Heterogeneous layered ultrafine-grained steel with excellent strength and toughness and preparation method thereof

Abstract

The present invention discloses a non-uniform layered ultrafine-grained steel with excellent strength and toughness and a preparation method thereof, and belongs to the technical field of steel material preparation and processing. The present invention utilizes the inherent microscopic Mn segregation phenomenon in the continuous casting billet, and prepares a novel two-phase ultrafine-grained microstructure with non-uniform layered distribution characteristics by designing the composition and the heat treatment route. Compared with the traditional uniform structure, the present invention achieves a synergistic improvement in strength and plasticity by rationally constructing a non-uniform layered organization, while also improving the fracture toughness, significantly optimizing the comprehensive service performance, and effectively meeting the urgent demand of modern industry for high-performance, cost-effective steel materials. In addition, the present invention utilizes the preparation process of conventional rolling and heat treatment equipment, which is also conducive to the large-scale promotion and application of the process, and has good industrial application value and market potential.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation and processing of steel materials, and in particular to a non-uniform layered ultrafine-grained steel with excellent strength and toughness and a preparation method thereof. Background Art

[0002] In modern industry, the demand for high-performance structural materials is growing, particularly in sectors such as marine engineering, aerospace, and automotive manufacturing, which place extremely high demands on material strength and ductility. Low-carbon microalloyed steels are widely used due to their excellent processability and affordability, but their increased strength often comes at the expense of ductility and toughness. Therefore, the production of low-carbon microalloyed steels with superior overall performance through process optimization and microstructure control has been a research hotspot in the industry.

[0003] Existing theoretical and experimental research indicates that grain refinement is a strengthening and toughening mechanism that can simultaneously improve the strength and toughness of steel materials. For example, Patent Application No. 202010240014.6 discloses a method for improving the plasticity of pipeline steel by achieving an acicular ferrite ultrafine-grained structure with a bimodal size distribution through controlled rolling and controlled cooling; Patent Application No. 202110314137.4 provides a method for producing ultrafine-grained low-alloy steel plates with excellent ultra-low temperature toughness, characterized by a dual gradient of crystal texture and grain size, using an intermediate billet ultra-rapid cooling process; and Patent Application No. 202210829899.2 utilizes a thermal cycle and deformation-induced ferrite mechanism to achieve the production of ultrafine-grained ferrite steel plates with a good balance of strength and ductility under low reduction ratio conditions.

[0004] However, some studies have pointed out that layered ultrafine-grained materials may have greater potential in plasticity, work hardening ability and structural stability than single ultrafine-grained materials due to their more widely distributed heterogeneous interfaces and unique grain shape. However, compared with the single non-ferrous metal materials commonly used to regulate layered ultrafine-grained structures, including Ti, Cu, etc., the more complex phase transformation mechanism and chemical composition in low-carbon microalloyed steels increase the difficulty of regulating layered ultrafine-grained structures. How to use existing technologies to prepare layered ultrafine-grained steels with excellent strength and toughness still faces challenges. Summary of the Invention

[0005] In view of this, the present invention provides a non-uniform layered ultrafine-grained steel with excellent strength and toughness and a preparation method thereof, which is mainly intended to solve the "inverted" contradiction between strength and plasticity and toughness of current low-carbon microalloyed steels during service. At the same time, the present invention utilizes the process flow of conventional rolling and heat treatment equipment to solve the current problems of complicated ultrafine-grained steel preparation process and high equipment requirements.

[0006] To achieve the above object, the technical solution provided by the present invention is as follows:

[0007] Disclosed is a non-uniform layered ultrafine-grained steel with excellent strength and toughness. The chemical composition, in percentage by mass, comprises: C: 0.04-0.10%, Si: 0.10-0.30%, Mn: 1.00-2.20%, P: ≤0.008%, S: ≤0.004%, Ni: 0.20-0.60%, Cu: 0.40-0.70%, Nb: 0.015-0.045%, Ti: 0.005-0.030%, and the remainder is Fe and unavoidable impurities. The RD-ND surface of the ultrafine-grained steel exhibits a non-uniform alternating distribution of ultrafine-grained layers and coarse-grained layers in a macroscopic view throughout the entire thickness.

