Method for eliminating surface color difference of cold-rolled dual-phase high-strength steel

Abstract

This invention relates to the field of cold-rolled high-strength steel production technology, specifically to a method for eliminating surface color differences in cold-rolled duplex high-strength steel. The method includes controlling the slab heating temperature and time, finishing and roughing temperatures, laminar flow cooling mode, pinch roll pressure, and coiling tension during the hot rolling process, and controlling the cold rolling pickling insertion amount, acid concentration, and strip speed during the cold rolling pickling process. This invention, by controlling specific processes in hot rolling and cold rolling pickling, is applicable to all thickness specifications and achieves the elimination of color differences at the beginning and end of cold-rolled duplex steel strips, reducing the color difference error rate to zero. This method requires no additional cost, is easy to implement, and eliminates color differences without affecting the original mechanical properties of the duplex steel.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled high-strength steel production technology, specifically to a method for eliminating surface color difference in cold-rolled duplex high-strength steel. Background Technology

[0002] Cold-rolled dual-phase high-strength steel is one of the main raw materials for automotive structural safety components and other parts. It possesses high strength and good formability, effectively absorbing energy generated during collisions, making it a beneficial material for lightweight, safe, and energy-efficient automotive development. Automotive parts have high surface finish requirements; color differences on the strip steel surface can affect the electrophoresis and coating production of parts, resulting in defective components.

[0003] In actual production, cold-rolled duplex high-strength steel has a high alloying element content in its chemical composition, and the hot-rolling coiling temperature is generally high. After hot rolling and before entering the coiler, the strip head and tail are in a state of tension loss. The strip head and tail shape is difficult to control, which easily leads to poor strip head shape and many intermediate waves. Uneven strip head and tail temperature, poor strip head and tail shape, and varying degrees of collision between the surface and the conveyor rollers, along with other comprehensive factors, can easily cause differences in the type, depth, and degree of iron oxide scale within 100 meters of the strip head and tail, resulting in blocky color differences. This color difference appears as dark gray in hot rolling and is not easily detected. After pickling, blocky black spots of various shapes appear at the strip head and tail. This color difference is concentrated in the middle of the strip width at the head and tail, and varies in shape and size, seriously affecting the quality of the strip. Therefore, it is very necessary to provide a method to eliminate the surface color difference of cold-rolled duplex high-strength steel. Summary of the Invention

[0004] To address the technical problem of color difference defects at the beginning and end of cold-rolled duplex steel, this invention provides a method for eliminating surface color differences in cold-rolled high-strength duplex steel. By controlling specific processes in hot rolling and cold pickling production, this method is applicable to all thickness specifications and achieves complete elimination of color differences at the beginning and end of cold-rolled duplex steel strips, reducing the color difference error rate to zero. This method requires no additional cost, is easy to implement, and eliminates color differences without affecting the original mechanical properties of the duplex steel.

[0005] The technical solution of this invention is as follows:

[0006] A method for eliminating surface color difference in cold-rolled duplex high-strength steel includes:

[0007] Hot rolling production control: The billet heating temperature is controlled at 1230-1300℃, the billet heating time is 230-290min, the hot rolling roughing mill exit temperature is 1080-1130℃, the finishing mill inlet temperature is 1050-1100℃, and the finishing mill exit temperature is 850-890℃. The laminar flow cooling mode adopts front-end centralized cooling. The tension of the pinch rolls during the hot rolling coiling process is 18-28KN, and the coiling tension at the head and tail of the strip is controlled at 18-25KN and 13-20KN, respectively.

[0008] Cold rolling and pickling production control: control the insertion amount of the bending roll of the tension leveler to be 20-30mm, the insertion amount of the straightening roll to be 15-26mm, the pickling acid temperature to be 70-95℃, the pickling speed of the strip head and tail to be 100-140m / min, the free acid concentration of No.1 acid tank to be 40-80g / L, and the free acid concentration of No.2 acid tank to be 100-130g / L.

[0009] Furthermore, high-pressure water descaling is performed before rolling, with a descaling water pressure of 25-45 MPa to ensure that the iron oxide scale is basically removed in one pass.

[0010] Furthermore, high-pressure water descaling is carried out between the roughing and finishing mills, with a descaling water pressure of 20-35 MPa to ensure that the secondary iron oxide scale is completely removed.

[0011] Furthermore, the hot-rolling coiling temperature is 560-660℃. Specifically, the hot-rolling coiling temperature for 590MPa grade duplex steel is 570-610℃, for 780MPa grade duplex steel it is 610-640℃, and for 980MPa grade duplex steel it is 620-650℃. This temperature is about 50-90℃ lower than the process temperature at which defects occur. Duplex steel is a phase transformation strengthening process, and the change in coiling temperature has virtually no impact on its original mechanical properties.

