A threaded steel bar with a yield strength of ≥1200MPa and a production method thereof

Abstract

A type of engineering machinery steel with excellent plate shape and yield strength ≥1200MPa, comprising the following components and wt%: C: 0.48~0.65%, Si: 0.80~1.55%, Mn: 1.00~1.65%, P≦0.035%, S≦0.035%, B: 0.0006~0.0038%, Cr: 0.60~1.10%, Zr: 0.50~0.75%, N 0.0045~0.0120%, V: not higher than 0.10% or Nb not higher than 0.05%, or a mixture of both, wherein the mixture of both satisfies the following condition: (V+1.6Nb) not less than 0.03%; molten iron pretreatment; converter smelting; LF furnace refining; casting into square billets; heating the billets; roughing and intermediate rolling; finishing rolling in two stages; cooling. The present invention provides a simple process for hot-rolled yield strength Rel≥1200MPa, Rm≥1330MPa, elongation A≥6%, and Att≥3.5%, and the finished threaded steel products have stable performance. Furthermore, by increasing the heating temperature, the rolling mill load can be reduced while significantly lowering the cost, which is conducive to improving production efficiency.

Description

Technical Field

[0001] This invention relates to a type of threaded steel and its production method, specifically to a type of threaded steel with a yield strength ≥1200MPa and its production method. It is particularly suitable for producing threaded steel with a diameter of 34-75mm, and is used for 1200MPa precision-rolled threaded steel for hydropower dams, urban overpasses, highway bridges, port terminals, railway bridges, etc. Background Technology

[0002] Precision-rolled threaded steel bars, also known as threaded steel bars for prestressed concrete, are straight steel bars with discontinuous external threads on the surface of the entire bar, without longitudinal ribs. These bars are directly connected using nuts and spiral sleeves, eliminating the need for welding and machining of the threads. They offer advantages such as simple connection and anchoring, strong adhesion, reliable tensioning and anchoring, convenient and fast construction, and material savings. Precision-rolled threaded steel bars are widely used in large-scale water conservancy and hydropower projects, continuous beams and large frame structures in industrial and civil buildings, highways, high-speed railways, large bridge cable trays, nuclear power plants, ground anchors, slope anchoring, and other extra-large buildings, frame structures, bridges, and culverts.

[0003] Precision rolled steel bars are considered high-tech steel products in China, with extremely stringent requirements for product grade and quality. Since weldability is not a concern, various micro-alloying or heat treatment processes can be employed to produce this steel. Based on the characteristics of precision rolled threaded steel bars, the connection device and finished product pass of the rolling mill need to be redesigned, and high precision control is required during the rolling process.

[0004] Compared with traditional ordinary bar rolling production lines, high-speed bar production lines can obtain higher precision finished products by using a roll ring mill. The finishing rolling zone uses single-strand twist-free low-temperature controlled rolling technology to achieve alloy reduction production. On the cooling bed, single bars are placed in a single groove for cooling and single-strand alignment to obtain a higher yield. Based on these advantages, high-speed bar production technology has developed rapidly in recent years.

[0005] There are currently two main types of high-speed bar production lines: one is a single-line high-speed bar production line, also known as a single high-speed bar production line, in which the roughing, intermediate, pre-finishing, and finishing mills all produce single-bar rolling. After finishing, the bar is fed into a cooling bed by a high-speed steel feeding system. The other is a double-line high-speed bar production line, also known as a double high-speed bar production line, in which the roughing and intermediate mills produce single-bar rolling, the pre-finishing mills produce two separate rolling processes, and after the pre-finishing mills, the bar is fed into two finishing mills simultaneously on two separate lines. Each line produces single-bar rolling in each finishing mill, and after finishing, the two lines are combined and fed into the cooling bed by a double high-speed steel feeding system that can support simultaneous loading of the two lines onto the cooling bed.

[0006] In the national standard GB / T 20065-2016, the chemical composition (smelting composition) of high-strength threaded steel bars is not explicitly specified, except for the control of S and P mass fractions ≤0.035%. The selection of chemical composition and alloying elements is left to the manufacturer to ensure that finished steel bars processed by different methods meet the mechanical performance requirements specified in the standard. High-strength threaded steel bars have added inspection items for relaxation, fatigue, and non-metallic inclusions, making their quality requirements significantly higher than those for ordinary threaded steel bars.

