Steel wire rod and method for manufacturing steel wire rod

Abstract

To provide a steel wire rod having uniform strain imparted to a surface layer thereof, and a method for manufacturing the steel wire rod.SOLUTION: The invention provides a steel wire rod that is excellent in cold shearing workability and has high hardness and toughness. The steel wire rod according to the present invention contains C in an amount of 0.10 mass% or more and 0.90 mass% or less, Si in an amount of 0.10 mass% or more and 3.00 mass% or less, Mn in an amount of 0.10 mass% or more and 2.00 mass% or less, and Cr in an amount of 0.10 mass% or more and 2.00 mass% or less, with a balance being Fe and unavoidable impurities. In a surface layer of the steel wire rod, a ratio of a maximum value to a minimum value of an elongated texture degree is 2.0 or less, the elongated texture degree being a proportion of blocks in a martensitic structure in which an angle β formed between a long axis of a block and a longitudinal direction of the steel wire rod is smaller than 18°.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel wire rod and a method for manufacturing the steel wire rod. [Background technology]

[0002] In recent years, when manufacturing steel wire rods with excellent strength and ductility, area reduction processing has been carried out in multiple passes using two or more grooved rolls.

[0003] In the grooved roll rolling method, when performing area reduction processing in multiple passes, the area reduction processing is performed using a roll having multiple grooves of different shapes (see, for example, Patent Document 1), or the area reduction processing is performed by changing the compression direction of the roll (see, for example, Patent Document 2), thereby introducing strain into the steel wire material and obtaining steel wire material with excellent strength and ductility. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-160628 [Patent Document 2] Japanese Patent Application Publication No. 8-99101 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, the strain is imparted to the surface layer of the steel wire material uniformly by changing the introduction angle of the steel wire material into the groove, and in Patent Document 2, the strain is imparted to the steel wire material uniformly by changing the compression direction of the roll.

[0006] However, in Patent Documents 1 and 2, the angle at which the steel wire is introduced into the groove and the change in the compression direction of the rolls are limited to 90°, and when performing area reduction processing in two or more passes, the presence of gaps between the rolls causes the locations at which burrs are generated on the surface layer of the steel wire to overlap, and the strain imparted to the surface layer of the steel wire by the area reduction processing becomes uneven, which may result in an insufficient effect on improving the durability of the steel wire.

[0007] The present invention has been made in view of the above, and an object of the present invention is to provide a steel wire rod in which a uniform strain is imparted to a surface layer, and a method for manufacturing the steel wire rod. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems and achieve the object, the steel wire rod of the present invention contains C in proportions of 0.10 mass% to 0.90 mass%, Si in proportions of 0.10 mass% to 3.00 mass%, Mn in proportions of 0.10 mass% to 2.00 mass%, Cr in proportions of 0.10 mass% to 2.00 mass%, and the remainder being Fe and unavoidable impurities, and is characterized in that the ratio of the maximum value to the minimum value of the elongated structure degree, which is the proportion of martensite structures in the surface layer of the steel wire rod where the angle β between the major axis of the block and the longitudinal direction of the steel wire rod is less than 18°, is 2.0 or less.

[0009] Furthermore, the steel wire rod according to the present invention contains C in an amount of 0.10 mass% or more and 0.90 mass% or less, Si in an amount of 0.10 mass% or more and 3.00 mass% or less, Mn in an amount of 0.10 mass% or more and 2.00 mass% or less, Cr in an amount of 0.10 mass% or more and 2.00 mass% or less, and the remainder being Fe and unavoidable impurities, and the ratio of the maximum value to the minimum value of equivalent strain in the cross-sectional surface layer at a position halfway along the entire length after rolling according to FEM analysis is 2.5 or less.

[0010] Furthermore, the steel wire rod according to the present invention is characterized in that, in the above invention, it contains Cu in an amount of 0.05 mass% or more and 0.40 mass% or less, Ni in an amount of 0.10 mass% or more and 0.70 mass% or less, and Ti in an amount of 0.05 mass% or more and 0.20 mass% or less.

[0011] In addition, in the steel wire rod according to the present invention, the ratio of the maximum value to the minimum value of equivalent strain in the cross-sectional surface layer at a position halfway along the entire length after rolling is 2.5 or less, as determined by FEM analysis.

[0012] Further, in the method for producing steel wire rod according to the present invention, a steel wire rod containing C in an amount of 0.10 mass% or more and 0.90 mass% or less, Si in an amount of 0.10 mass% or more and 3.00 mass% or less, Mn in an amount of 0.10 mass% or more and 2.00 mass% or less, Cr in an amount of 0.10 mass% or more and 2.00 mass% or less, with the balance being Fe and unavoidable impurities, is quenched and tempered, and the quenching step or the tempering step involves performing two or more passes of area reduction processing using a rolling mill having two or more rolls immediately after heating, and the area reduction processing is performed while the rolling mill or the steel wire rod is rotating so that burrs generated at the position of the joint of the rolling mill rolls on the surface layer of the steel wire rod do not coincide with the position of the joint of the rolling mill rolls that will perform the next or subsequent area reduction processing.

[0013] In addition, in the method for manufacturing steel wire rod according to the present invention, when the rolling rolls have two rolls, three or more passes of area reduction processing are performed, and when the rolling rolls have three or more rolls, two or more passes of area reduction processing are performed.

