A rare earth modified high-strength steel plate with stable thickness direction performance and a preparation method thereof

Abstract

The application belongs to the technical field of low-alloy high-strength steel manufacturing, and discloses a rare earth modified high-strength steel plate with stable thickness direction performance and a preparation method thereof. Through the composite control of elements such as C, Mn, Cr, Ni, Mo, Nb, V, Ti, B and Ce, and by limiting the proportional relationship between Ce / (O+S), Ti / N and (Cr+Mo) / Mn, the low-hazard of inclusions, the control of hardenability of the core of thick plate and the stability of the thickness direction structure are realized. The thickness of the obtained steel plate is 60mm-100mm, the volume fraction of tempered martensite at the 1 / 2 thickness position is greater than or equal to 80%, and the number ratio of all inclusions with a maximum size of less than or equal to 5μm in the rare earth modified high-strength steel plate with stable thickness direction performance is greater than or equal to 80%. The steel plate is suitable for the fields of engineering machinery, ocean engineering and large load-bearing structural parts.

Description

Technical Field

[0001] This invention relates to the field of low-alloy high-strength steel manufacturing technology, specifically to a rare-earth modified high-strength steel plate with stable properties in the thickness direction and its preparation method. Background Technology

[0002] Low-alloy high-strength steel plates are widely used in engineering machinery, marine engineering, wind power equipment, bridge structures, and large load-bearing structural components due to their high strength, good low-temperature toughness, and good overall performance. As equipment develops towards larger size, lighter weight, and higher safety, the thickness of steel plates is constantly increasing, placing higher demands on the strength, low-temperature toughness, and thickness-direction performance stability of thicker high-strength steel plates. For 60mm–100mm thick high-strength steel plates, differences in deformation penetration, cooling rate, phase transformation process, and microstructure evolution between the surface and half-thickness areas can easily lead to uneven microstructure and properties in the thickness direction. This is particularly evident in insufficient strength at the half-thickness area, decreased low-temperature impact toughness, and a greater performance difference between the surface and half-thickness areas.

[0003] Existing low-alloy high-strength steels are typically produced through composite alloying with elements such as C, Mn, Cr, Ni, Mo, Nb, V, Ti, and B, combined with controlled rolling, quenching, and tempering processes to obtain a strengthened microstructure dominated by martensite or bainite. Mn, Cr, Mo, and B improve the hardenability of the steel, which is beneficial for the formation of martensite in the core of thick steel plates; Ni helps improve low-temperature toughness; and microalloying elements such as Nb, V, and Ti can refine grains and promote precipitation strengthening. However, for steel plates with a thickness of 60mm–100mm, simply increasing the content of hardenability-enhancing elements such as Mn, Cr, and Mo, while beneficial for improving core microstructure transformation, can also easily lead to problems such as center segregation, increased carbon equivalent, carbide aggregation, and decreased low-temperature toughness. Therefore, current composition design struggles to simultaneously balance core hardenability, low-temperature toughness, and thickness-direction performance stability in thick plates.

[0004] Inclusion control is also a crucial factor affecting the low-temperature toughness and thickness-direction performance stability of thick high-strength steel plates. Common inclusions in steel, such as Al₂O₃, MnS, TiN, and Ti(C,N), when large in size, irregular in shape, or distributed in a sharp-angled or chain-like manner, can easily become crack initiation sources during low-temperature impact. Rare earth element Ce can react with O and S in steel to form Ce-containing oxides, Ce-containing sulfides, Ce-containing oxysulfides, or Ce-Al-OS composite inclusions, thereby improving the inclusion morphology. However, the effect of Ce is closely related to the O and S content in the steel; insufficient Ce addition leads to inadequate inclusion modification, while excessive Ce addition easily forms coarse rare earth inclusions or inclusion agglomeration. Therefore, current technologies that only control Ce content or simply perform rare earth treatment are insufficient to reliably achieve low-hazard inclusion levels. Furthermore, the matching relationship between Ti, N, and B also has a significant impact on the hardenability and low-temperature toughness of thick high-strength steel plates. When the Ti content is insufficient or the Ti / N ratio is too low, the Ti's effect on consolidating N is insufficient, and B easily combines with N to form BN, thus reducing the effectiveness of B in improving hardenability. When the Ti content is too high or the Ti / N ratio is too high, coarse TiN or Ti(C,N) inclusions are easily formed, increasing the initiation point for low-temperature impact cracks. Existing technologies do not pay enough attention to the synergistic effect of the Ti / N ratio on both the effectiveness of B and the tendency to form coarse TiN or Ti(C,N). Meanwhile, the rolling, quenching, and tempering processes of thick steel plates have a significant impact on the microstructure stability at the 1 / 2 thickness position. If the rolling reduction is insufficient, the core deformation penetration is inadequate, easily leading to coarsening of the core microstructure and uneven microstructure in the thickness direction. If the quenching and tempering regimes are unreasonable, it is difficult to stably obtain a microstructure dominated by tempered martensite at the 1 / 2 thickness position, resulting in increased differences in thickness direction strength and low-temperature toughness.

[0005] In summary, existing thick-gauge low-alloy high-strength steel plates still have the following shortcomings: First, in 60mm-100mm thick steel plates, insufficient hardenability and decreased microstructure stability are prone to occur at the half-thickness position, leading to fluctuations in properties in the thickness direction. Second, existing composition designs often focus solely on improving hardenability, lacking synergistic control over the relationship between Cr, Mo, and Mn and the microstructure stability of the thick plate's core. Third, existing rare earth treatments lack sufficient control over the matching relationship between Ce and O / S, easily leading to insufficient inclusion modification or coarsening of rare earth inclusions. Fourth, existing technologies lack sufficient control over the relationship between the Ti / N ratio and B effectiveness, as well as the formation of coarse TiN or Ti(C,N). Fifth, existing rolling, quenching, and tempering processes lack sufficient control over the microstructure stability at the half-thickness position and the performance difference between the surface and half-thickness positions. Therefore, it is necessary to develop a rare earth-modified high-strength steel plate with stable properties in the thickness direction for 60mm-100mm thick steel plates and its preparation method. Summary of the Invention

[0006] (I) The technical problems to be solved; This invention addresses the problem of uneven microstructure and properties in the thickness direction of existing 60mm-100mm thick low-alloy high-strength steel plates, providing a rare-earth modified high-strength steel plate with stable properties in the thickness direction and its preparation method. In the preparation of existing thick-gauge low-alloy high-strength steel plates, the deformation penetration, cooling rate, and phase transformation process differ between the surface layer and the half-thickness region. The half-thickness region is prone to problems such as insufficient hardenability, reduced tempered martensite proportion, increased inclusion damage, and decreased low-temperature impact toughness, resulting in a significant difference in yield strength and Charpy impact energy at -40℃ between the surface layer and the half-thickness region.

