing, friction with the die causes additional shear strain on the surface of the wire, resulting in the formation of a

[0111] crystal. It is known that the number of orientations increases. Since the wire surface is greatly affected by the wiredrawing conditions, EBSD analysis was performed at the center of the wire, where the influence of friction is small.

[0080] The condition for EBSD measurement is that the proportion of CI values ​​of 0.1 or more at all measurement points is 70% or more. The accelerating voltage and other measurement conditions are set. The interval between measurement points is called the step number, and this step number is basically 0.07 μm. However, in terms of EBSD performance, the measurement is performed with a step number of 0.07 μm. There are cases where the analysis software cannot process the data because there are too many fixed points. In such cases, the number of steps can be changed to an upper limit of 0.20 μm, but the number of crystal grains at a grain boundary setting angle of 2° should be kept within a range where the number of crystal grains at a grain boundary setting angle of 15° is 1.5 times or more. When the ratio of the number of crystal grains at an angle of 2° is less than 1.5 times, the area with large strain cannot be measured. , or the number of steps is too large and a part that is not a grain boundary is judged to be a grain boundary, so the measurement conditions are changed. The measurement range is the longitudinal measurement length, and since the object to be measured becomes elongated in the longitudinal direction due to wire drawing, the longitudinal direction of the crystal grain measured with a grain boundary setting angle of 15° and a CI value of 0.1 or more. The area is observed to be at least twice the maximum length of the crystal grains, and the area is observed to contain at least 30 crystal grains whose average crystal grain size (converted into the diameter of a circle with an area equal to the crystal grain area) is at least the average value.

[0081] Figure 19 shows a graph with true strain on the horizontal axis and the vertical axis on the vertical axis, where the cross-sectional area in the longitudinal direction including the central axis (longitudinal cross-sectional area) when a wire 13 with a diameter A0 and length L0 before wiredrawing is drawn to a diameter A (A > A0) and length L (L > L0) is the calculated rate of change (longitudinal cross-sectional area after wiredrawing / longitudinal cross-sectional area before wiredrawing), where the longitudinal cross-sectional area of ​​the wire 13 at diameter A0 is set to 1. True strain is a value calculated by 2ln(A0 / A) ("ln" is the natural logarithm) where A0 is the longitudinal cross-sectional area of ​​the wire 13 (heat-treated steel material 12) before wiredrawing and A is the longitudinal cross-sectional area of ​​the wire 13 after wiredrawing. The value of true strain increases as the degree of processing (diameter reduction rate) of the heat-treated steel material 12 by the wiredrawing device (die) increases.

[0082] Since the volume of the heat-treated steel material 12 does not change before and after wiredrawing, the length L of the heat-treated steel material 12 when it is drawn from diameter A0 to diameter A is expressed as (A0 / A) × L0. Furthermore, the ratio of the longitudinal cross-sectional area including the central axis when the heat-treated steel material 12 is drawn from diameter A0 to diameter A is A0 / A, and the length of the heat-treated steel material 12 is irrelevant. Therefore, the relationship between the true strain and the ratio of the longitudinal cross-sectional area is expressed as exp[0.5 × {2 × ln(A0 / A)}]. This formula is shown by the solid line in Figure 19. In calculations, the larger the true strain value (the thinner the wire diameter), the larger the longitudinal cross-sectional area of ​​the heat-treated steel material 12. The same applies to the grain size observed in the longitudinal cross section.

[0083] Figure 20 is a graph showing the relationship between true strain and average grain size for both the developed product and the conventional product, with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average grain size (converted into the diameter of a circle with the same area as the grain area) (μm) when the grain boundary setting angle is set to 15° in the EBSD analysis software. In Figure 20, the dashed line represents the developed product and the solid line represents the conventional product. For both the developed product and the conventional product, wire 13 (heat treated and drawn from the same steel material 11 (SWRH62A)) was used (as mentioned above, the difference between the developed product and the conventional product was due to the difference in the heat treatment). For the developed product, the measurement results for three types of wire13 made at molten lead17 temperatures of 565℃, 450℃, and 425℃ are shown. For the conventional product, The measurement results for one type of wire 13 prepared with the molten lead 17 at a temperature of 565°C are shown. Below the graph in 20, the wire types shown on the graph, the steel type for each wire type, the distinction between developed and conventional products, and the isothermal transformation temperature are shown (same below).

