KR101262454B1 - High strength wire rod and drawn wire rod for prestressed concrete and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a method for producing wire rods and wire rods used to manufacture PC strands widely used in civil construction, such as bridges, construction, anchors, etc. The purpose of the present invention is to add austenite to high carbon steel wire rods By miniaturizing the crystal grains, it is possible to secure sufficient tensile strength in the wire state without eliminating the subsequent incubation process (LP), and by adding Cr to refine the pearlite layer spacing to improve the freshness and solid solution through the addition of Si By increasing the reinforcing effect and improving the resistance to softening due to bluing heat treatment, there is no conventional problem, and to provide a wire rod for PC stranded wire having a high strength even in a wire rod state and a method of manufacturing the same.
In order to achieve the above object, the present invention is a weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3% P: 0.015 The technical gist of the steel sheet and steel wire for high strength PC stiffness, which is composed of% or less, S: 0.015% or less, balance Fe and other unavoidable impurities, is to be considered.

Description

High-strength PC stranded wire, wire rod and manufacturing method thereof {HIGH STRENGTH WIRE ROD AND DRAWN WIRE ROD FOR PRESTRESSED CONCRETE AND METHOD FOR MANUFACTURING THEREOF}

The present invention relates to a wire rod, a wire rod and a manufacturing method thereof used to manufacture PC strands widely used in civil construction, such as bridges, construction, anchors, and more particularly, without a subsequent incubation process. The present invention relates to a wire rod for high strength PC stranded wire, a wire rod, and a method of manufacturing the same, which have sufficient tensile strength, improve drawability, and prevent a decrease in strength during bluing heat treatment.

Typically, wire rods and wire rods for PC stranded wire require adhesion to concrete and excellent tensile strength.

Conventional processing methods for PC strands having a tensile strength of 2200 MPa are as follows.

First, molten steel having a carbon concentration of about vacancy is rolled into bloom or billet, and the billet is reheated at about 1000 to 1200 ° C. for about 2 hours, and then rolled into a wire having a diameter of 10 to 14 mm at a temperature between 950 to 1200 ° C. Next, the water is cooled to 800 to 900 ° C., and then wound in a ring to cool on a stelmor cooling stand. After reheating the cooled wire to the austenitic single phase zone, it is rapidly cooled to the pearlite transformation start temperature and maintained at constant temperature for several minutes to maximize the microstructure uniformity and maximize the fineness of the pearlite layer spacing to increase the strength of the wire as much as possible. Finally, to reduce friction during drawing, the surface of the heat-treated wire is coated with a lubricant, freshly processed to freshness to the final wire diameter, and high frequency heat treatment (blueing heat treatment) is used to remove residual stress so that excellent relaxation characteristics can be obtained. do.

The method of acquiring the strength of wire rod for PC stranded wire is to increase the strength of wire rod state, to increase the strength through LP heat treatment, and to increase the work hardening rate or the amount of drawn wire.

Conventionally, in order to increase the strength of wire rods for PC strands, the carbon content is mainly increased to increase the fraction of cementite or LP heat treatment to increase the cooling rate to refine the pearlite layer spacing, thereby increasing the strength of the wire state or processing. It was dependent on the method of increasing the curing rate. However, pursuing the increase in strength by increasing the work hardening rate is effective when the processing amount such as tire cord is very large, but it is not effective when the processing amount is relatively small like PC strands. Increasing has a problem of increasing the manufacturing cost due to the addition of the process and the problem of environmental pollution due to the use of lead (Lead).

In addition, increasing the amount of fresh processing can easily achieve strength, but seriously reduce the ductility of the material causes processing problems.

Recently, due to the increase in tensile strength required for the final product, high strength is required even in the state of wire before pre-processing. Acquiring the strength of new wire rod for PC stranded wire with high strength even in the state of wire without having the above problems. There was a need.

In one aspect of the present invention, by adding V to the high carbon steel wire rod to secure sufficient tensile strength in the wire state, by adding Cr to improve the fresh workability, increase the solid-solution strengthening effect through the addition of Si and blue heat treatment To improve the resistance to softening, and does not have a conventional problem and to provide a wire rod for PC stranded wire, a wire rod and a method of manufacturing them having a high strength even in the wire state.

