550MPa-grade low-yield-ratio steel plate for building structure and manufacturing method of 550MPa-grade low-yield-ratio steel plate

Abstract

The invention belongs to the technical field of steel plate manufacturing, and particularly relates to a 550MPa-grade low-yield-ratio steel plate for a building structure and a manufacturing method of the 550MPa-grade low-yield-ratio steel plate. The steel plate comprises the following chemical components in percentage by mass: 0.07-0.12% of C; 0.30% to 0.50% of Si; mn: 1.50 to 1.80%; less than or equal to 0.015% of P; s: < = 0.005%; 0.20% to 0.50% of Cr; 0.035% to 0.055% of Nb; 0.010-0.025% of Ti, and the balance of Fe and other inevitable elements. The low-carbon micro-alloying component design is adopted, the steel plate with the thickness of 20-50 mm is produced through the on-line controlled rolling and controlled cooling technology, the final yield strength of the steel plate is 550-690 MPa, the tensile strength is 670-830 MPa, the percentage elongation after fracture is larger than 17%, the yield ratio is not larger than 0.85, and the impact energy at the temperature of-40 DEG C is not lower than 120 J. Compared with the prior art, the invention has the advantages of small alloy addition amount, short production period, low cost, excellent performance and the like, and is suitable for the field of steel structures of high-rise buildings and the like with aseismic design requirements.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel plate manufacturing, and particularly relates to a 550MPa grade low yield ratio building structure steel plate and a manufacturing method thereof. BACKGROUND

[0002] In recent years, with the rapid development of the construction industry in China, various buildings with flexible modeling and innovative structure, such as high-rise and large-span buildings, are emerging in an endless stream. In order to ensure the safety of building structures, ordinary building structure steel plates cannot meet the requirements of high strength, earthquake resistance, low temperature resistance and other aspects in different application environments. Therefore, the 550MPa grade low yield ratio building structure steel plate has broad application prospects due to its excellent comprehensive performance.

[0003] At present, the production process of the yield strength 550MPa grade ultra-high strength steel plate mainly adopts quenching and tempering process. For example, the patent CN112813354 discloses a 550MPa grade high-strength thick steel plate for high-rise buildings and a preparation method. In order to meet the excellent low temperature toughness, 0.8% to 1.2% Ni is added in the composition design, and the yield strength 550MPa grade high-rise building steel plate is produced by adopting quenching + high temperature tempering process; the patent CN116640998A discloses a high-rise building steel Q550GJ steel plate and a manufacturing method thereof. In the composition design, the content of precious metal Mo is 0.15 to 0.25%, and the TMCP + tempering process is adopted in the process. The 550MPa grade high-rise building steel plate involved in these patents has the problems of high alloy cost, complex process flow and high production cost.

[0004] In order to solve the above problems of high alloy cost and complex production process, the TMCP (thermo-mechanical controlled rolling) technology is the most effective way. By adopting the TMCP technology, the addition amount of alloy elements in the steel can be reduced, the CEV (carbon equivalent) can be reduced, the steel plate has good welding performance, the TMCP process has simple production process and short production cycle, and subsequent heat treatment can greatly reduce energy consumption, which belongs to green manufacturing. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a 550MPa grade low yield ratio building structure steel plate with a thickness of 20-50mm and a manufacturing method thereof, which uses continuous casting billet as raw material and adopts online controlled rolling and controlled cooling process, to overcome the shortcomings of the existing high-strength steel adopting quenching and tempering or TMCP+T process.

[0006] The technical scheme adopted by the present application to solve the above problems is as follows: a 550MPa grade low yield ratio building structure steel plate, the chemical components are as follows in percentage by weight: C: 0.07~0.12%; Si: 0.30~0.50%; Mn: 1.50~1.80%; P: ≤0.015%; S: ≤0.005%; Cr: 0.20~0.50%; Nb: 0.035~0.055%; Ti: 0.010~0.025%, and the rest is Fe and other inevitable elements.

[0007] The steel plate is produced by adopting continuous casting billet through on-line controlled rolling and controlled cooling process, the yield strength of the steel plate is 550~690MPa, the tensile strength is 670~830MPa, the elongation after fracture is greater than 17%, the yield ratio is not greater than 0.85, and the impact energy at-40℃ is not less than 120J.

