Method for producing 460MPa-grade hot-rolled thin strip steel based on double-roller cast rolling

Abstract

The invention belongs to the field of steel production, and relates to a method for producing 460MPa-grade hot-rolled thin strip steel based on double-roller cast rolling. Molten steel is subjected to double-roller continuous casting and sub-rapid solidification to form a cast strip, is subjected to one-pass hot rolling to form a thin strip, is subjected to aerial fog cooling to 530-610 DEG C and is coiled, the coiled thin strip is placed in a cover type annealing furnace to be heated and subjected to heat preservation for 1-2 days, and a hot-rolled thin strip finished product is obtained after finishing. According to the method, a medium-carbon low-manganese low-silicon microalloy-element-free component system is adopted, the high-strength hot-rolled thin strip with the low yield ratio is produced through a short-process double-roller cast rolling production line, and the production energy consumption and the alloy cost are remarkably reduced. A bainite, ferrite and pearlite composite structure with the large grain size is obtained by combining the precise control aerial fog cooling process with the online waste heat annealing process design, the yield ratio is low in performance, the low-temperature toughness is good, and the strength uniformity is good. And the isotropy of the finished product is obviously superior to that of a traditional hot-rolled low-alloy product, and the finished product is suitable for producing thin-specification components through cold bending and punch forming.

Description

Technical Field

[0001] The present invention belongs to the field of steel production, and relates to a method for producing 460 MPa grade hot-rolled thin strip steel based on twin-roll casting and rolling. Background Art

[0002] Low-alloy high-strength steel has good welding and forming properties and relatively low alloy costs, and is widely used in fields such as logistics, construction, automobiles, and construction machinery. Against the backdrop of the global shortage of energy supply, the trend of automotive lightweighting is constantly developing, which also increases the demand for thin-gauge low-alloy high-strength steel with both high load-bearing capacity and low self-weight. However, currently, the 460 MPa grade low-alloy high-strength steel produced at home and abroad is mainly 1.5 - 20 mm hot-rolled plates and cold-rolled plates, and mostly adopts the process route of Nb and V microalloying + controlled rolling and controlled cooling. There are few reports on 460 MPa grade low-alloy high-strength hot-rolled thin strips with a thickness of less than 1.5 mm.

[0003] In the application of the automotive industry for hot-rolled thin strips, cold bending or stamping forming is mostly used. Different from ordinary carbon steel grades, high-strength steel is more likely to undergo springback, cracking, etc. under complex forming conditions, affecting the dimensional accuracy and load-bearing capacity of components. How to obtain a hot-rolled low-alloy material with both a low yield ratio, high processing performance, and uniform strength distribution is the main problem faced in this field currently.

[0004] Patent document 202110768831.3 discloses "A production method for reducing the yield ratio of 600 MPa grade low-alloy high-strength steel in the CSP process". The process route of this method includes smelting and continuous casting into slabs, rolling after furnace tapping, and laminar cooling. It mainly provides a cooling strategy by adopting sparse cooling to control the grain size and secondary phase precipitation after rolling, so as to achieve a low yield ratio of high-strength steel.

[0005] Patent document 201711050581.X discloses "A method for producing low-alloy high-strength steel based on the endless thin slab continuous casting and rolling process". This method smelts raw materials in a converter, RH furnace, and LF furnace in sequence, and the molten steel generates hot-rolled strip steel with different thicknesses through the ESP production line. The continuous casting process mainly adopts a low-carbon niobium-titanium microalloying composition. Nb forms extremely stable compounds with C and N elements, and the fine Nb(C, N) precipitated by deformation induction during the finishing rolling process inhibits austenite recrystallization to solve problems such as poor ductility and weldability of low-alloy high-strength steel.

[0006] Patent Document 202210987715.5 discloses "A Structural Steel with a Yield Strength of 460 MPa and Its Manufacturing Method". This method discloses a structural steel with a yield strength of 460 MPa and its manufacturing method. Through the above-mentioned composition design, medium carbon and lower manganese are used to design an appropriate carbon equivalent to ensure the welding performance of the hot plate. At the same time, by using Mg in the steel in the composition design to control the size and distribution of Ti-containing inclusions, the precipitation strengthening and fine grain strengthening effects of Ti are improved to make up for the role of the cancelled niobium alloy. This patent mainly solves the technical problems of high production cost and low elongation after fracture in the prior art. The finished product has a thickness range of 1.5 - 19.0 mm, and the elongation after fracture A is 22% - 32%.

