A 785mpa grade weldable rare earth steel and a method of making the same

Abstract

The present application relates to a kind of 785MPa grade easy-to-weld rare earth steel and its preparation method, belong to easy-to-weld rare earth steel technical field, solve the problem of the strength and toughness mismatch of high-strength steel, poor welding performance in prior art.The present application provides a kind of 785MPa grade easy-to-weld rare earth steel, each component is according to mass percentage ratio:C:0.03~0.06, Si:0.20~0.40, Mn:0.08~1.20, P≤0.005, S≤0.005, 0<Als≤0.03, Ni:2.50~4.00, Cr:0.5~1.0, Mo:0.5~0.8, Cu:1.00~1.70, Nb:0.025~0.041, Ti:0.01~0.03, N:0.0025~0.0060, Ce:0.003~0.007, the rest is Fe and inevitable impurity element.The easy-to-weld rare earth steel realizes the matching of strength and toughness, and the tensile strength R m ≥898MPa of steel plate prepared, yield strength Rp 0.2 ≥800MPa, elongation after break ≥21%, reduction of area ≥70%, charpy impact V type opening impact energy at-84 ℃ ≥189J and excellent welding performance.

Description

Technical Field

[0001] This invention relates to the field of alloy steel technology, and in particular to a 785MPa grade easy-to-weld rare earth steel and its preparation method. Background Technology

[0002] With the development of shipbuilding and marine engineering, as well as the energy and petrochemical industries, the demand for manufacturing complex structures and large components has led to increasingly higher requirements for the comprehensive performance of steel materials. Steel must possess a balance of high strength and high toughness. Furthermore, due to the demands for high efficiency and low cost in engineering construction, good weldability is essential. The addition of rare earth elements allows steel materials to maintain strength and toughness while reducing carbon content and carbon equivalent, thereby improving the weldability of high-strength steel. Currently, high-strength steel with a yield strength of 785 MPa uses traditional Cr, Mo, and V microalloying strengthening methods. While increasing strength, this also increases the carbon equivalent of the steel, making welding more difficult.

[0003] In summary, it is currently difficult to simultaneously satisfy strength, toughness, and good weldability in steel materials of this strength level. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a 785MPa grade easy-to-weld rare earth steel and its preparation method, in order to solve at least one of the problems in the prior art, such as the mismatch between strength and toughness and poor welding performance of steel of this strength grade.

[0005] This invention provides a 785MPa grade easy-to-weld rare earth steel, wherein the components of the easy-to-weld rare earth steel, by mass percentage, are: C: 0.03-0.06, Si: 0.20-0.40, Mn: 0.08-1.20, P≤0.005, S≤0.005, 0<Als≤0.03, Ni: 2.50-4.00, Cr: 0.5-1.0, Mo: 0.5-0.8, Cu: 1.00-1.70, Nb: 0.025-0.041, Ti: 0.01-0.03, N: 0.0025-0.0060, Ce: 0.003-0.007, with the remainder being Fe and unavoidable impurity elements.

[0006] Furthermore, the contents of C, Ti, Nb, and N satisfy the following relationship: 12[C]≤6[Ti]+40[Nb]+16[N], where [X] is the mass percentage content of the corresponding element X.

[0007] Furthermore, the contents of Ce, Mn, and S satisfy the following relationship: 14[Ce]≥[Mn] / 60+[S] / 3, where [X] is the mass percentage content of the corresponding element X.

[0008] Specifically, the microstructure of the easy-to-weld rare earth steel is a mixture of tempered lath martensite, bainite, and a second phase, wherein the second phase is composed of C, N, Nb, and Ti.

[0009] Specifically, the second phase has a size of 5–20 nm and is mainly distributed on the lath martensite matrix; the effective grain size of the easy-to-weld rare earth steel is 2–3 μm.

[0010] Specifically, the tensile strength R of the steel plate made of the easily weldable rare earth steel is... m ≥898MPa, yield strength Rp 0.2 ≥800MPa, elongation after fracture ≥21%, reduction of area ≥70%, Charpy impact V-shaped opening impact energy at -84℃ ≥189J, and crack rate of surface, cross section, and root of the test specimen in the room temperature oblique Y-type crack resistance test is 0%.

[0011] The present invention also discloses a method for preparing the aforementioned easy-to-weld rare earth steel, wherein the process steps of the preparation method are as follows: hot metal pretreatment, converter smelting, refining, vacuum degassing, continuous casting, finishing, heating, rolling and heat treatment.

