Method for preventing high-carbon tool steel hot rolled steel coil from being flattened

Abstract

The invention discloses a method for preventing a high-carbon tool steel hot rolled steel coil from being flattened, which comprises the following steps of: providing a continuous casting slab which comprises the following components in percentage by mass: 0.65 to 1.35 weight percent of C, 0 to 1.6 weight percent of Si, 0 to 1.2 weight percent of Mn, 0 to 1.2 weight percent of Cr, less than or equal to 0.030 weight percent of P, less than or equal to 0.030 weight percent of S and the balance of iron and inevitable impurities; the charging temperature of the continuous casting sheet billet is larger than or equal to 300 DEG C, the continuous casting sheet billet is fed into a heating furnace to be heated, the discharging temperature of the continuous casting sheet billet is 1150-1250 DEG C, and a first sheet billet is obtained; the first plate blank is subjected to rough rolling, the rough rolling and finish rolling temperature ranges from 1050 DEG C to 1150 DEG C, and an intermediate blank is obtained; the intermediate billet is subjected to finish rolling, the finish rolling temperature of finish rolling is 800-950 DEG C, and finish rolled steel is obtained; laminar cooling is conducted on the finish-rolled steel in a front-section centralized cooling mode, final fine adjustment water and front and rear side spraying of laminar cooling are closed, the temperature of the finish-rolled steel is reduced to the first target temperature to the phase transformation point, and cooled steel is obtained; and the cooled steel is coiled at the temperature of 590-690 DEG C, the coiling tension is increased by 10%-20% in the coiling process, and a hot rolled steel coil of the high-carbon tool steel is obtained.

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 a hot-rolled steel coil of a high carbon tool steel from flattening. Background Art

[0002] High carbon tool steel mainly refers to steel with a C content of ≥0.65wt.%, which has good hardenability and wear resistance. After proper heat treatment, it can obtain products with high strength, high hardness, high elastic limit and high fatigue limit. It is generally used to manufacture cutting tools, gauges, molds and wear-resistant parts. High carbon tool steel is generally used after quenching and tempering at medium temperature or in a state of avoiding quenching.

[0003] It is a more efficient method to produce high carbon tool steel wide coils in conventional hot rolling lines, such as Figure 1 As shown in the figure, in the current production process of hot-rolled high-carbon tool steel coils, thinner high-carbon tool steel is very prone to coil flattening, that is, the inner diameter of the coil is flattened from a round shape to an elliptical shape by its own weight. For general low-carbon steel grades, increasing the coiling tension will have a better effect of improving the flattening effect, but high-carbon tool steel has a higher carbon content and higher strength. At the same time, there are more carbides participating in the phase change, and the uneven volume expansion or contraction caused by the phase change will be more prominent. Simply increasing the tension cannot solve the problem of flattening. Flattening will cause the gap between the layers of the coil to fluctuate, and the surface of the strip will be severely scratched and scratched during the subsequent uncoiling. In more serious cases, the minimum inner diameter of the coil is too small, resulting in the inability to put the subsequent processes such as flattening on the machine, and the entire coil will be scrapped, which has a great impact on subsequent production.

[0004] The methods for preventing hot-rolled strip steel from flattening in the prior art are mainly aimed at hot-rolled strip steel with a tensile strength level of 500MPa to 600MPa, and are not suitable for preventing the formation of flattened coils for high-carbon tool steel. The prior art also discloses a seasonal flattening control method for thin-gauge high-carbon steel grades, which is based on thin-gauge medium-high carbon steel in a short-process production mode. Different processes can be used to prevent flattening according to different seasons, and the process needs to be changed repeatedly, which is not conducive to stable control of production. There are other methods, such as reducing the weight of the slab or improving the saddle of the steel coil, which can also reduce the flattening degree of the high-carbon tool steel coil. The former alleviates the flattening phenomenon by reducing the dead weight of the steel coil, but it will limit the upper limit of the weight of the finished steel coil, and cannot meet the coil weight requirements of some end users. At the same time, the small coil weight will lead to a low yield rate of subsequent deep processing, and its own limited performance is poor. The latter requires improvement of existing equipment, which is costly. In the production of thin-gauge high-carbon tool steel, the flattening problem occurs frequently and there is no good solution. Therefore, a new method for preventing the flattening of hot-rolled steel coils of high-carbon tool steel is urgently needed. Summary of the invention

