Method for controlling cross breaks in hot-rolled strip

Abstract

This invention discloses a method for controlling transverse creases in hot-rolled strip steel, comprising the following steps: S1 Heating process: The slab is heated in a heating furnace, and the temperature of the upper surface of the slab is controlled to be 20-25°C higher than that of the lower surface in the soaking section; S2 Roughing process: The heated slab is rolled into an intermediate slab using a roughing mill, and an insulation cover is placed in the intermediate roller table at the roughing mill exit; S3 Finishing process: The intermediate slab is rolled into finished strip steel using a finishing mill, and the cooling water between the stands and the reverse spray water between the stands are controlled, with the final rolling temperature set to meet the condition of being higher than the ferrite transformation temperature; S4 Cooling and coiling process: Laminar flow cooling is used to cool the strip steel to the coiling temperature and coil it into a coil. The cooling mode uses front-end cooling, and the flow rate of the upper and lower manifolds of the laminar flow cooling in each section is controlled. This invention can effectively eliminate the transverse crease problem caused by the temperature difference between the upper and lower surfaces of the rolled piece, and plays a positive role in reducing production process costs; it has the advantages of good transverse crease elimination effect, easy control, and low cost.

Description

Technical Field

[0001] This invention relates to the field of hot-rolled steel technology, and in particular to a method for controlling transverse folds in hot-rolled strip steel. Background Technology

[0002] Horizontal creases are a common surface defect in the leveling process of hot-rolled low-carbon steel coils for low-strength structural applications. This defect cannot be completely eliminated by cold forming processes such as stamping, bending, and rolling, severely affecting the surface quality of finished structural parts, often leading to quality objections from customers. Particularly in medium-width hot-rolling lines, as slabs become thicker and rolling lines become longer, the numerous cooling points designed during rolling (including descaling, side spraying, reverse spraying, inter-stand cooling water, and dust removal water) significantly reduce the surface temperature of the rolled piece, increasing the temperature difference between the upper and lower surfaces. This temperature difference easily causes C-shaped defects in the finished product, making the problem of horizontal creases in medium-width hot-rolled coils more prominent. Adjusting the composition and process—such as increasing the yield strength, yield ratio, or eliminating the yield plateau—often yields poor results and negatively impacts downstream processing. Furthermore, simply optimizing the bending roll parameters to improve the plate shape cannot eliminate this defect. Currently, the most effective measure is to add a leveling process, but this increases processing costs and extends delivery time. Other studies have explored controlling the temperature difference between the upper and lower surfaces of rolled pieces using descaling water. For example, Chinese patent application No. 202211294753.9, "A Method for Controlling the Temperature Difference Between the Upper and Lower Surfaces of Medium-Thick Plates During Slab Rolling," controls this temperature difference by adjusting the pressure of the upper and lower manifolds in the descaling manifold. The drawback of this method is that it reduces the processing capacity of the descaling manifold and easily leads to surface quality problems caused by the intrusion of iron oxide scale. Another patent application, No. 201910023192.0, "A Method for Controlling the Temperature Difference Between the Upper and Lower Surfaces of a Finishing Intermediate Slab," controls the temperature difference between the upper and lower surfaces of the intermediate slab by adding an edge heater between the roughing and finishing mills. The disadvantage of this method is that it requires increased equipment investment and cannot overcome the temperature difference between the upper and lower surfaces of the rolled piece in and after the finishing mill. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a method for controlling transverse creases in hot-rolled strip steel that can effectively eliminate transverse creases.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following steps: S1, heating process: heating the slab in a heating furnace, wherein the temperature of the upper surface of the slab is controlled to be 20-25°C higher than the temperature of the lower surface in the heat soaking section; S2. Rough rolling process: The heated slab is rolled into an intermediate billet using a rough rolling mill, and the intermediate roller table at the rough rolling mill exit is placed under a heat insulation cover. S3. Finishing rolling process: The intermediate billet is rolled into finished strip steel using a finishing mill, and the cooling water between the stands and the reverse spray water between the stands are controlled. The final rolling temperature setting value meets the condition of being higher than the ferrite transformation temperature. S4. Cooling and Coiling Process: Laminar flow cooling cools the strip to the coiling temperature and coils it into a coil. The cooling mode uses front-end cooling and controls the flow rate of the upper and lower manifolds of each laminar flow cooling section.

