Preparation method for producing economical steel plate through four-stage rolling

Abstract

The invention relates to a preparation method for producing an economical steel plate through four-stage rolling. The preparation method comprises the steps that molten steel containing all elements is smelted in a vacuum furnace, and refining is completed; casting the smelted molten steel into a casting blank; the continuous casting billet is heated, the heating temperature ranges from 1200 DEG C to 1250 DEG C, the heating time ranges from 1 h to 3 h, and after heating, the continuous casting billet is discharged out of a furnace to remove oxide scale; the casting blank subjected to soaking is subjected to four-stage rolling to obtain a steel plate, the first-stage controlled rolling process is austenite recrystallization area large-deformation rolling, the second-stage controlled rolling process is austenite non-recrystallization area rolling, and the third-stage controlled rolling process is austenite critical area rolling; the fourth-stage controlled rolling process is austenite and ferrite two-phase region high-temperature small-deformation rolling; the steel plate obtained after four-stage rolling is cooled to 270-320 DEG C at the cooling speed of 15-20 DEG C / s, and then air cooling is conducted to the room temperature; the problems that in a conventional controlled rolling and controlled cooling process, although a heat treatment link can be omitted, it is difficult to synchronously optimize the strength and toughness and refine the structure under the low alloy cost are solved.

Description

Technical Field

[0001] This invention belongs to the field of economical structural steel technology, and relates to a method for preparing economical steel plates through a four-stage rolling process. Background Technology

[0002] With the rapid development of energy structure transformation and infrastructure construction, modern large-scale steel structure projects (such as long-span bridges, high-rise buildings, and cryogenic pressure vessels) have placed more stringent requirements on key structural materials. These materials must not only possess high strength (yield strength ≥ 550 MPa), high toughness (impact energy ≥ 175 J at -80℃), and low yield strength ratio (≤ 0.75) under extreme low-temperature conditions (such as -50℃ to -80℃), but also excellent weldability (no preheating required before welding) and good corrosion resistance. Simultaneously, they must have low production costs and simplified processes to meet the demands of large-scale manufacturing. However, existing technologies generally employ a high-alloying process (such as increasing Ni, Mo, V, and Nb content) combined with offline quenching and tempering heat treatment (quenching + tempering). While this achieves a certain balance between strength and toughness, it suffers from high alloy costs, long production cycles, high energy consumption, and uneven microstructure caused by heat treatment, making it difficult to meet the current sustainable development requirements for lightweight and economical structural steel. For example, although the patent with publication number CN118360545A achieves a strength of 580MPa, it relies on post-rolling quenching and tempering, and its low-temperature toughness only meets the requirement of -50℃; although the patent with publication number CN117778898A has high strength, its yield strength ratio is too high (>0.9), resulting in insufficient seismic performance; and although the patent with publication number CN118166285A has a simple process, its strength is too low (yield strength is only 205~245MPa), limiting its application range.

[0003] To overcome the aforementioned technical bottlenecks, there is an urgent need to develop a novel method for preparing high-performance steel plates that integrates compositional design with controlled rolling and cooling processes. Current economical steel plates mostly employ conventional controlled rolling and cooling processes (such as two-stage or three-stage rolling), which, while eliminating the need for heat treatment, struggle to simultaneously optimize strength, toughness, and microstructure refinement at low alloy costs. Especially at high strength levels (yield strength ≥ 600 MPa), traditional processes are prone to grain coarsening and uneven precipitate distribution, leading to decreased low-temperature toughness and weldability. Summary of the Invention

[0004] In view of this, in order to solve the problem that the conventional controlled rolling and cooling process can eliminate the heat treatment step, but it is difficult to simultaneously optimize strength, toughness and microstructure refinement under low alloy cost, and at high strength level (yield strength ≥600MPa), the traditional process is prone to grain coarsening and uneven distribution of precipitates. The present invention provides a four-stage rolling method for producing economical steel plates. This process can effectively control the phase transformation path and form a uniform and fine multiphase microstructure, thereby simultaneously improving strength, low-temperature toughness and corrosion resistance.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for preparing economical steel plates using a four-stage rolling process includes the following steps:

[0007] S1. Smelt the molten steel containing each element in a vacuum furnace and complete the refining and alloying.

[0008] S2. Cast the molten steel into a billet;

[0009] S3. Heat the continuous casting billet to a temperature of 1200~1250℃ for 1~3 hours. After heating, remove the continuous casting billet from the furnace and remove the iron oxide scale.

[0010] S4. The homogenized billet is rolled in four stages to obtain steel plates. The first stage rolling process is controlled as follows: initial rolling temperature ≥1100℃, final rolling temperature ≥970℃, rough rolling 3~5 passes, cumulative reduction ≥55%; the second stage rolling process is controlled as follows: initial rolling temperature ≥950℃, finish rolling 2~3 passes, final rolling temperature ≥840℃, cumulative reduction ≥40%; the third stage rolling process is controlled as follows: rolling temperature Ac3~Ar3, cumulative reduction ≥30%; the fourth stage rolling process is controlled as follows: cumulative reduction ≥20%, pass reduction 10~15%, final rolling temperature ≥760℃.

[0011] S5. Cool the steel plate after the fourth stage of rolling in step S4 to 270-320 ℃ at a cooling rate of 15~20 ℃ / s, and then air cool to room temperature.

