An ultra-low boron content BS500B hot-rolled ribbed steel bar and a production method thereof

Abstract

The application discloses a kind of ultra-low boron content British standard B500B hot-rolled ribbed steel bars and production method thereof, and the chemical composition of hot-rolled ribbed steel bar is as follows in percentage by mass: C 0.18%~0.22%, Si 0.20%~0.50%, Mn 0.90%~1.20%, V 0.001%~0.003%, N 0.005%~0.012%, P≤0.035%, S≤0.035%, Cr≤0.15%, B≤3ppm, Cu≤0.20%, Ni≤0.15%, Mo≤0.08%, the balance is Fe and inevitable impurities.The application has the advantages that converter double-slag process is used, and only industrial silicon-manganese alloy is added during tapping without adding strong deoxidizer, the weak oxidizing property of molten steel is maintained, reducing slag is not formed in the whole argon blowing station, combined with low-boron refractory end control of continuous casting, the whole-process closed-loop boron control path of "deboronation-suppression of back-dissolution-prevention of boron increase" is formed.

Description

Technical Field

[0001] This invention relates to the field of metallurgical rolling technology for construction steel, and in particular to a hot-rolled ribbed steel bar with ultra-low boron content (British Standard B500B) and its production method. Background Technology

[0002] British Standard B500B hot-rolled ribbed steel bars comply with BS 4449:2005 standard and are widely used in British Standard construction projects in the UK, the Middle East, Southeast Asia, etc. They require a yield strength ≥500MPa and a strength-to-yield ratio ≥1.08, and also have clear constraints on carbon equivalent, weldability and anti-aging properties.

[0003] It is well known in the art that boron is an extremely strong hardenability element in steel, and trace amounts of boron (5~15 ppm) can significantly improve the hardenability and strength of steel. Therefore, existing B500B steel bar production technologies mostly adopt a boron-containing microalloying route to improve strength and reduce alloy costs.

[0004] However, for steel reinforcement in welded structures, especially in prefabricated buildings where flash butt welding and arc welding are widely used, boron tends to segregate at grain boundaries and combine with nitrogen during the welding thermal cycle to form boron nitrogen (BN) or boron carbonitridates. This leads to a decrease in the toughness of the weld heat-affected zone (HAZ) and a significant increase in its susceptibility to aging embrittlement. Technical specifications from the Hong Kong Housing Authority (HKHA) and some Middle Eastern owners' codes implicitly require high HAZ impact energy and long-term aging resistance. When boron content is ≥ 5 ppm, the HAZ toughness fluctuation can reach over 30%, making it difficult to meet the requirements of high-standard welded structures. Therefore, for such applications, the boron content in the finished product needs to be controlled at an extremely low level (≤ 3 ppm).

[0005] Currently, it is generally accepted in the metallurgical field that the production of ultra-low boron steel with B ≤ 3 ppm must adopt a long process of "converter - LF refining - (RH vacuum) - continuous casting". On the one hand, it is necessary to purchase special raw and auxiliary materials such as low-boron silicon manganese (B ≤ 15 ppm) and low-boron refining slag to reduce boron introduction from the source; on the other hand, it relies on LF refining for deep deoxidation, inclusion control and composition fine-tuning. There has long been a technical bias in this field: the boron impurity content of ordinary industrial silicon manganese alloys is as high as 150~250 ppm, and direct addition to molten steel will inevitably lead to excessive boron in the finished product. Therefore, the production of ultra-low boron steel must use low-boron alloys, and LF refining is a necessary process to ensure the purity and compositional accuracy of molten steel.

