CN118389957A - Light high-corrosion-resistance steel bar and preparation method thereof - Google Patents

Abstract

A light high corrosion resistant steel bar and a preparation method thereof belong to the field of steel bar manufacturing for building structures, wherein the steel bar comprises the following components, by mass, less than or equal to 0.05% of C, 4.5-8.5% of Cr, 2.0-6.0% of Al, 0.5-0.8% of Mo, 1.0-1.2% of Mn, 0.2-0.4% of Si, less than or equal to 0.006% of P, less than or equal to 0.006% of S, and the balance of Fe and impurities; the preparation method comprises smelting and LF-RH refining; and (5) hot continuous rolling or continuous casting rolling control and temperature control cooling. The density of the prepared light high corrosion resistant steel bar is reduced by 3% -9%, the chloride ion resistance is 16-20 times of that of the common steel bar HRB500, the relative corrosion rate is lower than 8.0% of that of the HRB500, and the steel bar has excellent corrosion resistance. The service performance is ensured, the long service life is realized, the whole cost is minimized, and the method has important practical significance for promoting the energy conservation, consumption reduction and emission reduction of related industries. Has wide application prospect in offshore engineering in severe marine environments such as near-open sea and the like.

Description

A lightweight and highly corrosion-resistant steel bar and a preparation method thereof

Technical Field

The invention relates to the technical field of manufacturing steel bars for building structures, in particular to steel bars for building structures in deep sea and harsh marine environments.

Background technique

The ocean is rich in resources, and accelerating the development of the ocean will ease the continuous consumption of land resources. Reinforced concrete structure is the most widely used structural form in infrastructure construction. The specific environmental factors of the ocean put forward many special requirements for the service performance of building structures. Especially in harsh environments such as high salt fog, high humidity and heat, and strong radiation, ordinary steel bars embedded in concrete structures are easily damaged by chlorides, which not only increases maintenance costs but also poses a risk of structural failure. In response to the problem of steel corrosion, protective measures and technologies such as cathodic protection, surface modification, and coating protection have a series of problems such as construction difficulties, easy aging and failure, and high maintenance costs. Therefore, considering the service performance and comprehensive cost, improving the corrosion resistance of the steel matrix is the fundamental way to solve the problem. Stainless steel bars have excellent corrosion resistance and can ensure the long-term service life of reinforced concrete structures. However, stainless steel is difficult to be applied in large-scale engineering projects because it contains too many alloy elements, has poor weldability, and has high initial production costs.

The development and application of lightweight and highly corrosion-resistant steel bars can not only extend the service life of buildings and reduce maintenance costs while ensuring the quality and safety of steel bars used in building structures, but also help save construction workload and material resource consumption. It has important practical significance for promoting energy conservation, consumption reduction and emission reduction in related industries.

Therefore, in order to overcome the deficiencies of the prior art, it is necessary to study lightweight and highly corrosion-resistant steel bars and preparation methods thereof to solve or alleviate one or more of the above problems.

Summary of the invention

The present invention aims to provide a lightweight and highly corrosion-resistant steel bar and a preparation method thereof. By adding elements such as Cr, Al and Mo, a lightweight and highly corrosion-resistant steel bar is designed and developed, and the density is reduced as much as possible while meeting the performance requirements of steel used in building structures in harsh marine environments, thereby reducing costs.

In order to achieve the above object, the present invention adopts the following technical solution:

A lightweight and highly corrosion-resistant steel bar, characterized in that the mass percentages of the components are: C≤0.05%, Cr 4.5-8.5%, Al 2.0-6.0%, Mo 0.5-0.8%, Mn 1.0-1.2%, Si 0.2-0.4%, P≤0.006%, S≤0.006%, and the rest are Fe and unavoidable impurities.

Furthermore, the density of the steel bar can be reduced by 3-9%, and the mechanical performance indicators are: yield strength ≥500 MPa, tensile strength ≥630 MPa, elongation after fracture A ≥20%, and total elongation under maximum force Agt ≥10.0%.

Furthermore, the corrosion resistance index of the steel bar is: in a typical concrete with an alkalinity of pH=13.0±0.2, the critical chloride ion concentration of the lightweight and highly corrosion-resistant steel bar is 3.20-3.80 mol/L, and its chloride ion resistance is 16-20 times that of ordinary steel bar HRB500.