[0012] Furthermore, in step S4, the first stage of the controlled rolling process is rolling with large deformation in the austenite recrystallization region, the second stage of the controlled rolling process is rolling in the austenite non-recrystallization region, the third stage of the controlled rolling process is rolling in the austenite critical region, and the fourth stage of the controlled rolling process is rolling with small deformation in the austenite + ferrite two-phase region at high temperature.

[0013] Further, in step S1, the elements in the molten steel are as follows by weight percentage: C: 0.06~0.08%, Si: 0.25~0.40%, Mn: 1.45~1.55%, Cu: 0.20~0.40%, Ni: 0.70~0.90%, Ti: 0.02~0.05%, V: 0.04~0.06%, Nb: 0.01~0.03%, Als: 0.020~0.045%, N: 0~0.004%, where 0.9≤Cu+Ni≤1.3, (Nb+Ti+V+Als) / N≥17.5, 5≤Si / Als≤9; the balance is Fe and unavoidable impurities.

[0014] Furthermore, the metallographic structure of the steel plate prepared in step S5 consists of 60-65% fine polygonal ferrite and 35-40% bainite, with fine nano-phases present.

[0015] Furthermore, the mechanical properties of the steel plate prepared in step S5 are as follows: yield strength R p0.2 ≥ 570 MPa, tensile strength R m ≥ 760 MPa, yield strength ratio R p0.2 / R m ≤ 0.75, elongation after fracture A ≥ 20%, impact energy at -80 ℃ KV2 ≥ 175 J.

[0016] Further, in step S1, the elements in the molten steel are as follows by weight percentage: C: 0.06~0.08%, Si: 0.25~0.40%, Mn: 1.45~1.55%, Cu: 0.20~0.40%, Cr: 0.15~0.35%, Ni: 0.70~0.90%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, Als: 0.020~0.045%, N: 0~0.004%, where 1.10≤Cu+Cr+Ni≤1.50, (Nb+Ti+Als) / N≥14, 5≤Si / Al≤9; the balance is Fe and unavoidable impurities.

[0017] Furthermore, the metallographic structure of the steel plate prepared in step S5 is 22-28% fine and uniform ferrite and 72-78% lath bainite.

[0018] Furthermore, the mechanical properties of the steel plate prepared in step S5 are as follows: yield strength R p0.2 ≥ 600 MPa, tensile strength R m ≥ 850 MPa, yield strength ratio R p0.2 / R m ≤ 0.75, elongation after fracture A ≥ 20%, impact energy at -80 ℃ KV2 ≥ 175 J.

[0019] Furthermore, in step S4, Ac3 is 830 ℃ and Ar3 is 780 ℃.

[0020] Furthermore, the carbon equivalent (Ceq), weld crack sensitivity index (Pcm), and corrosion resistance index (I) of the steel plate prepared in step S5 satisfy the following relationships: Ceq = C + A(C)[Si / 24 + Mn / 6 + Cu / 15 + Ni / 20 + (Cr + Mo + Nb + V) / 5 + 5B] ≤ 0.3%, where A(C) = 0.75 + 0.25tanh[20(C - 0.12)]; Pcm = C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≤ 0.2%; I = 26.01Cu + 3.88Ni + 1.20Cr + 1.49Si + 17.28P - 7.29Cu×Ni - 9.10Ni×P - 33.39Cu 2 ≥6.4%.

[0021] The beneficial effects of this invention are as follows:

[0022] 1. The four-stage rolling method for producing economical steel plates disclosed in this invention innovatively introduces a four-stage synergistic controlled rolling process: "large deformation rolling in the austenite recrystallization zone + rolling in the non-recrystallization zone + rolling in the critical zone + small deformation rolling in the two-phase zone." Combined with intermittent cooling technology, this method can achieve ultra-fine grains, dispersed precipitate distribution, and precise control of microstructure ratios without completely eliminating post-rolling heat treatment. This process can effectively regulate the phase transformation path to achieve microstructure refinement, multiphase microstructure design, and nanophase precipitation control, resolving the contradiction of reconciling high strength, high toughness, and low yield strength ratio. The austenite microstructure is homogenized and refined through large deformation rolling in the austenite recrystallization zone. Then, combined with rolling in the non-recrystallization zone, a large number of nucleation sites are introduced, the microstructure is refined, energy is stored, and the driving force of phase transformation is increased, providing energy preparation for the proeutectoid ferrite phase transformation at higher temperatures. Subsequently, the phase transformation is promoted by rolling deformation in the critical zone to produce fine ferrite, and the ferrite is directly broken up and the austenite is segmented. Combined with the redistribution of carbon, the dispersion and distribution of nanophases are promoted, achieving the ultimate refinement of the microstructure and the design of multiphase microstructure, providing microstructure guarantee for the high strength, high toughness, and high uniformity of the steel plate. Finally, the strength, plasticity, and toughness are finely adjusted and the shape and surface quality of the steel plate are optimized through small deformation rolling in the two-phase zone.

