KR20090071178A - High strength galvanized wire rod for bridge cable and manufacturing method thereof - Google Patents

Abstract

A galvanized steel wire for a high intensity bridge and a manufacturing method thereof are provided to reduce the construction cost by reducing the construction period and the cable weight. A manufacturing method of a galvanized steel wire for a high intensity bridge comprises: a step of hot-rolling the wire of 11~ 14mm at the temperature range of 950~1200°C after heating a billet at 1000~1250°C; a step of continuously cooling at 3~10°C / s, from cooling start temperature of 1050~1150°C to the pearlite transformation start temperature; a step of winding firstly at a reduction rate of 5~25%; a step of winding secondly at the reduction rate of 78~85% after a LP thermal process; and a step of zinc-plating at the temperature of 450°C or greater.

Description

High Strength Galvanized Wire Rod for Bridge Cable and Manufacturing Method Thereof}

The present invention relates to a manufacturing method for manufacturing a cable for bridges with a tensile strength of 2000MPa or more, and more particularly, to the primary wire drawing before the LP heat treatment, and to the galvanizing temperature of the high strength bridge galvanized steel wire. It relates to a manufacturing method.

Conventionally, steel wire processing for bridges is performed by pickling the surface of wire diameter 12 ~ 13mm by pickling, then coating lubricant to reduce friction during drawing process, and applying fine pearlite structure (sorbite) suitable for drawing process through heat treatment. It is made by wire drawing with Zn to make steel wire 5mm in diameter to improve the corrosion resistance.

The high strength of the bridge steel wire has been going on for a long time with the extension of the bridge. The steel wire which has the highest strength among the steel wires applied as the actual bridge steel wire is the 1800MPa class steel wire applied to the Japan Myeongsegae Bridge in 1998.

As shown in Fig. 2, the method of acquiring the strength of the steel wire for a bridge is as follows. There is this.

Up to 1800MPa class steel wire has been used the most to increase the strength of wire rod state by adding alloy elements such as carbon, silicon, chromium and vanadium for the high strength. In other words, a method of reducing the drop in strength during Zn plating has been applied. If the phase transformation is controlled by controlling the alloying elements, finer lamellar layer spacing can be obtained, thereby increasing the work hardening 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 such as steel wire for bridge. In addition, increasing the amount of hardening seriously degrades 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 rod before drawing. In other words, the strength imbalance of each part of the material due to the work hardening during the drawing was intensified, and thus, the drawing was not performed properly in many cases.

Even when LP heat treatment is applied to prepare a microstructure suitable for drawing, the thicker the wire diameter, the greater the difference in cooling rate between the inside and the outside, so that the microstructure is not uniform. And there is a problem causing bad salt characteristics of the final product.

It is an object of the present invention to provide a method for producing a high strength bridge galvanized steel wire by subjecting the primary wire drawing before the LP heat treatment and limiting the zinc plating temperature.

In order to achieve the above object, the high-strength bridge galvanized steel wire of the present invention is a weight%, C: 0.90 to 1.00%, Si: 1.0 to 1.6%, Mn: 0.4 to 0.6%, Cr: 0.2 to 0.4%, S: 0.015% or less (does not contain 0%), P: 0.015% or less (does not contain 0%) and the rest contains Fe and unavoidable impurities, tensile strength is 2000MPa or more, and the fracture surface of the steel wire It is characterized by a right angle to the longitudinal direction.

Furthermore, the method for producing a high strength bridge galvanized steel wire of the present invention is a step of hot rolling in the temperature range of 950 ~ 1200 ℃ with a wire diameter of 11 ~ 14mm after heating the billet formed in the composition range to 1000 ~ 1250 ℃. ; Continuous cooling at a cooling rate of 3 to 10 ° C./s from a cooling start temperature of 1050 to 1150 ° C. to a pearlite transformation start temperature; Primary freshness with a reduction rate of 5-25%; After the LP heat treatment step of the second fresh with a reduction rate of 78 ~ 85%; And galvanizing at a temperature of 450 ° C. or higher.

