A low-cracking-tendency core rod steel for continuous casting bloom rolling and a method for manufacturing the same

Abstract

A kind of low cracking tendency mandrel steel adopting continuous casting big square billet rolling and its manufacturing method, its component mass percentage is as follows: C 0.33~0.38%, Mn 0.40~0.60%, Si 0.80~1.25%, Cr 4.75~5.25%, Mo 1.10~1.20%, V 0.30~0.40%, Nb 0.035~0.06%, Ti 0.012~0.020%, H≤2.0ppm, the rest is Fe and other inevitable impurities.The application is designed by Nb-Ti microalloying and lower H content, combined with high-temperature heating and long-time holding, effectively ensures that the deformation amount in rolling process does not fall within the range of critical deformation amount, avoids the formation of local abnormal coarse structure caused by insufficient local recrystallization, avoids the formation of low plasticity and toughness structure, thereby reduces the cracking tendency under residual tensile stress and the cracking tendency of hydrogen-induced delayed cracking, at the same time, the interval time between rolling and annealing is greatly extended from original 24h to 72h, greatly reduces the cracking proportion in the process of waiting annealing after rolling, improves the yield, is beneficial to save cost, reduce energy consumption, improve efficiency.

Description

Technical Field

[0001] This invention relates to the field of mandrel steel manufacturing technology, specifically to a low-cracking-prone mandrel steel produced by continuous casting of large square billets and its manufacturing method. Background Technology

[0002] Mandrels need to be used repeatedly under high-temperature and stress conditions, thus requiring the addition of numerous alloying elements to improve thermal strength and resistance to thermomechanical fatigue, such as Cr, Mo, V, and Si. The high addition of these alloying elements results in a predominantly martensitic microstructure under quenching conditions, achieving a hardness of 55 HRC or higher. High-hardness martensite often has poor toughness, making it prone to crack induction and rapid crack propagation under internal residual stress or external impact, leading to macroscopic cracks and ultimately scrapping the mandrel steel, causing significant economic losses. This means that even after high-temperature cooling, mandrel steel can form a predominantly martensitic phase transformation structure, even under air-cooling conditions, essentially making it suitable for air-quenched steel. Therefore, it has a high tendency to crack, and a large proportion of cracks are likely to occur during production.

[0003] Chinese patent CN1616700A discloses "a steel that can be used to process mandrels for continuous rolling mills and its production process." The chemical composition of this material, by weight percentage, is: C 0.30-0.43%, Mn≤0.60%, Si 0.70-1.00%, P≤0.010%, S≤0.007%, Cr 4.50-5.50%, Mo 1.00-1.50%, V 0.7-1.00%, Nb 0-0.25%, Al≤0.015%, Ni 0.10-0.75%, Cu≤0.10%, [O]≤30ppm, [H]≤2ppm, with the balance being iron. By designing the composition, using electroslag remelting, and controlling the slag system selection, the impact performance and thermal fatigue performance of the mandrel steel are improved by reducing carbide banding segregation and avoiding secondary oxidation during casting.

[0004] Chinese patent CN1840286A discloses a "new manufacturing process for a steel pipe continuous rolling mandrel with limited movement," which employs the following steps: forging / rolling billet → full annealing → hot straightening → roughing / semi-finishing → dual-frequency quenching → secondary high-temperature tempering → belt grinding → chromium plating → dehydrogenation. This new process reduces processing difficulty, increases production efficiency, and improves material yield.

[0005] Chinese patent CN102162071A discloses a "limited-motion mandrel steel material for tube rolling and its manufacturing method," with the following chemical composition by mass percentage: C 0.40–0.48%, Si 0.40–0.80%, Mn 0.30–0.60%, P≤0.010%, S≤0.008%, Cr 2.30–3.00%, Mo 2.0–2.5%, V 1.0–1.5%, Ni 0.50–0.80%, Nb 0.04–0.10%, Al 0.015–0.035%, with the balance being Fe. The process steps include: vacuum refining in an electric arc furnace and ladle furnace, electroslag remelting, forging, annealing, and quenching and tempering. The mandrel life is improved by increasing the high-temperature yield strength and resistance to tempering softening of the mandrel steel material. By reducing the Cr content and increasing the Mo and V content, the formation of the high-temperature stable phases Mo2C and V4C3 is promoted, while the formation of the high-temperature unstable phases Cr7C3 and Cr is reduced. 23 The formation of C6 increases its service temperature; the addition of Nb refines the austenite grains and improves its ductility and toughness; the tempering temperature is close to its service temperature, resulting in better microstructural stability during use; and the addition of an appropriate amount of Ni further improves its ductility and toughness.