[0007] The technical problem this invention aims to solve is how to reduce the variation in microstructure and properties along the thickness direction of thick steel plates, under the condition that the steel plate thickness is 60mm to 100mm and the surface and half-thickness positions both meet the requirements for high strength and low-temperature toughness, through the synergistic design of composition ratio, inclusion control, core microstructure control, and heat treatment processes. Specifically, this invention controls the matching relationship between rare earth Ce and O and S in steel by limiting Ce / (O+S), causing inclusions to transform into spherical or near-spherical rare earth composite inclusions, thus reducing the harm of inclusion crack initiation; it controls the effect of Ti solidifying N, the effectiveness of B element, and the tendency to form coarse TiN or Ti(C,N) inclusions by limiting Ti / N; it coordinates the hardenability and segregation tendency of the core of the thick plate by limiting (Cr+Mo) / Mn; and it combines continuous casting billet heating, rolling reduction rate, quenching, and tempering processes to stably obtain a microstructure dominated by tempered martensite at the half-thickness position.

[0008] Therefore, this invention aims to solve the problems of unstable rare earth inclusion modification, unreasonable Ti / N matching, insufficient hardenability of the core, poor microstructure stability at the 1 / 2 thickness position, and difficulty in controlling the performance difference between the surface and 1 / 2 thickness positions in existing thick-gauge low-alloy high-strength steel plates. In this way, a rare earth modified high-strength steel plate can be obtained with yield strength, tensile strength, elongation after fracture, and impact energy at -40℃ all meeting the requirements, and with small differences in strength and toughness in the thickness direction.

[0009] (II) The technical solution of the present invention; A rare-earth-modified high-strength steel plate with stable properties in the thickness direction comprises the following chemical composition by mass percentage: C: 0.14%–0.18%, Si: 0.25%–0.45%, Mn: 0.90%–1.40%, Cr: 0.40%–0.80%, Ni: 0.10%–0.40%, Mo: 0.10%–0.30%, Nb: 0.010%–0.050%. V: 0.010%~0.050%, Al: 0.020%~0.080%, Ti: 0.010%~0.030%, B: 0.0005%~0.0030%, Ce: 0.002%~0.006%, N: 0.002%~0.008%, P≤0.015%, S≤0.003%, O≤0.004%, balance being Fe and unavoidable impurities; Among them, Ce, O, S, Ti, N, Cr, Mo, and Mn are all mass percentage contents, and satisfy the following conditions: Ce / (O+S)=0.10~1.50, Ti / N=2.5~4.5, (Cr+Mo) / Mn=0.45~0.95; The thickness of the rare earth modified high-strength steel plate with stable performance in the thickness direction is 60mm to 100mm. The matrix structure at the 1 / 2 thickness position is mainly tempered martensite, and the volume fraction of tempered martensite at the 1 / 2 thickness position is ≥80%, with the remainder being tempered bainite. The rare earth-modified high-strength steel plate with stable properties in the thickness direction contains rare earth composite inclusions, which are spherical or near-spherical; the proportion of all inclusions with a maximum size ≤5μm in the rare earth-modified high-strength steel plate with stable properties in the thickness direction is ≥80%; The yield strength difference between the surface and half-thickness positions of the rare earth modified high-strength steel plate with stable performance in the thickness direction is ≤70MPa, and the Charpy impact energy difference between the surface and half-thickness positions at -40℃ is ≤20J.

[0010] The rare earth composite inclusions are one or more of Ce oxides, Ce sulfides, Ce oxysulfides, and Ce-Al-OS composite inclusions; the aspect ratio of the two-dimensional projection of the rare earth composite inclusions on the metallographic or scanning electron microscope observation section is ≤2.0.

[0011] The proportion of large-angle grain boundaries with an orientation difference angle ≥15° at the 1 / 2 thickness position of the rare earth modified high-strength steel plate with stable performance in the thickness direction is ≥35%.

[0012] The yield strength of the rare earth modified high-strength steel plate with stable performance in the thickness direction is ≥950MPa at both the surface and half-thickness positions, the tensile strength is ≥1000MPa, the elongation after fracture is ≥14%, and the impact energy at -40℃ is ≥80J.

[0013] A method for preparing rare-earth modified high-strength steel plates with stable properties in the thickness direction includes the following steps: Step S1: Smelt and RH vacuum refine the molten steel, controlling the S content in the refined molten steel to be ≤0.003% and O content to be ≤0.004%; Step S2: Add Ce-containing rare earth alloy to the refined molten steel, control the Ce mass percentage content to be 0.002% to 0.006%, and Ce / (O+S) = 0.10 to 1.50, and transform irregular Al2O3 inclusions, MnS inclusions and Ti(C,N) inclusions into spherical rare earth composite inclusions or near-spherical rare earth composite inclusions by soft blowing argon. Step S3: The molten steel from step S2 is continuously cast into a continuous casting billet; the continuous casting billet is heated to 1180℃~1200℃ and held for 2 hours, and then rolled into a steel plate with a thickness of 60mm~100mm. Step S4: Heat the rolled steel plate to 900℃~950℃, hold at 1.5min / mm according to the thickness of the steel plate, and water cool to room temperature so that the volume fraction of tempered martensite in the microstructure after quenching at 1 / 2 thickness of the steel plate is ≥80%; Step S5: Temper the quenched steel plate at 600℃~640℃, hold at 3.0min / mm according to the thickness of the steel plate, and air cool to room temperature after tempering to obtain rare earth modified high-strength steel plate with stable performance in the thickness direction.

[0014] In step S3, the cumulative reduction rate of rough rolling is ≥50%, and the cumulative reduction rate of finish rolling is ≥30%.

[0015] In step S2, the Ce-containing rare earth alloy is added after deoxidation, desulfurization and composition adjustment are completed in RH vacuum refining, and before the RH vacuum is broken. After addition, it is soft-blown with argon for 5 min to 20 min.

[0016] (III) The role of each element in this invention; The roles of each chemical element in the steel of this invention and the reasons for limiting their content are as follows. Unless otherwise specified, the content of each element is a mass percentage.

[0017] C: 0.14%~0.18%. C is the main element that ensures the formation of martensite and the attainment of high strength after steel plate quenching. When the C content is below 0.14%, the martensitic strengthening is insufficient, and it is difficult to stably reach a yield strength of over 950MPa; when the C content is above 0.18%, the low-temperature toughness and weldability decrease, and the properties in the thickness direction fluctuate more.

[0018] Si: 0.25%~0.45%. Si mainly plays a role in deoxidation and solid solution strengthening, and helps to improve tempering stability. When the Si content is below 0.25%, the strengthening effect is insufficient; when it is above 0.45%, it easily reduces low-temperature toughness and weldability.

[0019] Mn: 0.90%~1.40%. Mn can improve hardenability and solid solution strengthening effect, which is beneficial to the core microstructure transformation of thick steel plates. When the Mn content is less than 0.90%, the hardenability at the 1 / 2 thickness position is insufficient; when it is higher than 1.40%, it is easy to aggravate the central segregation and banded structure, and increase the poor performance in the thickness direction.

