Method for preventing edge black-line defect from appearing on hot-rolled low-carbon steel plate after cold rolling

Abstract

A method for preventing an edge black-line defect from appearing on a hot-rolled low-carbon steel plate after cold rolling. The method comprises: 1) strictly controlling the chemical composition of a hot-rolled plate; 2) establishing an overall processing process flow for the steel plate; 3) controlling the spacing between charged hot-rolled intermediate slabs; 4) establishing the heating regime of heating during hot rolling, and taking temporary remedial measures when the in-furnace time exceeds the set in-furnace time; and 5) controlling a rough rolling procedure. By means of specifically establishing and controlling a rolling schedule for the hot rolling production of a low-carbon steel material for cold rolling, and making improvements in terms of preventing the overheating of slabs and taking temporary remedial measures when the slabs have stayed in the furnace for an overly long time, the product quality of cold-rolling-series low-carbon steel is effectively improved, quality degradation caused by edge black-line defects is reduced, and loss caused by the quality degradation is reduced.

Description

A method for preventing edge black line defects in hot-rolled low-carbon steel sheets after cold rolling. Technical Field

[0001] This invention relates to the field of hot rolling technology, and in particular to a method for preventing edge black line defects from occurring on hot-rolled low-carbon steel plates after cold rolling. Background Technology

[0002] Currently, the proportion of base materials supplied by hot rolling mills to downstream cold rolling processes is increasing year by year, reaching over 48%. Among them, the problem of edge black lines is particularly serious in low-carbon cold-rolled steels after cold rolling. This defect cannot be detected by surface inspection instruments after hot rolling, but after cold rolling, visible black lines appear on one or both sides of the steel coil, failing to meet delivery conditions and affecting contract delivery. According to statistics, the product downgrading rate due to edge black line defects accounts for a large proportion, reaching 0.418% of the supply, seriously affecting the hot rolling mill's "scrap and substandard" indicators. At the same time, the sale of downgraded products also leads to economic losses.

[0003] Taking the cold-rolled steel sheet with grade DC01 as an example, the metallographic structure of the black line defect in the intermediate billet is ferrite and a small amount of pearlite; the grain size in the coarse grain region is grade 5.2, and the grain size in the fine grain region is grade 7.4, with obvious coarse grain and mixed grain phenomena.

[0004] The specific reasons for the black lines on the edges include:

[0005] (1) When the billet is heated, if the time in the furnace is too long or the temperature at the exit of the furnace is too high, the temperature at the edge will be too high, and there will be a transverse temperature difference between the edge and the middle. The grains at the edge will grow rapidly, resulting in coarse grains and mixed grains.

[0006] (2) During the reversible roughing process, the temperature of the last roughing pass is low, which leads to the rolling of some recrystallization zone or hot brittle zone. As the deformation deepens, some recrystallized grains at the grain boundary grow abnormally, and their size is much larger than the original grains, resulting in uneven grains and mixed crystal phenomenon.

[0007] (3) If coarse grains or mixed grains exist at the edge of the strip, uneven deformation at the edge of the strip during rolling will cause edge cracks. In subsequent rolling processes, the longitudinal tensile stress will cause the metal at the edge corners to flatten onto the surface of the strip, thus forming black lines at the edge. Energy dispersive spectroscopy analysis results show that the main component at this location is iron oxide scale. Summary of the Invention

[0008] This invention provides a method to prevent edge black line defects in hot-rolled low-carbon steel plates after cold rolling. By specifically formulating and controlling the rolling process of cold-rolled low-carbon steel materials during hot rolling production, and by improving aspects such as preventing slab overheating and taking temporary remedial measures when slabs are in the furnace for excessively long periods, the product quality of cold-rolled low-carbon steel series steels is effectively improved, the quality degradation caused by edge black line defects is reduced, the contract delivery rate is increased, and the economic losses caused by quality degradation are reduced.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A method for preventing edge black line defects in hot-rolled low-carbon steel sheets after cold rolling includes taking the following measures during the hot rolling stage:

[0011] 1) The chemical composition of hot-rolled steel plates is strictly controlled using internal control standards, including controlling the following components within the specified range by mass percentage: C 0.010%~0.030%; Si≤0.03%; Mn 0.18%~0.25%; P≤0.018%; S≤0.010%; Alt 0.015%~0.048%; Als 0.015%~0.045%; Ti≤0.010%; Cu≤0.10%;