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

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

[0007] The continuous casting slab has a furnace entry temperature of ≥300°C, and is sent into a heating furnace by hot charging, and the continuous casting slab with a furnace entry temperature of ≥300°C is heated to a furnace exit temperature of 1150°C to 1250°C, thereby obtaining a first slab;

[0008] The first slab is rough-rolled by a four-roll rough rolling mill, and the final rough rolling temperature is 1050° C. to 1150° C. to obtain an intermediate slab of a preset thickness;

[0009] The intermediate billet is finished-rolled by a four-roller seven-stand finishing mill, and the finishing temperature of the finishing rolling is 800°C to 950°C to obtain finished rolled steel;

[0010] The finished rolled steel is laminar-cooled by front-stage centralized cooling, 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 the first target temperature and enters the phase change 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 continuous casting slab is 220 mm to 240 mm.

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

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

[0015] In some embodiments of the present application, in the step of rough rolling the first slab using a four-roll rough rolling mill, the preset thickness of the intermediate billet for rolling strip steel with a thickness of more than 3.0 mm is 30 mm to 56 mm, or when rolling a strip steel coil with a thickness of ≤3.0 mm, the intermediate billet is put into a hot coil box for coiling, and the preset thickness of the intermediate billet is ≤36 mm.

[0016] In some embodiments of the present application, a four-roll seven-stand finishing mill is used to perform finish rolling on the intermediate billet, which also includes: keeping the small vertical rolls and the finishing entrance side guide of the finishing mill under stress, and controlling the wedge shape of the strip during the finishing rolling process to be 0μm±20μm.

[0017] In some embodiments of the present application, the finish rolling of the intermediate billet by a four-roller seven-stand finishing mill also includes: setting the rolling convexity to 20 μm to 40 μm.

[0018] In some embodiments of the present application, when a four-roll seven-stand finishing mill is used to finish-roll the intermediate billet, it also includes: closing the inter-frame water of the front frame, allowing the inter-frame water of the rear frame to be normally opened, the cooling water of the working rolls of the front frame rolls of the finishing mill is normally opened, the cooling water of the working rolls of the rear frame rolls of the finishing mill is normally opened, and the water flow rate of the cooling water of the working rolls of the rear frame rolls is reduced by 20% to 30%.

[0019] In some embodiments of the present application, a four-roller seven-stand finishing mill is used to perform finishing rolling on the intermediate billet in an increasing speed rolling manner.

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

[0021] In some embodiments of the present application, coiling the cooled steel also includes: for steel coils with a strip thickness of ≤3.0 mm, after unloading, the steel coils are rotated 90° to the ground roller position, and then transferred to the walking beam, and finally the high-carbon tool steel coils are stored in the insulation pit for slow cooling to avoid being placed in the lower layer and causing flattening.

[0022] The method for preventing the 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 concentrated 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 of the laminar cooling and the front and rear side sprays 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 the uneven phase change and volume expansion of the finished steel during the coiling and warming process, preventing loosening and slipping between layers of the strip steel coil of the high-carbon tool steel, thereby preventing the occurrence of flat coils. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solution 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 diagram of a high carbon tool steel coil rolled by a conventional hot rolling line in the prior art.

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

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

[0027] Explanation of the accompanying drawings: 1. Flat-rolled steel coil; 2. Steel coil that has not been flat-rolled. 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 only 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 article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the statement "include..." do not exclude the presence of other identical elements in the process, method, article or device including the elements.

[0030] As described in the background technology section, the carbon content of high carbon tool steel often exceeds 0.65wt.%. Due to the high carbon content, the proportion of cementite in this steel structure is high. The final structure of hot rolling is generally pearlite + cementite. The hardness of cementite is very high, the plasticity and toughness are almost zero, and the rollability is poor. Therefore, the production difficulty of high carbon steel is much higher than that of general medium and low carbon steel. High carbon tool steel is more sensitive to temperature during hot rolling, the rolling force is large, the convexity, plate shape, etc. are not controlled, and there are more carbides participating in phase transformation in the structure during cooling. The structure of the steel material often changes from austenite to pearlite, and then is warmed up to austenite, and finally gradually cools down to form the final structure of pearlite + cementite. In the repeated phase transformation process, the steel coil will produce uneven volume expansion. When the thickness is thin, the strip steel is difficult to bear its own weight, resulting in the shape of the steel coil being elliptical due to its own gravity.

