KR20110014853A - High strength wire rod for prestressed concrete stranded wire and prestressed concrete stranded wire and manufacturing method thereof - Google Patents

Abstract

PURPOSE: A high-strength wire rod for a Prestressed Concrete strand, a PC strand, and a manufacturing method thereof are provided to improve drawability by making pearlite interlamellar spacing minute through isothermal heat treatment. CONSTITUTION: A high-strength wire rod for a Prestressed Concrete strand comprises C 0.80~0.95 weight%, Si 0.8~1.6 weight%, Mn 0.3~0.8 weight%, Cr 0.1~0.5 weight%, P less than 0.015 weight%, residual Fe, and inevitable impurities. A modified structure is composed of pearlite more than 95% and ferrite or proeutectoid cementite.

Description

High strength wire rod for prestressed concrete stranded wire and prestressed concrete stranded wire and manufacturing method

The present invention relates to a wire rod for high strength PC strands and a method for manufacturing the same, and in detail, to refine the pearlite layer spacing to improve the drawability and to prevent the decrease in strength during bluing heat treatment, the wire rod for high strength PC strands, PC strands and its manufacture It is about a method.

PC stranded wire is a steel used as tensioning material in prestressed concrete. It is manufactured by stranding PC steel wire and should have excellent adhesion and tensile strength. Brief description of the method for processing PC strands is as follows. Molten steel with a carbon concentration of vacancy composition 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 desired diameter. At this time, the finish rolling temperature is between 950 ~ 1200 ℃. After cooling to 800 ~ 900 ℃ and then wound in the form of a ring (ring) to cool on the stelmor cooling stand. After pickling the surface of 10 ~ 14mm wire rod, it is coated with lubricant to reduce friction during drawing process, and it is drawn to the final wire diameter through drawing process, and it shows excellent relaxation characteristics by removing residual stress through high frequency heat treatment (blueing heat treatment). To do that.

The method of acquiring the strength of the PC strand includes the method of increasing the strength of the wire rod state, the method of increasing the work hardening rate or the amount of drawing, and the method of suppressing the decrease in strength during the bluing heat treatment.

Conventionally, in order to increase the strength of PC strands, a method of increasing the carbon content to increase the fraction of cementite or increasing the cooling rate to refine the pearlite layer spacing, thereby increasing the strength of the wire state or increasing the work hardening rate. Relied on.

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 such as PC strand.

In addition, increasing the amount of fresh processing seriously lowers the ductility of the material, so strength can be easily achieved, but it causes processing problems.

In addition, as the tensile strength required for the final product increases, high strength is required even in the state of wire rods before drawing. In other words, the strength imbalance for each part of the material due to work hardening during the fresh processing is intensified and the fresh processing is often not properly performed, which causes a problem of poor quality of the final product.

The present invention improves the freshness by adding Cr to the high carbon steel wire and minimizing the pearlite layer spacing through constant temperature heat treatment, and increasing the solid-solution strengthening effect and the resistance to softening by bluing heat treatment through the addition of Si. To provide a wire rod for PC stranded wire and a method of manufacturing the same. In addition, it is intended to provide a fresh PC strand and a manufacturing method using the wire.

In one embodiment, the present invention provides, in weight percent, C: 0.80-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, P: 0.015% or less, S: 0.015% Hereinafter, the balance Fe and other unavoidable impurities, the microstructure relates to the wire rod for PC strands consisting of more than 95% of pearlite and the balance of ferrite or cementite cementite.

The pearlite is preferably composed 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 addition, the present invention relates to a high-strength PC strand, characterized in that the tensile strength of 2200 MPa or more as a PC strand produced using the wire rod as another embodiment.

In addition, the present invention, as another embodiment, in weight%, C: 0.80-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, P: 0.015% or less, S: A billet containing 0.015% or less, balance Fe and other unavoidable impurities is heated to 1000 to 1200 ° C., and the heated billet is hot rolled at 950 to 1200 ° C., and then the hot rolled wire is cooled to 800 to 900 ° C. and wound up. Then, the present invention relates to a method for manufacturing a high strength PC stranded wire rod comprising the step of cooling the wound wire to 350 ~ 450 ℃ at a cooling rate of 3 ~ 15 ℃ / s.