[0008] The ultrafine-grained steel of the present invention is produced by selecting specific compositional contents. The resulting ultrafine-grained steel exhibits a non-uniform alternation of ultrafine and coarse-grained layers on a millimeter-scale longitudinal cross-section. This microstructural specificity enables a superior balance of strength and toughness.

[0009] The reasons for designing the chemical composition of the ultrafine grain steel of the present invention are as follows:

[0010] The C element can improve strength through solid solution strengthening, precipitation strengthening, etc. When its content is too low, the strength level is difficult to ensure, while when its content is too high, it affects the toughness and welding performance. Therefore, under the condition of ensuring strength, in order to expand the scope of application, the present invention reduces the C content as much as possible and controls it within the range of 0.04-0.10%.

[0011] In addition to ensuring strength, Si is also an important deoxidizing element during smelting. To ensure this effect, Si should be added at a level of 0.10% or higher. However, excessive Si content can negatively impact the surface quality and toughness of the steel sheet. Therefore, in the present invention, the Si content is controlled within the range of 0.10-0.30% to ensure good toughness in the finished product.

[0012] Mn is an effective element for improving strength under low-carbon conditions. At the same time, it can reduce the austenite transformation temperature and refine the grain size. However, if its content is too high, it will easily cause severe segregation of the continuous casting billet and damage the low-temperature toughness. Therefore, the Mn content of low-carbon microalloyed steel is usually controlled below 2.5%. For example, in patent application No. 202210829899.2, the Mn content is limited to 1.20-1.60%. On the one hand, severe segregation is avoided. On the other hand, ultrafine ferrite and dispersed fine carbides are obtained through the fluctuation of the Mn element composition. In contrast, the present invention changes its thinking and focuses on the inherent Mn segregation phenomenon in the continuous casting billet. By utilizing its phase transformation characteristics, a novel dual-phase ultrafine grain microstructure with non-uniform layered distribution characteristics is prepared by designing a hot working route. Therefore, considering comprehensively, in order to ensure that there is an appropriate amount of Mn segregation in the continuous casting billet, the Mn content of the present invention should be controlled within the range of 1.00-2.20%.

[0013] On the basis of the above-mentioned components, through thermal and dynamic simulation tests, and taking full account of the economy of the alloying elements and their influence on the comprehensive performance, the present invention is designed to add four elements in a composite manner, namely: Ni: 0.20-0.60%, Cu: 0.40-0.70%, Nb: 0.015-0.045%, and Ti: 0.005-0.030%.

[0014] P and S are harmful elements. From the perspective of clean steel, the lower the better. Considering comprehensive economic efficiency, it is best to control the P content below 0.008% and the S content below 0.004%.

[0015] In some embodiments, considering the composite precipitation effect of Nb and Ti elements and the production cost, the Nb+Ti element content is controlled to be ≤0.045%; and since Cu and Ni are also stable supercooled austenite elements, they can affect the phase transformation characteristics of Mn segregation in the continuous casting billet, and the study also found that: the Mn+Ni+Cu content in ultrafine-grained steel has an important influence on the stability of the process and the qualified rate of the finished product. Under the same process flow conditions, further controlling the Mn+Ni+Cu content in ultrafine-grained steel to be ≤2.50% can ensure that it obtains more excellent and stable strength and toughness properties.

[0016] In some embodiments, there is chemical composition inhomogeneity between the ultrafine-grained layer and the coarse-grained layer, wherein the ultrafine-grained layer contains a higher content of Mn element than the coarse-grained layer.

[0017] In some embodiments, the volume fraction of the ultrafine crystal layer is 25-45%, the width of a single ultrafine crystal layer is 10-300 μm, and the average width is greater than 50 μm.

[0018] In some embodiments, the average grain size of the ultrafine-grained layer is 0.50-0.80 μm, and the grain size of the coarse-grained layer is less than 5.0 μm.

