Production method of hot rolled steel strip with high thickness precision

Abstract

The invention discloses a production method of a hot rolled steel strip with high thickness precision. The production method comprises the following steps of smelting, continuous casting, hot rolling and cooling. The off-line fixed length of the casting blank is 7-8 m; in the hot rolling step, reverse width rolling does not exceed 50 mm; according to the hot coil finished product thickness d, the intermediate billet thickness D and the hot coil head thickness abnormal length L, the descaling process of an intermediate billet head L0 area is determined; in the finish rolling process, the convexity C40 is smaller than 25 micrometers, the wedge shape W40 ranges from 0 micrometer to 10 micrometers, the tension of F6 and F7 ranges from 19 MPa to 21 MPa, and the speed compensation value of F6 ranges from-1.7% to-1.5%; through control of parameters in the continuous casting and hot rolling processes, C25 in the width direction of a full-rolled plate is controlled to be 33-40 microns, C40 in the width direction of the full-rolled plate is controlled to be 26-33 microns, W25 in the width direction of the full-rolled plate is controlled to be 20-25 microns, and W40 in the width direction of the full-rolled plate is controlled to be 17-20 microns, so that the thickness precision of the full-length direction and the thickness precision of the width direction of a hot-rolled steel strip produced

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 accuracy. Background Art

[0002] Saw blade steel is usually delivered in two states: hot rolled coil and cross-cut plate. The processing procedures of hot rolled coil in cold rolling mill are usually: hot rolled coil striping and trimming, then pickling, and then cold rolling and annealing according to the thickness of the finished product, and then subsequent heat treatment to improve the surface hardness. In order to improve the utilization rate of raw materials, avoid the use of targeted cold rolling due to the characteristics of the thickness distribution of striped coils and hot coil heads, middle and tails, and reduce unnecessary processing rates, it is bound to put forward higher requirements on the accuracy of the hot coil plate difference, that is, a smaller cold rolling pressure can achieve a finished product with a plate difference of 0.01-0.02mm.

[0003] When the delivery state is cross-cut boards, the cross-cut boards are laser cut according to the outer diameter of the saw blade, and then heat treated. In this process, there are two main ways to ensure the thickness accuracy of the finished product. One is to grind and calibrate both sides of the finished product. In addition to facilitating the control of thickness accuracy, it can also reduce the depth of the double-sided decarburization layer, which can improve the surface hardness and strength to a certain extent, but this approach leads to increased labor costs; the second is to increase the edge margin during the laser cutting process. Although the material yield rate is reduced to a certain extent, the grinding process is reduced. Compared with the first approach, the cost of the second approach is significantly reduced.

[0004] The special properties of saw blade steel require that the thickness accuracy of hot coils be higher than that of ordinary commercial materials. The thickness accuracy of hot coils mainly includes the thickness accuracy of the full length of the hot coils (especially the head of the hot coils) and the thickness accuracy in the width direction. The production methods of hot coils disclosed in the prior art mostly focus on the performance of the hot coils, and rarely pay attention to the thickness accuracy control of the hot coils. There is no production method for the thickness accuracy control of hot coils disclosed in the prior art.

[0005] For example, Chinese patent CN101773930A discloses a method for producing 65Mn hot-rolled steel plate, which is based on the thin slab continuous casting and rolling process, and mainly includes smelting, refining, thin slab continuous casting, soaking, high-pressure water descaling, hot rolling, cooling, and coiling steps. The superheat of molten steel T is 30℃<T≤45℃, the temperature of the casting entering the furnace T is 900℃≤T≤1050℃, the final rolling temperature T is 850℃≤T≤950℃, the coiling temperature T is 550℃≤T≤650℃, the liquid core pressure of the casting L is 2≤L≤20mm, and the depth of the single-side decarburization layer is 0.3-0.9% of the thickness of the steel plate. The chemical composition of the molten steel after alloying treatment in the refining process is: C: 0.62-0.70wt.%, Si: 0.17-0.37wt.%, Mn: 0.90-1.20wt.%, P≤0.035wt.%, S≤0.035wt.%, Cr≤0.25wt.%, Ni≤0.25wt.%, and the rest is Fe and inevitable impurities. The 65Mn hot-rolled steel plate produced by the method of the present invention has the characteristics of uniform structure and performance, high thermal stability and high strength, and the quality of the 65Mn hot-rolled steel plate has been significantly improved. However, the patent does not mention effective measures to improve the thickness accuracy of the hot-rolled coil. Summary of the invention