Heating of the billet is preferably carried out for 2 to 3 hours.

In addition, the present invention, as another embodiment, in weight%, C: 0.80-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, P: 0.015% or less, S: A billet containing 0.015% or less, balance Fe and other unavoidable impurities is heated to 1000-1200 ° C. and the heated billet is hot rolled at 950-1200 ° C., and the hot rolled wire is cooled to 800-900 ° C. and wound up. After cooling the wound wire to 350 ~ 450 ℃ at a cooling rate of 3 ~ 15 ℃ / s and reheating the cooled wire to 950 ~ 1150 ℃ and cooling the reheated wire 10 ~ 30 ℃ / s After cooling to the furnace pearlite transformation start temperature, the cooled wire rod is kept at a constant temperature for 1 to 3 minutes at the pearlite transformation start temperature, the wire rods kept at constant temperature are drawn, and the wire rod is stranded and blue at 200 to 450 ° C. It relates to a method for producing a high strength PC strand, comprising the step of heat treatment.

According to the present invention, it is possible to provide a wire rod with excellent drawing property, work hardening rate, and tensile strength, and by using the wire rod, it is possible to manufacture PC strands with a tensile strength of 2200 MPa or more. It is possible to lighten the structure, slim down the structure and shorten the construction period, thereby greatly reducing the construction cost.

In the present invention, by applying the constant temperature heat treatment to ensure the uniformity of the microstructure as much as possible, it is possible to maximize the miniaturization of the pearlite layer spacing through the constant temperature heat treatment by adding Cr. In addition, by increasing the content of Si may increase the solid-solution strengthening effect and have a resistance to softening due to the bluing heat treatment.

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

C (carbon): 0.80-0.95 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 0.80% 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, in order to increase the content of C in order not to impair the processability, it is preferable to secure the cooling rate sufficiently or to control the phase transformation by adding other alloy component systems together.

Si (silicon): 0.8-1.6 wt%

Si may be dissolved in ferrite, which is a base structure, to provide an effect of solid solution strengthening. However, Si is also effective in preventing strength degradation by preventing cementite structure collapse during bluing heat treatment. During bluing heat treatment, the steel wire temperature is raised to 300 ℃ or more. At this time, the cementite is spheroidized and coarsened to decrease the strength.Si addition of Si forms a concentrated layer of Si at the ferrite / cementite interface of the pearlite microstructure. The collapse of can be suppressed.

When the content of Si is less than 0.8% by weight, the above effect is insignificant. In addition, when the content of Si exceeds 1.6% by weight, decarburization is easily generated on the surface of the material during reheating for incubation and fresh workability is reduced, making it difficult to obtain the strength desired by the present invention.

Figure 1 shows the effect of the Si content change on the tensile strength of the fresh material at each bluing heat treatment temperature. When heat-treated at 200 to 500 ° C. for 60 seconds, the raw material including Si 1.3 to 1.6% by weight increased in strength up to 300 ° C. and began to drop in strength from 400 ° C., but the strength was maintained at 2200 MPa or more. When the heat treatment at 500 ℃ it can be seen that the strength drops sharply.

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 produced even at a slightly slow cooling rate. However, when it exceeds 0.8% by weight, Mn segregation occurs rather than a solid solution strengthening effect, and the grain boundary of the tissue on the surface of the material is easily oxidized during heat treatment, which adversely affects the product characteristics.

Cr (chrome): 0.1-0.5 wt%

Cr is finer perlite layer spacing, and the miniaturization effect is maximized by the constant temperature heat treatment to reduce the content of C can effectively increase the strength of the wire. Similarly to Si, there is an effect of minimizing the strength reduction 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, there is a concern that the hardenability during addition is greatly increased to produce martensite during the continuous cooling process.

