785MPa-grade easily-welded rare earth steel and preparation method thereof

Abstract

The invention relates to 785MPa-grade easily-welded rare earth steel and a preparation method thereof, belongs to the technical field of easily-welded rare earth steel, and solves the problems that in the prior art, the strength and toughness of high-strength steel are not matched, and the welding performance is poor. The invention provides 785MPa-grade easily-welded rare earth steel which comprises the following components in percentage by mass: 0.03 to 0.06 percent of C, 0.20 to 0.40 percent of Si, 0.08 to 1.20 percent of Mn, less than or equal to 0.005 percent of P, less than or equal to 0.005 percent of S, more than 0 and less than or equal to 0.03 percent of Als, 2.50 to 4.00 percent of Ni, 0.5 to 1.0 percent of Cr, 0.5 to 0.8 percent of Mo, 1.00 to 1.70 percent of Cu, 0.025 to 0.041 percent of Nb, 0.01 to 0.03 percent of Ti, 0.0025 to 0.0060 percent of N, 0.003 to 0.007 percent of Ce and the balance of Fe and inevitable impurity elements. According to the easily-welded rare earth steel, matching of strength and toughness is achieved, the tensile strength Rm of a prepared steel plate is larger than or equal to 898 MPa, the yield strength Rp0.2 is larger than or equal to 800 MPa, the percentage elongation after fracture is larger than or equal to 21%, the percentage reduction of area is larger than or equal to 70%, the Charpy impact V-shaped opening impact energy at the temperature of-84 DEG C is larger than or equal to 189 J, and the welding performance is excellent.

Description

Technical Field

[0001] The invention relates to the technical field of alloy steels, and in particular to a 785MPa grade easy-to-weld rare earth steel and a preparation method thereof. Background Art

[0002] With the development of shipbuilding, marine engineering and energy and petrochemical industries, driven by the demand for manufacturing complex structures and large components, the comprehensive performance requirements of steel materials are getting higher and higher, requiring steel to have high strength and high toughness. In addition, due to the requirements of high efficiency and low cost of engineering construction, steel needs to have good weldability. The addition of rare earth elements reduces the carbon content and carbon equivalent of steel materials while ensuring strength and toughness, thereby improving the weldability of high-strength steel. At present, high-strength steel with a yield strength of 785MPa uses traditional Cr, Mo, and V micro-alloying strengthening methods. While increasing the strength, it also increases the carbon equivalent of the steel and increases the difficulty of welding.

[0003] In summary, it is currently difficult to simultaneously meet the requirements of strength, toughness and good welding performance in steel materials of this strength level. Summary of the invention

[0004] In view of the above analysis, an embodiment of the present invention aims to provide a 785MPa grade easy-to-weld rare earth steel and a preparation method thereof, so as to solve at least one of the problems in the prior art of mismatch between strength and toughness of steel of this strength level and poor welding performance.

[0005] The invention provides a 785MPa grade easy-to-weld rare earth steel. The components of the easy-to-weld rare earth steel are as follows by mass percentage: 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, and the rest are Fe and unavoidable impurity elements.

[0006] Furthermore, the contents of C, Ti, Nb, and N satisfy the 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 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 characterized by a mixed microstructure of tempered lath martensite, bainite and a second phase, and the second phase is composed of C, N, Nb and Ti.

[0009] Specifically, the size of the second phase is 5 to 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 to 3 μm.

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

[0011] The invention also discloses a preparation method of the easy-to-weld rare earth steel, and the process steps of the preparation method are: molten iron 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 rough rolling section and a finishing rolling section;

[0013] The starting temperature of the rough rolling section is 1100-1180°C, and the rough rolling stage is at least 2 passes of low-speed large deformation rolling: the deformation of a single pass is 20-25%, the rolling speed is ≤1.5m / s, and the end temperature of the rough rolling is ≥950°C;

[0014] The starting temperature of the finishing rolling section is 800-950°C, and the accumulated deformation of the finishing rolling section is ≥35%; the finishing rolling is followed by accelerated cooling, with a cooling rate of 10-20°C / s and a final cooling temperature of ≤600°C.

