A method for pickling and producing low-temperature coiling hot-rolled dual-phase steel with no pits or pits on the surface

Abstract

The present invention relates to a low-temperature coiling type hot-rolled dual-phase steel pickling and production method with no pits and pits on the surface. After hot rolling, the dual-phase steel is coiled at low temperature, uncoiling, straightening machine, lifting and bending roller, tension and straightening unit descaling, three-stage turbulent pickling, rinsing, and coiling to obtain a dual-phase steel finished product with no pits and pits on the surface. Before uncoiling and during pickling, the straightening roller, lifting and bending roller and tension and straightening roller are cleaned irregularly to ensure that the cleanliness of each roller surface is ≤ Grade II. During pickling, the straightening unit and lifting and bending roller are not put into use, thereby reducing the shedding of iron sheets; when the strip enters the tension and straightening descaling unit, the tension and straightening rate is ≤0.2%, and the strip tension is 100-150KN; the temperature, Fe ion concentration, and acid concentration are controlled during segmented pickling. The present invention establishes a corresponding relationship between the amount of iron sheet falling off on the roller and the pitting and pitting defects, and utilizes it to optimize the core and key process parameters such as straightening, lifting and bending rolls, pulling and straightening, and pickling speed. The resulting process method has the advantages of simple operation, flexibility, low cost, and easy promotion.

Description

Technical Field

[0001] The invention relates to the technical field of hot-rolled dual-phase steel, and in particular to a pickling and production method of a low-temperature coiled hot-rolled dual-phase steel with no pits or pits on the surface. Background Art

[0002] Since its advent in the 1960s, duplex steel has been widely used in the production of automobile wheels due to its good strength-toughness ratio, low yield strength ratio (Rp0.2 / Rm), high elongation, high work hardening rate, no yield phenomenon, good forming performance and small rebound after forming. High-grade hot-rolled pickled duplex steel for automobiles has strict requirements not only on stamping performance, but also on surface quality. As an appearance part, wheels require zero defects on the surface after painting. However, painting has an amplifying effect on the defects of the pickled surface. After pickling, the hot-rolled duplex steel should have no defects such as residual iron oxide scale, pits, pitting, rust, etc., which puts higher requirements on the pickling and production process of hot-rolled duplex steel.

[0003] At present, the production and manufacturing of hot-rolled dual-phase steel mainly includes two technical routes, low-temperature coiling and medium-high temperature coiling. Among them, the medium-high temperature coiling type hot-rolled dual-phase steel mainly relies on stable austenite elements (such as Cr, Mo, etc.) to improve hardenability, and the steel coil is controlled in the austenite metastable phase-free transformation zone for coiling. Although this process has good surface quality, the alloy cost is relatively high. The low-temperature coiling type hot-rolled dual-phase steel adopts the composition design of C-Si-Mn-Cr, the coiling temperature is ≤150℃, and the strip completes the transformation of ferrite and martensite structure after final rolling and before coiling. Low-temperature coiling type dual-phase steel has the advantages of low alloy design content and cost economy, and is the mainstream production method of steel mills at present. For low-temperature coiling type dual-phase steel, since the strip needs to be cooled to a low temperature, the Leidenfrost effect exists on the surface of the strip, which has a great influence on the temperature of the strip surface and cannot be accurately controlled. In the actual production process, the coiling temperature is generally below 100℃. However, when the coiling temperature is low, the cooling water on the strip surface cannot be evaporated and is prone to rust. The iron scale structure is mainly FeO and FeO·(H2O) on the surface. This type of iron scale is loose and porous and easy to fall off. If the pickling process is not appropriate, it is easy to produce pitting and other defects on the strip surface, thereby reducing the competitiveness of the product. Therefore, in order to solve the problem of high surface quality requirements for low-cost pickled duplex steel with low temperature coiling, it is urgent to develop a new duplex steel production process and method with no pitting and pitting defects on the surface.

[0004] Chinese invention patent CN105256323B provides a continuous pickling method for hot-rolled dual-phase steel with various chemical compositions, which includes hot-rolled raw material uncoiling, laser welding, phosphorus removal in the straightening unit, turbulent pickling, rinsing and online leveling. In the phosphorus removal process of the straightening unit, the straightening elongation of the straightening unit is controlled at 1.0%-1.6%. In the turbulent pickling process, a single acid environment is used for pickling, the pickling temperature is 55-85°C, and the pickling is carried out in at least three stages. The pickling conditions of each stage are controlled as follows: during the first pickling process, Fe in the pickling tank 2+ Concentration ≤ 150g / L, the concentration of the monoacid is 50-120g / L; in the second pickling process, Fe 2+ Concentration ≤ 120g / L, the concentration of the monoacid is 60-160g / L; during the third pickling process, Fe 2+ The concentration is ≤80g / L, and the concentration of the monoacid is 80-200g / L. In the turbulent pickling process, the pickling speed is 40-90m / min. The rinsing process is carried out in at least three stages, and the temperature in the last stage of the rinsing tank is 40-50℃ and the pH value is ≥6.0. After rinsing, the conductivity of the solution in the rinsing tank is controlled to be ≤2.0us / cm. In the online leveling process, the rolling force of the leveling unit is controlled at 150-450 tons.

