Q550GJ steel plate for building structure and manufacturing method of Q550GJ steel plate

Abstract

The invention relates to a Q550GJ steel plate for a building structure and a manufacturing method of the Q550GJ steel plate, and belongs to the field of ferrous metallurgy. The steel plate comprises the following chemical components in percentage by weight: 0.05%-0.12% of C, 0.15%-0.45% of Si, 1.40%-1.70% of Mn, less than or equal to 0.002% of S, less than or equal to 0.015% of P, 0.20%-0.50% of Cr, 0.02%-0.05% of Alt, 0.02%-0.05% of Nb, 0.02%-0.05% of V, 0.01%-0.03% of Ti, 0.001%-0.003% of B and the balance of Fe and inevitable impurity elements, the carbon equivalent Ceq is less than or equal to 0.47%, and the carbon equivalent Pcm is less than or equal to 0.25%. The metallographic structure of the steel plate is a ferrite and bainite complex-phase structure. The production thickness of the steel plate is 50-80 mm. The invention relates to a Q550GJ steel plate for a building structure, which is produced by adopting a continuous casting and TMCP (Thermal Mechanical Control Processing) process, has the advantages of good internal quality, good matching mechanical properties, low yield ratio and excellent thickness direction performance, and can be applied to manufacturing and use of high-rise buildings and large-span buildings.

Description

Technical Field

[0001] This invention belongs to the field of steel plate manufacturing, specifically relating to a Q550GJ steel plate with a thickness >50-80mm for building structures and its manufacturing method. Background Technology

[0002] Q550GJ is a grade in the GB / T19879-2023 standard, mainly used in the manufacture of building steel structures. With the development of buildings towards higher heights and larger spans, steel plates for building structures are also developing towards greater thickness and higher strength and toughness to meet the needs of green and lightweight building structures. To ensure the safety of building structures, the design requires an impact strength of ≥120J at -20℃ and a yield strength ratio of no more than 0.85, and the maximum thickness of the steel plate reaches 100mm, far exceeding the maximum thickness specified in the standard. Due to limitations in equipment and process technology for medium and heavy plates, the TMCP process is used for production, making the matching of strength and toughness of the steel plate extremely difficult and resulting in extremely high production challenges. Currently, for ultra-high strength steel plates with a yield strength of 550MPa, domestic medium and heavy plate mills need to use heat treatment processes for production. Patent publication CN112813354A discloses a method for preparing a 550MPa-grade high-strength, high-heat-efficiency weldable thick steel plate for high-rise buildings. To achieve excellent low-temperature toughness, 0.8%–1.2% Ni is added to the composition, and the process employs quenching followed by high-temperature tempering to produce the 550MPa-grade steel plate for high-rise buildings. Patent publication CN116640998A discloses a Q550GJ steel plate for high-rise buildings and its manufacturing method, adding 0.15–0.25% Mo (a precious metal) to the composition, and employing TMCP (Transformer Metal Processing) followed by tempering. All of these inventions suffer from practical production problems, including high alloy costs, the need for heat treatment processes, and poor weldability, which to some extent limits their promotion and application in high-rise buildings. The invention patent document with patent publication number CN118854185A discloses a Q550GJ steel plate for earthquake-resistant high-strength buildings and its production method. It is produced using the TMCP process. The composition design and production process do not involve heat treatment. However, the steel plate is produced with a small thickness, with a maximum thickness of only 30mm, which cannot meet the demand of large-scale construction projects for thick, high-strength and tough steel plate materials. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a Q550GJ steel plate for building structures and its manufacturing method, which produces a thicker plate with good internal quality, good strength and toughness, low yield strength ratio, and excellent thickness direction performance, and can be applied to the manufacture and use of high-rise buildings and large-span buildings.

[0004] The technical solution adopted by the present invention to solve the above problems is as follows: a Q550GJ steel plate for building structures, wherein the chemical composition of the steel plate by weight percentage is C: 0.05-0.12%, Si: 0.15-0.45%, Mn: 1.40-1.70%, S: ≤0.002%, P: ≤0.015%, Cr: 0.20-0.50%, Alt: 0.02-0.05%, Nb: 0.02-0.05%, V: 0.02-0.05%, Ti: 0.01-0.03%, B: 0.001-0.003%, with the balance being Fe and unavoidable impurity elements.