[0012] It is worth noting that the rolling process includes a roughing section and a finishing section;

[0013] The starting temperature of the roughing section is 1100-1180℃, and the roughing stage involves at least two passes of low-speed large deformation rolling: the deformation per pass is 20-25%, the rolling speed is ≤1.5m / s, and the ending temperature of the roughing is ≥950℃.

[0014] The starting temperature of the finishing rolling section is 800-950℃, and the cumulative deformation of the finishing rolling section is ≥35%. After the finishing rolling is completed, the cooling is accelerated at a rate of 10-20℃ / s, and the final cooling temperature is ≤600℃.

[0015] Specifically, the superheat in the continuous casting process is ≤15℃ and electromagnetic stirring and / or light pressure process are used to control the center segregation of the continuous casting billet.

[0016] Specifically, the heating temperature of the heating stage is 1120-1180℃; the specific process of the heat treatment stage is as follows: the steel plate is held at 850-900℃ for 0.5-2 hours, cooled to room temperature by water, and tempered at 500-650℃.

[0017] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0018] 1. The easy-to-weld rare earth steel / plate provided by this invention has high strength, good toughness, and a high degree of matching between strength and toughness. This invention, through component design and formula optimization, fully utilizes the role of each component and avoids its defects through reasonable design, thus obtaining easy-to-weld rare earth steel with high strength and good toughness.

[0019] It is particularly worth emphasizing that C, N, Ti, and Nb can produce a synergistic effect and form a fine second phase. This second phase is dispersed in the tempered lath-like martensite matrix, which can refine the grains and strengthen the steel, effectively compensating for the strength loss caused by the reduction of C content in alloy steel. The synergistic effect is most fully realized when 12[C]≤(6[Ti]+40[Nb]+16[N]).

[0020] In addition to their inherent roles, other elements also play a synergistic role in the overall alloy system. Among them, the most prominent synergistic effects are:

[0021] Mn: Manganese is an essential element for ensuring the strength and toughness of steel. A certain amount of Mn can refine ferrite grains, increase austenite stability, expand the process window for controlled rolling and cooling, and also generate MnS to reduce the harmful effects of sulfur in steel. Ni: Nickel is an essential element for ensuring the strength and toughness of steel plates, improving hardenability and tempering stability, and ensuring corrosion resistance. It can also increase the thermal stability of precipitated phases. Mo: Molybdenum is an element for ensuring the strength of steel, while also improving the stability of carbides and refining grains. When molybdenum reacts with chromium... When elements such as manganese are present, it can reduce or inhibit temper brittleness caused by other elements, thereby improving the toughness and plasticity of steel; Ti: Titanium is a strengthening element that controls the precipitation of the second phase in steel. Titanium and carbon form fine precipitates that can be pinned at grain boundaries, playing a role in refining grains. It can also be dissolved into the solid solution of iron to play a certain strengthening role; Ce: Cerium can regulate the diffusion process of carbon in the aforementioned easy-to-weld rare earth steel, significantly refine carbonitrides, inhibit the growth of original austenite grains, thereby refining grains and improving strength.

[0022] The tensile strength R of the steel plate made of the easily weldable rare earth steel m ≥898MPa, yield strength Rp 0.2 ≥800MPa, elongation after fracture ≥21%, reduction of area ≥70%, Charpy impact V-shaped opening impact energy at -84℃ ≥189J.

[0023] 2. The easy-to-weld rare earth steel provided by this invention has good weldability, and the crack rate of the surface, cross section and root of the room temperature oblique Y-type crack-resistant test specimen is 0%.

[0024] The easy-to-weld rare earth steel provided by this invention is designed based on a low carbon content. While maintaining strength and toughness, the alloying elements affecting carbon equivalent are controlled, resulting in a Ceq ≤ 0.8, placing the alloy in the easy-to-weld zone, a significant reduction compared to existing steel grades. Furthermore, through control of rolling, cooling, and heat treatment processes, the precipitation of C, N, Nb, and Ti second phases is effectively controlled. This precipitation strengthens the steel and controls grain refinement, significantly improving its strength. Thus, the easy-to-weld rare earth steel exhibits significantly improved weldability while maintaining strength. The addition of the rare earth element cerium further enhances weldability by increasing the proportion of large-angle grain boundaries, thereby suppressing cracking during welding and improving weldability. In addition, the contents of elements detrimental to weldability, such as Si, P, Al, Ni, Cr, and Cu, are controlled at low levels to minimize their adverse effects on weldability.

