Steel material for high intensity shear reinforcement bar

Abstract

To reduce an addition amount of a hardenability alloy when producing a hot-rolled steel material (corresponding to bar steel D785) applicable to a high intensity shear reinforcement bar.SOLUTION: A product is produced in two ways. One method obtains the necessary strength and ductility through appropriate steel grades and wire rod rolling+controlled cooling, while the other uses an ordinary reinforcement bar producing facility and ensures similar mechanical properties under the same working conditions as the ordinary reinforcing bar. The latter requires a large amount of rare metal quenching alloys to compensate for insufficient cooling capacity. By changing the cross-sectional shape of the round steel to a rectangular shape with a semicircular end, the reduction in thickness and increase in specific surface area enhance the cooling rate despite being air-cooling, resulting in effects similar to controlled cooling. The required amount of quenching alloy is halved. An appropriate aspect ratio of the cross-section is between 3 and 10.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to high-strength shear reinforcement bars that assist in building high-rise and earthquake-resistant reinforced concrete structures. [Background technology]

[0002] Shear reinforcement is a type of auxiliary steel bar in the shape of a hoop that is arranged in multiple stages around a forest of main reinforcement bars in order to prevent shear failure in reinforced concrete structures. To build high-rise concrete structures and make them earthquake-resistant, it is necessary to strengthen the shear reinforcement bars. Densely packed reinforcement bars are effective for this purpose, but this hinders the flow of concrete and is therefore impractical. The incorporation of high-strength shear reinforcement bars maintains the same spacing as before, which is effective in mitigating this problem. The yield strength of the steel used for reinforcing bars will be increased from the conventional 300 MPa to over 800 MPa. In addition to strength, bending workability, weldability, and adhesion to concrete are also required as before.

[0003] Since this steel material is a new product for construction, it requires strong technical support not only for its manufacture but also for its use, and its development was carried out in an industry-academia collaborative project centered on the wire rod division of a major steel manufacturer. Although the technology was successful, demand was sluggish. Today, similar products are manufactured by small and medium-sized electric furnace manufacturers' bar steel mills, and the market is growing steadily.

[0004] Initially, it was assumed that controlled cooling of hot rolling was essential as a manufacturing process to achieve high strength, but conventional bar mills were not equipped with the necessary equipment and therefore could not manufacture the product, so controlled cooling at wire rod mills was applied. Although it became possible to obtain sufficient performance by appropriately combining the steel type and cooling conditions, it was later discovered that similar quality could be obtained depending on the alloy even with simple air cooling as the cooling conditions, and this was applied in the similar products mentioned above. It is easy to manufacture as it does not require the development and investment in controlled cooling equipment for bar mills, but the problem is that the amount of vanadium (V), an expensive rare metal, is 0.2 to 0.4% more alloyed than in wire rod, which continues to be a cost issue.

[0005] Patent Document 1 discloses a manufacturing method for wire rod with a yield stress of 700 MPa, which is a precursor to the product in question. It shows that the amount of expensive V added is kept to 0.1% or less by applying controlled cooling by immersion in boiling water after hot rolling. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Published Patent Publication 2001-181781 Summary of the Invention [Problem to be solved by the invention]

[0007] Nowadays, high-strength steel bars for shear reinforcement can be easily manufactured by ordinary bar mills, but because they are air-cooled after hot rolling, they require the addition of a large amount of the expensive hardenable alloy V. The problem to be solved by the present invention is to reduce the amount of V added. [Means for solving the problem]

[0008] The first aspect of the present invention is a steel material for high-strength shear reinforcement, the composition of which is: C: 0.10-0.20%, (Si+Mn+Cr); 2.0-3.5%; (V+Nb+Mo); 0.10-0.50%; The balance is made of Fe and unavoidable impurities by hot rolling. The microstructure consists of a mixture of ferrite + pearlite + bainite. This hot-rolled steel material for high-strength shear reinforcement has mechanical properties equivalent to those of high-strength shear reinforcement steel bars, with a yield strength of 785 MPa or more, a tensile strength of 930 MPa or more, an elongation of 8% or more, a bending strength of 180°C / 3D, and a diameter of 10 to 16 mm, and is characterized by its cross-sectional shape being semicircular at the ends and its cross-sectional aspect ratio being flat steel shape of 3 or more and 10 or less.