[0005] Furthermore, in step S3, the flow rate ratio of the cooling water between the finishing mill stands is controlled at 1:(1.2~1.3), and the cooling water between the finishing mill stands and the reverse spray water between the stands are shut off for the last three stands.

[0006] Furthermore, in step S3, the final rolling temperature setpoint FDT=T f +C. Where T is in the formula. f The ferrite transformation temperature is given; C is a constant, ranging from 25 to 30 °C.

[0007] Furthermore, in step S4, the flow rates of the upper and lower manifolds of each laminar cooling section are controlled as follows: the laminar cooling is activated in the ultra-fast cooling section and / or the coarse adjustment section, as well as the fine adjustment section. The flow rates of the upper and lower manifolds of the ultra-fast cooling section are activated at a ratio of 1:(1.5 to 1.6); the flow rates of the upper and lower manifolds of the coarse adjustment section are activated at a ratio of 1:(1.3 to 1.4); and the flow rates of the upper and lower manifolds of the fine adjustment section are activated at a ratio of 1:(1.2 to 1.3).

[0008] The beneficial effects of adopting the above technical solution are as follows: By systematically controlling the factors affecting the temperature difference of the rolled piece throughout the hot rolling process, and combining this with the principle of phase transformation in steel, the problem of transverse wrinkles caused by the temperature difference between the upper and lower surfaces of the rolled piece is effectively eliminated without affecting rolling stability and surface quality, thus playing a positive role in reducing production process costs. This invention has the advantages of good transverse wrinkle elimination effect, easy control, and low cost. Attached Figure Description

[0009] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0010] Figure 1 This is a typical surface quality diagram of the upper surface of the hot-rolled strip steel obtained in an embodiment of the present invention; Figure 2 This is a typical surface quality diagram of the lower surface of hot-rolled strip steel obtained in an embodiment of the present invention. Detailed Implementation

[0011] The chemical composition and mass percentage content of the steel grades applicable to this method for controlling transverse creases in hot-rolled strip are as follows: C 0.05–0.08%, Mn 0.15–1.0%, Als 0.015–0.050%, and may also contain Si ≤0.10%, Ti ≤0.02%, and / or B ≤0.002%, with the remainder being Fe and unavoidable impurities; the steel grades include SPHC / 330CL, 380CL, and SAE1006B. This method includes sequential heating, roughing, finishing, and cooling / coiling processes; the following equipment is used: a regenerative heating furnace, two roughing mills, a roughing mill exit insulation cover, seven finishing mills, laminar flow cooling equipment, and an underground coiler; the processes for each step are described below: S1. Heating process: The slab thickness is 200-230 μm. The slab is heated in a heating furnace. The slab tapping temperature is 1190-1250℃. The slab tapping temperature refers to the temperature of the upper surface of the steel slab. The soaking zone controls the temperature of the upper surface of the slab to be 20-25℃ higher than the temperature of the lower surface. S2. Rough rolling process: The slab is rolled into an intermediate slab by three passes of the R1 two-roll reversible rolling mill and three passes of the R2 four-roll reversible rolling mill. The intermediate roller table at the exit of the rough rolling mill, i.e. the exit of the R2 mill, is put into a heat preservation cover to prevent the surface temperature of the intermediate rolled piece from dropping.

[0012] S3, Finishing Rolling Process: The intermediate billet is rolled into finished strip steel using a finishing mill. The flow rate ratio of the upper and lower surface cooling water (ISC) of the finishing mill stand is adjusted according to the ratio of upper surface cooling water flow rate to lower surface cooling water flow rate = 1:(1.2~1.3). The cooling water (ISC) of the finishing mill stand and the back spray water between the stands are turned off in the last 3 stands. The final rolling temperature setpoint FDT is set according to the following formula (1) to meet the condition of being higher than the ferrite transformation temperature: FDT=T f +C (1); In the formula, T f is the ferrite transformation temperature, in °C; C is a constant, ranging from 25 to 30 °C.

