Preparation method of titanium microalloy reinforced B500B hot-rolled ribbed steel bar

Abstract

The invention provides a preparation method and application of a titanium microalloy reinforced B500B hot-rolled ribbed steel bar. The preparation method comprises the following steps that S1, a continuous casting blank is heated at the temperature of 1200-1250 DEG C, and the heated continuous casting blank is obtained; s2, rough rolling is conducted on the heated continuous casting billet, the initial rolling temperature is controlled to be 1050-1100 DEG C, finish rolling is conducted after controlled rolling and controlled cooling are adopted, the finish rolling temperature is controlled to be 930-980 DEG C, and then the continuous casting billet passes through a finished product rolling mill at a high speed; and S3, cooling the steel bar obtained by rolling the finished product to obtain the titanium microalloy reinforced B500B hot-rolled ribbed steel bar. Through proper component design and a controlled rolling and controlled cooling technology, preparation of the B500B hot-rolled ribbed steel bar with low alloy cost and stable performance is realized.

Description

Technical Field

[0001] This invention belongs to the field of steel continuous casting production technology, and particularly relates to a method for preparing titanium microalloyed B500B hot-rolled ribbed steel bars. Background Technology

[0002] In the field of construction steel, 500 MPa grade hot-rolled ribbed steel bars have become a key material for high-rise and important structural projects both domestically and internationally due to their excellent strength and seismic performance. The current standard GB 1499.2-2024 sets a high strength-to-yield ratio requirement (tensile strength / yield strength ≥ 1.25) for HRB500E seismic steel bars. This is typically achieved by adding higher contents of microalloying elements such as niobium (Nb) and vanadium (V) combined with controlled rolling and cooling processes. While this ensures mechanical properties, the alloy cost is high. In contrast, the strength-to-yield ratio requirement for B500B steel bars in standard BS 4449:2005 is only ≥ 1.08. Although its performance indicators are slightly lower than HRB500E, it can still meet the engineering needs of most non-high-intensity earthquake zones through reasonable composition design and process control. In recent years, with the continuous growth in market demand for 500 MPa grade steel bars, export-oriented enterprises urgently need to develop a B500B steel bar production technology that combines cost-effectiveness and performance stability. Existing studies have adopted niobium-titanium or vanadium-titanium composite microalloying strategies, which have optimized the strength and toughness of small-diameter steel bars or reduced alloy costs to some extent. However, the former still relies on the relatively expensive niobium element, while the latter focuses on specific specification ranges and is difficult to meet the 500 MPa strength requirement. Summary of the Invention

[0003] The main objective of this invention is to provide a method for preparing titanium microalloyed reinforced B500B hot-rolled ribbed steel bars, aiming to achieve low alloy cost and stable performance of B500B hot-rolled ribbed steel bars through appropriate composition design and controlled rolling and cooling technology.

[0004] To achieve the above objectives, this invention provides a method for preparing titanium microalloyed reinforced B500B hot-rolled ribbed steel bars, comprising the following steps: S1. The continuously cast billet is heated at 1200~1250℃ to obtain a heated continuously cast billet; S2. The heated continuous casting billet is first rough rolled, and the initial rolling temperature is controlled at 1050~1100℃. Then, it is finished rolled by controlled rolling and cooling, and the finishing rolling temperature is controlled at 930~980℃. Then, it is passed through the finishing mill at high speed. S3. Cool the steel bars obtained after rolling to obtain B500B hot-rolled ribbed steel bars reinforced with titanium microalloying.

[0005] According to a first aspect of the present invention, at least the following beneficial effects are achieved: The method for preparing B500B hot-rolled ribbed steel bars provided by this invention firstly involves heating the continuously cast billet at a high temperature of 1200~1250℃. This ensures that titanium (Ti) in the steel is fully dissolved in the austenite matrix, laying the foundation for subsequent precipitation strengthening. This temperature range avoids excessive grain coarsening while ensuring complete dissolution of microalloying elements, thereby obtaining fine and dispersed TiC precipitates during subsequent controlled cooling, effectively improving strength. Secondly, after rough rolling, water cooling is used, and the temperature before finishing rolling is precisely controlled at 930~980℃, which is between the non-recrystallized austenite region and the partial phase transformation region. Finish rolling within this temperature window promotes ferrite nucleation through the deformation energy storage of non-recrystallized austenite, refining the final microstructure. Furthermore, it inhibits grain growth and promotes the precipitation of Ti in the form of nanoscale carbonitrides during phase transformation, resulting in a significant synergistic effect of precipitation strengthening and grain refinement strengthening. Finally, the cooling path is controlled after finishing rolling to obtain a multiphase microstructure of bainite (B) + ferrite (F) + pearlite (P). This microstructure combines high strength with good ductility and toughness, enabling the steel bars to not only meet the stringent requirements of BS 4449-2005 standard for yield strength (≥520 MPa), strength-to-yield ratio (≥1.1), and total elongation at maximum force (≥10.0%), but also possess excellent bending properties and weldability.

