Self-tapping screw production process and material quality

Self-tapping screw production process and material quality

The self-tapping screw is a light structural fastener that can quickly drill into a certain thickness of fiberboard or ordinary low carbon steel steel under a certain pressure and speed. In order to achieve the above characteristics, first, the self-tapping screws should adopt the cold heading process to improve the strength, elastic limit and toughness of the product; secondly, the tip of the self-tapping screws should be pointed, such as the cross-slot double-line fast-wall panel screws. The tip angle of the head is 25 ° ~ 28 °; Third, the self-tapping screws have strict requirements on surface hardness, carburized layer depth, core hardness and torque. SWRCH22A is a typical steel grade for self-tapping screws. To satisfy the production process.

SWRCH22A is a typical steel grade for self-tapping screws. In order to meet the special requirements of the production process, the following requirements are put forward for the quality of the wire rod: 1 good cold heading performance, 5% cold heading pass rate 100%, 1/3 cold head pass rate > 95%; 2 good tip performance The pass rate of the tip is >95%; 3 the drawing performance is good, and the wire compression rate is >70% 4 to satisfy the grain size of the wire rod for carburizing and quenching treatment.

Self-tapping processing technology and raw material quality

The production of self-tapping screws is roughly divided into four processes: blanking, cold heading, twisting and finished carburizing and quenching. The relationship between the performance of the wire rod and the production and product quality of each process was analyzed.

1 blanking process

The usual process is: ∮6.5mm wire rod→mechanical shelling→cold drawing (four to six channels)→∮3.65～2.35mm steel wire→heat treatment (700°C～720°C/4～5.5h)→tank cold→out can → Finely smashed 3.55~2.25mm steel wire. The rough drawing reduction rate is 70-85%, and the fine drawing reduction rate is 10%.

The main problems affecting the production of this process are cold-drawn fracture of raw materials, fast hardening rate of steel wire, and high consumption of wire drawing die. The cold-drawn fracture is mainly caused by the large amount of oxide inclusions. Figure 1 shows the typical fracture morphology of the large-particle Al2O3 fracture; in addition, the steel surface defects (folding and micro-cracking) are also important causes of fracture.

Raw materials containing more oxide inclusions, during the cold drawing process, because the relative deformation rate of oxides is small, the matrix and oxide will not deform at the same time. This will inevitably be "off" under the action of tensile stress. The microcracks are formed in a manner, and the internal stress is highly concentrated at the microcracks, causing extended polymerization of microcracks, and finally causing fracture.

In addition, as the raw material decreases with the cold drawing rate, the sensitivity of the hardening to fracture is increased. The hardening rate of steel wire is related to the Si content in steel with the increase of silicon. The increase of σb and HB of steel and the decrease of δ and ψ are very significant.

In order to meet the requirements of the wire drawing process of the self-tapping nail making process, it is necessary to strictly control the Als/Alt>0.95, reduce the amount of large-particle Al2O3 inclusions, and reduce the Si content to below 0.04% as much as possible.

2 cold heading molding process

The waste that occurs in this process is a screw cap crack. From the analysis of the splitting morphology, the causes of cracking should be as follows.

1) Wire rod surface quality

This is the most intuitive factor affecting the formation of self-tapping screws. Defects such as cracks and folds that are locally present on the surface of the wire rod cause uneven distribution of the cracked scrap. The product with a larger wire diameter has a larger split ratio than the small screw.

2) Influence of inclusions

Due to the great reduction rate of the drawing, the internal inclusions of the original wire rod are moved to the near surface, causing the screw to form a "burst".

3) Influence of steel ball spheroidization effect

According to the fastener industry standard, the self-tapping nail production requirements after the spheroidizing treatment of the steel wire reach the level 4 or higher. Since some self-tapping nail production plants use the earth furnace for annealing, the furnace temperature control has a large fluctuation range and does not reach the spheroidization effect. The steel wire is poor in plasticity, and the nut is also cut at 45 degrees when it is cold.