In the present invention, by weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3%, P: 0.015% or less, S: It provides a high strength PC ductile steel composed of 0.015% or less, balance Fe and other unavoidable impurities.

At this time, the microstructure of the wire rod is preferably 10 µm or less in size of the old austenite grains, 95% or more of pearlite, and the remainder of ferrite or cementite cementite.

In addition, the pearlite is made of 85% to 90% ferrite and 10% to 15% cementite, the layer spacing of the pearlite is preferably in the range of 150nm or less.

In the present invention, by weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3%, P: 0.015% or less, S: Heating the billet containing 0.015% or less, the balance Fe and other unavoidable impurities to 1100-1200 ° C; Cooling the heated billet to a temperature range of 900 to 1000 ° C .; Hot rolling between 900-1000 ° C., cooling and winding the hot rolled wire to 800-900 ° C., and cooling the wound wire to 350-450 ° C. at a cooling rate of 5-15 ° C./s. It provides a method of manufacturing a high strength PC stranded wire including the step.

In the present invention, by weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3%, P: 0.015% or less, S: A heating step of heating the billet containing 0.015% or less, balance Fe and other unavoidable impurities to 1100 to 1200 ° C. for 2 to 3 hours; Hot rolling step of hot rolling the heated billet at 900 ~ 1000 ℃; Winding step of cooling and winding the hot rolled wire to 800 ~ 900 ℃; High strength PC stranded wire comprising a cooling step of cooling the wound wire to 350 ~ 450 ℃ at a cooling rate of 5 ~ 15 ℃ / s and a drawing step of drawing the wire at a cross-sectional reduction rate of 10 to 30% per drawing pass Provided is a method for producing a wire rod.

In this case, the manufacturing method may further include a step of heat-processing the blue wire (bluing) for 10 to 300 seconds in the temperature range of 200 ~ 450 ℃ after the drawing step.

According to the present invention, it is possible to manufacture a high-strength PC strand with a tensile strength of 2200 MPa or more without undergoing a lead patent process, the effect of improving the price competitiveness obtained by omitting the constant heat treatment process, and lead as environmental pollutant Eco-friendly effect can be obtained by not using it. In addition, by providing a high-strength PC stranded wire compared with the prior art, when applied to large concrete structures, it is possible to reduce the weight of the structure, slim and short construction period, there is an effect that can significantly reduce the construction cost.

1 is a view confirming the difference of the microstructure obtained by the V addition in the alloy components of Comparative Examples and Invention Examples by using a scanning electron microscope (a) is a case of Comparative Example 1 (b) is a case of Inventive Example 2 It is shown.
2 is a graph showing the effect of V content on the tensile properties of wire rods.
3 is a graph showing the effect of V content on the tensile properties of the final wire.

In the present invention, even after omitting the subsequent incubation (LP) process, V is added to refine the austenite grains so as to secure sufficient tensile strength in the wire rod state, and the addition of Cr is used to improve the perforated lamellar spacing. It is made finer, and the addition of Si increases the solid-solution strengthening effect and makes it resistant to softening due to bluing heat treatment.

Hereinafter, the present invention will be described in detail.

First, the component system and composition range of the wire rod of the present invention will be described in detail.

C (carbon): 0.85-0.95 weight%

C is mostly in the form of cementite in the roughened steel wire. Cementite forms a layered pearlite together with ferrite, which is higher in strength than ferrite, so as the fraction of cementite increases, the strength of the wire increases. Increasing the content of C increases the fraction of cementite and makes the layer spacing fine, which is very effective in increasing the strength of the wire rod.

In the present invention, in order to secure a tensile strength of 2200 MPa or more, the content of C is preferably included at 0.85% by weight or more. However, when the content of C exceeds 0.95% by weight, when the cooling rate is not sufficient, there is a problem that the cornerstone cementite is generated at the austenite grain boundary, thereby lowering the freshness. Therefore, the content of C is preferably limited to 0.85-0.95% by weight.