[0008] Reasons for limiting each chemical element and implementation principle C element: it is the cheapest and most effective strengthening element in steel, forms carbide with Nb, V, Ti, Cr and other alloying elements in steel, suppresses the growth of austenite grains in the continuous casting slab heating process, controls the austenite recrystallization zone, and strongly affects the phase transition behavior of austenite to ferrite. A certain amount of C is retained in the steel matrix to improve the strength of the steel, and the contribution to the tensile strength is greater, thereby reducing the yield ratio. Therefore, the content of C is controlled to be 0.07%~0.12%.

[0009] Si element: it is a common deoxidizing element, and can improve the strength of ferrite through solid solution strengthening. However, too much silicon element will promote the formation of M-A island structure in the welding heat affected zone, and is not conducive to the decomposition of M-A island structure, which deteriorates the welding performance. Therefore, the content of Si is controlled to be 0.3%~0.50%.

[0010] Mn element: it is a common solid solution strengthening element, and the controlled process is conducive to obtaining ideal bainite structure; the Mn element reduces the lower critical point (reduces the critical transformation temperature Ar3), thereby increasing the supercooling degree of austenite cooling, refining the structure and promoting the generation of bainite ferrite, and improving the strength and low temperature toughness of the steel. However, when Mn is solid-solved in ferrite to play a strengthening role, the effect of increasing yield strength is higher than that of increasing tensile strength, thereby increasing the yield ratio. Therefore, the content of Mn in the present application is 1.50%~1.80%.

[0011] Cr (Cr): It lowers the ferrite transformation point of steel, reduces the critical austenite transformation cooling rate, shifts the austenite isothermal transformation curve to the right, and affects the diffusion behavior of carbon and carbides in austenite and ferrite, thus facilitating the control of metallographic structure after accelerated cooling. As a medium-strong carbide-forming element, when precipitated as carbides, it reduces the toughness of the steel plate but increases the yield strength ratio. This invention, combined with the control of other carbide-forming element content, prevents the formation of chromium carbides in the steel, thus controlling Cr at 0.20%–0.50%.

[0012] Nitrogen (Nb) is a very effective grain-refining strengthening element that can effectively control the original austenite size and refine bainite grains, thereby improving the strength and toughness of steel plates. The limit range for Nb is 0.035% to 0.055%.

[0013] Ti: A strong nitrogen-fixing element, titanium nitrides can prevent grain growth in the billet during heating, rolling, and welding, improving the toughness of the base material and the weld heat-affected zone. Excessive Ti will cause coarsening of titanium nitrides, affecting the low-temperature toughness of the steel plate. Therefore, the limit range for Ti is 0.010~0.025%.

[0014] P and S elements: These are harmful elements. P can cause cold brittleness in steel, and S can form low-melting-point sulfides that cause hot brittleness. Therefore, their content must be strictly controlled: P ≤ 0.015% and S ≤ 0.005%.

[0015] In this invention, Cr (0.20~0.50%) is used to replace Mo for phase transition regulation, and Nb (0.035~0.055%) is used to enhance the grain refinement effect, thus avoiding the addition of precious metals.

[0016] The steel plate of this invention is produced using a thick continuously cast billet. The manufacturing process mainly includes smelting, continuous casting, heating, and online controlled rolling and cooling, specifically including the following steps: Smelting: The raw materials are sequentially processed through KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting to produce continuously cast billets that meet the composition requirements. The center segregation of the continuously cast billets is ≤0.5 grade (C category), and the center porosity is ≤0.5 grade. KR hot metal pretreatment effectively removes harmful elements such as sulfur from the hot metal. Converter smelting achieves basic smelting tasks such as decarburization of steel. LF refining further adjusts the composition and temperature and removes inclusions. RH vacuum degassing reduces the gas content in the steel and improves its purity. The continuous casting process controls center segregation and porosity, providing high-quality billets for subsequent rolling.

[0017] Heating: The continuous casting billet is heated using a high-temperature, long-duration, segmented heating method. The total heating time is 389~426 min. The temperature of the second heating section is 1200~1280℃, and the temperature of the soaking section is 1200~1260℃. The total heating time of the second heating section and the soaking section (collectively referred to as the high-temperature section) is not less than 180 min. High-temperature, long-duration heating is beneficial for the full solidification of alloying elements in the continuous casting billet, especially microalloying elements such as Nb and Ti. At the same time, it can eliminate casting stress, improve the plasticity of the billet, and create favorable conditions for subsequent rolling.

[0018] Online controlled rolling and cooling: The steel plate is obtained by two-stand rolling, roughing, finishing and cooling.

