The principle of heating furnace
The purpose of heating is to heat the billet to a uniform temperature suitable for rolling (austenite structure). After the temperature is increased, the plasticity of the steel is first increased, the deformation resistance is reduced, and the steel is easily deformed. For example, the deformation resistance of T12 steel at room temperature is about 600Mpa, and when heated to 1200 °C, the deformation resistance drops to about 30Mpa, which is only one-twentieth of the deformation resistance at room temperature. Steel with a suitable heating temperature can be rolled with a larger reduction, which can reduce equipment accidents caused by wear and impact, improve the productivity and operation rate of the rolling mill, and reduce rolling energy consumption. Secondly, heating can improve the internal structure and properties of the billet. The inhomogeneous structure and non-metallic inclusions are homogenized by the diffusion effect of high temperature heating. Heating temperature and uniformity are the signs of heating quality. When steel with good quality is heated, it is easy to obtain finished products with correct cross-sectional shape and accurate geometric dimensions.
2. Heating process
The heating temperature of the billet includes the surface temperature, the temperature difference along the section and the temperature difference along the length of the billet. The final heating temperature of the billet in the furnace is determined after considering the actual conditions such as the rolling process, the structural characteristics of the rolling mill and the structural characteristics of the furnace. The time required to heat to the specified temperature depends on the size of the billet, the type of steel, the temperature regime used and some other conditions.
The billet gets heat in the furnace by convection and radiation. The former is the furnace gas scouring the surface of the billet; the latter is the radiant heat of the furnace gas and the hot furnace lining. Our heating furnace is controlled in three sections along the length direction: the preheating section, the heating section and the soaking section. When the billet enters the preheating section of the heating furnace, the heat flow increases gradually. When the billet reaches the second heating section, the heat flow remains basically unchanged, and when the billet reaches the soaking section, the heat flow gradually decreases. In the soaking section of the billet, the surface temperature of the billet remains basically unchanged, while the temperature difference of the section gradually decreases, and the heat obtained on the surface of the billet diffuses to the interior by heat conduction. The smaller the heat flow transmitted to the surface of the billet, the larger the heating area, the smaller the section size of the billet, the greater the thermal conductivity of the steel, and the smaller the section temperature difference. Generally, the heating time of a billet with a large section is longer than that of a billet with a small section, and the heating time of alloy steel is longer than that of carbon steel.
3. Heating defects
Alloy steel cracking: At the beginning of heating (below 700℃), for high-carbon tool steel, high-manganese steel, bearing steel, high-speed steel and other steels with low thermal conductivity, if the heating rate is too fast, the surface temperature suddenly rises and the fracture surface If the temperature difference is too large, thermal stress will occur, resulting in cracks.
Overheating and over burning: If the heating temperature is too high or the residence time at high temperature is too long, the grains of the steel will grow too much, the connection between the grains will be weakened, and the steel will become brittle, which is called overheating. Cracks will occur when the overheated billet is rolled; even if the rolling does not crack, the mechanical properties of the finished product cannot meet the requirements. Normalizing overheated billets can be salvaged. Overheating further develops, the grains continue to grow, and the grain boundaries are oxidized or melted, and are often broken or cracked during rolling, which is called over burning. Overfired billets are irretrievable waste. When the rolling industry suddenly breaks down and stops rolling, overheating or over burning is prone to occur; if the temperature of high carbon steel is not properly controlled, it is also prone to overheating or over burning.
Oxidation and decarburization of billets: During the heating of billets in the furnace, the metal elements in the steel react with the oxidizing atmosphere in the furnace to produce iron oxide scale (the inner layer of the iron oxide scale is iron oxide, and the middle layer is iron tetroxide). , the outermost layer is ferric oxide). Decarburization is caused by carbon in the steel diffusing to the surface and reacting with the furnace atmosphere. Bearing steel, tool steel, spring steel and some other steel grades, the decarburization of steel is harmful. , the elasticity is reduced.
The oxidation and decarburization processes are carried out simultaneously, and they are all related to the heating conditions (temperature, time in the furnace, atmosphere in the furnace, and chemical composition of the billet). Generally speaking, oxidation and decarburization are not obvious when the temperature is less than 750℃. However, the increase is particularly rapid when the temperature is greater than 800 °C.