[0019] In some embodiments, the coarse-grained layer has a bimodal structural characteristic, wherein: 5-25% of the grains have a size smaller than 1.0 μm, and 25-45% of the grains have a size larger than 3.0 μm.

[0020] In some embodiments, the RD-ND surface has a gradient crystal texture in the through-thickness direction, wherein: <110> / / The proportion of ND texture gradually decreases from 45-65% in the upper surface to 15-30% in the core; <111> / / ND gradually increases from 5-15% on the upper surface to 25-45% in the core.

[0021] In some embodiments, the volume fraction of martensite in the ultrafine grain layer is 20-35%, and the martensite is mainly (112) <110> The twin martensite has a twin thickness range of 4 to 15 nm.

[0022] In some embodiments, the yield strength of the ultrafine-grained steel is 500-700 MPa, the tensile strength is 800-1000 MPa, and the uniform elongation is 18-24%.

[0023] In some embodiments, the fracture toughness K of the ultrafine grain steel is Q 200~300MPa·m 1 / 2 , crack growth toughness K SS 250~350MPa·m 1 / 2 .

[0024] The method for preparing the above-mentioned non-uniform layered ultrafine-grained steel with excellent strength and toughness includes the following process flow:

[0025] S1 hot metal pretreatment → S2 converter smelting → S3 LF refining → S4 RH vacuum treatment → S5 continuous casting → S6 heating → S7 rough rolling → S8 intermediate cooling → S9 finishing rolling → S10 final cooling;

[0026] The heating time of S6 is less than 3 hours;

[0027] The S7 rough rolling is performed 3 to 5 times with a reduction of 70 to 90%.

[0028] The intermediate cooling step S8 cools the steel billet to room temperature at a cooling rate of 80-150K / s;

[0029] The temperature of the S9 finishing rolling is in the austenite-ferrite two-phase region, the holding time is 1 to 3 hours, the rolling passes are 2 to 4 times, and the reduction is 60 to 80%;

[0030] The steel billet after the S10 final cooling is cooled to room temperature at a cooling rate of 50 to 70 K / s.

[0031] In some embodiments, the temperature of the S6 heating is 1323-1423K; the temperature of the S7 rough rolling is 1253-1323K; the temperature of the S9 finishing rolling is 993-1053K, and the final rolling temperature is 953-1013K.

[0032] In some embodiments, the S9 finishing rolling is asynchronous rolling, and the upper roller speed is 0.5-2 m / s, and the lower roller speed is 0.55-4 m / s during rolling.

[0033] In some embodiments, the ratio of the lower roller speed to the upper roller speed is 1.1 to 2.0.

[0034] In some embodiments, the cumulative reduction of the S7 rough rolling and the S9 finishing rolling is ≥90%.

[0035] In some embodiments, the thickness ratio of the steel plate after S7 rough rolling to the finished product is 3.5 to 5.0.

[0036] In some embodiments, the reduction amount in each pass during the S7 rough rolling and the S9 finishing rolling is the same.

[0037] According to the above technical solution, the present invention utilizes the inherent microscopic Mn segregation phenomenon in the continuous casting billet, and prepares a novel dual-phase ultrafine-grained microstructure with non-uniform layered distribution characteristics by designing a hot processing route. Compared with traditional steels with uniform microstructures, the present invention achieves a synergistic improvement in material strength and plasticity by rationally constructing a non-uniform layered organization in the material, while optimizing the fracture toughness and significantly improving the comprehensive service performance of the material, enabling it to maintain stable performance in a variety of complex environments, and meeting the urgent needs of modern industry for high-performance, cost-effective steel materials. In addition, the ultrafine-grained steel of the present invention uses conventional rolling and heat treatment equipment in the preparation process, which reduces dependence on special equipment, is conducive to the large-scale promotion and application of the process, and demonstrates important industrial application value and significant market potential.

[0038] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are specifically listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0040] Figure 1 is a process flow chart of some embodiments of the present invention;

[0041] Figure 2 is a schematic diagram of a non-uniform layered structure according to some embodiments of the present invention;

[0042] Figure 3 1 is a microscopic diagram of some embodiments of the present invention. DETAILED DESCRIPTION

[0043] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, but those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. If specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products or detection services that can be purchased commercially.

