A 700mpa grade ultra-low carbon medium-manganese high-strength high-toughness steel plate and a manufacturing method thereof

Abstract

The application discloses a 700MPa-grade ultra-low-carbon medium-manganese high-strength high-toughness 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: 0.30-1.60%, Mn: 3.50-4.50%, Al: 0-0.2%, P: <0.01%, S: <0.002%, and the balance of Fe and other inevitable impurities. The steel plate needs to meet the following conditions simultaneously: 3.52%≤(C+Mn)≤4.51%; (Nb+Ti+V): 0.04-0.16%. The yield strength of the steel plate at room temperature is greater than or equal to 690MPa, the tensile strength is greater than or equal to 1000MPa, the yield strength ratio is less than 0.7, the elongation reaches more than 18%, and the Charpy impact at room temperature is greater than or equal to 70J.

Description

Technical Field

[0001] This invention relates to the field of steel for engineering machinery, and in particular to a 700MPa grade ultra-low carbon medium manganese high strength and high toughness 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's matrix structure can be a multiphase structure consisting of bainite, martensite, and retained austenite, giving medium-manganese steel sheets high strength while also possessing good ductility and toughness. Furthermore, by rationally changing the heat treatment regime, different proportions of the above-mentioned structures can be obtained in the steel, thereby adjusting the mechanical properties of the steel sheet. Automotive steel is mostly thin-gauge material. In actual production processes, due to the large compression ratio or the use of cold rolling, the grains of the steel sheet are sufficiently refined during deformation, thus improving the strength of the steel sheet.

[0004] For medium and heavy plates used in engineering machinery, due to the large thickness of the steel plates, the compression ratio during rolling is small in the actual production process. The steel plates are usually produced by controlled rolling and controlled cooling. Therefore, during the production process, it is necessary to wait for the temperature to rise or after the steel plates are rolled, they need to undergo subsequent quenching and tempering heat treatment, which reduces production efficiency. At the same time, the addition of precious metal elements such as Cr, Ni, and Mo also increases the cost of alloying elements in the steel.

[0005] For example, Chinese Patent Publication No. CN104694822A discloses "A high-strength hot-rolled steel plate with a yield strength of 700MPa and its manufacturing method." The composition percentage of this steel plate is: C: 0.06~0.10%, Si: 0.05~0.40%, Mn: 1.20~1.80%, P≤0.015%, S≤0.01%, N≤0.006%, Al: 0.015~0.050%, Ti: 0.07~ 0.15%, Nb: 0.025~0.070%, Ca: 0.001~0.006%, balance is iron and unavoidable inclusions. The continuously cast billet is heated and held at 1220~1270℃ and then hot rolled. The rough rolling end temperature is 1020~1070℃. After waiting for the temperature to drop, it is finished rolled at 860~920℃. After rolling, it is accelerated cooled to 550~650℃ for coiling, and then cooled to 100℃ after more than 48 hours.

[0006] Chinese Patent Publication No. CN104532159A discloses "a high-strength steel with a yield strength of 700MPa and its production method". The composition of the steel plate is as follows: C: 0.06-0.13%, Si: 0.10-0.30%, Mn: 0.80-1.60%, Cr: 0.20-0.70%, Mo: 0.10-0.30%, Ni: 0-0.03%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, V: 0.01-0.03%, B: 0.0005-0.0030%, Al: 0.02-0.06%, Ca: 0.001-0.004%, N: 0.002-0.005%, P≤0.02%, S≤0.01%, O≤0.008%, with the remainder being Fe and unavoidable impurities. The above-mentioned steel plate production method includes rolling and coiling, quenching at Ac3+(30-80)℃, and then tempering at 400-550℃.

[0007] Judging from the patents for high-strength steel plates used in engineering machinery, most steel plates with a strength of 690MPa or higher are currently thin-gauge hot-rolled coils that require controlled rolling or subsequent quenching and tempering heat treatment. Due to their thinness, the reduction from slab to finished steel plate is relatively large during actual production, resulting in the steel plate achieving the strength level of 690MPa. However, there are no reports on low-cost, ultra-low carbon high-strength medium-thick plates that are directly air-cooled to room temperature after rolling. Summary of the Invention

[0008] The purpose of this invention is to provide a 700MPa grade ultra-low carbon medium manganese high strength and high toughness steel plate and its manufacturing method. The plate has a yield strength ≥690MPa, tensile strength ≥1000MPa, yield ratio <0.7, elongation of more than 18%, Charpy impact ≥70J at room temperature, and is low in cost and simple in production process.