[0012] Furthermore, the U-shaped winding is eliminated to avoid excessively high temperatures at the beginning and end, which would complicate the types and structures of iron oxide scale and make it difficult to remove in subsequent processes.

[0013] Furthermore, due to the high strength of duplex steel, proper hot rolling transition is necessary to avoid roll vibration or uneven rolling force caused by transition issues, which could lead to color differences. The strength transition involves using steel grades with a yield strength 100-250 MPa lower for the transition, and the specification transition involves ensuring that the thickness difference between transition coils does not exceed 0.6 mm and the width difference does not exceed 260 mm.

[0014] Furthermore, cold-rolled duplex high-strength steel is a series of cold-rolled duplex steels with a strength of 550MPa-980MPa.

[0015] Furthermore, cold-rolled duplex high-strength steel includes cold-rolled continuously annealed and / or galvanized series duplex steel.

[0016] Furthermore, in cold-rolled dual-phase high-strength steel, the C content is 0.070%-0.15%, the Si content is 0.3%-0.5%, the Cr content is 0.2%-0.7%, the Mn content is 0.16%-0.25%, and the Al content is 0.035%-0.075%.

[0017] The beneficial effects of this invention are as follows:

[0018] This invention optimizes the hot rolling process by considering aspects such as ensuring sufficient alloy melting and uniform slab temperature in hot-rolled slabs, thorough removal of iron oxide scale using high-pressure water in primary and secondary processes, optimizing transition material to ensure stable rolling, maintaining consistent rolling temperature, reducing head and tail temperatures to minimize the degree and complexity of iron oxide scale, reducing pinch roll pressure, reducing strip head and tail curling tension, and altering the laminar cooling mode. This reduces the severity of dark gray color difference defects in hot-rolled steel. Simultaneously, by optimizing the pickling process by reducing pickling insertion, lowering acid concentration, and increasing pickling cleaning capacity, the black spot color difference after pickling is essentially eliminated. Through multiple trials and the solidification of processes and parameters, a method for eliminating surface color differences in cold-rolled duplex high-strength steel has been finally developed.

[0019] This invention is applicable to both cold-rolled continuous annealed and galvanized series of dual-phase steel of various strength grades. It can be easily implemented in existing steel mill production lines without additional costs. This invention can reduce the re-judgment rate caused by color difference defects at the beginning and end of cold-rolled dual-phase steel coils to zero, avoiding re-judgment losses, and will not change the original mechanical properties of the finished product. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 It refers to the surface color difference defect morphology of the head and tail of hot-rolled strip steel.

[0022] Figure 2 It refers to the color difference defects on the surface of hot-rolled strip coils and the microstructure of the normal surface.

[0023] Figure 3 It refers to the morphology of the color difference on the surface of the head and tail of hot-rolled strip steel after cold rolling, pickling, and pickling.

[0024] Figure 4 This describes the improvement in surface color difference at the head and tail of the CR340 / 590DP hot-rolled strip in Example 1.

[0025] Figure 5 This describes the improvement in surface color difference of CR340 / 590DP strip after pickling and pickling in Example 1.

[0026] Figure 6 This describes the improvement in surface color difference at the head and tail of the CR420 / 780DP hot-rolled strip in Example 2.

[0027] Figure 7 This describes the improvement in color difference on the surface of the CR420 / 780DP strip after pickling and pickling in Example 2. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0029] After pickling, duplex steel produced by Rizhao Iron & Steel Group Co., Ltd. exhibited irregular black patches of varying sizes within a 90m range at the head and tail of the strip. Tracing the corresponding positions at the hot-rolling head and tail revealed that the color difference defect originated from the hot-rolling process. The hot-rolling process defect is characterized by irregular, dark gray, blocky color differences, with the following morphology: Figure 1 As shown, Figure 1 The area circled in the middle represents the dark gray color difference defect.

[0030] Through comparative analysis, the color difference was found to be caused by numerous wavy sections at the beginning and end of the hot-rolled coil, poor steel sheet shape, and slight surface breakage due to pressure or collisions from the conveyor rollers and clamping devices, as well as excessive coiling tension. The differences in temperature, oxide scale condition, and degree between the beginning and end, combined with these factors, resulted in blocky dark gray color differences at the head and tail of the hot-rolled strip. The surface microstructure of this color difference was porous and fractured. Figure 2 As shown.

[0031] Hot-rolled coils undergo pickling. The normal surface of hot-rolled coils has a dense iron oxide scale, making it difficult to pickle. However, at defective areas, the iron oxide scale is broken and easily washed away, resulting in over-pickling. Furthermore, pickling enlarges the defect area, causing it to appear as black patches with color variations. Figure 3 As shown.

[0032] To eliminate the surface color difference problem existing in the above-mentioned cold-rolled dual-phase high-strength steel, the present invention provides the following method:

[0033] The hot rolling process includes billet heating, pre-rolling high-pressure water descaling, rough rolling, high-pressure water descaling, finish rolling, laminar flow cooling, coiling, and coil cooling.

