Method of preventing flatness of high carbon tool steel hot rolled coils

Abstract

The application discloses a method for preventing flat coiling of high-carbon tool steel hot-rolled steel coil, which comprises the following steps: providing a continuous casting slab, which comprises the following components in percentage of mass: C: 0.65wt.%-1.35wt.%, Si: 0wt.%-1.6wt.%, Mn: 0wt.%-1.2wt.%, Cr: 0wt.%-1.2wt.%, P≤0.030wt.%, S≤0.030wt.%, and the rest is iron and inevitable impurities; feeding the continuous casting slab into a heating furnace for heating treatment, so that the inlet temperature of the continuous casting slab is greater than or equal to 300 DEG C, and the outlet temperature of the continuous casting slab is 1150 DEG C-1250 DEG C, thereby obtaining a first slab; performing rough rolling on the first slab, so that the final rolling temperature of the rough rolling is 1050 DEG C-1150 DEG C, thereby obtaining an intermediate blank; performing finish rolling on the intermediate blank, so that the final rolling temperature of the finish rolling is 800 DEG C-950 DEG C, thereby obtaining a finish-rolled steel material; performing laminar cooling on the finish-rolled steel material by adopting a front concentrated cooling mode, and closing the last finish-adjusting water and the front and rear side spray, so that the temperature of the finish-rolled steel material is reduced to a first target temperature to a phase transition point, thereby obtaining a cooled steel material; and coiling the cooled steel material at 590 DEG C-690 DEG C, and increasing the coiling tension by 10%-20% during the coiling process, thereby obtaining a high-carbon tool steel hot-rolled steel coil.

Description

Technical Field

[0001] The present application belongs to the technical field of high carbon steel processing, and in particular relates to a method for preventing flattening of hot rolled coils of high carbon tool steel. Background Art

[0002] High-carbon tool steel primarily refers to steels with a carbon content of 0.65 wt.% or greater. These steels exhibit excellent hardenability and wear resistance. After proper heat treatment, they can achieve high strength, hardness, elastic limit, and fatigue limit. They are generally used in the manufacture of cutting tools, gauges, molds, and wear-resistant parts. High-carbon tool steels are typically tempered at a moderate temperature after quenching or used in a condition where quenching is avoided.

[0003] It is a relatively efficient method to produce high carbon tool steel wide coils in conventional hot rolling lines, such as Figure 1 As shown, in the current production process of hot-rolled high-carbon tool steel coils, thinner high-carbon tool steels are prone to coil flattening, a phenomenon in which the coil's inner diameter is flattened from a round shape to an elliptical shape by its own weight. While increasing the coiling tension for conventional low-carbon steels can significantly improve flattening, high-carbon tool steel has a higher carbon content and higher strength, and it also contains more carbides that undergo phase transformation, making the uneven volume expansion or contraction caused by phase transformation more pronounced. Simply increasing the tension does not address the flattening issue. Flattening can cause the gaps between coil layers to fluctuate, resulting in severe scratches and scuffing on the strip surface during subsequent uncoiling. In more severe cases, the coil's minimum inner diameter is too small, preventing subsequent processes like flattening from being processed on the machine, rendering the entire coil scrapped and significantly impacting subsequent production.

[0004] Existing methods for preventing flattening of hot-rolled strip primarily target hot-rolled strip with a tensile strength of 500 MPa to 600 MPa and are not suitable for preventing flattening of high-carbon tool steel. The prior art also discloses a seasonal flattening control method for thin-gauge high-carbon steel grades. This method, based on a short-process production model for thin-gauge medium- and high-carbon steel, utilizes different processes to prevent flattening according to the season. This requires repeated process changes, which is detrimental to stable production control. Other methods, such as reducing slab weight or improving the saddle of the coil, can also reduce the degree of flattening in high-carbon tool steel coils. The former mitigates flattening by reducing the deadweight of the coil, but this limits the upper weight limit of the finished coil, failing to meet the coil weight requirements of some end users. Furthermore, a low coil weight results in a lower yield rate in subsequent further processing and inherently poor performance. The latter requires modifications to existing equipment, resulting in significant cost investment. Flattening is a frequent problem in the production of thin-gauge high-carbon tool steel, and no effective solution exists. Therefore, a new method for preventing flattening of hot-rolled high-carbon tool steel coils is urgently needed. Summary of the Invention

[0005] The embodiment of the present application provides a method for preventing flattening of hot-rolled high-carbon tool steel coils. Starting from the heating, rolling, cooling and coiling of hot rolling, it can prevent the flattening of hot-rolled high-carbon tool steel coils and reduce the loss of steel coils caused by flattening.

[0006] The present application provides a method for preventing flattening of a hot-rolled coil of high-carbon tool steel, comprising: providing a continuously cast slab of high-carbon tool steel, wherein the continuously cast slab comprises the following components in percentage by mass: C: 0.65 wt.% to 1.35 wt.%, Si: 0 wt.% to 1.6 wt.%, Mn: 0 wt.% to 1.2 wt.%, Cr: 0 wt.% to 1.2 wt.%, P ≤ 0.030 wt.%, S ≤ 0.030 wt.%, and the remainder being iron and unavoidable impurities;

[0007] The continuous casting slab is fed into a heating furnace at a temperature of ≥300°C, and is fed into the heating furnace by hot charging. The continuous casting slab having a furnace entry temperature of ≥300°C is heated to a temperature of 1150°C to 1250°C after exiting the furnace, thereby obtaining a first slab.

[0008] The first slab is rough-rolled by a four-roll roughing mill, with the roughing finishing temperature being 1050°C to 1150°C, to obtain an intermediate slab of a preset thickness;

[0009] The intermediate billet is finish-rolled by a four-roll seven-stand finishing mill, with the finishing temperature of the finishing rolling being 800°C to 950°C to obtain the finished steel.

