Wire rod and method for manufacturing same

Abstract

A wire rod according to the present invention comprises, in weight%, 0.10-0.30% of C, 0.05-0.30% of Si, 1.0-2.0% of Mn, 0.03-0.20% of Ti, 0.01-0.04% of Al, 0.001-0.020% of N, 0.1% or less of P, 0.1% or less of S, and the remainder of Fe and inevitable impurities, has a microstructure including ferrite in an area fraction of 60-95% and the remainder pearlite, and includes Ti-based carbide having an average size of 2-10 nm.

Description

Wire rod and method of manufacturing the same

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

[0002] Coiled rebar is a material used in the manufacture of reinforced concrete. The use of coiled rebar is inevitable due to factors such as rising labor costs resulting from economic advancement and the need to reduce construction costs.

[0003] Straight reinforcing bars, which are typically cut into lengths of 6 to 12 meters, are an inefficient shape in which the proportion of material consumed reaches 5 to 10 percent due to the reinforcing bar ends remaining during secondary processing and lap splicing.

[0004] Currently, in developed markets centered on Europe, coiled rebar utilizing automated processing equipment accounts for the majority of rebar diameters of D16 or less. In the domestic market, as it remains in a growth phase and the economy continues to shift toward a high-labor-cost structure, the utilization of coiled rebar is expected to increase rapidly.

[0005] One aspect of the present invention for solving the above-mentioned problem is to provide a wire rod and a method for manufacturing the same that can secure high strength and high ductility properties without a separate water cooling facility by controlling the alloy composition of the wire rod and controlling the heating temperature, cooling conditions, etc., to have a composite structure of ferrite and pearlite and to have Ti-based fine carbides.

[0006] The technical problems intended to be solved in this document are not limited to those mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which this invention belongs from the description below.

[0007] To achieve the above objective, a wire rod according to one embodiment of the present invention may contain, in weight percent, C: 0.10-0.30%, Si: 0.05-0.30%, Mn 1.0-2.0%, Ti: 0.03-0.20%, Al: 0.01-0.04%, N: 0.001-0.020%, P: 0.1% or less, S: 0.1% or less, and the remainder being Fe and other unavoidable impurities, and may contain a microstructure having an area fraction of 60-95% ferrite and the remainder being pearlite, and may contain Ti-based carbides with an average size of 2-10 nm.

[0008] According to one embodiment of the present invention, the number of the Ti-based carbides may be four or more per 1,000 nm × 1,000 nm area.

[0009] According to one embodiment of the present invention, the wire may have a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more.

[0010] The wire according to one embodiment of the present invention may have a different shape.

[0011] A method for manufacturing a wire rod according to one embodiment of the present invention may include the steps of: heating a billet containing, in weight percent, C: 0.10-0.30%, Si: 0.05-0.30%, Mn 1.0-2.0%, Ti: 0.03-0.20%, Al: 0.01-0.04%, N: 0.001-0.020%, P: 0.1% or less, S: 0.1% or less, and the remainder being Fe and other unavoidable impurities, at 1000-1250°C; multi-stage rolling the heated billet at 900-1000°C; winding the rolled wire rod into a ring shape at 800-950°C; and cooling the wound wire rod at a cooling rate of 0.05-1.0°C / s.

[0012] According to one embodiment of the present invention, the multi-stage rolling step may be performed sequentially in the stages of rough rolling, intermediate rolling, finishing rolling, and shape rolling.

[0013] According to one embodiment of the present invention, the wire rod after the cooling step has a microstructure comprising 60 to 95% ferrite in area fraction and the remainder pearlite, and may include Ti-based carbides with an average size of 2 to 10 nm.

[0014] According to one embodiment of the present invention, the number of the Ti-based carbides may be four or more per 1,000 nm × 1,000 nm area.

[0015] According to one embodiment of the present invention, after the cooling step, the wire rod may have a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more.

[0016] According to the present invention, by controlling the alloy composition of the wire rod and controlling the heating temperature and cooling conditions, the wire rod has a composite structure of ferrite and pearlite and has Ti-based fine carbides, thereby providing a wire rod and a method for manufacturing the same that can secure high strength and high ductility properties without a separate water cooling facility.