[0005] Chinese invention patent CN109112433B discloses a 590MPa grade cold-rolled dual-phase steel without surface streak defects and a production method. The composition of the steel plate is as follows by weight percentage: C: 0.050%-0.100%, Si: ≤0.25%, Mn: 1.20%-2.00%, P: ≤0.012%, S: ≤0.008%, Al: 0.05%-0.10%, Sb: 0.015%-0.050%, and the balance is Fe and other inevitable impurities. The production method of the steel plate includes smelting, continuous casting, hot charging heating, high-pressure water descaling and rough rolling, finishing rolling, laminar cooling, coiling, hot-rolled plate pickling and cold continuous rolling, continuous annealing, air cooling, and flat coiling. The specific process parameters are as follows: hot charging heating: the continuous casting billet is directly hot charged and sent to the heating furnace. The billet hot charging temperature is greater than 850°C, the furnace gas temperature in the preheating section is 900°C-1000°C, the preheating time is 20min-30min, the heating section temperature is 1230°C-1270°C, the heating time is 15min-25min, the soaking section temperature is 1230°C-1270°C, the soaking section insulation time is 70min-90min, and the furnace pressure is always in a slightly positive pressure state in the dynamic state; high-pressure water descaling and rough rolling Rolling process: After the continuous casting billet is output from the furnace, the rolling rhythm is controlled to roll a piece of steel billet within 1min-2min, and the high-pressure water is used for descaling for 4-6 times, and the nozzle pressure is 19MPa-21MPa; the front-stage laminar cooling is adopted, the coiling temperature is 600℃-650℃, and the thickness of the hot-rolled plate after rolling is 2.5mm-5.0mm; hot-rolled plate pickling and cold continuous rolling: turbulent hydrochloric acid pickling is adopted, the hydrochloric acid concentration is 15%-25%, the pickling temperature is 80℃-95℃, the strip is stretched and straightened, and the running speed is 50-70m / min; continuous annealing: the steel strip has a uniform heating temperature of 770℃-830℃ in the remote annealing furnace, a slow cooling temperature of 650℃-700℃, and a temperature of 260℃-290℃ after rapid cooling and over-aging. The 590MPa grade cold-rolled dual-phase steel produced by this method has a bright surface, no streak defects, good corrosion resistance and coating adhesion, and the mechanical properties of the steel plate are excellent.

[0006] Chinese invention patent CN110669913B discloses a hot-rolled pickled dual-phase steel for high-strength automobile wheels and a production method thereof. The specific production process is: slab continuous casting-heating-rolling-cooling-pickling. The steel contains C: 0.060%-0.090%, Si: 0.010%-0.090%, Mn: 1.10%-1.49%, P: 0.020%-0.040%, S≤0.003%, Als: 0.41%-0.59%, rare earth elements La+Ce: 0.020%-0.040%, and the rest is Fe and unavoidable impurities. During continuous casting of slabs, light reduction and electromagnetic stirring processes are used, and the thickness of the slab is 170-230mm; the slab is heated to 1180-1200℃ and kept warm for 1-2h; the reduction rate in the finishing rolling zone is >92%, the finishing rolling temperature is 760-820℃, and the constant rolling speed is 3.5-4.5m / s; after final rolling, the cooling mode of laminar cooling + ultra-fast cooling is adopted, the cooling rate of laminar cooling is 8-10℃ / s, and ultra-fast cooling is carried out after 640-660℃, and the ultra-fast cooling rate is >100℃ / s. After cooling the steel plate to ≤150℃, it is coiled and air-cooled to room temperature, and finally pickled. Before pickling, the steel plate is straightened with a straightening elongation of 0.2%-0.5%. The steel plate travel speed during pickling is 20-40m / min. The pickling process is tension pickling with a tension of 5-10KN. The tensile strength of the steel plate is 690-730MPa, the yield strength ratio is 0.55-0.59, and the elongation is ≥27.0%.

