A production method for hot-rolled steel strip with high thickness precision

Abstract

The invention discloses a method for producing a hot-rolled steel strip with high thickness precision. The method comprises the following steps: smelting, continuous casting, hot rolling, and cooling; the cast billet has a fixed length of 7 to 8 meters off the production line; in the hot rolling step, the reverse width rolling does not exceed 50 mm; a descaling process for the intermediate billet head L0 region is determined according to the thickness d of the hot coil finished product, the thickness D of the intermediate billet, and the abnormal thickness length L of the hot coil head; in the finishing rolling process, the convexity C40 is less than 25 μm, the wedge W40 is 0 to 10 μm, the F6 and F7 tensions are 19 to 21 MPa, and the F6 speed compensation value is -1.7 to -1.5%; by controlling the parameters of the continuous casting and hot rolling processes, the C25 in the width direction of the coil is controlled to be 33 to 40 μm, the C40 is controlled to be 26 to 33 μm, the W25 is controlled to be 20 to 25 μm, and the W40 is controlled to be 17 to 20 μm, thereby improving the thickness precision of the hot-rolled steel strip produced by the conventional hot rolling process in the full length and width directions.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production, and in particular relates to a production method of a hot-rolled steel strip with high thickness precision. Background Art

[0002] Saw blade steel is typically delivered in two states: hot-rolled coil and cross-cut plate. Hot-rolled coil is typically processed in the cold rolling mill as follows: slitting and trimming followed by pickling. Cold rolling and annealing are then performed according to the thickness of the finished product, followed by subsequent heat treatment to improve surface hardness. To improve raw material utilization, avoid targeted cold rolling due to the thickness distribution of slitting coils and hot coil headers, and reduce unnecessary processing, higher requirements are inevitably placed on the accuracy of the hot coil to cross-cut plate. Specifically, a smaller cold rolling reduction can achieve a cross-cut plate accuracy of 0.01-0.02mm.

[0003] When delivered in cross-cut form, the boards are laser cut according to the outer diameter of the saw blade and then heat treated. During this process, two main approaches are used to ensure the thickness accuracy of the finished product. The first is to polish and calibrate both sides of the finished product. This not only facilitates thickness control but also reduces the depth of the decarburized layer on both sides, improving surface hardness and strength to a certain extent. However, this approach results in increased labor costs. The second approach is to increase the edge allowance during the laser cutting process. Although this reduces the material yield, the reduction in polishing steps significantly reduces costs compared to the first approach.

[0004] The special properties of saw blade steel require higher thickness accuracy for hot coils compared to standard commercial steel. This accuracy primarily encompasses the overall thickness of the coil (especially at the coil's head) and the width. Existing hot coil production methods focus primarily on coil performance, with little attention paid to controlling thickness accuracy. Furthermore, no existing methods disclose methods for achieving this.

[0005] For example, Chinese patent CN101773930A discloses a method for producing 65Mn hot-rolled steel plate. This method is based on a thin slab continuous casting and rolling process and primarily includes smelting, refining, thin slab continuous casting, soaking, high-pressure water descaling, hot rolling, cooling, and coiling. The molten steel superheat, T, is 30°C < T, ≤ 45°C; the slab furnace entry temperature, T, is 900°C ≤ T, ≤ 1050°C; the finishing rolling temperature, T, is 850°C ≤ T, ≤ 950°C; the coiling temperature, T, is 550°C ≤ T, ≤ 650°C; the slab liquid core pressure, L, is 2 ≤ L, ≤ 20 mm; and the depth of the single-side decarburization layer is 0.3-0.9% of the steel plate thickness. The chemical composition of the molten steel after alloying during the refining process is: C: 0.62-0.70 wt.%, Si: 0.17-0.37 wt.%, Mn: 0.90-1.20 wt.%, P ≤ 0.035 wt.%, S ≤ 0.035 wt.%, Cr ≤ 0.25 wt.%, Ni ≤ 0.25 wt.%, with the remainder being Fe and unavoidable impurities. The 65Mn hot-rolled steel sheet produced using the method of the present invention exhibits uniform microstructure and properties, high thermal stability, and high strength, significantly improving the quality of the 65Mn hot-rolled steel sheet. However, the patent does not mention effective measures for improving the thickness accuracy of the hot-rolled coil. Summary of the Invention