[0014] In addition, in the method for producing a steel wire rod according to the present invention, in the above invention, the area reduction process is carried out while the rolling roll or the steel wire rod is rotated at a rotation angle calculated from the following formula. Rotation angle = 360° / (number of rolls x number of area reduction passes)

[0015] Further, in a method for producing a steel wire rod according to the present invention, a steel wire rod containing 0.10% by mass or more and 0.90% by mass or less of C, 0.10% by mass or more and 3.00% by mass or less of Si, 0.10% by mass or more and 2.00% by mass or less of Mn, 0.10% by mass or more and 2.00% by mass or less of Cr, with the balance being Fe and inevitable impurities, is quenched and tempered, and the quenching step or the tempering step is performed by inserting the steel wire rod having a circular cross section into slots of a rolling mill roll immediately after heating, so that the cross section has a polygonal shape having at least 10 or more interior angles, and The method includes a first step of reducing the area of ​​the steel wire material by rolling it into a steel wire material having a polygonal cross section, with at least four of the interior angles of the cross section being greater than 180°, and a second step of reducing the area of ​​the steel wire material having a polygonal cross section by rolling it into a steel wire material having a circular cross section, wherein in the first step, the diameter of the circumscribing circle of the steel wire material having a polygonal cross section is equal to or less than the diameter of the steel wire material having a circular cross section to be rolled, and the diameter of the inscribing circle is 75% to 95% of the diameter of the steel wire material having a circular cross section to be rolled, and in the second step, the steel wire material having a polygonal cross section or a rolling roll is rotated to reduce the area of ​​the steel wire material having a polygonal cross section to equal to or less than the diameter of the inscribing circle of the steel wire material having a polygonal cross section. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a steel wire rod having a uniform strain imparted to the surface layer. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram illustrating the measurement of the degree of elongation texture of a steel wire rod according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic view illustrating a first method for producing a steel wire rod using two reduction rolls according to an embodiment of the present invention. [Figure 3] FIG. 3 is a schematic diagram illustrating the area reduction process of a steel wire rod using two rolls in the prior art. [Figure 4] FIG. 4 is a diagram showing the calculation results of strain of the steel wire material subjected to area reduction processing by the process of FIG. [Figure 5] FIG. 5 is a diagram showing the calculation results of strain of the steel wire material subjected to area reduction processing by the process of FIG. [Figure 6] FIG. 6 is a schematic diagram illustrating the area reduction process of a steel wire rod in two passes using three reduction rolls. [Figure 7] FIG. 7 is a schematic diagram illustrating the area reduction process of a steel wire rod during three-pass area reduction process using three reduction rolls. [Figure 8] FIG. 8 is a schematic diagram illustrating a second method for producing a steel wire rod according to an embodiment of the present invention. [Figure 9] FIG. 9 is a schematic diagram of a rolling roll used in the manufacturing method of FIG. [Figure 10] FIG. 10 is a diagram showing the measurement results of strain of the steel wire rod after rolling with the rolling rolls of the steel wire rod rolling mill of FIG. [Figure 11] FIG. 11 is a diagram showing the correlation between the degree of elongation texture and the equivalent strain. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the accompanying drawings. Note that the drawings are schematic, and the relationship between the thickness and width of each part, the thickness ratio of each part, etc. may differ from the actual ones, and the drawings may also include parts with different dimensional relationships and ratios.

[0019] (Embodiment) The steel wire material according to the present invention contains C in proportions of 0.10 mass% to 0.90 mass%, Si in proportions of 0.10 mass% to 3.00 mass%, Mn in proportions of 0.10 mass% to 2.00 mass%, Cr in proportions of 0.10 mass% to 2.00 mass%, and the remainder being Fe and unavoidable impurities, and in the surface layer of the steel wire material, the ratio of the maximum to minimum values ​​of the elongation texture degree, which is the proportion of block grains whose angle β between the major axis and the longitudinal direction (rolling direction) of the steel wire material is less than 18°, is 2.0 or less.

[0020] <Material ingredients> The steel wire rod according to the present invention contains C in a proportion of 0.10% by mass or more and 0.90% by mass or less. C contributes to improving the strength of the steel wire rod. If the C content is less than 0.10% by mass, the effect of improving strength is not sufficiently obtained, resulting in insufficient fatigue resistance and sag resistance. If the C content exceeds 0.90% by mass, toughness decreases and cracking becomes more likely to occur.

[0021] The steel wire rod according to the present invention contains Si in a proportion of 0.10% by mass or more and 3.00% by mass or less. Si is effective in deoxidizing the steel wire rod and also contributes to improving strength and temper softening resistance. If the Si content is less than 0.10% by mass, the above effects cannot be sufficiently obtained. If the Si content exceeds 3.00% by mass, toughness decreases, making the wire rod more susceptible to cracking, and decarburization is promoted, resulting in a decrease in the wire rod surface strength.

[0022] The steel wire rod according to the present invention contains Mn in a proportion of 0.10% by mass or more and 2.00% by mass or less. Mn contributes to improving hardenability. If the Mn content is less than 0.10% by mass, it becomes difficult to ensure sufficient hardenability, and the effect of fixing S (MnS formation), which is harmful to ductility and toughness, becomes poor. Furthermore, if the Mn content exceeds 2.00% by mass, ductility decreases, and cracks and surface scratches are more likely to occur.

[0023] The steel wire rod according to the present invention contains 0.10% by mass or more and 2.00% by mass or less of Cr. Cr is effective in preventing decarburization, and contributes to improving strength and temper softening resistance, thereby improving fatigue resistance. Cr is also effective in improving sag resistance in warm conditions. If the Cr content is less than 0.10% by mass, the above effects cannot be sufficiently obtained. Furthermore, if the Cr content exceeds 2.00% by mass, toughness decreases, and cracks and surface scratches tend to occur.