Furthermore, the corrosion resistance performance index of the steel bar is: in a concrete environment pre-containing chloride (such as sea sand concrete mixed with seawater), the lightweight and highly corrosion-resistant steel bar can still maintain stable passivation.

Furthermore, the corrosion resistance performance index of the steel bar is: the high corrosion resistant steel bar is inspected according to YB/T 4367, the test solution adopts a sodium chloride solution with an initial concentration of 5.0% higher than the standard, the mass fraction specified in YB/T 4367 is 2.00%±0.05%, the solution temperature is 45°C±2°C, the humidity is 70%RH±10%RH, the cycle period is 60min±5min, and the immersion time is 12min±2min; the relative corrosion rate is calculated according to GB/T 34206-2017, and the relative corrosion rate is less than 8.0% of HRB500 in GB/T 1499.2-2018.

Furthermore, the steel bar corrosion resistance index is: the average thickness of the rust layer after 360 hours of corrosion is less than 100 μm, and the volume expansion is very small compared with the average thickness of the rust layer of HRB500 of 300 μm.

A method for preparing a lightweight and highly corrosion-resistant steel bar as claimed in claim 1, comprising the following steps:

S1. Use vacuum induction furnace to smelt according to the established element mass percentage;

S2, LF-RH refining;

S3, hot rolling or continuous casting controlled rolling;

S4, temperature control cooling.

Furthermore, the specific process of S1 includes: pre-desulfurizing the molten iron, then adding scrap steel, molten iron and pig iron into the converter for top and bottom composite blowing; using slag washing and bottom argon blowing throughout the steelmaking process, and the steelmaking temperature is greater than 1600°C; then adding deoxidizer, high carbon ferromanganese, ferrosilicon and ferrochrome into the ladle to complete preliminary deoxidation and alloying to improve efficiency.

Furthermore, the specific process of S2 includes: adding Cr, Al, Mn and Mo alloy elements to the ladle of the LF furnace, and performing bottom blowing throughout the process with an argon flow rate of 100-120 L/min to ensure the melting and homogenization of the alloy in the ladle, and the steel-out temperature is >1550°C; degassing and decarburization are carried out in the RH furnace, and at the same time, micro-carbon chromium-iron alloy is added to further alloy the molten steel so that the content of each element is controlled within an appropriate range; when the vacuum degree is <2 mbar, a net circulation treatment of >10 min is carried out, and the steel-out temperature is >1550°C.

Furthermore, the specific process of S3 includes: casting the molten steel refined by the RH furnace into a cast billet through a continuous casting machine under protective casting conditions; during the continuous casting process, the temperature is controlled at 1540-1560°C, and the pulling speed is controlled at 2.0-2.5 m/min; in the hot rolling process of S3, the heating furnace temperature is greater than 1150°C, and the start rolling temperature is 960-1100°C; the rolling process is controlled to include rough rolling and finish rolling, and the billet obtained by continuous casting is heated in a heating furnace, the heating temperature is greater than 1150°C, and the heating time is greater than 2 h to ensure that the alloy elements are fully dissolved back; during the rolling process, the rough rolling temperature is 960-1050°C, the finish rolling temperature is 830-940°C, the rolling speed is 15-20 m/s, and then rolled into steel bars with a diameter of 10-40 mm.

Furthermore, the specific process of S4 includes: accelerating cooling the rolled steel bar to below 400° C. at a cooling rate of >5° C./s, and then cooling it naturally on a cooling bed.

Compared with the prior art, the present invention can obtain the following technical effects: the lightweight high corrosion resistant steel bar of the present invention can reduce the density by 3% to 9%, and in a typical concrete environment, the chloride ion resistance is 16 to 20 times that of ordinary steel bar HRB500; in a concrete environment containing chloride (such as sea sand concrete mixed with seawater), the lightweight high corrosion resistant steel bar can still maintain stable passivation; in a harsh 5.0% sodium chloride environment, the relative corrosion rate of HRB500 is less than 8%, and the average thickness of the rust layer after corrosion for 360 hours is less than 100μm, which has a very small volume expansion compared with the average thickness of 300μm of the rust layer of HRB500. While ensuring the service performance requirements, it can not only achieve a long service life cycle, but also minimize the overall cost. It has important practical significance for promoting energy conservation, consumption reduction and emission reduction in related industries, and has broad application prospects in offshore engineering in harsh marine environments such as near and far seas.