[0006] To address the above technical issues, the present invention provides a method for producing hot-rolled steel strip with high thickness precision. By controlling the parameters of the continuous casting and hot rolling processes, the thickness precision of the hot-rolled steel strip produced by the conventional hot rolling process is improved, and the C25 in the width direction of the coil is controlled to be 33-40 μm, C40 to be 26-33 μm, W25 to be 20-25 μm, and W40 to be 17-20 μm. The technical solutions adopted by the present invention are as follows:

[0007] A method for producing a hot-rolled steel strip with high thickness precision, the method comprising the following steps: smelting, continuous casting, hot rolling, and cooling;

[0008] In the continuous casting step, the length of the cast billet off the line is 7 to 8 meters;

[0009] In the hot rolling step, the reverse width rolling does not exceed 50 mm;

[0010] In the hot rolling step, the descaling process of the intermediate billet head L0 region is determined based on the thickness d of the hot coil finished product, the thickness D of the intermediate billet, and the abnormal thickness length L of the hot coil head; wherein L0=d·L / D, d and D are both in mm, and L is in m;

[0011] The descaling process of the intermediate billet head L0 area is as follows: the pressure at the descaling manifold P = (17-18)(T1'-T2') / (T1-T2)·V1 / V2, in MPa; the descaling water volume V = (450-500)(T1'-T2') / (T1-T2)·V1 / V2, in m 3 / h; where T1 is the outlet temperature of the current roll R2, T2 is the temperature of the current roll before the secondary high-pressure descaling, T1' is the outlet temperature of the previous roll R2, T2' is the temperature of the previous roll before the secondary high-pressure descaling, V1 is the speed of the roller from the current roll R2 to the secondary high-pressure descaling, and V2 is the speed of the roller from the previous roll R2 to the secondary high-pressure descaling;

[0012] During the finishing rolling process, the crown value is less than 25μm, the wedge value is 0-10μm, the F6 and F7 tensions are 19-21MPa, and the F6 speed compensation value is -1.7--1.5%.

[0013] Furthermore, in the hot rolling step, the descaling process of the remaining length area of ​​the intermediate billet is as follows: the pressure at the descaling manifold is 18.5-19 MPa, the descaling water volume is 530-550 m 3 / h.

[0014] The working rolls of finishing mill F6 and F7 adopt SVT1.2 F67 roll profile, and the minimum roll profile is -0.36mm.

[0015] In the continuous casting step, the steel pouring process maintains a constant casting speed of 1.0 to 1.2 m / min, and dynamic soft reduction and electromagnetic stirring are used, with a reduction of 6.5 to 6.8 mm.

[0016] In the hot rolling step, the cold rolled base material is prevented from directly switching to hot coil during hot rolling production scheduling.

[0017] The steel grade of the hot-rolled steel strip is 65Mn steel; its chemical composition by weight percentage is C: 0.64% to 0.68%, Si: 0.20% to 0.30%, Mn: 0.90% to 1.10%, P: ≤0.015%, S: ≤0.008%, Alt: 0.010 to 0.030%; the rest is Fe and unavoidable inclusions.