[0015] Specifically, the overheat degree of the continuous casting process is ≤15°C and electromagnetic stirring and / or light pressing process is used to control the central segregation of the continuous casting billet.

[0016] Specifically, the heating temperature of the heating link is 1120-1180°C; the specific process of the heat treatment link is that the steel plate is kept at 850-900°C for 0.5-2h, water-cooled to room temperature, and the tempering temperature is 500-650°C.

[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 / steel plate provided by the present invention has high strength, good toughness, and a high matching degree between strength and toughness. The present invention obtains an easy-to-weld rare earth steel with high strength and good toughness by optimizing the composition design and formula, giving full play to the role of each component and avoiding its defects through reasonable design.

[0019] It is particularly worth emphasizing that C, N, Ti, and Nb can produce a synergistic effect and form a fine second phase, which is dispersed in the tempered lath martensite matrix, can play a role in grain refinement and strengthening, and can effectively make up for the strength loss caused by the reduction of C content in alloy steel. In particular, when 12[C]≤(6[Ti]+40[Nb]+16[N]) is met, the synergistic effect is most fully exerted.

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

[0021] Mn: Manganese is an essential element to ensure the strength and toughness of steel. A certain content of Mn can refine the ferrite grains, increase the stability of austenite, expand the process window of controlled rolling and controlled cooling, and produce MnS to reduce the harmful effects of S in steel; Ni: Nickel is an essential element to ensure the strength and toughness of steel plates, improve the hardenability and tempering stability of steel, and can ensure the corrosion resistance of steel and increase the thermal stability of the precipitated phase; Mo: Molybdenum is an element to ensure the strength of steel, while improving the stability of carbides and refining grains. When molybdenum is combined with chromium When elements such as titanium and manganese coexist, it can reduce or inhibit the temper brittleness caused by other elements, thereby improving the toughness and plasticity of the 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 the grain boundaries, which plays a role in refining the grains. It can also be dissolved in the solid solution of iron to play a certain strengthening role; Ce: The cerium element can regulate the diffusion process of carbon in the easy-to-weld rare earth steel, significantly refine carbonitrides, and inhibit the growth of original austenite grains, thereby refining the grains and improving the strength.

[0022] The tensile strength R of the steel plate made of the easy-to-weld rare earth steel m ≥898MPa, yield strength Rp 0.2 ≥800MPa, elongation after fracture ≥21%, section shrinkage ≥70%, -84℃ Charpy impact V-type opening impact energy ≥189J.

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

[0024] The easy-to-weld rare earth steel provided by the present invention is designed based on low C content. On the basis of meeting the requirements of strength and toughness, the alloy elements affecting the carbon equivalent are controlled so that Ceq≤0.8, so that the alloy is in the easy-to-weld zone, which is significantly lower than that of existing steels; at the same time, through the control of rolling, cooling and heat treatment processes, effective control is performed when the second phases of C, N, Nb and Ti are precipitated, and the strength of the steel is significantly improved through precipitation strengthening of the second phase and controlled grain refinement, so that the weldability of the easy-to-weld rare earth steel is significantly improved while maintaining the strength; at the same time, the rare earth element cerium is added, which has an excellent effect on improving the weldability of the steel, and the cracking tendency during the welding process is suppressed by increasing the proportion of large-angle grain boundaries, thereby improving the welding performance of the steel; in addition, the contents of elements harmful to the welding performance, such as Si, P, Al, Ni, Cr, Cu, etc. in the easy-to-weld rare earth steel are all controlled at a relatively low level, thereby avoiding their adverse effects on the welding performance as much as possible.

[0025] 3. The preparation method adopted by the present invention, through reasonable process design and parameter optimization, does not require the use of special equipment, and can be implemented using existing common smelting and processing equipment. By using a relatively low superheat of ≤15°C, electromagnetic stirring and / or light pressing processes are used to control the component segregation of the alloy, so that the composition of the continuous casting billet is more uniform; the grain size of the hot-rolled state is controlled by combining temperature control with rough rolling and finish rolling, providing a fine grain structure for heat treatment, and the precipitation of the second phase with a strengthening effect is controlled by heat treatment methods, which is helpful to form the final alloy structure, thereby facilitating further improvement of the strength and toughness of the easy-to-weld rare earth steel.