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

[0010] A 700MPa grade ultra-low carbon medium-manganese high-strength and high-toughness steel plate has the following composition by weight percentage: C: 0.06-0.12%, Si: 0.30-1.60%, Mn: 3.50-4.50%, Al: 0-0.2%, P < 0.003%, S < 0.002%, Nb: 0.01-0.08%, Ti: 0.01-0.03%, V: 0.02-0.06%, with the balance including Fe and other unavoidable impurities. The composition must satisfy the following: 3.92% ≤ (C+Mn) ≤ 4.51%; (Nb+Ti+V): 0.04-0.16%.

[0011] Preferably, the balance is Fe and other unavoidable impurities.

[0012] The microstructure of the steel plate described in this invention is 35-45% fine-grained ferrite + 50-55% lath martensite + a small amount of retained austenite.

[0013] The steel plate of the present invention has a yield strength ≥690MPa, tensile strength ≥1000MPa, yield ratio <0.7, elongation of ≥18%, and Charpy impact ≥70J at room temperature.

[0014] In the composition design of the steel of this invention:

[0015] Carbon (C): 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, enhancing the strength level of the steel plate through the interaction of other alloying elements. This allows the steel plate to possess high strength while maintaining excellent ductility, toughness, and weldability. Therefore, the carbon content in this invention is controlled between 0.06% and 0.12%.

[0016] 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 the steel plate. However, excessive content can lead to severe decarburization on the steel surface, reducing weldability. Therefore, the silicon content selected in this invention is between 0.30% and 1.60%.

[0017] Manganese (Mn): Manganese is a major element stabilizing austenite in steel. It can significantly shift the CCT curve of steel to the right, reduce the critical cooling rate of steel, improve the hardenability of steel plates, lower the eutectoid transformation temperature of steel, refine the ferrite and pearlite structure in steel, and ensure that the steel plate has high strength. Manganese is also a solid solution strengthening element in steel, which can effectively improve the strength of steel plates. However, excessive manganese content will also lead to an increase in the carbon equivalent of the steel plate, reduce the weldability of the steel plate, and increase the cost. Therefore, the manganese content in this invention is controlled at 3.50% to 4.50%.

[0018] Since carbon and manganese are both strongly stable austenitic elements, their content in steel must be strictly controlled during actual production. If we calculate based on the assumption that both C and Mn enrich into austenite during ferrite transformation, when the combined carbon and manganese content is low (below 3.52%), the austenite will undergo bainitic transformation during subsequent air cooling due to the higher martensitic transformation temperature, reducing the steel's strength. Conversely, when the combined carbon and manganese content is high (above 4.51%), the austenite will become further enriched with carbon near the phase interface during air cooling due to the initiation of martensitic transformation, increasing its chemical stability and preventing martensitic transformation. This also leads to a decrease in martensite content and a reduction in steel strength. Therefore, this invention requires adding limiting conditions to the C and Mn composition: 3.52% ≤ (C + Mn) ≤ 4.51%.

[0019] Aluminum (Al): Aluminum effectively prevents the formation of carbides 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 controls the aluminum content to 0-0.2%.

[0020] Vanadium (V): Vanadium is a strong carbide-forming element, playing a role in precipitation strengthening and grain refinement in materials. Because excessive Mn content in steel can easily lead to grain coarsening, adding trace amounts of vanadium helps refine the microstructure and improve alloy strength. Simultaneously, V carbide precipitation has a dispersion strengthening effect, further enhancing the strength of the steel. Therefore, this invention controls the vanadium content to 0.02–0.06%.

[0021] 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 austenite stability. Therefore, this invention controls the Ti addition amount to 0.01–0.03%.

[0022] 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 mass percentage of niobium is controlled between 0.01% and 0.08%; simultaneously, the mass percentages of niobium, vanadium, and titanium are controlled between (Nb+Ti+V): 0.04% and 0.16%.

[0023] The manufacturing method of the 700MPa grade ultra-low carbon medium manganese high strength and high toughness 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 slabs;

[0026] 2) Heating

[0027] The slab is heated to 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 810-840℃. 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 cooled online at a cooling rate of 0.25 to 0.80℃ / s to 20 to 30℃ above A1, and then directly air-cooled to room temperature after leaving the line.

[0032] Preferably, the thickness of the finished steel plate is 15-40mm.

[0033] In the manufacturing method of the 700MPa grade ultra-low carbon medium manganese high strength and high toughness steel plate of the present invention:

[0034] 1. Manganese plays an important role in controlling the rolling and cooling processes.

[0035] First, during the controlled rolling process of this invention, manganese can strongly shift the "C" curve of the steel plate to the right, delaying the pearlite-ferrite phase transformation of the steel plate and causing the ferrite phase transformation to move to the low temperature region, which is beneficial to refining the grain size of proeutectoid ferrite in the steel.

