Low-alloy high-strength steel plate with low crack sensitivity coefficient of 550MPa and manufacturing method of low-alloy high-strength steel plate

Abstract

The invention relates to a low-alloy high-strength steel plate with a low crack sensitivity coefficient of 550MPa and a manufacturing method thereof, and belongs to the technical field of metallurgy. The steel plate comprises the following chemical components: 0.05 to 0.09 percent of C, 0.10 to 0.30 percent of Si, 1.40 to 1.60 percent of Mn, 0.040 to 0.060 percent of Nb, 0.008 to 0.018 percent of Ti, 0.020 to 0.040 percent of Al, less than or equal to 0.0010 percent of B, 0.0005 to 0.0015 percent of Ca, less than or equal to 0.012 percent of P, less than or equal to 0.002 percent of S, less than or equal to 0.002 percent of O, less than or equal to 0.0035 percent of N, less than or equal to 0.0001 percent of H and the balance of Fe and inevitable impurity elements. The carbon equivalent CEV is less than or equal to 0.37%, and Pcm is less than or equal to 0.19%. The production thickness is 50-70 mm, and the microstructure is mainly a uniform acicular ferrite and bainite mixed structure. Elemental composition design, base metal structure, rolling deformation, small-water-amount cooling and the like are controlled, and it is guaranteed that the steel plate has high strength, high toughness, excellent cold bending performance and welding cracking resistance; and the steel plate can be widely applied to manufacturing of large-scale engineering machinery equipment or steel structures with strict requirements on formability and weldability, such as engineering machinery, high-rise buildings, bridges and ocean platforms.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to an alloy steel plate with a yield strength of 550 MPa. Background Art

[0002] High-strength steel plates with a yield strength of 550 MPa are mainly used in key stress-bearing parts of engineering machinery such as high-rise buildings, engineering crane buckets, concrete pump trucks, hydraulic supports for coal mine conveyors, etc., and have particularly high requirements for welding crack sensitivity, fracture toughness, surface quality, etc. Too high carbon equivalent can easily increase the risk of fracture during welding or application of key parts of engineering machinery, which to a certain extent puts forward stricter requirements for low carbon equivalent and low crack sensitivity coefficient in component design.

[0003] Chinese patent publication number CN 113604737 A discloses a Q550D high-strength steel plate and its preparation method, which uses relatively cheap nitrogen to reduce alloy costs. Excessive nitrogen in molten steel (≥0.01%) will cause star cracks on the surface of continuous casting billets, increase the cost of manual grinding, reduce the yield rate of casting billets and the yield rate of rolled plates; the steelmaking process does not undergo vacuum degassing and refining, shortening the production cycle. The hydrogen content in molten steel remains high, and hydrogen-induced delayed cracks are generated during the cutting and welding of steel plates, causing quality objections and affecting the customer's use cycle; there is no clear thickness specification for rolled steel plates. In contrast, the present application can effectively control N≤0.0035% and H≤0.00010% in molten steel, inhibit the generation of star cracks on the surface of continuous casting billets and hydrogen delayed cracks in steel plate cutting and welding, increase the yield rate of casting billets and the yield rate of rolled plates, and ensure the customer's use cycle.

[0004] Chinese patent publication number CN 114570898A discloses a method for producing a low-yield ratio low-alloy high-strength steel plate. The composition includes C: 0.09-0.18%, Nb: 0.3-0.5%, V: 0.3-0.8%, and Cu: 0.1-0.25%. The high content of carbon and other elements increases the carbon equivalent and crack sensitivity coefficient, which is not conducive to improving the weldability of the steel plate, and is prone to welding cracks. The production efficiency is low, and the addition of precious alloy element Cu further increases the raw material cost of the steel plate.

[0005] Chinese patent publication number CN 114672725 A discloses a TMCP-delivered Q550D steel for engineering machinery and its preparation method. It is a VN microalloyed thin-gauge steel for engineering machinery, C: 0.12-0.15%, N: 80-100ppm, carbon equivalent CEV: 0.39-0.44%, Pcm: 0.22-0.25%. The strength of the steel plate is increased, while the higher carbon equivalent is not conducive to improving the welding performance of the steel plate. In addition, the carbon content is in the peritectic steel region, and the degree of segregation of the mass center inside the continuous casting billet is easy to increase; high nitrogen is easy to cause star cracks on the surface of the continuous casting billet, increase the cost of manual grinding, reduce the yield rate of the casting billet and the yield rate of the rolled plate; the production thickness of the steel plate is only 12-20mm, and the thin plate limits the scope of application.

