Bainite high-tensile-yield-ratio high-strength steel bar and preparation method thereof

Abstract

The invention relates to a bainite high-tensile-yield-ratio high-strength steel bar and a preparation method thereof, belongs to the field of high-strength steel bars, and solves the problems of insufficient strength allowance and shock resistance of an existing steel bar. The bainite high-strength steel bar with the high strength-to-yield ratio comprises, by weight, 0.25%-0.30% of C, 0.60%-1.00% of Si, 1.30%-1.80% of Mn, 0.25%-0.30% of V, 0.6%-1.0% of Cr, 0.25%-0.35% of Mo, 0.002%-0.005% of B and the balance Fe and inevitable impurity elements, and the relation between the mass percentage content [C] of C in steel and the mass percentage content [Mo], [V] and [Cr] of Cr, V and Mo elements with fixed C atoms meets the condition that [C] / (0.96 [Cr] + 1.37 [Mo] + 0.78 [V]) is larger than or equal to 0.16 and smaller than or equal to 0.25. The high-strength steel bar has the characteristics of high strength, high plasticity, high tensile-to-yield ratio and the like at the same time.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel bars, and in particular to a bainitic high-strength yield ratio high-strength steel bar and its preparation method. Background Technology

[0002] As my country's construction industry continues to develop towards large-scale operations, large public buildings and high-rise buildings have increasingly complex structures, which places stringent requirements on the strength and comprehensive performance of steel reinforcement.

[0003] Upgrading the strength grade of reinforcing steel not only increases the safety of building structures but also saves materials. Compared with HRB335, HRB400, and HRB500, 600MPa grade reinforcing steel can save 73.3%, 44.4%, and 19.5% of steel consumption, respectively. If the existing reinforcing steel structure is replaced with 700MPa grade or higher, it is estimated that more than 80% of steel consumption can be saved. However, my country currently only has the HRB600 national standard, while foreign construction reinforcing steel has already reached the 700MPa level.

[0004] Patent application CN114990429A discloses a high-strength seismic-resistant steel bar HRB600E. The steel bar has the following alloy composition: C: 0.23-0.28%, Si: 0.45-0.80%, Mn: 1.30-1.50%, Cr: 0.15-0.30%, V: 0.10-0.15%, Nb: 0.015-0.030%, P: ≤0.035%, S: ≤0.035%, N: 0.015-0.025%, yield strength ≥600MPa, tensile strength ≥730MPa, total elongation at maximum force ≥9%, strength-to-yield ratio ≥1.25, and yield-to-standard ratio ≤1.30. As can be seen from the embodiments of this patent, the yield strength is only 630MPa and the strength-to-yield ratio is 1.27, which still does not meet the requirements for strength and seismic resistance.

[0005] Patent application CN118147522A discloses a 630MPa high-strength earthquake-resistant steel bar with the following chemical composition: C: 0.23-0.28%; Si: 0.50-0.70%; Mn: 1.50-1.60%; Nb: 0.010-0.017%; V: 0.135-0.145%; Ti: 0.007-0.018%; P≤0.030%; S≤0.030%; N: 0. 0.020~0.025%, Cr≤0.055%, Mo≤0.006%; the balance is Fe and unavoidable impurities, yield strength ≥645MPa, tensile strength ≥825MPa, strength-to-yield ratio ≥1.25%, elongation ≥16%, maximum total elongation ≥9.5%. In the example, the maximum yield strength is 677MPa, and the strength-to-yield ratio is also below 1.3, which still does not meet the increasingly higher requirements for strength and seismic performance.