[0013] S4. Cooling and Coiling Process: Laminar flow cooling cools the strip to the coiling temperature and coils it into a coil. The cooling mode uses front-end cooling, and the coiling temperature is 640-650℃. Laminar flow cooling is implemented using ultra-fast cooling section + fine adjustment section, coarse adjustment section + fine adjustment section, or ultra-fast cooling section + coarse adjustment section + fine adjustment section. The flow ratio of the upper and lower manifolds of each laminar flow cooling section is controlled as follows: the flow ratio of the upper and lower manifolds of the ultra-fast cooling section is 1:(1.5-1.6); the flow ratio of the upper and lower manifolds of the coarse adjustment section is 1:(1.3-1.4); and the flow ratio of the upper and lower manifolds of the fine adjustment section is 1:(1.2-1.3). Example 1

[0014] Steel grade: SPHC / 330CL, chemical composition and mass percentage: C 0.05%, Mn 0.3%, Als 0.05%, the remainder being Fe and unavoidable impurities; slab thickness 200mm, finished product width 1500mm, finished product thickness 2.5mm.

[0015] S1. Heating process: The slab is heated to 1250℃ for steel tapping. The temperature of the upper surface of the slab is controlled at 1250℃ and the temperature of the lower surface is controlled at 1230℃ in the soaking zone.

[0016] S2. Rough rolling process: After heating, the slab is rolled in three passes by the R1 two-roll reversible rolling mill and three passes by the R2 four-roll reversible rolling mill to become an intermediate rolled piece. The intermediate roller table at the R2 exit is put into a heat preservation cover.

[0017] S3, Finishing Rolling Process: The finishing mill rolls the intermediate billet into finished strip steel. The flow ratio of the inter-stand cooling water ISC is adjusted to 1:1.2. The ISC and backspray water between the stands are shut off for the last three stands of the finishing mill. SPHC's T f According to the classic formula T f =910-230×C-21×Mn-15×Ni+32×Mo+45×Si+13×W+104×V is calculated to be 892℃, and the final rolling temperature setpoint FDT=892+25=917℃.

[0018] S4. Cooling and Coiling Process: The cooling mode uses front-end cooling, with a coiling temperature of 640℃. Laminar flow cooling is applied to the ultra-fast cooling section and the fine-tuning section. The flow rates of the upper and lower manifolds in the ultra-fast cooling section are applied at a ratio of 1:1.5, and the flow rates of the upper and lower manifolds in the fine-tuning section are applied at a ratio of 1:1.2. The strip is cooled to the coiling temperature and then coiled into a coil.

[0019] Typical surface quality of the upper and lower surfaces (D) of the SPHC / 330CL hot-rolled strip obtained in this embodiment is shown in the figure. Figure 1 , Figure 2 ,Depend on Figure 1 and Figure 2 It can be seen that the obtained SPHC / 330CL hot-rolled strip has no transverse creases on either the upper or lower surface, and the surface quality is good. Example 2

[0020] Steel grade: 380CL, chemical composition and mass percentage: C 0.08%, Mn 1.0%, Als 0.015%, Si 0.10%, Ti 0.02%, the remainder being Fe and unavoidable impurities; slab thickness 230mm, finished product width 2000mm, finished product thickness 6.5mm.

[0021] S1. Heating process: The slab is heated to a tapping temperature of 1190℃. The temperature of the upper surface of the slab is controlled at 1190℃ and the temperature of the lower surface is controlled at 1165℃ in the soaking zone.

[0022] S2. Rough rolling process: After heating, the slab is rolled in three passes by the R1 two-roll reversible rolling mill and three passes by the R2 four-roll reversible rolling mill to become an intermediate rolled piece. The intermediate roller table at the R2 exit is put into a heat preservation cover.

[0023] S3, Finishing Rolling Process: The finishing mill rolls the intermediate billet into finished strip steel. The flow ratio of the inter-stand cooling water ISC is adjusted to 1:1.3. The ISC and backspray water between the stands are shut off for the last three stands of the finishing mill. SPHC's T f According to the classic formula T f =910-230×C-21×Mn-15×Ni+32×Mo+45×Si+13×W+104×V is calculated to be 875℃, and the final rolling temperature setting value FDT=875+30=905℃.

[0024] S4. Cooling and Coiling Process: The cooling mode uses front-end cooling, with a coiling temperature of 645℃. Laminar flow cooling is applied to the ultra-fast cooling section, coarse adjustment section, and fine adjustment section. The flow rates of the upper and lower manifolds in the ultra-fast cooling section are applied at a ratio of 1:1.6, in the coarse adjustment section at a ratio of 1:1.4, and in the fine adjustment section at a ratio of 1:1.3. The strip is cooled to the coiling temperature and then coiled into a coil.