[0006] Among existing publicly available technologies, patents related to ultra-low boron steel are all developed for high-end structural steel, gear steel, and other similar products. For example, Nanjing Iron & Steel Co., Ltd.'s invention patent CN121450867A discloses a "control method for smelting ultra-low boron steel in a converter plant," employing a full-process technology of KR desulfurization → converter double-slag smelting → LF double-slag refining → RH vacuum treatment. Although ordinary silicon-manganese alloy can be used for the steel output, it still relies on the LF process for dual control of low-basicity slag boron adsorption and high-basicity slag anti-redissolution, ultimately achieving B≤1.2ppm. Jiangsu Changqiang Iron & Steel Co., Ltd.'s invention patent CN117965839A discloses another method for smelting ultra-low boron steel in a converter, employing a converter → LF+VD → continuous casting process, explicitly requiring the use of low-boron alloys and low-boron auxiliary materials, with B≤2ppm. All of the above schemes indicate that existing ultra-low boron steel technologies invariably rely on the LF refining process. All of the above existing technologies are essentially ultra-low boron steel = low-boron alloy + LF refining. The underlying logic is that only LF refining can provide sufficient purity and compositional precision in molten steel, and low-boron alloys are the only option for controlling boron sources. However, after in-depth research, the inventors discovered a fundamental contradiction in this technical approach: the strong reducing atmosphere of LF refining (such as Al deoxidation) is precisely the direct thermodynamic driving force leading to the reduction and re-dissolution of B2O3 in the slag. Existing technologies attempt to offset this contradiction by using more expensive low-boron alloys and more complex LF operations, but have never considered fundamentally eliminating LF to completely remove the driving force of re-dissolution.

[0007] Furthermore, the strong reducing atmosphere of LF refining is a significant contributing factor to excessive boron levels. CN113528751A confirms that Al deoxidation during the LF reduction period leads to the reduction and re-dissolution of boron oxides in refractory materials and slag, resulting in an abnormally high boron level in the finished product. In other words, while LF refining can adsorb and remove some primary boron, its reducing atmosphere simultaneously reduces and re-dissolves boron already in the slag, creating a dynamic balance of "removal and re-dissolution," which significantly increases the difficulty of controlling boron levels—this also explains why existing ultra-low boron steel processes have to purchase expensive low-boron alloys and auxiliary materials to offset the risk of LF re-dissolution.

[0008] For products like construction steel bars that are produced in large quantities and widely used, long-process technology has significant drawbacks: LF reduction refining leads to slag boron remelting and boron control is difficult; at the same time, special low-boron raw materials are expensive and the long-process production cycle is long, increasing the cost per ton of steel by 30 to 50 yuan, which is seriously mismatched with the demand for low-cost, large-scale production of steel bars.

[0009] Existing publicly available patents for B500B steel rebar production generally employ boron-containing microalloying or conventional vanadium-nitrogen strengthening routes, none of which specify an ultra-low boron limit requirement of ≤3ppm (B), nor are there any records of short-process ultra-low boron control. Currently, there is no publicly available technology capable of stably producing hot-rolled ribbed steel rebar with a boron content ≤3ppm and meeting the British Standard B500B requirements without LF refining, using only ordinary industrial silicon-manganese, and without adding additional deoxidizers. A clear technological gap exists in this field. Summary of the Invention

[0010] The purpose of this invention is to provide an ultra-low boron content British Standard B500B hot-rolled ribbed steel bar and its production method. This method utilizes a converter double-slag process to oxidize and remove over 70% of the original boron in the molten iron. Only ordinary industrial silicon-manganese alloy is added at the tapping stage, without the addition of strong deoxidizers, maintaining the weak oxidizing properties of the molten steel. No reducing slag is generated throughout the argon blowing process, maintaining the top slag FeO+MnO≥2.0%, thus thermodynamically suppressing the reduction and re-dissolution of B2O3. Combined with end-stage control of low-boron refractory materials in continuous casting, a closed-loop boron control path is formed throughout the entire process: "chemical deboration—suppression of re-dissolution—prevention of boron increase."

