900mpa grade ultra-low carbon medium-manganese high-strength hot-rolled steel sheet and method of manufacturing the same

Abstract

The application discloses a 900MPa-grade ultra-low-carbon medium-manganese high-strength hot-rolled steel plate and a manufacturing method thereof.The chemical composition of the steel plate comprises the following components by weight percentage: C: 0.06-0.12%, Si: 0.6-1.6%, Mn: 4-6%, Al: 0-0.2%, P: <0.01%, S: <0.002%, Nb: 0.01-0.08%, Ti: 0.05-0.15%, Mo: 0.1-0.4%, and the balance of Fe and inevitable impurities; and the following conditions are simultaneously met: when C<=0.1%, Mn is 4-6%; and when C>0.1%, Mn<=5%. The yield strength of the steel plate at room temperature is greater than or equal to 880MPa, the tensile strength is greater than or equal to 1200MPa, the yield strength ratio is less than 0.75, the elongation is greater than or equal to 16%, and the Charpy impact energy at room temperature is greater than or equal to 60J; and the steel plate has low cost and simple production process.

Description

Technical Field

[0001] This invention relates to the field of steel technology for engineering machinery, and in particular to a 900MPa grade ultra-low carbon medium manganese high strength hot-rolled steel plate and its manufacturing method. Background Technology

[0002] In the performance requirements of thick plates used in fields such as engineering machinery, although there are no explicit restrictions on the carbon content and carbon equivalent of steel plates due to weldability requirements, a lower carbon equivalent has always been a goal pursued by researchers during the steel plate development process. Because of the low carbon content in the steel, the strength of the steel plate is insufficient, often requiring the addition of certain amounts of precious metal elements such as Cr, Ni, and Mo to improve its strength. Furthermore, considering the impact toughness of the steel plate, controlled rolling and controlled cooling processes are frequently employed in its production, which significantly increases the production cost.

[0003] Currently, medium-manganese steel products are widely used in automotive steel sheet manufacturing. Through appropriate heat treatment processes, the steel sheet matrix can achieve a multiphase structure consisting of bainite, martensite, and retained austenite, resulting in high strength while maintaining good ductility and toughness. However, this usually requires a suitable offline heat treatment process to obtain different proportions of the aforementioned microstructures within the steel, thereby adjusting the mechanical properties of the sheet. Furthermore, automotive steel is often produced in thin-gauge form. In actual production, due to high compression ratios or the use of cold rolling, the grain size of the steel sheet is sufficiently refined during deformation, further enhancing its strength.

[0004] For high-strength steel plates of medium and heavy thickness, the rolling compression ratio is small during actual production. Controlled rolling and controlled cooling are typically used in production, requiring either a waiting period before rolling or subsequent quenching and tempering heat treatment, which reduces production efficiency. Offline heat treatment also increases production costs. Furthermore, the addition of precious metals such as Cr, Ni, and Mo increases the alloying cost of the steel.

[0005] For example, Chinese patent CN110952020A discloses "An economical 900MPa grade ultra-high pressure tempered steel plate and its production method." The composition percentage of this steel plate is: C: 0.155~0.175%, Si: 0.1~0.3%, Mn: 1.20~1.40%, P≤0.013%, S≤0.003%, Nb: 0.020~0.030%, Ti: 0.010~0.020%, Cr: 0.20~0.40%. Mo: 0.30–0.40%, B: 0.0010–0.0020%, Als: 0.015–0.050%, with the balance being iron and unavoidable inclusions. The continuously cast billets are heated to 1230–1270℃ and then subjected to two-stage rolling. The roughing rolling begins at 1050–1100℃, followed by finishing rolling after reaching a thickness of three times the finished steel plate. The finishing rolling ends at 800–840℃, and the plate is then rapidly cooled to 650–700℃ and air-cooled to room temperature. The rolled steel plate is then quenched at 880–910℃ and tempered at 540–560℃.

