Method for controlling scale defects of hot-rolled high-strength pickled sheet

Abstract

The present application relates to a kind of methods for controlling hot-rolled high-strength pickled plate scale defects, including using descaling impact test method, checking each trace complete on test special slab, lap is not superimposed, and the gap between impact mark is ≤2mm;Production plan is from at least the 10th block in cycle, and no more than the 55th block of high-strength pickled plate is arranged;Control the sum of high-strength pickled plate second heating section and soaking section time is 100-180min;High-strength pickled plate rolling time interval is controlled at 140-210s;High-strength pickled plate is descaled in all directions, at least one pass descaling water is arranged when rough rolling;The use cycle of finishing mill work roll is less than or equal to 2 times;Increase finishing rolling lubricating oil flow;The present application greatly reduces the residual amount of hot-rolled high-strength pickled plate scale, thereby improving the surface quality of high-strength hot-rolled pickled plate, and providing strong guarantee for subsequent pickling to improve surface quality.

Description

Technical Field

[0001] This invention relates to the field of steel rolling technology, and in particular to a method for controlling iron oxide scale defects in hot-rolled high-strength pickled steel sheets. Background Technology

[0002] Pickled steel sheets are intermediate products made from high-quality hot-rolled steel coils. After the oxide layer is removed, the edges are trimmed, and the sheets are finished using a pickling unit, the surface quality and usage requirements are between those of hot-rolled and cold-rolled steel sheets. They are ideal substitutes for some cold-rolled steel sheets. High-strength pickled steel sheets refer to those with a yield strength of 420MPa or higher, mainly including the SMC series, QSTE series, and MP1500 series. Pickled steel sheets have very high requirements for surface quality, requiring a smooth surface without visible defects such as residual iron oxide scale. Otherwise, it will seriously affect subsequent stamping and painting, causing economic losses.

[0003] Iron oxide scale defects are a common surface defect formed during the hot rolling process of pickled steel sheets. They mainly include iron oxide scale indentation and pitting. When the iron oxide scale formed on the surface of the rolled piece is not completely removed, it is pressed into the surface of the strip steel during rolling. After pickling, it will form defects resembling landscape paintings, or pitting left after the iron oxide scale is washed off. From the root cause, iron oxide scale is mainly caused by the following three aspects: First, improper billet heating time and temperature, and excessively strong oxidizing atmosphere in the furnace, resulting in excessively thick iron oxide scale on the primary oxidation surface. Second, abnormal operation of the descaling device during hot rolling, such as descaling angle deviation, delayed water spraying in the descaling box, or blockage and leakage of descaling water nozzles, will result in secondary iron oxide scale indentation. Third, the oxide film on the surface of the hot rolling rolls may peel off, and the height of the plate or roller table may be unsuitable, which can easily cause scratches on the surface of the strip steel, increase roughness, and promote the indentation of iron oxide scale.

[0004] High-strength pickled steel sheets generally contain higher levels of silicon (Si) than low-strength pickled steel sheets, typically several to tens of times higher. The root cause of residual iron oxide scale defects on the surface of pickled steel sheets with higher Si content after pickling is the selective oxidation of Si during prolonged high-temperature heating of the slab. This oxidation forms the ferroolitic phase 2FeO·SiO2 (also known as ferrous silicate Fe2SiO4) at the interface between FeO and the steel matrix. This phase has a low melting point (1170℃) and, once molten, will infiltrate the steel in a wedge-shaped manner. When the temperature is below 1170℃, Fe2SiO4 solidifies and bonds tightly to the steel matrix, forming a complex and special scale layer at the interface between the scale and the steel matrix. This scale layer is difficult to remove completely during descaling. The surface of the high-strength pickled plate produced by Ansteel's hot-rolled 1780 line is relatively rough. After pickling, spindle-shaped, rice-grain-shaped, short strip-shaped, or long strip-shaped marks often form on the surface. The reversal rate of iron oxide scale defects reaches 8% of its total production, causing huge losses to the next process, cold rolling.

