A method for reducing surface stripes of high-strength hot-rolled pickled steel strip

Abstract

The present invention relates to a method for reducing the surface stripes of high-strength hot-rolled pickled strip steel, comprising the following aspects: the temperature of the slab in the preheating section of the heating furnace is 700-1000°C, and the time is controlled within 30-40 minutes; the temperature of the first heating section is 1100-1170°C, and the time is controlled within 50-60 minutes; the temperature of the second heating section is 1180-1250°C, and the time is controlled within 60-70 minutes; the temperature of the soaking section is 1150-1230°C, and the time is controlled within ≥35 minutes; the total furnace time is controlled within 180-230 minutes; the furnace discharge rhythm is controlled according to the thickness of the hot-rolled finished product; and hot rolling lubricating oil is sprayed onto the surfaces of the support roll and the working roll at the same time after the strip head bites the steel. The advantages are: avoiding cracks in the slab, improving the quality of the slab, and significantly improving the stripe defects of the hot-rolled pickled plate.

Description

Technical Field

[0001] The invention belongs to the field of hot-rolled pickled steel plate production, and in particular relates to a method for reducing surface stripes of high-strength hot-rolled pickled steel strip. Background Art

[0002] Hot-rolled pickled steel is an intermediate product between hot-rolled steel and cold-rolled steel. Using hot-rolled pickled steel to replace the corresponding cold-rolled steel can save costs for enterprises. Compared with ordinary hot-rolled steel, hot-rolled pickled steel removes the surface oxide scale, improves the surface quality of steel, and is convenient for welding, oiling and painting.

[0003] Among the many factors that affect the surface quality of strip steel, the effect of high-pressure water descaling is particularly important. However, the descaling effect is affected by aspects such as billet quality, heating quality, descaling equipment, descaling process and steel rolling process. Once quality problems caused by poor descaling occur, it is difficult to analyze and find, and it is necessary to systematically consider the influence of multiple factors. At present, most hot-rolled pickled plates have black and white striped "defects" on the surface after pickling. This "defect" has no obvious feel and does not affect the user's subsequent welding use, but it seriously affects the appearance quality of the product.

[0004] In the prior art, Chinese patent application number: 202111565020.X discloses a hot rolling method for solving the problem of stripe oxide on the surface of high-strength steel, which is sequentially descaled by the descaling equipment before the roughing mill, descaled by the roughing mill stand and rolled by the roughing mill, and then finely descaled before the finishing mill and rolled by the finishing mill, so as to increase the thickness of the intermediate billet oxide scale to about 100um and the temperature to above 1100°C. The surface of the intermediate billet with a higher temperature will drop in temperature during fine descaling, which is beneficial to the removal of the intermediate billet oxide scale by fine descaling, thereby achieving the purpose of reducing the surface stripes of high-strength steel. However, the generation of surface stripes of high-strength hot-rolled pickled strip is not only caused by the oxide scale, but also related to other factors, such as: heating process, heating furnace discharge rhythm, roller surface roughness of the finishing work roll, etc. Summary of the invention

[0005] To overcome the deficiencies of the prior art, the present invention aims to provide a method for reducing surface stripes on high-strength hot-rolled pickled strip, thereby reducing surface stripes on the hot-rolled pickled strip, improving the appearance quality of the hot-rolled pickled strip and reducing the defect incidence.

[0006] To achieve the above object, the present invention is implemented through the following technical solutions:

[0007] A method for reducing surface stripes of high-strength hot-rolled pickled steel strip, the process flow includes: smelting, continuous casting, hot delivery and hot charging, post-furnace descaling and rough rolling, finishing rolling, rough and finishing descaling, laminar cooling, cold rolling pickling, and coiling; specifically includes the following aspects:

[0008] 1) The temperature of the slab in the preheating section of the heating furnace is 700-1000℃, and the time is controlled within 30-40min; the temperature of the first heating section is 1100-1170℃, and the time is controlled within 50-60min; the temperature of the second heating section is 1180-1250℃, and the time is controlled within 60-70min; the temperature of the soaking section is 1150-1230℃, and the time is controlled within ≥35min; the total time in the furnace is controlled within 180-230min;

[0009] 2) Release rhythm:

[0010] The thickness of hot-rolled finished product is less than 2.0mm, and the furnace-out rhythm is ≥180s / piece;

[0011] The thickness of hot-rolled finished products is 2.0-2.5mm, and the furnace-out rhythm is ≥160s / piece;

[0012] The thickness of hot-rolled finished product is >2.5mm, and the furnace-out rhythm is ≥150s / piece;

[0013] 3) After the strip head bites the steel, hot rolling lubricating oil is sprayed onto the surface of the support roll and the working roll at the same time.