Of course, any product implementing the present invention does not necessarily need to achieve all of the above-mentioned technical effects at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a process flow chart of the present invention;

FIG2 is a photograph of the sample surface after being passivated by the concrete pore liquid (pH=13.5±0.2) and immersed for half a year in 1.0 mol/L sodium chloride (a-Example 1, b-Example 2, C-HRB500).

Detailed ways

In order to make the technical solutions and advantages of the present invention clearer, the specific embodiments of the present invention are described in detail below in conjunction with the diagrams. It should be noted that any feature description mentioned in this specification can be replaced by other equivalent or similar alternative feature descriptions. Unless otherwise specified, any feature description is only an example of the entire series of equivalent or similar purpose feature descriptions. The embodiments described are only used to explain and help understand the present invention, and are not used to specifically limit the present invention.

The present invention adopts the low-carbon alloying principle to design the steel composition, adopts converter smelting-refining-hot rolling or continuous casting controlled rolling process to prepare and shape, and adds a suitable temperature control cooling process to make the steel bar have good mechanical properties and corrosion resistance.

A lightweight high-corrosion-resistant steel bar, wherein the mass percentages of the components of the high-corrosion-resistant steel bar are: C≤0.05%, Cr 4.5-8.5%, Al 2.0-6.0%, Mo 0.5-0.8%, Mn 1.0-1.2%, Si 0.2-0.4%, P≤0.006%, S≤0.006%, and the rest are Fe and impurities;

The method for preparing the above-mentioned lightweight and highly corrosion-resistant steel bar comprises the following steps:

Step 1, smelting according to the above designed composition;

Step 2: LF-RH refining is performed on the molten steel. Cr, Al, Mn and Mo alloy elements are added to the ladle of the LF furnace, and degassing and decarburization are performed in the RH furnace. Micro-chromium-iron alloy is added to further alloy the molten steel, and the content of each element is controlled within the required range. The tapping temperature is greater than 1550°C.

Step 3, the continuous casting process includes: the temperature during the continuous casting process is controlled at 1540-1560°C, and the pulling speed is controlled at 2.0-2.5 m/min; the hot rolling process includes: the heating furnace temperature is greater than 1150°C, and the start rolling temperature is 960-1100°C; the rolling process is controlled: the heating temperature is greater than 1150°C, and the heating time is greater than 2 h to ensure that the alloy elements are fully dissolved back; during the rolling process, the rough rolling temperature is 960-1050°C, the finishing rolling temperature is 830-940°C, and the rolling speed is 15-20 m/s, and then rolled into steel bars with a diameter of 10-40 mm.

Step 4: The rolled steel bars need to be cooled under controlled temperature. Taking into account the performance index requirements, the rolled steel bars are cooled at a cooling rate of >5°C/s to below 400°C, and then cooled naturally on the cooling bed.

The prepared lightweight and highly corrosion-resistant steel bar can reduce the density by 3% to 9%, and its yield strength is ≥500 MPa, tensile strength is ≥630 MPa, elongation after fracture A is ≥20%, and total elongation under maximum force Agt is ≥10.0%. In a typical concrete environment, the chloride ion resistance is 16 to 20 times that of ordinary steel bar HRB500; in a concrete environment containing chloride (such as sea sand concrete mixed with seawater), the lightweight and highly corrosion-resistant steel bar can still maintain stable passivation; in a harsh 5.0% sodium chloride environment, the corrosion rate relative to HRB500 is less than 8%, and the average thickness of the rust layer after 360 hours of corrosion is less than 100μm, which has a very small volume expansion compared with the average thickness of 300μm of the rust layer of HRB500.

The mechanism of action of each alloy component in the lightweight and highly corrosion-resistant steel bar of the present invention is as follows:

C: The mechanical properties and corrosion resistance of steel bars are affected by the C element. If the C content is too high, it will precipitate in the form of carbides. Carbides will not only reduce the elongation and impact plasticity and have an adverse effect on mechanical properties, but also induce intergranular corrosion. Moreover, a high carbon content will consume the Cr in the matrix to form Cr-containing carbides, which will not only reduce the alloying effect, but also significantly reduce the corrosion resistance of the steel. Therefore, the C content in the matrix must be strictly controlled during the composition design of corrosion-resistant steel grades.

