A thick, high-strength and tough NM550 wear-resistant steel and its preparation method

Abstract

The present invention belongs to the technical field of iron and steel metallurgy and high-strength wear-resistant materials, and discloses a thick, high-strength and tough NM550 wear-resistant steel and its preparation method. Nb, Ti, and V synergistically form a multi-component nanocarbide (Nb, Ti, V)C main strengthening phase with an average particle size of 5nm to 50nm in a mass ratio of 1:1:2. During the controlled rolling and austenitization stages, Cr and Mo elements form fine carbides, inducing the preferential precipitation of the main strengthening phase and preventing cluster aggregation. Al primary deoxidation and Ce and Ca composite modification treatment are used to form spherical Ce-Ca-Al-O composite oxide inclusions with an average particle size of ≤2μm. Clean smelting, uniform temperature heating, temperature-controlled rolling, three-stage flow-controlled water cooling, and two-stage tempering treatment are used to achieve uniform microstructure control and multi-scale strengthening. Yield strength ≥1400MPa, tensile strength ≥1850MPa, elongation ≥15%, impact energy at -20℃ ≥40J, and surface hardness ≥560HBW.

Description

Technical Field

[0001] The present invention relates to the technical field of steel metallurgy and high-strength wear-resistant materials, and in particular to a thick, high-strength and tough NM550 wear-resistant steel and a preparation method thereof. Background Art

[0002] As industries like engineering machinery, mining equipment, and metallurgical manufacturing continue to demand higher performance from materials, wear-resistant steel has become the preferred material for key components operating under demanding conditions such as high wear and high loads. Due to its excellent wear resistance, high strength, and moderate toughness, this material is widely used in mining trucks, crushing equipment, buckets, and liners, significantly extending service life, reducing maintenance costs, and improving equipment reliability.

[0003] Currently, commonly used wear-resistant steels such as NM400, NM450, and NM500 have a typical Brinell hardness range of 400 to 500 HBW, which can meet the application requirements of general moderate wear conditions. However, when faced with more demanding service environments (such as continuous heavy loads, high impact, and fluctuating temperature conditions), traditional steel grades are gradually showing performance bottlenecks in terms of the comprehensive balance of strength, toughness, and wear resistance, making it difficult to support the long-term, high-intensity use requirements of high-end equipment structural components.

[0004] To meet the demands of more extreme usage scenarios, NM550 wear-resistant steel was developed. With a Brinell hardness of approximately 550 HBW, it offers higher strength and wear resistance than NM400 / NM500. It is widely used in high-wear and high-impact applications, such as mining crusher liners, loading equipment track plates, and metallurgical conveying equipment, demonstrating excellent wear resistance and service life. However, existing NM550 steel still faces many challenges in practical applications:

[0005] (1) Insufficient matching of strength and toughness: While high hardness brings about increased strength, it often leads to a decrease in the toughness of the core tissue, which is particularly prone to brittle fracture in low temperature or high impact environments;

[0006] (2) Problems of uneven performance and center segregation in thick plates: In thick steel plates above 80 mm, it is difficult to control the core structure, and performance gradients are prone to unevenness;

[0007] (3) Insufficient control of precipitation phase and inclusions: Traditional NM550 does not systematically control the microalloying precipitation strengthening phase and non-metallic inclusion morphology, which affects its crack resistance and service life.

[0008] Therefore, the current NM550 wear-resistant steel still needs to make a technical breakthrough in the high-strength-high-toughness-thick plate consistency control mechanism. To this end, the present invention provides a large-thickness high-strength-toughness NM550 wear-resistant steel and a preparation method thereof. By optimizing the alloy composition design, a (Nb, Ti, V) C ternary nanocarbide system with a mass ratio of 1:1:2 is constructed, and the Ce-Ca-Al composite modification and deoxidation strategy is combined to improve the degree of inclusion spheroidization, and supplemented by clean smelting, continuous casting billet heating and temperature equalization, controlled rolling, austenitizing treatment, three-stage zoned cooling and two-stage tempering heat treatment, it is achieved that the structural uniformity, tempering stability and low-temperature impact toughness of the steel plate are significantly improved on the basis of high hardness.

