Economical high-flange 900MPa-grade hot-rolled steel plate for automobile frame and production method

Abstract

The invention provides an economical high-flange 900MPa-grade hot-rolled automobile beam steel plate and a production method, and belongs to the technical field of hot-rolled automobile beam steel manufacturing. The steel plate is prepared from chemical components in percentage by mass as follows: 0.04%-0.10% of C, 0.06%-0.15% of Si, 1.3%-1.8% of Mn, smaller than or equal to 0.005% of S, smaller than or equal to 0.008% of P, 0.010%-0.030% of Nb, 0.06%-0.10% of Ti, larger than or equal to 0.025% of Als, smaller than or equal to 40 ppm of N and the balance of Fe and inevitable impurities. The production method comprises the working procedures of smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling. According to the invention, no expensive microalloy element Mo is added, the relative cost is relatively low, and by designing a reasonable controlled cooling process for different thickness specifications, good plasticity and toughness can be maintained while the strength of the steel plate is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of hot-rolled automobile beam steel manufacturing, and in particular relates to an economical high-flange 900MPa grade hot-rolled automobile beam steel plate and a production method. Background Art

[0002] In order to meet the demand for lightweight, automobile beam steel is constantly developing towards high strengthening. As the strength increases, the toughness of the steel grade gradually decreases, especially in dynamic load components, the fatigue strength is seriously affected. Because the automobile is under high stress bearing conditions for a long time, initial damage will inevitably occur in its structural parts and form microcracks. When the combination of the applied load and the length of the microcracks forces the stress intensity factor to exceed the threshold value of fatigue crack growth, the automobile parts will suffer non-plastic failure after a limited number of load cycles. Therefore, when pursuing the goal of high strength and lightweight, the plastic toughness of the steel must also be guaranteed.

[0003] Although the automobile beam steel plates developed and used with existing technologies have improved strength, their strength-plasticity product cannot reach a good level. In addition, due to the large amount of alloy elements added, the difficulty in processing, and the low yield rate, the cost of the steel plates has increased significantly, restricting the application of the material.

[0004] Chinese patent CN 105543666 B discloses a 960 MPa yield strength automobile beam steel and a production method thereof. Although the invention improves the safety performance of the vehicle and is conducive to the lightweight development of the vehicle, the invention adds the expensive microalloying element Mo, which greatly increases the cost of the automobile beam steel, which brings huge cost pressure to both enterprises and customers.

[0005] Chinese patent CN 109097699 A discloses a 900MPa grade hot-rolled automobile beam steel and a manufacturing method thereof, which mainly solves the shortcomings of the prior art in terms of plate shape and poor welding. However, the specification of this method is only 4.0-10.0mm, which does not completely cover all specifications of beam steel.

[0006] Chinese patent CN 113322413 B discloses a high fatigue performance 900MPa hot-rolled automobile beam steel strip and its preparation method, which solves the problem of poor fatigue performance of existing 900MPa hot-rolled automobile beam steel strips. However, the invention also adds expensive microalloying element Mo, which greatly increases the cost of automobile beam steel, bringing huge cost pressure to enterprises and customers.

[0007] In summary, the currently developed high-strength beam steel has high mechanical properties, but low plastic toughness, and cannot achieve a good match between high strength and high plastic toughness, thus not having good formability. Moreover, most of the high-elongation high-strength steels in the prior art need to undergo heat treatment processes after hot rolling to improve yield strength, tensile strength and elongation, which obviously increases production costs; most high-strength beam steels need to add expensive microalloying elements Mo to improve strength and maintain the plastic toughness of the steel, which greatly increases the cost of automobile beam steel, bringing huge cost pressure to both companies and customers, and is not conducive to large-scale industrial production and promotion. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide an economical high-flange 900MPa grade hot-rolled steel plate for automobile beams and a production method, which solves the problem that the strength, forming and cost of steel in the prior art cannot meet the lightweight service conditions of automobile beams.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is: an economical high-flange 900MPa grade hot-rolled steel plate for automobile beams, characterized in that the chemical composition of the steel plate and its mass percentage are: C: 0.04%-0.10%, Si: 0.06%-0.15%, Mn: 1.3%-1.8%, S≤0.005%, P≤0.008%, Nb: 0.010%-0.030%, Ti: 0.06%-0.10%, Als≥0.025%, N≤40ppm, and the rest are Fe and unavoidable impurities.