[0006] Patent KR100987347B1 discloses a high-strength, low-yield steel bar with high yield strength and low yield-to-yield ratio. Its chemical composition is: C: 0.1–0.4%, Si: 0.1–0.6%, Mn: 0.4–1.5%, P: less than 0.03%, S: 0.03%, Cu: less than 0.3%, V: less than 0.15%, with the remainder consisting of Fe and other unavoidable impurities. The surface portion is a soft phase, while the central portion is a mixed structure of ferrite and pearlite. The yield strength ranges from 400 to 750 MPa, while the yield ratio is less than 80%. In the examples, the highest yield strength of the steel is only 610 MPa, and the highest yield-to-yield ratio is only 1.38. The strength and yield-to-yield ratio still do not meet the requirements, and the preparation method is overly complex.

[0007] Patent application KR20220021060A discloses an ultra-high strength rebar containing 0.10% to 0.45% C, 0.5% to 1.0% Si, 0.40% to 1.80% Mn, 0.10% to 1.0% Cr, 0-0.2% V, 0-0.4% Cu, 0-0.5% Mo, 0.015% to 0.070% Al, greater than 0 and less than or equal to 0.25% Ni, greater than 0 and less than or equal to 0.1% Sn, greater than 0 and less than or equal to 0.05% P, and greater than 0 and less than or equal to 0.25% Ni, greater than 0 and less than or equal to 0.1% Sn, greater than 0 and less than or equal to 0.05% P, and greater than 0 and less than or equal to 0.25% Ni, greater than 0 and less than or equal to 0.1% Sn, greater than 0 and less than or equal to 0.05% P, and greater than 0 and less than or equal to 0.25% Ni, greater than 0 and less than or equal to 0.25% Sn, greater than 0 and less than or equal to 0.1% Sn, greater than 0 and less than or equal to 0.25% Ni, greater than 0 and less than or equal to 0.25% Sn, greater than 0 and less than or equal to 0.25% Cu ... The alloy contains 0.03% S, 0.005% to 0.02% N, and the balance is iron (Fe) and other unavoidable impurities, including a central portion and a surface portion formed on the outer circumference of the central portion, wherein the surface portion contains tempered martensite, and the microstructure of the central portion includes ferrite, pearlite and bainite, with a yield strength of 700 MPa or more and a strength-to-yield ratio of 1.25 or more. Although the strength level of this patent reaches 700 MPa, the strength-to-yield ratio is less than 1.28, which does not meet the higher seismic resistance requirements. At the same time, it contains a lot of alloying elements, making manufacturing complicated and costly.

[0008] Therefore, developing high-strength, high-yield-strength steel bars with excellent comprehensive performance is an inevitable trend in response to the development of the construction industry. Summary of the Invention

[0009] The purpose of this invention is to provide a bainitic high-strength-yield-ratio high-strength steel bar and its preparation method. This steel bar has the characteristics of high strength, high strength-yield-ratio and good strength-plasticity matching, so as to solve the problems of insufficient strength margin and seismic resistance of existing steel bars.

[0010] The technical solution adopted by the present invention to solve the above problems is as follows: a bainitic high-strength yield ratio high-strength steel bar, wherein the chemical composition of the steel bar by mass percentage is: C: 0.25-0.30%, Si: 0.60-1.00%, Mn: 1.30-1.80%, V: 0.25-0.30%, Cr: 0.6-1.0%, Mo: 0.25-0.35%, B: 0.002-0.005%, with the remainder being Fe and unavoidable impurity elements. The relationship between the mass percentage of C in the steel [C] and the mass percentages of Cr, V and Mo elements with fixed C atoms [Mo], [V], [Cr] conforms to the following: 0.16≤[C] / (0.96[Cr]+1.37[Mo]+0.78[V])≤0.25.

[0011] Furthermore, the microstructure of the steel reinforcement is mainly bainitic.

[0012] Furthermore, the bainite content in the steel reinforcement is 80-100% by volume, preferably 85-99% by volume, and more preferably 90-99% by volume.

[0013] Furthermore, the steel bar also contains 0-20 vol% ferrite and 0-20 vol% pearlite, preferably 0-15 vol% ferrite and 0-15 vol% pearlite, more preferably 0-10 vol% ferrite and 0-10 vol% pearlite.

