Control method for peak-shaped color difference defects on the surface of hot-rolled pickled steel

Abstract

The present invention discloses a method for controlling mountain-shaped color difference defects on the surface of hot-rolled pickled steel, comprising the steps of: controlling the depth of vibration marks in the continuous casting stage, consuming the vibration mark defect layer in the rotary descaling stage, controlling oxidation burning loss in the soaking stage, removing the oxide layer in the descaling stage before finishing rolling, controlling the pressing system in the rolling stage, controlling the structure of the iron oxide skin layer in the laminar flow and coiling cooling stage, controlling the integrity of the iron oxide skin layer in the flattening stage, and controlling the microscopic three-dimensional morphology of the surface during the pickling stage. The present invention can eliminate the mountain-shaped color difference defects on the surface of hot-rolled pickled steel by optimizing the continuous casting, descaling, heating, rolling, cooling, and subsequent flattening and pickling processes of the CSP (compact strip production) short process, and effectively improve the production quality of hot-rolled pickled steel.

Description

Technical Field

[0001] The invention relates to a steel metallurgical production process, in particular to a method for controlling mountain-shaped color difference defects on the surface of hot-rolled pickled steel. Background Art

[0002] Hot-rolled pickled steel is an intermediate product between hot-rolled strip steel and cold-rolled strip steel. It is an economical substitute for some hot-rolled plates and cold-rolled plates and is increasingly used in the automotive, home appliance and other industries. At present, hot-rolled pickled steel is produced by thin slab continuous casting and rolling process (short process), such as the mainstream CSP (compact strip production) process. The steel types produced include low carbon steel (LC steel), low alloy high strength steel (HSLA), automotive advanced high strength steel (AHSS), etc. Its various mechanical properties and application properties are equivalent to those of cold-rolled products of the same thickness, which can achieve "replacing cold with heat".

[0003] However, in actual production, the pickling surface quality of hot-rolled pickled steel is significantly different from that of traditional hot-rolled pickled products and cold-rolled products. The pickling surface of short-process thin-gauge pickled steel has a large number of regularly distributed peak-shaped color difference defects. The typical characteristics of the peak-shaped color difference defects on the surface of short-process thin-gauge hot-rolled pickled steel are: (1) Each peak-shaped color difference is distributed along the entire width of the strip, and there is a fixed spacing between the peak-shaped color difference strips, showing a clear periodicity, and the peak-shaped color difference spacing is negatively correlated with the thickness specification of the finished product; (2) The peak-shaped color difference defects exist on both the upper and lower plates, and the positions are basically corresponding; (3) The color of the peak-shaped defect area is whiter and brighter than the normal area, showing a certain reflective color difference; (4) The peak-shaped defect is sometimes accompanied by growth stripe color difference.

[0004] The surface micromorphology and microstructure of the peak-shaped color difference defect and the normal part are compared and analyzed. The periodic law of the peak-shaped defect and the negative correlation between the peak-shaped color difference spacing and the finished product thickness specification are determined. The peak-shaped defect is related to the vibration mark. The vibration mark is a horizontal concave shallow groove with uniform spacing and a certain depth formed on the surface of the ingot by the periodic vibration of the crystallizer. The depth of the vibration mark of the thin slab in the short process production line is generally 0.2-0.3mm. According to the formation mechanism of the vibration mark, a microscopic positive segregation zone of the solute elements (Mn, Si, etc.) of the molten steel will be generated at the bottom of the vibration mark. The segregation is unevenly distributed along the circumference of the vibration mark, and there are deeper / wider segregation areas. Or, due to the large pressure of the liquid slag on the initial solidified ingot shell during the vibration mark formation process, the carbon-rich layer in the liquid slag layer diffuses to the vicinity of the vibration mark to form a carburized zone. The carburized zone extends during rolling to form a peak shape and a growth strip, and the peak spacing is related to the reduction. The difference in composition between the defective area after rolling and the normal part leads to differences in phase change rules and final organization. This difference manifests as a difference in microscopic three-dimensional morphology after pickling, and is reflected as a peak-shaped color difference defect in the human eye.

