A hot-rolling casting blank heating process

Abstract

The present application relates to a kind of hot-rolled casting blank heating process, according to the content of Si element in casting blank, casting blank is divided into two categories, the silicon content of I class casting blank is 0.05%~0.75%, the silicon content of II class casting blank is greater than 0.75% less than or equal to 1.75%;The I class casting blank, the preheating temperature in heating furnace is ≤800 ℃, the soaking temperature is controlled at 1120-1140 ℃, the casting blank temperature is controlled at 1100-1120 ℃ when discharging or descaling;The II class casting blank, the preheating temperature in heating furnace is 830-1000 ℃, the soaking temperature is 1170-1200 ℃, the casting blank temperature is controlled at 1160-1180 ℃ when discharging or descaling.The present application starts from the structure and thickness of casting blank scale, according to the content of silicon element, the casting blank is classified, the preheating temperature, heating temperature, heating time and residual oxygen content are controlled, the furnace scale of easy broken, small adhesion is obtained, so as to improve the descaling efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to a hot-rolled casting billet production process, belonging to the technical field of metallurgical rolling process. BACKGROUND

[0002] The descaling of the hot-rolled casting billet before entering the rough rolling mill is mainly in the form of high-pressure water spraying. The high-pressure water generated by the high-pressure water pump forms a high-speed jet through the nozzle. Under the action of water erosion and stripping and thermal explosion effect, the furnace-born iron oxide scale on the surface of the billet rapidly falls off from its surface.

[0003] In the prior art, the descaling effect is mainly improved by adjusting the water pressure and the angle and number of nozzles. For example, CN112718889A provides a method for improving the descaling effect of a billet, the high-pressure water jetting direction is 30°-60° to the inclination angle α of the surface of the casting billet, and the high-pressure water pressure is 15 MPa-22 MPa. CN 113458164 A discloses a descaling method for small-section Cr, Ni, and Mo alloy continuous casting square billets. When high-pressure water is used for descaling, a detachable plug is used to block the nozzles on both sides and the upper side of the descaling ring to prevent mutual interference. The angle of the descaling nozzles and the annular water pipe is designed to be opposite to the rolling direction. The jet angle of the descaling nozzles is changed to 33-35°. CN 106140843 B discloses a high-pressure water descaling method for alloy steel plates. Low-temperature high-pressure water is used in the descaling box. By adjusting the nozzle parameters, the striking force and water volume per unit area of the plate surface are improved, and the iron oxide scale removal capacity is enhanced. Before the rough rolling mill, the high-pressure water striking position is changed by rotating 90°. The plate billet is slowly bitten into the gap between the upper and lower rollers, and the remaining scale is completely removed by controlling a small reduction and water pressure. After descaling, the plate billet is rotated 90° in the opposite direction to the initial position, and then normal rough rolling is performed.

[0004] With the development of technology, some descaling devices using ice particles, abrasives, and steel shots instead of high-pressure water have appeared. For example, CN102284523 A discloses a method for descaling a billet during rough rolling. Ice particles are sprayed onto the surface of the billet through a nozzle. This method can reduce the cooling of the billet during descaling and improve the descaling efficiency. CN 106799401 A discloses a hot billet descaling device and a method for descaling using the same. Compressed air is used to spray metal abrasives such as steel shots onto the surface of the billet, and the impact force of the abrasives is used to completely crush the iron oxide scale layer.

[0005] There are also some descaling techniques using mechanical methods, such as patting or impacting the surface of the casting blank, to achieve the purpose of removing the iron scale; CN 212469255 U discloses an auxiliary descaling device before rolling of the steel blank, which comprises a descaling roller and a reciprocating mechanism for driving the descaling roller to approach or move away from the steel blank, for repeatedly patting the surface of the steel blank before entering the high-pressure water descaling device, to break the furnace-born iron scale on the surface of the steel blank.

