Steel fiber for reinforcing cement hardened body and cement composition

Abstract

To provide a steel fiber for reinforcing a cement hardened body, which is improved in adhesion with the cement hardened body without using a shape where fiber balls may be formed, and to provide a cement composition.SOLUTION: A steel fiber for reinforcing a cement hardened body comprises a steel fiber and a plurality of recesses provided on an outer peripheral surface of the steel fiber, wherein the steel fiber has a cross-sectional area of 0.0100 to 0.5000 mm2, and each of the plurality of recesses has a length of 100 to 700 μm, a width of 100 to 400 μm, and a depth of 5.0 to 70.0 μm. The total area of the plurality of recesses on an outer peripheral surface of the steel fiber accounts for 3.0 to 50.0% of the total area of the outer peripheral surface of the steel fiber. The length of a recess refers to the longest diameter in a longitudinal direction of the steel fiber, the width of a recess refers to the longest diameter in a direction perpendicular to the longitudinal direction of the steel fiber, and the depth of a recess refers to the longest distance from a line drawn in the longitudinal direction of the steel fiber at a boundary between a surface of the steel fiber and the recess to a bottom surface of the recess perpendicular to a depth direction of the recess.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to steel fibers for reinforcing hardened cement paste and cement compositions. [Background technology]

[0002] Hardened cement concrete, which is made by mixing cement, sand, gravel, water, etc. and hardening it through a hydration reaction, is used as a strength component in various civil engineering and architectural structures because of its high degree of freedom in being able to be formed into any shape at the construction site and its economical efficiency.

[0003] Since the hardened cement paste alone has a low ability to withstand tensile or bending loads, it is generally used as reinforced concrete in the beams and columns of buildings and bridge girders where tensile loads are required. In places where reinforcement is difficult, steel fibers are sometimes mixed into the ready-mixed concrete before pouring in order to improve the strength of the hardened cement paste itself. As the cement and water harden through a hydration reaction, they compact and shrink aggregates such as sand and gravel, forming a strong bond between the steel fibers and the hardened cement paste, providing pull-out resistance and allowing the hardened cement paste itself to withstand a certain degree of bending and tensile loads.

[0004] For a long time, methods to ensure adhesion between steel fibers and hardened cement paste have been used, for example, by adding hooks or bends to both ends of the steel fibers or by giving the steel wire a crimp or other shape.

[0005] Patent Document 1 discloses a steel fiber for concrete reinforcement made of a strip-shaped or short wire-shaped steel piece, which has on its outer surface a number of spaced projections extending transverse to the longitudinal direction of the steel piece and projecting perpendicular or normal to this direction, with recesses at the portions contacting both ends of the projections and convex portions extending outward at the portions not contacting both ends of the projections.In Patent Document 1, the steel fiber was embedded in concrete and allowed to cure for a short period of time, and when force was applied in the longitudinal direction to measure the force required to pull the steel fiber out of the concrete, it was found that several times more force was required than with previously known steel fibers. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 57-94403 Summary of the Invention [Problem to be solved by the invention]

[0007] When using shape effects to improve the adhesion between steel fibers and hardened cement paste, the steel fibers can become entangled during feeding, forming clumps of fibers called "fiber balls," which can reduce workability. When fiber balls form, the fibers must be manually loosened before feeding, which poses the problem of increased construction costs, such as requiring additional workers. Another problem is that the fluidity of the concrete composition decreases.

[0008] An object of the present invention is to provide steel fibers for reinforcing hardened cement bodies, which suppress the formation of fiber balls without imparting hooks or bends to the steel fibers, and which also have improved adhesive strength to the hardened cement bodies, and a cement composition. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems. As a result, they have found that the adhesive strength with hardened cement paste can be improved by forming recesses of an appropriate shape and area ratio on the surface of steel fibers. The present invention is the result of further research and is summarized as follows.

[0010] (1) A steel fiber for reinforcing a hardened cement body, comprising a steel fiber and a plurality of recesses provided on the outer surface of the steel fiber, and the cross-sectional area of ​​the steel fiber is 0.0100 to 0.5000 mm 2 a steel fiber for reinforcing a hardened cement body, characterized in that each of the plurality of recesses has a length of 100 to 700 μm, a width of 100 to 400 μm, and a depth of 5.0 to 70.0 μm, and the total area of ​​the plurality of recesses on the outer peripheral surface of the steel fiber accounts for 3.0 to 50.0% of the total area of ​​the outer peripheral surface of the steel fiber. Here, the length of a recess means the longest diameter in the drawing direction of the steel fiber, the width of a recess means the longest diameter in a direction perpendicular to the drawing direction, the depth of a recess means the longest length from a line connecting the surface of the steel fiber and the boundaries at both ends of the recess to the bottom of the recess, which is perpendicular to the depth direction of the recess, and the drawing direction of the steel fiber means the extension direction of a wiredrawing groove formed on the surface of the steel fiber.

[0011] (2) The steel fiber for reinforcing hardened cement bodies according to (1), characterized in that, on a cross section obtained by cutting the recess along the longest diameter in the wiredrawing direction, the edge angle, which is the angle formed by a line drawn from the boundary between the surface of the steel fiber at both ends of the recess along the recess toward the depth side of the recess and a line drawn along the surface of the steel fiber, is 90 to 160°.