[0005] Chinese invention patent ZL201711393739.3 discloses a method for eliminating the color difference defect on the surface of non-oriented electrical steel sheet coil, comprising step 1: continuously casting the refined non-oriented electrical steel molten steel into thin slabs, during the continuous casting process, the crystallizer is put into protective slag, electromagnetic braking and vibration are adopted; step 2: the continuously cast slab enters the soaking furnace for heating, and then enters the hot rolling mill for rolling, and the coil is obtained to obtain a hot rolled coil; high-pressure water descaling is performed before entering the soaking furnace and the hot rolling mill; step 3: the hot rolled coil is pickled to remove the iron oxide scale, and then the cold rolling mill is used for cold rolling to the target thickness, and then annealed in the annealing furnace. In this patent, the protective slag remaining on the surface of the slab and related defects are the causes of the color difference defects. The method only involves reducing the intergranular corrosion of steel by the protective slag in the vibration mark by controlling the original continuous casting vibration, and removing the residual protective slag by descaling. It does not consider the influence of the vibration mark layer itself (including the segregation layer at the bottom of the vibration mark) on the defects, nor does it consider the consumption of the vibration mark layer depth in subsequent processes, thereby failing to fundamentally eliminate the peak-shaped color difference defects on the surface of hot-rolled pickled steel.

[0006] Chinese invention patent application CN202110322345.9 discloses a method for improving the surface color difference defects of high-grade non-oriented silicon steel, and specifically discloses: the continuous casting billet is subjected to hot rolling, normalizing, pickling, single-stand rolling and continuous annealing processes in sequence, and the continuous casting billet contains the following components by weight percentage: C <0.003%, Si1.6-2.0%, Mn 0.35-0.65%, Al 0.35-0.65%, P <0.02%, S <0.0025%, and the balance is Fe and other inevitable impurity elements; the coiling temperature of the hot rolling process is 620-640°C. In this patent, the structure and thickness of iron oxide scale are the causes of the surface color difference of high-grade non-oriented silicon steel, and the surface color difference defects of high-grade non-oriented silicon steel are improved by adjusting the hot rolling coiling temperature to reduce the iron oxide thickness on the strip surface and change the iron oxide structure on the strip surface. However, the peak-shaped color difference problem caused by continuous casting vibration marks cannot be solved. Summary of the invention

[0007] The object of the present invention is to provide a method for controlling mountain-shaped chromatic aberration defects on the surface of hot-rolled pickled steel, which can effectively control the occurrence rate of mountain-shaped chromatic aberration defects on the surface of hot-rolled pickled steel.

[0008] The present invention is achieved in that:

[0009] A control method for mountain-shaped color difference defects on the surface of hot-rolled pickled steel, the control method is implemented based on a thin slab continuous casting and rolling process, the thin slab continuous casting and rolling process includes the following steps: molten iron desulfurization → converter smelting → refining → thin slab continuous casting → soaking → finishing rolling → laminar cooling → coiling; and hot-rolled pickled steel is formed by leveling, pickling and oiling;

[0010] The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel comprises the following steps:

[0011] Step 1: Molten iron is subjected to desulfurization, converter smelting and refining processes to obtain molten steel, which is then continuously cast into thin slabs to form continuous casting billets. During the continuous casting production stage, the vibration parameters of the crystallizer are controlled to control the vibration mark depth of the continuous casting billet.

[0012] Step 2: Rotate and descale the surface of the continuous casting billet, and consume the vibration mark defect layer during the rotary descaling process;

[0013] Step 3: Soak the continuous casting billet after rotary descaling, and increase the oxidation consumption of the vibration mark defect layer during the soaking stage;

[0014] Step 4: performing secondary descaling on the continuous casting billet, and removing the iron oxide layer containing part of the vibration mark defect layer during the secondary descaling process;

[0015] Step 5: The continuous casting billet is rolled into a strip steel, and the reduction rate of the rolling stand is controlled during the rolling stage;

[0016] Step 6: Cooling and coiling the strip, and controlling the structure of the iron oxide layer on the surface of the strip during the cooling and coiling stages;

[0017] Step 7: Flatten the steel coil and destroy the integrity of the iron oxide layer on the surface of the strip during the flattening stage;

[0018] Step 8: After the strip is straightened, the strip is pickled, and the iron oxide layer on the surface of the strip is dissolved during the pickling stage. After pickling, the surface of the strip is rust-proofed and oiled to make hot-rolled pickled steel.