[0034] The billet heating temperature is 1230-1300℃, and the billet heating time is a relatively long 230-290min to ensure that the alloy is completely melted and the billet temperature is uniform, and the surface oxide scale is consistent, which is conducive to smooth rolling.

[0035] Before rolling, high-pressure water descaling is carried out, with a descaling water pressure of 25-45 MPa to ensure that the iron oxide scale is basically removed in one pass.

[0036] The exit temperature of the hot-rolled roughing mill is 1080-1130℃, the entry temperature of the finishing mill is 1050-1100℃, and the exit temperature of the finishing mill is 850-890℃. High-pressure water descaling is also carried out between the roughing and finishing mills, with a water pressure of 20-35MPa, to ensure that the secondary iron oxide scale is completely removed.

[0037] The laminar flow cooling mode adopts front-end centralized cooling, which makes the strip temperature drop rapidly and avoids the iron oxide scale from becoming more complex and structurally diverse at higher temperatures.

[0038] During the hot rolling coiling process, the pinch roll tension is 18-28 KN. Because the head and tail of the high-strength steel hot-rolled strip lose tension after finishing, and the middle section is severely wavy with poor strip shape, excessive pinch roll pressure can easily cause surface iron oxide scale to be crushed or pressed in, and the degree of this can be inconsistent. Furthermore, excessive coiling tension should be avoided, as it can cause friction between the head and tail coil layers, leading to color differences. The coiling tensions at the head (core) and tail (tail) of the strip are 18-25 KN and 13-20 KN, respectively. A lower coiling temperature of 560-660℃ is used during hot rolling. This lower temperature allows for better control of surface iron oxide scale. The types of scale are relatively simple and the structure is relatively stable. The hot rolling coiling temperature for 590MPa grade duplex steel is 570-610℃, for 780MPa grade duplex steel it is 610-640℃, and for 980MPa grade duplex steel it is 620-650℃. This temperature is about 50-90℃ lower than the process temperature at which defects occur. Duplex steel is a phase transformation strengthening process, so the change in coiling temperature has little impact on the original mechanical properties. The U-shaped coiling is eliminated to avoid excessively high head and tail temperatures, which would complicate the types and structures of iron oxide scale and make it difficult to remove in subsequent processes.

[0039] Duplex steel has high strength and requires proper transition to avoid roll vibration or uneven rolling force, which could lead to color differences. The strength transition is as follows: use steel grades with a yield strength 100-250MPa lower. The specification transition is as follows: the thickness difference between rolls should not exceed 0.6mm, and the width transition thickness difference should not exceed 260mm.

[0040] The cold rolling and pickling process includes uncoiling, pickling, rolling, and coiling to obtain cold-rolled coils. The cold-rolled coils then undergo continuous annealing and galvanizing processes to produce finished duplex steel.

[0041] The bending roller insertion depth of the tension leveler is 20-30mm, and the straightening roller insertion depth is 15-26mm, to ensure that the iron oxide scale is broken before pickling, but severe breakage should be avoided.

[0042] The acid temperature for pickling is 70-95℃, and the pickling speed, including both the head and tail of the tank, is 100-140 m / min. The free acid concentration in acid tank #1 is 40-80 g / L, and the free acid concentration in acid tank #2 is 100-130 g / L. Appropriate acid concentrations should be used to avoid over-pickling.

[0043] Example 1

[0044] Surface color difference was eliminated in cold-rolled CR340 / 590DP steel. The hot-rolled thickness of this steel is 3.5*1540mm, corresponding to a cold-rolled specification of 1.4*1500mm. The composition is: C content 0.075%, Si content 0.35%, Mn content 0.17%, Cr content 0.21%, and Alt content 0.040%. Q355B was used as the transition material for hot rolling, with a specification of 3.0mm*1500mm. The specific steps are as follows:

[0045] Hot rolling production control:

[0046] The slab is heated to 1250℃ for 246 minutes. Before entering the rolling mill, the slab undergoes high-pressure water descaling at a pressure of 35 kN. Following this, it undergoes roughing rolling at an exit temperature of 1120℃, finishing rolling at an inlet temperature of 1080℃, and a finish rolling exit temperature of 870℃. High-pressure water descaling is performed between the roughing and finishing mill stands at a pressure of 26 kN. Laminar flow cooling is used with centralized front-end cooling. The coiling temperature is 570℃, and U-shaped coiling is eliminated. The coiling pinch roll pressure is 23 kN, and the coiling tension at the head and tail of the strip is 23 kN and 17 kN respectively. After coiling, the strip is slowly cooled for 62 hours before cold rolling and pickling. The resulting hot-rolled coil exhibits significantly reduced color difference at the head and tail. Figure 4 .