Cr: The higher the Cr content, the more significantly the passivation ability and corrosion resistance of the steel are enhanced. Stable Cr oxides or hydroxides have excellent passivation properties, which can not only protect the matrix from medium corrosion, but also have cation selectivity, which can further inhibit the invasion of ^Cl- . At the same time, under alkaline conditions such as concrete environment, Cr oxides or _hydroxides can promote the transformation of α-FeOOH with larger volume expansion to _Fe3O4 with smaller volume expansion. Even if the steel bar corrodes, it can achieve smaller corrosion expansion in the concrete, reducing the degree of damage caused by structural corrosion failure.

Al: The addition of Al can significantly reduce the density of steel. Increasing the Al content by 1wt.% can reduce the specific gravity by about 1.5%. Al2O3 with higher electrochemical stability can not only prolong the passivation time of steel bars, but also effectively prevent further corrosion. The typical seawater corrosion-resistant steel 10CrMoAl proves that the Al2O3 protective film is both anti-corrosion and corrosion-resistant.

Mo: Mo has excellent pitting corrosion resistance. It will generate Mo-containing insoluble salts that are enriched in the corrosion product film, promote the formation of amorphous oxide film, and then inhibit pitting corrosion. If the Mo content is too low, the effect of improving the pitting corrosion resistance of steel is not obvious; if the Mo content is too high, the pitting corrosion resistance of steel is improved, but the cost will increase.

Mn: Mn is an austenite forming and solid solution strengthening element, which can improve strength. However, if the Mn content is too high, it will combine with S to form MnS inclusions, which will not only affect the plasticity, impact toughness and welding performance of the steel bar, but also increase the sensitivity to pitting corrosion. Therefore, the Mn content should be properly controlled within a reasonable range.

Si: Si dissolved in ferrite can play a role in solid solution strengthening. At the same time, it can inhibit the diffusion of C element in austenite, delay the phase transformation of ferrite and pearlite, and improve the yield strength and tensile strength of steel bars. However, too high Si content will reduce the plasticity of steel and deteriorate the welding performance of steel bars, so its content should be controlled within a reasonable range.

P and S: From the perspective of mechanical properties, P will reduce the plasticity and toughness of steel, and will cause problems such as quenching and tempering brittleness and cold brittleness, and will also have an adverse effect on hot workability and weldability. S will increase the sensitivity to hot and cold cracks, thereby adversely affecting the ability to resist hydrogen embrittlement. From the perspective of atmospheric corrosion, P is an anodic depolarizer that can accelerate the uniform dissolution of steel and the oxidation rate of Fe ²⁺ , thereby significantly improving the corrosion resistance of weathering steel. However, in addition, P and S are prone to cause component segregation, and inclusions containing P and S elements will induce localized corrosion such as pitting corrosion, so it is necessary to reduce its content in steel as much as possible.

The inevitable impurity elements mainly refer to O, H and harmful elements such as Pb, Sn, As, Sb, and Bi.

The present invention designs and develops lightweight high corrosion-resistant steel bars by adding elements such as Cr, Al and Mo, thereby reducing the overall engineering cost as much as possible while meeting the performance requirements of high corrosion-resistant steel bars for harsh marine environments.

The present invention provides a method for manufacturing lightweight and highly corrosion-resistant steel bars. The preparation steps of each embodiment of the method include: steelmaking, LF-RH refining, hot rolling or continuous casting controlled rolling, and temperature-controlled cooling.

Embodiment 1 and 2 specifically include the following steps:

Step 1: Smelting is performed according to the composition in Table 1, and the tapping temperature is > 1610°C, preferably 1610°C.

Step 2: In the ladle of the LF furnace, the argon flow rate is 100-120 L/min, preferably 100 L/min, and the tapping temperature is >1550°C, preferably 1560°C; in the RH furnace, when the vacuum degree is <2 mbar, a net circulation treatment is performed for >5 min, preferably 8 min, and the tapping temperature is >1550°C, preferably 1560°C. This step is to ensure that the alloy in the ladle is dissolved and homogenized, and the content of each element is controlled within the design range.