[0019] In step 1, the control method of the vibration mark depth of the continuous casting billet is: the negative slip time t n =0.029-0.033s, negative slip time ratio NSR=15.4-16.4%, vibration frequency f=297-317 times / min, amplitude S=5.8mm, waveform slope α=15-20%; the continuous casting speed in the continuous casting production stage is 4.3-4.5m / min, and the liquid core is used to control the thickness of the casting to be in the range of 65-70mm.

[0020] In the step 2, the method for consuming the vibration mark defect layer is: the descaling water forms a water jet through a rotating descaling nozzle and impacts the surface of the continuous casting billet at different angles and directions, the rotation speed of the rotating descaling nozzle is 500-700rpm, the working pressure of the descaling water is 380-400bar, so that the impact pressure of the water jet on the surface of the continuous casting billet is 7-8MPa.

[0021] In step 3, the oxidation loss control method of the vibration mark defect layer is: the continuous casting billet is heated and soaked in a soaking furnace, the soaking time is 35-45 minutes, the furnace outlet temperature of the continuous casting billet is controlled to be 1200-1220°C, and the excess air coefficient in the soaking furnace is 1.15-1.25.

[0022] In the step 4, the secondary descaling method is: descaling the surface of the continuous casting billet by high-pressure descaling water, and the working pressure of the descaling water is 300-380 bar.

[0023] In step 5, the reduction rate control parameters of the rolling stands are: the reduction rate of the first stand is 55-65%, and the reduction rate of the second stand is 50-60%.

[0024] In step 6, the strip cooling method is: laminar cooling to the coiling temperature before the strip is coiled, and ventilation cooling to room temperature after the strip is coiled; the coiling temperature is 620-650°C.

[0025] In the step 7, the leveling force is 180-220 tons, the leveling unwinding tension is 50-60 MPa, and the coiling tension is 100-120 MPa.

[0026] In step 8, the tensile straightening elongation of the steel strip is 1.0-1.5%; the pickling method of the steel strip is as follows: the pickling section is composed of a plurality of pickling tanks connected in series, the pickling medium is a mixed solution of a hydrochloric acid solution and a pickling inhibitor, the steel strip passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the steel strip; the temperature of the pickling tank is 75-90° C., according to the movement direction of the steel strip, the concentration of the hydrochloric acid solution in the first pickling tank is ≥50 g / l, and the concentration of the hydrochloric acid solution in the last pickling tank is 160-200 g / l; the total pickling time is 36-45 s, and the pickling speed is 120-150 m / min.

[0027] The mass of the pickling inhibitor accounts for 0.5-0.8% of the mass of the mixed solution. The pickling inhibitor is a mixed solution of hexamethylenetetramine, propargyl alcohol and water. In terms of mass percentage, the ratio of hexamethylenetetramine, propargyl alcohol and water is 25:5:70.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The present invention optimizes the CSP short-process technology by sequentially controlling the depth of vibration marks in the continuous casting stage, consuming the vibration mark defect layer in the rotary descaling stage, controlling the oxidation burning loss in the soaking stage, removing the oxide layer in the descaling stage before finishing rolling, controlling the reduction system in the rolling stage, controlling the structure of the iron oxide skin layer in the laminar flow and coiling cooling stage, controlling the integrity of the iron oxide skin layer in the leveling stage, and controlling the surface microscopic three-dimensional morphology in the pickling stage. The occurrence of mountain-shaped color difference defects on the surface of the strip is basically eliminated, and the surface quality of the strip can be improved on the basis of ensuring the mechanical properties and application performance of the strip, thereby meeting the application demand of "replacing cold with heat" of hot-rolled pickled steel plates for cold-rolled plates.

[0030] 2. The present invention is implemented based on the existing CSP short-process technology, does not increase production costs, and improves the economic benefits of hot-rolled pickled steel.