[0018] When the steel grade of the hot-rolled steel strip is 65Mn steel, high-carbon steel protective slag is used in the continuous casting step to reduce adhesion during the casting process; the liquidus temperature TL of this steel grade is 1475°C, and the target temperature of the tundish is controlled at 15 to 30°C above the liquidus temperature. The superheat of the molten steel is crucial to the structure of the ingot. Reducing the superheat can effectively increase the equiaxed crystal rate and reduce the thickness of the columnar crystals, resulting in central segregation and dispersion. The high superheat leads to a large temperature gradient at the solidification front of the ingot, and the directional heat transfer time is maintained for a long time, which is conducive to the development of columnar crystals, and inhibits the formation of equiaxed crystals, increasing central segregation. The casting process maintains a constant pulling speed of 1.0 to 1.2 m / min. The constant pulling speed is conducive to the stability of the internal quality of the ingot, increases the flow rate of the molten steel, and reduces the probability of large inclusions floating up and being removed. The present invention has a high carbon content and high crack sensitivity. Especially in the hot charging production mode, no manual cleaning is required. Low pulling speed is one of the important measures to reduce cracks in the corners. Dynamic soft reduction and electromagnetic stirring are used, with a reduction of 6.5 to 6.8 mm, which is beneficial to improving the internal quality of the ingot. In the hot rolling step, the ingot enters the heating furnace for heating, and the furnace outlet temperature is controlled at 1180 to 1200°C; the preheating and first heating period lasts for 70 to 80 minutes, with a heating rate of ≤5°C / min; the second, third and soaking periods last for 70 to 85 minutes, with a heating rate of ≥5.8°C / min. For medium and high carbon steel, the heating process directly affects the decarburization layer and thus the surface hardness of the strip. No alloying elements are added to the steel strip. A heating temperature of no more than 1200°C can fully austenitize the ingot. By controlling the furnace outlet temperature and the time in the furnace, the decarburization layer is ensured to be as low as possible. The start temperature of finishing rolling is controlled at 1000-1080℃, the cumulative reduction rate of finishing rolling is ≥90%, and the final rolling temperature is controlled at 860-900℃. In the finishing rolling stage, large deformation is accumulated to increase the deformation band and dislocation density in the deformed austenite, increase the phase deformation nucleus and refine the grain. In the cooling step, the rolled steel plate is coiled after laminar cooling. The cooling method is front-stage sparse cooling. The cooling rate is controlled at 8-10℃ / s. The coiling temperature of the cooled steel plate is controlled at 700-740℃, slightly higher than Ar1. After coiling, the phase transformation gradually shifts from the head and tail of the hot coil to the center. The support of the inner and outer layers can eliminate the collapse of the hot coil. After the hot coil is off the line, it is concentrated in a high-temperature zone for 72 hours for slow cooling to reduce internal stress and avoid defects such as edge cracks. By controlling the above-mentioned production process, a 65Mn steel having a microstructure of pearlite + a small amount of ferrite can be obtained, with a yield strength Rp0.2 of 442-475 MPa, Rm of 848-882 MPa, a yield strength ratio of 0.52-0.53, an elongation of 22.0-24.0%, and inclusion fineness controlled to 0.5 for B and 1.0 for D at most. The hot-rolled state performance is stable and the inclusion control results are good.

[0019] In the method for producing high-thickness-precision hot-rolled steel strip provided by the present invention, the cut-length of the cast slab off-line is 7-8 meters, meaning the cut-length is shortened. Simultaneously, while the intermediate slab thickness is increased, the intermediate slab length is reduced. This minimizes the temperature difference between the head and tail of the intermediate slab, ensuring good centering and stable wedge control during finishing rolling in the seven-stand hot continuous rolling mill, thereby improving thickness variation across the strip width. Furthermore, the rolling force fluctuations at the head, middle, and tail are minimal, facilitating improvements in thickness fluctuations at the head and tail, particularly at the head. This reduces the removal of steel strip that does not meet thickness precision during subsequent leveling or cold rolling, thereby improving the yield rate.

[0020] When scheduling hot rolling, the plan should avoid switching directly from cold-rolled base material to hot coils to ensure wedge control. At the same time, the reverse width should not exceed 50mm during rolling. The width of 65Mn is mostly below 1250mm. Maanshan Iron and Steel primarily produces it on the 1580 production line, which primarily produces cold-rolled base material. The strength difference between 65Mn and cold-rolled base material is significant. If the production schedule changes directly, there will be no transition, resulting in significant differences in mill stiffness, untimely model correction, and poor wedge control. At the same time, if the reverse width is too large and the roll shifting is not timely, the wedge will be difficult to control, which will in turn lead to large variations in coil thickness.