[0006] Chinese patent CN104227350A discloses a "method for preparing a mandrel with limited movement," characterized by the use of H13 steel and Ni steel as raw materials during refining, with the addition of 0.30–0.40% Ni, achieving high strength while significantly improving the impact toughness of the mandrel. The composition range of H13 steel is generally: C 0.32–0.45%, Mn 0.20–0.60%, P≤0.030%, S≤0.030%, Si 0.80–1.25%, Cr 4.75–5.50%, V 0.80–1.20%, and Mo 1.10–1.75%.

[0007] Chinese patent CN104998905A discloses "alloy steel, limiting mandrel and its production method". The steel composition is: C 0.27-0.31%, Si 0.60-0.80%, Mn 0.7-0.9%, P≤0.015%, S≤0.005%, Cr 3.50-3.70%, Ni 0.20-0.30%, V 0.40-0.50%, Mo 0.50-0.58%, Al≤0.025%, with the remainder being Fe and impurity elements.

[0008] ASTM A681 specifies the composition control range for H11 steel: C 0.33–0.43%, Mn 0.20–0.60%, P ≤0.030%, S ≤0.030%, Si 0.80–1.25%, Cr 4.75–5.50%, V 0.30–0.60%, and Mo 1.10–1.60%.

[0009] GB / T 1299 specifies the composition control range for 4Cr5MoSiV steel: C 0.33–0.43%, Mn 0.20–0.50%, P 0.025%, S ≤0.010%, Si 0.80–1.20%, Cr 4.75–5.50%, V 0.30–0.60%, and Mo 1.10–1.60%. For vacuum-degassed steel, non-metallic inclusions are inspected and graded according to Method A of GB / T 10561-2005, requiring fine inclusions to be A ≤2.5, B ≤2.5, C ≤1.5, and D ≤2.5; and coarse inclusions to be A ≤2.0, B ≤2.0, C ≤1.5, and D ≤2.0.

[0010] The mandrel steels mentioned in the above patents or documents all have a large amount of alloy added, which allows them to obtain a martensitic structure under air cooling conditions. After being cooled from high temperature to room temperature, the surface is subjected to residual tensile stress. In addition, the toughness of quenched (air-quenched) martensite is low, so it has a greater tendency to crack.

[0011] Most existing technologies focus on improving the fatigue life of mandrels, while paying less attention to improving the yield of intermediate mandrel products. The tendency of mandrel steel to crack during rolling is relatively high, and there is currently no good solution to the problem of large-scale cracking during production. Summary of the Invention

[0012] The purpose of this invention is to provide a mandrel steel with a low cracking ratio rolled from a continuously cast large billet and its manufacturing method, which can effectively solve the problems of high cracking tendency and low yield of mandrel steel, and is conducive to cost saving. It has a very good prospect for promotion and application value.

[0013] To achieve the above objectives, the technical solution of the present invention is as follows:

[0014] Steel is prone to cracking due to residual stress during the cooling process after being rolled into a circular cross-section. Residual stress arises because the cooling and phase transformation of the outer surface and the core are asynchronous. Generally, the temperature difference between the outer surface and the core causes uneven volume shrinkage, generating thermal stress, characterized by compressive stress on the surface and tensile stress in the core. The microstructure stress exhibits the opposite pattern; different phase transformations result in tensile stress on the surface and compressive stress in the core. Inclusions and pores in the core are prone to cracking under tensile stress, becoming macroscopic crack initiations; similarly, surface defects or low-ductility microstructures are prone to crack initiation under tensile stress, also becoming macroscopic crack initiations. Reducing or avoiding residual tensile stress can decrease the tendency to crack.