[0012] (3) The steel fiber for reinforcing hardened cement paste according to (1) or (2), characterized in that the plurality of recesses are formed in one or more rows with translational symmetry in the wiredrawing direction, and the interval between adjacent recesses in each row is 1.2 times or more the length of the recess and 1.50 mm or less.

[0013] (4) A steel fiber for reinforcing a hardened cement body according to any one of (1) to (3) above, characterized in that the angle between the drawing groove and the longitudinal direction of the steel fiber is 15° or more.

[0014] (5) A steel fiber for reinforcing hardened cement paste according to any one of (1) to (4), characterized in that the hardness of the recesses is 150 to 1000 HV and is 10 to 100 HV greater than the hardness of the steel fiber core.

[0015] (6) A steel fiber for reinforcing hardened cement paste according to any one of (1) to (5), characterized in that one or more rows of rough surface portions are provided on the outer surface of the steel fiber in the longitudinal direction of the steel fiber, and the rough surface portions have an Ra of 0.10 to 0.50 μm as specified in JIS B 0601:2013 and have translational symmetry.

[0016] (7) A cement composition containing the steel fiber for reinforcing hardened cement paste according to any one of (1) to (6) above. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide steel fibers for reinforcing hardened cement bodies, which can suppress the formation of fiber balls and the decrease in the fluidity of concrete compositions, and have improved adhesive strength to hardened cement bodies. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram illustrating one embodiment of a steel fiber for reinforcing a hardened cement body according to the present invention. [Figure 2] FIG. 2 is a diagram illustrating the wire drawing groove and spiral angle on the surface of the steel fiber. [Figure 3] FIG. 10 is a diagram illustrating a method for measuring the size of a recess. [Figure 4] 10A and 10B are diagrams illustrating a method for measuring the depth and edge angle of a recessed portion. [Figure 5] FIG. 2 is a diagram illustrating a method for measuring the area ratio of recesses. [Figure 6] FIG. 10 is a diagram illustrating a method for measuring the interval between recesses. [Figure 7]FIG. 2 is a diagram illustrating a method for measuring Vickers hardness. [Figure 8] FIG. 2 is a diagram illustrating a method for measuring Ra of a rough surface portion. [Figure 9] FIG. 2 is a schematic diagram illustrating another embodiment of the steel fiber for reinforcing hardened cement bodies of the present invention. [Figure 10] FIG. 1 is a diagram illustrating a method for measuring a pull-out force. DETAILED DESCRIPTION OF THE INVENTION

[0019] <Steel fiber for reinforcing hardened cement paste>

[0020] The steel fiber for reinforcing hardened cement paste of this embodiment comprises a steel fiber and a plurality of recesses provided on the outer peripheral surface of the steel fiber, and the cross-sectional area of ​​the steel fiber is 0.0100 to 0.5000 mm 2 wherein each of the plurality of recesses has a length of 100 to 700 μm, a width of 100 to 400 μm, and a depth, which is the length from the surface of the steel fiber to the bottom of the recess, of 5.0 to 70.0 μm, and the sum of the areas of the plurality of recesses on the outer peripheral surface of the steel fiber accounts for 3.0 to 50.0% of the area of ​​the entire outer peripheral surface of the steel fiber. In this specification, the "length" of a recess refers to the diameter of the recess, i.e., the longest diameter among the diameters in the wiredrawing direction of the steel fiber. The "width" of a recess refers to the diameter of the recess, i.e., the longest diameter among the diameters in the direction perpendicular to the wiredrawing direction of the steel fiber. The "depth" of a recess refers to the longest length, along the longest diameter in the wiredrawing direction, from a line connecting the surface of the steel fiber and the boundaries at both ends of the recess to the bottom of the recess, which is perpendicular to the depth direction of the recess. Furthermore, the "wiredrawing direction" refers to the extension direction of the wiredrawing groove formed on the surface of the steel fiber.

[0021] Fig. 1 is a schematic diagram illustrating one embodiment of the present invention. The embodiment shown in Fig. 1 will be described below. The present invention is not limited to the embodiment described below. A steel fiber 1 for reinforcing hardened cement bodies includes steel fibers 10, and the steel fibers 10 are provided with a plurality of recesses 11 on the outer peripheral surface. Wire drawing grooves are formed on the surface of the steel fiber for reinforcing hardened cement bodies.

[0022] Drawing grooves are formed on the surface of a round steel wire, which is a raw material for steel fibers to reinforce hardened cement paste, when it is produced. During cold drawing, a compressive force acts on the surface of the steel material being processed through the holes in the wiredrawing die. This circumferential stress forms multiple grooves on the surface of the cold-drawn round steel wire. These grooves are referred to as drawing grooves. The drawing grooves extend in the direction in which the steel material moves relative to the wiredrawing die, that is, in the longitudinal direction of the round steel wire being produced. As described above, since the drawing grooves are formed by circumferential stress, the wiredrawing dies used for round steel wire are not specially designed for forming drawing grooves in steel materials, but are publicly known dies. As will be described later, when torsional plastic deformation is imparted to steel fibers obtained by forming recesses in a round steel wire, the drawing grooves are formed in a spiral shape at an angle relative to the longitudinal direction of the steel fiber.

[0023] Figure 2 shows an example of a wiredrawing groove formed on the surface of a steel fiber. Figure 2 shows the surface of a steel fiber observed with an SEM, and the lines extending in an oblique direction observed on the surface of the steel fiber are wiredrawing grooves 21. In the example of Figure 2, multiple recesses are formed along the extension direction of the wiredrawing groove 21.

