Process for producing 65mn by thin slab continuous casting and rolling

Abstract

This invention discloses a process for producing 65Mn steel through continuous casting and rolling of thin slabs, belonging to the field of steel production. The method includes the following steps: steelmaking, casting and rolling, and slow cooling. The steel composition is controlled as follows: C: 0.62%-0.70%, Si: 0.17%-0.37%, Mn: 0.90%-1.20%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe and impurities. In the continuous casting process, the slab thickness is 90mm-110mm, the casting speed is controlled at 4.0m / min-4.8m / min, and the temperature difference of the slab cross-section is controlled at 40℃-65℃. In the finishing rolling process, the F3 work roll uses high-speed steel rolls, and the F4 and F5 work rolls use high-nickel-chromium rolls. The crown of the F3 work roll is set to be 200µm larger than that of other steel grades, and the load distribution of each stand follows a decreasing principle. Compared with existing technologies, this method improves the yield rate of straight hair curls.

Description

Technical Field

[0001] This invention is a divisional application of "A method for controlling the shape of high carbon steel in the continuous casting and rolling production of thin slabs" (2021108734569), which relates to a steel production method, particularly the process of continuous casting and rolling of 65Mn steel in thin slabs. Background Technology

[0002] Thin slab continuous casting and rolling of high carbon steel offers advantages such as high dimensional accuracy, no segregation in microstructure, uniform composition, and thinner limit specifications, saving on annealing and cold rolling costs. Downstream customers are primarily in the cold rolling industry, with the main processing flow being pickling (or slitting → pickling) → cold rolling → annealing → cold rolling → slitting. Products are widely used in hardware industries such as saw blades, measuring tapes, logistics tensioners, and ticket clips. Surveys indicate that customers generally recognize and are satisfied with the advantages of thin-gauge products and the control of finished product dimensional accuracy in thin slab continuous casting and rolling of high carbon steel. However, there are many quality objections or complaints regarding poor uncoiling shape in direct-coil production. Specific problems include severe distorted shape, which reduces the efficiency of slitting and pickling processes, and in severe cases, causes edge scraping, rotten edges, and uneven coiling during pickling, leading to a decline in customer product experience and satisfaction.

[0003] A literature search revealed that patent CN110624954A discloses a method for controlling the shape of hot-rolled thin-gauge high-strength weathering steel. Targeting the characteristics of weathering steel, this method primarily improves the shape of thin-gauge high-strength weathering steel by optimizing load distribution, adjusting the shifting and bending rolls, improving laminar cooling uniformity, and slowing the cooling of the coil after it leaves the production line. Patent CN111250549A discloses a method for precisely controlling the shape by adjusting the temperature of the emulsion sprayed onto the work rolls to change the crown of the work rolls. Patent CN110404978A discloses a method for dynamically controlling the shape of the entire strip length by combining the real-time speed and temperature of the strip and dynamically compensating for the target straightness of the strip. Patent CN111451294A discloses a method for improving the shape accuracy of hot-rolled strip steel by introducing a stiffness adjustment correction coefficient when calculating the roll gap value and rolling force load. In summary, the main principles of hot-rolled conventional strip shape control are to control the roll gap accuracy under mill load, reduce uneven deformation during rolling, or improve pre-calculation accuracy by optimizing secondary model calculations and compensating for stiffness coefficients. Additionally, measures such as controlling laminar cooling uniformity and slow cooling of the coil after it leaves the rolling mill are used to reduce the impact of internal stress on strip shape.

[0004] Due to the significant differences in layout and steel grade process characteristics between thin slab continuous casting and rolling production lines and traditional production lines, even when adopting the aforementioned shape control concepts and corresponding control points, shape defects still occur in the production of medium and high carbon steel due to bright strips or ribs on the coils. The shape defect rate of directly cast coils exceeds 30%, indicating poor control effectiveness. Therefore, it is urgent to research and develop a shape control method that conforms to the characteristics of thin slab continuous casting and rolling production lines and the process requirements of medium and high carbon steel. Summary of the Invention

[0005] The technical objective of this invention is to address the shortcomings of the prior art by providing a process for the continuous casting and rolling of 65Mn slabs, aiming to reduce the defect rate of straight-rolled coils, meet the raw material requirements of downstream processes such as pickling and slitting, and improve the user experience and customer satisfaction.