[0015] There are generally two reasons for quenching cracks: one is that the tensile stress exceeds the fracture strength of the material; the other is that although the internal stress is not too high (not exceeding the fracture strength of the material), there are internal defects in the material, such as non-metallic inclusions, carbide segregation, coarse second phases, or low plasticity and toughness structures.

[0016] This invention, through research, reveals that the primary form of cracking in mandrels rolled from continuously cast large square billets is longitudinal cracking. The cause of this longitudinal cracking is the formation of abnormally coarse, longitudinally flattened grains on the surface, which reduces the local plasticity and toughness (resistance to cracking) of the surface material. During conventional cooling, the mandrel surface is subjected to residual stress dominated by tensile stress, thus inducing surface cracks. Under the influence of stress, these cracks rapidly propagate along the abnormally coarse, longitudinally flattened, low-toughness grains on the surface, developing from microscopic cracks into macroscopic cracks. Therefore, this invention focuses on addressing the causes of cracking in mandrels rolled from continuously cast large square billets, including high hydrogen embrittlement sensitivity, high residual tensile stress on the surface, and the appearance of abnormally coarse, low-toughness surface structures.

[0017] Mandrels are a type of steel material. Steel undergoes recrystallization during hot rolling, but the conditions for recrystallization are related to the deformation temperature and the amount of deformation. Generally, as the deformation temperature decreases, the critical deformation required for complete dynamic recrystallization increases. When the heating temperature (deformation temperature) across the billet cross-section is uneven, different critical deformation amounts for complete dynamic recrystallization occur under the same deformation conditions. This results in some parts of the material undergoing deformation within the critical deformation amount during manufacturing, leading to insufficient recrystallization and the formation of coarse or abnormally large grain structures, thereby reducing the plastic deformation coordination and toughness of the local material.

[0018] Therefore, in terms of composition design, this invention promotes the precipitation of NbN and TiN by adding Nb and Ti elements, and controlling the composition of Nb and Ti in accordance with the heating temperature. This ensures that a suitable Nb and Ti ratio is selected at the corresponding heating temperature, which hinders grain growth, increases the recrystallization temperature, and raises the recrystallization temperature. This avoids the deformation process of some materials within the deformation temperature range of 1000-1250℃ being within the critical deformation amount, thereby avoiding insufficient recrystallization and effectively suppressing the formation of low-toughness grains with abnormally large and longitudinally flattened surfaces.

[0019] In addition, the present invention controls the H content in the steel to a low range, avoiding the possibility of increased H solubility at the grain boundary due to the formation of coarse grains on the surface during the manufacturing process. Under the action of hydrogen pressure and residual tensile stress on the surface, it reduces the tendency of hydrogen-induced delayed cracking on the surface.

[0020] Specifically, the low-cracking-prone mandrel steel rolled from continuously cast large square billets as described in this invention has the following chemical composition by mass percentage: C: 0.33–0.38%, Mn: 0.40–0.60%, Si: 0.80–1.25%, Cr: 4.75–5.25%, Mo: 1.10–1.20%, V: 0.30–0.40%, Nb: 0.035–0.06%, Ti: 0.012–0.020%, H ≤ 2.0 ppm, with the remainder being Fe and other unavoidable impurities, and simultaneously satisfying: Nb ≥ 10 ppm. [4.1-8500 / (T+273.15)] Ti≥10 [3.32 -8000 / (T+273.15)] , where T is the heating temperature.

[0021] Furthermore, the other unavoidable impurities include one or more of the following: P ≤ 0.010%, S ≤ 0.005%, As ≤ 0.01%, Sb ≤ 0.01%, Sn ≤ 0.015%, Pb ≤ 0.005%, and Bi ≤ 0.005%.

[0022] The mandrel steel described in this invention has a grain size ≥ 6.

[0023] In the microstructure of the mandrel steel of the present invention, the total number of fine inclusions of types A, B, C, and D is ≤3.0, and the total number of coarse inclusions is ≤2.5; the number of qualified grades of fine or coarse inclusions of types A, B, C, and D is ≤1.5.