[0007] The production methods of hot-rolled thin strips of high-processability low-alloy high-strength steel based on CSP process, ESP process and traditional hot strip rolling process disclosed in the above literature. In the prior art, in terms of the disclosed composition, on the basis of adding a higher content of Mn, a micro-alloy design of any combination of Nb, Ti, V or Nb, Ti, V is adopted, combined with precise control of hot rolling and spray cooling to meet the requirements of strength and toughness. Although this composition design solves the problems of poor plastic toughness and poor welding performance under some thickness specifications, it is not applicable to the production of hot-rolled thin strips with a thickness of 0.6 - 1.5 mm that are economical, have a low yield ratio, good isotropy and good low-temperature toughness. Summary of the Invention

[0008] Therefore, the purpose of the present invention is to provide a method for producing 460 MPa grade hot-rolled thin strip steel based on twin-roll casting to overcome the deficiencies of the prior art.

[0009] In the twin-roll casting and rolling process, under the condition of sub-rapid solidification when molten steel contacts the casting roll, alloying elements and residual harmful elements in the molten steel are not prone to segregation and precipitation, which is beneficial to improving the low-temperature toughness of the finished product. The molten steel will form a large-grain as-cast structure under the condition of sub-rapid solidification, and it is easy to obtain a high-strength and coarse hot-rolled structure during the first-pass hot rolling process. Therefore, compared with the traditional hot continuous rolling process route, when producing low-alloy steel with a yield strength of 460 MPa level, under the composition design of medium carbon, low manganese, low silicon and no micro-alloying elements, the coiling temperature does not need to be reduced to meet the performance requirements. The tensile properties of the finished product show a continuous yield characteristic, and the yield ratio is relatively low due to the influence of the large-grain size structure of the finished product matrix. The rolled strip can be coiled after air mist cooling to obtain a low-alloy hot-rolled coil. Further, the low-alloy steel produced by the twin-roll casting and rolling process of the present invention can be heated and kept warm by using the online waste heat and a bell-type annealing furnace after coiling, and the residual stress in the hot rolling process can be eliminated by online annealing. The minimum bending radius of the 90° bending of the produced low-alloy steel can reach 1 mm, and the minimum bending radius of the 180° bending can reach 1a. While achieving low alloy cost, low yield ratio, good strength uniformity and good isotropy, it has good forming performance, producing technical effects that are difficult to achieve by traditional hot continuous rolling products.

[0010] Specifically, the present invention adopts the following technical solutions:

[0011] According to the first aspect of the present invention, a method for producing 460 MPa grade hot-rolled thin strip steel based on twin-roll casting and rolling is proposed, and the method includes the following steps:

[0012] 1) Prepare molten steel by proportioning according to the following chemical components and mass percentage ratios:

[0013] C: 0.20 - 0.30%; Si: 0.08 - 0.30%; Mn: 0.20 - 1.00%;

[0014] P: ≤0.020%; S: ≤0.004%; Al: ≤0.003%;

[0015] N: ≤0.005%; Cr: ≤0.40%; the balance is Fe and inevitable impurities;

[0016] 2) Thin strip continuous casting:

[0017] Use the twin-roll casting and rolling process to carry out thin strip continuous casting on the molten steel obtained in step 1) to obtain a cast strip;

[0018] 3) Hot rolling:

[0019] Hot roll the cast strip obtained in step 2) through a single pass to obtain a rolled strip with a thickness of 0.6 - 1.5 mm, where the reduction ratio of hot rolling is 10 - 60%, and the hot rolling outlet temperature is 820 - 900 °C;

[0020] 4) Mist cooling:

[0021] Cool the rolled strip obtained in step 3) through multiple groups of mist cooling nozzles, where the header water pressure is controlled at 5.0 - 9.0 Bar, the air pressure is controlled at 1.5 - 4.0 Bar, and the total water flow rate is controlled at 800 - 3000 lpm;

[0022] 5) Temperature-controlled coiling:

[0023] Perform temperature-controlled coiling on the thin strip obtained in step 4) to obtain a hot coil, where the coiling temperature is controlled at 530 - 610 °C and the coiling tension is 20 - 60 MPa.

[0024] 6) Online annealing:

[0025] Use the waste heat of the steel coil to heat and hold the hot coil obtained in step 5) in a hood for annealing to obtain an annealed coil.

