steel having a top and bottom surface quality of grade FB

Abstract

The present application relates to the technical field of steel smelting, and in particular to a steel material with FB grade upper and lower surface quality, wherein the steel material is prepared by blowing water on the upper and lower surfaces in the finishing rolling step. The steel material has good upper and lower surface quality, can reach FB grade, and the upper and lower surfaces of the steel material do not adversely affect the appearance of subsequent painting or electroplating.

Description

Technical Field

[0001] This invention relates to the field of steel smelting technology, and more specifically, to steel with an upper and lower surface quality of FB grade. Background Technology

[0002] The steel used in related technologies, such as steel strips for automotive stamping, has high surface quality requirements. It is necessary to ensure that the surface of the steel does not have any defects that would affect the appearance after subsequent painting or electroplating.

[0003] However, the steel produced by the relevant technologies can only ensure that the upper surface of the steel has good surface quality, and it is difficult to ensure that both the upper and lower surfaces of the steel have good surface quality at the same time. Summary of the Invention

[0004] The purpose of this invention is to provide steel with an FB grade for both the upper and lower surfaces. The upper and lower surfaces of the steel are of good quality and can reach the FB grade. Furthermore, neither the upper nor lower surface of the steel will have an adverse effect on the appearance of subsequent painting or electroplating.

[0005] This invention is implemented as follows:

[0006] This invention provides a steel with an upper and lower surface quality of FB grade. The composition of the steel with an upper and lower surface quality of FB grade, by mass percentage, includes:

[0007] C≤0.0025%, Mn: 0.10-0.2%, S≤0.012%, P≤0.015%, Si≤0.030%, Als: 0.020-0.05%, Ti: 0.050-0.07%, B: 0.0002-0.0004%, with the balance being Fe and unavoidable impurities, and Mn / S≥12;

[0008] Methods for preparing steel with an upper and lower surface quality of FB grade include:

[0009] The processes include: smelting molten iron, KR desulfurization, LD converter steelmaking, refining, continuous casting, cold or hot conveying, and rolling; among which...

[0010] Refining includes at least one of LF refining and RH refining;

[0011] The composition of the molten iron must meet the following requirements: S≤0.030%, Cu≤0.050%, Cr≤0.050%, Ni≤0.050%.

[0012] In the KR desulfurization process, the sulfur content in the molten iron entering the furnace is ≤0.0020%, and the slag is removed until the bright metal surface of the molten iron is ≥95%, and the temperature of the molten iron entering the furnace is 1250-1380℃.

[0013] In the LD converter steelmaking process, argon gas is used throughout the bottom blowing process until tapping, and lime and fluorite slag are added for washing during tapping; the final control of the steel grade meets the following requirements: C content 0.04-0.06%, P content ≤0.010%, S content ≤0.0060%, N content ≤18ppm; aluminum blocks, MnFe and Ti are used for deoxidation and alloying during tapping;

[0014] In the continuous casting process, the pressure of the stopper rod argon gas is 0.3-0.4 MPa, and the flow rate of the stopper rod argon gas is 4.5-5.5 L / min; the pressure of the upper nozzle argon gas is 0.3-0.4 MPa, and the flow rate of the upper nozzle argon gas is 4.5-5.5 L / min; the pressure of the interplate argon gas is 0.3-0.4 MPa, and the flow rate of the interplate argon gas is 2.5-3.5 L / min.

[0015] The rolling process includes heating, roughing, finishing, laminar flow, and coiling; the finishing process also includes blowing water onto the upper and lower surfaces of the rolled steel.

[0016] In an optional implementation, during the RH refining step, the circulating gas flow rate during oxygen blowing is controlled to be 90-110 Nm³. 3 / h; the gas flow rate during cyclic decarbonization is 240-280 Nm³. 3 / h, time is 15-20min; at the end of decarburization, the oxygen content is controlled at 450-600ppm and the temperature is greater than 1565℃; the circulating gas flow rate during alloying is 200-240Nm 3 / h, the circulating gas flow rate during pure circulation after alloying is 120-150 Nm³. 3 / h, the circulating gas is nitrogen; the N content before RH refining and leaving the station is ≤40%.

[0017] In optional implementations, refining includes CAS refining, LF refining, and RH refining;

[0018] The tapping temperature of the casting furnace in the LD converter steelmaking stage is ≥1640℃; the arrival temperature of the casting furnace in the CAS refining stage is ≥1590℃; the outlet temperature of the casting furnace in the LF refining stage is 1635-1645℃; the inlet temperature of the casting furnace in the RH refining stage is 1620-1635℃; the outlet temperature of the casting furnace in the RH refining stage is 1595-1605℃; and the temperature from the ladle to the continuous casting platform is T. L +60±5, the temperature of the tundish in the casting furnace is T. L +30±10;

[0019] The tapping temperature of the continuous casting furnace in the LD converter steelmaking stage is ≥1640℃; the arrival temperature of the continuous casting furnace in the CAS refining stage is ≥1590℃; the outlet temperature of the continuous casting furnace in the LF refining stage is 1625-1635℃; the inlet temperature of the continuous casting furnace in the RH refining stage is 1610-1625℃; the outlet temperature of the continuous casting furnace in the RH refining stage is 1590-1600℃; and the temperature from the ladle to the continuous casting platform is T. L +60±5, the temperature of the tundish in the continuous casting furnace is T L +30±10;

[0020] Among them, T L This is the liquidus temperature of molten steel.

[0021] In an optional implementation, during the continuous casting step,

[0022] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×1250mm and less than or equal to 230mm×1400mm, and the temperature is greater than or equal to 1565℃ and less than or equal to 1575℃, the casting speed is controlled at 1.0m / min.

[0023] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×1250mm and less than or equal to 230mm×1400mm, and the temperature is greater than or equal to 1545℃ and less than 1565℃, the casting speed is controlled at 1.1m / min.

[0024] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×1250mm and less than or equal to 230mm×1400mm, and the temperature is less than 1545℃, the casting speed is controlled at 1.15m / min.

[0025] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×1100mm and less than 230mm×1250mm, and the temperature is greater than or equal to 1565℃ and less than or equal to 1575℃, the casting speed is controlled at 1.1m / min.

[0026] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×1100mm and less than 230mm×1250mm, and the temperature is greater than or equal to 1545℃ and less than 1565℃, the casting speed is controlled at 1.2m / min.

[0027] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×1100mm and less than 230mm×1250mm, and the temperature is less than 1545℃, the casting speed is controlled at 1.25m / min.

[0028] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×900mm and less than 230mm×1100mm, and the temperature is greater than or equal to 1565℃ and less than or equal to 1575℃, the casting speed is controlled at 1.2m / min.

[0029] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×900mm and less than 230mm×1100mm, and the temperature is greater than or equal to 1545℃ and less than 1565℃, the casting speed is controlled at 1.3m / min.

