In the process of steel structure from high temperature to low temperature, austenite generates pearlite and grows; When it is rapidly cooled from 950℃ to 550℃, it becomes uniform pearlite, which is composed of hard and brittle cementite phase and soft and tough ferrite in the same direction. However, for materials with good machinability, such as automobile plates, soft ferrite single phase will be produced.
If the tempering treatment can be omitted, simplifying the processing will bring great benefits to users. The "DLP" equipment mentioned above can play this role, that is, it can be evenly adjusted and cooled in a salt bath at 550℃, so that it can be transformed into pearlite when it is semi-finished. For the production of practical high-strength steel wire for concrete compression, Nippon Steel also handles it with "DLP" equipment, which creates conditions for users to save toughening treatment. In the production of steel wire for bridge, after toughening treatment, it is first "lubricated" by pickling and zinc phosphate film treatment, and then it is drawn in multiple sections at room temperature. That is, Φ13 mm hot rolled semi-finished wire rod is thinned to Φ 7 mm by cold drawing, and finally galvanized to improve corrosion resistance. However, there are many processing procedures for radial steel wire for tire reinforcement, that is, wire rod with a diameter of 5.5mm is used to draw a steel wire with a diameter of 3 mm, then it is toughened in the middle to draw a steel wire with a diameter of φ 1.5mm, finally it is toughened and brass plated (which can improve the adhesion with rubber), and finally it is drawn into five wires with a diameter of 0.3 mm.. The reason of intermediate toughening treatment is to prevent wire breakage due to poor toughness when drawing from 5.5mm to φ 1.5mm at one time. In short, when all steels are of high strength, ductility will decrease with the increase of strength, so the key to practical high strength limit lies in ductility, and the key technology of high carbon steel wire for high strength is how to maintain ductility.
(6) The smaller the diameter of steel wire, the higher the strength of high carbon steel wire.
The strength of steel wire is obviously related to the diameter of steel wire. For example, the diameter of steel wire for bridge is φ 5 ~ 7 mm, and its strength is below 2000MPa, while the strength of radial steel wire for tire with φ 0.2 ~ 0.4 mm is as high as 4000MPa. Through the high strength of steel wire, it is beneficial to reduce the construction cost and tire weight.
When the diameter of steel wire decreases, the strength increases with the increase of its thinning degree (deformation) due to the pressure exerted during wire drawing, which is its fundamental principle. Although there are some differences among different steel grades, the strength of steel wire with the strength of 1200 ~ 1500 MPa continues to improve after toughening treatment. The deformation of bridge steel wire is about 65438 0.5, while the deformation of radial steel wire is as high as 3.5 ~ 4. The relationship between processing deformation and strength is shown in Table 3.
Table 3 Relationship between processing deformation and strength of 0.82% C steel
Machining deformation (%)
1
2
three
four
five
Tensile strength (MPa)
1200
1700
2000
2800
3500
4300
This principle can be explained by the structural change of steel. The smaller the ferrite interval width (that is, the lamellar thickness), the higher the strength. Because the crystallization directions of ferrite and cementite of steel wire are random and irregular, and the crystallization of cementite with high strength and ferrite with good ductility becomes uniform according to the drawing direction through wire drawing, the thinner the steel wire, the smaller the thickness of the sheet and the higher the strength. For example, the particle size of ferrite in steel is φ 10 ~ 30um, which is only 0.5 ~ 0.8um as a "super metal (high strength steel)" in national project development, and the thickness of steel wire plate after toughening treatment is only about 0. 1um (1200 ~ 14500MPa.
It is the same phenomenon of steel that the crystallization direction is consistent after rolling, which can improve the strength. However, the crystallization of thin plates and other products only extends in the rolling direction, not in the width direction, so the grain size changes with the direction. However, the cold drawing die used in the wire drawing process uses strong pressure different from the rolling method to evenly extrude wires from all sides, so the crystallization can only develop in the wire drawing direction, and as a result, the lamellae are evenly thinned and the strength is rapidly improved. In order to exert strong pressure on ultra-high strength steel wire, ultra-high hardness diamond dies are often used in wire drawing.
(7) The key to high strength is the transformation of pearlite.
As mentioned above, pearlite of high carbon steel is much stronger than ferrite single phase of low carbon ordinary steel. It can be seen that pearlite is easy to obtain high strength under small wire drawing deformation, so it has become an important factor in industrialization. On the contrary, no matter how strong the pressure is applied, it is difficult to achieve high strength effect for cold drawing of pure iron.
