Method for inhibiting scale on surface of thick-specification high-carbon steel from being broken

Abstract

The invention discloses a method for inhibiting scale on the surface of thick-specification high-carbon steel from being broken, and belongs to the field of metal processing. The method comprises the steps of casting blank heating, high-pressure water descaling and rolling and controlled cooling. The high-carbon steel comprises the following chemical components in percentage by weight: 0.45-0.55% of C, 0.15-0.39% of Si, 0.5-0.8% of Mn, less than or equal to 0.035% of P, less than or equal to 0.035% of S and the balance of Fe and inevitable impurities. The invention solves the problems that the steel plate is not clear in identification, the follow-up material information tracking and the use of a user are influenced and the like due to the fact that the steel plate is high in carbon content and finish rolling temperature and relatively high in iron sheet growth speed in order to ensure the plate shape and in the follow-up hot straightening process, the iron sheet is broken, straightened and pressed in.

Description

Technical Field

[0001] This invention belongs to the field of metal processing, and specifically relates to a production method for suppressing the breakage of iron oxide scale on the surface of thick (60-100mm) high carbon steel. Background Technology

[0002] High-carbon steel, represented by 45# and 50# steel, can be used to manufacture plastic molding dies, mold frames, top columns, and some other parts with relatively low requirements for dimensional accuracy and surface roughness. As market demands for high-carbon steel become increasingly stringent, the demand for specifications with thicknesses of 60-100mm and above is growing. High-carbon steel, represented by 45# and 50# steel, is mainly produced through direct rolling of continuously cast billets. However, due to the high final rolling temperature and an FeO content of only 50%-60% (mass percentage), the steel plate surface subsequently exhibits iron oxide scale breakage, resulting in unclear surface markings, affecting user use and delivery, and significantly impacting production efficiency. During subsequent straightening, large areas of the steel plate may peel off, even being pressed into the steel plate matrix, causing surface defects and resulting in significant losses for downstream customers, including remedial work and project delays, and severely damaging the company's image and reputation. Therefore, it is necessary to develop a new process to improve the density of the iron oxide scale, reduce the breakage and detachment of the iron oxide scale from the steel plate, and address the problems of unclear surface quality and markings.

[0003] Currently, high-carbon steel plates with a thickness of 60-100mm are mainly produced using a high-temperature hot rolling + straightening process. Due to the final rolling temperature (above 1000℃) and high carbon content (≥0.40%), the scale growth rate is relatively fast. To ensure plate shape, the scale breaks and is pressed into the plate during the subsequent hot straightening process, resulting in unclear steel plate markings and affecting subsequent material information tracking and user use. Therefore, addressing the issue of scale breakage on the surface of high-carbon steel requires systematic optimization of its heating, rolling, and descaling processes. This optimization aims to ensure steel plate surface quality while controlling manufacturing costs and maintaining the rolling rhythm. Suppressing scale breakage in thick (60-100mm) high-carbon steel is a critical problem that urgently needs to be solved in its mass production.

[0004] Compared with existing technologies:

[0005] To date, there has been very little research, both domestically and internationally, on methods to suppress the breakage of high-carbon steel scale with a thickness of 60-100mm. Prior to this invention, patent publication number CN 119346638A disclosed a method for reducing the breakage of iron oxide scale on the surface of S355 series medium-thick plates. This method suppresses the formation and breakage of iron oxide scale by increasing the number of descaling passes (≥2 passes) and controlling the rolling temperature to ≤920℃. Furthermore, the design uses low-carbon steel (C≤0.20%), which severely restricts production efficiency, and the thickness spacing of the steel plates is not clearly defined.

[0006] The aforementioned patent documents disclose a method for reducing the breakage of iron oxide scale on the surface of S355 series medium and thick plates, but this method is not suitable for suppressing the breakage of iron oxide scale on the surface of high-carbon steel produced using high-temperature hot rolling and straightening processes. The technical solution provided by this invention effectively overcomes the above-mentioned shortcomings and solves the problem of iron oxide scale breakage after straightening at a thickness of 60-100mm after rolling due to rapid scale growth caused by high final rolling temperature when using continuously cast billets with a thickness of less than 300mm. This results in unclear steel plate markings, affecting subsequent material information tracking and user operation. Summary of the Invention

[0007] The purpose of this invention is to overcome the aforementioned technical problems and shortcomings, and to provide a method for suppressing the breakage of iron oxide scale on the surface of thick high-carbon steel. The steel plate can meet surface quality requirements without subsequent controlled rolling. This solves the problems of rapid iron oxide scale growth in such steel plates due to high carbon content and final rolling temperature, leading to scale breakage and straightening pressure during subsequent hot straightening to maintain plate shape, resulting in unclear steel plate markings, affecting subsequent material information tracking and user operation.