[0007] Chinese invention patent CN110000207A discloses a method for manufacturing high surface grade hot-rolled pickled dual-phase steel, which mainly includes the following steps: (1) controlling the heating temperature within a certain precision range; (2) performing full-pass rough rolling to remove scale and rolling at a specified speed for each pass; (3) controlling the coiling temperature according to a two-stage cooling temperature control, and controlling the coiling temperature, coiling tension, and side guide plate pressure within the required range; (4) sending the steel to the pickling process within the specified time range after the coiling is completed; (5) adopting a pickling and leveling process to ensure the pickling speed, corrosion inhibitor concentration, and drying speed. The dry distance, temperature and leveling reduction ratio are within the required range. The specific process parameters are as follows: the heating temperature is selected between 1230-1320℃, the heating time is ≥200 minutes, and the heating temperature is selected within the appropriate range to ensure that the alloy elements containing Mn and Cr form Mn and Cr alloy compounds on the surface of the slab during the heating process, and maintain easy peeling from the matrix within a certain temperature range and control the amount of generation; the rough rolling passes of each stand are 1+5 passes, and descaling is selected between the stands at each pass, the descaling passes are ≥5 passes, and the descaling impact force is maintained at ≥0.9N / mm 2 The rolling speed of each rough rolling pass is selected as R1: 1.5-2.2m / s, R 2-1 :2.3-3.5m / s, R 2-2:3.6-4.5m / s, R 2-3 :4.3-5.0m / s, R 2-4 :5.0-5.3, R 2-5 :5.2-5.5m / s; the rolling speed of each stand is more than 10% higher than the conventional rolling speed, which can ensure that the rolling time of each stand is shortened by more than 30s. The coiling temperature is controlled at ≤300℃, and a two-stage cooling process is adopted. The cooling temperature of about 1 / 2 of the total length of the plate and strip head is controlled at about 150℃, and the temperature of about 1 / 2 of the total length of the plate and strip tail is controlled within 300℃. The cooling water pressure is maintained above 0.35MPa, the coiling tension is controlled at 12-19MPa, and the coiling side guide pressure is controlled in the range of 6-12KN to ensure that the strip has a certain temperature after coiling, reduce the generation of Fe3O4 and the foreign matter falling into the surface and scratch defects during the coiling process. The pickling time after coiling is controlled within ≤48 hours, and the pickling process adopts pickling plus leveling process: the pickling speed is controlled at 60-120m / s, the corrosion inhibitor concentration is controlled at 10%-20%, the length of the drying section after pickling is controlled at 2-4 meters, and the drying temperature is ≥100℃. The leveling reduction rate is controlled at 0.3%-2%, and different leveling reduction rates are selected according to the different thickness specifications of the product.

[0008] Chinese invention patent CN102337547B discloses a pickling method for hot-rolled duplex stainless steel plate, which adopts double mixed acid pickling. First, the surface of the steel plate is shot blasted, and then the steel plate is sent to the pre-water washing section via a roller. After the steel plate is washed with water, it enters the pre-pickling section for pre-pickling. In the pre-pickling and final pickling sections, HNO3+HF mixed solution is used for pickling. In the pre-pickling stage, the pre-pickling solution is: HNO3, 140-180g / L; HF, 80-100g / L; metal ion content in the acid: 5-40g / L; in the final In the final pickling stage, the final pickling solution is: HNO3, 200-280g / L; HF, 55-75g / L; the metal ion content in the acid is 5-40g / L; the running speed of the steel plate is maintained at 1-3m / min, the acid solution temperature is controlled at 45±5℃, the steel plate directly enters the water washing section after the pre-pickling section, and the steel plate enters the final pickling section for pickling after the water washing section. The running speed of the steel plate is maintained at 1-3m / min, the acid solution temperature is controlled at 45±5℃, and the time for the steel plate to go through the pre-pickling and final pickling process sections is 11.3-34 minutes. After the steel plate exits the final pickling section, it is finally washed with water, then purged and dried by air curtain to complete the pickling.

[0009] A careful analysis and comparison show that the above patented technologies have the following problems and shortcomings: (1) These solutions are mainly considered from the perspective of steel grade development. The proposed hot rolling, pickling and cold rolling process requirements do not mention the pitting and pitting defects of the strip during the pickling process. Therefore, it is impossible to guarantee that there will be no pitting and pitting defects on the surface of the duplex steel after pickling; (2) Due to the special process of low-temperature coiling of economical duplex steel, the iron oxide structure on the surface of the strip is significantly different from that of other steel grades. Therefore, it is not possible to simply refer to the pickling process parameters of other steel grades. The existing patented technologies also do not involve the correspondence between the iron oxide structure and the pickling process parameters.