[0024] In the composition design of the mandrel steel described in this invention:

[0025] Carbon (C): plays a role in solid solution strengthening, precipitation strengthening, and martensitic transformation strengthening; its content should not be less than 0.33%. However, excessively high C content can lead to severe compositional segregation and banded structure, which is detrimental to preventing or mitigating the tendency for quenching cracks during cooling, and it easily combines with V to form V₂. x C y The brittle, hard, liquid-precipitated carbides reduce toughness (crack resistance), and the C content should not exceed 0.38%. Therefore, this invention requires controlling the C content between 0.33% and 0.38%.

[0026] Manganese (Mn): Mn is a solid solution strengthening element, and its content should not be less than 0.40%. Excessive content will lead to severe component segregation and banded structure, reducing thermal fatigue performance. Therefore, the content should not exceed 0.60%. Therefore, this invention controls the Mn content between 0.40% and 0.60%.

[0027] Silicon (Si): A deoxidizing element in steelmaking, it also improves the high-temperature oxidation resistance of steel. Si also contributes to tempering stability and hardenability, and its content should not be less than 0.80%. However, excessive Si content is detrimental to the toughness of the material. Therefore, this invention requires controlling the Si content between 0.80% and 1.25%.

[0028] Chromium (Cr): Cr mainly combines with C to form carbides at the lath interface, resulting in precipitation strengthening. It is also beneficial for resisting tempering softening, high-temperature oxidation resistance, and improving hardenability. Its content should not be less than 4.75%. However, excessive Cr content will lead to a decrease in material toughness. Therefore, this invention controls the Cr content between 4.75% and 5.25%.

[0029] Molybdenum (Mo): The addition of Mo plays a role in solid solution strengthening and improving hardenability. It also combines with C to form carbides Mo2C, resulting in precipitation strengthening and thus increasing strength. The content should not be less than 1.10%. However, excessive Mo content will reduce the diffusion rate of C, which is detrimental to the homogenization of composition during high-temperature heating and subsequent microstructure homogenization. Therefore, this invention controls the Mo content at 1.10–1.20%.

[0030] Vanadium (V): The addition of V is to combine with C to form carbides, resulting in precipitation strengthening, which improves strength, high-temperature strength, and refines grain size. The content should not be less than 0.30%. However, excessive V content will easily form vanadium (V₂O₃) with C. x C y In brittle and hard phase liquids, carbides precipitate, thus reducing toughness. Furthermore, once the V content exceeds a certain level, the beneficial effect of precipitation strengthening becomes insignificant, and it even has a negative effect of reducing toughness. Therefore, this invention controls the V content to 0.30–0.40%.

[0031] Niobium (Nb): Increasing the recrystallization temperature during rolling prevents incomplete recrystallization, which can lead to localized coarse grains and deteriorate ductility and toughness. The content should not be lower than 0.035%, but excessively high content can easily form Nb-rich inclusions, which are detrimental to toughness; therefore, the content should not exceed 0.06%. Furthermore, depending on the heating temperature, Nb ≥ 10 [4.1-8500 / (T+273.15)] This promotes the precipitation of NbN, ensures that at the appropriate heating temperature, it hinders grain growth, increases the recrystallization temperature, and prevents grain coarsening and the formation of abnormally large grains.

[0032] Titanium (Ti): It inhibits austenite grain growth and coarsening during high-temperature heating, preventing uneven deformation due to uneven deformation during subsequent rolling, which can lead to localized coarse grains and deteriorate plasticity and toughness. The content should not be less than 0.012%. However, excessively high content can easily form coarse TiN inclusions, which are detrimental to resistance to hydrogen embrittlement (hydrogen-induced delayed cracking). Therefore, the content should not exceed 0.020%. Thus, this invention controls the Ti content between 0.012% and 0.020%. Furthermore, depending on the heating temperature, Ti ≥ 10%. [3.32-8000 / (T+273.15)] This promotes the precipitation of TiN, ensures that at the appropriate heating temperature, it hinders grain growth, increases the recrystallization temperature, and prevents grain coarsening and the formation of abnormally large grains.

