1) The case of cold tensile stress control:
For steel bars of grades II, III, IV and No. 5 steel, after cold drawing, those used as prestressed steel bars should be controlled by cold tensile stress. However, after inspection of the steel bar after cold drawing, the maximum cold drawing rate exceeds the specified value of the specification, and another mechanical energy test is required.
2) Cold drawing rate measurement and control requirements:
When the cold drawing of steel bars is controlled by the cold drawing rate, the control value shall be determined by experiments. Requirements for the test and measurement: There should be no less than 4 test specimens in the same furnace and the same batch. Each test piece should be measured according to the cold tensile stress specified in the specification. The actual cold drawing rate of the batch of steel bars. If the strength of the steel bar is high and the average cold drawing rate is lower than 1%, the cold drawing rate of 1% should still be controlled when the steel bar is cold drawn.
3) Cold drawing control of different batches:
For the steel bars that are mixed and indistinguishable from the furnace batch, the cold drawing rate cannot be used to control the cold drawing, and the cold drawing rate and control stress of each steel bar must meet the specifications.
4) Cold drawing speed control:
In order to fully deform the steel bar, it is necessary to properly control the cold drawing speed. Generally, 0.5-1.0 is appropriate; at the same time, it is required that after the cold drawing reaches the specified stress and cold drawing rate, the drawing is stopped for 2-3 minutes, and then the steel bar is loosened and the cold drawing is ended, so as to give the steel bar sufficient time to deform.
5) Cold drawing control method:
During cold drawing, only cold drawing rate or cold drawing stress control is called single control; during cold drawing, cold drawing rate and cold drawing stress are applied at the same time, which is called double control. With single control, the construction is simple and convenient. However, for the steel bars with uneven material, it is impossible to test one by one; it is impossible to do the test one by one because it costs labor and materials, and some of the same steel bars have different cold drawing rates, and the cold drawing quality cannot be guaranteed. The dual control method can avoid the above problems. During cold drawing, if the control stress has reached the cold drawing rate and does not exceed the allowable value, it can be considered qualified; however, if the cold drawing rate has reached but the cold drawing stress has not reached the control stress, the strength of this kind of steel bar should be reduced. For prestressed reinforcement a dual control method must be used.