[0023] 2. The four-stage rolling method for producing economical steel plates disclosed in this invention combines four-stage rolling with rapid cooling to ultimately form a uniform and fine ferrite + bainite multiphase structure, thereby simultaneously improving strength, low-temperature toughness, and corrosion resistance, and achieving a low yield strength ratio. This four-stage rolling strategy not only resolves the contradiction between high strength, high toughness, and low yield strength ratio, but also significantly improves weldability by reducing carbon equivalent (Ceq≤0.3%) and crack sensitivity index (Pcm≤0.2), ultimately providing a necessary technical path for low-cost, short-process production of high-performance economical steel plates.

[0024] 3. The high-strength, tough, easily weldable, low-temperature pressure vessel steel plate produced by the four-stage rolling process disclosed in this invention, by eliminating the post-rolling heat treatment process, adopts a composition design system of Cu+Ni alloying and Ti+Nb+V micro-alloying, combined with a comprehensive control technology of a four-stage controlled rolling process and intermittent cooling process, which integrates rolling in the austenite recrystallization zone + rolling in the austenite non-recrystallization zone + rolling in the austenite critical zone + rolling in the (austenite + ferrite) two-phase zone, to obtain a structural steel plate with good comprehensive mechanical properties: yield strength R p0.2 ≥ 570 MPa, tensile strength R m ≥ 760 MPa, yield strength ratio R p0.2 / R m With a thickness of ≤0.75 mm, elongation after fracture A ≥ 20%, and impact energy at -80 ℃ KV2 ≥ 175 J, high-performance cryogenic pressure vessel steel plates are obtained. The entire process does not require post-rolling heat treatment, has a short production flow, simple and easy-to-control production process, low production cost, is suitable for large-scale production, and can be widely used in the construction of large cryogenic pressure vessels.

[0025] 4. The low-temperature resistant, high-performance, earthquake-resistant, and corrosion-resistant structural steel plate disclosed in this invention, produced based on a four-stage rolling process, eliminates the post-rolling heat treatment process. It employs a composition design system of Cu+Cr+Ni alloying and Ti+Nb micro-alloying, combined with a comprehensive control technology of a four-stage controlled rolling process (rolling in the austenite recrystallization zone + rolling in the austenite non-recrystallization zone + rolling in the austenite critical zone + rolling in the (austenite + ferrite) two-phase zone) and intermittent cooling process, resulting in a structural steel plate with excellent comprehensive mechanical properties: yield strength R... p0.2 ≥ 600 MPa, tensile strength R m ≥ 850 MPa, yield strength ratio R p0.2 / R m With a tensile strength ≤0.75, elongation after fracture A ≥ 20%, and impact energy KV2 ≥ 175 J at -80 ℃, this process yields high-performance, earthquake-resistant, and corrosion-resistant structural steel plates that are resistant to low temperatures. Furthermore, the entire process requires no post-rolling heat treatment, has a short production flow, is simple and easy to control, and has low production costs. It is suitable for large-scale production and can be widely used in the construction of large steel structures with strict requirements for seismic safety performance, such as long-span heavy-load bridges and high-rise buildings.

[0026] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0028] Figure 1 This is a matrix phase diagram of the high-strength, tough, easy-to-weld low-temperature pressure vessel steel plate produced by the present invention based on a four-stage rolling process.

[0029] Figure 2 This invention provides a phase diagram of precipitated phases in the production of high-strength, tough, easily weldable low-temperature pressure vessel steel plates based on a four-stage rolling process.

[0030] Figure 3 This is a metallographic diagram of the high-strength, tough, and easily weldable low-temperature pressure vessel steel plate produced by the present invention based on a four-stage rolling process.

[0031] Figure 4 This is an engineering stress-strain curve diagram of high-strength, tough, easy-to-weld low-temperature pressure vessel steel plates produced by the four-stage rolling process of this invention.

[0032] Figure 5 This is an SEM image of the impact fracture surface of high-strength, tough, easy-to-weld low-temperature pressure vessel steel plate produced by the four-stage rolling process of the present invention.

[0033] Figure 6 The image shows the metallographic structure of the low-temperature resistant, high-performance, earthquake-resistant, and corrosion-resistant structural steel plate produced by the four-stage rolling process of this invention.

[0034] Figure 7 This is an SEM image of the impact fracture surface of the low-temperature resistant, high-performance, shock-resistant, and corrosion-resistant structural steel plate produced by the four-stage rolling process of this invention.

[0035] Figure 8 This is an engineering stress-strain curve diagram of the low-temperature resistant, high-performance, earthquake-resistant, and corrosion-resistant structural steel plate produced by the present invention based on a four-stage rolling process. Detailed Implementation

[0036] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0037] A method for producing high-strength, tough, and easily weldable low-temperature pressure vessel steel plates based on a four-stage rolling process includes the following steps:

[0038] S1. Smelting molten steel containing various elements in a vacuum furnace and completing refining and alloying; specifically:

[0039] Referring to the chemical composition of the steel in the various embodiments and comparative examples shown in Table 1 below, 150 kg of molten steel with the chemical composition shown in Table 1 was smelted in a vacuum induction furnace and then refined and cast into steel ingots.

[0040] Table 1 shows the smelting chemical composition (mass percentage, %) of the steel plates in the embodiments of the present invention and the comparative steel plates.

[0041]

[0042] 0.9≤Cu+Ni≤1.3, Cu and Ni elements mainly exist in solid solution in austenite, which ensures the corrosion resistance and low-temperature toughness of the steel while reducing the A3 temperature.