[0031] The inventors of the present application have discovered through research that the generation of flat coils is mainly due to the uneven cooling of the strip 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, loosening and slippage between layers of the steel coil, and the greater the weight of the steel coil, the more obvious the flattening phenomenon.

[0032] In order to solve the problem of flattening of high carbon tool steel coils after hot rolling in the prior art, the embodiment of the present application provides a method for preventing flattening of high carbon tool steel hot rolled coils, which is suitable for thin-gauge high carbon tool steel coils. By optimizing the hot rolling process, cooling process, and coiling process, the subsequent flattening, rework, and scrapping are reduced, and the production cost is greatly reduced. Among them, steel with a C content of 0.65wt.% to 1.35wt.% is called high carbon steel.

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

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

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

[0036] S2, making the furnace entry temperature of the continuous casting slab ≥300°C, sending the continuous casting slab into the heating furnace by hot charging, heating the continuous casting slab with the furnace entry temperature ≥300°C, making the furnace exit temperature of the continuous casting slab 1150°C to 1250°C, and obtaining the first slab;

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

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

[0039] S5, laminar cooling the finished rolled steel by front-stage centralized cooling, and closing the final fine-tuning water of the laminar cooling and the front-side spray and the rear-side spray corresponding to the fine-tuning water, so that the temperature of the finished rolled steel is reduced to the first target temperature and enters the phase change point, thereby obtaining a cooled steel;

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

[0041] The method for preventing the 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 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 the uneven phase change and volume expansion of the finished steel during the coiling and warming process, preventing loosening and slippage between layers of the steel coil, thereby preventing the flattening of the hot-rolled coils of high-carbon tool steel.

[0042] It should be noted that hot charging refers to keeping the continuous casting slab at a certain temperature before loading it into the heating furnace. In the embodiments of the present application, the continuous casting slab needs to be kept at a temperature of ≥300°C before loading it into the heating furnace. Because high carbon steel is sensitive to temperature, cracks are prone to occur after the slab is cooled, and it is easy to crack or break during rough rolling. By using hot charging to send the continuous casting slab into the heating furnace for heating, cracking or breaking during rolling caused by cracks in the continuous casting slab can be effectively avoided. 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 220 mm to 240 mm. For example, the thickness of the continuous casting slab can also be 225 mm, 230 mm, 235 mm, or 238 mm.

[0044] In some embodiments of the present application, the continuous casting slab with a furnace temperature of ≥300°C is heated in a heating furnace for 200 to 300 minutes. For example, the heating treatment time may be 220 minutes, 240 minutes, 250 minutes, 260 minutes, 270 minutes, 280 minutes, 290 minutes, or any time therebetween, so that the continuous casting slab is fully heated to the temperature required for rough rolling.

[0045] In some embodiments of the present application, a four-roller rough rolling mill is used to perform 5 to 7 rough rolling passes on the first slab. The rough rolling mill is a complete set of equipment, and one pass is one pass of the continuous casting slab using the entire rough rolling mill. The thinner the rolling thickness of the preset intermediate slab is, the more rough rolling passes are.

[0046] In some embodiments of the present application, in the step of rough rolling the first slab using a four-roll rough rolling mill, the thickness of the intermediate billet for rolling strip steel with a thickness of more than 3.0 mm is set to 30 mm to 56 mm; for rolling strip steel coils with a thickness of less than 3.0 mm, the intermediate billet is put into a hot coil box for coiling, and the preset thickness of the intermediate billet is ≤36 mm. Corresponding to the thickness of the strip steel to be rolled, the thinner the thickness of the finished strip steel is, the thinner the thickness of the intermediate billet is.

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

[0048] In some embodiments of the present application, the finishing rolling of the intermediate billet by using a four-roller seven-stand finishing mill group also includes: keeping the small vertical roll of the finishing mill group and the inlet side guide of the finishing mill group under stress to improve the centering of the finished steel during the finishing rolling process; and controlling the wedge shape of the strip steel during the finishing rolling process to be 0μm±20μm to prevent the wedge shape from being too large and the strip steel from shifting to one side, resulting in poor coil shape. Thus, high-carbon tool steel with good coil shape can be stably and mass-produced to avoid uneven cooling of the hot-rolled steel coil of high-carbon tool steel during the subsequent cooling process due to excessive wedge shape, thereby avoiding uneven phase change and uneven volume expansion or contraction.