[0036] Secondly, during the cooling process of 0.25 to 0.80℃ / s after rolling, due to the high temperature, elements such as manganese and aluminum in the steel can still diffuse effectively. During the nucleation and growth of proeutectoid ferrite grains, they can continuously enrich 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.

[0037] Finally, during the controlled cooling stage, the austenite in the steel has completed the enrichment of carbon and manganese elements. The chemical composition of the austenite is almost twice that of the original steel, further improving its chemical stability and thus lowering the martensitic transformation temperature of the steel. When the steel plate cools to an even lower temperature, martensite begins to form in the steel. At this time, the manganese- and carbon-rich austenite, during the martensitic transformation, will dissolve in the newly formed martensite due to the inability (or extremely low) of manganese to diffuse, resulting in strong solid solution strengthening and significantly increasing the strength of the martensite. During the martensitic transformation, although other alloying elements cannot diffuse, carbon atoms, due to their small atomic size, still undergo short-range diffusion at this stage. The martensitic transformation is accompanied by carbon removal, at which point carbon atoms further enrich the untransformed austenite at the martensitic-austenite interface with carbon, improving the stability of the austenite and lowering its critical transformation temperature below room temperature. Ultimately, a small amount of residual austenite remains in the steel.

[0038] 2. The rolling process 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.

[0039] 3. For steel plates rolled to the finished thickness, the key manufacturing process of this invention involves online cooling at a rate of 0.25–0.80℃ / s to 20–30℃ above the A1 temperature, followed by air cooling to room temperature. Under this controlled cooling rate, the austenite in the steel can undergo sufficient proeutectoid ferrite phase transformation. The relatively slow cooling rate also provides ample time for carbon and manganese elements to enrich into untransformed austenite during the phase transformation. The purpose of controlled cooling followed by air cooling is to increase the cooling rate of the steel plate, preventing further proeutectoid ferrite transformation and effectively controlling the ferrite content to avoid excessive ferrite content that could reduce the plate's strength. During controlled cooling, the phase transformation of proeutectoid ferrite provides a chemical driving force for the reverse diffusion of elements such as C and Mn into nearby austenite, further improving the stability of the supercooled austenite. The element-enriched austenite, due to its increased stability during subsequent air cooling, experiences a decrease in its critical martensitic transformation temperature, increasing the supercooling of austenite and refining the dimensions of the martensitic laths. According to calculations based on the Holpage formula, refined strip dimensions improve the final strength properties of the steel plate.

[0040] 4. Because the microstructure of the steel plate described in this invention consists of 35-45% fine-grained ferrite, 50-55% lath martensite, and 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, when the ferrite deforms to a certain extent, the martensite in the steel reaches its yield point and deforms in cooperation with the ferrite that yielded first, continuously increasing the strength of the steel plate. With further deformation, local stress concentration occurs in the steel. The retained austenite, through phase transformation-induced plasticity, alleviates the local stress concentration and improves 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.

[0041] Currently available 690MPa high-strength steel products on the market are mostly designed with low-carbon, low-alloy compositions, adding certain amounts of precious metals such as chromium, nickel, and molybdenum to enhance strength and toughness. Due to the poor performance stability of TMCP (Transformed Metallic Concentrate) steel plates, users rarely accept TMCP-delivered products. In actual manufacturing processes, offline quenching and tempering heat treatment is often required for rolled steel plates to ensure relatively stable mechanical properties. This further extends the production cycle, increases manufacturing costs, and increases energy consumption, hindering carbon reduction in steel production. Furthermore, for quenched and tempered high-strength steel plates, the matrix structure (mostly tempered martensite) determines a high yield strength ratio, which is difficult to reduce through process adjustments.

[0042] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0043] 1. From the perspective of composition design: This invention, through the rational design of chemical composition, completely avoids the use of precious metal elements such as chromium, nickel, and molybdenum by adding manganese to traditional low-carbon alloy steel, thereby reducing the actual production cost of steel plates and saving precious metal resources. It utilizes the reverse diffusion of austenite-stabilizing elements such as C and Mn through the chemical concentration gradient between the ferrite and austenite phases to improve the chemical and mechanical stability of supercooled austenite and lower the martensitic transformation temperature.

[0044] 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.

[0045] 2. From the perspective of manufacturing process, the steel plate of the present invention is manufactured by combining controlled rolling and controlled cooling process with air cooling system, which significantly improves the production efficiency of steel plate and reduces energy consumption in the steel plate production process.

