Continuous casting process of vanadium steel
In the production process, vanadium steel, like other high-strength low-alloy steels, is mainly produced by continuous casting process, with relatively few die casting processes. Therefore, the continuous casting process is generally concerned. The quality of the continuous casting billet, such as the structure, surface defects, center segregation, uniformity of composition and structure, etc., have a certain influence on the performance and quality of the final product.
Improving and enhancing the solidification structure of continuous casting billets is an important part of obtaining high-performance final products. In general, the solidification structure of the continuous casting billet from the edge to the center is composed of small equiaxed crystal bands, columnar crystal bands and central equiaxed crystal bands. The small equiaxed crystal belt is located on the surface of the continuous casting billet. When the liquid steel enters the mold, the molten steel contacts the copper wall mold, and the cooling rate is very fast. A small equiaxed crystal belt is formed on the edge of the cast slab; the columnar crystal belt is located On the inside of the small equiaxed crystal belt, the formation process of the small equiaxed crystal belt is accompanied by the volume shrinkage of the billet. When the billet is separated from the copper wall, an air gap is formed, which reduces the heat transfer rate, and the cast billet forms a columnar crystal zone. ; The equiaxed crystal belt is located in the center of the continuous casting slab. As the solidification front progresses, the temperature gradient between the solidification layer and the solidification front gradually decreases, the width of the two-phase zone gradually increases, and the liquid phase temperature of the core of the slab drops to the liquid phase After the wire, the core begins to crystallize. Because the single phase of heat transfer in the core is not obvious, equiaxed crystals are formed, heat transfer is limited, and the crystal grains are larger than the chilled layer.
Preventing cracks in the cast slab during continuous casting is a very important issue for the quality of the cast slab. As the continuous casting molten steel cools and solidifies, phenomena such as liquid shrinkage, solidification shrinkage, and solid state shrinkage will occur. Among them, the amount of solid state shrinkage is relatively large, and thermal stress will be generated during the temperature drop, and structural stress will be generated during the phase transformation process. The generation of these internal stresses is the source of cracks in the slab. Therefore, solid state shrinkage has the greatest impact on the quality of the slab.
The defects of continuous casting slab mainly include the purity, surface quality, internal quality and appearance of the continuous casting slab. Among them, there are two main types: surface defects and internal defects. Surface defects are important defects that affect the production of continuous casting and the quality of continuous casting billets, including surface longitudinal cracks, transverse cracks, network cracks, subcutaneous inclusions and subcutaneous pores; internal defects mainly include central segregation, central looseness, middle cracks, and subcutaneous cracks. These internal defects are formed under the influence of factors such as the bulge of the cast slab, the bending and straightening of the liquid core, the thermal stress generated by the rise of the surface temperature of the cast slab, and the gaps of the dendrites filled with excessively enriched solute elements. of. These defects have a greater impact on the final quality of the rolled material, and it is impossible to eliminate them in the subsequent processing.
Among the surface defects of continuous casting slab, longitudinal cracks mostly occur in the center of the wide surface of the slab, and the billet mostly occurs at the corners. The main reason is that the thickness of the slab shell is uneven due to the uneven cooling strength in the mold. Where the stress is concentrated, when the stress exceeds the tensile strength of the blank shell, longitudinal cracks are produced. The transverse cracks on the surface of the continuous casting slab mostly appear at the troughs of the inner arc side of the casting slab, which are usually hidden and invisible. After metallographic inspection, they are in the ferrite network zone, which also happens to be the primary austenite grain boundary. It can also be observed that there are fine precipitated material points, which reduces the bonding force of the grain boundary and induces transverse cracks. Compared with the longitudinal cracks of the slab, it has a greater impact on the quality of the continuous casting slab. In the continuous casting process of high-strength low-alloy steel (HSLA) steel, transverse cracking is a common form of failure. Many researchers have carried out a lot of research work in this area. Although the research conditions and test methods are not the same, the test The results show a relatively consistent trend of change. When continuous casting steel shrinks in solidification and solid state, there is a plastic trough area at 700~950 ℃. When mechanical load (such as straightening) is applied to the surface of the cast slab, it is easy to induce transverse cracks. Therefore, it can be said that the occurrence of transverse cracks of continuous casting slab is closely related to the high temperature brittle zone of 700~950℃ of continuous casting steel.
Figure 1 shows the relationship between the reduction of area and temperature of continuous casting billets of different steel grades. Figure 1a shows the test results of Mintz and Abushosha. The preheating temperature is 1330℃; Figure 1b shows the test results of solidified samples. Although the test methods are different, the test results are basically the same. It can be seen from the figure that as the temperature decreases, the change trend of the reduction of area of C-Mn steel and niobium and vanadium microalloyed steel is the same. As the temperature decreases, the reduction of area of steel starts to decrease from 950°C When the temperature reaches 810~830℃, the plasticity drops to a trough, the temperature continues to decrease and the reduction of area begins to rise again, and the plasticity completely recovers near 750℃. There is a similar high temperature plasticity trough area in several different test steels.
