Views: 1 Author: Site Editor Publish Time: 2024-10-30 Origin: Site
First, heating defects of steel pipes
(I) Oxidation of steel pipes and its influencing factors: When steel is heated in a high-temperature furnace, the furnace gas contains a large amount of O2, CO2, and H2O. The surface layer of the steel pipe will oxidize. Oxidation not only causes direct losses of steel pipes - the yield rate decreases, but also rolling will press iron oxide chips into the surface of the steel pipe when the scale is not removed cleanly, causing pitting defects on the surface of the finished steel. If the oxide layer is too deep, the subcutaneous bubbles of the ingot will be exposed, resulting in waste after rolling. The thermal conductivity of the iron oxide scale is lower than that of steel, so the surface of the steel pipe is covered with the iron oxide scale, which doubles the deterioration of heat transfer conditions, reduces furnace productivity, and increases energy consumption. The factors affecting the oxidation of steel pipes are heating temperature, heating time, furnace gas composition, and steel pipe composition. Among these factors, the heating temperature, furnace gas composition, and steel pipe composition have a greater impact on the oxidation rate, while the heating time mainly affects the amount of burn loss of the steel pipe.
1. Influence of heating temperature: Since oxidation is a diffusion process, the influence of temperature is very significant. The higher the temperature, the faster the diffusion and the greater the oxidation rate. The oxidation rate of steel pipes at room temperature is very slow. It begins to change significantly above 600°C. When the steel temperature reaches above 900°C, the oxidation rate increases sharply.
2. Composition of steel pipes: For carbon steel, the amount of steel pipe burnout decreases with the increase of its C content. This is because after the C in the steel pipe is oxidized, part of it generates CO, which prevents the oxidizing gas from diffusing into the steel. Therefore, under the same heating conditions, the burnout of high-carbon steel is lighter than that of low-carbon steel pipes. Alloy elements such as Cr and Ni are easily oxidized into corresponding oxides, but because the thin layer of oxides they generate is very dense and stable, this thin oxide film prevents the internal matrix of the steel pipe from being re-oxidized. Therefore, chromium steel, chromium-nickel steel, chromium-silicon steel, etc. have good high-temperature oxidation performance.
3. Influence of heating time: Under the same conditions, the longer the heating time, the more oxidation and burning loss of the steel pipe, so the heating time should be shortened as much as possible.
(II) Decarburization: When the steel pipe is heated, based on the formation of iron oxide scale, due to the presence and diffusion of high-temperature furnace gas, the carbon atoms in the unoxidized steel pipe surface layer diffuse outward, and the oxygen atoms in the furnace gas also diffuse inward through the iron oxide scale. When the two diffusions meet, the carbon atoms are burned, resulting in the chemical composition of the unoxidized steel pipe surface layer being carbon-poor. This phenomenon is called decarburization.
1. Influence of heating temperature on decarburization: The effect of heating temperature on the thickness of the visible decarburization layer of the steel billet varies depending on the type of steel. Generally, as the heating temperature increases, the thickness of the visible decarburization layer increases significantly. However, for some steel types, the thickness of the decarburization layer begins to increase with the increase in temperature. However, after the heating temperature reaches a certain value, the thickness of the decarburization layer does not increase with the increase in temperature but decreases instead. Many steel types have similar rules. For these steel types, when selecting the heating temperature, the "peak" temperature range of this decarburization rate should be avoided as much as possible.
2. The effect of heating time on decarburization. The longer the heating time, the thicker the visible decarburization layer. Therefore, shortening the heating time, especially shortening the residence time of the billet in the furnace after the surface has reached a higher temperature, to achieve rapid heating, is an effective measure to reduce the decarburization of the billet.
3. The effect of furnace atmosphere on decarburization: The effect of furnace atmosphere on decarburization is fundamental. H2O, H2, O2, and CO2 in the furnace atmosphere can cause decarburization, while CO and CH4 can increase the carbon content of steel. Practice has proved that to reduce the thickness of the visible decarburization layer, heating in a strong oxidizing atmosphere is beneficial, because the oxidation of iron will exceed the oxidation of carbon, thus reducing the thickness of the visible decarburization layer.
