A method for identifying and controlling scratch defects of cold-rolled high-strength steel

Abstract

This invention relates to a method for identifying and controlling scratch defects in cold-rolled high-strength steel, belonging to the technical field of metallurgical control methods. The technical solution of this invention is: a method for statistically analyzing and identifying the morphological characteristics of scratches in different processes of continuous casting, hot rolling, and pickling, as well as corresponding process control schemes, to comprehensively control key control points in each process. The beneficial effects of this invention are: by providing a method for identifying the morphological characteristics of scratches in different processes, and a control scheme for scratches in different processes, it effectively controls scratch defects in finished products, improves product quality, reduces quality losses, and simultaneously increases contract fulfillment rates.

Description

Technical Field

[0001] This invention relates to a method for identifying and controlling scratch defects in cold-rolled high-strength steel, belonging to the technical field of metallurgical control methods. Background Technology

[0002] With the deepening implementation of the national strategy of "carbon peaking and carbon neutrality," the automotive manufacturing industry, as an important economic pillar, plays a crucial historical role in the journey of energy conservation and emission reduction. The trend towards lightweight vehicles inevitably requires the application of more and more advanced high-strength steels. Among advanced high-strength steels, duplex steel, due to its superior comprehensive properties such as low yield strength ratio, high elongation, and high work hardening index, is seeing its application ratio increase under the dual drive of automotive lightweighting and safety. Statistics show that duplex steel accounts for as much as 74% of the usage in C-class vehicles, indicating a significant market demand.

[0003] Cold-rolled duplex steel involves numerous production processes and a long workflow, resulting in a wide variety of surface defects. Among these, scratches are often unavoidable and are one of the main types of surface defects. Scratches can occur in multiple processes, including steelmaking, hot rolling, cold rolling, and continuous annealing. Chinese invention patent application number CN202010618379 proposes a production method for cold-rolled continuous annealing wide plates to improve short linear scratch defects. The thickness of the cold-rolled finished product is 1.0-2.0 mm, the thickness of the hot-rolled incoming material is 5.0-6.0 mm, the crown is controlled at 40-50 μm, the total reduction rate of acid rolling is 65-80%, the flatness and shape adjustment adopts symmetrical adjustment, and the single-sided rib wave is adjusted into double-sided rib wave or central rib wave by skew and bending rolls. The flatness is controlled at ≤10I. In the continuous annealing process, the tension difference between the transmission side and the operation side in the furnace is controlled at ≤10%, the hydrogen atmosphere content in the furnace is 5-10%, and the dew point of the heating zone is controlled at ≤-30℃. The longitudinal and transverse short linear scratch defects are improved by adjusting the shape, deviation and roughness of the cold-rolled continuous annealing wide plate. For example, the invention patent with patent application number CN202211052357 proposes a method to prevent scratches on cold-rolled sheets during abnormal shutdown of a continuous annealing furnace. After the annealing furnace area of ​​the continuous annealing unit is shut down, the holding temperature is controlled according to the thickness of the cold-rolled sheet. At the same time as the shutdown, the tension setting value of each section of the continuous annealing furnace area is reduced to 2-3 kN. If the annealing furnace area of ​​the continuous annealing unit is shut down for more than 30 minutes, the annealing furnace area is set to unstressed on the secondary HMI. At the same time, the compensation roller pin locks at the inlet and outlet of the annealing furnace area are manually opened. The compensation rollers move horizontally to compensate for the shrinkage of the cold-rolled sheet length. After manually opening the compensation roller pin locks, if the tension in the annealing furnace area still exceeds the set value by 50%, the inlet and outlet loopers are loosely fitted on the local operating table to further compensate for the shrinkage of the cold-rolled sheet length. This patent is to prevent scratch defects from occurring during the production of cold-rolled sheets by a continuous annealing unit.

[0004] Current patented technologies all focus on improving and controlling scratches in a single process, and there is no method for identifying the process that generates scratches. However, scratches in steelmaking, hot rolling, and pickling processes can be inherited by the final continuous annealing process. Therefore, it is particularly important to study a scratch identification and control method applicable to cold-rolled high-strength steel. Summary of the Invention

[0005] The purpose of this invention is to provide a method for identifying and controlling scratch defects in cold-rolled high-strength steel. By providing a method for identifying the morphological characteristics of scratches in different processes, as well as a control scheme for scratches in different processes, the invention effectively controls scratch defects in finished products, improves product quality, reduces quality losses, and increases contract fulfillment rates, thus effectively solving the aforementioned problems in the background art.