[0019] Rough rolling: The temperature range is 1170~1000℃, the single-pass reduction rate is not less than 15%, and the cumulative reduction rate is not less than 50%, to obtain an intermediate billet with a thickness of not less than 2.5h (h is the finished product thickness). The rough rolling stage is carried out with high reduction rolling in the austenite recrystallization zone, which can break up and refine the austenite grains, providing a fine austenite structure for subsequent finish rolling.

[0020] Finish rolling: The temperature range is 800~840℃, the reduction rate per pass is 15~20%, and the final rolling temperature is 760~780℃. Finish rolling is carried out in the non-recrystallized austenite region. Through deformation accumulation, the austenite grains are elongated, the grain boundary area is increased, and the formation of fine grain structure is promoted during the subsequent cooling process, thereby improving the strength and toughness of the steel plate.

[0021] High-temperature heating ensures the dissolution of microalloys, two-stage controlled rolling refines austenite, and precise cooling after rolling controls the multiphase structure, eliminating the need for a tempering process.

[0022] Post-rolling cooling: By controlling the speed after finishing mill and matching a suitable initial cooling temperature of 740~760℃, and by adjusting the water group, water volume, and roller speed, the final cooling and reheating temperatures are precisely controlled, ensuring that the final cooling temperature is lower than the bainite transformation end temperature, the reheating temperature of the steel plate is 440~540℃, and the average cooling rate is 8~15℃ / s. Rapid post-rolling cooling can inhibit the transformation of austenite to pearlite and promote the transformation of bainite, while refining the bainite structure. The formation of a small amount of ferrite helps to reduce the yield strength ratio, ultimately obtaining a bainite-dominant + small amount of ferrite multiphase structure, giving the steel plate excellent comprehensive mechanical properties. Through the "bainite + ferrite" multiphase structure design, high strength (yield ≥550MPa) is ensured while reducing the yield strength ratio (≤0.85), and low-temperature toughness is improved through TiN pinning grains.

[0023] Compared with the prior art, the present invention has the following beneficial effects: 1. The steel plate requires less alloying material, eliminating the need for precious metals such as Ni and Mo, thus reducing alloying costs. The production process employs online controlled rolling and cooling, replacing offline quenching and tempering or TMCP + tempering processes. This results in a shorter production flow, simpler procedures, shorter production cycle, energy savings, reduced emissions, and significantly improved production efficiency. This facilitates shorter delivery times, substantially lowers production costs, and promotes the widespread application of high-strength, low-yield-strength-ratio steel plates.

[0024] 2. The steel plate possesses high strength, with a yield strength of 550-690 MPa and a tensile strength of 670-830 MPa. It has a low yield-to-tensile strength ratio (less than 0.85), exhibiting good plastic deformation capacity and energy dissipation capacity under seismic loads, resulting in excellent seismic performance. It also demonstrates good toughness, with a Charpy impact value (single value) greater than 120 J at -40℃, exhibiting good low-temperature toughness and adaptability to cold-climate environments. Furthermore, its elongation after fracture is greater than 17%, indicating good plasticity. Its excellent overall performance makes it highly suitable for steel structures with seismic design requirements, including but not limited to the construction industry.

[0025] 3. Using the composition design and manufacturing method of this invention, the produced steel plate thickness can reach 20-50mm, which can meet the requirements of different building structures for steel plate thickness, and has good performance stability with small performance fluctuations between batches of steel plates.

[0026] 4. The steel plate has good weldability. Due to the low-carbon micro-alloying design, the carbon equivalent is reduced, making it less prone to welding cracks during the welding process. The heat-affected zone of the weld has excellent performance, ensuring the safety of the welded structure. Attached Figure Description

[0027] Figure 1. Typical microstructure (20 μm) of the steel plate in Example 3 of the present invention.

[0028] Figure 2. Typical microstructure (20 μm) of the steel plate of Comparative Example 1 of the present invention. Detailed Implementation

[0029] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0030] The core objective of this invention is to produce a 20-50mm thick, 550MPa grade low yield-to-tensile ratio steel plate for building structures using continuously cast billets, through the matching of composition and controlled rolling and cooling processes. The steel plate has a yield strength of 550-690MPa, a tensile strength of 670-830MPa, a yield-to-tensile ratio not exceeding 0.85, an elongation greater than 17%, and meets the requirement of a Charpy impact energy of not less than 100J at -40℃. The microstructure of the steel plate is bainite + ferrite. Example 1

[0031] The steel plate in this embodiment has a thickness of 20 mm. It is smelted from the following components by mass fraction: C: 0.11%, Si: 0.35%, Mn: 1.55%, P: 0.013%, S: 0.003%, Nb: 0.045%, Ti: 0.015%, Cr: 0.25%, with the remainder being Fe and other unavoidable elements.