[0010] The steel plate of the present invention has a tensile strength of 926-960 MPa, a yield strength of 832-869 MPa, an elongation of 16-23%, and a low-temperature impact energy of -20°C of ≥100 J when the thickness is greater than 4 mm. When the thickness is ≤4 mm, the hole expansion rate is 75-85%.

[0011] The metallographic structure of the steel plate of the present invention is acicular ferrite+pearlite+bainite.

[0012] The steel plate of the present invention has a grain size of 11-13.

[0013] The thickness specification of the steel plate of the present invention is 1.8-16mm.

[0014] The present invention also provides a method for producing the above-mentioned economical high-flange 900MPa grade hot-rolled steel plate for automobile beams, the production method comprising smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes; the hot rolling process comprises high-pressure water descaling, the descaling water pressure is 18-22MPa, the intermediate billet thickness after rough rolling is 30-55mm, the start temperature of finish rolling is 980-1030°C, and the final rolling temperature is 840-880°C.

[0015] In the slab continuous casting process of the present invention, the thickness of the continuously cast slab is 150mm-250mm.

[0016] In the heating process of the present invention, the heating temperature of the casting is 1200-1240° C. and the heat preservation time is 180-230 minutes.

[0017] The rough rolling process of the present invention adopts the 3+3 mode / 3+5 mode to carry out rough rolling on the heated cast billet in the austenite recrystallization zone, and when the steel plate thickness is 1.8mm≤<6.0mm, it is rolled into an intermediate billet of 40-50mm, and the deformation amount of the first pass is controlled to be ≥20%, the deformation amount of the last pass is ≥40%, and the cumulative deformation amount is ≥85%; when the steel plate thickness is 6mm≤≤16mm, it is rolled into an intermediate billet of 50-55mm, and the deformation amount of the first pass is controlled to be ≥20%, the deformation amount of the last pass is ≥40%, the cumulative deformation amount of the last two stands is ≥55%, and the cumulative deformation amount is ≥80%.

[0018] According to the cooling process of the present invention, after the finish rolling, the steel plate with a thickness of 1.8 mm ≤ and less than 6.0 mm is cooled in two stages at a rate of 90-110°C / s to quickly cool to 680-720°C, and then air-cooled for 6-10s and then cooled to 420-450°C at a rate of 60-80°C / s; the steel plate with a thickness of ≥6.0 mm is cooled by concentrated front-stage water cooling from back to front at a rate of 80-100°C / s to cool to 580-600°C; in the hot rolling coiling process, the coiling temperature is 420-600°C.

[0019] The beneficial effects of adopting the above technical scheme are: 1. The present invention adopts a simple chemical composition design and does not add expensive microalloying element Mo, so the cost is relatively low; 2. The present invention improves the strength of the steel plate while maintaining good plasticity and toughness by designing a reasonable controlled cooling process for different thickness specifications, thereby perfectly achieving a match between strength and toughness; 3. The present invention achieves coverage of commonly used thickness specifications of 900MPa-grade beam steel, solving the problem in the prior art that the strength, forming and cost of steel materials cannot meet the lightweight service conditions of automobile beams. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The metallographic structure diagram of the 900MPa grade beam steel of Example 1; Figure 2 Metallographic structure diagram of 900MPa grade beam steel of Example 2; Figure 3 The metallographic structure diagram of the 900MPa grade beam steel of Example 3; Figure 4 Metallographic structure diagram of 900MPa grade beam steel of Example 4; Figure 5Metallographic structure diagram of 900MPa grade beam steel of Example 5; Figure 6 Metallographic structure diagram of 900MPa grade beam steel of Example 6; Figure 7 Metallographic structure diagram of 900MPa grade beam steel of Example 7. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with specific implementation methods. Example 1

[0022] The thickness of the steel strip for the beam in this embodiment is 1.8 mm, and its chemical composition and mass percentage are shown in Table 1.

[0023] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0024] (2) Slab continuous casting process, the thickness of the continuous casting slab is 150 mm.

[0025] (3) Heating process: the heating temperature of the ingot is 1240℃ and the insulation time is 200min.

[0026] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+3 mode to form a 40 mm intermediate billet. The deformation amount of the first pass is controlled to be 25%, the deformation amount of the last pass is 40%, and the cumulative deformation amount is 85%.

[0027] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 20MPa, intermediate billet thickness after rough rolling is 40mm, finishing rolling starting temperature is 1030℃, and final rolling temperature is 880℃.

[0028] (6) Cooling process: After finishing rolling, double-stage cooling is adopted, and the steel is rapidly cooled to 680°C at a rate of 100°C / s, and then air-cooled for 8s and further cooled to 440°C at a rate of 60°C / s.