[0010] The finished rolled steel is laminar-cooled by using a front-stage centralized cooling method, and the final fine-adjusting water of the laminar cooling and the corresponding front-side spray and rear-side spray of the fine-adjusting water are closed, so that the temperature of the finished rolled steel is reduced to the first target temperature and enters the phase transition point, thereby obtaining cooled steel;

[0011] The cooled steel is coiled at a coiling temperature of 590° C. to 690° C. The coiling tension of the cooled steel is increased by 10% to 20% during the coiling process to obtain a hot-rolled steel coil of high-carbon tool steel.

[0012] In some embodiments of the present application, the thickness of the continuously cast slab is 220 mm to 240 mm.

[0013] In some embodiments of the present application, the continuous casting slab with an entry temperature of ≥300° C. is heated in a heating furnace for 200 to 300 minutes.

[0014] In some embodiments of the present application, a four-roll roughing mill is used to perform 5 to 7 roughing passes on the first slab.

[0015] In some embodiments of the present application, in the step of rough rolling the first slab by using the four-roller rough rolling mill train, the preset thickness of the intermediate slab for rolling the strip steel with a thickness of > 3.0 mm or above is 30 mm to 56 mm, or the preset thickness of the intermediate slab for rolling the strip steel coil with a thickness of ≤ 3.0 mm or below is ≤ 36 mm.

[0016] In some embodiments of the present application, the intermediate slab is precisely rolled by using the four-roller seven-stand precise rolling mill train, and the method further comprises: keeping the small stand roll of the precise rolling mill train and the guide on the entry side of the precise rolling mill in a stressed state, and controlling the wedge shape of the strip steel in the precise rolling process to be 0 μm ± 20 μm.

[0017] In some embodiments of the present application, the intermediate slab is precisely rolled by using the four-roller seven-stand precise rolling mill train, and the method further comprises: setting the rolling crown to be 20 μm to 40 μm.

[0018] In some embodiments of the present application, in the step of precisely rolling the intermediate slab by using the four-roller seven-stand precise rolling mill train, the method further comprises: closing the inter-stand water of the front stand, normally opening the inter-stand water of the rear stand, normally opening the working roll cooling water of the front stand roll of the precise rolling mill train, normally opening the working roll cooling water of the rear stand roll of the precise rolling mill train, and reducing the water flow of the working roll cooling water of the rear stand roll by 20% to 30%.

[0019] In some embodiments of the present application, the intermediate slab is precisely rolled by using the four-roller seven-stand precise rolling mill train in the mode of increasing rolling speed.

[0020] In some embodiments of the present application, the temperature of the precisely rolled steel material is reduced to a first target temperature, wherein the first target temperature is (Ar1 phase transition temperature - 30℃) to (Ar1 phase transition temperature - 50℃) of the high-carbon tool steel grade.

[0021] In some embodiments of the present application, the step of coiling the cooled steel material further comprises: for the steel coil with a strip steel thickness of ≤ 3.0 mm, rotating 90° after uncoiling to the ground roll position, and then transferring to the walking beam, and finally storing the steel coil of the high-carbon tool steel in the storage pit for slow cooling, so as to avoid being placed in the lower layer to cause flattening.

[0022] The method for preventing flattening of hot-rolled coils of high-carbon tool steel in the embodiment of the present application starts from the phase change characteristics of high-carbon tool steel. In the laminar cooling stage of the finished steel, the finished steel is laminarly cooled by adopting the front-stage centralized cooling method, so that the coiling temperature of the finished steel of the high-carbon tool steel quickly reaches the first target temperature, so as to enter the phase change point in advance and complete the phase change from austenite to pearlite as soon as possible, and close the final fine-tuning water and front and rear side sprays of the laminar cooling to ensure that the finished steel has a sufficiently long air cooling time in the later stage of laminar cooling, so that the strip is cooled as evenly as possible in the length and width directions, avoiding uneven phase change and volume expansion of the finished steel during the coiling and temperature recovery process, preventing loosening and slipping between layers of the strip coil of high-carbon tool steel, and thus preventing flattening. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0024] Figure 1 This is a flat coil product of a high-carbon tool steel coil rolled in a conventional hot rolling line using existing technology.

[0025] Figure 2 It is a flow chart of a method for preventing flattening of a hot-rolled high-carbon tool steel coil provided in an embodiment of the present application.

[0026] Figure 3 This is a product diagram of a high-carbon tool steel coil rolled using the method for preventing flattening of high-carbon tool steel hot-rolled coils of the present application.

[0027] Explanation of the accompanying figures: 1. Flat-rolled steel coil; 2. Steel coil without flat-rolling. DETAILED DESCRIPTION

[0028] The features and exemplary embodiments of various aspects of the present application will be described in detail below. In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present application, rather than to limit the present application. For those skilled in the art, the present application can be implemented without the need for some of these specific details. The following description of the embodiments is merely to provide a better understanding of the present application by illustrating the examples of the present application.

[0029] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, the elements defined by the phrase "comprising..." do not exclude the presence of other identical elements in the process, method, article, or device comprising the elements.