[0009] The material and process system proposed in this invention is suitable for large thickness specifications of 80mm to 120mm, and has broad application potential in extreme wear and impact service occasions such as mining machinery, engineering equipment, and marine engineering components. It can effectively make up for the technical defects of traditional NM550 in high-end applications. Summary of the Invention

[0010] In response to the problems of core softening, structural heterogeneity, and insufficient low-temperature toughness in existing NM550 steel with a large thickness of more than 80 mm, the present invention provides a thick, high-strength, and tough NM550 wear-resistant steel and a preparation method thereof. Nb, Ti, and V are synergistically formed in a mass ratio of 1:1:2 to form ternary nano-carbides (Nb, Ti, V) C with an average particle size of 5 nm to 50 nm, which are dispersed and precipitated in the matrix as the main strengthening phase. During rolling and austenitization, Cr and Mo elements form (Cr, Mo) C at the grain boundaries, which serve as inducing nuclei for the precipitation of (Nb, Ti, V) C, while also stabilizing the interface and inhibiting cluster precipitation. Ce, Ca, and Al undergo composite modification and deoxidation in a low-oxygen environment to generate spherical Ce-Ca-Al-O inclusions. These inclusions, while refining the grains and inhibiting crack sources, indirectly promote the dispersed precipitation and uniform distribution of the main strengthening phase by regulating local element diffusion and interfacial energy states.

[0011] The process combines clean smelting, continuous casting heating and temperature equalization, controlled rolling, austenitization, three-stage zoned cooling, and two-stage tempering heat treatment to enhance austenite nucleation and microstructure uniformity, ensuring consistent performance between the core and surface layers. The resulting steel plate is suitable for thicknesses ranging from 80mm to 120mm and exhibits comprehensive properties including yield strength ≥1400MPa, tensile strength ≥1850MPa, elongation ≥15%, impact energy ≥40J at -20°C, surface hardness ≥560HBW, and core Brinell hardness ≥530HBW. It is suitable for complex environments with high wear and impact, such as open-pit mining, offshore platforms, and tunneling.

[0012] To achieve the above objectives, the steel plate of the present invention has been systematically tested in terms of composition design, rolling process parameters, and microstructure control. The technical solutions of the present invention are as follows:

[0013] A thick, high-strength and tough NM550 wear-resistant steel, the chemical composition of which includes, by mass percentage, C: 0.30%-0.35%, Si: 0.20%-0.40%, Mn: 0.50%-0.70%, Cr: 0.50%-0.90%, Mo: 0.20%-0.40%, B: 0.001%-0.003%, Nb: 0.01%-0.03%, and Ti: 0.01%. ~0.03%, V: 0.02%~0.06%, Al: 0.05%~0.08%, Ce: 0.001%~0.003%, Ca: 0.001%~0.003%, S≤0.003%, P≤0.005%, N≤0.004%, O≤0.002%, among which Nb, Ti and V are added synergistically in a mass ratio of 1:1:2, and the rest are Fe and unavoidable impurities.

[0014] The main strengthening phase is ternary nano-carbide (Nb, Ti, V)C dispersed and precipitated in the crystal, and the particle size of (Nb, Ti, V)C is 5nm~50nm.

[0015] Cr and Mo form (Cr,Mo)C during hot rolling and austenitization, which induces the nucleation of the main strengthening phase (Nb,Ti,V)C near it and stabilizes its interface structure, preventing cluster precipitation and austenite grain coarsening.

[0016] The inclusions are mainly spherical (Ce, Ca)-Al-O oxides with uniform particle size distribution, and the absolute value of the organizational deviation along the thickness direction is controlled within 10%; the spherical (Ce, Ca)-Al-O oxides serve as indirect regulating factors for the precipitation of the main strengthening phase, improving the nucleation position and distribution uniformity of (Nb, Ti, V) C and promoting organizational consistency in the thickness direction; the average particle size of the spherical (Ce, Ca)-Al-O oxides is ≤2μm, and the spheroidization rate is ≥90%.

[0017] Thick, high-strength and tough NM550 wear-resistant steel is suitable for steel plates with a thickness of 80mm~120mm. Its yield strength is ≥1400MPa, tensile strength is ≥1850MPa, elongation after fracture is ≥15%, impact absorption energy at -20℃ is ≥40J, surface Brinell hardness is ≥560HBW, and core Brinell hardness is ≥530HBW.

[0018] The main strengthening phase of the thick, high-strength and tough NM550 wear-resistant steel is ternary nano-carbides (Nb, Ti, V) C dispersed and precipitated within the grains, with a particle size of 5 nm to 50 nm, which are used to improve yield strength and temper softening resistance. Cr and Mo elements form (Cr, Mo) C during hot rolling and austenitization, and the precipitation behavior of the main strengthening phase (Nb, Ti, V) C is regulated by heterogeneous nucleation induction and element diffusion control mechanisms. Al is used as a primary deoxidizer, and Ce and Ca are introduced for composite modification treatment to convert Al2O3 inclusions into spherical (Ce, Ca)-Al-O oxide inclusions with an average particle size of ≤2 μm and a spheroidization rate of ≥90%. The spherical (Ce, Ca)-Al-O oxide inclusions suppress crack sources and refine grains while improving the distribution density and spatial uniformity of the main strengthening phase precipitation by regulating the nucleation environment and grain boundary energy distribution.