[0043] like Figure 1 and Figure 2 As shown, (Nb+Ti+V+Als) / N≥17.5 ensures the formation of fine Ti(C, N) and Ti, Nb, V)C particles in the austenite matrix, inhibiting austenite grain growth, significantly refining austenite grains, lowering Ar3 and Ac3 temperatures, and utilizing the austenite critical region rolling and (austenite + ferrite) two-phase region rolling strain to induce VC to disperse and precipitate in the ferrite matrix, further refining ferrite grains and improving strength and low-temperature toughness.

[0044] 5≤Si / Al≤9. If Si / Al is too low, Al combines with N to form AlN during continuous casting, which precipitates along the grain boundaries and causes surface cracks in the billet. If Si / Al is too high, it is easy to cause a high residual oxygen content in the steel, forming pores, inclusions or loose defects.

[0045] Carbon (C) has a significant solid solution strengthening effect and can improve the hardenability of steel. However, too low a C content in steel is not conducive to strength improvement, while too high a C content is detrimental to the low-temperature toughness and weldability of steel. Therefore, the C content in this invention is controlled at 0.06~0.08%.

[0046] Si: One of the deoxidizing elements in steel. At the same time, silicon is also a non-carbide forming element. It exists in the steel matrix in solid solution form and has a certain solid solution strengthening effect. However, excessive silicon is not good for the low-temperature toughness of steel and increases the sensitivity to welding cold cracks and hot cracks. The Si content in this invention is controlled at 0.25~0.40%.

[0047] Mn (Mn) is a key element for improving strength and toughness. It significantly enhances the hardenability of steel, expands the austenite phase region, improves strength through microstructure refinement and promotes bainite transformation, and also has a certain solid solution strengthening effect. However, excessively high Mn content reduces the corrosion resistance of steel, exacerbates segregation and porosity in the center of the cast billet, and severely affects the low-temperature toughness and crack sensitivity of the weld heat-affected zone. In this invention, the Mn content is controlled at 1.45~1.55%.

[0048] Cu primarily functions as a solid solution and precipitation strengthener in steel, and also improves its corrosion resistance. During cooling, Cu precipitates fine, copper-rich clusters, significantly enhancing the steel's strength and optimizing the strength-toughness balance. However, excessively high Cu content can lead to Cu segregation at grain boundaries, causing hot brittleness and deteriorating the steel's low-temperature toughness. In this invention, the Cu content is controlled at 0.20–0.40%.

[0049] Ni primarily enhances the strength of steel through solid solution strengthening and improves its hardenability. Furthermore, Ni effectively improves the low-temperature toughness and corrosion resistance of steel. However, excessively high Ni content in steel can lead to reduced weld strength. Therefore, the Ni content should be controlled within the range of 0.70% to 0.90%.

[0050] Ti: Trace amounts of Ti can form fine TiN and Ti(C, N) inclusions, which can effectively suppress the coarsening of the original austenite grains in the pre-rolling homogenization and weld heat-affected zone, improve low-temperature toughness, and improve the weldability of steel. Too little titanium is not conducive to its aforementioned effects, while too much titanium easily forms large-sized TiN inclusions, reducing low-temperature toughness. Therefore, the Ti content range is 0.02~0.05%.

[0051] V (V): Primarily strengthens steel by forming fine V(C, N) precipitates, significantly enhancing its precipitation hardening effect while simultaneously refining grains by pinning austenite grain boundaries, thus improving the steel's strength and low-temperature toughness. Furthermore, V(C, N) can redissolve and precipitate in the weld heat-affected zone (HAZ), helping to refine the coarse-grained structure of the HAZ and improve its toughness. However, excessive V content leads to high V(C, N) content and coarsening, which deteriorates the steel's low-temperature toughness and weldability. Therefore, the V content range is 0.04%–0.06%.

[0052] Nitrogen (Nb): Primarily enhances the strength and toughness of steel through grain refinement and precipitation strengthening. Nb dissolved in austenite refines the recrystallized grains of austenite through the solute dragging effect and significantly improves hardenability, thus effectively refining the microstructure after phase transformation. Furthermore, Nb effectively increases the strength of steel by precipitating fine Nb(C,N) particles in the matrix. However, excessively high Nb content increases the cost of the steel. Therefore, the Nb content is controlled within the range of 0.01% to 0.03%.

[0053] Al is the main deoxidizing element in steel, and trace amounts of Al can reduce inclusion content. Furthermore, Al can refine austenite grains and improve the strength-to-yield ratio of steel. Additionally, Al combines with nitrogen to form AlN, reducing dislocation pinning and improving impact toughness. However, excessively high Al content can deteriorate the hot deformation properties of steel, making it prone to cracking during hot rolling. Therefore, the Al content in this invention is controlled at 0.02~0.045%.

[0054] P and S: Impurity elements in steel that significantly reduce ductility, toughness, and weldability, preventing surface cracking of billets and rolled products caused by copper grain boundary segregation and CuS (or Cu2S) precipitation. Therefore, in this invention, the contents of phosphorus and sulfur are controlled within the ranges of P < 0.015% and S < 0.005%, respectively.

[0055] Nitrogen (N): An important austenite-forming element in steel, it can form stable nitrides with elements such as Ti, V, and Nb, improving the strength and toughness of steel through grain refinement and precipitation strengthening. However, excessive residual N content in steel can lead to a loose macrostructure or the formation of pores, reducing toughness and weldability. Therefore, the N content in this invention is ≤0.004%.