[0006] Chinese patent publication number CN 112725703A discloses a low yield ratio Q550D high-strength steel plate and its manufacturing method. The invention adopts controlled rolling and controlled cooling + low temperature quenching + tempering process, but the production cost increases and the production cycle is long, which seriously affects the delivery time and the economic benefits are not significant. In addition, the thickness of the steel plate involved is only 20 to 30 mm.

[0007] Chinese patent publication number CN 113943890A discloses a low-energy consumption Q550D thick gauge steel plate and its production method. The same quenching and tempering process is used, which increases energy consumption costs and has a long production cycle. In addition, a low-carbon and high-manganese design (C: 0.075-0.095%, Mn: 1.70-1.60%) is adopted. The increase in Mn content can make up for the decrease in strength caused by the decrease in C content, but the Mn element is an element that is easy to segregate. Too high a content can easily cause the center segregation of the continuous casting billet to increase, and subsequent steel plate cutting and welding are prone to cracks, which in turn affects the safety of the steel plate. Summary of the invention

[0008] The purpose of the present invention is to provide a low-crack-sensitive low-alloy steel plate with a yield strength of 550 MPa and a manufacturing method thereof. Low carbon and low alloy achieve control of low carbon equivalent, ensure welding performance, reduce crack sensitivity, and achieve high strength of the steel plate in combination with the manufacturing method.

[0009] The technical solution of the present invention is as follows: the chemical composition of the steel plate of the present invention is C: 0.05-0.09%, Si: 0.10-0.30%, Mn: 1.40-1.60%, Nb: 0.040%-0.060%, Ti: 0.008%-0.018%, Al: 0.020-0.040%, B: ≤0.0010%, Ca: 0.0005-0.0015%, P: ≤0.012%, S: ≤0.002%, O: ≤0.002%, N: ≤0.0035%, H: ≤0.0001%, and the balance is Fe and unavoidable impurity elements; carbon equivalent CEV: ≤0.37%, Pcm≤0.19%.

[0010] The production thickness of the low-alloy high-strength steel plate with a yield strength of 550 MPa is 50 to 70 mm.

[0011] The reasons for limiting the element composition of the steel of the present invention are as follows: C: Carbon as a solid solution element can significantly improve the strength of the steel plate, but it has an adverse effect on the weldability, plasticity, and cold bending performance of the steel plate. Based on the requirements of the weldability matching and cold bending performance of the steel plate, the carbon content in the present invention is controlled to be 0.05-0.09%.

[0012] Si: It plays a role of solid solution strengthening in the steel. However, too high Si content will deteriorate the toughness of martensitic high-strength steel and increase the cold crack sensitivity of the steel plate. In the present invention, Si content is controlled between 0.10 and 0.30%.

[0013] Mn: improves hardenability, promotes martensitic transformation, and improves strength in the steel. However, Mn is a major segregation element. Excessive content of Mn will cause central segregation of the continuous casting billet to form MnS, which will have an adverse effect on the toughness, lamellar tearing resistance, and welding performance of the steel plate. The present invention stipulates that the amount of manganese added is within the range of 1.40-1.60%.

[0014] Nb: It can produce significant grain refinement, precipitation strengthening and precipitation strengthening effects. Nb dissolved in austenite can improve hardenability, and the carbide precipitation phase of Nb has the effect of refining grains and improving matrix strength. In the present invention, the Nb content is controlled to be 0.040-0.060%.

[0015] Ti: Ti (C / N) has a high dissolution temperature, usually above 1400°C, and has a strong pinning effect on the grain boundary at high temperature, preventing the coarsening of austenite grains during heating. The present invention requires rolling at high temperature, and the Ti content is controlled to be 0.008-0.018%.

[0016] Al: a deoxidizing and grain refining element. The present invention stipulates that the Al content is 0.020-0.040%.

[0017] Ca: inclusion modification element, which can react with long strips of MnS to generate spherical CaS, change the anisotropy of the steel plate, and can also modify the Al2O3 inclusions generated by Al deoxidation into spherical low-melting-point inclusions, promote the removal of inclusions, and improve the plastic toughness of the steel plate. In the present invention, the Ca content is controlled to be 0.0005-0.0015%.