4. The effect of the chemical composition of the steel pipe on decarburization: The higher the carbon content in the steel pipe, the easier it is to decarburize when heated. If the steel pipe contains elements such as aluminum, tungsten, and cobalt, the decarburization will increase; if the steel pipe contains elements such as chromium, manganese, and boron, the decarburization will decrease. Nickel, silicon, and vanadium do not affect decarburization. The types of steel that are prone to decarburization mainly include carbon tool steel, die steel, spring steel, ball bearing steel, high-speed steel, etc.
Measures to reduce decarburization: Measures to reduce the oxidation of steel pipes apply to reducing decarburization. For example, rapid heating, shortening the residence time of steel in the high-temperature area, correctly selecting the heating temperature, avoiding the decarburization peak range of steel pipes that are prone to decarburization; properly adjusting and controlling the atmosphere in the furnace, maintaining an oxidizing atmosphere in the furnace for steel that is prone to decarburization, and making the oxidation rate greater than the decarburization rate, etc.
(III) Overheating of steel pipes
If the heating temperature of the steel pipe exceeds the critical temperature AC3, the grains of the steel pipe begin to grow, and grain coarsening is the main feature of overheating. The higher the heating temperature and the longer the heating time, the more significant this grain growth phenomenon is. Excessive grain growth will reduce the mechanical properties of the steel pipe and easily produce cracks during processing. Especially in the angular parts of the ingot or the edge parts of the parts, cracks will occur during rolling, causing cracks in the finished product. Heating temperature and heating time have a decisive influence on grain growth. In the rolling operation, the heating temperature and the time the steel stays in the high temperature area should be mastered.
(IV) Overburning of steel pipes
When the steel is heated to a temperature higher than overheating, not only do the grains of the steel pipe grow, but the film around the grains begins to melt, and oxygen enters the gaps between the grains, causing the steel to oxidize, resulting in a significant reduction in the bonding force between the grains and deterioration of plasticity. In this way, the steel will crack during the pressure processing process, causing cracks in the finished steel. This phenomenon is overburning.
Second, the heating temperature and heating speed of the steel pipe
The heating temperature of the steel pipe refers to the surface temperature of the steel when it is heated and taken out of the furnace. The heating before rolling is to obtain good plasticity and smaller deformation resistance. The most suitable heating temperature should enable the steel to obtain the best plasticity and minimum deformation resistance, which is conducive to hot processing, increasing production, and reducing equipment wear and power consumption, but for heating high-quality steel, there are different heating processes according to different heating purposes. During the heating process of the steel pipe, due to the thermal resistance of the steel itself, there is inevitably a temperature difference between the inside and outside. The surface temperature always rises faster than the center temperature. At this time, the expansion of the surface is greater than the expansion of the center. In this way, the surface is under pressure and the center is under tension, so thermal stress is generated inside the steel pipe. The magnitude of the thermal stress depends on the magnitude of the temperature gradient. The faster the heating speed, the greater the temperature difference between the inside and outside, the greater the temperature gradient, and the greater the thermal stress. If this stress exceeds the rupture strength limit of the steel pipe, cracks will occur inside the steel pipe, so the heating speed must be limited to the range allowed by the stress. The stress in the steel pipe is only dangerous within a certain temperature range. Most steels are in an elastic state below 550°C and have relatively low plasticity. At this time, if the heating speed is too fast, the temperature stress exceeds the strength limit of the steel pipe, and cracks will appear. When the temperature exceeds this temperature range, the steel enters the plastic state. For low-carbon steel, a lower temperature may enter the generation range. At this time, even if a large temperature difference is generated, the stress will disappear due to plastic deformation and will not cause cracks. Therefore, the temperature stress limits the heating speed mainly at low temperatures (below 500°C). Generally speaking, the heating speed of low-carbon steel in the low temperature section is not limited. For high-carbon steel and alloy steel pipes, the low-temperature plasticity is poor and the thermal conductivity is low, so the heating speed in the low-temperature section should be controlled.