[0084] Referring to Figure 20, for the conventional product (solid line), the average grain size is relatively large at a grain boundary setting angle of 15°, and the average grain size increases as the true strain increases up to around 1.0 true strain. However, when the true strain exceeds 1.5, the average grain size decreases as the true strain increases. On the other hand, for the developed product (dashed line), the average grain size at a grain boundary setting angle of 15° is small (about 4 μm) in the state without wire drawing (true strain is 0.0), and It can be seen that even if the true strain is increased, that is, even if the heat-treated steel material 12 is subjected to wire drawing, the average crystal grain size at a grain boundary setting angle of 15° does not change as much as in the conventional product.

[0085] Figure 21 is a graph showing the relationship between true strain and average grain size for both the developed product and the conventional product, with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average grain size (μm) when the grain boundary setting angle is set to 5° in the EBSD analysis software. As with the graph shown in Figure 20, for both the developed product (dashed line) and the conventional product (solid line), the wire 13 (which is the same steel material 11 (SWRH62A) as described above) was heat treated and drawn. These are the measurement results for the product (heat treatment is different for the product).

[0086] When the grain boundary angle is set to 5°, the average grain size decreases as the true strain increases for both the conventional product (solid line) and the developed product (dashed line).The average grain size of the developed product is also smaller than that of the conventional product.

[0087] Figure 22 is a graph showing the relationship between true strain and average grain size for both the developed product and the conventional product, with the horizontal axis representing true strain and the vertical axis representing the measurement results of the average grain size (μm) when the grain boundary setting angle is set to 2° in the EBSD analysis software. As with the graphs shown in Figures 20 and 21, for both the developed product (dashed line) and the conventional product (solid line), wire 13 (developed as described above) was obtained by heat treating and wiredrawing steel material 11 (SWRH62A) of the same steel type. These are the measurement results for the difference in heat treatment between the developed product and the conventional product.

[0088] Even when the grain boundary angle was set to 2°, the average grain size decreased as the true strain increased for both the conventional product (solid line) and the developed product (dashed line).The average grain size of the developed product was also smaller than that of the conventional product.

[0089] Comparing the graph when the grain boundary angle was set to 15° (Fig. 20), the graph when the grain boundary angle was set to 5° (Fig. 21), and the graph when the grain boundary angle was set to 2° (Fig. 22), it can be seen that for the conventional product (solid line), the smaller the grain boundary angle, the smaller the change in average crystal grain size according to the degree of wiredrawing (degree of true strain).On the other hand, for the developed product (dashed line), it can be seen that the change in average crystal grain size is roughly the same regardless of the size of the grain boundary angle and the degree of wiredrawing (degree of true strain).

[0090] Figure 23 is a graph showing the relationship between true strain and average grain size for a number of developed and conventional products, with the horizontal axis representing true strain and the vertical axis representing the measurement results of average grain size (μm) when the grain boundary setting angle is 15° in EBSD analysis software. Figure 23 shows the relationship between true strain and average grain size when the grain boundary angle is 15° for wire 13 of steel types SWRH42A, SWRH62A, SWRH82A, SWRH82B, SWRS92A, 92A-Cr, 92B-Si, and 102A-Cr for the conventional product (solid line) and the developed product (dashed line).

[0091] As can be seen from Figure 23, for all of the various steel types, the average grain size at a grain boundary setting angle of 15° changes or fluctuates significantly when the true strain changes for the conventional product (solid line). In contrast, the average grain size changes little for the developed product (dashed line) even when the true strain is changed. The graph in Figure 23 also shows that for both the conventional and developed products, the larger the average grain size before wiredrawing (true strain 0), the greater the decrease in average grain size due to wiredrawing. Furthermore, for the conventional SWRH62A and conventional SWRH42A products, the true strain is large up to a true strain of 2.0. The average grain size becomes larger as the temperature increases.