[0006] In order to solve the above technical problems, 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 process, the thickness precision of the hot-rolled steel strip produced by the traditional hot rolling process is improved, and the width direction of the coiled plate C25 is controlled to be 33-40 μm, C40 is 26-33 μm, W25 is 20-25 μm, and W40 is 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 according to the finished hot coil thickness d, the intermediate billet thickness D and the abnormal length L of the hot coil head thickness; wherein L0=d·L / D, the units of d and D are both mm, and the unit of L is 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 secondary high-pressure descaling, T1' is the outlet temperature of the previous roll R2, T2' is the temperature of the previous roll before secondary high-pressure descaling, V1 is the roller speed from the current roll R2 to the secondary high-pressure descaling, and V2 is the roller speed from the previous roll R2 to the secondary high-pressure descaling;

[0012] During the finishing rolling process, the convexity 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, and the descaling water volume is 530-550 m 3 / h.

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

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

[0016] In the hot rolling step, the cold rolled base material is prevented from directly changing into hot coil when scheduling the hot rolling.

[0017] The steel type of the hot-rolled steel strip is 65Mn steel; its chemical composition weight percentage content 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 bonding during the casting process; the liquidus temperature of this steel grade TL=1475℃, and the target temperature of the middle package is controlled at 15-30℃ above the liquidus temperature. The superheat of 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 crystal, resulting in central segregation dispersion. The superheat is high, the temperature gradient at the solidification front of the ingot is large, and the directional heat transfer time is long, which is conducive to the development of columnar crystals, and inhibits the formation of equiaxed crystals and increases central segregation. The steel casting process maintains a constant pulling speed of 1.0-1.2m / min. The constant pulling speed is conducive to the stability of the internal quality of the ingot, the flow rate of the molten steel is increased, and the probability of floating and removing large inclusions is reduced. The present invention has a high carbon content and high crack sensitivity. Especially in the hot charging production mode, no manual cleaning is used. Low pulling speed is one of the important measures to reduce cracks in the corners. And dynamic light pressure reduction and electromagnetic stirring are used, and the pressure reduction is 6.5-6.8mm, which is conducive to improving the internal quality of the ingot. In the hot rolling step, the ingot enters the heating furnace for heating, and the furnace temperature is controlled at 1180-1200℃; the preheating and the first heating period are 70-80min, and the heating rate is ≤5℃ / min; the second, third and soaking periods are 70-85min, and the heating rate is ≥5.8℃ / min. For medium and high carbon steel, the heating process directly affects the decarburization layer and then affects the surface hardness of the strip. No alloying elements are added to the steel strip. The heating temperature of no more than 1200℃ can make the ingot fully austenitized. By controlling the furnace temperature and the furnace time, the decarburization layer is ensured to be as low as possible. 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. In the finishing rolling stage, the deformation band and dislocation density in the deformed austenite are increased by accumulating large deformation, and the phase deformation nucleus is increased to refine the grain. In the cooling step, the rolled steel plate is coiled after laminar cooling. The cooling method is sparse cooling in the front section. The cooling rate is controlled at 8-10°C / s. The coiling temperature of the cooled steel plate is controlled at 700-740°C, which is slightly higher than Ar1. After coiling, the phase change gradually shifts from the head and tail of the hot coil to the center. Through the support of the inner and outer layers, the collapse of the hot coil can be eliminated. After the hot coil is off the line, it is concentrated in the high temperature zone for 72 hours for slow cooling to reduce internal stress and avoid defects such as edge cracks on the edges. By controlling the above-mentioned production process, a 65Mn steel with a microstructure of pearlite + a small amount of ferrite can be obtained, and its yield strength Rp0.2: 442 ~ 475MPa, Rm: 848 ~ 882MPa, yield strength ratio: 0.52 ~ 0.53, elongation: 22.0 ~ 24.0%, inclusion control B fineness 0.5, D fineness up to 1.0, hot rolled state performance is stable, and inclusion control results are good.

[0019] In the production method of the hot-rolled steel strip with high thickness precision provided by the present invention, the fixed length of the cast billet off-line is 7-8m, that is, the fixed length of the cast billet is shortened, and the length of the intermediate billet is reduced under the premise of increasing the thickness of the intermediate billet, so that the temperature difference between the head and the tail of the intermediate billet can be reduced, and when entering the 7-stand hot continuous rolling and finishing rolling, the centering is good and the wedge control is stable, thereby improving the thickness difference in the plate width direction. At the same time, the rolling force fluctuation of the head, middle and tail is small, which is conducive to improving the thickness fluctuation of the head and tail, especially the head, and can reduce the removal of steel strips with inconsistent thickness accuracy during subsequent leveling or cold rolling, so as to improve the yield rate.

