Preparation method of tempered structure B500B hot-rolled ribbed steel bar

Abstract

The invention provides a preparation method of a tempered structure B500B hot-rolled ribbed steel bar. The preparation method comprises the following steps: S1, heating a continuous casting billet at 900-1100 DEG C to obtain a heated continuous casting billet; s2, rough rolling is conducted on the heated continuous casting billet, then finish rolling is conducted after water cooling is conducted, the temperature when finish rolling is conducted is controlled to be 900-940 DEG C, and then the continuous casting billet passes through a finished product rolling mill at a high speed; and S3, the steel bar obtained after finished product rolling is completed is cooled, and the tempered structure B500B hot-rolled ribbed steel bar is obtained. On the premise that expensive microalloy elements such as niobium and vanadium are not added, the full-specification B500B hot-rolled ribbed steel bar is stably produced through cooperative regulation and control of component optimization and a controlled rolling and controlled cooling process; and therefore, the requirements of high strength (the yield strength is 520-580 MPa), a reasonable strength-to-yield ratio (1.10-1.20), good plasticity (the maximum force total elongation is greater than or equal to 10%), low welding sensitivity (Ceq is less than or equal to 0.48%) and excellent surface quality are met at the same time.

Description

Technical Field

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

[0002] With the increasing demand for infrastructure construction, the market for 500 MPa grade hot-rolled ribbed steel bars continues to expand, among which B500B steel bars specified in European standard BS 4449-2005 have become an important category. Compared with HRB500E seismic steel bars in standard GB 1499.2-2024, B500B has significant differences in mechanical property requirements: it only requires a tensile strength to yield strength ratio (strength-to-yield ratio) ≥1.08, far lower than HRB500E's ≥1.25. This difference means that B500B does not need to rely on high strength, toughness, and ductility design, and therefore does not need to use expensive microalloying elements such as high content of niobium (Nb) and vanadium (V) for strengthening. Instead, the microstructure and properties can be controlled by optimizing hot rolling process parameters. However, in actual production, how to achieve stable control of B500B steel bar performance while taking into account cost-effectiveness without using or significantly reducing microalloying additions remains a technical challenge for the industry. On the one hand, while over-reliance on alloying can guarantee performance, it significantly increases raw material costs. On the other hand, adjusting conventional rolling processes alone often fails to simultaneously meet the comprehensive requirements for strength, plasticity, carbon equivalent, and surface quality across the entire specification range (especially ∮12mm~∮32mm), resulting in poor batch stability, high fuel consumption, and insufficient market competitiveness. Existing solutions propose stabilizing the performance of B500B steel bars by precisely controlling the rolling temperature; other solutions attempt to replace ferrovanadium with micro-nitrogen alloys, reducing alloy costs while improving performance margins. However, these solutions either still involve specific alloy adjustments or fail to systematically address the coordination issues between low-temperature heating, fast rolling rhythm, and controlled cooling paths, and still fall short in balancing low fuel consumption, absence of micro-alloying, applicability across all specifications, and uniform microstructure. Summary of the Invention

[0003] The main objective of this invention is to provide a method for preparing tempered B500B hot-rolled ribbed steel bars. This method aims to produce high-performance, high-stability B500B hot-rolled ribbed steel bars without relying on expensive microalloying, and simply through composition optimization and controlled rolling and cooling processes, thereby solving the problem of insufficient performance of existing products.

[0004] To achieve the above objectives, the present invention provides a method for preparing tempered B500B hot-rolled ribbed steel bars, comprising the following steps: S1. The continuously cast billet is heated at 900~1100℃ to obtain a heated continuously cast billet; S2. The heated continuous casting billet is first rough rolled at a temperature of 950~990℃, then water-cooled and then fine rolled at a temperature of 900~940℃, and then passed through a high-speed finishing mill. S3. Cool the steel bars obtained after rolling to obtain tempered B500B hot-rolled ribbed steel bars.