[0005] The carbon equivalent Ceq ≤ 0.47% is calculated using the formula: Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the element symbols represent the weight percentage of the corresponding element.

[0006] Pcm≤0.25%, the calculation formula is: C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B, where the element symbols represent the weight percentage of the corresponding element.

[0007] The steel plate of this invention has a production thickness of 50-80mm. The internal quality and mechanical properties of the steel plate meet the following requirements: yield strength: 540-670 MPa, tensile strength: 660-810 MPa, elongation after fracture ≥17%, yield-to-tensile ratio ≤0.85, average longitudinal Charpy impact absorption energy at -20℃ ≥120J, and average Z-direction tensile reduction of area ≥35%. The steel plate undergoes UT flaw detection to meet GB / T2970 Class I requirements.

[0008] This invention pertains to iron-based alloys. The main chemical elements in the steel used in this invention and their roles are as follows: C can significantly improve the strength of steel plates, and its contribution to tensile strength is greater than that to yield strength. Therefore, appropriately increasing the C content is beneficial to reducing the yield strength ratio. However, excessive C content is detrimental to low-temperature impact toughness and weldability. Considering both the mechanical properties and weldability of steel plates, the C content in this invention is set to 0.05-0.12%.

[0009] Si: It is mainly used as a reducing agent and deoxidizer in steelmaking. It has a certain solid solution strengthening effect. If the content is too high, it will lead to a decrease in the low-temperature toughness of the steel plate. Therefore, the Si content in this invention is set to 0.15-0.45%.

[0010] Mn: It is a good deoxidizer and desulfurizer in the steelmaking process, and it is also the cheapest element to improve the strength of steel plates. However, its effect on improving yield strength is greater than that on tensile strength. Therefore, excessive Mn content is not conducive to controlling the yield strength ratio and its use should be restricted. Taking into account the mechanical properties of steel plates and alloy costs, the Mn content in this invention is controlled at 1.40 to 1.70%.

[0011] Cr: It lowers the transformation temperature of austenite to ferrite and prolongs the phase transformation time, which is beneficial for accurately controlling the ratio of soft and hard phases during controlled cooling, thereby reducing the yield strength ratio. However, if its content is too high, it may lead to carbide coarsening or a decrease in welding performance. In this invention, Cr is controlled at 0.20% to 0.50%.

[0012] Nb and V form carbonitrides with C and N elements, which can delay the recrystallization of austenite and refine the ferrite grains. They can simultaneously improve the strength and toughness of steel plates. However, if their content is too high, it will be detrimental to controlling the yield strength ratio of the steel plate. Therefore, in this invention, the Nb content is controlled at 0.02 to 0.05% and the V content is controlled at 0.02 to 0.05%.

[0013] Ti: It forms carbonitrides with C and N elements, which can delay the recrystallization of austenite and refine the ferrite grains. It can simultaneously improve the strength and toughness of steel plates. At the same time, for Nb-containing steel, adding an appropriate amount of Ti can effectively prevent longitudinal cracks from forming on the surface of continuously cast billets. Therefore, the Ti content in this invention is controlled at 0.010 to 0.03%.

[0014] B significantly improves the hardenability of steel, can inhibit the nucleation of ferrite and pearlite, delay high-temperature transformation, and promote the formation of bainite during rapid cooling after rolling. It also has the effect of refining grains and improving the strength and toughness of steel plates. However, if its content is too high, it will affect toughness and weldability. Its content should be strictly limited. In this invention, the B content is controlled at 0.001 to 0.003%.

[0015] The present invention discloses a method for manufacturing Q550GJ steel plate for building structures, comprising the following: The steel plate of this invention is produced using a continuous casting slab + TMCP process. The manufacturing steps, in the order of production, are smelting, continuous casting, billet heating, rolling, slow cooling, flaw detection, finishing, and inspection. The specific production process is as follows: KR hot metal pretreatment → converter smelting → LF refining → RH refining → slab continuous casting → billet heating → steel plate rolling → steel plate slow cooling → flaw detection → finishing → inspection → warehousing.