[0009] A second invention is a steel material for high-strength shear reinforcement as described in the first invention, characterized in that the thickness at the width center is 0.6 to 0.9 times the diameter at both ends. Effect of the Invention

[0010] The effects of the present invention are as follows. In hot-rolled steel that is air-cooled after hot rolling, flattening the cross-sectional shape while maintaining the same cross-sectional area, i.e., the same strength, as conventional circular cross-section steel bars increases the circumference, increasing the surface area / volume (=circumference / cross-sectional area) ratio and increasing the cooling rate, resulting in a metallurgical effect equivalent to air blast cooling commonly used in controlled cooling of wire rods, and making it possible to reduce the amount of expensive hardenable alloy element V added.

[0011] Secondly, a general property of hot-rolled steel is that the larger the dimension, the lower the strength and ductility / toughness. However, the flattening of the present invention has an advantageous effect on elongation and bendability, which are at least no lower than those of conventional materials. [Brief description of the drawings]

[0012] [Figure 1] The cross-sectional shape of the shear reinforcement steel is shown. A is the current steel bar, and B is the product of the present invention with the same cross-sectional area as A. [Diagram 2] The figures show examples of surface irregularities in the shear reinforcement steel material of the present invention. A is flat, B is convex, and C is concave. [Diagram 3] The following shows examples of shear reinforcement (hoop-shaped) formed using the steel material of the present invention. A shows butt welding, B shows lap bending, C shows hook joint, and D shows lap winding. [Figure 4] 4 shows the cross-sectional dimensional relationship of the shear reinforcement steel material of the present invention. [Diagram 5] The figure shows the relationship between the cross-sectional aspect ratio and cross-sectional dimensions, surface area / volume (=perimeter / cross-sectional area) ratio, and cooling rate ratio relative to a circular cross-section when a circular cross-section is flattened while maintaining a constant cross-sectional area. [Figure 6] 4 shows another example of the cross-sectional shape of the shear reinforcement steel material of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The steel type used in the high-strength shear reinforcement of the present invention is basically in the same range of components as the current product, and in mass%, The basic component C is between 0.10% and 0.20%. Strengthening and hardenability auxiliary alloy (Si+Mn+Cr) is 2.0% or more and 3.5% or less. Hardenability and toughening alloy (V+Nb+Mo) is 0.05% or more and 0.50% or less, The balance is Fe and unavoidable impurities. C must be at least 0.1% for strength, and 0.2% or less is desirable for weldability. The billets of the above steel types are processed into steel materials of a desired shape by hot rolling in the same manner as reinforcing steel bars.

[0014] The shape of the high strength shear reinforcement steel material of the present invention will be described with reference to FIG. Figure 1A shows a current product, with a circular cross-sectional shape, but B1, B2, B3, and B4 of the present invention are rectangular with semicircular ends, and fall into the category of so-called flat steel. As opposed to the currently most commonly used 13mm diameter dimension, examples include 6mm x 24mm, 5mm x 28mm, and 4mm x 34mm, which have the same cross-sectional area. The effective range for the cross-sectional aspect ratio is 3 or more and 10 or less. The basis for this value will be explained later. The reason why a semicircular shape is specified is that in normal flat rolling, corners are sharpened, but since hardenable special steel is used as the material for high-strength reinforcing bars, sharp corners are inappropriate as they induce overcooling and undercooling. It is necessary to maintain uniform temperature within the cross section, so although a strict semicircular shape is not an essential condition, a semicircular shape is necessary. Flattening the cross-sectional shape while maintaining the same cross-sectional area increases the cooling rate of the steel through reduced thickness and increased surface area, inducing a similar effect to controlled cooling. High strength can be achieved by appropriately combining cooling strengthening with the hardenable alloy described above.