[0024] The wire drawing groove may be formed in the longitudinal direction of the steel fiber, or may be formed at an angle (hereinafter referred to as the "helical angle") to the longitudinal direction of the steel fiber to form a helical state. When the wire drawing groove is formed in the longitudinal direction of the steel fiber, the helical angle is 0°. By subjecting the steel wire to torsional plastic deformation to form a helical state, even if the cross section of the steel wire becomes irregular, the irregular shape is uniformly dispersed in the circumferential direction, improving the linearity of the steel fiber for reinforcing hardened cement paste as a whole. The helical angle may be 1° or more, 2° or more, 3° or more, 5° or more, 7° or more, or 10° or more. A helical angle of 15° or more or 20° or more is preferable from the viewpoint of dispersion of the irregular shape. If the helical angle is too large, the steel wire becomes more likely to break, so the helical angle is preferably 40° or less. The helical angle may be 35° or less, 30° or less, or 25° or less.

[0025] (cross-sectional area) Steel fibers have a cross-sectional area of ​​0.0100 to 0.5000 mm 2and the cross section is approximately circular. The term "approximately circular" may be a perfect circle, an ellipse, or a combination of a part of a circle with a straight line. In the steel fiber for reinforcing hardened cement body of this embodiment, at the position where the recess is formed, the cross section has a shape in which the recess portion is curvedly missing from the cross section of the steel fiber, but this is also included in the category of approximately circular. In the case of a shape other than a perfect circle, the maximum diameter length is 2.0 or less when the minimum diameter length is 1. When steel fibers are contained in a cement composition, it is preferable to contain a large amount of small diameter steel fibers to enhance the reinforcing effect. On the other hand, from the viewpoint of the fluidity of the cement composition, it is preferable to contain large diameter steel fibers. Taking into account the balance between the two, the cross section area is set to the above range. The cross section area of ​​the steel fiber is 0.0300 mm 2 Over 0.0600mm 2 Over 0.1100mm 2 Over 0.1700mm 2 Over 0.2000mm 2 or more, or 0.2600 mm 2 The cross-sectional area may be 0.4500 mm or more. 2 Below, 0.4000mm 2 Below, 0.3500mm 2 Less than or equal to 0.3000mm 2 It may be the following:

[0026] (shape of recess) In the steel fiber for reinforcing hardened cement body of this embodiment, a plurality of recesses 11 are provided on the outer peripheral surface of the steel fiber 10. By forming the recesses as described below, it is possible to improve the adhesive strength between the steel fiber and the hardened cement body while suppressing the formation of fiber balls and the decrease in the fluidity of the concrete composition.

[0027] Each of the plurality of recesses has a length of 100 to 700 μm, a width of 100 to 400 μm, and a depth of 5.0 to 70.0 μm.

[0028] The steel fibers for reinforcing hardened cement bodies of this embodiment are embedded in the hardened cement body for use. When embedded in the hardened cement body, the steel fibers are oriented in various directions, but are pulled out in the longitudinal direction of the steel fibers, and force is applied in the drawing direction (or in that direction if the steel fibers have been subjected to torsional plastic deformation). Therefore, the shape of the steel fibers in the drawing direction is important for improving the adhesive strength between the steel fibers and the hardened cement body. If the length of the recesses is too short, the effect of improving the adhesive strength between the steel fibers and the hardened cement body cannot be obtained. If the length of the recesses is too long, the steel fibers are more likely to break.

[0029] As described below, the recesses can be formed by rotating a roller with protrusions and pressing it against the steel fibers. In this case, the width and depth of the recesses are correlated. If the width of the recesses is too short and the depth is too shallow, the effect of improving the adhesion between the steel fibers and the hardened cement body cannot be obtained. If the width of the recesses is too long and the depth is too deep, the steel fibers are more likely to break starting from the recesses.

[0030] The length of the recess may be 125 μm or more, 155 μm or more, 210 μm or more, 270 μm or more, 300 μm or more, or 370 μm or more. The length of the recess may be 670 μm or less, 640 μm or less, 570 μm or less, 510 μm or less, or 480 μm or less. The width of the recess may be 125 μm or more, 145 μm or more, 190 μm or more, 240 μm or more, 260 μm or more, or 320 μm or more. The width of the recess may be 390 μm or less, 385 μm or less, 370 μm or less, 360 μm or less, or 350 μm or less. The depth of the recess may be 8.0 μm or more, 11.0 μm or more, 18.0 μm or more, 25.0 μm or more, 28.0 μm or more, or 36.0 μm or more. The depth of the recesses may be 66.0 μm or less, 63.0 μm or less, 57.0 μm or less, 50.0 μm or less, or 47.5 μm or less.

[0031] (area ratio of recessed areas) The ratio of the total area of ​​the plurality of recesses to the area of ​​the entire outer peripheral surface of the steel fiber (hereinafter also referred to as the "area ratio of recesses") is 3.0 to 50.0%. If the area ratio of recesses is too small, the effect of improving the adhesive strength between the steel fiber and the hardened cement body cannot be obtained. If the area ratio of recesses is too large, the steel fiber becomes more likely to break. The area ratio of recesses may be 4.0% or more, 5.0% or more, 9.0% or more, 10.0% or more, 15.0% or more, or 20.0% or more. The area ratio of recesses may be 45.0% or less, 40.0% or less, 30.0% or less, 25.0% or less, or 23.0% or less. The arrangement of the plurality of recesses is not particularly limited.