[0025] 3. The preparation method adopted in this invention, through reasonable process design and parameter optimization, does not require special equipment and can be implemented using existing common smelting and processing equipment. By using a low superheat of ≤15℃ and electromagnetic stirring and / or light pressing processes to control the compositional segregation of the alloy, the composition of the continuously cast steel billet is made more uniform. By controlling the temperature and combining rough rolling and finish rolling, the grain size in the hot-rolled state is controlled, providing a fine grain structure for heat treatment. By controlling the precipitation of the second phase with strengthening effect through heat treatment, it helps to form the final alloy microstructure, thereby further improving the strength and toughness of the weldable rare earth steel.

[0026] 4. The easy-to-weld rare earth steel provided by this invention has excellent comprehensive performance, good uniformity, outstanding weldability, and a relatively simple preparation process, making it suitable for large-scale production and widespread application.

[0027] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0028] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0029] Figure 1 The image shows the metallographic structure of the easily weldable rare earth steel plate in Example 1.

[0030] Figure 2This is a tensile curve of the easily weldable rare earth steel plate in Example 1;

[0031] Figure 3 The impact dynamometer diagram of the welded joint near the surface at -20°C in Example 1 is shown.

[0032] Figure 4 This is a photograph of the room temperature oblique Y-groove welding crack test of the easily weldable rare earth steel plate in Example 1. Detailed Implementation

[0033] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0034] This invention provides a 785MPa grade easy-to-weld rare earth steel, wherein the components of the easy-to-weld rare earth steel, by mass percentage, are: C: 0.03-0.06, Si: 0.20-0.40, Mn: 0.08-1.20, P≤0.005, S≤0.005, 0<Als≤0.03, Ni: 2.50-4.00, Cr: 0.5-1.0, Mo: 0.5-0.8, Cu: 1.00-1.70, Nb: 0.025-0.041, Ti: 0.01-0.03, N: 0.0025-0.0060, Ce: 0.003-0.007, with the remainder being Fe and unavoidable impurity elements.

[0035] The roles (including synergistic effects) and contents of each element are determined based on the following:

[0036] C: Carbon is a key element for improving strength and affecting weldability. Excessive carbon content has an adverse effect on the plasticity, toughness and weldability of steel, while excessively low carbon content will make the strength of steel insufficient and make it difficult for second-phase carbides to precipitate. Therefore, considering all aspects, the carbon content should be controlled between 0.03 and 0.06%.

[0037] Meanwhile, this invention fully utilizes the design concept of microalloying, considering that certain amounts of C and N can combine with Ti and Nb to form a relatively fine second phase, which can refine the grains and strengthen the steel. Through systematic research and theoretical analysis, the relationship between the content of C atoms and elements such as N, Nb, and Ti should satisfy: 12[C]≤(6[Ti]+40[Nb]+16[N]). This ensures the precipitation of the second phase while keeping the carbon content within acceptable limits. Here, "without exceeding limits" means that excessive carbon content will lead to a decrease in plasticity. However, if the carbon content is reduced, the strength of the steel cannot be guaranteed. Therefore, to improve strength, precipitation strengthening and grain refinement strengthening are necessary.

[0038] Silicon (Si): Silicon is a deoxidizing element. When the Si content is below 0.20%, molten steel is easily oxidized. It is also a solid solution strengthening element, which can improve the strength of steel to a certain extent. However, if the silicon content exceeds a certain amount, it will affect the toughness, plasticity, and weldability of the steel. The Si content should be controlled to be no more than 0.40%, and the control requirement for Si content is 0.20% to 0.40%.

[0039] Mn: Manganese is an essential element to ensure the strength and toughness of steel. A certain amount of Mn can refine ferrite grains, increase the stability of austenite, expand the process window of controlled rolling and cooling, and also generate MnS to reduce the harmful effects of S in steel. The Mn content control requirement is 0.08~1.20%.

[0040] P: Phosphorus is an impurity element in steel that can impair its toughness, plasticity, and weldability. Therefore, the P content should be controlled below 0.005%.

[0041] S: Sulfur is an impurity element in steel, which can form sulfide inclusions, becoming the starting point for corrosion, and it is important to avoid competitive precipitation of S and Ti. Therefore, the S content should be controlled below 0.005%.