[0006] As can be seen from the above, the main methods to improve the descaling effect of hot-rolled casting blanks can be summarized into three categories: 1) adjusting the water pressure size and nozzle angle and number; 2) replacing high-pressure water with ice particles, abrasives, and steel shots; and 3) using mechanical methods, such as patting or impacting the surface of the casting blank, to remove the iron scale, which are all considered from the perspective of the descaling equipment. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a hot-rolled casting blank heating process that can achieve efficient descaling. From the structure and thickness of the iron scale on the casting blank, the casting blank is classified according to the content of silicon element, the preheating temperature, the heating temperature, the heating time, and the residual oxygen content are controlled, to obtain furnace-born iron scale that is easy to break and has small adhesion, thereby improving the descaling efficiency.

[0008] The technical solution to solve the above technical problem is:

[0009] A hot-rolled casting blank heating process is provided, which is suitable for casting blanks of medium and low carbon steel, alloy structural steel, and high carbon steel. The chemical composition and mass percentage of the casting blank are as follows: C: 0.01% to 0.65%, Si: 0.05% to 1.75%, Mn: 0.10% to 1.00%, P≤0.020%, S≤0.020%, ALs 0.005% to 0.050%, Cr: 0.10-1.20%, Mo≤0.20%, Ti≤0.045%, B≤0.0025%, and the rest is iron and unavoidable impurities.

[0010] The above hot-rolled casting blank heating process classifies the casting blank into two categories according to the content of Si element in the casting blank. The silicon content of I-class casting blank is 0.05% to 0.75%, and the silicon content of II-class casting blank is greater than 0.75% and less than or equal to 1.75%.

[0011] For the I-class casting blank, the preheating temperature in the heating furnace is ≤800℃, the soaking temperature is controlled at 1120-1140℃, and the casting blank temperature is controlled at 1100-1120℃ when discharging or descaling;

[0012] For the II-class casting blank, the preheating temperature in the heating furnace is 830-1000℃, the soaking temperature is 1170-1200℃, and the casting blank temperature is controlled at 1160-1180℃ when discharging or descaling.

[0013] The hot-rolled casting blank heating process, the I-class casting blank and the II-class casting blank are rectangular casting blanks with a section size of 150mm*150mm-320mm*380mm, the total heating time in the heating furnace is controlled to be 1.5h-8h, the soaking time should not be more than 30% of the total heating time, and the residual oxygen content in the furnace is controlled to be 2-5%.

[0014] The hot-rolled casting blank heating process, the I-class casting blank is discharged from the furnace, the thickness of the furnace-born oxide scale is 100-280μm, the content of Fe3O4 in the oxide scale is 20-40%, and the interface between the oxide scale and the matrix does not have a single phase of Fe2SiO4.

[0015] The hot-rolled casting blank heating process, the I-class casting blank is discharged from the furnace, the thickness of the furnace-born oxide scale is 100-280μm, the content of Fe3O4 in the oxide scale is 20-40%, and the interface between the oxide scale and the matrix does not have a single phase of Fe2SiO4.

[0016] The hot-rolled casting blank heating process, the area ratio refers to the proportion of the area of a certain phase in the total area of the field of view when observed under a 100-fold field of view.

[0017] The hot-rolled casting blank heating process, the II-class casting blank is discharged from the furnace, the thickness of the furnace-born oxide scale is 250-600μm, the content of Fe3O4 in the oxide scale is 10-30%, and the interface between the oxide scale and the matrix has a phase of Fe2SiO4, and due to the discharge temperature being higher than 1140℃, the Fe2SiO4 remains in a molten state.

[0018] The hot-rolled casting blank heating process, the heating furnace is a walking beam type heating furnace for industrial production, at least two temperature zones, i.e. a preheating section and a soaking section, the preheating temperature and the soaking temperature can be controlled in different zones; the heating furnace can use blast furnace gas, coke oven gas or mixed gas for heating.

[0019] The hot-rolled casting blank heating process can realize efficient descaling, the heating system is optimized according to the content of Si element in the casting blank, the furnace-born oxide scale with different structures and thicknesses is obtained, and then the water jet descaling equipment is used to remove the oxide scale.