[0056] S2. Cast the molten steel into a billet;

[0057] S3. The continuously cast billet is hot-forged into a steel billet with a cross section of 150 mm × 150 mm. After hot forging, the steel billet is taken out of the furnace and the iron oxide scale is removed. The heating process parameters of the steel bars in each embodiment and the comparative embodiment are shown in Table 2 below.

[0058] Table 2: Heating process parameters of steel billets in the embodiments of the present invention and the comparative example steel billets

[0059]

[0060] S4. The homogenized billet is rolled in four stages to obtain steel plate. The rolling process parameters of the billets in each embodiment and the comparative example are shown in Table 3 and Table 4 below.

[0061] Table 3: First and second stage rolling process parameters of the steel in the embodiments and comparative examples of the present invention

[0062]

[0063] Table 4: Third and fourth stage rolling and cooling process parameters of the steels in the embodiments and comparative examples of the present invention

[0064]

[0065] S5. Cool the steel plate after the four-stage rolling in step S4. The cooling methods for the steel plates in each embodiment and the comparative example are as follows: Example 1: Cooled to 277°C at 15°C / s, then air-cooled to room temperature; Example 2: Cooled to 291°C at 17°C / s, then air-cooled to room temperature; Example 3: Cooled to 317°C at 18°C / s, then air-cooled to room temperature; Comparative Example 1: Cooled to 288°C at 19°C / s, then air-cooled to room temperature; Comparative Example 2: Cooled to 293°C at 16°C / s, then air-cooled to room temperature; Comparative Example 3: Cooled to 279°C at 15°C / s, then air-cooled to room temperature; Comparative Example 4: Cooled to 285°C at 17°C / s, then air-cooled to room temperature; Comparative Example 5: Water quenched to room temperature; Comparative Example 6: Air-cooled to room temperature.

[0066] The carbon equivalent and welding cold cracking sensitivity index of the rolled steel plate products of each embodiment and comparative example are shown in Table 5.

[0067] Table 5: Carbon equivalent (C) of the steel in the embodiments of the present invention and the comparative steel eq ) and welding cold cracking susceptibility index (P cm )

[0068]

[0069] The mechanical properties of the rolled structural steel plates of each embodiment and comparative example are shown in Table 6.

[0070] Table 6: Mechanical properties of the steel in the embodiments of the present invention and the comparative steel

[0071]

[0072] The carbon equivalent of Examples 1, 2, and 3 is 0.23~0.27, the welding cold cracking sensitivity index is 0.17~0.20, the yield strength is 578~587 MPa, the tensile strength is 781~788 MPa, the elongation is higher than 20%, the yield strength ratio is not higher than 0.74, and the average impact energy at -80 ℃ is 177~185 J. The metallographic structure of the rolled steel plate products of the examples is as follows. Figure 3 As shown, its microstructure mainly consists of fine, proportionally and rationally distributed polygonal ferrite and bainite, and the impact fracture morphology is as follows. Figure 5As shown, the numerous equiaxed dimples indicate good low-temperature toughness. The microalloying approach and optimized chemical composition design proposed in this patent, combined with a four-stage controlled rolling process (rolling in the austenite recrystallization zone + rolling in the austenite non-recrystallization zone + rolling in the austenite critical zone + rolling in the (austenite + ferrite) two-phase zone) and an intermittent cooling controlled rolling and cooling process, can obtain low-cost, high-strength, low-yield-strength ratio, low-temperature resistant, and easily weldable structural steel. However, while Comparative Examples 1-3 employed reasonable controlled rolling and cooling processes, Comparative Example 1 exhibited poor weldability and seismic resistance due to its excessively high carbon content; Comparative Example 2 had low levels of corrosion-resistant elements Cu and Ni, resulting in poor low-temperature toughness; and Comparative Example 3 lacked Ti+Nb+V microalloying, leading to lower strength and poorer low-temperature toughness. Comparative Examples 4 to 6 all employed reasonable composition designs. However, Comparative Example 4 did not undergo austenite critical zone rolling or high-temperature, low-deformation rolling in the austenite + ferrite two-phase region, resulting in coarse microstructure, an unreasonable ratio and distribution of ferrite and bainite, and failure of VC particles to precipitate effectively. Comparative Example 5 had an excessively high cooling rate, resulting in a large amount of martensite in its microstructure. Comparative Example 6 had an excessively low cooling rate, leading to coarsening of grains and precipitated phases. Consequently, the overall mechanical properties of Comparative Examples 4 to 6 were poor. Figure 4 The stress-strain curve of the steel plate manufactured in Example 1 shows its yield strength R. p0.2 ≥ 570 MPa, tensile strength R m ≥ 760 MPa, yield strength ratio R p0.2 / R m The result is ≤ 0.75, and the elongation after fracture A ≥ 20%, which proves its excellent room temperature mechanical properties.

[0073] A method for producing low-temperature resistant, high-performance, earthquake-resistant, and corrosion-resistant structural steel plates based on a four-stage rolling process includes the following steps:

[0074] S1. Smelting molten steel containing various elements in a vacuum furnace and completing refining and alloying; specifically:

[0075] Referring to the chemical composition of the steel in the various embodiments and comparative examples shown in Table 7 below, 150 kg of molten steel with the chemical composition shown in Table 7 was smelted in a vacuum induction furnace, and then refined and cast into steel ingots.