[0026] 7) Offline finishing:

[0027] Perform offline finishing on the annealed coil obtained in step 6) to obtain a finished coil.

[0028] According to the method for producing 460 MPa grade hot-rolled thin strip steel based on twin-roll casting and rolling of the present invention, preferably, in step 1), converter steelmaking, VD vacuum decarburization and deoxidation, and LF refining are adopted to obtain qualified molten steel, where the tapping temperature of the converter is controlled at 1600 - 1680 °C, the degassing time in the high-vacuum stage of the VD furnace is controlled at ≥15 min, and the total lime addition amount in the VD furnace and the LF furnace is controlled at not less than 500 kg.

[0029] According to the method for producing 460 MPa grade hot-rolled thin strip steel based on twin-roll casting and rolling of the present invention, preferably, in step 2), the molten steel is cast and rolled using twin-roll thin strip casting and rolling equipment, the free oxygen content in the tundish is 15 - 80 ppm, the H content in the tundish is ≤5 ppm, the starting casting temperature of the molten steel is 1560 - 1680 °C, the casting and rolling speed is 30 - 110 m / min, the molten steel is subjected to thin strip continuous casting under the protection of inert gas, and the thickness of the cast strip is 1.2 - 2.5 mm.

[0030] According to the method for producing 460 MPa grade hot-rolled thin strip steel based on twin-roll casting and rolling of the present invention, preferably, in step 6), annealing using the waste heat of the steel coil further includes: placing the low-alloy steel hot coil in a bell-type furnace for heat preservation for 1 - 2 days, where the temperature after entering the hood needs to be raised to the coiling temperature ±30 °C within 1 h, the temperature fluctuation inside the hood after entering the hood does not exceed 20 °C, and it is air-cooled to room temperature after leaving the hood.

[0031] According to the second aspect of the present invention, a 460 MPa grade low-alloy steel is proposed, and the 460 MPa grade low-alloy steel is produced using the aforementioned method.

[0032] For the 460 MPa grade low alloy steel according to the present invention, preferably, a 180° bending test is carried out on the low alloy thin strip obtained after offline finishing. When the bending radius ≥ 1a (where a is the thickness of the steel plate), no cracking will occur at the bending position.

[0033] For the 460 MPa grade low alloy steel according to the present invention, preferably, a 90° bending test is carried out on the low alloy thin strip obtained after offline finishing. When the bending radius ≥ 1.0 mm, no cracking will occur at the bending position.

[0034] For the 460 MPa grade low alloy steel according to the present invention, preferably, the transverse and longitudinal yield strengths of the finished low alloy thin strip obtained after offline finishing are ≥ 460 MPa, the tensile strength is ≥ 600 MPa, 0.68 ≤ yield ratio ≤ 0.75, the elongation after fracture is ≥ 14%, and among them, the transverse and longitudinal impact energies at -20 °C are ≥ 40 J.

[0035] For the 460 MPa grade low alloy steel according to the present invention, preferably, the difference in the average strength between the transverse and longitudinal directions of the finished product obtained after offline finishing is less than 30 MPa, the difference in elongation is less than 1.5%, and the difference in the average impact energy between the transverse and longitudinal directions at -20 °C does not exceed 10 J.

[0036] For the 460 MPa grade low alloy steel according to the present invention, preferably, the matrix metallographic microstructure of the low alloy steel finished product is mainly bainite, ferrite and pearlite, and the average grain size of ferrite grains is ≥ 100 μm.

[0037] Beneficial technical effects

[0038] Compared with the prior art, the technical concept and corresponding technical solutions of the present invention can at least obtain the following beneficial technical effects:

[0039] (1) When smelting molten steel by the method of the present invention, it is not necessary to add a large amount of solution strengthening elements such as manganese and silicon, nor is it necessary to add expensive micro alloy elements such as niobium and vanadium. The target strength can be achieved by virtue of the as-cast structure and the hot-rolled structure obtained by cooling, and the alloy cost is significantly reduced.

[0040] (2) The method provided by the present invention for producing an economic low yield ratio low alloy hot-rolled thin strip based on a twin-roll casting and rolling process does not need to pass through multiple hot rolling mills to reach the target thickness during the hot rolling production process, nor does it need to use a soaking furnace or a heating furnace to maintain the strip temperature. The online waste heat can be fully utilized for annealing, and both the equipment cost and production energy consumption are significantly lower than those of the traditional hot continuous rolling production line.