[0020] The theoretical analysis of the present application is as follows:

[0021] The effect of hot rolling slab descaling mainly depends on the adhesion of the oxide scale to the substrate, and the structure of the oxide scale determines the adhesion. In the structure of the oxide scale, the inner layer is a loose and porous FeO fine crystalline structure, the middle layer is a dense and crack-free Fe3O4 with a glass-like fracture, and the outer layer is a columnar crystalline structure of Fe2O3. Research shows that when the Fe3O4 content in the oxide scale structure is 40-70%, and the FeO is uniformly distributed in the form of islands in the Fe3O4, the oxide scale has good adhesion. The oxide scale with this structure is not easy to remove, so the Fe3O4 content in the oxide scale is controlled below 40%, the adhesion of the oxide scale is poor, and it is beneficial to efficient descaling.

[0022] In addition to the composition ratio of each phase in the oxide scale, the silicon element also affects the adhesion between the oxide scale and the substrate, and has a great influence on the descaling effect.

[0023] When silicon is contained in the steel, silicon is easy to combine with iron at high temperature to form Fe2SiO4. Fe2SiO4 is formed by the polymerization of FeO and SiO2, and its melting point is lower than that of its component oxides, about 1140℃. Experiments have proved that Fe2SiO4 exists between the oxide scale and the substrate, which destroys the flatness of the interface between the oxide scale and the substrate, and is not conducive to the removal of the oxide scale. Therefore, for type II slab, in order to reduce the adverse effects of Fe2SiO4, descaling should be carried out in the molten state of Fe2SiO4, and the adhesion between the oxide scale and the substrate is the smallest.

[0024] For type I slab, the growth rate of the oxide scale below 800℃ in the heating furnace hardly changes with time, and the oxidation rate increases in the temperature range of 800-1000℃. The soaking temperature is controlled below 1140℃ to form a relatively thin furnace-born oxide scale. When the furnace-born oxide scale meets the cooling water at a high temperature during discharge or descaling, it produces a large area of stress cracking and is immediately washed away after breaking.

[0025] For type II slab, the preheating temperature and the soaking temperature are both higher, and the formed oxide scale is thicker, which is structurally an inner layer of molten Fe2SiO4 and an outer layer of oxide scale. When the slab temperature is controlled at 1160-1180℃ during discharge or descaling, the Fe2SiO4 is still in a molten state, the adhesion to the substrate is small, the high-temperature furnace-born oxide scale meets the cooling water, produces a large stress cracking, and is immediately washed away after breaking.

[0026] The beneficial effects of the present application are:

[0027] The present application classifies the casting blank according to the content of silicon element, controls the preheating temperature, heating temperature, heating time and residual oxygen content, and obtains the furnace-born oxide scale which is easy to break and has small adhesion, so as to improve the descaling efficiency; is suitable for various steel grades, has wide application range, does not need to increase equipment investment, and can achieve higher descaling efficiency by using the existing descaling equipment, and the total area of residual oxide scale accounts for ≤1‰ of the surface area of the casting blank. DETAILED DESCRIPTION

[0028] The present application is further described in detail through specific examples. Examples 1-13 use walking beam heating furnace. Table 1 lists the chemical composition and mass percentage of the casting blank in Examples 1-13. In addition to the components in the table, other components are iron and unavoidable impurities.

[0029] Table 1

[0030]

[0031] Example 1

[0032] The casting blank used in this example is Class I casting blank, the cross-sectional size is 150mm*150mm, the total heating time of the casting blank in the heating furnace is 1.5h, the soaking time is 25min, and the residual oxygen content in the furnace is controlled at 5%.

[0033] The preheating temperature in the heating furnace is 700℃, the soaking temperature is controlled at 1130℃, and the casting blank temperature is controlled at 1110℃ when discharging or descaling. When the casting blank is discharged, the thickness of the furnace-born oxide scale is 100μm, the content of Fe3O4 in the oxide scale is 20%, and there is no Fe2SiO4 single phase at the interface between the oxide scale and the matrix.

[0034] The high-pressure water jet direction is inclined at an angle α of 60° to the surface of the casting blank, the high-pressure water pressure is 15 MPa-22 MPa, the furnace-born oxide scale meets the cooling water at a relatively high temperature, and large-area stress cracking occurs, and after breaking, it is immediately washed away. After water jet descaling, the total area of residual oxide scale accounts for 0.6‰ of the surface area of the casting blank.