[0020] When scheduling hot rolling, the plan connection should avoid the direct jump of cold-rolled base material to hot coil, so as to ensure wedge control. At the same time, the reverse width should not exceed 50mm during rolling. The width of 65Mn is mainly below 1250mm. Ma Steel basically produces it on the 1580 production line, and the 1580 production line mostly produces cold-rolled base materials. The strength difference between 65Mn and cold-rolled base materials is large. If the production schedule jumps directly, there will be no transition connection, the stiffness of the rolling mill will be greatly different, the model correction will not be timely, and the wedge control will be poor. At the same time, if the rolling reverse width is too large and the roller shifting is not timely, it will make the wedge difficult to control, which will lead to a large difference in the thickness of the coil.

[0021] After rough rolling, the ingot is formed into an intermediate billet. The temperature of the intermediate billet drops faster than that of the middle position. The rolling force compensation value calculated by the model is easy to reach the upper limit. When the rolling is completed in the area with lower temperature in the head, the compensation cancellation cannot offset the roll gap variable caused by the temperature difference. Therefore, 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 of the intermediate billet. The high-pressure water descaling machine has an adjustable upper water spray header and a fixed lower water spray header. According to 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, the "gradient descaling process" of the intermediate billet is determined. 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 convexity value is less than 25μm, and the wedge value is 0-10μm. The thickness accuracy in the width direction of the plate is controlled by small convexity and small wedge. At the same time, the working rolls of the finishing mill F6 and F7 use SVT1.2 F67 roll profiles, and the minimum convexity of the roll profile is -0.36mm. The thickness edge drop is improved by roll profile control. F6 and F7 tension: 19-21MPa, F6 speed compensation value: -1.7--1.5%. By reducing the tension and increasing the speed compensation value control means, the abnormal thinning of the center thickness of the hot coil head is avoided, which seriously reduces 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 ingot after rough rolling, the length of the intermediate ingot can be reduced, the temperature difference between the head and tail of the intermediate ingot can be reduced, the rolling stability can be improved, and the wedge shape can be well controlled.

[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 length 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 can be avoided, and the stability of the length and thickness of the hot rolled coil can be improved.

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

[0028] The present invention controls the hot coil width direction C25: 33-40 μm, C40: 26-33 μm, W25: 20-25 μm, W40: 17-20 μm through comprehensive control of the above processes, and the hot coil full-length thickness accuracy and width direction thickness accuracy 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 detection is carried out according to GB / T 4336 "Spark Source Atomic Emission Spectrometric Analysis Method for Carbon Steel and Medium and Low Alloy Steel (Conventional Method)".

[0032] Table 1 Chemical composition

[0033]

[0034]

[0035] The production method of 65Mn steel in each embodiment and comparative example of the present invention is as follows:

[0036] 1) Hot metal pretreatment: pre-slag removal and post-slag removal are required, and [S]≤0.0030% after hot metal desulfurization.

[0037] 2) Converter smelting: P is removed during the strengthening process, deoxidation and alloying are carried out during steel production, and strong stirring is carried out at the top and bottom of the argon station, and the strong stirring time is ≥4min.

[0038] 3) LF furnace refining: white slag operation to ensure full reduction of ladle top slag; adjust the chemical composition to the target value; [Ca] content is controlled at 15-30ppm, target: 20ppm, weak stirring time before and after calcium feeding line: 8-11min.

[0039] 4) Continuous casting: Use high carbon steel protective slag to reduce bonding during casting. The liquidus temperature of this steel is TL=1475℃, the target temperature of the tundish is controlled at 15-30℃ above the liquidus temperature, the casting process maintains a constant pulling speed of 1.0-1.2m / min, and dynamic light pressure and electromagnetic stirring are used, with a pressure reduction of 6.5-6.8mm. The fixed length of the billet off the line is 7-8m.

[0040] 5) Hot rolling production scheduling: Avoid direct change of cold-rolled base material to 65Mn hot-rolled coil in planning connection, and the width should not exceed 50mm during reverse rolling.

[0041] 6) Heating: The billet is heated in the heating furnace, and the temperature out of the furnace is controlled at 1180-1200℃. The time in the furnace is 140-150min. In the specific implementation process, the preheating and the first heating period are 70-80min, and the heating rate is ≤5℃ / min; the second, third and soaking periods are 70-85min, and the heating rate is ≥5.8℃ / min.

[0042] 7) Rolling: The rough rolling stage is carried out in the austenite recrystallization zone, the start temperature of the finishing rolling is controlled at 1000-1080°C, the cumulative reduction rate of the finishing rolling is ≥90%, and the final rolling temperature is controlled at 860-900°C.