[0011] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0012] A hot-rolled ribbed steel bar with ultra-low boron content (British Standard B500B), wherein the chemical composition of the hot-rolled ribbed steel bar, by mass percentage, is: C 0.18%~0.22%, Si 0.20%~0.50%, Mn 0.90%~1.20%, V 0.001%~0.003%, N 0.005%~0.012%, P≤0.035%, S≤0.035%, Cr≤0.15%, B≤3ppm, Cu≤0.20%, Ni≤0.15%, Mo≤0.08%, with the balance being Fe and unavoidable impurities.

[0013] Preferably, the carbon equivalent Ceq of the hot-rolled ribbed steel bar is C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.50%, and the microstructure of the steel bar is mainly composed of ferrite and pearlite, with the ferrite grain size ≥10.

[0014] Preferably, the hot-rolled ribbed steel bar has a yield strength ≥ 500 MPa, a tensile strength of 550~650 MPa, an elongation after fracture A ≥ 14%, a maximum force total elongation Att ≥ 5.0%, and a strength-to-yield ratio ≥ 1.10, meeting the requirements of BS 4449-2005 standard.

[0015] A method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars specifically includes the following steps: (1) The converter adopts the double slag method for smelting. During the middle stage of blowing, the oxidized and deboronized slag is poured out. The slag amount is controlled by double slag blocking during tapping. Industrial silicon-manganese alloy is added during the tapping process to complete silicon-manganese alloying and weak deoxidation. (2) The argon blowing station adopts weak argon blowing treatment. Vanadium-nitrogen alloy is added in the later stage of argon blowing to adjust the composition. After feeding the wire, soft blowing is carried out. (3) Low-boron tundish refractory material and low-boron mold protective slag are used for continuous casting, and the casting process is protected throughout; (4) The billet is rolled into finished steel bars through heating, controlled rolling and controlled cooling processes.

[0016] Preferably, in step (1), the molten iron entering the furnace is pre-desulfurized, and the sulfur content of the molten iron entering the furnace is ≤0.005%, and the boron content of the molten iron entering the furnace is ≤2ppm.

[0017] Preferably, in step (1), the deboronized slag is poured out when the mid-stage blowing temperature is 1420~1480℃, the slag discharge amount is ≤3kg / t steel, the converter endpoint C is ≥0.08%, the tapping temperature is 1640~1665℃, the endpoint P is ≤0.025%, and the boron oxidation removal rate is ≥70%.

[0018] Preferably, in step (1), the boron content of the industrial silicon-manganese alloy is 150~220ppm, the amount added per ton of steel is 11~12kg, and the acid-soluble aluminum content in the molten steel after tapping is ≤0.005%.

[0019] Preferably, in step (2), the total argon blowing time is ≥8 min, of which the soft blowing time is ≥5 min, the V / N mass ratio in the steel is controlled between 0.05 and 0.3, the mass fraction of FeO+MnO is ≥2.0%, and the soft blowing after wire feeding is ≥3 min.

[0020] Preferably, in step (3), the boron content of the low-boron tundish refractory material is ≤5ppm, the boron content of the low-boron crystallizer protective slag is ≤3ppm, the tundish superheat is 20~30℃, the casting speed is 2.0~2.4m / min, and the billet is a 150mm×150mm small square billet.

[0021] Preferably, in step (4), the billet heating temperature is 1100~1130℃, the holding time in the soaking section is ≥40min, the roughing rolling start temperature is 1050~1100℃, the final rolling temperature is 850~900℃, the post-rolling water cooling rate is 300~350℃ / s, and the upper cooling bed temperature is 500~550℃.

[0022] In summary, the present invention has the following beneficial effects: 1. This invention breaks down the ingrained perception and industry bias that "ultra-low boron steel must be refined using low-boron alloys + LF refining." While existing technologies also employ dual-slag and LF processes, their core idea remains optimization within the LF framework. This invention, however, takes a different approach, completely breaking free from the LF framework. It proposes and verifies for the first time a novel technical route—a short process without LF—proving that even without LF, ultra-low boron control can be stably achieved solely by controlling thermodynamic conditions.