[0014] Furthermore, the steel bar has a yield strength of 720MPa to 880MPa, a tensile strength of 1070 to 1330MPa, a strength-to-yield ratio of 1.45 or higher, an elongation after fracture of 13% or higher, and a maximum total elongation at maximum force (Agt) of 7.5% or higher.

[0015] Furthermore, the yield strength of the steel bar is 730MPa to 830MPa, the tensile strength is 1100 to 1250MPa, the strength-to-yield ratio is 1.45 to 1.6, the elongation after fracture is 13% to 20%, and the total elongation at maximum force (Agt) is 7.5% to 12%.

[0016] On the other hand, the present invention provides a method for preparing bainitic high-strength, high-yield-ratio steel bars, which includes the following steps: hot metal pretreatment, converter smelting, ladle refining, continuous casting, and continuous rolling. The preparation method specifically includes the following steps:

[0017] (1) Hot metal pretreatment: Hot metal pretreatment is used to reduce the content of sulfur impurity elements in the steel.

[0018] (2) Converter smelting + ladle refining: High-quality scrap steel is added for converter smelting. The steel composition and temperature are controlled through ladle refining, degassing, desulfurization and argon blowing in the later stage of refining to control the content of inclusions in the steel.

[0019] (3) Continuously cast into square or rectangular billets;

[0020] (4) Continuous rolling into steel bars.

[0021] The molten iron pretreatment includes KR desulfurization, after which the S content is less than 0.002%.

[0022] Furthermore, the continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the tundish is ≤25℃, and it enters the slow cooling pit after continuous casting.

[0023] Furthermore, in the continuous rolling process, the heating temperature is between 1100 and 1200°C, and the heating time is between 180 and 240 minutes.

[0024] Furthermore, in the continuous rolling process, the initial rolling temperature is 1000–1100℃, and the final rolling temperature is 900–1000℃.

[0025] Furthermore, in the post-rolling cooling process, air cooling is performed after rolling, with a cooling rate of 9-13 m / s.

[0026] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0027] 1. This invention controls the elemental ratio of Cr, Mo, V and C in steel to ensure the precipitation of carbides while keeping the carbon content within the limit, promotes the transformation of bainite, and ensures that the microstructure of the steel plate is dominated by bainite, thereby obtaining the required strength, strength-to-yield ratio and plasticity.

[0028] 2. This invention controls the rolling and cooling process of the steel plate. The heating temperature of the billet is between 1100 and 1200℃, the heating time is 180 to 240 minutes, the initial rolling temperature is between 1000 and 1100℃, the final rolling temperature is 900 to 1000℃, and the finished product rate is 9-13 m / s. Rolling under this process helps to suppress ferrite transformation and promote bainite formation.

[0029] 3. This invention achieves excellent mechanical properties of steel bars by controlling the types and contents of elements, especially the ratio of C to Cr, V and Mo, combined with rolling and cooling processes.

[0030] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained through the details specifically pointed out in the description and drawings. Attached Figure Description

[0031] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0032] Figure 1 This is a microstructure diagram of high-strength steel bars with high bainitic yield ratio. Detailed Implementation

[0033] This invention provides a bainitic high-strength-yield-ratio high-strength steel bar and its preparation method. The steel bar has the characteristics of high strength, high strength-yield-ratio and good strength-plasticity matching, which can solve the problem of insufficient strength grade of existing steel bars.

[0034] The high-strength steel bar provided by this invention has a yield strength of 720MPa to 880MPa, a tensile strength of 1070 to 1330MPa, a strength-to-yield ratio of 1.45 or higher, an elongation after fracture of 13% or higher, a maximum force total elongation Agt of 7.5% or higher, and a Ceq of 0.5 to 0.9.