[0017] Furthermore, compared to straight reinforcing bars, the material consumption rate due to secondary processing can be reduced, thereby enabling the production of coil reinforcing bars that can be economically applied to concrete structures. This provides a wire rod and a method for manufacturing the same.

[0018] The effects obtainable from the present invention are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

[0019] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0020] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0021] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense. For instance, singular expressions in this specification include plural expressions unless the context clearly indicates an exception.

[0022] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0023] The present invention relates to a wire rod that can be applied to concrete structures used in various construction sites, and by controlling the alloy composition and manufacturing process of the wire rod, it is possible to provide a wire rod with excellent strength and ductility without a tempcore process using a separate water cooling facility.

[0024] A wire according to one embodiment of the present invention will be described in detail below.

[0025] A wire rod according to one embodiment of the present invention comprises, in weight%, C: 0.10-0.30%, Si: 0.05-0.30%, Mn 1.0-2.0%, Ti: 0.03-0.20%, Al: 0.01-0.04%, N: 0.001-0.020%, P: 0.1% or less, S: 0.1% or less, and the remainder being Fe and other unavoidable impurities.

[0026] The reasons for limiting the compositional range of each alloying element are described below. Unless otherwise noted, units are weight percent.

[0027] The content of C can be 0.10~0.30%.

[0028] C is an element that must be added to ensure the strength of the steel. If the C content is less than 0.10%, the target strength of the present invention cannot be achieved, and if it exceeds 0.3%, the tensile strength increases excessively during cooling, which may reduce ductility or bendability. Therefore, it is desirable to maintain the C content at 0.10~0.30%, and more preferably at 0.12~0.29%.

[0029] The Si content can be 0.05~0.30%.

[0030] Si plays a role necessary for the deoxidation of steel, and at the same time, as a solid solution strengthening element, it can also be effective in improving yield strength. To effectively obtain these effects, it is desirable to add at least 0.05% of Si. However, if the Si content exceeds 0.30%, ductility and bendability may decrease. Therefore, it is desirable to maintain the Si content at 0.05~0.30%, and more preferably at 0.06~0.29%.

[0031] The Mn content can be 1.0~2.0%.

[0032] Mn is an element that strengthens steel by forming a substitutional solid solution within the matrix structure, and is a very useful element for improving the strength of steel. When the Mn content is less than 1.0%, there is almost no effect from Mn segregation, but it may be difficult to expect the effect of improving steel strength through solid solution strengthening. When it exceeds 2.0%, Mn segregation occurs rather than solid solution strengthening, which may have a detrimental effect on product characteristics. Therefore, it is desirable to maintain the Mn content at 1.0~2.0%, and more preferably at 1.2~1.9%.

[0033] The Ti content can be 0.03~0.20%.

[0034] Ti is an element added to suppress precipitation strengthening and grain coarsening. If the Ti content is less than 0.03%, it may be difficult to obtain the high strength targeted in the present invention, and if it exceeds 0.20%, coarse carbides are formed, which reduces ductility and may cause cracks during bending. Therefore, it is desirable to maintain the Ti content at 0.03~0.20%, more preferably at 0.05~0.20%, and most preferably at 0.12~0.18%.

[0035] The Al content may be 0.01~0.04%.

[0036] Al is widely used as a deoxidizer in the steelmaking process and is also effective in refining austenite grains through AlN formed by reacting with N. If the N content is less than 0.01%, the deoxidation effect is insufficient, and an excessive amount of impurities may remain in the steel. If it exceeds 0.04%, the formation of hard non-metallic inclusions such as alumina is excessive, which may exacerbate defects in the steel. Therefore, it is desirable to maintain the Al content at 0.01~0.04%, and more preferably at 0.02~0.03%.

[0037] The content of N can be 0.001~0.020%.

[0038] N is an element that is widely used instead of expensive alloying elements because it is effective in refining austenite grains through AlN formed by reacting with Al. If the N content is less than 0.00%, the number of nitrogen compounds is insufficient, which may reduce the austenite grain refining effect. If it exceeds 0.020%, the forging heat generated during cold forging causes the movement and proliferation of dislocations within the material, and free nitrogen becomes fixed to the dislocations, increasing deformation strength and potentially reducing die life.

[0039] The content of P may be 0.1% or less.