[0049] It should be noted that the small vertical rollers and the entrance side guides of the finishing mill are both part of the finishing mill. The vertical rollers are pairs of rollers located on both sides of the entrance of the finishing mill. They will squeeze the edges of the slabs that serve as intermediate billets to a certain extent, so that the intermediate billets can be finished rolled in the middle of the rollers of the finishing mill as much as possible. They are generally used to control the width of the strip of finished steel to improve the centering of the finished steel during the finishing rolling process. The entrance side guides of the finishing mill are generally located in front of the vertical rollers. When the intermediate billet of the rolled strip deviates, it will come into contact with the entrance side guides, and the intermediate billet will be transported to the middle of the rollers of the finishing mill under the guiding action of the entrance side guides. That is, the entrance side guides only correct the position and direction of the slabs that serve as intermediate billets to prevent the intermediate billets from deviating. Because the small vertical rollers and the entrance side guides both act on the edges of the intermediate billet, when both the small vertical rollers and the entrance side guides of the finishing mill are kept in a lightly stressed state, it means that the strip of finished steel produced from the intermediate billet through finish rolling is transported forward along a straight line and has good centering.

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

[0051] Exemplarily, the wedge shape of the steel strip during the finish rolling process is controlled to be 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-roller seven-stand finishing mill is used to perform finish rolling on the intermediate billet, and the rolling convexity is set to 20 μm to 40 μm.

[0053] It should be noted that rolling crown refers to the crown of the finished steel, which generally refers to the difference between the thickness in the middle of the strip width of the finished steel and the average thickness of the two sides. In the embodiment of the present application, the parameter C40 is used to characterize the crown of the finished steel, which specifically refers to the difference between the thickness in the middle of the strip width of the finished steel and the average thickness at 40 mm on both sides.

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

[0055] The method for preventing the flattening of the hot-rolled steel coil of the high-carbon tool steel in the embodiment of the present application optimizes the hot rolling process and sets the rolling crown in the finishing rolling to 20μm to 40μm, so as to increase the friction between the strip steel layers of the high-carbon tool steel coil, prevent the strip steel layers of the steel coil from loosening and slipping, and prevent the steel coil from flattening during the subsequent placement process, and cooperate with the adjustment of the subsequent cooling process of the finishing steel to further prevent the flattening of the hot-rolled steel coil of the high-carbon tool steel. And the finishing steel that meets the rolling crown will not cause serious scratches and scratches on the surface of the strip steel when unwinding.

[0056] In some embodiments of the present application, when a four-roller seven-stand finishing mill is used to finish-roll the intermediate billet, the following steps are further included: closing the inter-frame water of the front frame of the finishing mill, opening the inter-frame water of the rear frame normally, opening the working cooling water of the front frame rolls of the finishing mill normally, opening the working cooling water of the front frame rolls of the rear frame of the finishing mill normally, and reducing the water flow of the working cooling water of the rear frame rolls by 20% to 30%. For example, the water flow of the working cooling water of the rear frame rolls of the finishing mill can be reduced by 22%, 24%, 25%, 27%, 28%, and 29%.

[0057] It should be noted that the four-roller seven-stand finishing mill used in this application has seven stands, the front stands refer to the first and second stands in the direction of finishing rolling of the intermediate billet, and the rear stands refer to the third to seventh stands except the first and second stands. Among them, the water between the stands is used to spray the finishing steel to reduce the temperature of the finishing steel during the finishing rolling process. The cooling water of the working roll of the roll is located inside the roll, which is used to cool the roll to keep the roll in good shape and strength and to perform finishing rolling on the intermediate billet.

[0058] In some embodiments of the present application, a four-roller seven-stand finishing mill is used to finish-roll the intermediate billet, and the inter-stand water of the front stand of the finishing mill is reduced, the inter-stand water of the rear stand is opened normally, and the cooling water of the working roll of the front stand of the finishing mill is opened normally, and the flow rate of the cooling water of the working roll of the front stand of the finishing mill is increased to ensure that the rollers of the front stand have a better cooling effect, reduce the uneven wear and deformation of the working rolls of the front stand of the finishing mill, so as to better control the rolling crown of the smaller rolled steel. The cooling water flow rate of the working roll of the rear stand is appropriately reduced by 20% to 30%, reducing the tail temperature drop of the strip of the finished steel, while reducing the rolling force of the rear stand, and improving the stability of the finishing rolling.