[0046] 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 to directly cool 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. This often results in significant fluctuations in the performance of steel plates manufactured by the TMCP process, which often requires the use of heat treatment processes such as quenching and tempering to improve the stability of the mechanical properties of the steel plates.

[0047] The controlled cooling technology employed in this invention involves cooling the steel plate online from near A3 to 20-30°C above A1. This cooling range is relatively small, and the controlled cooling rate is low (0.25-0.80°C / s), resulting in lower requirements for the cooling capacity of the cooling equipment. Subsequent air cooling eliminates the need for offline heat treatment, significantly reducing energy consumption and production costs.

[0048] 3. From the perspective of the comprehensive performance of steel plates, steel plates produced by offline quenching and tempering not only increase the production process and increase production costs, but also result in a poor combination of the comprehensive performance (strength, toughness, yield strength ratio) of the final product, forcing users to settle for second best when choosing products.

[0049] The 700MPa 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

[0050] 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.

[0051] The components of the embodiments of the present invention are shown in Table 1.

[0052] 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 ingots or slabs with alloy compositions as shown in Table 1; the ingots or slabs are heated and rolled to the thickness of the finished steel plate using a controlled rolling process; after rolling, the steel plate is controlled-cooled and then air-cooled to room temperature. Specific process parameters are shown in Table 2.

[0053] The room temperature tensile properties and room temperature impact energy test results of the steel plate described in the embodiments of the present invention are shown in Table 3.

[0054] As can be seen from Tables 1 to 3, the 700MPa grade ultra-low carbon medium-manganese high-strength and high-toughness steel plate of this invention has a yield strength of over 690MPa, a tensile strength of over 1000MPa, a yield-to-tensile ratio of less than 0.7, and an elongation after fracture exceeding 18%, exhibiting excellent plastic deformation capacity and meeting the cutting, bending, and other processes required by conventional processing. At room temperature, its V-shaped Charpy impact energy reaches over 70J, demonstrating good impact resistance.

[0055] In summary, the 700MPa grade ultra-low carbon medium manganese high strength and high toughness steel plate involved in this invention has significant cost advantages, simple production process and good mechanical properties.

[0056] Table 1 Unit: Weight Percentage

[0057] Example 1 0.07 0.76 3.88 0.0052 0.0012 0.069 0.058 0.029 0.062 Example 2 0.06 0.32 4.42 0.0061 0.0016 0 0.051 0.015 0.058 Example 3 0.10 1.38 3.64 0.0046 0.0013 0.093 0.047 0.024 0.078 Example 4 0.12 1.59 3.77 0.0058 0.0015 0.194 0.061 0.017 0.044 Example 5 0.09 1.04 4.16 0.0078 0.0012 0.085 0.022 0.028 0.027 Example 6 0.08 0.35 3.52 0.0087 0.0011 0.077 0.034 0.013 0.015

[0058] Table 2

[0059]

[0060] Table 3

[0061]

Claims

1. A 700MPa grade ultra-low carbon medium-manganese high-strength and high-toughness steel plate, wherein the weight percentage composition is: C: 0.06~0.12%, Si: 0.30~1.60%, Mn: 3.50~4.50%, Al: 0~0.2%, P<0.003%, S<0.002%, Nb: 0.01~0.08%, Ti: 0.01~0.03%, V: 0.02~0.06%, with the balance including Fe and other unavoidable impurities, and must satisfy: 3.92%≤(C+Mn)≤4.51%; (Nb+Ti+V): 0.04~0.16%; The microstructure of the steel plate is 35-45% fine-grained ferrite + 50-55% lath martensite + a small amount of retained austenite; The steel plate has a yield strength ≥690MPa, tensile strength ≥1000MPa, yield ratio <0.7, elongation ≥18%, and Charpy impact ≥70J at room temperature.

2. The 700MPa grade ultra-low carbon medium manganese high-strength and high-toughness steel plate as described in claim 1, characterized in that, The balance is Fe and other unavoidable impurities.

3. The method for manufacturing 700MPa grade ultra-low carbon medium manganese high-strength and high-toughness steel plate as described in claim 1 or 2, characterized in that, include: 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 810~840℃. After reaching the temperature, it is rolled to the thickness of the finished steel plate. 4) Cooling The controlled cooling process is adopted, and the rolled steel plate is cooled online at a cooling rate of 0.25~0.80℃ / s to 20~30℃ above A1, and then directly air-cooled to room temperature after leaving the line.

4. The manufacturing method of the 700MPa grade ultra-low carbon medium manganese high-strength and high-toughness steel plate as described in claim 3, characterized in that, The thickness of the finished steel plate is 15~40mm.

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