[0030] As described in the background technology section, the carbon content of high-carbon tool steel often exceeds 0.65wt%. Due to its high carbon content, the proportion of cementite in this steel structure is high, and the final structure after hot rolling is generally pearlite + cementite. Cementite is very hard, with almost zero plasticity and toughness, and poor rollability. Therefore, the production of high-carbon steel is much more difficult than that of general medium- and low-carbon steel. High-carbon tool steel is more sensitive to temperature during hot rolling, with large rolling forces, and uncontrolled crown and flatness. In addition, during cooling, the structure contains a large number of carbides that participate in phase transformation. The steel structure often transforms from austenite to pearlite, which is then returned to austenite upon warming. Finally, it gradually cools down to form a final structure of pearlite + cementite. During this repeated phase transformation process, the steel coil will undergo uneven volume expansion. When the thickness is thin, the strip cannot withstand its own weight, resulting in the steel coil taking an elliptical shape due to its own gravity.

[0031] After research, the inventors of this application found that the formation of flat coils is mainly due to the uneven cooling of the strip steel in the width and length directions before coiling, which causes uneven phase change and uneven volume expansion or contraction when the steel coil is coiled, and loosening and slippage occur between the layers of the steel coil. The heavier the steel coil, the more obvious the flattening phenomenon.

[0032] To address the existing problem of flattening of high-carbon tool steel coils after hot rolling, the present invention provides a method for preventing flattening of hot-rolled high-carbon tool steel coils. This method is applicable to thin-gauge high-carbon tool steel coils. By optimizing the hot rolling, cooling, and coiling processes, it reduces subsequent flattening, rework, and scrapping, significantly reducing production costs. Steel grades with a carbon content of 0.65 to 1.35 wt.% are considered high-carbon steel.

[0033] The following first introduces the method for preventing flattening of high-carbon tool steel hot-rolled coils provided in the embodiments of the present application. Figure 1 A schematic flow chart of a method for preventing flattening of a hot-rolled coil of high-carbon tool steel provided in one embodiment of the present application is shown.

[0034] like Figure 1 As shown, the method for preventing flattening of a high carbon tool steel hot rolled coil comprises:

[0035] S1. Provide a continuously cast slab of high carbon tool steel, wherein the continuously cast slab comprises the following components, calculated by mass percentage: C: 0.65 wt.% to 1.35 wt.%, Si: 0 wt.% to 1.6 wt.%, Mn: 0 wt.% to 1.2 wt.%, Cr: 0 wt.% to 1.2 wt.%, P ≤ 0.030 wt.%, S ≤ 0.030 wt.%, and the remainder being iron and unavoidable impurities;

[0036] S2. The continuous casting slab is fed into a heating furnace at a temperature of ≥300° C., and is hot-charged to heat the continuous casting slab. The continuous casting slab having a furnace entry temperature of ≥300° C. is heated to a temperature of 1150° C. to 1250° C. after exiting the furnace, thereby obtaining a first slab.

[0037] S3, rough rolling the first slab using a four-roll rough rolling mill, with the rough rolling finishing temperature being 1050° C. to 1150° C., to obtain an intermediate slab of a preset thickness;

[0038] S4, using a four-roll seven-stand finishing mill to perform finish rolling on the intermediate billet, with the finishing rolling temperature being 800° C. to 950° C. to obtain a finish-rolled steel product;

[0039] S5. Laminar cooling is performed on the finished rolled steel using a front-stage centralized cooling method, and the final fine-adjusting water in the laminar cooling process and the corresponding front-side spray and rear-side spray are closed to reduce the temperature of the finished rolled steel to a first target temperature and enter a phase transition point, thereby obtaining a cooled steel.

[0040] S6. Coil the cooled steel at a coiling temperature of 590° C. to 690° C., increase the coiling tension of the cooled steel by 10% to 20% during the coiling process, and obtain a hot-rolled coil of high-carbon tool steel.

[0041] The method for preventing flattening of hot-rolled coils of high-carbon tool steel in the embodiment of the present application starts from the phase change characteristics of high-carbon tool steel. In the laminar cooling stage of the finished steel, the finished steel is laminarly cooled by adopting the front-stage centralized cooling method, so that the coiling temperature of the finished steel of the high-carbon tool steel quickly reaches the first target temperature, so as to enter the phase change point in advance and complete the phase change transformation as early as possible, and close the final fine-tuning water and front and rear side sprays of the laminar cooling to ensure that the finished steel has a sufficiently long air cooling time in the later stage of laminar cooling, so that the strip is cooled as evenly as possible in the length and width directions, avoiding uneven phase change and volume expansion of the finished steel during the coiling and temperature recovery process, preventing loosening and slippage between layers of the steel coil strip, and thus preventing flattening of the hot-rolled coils of high-carbon tool steel.

[0042] It should be noted that hot charging refers to charging the continuous casting slab into the heating furnace while maintaining a certain temperature. In the embodiments of the present application, the continuous casting slab needs to be charged into the heating furnace while maintaining a temperature of ≥300℃. Because high-carbon steel is sensitive to temperature, the slab is prone to cracking after cooling, and is prone to cracking or breaking during rough rolling. Using the hot charging method to send the continuous casting slab into the heating furnace for heating can effectively avoid cracking or breaking during rolling due to cracks in the continuous casting slab. At the same time, hot charging can also reduce the heat loss of the continuous casting slab and reduce production costs.

[0043] In some embodiments of the present application, the thickness of the continuous casting slab is 220mm-240mm. Exemplarily, the thickness of the continuous casting slab can also be 225mm, 230mm, 235mm, 238mm.

[0044] In some embodiments of the present application, the continuous casting slab with an entry temperature ≥300℃ is heated in the heating furnace for 200-300 minutes. Exemplarily, the heating time can be 220 minutes, 240 minutes, 250 minutes, 260 minutes, 270 minutes, 280 minutes, 290 minutes, or any length of time between any two of them, so that the continuous casting slab is heated to the required temperature for rough rolling.