[0059] In some embodiments of the present application, a four-roller seven-stand finishing mill is used to perform finish rolling on the intermediate billet in a speed-up rolling manner. Speed-up rolling means that the rolling speed will gradually increase after the strip is threaded. Because the temperature of the head and tail of the continuous casting slab is uniform after heating, but the head is rolled first and then the tail during rough rolling. Therefore, when the intermediate billet is rolled at a uniform speed, the temperature of the tail will be significantly lower than that of the head. The speed-up rolling can avoid the increase in rolling force caused by the decrease in the temperature of the tail of the intermediate billet of the rolled strip.

[0060] In the embodiments of the present application, the laminar cooling of the finished steel is performed by using the front-stage centralized cooling method, which means that the order of opening the cooling water in the laminar cooling section is 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 steel reach the required coiling temperature.

[0061] In some embodiments of the present application, the temperature of the finished rolled steel is reduced to a first target temperature, wherein the first target temperature is (Ar1 phase transformation temperature - 30°C) to (Ar1 phase transformation temperature - 50°C) of a high carbon tool steel. By reducing the temperature of the finished rolled steel to within this temperature range, the strip of the finished rolled steel can enter the phase transformation point in advance. The Ar1 phase transformation temperature refers to the phase transformation temperature at which austenite begins to transform into pearlite.

[0062] In the embodiments of the present application, the Ar1 phase transition temperature is the phase transition 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 warm up after coiling, resulting in the actual control of the cooling temperature of the finished rolled steel according to the Ar1 phase transition temperature, so that many structures of the cooled steel have not been transformed into pearlite after coiling, and will gradually undergo phase transitions later, resulting in flat coils. Therefore, the present application creatively controls the first target temperature at (Ar1 phase transition temperature - 30°C) to (50°C of the Ar1 phase transition temperature), so that the phase transition from austenite to pearlite in the cooled steel can be completed as much as possible, and no phase transition will occur later, so as to avoid volume changes caused by subsequent phase transitions and the resulting flat coils.

[0063] In some embodiments of the present application, the final fine-tuning water of laminar cooling and the corresponding front and rear sprays 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 fine-tuning water is generally the last group of cooling water manifolds for 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 fine-tuning water to blow away the last fine-tuning water on the finished steel. Shut down the last fine-tuning water for laminar cooling, that is, shut down the cooling water sprayed by the last group of manifolds, and 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 side spray and the rear side spray can reduce the uneven cooling of the finished steel in the width direction caused by the side spray, further improve the uniformity of the structure 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 during the coiling process is increased by 10% to 20% to increase the coiling tightness of the inner ring of the strip steel in the steel coil to obtain a hot-rolled steel coil of high-carbon tool steel. Exemplarily, the coiling temperature can 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, 690°C.

[0066] The method for preventing the flattening of hot-rolled high-carbon tool steel coils disclosed in the present application closes the inter-frame water of the front frame, opens the inter-frame water of the rear frame, and normally opens the cooling water of the rolling mill working rolls of the front frame and the rear frame, and combines the simultaneous operation with adjusting the flow rate of the cooling water of the rolling mill working rolls of the rear frame, 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 for preventing the flattening of hot-rolled high-carbon tool steel coils of the present application combines measures such as reducing various cooling water of the finishing rolling unit, selecting a laminar cooling mode, and controlling the water flow rate of the side spray, 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 cooling of the strip 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 strip 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 on 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, thereby changing the stress condition of the hot-rolled steel coil that has just come off the line, and having 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 hot-rolled high-carbon tool steel coils in the embodiment of the present application does not require improvement to existing equipment, has good operability, can prevent flattening of hot-rolled high-carbon tool steel coils, greatly improves the coil shape qualification rate of high-carbon tool steel coils, and reduces the flat coil rework rate of hot-rolled high-carbon tool steel coils from 38.9% before 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 various tools such as hardware and knives, 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 a hot-rolled steel coil of high carbon tool steel (SK85) from flattening, comprising:

[0076] Provide SK85 high carbon tool steel continuous casting slabs, the thickness of the continuous casting slabs is 240mm, and the composition is the above chemical composition;

