First, 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 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 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 burned.
1. The influence of heating temperature: Because 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, and it begins to change significantly above 600°C. When the steel temperature reaches above 900°C, the oxidation rate increases sharply. The relationship between the amount of iron oxide scale generated and the temperature is as follows.
2. Steel composition: For carbon steel, as its C content increases, the amount of steel burned 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, high-carbon steel burns less than low-carbon steel. 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 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 of the steel, so the heating time should be shortened as much as possible during heating.

(II) Decarburization: When steel is heated, based on the generation 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, the decarburization problem 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 effect of heating temperature on decarburization: The effect of heating temperature on the thickness of the visible decarburization layer of steel billet varies with different steel types. 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 not only does not increase but decreases with the increase in temperature. For example, the thickness of the decarburization layer of spring steel (60Si2Mn) increases rapidly with the increase of temperature before 1100℃ but decreases significantly with the increase of temperature after exceeding 1100℃, which indicates that there is a "peak" of decarburization rate near 1100℃. Many other steel types have similar rules. For these steel types, when choosing the heating temperature, try to avoid the "peak" temperature range of the decarburization rate.
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 steel on decarburization: The higher the carbon content in the steel, the easier it is to decarburize when heated. If the steel contains elements such as aluminum, tungsten, and cobalt, the decarburization will increase; if the steel 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.
5. Measures to reduce decarburization: Measures to reduce steel oxidation 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 that is 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, 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, 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 steel 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 rolling operations, 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 grain growth. 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 grains.

(IV) Overburning of steel: When steel is heated to a temperature higher than overheating, not only 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 pressure processing, 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 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 heating rate is as fast as possible, the heating time is short, and the oxidation and burning of steel are also reduced. However, increasing the heating speed is limited by some factors. In addition to the limitations 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 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 should be limited to the range allowed by the stress. The stress in steel is dangerous only 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 the plastic state. For low-carbon steel, the temperature may be lower and enter the production 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 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, the low-temperature plasticity is poor and the thermal conductivity is low, so the heating speed in the low-temperature section must be controlled.