[0010] Therefore, when dual-phase steel is pickled online, it is necessary to formulate a targeted pickling process in combination with its particularity. The present invention mainly focuses on low-temperature coiling economical dual-phase steel, analyzes the influence of pickling process parameters such as straightening, bending rolls, and straightening before pickling on surface pitting and pits, and provides a production method for high surface quality pickled dual-phase steel without pitting and pitting defects. Summary of the invention

[0011] The purpose of the present invention is to overcome the above-mentioned problems existing in the prior art. According to the production characteristics of low-temperature coiling economical dual-phase steel, combined with the actual situation that the structure of the iron oxide scale on the surface of the hot-rolled base material is mainly inner layer FeO + outer layer Fe2O3 and the iron scale is loose and easy to fall off, by controlling the process parameters such as straightening, lifting and bending rolls, pulling and straightening, and pickling speed, the shedding of iron scale on the surface of the steel plate is reduced to the greatest extent. At the same time, the corresponding relationship between the amount of iron scale shedding on the roller and the pitting and pitting defects is established, and the optimal pickling production process of low-temperature coiling economical dual-phase steel is determined. Finally, the pickled dual-phase steel products with no pitting and pitting defects on the surface are obtained in batches.

[0012] A method for pickling and producing hot-rolled dual-phase steel with no pits and pockmarks on the surface, the method comprising the following steps: hot-rolled steel plates are coiled to obtain steel coils through low-temperature coiling, the steel coils are uncoiled and sequentially passed through a straightening machine, a lifting and bending roller, a tension and straightening unit for descaling, pickling, rinsing, and then coiled again, finally obtaining hot-rolled dual-phase steel with no pits and pockmarks on the surface.

[0013] Furthermore, the low-temperature coiling temperature of the hot-rolled steel plate is ≤150°C, preferably 50-100°C.

[0014] Furthermore, the steel coil obtained by low temperature coiling does not need to be leveled and can directly enter the pickling process within 24 hours.

[0015] Furthermore, before unwinding the steel coil, the straightening rollers, bending rollers and straightening rollers in the pickling process need to be cleaned to ensure that the cleanliness of each roller surface is ≤ Grade II, that is, the amount of iron sheet falling off the roller surface is ≤3mg / cm 2 Cleaning each roller in the pickling process can effectively reduce the physical pressure pits formed on the strip surface due to the accumulation of iron scale on the roller surface.

[0016] Furthermore, after the steel coil is uncoiled, the straightening unit is not put into use, and the upper straightening roller is lifted to a high position to ensure that the strip surface does not contact the roller surface of the upper straightening roller. The magnitude of the straightening force is optimized and set with reference to formula (1).

[0017] Furthermore, the distance between the upper straightening roller and the strip surface after it is lifted is ≥8mm.

[0018] Furthermore, the bending rollers are not used after the steel coil is uncoiled, and no pressure is applied when the strip passes through, thereby reducing further shedding of the iron sheet due to the applied pressure.

[0019] Furthermore, when the strip enters the tensioning and leveling unit for descaling, the tensioning rate is ≤0.2%, and the strip tension is 100-150KN (refer to formula (2) for optimization setting). The optimization and setting of tensioning and leveling parameters are the key. Reasonable process parameters can prevent the large-scale shedding of the oxide scale on the surface of the strip and avoid defects such as pitting and pitting on the surface.

[0020] Furthermore, the pickling is carried out in a three-stage turbulent pickling tank.

[0021] Furthermore, the pickling temperature of the first stage is 70-80°C, the Fe ion concentration is 60-100g / L, and the acid concentration is 100-120g / L; the pickling temperature of the second stage is 70-80°C, the Fe ion concentration is 40-80g / L, and the acid concentration is 100-140g / L; the pickling temperature of the third stage is 70-80°C, the Fe ion concentration is 20-60g / L, and the acid concentration is 120-180g / L; the pickling speed is 110-160m / min.

[0022] Furthermore, after pickling, three-stage rinsing is performed, and deionized water is used in the rinsing stage, and the conductivity is ≤10um / cm; after rinsing, the strip is surface dried at a drying temperature of 105-130°C.

[0023] Furthermore, the surface quality of the strip steel needs to be monitored online during the entire pickling process. When pits or pits with a depth of ≥10um appear on the surface, the detached iron scale accumulated on the roller surface of the straightening unit, the lifting and bending roller surface, and the pull-leveling and descaling unit roller surface of the pickling process is cleaned again. The cleanliness of each roller surface after cleaning is ≤ Grade II. After the roller surface is cleaned, the pickling of the strip steel is continued with reference to the above process flow.