[0030] When the cross-sectional dimensions of the billet are greater than or equal to 230mm×900mm and less than 230mm×1100mm, and the temperature is less than 1545℃, the casting speed is controlled at 1.35m / min.

[0031] In an optional implementation, during the continuous casting step, the nozzle insertion depth is 130±5mm, and the protective slag is ultra-low carbon steel protective slag.

[0032] In an optional implementation, when a cold feeding step is used before rolling and the thickness of the finished steel obtained by rolling is 1.4-3.2mm, the time of the billet in the furnace during the heating step is ≥150min.

[0033] When a cold feeding step is used before rolling, and the thickness of the finished steel product obtained by rolling is greater than 3.2mm, the time of the billet in the furnace during the heating step is ≥160min.

[0034] When a hot conveying step is used before rolling, and the thickness of the finished steel product obtained by rolling is 1.4-3.2mm, the time of the billet in the furnace during the heating step is ≥140min.

[0035] When a hot conveying step is used before rolling, and the thickness of the finished steel product obtained by rolling is greater than 3.2 mm, the time of the billet in the furnace during the heating step is ≥150 min.

[0036] In an optional embodiment, during the heating step, the temperature of the first heating section is 1100-1200℃, the temperature of the second heating section is 1190-1260℃, the temperature of the soaking section is 1190-1240℃, and the furnace exit temperature is 1195-1205℃; wherein, in the second heating section and the soaking section, the temperature difference between the upper and lower surfaces of the billet is 10℃.

[0037] In the rough rolling step, the final rolling temperature is 1030±30℃;

[0038] In the finishing rolling process, when the thickness of the finished steel product is 1.4-3.2mm, the final rolling temperature is 930±20℃; when the thickness of the finished steel product is greater than 3.2mm, the final rolling temperature is 920±20℃.

[0039] During the winding process, the temperature is 710±20℃ and the pressure is 130±10N / mm. 2 .

[0040] In an optional implementation, the roughing step includes inlet dephosphorization and outlet dephosphorization, wherein the dephosphorization water pressure of either inlet or outlet dephosphorization is ≥20 MPa.

[0041] In an optional embodiment, the finishing rolling step includes seven rolling passes. When the thickness of the finished steel obtained by rolling is 1.4-3.0 mm, the reduction rate of the first pass is 29%, the reduction rate of the second pass is 31%, the reduction rate of the third pass is 26%, the reduction rate of the fourth pass is 23%, the reduction rate of the fifth pass is 21%, the reduction rate of the sixth pass is 17%, and the reduction rate of the seventh pass is 13%.

[0042] When the thickness of the finished steel obtained by rolling is 3.01-4.0mm, the reduction rate of the first pass is 28%, the reduction rate of the second pass is 30%, the reduction rate of the third pass is 26%, the reduction rate of the fourth pass is 23%, the reduction rate of the fifth pass is 21%, the reduction rate of the sixth pass is 17%, and the reduction rate of the seventh pass is 13%.

[0043] When the thickness of the finished steel obtained by rolling is 4.01-6.5mm, the reduction rate of the first pass is 25%, the reduction rate of the second pass is 27%, the reduction rate of the third pass is 25%, the reduction rate of the fourth pass is 23%, the reduction rate of the fifth pass is 21%, the reduction rate of the sixth pass is 17%, and the reduction rate of the seventh pass is 15%.

[0044] In the finishing rolling process, the interstand tension L1 between the first and second passes is 5 N / mm. 2 The inter-rack tension L2 between the second and third passes is 7 N / mm. 2 The inter-rack tension L3 between the third and fourth passes is 9 N / mm. 2 The inter-rack tension L4 between the fourth and fifth passes is 11 N / mm. 2 When the thickness of the finished steel obtained by rolling is 1.4-3.0 mm, the interstand tension L5 between the fifth and sixth passes is 14 N / mm. 2 When the thickness of the finished steel obtained by rolling is 3.01-6.5mm, the interstand tension L5 between the fifth and sixth passes is 13N / mm.2 When the thickness of the finished steel obtained by rolling is 1.4-3.0 mm, the interstand tension L6 between the sixth and seventh passes is 16 N / mm. 2 When the thickness of the finished steel obtained by rolling is 3.01-6.5mm, the interstand tension L6 between the sixth and seventh passes is 15N / mm. 2 .

[0045] In an optional embodiment, the step of purging water onto the upper and lower surfaces of the rolled steel specifically includes: when the thickness of the finished steel obtained by rolling is ≥3.00mm, purging water onto the upper and lower surfaces of the steel is turned on in the first, second, and third passes.

[0046] When the thickness of the finished steel obtained by rolling is ≥1.4mm and less than 3.00mm, the upper surface purging water and lower surface purging water of the steel are turned on in both the first and second passes.

[0047] Both the upper and lower surface purging water are purged using a purging water device. The purging water device includes a water supply manifold and nozzles connected to the water supply manifold. The nozzles are used to purge the water supplied by the water supply manifold onto the upper or lower surface of the steel. Specifically, the spray direction of the nozzle used to purge the upper surface of the steel is at an angle of 15±5° to the horizontal plane, and the corresponding water supply manifold is at an angle of 45±5° to the horizontal plane. The spray direction of the nozzle used to purge the lower surface of the steel is at an angle of 10±5° to the horizontal plane, and the corresponding water supply manifold is at an angle of 80-100° to the horizontal plane.

[0048] The water pressure of the upper and lower surface purging water is 0.8 ± 0.2 MPa.

[0049] The present invention has the following beneficial effects:

[0050] In the finishing rolling process of the steel of the present invention, oxygen scale on the upper and lower surfaces is removed by blowing water on them, thereby improving the quality of the upper and lower surfaces of the steel and ensuring that the quality of the upper and lower surfaces of the steel can reach the FB grade. Furthermore, the upper and lower surfaces of the steel will not have an adverse effect on the appearance of subsequent painting or electroplating. Attached Figure Description

[0051] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0052] Figure 1This is a diagram of the lower surface of the steel used in Comparative Example 1 of the present invention.

[0053] Figure 2 This is a diagram of the lower surface of the steel used in Comparative Example 2 of the present invention;

[0054] Figure 3 This is a diagram of the lower surface of the steel used in Comparative Example 3 of the present invention.