[0029] (7) Hot rolling coiling process, coiling temperature 440℃.

[0030] The steel strip for the beam obtained after hot rolling in this embodiment was tested and found to have a tensile strength of 958 MPa, a yield strength of 856 MPa, an elongation of 21%, an average hole expansion rate of 81% at a temperature of -20°C, a metallographic structure of acicular ferrite + pearlite + bainite, and a grain size of level 13. Example 2

[0031] The thickness of the steel strip for the beam in this embodiment is 3 mm, and its chemical composition and mass percentage are shown in Table 1.

[0032] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0033] (2) Slab continuous casting process, the thickness of the continuous casting slab is 200 mm.

[0034] (3) Heating process: the heating temperature of the ingot is 1240℃ and the insulation time is 200min.

[0035] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+3 mode to form a 50 mm intermediate billet. The deformation amount of the first pass is controlled to be 30%, the deformation amount of the last pass is 35%, and the cumulative deformation amount is 85%.

[0036] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 20MPa, intermediate billet thickness after rough rolling is 40mm, finishing rolling starting temperature is 1025℃, and final rolling temperature is 870℃.

[0037] (6) Cooling process: After finishing rolling, double-stage cooling is adopted, and the steel is rapidly cooled to 710°C at a rate of 110°C / s, and then air-cooled for 6s and further cooled to 450°C at a rate of 80°C / s.

[0038] (7) Hot rolling coiling process, coiling temperature 450℃.

[0039] The steel strip for the beam obtained after hot rolling in this embodiment was tested and found to have a tensile strength of 952 MPa, a yield strength of 846 MPa, an elongation of 23%, an average hole expansion rate of 85%, a phase structure of acicular ferrite + pearlite + bainite, and a grain size of level 13. Example 3

[0040] The thickness of the steel strip for the beam in this embodiment is 5 mm, and its chemical composition and mass percentage are shown in Table 1.

[0041] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0042] (2) Slab continuous casting process, the thickness of the continuous casting slab is 200 mm.

[0043] (3) Heating process: the heating temperature of the ingot is 1240℃ and the insulation time is 180min.

[0044] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+3 mode to form a 46 mm intermediate billet. The deformation amount of the first pass is controlled to be 30%, the deformation amount of the last pass is 45%, and the cumulative deformation amount is 90%.

[0045] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 20MPa, intermediate billet thickness after rough rolling is 46mm, finishing rolling starting temperature is 1010℃, and final rolling temperature is 860℃.

[0046] (6) Cooling process: After finishing rolling, double-stage cooling is adopted, and the steel is rapidly cooled to 720°C at a rate of 90°C / s, and then air-cooled for 10s and further cooled to 420°C at a rate of 75°C / s.

[0047] (7) Hot rolling coiling process, coiling temperature 420℃.

[0048] The steel strip for the beam obtained after hot rolling in this embodiment was tested and found to have a tensile strength of 955 MPa, a yield strength of 848 MPa, an elongation of 21%, and a Charpy impact absorption energy of the longitudinal specimen of the steel plate at a temperature of -20°C of 139 J, 142 J and 145 J. The phase structure was quasi-polygonal ferrite + pearlite + bainite, and the grain size was grade 12.5. Example 4

[0049] The thickness of the steel strip for the beam in this embodiment is 8 mm, and its chemical composition and mass percentage are shown in Table 1.

[0050] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0051] (2) Slab continuous casting process, the thickness of the continuous casting slab is 230 mm.

[0052] (3) Heating process: the heating temperature of the ingot is 1200℃ and the insulation time is 220min.

[0053] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+5 mode to form a 50 mm intermediate billet. The deformation amount of the first pass is controlled to be 25%, the deformation amount of the last pass is 40%, the cumulative deformation amount of the last two stands is 55%, and the cumulative deformation amount is ≥85%.

[0054] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 22MPa, intermediate billet thickness after rough rolling is 50mm, finishing rolling starting temperature is 1000℃, and final rolling temperature is 880℃.

[0055] (6) Cooling process: After finishing rolling, the steel is cooled by concentrated water cooling from the front section to the front, with a cooling rate of 80°C / s to 580°C.

[0056] (7) Hot rolling coiling process, coiling temperature 580℃.