[0042] Al: Acid-soluble aluminum is an excellent deoxidizer for steel and an effective element for refining grains. However, Al easily forms alumina inclusions, which is detrimental to steel forming and can easily become a source of crack initiation during welding, leading to a decrease in toughness. Therefore, it should be added as little as possible while ensuring the deoxidation effect of the steel. Thus, the acid-soluble aluminum in steel should be controlled at ≤0.03%.

[0043] Ni: Nickel is an essential element for ensuring the strength and toughness of steel plates, improving the hardenability and tempering stability of steel, and ensuring the corrosion resistance of steel. It can also increase the thermal stability of precipitated phases. However, a higher content increases the carbon equivalent of steel, which is not conducive to the weldability of steel. Therefore, the nickel content in steel is controlled between 2.50% and 4.00%.

[0044] Cr: Chromium is an element that ensures the corrosion resistance of steel, can improve the hardenability and tempering stability of steel, and the chromium compounds it forms can improve the strength and wear resistance of steel. It can also indirectly refine the grains to a certain extent. However, a higher content increases the carbon equivalent of steel, which is not conducive to the weldability of steel. Therefore, the chromium content in steel is controlled between 0.5% and 1.0%.

[0045] Mo: Molybdenum is an element that ensures the strength of steel and improves the stability of carbides. It can refine grains. When molybdenum coexists with elements such as chromium and manganese, it can reduce or inhibit temper brittleness caused by other elements, thereby improving the toughness and plasticity of steel. However, when the molybdenum content is high, the steel's resistance to deformation during hot working increases. Therefore, the molybdenum content in steel is controlled at 0.5% to 0.8%.

[0046] Cu: Copper has a certain solid solution effect on the grain boundaries and precipitates of steel, which can hinder grain boundary migration and phase transformation processes, thereby refining the grains and phase interfaces of steel. This helps to improve the microstructure and mechanical properties of steel, and can also improve the corrosion resistance of steel. However, a higher content increases the carbon equivalent of steel, which is not conducive to the weldability of steel. Therefore, the copper content in this invention is controlled at 1.00 to 1.70%.

[0047] Nb: Niobium acts as a grain-refining strengthening element in steel, refining the grain structure of steel by inhibiting grain growth. It forms carbonitride second phases with C and N. These second phases act as strengthening phases, which can significantly improve the strength and hardness of steel, while reducing the carbon equivalent of steel, thereby reducing the tendency to crack during welding. Therefore, the niobium content in this invention is controlled at 0.025 to 0.041%.

[0048] Nitrogen (N) can form second-phase carbonitrides with C, Nb, and Ti to improve the toughness of steel plates. However, excessive nitrogen content will preferentially form coarser titanium carbonitride particles with C and Ti, resulting in poor strengthening effect and reducing the effective strengthening titanium content in the steel. Therefore, the N content should be controlled between 0.0025% and 0.0060%.

[0049] Ti: Titanium is a strengthening element that controls the precipitation of the second phase in steel. Titanium forms fine precipitates with carbon, which can anchor at grain boundaries, refining the grains. It can also dissolve into the solid solution of iron, providing a certain strengthening effect. When the content exceeds a certain level, both strength and toughness will decrease to some extent. Therefore, the titanium content should be controlled between 0.01% and 0.03%.

[0050] The addition of cerium (Ce) has an excellent effect on improving the weldability of steel. It can increase the proportion of large-angle grain boundaries, thereby inhibiting the tendency of cracking during welding and improving the weldability of steel. In this steel, it can regulate the carbon diffusion process, significantly refine carbonitrides, inhibit the growth of original austenite grains, thereby refining the grains and improving strength. However, excessive addition will precipitate brittle phases, affecting the toughness of steel and thus affecting the overall performance of steel. The relationship between the mass percentage of Ce [Ce] in steel and the mass percentages of Mn [Mn] and S [S] is: 14[Ce]≥([Mn] / 60+[S] / 3), and the cerium content should be controlled between 0.003 and 0.007%.

[0051] Furthermore, the contents of C, Ti, Nb, and N satisfy the following relationship: 12[C]≤6[Ti]+40[Nb]+16[N], where [X] is the mass percentage content of the corresponding element X.

[0052] Furthermore, the contents of Ce, Mn, and S satisfy the following relationship: 14[Ce]≥[Mn] / 60+[S] / 3, where [X] is the mass percentage content of the corresponding element X.