[0044] The billet size obtained after the process flow S1 molten iron pretreatment → S2 converter smelting → S3 LF refining → S4 RH vacuum treatment → S5 continuous casting is 120×80×50mm 3 (length × width × thickness), the specific composition is shown in Table 1. The microstructure and properties of the steel billet will be regulated through the process flow of S6 heating → S7 rough rolling → S8 intermediate cooling → S9 finishing rolling → S10 final cooling.

[0045] For comparison, coarse-grained steel of Comparative Example 1 and ultrafine-grained steel of Comparative Example 2 with the same composition were prepared simultaneously. The steel billets of the two comparative examples were subjected to a long homogenization treatment at 1373K / 24h before rolling to minimize the segregation bands in the steel billets. However, in order to ensure a certain Mn segregation band content in the present invention, the heating time in process S6 should be less than 3h, as shown in FIG. Figure 1 As shown, the heating time in the embodiment is 2 hours to achieve uniform austenitization.

[0046] In the present invention, a large reduction technology is adopted in the S7 rough rolling stage, that is: through 3 to 5 rolling passes, the cumulative reduction is controlled to 70 to 90% to crush the austenite grains and prepare the structure for the subsequent acquisition of fine grain size; therefore, the rough rolling temperature is slightly higher and is controlled at 1253 to 1323K. Controlled cooling, as an important component of TMCP technology, is a key factor affecting the evolution of the microstructure of the steel billet after rolling. The cooling rate of the S8 intermediate cooling after rough rolling of the present invention is 80 to 150K / s. Under this cooling rate condition, due to the low C content and alloy element content in the steel plate, the hardenability is low, and after cooling to room temperature, a bainite-based matrix structure is formed; and because the steel billet has not undergone a long-term annealing treatment before hot rolling, it can be observed that there is an obvious Mn segregation band distributed along the rolling direction in the bainite matrix.

[0047] It was found that higher Mn content in the local microstructure of the steel lowers the phase transition point from ferrite to austenite and preferentially transforms to austenite during heating. Therefore, in the present invention, during S9 finish rolling, when the steel plate is heated for the second time to the austenite-ferrite two-phase region, local Mn-rich regions such as the Mn segregation band and grain boundaries will preferentially transform into austenite while retaining element segregation. Furthermore, after a holding time of 1 to 3 hours, austenite recrystallization will be suppressed in this region during rolling at a relatively low temperature, forming a grain morphology elongated along the rolling direction. At the same time, combined with a large reduction of 60 to 80% after 2 to 4 rolling passes and a cooling rate of 50 to 70 K / s in S10 final cooling, a mixed ultrafine-grained structure of martensite and ferrite cells is finally obtained; the micron-sized near-equiaxed ferrite in the coarse-grained layer originates from the recrystallization of the untransformed bainite matrix during the relatively low temperature warm rolling process, ultimately forming a unique bimodal structure of the coarse-grained layer, i.e., some fine ultrafine grains are distributed at the intersection of grain boundaries on the coarse-grained matrix.

[0048] In addition, the study found that texture gradient has an important influence on HDI strengthening. Unlike conventional rolling, the asynchronous rolling process causes the surface of the steel plate to bear greater shear strain, and the reasonable setting of the upper and lower roller speeds is conducive to forming an ideal texture gradient structure in the steel plate, thereby improving the strength of the steel plate. Therefore, in the present invention, the S9 finishing process adopts asynchronous rolling, preferably controlling the upper roller speed to 0.5-2m / s, the lower roller speed to 0.55-4m / s, and the ratio of the lower roller speed to the upper roller speed to 1.1-2.0, so as to obtain a gradient change in the crystal texture of the steel plate.

