Abstract: Cold-rolled steel wire rods are generally produced from low- and medium-carbon high-quality carbon structural steels and alloy steels. The process of controlled rolling and controlled cooling is adopted in the production process. It is required that the surface of the product should not have cracks, folds, scratches, ear, cold and cold. Defects such as cracking exist, and the final properties are guaranteed to have sufficient strength and good toughness and plasticity.
I. INTRODUCTION The steel grades of cold-rolled steel wire are generally low- and medium-carbon high-quality carbon structural steels and alloy steels. It is a kind of high-interchangeability in the use of metal plasticity and cold heading forming process at room temperature. The standard parts are made of steel and are mainly used for the manufacture of fasteners such as bolts, nuts, screws, self-tapping screws and various cold-formed parts. It is widely used in the automotive, mechanical equipment, construction and electrical industries.
Second, the common quality defects of cold heading steel The quality control of cold rolled steel wire is different from the corresponding low and medium carbon structural steel wire. Its final performance is to ensure sufficient strength and good plasticity and toughness, and its metallographic structure For ferrite plus pearlite, common quality defects of finished products are cracks, folds, scratches, crusting, delamination, inclusions, etc. When tanning on a cold heading machine, surface cracks sometimes appear on the blank, ie cold镦 cracking, in the production of standard parts, there is a process of rough deformation at normal temperature. The material has large local deformation, fast deformation speed and easy crack on the surface. There are three main types of cold forging cracking of cold heading steel: inclusion Cracks, scratch cracks and folding cracks, characterized by cracks in the size of millet grains on the surface of steel or parts, leaving small pits, oxide inclusions under the microscope, scratch cracks are characterized by longitudinal Deep, there are oxidation and decarburization in the deep crack, and the folding crack appears symmetrically on both sides of the steel or the part, and most of them are longitudinal cracks, and the crack inner wall is slightly oxidized and decarburized.
Third, the cause analysis
(1) The influence of billet quality and heating system on cold heading performance.
1. Blank quality inspection. The allowable deviation of the section length of the continuous casting billet is ±4.0mm, the length deviation of the billet length is +50mm; the difference of the diagonal length of the continuous casting billet is not more than 6mm, and the bending degree of the continuous casting billet is not more than 20mm per meter; The surface of the continuous casting slab shall be free from visible cracks, reconnections, tumbling, crusting, inclusions, scratches, indentations, abrasions, stomata, wrinkles, cold splashes, ears, bumps, and depths greater than 3 mm. The concave block and the hairline with a depth of more than 2 mm shall have no shrinkage holes and subcutaneous pores in the cross section.
2. Furnace temperature control.
The surface temperature difference with the billet is 30 °C higher in the middle of the length direction, the section temperature difference is less than 30 °C, the billet is discharged, and the scale is dewatered with high pressure water, and the water pressure is ≥180Mpa.
If the surface quality of the blank has serious defects or the furnace temperature is improperly controlled, it is difficult to completely eliminate it in the production process, and it is easy to cause a crack source in the cold forging process on the surface of the material.
(2) The influence of controlled rolling process measures on the cold heading performance of the product.
1. The influence of processing precision and installation quality of equipment and spare parts on cold heading performance.
(1) Front and rear guide grooves of 1#, 2#, 3# flying shears, loopers and looping sleeves of the looper device, empty guide channels, spinning pipes and spinning discs, air-cooled roller paths and Guide wheel, cloth cone nose cone and collecting drum, double core rod, etc. If the processing precision is not high, the installation is not in place or the maintenance is improper, resulting in severe friction between the equipment and the rolling stock, which is easy to cause scratch on the surface of the rolled piece. .
(2) There are steps or sharp burrs on the inner surface of the guide insert, the guide wheel does not rotate or the rotation is not flexible, the guide or guide groove or the empty guide tube is not centered, the rolling line or the wear is serious, etc. The surface of the wire is liable to cause surface scratches.
Scratches on the surface of the wire can cause the wire to crack or break at the scratch in the subsequent cold heading process due to the stress concentration at the tip.
2. The impact of production adjustment on the cold heading performance of the product. The production adjustment of the high line is the main factor affecting the cold heading performance of the high line. If the rolling production is improperly adjusted, the product may be folded or scratched during the rolling process, and the surface of the product is destroyed, so that the crack source of the upset cracking is easily formed. . If the rolling reduction is too large, or the rolling tension is improperly adjusted, or the rolling center line adjustment deviation is large, or the guide opening adjustment is not suitable, or the rolling hole type is improperly adjusted (wrong roller), etc. Etc., it is easy to produce bilateral or unilateral ears during the rolling process. If the subsequent rolling pass cannot be eliminated, the finished product will be folded, causing it to crack during cold upsetting.
3. The effect of controlled cooling process measures on the performance of wire products.
In controlled rolling, the cold rolling temperature is one of the main parameters determining the austenite grain size. It affects the microstructure transformation and the morphology of the transformation products during the transformation through the influence of the austenite grain size. The effect of finish rolling temperature on austenite grain size is: high finish rolling temperature, coarse grain; low final rolling temperature and fine grain. Due to the high rolling speed, the wire is always heated during the rolling of the pre-finishing mill, the finishing mill and the reduced-sizing unit. Generally, the wire is introduced into the NTM temperature, the RSM temperature, and the spinning temperature to a reasonable process temperature by means of water tank cooling and water cooling guide cooling.
The spinning temperature is the key parameter for controlling the onset temperature of the phase change. The temperature of the spinning temperature directly affects the stability of the supercooled austenite, which has an important effect on the performance. It mainly affects the performance of the product from two aspects: 1) The appearance of too cold austenite. Because the wire is heated at a high speed during water cooling, the temperature gradient on the wire is large, and the cooling is extremely uneven. If the wire temperature is too low, it is easy to cause supercooled austenite on the surface of the wire. If the air cooling speed is unreasonable, it will promote the upper shell. The production of celite or martensite severely destroys the structure of the material and reduces the plasticity and toughness of the material. 2) Abnormal growth of crystal grains . If the spinning temperature is too high, at a higher temperature, the recovery and recrystallization of the tissue occurs. If the holding time is long, the austenite grains grow abnormally at high temperatures inside the material, which promotes the formation of Wortite. It also affects the plasticity of the material.
The cooling rate after the wire is rolled into a roll is also the key to the quality of the product. It is mainly achieved by controlling the roller speed on the Steyr transporter, the opening position of the fan and the heat shield. Different austenite transformation structures can be obtained at different cooling rates of the wire near the phase transition temperature, and the cooling rate is too fast. After the cold austenite is formed, the high temperature transformation will produce upper bainite, and the low temperature transformation will produce martensite structure. The abnormally long high-temperature austenite structure promotes the formation of the Wei's body structure when the cooling rate is fast. The appearance of these bad tissues will seriously deteriorate the material structure, reduce the plasticity and toughness of the material, and thus affect the performance of the product.IV. Conclusion Through the analysis of the causes of the quality defects of cold heading steel, it provides a certain basis for further improving the wire rolling process and improving the quality of cold heading steel products. Sophisticated equipment, meticulous and meticulous operation, and scientific and rigorous craftsmanship are the three magic weapons for ensuring product quality.
references:
1) High-speed rolling mill wire production. Metallurgical Industry Press 2) Basic knowledge of rolling steel. Metallurgical Industry Press
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