[0018] P: A harmful element in steel, which is easy to segregate and has an adverse effect on the plasticity and toughness of the material. A high P content will increase the brittleness of the steel plate. The P content is limited to ≤0.012%.

[0019] S: Harmful element in steel, easy to segregate. It is easy to form MnS segregation inclusions with Mn element, resulting in delamination and cracking of steel plate. S limit: ≤0.002%.

[0020] O, N, H: harmful gas elements, high content, many inclusions, reduce the plastic toughness of the steel plate, and have a high risk of cracking of the steel plate. The present invention strictly controls the O content to ≤ 0.002%; the N content to ≤ 0.0035%; and the H content to ≤ 0.0001%.

[0021] CEV: The present invention adopts the carbon equivalent formula CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; carbon equivalent has a great influence on the strength and weldability of steel. High CEV has high strength, but reduced weldability; to ensure the weldability of the steel plate, the present invention controls CEV≤0.37%.

[0022] Pcm: The cold crack sensitivity coefficient formula adopted by the present invention is Pcm=C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+Mo / 15+V / 10+B / 5; the cold crack sensitivity coefficient has a great influence on the weldability of steel. The higher the Pcm, the lower the welding performance, and vice versa, the better the welding performance. The present invention controls Pcm≤0.19%.

[0023] The present invention further provides a method for manufacturing the above-mentioned low alloy steel plate with a yield strength of 550 MPa and low crack sensitivity, and the specific process involved is as follows: (1) Smelting: The following chemical composition is initially smelted by electric furnace or converter, and further fine-tuned by refining outside the furnace and VD or vacuum degassing treatment. After degassing, the molten steel is treated with trace amount of Ca and soft blowing of argon for a certain period of time. The chemical composition weight percentage is: C: 0.05-0.09%, Si: 0.10-0.30%, Mn: 1.40-1.60%, Nb: 0.040%-0.060%, Ti: 0.008%-0.018%, Al: 0.020-0.040%, B: ≤0.0010%, Ca: 0.0005-0.0015%, P: ≤0.012%, S: ≤0.002%, O: ≤0.002%, N: ≤0.0035%, H: ≤0.0001%, carbon equivalent CEV: ≤0.37%, Pcm: ≤0.19%.

[0024] (2) Continuous casting: It is preferred to use a 300-450mm cross-section continuous casting machine for low superheat argon protection pouring, and the superheat of the molten steel is controlled at 10-25°C; during pouring, the pulling speed fluctuation is controlled at ±0.05m / min, and the non-sinusoidal vibration of the crystallizer liquid surface is controlled at ±5mm; the protective slag layer thickness is 70-90mm, and the liquid slag thickness is 7-11mm; the low-multiple center segregation of the continuous casting billet is ≤C1.0 level, and the center looseness is ≤1.0 level.

[0025] (3) Heating: It is preferred to use a pusher type heating furnace to heat the continuous casting billet in sections to an equalizing temperature of 1200-1250°C, wherein the temperature of the first heating section is 650-750°C, the temperature of the second heating section is 750-1150°C, the temperature of the third heating section is 1200-1260°C, and the temperature of the fourth equalizing section is 1200-1250°C; the continuous casting billet is heated in the furnace for 8-14 min / cm, so that the alloy elements in the steel are fully dissolved to ensure the uniformity of the composition and performance of the final product.

[0026] (4) Rolling: After the continuous casting billet is removed from the furnace, it is descaled by high-pressure water and then subjected to rough rolling + finishing rolling in stages. In order to increase the rolling deformation of the core of the steel plate, the rough rolling is carried out in a high temperature zone, the starting temperature of the rough rolling is 1150-1210°C, the final rolling temperature is 1000-1100°C, the ratio of the thickness of the steel plate after rough rolling to the thickness of the finished product is controlled at 2-3 times, and the number of passes with a reduction rate of 15-20% is not less than 3, and the total reduction of the rough rolling pass is 50-70%; the starting temperature of the finishing rolling is 880-920°C, the final rolling temperature is 820-850°C, and the total reduction of the pass is 55%-65%.