[0021] After rough rolling, the ingot is formed into an intermediate billet. Since the cross-section of the intermediate billet cools down faster than the middle position, the rolling force compensation value calculated by the model is likely to reach the upper limit. When the rolling is completed in the area with lower temperature at the head, the compensation cannot offset the roll gap variation caused by the temperature difference, so the head thickness abnormality will appear. The finishing high-pressure water descaling machine is placed in front of the finishing mill to remove the iron oxide scale from the intermediate billet. The high-pressure water descaling machine has an adjustable upper water spray manifold and a fixed lower water spray manifold. The "gradient descaling process" for the intermediate billet is determined based on the thickness d of the hot coil finished product, the thickness D of the intermediate billet, and the abnormal length L of the hot coil head thickness. Through this process, the temperature uniformity of the intermediate billet in the length direction, especially the head, is improved after descaling.

[0022] During the finishing process, the crown value is less than 25μm, and the wedge value is 0-10μm. This low crown and wedge are used to control the thickness accuracy across the plate width. The work rolls of the finishing mills F6 and F7 use an SVT1.2 F67 profile with a minimum crown of -0.36mm. This profile control is used to improve the edge drop. The tensions for F6 and F7 are 19-21MPa, and the speed compensation for F6 is -1.7--1.5%. By reducing tension and increasing speed compensation, the center thickness of the hot coil head is prevented from being abnormally thinned, which could severely reduce the yield rate.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) By optimizing the fixed length of the ingot and the thickness of the intermediate billet after rough rolling, the length of the intermediate billet can be reduced, the temperature difference between the head and tail of the intermediate billet can be reduced, the rolling stability can be improved, and good wedge shape control can be achieved.

[0025] 2) Improve the temperature uniformity of the intermediate billet through the "gradient descaling process", thereby improving the thickness uniformity of the strip in the longitudinal direction.

[0026] 3) By well matching the rack tension and speed compensation, the abnormal thinning of the center thickness of the hot rolled coil head is avoided, and the stability of the length and thickness of the hot rolled coil is improved.

[0027] 4) Through the good transition of hot rolling production schedule, the excellent control of small crown and small wedge in the finishing rolling process, and the optimization of the roll profile of the F6 and F7 work rolls of the finishing mill, the thickness accuracy of the strip width direction is improved and the unnecessary processing rate in the cold rolling process is reduced.

[0028] The present invention controls the width direction C25 of the hot coiled plate to 33-40 μm, C40 to 26-33 μm, W25 to 20-25 μm, and W40 to 17-20 μm through comprehensive control of the above processes, and the thickness accuracy of the hot coil along the entire length and in the width direction are both high. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the metallographic structure diagram of the 65Mn steel produced by the present invention, and the metallographic structure is pearlite + a small amount of ferrite. DETAILED DESCRIPTION

[0030] The present invention will be described in detail below by taking the production of 65Mn steel as an example.

[0031] The chemical compositions of the embodiments and comparative examples of the present invention are shown in Table 1. The composition test was carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis of Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

[0032] Table 1 Chemical composition

[0033]

[0034] The production methods of 65Mn steel in the embodiments and comparative examples of the present invention are as follows:

[0035] 1) Hot metal pretreatment: Pre-slag and post-slag skimming are required, and [S] is required to be ≤ 0.0030% after hot metal desulfurization.

[0036] 2) Converter smelting: P is removed during the strengthening process, deoxidation and alloying are carried out during steel tapping, and strong stirring is carried out at the top and bottom of the argon station, and the strong stirring time is ≥4 minutes.

[0037] 3) LF furnace refining: white slag operation to ensure that the ladle top slag is fully reduced; adjust the chemical composition to the target value; [Ca] content is controlled at 15-30ppm, target: 20ppm, weak stirring time before and after feeding calcium line: 8-11min.

[0038] 4) Continuous Casting: High-carbon steel mold slag is used to reduce sticking during the casting process. The liquidus temperature of this steel grade, TL, is 1475°C, and the target tundish temperature is controlled 15-30°C above the liquidus temperature. During the casting process, a constant casting speed of 1.0-1.2 m / min is maintained, and dynamic soft reduction and electromagnetic stirring are used, with a reduction of 6.5-6.8 mm. The fixed length of the strand coming off the line is 7-8 m.

[0039] 5) Hot rolling production scheduling: Avoid direct transition of cold-rolled base material to 65Mn hot-rolled coil in planning connection, and the width should not exceed 50mm during reverse rolling.