[0026] 4. The easy-to-weld rare earth steel provided by the present invention has excellent comprehensive performance, good uniformity, outstanding weldability, and a relatively simple preparation process, and is suitable for large-scale production and wide promotion.

[0027] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0029] Figure 1 This is a metallographic structure morphology diagram of the easy-to-weld rare earth steel plate in Example 1;

[0030] Figure 2The tensile curve of the easy-to-weld rare earth steel plate in Example 1;

[0031] Figure 3 This is the -20°C impact work diagram near the surface of the welded joint in Example 1;

[0032] Figure 4 This is a photo of the room temperature inclined Y-groove welding crack test of the easy-to-weld rare earth steel plate in Example 1. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a 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 used to limit the scope of the present invention.

[0034] The invention provides a 785MPa grade easy-to-weld rare earth steel. The components of the easy-to-weld rare earth steel are as follows by mass percentage: 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, and the rest are Fe and unavoidable impurity elements.

[0035] The role (including synergistic effect) and content of each element are determined as follows:

[0036] C: Carbon is a key element to improve strength and affect weldability. Too high a carbon content will adversely affect the plasticity, toughness and weldability of steel. Too low a carbon content will make the strength of the steel fail to meet the requirements and make it difficult for the second phase carbide to precipitate. Therefore, considering all aspects, the C content should be controlled at 0.03-0.06%.

[0037] At the same time, the present invention makes full use of the design concept of microalloying, considering that a certain amount of C and N can be combined with Ti and Nb to form a relatively fine second phase, which can play a role in grain refinement and strengthening. After relatively 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]). The precipitation of the second phase can be guaranteed while ensuring that the carbon content does not exceed the standard. Not exceeding the standard here 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. At this time, if you want to increase the strength, you need to achieve it through precipitation strengthening and fine grain strengthening.

[0038] Si: Silicon is a deoxidizing element. When the Si content is less than 0.20%, the 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 at no more than 0.40%, and the control requirements of the Si content are: 0.20~0.40%.

[0039] Mn: Manganese is an essential element to ensure the strength, toughness and plasticity of steel. A certain content of Mn can refine the ferrite grains, increase the stability of austenite, expand the process window of controlled rolling and controlled cooling, and produce 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, which will damage the toughness, plasticity and weldability of steel. Therefore, the P content is controlled below 0.005%.

[0041] S: Sulfur is an impurity element in steel, which will form sulfide inclusions, become the starting point of corrosion, and avoid competitive precipitation of S and Ti. Therefore, the S content is controlled below 0.005%.

[0042] Als: Acid-soluble aluminum is an excellent deoxidizer for steel and an effective element for grain refinement. Al is easy to form aluminum oxide inclusions, which is not conducive to the forming of steel, and it is easy to become the source of crack initiation during welding, resulting in reduced toughness. Therefore, it is added as little as possible while ensuring the deoxidation effect of steel. Therefore, the acid-soluble aluminum in steel should be controlled at ≤0.03%.

[0043] Ni: Nickel is an essential element to ensure the strength and toughness of steel plates, improve the hardenability and tempering stability of steel, ensure the corrosion resistance of steel, and increase the thermal stability of the precipitated phase. 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 at 2.50-4.00%.

[0044] Cr: Chromium is an element that ensures the corrosion resistance of steel. It can improve the hardenability and tempering stability of steel. The chromium formed by it can improve the strength of steel and increase the wear resistance of steel. It can 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 at 0.5-1.0%.

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

[0046] Cu: Copper has a certain solid solution effect on the grain boundaries and precipitated phases of steel, which can hinder the migration of grain boundaries and the phase transformation process, thereby refining the grains and phase interfaces of steel, which 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 of the present invention is controlled at 1.00-1.70%.