[0045] In some embodiments of the present application, the first slab is rough rolled by a four-high roughing mill unit for 5-7 passes. The roughing mill unit is a complete set of equipment, and one pass is one time of rolling the continuous casting slab using the entire roughing mill unit. The thinner the rolling thickness of the preset intermediate slab, the more passes of rough rolling.

[0046] In some embodiments of the present application, in the step of rough rolling the first slab by a four-high roughing mill unit, the intermediate slab thickness of the strip steel with a rolling thickness of >3.0mm or more is set to 30-56mm; for the strip steel coil with a rolling thickness of ≤3.0mm or less, the intermediate slab is coiled into a hot coil box, and the preset thickness of the intermediate slab is ≤36mm. Corresponding to the thickness of the strip steel to be rolled, the thinner the thickness of the finished strip steel, the thinner the thickness of the intermediate slab.

[0047] It should be noted that the hot coil box is located at the entrance of the finishing mill unit and is a coreless coiling device for intermediate slabs. It can coil the intermediate slab rolled by the roughing mill unit, temporarily store the intermediate slab in the hot coil box, reduce the temperature drop of the intermediate slab, break through the length limit of the intermediate slab, and produce thinner and longer intermediate slabs. It is more commonly used when rolling high-strength steel and thin-gauge steel.

[0048] In some embodiments of the present application, when finishing the intermediate billet using a four-high, seven-stand finishing mill, the process further includes: maintaining the small vertical rolls of the finishing mill and the inlet side guide of the finishing mill in a stressed state to improve the centering of the finished steel during the finishing rolling process; and controlling the wedge shape of the strip during the finishing rolling process to be within 0μm±20μm to prevent the wedge shape from being too large, causing the strip to shift to one side and resulting in poor coil shape. This allows for stable, mass-produced high-carbon tool steel with good coil shape, thereby avoiding uneven cooling of the hot-rolled high-carbon tool steel coils during the subsequent cooling process due to excessive wedge shape, thereby avoiding uneven phase transformation and uneven volume expansion or contraction.

[0049] It should be noted that the small vertical rollers and entry side guides of the finishing mill are both part of the finishing mill. The vertical rollers are pairs of rollers located on either side of the finishing mill's entry. They squeeze the edges of the intermediate slab, ensuring that the intermediate slab is finished as close to the center of the mill's rolls as possible during finishing rolling. They are generally used to control the width of the finished steel strip and improve centering during the finishing rolling process. The entry side guides of the finishing mill are generally located in front of the vertical rollers. If the intermediate slab of the rolled strip deviates, it comes into contact with the entry side guides, guiding them toward the center of the finishing mill's rolls. In other words, the entry side guides simply correct the position and orientation of the intermediate slab to prevent it from deviating. Because the small vertical rollers and the entry side guides both act on the edges of the intermediate billet, when both the small vertical rollers and the entry side guides of the finishing mill are kept in a lightly stressed state, it means that the strip of finished steel produced by the intermediate billet through finish rolling is transported forward in a straight line and has good centering.

[0050] In the embodiments of the present application, the wedge shape is the difference in thickness between the two sides of the finished steel, generally expressed as W40, that is, the difference between the thickness at 40mm on the transmission side and the thickness at 40mm on the operating side. Since the rollers of the finishing mill have convexity changes, when the strip deviates seriously, the intermediate billet will not be rolled in the center of the rollers, so there will be a problem of the finished steel being wedge-shaped. When the wedge shape of the finished steel is large, subsequent use may cause uneven thickness of the steel plate, such as one side of the steel being thin and the other side being thick, making it unusable. The method of preventing flattening of hot-rolled high-carbon tool steel coils of the present application uses the above-mentioned small vertical rollers of the finishing mill to limit the stress state of the inlet side guide of the finishing mill, which can effectively prevent the strip from deviating and the uneven thickness of the finished steel.

[0051] Illustratively, the wedge shape of the steel strip during the finish rolling process is controlled within 0 μm±5 μm, 0 μm±8 μm, 0 μm±10 μm, 0 μm±12 μm, 0 μm±15 μm, or 0 μm±18 μm.

[0052] In the embodiment of the present application, a four-roll seven-stand finishing mill is used to perform finish rolling on the intermediate billet, and the rolling crown is set to 20 μm to 40 μm.

[0053] It should be noted that rolling crown refers to the crown of the finished steel product. Crown generally refers to the difference between the thickness at the center of the finished steel strip width and the average thickness at the two sides. In the examples of this application, the parameter C40 is used to characterize the crown of the finished steel product. Specifically, it refers to the difference between the thickness at the center of the finished steel strip width and the average thickness at 40 mm on both sides.

[0054] For example, the rolling crown can be 25 μm, 28 μm, 30 μm, 35 μm, or 38 μm, which can increase the friction between the steel strip layers of the steel coil, prevent loosening and slippage between the steel strip layers of the steel coil, and thereby prevent the steel coil from flattening.

[0055] The method for preventing flattening of a hot-rolled high-carbon tool steel coil according to the present invention optimizes the hot rolling process and sets the rolling crown during finish rolling to 20 μm to 40 μm. This increases the friction between the strip layers of the high-carbon tool steel coil, preventing loosening and slippage between the strip layers, thereby preventing the coil from flattening during subsequent placement. This method, combined with adjustments to the subsequent cooling process for the finished steel, further prevents flattening of the hot-rolled high-carbon tool steel coil. Furthermore, the finished steel that meets this rolling crown will not cause severe scratches or scrapes on the strip surface during uncoiling.

