KR20050087680A - Steel bar for prestressed concrete with improved fracture elongation of spot welded part - Google Patents

Abstract

The present invention relates to an improvement in elongation at break for spot welds of steel bars for high strength prestressed concrete, which is used as the tensile roots of high strength concrete piles requiring extremely low high temperature relaxation losses.

PC steel bar of the present invention, C 0.25 ~ 0.40wt%, Si 0.8 ~ 1.5wt%, Mn 0.60 ~ 2.0wt%, P + S 0.04wt% or less is contained as a basic component of the material, here Ti 0.01 ~ 0.05wt %, B 0.0005 ~ 0.0050wt%, Al 0.01 ~ 0.05wt% or Ti 0.01 ~ 0.05wt%, B 0.0005 ~ 0.0050wt%, 0.01 ~ 0.05wt% Al and 0.02 ~ 0.30wt% Cr And an unavoidable impurity, wherein the material steel wire having such a composition is continuously driven and heated to Ac 3 or more to quench and temper to obtain tensile strength of 1420 MPa or more and high temperature relactation loss of 15% or less. It is characterized by the elongation at break including spot welds being 5% or more as steel bars for prestressed concrete shown.

The steel of the invention maintains the relamination characteristics at high temperatures as compared to conventional steels, while the elongation at break of the spot weld is much improved, and the bending external force applied to the stretching process or pile to produce the PHC pile There is an advantage that there is little possibility of breaking from the back, there is an advantage that can produce a PC steel bar with ductility of the excellent spot welds.

Description

Steel bar for prestressed concrete with improved fracture elongation of spot welded part}

The present invention is a spot welded portion of a steel rod or steel wire (hereinafter referred to as a PC steel rod) for high strength prestressed concrete used as a tensile muscle of a high strength concrete pile (hereinafter referred to as PHC pile) requiring extremely low high temperature relaxation loss. It is about improving elongation at break for.

Various methods of manufacturing existing PHC piles are known. Looking at the most common PHC manufacturing method with reference to FIG.

As shown in the figure, a plurality of 7-13 mm PC steel rods (1) are used as the axial roots, and the common steel wire (MSRW8 of KSD3554, 3.2 mm) is wound around the axial roots with the spiral roots (2). After turning the spot contact between the axle and the spiral muscle to make the reinforcing bar, the reinforcing bar is embedded in the concrete by placing the concrete with the axle at 70% of the tensile strength.

After crushing the concrete by centrifugal molding after high temperature and high pressure curing (curing condition: After loading the concrete pile into the interior of the autoclave tank and raising the air pressure to 10 atm, after raising the temperature to 180 ℃ in 4 hours and then maintained for 3 hours By slow cooling to room temperature) to produce a PHC pile having an effective prestress amount of 80 Kgf / cm 2 or more.

Therefore, PC steel bar used for PHC pile should have a small loss of initial load (70% of tensile strength) introduced initially under high temperature and high pressure curing conditions to ensure sufficient effective free stress as PHC pile. At this time, the loss of the initial stress introduced is called relaxation loss, which is expressed by Equation 1 below.

PC steel bar for PHC pile is usually high strength with at least tensile strength of 1420MPa and high temperature relaxation value under the same laboratory conditions as high temperature and high pressure curing (experimental condition: after stretching 70% of tensile strength, the steel bar is heated up to 180 ℃ in 4 hours). After cooling for 3 hours, it is naturally cooled and the stress loss rate at 23 hours is measured to be a relaxation rate. A low recovery loss of 15% or less is required.

As an example of the manufacturing method of low-relaxation PC steel bar to achieve the above quality characteristics, Korean Patent Publication No. 92-5386 has 1.0-2.0wt% of Si content added much more than 0.15-0.35wt%. Low lylace with about 12-13% high temperature relactation loss by quenching by rapid heating and rapid cooling while continuously running the material steel wire and rapidly heating and quenching to a predetermined tempering temperature. It is said that PC rods can be manufactured.

However, the patent only focuses on improving the stress corrosion resistance and the high temperature relaxation loss of the PC steel bar by only increasing the amount of Si, and the brittleness of the weld after spot welding is not considered at all.

In another conventional method of manufacturing a low relaxation PC steel bar for producing high strength PC pile, Japanese Patent Publication No. Hei 3-31841 adds a large amount of Si to 1.6 to 2.0 wt% to a steel bar with a carbon content of 0.4 wt% or less. A technique for producing a PC steel bar having a high temperature relactation loss of 6% or less is performed by quenching after quenching and performing a warm working job which performs a small bending process (2% or less) during tempering.

However, the patent is to increase the Si content and to carry out warm knitting during tempering to further reduce the high temperature relactation loss to produce only a high strength pile of effective prestress 80Kg / ㎠ or more, the spot as the Si content is added in large quantities The problem of brittleness after welding is not considered at all.