[0031] The present invention can eliminate the mountain-shaped color difference defect on the surface of hot-rolled pickled steel by optimizing the continuous casting, descaling, heating, steel rolling, cooling, and subsequent leveling and pickling processes of the CSP short-process technology, thereby effectively improving the production quality of hot-rolled pickled steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a flow chart of a method for controlling mountain-shaped color difference defects on the surface of hot-rolled pickled steel according to the present invention;

[0033] Figure 2 It is a schematic diagram of the vibration curve of the crystallizer in the method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel of the present invention. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0035] Please see attached Figure 1 A method for controlling mountain-shaped color difference defects on the surface of hot-rolled pickled steel is disclosed. The control method is implemented based on a thin slab continuous casting and rolling process (CSP short process). The CSP short process includes the following steps: molten iron desulfurization → converter smelting → refining → thin slab continuous casting → soaking → finishing rolling → laminar cooling → coiling, and then leveling, pickling and oiling to form pickled steel.

[0036] The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel comprises the following steps:

[0037] Step 1: After desulfurization, converter smelting and refining, molten iron is subjected to the steps of obtaining molten steel that meets the composition requirements. The molten steel is continuously cast into thin slabs to form continuous casting billets. During the continuous casting production stage, the vibration parameters of the crystallizer are controlled to control the vibration mark depth of the continuous casting billet. The molten iron desulfurization, converter smelting and refining steps can adopt the conventional steps in the CSP short process, which will not be described here.

[0038] The control method of the vibration mark depth of the continuous casting billet is: the negative slip time t n =0.029-0.033s, negative slip time ratio (i.e. the ratio of negative slip time to vibration half period) NSR =15.4-16.4%, vibration frequency f =297-317 times / min, amplitude S =5.8mm, waveform deflection rate α =15-20%.

[0039] Negative slip time t n It refers to the time during which the downward movement speed Vm of the crystallizer exceeds the pulling speed Vc during the vibration cycle. The negative slip time can be express.

[0040] Wherein, f is the vibration frequency (times / min).

[0041] S is the amplitude (mm).

[0042] a is the waveform slope, when a=0 it is sinusoidal vibration; when a>0 it is non-sinusoidal vibration, the displacement curve lags on the crest and leads on the trough, and the negative slip time is shorter than that of sinusoidal vibration. The calculation formula of a is: Among them, A 1 A is the lag time of the non-sinusoidal waveform relative to the sine waveform to reach the maximum vibration displacement, 0 is one quarter of the vibration period. Figure 2 As shown in the figure, the vibration mark depth decreases with the decrease of negative slip time. By increasing the waveform slope and changing the vibration curve from a sine curve to a non-sinusoidal curve, the negative slip time can be effectively shortened. At the same time, the negative slip time can be shortened by reducing the amplitude and increasing the frequency.

[0043] The continuous casting speed in the continuous casting production stage is 4.3-4.5m / min, and the liquid core is pressed down to control the thickness of the ingot to be 65-70mm. Under the same finished product thickness, the subsequent rolling process can reduce the depth of the vibration mark layer to a thinner level, thereby reducing the generation of peak-shaped color difference defects.

[0044] Step 2: The surface of the continuous casting billet is subjected to rotational descaling, so that the surface of the continuous casting billet is in a cycle state of oxidation and descaling, and the vibration mark defect layer is consumed during the rotational descaling process.

[0045] The method for consuming the vibration mark defect layer is as follows: the descaling water forms a water jet through a rotating descaling nozzle and impacts the surface of the continuous casting billet at different angles and directions. The rotating speed of the rotating descaling nozzle is 500-700rpm, the working pressure of the descaling water is 380-400bar, and the impact pressure of the water jet on the surface of the continuous casting billet is 7-8MPa.

[0046] A descaling water jet with a relatively high pressure is used to perform rotational descaling on the surface of the continuous casting billet, so that the surface of the casting billet is in a cyclic state of oxidation and descaling. The parabolic law of steel oxidation dynamics (i.e. when the iron oxide scale is thinner, the thickness of the iron oxide scale increases faster) is utilized to remove a large proportion of the vibration mark defect layer on the surface of the continuous casting billet before entering the soaking furnace.