[0006] The technical solution of this invention is: a method for identifying and controlling scratch defects in cold-rolled high-strength steel, comprising the following steps:

[0007] (1) Identify scratches in cold-rolled high-strength steel. The identification rules are as follows: continuous casting scratches are black or yellowish-brown lines; hot rolling scratches are white lines with a slight or no feel, relatively thick, distributed in single or clustered lines, mainly distributed at the head and tail of the strip; pickling and rolling scratches are relatively thin white lines, which can be distributed in the middle or at the head and tail of the strip; continuous annealing scratches are metallic bright, with a slight or no feel, distributed throughout the strip or at the head and tail.

[0008] (2) During the continuous casting process, check the condition of the fan-shaped section rollers and bearings, repair sunken rollers and abnormal bearings to ensure their normal operation; and clean the rollers to avoid abnormal rotation and the adhesion of iron oxide scale and foreign matter.

[0009] (3) In the hot rolling process, the height of the transition beam at the entrance of the finishing mill is matched with the height of the pinch roll at the exit of the descaling mill before finishing mill; the height of one side of the F7 exit guide table is matched with the exit roller table; check and ensure that the laminar cooling roller table rotates normally; grind the coiling pinch roll before production to ensure the quality of the roll surface; control the clamping force of the guide plate when cleaning the coiling guide plate.

[0010] (4) During the pickling and rolling process, check and ensure that the surface of the steel platform is smooth; check the rotation of the roll system, restore the abnormal rolls that are not rotating, and regularly replenish the bearings with oil for lubrication; ensure the emulsion concentration is stable and control the roughness of the rolls;

[0011] (5) During the continuous annealing process, the damaged roller surface is repaired or replaced, the brush roller support roller is checked and ensured to rotate normally, the strip steel is kept under sufficient tension, and the leveling roller is replaced regularly to ensure the roughness.

[0012] In step (3), the elevation of the transition beam at the entrance of the finishing mill is more than 20mm lower than the lower pinch roll at the exit of the descaling machine before finishing mill; the elevation of the higher side of the F7 exit of the finishing mill is about 20mm lower than the exit roller table; when cleaning the coiling guide plate, the clamping force of the guide plate is controlled below 50KN to reduce the wear of the guide plate.

[0013] In step (4), the emulsion concentration is 1.5-3.0% for stands 1-4 and 0.3-1.0% for stand 5; the roll roughness is controlled below 1.0 μm for stands 1-4 and maintained at 2.5-3 μm for stand 5.

[0014] In step (5), the strip steel is kept under sufficient tension, the furnace section tension is controlled at 6-7 MPa; the leveling rollers are replaced regularly to ensure roughness, the looper tension is kept at 18-22 MPa, and the disc shear tension is kept at 25-30 MPa.

[0015] The beneficial effects of this invention are: by providing a method for identifying the morphological characteristics of scratches in different processes, as well as a control scheme for scratches in different processes, the invention effectively controls scratch defects in finished products, improves product quality, reduces quality losses, and increases contract fulfillment rate. Attached Figure Description

[0016] Figure 1 This is a macroscopic morphology diagram of the continuous casting scratches of the present invention;

[0017] Figure 2 This is a macroscopic morphology diagram of the hot rail scratches of the present invention;

[0018] Figure 3 This is a macroscopic morphology diagram of the acid track scratches of the present invention;

[0019] Figure 4 This invention describes the macroscopic morphology of continuous retraction scratches;

[0020] Figure 5 This is a metallographic diagram of the cross-section of the continuous casting scratch of the present invention;

[0021] Figure 6 This is a scanning electron microscope (SEM) image of the hot-rolled scratches of this invention.

[0022] Figure 7 This is the energy spectrum diagram of hot-rolled scratches in this invention;

[0023] Figure 8 This is a scanning electron microscope (SEM) image of the acid-rolled scratches of this invention.

[0024] Figure 9 Energy spectrum of acid-rolled scratches in this invention;

[0025] Figure 10 This is a scanning electron microscope image of the microstructure of the scratches from the continuous retraction of the present invention;

[0026] Figure 11 This is the energy spectrum of the continuous scratches of the present invention. Implementation

[0027] To make the purpose, technical solutions, and advantages of the invention's embodiments clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described are only a small part of the embodiments of the present invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.

[0028] A method for identifying and controlling scratch defects in cold-rolled high-strength steel, comprising the following steps:

[0029] (1) Identify scratches in cold-rolled high-strength steel. The identification rules are as follows: continuous casting scratches are black or yellowish-brown lines; hot rolling scratches are white lines with a slight or no feel, relatively thick, distributed in single or clustered lines, mainly distributed at the head and tail of the strip; pickling and rolling scratches are relatively thin white lines, which can be distributed in the middle or at the head and tail of the strip; continuous annealing scratches are metallic bright, with a slight or no feel, distributed throughout the strip or at the head and tail.