[0092] FIG. 24 is a graph in which the horizontal axis represents true strain and the vertical axis represents the average grain size (μm) at a grain boundary setting angle of 2°, with the dashed line representing the developed product and the solid line representing the conventional product.

[0093] Referring to Figure 24, when the grain boundary angle is set to 2°, the change in average grain size is small regardless of the magnitude of true strain, and the difference between the conventional product and the developed product is also small. True strain and average grain size have an approximately linear relationship, with the average grain size decreasing as the true strain increases. When the grain boundary angle is set to 2°, the average grain size is correlated with true strain.

[0094] Figure 25 is a graph with the horizontal axis representing true strain and the vertical axis representing integration degree, with the dashed line representing the developed product and the solid line representing the conventional product.

[0095] The vertical axis shows the degree of integration in the longitudinal direction (0101). This is a value calculated in SD, and it is calculated by taking the probability that the crystal orientation exists in a completely random state as 1, and calculating how many times the probability that the crystal orientation of the measured object exists. It is known that the

[0101] direction is oriented in the longitudinal direction when wire drawing is performed. The larger the true strain, the more concentrated the crystal orientation. The accumulation degree will be large. If the diameters of the heat-treated steel material 12 (wire before wire drawing) and the wire 13 after wire drawing are known, the "true strain" can be calculated. On the other hand, if the diameter of the heat-treated steel material 12 is unknown, the "accumulation degree" calculated by EBSD can be used as an index to determine the extent of wire drawing, although it is only a rough estimate. Hereinafter, the accumulation degree in the longitudinal direction

[0101] will be used.

[0096] Figure 26 shows the measurement results for several developed products and several conventional products, and is a graph with the horizontal axis being the average grain size at a grain boundary setting angle of 15° and the vertical axis being the GOS value / average grain size at a grain boundary setting angle of 15°. The conventional products are shown by a solid line, and the developed products by a dashed line.

[0097] The GOS (Grain Orientation Spread) value (also called the average GOS value) is the calculated average of the intra-grain orientation misorientation between two pixels within the same grain, and is used as an index of strain. As mentioned above, the grain boundary varies depending on the grain boundary setting angle, so changing the grain boundary setting angle will cause the GOS value to vary. The GOS value is also calculated using EBSD analysis software. The GOS value represents the misorientation over a wide range within the grain. The GOS value is a parameter that reflects the overall change in the crystal orientation of the grain, and is equivalent to the integration of the local misorientation (KAM) mentioned above. The GOS value does not depend on the number of steps, but when the twist of the crystal orientation per unit length is the same, it increases as the grain becomes larger. Below, the GOS value is the average value calculated from the area fraction within the measurement range. Use.

[0098] Referring to Figure 26, the developed product (dashed line) tends to have a larger GOS value / average crystal grain size ratio compared to the conventional product (solid line). By setting the grain boundary setting angle to 15° and calculating the GOS value / average crystal grain size, it is possible to roughly distinguish between the developed product and the conventional product.

[0099] Figure 27 shows the measurement results for several developed products and several conventional products, and is a graph with the horizontal axis representing the average grain size at a grain boundary setting angle of 2° and the vertical axis representing the GOS value / average grain size at a grain boundary setting angle of 2°. In Figure 27, the dashed line represents -0.18 x average grain size + 2.25.

[0100] When the grain boundary angle is set to 2°, the developed product has a larger GOS value / average grain size value than the conventional product at the same average grain size. Furthermore, the GOS value / average grain size value for the conventional product (solid line) is less than "-0.18 × average grain size + 2.25," while the GOS value / average grain size value for the developed product (dashed line) is greater than "-0.18 × average grain size + 2.25." The conventional and developed products can be distinguished by whether the GOS value / average grain size value measured when the grain boundary angle is set to 2° is greater than or less than -0.18 × average grain size + 2.25.

[0101] Figure 28 shows the measurement results for several developed products and several conventional products, with the horizontal axis representing the integration degree and the vertical axis representing the GOS value / average crystal grain size at a grain boundary setting angle of 2°. In Figure 28, the dashed line represents 0.06 x integration degree + 1.45.

