1. Heating defects of steel
During the heating process of steel, the temperature and atmosphere of the furnace must be properly adjusted. If the operation is improper, various heating defects will occur, such as oxidation, decarburization, overheating, overburning, etc. These defects affect the heating quality of steel, and in severe cases, cause waste products. Therefore, the process must be strictly implemented during the heating process to avoid the above defects.
(I) Oxidation of steel 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 gas used in the Sixth Rolling Plant is blast furnace gas, which is mainly composed of combustible components CO, H2, CH4 and non-combustible components CO2 and N2, of which CO accounts for about 30%, H2 and CH4 are very small, and blast furnace gas contains a large amount of CO2 and N2, accounting for about 60% to 70%). The surface layer of steel will be oxidized. Oxidation not only causes direct loss of steel - the yield rate decreases, but also rolling will press iron oxide chips into the surface of steel when the descaling is not clean, causing pitting defects on the surface of the finished steel. If the oxide layer is too deep, the subcutaneous bubbles of the steel ingot will be exposed, resulting in scrap after rolling. The thermal conductivity of the iron oxide scale is lower than that of steel, so the surface of the steel is covered with the iron oxide scale, which deteriorates the heat transfer conditions, reduces the furnace productivity, and increases energy consumption. The factors affecting the oxidation of steel are: heating temperature, heating time, furnace gas composition, and steel composition. Among these factors, heating temperature, furnace gas composition, and steel composition have a greater impact on the oxidation rate, while heating time mainly affects the amount of steel burnout.
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 at room temperature is very slow. It begins to change significantly above 600℃. When the steel temperature reaches above 900℃, the oxidation rate increases sharply. The relationship between the amount of iron oxide scale generated and the temperature is as follows: Steel temperature/℃ 900 1000 1100 1200 Burning loss ratio 1 2 3.5 7
2. Steel composition: For carbon steel, as its C content increases, the burning loss of steel decreases. This is because after the C in the steel is oxidized, part of it generates CO, which prevents the oxidizing gas from diffusing into the steel. Therefore, under the same heating conditions, the burning loss of high-carbon steel is lighter than that of low-carbon steel. Alloy elements such as Cr and Ni are easily oxidized to corresponding oxides, but because the thin oxide layer they generate is very dense and stable, this thin oxide film prevents the internal matrix of the steel from being reoxidized. 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 steel, so the heating time should be shortened as much as possible.
(ii) Decarburization: When steel is heated, on the basis of the formation of iron oxide scale, due to the presence and diffusion of high-temperature furnace gas, the carbon atoms in the unoxidized steel 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 surface layer being carbon-poor. This phenomenon is called decarburization. Carbon is one of the main elements that determine the properties of steel. Decarburization significantly reduces the mechanical properties of steel, such as hardness, wear resistance, fatigue strength, impact toughness, and service life. It has great harm to the quality of tool steel, ball bearing steel, spring steel, high carbon steel, etc., and even become scrap due to decarburization exceeding the regulations. Therefore, decarburization is one of the key issues in steel production. The factors affecting decarburization are the same as oxidation. The main factors affecting decarburization are heating temperature, heating time, and furnace atmosphere. In addition, the chemical composition of steel also has a certain influence on decarburization.
1. The influence of heating temperature on decarburization: The influence of heating temperature on the thickness of the visible decarburization layer of the steel billet varies with different steel types. Generally, with the increase in heating temperature, the thickness of the visible decarburization layer increases significantly. However, for some steel types, the thickness of the decarburization layer increases 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. For example, the thickness of the decarburization layer of spring steel (60Si2Mn) increases rapidly with the increase of temperature before 1100℃, but after exceeding 1100℃, the thickness of the decarburization layer decreases significantly with the increase of temperature. This shows that there is a "peak" of decarburization rate near 1100℃. There are many other steel types that have similar rules. For these steel types, when choosing 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 in order 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, thereby reducing the thickness of the visible decarburization layer.
4. Effect of the chemical composition of steel on decarburization: The higher the carbon content in steel, the easier it is to decarburize when heated. If the steel contains aluminum, tungsten, cobalt, and other elements, decarburization will increase; if the steel contains chromium, manganese, boron, and other elements, decarburization will decrease. Nickel, silicon, and vanadium have little effect on decarburization. The types of steel that are easy to decarburize mainly include carbon tool steel, die steel, spring steel, ball bearing steel, high-speed steel, etc.
Measures to reduce decarburization: Measures to reduce steel oxidation are basically applicable 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 that is easy to decarburize; properly adjusting and controlling the atmosphere in the furnace, maintaining an oxidizing atmosphere in the furnace for steel that is easy to decarburize, making the oxidation rate greater than the decarburization rate, etc.
(III) Overheating of steel: If the heating temperature of steel exceeds the critical temperature AC3, the grains of steel begin to grow. 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. If the grains grow too much, the mechanical properties of the steel will decrease, and cracks will easily occur during processing. Especially in the corners of the ingot or the edges of the parts, cracks will occur during rolling, causing cracks in the finished product. The heating temperature and heating time have a decisive influence on the growth of grains. During the rolling operation, the heating temperature and the time the steel stays in the high-temperature area should be controlled. Most alloying elements can reduce the trend of grain growth. Only carbon, phosphorus, and manganese can promote the growth of grains. Therefore, the thermal sensitivity of general alloy steel is lower than that of carbon steel, that is, the alloying elements play a role in refining the grains.
(IV) Overburning of steel: When the steel is heated to a temperature higher than the overheating temperature, not only do the grains of the steel grow, but also 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 rate of steel
The heating temperature of steel 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 small deformation resistance. The most suitable heating temperature should make the steel obtain the best plasticity and minimum deformation resistance, which is conducive to hot processing, increasing production, and reducing equipment wear and power consumption. However, for heating high-quality steel, different heating processes are used according to different heating purposes. The heating temperature of steel generally needs to be determined by referring to the phase diagram, plasticity diagram, and deformation resistance diagram of steel. The selection of heating temperature for carbon steel and low alloy steel is mainly based on the iron-carbon equilibrium phase diagram. The general heating temperature is 30-50℃ above AC3 of the iron-carbon equilibrium phase diagram and 100-150℃ below the solidus. The heating rate of steel refers to the degree of increase in the surface temperature of steel per unit of time. From the production perspective, it is hoped that the faster the heating rate, the better, and the shorter the heating time, the less oxidation and burning of steel. However, increasing the heating rate is limited by some factors. In addition to the limitation of furnace heating conditions, the problem of allowable temperature difference in steel should be considered in particular.