Figure 1 The relationship between the reduction of area and temperature of continuous casting billets of different steel grades
a- Preheating temperature is 1330℃; b- solidified sample
The high-temperature plasticity of C-Mn steel and niobium and vanadium microalloyed steel is different, which is mainly manifested in the width of the low-temperature plasticity valley and the temperature at which the plasticity begins to decrease. With the decrease of temperature, the reduction of area of C-Mn steel begins to decrease from about 900℃. When the temperature drops to about 820℃, the reduction of area of C-Mn steel is the lowest, and the plasticity trough appears. When the temperature continues to decrease, the reduction of area is again It starts to rise, and the plasticity of C-Mn steel is completely restored when the temperature drops to about 750°C. Therefore, for C-Mn steel, a trough is formed from 900 to 750°C.
Microalloying elements such as niobium and vanadium have an effect on the high-temperature ductility of continuous casting billets, and the most significant effect is niobium. It can be seen from Figure 1 that the upper critical temperature of niobium-containing steel is relatively high, about 1000 ℃, which is about 100 ℃ higher than that of C-Mn steel, and the lower critical temperature for complete plastic recovery is also reduced by nearly 100 ℃, so The range of niobium steel’s high temperature plastic trough area is obviously expanded, which shows that the niobium-containing steel is prone to transverse cracks when bending and straightening in a wider temperature range during continuous casting. N. Bannenberg also studied the high temperature thermoplasticity of niobium-containing steel. As shown in Figure 2, the effect of niobium on the thermoplasticity of steel is given. From the figure, it can be seen that the section shrinkage of the niobium-containing steel when the temperature is reduced to 1050°C The reduction rate of niobium-containing steel is the lowest at 830℃, and the reduction of area at the crack area continues to decrease, and the plasticity is fully recovered at 700℃. At the same time, the author puts forward the concept of a critical value for the reduction of area. It is stipulated that the reduction of area must be above 75% during the pouring process to avoid the occurrence of transverse cracks. Obviously, the critical range for cracking of C-Mn steel is very narrow, while the critical range for cracking of niobium steel is much wider. It is easy to produce transverse cracks when subjected to mechanical loads such as bending and straightening in this temperature range. Therefore, With the continuous cooling of the slab during continuous casting, the temperature of the surface, edges and corners of the slab must be bent and straightened before it drops to the critical range of 1000~700℃.
Figure 2 The effect of niobium on steel thermoplasticity
The high-temperature plasticity of vanadium-containing steel is close to that of C-Mn steel, and lies between C-Mn steel and niobium-containing steel. From the perspective of the width of the plastic trough zone, the width of the plastic trough zone of the vanadium microalloyed steel is slightly increased compared to the C-Mn steel in the high temperature direction; from the temperature range of the plastic trough zone, the vanadium microalloyed steel has a plastic trough zone. The temperature range of C-Mn steel is slightly extended to the high temperature direction, although the test results are not the same, the temperature range is extended to high temperature by about 50 ℃. From the above two aspects of the width of the plastic trough zone and its existence temperature range, the vanadium microalloyed steel is not sensitive to the cracks of the cast slab, and is relatively close to the C-Mn steel. The most sensitive is the niobium microalloyed steel. However, nitrogen has an effect on the high temperature plasticity of vanadium microalloyed steel, and many researchers have done a lot of research work on this. Mintz et al. studied the effect of vanadium and nitrogen content on the high temperature plasticity of 0.1%C-1.4%Mn-0.03%Al steel. For the convenience of comparison, the figure also shows the reduction of area of 0.03%Nb steel, as shown in Figure 3. Show. When the vanadium-nitrogen content is relatively low, that is, when the nitrogen content is less than 0.005%, and the vanadium content is less than 0.1%, the reduction of area of the steel at 850℃ is much higher than that of the 0.03%Nb steel. As the content of vanadium and nitrogen increases, the reduction of area at 850°C gradually decreases. When both nitrogen and vanadium are relatively high, that is, when the nitrogen content is greater than 0.01% and the vanadium content is greater than 0.1%, the high temperature plasticity of vanadium steel is only as good as 0.03% niobium. The steel is close, this phenomenon occurs because under continuous casting conditions, the coarsening rate of the precipitated V(C,N) is much faster than that of Nb(C,N), so the damage to high temperature plasticity is much smaller than that of niobium. Caused by. The increase of nitrogen content will promote the precipitation of V(C,N) particles in austenite and reduce the high temperature ductility of vanadium steel. For any steel, an increase in the volume fraction of precipitated particles will lead to a decrease in high temperature plasticity.
Figure 3 The effect of the product of the total content of V and N on the reduction of area at 850℃
A large number of research and test results show that in the continuous casting process, according to the width and temperature range of the high temperature plastic trough zone of various steels, the cooling rate of the secondary cooling zone is strictly controlled, and the smooth weak cooling is adopted to make the casting slab during bending and straightening. The surface temperature is higher than the precipitation temperature of carbonitrides, or higher than the γ→α phase transition temperature, or higher than the upper critical temperature for embrittlement, avoiding the plastic trough zone, regardless of vanadium steel, VN steel with high nitrogen and vanadium content, Niobium steel can prevent transverse cracks in the cast slab. In addition, from the use of a reasonable mold and high frequency, small amplitude, and good performance mold powder, maintain the stability of the mold liquid level, reduce the sulfur and phosphorus in the steel, inhibit the precipitation of carbonitrides, and make the Measures such as refining the austenite grains in the surface layer of the cast slab and reducing the crack sensitivity are also helpful to prevent the occurrence of transverse cracks in the cast slab.
Source: China Butt Weld Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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