[0020] Cr: 0.40%–0.80%. Cr can improve hardenability and tempering stability, which is beneficial for the formation of a martensite-dominated microstructure in the core of thick plates. When the Cr content is below 0.40%, its contribution to the hardenability of the core is insufficient; when it is above 0.80%, it tends to increase the carbon equivalent and the tendency for carbide segregation. Therefore, Cr should be controlled at 0.40%–0.80%.

[0021] Ni: 0.10%–0.40%. Ni is beneficial for improving low-temperature toughness and crack propagation resistance. When the Ni content is below 0.10%, the improvement in toughness is insufficient; when it is above 0.40%, the cost increases, and the further improvement on the thickness-direction performance stability of the present invention is limited.

[0022] Mo: 0.10%~0.30%. Mo can improve hardenability and tempering stability, and inhibit the formation of softened structure in the core of thick plates. When the Mo content is below 0.10%, the core structure stability is insufficient; when it is above 0.30%, it is easy to increase segregation and carbide coarsening tendency.

[0023] Nb: 0.010%~0.050%. Nb has the effect of refining austenite grains and precipitation strengthening, which is beneficial to improving strength and microstructure uniformity. When the Nb content is below 0.010%, the refining and strengthening effect is insufficient; when it is above 0.050%, coarse Nb(C,N) is easily formed, which impairs low-temperature toughness.

[0024] V: 0.010%~0.050%. V can form fine precipitates during tempering, improving the strength stability after tempering. When the V content is below 0.010%, precipitation strengthening is insufficient; when it is above 0.050%, it easily causes coarsening of the precipitates and increases costs.

[0025] Al: 0.020%~0.080%. Al is mainly used for deoxidation and can combine with N to inhibit grain growth. When the Al content is below 0.020%, the deoxidation effect is insufficient; when it is above 0.080%, more Al2O3 inclusions are easily formed, increasing the difficulty of inclusion control.

[0026] Ti: 0.010%~0.030%. Ti is mainly used to fix N, reduce BN formation, and ensure the effectiveness of B in improving hardenability. When the Ti content is below 0.010%, the fixed N is insufficient and the effectiveness of B decreases; when it is above 0.030%, coarse TiN or Ti(C,N) is easily formed, which becomes the source of low-temperature impact cracks.

[0027] B: 0.0005%~0.0030%. B can significantly improve the hardenability of low alloy steel, which is beneficial for the formation of a martensitic microstructure at the 1 / 2 thickness position of 60mm~100mm thick steel plates. When the B content is below 0.0005%, the effect of improving hardenability is insufficient; when it is above 0.0030%, it is easy to form borides or increase the tendency of grain boundary embrittlement.

[0028] Ce: 0.002%~0.006%. Ce is the key element for achieving low-hazard inclusions in this invention. Ce can react with O and S in steel to form Ce oxides, Ce sulfides, Ce oxysulfides, or Ce-Al-OS composite inclusions, causing irregular Al2O3, MnS, and other inclusions to transform into spherical or near-spherical shapes. When the Ce content is below 0.002%, the inclusion modification is insufficient; when it is above 0.006%, coarse rare earth inclusions or inclusion agglomeration are easily formed.

[0029] N: 0.002%~0.008%. N has a strong affinity for Ti, Al, and B. An appropriate amount of N is beneficial for the formation of fine nitrides and inhibits grain growth; when the N content is too high, coarse TiN or BN is easily formed, reducing the effectiveness of B and impairing low-temperature toughness.

[0030] P ≤ 0.015%. P tends to segregate at grain boundaries, reducing low-temperature toughness and weldability, therefore it should be controlled below 0.015%.

[0031] S≤0.003%. S easily forms MnS inclusions, and coarse or chain-like MnS can become the source of low-temperature impact cracks.

[0032] O ≤ 0.004%. Excessive O content will increase the number of oxide inclusions and consume rare earth Ce, thus weakening the inclusion modification effect.

[0033] Ce / (O+S) = 0.10~1.50. This ratio is used to control the matching relationship between Ce and O and S. When it is below 0.10, Ce is insufficient and the modification of inclusions is not sufficient; when it is above 1.50, Ce is excessive and it is easy to form coarse rare earth inclusions or inclusion agglomeration.

[0034] The Ti / N ratio is 2.5–4.5. This ratio is used to coordinate the role of Ti in securing N, the availability of B, and the tendency to form coarse TiN or Ti(C,N). Below 2.5, Ti is insufficient in securing N, and the availability of B decreases; above 4.5, the tendency to form coarse TiN or Ti(C,N) increases.

[0035] (Cr+Mo) / Mn = 0.45~0.95. This ratio is used to coordinate the core hardenability and segregation tendency of thick steel plates. When it is below 0.45, Cr and Mo do not contribute enough to the core hardenability and tempering stability; when it is above 0.95, Cr and Mo are relatively high, which can easily increase the adverse effects of carbon equivalent, segregation and carbides.

[0036] (iv) Compared with the prior art, the beneficial effects of the present invention include: 1. This invention limits the Ce content and Ce / (O+S) = 0.10~1.50, so that the rare earth Ce content matches the O and S content in the steel, promotes the transformation of irregular Al2O3, MnS and other inclusions into spherical rare earth composite inclusions or near-spherical rare earth composite inclusions, reduces the harm of coarse, sharp-angled or chain-like inclusions as low-temperature impact crack sources, and achieves low-hazard control of inclusions in thick high-strength steel plates.

[0037] 2. This invention, by limiting the Ti / N ratio to 2.5–4.5, coordinates the role of Ti in solidifying N, the effectiveness of B, and the tendency to form coarse TiN or Ti(C,N) inclusions. While ensuring that B improves hardenability, it suppresses the adverse effects of coarse nitrides and carbonitrides on low-temperature toughness, thus balancing the microstructure transformation and impact toughness of thick plates.

[0038] 3. By limiting (Cr+Mo) / Mn = 0.45 to 0.95, this invention coordinates the effects of Mn, Cr, and Mo on the core hardenability, tempering stability, and segregation tendency of thick steel plates, avoiding the reliance on high Mn, Cr, or Mo content to improve hardenability, thereby reducing the risk of center segregation, carbide agglomeration, and thickness-direction property fluctuations.

[0039] 4. This invention achieves stable microstructure dominated by tempered martensite at the 1 / 2 thickness position in 60mm-100mm thick steel plates through coordinated control of continuous casting billet heating, rough rolling and finish rolling reduction rates, quenching heating and holding, water cooling and tempering processes. This improves the stability of the core microstructure and reduces the microstructure difference between the surface position and the 1 / 2 thickness position.