[0030] (2) During the continuous casting process, check the condition of the fan-shaped section rollers and bearings, repair sunken rollers and abnormal bearings to ensure their normal operation; and clean the rollers to avoid abnormal rotation and the adhesion of iron oxide scale and foreign matter.

[0031] (3) In the hot rolling process, the height of the transition beam at the entrance of the finishing mill is matched with the height of the pinch roll at the exit of the descaling mill before finishing mill; the height of one side of the F7 exit guide table is matched with the exit roller table; check and ensure that the laminar cooling roller table rotates normally; grind the coiling pinch roll before production to ensure the quality of the roll surface; control the clamping force of the guide plate when cleaning the coiling guide plate.

[0032] (4) During the pickling and rolling process, check and ensure that the surface of the steel platform is smooth; check the rotation of the roll system, restore the abnormal rolls that are not rotating, and regularly replenish the bearings with oil for lubrication; ensure the emulsion concentration is stable and control the roughness of the rolls;

[0033] (5) During the continuous annealing process, the damaged roller surface is repaired or replaced, the brush roller support roller is checked and ensured to rotate normally, the strip steel is kept under sufficient tension, and the leveling roller is replaced regularly to ensure the roughness.

[0034] In step (3), the elevation of the transition beam at the entrance of the finishing mill is more than 20mm lower than the lower pinch roll at the exit of the descaling machine before finishing mill; the elevation of the higher side of the F7 exit of the finishing mill is about 20mm lower than the exit roller table; when cleaning the coiling guide plate, the clamping force of the guide plate is controlled below 50KN to reduce the wear of the guide plate.

[0035] In step (4), the emulsion concentration is 1.5-3.0% for stands 1-4 and 0.3-1.0% for stand 5; the roll roughness is controlled below 1.0 μm for stands 1-4 and maintained at 2.5-3 μm for stand 5.

[0036] In step (5), the strip steel is kept under sufficient tension, the furnace section tension is controlled at 6-7 MPa; the leveling rollers are replaced regularly to ensure roughness, the looper tension is kept at around 20 MPa, and the disc shear section tension is kept at around 28 MPa.

[0037] Theoretical Analysis:

[0038] The theoretical basis for judging and analyzing continuous casting scratches is as follows: The macroscopic morphology is black or yellowish-brown linear. The microscopic morphological characteristics are: normal areas contain ferrite + martensite, while defective areas contain a large amount of ferrite, very little carbide, no martensite, and the ferrite grains are coarser and have a higher ferrite proportion than normal areas, showing obvious decarburization. Therefore, it is judged that the defect is a result of the high-temperature environment of the hot rolling furnace at 1200℃. After rolling, the defect covers the substrate surface because the surface metal flow during hot rolling folds and covers the scratched grooves. Therefore, it is judged to be a slab scratch. Slab scratches refer to scratches on the slab surface produced in the continuous casting process. These scratches sometimes cannot be eliminated after hot rolling and cold rolling, and eventually appear after cold rolling annealing. Through investigation of the continuous casting process production site, the causes of billet scratches were found to be as follows: the sector section segment rollers of the continuous casting equipment were sunken, the bearing seats were higher than the roller surface, and they scraped the billet surface; the rollers did not rotate, which scratched the billet surface; in addition, there was the accumulation of iron oxide scale, residual steel or foreign objects adhering to the rollers, which scratched the billet.

[0039] The theoretical basis for judging and analyzing hot-rolled scratches: The macroscopic morphology consists of slightly palpable or imperceptible white linear defects, relatively coarse, distributed singly or in clusters, mainly at the beginning and end of the strip. Microscopic morphological characteristics include rolling textures and roughening pits within the scratches, indicating that this is the result of cold rolling followed by the last stand's roughening rolls. A distinct "sponge iron structure" is observed. This is because hot-rolled scratches are generated at high temperatures, producing iron oxide within the scratches. This oxide cannot be completely removed during pickling, and the residual iron oxide transforms into a reduced "sponge iron structure" after subsequent annealing. In the annealing furnace, the iron oxide scale is reduced by hydrogen gas, with the chemical reaction equation: 3H₂ + Fe₂O₃ → 2Fe + 3H₂O. Due to incomplete reduction, a small amount of iron oxide remains, and energy dispersive spectroscopy shows trace amounts of oxygen, thus classifying it as a hot-rolled scratch. Some scratches generated during the hot rolling process are minor and shallow, and can be eliminated in the subsequent cold rolling process, such as slight scratches caused by abnormal rotation of the laminar flow cooling rollers. However, some more severe scratches cannot be eliminated after subsequent cold rolling. The causes of scratches in the hot rolling process include: excessively high transition beam height at the finishing mill entrance scraping the strip, causing through-strip scratches; the strip head scraping the exit guide before tensioning at the finishing mill F7 exit, causing head scratches; and poor rotation of the laminar flow cooling rollers causing through-strip scratches. Steel scraps adhering to the surface of the coiling pinch rolls, combined with speed deviations in the head and tail lead-to-tail ratios, can cause head and tail scratches; small particles from wear on the coiling guide plates can be caught in the strip, causing scratches when there is misalignment between coil layers.