[0032] Each recess preferably has an edge angle of 90 to 160° at both ends, as observed when the steel fiber is cut along the longest diameter of the recess in the drawing direction of the steel fiber. If the edge angle is too small, the round steel wire may get caught on the roller during the manufacturing process described below, resulting in reduced productivity. Here, the edge angle refers to the angle between a line drawn along the surface of the steel fiber, drawn from the boundary between the surface of the steel fiber at both ends of the recess toward the depth of the recess, and a line drawn along the surface of the steel fiber. If the edge angle is too large, the effect of improving the adhesion between the steel fiber and the hardened cement paste tends to be reduced. The edge angle may be 95° or more, 100° or more, 105° or more, 115° or more, 120° or more, or 130° or more. The edge angle may be 155° or less, 150° or less, 145° or less, 140° or less, or 135° or less.

[0033] Even if recesses of the above-mentioned size and area ratio are formed on the surface of the steel fibers, the reinforcing fibers do not become entangled with each other when the steel fibers are fed, unlike when protrusions or convexities such as hooks or bends are formed. Furthermore, as will be shown in the examples described later, the effect of forming the recesses can improve the adhesive strength between the steel fibers and the hardened cement paste.

[0034] The length, width, depth, area ratio, and edge angle of each recess are measured using a microscope, laser microscope, etc. For example, a KEYENCE VK-X250 is used to measure the size of the recess at a magnification of 1000x. For large recesses, the observed images are linked together and measured.

[0035] Measurement of length and width will be explained with reference to Figure 3. Figure 3(a) is an image of a recess observed at 1000x magnification using a KEYENCE VK-X250. The "length" of the recess is the length of the longest diameter of the recess in the drawing direction of the steel fiber. The "width" of the recess is the length of the longest diameter in the direction perpendicular to the drawing direction of the steel fiber. In the example of Figure 3(a), the left-right direction on the page is the drawing direction of the steel fiber, which is the same as the longitudinal direction of the steel fiber. Figure 3(b) is a diagram explaining another example of measurement of length and width. Figure 3(b) is a schematic diagram explaining measurement of the length and width of a recess, similar to Figure 3(a), where the left-right direction on the page is the drawing direction of the steel fiber. Even if the shape of the recess is such that the side along the wiredrawing direction is shorter as shown in Figure 3(b), the shape of the steel fiber in the wiredrawing direction is important for improving the adhesive strength between the steel fiber and the hardened cement paste, as mentioned above. Therefore, as shown in Figure 3(b), the longest diameter of the steel fiber in the wiredrawing direction is defined as the "length" of the recess, and the longest diameter in the direction perpendicular to the wiredrawing direction is defined as the "width" of the recess. Figure 3(c) is also a schematic diagram explaining the measurement of the length and width of the recess, as with Figure 3(a). In the example of Figure 3(c), the extension direction of the wiredrawing groove is angled with respect to the longitudinal direction of the steel fiber, i.e., formed to have a positive helical angle. In such a case, as shown in Figure 3(c), the "length" of the recess is defined as the length of the longest diameter in the wiredrawing direction of the steel fiber, i.e., the direction of the helical angle with respect to the longitudinal direction, and the "width" of the recess is defined as the length of the longest diameter in the direction perpendicular to that direction. In this embodiment, the length and width of recesses within a 5 mm length of the steel fiber are measured, and the average value is used as the representative value.

[0036] The measurement of the depth and edge angle of the recess will be explained with reference to Figure 4. Figure 4(a) shows a depth profile obtained using a KEYENCE VK-X250 on a surface cut perpendicular to the surface of the steel fiber along the longest diameter of the recess in the drawing direction of the steel fiber. Figure 4(b) is a schematic diagram of the unevenness information obtained in the same manner as Figure 4(a). As shown in Figure 4(b), the "depth" of the recess is defined as the longest distance from a line drawn from the boundary between the surface of the steel fiber and the recess in the drawing direction of the steel fiber to the bottom of the recess perpendicular to the depth direction of the recess. If the distances from the surface of the steel fiber to the boundaries of the recess at both ends are different, the line drawn from the boundary farthest from the surface of the steel fiber is used as the reference. The "edge angle" is determined by drawing straight lines along the recess side (processed side) and the steel fiber side (unprocessed side) of the depth profile at the boundaries at both ends of the recess. The edge angle is defined as the average angle between the angles measured at both ends. In this embodiment, the depth and edge angle of recesses within a range of 5 mm in length of the steel fiber are measured, and the average value is used as the representative value.

[0037] The measurement of the area ratio of recesses will be explained with reference to Figure 5. Figure 5 is an image of recesses observed from directly above at 1000x magnification using a KEYENCE VK-X250. The recess area is adjusted visually, and the area of ​​the recesses is measured using the area measurement function of the device. For larger recesses, the observed images are linked together and measured. The area ratio is determined by measuring within a 5mm length of the steel fiber. Specifically, the areas of recesses within a 5mm length are measured using the method described above and added together to calculate the total area of ​​the recesses. Next, the surface area of ​​the steel fiber (circumference length x 5mm) is calculated using the average value of the diameters measured at any three points within the 5mm length. The area ratio of recesses is calculated by dividing the total area of ​​the recesses by the surface area of ​​the steel fiber.