[0053] Specifically, the microstructure of the weldable rare earth steel is a mixed microstructure composed of tempered lath martensite, bainite, and a second phase, wherein the second phase consists of C, N, Nb, and Ti. After quenching, a mixed microstructure of lath martensite / bainite is obtained. After high-temperature tempering, the lath martensite decomposes, the matrix softens, and some of the second phase precipitates, forming a mixed microstructure of tempered martensite / bainite and the second phase, which ensures an improved balance between strength and toughness in the steel.

[0054] Specifically, the second phase has a size of 5–20 nm and is mainly distributed on the lath martensite matrix; the effective grain size of the easily weldable rare earth steel is 2–3 μm. The high-density, fine-grained second phase controls the microstructure while simultaneously strengthening through precipitation, further refining the austenite grain size to achieve fine-grain strengthening of the alloy or dislocation strengthening by hindering dislocation movement.

[0055] Specifically, the steel plate made of the easy-to-weld rare earth steel has a tensile strength Rm≥898MPa, a yield strength Rp0.2≥800MPa, an elongation after fracture≥21%, a reduction of area≥70%, a Charpy impact V-shaped opening impact energy at -84℃≥189J, and a crack rate of 0% on the surface, cross section, and root of the oblique Y-type crack-resistant test specimen.

[0056] The present invention also discloses a method for preparing the aforementioned easy-to-weld rare earth steel, wherein the process steps of the preparation method are as follows: hot metal pretreatment, converter smelting, refining, vacuum degassing, continuous casting, finishing, heating, rolling and heat treatment.

[0057] Specifically, the pretreatment of molten iron mainly serves the purpose of desulfurization. During the converter smelting process, oxygen is blown to remove phosphorus and carbon, so that it reaches the range defined by this invention. Then, aluminum is used for deoxidation, and then it is transferred to the ladle furnace for refining. At the same time, alloy materials are added to adjust the composition to the target composition, so that the composition content of the molten steel reaches the preset value of the formula.

[0058] Specifically, the main function of vacuum degassing is to remove excess gas from molten steel. The vacuum conditions should meet the requirements of [H] < 1 ppm and [O] < 20 ppm to achieve a better degassing effect and avoid oxidation of alloy components. H is a harmful element / gas in steel. Due to its small molecular size, it is easy to aggregate and form hydrogen embrittlement, causing defects in steel.

[0059] Specifically, the superheat in the continuous casting process is ≤15℃ and electromagnetic stirring and / or light pressure process are used to control the center segregation of the continuous casting billet.

[0060] Specifically, the finishing process is mainly intended to prepare for the subsequent rolling process, including steps such as determining the billet size and inspecting for defects in the billet.

[0061] It is worth noting that the rolling process includes a roughing section and a finishing section;

[0062] The roughing stage begins at a temperature of 1100–1180℃ and involves at least two passes of low-speed, large-deformation rolling: the deformation per pass is 20–25%, the rolling speed is ≤1.5 m / s, and the finishing temperature of the roughing stage is ≥950℃. Controlling the heating temperature can control the growth of the original austenite grains. Rolling with a large reduction can more fully destroy the casting structure, eliminate defects in the casting and microstructure, and make the grains finer, thereby improving and enhancing the mechanical properties of the material.

[0063] The starting temperature of the finishing rolling section is 800-950℃, and the cumulative deformation of the finishing rolling section is ≥35%. After the finishing rolling, the temperature is accelerated and cooled at a rate of 10-20℃ / s, with a final cooling temperature ≤600℃. The main purpose of fine rolling is to control the austenite phase transformation process. Starting rolling in the non-recrystallized austenite region is beneficial for controlling the final rolling, cumulative deformation, and ensuring the control of the final rolling temperature. Accelerated cooling after finishing rolling can obtain good fine grains, retain a high dislocation density, and ensure the martensitic structure during subsequent heat treatment and quenching.

[0064] Specifically, the heating temperature in the heating stage is 1120–1180℃; the specific process of the heat treatment stage is as follows: the steel plate is held at 850–900℃ for 0.5–2 hours, water-cooled to room temperature, and tempered at 500–650℃. The quenching and tempering heat treatment process is to obtain a balance between strength and toughness. Quenching to obtain a martensitic structure can effectively guarantee the strength requirements of the material. Tempering can adjust the morphology of the martensite laths and provide a driving force for the precipitation of the second phase. Precipitation of the second phase (C, N, Nb, Ti) on the tempered martensite matrix, to a certain extent, inhibits grain growth, ensures grain refinement, and increases the proportion of large-angle grain boundaries, thus providing a guarantee for improving the strength-toughness balance of the steel.