[0047] Step 3: The continuous casting ingot after rotary descaling is evenly heated, and during the even heating stage, the oxidation loss of the vibration mark defect layer is increased by appropriately increasing the furnace temperature and extending the heating time.

[0048] The oxidation loss control method of the vibration mark defect layer is as follows: the continuous casting billet is heated and soaked in a soaking furnace, the soaking time is 35-45 minutes, the furnace outlet temperature of the continuous casting billet is controlled to be 1200-1220°C, the air excess coefficient in each furnace zone in the soaking furnace is 1.15-1.25, and the oxidation burning loss of the vibration mark defect layer can be increased by appropriately increasing the furnace outlet temperature and extending the heating time.

[0049] Step 4: The continuous casting billet is descaled for a second time before rolling, and the iron oxide layer containing part of the vibration mark defect layer is removed during the secondary descaling process.

[0050] The secondary descaling method is: descaling the surface of the continuous casting billet by high-pressure descaling water, the working pressure of the descaling water is 300-380bar, and the iron oxide skin layer formed on the surface of the continuous casting billet after heating is removed, and part of the vibration mark defect layer is oxidized into the iron oxide skin layer, and the iron oxide skin layer is further removed by secondary descaling. The greater the working pressure of the descaling water for secondary descaling, the better the descaling effect.

[0051] Step 5: The continuous casting slab is rolled into strip steel, and the reduction rate of the rolling stand is controlled during the rolling stage.

[0052] The rolling mill roll reduction control parameters are: the first roll reduction rate of the first roll F1 is 55-65%, and the second roll reduction rate of the second roll F2 is 50-60%. By increasing the reduction amount of the high temperature roll, a thicker vibration mark defect layer forms an iron oxide layer.

[0053] According to the moving direction of the continuous casting billet, by increasing the reduction rate of the first two stands, i.e. the first stand F1 and the second stand F2, the area-to-depth ratio of the vibration mark defect layer can be increased when the temperature of the continuous casting billet is high. Based on the high-temperature dynamics of iron oxide scale, the higher the temperature, the faster the growth of the iron oxide scale layer, so that a larger proportion of the vibration mark defect layer can form the iron oxide scale layer during the rolling stage.

[0054] Step 6: Cool and coil the strip, and control the structure of the iron oxide layer on the surface of the strip during the cooling and coiling stages to generate an iron oxide layer with a higher proportion of FeO.

[0055] The strip cooling method is as follows: the strip is cooled to the coiling temperature by laminar flow before coiling, and the strip is cooled to room temperature by ventilation after coiling; the coiling temperature is 620-650°C. The front-stage laminar rapid cooling of the strip adopts a higher CT (coiling temperature, i.e., coiling temperature) to allow more Fe ions in the steel to diffuse into the FeO layer, increase the proportion of FeO in the high-temperature iron oxide layer, and improve the thermodynamic stability of the Fe-rich FeO layer. During the ventilation cooling process of the steel coil, the FeO in the iron oxide layer does not undergo a eutectic reaction (i.e., 4FeO→Fe+Fe 3 O 4 ) and remain at room temperature, so that the iron oxide layer containing more FeO has a higher pickling rate.

[0056] Step 7: Flatten the steel coil and destroy the integrity of the iron oxide layer on the surface of the strip during the flattening stage.

[0057] The steel coil needs to go through unwinding, straightening, leveling, coiling and other processes in the leveling stage. The leveling force is 180-220 tons, the leveling unwinding tension is 50-60MPa, and the coiling tension is 100-120MPa. The use of a larger leveling unwinding tension can cause a large number of transverse cracks in the iron oxide layer under the action of tensile stress, destroying the integrity of the iron oxide layer and providing a large number of artificial pickling nuclei for the subsequent pickling process.