[0016] The specific operations for the main processes are as follows: (1) Smelting and casting process: The smelting and casting process adopts the conventional process that has been disclosed. The smelting raw materials are successively subjected to KR hot metal pretreatment, LF refining, RH vacuum degassing and continuous casting process. The distinguishing feature is that Ti is added first to fix nitrogen in the later stage of LF refining, and then B is added to prevent the formation of BN in the steel and enrichment at the grain boundary, which reduces the low temperature impact toughness of the steel plate.

[0017] (2) Heating process: The continuous casting billet adopts low temperature and long time segmented heating. On the basis of ensuring uniform heat penetration of the continuous casting billet, the coarsening of austenite grains is prevented. The total heating time is 400-450 min, the temperature of the second heating section is 1100-1200℃, the temperature of the soaking section is 1080-1180℃, and the total heating time of the second heating section and the soaking section is ≥200 min.

[0018] (3) Rolling process: Two-stage rolling is adopted. The initial rolling temperature of the roughing stage is ≥950℃, and a large reduction rolling process is adopted. At least two of the longitudinal rolling passes have a single-pass reduction of ≥40mm. The thickness of the intermediate billet is ≥2.5 times the thickness of the finished steel plate. The initial rolling temperature range of the finishing stage is 760~840℃, the cumulative reduction rate is ≥45%, and ACC water cooling is adopted after rolling. The water inlet temperature is controlled within the range of 730~760℃, and the final cooling and reddening temperature range is 350-500℃.

[0019] The principle behind the application scope of the above-mentioned process measures and process parameters in this invention is as follows: The technical challenge of using Q550GJ steel plate for building structures lies in how to control the microstructure through reasonable composition design and controlled rolling and cooling processes to obtain a fine and uniform ferrite + bainite multiphase structure, thereby achieving well-matched comprehensive mechanical properties.

[0020] This invention focuses on solving the aforementioned technical challenges through its composition and process design. The overall design concept employs a low-carbon composition, with appropriate amounts of alloying elements such as Si, Mn, Cr, Nb, V, Ti, and B. The production process utilizes low-temperature heating, low-temperature rolling, and low-temperature cooling to refine the ferrite matrix and bainite microstructure, thereby achieving a good balance of strength and toughness. By controlling the ratio of soft to hard phases, the yield strength ratio is effectively controlled, improving the steel plate's seismic resistance. The purpose of low-temperature heating is to prevent austenite grain coarsening. Low-temperature rolling in the finishing stage allows for sufficient flattening and elongation of the austenite microstructure, resulting in numerous uniform deformation bands within the austenite. Subsequent cooling produces a refined and uniform ferrite microstructure. By employing a low-temperature cooling process and appropriately increasing the ACC cooling rate, the bainite microstructure can be effectively refined, significantly improving the steel plate's strength without compromising its low-temperature impact toughness.

[0021] Compared with the prior art, the advantages of the present invention are: This invention relates to a Q550GJ steel plate for building structures, produced using a continuous casting billet + TMCP process. It possesses excellent internal quality, well-matched mechanical properties, a low yield strength ratio, and excellent thickness-direction properties, making it suitable for the manufacture and use of high-rise and large-span buildings. Testing shows that this Q550GJ steel plate for building structures with a thickness >50-80mm meets the following internal quality and mechanical properties: yield strength: 540-670 MPa, tensile strength: 660-810 MPa, elongation after fracture ≥17%, yield strength ratio ≤0.85, average longitudinal Charpy impact absorption energy at -20℃ ≥120J, and average Z-direction tensile reduction of area ≥35%. The steel plate meets the GB / T2970 Class I requirements for UT flaw detection. The metallographic structure of the steel plate is a ferrite + bainite multiphase structure, with typical morphologies detailed in the appendix. Figure 1 . Attached Figure Description

[0022] Figure 1 This is a typical metallographic structure of the Q550GJ steel plate of the present invention. Detailed Implementation