[0032] This embodiment describes the manufacturing method of 550MPa grade low yield strength ratio steel plate for building structures: Raw materials undergo KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a thick continuous casting billet. The center segregation of the continuous casting billet is ≤0.5 grade C, and the center porosity is ≤0.5 grade. The billet is cold-charged into a heating furnace, with a high-temperature section heating temperature of 1230℃ and a total furnace time of 395 minutes. A two-stage rolling process is employed: roughing starts at 1062℃, with a single-pass reduction rate of not less than 15% and a cumulative reduction rate of not less than 50%, resulting in an intermediate billet thickness of 50mm; finishing starts at 840℃, with a pass reduction rate of 15~20%, and a final rolling temperature of 767℃. The rolled steel plate undergoes rapid cooling via laminar flow cooling, with an initial water temperature of 747℃, a final cooling reheat temperature of 502℃, and an average cooling rate of 13.2℃ / s, followed by air cooling to room temperature. Example 2

[0033] The steel plate in this embodiment is 30mm thick. It is smelted from the following components by mass fraction: C: 0.10%, Si: 0.35%, Mn: 1.65%, P: 0.011%, S: 0.001%, Nb: 0.045%, Ti: 0.015%, Cr: 0.30%, with the remainder being Fe and other unavoidable elements.

[0034] This embodiment describes the manufacturing method of 550MPa grade low yield strength ratio steel plate for building structures: Raw materials undergo KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a thick continuous casting billet. The center segregation of the continuous casting billet is ≤0.5 grade C, and the center porosity is ≤0.5 grade. The billet is cold-charged into a heating furnace, with a high-temperature section heating temperature of 1235℃ and a total furnace time of 405 minutes. A two-stage rolling process is employed: roughing starts at 1062℃, with a single-pass reduction rate of not less than 15% and a cumulative reduction rate of not less than 50%, resulting in an intermediate billet thickness of 75mm; finishing starts at 840℃, with a pass reduction rate of 15~20%, and a final rolling temperature of 772℃. After rolling, the steel plate undergoes rapid cooling via laminar flow cooling, with an initial water temperature of 742℃, a final cooling reheat temperature of 512℃, and an average cooling rate of 10.3℃ / s, followed by air cooling to room temperature. Example 3

[0035] The steel plate in this embodiment is 50 mm thick. It is smelted from the following components by mass fraction: C: 0.09%, Si: 0.35%, Mn: 1.75%, P: 0.012%, S: 0.002%, Nb: 0.050%, Ti: 0.015%, Cr: 0.35%, with the remainder being Fe and other unavoidable elements.

[0036] This embodiment describes the manufacturing method of 550MPa grade low yield strength ratio steel plate for building structures: Raw materials undergo KR hot metal pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting to obtain a thick continuously cast billet. The center segregation of the continuously cast billet is ≤0.5 grade C, and the center porosity is ≤0.5 grade. The billet is cold-charged into a heating furnace, with a high-temperature section heating temperature of 1227℃ and a total furnace time of 407 minutes. A two-stage rolling process is employed: roughing starts at 1055℃, with a single-pass reduction rate of not less than 15% and a cumulative reduction rate of not less than 50%, resulting in an intermediate billet thickness of 125mm; finishing starts at 810℃, with a pass reduction rate of 15~20%, and a final rolling temperature of 775℃. The rolled steel plate undergoes rapid cooling via laminar flow cooling, with an initial water temperature of 754℃, a final cooling reheat temperature of 462℃, and an average cooling rate of 9.7℃ / s, followed by air cooling to room temperature. Comparative Example 1

[0037] The steel plate in this comparative example has a thickness of 50 mm and is smelted from the following components by mass fraction: C: 0.08%, Si: 0.31%, Mn: 1.57%, P: 0.013%, S: 0.001%, Nb: 0.013%, Ti: 0.013%, Cr: 0.46%, V: 0.027%, B: 0.0013%, with the remainder being Fe and other unavoidable elements.

[0038] This comparative example describes the manufacturing method of 550MPa grade low yield strength ratio steel plates for building structures: Raw materials are smelted in a converter, refined in a ladle, and continuously cast to obtain thick continuously cast billets. The billets are cold-charged into a heating furnace at 1222℃ for a total furnace time of 405 minutes. A two-stage rolling process is employed: roughing begins at 1063℃ with an intermediate billet thickness of 125mm, and finishing begins at 807℃, followed by finishing mill rolling at a final rolling temperature of 770℃. After a 72-second relaxation period, the rolled steel plate undergoes rapid laminar flow cooling at a water inlet temperature of 714℃, a final cooling temperature of 447℃, and an average cooling rate of 10.6℃ / s, followed by air cooling to room temperature.