[0005] According to the first aspect of the present invention, at least the following beneficial effects are achieved: This invention achieves stable production of high-performance, high-surface-quality B500B steel bars without adding expensive microalloying elements such as niobium (Nb) and vanadium (V) by synergistically controlling the steelmaking composition, heating regime, and post-rolling cooling path. Specifically, in the hot rolling process, this method heats the continuously cast billet at a low temperature of 900–1100℃ to effectively suppress austenite grain coarsening and reduce fuel consumption. Subsequently, through a controlled rolling and cooling path—rough rolling—inter-roll water cooling—finish rolling—finished product—the temperature entering the finish rolling stage is precisely controlled within the 900–940℃ range. This ensures sufficient dynamic recrystallization to refine the grains while avoiding coarse microstructure caused by high temperatures. This microstructure combines high strength and good plasticity, resulting in a smooth surface, intact transverse ribs, and excellent mechanical properties: yield strength of 520–580 MPa, tensile strength of 600–680 MPa, a strength-to-yield ratio stable at 1.10–1.20, and a maximum force elongation (Agt) of 10.0%–15.0%, fully meeting European Standard BS. The 4449-2005 standard sets forth various technical requirements for B500B steel bars (including chemical composition, carbon equivalent, bending performance, and strength-plasticity indices), achieving performance uniformity and batch stability across the entire specification range (nominal diameter 12–32 mm), significantly enhancing the product's market competitiveness and engineering applicability.

[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: 1.20%–1.40%, P≤0.030%, S≤0.030%, N≤0.012%, Cu≤0.20%, with the balance being Fe and unavoidable impurities.

[0007] This invention provides a method for preparing tempered B500B hot-rolled ribbed steel bars. By organically combining specific composition design with precise controlled rolling and cooling processes, a technological breakthrough is achieved in the stable production of high-performance B500B steel bars without the addition of microalloying elements such as niobium and vanadium. Specifically, in the steelmaking process, the chemical composition of the continuously cast billet is strictly controlled by mass percentage as follows: C 0.20%~0.22%, Si 0.30%~0.40%, Mn 1.20%~1.40%, P≤0.030%, S≤0.030%, N≤0.012%, Cu≤0.20%, with the balance being Fe and unavoidable impurities. This composition system ensures sufficient hardenability while controlling the carbon equivalent (Ceq) at a low level of ≤0.48%, balancing strength, plasticity, and weldability, and significantly reducing raw material costs. Based on this, the B500B hot-rolled ribbed steel bars produced have a smooth surface, complete transverse ribs, and excellent mechanical properties: yield strength of 520–580 MPa, tensile strength of 600–680 MPa, strength-to-yield ratio stable at 1.10–1.20, and maximum force elongation (Agt) of 10.0%–15.0%. Its chemical composition, carbon equivalent, and mechanical properties fully meet the requirements of European standard BS 4449-2005, and it is applicable to the full range of nominal diameters from 12 to 32 mm, exhibiting good batch stability and engineering applicability.

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

[0009] Under the aforementioned conditions, the high-speed finishing rolling process, in synergy with the temperature control path, significantly improves the microstructure uniformity and mechanical property stability of the product. On the one hand, the higher finishing rolling speed shortens the residence time of the steel in the high-temperature zone, effectively suppressing grain growth and premature ferrite precipitation, providing a fine and uniform parent phase for subsequent phase transformation. On the other hand, entering the finishing mill at a finishing rolling temperature window of 900–940℃, combined with a rolling speed of 10–15 m / s in the later stage, enhances deformation energy storage, promotes deformation-induced phase transformation, and is beneficial for improving strength and yield strength ratio.

[0010] In some embodiments, the rolling speed used in the finished product rolling process can be 10 m / s, 14 m / s, and 18 m / s, respectively.

[0011] In some embodiments, in step S2, the temperature of the continuously cast billet during rough rolling is 950–990°C.

[0012] In some embodiments, the temperature during rough rolling in step S2 is a specific point value, including but not limited to: 970°C, 975°C, and 980°C.

[0013] Under the aforementioned conditions, this temperature range is crucial for effectively connecting microstructure refinement with subsequent controlled rolling and cooling processes. On the one hand, it ensures sufficient dynamic and static recrystallization during rough rolling, significantly refining austenite grains and providing a fine and uniform microstructure foundation for the final product. On the other hand, it effectively prevents excessive grain growth, avoids coarsening of subsequent phase transformation products, and thus ensures a balance between the strength and toughness of the reinforcing steel.