[0024] The steel wire rod according to the present invention must contain C, Si, Mn, and Cr in the above-mentioned proportions, but may contain elements other than those mentioned above. In addition to C, Si, Mn, and Cr, the steel wire rod according to the present invention may contain Cu in proportions of 0.05% by mass to 0.40% by mass, Ni in proportions of 0.10% by mass to 0.70% by mass, and Ti in proportions of 0.05% by mass to 0.20% by mass.

[0025] Cu is effective in improving hardenability and can improve fatigue strength by dissolving in ferrite. If the Cu content is less than 0.05 mass%, the above effect cannot be sufficiently obtained. If the Cu content exceeds 0.40 mass%, cracks may occur during hot working. The steel wire rod according to the present invention preferably contains Cu in a proportion of 0.05 mass% or more and 0.40 mass% or less.

[0026] Ni is effective in improving hardenability, suppressing the formation of carbides, and improving fatigue strength. If the Ni content is less than 0.10 mass%, the effect of improving hardenability is insufficient. If the Ni content exceeds 0.70 mass%, not only will cost be a problem, but the amount of retained austenite will increase, reducing fatigue life. The steel wire rod according to the present invention preferably contains Ni in a proportion of 0.10 mass% or more and 0.70 mass% or less.

[0027] Ti combines with C and N to form carbides and nitrides, which act as hydrogen trapping sites, thereby suppressing hydrogen diffusion into the steel material, improving corrosion resistance and delayed fracture resistance, and improving strength and toughness through grain refinement and precipitation strengthening. If the Ti content is less than 0.05 mass%, the above effects cannot be fully achieved. If the Ti content exceeds 0.20 mass%, a large amount of TiN is formed, resulting in a decrease in fatigue strength. The steel wire rod according to the present invention preferably contains Cu in a proportion of 0.05 mass% or more and 0.20 mass% or less.

[0028] Examples of the steel wire material according to the present invention include, but are not limited to, SAE9254 and SUP7.

[0029] <Degree of elongation> In the steel wire material according to the present invention, in the surface layer, the ratio of the maximum value to the minimum value of the elongated structure degree, which is the proportion of all martensite blocks in which the angle β between the long axis of the block and the longitudinal direction (rolling direction) of the steel wire material is less than 18°, is 2.0 or less.

[0030] 1 is a diagram illustrating measurement of the degree of elongation structure of a steel wire rod according to an embodiment of the present invention. In the tempering process, when the steel wire rod according to the present invention is subjected to two or more passes of area reduction using a rolling mill having two or more rolls in a warm state immediately after the tempering heating, or when the steel wire rod is subjected to two or more passes of area reduction using a rolling mill having two or more rolls in a hot state immediately after the quenching heating, the rolling mill or the steel wire rod is rotated so that burrs generated at the position of the joint of the rolling mill rolls in the surface layer of the steel wire rod do not become the position of the joint of the rolling mill rolls that will perform the next or subsequent area reduction processes, and then quenched, whereby the longitudinal direction of the martensite blocks elongates in a direction approaching parallel to the longitudinal direction of the steel wire rod. The relationship between the angle β between the long axis of the elongated martensite block on the steel wire layer surface and the longitudinal direction of the steel wire and the surface strain was evaluated, and it was confirmed that the surface strain is uniformly imparted when the ratio of the maximum to minimum values ​​of the elongated structure degree, which is the proportion of blocks whose long axis is smaller than 18° and whose longitudinal direction is smaller than 18°, is 2.0 or less.

[0031] The angle β between the long axis of the block and the longitudinal direction of the steel wire was measured by measuring the angle β between the long axis of all martensite blocks in one field of view and the longitudinal direction of the steel wire in an electron microscope photograph taken at 5000x magnification of a cross section of the steel wire cut in the longitudinal direction. The degree of elongation structure is the percentage of blocks where the angle β between the long axis of the block and the longitudinal direction of the steel wire is less than 18°.

[0032] The angle β between the major axis of the block and the longitudinal direction of the steel wire is measured using an electron microscope photograph taken at the surface layer of the steel wire. In the steel wire according to the present invention, the ratio of the maximum to the minimum value of the elongated texture, which is the proportion of blocks where the angle β between the major axis of the block and the longitudinal direction of the steel wire is less than 18°, is 2.0 or less. If the ratio of the maximum to the minimum value of the elongated texture is greater than 2.0, the strain imparted to the surface layer of the steel wire becomes non-uniform, and the effect of improving the durability of the steel wire is not sufficient.

[0033] <Physical properties> In the steel wire rod according to the present invention, the ratio of the maximum value to the minimum value of equivalent strain in the cross-sectional surface layer at a position halfway along the entire length after rolling is 2.5 or less, as determined by FEM analysis. The ratio of the maximum value to the minimum value of equivalent strain being 2.5 or less improves the durability of the steel wire rod.

[0034] <Manufacturing method> (1) First manufacturing method A first manufacturing method of steel wire rod according to the present invention is a manufacturing method of steel wire rod, in which steel wire rod containing C in proportions of 0.10 mass% to 0.90 mass%, Si in proportions of 0.10 mass% to 3.00 mass%, Mn in proportions of 0.10 mass% to 2.00 mass%, Cr in proportions of 0.10 mass% to 2.00 mass%, with the balance being Fe and unavoidable impurities, is quenched and tempered, in which in the tempering process, two or more passes of area reduction are performed in a warm state using a rolling mill having two or more rolls immediately after tempering heating, or in the quenching process, two or more passes of area reduction are performed in a hot state using a rolling mill having two or more rolls immediately after quenching heating, and the area reduction is performed while the rolling mill or the steel wire rod is rotating so that burrs generated at the position of the joint of the rolling mill rolls on the surface of the steel wire rod do not coincide with the position of the joint of the rolling mill rolls that will perform the next or subsequent area reduction processing. Alternatively, in both the quenching step and the tempering step, immediately after heating, the steel sheet may be subjected to two or more passes of area reduction processing using a rolling mill equipped with two or more rolls.

