(7) The key to high strength is the transformation of pearlite
As mentioned above, the pearlite of high-carbon steel has much higher strength than the single ferrite phase of low-carbon ordinary steel. From this, it can be seen that pearlite is easy to obtain high strength under a small amount of drawing deformation, so it becomes an important factor for industrialization. On the contrary, it is difficult to achieve the effect of increasing the strength of pure iron no matter how strong the pressure is applied to cold drawing.
The mechanism by which pearlite can rapidly increase its strength through wire drawing is not yet fully understood. An important reason is that the “fine-grain strengthening” that thins the thickness of the lamellae after the crystallization is refined by wire drawing, and the “dislocation strengthening” that hardens the number of dislocations by processing have played an important role. , This is the same phenomenon that the place becomes hard when the steel wire is continuously bent.
For other organizations, such as cementite without grain boundaries before drawing, the strength of cementite can be improved after drawing to the nanometer level; there is also cementite that stabilizes the metal flower (Fe3C) It is decomposed by wire drawing, and the decomposed carbon adheres to dislocations to make it difficult to move, resulting in “solid solution strengthening” which increases the strength. In the past, it was only known that metal compounds would decompose under the action of a large external force. Recently, it has been discovered that all cementite is decomposed, which has attracted attention from all parties.
As a pioneer in the development of high-carbon steel wires, Nippon Steel takes the strength and elongation changes caused by cementite decomposition as an important research topic, and develops high-strength steel wires by studying its mechanism.
The reason why the decomposition mechanism of cementite is not ascertained is that iron is an excessively fine structure, and the cementite after strong processing is also an excessively fine structure of a few nanometers, which is difficult to observe with ordinary equipment, so its mechanism is difficult to explain. But now, through the “high resolution transparent microscope” that can analyze nanostructures and an atomic observer that can magnify 1 million times, the structure of single iron atoms, ferrite, and cementite can be clearly observed. , The research has made great progress and it is expected to be solved in the near future.
(8) The challenge of strength and extensibility
In order to make high carbon steel wire practical, not only the strength, but also the problem of insufficient elongation caused by breaking must be solved. From the perspective of the relationship between the two, when the strength of the steel wire for bridges exceeds 2000 MPa, its extensibility drops rapidly, that is, the highest practical strength should be balanced with the extensibility. From a technical point of view, the pure pursuit of strength can be further improved, but considering the significant decrease in extensibility, the ultimate strength of the radial steel wire is now only specified below 4000MPa.
A steel wire with high extensibility is subjected to a thermal break test by applying uniform pressure on the cross section of the steel wire. After dozens of twists, the vertical direction of the drawn wire breaks (normal breaking), but the steel wire with low extensibility deforms under torsion In the early stage, cracks (twisting) occurred along the vertical direction of the wire drawing. The occurrence of this phenomenon is known as an important reason for high strength. In addition, when the wire diameter is large, twisting occurs at about 2000 MPa, and when the wire diameter is small, it does not occur until 4000 MPa. This is called the “wire diameter effect”. There are many opinions on the causes of twisting cracks. According to research, the decomposition of cementite is the main cause.
(9) Minimize the high-strength steel processed by wire drawing
Use a processing process that takes into account both the strength and elongation of the steel. When strengthening the steel wire, first increase the strength by toughening treatment, and then increase the increase in strength per deformation (work hardening rate) by increasing the wire drawing (processing deformation) , As well as steel wire for bridges, some measures should be taken to suppress the strength drop caused by heating such as galvanizing (450℃) and bluing treatment.
While adopting the above methods to maintain high strength, it should also prevent the decrease in extensibility, that is, starting from the causal relationship that maintaining extensibility can avoid twisting, the test results prove that the toughened material is used to increase the strength and reduce the amount of wire drawing. The method of increasing the work hardening rate is more effective than increasing the amount of wire drawing to maintain the elongation. For example, when the final strength target is 2000 MPa, for low toughness treated materials (1000-1300 MPa level), twisting will easily occur when the target is reached by increasing the amount of drawing processing; if the 1400 MPa toughness treatment material is appropriately reduced It does not happen when measuring. It can be seen that the latter is more effective for maintaining the necessary extensibility under high strength. Precision spring
There are also many strengthening methods for toughening materials, and the representative method is alloying. That is, increasing the content of carbon, vanadium, chromium, silicon and other elements in the steel can increase the strength. Among them, the general basic method is to increase the carbon content; silicon can play a positive role in the solid solution strengthening of ferrite; chromium can make the thickness of the lamellae finer during the toughening treatment, so that the effect of increasing the strength is obvious. In addition, adding 0.2 to 0.5% of chromium to high carbon steel (containing C0.82%) can significantly increase the work hardening rate during wire drawing, so it is very helpful to increase the strength of high carbon steel wire. The application of meridian steel wire and bridge steel wire has been introduced in the previous article.