However, the PC steel bar is made of PC steel bar as the axle as shown in FIG. 1, and the ordinary steel wire is spot welded around the axle in a spiral shape to form a reinforcing bar and embedded in concrete to produce a PHC pile. When constructing reinforcing bar, when welding the intersection point of PC steel rod and ordinary steel wire, spot welding is usually performed through spiral current (ordinary steel wire) and shaft (PC steel bar) through electric current. Since silver current is difficult to flow, it is difficult to attach it unless a large amount of voltage is applied, and in this case, the welding influence part becomes large due to the large amount of applied voltage, so that the spot welding part breaks frequently when the initial tensile stress of PHC pile is introduced.

Therefore, as a countermeasure, users have recently demanded quality characteristics of 5% or more according to KSD3505 or JISG3137 including elongation at break of ordinary steel wire and spot welded PC steel bars. Since the situation cannot be satisfied, it is urgently required to develop a material meeting the quality characteristic requirements.

The present invention was devised in view of the problems posed by the conventional high-strength PC steel bar, the spot, while maintaining the mechanical properties, especially low temperature relamination characteristics of the conventional PC steel bar containing a large amount of Si, It is an object of the present invention to provide a PC steel bar for prestressed concrete which significantly improves the elongation at break of the weld weld.

PC steel bar of the present invention for achieving the above object, C 0.25 ~ 0.40wt%, Si 0.8 ~ 1.5wt%, Mn 0.60 ~ 2.0wt%, P + S 0.04wt% or less as a basic component of the material, 0.01 to 0.05 wt% Ti, 0.0005 to 0.0050 wt% B, 0.01 to 0.05 wt% Al, or 0.01 to 0.05 wt% Ti, 0.0005 to 0.0050 wt% B, 0.01 to 0.05 wt% Al, and 0.02 to 0.30 wt% Cr. Is added and the remainder is composed of iron and unavoidable impurities. The material steel having such composition is continuously quenched and heated to Ac3 point or more by quenching and tempering, and the tensile strength is 1420 MPa or more. A steel bar for prestressed concrete with a loss of 15% or less, characterized in that the elongation at break including spot welds is 5% or more.

Here, the main point of the present invention is to add a large amount of Si in order to secure a high temperature relase value of 15% or less, but when the large amount of Si is added, the loss of relase at a high temperature is significantly reduced, but instead a spot with a helix muscle. As the ductility of the weld is significantly lowered, it is intended to provide a PC steel rod with a new composition that can satisfy both the high temperature relamination characteristics and the spot weld ductility required as the quality characteristics required for the PC steel rod for high strength PHC pile.

PC steel bar of the present invention, by reducing the Si content of the material steel wire to improve the ductility of some welds, by adding Ti, B, Al and Cr to refine the size of the grains generated during spot welding and at the same time By specially regulating P and S forming embrittlement grain boundaries, not only the ductility of the spot weld is further improved, but also the increase in high temperature relaxation loss caused by the reduction of Si is complemented.

The role and component limitation reason for each component of the composition constituting the PC steel bar of the present invention as follows.

C: 0.25 to 0.40 wt%

C is the most important element necessary for increasing the strength during quenching. If it is less than 0.25wt%, the hardening effect by quenching is insufficient, so it is difficult to secure the predetermined strength required as a PC steel bar. Decreases the weldability.

Si: 0.8-1.5 wt%

Si is a very useful element for lowering the high temperature relaxation value in the present invention. At the time of less than 0.8 wt%, the inventors have conducted several studies and the high temperature relaxation rate is 15% or more, which is insufficient for the purpose of the present invention. In the case of 1.6wt% or more, the spot welding with the spiral roots does not flow well, so that the adhesive force is reduced, and the brittleness of the spot weld increases, so it is limited to 1.5% or less.

Mn: 0.60 to 2.0 wt%

Mn contributes to the improvement of strength during quenching, and is an element to compensate for the decrease in strength in low C and Si addition steels. Therefore, in order to expect the above effect, at least 0.60 wt% of addition is required, but if excessive addition deteriorates the ductility of the spot weld, the addition amount should not exceed 2.0 wt%.

Cr: 0.02 to 0.30 wt%

Cr is an element added to improve strength, quenching hardness, and toughness. At 0.02 wt% or less, the improvement of the properties is insignificant. When Cr is more than 0.30 wt%, the ductility of the spot weld is reduced, so it is 0.30 wt%. Do not exceed.

B: 0.0005 to 0.0050 wt%

B is an element that improves hardenability by forming boron carbide with a small amount of addition, and the effect of addition is unclear at less than 0.0005wt%, but rather hardenability is greater than 0.0050wt%.