[0065] Examples and Comparative Examples

[0066] The chemical composition of the embodiments and comparative examples of the present invention is shown in Table 1, with the balance being Fe and unavoidable impurities.

[0067] Table 1 Chemical composition of the examples and comparative examples

[0068]

[0069]

[0070] The steel plate manufacturing process is as follows:

[0071] Hot metal pretreatment—converter smelting—refining—vacuum degassing—continuous casting—finishing—heating—rolling—heat treatment;

[0072] Hot metal pretreatment: High-quality scrap steel is selected as raw material, and the hot metal undergoes deep desulfurization to reduce its sulfur content. Dephosphorization is also performed during the pretreatment process.

[0073] Converter smelting and refining: Using conventional processes in existing technologies, the refined molten steel reaches the preset formula values ​​(as shown in Table 1);

[0074] Vacuum (vacuum exhaust): [H] < 1 ppm, [O] < 20 ppm;

[0075] Continuous casting: Superheat ≤15℃, electromagnetic stirring and light reduction techniques were used to improve core segregation of the billet; low-magnification rating of the continuously cast billet was performed, and the results are shown in Table 2:

[0076] Table 2. Low-magnification rating results of continuously cast billets in the examples and comparative examples.

[0077] Example 1 0.5 0.5 0.5 Not seen Example 2 0.5 0.5 0.5 Not seen Example 3 0.5 0.5 0.5 Not seen Comparative Example 1 0.5 0.5 0.5 Not seen Comparative Example 2 0.5 0.5 0.5 Not seen Comparative Example 3 0.5 0.5 0.5 Not seen

[0078] Heating and rolling: The heating temperature is 1120~1180℃; the rolling process includes a roughing rolling section and a finishing rolling section;

[0079] The starting temperature of the roughing section is 1100-1180℃, and the roughing stage involves at least two passes of low-speed large deformation rolling: the deformation per pass is 20-25%, the rolling speed is ≤1.5m / s, and the ending temperature of the roughing is ≥950℃.

[0080] The starting temperature of the finishing rolling section is 800-950℃, and the cumulative deformation of the finishing rolling section is ≥35%. After the finishing rolling is completed, the cooling is accelerated at a rate of 10-20℃ / s, and the final cooling temperature is ≤600℃.

[0081] Table 3 Rolling process parameters for the examples and comparative examples

[0082] Rough rolling start temperature / ℃ 1180 1180 1100 1200 1200 1200 Roughing and finishing rolling temperatures / ℃ 1050 980 950 1100 1050 1150 Finishing rolling start temperature / ℃ 950 950 900 1000 980 1000 Finishing rolling temperature / ℃ 780 824 806 925 900 900 Rolling speed (m / s) 1.5 1.5 1.5 1.7 1.7 1.7 Roughing passes 4 3 3 2 2 2

[0083] Heat treatment: The steel plate is held at 850-900℃ for 50-80 minutes, the same as the examples and comparative examples. It is then water-cooled to room temperature and tempered at 500-650℃.

[0084] Table 4 Heat treatment process parameters for examples and comparative examples

[0085] Quenching temperature / °C 860 860 860 860 860 950 Furnace time / min 50 80 60 50 80 60 Quenching roller speed / m / min 4 1.5 2 4 1.5 2 Quenching water temperature / ℃ 11.4 11.8 11.8 11.4 11.8 11.8 Tempering temperature / °C 560 540 600 600 500 450 Furnace time / min 120 240 150 120 240 150

[0086] After heat treatment, the steel plates were sampled transversely and processed into tensile and impact specimens for mechanical property testing. The results are shown in Table 5.

[0087] Table 5. Mechanical property test results of the examples and comparative examples.

[0088]

[0089] The heat-treated steel plate was welded by gas shielded welding. The steel plate was cut into an X-shaped groove for welding. The welding was completed after 11 passes on both the top and bottom. The shielding gas was 95% Ar + 5% CO2. The welding heat input was 18 kJ / cm. The welding performance test results are shown in Table 6 (the impact notch locations include weld metal (WM), fusion line (FL), +1mm outside the line (FL+1mm), +2mm outside the line (FL+2mm), and +5mm outside the line (FL+5mm)).