[0035] In the quenching process, the wire is heated to the austenite region above the Ac3 point. Once heated to the austenite region, the wire is held for a sufficient time for the structure to transform into austenite. When performing area reduction in the quenching process, immediately after quenching, the wire is subjected to two or more passes of area reduction using a hot rolling mill equipped with two or more rolls, followed by rapid cooling. When performing two or more passes of area reduction, the rolls or the steel wire are rotated to prevent burrs from forming at the joints of the rolls on the surface of the steel wire from becoming the joints of the rolls that will be used for subsequent area reductions. The area reduction rate in the area reduction process is preferably 10% or more and less than 60%. The wire is then rapidly cooled from the austenite region to generate martensite.

[0036] In the quenching process, any cooling method can be used as long as it can transform austenite into martensite. For example, the steel wire material heated to the austenite region can be immersed in oil or water, or can be rapidly cooled by spraying water mist onto the steel wire material.

[0037] In the tempering process, immediately after the tempering heating, the steel wire is subjected to two or more passes of area reduction using a warm rolling mill equipped with two or more rolls, followed by quenching. When performing two or more passes of area reduction, the rolls or the steel wire are rotated to prevent burrs generated at the joints of the rolls on the surface layer of the steel wire from being positioned at the joints of the rolls that will be used for the next or subsequent area reduction. The area reduction rate in the area reduction is preferably 10% or more and less than 60%. Rapid cooling is preferably performed at 50°C / s or more. The rapid cooling method can be, for example, immersing the steel wire in oil or water, or spraying water mist onto the steel wire.

[0038] Before explaining the area reduction process according to the present invention, we will explain the area reduction process according to the prior art. Figure 3 is a schematic diagram illustrating the area reduction process in the tempering process of steel wire rods using a two-roll mill in the prior art. Figure 3 shows an example in which a three-pass area reduction process (φ17.0 → φ16.2 → φ15.4 → φ14.6) is performed using a two-roll mill, followed by a double-pass process without area reduction. A φ17.0 steel wire rod 1 is reduced to a φ16.2 steel wire rod 1a using a mill roll 10 consisting of two rolls 10a and 10b. Next, the φ16.2 steel wire rod 1a is reduced to a φ15.4 steel wire rod 1b' using a mill roll 20 consisting of two rolls 20a and 20b. Finally, the φ15.4 steel wire rod 1b' is reduced to a φ14.6 steel wire rod 1c' using a mill roll 30 consisting of two rolls 30a and 30b. Thereafter, double-pass processing without area reduction is performed to adjust the shape and distortion of the steel wire material. Note that, although processing without area reduction is preferable to adjust the shape and distortion of the steel wire material, processing without area reduction is not necessarily required.

[0039] As shown in Figure 3, gaps exist at the joints between the surface-reducing rolls 10, 20, and 30, causing burrs to form at the joints during the surface-reducing process. From the standpoint of applying uniform strain to the steel wire rod, the conventional technology rotates the surface-reducing rolls by 90° to perform the surface-reducing process. The surface-reducing roll 10 rolls from the top and bottom using rolls 10a and 10b. The surface-reducing roll 20 is rotated by 90° so that the joints between rolls 20a and 20b are positioned vertically and rolls from the left and right. The surface-reducing roll 30 is rotated by another 90° so that the joints between rolls 30a and 30b are positioned horizontally and rolls from the top and bottom.

[0040] In the area reduction process with the surface-reducing roll 10, a burr 2a is generated at the joint between the rolls 10a and 10b, in the area reduction process with the surface-reducing roll 20, a burr 2b is generated at the joint between the rolls 20a and 20b, and in the area reduction process with the surface-reducing roll 30, a burr 2c is generated at the joint between the rolls 30a and 30b. The burr 2a generated by the process with the surface-reducing roll 10 is smoothed to a slot shape (circular cross section) by the pressing force of the area reduction process with the surface-reducing roll 20, but the area reduction process with the surface-reducing roll 30 again generates a burr 2c at the position where the burr 2a was generated, and although the burr is smoothed in the subsequent process, a strong strain is generated at the position of the burr 2a (burr 2c), which has posed a problem in that the amount of strain imparted to the surface layer of the steel wire material is likely to be non-uniform.

[0041] In contrast, in the first manufacturing method of steel wire material of the present invention, the reduction processing is performed by rotating the rolling rolls or the steel wire material so that burrs that occur at the joints of two or more rolls on the surface of the steel wire material do not become the joints of the rolling rolls that will perform the reduction processing in the next or subsequent times, thereby making it possible to impart uniform strain to the surface of the steel wire material.

[0042] Fig. 2 is a schematic diagram illustrating a first manufacturing method of a steel wire rod using a two-roll mill according to an embodiment of the present invention. Fig. 2 shows an example of a three-pass area reduction process (φ17.0 → φ16.2 → φ15.4 → φ14.6) using two roll mills, similar to the conventional technique shown in Fig. 3, followed by a double-pass process without area reduction. A φ17.0 steel wire rod 1 is reduced to a φ16.2 steel wire rod 1a using a mill mill 10 consisting of two rolls 10a and 10b. The φ16.2 steel wire rod 1a is then reduced to a φ15.4 steel wire rod 1b using a mill mill 20 consisting of two rolls 20a and 20b. Finally, the φ15.4 steel wire rod 1b is reduced to a φ14.6 steel wire rod 1c using a mill mill 30 consisting of two rolls 30a and 30b. The double-pass process without area reduction is also similar to the conventional technique shown in Fig. 3.