[0056] In some embodiments of the present application, when finishing rolling the intermediate bar using a four-high, seven-stand finishing mill, the process further includes: shutting off the inter-stand water supply of the front stand of the finishing mill, leaving the inter-stand water supply of the rear stand normally open, opening the working cooling water of the rollers of the front stand of the finishing mill normally open, and opening the working cooling water of the rollers of the rear stand of the finishing mill normally open, and reducing the water flow of the working cooling water of the rollers of the rear stand by 20% to 30%. For example, the water flow of the working cooling water of the rollers of the rear stand of the finishing mill can be reduced by 22%, 24%, 25%, 27%, 28%, or 29%.

[0057] It should be noted that the four-high, seven-stand finishing mill used in this application has seven stands. The front stands refer to the first and second sets of stands in the direction of finishing rolling the intermediate billet, and the rear stands refer to the third through seventh sets of stands, excluding the first and second sets of stands. Inter-stand water is used to spray the finished steel to reduce its temperature during the finishing process. The working roll cooling water, located inside the rolls, cools the rolls to maintain their shape and strength, allowing the intermediate billet to be finished.

[0058] In some embodiments of the present application, a four-high, seven-stand finishing mill is used to finish-roll the intermediate bar. By reducing the inter-stand water flow in the front stand of the finishing mill, ensuring that the inter-stand water flow in the rear stand is normally open, and ensuring that the cooling water flow to the working rolls of the front stand of the finishing mill is normally open, the cooling water flow rate of the working rolls of the front stand of the finishing mill is increased, ensuring better cooling of the rollers of the front stand, reducing uneven wear and deformation of the working rolls of the front stand of the finishing mill, and better controlling the rolling crown of smaller rolled steel products. The cooling water flow rate of the working rolls of the rear stand is appropriately reduced by 20% to 30%, reducing the temperature drop at the tail end of the finished steel strip, while also reducing the rolling force of the rear stand and improving the stability of the finishing rolling process.

[0059] In some embodiments of the present application, a four-high, seven-stand finishing mill is used to finish-roll the intermediate slab using an accelerated rolling method. Accelerated rolling means that the rolling speed gradually increases after the strip is threaded. Because the temperature of the head and tail of the continuous casting slab is uniform after heating, the head is rolled first, followed by the tail during rough rolling. Therefore, when the intermediate slab is rolled at a uniform speed, the temperature of the tail will be significantly lower than that of the head. Accelerated rolling can avoid the increase in rolling force caused by the decrease in temperature of the tail of the intermediate slab during the rolling of the strip.

[0060] In the embodiments of the present application, the laminar cooling of the finished rolled steel is performed by adopting the front-stage centralized cooling method, which means that the order of opening the cooling water in the laminar cooling section is starting from the frontmost manifold of the laminar cooling to the rear manifolds, until the cooling effect of the manifolds makes the temperature of the finished rolled steel reach the required coiling temperature.

[0061] In some embodiments of the present application, the temperature of the finished steel is lowered to a first target temperature, wherein the first target temperature is (Ar1 phase transition temperature - 30°C) to (Ar1 phase transition temperature - 50°C) for high-carbon tool steel. Lowering the temperature of the finished steel to within this temperature range can cause the finished steel strip to reach the phase transition point earlier. The Ar1 phase transition temperature refers to the phase transition temperature at which austenite begins to transform into pearlite.

[0062] In the embodiments of the present application, the Ar1 phase transformation temperature is the phase transformation temperature from austenite to pearlite. However, due to the temperature difference between the core and the surface of the steel, the surface temperature of the cooled steel will return to normal after coiling, resulting in the actual control of the cooling temperature of the finished rolled steel according to the Ar1 phase transformation temperature. As a result, many structures of the cooled steel have not been transformed into pearlite after coiling, and will gradually undergo phase transformation later, resulting in flat coiling. Therefore, the present application creatively controls the first target temperature between (Ar1 phase transformation temperature - 30°C) and (50°C of the Ar1 phase transformation temperature), which can complete the phase transformation from austenite to pearlite in the cooled steel as much as possible, and no further phase transformation will occur later, thereby avoiding volume changes caused by subsequent phase transformations and the resulting flat coiling.

[0063] In some embodiments of the present application, the final fine-tuning water of the laminar cooling and the corresponding front-side spray and rear-side spray of the fine-tuning water are turned off to ensure that the strip has a sufficiently long air-cooling time after leaving the laminar cooling process, improve the temperature uniformity of the steel strip, complete the phase change before coiling, and thus obtain cooled steel.

[0064] It should be noted that the finishing water is generally the last group of cooling water spray headers in laminar cooling, which has 8 control water valves, and can more accurately control the temperature of the finished steel to the coiling temperature. Side sprays are installed before and after the finishing water to blow away the last finishing water on the finished steel. By shutting down the last finishing water of laminar cooling, that is, shutting down the cooling water sprayed by the last group of headers, the laminar cooling water can be concentrated to the front to cool the finished steel, so that the finished steel has enough time to return to temperature after cooling, so that the temperature of the finished steel is more uniform, and the steel structure is fully transformed. Synchronously shutting down the front and rear side sprays can reduce the uneven cooling of the finished steel in the width direction caused by the side sprays, further improve the structural uniformity of the finished steel, and allow sufficient time for the structure to be transformed in advance.

[0065] In some embodiments of the present application, the cooled steel is coiled at a coiling temperature of 590°C to 690°C, and the coiling tension of the cooled steel is increased by 10% to 20% during the coiling process to increase the tightness of the inner ring of the strip in the coil, thereby obtaining a hot-rolled coil of high-carbon tool steel. For example, the coiling temperature may be 595°C, 600°C, 610°C, 620°C, 628°C, 630°C, 632°C, 640°C, 650°C, 654°C, 660°C, 670°C, 680°C, 685°C, or 690°C.