Si (silicon): 0.8-1.6 wt%

Si is dissolved in ferrite, a matrix structure, and exhibits an effect of solid solution strengthening. In bluing heat treatment, it is effective in preventing strength degradation by preventing cementite structure collapse. In bluing heat treatment, the steel wire temperature rises above 300 ℃, and the cementite is spheroidized and coarsened. Therefore, the strength decreases. When Si is added, a Si-enriched layer is formed at the ferrite / cementite interface of the pearlite microstructure. Since the collapse of the tight is suppressed, the decrease in strength is suppressed.

When the content of Si is less than 0.8% by weight, the above effects are insignificant, and when the content of Si is more than 1.6% by weight, decarburization occurs easily on the surface of the material during reheating for incubation and the fresh workability is reduced. Since the target strength becomes difficult to obtain, it is preferable to limit the content of Si to 0.8 to 1.6% by weight.

Mn (manganese): 0.3-0.8 wt%

Mn is a very useful element that forms a solid solution in the matrix structure and solidifies it. Since it delays the pearlite transformation, it is preferable to add 0.3% by weight or more so that fine pearlite is easily formed even at a slightly slow cooling rate, and when it exceeds 0.8% by weight, Mn segregation occurs rather than a solid solution strengthening effect. Since the grain boundary of the tissue on the surface is easily oxidized and adversely affects the product properties, the Mn content is preferably limited to 0.3 to 0.8% by weight.

Cr (chrome): 0.1-0.5 wt%

Cr is finer perlite layer spacing, and incubation maximizes the miniaturization effect, the addition of Cr can effectively increase the strength of the wire even if the content of C is reduced. Similarly to Si, there is an effect of minimizing the decrease in strength by suppressing the segment of cementite during bluing heat treatment, and it is preferable to add 0.1% by weight or more in order to fully exhibit the effect. However, when the content of Cr exceeds 0.5% by weight, the hardenability is greatly increased, so that martensite may be generated during the continuous cooling process. Therefore, the content of Cr is preferably limited to 0.1 to 0.5% by weight.

V (vanadium): 0.1-0.3 wt%

When V is added, carbides precipitate during cooling, thereby suppressing austenite grain growth and maximizing grain refining effects. Therefore, when V is added, the tensile strength of the wire rod can be significantly increased without the subsequent incubation process. have. In order to fully exhibit the above effects, it is preferable to add 0.1% by weight or more, but when it is added in excess of 0.3% by weight, the hardenability is excessively increased and martensite may be generated during the continuous cooling process. It is preferable to limit it to 0.3 weight%.

S (sulfur): 0.015% by weight or less

S is an element that is inevitably contained in the manufacturing process, and it is preferable to manage it as low as possible because it precipitates at grain boundaries in the form of low melting point precipitates, causing hot embrittlement. In theory, it is possible to limit the content of S to 0%, but it is inevitable to be added in the manufacturing process, so it is important to manage the upper limit, the upper limit of the S content is preferably limited to 0.015% by weight.

P (phosphorus): 0.015% by weight or less

P is an element that is inevitably contained in the manufacturing process, and segregation between columnar tablets causes hot embrittlement, and it is preferable to manage P as low as possible because it causes cracks during cold drawing. In theory, it is possible to limit the content of P to 0%, but it is inevitable to be added in the manufacturing process, it is important to manage the upper limit, the upper limit of the content of P is preferably limited to 0.015% by weight.

The microstructure of the wire rod of the present invention has a size of the old austenite grains of 10 µm or less, 95% or more of pearlite as the main phase, and the balance of ferrite or cementite cementite. The pearlite is preferably composed of 85% to 90% ferrite and 10% to 15% cementite, which may not sufficiently secure the strength of the wire rod when the fraction of cementite in the pearlite is less than 10%. It is because there exists a problem that workability falls.

In addition, the layer spacing of the pearlite is preferably in the range of 150nm or less. This is because when the lamellar spacing of pearlite exceeds 150 nm, the strength of the wire rod and the work hardening rate are lowered, and thus the strength of the final PC strand cannot be secured due to insufficient increase in strength due to subsequent drawing.

Hereinafter, the manufacturing method of this invention is demonstrated.

The billet satisfying the above component system and composition range is heated at 1100 ~ 1200 ° C., and then the heated billet is hot rolled at 900 ~ 1000 ° C. and rolled into a wire of 10-14 mm.

