Heat-treatment-free 1670-mpa-grade hot-rolled prestressed steel bar and production method therefor

Abstract

Provided in the present invention are a heat-treatment-free 1670-MPa-grade hot-rolled prestressed steel bar and a production method therefor. The production method comprises a molten steel smelting step, a continuous casting step, a high-speed wire rolling step, an on-line salt bath quenching and aging step, and an uncoiling, shearing, and sizing step. In the present invention, the composition design and production process of the wire rod are optimized, the diameter size of a final steel bar is obtained by directly performing rolling, and post-rolling residual heat is used for directly performing heat treatment on-line to obtain optimal properties; a user can omit the two steps of drawing processing and quenching and tempering heat treatment; and the production method follows current green and low-carbon development trends in the industry and has good economic and social benefits.

Description

Heat treatment free type 1670MPa grade hot-rolled prestressed steel bar and production method thereof TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel metallurgy, and particularly relates to a heat treatment free type 1670MPa grade hot-rolled prestressed steel bar and a production method thereof. BACKGROUND

[0002] The steel bar for prestressed concrete (PC steel bar for short) is a kind of prestressed steel material developed by Japan High Frequency Heat Refining Corporation in the 1960s, and was approved for Japanese patent in 1964. It belongs to the intermediate strength grade in the prestressed strength grade. It has been widely used in prestressed components such as high-strength prestressed concrete centrifugal pipe pile, electric pole, viaduct pier, railway sleeper, etc. due to its characteristics such as high strength toughness, low relaxation, strong grip with concrete, good weldability, upsetting property and material saving (such as φ11mm PC steel bar can replace φ20mm hot-rolled steel bar), and has a very broad market. The main raw material of the PC steel bar is a no-twist controlled cold-hot rolled wire rod, and the main grades of the steel are 30MnSi and 35Si2Mn. After the wire rod is subjected to cold drawing, scoring and other steps, it needs to be subjected to quenching and tempering heat treatment to obtain a tempered sorbite structure, so as to obtain a tensile strength of 1420-1570MPa. Although the product and the production process can save materials, they need to be subjected to secondary heat treatment, which is high in energy consumption and low in production efficiency. SUMMARY

[0003] The present application proposes a heat treatment free type 1670MPa grade hot-rolled prestressed steel bar and a production method thereof, which can reduce or exempt the user from the two steps of drawing and quenching and tempering heat treatment.

[0004] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0005] A heat treatment free type 1670MPa grade hot-rolled prestressed steel bar, the chemical composition and weight percentage of which are as follows: C 0.32-0.38%, Si 1.20-2.20%, Mn 1.50-2.20%, Cr 0.30-0.80%, Ni≤0.30%, S≤0.010%, P≤0.020%, N≤0.0040%, O≤0.0020%, and the rest is Fe and other unavoidable impurities.

[0006] In the chemical composition of the present application:

[0007] C is the most important strengthening element in steel, in order to ensure the high tensile strength and good welding performance of the prestressed steel bar, the C content is limited to 0.32-0.38%.

[0008] Si is a strengthening element and deoxidizing element in steel, which helps to improve the anti-relaxation performance of the steel, and also has the effects of inhibiting carbide formation and promoting the formation of residual austenite, and the Si content is limited to 1.20-2.20% in the application.

[0009] Mn is a strengthening element in steel, which can improve the strength and hardenability of the steel, and ensure good work hardening rate of the wire rod, and the Mn content is limited to 1.50-2.20% in the application.

[0010] Cr can refine pearlite lamella and improve the stability of carbide, but excessive content increases the difficulty of controlling segregation, and the Cr content is limited to 0.30-0.80% in the application.

[0011] Ni is a strengthening element in steel, and can improve the toughness of the steel and the high temperature performance of the material, and in order to control the cost, the addition amount is limited to less than 0.30%.

[0012] S is easy to segregate at the grain boundary to make the grain boundary brittle, thereby reducing the strength and plasticity of the steel, and S will generate MnS with Mn, thereby reducing the solid solution strengthening effect of Mn, and the S content is limited to ≤0.010% in the application.