[0033] Hydrogen (H): For high-strength steels, the microstructure with decreasing sensitivity to hydrogen embrittlement is martensite, upper bainite, lower bainite, sorbite, pearlite, and austenite. Higher strength corresponds to higher sensitivity to hydrogen embrittlement and a lower allowable upper limit for the mass fraction of hydrogen in the material. Hydrogen in metals undergoes stress-induced diffusion, diffusing and accumulating from low-stress areas to high-stress areas, causing hydrogen embrittlement in the high-stress areas. The initial hydrogen content is crucial for the steel's resistance to hydrogen-induced delayed cracking; the lower the H content, the stronger the resistance. Higher initial hydrogen content necessitates subsequent measures such as slow cooling of continuously cast billets and rolled products to mitigate the problem to some extent; otherwise, the tendency for delayed cracking cannot be effectively reduced. Considering the impact on final performance and the increased cost and reduced production efficiency due to slow cooling, this invention controls the H content to not exceed 2 ppm.

[0034] For high-strength steel, material cleanliness requirements must be ensured. Impurity elements such as P, S, Sn, and As tend to segregate at grain boundaries, increasing the material's susceptibility to hydrogen embrittlement. Conversely, the formation of dispersed precipitates of a second phase strengthens the steel, resulting in lower hydrogen embrittlement sensitivity and higher fracture toughness and resistance to delayed fracture. Therefore, the upper limits of residual elements must be controlled. Thus, this invention controls the residual elements as follows: P ≤ 0.010%, S ≤ 0.005%, As ≤ 0.01%, Sb ≤ 0.01%, Sn ≤ 0.015%, Pb ≤ 0.005%, and Bi ≤ 0.005%.

[0035] The mandrel steel of this invention also requires control of inclusions: the total number of fine inclusion grades (A, B, C, D) ≤ 3.0; the total number of coarse inclusion grades ≤ 2.5; and the number of qualified grades for either the fine or coarse inclusion grades (A, B, C, D) ≤ 1.5. A large number and size of inclusions increase the tendency to crack under residual tensile stress and the tendency to develop hydrogen-induced delayed cracks; therefore, it is necessary to control the inclusion grades.

[0036] The grain size described in this invention is ≥6. Larger grain size means finer grains, larger grain boundary area, and lower residual element content and H content per unit area, which can reduce the tendency for hydrogen-induced delayed cracking.

[0037] The present invention also provides a method for manufacturing the low cracking tendency mandrel steel rolled from continuously cast large square billets, comprising the following steps:

[0038] 1) Smelting and casting

[0039] The materials are smelted according to the stated composition and continuously cast into large square billets with a cross-sectional area ≥320×425mm.

[0040] 2) Heating

[0041] Heating temperature T = 1250~1280℃, heating and holding time 4~8h;

[0042] 3) Rolling

[0043] Large billets are rolled into mandrel round bars with a diameter of 100-200 mm, with a forging ratio ≥5;

[0044] 4) Cool to room temperature;

[0045] 5) Annealing

[0046] The annealing temperature is 700-780℃, the annealing time is 3-8h, and after annealing, the product is cooled to room temperature. The maximum interval between rolling and annealing is 72h.

[0047] In the smelting and casting process, this invention controls the cross-sectional area of ​​the large billet to be ≥320×425mm to meet the requirements for producing large-diameter mandrel round steel with a diameter of 145~182mm, and ensures that the forging ratio (compression ratio) is ≥5. If the forging ratio is too small, the grains are coarse, the toughness is reduced, and it is sensitive to external surface defects. Cracks are easy to expand and develop into macroscopic large cracks, that is, easy to crack. Therefore, it is necessary to control the forging ratio to be ≥5.

[0048] In the heating process of the large billet of this invention, the addition of Nb and Ti prevents the deformation process of some materials from falling within the critical deformation range of 1000-1250℃, thus avoiding insufficient recrystallization and effectively suppressing the formation of abnormally coarse, longitudinally flattened, low-toughness grains. Setting the heating temperature at a relatively high range of 1250-1280℃, the addition of Nb and Ti can also suppress or prevent rapid grain coarsening during high-temperature heating, providing a good foundation for obtaining refined grains. This allows for high-temperature, long-term heating to improve heating uniformity, providing conditions for preventing grain coarsening. The composition design ensures the implementation of the process and the achievement of the target grain size.