[0006] For example, Chinese patent CN106319350A discloses a "rolling and heat treatment production method for high-strength steel with a yield strength of 900MPa". The composition of the steel plate is as follows: C: 0.10-0.20%, Si: 0.10-0.50%, Mn: 1.00-1.60%, P≤0.015%, S≤0.005%, Cr: 0.10-0.50%, Mo: 0.20-0.60%, Nb: 0.015-0.055%, V: 0.020-0.060%, Ti: 0.003-0.04%, Al: 0.02-0.07%, B: 0.0006-0.0025%, with the balance being Fe and unavoidable impurities. The above-mentioned steel plate production method includes heating and rolling the continuously cast billet at 1100-1250℃, with a final rolling temperature of 820-880℃, cooling the steel plate to 500-700℃ using laminar flow cooling at 10-25℃ / s and then coiling it, cross-cutting and straightening the steel coil with a temperature below 80℃ into steel plates, quenching it at 850-950℃, and then tempering it at 500-700℃.

[0007] Based on the patent information of the aforementioned high-strength steel plates, most 900MPa grade steel plates currently require controlled rolling or subsequent quenching and tempering heat treatment for production. There are no reports of low-cost ultra-low carbon high-strength steel plates manufactured through controlled rolling and controlled cooling. Summary of the Invention

[0008] The purpose of this invention is to provide a 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate and its manufacturing method. The plate has a yield strength ≥880MPa, tensile strength ≥1200MPa, yield ratio <0.75, elongation of ≥16%, and Charpy impact energy ≥60J at room temperature. Moreover, the alloy is inexpensive and the production process is simple. Compared with other medium and high alloy high-strength steels, it has significant comprehensive advantages.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] A 900MPa grade ultra-low carbon medium-manganese high-strength hot-rolled steel plate has the following chemical composition by weight percentage: C: 0.06–0.12%, Si: 0.6–1.6%, Mn: 4–6%, Al: 0–0.2%, P: <0.01%, S: <0.002%, Nb: 0.01–0.08%, Ti: 0.05–0.15%, Mo: 0.1–0.4%, with the balance including Fe and unavoidable impurities; and must simultaneously satisfy the following:

[0011] When C ≤ 0.1%, Mn: 4–6%;

[0012] When C > 0.1%, Mn ≤ 5%.

[0013] Furthermore, the balance consists of Fe and other unavoidable impurities.

[0014] The microstructure of the hot-rolled steel plate described in this invention is 10-15% fine-grained ferrite + 80-85% lath bainite + a small amount of retained austenite.

[0015] In the composition design of the 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate described in this invention:

[0016] Carbon (C): It ensures material strength; as carbon content increases, the strength of the steel plate significantly improves. However, excessive carbon content leads to decreased toughness and increased susceptibility to weld cracking, reducing weldability. This invention employs an ultra-low carbon design. Through interaction with Mn, the ferrite transformation start and end temperatures of the steel plate are maintained within a wide temperature range, improving on-site operability. When the carbon content exceeds 0.1%, to ensure the ferrite transformation start and end temperature range, the Mn content needs to be strictly controlled to not exceed 5%, otherwise the water-cooling process window will be reduced, hindering on-site control. Simultaneously, when the carbon content exceeds 0.12%, the Al temperature increases significantly, eliminating process feasibility. Therefore, this invention controls the C content between 0.06% and 0.12%.

[0017] Silicon (Si): Silicon has a solid solution strengthening effect, which can improve the corrosion resistance and high-temperature oxidation resistance of steel. The addition of Si can also effectively increase the elastic modulus of steel plates; however, excessive content can lead to severe decarburization on the steel surface, reducing weldability. Furthermore, after cooling to a low temperature range below 400°C, silicon and aluminum in the steel can effectively inhibit cementite precipitation, allowing more carbon in the steel to be utilized in stable, untransformed austenite, further improving the stability of austenite. Therefore, this invention limits the Si content to 0.6%–1.6%.

[0018] Manganese (Mn): Manganese is a key element for stabilizing austenite in steel and a core element of this invention. It significantly shifts the CCT curve of steel to the right, reduces the critical cooling rate, improves the hardenability of the steel plate, lowers the start and end temperatures of the eutectoid transformation, refines the ferrite and pearlite structure in the steel, and ensures high strength of the steel plate. In this invention, the manganese content must be strictly matched with the carbon content. When the carbon content is less than 0.1%, the end temperature of the eutectoid transformation of the steel decreases continuously with the increase of manganese, having little impact on process control. However, when the carbon content in the steel is greater than 0.1%, the manganese content cannot exceed 5%, otherwise it will severely compress the start and end temperature range of the eutectoid transformation, resulting in a significant reduction in ferrite content in the steel plate as described in this invention, and a decrease in the ductility and toughness of the steel plate. In addition, manganese is also a solid solution strengthening element in steel, which can effectively improve the strength of the steel plate. However, excessive manganese content will also lead to an increase in the carbon equivalent of the steel plate, reducing its weldability and increasing costs. Therefore, the carbon content of this invention is controlled at 4-6%.