Third, the heating system and heating time
That is, the steel billet is placed in three sections with different temperature conditions for heating, which are the preheating section, heating section, and soaking section. The three-stage heating system is a relatively complete heating system with many advantages. The steel billet is first preheated in the low temperature area. At this time, the heating speed is relatively slow, the temperature stress is small, and it will not cause danger. After the center temperature of the steel pipe exceeds 500℃, it enters the plastic range. At this time, it can be heated quickly until the surface temperature rises rapidly to the temperature required for leaving the furnace. At the end of the heating period, there is still a large temperature difference on the steel section, and it is necessary to enter the soaking period for soaking to reduce the temperature difference between the surface and the center. It should be noted that the heating system is not completely consistent with the furnace type of the heating furnace. The three-stage heating furnace can change the temperature distribution in the furnace by artificially adjusting the burner, thereby changing the formal distribution of the preheating section, heating section, and soaking section. The heating time is also related to the distribution of the billet in the furnace. The same billet has different heating times due to different heating areas under different charging steps. This is sometimes very important and cannot be ignored.
Fourth, the furnace pressure system
The furnace pressure system is also an important factor affecting the heating speed, heating quality, and fuel utilization of the billet. The pressure size and distribution in the heating furnace are one of the important means to organize the flame shape, adjust the temperature field, and control the atmosphere in the furnace. The furnace pressure of the heating furnace is usually referred to as the difference between the absolute pressure of the gas in the furnace and the atmospheric pressure outside the furnace, that is, the relative pressure. The distribution of the furnace pressure of the heating furnace along the length of the furnace varies with the furnace type, fuel combustion method, and operating system. Generally, the furnace pressure in the continuous heating furnace increases from the discharge side to the feed side, with a total pressure difference of 20 to 40Pa. In addition, due to the potential difference of the hot gas, there is also a vertical pressure difference in the heating furnace, which increases from bottom to top. The pressure difference per meter of furnace height within the normal operating temperature range is about 10Pa. Control benchmark and requirements for furnace pressure. To minimize oxidation and energy consumption in the furnace, the pressure in the heating furnace is generally required to be controlled at zero or slightly positive pressure. Since the pressure at each point in the heating furnace is different, the basic requirement for the furnace pressure system in actual production is to keep the pressure near the surface of the billet at the discharge end of the furnace at zero pressure or slightly positive pressure (about 0 to 20 Pa higher than the atmospheric pressure), while the airflow in the furnace is smooth, and strive to prevent fire at the tail of the furnace. When the furnace pressure is too high, a large amount of high-temperature gas will escape from the furnace, which not only worsens the working environment and makes operation difficult, but also shortens the life of the furnace and causes a lot of fuel waste.
Fifth, problems in the heating operation
① Inconsistent heating effect of steel billet: Since the capacity of the rolling mill is much greater than that of the heating furnace, the production mode is: rapid rolling of one furnace of steel billet - equalizing steel temperature - rapid rolling of one furnace of steel billet - reciprocating. This phenomenon is very serious when producing 220×260 billets. The time of equalizing steel temperature in the middle is as high as nearly 40 minutes/cycle. This causes the steel billets in the same furnace to stay at different temperatures for different times, that is, the heating effect is different. The steel billets that stay for a long time in the high-temperature section tend to overheat the surface, and the steel billets that stay for a long time in the low-temperature section are not heated thoroughly, and the interstitial atoms (C, N) cannot diffuse effectively, and it is easy to form a banded structure after rolling.
② High heating temperature at the tail of the furnace: During normal heating, the heating temperature at the tail of the furnace (heating stage I) is as high as 1050℃, which is very unfavorable for the heating of high-alloy steel pipes to be developed in the future.
③ High furnace pressure: Due to the insufficient capacity of the heating furnace, the operator increases the input gas and air volume, and the heat load of the heating furnace is always at the highest state. However, the exhaust gas discharge capacity is insufficient, that is, the input is much greater than the output, resulting in a high furnace pressure, and a large amount of high-temperature gas escapes from the furnace, which not only causes a waste of gas, but also the heat storage body cannot use the exhaust gas for preheating, resulting in the input gas and air not being well preheated, and the combustion is extremely incomplete. This creates a vicious cycle.