[0013] P is an impurity element in steel, which is easy to segregate at the grain boundary to make the grain boundary brittle, thereby reducing the strength and plasticity of the steel, and the P content is limited to ≤0.020% in the application.

[0014] N will cause the plasticity of the steel to deteriorate, increase the risk of delayed fracture of the wire rod, and high N content will cause AlN and TiN to coarsen, which is not conducive to fine-grain strengthening, and in addition, free nitrogen atoms in the steel wire will occur dynamic aging and reduce the plasticity of the steel wire, and the nitrogen content is limited to ≤0.0040% in the application.

[0015] O combines with alloying elements in steel to form non-metallic inclusions, which affects the strength and plasticity of the wire rod and the fatigue and corrosion resistance of the steel wire, and O needs to be limited to ≤0.0020%.

[0016] As a preferred, the chemical composition and weight percentage are: C 0.33-0.36%, Si 1.45-2.10%, Mn 1.55-2.15%, Cr 0.35-0.75%, Ni ≤0.20%, S ≤0.008%, P ≤0.012%, N ≤0.0036%, O ≤0.0008%, and the rest is Fe and other unavoidable impurities.

[0017] As a preferred, the diameter size of the steel bar is 5.5-16mm.

[0018] The application also provides a production method of the heat treatment-free 1670MPa-grade hot-rolled pre-stressed steel rod, which comprises the following steps: molten steel smelting, continuous casting, high-speed wire rolling, on-line salt bath quenching and aging, and uncoiling and cutting to a fixed size, wherein in the on-line salt bath quenching and aging step, the wire rod drawing temperature is 880-920℃, so that the wire rod is full austenite and has fine grain size, so as to promote phase transition and improve the final plasticity; then the wire rod is directly immersed in the salt bath for isothermal quenching, the salt bath temperature is Ms~420℃, and the time is 60-200s, so that the wire rod starts phase transition in the lower bainite region and the transition amount is more than 50%; and the wire rod after the on-line salt bath quenching is hotly collected and then enters an annealing furnace for on-line aging, the aging temperature is 300-400℃, and the time is greater than or equal to 60min, so that the bainite continues to phase transition completely and the internal stress is eliminated.

[0019] Preferably, in the continuous casting step, the superheat of the molten steel is 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the specific water consumption is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total amount of light reduction is 15.0±0.2mm; the low superheat and the casting speed control cooperate with the electromagnetic stirring and the light reduction to ensure that the continuous casting billet has good segregation, and the specific parameters can be set according to the equipment characteristics and the billet size. The continuous casting obtains a rectangular billet with a cross-sectional size of no less than 240mm×180mm.

[0020] Preferably, in the rolling step, the open rolling temperature is 1050-1100℃, the temperature entering the pre-finishing rolling mill is 910±20℃, the temperature entering the finishing rolling mill is 910±20℃, and the temperature entering the reducing mill is 910±20℃; the wire rod is drawn in a bright surface or a surface with ribs, and the diameter size of the wire rod is 5.5-16mm. The temperature in the rolling process is determined according to the characteristics of the production line, so that the billet is rolled in the full austenite state, and the austenite grain size is ensured to be fine and uniform.

[0021] Preferably, in the on-line salt bath quenching and aging step, the salt bath temperature is 350~405℃, and the time is 60-120s.

[0022] Compared with the prior art, the application has the following advantages and positive effects:

[0023] (1) According to the application, the diameter size of the final steel rod is directly obtained through the optimization of the wire rod composition design and the production process, and the best performance is obtained through the on-line heat treatment directly after the rolling and the use of the residual heat, so that the user can reduce or eliminate the two steps of drawing and quenching and tempering, which conforms to the development direction of the current industry of green and low carbon, and has good economic and social benefits.