[0040] The theoretical basis for judging and analyzing pickling and rolling scratches: The macroscopic morphology is fine white lines, which can be distributed in the middle or at the beginning and end of the strip. The microscopic morphological characteristics are: the scratches contain rolling marks and roughening pits. The obvious difference from hot rolling scratches is that there is no sponge iron structure in the scratches. Energy dispersive spectroscopy shows that only Fe element is present and there is no iron oxide, so it is judged to be a cold rolling scratch. Scratches in the pickling and rolling process are divided into equipment hard scratches and slippage scratches. The causes of scratches include: sharp edges on the steel platform, resulting in hard scratches; tension gauge roll not rotating, high-speed loss of rotation, or damage to the roll surface, resulting in hard scratches; excessive emulsion concentration, insufficient roll roughness, insufficient front tension, etc., causing the roll speed to be higher than the strip speed, resulting in roll slippage and slippage scratches.

[0041] The theoretical basis for judging and analyzing scratches in continuous rolling is as follows: The macroscopic morphology is a bright metallic color, with a slight or no tactile feel, and it is distributed in a continuous band or at the beginning and end. The microscopic morphological characteristics are: continuous along the rolling direction without interruption; clear scratch boundaries, significantly different from the surrounding normal areas; no rolling texture or roughening pits within the scratch; no sponge iron structure; and energy dispersive spectroscopy showing only Fe element, indicating it is a continuous rolling scratch. The causes of scratches in this continuous rolling process include: damage to the roller surface causing abrasions during acceleration and deceleration; poor rotation of the brush roller and support roller; insufficient tension in the furnace section causing strip swaying; slippage of the leveling exit guide roller; and severe wear and slippage of the exit section tension roller. Example

[0042] Table 1. Different scratch morphologies and process control in Examples 1-5

[0043] .

Claims

1. A method for identifying and controlling scratch defects in cold-rolled high-strength steel, characterized in that... Includes the following steps: (1) Identify scratches in cold-rolled high-strength steel. The identification rules are as follows: continuous casting scratches are black or yellowish-brown lines; hot rolling scratches are white lines with a slight or no feel, relatively thick, distributed in single or clustered lines, mainly distributed at the head and tail of the strip; pickling and rolling scratches are relatively thin white lines, which can be distributed in the middle or at the head and tail of the strip; continuous annealing scratches are metallic bright, with a slight or no feel, distributed throughout the strip or at the head and tail. (2) During the continuous casting process, check the condition of the fan-shaped section rollers and bearings, repair sunken rollers and abnormal bearings to ensure their normal operation; and clean the rollers to avoid abnormal rotation and the adhesion of iron oxide scale and foreign matter. (3) In the hot rolling process, the height of the transition beam at the entrance of the finishing mill is matched with the height of the lower pinch roll at the exit of the pre-finishing descaling mill; the height of one side of the F7 exit guide table is matched with the exit roller table; check and ensure that the laminar cooling roller table rotates normally; grind the coiling pinch roll before production to ensure the quality of the roll surface; when cleaning the coiling side guide plate, control the clamping force of the coiling side guide plate; the elevation of the transition beam at the entrance of the finishing mill is more than 20mm lower than the lower pinch roll at the exit of the pre-finishing descaling mill; the elevation of one side of the F7 exit is more than 20mm lower than the exit roller table; when cleaning the coiling side guide plate, control the clamping force of the coiling side guide plate to be below 50KN to reduce the wear of the coiling side guide plate. (4) During the pickling and rolling process, check and ensure that the surface of the steel platform is smooth; check the rotation of the roll system, restore the abnormal rolls that are not rotating, and regularly replenish the bearings with oil for lubrication; ensure the emulsion concentration is stable and control the roughness of the rolls; (5) During the continuous annealing process, the damaged roller surface is repaired or replaced, the brush roller support roller is checked and ensured to rotate normally, the strip steel is kept under sufficient tension, the furnace section tension is controlled at 6-7 MPa; the leveling roller is replaced regularly to ensure roughness, the looper tension is kept at 18-22 MPa, and the disc shear section tension is kept at 25-30 MPa.

2. The method for identifying and controlling scratch defects in cold-rolled high-strength steel according to claim 1, characterized in that: In step (4), the emulsion concentration is 1.5-3.0% for stands 1-4 and 0.3-1.0% for stand 5; the roll roughness is controlled below 1.0 μm for stands 1-4 and maintained at 2.5-3 μm for stand 5.

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