[0049] Based on the above-mentioned composition and process design, the present invention rationally constructs a non-uniform layered ultrafine-grained structure, which, on the one hand, can synergize the deformation mechanism to generate HDI stress, thereby obtaining an additional strengthening effect and ultimately improving the strength-plasticity matching of the steel plate; on the other hand, the layered structure of alternating coarse-grained / ultrafine-grained layers and the elongated grain morphology in the ultrafine-grained layers can effectively hinder crack propagation and void connection, thereby improving the energy dissipation rate, while the micron-sized ferrite in the coarse-grained layer has a higher plastic deformation tolerance and can form large-sized deep dimples, which consume a large amount of energy when the crack propagates. Ultimately, the above mechanisms work together to effectively improve the fracture toughness of the steel plate.

[0050] Reference Figure 1 The controlled rolling and controlled cooling process of the test steel in the embodiment of the present invention is specifically as follows:

[0051] The rough rolling temperature of S7 is about 1273K. After 4 rolling passes, the thickness of the steel billet is reduced to 10mm, with each pass reducing the thickness by 10mm and the cumulative reduction reaching 80%. Then, the cooling rate is controlled at about 100K / s to water cool the steel plate to room temperature, so that the steel plate obtains fine bainite structure under the conditions of large reduction and high cooling rate.

[0052] Furthermore, during the S9 finishing process, the steel plate is reheated to 1023K (austenite-ferrite two-phase region) and held at this temperature for 1.5 hours before being warmed on an asynchronous rolling mill. The top and bottom roll speeds during rolling are 1 m / s and 1.5 m / s, respectively. Three passes are performed, resulting in a final thickness of approximately 2.5 mm. Each pass achieves a 2.5 mm reduction, bringing the cumulative reduction for both roughing and finishing to 95%. To ensure a fine grain size and taking into account the mill load, the final rolling temperature is controlled at approximately 983K. The steel plate is then water-cooled to room temperature at a cooling rate of approximately 60K / s. It is important to note that the cooling rate of final cooling, as the final production step, significantly impacts the service performance of the steel plate. In the present invention, if the cooling rate is lower than 50K / s, on the one hand, it will affect the martensitic transformation of the final ultrafine-grained layer, and on the other hand, it will not be conducive to obtaining a fine grain size in the steel plate. When the cooling rate is higher than 70K / s, the service performance of the steel plate will be deteriorated due to the large quenching stress inside. Therefore, the cooling rate of the S10 final cooling should be controlled in the range of 50-70K / s.

[0053] Under the composition design system of the present invention, the overall grain size of the ultrafine-grained steel prepared by process optimization is relatively small. The non-uniform layered structure characteristics of the RD-ND plane in the full thickness macroscopic view of the experimental steel of the present invention are as follows: Figure 2 As shown in the figure, within the 2.5mm range of the longitudinal section, the ultrafine grain layer is unevenly distributed on it. Due to the different density of grain boundaries, the ultrafine grain layer appears "convex" in the SEM image after corrosion, as shown in the figure. Figure 3 As shown in (a), the ultrafine-grained layer is a mixed ultrafine-grained structure of martensite and ferrite cells; while the coarse-grained layer is mainly a micron-sized nearly equiaxed ferrite structure. The short arrows in the figure indicate that there are some tiny ultrafine grains distributed at the intersection of the grain boundaries on the coarse-grained matrix. Figure 3(b) Grain boundary map. Statistics show that the volume fraction of the ultrafine-grained layer is about 36%, and the average grain size is only 0.6μm, while the grain size of the coarse-grained layer is also less than 5.0μm. The coarse-grained layer also has a bimodal structure: some small ultrafine grains are distributed at the intersection of the grain boundaries on the coarse-grained matrix, of which ~10% of the grain size is less than 1.0μm, and ~34% of the grain size is greater than 3.0μm. In addition, the EBSD experimental results show that the crystal texture of the RD-ND plane in the full thickness direction of the experimental steel in the embodiment also has a gradient change: <110> / / The proportion of ND texture gradually decreases from 58% in the upper surface to 25% in the core; <111> / / ND gradually increases from 6% on the upper surface to 33% in the core; further TEM experimental results show that the ultrafine grain layer in the experimental steel of the embodiment contains a high density of dislocations, and the long ultrafine grains are mainly composed of nano / ultrafine martensite and ferrite cells, of which the volume fraction of martensite is about 29.3%, mainly (112) <110> The twin martensite has a twin thickness range of 4 to 15 nm.