[0024] (2) The tensile strength grade of the prestressed steel rod is increased to 1670 MPa, and the elongation is greater than 15%, and the strength plasticity cooperation is excellent, which can be used for coal mine support, large prestressed concrete buildings and other special needs. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 shows a scanning electron microscope picture of the microstructure of the wire rod of Example 1;

[0026] Figure 2 shows a scanning electron microscope picture of the microstructure of the wire rod of Example 2;

[0027] Figure 3 shows a scanning electron microscope picture of the microstructure of the wire rod of Example 3;

[0028] Figure 4 shows a scanning electron microscope picture of the microstructure of the wire rod of Comparative Example 1. DETAILED DESCRIPTION

[0029] In order to better understand the present application, the following will be specifically described in combination with the drawings and examples. Example 1

[0030] A production method of a heat treatment-free type 1670 MPa grade hot-rolled prestressed steel rod, comprising the following steps:

[0031] (1) Molten steel smelting step

[0032] The molten steel is smelted through the converter smelting and LF furnace refining steps, and the chemical composition of the molten steel at the end of smelting is shown in Table 1 in mass percent. The chemical composition of the continuous casting billet obtained by the continuous casting step, and the chemical composition of the finally prepared prestressed steel rod are consistent with the chemical composition of the molten steel at the end of the molten steel smelting step, and are shown in Table 1.

[0033] Among them, in the converter smelting step, the hot metal is sent into the converter to mix with scrap steel into molten steel, and desiliconization, dephosphorization, oxygen decarburization is carried out, and alloy is added into the ladle for deoxidation alloying when tapping; in the refining step, the molten steel after converter smelting is sent into the LF refining furnace for chemical composition adjustment and temperature control, and the inclusions in the molten steel are controlled by soft stirring, and the molten steel is tapped after the temperature and chemical composition reach the standard.

[0034] (2) Continuous casting step

[0035] In the continuous casting step, the superheat of the molten steel is controlled to be 20-25°C, the stirring current of the mold is 270±25A, the stirring frequency of the mold is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the water quantity per unit length of the continuous casting is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total amount of the light reduction is 15.0±0.2mm. The rectangular billet with a cross-sectional size of no less than 240mm×180mm is obtained in the continuous casting step, so as to ensure that the rod has sufficient compression ratio after being rolled into the wire rod to ensure the homogeneity of the wire rod.

[0036] (3) High-speed wire rolling step

[0037] In the rolling step, the rolling temperature is controlled to be 1050-1100°C by adjusting the heating temperature and time, the water tank flow is adjusted to make the temperature entering the pre-precise rolling be 910±10°C, the temperature entering the precise rolling mill be 900±10°C, and the temperature entering the reducing mill be 900±10°C, and the bright wire rod with a diameter of 5.5mm is obtained.

[0038] (4) Online salt bath quenching and aging step

[0039] In the online salt bath quenching and aging step, the wire drawing temperature is 900±10°C. After the wire drawing, the wire rod is directly immersed into the salt bath for isothermal quenching, the temperature of the salt bath is 350±2°C, and the time is 120s; the wire rod after the isothermal quenching of the salt bath is hotly collected and rolled, and then enters the temperature preservation corridor for online aging, the temperature is 350±10°C, and the time is 70min.

[0040] Unwinding, cutting and sizing step

[0041] After the wire rod is offline, the wire rod is unwound, straightened, and cut and sized according to the order requirements to obtain the prestressed steel rod. Example 2

[0042] A production method of a heat treatment-free type 1670MPa grade hot-rolled prestressed steel rod, comprising the following steps:

[0043] (1) Molten steel smelting step

[0044] The molten steel is smelted through the converter smelting and LF furnace refining steps, and the chemical composition of the molten steel at the end of the smelting is shown in Table 1. The chemical composition of the continuous casting billet obtained in the continuous casting step, and the chemical composition of the finally prepared prestressed steel rod are consistent with the chemical composition of the molten steel at the end of the smelting step, and are shown in Table 1.