[0012] 2) Develop the overall processing flow for steel plates, including hot metal pretreatment, converter smelting, ladle refining, continuous casting, hot rolling heating, roughing and finishing rolling, controlled cooling, coiling, cold rolling uncoiling, welding, pickling, cold continuous rolling, continuous annealing, leveling, and surface and performance inspection; the thickness of the hot-rolled slab is controlled at 220–240 mm; the furnace time during hot rolling heating is controlled at 180–220 minutes, and the furnace exit temperature is controlled at 1180–1250℃; the roughing mill exit temperature is controlled at 1020–1070℃; the finishing mill inlet temperature is controlled at 980–1050℃; the finishing mill final rolling temperature is controlled at 860–900℃; the coiling temperature is controlled at 660–710℃; and controlled cooling adopts a front-stage cooling layer cooling mode.

[0013] 3) The spacing between steel billets in hot-rolled intermediate billets is less than 60mm;

[0014] 4) Establish a heating system for hot rolling heating, and take temporary remedial measures for cases where the time spent in the furnace exceeds the set time.

[0015] 5) The roughing rolling adopts a 3+3 pass rolling mode. The side pressure ratio of the first and third passes of vertical roll E1 and the first and third passes of vertical roll E2 is 40%~42%, 30%~32%, 18%~20%, and 6%~12%, respectively. The effective contact surface height of the vertical roll is increased to not less than 375mm. Cooling water shielding devices are installed on both sides of the vertical roll to prevent the temperature drop of the intermediate billet edge.

[0016] Furthermore, the temporary remedial measures taken in measure 4) are as follows:

[0017] (1) Set the descaling water pressure to the maximum limit value;

[0018] (2) Reduce the thickness of the intermediate billet and increase the reduction rate of the rough rolling;

[0019] (3) The slab after exiting the heating furnace is put into the hot coil box;

[0020] (4) Mark the slabs that exceed the set time in the furnace on the HMI screen in the central control room, prompting the furnace operator to cool them down according to the corresponding cooling system, and pay special attention to the surface iron oxide scale and edge peeling or black line defects.

[0021] (5) The product grade is determined according to the product use. High-grade automotive cold-rolled products are reclassified as ordinary commercial products.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] By developing and controlling the rolling process of cold-rolled low-carbon steel in hot rolling production, and by improving measures to prevent slab overheating and take temporary remedial measures when slabs are in the furnace for too long, the product quality of cold-rolled low-carbon steel series has been effectively improved, the quality downgrade caused by edge black line defects has been reduced, the contract delivery rate has been increased, and the economic losses caused by quality downgrades have been reduced. Attached Figure Description

[0024] Figure 1 is a flow chart of the hot rolling production process described in this invention.

[0025] Figure 2 shows the heating temperature curves (including temperature curves for normal furnace time and ultra-long furnace time) during hot rolling heating as described in this invention.

[0026] Figure 3 is an example of how the slab with an ultra-long furnace time is marked on the HMI screen according to the present invention.

[0027] In the diagram: HF1~HF3 - Heating Furnace; HSB - Roughing Mill Descaling Machine; R1D - Descaling Machine Before R1; R2D - Descaling Machine Before R2; E1, E2 - Roughing Mill Front Vertical Rolls; RT - Roughing Mill Front High Temperature Gauge; R1, R2 - Roughing Mill; CB - Hot Coiling Box; CS - Flying Shear; FET - Finishing Mill Inlet High Temperature Gauge; FSB - Finishing Mill Descaling Machine; F1E - Finishing Mill Front Vertical Rolls; F1~F7 - Finishing Mill; FDT - Finishing Mill Outlet High Temperature Gauge; LCS - Laminar Flow Cooling Mechanism; CT - Coiling Preheater High Temperature Gauge; DC1~DC3 - Underground Coiler Detailed Implementation

[0028] The method for preventing edge black line defects in hot-rolled low-carbon steel plates after cold rolling, as described in this invention, includes taking the following measures during the hot rolling stage:

[0029] 1) The chemical composition of hot-rolled steel plates is strictly controlled using internal control standards, including controlling the following components within the specified range by mass percentage: C 0.010%~0.030%; Si≤0.03%; Mn 0.18%~0.25%; P≤0.018%; S≤0.010%; Alt 0.015%~0.048%; Als 0.015%~0.045%; Ti≤0.010%; Cu≤0.10%;

[0030] 2) Develop the overall processing flow for steel plates, as shown in Figure 1, including hot metal pretreatment, converter smelting, ladle refining, continuous casting, hot rolling heating, roughing and finishing rolling, controlled cooling, coiling, cold rolling uncoiling, welding, pickling, cold continuous rolling, continuous annealing, leveling, and surface and performance inspection. The thickness of the hot-rolled slab is controlled at 220–240 mm; the furnace time during hot rolling heating is controlled at 180–220 minutes, and the furnace exit temperature is controlled at 1180–1250℃; the roughing mill exit temperature is controlled at 1020–1070℃; the finishing mill inlet temperature is controlled at 980–1050℃; the finishing mill final rolling temperature is controlled at 860–900℃; the coiling temperature is controlled at 660–710℃; and controlled cooling adopts a front-stage cooling layer cooling mode.

