1. Cold drawing principle of steel bar
The principle of steel bar cold drawing is shown in Figure 4-24.
2. Cold drawing control
The cold drawing of steel bars can be controlled by controlling the cold drawing stress and cold drawing rate. See Table 4-7 for the control stress values of cold drawing.
3. Cold-drawn equipment
Steel cold drawing can be carried out by winch or long-stroke hydraulic jack. At present, winches are mostly used. When the winch is used for cold drawing, its main equipment consists of winch, pulley block, bearing structure, return device, measuring equipment and steel bar fixture, and its overall structure is shown in Figure 4-25.
4. Cold-drawn calculation of steel bar
The calculation of steel bar cold drawing includes the calculation of cold drawing force, elongation value, elastic retraction value and the selection of cold drawing equipment.
Ncon calculation of (1) cold tension. The calculation of cold drawing force has two functions: ① determining the reading of oil pressure gauge during cold drawing according to the control stress; (2) As the basis for selecting hoist.
The cold drawing force should be equal to the cross-sectional area As of steel bar before cold drawing multiplied by the control stress σcon during cold drawing, that is, ncon = As σ con
(2) Calculate the elongation value Δ l. The elongation value of steel bar should be equal to the product of the length l of steel bar before cold drawing and the cold drawing rate Δ of steel bar, that is Δ l = Δ l = Δ l.
(3) Calculate the elastic retraction value δ L 1. According to the elastic shrinkage δ 1 (generally around 0.3%), that is, δ L 1 = (L+δ L) δ 1.
The actual length of steel bar after cold drawing is: l' = l+δ l-δ l 1.
(4) Selection and calculation of cold-drawn equipment. The cold-drawing capacity of the equipment is greater than the maximum tension required for steel bar cold-drawing, and the friction resistance between the pulley and the ground and the resistance of the return device should also be considered, which is generally 1.2 ~ 1.5 times of the maximum tension.
Tension of winch and pulley block cold drawing equipment: q = t/k ′-f
Where t is the pulling force of the crane (kn);
F is the equipment resistance (kN), which is 5 ~10kn;
K' is the labor-saving coefficient of pulley block. K' can be calculated according to the following formula or looked up in Table 4-9.
k′=[f n- 1(f- 1)/(f n- 1)]
Where f is the resistance coefficient of a single pulley (bronze shaft sleeve is1.04);
N is the number of working lines of the pulley block.
The load-bearing structure can adopt ground anchor, and the force measuring device can adopt spring dynamometer, electronic scale or hydraulic jack with oil gauge. When at the tension end, the load Fp of dynamometer:
FP =( 1-K’)(N+F)
At the fixed end: FP = n-f
Where n is the cold tension (kN) of steel bar.
5. Precautions for cold-drawn steel bars
Matters needing attention of cold-drawn steel bars mainly include the following aspects:
(1) The stress control method should be adopted for prestressed reinforcement. For the steel bars that can't distinguish the heat number, it is not appropriate to control the cold drawing speed for cold drawing.
(2) The cold-drawing speed of steel bars should not be too fast, and it is generally appropriate to be 0.5 ~ 1.0 m/min. After being pulled to the specified control stress, it is necessary to pause 1 ~ 2 min before relaxing.
(3) Cold drawing of steel bars can be carried out at negative temperature, and the temperature should not be lower than -20℃. When the stress control method is adopted, the control stress of cold drawing is 30 MPa, which is higher than the value in Table 4-7. When the cold drawing rate is controlled, it is the same as normal temperature.
(4) When the stress control method is used to cold-draw steel bars, the dynamometer used should be maintained and calibrated regularly.
(5) After cold drawing, there should be no cracks or local shrinkage on the surface of the steel bar. Tensile test and cold bending test should be carried out according to the specification requirements, and its mechanical properties should comply with the provisions in Table 4-6.
(6) Pay attention to safety during cold drawing. It is forbidden to stand or walk at both ends of the steel bar to prevent the steel bar from breaking and rebounding.
(2) cold-drawn steel bar
Cold drawing is to pull φ 6 ~ 10 mm HPB235 round steel bar through the hole of the drawing die at room temperature (as shown in Figure 4-26).
There are vertical (as shown in Figure 4-27) and horizontal wire drawing machines for cold drawing. The diameter of the drum is generally 500 mm, and the cold drawing speed is about 0.2 ~ 0.3 m/s, which is easy to break the wire if the speed is too high.
The technological process of cold drawing of steel bars is: rolling head → peeling shell → entering the drawing die through lubricant for drawing. If steel bars need to be connected, butt welding should be adopted before cold drawing.
Generally, the number of cold drawing should not be too much. First, it affects production efficiency; Second, the steel wire will be brittle, which will affect the elongation. However, if the number of cold drawing is too small and the amount of compression is too large each time, it is also prone to wire breakage and equipment safety accidents. According to experience, the ratio of front steel wire diameter to rear steel wire diameter is generally 1: 1. 15. Such as Φ 8 to Φ 5 after cold drawing, Φ 8 →Φ7 →Φ6.3 →Φ5.7 →Φ5 in Cheng Kewei. If you pull from Φ 6.5 to Φ 4, it can be Φ 6.5 →Φ5.7 →Φ5 →Φ4.5 →Φ4.
The quality of cold-drawn low-carbon steel wire shall meet the requirements in Table 4- 10.
(3) Other processing methods of steel bars
In addition to cold working, the processing of steel bars also includes rust removal, straightening, cutting and bending. The shape and size of steel bar processing should meet the design requirements, and its deviation should meet the provisions of Table 4- 1 1.
1. Rust removal
Steel derusting can generally adopt the following two methods:
(1) Electric rust remover or manual wire brush, sand table, sand blasting and pickling can be used for a small amount of local rust removal of steel bars.
(2) A large number of steel bars can be derusted by cold drawing or straightening with a steel bar straightening machine.
straighten
Steel bar straightening should adopt mechanical method or cold drawing. For local twists and turns, bending or coiling reinforcement, it should be straightened before use. There are many methods for straightening steel bars, and the commonly used methods are winch straightening and straightening with straightening machine.
cut off
Before cutting, the length of steel bars of the same specification should be matched and arranged as a whole. Generally, the long material is cut first, and then the short material is cut to reduce the short head and loss. Steel bars can be cut by a steel bar cutter or a manual shearing device.
The above are collected and sorted by Zhong Da Consulting Company.
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