[0027] (5) Cooling: In order to avoid grain growth without generating large stress, a two-stage small water volume method is used to control cooling after rolling. The number of ACC and DQ laminar water groups is 17-20, the roller speed is 0.5-1.0 m / min, and the cooling rate is 12-20 °C / s; the cooling start temperature is 780-840 °C, the final cooling temperature is 500-600 °C, and the laminar upper and lower single water volume is 150-250 / 300-400 m 3 / h.

[0028] Compared with the prior art, the present invention has the following advantages: The invention provides a low-crack-sensitivity low-alloy steel plate with a yield strength of 550 MPa and a manufacturing method thereof. The steel plate has good resistance to welding cracking.

[0029] The present invention adopts a low-carbon, low-carbon equivalent, and low-crack sensitivity coefficient alloy design, eliminates the addition of precious alloy elements such as Ni, Mo, Cu, and V, and has a lower carbon equivalent (≤0.37%) and a lower crack sensitivity coefficient (≤0.19%) of the steel plate, which greatly reduces the cracking problem during steel plate welding. The controlled rolling and controlled cooling process is used to solve the problems of long process flow, high energy consumption, and large amount of alloy addition in the traditional quenching and tempering process. The controlled cooling process is optimized and the DQ+ACC two-stage small water volume method is used to control cooling, which solves the problem of uneven performance at the head and tail and in the transverse and longitudinal directions of the steel plate in the traditional controlled rolling and controlled cooling process.

[0030] In order to increase the rolling deformation of the core of the steel plate, the rough rolling is performed in a high temperature zone, and the rough rolling final rolling temperature is 1000-1100° C. The present invention makes full use of Nb / Ti microalloying to ensure grain refinement and improve toughness of the steel plate during high temperature and large deformation rolling.

[0031] Unlike the traditional two-stage rolling method that ensures the reduction in the finishing rolling stage, the rolling deformation in the present invention is mainly completed in the rough rolling stage. The ratio of the thickness of the steel plate after rough rolling to the thickness of the finished product is controlled to be less than 2 to 3 times, and the number of passes with a reduction rate of 15 to 20% is not less than 3.

[0032] The 50-70 mm steel plate of the present invention has a yield strength of ≥550 MPa, a tensile strength of ≥700 MPa, and an elongation of ≥20%; the low temperature Charpy impact energy of -40°C is ≥100 J; the steel plate has excellent cold bending performance, D=3a, and no cracks in 180° cold bending. The steel plate structure is mainly a mixed structure of fine and uniform acicular ferrite and bainite, which meets the application needs of Q550 steel plate for engineering machinery.

[0033] The method of the present invention can be extended and applied to other high-strength steel plates, such as high-strength steel for offshore shipbuilding, steel for high-rise buildings, steel for bridges, steel for engineering machinery, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1This is the metallographic structure at 1 / 4 of the thickness of the steel plate in Example 3, which is a mixture of fine and uniform acicular ferrite and bainite.

[0035] Figure 2 This is the metallographic structure at 1 / 2 of the thickness of the steel plate in Example 3, which is also a mixture of fine and uniform acicular ferrite and bainite. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below in conjunction with examples. The examples are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0037] The production process of the high-strength low-alloy steel plate in this embodiment is: converter smelting->LF refining->VD or RH vacuum degassing->Ca treatment->continuous casting->heating->controlled rolling and controlled cooling.

[0038] The method for manufacturing the low alloy steel plate with a yield strength of 550 MPa and low crack sensitivity of Examples 1-4 of the present invention comprises the following steps: (1) Smelting: The steel is smelted in a 150-ton converter, then sent to a LF furnace for refining and RH vacuum degassing, and then degassing for Ca treatment. The final composition of the molten steel is shown in Table 1.

[0039] (2) Continuous casting: Molten steel is cast into a 370mm thick continuous casting billet. The casting temperature is controlled at 10-25°C above the liquidus line. During casting, the pulling speed fluctuation is controlled within ±0.05m / min, and the non-sinusoidal vibration of the crystallizer liquid surface is controlled within ±5mm; the thickness of the protective slag layer is 70-90mm, and the thickness of the liquid slag is 7-11mm; the low-multiple center segregation of the continuous casting billet is ≤C1.0 level, and the center looseness is ≤1.0 level. The relevant process parameters are shown in Table 2.