[0102] When the grain boundary setting angle is set to 2°, the developed product has a larger GOS value / average crystal grain size value than the conventional product at the same integration level. Also, the GOS value / average crystal grain number for the conventional product (solid line) is "0.06 × integration level + 1.45 or less," while the GOS value / average crystal grain number for the developed product (solid line) is "0.06 × integration level + 1.45" or more. The conventional and developed products can be distinguished by whether the GOS value / average crystal grain size at a grain boundary setting angle of 2° is above or below the reference value calculated by "0.06 × integration level + 1.45."

[0103] Figure 29 shows the measurement results for several developed products and several conventional products, with the horizontal axis representing the GAM value at a grain size setting angle of 2° and a step number of 0.07 μm, and the vertical axis representing the GOS value / average crystal grain size at a grain boundary setting angle of 2°. The conventional products are shown by a solid line, and the developed products by a dashed line. Figure 29 also shows a dashed line representing -0.6 x average GAM value + 1.5.

[0104] The GAM (Grain Average Misorientation) value (also called the average GAM value) is the average value of the misorientation between adjacent pixels within a single grain, and is one of the indicators that shows the twist of the crystal orientation within the grain. The larger the GAM value, the more distorted the crystal lattice is. The GAM value varies depending on the distance between the measurement points (pixels) during measurement (expressed as the "number of steps"). The GAM value is calculated using EBSD analysis software. The average of m misorientations between measurement points within a grain is the GAM value. The GAM value, defined from the average of the local misorientations, is called the local misorientation KAM (Kernel Average Misorientation ) values ​​for each crystal grain, and its absolute value depends on the number of steps in the EBSD measurement. For wire 13 with non-uniform strain, the GAM value changes when the number of steps is changed, so in this example the number of steps is fixed at 0.07 μm. Hereinafter, the GAM value will be the average value calculated from the area fraction within the measurement range.

[0105] Referring to Figure 29, when the grain boundary setting angle is set to 2°, the developed product (dashed line) has a larger GOS value / average crystal grain size value compared to the conventional product (solid line). Furthermore, the conventional product and the developed product can be roughly distinguished by whether the GOS value / average crystal grain size value at a grain boundary setting angle of 2° is above or below the reference value (threshold value) of "-0.6 x GAM value + 1.5."

[0106] Figure 30 shows the measurement results of several conventional products, and is a graph with tensile strength (MPa) on the horizontal axis and hardness (Hv) on the vertical axis. Figure 30 shows the relationship between tensile strength and hardness for conventional wire 13 made from each of the steel types SWRH42A, SWRH62A, SWRH82A, SWRH82B, SWRS92A, 92A-Cr, 92B-Si, and 102A-Cr. Figure 30 plots wires that had normal torsion fracture surfaces in the torsion test.

[0107] Looking at the relationship between tensile strength and hardness in Figure 30, the hardness of conventional products is 0.2TS+88≦Hardness. The degree of cracking is within the range of ≦0.2TS+123 (TS is the tensile strength).

[0108] Figure 31 shows the measurement results of several developed products, and like Figure 30, is a graph with tensile strength (MPa) on the horizontal axis and hardness (Hv) on the vertical axis. Figure 31 also plots products that had normal torsion fracture surfaces in the torsion test.

[0109] In FIG. 31, the graphs shown in (a) and (b) are particularly explained. These are two types of developed wires 13 made from the same steel material 11 of steel type SWRH62A. However, the graph in Figure 31 shows the measurement results for two types of developed wire 13 manufactured by varying the heating conditions (including wire speed) and isothermal transformation temperature (temperature of molten lead 17) during patenting in the wire manufacturing equipment. In other words, the graph in Figure 31 shows that by adjusting the heating conditions or isothermal transformation temperature during patenting in the wire manufacturing equipment, wires 13 with variously adjusted relationships between tensile strength and hardness can be manufactured from the same starting wire material (steel material 11).