[0058] The pickling of crack-free scale is a nucleation and growth process. There is a slow pickling reaction incubation period, called "initial pickling-free time", which is the period of pickling nucleus formation. Scale breaking causes cracks in the scale. The density and size of the cracks increase with the increase of scale elongation. These cracks will play the role of artificial pickling nuclei during pickling. These cracks in the scale can make it easy for the acid to reach the steel base without nucleation, which significantly shortens the pickling incubation period (initial pickling-free time). For some cases with large scale elongation, the incubation period can even be completely eliminated.

[0059] Step 8: After the strip is straightened, the strip is pickled, and the iron oxide layer on the surface of the strip is dissolved during the pickling stage. After pickling, the surface of the strip is rust-proofed and oiled to make hot-rolled pickled steel.

[0060] The tensile elongation of the strip steel is 1.0-1.5%, which can further destroy the iron oxide layer on the surface of the strip steel, making it more conducive to being peeled off during the pickling process.

[0061] The pickling method of the strip steel is as follows: the pickling section is composed of several (preferably 3) pickling tanks connected in series, the pickling medium is a mixed solution of hydrochloric acid solution and pickling inhibitor, the strip steel passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the strip steel; the temperature of the pickling tank is 75-90°C; according to the movement direction of the strip steel, the concentration of the hydrochloric acid solution in the first pickling tank is ≥50g / l, and the concentration of the hydrochloric acid solution in the last pickling tank is 160-200g / l; the total pickling time is 36-45s, and the pickling speed is 120-150m / min. A high-concentration acid solution containing a pickling inhibitor is added, and the iron oxide layer on the surface of the strip steel is quickly dissolved in the pickling tank after pickling, so as to avoid the pickling speed difference between the vibration mark defect area and the normal area due to abnormal organization and composition, and finally cause the problem of microscopic three-dimensional surface difference.

[0062] The mass of the pickling inhibitor accounts for 0.5-0.8% of the mass of the mixed solution.

[0063] The pickling inhibitor is a mixed solution of hexamethylenetetramine, propargyl alcohol and water, and the ratio of hexamethylenetetramine, propargyl alcohol and water is 25:5:70 by mass percentage. The adsorption characteristics of hexamethylenetetramine and propargyl alcohol are used to form an adsorption film on the surface of the strip, which effectively prevents the electrochemical reaction between the pickling solution and the surface of the strip, greatly reduces the corrosion of the strip substrate, and further avoids the color difference caused by the inconsistent pickling speed between the vibration mark defect area and the normal area. The pickling inhibitor can also use other components that can form a film and prevent the pickling solution from corroding the strip substrate, which will not be repeated here.

[0064] The control method of the present invention can completely eliminate the mountain-shaped color difference defect on the surface of hot-rolled pickled steel.

[0065] Embodiment 1-3:

[0066] The control method of the present invention is used to produce hot-rolled pickled steel for automobile structure, the yield strength of the hot-rolled pickled steel is 500MPa, and the chemical composition is: by mass percentage, C: 0.05%, Si: 0.05%, Mn: 1.05%, S: 0.006%, P: 0.010%, Nb: 0.033%, Ti: 0.035%, N: 0.004%, Als: 0.035%. The hot-rolled pickled steel is produced based on the CSP short process, and the process includes: hot metal desulfurization → converter smelting → refining → thin slab continuous casting → soaking → 7-stand finishing rolling → laminar cooling → coiling, and then leveling, pickling and oiling to form the hot-rolled pickled steel.

[0067] Step 1: After the molten iron is desulfurized, smelted in a converter and refined, molten steel that meets the composition requirements is obtained, and the molten steel is continuously cast into thin slabs to form continuous casting billets, and the vibration parameters of the crystallizer are controlled during the continuous casting production stage to control the depth of the vibration marks on the surface of the continuous casting billets. The process parameters of the continuous casting production stage of Examples 1-3 are shown in Table 1.

[0068] Table 1 List of process parameters of continuous casting production stage of Examples 1-3 and Comparative Example 1 (using conventional CSP short process production)

[0069]

[0070] Step 2: The surface of the continuous casting billet is subjected to rotational descaling, and the vibration mark defect layer is consumed during the rotational descaling process.