[0035] In a preferred embodiment, the yield strength is 720 MPa to 850 MPa, more preferably 730 MPa to 820 MPa; the tensile strength is 1070 MPa to 1280 MPa, more preferably 1100 MPa to 1250 MPa; the strength-to-yield ratio is 1.45 to 1.6, preferably 1.48 to 1.58, more preferably 1.5 to 1.55; the elongation after fracture is 13 to 20%, preferably 13.5 to 18%, more preferably 14 to 17%; and the maximum force elongation (Agt) is 7.5 to 12%, preferably 7.8 to 10%, more preferably 8 to 9.5%. In a preferred embodiment, the carbon equivalent Ceq of the reinforcing steel is preferably in the range of 0.6 to 0.88, more preferably 0.7 to 0.85, wherein Ceq is specifically calculated as: Ceq = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15. A suitable range can ensure that the reinforcing steel has excellent yield strength and tensile strength, as well as good weldability.

[0036] The chemical composition of the steel reinforcement, by mass percentage, is as follows: C: 0.25-0.30%, Si: 0.60-1.00%, Mn: 1.30-1.80%, V: 0.25-0.30%, Cr: 0.6-1.0%, Mo: 0.25-0.35%, B: 0.002-0.005%, with the remainder being Fe and unavoidable impurity elements.

[0037] The reasons for limiting the composition of the billet in the steel bars and their preparation method in this invention will be explained. Hereinafter, only the percentage of mass in the composition will be used.

[0038] C: Carbon is an essential element for increasing strength, but it also reduces the weldability of materials. When the carbon content is below 0.25%, the strength of the steel plate will decrease, but excessive carbon content will adversely affect the weldability and low-temperature toughness of the steel plate. Considering both aspects, the carbon content should be controlled between 0.25% and 0.30%.

[0039] Meanwhile, to ensure sufficient precipitation strengthening, systematic experimental research and theoretical analysis have shown that the relationship between the C content and the contents of Cr, Mo, and V elements with fixed C atoms should satisfy: 0.16 ≤ [C] / (0.96 [Cr] + 1.37 [Mo] + 0.78 [V]) ≤ 0.25. Under this ratio, carbide precipitation can be guaranteed while ensuring the carbon content does not exceed the standard. "Not exceeding the standard" here means that excessive carbon content will lead to reduced plasticity. However, if the carbon content is reduced, the strength of the steel cannot be guaranteed. In this case, to improve strength, precipitation strengthening and grain refinement are necessary. Controlling the contents of C, Cr, Mo, and V within the range shown in the formula will generate suitable precipitates, thereby improving strength. If this ratio is too high or too low, the precipitates will not meet the requirements, reducing the strength and strength-to-yield ratio of the steel reinforcement.

[0040] Silicon (Si): As a deoxidizing element and a solid solution strengthening element, silicon can improve the strength of steel. However, excessive silicon content can reduce the low-temperature toughness and weldability of steel. Therefore, the Si content should be controlled between 0.60% and 1.00%.

[0041] Manganese (Mn) is an essential element for ensuring the strength and toughness of steel. It not only delays the ferrite phase transformation and refines ferrite grains, but also combines with sulfur (S) to form MnS, preventing the formation of FeS at grain boundaries and thus avoiding hot cracking. To obtain high-strength steel plates, the Mn content needs to be controlled above 1.30%. However, excessively high Mn content will cause central segregation, reducing toughness and weldability. Therefore, the Mn content should be controlled between 1.30% and 1.80%.

[0042] Cr: Chromium can significantly lower the transformation temperature of bainite, affect the C-curve of bainite transformation, and improve the strength of bainitic steel. Therefore, the Cr content is controlled between 0.6% and 1.0%.

[0043] Mo: Molybdenum can lower the bainite transformation temperature, promote bainite phase transformation, and shorten the bainite transformation time. In order to obtain a suitable bainite structure and strengthening effect, the content of Mo is controlled at 0.25% to 0.35%.