[0043] The rotation angle of the rolling rolls may be determined depending on the number of rolls and the number of passes of the area reduction process so that burrs generated at the joints of two or more rolls on the surface layer of the steel wire material do not coincide with the joints of the rolling rolls that will perform the area reduction process next or later. However, from the viewpoint of imparting uniform strain to the surface layer of the steel wire material, it is preferable to perform the area reduction process by rotating at a rotation angle calculated from the following formula. Rotation angle = 360° / (number of rolls x number of area reduction passes) From the viewpoint of uniformly applying strain to the surface layer of the steel wire material, it is preferable to rotate the wire at the same angle calculated by the above formula, but it is sufficient to rotate the wire so as not to be at the joint of the rolling rolls that perform the area reduction. For example, when performing area reduction in four passes using two rolling rolls and then double pass processing, the strain applied to the surface layer of the steel wire material can also be uniformized by rotating the wire from 0° to the 90° position (rotated 90°) to the 45° position (rotated 45° in the opposite direction from the 90° position) to the 135° position (rotated 90° from the 45° position).

[0044] In the area reduction process of the steel wire rod shown in Figure 2, the surface-reducing rolls 20 and 30 are each rotated by 60° so that burrs generated at the joint between two rolls on the surface of the steel wire rod will not be located at the joint between the rolling rolls that will perform the next area reduction process in subsequent area reduction processes. The surface-reducing roll 10 performs rolling from above and below using rolls 10a and 10b, and the surface-reducing roll 20 performs area reduction in an arrangement rotated so that the joint between rolls 20a and 20b is located at a position 60° from the joint between rolls 10a and 10b of the adjacent rolling roll 10 on the introduction side of the steel wire rod 1, and the surface-reducing roll 30 performs area reduction in an arrangement rotated so that the joint between rolls 30a and 30b is located at a position 60° from the joint between rolls 20a and 20b of the adjacent rolling roll 20 on the introduction side of the steel wire rod 1.

[0045] 2, as with the conventional technology, in the area reduction processing of the steel wire rod, burr 2a occurs at the joint between rolls 10a and 10b in the area reduction processing with surface-reducing roll 10, burr 2b occurs at the joint between rolls 20a and 20b in the area reduction processing with surface-reducing roll 20, and burr 2c occurs at the joint between rolls 30a and 30b in the area reduction processing with surface-reducing roll 30. However, burr 2a generated by processing with surface-reducing roll 10 is smoothed to a slot shape (cross-sectional shape: circular) by pressing in the area reduction processing with surface-reducing roll 20, and the position where burr 2c occurs by the area reduction processing with surface-reducing roll 30 is different from the position where burr 2a and burr 2b occur, so that it is possible to impart uniform strain to the surface layer of the steel wire rod compared to the conventional technology.

[0046] Fig. 4 is a diagram showing the calculation results of strain in a steel wire rod subjected to area reduction processing according to the process in Fig. 2, and Fig. 5 is a diagram showing the calculation results of strain in a steel wire rod subjected to area reduction processing according to the process in Fig. 3. The strain was calculated by FEM analysis using CAE software, as the equivalent strain in the cross section at a position halfway along the entire length after rolling.

[0047] As shown in Figures 4 and 5, it was confirmed that the steel wire material rolled by the process shown in Figure 2 has more uniform strain in the surface layer of the steel wire material than the steel wire material subjected to area reduction processing by the process shown in Figure 3.

[0048] The above describes a method of reducing the area of ​​a steel wire rod by rotating the joint position of the rolling rolls, but the same effect can be obtained by rotating the steel wire rod to be rolled for each area reduction process without rotating the rolling rolls.

[0049] Furthermore, although the above description has been given of a case where the rolling rolls have two rolls, similar effects can also be obtained in a rolling method using three or four rolls. Fig. 4 is a schematic diagram illustrating the area reduction of a steel wire rod during two-pass area reduction using three rolls. In two-pass area reduction using three rolls, a suitable rotation angle is 60° (rotation angle = 360° / (number of rolls × number of area reduction passes)), and after the area reduction using the rolls 10a, 10b, and 10c of the surface-reducing roll 10, the surface-reducing roll 20 preferably performs area reduction in such a manner that the position of the joint between the rolls 20a, 20b, and 20c is rotated 60° from the position of the joint between the rolls 10a, 10b, and 10c of the adjacent rolling roll 10 on the introduction side of the steel wire rod 1.

[0050] FIG. 7 is a schematic diagram illustrating a rolling method for a steel wire rod during three-pass area reduction processing using three reduction rolls. In three-pass area reduction processing using three rolling rolls, the preferred rotation angle is 40° (rotation angle = 360° / (number of rolls x number of area reduction processing passes)), and after area reduction processing by rolls 10a, 10b, and 10c of surface-reducing roll 10, surface reduction processing is performed by surface-reducing roll 20 in a rotated position so that the joint position of rolls 20a, 20b, and 20c is 40° from the joint position of rolls 10a, 10b, and 10c of the adjacent rolling roll 10 on the introduction side of the steel wire rod 1, and surface reduction processing is performed by surface-reducing roll 30 in a rotated position so that the joint position of rolls 30a, 30b, and 30c is 40° from the joint position of rolls 20a, 20b, and 20c of the adjacent rolling roll 20 on the introduction side of the steel wire rod 1.