[0019] A method for preparing thick, high-strength and tough NM550 wear-resistant steel comprises the following steps:

[0020] S1. Smelting and inclusion control;

[0021] The process uses converter smelting and LF-VD refining. Al-based primary deoxidized molten steel is used, and Ce and Ca are added to the molten steel at the end of LF-VD refining. The mixture reacts at a temperature of 1580°C to 1620°C for 20 to 30 minutes to form oxide inclusions enriched in Ce and Ca, with an average particle size of ≤2μm and a sphericity of ≥90%. The oxide inclusions enriched in Ce and Ca promote the refinement, precipitation, and spatial uniformity of the main strengthening phase (Nb, Ti, V)C by regulating the local element diffusion path and grain boundary energy state. O content in the molten steel is ≤0.002%, and N content is ≤0.004%. The molten steel is cast under full protection to form continuous casting billets.

[0022] S2, continuous casting billet heating and temperature equalization;

[0023] Heat the continuous casting billet in S1 to 1150°C~1250°C. During the heating process, ensure that the temperature difference between the 1 / 2 of the billet thickness and the billet surface is ≤30°C. Keep the temperature for 2.5~3.5 hours.

[0024] S3, controlled rolling;

[0025] The continuous casting slab after holding in S2 is subjected to controlled rolling, with the rough rolling and finishing rolling temperature being 980℃~1020℃, the rough rolling reduction ratio being ≥60%, the finishing rolling temperature being 820℃~860℃, the finishing rolling reduction ratio being ≥60%, and slowly cooled to room temperature after rolling to obtain a steel plate, with the average cooling rate being controlled at 0.5℃ / s~1.0℃ / s; this promotes the formation of fine carbides of Cr and Mo at the grain boundaries, providing a stable interface for subsequent (Nb, Ti, V)C nucleation, and at the same time providing a stable fine-grained structure for subsequent austenitization.

[0026] S4, austenitizing treatment;

[0027] The steel plate is heated to 870-890°C at a heating rate of ≤150°C / h, ensuring that the temperature difference between the 1 / 2 point in the thickness direction and the surface of the steel plate is ≤20°C, and the plate is kept warm for 3 minutes per millimeter of thickness; Cr and Mo form primary carbides in the phase boundary area, regulating the subsequent nucleation behavior of (Nb, Ti, V)C.

[0028] S5, three-stage zone cooling;

[0029] A three-stage water spray zone flow-regulated water cooling system is used, with a cooling rate of ≥20°C / s in the front section, 15°C / s~20°C / s in the middle section, and 8°C / s~13°C / s in the final section, ultimately cooling the steel plate after insulation in S4 to room temperature;

[0030] S6, double-stage tempering heat treatment;

[0031] The steel is then subjected to a double tempering heat treatment at 220°C and 160°C, with each stage lasting two minutes per millimeter of thickness. Air cooling to room temperature promotes multi-stage precipitation and residual stress release, improving structural stability. The result is a thick, high-strength, and tough NM550 wear-resistant steel.

[0032] The mechanical properties fluctuation in the thickness direction of the prepared large-thickness, high-strength and tough NM550 wear-resistant steel does not exceed 10%. The microstructure is composed of lath martensite, ternary nanocarbides (Nb, Ti, V) C and (Cr, Mo) C, and the volume fraction of retained austenite is controlled at 2% to 4%, constructing a multi-scale strengthening and crack passivation synergistic microstructure.

[0033] A thick, high-strength and tough NM550 wear-resistant steel is used in high-wear and high-impact load applications, including surface mining equipment liners, tunnel boring cutter heads, heavy-duty engineering track shoes and wear-resistant parts of offshore platforms.

[0034] To achieve the synergistic optimization of high strength, high toughness, microstructure uniformity, and low-temperature performance in thick, high-strength NM550 wear-resistant steel, the present invention establishes a multi-level coupling system based on "main precipitate phase, microalloying element regulation, rare earth inclusion induction, and multi-stage cooling control" in terms of strengthening mechanism. The specific mechanism is as follows:

[0035] 1. Principles of the reinforcement mechanism

[0036] (1) Ternary precipitation strengthening mechanism - the leading role of ternary nanocarbides;

[0037] This invention utilizes the synergistic design of Nb, Ti, and V in a 1:1:2 mass ratio, leveraging the complementary properties of the three microalloying elements in lattice structure, atomic radius, bonding energy, and diffusion capacity. This results in the formation of a ternary nanocarbide (Nb, Ti, V)C at the atomic scale, characterized by a stable structure, low interfacial energy, and high dislocation pinning capability. Compared to conventional single NbC or Nb-Ti-C systems, this ternary nanocarbide exhibits higher nucleation drive and thermal stability. It effectively pins dislocations, refines martensitic laths, and improves the yield strength and work hardening capability of the steel. During double tempering, the (Nb, Ti, V)C phase continuously precipitates, significantly enhancing temper softening resistance and hot service stability. Its multi-point dispersed structure improves microstructure refinement and thickness-direction consistency.