[0014] The composition of the strip is by weight percentage:

[0015] C: 0.1%~0.12%, Si: 1.0%~1.2%, Mn: 1.5%~1.8%, P≤0.005%, S≤0.005%, Cr: 0.15%~0.20%, Al: 0.02%~0.06%, N: 0~0.0045%, and the rest are Fe and unavoidable impurities.

[0016] The furnace pressure of the heating furnace is controlled at a slight positive pressure of 1.5-2Pa.

[0017] In step 1), the slab temperature entering the furnace is greater than 300°C. The slabs that cannot directly enter the heating furnace are placed in a slow cooling pit at 500-550°C for storage, and the storage time is within 72 hours.

[0018] The furnace temperature is controlled within ±20℃ of the target temperature.

[0019] In the post-furnace descaling and rough rolling, finishing rolling, rough and finishing rolling descaling:

[0020] The minimum amount of descaling water for rough rolling is 4 passes, and descaling water must be added at the entrance side of each pass of rough rolling;

[0021] The descaling water pressure of rough rolling is above 18MPa, the descaling water pressure of finishing rolling is above 23MPa, the rough rolling outlet thickness is 35-40mm; the width difference between slab and finished product is controlled at 200-260mm;

[0022] The laminar cooling and coiling:

[0023] The laminar cooling adopts the front-stage rapid cooling mode, the cooling rate is controlled at 30-40℃ / s, the coiling temperature is 580-620℃, and the final product thickness is 2.0-2.5mm.

[0024] The cold rolling pickling: the pickling solution is hydrochloric acid with a concentration of 20% to 25%, the pickling solution temperature is 75 to 80°C, and the straightening operation speed before pickling is 40 to 50 m / min.

[0025] The injection pressure of the hot rolling lubricating oil in step 2) is 2-4 MPa.

[0026] The surface roughness Ra of the support roll and the working roll is 0.8-1.2 μm.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The method of reducing the surface stripes of high-strength hot-rolled pickled strip steel is to control the time and temperature of each heating section of the heating furnace to avoid cracks in the slab, improve the quality of the slab, and significantly improve the stripe defects of the hot-rolled pickled plate. Control the rhythm of the furnace according to the thickness of the finished steel product to avoid ring-shaped shedding and strip iron sheets pressing into the strip. Spray hot rolling lubricating oil on the surface of the support roll and the working roll at the same time to improve the surface quality of the working roll, which can achieve hot rolling oil and roll gap cooling and improve the surface quality of the hot-rolled pickled plate. High-pressure water descaling control can remove the furnace iron scale to the greatest extent. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a comparison curve between the finishing roll cycle and roughness.

[0030] Figure 2 This is the roll surface roughness diagram after one cycle of the finishing rolling mill F1-F4.

[0031] Figure 3 This is a structural diagram of the arrangement of the high-pressure water descaling nozzles on both sides of the roughing mill.

[0032] Figure 4 It is the front view of the arrangement structure of the high-pressure water descaling nozzle.

[0033] Figure 5 It is a side view of the arrangement structure of the high-pressure water descaling nozzle.

[0034] Figure 3-Figure 5 Middle: 1-roughing mill 2-descaling header at the inlet side of the roughing mill 3-descaling header at the outlet side of the roughing mill 4-rolled product 5-descaling nozzle. DETAILED DESCRIPTION

[0035] The present invention is described in detail below in conjunction with the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0036] A method for reducing surface stripes of high-strength hot-rolled pickled steel strip, the composition of the high-strength steel strip is calculated by weight percentage:

[0037] C is 0.1% to 0.12%, Si is 1.0% to 1.2%, Mn is 1.5% to 1.8%, P≤0.005%, S≤0.005%, Cr is 0.15% to 0.20%, Al is 0.02% to 0.06%, N is 0 to 0.0045%, and the rest is Fe and unavoidable impurities.

[0038] The production process includes smelting, continuous casting, hot delivery and hot charging, post-furnace descaling and rough rolling, finishing rolling, rough and finishing rolling descaling, laminar cooling, and coiling process:

[0039] Hot rolling process formulation:

[0040] 1) Hot loading of slabs (furnace temperature > 300℃). Slabs that cannot be hot loaded need to be placed in a slow cooling pit at 500-550℃ for a storage time of less than 72 hours to avoid cracks in the slabs due to long storage time. The furnace time is guaranteed to be more than 180 minutes, but not more than 230 minutes; strictly control the heating time, heating temperature and furnace discharge temperature of each section in the furnace, see Table 1, the furnace discharge temperature is controlled at the target temperature ±20℃, and the furnace discharge target temperature is 1180~1220℃. The furnace pressure is controlled at a slightly positive pressure, and the air excess coefficient is 2.0, which can effectively reduce the FeSi2O4 thickness and gas consumption, and the gas unit consumption is reduced to 1.1GJ / t.