[0047] Nb: Niobium is used as a fine grain strengthening element in steel. It refines the grain structure of steel by inhibiting the growth of grains and forms a carbonitride second phase with C and N. These second phases can significantly improve the strength and hardness of steel as strengthening phases, while reducing the carbon equivalent of steel, thereby reducing the tendency of cracks generated during welding. Therefore, the niobium content of the present invention is controlled at 0.025-0.041%.

[0048] N: Nitrogen can form second-phase carbonitrides with C, Nb, and Ti to improve the toughness of steel plates. However, if the nitrogen content is too high, it will preferentially form coarse titanium carbonitrides with C and Ti, resulting in poor strengthening effects and a reduction in the effective strengthening titanium content in the steel. Therefore, the nitrogen content is controlled at 0.0025 to 0.0060%.

[0049] 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 the grain boundaries to refine the grains. They can also be dissolved in the solid solution of iron to play a certain strengthening role. When the content exceeds a certain level, its strength and toughness will decrease to a certain extent. Therefore, the titanium content should be controlled at 0.01-0.03%.

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

[0051] Furthermore, the contents of C, Ti, Nb, and N satisfy the 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 relationship: 14[Ce]≥[Mn] / 60+[S] / 3, where [X] is the mass percentage content of the corresponding element X.

[0053] Specifically, the structural characteristics of the easy-to-weld rare earth steel are a mixed structure of tempered lath martensite, bainite and a second phase, wherein the second phase is composed of C, N, Nb and Ti. After quenching, a mixed structure of lath martensite / bainite is obtained. After high-temperature tempering, the lath martensite structure decomposes, the matrix softens, and part of the second phase precipitates to form a mixed structure of tempered martensite / bainite structure and the second phase, which provides a guarantee for improving the strength and toughness matching of the steel.

[0054] Specifically, the size of the second phase is 5 to 20 nm, mainly distributed in the lath martensite matrix; the effective grain size of the weldable rare earth steel is 2 to 3 μm. The high number density and fine second phase controls the microstructure while precipitation strengthening, further refines the austenite grain size, achieves fine grain strengthening of the alloy, or hinders dislocation movement to achieve dislocation strengthening.

[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 cross-sectional shrinkage rate≥70%, a -84°C Charpy impact V-type opening impact energy≥189J, and the surface, cross-section, and root crack rates of the oblique Y-type anti-crack test specimen are 0%.

[0056] The invention also discloses a preparation method of the easy-to-weld rare earth steel, and the process steps of the preparation method are: molten iron pretreatment, converter smelting, refining, vacuum degassing, continuous casting, finishing, heating, rolling and heat treatment.

[0057] Specifically, the molten iron pretreatment mainly serves the purpose of desulfurization. In the converter smelting process, oxygen is blown to dephosphorize and decarbonize to make it reach the range specified by the present invention. Then aluminum is used for deoxidation. The molten iron is then transferred to the ladle furnace for refining, and alloy materials are added at the same time to adjust the composition to the target composition, so that the composition content of the molten steel finally reaches the preset value of the formula.

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

[0059] Specifically, the overheat degree of the continuous casting process is ≤15°C and electromagnetic stirring and / or light pressing process is used to control the central segregation of the continuous casting billet.

[0060] Specifically, the main purpose of the finishing is to prepare for the subsequent rolling process, including: determining the size of the steel billet, inspecting steel billet defects, etc.

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

[0062] The starting temperature of the rough rolling section is 1100-1180°C, and the rough rolling stage is at least 2 passes of low-speed large deformation rolling: the deformation of a single pass is 20-25%, the rolling speed is ≤1.5m / s, and the end temperature of the rough rolling is ≥950°C. Controlling the heating temperature can control the growth of the original austenite grains. The casting structure can be more fully destroyed by rolling with a large reduction, and the defects in the casting and microstructure can be eliminated, so that the grains are more refined, thereby improving and enhancing the mechanical properties of the material;

[0063] The starting temperature of the finishing rolling section is 800-950°C, and the accumulated deformation of the finishing rolling section is ≥35%; accelerated cooling is performed after the finishing rolling, the cooling rate is 10-20°C / s, and the final cooling temperature is ≤600°C. The fine rolling is mainly for regulating the austenite phase transformation process. Starting rolling in the austenite non-recrystallization zone is conducive to controlling the final rolling, accumulating deformation, and ensuring the control of the final rolling temperature. Accelerated cooling after finishing rolling can obtain good fine grains, retain a higher dislocation density, and ensure the martensitic structure state in the subsequent heat treatment quenching process.