[0054] The present invention utilizes the phase transformation characteristics of Mn segregation in the continuous casting billet. Through process control, the Mn segregation band region can be distinguished from the matrix region to obtain a different microstructure. Elemental analysis results show that this Mn segregation can still be observed in the final non-uniform layered structure of the test steel of the present invention. That is, the ultrafine-grained region of the bimodal structure in the ultrafine-grained layer and the coarse-grained layer can be observed to have a higher Mn content than the coarse-grained region. In addition, the research found that the content, distribution, and size of the ideal Mn segregation band obtained in the continuous casting billet through composition and process design are also closely related to the content, distribution, and size of the final ultrafine-grained layer. For example, similar to the Mn segregation band, the width of the ultrafine-grained layer in the test steel of the embodiment also varies and is unevenly distributed. Statistical results show that the width of a single ultrafine-grained layer ranges from 10 to 300 μm, with an average width of about 53 μm.

[0055] Through conventional process control, the microstructure of the coarse-grained steel in Comparative Example 1 is mainly a nearly equiaxed ferrite-martensite dual-phase structure with an average grain size of about 2.9 μm; the microstructure of the ultrafine-grained steel in Comparative Example 2 is mainly composed of a martensite matrix and cementite, the original austenite grain boundaries are vaguely visible, the width of the martensite laths is about 200 to 300 nm, and it has a high dislocation density.

[0056] The specific tensile properties of the test steels from the Examples and Comparative Examples are summarized in Table 2. As can be seen, the coarse-grained steel from Comparative Example 1 has the lowest strength, at 386 MPa, but the highest uniform elongation, reaching 13.9%. The ultrafine-grained steel from Comparative Example 2, with a single-phase martensite matrix, has the highest yield strength, at 751 MPa, but the lowest uniform elongation, at only 4.3%. The test steel from the Example, with its heterogeneous layered ultrafine-grained structure, exhibits the best strength-ductility match, with a yield strength of 545 MPa, approximately 41.2% higher than that of the coarse-grained steel from Comparative Example 1. Simultaneously, the uniform elongation of the test steel from the Example is 11.5%, approximately 2.7 times that of the ultrafine-grained steel from Comparative Example 2. Notably, the tensile strength of the test steel from the Example, reaching 880 MPa, is the highest of the three test steels and even 77 MPa higher than that of the ultrafine-grained steel from Comparative Example 2. This is due to its excellent work hardening ability.

[0057] According to the standard ASTM E1820, single-sided bending specimens were prepared along the rolling directions of three test steel plates to test the fracture toughness of the material, thereby effectively evaluating the damage tolerance of structural materials during crack initiation and propagation. As shown in the table, the test steel of the embodiment has the best crack initiation toughness, and its K Q 240.2 MPa·m 1 / 2 , which is 14.8% and 32.9% higher than that of the test steels in Comparative Example 1 and Comparative Example 2. When the crack Δa expands to about 0.8mm, the crack extension toughness K SS 262.4 MPa·m 1 / 2 and 240.7 MPa m 1 / 2 , while the K of the test steel in the embodiment is SS Increased to 302.4 MPa·m 1 / 2 The above results show that the experimental steel with heterogeneous layered ultrafine grain structure has the best fracture toughness.

[0058] Based on the above results, the test steel of the embodiment of the present invention has better strength-ductility-toughness matching and excellent comprehensive service performance.