[0045] In the converter smelting step, the molten iron is sent into the converter to mix with scrap steel into molten steel, and desiliconization, dephosphorization, oxygen blowing decarburization are carried out, and alloy is added into the ladle during tapping for deoxidization and alloying; in the refining step, the molten steel after the converter smelting is sent into the LF refining furnace for chemical composition adjustment and temperature control, and the inclusions in the molten steel are controlled through soft stirring, and the molten steel is tapped after the temperature and chemical composition reach the standard.

[0046] (2) Continuous casting step

[0047] In the continuous casting step, the superheat of the molten steel is controlled to be 20-25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the water amount during continuous casting is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total amount of light reduction is 15.0±0.2mm. The continuous casting step obtains a rectangular billet with a cross-sectional size of not less than 240mm×180mm, so as to ensure that the homogeneity of the rod is ensured after being rolled into a rod.

[0048] (3) High-speed wire rolling step

[0049] In the rolling step, the rolling temperature is adjusted to be 1050-1100℃ by adjusting the heating temperature and time, the water tank flow is adjusted to make the temperature entering the pre-precision rolling be 910±10℃, the temperature entering the precision rolling mill be 900±10℃, and the temperature entering the reducing and sizing mill be 900±10℃, and the bright rod with a diameter of 15mm is obtained.

[0050] (4) On-line salt bath quenching and aging step

[0051] In the on-line salt bath quenching and aging step, the wire drawing temperature is 900±10℃. After wire drawing, the rod is directly immersed in the salt bath for isothermal quenching, the salt bath temperature is 400±5℃, and the time is 60s; the rod after salt bath isothermal quenching is hot coiled, and then enters the temperature tunnel for on-line aging, the temperature is 380±10℃, and the time is 60min.

[0052] (5) Uncoiling, shearing and sizing step

[0053] After the rod is offline, it is uncoiled, straightened, and sheared and sized according to the order to obtain the prestressed steel rod. Example 3

[0054] A heat treatment-free type 1670MPa grade hot-rolled prestressed steel rod production method, comprising the following steps:

[0055] (1) Molten steel smelting step

[0056] The molten steel is smelted by sequentially performing a converter smelting step and an LF furnace refining step. The chemical composition of the molten steel at the end of the smelting step is shown in Table 1. In addition, the chemical composition of the continuous casting billet obtained in the continuous casting step and the chemical composition of the prestressed steel bar finally prepared are consistent with the chemical composition of the molten steel at the end of the molten steel smelting step, and are shown in Table 1.

[0057] In the converter smelting step, the molten iron is mixed with scrap steel into molten steel in the converter, and desiliconization, dephosphorization, oxygen blowing decarburization are performed, and alloying is performed by adding alloy to the ladle at the time of tapping. In the refining step, the molten steel after the converter smelting is sent to the LF refining furnace to adjust the chemical composition and temperature, and the inclusions in the molten steel are controlled by soft stirring, and the molten steel is tapped after the temperature and chemical composition reach the standard.

[0058] (2) Continuous casting step

[0059] In the continuous casting step, the superheat of the molten steel is controlled to be 20-25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the water volume during continuous casting is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total amount of light reduction is 15.0±0.2mm. The rectangular billet with a cross-sectional size of not less than 240mm×180mm is obtained in the continuous casting step to ensure that the billet has sufficient compression ratio after being rolled into a rod to ensure the homogeneity of the rod.

[0060] (3) High-speed wire rolling step

[0061] In the rolling step, the rolling temperature is controlled to be 1050-1100℃ by adjusting the heating temperature and time, the water tank flow is adjusted to make the temperature of the pre-precise rolling be 910±10℃, the temperature of the precise rolling machine be 900±10℃, and the temperature of the reducing and sizing machine be 900±10℃, and the rod with a diameter of 16mm is rolled, and the rib shape and size meet the requirements of GB1499.

[0062] (4) On-line salt bath quenching and aging step

[0063] In the on-line salt bath quenching and aging step, the wire drawing temperature is 900±10℃. After wire drawing, the rod is directly immersed in the salt bath for isothermal quenching, the salt bath temperature is 370±5℃, and the time is 100s. After isothermal quenching in the salt bath, the rod is hot coiled and then enters the temperature corridor for on-line aging, the temperature is 360±10℃, and the time is 90min.