[0024] The present invention mainly solves two major problems faced by existing hot-rolled dual-phase steel: first, the cooling rate of the economical dual-phase steel in the laminar flow stage is fast and the coiling temperature is low during low-temperature coiling (coiling temperature ≤ 150°C), and the high-temperature oxidized iron scale structure does not undergo phase transformation in time and is retained at room temperature. The iron scale structure of the dual-phase steel at room temperature is mainly inner layer FeO + outer layer Fe2O3. This iron scale is loose and easy to fall off. Due to the particularity of the process, it is impossible to improve the iron scale structure from the perspective of adjusting the process; second, due to the different iron scale structures of the dual-phase steel, the pickling process of other steel grades cannot be referred to. If inappropriate pickling process parameters such as straightening, lifting and bending rolls and pulling and straightening are used, the iron scale on the surface of the strip will further fall off and accumulate on the roller, thereby forming physically pressed pitting and pitting defects. The difficulty of the present invention lies in how to control the pickling process parameters such as straightening, bending roll lifting and tensioning, establish the corresponding relationship between the amount of iron oxide scale shedding, pickling process, pitting and pitting, and determine the optimal pickling production process for low-temperature coiling economical dual-phase steel, so as to obtain a pickled dual-phase steel product with no pitting and pitting defects on the surface.

[0025] Further research found that the inner layer FeO + outer layer Fe2O3 iron sheet thickness of the low-temperature coiled economical dual-phase steel surface is about 10-15um, the tensile strength of the inner layer FeO at room temperature is ≤10MPa and the hardness is 300HV, the outer layer Fe2O3 is basically non-plastic and the hardness reaches 1000HV. The organization of this dual-phase steel is soft phase organization F + hard phase organization M, where the surface hardness of the soft phase organization is HB F (Surface hardness of ferrite) is relatively low, so in order to prevent pitting and pitting of duplex steel during pickling, the following conditions must be met: the pressure per unit area of ​​the strip surface P 表面单位面积 <Surface hardness of ferrite HB F The pressure P per unit area on the strip surface can be calculated according to the following formula: 表面单位面积 :

[0026] P 表面单位面积 =F2 / M (1)

[0027] Among them: F2 is the straightening force of the pickling process, unit KN; M is the amount of iron scale falling off the strip surface, unit kg.

[0028] The amount of iron scale falling off the strip surface M is related to the tensile strength P of the strip cross section. 截 Related, P 截 The calculation formula is as follows:

[0029] P 截 =F1 / (b*t) (2)

[0030] Among them, F1 is the tension of the strip during pickling, unit KN; t is the thickness of the strip, unit mm; b is the width of the strip, unit mm.

[0031] When the strip section is subjected to tensile strength P 截 When the tensile strength of the inner layer FeO is greater than or equal to 10 MPa, the iron sheet will break and fall off. According to formulas (1)-(2), the optimal pickling process parameters such as straightening, bending rolls and straightening during pickling of the strip can be accurately obtained, and the corresponding relationship between the amount of iron oxide scale falling off, pickling process, pitting and pitting is established. The optimal pickling process parameters set accordingly can ensure that the surface of the duplex steel is free of pitting and pitting defects.

[0032] Compared with the existing similar technologies, the process of the present invention has the following significant advantages:

[0033] (1) Various problems in the pickling process of low-temperature coiling economical duplex steel were solved, and pickled duplex steel products with no pitting or pitting defects on the surface were successfully mass-produced, meeting the high-quality requirements of automotive customers;

[0034] (2) Based on the production characteristics of low-temperature coiling economical dual-phase steel and the structural characteristics of the inner layer FeO + outer layer Fe2O3 of the oxide scale on the surface of the hot-rolled base material, the corresponding relationship between the amount of scale shedding on the roller and the pitting and pitting defects was proposed and established for the first time. The core and key process parameters such as straightening, bending roll, tension leveling, and pickling speed were optimized and regulated to minimize the shedding of scale on the strip surface, and the optimal pickling production process for low-temperature coiling economical dual-phase steel was determined.