[0040] P is an impurity that can segregate at grain boundaries and become a major cause of hot embrittlement or reduced ductility, so it is desirable to control its content as low as possible. Theoretically, it is possible to limit the P content to 0%, but it is inevitably added during the manufacturing process. Therefore, it is important to manage the upper limit of P, and it is desirable to maintain the upper limit of P at 0.1%.

[0041] The S content may be 0.1% or less.

[0042] S is an impurity element and a low-melting-point element that precipitates at grain boundaries and causes hot embrittlement, so it is desirable to control its content as low as possible. Theoretically, it is possible to limit the S content to 0%, but it is inevitably added during the manufacturing process. Therefore, it is important to manage the upper limit of S, and it is desirable to maintain the upper limit of S at 0.1%.

[0043] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0044] The microstructure of a wire rod according to one embodiment of the present invention, which includes the alloy composition as described above, has a composite structure comprising 60 to 95% of ferrite in area fraction and the remainder being pearlite, and may include Ti-based carbides with an average size of 2 to 10 nm.

[0045] The above Ti-based carbides may be TiN, TiC, Ti(C,N), etc., and may be fine carbides with an average size of about 2 to 10 nm. If the average size of the carbides is less than 2 nm, the precipitation strengthening effect is small and the strength may be insufficient, and if it exceeds 10 nm, the precipitation strengthening effect is weakened due to coarse precipitates and the strength may be insufficient.

[0046] In addition, the number of the above Ti-based carbides may be four or more per 1,000 nm × 1,000 nm area. If the number of the above carbides is less than four, strength and ductility may be insufficient.

[0047] Here, the average size of the carbides refers to the average value of the size for each of the multiple unit carbides, and the number of carbides refers to the average value of the number of carbides present in a given unit carbide based on a cross-section of the unit carbide in a specific direction.

[0048] The wire rod of the present invention can secure high strength and high ductility without the application of rapid cooling water cooling equipment by appropriately controlling the composite structure of ferrite and pearlite and fine carbides as described above, and can significantly reduce the material consumption rate during secondary processing compared to straight reinforcing bars.

[0049] A wire rod according to one embodiment of the present invention may have a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more.

[0050] In addition, the wire according to one embodiment of the present invention may have a different shape, preferably a spiral shape in which spiral ribs are formed to wrap around the surface of the reinforcing bar, and such a shape may increase adhesion with concrete.

[0051] Next, a method for manufacturing a wire rod according to one embodiment of the present invention will be described.

[0052] A method for manufacturing a wire rod according to one embodiment of the present invention comprises the steps of: heating a billet containing, in weight percent, C: 0.10-0.30%, Si: 0.05-0.30%, Mn 1.0-2.0%, Ti: 0.03-0.20%, Al: 0.01-0.04%, N: 0.001-0.020%, P: 0.1% or less, S: 0.1% or less, and the remainder being Fe and other unavoidable impurities, at 1000-1250°C; multi-stage rolling the heated billet at 900-1000°C; winding the rolled wire rod into a ring shape at 800-950°C; and cooling the wound wire rod at a cooling rate of 0.05-1.0°C / s.

[0053] The reason for limiting the component range of each alloy composition above may be the same as described above, and each manufacturing step will be explained in more detail below.

[0054] First, a billet having the compositional components described above can be heated at 1000 to 1200°C for 1 to 3 hours.

[0055] When a billet satisfying the component system and composition range controlled in the present invention is heated within the above heating temperature range, the austenite single phase is maintained, the austenite crystal grains are not coarsened, and the remaining segregation, carbides, and inclusions can be effectively dissolved.

[0056] If the above heating temperature is less than 1000℃, sufficient heat cannot be transferred to re-dissolve segregation, carbides, etc., even by heating, and if it exceeds 1200℃, decarburization occurs on the surface of the billet and the austenite grains become very coarse, and after cooling, the ferrite grain size in the microstructure also becomes coarse, which may reduce ductility. Therefore, it is desirable to maintain the above heating temperature at 1000~1200℃.

[0057] In addition, if the heating time is less than 1 hour, the temperature of the billet to be heated cannot be uniform, and if it exceeds 3 hours, excessive decarburization may occur. Therefore, it is desirable to maintain the heating time at 1 to 3 hours.

[0058] Subsequently, the heated billet may undergo a multi-stage rolling step.