[0038] (2) Cr / Mo induced regulation mechanism - heterogeneous nucleation assistance and interface stabilization;

[0039] The Cr and Mo elements added in this invention form (Cr, Mo)C during hot rolling and austenitization, which is evenly distributed in the matrix, providing heterogeneous nucleation sites for the primary strengthening phase and regulating the nucleation environment. The (Cr, Mo)C interface induces the preferential precipitation of (Nb, Ti, V)C around it, improving the nucleation rate and uniformity. The Cr and Mo elements form microscopic enrichment zones in the nucleation area, enhancing the interfacial energy stability of the primary strengthening phase and suppressing clustering and coarsening. The Cr / Mo induction regulation mechanism significantly improves the spatial distribution density of the primary strengthening phase, effectively enhancing the structural integrity of the central microstructure of thick steel plates.

[0040] (3) Rare earth composite regulation mechanism - dual functions of inclusion modulation and nucleation guidance;

[0041] The present invention utilizes an Al primary deoxidation + Ce-Ca composite modification process. In the late stage of LF-VD refining, the O content in the molten steel is controlled to ≤0.002%, forming spherical (Ce, Ca)-Al-O oxides with an average particle size ≤2 μm and a spheroidization rate ≥90%. This process exerts the following dual strengthening effects: On the one hand, rare earth inclusions significantly spheroidize sharp-angle oxides, reduce crack sources, refine austenite grains, and improve low-temperature impact toughness; on the other hand, the spherical (Ce, Ca)-Al-O oxides act as heterogeneous nucleation inducers of the primary strengthening phase, regulating the surrounding grain boundary energy and diffusion pathways, inducing (Nb, Ti, V)C to preferentially form a dispersed precipitation structure near them. The spherical (Ce, Ca)-Al-O oxides synergize with the precipitated phase to establish an inclusion-precipitation coupling control mechanism, improving the uniformity of the thick steel plate microstructure along the thickness direction.

[0042] (4) Multi-stage cooling and tissue regulation mechanism - improved microstructural continuity;

[0043] In order to solve the problem of segregation in the heat treatment structure of thick steel plates, the present invention adopts a segmented cooling rate strategy and a slow cooling system to control the distribution of austenite grains and strengthening phases during the hot working and quenching stages. In the slow cooling stage after rolling, the cooling rate is controlled to 0.5℃ / s~1.0℃ / s, which promotes the preferential formation of (Cr,Mo)C and controls the growth of austenite grains, providing a refined matrix for the subsequent quenching structure. In the quenching stage, a flow-adjusted water cooling system with three water spray zones is used to form a temperature gradient matching the structural transformation rate, ensuring the synchronization of the martensitic structure transformation from the surface to the core. Combined with the double-stage tempering heat treatment, the multi-level precipitation of strengthening phases, the stress release of the structure and the reconstruction of the crack passivation structure are achieved, thereby improving the strength consistency and service stability of the entire plate thickness.

[0044] Beneficial effects of the present invention:

[0045] (1) Significantly improve the strength and toughness matching ability of thick plates;

[0046] Compared with traditional NM550 products, the present invention achieves a synergistic improvement in high strength (yield ≥ 1400MPa) and high toughness (-20°C impact absorption energy ≥ 40J), and is particularly suitable for high-impact and high-wear working conditions.

[0047] (2) Constructing a synergistic mechanism of nano-strengthening, grain refinement, and inclusion purification;

[0048] Through the synergy of multiple mechanisms such as ternary nanocarbide strengthening, Cr / Mo precipitation regulation, and Ce-Ca inclusion regulation, the comprehensive service performance of steel is significantly improved.

[0049] (3) Excellent performance stability and structural uniformity;

[0050] The difference in structure along the thickness direction is less than 10%, and the difference in hardness and strength between the core and the surface is significantly reduced, solving the problems of center softening and crack sensitivity in traditional thick plates.

[0051] (4) High process feasibility and clear industrialization path;

[0052] All process parameters can be achieved under industrial production conditions without relying on special equipment or high-cost processing methods, and have excellent promotion prospects and industrialization value. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a transmission electron microscope image of the finished steel plate obtained in Example 2 of the present invention.

[0054] Figure 2 This is a transmission electron microscope image of the finished steel plate obtained in Comparative Example 2 of the present invention.

[0055] Figure 3 This is the inclusion morphology of the finished steel plate obtained in Example 2 of the present invention.