Step 3: Under protective pouring conditions, the molten steel is poured into a continuous casting billet through a continuous casting machine, the temperature is controlled at 1540-1560 ° C, preferably 1540 ° C, and the pulling speed is controlled at 2.0-2.5 m/min, preferably 2.5 m/min. The billet obtained by continuous casting is heated in a heating furnace, the heating temperature is greater than 1150 ° C, preferably 1150 ° C, and the heating time is greater than 2.0 h to ensure that the alloy elements are fully dissolved, preferably 2.0 h. During the rolling process, the rough rolling temperature is 960-1050 ° C, preferably 1050 ° C, the finishing rolling temperature is 830-940 ° C, preferably 900 ° C, and the rolling speed is controlled at 15-20 m/s, preferably 15 m/s.

Step 4: The rolled steel bars need to be cooled under controlled temperature. The rolled steel bars are cooled at a cooling rate of >5°C/s to below 400°C, and then cooled naturally on a cooling bed.

The strength and plasticity test results of the lightweight and highly corrosion-resistant steel bars according to the above implementation method are shown in Table 2.

The corrosion resistance test of Examples 1 and 2 was carried out. In typical concrete (pH=13.5±0.2), the critical chloride ion concentration of Example 1 was 4.0 mol/L, and the critical chloride ion concentration of Example 2 was 3.70 mol/L. In a concrete environment containing chloride (such as sea sand concrete mixed with seawater), Examples 1 and 2 can still maintain stable passivation. According to the requirements for steel corrosion resistance in GB/T33953-2017, the test solution uses a sodium chloride solution with an initial concentration of 5.0% higher than the standard (the specified mass fraction is 2.00%±0.05%), the solution temperature is 45°C±2°C, the humidity is 70%RH±10%RH, the cycle period is 60min±5min, and the immersion time is 12min±2min; the corrosion rate of the comparison steel bar HRB500 is 4.81 g/m2·h at 72h, 5.23g/m2·h at 168h, and 6.38g/m2·h at 360h; taking Example 2 as an example, the corrosion rate of the corrosion-resistant steel bar is 0.08 g/m2·h at 72h, 0.25g/m2·h at 168h, and 0.49g/m2·h at 360h. According to GB/T 34206-2017, the relative corrosion rates are 1.7% (72h), 4.8% (168h) and 7.7% (360h) of HRB500 respectively. The relative corrosion rates of Examples 1 and 2 are less than 8.0% of HRB500. After 360h of corrosion, the average thickness of the rust layer is 92μm, which has a very small volume expansion compared with the average thickness of 286μm of the rust layer of HRB500.

It can be seen from the test results that Examples 1 and 2 have good comprehensive mechanical properties and excellent corrosion resistance. Figures 2a and 2b show the sample surfaces of Examples 1 and 2 taken after being passivated in the concrete pore liquid (pH = 13.5 ± 0.2) and soaked for half a year with 1.0 mol/L sodium chloride. No corrosion characteristics such as pitting were found, indicating that the steel bars are still in a passivated state. The Cr content of Example 1 is at the lower limit of the designed composition range, and the Al and Mo contents are at the upper limit. In Example 2, the Cr content is at the upper limit, and the Al and Mo contents are close to the lower limit. The results show that both have good Cl- corrosion resistance, and the corrosion resistance is enhanced with the increase of the corrosion-resistant alloy element content.

Embodiments 3 and 4 specifically include the following steps:

Step 1: Smelting is performed according to the composition in Table 1, and the tapping temperature is > 1600°C, preferably 1610°C.

Step 2: In the ladle of the LF furnace, the argon flow rate is 100-120 L/min, preferably 100 L/min, and the tapping temperature is >1550°C, preferably 1560°C; in the RH furnace, when the vacuum degree is <2 mbar, a net circulation treatment is performed for >5 min, preferably 8 min, and the tapping temperature is >1550°C, preferably 1560°C. This step is to ensure that the alloy in the ladle is dissolved and homogenized, and the content of each element is controlled within the design range.

Step 3, under protective pouring conditions, the molten steel is poured into a continuous casting billet through a continuous casting machine, the temperature is controlled at 1540-1560 ° C, preferably 1540 ° C, and the pulling speed is controlled at 2.0-2.5 m/min, preferably 2.5 m/min. The billet obtained by continuous casting is heated in a heating furnace, the heating temperature is greater than 1150 ° C, preferably 1150 ° C, and the heating time is greater than 2.0 h to ensure that the alloy elements are fully dissolved, preferably 2.0 h. Example 3 During the rolling process, the rough rolling temperature is 960-1050 ° C, preferably 960 ° C, the finishing rolling temperature is 830-940 ° C, preferably 870 ° C, and the rolling speed is controlled at 15-20 m/s, preferably 20 m/s. Example 4 Hot rolling process, the heating furnace temperature is greater than 1150 ° C, preferably 1200 ° C, and the start rolling temperature is 960-1100 ° C, preferably 1100 ° C.