[0015] Figure 2 shows examples of surface irregularities in the shear reinforcement steel material of the present invention. A is flat, B is convex, and C is concave. Shear reinforcement bars are arranged as hoops around the main bars, so it is not necessarily necessary to form nodes and ribs to strengthen the bond with the concrete, as with the main bars. The surface can be flat, but there are also uses other than hoops, and in some cases unevenness is required. There is no problem with forming unevenness. Rather, the surface area is several times that of a circle, so adhesion is greatly strengthened.

[0016] Figure 3 shows examples of forming the shear reinforcement steel material of the present invention into a hoop. A is butt welding 31, B is lap bending 32, C is hook joint 33, and D is lap winding 34. Each can be easily processed, but the hook type of C and lap winding of D have never been used with conventional circular cross-section steel bars. Flattening makes this easier.

[0017] Flattening increases the width of the reinforcing bars and reduces the spacing between the reinforcing bars in the axial direction. Normally, the spacing is set at 100 mm or more, so a reduction of about 20 mm does little to impede the flow of concrete.

[0018] Figure 4 shows the dimensional relationship when flattening the cross section of a steel bar with a diameter of 13 mm, which is the main dimension. The cross-sectional area S is a constant value shown by the following formula, and the relationship between the width W and thickness H is known, and the cross-sectional aspect ratio k = W / H can also be easily calculated. S=π / 4×H 2 +(WH)×H=constant ---(1) The cooling rate of an object is proportional to the surface area / volume, i.e., the circumference / cross-sectional area. If this is the rate coefficient R, then R=(π×H+2(WH)) / S ---(2) The ratio of the cooling rate coefficient of the flattened material to the cooling rate coefficient Ro for a standard circular cross section (usually 13 mm in diameter) (=R / Ro), i.e., the cooling rate ratio v, indicates the degree of increase in cooling rate due to flattening. v=R / Ro ---(3)

[0019] Figure 5 shows the relationship between the width W, thickness H, cooling rate coefficient R, and cooling rate ratio v to that of a cylinder with respect to the increase in the cross-sectional aspect ratio k. As can be seen from the figure, when k is 3 or more, v is 1.2 or more, and an increase in the cooling rate is expected, although not sufficient, and when k = 5.5, v is 1.5, which has the same cooling effect as air jet cooling, as will be described later. When H = 3 mm and k = 15, v is 2.3, and the cooling effect is further amplified. If it is thinner than this, problems with partial supercooling due to contact with the cooling table will occur, so a k value of 15 or less is safe for cooling. On the other hand, when k=15, the width increases from 13mm to 45mm, the spacing between the reinforcing bars decreases, and problems with concrete flow occur. It is appropriate to keep the k value below 10. When this numerical relationship was calculated for other rod diameter dimensions of 10 mm and 16 mm, it was found that there was almost no difference, and the appropriate cross-sectional aspect ratio was determined to be 3 or more and 10 or less.

[0020] The cooling rate of the bar will now be described. Mathematical analysis is relatively simple if we assume that the temperature distribution within the cross section is uniform. The Biot number Bio, a dimensionless number related to heat transfer, is effective in judging uniformity. If this value is 1 or less, it is considered sufficiently uniform, and if it is 10 or more, it is considered non-uniform. Roughly speaking, it is as follows, so uniformity can be assumed. Biot number Bio = heat transfer coefficient x dimension / solid thermal conductivity ≒100(kcal / m 2 h℃)×0.01(m) / 30(kcal / mh℃)≪1

[0021] If the temperature distribution within the cross section is uniform, the cooling rate dθ / dt of the cylinder and flat plate can be calculated using the following formula. Cylinder; dθ / dt=4α(θ-θo) / cρD ---(4) Flat plate; dθ / dt=2α(θ-θo) / cρD ---(5) θ; temperature, t; time, α; heat transfer coefficient, θo; ambient temperature, c; specific heat, ρ; density, D; dimension From the above two formulas, the cooling rate will not change even if the cylinder is pressed down to halve its dimensions while keeping the cross-sectional area constant, and flattened. Unless it is at least halved, no increase in the cooling rate can be expected. In reality, side cooling is effective, and even if the temperature is halved, the cooling rate is accelerated. The evaluation methods of (2) and (3) are valid.