[0076] Table 7 shows the smelting chemical composition (mass percentage, %) of the steel plates in the embodiments of the present invention and the comparative steel plates.

[0077]

[0078] 1.10≤Cu+Cr+Ni≤1.50, ensuring a reasonable amount of corrosion-resistant alloying elements to guarantee corrosion resistance and low-temperature toughness.

[0079] (Nb+Ti+Als) / N≥14 ensures the formation of fine TiN, NbC or composite precipitate particles, inhibits austenite grain growth, refines grains, expands the non-recrystallized austenite region, and improves strength and low-temperature toughness.

[0080] 5≤Si / Al≤9. If Si / Al is too low, Al combines with N to form AlN during continuous casting, which precipitates along the grain boundaries and causes surface cracks in the billet. If Si / Al is too high, it is easy to cause a high residual oxygen content in the steel, forming pores, inclusions or loose defects.

[0081] Carbon (C) has a significant solid solution strengthening effect and can improve the hardenability of steel. However, too low a C content in steel is not conducive to strength improvement, while too high a C content is very detrimental to the low-temperature toughness and weldability of steel. Therefore, the C content in this invention is controlled at 0.06~0.08%.

[0082] Si: One of the deoxidizing elements in steel. At the same time, silicon is also a non-carbide forming element. It exists in the steel matrix in solid solution form and has a certain solid solution strengthening effect. However, excessive silicon is not good for the low-temperature toughness of steel and increases the sensitivity to welding cold cracks and hot cracks. The Si content in this invention is controlled at 0.25~0.40%.

[0083] Mn (Mn) is a key element for improving strength and toughness. It significantly enhances the hardenability of steel, expands the austenite phase region, improves strength through microstructure refinement and promotes bainite transformation, and also has a certain solid solution strengthening effect. However, excessively high Mn content reduces the corrosion resistance of steel, exacerbates segregation and porosity in the center of the cast billet, and severely affects the low-temperature toughness and crack sensitivity of the weld heat-affected zone. In this invention, the Mn content is controlled at 1.45~1.55%.

[0084] Cu primarily functions as a solid solution and precipitation strengthener in steel, and also improves its corrosion resistance. During cooling, Cu precipitates fine, copper-rich clusters, significantly enhancing the steel's strength and optimizing the strength-toughness balance. However, excessively high Cu content can lead to Cu segregation at grain boundaries, causing hot brittleness and deteriorating the steel's low-temperature toughness. In this invention, the Cu content is controlled at 0.20–0.40%.

[0085] Cr: Solid solution Cr can significantly improve the hardenability of steel, refine ferrite grains significantly by lowering the phase transformation initiation temperature, and at the same time, Cr easily forms a dense oxide film, which can significantly improve the corrosion resistance of steel. However, excessive Cr content will form coarse Cr23C6 carbides at the grain boundaries, significantly deteriorating the toughness of the steel. In this invention, the Cr content is controlled at 0.15~0.35%.

[0086] Ni primarily enhances the strength of steel through solid solution strengthening and improves its hardenability. Furthermore, Ni effectively improves the low-temperature toughness and corrosion resistance of steel. However, excessively high Ni content in steel can lead to reduced weld strength. Therefore, the Ni content should be controlled within the range of 0.70% to 0.90%.

[0087] Ti: Trace amounts of Ti can form fine TiN and Ti(C, N) inclusions, which can effectively suppress the coarsening of the original austenite grains in the pre-rolling homogenization and weld heat-affected zone, improve low-temperature toughness, and improve the weldability of the steel. Too little titanium is not conducive to its aforementioned effects, while too much titanium easily forms large-sized TiN inclusions, reducing low-temperature toughness. Therefore, the titanium content range is 0.02~0.05%.

[0088] Nitrogen (Nb): Primarily enhances the strength and toughness of steel through grain refinement and precipitation strengthening. Nb dissolved in austenite refines the recrystallized grains of austenite through the solute dragging effect and significantly improves hardenability, thus effectively refining the microstructure after phase transformation. Furthermore, Nb effectively increases the strength of steel by precipitating fine Nb(C,N) particles in the matrix. However, excessively high Nb content increases the cost of the steel. Therefore, the Nb content is controlled within the range of 0.01% to 0.03%.

[0089] Al is the main deoxidizing element in steel, and trace amounts of Al can reduce inclusion content. Furthermore, Al can refine austenite grains and improve the strength-to-yield ratio of steel. Additionally, Al combines with nitrogen to form AlN, reducing dislocation pinning and improving impact toughness. However, excessively high Al content can deteriorate the hot deformation properties of steel, making it prone to cracking during hot rolling. Therefore, the Al content in this invention is controlled at 0.02~0.045%.

[0090] P and S: Impurity elements in steel that significantly reduce ductility, toughness, and weldability, preventing surface cracking of billets and rolled products caused by copper grain boundary segregation and CuS (or Cu2S) precipitation. Therefore, in this invention, the contents of phosphorus and sulfur are controlled within the ranges of P < 0.015% and S < 0.005%, respectively.