[0057] The steel strip for beam obtained after hot rolling in this embodiment has been tested to have a tensile strength of 935 MPa, a yield strength of 858 MPa, an elongation of 20%, and a Charpy impact absorption energy of the longitudinal specimen of the steel plate at a temperature of -20°C of 159 J, 168 J, and 163 J. The phase structure is quasi-polygonal ferrite + pearlite + a small amount of bainite, and the grain size is 12 levels. Example 5

[0058] The thickness of the steel strip for the beam in this embodiment is 10 mm, and its chemical composition and mass percentage are shown in Table 1.

[0059] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0060] (2) Slab continuous casting process, the thickness of the continuous casting slab is 230 mm.

[0061] (3) Heating process: the heating temperature of the ingot is 1225℃ and the insulation time is 210min.

[0062] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+5 mode to form a 50 mm intermediate billet. The deformation amount of the first pass is controlled to be 25%, the deformation amount of the last pass is 40%, the cumulative deformation amount of the last two stands is 60%, and the cumulative deformation amount is 90%.

[0063] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 20MPa, intermediate billet thickness after rough rolling is 50mm, finishing rolling starting temperature is 1050℃, and final rolling temperature is 875℃.

[0064] (6) Cooling process: After finishing rolling, the steel is cooled by concentrated water cooling from the front section to the front section at a cooling rate of 85°C / s to 600°C.

[0065] (7) Hot rolling coiling process, coiling temperature 600℃.

[0066] The steel strip for the beam obtained after hot rolling in this embodiment was tested and found to have a tensile strength of 938 MPa, a yield strength of 853 MPa, an elongation of 20%, and a Charpy impact absorption energy of the longitudinal sample of the steel plate at a temperature of -20°C of 167 J, 165 J and 169 J. The phase structure was quasi-polygonal ferrite + pearlite + a small amount of bainite, and the grain size was level 12. Example 6

[0067] The thickness of the steel strip for the beam in this embodiment is 12 mm, and its chemical composition and mass percentage are shown in Table 1.

[0068] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0069] (2) Slab continuous casting process, the thickness of the continuous casting slab is 250 mm.

[0070] (3) Heating process: the heating temperature of the ingot is 1230℃ and the insulation time is 190min.

[0071] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+5 mode to form an intermediate billet of 55 mm. The deformation of the first pass is controlled to be 20%, the deformation of the last pass is 50%, the cumulative deformation of the last two stands is 55%, and the cumulative deformation is 90%.

[0072] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 22MPa, intermediate billet thickness after rough rolling is 55mm, finishing rolling starting temperature is 980℃, and final rolling temperature is 840℃.

[0073] (6) Cooling process: After finishing rolling, the steel is cooled by concentrated water cooling from the front section to the front, with a cooling rate of 95°C / s to 595°C.

[0074] (7) Hot rolling coiling process, coiling temperature 595℃.

[0075] The steel strip for the beam obtained after hot rolling in this embodiment was tested and found to have a tensile strength of 935 MPa, a yield strength of 835 MPa, an elongation of 21%, and a Charpy impact absorption energy of the longitudinal specimen of the steel plate at a temperature of -20°C of 155 J, 146 J and 149 J. The phase structure was quasi-polygonal ferrite + pearlite + a small amount of bainite, and the grain size was level 12. Example 7

[0076] The thickness of the steel strip for the beam in this embodiment is 16 mm, and its chemical composition and mass percentage are shown in Table 1.

[0077] The production steps of the steel strip for beams in this embodiment include smelting, continuous casting, heating, rough rolling, finish rolling, cooling and coiling processes, and the specific process is as follows: (1) After converter smelting, LF refining and RH vacuum refining, the chemical composition of molten steel and its mass percentage are shown in Table 1.

[0078] (2) Slab continuous casting process, the thickness of the continuous casting slab is 250 mm.

[0079] (3) Heating process: the heating temperature of the ingot is 1235℃ and the insulation time is 205min.

[0080] (4) In the rough rolling process, the heated ingot is rough rolled in the austenite recrystallization zone using the 3+5 mode to form an intermediate billet of 55 mm. The deformation of the first pass is controlled to be 30%, the deformation of the last pass is 45%, the cumulative deformation of the last two stands is 65%, and the cumulative deformation is 90%.

[0081] (5) Hot rolling process, high-pressure water descaling, descaling water pressure 22MPa, intermediate billet thickness after rough rolling is 55mm, finishing rolling starting temperature is 990℃, and final rolling temperature is 855℃.

[0082] (6) Cooling process: After finishing rolling, the steel is cooled by concentrated water cooling from the front section to the front section at a cooling rate of 100°C / s to 580°C.