During the heating process of steel, 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. The magnitude of 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, cracks will occur inside the steel, so the heating speed must be limited to the range allowed by the stress. The stress in steel 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, and cracks will appear. When the temperature exceeds this temperature range, the steel enters a plastic state. For low-carbon steel, a lower temperature may enter the generation range. At this time, even if a large temperature difference occurs, the stress will disappear due to plastic deformation and will not cause cracks. Therefore, the limitation of temperature stress on heating speed is 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, 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
We implement a three-stage continuous heating system, that is, the 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 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 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 discharge. 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.
Heating time refers to the time required for the steel billet to be heated in the furnace to the temperature required for rolling under the specified temperature system. Heating time is a representation of the heating speed and is the sum of the time of the three stages of preheating, heating, and soaking. The following empirical formula can be used to heat steel billets in a general continuous heating furnace:
h=cs
h——Heating time, unit: hour
s——Steel thickness, unit: cm
c——Time required to heat each centimeter of steel, unit: hour/cm
For the heating system with double-sided heating, the c value is as follows
Low carbon steel c=0.05～0.075
Medium carbon steel and low and medium alloy steel c=0.075～0.1
High carbon steel and high alloy steel c=0.1～0.15
Advanced tool steel c=0.15～0.2
The heating time is also related to the distribution of steel billets in the furnace. The same steel billet, under different feeding steps, has different heating times due to different heating areas. This is sometimes very important and cannot be ignored in any case.

Fourth, the furnace pressure system
The furnace pressure system is also an important factor affecting the heating speed, heating quality, and fuel utilization of steel billets. 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 operation system. Generally, the furnace pressure in the continuous heating furnace increases from the discharge side to the feed side, and the total pressure difference is 20 to 40Pa. In addition, due to the potential difference of 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.
The control benchmark and requirements of the pressure in the furnace, in order to minimize oxidation and energy consumption in the furnace, the pressure in the heating furnace is generally required to be controlled to 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 billet surface at the furnace discharge end 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 furnace tail.
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 furnace life and causes a lot of fuel waste.

Fifth, problems in the heating operation
The heating effect of steel billets is inconsistent: Since the capacity of the rolling mill of the Sixth Rolling Mill is much greater than that of the heating furnace, the production mode of the Sixth Rolling Mill is: rapid rolling of one furnace of steel billets - equalizing steel temperature - rapid rolling of one furnace of steel billets - 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 banded structures are easily formed after rolling.
The heating temperature at the tail of the furnace is too high: When the Sixth Rolling Mill is heated normally, the heating temperature at the tail of the furnace (heating stage I) is as high as 1050℃, which is obviously very unfavorable for the heating of high-alloy steel to be developed in the future.
The furnace pressure is high: 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. A large amount of high-temperature gas escapes from the furnace, which not only wastes 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.

Sixth, the solution to the heating problem of the sixth rolling mill
(i) Control the rhythm of steel tapping, uniform heating, and uniform steel tapping: The heating time should be able to meet the requirements of different steel temperatures. In this way, all billets will obtain the same heating effect. Due to the uniform heating and slow rhythm of steel tapping, the heat load of the heating furnace can be moderately reduced, and the control of the furnace pressure is also beneficial.
(ii) The above measures have almost no effect on production