[0040] 6) Heating: The slab enters the heating furnace and is heated, with the exit temperature controlled at 1180-1200°C. The furnace temperature is 140-150 minutes. Specifically, the preheating and first heating phases last 70-80 minutes, with a heating rate of ≤5°C / min. The second, third, and soaking phases last 70-85 minutes, with a heating rate of ≥5.8°C / min.

[0041] 7) Rolling: The rough rolling stage is carried out in the austenite recrystallization zone, the finishing rolling start temperature is controlled at 1000-1080°C, the finishing rolling cumulative reduction rate is ≥90%, and the final rolling temperature is controlled at 860-900°C.

[0042] Determine the descaling process for the intermediate billet head L0 based on the finished hot coil thickness d, the intermediate billet thickness D, and the abnormal thickness length L of the hot coil head. Where L0 = d·L / D, where d and D are both in mm and L is in m.

[0043] The descaling process of the intermediate billet head L0 area is as follows: the pressure at the descaling manifold P = (17-18)(T1'-T2') / (T1-T2)·V1 / V2, in MPa; the descaling water volume V = (450-500)(T1'-T2') / (T1-T2)·V1 / V2, in m 3 / h; where T1 is the outlet temperature of the current roll R2, T2 is the temperature of the current roll before the secondary high-pressure descaling, T1' is the outlet temperature of the previous roll R2, T2' is the temperature of the previous roll before the secondary high-pressure descaling, V1 is the speed of the roller from the current roll R2 to the secondary high-pressure descaling, and V2 is the speed of the roller from the previous roll R2 to the secondary high-pressure descaling;

[0044] During the finishing rolling process, the crown value is less than 25μm and the wedge value is 0-10μm. At the same time, the working rolls of the finishing mill F6 and F7 use SVT1.2 F67 roll profile, and the minimum roll profile crown is -0.36mm; the tension of F6 and F7 is 19-21MPa, and the speed compensation value of F6 is -1.7--1.5%.

[0045] 8) Laminar cooling: The rolled steel plate is coiled after laminar cooling. The cooling method is front-stage sparse cooling. The cooling rate is controlled at 8-10℃ / s. The coiling temperature of the cooled steel plate is controlled at 700-740℃. After the hot coil is off the line, it is concentrated in a high temperature area for 72 hours for slow cooling.

[0046] The main process parameters of the steelmaking process of each embodiment of the present invention and the comparative example are shown in Table 2.

[0047] Table 2 Specifications of ingots and intermediate billets

[0048]

[0049]

[0050] The heating process parameters of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0051] Table 3 Heating process parameters

[0052]

[0053] The main process parameters of the rolling process of each embodiment of the present invention and the comparative example are shown in Table 4.

[0054] Table 4 Tension, compensation value and gradient descaling

[0055]

[0056]

[0057] The mechanical properties of the embodiments of the present invention and the comparative examples are shown in Table 5.

[0058] Table 5 Mechanical properties

[0059]

[0060] The inclusion results of the embodiments of the present invention and the comparative examples are shown in Table 6.

[0061] Table 6 Inclusion results of various embodiments of the present invention

[0062] Sample number A thick A thin B thick B thin C thick C thin D thick D thin Ds Banded tissue Example 1 0 0 0 0.5 0 0 0.5 0.5 0 1 Example 2 0 0 0 0.5 0 0 0 0.5 0 1 Example 3 0 0 0 0.5 0 0 0 0.5 0 0.5 Example 4 0 0 0 0.5 0 0 0 1 0 0.5 Comparative Example 1 0 0 0 0.5 0 0 0 0.5 0 1 Comparative Example 2 0 0 0 0.5 0 0 0 0.5 0 1

[0063] The depth of the decarburized layer of each embodiment of the present invention and the comparative example is shown in Table 7.

[0064] Table 7 Depth of decarburized layer in various embodiments of the present invention

[0065]

[0066] The thickness precision control conditions of the embodiments of the present invention and the comparative examples are shown in Table 8.