[0059] For example, it is preferable to roll the obtained billet in multiple stages at 900~1000℃ using rough rolling, intermediate rolling, and finish rolling to achieve a desired thickness and width, and then shape-roll it into a rebar shape at the final roll.

[0060] The shape of the above reinforcing bar may be a circular reinforcing bar with a circular cross-section, a deformed reinforcing bar with nodes in the cross-section, etc., depending on the shape of the cross-section, and preferably a deformed reinforcing bar. In addition, the above deformed reinforcing bar may be spiral, grid-shaped, transverse rib-shaped, protruding-shaped, two-row rib-shaped, three-row rib-shaped, etc., and preferably a spiral in which spiral-shaped ribs are formed to wrap around the surface of the reinforcing bar, thereby increasing the adhesion with concrete in a continuous spiral shape.

[0061] The above rolled wire can be lowered to a temperature of 800 to 950°C and then wound into a ring shape.

[0062] If the above winding temperature is less than 800℃, the strength of the material increases, making winding difficult, and if it exceeds 950℃, the microstructure becomes coarsened, making it difficult to obtain the desired mechanical properties.

[0063] After the above winding, a cooling step is performed.

[0064] It is best to proceed with cooling as slowly as possible to maintain strength variation within the coil.

[0065] If the above cooling rate is less than 0.05℃ / s, it may cause grain growth, making it difficult to obtain the desired strength, and if it exceeds 1.0℃ / s, the variation in tensile strength within the coil may increase, and it may be difficult to secure the desired ductility and bendability.

[0066] Therefore, it is preferable that the above cooling be performed at a rate of 0.05 to 1.0℃ / s, and more preferably at a rate of 0.06 to 0.91℃ / s.

[0067] During the cooling process after the above rolling, proeutectoid ferrite is first formed in the austenite with a hypoeutectoid composition, and then the residual austenite is transformed into a pearlite structure consisting of cementite and ferrite.

[0068] Consequently, the microstructure of the wire rod of the present invention has a composite structure consisting of ferrite with an area fraction of 60 to 95% and the remainder being pearlite, and may contain four or more Ti-based carbides with an average size of 2 to 10 nm per 1,000 nm × 1,000 nm area.

[0069] In addition, the wire of the present invention can secure mechanical properties such as a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more.

[0070] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto.

[0071] Examples

[0072] Steel with the alloy composition shown in Table 1 below was used as a sample, cast into a billet, reheated at 1000–1250°C for 2 hours, extracted, and hot-rolled into a wire rod with a diameter of D13. At this time, the finishing hot-rolling temperature was 900–1000°C, and then the wire rod was manufactured by winding it into a ring shape down to 850–950°C and cooling it at a cooling rate of 0.05–1.0°C / s.

[0073] Classification CSiMnTiAlN Cooling Rate (°C / s) Example 10.1 20.1 51.9 0.1 80.0 30.0 40.0 6 Example 20.29 0.0 61.2 0.1 20.0 30.0 40.5 6 Example 30.25 0.1 21.5 0.1 20.0 30.0 50.1 2 Example 40.2 20.29 1.6 0.1 40.0 20.0 40.1 5 Example 50.28 0.1 81.4 0.1 30.0 20.0 40.9 1 Comparative Example 10.09 0.1 51.8 0.1 90.0 30.0 40.0 6 Comparative Example 20.320.08 1.30.11 0.030.0050.51 Comparative Example 30.29 0.18 1.50.02 0.040.0040.88 Comparative Example 40.25 0.13 1.50.25 0.030.0040.12 Comparative Example 50.21 0.19 1.40.15 0.030.0042.1

[0074] The yield strength, tensile strength, elongation, average size of Ti-based carbides, and number of carbides of the wire rod manufactured above were measured, and the results are shown in Table 2 below.

[0075] Yield strength was measured according to the tensile test method for metal materials of the Korean Industrial Standard KS D 3504.

[0076] Tensile strength was measured according to the tensile test method for metal materials of the Korean Industrial Standard KS D 3504.

[0077] The elongation was measured according to the tensile test method for metal materials of the Korean Industrial Standard KS D 3504.

[0078] The average size of Ti-based carbides was determined by preparing a thin foil sample with a thickness of 50㎛ or less and measuring the number of carbides per unit area using a TEM instrument.