[0056] Figure 4 This is the inclusion morphology of the finished steel plate obtained in Comparative Example 3 of the present invention. DETAILED DESCRIPTION

[0057] Example 1

[0058] Prepare a thick, high-strength and tough NM550 wear-resistant steel with a finished plate thickness of 80 mm. The process steps are as follows:

[0059] According to the chemical composition of the large-thickness, high-strength and tough NM550 wear-resistant steel, smelting is carried out, and the chemical composition is recorded as follows by weight percentage: C: 0.30%, Si: 0.20%, Mn: 0.50%, Cr: 0.50%, Mo: 0.20%, B: 0.001%, Nb: 0.01%, Ti: 0.01%, V: 0.02%, Al: 0.05%, Ce: 0.001%, Ca: 0.001%, S≤0.003%, P≤0.005%, N≤0.004%, O≤0.002%, and the balance is Fe and other inevitable impurities; steel is smelted according to the component ratio, subjected to converter smelting and LF-VD refining, Al is used for primary deoxidation, Ce and Ca are added to the molten steel at the end of refining, and reacted at a temperature of 1600°C for 30 minutes to ensure the formation of high spheroidization rate inclusions, cast into continuous casting billets, control the inclusion size and oxygen and nitrogen content, O≤0.002%, N≤0.004%; heat the continuous casting billet to 1200℃ and keep it warm for 3 hours to ensure that the temperature difference between the center and the surface of the billet is ≤30℃; adopt a controlled rolling process to roll the heated continuous casting billet, including rough rolling and finish rolling stages, the rough rolling final rolling temperature is 980℃; the finish rolling final rolling temperature is 820℃; the hot rolled plate is slowly cooled to room temperature, the cooling rate is controlled at 0.8℃ / s, and then the air-cooled plate is heated to 880℃ and kept warm for 240min, and then water-cooled to room temperature, the cooling rate of the front section is 20℃ / s, the middle section is 15℃ / s, and the final section is 10℃ / s; then double tempering heat treatment at 220℃ and 160℃ is carried out in sequence, and each is kept warm for 160min before air cooling to room temperature.

[0060] After testing, the steel plate structure of the thick and high-strength NM550 wear-resistant steel is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 3%, the yield strength is 1460MPa, the tensile strength is 1880MPa, the impact energy at -20℃ is 45J, the elongation after fracture is 17.6%, the surface Brinell hardness is 566HBW, and the core Brinell hardness is 544HBW.

[0061] Example 2

[0062] Prepare a thick, high-strength and tough NM550 wear-resistant steel with a finished plate thickness of 100 mm. The process steps are as follows:

[0063] According to the chemical composition of the large-thickness, high-strength and tough NM550 wear-resistant steel, smelting is carried out, and the chemical composition is recorded by weight percentage as follows: C: 0.33%, Si: 0.30%, Mn: 0.60%, Cr: 0.70%, Mo: 0.30%, B: 0.002%, Nb: 0.02%, Ti: 0.02%, V: 0.04%, Al: 0.06%, Ce: 0.002%, Ca: 0.002%, S≤0.003%, P≤0.005%, N≤0.004%, O≤0.002%, and the balance is Fe and other inevitable impurities; the preparation and heat treatment process are compared with Example 1, except that the rough rolling finish rolling temperature is 1000°C, the fine rolling finish rolling temperature is 840°C, the austenitizing holding time is 300min, the holding time of each stage of the two-stage tempering heat treatment is 200min, and the other parameters are the same.

[0064] After testing, the structure of the steel plate is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 3%, the yield strength is 1490 MPa, the tensile strength is 1910 MPa, the impact energy at -20℃ is 44 J, the elongation after fracture is 17.7%, the surface Brinell hardness is 571 HBW, and the core Brinell hardness is 539 HBW.

[0065] Example 3

[0066] Prepare a thick, high-strength and tough NM550 wear-resistant steel with a finished plate thickness of 120 mm. The process steps are as follows:

[0067] According to the chemical composition of the large-thickness, high-strength and tough NM550 wear-resistant steel, smelting is carried out, and the chemical composition is recorded by weight percentage as follows: C: 0.35%, Si: 0.40%, Mn: 0.70%, Cr: 0.90%, Mo: 0.40%, B: 0.003%, Nb: 0.03%, Ti: 0.03%, V: 0.06%, Al: 0.08%, Ce: 0.003%, Ca: 0.003%, S≤0.003%, P≤0.005%, N≤0.004%, O≤0.002%, and the balance is Fe and other inevitable impurities; the preparation and heat treatment process are compared with Example 1, the rough rolling finish rolling temperature is 1020°C, the fine rolling finish rolling temperature is 860°C, the austenitizing holding time is 360min, the holding time of each stage of the two-stage tempering heat treatment is 240min, and the other parameters are the same. Among them, the purpose of increasing the final rolling temperature is to adapt to the rheological characteristics of high alloy content.