[0064] Specifically, the heating temperature of the heating link is 1120-1180°C; the specific process of the heat treatment link is that the steel plate is kept at 850-900°C for 0.5-2h, cooled to room temperature with water, and the tempering temperature is 500-650°C. The quenching and tempering heat treatment process is a process to obtain a match between strength and toughness. The martensitic structure obtained by quenching can well guarantee the strength requirements of the material. The tempering treatment can adjust the morphology of the martensitic lath and provide a driving force for the precipitation of the second phase, which precipitates on the tempered martensitic matrix. The precipitation of the second phase (C, N, Nb, Ti) prevents the growth of grains to a certain extent, ensures the refinement of grains, and increases the proportion of large-angle grain boundaries, which provides a guarantee for improving the strength and toughness matching of steel.

[0065] Examples and Comparative Examples

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

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

[0068]

[0069]

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

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

[0072] Hot metal pretreatment: Select raw materials, use high-quality scrap steel, and perform deep desulfurization on hot metal, so that the hot metal contains lower sulfur elements, and dephosphorization is performed during the pretreatment process. ;

[0073] Converter smelting and refining: using conventional processes in the prior art to make the refined molten steel reach the preset formula value (as shown in Table 1);

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

[0075] Continuous casting: superheat ≤ 15℃, electromagnetic stirring process and soft pressure reduction technology are used to improve the core segregation of the ingot; low-power rating of the continuous casting ingot is carried out, and the results are shown in Table 2:

[0076] Table 2 Low-power rating results of continuous casting slabs of the embodiments and comparative examples

[0077] General looseness (grade) Center looseness (grade) Ingot segregation (grade) Other low magnification defects 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 + rolling: the heating temperature is 1120-1180°C; the rolling process includes a rough rolling section and a finishing rolling section;

[0079] The starting temperature of the rough rolling section is 1100-1180°C, and the rough rolling stage is at least 2 passes of low-speed large deformation rolling: the deformation of a single pass is 20-25%, the rolling speed is ≤1.5m / s, and the end temperature of the rough rolling is ≥950°C;

[0080] The starting temperature of the finishing rolling section is 800-950°C, and the accumulated deformation of the finishing rolling section is ≥35%; the finishing rolling is followed by accelerated cooling, with a cooling rate of 10-20°C / s and a final cooling temperature of ≤600°C.

[0081] Table 3 Rolling process parameters of the embodiment and comparative example

[0082] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Rough rolling starting temperature / ℃ 1180 1180 1100 1200 1200 1200 Rough rolling and final rolling temperature / ℃ 1050 980 950 1100 1050 1150 Finish rolling starting temperature / ℃ 950 950 900 1000 980 1000 Finish 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 pass 4 3 3 2 2 2

[0083] Heat treatment: the steel plate is kept at 850-900°C for 50-80 min in both the embodiment and the comparative example, then water-cooled to room temperature, and the tempering temperature is 500-650°C.

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

[0085] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Quenching temperature / ℃ 860 860 860 860 860 950 Time in furnace / 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 / ℃ 560 540 600 600 500 450 Time in furnace / min 120 240 150 120 240 150

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

[0087] Table 5 Mechanical properties test results of examples and comparative examples

[0088]

[0089] The steel plate after heat treatment was welded by gas shielded welding. The steel plate was welded into an X-shaped groove. The welding was completed after 11 passes at the top and bottom. The shielding gas was 95% Ar + 5% CO2. The welding heat input was 18 kJ / cm2. The welding performance test results are shown in Table 6 (the impact notch position includes deposited metal (WM), fusion line (FL), outside the line + 1 mm (FL + 1 mm), outside the line + 2 mm (FL + 2 mm), and outside the line + 5 mm (FL + 5 mm)).