[0006] In some embodiments, the chemical composition of the continuously cast billet is as follows by mass percentage: C: 0.20%–0.22%, Si: 0.30%–0.40%, Mn: 0.80%–1.00%, P≤0.030%, S≤0.030%, N≤0.008%, Ti: 0.01%–0.03wt%, Cu≤0.20%, with the balance being Fe and unavoidable impurities.

[0007] In this invention, the above-mentioned composition design achieves multiple optimizations of strength, plasticity, weldability, and cost control from a metallurgical perspective: the carbon content is controlled in the range of 0.20% to 0.22%, which effectively limits the carbon equivalent (Ceq ≤ 0.40%) while ensuring the strength of the matrix, significantly improving the weldability and low-temperature toughness of the steel bars; silicon and manganese, as the main solid solution strengthening elements, are precisely matched in content, which can provide stable strength support without significantly deteriorating plasticity; titanium, as the core microalloying element, forms a large number of dispersed nanoscale TiC or TiN precipitates during the controlled rolling and cooling process, producing significant precipitation strengthening and grain boundary pinning effects, effectively refining the austenite and final ferrite-bainite structure.

[0008] In some embodiments, the finished product rolling process is completed at a speed of 10~18m / s.

[0009] Under the aforementioned controlled rolling conditions: on the one hand, the higher rolling speed can significantly shorten the residence time of austenite in the high-temperature region, effectively suppressing the static recrystallization and growth of austenite grains during the finishing rolling process, thereby retaining more deformation energy storage and fine, unrecrystallized austenite grains, providing a large number of nucleation sites for subsequent phase transformation, and promoting the formation of fine and uniform ferrite and bainite multiphase structures; on the other hand, the finishing rolling temperature control window of 930~980℃ combined with the high-speed finishing rolling of 10~18 m / s in the later stage helps to increase the deformation rate, enhance the dynamic strain-induced precipitation effect, and promote the precipitation of titanium (Ti) in the form of finer, more dispersed carbonitrides before or in the early stage of phase transformation, significantly improving the precipitation strengthening efficiency; at the same time, high-speed rolling can also reduce the uneven temperature drop on the surface of the rolled piece, improve the consistency of the structure and properties of the steel bars along the length direction, and improve dimensional accuracy and surface quality.

[0010] In some embodiments, in step S2, the continuous casting billet temperature at the start of rough rolling is 1050–1100°C.

[0011] In this invention, in step S2: on the one hand, completing the rough rolling at 1050–1100℃ fully utilizes the dynamic and static recrystallization mechanisms at high temperatures, effectively breaking down the as-cast structure, eliminating segregation, and obtaining uniform and fine austenite grains, laying a good foundation for the subsequent refinement of the microstructure in the finishing rolling stage; on the other hand, this temperature is higher than the critical temperature for the large-scale precipitation of titanium carbonitrides such as TiC (usually below 1000℃), ensuring that titanium elements remain in a solid solution state in austenite after rough rolling, avoiding premature precipitation to form coarse particles and lose strengthening potential, thus allowing it to precipitate as a nanoscale dispersed phase during the subsequent finishing rolling and cooling process at lower temperatures, maximizing the precipitation strengthening effect. In some embodiments, in step S3, the cooling method includes water cooling, and the water cooling temperature is 730–780℃.

[0012] In some embodiments, the nominal diameter of the obtained titanium microalloyed reinforced B500B hot-rolled ribbed steel bar is 12–32 mm.

[0013] In some embodiments, the Ceq of the continuously cast billet is ≤0.40%.

[0014] Under the above conditions, the hardening tendency of the heat-affected zone (HAZ) during the welding process can be effectively reduced, and the formation of brittle and hard structures such as martensite can be significantly suppressed, thereby greatly improving the welding performance and construction safety of B500B hot-rolled ribbed steel bars and meeting the stringent requirements of British Standard BS 4449-2005 for the weldability of high-strength steel bars. At the same time, without relying on high carbon or high manganese, the strength loss is compensated by fine grain strengthening and Ti precipitation strengthening, achieving the synergistic goal of "low Ceq + high strength".