[0068] After testing, the structure of the steel plate is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 4%, the yield strength is 1480 MPa, the tensile strength is 1890 MPa, the impact energy at -20℃ is 46 J, the elongation after fracture is 16.7%, the surface Brinell hardness is 567 HBW, and the core Brinell hardness is 534 HBW.

[0069] Examples 1 to 3 show that as the thickness increases, good structural uniformity and strong and tough properties can still be maintained by increasing the alloy content and appropriately extending the heat treatment time, verifying the scalability and practical value of the present invention in large-thickness steel plates.

[0070] Comparative Example 1 (excluding Nb, Ti, and V elements)

[0071] A thick, high-strength and tough NM550 wear-resistant steel was prepared. The finished steel plate had a thickness of 100 mm. Compared with Example 2, its chemical composition did not contain Nb, Ti, or V elements. The remaining elements were the same, and the rolling and heat treatment processes were also the same. This comparative example was used to verify the strengthening core effect of the ternary nanocarbide.

[0072] After testing, the structure of the steel plate is martensite with a small amount of bainite, the volume fraction of retained austenite is 5%, the yield strength is 1190MPa, the tensile strength is 1520MPa, the impact energy at -20℃ is 33J, the elongation after fracture is 13.6%, the surface Brinell hardness is 513HBW, and the core Brinell hardness is 457HBW.

[0073] Because the steel plate in Comparative Example 1 lacks the addition of Nb, Ti, and V, the dispersed precipitation of the ternary nanocarbide (Nb, Ti, V)C is absent, eliminating the crucial precipitation strengthening mechanism. This results in poor dislocation pinning and insufficient grain size control, which in turn reduces yield strength and work hardening capacity. Furthermore, the absence of microalloying elements weakens the inhibitory effect on austenite grain growth. Furthermore, the lack of microalloying makes it difficult to form an ideal hardened structure, resulting in a decrease in overall hardenability, a softening of the core structure, and a significant deterioration in mechanical properties.

[0074] Comparative Example 2 (Nb, Ti, V element mass ratio ≠ 1:1:2)

[0075] A thick, high-strength and tough NM550 wear-resistant steel was prepared. The thickness of the finished steel plate was 100 mm. Compared with Example 2, the chemical composition of the Nb, Ti and V elements were 0.03%, 0.03% and 0.02% respectively. The other elements were the same, and the rolling and heat treatment processes were also the same. This comparative example was used to verify the effect of poor strengthening phase distribution.

[0076] After testing, the structure of the steel plate is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 2%, the yield strength is 1380 MPa, the tensile strength is 1840 MPa, the impact energy at -20℃ is 24 J, the elongation after fracture is 11.9%, the surface Brinell hardness is 552 HBW, and the core Brinell hardness is 524 HBW.

[0077] Because the mass ratio of Nb, Ti, and V in the steel plate of Comparative Example 2 deviates from the designed ratio of 1:1:2, the nucleation and stability of the (Nb, Ti, V)C ternary nanocarbides are impaired. The composition and lattice structure of the ternary nanocarbides are inhomogeneous, resulting in large size fluctuations and unstable morphology of the strengthening phases formed. In some areas, coarse secondary phases or irregular precipitation bands even form, weakening their dislocation pinning ability. Furthermore, the spatial distribution density of the strengthening phases decreases, and the ability to coordinate deformation between grains deteriorates. This results in increased stress concentration and easier crack propagation under load, significantly reducing the material's fracture toughness and ductility.

[0078] Comparative Example 3 (excluding Ce and Ca elements)

[0079] A thick, high-strength and tough NM550 wear-resistant steel was prepared. The finished steel plate had a thickness of 100 mm. Compared with Example 2, its chemical composition did not contain Ce and Ca elements, and the remaining elements were the same. The rolling and heat treatment processes were also the same. This comparative example was used to verify the importance of inclusion control.

[0080] After testing, the structure of the steel plate is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 3%, the yield strength is 1460 MPa, the tensile strength is 1880 MPa, the impact energy at -20℃ is 18 J, the elongation after fracture is 11.3%, the surface Brinell hardness is 561 HBW, and the core Brinell hardness is 542 HBW.

[0081] Since Ce and Ca elements were not added to the steel plate of Comparative Example 3, and deoxidation was carried out solely by Al, the inclusions generated were mainly sharp-angled Al2O3 particles with uneven size distribution. They tend to aggregate at grain boundaries, forming potential crack sources. These highly rigid, non-deformable inclusions easily cause stress concentration under stress, inducing microcracks to initiate and propagate along grain boundaries, significantly weakening the impact toughness and ductile deformation capacity of the steel. In contrast, Ce and Ca elements can transform sharp-angled Al2O3 into spherical Ce-Ca-Al-O composite oxide inclusions through metamorphism, reducing interfacial energy and suppressing crack tip stress, thereby improving microstructural continuity and fracture toughness. Therefore, although the strength indicators of the steel plate of Comparative Example 3 remained stable, the plasticity and impact properties showed a significant decline, indicating that inclusion regulation plays a key role in the toughness of thick plates.