[0091] Nitrogen (N): An important austenite-forming element in steel, it can form stable nitrides with elements such as Nb and Ti, improving the corrosion resistance of steel. However, excessive residual N content in steel can lead to a loose macrostructure or the formation of pores, reducing toughness and weldability. Therefore, the N content in this invention is ≤0.004%.

[0092] S2. Cast the molten steel into a billet;

[0093] S3. The continuously cast billet is hot-forged into a steel billet with a cross section of 150 mm × 150 mm. After hot forging, the steel billet is taken out of the furnace and the iron oxide scale is removed. The heating process parameters of the steel bars in each embodiment and the comparative embodiment are shown in Table 8 below.

[0094] Table 8: Heating process parameters of steel billets in the embodiments of the present invention and the comparative example steel billets

[0095]

[0096] S4. The homogenized billet is rolled in four stages to obtain steel plate. The rolling process parameters of the billets in each embodiment and the comparative example are shown in Table 9 and Table 10 below.

[0097] Table 9: First and Second Stage Rolling Process Parameters of the Embodiments and Comparative Steels of the Invention

[0098]

[0099] Table 10: Third and fourth stage rolling and cooling process parameters of the steels in the embodiments and comparative examples of the present invention

[0100]

[0101] S5. Cool the steel plate after the four-stage rolling in step S4. The cooling methods for the steel plates in each embodiment and the comparative example are as follows: Example 4 is cooled to 278°C at 15°C / s and then air-cooled to room temperature; Example 4 is cooled to 294°C at 18°C / s and then air-cooled to room temperature; Example 6 is subsequently cooled to 317°C at 20°C / s and then air-cooled to room temperature; Comparative Example 7 is cooled to 271°C at 16°C / s and then air-cooled to room temperature; Comparative Example 8 is cooled to 315°C at 19°C / s and then air-cooled to room temperature; Comparative Example 9 is cooled to 299°C at 18°C / s and then air-cooled to room temperature; Comparative Example 10 is cooled to 303°C at 18°C / s and then air-cooled to room temperature; Comparative Example 11 is water-cooled to room temperature; and Comparative Example 12 is air-cooled to room temperature.

[0102] The carbon equivalent, welding cold cracking sensitivity index, and weathering index of the structural steel plate products rolled in each embodiment and comparative example are shown in Table 11, and the mechanical properties are shown in Table 12.

[0103] Table 11 shows the carbon equivalent (C) of the steel in the invention embodiments and the comparative steel. eq ), Welding cold cracking sensitivity index (P) cm ) and weather resistance index (I)

[0104]

[0105] Table 12 shows the mechanical properties of the steel in the invention embodiment and the comparative steel.

[0106]

[0107] The carbon equivalent of Examples 4, 5, and 6 is 0.25~0.29, the welding cold crack sensitivity index is 0.17~0.20, the weather resistance index is 6.4~7.1, the yield strength is 601~611 MPa, the tensile strength is 855~877 MPa, the elongation is higher than 23%, the yield strength ratio is not higher than 0.7, and the average impact energy at -80 ℃ is 177~201 J. This shows that the microalloying concept and optimized chemical composition design proposed in this patent, the four-stage controlled rolling process of rolling in the austenite recrystallization zone + rolling in the austenite non-recrystallization zone + rolling in the austenite critical zone + rolling in the (austenite + ferrite) two-phase zone combined with the controlled rolling and controlled cooling process of intermittent cooling, can obtain structural steel with low cost, high strength, low yield strength ratio, low temperature resistance, high corrosion resistance, and easy welding. However, Comparative Examples 7-9 and Comparative Example 8 employed reasonable controlled rolling and cooling processes. However, Comparative Example 7 had excessively high carbon content, resulting in poor welding and seismic resistance. Comparative Example 8 had low levels of corrosion-resistant elements Cu, Cr, and Ni, leading to low strength and poor corrosion resistance. Comparative Example 9 did not use Ti+Nb microalloying, resulting in low strength. Comparative Examples 10-12 all employed reasonable composition designs. However, Comparative Example 10 had excessively high initial rolling temperatures in the austenite critical zone rolling and (austenite + ferrite) two-phase zone rolling stages, resulting in unreasonable proportions, distribution, and sizes of ferrite and bainite. Comparative Example 11 had excessively high cooling rates, resulting in a large amount of martensite in its microstructure. Comparative Example 12 had excessively low cooling rates, leading to coarsening of grains and precipitates, resulting in poor overall mechanical properties for Comparative Examples 10-12.