[0031] 3) The spacing between steel billets in hot-rolled intermediate billets is less than 60mm;

[0032] 4) Establish a heating system for hot rolling heating, and take temporary remedial measures for furnace time exceeding the set furnace time (as shown in Figure 2);

[0033] 5) The roughing mill adopts a 3+3 pass rolling mode. The side pressure ratio of the first and third passes of vertical roll E1 and the first and third passes of vertical roll E2 is 40%~42%, 30%~32%, 18%~20%, and 6%~12%, respectively. That is, the side pressure ratio of the roughing mill vertical rolls is redistributed, reducing the side pressure ratio of vertical roll E1 and increasing the side pressure ratio of vertical roll E2. The effective contact surface height of the vertical rolls is increased, and the effective contact surface height of the vertical rolls is not less than 375mm. Cooling water shielding devices are installed on both sides of the vertical rolls to prevent the temperature drop of the intermediate billet edge.

[0034] Furthermore, the temporary remedial measures taken in measure 4) are as follows:

[0035] (1) Set the descaling water pressure to the maximum limit value;

[0036] (2) Reduce the thickness of the intermediate billet and increase the reduction rate of the rough rolling;

[0037] (3) The slab after exiting the heating furnace is put into the hot coil box;

[0038] (4) For slabs that exceed the set time in the furnace, mark them on the HMI screen in the central control room (as shown in Figure 3) to prompt the furnace operator to cool them down according to the corresponding cooling system, and pay special attention to the surface iron oxide scale and edge peeling or black line defects.

[0039] (5) The product grade is determined according to the product use. High-grade automotive cold-rolled products are reclassified as ordinary commercial products.

[0040] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0041]

Example 1

[0042] In this embodiment, a hot-rolled steel plate with grade DC01 and dimensions of 4.5mm (thickness) × 1250mm (width) is taken as an example.

[0043] The rolling production line is shown in Figure 1. HF1 to HF3 are three heating furnaces; HSB is the roughing mill descaling machine; R1D is the descaling machine before roughing mill R1; R2D is the descaling machine before roughing mill R2; E1 and E2 are the vertical rolls before roughing mill; RT is the pyrometer before roughing mill; R1 and R2 are two roughing mill stands; CB is the hot coil box; CS is the flying shear; FET is the pyrometer at the finishing mill inlet; FSB is the finishing mill descaling machine; F1E is the vertical roll before finishing mill; F1 to F7 are seven finishing mill stands; FDT is the pyrometer at the finishing mill outlet; LCS is the laminar flow cooling mechanism; CT is the pyrometer before coiling; and DC1 to DC3 are three underground coilers.

[0044] The chemical composition of the steel plate, which is subject to internal control standards, is strictly controlled. In this embodiment, the following parameters are controlled: C: 0.020%; Si: 0.02%; Mn: 0.21%; P: 0.008%; S: 0.008%.

[0045] The hot-rolled slab thickness is 230mm, and the slab is cold-loaded with a furnace temperature below 300℃. The slab loading spacing is 50mm, and the furnace time is controlled at 180-200 minutes; the temperature of the three heating sections is 1270-1280℃; and the furnace exit temperature is 1230-1250℃. The roughing mill descaling pressure is set at 19MPa, and descaling is done separately.

[0046] The thickness of the intermediate billet is 41mm.

[0047] The roughing process adopts a 3+3 pass rolling mode, with the side pressure ratio of the first and third passes of vertical roll E1 and the first and third passes of vertical roll E2 being 40%, 30%, 20%, and 10%, respectively; the rolls are not fed into the hot coil box.

[0048] The temperature of the steel plate entering the finishing mill is controlled at 990-1030℃. The final rolling temperature is controlled at 890℃, and the coiling temperature is controlled at 690℃.

[0049] In this embodiment, the rolling success rate of the steel plates was over 90%, no edge defects were found by the surface inspection instrument, and no black line defects appeared on the edges after the steel plates were cold rolled.