[0041] (3) The low-alloy hot-rolled thin strip provided by the present invention, during the production process, through precise control of the rolling temperature and laminar cooling process, under the controlled cooling rate, the matrix structure is rapidly cooled from the austenite region to the ferrite transformation region, forming a multiphase structure of pearlite, ferrite and bainite. The grain boundary area of the large-grained ferrite inherited from the as-cast structure is small, and the hindering and absorption effects of the grain boundaries on the dislocation slip under the action of strain are weakened, resulting in a relatively low yield ratio after hot rolling into coils. During the on-line annealing using the waste heat, the density of mobile dislocations decreases, the stress is further released, and the yield ratio further decreases.

[0042] (4) The low-alloy hot-rolled thin strip provided by the present invention has a low reduction ratio, precise control of aerosol cooling and long-time on-line annealing during the production process. Its isotropy and strength uniformity are significantly better than those of traditional hot-rolled and cold-rolled products, and it is suitable for producing thin-gauge components with complex shapes through cold bending and stamping forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description only relate to some embodiments of the present invention and do not limit the present invention.

[0044] Figure 1 It is a microstructural photograph of the finished coil obtained by off-line finishing after on-line annealing after temperature-controlled coiling in Example 1.

[0045] Figure 2 It is a microstructural photograph of the finished coil obtained by off-line finishing after on-line annealing after temperature-controlled coiling in Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] In order to make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0047] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the ordinary meanings understood by those of ordinary skill in the art to which the present invention pertains.

[0048] The following are embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention. All other embodiments that can be obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention shall fall within the protection scope of the present invention.

[0049] Generally speaking, the present invention provides a method for producing an economical low-alloy hot-rolled thin strip based on a twin-roll casting and rolling process, comprising the steps of:

[0050] 1) Smelting: melting steel by using specific chemical components and mass ratios for batching;

[0051] 2) Thin strip continuous casting: continuously casting the obtained molten steel by using a twin-roll casting and rolling process to obtain a cast strip;

[0052] 3) Hot rolling: hot rolling the obtained cast strip in one pass to obtain a rolled strip;

[0053] 4) Mist cooling: cooling the rolled strip by using multiple groups of mist cooling nozzles;

[0054] 5) Temperature-controlled coiling: performing temperature-controlled coiling on the thin strip to obtain a hot coil;

[0055] 6) Online annealing: heating and holding the hot coil in a hood by using the waste heat of the steel coil for annealing to obtain an annealed coil;

[0056] 7) Offline finishing: performing offline finishing on the annealed coil by using tension leveling and skin pass rolling processes to obtain a finished coil.

[0057] The present invention will be described in detail below in conjunction with specific embodiments.

[0058] Example 1

[0059] (1) Smelting: steelmaking in a converter, degassing in a VD furnace, and refining in an LF furnace to obtain molten steel with qualified composition, by weight percentage:

[0060] C: 0.23%; Si: 0.13%; Mn: 0.65%; P: 181 ppm; S: 21 ppm; Al: 16 ppm; N: 28 ppm; Cr: 21 ppm; the balance is iron and inevitable impurity elements. The tapping temperature of the electric furnace is 1600 °C. The vacuum degree in the high-vacuum stage of the VD furnace needs to be maintained at ≤ 67 Pa, the degassing time in the high-vacuum stage of the VD furnace is 30 min, the lime addition amount in the VD furnace is 500 kg, and the lime addition amount in the LF furnace is 150 kg.

[0061] (2) Thin strip continuous casting: continuously casting the molten steel with qualified composition by using a twin-roll caster under the protection of an inert gas to obtain a cast strip with a thickness of 1.95 mm. During the casting process, the free oxygen content in the tundish is 42 ppm, the H content in the tundish is 1.8 ppm, the pouring temperature of the molten steel is 1620 °C, and the casting and rolling speed is 56 m / min.

[0062] (3) Hot rolling: hot rolling the obtained cast strip in one pass to obtain a rolled strip; the obtained cast strip is hot rolled into a 1.5-mm thin strip in one pass with a reduction of 23%, the hot rolling temperature is 1030 °C, and the hot rolling outlet temperature is 850 °C.

[0063] (4) Mist cooling: The strip is cooled by multiple groups of mist cooling nozzles. The mist cooling outlet temperature is 580°C, the header water pressure is controlled at 6.5 Bar, the air pressure is controlled at 3.2 Bar, and the total water flow is 2100 lpm.