[0108] Figure 6 The image shows the metallographic structure of the steel plate manufactured in Example 4. The structure of the steel plate is mainly composed of fine and uniform ferrite and lower bainite, which is the fundamental reason for its good room temperature mechanical properties and low temperature toughness. Figure 7 The image shows the impact fracture surface SEM image of the steel plate manufactured in Example 4. The impact fracture surface has large and deep dimples, indicating that it mainly undergoes ductile fracture and has good low-temperature toughness. Figure 8 The stress-strain curve of the steel plate manufactured in Example 4 is shown, and its yield strength R is also shown. p0.2 ≥ 570 MPa, tensile strength R m ≥ 760 MPa, yield strength ratio R p0.2 / R m The result is ≤ 0.75, and the elongation after fracture A ≥ 20%, which proves its excellent room temperature mechanical properties.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing economical steel plates using a four-stage rolling process, characterized in that, Includes the following steps: S1. Smelt the molten steel containing each element in a vacuum furnace and complete the refining and alloying. S2. Cast the molten steel into a billet; S3. Heat the continuous casting billet to a temperature of 1200~1250℃ for 1~3 hours. After heating, remove the continuous casting billet from the furnace and remove the iron oxide scale. S4. The homogenized billet is rolled in four stages to obtain steel plate. The first stage rolling process is controlled as follows: initial rolling temperature ≥1100℃, final rolling temperature ≥970℃, rough rolling 3~5 passes, cumulative reduction ≥55%; The second stage of controlled rolling is as follows: initial rolling temperature ≥ 950℃, finishing rolling 2~3 passes, final rolling temperature ≥ 840℃, and cumulative reduction rate ≥ 40%; the third stage of controlled rolling is as follows: rolling temperature Ac3~Ar3, and cumulative reduction rate ≥ 30%; the fourth stage of controlled rolling is as follows: cumulative reduction rate ≥ 20%, pass reduction rate 10~15%, and final rolling temperature ≥ 760℃; S5. Cool the steel plate after the fourth stage of rolling in step S4 to 270-320 ℃ at a cooling rate of 15~20 ℃ / s, and then air cool to room temperature.

2. The method for preparing economical steel plates using a four-stage rolling process as described in claim 1, characterized in that, In step S4, the first stage of the controlled rolling process is rolling with large deformation in the austenite recrystallization region, the second stage of the controlled rolling process is rolling in the austenite non-recrystallization region, the third stage of the controlled rolling process is rolling in the austenite critical region, and the fourth stage of the controlled rolling process is rolling with small deformation in the austenite + ferrite two-phase region at high temperature.

3. The method for preparing economical steel plates using a four-stage rolling process as described in claim 2, characterized in that, In step S1, the elements in the molten steel are as follows by weight percentage: C: 0.06~0.08%, Si: 0.25~0.40%, Mn: 1.45~1.55%, Cu: 0.20~0.40%, Ni: 0.70~0.90%, Ti: 0.02~0.05%, V: 0.04~0.06%, Nb: 0.01~0.03%, Als: 0.020~0.045%, N: 0~0.004%, where 0.9≤Cu+Ni≤1.3, (Nb+Ti+V+Als) / N≥17.5, 5≤Si / Als≤9; the balance is Fe and unavoidable impurities.

4. The method for preparing economical steel plates using a four-stage rolling process as described in claim 3, characterized in that, The metallographic structure of the steel plate prepared in step S5 is 60-65% fine polygonal ferrite and 35-40% bainite.

5. The method for preparing economical steel plates using a four-stage rolling process as described in claim 4, characterized in that, The mechanical properties of the steel plate prepared in step S5 are: yield strength R p0.2 ≥ 570 MPa, tensile strength R m ≥ 760 MPa, yield strength ratio R p0.2 / R m ≤ 0.75, elongation after fracture A ≥ 20%, impact energy at -80 ℃ KV2 ≥ 175 J.

6. The method for preparing economical steel plates using a four-stage rolling process as described in claim 2, characterized in that, In step S1, the elements in the molten steel are as follows by weight percentage: C: 0.06~0.08%, Si: 0.25~0.40%, Mn: 1.45~1.55%, Cu: 0.20~0.40%, Cr: 0.15~0.35%, Ni: 0.70~0.90%, Ti: 0.02~0.05%, Nb: 0.01~0.03%, Als: 0.020~0.045%, N: 0~0.004%, where 1.10≤Cu+Cr+Ni≤1.50, (Nb+Ti+Als) / N≥14, 5≤Si / Al≤9; the balance is Fe and unavoidable impurities.

7. The method for preparing economical steel plates using a four-stage rolling process as described in claim 6, characterized in that, The metallographic structure of the steel plate prepared in step S5 is 22-28% fine and uniform ferrite and 72-78% lath bainite.

8. The method for preparing economical steel plates using a four-stage rolling process as described in claim 7, characterized in that, The mechanical properties of the steel plate prepared in step S5 are: yield strength R p0.2 ≥ 600 MPa, tensile strength R m ≥ 850 MPa, yield strength ratio R p0.2 / R m ≤ 0.75, elongation after fracture A ≥ 20%, impact energy at -80 ℃ KV2 ≥ 175 J.

9. The method for preparing economical steel plates using a four-stage rolling process as described in claim 1, characterized in that, In step S4, Ac3 is 830 ℃ and Ar3 is 780 ℃.

10. The method for preparing economical steel plates using a four-stage rolling process as described in claim 1, characterized in that, The carbon equivalent (Ceq), weld crack sensitivity index (Pcm), and corrosion resistance index (I) of the steel plate prepared in step S5 satisfy the following relationships: Ceq = C + A(C)[Si / 24 + Mn / 6 + Cu / 15 + Ni / 20 + (Cr + Mo + Nb + V) / 5 + 5B] ≤ 0.3%, where A(C) = 0.75 + 0.25tanh[20(C - 0.12)]; Pcm = C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≤ 0.2%; I = 26.01Cu + 3.88Ni + 1.20Cr + 1.49Si + 17.28P - 7.29Cu×Ni - 9.10Ni×P - 33.39Cu 2 ≥6.4%.

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