[0041] Table 1 Temperature of each section in the heating furnace and time of slab in each section

[0042]

[0043]

[0044] 2) Optimize the rolling plan arrangement, concentrate the pickling plates for production, arrange them in the first 50 blocks of the unit plan, and the F7 (finishing rolling) working roll rolling mileage is ≤40km. The finishing high-speed steel rolls must use newly ground rolls to ensure that the roll surface roughness Ra is 0.8~1.2μm to avoid local uneven oxidation caused by uneven wear.

[0045] Inspection and evaluation of the roughness of the roller surface after grinding: Grade I grinding wheel marks; the marks and feel are obvious, which means the grinding quality is "severely" unqualified roller (rework); Grade II grinding wheel marks; the marks are not obvious, but there is a feel, which means the grinding quality is unqualified roller (rework); the qualified roller is; there is no residual grinding wheel mark on the surface, the hand feels smooth, and the surface is smooth. Figure 1 , Figure 2 (The two pictures are the basis for determining the need to grind the rollers).

[0046] 3) After the strip head bites the steel, hot rolling oil is sprayed onto the surface of the support roll and the working roll at the same time, which improves the surface quality of the working roll. At the same time, it can not only realize the simultaneous use of hot rolling oil and roll gap cooling, but also does not need to stop the cooling water at the working roll inlet.

[0047] The ratio of oil-water mixture is 1000L water per 0.5L oil / min, and the injection pressure is 2MPa.

[0048] 4) Optimize the rhythm of steel tapping: reduce the rhythm of steel tapping to ensure that the finishing rolls are fully cooled and the oxide film is fully formed to avoid ring-shaped shedding and strip iron sheets being pressed into the strip. Since the lower the thickness, the higher the finishing rolling start temperature, and the easier it is to form oxide scale during finishing rolling, the rhythm of steel tapping is divided according to thickness:

[0049] The thickness of hot-rolled finished product is less than 2.0mm, and the furnace-out rhythm is ≥180s / piece;

[0050] The thickness of hot-rolled finished products is 2.0-2.5mm, and the furnace-out rhythm is ≥160s / piece;

[0051] The thickness of hot-rolled finished product is >2.5mm, and the furnace-out rhythm is ≥150s / piece;

[0052] 5) High-pressure water descaling: The rough rolling process is 6 passes, and the minimum descaling water input is 4 passes; the width difference between the slab and the finished product is controlled at 200-260mm, which can remove the raw iron scale to the greatest extent. The rough rolling descaling water pressure is above 18MPa, and the finishing descaling water pressure is above 23MPa. The rough rolling outlet thickness is 35-40mm, and the rough rolling time and descaling time are increased to provide the best conditions for finishing rolling. Figure 3 The inlet and outlet sides of the roughing mill are both equipped with descaling headers 2 and 3, and the descaling headers are equipped with descaling nozzles 5. When the roughing mill 1 adopts 6-pass rolling, the rolled piece 4 is respectively put into high-pressure water descaling before the 1st, 2nd, 4th and 6th passes of rolling.

[0053] See Figure 4 The high-pressure water descaling nozzles 5 at the entrance and exit of the four-roll roughing mill are staggered, the spray overlap amount D of adjacent descaling nozzles 5 is 7-10 mm, and the vertical height h2 of the descaling nozzles 5 is 150-250 mm, so that the descaling water fan-shaped surface is maximized, the number of descaling nozzles 5 installed is reduced, and at the same time, the impact force acting on the strip surface can meet the iron scale removal requirements.

[0054] See Figure 4, A is the spray distance of the descaling nozzle, B is the spray width of the descaling nozzle, C is the spray width in the rolling direction, D is the spray overlap of adjacent descaling nozzles, E is the spacing of the descaling nozzles, h2 is the vertical height of the descaling nozzle, h2 = 150 ~ 250mm; α is the spray angle of the descaling nozzle, α = 30 ~ 40°; β is the inclination angle of the descaling nozzle, β = 15°; γ is the deflection angle of the descaling nozzle relative to the axis of the water spray pipe, γ = 15°. During the rough rolling process, the iron sheet is in the same direction as the rolling direction and presents a willow leaf shape. The iron sheet has a low adhesion along the rolling direction. The above settings are conducive to the removal of the iron sheet.

[0055] 6) Cooling process and coiling temperature: Laminar cooling adopts the front-stage rapid cooling mode, and the cooling rate is controlled at 30-40℃ / s to avoid the generation of secondary iron scale in the back stage. The coiling temperature is 580-620℃, and the final product thickness is 2.0-2.5mm.