[0040] 5. The rare earth modified high-strength steel plate with stable thickness direction properties prepared by this invention has a yield strength ≥950MPa at both the surface position and the half-thickness position within the thickness range of 60mm to 100mm, a tensile strength ≥1000MPa, an elongation after fracture ≥14%, and an impact energy ≥80J at -40℃. At the same time, the yield strength difference between the surface position and the half-thickness position is ≤70MPa, and the Charpy impact energy difference at -40℃ is ≤20J. This can meet the requirements of engineering machinery, marine engineering and large load-bearing structural components for the strength, toughness and thickness direction property stability of thick-gauge high-strength steel plates. Attached Figure Description

[0041] Figure 1 Example 3 shows EBSD tissue at a surface location; Figure 2 The EBSD tissue at the 1 / 2 thickness position in Example 3; Figure 3 For Comparative Example 1, EBSD tissue is located in the surface layer; Figure 4 This is the EBSD tissue at the 1 / 2 thickness position for Comparative Example 1. Detailed Implementation

[0042] The present invention will be further described below with reference to embodiments and comparative examples. It should be understood that the following embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent substitutions or conventional adjustments made within the scope of the claims of the present invention should be included within the scope of protection of the present invention.

[0043] The steel plates obtained in the examples and comparative examples were tested for inclusions, microstructure, and mechanical properties. The morphology and size of inclusions were statistically analyzed using scanning electron microscopy and energy dispersive spectroscopy. Inclusions with a two-dimensional projection aspect ratio ≤2.0 on the cross-section observed by metallography or scanning electron microscopy were defined as spherical or near-spherical inclusions. The microstructure was observed and statistically analyzed using metallography, scanning electron microscopy, and EBSD. Grain boundaries with an orientation difference angle ≥15° were defined as large-angle grain boundaries. Tensile properties and low-temperature impact properties were tested by sampling at the surface and half-thickness positions of the steel plates, respectively. Low-temperature impact properties were tested using Charpy V-notch impact specimens at -40°C. The sampling direction of the tensile specimens was parallel to the rolling direction, and the sampling direction of the impact specimens was perpendicular to the rolling direction.

[0044] In this invention, the thickness of the steel plate is denoted as T. The surface location refers to the sampling area close to the rolled surface of the steel plate, specifically, a sampling position within a range of 3mm to 8mm from any rolled surface of the steel plate. The 1 / 2 thickness location refers to the central region along the thickness direction of the steel plate, i.e., a position approximately T / 2 away from any rolled surface of the steel plate, with sampling positions permitted within ±5mm of the thickness centerline. The yield strength difference and the Charpy impact energy difference at -40℃ between the surface location and the 1 / 2 thickness location are calculated based on the absolute value of the difference between their test values.

[0045] Example 1 This embodiment provides a rare-earth-modified high-strength steel plate with stable properties in the thickness direction, and the steel plate thickness is 60 mm. This embodiment illustrates that, under conditions of low alloy content and lower thickness limit, a high-strength steel plate with stable properties in the thickness direction can still be obtained by synergistic control of the Ce / (O+S), Ti / N, and (Cr+Mo) / Mn ratios, combined with rolling, quenching, and tempering processes.

[0046] The chemical composition of the steel plate in this embodiment, by mass percentage, is as follows: C: 0.14%, Si: 0.25%, Mn: 0.90%, Cr: 0.40%, Ni: 0.10%, Mo: 0.10%, Nb: 0.010%, V: 0.010%, Al: 0.020%, Ti: 0.010%, B: 0.0005%, Ce: 0.0020%, N: 0.0040%, P: 0.010%, S: 0.0015%, O: 0.0015%, with the balance being Fe and unavoidable impurities. Among these, Ce / (O+S) = 0.67, Ti / N = 2.50, and (Cr+Mo) / Mn = 0.56, all of which are within the scope defined by this invention.

[0047] The steel plate in this embodiment is prepared according to the following steps: S1, smelting and refining the molten steel, controlling the S content in the refined molten steel to be ≤0.003% and the O content to be ≤0.004%; S2, in the later stage of RH vacuum treatment during the refining stage (after deoxidation, desulfurization and composition adjustment are completed in RH vacuum refining, and before RH vacuum breaking), adding a Ce-containing rare earth alloy to the molten steel to make the Ce content in the finished steel 0.0020%, followed by soft argon blowing for 10 minutes to modify the irregular Al2O3, MnS and Ti(C,N) inclusions in the steel into spherical rare earth composite inclusions or near-spherical rare earth composite inclusions; S3, continuously casting the molten steel into a continuous casting billet. The continuously cast billet was heated to 1180℃ and held for 2 hours, then rolled into a steel plate with a thickness of 60 mm. The cumulative reduction rate of rough rolling was 55%, and the cumulative reduction rate of finish rolling was 35%. In step S4, the rolled steel plate was heated to 900℃ and held for 90 minutes at a rate of 1.5 min / mm according to the thickness of the steel plate. Then it was water-cooled to room temperature so that the microstructure of the steel plate after quenching at 1 / 2 thickness position was mainly tempered martensite. In step S5, the quenched steel plate was tempered at 600℃ and held for 180 minutes at a rate of 3.0 min / mm according to the thickness of the steel plate. After tempering, it was air-cooled to room temperature to obtain a rare earth modified high-strength steel plate with stable performance in the thickness direction.

[0048] Testing revealed that spherical or near-spherical inclusions accounted for 74% of all inclusions in the steel of this embodiment. In the rare-earth-modified high-strength steel plate with stable properties in the thickness direction, 82% of all inclusions had a maximum size ≤5μm. The volume fraction of tempered martensite at the surface and half-thickness locations of the steel plate was 96% and 89%, respectively. Large-angle grain boundaries with an orientation difference angle ≥15° accounted for 41% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1060MPa and 1015MPa, respectively; the tensile strengths were 1120MPa and 1070MPa, respectively; the elongation after fracture was 15.8% and 15.4%, respectively; and the impact energy at -40℃ was 98J and 90J, respectively. The difference in yield strength between the surface and half-thickness locations was 45MPa, and the difference in Charpy impact energy at -40℃ was 8J. The results show that this embodiment can still achieve low inclusion hazard, stable tempered martensite structure at the 1 / 2 thickness position, and control of performance differences between the surface and 1 / 2 thickness positions under low alloy content conditions.

[0049] Example 2 This embodiment provides a rare-earth-modified high-strength steel plate with stable properties in the thickness direction, and the steel plate thickness is 100mm. This embodiment is used to illustrate that under the conditions of upper thickness limit and high alloy content, the present invention can still ensure that the 1 / 2 thickness position obtains a microstructure dominated by tempered martensite, and achieve a small difference in strength and toughness between the surface position and the 1 / 2 thickness position.