[0067] Table 8 Thickness precision control of various embodiments of the present invention

[0068]

[0069]

[0070] In summary, the high-thickness, precision saw blade steel designed and produced according to the chemical composition, steelmaking, heating, and rolling processes provided by the present invention achieves a yield strength Rp0.2 of 442-475 MPa, Rm of 848-882 MPa, a yield strength ratio of 0.52-0.53, and an elongation of 22.0-24.0%. Inclusions are controlled to a fineness of 0.5 for B and a maximum fineness of 1.0 for D. The hot-rolled state exhibits stable performance and excellent inclusion control. Through continuous casting, hot rolling, and process parameter control, the thickness precision of saw blade steel produced by conventional hot rolling processes can be improved, achieving a widthwise C25 of 33-40 μm, a C40 of 26-33 μm, a W25 of 20-25 μm, and a W40 of 17-20 μm.

[0071] The above-mentioned detailed description of a method for producing a hot-rolled steel strip with high thickness precision with reference to the embodiment is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for producing a hot-rolled steel strip with high thickness precision, characterized in that: The production method comprises the following steps: smelting, continuous casting, hot rolling, and cooling; In the continuous casting step, the length of the cast billet off the line is 7 to 8 meters; In the hot rolling step, the reverse width rolling does not exceed 50 mm; In the hot rolling step, the descaling process of the intermediate billet head L0 region is determined based on the thickness d of the hot coil finished product, the thickness D of the intermediate billet, and the abnormal thickness length L of the hot coil head; wherein L0=d·L / D, d and D are both in mm, and L is in m; The descaling process of the intermediate billet head L0 area is as follows: the pressure at the descaling manifold P = (17~18)(T1'-T2') / (T1-T2)·V1 / V2, in MPa; the descaling water volume V = (450~500) (T1'-T2') / (T1-T2)·V1 / V2, in m 3 / h; where T1 is the outlet temperature of the current roll R2, T2 is the temperature of the current roll before the secondary high-pressure descaling, T1' is the outlet temperature of the previous roll R2, T2' is the temperature of the previous roll before the secondary high-pressure descaling, V1 is the speed of the roller from the current roll R2 to the secondary high-pressure descaling, and V2 is the speed of the roller from the previous roll R2 to the secondary high-pressure descaling; During the finishing rolling process, the crown value is less than 25μm, the wedge value is 0~10μm, the F6 and F7 tensions are 19~21MPa, and the F6 speed compensation value is -1.7~-1.5%.

2. The production method according to claim 1, characterized in that In the hot rolling step, the descaling process of the remaining length of the intermediate billet is as follows: the pressure at the descaling manifold is 18.5~19MPa, the descaling water volume is 530~550m 3 / h.

3. The production method according to claim 1, characterized in that In the continuous casting step, the steel pouring process maintains a constant casting speed of 1.0-1.2 m / min, and dynamic soft reduction and electromagnetic stirring are used, with a reduction of 6.5-6.8 mm.

4. The production method according to claim 1, characterized in that In the hot rolling step, the cold rolled base material is prevented from directly switching to hot coil during hot rolling production scheduling.

5. The production method according to claim 1, characterized in that The steel grade of the hot-rolled steel strip is 65Mn steel.

6. The production method according to claim 5, characterized in that In the hot rolling step, the ingot is heated in a heating furnace, and the furnace temperature is controlled at 1180-1200°C; the preheating and first heating period lasts 70-80 minutes, and the heating rate is ≤5°C / min; the second, third and soaking periods last 70-85 minutes, and the heating rate is ≥5.8°C / min.

7. The production method according to claim 5, characterized in that In the hot rolling step, the start temperature of finishing rolling is controlled at 1000-1080°C, the cumulative reduction rate of finishing rolling is ≥90%, and the final rolling temperature is controlled at 860-900°C.

8. The production method according to claim 5, characterized in that In the cooling step, the rolled steel plate is coiled after laminar cooling, the cooling method is front-stage sparse cooling, the cooling rate is controlled at 8-10°C / s, and the coiling temperature of the cooled steel plate is controlled at 700-740°C.

9. The production method according to claim 5, characterized in that After the hot coils come off the line, they are piled together and cooled slowly for 72 hours.

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