[0035] Example 2

[0036] The casting blank used in this example is Class I casting blank, the cross-sectional size is 320mm*380mm, the total heating time of the casting blank in the heating furnace is 8h, the soaking time is 120min, and the residual oxygen content in the furnace is controlled at 2%.

[0037] The preheating temperature in the heating furnace is 800°C, the soaking temperature is controlled at 1140°C, and the temperature of the cast blank when discharged or descaled is controlled at 1120°C. When the cast blank is discharged, the thickness of the furnace-born oxide scale is 280 μm, the content of Fe304 in the oxide scale is 30%, and there is 5% of Fe2SiO4 in the interface between the oxide scale and the matrix in the form of dots inside FeO.

[0038] The high-pressure water jetting direction is inclined at an angle α of 60° to the surface of the cast blank, the high-pressure water pressure is 15 MPa-22 MPa, the furnace-born oxide scale meets the cooling water at a higher temperature to produce a large area of stress cracking, and is washed away immediately after being broken, and after water jet descaling, the total area of the residual oxide scale accounts for 0.1‰ of the surface area of the cast blank.

[0039] Example 3

[0040] The cast blank used in this example is a Class I cast blank, the cross-sectional size is 160 mm*160 mm, the total heating time of the cast blank in the heating furnace is 3 h, the soaking time is 40 min, and the residual oxygen content in the furnace is controlled at 3%.

[0041] The preheating temperature in the heating furnace is 750°C, the soaking temperature is controlled at 1120°C, and the temperature of the cast blank when discharged or descaled is controlled at 1100°C. When the cast blank is discharged, the thickness of the furnace-born oxide scale is 170 μm, the content of Fe304 in the oxide scale is 40%, and there is 1% of Fe2SiO4 in the interface between the oxide scale and the matrix in the form of dots inside FeO.

[0042] The high-pressure water jetting direction is inclined at an angle α of 60° to the surface of the cast blank, the high-pressure water pressure is 15 MPa-22 MPa, the furnace-born oxide scale meets the cooling water at a higher temperature to produce a large area of stress cracking, and is washed away immediately after being broken, and after water jet descaling, the total area of the residual oxide scale accounts for 1‰ of the surface area of the cast blank.

[0043] Example 4

[0044] The cast blank used in this example is a Class I cast blank, the cross-sectional size is 200 mm*200 mm, the total heating time of the cast blank in the heating furnace is 4 h, the soaking time is 55 min, and the residual oxygen content in the furnace is controlled at 4%.

[0045] The preheating temperature in the heating furnace is 780°C, the soaking temperature is controlled at 1128°C, and the temperature of the cast blank when discharged or descaled is controlled at 1116°C. When the cast blank is discharged, the thickness of the furnace-born oxide scale is 205 μm, the content of Fe304 in the oxide scale is 35%, and there is 2% of Fe2SiO4 in the interface between the oxide scale and the matrix in the form of dots inside FeO.

[0046] The high-pressure water jetting direction is 60° to the surface of the casting blank, and the high-pressure water pressure is 15 MPa-22 MPa. The furnace-born oxide scale meets the cooling water at a higher temperature, and large-area stress cracking is generated, and the broken scale is immediately washed away. After the water jetting descaling, the total area of the residual oxide scale accounts for 0.5‰ of the surface area of the casting blank.

[0047] Example 5

[0048] The casting blank used in the example is a type I casting blank, and the cross-sectional size is 155 mm*155 mm. The total heating time of the casting blank in the heating furnace is 2.5 h, and the soaking time is 40 min. The residual oxygen content in the furnace is controlled to be 2%.

[0049] The preheating temperature in the heating furnace is 790°C, and the soaking temperature is controlled to be 1135°C. The temperature of the casting blank when discharged or descaled is controlled to be 1118°C. When the casting blank is discharged, the thickness of the furnace-born oxide scale is 190 μm, the content of Fe3O4 in the oxide scale is 30%, and there is 4% of Fe2SiO4 in the interface between the oxide scale and the matrix, which is distributed in the inner side of FeO in a dot shape.