[0008] The objective of this invention is achieved through the following technical solution: This invention provides a method for suppressing the breakage of iron oxide scale on the surface of high carbon steel of a certain thickness. The chemical composition of the steel plate (high carbon steel) includes, by weight percentage: C 0.45%~0.55%, Si 0.15%~0.39%, Mn 0.5%~0.8%, P≤0.035%, S≤0.035%, with the balance being Fe and unavoidable impurities.

[0009] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows: Carbon (C): The most economical and basic strengthening element in steel. It significantly improves the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content negatively impacts the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.45%–0.55%.

[0010] Mn: It enhances the strength of steel through solid solution strengthening, while compensating for the strength loss in steel plates caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to the acquisition of fine low-temperature phase transformation products, thus improving toughness. Alloy cost is also considered. Therefore, the Mn content range of this invention is designed to be 0.5% to 0.8%.

[0011] Si plays a role in deoxidation and improving the strength of the matrix in steelmaking. However, excessive Si content or improper heating temperature control can lead to the formation of FeO / Fe2SiO4 eutectoid phases that "pin" the surface. Therefore, the Si content in this invention is set at 0.15% to 0.39%.

[0012] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, due to considerations of smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.035% and 0.035%, respectively.

[0013] Furthermore, the steel plate has a thickness of 60-100mm, and is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of less than 300mm, with water as the cooling medium.

[0014] The above-mentioned method for suppressing the breakage of surface scale on thick high-carbon steel includes billet heating → high-pressure water descaling and rolling → controlled cooling; specifically, it includes the following steps: 1) Billet Heating: The billet is fed into a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace. The preheating section temperature range is 300–600℃ (to promote homogenization of the billet's microstructure and facilitate element diffusion), the heating section temperature range is 1170–1200℃, and the soaking section temperature range is 1050–1090℃. The heating section time is controlled at 2–3 hours, and the soaking section time is controlled at 0.5–1 hour. The air-fuel ratio is controlled at 1:2.1–2.3, ensuring that the upper surface temperature of the billet is 10–15℃ higher than the lower surface temperature (the heating section provides high temperature). Heating ensures temperature uniformity across all parts of the billet, improving the uniformity of lateral and longitudinal metal flow on both sides and at all locations of the steel plate. Simultaneously, controlling the air-fuel ratio ensures the temperature difference between the upper and lower surfaces of the billet is 10-15°C higher, preventing a rapid temperature drop on the upper surface after exiting the furnace, which would affect the shape of the rolled steel plate. Considering the billet composition, reducing the heating temperature in the soaking zone decreases energy consumption and lowers the final rolling temperature, preventing excessive Si content and improper heating temperature control that could lead to the formation of FeO / Fe2SiO4 eutectoid phases that "pin" the surface, affecting subsequent descaling and excessively thick and dense scale, thus preventing scale breakage during straightening.

[0015] 2) High-pressure water descaling and rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 2-2.5 minutes. The descaling machine pressure is 25-30 MPa (increasing the descaling pressure and time removes the primary iron oxide scale generated on the surface of high-carbon steel); a high-reduction rolling method is adopted, with a reduction rate of more than 20% in each of the first three rolling passes (the first three rolling passes use high reduction to avoid the impact on the equipment caused by the high rolling force in subsequent passes due to the low heating temperature of the billet); the last pass is used as a leveling pass, and position control is adopted. The roll gap of the leveling pass is increased by 1-5 mm based on the thickness of the finished steel plate, and the rolling force is 2000-5000 kN (using the last pass as a leveling pass avoids the subsequent hot straightening input, which would cause the iron scale to break and affect the surface quality and marking integrity of the steel plate).

[0016] 3) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 850–900℃ and a final cooling temperature range of 700–800℃. The number of controlled cooling manifold groups is 2–4, and the water flow rate per manifold is 100–150 m³. 3 / h, the manifold opening method is from back to front (the mill direction is front), the cooling rate is controlled at 5~10℃ / s (the steel plate is water-cooled to ensure that the steel plate experiences a rapid temperature drop, avoiding the fastest range of iron scale growth, while controlling the number of manifold opening groups, water volume and method to ensure that the iron scale on the steel plate surface is further densified before laminar flow cooling and does not break during the cooling process, and the iron scale is mainly FeO).