[0040] (3) Controlled rolling: The continuous casting billet obtained in step (2) is loaded into a pusher type heating furnace and heated to 1200-1250°C for 8-14 min / cm2 to fully dissolve the alloy elements in the steel to ensure the uniformity of the composition and performance of the final product. After the billet is removed from the furnace, it is descaled by high-pressure water and then subjected to two-stage controlled rolling of rough rolling + finishing rolling. In order to increase the rolling deformation of the core of the steel plate, the rough rolling is carried out in a high temperature zone, and the rough rolling final rolling temperature is 1000-1100°C. The ratio of the thickness of the steel plate after rough rolling to the thickness of the finished product is controlled to be 2-3 times, and the number of passes with a reduction rate of 15-20% is not less than 3, and the total reduction of the rough rolling pass is 50-70%; the starting rolling temperature of the steel plate finishing rolling is 880-920°C, the final rolling temperature is 820-850°C, and the total reduction of the pass is 55%-65%. The relevant process parameters are shown in Table 3.

[0041] (4) Controlled cooling: In order to avoid grain growth without generating large stress, the ACC+DQ two-stage small water volume method is used for cooling. The number of ACC and DQ laminar water groups is 17-20, the roller speed is 0.5-1.0 m / min, and the cooling rate is 12-20 °C / s; the cooling start temperature is 780-840 °C, the final cooling temperature is 500-600 °C, and the laminar upper and lower single water volume is 150-250 / 300-400 m 3 / h. Relevant process parameters are shown in Table 4.

[0042] The specific composition and manufacturing process parameters are shown in Tables 1 to 4. The corresponding mechanical properties of the steel plates of each example are shown in Table 5.

[0043] Figure 1 The microstructure metallographic photograph of the steel plate of Example 3 is given. The matrix structure of the steel plate is a mixture of fine and uniform acicular ferrite and bainite, which ensures that the steel plate has excellent strength, toughness and resistance to welding cracking.

[0044] The present invention adopts a high-cleanliness steelmaking continuous casting process, controlled rolling, and ACC+DQ two-stage small water volume controlled cooling process, and controls from the perspectives of chemical composition design, parent material structure, rolling deformation, small water volume cooling, etc., to ensure that the steel plate has high strength, high toughness, excellent cold bending performance and resistance to welding cracking. It can be widely used in the manufacture of large-scale engineering machinery equipment or steel structures such as engineering machinery, high-rise buildings, bridges, and offshore platforms that have strict requirements on formability and weldability.

[0045] Table 1 Chemical composition of high-strength steel plates in various examples (wt%) Example C Si Mn P S Al Nb V Ti B Ca O N H CEV Pc 1 0.06 0.25 1.51 0.008 0.001 0.030 0.050 0.003 0.012 0.0002 0.0016 0.0010 0.0029 0.0001 0.32 0.15 2 0.08 0.20 1.48 0.010 0.001 0.025 0.045 0.003 0.015 0.0003 0.0012 0.0008 0.0026 0.0001 0.34 0.16 3 0.06 0.23 1.53 0.007 0.001 0.032 0.048 0.004 0.012 0.0002 0.0012 0.0012 0.0030 0.0001 0.33 0.15 4 0.07 0.22 1.50 0.008 0.001 0.033 0.047 0.002 0.013 0.0002 0.0014 0.0011 0.0029 0.0001 0.33 0.16 Table 2 Continuous casting process parameters of high-strength steel plates in various embodiments Example Superheat, ℃ Pulling speed, m / min Thickness of protective slag layer, mm Liquid slag thickness, mm Center Segregation Loose center 1 18 0.57 75 8.5 C 0.5 0.5 2 15 0.55 80 9 C 0.5 0.5 3 16 0.55 78 8 C 0.5 0.5 4 15 0.56 75 8.5 C 0.5 0.5 Table 3 Controlled rolling process parameters of high-strength steel plates in various embodiments Example Product thickness specification, mm Rough rolling and final rolling temperature, ℃ Rough rolling ≥30mm pass Finish rolling start temperature, ℃ Finishing rolling temperature, ℃ 1 50 1090 4 900 840 2 57 1050 4 890 850 3 65 1030 3 890 830 4 70 1030 4 880 830 Table 4 Controlled cooling process parameters of high-strength steel plates in various embodiments Example Product thickness specification, mm Cooling start temperature, ℃ DQ+ACC laminar water group number, group Roller speed, m / min Cooling rate, ℃ / s Final cooling temperature, ℃ 1 50 820 118 0.64 17 560 2 57 827 18 0.65 18 530 3 65 810 20 0.58 19 500 4 70 815 20 0.55 19 520 Table 5 Mechanical properties of high-strength steel plates in various embodiments