[0038] (Arrangement of recesses) The arrangement of the recesses is not limited, but from the viewpoint of manufacturability, it is preferable to form them in one or more rows with translational symmetry in the drawing direction of the steel fiber. As described below, when steel fibers for reinforcing hardened cement paste are produced using a roller, the recesses are preferably formed with translational symmetry. Furthermore, it is preferable that the spacing between adjacent recesses in each row is at least 1.2 times the length of the recess and not more than 1.50 mm. The spacing between the recesses may be at least 1.3 times, at least 1.5 times, at least 1.7 times, at least 1.8 times, or at least 2.0 times the length of the recess. The spacing between the recesses may be 1.45 mm or less, 1.35 mm or less, 1.20 mm or less, 1.10 mm or less, or 1.00 mm or less.

[0039] The spacing between recesses when recesses are formed with translational symmetry in the drawing direction of the steel fiber is measured using a microscope, laser microscope, or the like. Referring to Figure 6, the measurement of the spacing between recesses when recesses are formed with translational symmetry in the drawing direction of the steel fiber will be described. Figure 6(a) shows the surface of a steel fiber on which recesses are formed, as observed with an SEM. The spacing between recesses is measured between the ends of adjacent recesses along the direction in which they are formed with translational symmetry. Figure 6(b) shows an example in which recesses are arranged in a spiral, as will be described later. In this case as well, the spacing between recesses is measured between the ends of adjacent recesses along the direction in which they are formed with translational symmetry.

[0040] (Hardness of the recess) The hardness of the recesses is preferably 150 to 1000 HV and 10 to 100 HV higher than the hardness of the steel fiber core. The hardness of the recesses may be 180 HV or more, 220 HV or more, 300 HV or more, 380 HV or more, 420 HV or more, or 510 HV or more. The hardness of the recesses may be 950 HV or less, 910 HV or less, 820 HV or less, 720 HV or less, or 670 HV or less. The difference in hardness between the recesses and the steel fiber core may be 15 HV or more, 20 HV or more, 30 HV or more, 40 HV or more, 45 HV or more, or 55 HV or more. The difference in hardness between the recesses and the steel fiber core may be 95 HV or less, 90 HV or less, 85 HV or less, 75 HV or less, or 70 HV or less.

[0041] Hardness is measured using a Vickers hardness tester. The formula in Table 2 of JIS Z2244-1:2020 (Vickers Hardness Test - Part 1, Test Method) is used to convert the test force and Vickers indentation shape into hardness. The formula in Table 2 applies to determining hardness even if the average diagonal length d of the Vickers indentation is less than 0.020 mm. Furthermore, even if the distance from the center of the indentation to the edge of the test piece is less than 2.5 times d, or if the center-to-center distance between two adjacent indentations is less than 3d, the measured d is used to convert into hardness using the formula in Table 2. The test force for the Vickers hardness test is 25 gf. The hardness symbol is HV0.025, but hereafter it will be abbreviated as HV.

[0042] A specific measurement method will be described with reference to Figure 7. Vickers hardness is measured at the recess and the center of the steel fiber on a cross section of the steel fiber that is parallel to the longitudinal direction of the steel fiber and includes the center line of the steel fiber. As shown in Figure 7(a), the hardness of the recess is measured at three points: a position 15 µm deep from the surface at the center position of the recess in the wiredrawing direction toward the center of the steel fiber, and a position moved ±50 µm from this position parallel to the wiredrawing direction. As shown in Figure 7(b), the hardness of the steel fiber center is measured at three points at 50 µm intervals in the longitudinal direction in an area half the diameter d of the sample. In this embodiment, three recesses and steel fiber centers within a 5 mm length of the steel fiber are randomly selected, and their hardness is measured, and the average of the hardness values ​​measured at a total of nine points at each of the recess and the steel fiber center is used as the representative value.

[0043] (Rough surface part) The steel fibers for reinforcing a hardened cement body of this embodiment may further have one or more rows of rough surface portions formed in the longitudinal direction on the outer peripheral surface of the steel fibers, in order to further improve the adhesive strength between the steel fibers and the hardened cement body. The rough surface portions preferably have an Ra (specified in JIS B0601:2013) of 0.10 to 0.50 μm measured in a direction parallel to the longitudinal direction of the steel fibers and have translational symmetry. If the Ra is too large, there is a tendency for the breakage of the steel fibers due to stress concentration to be a greater concern than the effect of improving adhesive strength. The rough surface portions are not essential and may be omitted. Furthermore, even if a rough surface portion is not intentionally formed, the surface of the steel fibers for reinforcing a hardened cement body may have a roughness Ra of more than 0 μm and less than 0.10 μm, which does not pose any problems. The width of the row of rough surface portions is not limited. The width of the row of rough surface portions may be, for example, 5% or more, 6% or more, 8% or more, 10% or more, 12% or more, or 15% or more of the circumferential length of the steel fibers. The width of the row may be, for example, 50% or less, 40% or less, 35% or less, or 30% or less of the circumferential length of the steel fibers. The Ra of the rough surface portion may be 0.11 μm or more, 0.13 μm or more, 0.17 μm or more, 0.21 μm or more, 0.23 μm or more, or 0.28 μm or more. The Ra of the rough surface portion may be 0.47 μm or less, 0.45 μm or less, 0.41 μm or less, 0.37 μm or less, or 0.35 μm or less. Furthermore, the rough surface portion may be formed so as to overlap with the above-mentioned recesses, or may be formed in a position that does not overlap with the recesses. Furthermore, when two or more rows of rough surface portions are formed, they may be formed so as to overlap with each other, or may be formed in positions that are separated from each other.