[0079] Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Average Size of Ti-based Carbides (nm) Number of Carbides (per 1,000 nm × 1,000 nm area) Example 1 5 1 2 5 5 4 1 6 4.5 6.3 Example 2 5 6 2 6 0 7 1 3 8.8 4.2 Example 3 5 3 5 5 8 6 1 4 5.1 6.1 Example 4 5 2 8 5 8 2 1 4 4.8 5.2 Example 5 5 5 1 5 9 3 1 3 6.9 4.5 Comparative Example 1 4 9 7 5 4 5 1 7 4.2 3.6 Comparative Example 2 5 7 5 6 5 6 1 1 8.9 4.2 Comparative Example 3 4 9 2 6 5 5 1 3 7.8 1.5 Comparative Example 4 5 2 6 5 8 2 9 5.6 4.3 Comparative Example 5 4 5 6 5 7 9 1 6 4.6 0.9

[0080] As shown in Table 2 above, Comparative Example 1 failed to produce sufficient strength due to an insufficient C content, while Comparative Example 2 satisfied the strength but showed relatively insufficient elongation due to an excessive C content. Additionally, Comparative Example 3 failed to produce sufficient strength due to a lack of Ti-based carbide water caused by an insufficient Ti content, and Comparative Example 4 showed inferior elongation due to the formation of coarse nitrides caused by an excessive Ti content. Meanwhile, in the case of Comparative Example 5, although it satisfied the alloy composition of the present invention, it was confirmed that the yield strength was insufficient because the cooling rate was too fast and a sufficient amount of Ti-based carbide water was not secured.

[0081] Meanwhile, in the case of Examples 1 to 5 satisfying the alloy composition, heating temperature, and cooling rate according to the present invention, it was confirmed that excellent mechanical properties were exhibited, satisfying a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more, by including at least 4 Ti-based carbides with an average size of 2 to 10 nm per 1,000 nm × 1,000 nm area.

[0082] Although embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be made by those skilled in the art to which the disclosed invention belongs without departing from the essence claimed in the claims.

Claims

1. In wt%, C: 0.10-0.30%, Si: 0.05-0.30%, Mn 1.0-2.0%, Ti: 0.03-0.20%, Al: 0.01-0.04%, N: 0.001-0.020%, P: 0.1% or less, S: 0.1% or less, and the remainder being Fe and other unavoidable impurities, Having a microstructure containing 60–95% area fraction of ferrite and the remainder of pearlite, Wire rod containing Ti-based carbides with an average size of 2 to 10 nm.

2. In Paragraph 1, A wire rod having four or more of the above-mentioned Ti-based carbides per 1,000 nm × 1,000 nm area.

3. In Paragraph 1, The above wire rod has a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more.

4. In Paragraph 1, The above wire is a wire with a different shape.

5. A step of heating a billet containing, in wt%, C: 0.10-0.30%, Si: 0.05-0.30%, Mn 1.0-2.0%, Ti: 0.03-0.20%, Al: 0.01-0.04%, N: 0.001-0.020%, P: 0.1% or less, S: 0.1% or less, and the remainder being Fe and other unavoidable impurities, at 1000-1250℃; A step of multi-stage rolling the above heated billet at 900~1000℃; A step of winding the above-mentioned rolled wire into a ring shape at 800~950℃; and A step of cooling the above-mentioned wound wire at a cooling rate of 0.05~1.0℃ / s; A method for manufacturing a wire rod including 6. In Paragraph 5, A method for manufacturing wire rods in which the above multi-stage rolling steps are performed sequentially in the stages of rough rolling, intermediate rolling, finishing rolling, and shape rolling.

7. In Paragraph 5, A method for manufacturing a wire rod having a microstructure after the cooling step comprising 60-95% ferrite in area and the remainder pearlite, and comprising Ti-based carbides with an average size of 2-10 nm.

8. In Paragraph 7, A method for manufacturing a wire rod having four or more of the above-mentioned Ti-based carbides per 1,000 nm × 1,000 nm area.

9. In Paragraph 5, A method for manufacturing a wire rod having a yield strength of 500 MPa or more, a tensile strength of 550 MPa or more, and an elongation of 12% or more after the cooling step above.