[0050] The chemical composition of the steel plate in this embodiment, by mass percentage, is as follows: C: 0.18%, Si: 0.45%, Mn: 1.40%, Cr: 0.80%, Ni: 0.40%, Mo: 0.30%, Nb: 0.050%, V: 0.050%, Al: 0.080%, Ti: 0.030%, B: 0.0030%, Ce: 0.0060%, N: 0.0080%, P: 0.012%, S: 0.0030%, O: 0.0040%, with the balance being Fe and unavoidable impurities. Among these, Ce / (O+S) = 0.86, Ti / N = 3.75, and (Cr+Mo) / Mn = 0.79, all within the scope defined by this invention. The steel plate in this embodiment is prepared according to the following steps: S1, smelting and refining the molten steel, controlling S≤0.003% and O≤0.004% in the steel; S2, during the refining stage, before breaking the vacuum at RH, adding a Ce-containing rare earth alloy to the molten steel to make the Ce content in the finished steel 0.0060%, followed by soft argon blowing for 20 minutes to modify the irregular Al2O3, MnS, and Ti(C,N) inclusions in the steel into spherical rare earth composite inclusions or near-spherical rare earth composite inclusions; S3, continuously casting the molten steel into a continuous casting billet, heating the billet to 1200℃ and holding it for 2 hours, and then... The steel plate was then rolled into a 100mm thick steel plate, with a cumulative reduction rate of 52% in rough rolling and 32% in finish rolling. In step S4, the rolled steel plate was heated to 950℃ and held at 1.5 min / mm thickness for 150 min, then water-cooled to room temperature, resulting in a quenched microstructure dominated by tempered martensite at half the plate thickness. In step S5, the quenched steel plate was tempered at 640℃ and held at 3.0 min / mm thickness for 300 min, then air-cooled to room temperature, yielding a rare-earth modified high-strength steel plate with stable properties in the thickness direction.

[0051] Testing revealed that spherical or near-spherical inclusions accounted for 70% of all inclusions in the steel of this embodiment. In the rare-earth-modified high-strength steel plate with stable thickness-direction properties, 80% of all inclusions had a maximum size ≤5μm. The volume fraction of tempered martensite at the surface and half-thickness locations of the steel plate was 93% and 86%, respectively. Large-angle grain boundaries with an orientation difference angle ≥15° accounted for 37% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1035MPa and 965MPa, respectively; the tensile strengths were 1095MPa and 1030MPa, respectively; the elongation after fracture was 14.8% and 14.3%, respectively; and the impact energy at -40℃ was 94J and 84J, respectively. The difference in yield strength between the surface and half-thickness locations was 70MPa, and the difference in Charpy impact energy at -40℃ was 10J. The results show that even when the steel plate thickness reaches 100 mm, the present invention can still meet the requirements of the present invention by synergistic control of the composition ratio, inclusions and heat treatment, so that the microstructure at the 1 / 2 thickness position and the performance difference between the surface position and the 1 / 2 thickness position meet the requirements of the present invention.

[0052] Example 3 This embodiment provides a rare-earth modified high-strength steel plate with stable properties in the thickness direction, and the steel plate thickness is 80mm. This embodiment is a typical embodiment of the present invention, used to illustrate that when Ce / (O+S), Ti / N, and (Cr+Mo) / Mn are in a better matching range, better inclusion control, microstructure stability at the 1 / 2 thickness position, and stability of properties in the thickness direction can be obtained.

[0053] The chemical composition of the steel plate in this embodiment, by mass percentage, is as follows: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0.0040%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. Among these, Ce / (O+S) = 0.89, Ti / N = 4.00, and (Cr+Mo) / Mn = 0.70, all of which are within the scope defined by this invention. The steel plate in this embodiment is prepared according to the following steps: S1, smelting and refining the molten steel, controlling the S content in the steel to be ≤0.003% and O content to be ≤0.004%; S2, during the later stage of RH vacuum treatment in the refining stage (after deoxidation, desulfurization and composition adjustment are completed in RH vacuum refining, and before RH vacuum breaking), adding a Ce-containing rare earth alloy to the molten steel to make the Ce content in the finished steel 0.0040%, followed by soft argon blowing for 15 minutes to modify the irregular Al2O3, MnS and Ti(C,N) inclusions in the steel into spherical rare earth composite inclusions or near-spherical rare earth composite inclusions; S3, continuously casting the molten steel into a continuous casting billet, and then continuously casting the billet into a continuous casting billet. The billet was heated to 1190℃ and held for 2 hours, then rolled into a steel plate with a thickness of 80 mm. The cumulative reduction rate of rough rolling was 56%, and the cumulative reduction rate of finish rolling was 34%. In step S4, the rolled steel plate was heated to 920℃ and held for 120 minutes at a rate of 1.5 min / mm according to the thickness of the steel plate. Then it was water-cooled to room temperature so that the microstructure of the steel plate after quenching at 1 / 2 thickness position was mainly tempered martensite. In step S5, the quenched steel plate was tempered at 620℃ and held for 240 minutes at a rate of 3.0 min / mm according to the thickness of the steel plate. After tempering, it was air-cooled to room temperature to obtain a rare earth modified high-strength steel plate with stable performance in the thickness direction.

[0054] like Figure 1 and Figure 2As shown, the proportion of spherical or near-spherical inclusions in the steel of this embodiment is 82%, and the proportion of all inclusions with a maximum size ≤5μm in the rare earth modified high-strength steel plate with stable properties in the thickness direction is 88%. The volume fraction of tempered martensite at the surface and half-thickness positions of the steel plate is 98% and 91%, respectively, and the proportion of large-angle grain boundaries with an orientation difference angle ≥15° at the half-thickness position is 43%. The yield strengths at the surface and half-thickness positions of the steel plate are 1050MPa and 990MPa, respectively; the tensile strengths are 1115MPa and 1050MPa, respectively; the elongation after fracture is 15.5% and 14.8%, respectively; and the impact energy at -40℃ is 99J and 88J, respectively. The difference in yield strength between the surface and half-thickness positions is 60MPa, and the difference in Charpy impact energy at -40℃ is 11J. The results show that this embodiment achieves better inclusion control, core structure stability, and surface and half-thickness position performance stability through the synergistic control of Ce / (O+S), Ti / N, (Cr+Mo) / Mn with rolling, quenching, and tempering processes.

[0055] Comparative Example 1 This comparative example is based on Example 3, except that Ce was omitted. This comparative example illustrates that without the addition of rare earth element Ce, inclusions such as Al2O3, MnS, and Ti(C,N) in the steel are difficult to fully modify, resulting in decreased low-temperature toughness. The chemical composition of the steel plate in this comparative example, by mass percentage, is as follows: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. Specifically, Ce / (O+S) = 0, Ti / N = 4.00, and (Cr+Mo) / Mn = 0.70. Except for the omission of Ce addition, the other components and process parameters of this comparative example are the same as those of Example 3.