[0015] In some embodiments, the resulting titanium microalloy reinforced B500B hot-rolled ribbed steel bars have a yield strength of 520–580 MPa, a tensile strength of 600–680 MPa, and a strength-to-yield ratio of 1.10–1.20.

[0016] This invention addresses the problem of high cost and insufficient economic efficiency in the production of B500B hot-rolled ribbed steel bars, which relies heavily on expensive microalloying elements such as niobium (Nb) and vanadium (V). It proposes a novel production method based on the combination of titanium (Ti) microalloying and refined controlled rolling and cooling (TMCP) processes. This method strictly adheres to the technical requirements of British Standard BS 4449-2005 for the mechanical properties and carbon equivalent (Ceq ≤ 0.40%) of B500B steel bars. It involves scientifically designing the chemical composition of the steel and introducing 0.01%–0.03% titanium. Based on this, the continuously cast billet is heated at 1200–1250℃ to promote full titanium dissolution. The rough rolling temperature is controlled at 1050–1100℃ to refine the austenite grains and prevent premature Ti precipitation. Subsequently, water cooling is used to precisely control the temperature before finishing rolling to 930–980℃, and the finished product is rolled at a high speed of 10–18 m / s. Finally, by controlling the upper cooling bed temperature, a multiphase microstructure of bainite + ferrite + pearlite (B+F+P) is obtained. This structure combines high strength, good plasticity, and excellent bending properties, resulting in highly stable mechanical properties across the entire specification range of nominal diameter from 12 to 32 mm. It also features a reasonable strength-to-yield ratio, excellent elongation, and no cracks when bent in both directions. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0018] Figure 1 , Figure 4 The images are magnified 7x microstructures of Example 1 and Comparative Example 1, respectively. Figure 2 , Figure 5 The images are magnified 200x microstructures of Example 1 and Comparative Example 1, respectively. Figure 3 , Figure 6 The images shown are magnified metallographic structures of Example 1 and Comparative Example 1, respectively, at 500x magnification.

[0019] The realization of the objective, functional characteristics and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0022] To further illustrate the present invention, the following examples are provided: Example 1 This embodiment is used to prepare titanium microalloyed reinforced B500B hot-rolled ribbed steel bars with a nominal diameter of 14 mm. The specific steps are as follows: S1. Provide a continuous casting billet with the chemical composition shown in Table 1. Feed the continuous casting billet into a walking beam furnace and heat it uniformly at 1230℃ to obtain a heated continuous casting billet. S2. The heated continuous casting billet is fed into the rolling mill for rough rolling. The temperature of the steel during rough rolling is controlled at 1070℃. Then, it is rapidly cooled by the water cooling device in the rolling mill. The temperature when entering the finishing mill is controlled at 965℃. The final forming is completed in the subsequent finished product mill. The final rolling speed is 18m / s, and hot-rolled ribbed steel bars with smooth surface and complete transverse ribs are obtained with a nominal diameter of 14 mm. S3. After the finished product is rolled, the steel bar is water-cooled by the post-rolling water cooling system, and the temperature is controlled at 760℃ when it is placed on the cooling bed. Then it is naturally air-cooled to room temperature on the cooling bed, and finally hot-rolled ribbed steel bar with bainite + ferrite + pearlite microstructure is obtained. The specific preparation process parameters are shown in Table 2.

[0023] Combination Figure 2 and Figure 3The metallographic micrographs shown (magnifications of 200× and 500×) reveal that, specifically at 200× magnification, ferrite appears as a bright white equiaxed or blocky distribution, while bainite exists in black or dark gray, needle-like or lamellar forms. These characteristics are more pronounced at 500× magnification, where finely layered pearlite structures are also visible. This indicates that grain refinement and microstructure optimization were successfully achieved by precisely controlling a series of process parameters, including heating temperature (1200–1250℃), roughing rolling temperature (1050–1100℃), finishing rolling temperature (930–980℃), and upper cooling bed temperature (730–780℃). Furthermore, although nanoscale TiC / TiN precipitates are not easily observed directly under an optical microscope, the uniformity of the microstructure and the degree of grain refinement suggest that titanium has formed fine, dispersed precipitates that act as grain boundary pinning agents and inhibit grain growth, thereby significantly enhancing the material's strength and stability.