[0049] When the heating temperature is below 1250℃, the temperature difference between the surface and the core of the billet is large during the rolling process, and the temperature is easily uneven in different parts of the cross section. This causes the deformation at certain locations of the cross section to fall into the range of critical deformation, resulting in local abnormal coarse structure and increasing the tendency to crack. Moreover, when the heating temperature is low, Nb and Ti play a role in hindering recrystallization throughout the entire rolling deformation process. The effect of grain refinement through deformation at high temperature during the rolling process is weakened, which is not conducive to grain refinement.

[0050] When the heating temperature exceeds 1280℃, austenite grains tend to grow and coarsen, which is detrimental to subsequent grain refinement to improve microstructure toughness. Furthermore, it reduces processing plasticity and deformation compatibility, easily leading to localized stress and strain concentrations. This also makes the formation of abnormally coarse microstructures in certain areas, increasing the tendency to crack. When the heating temperature is further increased, Nb and Ti cannot effectively inhibit or prevent grain coarsening; therefore, the heating temperature should not exceed 1280℃.

[0051] The heating and holding time is set between 4 and 8 hours. When the holding time is less than 4 hours, heat diffusion has not yet reached the core and achieved uniform temperature. The temperature at various parts of the cross-section has not yet reached uniformity, causing deformation at certain locations to easily fall within the critical deformation range, resulting in abnormally coarse microstructures and increasing the tendency to crack. When the holding time is more than 8 hours, austenite grains tend to grow and coarsen, which is not conducive to subsequent grain refinement to improve microstructure toughness. It also reduces processing plasticity and deformation coordination, easily leading to local stress and strain concentration, which also easily results in abnormally coarse microstructures and increases the tendency to crack. By controlling the grain size to ≥6 through the heating and rolling process, the larger the grain size, the finer the grains, the larger the grain boundary area, and the lower the residual element content and H content per unit area, the tendency to initiate hydrogen-induced delayed cracks can be reduced.

[0052] The cooling method after rolling can be either air cooling on an upper cooling bed or slow cooling in a stack, cooling to room temperature.

[0053] The annealing temperature of this invention is controlled at 700-780℃. If the annealing temperature is too high, the mandrel strength will decrease excessively, which is detrimental to subsequent use. If the annealing temperature is too low, a longer heat treatment time is required to relieve stress and reduce strength (hardness) to ensure subsequent roughing and finishing machining. The annealing time is 3-8 hours. If the annealing time is too long, the mandrel strength will decrease excessively, which is detrimental to subsequent use and increases production costs. If the annealing time is too short, the hardness will be too high, which is detrimental to subsequent roughing and finishing machining, and increases tool consumption, thus increasing costs.

[0054] In the mandrel steel described in this invention, Nb-Ti microalloying, with the Nb and Ti composition controlled in conjunction with the heating temperature, primarily promotes the precipitation of NbN and TiN. This ensures that at the appropriate heating temperature, grain growth is hindered, the recrystallization temperature is increased, and grain coarsening and the formation of abnormally large grains are prevented. Furthermore, the low H content design, combined with high-temperature heating and long-term heat preservation, effectively ensures that the deformation during rolling does not fall within the critical deformation range, avoiding the formation of abnormally large local structures due to insufficient local recrystallization and preventing the formation of low-ductility and low-toughness structures. This reduces the tendency to crack under residual tensile stress and the tendency for hydrogen-induced delayed cracking.

[0055] The beneficial effects of this invention are:

[0056] This invention, through research, has discovered that the main form of cracking in mandrels rolled from continuously cast large square billets is longitudinal cracking. The cause of this longitudinal cracking is the formation of abnormally coarse, longitudinally flattened grain structures on the surface. Therefore, in terms of composition design, this invention, on the one hand, increases the recrystallization temperature by adding Nb and Ti, and on the other hand, controls the composition of Nb and Ti in a way that matches the heating temperature, i.e., Nb ≥ 10. [4.1-8500 / (T+273.15)] Ti≥10 [3.32-8000 / (T+273.15)] (Where T is the heating temperature) promotes the precipitation of NbN and TiN, ensuring that at the corresponding heating temperature, grain growth is hindered and the recrystallization temperature is increased, avoiding the deformation process of some materials within the critical deformation range during heating, resulting in insufficient recrystallization; on the other hand, the H content in the steel is controlled within a low range to avoid the formation of coarse grains on the surface during manufacturing, which may lead to an increase in H solubility at the grain boundary; thus greatly reducing the proportion and tendency of longitudinal cracks and hydrogen-induced delayed cracking in the mandrel. At the same time, the rolling and annealing interval is greatly extended from the original 24h to 72h, significantly reducing the cracking ratio during the waiting annealing process after rolling, improving the yield, and helping to save costs.