[0056] like Figure 3 and Figure 4As shown, testing revealed that spherical or near-spherical inclusions accounted for only 45% of the steel in this comparative example, while inclusions with a maximum size ≤5μm accounted for 63%. The volume fraction of tempered martensite at the surface and half-thickness locations of the steel plate was 95% and 88%, respectively, and large-angle grain boundaries with an orientation difference angle ≥15° accounted for 42% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1045MPa and 985MPa, respectively; the tensile strengths were 1105MPa and 1040MPa, respectively; the elongation after fracture was 15.0% and 14.5%, respectively; and the impact energy at -40℃ was 66J and 59J, respectively. The difference in yield strength between the surface and half-thickness locations was 60MPa, and the difference in Charpy impact energy at -40℃ was 7J. The results show that the matrix structure and strength do not change much when Ce is not added, but the inclusions are not sufficiently reduced in terms of hazard. The proportion of inclusions with a maximum size ≤5μm in the steel is less than 80%, and the impact energy at -40℃ at the surface and half thickness positions is less than 80J, which does not meet the requirements of this invention for inclusion control and low-temperature toughness.

[0057] Comparative Example 2 This comparative example is based on Example 3, but with a reduced Ce content, resulting in both Ce content and Ce / (O+S) below the limits defined in this invention. This comparative example illustrates that insufficient Ce makes it difficult to adequately modify oxide and sulfide inclusions in steel. The chemical composition of the steel plate in this comparative example, by mass percentage, is as follows: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0.0003%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. The components were: Ce / (O+S)=0.07, Ti / N=4.00, and (Cr+Mo) / Mn=0.70. Except for the reduced Ce content, the other components and process parameters of this comparative example were the same as in Example 3.

[0058] Testing revealed that spherical or near-spherical inclusions accounted for 55% of the total inclusions in this comparative steel, while inclusions with a maximum size ≤5μm accounted for 68%. The volume fraction of tempered martensite at the surface and half-thickness locations of the steel plate was 95% and 85%, respectively. Large-angle grain boundaries with an orientation difference angle ≥15° accounted for 42% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1040MPa and 980MPa, respectively; the tensile strengths were 1100MPa and 1035MPa, respectively; the elongation after fracture was 15.1% and 14.4%, respectively; and the impact energy at -40℃ was 74J and 59J, respectively. The difference in yield strength between the surface and half-thickness locations was 60MPa, and the difference in Charpy impact energy at -40℃ was 15J. The results show that when Ce / (O+S) is less than 0.10, Ce is not well matched with O and S, the inclusion modification is insufficient, the proportion of inclusions with a maximum size ≤5μm in the steel is less than 80%, and the impact energy at -40℃ at the surface and half thickness positions is lower than the requirements of this invention.

[0059] Comparative Example 3 This comparative example is based on Example 3, but with an increased Ce content, resulting in both Ce content and Ce / (O+S) exceeding the limits defined in this invention. This comparative example illustrates that excessive Ce can easily lead to the formation of coarse rare earth inclusions or inclusion agglomerations, which conversely reduces low-temperature toughness. The chemical composition of the steel plate in this comparative example, by mass percentage, is: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0.0080%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. The proportions of Ce / (O+S) were 1.78, Ti / N were 4.00, and (Cr+Mo) / Mn were 0.70. Except for the increased Ce content, the other components and process parameters of this comparative example were the same as in Example 3.

[0060] Testing revealed that spherical or near-spherical inclusions accounted for 62% of the total inclusions in this comparative steel, while inclusions with a maximum size ≤5μm accounted for 58%. The volume fraction of tempered martensite at the surface and half-thickness locations of the steel plate was 92% and 85%, respectively. Large-angle grain boundaries with an orientation difference angle ≥15° accounted for 42% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1035MPa and 972MPa, respectively; the tensile strengths were 1095MPa and 1030MPa, respectively; the elongation after fracture was 14.6% and 14.0%, respectively; and the impact energy at -40℃ was 70J and 63J, respectively. The difference in yield strength between the surface and half-thickness locations was 63MPa, and the difference in Charpy impact energy at -40℃ was 7J. The results show that when Ce / (O+S) is higher than 1.50, the number of coarse rare earth inclusions or inclusion agglomeration increases, the proportion of inclusions with a maximum size ≤5μm decreases, and the impact energy at -40℃ at the surface and half thickness positions is lower than 80J, which cannot meet the requirements of this invention for low-hazard inclusions and low-temperature toughness.

[0061] Comparative Example 4 This comparative example is based on Example 3, but with a reduced Ti content, resulting in a Ti / N ratio below the range defined in this invention. This comparative example illustrates that when the Ti / N ratio is too low, Ti's ability to solidify N is insufficient, and the effectiveness of B element is reduced, leading to insufficient hardenability at the 1 / 2 thickness position of the thick steel plate. The chemical composition of the steel plate in this comparative example, by mass percentage, is: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.010%, B: 0.0015%, Ce: 0.0040%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. The values ​​of Ce / (O+S) were 0.89, Ti / N were 2.00, and (Cr+Mo) / Mn were 0.70. Except for the reduced Ti content, the other components and process parameters of this comparative example were the same as in Example 3.

[0062] Testing revealed that 80% of the comparative steel contained spherical or near-spherical inclusions, and 86% contained inclusions with a maximum size ≤5μm. The inclusion control level was similar to that of Example 3. However, due to the low Ti / N ratio, insufficient Ti to N, and reduced B effectiveness, the volume fraction of tempered martensite at the 1 / 2 thickness position decreased significantly. The volume fraction of tempered martensite at the surface and 1 / 2 thickness positions was 88% and 74%, respectively, and the proportion of large-angle grain boundaries with an orientation difference angle ≥15° at the 1 / 2 thickness position was 34%. The yield strengths at the surface and 1 / 2 thickness positions were 1010MPa and 920MPa, respectively; the tensile strengths were 1065MPa and 985MPa, respectively; the elongation after fracture was 14.8% and 13.8%, respectively; and the impact energy at -40℃ was 82J and 62J, respectively. The yield strength difference between the surface layer and the 1 / 2 thickness position is 90 MPa, and the Charpy impact energy difference at -40℃ is 20 J. The results indicate that when the Ti / N ratio is below 2.5, even with good inclusion control, insufficient boron availability and decreased core hardenability will result in a tempered martensite volume fraction below 80% at the 1 / 2 thickness position. Consequently, the strength, plasticity, and low-temperature impact toughness at the 1 / 2 thickness position are all below the requirements of this invention, and the yield strength difference between the surface layer and the 1 / 2 thickness position exceeds 70 MPa.

[0063] Comparative Example 5 This comparative example is based on Example 3, but with an increased Ti content, resulting in a Ti / N ratio higher than the range defined in this invention. This comparative example illustrates that an excessively high Ti / N ratio easily leads to the formation of coarse TiN or Ti(C,N) inclusions, increasing the risk of low-temperature impact cracking. The chemical composition of the steel plate in this comparative example, by mass percentage, is: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.030%, B: 0.0015%, Ce: 0.0040%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. The proportions of Ce / (O+S) were 0.89, Ti / N were 6.00, and (Cr+Mo) / Mn were 0.70. Except for the increased Ti content, the other components and process parameters of this comparative example were the same as in Example 3.