[0051] In the first manufacturing method of steel wire rod of the present invention, when performing two or more passes of area reduction processing using a rolling mill having two or more rolls, for example, two, three, or four rolls, the rolling mill or the steel wire rod is rotated to perform the area reduction processing so that burrs that occur at the joints of two or more rolls on the surface of the steel wire rod do not become the joints of the rolling mill rolls that will perform the next area reduction processing in the next or subsequent area reduction processing, thereby imparting uniform strain to the surface of the steel wire rod.

[0052] (2) Second manufacturing method Fig. 8 is a schematic diagram illustrating a second manufacturing method of a steel wire rod according to an embodiment of the present invention, and Fig. 9 is a schematic diagram of a rolling mill roll (first pass) used in the manufacturing method of Fig. 8. The second manufacturing method of Fig. 8 is an example of a manufacturing method of a steel wire rod in which a steel wire rod containing 0.10 to 0.90 mass% of C, 0.10 to 3.00 mass% of Si, 0.10 to 2.00 mass% of Mn, 0.10 to 2.00 mass% of Cr, the balance being Fe and unavoidable impurities, is quenched and tempered, in which a two-pass area reduction process (φ17.0 → concave-convex shape, circumscribed circle φ17.0, inscribed circle φ14.6 → φ14.6) is performed using two rolling rolls, followed by a double-pass process without area reduction (the latter stage of the double pass process without area reduction is not shown in Fig. 8). A steel wire rod 1 with a diameter of 17.0 mm is reduced in area by a rolling mill 10 consisting of two rolls 10a and 10b to form a steel wire rod 1a with a circumscribed circle diameter of 17.0 mm, an inscribed circle diameter of 14.6 mm, and an uneven surface. The uneven steel wire rod 1a is then reduced in area by a rolling mill 20 consisting of two rolls 20a and 20b to form a steel wire rod 1b with a diameter of 14.6 mm, and the steel wire rod 1b with a diameter of 14.6 mm is subjected to double-pass processing without area reduction. When transferring from the rolling mill 10 to the rolling mill 20 and from the rolling mill 20 to the rolling mill 20, the steel wire rods 1a and 1b are rotated 90° and inserted into the rolling mill rolls.

[0053] The cross-sectional shape of the slots of the first-pass rolling roll 10 shown in Figures 8 and 9 is a polygonal shape having 14 interior angles, of which six interior angles are vertices 10d greater than 180° and eight interior angles are vertices 10e less than 180°. Since the cross-sectional shape of the slots has six vertices 10d with interior angles greater than 180° and eight vertices 10e with interior angles less than 180°, multiple sharp convex portions with vertices 10d with interior angles less than 180° can be formed on the surface of the steel wire rod. The interior angles of the vertices 10e are preferably 120° or less. Furthermore, of the vertices 10d, 10e of the polygonal shape that forms the slots, it is preferable that the vertices 10d greater than 180° are not located at the joint between the two rolls 10a and 10b. In Figure 9, it can be seen that there is a small apex at the joint between rolls 10a and 10b, but this is due to the manufacturing process of rolls 10a and 10b and is not intentionally provided. Note that the cross-sectional shape of the slots in the first-pass rolling roll is a polygon with at least 10 apexes, and as long as four or more of the apexes of the polygon have an angle of greater than 180°, multiple sharp convex portions can be formed on the surface of the steel wire rod. In addition, the apexes of interior angles 10d and 10e and the joint between rolls 10a and 10b may be rounded.

[0054] In the slots of the rolling roll 10, it is preferable that at least four of the vertices 10d having an interior angle greater than 180° are arranged at positions at 45° around the rolling direction RD (Normal Direction) from the direction ND perpendicular to the rolling direction RD. By arranging at least four of the vertices 10d having an interior angle greater than 180° at positions at 45° around the rolling direction RD from the direction ND perpendicular to the rolling direction RD, it is possible to increase the strain imparted to the steel wire rod surface, and ultimately to impart uniform strain to the steel wire rod surface. In FIG. 9, the vertices 10d having an interior angle greater than 180° include vertex 10d1 arranged at a position at 45° around the rolling direction RD and vertex 10d2 arranged at a position at 90° around the rolling direction RD, but vertex 10d2 may have a flat shape instead of a vertex.

[0055] The diameter of the circumscribing circle of the slots of the rolling roll 10 is preferably equal to or smaller than the diameter of the steel wire rod with a circular cross section to be rolled, and the diameter of the inscribing circle is preferably 75% to 95% of the diameter of the steel wire rod with a circular cross section to be rolled. If the diameter of the circumscribing circle of the slots is larger than the diameter of the steel wire rod with a circular cross section to be rolled, the steel wire rod may not follow the shape of the slot, and a convex portion may not be formed. Furthermore, if the diameter of the inscribing circle is 75% or less of the diameter of the steel wire rod with a circular cross section to be rolled, the convex portion becomes large, which may make it difficult to impart uniform strain. If the diameter of the inscribing circle is 95% or more of the diameter of the steel wire rod with a circular cross section to be rolled, the number of steps in the area reduction process increases. The diameter of the inscribing circle is preferably 80% to 90% of the diameter of the steel wire rod with a circular cross section to be rolled.