[0057] In the manufacturing process of the mandrel, the present invention adopts a higher heating temperature and a longer heating and holding time to improve the temperature uniformity of the billet inside and outside, reduce the temperature difference between the surface and the core of the billet, and avoid concentrated deformation in the local high temperature area on the surface during rolling. This avoids the formation of abnormally coarse, longitudinally flattened, low-toughness grains due to local recrystallization.

[0058] The low cracking ratio mandrel steel rolled from continuously cast large square billets and its manufacturing method described in this invention can be effectively applied to the mass production of mandrel steel, significantly reducing the cracking ratio during the waiting annealing process after rolling. At the same time, it extends the allowable time interval for waiting annealing, which is very beneficial for production organization and arrangement when the material flow waiting time between upstream and downstream processes is long. It has good prospects for promotion and application value. Attached Figure Description

[0059] Figure 1 The image shows the microstructure of the steel in Example 4 of this invention.

[0060] Figure 2 This is a micrograph of the steel used in Comparative Example 6 of this invention;

[0061] Figure 3 This is a macroscopic photograph of the mandrel after rolling, which is the comparative example of this invention. Detailed Implementation

[0062] The following will further illustrate the low cracking tendency mandrel steel rolled from continuously cast large square billets according to the present invention based on specific embodiments. However, the specific embodiments and related descriptions do not constitute an improper limitation on the technical solution of the present invention.

[0063] The components of the embodiments and comparative examples of the present invention are shown in Table 1, with the balance being Fe and other unavoidable impurities. Specific process parameters for the embodiments and comparative examples of the present invention are shown in Table 2. Performance parameters for the embodiments and comparative examples of the present invention are shown in Table 3.

[0064] Figure 1 The image shows the microstructure of the steel in Example 4 of the present invention. As can be seen from the image, no abnormal megacrystalline structure was observed in the steel of the present invention.

[0065] Figure 2 The image shows a typical anomalous megacrystalline structure that appeared in Comparative Example 6 of this invention; Figure 3 The photograph shown is of Comparative Example 6 of this invention after the mandrel has been rolled. As can be seen from the photograph, the mandrel is severely cracked after rolling.

[0066] As can be seen from Tables 1, 2, and 3, compared to Examples 1-10, Comparative Examples 1-7 had chemical compositions or related process parameters that failed to meet the design requirements of this invention. The mandrels prepared in Comparative Examples 1-7 exhibited a significant proportion of post-rolling cracking.

[0067] In Comparative Example 1, 8% of the annealing intervals were too long, resulting in post-rolling cracking. This indicates that even if the composition and process meet the requirements of this invention, prolonged waiting after rolling can still lead to cracking.

[0068] In Comparative Example 2, 40% of the steel experienced post-rolling cracking due to excessively high levels of H, As, Sb, Sn, Pb, and Bi, as well as a long annealing interval.

[0069] In Comparative Example 3, abnormal giant grain structure appeared due to excessive heating time of steel before rolling. Although timely annealing measures were taken (annealing interval waiting time was only 24 hours), 50% of the steel still cracked after rolling.

[0070] In Comparative Example 4, abnormal giant grain structure appeared due to excessive heating temperature of the steel before rolling. Although timely annealing measures were taken (annealing interval waiting time was only 24 hours), 45% of the steel still cracked after rolling.

[0071] In Comparative Example 5, abnormal giant grain structure appeared due to the steel being heated too low before rolling. Although timely annealing measures were taken (the annealing interval was only 24 hours), 43% of the steel still cracked after rolling.