[0064] Testing revealed that spherical or near-spherical inclusions accounted for 58% of the total inclusions in this comparative steel, while inclusions with a maximum size ≤5μm accounted for 60%. This indicates that when Ti is relatively excessive, the number of coarse TiN or Ti(C,N) inclusions increases, and the size control of inclusions deteriorates. The volume fraction of tempered martensite at the surface and half-thickness locations of the steel plate was 91% and 84%, respectively. Large-angle grain boundaries with an orientation difference angle ≥15° accounted for 40% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1045MPa and 986MPa, respectively; the tensile strengths were 1105MPa and 1040MPa, respectively; the elongation after fracture was 14.7% and 14.1%, respectively; and the impact energy at -40℃ was 74J and 58J, respectively. The difference in yield strength between the surface and half-thickness locations was 59MPa, and the difference in Charpy impact energy at -40℃ was 16J. The results show that when Ti / N is higher than 4.5, coarse TiN or Ti(C,N) inclusions are prone to become low-temperature impact crack initiation sites. The proportion of inclusions with a maximum size ≤5μm in the steel is less than 80%, and the impact energy at -40℃ at the surface and half-thickness positions is lower than the requirements of this invention.

[0065] Comparative Example 6 This comparative example is based on Example 3, but with a reduction in Cr and Mo content, so that (Cr+Mo) / Mn is below the range defined in this invention. This comparative example illustrates that even if the individual contents of Mn, Cr, and Mo meet the range of this invention, if (Cr+Mo) / Mn is too low, the contribution of Cr and Mo to the hardenability and tempering stability of the core of the thick plate is still insufficient. The chemical composition of the steel plate in this comparative example, by mass percentage, is: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.40%, Ni: 0.25%, Mo: 0.10%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0.0040%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. The proportions of Ce / (O+S) were 0.89, Ti / N were 4.00, and (Cr+Mo) / Mn were 0.43. Except for the reduced Cr and Mo content, the remaining components and process parameters of this comparative example were the same as in Example 3.

[0066] Testing revealed that 80% of the inclusions in this comparative steel were spherical or near-spherical, and 86% were inclusions with a maximum size ≤5μm, indicating good inclusion control. However, due to the (Cr+Mo) / Mn ratio being below 0.45, Cr and Mo contributed insufficiently to the hardenability and tempering stability of the thick plate core, resulting in a reduced volume fraction of tempered martensite at the 1 / 2 thickness location. The volume fractions of tempered martensite at the surface and 1 / 2 thickness locations were 87% and 75%, respectively, with large-angle grain boundaries (≥15° orientation difference) accounting for 30% at the 1 / 2 thickness location. The yield strengths at the surface and 1 / 2 thickness locations were 1015MPa and 905MPa, respectively; the tensile strengths were 1070MPa and 970MPa, respectively; the elongation after fracture was 15.0% and 13.9%, respectively; and the impact energy at -40℃ was 86J and 66J, respectively. The yield strength difference between the surface layer and the half-thickness layer is 110 MPa, and the Charpy impact energy difference at -40℃ is 20 J. The results indicate that when (Cr+Mo) / Mn is below 0.45, the hardenability at the half-thickness layer of the steel plate is insufficient, and the volume fraction of tempered martensite is below 80%, leading to a decrease in strength, plasticity, and low-temperature toughness at the half-thickness layer, and causing the yield strength difference between the surface layer and the half-thickness layer to exceed 70 MPa.

[0067] Comparative Example 7 This comparative example is based on Example 3, but with an increased Cr and Mo content, resulting in (Cr+Mo) / Mn exceeding the range defined in this invention. This comparative example illustrates that even if the individual contents of Mn, Cr, and Mo meet the range of this invention, an excessively high (Cr+Mo) / Mn ratio can easily increase the risk of carbon equivalent, carbide segregation, or decreased toughness due to relatively high Cr and Mo content. The chemical composition of the steel plate in this comparative example, by mass percentage, is: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.80%, Ni: 0.25%, Mo: 0.30%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0.0040%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. The proportions of Ce / (O+S) were 0.89, Ti / N were 4.00, and (Cr+Mo) / Mn were 0.96. Except for the increased Cr and Mo content, the remaining components and process parameters of this comparative example were the same as in Example 3.

[0068] Testing revealed that 78% of the steel in this comparative example contained spherical or near-spherical inclusions, and 84% contained inclusions with a maximum size ≤5μm. The volume fraction of tempered martensite at the surface and half-thickness locations was 93% and 86%, respectively, while large-angle grain boundaries with an orientation difference angle ≥15° accounted for 42% at the half-thickness location. The yield strengths at the surface and half-thickness locations were 1060MPa and 1002MPa, respectively; the tensile strengths were 1120MPa and 1065MPa, respectively; the elongation after fracture was 14.2% and 13.6%, respectively; and the impact energy at -40℃ was 78J and 65J, respectively. The difference in yield strength between the surface and half-thickness locations was 58MPa, and the difference in Charpy impact energy at -40℃ was 13J. The results show that when (Cr+Mo) / Mn is higher than 0.95, the strength and hardenability of the steel plate can be maintained at a high level, but the plasticity and low-temperature impact energy decrease. Specifically, the elongation after fracture at the 1 / 2 thickness position is less than 14%, and the impact energy at -40℃ at the surface position and the 1 / 2 thickness position is less than 80J, which cannot meet the requirements of this invention for comprehensive strength and toughness.

[0069] Comparative Example 8 This comparative example is based on Example 3, with the same chemical composition. The only difference is that the cumulative reduction rate during rough rolling is reduced to 40%, while the remaining process parameters are the same as in Example 3. This comparative example illustrates that in steel plates with thicknesses of 60mm to 100mm, simply meeting the composition ratio design is insufficient to guarantee thickness-direction performance stability; a sufficient rough rolling reduction rate is also needed to improve core deformation penetration and microstructure uniformity.

[0070] The chemical composition of the comparative example steel plate, by mass percentage, is as follows: C: 0.16%, Si: 0.35%, Mn: 1.15%, Cr: 0.60%, Ni: 0.25%, Mo: 0.20%, Nb: 0.030%, V: 0.030%, Al: 0.050%, Ti: 0.020%, B: 0.0015%, Ce: 0.0040%, N: 0.0050%, P: 0.010%, S: 0.0020%, O: 0.0025%, with the balance being Fe and unavoidable impurities. Among these, Ce / (O+S) = 0.89, Ti / N = 4.00, and (Cr+Mo) / Mn = 0.70, all of which are within the scope defined in this invention. This comparative example was prepared according to the following steps: S1, smelting and refining molten steel, controlling S ≤ 0.003% and O ≤ 0.004% in the steel; S2, during the later stage of RH vacuum treatment in the refining stage, adding a Ce-containing rare earth alloy to the molten steel to make the Ce content in the finished steel 0.0040%, followed by soft argon blowing for 15 min; S3, continuously casting the molten steel into a continuous casting billet, heating the billet to 1190℃ and holding it for 2 h, and then rolling it into a steel plate with a thickness of 80 mm, wherein the cumulative reduction rate of rough rolling is 40% and the cumulative reduction rate of finish rolling is 34%; S4, heating the rolled steel plate to 920℃, holding it at 1.5 min / mm according to the steel plate thickness for 120 min, and then water cooling to room temperature; S5, tempering the quenched steel plate at 620℃, holding it at 3.0 min / mm according to the steel plate thickness for 240 min, and then air cooling to room temperature after tempering.