[0044] The measurement of the Ra of the rough surface will be described with reference to Figure 8. Ra can be calculated from the roughness curve. Figure 8(a) is a diagram illustrating an outline of the measured profile curve obtained by measuring the actual surface of steel fiber for reinforcing hardened cement paste. From the obtained measured profile, short wavelength components such as noise are removed to obtain the profile curve. Figure 8(b) shows an outline of the profile curve. Furthermore, long wavelength components (waviness), such as changes in the outer diameter of the steel fiber in the longitudinal direction, are removed to obtain the roughness curve. Figure 8(c) shows an outline of the roughness curve. Ra is calculated from the obtained roughness curve in accordance with JIS B0601:2013. The measured profile curve can be obtained using, for example, a KEYENCE VK-X250. Specifically, the measured profile curve is obtained at a magnification of 3000x. Next, a phase-compensated low-pass filter with a cutoff λs of 0.25 μm is applied to the obtained measured profile curve to obtain a profile curve from which short wavelength components have been removed. Next, the long-period waviness is removed with λc = 0.025 mm to obtain a roughness curve. From the obtained roughness curve, Ra is calculated according to JIS B0601:2013.

[0045] One embodiment of the steel fiber for reinforcing hardened cement paste of the present invention has been described above in detail, but the present invention is not limited to the above-described embodiment, and various design modifications can be made within the scope of the spirit of the present disclosure.

[0046] Figure 9 shows other examples of the form of steel fiber for reinforcing hardened cement paste of the present invention. Figure 9(a) shows an example in which two rows of recesses are formed on the outer surface of the steel fiber. Figure 9(b) shows an example in which a roughened surface portion is formed in addition to the recesses on the outer surface of the steel fiber. Figure 9(c) shows an example in which a roughened surface portion is formed in addition to the recesses on the outer surface of the steel fiber, where the recesses and the roughened surface portion overlap. Figure 9(d) shows an example in which two rows of recesses and a roughened surface portion are formed on the outer surface of the steel fiber, where the recesses and the roughened surface portion overlap.

[0047] Cement Composition By incorporating the steel fibers for reinforcing a hardened cement body of this embodiment described above into an unhardened cement composition, a cement composition can be obtained, and a reinforcing effect can be achieved for the hardened cement body. The method of use of the steel fibers for reinforcing a hardened cement body of this embodiment is not particularly limited, and they may be incorporated into an unhardened cement composition in the same manner as known steel fibers for reinforcing a hardened cement body. The content of the steel fibers for reinforcing a hardened cement body is not particularly limited, and may be appropriately set depending on the purpose. For example, the volume fraction may be 0.2 to 5.0%. The content of the steel fibers for reinforcing a hardened cement body may be 0.3% or more, 0.5% or more, 1.0% or more, 1.2% or more, or 1.5% or more. The content of the steel fibers for reinforcing a hardened cement body may be 4.5% or less, 4.0% or less, 3.5% or less, 3.0% or less, or 2.5% or less.

[0048] <Method for manufacturing steel fibers for reinforcing hardened cement paste> Next, an example of a method for producing the steel fiber for reinforcing hardened cement paste of this embodiment will be described.

[0049] The steel fiber for reinforcing hardened cement paste of this embodiment can be produced by cold drawing a round steel wire as a raw material and forming recesses on the surface of the steel wire.

[0050] (Steel wire) The round steel wire used as the raw material is not particularly limited, and may be, for example, a commercially available steel wire. The steel wire may be, for example, "Spring steel wires - Part 2: Cold drawn carbon steel wires (ISO specifications)" specified in JIS G 7305:2000, "Spring steel wires - Part 3: Oil tempered wires (ISO specifications)" specified in JIS G 7306:2000, "Piano wires" specified in JIS G 3522:2014, or "Hard steel wires" specified in JIS G 3521:2108.

[0051] Alternatively, a hot-rolled wire rod produced by a known method may be drawn using a cold drawing die to obtain a steel wire, and the steel fiber for reinforcing hardened cement paste of this embodiment may be produced using the obtained steel wire.

[0052] The steel wire as a raw material may be a brass-plated steel wire or a zinc-plated steel wire. Alternatively, a recess may be first formed on the surface of the steel wire, and then the surface may be brass-plated or zinc-plated under conditions that allow the shape of the formed recess to be maintained.

[0053] (Formation of recesses) The round steel wire is cold drawn to form recesses on the surface. For example, the processing is carried out by the following method.

[0054] A roller with a convex portion formed on it (a roller with a cylindrical pressing surface) is set into the cassette roller. The gap between the rolls is wide enough, and the above-mentioned round steel wire is passed from the entry side to the exit side, protruding about 100 mm as a "gripping allowance." The two roll gap adjustment bolts on the cassette are tightened evenly, and the roller presses down the round steel wire. This transfers the convex portion formed on the roller to the round steel wire, forming periodic concave portions on the surface of the round steel wire in the longitudinal direction.

[0055] The material of the roller is not particularly limited, but it is preferable to use a super-hard tool steel (WC-Co) or a tool steel (SKD, SKH), which is harder and less prone to wear than steel wire. The protrusions for forming the recesses in the steel fiber for reinforcing hardened cement paste of the present invention must be formed in advance on the pressing surface of the roller.