[0007] Fine-rolled threaded steel bars are classified according to their yield strength level, including five grades: PSB785, PSB830, PSB930, PSB1080, and PSB1200. Among them, PSB1200 is the grade with the highest strength, requiring a yield strength ≥1200MPa, tensile strength ≥1330MPa, elongation after fracture ≥6%, and total elongation at maximum force ≥3.5%.

[0008] Because there are no unified chemical composition requirements in the product standards for precision-rolled threaded steel, domestic steel mills use carbon steel microalloying strengthening technology for chemical composition design, generally employing hypoeutectoid steel with medium to low carbon content. The chemical composition design for precision-rolled threaded steel mainly includes two types: 40Si2MnV and Mn-based bainitic steel. 40Si2MnV is produced through a post-rolling residual heat treatment process, resulting in inconsistent internal and external microstructures and a large gradient in microstructure morphology from the surface to the core, leading to significant fluctuations in product performance. Mn-based bainitic precision-rolled threaded steel has a bainitic / martensite composite phase both internally and externally, with a uniform microstructure and relatively stable performance. However, due to the predominance of hard phase microstructure after rolling, the internal stress and residual stress are relatively high, requiring slow cooling in the pit or annealing heat treatment to eliminate stress cracks and reduce cracking tendency. Currently, both of these steel compositions only produce precision-rolled threaded steel bars with yield strengths of 785MPa to 1080MPa; there are no reports of industrial production of 1200MPa-grade high-strength precision-rolled threaded steel.

[0009] Therefore, developing and stably supplying 1200MPa grade high-strength precision-rolled threaded steel bars in large quantities will meet the market demand for high-strength steel and the needs of major engineering construction. High-strength steel can replace low-strength steel, which can reduce the overall steel consumption and has good economic and social benefits.

[0010] According to the search,

[0011] Chinese patent application CN202311374054.X (publication number CN 117230375A) discloses "A rare earth corrosion-resistant high-strength refined threaded steel bar and its processing method". The chemical composition by weight percentage is: C: 0.23-0.34%, Si: 0.7-1.2%, Mn: 1.4-1.8%, V: 0.01-0.05%, Lanthanum (La): 0.01-0.05%, Cerium (Se): 0.01-0.05%, Fe: balance. The method involves heating a steel billet containing rare earth elements to a rolling temperature and rolling it into a steel bar with regular threads; then cooling it to room temperature; and finally... Regularly threaded steel bars are fed into an induction heating furnace at a speed of 5-15 m / min and heated to 830-930℃. After induction heating, the bars are quenched directly with high-pressure water jet or quenching liquid without heat preservation. The quenching cooling rate is 200-250℃ / s, the quenching pressure reaches over 16MPa, and the quenching cooling time is 5-30 seconds, cooling the bar temperature below the Ms point. The quenched bars are then heated in a tempering furnace to 350-500℃. After tempering, the bars are water-cooled to 230℃ and then directly cooled to room temperature with water. The heads and tails of individual steel bars are connected by ring welding using carbon dioxide protective gas. Driven by a front traction drive system, the bars sequentially undergo heating, quenching, and tempering, before being pulled out for cooling by a rear traction drive system. The frequency difference between the front and rear traction drive systems is 25%-40%. Because the heads and tails of the bars are welded together, the bars move forward under the speed difference of the front and rear traction, ensuring quenching and tempering under micro-tension conditions, significantly reducing the relaxation rate of the steel bars. The paper describes a production process that uses online hot rolling followed by offline quenching and tempering. This process is complex, has low production efficiency, and is difficult to organize. Summary of the Invention

[0012] This invention addresses the shortcomings of existing technologies, such as high alloy content in steel bars, high energy consumption, poor cooling effect, and low production efficiency. It provides a hot-rolled 1200MPa precision-rolled rebar with a yield strength Rel≥1200MPa, Rm≥1330MPa, elongation A≥6%, and Att≥3.5%, which simplifies the process, ensures stable product performance, and produces rebar with a nominal diameter of 34-75mm, along with its production method.