[0071] Testing revealed that 80% of the spherical or near-spherical inclusions in this comparative steel were present, and 86% of the inclusions had a maximum size ≤5μm. The inclusion control level was similar to that of Example 3. However, due to the reduced cumulative reduction in rough rolling to 40%, the core deformation penetration was insufficient, leading to decreased uniformity of the microstructure in the thickness direction. The tempered martensite volume fractions at the surface and half-thickness positions of the steel plate were 91% and 82%, respectively, and the proportion of large-angle grain boundaries with an orientation difference angle ≥15° at the half-thickness position was 31%. The yield strengths at the surface and half-thickness positions were 1040MPa and 963MPa, respectively; the tensile strengths were 1100MPa and 1022MPa, respectively; the elongation after fracture were 15.0% and 14.0%, respectively; and the impact energy at -40℃ were 88J and 64J, respectively. The yield strength difference between the surface and half-thickness positions was 77MPa, and the Charpy impact energy difference at -40℃ was 24J. The results show that when the cumulative reduction rate of rough rolling is insufficient, even if the composition ratio and heat treatment regime meet the requirements, the proportion of large-angle grain boundaries at the 1 / 2 thickness position will decrease and the low-temperature impact energy will decrease due to insufficient core deformation penetration. Furthermore, the yield strength difference between the surface position and the 1 / 2 thickness position and the Charpy impact energy difference at -40℃ will exceed the requirements of this invention.

Claims

1. A rare-earth modified high-strength steel plate with stable properties in the thickness direction, characterized in that, The chemical composition, by mass percentage, includes the following: C: 0.14%–0.18%, Si: 0.25%–0.45%, Mn: 0.90%–1.40%, Cr: 0.40%–0.80%, Ni: 0.10%–0.40%, Mo: 0.10%–0.30%, Nb: 0.010%–0.050%, V: 0.010%–0.050%, Al: 0.020%–0.080%, Ti: 0.010%–0.030%, B: 0.0005%–0.0030%, Ce: 0.002%–0.006%, N: 0.002%–0.008%, P≤0.015%, S≤0.003%, O≤0.004%, with the balance being Fe and unavoidable impurities; Among them, Ce, O, S, Ti, N, Cr, Mo, and Mn are all mass percentage contents, and satisfy the following conditions: Ce / (O+S)=0.10~1.50, Ti / N=2.5~4.5, (Cr+Mo) / Mn=0.45~0.95; The thickness of the rare earth modified high-strength steel plate with stable performance in the thickness direction is 60mm to 100mm. The matrix structure at the 1 / 2 thickness position is mainly tempered martensite, and the volume fraction of tempered martensite at the 1 / 2 thickness position is ≥80%, with the remainder being tempered bainite. The rare earth-modified high-strength steel plate with stable properties in the thickness direction contains rare earth composite inclusions, which are spherical or near-spherical; the proportion of all inclusions with a maximum size ≤5μm in the rare earth-modified high-strength steel plate with stable properties in the thickness direction is ≥80%; The yield strength difference between the surface and half-thickness positions of the rare earth modified high-strength steel plate with stable performance in the thickness direction is ≤70MPa, and the Charpy impact energy difference between the surface and half-thickness positions at -40℃ is ≤20J.

2. The rare-earth modified high-strength steel plate with stable properties in the thickness direction according to claim 1, characterized in that, The rare earth composite inclusions are one or more of Ce oxides, Ce sulfides, Ce oxysulfides, and Ce-Al-OS composite inclusions; the aspect ratio of the two-dimensional projection of the rare earth composite inclusions on the metallographic or scanning electron microscope observation section is ≤2.

0.

3. The rare-earth modified high-strength steel plate with stable properties in the thickness direction according to claim 1, characterized in that, The proportion of large-angle grain boundaries with an orientation difference angle ≥15° at the 1 / 2 thickness position of the rare earth modified high-strength steel plate with stable performance in the thickness direction is ≥35%.

4. The rare-earth-modified high-strength steel plate with stable properties in the thickness direction according to claim 1, characterized in that, The yield strength of the rare earth modified high-strength steel plate with stable performance in the thickness direction is ≥950MPa at both the surface and half-thickness positions, the tensile strength is ≥1000MPa, the elongation after fracture is ≥14%, and the impact energy at -40℃ is ≥80J.

5. A method for preparing a rare-earth modified high-strength steel plate with stable properties in the thickness direction as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Step S1: Smelt and RH vacuum refine the molten steel, controlling the S content in the refined molten steel to be ≤0.003% and O content to be ≤0.004%; Step S2: Add Ce-containing rare earth alloy to the refined molten steel, control the Ce mass percentage content to be 0.002% to 0.006%, and Ce / (O+S) = 0.10 to 1.50, and transform irregular Al2O3 inclusions, MnS inclusions and Ti(C,N) inclusions into spherical rare earth composite inclusions or near-spherical rare earth composite inclusions by soft blowing argon. Step S3: The molten steel from step S2 is continuously cast into a continuous casting billet; the continuous casting billet is heated to 1180℃~1200℃ and held for 2 hours, and then rolled into a steel plate with a thickness of 60mm~100mm. Step S4: Heat the rolled steel plate to 900℃~950℃, hold at 1.5min / mm according to the thickness of the steel plate, and water cool to room temperature so that the volume fraction of tempered martensite in the microstructure after quenching at 1 / 2 thickness of the steel plate is ≥80%; Step S5: Temper the quenched steel plate at 600℃~640℃, hold at 3.0min / mm according to the thickness of the steel plate, and air cool to room temperature after tempering to obtain rare earth modified high-strength steel plate with stable performance in the thickness direction.

6. The preparation method according to claim 5, characterized in that, In step S3, the cumulative reduction rate of rough rolling is ≥50%, and the cumulative reduction rate of finish rolling is ≥30%.

7. The preparation method according to claim 5, characterized in that, In step S2, the Ce-containing rare earth alloy is added after deoxidation, desulfurization and composition adjustment are completed in RH vacuum refining, and before the RH vacuum is broken. After addition, it is soft-blown with argon for 5 min to 20 min.

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