Metal cold processing is an important part of steel industry production. There are many types of metal cold processing, such as: steel wire, steel wire rope, steel strand and other metal products, etc. The production of special steel wires has a history of nearly 70 years in my country. It is a typical industry in the metal cold processing industry.
With the continuous development of the industry, the varieties of special steel wires (hereinafter referred to as steel wires) have been increasing, and the quality and quality have been continuously improved. The old workshop-style production model is gradually replaced by semi-automated and automated production. However, no matter how the equipment changes, in terms of steel wire production, the production method of steel wire is actually two lines: continuous line and periodic line.
1 Basis for steel wire production
Steel wire production is mainly based on the product standard requirements to determine the production method. Product standards are the link between suppliers and customers. According to product standards, the use of reasonable technology and scientific management are the basic guarantees to ensure that the products are qualified.
In the production of steel wire, the same steel grade (same chemical composition) adopts different standards, and the production methods are different. Take T9A as an example: when the GB/T4357-89 elastic spring steel wire standard is implemented, continuous line production must be used, and the finished steel wire produced can be used to manufacture various springs and weave wire ropes. When implementing the GB/T5952-86 elastic tool steel wire standard, it is necessary to adopt the cycle line production, and the finished steel wire produced can be used for manufacturing tools. Such as: knives, drills, needles, etc. Because the steel wires produced by continuous and periodic lines have different microstructures, different mechanical properties, and different process properties, they have different uses.
It is worth clarifying: whether it is continuous line production or periodic line production, heat treatment, surface treatment, processing deformation, three elements are indispensable.
2 Steel wire production factors
2.1 Steel wire heat treatment
Heat treatment is one of the main elements of steel wire production. Some people outside the industry thought: “Wire drawing is cold processing, simple operation, no heat treatment, as long as there is a corresponding die for extrusion and drawing.” This is a misunderstanding of steel wire production. The so-called cold working refers to the deformation and production process carried out at room temperature. Before the steel wire is processed and deformed, heat treatment must be used to improve the internal quality of the steel, achieve the microstructure required by the standard, and obtain good physical shrinkage and ductility in order to smoothly process and deform. This is the important point of heat treatment.
The heat treatment of steel wire production has four steps: they are raw material heat treatment, intermediate heat treatment, pre-finished heat treatment, and finished product heat treatment (excluding products delivered in cold drawn state).
There are 8 main types of heat treatment:
(1) Complete annealing: The steel wire is heated to complete austenitization and slowly cooled to obtain a structure close to equilibrium;
(2) Incomplete annealing: the steel wire is heated to a temperature between Ac1 and Ac3 to achieve incomplete austenitization, followed by slow cooling;
(3) Spheroidizing annealing: heating the steel wire (wire rod) to above Ac1 temperature, so that the carbide of the steel reaches the spheroidizing state;
(4) Recrystallization annealing: the cold-worked and deformed steel wire is heated to above the recrystallization temperature and kept for a proper time to recrystallize the deformed grains into uniform equiaxed grains and eliminate cold work hardening;
(5) Bright annealing: the steel wire is annealed in a protective atmosphere or vacuum to prevent oxidation and keep the surface of the steel wire bright;
(6) Normalizing: Heat the steel wire to 30~50℃ above Ac3, keep it warm for an appropriate time, and then cool it;
(7) Soxtenitizing treatment (Pedentoff treatment): After austenitizing medium-carbon or high-carbon steel wire, quickly move it to an appropriate temperature (about 500℃) below Ar1 and cool it in the middle temperature or air. , To obtain the sorbite structure;
(8) Aging treatment: After the steel wire undergoes solution treatment or cold shrinkage deformation, it is kept at room temperature or a certain temperature to achieve the purpose of precipitation hardening (1).
According to the specific conditions of steel wire production, such as: wire status-raw materials, semi-finished products, pre-finished products, finished products; product standard requirements; it is very important to determine the type of heat treatment. Of course, the heating method directly affects the heat treatment quality. So far, there are many heat treatment and heating methods for cold working in my country, each with its own advantages.
2.1.1 Heating method
There are five heating methods:
(1) Coal-fired heating;
(2) Coal burning-local gas heating;
(3) Natural gas heating;
(4) Pipeline gas heating;
(5) Electric heating.
The first heating method has a lower cost, but the furnace temperature is not easy to stabilize, and the surface of the steel wire is prone to decarburization; the second heating method has a slight advantage over the first method; the third and fourth heating methods are superior. The temperature is relatively stable; the fifth heating method is the most ideal. Although the cost is higher, it has no pollution, stable furnace temperature, uniform heating, and the internal quality of the product will be greatly improved. It is the most wise choice for the production of high-grade steel wire.
2.1.2 The main purpose of heat treatment
There are four main purposes for heating wire rod (raw material) or steel wire:
One is sorbitization (grain refinement) to improve tensile strength, increase toughness and ductility;
The second is spheroidizing annealing to obtain a reasonable microstructure and increase shrinkage and ductility;
The third is recrystallization annealing to recrystallize deformed grains into uniform equiaxed grains, eliminate cold work hardening, and facilitate cold work again;
The fourth is softening annealing, no microstructure change, only to eliminate processing stress, meet the mechanical requirements (mostly used for finished steel wire).
2.2 Surface treatment
Surface treatment is the second major element of steel wire production. There are four procedures for surface treatment: one is to remove the oxide scale (oxide film) produced by heat treatment; the other is to neutralize, the purpose is to prevent the residual acidic substances from corroding the steel matrix; the third is to coat, to increase the carrier to ensure lubrication, there is Conducive to cold working deformation and ensure the surface quality of the steel wire; the fourth is to remove harmful impurities on the surface (including decoating) to ensure that the steel wire is corrosion-free and the surface is smooth and clean.
2.3 Cold working deformation
Cold working deformation (hereinafter referred to as drawing) is the third and most intuitive element of steel wire production. This element is the main process of steel wire production.