[0090] Table 6. Welding performance test results of the examples and comparative examples.

[0091]

[0092] It is evident that the steel plate prepared according to the composition design and production process of this invention can obtain a mixed structure of tempered martensite / bainite and second phase. At the same time, the steel plate can take into account both strength and toughness and weldability. Its yield strength reaches more than 800 MPa, tensile strength reaches more than 898 MPa, elongation is more than 21%, Charpy impact V-notch impact energy at -84℃ is ≥189 J, and it can be welded without preheating at room temperature.

[0093] Compared with the examples, the comparative examples have certain differences in composition and process, resulting in lower strength in Comparative Example 1, which cannot reach the mechanical performance level of the examples. In addition, it has a higher carbon equivalent, poor weldability, and poor impact performance of the welded joint, which cannot meet the welding performance requirements under room temperature conditions. Comparative Examples 2 and 3 meet the strength requirements, but cannot reach the toughness level of the examples, and have poor weldability, which cannot achieve room temperature welding.

[0094] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A 785MPa grade easy-to-weld rare earth steel, characterized in that, The components of the easily weldable rare earth steel, by mass percentage, are: C: 0.03–0.06, Si: 0.20–0.40, Mn: 0.8–1.20, P ≤ 0.005, S ≤ 0.005, 0 < Als ≤ 0.03, Ni: 2.50–4.00, Cr: 0.5–1.0, Mo: 0.5–0.8, Cu: 1.00–1.70, Nb: 0.025–0.041, Ti: 0.01–0.03, N: 0.0025–0.0060, Ce: 0.003–0.007, with the remainder being Fe and unavoidable impurity elements; Ceq ≤ 0.

8. The easily weldable rare earth steel has the following C, Ti, Nb, and N contents: 12[C]≤6[Ti]+40[Nb]+16[N], where [X] is the mass percentage content of the corresponding element X; The easy-to-weld rare earth steel described above has Ce, Mn, and S contents that satisfy the following relationship: 14[Ce]≥[Mn] / 60+[S] / 3, where [X] is the mass percentage content of the corresponding element X; The aforementioned easy-to-weld rare earth steel has a microstructure consisting of a mixture of tempered lath martensite, bainite, and a second phase; the second phase is composed of C, N, Nb, and Ti, with a size of 5–20 nm, and is mainly distributed on the lath martensite matrix; The aforementioned easy-to-weld rare earth steel produces steel plates with a tensile strength R. m ≥898MPa, yield strength Rp0.2≥800MPa, elongation after fracture≥21%, reduction of area≥70%, Charpy impact V-shaped opening impact energy at -84℃≥189J, crack rate of surface, cross section and root of the test specimen in the room temperature oblique Y-type crack resistance test is 0%, and it can achieve welding without preheating under room temperature conditions.

2. The easy-to-weld rare earth steel according to claim 1, characterized in that, The effective grain size of the easily weldable rare earth steel is 2–3 μm.

3. A method for preparing the easily weldable rare earth steel according to any one of claims 1 to 2, characterized in that, The process steps of the preparation method are as follows: molten iron pretreatment, converter smelting, refining, vacuum degassing, continuous casting, finishing, heating, rolling and heat treatment.

4. The preparation method according to claim 3, characterized in that, The rolling process includes a roughing section and a finishing section; The starting temperature of the roughing section is 1100-1180℃, and the roughing stage involves at least two passes of low-speed large deformation rolling: the deformation per pass is 20-25%, the rolling speed is ≤1.5m / s, and the ending temperature of the roughing is ≥950℃. The starting temperature of the finishing rolling section is 800-950℃, and the cumulative deformation of the finishing rolling section is ≥35%. After the finishing rolling is completed, the cooling is accelerated at a rate of 10-20℃ / s, and the final cooling temperature is ≤600℃.

5. The preparation method according to claim 3, characterized in that, In the vacuum degassing process, the vacuum conditions should meet the following requirements: [H] < 1 ppm, [O] < 20 ppm.

6. The preparation method according to claim 3, characterized in that, The superheat in the continuous casting process is ≤15℃, and electromagnetic stirring and / or light pressure processes are used to control the center segregation of the continuous casting billet.

7. The preparation method according to claim 3, characterized in that, The heating temperature of the heating stage is 1120-1180℃; the specific process of the heat treatment stage is as follows: the steel plate is held at 850-900℃ for 0.5-2 hours, cooled to room temperature by water, and tempered at 500-650℃.

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