[0014] In some embodiments, in step S3, the cooling method includes water cooling, and the temperature of the water cooling is 630~690°C.

[0015] In some implementations, the cooling method in step S3 includes water cooling, and the termination temperature of the water cooling (i.e., the temperature of the cooling bed on the steel bar) is a specific key point value, including but not limited to: 650°C, 660°C and 670°C.

[0016] Under the above conditions, the controlled cooling system is the core link to achieve the synergistic optimization of tempering structure control and strong plasticity. Terminating forced cooling within this temperature range can ensure that the surface of the steel bar receives a sufficiently fast cooling rate to promote the formation of fine grain structure. In the subsequent natural air cooling process, if the water cooling temperature is too low, the core heat will be insufficient, making it difficult to achieve effective self-tempering, which can easily lead to hard and brittle structure and reduced elongation.

[0017] In some embodiments, the nominal diameter of the resulting tempered B500B hot-rolled ribbed steel bar is 12–32 mm.

[0018] In some embodiments, the Ceq of the continuously cast billet is ≤0.48%.

[0019] Under the above conditions, if Ceq is too high, it will significantly increase the hardening tendency of the heat-affected zone, leading to easy cracking during welding and reducing engineering safety. Precisely controlling Ceq at a lower level of ≤0.48% ensures sufficient hardenability to achieve a yield strength of 520–580 MPa and a strength-to-yield ratio of 1.10–1.20, while also significantly improving the weldability and low-temperature toughness of the reinforcing steel. Simultaneously, it helps reduce the susceptibility to surface cracking during the rolling process.

[0020] In some embodiments, the resulting tempered 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.

[0021] This invention employs a process path combining low-temperature heating (900–1100℃) with segmented temperature-controlled rolling (roughing temperature 950–990℃ and finishing temperature 900–940℃), which significantly promotes grain refinement and improves the stability of the final product's performance. On the one hand, low-temperature heating effectively suppresses abnormal grain growth in continuously cast billets, providing a fine and uniform initial microstructure for subsequent hot rolling. On the other hand, controlling the temperature in the roughing stage within the 950–990℃ range ensures that deformation occurs in the austenite recrystallization zone. Through repeated dynamic and static recrystallization, the austenite grain size is significantly refined. Subsequently, the temperature is rapidly reduced to 900–940℃ by water cooling between the rolls before entering the finishing rolling stage, placing the steel at the boundary between the non-recrystallization zone and the phase transformation critical zone. At this point, high-density dislocations and deformation bands are preserved, which not only further hinders grain boundary migration but also provides numerous nucleation sites for phase transformations such as ferrite and bainite during the subsequent cooling process. The gradient temperature control strategy of low-temperature heating—fully recrystallized rough rolling—non-recrystallized finish rolling, combined with specific composition design (e.g., C 0.20%–0.22%, Mn 1.20%–1.40%) and post-rolling precise controlled cooling self-tempering process at 630–690℃, forms an organic whole. The synergistic effect of these two processes enables the steel bars to stably achieve a yield strength of 520–580 MPa, a strength-to-yield ratio of 1.10–1.20, and a maximum total elongation at maximum force of ≥10% even without Nb and V microalloying. More importantly, this synergistic mechanism has good adaptability to different specifications—by matching the finish rolling speed and cooling intensity, it can effectively compensate for changes in cooling rate caused by differences in cross-sectional dimensions, thereby achieving high consistency in microstructure and mechanical properties across the entire specification range. This fundamentally solves the technical problems of low strength-to-yield ratio and large batch fluctuations in traditional microalloy-free B500B steel bars.

[0022] This invention also provides an application of tempered B500B hot-rolled ribbed steel bars in construction engineering. Attached Figure Description

[0023] 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.

[0024] Figure 1 This is a microscope image (7x magnification) of the macroscopic metallographic structure obtained in Example 1. Figure 2 This is a microscopic image (500x magnification) of the edge metallographic structure obtained in Example 1. Figure 3This is a microscopic image (500x magnification) of the metallographic structure at a quarter position obtained in Example 1.

[0025] 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

[0026] 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.

[0027] 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.