[0013] Measures to achieve the above objectives:

[0014] A type of rebar with a yield strength ≥1200MPa, comprising the following components and weight percentages: C: 0.48~0.65%, Si: 0.80~1.55%, Mn: 1.00~1.65%, P≦0.035%, S≦0.035%, B: 0.0006~0.0038%, Cr: 0.60~1.10%, Zr: 0.50~0.75%, N 0.0045~0.0120%, V: not higher than 0.10% or Nb not higher than 0.05% or a mixture of both, wherein the mixture of both satisfies the following condition: (V+1.6Nb) not less than 0.03%, with the remainder being Fe and unavoidable impurities.

[0015] Preferably, the weight percentage content of C is 0.49% to 0.60%.

[0016] Preferably, the weight percentage content of Si is between 0.90% and 1.45%.

[0017] Preferably, the weight percentage content of Mn is 1.10 to 1.50%.

[0018] Preferably, the weight percentage content of B is 0.0006 to 0.0025%.

[0019] Preferably, the weight percentage content of Cr is 0.65% to 0.95%.

[0020] Preferably, the Nb content is between 0.015% and 0.035% by weight.

[0021] Preferably, the weight percentage content of Zr is 0.55% to 0.70%.

[0022] A method for producing a rebar with a yield strength ≥1200MPa, comprising the following steps:

[0023] 1) Perform hot metal pretreatment. After desulfurization, the hot metal should have the following composition: P < 0.020% and S < 0.020%.

[0024] 2) In converter smelting, control the final carbon content of the converter to be between 0.1% and 0.36%, and the tapping temperature to be ≥1650℃; when the molten steel has been tapped to 2 / 3, the alloy and carbon raiser should be added all at once.

[0025] 3) Perform LF furnace refining for no less than 25 minutes, with argon blowing throughout the refining process; after the composition is adjusted, exposed molten steel is prohibited.

[0026] 4) Cast into square billets, during which the casting process is protected by a long nozzle in a large ladle and an immersion nozzle in a crystallizer, and the superheat of the molten steel in the tundish is controlled at 20-35℃; the cross-sectional dimensions of the continuously cast square billet are not less than 200mm, and the ratio of the cross-sectional area of ​​the square billet to the cross-sectional area of ​​the thread is controlled to be not less than 9.0.

[0027] 5) When heating the billet, the temperature of the soaking zone should be controlled at 1100-1220℃, and the total heating time should be 60-90 minutes; and the temperature difference between the cross sections of the same billet should be controlled to be ≤35℃.

[0028] 6) Perform roughing and intermediate rolling, and control the initial rolling temperature at 1000-1060℃, and control the cumulative deformation rate of roughing and intermediate rolling at 15-50%.

[0029] 7) Perform finishing rolling in two stages, during which: control the temperature of entering finishing rolling stage I at 880-950℃, the temperature of entering finishing rolling stage II at 900-980℃, and the rolling speed of the last stand at 1.2-20.8m / s;

[0030] 8) Cool the bed by controlling the temperature of the bed to be between 320 and 380°C and cooling it to room temperature at a cooling rate of 3 to 7°C / s.

[0031] The role and mechanism of each component and main process in this invention

[0032] C: C is the cheapest and most effective strengthening element in steel materials. When dissolved in the matrix, it can play a solid solution strengthening role. If the C content is less than 0.65%, it is difficult to guarantee the yield strength and tensile strength of the steel bars. If the C content is higher than 0.48%, it will worsen the toughness of the steel. Therefore, the C content should be controlled between 0.48% and 0.65%, preferably between 0.49% and 0.60%.

[0033] Si exists in steel in a solid solution state to increase strength. It is also a deoxidizing element in steelmaking. Under the premise of ensuring that the strength and toughness of the steel bars are qualified, we should use as much cheap Si element as possible, but the content should not be too high to avoid reducing the toughness and plasticity of the steel. Therefore, it should be controlled at 0.80 to 1.55%, and preferably the Si content is 0.90 to 1.45%.