[0056] Fig. 10 is a diagram showing the calculation results of the strain of the steel wire rod after rolling with the rolling rolls of Fig. 8. The strain was calculated by FEM analysis using CAE software, and is the equivalent strain of the cross section at a position halfway along the entire length after rolling. As shown in Fig. 10 and Fig. 3, it was confirmed that the steel wire material rolled by the rolling rolls shown in Fig. 10 had more uniform strain in the surface layer of the steel wire material than the steel wire material subjected to area reduction processing by the process shown in Fig. 3.

[0057] As described above, the steel wire rod according to the present invention has excellent durability because the strain applied to the surface layer is uniform. The steel wire rod according to the present invention can be suitably used for spring applications, such as automobile suspension springs, valve springs, clutch damper springs, disc springs, stabilizer springs, and torsion bars. [Example]

[0058] (Evaluation method) -Elongation texture- A 17 mm square steel wire made of SAE9254 was heated at a quenching temperature of 900°C for 10 minutes, then placed in a water bath and cooled. It was then heated at 450°C for 10 minutes for the tempering process, and subjected to area reduction at a speed of 5 m / min according to the conditions shown in Table 1 to produce one-pass samples (each rotated 90°), two-pass samples (each rotated 90°), three-pass samples (each rotated 90°), and five-pass samples (each rotated 90°). The 90° rotation is the rotation pattern shown in Figure 3.

[0059] [Table 1]

[0060] The structure of any point on the sample was observed at 5000x magnification using an FE-SEM analyzer (Field Emission Scanning Electron Microscopy, JSM-7900F, manufactured by JEOL Ltd.), and an IPF map was obtained using an EBSD analyzer (Electron Backscatter Diffraction, 1500M-T1-GE-EX, OIM ver.8.1.0, manufactured by TSL Corporation). The angle β between the major axis of all blocks in one field of view and the longitudinal direction of the steel wire rod was measured by image analysis. The proportion of measured angles β smaller than 18° was calculated as the degree of elongation. In this specification, the term "surface layer" refers to the surface layer in the region excluding the decarburized layer located at the outermost surface.

[0061] -Significant distortion- For the above samples and the samples of Examples 1 and 2 and Comparative Example 1 described below, the equivalent strain in the cross section at the position halfway along the entire length after rolling was calculated by FEM analysis using CAE software.

[0062] - Approximate formula for elongation texture and equivalent strain - An approximate formula was calculated by linear approximation from the calculated values ​​of the elongation texture and equivalent strain for each sample. Figure 11 is a diagram showing the correlation between the elongation texture and equivalent strain, and the dotted line in the figure is the approximate formula for the elongation texture and equivalent strain. The elongation textures of Examples 1 and 2 and Comparative Example 1 were calculated by substituting the values ​​of the equivalent strain into the calculated approximate formula.

[0063] Example 1 SAE9254 steel (cross-sectional shape: 17φ round) was used. It was quenched at 900°C for 10 minutes and then placed in a water bath for cooling. It was then tempered at 450°C for 30 minutes. Following the process shown in Figure 2, it underwent area reduction at a speed of 5 m / min, was placed in a water bath for quenching, and the wire was then manufactured. FEM analysis was performed to calculate strain and the degree of elongation was calculated using an approximate formula. Figure 2 shows a three-pass reduction process (φ17.0 → φ16.2 → φ15.4 → φ14.6) using two rolls, followed by a double-pass process without area reduction, with a 60° rotation between each pass. Figure 4 shows the strain measurement results for the steel wire that underwent area reduction according to the process shown in Figure 2. 4, the minimum value of the equivalent strain after area reduction processing of the steel wire rod according to Example 1 was 0.73, the maximum value was 1.43, and the ratio of the maximum value to the minimum value was 1.93. In addition, the minimum value of the elongation texture degree obtained from the approximation formula was 0.39, the maximum value was 0.59, and the ratio of the maximum value to the minimum value was 1.52.

[0064] Example 2 SAE9254 steel (cross-sectional shape: 17φ round) was used. It was quenched at 900°C for 10 minutes and then placed in a water bath for cooling. It was then tempered at 450°C for 30 minutes. Following the process shown in Figure 8, it underwent area reduction at a speed of 5 m / min and was then placed in a water bath for quenching. FEM analysis was performed assuming the steel wire was produced to calculate strain and the degree of elongation was calculated using an approximate formula. Figure 10 shows a two-pass reduction process using two rolls (φ17.0 → irregular shape, circumscribed circle φ17.0, inscribed circle φ14.6 → φ14.6), followed by a double-pass process without area reduction, with a 90° rotation between each process. Figure 10 shows the strain measurement results for a steel wire subjected to area reduction according to the process shown in Figure 8. 10, the minimum value of the equivalent strain after area reduction processing of the steel wire rod according to Example 2 was 0.83, the maximum value was 2.00, and the ratio of the maximum value to the minimum value was 2.41. In addition, the minimum value of the elongation texture degree obtained from the approximation formula was 0.41, the maximum value was 0.75, and the ratio of the maximum value to the minimum value was 1.80.

[0065] (Comparative Example 1) SAE9254 steel (cross-sectional shape: 17φ round) was used. It was quenched at 900°C for 10 minutes and then placed in a water bath for cooling. It was then tempered at 450°C for 30 minutes. Following the process shown in Figure 3, it underwent area reduction at a speed of 5 m / min and was then placed in a water bath for quenching. FEM analysis was performed assuming the production of steel wire rod to calculate strain and the degree of stretching was calculated using an approximate formula. Figure 3 shows a three-pass area reduction process using two rolls (φ17.0 → φ16.2 → φ15.4 → φ14.6) followed by a double-pass process without area reduction, with a 90° rotation between each pass. Figure 5 shows the strain measurement results for the steel wire rod after area reduction according to the process shown in Figure 3. 5, the minimum value of the equivalent strain after area reduction of the steel wire rod of Comparative Example 1 was 0.54, the maximum value was 1.73, and the ratio of the maximum value to the minimum value was 3.20. The minimum value of the elongation texture degree obtained from the approximate formula was 0.33, the maximum value was 0.67, and the ratio of the maximum value to the minimum value was 2.02.