[0071] Step 3: Soak the continuous casting billet after rotary descaling, and control the oxidation loss of the vibration mark defect layer during the soaking stage. The process parameters of the rotary descaling stage and the soaking stage of Examples 1-3 are shown in Table 2.

[0072] Table 2 List of process parameters for the rotary descaling stage and the soaking stage of Examples 1-3 and Comparative Example 1

[0073]

[0074] Step 4: Descaling the continuous casting ingot for the second time, and removing the iron oxide layer containing part of the vibration mark defect layer during the second descaling process.

[0075] Step 5: The continuous casting slab is rolled into strip steel, and the reduction rate of the rolling stand is controlled during the rolling stage.

[0076] Step 6: Cool and coil the steel strip, and control the structure of the iron oxide layer on the surface of the steel strip during the cooling and coiling stages. The process parameters of the secondary descaling stage, rolling stage, and cooling and coiling stage of Examples 1-3 are shown in Table 3.

[0077] Table 3 Process parameters of the secondary descaling stage, rolling stage and cooling coiling stage of Examples 1-3 and Comparative Example 1

[0078]

[0079] Step 7: Level the steel coil and control the integrity of the iron oxide layer on the surface of the strip during the leveling stage.

[0080] Step 8: After the strip is straightened, the strip is pickled, and the iron oxide layer on the surface of the strip is dissolved in the pickling stage. After pickling, the surface of the strip is rust-proofed and oiled to make hot-rolled pickled steel. The pickling solution is a mixed solution of hydrochloric acid solution and a pickling inhibitor, and the mass of the pickling inhibitor accounts for 0.5-0.8% of the mass of the mixed solution. The pickling inhibitor is a mixed solution of hexamethylenetetramine, propargyl alcohol and water. In terms of mass percentage, the ratio of hexamethylenetetramine, propargyl alcohol and water is 25:5:70. The process parameters of the leveling stage, straightening and pickling stages of Example 1-3 are shown in Table 4.

[0081] Table 4 List of process parameters of the leveling stage, straightening and pickling stage of Examples 1-3 and Comparative Example 1

[0082]

[0083] The hot-rolled pickled steels of Examples 1-3 and Comparative Example 1 produced by the control method of the present invention have consistent mechanical properties and application performance. The parameter comparison of the surface inspection state of the vibration mark depth, the FeO ratio of the iron oxide layer and the peak-shaped color difference defect of Examples 1-3 and Comparative Example 1 is shown in Table 5.

[0084] Table 5 Quality parameters of Examples 1-3 and Comparative Example 1

[0085]

[0086] It can be seen from Table 5 that no mountain-shaped color difference defect occurs on the surface of the hot-rolled pickled steel of Examples 1-3 produced by the control method of the present invention.

[0087] It can be seen from Tables 1 to 5 that through the control of vibration mark depth in the continuous casting stage in step 1 → consumption of vibration mark defect layer in the rotary descaling stage in step 2 → control of oxidation burning in the soaking stage in step 3 → removal of oxide layer in the descaling stage before finishing rolling in step 4 → control of reduction system in the rolling stage in step 5 → control of iron oxide scale structure in the laminar flow and coiling cooling stage in step 6 → control of iron oxide scale integrity in the leveling stage in step 7 → control of surface microscopic three-dimensional morphology in the pickling stage in step 8, the CSP short process is optimized and the occurrence of peak-shaped color difference defects on the strip surface is effectively avoided.