[0024] Further analysis shows that the microstructure in the figure has no obvious coarse grains or crack defects, indicating that the entire production process is stable and controllable, which is crucial for ensuring consistent mechanical properties across the entire specification range. Therefore, Figure 1 Examples 2 and 3 not only verify the effectiveness of the titanium microalloying and controlled rolling and cooling process employed in this invention, but also demonstrate that it can produce high-performance steel reinforcement products that meet the requirements of BS4449-2005 standard. By comparing the data from the comparative examples and embodiments, the importance of specific process steps in obtaining the ideal microstructure is further highlighted, thereby strengthening the technological innovation of the patent and its practical application value.

[0025] In summary, Figure 1 The metallographic information provided in 2 and 3 is strong evidence of the feasibility of the technical solution of this invention, and also provides an important microscopic perspective for understanding the relationship between the production process and the final material properties.

[0026] Example 2 This embodiment provides a method for preparing titanium microalloyed reinforced B500B hot-rolled ribbed steel bars, which prepares titanium microalloyed reinforced B500B hot-rolled ribbed steel bars with a nominal diameter of 20 mm. The specific steps are as follows: S1. Provide a continuous casting billet with the chemical composition shown in Table 1. Feed the continuous casting billet into a walking beam furnace and heat it uniformly at 1230℃ to obtain the heated continuous casting billet.

[0027] S2. The heated continuous casting billet is fed into the rolling mill for rough rolling, and the steel temperature is controlled at 1080℃ at the beginning of rough rolling; then it is rapidly cooled by the water cooling device in the rolling mill, and the temperature is controlled at 970℃ in the finishing mill; the final forming is completed in the downstream finishing mill, and the final rolling speed is 14m / s, to obtain hot-rolled ribbed steel bars with smooth surface and complete transverse ribs, with a nominal diameter of 20 mm.

[0028] S3. After the finished product is rolled, the steel bar is water-cooled by the post-rolling water cooling system, and the temperature is controlled at 750℃ when it is placed on the cooling bed; then it is naturally air-cooled to room temperature on the cooling bed, and finally hot-rolled ribbed steel bar with bainite + ferrite + pearlite microstructure is obtained. The specific preparation process parameters are shown in Table 2.

[0029] Example 3 This embodiment provides a method for preparing titanium microalloyed reinforced B500B hot-rolled ribbed steel bars, which produces titanium microalloyed reinforced B500B hot-rolled ribbed steel bars with a nominal diameter of 32 mm. The specific steps are as follows: S1. Provide a continuous casting billet with the chemical composition shown in Table 1. Feed the continuous casting billet into a walking beam furnace and heat it uniformly at 1230℃ to obtain a heated continuous casting billet. S2. The heated continuous casting billet is fed into the rolling mill for rough rolling, and the steel temperature is controlled at 1085℃ at the end of the rough rolling. Then, it is rapidly cooled by the water cooling device in the rolling mill, and the temperature is controlled at 970℃ in the finishing mill. The final forming is completed in the downstream finishing mill, with a final rolling speed of 10m / s, to obtain hot-rolled ribbed steel bars with a smooth surface and complete transverse ribs, with a nominal diameter of 32mm. S3. After the finished product is rolled, the steel bar is water-cooled by the post-rolling water cooling system, and the temperature is controlled at 755℃ when it is placed on the cooling bed. Then it is naturally air-cooled to room temperature on the cooling bed, and finally hot-rolled ribbed steel bar with bainite + ferrite + pearlite microstructure is obtained. The specific preparation process parameters are shown in Table 2.

[0030] Comparative Example 1 This comparative example is used to prepare non-titanium microalloyed reinforced B500B hot-rolled ribbed steel bars with a nominal diameter of 20 mm. The specific steps are as follows: The chemical composition of this comparative example is shown in Table 1, and the specific preparation process parameters are shown in Table 2. The other conditions are the same as those in Example 2.

[0031] Test case The mechanical properties of the titanium microalloyed reinforced B500B hot-rolled ribbed steel bars prepared in the examples were tested, and the test results are shown in Table 3. Yield strength, tensile strength, strength-to-yield ratio, and maximum force elongation (Agt) were all determined according to the test methods specified in BS4449:2005 "Steel for the reinforcement of concrete – Weldable reinforcing steel – Bar, coil and decoiled product – Specification". Specifically, the tensile test was performed according to BS EN ISO 6892-1, and the bending and reverse bending properties were tested according to Annex B of BS4449:2005. All samples were taken from finished steel bars, and after standard sampling and processing, the tests were completed at room temperature. The results are shown in Table 3.