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 control it as low as possible because it precipitates at the grain boundary in the form of low melting point precipitates, causing hot embrittlement. In theory, it is possible to limit the content of S to 0%. It is inevitably added. It is important to manage the upper limit, the upper limit of the content of S 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 manufacturing, and segregates between columnar tablets, causing hot embrittlement, and causing cracks during cold drawing, so it is desirable to manage P as low as possible, and theoretically limit P content to 0%. It is possible, but inevitably 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.

In the microstructure of the wire rod of the present invention, it is preferable that pearlite is a main phase, 95% or more in terms of phase fraction, and the remainder is ferrite or cementite cementite. The pearlite is preferably made of 85 to 90% ferrite and 10 to 15% of cementite. If the cementite fraction of pearlite is less than 10%, the strength of the wire rod may not be sufficiently secured. There is a problem of this deterioration.

In addition, the layer spacing of the pearlite is preferably in the range of 150nm or less. When the lamellar spacing of pearlite exceeds 150 nm, the strength of the wire rod and the work hardening rate are lowered, so that the increase in strength due to the subsequent drawing is not sufficient, thereby making it difficult to secure the strength of the final PC strand.

Hereinafter, the manufacturing method of this invention is demonstrated.

Hot rolling stage

The billet which satisfies the above component system and composition range is heated to 1000 to 1200 ° C, and the heating time is preferably 2 to 3 hours. After the heated billet is preferably hot-rolled at 950 ~ 1200 ℃. It is preferable to roll with a wire of 10 ~ 14mm through the hot rolling step.

Cooling and winding stage

The hot rolled wire is preferably cooled to 800 ~ 900 ℃, wound in a ring form and cooled to 350 ~ 450 ℃ at a cooling rate of 3 ~ 15 ℃ / s.

Reheating stage

After reheating the cooled wire to 950 ~ 1150 ℃ it is preferable to cool to the pearlite transformation start temperature at a cooling rate of 10 ~ 30 ℃ / s.

Incubation step

Preferably, the cooled wire is kept at a constant temperature for 1 to 3 minutes at a pearlite transformation start temperature. Through this, it is possible to refine the pearlite layer spacing and obtain the target strength, and to suppress the variation of each part of the wire rod as much as possible. If the constant temperature holding time is less than 1 minute, the above effects are insignificant. If the constant temperature is longer than 3 minutes, the microstructure of the wire rod tends to coarsen. Moreover, in this invention, it is preferable to put in a bath of 560-640 degreeC, and to make constant temperature transformation (LP heat treatment).

Fresh stage

As the step of drawing the wire rod that is kept at a constant temperature, it is preferable to draw the wire by an appropriate pass schedule according to the characteristics of the wire rod to be targeted.

Blueing Heat Treatment Step

It is preferable to perform a high-frequency heat treatment of the drawing material at 200-450 ° C. for bluing, and to improve relaxation characteristics of the final product for 10-300 seconds in a temperature range of 200-450 ° C. During the final drawing process, it is possible to improve the relaxation characteristics by removing residual stress caused by too much drawing processing. Residual stress may not be sufficiently resolved at a temperature below 200 ° C., and strength may be degraded at a temperature above 450 ° C. resulting in the collapse of freshly processed pearlite microstructure. Therefore, the bluing heat treatment temperature is preferably limited to 200 ~ 450 ℃.

Hereinafter, the present invention will be described in detail through examples.

(Example)

Using a steel material that satisfies the component system and composition range as shown in Table 1 below, the sample is cast into a 50kg ingot, welded to a billet, reheated for about 2 hours at 1000 to 1200 ° C, and then extracted and hot rolled with a wire having a diameter of 11 mm. It was. At this time, the finish hot rolling temperature was 1000 ° C., after which water was cooled to 800 ° C. to 900 ° C., and then wound up in a ring to blow air on a roller conveyor at a cooling rate of 3 ° C. to 15 ° C./s. After incubation for 1 minute and 30 seconds at 580 ℃, fresh to 4.25mm wire diameter and then bluing heat treatment at 250 ℃.