At present, the mechanism of rapid strength improvement of pearlite by wire drawing is not completely clear. One of the important reasons is that "fine grain strengthening", which makes the crystal thin by wire drawing, and "dislocation strengthening", which increases the number of dislocations and hardens by processing, play an important role, which is the same as the hardening phenomenon of steel wire during continuous bending.
For other structures, such as "cementite fine grain strengthening", the strength of cementite without grain boundary before drawing can also be improved after drawing and refining to nanometer level; There is also cementite (Fe3C) which is decomposed and stabilized by wire drawing, and the decomposed carbon adheres to dislocation, making it difficult to move, thus improving the strength of "solid solution strengthening". Previously, it was only known that metal compounds would decompose under the action of large external forces. Recently, it was found that cementite was completely decomposed, which attracted attention from all sides. As a pioneer in developing high-carbon steel wire, Nippon Steel takes the change of strength and ductility caused by cementite decomposition as an important research topic, and develops high-strength steel wire by studying its mechanism.
The reason why the decomposition mechanism of cementite has not been clearly understood is that iron is ultrafine structure, and the cementite after strong processing is also ultrafine structure of several nanometers, which is difficult to observe with general instruments, so it is difficult to explain its mechanism. But now, through the "high-resolution energy transmission microscope" and the atomic observer with magnification of 6.5438+0 million times, the organization of single iron atom, ferrite and cementite can be clearly observed, which has made great progress and is expected to be solved in the near future.
(8) The challenge of the opposition between strength and ductility
In order to make high carbon steel wire practical, not only the strength problem, but also the problem of insufficient ductility caused by fracture must be solved. From the relationship between them, when the strength of steel wire for bridge exceeds 2000MPa, its ductility decreases rapidly, that is, the actual maximum strength should be balanced with ductility. From a technical point of view, if we simply pursue strength, we can further improve it, but considering the obvious decline in ductility, the ultimate strength of radial steel wire is only below 4000MPa.
When a uniform pressure is applied to the cross section of the steel wire for thermal fracture test, the steel wire with high ductility will break (normal fracture) in the vertical direction of drawing after dozens of twists, while the steel wire with low ductility will crack (twist) in the vertical direction of drawing at the initial stage of torsional deformation, which is called as an important reason affecting high strength. In addition, when the wire diameter is large, there will be torsional cracks around 2000MPa, while when the wire diameter is small, there will be no torsional cracks at 4000MPa, which is the so-called "wire diameter effect". There are different opinions about the causes of torsional cracking, and the decomposition of cementite is the main reason.
(nine) as far as possible to reduce the wire drawing processing of high strength steel.
When the steel wire is reinforced by a processing program that gives consideration to both steel strength and ductility, the strength is first improved by toughening, and then the strength increase per unit deformation (work hardening rate) is increased by increasing wire drawing (work deformation). For example, measures such as zinc plating (450℃) and bluing treatment are taken to suppress the strength decrease caused by heating.
Using the above method to maintain high strength, it is also necessary to prevent ductility reduction, that is, ductility reduction can be avoided from causality. The experimental results show that it is more effective to improve the work hardening rate by toughening high-strength wood and reducing the drawing amount than by increasing the drawing amount. For example, when the ultimate strength target is 2000MPa, materials with low toughness (1000 ~ 1300 MPa) are prone to torsional cracking when the target is reached by increasing the drawing amount; For the toughness treated material of 1400MPa, if the drawing amount is appropriately reduced, this situation will not occur. It can be seen that the latter is more effective for maintaining the necessary ductility at high strength.
There are also many ways to strengthen and toughen wood, and the representative method is alloying. That is, increasing the content of carbon, vanadium, chromium, silicon and other elements in steel can improve the strength. Among them, the general basic method is to increase carbon content; Silicon can play an active role in solid solution strengthening of ferrite; Chromium can make the thickness of lamellae thinner during toughening treatment, thus improving the strength obviously. In addition, adding 0.2 ~ 0.5% chromium to high carbon steel (containing 0.82%% C) will obviously improve the work hardening rate during wire drawing, so it is very beneficial to the high strength of high carbon steel wire. The application of radial steel wire and bridge steel wire has been introduced before.