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

[0078] The first slab with the surface iron oxide scale removed is subjected to 7 passes of rough rolling by a four-roll rough rolling unit, with the final rough rolling temperature being 1090°C, to obtain an intermediate slab with a thickness of 46 mm;

[0079] The intermediate billet is finished by a four-roller seven-stand finishing mill. The finishing temperature is 900℃ and the rolling crown is set to 25μm to increase the friction between the steel strip layers of the steel coil. The small vertical rolls of the finishing mill and the inlet side 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 the excessive wedge, resulting in 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 that the working rolls have a good 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 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%, 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;

[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 the front-stage centralized cooling method, so that the temperature of the finished steel is reduced to the first target temperature, that is, 640℃, so that the strip of the finished steel enters the phase change point in advance. At the same time, the final fine-tuning water of the laminar cooling and the front and rear sprays corresponding to the fine-tuning water of this section are closed to ensure that the strip has a sufficiently long air cooling time after 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 to obtain cooled steel;

[0081] The steel cooled to 640℃ is coiled, and the tension of the cooled steel is increased by 20% during the coiling process to increase the tightness of the inner ring of the strip in the steel coil, and obtain the hot-rolled steel coil of high-carbon tool steel. For steel coils with a strip thickness of ≤3.0mm, 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, and avoid being placed on the lower layer and causing flattening.

[0082] like Figure 3 As shown, the hot rolled steel coil of high carbon tool steel SK85 rolled by the method for preventing flattening of 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.

[0083] Example 2

[0084] 9SiCr is a high-carbon tool steel, widely used in various tools such as hardware and knives, 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 a high carbon tool steel (9SiCr) hot rolled steel coil from flattening, comprising:

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

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

[0088] The first slab with the surface iron oxide scale removed is subjected to 7 passes of rough rolling by a four-roll rough rolling unit, with the final rough rolling temperature being 1090°C, to obtain an intermediate slab with a thickness of 45 mm;

[0089] The intermediate billet is finished by a four-roller seven-stand finishing mill. The finishing temperature is 910℃ and the rolling crown is set to 25μm to increase the friction between the steel strip layers of the steel coil. The small vertical rolls of the finishing mill and the inlet side 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 the excessive wedge, resulting in 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. 2) inter-stand water (ISC), and open the inter-stand water of the rear stand except F1 and F2 normally, and open the cooling water of the working roll of the front stand of the finishing mill normally to ensure that the working roll has a good cooling effect and reduce uneven wear and deformation; the cooling water of the working roll of the rear stand of the finishing mill is turned on, and the water flow of the cooling water of the working roll 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, so as to obtain 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 the front-stage centralized cooling method, so that the temperature of the finished steel is reduced to the first target temperature, that is, 690℃, so that the strip of the finished steel enters the phase change point in advance. At the same time, the final fine-tuning water of the laminar cooling and the front and rear sprays corresponding to the fine-tuning water of this section are closed to ensure that the strip has a sufficiently long air cooling time after 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 to obtain cooled steel;

[0091] The steel cooled to 690℃ is coiled, and the tension of the cooled steel is increased by 20% during the coiling process to increase the tightness of the inner ring of the strip in the steel coil, and obtain the hot-rolled steel coil of high-carbon tool steel. For steel coils with a strip thickness of ≤3.0mm, 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, and avoid being placed on the lower layer and causing flattening.

[0092] The hot-rolled steel coil of high-carbon tool steel 9SiCr rolled by 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 the same batch and specification of continuous casting slabs as Example 2, including: 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., and its chemical composition is as follows: 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. Its processing method is somewhat different from the method of preventing the flattening of high-carbon tool steel hot-rolled steel coils in this application, including:

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

[0096] The continuous casting slab is sent into the heating furnace by hot charging, and the furnace-out temperature of the continuous casting slab is 1200° C. The total furnace-in time is 260 minutes, and the first slab is obtained; after the first slab is taken out of 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 with the surface iron oxide scale removed is subjected to five rough rolling passes by a four-roll rough rolling unit, with the final rough rolling temperature being 1120° C., to obtain an intermediate slab with a thickness of 55 mm.

[0098] The intermediate billet is finished by a four-roller seven-stand finishing mill, the finishing temperature is 920℃, and the rolling crown is set to 40μm; there is no restriction on the wedge requirements, all kinds of cooling water between the stands are turned on normally, the flow rate is automatically set by the system, and the rolling speed is automatically set at the second level;

[0099] The coiling temperature is set to 730℃, and the finish-rolled steel is cooled by central sparse cooling. 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 conventional settings.