[0028] To further illustrate the present invention, the following examples are provided: Example 1 This embodiment is used to prepare tempered 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 1030℃ to obtain the 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 970℃. Then, it is rapidly cooled by the water cooling device in the rolling mill. The temperature of the finishing mill is controlled at 955℃. The final forming is completed in the subsequent finishing 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 14mm. 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 650℃ when it is placed on the cooling bed. Then it is naturally air-cooled to room temperature on the cooling bed, and self-tempering is achieved by using the residual heat of the core. Finally, hot-rolled ribbed steel bars with a metallographic structure of tempered sorbite + bainite + ferrite + pearlite are obtained. The specific preparation process parameters are shown in Table 2.

[0029] Example 2 This embodiment provides a method for preparing tempered B500B hot-rolled ribbed steel bars, which produces tempered 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 1030℃ to obtain the 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 980℃ during rough rolling; then it is rapidly cooled by the water cooling device in the rolling mill, and the temperature is controlled at 960℃ during the finishing mill; the final forming is completed in the subsequent finishing mill, with a final rolling speed of 14m / s, to obtain hot-rolled ribbed steel bars with a smooth surface and complete transverse ribs, and a nominal diameter of 20mm. 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 660℃ when it is placed on the cooling bed. Then it is naturally air-cooled to room temperature on the cooling bed, and self-tempering is achieved by using the residual heat of the core. Finally, hot-rolled ribbed steel bars with a metallographic structure of tempered sorbite + bainite + ferrite + pearlite are obtained. The specific preparation process parameters are shown in Table 2.

[0030] Example 3 This embodiment provides a method for preparing tempered B500B hot-rolled ribbed steel bars, which produces tempered 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 1050℃ 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 975℃ during rough rolling; then it is rapidly cooled by the water cooling device in the rolling mill, and the temperature is controlled at 950℃ during the finishing mill; the final forming is completed in the subsequent 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, and 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 670℃ when it is placed on the cooling bed. Then it is naturally air-cooled to room temperature on the cooling bed, and self-tempering is achieved by using the residual heat of the core. Finally, hot-rolled ribbed steel bars with a metallographic structure of tempered sorbite + bainite + ferrite + pearlite are obtained. The specific preparation process parameters are shown in Table 2.

[0031] Comparative Example 1 Comparative Example 1 provides a method for preparing tempered B500B hot-rolled ribbed steel bars. The difference between this method and Example 1 is that the heating temperature of the continuously cast billet is increased to 1200℃, while all other process parameters remain the same. Under these conditions, the excessively high heating temperature leads to grain coarsening. Even if subsequent roughing and finishing rolling are carried out within the same temperature range, it is difficult to completely refine the already grown original grains through recrystallization. The coarse austenite structure transforms into coarse ferrite-pearlite or Widmanstätten structure during the subsequent controlled cooling process, inhibiting the effective formation of bainite and fine-grained tempered sorbite. The elongation decreases significantly, the yield strength fluctuates greatly, and the mechanical properties barely meet the core requirements of BS 4449-2005 for B500B steel bars.

[0032] Comparative Example 2 Comparative Example 2 provides a method for preparing tempered B500B hot-rolled ribbed steel bars, which differs from Example 1 in that the upper cooling bed temperature is controlled at 550°C, while the other conditions are the same as in Example 1.