Special attention is: for carbon steel, the heating temperature should not be higher than 1300℃. Above this temperature, the iron oxide scale on the surface of the billet will melt (the melting point of pure iron oxide scale is 1377~1565℃, and it will drop to 1300~1350℃ when it contains impurities). ℃), falling off the steel surface, exposing a new surface, and the burning loss increases rapidly. Second, when the heating temperature is greater than 1300°C, the billet is likely to be overheated or overburned. When producing other steel grades, more attention should be paid to the maximum heating temperature.
After the steel billet is oxidized and burned, it will inevitably affect the yield; the thermal conductivity of the iron oxide scale is very poor, which has an impact on the heating time of the billet; the iron oxide scale falls off and accumulates in the lower furnace, and manual cleaning is very difficult and the work is very hard.
4. Thermal system
The thermal system involves the situation of fuel and air entering the furnace, the combustion situation, the elimination of combustion products, and the recovery and utilization of waste heat. Thermal system includes temperature system, furnace pressure system and furnace combustion system.
Temperature system: see the table below for heating temperature and precautions
Heating temperature and precautions
ordinary low carbon steel
Ordinary high carbon steel
High carbon steel is easy to decarburize at high temperature and must be avoided
Austenitic stainless steel (including heat-resistant steel)
Steel with high nickel and chromium content has poor thermal conductivity, and the heating time is 0.5~1 times longer than that of carbon steel
Sulfur in combustion products and nickel in steel produce low melting point S, which is prone to cracks during processing, and low-sulfur fuel should be used
High deformation resistance, high temperature heating is required
A part of ferrite is formed at high temperature, and the heating temperature range is narrow. It should be uniformly heated and overheated for a long time at high temperature, and the grains will grow.
Ferritic and martensitic stainless steels
The deformation resistance is smaller than that of austenitic stainless steel, and the heating temperature is similar to that of carbon steel.
The temperature of low carbon and high chromium steel is easy to overheat when the temperature is above 1050℃, which affects the normal rolling.
high speed steel
Tungsten-containing high-speed steel has a narrow processing temperature range. It should be fully and uniformly heated. It is easy to overheat and decarburize at high temperature. Care must be taken to avoid poor thermal conductivity. The heating time is more than twice that of carbon steel.
Bearing steel, spring steel
Decarburization must be strictly avoided, and the high temperature section is rapidly heated
If the temperature is too low, the wear of the rolling mixed pass will be accelerated.
Win phosphorus high flow free cutting steel
If the temperature at the end of the rolled piece is low, cracks are prone to occur, so uniform heating is required.
Composite free-cutting steel
The sensing element for measuring the furnace temperature and steel temperature is a thermocouple, which measures the furnace temperature, which is generally more than 20 °C higher than the surface temperature of the billet.
5. Furnace pressure system:
The size and distribution of the furnace pressure in the heating furnace is an important means to adjust the temperature field, control the flame and the atmosphere in the furnace, it affects the heating speed and heating quality, and also affects the quality of fuel utilization. In particular, the furnace pressure at the discharge section of the furnace is particularly important.
The furnace pressure setting should be about 0~30Pa higher than the atmospheric pressure. If the furnace pressure is too large, the openings such as the charging port, the discharging port, and the observation hole will catch fire. The result is: (1) The loss of furnace gas increases, which increases the heat loss; (2) Harmful gases such as SO2 enter the workshop and pollute the working environment; (3) The furnace wall, nearby steel structure or mechanical equipment of the fire part is damaged or deformed. On the contrary, if the furnace pressure is too low, the absorption of cold air in the workshop will reduce (1) the furnace temperature and increase the fuel consumption; (2) the cooling of the billet by the low-temperature air will lead to uneven temperature;
6. Combustion system:
The basic requirement is to ensure that the fuel is completely burned in the furnace, and the air coefficient α should be small. Excessive air will lead to a large amount of flue gas, which will increase the amount of heat taken away; if the amount of air is insufficient, incomplete combustion will be formed in the furnace, which will also increase heat loss.