[0023] 2. This invention addresses the core challenges of slag boron re-dissolution caused by the reducing atmosphere of the LF furnace and the difficulty in controlling boron levels. Based on the thermodynamic principle of B2O3 reduction (2Al + B2O3 = Al2O3 + 2B, ΔG = -89.83 / T + 0.031), the invention proactively eliminates the LF reduction refining process, thereby removing the driving force for boron oxide re-dissolution from its thermodynamic origin. This permanently locks the primary boron removed from the converter's dual slags into the top slag discharge system.

[0024] 3. To achieve the synergy of ultra-low boron precision control, molten steel castability assurance, and stable compliance with B500B grade mechanical properties, and to address the implicit requirements of British standard projects such as the Hong Kong Housing Authority and the Middle East for welding HAZ toughness and anti-aging performance, under the constraint of B≤3ppm, vanadium-nitrogen microalloying and controlled rolling and cooling are used to strengthen the product in a synergistic way, taking into account both the high performance of the product and the needs of low-cost large-scale production. Detailed Implementation

[0025] The specific embodiments of the present invention will be further described below. These embodiments do not constitute a limitation on the present invention.

[0026] Example 1: The production of φ25mm ultra-low boron B500B steel bars adopts a short process of 120-ton converter-argon blowing station-continuous casting.

[0027] Chemical composition (mass percentage): C: 0.20%, Si: 0.32%, Mn: 1.02%, V: 0.001%, N: 0.010%, P: 0.024%, S: 0.023%, Cr: 0.07%, Cu: 0.09%, B: 2.4ppm, carbon equivalent (Ceq) = 0.44%. The boron content of the ordinary industrial silicon-manganese alloy used is 210ppm, and the amount added per ton of steel is 11kg.

[0028] Smelting process The molten iron is pre-desulfurized to S=0.004% before being fed into the furnace, and the boron content of the molten iron is 1.8ppm. The converter double-slag method was used for blowing, with the boron slag removed at a mid-term temperature of 1450℃, achieving a boron removal rate of 76%; the final carbon content was 0.10%, the tapping temperature was 1655℃, and the slag discharge rate was 2.5 kg / t. The steelmaking process involves adding only ordinary silicon-manganese alloy, without adding any deoxidizers or refining slag; The argon blowing station performed weak argon blowing for 10 minutes and soft blowing for 6 minutes, with the top slag showing FeO + MnO = 2.6%. Vanadium-nitrogen wire was fed to adjust the composition, followed by soft blowing for 4 minutes after wire feeding.

[0029] Continuous casting and rolling processes Continuous casting of 150mm×150mm small square billets, tundish superheat 26℃, casting speed 2.2m / min, low boron magnesium lining and low boron protective slag; The billet is heated to 1110℃ and held for 45 minutes; the roughing rolling temperature is 1070℃ and the final rolling temperature is 880℃. After rolling, the material is cooled by water at a rate of 310℃ / s, and the temperature of the upper cooling bed is 520℃.

[0030] Performance test results Yield strength Re = 520 MPa, tensile strength Rm = 585 MPa, strength-to-yield ratio Rm / Re = 1.125; Elongation after fracture A = 18.2%, total elongation at maximum force Agt = 7.0%; The 180° bending test (bending mandrel diameter 4d) passed, with no cracks. The boron content of the finished product was 2.4 ppm, which meets the requirement of B≤3 ppm.

[0031] Example 2: We produce ultra-low boron B500B steel bars with a diameter of φ32mm.

[0032] Chemical composition (mass percentage) The composition is as follows: C: 0.21%, Si: 0.38%, Mn: 1.12%, V: 0.002%, N: 0.011%, P: 0.026%, S: 0.021%, Cr: 0.09%, Cu: 0.11%, B: 2.7ppm, carbon equivalent (Ceq) = 0.46%. The boron content of the ordinary industrial silicon-manganese alloy used is 190ppm, and the addition rate is 12kg per ton of steel.