[0082] Comparative Example 4 (Three-stage water cooling not used after finishing rolling)

[0083] A thick, high-strength and tough NM550 wear-resistant steel was prepared. The finished steel plate had a thickness of 100 mm. Its chemical composition and heat treatment process were the same as those in Example 2. The only difference was that after rolling, it was not subjected to three-stage water cooling. Instead, it was cooled to room temperature using a rapid laminar cooling system with an average cooling rate of 20°C / s. This comparative example was intended to verify the necessity of three-stage controlled cooling.

[0084] After testing, the structure of the steel plate is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 1.5%, the yield strength is 1310 MPa, the tensile strength is 1720 MPa, the impact energy at -20℃ is 29 J, the elongation after fracture is 14.1%, the surface Brinell hardness is 534 HBW, and the core Brinell hardness is 497 HBW.

[0085] Comparative Example 4, which did not employ a three-stage controlled cooling process but instead used rapid water cooling directly after hot rolling, failed to effectively refine the austenite grains. This prevented sufficient precipitation of (Cr, Mo)C to form an interface-induced structure, affecting the uniform precipitation and stability of the primary strengthening phase (Nb, Ti, V)C. Furthermore, the uncontrolled cooling rate through the thickness exacerbated structural heterogeneity, resulting in a significant decrease in performance in the core of the steel plate and a significant difference in strength and toughness between the surface and the center. This demonstrates the crucial role of the three-stage controlled cooling process in regulating strengthening phases and maintaining structural consistency.

[0086] Comparative Example 5 (only one tempering at 220°C)

[0087] A thick, high-strength and tough NM550 wear-resistant steel was prepared. The finished steel plate had a thickness of 100 mm. Its chemical composition and rolling process were the same as those in Example 2. The only difference was that only one tempering at 220°C was performed during the tempering heat treatment. This comparative example was intended to verify the importance of the two-stage tempering heat treatment.

[0088] After testing, the structure of the steel plate is martensite, ternary nano-carbides (Nb, Ti, V) C and (Cr, Mo) C, the volume fraction of retained austenite is 3%, the yield strength is 1510 MPa, the tensile strength is 1820 MPa, the impact energy at -20℃ is 28 J, the elongation after fracture is 12.7%, the surface Brinell hardness is 565 HBW, and the core Brinell hardness is 520 HBW.

[0089] After the 160°C secondary tempering is eliminated, residual stresses cannot be fully released, and high dislocation density and unstable retained austenite remain in some quenched martensite structures, resulting in incomplete phase transformation, stress concentration, and increased sensitivity to crack initiation. Although a single 220°C tempering can partially increase surface hardness and slightly increase yield strength, the lack of a second stage of deep microstructure recovery and carbide reprecipitation results in insufficient precipitation strengthening and incomplete microstructure control during tempering, leading to a decrease in tensile strength, reduced elongation, and a significant reduction in low-temperature impact toughness. At the same time, stress release in the surface and core is asynchronous, increasing the gradient between microstructure and performance, limiting the overall toughness reserve, and creating a tendency towards temper embrittlement, affecting the service reliability and safety margin of the thick plate.

[0090] In addition to the above embodiments, adjustments to process parameters and component contents according to the present invention can also achieve the preparation of a thick, high-strength and tough NM550 wear-resistant steel of the present invention, exhibiting properties substantially consistent with those of the present invention. The above description of the present invention is illustrative, and it should be noted that any simple variations, modifications, or equivalent substitutions that can be made by those skilled in the art without inventive effort, without departing from the core of the present invention, fall within the scope of protection of the present invention.

Claims

1. A thick, high-strength and tough NM550 wear-resistant steel, characterized in that: Its chemical composition by mass percentage includes: C: 0.30%~0.35%, Si: 0.20%~0.40%, Mn: 0.50%~0.70%, Cr: 0.50%~0.90%, Mo: 0.20%~0.40%, B: 0.001%~0.003%, Nb: 0.01%~0.03%, Ti: 0.01%~0.03%, V: 0.02%~0.06%, Al: 0.05%~0.08%, Ce: 0.001%~0.003%, Ca: 0.001%~0.003%, S≤0.003%, P≤0.005%, N≤0.004%, O≤0.002%, among which Nb, Ti and V are added synergistically in a mass ratio of 1:1:2, and the rest are Fe and unavoidable impurities.