[0035] (3) The method of the present invention is simple to operate, flexible, low-cost, and easy to promote. Different process parameters can be set according to the surface iron sheet shedding situation, while ensuring that the pickling surface is free of pitting and pitting defects without reducing the pickling production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a photo of the surface quality of a low temperature coiled hot rolled dual phase steel product;

[0037] Figure 2 This is a photo of the iron scale structure and thickness on the surface of low-temperature coiled hot-rolled dual-phase steel;

[0038] Figure 3 The actual surface pits and cross-sectional microscopic morphology photos of the hot-rolled dual-phase steel strip after pickling produced by the original process;

[0039] Figure 4 This is the 3D profile of the pits on the surface of the hot-rolled dual-phase steel strip after pickling produced by the original process;

[0040] Figure 5 The following are photos of the surface quality and cross-sectional microstructure of the hot-rolled dual-phase steel produced by the new process of the present invention after pickling. DETAILED DESCRIPTION

[0041] In order to enable ordinary technicians in the field to fully understand the technical solutions and beneficial effects of the present invention, further description is given below in conjunction with specific embodiments and drawings.

[0042] Example 1

[0043] A dual-phase steel pickling process and production method with no pitting or pitting defects on the surface, the specific steps are as follows: the pickling process includes hot-rolled raw material uncoiling, straightening machine, lifting and bending rolls, tension and leveling unit scale breaking, three-stage turbulent pickling, rinsing, and coiling.

[0044] 1. Low-temperature coiled economical duplex steel (coiling temperature is set at 100°C) produced by hot rolling does not need to be leveled. Due to the low coiling temperature, the strip surface will have residual water and needs to be pickled within 24 hours to reduce the probability of electrochemical corrosion on the surface.

[0045] 2. Before unwinding the hot-rolled coil, first clean the straightening rolls, bending rolls and straightening rolls in the pickling process to ensure that the cleanliness of each roll surface is level II.

[0046] 3. After the steel coil is uncoiled, the straightening unit is not put into use. The upper straightening roller is lifted to a high position so that the distance between it and the surface of the strip is 8 mm to ensure that the surface of the strip does not contact the surface of the upper straightening roller.

[0047] 4. At the same time, the lifting and bending rollers are not put into use, and no pressure is applied when the strip passes through to prevent the iron sheet from falling off in this process.

[0048] 5. When the strip enters the tension and leveling unit for descaling, the setting of tension and leveling parameters is the key. In order to prevent the large-scale shedding of the oxide scale on the surface of the strip, the tension and leveling rate is required to be 0.2% and the strip tension is 100KN.

[0049] 6. The strip enters the three-stage turbulent pickling tank for pickling. The first pickling temperature is 70°C, the Fe ion concentration is 70g / L, and the acid concentration is 100g / L; the second pickling temperature is 75°C, the Fe ion concentration is 60g / L, and the acid concentration is 100g / L; the third pickling temperature is 75°C, the Fe ion concentration is 40g / L, and the acid concentration is 150g / L. The pickling speed is controlled at 120m / min, and the pickled strip is rinsed in three stages. Deionized water is used in the rinsing section, and the conductivity is 10um / cm. After rinsing, the surface is dried at a drying temperature of 110°C.

[0050] 7. During the production process, according to the online detection results of the pickling surface, when pits or pits with a depth of 10um appear on the surface of the dual-phase steel, it is necessary to clean the detached iron scale accumulated on the roller surface of the straightening unit, the lifting and bending roller surface, and the pull-leveling and descaling unit roller surface in the pickling process. After cleaning, the cleanliness of each roller surface is level II. After the roller surface is cleaned, continue to pickle the strip according to step 3 and its subsequent steps.

[0051] Example 2

[0052] A dual-phase steel pickling process and production method with no pitting or pitting defects on the surface, the specific steps are as follows: the pickling process includes hot-rolled raw material uncoiling, straightening machine, lifting and bending rolls, tension and leveling unit scale breaking, three-stage turbulent pickling, rinsing, and coiling.

[0053] 1. After hot rolling, the low-temperature coiled economical duplex steel (coiling temperature is set at 80°C) does not need to be leveled and is ready for pickling within 18 hours.

[0054] 2. Before unwinding the hot-rolled coil, first clean the straightening rolls, bending rolls and straightening rolls in the pickling process to ensure that the cleanliness of each roll surface is level II.

[0055] 3. After the steel coil is uncoiled, the straightening unit is not put into use. The upper straightening roller is lifted to a high position so that the distance between it and the surface of the strip is 10 mm to ensure that the surface of the strip does not contact the surface of the upper straightening roller.

[0056] 4. At the same time, the lifting and bending rollers are not put into use, and no pressure is applied when the strip passes through to prevent the iron sheet from falling off in this process.

[0057] 5. When the strip enters the tensioning and straightening unit for descaling, the tensioning and straightening rate is required to be 0.1% and the strip tension is 120KN.