[0088] The above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for controlling mountain-shaped color difference defects on the surface of hot-rolled pickled steel, which is implemented based on a thin slab continuous casting and rolling process, which includes the following steps: hot metal desulfurization → converter smelting → refining → thin slab continuous casting → soaking → finishing rolling → laminar cooling → coiling, and then flattening, pickling and oiling to form hot-rolled pickled steel; Its characteristics are: The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel comprises the following steps: Step 1: Molten iron is subjected to desulfurization, converter smelting and refining processes to obtain molten steel, which is then continuously cast into thin slabs to form continuous casting billets. During the continuous casting production stage, the vibration parameters of the crystallizer are controlled to control the vibration mark depth of the continuous casting billet. Step 2: Rotate and descale the surface of the continuous casting billet, and consume the vibration mark defect layer during the rotary descaling process; Step 3: Soak the continuous casting billet after rotary descaling, and increase the oxidation consumption of the vibration mark defect layer during the soaking stage; Step 4: performing secondary descaling on the continuous casting billet, and removing the iron oxide layer containing part of the vibration mark defect layer during the secondary descaling process; Step 5: The continuous casting billet is rolled into a strip steel, and the reduction rate of the rolling stand is controlled during the rolling stage; Step 6: Cooling and coiling the strip, and controlling the structure of the iron oxide layer on the surface of the strip during the cooling and coiling stages; Step 7: Flatten the steel coil and destroy the integrity of the iron oxide layer on the surface of the strip during the flattening stage; Step 8: After the strip is straightened, the strip is pickled, and the iron oxide layer on the surface of the strip is dissolved in the pickling stage. After pickling, the surface of the strip is rust-proofed and oiled to make hot-rolled pickled steel; In step 1, the control method of the vibration mark depth of the continuous casting billet is: the negative slip time t n =0.029-0.033s, negative slip time ratio NSR=15.4-16.4%, vibration frequency f=297-317 times / min, amplitude S=5.8mm, waveform deflection rate α=15-20%; the continuous casting speed in the continuous casting production stage is 4.3-4.5m / min, and the liquid core is used to control the thickness of the casting to be 65-70mm; In the step 3, the oxidation loss control method of the vibration mark defect layer is: the continuous casting billet is heated and soaked in a soaking furnace, the soaking time is 35-45 minutes, the discharge temperature of the continuous casting billet is controlled to be 1200-1220°C, and the excess air coefficient in the soaking furnace is 1.15-1.25; In step 5, the rolling mill roll reduction control parameters are: the rolling mill roll reduction rate is 55-65% for the first mill roll and 50-60% for the second mill roll; In step 6, the strip cooling method is: laminar cooling to the coiling temperature before the strip is coiled, and ventilation cooling to room temperature after the strip is coiled; the coiling temperature is 620-650°C.

2. The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel according to claim 1 is characterized by: In the step 2, the method for consuming the vibration mark defect layer is: the descaling water forms a water jet through a rotating descaling nozzle and impacts the surface of the continuous casting billet at different angles and directions, the rotation speed of the rotating descaling nozzle is 500-700rpm, the working pressure of the descaling water is 380-400bar, so that the impact pressure of the water jet on the surface of the continuous casting billet is 7-8MPa.

3. The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel according to claim 1, characterized in that: In the step 4, the secondary descaling method is: descaling the surface of the continuous casting billet by high-pressure descaling water, and the working pressure of the descaling water is 300-380 bar.

4. The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel according to claim 1, characterized in that: In the step 7, the leveling force is 180-220 tons, the leveling unwinding tension is 50-60 MPa, and the coiling tension is 100-120 MPa.

5. The method for controlling mountain-shaped color difference defects on the surface of hot-rolled pickled steel according to claim 1, characterized in that: In step 8, the tensile straightening elongation of the strip is 1.0-1.5%; the pickling method of the strip is: the pickling section is composed of a plurality of pickling tanks connected in series, the pickling medium is a mixed solution of hydrochloric acid solution and a pickling inhibitor, the strip passes through the pickling medium, and the flow direction of the pickling medium is opposite to the movement direction of the strip; the temperature of the pickling tank is 75-90°C, according to the movement direction of the strip, the concentration of the hydrochloric acid solution in the first pickling tank is ≥50g / L, and the concentration of the hydrochloric acid solution in the last pickling tank is 160-200g / L; the total pickling time is 36-45s, and the pickling speed is 120-150m / min.

6. The method for controlling the mountain-shaped color difference defect on the surface of hot-rolled pickled steel according to claim 5, characterized in that: The mass of the pickling inhibitor accounts for 0.5-0.8% of the mass of the mixed solution. The pickling inhibitor is a mixed solution of hexamethylenetetramine, propargyl alcohol and water. In terms of mass percentage, the ratio of hexamethylenetetramine, propargyl alcohol and water is 25:5:70.

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