2. The thick, high-strength and tough NM550 wear-resistant steel according to claim 1 is characterized in that: The main strengthening phase is ternary nano-carbide (Nb, Ti, V) C dispersed and precipitated in the matrix, and the particle size of (Nb, Ti, V) C is 5nm~50nm.

3. The thick, high-strength and tough NM550 wear-resistant steel according to claim 2 is characterized in that: Cr and Mo form (Cr,Mo)C during hot rolling and austenitization, which induces the nucleation of the main strengthening phase (Nb,Ti,V)C near it and stabilizes the (Nb,Ti,V)C interface structure, preventing cluster precipitation and austenite grain coarsening.

4. The thick, high-strength and tough NM550 wear-resistant steel according to claim 3 is characterized in that: The inclusions are mainly spherical (Ce, Ca)-Al-O oxides with uniform particle size distribution, and the absolute value of the organizational deviation along the thickness direction is controlled within 10%; the spherical (Ce, Ca)-Al-O oxides serve as indirect regulating factors for the precipitation of the main strengthening phase, improving the nucleation position and distribution uniformity of (Nb, Ti, V) C and promoting organizational consistency in the thickness direction; the average particle size of the spherical (Ce, Ca)-Al-O oxides is ≤2μm, and the spheroidization rate is ≥90%.

5. The thick, high-strength and tough NM550 wear-resistant steel according to any one of claims 1 to 4, characterized in that: Applicable to steel plates with a thickness of 80mm~120mm, with a yield strength ≥1400MPa, tensile strength ≥1850MPa, elongation after fracture ≥15%, impact absorption energy at -20℃ ≥40J, surface Brinell hardness ≥560HBW, and core Brinell hardness ≥530HBW.

6. A method for preparing the thick, high-strength and tough NM550 wear-resistant steel according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Smelting and inclusion control; The process uses converter smelting and LF-VD refining. Al is used for primary deoxidation of the molten steel. Ce and Ca are added to the molten steel at the end of LF-VD refining. The mixture reacts at a temperature of 1580°C to 1620°C for 20 to 30 minutes to form oxide inclusions enriched in Ce and Ca. The average particle size is ≤2μm and the sphericity is ≥90%. O content in the molten steel is ≤0.002% and N content is ≤0.004%. The molten steel is cast under full protection to form continuous casting billets. S2, continuous casting billet heating and temperature equalization; Heat the continuous casting billet in S1 to 1150℃~1250℃. During the heating process, ensure that the temperature difference between the 1 / 2 of the billet thickness and the billet surface is ≤30℃. Keep the temperature for 2.5~3.5 hours. S3, controlled rolling; The continuous casting slab after holding in S2 is subjected to controlled rolling, with the rough rolling and finishing rolling temperature being 980℃~1020℃, the rough rolling reduction ratio being ≥60%, the finishing rolling temperature being 820℃~860℃, the finishing rolling reduction ratio being ≥60%, and slowly cooled to room temperature after rolling to obtain a steel plate, with the average cooling rate being controlled at 0.5℃ / s~1.0℃ / s; S4, austenitizing treatment; Heat the steel plate to 870-890°C at a heating rate of ≤150°C / h, ensuring that the temperature difference between the 1 / 2 point in the thickness direction of the steel plate and the surface temperature of the steel plate is ≤20°C, and keep the temperature for 3 minutes per millimeter of plate thickness; S5, three-stage zone cooling; A three-stage water spray zone flow-regulated water cooling system is used, with a cooling rate of ≥20°C / s in the front section, 15°C / s~20°C / s in the middle section, and 8°C / s~13°C / s in the final section, ultimately cooling the steel plate after insulation in S4 to room temperature; S6, double-stage tempering heat treatment; Double tempering heat treatment at 220°C and 160°C was carried out in sequence. The holding time for each stage of tempering heat treatment was 2 minutes per millimeter of thickness. The steel was then air-cooled to room temperature to obtain high-strength and tough NM550 wear-resistant steel with large thickness.

7. The preparation method according to claim 6, characterized in that The mechanical properties fluctuation in the thickness direction of the prepared large-thickness, high-strength and tough NM550 wear-resistant steel does not exceed 10%. The microstructure is composed of lath martensite, ternary nanocarbides (Nb, Ti, V) C and (Cr, Mo) C, and the volume fraction of retained austenite is controlled at 2% to 4%, constructing a multi-scale strengthening and crack passivation synergistic microstructure.

8. An application of the thick, high-strength and tough NM550 wear-resistant steel according to any one of claims 1 to 5, characterized in that: The thick, high-strength and tough NM550 wear-resistant steel is used in high-wear and high-impact load situations, including open-pit mining equipment liners, tunnel boring cutter heads, heavy-duty engineering track plates and wear-resistant parts of offshore platforms.

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