[0050] The high-pressure water jetting direction is 60° to the surface of the casting blank, and the high-pressure water pressure is 15 MPa-22 MPa. The furnace-born oxide scale meets the cooling water at a higher temperature, and large-area stress cracking is generated, and the broken scale is immediately washed away. After the water jetting descaling, the total area of the residual oxide scale accounts for 0.5‰ of the surface area of the casting blank.

[0051] Example 6

[0052] The casting blank used in the example is a type I casting blank, and the cross-sectional size is 280 mm*380 mm. The total heating time of the casting blank in the heating furnace is 6 h, and the soaking time is 105 min. The residual oxygen content in the furnace is controlled to be 4%.

[0053] The preheating temperature in the heating furnace is 760°C, and the soaking temperature is controlled to be 1125°C. The temperature of the casting blank when discharged or descaled is controlled to be 1105°C. When the casting blank is discharged, the thickness of the furnace-born oxide scale is 265 μm, the content of Fe3O4 in the oxide scale is 25%, and there is 2% of Fe2SiO4 in the interface between the oxide scale and the matrix, which is distributed in the inner side of FeO in a dot shape.

[0054] The high-pressure water jetting direction is 60° to the surface of the casting blank, and the high-pressure water pressure is 15 MPa-22 MPa. The furnace-born oxide scale meets the cooling water at a higher temperature, and large-area stress cracking is generated, and the broken scale is immediately washed away. After the water jetting descaling, the total area of the residual oxide scale accounts for 0.4‰ of the surface area of the casting blank.

[0055] Example 7

[0056] The slab used in this example is a Class I slab, with a cross-sectional size of 160 mm x 160 mm. The total heating time of the slab in the heating furnace is 2 h, and the soaking time is 35 min. The residual oxygen content in the furnace is controlled at 5%.

[0057] The preheating temperature in the heating furnace is 740°C, and the soaking temperature is controlled at 1120°C. The slab temperature is controlled at 1112°C when the slab is discharged or descaled. When the slab is discharged, the thickness of the furnace-born scale is 150 μm, the Fe304 content in the scale is 35%, and there is 3% of Fe2SiO4 distributed in the FeO in a dotted pattern at the interface between the scale and the substrate.

[0058] The high-pressure water jetting direction is inclined at an angle α of 60° to the surface of the slab, and the high-pressure water pressure is 15 MPa to 22 MPa. The furnace-born scale meets the cooling water at a relatively high temperature, resulting in a large area of stress cracking. After being broken, the scale is immediately washed away. After water jet descaling, the total area of the residual scale accounts for 0.8‰ of the surface area of the slab.

[0059] Example 8

[0060] The slab used in this example is a Class I slab, with a cross-sectional size of 160 mm x 160 mm. The total heating time of the slab in the heating furnace is 2 h, and the soaking time is 35 min. The residual oxygen content in the furnace is controlled at 5%.

[0061] The preheating temperature in the heating furnace is 740°C, and the soaking temperature is controlled at 1120°C. The slab temperature is controlled at 1112°C when the slab is discharged or descaled. When the slab is discharged, the thickness of the furnace-born scale is 150 μm, the Fe304 content in the scale is 35%, and there is 3% of Fe2SiO4 distributed in the FeO in a dotted pattern at the interface between the scale and the substrate.

[0062] The high-pressure water jetting direction is inclined at an angle α of 60° to the surface of the slab, and the high-pressure water pressure is 15 MPa to 22 MPa. The furnace-born scale meets the cooling water at a relatively high temperature, resulting in a large area of stress cracking. After being broken, the scale is immediately washed away. After water jet descaling, the total area of the residual scale accounts for 0.8‰ of the surface area of the slab.

[0063] Example 9

[0064] The slab used in this example is a Class II slab, with a cross-sectional size of 155 mm x 155 mm. The total heating time of the slab in the heating furnace is 2 h, and the soaking time is 30 min. The residual oxygen content in the furnace is controlled at 4%.