Step 4: The rolled steel bars need to be cooled under controlled temperature. The rolled steel bars are cooled at a cooling rate of >5°C/s to below 400°C, and then cooled naturally on a cooling bed.

The strength and plasticity test results of the lightweight and highly corrosion-resistant steel bars produced according to the above implementation method are shown in Table 4.

The corrosion resistance test of Examples 3 and 4 was carried out. In typical concrete (pH=13.5±0.2), the critical chloride ion concentration of Example 3 was 3.80 mol/L, and the critical chloride ion concentration of Example 4 was 3.60 mol/L. In a concrete environment containing chloride (e.g., sea sand concrete mixed with seawater), Examples 3 and 4 can still maintain stable passivation. According to the requirements for steel bar corrosion resistance in GB/T33953-2017, the test solution uses a sodium chloride solution with an initial concentration of 5.0% higher than the standard (the specified mass fraction is 2.00%±0.05%), the solution temperature is 45°C±2°C, the humidity is 70%RH±10%RH, the cycle period is 60min±5min, and the immersion time is 12min±2min; taking Example 3 as an example, the corrosion rate at 72h is 0.09 g/m2·h, the corrosion rate at 168h is 0.27g/m2·h, and the corrosion rate at 360h is 0.50g/m2·h. According to GB/T 34206-2017, the relative corrosion rates are 1.9% (72h), 5.2% (168h) and 7.8% (360h) of HRB500 respectively. The relative corrosion rates of Examples 3 and 4 are less than 8.0% of HRB500. After 360 hours of corrosion, the average thickness of the rust layer is 97 μm, which has a very small volume expansion compared with the thickness of the HRB500 rust layer.

Example 3 is prepared by controlled rolling, and Example 4 is prepared by hot continuous rolling. The mechanical test results show that the comprehensive mechanical properties of Examples 3 and 4 are good. The corrosion test results show that the preparation process has little effect on the corrosion performance, and Examples 3 and 4 have excellent corrosion resistance.

In summary, within the composition range and process range designed by the present invention, the solution can achieve the expected effect.

Compared with the prior art, the present invention has the following advantages:

The present invention makes full use of the advantages of the alloying design concept, and designs and develops lightweight and highly corrosion-resistant steel bars by adding elements such as Cr, Al and Mo. The steel bar density can be reduced by 3-9%, and the mechanical performance indicators are: yield strength ≥500 MPa, tensile strength ≥630 MPa, elongation after fracture A ≥20%, and total elongation under maximum force Agt ≥10.0%. In typical concrete with an alkalinity of pH=13.0±0.2, the critical chloride ion concentration of the lightweight and highly corrosion-resistant steel bars is 3.20-3.80 mol/L, and its chloride ion resistance is 16-20 times that of ordinary steel bars HRB500, and the relative corrosion rate is less than 8.0% of HRB500 in GB/T 1499.2-2018.

The lightweight and highly corrosion-resistant steel bars of the present invention not only meet the service performance requirements of building structure engineering in harsh marine environments, but also have the advantages of sustainability and economy, and can effectively save costs and thus bring greater economic benefits.

Table 1 Elemental composition of Examples 1 and 2 (wt%)

Table 2 Mechanical properties of Examples 1 and 2

Table 3 Elemental composition of Examples 3 and 4 (wt%)

Table 4 Mechanical properties of Examples 3 and 4

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Claims (9)