[0019] Aluminum (Al): Aluminum effectively prevents the formation of cementite in steel, facilitates the solid solution of carbon in austenite, and improves the stability of austenite during cooling, which is highly beneficial for improving the toughness of steel plates. However, if the aluminum content in steel is too high, it will increase the difficulty of smelting and casting, raise manufacturing costs, and cause excessive oxide formation, thus deteriorating the quality of the steel plate. Therefore, this invention limits the Al content to 0-0.2%.

[0020] Titanium (Ti): Titanium is a strong carbide-forming element, capable of forming TiN in steel. As a nucleation site for austenite, it refines the austenite grains. In this invention, the addition of trace amounts of Ti primarily serves to refine the austenite grains. Excessive Ti content leads to the precipitation of TiC, consuming carbon (C) in the steel, reducing the solid solubility of C in austenite, and consequently decreasing the stability of austenite. Therefore, this invention controls the Ti content to be 0.05–0.15%.

[0021] Niobium (Nb): Niobium is a strong carbide-forming element. In steel, during high-temperature rolling deformation, it can refine rolled austenite grains through deformation-induced precipitation. Simultaneously, the precipitated carbides pinning dislocations also contribute to precipitation strengthening. Too low a Nb content results in weak precipitation strengthening and grain refinement effects in steel. As the niobium content increases, its strengthening effect strengthens; however, when the niobium content is too high, its precipitation strengthening and grain refinement effects become insignificant. Therefore, in this invention, the Nb content is controlled at 0.01–0.08%.

[0022] Molybdenum (Mo): In steel, molybdenum improves the hardenability and hot strength of steel plates, enabling deeper and more thorough hardening of thick steel plates with larger cross-sections. Molybdenum in steel reduces the tendency for carbides to form a continuous network at grain boundaries, reducing retained austenite and relatively increasing the hardness and wear resistance of the steel matrix. Similarly, molybdenum is a precious metal, and due to its high cost, the Mo content is limited to 0.1%–0.4% in this invention.

[0023] The manufacturing method of the 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate of the present invention includes the following steps:

[0024] 1) Smelting and casting

[0025] The above-mentioned components are smelted and cast into steel ingots or continuously cast billets;

[0026] 2) Heating

[0027] Heating of steel ingots or continuously cast billets at a temperature of 1140–1160℃;

[0028] 3) Rolling

[0029] The controlled rolling process is adopted, and the steel ingot is rolled after being opened; or, the continuously cast billet is rolled directly. The initial rolling temperature is 1080-1100℃. The steel plate is first rolled to a thickness of 3-5 times that of the finished steel plate and then heated to the temperature range of 850-870℃. After reaching the temperature, it is rolled to the thickness of the finished steel plate.

[0030] 4) Cooling

[0031] A controlled cooling process is adopted, in which the rolled steel plate is water-cooled online to 20-30°C above A1, and then directly stacked to room temperature after leaving the line.

[0032] In the manufacturing method of the 900MPa grade ultra-low carbon medium manganese high strength hot-rolled steel plate of the present invention:

[0033] 1. Manganese plays an important role in the slow cooling process after water cooling.

[0034] Firstly, in the early stage of stack cooling, due to the high temperature, the diffusion of elements such as manganese and aluminum in the steel can still proceed effectively. During the nucleation and growth of proeutectoid ferrite grains, these elements can continuously be enriched in the untransformed austenite around the ferrite grains through reverse diffusion, thereby increasing the content of alloying elements such as carbon and manganese in the austenite, improving the stability of the supercooled austenite, and reducing the bainite transformation temperature.

[0035] Secondly, in the latter half of the stacking cooling process (when the temperature drops below 400℃), the austenite in the steel undergoes a bainitic transformation. During this process, manganese hardly diffuses, resulting in a higher manganese content in the bainitic laths than the originally designed average manganese content in the steel. This provides solid solution strengthening, and the refined bainitic laths further enhance the steel's strength. While other alloying elements cannot diffuse during the bainitic transformation, carbon atoms, due to their smaller size, still diffuse at this stage. Bainite formation is accompanied by carbon removal, which further enriches the untransformed austenite at the phase interface, improving its stability and lowering its critical transformation temperature below room temperature. Ultimately, a small amount of residual austenite remains in the steel.