[0064] (5) Temperature-controlled coiling: The thin strip is temperature-controlled coiled to obtain a hot coil, with a coiling temperature of 560 °C and a coiling tension of 34 MPa.

[0065] (6) Online annealing: The hot coil is placed in a hood for annealing using the residual heat of the steel coil to obtain an annealed coil. The temperature must be raised to 570°C within 1 hour after entering the hood. The temperature fluctuation in the hood after entering the hood shall not exceed 20°C. The hot coil is placed in a hood furnace for 2 days of insulation and air-cooled to room temperature after leaving the hood.

[0066] (7) Offline finishing: The hot coil is finished by tension leveling and leveling process to obtain the finished coil. The tension leveling tension is 300kN, the tension leveling elongation is 0.4%, the leveling rolling force is 1200kN, and the bending roll force is 240kN.

[0067] Table 1 Mechanical properties of low alloy steel obtained in Example 1

[0068]

[0069] Figure 1 This is a microstructure photograph of the finished coil obtained in Example 1 after online annealing after temperature-controlled coiling and offline finishing.

[0070] Example 2

[0071] (1) Smelting: Converter steelmaking, VD furnace degassing, LF refining, to obtain molten steel with qualified composition, in terms of weight percentage:

[0072] C: 0.26%; Si: 0.17%; Mn: 0.74%; P: 114ppm; S: 24ppm; Al: 15ppm; N: 35ppm; Cr: 19ppm; the remainder is iron and unavoidable impurity elements. The tapping temperature of the converter is 1640℃, the vacuum degree of the VD furnace needs to be maintained at ≤67Pa in the high vacuum stage, the degassing time of the VD furnace in the high vacuum stage is 35min, the amount of lime added to the VD furnace is 400kg, and the amount of lime added to the LF furnace is 250kg.

[0073] (2) Thin strip continuous casting: The molten steel with qualified composition is cast into thin strip by twin-roll casting machine under the protection of inert gas to obtain a cast strip with a thickness of 1.80 mm. During the casting process, the free oxygen content of the middle package is 50 ppm, the H content of the middle package is 2.5 ppm, the pouring temperature of the molten steel is 1605°C, and the casting speed is 63 m / min.

[0074] (3) Hot rolling: The obtained cast strip is hot rolled in one pass to obtain a rolled strip; the obtained cast strip is hot rolled in one pass with a reduction of 33% to roll into a thin strip of 1.2 mm, the hot rolling temperature is 1025 °C, and the hot rolling outlet temperature is 850 °C.

[0075] (4) Mist cooling: The rolled strip is cooled by multiple groups of mist cooling nozzles, the mist cooling outlet temperature is 610 °C, the header water pressure is controlled at 5.9 Bar, the air pressure is controlled at 2.8 Bar, and the total water flow rate is 1450 lpm;

[0076] (5) Temperature-controlled coiling: The thin strip is subjected to temperature-controlled coiling to obtain a hot coil, the coiling temperature is 590 °C, and the coiling tension is 34 MPa.

[0077] (6) In-line annealing: The hot coil is annealed by using the waste heat of the steel coil to enter the hood for heat preservation to obtain an annealed coil. The temperature after entering the hood needs to be raised to 600 °C within 1 h, the temperature fluctuation in the hood after entering the hood does not exceed 20 °C, the hot coil is placed in the bell-type furnace for heat preservation for 1.5 days, and it is air-cooled to room temperature after leaving the hood.

[0078] (7) Off-line finishing: The hot coil is finished by using the tension leveling and skin pass rolling processes to obtain a finished coil. The tension leveling tension is 250 kN, and the tension leveling elongation is 0.2%. The skin pass rolling force is 1200 kN, and the work roll bending force is 200 kN.

[0079] Table 2 Mechanical properties of the low-alloy steel obtained in Example 2

[0080]

[0081] Figure 2 is the microstructural photograph of the finished coil obtained after temperature-controlled coiling, in-line annealing, and off-line finishing in Example 2.