[0005] In patent CN102169063A, "Rapid Detection Method for Iron Oxide Scale Defects on the Surface of Hot-Rolled Pickled Steel Plates," a sample plate of 150×150mm is first cut from one-third of the steel plate's edge using a shearing machine. The center of each sample plate is 170mm from the edge of the steel plate. 31% hydrochloric acid at room temperature is added to an enamel-lined container, and the container is heated in an electric furnace for 5-10 minutes until the acid temperature reaches 70-80℃. The cut samples are then placed in the enamel-lined container for pickling for 1 minute. This method can quickly and effectively detect iron oxide scale defects on the surface of hot-rolled pickled steel plates and is convenient to use. However, it can only be used for detecting iron oxide scale defects on hot-rolled pickled steel plates and cannot control the iron oxide scale on the surface of hot-rolled pickled steel plates.

[0006] The literature "Analysis of Causes and Control Measures for Iron Oxide Scale Defects in Hot-Rolled Pickled Steel Plates (Rolling Steel, 2020, 37(06):95-99)" analyzes and investigates the influencing factors in the hot rolling process to identify sheet-like, strip-like, landscape-pattern-like, and rough-edged iron oxide scale defects that hot-rolled steel plate inspection instruments cannot effectively identify, and obtains the causes of the defects. Repeated heating of slabs in the furnace will increase the amount of iron oxide scale generated, easily leading to sheet-like iron oxide scale defects after pickling. When the descaling water pressure, spray angle, spray surface overlap, and descaling passes are insufficient in their ability to remove secondary iron oxide scale, strip-shaped and landscape-like iron oxide scale defects are easily generated after pickling. At the same time, long rolling distances of roughing work rolls and excessively high intermediate billet temperatures can also have a certain impact on landscape-like iron oxide scale defects. When the final rolling or coiling temperature of hot-rolled strip is high and the sheet shape of thin strip is poor, rough iron oxide scale defects will be caused on the edges of the strip after pickling. To address this, the slab heating process, roughing and descaling processes, and finishing and laminar cooling processes have been optimized, which has greatly reduced the incidence of iron oxide scale defects in pickled plates and improved the surface quality of the products. However, although the causes of iron oxide scale defects are described in detail, the specific process optimization process and control measures are not disclosed, so the problem of iron oxide scale defects in hot-rolled pickled plates cannot be effectively solved. Summary of the Invention

[0007] This invention provides a method for controlling iron oxide scale defects in hot-rolled high-strength pickled steel sheets. Under the premise of ensuring that the descaling water system is fault-free, the residual amount of iron oxide scale in hot-rolled high-strength pickled steel sheets is greatly reduced by increasing the flow rate of lubricating oil in the finishing mill, controlling the production schedule, heating time, furnace exit temperature, rolling time interval, number of descaling water input passes, and the material and service life of the rolls, thereby improving the surface quality of the hot-rolled pickled steel sheets.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for controlling iron oxide scale defects in hot-rolled high-strength pickled steel sheets includes the following:

[0010] 1) using the method of descaling impact test, check the test special slab on each trace complete, lap without superimposed, and the impact mark gap ≤2mm;

[0011] 2) production plan is from at least the 10th block in the cycle to arrange production high strength pickling plate, no more than 55 blocks;

[0012] 3) control high strength pickling plate two heating section and soaking section time sum is 100-180min;

[0013] 4) high strength pickling plate rolling time interval control in 140-210s;

[0014] 5) to high strength pickling plate all-round descaling, when arranging at least one pass descaling water for pickling plate rough rolling;

[0015] 6) the service life of the work roll of finishing mill is less than or equal to 2 times;

[0016] 7) increase the rolling lubricating oil flow of finishing mill, the rolling lubricating oil flow of the first finishing mill is controlled to be 40%-45% of the maximum flow, the rolling lubricating oil flow of the second to fourth finishing mill is controlled to be 60%-65% of the maximum flow, and the rolling lubricating oil flow of the fifth finishing mill is controlled to be 70%-75% of the maximum flow.