[0023] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention. Example 1

[0024] The Q550GJ steel plate in this embodiment has a thickness of 80mm. Its chemical composition by weight percentage is: C: 0.07%, Si: 0.35%, Mn: 1.55%, S: 0.001%, P: 0.015%, Cr: 0.45%, Alt: 0.035%, Nb: 0.045%, V: 0.035%, Ti: 0.015%, B: 0.0015%, with the balance being Fe and unavoidable impurity elements. The carbon equivalent Ceq is 0.44%, calculated using the formula: Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. Pcm is 0.21%, calculated using the formula: C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B.

[0025] The manufacturing process of this steel plate is as follows: The steel plate of this invention is produced using a continuous casting slab + TMCP process. The manufacturing steps, in the order of production, are smelting, continuous casting, billet heating, rolling, slow cooling, flaw detection, finishing, and inspection. The specific production process is as follows: KR molten iron pretreatment → converter smelting → LF refining → RH refining → 370mm slab continuous casting → billet heating → steel plate rolling → steel plate slow cooling → flaw detection → finishing → inspection → warehousing.

[0026] The specific operations for the main processes are as follows: (1) Smelting and casting process: The smelting and casting process adopts the conventional process that has been disclosed. The smelting raw materials are successively subjected to KR hot metal pretreatment, LF refining, RH vacuum degassing and continuous casting process. The distinguishing feature is that Ti is added first to fix nitrogen in the later stage of LF refining, and then B is added to prevent the formation of BN in the steel and enrichment at the grain boundary, which reduces the low temperature impact toughness of the steel plate.

[0027] (2) Heating process: The continuous casting billet adopts low temperature and long time segmented heating. On the basis of ensuring uniform heat penetration of the continuous casting billet, the coarsening of austenite grains is prevented. The total heating time is 440 min. The temperature of the second heating section is 1100~1200℃, the temperature of the soaking section is 1080~1180℃, and the total heating time of the second heating section and the soaking section is 230 min.

[0028] (3) Rolling process: Two-stage rolling is adopted. The roughing stage starts at a rolling temperature of 952℃ and adopts a large reduction rolling process. Two of the longitudinal rolling passes have a single-pass reduction of ≥40mm, namely 45mm and 50mm. The thickness of the intermediate billet is 200mm. The finishing stage starts at a rolling temperature of 780℃ and the cumulative reduction rate is 60%. After rolling, ACC water cooling is adopted, the water inlet temperature is 755℃, and the final cooling and reddening temperature range is 420℃.

[0029] The 80mm thick Q550GJ steel plate for building structures, produced by the above manufacturing process, meets the GB / T2970 Class I requirements after UT flaw detection and has well-matched comprehensive mechanical properties, as detailed in Table 1. Example 2

[0030] The Q550GJ steel plate in this embodiment has a thickness of 60mm. Its chemical composition, by weight percentage, is: C: 0.07%, Si: 0.32%, Mn: 1.60%, S: 0.001%, P: 0.012%, Cr: 0.35%, Alt: 0.032%, Nb: 0.040%, V: 0.030%, Ti: 0.015%, B: 0.0012%, with the balance being Fe and unavoidable impurity elements. The carbon equivalent (Ceq) is 0.43%, and the pcm is 0.20%. The manufacturing process of this steel plate is as follows: The steel plate of this invention is produced using a continuous casting slab + TMCP process. The manufacturing steps, in the order of production, are smelting, continuous casting, billet heating, rolling, slow cooling, flaw detection, finishing, and inspection. The specific production process is as follows: KR molten iron pretreatment → converter smelting → LF refining → RH refining → 370mm thick slab continuous casting → billet heating → steel plate rolling → steel plate slow cooling → flaw detection → finishing → inspection → warehousing.

[0031] The specific operations for the main processes are as follows: (1) Smelting and casting process: The smelting and casting process adopts the conventional process that has been disclosed. The smelting raw materials are successively subjected to KR hot metal pretreatment, LF refining, RH vacuum degassing and continuous casting process. The distinguishing feature is that Ti is added first to fix nitrogen in the later stage of LF refining, and then B is added to prevent the formation of BN in the steel and enrichment at the grain boundary, which reduces the low temperature impact toughness of the steel plate.