[0082] The above are only the specific embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for producing 460MPa grade hot-rolled thin strip steel based on twin-roll casting, characterized in that: The method comprises the following steps: 1) Prepare molten steel according to the following chemical composition and mass percentage ratio: C: 0.20~0.30%; Si: 0.08~0.30%; Mn: 0.20~1.00%; P: ≤0.020%; S: ≤0.004%; Al: ≤0.003%; N: ≤0.005%; Cr: ≤0.40%; the balance is Fe and unavoidable impurities; 2) Thin strip continuous casting: The molten steel obtained in step 1) is subjected to thin strip continuous casting using a twin-roll casting process to obtain a cast strip; 3) Hot rolling: The cast strip obtained in step 2) is subjected to one-pass hot rolling to obtain a rolled strip with a thickness of 0.6-1.5 mm, wherein the hot rolling reduction rate is 10-60% and the hot rolling outlet temperature is 820-900° C.; 4) Aerosol cooling: The rolled strip obtained in step 3) is cooled by multiple groups of aerosol cooling nozzles, wherein the manifold water pressure is controlled to be 5.0-9.0 Bar, the air pressure is controlled to be 1.5-4.0 Bar, and the total water flow rate is controlled to be 800-3000 lpm; 5) Temperature controlled coiling: The thin strip obtained in step 4) is temperature-controlled coiled to obtain a hot coil, wherein the coiling temperature is controlled at 530-610° C. and the coiling tension is 20-60 MPa. 6) Online annealing: The hot coil obtained in step 5) is heated by using the residual heat of the steel coil and kept warm for annealing to obtain an annealed coil. 7) Offline finishing: The annealed coil obtained in step 6) is subjected to off-line finishing to obtain a finished coil.

2. The method for producing 460MPa grade hot rolled thin strip steel based on twin roll casting according to claim 1, characterized in that: In step 1), converter steelmaking, VD vacuum decarburization and deoxidation, and LF refining are used to obtain molten steel with qualified composition, wherein the converter tapping temperature is controlled to be 1600-1680°C, the degassing time in the high vacuum stage of the VD furnace is controlled to be ≥15min, and the total amount of lime added to the VD furnace and the LF furnace is controlled to be not less than 500kg.

3. The method for producing 460MPa grade hot rolled thin strip steel based on twin roll casting according to claim 1, characterized in that: In step 2), the molten steel is cast and rolled using a twin-roll thin strip casting and rolling equipment, the free oxygen content in the middle package is 15-80ppm, the H content in the middle package is ≤5ppm, the pouring temperature of the molten steel is 1560-1680℃, the casting and rolling speed is 30-110m / min, and the molten steel is continuously cast into thin strips under the protection of inert gas, and the thickness of the cast strip is 1.2-2.5mm.

4. The method for producing 460MPa grade hot rolled thin strip steel based on twin roll casting according to claim 1, characterized in that: In step 6), utilizing the residual heat of the steel coil for annealing further comprises: Place the low alloy steel hot coil in a bell-type furnace for insulation for 1-2 days. The temperature after entering the bell must be raised to the coiling temperature ±30℃ within 1 hour. The temperature fluctuation inside the bell after entering the bell should not exceed 20℃. After leaving the bell, air cool to room temperature.

5. A 460MPa grade low alloy steel, characterized in that: The 460 MPa grade low alloy steel is produced by the method according to any one of claims 1 to 4.

6. The 460MPa grade low alloy steel according to claim 5, characterized in that: The low alloy thin strip obtained after offline finishing was subjected to a 180° bending test. When the bending radius was ≥1a, the bending part would not crack, where a was the thickness of the steel plate.

7. The 460MPa grade low alloy steel according to claim 5, characterized in that: The low alloy thin strip obtained after offline finishing was subjected to a 90° bending test. When the bending radius was ≥1.0mm, there would be no cracking at the bending point.

8. The 460MPa grade low alloy steel according to claim 5, characterized in that: The finished low alloy thin strip obtained after offline finishing has a transverse and longitudinal yield strength of ≥460MPa, a tensile strength of ≥600MPa, a yield strength ratio of 0.68≤≤0.75, and an elongation after fracture of ≥14%. Among them: -20℃ transverse and longitudinal impact energy ≥40J.

9. The 460MPa grade low alloy steel according to claim 5, characterized in that: The difference in average strength in the transverse and longitudinal directions of the finished product obtained after offline finishing is less than 30MPa, the difference in elongation is less than 1.5%, and the difference in average impact energy in the transverse and longitudinal directions at -20°C does not exceed 10J.

10. The 460 MPa grade low alloy steel according to any one of claims 5 to 9, characterized in that: The metallographic microstructure of the finished low alloy steel matrix is ​​mainly bainite, ferrite and pearlite, and the average ferrite grain size is ≥100μm.

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