[0044] Vanadium (V) is a commonly used microalloying element with significant precipitation strengthening effects. It can form VC precipitates with carbon (C), thus achieving precipitation strengthening. Therefore, the V content is controlled at 0.25–0.30%.

[0045] B: Boron is an element that significantly improves hardenability. It can delay ferrite transformation, inhibit ferrite nucleation, and promote bainite transformation. To meet minimum requirements, the boron content should be above 0.002%, while excessive boron will affect the toughness of the steel and should be controlled below 0.005%. Therefore, the boron content should be controlled between 0.002% and 0.005%.

[0046] For other impurities in the steel bars, as long as the requirements for pure steel are met, the P content is below 0.03%, preferably below 0.01%, the S content is below 0.02%, preferably below 0.01%, more preferably below 0.005%, and particularly preferably below 0.002%. For the impurity N element, its content is below 0.003%, and for the impurity H element, it is below 5 ppm.

[0047] The present invention discloses a bainitic high-strength, high-yield-ratio steel bar and its preparation method, the specific process of which is as follows:

[0048] (1) Hot metal pretreatment: The sulfur content in steel is controlled by hot metal pretreatment. After desulfurization, the S content is less than 0.002%. KR desulfurization or powder injection desulfurization can be used.

[0049] (2) Converter smelting + ladle refining: High-quality scrap steel is added for converter smelting. The steel composition and temperature are controlled through ladle refining process, as well as degassing, desulfurization and argon blowing in the later stage of refining to control the content of inclusions in the steel.

[0050] (3) Continuous casting into square or rectangular billets: The continuous casting process adopts full-process protective casting, the target superheat of the molten steel in the tundish is ≤25℃, and after continuous casting, it enters the slow cooling pit for treatment.

[0051] (4) Continuous rolling into reinforcing bars: The heating temperature of the cast billet is between 1100 and 1200℃, the heating time is 180 to 240 minutes, the initial rolling temperature is between 1000 and 1100℃, the final rolling temperature is 900 to 1000℃, and the billet is air-cooled after rolling at a cooling rate of 9-13 m / s. The reinforcing bars can specifically be straight cracked reinforcing bars.

[0052] It should be noted that rolling under this process helps to suppress ferrite transformation and promote bainite formation.

[0053] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0054] Example

[0055] This embodiment discloses nine types of high-strength steel bars (1#-9#).

[0056] All steels from #1 to #8 are produced using the same process: (1) KR desulfurization; (2) converter smelting + ladle refining; (3) continuous casting into square billets: the target superheat of the molten steel in the ladle is ≤25℃, and after continuous casting, it is placed in a slow cooling pit for treatment; (4) continuous rolling into straight steel bars: the heating temperature of the billet is between 1100 and 1200℃, the heating time is 180 to 240 minutes, the initial rolling temperature is between 1000 and 1100℃, the final rolling temperature is 900 to 1000℃, and after rolling, it is air-cooled with a cooling rate of 9-13 m / s.

[0057] The elemental composition mass percentages of steels #1 to #5 and #9 all meet the requirements of this invention. The elemental composition mass percentages of steels #6 and #7 meet the requirements of this invention but do not conform to the formula 0.16≤[C] / (0.96[Cr]+1.37[Mo]+0.78[V])≤0.25. The elemental composition mass percentage of steel #8 does not meet the requirements of this invention. The differences in their elemental composition are shown in Table 1. The microstructure of steel #1 is shown below. Figure 1 As shown.

[0058] The elemental composition (mass percentage) of steels #1 to #7 and #9 all conform to the following: C: 0.25–0.30%, Si: 0.60–1.00%, Mn: 1.30–1.80%, V: 0.25–0.30%, Cr: 0.6–1.0%, Mo: 0.25–0.35%, B: 0.002–0.005%. The process parameters of steels #1 to #7 all meet the requirements of this invention, while the process parameters of steel #9 do not meet the requirements of this invention. The differences in their process parameters are shown in Table 2.