[0032] (2) Heating process: The continuous casting billet adopts low temperature and long time segmented heating. On the basis of ensuring uniform heat penetration of the continuous casting billet, the coarsening of austenite grains is prevented. The total heating time is 420 min. The temperature of the second heating section is 1100~1200℃, the temperature of the soaking section is 1080~1180℃, and the total heating time of the second heating section and the soaking section is 210 min.

[0033] (3) Rolling process: Two-stage rolling is adopted. The roughing stage starts at a rolling temperature of 960℃ and adopts a large reduction rolling process. Two of the longitudinal rolling passes have a single-pass reduction of ≥40mm, namely 50mm and 52mm. The thickness of the intermediate billet is 160mm. The finishing stage starts at a rolling temperature of 760℃ and the cumulative reduction rate is 62%. After rolling, ACC water cooling is adopted, the water inlet temperature is 745℃, and the final cooling and reddening temperature range is 442℃.

[0034] The 80mm thick Q550GJ steel plate for building structures, produced by the above manufacturing process, meets the GB / T2970 Class I requirements after UT flaw detection and has well-matched comprehensive mechanical properties, as detailed in Table 1.

[0035] Table 1 Mechanical properties of steel plates from various embodiments

[0036] 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 Q550GJ steel plate for building structures, characterized in that: The chemical composition of the steel plate, by weight percentage, is: C: 0.05-0.12%, Si: 0.15-0.45%, Mn: 1.40-1.70%, S: ≤0.002%, P: ≤0.015%, Cr: 0.20-0.50%, Alt: 0.02-0.05%, Nb: 0.02-0.05%, V: 0.02-0.05%, Ti: 0.01-0.03%, B: 0.001-0.003%, with the balance being Fe and unavoidable impurity elements. The carbon equivalent Ceq ≤0.47% and Pcm ≤0.25%. The metallographic structure of the steel plate is a ferrite + bainite complex structure.

2. The steel plate according to claim 1, characterized in that: The production thickness of the steel plate is 50-80mm.

3. The steel plate according to claim 1, characterized in that: The internal quality and mechanical properties of the steel plate meet the following requirements: yield strength: 540~670Mpa, tensile strength: 660~810Mpa, elongation after fracture ≥17%, yield strength ratio ≤0.85, average longitudinal Charpy impact absorption energy at -20℃ ≥120J, average Z-direction tensile reduction of area ≥35%, and the steel plate meets the requirements of GB / T2970 Class I for UT flaw detection.

4. A method for manufacturing the steel plate of claim 1, characterized in that: The process includes steel smelting, continuous casting, billet heating, rolling, and slow cooling. Steel smelting involves refining steel according to the elemental composition of the steel plate. Continuous casting is used to pour the molten steel into billets. Heating the billets austenitizes the microstructure. The heated billets are then rolled into the final steel plate using a two-stage rolling process. In the roughing stage, the initial rolling temperature is ≥950℃, employing a high-reduction rolling process. At least two longitudinal rolling passes have a single-pass reduction of ≥40mm; the intermediate billet thickness is ≥2.5 times the thickness of the finished steel plate. The finishing stage is a low-temperature rolling process, with an initial rolling temperature of 760–840℃, a cumulative reduction rate of ≥45%, and ACC water cooling after rolling. The steel plate's water immersion temperature is controlled at 730–760℃, and the final cooling and reddening temperature is 350–500℃.

5. The method according to claim 4, characterized in that: Steelmaking includes KR hot metal pretreatment, LF refining, and RH vacuum degassing. In the later stage of LF refining, Ti is added first to fix nitrogen, and then B is added.

6. The method according to claim 4, characterized in that: The billet is heated using a segmented heating process, with a total heating time of 400–450 min. The temperature of the second heating section is 1100–1200℃, and the temperature of the soaking section is 1080–1180℃. The total heating time of the second heating section and the soaking section is ≥200 min.

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