[0066] The method for preventing flattening of hot-rolled high-carbon tool steel coils of the present application closes the inter-stand water of the front stand, opens the inter-stand water of the rear stand, and normally opens the cooling water of the working rolls of the front stand and the rear stand, and combines the operations of adjusting the flow rate of the cooling water of the working rolls of the rear stand at the same time, which can reduce the temperature drop of the strip of finish-rolled steel and reduce the rolling force, so that the rolling process can be carried out stably.

[0067] The method of preventing flattening of hot-rolled high-carbon tool steel coils in the present application combines measures such as reducing various cooling water of the finishing mill, selecting a laminar cooling mode, and controlling the water flow of side sprays, so that the thickness and uniformity of the rolled high-carbon tool steel strip are relatively consistent, and the strip is cooled uniformly in the length and width directions, thereby improving the problem of uneven strip cooling and avoiding uneven phase change and uneven volume expansion or contraction of the coil formed by coiling the cooled steel.

[0068] In some embodiments of the present application, coiling the cooled steel also includes: for steel coils with a thickness of ≤3.0 mm, after unloading, they are rotated 90° to the ground roller position and then transferred to the walking beam, stored in the insulation pit for slow cooling, to avoid being placed in the lower layer and causing flattening.

[0069] It should be noted that the walking beam is a device for transporting steel coils, and the ground roller is a pair of rollers specially used for rotating steel coils. It is located between the coiling equipment and the walking beam. When the steel coil is placed on it, the flat steel coil can be rotated 90° in the direction of the steel coil bending, which changes the stress condition of the hot-rolled steel coil that has just come off the line. It has a certain improvement effect on the steel coil that has been slightly flattened, and then it can be operated to the subsequent process.

[0070] Compared with the prior art, the method for preventing flattening of high-carbon tool steel hot-rolled coils in the embodiment of the present application does not require improvement to existing equipment, has good operability, can prevent flattening of high-carbon tool steel hot-rolled coils, greatly improves the coil shape qualification rate of high-carbon tool steel coils, and reduces the flat coil rework rate of high-carbon tool steel hot-rolled coils from 38.9% before the improvement to 6.25%.

[0071] The technical solution of the present application is further illustrated below through specific embodiments and comparative examples.

[0072] Examples

[0073] Example 1

[0074] SK85 is a high-carbon tool steel widely used in hardware, cutting tools and other tools, such as turning tools, planers, utility knives, paper cutters, measuring tools, etc. Its chemical composition, in terms of mass percentage, is: C: 0.85wt.%, Si: 0.24wt.%, Mn: 0.36wt.%, Cr: 0.19wt.%, P content 0.015wt.%, S content 0.009wt.%, and the rest is iron and unavoidable impurities.

[0075] This embodiment provides a method for preventing flattening of a hot-rolled high-carbon tool steel (SK85) coil, comprising:

[0076] Provide SK85 high carbon tool steel continuous casting slabs with a thickness of 240 mm and the chemical composition as described above;

[0077] The continuous casting slab is fed into a heating furnace at a temperature of 480° C. and is hot-charged. The continuous casting slab is heated in the heating furnace until its outlet temperature reaches 1190° C. The total time in the furnace is 280 minutes, thereby obtaining a first slab. After the first slab is discharged from the furnace, it is subjected to rough descaling to obtain the first slab with its surface oxide scale removed.

[0078] The first slab, after removing the surface iron oxide scale, was subjected to seven rough rolling passes using a four-roll rough rolling mill. The final rough rolling temperature was 1090°C, and an intermediate slab with a thickness of 46 mm was obtained.

[0079] The intermediate billet is finish-rolled using a four-high seven-stand finishing mill. The finishing temperature is 900°C and the rolling crown is set to 25μm to increase the friction between the strip layers of the coil. The small vertical rollers of the finishing mill and the inlet guide of the finishing mill are kept under stress to improve the centering of the rolling. The strip wedge is controlled between 0μm±20μm to prevent the steel coil from shifting to one side due to an excessive wedge, resulting in a poor coil shape. At the same time, the first unit (F1) and the second unit (F2) of the front stand of the finishing mill are closed. ) and open the inter-stand water (ISC) of the rear stands except F1 and F2 normally. The cooling water of the working rolls of the front stand of the finishing mill is opened normally to ensure a good cooling effect of the working rolls and reduce uneven wear and deformation. The cooling water of the working rolls of the rear stand of the finishing mill is opened normally, and the water flow of the cooling water of the working rolls of the rear stand of the finishing mill is reduced by 20%. The rolling force of the rear stand is reduced, and the speed rolling is increased to reduce the temperature drop at the tail end and improve the rolling stability, thereby obtaining finished rolled steel.

[0080] After checking, the Ar1 temperature of SK85 steel is 690℃, so the coiling temperature is set to 640℃. The laminar cooling of the finished steel is carried out by using the front-end centralized cooling method to reduce the temperature of the finished steel to the first target temperature, that is, 640℃, so that the finished steel strip reaches the phase change point in advance. At the same time, the final fine-tuning water of the laminar cooling and the corresponding front and rear side sprays of the fine-tuning water are turned off to ensure that the strip has a sufficient amount of air cooling time after exiting the laminar cooling process, improve the uniformity of the steel strip temperature, and further ensure that the steel coil completes the phase change before coiling, thus obtaining cooled steel.

[0081] After cooling to 640°C, the cooled steel is coiled. During the coiling process, the tension is increased by 20% to tighten the inner coil of the strip, resulting in a hot-rolled coil of high-carbon tool steel. For coils ≤3.0mm thick, after unloading, they are rotated 90° to the ground roller position and then transferred to the walking beam. They are then stored in a holding pit for slow cooling to avoid flattening on the lower level.