[0072] In Comparative Example 6, no Nb or Ti was added to the steel composition, and an abnormal giant grain structure appeared. Although timely annealing was taken (the annealing interval was only 24 hours), 40% of the steel experienced post-rolling cracking.

[0073] In Comparative Example 7, although Nb and Ti were added to the steel composition, the requirement of Nb ≥ 10 was not met because the additions were not specifically tailored to the heating temperature. [4.1-8500 / (T+273.15)] Ti≥10 [3.32-8000 / (T+273.15)] The composition control requirements (where T is the heating temperature) resulted in abnormal giant crystal structure. Although timely annealing measures were taken (the annealing interval was only 24 hours), 30% of the cracks occurred after rolling.

[0074] In summary, the low-cracking-ratio mandrel steel produced by continuous casting of large square billets as described in this invention, through reasonable chemical element composition design and optimized process, can effectively avoid or reduce the cracking ratio after rolling. It can be applied to the mass production of mandrel steel, significantly reducing the cracking ratio during the post-rolling annealing process and extending the allowable annealing time interval, demonstrating good prospects for promotion and application value.

[0075] It should be noted that the scope of protection of the prior art in this invention is not limited to the embodiments given in this application. All prior art that does not contradict the solution of this invention, including but not limited to prior patent documents, prior publications, prior public uses, etc., can be included in the scope of protection of this invention.

[0076] It should also be noted that the above examples are merely specific embodiments of the present invention, and the present invention is obviously not limited to the above embodiments, with many similar variations. All modifications that can be directly derived or conceived by those skilled in the art from the content disclosed in this invention should fall within the protection scope of this invention.

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[0078]

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Claims

1. A low cracking tendency core rod steel for continuous casting bloom rolling, having a chemical composition in mass percent of: C: 0.33-0.38%, Mn: 0.40-0.60%, Si: 0.80-1.25%, Cr: 4.75-5.25%, Mo: 1.10-1.20%, V: 0.30-0.40%, Nb: 0.035-0.06%, Ti: 0.012-0.020%, H≤2.0ppm, the balance being Fe and other unavoidable impurities; and simultaneously satisfying: Nb≥10 [4.1-8500 / (T+273.15)] , Ti≥10 [3.32-8000 / (T+273.15)] , wherein, T represents the heating temperature during the pre-rolling heating stage.

2. The low-cracking-prone mandrel steel rolled from continuously cast large square billets as described in claim 1, characterized in that, The other unavoidable impurities include one or more of the following: P≤0.010%, S≤0.005%, As≤0.01%, Sb≤0.01%, Sn≤0.015%, Pb≤0.005%, and Bi≤0.005%.

3. The low-cracking-prone mandrel steel rolled from continuously cast large square billets as described in claim 1 or 2, characterized in that, The grain size of the mandrel steel is ≥6.

4. The low-cracking-prone mandrel steel rolled from continuously cast large square billets as described in claim 1, 2, or 3, characterized in that, The total number of fine inclusions of type A, B, C, and D in the microstructure of the mandrel steel is ≤3.0, and the total number of coarse inclusions is ≤2.5; the number of qualified grades of fine or coarse inclusions of type A, B, C, and D is ≤1.

5.

5. The method for manufacturing low-cracking-prone mandrel steel rolled from continuously cast large square billets as described in any one of claims 1 to 4, characterized in that, Includes the following steps: 1) Smelting and casting The material is smelted according to the composition described in claim 1 or 2 and continuously cast into a large square billet with a cross-sectional area ≥320×425mm. 2) Heating Heating temperature T = 1250~1280℃, heating and holding time 4~8h; 3) Rolling Large billets are rolled into mandrel round bars with a diameter of 100~200mm, with a forging ratio ≥5; 4) Cool to room temperature; 5) Annealing The annealing temperature is 700~780℃, the annealing time is 3~8h, and after annealing, the product is removed from the furnace and cooled to room temperature. The maximum interval between rolling and annealing is 72h.

6. The method for manufacturing low-cracking-prone mandrel steel rolled from continuously cast large square billets as described in claim 5, characterized in that, In step 4), the cooling method is either air cooling on a cooling bed or slow cooling by stacking in an insulated pit.

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