[0006] The technical solution of this invention to solve its technical problem is: a process for producing 65Mn by continuous casting and rolling of thin slabs, the method steps including: steelmaking process, casting and rolling process, and slow cooling process; the casting and rolling process includes continuous casting, rough rolling, induction heating, finish rolling, layer cooling and coiling; characterized in that: the composition of molten steel is controlled as follows: C: 0.62%-0.70%, Si: 0.17%-0.37%, Mn: 0.90%-1.20%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe. And impurities; in the continuous casting process, the billet thickness is 90mm-110mm, the casting speed is controlled within the range of 4.0m / min-4.8m / min, and the temperature difference of the billet cross section is controlled within the range of 40℃-65℃; in the finishing rolling process, the F3 work roll adopts high-speed steel roll, and the F4 and F5 work rolls adopt high-nickel-chromium roll; the crown of the F3 work roll is set to be 200um larger than that of other steel grades, and the load distribution of each stand adopts the principle of decreasing sequentially; in the laminar cooling process, a set of cooling water manifolds in the laminar cooling section is turned on, the upper and lower valves are opened at 100%, and the steel strip is cooled to 590±10℃ through the laminar flow section.

[0007] Furthermore, in the aforementioned roughing process, the thickness of the intermediate billet is set to 8mm-20mm.

[0008] Furthermore, the exit temperature of the roughing mill is controlled at 800℃-950℃.

[0009] Furthermore, the outlet temperature of the aforementioned IH induction heating furnace is set at 1100℃-1250℃.

[0010] Furthermore, the exit temperature of the aforementioned finishing mill is controlled at 780℃-880℃.

[0011] Compared with the prior art, the present invention has the following outstanding advantages:

[0012] 1. This invention addresses the issue of bright stripe defects in steel coils, meeting the characteristics of production lines and the process requirements for medium and high carbon steel, reducing the defect rate of directly rolled coils to less than 10%.

[0013] 2. This invention innovatively uses the method of opening an appropriate amount of cooling water in the layer to cause the strip steel to undergo a phase transformation in advance before coiling, which effectively solves the problem of poor plate shape caused by the bright strip at the edge of the steel coil in the continuous casting and rolling production of thin slabs, and improves the good plate shape rate of direct-cast coils.

[0014] 3. This invention not only improves the user experience and customer satisfaction, but also provides a strong guarantee for maintaining China's leading advantage in the rolling of high carbon steel in thin slabs. Detailed Implementation

[0015] The present invention will be further described below with reference to specific embodiments.

[0016] This invention provides a method for controlling the shape of high-carbon steel in the continuous casting and rolling production of thin slabs.

[0017] The specific production steps include:

[0018] (1) Steelmaking process.

[0019] The raw materials are smelted in a decarburizing furnace and a refining furnace in sequence, and the composition of the steel produced meets the standard requirements.

[0020] (2) Continuous casting process.

[0021] The qualified molten steel is poured into continuous billets. The billet thickness is 90mm-110mm, the casting speed during the continuous casting stage is controlled within the range of 4.0m / min-4.8m / min, and the temperature difference of the billet cross section is controlled within 70℃.

[0022] The reason this invention controls the casting speed during the continuous casting stage to be between 4.0 m / min and 4.8 m / min is that in a thin slab continuous casting and rolling production line, the continuous casting process and the rolling process are rigidly connected, without the traditional heating buffer. The casting speed is the foundation of the entire production line's speed. Medium and high carbon steel has high resistance to hot deformation, large rolling load, and strong shape sensitivity, making production difficult. Therefore, the casting speed must be controlled between 4.0 m / min and 4.8 m / min; too high or too low a speed will have adverse effects. Furthermore, the casting speed affects the roll rotation speed, which in turn affects the cooling efficiency of the roll cooling water per unit time, thus affecting the roll thermal crown and rolling stability.

[0023] The reason this invention controls the temperature difference across the billet section to within 70°C is that the induction heating furnace in the thin slab continuous casting and rolling production line uses surface skin heating, which has limited heating depth and short heating time. Larger billet temperature differences (>70°C) cannot be effectively mitigated by induction heating, leading to uneven wear of the finishing rolls, affecting the smoothness of the section control, and exacerbating uneven deformation of the strip.

[0024] (3) Rough rolling process.

[0025] The thickness of the intermediate billet is set to 8mm-20mm.