[0082] like Figure 3 As shown, the hot rolled coil of high carbon tool steel SK85 rolled by the method for preventing flattening of high carbon tool steel hot rolled coil of the present application has a good coil shape without flattening, and does not need to be repaired or cut and scrapped.

[0083] Example 2

[0084] 9SiCr is a high-carbon tool steel widely used in hardware, cutting tools and other tools, such as turning tools, planers, utility knives, paper cutters, measuring tools, etc. Its chemical composition, in terms of mass percentage, is: C: 0.92wt.%, Si: 1.38wt.%, Mn: 0.46wt.%, Cr: 1.05wt.%, P content 0.012wt.%, S content 0.008wt.%, and the rest is iron and unavoidable impurities.

[0085] This embodiment provides a method for preventing flattening of a hot-rolled high-carbon tool steel (9SiCr) coil, comprising:

[0086] Providing a continuous casting slab of 9SiCr high carbon tool steel, the continuous casting slab having a thickness of 240 mm and a composition having the above chemical composition;

[0087] The continuous casting slab is fed into a heating furnace at a temperature of 480° C. and is hot-charged. The continuous casting slab is heated in the heating furnace at a temperature of 480° C. until the slab is discharged from the furnace at a temperature of 1200° C. The total time in the furnace is 260 minutes, thereby obtaining a first slab. After the first slab is discharged from the furnace, it is subjected to rough descaling to obtain the first slab with surface iron oxide scale removed.

[0088] The first slab, from which the surface oxide scale is removed, is subjected to seven rough rolling passes using a four-roll roughing mill. The final rough rolling temperature is 1090°C, resulting in an intermediate slab with a thickness of 45 mm.

[0089] The intermediate billet is finish rolled using a four-high seven-stand finishing mill. The finishing temperature is 910°C and the rolling crown is set to 25μm to increase the friction between the steel layers of the coil. The small vertical rollers of the finishing mill and the inlet guide of the finishing mill are kept under stress to improve the centering of the rolling. The strip wedge is controlled between 0μm±20μm to prevent the steel coil from deviating to one side and causing poor coil shape due to excessive wedge. At the same time, the first unit (F1) and the second unit (F2) of the front stand of the finishing mill are closed. 2) Inter-stand water (ISC), and open the inter-stand water of the rear stand except F1 and F2 normally. The cooling water of the working rolls of the front stand of the finishing mill is opened normally to ensure that the working rolls have a better cooling effect and reduce uneven wear and deformation; the cooling water of the working rolls of the rear stand of the finishing mill is opened, and the water flow of the cooling water of the working rolls of the rear stand of the finishing mill is reduced by 20%, reducing the rolling force of the rear stand, adopting speed-up rolling, reducing the temperature drop at the tail, and improving the stability of rolling, thereby obtaining finished rolled steel;

[0090] After checking, the Ar1 temperature of 9SiCr steel is 730℃, so the coiling temperature is set to 690℃. The laminar cooling of the finished steel is carried out by using the front-end centralized cooling method to reduce the temperature of the finished steel to the first target temperature, i.e. 690℃, so that the finished steel strip reaches the phase transition point in advance. At the same time, the final fine-tuning water of the laminar cooling and the corresponding front and rear side sprays of the fine-tuning water are turned off to ensure that the strip has a sufficient amount of air cooling time after exiting the laminar cooling process, improve the uniformity of the steel strip temperature, and further ensure that the steel coil completes the phase transition before coiling, thus obtaining cooled steel.

[0091] After cooling to 690°C, the cooled steel is coiled. During the coiling process, the tension is increased by 20% to tighten the inner ring of the strip, resulting in a hot-rolled coil of high-carbon tool steel. For coils with a thickness of 3.0mm or less, after unloading, they are rotated 90° to the ground roller position and then transferred to the walking beam. They are then stored in a holding pit for slow cooling to avoid flattening on the lower level.

[0092] The hot-rolled steel coil of high-carbon tool steel 9SiCr rolled by adopting the method for preventing flattening of the hot-rolled steel coil of high-carbon tool steel of the present application has a good coil shape without flattening, and does not need to be repaired or cut and scrapped.

[0093] Comparative Example 1

[0094] This comparative example uses a continuous casting slab of the same batch and specifications as Example 2, including: 9SiCr is a high-carbon tool steel widely used in hardware, cutting tools, and various other tools, such as turning tools, planers, utility knives, paper cutters, and measuring tools. Its chemical composition, expressed in mass percentage, is: C: 0.92 wt.%, Si: 1.38 wt.%, Mn: 0.46 wt.%, Cr: 1.05 wt.%, P content 0.012 wt.%, S content 0.008 wt.%, and the remainder is iron and unavoidable impurities. The processing method thereof differs from the method for preventing flattening of hot-rolled high-carbon tool steel coils of the present application, including:

[0095] Providing a continuous casting slab of 9SiCr high carbon tool steel, the continuous casting slab having a thickness of 240 mm and a composition having the above chemical composition;

[0096] The continuous casting slab is fed into a heating furnace by hot charging, with the furnace temperature of the continuous casting slab being 1200°C and the total furnace time being 260 minutes, thereby obtaining a first slab. After the first slab is discharged from the furnace, it is subjected to rough descaling in sequence to obtain the first slab with the surface oxide scale removed.

[0097] The first slab, from which the surface iron oxide scale is removed, is subjected to five rough rolling passes using a four-roll rough rolling mill. The final rough rolling temperature is 1120°C, and an intermediate slab with a thickness of 55 mm is obtained.