Ti: 0.01 to 0.05 wt%

Ti coexists with B to increase the quenching strength, but it is effective in refining the grain size of austenite, and in particular, it results in precipitation of carbon and nitride with low solubility during spot welding, resulting in higher transformation point. The effect is great.

However, when the amount is less than 0.01 wt%, the effect is insufficient. When the amount exceeds 0.05 wt%, the inclusions increase, which lowers various required physical properties.

Al: 0.01 to 0.05 wt%

Al combines with nitrogen and has an effect of inhibiting grain growth of austenite, and therefore, has a large effect of suppressing crack generation in spot welds when the material is tensioned. However, when a large amount is contained, a lot of hard aluminum oxide-based inclusions are generated, and even the portion other than the spot weld portion rapidly lowers the ductility. Therefore, in order to achieve the object of the present invention, it is most suitable to add about 0.01 to 0.05 wt% of Al.

P + S: 0.040 wt% or less

P and S are unavoidable impurity elements. In the case of P, solid solution is used in ferrite to improve hardness and tensile strength. However, P and S solidify to Fe ₃ P due to segregation during solidification, and diffuse to high temperature heating due to segregation at grain boundaries. Since this is difficult, the impact value and ductility are significantly reduced. In addition, when a large amount of S is added, the inclusions of FeS and MnS segregate at the grain boundaries in combination with iron and manganese, and thus, the grain boundary embrittlement is very likely to promote cracking when the spot is welded.

Therefore, the content of P and S should be managed as low as possible, and to achieve the object of the present invention, it is most appropriate to manage P + S at less than 0.040 wt%.

Example

The present inventors maintain C-Mn-PS, which is a basic component system as a PC steel bar, at about the same level, and each material steel wire having a wire diameter of 10 mm with or without Ti, B, Al, and Cr added while changing the Si content (wire rod) ) To 9.0mm, quenched by rapid heating over Ac3 transformation point temperature while continuously running the steel wire using high frequency induction heating device, and rapid heating to predetermined tempering temperature so that tensile strength is 1470 ~ 1500MPa The sample was prepared by rapid cooling.

Table 1 shows the tensile and high temperature relase test results after spot welding of the additive chemical composition of each sample used in this example and each sample prepared as described above.

TABLE 1

Here, the tensile test results for the steel wire manufactured in the above composition, 3.2mm ordinary steel wire (MSRW8 of KSD3554) in the spot of continuous continuous welding at 3000A, 2 cycles at 100mm intervals, the ordinary steel wire attached to the steel wire Is a value obtained by performing a tensile test on a test piece obtained by cutting a steel wire so that at least three spot welds exist.

At this time, the gauge length for breaking elongation was 8 times the wire diameter according to KSD3505, the relevant standard for PC steel bars, and the measurement of breaking elongation was carried out for the inclusion of spot welds.

However, when breaking at the spot welding part and breaking at the base material part, it was judged to be excellent in ductility of the spot welding part, and the value was recognized as it is.

In addition, the measurement of the high temperature relaxation value is obtained by stretching 70% of the sample prepared as the tensile test specimen with respect to the nominal tensile strength (1420MPa) of KSD3505 standard, and then raising the temperature to 180 ° C. at 4 hours at room temperature and then 3 hours. After holding, the mixture was naturally cooled and the stress loss rate at 23 hours was measured to obtain a relaxation value.

The present inventors were able to confirm the following facts from the above results.

In the case of Samples 1 to 4 in which the amount of Si is less than 0.80 wt%, the elongation at break of the spot weld is sufficiently satisfactory due to the low amount of Si. However, even if Ti, B, and Al are added, the high temperature relaxation rate is increased in the PHC pile. It does not meet the requirements below 15%.

In the Si content range of 0.80 to 1.5wt% of Si in the PC steel bar according to the present invention, in the case of Sample-5 having 0.91wt% of Si, the elongation at break is 5% or more, which satisfies the required target value, Only the lock value exceeds the requirement of 15%.

On the other hand, in the case of Sample-6 having an equivalent amount of Si (0.89%), Ti, B, and Al were added, which not only satisfies the high temperature relaxation rate of 15% or less, but also the elongation at break of 5% or more. The effects of such addition of Ti, B and Al are similarly confirmed in Samples 8 to 12.

Samples 13 to 15 are intended to determine the effects of addition of Ti, B, and Al together with Cr, but as a result, the addition of Cr further reduces the high temperature recovery loss. , The improvement effect on the elongation at break of spot welds was hardly seen.

Addition of Ti, B and Al dominates the improvement of elongation at break of spot welds.