[0055] Figure 4 This is a diagram of the lower surface of the steel used in Comparative Example 4 of the present invention;

[0056] Figure 5 This is a diagram showing the lower surface of the steel material in Embodiment 1 of the present invention;

[0057] Figure 6 The second illustration shows the lower surface of the steel material in Embodiment 1 of the present invention;

[0058] Figure 7 Figure 3 shows the lower surface of the steel material in Embodiment 1 of the present invention;

[0059] Figure 8 Figure 4 shows the lower surface of the steel material in Embodiment 1 of the present invention. Detailed Implementation

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0061] This invention provides a steel with an upper and lower surface quality of grade FB. The steel composition, by mass percentage, includes: C ≤ 0.0025% (e.g., 0.0025%, 0.0024%, 0.0023%, 0.0022%, etc.), Mn: 0.10-0.2% (e.g., 0.10%, 0.12%, 0.16%, 0.18%, 0.20%, etc.), S ≤ 0.012% (e.g., 0.012%, 0.011%, 0.010%, etc.), P ≤ 0.015% (e.g., 0.015%, 0.014%, 0.013%, 0.012%, etc.), and Si ≤ 0.030% (e.g., 0.030%, 0.028%, 0.02%). 7%, 0.025%, 0.023%, etc.), Al: 0.020-0.05% (e.g., 0.020%, 0.025%, 0.030%, 0.040%, 0.045%, 0.05%, etc.), Ti: 0.050-0.07% (e.g., 0.050%, 0.055%, 0.060%, 0.065%, 0.070%, etc.), B: 0.0002-0.0004% (e.g., 0.0002%, 0.00025%, 0.0003%, 0.00035%, 0.0004%, etc.), with the balance being Fe and unavoidable impurities, and Mn / S ≥ 12 (e.g., 12, 12.5, 12.8, 13, etc.). Among them, reducing the content of Mn and Si can reduce the formation of oxygen scale.

[0062] Methods for preparing steel with an upper and lower surface quality of FB grade include:

[0063] The process includes smelting molten iron, KR desulfurization, LD converter steelmaking, refining, continuous casting, cold or hot conveying, and rolling; wherein refining includes at least one of LF refining and RH refining; and rolling includes billet inspection, heating, rough rolling, finish rolling, laminar flow, and coiling.

[0064] The following will provide a detailed explanation of each process step.

[0065] In the process of smelting molten iron, the composition of the molten iron must meet the following requirements: S ≤ 0.030% (e.g., 0.030%, 0.028%, 0.025%, 0.020%, 0.015%, 0.012%, 0.011%, 0.010%, etc.), Cu ≤ 0.050% (e.g., 0.050%, 0.045%, 0.040%, 0.045%, 0.030%, 0.028%, 0.025%, 0.020%, 0.015%, 0.012%, 0.011%, 0.010%, etc.), Cr ≤ 0. 0.050% (e.g., 0.050%, 0.045%, 0.040%, 0.045%, 0.030%, 0.028%, 0.025%, 0.020%, 0.015%, 0.012%, 0.011%, 0.010%, etc.), Ni≤0.050% (e.g., 0.050%, 0.045%, 0.040%, 0.045%, 0.030%, 0.028%, 0.025%, 0.020%, 0.015%, 0.012%, 0.011%, 0.010%, etc.).

[0066] Furthermore, the molten iron needs to undergo KR deep desulfurization treatment, with S in the molten iron entering the furnace ≤ 0.0020% (e.g., 0.0020%, 0.0018%, 0.0017%, 0.0015%, etc.), and slag is removed until the bright metallic surface of the molten iron is ≥ 95% (e.g., 95%, 96%, 97%, etc.); the temperature of the molten iron entering the furnace is 1250-1380℃ (e.g., 1250℃, 1260℃, 1270℃, 1280℃, 1290℃, 1300℃, 1310℃, 1320℃, 1330℃, 1340℃, 1360℃, 1370℃, 1380℃, etc.).

[0067] In the LD converter steelmaking process, the bottom blowing is done with argon gas throughout until the steel is tapped. The converter should maintain the tapping port well to prevent steel spillage and ensure good slag blocking effect. Lime and fluorite slag washing are added during tapping. The amount of lime and fluorite added can be selected according to the needs. For example, lime can be added at 300±10 kg / t and fluorite can be added at 100±10 kg / t, etc., without specific limitations.

[0068] The final control of the steel grade meets the following requirements: C content is 0.04-0.06% (e.g., 0.04%, 0.05%, 0.06%), P content is ≤0.010% (e.g., 0.010%, 0.008%, 0.007%), S content is ≤0.0060% (e.g., 0.006%, 0.005%, 0.004%), and N content is ≤18ppm (e.g., 18ppm, 17ppm). During tapping, aluminum blocks, ferromanganese, and Ti are used for deoxidation and alloying.

[0069] The amount of aluminum blocks and Ti added can be selected as needed. For example, the amount of aluminum blocks added can be 2.5 kg / t, and the amount of Ti added can be 1.1 kg / t.

[0070] Furthermore, the aluminum block contains ≥99% aluminum, the weight ratio of Mn to C in MnFe can be 78:1.5, the reference recovery rate of Mn can be 90%, and the reference recovery rate of C can be 80%; the addition of Ti can be the addition of Ti-containing alloying material, wherein the Ti content is ≥70%, and the reference recovery rate of Ti can be 85%.

[0071] The refining steps in this invention may include LF refining and RH refining; of course, in a preferred embodiment, refining includes CAS refining, LF refining, and RH refining. In other embodiments, it may include only LF refining or RH refining.

[0072] Furthermore, as shown in Table 1, the tapping temperature of the start-up furnace and the continuous casting furnace in the LD converter steelmaking step is ≥1640℃ (e.g., 1640℃, 1650℃, 1660℃, etc.), and the arrival temperature of the start-up furnace and the continuous casting furnace in the CAS refining step is ≥1590℃ (e.g., 1590℃, 1600℃, 1610℃, etc.).

[0073] In the LF refining process, the outlet temperature of the casting furnace is 1635-1645℃, for example: 1635℃, 1638℃, 1640℃, 1642℃, 1645℃, etc.; the outlet temperature of the continuous casting furnace is 1625-1635℃, for example: 1625℃, 1628℃, 1630℃, 1632℃, 1635℃, etc.

[0074] In the RH refining process, the circulating gas flow rate during oxygen blowing is controlled to be 90-110 Nm. 3 / h, for example: 90Nm 3 / h、95Nm 3 / h, 100Nm 3 / h, 105Nm 3 / h、110Nm 3 / h, etc.; the gas flow rate during cyclic decarbonization is 240-280 Nm³. 3 / h, for example: 240Nm 3 / h, 250Nm 3 / h, 260Nm 3 / h, 270Nm 3 / h, 280Nm 3The time is 15-20 minutes, e.g., 15 min, 16 min, 17 min, 18 min, 19 min, 20 min, etc.; at the end of decarburization, the oxygen content is controlled at 450-600 ppm (e.g., 450 ppm, 480 ppm, 500 ppm, 530 ppm, 550 ppm, 580 ppm, 600 ppm, etc.) and the steel temperature is greater than 1565℃ (e.g., 1565℃, 1570℃, 1575℃, 1580℃, etc.); the circulating gas flow rate (nitrogen) during alloying is 200-240 Nm. 3 / h (e.g., 200Nm) 3 / h、210Nm 3 / h, 220Nm 3 / h、230Nm 3 / h, 240Nm 3 The circulating gas flow rate during pure circulation after alloying is 120-150 Nm³ / h, etc. 3 / h (e.g., 120Nm) 3 / h, 125Nm 3 / h, 130Nm 3 / h, 135Nm 3 / h, 140Nm 3 / h, 145Nm 3 / h, 150Nm 3 / h etc). The N content before RH refining is ≤40% (e.g., 40%, 35%, 30%, 25%, 20%, etc.).