[0017] Furthermore, the thickness of the cast billet is less than 300 mm.

[0018] By adopting the above-mentioned composition, heating, rolling, and cooling process scheme, the shortcomings of existing technologies are overcome. This solves the problems of rapid scale growth in this type of steel plate due to its high carbon content and final rolling temperature, which leads to scale breakage and pressure during subsequent hot straightening to maintain plate shape. This also results in unclear steel plate markings, affecting subsequent material information tracking and user operation. The scale breakage rate on the steel plate surface is reduced to less than 1%.

[0019] The beneficial effects of this invention are: 1. Limiting the temperature and time of the billet in the preheating, heating, and soaking zones promotes the homogenization of the billet structure and facilitates the full diffusion of elements. High-temperature heating in the heating zone ensures temperature uniformity across the billet, improving the uniformity of lateral and longitudinal metal flow on both sides and at all locations of the steel plate. Simultaneously, controlling the air-fuel ratio ensures a consistent temperature difference on the upper surface of the billet, preventing rapid temperature drop after exiting the furnace and affecting the shape of the rolled steel plate. Based on the billet composition, reducing the heating temperature in the soaking zone decreases energy consumption and lowers the final rolling temperature. This prevents excessive Si content and improper heating temperature control, which could lead to the formation of FeO / Fe2SiO4 eutectoid phases that "pin" the surface, affecting subsequent descaling and causing excessively thick and dense scale, resulting in scale breakage during straightening.

[0020] 2. By increasing the descaling pressure and time, the primary iron oxide scale generated on the surface of high-carbon steel is effectively removed. A high reduction rate rolling method is adopted to avoid impact on the equipment caused by the low heating temperature of the billet. Based on the traditional rolling method, the last pass is used as a plate shape control and leveling pass. Position control is adopted, and the roll gap of the leveling pass is increased by 1-5mm based on the finished product thickness. The rolling force is 2000-5000kN. Without affecting the rolling efficiency, the last pass is used as a leveling pass to avoid subsequent hot straightening, which would cause the iron oxide scale to break and affect the surface quality and marking integrity of the steel plate.

[0021] 3. By water cooling the steel plate, a rapid temperature drop is ensured, avoiding the fastest growth range of iron scale (850-900℃). At the same time, the number of manifold opening groups, water volume and method are controlled to ensure that the iron scale on the steel plate surface is further densified before cooling and does not break during the cooling process. The iron scale is mainly FeO and supplemented by Fe3O4, with proportions of 80%-90% and 10%-20% respectively.

[0022] 4. By adopting the above-mentioned composition, heating, rolling, and cooling process, the shortcomings of existing technologies are overcome. This solves the problems of rapid scale growth in this type of steel plate due to its high carbon content and final rolling temperature, leading to scale breakage and pressure during subsequent hot straightening to maintain plate shape. This also results in unclear steel plate markings, affecting subsequent material information tracking and user operation. The scale breakage rate on the steel plate surface is reduced to less than 1%. This significantly improves surface quality and production efficiency. Detailed Implementation

[0023] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0024] A method for suppressing the breakage of iron oxide scale on the surface of high-carbon steel of a certain thickness, wherein the chemical composition of the steel plate, by weight percentage, includes: C 0.45%–0.55%, Si 0.15%–0.39%, Mn 0.5%–0.8%, P≤0.035%, S≤0.035%, with the balance being Fe and unavoidable impurities. The steel plate has a thickness of 60–100 mm and is produced using continuously cast billets with a thickness of less than 300 mm on a medium-thick plate reciprocating rolling mill, with water as the cooling medium.

[0025] The method includes billet heating → high-pressure water descaling and rolling → controlled cooling; specifically, it includes the following steps: 1) Billet heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 300-600℃, the temperature range of the heating section is 1170-1200℃, and the temperature range of the soaking section is 1050-1090℃. The heating time is controlled at 2-3 hours, and the soaking time in the furnace is controlled at 0.5-1 hour. The air-fuel ratio is controlled at 1:2.1-2.3 to ensure that the temperature of the upper surface of the billet is 10-15℃ higher than that of the lower surface.