[0017] Further, in the 3) for high strength pickling plate slab heating, control the atmosphere in the two heating section and soaking section heating furnace to be oxidizing atmosphere, the actual air-fuel ratio of the two heating section and the soaking section is 2.1-2.2, when the slab is discharged, the discharge temperature and the target value error is controlled to be between-10-30℃.

[0018] Further, the rolling time interval is controlled from the first three slabs of high strength pickling plate.

[0019] Further, when 1.8mm≤high strength hot rolled pickling plate thickness≤2.1mm, the rough rolling mill inlet is opened 1-2 passes of descaling water, 2.1mm<high strength hot rolled pickling plate thickness≤2.6mm, and the width of high strength hot rolled pickling plate≥1360mm, the rough rolling mill inlet is opened at least 2 passes of descaling water.

[0020] Compared with the prior art, the beneficial effects of the present application are:

[0021] 1) ensure that the descaling water system is fault-free, respectively through increasing the rolling lubricating oil flow of finishing mill, controlling the production plan, heating time, discharge temperature, rolling time interval, descaling water input pass, roll material and service life, greatly reducing the residual amount of iron oxide scale of hot rolled high strength pickling plate;

[0022] 2) greatly improve the hot-rolled high-strength pickling plate surface scale type problem, improve the hot-rolled pickling plate surface quality. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is the invention described by the scale removal water nozzle impact test plate mark qualified schematic diagram.

[0024] Figure 2 is the invention described by the hot-rolled high-strength pickling plate surface unevenness caused by incomplete scale removal water nozzle scale removal schematic diagram.

[0025] Figure 3 is the invention described by the improved heating process before the high-strength pickling plate with defects scale oxide layer composition and content schematic diagram.

[0026] Figure 4 is the invention described by the improved heating process after the high-strength pickling plate without defects scale oxide layer composition and content schematic diagram.

[0027] In the figure: 1. special plate blank for scale removal impact test 2. impact mark 3. scale removal water nozzle 4. high-strength pickling plate hot-rolled rolling piece 5. pit 6. protrusion DETAILED DESCRIPTION

[0028] The specific embodiments of the present application will be further described below in conjunction with the accompanying drawings:

[0029] A method for controlling the scale of hot-rolled high-strength pickling plate, comprising the following contents:

[0030] 1) Confirm that the scale removal water system is fault-free, and before the production cycle of hot-rolled high-strength pickling plate, confirm that the scale removal box before the width press, the scale removal box before the finishing rolling, the scale removal water system at the entrance and exit of the rough rolling, and the scale removal water system between the first four finishing rolling mills are fault-free;

[0031] See Figure 1 , the scale removal impact test is carried out on the special plate blank 1 for the scale removal water nozzle 3 of the hot-rolled production line, the impact mark 2 of the scale removal impact test needs to cover the entire transverse distance of the plate blank, the impact mark 2 is complete, the overlapping part is not superimposed, the gap is uniform and the gap is ≤2mm, if the impact mark 2 appears the phenomenon of poor overlapping, such as bending or uneven gap of the impact mark 2, the strip surface will be left with scale after rolling, which needs to be immediately adjusted to the permeability and angle of the rolling mill scale removal water nozzle 3 to eliminate the adverse effects;

[0032] See Figure 2, high strength pickling plate hot rolling piece 4 surface uneven caused by pickling water pickling incomplete schematic diagram, high strength pickling plate hot rolling piece 4 surface treatment before if there is pit 5 or have protrusions 6, pickling water pickling 3 will exist dead angle, cause high strength pickling plate hot rolling piece 4 surface iron oxide scale formation, therefore need to high strength pickling plate hot rolling piece 4 all-round pickling.

[0033] 2) high strength pickling plate production plan is from at least the 10th block in the cycle, concentrated production when the total number is not more than 55, to prevent because the rolling block number is too much, causing the roll surface state deterioration, thus on the strip surface produce adverse effects.