[0075] The inlet temperature of the casting furnace in the RH refining step is 1620-1635℃ (e.g., 1620℃, 1625℃, 1628℃, 1630℃, 1632℃, 1635℃, etc.), and the outlet temperature of the casting furnace in the RH refining step is 1595-1605℃ (e.g., 1595℃, 1598℃, 1600℃, 1603℃, 1605℃, etc.). The inlet temperature of the continuous casting furnace in the RH refining step is 1610-1625℃ (e.g., 1610℃, 1615℃, 1618℃, 1620℃, 1625℃, etc.), and the outlet temperature of the continuous casting furnace in the RH refining step is 1590-1600℃ (e.g., 1590℃, 1595℃, 1598℃, 1600℃, etc.).

[0076] The temperature of the ladle from the start-up furnace and the continuous casting furnace to the continuous casting platform is T. L +60±5, the temperature of the tundish in the continuous casting furnace is T L +30±10; where T L This refers to the liquidus temperature of molten steel. It should be understood that the liquidus temperature will change to some extent as the composition of the steel changes. (T) LIt can be approximately 1532±2℃.

[0077] Table 1

[0078]

[0079] In the continuous casting process, the argon gas control mechanism in the tundish is shown in Table 2. The pressure of the stopper rod argon gas is 0.3-0.4 MPa (e.g., 0.3 MPa, 0.32 MPa, 0.35 MPa, 0.38 MPa, 0.40 MPa, etc.), and the flow rate of the stopper rod argon gas is 4.5-5.5 L / min (e.g., 4.5 L / min, 4.8 L / min, 5.0 L / min, 5.2 L / min, 5.5 L / min, etc.). The pressure of the argon gas at the top nozzle is 0.3-0.4 MPa, e.g., 0.3 MPa, 0.32 MPa, 0.35 MPa, 0.38 MPa, 0.40 MPa, etc. The pressure of argon gas between plates is 0.3-0.4 MPa (e.g., 0.3 MPa, 0.32 MPa, 0.35 MPa, 0.38 MPa, 0.40 MPa), and the flow rate of argon gas between plates is 2.5-3.5 L / min (e.g., 2.5 L / min, 2.8 L / min, 3.0 L / min, 3.2 L / min, 3.5 L / min).

[0080] Table 2

[0081]

[0082] Furthermore, the control of temperature and casting speed during continuous casting can be found in Table 3.

[0083] Table 3

[0084]

[0085] Furthermore, in the continuous casting process, the nozzle insertion depth is 130±5mm, and the protective slag is ultra-low carbon steel protective slag.

[0086] In the rolling process, the heating process, the temperature of the final rolling of rough rolling, the temperature of the final rolling of finish rolling, and the coiling temperature can all be referred to Table 4. Among them, by focusing the heating (i.e. heating at the upper limit of the furnace gas temperature) and keeping the furnace time as low as possible, i.e. controlling the furnace time to be relatively short, the amount of primary furnace oxygen scale in the furnace can be reduced.

[0087] Table 4

[0088]

[0089]

[0090] According to Table 4, in the heating process, the temperature of the first heating section is 1100-1200℃, the temperature of the second heating section is 1190-1260℃, the temperature of the soaking section is 1190-1240℃, and the furnace exit temperature is 1195-1205℃. Among these, the temperature difference between the upper and lower surfaces of the billet is 10℃ in the second heating section and the soaking section.

[0091] In the rough rolling step, the thickness of the incoming material can be controlled at 33-36mm, 38mm, and 43mm. Different thicknesses of finished products can be rolled to correspond to different incoming material thicknesses. For example, for a cast billet with an incoming material thickness of 33-36mm, the rolled finished product thickness can be 1.4-3.0mm; for a cast billet with an incoming material thickness of 38mm, the rolled finished product thickness can be 3.01-4.00mm; and for a cast billet with an incoming material thickness of 33-36mm, the rolled finished product thickness can be 4.01-6.50mm.

[0092] Optionally, the width of the rough-rolled strip = the width of the final-rolled strip + (8~12mm).

[0093] Optionally, the head and tail can be cut off during rough rolling, and the length of the head and tail cut off can be ≥100mm.

[0094] Optionally, the roughing process includes inlet dephosphorization and outlet dephosphorization, with the dephosphorization water pressure of either the inlet or outlet dephosphorization being ≥20 MPa; and the dephosphorization process is carried out in no less than 4 passes.

[0095] The finishing rolling process includes seven rolling passes. Referring to Table 5, different reduction rates can be set for each pass depending on the thickness of the final rolled product (the thickness of the rolled steel).

[0096] Table 5

[0097]

[0098] Optionally, the mill limiting parameters for finishing rolling are shown in Table 6.

[0099] Table 6

[0100] 0-0.25 9.5 20 15

[0101] In the finishing rolling process, the tension settings between stands for each pass are shown in Table 7.

[0102] Table 7

[0103]

[0104] In this invention, to improve the surface quality of the finished steel, the finishing rolling step includes purging water onto the upper and lower surfaces of the rolled steel. By purging water onto the upper and lower surfaces, oxygen scale is removed, improving the quality of the upper and lower surfaces of the steel and ensuring that the quality of the upper and lower surfaces of the steel reaches the FB grade. Furthermore, the upper and lower surfaces of the steel will not adversely affect the appearance of subsequent painting or electroplating.

[0105] Furthermore, the step of purging water onto the upper and lower surfaces of the rolled steel specifically includes: when the thickness of the finished steel obtained by rolling is ≥3.00mm, purging water onto the upper and lower surfaces of the steel is turned on in the first, second, and third passes; when the thickness of the finished steel obtained by rolling is ≥2mm and less than 3.00mm, purging water onto the upper and lower surfaces of the steel is turned on in the first and second passes.

[0106] Both the upper and lower surface purging water are purged using a purging water device. The purging water device includes a water supply manifold and nozzles connected to the water supply manifold. The nozzles are used to purge the water supplied by the water supply manifold onto the upper or lower surface of the steel. Specifically, the spray direction of the nozzle used to purge the upper surface of the steel is at an angle of 15±5° to the horizontal plane, and the water supply direction of the water supply manifold corresponding to the upper surface purging water nozzle is at an angle of 45±5° to the horizontal plane. The spray direction of the nozzle used to purge the lower surface of the steel is at an angle of 10±5° to the horizontal plane, and the water supply direction of the water supply manifold corresponding to the lower surface purging water nozzle is at an angle of 80-100° to the horizontal plane. This setup allows water to be blown more stably and reliably onto the upper and lower surfaces of the steel, removing secondary oxide scale and improving the quality of both surfaces. Specifically, the nozzles used to blow water onto the lower surface of the steel spray water directly onto the bent steel strip after the rolling mill stand is looped. Because the strip is bent on the loop, the secondary oxide scale becomes softer due to deformation. Under the pressure of the downward blowing water, the secondary oxide scale falls off, thus reliably improving the surface quality of the lower surface.