[0058] 6. The strip enters the three-stage turbulent pickling tank for pickling. The pickling temperature of the first stage is 75°C, the Fe ion concentration is 80g / L, and the acid concentration is 120g / L; the pickling temperature of the second stage is 75°C, the Fe ion concentration is 80g / L, and the acid concentration is 110g / L; the pickling temperature of the third stage is 75°C, the Fe ion concentration is 60g / L, and the acid concentration is 150g / L. The pickling speed is controlled at 140m / min, and the pickled strip is rinsed in three stages. Deionized water is used in the rinsing stage, and the conductivity is 8um / cm. After rinsing, the surface is dried at a drying temperature of 120°C.

[0059] 7. During the production process, according to the online detection results of the pickling surface, when pits or pits with a depth of 12um appear on the surface of the dual-phase steel, the accumulated iron scale on the roller surface of the straightening unit, the roller surface of the lifting and bending roller, and the roller surface of the tension and descaling unit in the pickling process needs to be cleaned. After cleaning, the cleanliness of each roller surface is Class I. After the roller surface is cleaned, continue to pickle the strip according to step 3 and its subsequent steps.

[0060] Example 3

[0061] A dual-phase steel pickling process and production method with no pitting or pitting defects on the surface, the specific steps are as follows: the pickling process includes hot-rolled raw material uncoiling, straightening machine, lifting and bending rolls, tension and leveling unit scale breaking, three-stage turbulent pickling, rinsing, and coiling.

[0062] 1. After hot rolling, the low-temperature coiled economical duplex steel (coiling temperature is set at 50°C) does not need to be leveled and is ready for pickling within 12 hours.

[0063] 2. Before unwinding the hot-rolled coil, first clean the straightening rolls, bending rolls and straightening rolls in the pickling process to ensure that the cleanliness of each roll surface is level I.

[0064] 3. After the steel coil is uncoiled, the straightening unit is not put into use. The upper straightening roller is lifted to a high position so that the distance between it and the surface of the strip is 10 mm to ensure that the surface of the strip does not contact the surface of the upper straightening roller.

[0065] 4. At the same time, the lifting and bending rollers are not put into use, and no pressure is applied when the strip passes through to prevent the iron sheet from falling off in this process.

[0066] 5. When the strip enters the tensioning and straightening unit for descaling, the tensioning and straightening rate is required to be 0.2% and the strip tension is 120KN.

[0067] 6. The strip enters the three-stage turbulent pickling tank for pickling. The first pickling temperature is 75°C, the Fe ion concentration is 80g / L, and the acid concentration is 100g / L; the second pickling temperature is 75°C, the Fe ion concentration is 60g / L, and the acid concentration is 110g / L; the third pickling temperature is 80°C, the Fe ion concentration is 40g / L, and the acid concentration is 160g / L. The pickling speed is controlled at 150m / min, and the pickled strip is rinsed in three stages. Deionized water is used in the rinsing section, and the conductivity is required to be 6um / cm. After rinsing, the surface is dried at a drying temperature of 125°C.

[0068] 7. During the production process, according to the online detection results of the pickling surface, when pits or pits with a depth of 15um appear on the surface of the dual-phase steel, it is necessary to clean the detached iron scale accumulated on the roller surface of the straightening unit, the lifting and bending roller surface, and the pull-leveling and descaling unit roller surface in the pickling process. After cleaning, the cleanliness of each roller surface is level II. After the roller surface is cleaned, continue to pickle the strip according to step 3 and its subsequent steps.

[0069] Comparative Example 1 (Original Process)

[0070] The hot-rolled dual-phase steel of the same composition is coiled at low temperature (CT: 180±20℃). In this temperature range, the cooling of the strip surface has the Leidenfrost effect, which cannot be precisely controlled. The temperature fluctuates greatly, causing the strip to have certain waves. Therefore, the complete pickling and production steps are as follows: uncoiling, leveling, re-coiling, uncoiling, straightening machine, lifting and bending roller, tension and leveling unit descaling, three-stage turbulent pickling, rinsing, and coiling.

[0071] After hot rolling, the low-temperature coiled duplex steel is put on the leveling machine according to the leveling plan (generally after 5 days of rolling) to eliminate the flatness and improve the surface quality. The rolling force of the leveling machine is 200-300 tons to effectively improve the flatness, but too large a rolling force will cause physical pits to form on the surface of the strip.

[0072] After the steel coil is flattened, it is sent to the pickling line for pickling according to the transportation plan (usually 2 days). When the steel coil is pickled, in order to ensure the quality of the plate shape, the straightener, bending roller, and straightening unit are all put into use. The straightener is 100-200t, and the straightening rate is ≥0.5%, which further increases the incidence of pits formed by the iron sheet on the surface of the strip being pressed into the matrix. When the strip enters the three-stage turbulent pickling tank for pickling. There are no special requirements for the pickling temperature, Fe ion concentration, acid concentration, and pickling speed of each section.