[0110] Referring to Figure 31, the developed product is made of steel type SWRH62A. was particularly noticeable, but the relationship between tensile strength and hardness was within the range indicated by 0.16TS + 90 ≦ hardness ≦ 0.16TS + 290 (TS is tensile strength). Furthermore, as mentioned above, it was found that the relationship between tensile strength and hardness of the developed product can be controlled by changing the heating conditions and isothermal transformation temperature during patenting in the wire manufacturing equipment. Comparing the graphs shown in (a) and (b) in Figure 31, for example, if the tensile strength is 2,100 MPa, the wire 13 shown in graph (a) has a hardness of 570 Hv, while the wire 13 shown in graph (b) has a hardness of about 480 Hv. Graph (b) The wire 13 shown in graph (a) has a similar tensile strength to the wire 13 shown in graph (a), but has superior toughness.

[0111] Figure 32 shows the true strain of wire 13 of steel type SWRH62A, with the horizontal axis being the true strain and the vertical axis being the tensile strength (MPa). The work hardening curve is shown.

[0112] In Figure 32, all of the specimens were made from steel type SWRH62A, but the temperature of the molten lead 17 was different. The graphs show the wire 13 of three developed products (all broken lines) and two conventional wire 13 products (all solid lines) made from steel type SWRH62A but with different molten lead 17 temperatures. The conventional wire 13, which was made with a lead furnace temperature of 450°C, had the same true strain as the remaining four wires even when the true strain was increased. It can be seen that the tensile strength is not as improved as that of the conventional Wire 13. In the case of the developed product, whether the temperature of the molten lead 17 is made at 450°C or at an even lower temperature of 425°C, the tensile strength of the produced Wire 13 improves as the true strain increases. In other words, the tensile strength of the conventional Wire 13 decreases when the temperature of the molten lead 17 is lowered, but the tensile strength of the developed Wire 13 does not decrease even when it is made using molten lead 17 at a lower temperature. In other words, in the case of the developed product, the tensile strength of the conventional Wire 13 decreases when the temperature of the molten lead 17 is lowered It is possible to obtain wire 13 with excellent tensile strength even when the temperature is lowered to 425°C. Compared to when the temperature of molten lead 17 is set to 565°C, by setting the temperature of molten lead 17 at 425°C, it is possible to reduce the heat loss from the bath 16, and fuel costs can be reduced by approximately 20%. In other words, the developed product has better energy efficiency than conventional products because the tensile strength does not decrease even when using molten lead 17 at a low temperature.

[0113] FIG. 35 shows the work hardening curves of wire 13 made of steel grades SWRH42A and SWRH62A.

[0114] Figure 33 shows graphs of four developed products (all dashed lines) created with different temperatures of molten lead 17, and graphs of two conventional products (all solid lines) created with different temperatures of molten lead 17.

[0115] When comparing developed Wire 13 of the same steel type, whether it is SWRH42A or SWRH62A, the tensile strength of Wire 13 made with molten lead 17 at 450°C is superior to the tensile strength of Wire 13 made with molten lead 17 at 565°C. In other words, the tensile strength of the developed product can be controlled by controlling the temperature of the molten lead 17, and tensile strength can be improved by using molten lead 17 at a lower temperature. Furthermore, when comparing the developed Wire 13 with conventional Wire 13 using the same steel type and the same molten lead 17 temperature, the graph in Figure 33 also shows that the developed product has superior tensile strength to the conventional product.

[0116] FIG. 34 shows the work hardening curves of wire 13 made of steel types SWRH82A and SWRH82B.

[0117] For example, when comparing the work hardening curve of the developed product of steel type SWRH82A, which was made in a lead furnace temperature of 450°C, with the work hardening curve of the developed product of steel type SWRH82B (which contains a high amount of manganese), which was also made in a lead furnace temperature of 450°C, they are almost identical. Similarly, when comparing the work hardening curve of the developed product of steel type SWRH82A, which was made in a lead furnace temperature of 565°C, with the work hardening curve of the developed product of steel type SWRH82B, which was also made in a lead furnace temperature of 565°C, they are almost identical. On the other hand, for conventional products, the work hardening curve of steel type SWRH82B The slope of the work hardening curve is slightly larger than that of the steel type SWRH82A, and it has superior tensile strength. This means that the developed product does not need to add expensive alloying elements (such as manganese) to increase tensile strength. The developed product achieves high strength without using steel types that contain expensive alloying elements (such as manganese and chromium) to increase strength, which allows for cost reductions.