[0043] According to the thickness d of the hot coil finished product, the thickness D of the intermediate billet and the abnormal length L of the thickness of the hot coil head, the descaling process of the intermediate billet head L0 area is determined; wherein L0 = d·L / D, the units of d and D are both mm, and the unit of L is m;

[0044] 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 secondary high-pressure descaling, T1' is the outlet temperature of the previous roll R2, T2' is the temperature of the previous roll before secondary high-pressure descaling, V1 is the roller speed from the current roll R2 to the secondary high-pressure descaling, and V2 is the roller speed from the previous roll R2 to the secondary high-pressure descaling;

[0045] During the finishing process, the convexity value is less than 25μm, and the wedge value is 0-10μm. At the same time, the working rolls of the finishing mills F6 and F7 use SVT1.2 F67 roll profile, and the minimum roll profile convexity is -0.36mm; the tension of F6 and F7 is 19-21MPa, and the speed compensation value of F6 is -1.7--1.5%.

[0046] 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°C / s. The coiling temperature of the cooled steel plate is controlled at 700-740°C. After the hot coil is off the line, it is concentrated in a high temperature area for 72 hours for slow cooling.

[0047] The main process parameters of the steelmaking process of each embodiment of the present invention and the comparative example are shown in Table 2.

[0048] Table 2 Specifications of ingot and intermediate billet

[0049]

[0050]

[0051] The heating process parameters of the embodiments of the present invention and the comparative examples are shown in Table 3.

[0052] Table 3 Heating process parameters

[0053]

[0054] The main process parameters of the rolling process of each embodiment of the present invention and the comparative example are shown in Table 4.

[0055] Table 4 Tension, compensation value and gradient descaling

[0056]

[0057]

[0058] The mechanical properties of the embodiments of the present invention and the comparative examples are shown in Table 5.

[0059] Table 5 Mechanical properties

[0060]

[0061] The inclusion results of the embodiments of the present invention and the comparative examples are shown in Table 6.

[0062] Table 6 Inclusion results of various embodiments of the present invention

[0063]

[0064]

[0065] The depth of decarburized layer of each embodiment of the present invention and comparative example is shown in Table 7.

[0066] Table 7 Decarburization layer depth of various embodiments of the present invention

[0067]

[0068] The thickness precision control conditions of the embodiments of the present invention and the comparative examples are shown in Table 8.

[0069] Table 8 Thickness precision control of various embodiments of the present invention

[0070]

[0071] In summary, the high-thickness precision saw blade steel designed and produced according to the chemical composition, steelmaking, heating, and rolling process provided by the present invention has a yield strength Rp0.2: 442-475MPa, Rm: 848-882MPa, a yield strength ratio: 0.52-0.53, and an elongation of 22.0-24.0%. The inclusion control B fineness is 0.5, and the maximum D fineness is 1.0. The hot rolling state performance is stable and the inclusion control result is good. Through the control of continuous casting, hot rolling process and process parameters, the thickness accuracy of the saw blade steel produced by the traditional hot rolling process can be improved, and the width direction of the coil is C25: 33-40μm, C40: 26-33μm, W25: 20-25μm, and W40: 17-20μm.

[0072] 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 accuracy, 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 according to the finished hot coil thickness d, the intermediate billet thickness D and the abnormal length L of the hot coil head thickness; wherein L0=d·L / D, the units of d and D are both mm, and the unit of L is 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 secondary high-pressure descaling, T1' is the outlet temperature of the previous roll R2, T2' is the temperature of the previous roll before secondary high-pressure descaling, V1 is the roller speed from the current roll R2 to the secondary high-pressure descaling, and V2 is the roller speed from the previous roll R2 to the secondary high-pressure descaling; During the finishing rolling process, the convexity 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 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.

3. The production method according to claim 1, characterized in that The working rolls of finishing mills F6 and F7 adopt SVT1.2 F67 roll profile, and the minimum roll profile crown is -0.36mm.

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

5. The production method according to claim 1, characterized in that: In the hot rolling step, the cold rolled base material is prevented from directly changing into hot coil when scheduling the hot rolling.

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

7. The production method according to claim 6, 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 for 70-80 minutes, and the heating rate is ≤5°C / min; the second, third and soaking periods last for 70-85 minutes, and the heating rate is ≥5.8°C / min.

8. The production method according to claim 6, characterized in that: In the hot rolling step, the finishing rolling start temperature is controlled at 1000-1080°C, the finishing rolling cumulative reduction ratio is ≥90%, and the final rolling temperature is controlled at 860-900°C.

9. The production method according to claim 6, 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.

10. The production method according to claim 6, 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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