[0036] 2. The steel plate rolling of this invention adopts a two-stage rolling process. In the first stage of rolling, due to the high temperature, the austenite grains are refined through dynamic recrystallization of austenite. In the second stage of rolling, which is in the non-recrystallization temperature range, the deformation degree of austenite grains is increased through large deformation, which increases the defect density in the microstructure. Rolling defects such as dislocations and vacancies become nucleation points for ferrite, thereby increasing the nucleation sites of ferrite in the steel after cooling. This provides a favorable microstructure guarantee for the formation of refined ferrite grains during the subsequent stacking cooling process after cooling to 20-30°C above the A1 temperature.

[0037] 3. For steel plates rolled to the finished thickness, online water cooling to 20-30°C above the A1 temperature followed by online stack cooling to room temperature is one of the core technologies of this invention. This is primarily to increase the supercooling of deformed austenite, further increasing the driving force for ferrite nucleation, refining ferrite grains, and improving the yield strength of the finished steel plate. The purpose of stack cooling is to control the ferrite content in the steel by reducing the cooling rate, allowing C, Mn, and other elements to undergo reverse diffusion into austenite during ferrite grain nucleation and growth, further improving the stability of the supercooled austenite. During subsequent slow cooling, the element-enriched austenite exhibits increased stability, causing its bainite transformation temperature to shift to a lower temperature range, increasing the supercooling of austenite. When the bainite transformation occurs in the lower temperature range, the high supercooling significantly increases the nucleation rate of the bainite transformation. This increased nucleation location significantly refines the size of the bainite laths. According to the Hall-Page formula, the refined lath size improves the final strength properties of the steel plate.

[0038] 4. Because the microstructure of the steel plate described in this invention consists of 10-15% fine-grained ferrite + 80-85% lath bainite + a small amount of retained austenite, the ferrite, as the soft phase, yields first during deformation, resulting in a lower yield strength. As deformation progresses, the bainite in the steel cooperates with the initially yielding ferrite in deformation, continuously increasing the strength of the steel plate. When deformation reaches a certain level and local stress concentration occurs, the retained austenite in the steel alleviates the local stress concentration through phase transformation-induced plasticity, improving the steel plate's ability to continue deforming. Simultaneously, the retained austenite transforms into martensite, which has a relatively high carbon and manganese content and high strength, ultimately increasing the steel plate's fracture strength. In summary, this microstructure effectively reduces the yield strength ratio of the steel plate, improves its deformation capacity, and enhances the safety performance of building structures and engineering machinery when resisting deformation.

[0039] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0040] 1. This invention, through the rational design of chemical composition, reduces the amount of precious metals such as chromium, nickel, and molybdenum used in traditional low-carbon alloy steel by adding manganese. It utilizes the reverse diffusion of austenite-stabilizing elements such as C and Mn across the chemical concentration gradient between the ferrite and austenite phases to improve the chemical and mechanical stability of supercooled austenite and lower the bainite transformation temperature.

[0041] Compared to other medium-high manganese steel products currently on the market, the amount of manganese added in the steel of this invention is significantly reduced. This is mainly achieved by effectively improving the utilization rate of Mn through the reverse diffusion effect of the element, thereby reducing the alloy cost of the steel plate. Through the mutually restrictive combination of C and Mn elements, the critical transformation temperature of the steel plate can be significantly reduced, which is beneficial to refining the microstructure. On the other hand, the temperature range between A1 and A3 of the steel plate is expanded, improving the feasibility of on-site production.

[0042] 2. Currently, most of the controlled rolling and controlled cooling technologies used in medium and heavy plate products employ a strong cooling process during the cooling stage, directly cooling the steel plate to room temperature or between 400 and 500°C. The temperature drop range is relatively large, and the strong cooling rate places high demands on the cooling capacity of the equipment. Then, heat treatment processes such as quenching and tempering or tempering are used to improve the strength and toughness of the steel plate.

[0043] The controlled cooling technology employed in this invention cools the steel plate online from near A3 to 20-30°C above A1. This narrow cooling range places lower demands on the cooling capacity of the equipment. Subsequent stacking cooling eliminates the need for offline heat treatment, significantly reducing energy consumption and manufacturing costs. Furthermore, the steel plate is directly transported to the stacking cooling area after cooling, without occupying cooling bed space, thus improving the production efficiency of the rolling mill line.