[0065] The preheating temperature of the casting blank in the heating furnace is 830°C, the soaking temperature is 1170°C, and the temperature of the casting blank when discharged or descaled is controlled at 1160°C. When the casting blank is discharged, the thickness of the furnace-born oxide scale is 310 μm, the content of Fe304 in the oxide scale is 10%, and Fe2SiO4 phase exists at the interface between the oxide scale and the matrix, and Fe2SiO4 keeps in molten state and has small adhesion to the matrix.

[0066] The high-pressure water jetting direction is inclined to the surface of the casting blank at an angle of 60°, the pressure of the high-pressure water is 15 MPa-22 MPa, the high-temperature furnace-born oxide scale meets the cooling water and cracks under the stress, and is then washed away after being broken, and after the water descaling, the total area of the residual oxide scale accounts for 0.4‰ of the surface area of the casting blank.

[0067] Example 10

[0068] The casting blank used in the example is a type II casting blank, the cross-sectional size is 320 mm*380 mm, the total heating time of the casting blank in the heating furnace is 8 h, the soaking time is 130 min, and the residual oxygen content in the furnace is controlled at 2%.

[0069] The preheating temperature of the casting blank in the heating furnace is 1000°C, the soaking temperature is 1200°C, and the temperature of the casting blank when discharged or descaled is controlled at 1180°C. When the casting blank is discharged, the thickness of the furnace-born oxide scale is 600 μm, the content of Fe304 in the oxide scale is 30%, and Fe2SiO4 phase exists at the interface between the oxide scale and the matrix, and Fe2SiO4 keeps in molten state and has small adhesion to the matrix.

[0070] The high-pressure water jetting direction is inclined to the surface of the casting blank at an angle of 60°, the pressure of the high-pressure water is 15 MPa-22 MPa, the high-temperature furnace-born oxide scale meets the cooling water and cracks under the stress, and is then washed away after being broken, and after the water descaling, the total area of the residual oxide scale accounts for 0 of the surface area of the casting blank.

[0071] Example 11

[0072] The casting blank used in the example is a type II casting blank, the cross-sectional size is 320 mm*380 mm, the total heating time of the casting blank in the heating furnace is 8 h, the soaking time is 130 min, and the residual oxygen content in the furnace is controlled at 2%.

[0073] The preheating temperature of the casting blank in the heating furnace is 1000°C, the soaking temperature is 1200°C, and the temperature of the casting blank when discharged or descaled is controlled at 1180°C. When the casting blank is discharged, the thickness of the furnace-born oxide scale is 600 μm, the content of Fe304 in the oxide scale is 30%, and Fe2SiO4 phase exists at the interface between the oxide scale and the matrix, and Fe2SiO4 keeps in molten state and has small adhesion to the matrix.

[0074] The high-pressure water jetting direction is 60° to the surface of the casting blank, and the high-pressure water pressure is 15 MPa-22 MPa. The high-temperature furnace oxide skin meets the cooling water, and a large stress cracking is generated, and is broken and then washed away. After the water jetting descaling, the total area of the residual oxide skin accounts for 0.8‰ of the surface area of the casting blank.

[0075] Example 12

[0076] The casting blank used in the example is a type II casting blank, and the cross-sectional size is 280 mm*280 mm. The total heating time of the casting blank in the heating furnace is 7 h, and the soaking time is 100 min. The residual oxygen content in the furnace is controlled to be 3%.

[0077] The preheating temperature of the casting blank in the heating furnace is 950℃, and the soaking temperature is 1180℃. The temperature of the casting blank is controlled to be 1170℃ when it is discharged or descaled. When the casting blank is discharged, the thickness of the furnace oxide skin is 540 μm, the content of Fe3O4 in the oxide skin is 27%, and Fe2SiO4 phase exists at the interface between the oxide skin and the matrix. Fe2SiO4 keeps in a molten state, and has a small adhesion to the matrix.

[0078] The high-pressure water jetting direction is 60° to the surface of the casting blank, and the high-pressure water pressure is 15 MPa-22 MPa. The high-temperature furnace oxide skin meets the cooling water, and a large stress cracking is generated, and is broken and then washed away. After the water jetting descaling, the total area of the residual oxide skin accounts for 0.2‰ of the surface area of the casting blank.