According to the present invention, steel wire for bridges with a tensile strength of 2000MPa or more can be manufactured, so that it can be applied to the construction of ultra long cable bridges with a center span of 2000m or more, which can reduce the weight of the cable and shorten the construction period. It can have a significant effect on the construction cost reduction.

The present invention has been made to solve the problem that the processability is not secured due to the high strength and ultimately the salting property of the final product is not improved when the strength of the wire rod is added by adding an alloy component in the existing steel wire.

Products that do not have salt characteristics will cause delamination, which occurs along the axial direction of the wire during salt testing. The reason that the salt property is not secured in the final product is that as the strength of the wire increases, the load applied during the drawing process increases, which may cause cracks in non-uniform microstructures of the material or external defects that may be generated by drawing problems. This is because the probability increases. Therefore, if the microstructure of the material is more uniform and the amount of drawing is not reduced, it is difficult to secure the salt characteristics after drawing the high strength wire.

Conventionally, when reducing the amount of drawing, it was not applied because it is difficult to secure the strength of the final product. However, in the present invention, a small amount of drawing is applied before heat treatment to reduce the wire diameter of the wire, and thus, a faster cooling rate and a uniform microstructure through LP heat treatment. By securing the steel wire with improved strength, it is possible to reduce the amount of drawing and finally to produce a steel wire for bridges with excellent strength and salt characteristics.

Hereinafter, the reason for limitation of the composition range of this invention is demonstrated concretely.

C: 0.90 to 1.00% (hereinafter, weight%)

Carbon (C) is formed of cementite in the roughened steel wire for bridges. 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. In addition, the more homogeneous and finer the entire layered structure, the higher the strength of the wire rod. Increasing the carbon content increases the fraction of cementite and makes the lamellar spacing fine, which is very effective in increasing the strength of the wire rod. In the present invention, in order to aim at a tensile strength of 2000MPa or more, more than 0.90% of carbon was added to seek strength improvement. However, if the carbon content exceeds 1.00%, if the cooling rate is not enough, there is a problem that the workability is poor by forming the cornerstone cementite at the austenite grain boundary. Therefore, in order to increase the content of carbon in order not to impair the processability, the austenitic-pearlite phase transformation kinetics should be able to be controlled by sufficiently securing the cooling rate or adding other alloying components together.

Si: 1.0-1.6%

Silicon (Si) is known to have an effect of suppressing strength degradation by inhibiting the collapse of cementite structure during zinc plating, although it is dissolved in ferrite, a matrix structure. It is reported that the strength reduction suppression mechanism is due to the formation of Si-concentrated layers at the ferrite / cementite interface in pearlite, thereby inhibiting the collapse of cementite. In other words, the steel wire temperature during the galvanizing process rises to 400 ° C. or more. At this time, the lamellar cementite in the steel wire is granulated to nano size and coarsened, thereby decreasing strength and ductility. Silicon is concentrated on the surface of lamellar cementite and prevents coarsening of the lamellar cementite nanoparticles due to temperature increase, thereby preventing the decrease in strength and maintaining high ductility. The content of silicon is less than 1.0%. The effect is minimal when added. However, when the silicon content exceeds 1.6%, carbon is easily oxidized and removed from the surface of the material during heat treatment, which is likely to cause delamination during the drawing process.

Mn: 0.4-0.6%

Manganese (Mn) is a very useful element that forms a solid solution in a matrix to enhance the solid solution. Since Mn delays pearlite transformation, an amount of 0.4% or more is added so that fine pearlite is easily produced even at a slow cooling rate. However, when added in excess of 0.60%, the grain boundary of the tissue on the surface of the material is easily oxidized during the generation of manganese segregation and heat treatment rather than the solid solution strengthening effect, which adversely affects the product characteristics. Therefore, manganese is preferably added at 0.60% or less.