[0034] Mn: Mn dissolves in ferrite to increase the strength of steel, lowers the transformation temperature of supercooled austenite, and refines ferrite grains, which is beneficial to improving the strength and toughness of precision-rolled steel bars. However, Mn is also an element that is prone to segregation, and the higher the content, the easier it is to segregate. Therefore, the content is controlled at 1.00-1.65%, and preferably 1.10-1.50%.

[0035] P and S: P and S are harmful elements in this steel. Generally, the lower their content, the better. Considering the actual control of steelmaking and production cost factors, P ≤ 0.035% and S ≤ 0.035% are controlled.

[0036] B: In alloy steel, B mainly improves the hardenability of the steel, but the inherent properties of reinforcing bars do not involve hardenability. In this invention, B is a strong nitride-forming element, and the formation of fine and dispersed BN can produce a precipitation strengthening effect, increasing the strength of the steel. It also has a strong affinity for O, which can reduce free oxygen and nitrogen in the steel, thus helping to eliminate the aging phenomenon of reinforcing bars. At the same time, adding trace amounts of hardenability-enhancing elements can compensate for the lack of online controlled cooling capability and accelerate the microstructure transformation. However, if the B content is too high, it will deteriorate the strength and toughness of the steel. Therefore, the content is controlled at 0.0006-0.0038%, preferably 0.0006-0.0025%.

[0037] Cr: It can lower the transformation temperature of supercooled austenite, refine ferrite grains, refine pearlite lamellae, and improve the strength of steel bars. However, if the content is too high, it will deteriorate the toughness of steel. Therefore, the content is controlled at 0.60 to 1.10%, and preferably the Cr content is 0.65 to 0.95%.

[0038] Zr: Zirconium can degas and refine grains, but excessive zirconium content will increase smelting difficulty and production cost, and cause local segregation, which will deteriorate the performance of steel. Therefore, the content is controlled at 0.50-0.75%, preferably 0.55-0.70%.

[0039] Nitrogen (N): Nitrogen is a gaseous element that combines with microalloying elements such as V and NbN in steel to form second-phase particles such as VN and NbN, enhancing precipitation strengthening and grain refinement, significantly improving the strength and toughness of the steel. However, as a gaseous element, nitrogen content is relatively unstable. Insufficient nitrogen makes it difficult to guarantee the aforementioned strengthening effect, while excessive nitrogen reduces the toughness of the steel and increases the difficulty of smelting. Therefore, the nitrogen content is controlled between 0.0045% and 0.0120%, preferably between 0.0055% and 0.0095%.

[0040] V, Nb, and Ti: These are the most common microalloying elements. All three are strong nitride and carbide forming elements. During coil rolling, they generate fine, dispersed second-phase particles, producing a strong precipitation strengthening effect and refining the grain size, thus improving the strength and toughness of the steel. However, excessive V, Nb, and Ti content can lead to increased precipitate size, potentially worsening these effects, especially with large-particle, hard TiN. Multiple tests have shown that considering the interactions between two or three elements, controlling V to 0–0.10%, Nb to 0–0.05%, and simultaneously satisfying 2.0 ≤ (V + 2Nb + 2Ti) / N ≤ 4.6, achieves good results.

[0041] High compression ratio rolled finished products: This invention uses continuous casting billets with cross-sectional dimensions of 200mm×200mm, 220mm×220mm and above for rolling. Compared with the general rolling of 150mm×150mm and 160mm×160mm continuous casting billets, it has a larger compression ratio. Rolling with large deformation can increase the compression rate of the billet, which is beneficial to improving the fineness and uniformity of the micrograin of the steel bar, and is beneficial to improving the strength and toughness of the steel. In addition, it can increase the nucleation and growth of controlled-cooling martensite.

[0042] The reason why this invention controls the converter endpoint C to be between 0.1% and 0.36% and the tapping temperature to be ≥1650℃, and adds all the alloys and carburizing agents at once when the molten steel has been tapped to 2 / 3, is to ensure precise control of the content of alloying elements Si, Mn, Al, V, Zr and C. If added in advance, the alloying elements will oxidize prematurely, resulting in a low yield. If added in a later manner, the alloying lumps will not have enough time to melt into the molten steel, resulting in uneven composition and even the presence of foreign metals.