[0056] 7) Cold rolling and pickling: The concentration of pickling hydrochloric acid is 20% to 25%, the temperature of the pickling solution is 75 to 80°C, and the straightening speed before pickling is 40 to 50 m / min.

[0057] Examples 1-3 produce 440MPa, 550MPa, and 700MPa high-strength pickling products, respectively. The strip steel compositions and production process parameters thereof are shown in Tables 1 to 4.

[0058] Table 1 Composition of steel strip in the example

[0059] Example C Si Mn P S Cr Al N 1 0.1％ 1.0％ 1.5％ 0.003％ 0.002％ 0.15％ 0.02％ 0.001％ 2 0.11％ 1.1％ 1.6％ 0.002％ 0.003％ 0.18％ 0.04％ 0.002％ 3 0.12％ 1.2％ 1.7％ 0.004％ 0.002％ 0.20％ 0.06％ 0.004％

[0060] Table 2 Example heating process parameters

[0061]

[0062] Table 3 Rough rolling process parameters of the embodiment

[0063]

[0064] Table 4 Example finishing and pickling process parameters

[0065]

[0066] It can be seen that the stripe defects of hot-rolled pickled plates have been solved.

Claims

1. A method for reducing surface stripes on high-strength hot-rolled pickled steel strip, characterized in that: The composition of the strip is by weight percentage: C: 0.1%~0.12%, Si: 1.0%~1.2%, Mn: 1.5%~1.8%, P≤0.005%, S≤0.005%, Cr: 0.15%~0.20%, Al: 0.02%~0.06%, N: 0~0.0045%, the rest is Fe and unavoidable impurities; The process includes: smelting, continuous casting, hot delivery and hot charging, post-furnace descaling and rough rolling, finishing rolling, rough and finishing descaling, laminar cooling, cold rolling pickling, and coiling; specifically includes the following aspects: 1) The temperature of the slab in the preheating section of the heating furnace is 700-1000℃, and the time is controlled within 30-40min; the temperature of the first heating section is 1100-1170℃, and the time is controlled within 50-60min; the temperature of the second heating section is 1180-1250℃, and the time is controlled within 60-70min; the temperature of the soaking section is 1150-1230℃, and the time is controlled within ≥35min; the total time in the furnace is controlled within 180-230min; 2) After the strip head bites the steel, the hot rolling lubricating oil is sprayed onto the surface of the support roll and the working roll at the same time; the surface roughness Ra of the support roll and the working roll is 0.8~1.2μm; 3) Release rhythm: The thickness of hot-rolled finished product is less than 2.0mm, and the furnace-out rhythm is ≥180s / piece; The thickness of hot-rolled finished products is 2.0-2.5mm, and the furnace-out rhythm is ≥160s / piece; The thickness of hot-rolled finished product is >2.5mm, and the furnace-out rhythm is ≥150s / piece; In the post-furnace descaling and rough rolling, finishing rolling, rough and finishing rolling descaling: The minimum amount of descaling water input for rough rolling is 4 rough rolling passes; The descaling water pressure of rough rolling is above 18MPa, the outlet thickness of rough rolling is 35-40mm; the width difference between slab and finished product is controlled at 200-260mm; the descaling water pressure of finishing rolling is above 23MPa; The laminar cooling and coiling: The laminar cooling adopts the front-stage rapid cooling mode, the cooling rate is controlled at 30-40℃ / s, the coiling temperature is 580-620℃, and the final product thickness is 2.0-2.5mm.

2. A method for reducing surface stripes on high-strength hot-rolled pickled steel strip according to claim 1, characterized in that: The furnace pressure of the heating furnace is controlled at a slight positive pressure of 1.5-2Pa.

3. The method for reducing surface stripes of high-strength hot-rolled pickled steel strip according to claim 1, characterized in that: The furnace entry temperature of the slab is greater than 300°C. The slab that cannot directly enter the heating furnace is placed in a slow cooling pit at 500-550°C for storage, and the storage time is within 72 hours.

4. A method for reducing surface stripes on high-strength hot-rolled pickled steel strip according to claim 1, characterized in that: The furnace temperature is controlled within ±20℃ of the target temperature.

5. The method for reducing surface stripes of high-strength hot-rolled pickled steel strip according to claim 1, characterized in that: The cold rolling pickling: the pickling solution is hydrochloric acid with a concentration of 20% to 25%, the pickling solution temperature is 75 to 80°C, and the straightening operation speed before pickling is 40 to 50 m / min.

6. A method for reducing surface stripes on high-strength hot-rolled pickled steel strip according to claim 1, characterized in that: The injection pressure of the injection hot rolling lubricating oil is 2-4 MPa.

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