[0025] The 380CL hot-rolled strip obtained in this embodiment has no transverse creases on either the upper or lower surface, and the surface quality is good. Example 3

[0026] Steel grade: SAE1006B, chemical composition and mass percentage: C 0.05%, Mn 0.15%, Als 0.035%, Si 0.02%, B 0.002%, the remainder being Fe and unavoidable impurities; slab thickness 220mm, finished product width 1900mm, finished product thickness 2.0mm.

[0027] S1. Heating process: The slab is heated to 1230℃ for steel tapping. The temperature of the upper surface of the slab is controlled at 1230℃ and the temperature of the lower surface is controlled at 1208℃ in the soaking zone.

[0028] S2. Rough rolling process: After heating, the slab is rolled in three passes by the R1 two-roll reversible rolling mill and three passes by the R2 four-roll reversible rolling mill to become an intermediate rolled piece. The intermediate roller table at the R2 exit is put into a heat preservation cover.

[0029] S3. Finishing Rolling Process: The finishing mill rolls the intermediate billet into finished strip steel. The flow ratio of the inter-stand cooling water ISC is adjusted to 1:1.25. The ISC and backspray water between the stands are shut off for the last three stands of the finishing mill. (SAE1006B T) f According to the classic formula T f=910-230×C-21×Mn-15×Ni+32×Mo+45×Si+13×W+104×V is calculated to be 896℃, and the final rolling temperature setting value FDT=896+28=924℃.

[0030] S4. Cooling and Coiling Process: The cooling mode uses front-end cooling, with a coiling temperature of 650℃. Laminar flow cooling is applied to the coarse adjustment section and the fine adjustment section. The flow rates of the upper and lower manifolds in the coarse adjustment section are applied at a ratio of 1:1.3, and the flow rates of the upper and lower manifolds in the fine adjustment section are applied at a ratio of 1:1.25. The strip is cooled to the coiling temperature and then coiled into a coil.

[0031] The SAE1006B hot-rolled strip obtained in this embodiment has no transverse creases on either the upper or lower surfaces, and its surface quality is good.

Claims

1. A method for controlling transverse creases in hot-rolled strip steel, characterized in that, The process includes the following steps: S1, heating process: the slab is heated in a heating furnace, and the temperature of the upper surface of the slab is controlled to be 20-25°C higher than that of the lower surface in the heat soaking section; S2. Rough rolling process: The heated slab is rolled into an intermediate billet using a rough rolling mill, and the intermediate roller table at the rough rolling mill exit is placed under a heat insulation cover. S3. Finishing rolling process: The intermediate billet is rolled into finished strip steel using a finishing mill, and the cooling water between the stands and the reverse spray water between the stands are controlled. The final rolling temperature setting value meets the condition of being higher than the ferrite transformation temperature. The flow rate ratio of the cooling water between the stands of the finishing mill is controlled at 1:(1.2~1.3). The cooling water between the stands and the reverse spray water between the stands are turned off in the last 3 stands of the finishing mill. S4. Cooling and Coiling Process: Laminar flow cooling cools the strip to the coiling temperature and coils it into a coil. The cooling mode uses front-end cooling, and the flow rates of the upper and lower manifolds of each laminar flow cooling section are controlled. The flow rates of the upper and lower manifolds of each laminar flow cooling section are controlled as follows: for laminar flow cooling, the flow rates of the upper and lower manifolds of the ultra-fast cooling section and / or the coarse adjustment section and the fine adjustment section are 1:(1.5~1.6); the flow rates of the upper and lower manifolds of the coarse adjustment section are 1:(1.3~1.4); and the flow rates of the upper and lower manifolds of the fine adjustment section are 1:(1.2~1.3).

2. The method for controlling transverse wrinkles in hot-rolled strip steel according to claim 1, characterized in that: In step S3, the final rolling temperature setpoint FDT=T f +C, where T f is the ferrite transformation temperature; C is a constant, ranging from 25 to 30℃.

3. A method for controlling transverse wrinkles in hot-rolled strip steel according to claim 1 or 2, characterized in that: In step S1, the slab tapping temperature is 1190–1250℃.

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