[0083] (7) Hot rolling coiling process, coiling temperature 580℃.

[0084] The steel strip for the beam obtained after hot rolling in this embodiment was tested and found to have a tensile strength of 938 MPa, a yield strength of 846 MPa, an elongation of 21%, and a Charpy impact absorption energy of the longitudinal specimen of the steel plate at a temperature of -20°C of 145 J, 156 J and 152 J. The phase structure was quasi-polygonal ferrite + pearlite + a small amount of bainite, and the grain size was level 12.

[0085] Table 1 Chemical composition of steel plates of Examples 1-7 and their mass percentage (%)

[0086] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. An economical high flange 900MPa grade hot-rolled steel plate for automobile beams, characterized in that: The chemical composition of the steel plate and its mass percentage are: C: 0.04%-0.10%, Si: 0.06%-0.15%, Mn: 1.3%-1.8%, S≤0.005%, P≤0.008%, Nb: 0.010%-0.030%, Ti: 0.06%-0.10%, Als≥0.025%, N≤40ppm, and the rest are Fe and unavoidable impurities.

2. The economical high flange 900MPa grade hot-rolled steel plate for automobile beam according to claim 1, characterized in that: The steel plate has a tensile strength of 926-960 MPa, a yield strength of 832-869 MPa, an elongation of 16-23%, a low-temperature impact energy of -20°C of ≥100 J when the thickness is >4 mm, and a hole expansion rate of 75-85% when the thickness is ≤4 mm.

3. The economical high flange 900MPa grade hot-rolled steel plate for automobile beam according to claim 1 or 2, characterized in that: The metallographic structure of the steel plate is acicular ferrite+pearlite+bainite.

4. The economical high flange 900MPa grade hot-rolled steel plate for automobile beam according to claim 1 or 2, characterized in that: The steel plate has a grain size of 11-13.

5. The economical high flange 900MPa grade hot-rolled steel plate for automobile beam according to claim 1 or 2, characterized in that: The thickness specification of the steel plate is 1.8-16mm.

6. The method for producing an economical high flange 900 MPa grade hot-rolled steel plate for automobile beams according to any one of claims 1 to 5, characterized in that: The production method comprises smelting, continuous casting, heating, rough rolling, finishing rolling, cooling and coiling processes; in the hot continuous rolling process, high-pressure water descaling is performed, the descaling water pressure is 18-22 MPa, the intermediate billet thickness after rough rolling is 30-55 mm, the finishing rolling starting temperature is 980-1030° C., and the final rolling temperature is 840-880° C.

7. The method for producing an economical high flange 900MPa grade hot-rolled steel plate for automobile beams according to claim 6, characterized in that: In the slab continuous casting process, the thickness of the continuous casting slab is 150mm-250mm.

8. The method for producing an economical high flange 900MPa grade hot-rolled steel plate for automobile beams according to claim 6, characterized in that: In the heating process, the heating temperature of the ingot is 1200-1240°C and the temperature is kept for 180-230 minutes.

9. The method for producing an economical high flange 900MPa grade hot-rolled steel plate for automobile beams according to any one of claims 6 to 8, characterized in that: The rough rolling process adopts the 3+3 mode / 3+5 mode to carry out rough rolling on the heated cast billet in the austenite recrystallization zone, and when the steel plate thickness is 1.8mm≤<6.0mm, it is rolled into an intermediate billet of 40-50mm, and the deformation amount of the first pass is controlled to be ≥20%, the deformation amount of the last pass is ≥40%, and the cumulative deformation amount is ≥85%; when the steel plate thickness is 6mm≤≤16mm, it is rolled into an intermediate billet of 50-55mm, and the deformation amount of the first pass is controlled to be ≥20%, the deformation amount of the last pass is ≥40%, the cumulative deformation amount of the last two stands is ≥55%, and the cumulative deformation amount is ≥80%.

10. The method for producing an economical high flange 900MPa grade hot-rolled steel plate for automobile beams according to any one of claims 6 to 8, characterized in that: In the cooling process, after the finish rolling, the steel plate with a thickness of 1.8mm≤steel plate<6.0mm is cooled by double stage at a rate of 90-110℃ / s to quickly cool to 680-720℃, and then air-cooled for 6-10s and further cooled to 420-450℃ at a rate of 60-80℃ / s; the steel plate with a thickness of ≥6.0mm is cooled by concentrated front-stage and open water from back to front at a rate of 80-100℃ / s to cool to 580-600℃; in the hot rolling coiling process, the coiling temperature is 420-600℃.

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