[0026] 2) High-pressure water descaling and rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 2-2.5 minutes, with a descaling machine pressure of 25-30 MPa; a high-reduction rolling method is adopted, with a reduction rate of more than 20% in each of the first three rolling passes. The last pass is used as a leveling pass, with position control. The roll gap of the leveling pass is increased by 1-5 mm based on the thickness of the finished steel plate, and the rolling force is 2000-5000 kN.

[0027] 3) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature range of 850–900℃ and a final cooling temperature range of 700–800℃. The number of controlled cooling manifold groups is 2–4, and the water flow rate per manifold is 100–150 m³. 3 / h, the manifold is opened from back to front (forward in the mill direction), and the cooling rate is controlled at 5~10℃ / s; then air-cooled to room temperature.

[0028] Examples 1-6 Table 1 shows the chemical composition of the steel in the examples; Table 2 shows the heating regime of the billet and the high-pressure water descaling process before rolling of the continuously cast billet; Table 3 shows the rolling method of the steel in the examples; Table 4 shows the cooling process and surface quality of the steel in the examples.

[0029] Table 1 Chemical composition (wt, %) of the examples

[0030] Note: Impurity elements in steel: P≤0.035%, S≤0.035%.

[0031] Table 2 Heating regime of steel billets and high-pressure water descaling process before rolling of continuously cast billets in the examples

[0032] Table 3 Rolling method of steel in the examples

[0033] Table 4 Cooling process and surface quality of steel in the examples

[0034] Therefore, compared with the prior art, the purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide a production method for suppressing the breakage of iron oxide scale on the surface of thick (60-100mm) high-carbon steel. By optimizing the composition, heating, rolling, and cooling processes, the shortcomings of the prior art are overcome, and the problems of rapid iron oxide scale growth due to the high carbon content and final rolling temperature of such steel plates, leading to scale breakage and straightening pressure during subsequent hot straightening to maintain plate shape, resulting in unclear steel plate markings, affecting subsequent material information tracking and user use, are addressed. The iron oxide scale breakage rate on the steel plate surface is reduced to less than 1%.

[0035] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for suppressing the breakage of iron oxide scale on the surface of thick high-carbon steel, characterized in that, Includes the following steps: 1) Billet heating: The billet is sent into the heating furnace for heating. The billet passes through the preheating section, heating section and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 300-600℃, the temperature of the heating section is 1170-1200℃, and the temperature of the soaking section is 1050-1090℃. The heating time is controlled at 2-3 hours, and the soaking time in the furnace is controlled at 0.5-1 hour. The air-fuel ratio is controlled at 1:2.1-2.3 to ensure that the temperature of the upper surface of the billet is 10-15℃ higher than that of the lower surface. 2) High-pressure water descaling and rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 2-2.5 minutes, with a descaling machine pressure of 25-30 MPa; a high-reduction rolling method is adopted, with a reduction rate of more than 20% in each of the first three rolling passes; the last pass is used as a leveling pass, with position control adopted, and the roll gap of the leveling pass is increased by 1-5 mm based on the thickness of the finished steel plate, with a rolling force of 2000-5000 kN; 3) Controlled cooling: Laminar flow cooling is adopted, with an initial cooling temperature of 850-900℃ and a final cooling temperature of 700-800℃. The number of controlled cooling manifold groups is 2-4, and the water flow rate per manifold is 100-150m³. 3 / h, the mill direction is forward, the manifold opening method is from back to front, and the cooling rate is controlled at 5~10℃ / s.

2. The method according to claim 1, characterized in that, The chemical composition of the high-carbon steel, by weight percentage, includes: C 0.45%–0.55%, Si 0.15%–0.39%, Mn 0.5%–0.8%, P≤0.035%, S≤0.035%, with the balance being Fe and unavoidable impurities.

3. The method according to claim 1, characterized in that, The thickness of the high-strength steel is 60-100 mm.

4. The method according to claim 1, characterized in that, The thickness of the cast billet is less than 300 mm.

5. The method according to claim 1, characterized in that, The high-strength steel is produced using cast billets on a medium-thick plate reciprocating rolling mill, with water as the cooling medium.

6. The method according to claim 1, characterized in that, The iron oxide scale on the surface of the high-strength steel is mainly composed of FeO and supplemented by Fe3O4, with FeO accounting for 80% to 90% and Fe3O4 accounting for 10% to 20% by mass percentage.

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