Claims

1. A 550MPa grade low alloy high strength steel plate with low crack sensitivity coefficient, characterized in that: The chemical composition by mass percentage is C: 0.05~0.09%, Si: 0.10~0.30%, Mn: 1.40~1.60%, Nb: 0.040%~0.060%, Ti: 0.008%~0.018%, Al: 0.020~0.040%, B: ≤0.0010%, Ca: 0.0005~0.0015%, P: ≤0.012%, S: ≤0.002%, O: ≤0.002%, N: ≤0.0035%, H: ≤0.0001%, the balance is Fe and unavoidable impurity elements; carbon equivalent CEV: ≤0.37%, Pcm≤0.19%.

2. The steel plate according to claim 1, characterized in that: The production thickness of the steel plate is 50-70 mm, and the microstructure is mainly a mixed structure of uniform acicular ferrite and bainite.

3. The steel plate according to claim 1, characterized in that: The yield strength of the steel plate is ≥550MPa, the tensile strength is ≥700MPa, and the elongation is ≥20%; -40℃ low temperature Charpy impact energy ≥100J; the steel plate has excellent cold bending performance, D=3a, and no cracks in 180° cold bending.

4. A method for manufacturing the steel plate according to claim 1, characterized in that: include:

1. Smelting The steel is first smelted in an electric furnace or converter, and further fine-tuned by refining outside the furnace and vacuum degassing. After the steel is degassed, Ca treatment is performed and the steel is allowed to stand for soft blowing of argon.

2. Continuous Casting The molten steel is cast into billets, and the low-center segregation of the continuous casting billet is ≤C1.0 level, and the center porosity is ≤1.0 level; 3. Heating The continuous casting billet is heated in the furnace, and the structure is completely austenitized, so that the alloy elements in the steel are fully dissolved; 4. Rolling After the continuous casting billet is removed from the furnace, it is descaled by high-pressure water and then subjected to rough rolling + finishing rolling in stages: in order to increase the rolling deformation of the core of the steel plate, the rough rolling is carried out in a high temperature zone, the starting rolling temperature of the rough rolling is 1150-1210°C, the final rolling temperature is 1000-1100°C, the ratio of the thickness of the steel plate after rough rolling to the thickness of the finished product is controlled at 2-3 times, and the number of passes with a reduction rate of 15-20% is not less than 3, and the total reduction of the rough rolling pass is 50-70%; the starting rolling temperature of the finishing rolling is 880-920°C, the final rolling temperature is 820-850°C, and the total reduction rate of the pass is 55%-65%; 5. Cooling After rolling, a two-stage small water volume method is used to control cooling. The number of ACC and DQ laminar water groups is 17-20, the roller speed is 0.5-1.0m / min, and the cooling rate is 12-20℃ / s; the cooling start temperature is 780-840℃, the final cooling temperature is 500-600℃, and the laminar upper and lower single water volume is 150-250 / 300-400m 3 / h.

5. The method according to claim 4, characterized in that: Step 2: Continuous casting uses a 300-450mm section thick continuous casting machine for low superheat argon protection pouring, and the superheat of the molten steel is controlled at 10-25°C; during pouring, the pulling speed fluctuation is controlled within ±0.05m / min, and the non-sinusoidal vibration of the crystallizer liquid surface is controlled within ±5mm; the protective slag layer thickness is 70-90mm, and the liquid slag thickness is 7-11mm.

6. The method according to claim 4, characterized in that: Step three, use a pusher type heating furnace to heat the continuous casting billet in sections to an even heating temperature of 1200-1250°C, wherein the temperature of the first heating section is 650-750°C, the temperature of the second heating section is 750-1150°C, the temperature of the third heating section is 1200-1260°C, and the temperature of the fourth even heating section is 1200-1250°C; the continuous casting billet is heated in the furnace for 8-14 min / cm.

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