[0022] The required amount of hardenable alloy (V+Nb+Mo) is explained. The metal structure (mainly bainite with ferrite and pearlite mixed in) and alloy composition required for hot-rolled steel for high-strength shear reinforcement (yield stress 785 MPa or more) are well known. Naturally, the cooling rate is closely related to this. As the manufacturing method shifted from wire rod rolling + controlled cooling to bar rolling + air cooling, the cooling deficiency was compensated for by increasing the amount of alloy, which is about 0.4%. If the type of alloy to be added is a strengthened alloy (Si+Mn+Cr), the cost burden is not large, but the problem is that a large amount of hardenable alloys, mainly V, are added. The unit price of the latter is 20 to 30 times that of the former. There is also a resource risk of rare metals, so reduction is a significant issue. As mentioned above, in the present invention, a cooling rate equivalent to air blast cooling of wire can be obtained, so the amount of alloy required can be the same as that of current wire (hardenable alloy amount ≒ 0.2%). It is possible to halve the amount of alloy.

[0023] FIG. 6 shows a more desirable cross-sectional shape. Even if the thermal uniformity is high, if the cross-sectional aspect ratio is large, the cooling of the center will lag behind the end parts. This problem can be improved by making the thickness of the center smaller than the thickness (diameter) of the end parts. The desired thickness of the center is 0.6 to 0.9 times that of the end. If it is less than 0.6 times, the cooling will be excessive, and if it is more than 0.9 times, the effect will be weak. EXAMPLES

[0024] Table 1 summarizes the comparison between the commercially available product and the present invention.

[0025] [Table 1]

[0026] Comparative Example A is a wire rod, and controlled cooling is applied immediately after hot rolling. The cooling method is about twice as fast as air cooling because of the use of air blast. The amount of hardenable alloy added is about 0.2%. Comparative Example B is also a wire rod, the cooling method is immersion in boiling water, the cooling rate is increased to about 2.5 times that of air cooling, and the required amount of hardening alloy is 0.05%. Comparative Example C is a steel bar produced in an existing reinforcing bar mill, and production has been increasing rapidly recently because no special equipment is required, but because it is air-cooled, about 0.4% of a hardenable alloy is required. The present invention is air-cooled like the above, but the cooling is strengthened by the reduction in thickness and the increase in surface area, which is comparable to air blast cooling. The amount of alloy required for hardening is at the same level as Comparative Example A. Even with air cooling, it is possible to reduce the amount of alloy compared to the current method. [Industrial Applicability]

[0027] The present invention allows not only steel bar manufacturers but also flat steel manufacturers to enter the market for high strength shear reinforcement bars, where demand is growing. [Explanation of symbols]

[0028] 1;center 2;end 21;flat 22;convex section 23;concave section 31;butt weld 32;bend fastener 33;hook connection 51;end 52;center

Claims

1. A steel material for high-strength shear reinforcement, the composition of which is given by mass% as follows: C; 0.10 to 0.20%, (Si+Mn+Cr); 2.0 to 3.5%, (V+Nb+Mo); 0.10 to 0.50%, The balance is made of Fe and unavoidable impurities by hot rolling. The microstructure consists of a mixture of ferrite, pearlite and bainite. A hot-rolled steel material for high-strength shear reinforcement, characterized in that it has mechanical properties equivalent to those of a high-strength shear reinforcement steel bar having a yield strength of 785 MPa or more, a tensile strength of 930 MPa or more, an elongation of 8% or more, a bending strength of 180°C / 3D, and a diameter of 10 to 16 mm, and that its cross-sectional shape is semicircular at its end and that it has a cross-sectional aspect ratio of 3 to 10 in the form of flat steel.

2. 2. A steel material for high strength shear reinforcement according to claim 1, characterized in that the thickness at the width center is 0.6 to 0.9 times the diameter at both ends.

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