[0039] The mass percentage of each element in the chemical composition of each embodiment and comparative example of the present invention is shown in Table 1, the main process parameters are shown in Table 2, and the mechanical properties of the steel plate were tested, and the results are shown in Table 3. Table 1 shows the main chemical components of the various embodiments and comparative examples of the present invention.

[0040]

[0041] Note: — indicates no addition; Ceq=[C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15] Table 2 shows the slab rolling processes of various embodiments and comparative examples of the present invention.

[0042] Table 3 shows the tissue types and conventional mechanical properties of the various embodiments and comparative examples of the present invention.

[0043]

[0044] As shown in Table 3, the steel plates obtained in Examples 1 to 3 exhibit yield strength ≥ 599 MPa, tensile strength ≥ 710 MPa, yield-to-tensile ratio ≤ 0.84, elongation ≥ 20%, impact energy at -40℃ ≥ 162 J, and Z-direction reduction of area ≥ 53%, all exceeding the requirements specified in standard GB / T 19879-2023. They demonstrate good performance stability and can meet the usage requirements of high-rise buildings. Compared with the comparative examples, they have advantages such as lower alloy content, higher strength, and ease of production. Specifically, they do not require relaxation rolling, a production method that affects the production rhythm; excellent comprehensive mechanical properties are achieved solely through the TMCP process. The overall technical indicators far exceed those of existing technologies.

[0045] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A steel plate for building structures with a low yield strength ratio of 550MPa, characterized in that, By weight percentage, including C: 0.07~0.12%; Si: 0.30~0.50%; Mn: 1.50~1.80%; P: ≤0.015%; S: ≤0.005%; Cr: 0.20~0.50%; Nb: 0.035~0.055%; Ti: 0.010~0.025%, the remainder being Fe and other unavoidable elements.

2. The 550MPa grade low yield strength ratio steel plate for building structures according to claim 1, characterized in that: The steel plate has a thickness of 20~50mm and a metallographic structure that is mainly bainite with a small amount of ferrite.

3. The 550MPa grade low yield strength ratio steel plate for building structures according to claim 1, characterized in that: The steel plate has a yield strength of 550~690MPa, a tensile strength of 670~830MPa, an elongation after fracture of more than 17%, a yield strength ratio of not more than 0.85, and an impact energy of not less than 120J at -40℃.

4. A method for manufacturing a 550MPa grade low yield strength ratio steel plate for building structures as described in claim 1, characterized in that, The production process, which uses thick continuous casting billets, includes the following steps: converter smelting, LF refining, RH vacuum degassing, continuous casting, heating, rolling, and online controlled cooling.

5. The method for manufacturing a 550MPa grade low yield strength ratio steel plate for building structures according to claim 4, characterized in that: In the continuous casting process, the center segregation of the continuously cast billet is Class C ≤ 0.5, and the center porosity is ≤ 0.

5.

6. The method for manufacturing a 550MPa grade low yield strength ratio steel plate for building structures according to claim 4, characterized in that: In the heating process, high-temperature, long-duration segmented heating is adopted, with a total heating time of 389 min to 426 min. The temperature of the second heating section is 1200 to 1280℃, and the temperature of the heat soaking section is 1200 to 1260℃. The total heating time of the second heating section and the heat soaking section is not less than 180 min.

7. The method for manufacturing a 550MPa grade low yield strength ratio steel plate for building structures according to claim 4, characterized in that: The method includes the following steps: In the rolling process, a controlled rolling process is adopted, which is rolled in two stages with a compression ratio greater than 7 times. The rough rolling start temperature is ≥1000℃. This stage adopts a low-speed, high-reduction rolling mode, with a single-pass reduction rate of not less than 15% and a cumulative reduction rate of not less than 50%. The finishing rolling waiting thickness is 2.5h, where h is the finished thickness. The second rolling start temperature is 800~840℃, the pass reduction rate is 15~20%, and the final rolling temperature is 760~780℃.

8. The method for manufacturing a 550MPa grade low yield strength ratio steel plate for building structures according to claim 4, characterized in that: In the cooling process, after rolling, the rolls are cooled by laminar flow using ACC or DQ+ACC, with an inlet water temperature of 740~760℃, a final cooling temperature of 440~540℃, an average cooling rate of 8~15℃ / s, and then air-cooled to room temperature.

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