[0033] Smelting process The molten iron is pre-desulfurized to S=0.003% before being fed into the furnace, and the boron content of the molten iron is 1.7ppm. The converter double-slag method was used for blowing, with the boron slag removed at an intermediate temperature of 1430℃, achieving a boron removal rate of 72%; the final carbon content was 0.09%, the tapping temperature was 1650℃, and the slag discharge rate was 2.8 kg / t. The steel is produced with only ordinary silicon-manganese alloy, without any additional deoxidizer; Argon blowing station blows argon for 12 minutes, soft blowing for 7 minutes, top slag FeO+MnO=2.3%; vanadium nitrogen feed line adjusts composition.

[0034] Continuous casting and rolling processes The tundish superheating temperature for continuous casting is 24℃, and the casting speed is 2.1m / min; The billet heating temperature is 1110℃, the final rolling temperature is 870℃, the cooling rate is 315℃ / s, and the upper cooling bed temperature is 535℃.

[0035] Performance test results Yield strength Re = 530 MPa, tensile strength Rm = 600 MPa, strength-to-yield ratio Rm / Re = 1.132; Elongation after fracture A = 17.5%, total elongation at maximum force Agt = 6.7%; The 180° bending test (bending mandrel diameter 7d) passed; The boron content of the finished product was 2.7 ppm, which meets the requirement of B≤3 ppm.

[0036] Comparative Example 1 (Conventional LF refining process, same as ordinary silicon-manganese) Using the same raw materials, composition targets and ordinary silicon-manganese alloy as in Example 1, the process flow is the conventional route of "converter-LF refining-continuous casting". LF refining produces white slag for reduction treatment, and the white slag is held for 15 minutes.

[0037] Test results: The boron content of the finished product is 13.6 ppm, far exceeding the limit of 3 ppm; the yield strength is 525 MPa and the tensile strength is 590 MPa, which meets the mechanical properties, but the boron content does not meet the requirements for ultra-low boron.

[0038] Comparative conclusion: LF reduction refining causes a large amount of boron oxides in the slag to dissolve back into the molten steel. Even with the same raw materials, the boron content in the finished product will increase several times, proving that eliminating LF is the key means to achieve ultra-low boron control.

[0039] Comparative Example 2 (same as short process, using low boron silicon manganese) The same short-process technology as in Example 1 was used, except that ordinary silicon manganese was replaced with low-boron silicon manganese alloy (boron content 15ppm), and the amount added per ton of steel was 11kg.

[0040] Test results: The boron content of the finished product is 1.8 ppm, and the mechanical properties are basically the same as those in Example 1; however, the procurement cost of low boron ferrosilicon manganese is 280 yuan / ton higher than that of ordinary ferrosilicon manganese, and the cost of ton steel alloy increases by about 3 yuan.

[0041] Comparative conclusion: This invention achieves boron control effects similar to low-boron silicomanganese using ordinary silicomanganese, with boron content meeting the requirement of ≤3ppm, while significantly reducing raw material costs and possessing significant economic advantages.

[0042] The following table shows the chemical composition, process parameters, and finished product performance test results for each embodiment and comparative example.

[0043] Table 1 Chemical composition (mass fraction) of each sample

[0044] Table 2 Main process parameters of each sample

[0045] Table 3. Performance test results of each sample.

[0046] By comparing Comparative Example 1 with Example 1, the core mechanism of this invention can be clearly revealed: During the LF refining process, although the reducing slag can absorb some inclusions, its strong reducing atmosphere (established by deoxidizers such as Al and Si) causes B2O3 in the slag to undergo a reduction reaction (2Al + B2O3 → Al2O3 + 2B), causing the removed boron to return to the molten steel, forming a dynamic equilibrium of simultaneous removal and return, ultimately leading to uncontrolled boron content. This invention completely cuts off this return path, maintaining the weak oxidizing properties of the top slag, keeping B2O3 in a thermodynamically stable state, preventing its reduction, thereby achieving precise, unidirectional control of boron content. This is the essential difference between this invention and all existing ultra-low boron steel technologies.