[0033] Table 1 Chemical composition of each embodiment (wt%, balance Fe) Table 2 Rolling process parameters for each embodiment Test case The mechanical properties of the tempered B500B hot-rolled ribbed steel bars prepared in the examples were tested, and the results are shown in Table 3. Yield strength, tensile strength, strength-to-yield ratio, and total elongation at maximum force (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, tensile tests were performed according to BS EN ISO 6892-1, and 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. Table 3 Mechanical properties of each embodiment Mechanical properties of the tempered B500B hot-rolled ribbed steel bars with ∮14 mm, ∮20 mm, and ∮32 mm diameters prepared in Examples 1-3 were tested, and the results are shown in Table 3. All samples exhibited high strength, good plasticity, and a stable strength-to-yield ratio, fully meeting the technical requirements of European Standard BS 4449-2005 for the B500B grade. Specifically: Example 1 (∮14 mm): Under the process conditions of heating temperature 1100℃, roughing temperature 970℃, finishing temperature 955℃, upper cooling bed temperature 650℃, and final rolling speed 12 m / s, a yield strength of 520 MPa, tensile strength of 620 MPa, strength-to-yield ratio of 1.19, and total elongation at maximum force of 11.0% were obtained. These results indicate that low-temperature rapid rolling and precise controlled cooling effectively refined the grains and promoted the formation of tempered sorbite and bainite, achieving a good match between strength and ductility in small-size products. Example 2 (∮20 mm): Using a slightly higher roughing rolling temperature (980℃) and finishing rolling temperature (960℃), combined with an upper cooling bed temperature of 670℃, a yield strength of 550 MPa, tensile strength of 630 MPa, a strength-to-yield ratio of 1.15, and an elongation of 10.0% were obtained. This shows that for medium-sized specifications, appropriately increasing the rolling temperature window helps ensure deformation uniformity, while still achieving sufficient phase transformation strengthening through post-rolling controlled cooling to ensure stable and compliant performance. Example 3 (∮32 mm): As a representative of large-sized specifications, its cooling rate is relatively slow, which easily leads to coarsening of the microstructure. This invention successfully suppressed excessive ferrite precipitation by controlling the roughing rolling temperature at the upper limit of 975℃, the finishing rolling temperature at 950℃, and using a relatively low upper cooling bed temperature of 690℃, ultimately obtaining a yield strength of 540 MPa, tensile strength of 645 MPa, a strength-to-yield ratio of 1.19, and an elongation of 10.0%. The achievement of high strength-to-yield ratio and high tensile strength proves that the process can still effectively control the microstructure to a fine tempered S+B+F+P mixed microstructure in large-size products, avoiding the problem of low strength-to-yield ratio commonly found in traditional microalloying processes.

[0034] Comparative Example 1, using a high heating temperature (1150-1200℃), showed that although the mechanical properties were basically qualified, the toughness was significantly worse, microcracks appeared during cold bending, and the elongation was also below the lower limit. Comparative Example 2, using a low upper cooling bed temperature (550℃), showed that the yield strength exceeded the standard, and the strength fluctuated greatly.

[0035] In summary, by optimizing the basic composition (Ceq ≤ 0.45%), using low-temperature heating of ≤1100℃, controlling the temperature range of roughing and finishing rolling, matching the high speed of final rolling (10–18 m / s), and precisely controlling the temperature of the upper cooling bed (650–690℃), this invention successfully prepared B500B hot-rolled ribbed steel bars with uniform microstructure, stable performance, and excellent comprehensive mechanical properties in the full specification range of ∮12–∮32 mm without adding microalloying elements such as Nb and V. This significantly improves the market competitiveness and export applicability of the product.

[0036] 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 tempered structure B500B hot-rolled ribbed steel bar, characterized by, Includes the following steps: S1. The continuously cast billet is heated at 900~1100℃ to obtain a heated continuously cast billet; S2. The heated continuous casting billet is first rough rolled at a temperature of 950~990℃, then water-cooled and then fine rolled at a temperature of 900~940℃, and then passed through a high-speed finishing mill. S3. Cool the steel bars obtained after rolling to obtain tempered B500B hot-rolled ribbed steel bars.

2. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, characterized by, The chemical composition of the continuously cast billet, by mass percentage, is C: 0.20%–0.22%, Si: 0.30%–0.40%, Mn: 1.20%–1.40%, P≤0.030%, S≤0.030%, N≤0.012%, Cu≤0.20%, with the balance being Fe and unavoidable impurities.

3. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, characterized by, The finished product rolling process is completed at a speed of 10~18m / s.

4. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, characterized by, In step S2, the temperature of the continuously cast billet at the end of the rough rolling is 950-990℃.

5. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, wherein In step S3, the cooling method includes water cooling, and the temperature of the water cooling is 630~690℃.

6. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, wherein The nominal diameter of the tempered B500B hot-rolled ribbed steel bars is 12–32 mm.

7. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, wherein The Ceq of the continuously cast billet is ≤0.48%.

8. The method of producing a tempered structure B500B hot-rolled ribbed steel bar according to claim 1, wherein The resulting tempered 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.

9. The application of a tempered B500B hot-rolled ribbed steel bar prepared by the method described in any one of claims 1 to 8 in construction engineering.

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