Cr: 0.2 ~ 0.4%

Chromium (Cr) refines the lamellar spacing and, like Si, suppresses the segmentation of cementite during zinc plating, thereby minimizing the strength reduction. However, Cr is expensive and there is a fear that martensite is generated during the continuous cooling process by greatly increasing the hardenability when added in excess of 0.4%. In addition, there is a fear of martensite formation when Si is also present as an element that greatly increases the hardenability.

S: 0.015% or less (not including 0%)

When sulfur (S) is added in excess of 0.015%, it is preferable to manage as low as possible because it precipitates at grain boundaries in the form of low melting point precipitates, causing hot embrittlement.

P: 0.015% or less (does not include 0%)

When phosphorus (P) is added in excess of 0.015%, it is segregated between columnar tablets to cause hot embrittlement, and it is preferable to manage it as low as possible because it causes cracking during cold working.

In addition to the above components, the remainder includes Fe and unavoidable impurities.

Hereinafter, the manufacturing method of this invention is demonstrated.

(1) hot rolling stage

Hot rolling is performed by a general method. Preferably, the billet is heated at a temperature of 1000 to 1250 ° C for 1.5 to 2.5 hours, and then a strain rate of about 10 ² / s at a temperature of 950 ° C to 1200 ° C. ) Is rolled into a wire having a diameter of 11 to 14 mm.

(2) cooling stage

Hot rolling is carried out by a general method, and preferably continuously cooled at a cooling rate of 3 to 20 ° C./s from a cooling start temperature of 1050 to 1150 ° C. to a pearlite transformation start temperature.

(3) First Fresh Stage

This is the first drawing step with reduction rate of 5 ~ 25%. In the case of drawing up to 5mm wire diameter, there is too much processing hardening, which is a problem for the salt characteristics of the final product, so that the final processing amount is reduced and the microstructure is more dense.

(4) LP heat treatment and 2nd fresh step

After the LP heat treatment, the secondary drawing is performed with a reduction ratio of 78 ~ 85% .The final drawing is large, so there is a large amount of processing, so when processing with the current microstructure, there is a risk of disconnection during drawing. This is called LP (Lead Patenting) heat treatment. In the present invention, after reheating to 1000 ℃ or more, it is put into a bath of 540 ~ 640 ℃ to make constant temperature transformation.

(5) galvanizing step

In order to give corrosion resistance to the final product, it is zinc plated at temperature above 450 ℃. Due to the too much amount of work hardening during the final drawing process, the ductility of the product becomes a problem, the zinc plating is performed at 450 ℃ or more to obtain a ductile recovery effect.

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

(Example)

By weight, a billet having a composition of C: 0.94%, Si: 1.0%, Mn: 0.5%, Cr: 0.25%, S: 0.012%, P: 0.013% is heated at a temperature of 1000 to 1250 ° C for 1.5 to 2.5 hours. Then, it is rolled with a wire diameter of 12 mm at a strain rate of about 10 ² / s at a temperature of 950 ° C to 1200 ° C. The hot rolled wire is quenched with water for about one second to the winding temperature of 850 ° C, and then wound in a coil shape. And the wound wire is cooled at a cooling rate of 3 ℃ / s ~ 20 ℃ / s.

When the wire is manufactured, the surface treatment is performed immediately after the LP heat treatment is a conventional method, in the present invention, after the surface treatment to reduce the wire diameter of the wire by applying fresh, the LP heat treatment. Table 1 compares the tensile properties measured after the LP treatment with the existing wire diameter of 12mm wire and the tensile properties when the LP treatment after fresh with 10.5, 11, 11.5mm wire diameter.

Specimen size Mechanical properties Primary freshness reduction rate (%) Tensile Strength (MPa) Cross section reduction rate (%) Wire rod Diameter 12.0mm - 1342 27 LP heat treatment material Diameter 10.5mm 23 1454 37 Diameter 11.0mm 16 1443 35 Diameter 11.5mm 8 1441 36

As shown in Table 1, even if the constant temperature transformation at the same temperature through the LP heat treatment can be seen that the finer lamellar structure is obtained because the cooling rate of the wire increases as the wire diameter becomes thinner. This fine lamellar structure increases the work hardening rate so that the strength can be secured in the final wire even though the amount of drawing is reduced.