[0047] Comparative analysis conclusions 1. Comparison of Examples 1 and 2 with Comparative Example 1: Under the premise of using the same ordinary ferrosilicon manganese raw materials, the boron content of the finished product of the conventional LF refining process is as high as 13.6 ppm, far exceeding the limit of 3 ppm. However, the boron content of the LF-free short-process of the present invention is stably controlled within 3 ppm, and the mechanical properties still fully meet the standards. This proves that the LF reducing atmosphere is the core reason for the slag boron re-dissolution and boron exceeding the standard. Eliminating LF is the key means to achieve ultra-low boron control.

[0048] 2. Comparison of Example 1 and Comparative Example 2: The present invention can achieve a boron control effect close to that of low-boron silicon manganese using ordinary silicon manganese, both meeting the requirement of ≤3ppm, and the mechanical properties are basically the same; however, the purchase cost of ordinary silicon manganese is 280 yuan / ton lower than that of low-boron silicon manganese, which can reduce the cost of steel alloy by about 3 yuan per ton, while eliminating the cost of LF process, resulting in significant comprehensive economic benefits.

[0049] 3. The mechanical and technological properties of both sets of embodiments meet the requirements of BS 4449-2005 standard B500B level, proving that the process of the present invention can stably achieve the product performance standard under ultra-low boron constraint.

[0050] This invention, for the first time in the steel metallurgical field, proposes an unconventional technical approach: "using the elimination of the LF process as the core means to achieve ultra-low boron control." The experimental data from Comparative Example 1 irrefutably demonstrates that, under the same raw material conditions, the conventional LF process results in a boron content as high as 13.6 ppm, while the LF-free process of this invention achieves a boron content of only 2.4 ppm. This stark contrast not only confirms that the LF reducing atmosphere is the primary cause of excessive boron levels but also highlights the non-obviousness and inventiveness of this invention's "actively avoiding LF" technical solution. It is not a simple improvement on existing technology but a fundamental transformation of traditional technical approaches.

[0051] The process is greatly simplified, and production costs are significantly reduced. The LF refining process is eliminated, saving on refining electricity consumption, electrode consumption, and refining slag costs. At the same time, there is no need to purchase expensive low-boron special alloys, reducing the production cost per ton of steel by 30-50 yuan and shortening the production cycle by more than 20%, making it suitable for the needs of large-scale, low-cost production of construction steel bars.

[0052] The boron control effect is stable and the performance fully meets the standards. Through closed-loop management of the entire process of "converter oxidation deboring - weak oxidation slag suppression of remelting - end-of-line refractory controlled boron increase", the boron content of the finished product is stably controlled within 3ppm; by adopting vanadium-nitrogen microalloying and controlled rolling and cooling synergistic strengthening, the yield strength is stable at 510~540MPa, the strength-to-yield ratio is ≥1.12, and the plasticity and bending properties all meet the BS 4449-2005 standard, with specifications covering φ12~φ40mm.

[0053] It features excellent weldability and anti-aging properties due to its ultra-low boron content. This avoids boron, carbon, and nitride grain boundary segregation, resulting in minimal fluctuations in the toughness of the weld heat-affected zone. The carbon equivalent is controlled at 0.42%~0.48%, ensuring excellent weldability. At the same time, it completely solves the embrittlement problem caused by boron precipitation during aging, ensuring stable performance over long-term service.

[0054] It has strong production adaptability and is easy to promote. No new equipment is required, and it can be implemented on a conventional converter-continuous casting-bar rolling production line. It has a wide process parameter window, good production stability, and is easy to promote industrially.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art can make various modifications or equivalent substitutions to the present invention within the scope of its essence and protection. Such modifications or equivalent substitutions should also be considered to fall within the protection scope of the present invention.