[0059] Table 11#-9# High-strength steel bars, chemical composition (wt, %)

[0060] Steel serial number C Si Mn Cr Mo V B 1# 0.30 0.80 1.30 0.77 0.25 0.33 0.002 2# 0.25 0.60 1.50 0.63 0.30 0.30 0.004 3# 0.26 1.00 1.45 0.60 0.38 0.30 0.002 4# 0.27 0.76 1.70 1.00 0.31 0.35 0.004 5# 0.27 0.91 1.67 0.86 0.35 0.25 0.005 6# 0.25 0.79 1.63 0.95 0.34 0.29 0.003 7# 0.30 0.84 1.40 0.60 0.25 0.25 0.003 8# 0.23 0.66 1.35 0.53 0.44 0.17 0.001 9# 0.27 0.73 1.66 0.73 0.28 0.27 0.004

[0061] Table 2. Process parameters for high-strength steel bars #1-#9

[0062]

[0063]

[0064] The mechanical properties of high-strength steel bars #1-#9 are shown in Table 3.

[0065] Table 3 Mechanical properties of high-strength steel bars #1-#9

[0066]

[0067] Comparison shows that the smelting methods and process parameters of steel bars #1 to #8 are the same or similar. The elemental composition mass percentages and relationships between elements of steel bars #1 to #5 all meet the requirements of this invention. Therefore, the yield strength, tensile strength, elongation after fracture, and total elongation at maximum force of steel bars #1 to #5 all meet the requirements. In addition, the strength-to-yield ratio is also above 1.45 and above 1.5, achieving unexpected results and exhibiting excellent strength, toughness, and seismic resistance. The elemental composition percentages of #6 and #7 steel bars meet the requirements of this invention but do not conform to the formula 0.16≤[C] / (0.96[Cr]+1.37[Mo]+0.78[V])≤0.25. Specifically, the calculated percentage for #6 steel bars is 0.156, which is less than 0.16, resulting in a significant decrease in yield strength and tensile strength, and a strength-to-yield ratio less than 1.45. The calculated percentage for #7 steel bars is 0.269, which exceeds 0.25, leading to lower yield strength and mechanical properties, and a strength-to-yield ratio that also fails to meet the requirements. Analysis indicates that the ratios of C to Cr, Mo, and V need to be controlled within a suitable range to obtain appropriate carbide precipitates, ensuring the strength and strength-to-yield ratio of the steel bars. If the calculated percentage is too low, insufficient carbide precipitates will not form, resulting in unsatisfactory strength and yield strength ratio. If the calculated percentage is too high, the corresponding Cr, Mo, and V content will be lower, again resulting in unsatisfactory carbide precipitates. Therefore, the relevant properties also fail to meet the standards. The elemental composition percentage of #8 rebar does not meet the requirements of this invention, and the content of bainite-forming elements is insufficient. Using the same process, the bainite content in the rebar's microstructure does not meet the requirements, resulting in more ferrite and failing to achieve the desired strengthening effect. Although the elongation and total elongation at maximum force of #8 rebar meet the requirements, its yield strength, tensile strength, and strength-to-yield ratio are significantly lower than those of #1 to #5, making it difficult to meet the strength requirements. The elemental composition percentage of #9 rebar is similar to that of #1 to #5, all meeting the requirements of this invention. However, the manufacturing process parameters of #9 rebar do not meet the requirements, specifically the initial rolling temperature, final rolling temperature, and cooling rate. Ultimately, the bainite content in the rebar is insufficient, resulting in more pearlite. This leads to a significant decrease in the elongation at fracture, total elongation at maximum force, and strength-to-yield ratio of #9 rebar compared to #1 to #5, making it difficult to meet the requirements.