[0064] (5) Uncoiling and cutting step

[0065] After the rod is offline, it is uncoiled, straightened, and cut to size according to the order requirements to obtain the prestressed steel bar.

[0066] Comparative Example

[0067] The comparative example adopts a conventional 35Si2Mn composition, and a conventional smelting-rolling-Stelmor cooling process-ageing-drawing-notching-quenching and tempering heat treatment process to produce a steel bar with a diameter of 15 mm. The specific steps are as follows:

[0068] (1) Molten steel smelting step

[0069] The molten steel is smelted through the sequentially performed converter smelting and LF furnace refining steps. The chemical composition of the molten steel at the end of smelting is shown in Table 1 in terms of mass percentage.

[0070] In the converter smelting step, the molten iron is sent into the converter to be mixed with scrap steel into molten steel, and desiliconization, dephosphorization, oxygen blowing decarburization are performed, and at the time of tapping, alloy is added to the ladle for deoxidization and alloying. In the refining step, the molten steel after the converter smelting is sent into the LF refining furnace for chemical composition adjustment and temperature control, and the inclusions in the molten steel are controlled through soft stirring, and after the temperature and chemical composition reach the standard, the molten steel is tapped.

[0071] (2) Continuous casting step

[0072] In the continuous casting step, the superheat degree of the molten steel is controlled to be 20-25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the water amount for continuous casting is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, and the total amount of light reduction is 15.0±0.2mm. The rectangular billet with a cross-sectional size of not less than 240mm×180mm is obtained through the continuous casting step, so as to ensure that the homogeneity of the rod is ensured after being rolled into a rod.

[0073] (3) High-speed wire rolling step

[0074] In the rolling step, the rolling temperature is adjusted to be 1050-1100℃ by adjusting the heating temperature and time, the water tank flow is adjusted to make the temperature entering the pre-precision rolling mill be 920±10℃, the temperature entering the precision rolling mill be 920±10℃, and the temperature entering the reducing and sizing mill be 920±10℃, and the smooth rod with a diameter of 16mm is rolled.

[0075] (4) Stelmor cooling and ageing step

[0076] In the Stelmor cooling step, the wire drawing temperature is 910±10℃, and after wire drawing, fan cooling and slow cooling with a heat preservation cover are adopted, specifically, the first to eighth fans are opened at 100%, the ninth to twelfth fans are opened at 50%, and subsequent slow cooling with the heat preservation cover is adopted. The ageing step is to place at room temperature for 15 days.

[0077] (5) Drawing-notching-tempering step

[0078] The wire rod after aging is drawn by 2 passes to 15 mm, the notched steel wire is inductively heated to 920°C, water cooled, inductively heated to 550-600°C, and finally cut to size to obtain the final product.

[0079] Table 1 Chemical composition of molten steel at the end of smelting of examples 1-3 and comparative examples

[0080]

[0081] The steel rods prepared in examples 1-3 and comparative examples are sampled and subjected to metallographic structure detection and mechanical property detection according to the same test method, and the specific test method and detection results are as follows:

[0082] (1) Metallographic structure detection: samples with a length of 10 cm are taken from one end of the steel rod, and metallographic samples are prepared, after mechanical grinding and polishing, nitric acid alcohol etching, and observation under a metallographic microscope, the metallographic structure pictures of the steel rod samples prepared in examples 1-3 and comparative examples are shown in figures 1 to 4. As can be seen from the figures, figures 1-3 are quasi-bainite structure, also called carbide-free bainite, which is composed of fine lath bainite ferrite and residual austenite film. In the chemical composition system designed in the present application, a large amount of Si can inhibit the formation of carbide, so that the wire rod is rapidly cooled to the lower bainite transformation temperature zone after spinning, a large amount of bainite ferrite is formed, but no carbide is precipitated, so that the untransformed austenite is enriched with carbon and stabilized to room temperature. The traditional process shown in figure 4 is a typical tempered sorbite structure.