7. Operation of the furnace
Walking beam regenerative heating furnace is a complex mechanized and automated thermal equipment. It consists of furnace body (stepping system), billet transportation facilities, pipeline systems for conveying various media (blast furnace gas, converter gas, air, nitrogen, compressed air, water, hydraulic oil, lubricant), fuel combustion facilities, Emissions of combustion products and waste heat recovery facilities, control systems for thermal systems, cooling systems for various components, and removal facilities for scale generated during heating, etc. Only when the production personnel have a good understanding and mastery of this set of process equipment can they be properly maintained and operated, and the heating furnace can have good indicators. The work of the production personnel involves: charging the billet, transporting the billet in the furnace, releasing the billet, the heating system of the billet and the thermal system of the furnace; cleaning of the oxide scale; accident shutdown; furnace etc.
The operation of the heating furnace mainly pays attention to the following aspects:
1. The control of the thermal system is usually automatic in the early stage when all the regulators are put into operation, and the setting of the regulator comes from the process specification. Manual operation is used after the automatic adjuster fails or is overhauled, and manual operation can be used in the event of an accident or damage to the automatic components.
2. In order to maintain the long-term effective use of the furnace, regular maintenance and attention should be paid to its use status; regular cleaning of the iron oxide scale; regular inspection of the water volume, water temperature and water pressure of the water-cooled components in the furnace.
3. The air supply will be gradually interrupted when the fan fails or the power is suddenly cut off. At this time, the air, gas valve and the main gas valve on the burner should be quickly turned off.
Oven: The heating furnace needs to remove the moisture in the masonry before production, after major or minor repairs, that is, the oven. Oven time should consider many factors. Here are some reference data: before the production of a 150t walking beam heating furnace, it takes 36 hours to rise from room temperature to 150 °C, and the temperature is kept for 60 hours; the temperature rises to 350 °C at a heating rate of 15 °C/h, and the temperature is kept for 60 hours; then 15 °C/h The temperature was raised to 600 °C at a heating rate of 1 h and kept for 48 h; then the temperature was raised to 8000 °C at a heating rate of 20 °C/h, and the temperature was kept for 36 h; then the temperature was raised to 1200 °C at a heating rate of 20 °C/h, and the temperature was kept for 36 h. The total oven time was 14 days. The overhauled oven was heated to 200 °C and held for 4 h; heated to 700 °C at a heating rate of 20 °C/h and then held for 4 h; and then heated to 1200 °C at a heating rate of 50 °C/h and held for 4 h. The total oven time is 2 days, which can be extended to 3 days if the time is rich. After minor repairs, the oven was heated to 600°C at a heating rate of 25°C/h, and then kept for 6 hours; and then heated to 1200°C at a heating rate of 50°C/h, and then kept for 4 hours. Total oven time is about 2 days.
Ignition: The gas pipeline in front of the furnace that is newly built, repaired, or has been out of use for a long time must be cleared of all the air in the pipeline before the gas is passed. During operation, pay attention to whether the pipeline and its accessories are leaking.
The operation of driving the air in the pipeline and its accessories to the atmosphere outside the workshop is called release, that is, the air is now driven by nitrogen (or steam), and then the nitrogen (or steam) is driven by gas. The release is carried out in sections, and is from the main pipe to each main pipe to each branch pipe and finally to the gas pipe before the burner. Pass nitrogen for a specified time and confirm that the replacement is good, stop feeding nitrogen, and then pass gas and drive nitrogen out of the venting pipe. After this step is completed, take samples from the sampling pipe at the end of the gas pipeline and perform a burst test. The vent valve can be closed and this section is considered to be vented. If it fails, it needs to be driven out for a period of time and the burst test is carried out until it is qualified.
The burst test is carried out in a drum with a diameter of 70~100mm and a length of about 300mm. Light a small fireball on the ground at a safe place a little far from the gas site, move the mouth of the cylinder filled with the sample to the fireball, open the lid of the cylinder and light the sample in the cylinder. If it burns to the bottom of the cylinder, it is considered to be qualified; if it burns quickly after ignition, it means that there is still some air; after ignition, it burns rapidly or produces a detonation, indicating that the sample is in the explosion range. The latter two cases should be Continue to disperse, and make another burst test.
When igniting before the burner, firstly supply air to the burner (the air butterfly valve is opened by 20%~30%), then open the gas valve in front of the burner (open 1/3~1/2), and use the oily cotton yarn that burns vigorously. Ignition, and then gradually adjust the amount of gas and air. If the gas is not ignited after spraying, the gas valve in front of the burner should be closed immediately, and the ignition should be restarted after the combustible mixture in the furnace is removed.