[0044] 3. Currently, most medium and heavy plate high-strength steel products either have complex alloy composition designs leading to excessively high costs; or use offline quenching and tempering processes in production, increasing production steps and costs; or the overall performance (strength, toughness, yield strength ratio) of the final product is not well-matched, forcing users to settle for second-best when choosing products.

[0045] The 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate of this invention maintains excellent plasticity and toughness while possessing high strength. Furthermore, through reasonable microstructure ratio, it effectively reduces the yield strength ratio of the steel plate, improves the steel plate's resistance to deformation and safety during service, and has significant comprehensive advantages compared to products currently on the market. Detailed Implementation

[0046] The present invention will be further described below with reference to the embodiments. However, the present invention is not limited to the following embodiments, and various technical solutions derived therefrom should be within the protection scope of the present invention.

[0047] The components of the embodiments of the present invention are shown in Table 1, Table 2 shows the process parameters of the embodiments of the present invention, and Table 3 shows the performance parameters of the steel of the embodiments of the present invention.

[0048] The manufacturing process of this invention is as follows: smelting is carried out in an electric furnace or converter, and the steel is cast into steel ingots or continuous casting billets; the steel ingots or billets are heated to 1150°C, the initial rolling temperature of the steel plate is 1100°C, and the plate is rolled to a thickness of 3 to 4 times the finished product thickness. The plate is then heated to a temperature range of 850 to 870°C to begin the next stage of rolling; after the steel plate is rolled to the finished product thickness, it is immediately water-cooled online to 20 to 30°C above the A1 temperature, and then cooled offline to room temperature.

[0049] Table 1 Unit: Weight Percentage

[0050]

[0051]

[0052] Table 2

[0053]

[0054] Table 3

[0055]

[0056] As can be seen from Tables 1 to 3, the ultra-low carbon medium manganese high-strength steel plate produced by the manufacturing method described in this invention has a yield strength of over 880 MPa, a tensile strength of over 1200 MPa, and an elongation after fracture of over 16%.

[0057] Through its rational composition and process design, this invention produces a room-temperature microstructure consisting of 10-15% fine-grained ferrite, 80-85% lath bainite, and a small amount of retained austenite. The ratio of soft to hard phases results in a yield strength ratio of less than 0.75, giving it excellent plastic deformation capabilities and meeting the requirements of conventional processing such as cutting and bending. At room temperature, its V-shaped Charpy impact energy reaches over 60J, demonstrating good impact resistance.

[0058] In summary, the 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate involved in this invention has significant cost advantages, simple production process and good mechanical properties.

Claims

1. A 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate, the chemical composition by weight percentage being: C: 0.06~0.12%, Si: 0.6~1.6%, Mn: 4~6%, Al: 0~0.2%, P<0.01%, S<0.002%, Nb: 0.01~0.08%, Ti: 0.05~0.15%, Mo: 0.1~0.4%, the balance being Fe and other unavoidable impurities, and must satisfy: When C ≤ 0.1%, Mn: 4~6%; When C > 0.1%, Mn ≤ 5%; The microstructure of the hot-rolled steel plate is 10-15% fine-grained ferrite + 80-85% lath bainite + a small amount of retained austenite; the hot-rolled steel plate has a yield strength ≥880MPa, tensile strength ≥1200MPa, yield ratio <0.75, elongation of ≥16%, and Charpy impact energy ≥60J at room temperature.

2. The 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable impurities.

3. The method for manufacturing 900MPa grade ultra-low carbon medium manganese high-strength hot-rolled steel plate as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Smelting and casting Smelting and casting into slabs according to the composition described in claim 1 or 2; 2) Heating The slab is heated to 1140~1160℃; 3) Rolling The controlled rolling process is adopted, and the steel ingot is rolled after being opened; or, the continuously cast billet is rolled directly. The initial rolling temperature is 1080~1100℃. The steel plate is first rolled to a thickness of 3~5 times that of the finished steel plate and then heated to the temperature range of 850~870℃. After reaching the temperature, it is rolled to the thickness of the finished steel plate. 4) Cooling A controlled cooling process is adopted, in which the rolled steel plate is water-cooled online to 20~30℃ above A1, and then directly stacked to room temperature after leaving the line.

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