[0118] Figure 35 shows the steel types SWRH92A, 92A-Cr (chromium added), and 92B-Si (high manganese content). 35 shows the work hardening curve of the wire 13 (chromium and silicon added). It can also be seen that the developed product does not need to add expensive alloy elements (chromium, silicon, etc.) to increase the tensile strength.

[0119] Figure 36 shows the work hardening curve of steel type 102A-Cr. The developed product has a higher tensile strength than the conventional product. It can be seen that the quality increases.

[0120] Compare Figures 32 to 36 from the perspective of carbon content. When comparing the developed product with a conventional product of the same steel type, the lower the carbon content (see Figure 32, for example), the steeper the slope of the work-hardening curve of the developed product is, and the higher the tensile strength tends to be. Conversely, the higher the carbon content (see Figure 36, for example), the closer the slope of the work-hardening curve of the developed product is to that of the conventional product. However, when focusing on the tensile strength immediately after heat treatment (when true strain is 0), the developed product has a higher tensile strength for both steel types, demonstrating that the tensile strength of the developed product is superior to that of the conventional product.

[0121] When the temperature of molten lead 17 is set to 565°C, the difference in the slope of the work hardening curve between the developed product and the conventional product is not as great as when the temperature of molten lead 17 is set to 450°C. Even in this case, when comparing the same steel type, the developed product has a higher tensile strength than the conventional product, and the developed product has superior tensile strength to the conventional product.

[0122] The developed product can produce wire 13 with a variety of tensile strengths and hardnesses using fewer steel types (fewer types of steel 11) as starting materials than conventional products, which makes it easier to manage wire manufacturing factories. Also, by changing the isothermal transformation temperature (temperature of molten lead 17), it is possible to achieve higher strength than conventional products with the same true strain. [Explanation of symbols]

[0123] 11 Steel materials 12 Heat-treated steel 13 wires 14 Power supply 15 Power supply roll 16 Bathtub 17 Molten Lead 22,31 Dice

Claims

1. A drawn wire material obtained by drawing a heat-treated steel material containing, in mass%, C: 0.38 to 1.05%, Mn: 0.0 to 1.0%, Cr: 0.0 to 0.50%, and Si: 0.0 to 1.5%, with the balance being Fe and unavoidable impurities, The GOS value / average grain size at a grain boundary setting angle of 2° and a step number of 0.07 μm is −0.6 × GAM value + 1.5 or more. Drawn wire material.

2. A process for preparing a steel material containing, by mass%, 0.38 to 1.05% C, 0.0 to 1.0% Mn, 0.0 to 0.50% Cr, and 0.0 to 1.5% Si, with the balance being Fe and unavoidable impurities; and a process for directly heating the steel material to 800°C or higher and 975°C or lower by passing an electric current through the steel material. a step of cooling the heated steel material by passing the steel material through a bath containing a cooling medium capable of performing isothermal transformation; The cooled steel material is drawn into a wire, The heating process has the largest temperature gradient in the final stage of heating, and immediately after the steel material reaches a predetermined maximum heating temperature, the heated steel material is introduced into the cooling medium, thereby starting cooling without maintaining the predetermined maximum heating temperature, Produce a drawn wire material in which the GOS value / average grain size at a grain boundary setting angle of 2° and step number of 0.07 μm is -0.6 × GAM value + 1.5 or more. Manufacturing method for drawn wire material.

3. A process of heating a steel material containing, in mass%, 0.38 to 1.05% C, 0.0 to 1.0% Mn, 0.0 to 0.50% Cr, and 0.0 to 1.5% Si, with the balance being Fe and unavoidable impurities, from room temperature to a maximum heating temperature of 800°C or higher and 975°C or lower within several seconds; A process of cooling the heated steel material to 620°C or less without maintaining the maximum heating temperature; The cooled steel material is drawn into a wire, Produce a drawn wire material in which the GOS value / average grain size at a grain boundary setting angle of 2° and step number of 0.07 μm is -0.6 × GAM value + 1.5 or more. Manufacturing method for drawn wire material.

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