[0107] The water pressure of the upper and lower surface purging water is 0.8 ± 0.2 MPa.

[0108] It should be noted that the number and spray specifications of the nozzles used for purging water in each pass can be set as needed. For example, each pass can be equipped with 7 nozzles spaced 200 mm apart and spraying water with a specification of 150 mm, used to purge water onto the upper surface; and 8 nozzles spaced 150 mm apart and spraying water with a specification of 250 mm, used to purge water onto the lower surface.

[0109] Optionally, the nozzle for blowing water onto the lower surface of the steel is positioned between the looper and the guide of the frame.

[0110] The present invention employs laminar flow cooling, and the cooling rate can be 100℃ / s, without any specific limitation.

[0111] In this invention, the winding temperature is 710±20℃, and the winding pressure can be 130±10 N / mm. 2 For example: 120N / mm 2 125N / mm 2 130N / mm 2 135N / mm 2 140N / mm 2 wait.

[0112] The present invention will be further described in detail below with reference to the embodiments.

[0113] Example 1

[0114] The composition of steel with an FB surface finish, expressed as a percentage by mass, includes:

[0115] C: 0.0025%, Mn: 0.10%, S: 0.012%, P: 0.015%, Si: 0.030%, Als: 0.020%, Ti: 0.050%, B: 0.0002%, with the balance being Fe and unavoidable impurities, and Mn / S = 12.

[0116] The preparation methods include: smelting molten iron, KR desulfurization, LD converter steelmaking, refining, continuous casting, cold or hot delivery, and rolling.

[0117] The composition of the molten iron is as follows: S: 0.030%, Cu: 0.050%, Cr: 0.050%, Ni: 0.050%. The molten iron undergoes KR deep desulfurization treatment, and the [S] content of the molten iron entering the furnace is ≤0.0020%. Slag is removed until the metallic brightness of the molten iron reaches 95%. The temperature of the molten iron entering the furnace is 1250℃.

[0118] Argon gas is used for bottom blowing in the converter until the steel is tapped.

[0119] Maintain the converter taphole properly to prevent slag spillage and ensure good slag blocking effect; add 300 kg of lime and 100 kg of fluorite for slag washing after tapping. The final steel grade control should meet the following requirements: C content 0.04%, P content 0.010%, S content 0.0060%, N content ≤18 ppm; during tapping, use 2.5 kg / t aluminum blocks, 0.22 kg / t ferromanganese, and 1.1 kg / t Ti for deoxidation and alloying.

[0120] LF refining, first casting furnace outlet temperature: 1645℃, continuous casting furnace outlet temperature: 1635℃.

[0121] RH refining, the circulating gas flow rate during oxygen blowing is 90 Nm³. 3 / h, the gas flow rate during cyclic decarbonization is 240 Nm 3 / h, time controlled within 15min; oxygen content controlled at 450ppm and molten steel temperature at 1570℃ at the end of decarburization; circulating gas flow rate of 200m³ / h during alloying. 3 / h, after alloying, the circulating gas flow rate during pure circulation is 120 Nm³. 3 / h, the circulating gas is nitrogen; the N content before leaving the RH refinery is 40%.

[0122] Temperature control:

[0123]

[0124] Continuous casting, argon gas control in the tundish:

[0125]

[0126] Temperature and pulling speed:

[0127] 230×1250 Pulling speed, m / min 1.0 1.1 1.15

[0128] Sprue insertion depth and protective slag:

[0129] 130 Ultra-low carbon steel protective slag

[0130] Rolling, slab inspection → heating furnace firing → rough rolling → finish rolling → laminar flow → coiling.

[0131] Heating furnace for steelmaking:

[0132]

[0133]

[0134] The incoming material thickness for rough rolling is 33mm; during rough rolling, the head and tail are cut off, and the length of each cut-off head and tail is 100mm. Rough rolling includes inlet dephosphorization and outlet dephosphorization, and the dephosphorization water pressure for inlet and outlet dephosphorization is 20MPa; the dephosphorization process is carried out in 4 passes.

[0135] Finishing rolling, reduction distribution:

[0136]

[0137] Limitations of finishing mills:

[0138] 0.25 9.5 20 15

[0139] Tension settings between racks for each pass:

[0140]

[0141] The upper and lower surfaces of the rolled steel are purged with water. The upper and lower surfaces of the steel are purged with water in the first, second and third passes.

[0142] The nozzles used to purge water onto the upper surface of the steel have a spray direction at a 15° angle to the horizontal plane, and the water supply manifold corresponding to the nozzles for purging water onto the upper surface has a water delivery direction at a 45° angle to the horizontal plane; the nozzles used to purge water onto the lower surface of the steel have a spray direction at a 10° angle to the horizontal plane, and the water supply manifold corresponding to the nozzles for purging water onto the lower surface has a water delivery direction at an 80° angle to the horizontal plane.

[0143] The water pressure for both the upper and lower surface purge water is 0.8 MPa.

[0144] Example 2

[0145] The composition of steel with an FB surface finish, expressed as a percentage by mass, includes:

[0146] C: 0.0020%, Mn: 0.20%, S: 0.010%, P: 0.013%, Si: 0.025%, Als: 0.050%, Ti: 0.070%, B: 0.0004%, with the balance being Fe and unavoidable impurities, and Mn / S = 13.

[0147] The preparation methods include: smelting molten iron, KR desulfurization, LD converter steelmaking, refining, continuous casting, cold or hot delivery, and rolling.

[0148] The composition of the molten iron is as follows: S: 0.025%, Cu: 0.045%, Cr: 0.040%, Ni: 0.040%. The molten iron undergoes KR deep desulfurization treatment, and the [S] content of the molten iron entering the furnace is ≤0.0015%. Slag is removed until the metallic brightness of the molten iron reaches 97%. The temperature of the molten iron entering the furnace is 1380℃.

[0149] Argon gas is used for bottom blowing in the converter until the steel is tapped.

[0150] Maintain the converter taphole properly to prevent slag spillage and ensure good slag blocking effect; add 300 kg of lime and 100 kg of fluorite for slag washing after tapping. The final steel grade control should meet the following requirements: C content 0.05%, P content 0.008%, S content 0.0050%, N content ≤15 ppm; during tapping, use 2.0 kg / t aluminum blocks, 1.04 kg / t ferromanganese, and 1.0 kg / t Ti for deoxidation and alloying.