The neck of Samples 16-19 with Si content exceeding 1.5wt%, as mentioned above, the high temperature relactation loss is reduced by adding a large amount of Si, but the brittleness of spot welds is significantly increased, so that Ti, B and Al It can be seen that even if the addition effect of the elongation at break is very poor.

In addition, when P + S is 0.040 wt% or more from Sample-7 and Sample-15, even when Ti, B, Al, Cr, etc. are added, the elongation at break of the spot weld is sharply lowered compared to the equivalent component system to achieve the object of the present invention. I could not confirm clearly.

The above facts are summarized as shown in FIG. 2 and FIG. Figure 2 is a measurement of high temperature relaxation loss by changing only the amount of Si without adding Ti, B, Al to the basic component system of the present invention, in order to obtain 15% or less, which is a requirement of PC steel bar for PHC pile, at least 1.0 It is well understood that more than% Si should be added. On the other hand, when Ti, B, and Al were added, as shown in Table 1, when only 0.80 wt% or more of Si was obtained, high temperature relactation loss of 15% or less could be obtained.

On the other hand, Figure 3 is a graph comparing the elongation at break including spot welds with respect to the invention material added Ti, B and Al and the comparative material not added, the case of the invention material exhibits much higher elongation at break than the comparative material, PC It was found that 5%, a requirement for steel bars, was fully satisfied. In addition, the addition of Ti, B, and Al on the elongation at break does not have a significant improvement effect at less than 0.80 wt% of Si amount, but has a significant improvement effect at 0.80 to 1.50 wt% of Si content range according to the present invention. When it exceeds 1.5wt% it could be confirmed that the effect is sharply lowered.

In conclusion, in the present invention, the amount of Si embrittlement of the weld to improve the break elongation of the spot weld of the PC steel bar is 1.0-2.0 wt% or 1.6- Adjust from 0.8wt% to 1.5wt% at 2.0wt%, and maintain P + S below 0.04wt% to improve spot weldability and break elongation at break. In addition, when Ti, B, and Al are added, the crystal grains of the weld are fine, and fine carbon and nitride are effectively precipitated to suppress cracking of the spot weld. It was clearly confirmed that extremely superior PC steel rods could be manufactured.

On the other hand, in the case of carrying out the warm-weaving during tempering in order to further reduce the high temperature relactation loss as in the conventional known art (Japanese Patent Publication No. 3-31841) mentioned above, the material of the present invention is used in the examples. As can be predicted, PC bars with extremely high elongation at break including spot welds with lower high temperature relaxation loss can be obtained.

As described above, the steel of the present invention is a process of tensioning to produce a PHC pile with much improved elongation at break of the spot weld, while maintaining the relamination characteristics at high temperatures as compared with conventional steels. In addition, there is an advantage that there is little possibility of being broken from the bending external force applied to the pile, etc., and the present invention is 7.6 to 8.3%, which is about 2.3 to 2.5 times superior to 3.3% of the elongation at break of the conventional material (Sample-18). There is an advantage that can produce a PC steel bar having a ductility of the spot welds.

1 is a schematic structural diagram of a reinforcing bar for manufacturing a high strength prestressed concrete pile,

(A) is a side view,

(B) is front view.

Figure 2 is a graph showing the change in high temperature relaxation loss according to the Si content of the PC steel bar not added Ti, B, Al.

Figure 3 is a graph showing the difference in elongation at break including spot welds according to the Si content for the present invention material containing Ti, B, Al and a comparative material not containing.

((Explanation of symbols for main parts of drawing))

1. PC rod 2. Spiral root

3. Spot weld

Claims (4)

C 0.25-0.40wt%, Si 0.8-1.5wt%, Mn 0.60-2.0wt%, P + S 0.040wt% or less, Ti 0.01-0.05wt%, B 0.0005-0.0050wt%, Al 0.010-0.050wt% The steel rod for prestressed concrete having excellent high temperature relamination characteristics, characterized in that the residual portion is made of iron and unavoidable impurities, and the elongation at break including spot welds is 5% or more. The steel bar for prestressed concrete having excellent high temperature relamination characteristics according to claim 1, wherein the steel bar has a tensile strength of 1420 MPa or more. C 0.25 ~ 0.40wt%, Si 0.8 ~ 1.5wt%, Mn 0.60 ~ 2.0wt%, P + S 0.040wt% or less, Ti 0.01 ~ 0.05wt%, B 0.0005 ~ 0.0050wt%, Al 0.010 ~ 0.050wt%, A steel bar for prestressed concrete having excellent high temperature relaxing properties, containing Cr 0.02 to 0.30wt%, the remainder being made of iron and unavoidable impurities, and having an elongation at break of not less than 5% including spot welds. 4. The steel bar for prestressed concrete having excellent high temperature relamination characteristics according to claim 3, wherein the steel bar has a tensile strength of 1420 MPa or more.