[0050]

Example 2

[0051] In this embodiment, a hot-rolled steel plate with grade SGCC and dimensions of 2.85mm (thickness) × 1250mm (width) is used as an example. The rolling production line used is the same as in Example 1.

[0052] The chemical composition of the steel plate, which is subject to internal control standards, is strictly controlled. In this embodiment, the following parameters are controlled: C: 0.030%; Si: 0.026%; Mn: 0.25%; P: 0.006%; S: 0.0074%.

[0053] The hot-rolled slab thickness is 230mm. The slab is loaded into the furnace at a temperature of 400–600℃. The slab loading spacing is 50mm, and the furnace time is controlled at 180–190 minutes. The temperature of the three heating sections is 1240–1260℃; the furnace exit temperature is 1210–1230℃. The roughing mill descaling pressure is set at 19MPa, and descaling is performed separately.

[0054] The thickness of the intermediate billet is 40mm.

[0055] The roughing process adopts a 3+3 pass rolling mode, with the side pressure ratios of the first and third passes of vertical roll E1 and the first and third passes of vertical roll E2 being 41%, 32%, 19%, and 8%, respectively; the rolls are then fed into the hot coil box.

[0056] The temperature of the steel plate entering the finishing mill is controlled at 980-1020℃. The final rolling temperature is controlled at 900℃, and the coiling temperature is controlled at 700℃.

[0057] In this embodiment, the rolling success rate of the steel plates was over 90%, no edge defects were found by the surface inspection instrument, and no black line defects appeared on the edges after the steel plates were cold rolled.

[0058] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preventing the occurrence of edge black line defects after cold rolling of a hot-rolled low-carbon steel sheet, characterized by, The measures taken in the hot rolling stage include: 1) The chemical composition of the hot-rolled steel plate is strictly controlled by using internal control standards, including controlling the following components within the limited range in terms of mass percentage: C 0.010%~0.030%; Si≤0.03%; Mn 0.18%~0.25%; P≤0.018%; S≤0.010%; Alt 0.015%~0.048%; Als 0.015%~0.045%; Ti≤0.010%; Cu≤0.10%; 2) The overall processing flow of the steel plate is formulated, including hot metal pretreatment, converter smelting, secondary refining, continuous casting, hot rolling heating, rough rolling and finish rolling, controlled cooling, coiling, cold rolling uncoiling, welding, pickling, cold continuous rolling, continuous annealing, skin passing, surface and performance inspection; wherein the thickness of the hot rolling billet is controlled within 220~240mm; the in-furnace time during hot rolling heating is controlled within 180~220 minutes, and the discharge temperature is controlled within 1180~1250℃; the rough rolling exit temperature is controlled within 1020~1070℃; the finish rolling entrance temperature is controlled within 980~1050℃; the finish rolling final rolling temperature is controlled within 860~900℃; the coiling temperature is controlled within 660~710℃; the controlled cooling adopts the layer cooling mode of the front section cooling; 3) The hot rolling intermediate billet has a steel spacing of less than 60mm; 4) The heating schedule during hot rolling heating is formulated, and temporary remedial measures are taken for the in-furnace time exceeding the set in-furnace time; 5) The rough rolling adopts a 3+3 pass rolling mode, and the side pressure ratio distribution of the first pass and the third pass rolling of the vertical roller E1 and the first pass and the third pass of the vertical roller E2 is 40%~42%, 30%~32%, 18%~20%, 6%~12%; the effective contact surface height of the vertical roller is increased, and the effective contact surface height of the vertical roller is not less than 375mm; the cooling water shielding device is arranged on both sides of the vertical roller to prevent the temperature drop of the edge of the intermediate billet.

2. The method for preventing edge black line defects in hot-rolled low-carbon steel plates after cold rolling according to claim 1, characterized in that, In the measure 4), the temporary remedial measures taken are as follows: (1) The descaling water pressure is set to the highest upper limit value; (2) The thickness of the intermediate billet is reduced, and the reduction rate of the rough rolling is increased; (3) The billet after leaving the heating furnace is put into the hot coiling box; (4) The billet exceeding the set in-furnace time is marked on the HMI screen in the central control room, prompting the heating furnace operator to give cooling treatment according to the corresponding cooling schedule, and focusing on the surface iron oxide scale and the edge skin or black line defects; (5) The product grade is determined according to the product use, and the cold-rolled product of the high-grade automobile plate is changed to the ordinary commercial product.

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