1. A lightweight and highly corrosion-resistant steel bar, characterized in that the mass percentage of each component is: C≤0.05%, Cr 4.5-8.5%, Al 2.0-6.0%, Mo 0.5-0.8%, Mn 1.0-1.2%, Si 0.2-0.4%, P≤0.006%, S≤0.006%, and the rest is Fe and impurities. 2. The lightweight and highly corrosion-resistant steel bar according to claim 1 is characterized in that the lightweight and highly corrosion-resistant steel bar can reduce the weight by 3 to 9%, the yield strength is ≥500 MPa, the tensile strength is ≥630 MPa, the elongation after fracture A is ≥20%, and the total elongation under maximum force Agt is ≥10.0%. 3. The lightweight and highly corrosion-resistant steel bar according to claim 1 or 2 is characterized in that the corrosion resistance performance index of the steel bar is: in a typical concrete with an alkalinity of pH=13.0±0.2, the critical chloride ion concentration of the lightweight and highly corrosion-resistant steel bar is 3.20-3.80 mol/L, and its chloride ion resistance is 16-20 times that of ordinary steel bar HRB500; in a concrete environment containing chloride, the lightweight and highly corrosion-resistant steel bar can still maintain stable passivation. 4. The lightweight and highly corrosion-resistant steel bar according to claim 1 or 2 is characterized in that, according to the requirements for steel bar corrosion resistance in GB/T 33953-2017, the highly corrosion-resistant steel bar is tested in accordance with YB/T 4367, the test solution adopts a sodium chloride solution with an initial concentration of 5.0% higher than the standard, the mass fraction specified in YB/T 4367 is 2.00%±0.05%, the solution temperature is 45°C±2°C, the humidity is 70%RH±10%RH, the cycle period is 60min±5min, and the immersion time is 12min±2min; the relative corrosion rate is calculated according to GB/T 34206-2017, the relative corrosion rate is less than 8.0% of HRB500 in GB/T 1499.2-2018, and the average thickness of the rust layer after corrosion for 360h is less than 100μm, which has a very small volume expansion compared with the rust layer with an average thickness of 300μm of HRB500. 5. A method for preparing the lightweight and highly corrosion-resistant steel bar according to claim 1 or 2, characterized in that it comprises the following steps: S1. Use vacuum induction furnace to smelt according to the established element mass percentage; S2, LF-RH refining; S3, hot rolling or continuous casting controlled rolling; S4, temperature control cooling. 6. The method for preparing lightweight and highly corrosion-resistant steel bars according to claim 5 is characterized in that the specific process of S1 includes: pre-desulfurizing the molten iron, then adding scrap steel, molten iron and pig iron into the converter for top and bottom composite blowing; using slag washing and bottom argon blowing throughout the steel tapping process, and the steel tapping temperature is greater than 1600°C; then adding deoxidizer, high carbon ferromanganese, ferrosilicon and ferrochrome into the ladle to complete preliminary deoxidation and alloying to improve efficiency. 7. The method for preparing lightweight and highly corrosion-resistant steel bars according to claim 5 is characterized in that the specific process of S2 includes: adding Cr, Al, Mn and Mo alloy elements to the ladle of the LF furnace, and performing bottom blowing throughout the process at an argon flow rate of 100 to 120 L/min to ensure the melting and homogenization of the alloy in the ladle, and the steel-out temperature is >1550°C; degassing and decarburization are carried out in the RH furnace, and micro-carbon chromium-iron alloy is added to further alloy the molten steel so that the content of each element is controlled within an appropriate range; when the vacuum degree is <2 mbar, a net circulation treatment of >10 min is carried out, and the steel-out temperature is >1550°C. 8. The method for preparing a lightweight and highly corrosion-resistant steel bar according to claim 5 is characterized in that the specific process of S3 includes: casting the molten steel refined in the RH furnace into a cast billet through a continuous casting machine under protective casting conditions; the temperature during continuous casting is controlled at 1540-1560°C, and the pulling speed is controlled at 2.0-2.5 m/min; in the hot rolling process of S3, the heating furnace temperature is greater than 1150°C, and the start rolling temperature is 960-1100°C; the rolling process is controlled to include rough rolling and finish rolling, and the steel billet obtained by continuous casting is heated in a heating furnace, the heating temperature is greater than 1150°C, and the heating time is greater than 2 h to ensure that the alloy elements are fully dissolved back; during the rolling process, the rough rolling temperature is 960-1050°C, the finish rolling temperature is 830-940°C, the rolling speed is 15-20 m/s, and then rolled into steel bars with a diameter of 10-40 mm. 9. The method for preparing lightweight and highly corrosion-resistant steel bars according to claim 5, characterized in that the specific process of S4 comprises: accelerating cooling the rolled steel bars to below 400°C at a cooling rate of >5°C/s, and then naturally cooling them on a cooling bed.