[0056] (Torsion) After the recesses are formed, the steel fibers for reinforcing hardened cement paste may be twisted. However, twisting is not essential. By subjecting the steel fibers to torsional plastic deformation, the extension direction of the wire-drawing grooves becomes a direction (wire-drawing direction) at an angle to the longitudinal direction of the steel fibers. Furthermore, the formed recesses are arranged in a spiral in the wire-drawing direction of the steel fibers. Furthermore, by twisting, the curvature imparted when manufacturing the round steel wire and the small bends inevitably formed by flat pressing are uniformly dispersed in the longitudinal and circumferential directions, greatly improving the apparent straightness of the twisted wire. The degree of twist is not particularly limited, but as mentioned above, a helix angle of 0 to 40° is preferred.

[0057] (Formation of rough surface) Furthermore, the outer peripheral surface of the steel fiber for reinforcing a hardened cement body may be provided with the above-mentioned rough surface portion. The formation of the rough surface portion is not essential. The method for forming the rough surface portion is not particularly limited, and the steel fiber for reinforcing a hardened cement body may be cut or polished in the longitudinal direction to form the rough surface portion. Alternatively, the rough surface portion may be formed by cutting or polishing the round steel wire, which is the raw material.

[0058] The steel fibers for reinforcing hardened cement paste produced as described above may be cut to a desired length by any known cutting method.

[0059] By the above-mentioned method, it is possible to produce steel fibers for reinforcing hardened cement paste that have a substantially circular cross section and are substantially linear. -1 The term "curvature" refers to steel fibers with a small curvature. Steel fibers may be bundled together and cut together. If the curvature is small, the length variation will be small, which is preferable. The curvature is 0.04 m. -1 Below, 0.03m -1 Below, 0.02m -1 Less than or equal to 0.01m -1 The curvature can be measured by cutting the steel fiber to a length of 1000 mm, throwing it on a smooth floor with little friction, and measuring the arc-like shape that is naturally formed.

[0060] The above describes one example of a method for producing steel fibers for reinforcing hardened cement bodies. The above conditions are merely an example for effectively producing steel fibers for reinforcing hardened cement bodies according to the present invention, and do not limit the processing conditions such as the area reduction rate and die angle at each stage of drawing to produce the steel wire of the present invention. [Example]

[0061] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples.

[0062] The round steel wire used as the raw material was a "piano wire" specified in JIS G 3522:2014 with the cross-sectional area shown in Table 1, and steel fibers for reinforcing hardened cement paste were manufactured using this.

[0063] First, a roller with convex portions formed was set in a cassette roller, and a round steel wire was passed from the inlet to the outlet, with one roller pressing down the round steel wire. This resulted in the formation of one to four rows of translationally symmetrically spaced concave portions in the longitudinal direction on the surface of the round steel wire, yielding steel fiber for reinforcing hardened cement paste. Note that a "-" next to the "number of translationally symmetrical rows" in Table 1 means that the concave portions formed did not have translational symmetry.

[0064] After forming the recesses, in some cases, the steel fiber for reinforcing hardened cement paste was passed through a chuck with a tubular rotating shaft and a through-hole, hooked onto a hook, and rotated along with the chuck to impart torsional plastic deformation. Those with a "helical angle" of more than 2° shown in Table 1 are examples in which torsional plastic deformation was imparted.

[0065] In some cases, the surface of the steel fiber was polished in the longitudinal direction to form a single row of roughened surfaces. The width of the roughened surface was 10% of the circumference. Table 1 shows examples with an "Ra" of 0.10 μm or more.

[0066] The steel fibers for reinforcing hardened cement paste obtained as described above were evaluated for the shape of the recesses formed therein as follows.

[0067] (recess shape) The recesses were observed at 1000x magnification using a KEYENCE VK-X250, and the length, width, and area ratio of the recesses were determined using the method described above. A depth profile was also obtained using the method described above to determine the recess depth and edge angle. Furthermore, the spacing between the recesses was determined using SEM observation. Note that the recesses marked with "-" in Table 1 did not have translational symmetry and had large spacing variations, so they were not evaluated.

[0068] (Hardness) Using a Vickers hardness tester, the hardness of the recessed portion and the hardness of the steel fiber center were measured using the method described above, and the hardness of the recessed portion and the difference in hardness were calculated. If the difference in hardness is positive, it indicates that the hardness of the recessed portion is greater.

[0069] Furthermore, in order to confirm whether the obtained steel fibers for reinforcing hardened cement paste have the desired properties, the pull-out strength was measured as follows.

[0070] (Pull-out force measurement) The adhesion between the steel fiber for reinforcing hardened cement paste and the hardened cement paste was evaluated by pull-out force measurement. The pull-out force measurement will be explained with reference to Fig. 10. Using 100 mm of the obtained steel fiber 101, 15 mm was cured with mortar 102 as shown in Fig. 10, and a pull-out test was carried out using a tensile tester 103. The evaluation items were the maximum load and the work load up to a movement distance of 5 mm (the integral value of the load over the pull-out distance).

[0071] The maximum load and workload were evaluated as follows based on how many times they were compared with an unprocessed material made of the same material as each steel fiber but with no recesses formed, and are shown in Table 1.