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

[0091]

[0092] It can be seen that the steel plate prepared according to the composition design and production process of the present invention can obtain a mixed structure of tempered martensite / bainite structure and the second phase, and the performance of the steel plate can take into account both strength and toughness and weldability, with a yield strength of more than 800 MPa, a tensile strength of more than 898 MPa, an elongation of more than 21%, a -84°C Charpy impact V-opening impact energy ≥189 J, and can achieve non-preheating welding at room temperature.

[0093] Compared with the embodiments, the comparative examples have certain differences in composition and process, resulting in lower strength of comparative example 1, which cannot reach the mechanical property level of the embodiments, and its carbon equivalent is high, the weldability is poor, and the impact performance of the welded joint is poor, which cannot meet the welding performance requirements under room temperature conditions; the strength of comparative examples 2 and 3 meets the requirements, but cannot reach the toughness level of the embodiments, and the weldability is poor, and room temperature welding cannot be achieved.

[0094] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A 785MPa grade easy-to-weld rare earth steel, characterized in that: The components of the easy-to-weld rare earth steel are calculated by mass percentage as follows: 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, and the rest are Fe and unavoidable impurity elements.

2. The easy-to-weld rare earth steel according to claim 1, characterized in that: The contents of C, Ti, Nb and N satisfy the relationship: 12[C]≤6[Ti]+40[Nb]+16[N], where [X] is the mass percentage content of the corresponding element X.

3. The easy-to-weld rare earth steel according to claim 1, characterized in that: The contents of Ce, Mn and S satisfy the relationship: 14[Ce]≥[Mn] / 60+[S] / 3, where [X] is the mass percentage content of the corresponding element X.

4. The easy-to-weld rare earth steel according to claim 1, characterized in that: The structural characteristics of the easy-to-weld rare earth steel are a mixed structure of tempered lath martensite, bainite and a second phase; the second phase is composed of C, N, Nb and Ti.

5. The easy-to-weld rare earth steel according to claim 4, characterized in that: The size of the second phase is 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.

6. The easy-to-weld rare earth steel according to claim 1, characterized in that: The tensile strength R of the steel plate made of the easy-to-weld rare earth steel is m ≥898MPa, yield strength Rp 0.2 ≥800MPa, elongation after fracture ≥21%, section shrinkage ≥70%, -84℃ Charpy impact V-type opening impact energy ≥189J, room temperature inclined Y-type crack resistance test specimen surface, cross section and root crack rate is 0%.

7. A method for preparing the easily weldable rare earth steel according to any one of claims 1 to 6, characterized in that: The process steps of the preparation method are: molten iron pretreatment, converter smelting, refining, vacuum degassing, continuous casting, finishing, heating, rolling and heat treatment.

8. The preparation method according to claim 7, characterized in that: The rolling process includes a rough rolling section and a finishing rolling section; The starting temperature of the rough rolling section is 1100-1180°C, and the rough rolling stage is at least 2 passes of low-speed large deformation rolling: the deformation of a single pass is 20-25%, the rolling speed is ≤1.5m / s, and the end temperature of the rough rolling is ≥950°C; The starting temperature of the finishing rolling section is 800-950°C, and the accumulated deformation of the finishing rolling section is ≥35%; the finishing rolling is followed by accelerated cooling, with a cooling rate of 10-20°C / s and a final cooling temperature of ≤600°C.

9. The preparation method according to claim 7, characterized in that: The overheat degree of the continuous casting process is ≤15°C and electromagnetic stirring and / or light pressing process is used to control the central segregation of the continuous casting billet.

10. The preparation method according to claim 7, characterized in that: The heating temperature of the heating link is 1120-1180°C; the specific process of the heat treatment link is that the steel plate is kept at 850-900°C for 0.5-2h, water-cooled to room temperature, and the tempering temperature is 500-650°C.

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