[0073] After pickling, the steel coils are blocked and released according to the test results of the pickling surface inspection. At the same time, if the surface pits and pits are serious, the steel coils will not be arranged for pickling in the future, which seriously affects the production efficiency and quality of hot-rolled dual-phase steel.

[0074] Figure 1-2 This is the observation of low temperature coiled hot rolled dual phase steel before pickling, which is a description of the incoming material. Figure 1 It shows that after adopting the low-temperature coiling process, watermarks appear on the uncoiled surface of the hot-rolled steel coil; Figure 2 It shows that after adopting the low-temperature coiling process, the iron oxide scale on the surface of the strip is mainly FeO, with a thickness of 7-10um.

[0075] Figure 3-4 The actual quality and depth of pits of the hot-rolled dual-phase steel after pickling produced by the original process. As can be seen from the figure, there are obvious spots and pits of different sizes on the surface of the product produced by the original process, which does not meet the customer's quality requirements.

[0076] Figure 5 The actual quality and cross-sectional morphology of the hot-rolled dual-phase steel after pickling produced by the new process of the present invention. As can be seen from the figure, no pits or pits are found on the surface of the product produced by the new process. Compared with the original process, the surface quality of the product produced by the new process is significantly improved.

Claims

1. A method for producing low temperature coiling hot rolled dual phase steel with no pits and pits on the surface, characterized in that: The method comprises the following steps: hot-rolled steel sheets are coiled at a low temperature of ≤150°C to obtain steel coils, the steel coils are uncoiled and successively passed through a straightening machine, a lifting and bending roller, a tension and straightening unit for descaling, pickling, rinsing, and then coiled again, and finally hot-rolled dual-phase steel with no pits and pits on the surface is obtained, and the pits and pits on the surface of the hot-rolled dual-phase steel have a depth of less than 10um; the dual-phase steel must meet the following conditions in order to produce no pits and pits during the pickling process: the pressure P per unit area on the surface of the strip steel 表面单位面积 <Surface hardness of ferrite HB F , the pressure P 表面单位面积 =F2 / M, F2 is the straightening force of the pickling process, unit KN; M is the amount of iron scale falling off the strip surface, unit kg; the amount of iron scale falling off the strip surface M is related to the tensile strength P of the strip section 截 Related, P 截 =F1 / (b*t), F1 is the tension of the strip during pickling, unit KN; t is the thickness of the strip, unit mm; b is the width of the strip, unit mm; when the strip section is subjected to tensile strength P 截 ≥When the maximum tensile strength of the inner layer FeO is 10MPa, the iron sheet will break and fall off; before the steel coil is unrolled, the straightening rollers, bending rollers and straightening rollers in the pickling process must be cleaned to ensure that the cleanliness of each roller surface is ≤ Grade II; after the steel coil is unrolled, the straightening unit is not put into use, and the upper straightening roller is lifted to a high position so that the distance between it and the strip surface is ≥8mm to ensure that the strip surface does not contact the upper straightening roller surface; after the steel coil is unrolled, the bending roller is not put into use, and no pressure is applied when the strip passes; when the strip enters the straightening unit and breaks the scale, the straightening rate is ≤0.2%, and the strip The tension is 100-150KN; the pickling is carried out in a three-stage turbulent pickling tank, the first stage pickling temperature is 70-80°C, the Fe ion concentration is 60-100g / L, and the acid concentration is 100-120g / L; the second stage pickling temperature is 70-80°C, the Fe ion concentration is 40-80g / L, and the acid concentration is 100-140g / L; the third stage pickling temperature is 70-80°C, the Fe ion concentration is 20-60g / L, and the acid concentration is 120-180g / L; the pickling speed is 110-160m / min.

2. The method according to claim 1, characterized in that: The steel coils obtained by low-temperature coiling do not need to be flattened and can be pickled directly within 24 hours.

3. The method according to claim 1, characterized in that: After pickling, three-stage rinsing is carried out, and deionized water is used in the rinsing stage, and the conductivity is ≤10um / cm; after rinsing, the strip is surface dried at a drying temperature of 105-130℃.

4. The method according to claim 1, characterized in that: The surface quality of the strip needs to be monitored during the entire pickling process. When pits or pockmarks with a depth of ≥10um appear on the surface, the roller surfaces of the straightening unit, the lifting and bending rollers, and the tensioning and descaling unit in the pickling process should be cleaned again to ensure that the cleanliness of each roller surface is ≤Level II. After the roller surface cleaning is completed, the strip pickling can continue.

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