[0047] Cold rolling and pickling production control:

[0048] The bending roll insertion depth of the tension leveler is 20mm, and the straightening roll insertion depth is 18mm. The strip enters the pickling bath at a temperature of 76℃. The free acid concentration in pickling tank #1 is 55g / L, and the free acid concentration in pickling tank #2 is 130g / L. The strip head and tail pass through the pickling tank at a speed of 108m / min. Afterward, it is cold-rolled to obtain a 1.4mm cold-rolled coil. The color difference defects in the obtained cold-rolled sample coil are basically eliminated. Figure 5 .

[0049] Cold-rolled and annealed or subjected to performance adjustments, the finished dual-phase steel is obtained.

[0050] Example 2

[0051] Surface color difference was eliminated in cold-rolled CR4200 / 780DP steel. The hot-rolled thickness of this steel is 4.5*1335mm, corresponding to a cold-rolled specification of 1.7*1300mm. The composition is: C content 0.12%, Si content 0.38%, Mn content 0.19%, Cr content 0.31%, and Al content 0.060%. HC340LA was used as the transition material for hot rolling, with a specification of 4.0mm*1290mm. The specific steps are as follows:

[0052] Hot rolling production control:

[0053] The slab is heated to 1280℃ for 266 minutes. Before entering the rolling mill, the slab undergoes high-pressure water descaling at a pressure of 30 kN. Following this, it undergoes roughing rolling at an exit temperature of 1130℃, finishing rolling at an inlet temperature of 1100℃, and a finish rolling exit temperature of 890℃. High-pressure water descaling is performed between the roughing and finishing mill stands at a pressure of 22 kN. Laminar flow cooling is used with centralized front-end cooling. The coiling temperature is 630℃, and U-shaped coiling is eliminated. The coiling pinch roll pressure is 22 kN, and the coiling tension at the head and tail of the strip is 20 kN and 14 kN respectively. After coiling, the strip is slowly cooled for 68 hours before cold rolling and pickling. This results in a significant reduction in color difference at the head and tail of the hot-rolled coil. Figure 6 .

[0054] Cold rolling and pickling production control:

[0055] The bending roll insertion depth of the tension leveler is 24mm, and the straightening roll insertion depth is 22mm. The strip enters the pickling bath at a temperature of 85℃. The free acid concentration in pickling tank #1 is 55g / L, and the free acid concentration in pickling tank #2 is 130g / L. The strip head and tail pass through the pickling tank at a speed of 110m / min. Afterward, it is cold-rolled to obtain a 1.7mm cold-rolled coil. The color difference defects in the obtained cold-rolled sample coil are basically eliminated. Figure 7 .

[0056] Cold-rolled and annealed or subjected to performance adjustments, the finished dual-phase steel is obtained.

[0057] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for eliminating surface color difference of cold-rolled dual-phase high-strength steel, characterized in that, include: Hot rolling production control: The billet heating temperature is controlled at 1230-1250℃, the billet heating time is 230-290min, the hot rolling roughing mill exit temperature is 1080-1130℃, the finishing mill inlet temperature is 1050-1080℃, and the finishing mill exit temperature is 870-890℃. The laminar flow cooling mode adopts front-end centralized cooling. The tension of the pinch rolls during the hot rolling coiling process is 18-28KN, and the coiling tension at the head and tail of the strip is controlled at 18-25KN and 13-20KN, respectively. Cold rolling and pickling production control: control the insertion amount of the bending roll of the tension leveler to be 20-30mm, the insertion amount of the straightening roll to be 18-26mm, the pickling acid temperature to be 70-95℃, the pickling speed of the strip head and tail to be 100-140m / min, the free acid concentration of No. 1 pickling tank to be 40-80g / L, and the free acid concentration of No. 2 pickling tank to be 100-130g / L; The cold-rolled duplex high-strength steel is a 590MPa grade cold-rolled duplex steel. The hot rolling temperature of the 590MPa grade cold-rolled duplex steel is 570-610℃. In the hot-rolled transition, the strength transition is carried out by selecting steel grades with a yield strength level 100-250MPa lower, and the specification transition is that the thickness difference between transition coils does not exceed 0.6mm and the width difference does not exceed 260mm.

2. The method of claim 1, wherein, Before rolling, high-pressure water descaling is performed, with a descaling water pressure of 25-45 MPa.

3. The method of claim 1, wherein, High-pressure water descaling is carried out between the roughing and finishing mills, with a descaling water pressure of 20-35 MPa.

4. The method as described in claim 1, characterized in that, In cold-rolled dual-phase high-strength steel, the C content is 0.070%-0.15%, the Si content is 0.3%-0.5%, the Cr content is 0.2%-0.7%, the Mn content is 0.16%-0.25%, and the Al content is 0.035%-0.075%.

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