[0151] LF refining, first casting furnace outlet temperature: 1635℃, continuous casting furnace outlet temperature: 1625℃.

[0152] RH refining, the circulating gas flow rate during oxygen blowing is 110 Nm³.3 / h, the gas flow rate during cyclic decarbonization is 280 Nm 3 / h, time controlled within 20min; oxygen content controlled at 600ppm and molten steel temperature at 1576℃ after decarburization; circulating gas flow rate of 240m³ / h during alloying. 3 / h, after alloying, the circulating gas flow rate in pure circulation is 150 Nm³. 3 / h, the circulating gas is nitrogen; the N content before leaving the RH refinery is 25%.

[0153] Temperature control:

[0154]

[0155] Continuous casting, argon gas control in the tundish:

[0156]

[0157] Temperature and pulling speed:

[0158]

[0159]

[0160] Sprue insertion depth and protective slag:

[0161] 135 Ultra-low carbon steel protective slag

[0162] Rolling, slab inspection → heating furnace firing → rough rolling → finish rolling → laminar flow → coiling.

[0163] Heating furnace for steelmaking:

[0164]

[0165] The incoming material thickness for rough rolling is 38mm; during rough rolling, the head and tail are cut off, and the length of each cut-off head and tail is 100mm. Rough rolling includes inlet dephosphorization and outlet dephosphorization, and the dephosphorization water pressure for inlet and outlet dephosphorization is 30Mpa; the dephosphorization process is carried out in 5 passes.

[0166] Finishing rolling, reduction distribution:

[0167]

[0168] Limitations of finishing mills:

[0169] 0.15 9.5 20 15

[0170] Tension settings between racks for each pass:

[0171]

[0172] The upper and lower surfaces of the rolled steel are purged with water. The upper and lower surfaces of the steel are purged with water in the first, second and third passes.

[0173] The nozzles used to purge water onto the upper surface of the steel have a spray direction at a 20° angle to the horizontal plane, and the water supply manifold corresponding to the nozzles for purging water onto the upper surface has a water delivery direction at a 40° angle to the horizontal plane; the nozzles used to purge water onto the lower surface of the steel have a spray direction at a 15° angle to the horizontal plane, and the water supply manifold corresponding to the nozzles for purging water onto the lower surface has a water delivery direction at a 100° angle to the horizontal plane.

[0174] The water pressure for both the upper and lower surface purge water is 1.0 MPa.

[0175] Example 3

[0176] The composition of steel with an FB surface finish, expressed as a percentage by mass, includes:

[0177] C: 0.0015%, Mn: 0.15%, S: 0.011%, P: 0.011%, Si: 0.020%, Als: 0.030%, Ti: 0.060%, B: 0.0003%, with the balance being Fe and unavoidable impurities, and Mn / S = 15.

[0178] The preparation methods include: smelting molten iron, KR desulfurization, LD converter steelmaking, refining, continuous casting, cold or hot delivery, and rolling.

[0179] The composition of the molten iron is as follows: S: 0.030%, Cu: 0.050%, Cr: 0.050%, Ni: 0.050%. The molten iron undergoes KR deep desulfurization treatment, and the [S] content of the molten iron entering the furnace is ≤0.0020%. Slag is removed until the metallic brightness of the molten iron reaches 95%. The temperature of the molten iron entering the furnace is 1300℃.

[0180] Argon gas is used for bottom blowing in the converter until the steel is tapped.

[0181] Maintain the converter taphole properly to prevent slag spillage and ensure good slag blocking effect; add 300 kg of lime and 100 kg of fluorite for slag washing after tapping. The final steel grade control should meet the following requirements: C content 0.05%, P content 0.010%, S content 0.0050%, N content ≤18 ppm; during tapping, use 2.5 kg / t aluminum blocks, 0.52 kg / t ferromanganese, and 1.1 kg / t Ti for deoxidation and alloying.

[0182] LF refining, first casting furnace outlet temperature: 1640℃, continuous casting furnace outlet temperature: 1630℃.

[0183] RH refining, the circulating gas flow rate during oxygen blowing is 100 Nm³. 3 / h, the gas flow rate during cyclic decarbonization is 260 Nm³. 3 / h, time controlled within 18min; oxygen content controlled at 500ppm and molten steel temperature at 1572℃ at the end of decarburization; circulating gas flow rate of 220m³ / h during alloying. 3 / h, after alloying, the circulating gas flow rate in pure circulation is 130 Nm³. 3 / h, the circulating gas is nitrogen; the N content before leaving the RH refinery is 30%.

[0184] Temperature control:

[0185]

[0186] Continuous casting, argon gas control in the tundish:

[0187]

[0188] Temperature and pulling speed:

[0189] 230×900 Pulling speed, m / min 1.2 1.3 1.35

[0190] Sprue insertion depth and protective slag:

[0191] 125 Ultra-low carbon steel protective slag

[0192] Rolling, slab inspection → heating furnace firing → rough rolling → finish rolling → laminar flow → coiling.

[0193] Heating furnace for steelmaking:

[0194]

[0195] The incoming material thickness for rough rolling is 43mm; during rough rolling, the head and tail are cut off, and the length of each cut-off head and tail is 100mm. Rough rolling includes inlet dephosphorization and outlet dephosphorization, and the dephosphorization water pressure for inlet and outlet dephosphorization is 25Mpa; the dephosphorization process is carried out in 6 passes.

[0196] Finishing rolling, reduction distribution:

[0197]

[0198] Limitations of finishing mills:

[0199] 0.20 9.5 20 15

[0200] Tension settings between racks for each pass:

[0201]

[0202] The upper and lower surfaces of the rolled steel are purged with water. The upper and lower surfaces of the steel are purged with water in the first, second and third passes.

[0203] The nozzles used to purge water onto the upper surface of the steel have a spray direction at a 10° angle to the horizontal plane, and the water supply manifold of the nozzles for purging water onto the upper surface has a water delivery direction at a 50° angle to the horizontal plane; the nozzles used to purge water onto the lower surface of the steel have a spray direction at a 5° angle to the horizontal plane, and the water supply manifold of the nozzles for purging water onto the lower surface has a water delivery direction at a 90° angle to the horizontal plane.

[0204] The water pressure for both the upper and lower surface purge water is 0.6 MPa.

[0205] Comparative Example 1

[0206] The difference between Comparative Example 1 and Example 1 is that the Mn content is 0.3% and the Si content is 0.45%; the process parameters are the same as in Example 1.

[0207] Comparative Example 2

[0208] The difference between Comparative Example 2 and Example 1 is that the pressure of the water used to purge the upper and lower surfaces is 0.3 MPa, and the process parameters are the same as those in Example 1.