Next, the hot rolled wire is cooled to 800 ~ 900 ℃ and then wound in a ring (Ring) form is preferably cooled to 350 ~ 450 ℃ at a cooling rate of 5 ~ 15 ℃ / s. If the cooling rate is cooled to less than 5 ℃ / s, the strength of the wire rod is not sufficiently secured by the growth of crystal grains, and if the cooling rate exceeds 15 ℃ / s, a low temperature structure such as martensite is generated to create a ductility of the wire rod Since the cooling rate is limited to 5 to 15 ° C./s, and cooling to 350 to 450 ° C., cooling to 350 to 450 ° C. is preferable since the transformation of the microstructure can be sufficiently completed to obtain the desired physical properties.

In the present invention, since the desired tensile strength can be obtained without the incubation process by adding V, omits the incubation process (LP), which was an essential process for manufacturing 2200MPa class PC stranded wire products, and 10 to 30% per draw pass. By applying the fresh working amount of the reduction rate, the wire can be drawn to obtain a wire having the target strength.

Additionally, in order to improve the relaxation characteristics of the final product, the drawing material may be heat treated with blue for 10 to 300 seconds in a temperature range of 200 to 450 ° C. During the final drawing process, it is possible to improve the relaxation characteristics by removing residual stress caused by too much drawing volume. The residual stress may not be sufficiently resolved at a temperature of less than 200 ° C., and the strength may be degraded by causing collapse of the freshly processed pearlite microstructure at a temperature exceeding 450 ° C., so the bluing heat treatment temperature is 200 to 450 ° C. It is limited.

Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the following examples.

Using a steel material that satisfies the component system and composition range as shown in Table 1 below, the sample was cast into a 50kg ingot, and the billet was reheated at about 1100 to 1200 ° C. for about 2 hours, and then extracted and finished with a wire having a diameter of 13 mm. Hot-rolled to ° C. Thereafter, water was cooled to 800 to 900 ° C., and then wound in a ring to blow air to 350 to 450 ° C. at a cooling rate of 5 to 15 ° C./s on a roller conveyor. Blueing heat treatment was performed at.

division C (% by weight) Si (% by weight) Mn (% by weight) Cr (% by weight) V (% by weight) Remarks Inventory 1 0.93 1.50 0.40 0.25 0.1 Inventive Example 2 0.93 1.51 0.40 0.25 0.2 Comparative Example 1 0.93 1.50 0.40 0.25 - Comparative Example 2 0.93 1.51 0.40 0.26 0.05 Comparative Example 3 0.93 1.49 0.40 0.25 0.35 Martensite formation

In the embodiment carried out as described above by measuring the austenite grain size (AGS), tensile strength (TS), cross-sectional reduction rate (RA) and tensile strength and cross-sectional reduction rate of the wire material after the wire processing to the following Table 2 and Figure 2 and 3 are shown.

In addition, scanning electron micrographs of the invention examples and comparative examples were observed and the results are shown in FIG. 1. (A) of Figure 1 is a microstructure of Comparative Example 1 (b) is a microstructure of Inventive Example 2 in the case of the microstructure of the invention example austenite grains are very fine compared to the microstructure of the comparative example due to the addition of V Able to know.

In addition, Comparative Example 1, in which V was not added, was subjected to an incubation process. This constant heat treatment reheats the wire to 950-1150 ° C, cools the reheated wire to the pearlite transformation start temperature at a cooling rate of 10-30 ° C / s, and then cools the cooled wire at the pearlite transformation start temperature for 1 to 3 minutes. After maintaining constant temperature, it proceeded to the process of air cooling.

In addition, the incubated wire rod was drawn. The tensile strength and the RA value of the wire rod and the wire rod treated with incubation as described above are shown in Table 3 below.

division Wire rod Sinseonjae AGS (μm) TS (MPa) RA (%) TS (MPa) RA (%) Inventory 1 9 1478 25 2207 43 Inventive Example 2 6 1501 22 2233 40 Comparative Example 1 32 1334 28 1936 49 Comparative Example 2 19 1426 24 2112 48 Comparative Example 3 5 1581 8 - -

division
Wire rod LP heat treatment material Sinseonjae TS (MPa) RA (%) TS (MPa) RA (%) TS (MPa) RA (%) Comparative Example 1 1334 28 1431 31 2217 47