[0026] If a thinner intermediate billet is selected, the roughing rolling load will increase significantly, and the centerline of the roughing mill exit will fluctuate considerably. The instability of the roughing mill will simultaneously affect the stability of the finishing and coiling areas. If a thicker intermediate billet is selected, on the one hand, it will directly increase the finishing rolling load, directly affecting the stability of the finishing mill; on the other hand, it will cause vibration of the finishing mill, resulting in vibration marks on the strip surface.

[0027] The exit temperature of the roughing mill is controlled at 800℃-950℃.

[0028] (4) Induction heating.

[0029] The outlet temperature of the IH induction heating furnace is set to 1100℃-1250℃.

[0030] (5) Finishing rolling process.

[0031] The exit temperature of the finishing mill is controlled above 780℃.

[0032] The reason this invention controls the roughing mill exit temperature at 800℃-950℃, the IH induction heating furnace exit temperature at 1100℃-1250℃, and the finishing mill exit temperature above 780℃ is that a higher finishing rolling temperature ensures that medium and high carbon steel is always rolled in the austenitic region, guaranteeing the accuracy of the secondary load model calculation and enhancing rolling stability. Furthermore, since the maximum heating capacity of the IH induction heating is around 300℃, it is necessary to ensure that the roughing mill exit temperature is above 800℃.

[0033] The F3 work roll uses high-speed steel, while the F4 and F5 work rolls use high-nickel-chromium rolls. The reason this invention requires the latter two rolls, F3 and F4 / F5, to be made of high-speed steel and high-nickel-chromium, is that medium- and high-carbon steel has a high alloy content and a large coefficient of thermal friction, resulting in relatively faster roll wear. Using high-speed steel for F3 improves the roll's resistance to uneven wear and its roll shape retention. Furthermore, the centerline stability of medium- and high-carbon steel is relatively poor compared to other steel grades, posing a risk of scrap. Using high-nickel-chromium rolls for F4 and F5 reduces the probability of roll chipping and spalling in the event of an accident (high-nickel-chromium rolls have stronger resistance to crack propagation than high-speed steel rolls), preventing further escalation of the accident.

[0034] The F3 work roll crown is set 200µm larger than that used for other steel grades, and the load distribution across each stand follows a decreasing principle. The reason this invention requires a slightly larger F3 work roll crown compared to other steel grades is that the outermost edges of the medium-high carbon steel billet have a much lower temperature than the middle, resulting in a smaller thermal crown at the corresponding positions on both sides of the roll. This means the actual load-bearing roll gap on both sides is larger than in the middle, which is detrimental to controlling edge warping on both sides of the strip cross-section. Increasing the F3 roll crown effectively compensates for the actual roll gap shape on the machine, reducing the tendency for edge warping. Furthermore, the decreasing load distribution across the stands is primarily to adhere to the principle of proportional crown control, ensuring uniform deformation at all points on the cross-section.

[0035] (6) Layer cooling and winding.

[0036] During normal production, the layer cooling process is air cooling. If the steel coil shows a bright band, turn on a set of cooling water manifolds in the later stage of layer cooling, with the water valve opening ratio being 30%-100%; control the coiling temperature to be no lower than 580℃.

[0037] The rationale behind the above layer cooling and coiling control is as follows: Since the downstream process for medium and high carbon steel is mainly cold rolling, to reduce strength and facilitate cold rolling processing by customers, the layer cooling process is normally set to air cooling. However, in actual production, some steel coils often exhibit bright strips or ribs at the edges, resulting in poor uncoiling shape of the straight-rolled coil. For bright strips or ribs, the conventional rolling line control strategy mainly focuses on controlling the smoothness of the finish mill exit section and controlling local high points. However, when producing medium and high carbon steel on a thin slab continuous casting and rolling line, even with smooth section control, bright strips or ribs can still occur, and the position of the bright strip does not correspond to the position of the high point on the section. Controlling the coiling temperature below the pearlite phase transformation point of medium and high carbon steel ensures that the strip undergoes a phase transformation before coiling. This effectively avoids the problem of bright strips caused by the volume expansion of the strip due to the pearlite phase transformation during coiling, which results in the edge of the strip being pressed against the chamfered corners of the coiler's pinch rolls.

[0038] Therefore, when a bright strip appears on the steel coil, the cooling water in a set of manifolds in the later stage of the laminar flow cooling can be turned on (valve ratio 20%-100%) to reduce the temperature by an additional 10℃-20℃, so that coiler #1 becomes coiler #2 and coiler #2 becomes coiler #3. However, considering that the laminar flow cooling will cause the strip strength to increase to a certain extent, which is not conducive to cold rolling, the coiling temperature must be controlled not to be lower than 580℃.