[0098] The intermediate billet is finish-rolled using a four-high, seven-stand finishing mill. The final finishing temperature is 920°C, and the rolling crown is set to 40μm. There are no restrictions on wedge shape requirements. All cooling water between stands is turned on normally, with the flow rate automatically set by the system, and the rolling speed is automatically set to the second level.

[0099] The coiling temperature is set to 730℃, and the finished rolled steel is cooled by the middle sparse cooling method. The laminar cooling fine-tuning water and all side sprays are turned on normally, and the coiling parameters such as tension are controlled according to the conventional settings.

[0100] As shown in Table 1, the hot-rolled coils of high-carbon tool steel 9SiCr produced by the method of Comparative Example 1 had poor coil shape and serious flattening, and two coils with a thickness of less than 3.0 mm were scrapped.

[0101] Table 1

[0102] Comparative case 9SiCr process comparison results Number of coils with thickness below 3.0 mm Number of flat coils Number of downgraded or rejected coils Example 2 Test coil 7 0 0 Comparative example 1 Original process 9 5 2

[0103] It can be concluded from Table 2 that, compared with the unimproved processing method, the method of the present application for preventing flattening of high-carbon tool steel hot-rolled coils can effectively prevent flattening of high-carbon tool steel hot-rolled coils without the need for rework or cutting and scrapping, thereby reducing the losses caused by flattening.

[0104] The above description is only a specific embodiment of the present application. Those skilled in the art will clearly understand that for the convenience and brevity of description, the specific working process described above can refer to the corresponding process in the aforementioned method embodiment, and will not be repeated here. It should be understood that the scope of protection of the present application is not limited to this. Any person skilled in the art can easily think of various equivalent modifications or replacements within the technical scope disclosed in this application, and these modifications or replacements should be included in the scope of protection of this application.

Claims

1. A method for preventing flattening of hot-rolled high-carbon tool steel coils, characterized in that: include: A continuously cast slab of high carbon tool steel is provided, wherein the continuously cast slab comprises the following components in percentage by mass: C: 0.65 wt.% to 1.35 wt.%, Si: 0.24 wt.% to 1.6 wt.%, Mn: 0.36 wt.% to 1.2 wt.%, Cr: 0 wt.% to 1.2 wt.%, P≤0.030 wt.%, S≤0.030 wt.%, and the remainder being iron and unavoidable impurities; The continuous casting slab is fed into a heating furnace at a temperature of ≥300°C, and is fed into the heating furnace by hot charging. The continuous casting slab having a temperature of ≥300°C is heated to a temperature of 1150°C to 1250°C after being discharged from the furnace, thereby obtaining a first slab. Rough rolling the first slab using a four-roll roughing mill, with the final roughing rolling temperature being 1050° C. to 1150° C., to obtain an intermediate slab of a preset thickness; The intermediate billet is finish-rolled by a four-high seven-stand finishing mill, with the finishing rolling temperature being 800° C. to 950° C., the small vertical rolls of the finishing mill and the inlet side guide of the finishing mill are kept in a stressed state, and the wedge shape of the strip steel is controlled to be 0 μm±20 μm during the finishing rolling process; the inter-stand water of the front stand is closed, and the inter-stand water of the rear stand is normally opened; the cooling water of the working rolls of the front stand rolls of the finishing mill is normally opened, the cooling water of the working rolls of the rear stand rolls of the finishing mill is normally opened, and the water flow of the cooling water of the working rolls of the rear stand rolls is reduced by 20% to 30%, thereby obtaining a finished steel product; The finished rolled steel is subjected to laminar cooling by using a front-stage centralized cooling method, and the final fine-adjusting water of the laminar cooling and the front-side spray and the rear-side spray corresponding to the fine-adjusting water are closed, so that the temperature of the finished rolled steel is reduced to a first target temperature and enters a phase transition point, thereby obtaining a cooled steel; The cooled steel is coiled at a coiling temperature of 590° C. to 690° C., and the coiling tension of the cooled steel is increased by 10% to 20% during the coiling process to obtain a hot-rolled steel coil of high-carbon tool steel.

2. The method according to claim 1, characterized in that Meet at least one of the following requirements: The thickness of the continuous casting slab is 220 mm to 240 mm; The time for heating the continuous casting slab with an entry temperature of ≥300° C. in the heating furnace is 200 to 300 minutes.

3. The method according to claim 1, characterized in that The four-roll rough rolling mill is used to perform 5 to 7 rough rolling passes on the first slab.

4. The method according to claim 1, wherein In the step of rough rolling the first slab using a four-roll rough rolling mill, for strip steel with a rolling thickness of >3.0mm, the preset thickness of the intermediate billet is 30mm to 56mm; for strip steel with a rolling thickness of ≤3.0mm, the intermediate billet is put into a hot coil box for coiling, and the preset thickness of the intermediate billet is ≤36mm.

5. The method according to claim 1, wherein The method of finishing rolling the intermediate billet by using a four-roller seven-stand finishing mill group further includes setting the rolling crown to be 20 μm to 40 μm.

6. The method according to claim 1, characterized in that The four-roll seven-stand finishing mill is used to perform finishing rolling on the intermediate billet in a speed-increasing rolling manner.

7. The method according to claim 1, characterized in that The temperature of the finish-rolled steel is lowered to a first target temperature, wherein the upper limit of the first target temperature is the Ar1 phase transformation temperature of high carbon tool steel minus 30°C, and the lower limit of the first target temperature is the Ar1 phase transformation temperature of high carbon tool steel minus 50°C.

8. The method according to claim 1, characterized in that The coiling of the cooled steel also includes: for steel coils with a thickness of less than 3.0 mm, after unloading, rotating 90 degrees to the ground roller position and then transferring to the walking beam, and finally storing the high-carbon tool steel coils into the insulation pit for slow cooling.

Images