[0043] The reason why the temperature of the homogenization zone is controlled at 1100-1220℃ and the total heating time is 60-90min, and the temperature difference of the same billet cross section is controlled at ≤35℃, is that the use of homogenization forging at a high temperature of 1100-1180℃ and a long heating time is conducive to the uniform diffusion of each component in the steel, reducing segregation, and is beneficial to the uniformity of microstructure and the stability of performance.

[0044] The reason why the present invention controls the initial rolling temperature at 1000-1060℃ during roughing and intermediate rolling, and controls the cumulative deformation rate of roughing and intermediate rolling at 15-50%, is that using this initial rolling temperature can ensure that the billet is fully austenitized in the heating furnace and undergoes austenitic deformation during subsequent rolling, which is beneficial to the uniformity of composition and structure. Controlling the cumulative deformation rate can ensure the uniform refinement of the microstructure.

[0045] The reason why the temperature of the first stage of finishing mill is controlled at 880-950℃, the temperature of the second stage of finishing mill is controlled at 900-980℃, and the rolling speed of the last stand is controlled at 1.2-20.8m / s is that the finishing mill still uses the austenitic region for rolling deformation, which is beneficial to ensuring the appropriate strength and plasticity of the finished steel. The speed of the last stand is controlled at 1.2-20.8m / s because different rolling speeds are used for the 34-75mm specifications. On the one hand, this plays a role in deformation-induced strengthening, and on the other hand, it ensures smooth production and increases hourly output.

[0046] The reason why the temperature of the steel entering the cooling bed is controlled at 320-380°C is to obtain a martensitic structure by strong cooling after rolling, so as to have higher strength and suitable plasticity. If the temperature of the steel entering the cooling bed is too low, the plasticity of the steel will be insufficient, and if it is too high, the strength of the steel will be insufficient.

[0047] Compared with the prior art, the present invention has a simple process and stable performance of precision rolled threaded steel products under the conditions of hot rolling yield strength Rel≥1200MPa, Rm≥1330MPa, elongation A≥6%, and Att≥3.5%. Furthermore, by increasing the heating temperature, the rolling mill load can be reduced while significantly reducing costs, which is conducive to improving production efficiency. Attached Figure Description

[0048] Figure 1 This image shows the metallographic structure of the threaded part of the present invention as tempered troostite. Detailed Implementation

[0049] The present invention will now be described in detail:

[0050] Table 1 is a list of chemical composition values ​​for each embodiment and comparative example of the present invention;

[0051] Table 2 is a list of process parameters for the smelting process in each embodiment and comparative example of the present invention;

[0052] Table 3 is a list of rolling process parameters for each embodiment and comparative example of the present invention;

[0053] Table 4 shows the experimental results of various embodiments and comparative examples of the present invention.

[0054] The various embodiments of the present invention are produced according to the following steps.

[0055] 1) Perform hot metal pretreatment. After desulfurization, the hot metal should have the following composition: P < 0.020% and S < 0.020%.

[0056] 2) In converter smelting, control the final carbon content of the converter to be between 0.1% and 0.36%, and the tapping temperature to be ≥1650℃; when the molten steel has been tapped to 2 / 3, the alloy and carbon raiser should be added all at once.

[0057] 3) Perform LF furnace refining for no less than 25 minutes, with argon blowing throughout the refining process; after the composition is adjusted, exposed molten steel is prohibited.

[0058] 4) Cast into square billets, during which the casting process is protected by a long nozzle in a large ladle and an immersion nozzle in a crystallizer, and the superheat of the molten steel in the tundish is controlled at 20-35℃; the cross-sectional dimensions of the continuously cast square billet are not less than 200mm, and the ratio of the cross-sectional area of ​​the square billet to the cross-sectional area of ​​the thread is controlled to be not less than 9.0.

[0059] 5) When heating the billet, the temperature of the soaking zone should be controlled at 1100-1220℃, and the total heating time should be 60-90 minutes; and the temperature difference between the cross sections of the same billet should be controlled at ≤35℃.