[0079] Example 13

[0080] The casting blank used in the example is a type II casting blank, and the cross-sectional size is 150 mm*150 mm. The total heating time of the casting blank in the heating furnace is 2.5 h, and the soaking time is 45 min. The residual oxygen content in the furnace is controlled to be 3%.

[0081] The preheating temperature of the casting blank in the heating furnace is 850℃, and the soaking temperature is 1180℃. The temperature of the casting blank is controlled to be 1160℃ when it is discharged or descaled. When the casting blank is discharged, the thickness of the furnace oxide skin is 300 μm, the content of Fe3O4 in the oxide skin is 18%, and Fe2SiO4 phase exists at the interface between the oxide skin and the matrix. Fe2SiO4 keeps in a molten state, and has a small adhesion to the matrix.

[0082] The high-pressure water jetting direction is 60° to the surface of the casting blank, and the high-pressure water pressure is 15 MPa-22 MPa. The high-temperature furnace oxide skin meets the cooling water, and a large stress cracking is generated, and is broken and then washed away. After the water jetting descaling, the total area of the residual oxide skin accounts for 0.3‰ of the surface area of the casting blank.

Claims

1. A heating process for hot-rolled billets, characterized in that: The hot-rolled billet is a billet made of medium-low carbon steel, alloy structural steel, or high carbon steel. The chemical composition and mass percentage of the billet are as follows: C: 0.01%–0.65%, Si: 0.05%–1.75%, Mn: 0.10%–1.00%, P≤0.020%, S≤0.020%, Als 0.005%–0.050%, Cr: 0.10–1.20%, Mo≤0.20%, Ti≤0.045%, B≤0.0025%, with the remainder being iron and unavoidable impurities. Based on the Si content, the billets are divided into two categories: Category I billets have a silicon content of 0.05%–0.75%, and Category II billets have a silicon content greater than 0.75% and less than or equal to 1.75%. The heating process for the Type I billet is as follows: preheating temperature in the heating furnace is ≤800℃, the soaking temperature is controlled at 1120-1140℃, and the billet temperature is controlled at 1100-1120℃ when exiting the furnace or descaling; the heating process for the Type II billet is as follows: preheating temperature in the heating furnace is 830-1000℃, the soaking temperature is 1170-1200℃, and the billet temperature is controlled at 1160-1180℃ when exiting the furnace or descaling. When the Type I billet is tapped from the furnace, the thickness of the furnace-grown iron oxide scale is 100-280 μm, the Fe3O4 content in the iron oxide scale is 20-40%, and there is no Fe2SiO4 separate phase at the interface between the iron oxide scale and the matrix, or the interface between the iron oxide scale and the matrix contains Fe2SiO4 phase with an area ratio not exceeding 5% distributed in a dotted manner inside the FeO; when the Type II billet is tapped from the furnace, the thickness of the furnace-grown iron oxide scale is 250-600 μm, the Fe3O4 content in the iron oxide scale is 10-30%, the Fe2SiO4 phase exists at the interface between the iron oxide scale and the matrix, and the Fe2SiO4 remains in a molten state.

2. The hot-rolled billet heating process as described in claim 1, characterized in that: The Class I and Class II billets are rectangular billets with cross-sectional dimensions of 150mm*150mm~320mm*380mm. The total heating time in the heating furnace is controlled between 1.5h and 8h, the soaking time shall not exceed 30% of the total heating time, and the residual oxygen content in the furnace shall be controlled between 2% and 5%.

3. The hot-rolled billet heating process as described in claim 1, characterized in that: The area ratio refers to the proportion of the area of ​​a certain phase to the total area of ​​the field of view when observed at 100x magnification.

4. The hot-rolled billet heating process as described in claim 1 or 2, characterized in that: The heating furnace is an industrial production walking beam heating furnace, which includes at least two temperature zones, namely a preheating zone and a soaking zone. The preheating temperature and the soaking temperature can be controlled in zones. The heating furnace uses blast furnace gas, coke oven gas or mixed gas for heating.

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