[0059] Table 1 Chemical composition of three test steels (mass fraction / %)

[0060]

[0061] Table 2 Room temperature mechanical properties of three test steels

[0062]

[0063] The terms "including" and "having" and any variations thereof in the description and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatus.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A non-uniform layered ultrafine grain steel with excellent strength and toughness, characterized in that: The chemical composition of the ultrafine-grained steel comprises, by mass percentage, C: 0.04-0.10%, Si: 0.10-0.30%, Mn: 1.00-2.20%, P: ≤0.008%, S: ≤0.004%, Ni: 0.20-0.60%, Cu: 0.40-0.70%, Nb: 0.015-0.045%, Ti: 0.005-0.030%, and the remainder is Fe and inevitable impurities. The chemical composition of the ultrafine-grained steel is such that Nb+Ti: ≤0.045% and Mn+Ni+Cu: ≤2.50%. The RD-ND surface of the ultrafine-grained steel shows a non-uniform alternating distribution of ultrafine-grained layers and coarse-grained layers in a macroscopic view throughout the thickness. There is chemical composition inhomogeneity between the ultrafine-grained layers and the coarse-grained layers, wherein the ultrafine-grained layers contain a higher content of Mn than the coarse-grained layers. The preparation process of the ultrafine grain steel includes: S1 molten iron pretreatment → S2 converter smelting → S3 LF refining → S4 RH vacuum treatment → S5 continuous casting → S6 heating → S7 rough rolling → S8 intermediate cooling → S9 finishing rolling → S10 final cooling; The S6 heating temperature is 1323-1423K and the time is less than 3 hours; the S7 rough rolling temperature is 1253-1323K, the rolling passes are 3-5 times, and the reduction is 70-90%; The intermediate cooling step S8 cools the steel billet to room temperature at a cooling rate of 80-150K / s; The S9 finishing rolling is asynchronous rolling, the temperature is 993-1053K, in the austenite-ferrite two-phase region, the holding time is 1-3h, the final rolling temperature is 953-1013K, the rolling passes are 2-4 times, the reduction is 60-80%, the upper roll speed is 0.5-2m / s, the lower roll speed is 0.55-4m / s, and the ratio of the lower roll speed to the upper roll speed is 1.1-2.0; The steel billet after the S10 final cooling is cooled to room temperature at a cooling rate of 50 to 70 K / s.

2. The ultrafine grain steel according to claim 1, characterized in that: The volume fraction of the ultrafine crystal layer is 25-45%, the width of a single ultrafine crystal layer is 10-300 μm, and the average width is greater than 50 μm.

3. The ultrafine grain steel according to claim 1, characterized in that: The average grain size of the ultrafine-grained layer is 0.50 to 0.80 μm, and the grain size of the coarse-grained layer is less than 5.0 μm.

4. The ultrafine grain steel according to claim 3, characterized in that: The coarse-grained layer has a bimodal structural feature, wherein 5-25% of the grains have a size smaller than 1.0 μm, and 25-45% of the grains have a size larger than 3.0 μm.

5. The ultrafine grain steel according to claim 1, characterized in that: The crystal texture of the RD-ND surface changes gradually in the full thickness direction, wherein: <110> / / The proportion of ND texture gradually decreases from 45-65% in the upper surface to 15-30% in the core; <111> / / ND gradually increases from 5-15% on the upper surface to 25-45% in the core.

6. The ultrafine grain steel according to claim 1, characterized in that: The volume fraction of martensite in the ultrafine grain layer is 20-35%, and the martensite is mainly (112) <110> The twin martensite has a twin thickness range of 4 to 15 nm.

7. The ultrafine grain steel according to claim 1, characterized in that: The yield strength of the ultrafine-grained steel is 500-700 MPa, the tensile strength is 800-1000 MPa, and the uniform elongation is 18-24%.

8. The ultrafine grain steel according to claim 1, characterized in that: The crack initiation toughness K of the ultrafine grain steel Q 200~300MPa·m 1 / 2 , crack growth toughness K SS 250~350MPa·m 1 / 2 .

9. The ultrafine grain steel according to claim 1, characterized in that: The cumulative reduction of the S7 rough rolling and the S9 finishing rolling is ≥90%.

10. The ultrafine grain steel according to claim 1, characterized in that: The thickness ratio of the steel plate after S7 rough rolling to the finished product is 3.5 to 5.

0.

11. The ultrafine grain steel according to claim 1, characterized in that: The reduction in each pass during the S7 rough rolling and S9 finishing rolling is the same.

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