[0072] Maximum Load ◎: 1.6 times or more of unprocessed material ○: 1.4 times or more but less than 1.6 times the thickness of unprocessed material ×: Less than 1.4 times the unprocessed material

[0073] [Workload] ◎: 1.5 times or more the thickness of unprocessed material ○: 1.3 times or more but less than 1.5 times the thickness of unprocessed material ×: Less than 1.3 times the unprocessed material

[0074] In this example, if both the maximum load and the workload were rated "○" or higher, it was determined that the adhesive strength with the hardened cement body was improved and the problem of the present invention was solved.

[0075] The results are shown in Table 1.

[0076] [Table 1]

[0077] No. B1 is a comparative example in which the recesses formed on the surface of the steel fiber are long. It is believed that the effect of improving adhesive strength was reduced because the recesses were too long. As a result, the desired results were not obtained for both the maximum load and the amount of work in the pull-out force measurement.

[0078] No. B2 is a comparative example in which the recesses are wide and deep. It is believed that the steel fibers broke because the recesses were too wide and deep, preventing the improvement in adhesive strength. As a result, the desired results were not obtained for both the maximum load and the amount of work in the pull-out force measurement.

[0079] No. B3 is a comparative example in which the recess length is short. It is believed that the steel fiber broke due to the notch effect caused by the short recess length. As a result, the desired results were not obtained for both the maximum load and the amount of work in the pull-out force measurement.

[0080] No. B4 is a comparative example in which the recesses are narrow and shallow. It is believed that the narrow and shallow recesses did not improve adhesive strength. Therefore, the desired results were not obtained for both the maximum load and the workload in the pull-out force measurement.

[0081] No. B5 is a comparative example in which the area ratio of the recesses is small. It is believed that the small area ratio of the recesses does not improve the adhesive strength. As a result, the desired results were not obtained for both the maximum load and the workload in the pull-out force measurement.

[0082] No. B6 is a comparative example in which the recesses are shallow. It is believed that the shallow recesses did not improve the adhesive strength. As a result, the desired results were not obtained for both the maximum load and the workload in the pull-out force measurement.

[0083] On the other hand, Nos. A1 to A15 are examples of the present invention, and desired characteristics were obtained in both the maximum load and workload in the pull-out force measurement.

[0084] In addition, when the steel fibers were supplied, it was confirmed whether or not "fiber balls" formed, in which the steel fibers were entangled with each other, and if no "fiber balls" were formed, it was marked with "○", and if fiber balls were formed, it was marked with "×". The formation of "fiber balls" was not confirmed in any of Nos. A1 to A15 and B1 to B6. [Explanation of symbols]

[0085] 1 Steel fibers for reinforcing hardened cement paste 10 Steel Fiber 11 Recess 21 Wire drawing groove 101 Steel Fiber 102 Mortar 103 Tensile testing machine

Claims

1. Steel fibers and A plurality of recesses provided on the outer peripheral surface of the steel fiber A steel fiber for reinforcing a hardened cement body, comprising: The cross-sectional area of ​​the steel fiber is 0.0100 to 0.5000 mm 2 and Each of the plurality of recesses has a length of 100 to 700 μm, a width of 100 to 400 μm, and a depth of 5.0 to 70.0 μm; The total area of ​​the plurality of recesses on the outer peripheral surface of the steel fiber accounts for 3.0 to 50.0% of the total area of ​​the outer peripheral surface of the steel fiber. A steel fiber for reinforcing hardened cement paste, characterized by: Here, the length of the recess means the length of the longest diameter in the drawing direction of the steel fiber, the width of the recess means the length of the longest diameter in a direction perpendicular to the drawing direction, the depth of the recess means the longest length along the longest diameter in the drawing direction from a line connecting the surface of the steel fiber and the boundaries at both ends of the recess to the bottom surface of the recess perpendicular to the depth direction of the recess, and the drawing direction of the steel fiber means the extension direction of the wiredrawing groove formed on the surface of the steel fiber.

2. 2. The steel fiber for reinforcing a hardened cement body according to claim 1, characterized in that, on a cross section obtained by cutting the recess along the longest diameter in the wiredrawing direction, an edge angle is formed between a line drawn from the boundary between the surface of the steel fiber at both ends of the recess along the recess toward the depth side of the recess and a line drawn along the surface of the steel fiber, and the edge angle is 90 to 160°.

3. 3. The steel fiber for reinforcing hardened cement paste according to claim 1, wherein the plurality of recesses are formed in one or more rows with translational symmetry in the wiredrawing direction, and the interval between adjacent recesses in each row is 1.2 times or more the length of the recesses and 1.50 mm or less.

4. 3. The steel fiber for reinforcing a hardened cement body according to claim 1, wherein the angle between the drawing groove and the longitudinal direction of the steel fiber is 15 degrees or more.

5. 3. The steel fiber for reinforcing hardened cement paste according to claim 1, wherein the hardness of the recessed portion is 150 to 1000 HV, and is 10 to 100 HV greater than the hardness of the steel fiber core.

6. 3. The steel fiber for reinforcing hardened cement bodies according to claim 1 or 2, characterized in that one or more rows of rough surface portions are provided on the outer peripheral surface of the steel fiber in the longitudinal direction of the steel fiber, and the rough surface portions have an Ra of 0.10 to 0.50 μm as specified in JIS B 0601:2013 and have translational symmetry.

7. A cement composition containing the steel fiber for reinforcing hardened cement paste according to claim 1 or 2.

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