[0032] Comparative Example 1 shows tempered sorbite on the surface of the reinforcing steel, meaning the metallographic structure consists of tempered sorbite + bainite + ferrite + pearlite. Figure 4-6 As shown, the same 500MPa strength as the titanium microalloy reinforcement in the example is achieved through high manganese alloy and microstructure strengthening. The alloy cost is about 20 yuan / ton higher than that of the example steel.

[0033] Table 3 Mechanical properties of each embodiment The present invention provides a production method for titanium microalloyed B500B hot-rolled ribbed steel bars, which achieves a balance of high performance, high stability and low cost within a full range of hot-rolled specifications through systematic integration of composition design and full-process thermomechanical control technology. Specifically, this method precisely controls the heating temperature of the continuously cast billet at 1200–1250℃ to ensure that titanium is fully dissolved in the austenite matrix; the temperature of the rough rolling stage after tapping is controlled at 1050–1100℃, which effectively refines the austenite grains and avoids premature precipitation of titanium; then, the temperature of the finishing rolling is rapidly controlled to 930–980℃ by water cooling, so that the steel is in the non-recrystallization region of austenite, accumulating deformation energy to promote subsequent phase transformation refinement; the subsequent finished product rolling process is completed at a high speed of more than 10 m / s (preferably 10–18 m / s), which further inhibits grain growth and enhances the strain-induced precipitation effect; after rolling, the temperature of the upper cooling bed is precisely controlled at 730–780℃ by water cooling, creating ideal conditions for bainite transformation and nano-scale TiC / TiN precipitation. The synergistic effect of the aforementioned high-temperature heating and refined controlled rolling and cooling processes significantly promoted the solid solution-precipitation behavior of titanium microalloys, realizing a multiple strengthening mechanism of grain refinement, precipitation strengthening, and multiphase microstructure regulation. The resulting B500B hot-rolled ribbed steel bars have a smooth surface and uniform microstructure, with a metallographic structure of a multiphase combination of bainite (B), ferrite (F), and pearlite (P). Their excellent and stable mechanical properties fully meet all the technical requirements of British Standard BS 4449-2005 for B500B steel bars in terms of strength, ductility, weldability, and carbon equivalent.

[0034] In summary, by precisely controlling the Ti content and Ceq, and combining high-temperature solution treatment, fine rolling in the non-recrystallization zone, high-speed rolling, and precise controlled cooling, this invention successfully activates the fine-grain strengthening and precipitation strengthening potential of titanium, achieving high stability, high pass rate, and excellent comprehensive performance of mechanical properties of all specifications of B500B steel bars, fully meeting the requirements of BS 4449-2005 standard.

[0035] In summary, the above-described technical solutions of the present invention are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention's specification and drawings under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar, characterized by, The method comprises the following steps: S1. heating the continuous casting billet at 1200-1250℃; S2. rough rolling the heated continuous casting billet, controlling the rough rolling temperature to be 1050-1100℃, then controlling rolling and cooling to finish rolling, controlling the finish rolling temperature to be 930-980℃, and then high-speed passing through a finished rolling mill; S3. controlling cooling of the finished rolled steel bar to obtain a titanium micro-alloyed B500B hot-rolled ribbed steel bar.

2. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, The chemical composition of the continuous casting billet is as follows in mass percentage: C: 0.20%-0.22%, Si: 0.30%-0.40%, Mn: 0.80%-1.00%, P≤0.030%, S≤0.030%, N≤0.008%, Ti: 0.01%-0.03wt%, Cu≤0.20%, and the balance being Fe and inevitable impurities.

3. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, In the finished rolling process, the speed is 10-18m / s.

4. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, In step S2, the rough rolling temperature is controlled to be 1050-1100℃, and the finish rolling temperature is controlled to be 930-980℃.

5. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, In step S3, the cooling method comprises water cooling, and the temperature after water cooling is 730-780℃.

6. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, The nominal diameter of the obtained titanium micro-alloyed B500B hot-rolled ribbed steel bar is 12-32mm.

7. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, The Ceq of the continuous casting billet is ≤0.40%.

8. The method of producing a titanium micro alloyed B500B hot rolled ribbed steel bar as claimed in claim 1, wherein, The yield strength of the obtained titanium micro-alloyed B500B hot-rolled ribbed steel bar is 520-580MPa, the tensile strength is 600-680MPa, and the strength-yield ratio is 1.10-1.

20.

9. Application of the titanium micro-alloyed B500B hot-rolled ribbed steel bar prepared by the method of any one of claims 1-8 in building engineering.

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