[0039] (7) Slow cooling.

[0040] After being rolled up, the product is stacked in the warehouse and allowed to cool slowly for at least 48 hours.

[0041] To better compare the process of this application with the prior art, comparative experiments were conducted.

[0042] (1) Production of 50 steel by continuous casting and rolling of thin slabs

[0043] Examples 1-4 employ the method of the present invention, with high-speed steel rolls used for work roll F3 and high-nickel-chromium rolls used for work rolls F4 and F5; Comparative Example 1 employs the group's low-carbon steel plate shape control production method.

[0044] The process flow for each group is as follows: blast furnace → molten iron pretreatment → decarburization furnace → LF refining → continuous casting → rough rolling → induction heating → finish rolling → stratified cooling → coiling → slow cooling → finished product.

[0045] All groups of raw materials, such as molten iron and scrap steel, are sequentially smelted in a decarburizing furnace and refined in an LF furnace. The composition of the molten steel is controlled as follows: C: 0.47%-0.55%, Si: 0.17%-0.37%, Mn: 0.50%-0.80%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe and impurities.

[0046] In Examples 1-4, the process flow for producing 50 steel by continuous casting and rolling of thin slabs is as follows: blast furnace → hot metal pretreatment → decarburization furnace → LF refining → continuous casting → rough rolling → induction heating → finish rolling → layer cooling → coiling → slow cooling → finished product.

[0047] In this embodiment, molten iron and scrap steel are sequentially smelted in a decarburizing furnace and refined in an LF furnace. The composition of the molten steel is controlled as follows: C: 0.47%-0.55%, Si: 0.17%-0.37%, Mn: 0.50%-0.80%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe and impurities.

[0048] 1. Hot-rolled strip steel of different thicknesses is produced by continuously casting, roughing, finishing, coiling, layer cooling, and slow cooling processes of molten steel refined by LF through a continuous casting and rolling production line.

[0049] 2. Continuous casting speed: 4.0m / min-4.8m / min, billet thickness: 110mm, billet cross-sectional temperature difference: 50℃-68℃.

[0050] 3. The billet is rolled in three roughing passes, with the intermediate billet thickness being 8mm-20mm and the roughing exit temperature being 800℃-950℃.

[0051] 4. The rough-rolled intermediate billet is heated in an induction heating (IH) furnace, and the IH outlet temperature is controlled at 1100℃-1250℃;

[0052] 5. The induction-heated intermediate billet is rolled into hot-rolled strip steel with a thickness of 1.5mm-3.5mm through 5 passes of finishing rolling, and the finishing rolling exit temperature is controlled at 780℃-880℃;

[0053] 6. Turn on the cooling water of a set of manifolds in the laminar flow section, with the upper and lower valves opening at a ratio of 30%-80%. Cool the steel strip to 590±10℃ in the laminar flow section, and then send it to the coiler to be coiled into a steel coil.

[0054] 7. Release the steel coil after it has cooled slowly for 48 hours.

[0055] The mechanical properties of the produced 50 steel are shown in the table below:

[0056]

[0057] Examples 1-4 produced a total of 59 coils of 50 steel. The appearance quality was normal, with no bright stripes. The uncoiling of some steel coils in the factory was of good shape, and no customer objections or complaints were received regarding the shape. The cooling water did not have a significant impact on the performance.

[0058] (2) Production of 65Mn by continuous casting and rolling of thin slabs

[0059] Examples 5-8 employ the method of the present invention, with high-speed steel rolls used for work roll F3 and high-nickel-chromium rolls used for work rolls F4 and F5; Comparative Example 2 employs the group's low-carbon steel plate shape control production method.

[0060] The process flow for each group is as follows: blast furnace → molten iron pretreatment → decarburization furnace → LF refining → continuous casting → rough rolling → induction heating → finish rolling → stratified cooling → coiling → slow cooling → finished product.

[0061] All groups of raw materials, such as molten iron and scrap steel, are sequentially smelted in a decarburizing furnace and refined in an LF furnace. The composition of the molten steel is controlled as follows: C: 0.62%-0.70%, Si: 0.17%-0.37%, Mn: 0.90%-1.20%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe and impurities.