Table 2 below shows changes in mechanical properties after drawing in the case of drawing in different wire diameters.

Change of diameter Secondary freshness reduction rate (%) Tensile Strength (MPa) Cross section reduction rate (%) Diameter 10.5mm → Diameter 4.92mm 78 2041 46 Diameter 11.0mm → Diameter 4.92mm 80 2099 41 Diameter 11.5mm → Diameter 4.92mm 82 2120 43

As shown in Table 2, it can be seen that the strength increases as the reduction rate is increased. From the reduction rate of 78% or more, the strength is higher than 2000 MPa.

Table 3 shows the tensile and salt characteristics after the final Zn plating. Delamination occurrence of the following Table 3 was evaluated by whether the fracture surface of the steel wire is perpendicular to the longitudinal direction of the steel wire. Delamination does not occur when the fracture surface is perpendicular to the longitudinal direction of the steel wire, and this is represented by 'X', and when it is not perpendicular, delamination occurs and is represented by 'O'.

division Plating temperature (℃) Wire diameter change (secondary freshness reduction rate%) Tensile Strength (MPa) Cross section reduction rate (%) Salted salt (time) Delamination occurrence Comparative Steel 1 400 or more ~ less than 450 Diameter 10.5mm → Diameter 4.92mm (78%) 2004 36 13 O Comparative Steel 2 Diameter 11.0mm → Diameter 4.92mm (80%) 2042 40 7 O Comparative Steel 3 Diameter 11.5mm → Diameter 4.92mm (82%) 2089 42 12 O Comparative Steel 4 450 or more ~ 500 or less Diameter 10.5mm → Diameter 4.92mm (78%) 1963 33 24 X Comparative Steel 5 Diameter 11.0mm → Diameter 4.92mm (80%) 1998 34 6 O Inventive Steel 1 Diameter 11.5mm → Diameter 4.92mm (82%) 2033 29 25 X

As shown in Table 3, when considering that the salt value should be generally obtained about 14 times or more, it can be seen that all of the salt value is poor and delamination occurs under the plating temperature below 450 ° C. Under the condition of 450 ° C. or higher, no salt salt defect occurred except for Comparative Steel 5. In particular, invented steel 1 is considered to be the most suitable condition for the production of steel wire for bridges, given that its tensile strength is higher than 2000 MPa, its salt value is high and its delamination does not occur at all.

1 is a view showing a comparison between the existing steel wire heat treatment and processing process for the bridge and the new heat treatment and machining process of the present invention.

Figure 2 is a view showing how the strength changes as the steel wire manufacturing process progresses for the bridge.

Claims (2)

By weight%, C: 0.90 to 1.00%, Si: 1.0 to 1.6%, Mn: 0.4 to 0.6%, Cr: 0.2 to 0.4%, S: 0.015% or less (not including 0%), P: 0.015% Galvanized steel for high strength bridges characterized by the following (not including 0%) and the remainder containing Fe and unavoidable impurities, tensile strength of 2000 MPa or more, and the fracture surface of the steel wire being perpendicular to the longitudinal direction of the steel wire. Gold wire. By weight%, C: 0.90 to 1.00%, Si: 1.0 to 1.6%, Mn: 0.4 to 0.6%, Cr: 0.2 to 0.4%, S: 0.015% or less (not including 0%), P: 0.015% (Not containing 0%) and the remainder are billets composed of Fe and unavoidable impurities. Hot-rolling at a temperature range of 950-1200 ° C. with a wire of 11-14 mm in diameter after heating to 1000-1250 ° C .; Continuous cooling at a cooling rate of 3 to 10 ° C./s from a cooling start temperature of 1050 to 1150 ° C. to a pearlite transformation start temperature; Primary freshness with a reduction rate of 5-25%; After the LP heat treatment step of the second fresh with a reduction rate of 78 ~ 85%; And Method for producing a high strength bridge galvanized steel wire, characterized in that consisting of a step of galvanizing at a temperature of 450 ℃ or more.

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