[0100] As shown in Table 1, the hot-rolled steel coils of high carbon tool steel 9SiCr produced by the method of Comparative Example 1 had poor coil shape and serious flat coils, and 2 coils with a thickness of less than 3.0 mm were scrapped.

[0101] Table 1

[0102] Comparative Cases 9SiCr process comparison results Number of rolls with thickness below 3.0mm Number of flat rolls Number of papers downgraded or scrapped Example 2 Test Paper 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 preventing the flattening of high-carbon tool steel hot-rolled coils in the present application can effectively prevent the flattening of high-carbon tool steel hot-rolled coils without the need for rework or cutting and scrapping, thereby reducing the losses caused by flat coils.

[0104] The above is only a specific implementation method of the present application. Those skilled in the art can clearly understand that for the convenience and simplicity 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 protection scope of the present application is not limited to this. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed in the present application, and these modifications or replacements should be included in the protection scope of the present application.

Claims

1. A method for preventing flattening of a hot-rolled high-carbon tool steel coil, characterized in that: include: A continuous casting slab of high carbon tool steel is provided, wherein the continuous casting slab comprises the following components in percentage by mass: C: 0.65wt.% to 1.35wt.%, Si: 0wt.% to 1.6wt.%, Mn: 0wt.% to 1.2wt.%, Cr: 0wt.% to 1.2wt.%, P≤0.030wt.%, S≤0.030wt.%, and the remainder is iron and unavoidable impurities; The continuous casting slab is fed into a heating furnace with a temperature of ≥300°C, and the continuous casting slab is fed into a heating furnace by hot charging, and the continuous casting slab with a temperature of ≥300°C is heated to a temperature of 1150°C to 1250°C to obtain a first slab; The first slab is rough-rolled by a four-roll rough rolling mill, and the final rough rolling temperature is 1050° C. to 1150° C. to obtain an intermediate slab of a preset thickness; The intermediate billet is subjected to finish rolling by a four-roller seven-stand finishing rolling mill, with the finishing temperature of the finishing rolling being 800° C. to 950° C., to obtain a finished rolled steel product; The finished rolled steel is laminar-cooled by front-stage centralized cooling, and the final fine-tuning water of the laminar cooling and the front-side spray and the rear-side spray corresponding to the fine-tuning water are closed, so that the temperature of the finished rolled steel is reduced to the first target temperature and enters the phase change point, thereby obtaining a cooled steel; 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.

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 heating time of the continuous casting slab with the furnace entry temperature ≥300°C in the heating furnace is 200 minutes to 300 minutes.

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

4. The method according to claim 1, characterized in that: 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, characterized in that The method of performing finish rolling on the intermediate billet by using a four-roller seven-stand finishing mill group also includes: keeping the small vertical rolls of the finishing mill group and the inlet side guide of the finishing mill group under stress, and controlling the wedge shape of the strip steel to be 0μm±20μm during the finishing rolling process.

6. The method according to claim 1, characterized in that The method of finishing rolling the intermediate billet by using a four-roller seven-stand finishing mill group also includes: setting the rolling convexity to 20 μm to 40 μm.

7. The method according to claim 1, characterized in that The method of using a four-roller seven-stand finishing mill group to finish roll the intermediate billet also includes: closing the inter-frame water of the front frame and allowing the inter-frame water of the rear frame to be normally opened; the cooling water of the working rolls of the front frame rolls of the finishing mill group is normally opened, the cooling water of the working rolls of the rear frame rolls of the finishing mill group is normally opened, and the water flow rate of the cooling water of the working rolls of the rear frame rolls is reduced by 20% to 30%.

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

9. The method according to claim 1, characterized in that: The temperature of the finish-rolled steel material is lowered to a first target temperature, wherein the first target temperature is (Ar1 phase transformation temperature-30° C.) to (Ar1 phase transformation temperature-50° C.) of a high carbon tool steel grade.

10. The method according to claim 1, characterized in that The coiling of the cooled steel also includes: for steel coils with a strip thickness of less than 3.0 mm, after unloading, the steel coils are rotated 90 degrees to the ground roller position and then transferred to the walking beam, and finally the steel coils of high carbon tool steel are stored in the insulation pit for slow cooling.

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