At present, in order to reduce CO2 emissions, automobile manufacturers are generally adopting measures to reduce fuel consumption by lightening the car body. Recently, high-strength aluminum alloy materials composed of aluminum with a small specific gravity are used, and the simple evaluation of the materials used uses the specific strength per unit weight. (Tensile strength/specific gravity).
Steels of 800 to 1200 MPa, which are generally called high-strength steels, have less specific strength than aluminum alloys, but steel materials have various structures. As methods to increase the strength of steels, there are “fine grain strengthening”, “solid solution strengthening”, and Organizational control techniques such as precipitation strengthening” and “dislocation strengthening”. Taking fine grain strengthening as an example, the following is an explanation. The method of refining the crystals of usually 20-30um ferrite to increase its strength, especially the effect of increasing the strength when the grain diameter is refined below 1um (see Table 2). The combination of microstructure technology and processing and heat treatment may achieve ultimate strength. Based on this, “ultra-high-strength steel” that rivals aluminum alloys has been developed.
Table 2 The relationship between ferrite grain refinement and strength
Grain diameter (um) 1251020304050
0.2C% strength (MPa) 920 700 400 325 280 240 230 220
Solid solution strengthening refers to the hardening phenomenon caused by the increase of a large amount of intrusive atoms such as C and N and substitution elements such as Si and Mn. Precipitation strengthening is due to the increase of compounds, and dislocation strengthening is due to the increase in the number of dislocations in the steel produced by processing. The hardening phenomenon caused.
(5) High carbon steel wire pursuing the ultimate strength
In rod and wire products, the material that pursues ultimate strength is high-carbon steel wire. The process technology of steel wire for bridge is described as follows.
Secondary processing companies require toughening treatment for hot-rolled semi-finished products to improve their strength while requiring good workability. This technology obtained a British patent in the 19th century. This kind of heat treatment adopts isothermal and homogenized heat treatment in a metal bath with good thermal conductivity to transform the ferrite and cementite structure existing in the steel into austenite at room temperature, and then transform it into austenite by rapid cooling Pearlite (Layered structure composed of cementite and ferrite). In the pearlite structure produced by this method, the strength of the wire is determined by the spacing between cementites (that is, the thickness of the lamellae). The smaller the thickness, the higher the strength. If it is cooled to room temperature without toughening treatment, the thickness of the lamellae is not uniform and the drawing processability is reduced, and the final strength is also reduced. For this reason, toughening treatment is an indispensable process for the production of steel that requires high strength.
In the process of the steel structure from high temperature to low temperature, austenite forms pearlite and grows up; but when it is rapidly cooled from 950°C to a low temperature of 550°C, it becomes uniform pearlite, which changes from hard and brittle cementite phase. It is formed side by side with the soft and good extensibility ferrite phase in the same direction; while for automotive plates and other materials with good workability, a softer single ferrite phase is formed.
If the toughening treatment can be omitted, it will bring great benefits to the user to simplify the processing. The above-mentioned “DLP” equipment can play this role, that is, uniformly adjust the cooling in a salt bath at 550°C to make it a semi-finished product When it turns into pearlite. For the production of high-strength steel wire for concrete shrinkage, Nippon Steel also uses “DLP” equipment processing to create conditions for users to eliminate toughening processing. In the production of steel wire for bridges, after toughening treatment, it is first subjected to pickling and zinc phosphate film treatment for “lubrication” treatment after being toughened, and then drawing is performed in multiple stages at room temperature. The hot-rolled Φ13mm semi-finished wire rod is cold drawn to Φ7mm, and finally galvanized to improve the corrosion resistance. However, the radial steel wire for tire reinforcement has many processing steps, that is, it uses Φ5.5mm wire, which is drawn into a Φ3mm steel wire. After intermediate toughening treatment, it is then drawn to a Φ1.5mm steel wire, and then finally toughened. Treatment and brass plating (which can improve the adhesion to rubber) treatment, and finally wire drawing to Φ0.3mm and composed of 5 pieces. The reason for the intermediate toughening treatment is to prevent wire breakage due to poor toughness when the wire is drawn from Φ5.5mm to Φ1.5mm at a time. In short, when all steels become higher in strength, their extensibility decreases as their strength increases. Therefore, the key to practical high-strength limit is extensibility. The key technology of high-strength high-carbon steel wire is also how to maintain extensibility.
(6) High carbon steel wire with finer diameter and higher strength
The strength of the steel wire has an obvious relationship with the wire diameter. For example, the wire diameter of the steel wire for bridges is Φ5～7mm, and its strength is below 2000MPa, while the radial steel wire for tires with a wire diameter of Φ0.2～0.4mm has a strength of about 4000MPa. . By increasing the strength of the steel wire, it is helpful to reduce the construction cost and reduce the weight of the tire.
When the diameter of the steel wire is reduced, due to the pressure applied during the wire drawing process, the strength will increase correspondingly as the degree of thinning (deformation processing amount) increases, which is the fundamental principle. Although there are certain differences between different steel grades, the strength of the steel wire with a strength of 1200～1500MPa after toughening treatment continues to increase when it continues to be drawn. The deformation of steel wire for bridges is about 1.5, and the deformation of meridian steel wire is as high as 3.5 to 4. The relationship between the deformation and strength of the processing is shown in Table 3.
Table 30. Relationship between processing deformation and strength of 82%C steel
Machining deformation (%) 012345
Tensile strength (MPa) 120017002000280035004300
This principle can be explained by the changes in the structure of steel. The smaller the width of the ferrite interval (that is, the thickness of the lamella), the higher the strength. Because the steel wire that has just been toughened, the crystal directions of ferrite and cementite are random and irregular. The crystals of high-strength cementite and good elongation ferrite are processed by wire drawing. The direction becomes uniform, so the thinner the steel wire, the smaller the thickness of the sheet and the higher the strength. For example, the grain size of ferrite in steel is Φ10～30um, which is only 0.5～0.8um as the “super metal (high-strength steel)” in national project development; while the thickness of steel wire after toughening is only It is about 0.1um (1200～1500MPa). The most advanced radial steel wire becomes 0.01um after about 20 times of drawing, and the corresponding strength also rises to 4500MPa.
It is a common phenomenon of steel materials to increase the strength if the crystallographic direction after rolling is the same, but the crystals only extend in the rolling direction and not in the width direction during rolling of products such as thin plates. Therefore, the crystal grain size varies with the direction. Also different. The cold drawing die used in the wire drawing process uses a strong pressure different from the rolling method to uniformly squeeze the wire from all around, so the crystallization can only develop in the drawing direction. As a result, the lamellae are uniform and the thickness is reduced, which makes the strength Improve quickly. In order to apply strong pressure to ultra-high-strength steel wire, ultra-high hardness diamond molds are often used when drawing. Stainless steel spring