[0083] (2) Mechanical property detection: according to the standard test method and definition of GB / T228, the tensile testing machine is used to test the mechanical properties of the steel rod samples, and the tensile strength and reduction of area of the steel rod samples prepared in examples 1-3 and comparative examples are shown in table 2. As can be seen from table 2, the tensile strength of the prestressed steel rod prepared in examples 1-3 is increased to 1670 MPa grade, and the elongation is greater than 15%, and the strength and plasticity are excellent. It can be seen that under the combined action of alloying elements and microstructure, the tensile strength and plasticity of the steel rod prepared by the present application are greatly improved compared with the traditional process, and the fine lath bainite ferrite in the microstructure ensures that the steel rod has very high tensile strength, and the residual austenite and Ni and other alloying elements ensure good plasticity.

[0084] Table 2 Performance parameters of steel rod prepared in examples 1-3 and comparative examples

[0085]

[0086] The above merely describes preferred embodiments of the present application, but is not intended to limit the present application to other forms, any person skilled in the art can make changes or modifications to the above disclosed technical contents into equivalent embodiments with equivalent changes, and apply to other fields, but any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present application without departing from the technical solution content of the present application still belongs to the protection scope of the present application technical solution.

Claims

1. A non-heat treated 1670 MPa grade hot-rolled prestressed steel bar, characterized in that, The chemical composition and weight percentage are: C 0.32-0.38%, Si 1.20-2.20%, Mn 1.50-2.20%, Cr 0.30-0.80%, Ni≤0.30%, S≤0.010%, P≤0.020%, N≤0.0040%, O≤0.0020%, and the rest is Fe and other inevitable impurities.

2. The non-heat treated 1670 MPa grade hot rolled prestress steel bar according to claim 1, characterized in that, The chemical composition and weight percentage are: C 0.33-0.36%, Si 1.45-2.10%, Mn 1.55-2.15%, Cr 0.35-0.75%, Ni≤0.20%, S≤0.008%, P≤0.012%, N≤0.0036%, O≤0.0008%, and the rest is Fe and other inevitable impurities.

3. The non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1, wherein: The diameter size of the steel rod is 5.5-16mm.

4. A method for producing a heat treatment free type 1670 MPa grade hot rolled prestressed steel bar, characterized in that, The steel rod is prepared by a molten steel smelting step, a continuous casting step, a high-speed wire rod rolling step, an on-line salt bath quenching and aging step, and a decoiling and cutting step, wherein in the on-line salt bath quenching and aging step, the wire rod drawing temperature is 880-920℃, and then the wire rod is directly immersed in a salt bath for isothermal quenching, the salt bath temperature is Ms~420℃, and the time is 60-200s; the wire rod after on-line salt bath quenching is hot coiled, and then enters an annealing furnace for on-line aging, the aging temperature is 300-400℃, and the time is≥60min.

5. The process for production of non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1 wherein: In the continuous casting step, the superheat of the molten steel is 20~25℃, the stirring current of the crystallizer is 270±25A, the stirring frequency of the crystallizer is 3±0.5Hz, the casting speed is 1.2±0.05m / min, the water volume ratio is 0.22±0.01L / kg, the end stirring current is 450±25A, the end stirring frequency is 8±0.5Hz, the total amount of light reduction is 15.0±0.2mm, and the rectangular billet with a cross-sectional size not less than 240mm×180mm is obtained by continuous casting.

6. A process for production of non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1 wherein: In the rolling step, the open rolling temperature is 1050-1100℃, the temperature entering the pre-precision rolling mill is 910±20℃, the temperature entering the precision rolling mill is 910±20℃, and the temperature entering the reducing mill is 910±20℃, and the wire rod with smooth surface or surface rib is obtained by rolling, and the diameter size of the wire rod is 5.5-16mm.

7. The process for production of non-heat treated 1670 MPa grade hot rolled prestress steel bar as claimed in claim 1 wherein: In the on-line salt bath quenching and aging step, the salt bath temperature is 350~405℃, and the time is 60-120s.

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