Claims

1. A hot-rolled ribbed steel bar with ultra-low boron content (British Standard B500B), characterized in that, The chemical composition of the hot-rolled ribbed steel bars, by mass percentage, is as follows: C 0.18%~0.22%, Si 0.20%~0.50%, Mn 0.90%~1.20%, V 0.001%~0.003%, N 0.005%~0.012%, P≤0.035%, S≤0.035%, Cr≤0.15%, B≤3ppm, Cu≤0.20%, Ni≤0.15%, Mo≤0.08%, with the balance being Fe and unavoidable impurities.

2. The ultra-low boron content British Standard B500B hot-rolled ribbed steel bar according to claim 1, characterized in that: The carbon equivalent Ceq of the hot-rolled ribbed steel bar is C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.50%, and the microstructure of the steel bar is mainly composed of ferrite and pearlite, with the ferrite grain size ≥10.

3. The ultra-low boron content British Standard B500B hot-rolled ribbed steel bar according to claim 1, characterized in that: The hot-rolled ribbed steel bars have a yield strength ≥500MPa, tensile strength 550~650MPa, elongation after fracture A ≥14%, total elongation at maximum force Agt ≥5.0%, and strength-to-yield ratio ≥1.08, meeting the requirements of BS 4449-2005 standard.

4. A method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to any one of claims 1 to 3, characterized in that, Specifically, the following steps are included: (1) The converter adopts the double slag method for smelting. During the middle stage of blowing, the oxidized and deboronized slag is poured out. The slag amount is controlled by double slag blocking during tapping. Industrial silicon-manganese alloy is added during the tapping process to complete silicon-manganese alloying and weak deoxidation. (2) The argon blowing station adopts weak argon blowing treatment. Vanadium-nitrogen alloy is added in the later stage of argon blowing to adjust the composition. After feeding the wire, soft blowing is carried out. (3) Low-boron tundish refractory material and low-boron mold protective slag are used for continuous casting, and the casting process is protected throughout; (4) The billet is rolled into finished steel bars through heating, controlled rolling and controlled cooling processes.

5. The method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to claim 4, characterized in that: In step (1), the molten iron entering the furnace is pre-desulfurized, and the sulfur content of the molten iron entering the furnace is ≤0.005%, and the boron content of the molten iron entering the furnace is ≤2ppm.

6. The method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to claim 4, characterized in that: In step (1), when the mid-stage blowing temperature is 1420~1480℃, the deboronized slag is poured out, the slag amount at tapping is ≤3kg / t steel, the converter endpoint C≥0.08%, the tapping temperature is 1640~1665℃, the endpoint P≤0.025%, and the boron oxidation removal rate is ≥70%.

7. The method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to claim 4, characterized in that: In step (1), the boron content of the industrial silicon-manganese alloy is 150~220ppm, the amount added per ton of steel is 11~12kg, and the acid-soluble aluminum content in the molten steel after tapping is ≤0.005%.

8. The method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to claim 4, characterized in that: In step (2), the total argon blowing time is ≥8 min, of which the soft blowing time is ≥5 min. The V / N mass ratio in the steel is controlled to be 6~8:1 (the vanadium and nitrogen content is adjusted when feeding vanadium-nitrogen alloy wire, and the V / N mass ratio in the steel is controlled to be no greater than 0.5). The FeO+MnO mass fraction is ≥2.0%, and the soft blowing time after feeding the wire is ≥3 min.

9. The method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to claim 4, characterized in that: In step (3), the boron content of the low-boron tundish refractory material is ≤5ppm, the boron content of the low-boron crystallizer protective slag is ≤3ppm, the tundish superheat is 20~30℃, the casting speed is 2.0~2.4m / min, and the billet is a 150mm×150mm small square billet.

10. The method for producing ultra-low boron content British Standard B500B hot-rolled ribbed steel bars according to claim 4, characterized in that: In step (4), the billet heating temperature is 1100~1130℃, the holding time in the soaking section is ≥40min, the rough rolling start temperature is 1050~1100℃, the final rolling temperature is 850~900℃, the water cooling rate after rolling is 300~350℃ / s, and the upper cooling bed temperature is 500~550℃.

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