[0209] Comparative Example 3

[0210] The difference between Comparative Example 3 and Example 1 is that the spray direction of the nozzle used to purge water onto the upper surface of the steel is at a 35° angle to the horizontal plane, and the water supply direction of the water supply manifold corresponding to the nozzle for purging water onto the upper surface is at a 45° angle to the horizontal plane; the spray direction of the nozzle used to purge water onto the lower surface of the steel is at a 25° angle to the horizontal plane, and the water supply direction of the water supply manifold corresponding to the nozzle for purging water onto the lower surface is at an 80° angle to the horizontal plane; the process parameters are the same as in Example 1.

[0211] Comparative Example 4

[0212] The difference between Comparative Example 4 and Example 1 is that the spray direction of the nozzle used to purge water onto the upper surface of the steel is at a 35° angle to the horizontal plane, and the water supply direction of the water supply manifold corresponding to the upper surface purge water nozzle is at a 55° angle to the horizontal plane; the spray direction of the nozzle used to purge water onto the lower surface of the steel is at a 25° angle to the horizontal plane, and the water supply direction of the water supply manifold corresponding to the lower surface purge water nozzle is at a 45° angle to the horizontal plane; the pressure of the purge water on both the upper and lower surfaces is 0.3 MPa; and the process parameters are the same as in Example 1.

[0213] according to Figure 1-8 It is understood that Embodiment 1 of the present invention can effectively remove the oxide scale from the lower surface of steel, while Comparative Examples 1-4 cannot remove the oxide scale from the lower surface of steel. Moreover, the oxide scale on the lower surface can only be effectively reduced when the composition and the water pressure and angle of the purging water on both the upper and lower surfaces meet the requirements of this disclosure. Figure 1As shown on the lower surface of the steel in Comparative Example 1, although both the upper and lower water-sweeping processes were carried out in accordance with this disclosure, the composition did not meet the requirements of this disclosure, and it still had a certain amount of oxygen scale. Figure 2 As shown on the lower surface of the steel in Comparative Example 2, the composition meets the requirements of this disclosure, but the pressure of the water used for sweeping the upper and lower surfaces is significantly lower than that of this disclosure, and the lower surface of the steel has a large amount of oxygen scale. Figure 3 As shown on the lower surface of the steel in Comparative Example 3, the composition meets the requirements of this disclosure, and the pressure of the upper and lower water sweeping also meets the requirements of this disclosure. However, the angle of the upper and lower water sweeping does not meet the requirements of this disclosure, and the lower surface of the steel has a large amount of oxygen scale. Figure 4 As shown in the lower surface of the steel in Comparative Example 4, the pressure of the water sweeping from top to bottom is significantly lower than that of this disclosure, and the angle of the water sweeping from top to bottom does not meet the requirements of this disclosure. The lower surface of the steel has a large amount of oxygen scale.

[0214] In summary, the steel of the present invention with an FB grade surface quality is improved by blowing water onto the upper and lower surfaces to remove oxygen scale, thereby ensuring that the quality of the upper and lower surfaces of the steel can reach the FB grade. Furthermore, the upper and lower surfaces of the steel will not adversely affect the appearance of subsequent painting or electroplating.

[0215] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A type of steel with an upper and lower surface quality of grade FB, characterized in that, The composition of the steel with an FB surface quality, expressed as a percentage by mass, includes: C≤0.0025%, Mn: 0.10-0.2%, S≤0.012%, P≤0.015%, Si≤0.030%, Als: 0.020-0.05%, Ti: 0.050-0.07%, B: 0.0002-0.0004%, with the balance being Fe and unavoidable impurities, and Mn / S≥12; The method for preparing the steel with an upper and lower surface quality of grade FB includes: The processes include: smelting molten iron, KR desulfurization, LD converter steelmaking, refining, continuous casting, cold or hot conveying, and rolling; among which... The refining includes at least one of LF refining and RH refining; The composition of the molten iron satisfies: S≤0.030%, Cu≤0.050%, Cr≤0.050%, Ni≤0.050%; In the KR desulfurization step, the S in the molten iron entering the furnace is ≤0.0020%, and the slag is removed until the bright metal surface of the molten iron is ≥95%, and the temperature of the molten iron entering the furnace is 1250-1380℃. In the LD converter steelmaking process, the bottom blowing is done with argon gas throughout until the end of tapping, and lime and fluorite slag are added for washing during tapping; the steel grade final control meets the following requirements: C content is 0.04-0.06%, P content is ≤0.010%, S content is ≤0.0060%, and N content is ≤18ppm; aluminum blocks, ferromanganese, and Ti are used for deoxidation and alloying during tapping; In the continuous casting step, the pressure of the stopper rod argon gas is 0.3-0.4 MPa, and the flow rate of the stopper rod argon gas is 4.5-5.5 L / min; the pressure of the upper water inlet argon gas is 0.3-0.4 MPa, and the flow rate of the upper water inlet argon gas is 4.5-5.5 L / min; the pressure of the inter-plate argon gas is 0.3-0.4 MPa, and the flow rate of the inter-plate argon gas is 2.5-3.5 L / min. The rolling process includes: heating, roughing, finishing, laminar flow, and coiling; wherein, the finishing step further includes blowing water onto the upper and lower surfaces of the rolled steel. Both the upper and lower surfaces are purged using a purging water device, which includes a water supply manifold and nozzles connected to the water supply manifold. The nozzles are used to purge the water supplied by the water supply manifold onto the upper or lower surface of the steel. Specifically, the nozzles used to purge the water onto the upper surface of the steel have a spray direction at an angle of 15±5° to the horizontal plane, and the water supply direction of the water supply manifold has a corresponding angle of 45±5° to the horizontal plane. The nozzles used to purge the water onto the lower surface of the steel have a spray direction at an angle of 10±5° to the horizontal plane, and the water supply direction of the water supply manifold has a corresponding angle of 80-100° to the horizontal plane. In the heating step, the temperature of the two heating sections on the upper surface of the billet is 1200-1260℃, the temperature of the two heating sections on the lower surface is 1190-1250℃, and the temperature difference between the upper and lower surfaces of the billet is 10℃. The temperature of the soaking zone on the upper surface of the billet is 1200-1240℃, the temperature of the soaking zone on the lower surface is 1190-1230℃, and the temperature difference between the upper and lower surfaces of the billet is 10℃. The step of purging water on the upper and lower surfaces of the rolled steel specifically includes: when the thickness of the finished steel obtained by rolling is ≥3.00mm, purging water on the upper and lower surfaces of the steel is turned on in the first, second and third passes. When the thickness of the finished steel obtained by rolling is ≥1.4mm and less than 3.00mm, the upper surface purging water and lower surface purging water of the steel are turned on in both the first and second passes. The water pressure of the upper surface purge water and the lower surface purge water is 0.8 ± 0.2 MPa.

2. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, In the RH refining step, the circulating gas flow rate during oxygen blowing is controlled to be 90-110 Nm. 3 / h; the gas flow rate during cyclic decarbonization is 240-280 Nm³. 3 / h, time is 15-20min; at the end of decarburization, the oxygen content is controlled at 450-600ppm and the steel temperature is greater than 1565℃; the circulating gas flow rate during alloying is 200-240Nm 3 / h, the circulating gas flow rate during pure circulation after alloying is 120-150 Nm³. 3 / h, the circulating gas is nitrogen; the N content before RH refining and leaving the station is ≤40%.

3. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, The refining process includes CAS refining, LF refining, and RH refining. The tapping temperature of the casting furnace in the LD converter steelmaking step is ≥1640℃; the arrival temperature of the casting furnace in the CAS refining step is ≥1590℃; the outlet temperature of the casting furnace in the LF refining step is 1635-1645℃; the inlet temperature of the casting furnace in the RH refining step is 1620-1635℃; the outlet temperature of the casting furnace in the RH refining step is 1595-1605℃; and the temperature from the ladle to the continuous casting platform is T. L +60±5, the temperature of the tundish in the casting furnace is T. L +30±10; The tapping temperature of the continuous casting furnace in the LD converter steelmaking step is ≥1640℃; the arrival temperature of the continuous casting furnace in the CAS refining step is ≥1590℃; the outlet temperature of the continuous casting furnace in the LF refining step is 1625-1635℃; the inlet temperature of the continuous casting furnace in the RH refining step is 1610-1625℃; the outlet temperature of the continuous casting furnace in the RH refining step is 1590-1600℃; and the temperature from the ladle to the continuous casting platform is T. L +60±5, the temperature of the tundish in the continuous casting furnace is T L +30±10; Wherein, the T L This is the liquidus temperature of molten steel.

4. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, In the continuous casting step, When the cross-sectional dimensions of the cast billet are greater than or equal to 230mm × 1250mm and less than or equal to 230mm × 1400mm, the casting speed is controlled at 1.0m / min when the temperature is greater than or equal to 1565℃ and less than or equal to 1575℃; 1.1m / min when the temperature is greater than or equal to 1545℃ and less than 1565℃; and 1.15m / min when the temperature is less than 1545℃. When the cross-sectional dimensions of the cast billet are greater than or equal to 230mm × 1100mm and less than 230mm × 1250mm, the casting speed is controlled at 1.1m / min when the temperature is greater than or equal to 1565℃ and less than or equal to 1575℃; 1.2m / min when the temperature is greater than or equal to 1545℃ and less than 1565℃; and 1.25m / min when the temperature is less than 1545℃. When the cross-sectional dimensions of the cast billet are greater than or equal to 230mm × 900mm and less than 230mm × 1100mm, the casting speed is controlled at 1.2m / min when the temperature is greater than or equal to 1565℃ and less than or equal to 1575℃; 1.3m / min when the temperature is greater than or equal to 1545℃ and less than 1565℃; and 1.35m / min when the temperature is less than 1545℃.

5. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, In the continuous casting step, the nozzle insertion depth is 130±5mm, and the protective slag is ultra-low carbon steel protective slag.

6. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, When the cold feeding step is used before rolling and the thickness of the finished steel obtained by rolling is 1.4-3.2mm, the time of the billet in the furnace during the heating step is ≥150min. When the cold feeding step is used before rolling and the thickness of the finished steel obtained by rolling is greater than 3.2mm, the time of the billet in the furnace during the heating step is ≥160min. When the hot conveying step is used before rolling, and the thickness of the finished steel obtained by rolling is 1.4-3.2mm, the time of the billet in the furnace during the heating step is ≥140min. When the hot conveying step is used before rolling, and the thickness of the finished steel obtained by rolling is greater than 3.2 mm, the time of the billet in the furnace during the heating step is ≥150 min.

7. The steel with an upper and lower surface quality of FB grade according to claim 6, characterized in that, In the heating step, the temperature of the first heating section is 1100-1200℃; the furnace exit temperature is 1195-1205℃; wherein, in the second heating section and the soaking section, the temperature difference between the upper and lower surfaces of the billet is 10℃. In the rough rolling step, the final rolling temperature is 1030±30℃; In the finishing rolling step, when the thickness of the finished steel obtained by rolling is 1.4-3.2mm, the final rolling temperature is 930±20℃; when the thickness of the finished steel obtained by rolling is greater than 3.2mm, the final rolling temperature is 920±20℃. In the winding step, the temperature is 710±20℃ and the pressure is 130±10N / mm. 2 .

8. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, The roughing process includes inlet dephosphorization and outlet dephosphorization, wherein the dephosphorization water pressure of either the inlet dephosphorization or the outlet dephosphorization is ≥20 MPa.

9. The steel with an upper and lower surface quality of FB grade according to claim 1, characterized in that, The finishing rolling process includes seven rolling passes. When the thickness of the finished steel obtained by rolling is 2-3.0 mm, the reduction rate of the first pass is 29%, the reduction rate of the second pass is 31%, the reduction rate of the third pass is 26%, the reduction rate of the fourth pass is 23%, the reduction rate of the fifth pass is 21%, the reduction rate of the sixth pass is 17%, and the reduction rate of the seventh pass is 13%. When the thickness of the finished steel obtained by rolling is 3.01-4.0mm, the reduction rate of the first pass is 28%, the reduction rate of the second pass is 30%, the reduction rate of the third pass is 26%, the reduction rate of the fourth pass is 23%, the reduction rate of the fifth pass is 21%, the reduction rate of the sixth pass is 17%, and the reduction rate of the seventh pass is 13%. When the thickness of the finished steel obtained by rolling is 4.01-6.5mm, the reduction rate of the first pass is 25%, the reduction rate of the second pass is 27%, the reduction rate of the third pass is 25%, the reduction rate of the fourth pass is 23%, the reduction rate of the fifth pass is 21%, the reduction rate of the sixth pass is 17%, and the reduction rate of the seventh pass is 15%. In the finishing rolling step, the interstand tension L1 between the first and second passes is 5 N / mm. 2 The inter-rack tension L2 between the second and third passes is 7 N / mm. 2 The inter-rack tension L3 between the third and fourth passes is 9 N / mm. 2 The inter-rack tension L4 between the fourth and fifth passes is 11 N / mm. 2 When the thickness of the finished steel obtained by rolling is 2-3.0 mm, the interstand tension L5 between the fifth and sixth passes is 14 N / mm. 2 When the thickness of the finished steel obtained by rolling is 3.01-6.5mm, the interstand tension L5 between the fifth and sixth passes is 13N / mm. 2 When the thickness of the finished steel obtained by rolling is 2-3.0 mm, the interstand tension L6 between the sixth and seventh passes is 16 N / mm. 2 When the thickness of the finished steel obtained by rolling is 3.01-6.5 mm, the interstand tension L6 between the sixth and seventh passes is 15 N / mm. 2 .

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