[0066] It was confirmed that the surface layer strain of the steel wire rod according to the present embodiment is uniform from the numerical values ​​of the equivalent strain in Examples 1 and 2 and Comparative Example 1. It was also confirmed that the surface layer strain is uniform when the ratio of the maximum value to the minimum value of the degree of elongation texture is 2.0 or less. [Explanation of symbols]

[0067] 1, 1a, 1a', 1b, 1b', 1c, 1c', 1d, 1d' Steel wire rod 2a, 2b, 2c Bali 10, 20, 30 rolling mill rolls 10a, 10b, 10c, 20a, 20b, 20c, 30a, 30b, 30c rolls 10d, 10e vertices

Claims

1. A steel wire rod containing C in an amount of 0.10 mass% or more and 0.90 mass% or less, Si in an amount of 0.10 mass% or more and 3.00 mass% or less, Mn in an amount of 0.10 mass% or more and 2.00 mass% or less, Cr in an amount of 0.10 mass% or more and 2.00 mass% or less, and the balance being Fe and unavoidable impurities, A steel wire material characterized in that in the surface layer of the steel wire material, the ratio of the maximum value to the minimum value of the elongated structure degree, which is the proportion of blocks in the martensite structure where the angle β between the major axis of the block and the longitudinal direction of the steel wire material is less than 18°, is 2.0 or less.

2. A steel wire rod containing C in an amount of 0.10 mass% or more and 0.90 mass% or less, Si in an amount of 0.10 mass% or more and 3.00 mass% or less, Mn in an amount of 0.10 mass% or more and 2.00 mass% or less, Cr in an amount of 0.10 mass% or more and 2.00 mass% or less, and the balance being Fe and unavoidable impurities, A steel wire material according to claim 1, wherein the ratio of the maximum value to the minimum value of equivalent strain in the cross-sectional surface layer at a position halfway along the entire length after rolling is 2.5 or less according to FEM analysis.

3. 3. The steel wire rod according to claim 1, comprising Cu in an amount of 0.05 mass% or more and 0.40 mass% or less, Ni in an amount of 0.10 mass% or more and 0.70 mass% or less, and Ti in an amount of 0.05 mass% or more and 0.20 mass% or less.

4. 2. The steel wire rod according to claim 1, wherein the ratio of the maximum value to the minimum value of equivalent strain in the cross-sectional surface layer at a position halfway along the entire length after rolling is 2.5 or less according to FEM analysis.

5. A method for producing a steel wire rod, comprising quenching and tempering a steel wire rod containing 0.10 mass% or more and 0.90 mass% or less of C, 0.10 mass% or more and 3.00 mass% or less of Si, 0.10 mass% or more and 2.00 mass% or less of Mn, 0.10 mass% or more and 2.00 mass% or less of Cr, with the balance being Fe and unavoidable impurities, The quenching or tempering process includes performing area reduction processing by two or more passes using a rolling mill having two or more rolls immediately after heating, The reduction process is a method for manufacturing steel wire material in which the reduction process is performed while the rolling rolls or the steel wire material are rotating so that burrs that occur at the joints of the rolling rolls on the surface of the steel wire material do not become the joints of the rolling rolls that will perform the next or subsequent reduction process.

6. 6. The method for producing a steel wire rod according to claim 5, wherein when the rolling rolls have two rolls, three or more passes of area reduction processing are performed, and when the rolling rolls have three or more rolls, two or more passes of area reduction processing are performed.

7. 7. The method for producing a steel wire rod according to claim 5, wherein the area reduction is performed while the reduction roll or the steel wire rod is rotated at a rotation angle calculated from the following formula: Rotation angle = 360° / (number of rolls x number of area reduction passes)

8. A method for producing a steel wire rod, comprising quenching and tempering a steel wire rod containing 0.10 mass% or more and 0.90 mass% or less of C, 0.10 mass% or more and 3.00 mass% or less of Si, 0.10 mass% or more and 2.00 mass% or less of Mn, 0.10 mass% or more and 2.00 mass% or less of Cr, with the balance being Fe and unavoidable impurities, The quenching step or tempering step is a first step of inserting a steel wire rod having a circular cross section into a slot of a rolling roll immediately after heating, and performing area reduction rolling to obtain a steel wire rod having a polygonal cross section having at least 10 interior angles, wherein 4 or more of the interior angles of the polygonal cross section are greater than 180°; A second step of reducing the cross section of the steel wire rod having a polygonal cross section by rolling it into a circular cross section; wherein the first step is a step in which a diameter of a circumscribing circle of the steel wire rod having a polygonal cross section is equal to or smaller than a diameter of the steel wire rod having a circular cross section to be rolled, and a diameter of an inscribing circle is equal to or larger than 75% and equal to or smaller than 95% of a diameter of the steel wire rod having a circular cross section to be rolled, The second step is a method for manufacturing a steel wire rod, in which the steel wire rod having a polygonal cross section or a rolling roll is rotated to reduce the area of ​​the steel wire rod having a polygonal cross section to a diameter equal to or smaller than the inscribed circle of the steel wire rod having a polygonal cross section.

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