Tensile strength (TS) and cross-sectional reduction rate (RA) of the wire rods before and after incubation were measured and the tensile strength and cross-section reduction ratio of the wire rods after drawing were measured and shown in Table 2 below.

division C (% by weight) Si (% by weight) Mn (% by weight) Cr (% by weight) Remarks Inventive Example 1 0.91 1.29 0.50 0.31 Inventive Example 2 0.89 1.58 0.48 0.31 Comparative Example 1 0.90 0.62 0.51 0.30 Comparative Example 2 0.89 1.30 0.51 - Comparative Example 3 0.90 1.30 0.49 0.8 Martensite formation

division Wire rod before incubation Wire rod after incubation Sinseonjae TS (MPa) RA (%) TS (MPa) RA (%) TS (MPa) RA (%) Inventive Example 1 1335 25 1474 31 2268 48 Inventive Example 2 1378 21 1493 28 2305 46 Comparative Example 1 1257 28 1319 35 2085 54 Comparative Example 2 1245 27 1310 31 1976 53

In Comparative Example 1, the content of Si was 0.62% by weight, and thus the present invention could not prevent the strength from lowering due to the content of Si intended to be lowered. Therefore, the tensile strength of the wire rod was measured to be low. It was measured below.

Comparative Example 2 did not contain Cr, so the strength improvement effect of the wire rod was insignificant, and the tensile strength of the wire rod was confirmed to be lower than that of the invention example.

In Comparative Example 3, since the content of Cr is greater than the content of Cr intended for the present invention, the hardenability is increased to form martensite.

Inventive examples 1 and 2 satisfy the component system and composition range controlled in the present invention, it can be seen that the tensile strength and the reduction ratio of the wire rod correspond to the intended purpose of the present invention.

1 is a graph showing the tensile strength of the bluing heat treatment temperature according to the content of Si.

Claims (7)

By weight, C: 0.80-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, P: 0.015% or less, S: 0.015% or less, balance Fe and other unavoidable impurities Including, the microstructure of the pearlite 95% or more and the remainder of ferrite or cementite cemented wire rod for high strength PC strands. The wire rod of claim 1, wherein the pearlite comprises 85 to 90% of ferrite and 10 to 15% of cementite. The wire rod for high-strength PC strand according to claim 1, wherein the layered spacing of pearlite is in a range of 150 nm or less. PC strand produced by using the wire of any one of claims 1 to 3, the high strength PC strand is characterized by a tensile strength of 2200MPa or more. By weight, C: 0.80-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, P: 0.015% or less, S: 0.015% or less, balance Fe and other unavoidable impurities Heating the billet comprising 1000 to 1200 ° C; Hot rolling the heated billet at 950˜1200 ° 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 3 ~ 15 ℃ / s. 6. The method of claim 5, wherein the heating of the billet in the heating step is carried out for 2 to 3 hours. By weight, C: 0.80-0.95%, Si: 0.8-1.6%, Mn: 0.3-0.8%, Cr: 0.1-0.5%, P: 0.015% or less, S: 0.015% or less, balance Fe and other unavoidable impurities Heating the billet comprising 1000 to 1200 ° C; Hot rolling the heated billet at 950˜1200 ° C .; Cooling and winding the hot rolled wire at 800 to 900 ° C; Cooling the wound wire to 350 to 450 ° C. at a cooling rate of 3 to 15 ° C./s; Reheating the cooled wire at 950-1150 ° C .; Cooling the reheated wire rod to a pearlite transformation start temperature at a cooling rate of 10 to 30 ° C./s; Maintaining the cooled wire at constant temperature for 1 to 3 minutes at a pearlite transformation start temperature; Drawing the wire rod that is kept at a constant temperature; And The method of manufacturing a high-strength PC stranded wire comprising the step of bluing (heating) the wire to the stranded wire and 200 ~ 450 ℃.

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