[0062] In Examples 5-8, the process flow for producing 65Mn by continuous casting and rolling of thin slabs is as follows: blast furnace → hot metal pretreatment → decarburization furnace → LF refining → continuous casting → rough rolling → induction heating → finish rolling → layer cooling → coiling → slow cooling → finished product.

[0063] In this embodiment, molten iron and scrap steel are sequentially smelted in a decarburizing furnace and refined in an LF furnace. The composition of the molten steel is controlled as follows: C: 0.62%-0.70%, Si: 0.17%-0.37%, Mn: 0.90%-1.20%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe and impurities.

[0064] 1. Hot-rolled strip steel of different thicknesses is produced by continuous casting, roughing, finishing, coiling, and slow cooling processes of molten steel refined by LF through a continuous casting and rolling production line.

[0065] 2. Continuous casting speed: 4.0m / min-4.8m / min, billet thickness: 110mm, billet cross-sectional temperature difference: 40℃-65℃.

[0066] 3. The billet is rolled in three roughing passes, with the intermediate billet thickness being 8mm-20mm and the roughing exit temperature being 800℃-950℃.

[0067] 4. The rough-rolled intermediate billet is heated in an induction heating (IH) furnace, and the IH outlet temperature is controlled at 1100℃-1250℃;

[0068] 5. The induction-heated intermediate billet is rolled into hot-rolled strip steel with a thickness of 1.8mm-3.5mm through 5 passes of finishing rolling, with the finishing rolling exit temperature controlled at 780℃-880℃;

[0069] 6. Turn on the cooling water of a set of manifolds in the laminar flow section, with the upper and lower valves open at 100%. Cool the steel strip to 590±10℃ in the laminar flow section, and then enter the coiler to be coiled into a steel coil.

[0070] 7. Release the steel coil after it has cooled slowly for 48 hours.

[0071] The mechanical properties of the produced 65Mn steel are shown in the table below:

[0072]

[0073] Examples 5-8 produced a total of 101 coils of 65Mn. The appearance quality was normal, with no bright stripes. The uncoiling shape of the coils tracked in the factory was good, and no customer objections or complaints were received regarding the shape. The cooling water did not have a significant impact on the performance.

[0074] It should be noted that the specific embodiments of the present invention have been described in detail. For those skilled in the art, various obvious changes made to it without departing from the spirit and scope of the present invention are within the protection scope of the present invention.

Claims

1. A process for producing 65Mn by continuous casting and rolling of thin slabs, comprising the following steps: The steelmaking process includes a casting and rolling process and a slow cooling process; the casting and rolling process includes continuous casting, roughing, induction heating, finishing, layer cooling, and coiling; characterized in that the steel composition is controlled as follows: C: 0.62%-0.70%, Si: 0.17%-0.37%, Mn: 0.90%-1.20%, P≤0.035%, S≤0.030%, Cr≤0.20%, Ni≤0.30%, Cu≤0.25%, with the balance being Fe and impurities; the billet thickness in the continuous casting is 90mm-11mm. 0mm, the casting speed is controlled within the range of 4.0m / min-4.8m / min, and the temperature difference of the billet section is controlled within the range of 40℃-65℃; in the finishing rolling process, the F3 work roll adopts high-speed steel roll, and the F4 and F5 work rolls adopt high-nickel-chromium roll; the crown of the F3 work roll is set to be 200um larger than that of other steel grades, and the load distribution of each stand adopts the principle of decreasing sequentially; in the laminar cooling process, a set of cooling water manifolds in the laminar cooling section is turned on, and the opening ratio of the upper and lower valves is 100%, so that the steel strip is cooled to 590±10℃ through the laminar flow section.

2. The process for producing 65Mn by continuous casting and rolling of thin slabs according to claim 1, characterized in that: In the roughing process, the thickness of the intermediate billet is set to 8mm-20mm.

3. The process for producing 65Mn by continuous casting and rolling of thin slabs according to claim 1, characterized in that: The exit temperature of the roughing mill is controlled at 800℃-950℃.

4. The process for producing 65Mn by continuous casting and rolling of thin slabs according to claim 3, characterized in that: The outlet temperature of the IH induction heating furnace is set to 1100℃-1250℃.

5. The process for producing 65Mn by continuous casting and rolling of thin slabs according to claim 4, characterized in that: The exit temperature of the finishing mill is controlled at 780℃-880℃.

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