[0060] 6) Perform roughing and intermediate rolling, and control the initial rolling temperature at 1000-1060℃, and control the cumulative deformation rate of roughing and intermediate rolling at 15-50%;

[0061] 7) Perform finishing rolling in two stages, during which: control the temperature entering finishing rolling stage I at 880–950℃, and the temperature entering finishing rolling stage II at…

[0062] The rolling temperature is 900–980℃, and the rolling speed of the last stand is 1.2–20.8 m / s.

[0063] 8) Cool the bed by controlling the temperature at 320-380℃ and cooling it to room temperature at a cooling rate of 3-7℃ / s.

[0064] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0065]

[0066]

[0067] Table 2. List of process parameters for the smelting process of various embodiments and comparative examples of the present invention.

[0068]

[0069] Table 3. List of rolling process parameters for each embodiment and comparative example of the present invention.

[0070]

[0071]

[0072] Table 4 Performance test results of various embodiments and comparative examples of the present invention

[0073]

[0074] Note: The thread diameter in all the above embodiments is in the range of 34-75mm.

[0075] As can be seen from Table 4, compared with the comparative example, the steel of the present invention in the embodiment has higher yield strength, tensile strength and more suitable elongation and other plastic properties, while reducing the total cost of use.

[0076] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A type of rebar with a yield strength ≥1200MPa, comprising the following components and weight percentages: C: 0.48~0.65%, Si: 0.80~1.55%, Mn: 1.00~1.65%, P≤0.035%, S≤0.035%, B: 0.0006~0.0038%, Cr: 0.60~1.10%, Zr: 0.50~0.75%, N 0.0045~0.0120%, V: not higher than 0.10% or Nb not higher than 0.05%, or a mixture of both, wherein the mixture of both satisfies the following condition: (V+1.6Nb) not less than 0.03%, with the remainder being Fe and unavoidable impurities; Production method: 1) After pretreatment of molten iron and desulfurization, the molten iron has the following properties: P < 0.020% and S < 0.020%. 2) Converter smelting: control the final C content at the converter endpoint between 0.1% and 0.36%, and the tapping temperature ≥1650℃; when 2 / 3 of the molten steel has been tapped, The alloy and carburizer should be added all at once; 3) Refine in an LF furnace for at least 25 minutes, with argon blowing throughout the refining process; after the composition is adjusted, molten steel must not be stored. In the phenomenon of nudity; 4) Cast into square billets, during which the casting process is protected by a long nozzle in a large ladle and an immersion nozzle in a crystallizer, and the superheat of the molten steel in the tundish is controlled at 20-35℃; the cross-sectional dimensions of the continuously cast square billet are not less than 200mm, and the ratio of the cross-sectional area of ​​the square billet to the cross-sectional area of ​​the thread is controlled to be not less than 9.

0. 5) When heating the billet, the temperature of the soaking zone should be controlled at 1100-1220℃, and the total heating time should be 60-90 min; and the temperature difference between the same billet cross sections should be controlled at ≤35℃. 6) Perform roughing and intermediate rolling, and control the initial rolling temperature between 1000 and 1060℃, and control the cumulative deformation rate of roughing and intermediate rolling between 15% and 50%; 7) Perform finishing rolling in two stages, during which: control the temperature of entering finishing rolling stage I at 880-950℃, the temperature of entering finishing rolling stage II at 900-980℃, and the rolling speed of the last stand at 1.2-20.8m / s; 8) Cool the bed by controlling the temperature of the bed to be between 320 and 380°C and cooling it to room temperature at a cooling rate of 3 to 7°C / s.

2. The rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The weight percentage of C is between 0.49% and 0.60%.

3. A rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The weight percentage content of Si is between 0.90 and 1.45%.

4. A rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The weight percentage content of Mn is 1.10 to 1.50%.

5. A rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The weight percentage content of B is between 0.0006 and 0.0025%.

6. A rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The weight percentage content of Cr is between 0.65% and 0.95%.

7. A rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The Nb content is between 0.015% and 0.035% by weight.

8. A rebar with a yield strength ≥1200MPa as described in claim 1, characterized in that: The Zr content is between 0.55% and 0.70% by weight.

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