As shown in Table 2, Figures 2 and 3 Comparative Example 1 is a V-free addition, the tensile strength of the wire rod is relatively lower than 100MPa compared to the V-added steel, the tensile strength of the wire rod after drawing did not reach 2000MPa In Comparative Example 2, the content of V was 0.05% by weight, which is lower than the content of V of the present invention, and thus the tensile strength of the wire rod was low, and the tensile strength of the wire rod after drawing was also lower than 2200 MPa. In Comparative Example 3, the content of V was 0.35% by weight, so that the hardenability was increased more than the content of V of the present invention to form martensite, and the ductility was very low, so that the fresh processing process was not applied. On the other hand, Inventive Examples 1 and 2 in accordance with the present invention can be seen that the tensile strength of the wire rod after drawing the tensile strength of 1478 ~ 1501MPa is also high as 2207 ~ 2233MPa.

Therefore, according to the V added from the above embodiment, not only the tensile strength of the final wire rod as well as the tensile strength of the wire rod before drawing was improved, and it was confirmed that the addition amount is appropriately 0.05 to 0.35% by weight.

On the other hand, as shown in Table 3, Inventive Example 1 and Inventive Example 2 not only have the required tensile strength, but also has a higher tensile strength value than the tensile strength of the incubated wire rod, even though not subjected to the same heat treatment process as Comparative Example 1 It was confirmed that the wire rod had a high strength even before the wire processing, and as a result, the tensile strength of the wire rod had a value of 2200 MPa or more intended in the present invention.

In addition, in this embodiment, the rate of cross-sectional reduction (RA) was measured, which is a measure of the change in the cross-sectional area of the tensile test specimen that was broken during the tensile test of the material. Referring to FIG. 2, in Comparative Example 1, the tensile strength is too low, while in Comparative Example 3, the tensile strength is high while the ductility is too low. On the other hand, in the invention example, the cross-sectional reduction rate does not drop so much without deteriorating the ductility of the wire rod. It can be seen that the tensile strength can be increased to the maximum. In addition, referring to FIG. 3, in the case of the invention, it was confirmed that the ductility was not greatly reduced while the tensile strength of the wire rod was 2200 MPa or more.

Claims (8)

By weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3%, P: 0.015% or less, S: 0.015% or less Wire rod for high-strength PC strands, including the balance Fe and other unavoidable impurities, the tensile strength of 1450MPa or more, and the size of the old austenite grains of 10㎛ or less.
delete The method according to claim 1,
The microstructure of the wire rod is a high-strength PC stranded wire rod composed of more than 95% pearlite and the balance ferrite or cementite cementite.
The method according to claim 3,
The pearlite is a wire rod for high strength PC stranded wire consisting of 85% to 90% ferrite and 10% to 15% cementite.
The method according to claim 3,
The layered spacing of the pearlite is 150nm or less wire rod for high strength PC strands.
By weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3%, P: 0.015% or less, S: 0.015% or less Heating the billet containing the balance Fe and other unavoidable impurities to 1100-1200 ° C .;
Hot rolling the heated billet at 900˜1000 ° C .;
Cooling and winding the hot rolled wire at 800 to 900 ° C; And
The method of manufacturing a high-strength PC stranded wire rod comprising the step of cooling the wound wire to 350 ~ 450 ℃ at a cooling rate of 5 ~ 15 ℃ / s.
By weight%, C: 0.85-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, V: 0.1-0.3%, P: 0.015% or less, S: 0.015% or less A heating step of heating the billet containing the balance Fe and other unavoidable impurities to 1100 to 1200 ° C;
Hot rolling step of hot rolling the heated billet at 900 ~ 1000 ℃;
Winding step of cooling and winding the hot rolled wire to 800 ~ 900 ℃;
Cooling step of cooling the wound wire to 350 ~ 450 ℃ at a cooling rate of 5 ~ 15 ℃ / s; And a drawing step of drawing the cooled wire at a cross-sectional reduction rate of 10 to 30% per drawing pass.
The method of claim 7,
After the drawing step, the method of producing a high-strength PC stranded wire material further comprising the step of bluing (bluing) heat treatment for 10 to 300 seconds in the temperature range of 200 ~ 450 ℃.

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