[0068] Comparison shows that this invention achieves excellent mechanical properties of steel bars based on the elemental composition and rolling and cooling process. The yield strength of the steel bars is 720MPa~880MPa, the tensile strength is 1070~1330MPa, the strength-to-yield ratio is above 1.45, the elongation after fracture is above 13%, the total elongation at maximum force (Agt) is above 7.5%, and the carbon equivalent (Ceq) is 0.5-0.9.

[0069] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A bainitic high-strength, high-yield-ratio steel bar, characterized in that, The chemical composition of the steel reinforcement, by mass percentage, is as follows: C: 0.25–0.30%, Si: 0.60–1.00%, Mn: 1.30–1.80%, V: 0.25–0.30%, Cr: 0.6–1.0%, Mo: 0.25–0.35%, B: 0.002–0.005%, with the remainder being Fe and unavoidable impurity elements. The relationship between the mass percentage of C in the steel reinforcement [C] and the mass percentages of Cr, V, and Mo elements with fixed C atoms [Mo], [V], [Cr] conforms to the following: 0.16 ≤ [C] / (0.96 [Cr] + 1.37 [Mo] + 0.78 [V]) ≤ 0.

25.

2. The bainitic high-strength, high-yield-ratio steel reinforcement according to claim 1, characterized in that, The microstructure of the steel reinforcement is mainly bainitic, with a bainite content of 80-100% by volume.

3. The bainitic high-strength, high-yield-ratio steel reinforcement according to claim 1, characterized in that, The steel bars have a yield strength of 720MPa to 880MPa, a tensile strength of 1070 to 1330MPa, a strength-to-yield ratio of 1.45 or higher, an elongation after fracture of 13% or higher, a maximum total elongation at maximum force (Agt) of 7.5% or higher, and a Ceq of 0.5 to 0.

9.

4. The bainitic high-strength, high-yield-ratio steel reinforcement according to claim 3, characterized in that, The steel bars have a strength-to-yield ratio of 1.45 to 1.6, an elongation after fracture of 13% to 20%, and a maximum force total elongation Att of 7.5% to 12%.

5. A method for preparing bainitic high-strength, high-yield-ratio, high-strength steel bars, characterized in that, The method for preparing the bainitic high-strength, high-yield-ratio, high-strength steel bars according to any one of claims 1 to 4 comprises the following process steps: hot metal pretreatment, converter smelting, ladle refining, continuous casting, and continuous rolling. The specific preparation method includes the following steps: (1) Hot metal pretreatment: Hot metal pretreatment is used to control the sulfur content in steel; (2) Converter smelting + ladle refining: High-quality scrap steel is added for converter smelting. The steel composition and temperature are controlled through ladle refining, degassing, desulfurization and argon blowing in the later stage of refining to control the content of inclusions in the steel. (3) Continuously cast into square or rectangular billets; (4) Continuous rolling into steel bars.

6. The method for preparing bainitic high-strength, high-yield-ratio steel bars according to claim 5, characterized in that, The molten iron pretreatment includes desulfurization to a sulfur content of less than 0.002%.

7. The method for preparing bainitic high-strength, high-yield-ratio steel bars according to claim 5, characterized in that, The continuous casting process adopts full-process protective casting, with the target superheat of the molten steel in the tundish ≤25℃, and after continuous casting, it enters the slow cooling pit for treatment.

8. The method for preparing bainitic high-strength, high-yield-ratio steel bars according to claim 5, characterized in that, The heating temperature is between 1100 and 1200℃, and the heating time is between 180 and 240 minutes.

9. The method for preparing bainitic high-strength, high-yield-ratio steel bars according to claim 5, characterized in that, The initial rolling temperature is 1000℃~1100℃, and the final rolling temperature is 900~1000℃.

10. The method for preparing bainitic high-strength, high-yield-ratio steel bars according to claim 9, characterized in that, After rolling, the product is cooled, and the finished product rate is 9-13 m / s.

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