[0034] 3) when heating high strength pickling plate slab, control the sum of the second heating section and soaking section time is 100~180min, and the atmosphere in the heating furnace is oxidizing atmosphere, the actual air-fuel ratio of the second heating section and the soaking section is 2.1~2.2, when the slab out of the furnace, the temperature and the target value error control in-10~30℃ between, prevent because rolling too fast, causing the work roll surface at high temperature for a long time, make the roll surface state deterioration rapidly, and many pieces of steel simultaneously on the production line will also cause pickling water pressure drop, affect the pickling effect.

[0035] 4) from the first three pieces of high strength pickling plate slab rolling control rolling time interval is 140~210s.

[0036] 5) high strength pickling plate all-round pickling, HSB high pressure water pickling and FSB finishing high pressure water pickling for double pickling, when 1.8mm≤high strength hot rolled pickling plate thickness≤2.1mm, rough rolling mill entrance open 1~2 pass pickling water, 2.1mm

[0037] 6) high strength pickling plate hot rolling first rough rolling mill R1 work roll selects 60CrNiMo alloy steel roll, the second rough rolling mill R2 work roll selects high Cr steel roll, the first four finishing mill F1~F4 work roll selects high speed steel roll, the last three finishing mill F5~F7 work roll selects infinite hard centrifugal cast iron roll, and the use cycle of finishing mill work roll is less than or equal to 2 times, ensure the roll surface smooth and complete before entering the machine.

[0038] 7) increase the finishing rolling lubricating oil flow, the first finishing mill F1 rolling lubricating oil flow control is 40% of the maximum flow, the second to the fourth finishing mill F2~F4 rolling lubricating oil flow control is 60% of the maximum flow, the fifth finishing mill F5 rolling lubricating oil flow control is 70% of the maximum flow.

[0039] see Figure 3 and Figure 4The composition of the surface scale of the high-strength pickling plate is determined by using XRD. Before the heating parameters are improved, the determination result is that the matrix α-Fe accounts for 41.0%, the intermediate layer Fe3O4 accounts for 29.2%, the surface layer Fe2O3 accounts for 11.2%, and the bottom layer FeO accounts for 18.7%, and the pickling plate has the scale defect. After the heating parameters are improved, the determination result is that the matrix α-Fe accounts for 32.4%, the intermediate layer Fe3O4 accounts for 49.3%, the surface layer Fe2O3 accounts for 9.7%, and the bottom layer FeO accounts for 8.6%, and the pickling plate does not have the scale defect. Although the Si content of the high-strength pickling plate is higher than that of the low-strength pickling plate, the content is still low, and the XRD equipment cannot detect it, but the content of Fe2SiO4 can be determined from the content of FeO combined with the Si element. If the heating time of the slab in the heating furnace is too long and the temperature is too high, the amount of O element diffused to the matrix will increase, the content of FeO in the bottom layer of the surface scale of the slab will increase, and the content of Fe2SiO4 will increase, so that the scale is difficult to be removed.

[0040] The following examples are implemented on the premise of the technical scheme of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples. The methods used in the following examples are all conventional methods unless otherwise specified.

[0041]

Example

[0042] Before the production cycle of the hot-rolled high-strength pickling plate, the descaling box in front of the setting width press, the descaling box at the inlet of the finishing mill, the descaling water system at the inlet and outlet of the rough rolling mill, and the descaling water system between the first four stands of the finishing mill are confirmed to be in a fault-free state, and the descaling impact test is periodically carried out on the descaling water nozzles 3 at each position of the production line. The high-pressure water descaling HSB, the rough rolling mill R1, the rough rolling mill R2, the high-pressure water descaling FSB before the finishing mill, and the finishing mill F1-F4 are used to descale the whole plate surface. The chemical composition of the hot-rolled high-strength pickling plate is shown in Table 1.

[0043] Table 1

[0044] Example Steel grade C Si Mn P S Als Cu Cr Ni Nb V Ti Mo Ca B N 1 MP1500H 0.233 0.264 1.308 0.01 0.001 0.045 0.044 0.022 0.171 0.011 0.024 0.001 0.039 0.001 0.0007 0.0024 2 S420MC 0.063 0.117 0.871 0.013 0.005 0.034 0.005 0.035 0.007 0.04 0.001 0.002 0.001 0.0004 0.0001 0.0031 3 S500MC 0.081 0.116 1.551 0.012 0.006 0.042 0.012 0.024 0.007 0.047 0.001 0.036 0.001 0.0019 0.0001 0.003 4 QSTE420TM 0.063 0.088 0.893 0.012 0.003 0.048 0.008 0.035 0.006 0.039 0.001 0.001 0.005 0.0015 0.0001 0.0023 5 QSTE500TM 0.076 0.095 1.578 0.011 0.001 0.045 0.01 0.026 0.008 0.046 0.001 0.04 0.017 0.0001 0.0001 0.0029

[0045] The hot rolling control parameters of the hot-rolled high-strength pickling plate are shown in Table 2.

[0046] Table 2

[0047]

[0048] The descaling water input condition of the hot-rolled high-strength pickling plate is shown in Table 3.

[0049] Table 3

[0050]

[0051] The roll parameters for the high-strength pickled sheet hot-rolled piece 4 are shown in Table 4:

[0052] Table 4

[0053] Block number in cycle [rl work roll] [rl work roll] [CAT] F1-F4 work rolls [CAT] F5-F7 work rolls 10 60CrNiMo High Cr steel High speed steel High Ni Cr centrifuge 50 60CrNiMo High Cr steel High speed steel High Ni Cr centrifuge 11 60CrNiMo High Cr steel High speed steel High Ni Cr centrifuge 53 60CrNiMo High Cr steel High speed steel High Ni Cr centrifuge 24 60CrNiMo High Cr steel High speed steel High Ni Cr centrifuge

[0054] After the improvement by the method of the present application, the rejudgment rate of the high-strength pickled sheet due to the iron oxide scale type defects is reduced from about 8% to less than 1%, and the defects are obviously controlled.

Claims

1. A method of controlling scale-like defects of hot-rolled high-strength pickled sheets, characterized by, The following is included: 1) using descaling impact test method, check the test special slab on each trace complete, lap no superimposed, and the impact mark gap ≤2mm; 2) production plan for at least the 10th from the cycle to start production of high strength pickling plate, not more than 55; 3) control high strength pickling plate two heating segment and soaking segment time is 100~180min; control two heating segment and soaking segment heating furnace atmosphere is oxidizing atmosphere, two heating segment and soaking segment of the actual air-fuel ratio is 2.1~2.2, when the slab out of the furnace, the out of the furnace temperature and the target value error control in-10~30℃ between; 4) high strength pickling plate rolling time interval control in 140~210s; 5) to high strength pickling plate all-round descaling, when the pickling plate rough rolling arrange at least one pass descaling water; 6) the use cycle of finishing mill work roll is less than or equal to 2 times; 7) increase the finishing rolling lubricating oil flow, the first finishing rolling lubricating oil flow control for the maximum flow of 40%~45%, the second to the fourth finishing rolling lubricating oil flow control for the maximum flow of 60%~65%, the fifth finishing rolling lubricating oil flow control for the maximum flow of 70%~75%.

2. The method of claim 1, wherein the method is characterized by, From the first three slab of high strength pickling plate start to control the rolling time interval.

3. The method of claim 1, wherein the method is characterized by: When 1.8mm≤high strength hot rolled pickling plate thickness≤2.1mm, rough rolling mill entrance open 1~2 pass descaling water, when 2.1mm< high strength hot rolled pickling plate thickness≤2.6mm, and high strength hot rolled pickling plate width≥1360mm, rough rolling mill entrance open at least 2 pass descaling water.

Images